Abstract
The Northern Range of Trinidad is an east-west trending mountainous exposure of metamorphic rocks located in the Caribbean-South American plate boundary zone. With a maximum elevation of ~1km, the Northern Range is the only place in Trinidad were metamorphic rocks have been exhumed to the surface. Prior to ca. 10 Ma the Caribbean plate was obliquely converging relative to the South American plate and began creating the geologically young mountains that we see today. However, ca. 5-10 Ma, the Caribbean plate shifted from oblique convergence to dextral shearing (strike-slip faulting), and near Trinidad is currently moving at ~20mm/year toward N86°E ± 2° with respect to the South American plate. Using apatite fission-track methods, we test whether the Northern Range rocks were exhumed before or after this shift in plate motion to better understand the processes by which they were exhumed (unburied). Samples were collected over the past decade, and then the fission-track analysis was performed at Apatite to Zircon Inc. in the summer of 2008. We studied apatite fission tracks in fifteen samples: fourteen from the Northern Range, and one from the Paria Peninsula in Venezuela. All samples gave reset fission-track ages, between ~4 Ma and ~20 Ma. We recognize a spatial pattern in the ages: older fission-track ages (ca. 13 to 18 Ma) are located in the eastern Northern Range and younger fissiontrack ages (ca. 5 to 6 Ma) are located in the western Northern Range. The younger fission-track ages in the western Northern Range postdate oblique convergence of the Caribbean plate motion relative to South America. The older fission-track ages in the east predate the shift in plate motion. Inverse modeling using the HeFTy program also shows that exhumation occurred at a faster rate (ca. 15ºC/m.y.) in the west than in the east (ca. 5ºC/m.y.). Our interpretation is that a thick, buoyant crustal root is present in the western Northern Range and this resulted in isostatic uplift and associated erosion, exhuming rocks of the western rocks long after oblique convergence (transpression) shut off.