THERMAL EVOLUTION OF MESOZOIC RIFT BASINS AND POST-RIFT MAGMATISM IN THE MID-ATLANTIC

Early Mesozoic rift basins along the Eastern North America Margin (ENAM) developed as the forerunner to the opening of the Atlantic Ocean. In the mid-Atlantic region, these basins are primarily half-grabens that are localized along pre-existing Paleozoic fault zones. We use zircon U-Pb and (U-Th)/He double dating in both basin sedimentary rocks and older rocks from footwall and hanging wall domains to evaluate sediment provenance and infer the subsequent low-temperature exhumation/erosion history across the ENAM rift system. Additionally, we use using U-Pb zircon and apatite geochronology to date a suite of post-rift dikes providing a temporal framework for post-Triassic events.

Early syn-rift deposits from the Culpeper, Newark, Scottsville, and Gettysburg basins are sourced from local footwall and hanging wall drainages influenced by preexisting topography and local fault propagation. Detrital zircon U-Pb data are consistent with footwall drainage networks that provide significant sediment inputs from terranes to the west. Detrital inputs from the hanging wall block (to the southeast) in large proximal rift basins include a component of Pan-African sources that is absent from rift shoulder basins to the west. Zircon (U-Th)/He ages obtained from the Triassic syn-rift deposits and a trasverse across the footwall and hanging wall blocks range from ~205-150 Ma with a slight decrease in ages towards the basin in both the footwall and hanging wall. The absence of older zircon (U-Th)/He zircon ages (Paleozoic to early Triassic) in these basins indicates that post-depositional burial and heat flow was sufficient to reset the zircon (U-Th)/He system.

In central and western Virginia, a suite of rhyolitic to phonolitic dikes cut Proterozoic to Early Paleozoic metamorphic rocks in the eastern Blue Ridge and Paleozoic strata in Valley & Ridge. U-Pb zircon and apatite geochronology yield late Jurassic ages (150 to 160 Ma) for these dikes suggesting that the region was also affected by a post-rift thermal event. We infer these dikes to represent off-axis rift margin magmatism related to the nascent opening of the Atlantic Ocean that’s focused along the continental extension of oceanic fracture zones.