origin of the betic-rif mountain belt

8/10/2019 Origin of the Betic-Rif mountain belt

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TECTONICS,OL.16,NO.3,PAGES04-522,UNE 997

Origin of the Betic-Rif mountain belt

Lidia Lonergan

Department of Geology,mperial College,London

Nicky White

Bu]laxdLaboratories, niversity f Cambridge, ambridge,

Abstract. In recentyears, he originof the Betic-

Rif orocline has been the subject of considerable e-

bate. Mudi of this debate has, focused on mechanisms

required o generate apid ate-orogenicxtension ith

coevalshortening.Here we summarizehe principal

geological nd geophysicalbservationsnd propose

model for the Miocene evolution of the Betic-lq. f moun-

tain belts, which is compatiblewith the evolutionof

the rest of the westernMediterranean. We regard

palaeomagneticata, which ndicate hat there have

been arge otations boutverticalaxes, ndearthquake

data, whichshow hat deepseismicity ccurs eneath

the Alboran Sea, to be the mostsignificant ata sets.

Neither data set s satisfactorily ccountedor by mod-

els which invoke convective removal or delamination of

lithospheric antle.Existing eologicalndgeophysical

observations re, however, ntirely consistentwith the

existence f a subductiononewhich olledor peeled

back until it collidedwith North Africa. We suggest

that thisancient ubductinglabconsequentlyplit nto

two fragments, neof whichhas continuedo roll back,

generatinghe Tyrrhenian eaand orminghepresent-

dayCalabnan rc Theother lab ragmentolled ack

to the west, generatinghe AlboranSeaand the Betic-

Rif oroctine uring he early to middleMiocene.

Introduction

Extensionalasinsssociatedithconvergentlate

boundaries are common n the western Pacific Ocean

and n the Mediterraneanea e.g.,Vine andSmith,

1981]. Whilst manyof the PacificOceanbasins ave

clearly formed as a result of back arc extension aused

by subductionone ollback,he originof geometri-

cally similar basins n the westernMediterranean ea

has proved more controversial. Much of the current

debate s focused n the originof the AlboranSea o-

getherwith he surroundingetic-Pfif ountainelts,

the most westerlyof the Alpine mountainchainsof

southernuropeFigure ). Thisoroclineeveloped

Copyright 1997 by the American Geophysical nion.

Paper mlmber 96T003937.

0278-7407/97/96T0-03937512.00

504

during,and partly in responseo, Late Mesozoico Ter-

tiary convergenceetween fricaand beria. In recent

years,hreeprincipalectonicmodels avebeen ro-

posedo accountor ts geometryndevolution:1)

irapidwestward otion f a rigidAlboranmicroplate,

(2) subductmnone ollback, nd 3) radialextensional

collapse ausedby rapid convective emoval or delann-

nationof hthosphencmantle.

Herewe first summarizehe importantgeological

and geophysical bservationshat must be accounted

for by any model for the evolution of the westernend

of the Mediterranean Sea. The limitations of models

which rely on the rapid, convective emoval of litho-

sphericmantleshall hen be discussedWe argue,m-

stead, hat the most mportant observations re consis-

tent with a short, steep east dippingsubduction one

whichpeeledor rolledback at a rate of between50 and

100mm/yr to the westduring he early Miocene ntl

subduetable ceanic ithospherewas exhaustedn the

vicinity of Gibraltar.

Traditionally, rocks of the Betic-lq.iforogenhave

been divided nto External Zones, nternal Zones,and

Flysch appesFigure2). In the BeticCordillera,he

External Zone consists f Mesozoicand Tertiary sedi-

mentary ockswhichweredepositedn basinal Subber-

ics) andshelf Prebetics) nvironmentsn the Iberian

margin of the Tethyan Ocean. These rocks werede-

formed by northwestdirected, thin skinned thrusting

and foldingduring he early to late Miocene Garcia-

Hernandez et al., 1980; Banks and Warburton, 1991;

Allertonet al., 1993]. Shortening as accompaniedy

development f the Guadalquivir orelandbasin. The

Internal Zone, located arther south, consists f moun-

tain rangesof metamorphosed alaeozoic nd Mesozoic

rocks eparated y Neogenentermontaneasins.These

metamorphicockswereaffected y Palaeogene-early

Miocenepenetrativedeformation nd regionalmeta-

morphism.

A threefold division of the lq.if belt of North Africa

isgenerallycceptedWild,1983] Figure ). Its Infer-

nal Zonecontainsmetamorphicocksbroadlysimilar

to thoseof the Betic Cordillera,although he nomen-

claturediffers. The intermediate lysch Zone, nfor-

mally nown s heFlysch appes,onsists ainly f

Early Cretaceouso early Miocenedeepmarineclas-


2/19

LONERGAN AND WHITE: ORIGIN OF THE BETIC-RIF MOUNTAIN BELT 505

Neogenehrustelts

IMajorhr,

Alborane

5 lo 1' 2O 25

. s,,km..1

Figure 1. Map of MediterraneanSea showing rincipal hrust belts and Neogene xtensional asins s

referred to in text. Area known as westernMediterranean ncludes:TyrrhenianSea, ValenciaTrough,

Alboran Sea, and surroundingAlpine mountainchains.Thrust beltshavebeensimplifiedby division nto

two parts: "Internal Zones"deformedmainly in Palaengene,ncludingmetamorphicocks,and "External

Zones"which are thin-skinnedand formed n Neogene.Regions hat haveundergone eogene xtension

(westernMediterranean, egeanSea, and Pannonian asin)are all surroundedy thrustbelts. Note

marked curwtture of the Betic-Pdf mountain chains and the Calabrian Arc at each end of the western

Mediterranean.

tic depositsWildi, 1983]. The ExternalZoneof the

Rif is made up of Mesozoicand Tertiary sedimentary

rocksdepositedon the North African margin. Dur-

ing the early Miocene, ts Internal Zone was thrust

onto he Flyschnappes.Continuedconvergenceuring

the Miocene deformed the External Zones into a thin-

skinnedoldand thrustbelt [Andrieux, 971,Frizonde

Lametie, 987, Morley, 1988]. Continental rustbe-

neath he Alboran Sea itself consists f metamorphic

rockswhichare similar to thoseof the adjacent nter-

halZonesP atlei a .,1996].

Key Observations

Coeval Extension and Shortening

The Alboran domain and adjacent nternal Zones

underwent considerable extension at the same time as

northwest,south, and southwest ergent hrusting oc-

curred in the External Zones of the Betic and Rif moun-

tains,respectivelyFigure3) [FrizondeLamolte,1987,

Garcla-Daeias el aL, 1988, P all and Vissers, 1989,

Banks and Warburton,1991, Fayre el a ., 1991, Garcia-

Dueas el a ., 1992, Comasel a ., 1992,Lonerganel a .,

1994, Jabaloy l aL, 1993, Walls el a ., 1993, P alzman

el a ., 1993]. Close o Gibraltar, hrustings directed

westwardBalanyd ndGarcia-Due6as,987,1atzman

e a ., 1993,P all el a ., 1995].The ENE striking nter-

hal/External oneboundarynd he Crevillcnteault

of the Betics ogetherwith the NE strikingJebhaand

Nekor aultsof the Rifwereoriginally nterpreted s the

northern and southern boundaries of a westward mov-

ing AlborannficroplateAndrieuz l a ., 1971,eblanc

and O ivier,1984]. However, ecentstructuralstudies

haveshownhat oblique onvergenceccurred long he

Internal/Externaloneboundary uring arlyMiocene

timesLonergan.l a ., 994, ertonl a .,1993].


