1982 Abstract of paper presented at the 78th Annual Mtg., Cordilleran Section, Geological Society of America Abstr. with Prog., v. 14, p. 166.

Late Tertiary clockwise oroflexural bending in southern Nevada – Paleomagnetic support of a geologic hypothesis

S. L. Gillett, Sierra Geophysics, Inc.
D. R. Van Alstine, Sierra Geophysics, Inc.

The Las Vegas Shear Zone, a major tectonic element in southern Nevada, is one of several northwesterly trending, right-lateral fault zones in southern Nevada and adjacent California that were active in late Tertiary time. Clockwise tectonic bending of nearby mountain ranges ("oroflexural bending"; Albers, 1967) was proposed to be associated with this shear zone, based on consistent bends in isopach trends, Mesozoic structures, and topographic expression as they near the shear zone. In the Desert Range, north of the shear zone, Lower Paleozoic miogeoclinal limestones yield a characteristic, reversed-polarity magnetization that is probably late Paleozoic and a secondary, normal-polarity magnetization that is probably Late Cretaceous. These components can be identified as linear segments on vector diagrams, as their blocking temperature spectra are distinct. Both magnetizations yield well-grouped directions, probably reside in detrital magnetite, and appear to be VPTRM's imposed during times of regional uplift. At widely spaced sites along the range, the declinations of both components show a systematic clockwise rotation, consistent with oroflexural bending. However, the Cretaceous component indicates about 40% more vertical-axis rotation than the Paleozoic component. At the southernmost site, for example, the Cretaceous component suggests a rotation of 70 ± 9°, whereas the Paleozoic component shows 44 ± 5°. This proportional difference probably results from imposition of the Cretaceous component on dipping strata, so that a simple rotation about strike leads to an incorrect declination. Hence, this consistent difference probably reflects an "apparent" rotation (MacDonald, 1980), and shows that magnitudes of vertical-axis rotations cannot always be determined from declination differences. The Paleozoic component is preferred as the estimator of vertical-axis rotation, as the regional geology suggests these strata were essentially horizontal throughout the Paleozoic.