We stand at a critical juncture in the evolution of the Pacific/North America plate boundary system. This major transform plate boundary system that began 12-14 million years ago has both grown and migrated inland, generating distributed shear across California and the Great Basin. The evolution continues to the present. Although a map of the Basin and Range suggests spatially uniform extension, geodesy reveals a recent focusing of deformation near its western and eastern edges. This, in turn, suggests dramatic differences in the material properties within the crust and mantle across the Basin and Range that are probably thermally induced.
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Crustal Strain and Deformation
How the solid Earth responds to deformational forces is a topic of considerable uncertainty. The solid Earth is compositionally heterogeneous, and its ability to carry and transmit stress varies widely with composition, mineralogy, pressure, temperature, deformation history, and the presence or absence of fluids. Variations in these properties determine where the crust will deform broadly and slowly or, alternatively, break locally and rapidly in a damaging earthquake. To understand how the crust will respond to tectonic and/or volcanic forces requires answers to several key questions.
Deformation within the Pacific/North American plate boundary zone occurs over a wide range of spatial and temporal scales. On the largest scale, shortening that produced the North American Cordillera is giving way to shear and extension that is now pulling this region away from the stable continent. This provides us with an actively deforming natural laboratory that should greatly increase our understanding of plate-boundary processes.
Because these processes have dominated North American geology over the last several billion years, the North American continent is an excellent place to unravel the general evolution of our continent.
Upper left: Calculated coseismic Coulomb stress changes caused by fault slips associated with the 1992 Joshua Tree (JT), Landers (L), and Big Bear (BB) quakes. Lower right: same as above but stresses are shown for the top surface and a cut plane along the San Andreas Fault. Upper right: calculated combined stress changes associated with the 1992 quakes, the 1999 Hector Mine quake, and postseismic relaxation of a viscous lower crust layer from 1992 through 2020. Bottom right: same as above but stresses are shown for the top surface and a cut plane along the San Andreas Fault. Figure from: Freed, A. M. and J. Lin, 2002, Accelerated stress buildup on the southern San Andreas fault and surrounding regions caused by Mojave Desert earthquakes, Geology, in press.
Research Questions
- How do crust and mantle rheology vary with rock type and with depth?
- How does lithospheric rheology change in the vicinity of a fault zone?
- What is the distribution of stress in the lithosphere?
- What types of transient movements occur in the lithosphere?
- What is the role of non-tectonic processes in creating lithospheric stress? >
- How do faults interact with one another?