Terrane transfer: Lessons from Baja California

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Terrane transfer Lessons from Baja
California
C. Plattner R. Malservisi LMU München,
Germany T. Dixon Univ. Miami, FL, USA F.
Suarez-Vidal CICESE, Mexico R.
Govers Universiteit Utrecht Netherlands G.
Iaffaldano Harvard University USA
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Baja California is a typical example for active
terrane transfer
NA
PA
Fletcher et al., 2007
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Motivation
While large plates are driven by mantle
convection...
... terranes are likely dragged by neighbouring
plates through lithospheric coupling forces.
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Lithospheric coupling forces
NA
PA
Fletcher et al., 2007
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Studying Baja transfer by partial coupling with
the Pacific plate

• Quantify the present-day plate kinematics from
  GPS
• Quantify the coupling (dynamics) from numerical
  modelling
• Implications for NAM - PAC plate boundary region

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Kinematics from GPS
Baja California Permanent Campaign GPS
network LMU München, Univ. Miami,
CICESE Pacific plate Permanent
GPS network (Rigid plate motion from Plattner et
al., 2007) North America plate (Rigid plate
motion from Sella et al., 2006)
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• GPS data processing using GIPSY/ OASIS II
  provides
• daily position estimates and errors in IGb00
  reference frame.
• Velocity from linear regression of
  position-time-series.

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Rigid plate kinematics from GPS
The motion of the plate is represented by a Euler
vector, through which the velocity V at every
point on this plate is expressed as V ? x
P V GPS velocity vector P GPS Position
vector ? Euler vector
V(Model) ? R sin d
Great circle distance from Euler pole deg
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Rigid plate motions - example from Pacific plate
Sensitivity test on stable Pacific plate
reference frame Jackknife F-test We
ighting
Increased relative weighting on GUAZ
Increased relative weighting on CHAT
Best-fit solution excluding GUAZ CHAT
Plattner et al. 2007
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Rigid Baja California microplate
Test geologically undeformed microplate as rigid
block
Average residual 1.308 mm/yr
Contraction 10-16 s-1
Average residual 1.708 mm/yr
Plattner et al. (GJI, 2007)
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Rigid Baja California microplate
Test for bias from elastic strain accumulation
at faults affecting the rigid plate calculation
using a block model (McCaffrey, 2003)
v (V/pi) tan 1 (x/D)
PUT A STRAIN ACCUMULATOIN ATAN PLOT WITH GPS (EG
FROM MATTHIAS FITTING ROUTINE)
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From plate kinematics to dynamics Numerical model
should reproduce the geodetic rigid plate
motions Pacific (PAC) - Baja California - North
America (NAM) (Plattner et al., 2007).
Geodetic Sierra Nevada motion from Psencik et
al., 2006
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2D spherical shell model simulates BAJA dragged
by Pacific plate
G-TECTON Govers and Meijer, 1998
FE Model with plane stress spherical elements
100 km lithosphere thickness Viscosity 1023
Pa.s
s
Plattner et al., 2007
Fault with strike-slip motion and differential
shear stress
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Scale shear stress from zero until geodetic plate
motion is fitted
FEM GPS
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Scale shear stress from zero until geodetic plate
motion is fitted
FEM GPS
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Geodetic plate relative motion is fitted with 10
MPa
Plattner et al., in press _at_ Geology
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Geodetic plate relative motion is fitted with 10
MPa
Plattner et al., in press _at_ Geology
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Rigid plate motions and geologic deformation
validate the model
Fit velocity vectors in rigid microplate Fit
velocity magnitudes in deformation zone
Geodetic and neotectonic fault rates as
summarized in Becker et al., 2005
Plattner et al., Geology, in press
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Influence of incomplete decouping in the Gulf of
California (current status in northern GULF
unknown) decrease of motion of BAJA with respect
to NAM
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Influence of incomplete decoupling in the Gulf of
California
For higher stresses the Pacific - Baja plate
boundary is continuously deforming, instead of
localized faulting.
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Implications of a constant shear stress over the
last gt6 Myrs
Gravitational coupling suggest constant shear
stress. Rupture of Gulf of California should have
increased Baja - NAM motion, and slowed
Pacific-BAJA motion.
Fletcher et al., 2007
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Western NAM imposes resisting forces for BAJA
motion Fault evolution since 3 Ma suggests
decreasing resisting forces

In the past lower velocity of BAJA w.r.t. NAM
than today?
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Western NAM imposes resisting forces for BAJA
motion Fault evolution since 3 Ma suggests
decreasing resisting forces

