(U-TH)/HE THERMOCHRONOLOGY AND THERMAL HISTORY MODELING REVEALS EARLY CRETACEOUS DEFORMATION ACROSS THE MESOZOIC HARTFORD BASIN, CONNECTICUT

The Hartford Basin (HB) in Connecticut is one of several Mesozoic extensional basins in eastern North America associated with the opening of the Atlantic Ocean. The depositional history and stratigraphy of these basins have been extensively characterized, but the thermal consequences of footwall erosion, sediment accumulation, and subsequent exhumation are not well established. Here we provide new apatite and zircon (U-Th)/He thermochronology from six samples in both the HB sedimentary rocks and the crystalline footwall of the HB-bounding Eastern Border fault (EBf) to provide new constraints on the thermal history from active rifting through early development of the Atlantic passive margin.

Crystalline footwall samples yield a spread of zircon dates between ~144–230 Ma and a narrower range of apatite dates between ~147-207 Ma. In contrast, HB clastic samples yield apatite dates between ~110-207 Ma and zircon dates suggesting inherited ages and limited basin reheating. Preliminary thermal history models of the crystalline footwall samples include added constraints from available ⁴⁰Ar/³⁹Ar cooling ages. Our results from two samples suggest monotonic cooling from high-grade conditions during the Permian Alleghanian orogeny through the Early Jurassic, followed by cooling to temperatures of <40 °C by ~140 Ma. Results from a third sample also suggest monotonic cooling from the Permian to Late Triassic-Early Jurassic but show slower protracted cooling to temperatures of only <150 °C by ~130 Ma. Preliminary thermal history models for two of the three HB samples show general agreement and require between 50–200 °C of post-depositional heating, then cooling to <50 °C by ~130–120 Ma. Weighted mean paths for both models indicate peak heating may have occurred by ~165 Ma. Modeling results from the third HB sample are in contrast, requiring less than ~60 °C post-depositional heating. These results suggest that the rocks of the hanging wall and footwall of the EBf first reached equal crustal level in the Early Cretaceous (~140–120 Ma) at temperatures < ~40–50 °C, consistent with available apatite fission track ages and continued deformation across the EBf into the Cretaceous.