The Hawasina Nappes: stratigraphy, palaeogeography andstructural evolution of a fragment of the south-Tethyan passive

continental margin

F. BECHENNEC', J . LE METOUR' , D. RABU' , CH . BOURDILLON-DE­GRISSAC1

, P. DE WEVER2 , M. BEURRIER' & M. VILLEY'

1 Service Geologique National, BRGM, BP 6009, 45060 Orleans Cedex,France)

2 CNRS UA 319, Universite Pierre et Marie Curie, T 15-16,75252 ParisCedex 05, France

Abstract: Lithostratigraphic and biostratigraphic revision of the Hawasina Nappes in theeastern and central Oman Mountains , including the redefinition of the Hamrat DuruGroup and the definition of the three new groups , the Al Aridh, Kawr and Umar Groups,has led to a new interpretation of the palaeogeographic and structural evolution of theso uth-Te thyan continental margin . Margin history began in the Late Permian with a phaseof extension and rifting , accompanied by considerable magmatic acti vity , and led to thedevelopment of the Hamrat Duru Basin separating the Arabian Platform to the so uthfrom the Baid Platform to the north. In the Middle to Late Triassic renewed extension ledto rifting , again accompanied by important magmatic activity, with the break-up of theBaid Platform , and the development of the AJ Aridh Trough, the Misfah Horst and theUmar Basin . These Permian and Triassic tectonic units together constituted the HawasinaBasin. A third phase of extension during the Late Tithonian-Berriasian caused a generalfoundering of the continental margin . The development of the Hawasina Basin terminatedin the Santonian, when compression initiated the firs e of obduction that closed thebasin . Ongoing obduction, during the Campanian, le.hrusting of the Hawasina andthe Samail ophiolite nappes onto the Arabian Platfori ., v J gravitational mechanisms.

Mapping of the eastern and central parts of theOman Mountains at the scale of 1 : 100000 hasled to a revision of the stratigraphy of theHawasina Nappes including important changeswith respect to the stratigraphy established byGlennie et al. (1974), and to a lesser degree thatof Searle et al. (1980). Four lithostratigraphicgroups (Table 1) have been either redefined(the Hamrat Duru Group) or newly defined(the Al Aridh, Kawr and Umar Groups;Bechennec 1987; De Wever et al. 1988, 1990),with the definition of various formations andmembers within the sedimentary and volcanicsequences constituting the groups (Bechennec1987; Bechennec et al. 1988). These depositswere laid down between the Late Permian andthe Early Senonian in the various morphostruc­tural zones of the Hawasina Basin, which are,from \he Arabian platform towards the internalzones, the Hamrat Duru Basin , the Baid (relict)Horst , the Al Aridh Trough, the Misfah Horstand the Umar Basin (Figs 1 & 2).

Palaeogeographic and structural evolutionof the Hawasina Basin

Development of the Hamrat Duru Basin inthe Late Permian

A phase of rifting and extension that com­menced during the Murghabian (Late Permian)along the northern part of Gondwana resultedin the development of a broad intracontinentalbasin, the Hamrat Duru Basin, bounded to thesouthwest by the Arabian Platform and to thenortheast by the Baid Platform.

Magmatic activity occurred during this phasealong the margins of the two platforms (SaiqFormation, Le Metour 1987; Baid Formation,Bechennec 1987, Bechennec et al. 1988) and, inparticular, in the developing basin, (the LowerMember of the Al Jil Fm, Bechennec et al.1986a; Bechennec 1987). Volcanism was essen­tially alkaline, with minor transitionalcomponents , and characterizes an intraconti-

From ROBERTSON, A . H. F ., SEARLE, M. P . & RIES, A . C. (eds) , 1990,The Geology and Tectonics of the Oman Region .Geological Society Special Publication No 49, pp 213-223

213

N.........Table 1. Lithostratigraphy of the Hawasina Nappes. After Bechennec et al. (1988) modified.

