No abstract

Complete Bouguer gravity, aeromagnetic, and ground magnetic data interpreted in light of geologic mapping and well data within a 30-minute quadrangle centered near Cholame, Calif., express highly contrasting subsurface rock and structural patterns on opposite sides of the San Andreas fault. The geologic terrane northeast of the fault is composed of pervasively sheared Franciscan sedimentary and volcanic rocks of Mesozoic age and ultramafic rocks overlain by a thick series of severely deformed Cretaceous and Tertiary sedimentary rocks and locally deformed Quaternary valley sediments. The terrane southwest of the fault has a basement of Mesozoic plutonic and metamorphic rocks overlain by gently folded Tertiary sedimentary rocks and Quaternary valley sediments. Northeast of the San Andreas fault the geophysical data reveal the presence of (1) a major gravity high between Parkfield and Cholame, associated with relatively dense, nonmagnetic Franciscan and Cretaceous miogeosynclinal rocks overlain by Cenozoic rocks in the northeastern Diablo Range and in contact with low-density Cenozoic rocks southwest of the fault, (2) an elongate magnetic ridge and gravity trough at Table Mountain, probably generated by a thick tabular body of serpentinite dipping steeply north-northeast, inferred to be the main source of the serpentinite extrusion along this mountain, (3) a major elliptical magnetic high in an area of low density gradient near Palo Prieto Pass, probably associated with a large, deeply buried serpentine-rich body bounded on the southwest by the vertical San Andreas fault and on the northeast by a northeast-dipping contact or fault with nonmagnetic rocks, and (4) a northwest-trending gravity low over the Kettleman Plain, probably produced by a thickening of low-density Cenozoic sedimentary rocks near the axis of a major syncline. Southwest of the San Andreas fault the data show (1) a local gravity high north of Red Hills, associated with an uplift of plutonic and metamorphic rocks in these hills on the northeast side of the San Juan fault, (2) a prominent southwest-sloping northwest-trending gravity gradient southeast of Red Hills, which suggests a buried basement fault or a steep subsurface contact between basement and sedimentary rocks extending from the San Juan fault southeastward to the San Andreas fault near the southern end of the quadrangle, (3) a discontinuous north west-trending gravity trough east and north of Shandon, probably associated with a depression of the basement surface under an area near San Juan Creek, and (4) a broad northeast-trendinf gravity plateau west-southwest of Shandon, probably associrted with a thinning of Cenozoic sedimentary rocks overlying r. shallow basement surface that slopes from the extensive basement exposures of the La Panza Range southwest of the quadrangle.

_ ____________________ Introduction. _____________________________________ General geology. _ _____ _______ Basement rocks______________________ Ultramafic rocks ___________________. Franciscan Formation______________ Granitic rocks____-_________________ Superbasement sedimentary rocks-___________ Paleocene and Upper Cretaceous deposits ___ Tierra Redonda and Vaqueros Formations_____ Volcanic rocks-________________________ Monterey Shale.

Complete Bouguer gravity coverage of 390 stations and general geologic mapping were compiled for the Cape San Martin, Bryson, Piedras Blancas, and San Simeon quadrangles, California. These quadrangles constitute a 30-by 30-minute rectangle covering approximately 600 square miles of land area, most of which is in the rugged Santa Lucia Range. Two distinct basement units underlie the map area. In the northeast part, the granitic-metamorphic Salinian block constitutes the basement. The eugeosynclinal Franciscan Formation, however, underlies most of the map area and here constitutes all but a small part of the Santa Lucia Range. The Nacimiento fault is commonly believed to separate these major basement blocks. Overlying both basement units is a sequence of Cretaceous and Tertiary marine deposits and the nonmarine Paso Robles Formation. Detailed gravity data indicate that the unserpentinized core of the Burro Mountain ultramafic body has a subsurface volume no greater than 1 to 2 cubic kilometers and extends no deeper than 2,000 to 3,000 feet. Aeromagnetic data seem to preclude a large volume of subsurface serpentinite. The major features defined by regional gravity data include: (1) a rather even gradient of 3 milligals per mile in the entire southern half of the area which probably reflects deep structure of the continental margin, (2) a 10-milligal high coincident with the topographic mass of the Santa Lucia Range which suggests a density of over 2.8 grams per cubic centimeter for this mass, (3) a broad gravity low associated with Lockwood Valley which suggests that the valley is underlain by as much as 7,000 feet of low density sediments, and (4) a conspicuous gravity gradient of up to 20 milligals per mile which cuts diagonally across the entire Bryson quadrangle and represents a fault which vertically displaces the basement surface 5,000 to 10,000 feet. The most significant structural feature of the map area, the contact between the Franciscan and Salinian basement blocks, shows little or no gravity expression.