Neogene Deformation History of Western North America and Volcanism in Coastal California
Animated tectonic reconstruction at regional and close-up scales
North America is kept stationary in these models. Gridded areas highlight larger rigid blocks, non-gridded areas are treated as non-rigid crust. Thick orange lines show the location of the fastest moving faults at any given time. Colored stripes offshore depict age and motion of the oceanic Pacific plate through time.
These movies animate a model we created to both reconstruct the geography of California through time and to aid predictions about how it will change in the future. Western California is cut by a major plate tectonic boundary called the San Andreas transform boundary. This broad boundary is composed of many faults that divide the land into separate pieces called fault blocks. The fault-bounded blocks create earthquakes as they jostle each other while being rafted along as pieces of crust caught within the San Andreas transform boundary. Some faults that compose this boundary moved rapidly millions of years ago but now move slowly or not at all. Other, younger faults are moving rapidly today and are a potential source of damaging earthquakes.
As you watch these movies, you can see faults and the fault blocks they bound start to move at various times in the past. Motion will slow on some blocks as the location of most rapid fault slip (thick orange line) jumps eastward. Each movie frame is a snapshot in time, numbered in millions of years before present (Ma). Watch for the initial fault to form near the base of the continental slope (thin blue line) about 28.5 Ma. A second fault segment forms to the north about 2 million years later, and the two segments grow towards each other and join. About 19 Ma, this initial trace of the San Andreas transform boundary is abandoned and a new trace forms near the present-day coastline. About the same time, a new segment forms near the western coastline of Baja California. The misalignment of these two segments triggers rotation of the Transverse Ranges in central California. About 12.4 Ma, the most active trace of fault system jumps eastward to its present-day location, known as the San Andreas fault. At this time, the fault extends south through the nascent Gulf of California, and Baja California begins to tear away from mainland Mexico. More detailed information can be found in the Journal Abstract and Synopsis below.
The geologic record of coastal California includes evidence of numerous volcanic centers younger than 30 Ma that do not appear to have erupted in an arc setting. By correlating these volcanic centers with specific slab windows predicted from analysis of magnetic anomalies on the Pacific plate, we add new constraints to tectonic reconstructions since 30 Ma. Our correlations— such as erupting the Morro Rock-Islay Hill complex south of the Pioneer fracture zone and the Iversen Basalt south of the Mendocino fracture zone—require larger displacements within western North America than advocated by most previous authors. Specifically, we infer at least 315 km of motion between the Sierra Nevada and rigid North America at an azimuth of about N60°W, and at least 515 km between Baja California and rigid North America in a similar direction. A consequence of inferring a large displacement of Baja California is that the Pacific-North American plate boundary must have developed most of its current form prior to 10 Ma. We interpret a slab window developing between Cocos and Monterey plates after 19 Ma that reconstructs under nearly all of the southern California volcanic centers dated at 18-14 Ma. Most of the sedimentary basins associated with volcanic rocks show brief periods of rapid subsidence synchronous with volcanism, followed by slow subsidence of variable but often extended duration, consistent with rapid extension of cold lithosphere over recently introduced hot asthenosphere.
Our goal is to determine whether correlating the coastal volcanic rocks with the slab windows has any unreasonable implications that would require revising generally accepted ideas about relative motions in the late Cenozoic. To this end, we define our model for North American deformation more rigorously than in previous studies by specifying finite rotations on a sphere among large numbers of rigid blocks, using the same mathematical description as motions of the major plates. We combine a global circuit of major plate motions, similar to that used by other recent authors, with this new model that describes relative motions within North America using finite rotations. The main advantage of our technique is that the position of a continental fragment relative to the Pacific plate is an exact prediction of the kinematic model; relative positions of volcanic centers and slab windows can be used as direct tests of a set of relative motion parameters. Furthermore, with all of the reconstruction performed by parameterized modeling, internal consistency of the reconstructions is tested much more rigorously than in reconstructions with a substantial component of freehand drafting.
Our model for North America includes a few major blocks generally recognized as mobile but nearly rigid, namely the Colorado Plateau, Sierra Nevada-Great Valley, Baja California, and the Sierra Madre Occidental (see Figure 4 in full publication). Close to the coast, especially in the areas where the volcanic rocks are observed, we keep track of many individual blocks bounded by major to moderate faults. For the inland deformation zones, commonly characterized by basin and range extension, we ignore most of the individual faults and fault blocks, considering only the motion between the major blocks and the North America craton.
Though some of the resulting reconstructed positions differ from what has commonly been published, we find no serious problems with these positions. We see our interpretation of a moderate increase in motion of the Sierra Nevada relative to the Colorado plateau or North America as a refinement of previous work, not requiring serious reinterpretations, and not clearly preferable to potential revisions in the plate circuit. Previous estimates of the original position of Baja California have been bimodal, with a majority advocating about 300 km of motion and a minority advocating about 500 km. Our reconstruction of the past positions of coastal volcanic units from 27 to 12 Ma strongly supports the minority view of at least 500 km of displacement. A direct consequence of this interpretation is that the approximate geometry of the current Pacific-North American plate boundary in California and western Mexico developed at about 10-12 Ma.
Within moderate uncertainties, there is no need to modify either the global plate circuit or the reversal time scale to satisfy the constraints derived from the slab window correlations. Of course, this consistency does not necessarily imply that either our reconstruction, the plate circuit or the time scale is correct, and better testing of the reconstruction will be possible as independent information refines the time scale and plate circuit.
The full publication is currently available on the AGU Recent Articles with Dynamic Content web page. Copyright  American Geophysical Union.