NEW SHIPBOARD GRAVITY AND MAGNETIC SURVEYS ON CHESAPEAKE BAY REFINE IMPACT CRATER STRUCTURE

We present high-resolution shipboard gravity and magnetic data collected over the Chesapeake Bay impact crater during the summers of 2002-2003. These data, in combination with previous aeromagnetic surveys and the NIMA gravity database shed new light on the crater's structure. Using mostly multichannel seismic reflection records, USGS workers have previously described the Chesapeake Crater as a complex peak-ring crater with outer escarpments forming a rim of diameter of ~90 km, and an inner peak ring of diameter 35-40 km. Scattered ship track data over the inner ring show it is associated with a gravity field low, attributed to the excavation of crystalline rock during the impact process and subsequent infill with breccia and sediment. We have mapped the magnetic field and gravity over 800 km² of the Chesapeake Crater, including most sections of the inner ring which lie beneath the Bay. The most prominent feature of the gravity data is a ring-shaped high along the peak ring. Anomalies directed toward the center of the crater reach -15 mgal, suggesting a 0.5-1 km thick density contrast of ~400-750 kg/m³. Though the free-air anomaly also decreases slightly outward from the peak ring, it is uncertain to what degree this decrease is created by the presence of tilted fault blocks, or other larger scale basement density anomalies. Toward the center of the crater, there is an increase of 2-4 mgal, consistent with central peak topography there. Magnetic anomaly data show two ring-shaped positive anomalies near the center of the crater, one close to the above central gravity anomaly, and the other ashore, near the town of Cape Charles. Over the scale of the crater, the magnetic field shows a ~400 nT low over much of the inner ring of the crater, surrounded by more positive anomalies on land to the east and west. The low may be due to the loss of magnetization during shock, heating, and brecciation of the underlying crystalline rock. Over broader regional scales, both the gravity and magnetic field data show northwest-southeast-trending lineations which most likely reflect earlier orogenic basement features. Notably, a significant portion of the crater peak ring coincides with these lineations, which may amplify the peak-ring gravity anomaly. This overlay suggests that basement structure played a role in the formation of the peak ring.