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 age's (FTA) are repor ted from the Piedmont Province of Pennsylvania, Delaware, and Maryland , the Reading Prong of New Jersey, and the Newark Basin of Pennsylvani a. With the exception of one sphene sample, FTA fall within a relative ly 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 tra ct of crystalline basement (similar to 20,000 km(2)) and within Upper Triassic sediments in the Newark Basin indicates Late Triassic-Early J urassic total resetting of the fission track clocks followed by region al cooling between sphene (-275 +/- 25 degrees C) to zircon (similar t o 220 +/- 40 degrees C) closure temperatures at rates of similar to 4- 8 degrees C/Ma. Fourteen Rb-Sr whole rock-mica ages, together with pre viously reported Ar-40/Ar-39 biotite and hornblende ages, generally in dicate no regional Mesozoic resetting. Thus, the maximum temperature e xperienced by the samples was similar to 300 degrees C. Sphene FTA clo sely match ages of similar to 201 Ma for Early Jurassic magmatism in t he Newark Basin; hence the thermal event was probably associated with this activity. However, since most of the basement is largely devoid o f such igneous rocks, the thermal effect of magmatism alone may not ha ve been responsible for the total resetting of the FT clocks. In this respect, the role of possible regional geotherm elevation related to r ift-related lithospheric thinning in Late Triassic time requires furth er investigation. The FTA, taken together with previously reported den udation history and thermal modeling of the Newark Basin, suggests tha t the thermal event proceeded under paleothermal gradients of greater than or equal to 55-60 degrees C/km. The zircon FTA pattern suggests t hat non-uniform cooling occurred between blocks bounded by reactivated Paleozoic faults.