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San Andreas Fault (Tectonic Evolution (Early transform faulting (Formed…
San Andreas Fault
Tectonic Evolution
~30 Ma -
Pacific Plate first impinged on N. America
∴ Two triple junctions formed: Mendocino & Rivera
= Point where 3 plates meet: Pacific, N. America & Juan de Fuca (M) or Cacos (R)
~20 Ma -
Triples junctions migrated NW & SE
Today these TJs are 2500 km apart
(Mendocino & Rivera TJS)
Early transform faulting
Formed lengthening boundary btwn Pac & N. Am
= Right lateral (Dextral)
Further West of modern SAF (not SAF)
Transform faulting stepped Eastward
within the N. American plate
∴ forming Northern section of modern SAF
S. Section
Modern SAF
formed by similar E jumps
caused opening of Gulf of Mexico
Plate
Tectonic Context
SAF = part of very complex plate boundary on Western USA
Complexity includes:
Aluetian thrust to NW
Spreading ridges binding SAF
Strike slip motion connect/ accomodate spreading portions
Between N. American & Pacific Plates
SAF = 1300 km segment
Geometry
Non-linear
- not straight & occupies multiple planes
Right - lateral
- dextral strike slip fault
Width = up to 130 km
Comprises many smaller faults
Modern SAF
Most active portion (dominant slip)
-->
now in N. American plate rocks
Separates Mendocino TJ from Gulf of California
Originated ~4 Ma
Boundary btwn Pac & N. Am?
Not strictly boundary
True in sense
Rocks on W side SAF
= moving in concert with Pacific plate
False in sense
Rocks = displaced fragments of N. American plate
World
stress
map
Orientation of σH
represented as lines
colour coded to tectonic regime
Constructed using stress msmts from around world
Data collection
best (denser)
plate boundaries
Europe / W USA / Himalayas
Determining stress orientation
Geodetic data
Borehole
msmts
deform actively ∴ reveal stress orientation
Public Awareness
Noticed after 1906 earthquake
Geologic study after revealed
--> fault ruptured in systematic way
SAF Relative Motions
Pacific Plate
Faster ~ 8-10 cm/yr NW
N American
= MUCH SLOWER 2.5 cm/yr
SAF Region
First Active
~ 30 Ma = Late Oligocene
Modern SAF ~ 4 Ma