LATE QUATERNARY OFFSET ALONG THE SIERRA MADRE FAULT REVEALED BY HIGH RESOLUTION TOPOGRAPHIC DATA

The Sierra Madre fault (SMF) system bounds the northern edge of the Los Angeles metropolitan area, posing a seismic hazard to a large population. This zone of reverse faulting is one of the largest continuous structural features accommodating contraction in the western Transverse Ranges. The SMF connects the San Andreas and San Jacinto strike-slip fault zones in the east to structures with apparently rapid slip rates in the Ventura basin area to the west. Existing estimates of slip rate along the faults of the Sierra Madre system permit models with slip rates that increase either east or west, or alternatively, the central part of the system could slip most rapidly, as one might expect for an integrated reverse fault system. Better resolution of the spatial variation of late Quaternary slip provides an important constraint in resolving potential tectonic controls for SMF slip and the degree of continuity between previously recognized segments of the broader fault system.

Alluvial fan surfaces along the rangefront of the San Gabriel Mountains have been offset across the Sierra Madre fault. In many areas, these fans span all or most of the active strands of the SMF zone, and the integrated deformation from surface-rupturing earthquakes since the fan surfaces were abandoned has been preserved. These offset fans were recently imaged by airborne LiDAR data acquired along the rangefront. In this contribution, we combine previously published geologic and geomorphic mapping with new high resolution topographic data from lidar to better resolve the pattern of deformation along the SMF. We focus on the Central Sierra Madre fault, which lies between the better-studied San Fernando and Cucamonga faults, and cuts many fans along the rangefront. Preliminary analysis of the LiDAR data indicates prominent fan surfaces that are vertically displaced by ~20 m. Correlative late Quaternary fan surfaces elsewhere in the Transverse Ranges span ages from ~ 10 to 60 ka, resulting in a relatively large range of uncertainty for the slip rate of the Central Sierra Madre fault over the time scale since the fans were abandoned. Direct dating of the fan surfaces along the central Sierra Madre fault using terrestrial cosmogenic nuclide dating and/or other techniques will complement existing trench-based estimates of late Quaternary slip along this fault zone.