RAPID EXHUMATION OF CRETACEOUS ARC ROCKS ALONG THE BLUE MOUNTAIN RESTRAINING BEND, JAMAICA, USING APATITE (U-TH)/HE THERMOCHRONOMETRY

The Blue Mountain restraining bend (BMRB) in eastern Jamaica, bounded to the south by the left lateral Enriquillo-Plantain Garden fault zone (EPGFZ), is an ideal setting for studying the relationship among oblique plate motion, complex fault geometry, and crustal shortening. The BMRB has the archetypical topographic form for a restraining bend, with topography being highest directly along the fault and tapering gradually away. Although evidence for rapid rock uplift is clear through geomorphic indicators, the pattern and history of rock uplift and crustal shortening have not yet been constrained. We present new apatite and zircon (U-Th)/He ages (AHe and ZHe), representing exhumation from ~2-6 km depth. Twenty bedrock samples of metamorphic, sedimentary basin-fill, and intermediate plutonic origins were collected, of which sixteen had datable apatite. AHe ages range from ~1 Ma along the main fault to ~6 Ma within the core of the range, spanning 1.5 km relief and yielding an age-elevation relationship that implies Neogene exhumation at 0.4 mm/yr. The youngest sample implies exhumation accelerated in the Quaternary, however, and may have been as rapid as ~2 mm/yr. Preliminary ZHe data for one sample yield an age of 17 Ma with comparable exhumation rates since at least this time. These data indicate that the BMRB formed in the late Cenozoic and is experiencing active rock uplift. Based on the pattern of AHe ages, the most plausible kinematic explanation for this rock uplift is crustal shortening and oblique slip directly along the EPGFZ. Given that total strike-slip motion along the EPGFZ is ~5-7 mm/yr, the rapid rock uplift of the BMRB implies a significant fraction of plate motion is accommodated through shortening. This is in contrast to other well-known transpressive systems, such as the San Andreas Fault, where strike-slip rates of 2-3 cm/yr along restraining bends with comparable obliquity to plate motion correspond to slower rock uplift rates relative to the BMRB. We hypothesize that the more efficient transpressive deformation in the BMRB results from the highly erosive conditions present in Jamaica, enabling a higher fraction of plate motion to be accommodated by rock uplift and associated erosional mass transfer. This implies that transpressive systems may manifest differently depending on climatic setting.