WEBVTT Kind: captions Language: en-US 00:00:03.149 --> 00:00:05.600 Okay. Good morning, everyone. 00:00:05.600 --> 00:00:09.900 And thanks for attending this week’s ESC seminar. 00:00:11.080 --> 00:00:14.900 I wanted to remind everyone a few quick things. 00:00:14.900 --> 00:00:18.540 Once again, when you join, please mute your microphone 00:00:18.540 --> 00:00:21.400 and turn off your video. 00:00:22.620 --> 00:00:27.960 Reminder to check if you’re on the VPN, and get off if you’re on the VPN. 00:00:29.720 --> 00:00:35.880 For today’s seminar, as usual, we’ll take questions at the end in the chat window. 00:00:35.880 --> 00:00:40.520 So if you think of questions as you go along, you can use the chat. 00:00:40.520 --> 00:00:45.120 And then, at the end, as questions – as we do more questions, you can 00:00:45.120 --> 00:00:48.540 also turn off your video – turn on your video and 00:00:48.540 --> 00:00:52.690 unmute yourself to ask the question to our speaker directly. 00:00:52.690 --> 00:00:58.470 Finally, I wanted to announce that next week, we will have – our speaker 00:00:58.470 --> 00:01:02.300 is Professor Eduardo Miranda from Stanford University. 00:01:02.300 --> 00:01:08.140 So next week, on June 17th, at 10:30 Pacific, we’ll have 00:01:08.140 --> 00:01:12.660 Professor Miranda give next week’s seminar. 00:01:12.660 --> 00:01:18.180 And with that, I will pass the microphone off to Steve DeLong 00:01:18.180 --> 00:01:21.820 to introduce this week’s speaker. Thank you. 00:01:21.820 --> 00:01:24.620 - Hi, everyone. Glad you joined us today. Thanks for that. 00:01:24.620 --> 00:01:28.560 It’s my pleasure to introduce Chad Trexler as our seminar speaker 00:01:28.560 --> 00:01:32.520 this week. Chad just joined the USGS a few weeks ago. 00:01:32.520 --> 00:01:34.850 Chad and I actually haven’t seen each other face to face since, 00:01:34.850 --> 00:01:37.540 I think, like, last November at GSA or something. 00:01:37.540 --> 00:01:40.580 So a weird time to join the USGS, but certainly glad he did. 00:01:40.580 --> 00:01:43.960 Chad comes to us from getting his Ph.D. at the University 00:01:43.960 --> 00:01:48.360 of California-Davis in 2018. before that, he got his undergrad 00:01:48.360 --> 00:01:52.600 at Whitman College up in Washington in 2011. 00:01:52.600 --> 00:01:56.299 For the past couple years, Chad has been teaching geology 00:01:56.299 --> 00:02:01.500 at Ohio Wesleyan University. And I first became aware of Chad 00:02:01.500 --> 00:02:06.750 and his work after the 2014 Napa earthquake, when he and Alex Morelan, 00:02:06.750 --> 00:02:11.719 who is now at the California Geological Survey, went out in the – you know, 00:02:11.719 --> 00:02:15.349 in the hours right after that earthquake and collected some novel data 00:02:15.349 --> 00:02:20.650 using low-cost, super easy ground-based photogrammetry, 00:02:20.650 --> 00:02:24.160 which is now a paper in GRL. 00:02:25.100 --> 00:02:28.400 And so Chad’s got a lot of experience on multiple time scales. 00:02:28.400 --> 00:02:32.880 And what he’s going to talk about today is some of his Ph.D. research. 00:02:32.880 --> 00:02:37.400 And the title of his talk is Exploring the Transition from Subduction to 00:02:37.409 --> 00:02:42.019 Slab Breakoff – Structural Insights from the Western Greater – 00:02:42.019 --> 00:02:44.409 Western Greater Caucasus Mountains. 00:02:44.409 --> 00:02:46.560 So, take it away, Chad. Thanks. 00:02:47.140 --> 00:02:49.120 - Thanks, Steve. 00:02:51.120 --> 00:02:55.140 [Silence] 00:02:55.320 --> 00:02:58.820 And thank you all for joining in today. 00:02:58.820 --> 00:03:01.880 I know there’s a lot of things going on right now – some important 00:03:01.880 --> 00:03:04.820 conversations, including some that are not science-related. 00:03:04.820 --> 00:03:10.719 And so I appreciate you taking the time to listen to something a little different 00:03:10.719 --> 00:03:14.529 than maybe is normal for this particular seminar. 00:03:14.529 --> 00:03:20.079 As Steve mentioned, I did a lot of my Ph.D. work in the 00:03:20.080 --> 00:03:24.340 Greater Caucasus Mountains, mostly in the Republic of Georgia. 00:03:24.340 --> 00:03:28.780 And that’s what I was hoping to share with you today. 00:03:28.780 --> 00:03:32.489 So this is, as I mentioned, part of my Ph.D. work. 00:03:32.489 --> 00:03:35.620 So collaborators on this work include Eric Cowgill, who was my 00:03:35.620 --> 00:03:39.800 Ph.D. adviser; Dylan Vasey, who is currently a Ph.D. student in the 00:03:39.800 --> 00:03:42.900 department there; along with Nathan Niemi and the University of Michigan 00:03:42.900 --> 00:03:49.200 and an assortment of collaborators in Georgia, most notably, Tea Godoladze. 00:03:50.740 --> 00:03:54.139 So for those of you who aren’t particularly familiar with the Greater 00:03:54.139 --> 00:03:59.139 Caucasus, I figured I’d start with a little bit of geographic setting here. 00:03:59.139 --> 00:04:03.620 So this map is one of the Alpine-Himalayan Orogenic Belt. 00:04:03.620 --> 00:04:07.329 It’s a mountain belt that wraps nearly halfway around the world, 00:04:07.329 --> 00:04:13.589 starting in the Alps in central Europe and continuing through much of the 00:04:13.589 --> 00:04:18.669 Middle East and all the way around to the Himalaya between India and Asia. 00:04:18.669 --> 00:04:23.400 The Greater Caucasus [background noises] middle … 00:04:25.340 --> 00:04:27.120 … outlined here in the yellow box. 00:04:27.140 --> 00:04:31.140 And that’s where we’re going to zoom in for the next slide. 00:04:31.860 --> 00:04:33.400 Maybe not quite the next slide. 00:04:33.419 --> 00:04:35.800 We will zoom in on that box at some point. 00:04:35.800 --> 00:04:41.150 So the Greater Caucasus themselves represent potentially an earlier stage 00:04:41.150 --> 00:04:45.160 of continental collision than anywhere else in the Alpine-Himalayan Belt. 00:04:45.160 --> 00:04:48.720 This is for two reasons. One is that the uplift 00:04:48.729 --> 00:04:52.780 actually starts more recently. So as, for example, as compared to 00:04:52.780 --> 00:04:57.009 the Himalaya, which started uplifting about 50 to 30 million years ago, 00:04:57.009 --> 00:05:02.480 the Greater Caucasus seemed to start uplifting more like 30 million years ago. 00:05:02.480 --> 00:05:06.080 And then they also have a much slower convergence rate 00:05:06.080 --> 00:05:09.100 overall than the Himalaya. 00:05:09.100 --> 00:05:13.700 Again, to compare, the Himalaya are converging – or, India is converging 00:05:13.710 --> 00:05:16.169 with Eurasia at about 50 millimeters a year. 00:05:16.169 --> 00:05:19.699 And in the Arabia/Eurasia collision, the rate is more like 20 millimeters a year. 00:05:19.700 --> 00:05:25.860 So both of these combine to give us an earlier snapshot in time than we see 00:05:25.860 --> 00:05:30.520 elsewhere along the collisional belt. And this means they’re giving us 00:05:30.520 --> 00:05:34.820 an earlier snapshot in the evolution of such orogenic systems. 00:05:36.080 --> 00:05:37.880 All right. So now we’re zoomed in on that yellow box. 00:05:37.890 --> 00:05:42.190 This is a simplified tectonic map of the Greater Caucasus. 00:05:42.190 --> 00:05:45.310 So the Black Sea is in the lower-left corner. 00:05:45.310 --> 00:05:50.849 The Caspian is in the upper right. And Russia is sort of the northern 00:05:50.849 --> 00:05:54.860 quadrant – northwestern quadrant of this map. 00:05:54.860 --> 00:05:57.460 And then Georgia is in the west. Azerbaijan is in the east. 00:05:57.479 --> 00:06:01.210 Then we have little bits of Turkey, Armenia, and Iran along the 00:06:01.210 --> 00:06:05.349 southern border. So the different colors here are showing you broad tectonic 00:06:05.349 --> 00:06:09.810 stratigraphic units. And there’s a lot of different things going on here, 00:06:09.810 --> 00:06:12.520 including some very small text, but I wanted to highlight a few things 00:06:12.520 --> 00:06:15.300 specifically so that we are sort of all starting on the same page 00:06:15.310 --> 00:06:19.300 as we dig into this orogen a little bit. 00:06:19.300 --> 00:06:23.530 So the northern margin of the Greater Caucasus is something 00:06:23.530 --> 00:06:26.449 called the Scythian Platform. And this is, for our purposes, 00:06:26.449 --> 00:06:33.600 stable Eurasia. So this is the structural backstop that Arabia is colliding with. 00:06:33.600 --> 00:06:37.280 Exposed in the Caucasus themselves, we’re looking primarily at 00:06:37.289 --> 00:06:40.340 Mesozoic-age shelf sedimentary rocks. 00:06:40.340 --> 00:06:48.550 So carbonates and [garbled audio] turbidite-type deposits. 00:06:48.550 --> 00:06:51.980 Also wanted to point out the blob of red there in the west. 00:06:51.980 --> 00:06:55.440 It’s labeled Variscan Basement. That’s Variscan-age crystalline 00:06:55.440 --> 00:07:00.200 basement of varying types, but it seems to be, you know, the – 00:07:00.200 --> 00:07:02.180 sort of the crystalline core of the ring. 00:07:02.200 --> 00:07:07.380 Just interesting only exposed in the west and not further east along the range. 00:07:08.290 --> 00:07:13.800 The Greater Caucasus themselves, the high peaks in the range are 00:07:13.819 --> 00:07:16.810 made up of rocks that were deposited in what is known as the Greater 00:07:16.810 --> 00:07:20.680 Caucasus Basin, which was a marine basin in the Mesozoic age. 00:07:20.680 --> 00:07:24.129 There are some volcanoclastic and volcanic rocks exposed in the west, 00:07:24.129 --> 00:07:29.000 but primarily this portion of the range is deep marine, 00:07:29.000 --> 00:07:31.200 distal sedimentary deposits. 00:07:31.200 --> 00:07:35.280 And this is where most of the deformation is as well. 00:07:36.800 --> 00:07:39.600 Moving further south, the southern flank of the range 00:07:39.600 --> 00:07:41.260 has an active fold/thrust belt. 00:07:41.270 --> 00:07:43.660 And this is where I spent most of my time working. 00:07:43.660 --> 00:07:47.380 This is where we’re going to focus a good chunk of our discussion today. 00:07:47.380 --> 00:07:51.520 There’s a fold/thrust belt that runs nearly the full length of the southern 00:07:51.520 --> 00:07:54.499 side of the Greater Caucasus, and this is where the active shortening is being 00:07:54.500 --> 00:07:57.600 accommodated, both in the west, in the Rioni Basin, where we’re going to 00:07:57.600 --> 00:08:02.800 focus, and also in the east in the Kura Basin where much of the 00:08:02.800 --> 00:08:05.860 prior work on active tectonics in the Caucasus region 00:08:05.860 --> 00:08:09.640 has been done by Adam Forte and others. 00:08:11.220 --> 00:08:16.420 And then finally, south of that foreland basin, there’s a region 00:08:16.420 --> 00:08:24.090 called the Lesser Caucasus. This is the mountain range to the 00:08:24.090 --> 00:08:29.289 south of the Greater Caucasus. And this range is primarily 00:08:29.289 --> 00:08:34.490 made up of arc volcanic rocks. So these are the southern margin 00:08:34.490 --> 00:08:38.620 of the Greater Caucasus basin in the Mesozoic as well as being 00:08:38.620 --> 00:08:43.200 the southern edge of the Caucasus region in the modern setting. 