Daniel D. Schultz-Ela

Mechanics of active salt diapirism

Abstract An active diapir forcefully intrudes its overburden, driven by diapir pressure that overcomes the resistance of the overburden strength. Possible causes for the driving pressure are differential loading of the source layer and a density contrast with the overburden. Possible resisting forces derive from the mass of the roof block and resistance to the faulting and folding that accommodate the intrusion. For a typical density contrast between salt and a sedimentary overburden, a simple force balance demonstrates that the diapir height must be more than two-thirds to three-quarters the thickness of the surrounding overburden to initiate substantial active diapirism. Narrow diapirs must be even taller. Physical and numerical modeling show that typical structures of active diapirism are a central crestal graben flanked by relatively unstrained flaps that rotate upward and outward. Curved reverse faults can separate the flaps from the regional overburden. Normal faults in the crestal graben propagate downward, with new faults created farther outward as the roof arches over the rising diapir. Thin, wide roofs develop multiple grabens separating a relatively flat roof from the rotating flaps. In cases leading to emergence, the piercing diapir evolves from a pointed crest to a progressively rounded and widening crest as the flaps rotate outward. The numerical models show that overburden flexure contributes to early formation of active diapir structures, but piercement does not continue to emergence unless the diapir is very wide or tall relative to the overburden thickness. Active diapirism becomes progressively more difficult for diapir crests ranging from a rectangle, a round-cornered rectangle, a semicircle, to a triangle with a pointed crest.

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.

Origin of drag folds bordering salt diapirs

Drag folds bordering salt diapirs are commonly attributed to shear from rising salt. Finite-element models demonstrate that shear deformation is only significant for extremely weak overburdens, such as those having high overpressure and ductile interlayers. Protrusions of overburden into and onto the diapir are most susceptible to folding. So-called drag folds are much more likely to initiate as drape folds of strata onlapping a downbuilding diapir, here termed flap folds for clarity. Potential for flap folding is greatest where the salt/overburden contact dips moderately and for episodic or variable timing and thickness of deposition. Moderate dip allows strata to onlap far across the diapir crest and be carried upward into a high-amplitude fold with the rising salt. Depositional hiatuses allow time for such rise without additional layers that would strengthen the onlapping wedge. Previously folded strata may block onlap of later layers. Stretching and rotation of the layers adjacent to steep parts of the diapir may cause disaggregation, slumping, and debris deposits that can be overridden as the diapir crest spreads laterally onto the overburden surface. Large changes in the relative rates of salt rise and sediment deposition can create cycles of onlap, flap folding, and spreading.

Excursus on gravity gliding and gravity spreading

Abstract The terms ‘gravity gliding’ and ‘gravity spreading’ have long been used to describe deformation driven by gravity alone. However, the traditional definitions of these terms cannot be applied unambiguously in many situations. The primary difficulties arise because rocks are not ideally rigid, detachment surfaces may not be planar, substrates may be deformable, and rock bodies do not deform in isolation. The term ‘gravity spreading’ is still useful if it is simply defined as gravity-driven lateral extension and vertical contraction, regardless of basal slope and coherence of the body. I suggest that the term ‘gravity gliding’ should be used rarely, and only if the defining characteristics are clearly stated and understood. In most cases, more detailed descriptions should be used instead of, or in addition to, either of these terms to capture the behavior of rock masses deforming under gravity.

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.

Restoration of cross-sections to constrain deformation processes of extensional terranes☆

Abstract Restoring balanced cross-sections constrains the possible deformation processes affecting extensional terranes with a ductile substratum, such as salt, and syndeformational sedimentation. Cross-sections are divided into fault blocks and shear domains, each of which can be undeformed with a different set of parameters. The success of restorations is judged by compatibility between the blocks and the reasonableness of the restored structures. Fault blocks in physical models restore with simple shear at about 60° dipping in the direction which removes extension, corresponding to an antithetic orientation in the hangingwalls of normal growth faults and subparallel to the fault in the footwall. Other components of the deformation are rigid rotation, translation and decompaction. Shear angles inferred from natural examples are more varied, ranging from vertical to moderately inclined, nearly always dipping in a direction that removes extension.

