Reinterpretation of onshore and offshore geologic mapping, examination of a key offshore well core, and revision of cross-fault ties indicate Neogene dextral strike slip of 156 ± 4 km along the San Gregorio–Hosgri fault zone, a major strand of the San Andreas transform system in coastal California. Delineating the full course of the fault, defining net slip across it, and showing its relationship to other major tectonic features of central California helps clarify the evolution of the San Andreas system. San Gregorio–Hosgri slip rates over time are not well constrained, but were greater than at present during early phases of strike slip following fault initiation in late Miocene time. Strike slip took place southward along the California coast from the western fl ank of the San Francisco Peninsula to the Hosgri fault in the offshore Santa Maria basin without significant reduction by transfer of strike slip into the central California Coast Ranges. Onshore coastal segments of the San Gregorio–Hosgri fault include the Seal Cove and San Gregorio faults on the San Francisco Peninsula, and the Sur and San Simeon fault zones along the flank of the Santa Lucia Range. Key cross-fault ties include porphyritic granodiorite and overlying Eocene strata exposed at Point Reyes and at Point Lobos, the Nacimiento fault contact between Salinian basement rocks and the Franciscan Complex offshore within the outer Santa Cruz basin and near Esalen on the flank of the Santa Lucia Range, Upper Cretaceous (Campanian) turbidites of the Pigeon Point Formation on the San Francisco Peninsula and the Atascadero Formation in the southern Santa Lucia Range, assemblages of Franciscan rocks exposed at Point Sur and at Point San Luis, and a lithic assemblage of Mesozoic rocks and their Tertiary cover exposed near Point San Simeon and at Point Sal, as restored for intrabasinal deformation within the onshore Santa Maria basin. Slivering of the Salinian block by San Gregorio–Hosgri displacements elongated its northern end and offset its western margin delineated by the older Nacimiento fault, a sinistral strike-slip fault of latest Cretaceous to Paleocene age. North of its juncture with the San Andreas fault, dextral slip along the San Gregorio–Hosgri fault augments net San Andreas displacement. Alternate restorations of the Gualala block imply that nearly half the net San Gregorio–Hosgri slip was accommodated along the offshore Gualala fault strand lying west of the Gualala block, which is bounded on the east by the current master trace of the San Andreas fault. With San Andreas and San Gregorio–Hosgri slip restored, there remains an unresolved proto–San Andreas mismatch of ∼100 km between the offset northern end of the Salinian block and the southern end of the Sierran-Tehachapi block. On the south, San Gregorio–Hosgri strike slip is transposed into crustal shortening associated with vertical-axis tectonic rotation of fault-bounded crustal panels that form the western Transverse Ranges, and with kinematically linked deformation within the adjacent Santa Maria basin. The San Gregorio–Hosgri fault serves as the principal link between transrotation in the western Transverse Ranges and strike slip within the San Andreas transform system of central California.

ABSTRACT The Half Moon Bay oil field was first developed in the 1880’s and further drilling has been done in each “oil boom” of the past 100 years. The field has produced an estimated 58,000 barrels of oil from about 19 wells within a maximum area of 155 acres. Recent peak field production was 11 B0PD in 1985. Efforts to develop new production have been severely limited by Coastal Zone restrictions. The main reservoir in the Half Moon Bay oil field consists of very thin sandstone layers within lower Pliocene (and upper Miocene?) mudstones of the Purisima Formation, at depths of 240 to 3,085 ft (73 to 940 m). Oil is high gravity: 43°-55° API. Petroleum is trapped on two separate structural features. Recent drilling has been concentrated in the Verde area on the northwest-trending Purisima anticline, where the Purisima Formation overlies a thick upper Miocene sequence. The potential for Monterey Shale production on that structure has not been adequately evaluated. Most of the earlier wells and production were located to the northeast in the Purisima Creek area in a fault block on which the Purisima Formation lies unconformably on lower Miocene and Eocene beds. The Eocene Butano Sandstone in that block has produced a very minor amount of oil. The northwest-striking faults which bound that block had significant pre-Pliocene offset. They appear to extend north to join the San Gregorio fault, and south to merge with a previously mapped major fault in the La Honda area. Similarities between the stratigraphic sections in the Half Moon Bay and Point Reyes areas support earlier estimates of about 44 mi (70 km) of right slip along the San Gregorio fault since the end of Miocene time. [Note: this paper is partially excerpted from A. J. Horn (1983), “The Resurrection of the Half Moon Bay Oil Field, San Mateo County, California”. Because that article emphasized the Purisima Creek area of the oil field, additional sources have been utilized to extend and update its coverage.]