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5O6

LONERGANNDWHITE:ORIGINOFTHEBETIC-RIFMOUNTAIN ELT

Iberianoreland -:'-:. -'?'"- .> Forelandasis

" ' L''' :;"'z:"fVFE .....one

7 Flyschappes

NeogeneVolcanism

Calc-alkaline

(Shosmte,)

" 10Ma .':" ( :;.' . .. : ' Strike-slipault

J.F 12 NF - 9-5 a ::"...

- s-s-s/vfricanoreland

NENekorault

60--,1 x'"'- o o o o 341NiF JebhaFault

'igure 2. Map of Alboran Sea and adjacentmountain belts. Main tectonicsubdivisions f Betic and

Pdf mountains re shown ogetherwith locations nd compositions f Neogene olcanic ocks.

Wide-angleseismic ata modelingshows hat the

crust varies from 40 km thickness at the Inter-

hal/External Zonebonudary n the Betics o about 15

km thicknessn the centerof the AlboranSea [Banda

and Artsot[e,1980,Torn[ and Banda, 992].Setsuric

reflection profileswhich traverse he Aiboran Sea also

showevidence f large-scaleormal aulting Wattset

al., 1993,Comas t al., 1992,Mauffret t al., 1992].

Analysisof Neogene ubsidencesingwell-log nforma-

tion from he northernmargin f thebasin ives tretch-

ing factors f 1.3-1.6 Wattset al., 1993].

There is also good evidence or extension both

within hemetamorphicocks f the nternal onese.g.,

Garcfa-Dueas t al.,1992,Jabaloy tal., 1993,Crespo-

Blanc et al., 199't]and within the well-exposedeo-

genebasiuswhichoccuronshore pain e.g.,Montenat

et al., 1987,Mora-Gluckstadt,993].The existence f

large amountsof crustalextensionwithin the metamor-

phosed Internal Zones is inferred from the structural

juxtaposition of rocksof signilictmtly ifferentmeta-

morphicgradealongmajor extensional hear ones uch

as the Boricmovementone Plattand Vissers,989]

and the Alpnjarride/Malaguide ontact Aldayae at,

1991, onergan ndPlatt, 1995].

Rapid exhumationof metamorphic ocks n the In-

ternal Zones of the Betics has been documentedusing

both radiometricdating and stratigraphicprovenance

studies Zeck et al., 1992, Moni[ et al., 1991, 1994;

Loner[an ndMange-Rajetzky,994].Exhumationnd

rapid coolingof metamorphic ocksoccurred rom ear-

liest Miocene times to ,-, 15 Ma, coeval with extension.

Rotations

Palaeomagnetic tudies confirm that thrust sheets

vithin the External Zones of the Bette mountains have

rotatedclockwise bout verticalaxes by up to 130

post-Oligocene,robablyn the MioceneOscte t al.,

1988,1989;Platzman,1992, Allerton et al., 1993, Platt

ctal., 1995].Even arger otations,-, 200 ) havebeen

documentedn Malaguide ockson the northernmar-

gin of the Iuternal Zone n the easternBeticsJAilerton

et al., 1993]. n the Rif, counterclockwiseotatious f

,-, 100 havebeenmeasuredPlat-'mant al., 1993].

Mostof the publishedalacomagneticata for the


4/19

LONERGAN ND WHITE:ORIGINOFTHE BETIC-RIFMOUNTAIN ELT 507

$triksllp Alkali

Pllocene m hortening Basalts

$--Messlnlan K'enriched--

15- Langhian --,,, o

Burdlgalian

4qu#anlan

Oligacene

35

Albaran Internal External Valcanlsm

Sea Zane Zane

Figure3. Titrangof Neogene ventsn BetteCmdilleraand

Alboran Sea

Behc and Rlf mountruns were collected from Am-

momtcoosso nd capas o3asacies imestonesf Up-

perJurassic nd Upper Cretaceous ges, espectively.

Teetomeotstransmust haveoccurredafter deposition

and re ikely o havebeenassociated ith foldingand

thrurang uring he early o middleMioceneAllerton,

1994] The Alletlon et al. [1993]studywas directed

at hmmg palaeomagnetic otations in a suite of rocks

rangingn age rom Triassic o upper Miocene ocatedat

thenorthernmarginof the nternal Zonesm the eastern

Betks. Triassic,Jurassic, nd upper Oligocene ocks

wthm a thrust stack all show similar amounts of ro-

tabon ,.. 200),but Oligo-Miocene arls rom the as-

sociatedorelandbasin are only rotated by 140. This

resulted Allerton et al. [1993] o concludehat ro-

tationstarted n the upper Oligocene nd continued

while orelandbasin sediments ere being deposited.

Uncleformedpper Miocenemarls from the adjacent

lntermontane asin are not rotated, providingan np-

per time constraint on the rotations. If rotation started

In the late Oligocene, he maximum time interval for

theobserved00 rotation s -. 30 Ms, givinga mini-

mum otation ate of ~ 7/Ms. However,f we assume

thatrotationwas contemporaneousth contractional

deformationlong he Internal/Externalonebound-

ary, henrotationmusthaveceased y Langhmnimes

( 15 Ma [Lonergant al., 1994]),suggestingrota-

tion ate of ~ 13/Ma In the Subbetmof the eastern

Beiges,he maximum rotatmnsare 1300 Here shorten-

ingcommencedn the early Miocene ~ 22 Ma), and

ff we assumeotations ook placebetweenhe early

Mmcene nd the present ay, the minirnumotation

raes 6/Ma. If rotational deformations accommo-

dated long hrusts ctive n the Subbetic uring he

early-middleiocene22-15Ma), then he rotation

ratescouldhave been as hgh as 18/Ma. Rotation

ratesof 6-18/Ma re hesame rderof magnitudes

those roposedy Kissel ndLa 1988]orpartsof he

Aegean egion

Volcanism

Volcanismhas both accompanied nd postdated

Neogene xtension: alc-alkaline,otassic, nd basaltic

volcanism is scattered across he eastern sector of Alb-

oranSeaandBetic-R.ifystemsFigures and3) [Bel-

Ion, 1981,Hernandez ndBellon,1985,Hernandez t

al., 1987]. The earliestgneous ctivity s a basalttc

dyke warm t 22 Ma (K/Ar dating)ocatedn the

centralandwesternnternalZones f the Betics Tor-

res Rolddn t al., 1986]. The earliest olcanicocks

are mostly calc-alkaline nd in Spain are restricted o

the SerradeGata-Carbonerasrea. n theeasternRlf,

calc-alkalineolcanicocks re morewidespread,ccur-

ring in Ras Taraf, TrosFourches, nd in the western

Tell nearOran. In southern pain, he datesobtained

for this volcanic tate ange rom 15 o 7 Ma, whereasn

the Rif-Tellmountains,atesangerom13-8Ma [Bel-

Ion,1981,Hernandezt al.,1987] Offshore, lboran s-

land tsellisa calc-alkallnicolcanic difice.Early work,

quotedby Bellon 1981], uggestedn ageof 245 Ma,

but more ecentworkby Apamcot al. [1991] ates he

volcanism t 18-7 Ma, which s in moregeneralagree-

ment with the agesobtained or onshore uitesof simi-

lar compositionn bothSpain ndNorthAfrica Figure

2). Relativelyargeamounts f volcanismre inferred

offshorerom both magnetic ata and dredging, hese

volcanic rocks are also assumed to be calc-alkaline m

colnpositionHernandezt al., 1987].

