QUATERNARY EXHUMATION OF THE VERDUGO MOUNTAINS, LOS ANGELES BASIN, CONSTRAINED BY LOW-TEMPERATURE THERMOCHRONOMETRY

The San Gabriel Mountains, situated north of the Los Angles basin, are a major block of the transverse ranges in Southern California, and have relatively well-constrained exhumation rates based on apatite fission-track and (U-Th)/He data. The Verdugo Mountain Block (VMB) abuts the San Gabriel Mountains to the south and is part of a generally southward-propagating zone of contractional deformation related to the “Big Bend” region of the San Andreas fault. The VMB is an E-W trending antiform that is ~15 km long and ~6 km wide, has a maximum elevation of 953 m, and is composed mainly of late Mesozoic quartz monzonite-granodiorite. The VMB is bound to the north by the Sierra Madre thrust fault, located between the Verdugo and western San Gabriel Mountain blocks, and the Verdugo fault to the south. These faults are part of the complex fault system that generally youngs southward away from the San Gabriel Mountains and accommodates much of the recent deformation in the basin. The Verdugo fault is completely covered by sediments, thus it is unknown how active it has been in the Quaternary; evaluating exhumation of the VMB may help place constraints on Verdugo fault displacement rates. An initial assessment of the VMB suggests it is a young structure and exhumation has been recent (~1-4 Ma). Meigs et al. (2003) used strata and structural data to estimate 1.5-2.0 km of exhumation at a rate of 0.9 km/My in the Quaternary. Work in progress using apatite (U-Th)/He and fission-track analysis on samples strategically collected across and along the VMB, as well as geomorphic analysis, will test the hypothesis of rapid Quaternary exhumation and may place further constraints on the minimum Quaternary exhumation magnitude of the Verdugo Mountains.

Meigs, A.,Yule, D., Blythe, A., and Burbank, D., 2003, Implications of distributed crustal deformation for exhumation in a portion of a transpressional plate boundary, Western Transverse Ranges, Southern California. Quaternary International, v. 101-102, p. 169-177.