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WNW-directed obduction of the Batain Group on the E-Oman continental margin at the Cretaceous-Tertiary boundary. Schreurs, G.; Immenhauser, A.M.

published in Tectonics 1999 DOI (link to publisher) 10.1029/1998TC900020 document version Publisher's PDF, also known as Version of record

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citation for published version (APA) Schreurs, G., & Immenhauser, A. M. (1999). WNW-directed obduction of the Batain Group on the E-Oman continental margin at the Cretaceous-Tertiary boundary. Tectonics, 18(1), 148-160. https://doi.org/10.1029/1998TC900020

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Download date: 04. Oct. 2021 TECTONICS, VOL. 18, NO. 1, PAGES 148-160, FEBRUARY 1999

West-northwestdirected obduction of the BatainGroup on the easternOman continentalmargin at the Cretaceous-Tertiaryboundary

Guido Schreurs

GeologicalInstitute, University of Bern,Bern, Switzerland

Adrian Immenhauser

Facultyof EarthSciences, Vrije Universiteit,Amsterdam, Netherlands

Abstract. The Batain coast area in eastern Oman is dominated 1. Introduction by allochthonousPermian to Late Maastrichtiansedimentary and volcanicrocks (Batain Group), unconformably overlain The Batain coast area is situated in northeasternOman and by neoautochthonousTertiary sediments.The allochthonous coversabout 4000 squarekilometers (Figure 1). It is bounded rocks of the Batain coastwere previouslyattributed to the to the north by the Gulf of Omanand to the eastby the Hawasina complex, the Permian to Coniacian/Santonian ArabianSea. The QuaternaryWahibah Sands separate the sedimentaryinfill of the neo-TethyanHawasina basin off Batain coast area from interior Oman. The area has a low to northernOman. Previous structural interpretations suggested moderaterelief and is dissectedby numerous,roughly E-W thatthe BatainGroup, along with the Hawasinacomplex and trendingwadis. Recentsand and gravel depositscover the Semail ophiolite, was obducted in the Coniacian to extensiveparts of thearea. The Batain Plain is dominatedby Campanian from NE to SW onto the northern Oman allochthonousPermian to Maastrichtiansedimentary and continentalmargin. Results of our work in the Batain area volcanic rocks. These rocks documentthe evolution of a differ from previousinterpretations, with most significant basinoff the easterncoast of Omanand are referredto as the differencesconcerning timing and directionof obduction.Our Batain Group [Immenhauseret al., 1998]. They are results show that WNW directed tectonic movements formed unconformablyoverlain by autochthonousupper Paleocene a fold-and-thrust belt and led to the obduction of to Miocene marine and continentalsediments. The rocks of allochthonousrocks onto the eastOman continental margin theBatain Group were obducted onto the continental margin during latest Maastrichtian/earliest Paleocene times. This is of Oman,parts of whichare exposed near Jabal Ja'alan (Figure coevalwith emplacementof ophioliticfragments along the 1). Here Precambriangneisses, migmatites, and schists were easterncoast of Oman (easternophiolite belt) but is about 15- intrudedby UpperProterozoic plutonic rocks and dykes 20 Myr later than emplacementof Hawasinacomplex and [Rogeret al., 1991;Wiirsten et al., 1991]and covered by Semail ophiolite in northern Oman. Posternplacement neoautochthonousMaastrichtian and Tertiary sediments [Le structuralevolution during the Tertiaryinvolved intraplate Mdtouret al., 1995].On the basisof reflectionseismic data, extension, possibly reflecting the Red Sea/Gulf of Aden welldata, geochemical data, and surface geology, Beauchamp opening,and late Tertiaryshortening related to convergence et al. [1995] suggested a hidden Cretaceous rift basin betweenArabia and Eurasia.Late Tertiary contractional (MasirahGraben) underlying the allochthonous sequence of deformationresulted in refoldingof the Batainnappes and in the Batain coast.The Cretaceousrift-related normal faults are foldingof the overlyingTertiary sediments. A palinspastic believedto havebeen reactivated in the late Tertiary because reconstruction of the Batain area indicates that the Permian to of riftingof theArabian Sea/Gulf of Aden[Beauchamp et al., UpperCretaceous sediments were formerlydeposited in the 1995] Batainbasin, a part of the proto-IndianOcean, along the Previousstudies interpreted the allochthonousrocks of present-dayeastern Oman margin. This leadsus to propose theBatain Plain as part of theHawasina complex exposed in that Permian breakup of Gondwanaland created both the OmanMountains [Glenhie et al., 1974;Cooper 1990; continentalmargins of Omanand led to the openingof two Shackletonet al., 1990;Roger et al., 1991;Bdchennec et al., major basins:the neo-TethyanHawasina basin in the north 1992;Wyns et al., 1992].The Hawasinacomplex represents andthe proto-IndianOcean Batain basin in the east,the latter thePermian to Coniacian/Santoniansedimentary infill of the separatingArabia from greaterIndia. formerneo-Tethyan Hawasina basin. This paleodepositional realmwas considered to be locatedalong the northernOman marginand bounded to thenorth by a spreadingridge. In the scenarioof the authorscited above, the rocks of the Batain Group along with sediments of the Hawasina basin Copyright1999 by the American Geophysical Union. (Hawasinacomplex) and ophiolites (Semail ophiolites) were thrusttoward the SW and obductedonto the northernOman Papernumber 1998TC900020. continentalmargin during the Coniacianto Campanianat 0278-7407/99/1998TC900020512.00 about 85-80 Ma.

148 SCHREURSAND IMMENHAUSER: OBDUCTION ON EASTERN OMAN MARGIN 149

. .58,:,.? 60? ?:'4...... * '::::......

s

Al.:.:!H.'ad•d

A1 'Kin Jabat

WAHIBA, SANDS

• Eastern ! i• OphioliteBelt o• • NappesBatain .• OphioliteSemail • .•.• NappesHawasina ...... [--'] Quaternary

/ /AxisNormal of fault• [• ...... TertiaryCarbonates •. structural -- highs :-¾•CarbonatesCretaceous Thrustfront "=:• Huqf-Haushi Thrust(inferred)fro'nt<• ":5• 'ProterozoicBasement A JabalJa'alan MasirahGraben Sealevel B.

