Diverse Pliocene-Quaternary tectonics in a transform environment, San Francisco Bay region, California

Abstract

The San Francisco Bay region occupies part of the diffuse transform boundary zone between the Pacific and North American plates. Although dextral strike-slip faulting is dominant, the plate motion is expressed in a variety of ways. Some strike-slip faults are parallel with the plate boundary, but some are slightly oblique. The major strike-slip faults are zones in which an interplay occurs between strands, some of which are en echelon. This interplay may be responsible for some apparent pull-apart basins and presumed normal faults along the Calaveras fault zone. In extensive areas between strike-slip fault zones, there are many compressional structures—folds and reverse faults—that are Pliocene-Quaternary in age, hence largely coeval with the strike-slip faults. Some of the compressional structures are oblique to, and others are parallel with, the major strike-slip faults. We have divided the San Francisco Bay region into domains on the basis of the types, orientations, and relationships of structures, or, in one case, lack of strong deformation. Each domain has responded in its own way to the regional northwest-southeast dextral shear imposed by the relative plate motion. In an attempt to understand the origin of various structures and the interrelationships among them, we compared established models with our observations. Some observed geometric relationships agree quite well with classical Coulomb-Anderson and simple shear models, but others require further explanation. Models based on fault interaction seem to apply to certain cases. For example, the East Bay Hills domain, which is between the left-stepping Calaveras and Hayward–Rodgers Creek fault zones, is under compression resulting from interaction between the two strike-slip zones.