ABSTRACT Well developed flights of marine terraces along the central California coastline, provide horizontal datums that record the neotectonics of the coastal area. The San Gregorio fault zone, a major branch off of the San Andreas fault deforms and offsets these marine terraces in coastal San Mateo County, and unambiguously offsets the marine terrace shoreline angles horizontally at Point Ano Nuevo. The fault zone is undergoing right lateral strike slip movement with 10-15 degrees of convergence, with the majority of the movement occurring along the two primary faults within the zone, the Frijoles and Coastways faults. Analysis of the offset of the two lowest marine terraces (Santa Cruz and Western terraces) indicates the rate of movement has been essentially constant throughout most of the late Pleistocene (past 230,000 years) at 6-10 mm/yr. Analysis of deranged stream patterns of two small creeks at Point Ano Nuevo suggests that horizontal slip has ranged between 4-8 mm/yr over the past 105,000 years.

ABSTRACT The central Monterey Bay region (Figure 1) has been a stable to subsiding depocenter and the locus of fluvial, alluvialfan, and eolian activity throughout the Quaternary. It and adjacent tectonically uplifting areas provide an opportunity to study the development of marine and non-marine depositional systems as a function of fluctuating sea level and tectonic setting.

ABSTRACT In this paper we synthesize geomorphic, seismic, and tectonic data in order to present a scenario for the evolution of the Santa Cruz Mountains that is genetically tied to the proximity of the Santa Cruz bend in the San Andreas fault. We use the marine terraces of the Santa Cruz coast to infer a repeat time for the Loma Prieta earthquakes of 700 years. When combined with the 1.3-1.9 m of dextral slip in the Loma Prieta event, this recurrence interval cannot accommodate the long term right lateral motion along the San Andreas fault, and other events must be called upon. We discuss possible candidates for these other events, and use the topographic signature of the Santa Cruz Mountains, the terrace elevations, the pre-1989 seismicity, and the Loma Prieta aftershocks to develop a picture of multiple faults affecting the long term history of the area. In the first section of the paper, we demonstrate that the vertical displacement pattern associated with the October 1989 Loma Prieta earthquake is consistent with long term terrace uplift over 60 km of coastline and time scales of >200 ky (ky=1000 years). The terrace ages and elevations, and the magnitude of the vertical uplift in the Loma Prieta event, constrain long-term average recurrence intervals of Loma Prieta type events to be of order 660-720 years. In the second section of the paper, we consider the northern Santa Cruz Mountains. Due to spatial variations in the vertical strain field dictated by the lateral extent of rupture and the 70°SW dip of the rupture plane, the uplift rates associated with repeated Loma Prieta type events reach their maximum 1-2 km to the west of the trace of the San Andreas fault. Over the million year time scales necessary to build the northern mountains, the evolving topography slips significantly toward the NW relative to the uplift pattern. The topography reaches a maximum altitude just beyond the bend, and then succumbs to erosion, decaying to half of its maximum height over a length scale of 30 km, or about 2-3 My (My= million years) at present slip rates. The topography of the northern Santa Cruz Mountains in the vicinity of the San Andreas fault is consistent with the vertical strain field and the inferred recurrence interval of the 1989 Loma Prieta earthquake, with the long-term slip rate on the San Andreas, and with the rates at which the geomorphic processes modify this topography. In contrast to the northern Santa Cruz Mountains, maximum altitudes of the mountains on the east side of the San Andreas Fault in the southern Santa Cruz Mountains (in particular Loma Prieta Peak itself) coincide with the middle of the restraining bend. This massif experienced slight subsidence during the recent earthquake, strongly suggesting that other seismic events with different uplift patterns are needed to explain the topography of the southern Santa Cruz Mountains. In the third section of this paper, we will address the possibility that repeated events on the Sargent-Berrocal Fault system are responsible for the southern Santa Cruz Mountain topography. We therefore propose a seismic scenario for the generation of the Santa Cruz Mountains that includes repeated Loma Prieta events (which accomplish both uplift of the northern range and dextral slip), repeated 1906 events (which accomplish dextral slip), and repeated events associated with the Sargent-Berrocal fault system (which accomplish both uplift of the southern range and dextral slip).

