IMPLICATIONS FOR REGIONAL TECTONICS GAINED FROM STUDYING THE CHESAPEAKE BAY IMPACT STRUCTURE

The Chesapeake Bay Impact Structure was generated 35 MA when an asteroid or comet impacted the eastern seaboard of the North America. The impact generated a crater approximately 100 km across that underlies southern Chesapeake Bay. Because the impact occurred on the shelf the immediate fill and post-impact sediment records are both remarkably complete. The crater was located in a transitional zone between a less tectonically active passive margin north of the impact site, and a more active margin to its' south. Thus the crater and its immediate surroundings provide a uniquely useful location for studying both the regional tectonics and any tectonic effects due to the impact event. In this study, modeling of the area was performed using backstripping. We used data from existing boreholes which dot the area to develop a preliminary understanding of the complex tectonic evolution of the area as a precursor to further studies. Five distinct events were documented in the modeling results. The first event is the actual impact event, and represents the excavation of the crater. The second event, immediately post-impact, is a small subsidence event, which decreases in magnitude with time. We suggest that this event is an artifact of compaction of the rapidly emplaced breccia layer located within the crater. The third tectonic event is only recorded within the crater, and shows a rapid uplift of the area. We suggest that this event is regional in scale, but was only recorded within the crater due to its' deeper marine environment. The fourth tectonic event occurred between 20 and 15 MA and represents a period of increased subsidence. Interestingly, this event appears to have been controlled by location, with the locations furthest north recording both the earliest initiation, and the greatest magnitudes of subsidence. The fifth event first appears in cores between 10 MA and 7 MA, and represents a renewed increase in subsidence rates. In this case the event began in the cores furthest south and shifted northward over time.