00:08:44.900 --> 00:08:48.880 So, in terms of active tectonics, and the reason we’re thinking about 00:08:48.880 --> 00:08:54.700 the Caucasus today, the reason they’re important in the region is 00:08:54.700 --> 00:08:58.760 that they are accommodating nearly all of the orogen-perpendicular 00:08:58.760 --> 00:09:04.910 shortening between Arabia and Eurasia. So I’ve got a little schematic map here 00:09:04.910 --> 00:09:11.740 in the lower right showing, more or less, what I mean when I say that. 00:09:13.000 --> 00:09:15.860 The convergence between Arabia and Eurasia is somewhat oblique, 00:09:15.860 --> 00:09:21.440 and it’s partitioned almost completely between thrusting in the Caucasus 00:09:21.450 --> 00:09:27.070 region of the northern edge and then lateral extrusion of the 00:09:27.070 --> 00:09:30.020 Anatolian Plateau in Turkey along the North Anatolian Fault 00:09:30.020 --> 00:09:33.310 and East Anatolian Fault. 00:09:33.310 --> 00:09:36.440 This is supported by some geologic data, 00:09:36.440 --> 00:09:41.180 but it’s primarily based on geodetic data. 00:09:43.200 --> 00:09:46.780 So all this is to say that the Greater Caucasus, although they are located 00:09:46.780 --> 00:09:50.860 at the northern edge of this margin inclusion zone are playing a significant 00:09:50.860 --> 00:09:54.680 role in how it works currently. But there are still some pretty 00:09:54.680 --> 00:10:00.960 significant questions about how that role evolves through time and 00:10:00.960 --> 00:10:06.240 how it is – sort of manifest in the modern mountain belt. 00:10:07.600 --> 00:10:13.240 The biggest question, it turns out, revolves around this Greater Caucasus 00:10:13.240 --> 00:10:18.340 Basin – this Mesozoic-age marine basin that has now been inverted to form the 00:10:18.340 --> 00:10:22.720 Greater Caucasus Mountains, which are the largest mountain range in Europe. 00:10:23.680 --> 00:10:27.940 There are two different end member models that have been proposed. 00:10:27.950 --> 00:10:33.270 On one end, we have a large basin model, or sometimes referred to as 00:10:33.270 --> 00:10:38.080 the subduction model, which is the one that basically says that this 00:10:38.080 --> 00:10:42.640 basin was significant in width – you know, a couple hundred 00:10:42.640 --> 00:10:44.500 kilometers, maybe more. 00:10:44.510 --> 00:10:47.840 And that the growth of the mountain range – the growth of 00:10:47.840 --> 00:10:51.120 the Greater Caucasus themselves is driven by closure of that basin’s 00:10:51.120 --> 00:10:55.500 subduction beneath the Greater Caucasus, and, 00:10:55.500 --> 00:11:00.060 ultimately, continental collision and slab detachment. 00:11:00.070 --> 00:11:04.370 The other end member model that is used to describe this basin inversion 00:11:04.370 --> 00:11:08.520 into a mountain range we’ll refer to here as the small basin model. 00:11:08.520 --> 00:11:13.730 And this model is somewhat simpler and says that the basin was maybe 00:11:13.730 --> 00:11:17.120 not all that big to begin with – less than 100 kilometers across – 00:11:17.120 --> 00:11:20.640 and that, in order to build the modern Greater Caucasus, 00:11:20.640 --> 00:11:25.710 you simply shortened that. And so you don’t have to 00:11:25.710 --> 00:11:29.030 have subduction. You don’t have slab detachment. 00:11:29.030 --> 00:11:31.150 And the growth of the mountain range is basically 00:11:31.150 --> 00:11:35.310 just due to pure shear thickening of the crust. 00:11:35.310 --> 00:11:39.700 These models make some pretty significantly different predictions 00:11:39.700 --> 00:11:44.520 that allow us to test between them. One of those, as I mentioned, 00:11:44.520 --> 00:11:48.490 is the width of the basin – the Mesozoic-age basin that has 00:11:48.490 --> 00:11:52.980 been inverted. The large basin model requires that the basin was wide 00:11:52.980 --> 00:11:55.420 enough for subduction to have initiated. 00:11:55.420 --> 00:11:58.360 This number is a little bit squishy, but at minimum, has to be 00:11:58.360 --> 00:12:02.160 several hundred kilometers. In comparison, the small basin model 00:12:02.170 --> 00:12:03.970 has no minimum width requirement. 00:12:03.970 --> 00:12:08.420 The basin could be 100 kilometers, or even less, in width. 00:12:09.890 --> 00:12:13.680 The other significant prediction for our purposes today is that there 00:12:13.680 --> 00:12:21.060 actually should be a change in shortening rate 00:12:21.060 --> 00:12:24.480 [audio garbled] 00:12:24.480 --> 00:12:26.260 depending on which model you’re looking at. 00:12:26.260 --> 00:12:28.920 So, with the large basin model, we should see a decreased shortening 00:12:28.930 --> 00:12:32.800 rate associated with continental collision and the detachment 00:12:32.800 --> 00:12:36.730 of the subducting slab underneath the Greater Caucasus. 00:12:36.730 --> 00:12:41.210 The small basin model doesn’t have any such change in mechanism. 00:12:41.210 --> 00:12:45.710 And so, as a result, there should be a constant rate and time 00:12:45.710 --> 00:12:51.120 with no change in that shortening rate. 00:12:53.180 --> 00:12:56.120 [Silence] 00:12:56.120 --> 00:12:59.280 So, with those key differences, the nice thing about these two that I’ve 00:12:59.280 --> 00:13:03.860 highlighted is that these should both be recorded in the surface geology. 00:13:03.860 --> 00:13:10.860 Specifically, again, we’re looking for the signal for whether there’s a large 00:13:10.861 --> 00:13:16.430 magnitude of shortening and whether we see a change in rate of shortening 00:13:16.430 --> 00:13:21.080 through time that would be an indicator of slab detachment. 00:13:24.560 --> 00:13:27.580 So I’m going to reframe that slightly into a couple of motivating questions 00:13:27.580 --> 00:13:32.060 that will spiral around for the rest of my talk. 00:13:32.060 --> 00:13:35.640 Those questions are the following. One, what is the structural architecture 00:13:35.650 --> 00:13:39.510 of the range? Where are the faults? What do they look like? 00:13:39.510 --> 00:13:42.440 Two, how much shortening do they accommodate? 00:13:42.440 --> 00:13:48.080 And then, three, what is that rate, and does it change through time? 00:13:50.500 --> 00:13:57.640 To start this work, I initially just started poring over existing geologic maps. 00:13:57.640 --> 00:14:02.900 Most of the maps of the region were made in the ’50s and ’60s when Georgia 00:14:02.910 --> 00:14:05.830 was part of the Soviet Union. And the interesting thing about 00:14:05.830 --> 00:14:09.850 Soviet geologists is that, while they did a lot of impeccable biostratigraphy, 00:14:09.850 --> 00:14:13.860 they did not pay much attention to structural data. 00:14:13.860 --> 00:14:20.220 Here is a piece of a 1-to-200,000 scale geologic map of the Caucasus region. 00:14:20.220 --> 00:14:23.150 And it’s very small here, but the thing I want to point out is that, on this 00:14:23.150 --> 00:14:26.880 entire chunk of map, which is probably 100 kilometers by 00:14:26.880 --> 00:14:31.280 80 kilometers, there are two strike-and-dip symbols. 00:14:31.280 --> 00:14:34.540 They’re located down there on that little fold belt in the middle, 00:14:34.540 --> 00:14:37.320 and they don’t have numbers on them. 00:14:37.320 --> 00:14:44.540 So it’s pretty hard to get any useful data from these maps 00:14:44.540 --> 00:14:49.170 without going out into the field and collecting more. 00:14:50.940 --> 00:14:55.420 Similarly, the published cross-sections are minimally useful. 00:14:55.430 --> 00:14:57.820 Here’s an example. This is a cross-section that actually 00:14:57.820 --> 00:15:01.560 was published in 2011. And, while it does give you a sense of 00:15:01.560 --> 00:15:04.740 what things look like at the surface, there was no effort made to project 00:15:04.740 --> 00:15:09.170 things at depth or understand the geometries that are linking the 00:15:09.170 --> 00:15:14.920 structures that you see at the surface. So it’s pretty hard to interpret what that 00:15:14.920 --> 00:15:20.420 might mean for the actual architecture of the whole orogen and how it formed. 00:15:22.820 --> 00:15:28.420 Because of these shortcomings, the estimates of basin width that have been 00:15:28.430 --> 00:15:32.600 published – so the size of this Greater Caucasus Basin – are widely variable. 00:15:32.600 --> 00:15:35.340 They span an order of magnitude. 00:15:35.340 --> 00:15:39.840 Depending on what techniques were used, the shortest estimates 00:15:39.840 --> 00:15:46.110 are something like 25 kilometers of basin width based on limited structural 00:15:46.110 --> 00:15:52.740 data, to over 1,000 kilometers of basin width based on paleomag 00:15:52.740 --> 00:15:59.420 estimates of collisional shortening across the whole Caucasus system, 00:15:59.420 --> 00:16:00.960 including the Lesser Caucasus. 00:16:00.960 --> 00:16:07.140 So we’re pretty sure it’s between those two, but the estimates are quite variable. 00:16:08.220 --> 00:16:11.220 And it’s important to note that, as I previously mentioned, since there’s 00:16:11.230 --> 00:16:18.440 no structural data, the sort of baseline estimate of basin width from something 00:16:18.440 --> 00:16:24.430 like a balanced cross-section hasn’t been attempted yet for this orogen. 00:16:24.430 --> 00:16:27.830 And this – if such a thing were produced, would provide 00:16:27.830 --> 00:16:30.640 a minimum bound on basin width. 00:16:32.660 --> 00:16:35.860 Just to drive this point home even further, not only is this a problem for 00:16:35.860 --> 00:16:40.000 understanding how wide the basin was and whether subduction could have 00:16:40.000 --> 00:16:47.600 initiated it, we also need basin width in order to estimate total magnitude 00:16:47.610 --> 00:16:52.340 of shortening and therefore shortening rate over time. 00:16:52.340 --> 00:16:56.410 So getting a handle on how wide this basin was is really important 00:16:56.410 --> 00:17:01.720 for understanding the evolution of this mountain belt through time. 00:17:04.560 --> 00:17:07.640 So, moving forward, we’re going to focus on these three questions 00:17:07.640 --> 00:17:11.580 again that I introduced earlier. What’s the structural architecture? 00:17:11.580 --> 00:17:15.940 How much shortening is there? And how fast is it being accommodated? 00:17:15.940 --> 00:17:19.040 To look at that, we’re going to focus primarily on the central 00:17:19.050 --> 00:17:24.600 and southern Greater Caucasus. So that marine basin sedimentary 00:17:24.600 --> 00:17:29.950 rocks section and then the active fold/thrust belt to the south. 00:17:30.960 --> 00:17:34.180 And, in doing so, I’m going to focus on two traverses. 00:17:34.180 --> 00:17:37.230 So the Enguri Traverse along the Enguri River in western Georgia 00:17:37.230 --> 00:17:38.680 is where we’re going to spend most of our time. 00:17:38.680 --> 00:17:41.700 And then, depending on how the time is going toward the end, we’ll spend 00:17:41.700 --> 00:17:45.530 a little bit of time talking about the Aragvi and Terek Traverse 00:17:45.530 --> 00:17:47.740 across the central Greater Caucasus. 