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The geometry of a part of the Mexican Ridges foldbelt in the southwestern Gulf of Mexico as detailed by multifold seismic data suggests an origin by large-scale gravity sliding. Here, apparently competent, lower Tertiary-Holocene strata, probably sandstones and shales, have moved downslope along an underlying deformed zone probably consisting of weak and incompetent Upper Cretaceous-lower Tertiary geopressured shales. Gravity-generated horizontal compressive stresses apparently caused uniform folding and imbricate thrusting within the competent strata. This deformation is probably an important mechanism for overcoming resistance to basal sliding within the incompetent layer. Large-scale submarine sliding is also suggested by the grouping of the fold trends into two large obate areas. Sliding apparently has continued periodically since about middle Tertiary time. This massive sliding may have been triggered by periods of uplift and seaward tilting and perpetuated by sediment loading in a large growth-fault system just landward of the main foldbelt along the upper slope.
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