PALEOMAGNETIC CONSTRAINTS ON SHORTENING ACROSS THE CENTRAL RANGE FAULT ZONE, TRINIDAD

The Central Range fault zone is located in Trinidad and formed as a restraining bend when Caribbean-South American plate motion stepped south off of the El Pilar fault. GPS data indicate transpressional kinematics are presently active. The zone, however, was also the locus of contractional deformation in the Late Miocene. We attempt to calculate the amount of finite strain that has accumulated since the Miocene using a preliminary paleomagnetic data from Miocene sediments in the Central Range. Data from the Middle Miocene Tamana Formation suggest counterclockwise rotation in one location and no statistically significant rotation in another. These sites, however, fail the fold test indicating that they record post folding (i.e. post Middle Miocene) rotation. In contrast, a small subset (N = 6) of Late Miocene sandstones and shales of the Manzanilla and Springvale formations pass the fold and reversal tests. As a group they suggest 35±11˚ of clockwise rotation. We interpret the contradictory senses of rotation to be due to rheological variations. The well-lithified Tamana Formation may have rotated as rigid crustal blocks, while the younger, weakly lithified sediments may have deformed as part of a continuously flowing medium. Interpretation of these rotational data in terms of amounts of plate motion requires an understanding of the kinematics of deformation. GPS data suggest modern transpressional kinematics with an angle of oblique convergence of 20˚. The orientation and geometry of folds in this part of the borderlands, suggests an angle of oblique convergence of approximately 35˚. The paleomagnetic results combined with this range of potential transpressional scenarios suggests 28-68% shortening. This amount of shortening applied to a zone 10 km wide centered on the Central Range fault, suggest 4-18 km of contraction and 12-31 km of total plate motion. The GPS determined rate of motion between the Caribbean and South American plates is approximately 20 mm/yr and could account for these strains in 1 to 2 million years. This is consistent with the interpretation that modern transpressional kinematics may have been active in the Central Range fault zone as early as the Pliocene.