san andreas fault -...
TRANSCRIPT
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San Andreas Fault-Weak or Strong?
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Background
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Background
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Background
Under strike-slip regime, expect stresses to have characteristic orientation
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BackgroundIt can be more complicated, however… distributed vs. partitioned shear
Miller, (1998)
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Weak FaultLow heat flow in shallow depth
Fault-normal horizontal compressionUplift of coast rangeEarthquakes’ focal mechanismReverse faulting and folding
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Heat flow near fault
Lachenbruch, Sass,1992
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Horizontal Compression
Coast range
Calaveras fault
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Folds and reverse faulting
Mount,Suppe 1987
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Orientation of maximum horizontal compression
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Lachenbruch, Sass,1992
Basal traction
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Basal shear stress
Zoback,1991
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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
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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?
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Strong FaultStress measurements were inconclusive or misinterpreted
Active structures have rotated into their current position
Expected heat flow anomaly dissipated by fluid flow
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Stresses
Weak: maximum compressive stresses nearly perpendicular to SAF everywhere
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StressesStrong: Stresses rotate on approach to the fault to agree with Anderson – Byerlee ideas
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Structures
Weak: Stress thought to be perpendicular to fault based on active structures.
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Strong: Structures formed earlier and have rotated into parallel with fault
Miller (1998)
Structures
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Heat FlowWeak: Not enough heat flow for expected “strong” shear stresses
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Heat Flow
Aside: Strong fault models of transpressionpredict increasing shear stress near fault
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Heat FlowStrong: Heat flow anomaly dissipated by gravity induced fluid flow
Williams and Narasimhan (1989)
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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?