Abstract

Fission-track (FT) data from the Late Triassic-Early Jurassic Newark rift basin indicate long-lived convective downwelling along the border fault. Zircons and apatites from the basin and the surrounding basement yield FT ages averaging ∼180 Ma and ∼140-150 Ma, respectively (i.e., younger than the age of the host rocks). However, along the border fault of the basin, zircon ages exceed 300 Ma and apatite ages are as high as 214 Ma (i.e., older than the depositional ages). The proximity of samples heated to >220 ±40 °C, which reset zircons, and those never heated above 120 ±20 °C across a zone 5-10 km wide indicates an anomalous zone of low temperatures that resulted from the downwelling of water at the border faults. The FT results and other indicators of Hydrothermal flow attest to the circulation of relatively high temperature (100-250 °C) fluids in a pervasive Jurassic hydrothermal convection system. The fluid-convection downwelling along the border fault of the basin may be the consequence of the high topography that bordered the Newark basin in Triassic and Jurassic time. Hydrothermal flow ended by the time coastal-plain sediments covered the basin. Hydrothermal fluids circulating in extensional environments are potentially very important in modifying the temperature structure of the basins and crust.

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