AGE HAII1RAT DUHU GHOUl' AL AIUDII GHOUl' KAWll.GROUP UII1AH GROUP

Late Turon..early Sen.~-0

ENayid Fm . Na <ca rbonate turbiditc)

lI1usallah Fm . Safll Fm. Sf'(micr it ic clay ey limestone)

Late-mid , Triassic

---------------------------1 I

_________ " lI1atbatFm.

~

1:0eno::r:enzzeno~

~

UmL (rni critic, s par ry limestone)

UmLC (m icritic Iim estone-rad iolarite)

Vm C (rad iolari te -shale)

AqilFm.

UmV ,.z (basa lt -a nd es ite )

Urn Vs (trach y te -granophy re)

Mf<platfo rm carbonate) r- UmR (olistolith ofplatIorm carbonate)

Mm (reef lim estone)Misfah Fm . -- --- ------------~ Sinni Fm.

MfY tandesitc -basatt)

Nadan Fm . Nd (mi critic lime stone,majolica)

Ar, (breccia-ca rbonate tu rbidi te ­

radiolarite)

Ar, V (ba salt)

Ar s (radiclarjte-megabreccia)ArR (olistolith of pla tform carbonate)

ArR (olistolith ofplatIorm ca rbonate)

Ar. V (andesitc-g ranophyre)

!'! !.C_<!:':.d~,:!~r~t~~:'::~.i!) _Ar.L (carbonate turbidite)

Sayfam Fm.

Iluwaydah Fm .

Arz (rad iola r ite -megabreccia )Arz V (hy aloclustite-undcsite)

AI Ghafat Fm.

SizC (silicified limestone)

Si, (silicifled limestone)

Sill - Si, LC (silicified Iimestone-rnicr tte)

Siz (carbonate turbiditc)

MbR <olistolith ofp latIo rm carbonate)

Mb, (carbonate t urb idite )

Mb, V (basa lt-hyaloclastite)

AizC (ra diolarite-shale)

AizS <sandstone)

MbzC-MbzLC (radiclarite-carbona te turl

MbzSt (siltstone-sha le-sandstone turb idi te )

Mbz (sandstone tu rb idite)

MbzL<calcareous sandstone turbidite)

Guwayza Fm . (Gw) (oolitic turbidite)

OWR (olistolith of platIorm carbonate)

Sid'rFm.

.,;

3-e"~u

~~E"­:I:

~:I:<?;

-- - ------+I-~--------------

Lias

Burremian

Dagger-Maim

Late Cenomanian

Tithonian-Berr.

Ea rl y TriassicHa l l s t a t facies

AIJi IFm. Ai, <carbonate tu rbidite)

Ai,Sh (megabreccia-radiolarite)

Ai, V tbasalt-andeslte-ke ratophyre)

Bd (platform carbonate)

Late PermianBaid Fm .

lldV (basalt)

r-..'

::r:>­~»enZ»z»"tI-emen

t a d .cc arn.

- ,

I-::' ~

I-.; ............. '

.~ci2~-

Unit Umar

\, ~3':f~~ Haut .-E.arlyApt.

\iriil "-

~ ,

\u.:

Nor.-Rh et.

~Rad i o l a r i t e s

I :::-:::~ I s ha le s

~Cla yey lim estones

~Pela g ic limestones _ (M :Maiolica)

~Si l i ci tied limestones

:.--:..... ~. t ·

8 Flint

-- --- .

~~}X~I--m

CD

~ !L

11Unit Misfah

I/

I/

AI Aridh

®

~.1'1'"'''''·';' -,'':iL~~ '' '<, .. ~ ~ M'd\dle Turonian

_- Late Alb - -• w..: ~ Top 0:Late Cenom

~ ". M \.~ ~_ / ~.~Tith .-BerriaSla'& .. ' ,'."'"" "~ : , : -, : . ' ~ .. ' l ' ''" s- :S, Lale T' / 1 ~ \i Ith .• Haut . I I -,. . ~~

$r

"~.

Unit

k';~';\:~l a reccia

[QJrr iassic l Meg ablocks[Q] perml an)

~limestones J

Turbidites. "... san dstones

"

-,

~

,'------- ~.' :>

" •. CD

- - - - , Hallstatt kri"';6 ' j ,

, u~.,,_. -"I~'" ,."",oJultian ' ~ Nor. " .\ g ~~:~~=_ _. u:

, -,,

late Tith .-Ber.

