Please enable JavaScript.
Coggle requires JavaScript to display documents.
Low Strength San Andras Fault Gouge (Lockner Findings (extremely low…
Low Strength San Andras Fault Gouge
San Andreas Motion
accomodates ~28-34 mm/yr right lateral motion Pacific plate wrt N. American
2 distinct locked segments in Cali w/seismic history
separated by 150 km creeping zone
SAFOD
NW of Parkfield, Cali
Core recovered from across the actively deforming section at a vertical depth ~ 2.7km
near South end of creeping segment
Lockner
Findings
lab strength msmts of fault core materials at in situ conditions
demonstrate SAF profoundly weak @ this locality & depth
due to presence of saponite
- a very weak smectite clay mineral
extremely low strength of foliated gouge in otherwise strong crust
= sufficient to explain the observed orientation
of σ1 at a high angle wrt the fault strike
Control creeping portions SAF
deformation of creeping portions of the SAF may be controlled by the
=presence of weak clay minerals
rather than high fluid pressure / other proposed mechanisms
msmts of fault core & borehole observations suggest an intrinsically weak SAF in an otherwise strong crust
Studied SAF Structure
show 200 m wide damage zone
comprising 2 actively creeping strands
Central deforming zone (CDZ),
SW Deforming Zone (SDZ)
Measurements of frictional strength of core material
Deforming
Zones
Highly foliated, incohesive fault gouge
associated with SDZ & CDZ
foliated gouge zones
porphyroclasts of serpentinite & sedimentary rock
dispersed in a matrix of Mg-rich clays
proposed formation gouge zones
shearing-enhanced metasomatic reactions between
serpentinite, tectonically entrained within the fault,
adjoining sedimentary rocks
Observations
very low measured strengths within
the two actively deforming shear zones.
all foliated gouge msmts yield μ<0.21
attribute low strengths to
abundance of the extremely weak mineral saponite
60–65%wt saponite in the foliated gouge matrix
Strength
even lower?
Rock fabric that localizes weak minerals
--> can lower frictional strength
wrt strength of ground & mixed samples
SAFOD foliated gouge may be even weaker
in its undisturbed state
SAF heat
flow paradox
If Byerlee's satisfied (μ = 0.6)
frictional heating of fault during equakes
& stable fault creep should result in
SHmax near fault should be oriented ~30° to fault trace
increased temp & heat flow adjacent to fault zone
Neither of above
are observed
no evidence of heatfow anomaly along SAF creeping section
borehole stress observations confirm SHmax is at a high angle to fault trace
Further evidence
low frictional strength (m<0.15)
of foliated gouge material inSZs
positive dependence of strength on slip rate of fault gouge
--> supports deformation by creep
SAF Stress
State Model
Whilst host rock shear stress = high
Principal stresses rotate within shear zone
--> to accomodate weaker material (fault gouge)
∴ Fault: high normal stress & low shear stress