Abstract: Compressive Structures Along the San Andreas Fault in Northern California: A Relationship Between Miocene and Quaternary Tectonics and Implications for Seismic Hazard

Michael Angell, N. Timothy Hall

Seismic source characterization and tectonic models of the northern San Andreas fault (SAF) are improved by evaluating the geometry and kinematics of associated compressive structures at three locations in the San Francisco Bay Area: Pajaro Gap (southern Santa Cruz Mts.), Palo Alto (eastern foothills of the northern Santa Cruz Mts.), and San Bruno (northern San Francisco Peninsula). Cross sections are constructed to mid-crustal depths using quantitative balancing techniques that incorporate surface geologic, shallow borehole, gravity, seismic refraction, and seismicity data. The sections extend northeast from the SAF across regions of uplifted, folded, and faulted Quaternary sediments and are "pinned" at the northeast end in undeformed Quaternary deposits at elevations &l ;35 m.

These cross sections reveal a southwest-dipping reverse or right-reverse fault system underlain by a downward-steepening "floor thrust" that joins the SAF along a branchline at depth. The leading edge of this fault is a blind thrust tipline beneath a late Quaternary deformation front that defines the northeast margin of faulting related to the SAF proper. Cross-sectional plots of seismicity parallel to the Palo Alto and Pajaro Gap sections are consistent with a northeast-tapering, actively deforming thrust wedge along the eastern margin of the SAF. Fault/fold relationships, uplift rates, and the regional budget for fault-normal strain indicate total contractional strains are low (~10% of the lateral slip) and suggest reverse slip rates on the order of 0.5 to 1.0 mm/yr.

The Palo Alto section restores to a Miocene half graben, indicating a pre-existing basement structure controls the location and geometry of contraction for that location. Contraction of the half-graben geometry can explain the local observation of structural and stratigraphic inversion of an elevated Tertiary syncline overthrusting a lowland valley underlain by Franciscan basement at the near surface. Tectonic inversion of pre-existing extensional structures may help to explain similar regional observations elsewhere in the Coast Ranges (e.g. Southern Santa Cruz Mts., East Bay Hills, San Luis Range).

The floor thrusts to the larger fault systems have sufficient area to generate earthquakes greater than M6, are therefore capable of producing coseismic ground rupture, and may be independent seismic sources. However, because the magnitude of total contractional strain observed in the geologic record for these regions is low, and because the floor thrust and overlying faults in the hanging wall slip during large events on the SAF, as indicated by observations of secondary contractional deformation along the range front following the 1989 Loma Prieta earthquake, coseismic secondary faulting during large magnitude "characteristic" events on the SAF, may account for all of the local SAF-normal contraction, precluding the need for independent large magnitude events on the compressive structures.

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