Peter LaFemina: Penn State

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Forearc Motion and Cocos Ridge Collision in
Central America

• Peter LaFemina Penn State
• T. Dixon, R. Govers, M. Protti, E. Norabuena, H.
  Turner, A. Saballos, G. Mattioli W. Strauch

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Changes in Seismogenic Zone Coupling Along the
Middle America Margin?
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Outline

• Tectonic Background
• Previous Geodetic Research
• Regional GPS Velocity Field
• Modeling Interseismic Strain Accumulation
• Cocos Ridge Collision
• Conclusions

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Cocos - Caribbean Interaction Central America

• Subduction of Cocos plate - Caribbean/Panama
  75-91 mm yr-1
• Along strike age morphology
• EPR-CNS-1 initiation of CNS 23 Ma
• CNS-1 - CNS-2 19 Ma seamount domain
• Oblique convergence at C. Nicoya Peninsula
  northward
• Forearc sliver transport
• CNS-2 - Cocos Ridge (13.5 - 14 Ma 20 km crustal
  thickness Walther, 2000) since gt0.5 Ma
• SE migration of PTJ and NW migration Cocos Ridge,
  seamounts triple junctions

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Cocos - Caribbean Interaction Central American
SEIZE

• Local and global seismic tomography and
  relocation studies
• Contortion of slab along CNS-1 - CNS-2 boundary
• No slab inboard of CR gt60 km
• CNS-2 Caribbean crust are under-thrusting
  Panama block
• Crustal shortening across Fila Costena NPDB
• Uplift of Cordillera de Talamanca
• Panama Block (?) boundaries
• Central Costa Rica Deformed belt
• North Panama Deformed belt
• Forearc sliver transport
• Bookshelf faulting in Nicaragua
• Costa Rica El Salvador (?)

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Convergence Obliquity
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Previous Geodetic Research

• Dixon, 1993
• Lundgren, et al. 1999
• Coupling at Nicoya P. - 100 locking SE NW
• McCaffrey, 2002
• Fore arc 5.8 5.5 mm yr-1
• Norabuena, et al. 2004
• Coupling at Nicoya P. - 14 2 km 39 6 km
• Fore arc 8 3 mm yr-1
• Turner, et al. 2007
• Fore arc 15 mm yr-1 (Nic.)

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Regional GPS Velocity Field for Central America

• ITFR00 relative to stable Caribbean (Lopez et
  al. 2005)
• 3-9 yr time series at gt60 sites
• Strain accumulation associated with Cocos Plate
  subduction
• Nicoya Osa Peninsulas
• Nicaragua!?
• Aseismic slow slip - Nicoya
• Forearc sliver transport
• Symmetric relative to Cocos Ridge
• Central Costa Rica to Nicaragua/El Salvador
• Northwest Panama

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Model Domain
McCaffrey (2002 Plate Boundary Zones)
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Resolution Test
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Modeling

• DEFNODE (McCaffrey, 1996)
• GPS velocities and earthquake slip vectors
• Joint inversion for coupling on faults and rigid
  block motion
• Coupling ratio of locked slip to total plate
  rate
• 3 - model types
• Constrained down-dip coupling - (2)
• No constraints

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Phi Constrained Down-dip
Effective Transition Zone
Linear Decrease in Phi
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No Phi Constraints

• All models predict phi 0.5 at Osa and Nicoya
  Peninsulas
• Broad region of coupling at Osa P.
• Pattern of phi at Nicoya similar to results of
  Norabuena et al. (2004)
• Along strike variability in phi?
• Increase in forearc translation from C. Costa
  Rica to N. Nicaragua
• No models predict motion away from CR

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Collision vs. Subduction The Models

• Corrigan et al. 1990
• Jacob et al. 1991
• Gardner et al 1992
• Taylor et al. 1995
• Calmant et al. 2003

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CNS-2 - Cocos Ridge
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CNS-2 - Cocos Ridge Collision

• FEM of Ridge Collision (GTECTON)
• Spherical shell elements
• Variable rheology b/w forearc/arc (viscoelastic)
  and Caribbean crust (elastic)
• Low-friction faults along arc aid in
  accommodating forearc motion
• North fixed East and west free
• Velocity boundary condition - CNS-2 - Cocos Ridge
  crust
• Captures main features of velocity field and
  long-term deformation
• High rates inboard CR
• Forearc transport

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CNS-2 - Cocos Ridge Collision
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Conclusions

• Elastic strain accumulation MAT, NPDB and
  forearc
• Phi 0.5 at Nicoya and Osa P.
• Broad zone of coupling at Osa P.
• Can not constrain coupling along Nicaraguan
  segment
• Fore arc translation symmetrical about Cocos
  Ridge to northwest and southeast
• Increasing rate toward NW

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Relative Plate Motions
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Conclusions

• Elastic strain accumulation MAT, NPDB and
  forearc
• Partitioning of strain between plate boundary and
  fore arc (80 mm/yr) and North Panama deformed
  belt Caribbean (10 mm/yr Implies 10 mm/yr
  along CCRDB)
• Fore arc translation symmetrical about Cocos
  Ridge to northwest and southeast
• Increasing rate toward NW
• NW to SE transition from Cocos subduction to
  collision and Caribbean subduction (i.e.,
  subduction polarity reversal)!

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Long-term Deformation?

• No. Elastic strain accumulation!
• However this leads to long-term permanent
  deformation over many earthquake cycles

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What Next?

• Increased density of EGPS and CGPS
• 12 CGPS on Nicoya
• Expand to Osa, NPDB CCRDB
• Integration of geophysical, geological and
  geochemical data in a geodynamic models

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Acknowledgements

• NSF - MARGINS
• OVSICORI V. Gonzalez, E. Hernandez
• INETER A. Saballos, W. Strauch
• UM-RSMAS G. Schmalzle
• University of Arkansas H. Turner, G. Mattioli

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Convergence Obliquity

• Convergence azimuth (DeMets, 2001 DeMets et al.
  1990)
• EQ slip azimuth (Mw gt6.0 gt60 km HCMT)
• Trench normal
• Oblique NW of C. Nicoya (gt20 Nica.)
• Normal C. Costa Rica

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Cocos Ridge Collision, Coupling and Strain
Partitioning

• Interseismic velocity field measured by GPS
  geodesy
• Subduction model versus collision/strain
  partitioning models
• Plate Boundary - Osa segment
• Fila Costena (forearc) and North Panama Deformed
  Belt (back arc)
• Regional collision model - FEM

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Cocos - Caribbean Plate Interaction Subduction
Collision

• Regional GPS velocity field, geologic
  observations and geodynamic models Nicaragua,
  Costa Rica Panama
• Coupling inboard Cocos Ridge - Osa P.
• Shallow locking of seismogenic zone (lt25 km)
• Coupling along Nicoya segment Nicaraguan
  segment?
• Region is characterized by trench-parallel motion
• Oblique convergence cannot be the primary driver
  for this pattern
• A model for Cocos Ridge collision may be more
  appropriate than oblique subduction

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McCaffrey (2002 Plate Boundary Zones)
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Model Domain
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50 mm/a