Post-amalgamation basins of the NE Arabian shield and ...rjstern/egypt/PDFs/General/JohnsonPCR03.pdf · Gondwana supercontinent that resulted from East and West Gondwana convergence

Precambrian Research 123 (2003) 321–337

Post-amalgamation basins of the NE Arabianshield and implications for Neoproterozoic IIItectonism in the northern East African orogen

Peter R. Johnson∗Saudi Geological Survey, P.O. Box 54141, Jiddah 21514, Saudi Arabia

Received 1 April 2001; received in revised form 20 November 2001; accepted 31 January 2002

Abstract

The development and structure of Neoproterozoic III volcanosedimentary basins in the northeastern part of the Arabian shieldrecord periods of uplift, erosion, extension, subsidence, compression, and strike-slip faulting that postdated the completion ofterrane amalgamation in the northern East African orogen (EAO) and predated the initiation of a passive margin on the northernflank of the Gondwana supercontinent. The oldest basins, dating from<670 to >650 Ma, include as much as 8000 m sandstone,conglomerate, bimodal volcanic rocks, and limestone, and were deposited in a large area (approximately 72,000 km2) of locallyfault-controlled subsidence (Murdama basin) and in narrow grabens (Bani Ghayy basins). The regional unconformity at the baseof the basins bevels the newly amalgamated terranes and exposes greenschist-, and locally amphibolite-, and granulite-grademetamorphic rocks, indicating possibly >10 km uplift and erosion in parts of the region prior to deposition. The basins wereclosed and inverted by folding at about 650 Ma and unconformably overlain by rocks of the Jurdhawiyah group and Hibshiformation, which were deposited between 640 and 620 Ma in fault-controlled basins following a further phase of uplift anderosion. The Jurdhawiyah and Hibshi basins closed and inverted during subsequent north–south shortening and north- andsouth-vergent reverse faulting. A final phase of Neoproterozoic III basin formation resulted in deposition of the Jibalah group(580–570 Ma) in small, isolated, pull-apart basins caused by strike- and dip-slip movements on faults of the Najd fault system.

The Murdama and Bani Ghayy basins contain shallow-marine clastic and carbonate deposits indicating that seaways penetratedthe core of the EAO soon after terrane amalgamation, which implies that much of the region was at a low elevation soon afterterrane amalgamation and orogeny. This limits the extent to which gravity-driven extensional collapse may have been a factor inthe Neoproterozoic III tectonic evolution of the region, unlike other parts of the EAO where orogenic extensional collapse wassignificant. The Jurdhawiyah, Hibshi, and Jibalah basins were probably isolated fault-controlled lakes. The northerly trend ofMurdama and Bani Ghayy folds implies bulk east–west shortening, which is consistent with the model of east–west Gondwanaconvergence conventionally applied to the EAO. East–west convergence conceivably accounts, additionally, for the creation ofthe Jurdhawiyah and Hibshi basins as a result of concomitant northward extension or tectonic escape. It would also account forthe simultaneous creation of the Murdama and Bani Ghayy basins, themselves, linking the two as foreland basins at subsidedand extended parts of a newly amalgamated crust in the center of the study area that was downflexed by the overthrusting of anophiolite complex and other terranes from the east.© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Foreland basin; Graben; Extension; Shortening; Arabian shield; East African orogen

∗ E-mail address: [email protected] (P.R. Johnson).

0301-9268/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0301-9268(03)00074-3

322 P.R. Johnson / Precambrian Research 123 (2003) 321–337

1. Introduction

Deformed, but barely metamorphosed, post-amal-gamation volcanosedimentary basins are a distinc-tive component of the geology of the northern EastAfrican orogen (EAO) in the Arabian-Nubian shield.Thirty or more basins are known in the region (Fig. 1)and are particularly extensive in the northeastern partof the Arabian shield (Fig. 2). The basins are mostlyNeoproterozoic III (650–540 Ma) in age, rangingfrom <670 to about 580 Ma, and vary in size, in theirpresent deformed states, from aggregates of basinsextending over as much as 72,000 km2 to small iso-lated basins of 200 km2. They are unconformableon the juvenile volcanic-arc rocks that make up the

Fig. 1. Neoproterozoic III post-amalgamation volcanosedimentary basins in the Arabian shield. HZF: Halaban-Zarghat fault zone; ARF:Ar Rika fault zone; RF: Ruwah fault zone. Inset shows the location of the Arabian shield in the Arabian Peninsula. The box outlines thearea ofFig. 2.

