SEPARATING REGIONAL UPLIFT FROM LOCAL UPLIFT IN AN ACTIVELY DEFORMING COMPRESSIONAL MOUNTAIN RANGE; THE WESTERN TRANSVERSE RANGES OF CALIFORNIA

The elevated topography of contractional mountain belts typically results from reverse or thrust faults along the fronts of the mountain belt, and from faults and folds within the mountain belt. But there is often a regional component of uplift in contractional mountain belts that cannot be attributed to specific structures seen at the surface of the Earth. Separating the contributions of local uplift from regional uplift is useful for understanding the processes driving mountain building, the causes of topographic variations across a mountain belt, and the seismic hazards in a region. The western Transverse Ranges (WTR) of California are an example of an active contractional mountain belt that is deformed by multiple faults and folds. We use fluvial and marine terraces as markers to document rock uplift patterns and rates within the WTR, and to separate the component of local uplift along individual faults and folds from the regional “background” component of uplift that must be due to deeper processes. Geomorphic mapping, long-valley profiles of active channels and terraces, reconstruction of deformed late Quaternary sedimentary units, and postIR-IRSL luminescence dating were used to calculate rock uplift rates and fault slip rates across a 100 km by 50 km area of the WTR. Rock uplift rates in the hanging walls of reverse faults and from fault-cored anticlines range from 0.5 to 4.0 mm/yr since 85 ka. Rates in the footwall blocks of these major reverse faults show that there is also a regional uplift rate of 0.7 to 1.3 mm/yr since 85 ka, possibly due to slip along a regional detachment that has been interpreted to underlie the WTR by multiple studies. Evaluation of rock uplift rates over three separate time intervals, based on three regionally extensive terrace deposits, show that localized uplift rates varied through time as individual faults turned on and off. The Baseline-Los Alamos and Santa Ynez River faults, for example, were active between 85 ka and 35 ka, but do not displace terraces younger than 35 ka. In contrast, there is no detectable variation in regional uplift rates measured in the footwall blocks over the past 85 ka. This deformation pattern suggests that deeper crustal shortening and vertical motion may be constant across the mountain belt, whereas associated deformation in the shallow crust may vary spatially and temporally over intermediate timescales.