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ABSTRACT

Scaled physical models illustrate the importanceof progradation as a trigger for salt tectonics andformation of allochthonous sheets. Regional exten-sion and contraction were excluded in the models.

In our experiments, prograding wedges above atabular, buoyant salt layer with a flat base expelledthe salt basinward, forming the following struc-

tures proximally to distally: (1) sigmoidally distort-ed initially planar wedges, (2) relict salt pillows andsalt welds, (3) basinward-dipping expulsionrollover and crestal graben, (4) rollover syncline,(5) landward-facing salt-cored monocline, and (6)distal inf lated salt layer. This deformation zoneamplified and advanced basinward during progra-dation; however, no diapiric salt structures formed.

Over a buoyant salt layer whose basement hadsteps facing landward, progradation initially formeda broad anticline where salt flow was restrictedacross each basement step. Distal aggradationpinned the anticline and enhanced differential load-ing. The anticline actively pierced its crest, which

had been thinned by faulting and erosion. There-after, the diapir grew passively, locally sourcingallochthonous salt sheets. This deformation cyclerepeated over each basement step so that the age,amplitude, complexity, and maturity of salt-relatedstructures decreased basinward.

As each allochthonous salt sheet was buried andevacuated by sediment loading, arcuate peripheralnormal faults formed along the sheets trailing

edge, detached wrench faults formed along its lat-eral edges, and active piercement at its leadingedge allowed the sheet to break out and climbstratigraphic levels. This process formed a multi-tiered complex of salt sheets that migrated basin-ward with time.

Restorations of examples from various salt tecton-ic provinces support our model results. Immenselandward-dipping pseudofaults could arise entirelyby salt expulsion rather than regional extension.Diapiric families and peripheral sinks are reinter-preted as the result of progradation squeezing saltbasinward and laterally.

INTRODUCTION

Salt structures can be triggered by a variety ofmechanisms. Several studies have emphasized therole of regional extension in the initiation andgrowth of salt structures (Duval et al., 1992;Vendeville and Jackson, 1992; Demercian et al.,1993; Jackson and Vendeville, 1994). The trigger-ing effect of sedimentary differential loading hasbeen invoked more often (e.g., Trusheim, 1960;Dailly, 1976; Seni and Jackson, 1983a, b; Jacksonand Talbot, 1986, 1991; Worrall and Snelson, 1989;Nelson, 1991) but has received less rigorous atten-tion, especially by workers using scaled physicalmodeling. Lateral variations of thickness or densityof overburden induce sedimentary differential load-ing, which triggers two types of deformation thatoccur separately or together. First, the thickest sed-iments sink deepest into the underlying salt,expelling it laterally, which thins or entirely evacu-ates the salt, forming a weld. Second, the sediment

398 AAPG Bulletin, V. 81, No. 3 (March 1997), P. 398423.

Copyright 1997. The American Association of Petroleum Geologists. Allrights reserved.

1Manuscript received October 2, 1995; revised manuscript received May9, 1996; final acceptance October 16, 1996.

2Bureau of Economic Geology, University of Texas at Austin, andDepartment of Geological Sciences, University of Texas at Austin, Austin,Texas 78713.

3Bureau of Economic Geology, University of Texas at Austin, Austin,Texas 78713.

All modeling was done at the Applied Geodynamics Laboratory of theBureau of Economic Geology, with financial support by grant number 3658-178 from the Texas Advanced Technology Program and from the followingcompanies: Agip S.p.A, Amoco Production Company, Anadarko PetroleumCorporation, ARCO Exploration and Production Technology, BP Exploration,Chevron Petroleum Technology Company, Conoco and Dupont, Exxon

Production Research Company, Louisiana Land and Exploration Company,Marathon Oil Company, Mobil Research and Development Corporation,Petroleo Brasileiro S.A., Phillips Petroleum Company, Socit Nationale ElfAquitaine Production, Statoil, Texaco, and Total Minatome Corporation. TheDepartment of Geological Sciences and the Geology Foundation at theUniversity of Texas at Austin and Phillips Petroleum Company providedadditional financial support for Hongxing Ge. Dan Schultz-Ela helped usdepth convert and restore seismic sections. Mark Rowan, Mike Hudec, LeeFairchild, Sharon Mosher, and Tucker Hentz provided invaluable suggestionsfor improving the paper. Publication authorized by the Director, Bureau ofEconomic Geology, University of Texas at Austin.

Kinematics and Dynamics of Salt TectonicsDriven by Progradation1

Hongxing Ge,2 Martin P. A. Jackson,3 and Bruno C. Vendeville3


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wedge itself can spread seaward by means of exten-sional faulting on the shelf and upper continentalslope and shortening by thrusting and folding inthe lower slope.

Progradation of alluvial fans and deltas along themargins of many terrigenous clastic basins is a com-mon and effective way to create differential load-ing. A prograding system commonly comprises a

fairly flat shelf, a shelf break, a middle and upperslope dipping between 0.3 and 5 (e.g., Galloway,1989; Worrall and Snelson, 1989), and a gently dip-ping prodeltaic zone of slow, fine sedimentation.Salt diapirs are common in prograding settings,such as in the Paradox Basin (Baars, 1966; Ge,1996), Gulf of Mexico (Humphris, 1979; Woodburyet al., 1980; Worrall and Snelson, 1989; Wu et al.,1990), west Africa (Duval et al., 1992; Lundin,1992), and Brazil (Mohriak et al., 1995). However,little is known about the kinematics and dynamicsof this relation. Apart from brief descriptions ofonlaps rotated into apparent downlaps owing towithdrawal of a laterally migrating mound of salt

(Jackson and Cramez, 1989; Wu et al., 1990), weknow of no published criteria to distinguish therole of progradation from other mechanisms capa-ble of triggering diapirs.

Rettger (1935) was the first to experiment withsedimentary differential loading. He loaded a sandwedge onto a uniformly thick layer of laminatedclay under water. Lateral f low of clay thinned itbelow the thickest part of the wedge. A listric nor-mal growth fault formed below the sand wedge,and minor folds formed in front of the wedge.Synkinematic progradation effects were not exam-ined. Similar structures were experimentally pro-duced by McKee and Goldberg (1969), who usedsand wedges dipping at 30 to create serial foldsand thrusts in the underlying mud sequences bydifferential loading. However, the experimentswere not scaled to simulate salt tectonics.

Centrifuged models by Jackson and Cornelius(1987) and Talbot (1992) simulated progradationon syndepositional salt structures. They used stiff,viscous, or power-law fluids to simulate overburdenand viscous fluids to simulate salt. Salt structureswere formed by differential loading and gravityspreading, but the fluid behavior of the overburdenis unrealistic for the shallow crust where salt tec-tonics is concentrated (Vendeville and Jackson,1993; Weijermars et al., 1993). Moreover, for practi-cal reasons, the models used either tabular or unre-alistically steeply dipping prograding increments.Nevertheless, these experiments produced someuseful insights that were corroborated by our mod-eling. First, salt was expelled to distal regions frombeneath the prograding wedges. In front of theadvancing wedges, the source layer was greatlythickened over a broad area, rather than forming a

diapir next to the progradation front. Second, bnal salt pinch-out and basement steps provideffective barriers to lateral salt flow, causing losalt accumulation and diapirism. The modelresults were compared with allochthonous sheets of the Gulf of Mexico region.

Experiments by Vendeville et al. (1994) exined the effects of sand wedges prograding ove

viscous silicone analog for salt. Because the silicolayer was thin or denser than the overburddeformation was dominated by extension and s ward spreading of the sedimentary wedExtension formed turtle structures separatedsmall, sagging diapirs in the footwalls of asymmric grabens.

In contrast, our study uses physical modelsinvestigate the effect of prograding wedges advaing over a thick, buoyant salt layer, a setup that tically opposes spreading and extension of the sment wedge. New concepts illustrate hprogradation triggers salt tectonics and how demation is dominated by sediment subsidence a

lateral expulsion of salt where the salt is thick amuch less dense than its overburden. We demstrate how basement steps and distal aggrading siments control diapir location. The modelresults are used to reinterpret salt tectonprovinces in the South Atlantic, Gulf of Mexiand Germany.

METHODOLOGY

Experimental Materials

Physical models are simplified laboratory recas of natural systems. Their usefulness increawith the degree to which they are dynamicascaled to their geologic equivalents. Dynamic sing theory has matured from the solid foundatlaid by Hubbert (1937) and has been successfuadapted to salt tectonics (e.g., Ramberg, 19Vendeville et al., 1987; Weijermars et al., 1993).

Our models use dry quartz sand to simulate btle sedimentary rocks. The sand shows near-perfMohr-Coulomb behavior and deforms along narrshear zones, which mimic fault zones (Man1988; Krantz, 1991). The sand has negligible cosion, an angle of internal friction of about 30, a density of about 1700 kg/m3. Sand is a good alog for most sedimentary rocks in the upper conental crust, which obey Mohr-Coulomb behav(Byerlee, 1978; Weijermars et al., 1993).

