REFINING BLOCK UPLIFT AND EXHUMATION OF THE SAN GABRIEL MOUNTAINS, CA, USING LOW-TEMPERATURE THERMOCHRONOLOGY

One of the challenges in studying landscape evolution is that the forces that create and destroy topography (e.g., climate, fault locations and slip rates) continually evolve, and current topography preserves a limited and patchy record of past events. The San Gabriel Mountains of Southern California provide an ideal natural laboratory for addressing questions regarding the topographic evolution of an active mountain range because a large data base already exists on the faults bounding and within the range and on the long and short term exhumation rates of individual fault blocks within the range (e.g. Blythe et al., 2000; DiBiase et al., 2010). Here we present new apatite fission-track (AFT) data for 13 samples from the San Gabriel Mountains (SGM) of southern California to better constrain the low-temperature thermal history of individual fault blocks. We collected samples along three separate elevation transects within fault-bounded blocks in the central part of the range. The westernmost transect consists of 5 samples collected from the central part of the Western San Gabriel block (WSGB); the AFT ages ranged from 20.9 to 13.5 Ma for 5 samples collected over 1.1 km of elevation, with the youngest ages from the lowest elevations. The central transect, consisting of 5 samples, spans ~1.7 km of elevation at the eastern edge of the WSGB; the AFT ages from these samples range from 44.4 to 17.8 Ma with the youngest age at the lowest elevation. The youngest ages from our dataset come from a transect of 3 samples collected over 1.8 km of elevation along the western boundary of the Mount Baldy block (MBB), and are the same age within error (~8 Ma). From these new data, as well as from previously published AFT, (U-Th)/He and cosmogenic analyses, we conclude the following: (1) the WSGB is a relict plateau surface from ~60 Myr, that underwent uplift in the past 13 Myr, but with minimal erosion, and (2) the MBB has undergone a substantial amount (>2 km) of exhumation in the last 8 Ma. We are currently acquiring AFT lengths and (U-Th)/He analyses for these samples, in order to obtain individual thermal histories of the blocks within the SGM.