Figure 1. (a) Simplifiedgeological map of northeasternOman, (b) Schematicsection across the Batainarea. Locationof sectionis given in Figure la. 150 SCHREURS AND IMMENHAUSER: OBDUCTION ON EASTERN OMAN MARGIN

Ophiolites also build Masirah Island, situated over 100 km to the SSEof the Batainarea (Figure 1). The uppermost Formationsdistal proximal(A1v• Group Units Lithology -- Pliocene Jurassic/lowermostCretaceous Masirah ophiolites, however, I!lllllllllllllllll I [ I I I [ I I [ I I Miocene formedearlier than the mid-CretaceousSemail ophiolite in the Fars 5Fms I I I I I I I I • I -- OmanMountains [Beurrier, 1987; Immenhauser, 1995] and I i I i I [ I I I I --Oligocene were obducted in a NW direction onto the eastern Oman Dhofar anuwayropuqm,1111111• I I I I I I I I I I [ I [ I I I [ --Eocene margin [Immenhauser, 1995; Gnos et al., 1997; Peters and DAMM./RUS I [ I I t [ I [ I [ -- 50 Mercolli, 1997, 1998)]. Paleomagneticstudies of the Masirah HadramautUMM ER RADH I [ I I I I I [ [ [ [ [ [ [ ] I [ I I [ [ --Paleocene [ i i i i i i i i i i [ [ [ i i i • ...... ophiolites corroboratedthese conclusionsand ruled out the mrumaSims./Hasad ...... •( : ."•::.•:••_•:•i ..... - - possibilityof a commonor relatedorigin with respectto the Semail ophiolite [Gnos and Pertin, 1996]. The Masirah I I I I II I I FayS-'"•&•&••;4--Late " "' -- Cret. Island ophiolitesalong with other fragmentsof oceanic lithospherealong the easternOman margin (Ra's Madrekah, northof SawquirahBay, seeFigure 1) weretermed the eastern • Fm mnmn•••••• vvvvvy mnmn.mn • --Early Cret. ophiolitebelt, and it was postulatedthat emplacementwas • • X.....• • •'.•-• •T--•-- related to transpressional motions between the Indo- Pakistaniand the Afro-Arabianplates at about65 Ma [Gnos et al., 1997]. m ..... M&GL+•½•- Jurassic It now becameapparent that we shouldfollow up the new data obtainedfrom Masirah Island by investigatingthe 5 / unditt Batain coast area. The stratigraphy,sedimentology, and depositionalenvironment of the BatainGroup is discussedin detailby Immenhauseret al. [1998].In thepresent paper we • / glJil '2• • X ••2•• - Triassic documentresults of structuralstudies in the Batainarea, which differ in several aspectsfrom previousstructural • • • ' - 250 interpretations.Most significant differences concern timing of obductionand associatedtectonic transport direction. Our • QarariU ['•- Vemian structural and kinematic findings, supported by biostratigraphicand sedimentologicdata [Immenhauseret al., 1998],allow us to proposea new interpretationof the Pillow basalts Lst congl. Oolitic 1st Manganese structuralevolution of the Batainarea and to brieflyoutline its palinspasticimplications. Nodular Halobia Radiolarian Shallow marine limestone limestone limestone limestone

2. Overview Radiolarites Quartz Volcanic Quartz siliceous sandstone ashes limestone 2.1. Batain Group- Stratigraphic Framework Figure2. Stratigraphyof theBatain Group and overlying Establishingthe stratigraphyof the Batain Group is Tertiary sediments. complicatedby intensefolding and faulting.Well-exposed successions with minor to little tectonic disturbance are limited, and sectionsare generallyexposed as thrust slices deposits;(4) the EarlyJurassic to EarlyOxfordian Guwayza only. A further complicationin settingup a stratigraphic Formationwith high-energyplatform detritus; (5) the mid- framework is strong lateral facies variationswithin what is Jurassic to earliest Cretaceous Ruwaydah Formation, consideredone and the samestratigraphic unit. Nevertheless, interpretedas a fragmentof a seamount;(6) the Oxfordianto well-datedsections and carefulmapping allow us to establish Coniacian-SantonianWahrah Formation, mainly radiolarites an overall stratigraphy.New biostratigraphicdata were with locallyvolcanic intercalations; and (7) the Santonianto largely obtainedfrom radiolarian-richlithologies and are uppermostMaastrichtian Fayah Formationbuilt by flysch- reportedby Immenhauseret al. [1998]. These data were type sedimentsand olistostromes(Table 1). Immenhauseret combinedwith biostratigraphicresults of previousstudies al. [1998] show that the large-scalestratigraphic and [Shackletonet al., 1990;Roger et al., 1991;Bichennec et al., sedimentologic pattern found in the allochthonous units 1992; I4/ynset at., 1992]. Figure2 givesan overviewof the exposedin the Batainarea is onlypartly comparable to what sedimentaryand volcanicrocks in the Batain Group.For wasdescribed from the HamratDuru Groupin the Hawasina reasonsof simplicity,stratigraphic nomenclature is largely complexby Glennieet al. [1974]and Bdchennec [1987]. For afterRoger et al. [ 1991]; Bichennecet al. [ 1992],and I4/yns moredetails on the stratigraphicframework of the Batainarea et at. [ 1992]. thereader is referredto Immenhauseret al. [1998].The Fayah The Batain Group consistsof (1) the Permian Qarari Formationis foundalong the entireeastern Oman margin but Formationdeposited in the toe of a slope setting;(2) the has no equivalentin the Oman Mountains.On Masirah Island UpperPermian to UpperLiassic A1 Jil Formationcomprising the Fayah Formationis sandwichedbetween ophiolite periplatform detritus and very coarse breccias;(3) the nappes and contains Coniacian to latest Maastrichtian Scythianto Norian Matbat Formationformed by slope nannoplankton[Immenhauser, 1995, 1996]. It overliesthe SCHREURSAND IMMENHAUSER:OBDUCTION ON EASTERNOMAN MARGIN 151

Table 1. Summaryof Formationsand Units ContainedWithin the Batain Group,Their DepositionalAge, Faciesand Interpreted Depositional Environment

Formation/Unit DepositionalAge Facies DepositionalEnvironment QarariFormation Permian argillaceousgrey limestonesin part on toe of slopesetting below the storm volcanic basement wave base but above the carbonate compensation depth (CCD)