We used a viscous silicone polymer (RG20, a traparent silicone polydimethysiloxane [PDMS]), angray, dense (1390 kg/m3), viscous Silbione gumsimulate damp rock salt that is assumed to be a pfect Newtonian viscous fluid, with a viscosity rang

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from 1017 to 1019 Pas, depending mainly on grainsize (van Keken et al., 1993). The silicone, RG20, isan almost perfect Newtonian fluid, with a viscosityof about 6 104 Pas at a strain rate of 104/s atroom temperature and a density of about 950kg/m3. Its properties are very similar to those ofother PDMS products, such as SGM36, that havebeen used frequently to model salt tectonics(Weijermars, 1986; Weijermars et al., 1993). Thegray, dense Silbione gum also has Newtonian vis-cous behavior. Its viscosity was reduced by a sol-vent to a value estimated to be similar to that of theother silicone. These two salt analogs were used indifferent experiments to investigate the influenceof density contrast on salt tectonics.

The scale ratios between models and geologicequivalents (prototypes) are * 0.50.8 (modelmaterials had densities slightly lower than those of

natural rocks, but the density difference in themodel was higher than in nature), g* = 1 (modelsand geologic prototypes both deformed in a naturalgravity field), and l* = 105 (1 cm in models repre-sents approximately 1 km in nature). The ratios oftime (t*) and associated rates of deformation ( ),progradation ( ) and aggradation ( *) dependedon the viscosity ratio (*). Because natural rock salthas a wide range of viscosity values (van Keken etal., 1993), these time-relevant model ratios alsovary widely, so their natural equivalents are not par-ticularly meaningful.

Hereafter, the silicone analogs for salt will bereferred to as salt for brevity.

Experimental Procedures

All models were contained in a rigid box 80 cmlong and 40 cm wide. Figure 1 is a schematic crosssection of undeformed models to show the initialgeometry of the salt basin and overburden strata. Inthe first experiment (model 253), both top and

base of the source layer were flat (unlike thestepped base shown in Figure 1). The salt sourcelayer was uniformly 1.8 cm thick. In the otherexperiments (models 263, 270, and 272), the top ofthe salt was flat, but the base had two steps dip-ping 60 landward. The steps simulated inactivenormal fault scarps in a block-faulted basin filled bysalt. Step 1 was 1.0 cm high, and step 2 was 0.5 cmhigh. The proximal salt was 2.0 cm thick andthinned across each basement step to 0.5 cm distal-ly (Figure 1). The models were then covered by aprekinematic overburden layer uniformly 0.30.6 cmthick.

Progradational and aggradational layers werethen added episodically to simulate synkinematicsedimentation (Figure 1). The progradational,wedge-shaped sediments had either a clinoformalslope (layers 2, 3, and 4) or a clinoform with aproximal shelf (layers 5, 6, 7, 10, and 11), or aproximal bypass surface (layers 8 and 9). All clino-forms were initially planar and dipped at 5. Thewedges prograded from right to left and down-lapped basinward (left). We used linear fronts inour models. Although the progradation front of anindividual delta or alluvial fan is commonly lobateand curved, single-sourced lobes can shift or coa-lesce to form a regionally linear or nearly linearprogradation front. The Cenozoic deltaic continen-tal margins of the northwestern Gulf of Mexico(Winker, 1982), the Cenozoic Ogallala Formation ofthe piedmont alluvial deposits in the Great Plainsprovince of the central United States (Frye, 1971),

and the present a lluvial fans in Death Valley,California (Bloom, 1991), are typical examples ofsediment wedges with nearly linear fronts. In ourmodels, the mean rates of progradation rangedfrom 0.5 to 5.3 cm/day (Figure 2). The distalaggrading layers, however, were tabular exceptwhere they thinned and onlapped landward againstgrowing salt structures (Figure 1). They aggradedconcurrently with only a few progradation wedges.

400 Progradation and Salt Tectonics

Figure 1Schematic, verticallyexaggerated cross section ofexperimental setup anddepositional systems. The baseof the tabular salt layer of model253 is shown as a dashed whiteline. Aggrading layers arenumbered the same as coevalprograding wedges.

D*

P*

A


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Figure 2Plot of distance ofprogradation front from proximend wall of models (xaxis) vs. t(yaxis). The mean rates ofprogradation ( ) were calculate

by excluding the first wedge.A steep slope indicates slow .The inset (upper right) showsthe complete plot for model 272at reduced scale.

The mean rates of aggradation ranged from 0 to0.3 cm/day.

Restorations

All restorations in this paper used the software,Restore (Schultz-Ela; 1992; Schultz-Ela andDuncan, 1994). Model sections required no depthconverting or decompacting. Seismic sections wereapproximately depth-converted before restorationwhere possible, using velocity profiles selected tofit the age and lithology of the strata, then adjustedif necessary to conform with other authors depthconversions or well data. Decompaction assumed ashale/sand ratio of 3:1 to 3:2. During backstripping,we unfolded strata using vertical shear because ofthe paucity of faults and because, in the models atleast, no regional extension occurred.

We use the term salt expulsion wherever salt isexpelled along strike or downdip by overburdendifferential loading, especially by progradingwedges. This process may or may not involve achange of the cross-sectional area of salt. We usethe term salt reduction where salt area has obvi-

ously been lost from a cross section by any process.

EXPERIMENTAL RESULTS

Progradation Over a Tabular Salt Layer

Model 253 investigated progradation over a tabu-lar salt layer. Figure 3 shows the initial setup of the

three uniform prekinematic layers (including sand the chronostratigraphy for the overlying skinematic wedges (layers 210). Each thick, hzontal line represents the length of a weddeposited episodically to prograde from righleft at an average rate of 1.4 cm/day for 20 days.aggrading layers were deposited. Blank regions rresent nondepositional hiatuses.

Structural Style and Kinematics

The final structure (Figure 4) of the deformmodel comprised two domains separated blandward-facing monocline: (1) a proximal regwhere prograding wedges overlay a residual lsalt pillow and salt weld, and (2) a distal regwhere the source layer was uniformly thickenedthe proximal end wall, an old reactive diapir (lowright in Figure 4b) flanked by normal faults wburied under the prograding wedges.

The synkinematic wedges downlapped onto prekinematic layer where salt was largely evacued, whereas the wedge tips onlapped against monocline. The deformed offlapping wedges w

sigmoidal; proximal planar surfaces steepened tally, forming a rollover antiform. Unlike a rolloin the hanging wall of a listric normal fault, rollover in our model was not induced by extsion, but by basinward expulsion of salt benethe prograding wedges. To sharpen this differenwe refer to this structure as an expulsion rollovekeystone graben formed at the crest of the expsion rollover. Distal faults in the graben dipp

Ge et al.

Time(day)

Distance from proximal edge (cm)

Model 253, P= 1.4 cm/dayModel 263, P= 1.6 cm/dayModel 270, P= 5.3 cm/dayModel 272, P= 0.5 cm/day

s1

s2

s1

s2

Basement step 1

Basement step 2

s1

s2

ProximalDistal Progradation

Time(day)

10203040506070 0

Distance (cm)

Model 272

s2

s1

20

40

80

100

60

003570

20

15

10

5

0

P

P


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more gently than proximal faults because of rota-tion as the wedges bent. The sigmoidal clinoformsthickened distally and then thinned, forming arollover syncline above the monocline.

Expulsion Rollover

An expulsion rollover (Figure 4) superficiallyresembles a fault rollover in the hanging wall of a


Distal Proximal

Progradation

Basement

0

4

8

12

16

20

0 5 10 cm

Prekinematic layer1 Synkinematic layers2-10Viscous layer (salt analog)

P= 1.4 cm/day

A = 0

Postkinematic layer11

Time

(day)

Progradation frontStarved basin

123

4

5

6

7

8

9

10

11

Figure 3Chronostratigraphy for model 253. The lowermost three layers show the initial setup. Each episodicallydeposited synkinematic layer is plotted as a numbered, thick, horizontal line. Layer numbers correspond to thosein following cross sections (Figures 4, 6). The uppermost layer (11) is the postkinematic layer deposited after theexperiment ended. = mean rate of distal aggradation; = mean rate of progradation.A

P

Regionaldatum

Earlydepocenter

Rolloversyncline

Expulsion rollover

Salt thickens andlifts overburden

Basement

11Proximal

faults

(b)

Rollingmonocline

Salt weld Relict pillow

Distal Proximal

334455

6789

101111

22

11111 Future layerSynkinematic layers2-10Prekinematic layer1Viscous layer (salt analog)

10 cm0

(a)

5 cm0

P = 1.4 cm/day

Figure 4(a) Close-up of a cross section through model 253. The anticline in front of the wedges was an artifactinduced by salt sagging during section cutting; the actual structure was a monocline. (b) Tracing of a regional crosssection of model 253. The dashed horizontal line represents the top of the initially tabular salt layer. The rollingmonocline, rollover syncline, and rollover all migrated basinward in tandem with the progradation front. Large

black dots show data points plotted in Figure 5a.


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listric growth normal fault. However, dip analysisof the rollover (rollover plot) provides an objec-tive and seemingly robust technique to differenti-ate them.

Figure 5a plots the maximum dip of each wedgein the expulsion rollover vs. distance from the

present progradation front in Figure 4b.the rollover syncline, the dip was measured on basinward-dipping flank. The dip of the youngwedge (layer 11 in Figure 4b) retained the ini

depositional slope of 5. The rollover plot is shaplike an asymmetric hill (Figure 5a). Dips rapiincrease away from the front to a maximum athen decrease less rapidly. The maximum dip cosponds to where the thickest salt was expellBecause the salt plateau thickened over time, thickest salt was below the youngest rollover scline. This maximum migrated basinward with tias each new syncline formed. The oldest wedpreserved the initial 5 depositional slope and nar shape even through they were folded aunfolded by the monocline that rolled throuthem. Subtle residual salt pillows affected the dFor example, the dip of layer 2 in Figure 4b is ab3slightly less then the original 5becauspillow was left beneath it.