,~- -- - -- .- ­.Late Turon .- con iac .

~- - - - - _ ..Cen :'-Early Turon .

"l .Bar r. - Ap t ian

~Tida l carb onates

~: .: :,:, Hyaloc lastites

~ Pillow ]volcaniles" ~~,,~ Massiv eJ., Gabb ro

CD

NE

'fI;jUni t HD2,.... <D

Unit HDa

'. u.. ~

':~ o'.~:>"

E~ ,.

CDHD1Uni t

50m

~

'0

v;

Sw

ARABIAN ~ HAWASINA BASIN(j) Q) CV @ ® L....0.----..ill__

''''''lli '" l l1f1JrrrT HAMRAT OURU ~~ ," u.o

T11rrrm IJII I1 /JIJJ JI IJJJllI lI 11/ MISFAH ' II~AI

ARIOH

'-I 'V;'" ... - -

-~ ~ .

-i ~~~~ \:::\ ~'. '

Fig . 1. Stratigraphic sections of the major Hawasina tectonic units in the central and east ern Oman Mountains and their pro-obduction position in the Hawasina basin. N......Vl

216 F. BECHENNEC ET AL.

ARABIAN PF --- ------ H A WA SINA BASIN TETHYS

_ ____=220km 15_k_m 40km 50km 110km

HAMRAT DURU BAS IN BAlD Al ARIDH MISFAHHORST TROUGH HORST

UM AR BASIN

7Z f~~ ~:~'• ! .. I ;; 0"- /" -, I

. " .. . ~ ... . > - ~ ~- --. . . ~ . ' :0(?: - ~G. - - - ::;- - - ?i'iOIv.~-" -.-- - '.-\~ . .. :~~ ---~ --. . ~ :'iJQ 'E'9 j'.' " . ' ., : . : ' : . ' .~

~Pre - Pe rm ia n~basemenl

TI DA L SEOI MEN TS

fL""TlTriassicw:::::ncarbonal e

~Iale Permian~carbona le

BATHYA L SEO IM ENT$

!k:dradio lar il e

I;'-~ :,I l u r b id i l e _ brec c i a ~ volcanic rocks= oceanic~ lilhosphere

Fig. 2. Palinspatic reconstruction of the Hawasina basin for the Middle to Late Triassic .

nental basin. However, locally, particularly inthe Hawasina Window, it also had MORBtholeiite character (Bechennec 1987) , a featurethat most probably reflects important crustalthinning in the northwestern Hamrat DuruBasin.

The development of th e Hamrat Duru Basinwas also characterised by, from the Murghabianonwards, bathyal sedimentation with the depo­sition of pelagic facies and turbiditic carbonateswith minor breccias and megabreccias contain­ing fragments of reworked shelf deposits fromthe two bordering platforms (Murghabian radio­larites and turbidites of the Lower Member ofthe Al Jil Fm.; Bechennec 1987; De Wever etal. 1988, 1989, 1990).

Development of the Ai Aridh Trough, theMisfah. Horst and the Umar Basin in theMiddle of Late Triassic

A second phase of rifting and extension duringthe Middle to Late Triassic primarily affectedthe Baid Platform, which was broken up andturned into a seamount or submarine plateau(Hallstatt facies of T. Calon, 1988, pers.comm.) . This platform relict neverthelessplayed an important role in the subsequentsedimentary history, as it prevented the influxof terrigenous detritus from the Arabian Plat­form into the internal zones of the HawasinaBasin. The Al Aridh Trough , the Misfah Horstand the Umar Basin that were also developed atthi s time , thus remained free of terrigenousmaterial, which was restricted to the HamratDuru Basin (Fig. 2).

An important magmatic episode was associ­ated with this phase of extension , in particularin the Umar Basin (Sinni Fm .), on the Misfah

Horst (Lower Member of the Misfah Fm.) , inthe Al A ridh Trough (Sayfam Fm. ) and in th emost distal parts of the Hamrat Duru Basin(Lower Member of the Matbat Fm). The volcan­ism was in ge ne ral of within-plate alkaline type ,although the sequences in the Sinni Formationshow an evolution towards transitional andlocally to MORB tholeiitic types (Beche nnec1987) . This suggests a marked crustaI thinningin the internal parts of the Umar Bas in , whichprobably was continuous with the Neo-Tethyansea-floor , whose formation by sea-floor spread­ing began at this tim e (Whitechurch et al. 1984).

