HR: 0830h
AB: The boundary between the North American and Caribbean plates is characterized primarily by left-lateral motion along predominantly east-west striking faults. Seismicity and marine geophysical survey data are consistent with at least two, and possibly three, microplates in the diffuse boundary zone in the northeastern Caribbean: (1) the Gonave, (2) the Hispaniola, and (3) the Puerto Rico-northern Virgin Islands (PRVI). We discuss results from GPS geodetic measurements acquired since 1994 to test the microplate hypothesis and constrain PRVI neotectonics. GPS-derived velocities are analyzed with respect to both North American and Caribbean plate reference frames. At the 95\% confidence level, GPS velocities for sites in PRVI are equivalent, with the exception of one site (GORD) in the eastern British Virgin Islands. Intrablock displacement therefore is less than a few millimeters per year and deformation associated with North American-Caribbean relative plate motion is limited to the block-bounding structures. The velocity of the eastern British Virgin Islands (GORD) closely approximates that of St. Croix (CRO1) on the rigid Caribbean plate. We infer that PRVI is attached to the Caribbean at its eastern edge. Motion of PRVI relative to North America is slower than that between the rigid Caribbean and North American plates, precluding eastward tectonic escape of PRVI within the plate boundary zone. GPS velocities predict left-lateral transpression across the Puerto Rico trench and the Muertos trough south of Puerto Rico, left-lateral transtension across the western Anegada passage, and east-west opening of a few millimeters per year across the Mona rift. This deformation pattern is not compatible with rotation of a rigid PRVI about a vertical axis located in the southeast of the island of Puerto Rico but is consistent with northeastward translation of PRVI relative to North America and eastern Hispaniola. Comparison of GPS velocities for PRVI with respect to North America with total North America-Caribbean relative motion suggests up to 85\% of North American-Caribbean plate motion is accommodated by the Puerto Rico trench and offshore faults north of Puerto Rico, which pose the greatest seismic hazard.
AN: T11B-04 INVITED
TI: Neotectonics of Puerto Rico and the Virgin Islands, Northeastern Caribbean, From GPS Geodesy: Implications for Seismic Hazard
AU: * Jansma, P E
EM: pam@geology.uprm.edu
AF: Department of Geosciences, University of Arkansas, Fayetteville, AR 72701 United States
AU: Mattioli, G S
EM: glen@geology.uprm.edu
AF: Department of Geosciences, University of Arkansas, Fayetteville, AR 72701 United States
AU: Mattioli, G S
EM: glen@geology.uprm.edu
AF: Department of Geology, University of Puerto Rico, Mayaguez, PR 00681 United States
AU: Lopez, A
AF: Department of Geology, University of Puerto Rico, Mayaguez, PR 00681 United States
AU: DeMets, C
AF: Department of Geology & Geophysics, University of Wisconsin, Madison, WI 53706 United States
AU: DeMets, C
AF: Geosciences Azur, CNRS, Sophia Antipolis, 06560 France
AU: Dixon, T H
AF: Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, FL 33149 United States
AU: Mann, P
AF: Institute for Geophysics, University of Texas, Austin, TX 78759 United States
AU: Calais, E
AF: Geosciences Azur, CNRS, Sophia Antipolis, 06560 France
DE: 7223 Seismic hazard assessment and prediction
DE: 8150 Plate boundary--general (3040)
DE: 8158 Plate motions--present and recent (3040)
DE: 1206 Crustal movements--interplate (8155)
DE: 1243 Space geodetic surveys
SC: T
MN: Fall Meeting 2000