The plot of a fault rollover in the hanging wala simple listric normal growth fault is quite difent: there is no decrease of dip away from the pent progradation front. Instead, the dips of hangwall cutoffs abruptly then gradually increasereach a maximum value away from the surftrace of the listric normal fault both in physimodels (Figure 5b) and in nature (Figure 5c). Tconstant maximum dip far from the fault corsponds to flattening of the underlying fault. Tmaximum dip depends on the initial sedimentat

slope, initial fault dip, and dip of the sole fa(Crans et al., 1980; Jackson and McKenzie, 19Vendeville, 1991). This type of analysis shouldcarried out only on synkinematic strata, which distinguished from prekinematic strata by havlocal lateral variation in thickness.

Listric faults having multiple ramps and flyield more complex rollover plots (Figure 5Both plots include a tail of data points fr

Ge et al.

Dipofrolloverflank

Distance from progradation front (cm)

(a) Model expulsion rollover

Dipofrolloverflank

(c) Geologic fault rollover

Dipofrolloverflank

Distance from master fault (cm)

(b) Model fault rollover

0 10 20 305 15 25

Dipofrolloverflank

Distance from master fault (cm)

(d) Model fault rollover

0

30

60

0

90

30

60

-20

A

B

A

B

A

B

Gulf of GuineaFault system 3, TXSnake Mountain, NV

Brazos Ridge 1, TXBrazos Ridge 2, TXGrand Isle, LA

Distance from master fault (km)

0 105 15

Ramp

Flat

Ramp

FlatFlat

Flat

Ramp FlatlatFlat RampampRamp Flat

90

0

30

60

0

10

20

0 5 10 15 20 25 30

0 5 10 15 20 25

Figure 5Rollover plots of the maximum dipsrollovers. (a) Experimental expulsion rollover (mo253). (b) Experimental fault rollovers associated wsimple listric normal faults (profile A, model 188B. C. Vendeville and S. T. Lin; profile B, experiment

by McClay, 1990). (c) Six geologic fault rollovers. BraRidge, Texas (Christensen, 1983), and Snake MountNevada (Robison, 1983), are depth-converted sectiofault system 3, Texas (Bruce, 1983); Gulf of Gui

(Gaullier et al., 1993); and Grand Isle, Louisiana (Lberg, 1983), are seismic time sections. (d) Experimefault rollovers having multiple ramp/flat fault geom(profile A, experiment E10, McClay, 1989; profilexperiment E48, McClay, 1990). Open data symbols icate prekinematic (nondiagnostic) and filled data s

bols indicate synkinematic (diagnostic) strata.


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wedge 4 tilted basinward more steeply than its ini-tial slope of 5. This steepening set the pattern forall younger wedges that also accumulated over themonocline between the inf lated plateau and thesubsiding wedge front. The depocenter shifted tothe frontal region above the monocline (Figure 6c)as a rollover syncline (layer 5+6+7). Lack of saltsupply hindered further rise of the proximal reac-tive diapir.

Lack of proximal salt meant that subsequentwedges thickened and deformed mostly at theirdistal parts. Basinward expulsion of salt belowthe thickened wedges caused the monocline toroll forward by folding, then unfolding, the over-burden (Figure 6ce). Evacuation of salt landwardof the monocline resulted in maximum subsi-dence of the overburden at the tip of the youngerwedges (Figure 6d, e). Originally straight wedges

became increasingly bent into sigmoidal cliforms basinward. A keystone graben formedlocal stretching of a migrating expulsion rolloanticline. The primary salt weld also propagabasinward in the wake of the monocline.

Although the salt layer in the distal regiinflated over time, no diapirs or even salt-coanticlines formed there. This supports Nelso(1991) proposal that progradation over tabusalt is an ineffective initiator of diapirism whthe salt is buried by a constant-thickness overbden in the distal part of the basin.

Progradation Over Stepped Salt Basins

A combination of basinward salt thinning alandward-dipping, subsalt nor mal faults co

Ge et al.

Distal Proxim

Progradation

Step 2 0

Time(day)

A = 0.02 cm/day

0

10

20

30

40

50

60

70

80

90

10

Prekinematic layer1 Synkinematic layers2-24 Postkinematic la25

P= 0.5 cm/day

Step 1Basement

Bypass

Progradation front

5

10

15

20

25

12

9

76

11

Viscous layer (salt analog)

Figure 7Chronostratigraphy of model 272. The lowermost three layers show the initial setup of the experimEach episodically deposited synkinematic layer is plotted as a numbered, thick, horizontal line; proximal wedextend from the right, whereas distal aggrading layers extend from the left. The distal bypass region represen

nondepositional hiatus. Layer numbers correspond to those in the following cross section. See Figure 3 for adtional explanation.


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provide an effective buttress against lateral saltflow. The following three models examine howsuch basement steps can influence salt tectonicsdriven by progradation.

Structural Style and Kinematics

Slow Progradation Over ModeratelyBuoyant Source Layer

In model 272, the density of the salt analogwas 1390 kg/m3, compared with 1700 kg/m3 forsand. The prekinematic layer was 0.5 cm thick.The model deformed over 97 days with 24 syn-kinematic wedges at an average progradation rateof 0.5 cm/day, the lowest of all models (Figure 2).Only four aggrading layers (6, 7, 9, and 12) wereadded distally at an average rate of 0.02 cm/day(Figures 2, 7).

A vertical section of the deformed model isillustrated in Figure 8. As in model 253, the oldest

prograding wedges (215) were deformed into anexpulsion rollover above a salt weld. However, asalt anticline (rather than a monocline) formedabove basement step 1. Thinning of the prekine-matic layer in the proximal flank of the salt anti-cline was caused by erosion (sand was scraped off)of the former crest that subsequently migrated bas-inward. The salt never extruded laterally, althoughparts of the salt crest were often exposed.

The distal region was little deformed. Synkine-matic aggrading layers (6, 7, 9, and 12) thinnedtoward and onlapped against the distal flank ofthe salt anticline. Young sediment wedges 1624prograded across the anticline and buried it.Overburden above basement step 2 tilted proxi-mally. Despite continued differential loading, onlya subtle salt pillow formed above basement step 2because the overburden was too thick to bendmuch (Figure 8b). Wedges that downlapped ontoaggrading layer 12 preserved their oblique clino-formal geometry.


P= 0.5 cm/day

Viscous layer (salt analog) Synkinematic layers2-24

Basement

Step 2

Step 1

1

55

10010

1551520020

234

6

7

89

11

1213

14166161771788189919

21121222233234424

1

6

7

9

12

ProximalDistal

V E = 1

V E = 3X

Salt weld

(a)

(b)

Downlap surface

Progradation

Early crest

Early

cre

st

10 cm0

10 cm0 Prekinematic layer1

A = 0.02 cm/day

Figure 8Regional cross sections of model 272 showing depositional patterns and structural style. (a) True scale,(b) vertically exaggerated by 3.


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Progradation Over Highly BuoyantSource Layer

Models 270 (Figures 9, 10) and 263 (Figures 11,12) had setups and depositional histories similarto those of model 272, but the density of the saltwas only 950990 kg/m 3 . The mean rates ofprogradation were 5.3 cm/day for 8 days in model270 and 1.6 cm/day for 24 days in model 263.Although the deformed models had similar overallstructural styles, the detailed salt structures var-ied greatly, even between different sections in thesame model.

Model 270 was tilted 1 landward before defor-mation to simulate the slightly dipping basementtypical of foreland basins and half grabens. Tiltingcaused gravity-driven extension at the distal endwhere a salt roller formed (left end of Figure 10).This extension was balanced by buckling theprekinematic layers in front of the progradingwedges. These minute buckle folds were maskedby younger deformation and became invisible.Proximal gravity gliding stopped early becausethe overburden thickened rapidly there. The

effect of tilting was negligible compared withthat of progradation.Salt structures were well developed above

basement step 1, but were developed only spo-radically next to step 2 (Figure 10). Above step1, salt structures varied from a salt anticline(Figure 10a), to a salt wall (Figure 10bd), to asalt tongue and salt sheet (Figure 10eh). Thesechanges along strike resulted partly from the

initial model setup, but were mainly controlby synkinematic deposition. The prekinemalayer was 0.51.0 mm thicker and the soulayer 0.51.0 mm thinner in section (a) thansection (h). This differential loading caused to f low along strike. As in model 272, progring wedges and distal aggradation localized a anticline above step 1. The anticline plungbeneath the thickest overburden (Figure 10and younger wedges prograded over and burthe anticline where it was lowest. Salt laflowed from this area, and the anticline graduly subsided. Eventually, only a relict, salt-coanticline was preserved above step 1 (Fig10a). However, the sigmoidal expulsion rollounambiguously recorded the thickening, ththinning, of the salt.

A salt wall with large overhangs dominated structure above basement step 1 in Figure 10bThe stem was pinched off locally (Figure 1when the basinward, inflated diapiric f lasagged as the supporting salt escaped by extsion. A salt tongue and sheet formed above bament step 1 where salt was initially thick(Figure 10eh). The basal contact of the advancsalt sheet between steps 1 and 2 has a flat/rageometry, where the base of salt climbs basinwoverall but locally cuts back landward wherevepulse of sediment accumulated.

