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U.S. Geological Survey
525 South Wilson Ave.
Pasadena, California 91106
(K.J.K.)
U.S. Geological Survey
Department of Earth Sciences, University of California–Riverside
Riverside, California 92521
(D.M.M.)
Desert Research Institute
2215 Raggio Parkway
Reno, Nevada 89512
(S.G.W.)
U.S. Geological Survey
345 Middlefield Road.
Menlo Park, California 94025
(R.W.S.)
The San Timoteo badlands is an area of uplift and erosional dissection that has formed as a result of late Quaternary uplift along a restraining bend in the San Jacinto fault, of the San Andreas fault system in southern California. This bend currently is located in a region where late Quaternary deposits and associated surfaces have formed in lower San Timoteo Canyon. We have used morphometric analysis of these surfaces, in conjunction with computer modeling of deformational patterns along the San Jacinto fault, to reconstruct spatial and temporal variations in uplift along the bend. Morphometric techniques used include envelope/subenvelope mapping, a gradient-length index along channels, and denudation values. Age control is determined using a combination of thermoluminescence (TL) and near infrared optical simulation luminescence dating (IROSL) and correlation of soil-development indices. These approaches are combined with an elastic half-space model used to determine the deformation associated with the fault bend. The region of modeled uplift has a similar distribution as that determined by morphometric techniques.
Luminescence dates and soil-correlation age estimates generally agree.
Based on soil development, surfaces within the study area were stabilized at
approximately 300–700 ka for Q3, 43–67 ka for Q2, and
27.5–67 ka for Q1. Luminescence ages (both TL and IROSL) for the
formation of the younger two surfaces are 58 to 94 ka for Q2 and 37 to 62 ka
for Q1 (ages reported to 
uncertainty). Periods of uplift were
determined for the surfaces in the study area, resulting in approximate uplift
rates of 0.34 to 0.84 m/ka for the past 100 ka and 0.13 to 1.00 m/ka for the
past 66 ka. Comparison of these rates of uplift to those generated by the
model support a higher rate of lateral slip along the San Jacinto fault than
commonly assumed (greater than 20 mm/yr, as compared to 8–12 mm/yr
commonly cited). This higher slip rate supports the proposal that a greater
amount of slip has transferred from the San Andreas fault to the San Jacinto
fault than generally held. The San Jacinto fault may have accommodated a
significant portion of the plate boundary slip during the past 100 ka.
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