Regional extent of Great Valley basement west of the Great Valley, California: Implications for extensive tectonic wedging in the California Coast Ranges

Interpretation and modeling of the magnetic field of central California indicate that the magnetic basement of the forearc deposits of the Great Valley sequence extends westward beneath the coeval subduction-related rocks of the Franciscan Complex. The basement surface slopes gently to the west, reaching midcrustal depths (15-19 km) at distances of 50-100 km west of the Great Valley. This magnetic basement is disrupted by the Hayward-Rodgers Creek Fault system and is cut by the San Andreas Fault at the south end of the Great Valley and possibly throughout much of central California. The widespread presence of the Great Valley basement beneath rocks of the Franciscan Complex implies that the basement is more extensive than proposed in earlier interpretations based on seismic studies near the Franciscan Complex-Great Valley sequence contact. This result forces major modifications to ideas concerning this fossil subduction complex and other subduction zones. The eastern boundary fault of the Franciscan Complex (Coast Range Fault) is not (and never was) a subduction zone thrust fault but rather was originally a roof thrust (wedge-roof fault) formed above the eastward wedging mass of Franciscan Complex intruded along the top of the basement beneath the Great Valley deposits. This tectonic interpretation offers a solution for the question of how high-pressure metamorphic rocks of the Franciscan Complex were juxtaposed at the Coast Range Fault against low-pressure metamorphic rocks of the Great Valley sequence. This interpretation also implies an older flat-lying thrust fault (wedge-floor fault) that forms the top of magnetic basement between the active San Andreas and Hayward Faults at depths of 15-17 km. This older thrust fault may today transfer strain between the two young strike-slip faults, possibly explaining the apparent coupling of major nineteenth century earthquakes on these two faults. The former east dipping subduction zone along which the rocks of the Franciscan Complex accumulated must lie west of the western limit of the Great Valley magnetic basement.

[1]  K. Furlong,et al.  Seismic Evidence for a Lower-Crustal Detachment Beneath San Francisco Bay, California , 1994, Science.

[2]  W. Ellsworth,et al.  Seismotectonics of the San Andreas Fault System Between Point Arena and Cape Mendocino in Northern California' Implications for the Development and Evolution of a Young Transform , 1993 .

[3]  V. Ramirez.  Evolution of the Coast Range thrust as interpreted from seismic reflection data , 1993 .

[4]  M. T. Naney,et al.  Petrologic and geophysical studies of an artificial magma , 1992 .

[5]  O. E. S. Party.  Geology and Tectonics of the Chile Triple Junction , 1992 .

[6]  I. Wong.  Contemporary seismicity, active faulting and seismic hazards of the coast ranges between San Francisco Bay and Healdsburg, California , 1991 .

[7]  M. Lisowski,et al.  Historic creep rate and potential for seismic slip along the Hayward Fault, California , 1991 .

[8]  I. Wong.  Seismotectonics of the Coast Ranges in the vicinity of Lake Berryessa, northern California , 1990 .

[9]  A. Griscom,et al.  Tectonics of formation, translation, and dispersal of the Coast Range Ophiolite of California , 1988 .

[10]  W. Holbrook,et al.  The crustal structure of the axis of the Great Valley, California, from seismic refraction measurements , 1987 .

[11]  A. Jayko,et al.  Attenuation of the Coast Range Ophiolite by extensional faulting, and nature of the Coast Range “Thrust,” California , 1987 .

[12]  M. Zoback,et al.  A transect across the Mesozoic accretionary margin of central California , 1987 .

[13]  A. Jayko,et al.  Significance of Klamath rocks between the Franciscan Complex and Coast Range Ophiolite, northern California , 1986 .

[14]  W. Mooney,et al.  Two-dimensional velocity structure along the synclinal axis of the Great Valley, California , 1986 .

[15]  R. Jachens,et al.  Three‐dimensional geometry of the Gorda Plate beneath northern California , 1983 .

[16]  W. Mooney,et al.  Crustal structure of the Diablo and Gabilan Ranges, central California: A reinterpretation of existing data , 1982 .

[17]  A. Lachenbruch,et al.  Heat flow and energetic of the San Andreas fault zone , 1980 .

[18]  W. Connelly,et al.  Blueschists of the Kodiak Islands, Alaska: An extension of the Seldovia schist terrane , 1977 .

[19]  L. A. Raymond.  Tesla-Ortigalita Fault, Coast Range Thrust Fault, and Franciscan Metamorphism, Northeastern Diablo Range, California , 1973 .

[20]  M. Lanphere,et al.  Tectonic Significance of Mineral Ages of Blueschists near , 1973 .

[21]  W. Ernst.  Tectonic contact between the Franciscan Mélange and the Great Valley Sequence—Crustal expression of a Late Mesozoic Benioff Zone , 1970 .

[22]  W. Hamilton.  Mesozoic California and the Underflow of Pacific Mantle , 1969 .

[23]  V. Baranov.  A NEW METHOD FOR INTERPRETATION OF AEROMAGNETIC MAPS: PSEUDO‐GRAVIMETRIC ANOMALIES , 1957 .

[24]  V. Vacquier,et al.  Interpretation of Aeromagnetic Maps , 1951 .

[25]  R. Jachens,et al.  The surface of crystalline basement, Great Valley and Sierra Nevada, California: A digital map database , 1995 .

[26]  T. Hildenbrand,et al.  Total intensity magnetic anomaly map of Missouri , 1991 .

[27]  D. S. Harwood,et al.  Late Cenozoic tectonism of the Sacramento Valley, California , 1987 .

[28]  D. Daniels,et al.  Aeromagnetic and generalized geologic map of parts of central California , 1978 .

[29]  J. W. Cady.  Magnetic and gravity anomalies in the Great Valley and western Sierra Nevada metamorphic belt , 1975 .

[30]  W. E. McCaslin.  Aeromagnetic map of the southern part of the San Francisco Bay region, California , 1971 .