ch01 (tectonic salt)

Jackson, M. P. A., 1995, Retrospective salt tectonics, in M. P. A. Jackson, D. G.Roberts, and S. Snelson, eds., Salt tectonics: a global perspective: AAPGMemoir 65, p. 1Ð28.

Chapter 1

Retrospective Salt Tectonics

Abstract

The conceptual breakthroughs in understanding salt tectonics can be recognized by reviewing the history ofsalt tectonics, which divides naturally into three parts: the pioneering era, the fluid era, and the brittle era.

The pioneering era (1856Ð1933) featured the search for a general hypothesis of salt diapirism, initially domi-nated 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 buoy-ancy versus orogeny were debated, contact relations were characterized, salt glaciers were discovered, and theconcepts of downbuilding and differential loading were proposed as diapiric mechanisms.

The fluid era (1933Ð~1989) was dominated by the view that salt tectonics resulted from Rayleigh-Taylor insta-bilities 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 reconstructionswere attempted; salt upwelling below thin overburdens was recognized; internal structures of mined diapirswere discovered; peripheral sinks, turtle structures, and diapir families were comprehended; flow laws for drysalt were formulated; and contractional belts on divergent margins and allochthonous salt sheets were recog-nized. 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, andthe internal structure of convecting evaporites and salt glaciers. The 1980s revealed salt rollers, subtle traps, flowlaws for damp salt, salt canopies, and mushroom diapirs. Modeling explored effects of regional stresses ondomal 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 renaissancein salt tectonic research.

Blossoming about 1989, the brittle era is actually rooted in the 1947 discovery that a diapir stops rising if itsroof becomes too thick. Such a notion was heretical in the fluid era. Stimulated by sandbox experiments andcomputerized reconstructions of Gulf Coast diapirs and surrounding faults, the onset of the brittle era yieldedregional detachments and evacuation surfaces (salt welds and fault welds) along vanished salt allochthons, rafttectonics, shallow spreading, and segmentation of salt sheets. The early 1990s revealed rules of section balanc-ing for salt tectonics, salt flats and salt ramps, reactive piercement as a diapiric initiator resulting from tectonicdifferential loading, cryptic thin-skinned extension, influence of sedimentation rate on the geometry of passivediapirs 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, andmock turtle structures.

1

M. P. A. JacksonBureau of Economic GeologyThe University of Texas at AustinAustin, TexasU.S.A.

The truth is that whoever touches this enticing sub-ject...is bound to indulge freely in speculation. The problemis so broad, the factors involved are so numerous, and thework to be done with regard to salt structures is so greatthat we cannot...[restrict our speculation to the narrow]limits of exact knowledge.

ÑEverett DeGolyer, 1925

Although this is no place in which to describe the adven-tures of a petroleum geologist it may, perhaps, be said thatthe carrying out of the geological work referred to was great-ly hampered owing to much of the time being spent as aprisoner in the hands of Italian, Turk and Arab.

ÑArthur Wade, mapping salt domes, Red Sea coast of Arabia, 1912

PREAMBLE

The Hedberg International Research Conference, heldin September 1993, was attended by a diverse group from15 countries. The range of salt basins described wasequally wide. Moreover, the lexicon of salt tectonics wasevolving rapidly. For all these reasons, a review of termi-nology was needed so that all attendees could attach thesame meaning to the same word. In addition, to facilitatescientific progress at the conference, the basic conceptualframework of salt tectonics needed to be reviewed. Byidentifying the main supports of the conceptual structure,as well as the components that were weak or missing,new breakthroughs during the conference were morelikely to be recognized.

Accordingly, an opening presentation was designed toreview both terminology and concepts. The oral reviewhad two parts: (1) a retrospective contemplation of thepast and (2) a prospective look ahead at unsolved prob-lems. An attempt to look into the future can be a stimu-lating oral presentation, for views ahead are alwaysthought provoking (e.g., Talbot, 1992). However, conjec-tures about the future become swiftly outdated in a rapid-ly evolving research field, and new problems continuallyadd to such a list. Thus, this chapter deals strictly with theconceptual evolution of salt tectonics up to early 1993,before abstracts for the Hedberg conference were submit-ted. Chapters in this volume present much of the progresssince that time.

The literature of salt tectonics is vast. Abibliography ofdiapirism and diapirs compiled by Braunstein andOÕBrien (1968) contained about 1800 entries; since 1968,this number may have doubled. Any review attemptingto document most of these contributions would be intol-erably weighty and tedious to read. The approach here isto highlight the minority of papers that seem to havewielded major influence on salt tectonics by elucidatingor catalyzing major conceptual breakthroughs. Many ofthese achievements were impelled or permitted by tech-nological breakthroughs, such as seismic processing andmodeling techniques. However, to prevent this reviewfrom sinking under its own weight, this technologicalaspect is all but ignored. Dates quoted are those of thefinal emergence of an idea as a published paper (orabstract if no paper followed soon after). For papers orig-inating from academia, publication is typically delayedby 1Ð3 years. For papers from industry, the lag time topublication may be 3Ð5 years, although delays of 10 yearsor more are not uncommon.

Readers wishing to explore the early literature arereferred to the reviews by DeGolyer (1925), Rios (1948),Lotze (1957), and Braunstein and OÕBrien (1968), whoseconveniently spaced bibliographies provided invaluablesignposts to papers previously unknown to me.

Despite the moderating and broadening efforts of thosecalled upon to review this paper and my own efforts, itremains a personal view of salt tectonics. As such, thereview is incomplete, the selection of papers is biased, andthe weight assigned to various ideas is subjective.

THREE ERAS OF SALT TECTONICS

From the perspective of the 1990s, three eras of greatlydisparate length encapsulate progress in understandingsalt tectonics. The pioneering era (1856Ð1933) featured thesearch for a general hypothesis for salt diapirism. Thefluid era (1934Ð~1989) overwhelmingly saw salt tectonicsas analogous to the overturn of two fluids with initiallyinverted densities. The brittle era (~1989Ðpresent) treatsthe overburden as a strong, brittle encasement whosedeformation controls the style of salt tectonics.

Parallel with this scientific evolution was the shift infocus from salt domes to salt tectonics. Salt domes werescrutinized in early decades because they are the mostwidespread near-surface expression of salt tectonics. Asthe subsurface was increasingly probed by seismic toolsand wells, the picture widened to salt tectonics as awhole, comprising salt source layer, salt structures of alltypes, underlying basement, and overlying overburden,commonly stretched or compressed by regionallyimposed lateral forces. This paper includes under theterm salt diapir any structurally discordant body of salt,regardless of its emplacement mechanism.

THE PIONEERING ERA (1856Ð1933)

Salt domes have been known and mined for millenniafor their prized reserves of preservative Òwhite goldÓ inplaces such as the arid coasts of the Persian Gulf and RedSea. Solution mining of rock salt began about 3000 yearsago in Poland and was probably also widespread else-where, given the simple technology required to extractthis precious commodity. Starting 700 years ago, a vastcathedral, comprising 200 km of galleries and headings,was carved from salt in the bottom of the still-producingWieliczka mine in southern Poland.

However, in the scientific sense, the pioneering era ofsalt tectonics opened with the first recorded discovery ofa salt dome in the geologic literature. In 1856, Villedescribed a salt mountain, Ran el Melah, exposed nearDjelfa in the Saharan Atlas of Algeria (Figure 1). He rec-ognized Ògeyser-likeÓ forceful emplacement of the salt.Soon afterward, the first subsurface salt dome was dis-covered in Louisiana when a brine well at Petite Anse,one of the coastal Five Islands, struck salt in 1860 (Tho-massy, 1863).

The study of salt tectonics became established amongthe hundreds of salt domes exposed in central Europe,especially in the Carpathians. As long ago as 1871, saltbodies here were known to be intrusions emplaced dis-cordantly against their country rocks (Figure 2) (Posepny,1871). However, it was another 40 years before Mrazec(1907) coined the term diapir for folds cored by piercingsalt. Even at the dawn of the pioneering era, Posepny(1871) recorded two other characteristics of salt domes: (1)angular unconformities in flanking strata and (2) similarstyle folds in their interiors (e.g., Vizakna, near Salzburg,Austria).

2 Jackson

Unconstrained Speculation

Geologic understanding was first hindered, then dri-ven, by economics. Before abundant hydrocarbons wereknown to be associated with salt domes, fragmentaryknowledge about them came entirely from rare, fortu-itous exposures. Because of this dearth of data, specula-tion ran wild and no hypothesis was too bizarre to omit.ÒAs is always the case when exact information is tooscarce to limit imagination seriously, widely different the-ories of origin were promulgated. Because the chief testfor excellence in theory is that it shall not violate knownfact, it is not surprising that a wide variety of theoriesshould have been put forwardÓ (DeGolyer, 1925, p. 835).Working on an edifice built by a century of intensiveresearch on salt tectonics, it is easy for us today to dismissmany of these hypotheses, which were necessarily basedon slender evidence. However, field data were scarce,hard won, and frequently dangerous to collect, as exem-plified by the opening quote from Wade (1931).