older terranes of the Arabian shield, which assem-bled during a process of transpressional subductionand ocean-basin closures culminating in Saudi Ara-bia in the Nabitah orogeny (680–640 Ma) (Johnsonand Kattan, 2001; Stoeser and Stacey, 1988). Thebasins are unconformably overlain by Lower Pale-ozoic passive-margin siliclastic rocks, which weredeposited on the EAO and the end-NeoproterozoicGondwana supercontinent that resulted from East andWest Gondwana convergence (Stern, 1994), and areintruded by large volumes of late- to post-orogenicgranitoids.

EAO post-amalgamation depositional basins arerare outside the Arabian-Nubian shield and Neo-proterozoic III events elsewhere in the orogen

P.R. Johnson / Precambrian Research 123 (2003) 321–337 323

Fig. 2. Simplified geologic map of the northeastern Arabian shield showing post-amalgamation basins and selected late- to post-tectonicgranites. Arrows show sense of shear on faults: HZF: Halaban-Zarghat fault zone; ARF: Ar Rika fault zone; RF: Ruwah fault zone. Insetshows principal terranes in the region: As: Asir; Af: Afif; H: Ha’il; D: Ad Dawadimi; R: Ar Rayn.

include the different processes of synorogenic duc-tile shearing and granulite-grade metamorphism inMadagascar (Martelat et al., 2000; de Wit et al.,2001); uplift and exhumation of granulite basement

in Tanzania (Maboko et al., 1989); and uplift ofgneiss domes and metamorphic core complexes inEgypt and Sinai (Blasband et al., 1997; Fowler andOsman, 2001; Fritz et al., 1996; Greiling, 1997). The


post-amalgamation basins, in contrast, reflect stagesof late- to post-orogenic erosion, subsidence, depo-sition, and deformation. These stages tectonicallydifferentiate the northeastern Arabian shield fromother parts of the EAO, which were undergoing con-temporary intense deformation and metamorphism,and constitute strong evidence that orogeny in theEAO was diachronous. The objective of this paper isto review the geologic history of these basins so asto draw attention to a distinctive feature of the EAOthat has hitherto lacked emphasis in the internationalliterature and to outline some of the constraints ontectonic modeling of the region that are implied bythe stratigraphy and structure of the basins.

2. Amalgamated terranes

The post-amalgamation basins overlie a collageof as many as five volcanic-arc terranes (Fig. 2,inset). The Afif terrane is the largest (Stoeser andCamp, 1985) comprising (1) the Khida subterrane,a small Archean-Paleoproterozoic continental mi-croplate in the southeast; (2) the Siham subterrane,a ∼750–745 Ma (Agar et al., 1992) volcanic arc de-posited against the Khida terrane in the west; (3) the

Fig. 3. Schematic east–west cross-section showing the relation of post-amalgamation basins in the northeastern Arabian shield to underlyingamalgamated terranes, sutures, and faults. Trends lines are a generalized indication that the layered rocks in the Tarib and Siham subterranesare mostly steeply dipping.

Suwaj subterrane, a∼700–670 Ma (Cole and Hedge,1986) magmatic arc in the east; and (4) the Nuqrahsubterrane, a∼850–829 Ma (Calvez et al., 1983;Pallister et al., 1988) volcanic arc in the northwestlargely outside the area ofFig. 2. The Asir compositeterrane, to the south and west, includes the An Nimas(>790 Ma) and Tarib (785–720 Ma) arcs (Stoeser andStacey, 1988). The Ad Dawadimi terrane is charac-terized by chlorite-sericite schist and phyllite (Abtformation;≥690 Ma) and mafic–ultramafic rocks ex-emplified by the Halaban ophiolite (∼695 Ma;Staceyet al., 1984). The Ha’il and Ar Rayn terranes arevolcanic arcs of uncertain age to the north and east.

Johnson and Kattan (2001)provisionally estimatedthat the Afif terrane was assembled between 720 and680 Ma and was sutured to the Asir terrane alongthe Ruwah fault zone at or soon after 680 Ma. TheAfif and Ad Dawadimi terranes were sutured alongthe Halaban suture zone (marked by the Halabanophiolite) (Fig. 2) by 680 Ma, the time of obductionof the Halaban ophiolite and metamorphism of thesub-ophiolite sole complex (Al-Saleh et al., 1998).The Ad Dawadimi and Ar Rayn terranes were prob-ably in contact by 650–640 Ma (Stacey et al., 1984),and terranes in the western part of the Arabian shieldwere assembled earlier, between 780 and 700 Ma.


