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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.

The Ascension-Monterey Canyon system, one of the largest submarine canyon systems in the world, is located offshore central California. The system is composed of two parts which contain a total of six canyons: 1) the Ascension part to the north, which includes Ascension, Ano Nuevo and Cabrillo Canyons, and 2) the Monterey part to the south, which includes Monterey Canyon and its distributaries, Soquel and Carmel Canyons. These six canyons have a combined total of 16 heads: one head each for Ascension, Soquel and Monterey Canyons, two heads for Ano Nuevo Canyon, three heads for Carmel Canyon, and eight heads for Cabrillo Canyon. Ascension, Ano Nuevo and Cabrillo Canyons coalesce in 2,300 m of water to form the Ascension Fan-Valley. Soquel and Carmel Canyons join Monterey Canyon at depths of 915 m and 1,900 m, respectively, to form Monterey Fan-Valley (the main channel of the system). Ascension Fan-Valley joins Monterey Fan-Valley on the proximal part of Monterey Fan in 3,290 m of water. The Ascension-Monterey Canyon system has a long and varied history. The ancestral Monterey Canyon originated in early Miocene time, cutting east-west into the crystalline basement of the Salinian block (possibly subaerially), somewhere near the present location of the Transverse Range of California. Since that time (∼21 Ma), the Salinian block, riding on the Pacific Plate, moved northward along the San Andreas fault zone. During this period of transport the Monterey Bay region was subjected to several episodes of submergence (sedimentation) and emergence (erosion) that alternately caused sedimentary infilling and exhumation of Monterey Canyon. The present configuration of the Ascension-Monterey Canyon System is the result of tectonic displacement of a long-lived submarine canyon (Monterey Canyon), with associated canyons representing the faulted offsets of past Monterey Canyon channels. Slivering of the Salinian block along several fault zones trending parallel or sub-parallel to the San Andreas fault zone (the Ascension fault and the Palo Colorado-San Gregorio fault zone, in particular) displaced to the north the westerly parts of Monterey Canyon. In this manner Monterey Canyon “fathered” Cabrillo Canyon, Ano Nuevo Canyon, Ascension Canyon and Pioneer Canyon, along with an unnamed canyon located between Ascension and Pioneer Canyons. Tectonics continue to dictate the morphology and processes active in the system today. The Palo Colorado-San Gregorio fault zone marks the continental shelf boundary in the Monterey Bay region and divides the canyon system into two parts, the Ascension and Monterey parts. The Monterey Canyon part has a youthful, V-shaped profile while the Ascension part, except for the heads that notch the shelf, and both fan-valleys exhibit more mature, U-shaped profiles. Earthquakes stimulate mass-wasting on the continental slope; most of the Ascension part of the system now receives its sediment from this source. The Monterey part, however, intercepts sediments carried by longshore transport and is the main regional conduit for terrestrial sediment transport to the abyssal plain.