Choffat (1882) proposed the term tiphonique (aftermythological Typhon) to describe intrusions of saliferousand gypsiferous marl in Portugal. ChoffatÕs (1882) sectionacross the valley of the Serra del Rei recorded the over-turned collar of sediments surrounding the diapir, butVeatch (1899, 1902) seems to have been the first to recog-nize doming of surrounding strata as an inherent charac-teristic of most salt domes.

Initially, the most prevalent view of their origin wasthat salt domes represented residual outlying islands ofsalt surrounded by the deposits of younger seas (e.g.,Lockett, 1871; Hildgard, 1872). Others suggested that saltdomes originated by local folding (Kennedy, 1892; Harrisand Veatch, 1899) or plutonic emplacement (Lerche,1893).

The spectacular blowout of hydrocarbons from the caprock of Spindletop, Texas, in 1901 not only initiated massproduction of petroleum but also provided a powerfuleconomic incentive to obtain subsurface data. The quan-tity of available information exploded. Within a mere

Chapter 1ÑRetrospective Salt Tectonics 3

Figure 1—The first salt diapir described in the geologic literature (Ville, 1856). The panorama shows the southern contact ofthe 100-m-high “Ran el Melah” (Rocher de Sel de Djelfa), a 1.4-km-wide plug of Triassic salt in the northern fringe of theSaharan Atlas range, Algeria. The diapir’s contact is marked by the white line. North of the diapir (not shown) is themegabreccia residue of a wasted salt glacier.

Figure 2—Posepny’s (1871)schematic cross section cap-tures some fundamentalcharacteristics of salt diapirs:inside the diapir, increasingconformity of evaporite layerstoward the diapiric contact;outside the diapir, markeddiscordance of most flankingstrata, onlap of deep strataagainst the diapir, and anoverturned collar around theshoulders of the diapir.

5 years, some 50 salt domes were producing oil or gas inthe Gulf Coast region. Exploration focused on cap rockdeposits of hydrocarbons, sulfur, anhydrite, and gypsum.Hypotheses on origin became more constrained andfocused on the origin of these resources from the domeitself. Ideas centered on processes of deposition fromascending hydrothermal brines that also carried oil, sul-fur, and possibly volcanic gases (e.g., Hill, 1902; Hayesand Kennedy, 1903). Some notions involved an expansiveforce of crystallization that was already being attributedto calcareous nodules (Fenneman, 1906; Harris, 1907).This hypothesis contained the embryonic idea of steady-state equilibrium in the growth of salt domes: domes roseby expansive crystallization at their base while their crestswere continuously truncated by dissolution and theirroofs were lifted and tilted. Others suggested shallowmagmatic laccoliths just below the domes, regional iso-static adjustments, and local osmotic pressures for the ori-gin of salt domes. ÒThroughout all of these periods, likethe chorus in a Greek Tragedy, run constantly recurringtheories of dome-origin as a result, direct or indirect, ofvolcanic activity...because structurally an igneous plug isthe only geological feature known to us that even remote-ly resembles a salt domeÓ (DeGolyer, 1925, p. 836, 863).

Mechanical Control of Salt Diapirism

This free-ranging debate continued in North Americauntil checked by data pouring in from hydrocarbon dis-coveries in the flanking sands of salt domes. Structuralaspects of salt domes were brought to the forefront, andthis focused attention on mechanical means of emplace-ment. Ideas of structural control inexorably displaced theprevailing paradigm of in situ deposition of salt. Duringthe 1910s, most geologists accepted that salt domes andanticlines represented intrusions of evaporites derivedfrom normally bedded sequences. For example, afterexamining German, Romanian, and Gulf Coast domes,Van der Gracht (1917) proposed that domes were pushedup as intrusive plastic masses through weaker spots inthe overburden. Van der GrachtÕs proposal explained thevariety of known salt structures from stocks to anticlines.It was supported by (1) rock mechanics data that showedductile creep of salt (e.g., Johnston and Adams, 1913), (2)contortions within salt structures, and (3) tilting anddoming of adjoining strata.

Meanwhile, the field of salt tectonics in Europe hadmoved far ahead. The wide variety of salt structures inRomania, North Africa, Iberia, and Germany providedEuropean geologists with a more panoramic view of salttectonics than that available in America. Europeans tend-ed to study salt diapirs as part of their tectonic settingrather than regarding them as local anomalies in an oth-erwise unstructured plain such as the U.S. Gulf Coast. Bythe late 1910s, two contesting indigenous hypothesesdominated thinking in Europe and rapidly diffused toAmerica: buoyancy (vertical forces caused by the weightof overlying strata) and regional contraction (orogenic lat-eral forces).

Buoyancy

Lachmann (1910) originated the buoyancy hypothesisfor salt diapirism, but he lacked any physical explanationof the process. Arrhenius (1913) took up this challengeand inferred the effects of gravity under conditions ofdensity inversion. Salt was driven upward like a risingoil drop by the sinking of a denser fluid overburden.Arrhenius ascribed the folds observed within diapirs todifferential collapse of solution cavities in the salt. Stille(1925) felt that the buoyancy hypothesis implied that saltstocks in general would have to be surrounded by soft,wet clay or ground water. Nevertheless, in two series ofexemplary experiments based on earlier ones by Torreyand Fralich (1926), Escher and Kuenen (1929) were ableto reproduce all known shapes and attitudes of folds inGerman salt domes by pressure forcing a viscous strati-fied sequence up through a circular opening in a rigidoverburden. The realism of their models convinced themto favor the gravity-driven hypothesis, although theycould not exclude regional shortening as an agent. Bythis time, torsion balance surveys had revealed gravityminima over many salt domes in the Gulf Coast(Nettleton, 1955), which were compatible with theinferred low density of salt.

Regional Contraction

In Romania, where diapirs formed the discordantintrusive cores of antiforms in a foldbelt, a tectonic originwas compelling. The same was found to be true in Spainand Morocco, where excellent exposures enabledYovanovitch (1922) to construct a cross section throughthe Rif nappes that contained virtually all the elements ofcontractional salt tectonics known today (Figure 3) (com-pare with Cobbold et al., 1995; Harrison, 1995; Letouzeyet al., 1995; Sans and Verg�s, 1995, all in this volume).Even in Germany, enough regional deformation was pre-sent to ensure that prevailing hypotheses on salt struc-tures were dominated by a tectonic engine. Stille (1910,1925) advocated orogenic shortening as the cause of saltfolding and upwelling in Saxony. Stille (1917) identified acharacteristic style of folds decoupling over PermianZechstein salt in northern Saxony, having tight antiformsseparated by broad, flat-bottomed synforms (Figure 4).This ejective style contrasted with the dejective fold stylefarther south, in which tight synforms were separated bybroad, flat-topped antiforms.

StilleÕs (1925) signal contribution was to emphasizethat German salt structures formed a broad spectrum ofstructures intermediate between two end-members: (1)the salt-cored anticline produced by orogenic folding of asequence underlain by salt and (2) the salt stock. Such acontinuity implied to him a common origin (althoughthis could also be interpreted as the varying dominance oftwo different processes, such as gravity and orogeny).During episodic contraction, salt stocks were upthrustinto faulted rather than folded overburden in a styleintermediate between folding and igneous intrusion

4 Jackson

(Figure 5). Noting that the salt was much more deformedthan the surrounding strata, he accurately ascribed thisdifference to the Òuncommonly mobileÓ salt. In accordwith modern views, Stille (1925) regarded the saltÕs highmobility as more significant than its low density. Indeed,Lohest (1921) had already physically modeled similardecoupling of realistic folds and thrusts over a lubricatinglayer of grease. Timing seemed to support StilleÕs hypoth-esis. Generally the rise of salt structures seemed to corre-late with known episodes of Saxon folding. DeGolyer(1925) also favored the idea of lateral stresses causing therise of salt domes, but because of the lack of observedregional folding in the Gulf Coast, he had to infer pre-Cretaceous, probably Paleozoic, orogenic emplacementof the salt domes.

Stille (1925) was one of the first to recognize the exis-tence of diapiric stems, vertically pinched-off diapirs, and

laterally pinched-off source layers (Figure 5). StilleÕs oro-genic engine fell out of favor from the 1930s onward, butrecent (1990s) ideas on the role of inversion in the defor-mation of Zechstein salt in northwestern Europe suggestthat some salt structures may have been initiated or mod-ified along the lines of StilleÕs hypothesis (Coward andStewart, 1995).


Figure 3—Yovanovitch’s (1922) astonishingly perceptive cross section in the Rif nappes of Alpine age in Morocco showsalmost all the important characteristics of contractional salt tectonics: inclined and squeezed walls of Triassic salt, diapiricfold cores, thin smears of salt in the hanging walls of imbricated thrust nappes, narrow antiforms separated by broad flat-bottomed synforms, and diminution of shortening away from the hinterland. The 10× vertically exaggerated section trendswest-southwest from Ouezzane.

Figure 4—Stille (1917) identifies the characteristic geo-metries of Saxon folds decoupling over PermianZechstein salt: (a) ejective style of narrow antiforms andbroad, flat-bottomed synforms; (b) a kongruente styleintermediate between the two end-members; and (c)dejective style of narrow synforms and broad, flat-toppedantiforms.

