Strain SAF
Strain Partitioning
= Internal decomposition of total strain across a deformed zone
into zones/domains of different strain types
Why?
Natural rocks = anisotropic & heterogeneous
∴ strain is unevenly distributed throughout a deformed rock volume
Transpression / Transtension
= Fault / shear zone deviating from simple shear
∴ combination of
1. strike-slip motion (simple shear)
2. shortening or extension
(⊥ fault plane/structure being sheared)
Transpression - shortening (compression)
Transtension - extension ⊥ shear zoe
e.g. Strain Partitioning
If a weak fault / shear zone forms
Fault
Surrounding volume
must accomodate coaxial strain components
to balance strain imposed by boundary conditions
will localise the simple shear component
SAF Strain Partitioning
Pacific plate motion wrt SAF
comprises shear & pure components
Simple shear component
Accomodated in strike slip (dominant & parallel)
Pure shear component
accomodated by fold / faults
compression / extension esp @ fault bends
in adjacent units oblique to SAF
SAF Stepover Complications
Pronounced bend in SAF to left
Left lateral stepover near Los Angeles
Extensional Garlock fault accomodates this
Garlock = extension fault ⊥ SAF
Problem: Extension produces material
S of Garlock Fault
Thrusts accommodate new material either side of extension
Clockwise rotation also occurring
Teeth on
thrust
represent hanging wall
i.e. overriding unit / block
SA & Garlock Fault Systems
Very seismically active area
Active faulting in extensional & compressional domains
Clockwise rotation of whole system
SAF Highly Segmented
Not a straight line
Segments behave very differently
Locked
segments
only release seismic energy via major equakes
Creeping
segments
only generate microseismicity
steady movement & strain dissipation
Segmentation revealed by surface expression map
Steady
state creep
Steadily moving
Only generates microseismicity
Steady dissipation of strain energy
Constant gradual displacement
∴ Energy pumped into system meets little resistance