Relation of Subsalt Structures to Suprasalt Structures During Extension

Recent discoveries of hydrocarbons beneath allochthonous salt sheets have heightened interest in the location and geometry of subsalt structures, including faults offsetting the base of salt sheets. Salt extrudes to form flat-topped sheets with discordant or stepped lower contacts that then deform in response to sedimentation and regional or local stretching. We simulate the interacting growth of subsalt and suprasalt structures during regional extension using two-dimensional finite-element models. Fault arrays above and below salt comprise combinations of three characteristic patterns: simple grabens, reactive grabens, and drape grabens. Thickness variations in the roof of a salt sheet strongly influence the initial location of suprasalt faults. Fast extension or thin sa t enhances coupling between subsalt and suprasalt structures, summarized by these predictive guidelines: (1) subsalt faults are laterally offset from suprasalt drape grabens, and the offset and graben symmetry increase as coupling decreases, (2) symmetric reactive grabens tend to overlie zones of no subsalt faulting, and (3) suprasalt sags with little faulting overlie subsalt grabens. Flow in a deeper salt layer increases structural relief beneath a shallower sheet but decreases surface relief. The modeling exposes several possible interpretation pitfalls concerning true and apparent regional elevations, apparent feeder stems, subsalt ramps and flats, and effects of salt withdrawal vs. extension.

Strain in an Archean greenstone belt of Minnesota

Abstract We measured strain at more than 60 locations in metasedimentary and metavolcanic rocks of the Vermilion district, an E-W trending Archean greenstone belt in Minnesota. Strain ellipsoid orientations and shapes correlate strongly with N-S location in the belt, but magnitudes do not. Flattening strains occur near the present Vermilion fault (which bounds the greenstone belt to the north) with constrictional strains to the south. The observed strain patterns can be mathematically modeled by deformation paths which produce the flattening strains (with west plunging λ1 axes) by dextral shear of the constrictional strains (with east plunging λ1 axes). Using reasonable geologic constraints, the shear plane must dip to the north with a subhorizontal shear direction. Structures throughout the district also indicate dextral shear. A geometrical finite element program uses the measured strains to destrain the rocks and find the configuration which most closely satisfies strain compatibility equations. The linear E-W strain patterns and minor rotations about horizontal axes during the deformation preclude origin of the greenstone belt by infolding and shear off the flanks of a rising granitic diapir. By accounting for rotations which result in the (deformed) curvature of the original surface, a true estimate of 50% N-S shortening across the belt can be made. The data and deformation models favor the origin of the Vermilion district rocks at a convergent margin, most likely as a N-dipping subduction zone complex with shallow slab dip. The origin of the constrictional strains remains enigmatic.

Mechanics of graben evolution in Canyonlands National Park, Utah

Results of numerical models and field observations of regularly spaced grabens in Canyonlands National Park, Utah, demonstrate that salt flow beneath a brittle overburden accommodated recent and ongoing westward gravity spreading. Erosion of the Colorado River canyon differentially loaded the underlying viscous salt. In our models, the overlying brittle strata flexed downward toward the canyon, initiating faults near the surface that propagated downward toward the salt contact. Modeled grabens developed sequentially away from the canyon (eastward) as salt was expelled from beneath undeformed strata. After their eastern boundary faults broke through, horst blocks tilted in the opposite direction of initial flexure, resulting in increased symmetry of older grabens closer to the canyon. Continued extension formed a reactive diapir beneath each graben. Field observations show that multiple faults bound grabens, indicating reactive diapirs beneath them. Topographic profiles and surveyed points along a stratigraphic layer show that horst blocks subsided as salt migrated toward the river canyon and into the diapirs. Field data from less evolved horsts imply that individual horst blocks responded to differential loading by progressive flexure and tilt, similar to the models. Horst-block flexures also vary along strike, and localized folds and faults formed where fault displacement changes abruptly.

Salt‐related structures in the Gulf of Mexico: A field guide for geophysicists

In finding economic quantities of oil and gas below a salt sheet, the Mahogany discovery well in Ship Shoal South Addition, offshore Louisiana, dramatically boosted exploration interest in the Gulf of Mexico’s subsalt play in shallow water. Interest was initially aroused by the earlier deep‐water discovery below a salt sheet forming part of “Mickey Mouse,” a cluster of pancake‐shaped salt sheets in Mississippi Canyon Block 211. A major technologic obstacle in this play has been the difficulty of imaging the base of salt sheets and the subsalt structures targeted for exploration. Some recent articles have showcased the spectacular fidelity of 3-D seismic data sets collected on closely spaced survey lines and processed by techniques such as 3-D prestack depth migration and 3-D dip moveout followed by 3-D turning‐ray Kirchhoff migration. However, at present the cost of this technology limits its use to specific, rigorously selected targets.