Michael R. Hudec

Advance of allochthonous salt sheets in passive margins and orogens

Allochthonous salt sheets advance in four ways: (1) extrusive advance, (2) open-toed advance, (3) thrust advance, and (4) salt-wing intrusion. These mechanisms are determined primarily by the geometry and thickness of the roof that overlies the advancing sheet. An extrusive sheet spreads without a roof or with a roof of negligible mechanical strength. An open-toed sheet is partially covered by a mechanically significant roof but has an extrusive toe. An overthrusting sheet advances along a thrust fault at its leading edge, carrying its roof with it. A salt wing intrudes from the flank of a diapir into a shallower salt layer.Extrusive, open-toed, and overthrusting salt sheets are found in both passive margins and orogens. A sheet typically evolves through two or more of these mechanisms. Three lineages, or evolutionary paths, are common. Plug-fed extrusions emanate from the top of a salt dome or salt wall. Plug-fed thrusts form the base of the hanging wall of thrust faults that are rooted in the tops of salt domes or salt walls. Finally, source-fed thrusts initiate as thrust faults rooted in the autochthonous salt layer. Source-fed thrusts form the largest individual salt sheets, some covering thousands of square kilometers. Salt-wing intrusions form only under special circumstances, so they are not part of the three major lineages. These intrusions are restricted to compressionally inverted basins containing multiple salt layers and are known only in the Zechstein salt basin.

Regional restoration across the Kwanza Basin, Angola: Salt tectonics triggered by repeated uplift of a metastable passive margin

Restoration of a 375-km (230-mi)-long section across the Kwanza Basin, Angola, shows three stages of deformation detaching on Aptian salt, each caused by basement tectonics. First, tilting related to postrift thermal subsidence initiated early Albian deformation, shortly after salt deposition ended. Deformation waned in the late Albian, probably because of thinning of salt lubricant beneath the extensional province. The second phase of deformation was triggered by hitherto unrecognized crustal uplift beneath the continental rise around 75 Ma (Campanian). Uplift led to salt extrusion and seaward advance of the Angola salt nappe over the abyssal plain. Exposure of the nappe toe removed the buttress provided by abyssal-plain cover, which rejuvenated seaward translation. Third, Miocene basement uplift below the shelf steepened the bathymetric slope and greatly accelerated downslope translation. This deformation is now slowing because accelerated sedimentation on the abyssal plain reduced the relief of the system and blocked salt-nappe advance.

The paradox of minibasin subsidence into salt: Clues to the evolution of crustal basins

Why do salt-floored minibasins subside? An almost universal explanation is that salt is forced from beneath the sinking basin by the weight of its sedimentary fill. This explanation is valid if the average density of the basin fill exceeds that of salt, which in the Gulf of Mexico needs at least 2300 m of siliciclastic fill to ensure enough compaction. However, most minibasins start sinking when they are much thinner than this. Some mechanism other than density inversion must explain the early history of these minibasins. Conventional understanding of minibasin subsidence is thus incomplete. Here, we identify five alternatives to density-driven subsidence of minibasins. (1) During diapir shortening, the squeezed diapirs inflate, leaving the intervening minibasins as bathymetric depressions. (2) In extensional diapir fall, stretching of a diapir causes it to sag, producing a minibasin above its subsiding crest. (3) During decay of salt topography, a dynamic salt bulge subsides as upward flow of salt slows, which lowers the salt surface below the regional sediment surface. (4) During sedimentary topographic loading, sediments accumulate as a bathymetric high above salt. (5) Finally, subsalt deformation affecting the base of salt may produce relief at the top of salt. Each mechanism (including density-driven subsidence) produces a different bathymetry, which interacts with sediment transport to produce different facies patterns in each type of minibasin. The particular mechanism for minibasin subsidence depends on the tectonic environment, regional bathymetry, and sedimentation rate. Their spatial variation on a continental margin creates provinces in which a given minibasin style is dominant. An appreciation of subsidence mechanisms should thus improve our understanding of minibasin fill patterns and allow genetic comparisons between minibasins. The mechanics of a minibasin sinking into fluid salt is in many ways analogous to a crustal basin sinking into a fluid asthenosphere. However, minibasins lack the complex rheologies, thermal histories, and compositional variations that make study of crustal basins so challenging. Minibasins are thus natural analogs and have the potential to elucidate fundamentals of subsidence mechanics.