Figure 5—Stille’s (1925) explanation of (a) “harmonious”folding of beds of similar strength and (b) salt tectonicsas an extreme form of “inharmonious” folding of asequence including a highly mobile bed c (blackened forcontrast). Both parts show contrasting modes ofemplacement by faulting and part (b) also shows ductilestretching, pinching of a diapir stem, and laterallypinched-off source layers.

Erosional Differential Loading

Differential loading refers to lateral variations in thethickness, density, or strength of overburden strata abovesalt. Three types of differential loading would eventuallybe recognized. The first of these was that due to erosion-al removal of overburden (Harrison, 1927). The mostspectacular example cited by Harrison was the deep,meandering canyon cut by the Colorado River in Utah(Figure 6). Erosional unloading of the floor of the canyonlocally decreased the pressure on the salt. The weight ofthe canyon walls on the underlying PennsylvanianParadox Salt squeezed salt sideways into the low-pres-sure zone below the floor of the canyon, where saltbulged up to form a meandering anticline. In severalplaces, evaporites broke through the erosionally thinnedoverburden to emerge as gypsum diapirs.

Salt Fountains and Salt Glaciers

Steady-state diapirism requires a balance between thesupply of salt from below and the loss of salt at the sur-face. Escher and Kuenen (1929) inferred that theRomanian diapirs must still be rising in order to remainas mounds of rock salt exposed to a moist climate withrainfall of 80Ð90 cm per year. Apparently unknown tothem, geologists in the Middle East were directly observ-ing the spectacular effects of an oversupply of salt. Lees(1927) and De B�ckh et al. (1929) described salt plugs inthe Zagros of southern Persia that emitted salt extrudingdownhill (Figure 7); they called these salt glaciers becausethey resembled ice glaciers. Harrison (1931) documentedthe salt plugs and their glaciers in considerable detail andspeculated on the processes of emplacement. He seems tohave been the first to deduce that the salt plugs emergedas early as the Late Cretaceous because these stratainclude conglomerates containing clasts of distinctiveHormuz igneous rocks. Noting that the results of defor-mation experiments were incompatible with the existenceof salt glaciers, Harrison presciently speculated that flowcould be aided by traces of water in the salt. Wade (1931,p. 358) went further: ÒHeavy rains are rare in the ranges[of Zeit somewhere by the northern Red Sea], but at suchtimes the flow [of gypsum glaciers] is noticeably quick-

ened.Ó Wade even attempted to measure the flow rate ofglacial gypsum, and Bailey (1931) suggested doing thesame for salt glaciers by staking them. He also originatedthe metaphor of extruding salt diapirs as Òslow-motionfountains.Ó

Depositional Differential Loading

Depositional differential loading is the second type ofdifferential loading. Bailey (1931) was apparently one ofthe first to propose it as a mechanism for triggeringdiapirism in regions of Òorogenic tranquillity.Ó Sub-sequently, Rettger (1935) modeled the effects of a wedge-shaped differential load above a mobile substrate(Figure 8). The substrate was squeezed laterally andwelled up along the periphery of the deltaic differentialload.

Downbuilding

Until the early 1930s, the consensus was that saltdiapirs rise discordantly, piercing thick, previouslydeposited piles of sedimentary rock. This penetrationrequires upward displacement or stoping of a huge vol-ume of country rock. In modern parlance, such diapirshave a severe room problem. Wade (1931) was skeptical

6 Jackson

Figure 6—Upwelling of Pennsylvanian Paradox evaporites(patterned) by erosionally induced differential loadingbelow the Colorado Canyon, Utah (from Harrison, 1927).

Figure 7—G. M. Lees’ field sketch of Kuh-e-Anguru salt plug, Iran, visited in the early 1920s. Along with an accompanyingphotograph (not shown) of Kuh-e-Namak (Dashti), this was the first illustration of an identified salt glacier (De Böckh et al.,1929). The glacier overflows Cretaceous–Oligocene carbonates; two Eocene inliers (c) project through the flow.

that Òputty nailsÓ could be driven through woodenboards. However, surprisingly few geologists seem tohave been bothered by the severe mechanical difficultiesrequired for diapiric piercement through great thickness-es of preexisting strong overburden. At this point, a fun-damental conceptual breakthrough was made.

Previously, diapirs were thought to have grownupward from a relatively deep, static base. Barton (1933)imaginatively proposed that a diapir actually grewdownward from a relatively shallow, static crest, aprocess he called downbuilding (Figure 9). Barton envis-aged a dome growing syndepositionally. The diapirÕscrest remains at or near the surface of sedimentationwhile its base sinks together with the surrounding, sub-siding strata. Because the diapirÕs crest is continuallyemergent, no country rocks are displaced or lifted. Thediapir has no room problem even though its contacts arediscordant to the surrounding strata. By downbuilding, adiapir can appear to penetrate or pierce many kilometersof strong overburden because the latter is deposited onlyalong the flanks of the diapir (disregarding any ephemer-al veneer of sediments over the crest). To support his idea,Barton (1933) discussed geologic observations andmechanics. Some of the mechanical reasoning in this sub-stantial paper is debatable (for a discussion, see Jackson etal., 1988), but the essential validity of the downbuildinghypothesis was so apparent that it was soon overwhelm-ingly accepted by American geologists. It diffused toEurope far more slowly, possibly because it was incom-patible with the hypothesis of regional contraction, forwhich the evidence was strong in Europe.

Thus, by the early 1930s, far-ranging intellectualexploration had recognized many important facets of salttectonics. In the next era, some of these hypothesesbecame unfashionable in the shadow of a commandingand durable new hypothesis.

THE FLUID ERA (1933Ð~1989)

Not many earth scientists have been able to develop ahypothesis that dominated their subdiscipline for 55years and also laid the foundations of a theory that even-tually supplanted their own. This double feat of inaugu-rating the fluid era and laying the foundation for the suc-ceeding brittle era was achieved by Nettleton (1934) andNettleton and Elkins (1947). In his investigation of thefluid mechanics of salt domes, Nettleton (1934) assumedthat when scaled down, salt and its overburden could berepresented by two viscous fluids of negligible strength(oil and syrup). A transparent cylinder containing a stablearrangement of dense fluid beneath less dense fluid wasinverted, and the overturned fluids slowly returned toequilibrium as the denser fluid sank (Figure 10).


Figure 8—Sedimentary differential loading demonstrated by a model of a subaqueous delta of sand, which results in thesqueezing of a mobile substratum of laminated clay from beneath A to B (abridged from Rettger, 1935).

Figure 9—Barton’s (1933) concept of diapiric down-building.

Nettleton (1934) demonstrated that gravity alone couldgenerate diapir-like shapes and a surrounding peripheralsink from an undeformed source layer. Eventual evacua-tion of buoyant ÒsaltÓ from the peripheral sink could cutoff the supply of ÒsaltÓ to the base of the diapir. Nettle-tonÕs basic hypothesis and assumptions prevailed for thenext 55 years, even though he himself was to questionthem only 13 years later. NettletonÕs modeling approachwas widely applied, especially by Hans Ramberg.RambergÕs prodigious and elegant research, which beganin the 1960s and was summarized in his books (1967,1981), propelled the fluid approach ever higher. His workbroke new ground on almost every facet of gravity tec-tonics, but it is probably of greater relevance to crustal tec-tonics than to salt tectonics.

The fluid hypothesis was widely applied because itwas simple and because it focused on the progressive

change in shape of the salt source layer and diapir. Had itbeen more explicit about structures in the overburden, thehypothesis would not have been so durable.

Palinspastic Restorations

Exactly when the first palinspastic reconstruction waspublished is uncertain. An early example of a detailedrestoration is by Rios (1948) (Figure 11). He graphicallyremoved the contractional overprint of Alpine orogeny toreveal the Oligocene form of a basin floored by TriassicKeuper evaporites. His restoration shows how the deep-est part of the original basin became the most upliftedpart during subsequent regional contractionÑa processtermed inversion since 1978 (Coward and Stewart, 1995;Letouzey et al., 1995).

D�collement Styles and Downward Salt Bulges

By the 1930s, geologists were familiar with the ideathat deformation in the cover could be very different fromthat in the underlying basement if separated by evapor-ites. For example, de Cizancourt (1934) distinguishedbetween Romanian style thick-skinned contraction andNorth African style thin-skinned contraction, in whichthe base of the evaporite was roughly planar over a non-deforming basement but the top of the salt was stronglydeformed by incorporation within folds and thrust faultsin the deformed cover. He also proposed that diapirscould be laterally squeezed until they expressed all thesalt from within them.

Later, in Persia, the geometry of a new type of decou-pling both above and below the evaporites became clear.Elaborating on earlier observations by Lees (1938),OÕBrien (1957) described Miocene salt that decoupledupper cover from lower cover in such a way that saltflowed into overlying anticlines and underlying syn-clines. This segregation formed salt bulges laterally sepa-rated by much thinner salt. Locally, the salt bulgesbecame diapiric and broke through their thrusted cara-pace.

Peripheral Sinks

Elements of StilleÕs (1925) idea of salt tectonics drivenby orogeny still lingered in Germany in the 1950s. Forexample, Richter-Bernburg and Schott (1959) interpretedvortex structures within the salt as indicating the suddenrelease of compressional forces during eruptive emplace-ment of salt intrusions during orogenic phases.

