structural segmentation, inversion, and salt tectonics on a passive margin: evolution of the inner...

For permission to copy, contact [email protected] 2002 Geological Society of America1222

GSA Bulletin; October 2002; v. 114; no. 10; p. 1222–1244; 17 figures; 1 table.

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

Michael R. Hudec*Martin P.A. JacksonBureau of Economic Geology, University of Texas, Box X, University Station, Austin, Texas 78713-8924, USA

ABSTRACT

The Kwanza Basin, Angola, is dividedinto the Inner and Outer Kwanza salt ba-sins, separated by a chain of synrift base-ment highs on which Aptian (112–122 Ma)salt is thin or absent. North- to northwest-trending basement structures in the InnerKwanza Basin have repeatedly been reacti-vated since Neocomian (144–127 Ma) rifting.Reactivation formed three northwest-strikingfold-and-thrust belts near basement uplifts.The thrust belts are bounded by northeast-striking rift-related transfer-fault zonesthat were apparently reactivated duringsubsequent shortening. Three episodes ofpostrift, basement-involved shortening aredocumented in the Inner Kwanza Basin: (1)Albian–early Cenomanian (112–96 Ma), (2)Senonian (89–65 Ma), and (3) Oligocene–Holocene (34–0 Ma). We relate the Albian–early Cenomanian event to ridge push, theSenonian event to global-plate reorganiza-tion, and Oligocene–Holocene shortening touplift of the African superswell.

Structural segmentation of the InnerKwanza Basin controlled the evolution ofsalt structures. Adjacent to basement up-lifts, diapirs were initiated as buckle folds.Some anticlines were unroofed by erosionand evolved into passive salt walls. Else-where, broad salt walls were triggered byeither detached extension or basement-block uplift. These walls grew until they ex-hausted their supply of salt. Thereafter, dis-solution rates exceeded rates of salt inflow,so the walls began to subside. Withdrawalof salt from the walls produced the elongatesedimentary troughs for which the basin isfamous. Trough fill ranges in age from Cen-omanian to Pliocene, and this age varies

*E-mail: [email protected].

greatly from trough to trough and alongstrike within troughs.

Keywords: Angola, basement tectonics,Kwanza Basin, passive margins, salt tecton-ics, segmentation.

INTRODUCTION

Deformation of Passive Margins

As the name implies, passive margins havelong been considered tectonically inert sites ofsediment accumulation and progressive sub-sidence. Although the cover may be involvedin complex gravity-driven deformation (e.g.,Bryant et al., 1968; Humphris, 1978; Worralland Snelson, 1989), basement is normallymodeled as steadily subsiding in response tothermal contraction and sediment loading(e.g., Steckler and Watts, 1978; Watts et al.,1982; Steckler et al., 1988; Lavier et al.,2000).

An increasing number of exceptions to thisorthodoxy have been reported, however. Forexample, basement-involved shortening anduplift have affected the divergent continentalmargins of Norway (Blystad et al., 1995; Doreand Lundin, 1996), the British Isles (Roberts,1989; Boldreel and Andersen, 1993), easternCanada (Sinclair, 1995; Withjack et al., 1995),the southeastern United States (Behrendt etal., 1983; Talwani, 1990; Weems and Lewis,2002), Brazil (Cobbold et al., 2001), west-centralAfrica (Teisserenc and Villemin, 1989; Dailly,2000), Morocco (Laville and Pique, 1992),and northwest Australia (Karner and Driscoll,1999a). Some authors have attributed passive-margin shortening to transmission of stressfrom distant collisions or subduction zones(e.g., Roberts, 1989; Ziegler, 1989; Cobboldet al., 2001). Others have suggested that thistype of deformation need not be tied to con-

vergent plate boundaries (e.g., Withjack et al.,1995; Dore and Lundin, 1996; Dailly, 2000).

Recognition of widespread inversion is asignificant advance in our understanding ofpassive-margin processes, but a number of un-answered questions remains. What controlsthe timing of inversion? What are the mech-anisms? Why do some passive margins short-en but others do not? Why is inversion typi-cally variable along a single margin?

We report the results of an investigationinto the geologic evolution of part of theKwanza Basin, Angola (Fig. 1). We argue thatthe Kwanza Basin has undergone three epi-sodes of basement-involved shortening sincerifting. Deformation appears to have been con-trolled by reactivation of zones of transferfaults, hereafter called ‘‘transfer zones’’ thatformed during Gondwana breakup and initialopening of the South Atlantic during the EarlyCretaceous.

The Kwanza Basin

The Kwanza Basin formed during Neocom-ian rifting of the South Atlantic margin of Af-rica, which began in the Kwanza region at ca.144–140 Ma and proceeded to continentalseparation by 127–117 Ma (Fig. 2; Brice etal., 1982; Teisserenc and Villemin, 1989; Gui-raud and Maurin, 1992; Karner and Driscoll,1998). Synrift argillaceous sandstones of theLower (Red) Cuvo Formation are separatedfrom the better-sorted sandstones of the Upper(Gray) Cuvo Formation by a breakup uncon-formity. During the late Aptian, shortly aftercontinental separation, restricted marine con-ditions led to the precipitation of evaporites inthe Kwanza Basin, synchronous with evapo-rite deposition in other basins on both sides ofthe South Atlantic (Evans, 1978). Oceanic cir-culation increased at the end of salt deposi-tion, leading to the establishment of the Pindacarbonate ramp, which persisted throughout

Geological Society of America Bulletin, October 2002 1223

EVOLUTION OF THE INNER KWANZA BASIN, ANGOLA

Figure 1. Regional tectonic setting of the Kwanza Basin, Angola.

the Albian. The Kwanza Basin has been dom-inated by clastic deposition since Cenomaniantime.

Salt-related structural styles separate theKwanza Basin into three parts (Fig. 3). TheInner and Outer Kwanza Basins originallycontained thick (locally .1 km) evaporitesuccessions. The two basins were separatedfrom one another by the Flamingo, Ametista,and Benguela Platforms, a series of basementhighs on which salt was thin or absent. Thesehighs are roughly coincident with positiveanomalies on free-air gravity (Sandwell andSmith, 1997) and proprietary magnetic maps.In the gap between the Ametista and BenguelaPlatforms, the boundary between Inner andOuter Kwanza Basins is indistinct.

Exploration in the Outer Kwanza Basin hasaccelerated greatly in the past 15 yr, yieldingtens of thousands of kilometers of new two-and three-dimensional seismic data. Analysis

of these data has led to major advances in un-derstanding of the basin (e.g., Spathopoulos,1996; Henry and Abreu, 1998; Peel et al.,1998; Uncini et al., 1998; Cramez and Jack-son, 2000; Danforth et al., 2000; Goldhammeret al., 2000; Marton et al., 2000; Hudec et al.,2001; Jackson et al., 2001). Conversely, inter-est in the Inner Kwanza has dwindled—a leg-acy of ongoing civil war and declining re-serves. Most newer studies of the InnerKwanza Basin are based on outcrop data (e.g.,Marzoli et al., 1999; Castellano et al., 2000;Ciampo et al., 2000; Duarte-Morais andSgrosso, 2000; Morais et al., 2000) or on lim-ited subsurface information (e.g., Duval et al.,1992; Lunde et al., 1992; Lundin, 1992; Bur-wood, 1999). No syntheses of the InnerKwanza Basin have been published since thelandmark papers of Brognon and Verrier(1966) and Masson (1972).

Data Sources

Because few new subsurface data have beenacquired in the Inner Kwanza Basin since1972, much of our work involved compilationand reinterpretation of existing data. Our mostimportant resource was the surface geologicmap (Total-Sonangol, 1987), based on map-ping completed in 1972. Digital topographywas from the GLOBE Project (Hastings et al.,1999). Subsurface data in the form of crosssections, structure maps, isochore maps, andfacies maps were extracted from Brognon andVerrier (1966), Masson (1972), Verrier andCastello Branco (1972), and Duval et al.(1992). Additional proprietary maps and sec-tions came from internal company reports sup-plied by Shell, Texaco, TotalFinaElf, and BHP.TotalFinaElf provided several proprietary wellreports, and Shell supplied additional welldata. WesternGeco made available several

1224 Geological Society of America Bulletin, October 2002

HUDEC and JACKSON

Figure 2. Chart showing Inner Kwanza Basin stratigraphy and timing of tectonic, thermal, and deformational events. Timing of thermalevents compiled from Cahen et al. (1984) and Walgenwitz et al. (1990). Timing of deformation in the Gonga fold belt from Goldhammeret al. (2000).



Figure 3. Basin configuration during Aptian salt deposition. The Flamingo, Ametista, andBenguela Platforms were high blocks that separated the Inner and Outer Kwanza Basins.Numbered polygons denote offshore exploration blocks.

seismic lines across the present continentalshelf. Free-air gravity data on the continentalshelf are from Sandwell and Smith (1997) asprocessed by EDCON, Inc., Bouguer gravitydata from Karner and Driscoll (1999b), andmagnetic data from a proprietary industrysource. Locations of well, seismic, and cross-section control are shown in Figure 4. Datacontrol was densest in the northern part of theInner Kwanza Basin and sparse in the south.

SEGMENTATION OF THE INNERKWANZA BASIN

Maps of the Kwanza Basin show right-handjogs in the coastline at Ponta das Palmerinhas,Cabo Ledo, Cabo do Sao Braz, Cabo das TresPontas, and Cabo do Morro (Fig. 5). Thesharpness, consistency in spacing, and senseof offset of the jogs suggest structural control.

Previous workers have interpreted northeast-striking fault zones offsetting the coastline(e.g., Instituto Nacional de Geologia, 1988;Duval et al., 1992; Spathopoulos, 1996; Gold-hammer et al., 2000; Marton et al., 2000). Weconcur with this interpretation, although ourfault geometries differ in detail from previ-ously published versions (Fig. 5). The onshoreposition of the faults is based largely on sur-face geology; the offshore is based on linea-ments in gravity (Sandwell and Smith, 1997;Karner and Driscoll, 1999b), magnetics (pro-prietary data), and base-salt structure (Figs. 6,7).

