--> Abstract: Dextral Translation and Progressive Emergence of the Pleistocene Merced Basin and Implications for Timing of Initiation of the San Francisco Peninsula Segment of the San Andreas Fault, by J. V. Hengesh and J. Wakabayashi; #90958 (1995).

Datapages, Inc.Print this page

Abstract: Dextral Translation and Progressive Emergence of the Pleistocene Merced Basin and Implications for Timing of Initiation of the San Francisco Peninsula Segment of the San Andreas Fault

James V. Hengesh, John Wakabayashi

The Merced and Colma formations were deposited in a basin offshore of the San Francisco peninsula (SFP) that has subsided at a rate of about 1.3 mm/yr between 1.6 and 0.4 Ma, and 0.8 mm/yr since about 0.4 Ma. These formations are believed to result from transgressive and regressive cycles related to eustatic sea-level fluctuations (Clifton et al., 1988; Hall, 1966). Outcrops on the SFP and Marin peninsula (MP) demonstrate that, although originating in a rapidly subsiding basin, these deposits have emerged and are currently exposed at ~300-700 ft. While inferred extensional faulting associated with subsidence is consistent with normal fault focal mechanisms identified offshore of SFP, it appears inconsistent with contractional deformation observed adjacent to the San Andre s fault (SAF) on the SFP and MP. These observations imply that either the locus and style of deformation affecting the Merced fm has changed from local subsidence to uplift, or that the regional tectonic framework has changed in the last 1.6 My. The distribution of seismicity suggests that local extension is an active process offshore of SFP, therefore, we believe that a pull-apart structure has persisted since Merced time and is migrating northward with a minor bend in the SAF relative to the SFP east of the SAF. Merced basin deposits trailing on the SFP east of the SAF are now being progressively uplifted and deformed as a result of northward migrating contraction along the SAF on the SFP. The high rate of lateral translation of the locus of extension explains why an extremely deep sed mentary basin (e.g. Owens Valley) has not formed as a result of apparent high subsidence rates. We speculate that the Merced fm was deposited in the pull-apart during sealevel high-stands and preferentially preserved due to structural relief, and that relief in the pull-apart develops during sealevel low-stands (period of non-deposition).

Limestone-bearing Franciscan nappes are displaced 27 km by the SFP SAF segment. Assuming a 20 mm/yr slip rate, this segment may have initiated movement around 1.35 Ma. The age of the basal Merced (1.2-1.6 Ma) suggests there may be a genetic link between the onset of pull-apart basin formation and initiation of movement on the SFP SAF segment.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California