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 subdomains
• 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
Depth of
Seismicity
• Red, deep
(30.0 km)
• Blue, shallow
• Red, deep
(20.0 km)
• Blue, shallow
Brittle to ductile transition
(350°c isotherm)
Photo micrographs of deformed
rocks
• (a) Loma Prieta Creek
• (b) Sierra foothills
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
Summary
• Neogene compressive deformation extends
throughout the Coast Ranges.
• Major folds and attendant thrusts
• Transpressive dip-slip bound crustal blocks
have undergone uplift or subsidence.
• Different from the standard plate tectonic
model