Thus, the Hawasina Basin possessed its princi­pal morphological and structural features fromthe Middle to Late Triassic onward , and was toretain them until the Early Senonian.

Sedimentation in the Hawasina Basin fromthe Middle Triassic to the Late Iurassic

During this period the Hamrat Duru Basin wascharacterized by the deposition of a coarsening­upward megasequence indicating a phase ofsediment progradation into the basin. Thismegasequence begins with a series of radio­larites to siliceous micrites of Carnian-Norianage (Upper Member of the Al Jil Fm. ;Bechennec 1987) documenting both deepeningof the basin related to crustaI extension and adecrease in th e influx of terrigenous detritusfrom the Arabian Platform , which was experi­encing evaporitic conditions at this time (Murris1980; Le Metour 1987; Rabu 1987) .

This siliceous sequence was followed in LateTriassic time (Bernoulli & Weissert 1987 ;Bernoulli et al. 1990) by a seqence of radiolaria­and bivalve-bearing micrites associ ated withfine-grained calcareous turbidites (Lower

HA WASINA NAPPES 217

Member of the Matbat Fm.) . In the Jurassic,two major turbidite sequences were deposited;the first one , Liassic, in age, is characterized inparticular by a quartzose facies (Upper Memberof the Matbat Fm.), and the second one, Middleto Late Jurassic, by limestones containing re­worked oolites (Guwayza limestones, Glennieet al. 1974; Cooper 1987; Guwayza Fm .Bechennec 1987). The paralle lism between thesequences on the Arabian Platform and those inthe Hamrat Duru Basin is remarkable andunderlines the fact that the platform was theprincipal source for the sediments of the HamratDuru Basin (Bechennec 1987; Bechennec et al .1988; Le Metour 1987; Rabu 1987). From theend of the Liassic the sed iments of the east­central part of the Hamrat Duru Basin becameenriched in silica: the oolitic turbidites equiv­alent of the Guwayza Formation, are silicifiedto varying degrees, and there are intercalationof radiolarites (Wahrah Fm., Glennie et al.Bechennec 1987). This local differentiation con­tinued into the Cretaceous.

The relict horst of the Permian Baid Platformbounded the Hamrat Duru Basin to its northeastand protected the internal zones (AI AridhTrough, and Umar Basin) of the HawasinaBasin from the influx of clastic material fromthe Arabian Platform. In the Middle to LateTriassic, after the peak of magmatic activity, onthe Misfah Platform a sequence of shallow­water carbonates was deposited (Upper Mem­ber of the Misfah Fm.) whose thickness (>800m) reflects rapid subsidence during the LateTriassic. However, during the Jurassic, car­bonate sedimentation was extremely reducedon the horst, which was even emerged at sometimes, as witnessed by the microkarsts and fer­romanganiferous encrustations at the top of theMisfah Formation. In contrast, in the Al AridhTrough and the Umar Basin, pelagic sedimentswere deposited, with intercalations of brecciasand megabreccias (Sayfam Fm. of the Al AridhGroup, Aqil Fm. of the Umar Group) . Thesefragmental deposits yield almost exclusivelyblocks and megablocks of shallow-water carbon­ates from the bordering horsts, Late Permianand Late Triassic in age in the Al Aridh Trough,and Late Triassic in age in the Umar Basin.

Subsidence of the continental marginduring the Late Tithonian - Berriasian

A new phase of regional extension occurredduring the Late Tithonian - Berriasian. On thecontinental margin this was reflected by subsid­ence of the edge of the Arabian Platform, witha consequent 250 km southwestward retreat of

the continental slope (Le Metour 1987; Rabu1987; Rabu et al. 1990), and by a generaldeepening of the Hawasina Basin, whose mainmorphological and structural features were ,however, preserved (Bechennec 1987; Bechen­nec et al. 1988).