Several lines of evidence suggest that thesefaults involve basement. First, the jog at Cabodo Morro coincides with a flip in the structuralpolarity of the synrift normal-fault system (Je-ronimo et al., 1998). Second, gravity maps ofthe Kwanza continental shelf (Sandwell and

Smith, 1997; Karner and Driscoll, 1999b)show northeast-striking lineaments extendingseaward from the jogs in the coastline. Third,several of the lineaments are coincident withmonoclinal ramps in the base of salt (Fig. 3).Finally, seismic lines typically show abruptthickening of presalt sedimentary rocks (Low-er Cuvo Formation and Upper Cuvo Forma-tion, Fig. 2) across the lineaments.

On the basis of this evidence, we suggestthat the faults originated as rift-related transferfaults during Late Jurassic–Early Cretaceouscontinental breakup. Their strikes vary but aregenerally subparallel to the inferred directionof rifting of the South Atlantic margin. Similarstructures have been interpreted on the con-jugate Brazilian margin (Meisling et al.,2001).

The geologic map of the Inner Kwanza Ba-sin (Fig. 5) shows the influence of the transferzones on surface geology, especially theabrupt changes in geologic units exposed (Ta-ble 1). These changes die out toward the east-ern edge of the basin, where a west-dippingband of Cretaceous rocks extends continuous-ly along the entire basin margin. The conti-nuity of these units across province boundar-ies and the lack of northeast trends inbasement rocks immediately to the east of theKwanza Basin (Reis, 1972; Rodrigues, 1972;de Boorder, 1982; de Carvalho, 1982) suggestthat the transfer zones die out landward withinthe Inner Kwanza Basin.

MAJOR STRUCTURES OF THE INNERKWANZA BASIN

Basement Uplifts

Data quality in the Inner Kwanza Basin pre-cludes detailed mapping of the top of crystal-line basement, but first-order features can beinterpreted from structure maps contoured onthe base of Aptian salt (Figs. 6, 7). The InnerKwanza Basin contains four major upliftsbounded by transfer zones on at least one side.The presence of large salt-related structures ontop of these uplifts distinguishes them fromthe thin-salt Flamingo, Ametista, and Bengue-la Platforms that separate the Inner and OuterKwanza Basins.

Subsalt units have been penetrated by wellson three of the four uplifts (Brognon and Ver-rier, 1966, their Fig. 7). The Cacuaco, CaboLedo, and Morro Liso uplifts are mostly de-void of Cuvo sediments, and salt lies directlyon either Precambrian crystalline rocks or un-dated volcanic rocks. These volcanic rockscould have been basement to the Cuvo orcould constitute a lateral equivalent of the


HUDEC and JACKSON

Figure 4. Subsurface database used in this paper.

Cuvo sedimentary rocks. We favor the formerinterpretation because offshore wells have en-countered Upper or Lower Cuvo resting onthick volcanic sections. The subcrop of vol-canic rocks below salt is thus interpreted as aproduct of synrift uplift and nondeposition orerosion of Cuvo rocks. Consistent with thisinterpretation, Brognon and Verrier (1966) re-ported coarse sandstone and conglomerate insubsalt sections near the uplifts, which mayrepresent fanglomerates shed from emergenthighs.

The presence of salt-related structures atopthe basement blocks indicates that theyformed lows during salt deposition, but upliftappears to have recommenced shortly there-after. The best evidence for this comes fromthe Cabo Ledo uplift, where Brognon and Ver-rier’s (1966) detailed facies and isopach mapsshow consistent thinning of upper Aptian–Albian units onto the basement high. A sec-ond period of reactivation occurred in the Ter-

tiary, documented by thinning of Oligocene–Miocene strata onto the Cabo Ledo uplift fromthe seaward side (Fig. 8). The present-daycoastline is marked by 100-m-tall sea cliffscontaining Pliocene marine strata (Morais etal., 2000), suggesting that the second phase ofuplift continued at least through the Pliocene.The sheerest sea cliffs are present above theCacuaco uplift.

The total magnitude of block uplift is de-fined by the observation that crests of theCabo Ledo, Morro Liso, and Cacuaco upliftsall lie 600–1000 m above the platforms thatseparated the Inner and Outer Kwanza Basins(Figs. 6, 7B). The platforms were higher thanthe Inner Kwanza Basin during salt deposi-tion, so this relief provides a minimum esti-mate of total uplift.

Sedimentary Troughs

The postsalt succession in the Inner Kwan-za Basin includes five elongate fault-bounded

sedimentary basins, 10–25 km wide (Fig. 9).These troughs are probably the best-knowngeologic features of the Inner Kwanza Basin(e.g., Masson, 1972; Verrier and CastelloBranco, 1972; Burollet, 1975; Duval et al.,1992; Lundin, 1992). Albian strata are missingfrom the bases of the troughs. The troughs arecored by thicker-than-normal stratigraphic se-quences, many of which now form turtle an-ticlines (Figs. 10, 11). Previous workers (e.g.,Duval et al., 1992; Lundin, 1992) contendedthat trough fill is exclusively Tertiary, but welldata indicate that some troughs began formingas early as the Cenomanian (Figs. 10, 12).

The map pattern of sedimentary troughs inthe Inner Kwanza Basin is related to bothtransfer zones and basement uplifts (Figs. 7C,9). All of the troughs have at least one bound-ary defined by transfer zones, and two troughs(Muxate and Sangano) trend along transferzones. Transfer zones also separate provincesin which sedimentary troughs are present(Quenguela, Kissama, and Morro Liso prov-inces; Figs. 9–11) from provinces in whichtroughs are absent (Muxima and Porto Am-boim; Fig. 8). Many of the troughs overliesubsalt highs for at least part of their length(Fig. 7C): The Morro Liso Trough overlies theMorro Liso uplift, the Funda Trough coversthe Cacuaco uplift, and the Quenguela Troughoverlies a small, unnamed ridge in the base ofsalt. Other troughs flank basement uplifts(e.g., Sangano, Muxate, and Praia Troughs).

Contractional Fold Belts

Most previous workers have interpretedstructures in the Inner Kwanza Basin as theresult of some combination of extension andhalokinesis (e.g., Brognon and Verrier, 1966;Masson, 1972). More recently, Duval et al.(1992) and Cramez et al. (2000) interpretedseveral structures along the coast to be relatedto shortening. We expand on this interpreta-tion and recognize three fold belts in the InnerKwanza Basin: the Coastal, East Kwanza, andGonga fold belts (Fig. 9). We describe the foldbelts, justify a contractional interpretation,give evidence of their timing, and present atectonic mechanism.

Coastal Fold BeltThe Coastal fold belt comprises elongated

folds, both onshore and offshore in Muximaprovince (Fig. 9). Cross sections (Brognonand Verrier, 1966; Masson, 1972) indicate thatthe folds are cored by salt, mostly as pierce-ment diapirs but also as concordant salt anti-clines (Fig. 8). Many of the salt structures arelocalized above steps in the base of salt. Col-



Figure 5. Geologic map of the Inner Kwanza Basin, Angola, showing approximate location of rift-related transfer zones (red). Blacklines show the location of cross sections A–A9 to D–D9. Black rectangle shows the location of Figure 13. Surface geology simplified fromTotal-Sonangol (1987).


HUDEC and JACKSON

Figure 6. Structure map contoured on the base salt in the Inner Kwanza Basin. Shaded areas are basement uplifts.

lapse grabens have formed over the crests ofsome of the folds (e.g., Pitchi-Tuenza, Fig. 8).Total shortening in the Coastal fold belt is;4.4 km (6%). Most anticlines in the Coastalfold belt have been drilled, resulting in minor

oil discoveries at Galinda and Tobias fields(Fig. 9).

The Coastal fold belt is ;60 km long by60 km across. The fold belt terminates abrupt-ly against the Cabo Ledo transfer zone in the

north and more diffusely against the Sao Braztransfer zone in the south. Individual foldstrend northwest, slightly oblique to the coast.Fold culminations are concentrated near trans-fer zones (Fig. 9).



Figure 7. Views of the Inner Kwanza Basin. All images generated with EarthVision. (A) Perspective view north-northeast of top GrayCuvo Formation (base salt) surface. Vertical exaggeration 310; colors are elevation contours, ranging from 23600 m (purple) to 1500m (red). Basement highs are labeled; transfer zones form northeast-trending lineaments and steps. (B) Perspective view northeast ofbase-salt surface draped by presalt structural elements. Vertical exaggeration 310. White line is the projected coastline; red lines aretransfer zones. Yellow pattern shows areas of zero Cuvo thickness, where salt was deposited directly on basement. Blue pattern showsthe location of basement platforms. (C) Orthographic projection of base-salt surface draped by postsalt structural elements. White lineis the projected coastline; red lines are transfer zones. Purple, green, and yellow patterns show outlines of anticlines in the East Kwanza,Coastal, and Gonga fold belts, respectively. Orange pattern shows Tertiary troughs. (D) Perspective view east at topography in theKwanza Basin region, generated from GLOBE project data (;1 km spacing, Hastings et al., 1999). Vertical exaggeration 315. TheMalange uplift is an eroded transpressional horst of Archean rocks. Colors are elevation, ranging from 0 m (purple) to 16001 m (red).White line shows the coast; orange line marks the east edge of the Inner Kwanza Basin; green lines delineate anticlinal traces in theEast Kwanza fold belt.


HUDEC and JACKSON

TABLE 1. INFLUENCE OF SEGMENTATION IN THE INNER KWANZA BASIN

Province

Quenguela Muxima Kissama Morro Liso Porto Amboim

Level of surface exposure Oligocene-Miocene Eocene-Cretaceous Oligocene-Eocene Oligocene-Eocene CretaceousSedimentary troughs Yes No Yes Yes NoFold belts along coast No Coastal fold belt No No Gonga fold beltEast Kwanza fold belt No Mild Moderate Moderate IntenseBasement uplifts Caucuaco Cabo Ledo None Morro Liso Pequena

Duval et al. (1992) and Cramez et al. (2000)interpreted structures in the Coastal fold beltto be related to horizontal shortening. We rec-ognize four lines of evidence to support thisinterpretation: First, the flanks of several an-ticlines are offset by reverse faults (e.g., theTobias anticline, Brognon and Verrier, 1966).Second, diapirs are narrow in cross sectionand appear squeezed (Fig. 8). Third, all of thesalt-cored folds deform thick (up to 2200 m)prekinematic roofs, a geometry that is diag-nostic of shortening above salt (Vendevilleand Nilsen, 1995). Finally, small thrusts havebeen mapped on the surface and in boreholesin the area around Cabo Ledo (C. Cramez,2001, personal commun.).

