Conceptual Breakthroughs in Salt Tectonics: A Historical Review, 1856-1993

This report subjectively reviews the chief conceptual breakthroughs in understanding salt tectonics. Each discovery and its implications are presented as simply as possible. The history of salt tectonics divides naturally into three parts, defined here as the Pioneering Era, the Fluid Era, and the Brittle Era. The Pioneering Era (1856 to 1933) featured the search for a general hypothesis of salt diapirism, which was initially dominated by erroneous notions of igneous activity, residual islands, in situ crystallization, osmotic pressures, and expansive crystallization. Gradually data from oil exploration constrained speculation. The effects of buoyancy versus orogeny were debated; contact relations were characterized; the spectacular efficiency of salt as a geologic lubricant became evident in fold belts; salt glaciers and flow sheets were discovered; palinspastic restorations were attempted; and the concepts of downbuilding and differential loading were proposed as diapiric mechanisms. The Fluid Era (1933 to ~1989) was dominated by the view that salt tectonics resulted from Rayleigh-Taylor instabilities in which a dense fluid overburden having negligible yield strength sinks into a less-dense fluid salt layer, displacing it upward. Density contrasts, viscosity contrasts, and dominant wavelengths were emphasized, whereas strength and faulting of the overburden were ignored. During this era, salt bulges between extremely disharmonic fold trains were investigated; salt upwelling below thin overburden was identified; internal structures of mined diapirs were discovered; peripheral sinks, turtle structures, and diapir families were conceived; flow laws for dry salt were formulated; and contractional belts on divergent margins and allochthonous salt sheets were determined. The 1970’s revealed intrasalt minibasins, finite strains in diapirs, the basic driving force of salt allochthons, the possibility of thermal convection in salt, direct measurement of salt glacial flow stimulated by rainfall, and the internal structure of convecting evaporites and salt glaciers. The 1980’s revealed salt rollers, subtle traps, flow laws for damp salt, salt canopies, and mushroom diapirs; modeling explored effects of (1) regional stresses on domal faults, (2) spoke circulation, and (3) combined Rayleigh-Taylor instability and thermal convection. By this time, the awesome implications of increased potential for hydrocarbon reservoirs below allochthonous salt sheets had stimulated a renaissance in salt tectonics research. Blossoming about 1989, the Brittle Era (~1989 to present) is actually rooted in the 1947 discovery that a diapir stops rising if its roof becomes too thick. Such a notion was heretical in the Fluid Era. Stimulated by sandbox experiments and computerized reconstructions of Gulf Coast diapirs and surrounding faults, the onset of the Brittle Era unveiled regional detachments and evacuation surfaces (salt welds and fault welds) along vanished salt allochthons, raft tectonics, and shallow spreading and segmentation of salt sheets. The early 1990’s revealed rules of section balancing for salt tectonics; salt flats and salt ramps; reactive Piercement as a diapiric initiator resulting from tectonic differential loading; cryptic thin-skinned extension; the influence of sedimentation rate on the geometry of passive diapirs and extrusions; the importance of critical overburden thickness to the viability of active diapirs; fault segmented sheets; roho systems and counterregional fault systems; subsiding diapirs; extensional turtle structure anticlines; and mock-turtle structures. "The truth is that whoever touches this enticing subject . . . is bound to indulge freely in speculation. The problem is so broad, the factors involved are so numerous, and the work to be done with regard to salt structures is so great that we cannot . . .[restrict our speculation to the narrow] limits of exact knowledge."—Everett DeGolyer, 1925 "Although this is no place in which to describe the adventures of a petroleum geologist it may, perhaps, be said that the carrying out of the geological work referred to was greatly hampered owing to much of the time being spent as a prisoner in the hands of Italian, Turk and Arab." —Arthur Wade, mapping salt domes on the Red Sea coast of Arabia in 1912