In the Hamrat Duru Basin , the foundering ofthe Arabian Platform resulted in the depositionof very coarse-grained breccias in the mostproximal facies near the top of the GuwayzaFormation. Subsequently, the retreat of thecontinental slope caused by a reduction in clasticinflux, and pelagic sedimentation became pre­dominant (Lower Member of the Sid'r Fm.)with a tendency to silicification of the clasticfacies due to the concomitant deepening of thebasin.

This extensional phase also reactivated thehorsts in the internal zones of the HawasinaBasin. Thus the most distal facies near the topof the Guwayza Formation are characterized bylarge blocks of reworked shallow-water carbon­ates Late Permian in age from the relict BaidHorst. Similarly breccias and megabreccias withreworked blocks and megablocks of Permianand Triassic shallow-water carbonates becameparticularly common in the Al Aridh Troughduring this period (Buwaydah and Al GhafatFms .). This trough, was situated between thePermian and Triassic platforms and was also thesite of magmatic activity (Buwaydah Fm.;Bechennec et al. 1986b; Bechennec 1987) whosewithin-plate alkaline character reflects the conti­nental nature of the basement to the HawasinaBasin.

The Misfah Horst, until this time barelysubmerged, also subsided during the LateTithonian - Berriasian, and the carbonate shelffacies was succeeded by the bathyal Maiolicafacies of the Nadan Formation (Glennie et al.1974). Pelagic sedimentaion continued in theUmar Basin.

Cretaceous sedimentation in the HawasinaBasin

Shallow-water carbonate sedimentation pro­graded during the Valanginian over the foun­dered part of the Arabian Platform, completelycovering it again by Barremian times (LeMetour 1987; Rabu 1987, 1990). In the HamratDuru Basin this progradation is reflected byrenewed to carbonate turbidite depositionduring the Barremian - Aptian (Middle Memberof the Sid'r Fm.) (Bechennec 1987).

A new episode of siliceous sedimentationdeveloped during the Albian-Cenomanian(Upper Member of the Sid'r Fm.), perhaps

2 18 F. BECH EN NEC ET AL.

co nnec te d with ano the r period of exte nsion,th at most probabl y was resp onsibl e for se a-floorspre ading in th e Sa mail Basin . However ca rbo n­ate turbidi te sedime nta tio n se t in again du ringthe Lat e Ce no ma nian - Early T uronia n (NayidFm .: Beche nnec 1987), and co ntinue d until a tleast th e Lat e Turonian - Early Se no nian . Theseturbidites are fai rly fine-grain ed and co ntai nc1asts o f reworked she lf deposit s from theA ra bian Platform . A t th e sa me time, acti vefaulting along the edge o f th e plat form led toth e deposition of channelized breccias andmegabreccias, both on th e continental slope(M ember 0 of the Mayhah Fm . ; Watts &Garrison 1986) and in the mo st proximal partsof th e Hamrat Duru Basin (N ayid Fm. in th eHawasina Window, Bechennec 1987 andR iyamah Fm . Cooper 1987 ; Robertson 1987) .

The A I Aridh T rough was chara cterised, atleast until th e lat e Turonian -early Sen onian , byessentially pel agic se dime nta tio n (AI G hafa tand Musallah Fms. ) with th e int ercalat ion ofbr ecci as and megabreccias with reworked , Lat ePermian and Triasic sha llo w-wate r ca rbo na tesderived from the two bordering horsts.Mag matic activity, altho ugh limited , co ntinue din this zo ne and is represented by intercala tio nsof ba salt , ande site and tr achyte , with micro­ga bbro and granophyr e (M usa lla h Fm. :Bechennec 1987).

The Misfah Horst remained in a bathyal en­viro nme nt throughout th e Cret aceous. Afterth e deposition of th e Maiolica facies sedimentsduring the Tithonian-Valanginian , sedime n­tation ceased from the Valanginian to the begin­ning of the Middle Cenomanian, when pelagicsediments resumed for at least until the MiddleTuronian.

Sedimentation in th e Umar Basin remainedessentially radiolari an through most of th eCre tace ous.

