Wide Plate Margin Deformation, Southern Central America and Northwestern South America,
CASA GPS OBSERVATIONS

Robert Trenkamp,
James N. Kellogg,
Jeffrey T. Freymueller,
Hector Mora P.

ABSTRACT:

Global Positioning System (GPS) data from southern Central America and northwestern South America were collected during 1991, 1994, 1996, and 1998 in Costa Rica, Panama, Ecuador, Colombia and Venezuela. These data reveal wide plate boundary deformation and escape tectonics occurring along an approximately 1400 km length of the North Andes; locking of the subducting Nazca plate and strain accumulation in the Ecuador-Colombia forearc; ongoing collision of the Panama arc and Colombia; and convergence of the Caribbean plate with Panama and South America. Elastic modeling of observed horizontal displacements in the Ecuador forearc is consistent with partial locking (50%) in the subduction zone and partial transfer of motion to the overriding South American plate. The deformation is hypothesized to reflect elastic recoverable strain accumulation associated with the historic seismicity of the area and active faulting associated with permanent shortening of 6 mm/a. Deformation associated with the Panama-Colombia collision is consistent with elastic strain accumulation on a fully locked Atrato-Uraba Fault Zone (AUFZ) suture.

CASA RESULTS 1991-1998

Figure 1. Station velocity vectors relative to stable South America at 95% confidence using the data from the 1991, 1994, 1996, and 1998 CASA campaigns. The Cocos Island vector (COCO) was calculated relative to South America using a vector reported in Freymueller et al.(1993). Location of the COCO vector has been shifted to maintain the clarity of the figure. ( Vector table)

Oblique Subduction and "Escape" - North Andes - rapid subduction of the Nazca plate and Carnegie aseismic ridge (67 ± 6 mm/yr) at the Ecuador trench relative to stable South America is oblique to the Colombia - Ecuador margin resulting in shortening perpendicular to the North Andean margin and lateral "escape" (6 ± 2 mm/yr) to the northeast.

Kinematics and Mechanics of an Island Arc - Continental Collision - The surprising ongoing collision of the oceanic Panama arc with stable South America at approximately 29 mm/yr indicates that the Panama arc is behaving as an approximately rigid indenter with the wide continental plate boundary zone in Colombia compressing normal to the suture zone (Atrata - Uruba fault zone, AUFZ). Deformation extends into the continental crust up to 600 km from the suture.

Southwestern Caribbean Plate Subduction - Displacement vectors show oblique ESE convergence of 19 ± 2 mm/yr between the Caribbean and South America plates which supports the controversial prediction of a Benioff zone produced by slow amagmatic subduction of Caribbean lithosphere.

CASA RESULTS 1991-1998 NEAR THE AREA OF THE 1906-1979 EARTHQUAKE SEQUENCE
Earthquake Strain Accumulation - Ecuador Trench - 26 ± 4 mm/yr short ening since 1991 at the coastal sites at Muisne and Esmeraldas, Ecuador, hypothesized to reflect three modes of deformation roughly parallel to the convergence direction
  1. motion due to elastic strain accumulation associated with the seismic cycle at the locked subduction zone;
  2. motion due to aseismic slip in the subduction zone;
  3. motion due to permanent shortening associated with active folding and faulting in the inter-Andean and sub-Andean fault systems several hundred km east of the coast.

MODELING STRAIN AT THE ECUADOR-COLOMBIA TRENCH AND PANAMA-COLOMBIA COLLISION ZONE:
(Ecuador-Colombia trench strain accumulation

(Panama Colombia Collision

MAJOR TECTONIC FEATURES OF THE CASA AREA

PROFILES A-A' and B-B' ARE THE PROFILES USED FOR MODELS

TECTONIC PLATES AND MAJOR EARTHQUAKES
THE CASA PROJECT AREA

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