A secondbroad state of dominantly potassium-

enrichedrocks with a wide variety of compositions

(shoshouitico lamprotlc) ereeruptedn Spainbe-

tween 8 and 5 Ma and in North Africa between 9 and

4 Ma. The lamproitic volcanic ocks are very widely

scatteredFigure2) In southern pain, he youngest

lavas are Plio-Quarternary alkaline basaltswhich were

erupted near Cartagena. The youngestavas n North

Atnca m'e also alkalinebasalts, ranging n age from

Messinian5.9 Ms) to Quarternary.

In the past, both north and south dipping sub-

ductran zoneshave been postulatedbetweenSpmn

and Africa [e.g., Torres-Rolddnt al., 1985, Sanz de

Galdeano,990].However,t is clear romFgure that

suchsubduction ones re not compatible ith the spa-

tial distribution of volcanism. More recently, French

workers Hernandez t al., 1987, de Larouzre et al.,

1988, Montenatet at., 1987],noting he spatialrela-

tionshipbetween olcanic ocks nd ateNeogene trike-

shp fault systemsn SE Spain Carboneras-Palomares

faults), haveattempted o relate the volcanismo a

"trans-Alboran" shear zone. Seismic reflection data in

the AlboranSea [e.g., Watts el al., 1993]provideno


5/19

508 LONERGANND HITE:RIGINF HE ETIC-RIFOUNTAINELT

344' 348' 352' 356' O' 4' 8' 12' 16 20'

- 19 "

44' .. .. _,,.,_.,, '. "' ' 44'

' ' ' i ' 40'

eO" , ' 81 - / %

. 851027

36'

L'-':

2'

344' 348' 352" 356' O' 4' 8' 12' 16' 20'

Figure 4. General eismicitynd ocalmechanismolutions f arge Mb >_5.5) earthquakeshroughout

westernMediterranean egion.Solutions rom the period 1977-1995were taken from Harvard Centroid

MomentTensor CMT) catalogue. or the regionwestof 10 E, oldersolutionswere aken rom review

by Anderson1985]. When possible, MT solutions avebeenverifiedby comparison ith solutions

determined y conventionalaveformnversion. he dateof eachevent year,month,andday) is given

aboveeachbeachball, the compressionaluadrants f whichare shaded.

evidence or a continuous hear zoneoffshore.The pat-

tern of early calc-alkaline olcanismn the AlboranSea

is not controlled by the location of strike-slip faults.

Existing offshore ata suggest hat the trend of calc-

alkaline volcanic ockscrudely ollows he strikeof the

orocline.

Selsnticity

Bafornet aL [1990] ummarizeata for threedeep

earthquakeswhichoccurred eneathGrinreda his cen-

tury. Theseearthquakesestify to the existence f ma-

terial which s coldand rigid enougho produce udden

releases f strainenergy t considerableepths eneath

the Alboran Sea. The best knownand largestof these

earthquakesccurredn 1954 Mb = 7.1, Figure ) at a

depthof ~ 630 4- 4 kin. The P axes or all threeearth-

quakes ip steeplyeast [Changand Kanamori,1976,

Grimisonand Caen,1986,Bufornet al., 1988,1990].

Good station distribution throughout Europe mean8

that tile nodal plane strike of the 1954 event is well

determined24- 5 E). Intermediate-depthnd shal-

lowseismicity ccurshroughoutheregion, he biggest

events ndicating oblique convergence etweenAfrica

and Europe Figure4). In the AlbortmSea, nterme-

diate events do not occur below 150 kin, and there s

also smaller apbetween 0 and 100km [Sebcr l al.,

19961.

Grimson nd Caen 1986]and Bufornei al. [1990]

pointout that the presencef a steep, astdipping, lab

is verydifficult o reconcile ith the Neogene ndRe-

centplatemotionvectors.Thesevectors re oriented

north-south to northwest-southeast and have been used

to invoke he existence f a north or south dipping ub-

ductionzone betweenNorth Africa and Europe. The

above authors also argue that acceptedplate recon-

structions recludehe existence f 600 km of oceanic

crust betweenAfrica and Iberia during the Tertiary

[e.g.,Dewey t al., 1989].

This geometricproblemhas ueledmore recentspec-

ulation that the deep earthquakesare evidence or a

blockof ithospheric antlewhichhasbeenconvectively

removedGrimsonndcaen,1986]. eber i al. [1996]

suggesthat the distributionof relocated ntermediate-

depthseismicitys consistent ith activedelamination.

Unfortunately,heir analysisgnoreshe existencef

the deep 1954 event, which dominates he total seis-

mic moment eleasewithin the region. Furthermore,

their inferencehat a largezoneof upwelled stheno-


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LONERGAN AND WHITE ORIGIN

sphericaterialccursetween0and 0kmdepths

incompatibleith models f melt generationy adia-

batic ecompressionf the asthenosphere.

The presencef an eastdippingP axiswithin an

eastwardippingWadati-Benioffone s entirelyconsis-

tentwith the existence f a N-S strikingsubducted lab.

The argeseismic ap between150 and 600 km sug-

gestshat hiscold ubductedlab asnowdetached,

inagreementith ectonicvidencehat 12-15Ma have

elapsedince ubduelionerminated.Wealso ote hat,

in fact, there s little difficultym subducting 00 km

of old oceanic ithosphere f the subduelionzone rolled

backroma positionwestof the currenteasternmargin

of beria toward Gibraltar.

Timing and Plate Motions

Foreland-directedhrusting, coevalwith extension,

occursn all threesides f the oroclineFigure3). On

theSpanishmainland, typical foreland-basin tyle sub-

sidence ccurs within the Guadalquivir basin. This

basincan be traced from the Prebetic arc in the east,

through ibraltarandonto he Moroccanmarginwhere

it is called the Rharb basin. Seismic reflection data

showhat deformed nits the Flysch/External ones)

occurup to 250 km west of Gibraltar, probably rep-

resentinghe most westerlymanifestation f shorten-

lug ajal t al., 1975]. Usingbathymetric ata, a3at

e al. [1975] lsoproposehat shortening, anifest s

ohsostromes,xtends up to 500 km west of Gibraltar

and that Tortonian sedmentaryrocks are shortened.

SinceatestTortonianimes 8-9 Ma), the Africa-

Eurasiaplate convergence ector has changed rom

north-southo northwest-southeastDewey l al, 1989,

Mazzoh nd Helman, 1994]. As a consequence,ex-

tral oblique hortenings now occurring etweenNorth

Africa and Iberia. This overall motion is manifestby

strike-slipaulting, local folding, thrust faulting, and

structural nversionof previous normal faults which

affected sediments within both the Alboran Sea and

withinonshore eogeneasinsWoodsidendMaldon

ado, 1992, Walls el al., 1993, de Larouzre et al.,

1988].Since he Pliocene,he onshore eogene asins

in southern painhavebeen apidyuplifted,attesting

to continued onvergenceetweenAfrica and Eurasia

and/orslabbreak-off.

