Earthquakes and erosion are two of the major Earth processes that affect civilization. Geologists now understand that plate tectonics, which causes earthquakes, and the erosive and depositional processes at the Earth's surface are strongly interrelated; each set of processes affects the other in a what is known as a coupled system. Making new discoveries in the realm of natural hazards therefore requires holistic, integrated study of complex processes spanning a range of length and time scales.

The actions of plate tectonics and surficial processes are strongly coupled and readily apparent where fault motions provide sources and sinks for sediment. The upward motion of rock in mountains creates a mass source to be eroded, while the downward drop of valleys provides the hole where sediments can accumulate in thick layers. Geologists track these interactions by measuring the rates of mountain uplift and comparing them to sediment accumulation, while analyzing the architecture and mechanics of the intervening, causative faults.

In this research project, we used the San Andreas fault in southern California as a test case for learning more about how mountain uplift is balanced by sediment deposition via vertical fault motions. The southern San Andreas fault is well known as a potential source of future earthquakes, but is also understood to have a long, complex history that involves many other structures. We have discovered that periods of extension, or the pulling-apart of the Earth’s crust, occurred in the earlier phases of motion on the San Andreas fault, resulting in just the type of motions that provide a source and sink for sediment.

We have measured the growth of a source of sediment as the uplift of a chain of mountains around the Salton Sea known as the Santa Rosa, San Jacinto, and Little San Bernardino Mountains. Portions of these mountains uplifted during a phase of extension on a gently-inclined fault several million years ago. Today, the mountains continue to uplift due to extension on more steeply-inclined faults that lie between the San Andreas and San Jacinto faults. The uplift of these mountains has contributed a significant fraction of sediment that has been deposited in the adjacent Coachella Valley, where the San Andreas fault cuts through an oblique rift in the crust that connects to the Gulf of California to the south. Although the majority of the kilometers-deep sediment fill in the valley has been provided by the Colorado River, the uplifting mountains have provided the trap for the sediment, and in turn have enabled the bottom of the valley to hover near sea level, thus preventing northward penetration of the Gulf of California into the continent.

In addition to shaping the landscape that we see today, the vertical motion of these mountain blocks and adjacent sedimentary basins have influenced the state of stress and geometry of faults in the San Andreas system via feedbacks in a coupled evolution. The multi-phase, complex history of vertical motions in this area is surprising, given the simple notion of the San Andreas fault as a strike-slip boundary involving lateral (horizontal) sliding of tectonics plates against each other. These vertical motions have also influenced the modern connectivity of active faults in the region, and therefore have been in important control on the size, distribution, and timing of future damaging earthquakes in the Los Angeles area.

By studying the development of mountains, basins, and structures over millions of years, we have therefore gained new understanding of how plate tectonics interacts with surface processes and how together these processes lead to and influence the natural hazards we face today.

Last Modified: 05/02/2017
Modified by: James A Spotila