A1 Jil Formation Late Permian to late Liassic shallow marine calcarenites, periplatform detritus and very conglomerates and megabreccia on coarseplatform breccias pillow basaltsand siliceousradiolarites

Matbat Formation Scythianto Norian radiolarian limestonesand clays overlain lower slopesetting, possibly reached by calcarenites, Halobia limestones, by heavy stormwaves shales and siliciclastic carbonates

Guwayza Formation Early Jurassicto early Oxfordian graded oolitic calcarenites,fine-grained turbiditesshed from a high energy bio-pelmicrites and calcareous platform conglomerates

RuwaydahUnit Mid-Jurassic to earliest Cretaceous breccias with volcanic and sedimentary dismembered seamount structure clasts, thinly bedded volcanoclastic calcarenites, basaltic and andesitic pillows and flows, radiolarian shales, Ammonitico rosso facies and coarse shallow marine limestone breccias

Wahrah Formation Oxfordian to Coniacian/Santonian radiolarian micrites and clays, open marine deposits below the "porcellanites"and ribbon cherts CCD, upwelling-radiolarites

FayahFormation Coniacian-Santonian to latest siliciclastic limestones with derived flysch deposits Maastrichtian boulders of continental basement, shales and clays and coarsedebris flows

Ra's Madrekah ophiolite and yields Late Maastrichtian deformationbecame increasingly complex northward. Locally, foraminifera [Gnos et al., 1997]. In the Batain area the Fayah they distinguishedearlier structures,which preceded the Formation ("Fayah Sandstone"of Shackletonet al. [1990]) main NNE trending structures.Most of the Batain coast area, was dated Coniacian to Campanian near Jabal Ja'alan but however, was interpreted as a "melange", which was otherwise was dated Campanian to Maastrichtian by consideredto be "primarilytectonic but probablycomposite Shackletonet al. [1990], Late Maastrichtianby Bdchennecet in origin" by Shackletonet al. [1990]. They consideredthat al. [1992], and latest Maastrichtian by Roger et al. [1991]. the main deformationin the Batain area was of pre-Tertiary The age of the Fayah Formationand the deformationregistered and probablypre-Late Maastrichtianage. Shackletonet al. in it play an important role in the timing of tectonic events [1990] correlatedthe Batain "melange" with the Hawasina along the eastOman Margin. "melange"(Haybi complex sensuRobertson et al. [1990] in the Oman Mountains,which structurallyunderlies the Semail ophiolite. Shackleton and Ries [1990] concluded that 2.2. Previous Structural Studies emplacementof the Batain "melange" and formation of the Structural investigations of the Batain coast area were Batain fold-and-thrustbelt were the resultsof relative plate carried out by Shackletonet al. [1990] and by geologistsof motions toward the SW during the Late Cretaceous,coeval the Bureaude RecherchesG6ologiques et MiniSres(BRGM) with the obductionof the Semail ophiolite and the Hawasina as part of their detailed mapping program of Oman, which complex in northern Oman. They considered Tertiary resultedin the publication of maps NF 40-12, A1 Ashkharah deformationto be weak. StrongerTertiary deformationwas with a scaleof 1:250,000 [Bdchennecet al., 1992], NF 40-08, describednear the Jabal Ja'alan Uplift [Filbrandt et al., Sur with a scaleof 1:250,000 [Wynset al., 1992], and NF 40- 1990]. An analysisof Neogene and Quaternarydeformation 8E, Ja'alanwith a scaleof 1:100,000 [Rogeret al., 1991] with and associated paleostress fields in the eastern Oman accompanyingexplanatory notes. Geologists of the Metal Mountainswest of Jabal Ja'alanwas carried out by Carbon Mining Agency of Japan studied manganese-bearing [1996], who concluded that deformation since middle-late radiolarites of the Wahrah Formation and their structures in Miocenetimes occurredunder NE-SW directedcompression. parts of the centraland northernBatain area (Metal Mining Bdchennecet al. ([1992], Roger et al. [1991], and Wynset Agency of Japan,Tokyo, unpublishedreport, 1982). al. [1992] arrived at similar conclusionsas Shackletonet al. Shackletonet al. [1990] describedNNE trending, west [1990] for the Late Cretaceousdeformation event. They also vergentstructures in the southernBatain area (referredto as postulated a Late Cretaceous obduction-relateddeformation the Batain fold-and-thrust belt) and mentioned that event (Coniacian- Early Campanian,i.e., at about 85-80 Ma, 152 SCHREURSAND IMMENHAUSER: OBDUCTION ON EASTERN OMAN MARGIN their "Eoalpine phase"), which not only affected the Semail mechanicalstratigraphy. Structures are best observedin well- ophiolite and the Hawasina complex in northern Oman but layered radiolarian cherts and shales of the Wahrah also affected Permian to mid-Cretaceoussediments along the Formation.First-phase deformation generally produces tight Batain coast. Bichennec et al. [1992] associated extensive folds whoseaxial planesare usuallymoderately to steeply internal deformationin the Upper Jurassicto mid-Cretaceous dipping(Figures 3b and 5). Associatedthrusts strike parallel Wahrah Formation with SW directed obduction. Westward to the trend of the fold axes, and their dip is either directed thrusting and associated tight N-S to NNE-SSW subhorizontalor subparallelto the dip of the axial plane trending folding in the Maastrichtian Fayah Formation (Figures3c and 6). First-phasefolds are oftenasymmetric with (which in their opinion uncomformably overlies deformed a long limb and a steepto overturnedshort limb. First-phase Permian to mid-Cretaceous sediments) was considered by foldsin the WahrahFormation vary in scale,with amplitudes Bichennec et al. [1992] to reflect a later phaseof WNW-ESE and wavelengthsranging from 1 m to severalhundred meters. directed tectonic compression during the Late Cretaceous Folds in more competentlithologies, such as the siliciclastic and/or early Tertiary (their "Laramide" phase). Finally, and calcareousrocks of the Matbat, Guwayza,and Fayah Bichennecet al. [1992] mentionedminor open folding, which formationsgenerally have largeramplitudes and wavelengths locally deformed "Laramide phase" thrust sheets and was up to kilometer scale. consideredto be of late Miocene age (their "Alpine phase"). Thrust contacts in the Batain fold-and-thrust belt are usually characterizedby tectonic brecciasand are sometimes associated with occurrencesof gypsum. Discrete thrust 3. Structures of the Batain Area planeswith slickensideshave also been observed(Figure The apparent chaotic and irregular distribution of 3c). The NW-WNW directed sense of movement on thrusts lithologies in the Batain coast area led Shackleton et al. correspondsto the one inferred from the vergence of [1990] to proposethat most of the area is underlainby a asymmetricfirst-phase folds. "melange". However, in our opinion there is far more An exampleof an areadominated by first-phasefolding and structural coherence than hitherto believed. On the basis of thrustingis locatedNNW of Musawa(Figure 7). A tectonic overprinting relationships we distinguish two major contactdipping 50ø to the east and markedby cataclastic contractionalevents in the Permianto Upper Maastrichtian deformationseparates the GuwayzaFormation from sediments rocksof the BatainGroup, separated from one anotherby an of the WahrahFormation to the west.Younging criteria are extensional event. The structural evolution of the ubiquitous in the Guwayza Formation and allow us to allochthonous rocks of the Batain area can be described in classifythe large-scalefolds as upwardfacing synclines and termsof obduction-relateddeformation and postemplacement anticlines. Fold axes plunge about 35ø to north to NNE. deformation and is discussed accordingly. Map-scale Whereasthe northernpart of Figure 7 is characterizedby structural features are shown in Figure 3 along with upwardfacing folds with subverticalto steeplyeast dipping stereographicplots of major structuralelements. Figure 4 axial planes,the southernpart showsdominantly overturned givesa moredetailed structural overview of the centralpart of asymmetric,west vergent folds. the Batain area. Figure 8 shows how east dipping thrusts repeat stratigraphicunits of the Batain Group in an imbricatestack, 3.1. Obduction-Related Deformation each slice having a thickness of about 50-100 m. Internal deformationin the individual thrust slices is especially (First Phase of Shortening) evidentin Wahrah, Matbat, and Guwayzaformations. From Surface mapping reveals that a thin-skinned fold-and- the WNW vergentfolds, the WNW directedfacing directions, thrustbelt, building the Batain Group, formed duringa first and the east dipping thrust surfaces, a WNW-directed phaseof contractionaldeformation. This phaserepresents the transportdirection is inferred. main deformationevent of the BatainGroup. It resultedin the On the basis of interpretations of seismic sections, most conspicuousstructural features along the Batain coast Beauchampet al. [1995] suggestedthat the allochthonous which consistof thin thrust imbricatesthat generallyshow rocksof the Bataincoast are underlainby a low-anglebasal intenseinternal folding. Becauseof limited vertical relief and detachment at depth. The basal detachment is nowhere lack of continuousoutcrop in the Batain coastarea, the total exposedat the surface, except on the southeasternflank of thicknessof the stacked sedimentaryand volcanic rocks is Jabal Ja'alan (Figure 4). Here allochthonousrocks of the difficult to assess. An estimate of 1.5-2 km for the entire Batain complex are thrust on top of Maastrichtianslope stackedsequence is inferredfrom seismicsections given by depositsof the HasadFormation (age of HasadFormation after Beauchampet al. [1995]. It is importantto emphasizealready Roger et al. [1991]), which represents part of the at this stagethat this phase of deformationaffects the entire autochthonous cover of the Jabal Ja'alan Proterozoic Batain Group, including the Fayah Formation to which basement.The autochthonousMaastrichtian and Tertiary Shackleton et al. [1990] attributed a Santonian to sedimentary cover overlying parts of the Jabal Ja'alan Maastrichtian age. Proterozoic basement is not affected by the intense The overall trend of the fold-and-thrustbelt is roughly deformation as seen in the Batain area to the east. This NNE-SSW, and the movement direction is WNW to NW suggeststhat the exposed basal detachment east of Jabal (Figures 3 and 4). Deflections of this trend are the result of Ja'alan forms the westernmost limit of the Batain thrust front. younger, postemplacementdeformation and will be discussed The continuation of this thrust front to the north is hidden in section 3.2. Deformation style is strongly dependenton beneath Tertiary sediments, whereas its southern SCHREURS AND IMMENHAUSER: OBDUCTION ON EASTERN OMAN MARGIN 153

