Sedimentary Archives of Active Tectonics
Wherever the Earth’s surface sinks down in response to applied tectonic forces, sediments are deposited to fill the space created by subsidence. Examples of actively subsiding basins include the Ganges River plain in India, the Mississippi Delta, and the San Francisco Bay. As sediments accumulate they store a record of the local environment, depositional processes, and changing climate. Sedimentary deposits also contain a wealth of information about ancient faults, structures, and regional tectonic forces that drive basin subsidence. When sediments are later revealed by uplift and erosion, stratigraphers can extract information from the deposits to reconstruct histories of landscape evolution, climate change, and crustal deformation in tectonically active regions.
Becky Dorsey and her students use sediments to study the San Andreas fault system in southern California and NW Mexico, where it makes up the active plate boundary between the Pacific and North American plates. Because the shape of the plate boundary is highly irregular, the crust is subjected to complex deformation as the two plates grind past each other. We want to know when faults have turned on and off and how their behavior has changed through time, to help us understand controls on crustal deformation and landscape evolution. Sedimentary rocks in this region preserve a record of alternating basin subsidence and uplift over the past ~8 million years, reflecting a complex history of fault initiation, growth, and destruction.The Fish Creek – Vallecito Basin contains a remarkably well exposed record of these processes.
Basin analysis also has yielded new insights into the birth and evolution of the Colorado River. As the Pacific plate in southern and Baja California moves obliquely away from North America, this motion has opened up a deep gash, or “rupture” in the old continent that represents the early stage of a new ocean basin (see image on right). The Colorado River first entered the Salton Trough lowland about 5.3 million years ago, concluding a major integration event that completely reorganized river drainages in the Colorado Plateau region. Vigorous erosion by the Colorado River has transferred a large volume of crust from the stable continental interior to deep basins embedded in the active plate boundary over the past 5-6 million years (see related Paper). Thus we see that processes of fluvial erosion and sedimentation in this setting play a major role in regional-scale recycling of the Earth’s crust.