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ABSTRACT: Depositional and Deformational History of the Franciscan Complex, Northernmost California

K. R. Aalto

Pervasive extensional shear fractures and curvilinear arrays of clay- and silt-filled veins in Franciscan Complex melanges and turbidites formed when Franciscan sediments were unlithified. Sandstone dikes both crosscut and follow fractures. Several scales of extensional faulting account for the juxtaposition of turbidites of different facies and/or with varying degrees of stratal disruption, the formation of sandstone lozenges and pinch-and-swell structures, and the formation of scaly foliation within the matrix of melange units. Within turbidites, the upper laminated portions of beds commonly contain abundant listric microfaults and the more massive lower portions of beds contain sediment-filled vein arrays. Veining and faulting occurred concurrently and resulted in diff rential extension of upper verses lower portions of beds. The finer sediment in veins reflects both cataclasis and filtering in of clay and silt from vein walls.

Most Franciscan rocks record an early pervasive, layer-parallel flattening strain, which may be related to the gravitational collapse of late Mesozoic Franciscan inner trench slope sediments that accompanied accretionary prism expansion resulting from underplating. However, some turbidites record noncoaxial extension that resulted from downslope creep of sediments. At Crescent City, sediment creep resulted in oversteepening of the Franciscan inner trench slope, which, in turn, may have triggered large-scale failure of slope materials resulting in the emplacement of the Crescent City olistostrome. The olistostrome crops out for 12 km along the coast, is up to 600 m thick, is in depositional contact with turbidites, and contains chiefly sandstone, greenstone, chert olistoliths up to 200 m across, and zones of slumpfolded turbidites.

Later internal shortening of the Franciscan accretionary wedge resulted in the formation of large isoclinal folds and discrete out-of-sequence contractional faults and shear zones, which may have resulted from the lateral emplacement of nappes of older more-elevated Franciscan rocks westward over younger, more recently accreted Franciscan. This deformation reflects cessation of subduction and consequent gravitational collapse of the wedge.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990