Anomalous Mesozoic thermal regime, central Appalachian Piedmont: Evidence from sphene and zircon fission-track dating

A total of 17 sphene and 45 zircon fission track ages (FTA) are reported from the Piedmont Province of Pennsylvania, Delaware, and Maryland, the Reading Prong of New Jersey, and the Newark Basin of Pennsylvania. With the exception of one sphene sample, FTA fall within a relatively restricted age range: sphene 190-217 Ma (average 199 Ma) and zircon 143-218 Ma (average 184 Ma). Near concordance of FTA over a large tract of crystalline basement (-20,000 km2) and within Upper Triassic sediments in the Newark Basin indicates Late Triassic-Early Jurassic total resetting of the fission track clocks followed by regional cooling between sphene (-275 + 25°C) to zircon (--220 + 40°C) closure temperatures at rates of -4-8°C/Ma. Fourteen Rb-Sr whole rock-mica ages, together with previously reported 40Ar/39Ar biotite and hornblende ages, generally indicate no regional Mesozoic resetting. Thus, the maximum temperature experienced by the samples was -300°C. Sphene FTA closely match ages of -201 Ma for Early Jurassic magmatism in the Newark Basin; hence the thermal event was probably associated with this activity. However, since most of the basement is largely devoid of such igneous rocks, the thermal effect of magmatism alone may not have been responsible for the total resetting of the FT clocks. In this respect, the role of possible regional geotherm elevation related to rift-related lithospheric thinning in Late Triassic time requires further investigation. The FTA, taken together with previously reported denudation history and thermal modeling of the Newark Basin, suggests that the thermal event proceeded under paleothermal gradients of -55-60°C/km. The zircon FTA pattern suggests that non-uniform cooling occurred between blocks bounded by reactivated Paleozoic faults.