00:17:47.740 --> 00:17:52.080 The locations of these traverses are determined mostly by access. 00:17:53.400 --> 00:17:56.020 Before we get too [audio garbled], want to show you guys a couple of 00:17:56.021 --> 00:18:01.100 pictures of Georgia, just for interest’s sake, since I imagine many of you 00:18:01.100 --> 00:18:05.320 have not been to Georgia. It’s a fascinating place to travel. 00:18:05.320 --> 00:18:08.480 Beautiful countryside. Wonderful food. 00:18:08.480 --> 00:18:12.280 These are a couple pictures of the capital city of Tbilisi. 00:18:12.280 --> 00:18:16.970 Which actually has been the interest of some seismic hazard studies recently. 00:18:16.970 --> 00:18:20.720 There was a magnitude 5 earthquake just outside of Tbilisi a handful 00:18:20.720 --> 00:18:25.540 of years ago that triggered a lot of research. 00:18:27.200 --> 00:18:31.280 Georgia has its own language. This is what it looks like, both in 00:18:31.280 --> 00:18:35.120 the Georgian alphabet and then in a transliterated Latin alphabet. 00:18:35.120 --> 00:18:38.620 You can see that, even when you can read the letters, it isn’t necessarily more 00:18:38.620 --> 00:18:42.190 pronounceable for those of us who grew up speaking English. 00:18:42.190 --> 00:18:47.160 This is the Tskhenistskali River, which translates to “horse water.” 00:18:49.080 --> 00:18:52.800 Here’s a picture of one of the field houses we stayed in and 00:18:52.800 --> 00:18:56.240 a man cooking traditional Georgian dumplings. 00:18:57.960 --> 00:19:01.700 Interesting things you find on the road include sheep roadblocks 00:19:01.710 --> 00:19:04.310 and Soviet trucks carrying mysteriously large boulders. 00:19:04.310 --> 00:19:07.640 This picture was taken just south of the border with Russia. 00:19:07.640 --> 00:19:10.560 The rock was traveling to Russia. 00:19:10.560 --> 00:19:14.550 I don’t know why they needed that specific boulder, but it certainly 00:19:14.550 --> 00:19:16.920 took a lot of effort to move. 00:19:18.100 --> 00:19:19.880 And that’s when there are roads. 00:19:19.880 --> 00:19:26.550 Often the roads are in much poorer repair than the ones with sheep on them. 00:19:26.550 --> 00:19:31.300 So the infrastructure certainly makes field work an interesting proposition. 00:19:31.300 --> 00:19:35.150 But the landscapes are beautiful. The exposures are sometimes limited 00:19:35.150 --> 00:19:38.480 due to vegetation or because the hillsides are very steep. 00:19:38.480 --> 00:19:40.560 Sometimes there are reservoirs in the way. 00:19:40.560 --> 00:19:43.320 But, when you can see the rocks, they’re quite spectacular. 00:19:43.320 --> 00:19:47.960 This is a picture from the crystalline core of the range looking south. 00:19:47.960 --> 00:19:52.420 So we’re standing in the crystalline rock looking south into the Mesozoic section. 00:19:54.080 --> 00:19:58.040 And there are mountainside-scale folds. 00:19:59.480 --> 00:20:06.440 And spectacular active river channels – bedrock river channels with big terraces 00:20:06.440 --> 00:20:09.290 abandoned on them. 00:20:09.290 --> 00:20:13.480 And then, because the region has been populated for several thousand years, 00:20:13.480 --> 00:20:17.300 you get exciting things like castles built on active fold/thrust belts. 00:20:17.300 --> 00:20:21.660 This is a location we’ll return to in a little bit. 00:20:21.660 --> 00:20:24.460 So back to the geology. 00:20:24.460 --> 00:20:26.820 Focus first on the structural architecture of the range. 00:20:26.820 --> 00:20:29.880 And, again, we’re going to look primarily today at the western 00:20:29.880 --> 00:20:32.720 Greater Caucasus along the Enguri River. 00:20:34.220 --> 00:20:39.480 So I’m going to spend a bit of time here. I’m going to go through it relatively 00:20:39.480 --> 00:20:42.760 quickly, but I do want to show you the types of data we’re working with. 00:20:42.760 --> 00:20:47.860 So we’re going to walk through this traverse from south to north. 00:20:47.860 --> 00:20:51.620 And I’ll talk a little bit about the rocks that are exposed, what the structural 00:20:51.620 --> 00:20:57.670 geology looks like, and sort of where we are in this stratigraphic column. 00:20:57.670 --> 00:21:02.320 As I do this, wanted to point out that I will be building a strat column – 00:21:02.320 --> 00:21:06.200 a tectonic stratigraphic column in the upper-right corner. 00:21:06.200 --> 00:21:08.360 And then I’ll be showing you some structural data as well 00:21:08.370 --> 00:21:12.980 in the form of stereonets. They have been rotated – 00:21:12.980 --> 00:21:17.540 the stereonets have been rotated to align with the orientation of 00:21:17.540 --> 00:21:20.771 the map that I’m showing you. So remember that north is 00:21:20.780 --> 00:21:24.240 to the right and south is to the left. 00:21:26.220 --> 00:21:29.920 So, as I said, we’re going to start at the south end and work north. 00:21:29.920 --> 00:21:35.040 So we’re starting in the active fold/thrust belt in the foreland basin. 00:21:35.040 --> 00:21:39.850 In this particular location, there are some deformed river terraces that 00:21:39.850 --> 00:21:42.360 suggest that rivers are active. I forgot to update this slide, 00:21:42.360 --> 00:21:44.539 but that should say, Trexler et al., 2020. 00:21:44.540 --> 00:21:48.080 That paper is now out in EPSL. 00:21:49.080 --> 00:21:54.960 And these structures that [inaudible] in the foreland fold/thrust belt seem to 00:21:54.970 --> 00:21:58.500 connect to the main range by a shallow detachment beneath the 00:21:58.500 --> 00:22:03.160 northern part of the basin. This is constrained by seismic reflection data. 00:22:04.100 --> 00:22:09.460 As we move northward into the mountains, we go through a dramatic 00:22:09.460 --> 00:22:13.580 range front of flatirons. These are primarily cretaceous 00:22:13.580 --> 00:22:19.130 carbonate south-dipping flatirons. Interestingly, there is no fault at 00:22:19.130 --> 00:22:23.220 the surface at the range front. And so we have a – what appears to 00:22:23.220 --> 00:22:26.970 be a hanging wall ramp sitting on a footwall flat that forms that range front. 00:22:26.970 --> 00:22:30.710 So, in this picture, if we’re standing at the range front looking south, 00:22:30.710 --> 00:22:33.500 annotated – shown sort of the orientation of the bedding, 00:22:33.500 --> 00:22:37.280 so we’re looking out toward that foreland fold/thrust belt. 00:22:37.960 --> 00:22:42.360 Moving into the range, we would go down into the 00:22:42.370 --> 00:22:47.010 Mesozoic sedimentary section. There are lots of volcanic breccias 00:22:47.010 --> 00:22:51.010 and debris flows, but as we get further down in the section, we get further 00:22:51.010 --> 00:22:53.590 and further away from the source of these things, and we get into 00:22:53.590 --> 00:22:58.400 deeper more distal sediments. And broadly, things here are 00:22:58.400 --> 00:23:01.970 relatively moderately dipping with a wide range of dips, 00:23:01.970 --> 00:23:08.900 but the strikes are pretty consistently just northwest of east-west. 00:23:11.220 --> 00:23:18.100 Moving further north and further down-section, there are some pillow 00:23:18.110 --> 00:23:21.620 lavas and other volcaniclastics, but really primarily things are made up 00:23:21.620 --> 00:23:25.580 of marine facies sedimentary rocks. There are some nice sedimentary 00:23:25.580 --> 00:23:28.440 structures that help us determine facing direction, which it turns out is actually 00:23:28.440 --> 00:23:32.610 very important because, in this zone, and everywhere north of it, bedding 00:23:32.610 --> 00:23:36.900 is no longer moderately dipping. It’s actually sub-vertical in most places. 00:23:40.580 --> 00:23:44.940 Moving further north, the bedding orientations stay pretty consistently 00:23:44.940 --> 00:23:49.780 sub-vertical. We start to get into low-grade metamorphic rocks. 00:23:49.780 --> 00:23:51.740 We reach phyllite grade, which is actually about as high as 00:23:51.740 --> 00:23:55.360 anything gets in the Greater Caucasus. 00:23:55.360 --> 00:23:59.220 Again, there’s some nice preserved sedimentary structures. 00:23:59.220 --> 00:24:02.510 And then finally, in the core of the range, we reach the 00:24:02.510 --> 00:24:06.570 crystalline basement backstop. This is that Variscan-age crystalline 00:24:06.570 --> 00:24:12.179 core of the range that defines sort of the northern limit of 00:24:12.180 --> 00:24:15.280 deformation in the western Greater Caucasus. 00:24:15.960 --> 00:24:20.640 Here’s a picture – the field photo on the right is from a similar location 00:24:20.640 --> 00:24:23.220 to that previous one I showed you when I said I was – we were looking 00:24:23.220 --> 00:24:28.300 south from the crystalline core. And I’ve annotated it to show the 00:24:28.300 --> 00:24:34.740 location of that south-bounding fault on this crystalline basement. 00:24:34.740 --> 00:24:37.200 The kinematics of this structure are unclear, but it does appear to 00:24:37.200 --> 00:24:42.320 put crystalline rock in the hanging wall juxtaposed over Mesozoic marine 00:24:42.320 --> 00:24:46.040 sedimentary rock. We don’t know the timing of activity on the structure. 00:24:46.040 --> 00:24:49.980 It could be Cenozoic active, or it could be much older than that. 00:24:52.400 --> 00:24:56.980 So, just to recap that – I knew we went through it relatively quickly, 00:24:56.990 --> 00:25:00.330 first I want to point out that, although I have noted the locations of some 00:25:00.330 --> 00:25:03.250 major structures here, a few of these faults are actually directly observed 00:25:03.250 --> 00:25:07.490 in the field instead we're inferring their locations from stratigraphic truncations, 00:25:07.490 --> 00:25:12.500 abrupt structural changes, things like that. The exposure just is limited enough 00:25:12.500 --> 00:25:17.220 that you don’t actually get to see these things on the ground all that often. 00:25:17.220 --> 00:25:20.810 But regardless of that, since significant shortening is required to create the 00:25:20.810 --> 00:25:24.030 geometry we see in the field, the bedding is extremely steeply dipping, 00:25:24.030 --> 00:25:29.170 particularly in the northern zones. And even where we don’t have faults, 00:25:29.170 --> 00:25:34.530 we have extensive folding to accommodate that deformation. 00:25:34.530 --> 00:25:39.640 The cross-section along the bottom of this slide is actually true vertical scale. 00:25:39.640 --> 00:25:41.800 I haven’t tried to [inaudible] this particular one. 00:25:41.800 --> 00:25:45.920 I’m just showing what is happening at the surface, but you can see that, 00:25:45.920 --> 00:25:51.679 particularly in the dark blue units there in the middle, things are almost vertical 00:25:51.680 --> 00:25:58.820 and extremely deformed from the original orientation and shape. 