The largest post-amalgamation basin described here,the Murdama basin, overlies the cryptic join be-tween the Siham and Suwaj subterranes of the Afifterrane (Fig. 3), and is subparallel to and 0–25 kmwest of the Halaban suture and overthrust Halabanophiolite.

3. Murdama group basins

The Murdama basin, occupied by sedimentaryand subordinate volcanic rocks of the Murdamagroup (<670 to >650 Ma;Cole, 1988), is a com-posite structure composed of separate areas referredto as the Maraghan, Mushrifah, Urayk, Salam, andMaslum basins (Fig. 2). The base of the Murdamagroup is a regional unconformity at which expo-sures of greenschist- to amphibolite-, and locally,granulite-facies metamorphosed volcanic and plu-tonic rocks belonging to the underlying Suwaj andSiham subterranes (Cole, 1988) indicate that as muchas 10–15 km uplift and erosion occurred prior to Mur-dama deposition. Folds and strike-slip faults deformthe Murdama group, and later intrusions and youngerbasins interrupt continuity of exposure. However, localfacing directions and the presence of basal conglom-erates ledCole (1988)to infer that the present-daybasin margins approximate the original margins. Re-moving the effect of post-Murdama strike-slip faulting(Fig. 4) outlines a composite basin more than 600 kmlong and 120 km wide, part of which is located be-neath Phanerozoic cover east of the shield (Johnsonand Stewart, 1995), making the Neoproterozoic Mur-dama basin comparable in size to other orogenicdepositional basins such as the Mesozoic Pannonianbasin in the northern Carpathian foreland (Horvath,1993) and the Himalayan collisional successor basinsin northwestern China (Graham et al., 1993). Al-though folding creates uncertainty about the exactmeasure, the group is as much as 8000 m thick. Givenits surface area, it clearly contains a vast amount ofdetrital material, which implies that the underlyingvolcanic-arc terranes underwent major erosion anddegradation following Nabitah orogeny. The group isintruded by granitoid plutons belonging to the Kilabsuite (∼650 Ma), the Idah suite (∼620–615 Ma), andthe Abanat suite (585–570 Ma) (Cole and Hedge,1986).

The Murdama consists of sandstone, conglomer-ate, subordinate siltstone, limestone, and volcanicrocks regionally metamorphosed to the lower green-schist facies (Cole, 1988; Greene, 1993; Pellaton,1984). Sandstone, assigned to the Zaydi formationin the Maslum basin (Fig. 5) and the Maraghan for-mation in the basins to the north, is monotonous,fine-to-medium-grained, gray-green lithic (volcanic)arenite and arkosic arenite exposed in planar bed-ding units a few centimeters to a meter or more thick(Fig. 6A). Bedding is well developed and continu-ous on strike for hundreds of meters. The sandstoneis poorly sorted and has angular grains of volcanicrock, feldspar, quartz, and feldspar-quartz aggregatesin a matrix of epidote, chlorite, calcite, clay miner-als and unidentified, probably very fine-grained rockfragments that makes up 1–30% of the rock. Thesandstone plots in the provenance fields of slightly tostrongly dissected magmatic arcs (Fig. 7), consistentwith the superimposition of the Murdama group onnewly amalgamated volcanic-arc terranes. Sedimen-tary structures include planar cross-bedding, ripplecross-lamination, planar lamination, grading, andscour and fill features. Fine-grained sandstone gradeslocally into siltstone and coarse-grained sandstonegrades into pebbly sandstone and conglomerate, andthe rocks form upward-fining cycles of sandstone,siltstone, and shale less than one to several metersthick (Wallace, 1986). Indistinctly bedded to massive,polymict conglomerate is common at the base of thesandstone sequence and as interbeds higher in the se-quence. The conglomerate is mainly clast supportedand is composed of subangular to subrounded pebbles,cobbles, and rare boulders of metaandesite, metadior-ite, metadacite, marble, varied felsic metavolcanicrocks, granodiorite, granite, and mica schist (Cole,1988).

