San Andreas Fault

San Andreas Fault
-Weak or Strong?
Background
Background
Background
Under strike-slip regime, expect stresses to have
characteristic orientation
Background
It can be more complicated, however… distributed
vs. partitioned shear
Miller, (1998)
Weak Fault
Low heat flow in shallow depth
Fault-normal horizontal compression
Uplift of coast range
Earthquakes’ focal mechanism
Reverse 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
Basal traction
Lachenbruch, Sass,1992
Basal shear stress
Zoback,1991
Implications of Transpressive Wrench Tectonics
Younger folds would be close to
wrench orientation
Older folds would be closely
parallel to the fault
Miocene folds’ orientations are
close to wrench orientation
Pleistocene 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 temperaturesensitive ductile zone beneath the fault?
Strong Fault
Stress 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
Stresses
Strong: 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.
Structures
Strong: Structures
formed earlier and
have rotated into
parallel with fault
Miller (1998)
Heat Flow
Weak: Not enough heat flow for expected
“strong” shear stresses
Heat Flow
Aside: Strong fault models of transpression
predict increasing shear stress near fault
Heat Flow
Strong: Heat flow anomaly dissipated by gravity
induced fluid flow
Williams and Narasimhan (1989)
Open Questions
Is 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?