Total tectonic subsidence: A parameter for distinguishing crust type at the U.S. Atlantic Continental Margin

Abstract

Total tectonic subsidence (TTS) is the difference between the prerifting continental crust elevation and the present, sediment‐unloaded, basement depth in a sedimentary basin. TTS is calculated using observations of bathymetry and basement depth and assumptions about sediment density and the loading response of the lithosphere. I use the extensional model for sedimentary basin formation to interpret the TTS of a passive continental margin. That model predicts that the TTS on an old margin should define the major crust‐type boundaries and indicate the lateral distribution of extension within the extended continental crust. TTS analysis of the U.S. Atlantic continental margin indicates that the boundary between extended continental and oceanic crusts lies along the East Coast Magnetic Anomaly (ECMA) to the north of the Blake Spur Fracture Zone (BSFZ), and along the Blake Escarpment to the south. There is no TTS anomaly associated with the Blake Spur Magnetic Anomaly which seems to be underlain by normal oceanic crust. The ECMA appears to be associated with a TTS minimum, but I attribute that to artificially shallow estimates of depth to magnetic basement under the ECMA. TTS estimates of extended continental crust thickness agree well with gravity models except where the problem with magnetic basement depth estimates interferes. The amount of crust extension prior to seafloor spreading varies along the strike of the margin. The most abrupt change is at the BSFZ. Across the BSFZ the amount of extension varies by 160 km, but the volume of continental material does not vary. The extension variation may be caused by lateral changes in the gross rheology of the continental lithosphere. In the case of the BSFZ it would be the suture of more “ductile” pre‐late Paleozoic African craton with more “brittle” North American craton that could cause the abrupt change.