Volcanic rocks crop out at the base of the groupon the southwestern side of the Maslum basin (AtTuwawiyyah formation;Fig. 4) and are interbeddedwith sandstone higher in the succession. Conven-tionally, the basal volcanic rocks are interpreted asa pre-Murdama unit, but transitional interfingeringbetween volcanic rocks and typical Murdama-groupepiclastic rocks (Bois et al., 1975; Johnson, 1996) jus-tifies grouping the volcanic rocks with the Murdama.The At Tuwawiyyah formation includes rhyolite, an-desite, local marble, chert, and conglomerate, overlain


Fig. 4. Palinspastic sketch map of depositional basins in the northeastern Arabian shield removing the effect of sinistral shear on theHalaban-Zarghat and Ar Rika faults, using the contacts of the Bani Ghayy and Murdama groups to guide the amount of restoration.


Fig. 5. Stratigraphic columns for the lower part of the Murdama group on either side of the Maslum basin at Jabal Murdama (afterLetalenet, 1974) and Jabal Farida (afterGreene, 1993). Locations of the jabals (mountains) are shown inFig. 2.

by interbedded rhyolite, rhyolite breccia, ignimbrite,dacite, andesite, basalt, and volcaniclastic sedimen-tary rocks. The rocks have a fairly mature calc-alkalicand high-K calc-alkalic geochemical signature andhave been compared to present-day transitionalvolcanic-arc sequences such as the Indonesian Sundaarc (Roobol et al., 1983). Their bimodal character isnoteworthy and may be evidence for extension andrifting during the initiation of Murdama deposition.

Lenses of limestone tens of meters to 300 m thickoccur in the Maraghan basin and limestone, morethan 1000 m thick (locally named the Farida forma-

tion; Fig. 5), is interbedded with basal conglomer-ate and sandstone or rests directly on sub-Murdamarocks at the eastern margin of the Maslum basin. Thelimestone is commonly well bedded in layers severalmillimeters to tens of centimeters thick, and includestan, fine-grained micrite, dark brown, siliceous impurelimestone, black carbonaceous limestone, and tan stro-matolitic limestone.

Because of the predominance of planar-bedded,poorly sorted sandstone,Greene (1993)proposed thatthe group originated in a deltaic environment inter-spersed with conglomerate-bearing alluvial channels.


Fig. 6. (A) Outcrop-scale view of gently dipping Murdama group sandstone. (B) Bani Ghayy group polymict conglomerate showing typicalappearance of pebble–cobble clasts. (C) Homoclinal, east-dipping alternating beds of sandstone and conglomerate in the Jibalah group,Antaq basin. (D) Wavy bedding of algal-mat limestone in the Jibalah group, Jabal Jibalah type area.

A particularly thick wedge of conglomerate (theTimiriyat conglomerate) represents an east–westchannel north of the Shara fault (Fig. 8) along thesouthern margin of the Maraghan basin (Cole, 1988).The thick, locally stromatolitic Farida formation andother carbonates identify marine environments alongthe eastern side of the Maslum basin and to thenorth, and fine-grained sandstone, siltstone, and shaledeposited in near-shore mud-flat and broad-channelenvironments with carbonate mud and algal buildupsevidence lagoonal, lacustrine, or shallow-marine con-ditions in the Maraghan basin (Wallace, 1986).

Gently plunging, open, upright, north-trending foldswith locally vertical limbs deform the Murdama group(Fig. 8). The folds are associated with a subverticalaxial-plane cleavage and their axes are a few meters

to several kilometers apart. En echelon folds at thesouthwestern margin of the Maslum basin (Fig. 9)probably developed during a period of ductile shear-ing on the adjacent Ar Rika fault zone, one of themany northwest-trending sinistral transcurrent Najdfaults that cut the northern Arabian shield (Moore,1979; Johnson and Kattan, 1999). The trends of thefolds indicate deformation by bulk east–west shorten-ing, a stress orientation that would be compatible withsinistral shearing.

4. Bani Ghayy group basins

The Bani Ghayy group (Agar, 1986a) crops outin fault-controlled basins west and southwest of the


Fig. 7. Triangular plot of framework mineralogy of Murdamagroup sandstone from the Maslum basin showing the relationof the Murdama rocks (shaded field—afterGreene, 1993) to theprovenance fields determined byDickinson and Suczek (1979)andDickinson (1985). (Q: total quartz; F: total feldspar; L: unstablelithic fragments).

