Although active diapirs must deform the overburdens they pierce, the shape of passive (downbuilt or syn-depositional) diapirs is formed or molded by their overburdens. Molding of salt diapirs is simplified here to profiles of diapirs entirely downbuilt in effectively rigid overburden. The dips of salt-sediment contacts are shaped by the interaction of two processes: local net accumulation of overburden (A = deposition minus compaction) at rate A and the net increase in relief of salt structures (R = salt rise minus dissolution) at rate R. Steady kinematic molding ratios, R / A, forward model realistic dips of molded salt contacts, a, at particular depths using R / A or A / R = tan a/2. Rising or falling ratios of incremental molding forward model complete diapir profiles. Conversely, molding histories can be read by backstripping profiles of downbuilt diapirs.
Salt diapirs are downbuilt in a field of downbuilding (100 > R / A > 0.01), that is bounded by burial and extrusion. Within this range, aggradation faster than salt can rise (R / A < 1) molds tapering (narrowing-upward) top contacts of salt. Accumulation of overburden slower than salt rises ( R / A > 1) molds flaring (widening-upward) salt contacts. Below this range (where R / A < -0.01), the top contact of the salt is eclipsed (temporarily buried to depths from which it can still upbuild) or even occluded (buried below its critical roof thickness and thus unable to rise again autonomously). Occluded salt is either dissolved at depth or rises in reactivated diapirs after exhumation or faulting of overburden that is not rigid. Where R / A > -100, salt emerges like a fountain and extrudes sheets of allochthonous salt. Extruded salt is recycled back into the ocean by dissolution at the surface or after burial and reactivation in another cycle.
Figures & Tables
Salt Tectonics: A Global Perspective
The conceptual breakthroughs in understanding salt tectonics can be recognized by reviewing the history of salt tectonics, which divides naturally into three parts: the pioneering era, the fluid era, and the brittle era.
The pioneering era (1856-1933) featured the search for a general hypothesis of salt diapirism, initially dominated by bizarre, 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, salt glaciers were discovered, and the concepts of downbuilding and differential loading were proposed as diapiric mechanisms.
The fluid era (1933–1989) 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, palinspastic reconstructions were attempted; salt upwelling below thin overburdens was recognized; internal structures of mined diapirs were discovered; peripheral sinks, turtle structures, and diapir families were comprehended; flow laws for dry salt were formulated; and contractional belts on divergent margins and allochthonous salt sheets were recognized. The 1970s revealed the basic driving force of salt allochthons, intrasalt minibasins, finite strains in diapirs, 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 1980s revealed salt rollers, subtle traps, flow laws for damp salt, salt canopies, and mushroom diapirs. Modeling explored effects of regional stresses on domal faults, spoke circulation, and combined Rayleigh-Taylor instability and thermal convection. By this time, the awesome implications of increased reservoirs below allochthonous salt sheets had stimulated a renaissance in salt tectonic research.
Blossoming about 1989, the brittle era 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 yielded regional detachments and evacuation surfaces (salt welds and fault welds) along vanished salt allochthons, raft tectonics, shallow spreading, and segmentation of salt sheets. The early 1990s 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, 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, counter-regional fault systems, subsiding diapirs, extensional turtle structure anticlines, and mock turtle structures.