At the same time, this orogenic linkage was beingeclipsed by methodical interpretations of seismic datafrom the rejuvenated exploration of oil and gas inGermany. Trusheim (1957, 1960) saw most German saltstructures as resulting from autonomous, isostatic rise ofsalt, which he termed halokinesis. This idea was old, buthis technique of analysis was novel. His was the first sys-

8 Jackson

Figure 10—Dawn of the fluid era. Nettleton’s (1934) gravi-tational overturn of a fluid-fluid system initially comprisingcorn syrup (white) overlying less dense crude oil (black)(abridged from Nettleton, 1934).

tematic attempt to infer the history of salt flow from thesedimentary record of the surrounding strata (Figure 12).Trusheim (1957, 1960) used lateral thickness changes instrata deposited during salt flow to infer when underly-ing salt was flowing laterallyÑan interpretive techniquethat survives today. He recognized the distinctive geom-etry of primary peripheral sinks, which form around pil-lows, and secondary peripheral sinks, which formaround diapirs. From these characteristics, he was able tochart the structural evolution of salt diapirs entirely fromthe sedimentary record (Figure 12). He also recognizedthe structural inversion (Òtransformation of structuralreliefÓ) that accompanies the transformation of a pillowinto a diapir and forms turtle structure anticlines.

TrusheimÕs hypothesis provided geologists with arational basis for interpreting salt structures, and hismethod was unquestioningly applied for severaldecades. In retrospect, two of his assumptions look weak:that diapirs must necessarily evolve through a precedingpillow stage and that a pillow forms solely by buoyancyhalokinesis, even where the overburden is thick andlargely uniform (compare Coward and Stewart, 1995).

Trusheim (1960) also recognized that extensional struc-tures could be overprinted by contractional structures,which he attributed to collapse of salt structures. Today,basement-involved inversion would be a more popularexplanation of the contractional reactivation.

Internal Structures of Diapirs

Because German salt diapirs have been exploitedlargely for scarce potash rather than rock salt, mappingtheir internal structure and stratigraphy was essential.Conversely, U.S. Gulf Coast salt diapirs were minedlargely for rock salt. Their internal structures were thuslong neglected except for safety-related features such asfractures and gas pockets. The Hans Cloos school inGermany had developed mapping techniques for study-ing the igneous and deformational structures of plutons.Balk (1949, 1953) transferred these techniques to theGrand Saline dome (Texas) and Jefferson Island dome(Louisiana). Mapping of mined cavern roofs betweenrock pillars revealed steeply plunging folds with steeplydipping curved axial surfaces. These were the curtain folds


Figure 11—An early example of detailed palinspastic restoration of salt tectonics in the Pyrenees, without vertical exagger-ation (redrawn from Rios, 1948). A basin floored by Triassic Keuper evaporites (bottom) became inverted during Alpinecontraction (top). Another interpretation might show the diapir growing passively throughout the Cretaceous, periodicallyextruding to form the lateral flanges depicted, and more normal faults in the restored section because of null points alongthe faults in the upper section.

described in Germany by Stier (1915) and simulated bythe experiments of Escher and Kuenen (1929). A fewexposures had closed folds like eyes; these were subse-quently attributed to constrictional sheath folds (Talbot andJackson, 1987). Balk (1949, 1953) also noted that the bed-ding defined by disseminated anhydrite curved into par-allelism with the external contact of the diapirs and thatfolds tightened toward the contact. Both observationspointed to what would eventually be called an externalshear zone (Kupfer, 1976) in the outermost parts of thediapir.

Balk also advanced knowledge on smaller scale struc-tures in these domes. Fabric studies showed that dissem-inated anhydrite and spindle-shaped aggregates of anhy-drite defined a strong mineral lineation throughout mostof the mine in the Grand Saline dome (Balk, 1949). Thelineation was axial to the steeply plunging curtain folds,all within 20¡ of the vertical. Halite grains were less

strongly lineated because they had recrystallized. Balkalso stressed the rarity of macroscopic fluid inclusions,fractures, faults, and foreign inclusions.

Early Initiation of Salt Upwelling

By the 1960s, views on the mechanical properties ofrock salt that were derived from experiments began toconflict with real-world observations. Experiments on thedislocation creep of dry salt (e.g., Handin and Hager,1958) had indicated that salt could only flow at tempera-tures above 205¡C, corresponding to a burial depth ofmore than 7 km (Gussow, 1968). This belief conflictedwith (1) the long domal growth histories deduced byTrusheim (1960), and (2) the concept of downbuilding(Barton, 1933). More tellingly, salt glacial flow could onlybe reconciled with the experimental data if salt extrusionwas rapid and red-hot (Gussow, 1968). Yet, many salt

10 Jackson

Figure 12—The sedimentary record begins to be systematically deciphered by Trusheim (1960). A flat-lying sequence con-taining Permian Zechstein salt (bottom) evolves into a pillow surrounded by primary peripheral sinks (Dogger time), theninto a diapir and adjoining secondary peripheral sinks (top of Lower Cretaceous), and finally into a postdiapir stage over-lain by Tertiary peripheral sinks (top). Adjoining turtle structure anticlines are formed as the flanks of the primary sink sub-side over the flanks of the deflating pillow.

glaciers in Iran were obviously still flowing at present-day surface temperatures (Kent, 1958, 1966). Also, seismicdata (Figure 13) indicated early upwelling and flow ofsalt at shallow depths. In the Gulf Coast interior basins,salt flowed under overburdens ~350 m thick (Rosenkransand Marr, 1967; Hughes, 1968) and in the North Sea Basinbelow overburdens ~610 m thick (Brunstrom andWalmsley, 1969). This evidence took the form of onlaps,truncations, and lateral thickness changes against domeflanks; reefs were predicted and discovered on domecrests.

Salt Nappe

By the late 1960s, two distinct concepts in salt tectonicswere well established but perceived to be unrelated. First,the fold and thrust belts in Europe, North Africa, andIran, for example, were known to have involved large tec-tonic translations over evaporites. Second, allochthonoussalt masses were known to exist (e.g., Lotze, 1934) butwere generally referred to as ÒoverhangsÓ or ÒburiedextrusionsÓ and regarded as merely local anomalies in aworld of vertical salt diapirism.

In 1969, these concepts began to be linked. The hori-zontal component of salt tectonics was brought to theforeground for the first time with new data from theSigsbee ScarpÑa lobate, arcuate scarp separating the con-tinental rise from the continental slope of the northernGulf of Mexico. The scarp was first interpreted by Ewingand Antoine (1966) as a deformation front: salt welled upvertically at the base of the continental slope because ofbasinward flow of salt confined within the autochtho-nous layer. Loading by landward sediments caused thisflow, and continued episodes of loading caused new saltwalls to rise seaward of older ones.

After Ewing and Antoine (1966), increasingly daringconcepts evolved to explain the scarpÕs origin. Amery(1969) first glimpsed saltÕs underworld by identifying anallochthonous salt tongue below the Sigsbee scarp(Figure 14). The salt itself was dimly imaged and recog-nized mainly as a high-velocity anomaly, which pulled

up underlying reflectors of strata overridden by the salttongue. This velocity effect was augmented by the waterwedge above the overlying scarp. Amery (1969, 1978),too, thought that salt was flowing basinward, mainlywithin the autochthonous layer but locally as a 7-km-wide lateral tongue below the scarp. He also favored anextrusive origin for the tongue in which a thin (200Ð300m) veneer of sediments overlay denser salt. The mechan-ics of salt drive remained obscure; these authors favoredan ill-defined blend of gravity gliding and sedimentarydifferential loading.

Humphris (1978), in turn, emphasized the importanceof the differential load applied by the prograding conti-nental margin in driving salt laterally (Figure 15). Headvocated two new concepts. First, lateral extrusion ofsalt occurred for tens of kilometers across younger strata(rather than mere lateral flow of salt within an autochtho-nous layer), which required equally large overthrustingof the overburden, like a giant thrust nappe riding on salt.Second, salt structures landward of the salt nappe weremerely remnants of the salt nappe left after salt was large-ly displaced basinward to the front of the salt sheet.

Contractional Belts on Divergent Margins

Reyer (1888) hypothesized that gravity gliding aloneover a weaker layer could cause thin-skinned contractiondowndip of an extensional zone. Similar fold and thrustbelts were produced by modeling (Nettleton and Elkins,1947). Such contractional belts were eventually discov-ered in deep water on divergent continental slopes suchas the Gulf of Mexico, first over a Paleogene shale d�colle-ment in the Mexican Ridges (Bryant et al., 1968; Garrisonand Martin, 1973) then over autochthonous Jurassic saltin the Perdido foldbelt (Blickwede and Queffelec, 1988).

Finite Strain in Diapirs

Structures within diapirs were further quantified bystrain modeling in the 1970s. These studies were aimedprimarily at understanding the internal structures of


Figure 13—Seismic evidenceof upwelling of JurassicLouann Salt below a thin over-burden of Smackover carbon-ates (central Mississippi).Buckner reflectors onlapagainst domed Smackoverreflectors on the flanks of the1.8-km-high pillow (abridgedfrom Rosenkrans and Marr,1967).