00:26:01.600 --> 00:26:07.210 To bring a little bit more information to bear on the question of deformation 00:26:07.210 --> 00:26:10.490 in the range, I also did a little bit of low-temperature thermochronology 00:26:10.490 --> 00:26:14.760 to constrain exhumation depth. So, for those of you who aren’t familiar 00:26:14.760 --> 00:26:18.920 with this, or haven’t thought about it in a while, just a reminder that 00:26:18.920 --> 00:26:22.560 low-temperature thermochronometers are recording the age at which a 00:26:22.560 --> 00:26:27.340 particular mineral grain passes through a closure temperature window. 00:26:27.340 --> 00:26:30.300 So they give us an age and a temperature. 00:26:30.310 --> 00:26:33.050 Then we can calculate a cooling rate of that particular rock. 00:26:33.050 --> 00:26:36.440 And, since there are different thermochronometer systems, we can get 00:26:36.440 --> 00:26:43.520 a time/temperature history if we can find multiple systems in a single rock. 00:26:44.380 --> 00:26:47.860 On top of that, if we assume a geothermal gradient, 00:26:47.860 --> 00:26:50.610 then you can use temperature as a proxy for depth in the crust, 00:26:50.610 --> 00:26:53.840 and that can give you an exhumation rate through time. 00:26:55.050 --> 00:26:58.040 For our purposes today, we’re not going to focus too much on the details there. 00:26:58.040 --> 00:27:00.660 Instead, what we’re going to look for is simply whether a 00:27:00.660 --> 00:27:03.120 thermochronometer system has been reset. 00:27:03.120 --> 00:27:06.940 In other words, was the rock that we’re looking at exhumed from 00:27:06.950 --> 00:27:10.540 depths great enough to exceed the closure temperature 00:27:10.540 --> 00:27:14.350 of that particular thermochron system? 00:27:14.350 --> 00:27:16.570 In the case today, because this mountain range is quite young, 00:27:16.570 --> 00:27:23.500 we’re going to look at closure temperatures reset in the last 10 Ma. 00:27:23.500 --> 00:27:25.490 And the two systems we’re going to focus on are apatite and 00:27:25.490 --> 00:27:27.260 uranium/thorium-helium, which has a closure temperature 00:27:27.260 --> 00:27:30.260 of about 70 degrees C, and apatite fission track, 00:27:30.260 --> 00:27:33.040 which has a closure temperature of about 110 degrees C. 00:27:33.060 --> 00:27:38.720 We just translate to 2-1/2 and 4 kilometers of exhumation, respectively. 00:27:40.049 --> 00:27:42.460 So I’m going to show you a bunch of data on the next slide. 00:27:42.470 --> 00:27:44.929 In order to help you sort of tease out what’s going on there, 00:27:44.929 --> 00:27:48.750 I’ve color-coded it. So anything that is orange has been 00:27:48.750 --> 00:27:52.730 reset in the last 10 million years, suggesting that it is exhuming 00:27:52.730 --> 00:28:00.240 quite rapidly. Anything that is a cooler color has an older closure temperature, 00:28:00.240 --> 00:28:06.500 suggesting a slower exhumation rate and/or just less exhumation overall. 00:28:08.600 --> 00:28:12.360 So here is that assortment of thermochron samples. 00:28:12.360 --> 00:28:14.590 This is a compilation of my own and some public – 00:28:14.590 --> 00:28:18.130 previously published ones. And I’ve broken them apart 00:28:18.130 --> 00:28:23.230 into different zones based on their structural location in the orogen. 00:28:23.230 --> 00:28:25.530 So I have the same cross-section we were looking at before along 00:28:25.530 --> 00:28:31.500 the bottom with sample locations noted by the white stars. 00:28:32.770 --> 00:28:38.160 And the pattern I want to point out here is that the 00:28:38.170 --> 00:28:45.830 apatite uranium/thorium-helium samples are reset north of the Khaishi 00:28:45.830 --> 00:28:50.970 Fault in the center of this profile. And the apatite fission track ages 00:28:50.970 --> 00:28:59.370 are actually reset further north than the apatite helium, suggesting that the 00:28:59.370 --> 00:29:02.500 exhumation in the core of the range to the northern end of this section 00:29:02.500 --> 00:29:06.710 is greater than further south. It seems to be a gradient 00:29:06.710 --> 00:29:09.460 moving from south to north. And, actually, I don’t have it on 00:29:09.460 --> 00:29:12.930 this slide, but recent work by Dylan Vasey that just came out in 00:29:12.930 --> 00:29:17.360 Tectonics a couple of months ago, fleshes out this story a bit more by 00:29:17.360 --> 00:29:22.049 adding some zircon uranium/thorium- helium and zircon fission track 00:29:22.049 --> 00:29:27.030 to really show this gradient of increasing exhumation going 00:29:27.030 --> 00:29:30.100 from south to north in the western Greater Caucasus. 00:29:33.480 --> 00:29:37.760 So, to sum all of that up, in response to this first question of the structural 00:29:37.760 --> 00:29:42.290 architecture of the range, the mapping we’ve done documents these discrete 00:29:42.290 --> 00:29:45.830 tectonostratigraphic domains, which are separated by major structures. 00:29:45.830 --> 00:29:49.590 It looks broadly like a south-vergent imbricate thrust stack. 00:29:49.590 --> 00:29:52.540 And structural depth increases from south to north. 00:29:55.600 --> 00:29:59.360 We also can use these observations to make an estimate of shortening 00:29:59.360 --> 00:30:02.360 accommodated in the range by building a balanced cross-section. 00:30:02.360 --> 00:30:05.320 So that’s what we’re going to do next. 00:30:05.320 --> 00:30:10.100 Because I’m dealing with relatively limited data and a very large scale – 00:30:10.110 --> 00:30:16.211 this profile is about 150 kilometers long – I had to 00:30:16.211 --> 00:30:18.700 make some assumptions regarding the geometry of the structures. 00:30:18.700 --> 00:30:23.320 So, one, we’re assuming that the structural style is thin-skinned. 00:30:23.320 --> 00:30:26.760 Two, we’re assuming that it has fault-bend-fold geometry. 00:30:26.760 --> 00:30:30.680 For those of you who want a reminder on what a fault-bend-fold looks like, 00:30:30.680 --> 00:30:33.480 here is a nice example of fault-bend-fold growing. 00:30:33.480 --> 00:30:38.929 The key details here are that you have a fixed backlimb dip that 00:30:38.929 --> 00:30:43.630 leads to a specific orientation of the forelimb. 00:30:43.630 --> 00:30:48.160 In the case of the balanced cross- section I’m going to show you next, 00:30:48.160 --> 00:30:52.040 we’re operating under the assumption that all ramps and backlimbs are 00:30:52.060 --> 00:30:54.820 dipping at 30 degrees to the north, and all forelimbs are dipping 00:30:54.820 --> 00:30:58.200 at 50 degrees to the south. This means that any geometry 00:30:58.200 --> 00:31:01.549 we see at the surface I’ve tried to reproduce using 00:31:01.549 --> 00:31:05.040 only fault-bend-folds with those geometries. 00:31:06.620 --> 00:31:09.860 And then finally, we had to make some assumptions with regard to 00:31:09.860 --> 00:31:15.640 what to do with stratigraphic units along strike and along perpendicular 00:31:15.640 --> 00:31:19.429 strike along the section. Because we’re going for a shortening 00:31:19.429 --> 00:31:22.820 estimate, I decided it was probably best to make sure that all estimates 00:31:22.820 --> 00:31:27.410 were either minima or maxima in our – since we’re trying to constrain the 00:31:27.410 --> 00:31:30.940 minimum size of the basin, I decided to make assumptions that would 00:31:30.940 --> 00:31:35.230 minimize my shortening estimates. So I’m assuming that units are laterally 00:31:35.230 --> 00:31:40.200 continuous along the full section, all the way across this Mesozoic-age basin. 00:31:40.200 --> 00:31:44.330 I know that is an unreasonable assumption, but the thinking is that, 00:31:44.330 --> 00:31:46.940 if I assume they are laterally continuous, then, if they pinch out, 00:31:46.940 --> 00:31:50.320 that would only provide additional space in my cross-section 00:31:50.320 --> 00:31:54.640 that I would have to fill, and thus my estimate is a minimum. 00:31:54.640 --> 00:32:00.880 Similarly, I’m assuming that these units are their maximum possible thickness 00:32:00.880 --> 00:32:05.360 based on my field observations across the entire section. 00:32:05.360 --> 00:32:10.320 So, again, I think that’s probably an unreasonable assumption in detail, 00:32:10.320 --> 00:32:16.280 but the large scale, for the purposes of our test here, if we assume the units 00:32:16.280 --> 00:32:20.180 are their maximum thickness, then, if they are thinner than that in any 00:32:20.180 --> 00:32:24.680 location, that, again, provides additional space that we have to fill, and thus the 00:32:24.680 --> 00:32:28.400 shortening estimate would increase. So our estimate is a minimum. 00:32:29.080 --> 00:32:32.780 So, with those caveats, I’m going to show you that cross-section. 00:32:32.780 --> 00:32:36.560 Here’s a strip map across the whole orogen. 00:32:36.560 --> 00:32:39.500 So the field observations are in the southern half. 00:32:39.500 --> 00:32:43.470 On the left side, they stop there at about the black line because that’s 00:32:43.470 --> 00:32:49.740 where the Russian border is and access is much more limited to the north of that. 00:32:49.740 --> 00:32:52.280 But I did want to point out that, on the northern flank of the range 00:32:52.299 --> 00:32:55.800 in this particular location, it seems to be a gently north-dipping homocline 00:32:55.800 --> 00:33:01.860 with no apparent major structures exposed based on published mapping. 00:33:01.860 --> 00:33:04.870 So I feel relatively confident I am constructing 00:33:04.870 --> 00:33:07.120 a balanced cross-section through here. 00:33:07.120 --> 00:33:08.740 So here’s what the cross-section looks like. 00:33:08.740 --> 00:33:12.840 This is a 1-to-1 vertical scale. 00:33:13.860 --> 00:33:16.860 And you can see that I’ve had to play some interesting games with duplexes 00:33:16.860 --> 00:33:21.160 at depth in order to produce the geometries we see at the surface. 00:33:21.160 --> 00:33:24.300 Once we have this balanced cross-section, we can pull it apart, 00:33:24.300 --> 00:33:30.840 step by step, restore it, and calculate a total magnitude of shortening. 00:33:30.840 --> 00:33:33.260 In this particular case, along this cross-section, we have about 00:33:33.260 --> 00:33:38.860 170 kilometers of shortening. In addition, we have a couple of major 00:33:38.860 --> 00:33:42.880 structures identified in the field – the Khaishi Fault, which is located here, 00:33:42.880 --> 00:33:46.280 and the Ushba Fault, which is the crystalline basement structure that 00:33:46.280 --> 00:33:50.460 I pointed out, that have no direct stratigraphic correlation across them. 00:33:50.470 --> 00:33:52.500 And this means that they could accommodate significantly 00:33:52.500 --> 00:33:56.169 more displacement than I have put across them here, 00:33:56.169 --> 00:33:59.460 which would increase that minimum amount of shortening. 00:34:04.040 --> 00:34:08.520 So, to return to our framing questions here, restored balanced cross-sections 00:34:08.520 --> 00:34:14.140 along this traverse provide a minimum estimate of 170 kilometers of 00:34:14.140 --> 00:34:18.420 shortening, although that total may be much larger. 00:34:18.429 --> 00:34:20.839 And this serves the minimum bound for basement width. 00:34:20.839 --> 00:34:23.220 Right, the basin could not have been narrower than 00:34:23.220 --> 00:34:24.450 the amount of shortening we see. 00:34:24.450 --> 00:34:28.600 It could have been more if we are not recording all of the shortening. 