J ...... } Recent smxland gravel "• Tertiaryvolcmics

Au•ocllhonousTertiary

•N•%N coverA u•ocll ofhonous Prot•-. M basementamtncht .'"•."'x""-.]JabaIJa'altm Prolero- ,' / ,.' I zoicbase•rent

BatainGroup.

Bai.c vokm.ics

[----• (Smtm'•Fa)t•hYonnation t• Maastrid•t.imO ::.•:•,•...... x::•.,-• Wahrah Formation

S•miau)

'¾--7.ff•..•.•x-•,•"• Nhtbat,FOrmadorn Guw'a)e:a, &Qarari A1 JilUnit

Legendstructures Ax"mltrace traje•ories •:,x.,• .• •.-: • offi.rst-phase folds

%,..,'eV •,/ Thrustsfirstsphaseassociatedfoils with ... . - ,•. -, • • .f• -,, .•. First-phasemoverre. nt direction

Fault

•"'.{:" .;•,'"%- Arabian Sea Axbitrace of second- phasefold

u-•'4'00••....t• •/ ..... :??/ PoleIo tTm;.vp*•--•e re•me fault

N N

- Pole•a •.cor•t-pha•citxial plane ø Scorn.t-phase foil axis

Figure 3. Major structuresof the Batain area. Faulting and folding in JabalJa'alan area and Tertiary sediments are mainly after Roger et al. [1991] and B•chennecet al. [1992]. (a) Structuralmap of the Batain area. (b) Poles to first-phaseaxial planes.(c) Poles to first-phasereverse faults and orientationof slickensideson thesefaults. (d) Orientationof first-phasemovemere directions. (e) Poles to second-phaseaxial planesand orientationof second-phasefold axes. Structuraldata are plottedon Schmidtnet, lower hemisphereprojection. 154 SCHREURSAND IMMENHAUSER:OBDUCTION ON EASTERNOMAN MARGIN

North Ja'alan

Fault -.• ß ::

.

' •'%d'• ...... ::.ß

2460, :.

