Calcite and quartz microstructural geothermometry of low-grade metasedimentary rocks, Northern Range, Trinidad

The Northern Range of Trinidad is a mountainous exposure of deformed metasedimentary rocks with Mesozoic depositional ages and Tertiary metamorphic ages located in the Caribbean–South American plate boundary zone. These rocks lack precise mineralogical indicators of metamorphic grade. Using temperature-sensitive quartz and calcite microstructures, and fission track data, we identify, describe, quantify, and map a nearly continuous spectrum of relatively low (>l50°C) to relatively high (250–400°C) deformation temperatures from east to west across the range. Average estimated rock exhumation rates also increase systematically along this trend. The analysis illuminates a sharp, east–west-trending, major thermal discontinuity (i.e. a fault) along the southern boundary of the range. Kinematic analysis of faults and shear bands adjacent to this boundary and modeling indicate that the fault probably dips southward at 80–85° and accommodated normal dip-slip at rates that decreased from west to east. Earlier experimental studies at high strain rates illustrated a strong strain rate dependency for quartz microstructural transitions. Our work indicates a possible strong temperature control for quartz microstructural transitions at natural strain rates.