00:34:30.120 --> 00:34:33.260 And then, last but not least, I want to talk a little bit about shortening rate. 00:34:33.260 --> 00:34:38.859 And, to do that, first we can think a little bit about what we can do 00:34:38.859 --> 00:34:42.039 with the combination of the balanced cross-sections and the low temperature 00:34:42.039 --> 00:34:46.460 thermochronology data I showed you. As I alluded to earlier, if you have 00:34:46.460 --> 00:34:49.669 multiple thermochronometers in the individual rock that have different 00:34:49.669 --> 00:34:51.649 closure temperatures, and therefore require different ages, 00:34:51.649 --> 00:34:55.079 then you can construct a time/temperature history, 00:34:55.080 --> 00:34:58.580 which is analogous to a time exhumation history. 00:34:58.580 --> 00:35:00.640 And a couple of previous studies have done this. 00:35:00.640 --> 00:35:04.410 Here are two examples. On the left from Vincent et al., 00:35:04.410 --> 00:35:07.720 published in 2007. And on the right from Avdeev 00:35:07.720 --> 00:35:12.520 and Niemi, published in 2011. And both of these plots have 00:35:12.529 --> 00:35:16.279 temperature on the vertical axis and time going from old to young 00:35:16.279 --> 00:35:19.529 on the horizontal axis from left to right. 00:35:19.529 --> 00:35:24.109 And they both seem to show exhumation beginning in the orogen 00:35:24.109 --> 00:35:29.069 about 30 million years ago, marked by the orange bars. 00:35:29.069 --> 00:35:34.609 So if we take this exhumation initiation at 30 Ma and assume that all of the 00:35:34.609 --> 00:35:40.999 shortening that we record along this cross-section has occurred since that 00:35:40.999 --> 00:35:45.520 exhumation begins, which seems like a somewhat reasonable assumption, 00:35:45.520 --> 00:35:49.619 then that gives us an average shortening rate over the western Greater Caucasus 00:35:49.619 --> 00:35:53.589 over the last 30 million years of at least 6 millimeters a year. 00:35:53.589 --> 00:35:57.700 It could be larger than that if our shortening estimate is [inaudible]. 00:35:58.980 --> 00:36:01.380 So long-term, we’re looking at something like 6 millimeters 00:36:01.380 --> 00:36:04.760 a year of shortening across the western Greater Caucasus. 00:36:06.600 --> 00:36:09.660 And remember that the question we started with was whether 00:36:09.660 --> 00:36:12.519 that shortening rate changes with time. 00:36:12.519 --> 00:36:18.390 So, in order to compare over time, we need to have estimates of 00:36:18.390 --> 00:36:21.180 shortening rate at different time scales. 00:36:21.180 --> 00:36:24.080 So if the long-term rate is 6 millimeters a year, what’s happening now? 00:36:24.089 --> 00:36:27.529 What’s the short-term geologic shortening rate? 00:36:27.529 --> 00:36:29.549 To look at that question, we’re going to move to – 00:36:29.549 --> 00:36:32.600 back to the deformed terraces in the foreland basin. 00:36:35.049 --> 00:36:38.780 So we’re looking here at what’s known as the Tsaishi Fold. 00:36:38.780 --> 00:36:43.480 This is in the Rioni Basin. In this particular photo, 00:36:43.480 --> 00:36:46.170 we’re looking at a Jordan Orthodox Church. 00:36:46.170 --> 00:36:49.790 You can see, up on the hillside on the left there, there is the ruins of one. 00:36:49.790 --> 00:36:53.319 This particular church was actually destroyed by an earthquake in 1614, 00:36:53.320 --> 00:36:58.060 and they built the new one down on the flat there in the middle of the photo. 00:37:00.340 --> 00:37:06.880 So that fold/thrust belt is the bar of green and brown at the southern edge 00:37:06.880 --> 00:37:09.720 of the white box that’s outlined here. And that white box is showing the 00:37:09.720 --> 00:37:12.400 location of the traverse we just spent some time thinking about 00:37:12.400 --> 00:37:13.769 with that balanced cross-section. 00:37:13.769 --> 00:37:17.320 And the spot I want to focus on in particular is outlined in red. 00:37:17.320 --> 00:37:20.890 There are these river terraces along the Enguri that seem to record some 00:37:20.890 --> 00:37:24.540 deformation. So we’re going to zoom in on that red box. 00:37:26.240 --> 00:37:29.579 The reason we’re focusing in on river terraces, as many of you guys are 00:37:29.579 --> 00:37:33.980 probably familiar, is they act as great strain markers. 00:37:33.980 --> 00:37:38.349 What I mean by that is that they form in a very specific orientation. 00:37:38.349 --> 00:37:42.819 We know how they form, and so, if they do not match our expectations, 00:37:42.819 --> 00:37:45.190 then we can back out what they might have looked like 00:37:45.190 --> 00:37:49.160 initially and calculate an amount of deformation. 00:37:49.160 --> 00:37:53.200 Here is a photo of these river terraces just to give you a sense of scale. 00:37:53.200 --> 00:37:55.980 So we’re at the location marked 1 on that map looking north 00:37:55.980 --> 00:37:58.039 toward the range front. You can see the snow-capped peaks 00:37:58.039 --> 00:38:03.880 of the central Greater Caucasus there – excuse me – in the north. 00:38:03.880 --> 00:38:07.420 And these river terraces are sort of arrayed out in front of us. 00:38:07.420 --> 00:38:11.019 We’re actually standing at the top of one of these terrace risers. 00:38:11.019 --> 00:38:13.930 So these things are quite large. 00:38:13.930 --> 00:38:17.769 The riser heights are often 100 meters or more, 00:38:17.769 --> 00:38:21.540 particularly up toward the northern range front. 00:38:24.560 --> 00:38:27.619 So here’s a picture from the range front looking south. 00:38:27.619 --> 00:38:32.339 We’re looking back toward the location where that previous photo was taken. 00:38:32.339 --> 00:38:35.670 And I wanted to remind you here that the Rioni Basin – all of these terraces 00:38:35.670 --> 00:38:38.960 are underlined by an active low-angle shallow detachment. 00:38:38.960 --> 00:38:42.290 Which means that any movement along that detachment should be 00:38:42.290 --> 00:38:46.560 recorded in these terraces. 00:38:46.560 --> 00:38:50.220 So there’s the location of that previous photo and several of these 00:38:50.220 --> 00:38:53.700 terrace flights outlined in white, just to point out what we’re looking at, 00:38:53.700 --> 00:38:56.120 again, to give you a sense of scale. 00:38:57.400 --> 00:39:03.460 So let’s look at these terraces and their geometries. 00:39:03.470 --> 00:39:08.700 This is a plot of longitudinal profile elevation of these terraces. 00:39:08.700 --> 00:39:13.860 We’re looking here at the T3 and T4 surfaces in orange and yellow, 00:39:13.860 --> 00:39:19.020 respectively, going from north on the left to south on the right. 00:39:19.029 --> 00:39:23.260 And you can see that they’re relatively graded, more or less, with the modern 00:39:23.260 --> 00:39:26.280 Enguri River, which is shown there in blue. 00:39:28.040 --> 00:39:34.440 Now I’m going to add the T5 terrace. It’s going to show up on the right edge. 00:39:34.440 --> 00:39:36.880 And you notice right away that this terrace is doing something very 00:39:36.890 --> 00:39:41.520 different, particularly at the south end. Rather than being oriented with the 00:39:41.520 --> 00:39:47.800 modern terrace surface, it suddenly is dipping back toward the mountain front. 00:39:47.800 --> 00:39:51.540 So let’s focus in on that a bit more. 00:39:51.549 --> 00:39:57.769 This is pretty clearly evidence of active deformation. 00:39:57.769 --> 00:40:03.049 And so we can use the geometry of that deformed terrace surface 00:40:03.049 --> 00:40:08.040 to understand what the geometry of the fold is, or the faulting, 00:40:08.040 --> 00:40:11.360 beneath it that caused that deformation. 00:40:14.280 --> 00:40:18.380 So we tried a couple of different fault models to see if we could 00:40:18.380 --> 00:40:21.200 reproduce this shape. The first model we tried was just 00:40:21.200 --> 00:40:25.400 a simple step-up. This is, again, a nice little fault-bend-fold. 00:40:25.400 --> 00:40:29.960 And, in this model, the width of the deformed zone is 00:40:29.960 --> 00:40:34.059 directly comparable to the amount of slip on the fault. 00:40:34.059 --> 00:40:36.119 Because that deform zone is formed by a translation 00:40:36.120 --> 00:40:39.520 of material from the flat to the ramp. 00:40:40.640 --> 00:40:45.440 The problem with that is, when you look at the horizontal scale on this plot – 00:40:45.440 --> 00:40:48.140 and I apologize that the little kilometers label got cropped off, 00:40:48.140 --> 00:40:52.920 but this is in kilometers. So that deformed zone is 00:40:52.920 --> 00:40:56.000 4 to 6 kilometers wide, which implies that, if this is the 00:40:56.000 --> 00:41:00.180 fault geometry we’re looking at, that requires 4 to 6 kilometers of slip. 00:41:00.180 --> 00:41:05.660 Now, this is a river terrace. So that is probably an unrealistic 00:41:05.660 --> 00:41:11.100 amount of slip to have occurred at any sort of semi-normal shortening rate, 00:41:11.100 --> 00:41:15.660 assuming the terrace is less than 100,000 years or so old. 00:41:17.520 --> 00:41:20.700 Perhaps more importantly, this geometry is also inconsistent 00:41:20.710 --> 00:41:25.049 with the bedrock geology along strike of the fold. 00:41:25.049 --> 00:41:28.080 So here’s a geologic map. 00:41:28.080 --> 00:41:30.820 And you can see that, unlike the simple step-up model, 00:41:30.829 --> 00:41:34.410 which predicts a backlimb and then a flat, but no forelimb, in that fold, 00:41:34.410 --> 00:41:39.920 we have a nice south-dipping forelimb on the southern margin of this anticline, 00:41:39.920 --> 00:41:46.660 suggesting that this model doesn’t really match the data that we see in the field. 00:41:48.840 --> 00:41:52.779 The second model we tried was a simple listric fault. 00:41:52.779 --> 00:41:55.969 So, rather than having a step-up where the fault goes from flat 00:41:55.969 --> 00:42:04.479 to a consistent dipping ramp, instead we have a curved ramp. 00:42:04.479 --> 00:42:07.910 And this solves our width of deformed zone problem because, 00:42:07.910 --> 00:42:11.859 rather than the amount of slip defining the width of that zone, the width 00:42:11.860 --> 00:42:16.780 is defined by the radius of curvature and the depth to the detachment. 00:42:17.560 --> 00:42:21.880 So this solves that problem by requiring much less slip. 00:42:21.890 --> 00:42:24.890 The problem is, it doesn’t really very accurately capture the shape 00:42:24.890 --> 00:42:30.740 of the deformation we see, which has this nice convex curvature. 00:42:31.640 --> 00:42:35.580 So it turns out that the happy medium is actually a combination of the two. 00:42:35.580 --> 00:42:39.960 We have a fault-bend-fold with a listric hinge zone. 00:42:40.900 --> 00:42:46.270 So this allows us to have the small amount of slip producing 00:42:46.270 --> 00:42:51.220 that wide deformation zone, as we have with a listric fault. 00:42:51.220 --> 00:42:55.680 But it produces that convex shape that we see at the surface – 00:42:55.680 --> 00:42:58.660 or, in the terrace surface. 