Crustal structure and rift architecture of the Georges Bank, U.S. Atlantic margin

Detailed crustal structure and rift geometry of the Georges Bank, U.S. Atlantic margin, were constructed using a comprehensive seismic reflection dataset to investigate rift-related extension that preceded the early Jurassic magmatic seafloor spreading. The proposed crustal structure is based on four data sources: (1) 12 recently released dense grid (<6 km line spacing) multi-channel 2D seismic reflection surveys that were collected during the 1970's to 1990's and span over 29400 km in length (2) reprocessed USGS legacy seismic data with a 2470 km total line length, (3) two offshore test wells and (4) two published refraction lines, located in the Gulf of Maine, NW of Georges Bank. These closely-spaced data reveal multiple normal faults that displace basement and bound half grabens and horsts. From these datasets, detailed two-way-time structural maps are created for the post-rift unconformity, top basement, and "Moho". The maps were used for calculations of crust and syn-rift thickness and for quantitative directional analyses of basement and rift-basin faults. The distribution of crustal thickness reveals a general WNW to ESE decrease in crustal thickness toward the shelf edge, which is oriented orthogonal to the NNE-SSW strike of most of the observed rift basins. The largest of the rifts is an elongate depression with up to 2.5 seconds (two-way travel time) of syn-rift strata that becomes deeper to the south toward the shelf edge and is underlain by a thinner crust. This depression may be the result of a failed rift that predated the magmatic seafloor spreading, which occurred farther east. Previously unrecognized rift basins that trend more easterly are reported here for the first time and form a second rift orientation. We suggest two possible scenarios to explain the bimodal distribution of basement faults direction: (1) two phases of rifting with different stress regimes, or (2) one stress regime applied on a bimodal inherited crustal fabric.