David A. Ponce

Introduction

The Hayward Fault, regarded as one of the most hazardous faults in northern California, extends for about 90 km from the Warm Springs District of Fremont in the southeast to San Pablo Bay in the northwest (Fig. 1--Geologic map). Beneath San Pablo Bay the Hayward Fault steps over to the east and continues along the Rogers Creek Fault. The Hayward Fault is a NW trending, predominantly right-lateral strike-slip fault zone that forms the western boundary of the East Bay Hills.

A number of on-going and important issues related to the earthquake hazard potential of the San Francisco Bay area remain unresolved and the Hayward Fault itself is of special interest because the combined earthquake probability of 27% along the Hayward-Rodgers Creek Fault System is the greatest in the region. The Hayward Fault itself is enigmatic in that it both creeps to a depth of about 5 km along its entire length and is capable of producing large earthquakes, evidenced by the October 21, 1868 M6.8 earthquake that was known as the "Great San Francisco Earthquake" prior to the 1906 earthqauke.

It is because of these emerging and sometimes conflicting findings that a study of the Hayward Fault in the vicinity of a gabbro body near San Leandro, a known and quantifiable feature, provides a natural laboratory to advance our knowledge on the geometry and evolution of a major strike-slip fault. Our study deals with the relationship of the active recent trace of the Hayward Fault and the long-term Hayward Fault Zone. Geophysical data along the Hayward Fault are used to investigate the nature and origin of a large gabbro body, hereafter referred to as the San Leandro gabbro. Gravity and magnetic methods are particularly well suited to the study of these rocks because of their unusual physical properties that are, in general, strongly magnetic and more dense than surrounding rocks.

Modeling Subsurface Geology and Structure

Geophysical modeling suggests a much greater horizontal and vertical extent of the San Leandro gabbro than previously reported (Fig. 2). Modeling indicates that the gabbro extends from the westernmost exposures west of the Hayward Fault to the Chabot Fault on the east. The Chabot Fault appears to preferentially follow the eastern boundary of the San Leandro gabbro body. Higher magnetizations and lower densities to the northwest suggest that the causative body may be more altered to the northwest. Modeling also suggests that the uppermost parts of the gabbro may be fractured or weathered, reflected in a lower model density for these parts of the gabbro. Our models reveal that the San Leandro gabbro dips about 75 to 80 degrees to the east and extends to a depth of about 6 to 8 km. Magnetic models also indicate that several moderately magnetic and probably ultramafic rock bodies occur west of the Hayward Fault Zone and below the offshore San Leandro basin. These smaller tabular bodies are probably serpentinite rather than gabbro because they do not have an associated gravity high.

Evolution of the Hayward Fault Zone

In late Jurassic time (Fig. 3a), choking of the subduction zone along the western margin of North America with island arc terranes probably caused it to step west of the future site of the Coast Ranges, leaving behind or stranding a relatively flat-lying section of oceanic crust. Alternatively, this oceanic crust may have been obducted onto the North American plate . The westward stepping subduction zone may have moved in a series of steps rather than a single step further complicating and disrupting the stranded oceanic crust . By Late Cretaceous time the Franciscan Complex and the Great Valley sequence were already juxtaposed and the subduction-accretion of the Franciscan Complex resulted in major tectonic crustal thickening. In Paleocene to Eocene (Fig 3b) time this over-thickened crust was attenuated and extended by crustal collapse that may have been related to a change in the velocity or angle of the subducting slab. By Early Tertiary time (Fig. 3c), one or more low-angle wedge thrusts developed and scooped-up ophiolitic and Great Valley Sequence rocks, possibly porpoising in and out of the oceanic crust, forming a discontinuous belt of ophiolitic rocks. Ultimately, these ophiolitic rocks were brought to higher levels in the crust as a result of transpressional plate motions. After repeated episodes of extension and attenuation, our model suggests that the roof thrust was brought to a near-vertical position and combined with thrusting, folding, and strike-slip faulting created the present structure (Fig. 3d). Thus, the Hayward Fault along the westernmost edge of the San Leandro gabbro may once have been similar to the ancestral Coast Range Fault that has since been reactivated as a near-vertical strike-slip fault.

Conclusions

For Additional Information See The Following Articles:

Western Earth Surface Processes Team || GUMP || USGS Geophysics Hub

Western Region Geology || USGS Home || Search USGS

This page is maintained by Webmaster, Menlo Park, CA