UTIG RESEARCH PROJECTS ARCHIVE

Cracks Along the Outer Shelf FLYER

PI's:  Neal Driscoll, WHOI, John Goff, UTIG, and Jeffrey Weissel, LDEO

Cruise recently completed (May 6 - May 20, 2000) - photos now available.

See UTIG press release

Proposal Project Summary

A system of en echelon cracks, resembling small offset normal faults, has recently been discovered in NOAA gridded bathymetry and GLORIA sidescan imagery off Virginia and North Carolina. These features are located along the outer continental shelf in water depths ranging from 100-250 m between Norfolk canyon and the Albemarle-Currituck submarine slide to the south. The existing gridded bathymetry and GLORIA data are sufficient to permit recognition of the cracks. However, the swath data fail to image the detailed morphology of the cracks. Furthermore, existing seismic reflection profiles and 3.5 kHz echosounding data fail to provide any information on the sub-surface of these features. Without such detailed observations, it is not possible to identify the processes responsible for the formation of these en echelon cracks, nor assess their potential for future slope failure.

Our working hypothesis is that these en echelon cracks represent the initial stage of slope failure of the outer shelf and upper slope, and are destined to become headwalls of future submarine landslides (e.g., Albermarle-Currituck slide). The impetus for studying these features is two fold. First, new knowledge about these examples of incipient slope instability and landsliding will lead to a greater understanding of how cycles of broad-scale mass wasting, canyon cutting, and sedimentation interact to shape the observed morphology of the Atlantic continental margin. Second, from a societal perspective it is important to understand these features well because any submarine landslides nucleating on this en echelon crack system and occurring on human time scale pose a small but not negligible danger to populations along the adjacent coast, owing to tsunami generation.

For these reasons we propose a series of marine geological and geophysical investigations of the newly discovered features. In Phase I we will acquire detailed, high-resolution vertical-incidence imagery of the seafloor in the vicinity of the en echelon cracks along the outer shelf and upper slope off Virginia and North Carolina. The survey, which will be conducted on a 14-day leg of R/V Cape Hatteras operating out of Beaufort, NC, will utilize the new, deeply-towed WHOI SUBSCAN chirp sonar and sidescan system. The overall objective of Phase I field work is more accurate mapping of the aerial distribution of these features and their subsurface geometry, in order to evaluate our working hypothesis that the crack system indicates nascent slope failure along the headwalls of the landslides and the shape of the upper part of the potential failure surface. In addition, we will further examine existing and soon-to-be-released NOAA gridded bathymetric data along the US Atlantic margin for evidence of past catastrophic failure and subsequent slope canyon evolution. The support from NSF requested in this proposal covers the implementation of the Phase I effort.

Figures:  System of en echelon cracks, resembling small offset normal faults, recently discovered off Virginia and North Carolina between Norfolk canyon and Albemarle-Currituck submarine slide (NOAA gridded bathymetry).  At top, Three-dimensional perspective of continental margin off southern Virginia and North Carolina looking in direction aligned with shelf edge. Shading imitates illumination of sea floor by artificial sun in northwest. Upper Currituck failure surface is smooth area in the foreground, and en echelon crack system is seen farther north on outermost shelf. Inset shows three representative bathymetric profiles over cracks, and their locations are noted on perspective image.  Below, shaded relief images in two-dimensions, with inset A showing an enlargement of en echelon cracks and inset B a location map.