Neoproterozoic allochthonous salt tectonics during the Lufilian orogeny in the Katangan Copperbelt, central Africa

The Neoproterozoic Katangan rocks in the Copperbelt of Shaba (Democratic Republic of the Congo) and Zambia contain the worlds largest concentration of stratiform copper-cobalt ores. We propose that during the Lufilian orogeny, the Katangan basin was radically transformed first by extrusion of allochthonous evaporites, then by orogenic shortening. Salt tectonics plausibly explains spectacular breccias underlying 25,000 km2 and containing gigaclasts up to 10 km long. The brecciated Shaban Roan Supergroup—the lowermost Katangan unit—forms regional detachments and diapirs. The former existence of Roan evaporites is indicated by sabkha facies, crystals, and pseudomorphs of gypsum and anhydrite, stratigraphic gaps underlain by collapse breccias, chloride inclusions in ores, hydrothermally propylitized ore hosts, and saline springs. Salt tectonics began during deposition of the Roan Supergroup between 1050 and 950 Ma. In mid-Roan time, small walls and extrusions of evaporite-gigabreccia began to be emplaced. In early Kundelungu time (940–850 Ma), evaporitic diapirs enlarged. Lufilian deformation began between 850 and 650 Ma by laterally squeezing diapirs to form salt welds. Then, a large sheet of commingled Roan evaporite and carbonate-dominated sediments and ores was extruded northward by ∼65 km. Extrusion was fast enough to blanket a uniform preorogenic footwall unit without overriding any synorogenic deposits. Continued shortening then emplaced large thrust sheets, lubricated by the preexisting salt-sediment extrusive sheet. Restoration suggests that the Lufilian foreland in this area shortened from 193 km to 85 km. The depositional northern edge of the evaporite basin controlled the shape of the Outer Lufilian Arc. We infer three original salt provinces. The large-scale salt tectonics inferred here has implications for ore genesis and may yet be recognized in other Precambrian basins.

Structural segmentation, inversion, and salt tectonics on a passive margin: Evolution of the Inner Kwanza Basin, Angola

The Kwanza Basin, Angola, is divided into the Inner and Outer Kwanza salt basins, separated by a chain of synrift basement highs on which Aptian (112–122 Ma) salt is thin or absent. North- to northwest-trending basement structures in the Inner Kwanza Basin have repeatedly been reactivated since Neocomian (144–127 Ma) rifting. Reactivation formed three northwest-striking fold-and-thrust belts near basement uplifts. The thrust belts are bounded by northeast-striking rift-related transfer-fault zones that were apparently reactivated during subsequent shortening. Three episodes of postrift, basement-involved shortening are documented in the Inner Kwanza Basin: (1) Albian–early Cenomanian (112–96 Ma), (2) Senonian (89–65 Ma), and (3) Oligocene–Holocene (34–0 Ma). We relate the Albian–early Cenomanian event to ridge push, the Senonian event to global-plate reorganization, and Oligocene–Holocene shortening to uplift of the African superswell. Structural segmentation of the Inner Kwanza Basin controlled the evolution of salt structures. Adjacent to basement uplifts, diapirs were initiated as buckle folds. Some anticlines were unroofed by erosion and evolved into passive salt walls. Elsewhere, broad salt walls were triggered by either detached extension or basement-block uplift. These walls grew until they exhausted their supply of salt. Thereafter, dissolution rates exceeded rates of salt inflow, so the walls began to subside. Withdrawal of salt from the walls produced the elongate sedimentary troughs for which the basin is famous. Trough fill ranges in age from Cenomanian to Pliocene, and this age varies greatly from trough to trough and along strike within troughs.

Mesozoic diapirism in the Pyrenean orogen: Salt tectonics on a transform plate boundary

The Mesozoic history of the Pyrenean region was dominated by the opening of the North Atlantic Ocean and the left-lateral movement of Iberia relative to Europe. Most deformation was tensional or transtensional, but this style was interrupted by several short compressional events, perhaps related to bends in the transform plate boundary or minor changes in plate movement directions. These compressional events were recorded by deformation adjacent to and above salt structures, which were the weakest parts of the system and so served as especially sensitive strain barometers.The alternation of extension and shortening controlled the location and style of salt structures. Evaporites were initially deformed during the latest Jurassic–Early Cretaceous Neocimmerian transpression, which formed salt-cored anticlines above west-northwest–east-southeast–trending basement faults. Transtension in the Aptian–Albian caused salt domes to pierce to the surface at the intersections of these anticlines with reactivated northwest-southeast– and northeast-southwest–trending basement structures. Transtension was at least locally interrupted by brief periods of transpression at the Aptian–Albian and Albian–Cenomanian boundaries. Transpression shortened the diapirs, causing rotation, uplift, and erosion of beds near the salt. Most diapirs were buried during the Late Cretaceous. In some cases, postburial hydrothermal circulation dissolved much of the halite, causing the diapirs to sag and the basins to form in roof strata. Finally, the entire region was shortened during the Late Cretaceous–Tertiary Pyrenean orogeny, which greatly distorted many of the preexisting geometries.