Models for Generating Late Orogenic

Extension

The paradoxof coeval hortening nd extensioned

Dewey1988], latl and Vzssers1989], ndDoblas nd

Oyarzun1989]o proposehat the Alboran eaand

surrounding ountainchainsormedby extensionalol-

lapse f a previouslyhickened rustand ithospheric

OF THE BETIC-RIF MOUNTAIN BELT 509

mantle. Their work ollowshe earliersuggestionf

Grmson ndChen 1986]whoused eleseismicarth-

quakedata to infer he existencef a detachedrag-

ment of lithospheric antle. Severalotherworkers ave

elaboratedhesedeas Platzman, 992,Plat andEng-

land,1994,Vsserst al., 1995]. Alongwith the TI~

betan plateau, he Betic-Rifmountainbelt is now re-

gardedasan mportant xample f extensionalollapse

driven by rapid, convectiveemovalor delamination f

thickenedithospheric antle Housemant ai., 1981,

England ndHouseman,989].Herewebriefly eview

two principalmechanisms hich have beenused o ac-

count for the main observations n the western Mediter-

ranean: convective removal or delamination of the htho-

sphericmantle and subduelion one rollback.We shall

not consider models which invoke westward motion of

an Alboran "microplate"within a zoneof overall con-

vergence etween fricaand Europe Andreux t al.,

1971,Leblanc nd Ohwer,1984]. Althoughhis is an

appealingmechanismor producinghe dramaticoro-

elihal bendabout he Straits of Gibraltar, t is evident

that the Alborandomain id not behaveigidlyduring

the Miocene.

ConvectiveRemoval/Delamination Models

Using fluid dynamicalarguments,Parsons and

McKenzie1978] howedhat thevigorouslyonvecting,

adiabatic nteriorbeneath he oceanic late s separated

from the mechanical oundary ayer of the lithosphere

(~ 80 km hick)by a thermal oundaryayer ~ 50 km

thick). This thermalboundaryayer hasa lowervis-

cosity han the mechanical ounday ayer and acts to

maintain the thickness f the lithosphere t 120 km by

periodicallyverturningEngland,993].Subsequently,

Housemant al. [1981] arriedout a rangeof numer-

ical experiments hichshowedhat if an equilibrated

lithospheric late s nstantaneouslyhickened,hen he

thermalboundary ayerbecomes nstable nd dropsoff

on timescales f about 10 Ma for geologicallyealistic

Raleighnumbers.n their calculations,heysystemati-

callyvaried he thicknessf the thermal oundaryayer

by 1 orderof magnitude.

The principal difficulties with convectivere-

moval/delaminationodelsre wofoldFirst, he ime

taken for delamination to occur is sensitive to the vis-

cositystructure f the lithosphere.Buckand Toksdz

[1983] avecarried uta much maller umber f nu-

merical xperiments,nd heyconcludehat onlya thin

one at the base of the thermal boundary ayer is re-

movedduringconvectiveverturn.This disagreement

is a consequencefuncertaintiesn the heologicalrop-

ertiesof the upper mantle. Furthermore,emovalof

lithospheric antle s an inefficientmeansor generat-

ingsubstantialotential nergy ifferencesecausehe

densitycontrastbetweenithospheric antle and as-


7/19

$10 LONERGAN AND WHITE: ORIGIN OF THE BETIO-RIF MOUNTAIN BELT

thenospheressmall,-,0.13Mgm-a). Second,ouse-

manei al. [1981] ssumedhat shorteningf the itho-

spherewasboth uniform nd nstantaneous.f, how-

ever,shortening ccurs t geologicallyealistic train

rates, he thermalboundaryayerwill probably on-

vectivelyoverturn on a continualbasis.

If convectivenstabilityof a thickenedhermal

boundaryayercan akeplace ufficientlyapidly nd

on a largeenough cale, hen hismodel or the Betic-

Rff area s appealing. owever,here re wo mpor-

tant setsof observationshich renoteasily xplained

by a combinationf ithospherichickeningnd adial

extensionalollapse. irst,shorteningasonlybeen

takenup on hree ides f heorogennd s n nosense

radial Figures and2). It is unclearow he ree ast-

ernedge astof theAlboran ea anbeaccountedor,

except y nferringhatextensionalollapseenerated

the whole f thewestern editerraneanegion. his

inference equiresevidenceor coeval utwarddirected

thrustingall around he westernMediterranean ea.

Second, o seriousttempthasbeenmade o account

for he arge, lockwisealaeomagneticeclinationsb-

served n the ExternalZonesof the Betic mountains

and for the comparable ounterclockwiseeclinations

within heRif. Platzman1992]uggestshat heseo-

tationscouldbe incorporatedntoan extensionalol-

lapsemodel, lthoughhe oncludeshat hepalaeo-

magneticdata requiresomewestwardmotionof the

Alboran egion. n summary,arge ertical-axisota-

tions re hecrucialbservationhichannotasily

be accountedor by convectiveemoval r delamina-

tion. McKenzzendJackson1983]howedhat he

spreadingf eithera circular r elliptical lobdoes ot

generate ny rigid body otation.

Potential Energy Calculations

If significantonvectiveemovalccurs,t willgen-

eratearge mountsfgravitationalotentlalnergy.t

has ong een ecognizede.g., effreys,952, ott nd

Dean, 972]hatwhilsthe ithospheres n approx-

mate sostatic quilibrium,onsiderableateralvaria-

tion in densitymeanshat the stress alances not

complete: orizontal evlatorictressesanbe of the

order f 100MPa. If thegravitationalotentialn-

ergyof a region fcontinentalithospherexceedshat

of tssurroundings,henhat egionillbesubjected

to extensionaleviatorictresses.egionsithgreater

potentialnergyre suallytgreaterurfacelevation,

but inceotentialnergyependsponhe ensityis-

tribution ithin he ithosphere,levationlone annot

beusedo calculateotentialnergyifferences.t has

beenhownhatdifferencesn hedepth-averagedori-

zontal eviatorictressesor wo ithosphericolumnsn

isostaticquilibriumreproportionalo differencese-

tweenheir ravitationalotentialnergiesMolnarnd

Lyon-Caen,988]. ithosphericolumnsrebalanced

with eferenceo mld-oceamcidges heret isreason.

able o assumehathorizontalndvertical tressesre

equal i.e., a --=

Extensionalollapsef hecontinentalithosphere

thoughto be riggeredyarapidncreasen thepoten.

tialenergyfa lithosphericolumnPla andEngland,

1994]. his nferences based pon imple alculations

which how hat if the bottompart of thickenedtho-

sphericmantle s suddenlyemoved, rapid ncrease

in potentialnergyccursFiguresa and5band p-

pendix). This increase artly manifeststself asan n-

creasen surface levation.t is generally ssumedhat

rapdncreasesn potentialnergy 5 x 101-"m-1)

will give ise to extensional ollapse

The difficultyn using otential nergy alculations

to argue n favorof extensional ollapsemodelss that

the continentalithospheres clearly able to sustain

largedifferencesn potential nergywithoutgenerat-

ingextension.orexample,hepotential nergy iffer-

ence etweenouthernfricaand he surroundingld

ocean asinss as argeas that expected y convective

removalalculationscompareigures b and5d) In

fact,differencespotential nergymayhave opar-

ticular ignificance:t is more mportanto developn

understandingf mechanismshich llow tored oten-

tialenergyo bereleasedatherhan nterpretoten-

hal energyifferenceserse. Evenf large tfferences

in potentialnergyid ead o extensionalollapse,t s

not clearwhyextensionhould ontinue fter hmkened

continentalrust asbeen hinned nd ts upper ur-

face estoredo sea evel,unless ubsidenceelow ea

level s entirely aused y thermal ontraction

Subduerion Zone Rollback Models

At many sland rcs, he subductinglab etreats

away rom he arc n the hotspotrameof reference

[Molnarnd iwaler,978, hase,978, ewey,980].

