Abstract: Origin of Minibasins by Multidirectional Extension Above a Spreading Lobe of Allochthonous Salt
M. P. A. Jackson, B. C. Vendeville
In the Gulf of Mexico, salt tectonics on the Louisiana continental slope and lower shelf is dominated by intrasalt minibasins that sink into allochthonous salt canopies. The origin of minibasins seemed obvious in the 1980s, when both overburden and salt were widely assumed to behave as extremely viscous, strengthless fluids. Two tabular fluid layers overturn by spoke circulation if densities are inverted. This creates a cellular pattern in which dense, downwelling minibasins are surrounded by less-dense, upwelling polygons.
This interpretation now seems untenable. The 1990s have brought to salt tectonics a greater appreciation of overburden as a strong, brittle, faultable medium encasing weak, flowing salt. Spoke circulation involving a brittle overburden is impossible. Instead, physical modeling, combined with bathymetric images of the Louisiana slope, indicates that minibasins are initiated by multidirectional lateral extension in the clastic skin mantling the leading part of a salt canopy on the lower continental slope. Radial and concentric extension in the lobate, spreading front interact to form a cellular network of polygonal grabens. Diapirs rise reactively into the grabens to form a polygonal network of low salt walls. Where extension is greatest, the overburden is thinnest. Wall culminations ac ively pierce the thinnest overburden and emerge as passive diapirs, which cannibalize salt from wall depressions. As the minibasins deepen and merge laterally, the cellular pattern degrades to a more chaotic one typical of "Louisiana style" salt tectonics: arcuate growth faults and salt welds locally pierced by salt stocks and arcuate salt walls.
AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France