Published seismic lines and seismic-basedcross sections across the Coastal fold belt(Fig. 9; Masson, 1972; Duval et al., 1993)show evidence for several phases of defor-mation (Fig. 2). First, Albian strata thin overthe Pitchi anticline, suggesting an Early Cre-taceous initiation of some of the folding. Sec-ond, all of the diapirs near the seaward end ofthe fold belt appear to have overhangs nearthe top of the Cretaceous, suggesting activesalt extrusion at that time. A common causeof salt extrusion is lateral shortening of thefeeder, so the extrusion may record a secondphase of shortening during the Senonian. Sen-onian shortening could also explain a phase ofLate Cretaceous uplift documented in the areaby apatite fission-track analysis (Harris et al.,2002). Third, Oligocene–Holocene strata thinoffshore over the crests of folds.

East Kwanza Fold BeltTo our knowledge, the East Kwanza fold

belt has not been recognized previously. Itruns parallel to the eastern margin of the InnerKwanza Basin, near the contact with pre-Aptian basement (Fig. 9). Our data in the EastKwanza fold belt are restricted to a geologicmap (Total-Sonangol, 1987) and two crosssections through the western half of the belt(Fig. 4; Masson, 1972). Our conclusions arethus more speculative than those for the better-known Coastal fold belt.

The East Kwanza fold belt terminates in thenorth against the Cabo Ledo transfer zone and

extends to the southern end of the InnerKwanza Basin. Fold intensity in the EastKwanza fold belt decreases northward fromPorto Amboim province, which is intenselyfolded, to Quenguela province, which is notfolded. Most of the anticlines adjoin faults.Folds are elongate and have wavelengths of2–10 km and strike lengths of 5–100 km.Cross sections in Masson (1972) indicate thatsome of the folds near the western edge of theEast Kwanza fold belt are cored by salt, asshown in Figure 8. The eastern part of the foldbelt, however, lies beyond the evaporite basin.Total shortening across the East Kwanza foldbelt in Muxima province is estimated at 1.5km (2%).

We interpret the East Kwanza fold belt ascontractional for two reasons. First, down-plunge viewing suggests that several folds arein the hanging walls of thrust faults (Fig. 13).Second, Masson’s (1972) cross sections acrossthe western part of the fold belt in Muximaand Porto Amboim provinces show that thesalt-cored folds deform a thick (at least 2300m), prekinematic roof, a geometry diagnosticof shortening, according to numerical andphysical modeling (Schultz-Ela et al., 1993;Vendeville and Nilsen, 1995; Fig. 9).

We have evidence of only one episode ofdeformation in the East Kwanza fold belt butcannot exclude the possibility of other con-tractional episodes. Eocene units are every-where folded, and units as young as Miocenecore the Calomboloca syncline (Figs. 5, 9).Thus, we infer that most of the shortening inthe fold belt is Tertiary, probably coeval withthe Oligocene–Holocene event in the Coastalfold belt. Apatite fission-track analysis (Harriset al., 2002) indicates uplift events in both themiddle and late Tertiary, suggesting that short-ening may have been polyphase. Additionaldata may reveal earlier phases of deformation.

Gonga Fold BeltGoldhammer et al. (2000) conducted a de-

tailed sequence-stratigraphic and structural studyof the offshore Kwanza Basin. They interpretedan early Cenomanian phase of basement-involved shortening in the southern Kwanza Ba-sin marked by folding, uplift, and subaerial ex-

posure of Albian carbonates. Post-Cenomanianstrata of uniform thickness overlie these struc-tures. The Gonga fold belt, as we term it, lieson the Benguela Platform, in a salt-free area.It is bounded to the north by the Denda trans-fer zone and extends ;25 km south of ourmap area (R. Goldhammer, 2000, personalcommun.).

Arguments for Basement InvolvementWe lack direct evidence of basement short-

ening in the Inner Kwanza Basin. However, astrong circumstantial case can be built. First,we list arguments that favor uplift and tran-spression of basement. Next, we present ob-servations that argue against the alternativedetached origin for the structures.

The following lines of evidence supportbasement shortening:

1. All the fold belts adjoin basement up-lifts, and folds decrease in intensity away fromthe uplifts. Also, the northward decrease in in-tensity of deformation in the East Kwanzafold belt coincides with a northward decreasein magnitude of uplift of the African craton asexpressed by topography (Fig. 7D).

2. Basement uplifts are similar in age tothe adjacent fold belts (Fig. 2). The best-determined times of basement uplift areOligocene–Holocene for the Cabo Ledo up-lift and Oligocene–Miocene for the Africancraton (Partridge and Maud, 1987; Burke,1996; Partridge, 1997; Gurnis et al., 2000;Fig. 9). These match the time of greatestshortening in the Coastal and East Kwanzafold belts. Older phases of deformation areless well determined, but Albian initiation ofstructures in the Coastal fold belt coincidedwith thinning of strata onto the Cabo Ledouplift. We lack evidence for Senonian move-ment of the Cabo Ledo uplift, but shorteningin the Coastal fold belt was broadly coevalwith rise of the African craton in Namibia(Gallagher and Brown, 1999).

3. All three fold belts terminate at transferzones, which we interpret to involve base-ment. Also, culminations of individual foldstend to cluster near transfer zones (Fig. 9),suggesting that offset on the transfer zonesmay have caused deformation. This view is



Fig

ure

8.G

eose

cre

stor

atio

nof

cros

sse

ctio

nC

–C9

acro

ssM

uxim

apr

ovin

ce.

The

offs

hore

part

ofth

ecr

oss

sect

ion

was

inte

rpre

ted

from

unpu

blis

hed

seis

mic

data

.T

heon

shor

epa

rtw

asde

rive

dfr

oma

seis

mic

-bas

edcr

oss

sect

ion

inM

asso

n(1

972)

,m

odifi

edac

cord

ing

tosu

rfac

ege

olog

y(T

otal

-Son

ango

l,19

87)

and

tops

from

six

wel

lsal

ong

the

line

ofse

ctio

n.R

esto

rati

ondo

esno

tin

clud

ede

com

pact

ion,

beca

use

orig

inal

thic

knes

sof

the

now

-ero

ded

sect

ion

isno

tkn

own.

All

bloc

ksw

ere

rest

ored

byfle

xura

lsl

ip.

Loc

atio

nsh

own

inF

igur

e5.

(A)

Res

tora

tion

toth

een

dof

the

Cre

tace

ous;

vert

ical

exag

gera

tion

33.

The

boun

dary

betw

een

the

Inne

ran

dO

uter

Kw

anza

Bas

ins

show

sas

ash

arp

incr

ease

insa

ltan

dC

reta

ceou

sse

dim

ent

thic

knes

sfr

omth

eA

met

ista

Pla

tfor

mto

the

Inne

rK

wan

zaB

asin

.(B

)P

rese

nt-d

aycr

oss

sect

ion;

vert

ical

exag

gera

tion

33.

Hor

izon

spr

ojec

ted

abov

eth

eto

pogr

aphi

csu

rfac

ela

ckfil

lpat

tern

.Olig

ocen

e–H

oloc

ene

base

men

tsh

orte

ning

inve

rted

the

wes

ted

geof

the

Inne

rK

wan

zaB

asin

and

form

edth

eC

oast

alfo

ldbe

lt.

The

east

ern

edge

ofth

eba

sin

was

also

shor

tene

d,le

adin

gto

uplif

t,fo

ldin

g,an

dth

rust

ing

inth

eE

ast

Kw

anza

fold

belt

.(C

)P

rese

nt-d

aycr

oss

sect

ion;

nove

rtic

alex

agge

rati

on.


HUDEC and JACKSON

Figure 9. Map of fold belts and sedimentary troughs in the Inner Kwanza Basin. Location of offshore structures in Gonga fold beltfrom Goldhammer et al. (2000).



consistent with the interpretation of Cramez etal. (2000), who suggested that Neogene short-ening in the Coastal fold belt involved tran-spression and inversion along preexistingbasement structures.

4. Basement-involved contractional struc-tures are well documented in the Outer Kwan-za Basin. Goldhammer et al. (2000) favored abasement-involved model for the Gonga foldbelt, which they related to transpressional re-activation of the Denda transfer zone (R.Goldhammer, 2000, personal commun.). An-other example is the Cegonha anticline, whichstraddles the Luanda transfer zone (Fig. 9).The Cegonha anticline is cored by a squeezeddiapir, above which arched Neogene strata aretruncated at the ocean floor (Duval et al.,1993). Given the location of the anticline onthe continental shelf and its position along amajor transfer zone, we agree with Cramez etal. (2000) that the structure formed by tran-spressional reactivation of the Luanda transferzone.

The only alternative to basement involve-ment is that the fold belts were part of a de-tached gravity-driven system restricted to theInner Kwanza Basin (Duval et al., 1992).However, there are no exposed extensionalfault systems large enough to balance theshortening. Even if an extensional systemwere formerly present (for example, east ofthe present erosional limit of the basin), a de-tachment model fails to account for the posi-tion of the fold belts. Shortening is concen-trated on both margins of the basin, ratherthan in the intervening low area, as would beexpected for a contractional toe (Fig. 7C).Also, contractional structures are present onboth sides of the Cabo Ledo uplift, instead ofonly on the east side as would be expected ifshortening were being fed by translation fromthe east.

These arguments strongly suggest that base-ment shortening was involved in the fold beltsof the Inner Kwanza Basin. We have drawnand restored our cross sections on the basis ofthat interpretation (Figs. 8, 13). This inferenceis consistent with increasing recognition ofpostrift basement offsets elsewhere along thesouthwest African margin. Discussions of themargin in the late 1980s–early 1990s rarelymentioned basement structures, and all defor-mation was implicitly or explicitly assumed tobe detached above salt (e.g., Duval et al.,1992; Lundin, 1992). Scheevel and Dale(1993) first proposed basement involvement inextensional deformation, and this concept hasbeen increasingly accepted (e.g., Marton et al.,1998; Peel et al., 1998; Cramez and Jackson,

2000). We are extending this concept even fur-ther to include basement shortening.

