Paleostress evolution during the exhumation of high-P marbles, Samaná Complex, northern Hispaniola

Abstract

The marble of the Samaná complex presents a widespread foliation formed during its exhumation following a general decompressive strain path from high pressure (2.0>P>0.7 GPa) and low temperature (<500 ºC) conditions. The foliation is plano-linear and blastomylonitic. Deformation distribution is highly heterogeneous. Calcite preferred orientation is poor, even though the marble has a well-defined tectonic fabric. The blastomylonitic fabric is masking an earlier tectonic fabric. Cathodoluminescence images reveal that intense fracturing formed prior to foliation development in the marbles. The thermodynamic modelling of mineral phase transformations during prograde metamorphism indicate an increase in water content (1.2%<w.t.H2O<1.8%) that may have involved an increase in fluid pressure and triggered rock embrittlement and subsequent exhumation. Stress drops after a cataclastic event, as well as grain-size reduction by abrasion, may have activated dissolution-precipitation processes along cataclastic bands. Differential stress |σ1-σ3| increased as exhumation progressed after the cataclastic event. Estimates of paleostress based on calcite mechanical twinning indicate values of |σ1-σ3| >350 MPa during deformation. In contrast, mean flow stress during grain-boundary migration is estimated in |σ1-σ3| <150 MPa. The high paleostress record and microstructures of the marble are consistent with the high exhumation rate calculated (>110 MPa Ma-1). All of these data suggest that exhumation always occurred near the brittle-ductile regime of deformation.