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An Overview of Allochthonous Salt Tectonics


In this overview, the fundamentals of allochthonous salt tectonics are presented by posing several questions. First, why does allochthonous salt form? In many cases, allochthonous salt forms at or just beneath the surface because the supply of salt to the near surface outpaces sediment accumulation and thus moves more laterally. In others cases, it forms at depth because the salt is a décollement from which thrust faults emanate. Second, how is lateral emplacement initiated? Salt may gradually flare into allochthonous geometries due to a progressive increase in the ratio of salt-supply rate to sediment-accumulation rate. Often, however, there is an abrupt transition from a steep diapir to a subhorizontal sheet because erosion or slumping of the diapir roof frees up the salt to suddenly flow laterally. Third, how does allochthonous salt advance? For shallow salt, it depends on the depositional environment: in nonmarine settings, extrusive advance of a salt glacier occurs, whereas in marine settings the salt and its roof advance on short thrusts that link the tip of the salt to the sea floor. For deep salt, advance is simply the result of increased displacement on the thrust carrying salt in its hanging wall. Fourth, what are the different styles of allochthonous salt? Allochthonous salt includes salt bulbs/pancakes and their equivalent salt-stock canopies, salt tongues and salt-tongue canopies, source-fed thrusts and salt nappes, and small salt wings. Fifth, how do sheets and canopies evolve? During and after emplacement, sheets and canopies are typically modified by some combination of: a) sedimentary loading; b) gravitational failure of the overburden, with attendant extension, translation, and contraction; c) regional shortening; and d) evacuation, extension, or contraction at a deeper salt level. Finally, how are different styles of allochthonous salt distributed? Source-fed thrusts and salt nappes are confined to contractional provinces, i.e., convergent-margin fold-and-thrust belts and passive-margin deepwater domains. Delamination wings are also restricted to contractional settings, whereas other wings may flank diapirs in any environment. Salt bulbs/pancakes and salt-stock canopies, although more common in areas of shortening, are also found in other provinces, but salt tongues and salt-tongue canopies are mostly confined to slope domains of passive margins.