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Transtension in the brittle field: The Eastern California Shear Zone

UTIG Seminars

Transtension in the brittle field:
The Eastern California Shear Zone

John Dewey
Natural History Museum, London

When: Wednesday, April 03, 2013, 10:30 a.m. to 11:30 a.m.
Join us for coffee beginning at 10:00 a.m.
Where: Seminar Conference Room, 10100 Burnet Road, Bldg 196-ROC, Austin, Texas 78758
Host: Ian Dalziel, UTIG
image from Dr. Dewey's talk

A major problem of structural geology is how biaxial, uniaxial, and triaxial strains in the brittle upper crust are effected by fault systems. Transtension generates noncoaxial constriction and poses difficult problems of wrench/normal fault-combination block rotation and strain. Normal faults rotate around vertical and horizontal axes to give oblique slip, and wrench faults rotate around vertical axes. Kinematic problems arise because blocks of varying size and shape bounded by normal and wrench faults rotate at different rates about vertical and horizontal axes while shortening or lengthening. Compatibility problems may be solved by block margin deformation, by bulk block strain, by buckling of faults, by volume increase with holes opening at block intersections, by alternating periods of normal and wrench faulting, by discrete arrays of normal and wrench faults. The Coso transtensional region, in southern California is active, the horizontal strain rates are high, and there is a huge amount of seismic, heatflow, fluid flow, and borehole and surface geologic data. Between the Sierra Nevada and the Argus Range, the transport direction (from GPS) is roughly NNW at about 10 mmyr. This generates triaxial constriction with an instantaneous stretching direction roughly WNW and a horizontal strain rate of about 10-14 sec-1. Constriction is modeled by a combination of NNE normal faults, NE wrench faults, and WNW folds and thrusts, which rotate clockwise with vorticity, and N wrench faults that rotate counterclockwise against vorticity, a pattern of faulting, folding, and bulk strain recorded closely by fault slip data from earthquakes and field observations. In deformation zones, GPS and moment tensor sum data indicate very smooth velocity fields, which in turn means that the commonly accepted view of the rotation of large rigid blocks cannot be correct; instead, the upper crust behaves as a "continuum rubble" of very small "blocks."