:•..;.•!•'*.•-..... ':'" •'": JabalJa'alan

A1A,yn Bani Bu Ali

F

Thrust assoc hted with Autochthonouscover of Jaball JMaastrichtiana'alan Bedding Z first-phase'fokling Firsts-phasemovement "'•...... ".'Aut6chthonous Tertiary'..•.•-•q JabalProterozoic Ja'alan basement Axialtrace offirst-phase folds direction Batain Group First-Phasesynform,anti fom• Axial trace of second- phasefold --• (SamonianFayahFormation.toMaastrichtian) / Basic volcanics WahrahFormation Matbat& Guwayza synform,First-Phaseantiformoverturned Second-phasefold axis Samonian) Triassicto Jurassic) First-phasefoldaxis Sec•d-phaseax/al plane fill!Ill!I'll](mid-JurassicRuwaydah toUnit? Earlz:iCret..) (PermianAIJil Formation toEarly Jurassic) First.Phase axialplane Fault

Figure 4. Structuralmap of the centralpart of the Batain area.Location of map is given in Figure 3. Structures in Jabal Ja'alanarea and Tertiary sedimentsare mainly after Roger et al. [ 1991].

continuation is covered by the Quaternary Wahiba sands. 3.2. Posternplacement Deformation However, small exposuresof the Wahrah Formation (Batain The Batain fold-and-thrust belt was refolded during a Group) are present in the intertidal zone of the Hikman secondphase of shortening.This phaseis less prominentthan Peninsula [cf. Gnos et al., 1997, and references therein]. the earlier phase of intense folding and thrusting. Second- Ophiolites in the southern Batain area (e.g., near AI phasefolds are generallyopen and have wavelengthsvarying Ashkharah and about 10 km NE of Jabal Fayah) are as yet betweenseveral meters and several kilometers(Figures 3 and undated.They are petrographicallydifferent from the Masirah 4). The orientation of second-phasefold axes dependson Island ophiolites or the small ophiolitic fragment at Ra's beddingorientation after first-phasefolding. Since the overall Madrekah forming the eastern ophiolite belt of Oman. The orientationof beddingis generally moderateto steepbecause contactswith the surroundingsediments of the Batain Group of upright or overturned first-phase folding, second-phase are not exposed.One possibilityis that these ophioliteswere fold axes are mostly moderatelyto steeplyplunging (Figure accretedduring Proterozoic times and now form part of the E- 3e). The dip of second-phaseaxial planes is subvertical,and Oman continental basement exposed in a tectonic window its overall strike varies from NW-SE to NNW-SSE trending and overlain by the basal detachmentof the Batain fold-and- in the northernpart of the Batain complexto N-S trendingin thrust belt. Alternatively, the ophiolites could be of Permian the southernpart. or Mesozoic age and form part of the Batain fold-and-thrust An example of interference structures attributed to belt. superimposition of two folding phases occurs in an area SCHREURS AND IMMENHAUSER: OBDUCTION ON EASTERN OMAN MARGIN 155

-<

Figure 5. Photographof latestMaastrichtian/earliest Paleocene chevron-type folds in radiolarianmicrites, clays,and ribboncherts of the WahrahFormation. Cliff is about15 m high.

Figure6. LatestMaastrichtian/earliest Paleocene obduction-related folding and thrusting in rocks of the Wahrah Formation. Inferred movementdirection is SW (right-handside of photograph).Note that this directionis the resultof Late Tertiaryrefolding. Width of photographis approximately10 m. 156 SCHREURSAND IMMENHAUSER:OBDUCTION ON EASTERNOMAN MARGIN 2ooA A'"'"2.00 0m ::'":' [0m

0 l. km

2454-

B B' 200...... "FN 200 0 m 0 m

Youngingdirection / 68 Orientationof bedding(with dip angle) First-phasefold axis(with plunge) + First-phaseanticline + First-phasesyncline Overturnedanticline (first phase)

Overturnedsyncline (first phase) Reversefault(saw I Quaternary te•thon upper block) Fault ...... •Coniacian/Santonian)Wahrah Fm(Oxfordian to 2452 UTlVlcoordinates ::•7•:'1:7•.•71•.i.•1117•:id - GuwayzaFm(Jurassic)

Figure 7. Map showinglatest Maastrichtian/earliestPaleocene obduction-related folding and associated thrustingin Wahrahand Guwayzaformations. Insets show two crosssections, whose locations are givenon the map.

about 10 km WSW of Sal (Figure 9). Here volcanic and directed thrusts also occur within the Wahrah Formation. sedimentaryrocks of the Matbat Formationare thrust on top Minor parasitic,gentle, second-phasefolding in the Wahrah of rocks of the Wahrah Formation. The contact between the Formation overprints tight to isoclinal first-phase folds two formations is marked by a tectonic breccia and is (meter to decameterscale), whose overall fold axes azimuths deformed by large-scale, second-phaseopen folding. Fault mimic closelythe strike of the tectoniccontact. First-phase surfacesdip at about 50ø toward the east to SE, and fault axial planesof tight, west vergentfolds are subparallelto the striae are roughly downdip toward east to SSE. Deduced tectonic contact. Within the Matbat Formation, volcanic movement directions from slickensides are toward the WNW rocksoccur, whose contacts with the surroundingsediments to NW (e.g., at universal transverse mercator (UTM) are stratigraphic.These volcanic rocks outline the core of a coordinates 7.59.982/24.45.053). Small-scale west to NW first-phase,upward facing antiform,which was subsequently SCHREURSAND IMMENHAUSER: OBDUCTION ON EASTERNOMAN MARGIN 157

