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 An extensive program of slope-stability studies has been concluded in the San Francisco Bay region in California. Work to date has resulted in the publication of estimates of landslide damage; an estimated-landslide-abundance map of the region; new slope maps prepared by photo-mechanical processes; photointerpretive maps of landslide, colluvial, and other surficial deposits; and maps of relative slope stability. These studies indicate that landsliding is a major slopeerosion process in the region, that the damage resulting from landsliding is very great, and that additional development in the upland parts of the region should not be undertaken without careful evaluation of slope stability.

Abstract The basic geologic framework of the Yukon-Tanana upland, Alaska, a mountainous region of about 30,000 sq mi (77,700 sq km) between the Yukon and Tanana Rivers, was delineated primarily by L. M. Prindle and J. B. Mertie, Jr., in the early part of this century. The subsequent recognition of large-scale offset along the Tintina fault, which bounds the eastern upland on the north, has required a reconsideration of the regional stratigraphic and structural relations. The northwestern part of the upland is predominantly underlain by a sedimentary sequence consisting of rocks which range in age from Cambrian to Mississippian. Cretaceous and Tertiary sedimentary rocks unconformably overlie the older sequence. The Cambrian is apparently underlain by a thick section of grits, quartzites, phyllites, and quartz-mica schists. Pre-Silurian volcanic rocks, mafic and ultramafic rocks of probably Devonian age, and Permo-Triassic diabase and volcanic rocks are also present. These sedimentary and igneous rocks are cut by granitic plutons of Cretaceous and Tertiary age. The central and eastern parts of the upland are underlain by a metamorphic complex of rocks which range from lower-greenschist to amphibolite facies. Fossils date the parent sediments of some greenschist-facies rocks as Paleozoic. Radiometric dates from several localities in the metamorphic complex indicate that Precambrian, Ordovician, and Jurassic-Cretaceous thermal events are recorded in the metamorphic history. Mesozoic granodiorite and quartz monzonite batholiths and smaller granitic plutons of Mesozoic and Tertiary age intrude the crystalline schists. Locally, unmetamorphosed Cretaceous and/or Tertiary sedimentary rocks are in unconformable or fault contact with the older rocks. Tertiary volcanic rocks ranging in composition from rhyolife to basalt overlie the older rocks in small but significant parts of the eastern upland. Ultramafic intrusions, mostly small and serpentinized, also occur. Work has progressed to the point where the sedimentary rocks in the upland can reasonably be correlated with those in other parts of Alaska, but interregional correlation of the metamorphic terranes must await additional clarification of structural and petrologie relations.