HR: 09:15h
AN: U21E-06    [Abstracts]
TI: Seismic Evidence for Complex, Multi-layered, Deep Structure of the Chesapeake Bay Impact Structure, Delmarva Peninsula, Virginia, USA
AU: * Catchings, R D
EM: catching@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Powars, D S
EM: dspowars@usgs.gov
AF: U.S. Geological Survey, 926A National Center, Reston, VA 20192, United States
AU: Goldman, M R
EM: goldman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Gohn, G S
EM: ggohn@usgs.gov
AF: U.S. Geological Survey, 926A National Center, Reston, VA 20192, United States
AU: Horton, J W
EM: worton@usgs.gov
AF: U.S. Geological Survey, 926A National Center, Reston, VA 20192, United States
AU: Rymer, M J
EM: mrymer@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Gandhok, G
EM: ggandhok@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Hole, J A
EM: hole@vt.edu
AF: Virginia Tech Geosciences, 4044 Derring Hall, Blacksburg, VA 24061, United States
AB: Seismic velocity and reflection images from a series of seismic reflection and refraction profiles across the Chesapeake Bay impact structure (CBIS), Delmarva Peninsula, Virginia, show that the buried, ~35-m.y.-old CBIS is a complex, central-peak crater with well-defined structural features. These features include (with increasing radial distance) a flat-topped central uplift (peak), a multi-layered moat, a collapsed transient-crater margin (present central-crater margin or inner rim), and a shallowly deformed annular trough. The radial widths of the central uplift, moat, and collapsed transient-crater margin are about 5 km, 13 km, and 2 km, respectively. These widths indicate a final central-crater diameter (inner-rim diameter or collapsed transient-crater diameter) of about 40 km. We use tomographic P-wave velocity images and low-fold seismic reflection images to infer the structure and composition of rocks within the upper 5 km of the crust along a northern radius of the impact structure. By combining the seismic images with published data from two boreholes located along the seismic profile, we interpret the structure to include (from about 3.5 km depth upward): a complex of variably reflective and fractured rocks of varying compositions (units 1 through 4); poorly reflective crystalline-rock breccias of the central uplift (unit 5), strongly reflective rocks of varying compositions (unit 6); mixed crystalline-rock and sediment breccias (unit 7), and a strongly reflective section of slumped sediment megablocks and ocean-resurge sedimentary breccia (unit 8). The entire impact structure is covered by about 350 to 450 m of essentially undeformed post- impact sediments (unit 9). Units 8 and 9 thicken and dip into the moat while thinning over the central uplift and over the raised inner rim. Unit 7 and the lower part of unit 8 are interpreted as debris from the collapsed transient-crater wall.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7250 Transform faults
DE: 8107 Continental neotectonics (8002)
DE: 8175 Tectonics and landscape evolution
SC: Union [U]
MN: 2007 Fall Meeting