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Hi, everyone. My name is Matt Tarling.
I’m a postdoc at McGill University.
And today we’ll be going a little further afield than northern California, looking
at earthquake rupture signatures in a serpentinite shear zone in New Zealand.
So fossil earthquakes in serpentinite.
I’ll be presenting what we believe to be the first reported evidence
for an earthquake signature in a natural serpentinite shear zone
and then briefly looking at some numerical modeling which confirms the
hypothesis and sets some bounds on the earthquake magnitude responsible.
So serpentinites are ultramafic rocks that form from the hydration
of mantle material, such as peridotite.
So this occurs wherever there’s the opportunity for water to interact
with anhydrous ultramafics. This occurs in subduction zones,
in transform faults, both oceanic and, in some instances, continental,
as well as mid-ocean ridges and ophiolites, among others.
There, however, remain some pretty big questions with regard to
serpentinite’s slip behavior, which we need to answer if we
want to really properly understand the rheology of these tectonic settings.
One important observation that’s being made is this association between
the presence of frictionally weak serpentinite and serpentinite-associated
minerals and active creeping fault segments.
Which raises an interesting question. Do earthquake ruptures nucleate or
propagate through serpentinite-bearing fault segments in shear zones?
And then, from the perspective of the field geologist, if we look at
an exhumed example of a serpentinite shear zone,
what would we find in the rock record?
What would be the signature of a fossil earthquake in serpentinite?
And, more broadly speaking, in most rocks, the signature of
an earthquake is either related to a thermal process due to coseismic
frictional heating or a mechanical process, such as pulverization
or cataclasis. But really the golden standard when it comes to earthquakes
in the rock record are frictional melts known as pseudotachylytes.
We have an example here where this black line is the fault surface,
and the black rock is pseudotachylyte. So a frictional melt that occurred
due to heating during the earthquake. And here we have an injection vein.
However, pseudotachylyte has never been found in serpentinite.
But we can get some clues from experimental studies.
So there are many high-velocity shear experiments which have been done
at seismic slip rates which can give us some hints as to what a fossil
earthquake might look like, at least from a lab perspective.
And what they show when we look at the products of these experiments
is that the serpentinite dehydrates, as it might be expected,
converting back into those primary ultramafic minerals, such as olivine,
as well as amorphizing and, in some instances, possibly melting.
So, equipped with that information we’ll turn to the field and look at
a natural example of an exhumed serpentinite shear zone in New Zealand.
So this is the Livingstone Fault. The Livingstone Fault is a trans-crustal
plate boundary that extends over 500 kilometers from the South Island,
cut by the Alpine Fault, and then through to the North Island.
And it juxtaposes the continental quartzofeldspathic schists here
on the right against the ultramafic Dun Mountain ophiolite,
so peridotite and serpentinized peridotite.
That’s these brown-reddish rocks on the left.
And then, between the two, we have the Livingstone Fault.
So the Livingstone Fault is characterized by a scaly serpentinite
mélange that extends up to about 400 meters wide and is really
characterized by a pervasive anastomosing foliation.
So this is a foliation where we have slip surfaces at the millimetric
up to about the meter scale defining lenticular domains of serpentinite.
And it’s pretty consistent and pervasive all the way through
this package of rock.
Embedded within that – within that anastomosing matrix,
we have larger lithons, which you can see here along the ridge.
We’re looking down here at a mountain pass exposure.
So we have these lithons that are forming spines along the ridge,
and they consist of massive serpentinite lenses as well as rodingite,
a type of metagabbro that forms in serpentinite.
Here’s one of these rodingite pods. So they tend to be self-similar
with the fabric, so lenticular-shaped. And they’re embedded within this
serpentinite shear zone and typically tend to be coated with serpentinite or
even characterized by slip surfaces that go around them.
And these slip surfaces are something pretty interesting.
About the Livingstone Fault and probably most serpentinite shear zones
is that – is that we often find that, contrasting with this broad distribution
of strain in this scaly serpentinite, there are a great number of
sharp, discrete fault surfaces.
Some are which – some of which are along the margins of pods,
or in some cases, they cut through that scaly anastomosing fabric.
So we have an example here on the bottom right of
a slip surface on a rodingite.
And, on the left, this is a polished slip surface of the edge of a chromitite pod.
One of these sharp slip surfaces that we find both on the margins of pods
and cutting this anastomosing fabric that are particularly interesting
are magnetite-coated, or magnetite-bearing slip surfaces.
We have an example here where we have patchy magnetite on the
edge of a massive serpentinite pod. And if we take a closer look,
we find that the – we have here at the top in this photomicrograph,
we have that magnetite slip surface, that polished surface being the top.
And, below it, we have that serpentinite where we have that
characteristic anastomosing fabric. So these magnetite veins – in this case,
these little veinlets, delineate the lenticular domains of serpentinite.
And, if we take a much closer look again at the magnetite layer,
so on that principal slip surface.
This is that polished surface that we were looking at in the previous photos.