A younger salt anticline and diapir formed nexbasement step 2 (Figure 10fh). Its proximal ovhang coalesced with the salt sheet from the ol

Ge et al.

Prekinematic layer1 Synkinematic layers2-13

P= 5.3 cm/day

A = 0.3 cm/day

Viscous layer (salt analog) Postkinematic la14

10 c0

0

2

4

6

8

Time(day)

Step 1Step 2Basement

Proxima

Progradation

DistalProgradation front

11

55

10010

14414

11

33

55

77 88

Figure 9Chronostratigraphy of model 270. See Figures 3 and 7 for additional explanation.


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A = 0.3 cm/day

Progradation


ProximalDistal

(b)

(d)

(e)

(f)

(g)

(h)

(a)

(c)

Salt ramp Salt flat

P= 5.3 cm/day


70 60 50 40 30 20 10 0

Step 1Step 2


Salt wall

Detached salt diapir

Salt sheet

Salt weld

Relict pillow

Allochthonous salt

Salt anticline Salt canopy

Allochthonous salt Relict salt anticline

Downlap surface

112334567

89

00101111 1221213313

Basement11 33 55

77 88

Rollover and crestal faults

Salt diapir with overhangs

Salt roller

Rotated downlaps

Foundered roof

Figure 10Serial nonexaggerated cross sections of model 270 at the end of the experiment. Salt structures increasein maturity, complexity, and area from (a) to (h). Each section was 3 cm apart.


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diapir above step 1 to form a salt canopy (Jacksonand Talbot, 1989). A slab of overburden founderedand was engulfed in the salt (Figure 10g, h).

In the overburden, an expulsion rollover, crest-al graben, and salt weld dominated proximally(Figure 10). Along the distal flank of the diapirabove step 1, the aggrading layers tilted land-ward, rotating the downlap surface on the top oflayer 8 (Figure 10a) to become an apparent onlapsurface (Figure 10b). The wedge sediments in thedistal flank of the diapir above step 1 apparentlyonlapped against and thinned toward the anti-cline next to step 2. Because of this geometry,these strata could be misinterpreted as a sec-ondary peripheral sink (Trusheim, 1960). How-ever, the process is distinctively different. Thedomeward thickening here was primarily due toprogradation rather than diapirically induced saltwithdrawal. Furthermore, rol lover synclinesmigrated away from the diapir in contrast to sec-ondary or tertiary peripheral sinks that generallymigrate toward the rising diapir. We return to thissignificant difference in the section dealing withgeologic analogs. Little deformation occurred dis-tally of step 2.

In the slowly prograding model 263 (Figures11, 12), which had similar a setup to models 270and 272, a subsided salt anticline was preservedabove basement step 1. The tiny notch in layer 1in the proximal f lank of the relict anticline

(Figure 12bh) marked early reactive piercembelow crestal faults, although the salt did nemerge. The expulsion rollover and the onlapsaggradational sedimentation in the distal barecord the early growth and subsidence of anticline.

In Figure 12ik, where the source layer was tially thickest, a diapir above step 1 emerged aextruded a salt sheet laterally as far as section This salt sheet evolved into a multitiered shcomplex (Figures 12ik, 13). Each tier wbounded by normal faults at the trailing edgethe evacuated sheet (TFS1TFS3 in Figure 13) aby buckle folds (Figure 12j), frontal monocl(Figure 12k), active diapirs, and breakoutshigher stratigraphic levels at the leading edgethe sheet.

A second salt structure formed above ste(Figure 12). The anticline initiated early befdeposition of the first aggrading layer 5. This lathinned widely over the anticline above step 2 toward the anticline above step 1 (Figure 12ai, j) because these two folds were initially a sinbroad anticline, which was then divided by pgrading sediments. Here, there was time for to escape basinward of step 1 to form large distructures because the progradation rate wmuch slower than in model 270, where salt wtrapped against step 1. The crest of the old acline above step 2 was eroded, which enhanc

Ge et al.

Time(day)

Step 2 Step 1Basement

Progradationfront

A = 0.2 cm/day

P= 1.6 cm/dayProximaDistal

0

4

8

12

16

20

Viscous layer (salt analog) Prekinematic1 Synkinematic layers2-16 Postkinematic lay17

0 10 cm

Progradation

11

55

1001015515

17717Bypass

6655

11

14414 15515

Figure 11Chronostratigraphy of model 263. See Figures 3 and 7 for additional explanation.


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active, then passive, diapirism during depositionof layers 615. The youngest aggrading layer 16unconformably rested on the truncated flanks of

the diapir above step 2 (Figure 12eh). Anexpulsion rollover also formed in the proximalflank of this diapir.


Allochtho

ProximalDistal


Rollover and crestal faults

70 60 50 40 30 20 10 0


Step 2 Step 1

Salt canopy

Salt-sheet complexSalt anticline

Passive wallTruncation

Salt anticline Allochthonous salt

Evacuated allochthonous sheet

(b)

Rotated downlap

(a)

Progradation

Downlap surface Onlap

Relict salt anticline

Rotated downlap

TFS

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

Basement

Salt weld

11223456678990010

1111221233134414551566161551514414665511

Onlap

Buckle fold Salt rampSalt flat

A = 0.2 cm/dayP= 1.6 cm/d


Figure 12Serial nonexaggeratedcross sections of model 263 atthe end of the experiment. Eachsection was 2 cm apart except

between (j) and (k), which were4 cm apart. As in Figure 11, saltstructures increase in maturity,complexity, and area from(a) to (k).


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Dynamics of Salt Tectonics Driven byProgradation Over Stepped Salt

A conceptual model based on models 263, 270,

and 272 shows the evolution of salt structures assediment wedges prograded over a distally thinningsalt basin with basement steps dipping landward(Figure 14). Initially, a uniform prekinematic layer 1covered the stepped source layer. Synkinematicwedge 2 initially dipped 5 (Figure 14a). Distalaggradation was slow. Early on, differential loadingby the wedge expelled underlying salt basinward,as in the model without basement steps (Figure 6).However, the lateral flow of salt was restricted bythe basement step. Thus, salt backed up there,forming a broad, asymmetric salt-cored anticline(Figure 14b). As the salt anticline rolled forwardacross step 1, the source layer thickened in its dis-tal flank. Aggrading layer 4 thinned and onlappedagainst the anticline. These distal aggrading sedi-ments had two major influences: (1) they resistedbasinward rolling of the anticline by buttressing itsleading flank, so that the influx of more salt causedthe anticline to amplify, and (2) the onlapping lay-ers created differential loading that expelled distalsalt to feed the growing anticline. In nature, aggra-dation is less episodic than in our experiments, but,even so, the raised bathymetry above the distalinflated salt could prevent appreciable distal aggra-dation until late in the history, as in our followingexamples from the Gulf of Mexico and SantosBasin.

Arching of the anticline stretched its outer arc toform crestal faults, which initiated reactivediapirism above step 1. Erosion of the crest causedfurther thinning. After emerging, salt remained atthe surface during sedimentation of layer 5, and thediapir grew passively into a tall salt wall by down-building (Figure 14c) (Nelson, 1989; Jackson andTalbot, 1991). The passive diapir could extrude to

form a salt sheet (Figure 12d) and even an evacued salt-sheet complex. However, rapid progration could bury and depress the old anticl(Figures 10a, 12ah).

Progradation of wedge 5 across the diapir oveing step 1 triggered a younger anticline above next constriction, basement step 2 (Figure 14Again, repeated crestal faulting, erosion, and skinematic sedimentation promoted passive dpirism above step 2, while the diapir above stecontinued downbuilding or extrusion (Fig14df). Complete evacuation of the source lacaused the overburden to ground onto the bament, forming a basinward-younging salt weExpulsion rollovers formed against the proximflanks of each diapir. The amplitude of the rolloadjoining step 2 was smaller than that adjoinstep 1 (Figures 12, 14f) because the source lainitially thinned distally. However, the initially nar wedges still deformed to sigmoidal clinoforas was discussed in model 253 (Figure 6a).

In this way, progradation created a series of srelated structures subparallel to the progradatfronts and basement steps. The ages of progration fronts and salt-related structures decreabasinward. Accordingly, the amplitude, complexand maturity of salt structures decreased baward. However, as the overburden thickened the source layer thinned distally, diapir initiatbecame more difficult.

Evacuation of Salt Sheets DuringProgradation

As each extruded sheet was rapidly burieddeformed under the influence of basinward gravspreading and differential loading by new wedgNormal faults formed along the salt sheets trailedge, whereas active piercement at its leading e

Ge et al.

Figure 13Tracing of thestructure above step 1 inFigure 12i showing the salt-shecomplex formed by sedimentprogradation. TFS1TFS3 denotperipheral faults at trailing edgof salt sheets. LEB1LEB3 denotleading edge breakouts of saltsheets where these climbed

stratigraphic section.


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allowed salt to climb to a higher stratigraphic sec-tion over the basal cutoffs and spread glacially dur-ing each nondepositional or erosional hiatus.Repetition of the sometimes coeval processes ofburial, sheet evacuation, sheet climb, and sheet

spreading formed a multitiered complex of saltsheets (Figure 13).

