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ABSTRACT

The tectonic and structural evolution of the Monterey Bay region of central California is complex and diverse. Onshore and offshore geologic investigations during the past two decades indicate that the region has been subjected to at least two different types of tectonic forces; to a pre-Neogene orthogonal converging plate (subduction) and a Neogene-Quaternary obliquely converging plate (transform) tectonic influence. Present-day structural fabric, however, appears to have formed during the transition from a subducting regime to transform regime and since has been modified by both strike-slip and thrust movement.

Monterey Bay region is part of an exotic allocthonous structural feature known as the Salinian block or Salinia tectonostratigraphic terrane. This block is proposed to have originated as part of a volcanic arc a considerable distance south of its present location, somewhere between the Transverse Range (being the displaced segment of the southern Sierra-Nevada Mountain Range) and the latitude of Central America. It consist of Cretaceous granodiorite basement with an incomplete cover of Tertiary strata. Paleogene rocks are scarce, evidently stripped from the block during a time of emergence in the Oligocene time.

The Salinian block is presently located on the Pacific plate at the Pacific and North American plates’ active tectonic boundary. This boundary shifted to a transform margin approximately 21 Ma when the Mendocino triple-junction passed through the Monterey Bay region. Since that time the Salinian block has been moving northward along the San Andreas fault zone and basin and ridge topography was generated within the strike-slip faults of the San Andreas fault system. Sometime between 5 and 3.5 Ma, due to the shift in the direction of Pacific plate motion and the development of a more orthogonal convergence between the Pacific and North American plates, compressional forces became more pronounced in the region. The 1979 Loma Prieta earthquake and recently reprocesses multichannel seismic-reflection data offshore indicate that the the Monterey Bay region is presently being subjected to both strike-slip (wrench) and thrust (compressional) type tectonic forces.

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