SALT TECTONICS IN THE INNERKWANZA BASIN

The importance of salt deformation haslong been recognized in the Inner Kwanza Ba-sin (e.g., Brognon and Verrier, 1966; Masson,1972), but salt tectonics there has never beencomprehensively reviewed. All previous stud-ies focused on Quenguela province. Earlyworkers interpreted diapirs to have grown inthe absence of horizontal translation (e.g.,Masson, 1972; Verrier and Castello Branco,1972), whereas others have inferred largeamounts of extension (e.g., Burollet, 1975;Duval et al., 1992; Lundin, 1992).

In this section, we examine the distributionof salt structures in the entire Inner KwanzaBasin and discuss regional variations in struc-tural style, timing, and evolution. The historyof salt movement differs greatly from prov-ince to province. We conclude that basementconfiguration controlled the location and styleof salt tectonism.

Initiation and Growth of Salt Diapirs

An isochore map of the Albian Pinda Group(Fig. 14) shows large variations in sedimentthickness. Where Pinda strata are missing, weinfer extrusive salt walls. The relationshipsshown on this map suggest that salt diapirismbegan very early in the Inner Kwanza Basin.

Coastal Fold BeltTwo types of salt structures are present in

the Coastal fold belt: Salt anticlines and saltwalls (Fig. 8). The salt walls are located aboveand adjacent to the Cabo Ledo uplift, whereasthe salt anticlines are located farther from theuplift.

We suggest that basement contraction as-sociated with uplift of the Cabo Ledo blockproduced salt-cored buckle folds in Albianstrata (Fig. 15A). These folds had their great-est amplitude near the Cabo Ledo uplift. Dur-ing the Albian, the Inner Kwanza Basin wasthe site of inner-ramp-facies deposition in wa-ter depths of only a few meters, so even mod-est buckling was enough to raise Albian strataand expose them to subaerial erosion. Higher-amplitude folds near the uplift were eroded allthe way down to salt, leading to the establish-ment of passive salt walls. Lower folds fartheraway were eroded less deeply and were pre-served as buckle folds.

Outside the Coastal Fold BeltThe Albian salt highs shown in Figure 14

outside of the Coastal fold belt (i.e., in theQuenguela, Kissama, and Morro Liso prov-inces) have since deflated and are now occu-pied by the sedimentary troughs previouslydescribed (Fig. 9). These walls were 2–3 kmtall and 15–25 km wide, giving them an ex-tremely broad, squat appearance (e.g., Fig.11B). They were 5–10 times larger in mapview than the walls of the Coastal fold beltand represent some of the widest salt wallsknown anywhere. To conveniently distinguishbetween the two sets of structures, we refer tothe structures outside of the Coastal fold beltas ‘‘broad walls.’’ Broad walls were not as-sociated with fold belts, although there is aweak correlation with the positions of base-ment uplifts (Fig. 7C). All of the broad wallstrended parallel to base-salt structural contours(Fig. 7C).

The early history of the broad walls is notwell understood. Major differences in struc-tural style between the broad walls and theCoastal fold belt suggest different modes oforigin. We present two speculative hypothesesfor broad-wall formation.

The first possibility is that the broad wallswere initiated when thin slabs of dense over-lying carbonate rocks broke up and founderedinto the salt during the early Albian (Fig.15B). The Albian cover could have been dis-rupted by an early phase of minor detachedextension, the magnitude of which is restrictedby evidence for limited Albian shortening.The absence of broad walls in Muxima andPorto Amboim provinces is explained byblocking of seaward translation by the CaboLedo and Pequena coastal uplifts. The hy-pothesis does not explain the coincidence ofbroad walls with subsalt structural highs (Fig.7B).

A second possibility is that the broad wallsformed as salt swells draped above basementuplifts (Fig. 15C). Uplift raised Albian strataabove sea level and erosionally unroofed thesalt, creating very wide passive diapirs. Thishypothesis explains the broad, linear shape ofthe salt walls and the coincidence with subsalthighs. The hypothesis does not explain the ab-sence of broad walls in Muxima and PortoAmboim provinces.

Fall of Salt Diapirs

Patterns of Cenomanian–Holocene deposi-tion are closely related to Albian salt tecton-ism (Figs. 16, 17). Areas lacking preexistingsalt diapirs have uniform sediment thickness-es, but areas with salt structures show much


HUDEC and JACKSON



Figure 11. Geosec restoration of cross section B–B9 (location in Fig. 5) across the MorroLiso Trough, Morro Liso province. Section modified from Masson (1972). Restorationincludes decompaction. All blocks were restored by vertical shear. (A) End of Aptian salt(black) deposition. The Morro Liso area is interpreted as a local sag or graben near theeast edge of the Inner Kwanza Basin. (B) Initiation of Morro Liso salt wall, accompaniedby minor basement uplift. Adjacent blocks were nearly grounded, restricting salt flow. (C)The Morro Liso Trough began to subside. (D) Minor basement uplift during the Senonian.Subsidence into the western half of the Morro Liso salt wall continued. (E) The MorroLiso Trough grounded against the base of salt, causing the depocenter to shift to the east.(F) Horizons projected above the topographic surface lack fill pattern. Only a selvage ofsalt remains of the Morro Liso salt wall. Basement shortening in the Oligocene–Holoceneraised the Morro Liso uplift beneath the Morro Liso Trough. The eastern half of thesection shows the western edge of the East Kwanza fold belt.

Figure 10. Geosec restoration of cross section A–A9 (location in Fig. 5) across Quenguela province. The offshore part of the cross sectionwas interpreted from unpublished seismic data. The onshore part was derived from seismic-based cross sections in Duval et al. (1992) andMasson (1972), extensively modified according to surface geology (Total-Sonangol, 1987) and tops from 15 wells along the line of section.Restoration includes decompaction. All blocks were restored by vertical shear. Black arrows indicate locations of uplift. (A) End of Aptiansalt (black) deposition. The Inner Kwanza Basin was an interior salt basin, separated from the Outer Kwanza Basin (off section to thewest) by the Flamingo Platform. (B) Initiation of the Praia, Quenguela, and Funda salt walls. (C) Subsidence of blocks between the wallsled to near-grounding, which restricted salt supply to the walls. Salt dissolution initiated subsidence of the Quenguela Trough. (D) Subsi-dence continued in the Quenguela Trough and began on this line of profile in the Funda Trough. Regional uplift began in western InnerKwanza Basin. (E) Uplift continued in western Inner Kwanza Basin. Funda and Quenguela Troughs largely grounded. (F) Onset of large-scale seaward translation of blocks at the west end of the section, localized by uplift in the western Inner Kwanza Basin. Most extensionwas focused on the Praia Trough; minor extension occurred in the Quenguela Trough. Uplift began in the eastern Inner Kwanza Basin.(G) Seaward translation continued. Uplift of the eastern Inner Kwanza Basin and Flamingo Platform. (H) Present-day section, showingwell control. Uplift continues in eastern Inner Kwanza Basin and Flamingo Platform. (I) Present-day section without vertical exaggeration.N

more thickness variation. All of the Albianbroad walls began to deflate during the LateCretaceous or Tertiary and are now replacedby the sedimentary troughs discussed previ-ously. Commonly, the only remnants of saltare thin selvages along salt welds—the ‘‘ci-catrices salifieres’’ of Masson (1972). Shiftingpatterns of salt withdrawal within the diapirscreated turtle structures within the troughs(Figs. 10, 11). Evacuation of salt was incom-plete from the fact that relict salt diapirs re-main in the centers of some troughs (propri-etary data).

Two hypotheses explain sedimentarytroughs in the Inner Kwanza Basin: extension-induced collapse and dissolution-inducedcollapse.

Hypothesis Involving Extension-InducedCollapse

Troughs could have formed by stretching oforiginally narrower diapirs during the Tertiary(Burollet, 1975; Duval et al., 1992; Lundin,1992). Lundin (1992) proposed that basin tilt-ing related to Tertiary uplift of the Africancraton triggered the extension of the walls.This model elegantly explains the initiation ofdiapir fall and links Inner Kwanza Basintroughs to similar structures outside of the In-ner Kwanza Basin that are clearly extensionalin origin.

We consider that extension-induced col-lapse can account for evolution of the Mio-cene–Pliocene Praia Trough, located on theFlamingo Platform (Fig. 10). However, sev-eral lines of argument suggest that the modelis less applicable in the Inner Kwanza Basin.

First, the extension-induced collapse hy-pothesis fails to explain the diachroneity andtiming of trough development (Fig. 12). Welldata indicate that troughs formed during al-most the entire history of the Inner KwanzaBasin (Fig. 2). Parts of the Funda and MorroLiso Troughs subsided continuously from the


HUDEC and JACKSON

Figure 12. Age of trough fill in the Inner Kwanza Basin.

Cenomanian through the early Miocene, a pe-riod of 80 m.y. during which little evidencefor horizontal translation was recorded any-where else in the Kwanza Basin. Individualtroughs also vary greatly in age along strike,a feature typical of salt withdrawal but less soof detached extension. For example, the FundaTrough is of Late Cretaceous age at its north-ern end and of Oligocene–Miocene age farthersouth (Fig. 12).

Next, we cannot see any gravitational driv-ing mechanism for seaward translation in theInner Kwanza Basin over most of its history.

The base of salt dips landward into the InnerKwanza Basin from the present coastline (Fig.6), and restorations (Figs. 8, 10) suggest thatthis landward dip was even more pronouncedin the past. Any seaward translation wouldtherefore have to have been driven by topo-graphic slope rather than decollement dip.However, proprietary paleontologic data indi-cate that shallow-marine deposition persistedin the western Inner Kwanza Basin until thePliocene, which precludes a major spreadingplateau during almost all of trough formation.Major uplift of the eastern part of the basin

did begin in the middle Miocene, but this up-lift is (1) too late to explain the first 80 m.y.of trough development and (2) east of most ofthe troughs.

Finally, any extension-induced collapseentails ;25 km of extension in the InnerKwanza Basin to open the Albian gaps onthe Quenguela and Funda Troughs (Fig. 10).A regional restoration across the entireKwanza Basin shows that this amount oftranslation is not accommodated downdip(Hudec and Jackson, 2002). After all the ex-tension and shortening in the offshore areaare restored, there is only ;3 km of exten-sion that could have come from the InnerKwanza Basin.