12 Jackson

Figure 14—The first knownillustration of allochthonoussalt (from Amery, 1969). Theoriginal sparker profile (tooindistinct to reproducehere) across the Sigsbeescarp, Gulf of Mexico, wasinterpreted in (B) time and (C) depth to illustrate awedge of extrusiveallochthonous salt over flat-lying reflectors.

gneiss domes, but they provided the first strain mapswithin model diapirs. Fletcher (1972) investigated theinternal strains of gneiss domes and anticlines by analyt-ical modeling. Dixon (1975) measured the three-dimen-sional finite strains within centrifuged diapiric walls. Heshowed that in the crest of the diapir, initial verticalstretching was followed by vertical flattening. Other partsof his diapirs also underwent what appeared to bepolyphase deformation as the diapir evolved through asingle overturn. This partly explains the notoriously com-plex folding observed in salt mines.

Thermal Convection and Salt Tectonics

All hypotheses of salt diapirism invoked surroundingclastic rocks until Talbot (1978) proposed that the highthermal expansion of halite would enable salt diapirs torise as thermal plumes through salt. Because the thermalexpansion of halite in a geothermal gradient is greaterthan its elastic compressibility (bulk modulus) due toconfining pressure, hot, deep salt would expand, losedensity, and rise. Cooler, denser salt sinking from theupper salt contact would replace it. Such a convectingsystem could operate even without overburden, althoughthermal effects could be enhanced by an insulating over-burden. Talbot (1978) speculated that a salt layer morethan 2 km thick in the Danakil Depression of Ethiopiaalong the East African rift system could be convectivelyoverturning as a result of the high geothermal gradient.

Thermal convection was invoked on a smaller scale forevaporites in Boulby potash mine, England. Talbot et al.(1982) convincingly explained hundreds of recumbentlobes of sylvinite as the products of increasingly thorough

convective stirring on two scales, roughly 100 m and 400m wide in axial section. The evaporites were estimated tohave deformed under temperatures as high as 170Ð390¡C.The recumbent sylvinite lobes were later intruded by sill-like sheets of rock salt.

Salt transmits heat more efficiently by conduction thanby thermal convection due to expansion. Diapirs act asheat chimneys for conduction, locally cooling their deepsurroundings and locally heating their shallow surround-ings. Such conductive perturbations could be expected toshift oil and gas windows in flanking strata up or down.However, such perturbations are difficult to modelbecause thermal convection in surrounding pore fluidstypically swamps the thermal effects that might be attrib-uted to conductive salt.

Flow Rates and Salt Budget of a Salt Glacier

In the late 1970s, unique fieldwork was also being car-ried out on twin salt glaciers in southwestern Iran. Earlier,Harrison (1930) and Kent (1958, 1966) had provided qual-itative geologic evidence that some Iranian salt glacierswere still flowing and wasting at air temperatures of only10Ð45¡C. However, a real understanding of the process ofextrusion took another decade. Wenkert (1979) calculatedthe flow rates of five Iranian salt glaciers by assuming asteady-state balance between glacial flow and dissolu-tion. Calculated flow rates for dry salt were much slowerthan the extrusive rates he had calculated, so Wenkert(1979) suggested that moisture provided by rainfallallowed the salt to flow by intergranular liquid diffusionat the calculated faster rate.

Nearly 50 years after direct measurement of salt glac-iers was suggested by Bailey (1931), Talbot and Rogers(1980) finally established a rate of glacial flow by repeat-edly surveying markers painted on the northern glacierof Kuh-e-Namak (Dashti, Iran). Their monitoring pro-gram was aborted by the Iranian revolution. However,incomplete data indicated that in the dry season the glac-ier oscillated back and forth diurnally as it heated andcooled. Conversely, during brief periods after rainfall,the glacier flowed downhill as much as 0.5 m per day.This gain was later slightly reduced as the glacier (orÒnamakierÓ) dried and shrank.

Talbot and Jarvis (1984) meticulously investigated thesalt budget of the Kuh-e-Namak extrusive dome and cal-culated that it approximated a 1-km-high, parabolic vis-cous fountain rising at almost 17 cm/year and spreadingextrusively under its own weight at a rate of 2 m/year.These rates were some 40 to 80 times faster than existingestimates for the rise of salt domes and suggested thatwater had a marked softening effect that enhanced theflow of salt. This evidence of rapid flow would prove tobe especially significant to the origin of allochthonous saltsheets.

Internal Structures of Salt Glaciers

TalbotÕs (1979, 1981) thorough work on salt glaciersincluded an investigation of their mesoscopic and micro-scopic structure. He found that the Kuh-e-Namak salt

glacier was a nappe complex of recumbent sheath foldsbounded by subhorizontal shear zones (Figure 16).Approaching a 20-m-high bedrock obstruction, the flow-ing salt decelerates and thickens, forming asymmetricflow folds and crenulations as streamlines are deflectedacross the color bands. Over the obstruction, glacial flowaccelerates and tightens these folds into isoclinal sheathfolds. The process of folding and tearing is cyclicallyrepeated as the glacier encounters each of about 15bedrock scarps. Farther downstream, the flowing saltbecomes sufficiently weakened by intense mylonitizationfor the glacier to undulate over obstructions withoutforming sheath folds.

Perturbation of Doming Stresses by Regional Stresses

As the roof of a rising diapir arches upward, it stretch-es by normal faulting. Partly this fault pattern reflects theshape of the diapir. The physical models of Link (1930)and Parker and McDowell (1955) confirmed that subradi-al faults form above diapiric plugs, whereas subparallelfaults formed above diapiric walls. In both plugs andwalls, most faults were curved rather than straight. Otherauthors had also speculated whether the regional stresspattern could control the orientation of domal faults(Balk, 1936; Cloos, 1968). However, it was Withjack andScheiner (1982) who first systematically investigated the


Figure 15—Sedimentary differential loading as the driving mechanism for the allochthonous Sigsbee salt nappe, Gulf ofMexico (from Humphris, 1978).

combined effects of diapiric shape and regional stressesduring gentle doming by clay and analytical modeling.Without regional strain, only normal faults developedover diapirs. In their models of regional extension, all nor-mal faults were roughly perpendicular to the extensiondirection; strike-slip faults trending 60¡ from the exten-sion direction formed in restricted sectors near theperiphery of circular domes. During regional shortening,normal faults developed subparallel to the shorteningdirection, and both strike-slip faults (30¡ from the short-ening direction) and reverse faults (perpendicular to theshortening direction) formed in restricted sectors on theperiphery of the domes.

Salt Rollers

In the 1980s, extensional salt tectonics began to berevealed in more detail. For about three previous decades(e.g., Quarles, 1953), the lower footwalls of some normalfaults were known to comprise ridges of mobile salt orshale. These diapiric ridges were thought to have initiat-ed the faults above them. However, these diapirsremained unnamed until Bally (1981) featured seismicillustrations of them, labeled Òsalt rollersÓ; he did notcomment on the origin of the term, which remainsobscure. Salt rollers are low-amplitude, asymmetric saltstructures comprising two flanks: one flank in con-formable stratigraphic contact with the overburden andthe other in normal-faulted contact with the overburden.Salt rollers are now thought to form entirely by regionalextension, typically thin skinned and gravity driven. (Inthis review, ÒregionalÓ deformation denotes a scale largerthan a single diapir, regardless of whether the basementis involved.)

Subtle Traps

Salt domes had long been known to create structuraltraps over their crests and against their flanks. By the late1960s, structural traps were well-established targets and

had been classified (e.g., Halbouty, 1967). Seni andJackson (1983a) focused on an equally wide range of sub-tle, facies-related traps associated with halokinesis; someof these subtle halokinetic traps were as much as 20 kmfrom the actual diapir. In synthesizing and analyzing thesedimentologic and tectonic history of all 16 shallow saltdomes in the East Texas Basin, they found that subtlefacies changes characterized certain depositional systemsat certain stages of dome growth. Applying statisticaltechniques to volumetric data from about 2000 wells, Seniand Jackson (1983b) were able to quantify crudely theindividual and collective growth histories, gross and netsalt flow rates, and strain rates of diapirs. Maximumgrowth rates were greatly in excess of regional aggrada-tion rates, implying considerable outflow and dissolutionof salt at the surface.

Flow Law for Damp Salt

Water had long been known to soften salt by the Joffeeeffect (Kleinhanns, 1914), but until the 1980s, experimen-tal rock mechanics had focused on the rheology of dryrock salt, known to deform by dislocation creep (Carterand Hansen, 1983). Stimulated by TalbotÕs correlation ofglacial flow with rainfall, Urai and co-workers investigat-ed the effect of trace amounts of water on the evaporitesbischofite, carnallite, and halite (Urai, 1983, 1985; Spiers etal., 1986; Urai et al., 1986). They found that damp evapor-ites could deform by solution-transfer creep under muchlower stresses and much more rapidly than the disloca-tion creep characteristic of dry evaporites. This explainedthe extraordinarily high flow rates of damp glacial salt.

