San Andreas Fault - science.earthjay.com€¦ · San Andreas Fault-Weak or Strong? Background. Background. Background Under strike-slip regime, expect stresses to have characteristic

San Andreas Fault-Weak or Strong?

Background

Background

Background

Under strike-slip regime, expect stresses to have characteristic orientation

BackgroundIt can be more complicated, however… distributed vs. partitioned shear

Miller, (1998)

Weak FaultLow heat flow in shallow depth

Fault-normal horizontal compressionUplift of coast rangeEarthquakes’ focal mechanismReverse faulting and folding

Heat flow near fault

Lachenbruch, Sass,1992

Horizontal Compression

Coast range

Calaveras fault

Folds and reverse faulting

Mount,Suppe 1987

Orientation of maximum horizontal compression

Lachenbruch, Sass,1992

Basal traction

Basal shear stress

Zoback,1991

Implications of Transpressive Wrench Tectonics

Younger folds would be close towrench orientationOlder folds would be closelyparallel to the fault

Miocene folds’ orientations areclose to wrench orientationPleistocene folds are nearly parallel

Mount,Suppe 1987

Open Questions

Was it always weak or did it weaken as it evolved?If it evolved,did driving forces persist to generate basal traction?What is the weakening caused by?What is the role of shearing-heating and weakening in the temperature-sensitive ductile zone beneath the fault?

Strong FaultStress measurements were inconclusive or misinterpreted

Active structures have rotated into their current position

Expected heat flow anomaly dissipated by fluid flow

Stresses

Weak: maximum compressive stresses nearly perpendicular to SAF everywhere

StressesStrong: Stresses rotate on approach to the fault to agree with Anderson – Byerlee ideas

Structures

Weak: Stress thought to be perpendicular to fault based on active structures.

Strong: Structures formed earlier and have rotated into parallel with fault

Miller (1998)

Structures

Heat FlowWeak: Not enough heat flow for expected “strong” shear stresses

Heat Flow

Aside: Strong fault models of transpressionpredict increasing shear stress near fault

Heat FlowStrong: Heat flow anomaly dissipated by gravity induced fluid flow

Williams and Narasimhan (1989)

Open QuestionsIs it reasonable to say that the entire fault is weak or strong?If the SAF is weak, does that mean that all nearby faults are also weak?If stresses change rapidly in the immediate vicinity of the fault, might they also change along length?Can rotation of structures be detected geodetically?Will aridity/depth to water table affect the fluid flow argument?