Hypothesis Involving Dissolution-InducedCollapse

To produce the troughs of the Inner KwanzaBasin, we prefer an alternative origin—dissolution-induced collapse of the underlyingbroad walls (Masson, 1972; Verrier and Cas-tello Branco, 1972). In this hypothesis, troughsubsidence was triggered by depletion of salt.Without new salt imported from adjoining ar-eas, salt dissolving at the crest of the wallcould no longer be replaced, so the diapir be-gan to subside. We envision that parts of thebroad walls remained exposed during subsi-dence, allowing a vent for salt to flow outfrom under the troughs and be dissolved at thesurface (e.g., Figs. 10, 11). The troughs havenow all finished subsiding (Fig. 2), and anyremaining caprock is presumed to have beenburied under subsequent Miocene–Holocenestrata.

Dissolution accounts for the subsidencehistory and is consistent with the argumentsagainst extension already described. Themain difficulties for the hypothesis are theneed for large initial volumes of salt to formthe broad walls and equally large volumes ofsalt dissolution to later deflate and removethe walls. Preserved salt walls of similar gi-ant scale have also been interpreted fromtwo-dimensional seismic data in the Whale,Horseshoe, and Jeanne d’Arc Basins on theAtlantic margin of Canada (Balkwill and Le-gall, 1989).

Basement Controls on Salt Tectonism

The most important basement structures inthe Kwanza Basin are the Flamingo, Ametis-ta, and Benguela Platforms, which divide thebasin into an interior salt province (the InnerKwanza Basin) and an exterior continentalmargin (platforms plus the Outer KwanzaBasin). Regional restorations (Hudec and



Figure 13. Map and cross sections of the northern East Kwanza fold belt. Location of the map area shown in Figure 5. (A) Map of thenorthern part of the East Kwanza fold belt, showing fold and fault patterns interpreted to be contractional. (B) Cross section D–D9. (C)Eastern part of cross section C–C9 (complete cross section shown in Fig. 8).


HUDEC and JACKSON

Figure 14. Albian isochore map in the Inner Kwanza Basin. Letters refer to the names of salt walls inferred to have been exposed atthe end of the Albian. Letters refer to the names of salt walls inferred to be exposed at the end of the Albian: F—Funda, M—Muxate,ML—Morro Liso, P—Praia, and Q—Quenguela.

Jackson, 2002) suggest that the Inner andOuter Kwanza Basins evolved largely inde-pendently. Salt tectonics in the exterior con-tinental margin is dominated by large-scale

seaward translation of the cover, which hasproduced a kinematically linked system ofextension and contraction at the landwardand seaward ends of the basin, respectively

(Hudec et al., 2001). By contrast, we inter-pret structures in the Inner Kwanza Basin tohave formed in the absence of significant lat-eral translation.



Figure 15. Mechanisms for initiating diapirism in the Inner Kwanza Basin. (A) Basementshortening reactivates some subsalt normal faults to accommodate reverse movement,uplifting graben blocks. Coeval shortening in the postsalt cover produces buckle folds thatare most intense near the uplift. Anticlines are unroofed by erosion, triggering salt walls.(B) Minor seaward translation disrupts thin cover above the salt. Dense carbonate blocksfounder into salt, allowing salt to reach the surface. Cover thickness exaggerated for clar-ity. (C) Basement shortening inverts some subsalt grabens, arching the overlying saltupward into broad plateaus. Drape folds over these plateaus are unroofed, leaving salt atthe surface.

TECTONIC SYNTHESIS

A controversial aspect of our model is theinterpretation of multiple episodes of base-ment shortening in the Kwanza Basin. Base-ment shortening has never before been pro-posed for the Angolan passive margin. In thissection we speculate on the causes of theseinversion events by addressing two majorquestions. First, was the Kwanza Basin moresusceptible to basement shortening than wereadjoining areas? Second, what triggered thepulses of basement shortening?

Evidence of Weak Lithosphere Beneaththe Kwanza Basin

The Angolan margin has probably beenthermally weakened and thus susceptible toreactivation for much of its history. Synriftvolcanism was voluminous in the Inner Kwan-za Basin (Brognon and Verrier, 1966). Welldata show hundreds of meters of subsalt basaltflows beneath the present continental shelf (e.g.,Jackson et al., 2000). Marzoli et al. (1999) sug-gested that the synrift Parana-Etendeka flood-basalt province extended at least as far north

as Sumbe (formerly Novo Redondo) in thesouthern Kwanza Basin (Fig. 1). Areas aroundthe Kwanza Basin remained magmatically ac-tive throughout the Late Cretaceous. Ceno-manian volcanic rocks near Sumbe (Marzoliet al., 1999) form part of a volcanic chain thattrends northwest from Sumbe into the OuterKwanza Basin and beyond (Fig. 1). The restof these volcanoes are built on oceanic crustof probable late Aptian–early Albian age. Nu-merous carbonatites, nepheline syenites, andalkalic ring complexes of Albian to Maastrich-tian age are present south and northeast of theKwanza Basin (Cahen et al., 1984). Watts andMarr (1995) have interpreted free-air gravitydata to indicate that the Kwanza Basin re-mains one of the weakest parts of the south-west African margin.

Also weakening the lithosphere of theKwanza Basin is its position along the trendof the Malange uplift, which is bounded bymajor Pan-African Neoproterozoic shearzones at the southern end of the West Congofold belt (Figs. 1, 7D). Many authors havenoted that Pan-African mobile belts tend tolocalize later deformation (e.g., Kennedy,1964, 1965; Maurin and Guiraud, 1993;Burke, 1996) and may have done so in theKwanza Basin. Significantly, the Cabo Ledouplift, the biggest and most active basementstructure in the Inner Kwanza Basin, lies di-rectly along strike of the Malange uplift.

Repeated inversion has also been proposedfor the Brazilian continental margin by Cob-bold et al. (2001). They argued that mantleupwelling thermally weakened the Brazilianmargin, allowing it to fail under far-fieldstresses, perhaps related to Andean orogenies.Contractional structures in coastal Brazil in-clude folds, faults, and uplift of several largemountain ranges. Similar structures are pre-sent in and around the Kwanza Basin. On bothcontinental margins, rift-related transfer zoneslinked to oceanic fractures were reactivatedduring the deformation (Cramez et al., 2000;Cobbold et al., 2001).

Triggers for Shortening

Passive margins are horizontally com-pressed as a result of gravitational collapse ofmid-ocean ridges (e.g., Dewey, 1988; Zoback,1992). However, ridge push alone does not ex-plain the occurrence of inversion structures onpassive margins—ridges are always pushing,but margins invert episodically or not at all.Periods of inversion must therefore marktimes of increased stress, decreased strength,or both. In this section we discuss possible


HUDEC and JACKSON

Figure 16. Cenomanian–Eocene (Iabe Group) isochore map in the Inner Kwanza Basin. Isochores are projected where the top of theIabe has been erosionally removed. Letters refer to salt walls inferred to have been exposed at the end of the Eocene: F—Funda, P—Praia, and Q—Quenguela.



Figure 17. Oligocene–Pliocene isochore map in the Inner Kwanza Basin.

triggers for the three inferred episodes ofshortening in the Inner Kwanza Basin.

We interpret Albian–early Cenomanianshortening to be the result of ridge push on athin, hot, and weak continental margin. As

documented previously in this paper, rifting inthe Kwanza Basin was accompanied by abun-dant magmatism, which apparently weakenedthe margin so that it was incapable of resistingstresses generated by the newly formed Mid-

Atlantic Ridge. Many passive-margin inver-sion structures worldwide formed just after therift-drift transition (e.g., Dewey, 1988; Bott,1992; Withjack et al., 1995; Dore and Lundin,1996). Cooling and strengthening of the An-


HUDEC and JACKSON

golan margin eventually ended this phase ofdeformation.

We suggest that Senonian shortening in theCoastal fold belt was related to a plate reor-ganization at chron 34 (84 Ma, the ‘‘Santonianevent’’) that changed the spreading directionbetween the African and South Americanplates (Nurnberg and Muller, 1991; Binks andFairhead, 1992). This reorganization led towidespread basement shortening in the north-ern half of the African plate. Santonian inver-sion structures have previously been mappedas far south as the Gabon Basin (Whiteman,1982; Teisserenc and Villemin, 1989; Gui-raud, 1991; Genik, 1992; Guiraud and Bos-worth, 1997; Bosworth et al., 1999; Dailly,2000). Shortening in the Kwanza Basin mayhave been facilitated by a second phase ofmagmatic weakening in the Late Cretaceous(Fig. 2). The lack of such magmatism in sur-rounding areas may explain the absence ofSantonian contractional structures in the ad-jacent Lower Congo and Benguela Basins.

Finally, we attribute Oligocene–Holoceneshortening to rise of the African superswell, abroad 500- to 1000-m-high uplift that extendsfrom the southeast Atlantic Ocean to northeastAfrica (Fig. 1; Bond, 1978; Sahagian, 1988;Nyblade and Robinson, 1994). Research onthis dynamic bulge in the African plate sug-gests that southern Africa is being actively up-lifted by an inclined plume in the middle ofthe lower mantle (Gurnis et al., 2000). Theproposed link between rise of the superswelland shortening in the Inner Kwanza Basin isbased on (1) an Oligocene–Holocene age ofboth events (Partridge and Maud, 1987;Burke, 1996; Gurnis et al., 2000), and (2) theposition of the Inner Kwanza Basin on thenorthwest rim of the superswell, at a majorbreak in regional elevation. Shortening mighthave been produced in response to gravityspreading of the uplifted continental interior.Alternatively, an inclined mantle plume mightbe capable of generating or enhancing hori-zontal stresses in the overlying lithosphere.

The analysis herein suggests at least threemechanisms for basement shortening on pas-sive margins, all of which can be enhanced bythermal weakening: (1) ridge push, (2) changein spreading direction, and (3) uplift of thecraton.

