EVALUATING UNROOFING AND LINKS WITH VOLCANISM IN AN ELEVATED AND ERODED ARC, CORDILLERA DE TALAMANCA, COSTA RICA: CONSTRAINTS FROM PRELIMINARY (U-TH)/HE THERMOCHRONOMETRY

The Cordillera de Talamanca of southern Costa Rica is an anomalous section of the Central American Volcanic Arc. Active arc-volcanism ceased ~10 Ma, and the region currently occupies an ~170 km long volcanic gap. Calc-alkaline intrusives, the roots of eroded volcanoes that constrain the termination of volcanism, now crop out at elevations as high as 4 km, indicating substantial post-10 Ma uplift. The range lies inboard of where the Cocos Ridge is currently subducting, and debate exists over the relationships between the onset of ridge subduction, termination of volcanism, and elevation gain of the range. The nature of these connections is important for understanding the tectonic and geodynamic controls on the evolution of arc systems.

Here we use preliminary apatite (U-Th)/He thermochronometry data to place constraints on the cooling (75-40 ºC) and erosion history of the Cordillera de Talamanca. We acquired apatite (U-Th)/He data for five granitic samples collected from an elevation range of 750 to 1635 m, near the city of Buenos Aires in the Ujarraz and Fila Aguacate region. Two samples from < 900 m yielded mean dates from 4.0 to 0.9 Ma. Three samples from > 900 m yielded indistinguishable mean dates from 6.8-7.7 Ma. When combined with previously published apatite fission-track (AFT) data from the same area, the results suggest accelerated cooling in late Miocene time and continued cooling in the Plio-Pleistocene. The Plio-Pleistocene cooling signal partly overlaps with volumetrically minor 2.8 to 0.9 Ma adakite-like volcanics in the range. Cooling induced by late Miocene unroofing is consistent with an angular unconformity to the north that indicates tilting and erosion between 11 and 5 Ma, and with evidence for widespread 8-10 Ma deformation offshore of northwestern Costa Rica. Late Miocene unroofing and deformation may be due to the inferred onset of Cocos Ridge subduction at ca. 8 Ma, as supported by geochemical evidence for the subsequent appearance of a hot spot signature in central Costa Rica arc volcanism. If the 8 Ma estimate for Cocos Ridge collision is correct, then our preliminary results suggest it could be causally related to subsequent unroofing of the range. Ridge collision could not as easily explain an earlier cessation of volcanism at 10 Ma, however, implying a more complex relationship among these events.