EXHUMATION OF THE VERDUGO MOUNTAINS, SOUTHERN CALIFORNIA: CONSTRAINTS FROM LOW-TEMPERATURE THERMOCHRONOLOGY AND GEOMORPHIC ANALYSIS

The Verdugo Mountains are part of a crustal block that accommodates contractional strain along a south-vergent blind thrust fault within the northern Los Angeles basin. Independent evidence from low-temperature thermochronometers, geomorphic indices, and stratigraphic relations are used to constrain the spatial patterns of uplift, exhumation, and lateral growth of the Verdugo Mountains. Apatite (U-Th)/He (AHe) ages from granitic samples collected across the 950 m high Verdugo antiform range from 8 to 78 Ma. A break in slope in the AHe age profile ca. 17 Ma marks a phase of rapid cooling and exhumation. The timing of a possible second phase of cooling since about 8 Ma is poorly constrained by the AHe data, but depth extrapolation of the elevation - AHe age profile suggests approximately 2 km of erosion since deposition of the 2.3 - 0.5 Ma Saugus Formation. The exhumation amount from the AHe ages and age of the uplifted and eroded Saugus Formation indicates an exhumation rate of 1 to 4 mm/yr. High slope angles of 27 degrees and intermediate to high hypsometric integral values suggest relatively high uplift rates. Mountain-front sinuosity and valley height to valley floor width ratios along the southern flank of the Verdugo Mountains anticline are low and systematically decrease eastward from 2.42 to 1.22 and 1.37 to 0.34, respectively. These geomorphic data also indicate rapid uplift, as well as eastward propagation, of the antiform. The initial phase of exhumation at ~17 Ma documented by the AHe data is similar to adjacent crustal blocks of the San Gabriel Mountains to the north and may reflect the regional tectonic shift from transrotation to transtension. Stratigraphic relationships, AHe ages, and geomorphic indices suggest recent faulting, exhumation, and uplift in a Plio-Quaternary transpressional regime. The Plio-Quaternary rapid uplift and exhumation of the Verdugo Mountain block suggest that the range-front fault system has stepped southward into the basin and that the basin-bounding fault may now be the Verdugo Fault.