From the closure of the Ha wasina Basin tothe fo rmation of an Eo -Alpine chain

Lithological , sedime ntologica l, biostratigraphi­cal and stru ctural data su ggest th at the maintectonic units that constitute th e HawasinaNappes correspond approxima te ly to th e princi­pal palaeogeographical domains of the Ha­wasina Basin. It also appears to the autho rs(contrary to -Bern o ulli .& Weiss ert 1987) thatthe units from more internal situations overridethose of more exte rn a l origin (in regard to th eA ra bian platform) . The H awasina Nappes canbe grouped into low er and upper tectonic mega­units that consist respectively of rocks derivedfrom th e Hamrat Duru Basin and Baid Horst ,

and from the A I Aridh Trough, the MisfahHo rst and the Umar Basin. Each mega-tectonicunit is itself divi de d into several mai n units , th estructura l edifice co mp rising, from base to top ,seven pri ncipal units (Bechennec 1987;Bechennec et al . 1988) (Figs 3-5) :1. The Hamrat D uru tec to nic unit 1 (H D l )

co nsists of the most pro xim al facies of th eHamrat Duru Group. It is thrust over the'Autoc ht ho no us' U nit B ( Le Metour 1987 ,1990) , and mainl y crops out at Birkat a lMawz, so uth of A I Aja l and in th eHawasina Wind ow .

2 . T he Hamrat Duru tect onic unit 2 (HD2)

co mprises the proximal to subpro ximalfacies of th e Hamrat Duru Group, and isthrust over eithe r unit HD l o r the 'Autoch­th on ous' U nit B , cropping out es pe cially inthe Hamrat Duru R an ge , in Jabals G huba rand Khatma, in so ut he rn Jabal Safra, andin th e Zukayt a rea .

3. T he Hamrat Duru tectonic unit 3 (H D 3)

co mpr ises th e subdista l facies of th e H amratDuru Group. It is locall y co ntinuo us withunit HD2 but more co mmonly it is thrustover it o r over th e 'Autoc htho nous' U nitB. This unit (H D 3) crops o ut essent ia lly inJab als Wahrah and H ammah , ce ntral JabalSafra and in th e A I Ayn and Jabal Misth­Jab al Misfah areas .

4. T he Hamrat Duru tectonic unit 4 (HD4 )

includes th e facies of th e Hamrat DuruGroup that were th e most distal relative toth e Arabian Platform, but th e most proxi­mal relative to the Baid Horst , a few relicsof which are incorporated into it. It isthrust over e ithe r th e preceding tectonicunits or over the 'A utochtho no us' Unit B,and mainly cro ps out in th e Doqal area andnortheast of th e H awasin a Window, in th eNakhl , Taww , Bid-Bid , Hayl , Baid ,Mudayb i and Wadi A ndam areas , and innorthern Jabal Safra .

5. The Al A ridh tectonic unit comprises th erocks of th e A l A rid h Group. It is thrustover all th e above-me ntio ne d units or re stsdirectl y on th e 'A utochthonous' Unit Band only occurs along the southe rn flank ofthe Oman Mountains , wh ere it forms anarrow but fairly continuous band.

6. The Misfah tectonic unit co nsists esse ntiallyof th e Triassic of the Misfah Formationwith certain faci es of th e Umar Group. It isthrust over th e prec eding units .

7. The Umar tectonic unit comprises the UmarGroup. It is thrust over all the precedingunits and locally onto the 'A utochtho nous'Unit B . It is itself overthrust by the ophio-

HA WASIN A NAPPES 219

sw NE

HAWASINA WINDOW BATINAHAI Aridh J. Abyad Doqal

~...~AUTOCHTHONOUS UNIT SUMEINI UNIT HAWASINA NAPPES SAMAIL NAPPE snkm 25 0

. Umar unit

D HD4

F·",;:,;I HD3

["' <J HD!

