EOCENE ARC-CONTINENT COLLISION, EXHUMATION, AND DETRITAL ZIRCON GEO- AND THERMOCHRONOMETRIC PROVENANCE RECORD IN WESTERN PUERTO RICO

The arc-continent collision of the oceanic Caribbean plate and the North American plate in the Late Eocene marks the onset of island arc accretion, plate fragmentation, and the cessation of volcanism along the NE Caribbean plate. On the island of Puerto Rico, the arc-continent collision resulted in significant transpressional deformation along the Southern and Northern Puerto Rico fault zones, recorded by an island-wide unconformity. Although the deformational styles and kinematic reconstruction of these fault zones are well established, the timing of inception and activity, magnitude of related exhumation, and their temporal relations to the cessation in volcanism are only partially understood. Furthermore, little is known about the evolution of sediment dispersal patterns and provenance in response to this event. New detrital zircon (DZ) geo-thermochronometric data from pre- and post- arc-continent collisional strata in western Puerto Rico reveal >5 km of rapid unroofing and exhumation of the accreted island-arc between the early Eocene and early Oligocene. The absence of DZ U-Pb ages younger than ~44 Ma in the Eocene Rio Culebrinas and Oligocene San Sebastian Fms. confirms the cessation in major island arc volcanism by the middle Eocene. Double-dated (U-Th)/(He-Pb) DZ ages, from the San Sebastian Fm., consistently yielded early Eocene (~53 Ma) exhumational cooling ages indicating that the onset of island arc exhumation predates the culmination of active arc magmatism. DZ U-Pb ages from the Rio Culebrinas Fm. indicate sediment derivation mainly from the active volcanic arc with subordinate age components at ~.6, 1.0-1.3, 1.8, and 2.6-2.8 Ga. The sediment provenance for these old ages is controversial, but they resemble DZ U-Pb ages found in N. and S. American cratons and basins. Potential sediment derivation and transport of these detritus could come from via sub-marine distal basin-floor fans, along fore-arc and subduction trench axes from S. America or by sediment recycling of older pre-arc or island arc complexes and basins previously linked to America. In contrast, DZ U-Pb ages from Oligocene strata appear to be derived from recycling of Late Cretaceous to Eocene strata and volcanic rocks that constitute the core of the island and were exhumed and unroofed during the late Eocene arc-continent collision.