Transfer-zone reactivation played a majorrole in all three Kwanza shortening events, asis common in passive-margin inversion (e.g.,Dore and Lundin, 1996; Dailly, 2000). Thisreactivation suggests that variations in stressesgenerated at the mid-ocean ridge can be trans-mitted to the continental margin via slip alongoceanic fracture zones. Transfer zones on the

margin are reactivated either because they aredirect extensions of the fractures or becauseslip is transferred at relay zones near theocean/continent boundary. In the Outer Kwan-za Basin, thick sediment and salt obscure anyconnection between fracture zones and trans-fer zones, so their relationship is speculative.

Implications

A common assumption of palinspastic res-toration on passive margins is that regionalsubsidence is controlled only by thermal andisostatic effects (e.g., Rowan, 1993; Lavier etal., 2000). The only independent variables arethe lithospheric- and crustal-thinning profiles,the flexural rigidity, and the sedimentation his-tory. Thus, backstripping is used to calculatecrustal thinning directly from subsidence anal-ysis (e.g., Steckler and Watts, 1978; Steckleret al., 1988; Watts and Stewart, 1998; Bel-lingham and White, 2000). However, these as-sumptions do not hold in the Kwanza Basinbecause inversion has produced crustal upliftsthat have nothing to do with either thermalsubsidence or isostasy. The magnitude, tim-ing, and spatial distribution of these uplifts arepoorly determined, which adds uncertainty toany subsidence-based analysis. Affected anal-yses include palinspastic restorations, back-stripping, thermal-maturation calculations,and paleobathymetry estimations.

CONCLUSIONS

1. The Kwanza Basin is subdivided into In-ner and Outer Kwanza salt basins, separatedby synrift basement highs on which Aptiansalt is thin or absent.

2. Strike-perpendicular transfer zones seg-mented the Inner Kwanza Basin into fiveprovinces during Early Cretaceous rifting.These transfer zones were reactivated duringlater shortening. Basement geometry, structur-al elevation, and salt-related structural style allchange across province boundaries.

3. Three contractional fold belts are inter-preted in and around the Inner Kwanza Basin.The East Kwanza fold belt runs parallel to theeastern margin of the Kwanza Basin and isdetached above salt in its western part. TheCoastal fold belt is located above and adjoin-ing the Cabo Ledo uplift. This fold belt is de-tached above thicker salt and contains severalsqueezed diapirs. The Gonga fold belt lies atthe southern end of the Kwanza Basin, in asalt-free area.

4. The fold belts result from at least threeepisodes of basement-involved shortening.Their ages are poorly determined, but best es-

timates are (a) Albian–early Cenomanian, (b)Santonian, and (c) Oligocene–Holocene.Basement shortening also uplifted basementblocks within the Kwanza Basin.

5. Two distinct styles of salt structures inthe Inner Kwanza Basin are related to differ-ences in basement architecture. In the Coastalfold belt, unroofing of salt-cored buckle foldsled to the emergence of narrow salt walls.Elsewhere in the basin, salt flowed into broad,elongate salt walls. The broad walls also wereinitiated during the Albian, probably by eitherfoundering of the roof or subsalt block uplift.

6. Withdrawal and dissolution of salt frombroad walls led to the formation of sedimen-tary troughs. Trough fill ranges in age fromCenomanian to Pliocene and is strongly diach-ronous from trough to trough and along strikewithin troughs. Only the more distal troughsformed by regional extension.

7. Weakening of the lithosphere probablyfacilitated basement inversion in the KwanzaBasin. We suggest that Albian shortening wasrelated to ridge-push forces, Senonian inver-sion to plate reorganization, and Oligocene–Holocene activity to mantle-driven uplift ofthe African superswell.

ACKNOWLEDGMENTS

We are grateful to Fina for their pioneering pub-lications in the Inner Kwanza Basin and to Totaland Sonangol for their outstanding geologic map.We also thank Sonangol for their help in publicationof this paper. We benefited greatly from discussionswith Peter Cobbold, Carlos Cramez, Bob Goldham-mer, Kris Meisling, Martin Norvick, and Frank Peel.We thank Ian Davison, Peter Cobbold, and PaulMann for their helpful and constructive reviews. Di-agrams were drafted by John T. Ames, under thedirection of Joel L. Lardon, and by the authors. Pub-lication was authorized by the Director, Bureau ofEconomic Geology, the John A. and Katherine G.Jackson School of Geosciences, University of Texasat Austin. Shell Angola, TotalFinaElf, BHP, andTexaco supplied proprietary data. We thank Para-digm Geophysical for use of Geosec-2D and Dy-namic Graphics for use of EarthVision.

The project was funded by the Applied Geodyn-amics Laboratory consortium, which comprises thefollowing oil companies: Amerada Hess Corpora-tion, Anadarko Petroleum Corporation, BHP Petro-leum (Americas), BP Amoco Production Company,Burlington Resources, ChevronTexaco, Conoco,ENI-Agip S.p.A., Enterprise Oil PLC, ExxonMobilUpstream Research Company, Marathon Oil Com-pany, PanCanadian Petroleum, Petroleo Brasileiro,S.A., Phillips Petroleum Company, TotalFinaElf,and Woodside Energy.

REFERENCES CITED

Balkwill, H.R., and Legall, F.D., 1989, Whale Basin, off-shore Newfoundland: Extension and salt diapirism, inTankard, A.J., and Balkwill, H.R., eds., Extensionaltectonics and stratigraphy of the North Atlantic mar-



gins: American Association of Petroleum GeologistsMemoir 46, p. 233–245.

Behrendt, J.C., Hamilton, R.M., Ackermann, H.D., Hen-ry, V.J., and Bayer, K.C., 1983, Marine multichannelseismic-reflection evidence for Cenozoic faultingand deep crustal structure near Charleston, SouthCarolina: U.S. Geological Survey Professional Paper1313-J, p. J1–J29.

Bellingham, P., and White, N., 2000, A general inversemethod for modelling extensional sedimentary basins:Basin Research, v. 12, p. 219–226.

Binks, R.M., and Fairhead, J.D., 1992, A plate tectonic set-ting for Mesozoic rifts of west and central Africa: Tec-tonophysics, v. 213, p. 141–151.

Blystad, P., Brekke, H., Faerseth, R.B., Larsen, B.T., Skog-seld, J., and T rudbakken, B., 1995, Structural ele-øments of the Norwegian continental shelf: NorwegianPetroleum Directorate Bulletin, v. 8.

Boldreel, I.O., and Andersen, M.S., 1993, Late Paleoceneto Miocene compression in the Faeroe-Rockall area,in Parker, J.R., ed., Petroleum geology of northwestEurope: Proceedings of the fourth conference: Geo-logical Society [London], p. 1025–1034.

Bond, G., 1978, Evidence for late Tertiary uplift of Africarelative to North America, South America, Australiaand Europe: Journal of Geology, v. 86, p. 47–65.

Bosworth, W., Guiraud, R., and Kessler, L.G., II, 1999, LateCretaceous (ca. 84 Ma) compressive deformation ofthe stable platform of northeast Africa (Egypt): Far-field stress effects of the ‘‘Santonian event’’ and originof the Syrian arc deformation belt: Geology, v. 27,p. 633–636.

Bott, M.H.P., 1992, The stress regime associated with con-tinental break-up, in Storey, B.C., Alabaster, T., andPankhurst, R.J., eds., Magmatism and the causes ofcontinental break-up: Geological Society [London]Special Publication 68, p. 125–136.

Brice, S.E., Cochran, M.D., Pardo, G., and Edwards, A.D.,1982, Tectonics and sedimentation of the south Atlan-tic rift sequence: Cabinda, Angola, in Watkins, J.S.,and Drake, C.L., eds., Studies in continental margingeology: American Association of Petroleum Geolo-gists Memoir 34, p. 5–18.

Brognon, G.P., and Verrier, G.R., 1966, Oil and geology inCuanza Basin of Angola: American Association of Pe-troleum Geologists Bulletin, v. 50, p. 108–158.

Bryant, W.R., Antoine, J., Ewing, M., and Jones, B., 1968,Structure of Mexican continental shelf and slope, Gulfof Mexico: American Association of Petroleum Ge-ologists Bulletin, v. 52, p. 1204–1228.

Burke, K., 1996, The African plate: South African Journalof Geology, v. 99, p. 339–409.

Burollet, P.F., 1975, Tectonique en radeaux en Angola: Bul-letin de la Societe Geologique de France, v. 17,p. 503–504.

Burwood, R., 1999, Angola: Source rock control for thelower Congo coastal and Kwanza Basin petroleumsystems, in Cameron, N.R., Bate, R.H., and Clure,V.S., eds., The oil and gas habitats of the South At-lantic: Geological Society [London] Special Publica-tion 153, p. 181–194.

Cahen, L., Snelling, N.J., Delhal, J., and Vail, J.R., 1984,The geochronology and evolution of Africa: Oxford,Clarendon Press, 512 p.

Castellano, M.C., Duarte-Morais, M.L., Putignano, M.L.,and Sgrosso, I., 2000, High-density turbidite facies inthe Miocenic deposits cropping out around Cabo Ledo(Kwanza Basin–Angola) [abs.]: Angolan Associationof Geologists Geoluanda 2000 International Confer-ence, Luanda, Angola, May 21–24, p. 35.

Ciampo, G., Di Donato, V., Duarte-Morais, M.L., Putig-nano, M.L., Sgrosso, I., and Buta Neto, A., 2000,Stratigraphic and sedimentologic characters of the Lu-anda Formation, in the succession cropping out in thesurroundings of Luanda (Kwanza Basin, Angola)[abs.]: Angolan Association of Geologists Geoluanda2000 International Conference, Luanda, Angola, May21–24, p. 36.

Cobbold, P.R., Meisling, K.E., and Mount, V.S., 2001, Re-activation of an obliquely-rifted margin, Campos andSantos Basins, southeastern Brazil: American Asso-

ciation of Petroleum Geologists Bulletin, v. 85,p. 1925–1944.

Cramez, C., and Jackson, M.P.A., 2000, Superposed defor-mation straddling the continental-oceanic transition indeep-water Angola: Marine and Petroleum Geology,v. 17, p. 1095–1109.

Cramez, C., Jackson, M.P.A., Fraenkl, R., and Sikkema, W.,2000, Contractional regimes in offshore and onshoreAngola: Geometry, distribution, timing, and origin[abs.]: Angolan Association of Geologists Geoluanda2000 International Conference, Luanda, Angola, May21–24, p. 38.