This complex of salt sheets migrated basinwardalong with the progradation front (Figures 12ik,13) (Ge et al., 1994; Ge, 1996). Extrusions flowed


Salt weld

Step 1Step 2

Semi-starved basin

Passive wallExpulsion rollover

Salt anticlineSlow aggradation

Salt anticlineStarved

Diapir with overhang

Truncated crestSlow aggradation

Passive wall

Salt sheet

22

223344

44

223

445

2

344

5

2

3

445

2

344

5

6

67

644

6 44

7

644

44 5

5

5

56

7

Prekinematic layer1 Synkinematic layers2-7Source layer

20 cm0

Slow aggradation

Slow aggradation

Progradation

Distal Proximal

Basement

(a)

(b)

Trailing fault system

11

11

11

11

11

11

(c)

(d)

(e)

(f)

Figure 14Cross sections showing the schematic evolution of salt structures during progradation above a salt basin

underlain by stepped basement.


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in any direction, depending on the local slope,which was commonly landward into the adjoiningrollover syncline. However, the breakout sites,where salt climbed stratigraphic section, systemati-cally shifted basinward. That may be a useful ruleof thumb for interpreting similar structures on seis-mic profiles.

In map view (Figure 15), the traces of the trailingfaults (TFS) were arcuate in contrast to the straightkeystone faults (KFS) because they soled out on therounded trailing edges of salt sheets. This extensionat the trailing edge of the sheet was balanced at theleading edge by mild buckling (Figure 12j), activediapiric piercement, contraction of the diapir neck,and transfer of allochthonous salt to higher strati-graphic levels.

Subtle wrench structures formed along the sidesof the roof over a sheet because of its basinwardmovement relative to the adjoining overburden(Figure 15). All these structures were linked duringsheet evolution.

GEOLOGIC ANALOGS

Salt structures form during progradation in msalt basins, but have been overprinted by eitextension on divergent margins or shorteningforeland basins. The closest analogs of our modshould be salt structures in progradational envirments that have been minimally affected by ot

processes. The following examples from varibasins have salt structures strikingly similar to model structures. We therefore suggest that salt tonics in those basins was driven by progradatio

Santos Basin

Figure 16 presents two seismic sections of salt-related structures in the northern Santos Baoffshore Brazil. The salt is Aptian (Demercian et1993; Mohriak et al., 1995; Szatmari et al., 19and the subsalt basement is fairly flat. A girollover downlaps onto a flat salt weld. The di

part of the rollover overlies a diapiric wDemercian et al. (1993) inferred that the rolloveFigure 16a formed during Late Cretaceous extsion as a fault rollover against the Cabo Frio faullandward-dipping listric growth fault (left cenof Figure 16a). The fault zone created a lategap in the lowermost Albian sequence of mthan 25 km (Demercian et al., 1993; Szatmari et1996). In contrast, Szatmari et al. (1996) attributhe rollover to the bending of the Upper Ctaceous prograding wedges during evacuationthe underlying salt. However, they too interprean extensional listric fault bounding the rollothat hindered seaward progradation. In contrMohriak (1995) and Mohriak et al. (1995) proposeveral hypotheses and favored a combinationprogradation and extension.

The structures in Figure 16 closely resemble model results (Figures 4, 6). In this section, we tthe hypothesis that the Cabo Frio fault zone arelated structures in the northern Santos Baresulted primarily from progradation and salt expsion rather than from regional extension.

Rollover plots (Figure 16c) of two adjoining li(Figure 16a, b) support an origin by progradatand salt withdrawal rather than by regional extsion. Each plot has the shape of an asymmetric

characteristic of an expulsion rollover (compFigure 5a), unlike the progressive dip increase tycal of fault rollover plots (Figure 5b, c). The smhollows in the broad crest of plot B coincide wthe relict pillows, where incomplete evacuationsalt resulted in less bending of the overburden.

We tried restor ing these lines incorporatregional extension, but were unable to produplausible solutions. Hypothetical extension eit

Ge et al.

Figure 15Map of model 263 after 19 days showinglinked systems with basinward-spreading salt sheets.

Ticks and hachures indicate downthrown side of nor-mal faults. LEM = leading edge monocline; TFS = trail-ing fault system; KFS = keystone fault system; WFS =

wrench fault system. See Figure 13 for cross-sectionalstructure.


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Ge et al.

Figure 17(ah) Evolution of the Cabo Frio fault zone, Santos Basin, offshore Brazil. (i) Speculative future deopment of a landward-dipping normal fault produced by removal of underlying salt on the proximal flank of diapir; such a structure is the analog of that in Figure 16a. The nonexaggerated present depth section (h) was c

verted from the seismic section in Figure 16b using a velocity profile from the Gulf of Mexico shelf (Schultz-Ela Duncan, 1994) slightly adjusted to correspond with the depth section in Mohriak et al. (1995). Wedges wassumed to initially dip at 1. Thin abyssal sediments above the starved distal salt plateau were omitted in (ad).


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deposited on the distal plateau above the inflatedsalt. This condensed section represents the present-day Albian gap. Evacuation from the thickened salt

layer formed an expulsion rollover and rolloversyncline (Figure 17c), both of which increased inamplitude and migrated seaward (Figure 17ah).


Thickened Louann Salt

(a)Progradation

North-NortheastSouth-Southwest

(b)

Depth(km)

(f) Present

(e) End Miocene

(d) Cenomanian

(c) Expulsion rollover

Rotated onlap

Relict pillow

Inverted rollover syncline

Rollover syncline

Future fault

0

5

10

TPPTP

TMOMOTMOTLMLMTLM

TLCLCTLC

TUMUMTUM

10 km0Top middle Oligocene (30 Ma)

Top Lower Cretaceous (94 Ma)

TMMTMTLCLCTLC

Top Pliocene (1.6 Ma)Top Miocene (5.5 Ma)

Top lower Miocene (15.5 Ma)

TPPTPTUMUMTUMTLMLMTLM

Rollover syncline

Figure 18Reconstruction of a depth section from the northeastern Gulf of Mexico (Wu et al., 1990, their foldout3c). Prograding wedges between the Upper Jurassic and the Lower Cretaceous were slightly reinterpreted based onthe original seismic line (Wu et al., 1990, their foldout 3a, b). Wedges were restored to 1 initial dip; other horizons

were restored to 0.5 initial dip. Section (c) was slightly shortened compared with section (d) to compensate forextensional faulting. Minor faults were omitted.


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Complete salt evacuation created a salt weld thatpropagated seaward as the expulsion rolloveradvanced. Incomplete salt evacuation, possiblybecause of rapid progradation, left two residual pil-lows (Figure 17a, e). Wedges that initially onlappedagainst the proximal flank of the rolling monoclinelater rotated to form apparent downlaps onto thesalt weld (Figure 17c).

A passive diapir was pushed up by expelled saltahead of the prograding wedges during Maas-trichtian time (Figure 17e), possibly because ofshifting depocenters. The diapir was probably pre-vented from migrating farther basinward by theadditional differential loading from the extreme dis-tal sediments. The diapir grew passively by main-taining its crest near the surface during sedimenta-tion. The contact between the diapir and theproximal rollover is diapiric rather than faulted(Figures 16b; 17g, h). Forward modeling of thepresent structure (Figure 17h) indicates how thisdiapiric f lank could evolve in the future (Figure17i). If salt were expelled seaward or along strike,the diapir and its roof would subside. Being brittle,the roof can subside only by faulting. We carriedout this forward modeling (Figure 17i) because analmost identical landward-dipping fault along strikeis imaged in a neighboring seismic profile (Figure16a) (Demercian et al., 1993). No regional exten-sion is necessary to form the fault, although wecannot eliminate the possibility of some extension.Similar arguments for an origin by salt reductionrather than by regional extension have been madeafter restoring similar landward-dipping faults inthe Gulf of Mexico (Diegel et al., 1993, 1995;Schuster, 1993, 1995).

Gulf of Mexico Basin

The Gulf of Mexico Basin contains some of the

most numerous and complex salt structures in theworld (e.g., Worrall and Snelson, 1989). Sedimentprogradation has been proposed to provide differ-ential loading that deformed both autochthonousand allochthonous salt (e.g., Humphris, 1979; Wuet al., 1990). This proposed effect is generallyobscured by the enormous thickness of the over-burden, by extensional and contractional tectonics,and by emplacement of allochthonous salt sheets.

In most parts of the Gulf of Mexico, progradatioeffects can only be displayed by accurate restotions of regional sections (Worrall and Snels1989; Diegel et al., 1995; Peel et al., 1995).

However, an exceptionally clear example of relation between progradation and salt tectonicthe northeastern Gulf of Mexico is illustrated bseismic line published by Wu et al. (1990, thfoldout 3), which we have slightly reinterpretdepth-converted, and restored (Figure 18). Quitative reconstruction by Wu et al. (1990, their ures 7, 10) suggested that Upper Jurassic aLower Cretaceous pseudo-clinoforms were inily subhorizontal slope sediments that rotated agrounded onto basement after basinward wdrawal of Louann Salt.

Our reconstruction (Figure 18a) shows that tially oblique (dipping 12) Upper Jurassic pgrading wedges expelled most of the underlysalt and mounded it into a distal salt plateau, whsalt was apparently thickened to about 4 km. Amodel 253 (Figures 4, 6), oblique wedges wdeformed into sigmoidal clinoforms as underlysalt was expelled seaward (Figure 18bd). Origally onlapping wedges were rotated to appardownlaps onto largely evacuated salt. A rollosyncline formed above the landward-facing rollmonocline. However, unlike in the Santos Ba(Figure 17) and our models, salt subsequenthinned rather than thickened distally (Fig18bd). Salt must have migrated beyond or outhis plane of section.

