PARTIAL RESETTING OF FISSION TRACKS IN DETRTIAL ZIRCON: DATING LOW TEMERPATURE EVENTS IN THE HUDSON VALLEY (NY)

Thermal resetting of fission tracks in zircon is a function of temperature, time, and radiation damage. While the thermal stability of crystalline zircon is fairly well known, much less is known of the effect of radiation damage on thermal stability. In a 10 Myr heating event, the lower limit of the Partial Annealing Zone (PAZ) for undamaged grains may be ~240°C, but as low as 180-200°C for undamaged grains. This lower bound for radiation-damaged grains is important for zircons in sedimentary strata brought to elevated temperatures (buried and/or heated). In such a detrital suite, damage varies from grain to grain, and therefore heating of these grains results in variable resetting. We dated zircons from the Ordovician Austin Glen Fm. (Catskill) and the Silurian Shawangunk Cg. (Minneweska). Cooling ages can be divided into three populations: a) reset in the Early Jurassic (~185 Ma); b) reset or partially reset in the late Paleozoic (c. 275 to 322); and; c) unreset to partially reset in the early Paleozoic. These FT data clearly show that the Shawangunk Cg. experienced an Early Jurassic thermal event. Published Vitrinite Reflectance, Conodont Alteration Index, and Illite crystalinity values suggest that rocks in this part of the Hudson Valley experienced temperatures of ~180-220°C. Published interpretations of this pattern of thermal maturation call on 8 km of burial heating by a thick package of Carboniferous strata. Instead, we suggest that Early Jurassic heating and an elevated geothermal gradient of ~50°C/km (recognized in the Newark and the Hartford basins) in the mid-Hudson Valley could explain this thermal pattern. In this case, burial by Devonain strata, perhaps tectonically thickened in this area, can account for the required overburden and subsequent thermal maturation occurred much later in the Mesozoic. This interpretation would suggest that an elevated geotherm and thermal overprinting in the Mesozoic affected a much larger area in NY and New England than is traditionally assumed.