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Transpressive evolution across the San Andreas fault system and
the California Coast Ranges
By
Rick Ford
Neogene transpressive evolution of the California Coast Ranges
By David L. Jones, Russell Graymer, Chi Wang, T.V.McEvilly, and
A.Lomax
Present tectonic motion across the Coast Ranges and San Andreas fault system in central California
By Donald F. Argus and Richard G. Gordon
Introduction
• Coast Ranges and San Andreas fault system• SAF generally considered to be a vertical structure • Strike slip dominant• Coast Range-wide midcrustal decollement at the
base of the seismogenic zone• Crustal shortening• Neogene structures and seismic evidence indicate
compressive deformation is still active
Central Coast Ranges
• Complex structural evolution
• Dominated by strike-slip displacements
• Compressive deformation
• North of Parkfield
• Several major fault strands diverge eastward
• Discrete tectonic blocks
• Different stratigraphic histories
San Andreas system• Major strands
bounded by subparallel Neogene imbricate fold and thrust belt
• Root with strike slip faults
• East and west vergent compressive domains
Complex System
• Major folds and attendant thrusts• Stratigraphic differences define six fault bounded sub-
domains• West vergent east dipping=blind thrusts• East vergent west dipping = imbricate fans
Imbricate Transpressive faults
• JKF, Franciscan• Jo, Ophiolite• MzCz, Marin strata• Ucz, non-marine
strata and minor volcanic rocks
• Qts, Silver Ck gravels
• Qtp, Packwood gravels
• Qti, Irvington gravels
•JKF, Franciscan
•Jo, Ophiolite
•MzCz, Marine strata
•Ucz, non-marine strata and minor volcanic rocks
•Qts, Silver Ck gravels
•Qtp, Packwood gravels
•Qti, Irvington gravels
Seismic Profile
• Wedge faults and decollement beneath the Great Valley Sequence
• Notice folds and faults don’t penetrate the the lower plate
Midcrustal Decollement
• Faults don’t penetrate the decollement• San Andreas is not a plate boundary• Base of the seismogenic zone Deformation of upper crust
produced by drag• Ductile flow
Implications for a new model
• Lateral movements of tectonic blocks on the basal decollement require dip-slip fault displacements
• Recurrence intervals on dip-slip faults may be much greater than on active strike slip faults so large earthquakes may occur on faults deemed to be inactive
• New faults may form far from previously recognized active fault zones due to readjustment along the basal decollement
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