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Late Pliocene-Pleistocene Evolution of the Little Pine Fault and Its Function on the Control of Sedimentation during Basin Formation: An Examination of the Late Pliocene-Pleistocene Paso Robles Formation, Santa Maria Basin, California

Lee, Ricky A.*1
(1) Geological Sciences, California State University Long Beach, Long Beach, CA.

The late Pliocene through Pleistocene Paso Robles Formation (PRF) of the Santa Maria Basin (SMB), California, is an ideal candidate to study the results of syn-tectonic basin deposition along a reverse-slip fault system, such as the Little Pine Fault (LPF). The Little Pine Fault is a northwest trending, north-dipping, oblique-slip thrust fault that steepens with depth, faults the Franciscan Complex mélange, and separates the Little Pine Mountain block from the San Marcos block; however, the question remains as to how much oblique-slip and/or reverse-slip has occurred during the Pleistocene, including their temporal and spatial relationships. This study will focus on the affects of fault evolution on basin sedimentation, and will be conducted using stratigraphic and geomorphic data gathered through field mapping.

Stratigraphic columns were measured at 11 locations along the LPF, and were examined for their stratigraphic character at the horizon where Franciscan-type clasts begin to appear. Utilized as a marker, the first-appearance of Franciscan-type clasts is presumed to signify the initial exhumation of Franciscan rocks northeast of the LPF. The percent composition of Franciscan-type clasts was also compared with two other depositional settings: recent alluvial deposits and active drainages. Using this comparison to establish relative temporal constraints on the latest episodes of uplift along the LPF, it was determined that the Franciscan units within the San Rafael Mountains were minimally exposed during the latest Pleistocene PRF deposition, and slightly more exposed during the deposition of Quaternary alluvium. In addition to this, a DEM (digital elevation model) study found that most of the drainages across the fault zone are consistent with a mostly reverse scenario of slip along the LPF, particularly within the southeastern portion of the LPF.

This study suggested that the LPF acted in a primarily reverse-slip manner, particularly within the southeastern-most portion of the fault zone. The present-day shape of the LPF is also not consistent with a strike-slip scenario, at least since the end of the Pleistocene. The LPF system is currently active, suggested by the increasing appearance of Franciscan-type clasts within the alluvium deposits, and an abundance of Franciscan rocks within active drainages.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California