Structural development and active tectonics of the Santa Maria Basin

The purpose of this study is to investigate the influence of vertical-axis rotation on the structural style of an actively deforming basin in southern California and to determine the kinematics and amount of Quaternary activity of the faults and folds present in the Santa Maria Basin. Completed work (see below) has focused mainly on the kinematic history of the Little Pine Fault and evidence of vertical axis rotation in the area. Current and future work includes 1.measuring the amount of small-scale shortening across the area that has not been accounted for in previous studies, and 2. expanding on the quantification of active deformation using mapping, surveying, and OSL dating of Quaternary terraces. Much of the work has been funded by a grant from the Petroleum Research Fund.

This work was broken up into several different components that have been evaluated primarily by my graduate students:

Yannick Wirtz is testing the idea that shortening across the Santa Maria Basin may be significantly greater than is represented in the large-scale restored cross-sections by Namson and Davis (1988). To do this, he is measuring the amount of shortening across the Santa Maria Basin at a variety of scales, focusing on retrodeforming new 2 to 4 km long cross-sections for parts of the basin as well as measuring shortening at the outcrop scale.

Ani Pytlewski has started to map Pleistocene fluvial terraces along the Santa Ynez river and will be dating them with OSL to infer the amount and rate of regional uplift, as well as constrain the amount and rate of Pleistocene slip along the Santa Ynez fault.

Andrew Farris is mapping, describing, and dating terraces (OSL) along Zaca Creek to look for and quantify Quaternary deformation and regional uplift in the area.

Previous students:

- Mike Cannon collected kinematic data along the Little Pine fault (the major fault that marks the north side of the Santa Maria Basin) and found that the fault has experienced variable styles of slip, both along strike and through time. His work suggests that there may have been a late Miocene history of right-lateral slip along the fault, but since Pliocene or early Pleistocene time the fault has primarily moved as a reverse fault that dies out to the northwest in the Zaca Creek area.

- Rick Lee looked at the sedimentology of the Pleistocene Paso Robles formation, as well as the Quaternary terrace material and active alluvium to evaluate the slip history of the Little Pine fault. He also found evidence that the Little Pine fault has experienced primarily reverse slip since Pliocene time.

- Todd Tyler mapped and dated (using OSL) terraces in Santa Cruz Creek to measure regional uplift and Quaternary activity of faults and folds that cross Santa Cruz Creek in the eastern Santa Maria Basin. He found that the Los Alamos-Baseline fault offsets a 30 Ka terrace by about 30 m (roughly 1 mm/yr slip rate) and that the height of the Quaternary terraces above the active channel suggest a regional uplift rate of between 1 to 1.8 mm/yr.

- Jon Guillaume collected paleomagnetic data from Miocene rocks to see if the Santa Maria Basin experienced large scale clockwise rotation like the western Transverse Ranges to the south. Unfortunately all the samples collected from the Monterey and Siquoc formations were re-magnetized, so we are still unsure how much rotation the rocks in the basin have experienced.