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Tectonic Reconstruction of the Southern San Andreas Fault System Using Segments of the Chocolate Mountains Anticlinorium in the San Gabriel Mountains, Southern California, U.S.A.

  • Author(s): Coffey, Kevin Thomas
  • Advisor(s): Ingersoll, Raymond V
  • et al.
Abstract

The San Gabriel Mountains of southern California contain anticlinoria of schist thought to correlate with one another, and with the Chocolate Mountains anticlinorium to the east. These correlations provide cumulative-slip estimates for the Punchbowl and southern San Andreas faults, respectively; however, the validity of these correlations, as well as the original orientations of these anticlinoria, is debated. This dissertation presents four related studies, each addressing a different aspect of this problem. Chapter 1 presents sedimentologic and structural data verifying that the two anticlinoria of the San Gabriel Mountains were continuous prior to 40-50 km of dextral slip on the Punchbowl fault, and casting doubt on the 80-110 km of dextral slip on the San Francisquito-Fenner-Clemens Well fault suggested by some prior studies. Chapter 2 presents structural data from the schist of the larger anticlinorium of the San Gabriel Mountains, which supports its correlation with the Chocolate Mountains anticlinorium, and thus estimates of approximately 240 km of dextral slip on the southern San Andreas fault. New data are combined with previously published structural and paleomagnetic data, which have been interpreted as implying different architectures for the anticlinorium, suggesting a model reconciling the datasets, and implying an originally linear, approximately northeast/southwest trend of the anticlinorium. Chapter 3 proposes a strain-partitioning model for the anomalously oriented Mojave segment of the southern San Andreas fault, in which slip south of this segment is partitioned within this segment into a strike-slip component along the San Andreas fault itself, and a shortening component beside it. This model, together with a previous study’s insight, brings otherwise contradictory cumulative-slip estimates of approximately 240 km and 160 km for the southern San Andreas fault into agreement. Chapter 4 presents new fault-kinematic data, and sequential reconstructions of the San Gabriel block as maps and cross sections, suggesting that the anticlinorium is in the hanging wall of a detachment fault, rather than the exhumed footwall as previously suggested. New geochronologic and geochemical data support correlation of the anticlinoria and constrain the onset of extension to ca. 26-25 Ma.

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