E"EE Ter t iary

[J] Perm. -Cret ac . I ::-:_:-~l AI Ar idh unit

IH,~"I 0"," "";1'

\

...\

\

""-Thrust fault

Fig. 3. Tectonic cross section across the western part of the central Om an Mountains showing the stackingsequence of the main allochthonous units and the late-obduction , NNE-facing, back-foldin g and back thrusting(location on Fig. 5).

sw ~ . _ - . _ --- - - - - - - -- - - .-- . . -_ . - --_ . . - _.I

_.-_.. _.__ .- _ . - '.-. NE

..

'> . BASIN

'\ . Ad ,370 Km

~ Rd: 90 Km

<," ~ Ad ,540 Km

-. Rd : 200Km-,

\

\

Um

~, ....\ \\ \ Th rust fault

"'-. \ majorvrn inor

Ad : Ab solute I .

I'" d isplacement

Rd : Relative

SA MAIL NAP PE

Qie Crusta l seq u en c e

~Mantle seq ue nc e.... .... .. Metamorphic so le

b.::::=::jurnar units

HAWASINA NAPPES

~;\~Ham ra t Duru units

[ZZl MiSfah units

I-~-~ i~-: I A I Ar idh un its

SUMEINI UNITS

D s ase me nt

AUTOCHTHONOUS

UNITS

o Per m ian .Cret aceou s

o

Fig. 4. Diagrammatic representation of the main Hawasina units in their palinspastic position . For some ofthem the position of the overthrust front , on the Arabian platform, is indicated , and the approximatedisplacement with respect to their original position (Ad) or to the underlying unit (Rd) .

lites of the Samail Nappe or its meta­morphic sole and has for this reason beenparticularly sliced up.

In the west-central part of the Oman Moun­tains this order of superposition has locally beenreversed by back-folding. The order also varieslocally as a result of inter-unit imbrication, inparticular on the north flank of Jabal Akhdar.

It is possible , by kinematic reconstruction(D ahlstrom 1969; Hossack 1979), to estimatethe original extent of the main tectonic units ,and from there to obtain an idea of the order ofthe original width of the various palaeogeo­graphical units of the Hawasina Basin. Thewidth of the Hamrat Dum Basin, together withthe relict Baid Horst , is estimated at 240 km

o Location at theC ;> structural;::...-" c r oss-sec t i ons

/ Normal f ault

~ Thrus t

J.Akhdar

I I I / 1

/CO:A:ST II / I

I I / I I

I ~/ \ II / n \, II .

-' c

J. Ak hd ar

______rk m

mukrn 0

E

~

so le

(1104

) t ect oni c um t

(1103

) t ectonic uni t

( 11 0 2) tec to nic uni t

t ect on i c un i t

NNo

~

COmo::r:mzZmom-J);>r

Fig. S. Structural sketch map of the centra l and eastern Oman Mountains.

HAWASINA NAPPES 221

(Beche nnec 1987), that of the Al A rid h Troughat 40 km , the Misfah Horst a t 55 km and theUmar Basin a t a minimum of 110 km , giving amin imum width for the Hawasina Basi n as awho le of about 450 km.

Taking into co nside ratio n the present posit ionof their southe rn fro nt on the Oman platform(Fig. 5), it is clear th at the vario us tectonic unitshave und ergone co nside rable displacement(Fig. 4). This is a ll th e mo re re marka ble in viewof th eir thin-sk inned nature and the fac t thatthey have suffe red re latively littl e in te rn al defor­mation . T hese feat ures co uld prompt the sugges­tion that , inst ead of compressive stress appliedfro m the rear (Mur re I 198 l; Merl e 1984) , gravi­tational forces were res po nsible for emplace­ment of the tect oni c units. This wo uld bemechani cally co mpatible with the lar ge displ ace­ment and the sty le of deformat ion. However ,the geometry of th rus t plan es at the base of thevarious units is, in ge nera l, not compa tible withsimple gravita tiona l sliding. T his would involvebasal truncations cutt ing ac ross the stra tigra phytow ards the rear of th e units, sliced-o ff fo ldsfo rming tec to nic re mnants, and increased defor­mati on towards the fro nt of the units. Structuralana lysis of the Hawasina Na ppes inst ead showsthat development of the main tect oni c units andtheir emplace me nt o nto the A rabian Platformtook place in three stages that involved differentmechani sms.