The structural facing of these first-phase folds is upward. These structuresare earlier than the large-scale(see schematic inset in Figure t0) open antiform which forms the Fayah Dome [cf. Shackletonet al., 1990] and plungessouthward in its southernpart. The overprintingrelationships are such that on overturnedlimbs, the structuralfacing becomesdownward with respect to second-phasefolding. Similar overprinting relationshipsoccur in other parts of the Batain area, for example near UTM coordinates 7.41.639/23.87.061, where Figure 8. Section showing a stacked sequence of thin clear younging directions in sediments of the Fayah imbricates with internal deformation, formed during WNW- Formation allow us to distinguishan anticlinal synform and ward directed obduction of rocks of the Batain Group. synclinal antiform pair. Highest point has universal transverse mercator (UTM) The Tertiary sedimentsof the Batain coast lack the intense coordinates: 7.66.785/24.28.480. first-phase folding and thrusting. Instead, they uncomformablyoverlie intenselyfolded and thrustedPermian to Upper Cretaceous sediments and are considered refolded by open second-phasefolds. The latter folds have neoautochthonousto these. The Tertiary cover itself is axial plane traces dipping steeplyto the ENE and fold axes deformedby gentle N-S to NW-SE trendingfolds (Figure 3), generallyplunging about 400-50ø toward the SW to SSW. with subhorizontal fold axes and subvertical axial planes. Another example of first-phase folds overprinted by a These folds are correlated with similar trending second-phase secondphase occurs in the area of Jabal Fayah (Figure t0). fold axial traces in the underlying Batain complex. The Just south of this hill top, the sediments of the Fayah difference in fold axis plunge can be explained by the pre- Formationare deformed by open to tight 50 to tOO-m-scale second-phasegeometric configuration of the strata, that is, first-phasefolds that plunge about 200-30ø to the west. The intenselyfolded and thrustedPermian to upper Maastrichtian southernmostvisible fold (Figure t0) is overturned toward sedimentswith steep limbs, overlain by horizontal Tertiary the north, and its axial plane dips at about 50ø to the south. sediments.Folding of such a geometric dispositionresulted in the generally steeply plunging second-phasefold axes in the Permian to Maastrichtian sediments and subhorizontal fold axes in the Tertiary cover. Throughoutthe entire area, numerousextensional normal faults crosscut the Batain fold-and-thrust belt on outcrop •i 0, 50,0m scale. Locally, they occur in conjugatesets. Their orientation Recentsand and gravel •i•:,:,A•/]•-•--•••..'•,•-...... ,.•.•.....:••••-•••••••••••••• i ' is variable because of reorientationby second-phasefolding. In areas with minor second-phaseoverprinting, two sets of conjugate faults can be distinguished:one set is dominant and trends NNE-SSW, whereas the other set trends E-W. Early Tertiary sediments also show evidence for normal faulting, and the extensional phase is therefore placed in Tertiary times, precedingthe secondphase of shortening.It is

A JabalFayah (1.9'0 m) B

/

27'1-20 270--19 17(ti50 "K? 5?kinagingdirection phasefold 170/75 Dip dim'eft.on•md dip of fi.r,sl•ph•x:•'a'x•I plane •5/ First-phaseAxial. planefoldoffirst-•..is 1• AxialSecond•.phasetraceoffirsbphmseantifo• fold 271-20 Azimuthand plunge of fi•bp•.:• fold axis • F.irst,ph•sethrust surface. • Second-Phase synform FayahFm• i:'•:•+ Fimbphasesynctine ph.•eføld ••.mst surface(saw onu.p•r blemk) 42:Second-phase foldaxis •53Stdaeonthrustsurface •70•-i• ...... plane of second-

Figure 9. Map of overprintingpattern which is interpreted Figure 10. N-S sectionthrough Jabal Fayah in the southern as the result of superpositionof two folding phases,the first part of the Batain area. UTM coordinatesof summit of Jabal straddling the Cretaceous/Paleogene transition and the Fayah are 7.45.725/23.97.200. Inset showsschematically the secondduring the late Tertiary. overprintingpattern in map view. 158 SCHREURSAND IMMENHAUSER: OBDUCTION ON EASTERN OMAN MARGIN

BatainArea i BatainArea : 'BatainArea ', MasirahGraben ', MasirahIsland (thispaper) : [Shackleton1990] etal., ': [Bdchennec 1992]et al., : [Beauchamp1.995:] etal., 'i [Peters1997]et aL, 2 Pleistocene 0 i ;...... i...... i 5 Ptiocene...... :•...... Mos an.... '•--'0b•--f61•hg-Uf"m .... -•:...... ;__ open ';';¾'•'oia]n• ...... :-•'Invemon, I n'•ar'j•belofJa'alanfaults: •'"...... '"l ooen folding' "" .....To, .1-Tertiarysediment.s-'•...... _:...... ',;l- - (,, Alpine : ß - phase)-:i.and;.. -• .... HugVUaushi-:-• -:- o•Temav- 15 Miocene ::•kan ! refoid.i•'•fBa.tai•i ...... ':...... {*l.•li•tl f6tding of-•- ..... sediments •-6•;-' '• 'fold-an&thrust'belt':...... ?...... y•-temary -,5--•...... :•:-...... :•.:- =•.-'--:-'{ ...... ';...... •1...... sediments +i ...... Aqu• .... i i :: ' !earlyOligøcene) • Pri I ' _ ': i "'•• W•'-E-SE{ ...... Lut : • • 1 reactivationof Eocene ' • : • Mesozoic __•

55 Ypr.. First-phaseBatain' .•: , riff-relatedihults.', ...... ;... Tha fold-and-thrust • ':l westdirected • • I Iptraoyea•l?. pateocene -. 'belt(WNW to NW'• ...... ?• thrusting •...... •--• •hr•{• Jotl•e½- 65 Dan i .... , , ß , ' '• t---[ar•mi•W,,hase • '.m o'y1• w alrecte.o • mrectea" o-•uchu•v; ß...... : Fmmati0'fi"6f' '. ß :;::-- •- •" a..... •., '•• •_' ...... :, • obducfi.o.n -- •aa•. Strati-ra-hico}:'•;•Fm a-e• ..... :_•.i• thrustBatain beltt•Id-an.d-; & i...... SWdirected [-•-! • SWobduction directed of .... •:...... S•at,graph,c :...... ::---:"•:'• Senonian'•fi•cam• • •t[Shackleton al i990• ...... i?.ffi•• directed.mdan.ge (SW' J;•• obduction("Eoalpine" phase)}i.... • 5røcksallochthonous ' ' ;SiJ'•i ageUnitofFayah •-•

Figure 11. Deformationtimetable summarizing the resultsof our work and comparingit with previous interpretationsof Batain coast,Masirah Graben, and MasirahIsland.