Thispeeling r rolling ackof a slabgeneratesnd

maintainsback arc extension.At continent-oceanu}>

ductionones,ollbacks probablyheprincipal eans

bywhich otentialnergytoredn thickenedontinen-

tal lithospheres released.

Rollbacks a naturalconsequencef subducting

oldoceanicithospherehichs colder, nd herefore

denser,han hemantlehroughhicht sinksLeP-

chon ndAnteher, 981].Within hesubductinglab,

thevertical egativeuoyancyorce canbe resolved

into wo omponents:ne omponentt right ngleso

thedippinglabR anda secondomponentlonghe

slab (Figurea) If theslabs nstatic quilibrium,

is balancedy viscousorces n the twosurfacesf the

slab nd y esistiveorcest theslabip. Theonlyway

in which canbesupporteds f thepressureithin

theasthenosphericantleeneathhedippinglabs


8/19

LONERGAN AND WHITE: ORIGIN OF THE BETIC-RIF MOUNTAIN BELT $11

a)RI [U.nforrp.

Lithosphere m

c)

Reference

b) Continent-ontinent

i ..............o

o . -

d) Ocean - Continent

o . .... i .

Figure 5. Potential nergy alculationsor different olumnsf lithosphere.a) cartoonllustrating

two end-membersor which potential energydifferences calculated.Reference olumnof continental

lithosphereith30 km thickcrust s n isostaticquilibrium ithstandard id-oceanidge olumn.b)

Calculated otential nergy ifferencex 10 3 Nm ) asa function f hickeningor woend-members

shownn Figure5a. Uppercurve s labeled convectiveemovalS:ithospherehich asbeen hickened

by factor but lowerpart of lithosphereasbeen emovedo maintainithospherichicknesst 125

km. Lowercurve s labeled"uniform hickening":ithospherellowedo thicken niformly. f convec-

tive removalof thickenedithosphericmantleoccurs t f = 1.8, then the path taken s shown y solid

arrowheads.A valueof +0.5 is maximum evelof potentialenergy ifferenceikely o be sustainablen

Earth. (c) and d) Potentialnergy alculationsorsouthernfrica omparedithsurroundingcean

basins.Crustal thicknessaken from Qiu [1995].

greater han the pressure bove he slab. This pressure

differences superimposed pon the hydrostaticpres~

suregradientand so will tend to drive mantlemate-

ria} romhigh o low pressurehus acilitating ollback

(Figures b and 6c). The buoyancyorces eneratedy

slabs reatlyexceedother convectiveorceswithiu the

mmitle,aud so motion will largely be controlled y the

densityexcess f the slab. The component f velocity

that is normal to the slab is determined by the ease

with which material can flow around the slab. Flow

cmieasilyoccur n the strikedirection i.e., around he

edges f the slab),'ut the increase f viscosity ith

depthmeans hat flow around he tip of the slab is

moredifficult. Hence he normalcomponent f velocity

isgreatest t subduction ones hat havea shortstrike

length< 1000km).

When subduction zone rollback is initiated at a

continent-oceanargin, t is unlikely hat the exten-

sional deviatetic stresseswithin thickened continental

lithospherean be trmismittedhrough igid oceanic

plate to the mid-oceanidge. As a result,continen-

tal lithosphere ill collapsey rapidextensionnto the

spaceprovided.

Estimatesor rollback elocity aryconsiderablyut.

aregenerally00-200 un/yr Dyerkint al., 1993].n

the Aegean,ePichoaandAneljer1981]used he

relativemotionsof Africa, Europe,andTurkey to infer

that the 11ellenic rench olledbackat - 40 mm/yr.

Vine and$'mth1981] uote50 mm/yr for the Scotia

Arc. Modern estimatesbasedupon Global Position-

ing SystemGPS) dataaudmomentensornversiou

of earthquake ata yield higher alues.Bilhris el al.

[1991] btained10-t-28mm/yr ron n analysisf

GPS data in the Aegem. he highestollbackveloci-

ties are from the qbnga-Kermadecreuchwhere Betas

et al. [1995] arried ut a series f GPS campaigns


9/19

512 LONERGANNDWHITE:RIGINF HE ETIC-RIFOUNTAINELT

SLibducfior'oP

/

oll-back / F

/ Extension

Figure 6. Cartoonswhich llustrate dealized inematic volutionof subduction one ollback: (a) cross

section efore ollback ommences,b) cross ection fterrollback, nd (c) plan view of oroclineshowing

juxtaposition of shorteningand extension ogether with systematic vertical-axis rotations.

and obtained 160 mm/yr for the opening f Lau basin

with convergenceatesof 240 mm/yr acrosshe Tonga

rencll.

Once rollback commences, t should continue until

the systemcompletely uns out of denseoceanic itho-

sphere. Thus there are two principal ways by which

rollbackceases. irst, t ceases hen he retreating ub-

duction zone collideswith mid-oceanicidge. Thns it

is likely that the Scotia Arc will continue to roll back

until it meets the mid-Atlantic idge. Second,when

the arc collides ith continentalithospherei.e., dense

oceanic lithosphere s completelyconsumed) ollback

ceases. his more nteresting ase s of especialmpor-

tance within continent-ocean-continent collisions where

the originalpassive ontinental arginsorm irregu-

lar boundariese.g.,during he closure f the Tethyan

Ocean). When an arc collideswith an irregularcon-

tincntal boundary,he subductinglab will probably

split into two partswith eachsegment ontinuingo

roll back.

As well as generating apid back arc extension

within a zone of overallconvergence,ollback s also

an excellentmeans orgeneratingapid otations bout

vertical axes. The classic xamples ome rom south-

east Asia where palaeomagneticeclination nomalies

are associatedith heopeningf manymarginal eas

(see arrard nd$asajima1980]or summary).Palaeo-

nmgnetic tudies rom many of these sland arcsclearly

demonstrate he role of rapid block otations aboutver-

tical axes n backarc basins.The largest otationsoccur

in the Japan, Ryukyu, and Marianas basinswhere rota-

tions of up to 80 haveoften been observedand where

major diffcrencesn the amountof rotation experienced

by differentsegments f the same arc occur. For ex-

ample, palaeomagneticata from southwest apan s

plentiful and clearly demonstratesclockwise otations

of 380-47 Most of this rotation occurred between 16

and14Ma [e.g.,Olofujiel al., 1991]. n contrast, orth-

east Japan rotated counterclockwisey ~ 32. These

estimates ield an angularvelocityof 20/Ma. It is n-

teresting o note that the physiography f the Japmi

Sea s a mirror mageof that of the westernMediter-

ranean Figure7).

Subduction Rollback in the Alboran Sea

We contend hat the Neogene tructureand evolu-

tion of the Betic-Rifmountainbelts are bestexplained

by the westward ollbackof a short east dipping nl>

duction one Frizonde Lamolleel al. 1991, Royden,

1993].Sinceate Oligoceneimes, he Africanplatehas


10/19

LONERGAN AND WHITE: ORIGIN OF THE BETIC~RIF MOUNTAIN BELT

A 5OOm

Bathymetry

Figm'e 7. Bathymetryof (a) westernMediterranean,b)

SouthChina Sea, and (c) Japan Sea. Note similar phys-

iographies.

convergedith Iberia by about 300 km [Dewey t ai.,

1989].