The salt diapir appears to have been initiatedthe middle Cretaceous (Figure 18c, d). What tgered this diapiric wall? Uniform thicknessesJurassic and Lower Cretaceous overburden adjoing the diapir eliminate differential loading aplausible explanation. The absence of evidenceany underlying basement fault also eliminatestepped base of salt as a likely cause. Instead,

propose that the diapir was initiated reactivelyregional extension. The hypothetical landwadipping fault depicted in Figure 18c is not recogable in the present-day section. However, alostrike (Wu, 1993, his lines D2-D6, plate 1), tdiapiric wall passes into a major nor mal fawhose growth is recorded by thickened midCretaceous sediments in the proximal flank (Fig18d). The diapir grew passively during the ea

Ge et al.

ProgradationNortheastSouthwest

Relict pillowSalt flow

TriassicAnhydrite and younger saltZechstein Salt

Figure 19Relation betweenTriassic prograding wedges andunderlying salt structures in theMansfeld Basin, Germany (after

Trusheim, 1960). Compare withthe similar model 253 in Figure


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Miocene. Rapid seaward spreading of salt duringthe late Miocene formed an allochthonous saltsheet (Figure 18e).

The experimental arcuate trailing fault systemdetaching on the landward periphery of evacuatedsalt sheets (Figures 13, 15) may have a counterpartin some of the seaward-dipping, arcuate growthfaults of offshore Louisiana (Worrall and Snelson,1989), where a series of arcuate faults define thelandward edge of some sheets.

North German Basins

The famous salt structures in northern Germanyhave traditionally been attributed to halokinesiswithout significant lateral tectonic forces (Trus-heim, 1960). Trusheim envisaged that during sedi-mentary differential loading, primary peripheralsinks developed during the pillow stage, whereassecondary and tertiary sinks formed during dia-pirism. Sannemann (1968) coined the term salt-stock family to describe how daughter andgranddaughter diapirs could develop in succes-sion from a central mother diapir as peripheralsinks migrated outward. This outward migrationwas envisaged by Trusheim (1960, p. 1523) as due

to a large-scale rhythmical phenomenon like rip-ples from a stone thrown into water.In contrast, based on structural geometry similar

to those in our experiments, we think that theGerman salt structures and diapiric families were ini-tiated by prograding sediment wedges. Trusheims(1960) turtle structures were ascribed to inversion ofthe primary peripheral sink as an adjoining pillowevolved into a diapir. We reinterpret this sink as an

expulsion rollover. Moreover, the sequential forma-tion of daughter and granddaughter diapirs isprobably a response to the progradational shift ofdepocenters between the Keuper and the earlyTertiary (Trusheim, 1960, his figure 20).

The Mansfeld Basin is a fine example of Triassicprograding wedges expelling underlying salt south-westward (Figure 19) in a form closely resemblingour models (Figures 4, 6). Laterally expelled salt accu-mulated as a broad pillow near the salt pinch-out atthe southwest margin. A salt pillow was overriddenby progradation and trapped in the northeast margin.Trusheim (1960) saw this geometry as evidence forearlier mobilization of deeper salt in the center of thesaucer-shaped basin.

In the Lower Saxonian Basin (Figure 20) (Trus-heim, 1960), the oldest Triassic overburden (Figure20, layer 2) is uniformly thick and thus prekinematic.The overlying Lias (Figure 20, layer 3) varies in thick-ness, but the thinnest section is on the present limbof the monocline (Figure 19a). This anomaly proba-bly records the former crest of the monocline, as inmodel 272 (Figure 8); the crest later became its limbas the fold rolled beneath it.

Northnortheastward progradation of theMiddle Jurassic Dogger (Figure 20, layer 4) toLower Cretaceous Neocomian (Figure 20, layer 7)

wedges uniformly thickened Zechstein salt. Thisformed a south-southwestwardfacing monoclinethat separated a salt plateau in front of thewedges. On the distal salt plateau, sedimentscoeval with the prograding wedges were sparse.In the proximal f lank of the monocline, rolloversynclines (double-ended arrows in Figure 19a) cre-ated by salt expulsion migrated basinward. As inmodel 250 (Figures 4, 6), the thickened salt did


Figure 20Cross sections of a saltstructure in Lower Saxonian Basin,Germany. Restored to the (a)pre-Albian and (b) present-daystructure (both after Trusheim,1960). The finely stippled stratafrom the Dogger to the Neocomian

were interpreted as a primaryperipheral sink by Trusheim

(1960). In contrast, we interpretthem as wedges progradingnorth-northeastward. Doublearrows denote rollover synclinesthat also advanced basinward.


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not pierce the uniform thickness of the Albiansediments. In contrast to Trusheims (1960) inter-pretation, the section shows no record of signifi-cant diapirism in the form of a salt-filled gap in

the restoration or diagnostic thickness chanabove the incipient diapir.

The Albian (Figure 20, layer 8) is fairly uniformthickness, which suggests that progradation en

Ge et al.

Figure 21Restored evolution ofGorleben salt wall, Germany. (ae)Initiation by the Middle TriassicMuschelkalk to Upper Jurassic Mal

wedges (fine stipple) progradingnorthwestward, and (fi) passive aglacial growth. The present structu(j, after Trusheim, 1960) was notdepth converted or decompacted

because the traveltime scale isunknown. The wedges were restorto 5 initial dip in (cf), which issmaller than the present-day 810dips (in time) of the deformedKeuper reflections.


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or migrated further basinward at this time. In con-trast, the overlying Upper Cretaceous is also greatlythickened on each side of the incipient diapir andon the proximal side of the turtle structure to thesouth-southwest. Accordingly, we interpret theUpper Cretaceous on the proximal f lank of thediapir not as a rollover syncline, but as a peripheralsink formed in response to salt reduction. The

incipient diapirs crestal graben is bounded byreverse faults. Because of the great thickness of thediapirs roof compared with its height, we ascribethese reverse faults not to active diapirism (Schultz-Ela et al., 1993) but to basin inversion during theLate Cretaceous. Salt probably migrated laterallyout of section, deforming the progradation wedgesinto a turtle structure anticline when the overbur-den was welded onto the basement (Figure 19b).

In contrast, Middle Triassic Muschelkalk toUpper Jurassic Malm wedges prograding north-westward over a subtle basement ramp created theGorleben salt wall (Figure 21). An incipient saltanticline with an eroding crest was inflated by salt

expelled from beneath the Muschelkalk sediments(Figure 21b). Lateral salt flow was restricted overthe basement ramp, which, together with distalaggradation, probably stabilized the position of theanticline. As the Gorleben anticline amplified, saltinflated the distal plateau (Figure 21cf), whichwas sediment-starved because of its elevation andbecause the anticline dammed the prograding

wedges. Crestal erosion and active diapirism(Figure 21e) created the Gorleben passive diapirduring the Late Triassic (Figure 21f), which spreadas a salt glacier (Figure 21g, h). Salt extruded land-ward (southeast) because the proximal anticlinalflank sank faster than the distal flank owing to sedi-mentary loading; similar land ward extrusionoccurred in our models (diapir above step 2 inFigure 12eh). The distal salt partly reversed flowto feed the extrusion because of distal aggradation.

Evacuation of salt caused the proximal overbur-den to rotate counterclockwise. The prekinematicBuntsandstein was partly unfolded and weldedonto the basement; the wedges distorted into sig-moidal clinoforms and formed an expulsionrollover during salt reduction (Figure 21gj). Theproximal weld below the wedges propagated bas-inward from the Early Jurassic to the Tertiary(Figure 20dj), whereas the weld below the distalaggrading sediments formed during the Tertiary(Figure 21j).

CONCLUSIONS

Physical models indicate that progradation alonecan initiate salt anticlines that can mature intosalt walls, stocks, glaciers, or linked, multitiered

complexes of evacuated sheets and salt welds. Theprograding wedges expelled the salt basinward bydifferential loading regardless of density relations inways that reveal the most subtle interplay betweensedimentation and lateral salt flow.

Over a flat base of salt, expelled salt evenlyinflated the salt layer in front of the wedges, form-ing a sediment-starved distal salt plateau. De-

formation was concentrated in the frontal region ofthe wedges in three adjoining and migratingdomains, comprising in a seaward direction: (1) ini-tially straight, oblique wedges were folded into sig-moidal clinoforms, forming an expulsion rollovercontaining crestal grabens, and initial onlaps wererotated into apparent downlaps by salt evacuation;(2) a rollover syncline; and (3) a progressivelysteepening landward-facing limb of a salt-cored,rolling monocline. These deformation zones ampli-fied and advanced basinward together with theprogradation front. Complete evacuation of salt leftbasinward-propagating salt welds below thewedges; incomplete evacuation left salt pillows as

residual highs; however, no diapirs formed.Over a salt basin with basement steps represent-

ing landward-facing fault scarps, the progradingwedge initially formed structures similar to those inthe model with a flat basement. However, furtherlateral flow of salt was restricted by each basementstep, causing salt to accumulate above the steps asa broad, salt-cored anticline, especially whereprogradation was too fast for salt to escape sea-ward. Aggrading distal sediments pinned the saltanticline by hindering its basinward migration andenhanced salt upwelling by providing additionaldifferential loading. The salt anticline evolved intoan active, and then a passive, diapir. An expulsionrollover formed in the proximal flank of the diapir.This deformation cycle was repeated as thewedges prograded across each basement step, cre-ating a series of salt structures next to major base-ment steps. Again, the deformation zonesadvanced basinward so that the age, amplitude,complexity, and maturity of salt-related structuresdecreased basinward.