We find that we have a magnetite layer of about – of a few hundred microns,
maybe up to a millimeter in some places, which has
a great number of inclusions within that magnetite surface.
And it’s these inclusions that we’re going to take a closer look at.
And the reason being is that it appears that these inclusions,
within the magnetite layers, preserve evidence for progressive
amorphization and dehydration, which we interpret as being –
as having occurred due to the passage of a coseismic thermal pulse.
So we’ll start – in this little diagram, we’re looking at the top of the slip
surface here, and then going further away from the slip surface into
the surrounding serpentinite.
So we’ll start in the surrounding serpentinite below the slip surface.
And what we find here is – well, it’s serpentine that is pretty
characteristic of most of the fault. It’s crystalline serpentine.
So we’re looking at lizardite and chrysotile.
And it’s pretty typical of the ambient deformation temperature of the
Livingstone Fault, so somewhere around 300 degrees.
Now, stepping into that magnetite layer, we find something a little bit different.
The same lizardite and chrysotile are present, but there appear to be
poorly crystalline or even amorphous, in some cases.
And, if this was due to a thermal process, this probably would indicate
a temperature of about 570 degrees. So these temperatures that
I’m giving you here are from a compilation of temperatures
reported in the literature that I’ve averaged out.
Taking one step further towards that principal slip surface, so further up
into that magnetite layer, we find the appearance of interlayered talc
and serpentine, along with amorphous serpentine and poorly crystalline
serpentine, indicating that serpentine is likely breaking down.
This is indicative of a temperature of probably around 620 degrees.
Another step further towards that principal slip surface,
we see the appearance of poorly crystalline nanogranular olivine,
which co-exists with amorphous serpentine
as well as that talc that we saw previously.
If this is due to a thermal process,
this indicates a temperature of about 700 degrees.
Yet another step towards that principal slip surface,
and the talc that we saw previously is no longer present.
And the assemblage largely consists of moderately crystalline
olivine and amorphous material.
This assemblage, taken with the disappearance of talc, presumably due
to thermal dehydration, would indicate a temperature of about 800 degrees.
If we go yet another step towards that principal slip surface,
we now find that these inclusions contain almost exclusively
fairly well-crystalline nanogranular olivine.
So we’re looking at nanograins of olivine in the 20- to 200-nanometer
range. And this is indicative of a temperature of about 833 degrees.
And finally, within microns of that top slip surface, we find
the appearance of enstatite. So now the assemblage consists
of fairly well-crystalline nanogranular olivine along with enstatite, which,
if occurred due to a thermal process from the dehydration of serpentine,
would indicate a temperature of about 925 degrees.
These observations appear to be consistent with the
progressive amorphization and dehydration of serpentine.
So, at lower temperatures, serpentine dehydrates to talc and olivine, releasing
water, while at higher temperatures, it dehydrates to forsterite and enstatite.
If we compare these observations made on this natural example,
we find that they’re strikingly similar, at least in appearance, to those that
we saw from those high-temperature experiments.
So this nanogranular olivine we have in the natural rock compared to
the nanogranular olivine in the high-velocity shear experiments
as well as that enstatite which co-exists with olivine.
And so here we have a cartoon representation of this magnetite layer.
So the central part is that magnetite. On either side, we have serpentinite.
And then we have that principal slip surface right here.
And I’ve placed each of these inclusion assemblages as a function of their
distance from the slip surface as well as the coseismic temperature that they
could potentially represent. And we see that they form what
looks to be a nice curve going away from that slip surface.
And, in order to get a sense of whether that might be consistent with
an earthquake rupture, I performed some numerical modeling.
Now, I’m not going to go into intimate detail into the modeling.
It suffices to say that it was important to take into account all of the physical
processes that occur during frictional heating and phase transformation
of serpentinite. So we have the frictional heating itself, the dehydration
of serpentinite, there’s an energy cost to that phase change, the production
of porosity, the release of water, and the pressurization –
the thermal pressurization of that water, which in turn
reduces the effectiveness of frictional heating.
Now, running that model for a range of slip distances, we find that this
curve that the inclusion assemblages represents going away from the
principal slip surface is pretty well constrained by the thermal maxima
that might occur due to coseismic frictional heating.
So that’s this red line here.
And this sets a nice lower bound, so magnitude 2.7.
So that’s a pretty reasonable lower bound to the assemblages observed.
And the upper bound is pretty well constrained by a magnitude 4.
That is the thermal – the thermal maximum due to a magnitude 4
earthquake. So I think it’s fairly convincing that what we’re looking at
here truly is an earthquake fossil – a fossil earthquake due to
an earthquake in a serpentinite shear zone.
And so, to conclude, I do believe that we have fairly convincing evidence
for earthquake rupture and coseismic dehydration of serpentine in a natural –
in a natural serpentinite fault. And, on this basis, I would suggest that
you may not be able to discount the possibility that an earthquake
could propagate through a serpentinite-bearing shear zone
based on the mere presence of serpentinite itself. Thank you.
If you’d like to find out more, please do go read our paper.
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