Murdama group (Fig. 2). The basins are narrow(20–50 km wide) and about 350 km long and thecontained assemblages are in excess of 6000 m thick(Agar, 1986a). SHRIMP U–Pb zircon dating gives acrystallization age of about 650 Ma for Bani Ghayyrhyolite from the Hadhah basin (J. Doebrich, writ-ten communication, 2001) and the group is olderthan previously thought. Earlier studies obtained aconventional U–Pb zircon age of 620± 5 Ma fromrhyolite (Stacey and Agar, 1985) and a three-pointRb–Sr whole rock isochron of 620± 95 Ma fromandesite and basalt (Fleck et al., 1980). The groupis intruded by granitoids belonging to the Hamlbatholith (650–600 Ma;Agar et al., 1992; Aleinikoffand Stoeser, 1988).

The Mujayrib basin contains upward-finingpolymict conglomerate, tuffaceous graywacke, andsiltstone, volcanic rocks and thin-bedded limestone.The conglomerates are massive, clast-supporteddeposits composed of subangular to rounded peb-bles, cobbles, and boulders (Fig. 6B). Clasts aremainly locally derived Siham-group volcanic rocks,Siham-associated diorite, granodiorite, and gran-ite, and intraformational volcanic rocks. Graywackeis well bedded and together with siltstone formsupward-fining, cross- and planar-laminated deposi-

tional units 10 and 50 cm thick that represent truncatedBouma cycles (Agar, 1986a). The volcanic rocks area bimodal sequence as much as 1000 m thick of por-phyritic rhyolite flows, sills, and tuffs and tholeiitic,high-alumina, and alkali basalt and andesite (Agar,1986a). Limestone units are also as much as 1000 mthick (Pellaton, 1984).

The Bani Ghayy group in the Hadhah basin in-cludes a bimodal sequence of basaltic–andesite andrhyodacite–rhyolite flows, tuffs, breccia, and ag-glomerate interbedded with volcanic conglomerate,conglomerate, wacke, quartz arenite, and siltstone,and contains a carbonate sequence 150–800 m thick(Kattan and Harire, 2000) (Fig. 10). The conglom-erates are commonly chaotic deposits of subroundedto subangular cobbles and boulders composed of an-desite, dacite, diorite, granodiorite, and granite. Thecarbonates include white, gray, and brown calc-rudite,calc-arenite, oolitic limestone, stromatolitic lime-stone, dolomite, and calcareous sandstone (Kattanand Harire, 2000). On the basis of the bimodal char-acter of the volcanic rocks and the fanglomeratecharacter of the coarse-grained clastic rocks,Agar(1986a)concluded that the basins were fault-boundedgrabens (Agar, 1986a) although Kattan and Harire(2000) discern a subduction-related influence on thevolcanic-rock geochemistry. The clastic rocks resem-ble sub-aqueous deposits and the local abundance ofstromatolitic carbonate rocks indicates marine condi-tions (Agar, 1986a).

The margins of both basins are intruded by youngerplutons, modified by post-Bani Ghayy faults and, inthe case of the Hadhah basin, extensively coveredby eolian sand so that the structure of the marginsis not fully established. However, strongly developednorth–northwest trending aeromagnetic lineamentsclose to some of the exposed basin margins are be-lieved to reflect bounding faults and one boundingfault at the eastern margin of the Mujayrib basin is ex-posed as a steeply east-dipping serpentinite-decoratedreverse fault (Agar, 1988). Folds in the BaniGhayy group are indicative of east–west short-ening similar to that implied by folding in theMurdama basins. They comprise open-to-isoclinalnorth-trending anticlines and synclines<1 to >5 kmapart (Fig. 11) that have steeply east-dippingaxial-plane cleavages (Agar, 1988; Kattan and Harire,2000).


Fig. 8. Folds in post-amalgmation basins in the northeastern Arabian shield, with stereonet plots (lower hemisphere) of poles to beddingillustrating the northerly trend and plunge of folds in the Murdama group and the westerly trend of folds in the Jurdhawiyah group (Idayribasin). AF: Ata fault; LF: Lughfiyah fault; RF: Raha fault; SF: Shara fault. Structural data afterDelfour (1979), Williams (1983), Letalenet(1979), Pellaton (1984), andCole (1988).