00:42:58.660 --> 00:43:02.280 And it matches the bedrock geology quite well. 00:43:02.280 --> 00:43:05.460 So here’s a more detailed version of that model. 00:43:07.520 --> 00:43:11.640 What I want to point out here is the little orange arrows. 00:43:11.640 --> 00:43:13.260 I hope you can see them. 00:43:13.260 --> 00:43:16.360 They ended up a little smaller than I intended. 00:43:16.369 --> 00:43:20.569 But the important detail here is that, depending on where your particle starts 00:43:20.569 --> 00:43:25.060 as it moves through this fold, it has a very different path. 00:43:25.680 --> 00:43:28.640 And this produces that convex shape of deformation that 00:43:28.640 --> 00:43:31.800 we see in the terrace surface. 00:43:33.560 --> 00:43:37.820 And finally, this accurately reproduces the bedrock fold along strike. 00:43:37.829 --> 00:43:46.410 Here is a balanced cross-section that I created using Trishear showing that 00:43:46.410 --> 00:43:51.270 fold with some bedrock field observations shown there with little 00:43:51.270 --> 00:43:57.700 dip ticks in the cross-section and using the geometry of the deformed terrace 00:43:57.700 --> 00:44:01.300 surface, and it recreates what we see in the field quite well. 00:44:03.300 --> 00:44:06.480 The nice thing about this model is, once we’ve figured all of that out, 00:44:06.480 --> 00:44:09.670 it actually is quite easy to calculate an amount of slip. 00:44:09.670 --> 00:44:15.670 Because all you need is the fault dip at the surface and the amount of uplift 00:44:15.670 --> 00:44:18.749 recorded by the terrace surface, assuming you can – assuming you can 00:44:18.749 --> 00:44:22.940 get out of that [inaudible] zone and onto the linear fault up-dip of it. 00:44:25.140 --> 00:44:28.340 So we can measure that uplift from the deformed terrace. 00:44:28.340 --> 00:44:33.120 In this case, it’s about 60 meters – 59 plus or minus a little bit. 00:44:34.009 --> 00:44:36.700 And we can estimate the fault dip based on the bedrock geometry 00:44:36.700 --> 00:44:39.040 of the fold from the structural observations. 00:44:39.040 --> 00:44:41.859 In this case, it’s dipping at about 35 degrees. 00:44:41.859 --> 00:44:46.609 And so we can use that to calculate an amount of slip, which here is 00:44:46.609 --> 00:44:53.100 just over 100 meters of slip since the formation of that terrace surface. 00:44:56.789 --> 00:45:00.040 So in order to make that a rate, we needed some age control. 00:45:00.050 --> 00:45:03.920 Getting Quaternary geochron in Georgia looks a little bit different 00:45:03.920 --> 00:45:08.089 than it does in the U.S. [chuckles] We – in order to collect some 00:45:08.089 --> 00:45:11.489 OSL samples, had to go to this guy who owned a taxi shop and also 00:45:11.489 --> 00:45:18.480 had a saw and some old fence posts that we could use as light opaque 00:45:18.480 --> 00:45:20.840 sampling tubes. And then we hired the guy 00:45:20.859 --> 00:45:26.310 in the teal shirt for 90 lari, which is the equivalent of about $25, 00:45:26.310 --> 00:45:30.840 to bring his backhoe down to this soccer field and dig a hole for us. 00:45:32.020 --> 00:45:38.210 So we dug a hole down to the gravel terrace surface, took a couple 00:45:38.210 --> 00:45:42.060 of samples, including several beneath the paleosol horizon, 00:45:42.060 --> 00:45:45.460 to get an age of this terrace surface, and it came out at just under 00:45:45.460 --> 00:45:49.000 100,000 years old – about 94,000 years old, give or take. 00:45:50.900 --> 00:45:54.260 So if we combine that with the amount of slip since the deposition 00:45:54.269 --> 00:45:58.969 of the terrace surface, this gives us an average shortening rate 00:45:58.969 --> 00:46:03.460 since the deposition of the terrace of just over a millimeter a year. 00:46:06.200 --> 00:46:09.620 Now, if we remember what the long- term rate was, it’s very different. 00:46:09.630 --> 00:46:13.819 We were looking at 6 millimeters a year or more over the last 00:46:13.820 --> 00:46:18.300 30 million years, on average. And, at this foreland structure, 00:46:18.300 --> 00:46:21.260 we’re only seeing about a millimeter a year. 00:46:24.300 --> 00:46:27.549 So the last piece I want to bring to this particular question is, what’s happening 00:46:27.549 --> 00:46:32.860 with the modern geodetic signal? Is this rate truly a dramatic decrease? 00:46:32.860 --> 00:46:36.240 Or is this fault that we’re looking at simply not capturing 00:46:36.240 --> 00:46:38.700 all of the modern shortening? 00:46:39.500 --> 00:46:44.479 So some work by our Georgian collaborators in the last couple years 00:46:44.479 --> 00:46:48.630 has put together an elastic dislocation model of some GPS data across the 00:46:48.630 --> 00:46:53.360 Caucasus region, including a transact across the western Greater Caucasus. 00:46:53.360 --> 00:46:59.460 And their data suggests that the – or, sorry, their modeling suggests that 00:46:59.460 --> 00:47:03.460 the data are best fit by a north-dipping structure beneath the Greater Caucasus, 00:47:03.460 --> 00:47:07.240 which is, more or less, consistent with our structural observations. 00:47:08.040 --> 00:47:11.680 And, across that structure, they model something in the ballpark 00:47:11.680 --> 00:47:16.510 of 3 to 5 millimeters a year – 4 plus or minus 1 – 00:47:16.510 --> 00:47:20.300 on a shallow north-dipping structure. 00:47:20.300 --> 00:47:25.940 Now, they’re only modeling up to a depth of 10 kilometers. 00:47:25.940 --> 00:47:31.670 And they’re not trying to think about what happens toward the surface. 00:47:31.670 --> 00:47:36.500 So some of that slip we can feed out onto the Tsaishi structure in the 00:47:36.500 --> 00:47:41.000 Rioni fold/thrust belt, where we just talked about the river terraces. 00:47:41.009 --> 00:47:44.749 But that is only depending on where you are within those 00:47:44.749 --> 00:47:50.930 margins of error – 25 to 50% of the geodetic shortening rate. 00:47:50.930 --> 00:47:53.869 So there are a couple of mechanisms to explain this discrepancy. 00:47:53.869 --> 00:47:57.380 One is that there are additional active structures, either within 00:47:57.380 --> 00:48:04.190 the range or potentially within the fold/thrust belt. 00:48:04.190 --> 00:48:07.020 There are some seismic data, which unfortunately are proprietary, 00:48:07.020 --> 00:48:11.719 and I have not actually seen, that suggest that there may be 00:48:11.719 --> 00:48:15.380 some duplexing at depth in that fold/thrust belt that 00:48:15.380 --> 00:48:20.260 could accommodate some of this additional modern shortening. 00:48:20.260 --> 00:48:23.480 Needless to say, more work is needed to really constrain this, 00:48:23.500 --> 00:48:26.620 but we do seem to have a discrepancy. 00:48:30.320 --> 00:48:33.160 So, long-term shortening rate over the last 30 million years 00:48:33.160 --> 00:48:36.680 is something like 6 millimeters a year. The short-term geologic rate, 00:48:36.680 --> 00:48:41.569 as best we can determine from the structure we looked at, 00:48:41.569 --> 00:48:44.719 is just over 1 millimeter a year. And then the geodetic rate 00:48:44.720 --> 00:48:48.120 is something closer to 4 millimeters a year. 00:48:49.140 --> 00:48:52.880 Real quick, I’m going to barrel through the Aragvi Traverse, which is 00:48:52.880 --> 00:48:54.920 in the central Greater Caucasus. I’m not going to show you the 00:48:54.920 --> 00:48:58.580 details here. I just want to sort of prove to you that I did similar work 00:48:58.580 --> 00:49:00.800 in the central Greater Caucasus. 00:49:00.800 --> 00:49:03.540 Because I want to leave some time for questions. 00:49:03.540 --> 00:49:06.660 But here’s the balanced cross-section that I produced. 00:49:06.660 --> 00:49:09.249 You can see that, broadly, it looks pretty similar. 00:49:09.249 --> 00:49:13.930 We have an imbricate thrust stack that is southward-vergent. 00:49:13.930 --> 00:49:16.119 Things are pretty steeply dipping in the core of the range 00:49:16.120 --> 00:49:18.640 and shallower as you move south. 00:49:20.460 --> 00:49:24.039 We can restore this balanced cross-section as well, and we get 00:49:24.040 --> 00:49:28.320 a similar amount of shortening – something like 200 kilometers. 00:49:29.630 --> 00:49:33.880 There are three major structures along this one that could accommodate 00:49:33.880 --> 00:49:38.989 additional shortening because there’s no traceable units across them. 00:49:38.989 --> 00:49:43.779 And then finally we had much worse luck getting low-temperature 00:49:43.779 --> 00:49:45.530 thermochronology along this traverse. 00:49:45.530 --> 00:49:50.560 I processed about 20 samples but only got three dates. 00:49:51.180 --> 00:49:57.340 But everything that we got is reset. So, unlike the western traverse, 00:49:57.349 --> 00:50:02.940 here we seem to have exhumation at least to 00:50:02.940 --> 00:50:06.680 4 kilometers for the full length of the traverse. 00:50:07.760 --> 00:50:12.500 All right. So let’s sort of wrap all this up and wrap it together to 00:50:12.509 --> 00:50:15.910 some orogen-scale interpretations of what might be going on. 00:50:15.910 --> 00:50:18.569 The first thing I want to point out is the structural style is broadly 00:50:18.569 --> 00:50:21.719 consistent along strike. Although we’re looking at different 00:50:21.719 --> 00:50:25.359 tectonostratigraphic units along the two traverses, which are separated 00:50:25.359 --> 00:50:29.240 by 200 kilometers or so, the major structural boundaries 00:50:29.240 --> 00:50:32.979 actually seem to be continuous between the two. 00:50:34.140 --> 00:50:38.940 Here’s another way of looking at that. This is the full Caucasus mountain belt. 00:50:38.940 --> 00:50:43.100 Again, it’s the Black Sea on the left and the Caspian on the right. 00:50:43.100 --> 00:50:47.320 And the two traverses that we’ve been looking at outlined in black. 00:50:47.320 --> 00:50:49.520 And all of the colored lines on here are faults. 00:50:49.529 --> 00:50:54.779 And I’ve color-coded them by the ages of units on either side of them, 00:50:54.779 --> 00:50:59.069 just as a way to get at roughly whether these things are active or not. 00:50:59.069 --> 00:51:03.029 So, if they’re cold colors – blue and green – that means 00:51:03.029 --> 00:51:05.769 that they have Mesozoic rocks in the footwall and 00:51:05.769 --> 00:51:09.080 either Paleozoic or Mesozoic rocks in the hanging wall. 00:51:09.080 --> 00:51:12.860 And if they are orange or red, they have Cenozoic rocks in the 00:51:12.869 --> 00:51:17.559 footwall and either Mesozoic or Cenozoic rocks in the hanging wall. 00:51:17.559 --> 00:51:24.209 So this is consistent with our field observations of structures 00:51:24.209 --> 00:51:28.400 that seem to be propagating toward the flanks of the orogens, 00:51:28.400 --> 00:51:32.860 particularly in the west. We seem to be southward-propagating. 00:51:32.860 --> 00:51:37.109 There’s an active fold/thrust belt in the central part of the range on the northern 00:51:37.109 --> 00:51:40.890 flank as well as on the southern flank. Unfortunately, this is also in Chechnya 00:51:40.890 --> 00:51:44.989 and Dagestan, which are not places that Americans can go these days. 00:51:44.989 --> 00:51:49.360 So it is going to remain a bit of an enigma, at least for the time being. 