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Abstract: Experimental and Geologic Evidence for Rejuvenation of Salt Diapirs by Horizontal Contraction

B. C. Vendeville, K. T. Nilsen, J. T. Johansen

Tectonic experiments and seismic data from the Nordkapp basin (Southern Barents Sea) illustrate how salt diapirs that have already depleted their source layer can later rise anew when horizontally shortened. Contraction can be due to regional compression or to downslope gravity gliding during basin subsidence.

Rock salt is much weaker that overburden sediments, so horizontal compression preferentially shortens the diapirs rather than the stronger, adjacent overburden. Horizontal contraction squeezes salt out of diapir stems, thus forcing diapirs to rise actively and deform their roof. Deformation varies with the initial diapir width: during horizontal contraction of initially narrow diapirs, their height increases much more than their width decreases; Reactivated, narrow diapirs rise by arching their roof, folding their shoulders, and forming a crestal graben, leaving no clues that their growth was induced by horizontal contraction. By contrast, initially wide diapirs rise less than they narrow; their roofs are deformed mostly by thrust faults and monoclinal folds.

Because shortening is mainly accommodated by squeezing the diapir stems rather than folding or faulting the adjacent overburden, the shortening episode usually remains cryptic and liable to be overlooked. In absence of direct structural clues for shortening (e.g., folds or thrusts, or reverse faults in overburden blocks between diapirs), geologists should suspect contraction- related diapir growth wherever diapirs grew by short pulses interspersed between long periods of immobility, diapirs rose after source layer depletion, or where buried diapirs were able to deform even thick roofs.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France