Mesozoic structural and metamorphic history of the central Ruby Mountains metamorphic core complex, Nevada

The central Ruby Mountains experienced relatively little deformation during Tertiary uplift of the Ruby Mountains-East Humboldt Range metamorphic core complex, and so Mesozoic fabrics are well preserved. Three penetrative deformational events and two amphibolite-facies metamorphic events have been identified, all of which are interpreted to have occurred during emplacement of a suite of two-mica granites dated at 153 ± 1 Ma (Late Jurassic). Phase equilibria indicate that there was an increase in both temperature and pressure between M 1 (410-580 °C, 2.6-3.75 kb) and M 2 (540-660 °C, 3.5-4.7 kb). Peak M 2 pressures suggest a Jurassic paleodepth of 13-18 km, 3-8 km more than the known thickness of the overlying stratigraphic column. The Late Jurassic therefore appears to have been a time of major crustal shortening and thickening in northeastern Nevada. In contrast, there are no Cretaceous penetrative fabrics in the central Ruby Mountains, suggesting that the Sevier orogeny was not a significant event in the area. This lack of Cretaceous deformation in the mid-crustal rocks of the Ruby Mountains argues against recent models that link shortening in the Sevier belt to emplacement of the Sierra Nevada batholith via a regional mid-crustal shear zone extending across the hinterland.

Structure and evolution of Upheaval Dome: A pinched-off salt diapir

Upheaval Dome (Canyonlands National Park, Utah) is an enigmatic structure previously attributed to underlying salt doming, cryptovolcanic explosion, fluid escape, or meteoritic impact. We propose that an overhanging diapir of partly extrusive salt was pinched off from its stem and subsequently eroded. Many features support this inference, especially synsedimentary structures that indicate Jurassic growth of the dome over at least 20 m.y. Conversely, evidence favoring other hypotheses seems sparse and equivocal. In the rim syncline, strata were thinned by circumferentially striking, low-angle extensional faults verging both inward (toward the center of the dome) and outward. Near the domes core, radial shortening produced constrictional bulk strain, forming an inward-verging thrust duplex and tight to isoclinal, circumferentially trending folds. Farther inward, circumferential shortening predominated: Radially trending growth folds and imbricate thrusts pass inward into steep clastic dikes in the domes core. We infer that abortive salt glaciers spread from a passive salt stock during Late Triassic and Early Jurassic time. During Middle Jurassic time, the allochthonous salt spread into a pancake-shaped glacier inferred to be 3 km in diameter. Diapiric pinch-off may have involved inward gravitational collapse of the country rocks, which intensely constricted the center of the dome. Sediments in the axial shear zone beneath the glacier steepened to near vertical. The central uplift is inferred to be the toe of the convergent gravity spreading system.

Geomechanical modeling of stresses adjacent to salt bodies: Part 1—Uncoupled models

We compare four approaches to geomechanical modeling of stresses adjacent to salt bodies. These approaches are distinguished by their use of elastic or elastoplastic constitutive laws for sediments surrounding the salt, as well as their treatment of fluid pressures in modeling. We simulate total stress in an elastic medium and then subtract an assumed pore pressure after calculations are complete; simulate effective stress in an elastic medium and use assumed pore pressure during calculations; simulate total stress in an elastoplastic medium, either ignoring pore pressure or approximating its effects by decreasing the internal friction angle; and simulate effective stress in an elastoplastic medium and use assumed pore pressure during calculations. To evaluate these approaches, we compare stresses generated by viscoelastic stress relaxation of a salt sphere. In all cases, relaxation causes the salt sphere to shorten vertically and expand laterally, producing extensional strains above and below the sphere and shortening against the sphere flanks. Deviatoric stresses are highest when sediments are assumed to be elastic, whereas plastic yielding in elastoplastic models places an upper limit on deviatoric stresses that the rocks can support, so stress perturbations are smaller. These comparisons provide insights into stresses around salt bodies and give geoscientists a basis for evaluating and comparing stress predictions.

Evolution of the Cretaceous Astrid thrust belt in the ultradeep-water Lower Congo Basin, Gabon

The Lower Congo Basin contains the greatest salt-based fold and thrust belt off Africas Atlantic margin. Our study area in the Anton Marin and Astrid Marin exploration blocks is in the northern part of the basin. Gravity-driven tectonic shortening began soon after the Aptian salt deposition, forming gentle, west-trending, salt-cored anticlines, which, together with salt diapirs, created a template for later thrusting. In the Late Cretaceous, a thrust front propagated landward into the study area, and thrusts formed above salt anticlines and diapirs. Formation of a hanging-wall wedge of growth strata was recorded when each thrust fault ruptured the seabed. Thrusting began after widespread salt thinning, as autochthonous salt was expelled into older, passive diapirs. Thinning stiffened the detachment, so that thrusts verge strongly seaward. Structural restorations, dip-corrected isochron maps, and fault-activity graphs all show that the landward edge of the thrust belt propagated landward. Three main pulses of shortening episodically reactivated thrust faults as the thrust front broke landward. As thrusting culminated, precursor passive diapirs were squeezed and extruded small allochthonous sheets. Translation culminated in major erosional scouring, from which we infer epeirogenic slope steepening in the Late Cretaceous. As shortening spread updip into the previously extensional domain during the Late Cretaceous to Paleogene, older extensional faults were inverted, and new extensional faults formed orthogonally, parallel to the regional paleoslope. The structural pattern, created in the Late Cretaceous when the paleoslope dipped southward, remains recognizable in the little-deformed Neogene strata, although the present continental slope dips westward.