00:51:52.400 --> 00:51:54.099 Among these structures, there are a couple that seem to 00:51:54.099 --> 00:51:57.309 continue along strike. For example, we can connect 00:51:57.309 --> 00:52:01.450 structures that we see along both of the traverses that I studied 00:52:01.450 --> 00:52:06.519 along this feature that’s been called the Main Caucasus Thrust. 00:52:06.519 --> 00:52:09.799 And then, further south than that, there’s another structure, 00:52:09.800 --> 00:52:13.460 the Racha-Lekhumi Fault, that connects, again, 00:52:13.460 --> 00:52:18.240 faults that we see both in the west and central part of the Caucasus. 00:52:21.680 --> 00:52:25.520 The shortening magnitudes along strike are also quite similar. 00:52:26.920 --> 00:52:29.320 On the Enguri, we saw 170 kilometers. 00:52:29.320 --> 00:52:31.059 On the Aragvi, we saw about 200 kilometers. 00:52:31.059 --> 00:52:35.180 And we can use these to provide a minimum bound on the amount 00:52:35.180 --> 00:52:38.650 of shortening accommodated across the Greater Caucasus. 00:52:38.650 --> 00:52:41.969 So they’re by no means providing a maximum. 00:52:41.969 --> 00:52:43.970 We could hide a whole lot more shortening in there, depending on 00:52:43.970 --> 00:52:48.860 how much material has been eroded off the top, but we have at least 00:52:48.860 --> 00:52:51.420 somewhere on the order of 200 kilometers of shortening 00:52:51.420 --> 00:52:55.140 required to have been accommodated by this orogen. 00:52:56.749 --> 00:53:00.580 And finally, let’s think again about the rate through time. 00:53:00.589 --> 00:53:06.260 This is a little bit of repetition, but I wanted to sort of bring this point home 00:53:06.260 --> 00:53:12.380 with a nice little plot showing that change in shortening rate through time. 00:53:12.380 --> 00:53:15.700 So, in blue on this plot, we have the long-term rate somewhere 00:53:15.710 --> 00:53:20.101 in the ballpark of 6 to 7 millimeters a year as a minimum. 00:53:20.101 --> 00:53:25.849 And so, for the last 30 million years, the geologic rate on the short term, 00:53:25.849 --> 00:53:31.160 looking at the last 100,000, is something like 1 millimeter a year. 00:53:31.160 --> 00:53:34.170 Which is substantially less than the geodetic rate, 00:53:34.170 --> 00:53:37.500 which is more like 3 to 5 millimeters a year. 00:53:40.360 --> 00:53:48.300 So there are a couple implications here. One is that we could potentially make 00:53:48.309 --> 00:53:54.760 these rates match a bit better by playing with our age bounds a little bit. 00:53:54.760 --> 00:53:59.049 For example, if the shortening began earlier, say something like 00:53:59.049 --> 00:54:03.619 40 million years ago, then we could make that long-term geologic 00:54:03.620 --> 00:54:08.340 rate fall within the error of the modern geodetic rate. 00:54:11.460 --> 00:54:17.440 However, if this signal is real – this decrease in rate over time, 00:54:17.440 --> 00:54:20.640 particularly between the long-term geologic rate and the geodetic rate, 00:54:20.650 --> 00:54:23.920 because we have ways to explain the short-term geologic rate and 00:54:23.920 --> 00:54:28.069 modern geodetic rate discrepancy. But if this long-term decrease 00:54:28.069 --> 00:54:32.100 in rate is real, then we need a mechanism to explain it. 00:54:32.960 --> 00:54:36.700 And I’m going to suggest that that mechanism is slab detachment. 00:54:36.700 --> 00:54:40.109 So here’s a summary of everything we just covered. 00:54:40.109 --> 00:54:43.380 I’m not going to read through it again. 00:54:44.609 --> 00:54:46.980 But I do want to return to those models we started with. 00:54:46.989 --> 00:54:48.269 So remember we had two models – 00:54:48.269 --> 00:54:51.719 a large basin model and a small basin model. 00:54:52.780 --> 00:54:55.600 And the big differences between them in terms of their predictions 00:54:55.609 --> 00:55:00.880 for the history of the orogen are the size of the basin and 00:55:00.880 --> 00:55:04.349 what the shortening rate does through time. 00:55:04.349 --> 00:55:08.970 And the observations that we’ve just gone through are that we have 00:55:08.970 --> 00:55:12.699 about 200 kilometers of shortening and that we seem to have a decrease 00:55:12.699 --> 00:55:18.170 in rate from the long-term geologic rate to the modern orogen. 00:55:18.170 --> 00:55:21.210 So these results are, between these two models, much more compatible 00:55:21.210 --> 00:55:24.060 with this subduction and slab breakoff model. 00:55:27.529 --> 00:55:31.319 The last thing I wanted to point you toward is some seismicity 00:55:31.319 --> 00:55:38.849 data sets that potentially show us some of the same story. 00:55:38.849 --> 00:55:43.569 So here are two profiles across the Greater Caucasus. 00:55:43.569 --> 00:55:49.200 On both of these, the south is on the left and the north is on the right. 00:55:49.200 --> 00:55:53.480 And they’re showing, in true scale – the topography is exaggerated, 00:55:53.480 --> 00:55:58.520 but the dots are showing true-scale depth of seismicity with earthquakes 00:55:58.530 --> 00:56:02.749 scaled by magnitude. And what I want to point out is 00:56:02.749 --> 00:56:07.039 a distinct lack of earthquakes deeper than 40 kilometers or so 00:56:07.039 --> 00:56:14.010 in the west and a nice zone of deep earthquakes – that looks potentially 00:56:14.010 --> 00:56:17.180 similar to a dipping slab – in the central part of the range. 00:56:17.180 --> 00:56:22.299 And this actually continues into the eastern part of the orogen as well. 00:56:22.299 --> 00:56:27.059 So there’s potentially seismological evidence of a subducted slab 00:56:27.060 --> 00:56:29.540 in the eastern part of the range but not in the west. 00:56:29.540 --> 00:56:36.460 And so we suggest that this is showing us this ongoing slab 00:56:36.460 --> 00:56:40.700 detachment that’s propagating from west to east along the orogen. 00:56:42.420 --> 00:56:46.140 And just to tie things up here, I have a nice little video. 00:56:46.150 --> 00:56:48.539 It worked when we tested it earlier, so I hope it works here, 00:56:48.540 --> 00:56:51.780 but this is that same seismicity data set. 00:56:52.820 --> 00:56:57.960 And what we are seeing are these earthquakes with real depth. 00:56:57.970 --> 00:57:01.329 They’re color-coded by depth as well, so we’re going to look along the 00:57:01.329 --> 00:57:05.789 orogen from west to east and see that nice dipping slab. 00:57:05.789 --> 00:57:09.420 I’ll see if I can pause it here so you can see it here. 00:57:09.420 --> 00:57:12.829 So hopefully you can kind of begin to pick out this zone – 00:57:12.829 --> 00:57:15.119 north-dipping zone of deep earthquakes beneath the range. 00:57:15.119 --> 00:57:21.039 And we think this may be evidence of this relic-ed slab 00:57:21.040 --> 00:57:24.000 that is in the process of detaching. 00:57:26.220 --> 00:57:30.660 [Silence] 00:57:30.660 --> 00:57:36.340 So this is exciting because slab detachment is sort of this necessary 00:57:36.340 --> 00:57:40.719 fundamental step in the transition from subduction to continental collision, 00:57:40.719 --> 00:57:44.489 but modeling suggests that the surface response to that – what the geology 00:57:44.489 --> 00:57:49.199 looks like at the surface – occurs quite quickly, often within a million years. 00:57:49.199 --> 00:57:53.569 And so it may be be quite ephemeral. And so, if the Greater Caucasus are 00:57:53.569 --> 00:57:57.079 actively undergoing slab breakoff, then this could provide an important 00:57:57.079 --> 00:58:00.599 window into what the surface looks like during that process, 00:58:00.600 --> 00:58:05.170 and therefore what the geology record might be of such a process. 00:58:06.880 --> 00:58:11.040 And with that, I will happily take any questions that anybody might have. 00:58:12.560 --> 00:58:15.700 - Thank you very much, Chad, for an excellent talk. 00:58:16.800 --> 00:58:19.840 Are there any questions for Chad? If you have questions, please either 00:58:19.840 --> 00:58:25.700 type it into the meeting chat or unmute yourself and just directly ask. 00:58:27.940 --> 00:58:30.880 [Silence] 00:58:30.880 --> 00:58:33.780 - Yeah, Chad, this is Tom Hanks. 00:58:36.160 --> 00:58:41.260 And I was interested in the T4 terrace. 00:58:41.260 --> 00:58:43.479 - Yeah. - Which is right across the river 00:58:43.480 --> 00:58:46.680 from the T5 terrace. - Mm-hmm. 00:58:46.680 --> 00:58:51.080 - And yet shows no deformation at all. - Yeah, so let me go back 00:58:51.089 --> 00:58:55.499 to that slide real quick. - And then the question is, why not, 00:58:55.499 --> 00:59:02.469 or is there a different elevation of that terrace or a different age or what? 00:59:02.469 --> 00:59:04.660 - Yeah. So … 00:59:06.700 --> 00:59:08.300 Good catch. 00:59:10.440 --> 00:59:12.040 Just going to go back [inaudible]. 00:59:12.040 --> 00:59:16.619 So, talking about the difference between the T5 terrace and the T4. 00:59:16.620 --> 00:59:19.680 It turns out, when you look just at the T4, and unfortunately I don’t have 00:59:19.680 --> 00:59:24.580 a slide of it by itself, but it also has a little bit of deflection. 00:59:24.580 --> 00:59:29.100 It’s much less than the T5 surface. But the other important difference 00:59:29.109 --> 00:59:33.959 is that the T5 is a strath terrace and the T4 is a fill terrace. 00:59:33.959 --> 00:59:38.509 So our interpretation is that the T4 is actually much younger and has been – 00:59:38.509 --> 00:59:41.690 has filled back up to the elevation of the T5 but has recorded 00:59:41.690 --> 00:59:43.960 much less deformation. 00:59:44.769 --> 00:59:47.779 We don’t have any independent age control on the T4 surface, unfortunately. 00:59:47.779 --> 00:59:52.569 It’s all very anthropogenically modified, and we weren’t able to 00:59:52.569 --> 00:59:55.640 find a sampling location that we were happy with. 00:59:56.520 --> 00:59:59.819 But that is our interpretation. And I am happy to – I have 00:59:59.819 --> 01:00:02.201 some other figures that I’m happy to send you if you’re 01:00:02.201 --> 01:00:05.699 interested in sort of how we defend that. 01:00:06.300 --> 01:00:08.200 - Good. 01:00:08.200 --> 01:00:16.660 Surface exposure ages help you get the age of it? Or as the fill terrace 01:00:16.660 --> 01:00:22.450 with gravels coming down the river and then the channel cuts down and 01:00:22.450 --> 01:00:28.180 weaves and stuff, and the exposure age should at least give you some idea. 01:00:28.180 --> 01:00:31.900 - Yeah. That’s a – that’s a good point. We thought about that a little bit. 01:00:31.900 --> 01:00:34.809 The problem is, we’re in a pretty difficult sort of age window. 01:00:34.809 --> 01:00:39.150 We’re much too young for something like cosmogenic. 01:00:39.150 --> 01:00:43.020 And so the trick is – again, is finding a piece of the terrace 01:00:43.020 --> 01:00:46.840 that seems to be relatively undisturbed. 