Joint 118th Annual Cordilleran/72nd Annual Rocky Mountain Section Meeting - 2022

Paper No. 27-4
Presentation Time: 2:30 PM


BENNETT, Scott1, DARIN, Michael2, MILLER, David M.3, DORSEY, Rebecca J.4, MAVOR, Skyler5, NURIEL, Perach6, THOMPSON, Lisa7, OSKIN, Michael E.8, BEARD, L. Sue9, CYR, Andrew10, BRICKEY Sr., Timothy11 and UMHOEFER, Paul J.11, (1)Geology, Minerals, Energy, and Geophysics Science Center, U.S. Geological Survey, 2130 S.W. Fifth Avenue, Portland, OR 97201, (2)Nevada Bureau of Mines & Geology, University of Nevada, Reno, 1664 N. Virginia St, MS 0178, Reno, NV 89557-0178, (3)Geology, Minerals, Energy, & Geophysics Science Center, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, (4)Department of Earth Sciences, University of Oregon, Eugene, OR 97403, (5)Geosciences, Colorado State University, 400 University Ave., Fort Collins, CO 80523-0001; U.S. Geological Survey, 2255 N Gemini Dr. 86001, Flagstaff, AZ 86001, (6)The Geological Survey of Israel Geochemistry, 32 Yeshayahu Leibowitz St., Jerusalem, 9692100, ISRAEL; Oregon State University, Corvallis, OR 97331, (7)Arizona Geological Survey, 1955 East 6th Street, P.O. Box 210184, Tucson, AZ 85721, (8)Earth and Planetary Sciences, University of California, Davis, Davis, CA 95616, (9)U.S. Geological Survey, 2255 N. Gemini Drive, Flagstaff, AZ 86001, (10)U.S. Geological Survey, Geology, Minerals, Energy and Geophysics Science Center, PO Box 158, 350 N. Akron Road, Moffett Field, CA 94035, (11)School of Earth and Sustainability, Northern Arizona University, Flagstaff, AZ 86011

The eastern California shear zone (ECSZ) is a significant component of the distributed Pacific-North America plate boundary. However, the true width, longevity, and total displacement of this shear zone is largely underappreciated. Here we summarize and reconstruct published fault slip data that characterize the full extent, lifespan, and evolution of the ECSZ. GPS data, seismicity, and neotectonic studies define the active ECSZ as a ~100 km-wide, NW-trending belt of active faults and rotating blocks that accommodates ~25% of dextral plate boundary shear. Geologic mapping indicates >50-75 km of total dextral displacement across this zone. However, post-middle Miocene NW-directed dextral shear also occurred on structures both west and east of the active ECSZ, including the El Mirage fault, Stateline fault, and distributed faults in SE CA and SW AZ. Our animated tectonic reconstruction highlights how an additional 45-60 km of post-middle Miocene dextral shear occurred across a ~100 km-wide, NW-trending belt that obliquely intersects the lower Colorado River between Lake Mead and Yuma, AZ, entirely inboard (east) of the active ECSZ.

Recent U-Pb dating of fault-zone opal in the active western ECSZ, U-Pb dating of fault-zone calcite in the mostly inactive eastern ECSZ, and stratigraphic studies in the southern Blythe basin all show that faults related to NW-directed shear were active across a broader ECSZ during late Miocene time. This suggests that distributed dextral plate boundary strain has localized into a narrower zone through time, now mostly focused into the western half of the ECSZ. This contrasts with a model that relates inactivity of strike-slip faults in the east to a conveyor belt that moves brittle faults eastward and out from above a fixed-width ductile shear zone.

Although the active ECSZ is a ~100 km-wide zone of active deformation, the real ECSZ initiated in late Miocene time (ca. 10 Ma) and was originally a >200 km-wide zone of distributed deformation from the San Andreas Fault to the CA-NV border and southwestern AZ. Late Miocene development of the ECSZ occurred in unison with that of the Walker Lane to the north and Gulf of California shear zone to the south, forming a >2,000 km-long, kinematically-linked belt of distributed dextral shear related to the evolving Pacific-North America plate boundary.