Dailly, P., 2000, Tectonic and stratigraphic development ofthe Rio Muni Basin, equatorial Guinea: The role oftransform zones in Atlantic Basin evolution, in Moh-riak, W., and Talwani, M., eds., Atlantic rifts and con-tinental margins: American Geophysical Union Geo-physical Monograph 115, p. 105–128.

Danforth, A., Henry, S., and Abreu, V., 2000, Paleogeog-raphy of the presalt and salt basins of the Angolancontinental margin: American Association of Petro-leum Geologists Annual Meeting Abstracts, v. 9,p. A35.

de Boorder, H., 1982, Deep-reaching fracture zones in thecrystalline basement surrounding the West Congo sys-tem and their control of mineralization in Angola andGabon: Geoexploration, v. 20, p. 259–273.

de Carvalho, H., 1982, Geologia de Angola, Sheet 1: La-boratorio Nacional de Investigacao Cientifica Tropical(Junta de Investigacoes Cientificas do Ultramar), Scale1:1,000,000.

Dewey, J.F., 1988, Lithospheric stress, deformation, andtectonic cycles: The disruption of Pangaea and the clo-sure of Tethys, in Audley-Charles, M.G., and Hallam,A., eds., Gondwana and Tethys: Geological Society[London] Special Publication 37, p. 23–40.

Dore, A.G., and Lundin, E.R., 1996, Cenozoic compres-sional structures on the northeast Atlantic margin: Na-ture, origin and potential significance for hydrocarbonexploration: Petroleum Geoscience, v. 2, p. 299–311.

Duarte-Morais, M.L., and Sgrosso, I., 2000, The strati-graphic succession exposed in the central-southernpart of the Kwanza Basin (Angola) [abs.]: AngolanAssociation of Geologists Geoluanda 2000 Interna-tional Conference, Luanda, Angola, May 21–24, p. 55.

Duval, B., Cramez, C., and Jackson, M.P.A., 1992, Rafttectonics in the Kwanza Basin, Angola: Marine andPetroleum Geology, v. 9, p. 389–404.

Duval, B., Cramez, C., Schultz-Ela, D.D., and Jackson,M.P.A., 1993, Extension, reactive diapirism, salt weld-ing, and contraction at Cegonha, Kwanza Basin, An-gola: American Association of Petroleum GeologistsHedberg Research Conference on Salt Tectonics,Bath, England, September 13–17, 1993, 2 p. extendedabstract.

Evans, R., 1978, Origin and significance of evaporites inbasins around Atlantic margin: American Associationof Petroleum Geologists Bulletin, v. 62, p. 223–234.

Gallagher, K., and Brown, R., 1999, The Mesozoic denu-dation history of the Atlantic margins of southern Af-rica and southeast Brazil and the relationship to off-shore sedimentation: in Cameron, N.R., Bate, R.H.,and Clure, V.S., eds., The oil and gas habitats of theSouth Atlantic: Geological Society [London] SpecialPublication 153, p. 41–53.

Genik, G.J., 1992, Regional framework, structural and pe-troleum aspects of rift basins in Niger, Chad, and theCentral African Republic (C.A.R.): Tectonophysics,v. 213, p. 169–185.

Goldhammer, R.K., Lawrence, R.M., Forero, H., Hussey, E.,and Gaudoin, D., 2000, Cretaceous tectono-stratigraphicand paleogeographic evolution of the southern KwanzaBasin, Angola [abs.]: American Association of Petro-leum Geologists Annual Meeting Abstracts, v. 9,p. A55.

Guiraud, M., 1991, Mechanisme de formation du bassin Cre-tace sur decrochements multiples de la Haute-Benoue(Nigeria)—Formation mechanism of the Cretaceous ba-sin on multiple strike-slip faults in the upper Benue Val-ley, Nigeria: Bulletin des Centres de Recherches Explo-ration-Production Elf-Aquitaine, v. 15, p. 11–67.

Guiraud, R., and Bosworth, W., 1997, Senonian basin in-

version and rejuvenation of rifting in Africa and Ara-bia: Synthesis and implications to plate-scale tecton-ics: Tectonophysics, v. 282, p. 39–82.

Guiraud, R., and Maurin, J.-C., 1992, Early Cretaceous riftsof western and central Africa: An overview: Tecton-ophysics, v. 213, p. 153–168.

Gurnis, M., Mitrovica, J.X., Ritsema, J., and van Heijst, H.-J., 2000, Constraining mantle density structure usinggeological evidence of surface uplift rates: The caseof the African superplume: Geochemistry GeophysicsGeosystems, v. 1, paper number 1999GC000035.

Harris, N.B., Hegarty, K.A., Green, P.F., and Duddy, I.R.,2002, Distribution, timing and intensity of major tec-tonic events on the west African margin from Gabonto Namibia: Results of a regional apatite fission trackstudy [abs.]: American Association of Petroleum Ge-ologists Annual Meeting Official Program, v. 11,p. A72.

Hastings, D.A., Dunbar, P.K., Elphingstone, G.M., Bootz,M., Murakami, H., Maruyama, H., Masaharu, H., Hol-land, P., Payne, J., Bryant, N.A., Logan, T.L., Muller,J.-P., Schreier, G., and MacDonald, J.S., eds., 1999,The Global Land One-kilometer Base Elevation(GLOBE) digital elevation model, version 1.0: Na-tional Oceanic and Atmospheric Administration, Na-tional Geophysical Data Center, CD-ROM.

Henry, S.G., and Abreu, V., 1998, Marine transgressions inthe presalt of the South Atlantic: New models for rift-ing and continental breakup: American Association ofPetroleum Geologists Annual Meeting Expanded Ab-stracts, v. 7, 5 p. extended abstract.

Hudec, M.R., and Jackson, M.P.A., 2002, Estranged neigh-bors: Independent tectonic evolution of the onshoreand offshore Kwanza salt basins, Angola [abs.]:American Association of Petroleum Geologists 2002Annual Meeting.

Hudec, M.R., Jackson, M.P.A., Binga, L.F., Da Silva, J.C.,Fraenkl, R., and Sikkema, W., 2001, Regional resto-ration in the offshore Kwanza Basin, Angola: Linkedzones of extension, translation and contraction [abs.]:American Association of Petroleum Geologists An-nual Meeting Official Program, v. 10, p. A95.

Humphris, C.C., Jr., 1978, Salt movement on continentalslope, northern Gulf of Mexico, in Bouma, A.H.,Moore, G.T., and Coleman, J.M., eds., Framework, fa-cies, and oil-trapping characteristics of the upper con-tinental margin: American Association of PetroleumGeologists Studies in Geology No. 7, p. 69–86.

Instituto Nacional de Geologia, 1988, Carta geologia deAngola: Instituto Nacional de Geologia, scale 1:1,000,000.

Jackson, M.P.A., Cramez, C., and Fonck, J.-M., 2000, Roleof subaerial volcanic rocks and mantle plumes in cre-ation of South Atlantic margins: Implications for salttectonics and source rocks: Marine and Petroleum Ge-ology, v. 17, p. 477–498.

Jackson, M.P.A., Hudec, M.R., Fraenkl, R., Sikkema, W.,Binga, L., and Da Silva, J., 2001, Minibasins trans-lating down a basement ramp in the deepwater mono-cline province of the Kwanza Basin, Angola [abs.]:American Association of Petroleum Geologists An-nual Meeting Official Program, v. 10, p. A99.

Jeronimo, P., Goncalves, F., and Inkollu, M., 1998, Riftingand its bearing on the hydrocarbon system of the mar-gin of Angola, west Africa: American Association ofPetroleum Geologists International Conference andExhibition, Extended Abstracts Volume, Rio de Ja-neiro, Brazil, November, p. 344–345.

Karner, G.D., and Driscoll, N.W., 1998, Tectonic setting ofthe Marnes-Noires/Falcao source rocks of the Congoand Angolan continental margins [abs.]: AmericanAssociation of Petroleum Geologists Bulletin, v. 82,p. 1928.

Karner, G.D., and Driscoll, N.W., 1999a, Style, timing anddistribution of tectonic deformation across the Ex-mouth Plateau, northwest Australia, determined fromstratal architecture and quantitative basin modelling,in Mac Niocaill, C., and Ryan, P.D., eds., Continentaltectonics: Geological Society [London] Special Pub-lication 164, p. 271–311.

Karner, G.D., and Driscoll, N.W., 1999b, Tectonic andstratigraphic development of the west African and


HUDEC and JACKSON

eastern Brazilian margins: Insights from quantitativebasin modeling, in Cameron, N.R., Bate, R.H., andClure, V.S., eds., The oil and gas habitats of the SouthAtlantic: Geological Society [London] Special Publi-cation 153, p. 11–40.

Kennedy, W.Q., 1964, The structural differentiation of Af-rica in the Pan-African (6500 m.y.) tectonic episode:Leeds, UK, University of Leeds, Research Institute forAfrican Geology Annual Report of Scientific Results,(Session 1962–1963), p. 48–49.

Kennedy, W.Q., 1965, The influence of basement structureon the evolution of the coastal (Mesozoic and Tertiary)basins of Africa, in Ion, D.C., ed., Salt basins aroundAfrica: London, Institute of Petroleum, p. 7–16.

Lavier, L.L., Steckler, M.S., and Brigaud, F., 2000, An im-proved method for reconstructing the stratigraphy andbathymetry of continental margins: Application to theCenozoic tectonic and sedimentary history of the Con-go margin: American Association of Petroleum Ge-ologists Bulletin, v. 84, p. 923–939.

Laville, E., and Pique, A., 1992, Jurassic penetrative de-formation and Cenozoic uplift in the central High At-las (Morocco): A tectonic model: Geologische Rund-schau, v. 80, p. 157–170.

Lunde, G., Aubert, K., Lauritzen, O., and Lorange, E.,1992, Tertiary uplift of the Kwanza Basin in Angola,in Curnelle, R., ed., Geologie africaine: Bulletin desCentres de Recherches Exploration-Production Elf-Aquitaine, Memoire 13, p. 99–117.

Lundin, E.R., 1992, Thin-skinned extensional tectonics ona salt detachment, northern Kwanza Basin, Angola:Marine and Petroleum Geology, v. 9, p. 405–411.