quite possiblethat large-scaleextensional structures exist in sedimentsof this formation are largely interpretedas flysch the Batain area as shown for the subsurfaceby Beauchamp et deposits,alternating with debris flows and grain flows in a al. [1995]. This, however, cannotbe substantiatedbecause of deep water environment [Immenhauser et al., 1998]. The the scatteredand discontinuousnature of outcrops. climax of turbidite deposition occurred in Maastrichtian and especially late Maastrichtian times [Bdchennecet al., 1992] as manifested by very thick sandstonesuccessions (e.g., at 4. Structures of the Batain Area - Discussion least several hundredsof meters near Jabal Fayah). Chaotic A summaryof the proposedtiming of deformationevents in debris flows with decameterblocks (e.g., near Sal, see Figure the Batain complex is given in Figure 11 along with a 3) have also been attributed a Late Maastrichtianage [Wyns comparisonto timetablesproposed in previousstudies. et al., 1992]. The continentalsource of the Fayah Formationis evident from abundant detritic minerals, such as quartz, 4.1. Timing of Obduction biotite, and muscovite, and derived pebbles of continental During a first phase of deformationthe Permian to Upper basement.Our structural investigations indicate clearly that Cretaceous sediments and basic volcanic rocks of a basin the Fayah Formation has undergonethe same emplacement- along the Oman continentalmargin were detachedfrom their related first-phase deformation as all the other rocks of the oceanicsubstratum and affected by intensedeformation. This Batain Group. This interpretation is fundamentallydifferent deformation resulted in a thin-skinned fold-and-thrust belt with respect to previous authors [Roger et al., 1991; and led to the obductionof the Batain Group onto the Oman Bdchennecet al., 1992; Wyns et al. 1992], who suggested continentalmargin. With regard to the timing of emplacement that deposition of the Fayah sediments postdates the deformationrecorded in the Fayah Formation plays a key emplacementof other allochthonsalong the easternOman role. Equivalentsof the Fayah Formation are missing in the margin. Hawasina complex of the Oman Mountains [Shackletonet al., Emplacement-related deformation is absent in Paleocene 1990]. On Masirah Island [Immenhauser, 1996] and on Ra's and younger deposits. These Tertiary sediments Madrekah [Gnos et al., 1997], however, the Fayah Formation unconformablyoverlie the Batain Group. This placesthe age has clearly been identified and was attributed a of obduction of the Batain Group on the Oman continental Coniacian/Santonian to late Maastrichtian age margin in latest Maastrichtian and/or early Paleocenetimes, [Immenhauser, 1995; Gnos et al., 1997]. Deposition of these that is at about 65 Ma. Obductionis coeval with emplacement flysch sedimentsis consideredcoeval to the earliest stagesof of the Masirah ophiolite [Peters et al., 1997], but about 15- the tectonic phase which ultimately led to the obductionof 20 Ma later than emplacement of the Hawasina the easternophiolite belt [Gnos et al., 1997]. complex/Semail ophiolite in the Northern Oman Mountains, The Fayah Formation in the Batain area has a Santonianto which occurred during early Campanian times (82-80 Ma Late Maastrichtian age [Shackleton et al., 1990]. The [Glennie et al., 1974]). SCHREURS AND IMMENHAUSER: OBDUCTION ON EASTERN OMAN MARGIN 159

Hawasina Complex & SeroallNappes SW directed obduction in Campanian'times(80 Ma)

•:i:• WN:'WC'oml•iex.O•)di•eeted:.o'bduction:in latest :•.•h'tiaa.:- ofBata'm :•'•[• .to.Paleoee•e"times. {65.Ma:), coeval with.• di-.reet.•':.•ucti .onof a. 80 Ma

. .. b. 65 Ma •"-EasteraO'::•..•'øia.'•...i•t m): ......

Figure 12. Sketch maps summarizing timing and direction of obduction on the northeasternArabian peninsula.(a) SW directedobduction of Hawasina complex and Semail ophiolite at 80 Ma. (b) WNW to NW directed obductionof Batain Group and Masirah ophiolite at 65 Ma.

4.2. Direction of obduction underlies the Batain fold-and-thrust belt, was reactivated during Tertiary extension.This extensionin the Batain area is Removing the effects of postnappeTertiary deformation tentatively linked to extensional intraplate deformation reveals structures that are characteristic of a thin-skinned reflectingthe Gulf of Aden rifting and progressiveopening, fold-and-thrustbelt: WNW vergent folds with subhorizontal which commencedin late Eocenetimes [e.g., Beydoun,1982; fold axes and east to SE dipping axial planes, upward to Hempton, 1987; Platel and Roger, 1989]. Alkali olivine WNW directed structural facing directions and thrust basalts in the Batain area have been dated at 44-37 Ma surfaceswhich are either subhorizontalor subparallelto axial [Bdchennecet al., 1992] and are most likely associatedwith planes and show roughly downdip striae. All structural field this rifting event. Mesozoic normal faults associatedwith the criteria point toward the presenceof a more or less linear, rifting stagealong the easternOman marginwere reactivated NNE-SSW trending, fold-and-thrust belt with a WNW during Tertiary extension [Beauchampet al., 1995] and directedtransport direction. Evidence for an earlier major, SW probablyfacilitated magma ascentalong these deep-seated directeddeformation phase in the Batain area as postulatedby faults. Roger et al. [1991], Bdchennecet al. [1992], and Wynset al. The Batain Group was refolded by a later deformation [1992] is missing. Shackleton et al. [1990] also suggested event, which also affected the overlying neoautochthonous the presenceof folds preceding the formation of the Batain Tertiary sedimentarycover to which Roger et al. [1991] fold-and-thrust belt. They underestimatedthe importance of attributeda late Paleoceneto early Mioceneage. The NW-SE Tertiary folding, however, and our field work shows that to N-S trendingopen second-phasefolding thus reflectslate these structurescan be explainedby Tertiary overprintingof Miocene-Pliocene roughly NE-SW to E-W directed first-phasefolds. shortening.This secondphase of shortening(for rocksof the Our structuralevidence, supportedby sedimentologicand Batain Group) is responsiblefor the map-scalechanges in biostratigraphicdata (presentedin detail by Immenhauser et orientation of first-phase fold axes, axial planes, and al., [1998]) indicatesthat emplacementof the sedimentaryand movementdirections. Neogene shorteningis thoughtto be volcanic rocks of the Batain Group occurred from related to convergencebetween Arabia and Eurasia [cf. approximately ESE to WNW (in present-day coordinates) Hempton, 1987; Carbon, 1996]. onto the easternOman continentalmargin (Figure 12). Regional changes in the trend of late Miocene fold axial tracesmight be relatedto and controlledby majorpreexisting normal faults. The WNW-ESE trendingNorth Jabal Ja'alan 4.3. PosternplacementEvolution During the Tertiary fault is considered a former normal fault, which was The postemplacementTertiary evolution of the Batain area reactivatedduring late Tertiary shortening[Filbrandt et al., consisted of an extensional and a contractional event. 1990; Carbon, 1996]. The left-stepping en echelon Extension in the Batain area is evident from numerous arrangementof N-S trendingfolds affectingTertiary sediments mesoscale brittle normal faults which crosscut both the along the North Jabal Ja'alan fault suggests sinistral Batain Group and overlying Tertiary sediments.Orientation transpressivemovement as already indicatedby Filbrandt et of conjugate sets of normal faults in areas which have not al. [1990] and by Carbon [1996]. Major NNE-SSW trending been reorientedsignificantly by later shorteningare roughly Mesozoic normal faults underlie the Batain Group NNE-SSW and E-W and are similar to Tertiary normal faults [Beauchamp et al., 1995]. These faults are associatedwith on Masirah Island [Marquer et al., 1995]. Beauchampet al. continentalextension and breakupleading to the formationof [1995] consider that the CretaceousMasirah Graben, which a passivecontinental margin in eastOman. One of thesemajor 160 SCHREURSAND IMMENHAUSER: OBDUCTION ON EASTERN OMAN MARGIN