An iinportant argument n favor of a subduction

rollback model iu the western Mediterranean is that the

boundary eparating xtensionrom shorteningollows

the oroclinal bend around the Straits of Gibraltar. The

Flyschnappesand External Zone "olistostrornes"est

ofGibraltarmaybe analogotiso the mbricatedrench

sedimentswhich are associated with arcs. If so, their

location nd strike can be used to pinpoint the final

position f the inferrednorth-south trikingsubduction

zone.Continentalithosphere t the Straitsof Gibraltar

hasprobably reventedollback romproceedingny

farther. n contrast,Royden 1983]suggestshat roll-

backmay havecontinued s far west as he Horseshoe

Seamounts.

As describedabove, volcanism s relatively young

when ompared ith deformation f InternalZones. t

is coeval ith and youngerhan extensionnd s dif-

fusely located within the Alboran Sea, onshoreNorth

Africa, and n southeast pain. Sucha pattern is diti-

chit to attribute to either a north or southdippingsub-

ductionzone,bul it is compatiblewith an east dipping

subduction onewhid rapidly rolled back o the west.

The rangeof cronpositionf volcanic ocks rom calk-

alkaline o alkalineand basaltic s typicalof small back

arc basins where both arc and extensional volcanism oc-

cur. In the Alboran Sea, no calk-alkaline olcanic ocks

havebeen eportedbetween he Straitsof Gibraltar and

,.040 west Figure2). Assuminghat the final surface

positionof the Alboran subductionzonewassomewhere

between he Straits of Gibraltar md the westerly imit

of deformationthe "olistostromes")nd hat the calk-

alkalinevolcanicedificesn the Alboran Sea represent

the dismembered rernains of a small arc, then the fi-

nal arc-trenchgap was of the order of 200 kin. This

distance s similar to the arc-trenchgap between the

Aeolian Islands and the Calabrian trench. the lack of

a clearly definedvolcanicarc in the Alboran Sea may

be attributed o oneof twocauses:1) late Miocene o

Recentstrike-slip aulting postdating he cessation f

subduction nd thus disrnemberinghe arc, or (2) an

originally arrowsnbductionone hat moved apidly,

preventing large,stationary rc edificerom forming.

In summary, he geology f the Betic Rif orocline

and formation of the Alboran Sea can be accounted for

by a north-south trikingsubduction onewhichorigi-

nally dippedeast. Rocks hat now makeup the base-

ment of the Alboran Sea and the Internal Zones of the

Betic and Rif mountains riginally ay to the eastof the

subduction oneand formeda collisional edge.As the

subduction onerolled back, thickened rustbehind the

subduction oneextended apidly to fill the spacegen-

erated by the retreating ubduction one. Rollbackof

the subduction zone and outward displacementof the

extendingnternalZones-Alboranea mpinged n the

passivemargins f both he IberianandAfricanplates,

causinghe onsetof oblique hrusting nd rotations n

the External Zones of the Betms and Rif mountains in

the early Miocene.

Shorteningn the ExternalZones ndextensionn

the Alboran Sea erminatedby the late Tortonian (Fig-

ures and8), suggestinghat thesubductinglabhad

rolled back as far as the Straits of Gibraltar by this

tirnc and that the Alboran Sea basin had achieved

its present-dayimensions.s the narrow ubduction

zonepeeledback, rotationaldeformatim n the mar-

ginswas akenupby oblique-sliphrustaultingn the

ExternalZones.Large otations re probably lsoas-

sociatedwith major extensionalaults n the Internal

Zones and Alboran Sea, similar to thoseobserved n

the Aegean.The onlypublished alaeomagncticata

for the Internal Zones come from the Cabo de Gata

volcanic rovincewhere otations re associated ith


11/19


12/19

LONERGAN ND WHITE:ORIGINOFTHE BETIC-RIFMOUNTAIN ELT

c. 5Ma

---;E:._...E:_--%E'-:--

berian & African

PassiveMargins

a

--] nternalones ShorteningAiriralateollonector

wrt Europe)

E]Flyschasins ' Blockolations

r BackrcxtendingSubductiononeubductlononehrustront

egion rollbackirection

E Externalones

Fiur 9.


13/19

516 LONERGANNDWHITE' RIGINFTHEBETIC-RIFOUNTAINELT

tgnyetal., 1981,Vghott ndLangenhem,995] Fig-

ure 9a)

There is general greementhat the the Balearic

and north Tyrrhenianbasins nitiated n a backarc set-

ting behind he north dipping ubductionone n the

earlyMioceneCohen, 980, ehault tal., 1985,Mal-

mvevno nd tyan, 1986,Kastensetal., 1988,Deweye

al., 1989]. As a consequencef this extension, orsica

and Sardinia rotated counterclockwise,ollidedwith

continentalAdria, and initiated the formation of the

ApenninesAlvarez tal, 1974].Continued outhand

southeast directed rollback of the subduction zone led

to the formationof the southernTyrrhenianSea and

the CainbrianArc (seeDeweyetal., [1989] or fur-

ther details). As shownn Figure8, the oldestdoc-

umented extension m the western Mediterranean area

occurs onshore in southern France where the narrow

Ohgocene rabenof Camargue nd Provence avean

east-westtrike Rehault tal., 1985].The extensional

grabenm westernSardiniawhichextends rom Sassari

to Cagliari owhasa north-southtrke, ut duringhe

late Oligocene-early iocene, hsbasinwasparallel o

theGulfof Lions rabentehault tal., 1985].Byearly

Mioceneimes 23-20Ma), extensiontartedn theVa-

lenciaTrough,Balearicbasin,Algero-Provenqalasin,

and Alborandomain, uggestinghat the samemoch-

arosin s responsibleor the formationof thesebasins.

The onsetof rifting n the Valencia roughgave ise

to clockwiseotation f the BalearicslandsPargs t

al, 1992].The dispersionf the otherunitsultimately

led o the ormation f newoceanicrustn theAlgero-

Provencalasin 21-18Ma [Rehault tal., 1985]).

We suggesthat later collision f the Kabylies onti-

nental ragments ith North Africa orced hissubduc-

tionzone o dividento wosegments:short egment

oriented orth-south hich ubsequentlyorearoundo

form hepresent-daylboran ea nda longer egment

whichcontinuedo evolve ntil the present ay, orm-

ing theTyrrhenian eaand he Cainbrian rc Figure

9b). By 18 Ma, the rotationof Corsica nd Sardina

was omplete,horteningadcommencedn thenorth-

ernApennines,nd hecollisionf theKabyliesith

Africa adcommencedCohen,980,Wildi, 983,

carlei al.,1994]. horteningas lso nderway he

External ones f the Betics ndRif. Hencehesub-

ductiononemusthavebeen pproximatelyocateds

sketchedn Figure10b. Continuedonvergenceed o

theevolutionf two ncreasinglyooped ranchesf he

originalubductiononen theAlboran ndTyrrenhaa

seas,espectively.y Tortonianimes, heAlboranea

hadevolvedo its current izeand shape.At theCal-

abrianArc,rollback ascontinuedhroughouthe ate

Mocenentil he present ay (Figure b) [Anderson

andJackson,987]. hecurrent ositionndasymme-

try of the fiyschbasins rom Scily around o the

n thewesternMediterraneanupportshis subductmn

zone eometry,rovidedhat the Flysch appesepre-

sent he deformed epositsof a trench-forearc asin.