Mushroom Diapirs

The term mushroom-shaped has long been looselyapplied to diapirs shaped like light bulbs, despite the factthat actual mushrooms are not shaped like bulbs. Trulymushroom-shaped diapirs have a bulb fringed by one ormore pendant peripheral lobes. These lobes are common-

14 Jackson

Figure 16—Glacial structurescreated by nonuniform flow ofsalt (left to right) over anobstructing bedrock step inKuh-e-Namak’s northern saltglacier, Dashti, Iran. Structuralzones are regenerated overeach bedrock step in the upperglacier: (1) flow folds, (2)crenulations, (3) salt veins, and(4) static salt (from Talbot,1981).

ly contained entirely within the diapir (internal mush-room), but in rare cases may enclose infolded countryrock (external mushroom). Mushroom diapirs were firstnumerically modeled in two dimensions by Daly (1967).By the 1980s, mushroom diapirs had been physicallymodeled in three dimensions (Jackson and Talbot, 1989a)and found to be identifiable by crescentic folds in hori-zontal section and by downward-facing folds in verticalsection. These features were recognized in at least 24external mushroom diapirs of the Great Kavir in centralIran (Jackson and Talbot, 1989a; Jackson et al., 1990). In aminority of these external mushroom domes, the periph-eral flanges rolled inward and upward to form vortexstructures. Mining data from several German diapirswere also reinterpreted to indicate internal mushroomand vortex structures (Jackson and Talbot, 1989a).

Salt Canopies

Laterally spreading salt sheets have the potential toeventually coalesce with their neighbors. Curiously,though, such structures were not described or even pre-dicted until the 1980s. Correa Perez and Gutierrez yAcosta (1983) referred to a laterally intrusive sheet of saltcoalesced from two feeders in the Salina Basin of coastalMexico (Figure 17a). They speculated that the allochtho-nous salt could be continuous throughout a wider areaoffshore. In the Great Kavir of central Iran, an exposed,coalesced cluster of 12 salt domes was independentlymapped and described as a salt canopy (Figure 17b)(Jackson and Cornelius, 1985; Jackson et al., 1987, 1990;Jackson and Talbot, 1989b). A cellular structure resulting

from the incorporation of two distinct evaporite unitsindicates that this salt canopy is a composite structure.Salt canopies of vast extent comprising probably hun-dreds of feeder stalks are now known to characterize theGulf of Mexico (e.g., see Diegel et al., 1995; Peel et al.,1995; Rowan, 1995; Schuster, 1995).

Rayleigh-Taylor Acme

The fluid era initiated by Nettleton (1934) drew to aclose in the late 1980s even as modeling of Rayleigh-Taylor instability (sinking of denser fluid into less densefluid) reached new heights of sophistication.

Two topics were investigated by finite-element model-ing in two dimensions. Schmeling (1987) systematicallyinvestigated how the final wavelength and geometry offluid upwellings could be influenced by initial irregulari-ties in the interface between the overturning fluids.Earlier modelers tended to unquestioningly assume thatthe spacing of diapirs could be predicted solely from vis-cosities, thicknesses, densities, and boundary conditions.Schmeling showed that the final wavelength could be upto four times larger than predicted, depending on thespacing of the initial irregularities. Schmeling (1988)investigated the overturn of fluids whose densities dif-fered owing to both composition and temperature.Various combinations of fluids and temperatures couldstabilize after a single overturn, or repeatedly overturn inthe same directions, or even repeatedly overturn inreversed directions each time.

Talbot et al. (1991) investigated the three-dimensionalpatterns of Rayleigh-Taylor overturn in detail. Combin-


Figure 17—(a) Coalesced salt bodies were first deducedas two-dome structures on the basis of gravity modelingalong the coastal Salina Basin, southwest Gulf of Mexico(from Correa Perez and Gutierrez y Acosta, 1983). (b)Other salt bodies were then mapped as a twelve-domecoalesced structure called a salt canopy in the Great Kavirof central Iran (from Jackson et al., 1987).

ing physical modeling and geologic observations, theyshowed that where not perturbed by edge effects or lat-eral surface forces, fluids overturn in a complex spokepattern. Two sets of laterally and vertically interlockingspokes carry fluids of different densities along the topand bottom boundaries into the nearest rising or sinkingplume. Each plume flows to the opposite boundary fromwhere it started, then expands into a broad bulb betweenspokes of the other fluid. This geometry implies thatbelow dense fluid overburdens, less dense fluid diapirsrise from the triple junctions of deep polygonal ridges intheir source layer.

THE BRITTLE ERA (~1989 TO PRESENT)

About 1989, the scientific tide changed. Salt tectonicsbegan to be increasingly approached as a system involv-ing a strong, brittle, fractured overburden rather than aweak, fluid one. Experiments with fluid overburdenshave continued into the 1990s, but they now seem to berelevant only for understanding crust at or below the brit-tle-ductile transition. Because petroleum explorationfocuses on brittle crust in and above the oil and gas gen-eration windows, a new set of salt tectonics models hasappeared. No single conceptual development triggeredthis paradigm shift from the fluid era to the brittle era, sothere is no precise starting date. However, two break-throughs seem to have been catalysts: (1) revitalizing anold modeling approach and (2) developing a new tool forstructural restoration.

Physical Modeling Using BrittleOverburdens

The roots of this approach are slender but deep.Concerned that NettletonÕs (1934, 1943) fluid-fluid modelswere not reproducing the pervasive faulting seen aroundGulf Coast salt domes, Nettleton and Elkins (1947) simu-lated brittle overburden with granular materials overlyinga viscous substratum representing salt. In contrast tomodels of fluid-fluid systems, where the rise of diapirs isaccelerated by thicker overburdens, they found that therise of diapirs was inhibited if an overburden of finitestrength exceeded a certain critical thickness. Hubbert(1951) provided the theoretical imprimatur for simulatingbrittle deformation of rock with dry sandÑas he had forNettletonÕs fluid-fluid approach (Hubbert, 1937).

In the most heroic undertaking of geologic modelingever published, Parker and McDowell (1955) constructedabout 800 physical models of diapirs piercing both gran-ular and liquid overburdens. They confirmed that diapir-ic growth could be terminated much more easily by addi-tional sedimentation than by exhausting the source layer.Bishop (1978) argued on theoretical grounds that thestrength of the overburden and sedimentation rates andstyles played an all-important role in salt tectonics, a viewthat would receive greater support today.

The modeling approach of using dry sand over a vis-cous fluid was revitalized by using paraffin wax or sili-cone to simulate salt (McGill and Stromquist, 1979;Vendeville et al., 1987; Vendeville and Cobbold, 1987, 1988;Vendeville, 1988, 1989; Cobbold et al., 1989). This model-ing focused on extension over a ÒsaltÓ substratum andproduced a far more realistic range of ÒsaltÓ structuresthan previously accomplished (Figure 18). Vendeville andCobbold (1988) discovered that a wide range of structuralstyles could result simply from varying the aggradationrate. Salt rollers were shown to be the response of salt flowto extension rather than the cause of the extension abovethem. Vendeville (1988) also demonstrated how monocli-nal flexure of a salt-based sequence above a basement nor-mal fault formed a graben in the overburden some dis-tance away from the basement fault.

Computerized Reconstruction

A contrasting impetus for the brittle era was the intro-duction of computerized palinspastic reconstructions andsection balancing to salt tectonics. Although balanced sec-tion construction did not originate in the easternCanadian Rockies, it was first widely applied there(Douglas, 1950; Bally et al., 1966). This provenance en-sured that, as the technique propagated in the 1970s and1980s, it remained rooted in the contractional style of foldand thrust belts. Then Gibbs (1983) adapted the conven-tions of balancing to extensional terranes, and Worralland Snelson (1989) adapted them to salt tectonics (Figure19). Their structural reconstructions in the Gulf of Mexicowere accurately carried out using proprietary softwareand depth-converted seismic data without vertical exag-geration. Strata could be progressively decompacted,backstripped, unfolded, and unfaulted. The widths ofgaps and overlaps between fault blocks rigorously con-strained the restoration.

Worrall and SnelsonÕs (1989) restorations indicated thefollowing: (1) emplacement of many restored domes wasaccompanied by faulting; (2) downbuilding was a domi-nant mechanism of dome growth; and (3) broad, flat-topped salt structures were covered by thin, bathyal shaleveneers that during progressive subsidence and steepen-ing became shale sheaths over the progressively narrow-ing, subsiding passive domes.

Hossack and McGuinness (1990) highlighted impor-tant ambiguities in section balancing that are peculiar tosalt tectonics. Both extension and salt withdrawal arecapable of lowering overburden strata below their region-al elevation. Thus, accommodation space for new sedi-ments can be created either by extension or by salt with-drawal. Because of this ambiguity, distinguishing exten-sion from withdrawal is nontrivial in section balancing.An analogous ambiguity results because both contractionand some types of salt emplacement raise overlying strataabove their regional elevation (Hossack, 1995).

Contraction in the deep-water foldbelts was accurate-ly estimated to be much less than updip extension ofequivalent age. This discrepancy suggested a role for dis-

16 Jackson

placed allochthonous salt in accommodating updipextension (Jackson and Cramez, 1989; Worrall andSnelson, 1989; Diegel and Cook, 1990; Hossack andMcGuinness, 1990; Hossack, 1995; Peel et al., 1995).

