Transpressive evolution across the San Andreas fault system and the California Coast Ranges By Rick Ford.

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

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