Why do salt-fl oored minibasins subside? An almost universal explanation is that salt is forced from beneath the sinking basin by the weight of its sedimentary fi ll. This explanation is valid if the average density of the basin fi ll exceeds that of salt, which in the Gulf of Mexico needs at least 2300 m of siliciclastic fi ll 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 fi ve alternatives to density-driven subsidence of minibasins. (1) During diapir shortening, the squeezed diapirs infl ate, 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 fl ow 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 fi ll patterns and allow genetic comparisons between minibasins.The mechanics of a minibasin sinking into fl uid salt is in many ways analogous to a crustal basin sinking into a fl uid 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.

No abstract

No abstract

No abstract

No abstract

Weakly metamorphosed Archean sedimentary and volcanic rocks of the Vermilion district, northern Minnesota, occupy an east-west-trending belt between gneisses of the Vermilion granitic complex to the north and the Giants Range batholith to the south. All the measured strain, a foliation, and a mineral lineation in this belt are attributed to the "main" phase of deformation (D,). Foliation strikes parallel to the belt and dips steeply, and the mineral lineation plunges moderately to steeply east or west and is parallel to the maximum stretching direction, X, and subparallel to fold hinges. An earlier, possibly nappeforming, event (D,) left little evidence of fabric in the Vermilion district.A number of features indicate that the D, deformation involved a significant component of dextral strike-slip shear in addition to north -south compression. They include ductile shear zones with sigmoidal foliation patterns, shear bands, asymmetric pressure shadows, and the fact that the asymmetry of the F, folds is predominantly Z. Other features are more simply explained by a deformation involving simple shear. The S, cleavage is locally folded, and a new spaced cleavage developed in an orientation similar to that of the old cleavage away from the folds. We consider this the result of a process of continuous shear, with perturbations of flow resulting in folding of S, and the development of a new foliation axial planar to the folds. The same type of perturbation can lead to the juxtaposition of zones of constrictional and flattening strains, a distinctive feature of the rocks of the Vermilion district otherwise hard to account for. The strain pattern requires a north-south component of shortening in addition to shear. The D, deformation in the Vermilion district can therefore be characterized as one of transpression: oblique compression between two more rigid lithospheric blocks to the north and south.Dans le district de Vermilion, au Minnesota nord, les roches volcaniques et skdimentaires archkennes, faiblement mktamorphiskes, occupent une ceinture orientke est-ouest comprise entre les gneiss du complexe granitique de Vermilion au nord et le batholite de Giants Range au sud. Toutes les contraintes mesurkes, la foliation et la linkation minkralogique observkes dans cette ceinture sont attribukes B la phase de dtformation "principale" dksignke (D,). La foliation prtsente une direction parallble a l'orientation de la ceinture et est fortement inclinke; la linkation minkralogique plonge modkrkment a abruptement vers l'est ou l'ouest, et elle est parallble 21 la direction d'ktirement maximum, X, et subparall&le aux charnibres des plis. Une fabrique peu dkveloppke dans le district de Vermilion rksulte d'un kvenement plus ancien (D,), possiblement une formation de nappes.Les zones de cisaillement ductile avec foliation de style sigmoide, les bandes de cisaillement, les ombres de pression asymktriques et la prkdominance de l'asymetrie Z dans les plis F, impliquent dans le cas de D,, non seulement une compression nord-sud, mais en plu...

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 dome's core, radial shortening produced constrictional bulk strain, forming an inwardverging 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 dome's 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.

No abstract