Major Detachments along Vanished Salt

Worrall and Snelson (1989) stressed that growth faultswere not merely surficial basinward slumps produced bygravity gliding. Rather, the growth faults extend and theirhanging wall strata expand by displacing salt duringregional-scale gravity spreading. Extension provides lat-eral accommodation space, and displacement of salt ormobile shale at depth creates vertical accommodationspace. Some major growth faults terminate downwardagainst laterally pinching-out salt (Figure 20). Restor-ations indicated that the salt was formerly thicker inmany places. This finding led to a radical concept: manymajor extensional detachments in the Gulf Coast Basinfollow vanished sheets of displaced allochthonous salt.Although presented speculatively, this hypothesis is nowwidely accepted and supported by newer seismic data(see Diegel et al., 1995; Peel et al., 1995; Rowan, 1995;Schuster, 1995).

Segmentation of Salt Sheets

Humphris (1978) envisaged that the updip parts of saltsheets could be segmented by sedimentary loading.Much of the salt displaced by this loading moved basin-ward and supplied salt for the downdip leading marginof the allochthonous salt mass (Figure 15).

Worrall and Snelson (1989) documented the vast sizeof the Sigsbee salt nappe on the Louisiana slope. Deltaicsystems loaded the rear of the nappe to produce mini-basins, some of which displaced all the salt below them.Strata at the base of the minibasins commonly rest direct-ly on an evacuated salt surface underlain by significantlyolder Cenozoic strata, creating a sedimentary hiatus rep-resenting the time interval when allochthonous salt wasat or just below the sea floor. Worrall and Snelson (1989)also identified a ÒLouisiana-styleÓ (including easternmostTexas) of growth faulting: short, arcuate faults dip bothlandward and basinward and are spatially associatedwith abundant shallow salt structures of irregular shape.They attributed this structural style to uneven (in timeand space) loading by irregular, shifting deltaic systems.This irregular load could only inefficiently displace saltbasinward, unlike the sweeping efficiency of long, sub-


Figure 18—Dawn of the brittle era. Several structural styles were simulated during gravity gliding experiments by variablesedimentation rates of dry sand overburdens and silicone source layers by Vendeville and Cobbold (1987).

parallel ÒTexas-styleÓ faults dipping basinward. Sumneret al. (1990) described the circular to elliptical minibasinsfull of ponded sediments enclosed by the arcuateLouisiana-style growth faults. These minibasins haveformed largely by salt withdrawal rather than by region-al extension.

Raft Tectonics, Salt Welds, and Fault Welds

Burollet (1975) had postulated that the Angolan mar-gin extended by gravity gliding over a thin layer of lubri-cating salt (Figure 21, top). As it stretched, the overburdenbroke into diverging raft-like blocks (radeaux) separated

by widening grabens or half-grabens. These fault-bound-ed depocenters rapidly filled with younger sediments.Some of the rafts were thought to have been originallyseparated by diapiric salt walls. The walls subsidedbelow deepening depocenters until only remnants wereleft (Figure 21, top). Burollet also mentioned a salt scar(cicatrice salif�re). This was a residual smear of salt left bydiapiric subsidence. The scar formed along the subverti-cal boundary between a gliding block and the adjoiningfault-bounded depocenter.

Such smears of salt began to be recognized elsewhere.Worrall and Snelson (1989) noted Òsalt evacuation sur-facesÓ in the Gulf of Mexico. Jackson and Cramez (1989)

18 Jackson

Figure 19—Another catalyst for the brittle era. The first computerized restorations in salt tectonics, as applied to the Wandaand Corsair fault systems in the northern Gulf of Mexico (from Worrall and Snelson, 1989).

surveyed a wide range of enigmatic residual structuresthey called Òsalt welds,Ó introducing the symbol of paireddots to denote it on sections and maps (Figure 21, below).A salt weld joins strata originally separated by depleted orvanished salt, consisting of thin salt or brecciated insolu-ble residue. A weld is typically recognizable by structuraldiscordance or local inversion above it. Salt welds can bedivided into primary (formed by removal of autochtho-nous salt), secondary (formed by removal of steep-sideddiapirs, such as BurolletÕs salt scar), or tertiary (formed byremoval of gently dipping allochthonous salt). Hossackand McGuinness (1990) extended the concept to faultwelds, equivalent to a salt welds along which there hasbeen significant shear.

Examples of salt welds are common in raft tectonics.Duval et al. (1992) demonstrated that raft tectonics in theKwanza Basin took place in two phases: the first formedsmall, asymmetric, rotated rafts when the overburdenwas only a few hundred meters thick; the second formedlarge, nonrotated glide blocks, which themselves com-prise smaller rafts. Duval et al. (1992) also compared theroles of contraction, allochthonous salt emplacement, andsea floor spreading in creating the space necessary forthin-skinned extension.

Shallow Emplacement of Salt Sills

Broad lateral flanges of salt in Algeria were interpretedas intrusive salt sills by Ehrmann (1922) (Figure 22). Muchlater, analysis of high-quality seismic data from more

than 50 salt sheets in the Gulf of Mexico suggested thatthese had been emplaced intrusively like sills from near-surface diapirs into shallow, low-density, mostly clay-richsediments (Nelson and Fairchild, 1989; Nelson, 1991)(Figure 23). These data showed reflectors onlapping athin clastic carapace over the bulging salt sheet. The thick-ness of this carapace averaged only about 120 m.Sediments deeper than 300 m were estimated to be toostrong to admit sills of salt. Nelson and Fairchild (1989)also deduced that after emplacement, many sheets werefurther inflated by salt (Fletcher et al., 1995).

Shapes of Passive Diapirs and SubmarineSalt Glaciers

Vendeville and Jackson (1990, 1991, 1992a) proposedthat the cross-sectional shape of a passive (downbuilt)diapir was the result of interplay between sedimentationand upwelling salt. Specifically, the dip of the saltÐsedi-ment contact was proportional to the ratio between thelocal aggradation rate and the net rise (or lateral spread-ing) rate of diapiric salt (the net rise rate being the grossrise rate minus any dissolution or erosion of the salt). Forpassive diapirs not cut by faults, this simple relationcould account for the variation in time and space of saltstructures from conical to steep-sided to overhanging totongues to sheets. In effect, allochthonous sheets wouldbe an extreme end-member of the above process, spread-ing rapidly at the surface during periods of extremelyslow local aggradation (Talbot, 1995).


Figure 20—Major detachments,such as the Wilcox fault zone,created accommodation spacebecause salt withdrew frombeneath them, culminating incomplete loss of salt. This led tothe concept that many majordetachments in the Gulf ofMexico follow vanished sheets ofautochthonous or allochthonoussalt (from Worrall and Snelson,1989).

Truncation of strata below salt tongues supported theview that many allochthonous sheets were extrusive. Inthe Gulf of Mexico, extrusion would necessarily havebeen underwater. Submarine salt glaciers were not a newidea. Trusheim (1960) speculated that greatly overhungdiapirs were the remains of salt glaciers that extrudedunder water. He also described how solution brecciaswere intercalated in the top of the salt like a moraine.Talbot and Jarvis (1984) reported a marine veneer on the

Kuh-e-Namak salt extrusion, which clearly implied sub-marine extrusion. The mechanics of submarine salt glaci-ers is investigated in detail by Fletcher et al. (1995), nowthe ruling hypothesis for emplacement of salt sheets.

Convergent and Divergent Gravity Gliding

Gravity gliding operates downslope, roughly normalto the shelf break. Parallel gliding on a homoclinal shelf

20 Jackson

Figure 21—The concept of salt welding. (Top)Extensional breakup of overburden into sepa-rate rafts (1a–c) and the filling and inversionof a depocenter over a subsiding salt wall(2b–c) (from Burollet, 1975). (Bottom) Similarstructural discordances produced by listricdécollement fault (left), combined salt weldand décollement fault (center), and nonexten-sional salt weld (right) (from Jackson andCramez, 1989).

produces mainly two-dimensional structures: an updipdomain of extension separated from a downdip domainof contraction by an undeformed domain of translation(Crans et al., 1980; Letouzey et al., 1995). Cobbold andSzatmari (1991) focused on the more common type ofcoastline, which is irregular. They found that glidingtends to be divergent off coastal salients but convergentoff coastal reentrants. Divergent gliding produces strike-parallel extension, whereas convergent gliding producesstrike-parallel contraction, neither of which can berestored by simple section balancing. Similar complexi-ties also apply to linked zones of extension and contrac-tion in gravity-spreading systems.

Reactive Diapirism and RegionalExtension

Beginning with Seidl (1926), several geologists havespeculated that regional extension could initiate dia-pirism, but mechanical explanations were lacking untilillustrated by physical modeling (Jackson and Vendeville,1990; Vendeville and Jackson, 1992a). Diapiric walls rosebeneath grabens created by regional extension at ratesentirely determined by the extension rate (Figure 24).Extensional faulting thinned and weakened the overbur-den locally, creating tectonically induced differentialloading. By this means, reactive diapirsÑas they werecalledÑcould rise beneath overburdens of any thickness,density, or lithology. Once diapirs were near enough tothe surface, they would typically transmute into activediapirs and forcefully intrude their thin roofs and reachthe surface. Once there, diapirs would typically evolve tothe passive rise stage (Nelson, 1989). Thereafter, the localaggradation rate, extension rate, or contraction ratewould determine the structural style and mode of growth(Fletcher et al., 1995; Letouzey et al., 1995; Nilsen et al.,1995; Talbot, 1995).