normal faults representsthe easternlimit of the Jabal Ja'alan implications.The paleodepositionalrealm of the sedimentary uplift andforms the westernlimit of the MasirahGraben (see and volcanicrocks in the Batain Group was formerly located section in Figure 1). These normal faults were reactivated ESE with respectto the present-dayeastern Oman margin. duringearly Tertiaryextension [Beauchamp et al., 1995].It This depositionalrealm is referredto as the "Batainbasin" by is postulatedthat NE-SW directedshortening during the late Immenhauseret al. [1998] in order to distinguishit from the Tertiary would lead to dextral transpressivereactivation of Hawasina basin, whose paleogeographicrealm was to the the NNE-SSWtrending normal faults and would cause right- north of the northern Oman margin. Thus the Permian to steppingen echelonfolding, trending roughly N-S, in the Upper Cretaceousrocks of the Batain Group were formerly immediate vicinity of these former normal faults. Such an depositedin a basin along easternOman that separatedthe overprintingmight explainthe N-S orientedlate folding in ArabianPlate from greaterIndia. This leadsus to proposethat the southernBatain coast area. A similarorigin is proposed the Permian breakup of Gondwanaland created both for N-S trendingfolds affectingTertiary sedimentsthat have continentalmargins of Oman and led to the openingof two beendescribed farther south on MasirahIsland by Moseley major basins: the neo-TethyanHawasina basin in the north [1990] and by Immenhauser[1995]. and the proto-Indian Ocean Batain basin in the east. Tertiary postemplacement extensional structures are 5. Summary and Conclusions tentatively linked to extensional intraplate deformation reflecting the Gulf of Aden rifting. Late Tertiary, On the basisof our structuralinvestigations and combined approximately NE-SW directed shortening refolded the with sedimentologicand stratigraphicdata reported by Batain fold-and-thrust belt and is most likely related to Immenhauseret al. [1998], and publisheddata from the convergencebetween Arabia and Eurasiaas a consequenceof Bataincoast and surrounding areas, we presentthe following the onset of oceanicseafloor spreading in the Gulf of Aden major conclusions. during the late Miocene. It is suggested that basement A thin-skinnedfold-and-thrust belt (Batain Group) is anisotropiescreated during Permo-Mesozoicand/or Tertiary formed during obduction-relateddeformation and affects all extensionexerted an importantcontrol on late Tertiary fold allochthonousPermian to Upper Maastrichtianrocks in the orientations.Major preexistingnormal faults are believedto Bataincoast area. Emplacement of the BatainGroup onto the have been reactivatedduring transpressionin late Tertiary Oman passivecontinental margin occurredduring latest times. Cretaceousand earliest Paleocene times (i.e., at about65 Ma). The intense obduction-relateddeformation followed by This is coeval with obductionof the Masirah ophiolite postemplacementdeformation overprinted an alreadycomplex [Peterset al., 1997]but is muchlater than emplacement of the paleogeographicrealm [cf. Immenhauseret al., 1998]. This Hawasinacomplex and the overlyingSemail ophiolite in the resulted locally in apparently irregular, disjointed, and Oman Mountains, which is Early Campanian(82-80 Ma chaotic structures, especially in areas where continuous [Glennie et al., 1974]). It is possiblethat in the northern outcrop is lacking. However, there is far more structural Batain area, rocks of the Hawasina nappesand/or Semail coherencein the Batain area than previouslydocumented and ophiolite are hidden in the subsurface beneath the Batain we feel that the term "melange"as suggestedby Shackletonet Group. al. [1990] for the Batain coast should be abandoned. The allochthonous units in the Batain area were obducted fromESE to WNW ontothe easternOman continental margin (Figure12). This is in clearcontrast to the transportdirection Acknowledgments.Research was supported by the SwissNational ScienceFoundation (project 20-33562.92). Fieldwork in theBatain area of the Hawasinacomplex and the Semail ophiolitein the waspossible due to the helpof the Ministryof Oil andGas of Oman, Oman Mountains,which is generallyinferred as SW directed especiallyMohammed Bin HusseinBin Kassimand Hilal AI Azri, [e.g. Glennie et al., 1974; Allemann and Peters, 1972; Directorof the GeologicalSurvey, who provided logistical help and Robertsonet al., 1990]. support.Discussions with Marc Hauser, Ramon Loosveld,Albert The differencesin both timing and directionof obduction Matter, and Tjerk Peters are gratefully acknowledged.Marco Herweghand Ivan Mercolli are thankedfor helpful commentson between the allochthonous units in the Batain coast and earlierversions of this paper.The manuscriptbenefited from reviews thosein the Hawasinacomplex have importantpalinspastic by Heiko Oterdoomand Francois Roure.

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