THE CHESAPEAKE BAY IMPACT STRUCTURE: DIFFERENTIATING REGIONAL TECTONICS FROM IMPACT SIGNATURES

The Chesapeake Bay Impact structure was formed 35 Ma when a meteorite impacted the eastern coast of North America. This impact occurred in a transitional area between the tectonically inactive passive margin to the north, and the tectonically active margin to the south. The impact occurred in a shallow marine environment, which lead to near instantaneous burial and preservation of the impact crater, followed by a return to normal deposition. Thus, a nearly complete and detailed record of post-impact deposition was preserved. This study focuses on the application of backstripping to the Chesapeake Bay Impact Structure to better understand the short and long-term effects of such impact events on the lithosphere, as well as to better understand the tectonic activity along the eastern margin of North America. We use analysis of core holes from across the area to model the regional tectonic history, and core holes from inside the crater to compare the subsidence of the impact crater with the regional history. The results of the modeling show several distinct events. The first event appears only inside the crater itself, and shows a higher than expected subsidence for the first 5 m.y. after impact. The rapid subsidence may have resulted from compaction of the breccia lens, which would be deposited in an under-compacted state due to the speed of deposition. In addition, the heating of the lithosphere due to impact generates an increased thermal gradient, and as such, higher rates of thermal cooling post-impact. The second event is localized to one core, which is located on an upthrown fault block. This second event appears to track the uplift of the fault block, followed by its inversion. This event appears to record the evolution of that block. The last event shows an increase in subsidence that occurs over the entire area. This event occurs between about 7 Ma and 2 Ma, and appears to be a regional event; with possible ties to activity further south.