Pseudotachylyte: Reading the Record of Paleoseismicity in Low-Angle Normal Faults

Dana Marion Smith
University of Wisconsin-Madison, Department of Geoscience Madison, WI 53706
[email protected]

Pseudotachylyte- rapidly quenched frictional melt generated through seismic failure- has been noted within low-angle normal fault systems (LANFs, dip <30°) but not extensively studied. We have documented significant pseudotachylyte exposures in both the South Mountains and Catalina-Rincon metamorphic core complexes of Arizona. In both field areas, generation veins range from 1 mm to 3 cm thick, are locally parallel to host rock foliations, and are part of meterscale networks containing multiple veins. Veins have shallow 7-24° dips. Though pseudotachylytes' full lateral extent could not be determined due to outcrop morphology, a minimum length of 50 m oblique to strike is estimated. Such abundant pseudotachylyte exposure implies significant and repeated seismicity.

If generation surfaces have maintained their seismicity-coeval orientation this would have major repercussions for traditional fault mechanics, which contradicts slip along faults dipping <30° in extensional regimes. To reconstruct fault paleogeometries, we are applying a novel fault rotation test. The cornerstone of this test is a comparison of two magnetic remanence vectors, one resolved through pseudotachylyte demagnetization, and a second representing the vector expected if no rotation has occurred. This second vector is determined through correlating a sample's 40Ar/39Ar age with its coeval magnetic north location. Any discrepancy between vector orientations is interpreted to represent rotation of the fault since its active phase. As preliminary data demonstrate a wide range of magnetic characteristics for collected samples, we anticipate this test will only be successfully applied toward some sites.

AAPG Search and Discovery Article #90157©2012 AAPG Foundation 2012 Grants-in-Aid Projects