01:00:49.949 --> 01:00:52.360 - How young do you think the T4 terrace is? 01:00:52.360 --> 01:01:02.220 - So if the T5 is 100,000 years old, my guess, based on sort of the 01:01:02.220 --> 01:01:07.500 regional climate signals in the Caucasus – [inaudible], etc., 01:01:07.500 --> 01:01:13.359 is that that T4 surface is probably on the ballpark of 10,000 to 20,000. 01:01:13.359 --> 01:01:19.519 But that’s a back-of-the-envelope guess. And that’s consistent with the amount 01:01:19.519 --> 01:01:23.289 of deformation we see in it. If we try to – if we basically say that 01:01:23.289 --> 01:01:27.589 our rate is consistent over the last 100,000 years, and you look at 01:01:27.589 --> 01:01:30.180 about a fifth of that amount of shortening, that’s consistent with 01:01:30.180 --> 01:01:34.360 the amount of sort of warping of the T4 surface that we see. 01:01:37.820 --> 01:01:40.340 - Do we have any other questions? 01:01:42.420 --> 01:01:46.520 [Silence] 01:01:46.520 --> 01:01:50.840 - This is Walter Mooney. I have a question. [echoing] 01:01:52.260 --> 01:01:54.740 Can you hear me? - Yep. 01:01:54.749 --> 01:01:59.400 - So do you think the Caucasus are kind of a classic orogen? 01:01:59.400 --> 01:02:04.049 Does it look – does it look just like the Swiss Alps, which are very 01:02:04.049 --> 01:02:08.360 well-studied and good for comparison, or other orogens? 01:02:08.360 --> 01:02:12.419 Or is there something unique about this orogen? 01:02:12.419 --> 01:02:17.410 - So I think that the unique thing about the Caucasus is the time window 01:02:17.410 --> 01:02:20.189 that we’re capturing. If I were put on the spot, 01:02:20.189 --> 01:02:23.960 I would say that they probably are relatively similar to the Himalaya and 01:02:23.960 --> 01:02:28.800 the Alps, but they are much, much, much earlier in their evolution. 01:02:29.540 --> 01:02:31.520 And that’s why it’s exciting. 01:02:31.520 --> 01:02:36.960 We’re looking sort of at the proto stage of classic alpine collision. 01:02:39.000 --> 01:02:43.340 - One comment is you suggested that there’s a slab in the central 01:02:43.349 --> 01:02:50.719 and eastern Caucasus and a lack of a slab in the west. 01:02:50.719 --> 01:02:53.720 I made a quick look at the tomographic models. 01:02:53.720 --> 01:02:55.080 That’s another way of looking at it. - Yeah. 01:02:55.080 --> 01:02:56.980 - The seismicity is one way of looking for slabs. 01:02:56.980 --> 01:03:03.239 Another way is looking for high velocities in the global tomography. 01:03:03.239 --> 01:03:07.930 - Mm-hmm. - And I agree that it – to the west, 01:03:07.930 --> 01:03:11.100 there doesn’t seem to be a slab in the tomography. 01:03:11.100 --> 01:03:13.500 It’s not so clear on the east, either, but … 01:03:13.500 --> 01:03:15.600 - Yeah. - … it’s permissible. 01:03:15.600 --> 01:03:18.900 - Right. So there’s a – sorry. - [inaudible] 01:03:20.620 --> 01:03:27.880 - There is a relatively recently funded NSF project with some of our Georgian 01:03:27.880 --> 01:03:36.329 collaborators and Eric Sandvol at Missouri, who are doing a local seismic 01:03:36.329 --> 01:03:40.069 deployment trying to get a higher resolution tomographic model. 01:03:40.069 --> 01:03:45.279 And they’re looking specifically for that. So hopefully we will learn more soon. 01:03:45.279 --> 01:03:48.799 I agree that the global tomography models are – 01:03:48.800 --> 01:03:52.680 they sort of permit the idea, but they are pretty agnostic. 01:03:53.780 --> 01:03:56.640 - I agree with you. Thank you. Thanks for your talk. 01:03:59.020 --> 01:04:01.520 - Any more questions? 01:04:03.020 --> 01:04:15.960 [inaudible speaking in background] 01:04:15.960 --> 01:04:19.740 - I’ll try once again, Chad. This is Tom Hanks again. 01:04:21.200 --> 01:04:27.490 This subduction of continental crust – I’ve never really thought very much 01:04:27.490 --> 01:04:31.420 of it, but you’re pushing low-density stuff … 01:04:31.420 --> 01:04:35.460 - Mm-hmm. - … into higher-density stuff. 01:04:35.460 --> 01:04:40.740 And at least simple physics say that shouldn’t work too well. 01:04:40.740 --> 01:04:44.579 - Right. - So how do you – how do you initiate 01:04:44.579 --> 01:04:52.369 this and drive it as opposed to just … - Just [inaudible] … 01:04:52.369 --> 01:04:59.400 - … doing the [inaudible] making, you know, big hill of stuff, you know, 01:04:59.400 --> 01:05:06.800 that sits on a – presumably a colder upper mantle. 01:05:06.800 --> 01:05:13.320 - Yeah. That’s a great point. So the other sort of regional geology 01:05:13.329 --> 01:05:18.539 detail that I skipped at the very beginning is that the Black Sea and 01:05:18.539 --> 01:05:23.549 the South Caspian both have relic-ed oceanic crust in them. Now. 01:05:23.549 --> 01:05:27.650 So the interpretation is that the Greater Caucasus Basin in the 01:05:27.650 --> 01:05:32.820 Mesozoic was underlain by oceanic crust, and that’s what subducted. 01:05:32.820 --> 01:05:35.999 But there is some debate over whether that’s actually the case. 01:05:35.999 --> 01:05:41.229 And that’s sort of the argument that the folks in the small basin pure shear 01:05:41.229 --> 01:05:44.869 thickening camp make is that we don’t know what the crust looked like. 01:05:44.869 --> 01:05:50.700 And if it was continental crust, then you wouldn’t expect that to subduct. 01:05:52.100 --> 01:05:54.420 - I should also say that’s a very interesting talk – 01:05:54.430 --> 01:05:59.749 not the sort of the thing we hear too often in the Earthquake 01:05:59.749 --> 01:06:05.089 Science Center, so thanks for bringing it to us. 01:06:05.089 --> 01:06:06.820 - Thank you. 01:06:08.200 --> 01:06:11.340 - Chad, there is a question from [inaudible]. 01:06:11.340 --> 01:06:15.599 He says, thanks, Chad. Is it possible that the difference 01:06:15.599 --> 01:06:19.170 between the shorter-term rate and middle-term rate is due to the presence 01:06:19.170 --> 01:06:24.100 of additional structures which weren’t reflected by the deformation of T5? 01:06:24.100 --> 01:06:28.120 Sorry if you addressed this during your talk and I missed it. 01:06:28.120 --> 01:06:31.959 - That’s a great question, and the answer is yes. 01:06:31.959 --> 01:06:35.299 There is a good chance that there are additional structures. 01:06:35.299 --> 01:06:39.499 If there are additional structures that are active, the trick is 01:06:39.499 --> 01:06:42.999 figuring out where they are. They don’t appear to be in the 01:06:42.999 --> 01:06:48.160 foreland basin because we have terrace profile data and river profile 01:06:48.160 --> 01:06:53.430 data across the whole northern half of the Rioni foreland basin. 01:06:53.430 --> 01:06:57.369 And there’s no signal that we see of active deformation. 01:06:57.369 --> 01:07:02.299 The place that it might be if it’s not in that foreland belt 01:07:02.299 --> 01:07:07.430 is in the main range itself. And that gets much harder to tease out, 01:07:07.430 --> 01:07:14.249 in part because of the relief and the exhumation and erosion rates. 01:07:14.249 --> 01:07:17.769 They’re quite high. We’re right downwind of the 01:07:17.769 --> 01:07:20.710 eastern Black Sea, so there’s a lot of precipitation, particularly 01:07:20.710 --> 01:07:23.440 in the western end of the Greater Caucasus. 01:07:23.440 --> 01:07:27.779 And so that signal could pretty easily be buried. 01:07:27.779 --> 01:07:32.730 But that is certainly where I would look if I were to – if I were given 01:07:32.730 --> 01:07:35.150 a bunch of money right now to go back and try to figure out where 01:07:35.150 --> 01:07:38.100 the rest of that shortening is going. That’s where I would look. 01:07:39.880 --> 01:07:42.280 - Any more questions? 01:07:44.660 --> 01:07:47.980 - Yeah. I have one. This is Belle Philibosian. 01:07:47.980 --> 01:07:50.000 Really enjoyed your talk. 01:07:50.009 --> 01:07:56.229 Sort of a follow-up to the previous questions, in that western cross-section, 01:07:56.229 --> 01:08:01.589 you have this – you have, in the northern part, these much higher 01:08:01.589 --> 01:08:03.789 or younger exhumation rates. 01:08:03.789 --> 01:08:09.160 And then, slower or lower exhumation rates in the south. 01:08:09.160 --> 01:08:13.339 So, you know, despite the fact that your one identified active structure 01:08:13.340 --> 01:08:19.700 is the one in the south. [laughs] So do you think that suggests – 01:08:19.700 --> 01:08:23.460 do you think that’s evidence that there is another active structure in the 01:08:23.469 --> 01:08:26.400 core of the range that’s causing that difference in exhumation? 01:08:26.400 --> 01:08:31.020 Or how do you reconcile that with the whole picture? 01:08:31.020 --> 01:08:35.680 - Yeah. Great point. If I can – let me turn on sharing 01:08:35.680 --> 01:08:39.420 again and bring up that cross-section one more time. 01:08:41.480 --> 01:08:45.000 And hopefully I can make that a bit clearer. 01:08:45.000 --> 01:08:52.520 So our interpretation – you’re correct that the exhumation suggests that – 01:08:52.520 --> 01:08:55.279 I mean, the exhumation is in the core of the range, and the active shortening 01:08:55.279 --> 01:08:58.859 is on the southern flank. Our interpretation in order to explain 01:08:58.859 --> 01:09:03.259 that is that there’s crustal ramp in the core of the range that things are 01:09:03.260 --> 01:09:08.920 getting uplifted over. And then they’re basically translating along a flat. 01:09:09.440 --> 01:09:15.220 And so the uplift is disconnected from the surface shortening. 01:09:18.380 --> 01:09:22.960 One way that some of our collaborators have been trying to tease that out is 01:09:22.960 --> 01:09:28.000 by looking at [inaudible] average erosion rates across the orogen. 01:09:28.000 --> 01:09:33.260 It turns out it’s pretty hard to do with Mesozoic flysch [laughs] 01:09:33.260 --> 01:09:36.009 because there isn’t a whole lot of quartz. 01:09:36.009 --> 01:09:41.650 But Adam Forte has been trying his hardest, and the beginnings of those 01:09:41.650 --> 01:09:43.989 results are just sort of starting to come together. 01:09:43.989 --> 01:09:49.389 And it basically looks like there’s no change in erosion rate across any 01:09:49.389 --> 01:09:53.139 of these major structures in the core of the range, which suggests that 01:09:53.139 --> 01:09:56.320 they don’t seem to be active and that we would probably 01:09:56.320 --> 01:10:01.540 expect some difference across them if that were the case. 01:10:01.540 --> 01:10:04.889 So that’s more or less consistent with this idea of things uplifting 01:10:04.889 --> 01:10:08.679 over a ramp in the range and then translating over a flat 01:10:08.680 --> 01:10:12.820 as they sort of erode as they move southward. 01:10:14.760 --> 01:10:16.620 - All right. - Great. Thanks. 01:10:16.620 --> 01:10:21.840 I guess that doesn’t help you with where to put that missing [inaudible]. 01:10:21.840 --> 01:10:24.200 - No. Unfortunately it doesn’t. 01:10:26.020 --> 01:10:28.300 - All right. Thanks, again, Chad. This has been a great talk. 01:10:28.309 --> 01:10:30.610 Thank you very much, and thanks, everybody, for joining us. 01:10:30.610 --> 01:10:32.280 And see you next week.