T he first, ' pre-early obd uct ion' stage , devel­ope d in a co mp ress ive se tt ing within the broade rfra me wo rk of the Ne o-Tethys and its mar gin.Following A lbian - Ce no ma nian accre tio n of theSamail Basin along a spreading axis (Be urr ie r1987) , northeast-direct ed intraoceanic sub­duction developed during the Ce noma nian­Turonian , between the young ocean ic crust ofthe Sam ail Basin and olde r, Earl y Cre taceousNeo-Tethya n oceanic crus t attached to theA ra bian margin. The evolution of thi s settingled to th e beginning of continental subductio n,for which the main evide nce is the contine ntalbiotite granites and the oceanward-ve rg ingstru ctures with associated high-pressure meta­morphism in the Sai h Hat at and Jab al A khdar(Le Met our et al. 1986, 1990; Le Metour 1987 ;Rabu 1987) . This co ntine nta l subduction led , inth e Turonian-Coniacian, to blocking of thesubductio n syste m and th e form ation of intra­oceanic det achments. It was from thi s zone ofdetachment , beh ind the subd uctio n zone , tha tth e Sam ail Nappe was derived; it th en reachedth e int ernal zo nes , of th e Hawasin a Basin duringth e Santon ian -Campanian , with resultant meta­morphism of the Umar Group rocks. Duringthis period , in the Hawasina Basin , the majorthrusts se pa ra ting the pr inc ipal tectonic units

were initia ted . T he ir geome try is co nsiste nt withco mpressive tectoni sm , and they began in themost in ternal zo nes (piggy-back thrusts); thus ,alt hough at this stage the displace me nts re­ma ined re lat ively small, the Ha wasin a suc­cession was already decoupled from itsbasement.

The seco nd stage, of ' true' obduction , gaverise to the major d isplacem ent of the principaltectonic un its an d the ir emp lace me nt onto theArabia n platform. T his took place duri ng theEarly Ca mpa nia n - Maastrichtian . T he firs t un itto be emplaced on the platform was the HamratDu ru tecton ic mega-unit , in whcih fea tures ofgravity stre tching can be see n. T hus, th eso uthe rn fron t of the most proximal un it , HD l ,

was ove rridde n by the so uthe rn fro nt of thesub-proxima l unit , HD2 , itse lf overridde n bysub-d istal un it HO) (Figs 3-5). These variousuni ts we re the n truncat ed by the A l A ridhtecton ic un it , whose so uthe rn fro nt is orienta te dtran sverse to the fronts of the preceding units .The sa me is true fo r the Misfah , Umar andSa ma il un its , so that it appears that the maintectonic un its were emplaced ind ividually o ntothe platform, the latest to be tr anspo rted beingthe most internal o nes (o ut-of-seq ue nce thrusts ,Searle 1985; Bech ennec 1987) . However , at alate stage, due to isostatic uplift followingco ntine nta l subduction, the met amorphoseautoc hto n formed ascending fro nta l and lat eralra mps ( Bec hen nec 1987; Le Me to ur 1987) . Themain co nse q ue nces of this were the for matio nof so uth-verging fo lds, with an associa te d cleav­age, in the last-emplaced units , the developmentof 's ubtractive ' slices in the 'autochthon', intra­unit imbrication and , finally halting ofo bd uctio n.

The third, ' la te obduction' stage, is rep­resented by northeast-verging backfolds, th atare particularl y we ll developed in the west ­ce ntra l part of the Oman Mountains (Glennie etal. 1974 , Graham 1979 , Searle & Cooper 1986),with associ at ed schi sto sity , greenschist faciesmetam orphism , fracture cleavage (Beche nnec1987) .

After the emplace me nt of the vario us tectonicunits on the A rabian Platform followed by th eea rly Maa strichtian transgression , a final phaseof folding , on axe s trending N30o-60° termin­at ed deformation of the Eo-Alpine (Le Metour1987; Rabu 1987) belt.

Conclusions

The Hawasina Basin developed in two mainstages. A first phase of rifting and exte nsio nduring th e Late Permian initiated the formationof the H amrat Duru Basin , which presents th e

HAWASINA NAPPES 223

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