Interaction between spreading salt canopies and their peripheral thrust systems

Abstract

Gravity spreading of a buried salt canopy produces thrust faults around the canopy rim. These thrusts may form by shear along the base of the salt, by frontal rolling of the canopy, by expulsion of underlying sediments, or by submarine landsliding off the canopy's frontal escarpment. Each mechanism has a characteristic structural style, providing clues to processes of shortening in canopy-margin thrust systems. Canopy-margin thrust systems along the Sigsbee Escarpment in the deepwater Gulf of Mexico may have three parts. First, nearly all canopy-margin systems include a roof-edge thrust, which cuts upsection from the tip of the salt canopy and separates condensed strata in the canopy roof from thicker sections on the adjoining peripheral plain. Second, roughly one-third of roof-edge thrusts overlie an imbricate wedge, which telescopes sediments on the peripheral plain above a bedding-plane décollement. Third, in a few areas, salt-roof thrusts shorten the roof of the salt sheet. Because roof-edge thrusts and imbricate wedges both form by shear along the base of the salt allochthon, their abundance along the Sigsbee Escarpment implies that basal shear is the primary mode of advance there. Despite the dominance of basal shear, however, the Sigsbee Escarpment exhibits a wide range of structural styles. We speculate that this range reflects variations in relief on the canopy margin, thickness of the roof above the canopy toe, strength of the base-salt zone, and strength of sediments on the peripheral plain.