Structural Styles and Evolution of Allochthonous Salt, Central Louisiana Outer Shelf and Upper Slope
Published:January 01, 1995
Mark G. Rowan, 1995. "Structural Styles and Evolution of Allochthonous Salt, Central Louisiana Outer Shelf and Upper Slope", Salt Tectonics: A Global Perspective, M.P.A. Jackson, D.G. Roberts, S. Snelson
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Seismic interpretation and section restoration are combined with recent models of salt deformation to describe the geometry and evolution of allochthonous salt from the central Louisiana outer shelf and upper slope. Scattered salt bodies are connected by a complex system of diachronous salt welds or remnant salt having two end-member geometries: (1) regionally extensive, subhorizontal sheets bounded by north-dipping (counter-regional) feeders and characterized by common listric growth faults that may accommodate significant extension; and (2) elliptical depressions bounded by dipping salt welds and arcuate growth faults that accommodate little extension.
Most salt bodies in the study area were emplaced at or near the sea floor and grew by downbuilding (passive diapirism). Reactive and active diapirs are rare. The former are confined to the updip margins of shallow salt sheets, and the latter may occur basinward of major salt-withdrawal minibasins. Many salt bodies along the downdip margins of sheets have been modified by contraction.
Two end-member evolutionary models account for the range of observed structural styles. In “counter-regional” systems, which are more typical of the shelf, salt rises through south-leaning feeder stocks and flows both downdip and along strike to form allochthonous sheets. In “salt stock canopy” systems, which are more typical of the upper slope, bulb-shaped salt stocks expand outward and form salt canopies. Subsequent gravitational collapse and sedimentary loading form bowl-shaped minibasins, from which salt is displaced into allochthonous tongues and remnant salt bodies.
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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.