Joint Meeting Pacific Section, AAPG & Cordilleran Section GSA April 29–May 1, 2005, San José, California
Neogene Fault Kinematics of the Santa Barbara Coastal Plain Support Transrotational Model for Western Transverse Ranges
Scott A. Minor and Karl S. Kellogg
U.S. Geol Survey, Box 25046, MS 980, Denver, CO 80225, [email protected]
The Santa Barbara coastal plain, located in the western Transverse Ranges (WTR) of southern California, is underlain by numerous dominantly WNW-trending folds and partly buried reverse faults of the late Neogene Santa Barbara fold and fault belt (SBFFB) (Keller and Gurrola, 2000). To better understand the tectonic evolution of the SBFFB, we collected kinematic data (slip-surface orientation, slickenline rake, and slip sense) from >200 slip surfaces of small (<5m) and rare large (10 to >100m) displacement faults within sedimentary rocks ranging in age from Eocene to middle Pleistocene.
Many measured slip surfaces throughout the sedimentary sequence strike similar to the dominant WNW trend of large-scale faults and folds within the SBFFB and exhibit reverse, oblique-reverse, or strike-slip kinematic indicators. In Miocene and older rocks, individual WNW-striking fault surfaces show kinematic evidence of both dextral and sinistral strike-slip movement. These older rocks also contain N-striking, steeply dipping, dextral oblique-slip faults; ENE-striking, sinistral and dextral oblique-slip faults; and rare, older, mainly ENE- and WNW-striking, normal-slip faults that are overprinted by strike-, oblique-, or reverse-slip striae. In some areas, reverse faults contain multiple generations of slickenlines that exhibit progressive shifts in rake. Oblique-slip faults commonly restore to nearly pure normal-slip or strike-slip faults by back tilting bedding to horizontal, suggesting that much of the folding and associated reverse faulting in the SBFFB were preceded by normal- and strike-slip faulting. Folded middle Pleistocene marine sediments exposed in coastal anticlinal hills contain younger ENE-striking, reverse-slip and sinistral-reverse-slip faults that cut obliquely across the dominant WNW structural trend of the older folds and faults. These “cross” faults are subparallel and kinematically similar to the ~20-km-long More Ranch fault, which cross cuts several large WNW-trending folds and faults. These results are consistent with previous tectonic models that invoke large (up to ~90o), Neogene, clockwise vertical-axis rotations of crustal fault blocks in the WTR accompanied by a gradual change from transtensional to transpressional fault kinematics (e.g., Luyendyk, 1991).
Posted with permission of The Geological Society of America; abstract also online (http://gsa.confex.com/gsa/2005CD/finalprogram/abstract_85113.htm). © Copyright 2005 The Geological Society of America (GSA).