Volcanism

Figure 10b summarizes he age and distribution f

Neogene olcanic ocks n the westernMediterranean

A similar pattern to that already described or the

Betic-Rif mountain chain is evident. Volcanism accom-

panied ndpostdatedhe extension pisode,heearliest

volcamsmending o be calk-alkalineollowed y more

alkalinepotassium-enriched)olcanismcompareg-

ures8 and 10).

The earliestdocumented eogene olcanismn the

western Mediterranean area is calk-alkaline and occurs

on both the western ideof Sardinia 24-13 Ma), on

the margins f the LigurmnBasin 34-20 Ma) [Bellon,

1981, ehault tal., 1985],and n the Valencm rough

These olcanic ocks re related o the earliestphasesf

backarc evolutionwhichbegan s he Gulf of Lons,he

Ligurian,and the ValenciaTroughbasinsopened, nd

as Sardiniaand Corsicabegan their counterclockwise

rotation. In the ValencmTrough,considerable uanm

ties of volcanic rock have been encountered in wells and

canbe nferred romseismic ata [Maillard ei ak, 1992,

Figure . Reconstructrunsilustrtingeogenevolutionfwesternediterranean.a)LateOligocene,

prior oopeningfwesternedlterraneaea ybck rc xtension.redominantlyetamorphicocks

thatmake p nternal onesf Betic-Rifrocline,abyliesf NorthAfrica,Calabria, orsica,nd

SardiniaereocatedehindorthoNWdippingubductionone. lyschepositsCretaceous-early

Miocene)hat an e rcedrom icilylogheNorth frican argino hewesternetics ay ave

beenepositedn the rench-foreaxcasinssociatedith his ubductionone.b)Subductionone

rolledback,CorsicandSaxdlniaotated ounterclockwise,ndBalearicslandsotated lockwises

both iguriannd alenciasinsormed.ymiddleioceneimes.-,18Ma),Kabyliesad ollided

with the northern axgin f Armco,nd hesubductlonone ow ormedwo branches. he narrow

westerlyegmentetreatedapidlyo ormheAlboranea, ausinghorteningnd otationn surround-

ingmount,finelts.Oceanicrustormedn Algero-Provenalasin. heeasternranch f thewestern

Mediterreanubductionystemontinuedoevolventilhe resentay,ormingheTyrrhenianea

andCalabrianrc.ReconstructedositionfsubductiononeorTyrrheniaaaea nd late ectorsre

fromDewey tal. [1989].


14/19

LONERGAN AND WHITE: ORIGIN OF THE BETIC-RIF MOUNTAINBELT

Neogeneceanic

[E Assumedethyian

i .... nicmst

4

Figur 10.


15/19

518

LONERGANAND WHITE ORIGIN

Marllard ndMauffret, 993]. hemajority f thevol-

canicactivity wasassociatedith the opening f the

basin n the earlyMiocene nd,where ampled,hese

voltanitsare dominantly alk-alkalinen composition

A second hase f alkaline olcanismccurredn the

latestMiocene ndPlio-Quarternaryoth n theValen-

ciaTrough8-1 Ma)andonSardima4-2 Ma). On he

northernmargin fAfrica n theKabylie ndTell anges

of Algeria ndTunisia, olcanicocks remainly alk-

alkaline n compositionnd span he middleMiocene

(i.e., both during nd after collisionf the Kabyhes

with North Africa Bellon, 981].A smallamount f

late Miocene alkaline volcanism also occurs.

In the Cainbrian Arc-Tyrrheman Sea-Apennines

system, olcanisms notablyyounger ut againclearly

linked to the back arc evolutionof the Tyrrhenian Sea.

The earliest volcanism ccurson the easternedgeof

Corsica associated with the oldest northern end of the

TyrrhenianSea. Subsequently,olcanismn the north-

ern Apennines ecame oungerromeast o westand

is spatiallyassociated ith the eastwardmigrationof

extension$erw t al., 1993].Apennineolcanicocks

are heterogeneous,ut they all generally howan en-

richment n potassium. The Aeohan slandsand as-

sociated eamounts omprisehe volcanicarc associ-

ated with the Cainbrian ubductionone Ellamet al.,

1989].This archasbeen ctiveor he ast 1.3Ma, and

the volcanic ocks xhibita rangeof compositionsrom

tholeiitic to calkalkahne and shoshonitic. A modest K-

enrichedrendwth ime canbe observedBeccaluvat

al, 1981].

The Tyrrenhian Sea has two basalt-floored ub-

basins, he Vaslovand Marsili basins Figure 10b).

Both ormedvery apidlybetweenhe ate Miocene nd

the Pecent. Basalticcrust formationwas widespread

during the Pliocene n the Vaslovbasin, and in the

Marsillbasin,basaltic rust ormation egan t -- 2 Ma

[Kastenst al., 1988,Kastens ndMuscle,990] Hence

there is clear evidencehat the Tyrrhenianbasin s

younger oward he southeast, spredicted y subduc-

tion rollback models. Two tholeiitic basaltic seamounts

occur n the southern yrrhenian ea n the areapre-

sumed o be underlain y oceanic rust. The composi-

tion of basalts both drilled on the seafloor and collected

OF THE BETIC-RIF MOUNTAIN BELT

fromhe eamountsn heTyrrhenianea averange

of chemistriesrom alkali to talc-alkalineaffinities.

The ange f compositionfNeogeneolcanicocks

throughouthe westernMediterraneans typical f

smallbackarc basinswherearc and extensional01-

canism verlapn a diffusemanner ndwhere oltanit

arcssensu trlctohavea limitedspatialdistribution

Striking imilaritiesxistat the eastern ndwestern

extremitiesf the westernMediterraneanea TheAe-

olian olcanic rc s ocated 300km northwestf he

currentpositionof the Cainbriansubduction one.

Roations

Conmderable alaeomagnetic ata have been

letted around he wesi,rn Mediterranean,androtations

about verticM axeson a variety of scaleshave been

umented. A generahzed ummary of the main trends

andsense f rotationss shownn Figure10a Earl

workm the region oncentratedn usingpalaeomag-

neticdata o constructolarwanderingurvesor igid

microplates ithin the Tethyansuture ealm e g., the

Adna microplate), ut it rapidlybecame pparenthat

superimposedn the movementof microplateshere

were local small-scale relative rotations of fault blocks

wthin he deformedegionseg., Charmell, 986].At

the largestscale, he onsetof the south-southeastefiy

mlgratmn f the subductmnone n late Ohgocenend

early Miocene ed to the openingof the Ligurianbasin

and counterclockwise rotation of Corisca and Sardmla

[Montgny t al., 1981]. The asymmetric pening f

the ValenciaTrough presumablygeneratedclockwise

rotation of the Baleuric slands A recent synthesisf

palaeomagneticata from the Baleuric slands ndad-

jacentareasonshoreberia by Pargs et al. [1992]den-

tify three rotational components. The youngest ota-

tions 20) on Mallorcaand Menorcaoccur n upper

Miocene rocks and are attributed to rotations on listnc

faults that occurred m the late Miocene and Pliocene

After subtractinghe effects f late Miocene-Pliocene

rotations, he magneticdirections n the Balearms-

landshavebeen rotated clockwise y variableamounts

whencomparedo the Spanishmargin. Pars et

[1992] ttribute heseotationso differentialotation

thrustsheetsn the early Miocene, ncludinga compo-

Figure10. a)GeneralizedummaryfsensendmagnitudefNeogene-recentotationsbout e ic

axes 'om alaeomagneticata.Tming f rotation iven t talics eneath mount f rotation.Dotted

lineshowsriginalositionsfSardinig,orsica,ndBalegncslands. xceptorCorsica,Maeomag-

netic nalysesecordariableotationsssocigtedithshorteningtructures.ote hat n many ases,

dgt rom umeroushrust heetsresummarizedt one oint.Dataarecompiledrom eferencesis-

cussedtext.Ornamentormountaineltssgivenn Figure (b) Age rid lstributlonfvolcarat

rocksn westernediterranegn:alk-alkalineolcanicocks redistinguishedromalkalineolcanic

rockscompiledrom ellon1981],ernandeznd ellon1985],ernandeztal. 1987],ehaultl al.