ABSTRACT 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, Año 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 Año Nuevo Canyon, three heads for Carmel Canyon, and eight heads for Cabrillo Canyon. Ascension, Año 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, Año 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.

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.

ABSTRACT Tertiary strata of the La Honda basin are exposed in the Santa Cruz Mountains along the central California coast south of San Francisco. The basin fill has a composite thickness of more than 14,500 m and consists of sedimentary and volcanic rocks that in places rest on granitic basement rocks of the Salinia terrane. Paleogene strata are mainly turbidite sandstone and hemipelagic mudstone that accumulated in deep-sea fan and basin plain environments at lower bathyal to abyssal depths. Neogene rocks are mainly shallow-marine shelf sandstone and upper to middle bathyal siliceous mudstone. Both Paleogene and Neogene strata exhibit rapid lateral variations in thickness and facies, several local and regional unconformities, numerous folds, and ubiquitous faults. The complicated geology and geologic history of the La Honda basin reflect the fact that, throughout its history, the basin has been located at or near the tectonically active plate boundary between the North American continent and various oceanic plates of the Pacific basin. The La Honda basin originated during the Paleocene, perhaps during an episode of wrench tectonism associated with oblique subduction and arrival of the Salinia terrane. Major restructuring of the basin during the Oligocene—including uplift and erosion of the basin margins, movement along the Zayante-Vergeles fault, and deposition of two sand-rich deep-sea fans—apparently resulted from the approach of the Farallon-Pacific spreading ridge and its collision with the California continental margin. During the late Oligocene and early Miocene, widespread volcanism and marine transgression accompanied an episode of regional transtension along the San Andreas fault system. Deposition of shallow-marine sandstones and deeper-water siliceous mudstones occurred during much of the Miocene and Pliocene but was interrupted at least three times by brief episodes of uplift and erosion associated with transpressional wrench tectonism along the San Andreas fault. Marine deposition ended and uplift of the modern Santa Cruz Mountains began during the late Pliocene in response to the most-recent episode of regional transpression. Five small oil fields in the La Honda basin have produced a total of 1.7 million barrels of oil and 300 million cubic feet of gas, mostly from reservoirs in Eocene turbidite sandstone and Miocene limestone.

ABSTRACT Opal-CT is the only diagenetic silica phase present in the Santa Cruz Mudstone within a 16 Km long area extending from the city of Santa Cruz north to Davenport, California. The d(101)-spacing values of opal-CT range from 4.11 Angstroms in outcrops within the city of Santa Cruz area, to 4.06 Angstroms in outcrops north of Santa Cruz. Associated with the decrease in d-spacings, the morphology of crystallites making up rosettes of opal-CT (lepispheres) varies from feathery in high d-spacing samples to spiny in low d-spacing samples. The extent of silica diagenesis in the Santa Cruz Mudstone suggests the unit should have reached burial depths of a minimum of 1000 m, in comparison to data obtained from other biogenic siliceous units. Even though present thicknesses of the Santa Cruz Mudstone within the study area are only a few hundreds of meters, a rapid thickening of the Santa Cruz Mudstone northwest of Santa Cruz suggests that extensive erosion of the Santa Cruz Mudstone preceded the deposition of the overlying Purisima Formation and Pleistocene deposits.

ABSTRACT The Monterey Formation at Pt. Reyes, in the Bodega basin, and Pt. Año Nuevo, in the Outer Santa Cruz basin, consists mainly of porcelanite and black quartz chert interbedded with mudstone, siltstone and dolomite. Diatom biostratigraphic analysis of the two sections has resulted in refined age estimates. The Monterey Formation at Pt. Reyes and Pt Año Nuevo are similar in age and range from approximately 15.0 to 13.4 Ma and 14.7 to 13.7 Ma, respectively. The age of the base of the Pt. Reyes and Pt. Año Nuevo sections strongly contrasts with the age of the base of chert-bearing intervals in the Santa Maria basin, where diatom biostratigraphy suggests an age approximately 2 million years younger than intervals in the north-central California Basins. This distribution implies that the onset of upwelling along the California margin was strongly diachronous, with older chertbearing intervals occurring in the more northerly latitudes. The chert-bearing sections in all three basins correlate with middle Miocene periods of high-latitude cooling. However, penetration of cooler water masses, increased upwelling, and biosiliceous sedimentation during the early middle Miocene were probably highly variable across latitudes, affecting the north-central California margin earlier than the south-central margin.