NEW CONSTRAINTS ON THE BAHAMA BANK-NORTHEAST CARIBBEAN COLLISION: APATITE AND ZIRCON (U-TH)/HE AGES FROM PUERTO RICO AND SURROUNDING ISLANDS

New apatite and zircon (U-Th)/He thermochronometric data, acquired from structural and igneous provinces in Puerto Rico, provide insights on the transition between the end stage of igneous island arc activity and the timing of collision with North America. Zircon He ages of 55.5 to 49.2 Ma (n=5) in southwest and 49.0 to 40.2 Ma (n=4) in northern Puerto Rico show that exhumation started in the Early Eocene. The spatial distribution of these ages suggests syn-collision arching, where exhumation was concentrated along the paleo east-west axis of the island. We interpret the He ages as recording an Early Eocene collision of Puerto Rico with the Bahama Bank at ca. 55-50 Ma, earlier than previous estimates. The collision thus coincides with the end of major magmatism in Puerto Rico at ca. 48 Ma based on zircon U-Pb ages (Pérez, 2008). A second cooling phase recorded by zircon at ~40 Ma in the southern Central Igneous Province suggests that the paleo-north experienced faster cooling and similar exhumation rates due to the proximity to the northern arc boundary. In contrast, zircon He ages from the southern part of the island record Early Oligocene cooling at ~34 Ma, related to the initiation of the Muertos Trough. Along the northern part of the island, apatite (U-Th)/He ages of 32.1 to 31.3 Ma (n=3) record cooling related to active subduction in the south and its possible northern flexural response. Rocks from islands east of Puerto Rico (Culebra and the British Virgin Islands) yield Early Oligocene to Early Miocene (29.7 to 17.7 Ma) apatite He ages, younger than most apatite ages from Puerto Rico (52.2 ± 3.1 Ma to 15.1 ± 0.9 Ma). The disparate apatite ages suggest that the islands east of Puerto Rico were structurally decoupled from the main island during the period of extension in the Anegada Passage. Estimates of exhumation rates vary by location and time. Assuming a typical geothermal gradient (20°C/km) during Eocene to Oligocene times, calculated exhumation rates range from 0.32-0.20km/Ma. These inversed-modeled thermochronometric data supply new temporal and thermal constraints on the tectonic variations and interactions of the main structural blocks in the island, which allow refinement of models related to the regional evolution of Puerto Rico.