Rollover plots, introduced here, provide anobjective way of distinguishing expulsion rolloveranticlines formed by salt expulsion during progra-dation from conventional rollover anticlines in thehanging walls of listric growth faults formed byregional extension. Expulsion rollovers in synkine-matic strata have a plot shaped like an asymmetrichill, whereas fault rollovers above simple listricfaults have plots that abruptly, then gradually,increase away from the surface fault trace. Rolloveranalysis can be applied to model results and to seis-mic sections in both time and depth.

Glacial extrusions from emergent diapirs flowedin any direction, depending on the local slope,



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including landward into the adjoining rollover syn-cline. However, the breakout sites, where saltclimbed stratigraphic section, systematically shiftedbasinward because of progradation, as in the Gulfof Mexico.

During differential loading by new wedges, a saltsheet carried its overburden basinward by gravityspreading as it became evacuated of salt, creating

arcuate peripheral normal faults that soled out onthe rounded trailing edge of salt sheets. Subtlewrench faults in the roof overlay the sides of thesheet. Extension at the sheets trailing edge wascontractionally balanced at its leading edge by sub-tle buckling, active diapirism, contraction of diapir-ic necks, and upward transfer of allochthonous salt.Repetition of burial, sheet evacuation, sheet climb,and sheet spreading created a multitiered complexof salt sheets that migrated seaward.

Consistency of restored geologic sections fromaround the world with model geometries allows usto propose a dif ferent explanation for some saltstructures. In the northern Santos Basin (Brazil),

the 25-km-wide missing section, known as theCabo Frio fault zone, is reinterpreted to resultalmost entirely from progradation rather than fromregional extension. The Gulf of Mexico Basin, too,contains an excellent example of a rolling deforma-tion front pushed by progradation; a diapir wastriggered by regional extension of uniformly thinstrata over the distal salt plateau. Some of theLouisiana-style seaward-dipping, listric growth faultsystems are interpreted to sole out on the landwardmargins of arcuate, buried salt extrusions thatbecame largely evacuated by progradation. In theZechstein Basin (northern Germany), classical fea-tures such as diapiric families and peripheral sinksare reinterpreted as the result of progradationsqueezing salt laterally.

REFERENCES CITED

Baars, D. L., 1966, Pre-Pennsylvanian paleotectonicskey to basinevolution and petroleum occurrences in Paradox basin, Utahand Colorado: AAPG Bulletin, v. 50, p. 20822111.

Bloom, A. L., 1991, Geomorphology: a systematic analysis of lateCenozoic landforms (2d ed.): Englewood Cliffs, New Jersey,Prentice-Hall, 532 p.

Bruce, C. H., 1983, Shale tectonics, Texas coastal area growthfaults, in A. W. Bally, ed., Seismic expression of structuralstyles: AAPG Studies in Geology Series 15, v. 2, p. 2.3.1/12.3.1/6.

Byerlee, J. D., 1978, Friction of rocks: Pure and AppliedGeophysics, v. 116, p. 615626.

Christensen, A. F., 1983, An example of a major syndepositionallistric fault, inA. W. Bally, ed., Seismic expression of structuralstyles: AAPG Studies in Geology Series 15, v. 2, p. 2.3.1/362.3.1/40.

Cobbold, P. R., P. Szatmari, L. S. Demercian, D. Coelho, and E. A.Rossello, 1995, Seismic experimental evidence for thin-skinnedhorizontal shortening by convergent radial gliding on evapo-rites, deep-water Santos Basin, in M. P. A. Jackson, R. G.

Roberts, and S. Snelson, eds., Salt tectonics: a global perstive: AAPG Memoir 65, p. 305321.

Crans, W., G. Mandl, and J. Haremboure, 1980, On the theorgrowth faulting: a geomechanical delta model based on grsliding: Journal of Petroleum Geology, v. 2, p. 265307.

Dailly, G. C., 1976, A possible mechanism relating progradagrowth faulting, clay diapirism and overthrusting in a regsive sequence of sediments: Canadian Petroleum GeoBulletin, v. 24, p. 92116.

Demercian, S., P. Szatmari, and P. R. Cobbold, 1993, Style and

tern of salt diapirs due to thin-skinned gravitational glidCampos and Santos basins, offshore Brazil: Tectonophy

v. 228, p. 393433.Diegel, F. A., J. F. Karlo, D. C. Schuster, R. C. Shoup, and P

Tauvers, 1993, Cenozoic structural evolution and tectonosgraphic framework of the northern Gulf Coast continental gin (abs.): AAPG Annual Convention, Program with Abstr

v. 2, p. 91.Diegel, F. A., J. F. Karlo, D. C. Schuster, R. C. Shoup, and P

Tauvers, 1995, Cenozoic structural evolution and tectstratigraphic framework of the northern Gulf Coast continemargin, in M. P. A. Jackson, R. G. Roberts, and S. Snelson, Salt tectonics: a global perspective: AAPG Memoirp. 109151.

Duval, B., C. Cramez, and M. P. A. Jackson, 1992, Raft tectonithe Kwanza basin, Angola: Marine and Petroleum Geol

v. 9, p. 389404.

Frye, J. C., 1971, The Ogallala Formationa review, in Ogaaquifer symposium: Texas Technology University InternatiCenter for Arid and Semi-Arid Land Studies Special Reporp. 514a.

Galloway, W. E., 1989, Genetic stratigraphic sequences in banalysis II: application to northwest Gulf of Mexico Cenobasin: AAPG Bulletin, v. 73, p. 143154.

Gaullier, V., J. P. Brun, G. Gurin, and H. Lecanu, 1993, Rafttonics: the effects of residual topography below a salt dcment: Tectonophysics, v. 228, p. 363381.

Ge, H., 1996, Kinematics and dynamics of salt tectonics inParadox basin, Utah and Colorado: field observationsscaled modeling: Ph.D. dissertation, University of Tex

Austin, Austin, Texas, 317 p.Ge, H., M. P. A. Jackson, and B. C. Vendeville, 1994, Experim

initiation of salt structures and salt sheet emplacement by grading sediment wedges (abs.): Geological Society of LonSalt Tectonics Meeting, September 1994, Programme

Abstracts, p. 5152.Hubbert, M. K., 1937, Theory of scale models as applied to

study of geologic structures: Geological Society of AmeBulletin, v. 48, p. 14591520.

Humphris, C. C., Jr., 1979, Salt movement on continental slnorthern Gulf of Mexico: AAPG Bulletin, v. 63, p. 782798

Jackson, J., and D. McKenzie, 1983, The geometric evolutionormal fault systems: Journal of Structural Geology, vp. 471482.

Jackson, M. P. A., and R. R. Cornelius, 1987, Stepwise centrimodeling of the effects of differential sediment loading ondeformation of salt structures, in I. Lerche and J. J. OBreds., Dynamical geology of salt and related structures: OrlaFlorida, Academic Press, p. 163259.

Jackson, M. P. A., and C. Cramez, 1989, Seismic recognition owelds in salt tectonics regimes, in Gulf of Mexico salt tecics, associated processes and exploration potential: Gulf CSEPM Foundation Tenth Annual Research Conference Progand Abstracts, p. 6671.

Jackson, M. P. A., and C. J. Talbot, 1986, External shapes, strates, and dynamics of salt structures: Geological Societ

America Bulletin, v. 97, p. 305323.Jackson, M. P. A., and C. J. Talbot, 1989, Salt canopies, in Gu

Mexico salt tectonics, associated processes and explorapotential: Gulf Coast Section of SEPM Foundation Tenth AnResearch Conference Program and Abstracts, p. 7278.

Ge et al.


25/26

Jackson, M. P. A., and C. J. Talbot, 1991, A glossary of salt tecton-ics: University of Texas at Austin, Bureau of Economic GeologyGeologic Circular 91-4, 44 p.

Jackson, M. P. A., and B. C. Vendeville, 1994, Regional extensionas a geologic trigger for diapirism: Geological Society of

America Bulletin, v. 106, p. 5773.Krantz, R. W., 1991, Measurements of friction coefficients and

cohesion for faulting and fault reactivation in laboratory mod-els using sand and sand mixtures: Tectonophysics, v. 188,p. 203207.

Larberg, G. M. B., 1983, Contra-regional faulting: salt withdrawalcompensation, offshore Louisiana, Gulf of Mexico, inA. W.Bally, ed., Seismic expression of structural styles: AAPG Studiesin Geology Series 15, v. 2, p. 2.3.2/422.3.2/44.

Lundin, E. R., 1992, Thin-skinned extensional tectonics on a saltdetachment, northern Kwanza basin, Angola: Marine andPetroleum Geology, v. 9, p. 405411.

Mandl, G., 1988, Mechanics of tectonic faulting: models and basicconcepts: Amsterdam, Elsevier, 407 p.

McClay, K. R., 1989, Physical models of structural styles duringextension, inA. J. Tankard and H. R. Balkwill, eds., Extensionaltectonics and stratigraphy of the north Atlantic margins: AAPGMemoir 46, p. 95110.

