Salt-Influenced Structures in the Mesozoic-Tertiary Cover of the Southern North Sea, U.K.
Published:January 01, 1995
Mike Coward, Simon Stewart, 1995. "Salt-Influenced Structures in the Mesozoic-Tertiary Cover of the Southern North Sea, U.K.", Salt Tectonics: A Global Perspective, M.P.A. Jackson, D.G. Roberts, S. Snelson
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A structural model encompassing the southern North Sea Basin west of the Central Graben has been developed that combines gravity gliding of the postsalt cover with basement tectonics. The basin differs from many salt basins in that it forms a closed system. Section construction and balancing through the cover of the North Sea need to take into account thin-skinned and thick-skinned extensions and contractions. The North Sea salt formed in Permian time in two large oval basins separated by the Mid North Sea High. The shape of these basins reflects variable patterns of thermal subsidence. Subsequent salt tectonics was governed by local graben structures and by regional uplift and subsidence.
Rifting initiated during the Triassic and allowed reactive and locally passive diapirs to develop in the post-salt cover. In the southern North Sea, the Dowsing graben system in the cover is offset from the Dowsing fault zone below the salt. This offset in extensional structures probably relates to the salt thickness and to the position of the surface hinge line that controlled the onset of gravity gliding in the postsalt section. Gravity gliding of the cover into the Triassic-Jurassic Sole Pit trough and away from zones of rift flank uplift was associated with Late Jurassic-Early Cretaceous extension in the Central North Sea; gliding caused asymmetric compressional pillows to develop downslope. Gravity spreading of the cover during the Late Cretaceous-early Tertiary was associated with tilting during thermal subsidence of the southern North Sea Basin, enhanced by pulses of tectonic inversion in the southern North Sea basement. The resultant glide tectonics formed new small grabens upslope and compressional pillows downslope. Where the compressional pillows were eroded sufficiently or faulted later, the salt broke through the thinned cover to produce new active and then passive diapirs, which drained the pillows to produce new rim synclines.
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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.