Constant velocity since 3 Ma! Coupling (driving
force) must have decreased!
In the past lower velocity of BAJA w.r.t. NAM
than today?
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SUMMARY
Velocity of BAJA w.r.t. NAM constant since at
least 3.0 Ma
Pacific - BAJA coupling
20 MPa (without Gulf ridge push)
10 MPa
Resisting forces Driving forces
Gulf ridge push
12 Ma 9 Ma 6 Ma 3 Ma 0
Decrease of resisting forces in western NAM from
shear zone evolution between 3Ma and present must
equal the decrease of coupling minus ridge push
to obtain constant Velocity of BAJA w.r.t. NAM
plate.
Gulf of California weakening
Resisting forces in western NAM decrease as
Basin and Range faults are activated as
strike-slip faults
Assumption Gulf completely ruptured at 3 Ma
(no more resisting forces)
If coupling forces decrease to zero, BAJA motion
will only be driven by ridge-push, BAJA velocity
depends on ridge push vs. resisting forces in
western NAM.
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Reactivation of Basin and Range normal faults as
strike-slip faults - a response from BAJA
motion?
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Effects of the pre-existing weakness in the Basin
and Range area on the Eastern California Shear
Zone formation
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Present-day deformation in northern and southern
BAJA from GPS
Residual and relative motion w.r.t. rigid BAJA
Northern BAJA Elastic halfspace and
viscoelastic Coupling modeling (Dixon et al.,
2002).
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Approach for southern BAJA deformation study
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CONCLUSIONS
Geologically rigid BAJA microplate shows
contraction of 10-16s-1. Modelled fault slip
rates in northern BAJA are strongly depend on
seismic cycle assumption (southern BAJA
deformation not yet constraint). BAJA moves in
same direction as PAC plate (w.r.t. NAM) but at a
10 slower rate (partial coupling). Coupling
stresses along the Pacific BAJA plate boundary
reproduces the observed kinematics of
BAJA. Coupling has decreased since 3Ma - likely
since coupling initiated Before rupture of BAJA
(Protogulf extension) higher slip rates west of
BAJA. BAJA motion strongly affects NAM - PAC
plate boundary region, in particular the Sierra
Nevada microplate motion, reactivation of normal
faults in the Basin and Range as strike-slip
faults and shear zone fromation around the
restraining bend. Pre-existing weakness from the
Basin and Range has influenced southern ECSZ
formation.
USGS
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Evidence for presence of coupling zone
High phase velocity zone supports presence of
lithospheric coupling along Western Baja
California (Zhang et al., 2007).
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Results from GPS study Rigidity of Baja
California microplate
Residual velocity 1.708 mm/yr
Convergent strain between northern and southern
network 10-16 s-1
Plattner et al. (GJI, 2007)
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Pure kinematic reference model
Broad scale needs simplification of plate and
microplate boundaries Test if fault geometry and
assumptions do not violate the kinematics (does
the model reproduce the kinematic input?)
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Pure kinematic reference model
The model is self-consistent and reproduces the
geodetic fault velocity and GPS velocity vectors
within the microplate.
GPS FEM
FEM GPS
w.r.t. fixed NA
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Lithospheric coupling model
FE code G-Tecton (Govers, 1993) as elastic, 2D,
shell Homogeneous rheology.
R. B. Smith et al., 2001
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Relative motion of BAJA with respect to Pacific
plate
Velocity field of Baja California w.r.t. rigid
Pacific plate
Plattner et al. (GJI, 2007)
Rigid plate relative motions
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Time-dependent interseismic deformation
Viscoelastic half space model Savage and
Lisowski, 1998 Interseismic variation of strain
field (earthquake cycle) Relaxation time
t2?/µ EQ Reccurance time T Elastic layer
thickness h
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Comparison of model results
Elastic Halfspace Viscoelastic
Halfspace Following Dixon
et al., 2002
v (V/pi) tan 1 (x/D)
AB 71 mm/yr SM 11 mm/yr
AB 31 mm/yr SM 51 mm/yr
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Elastic interseismic deformation
Elastic half space model Savage and Beaufort,
1973 Deformation as average over interseismic
period v (V/pi) tan 1 (x/D) v GPS
velocity V Far field plate motion x Distance
from fault D Locking depth of fault
Show a picture with a transect across ABF, SMV or
whereever. (since here no more Block modeling!!!)
Dixon et al., 2002