McClay, K. R., 1990, Extensional fault systems in sedimentarybasins: a review of analogue studies: Marine and PetroleumGeology, p. 206233.

McKee, E. D., and M. Goldberg, 1969, Experiments on formation

of contorted structures in mud: Geological Society of AmericaBulletin, v. 80, p. 231244.

Mohriak, W. U., 1995, Salt tectonics structural styles: contrasts andsimilarities between the south Atlantic and the Gulf of Mexico,in C. J. Travis, B. C. Vendeville, H. Harrison, F. J. Peel, M. R.Hudec, and B. F. Perkins, eds., Salt, sediment and hydrocar-bons: Gulf Coast Section of SEPM Foundation Sixteenth AnnualResearch Conference, p. 177191.

Mohriak, W. U., et al., 1995, Salt tectonics and structural styles inthe deep-water province of the Cabo Frio region, Rio de

Janeiro, Brazil, in M. P. A. Jackson, R. G. Roberts, an d S.Snelson, eds., Salt tectonics: a global perspective: AAPGMemoir 65, p. 273304.

Nelson, T. H., 1989, Style of salt diapirs as a function of the stageof evolution and nature of the encasing sediments, in Gulf ofMexico salt tectonics, associated processes and explorationpotential: Gulf Coast Section of SEPM Foundation Tenth AnnualResearch Conference, p. 109110.

Nelson, T. H., 1991, Salt tectonics and listric-normal faulting, inA.Salvador, ed., The Gulf of Mexico Basin: Geological Society of

America, The Geology of North America, v. J, p. 7389.Peel, F. J., C. J. Travis, and J. R. Hossack, 1995, Genetic structural

provinces and salt tectonics of the Cenozoic offshore U.S. Gulfof Mexico: a preliminary analysis, in M. P. A. Jackson, R. G.Roberts, and S. Snelson, eds., Salt tectonics: a global perspec-tive: AAPG Memoir 65, p. 153175.

Ramberg, H., 1981, Gravity, deformation and Earths crust in theo-ry, experiments and geological application (2d ed.): London,

Academic Press, 452 p.Rettger, R. E., 1935, Experiments on soft-rock deformation: AAPG

Bulletin, v. 19, p. 271292.Robison, B. A., 1983, Low-angle normal faulting, Marysriver

Valley, Nevada, in A. W. Bally, ed., Seismic expression ofstructural styles: AAPG Studies in Geology Series 15, v. 2,p. 2.2.2/122.2.2/16.

Sannemann, D., 1968, Salt-stock families in northwesternGermany, inJ. Braunstein and G. D. OBrien, eds., Diapirismand diapirs: AAPG Memoir 8, p. 261270.

Schultz-Ela, D. D., 1992, Restoration of cross-sections to constraindeformation processes of extensional terranes: Marine andPetroleum Geology, v. 9, p. 372388.

Schultz-Ela, D. D., and K. Duncan, 1994, Users manual and soft-ware for Restore (version 3.04): Bureau of EconomicGeology, University of Texas at Austin, Austin, Texas, 97 p.

Schultz-Ela, D. D., M. P. A. Jackson, and B. C. Vendeville, 1993,Mechanics of active salt diapirism: Tectonophysics, v. 228,p. 275312.

Schuster, D. C., 1993, Deformation of allochthonous salt and evo-lution of related structural systems, eastern Louisiana GulfCoast (abs.): AAPG Annual Convention, Official Program, v. 2,p. 179.

Schuster, D. C., 1995, Deformation of allochthonous salt and evo-lution of related salt-structural systems, eastern Louisiana GulfCoast, in M. P. A. Jackson, R. G. Roberts, and S. Snelson, eds.,

Salt tectonics: a global perspective: AAPG Memoir 65,p. 177198.

Seni, S. J., and M. P. A. Jackson, 1983a, Evolution of salt structures,East Texas diapir province, part 1: sedimentary record ofhalokinesis: AAPG Bulletin, v. 67, p. 12191244.

Seni, S. J., and M. P. A. Jackson, 1983b, Evolution of salt structures,East Texas diapir province, part 2: patterns and rates of haloki-nesis: AAPG Bulletin, v. 67, p. 12451274.

Szatmari, P., M. C. M. Guerra, and M. A. Pequeno, 1996, Genesis oflarge counter-regional normal fault by flow of Cretaceous saltin the South Atlantic Santos Basin, Brazil, in G. I. Alsop, D. J.Blundell, and I. Davison, eds., Salt tectonics: Geological Societyof London Special Publication 100, p. 259264.

Talbot, C. J., 1992, Centrifuged models of Gulf of Mexico profiles:Marine and Petroleum Geology, v. 9, p. 412432.

Trusheim, F., 1960, Mechanism of salt migration in northernGermany: AAPG Bulletin, v. 44, p. 10681093.

van Keken, P. E., C. J. Spiers, A. P. Van Den Berg, and E. J. Muyzert,1993, The effective viscosity of rocksalt: implementation ofsteady state creep laws in numerical models of salt diapirism:Tectonophysics, v. 225, p. 457475.

Vendeville, B. C., 1991, Mechanisms generating normal fault cur-vature: a review illustrated by physical models, in A. M.Yielding and B. Freeman, eds., The geometry of normal faults:Geological Society of London Special Publication 56,p. 241249.

Vendeville, B. C., and M. P. A. Jackson, 1992, The rise of diapirsduring thin-skinned extension: Marine and Petroleum Geology,

v. 9, p. 331353.Vendeville, B. C., and M. P. A. Jackson, 1993, Some dogmas in salt

tectonics challenged by modeling (abs.): AAPG HedbergInternational Research Conference on Salt Tectonics, Bath,England, September 1993, p. 263265.

Vendeville, B. C., P. R. Cobbold, P. Davy, J. P. Brun, and P.Choukroune, 1987, Physical models of extensional tectonics at

various scales, in M. P. Coward, J. F. Dewey, and P. L.Hancock, eds., Continental extensional tectonics: GeologicalSociety of London Special Publication 28, p. 95107.

Vendeville, B. C., H. Ge, and M. P. A. Jackson, 1994, Experimentaldeformation of prograding sedimentary wedges above a vis-cous source layer (abs.): AAPG Annual Convention, OfficialProgram, v. 3, p. 276.

Weijermars, R., 1986, Flow behaviour and physical chemistry ofbouncing putties and related polymers in view of tectonic labo-ratory applications: Tectonophysics, v. 124, p. 325358.

Weijermars, R., M. P. A. Jackson, and B. C. Vendeville, 1993,Rheological and tectonic modeling of salt provinces: Tectono-physics, v. 217, p. 143174.

Winker, C. D., 1982, Cenozoic shelf margins, northwestern Gulf ofMexico basin: Gulf Coast Association of Geological SocietiesTransactions, v. 32, p. 427448.

Woodbury, H. O., I. B. Murray, Jr., and R. E. Osborne, 1980,Diapirs and their relation to hydrocarbon accumulation, in

A. D. Miall, ed., Facts and principles of world petroleum occur-rence: Canadian Society of Petroleum Geologists, p. 119142.

Worrall, D. M., and S. Snelson, 1989, Evolution of the northernGulf of Mexico, with emphasis on Cenozoic growth faultingand the role of salt, inA. W. Bally and A. R. Palmer, eds., TheGeology of North Americaan overview: Boulder, Colorado,Geological Society of America, v. A, p. 97138.

Wu, S., 1993, Salt and slope tectonics, offshore Louisiana: Ph.D.



26/26

thesis, Rice University, Houston, Texas, 251 p.Wu, S., A. W. Bally, and C. Cramez, 1990, Allochthonous salt struc-

ture and stratigraphy of the northeastern Gulf of Mexico, II: structure: Marine and Petroleum Geology, v. 7, p. 3433

Ge et al.

Hongxing Ge

Hongxing Ge received his B.S.degree from Nanjing University,Peoples Republic of China, in1985, and his M. S. degree fromColorado State University in 1990.He completed his Ph.D. at theUniversity of Texas at Austin in1996. His dissertation on Paradoxbasin salt tectonics was nominatedfor the Outstanding Dissertation

Award. He is currently a postdoc-toral fellow at the Bureau of Economic Geology. Hisinterests include salt tectonics, tectonic modeling, strainanalysis, and petroleum geology.

Martin Jackson

Martin Jacksons early careerincluded lunar structures, mineralexploration, and Precambrian geol-ogy. He received his Ph.D. fromthe University of Cape Town in1976, and joined the Bureau ofEconomic Geology in 1980, wherehe directs the Applied GeodynamicsLaboratory funded by a consortiumof oil companies. A recipient of

AAPGs Sproule Award (with S. J.Seni) and Matson Award, he lectured in AAPGsStructural Geology School, was an AAPG Distinguished

Lecturer, and served 6 years as associate editor for theAAPG Bulletin and GSA Bulletin.

Bruno C. Vendeville

Bruno C. Vendeville receivedhis Ph.D. from the Universit deRennes, France, in 1987. He spe-cializes in experimental modelingof tectonic processes with empha-sis on gravity tectonics, salt tecton-ics, and extensional tectonics. He

was the co-recipient of two honor-able mentions from the SEPM/AAPG(1993) and AAPG (1990) withM. P. A. Jackson. He is currently aresearch scientist at the Bureau of Economic GeoloUniversity of Texas at Austin.

ABOUT THE AUTHORS

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