James R. Metcalf

Thermochronology constraints on Miocene exhumation in the Central Range Mountains, Trinidad

The Central Range fault zone is a continental transform that accommodates most of the present-day slip between the Caribbean and South American plates in Trinidad. Global positioning system data and paleoseismic work suggest that this zone is active today and has been active for at least the past several thousand years. The modern fault zone overprints a middle Miocene fold-and-thrust belt; therefore, the strain recorded in the Central Range fault zone is the sum of both middle Miocene and more recent events. Thermochronology data from Eocene and Oligocene sandstones in the Central Range were collected to evaluate the timing of exhumation driven by crustal shortening and thickening. (U-Th)/He zircon analysis of subhedral zircons collected from eight samples indicated that the burial temperatures of these sedimentary rocks did not exceed ∼180 °C, suggesting that these grains record detrital (U-Th)/He dates. Apatite fission-track (AFT) analysis of 10 samples yielded mixed results, with cooling ages ranging from 30 to 15 Ma; however, most sites failed the χ2 test, suggesting that multiple age cohorts are present. Pooled AFT ages suggest that rocks presently at the surface were exhumed through their AFT closure temperature ca. 11−18 Ma. Cooling and exhumation thus most likely resulted from middle Miocene shortening across the fold-and-thrust belt in response to early oblique convergence between the Caribbean and South America plates. Post-Miocene deformation associated with more recent transform tectonics has therefore resulted in more limited (<∼4 km), if any, exhumation.

Late accretion to the Moon recorded in zircon (U–Th)/He thermochronometry

Abstract We conducted zircon (U–Th)/He (ZHe) analysis of lunar impact-melt breccia 14311 with the aim of leveraging radiation damage accumulated in zircon over extended intervals to detect low-temperature or short-lived impact events that have previously eluded traditional isotopic dating techniques. Our ZHe data record a coherent date vs. effective Uranium concentration (eU) trend characterized by >3500 Ma dates from low (≤75 ppm) eU zircon grains, and ca. 110 Ma dates for high (≥100 ppm) eU grains. A progression between these date populations is apparent for intermediate (75–100 ppm) eU grains. Thermal history modeling constrains permissible temperatures and cooling rates during and following impacts. Modeling shows that the data are most simply explained by impact events at ca. 3950 Ma and ca. 110 Ma, and limits allowable temperatures of heating events between 3950–110 Ma. Modeling of solar cycling thermal effects at the lunar surface precludes this as the explanation for the ca. 110 Ma ZHe dates. We propose a sample history characterized by zircon resetting during the ca. 3950 Ma Imbrium impact event, with subsequent heating during an impact at ca. 110 Ma that ejected the sample to the vicinity of its collection site. Our data show that zircon has the potential to retain 4He over immense timescales (≥3950 Myrs), thus providing a valuable new thermochronometer for probing the impact histories of lunar samples, and martian or asteroidal meteorites.