These experiments also showed that large amounts ofregional extension could be hidden remarkably wellwithin a cross section containing salt structures. Apassivediapiric wall (which reaches the surface) typically accom-modates most or all of the regional extension simply bywidening, whereas the adjoining blocks of overburdenremain largely undeformed because of their greaterstrength. In contrast, for a reactive diapiric wall (whichdoes not reach the surface), deep extension is accommo-dated partly by widening of the diapir; shallow extensionis accommodated by faulting of its roof.


Figure 22—Ehrmann (1922) interpreted broad lateralflanges of Triassic salt (t1, chevron pattern) as intrusivesalt sills in the Gulf of Bejaïa (Bougie), Kabylie, Algeria.

Figure 23—Onlap of frontal reflectors above a thin roof averaging only 100 m thick (black bars on left) was the principal evidence of shallow emplacement of allochthonous salt tongues (after Nelson, 1991).

Diapiric Subsidence, Turtles, and MockTurtles

The experiments cited in the preceding sectionshowed that regional extension may cause a diapiric wallto widen between diverging blocks of overburden(Figure 25). This widening increases the demand on saltsupply even though the diapir crest may not be rising. Asthe salt supply rate diminishes, the source layer thins dueto extension and depositional loading, leading to redistri-bution of salt. Eventually, as the salt supply rate becomestoo low, the roof of the widening diapir begins to sag,potentially forming a graben flanked by residual horns ofsalt (Figure 25E).

Turtle structures can also form. Trusheim (1960) associ-ated turtle structure anticlines with the subsidence of pil-low flanks as the pillow crests became diapiric and brokethrough their roofs (Figure 12); extension was not requiredto account for these structures. The experiments ofVendeville and Jackson (1992b) showed that turtle struc-ture anticlines could also be formed by regional extension

as diapiric flanks subsided (Figure 25CÐE). BurolletÕs(1975) concept of the crest of a diapir subsiding into tworelict salt diapirs was also validated by these experimentsand by reconstruction (Schultz-Ela, 1992). The synclinaldepocenter separating these diapiric relics grounded onthe basement then inverted into an anticline. This anticlinewas termed a mock turtle structure to distinguish it from aturtle structure, from which it differed by forming above adiapir (rather than between diapirs) and by missing strati-graphic section at its base (Figure 25E).

POSTSCRIPT

The Hedberg conference that led to this book was con-vened to disseminate, evaluate, and consolidate the manynew ideas in salt tectonics that were circulating in theearly 1990s. The conference was fortuitously held on theeve of a spectacular renaissance of industry interest in salttectonics caused by a burgeoning subsalt play in the Gulfof Mexico. Amid the dust raised by this exploration boomand by the new, only partially tested ideas of the last fewyears, it is a reviewerÕs exacting task to distinguish wheatfrom chaff long before these become sifted naturally.Added to the difficulty of recognizing the durability andquality of a new concept is the inherent bias of an activeresearcher compared with a detached observer.

At present, the fluid era and the brittle era are glaring-ly contrasted in black-and-white opposites: fluid versusbrittle, density versus strength, gravity forces versus lat-eral forces, and so on. This counterpoint is typical of anyearly period, when a new (brittle) paradigm is pushed toits limits to explore its implications. Useful pieces fromthe previous (fluid) paradigm may lie discarded, likeStilleÕs orogenic engine, which rusted for decades andwas then refurbished with the recognition of inversion-related salt tectonics.

It is premature for the brittle era to be viewed in fullperspective, but some brief reflections on shades of grayin two aspects of salt tectonics soften strong contrasts andshould check any overweening confidence that we fullyunderstand how salt tectonics works.

In theory, a supercomputer groaning under the weightof appropriate algorithms and incorporating a realistical-ly large number of variables could simulate a full range ofsalt tectonics by forward modeling. Although such a sim-ulation is valid and could be repeatedly calibrated againstnatural structures, it could never actually verify thehypotheses built into the algorithms (Oreskes et al., 1994).Moreover, such a model cannot be built by even the mostbrilliant minds because we lack full data to be entered.Most lacking may be data on how overburden deforms.On geologic time scales, salt is a Newtonian viscous orpower law fluid, whereas shale seems to deform by per-vasive brittle shear. Yet both diapiric rock types can yieldsimilar structural stylesÑapparently because they share acommon overburden. The style of tectonics seems to becontrolled by the overburden rather than by the compo-sition of the diapiric rock.

22 Jackson

Figure 24—Rise of a model reactive diapir due to tectonicdifferential loading induced by a graben formed by region-al extension. Both the source layer and the overburdenwere initially tabular, as shown in the lower section. Therestoration did not unstrain the fault blocks, demonstrat-ing the local distortion within them; gaps are shownhachured (from Vendeville and Jackson, 1992a).

What do we know of this overburden? Current mod-eling of salt tectonics is dominated by an isotropic over-burden deforming as a brittle but previously broken rock,governed by ByerleeÕs law (e.g., Byerlee, 1978). This pre-fractured state is popular among modelers because itsbehavior is well understood and is robustly uniform for alarge range of rocks (excluding shale). Moreover,ByerleeÕs law is a conservative approach: less deviatoricstress is required to deform rock already fractured in awide range of orientations than to generate new fractures.However, overburdens accumulate in complex variety.What if the overburden itself contains halite, as in theGreat Kavir or offshore Yemen? What are the differentroles of disseminated halite or layered halite in weaken-ing the overburden? What proportion of halite, and otherevaporites, is necessary before we should treat the over-burden as fluid? And what of shale? Overpressuring isknown to promote fracturing in shale, but what if shale isprefractured? What is the role of diagenesis and com-paction (downward, upward, and lateral)? Without thesedata, our knowledge of salt tectonics is broad but skeletal.

Then there is the third dimension. At the close of thefluid era, the long-ignored three-dimensionality of fluids

overturning by spoke circulation was finally examined indetail. We should not have to wait that long for the brittleequivalent. Restrained by the two-dimensional nature ofmuch seismic data, of section restoration, and of numeri-cal modeling, our knowledge of the full three-dimension-al world of salt tectonics lags far behind the two-dimen-sional representation. Technology is allowing increasedexploration of the third dimension using seismic datavolumes and three-dimensional processing, visualiza-tion, and structural restoration. As we better comprehendthree-dimensional interactions, we should improve ourinterpretation of currently ambiguous processes such assalt withdrawal versus extension.

The history of salt tectonics reveals a familiar patternof science. Once-dominant paradigms are supplanted bynew ones. Next it will be the turn of the brittle paradigmto be dismantled and its durable parts recycled. Visionarypioneers are often excoriated by their peers but are even-tually venerated for the same hypothesis. Looking backover the last century to the first dim perceptions of salttectonics, we see a scientific landscape strewn not onlywith outmoded ideas but also with the conceptual seedsof the next revolution in salt tectonics.


Figure 25—The rise and fall of diapirs duringthin-skinned regional extension creates turtlestructure anticlines, mock turtles, keystonegrabens, and fault welds (from Vendeville andJackson, 1992b).

Acknowledgments I am grateful to the following colleagueswho kindly supplied copies of antique papers in Spanish,French, and German: Bernhard Knipping (Clausthal), CecilioQuesada (Madrid), Antonio Ribeiro (Lisbon), and Maura Sans(Barcelona). I also thank BP Exploration for inviting me to par-ticipate in their salt tectonic field trip to Algeria, which includ-ed the salt dome shown in Figure 1; Chris Talbot for providingan invaluable and creative perspective on the whole paper butespecially the postscript; Carlos Cramez for supplying the paperby Oreskes et al.; David Stephens for photographing all theillustrations; Kirt Kempter for digitally stitching Figure 1;Hongxing Ge for redrawing Figure 11; Bruno Vendeville andGiovanni Guglielmo for translating certain words; AmandaMasterson and Tucker Hentz for stylistic improvements; andDan Schultz-Ela, Tom Nelson, and especially Sig Snelson forrefereeing and polishing the paper. Research was funded by theTexas Advanced Research Program and by the AppliedGeodynamics Laboratory consortium comprising Agip S.p.A.,Amoco Production Company, Anadarko Petroleum Corpor-ation, Arco Oil and Gas Company and Vastar Resources, Inc.,BP Exploration Inc., Chevron, Conoco Inc. and Du PontCorporation, Exxon Production Research Company, TheLouisiana Land and Exploration Company, Marathon OilCompany, Mobil Research and Development Corporation,Petroleo Brasileiro S.A., Phillips Petroleum Company, Soci�t�Nationale Elf Aquitaine Production, Statoil, Texaco Inc., andTotal Minatome Corporation. This paper is published by per-mission of the Director, Bureau of Economic Geology.

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