Causes 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.