[1985],aillardndMauffret1993],llamtal. 1989],eccaluvatal. 1981],err tal [1993],nd

Kastens t al. [1988]).


16/19

LONERGAN ND WHITE ORIGINOF THE BETIC-RIFMOUNTAIN ELT $19

nent f microplate otation of the wholeof the Balearic

Islands.

The Betic and Rif Externalzonesand the Apen-

tone ndSicilian hrustbeltsall havevariablemagni-

tude otations ssociatedith thrusting Plazmaa i

d., 1993,Allrtoa e al., 1993,Jackson,990,Oldow

d., 1990, cheepevst al., 1993,AlleyIon, 994].Sense

of otation s reversed n the opposing ides f both the

Gibraltar nd CainbrianArcs. In both Sicily and in

theBetifs, a decreasen the amount of rotation is ob-

servedowardhedistalpartsof the hrustbelts Aller-

tone al., 1993, Oldowe al., 1990]. We note n pass-

mg hatdetailed alaeomagneticata roin he central

Apennineslaysupport he existencen someocations

ofsmallMiocene-Recentlockwiseotations Maei et

al, 1995].Rotations ssociated ith thrusting ndicate

obliqueonvergencend so he edges f the retreating

subductiononeare not definedby large-scale rench

faults A combination of transcurrent motion caused

byrollback nd shortening aused y continmng late

covergences clearlyan efficientmechanismor gener-

ating blique onvergence,specially t the edges f an

arcuate subduction zone.

Over he last 10 years,a similar pattern of rotations

has been documented n other parts of the Mediter-

ranean ea. In the Aegean,Ksse and La. [1988] nd

$peraazai a [1995]havemeasuredotations ssoci-

atedwith both thrust and normal aulting. A testable

predictionof the rollback model arises rom the fact

that the collision f the Kabylieswith the African mar-

ginwas orthogonaland that large rotations of thrust

sheetsshould not have occurred within the Tell moun-

tains The largest otationsare to be expected n the

most azcuate zones of the western Mediterranean

Conclusions

The generalmodel for the westernMediterranean

sea hat we have discussedere is implicit in the work

ofRehauli t al. [1985] Wehave xtendedheir deas o

includehe Alboran Sea and shown hat new geophysi-

cal,structuraland palaeomagnetic ata are consistent

with subduction zone rollback rather than with convec-

tive removalor delaminationof thickened ithospheric

mantle. Thus late-orogenicxtension, s documented

m the AlboranSea, s bestregarded s a logicalconse-

qnence f a rollbackmechanismwhereby hickenedcon-

tmentalcrust extends apidly as the subduction one

retreats.

Neogenemountain belts throughout he Mediter-

ranean egionare characterized y coevalshortening

and extension uring the late stagesof orogenesis.

Weargue,nl agreement ith somepreviousworkers

to.g,Roy(lea,993],hat apid xtensionf previously

shortenedrustwithin a convergentetting s an in-

evitableconsequencef thewell-known ubductionoll-

backmechanism. onvergenceetweenheAfricanand

Europeanplatescommencedn the Upper Cretaceous

and continuedhroughouthe Tertiary. This short-

eningproducedmany Palneogeneollisionalmountain

belts e.g., heAlborandomain,Kabylies,Calabria,and

Corsica-Sardinia)t thesameime,continuingorth-

wardsubductionf remants f theTethyanOcean,with

associated ubductionollback, ed to the onset of ex-

tension hroughout hesecollisional difices nd caused

their dispersiono the margins f the currentwestern

Mediterranean ackarc basin n the Neogene.We see

no reason to believe that the Alboran Sea at one end

of the Mediterraneanasevolvedn a fundamentally

differentway rom the Aegean eaat the other end.

Further work is needed to understand how a sub-

duction one tarts o roll back. t is alsounclearwhy

somesubduction ones hould emain static over long

periods f geologicalime, orming elatively imple in-

ear mountain chains,and others such as those of the

Tethyan belt and the westernPacificare very mobde,

becomearcuate, and lead to the formation of complex

orogense g., Figure7). Short,highlymobilesubduc-

tion zones hat havegenerated rcuatemountainsys-

temsmay be difficult o detect n the geologicalecord

becauseheymaynot formwell-developedolcanic rcs.

Once he backarcbasin losesn the continuingycle f

convergence,here may be little evidence f the former

existence of such a subduction zone.

Appendix

The gravitational potential energy calculations

shown n Figure5 werecarriedout using he approach

describedy Englaad ndHousemaa1988].We briefly

describe a corrected version of their formulation which

we have used o calculateFigure 5. All symbolsand

their valuesare identical o thoseusedby England and

Houseman1988].

Crust and lithospheric antleare assumedo have

uniform compositions,nd the density n each case s

givenby

= o(1 - ,') (1)

whereT is the temperaturen degrees elsius nd a is

the coefficient f thermal expansion.The variablePo s

the densityof eithercrustor mantleat 0C.We assume

that the geotherms inear hrough lithospheref orig-

inal thicknessa and that the temperaturebeneath the

hthospheres T.

Let us consider the case of thickened continental

lithospheren isostatic quilibriumwith a standardcol-

umn of continentalithosphereFigure5). It' the crust

and lithrsphere re thickened y factors and 7, re-

spectively,hen sostatic alance etweenhe thickened

column nd the referenceolumnmeanshat Ae , the


17/19

$20 LONERGANNDWHITE:RIGINF HE ETIC-RIFOUNTAINELT

elevationifferenceetweenhe wocolumns,sgiven terminedy ntegratingverhestress istributions

a function of depth:

Since r -- pgz, the densityprofiles,whicharedeter-

minedusingEquation 1), have o be integrated.

Acknowledgments. We are very grateful to JohnPlatt for

organizingnexcellentield rip to theBetteMountainsunn[

Easter1994. We also hank R. Edwards,D. Lyness,D. McKen-

(2) zie,nd.Qiuorheirelp..Dewey,Platt,.Robertson

and an anonymousefereeprovided houghtful eviews ut the

the differencen gravitationalotential nergy e- usualisclaimerpplies.hiss Cambridgearth cienceson-

tween he thickenedeferenceithospheres,an be de- tribution846

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Eastern Alboran Sea, GaGMar. elt

12, 111-116,1992.

Zeck, H P., P Moni I.M. Villa, andB T

Hansen,Veryhigh ratesof coolingnd

uplift in the Alpine belt of the Betlc

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0, 79-82,1992

L. Lonergan, Department of Geology,

Impeltat College,London, SW7 2BP,Eng-

land (eraall:1 [email protected]).

N White, Bullard Laborstoiles, Mad-

ingley Road, Cambridge, CB3 0EZ, UK

(email:[email protected]).

(Received amh29,1996;

revised ecember,1996;

acceptedecember4,1996

origin of the betic-rif mountain belt

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