Print this pageCauses and Structural Characteristics of Thin-Skinned Inversion during Gravity Gliding or Spreading above Salt or Shale
RAILLARD, S., B. C. VENDEVILLE, G. GUERIN, and T. MAUDUIT
In salt- or shale-bearing continental margins, the sedimentary section is commonly deformed by two gravity-driven mechanisms. Gravity gliding occurs down the continental slope and is controlled by the tilting of the basement top. Gravity spreading involves both slope and shelf and is controlled by the surface slope of the prograding sedimentary wedge. Both processes create a simple structural zonation comprising three main domains: (1) an upslope domain, representing the trailing edge of the gliding/spreading unit undergoing extension; (2) a midslope domain where sediments are passively translated downslope and do not deform significantly; and (3) a downslope domain, representing the toe of the gliding unit, undergoing contraction. However, this simple structural pattern applies only at a given time during margin evolution. Several geologic processes can cause the location of each structural domain to shift through time and reactivate or invert older structures: (1) updip propagation of the contractional toe zone can eventually invert older diapirs and normal faults previously located in the extensional domain, (2) salt withdrawal at the base of the slope can increase the gliding potential in the previously contractional toe zone and reactivate folds and thrusts in extension, and (3) shifts in sediment supply caused by sea-level changes or basement subsidence can trigger inversion. For example, progradation or a sea-level drop shifts the sediment supply seaward and thereby superimposes extension onto older folds and thrusts.
We use seismic examples from offshore West Africa and experimental tectonic models to illustrate the geometry and mechanism of such contraction-extension or extension-contraction salt structures.