Marton, G., Tari, C., and Lehmann, C.T., 1998, Evolution ofsalt-related structures and their impact on the postsaltpetroleum systems of the lower Congo Basin, offshoreAngola: American Association of Petroleum GeologistsInternational Conference and Exhibition, Extended Ab-stracts Volume, Rio de Janeiro, Brazil, p. 834–367.

Marton, L.G., Tari, G.C., and Lehmann, C.T., 2000, Evo-lution of the Angolan passive margin, west Africa,with emphasis on postsalt structural styles, in Moh-riak, W.U., and Talwani, M., eds., Atlantic rifts andcontinental margins: American Geophysical UnionGeophysical Monograph 115, p. 129–149.

Marzoli, A., Melluso, L., Morra, V., Renne, P.R., Sgrosso,I., D’Antonio, M., Duarte Morais, L., Morais, E.A.A.,and Ricci, G., 1999, Geochronology and petrology ofCretaceous basaltic magmatism in the Kwanza basin(western Angola), and relationships with the Parana-Etendeka continental flood basalt province: Journal ofGeodynamics, v. 28, p. 341–356.

Masson, M.P., 1972, L’exploration petroliere en Angola:Revue de l’Association Francaise des Techniciens duPetrole, v. 212, p. 21–34.

Maurin, J.-C., and Guiraud, R., 1993, Basement control inthe development of the Early Cretaceous West andCentral African rift system: Tectonophysics, v. 228,p. 81–95.

Meisling, K.E., Cobbold, P.R., and Mount, V.S., 2001, Seg-mentation of an obliquely rifted margin, Campos andSantos Basins, southeastern Brazil: American Asso-ciation of Petroleum Geologists Bulletin, v. 85,p. 1903–1924.

Morais, M.L., Putignano, M.L., Sgrosso, I., and Valente,A., 2000, Stratigraphical and sedimentological fea-tures of the Neogene to Quaternary succession aroundLuanda (Angola, southwestern Africa): Africa Geo-science Review, v. 7, p. 19–38.

Nurnberg, D., and Muller, R.D., 1991, The tectonic evo-lution of the South Atlantic from Late Jurassic to pre-sent: Tectonophysics, v. 191, p. 27–53.

Nyblade, A.A., and Robinson, S.W., 1994, The African su-

perswell: Geophysical Research Letters, v. 21,p. 765–768.

Partridge, T.C., 1997, Late Neogene uplift in eastern andsouthern Africa and its paleoclimatic implications, inRuddiman, W.F., ed., Tectonic uplift and climatechange: New York, Plenum, p. 63–86.

Partridge, T.C., and Maud, R.R., 1987, Geomorphic evo-lution of southern Africa since the Mesozoic: SouthAfrican Journal of Geology, v. 90, p. 179–208.

Peel, F., Jackson, M., and Ormerod, D., 1998, Influence ofmajor steps in the base of salt on the structural styleof overlying thin-skinned structures in deep water An-gola: American Association of Petroleum GeologistsInternational Conference and Exhibition, ExtendedAbstracts Volume, Rio de Janeiro, Brazil, November,p. 366–367.

Reis, B., 1972, Kimberlite distribution in Angola and itstectonic control: Montreal, Canada, International Geo-logical Congress, 24th, Abstracts, v. 24, p. 146.

Roberts, D.G., 1989, Basin inversion in and around theBritish Isles, in Cooper, M.A., and Williams, G.D.,eds., Inversion tectonics: Geological Society [London]Special Publication 44, p. 131–150.

Rodrigues, B., 1972, Conclusions on the distribution of al-kaline and carbonatitic complexes of Angola and Bra-zil: Revista da Faculdade de Ciencias, Universidadede Lisboa, Serie C: Ciencias Naturais, v. 17,p. 89–108.

Rowan, M.G., 1993, A systematic technique for the se-quential restoration of salt structures: Tectonophysics,v. 228, p. 331–348.

Sahagian, D., 1988, Epeirogenic motions of Africa as in-ferred from Cretaceous shoreline deposits: Tectonics,v. 7, p. 125–138.

Sandwell, D.T., and Smith, W.H.F., 1997, Marine gravityanomaly from Geosat and ERS 1 satellite altimetry:Journal of Geophysical Research, v. 102,p. 10,039–10,054.

Scheevel, J.R., and Dale, C.T., 1993, Models of the evo-lution of the lower Congo Basin, offshore Cabinda(Angola) and Zaire: American Association of Petro-leum Geologists Annual Meeting Abstracts, v. 2,p. 178.

Schultz-Ela, D.D., Jackson, M.P.A., and Vendeville, B.C.,1993, Mechanics of active salt diapirism: Tectono-physics, v. 228, p. 275–312.

Sinclair, I.K., 1995, Transpressional inversion due to epi-sodic rotation of extensional stresses in Jeanne d’ArcBasin, offshore Newfoundland, in Buchanan, J.G., andBuchanan, P.G., eds., Basin inversion: Geological So-ciety [London] Special Publication 88, p. 249–271.

Spathopoulos, F., 1996, An insight on salt tectonics in theAngola Basin, South Atlantic, in Alsop, G.I., Blundell,D.J., and Davison, I., eds., Salt tectonics: GeologicalSociety [London] Special Publication 100, p. 153–174.

Steckler, M.S., and Watts, A.B., 1978, Subsidence of theAtlantic-type continental margin off New York: Earthand Planetary Science Letters, v. 42, p. 1–13.

Steckler, M.S., Watts, A.B., and Thorne, J.A., 1988, Sub-sidence and basin modeling at the U.S. Atlantic pas-sive margin, in Sheridan, R.E., and Grow, J.A., eds.,The Atlantic continental margin, U.S.: Boulder, Col-orado, Geological Society of America, Geology ofNorth America, v. I-2, p. 399–416.

Talwani, P., 1990, Neotectonics in the southeastern UnitedStates with emphasis on the Charleston, South Caro-lina, area, in Krinitzsky, E.L., and Slemmons, D.B.,eds., Neotectonics in earthquake evaluation: Geolog-ical Society of America Reviews in Engineering Ge-ology, v. 8, p. 111–129.

Teisserenc, P., and Villemin, J., 1989, Sedimentary basinsof Gabon—geology and oil systems, in Edwards, J.D.,

and Santogrossi, P.A., eds., Divergent/passive marginbasins: American Association of Petroleum GeologistsMemoir 48, p. 117–199.

Total-Sonangol, 1987, Carte geologique du bassin duKwanza, Angola: Scale 1:250,000.

Uncini, G., Mario, B., and Antonio, G., 1998, Neocomi-an—Upper Aptian presalt sequence of southernKwanza Basin: A regional view: American Associa-tion of Petroleum Geologists International Conferenceand Exhibition, Extended Abstracts Volume, Rio deJaneiro, Brazil, November, p. 346.

Vendeville, B.C., and Nilsen, K.T., 1995, Episodic growthof salt diapirs driven by horizontal shortening, in Trav-is, C.J., Harrison, H., Hudec, M.R., Vendeville, B.C.,Peel, F.J., and Perkins, B.F., eds., Salt, sediment andhydrocarbons: Gulf Coast Section, Society of Eco-nomic Paleontologists and Mineralogists, SixteenthAnnual Research Conference, p. 285–295.

Verrier, G., and Castello Branco, F., 1972, La fosse Tertiaireet le gisement de Quenguela-Nord (Bassin du Cuan-za): Revue de l’Institut Francais du Petrole et Annalesdes Combustibles Liquides, v. 27, p. 51–72.

Walgenwitz, F., Pagel, M., Meyer, A., Maluski, H., andMonie, P., 1990, Thermo-chronological approach toreservoir diagenesis in the offshore Angola Basin: Afluid inclusion, 40Ar-39Ar and K-Ar investigation:American Association of Petroleum Geologists Bul-letin, v. 74, p. 547–563.

Watts, A.B., and Marr, C., 1995, Gravity anomalies and thethermal and mechanical structure of rifted continentalmargins, in Banda, E., Torne, M., and Talwani, M.,eds., Rifted ocean-continent boundaries: Boston, Klu-wer Academic Publishers, p. 65–94.

Watts, A.B., and Stewart, J., 1998, Gravity anomalies andsegmentation of the continental margin offshore westAfrica: Earth and Planetary Science Letters, v. 156,p. 239–252.

Watts, A.B., Karner, G.D., and Steckler, M.S., 1982, Lith-ospheric flexure and the evolution of sedimentary ba-sins: Royal Society of London Philosophical Trans-actions, Ser. A, v. 305, p. 249–281.

Weems, R.E., and Lewis, W.C., 2002, Structural and tec-tonic setting of the Charleston, South Carolina, region:Evidence from the Tertiary stratigraphic record: Geo-logical Society of America Bulletin, v. 114, p. 24–42.

Whiteman, A., 1982, Nigeria: Its petroleum geology, re-sources and potential: London, Graham and Trotman,394 p.

Withjack, M.O., Olsen, P.E., and Schlische, R.W., 1995,Tectonic evolution of the Fundy rift basin, Canada:Evidence of extension and shortening during passivemargin development: Tectonics, v. 14, p. 390–405.

Worrall, D.M., and Snelson, S., 1989, Evolution of thenorthern Gulf of Mexico, with emphasis on Cenozoicgrowth faulting and the role of salt, in Bally, A., andPalmer, A., eds., The geology of North America—Anoverview: Boulder, Colorado, Geological Society ofAmerica, Geology of North America, v. A, p. 97–138.

Ziegler, P.A., 1989, Evolution of the North Atlantic—Anoverview, in Tankard, A.J., and Balkwill, H.R., eds.,Extensional tectonics and stratigraphy of the North At-lantic margins: American Association of PetroleumGeologists Memoir 46, p. 111–129.

Zoback, M.L., 1992, First- and second-order patterns ofstress in the lithosphere: The World Stress Map Pro-ject: Journal of Geophysical Research, v. 97,p. 11,703–11,728.

MANUSCRIPT RECEIVED BY THE SOCIETY 6 AUGUST 2001REVISED MANUSCRIPT RECEIVED 6 MARCH 2002MANUSCRIPT ACCEPTED 22 MARCH 2002

Printed in the USA

structural segmentation, inversion, and salt tectonics on a passive margin: evolution of the inner...

Documents