5. Jurdhawiyah group basins

The Jurdhawiyah group, indirectly dated as640–625 Ma (Cole and Hedge, 1986), crops out inthree, partly fault-controlled, depositional basins(Fig. 2). The Idayri basin is bisected by thenorth-vergent Lughfiyah reverse fault and is boundedon the north by the Shara fault (Fig. 8), a possiblegrowth fault that governed rapid deposition of con-glomerate high in the Jurdhawiyah section (Cole,1988). The Safih basin is bounded on the northwestby a cryptic fault evidenced by an alignment of plugsand stocks. The An Nir basin is an inward-dippingregion of subsidence. The group rests unconformably

on folded Murdama group rocks and in the Safihbasin oversteps the contact between the Murdamagroup and the older Suwaj terrane. Granitoid rocks ofthe Idah (∼620–615 Ma) and Abanat (585–570 Ma)suites intrude the group.

The group includes conglomerate, volcanic aren-ite, and subordinate andesite, dacite, and rhyodaciteflows; limestone does not occur. The conglomer-ate comprises poorly sorted, angular to subrounded,cobble to boulder clasts (up to 1 m in diameter)of andesite, dacite, rare Murdama-group sandstoneand limestone, Suwaj-terrane rocks, and fragmentseroded from contemporary volcanoes and plugs andstocks of gabbronorite and andesite porphyry. The


Fig. 9. Detail of en-echelon folds in the Murdama group in the southern part of the Maslum basin adjacent to the Ar Rika fault. Inset ispart of an aerialphoto mosaic of the area outlined by the dashed line showing the outcrop character of the folds.

volcanic arenite, largely andesitic in composition,is medium to coarse grained and well bedded. Thebeds are 5–20 cm thick and continuous on strike forhundreds of meters. Other sedimentary structures in-clude channeling, cut-and-fill, ripple marks, raindropimprints, and small-scale cross-bedding. The mostabundant volcanic rocks, particularly in the Safihand Idayri basins, are structureless-to-amygdaloidalandesite flows that grade into coarse, fragmentalbreccia (Cole, 1988). Rhyodacite tuff breccia and

lapilli tuff locally crop out in lenses up to 100 mthick.

The Jurdhawiyah group was affected by moderatedeformation in the Idayri basin, in which north–southshortening created subhorizontal, chevron-styleeast-trending anticlines and synclines spaced about1 km apart and caused north-vergent reverse fault-ing (the Lughfiyah fault shown inFig. 8). Folds inthe Safih and An Nir basins are less prominent andbedding dips are mostly between 5 and 30◦.


Fig. 10. Stratigraphic column of the Bani Ghayy carbonate sequence at Jabal Hadhah in the northcentral part of the Hadhah basin (afterKattan and Harire, 2000).

6. Hibshi basin

The Hibshi basin is a northeast-trending faulted syn-cline, 2–30 km wide and more than 100 km long, thatseparates the Maraghan basin from the Ha’il terrane(Williams et al., 1986). The basin is filled by the Hibshiformation (632±5 Ma;Cole and Hedge, 1986), a suc-cession of volcanic, volcaniclastic, and epiclastic rocksbroadly correlative with the Jurdhawiyah group. It isunconformable on the Ha’il terrane to the north andfaulted against the Maraghan basin to the south. Theformation is ≥5000 m thick, implying major subsi-dence in whatWilliams et al. (1986)inferred to bea fault-controlled continental basin. The formation is

intruded by plutons of the Idah suite (∼620–615 Ma),and is moderately deformed resulting in bedding in-clinations of 40–85◦.

The rocks make up two volcanic centers. One,predominantly felsic, underlies the central part ofthe basin and contains basal polymict conglomerate,0–100 m thick, in local channels cut in the rocks ofthe Ha’il terrane, fine- to medium-grained, poorly tomoderately sorted arkose, lithic (volcanic) arenite,lithic graywacke, maroon to green siltstone, daciticand rhyolitic welded and ash-fall tuffs, and daciteand andesite flow rocks and breccia. The conglom-erate is crudely bedded and composed of locally de-rived clasts of diorite, andesite, dacite, and Murdama


Fig. 11. Folds and faults in Bani Ghayy group rocks in the northernpart of the Hadhah basin and stereoplots (lower hemisphere) ofpoles to bedding at selected locations. Data afterKattan and Harire(2000).

sandstone that coarsen upward from pebbles to boul-ders in an arkosic matrix. A bimodal volcanic centercomprising rhyolite flows and tuffs and massive basaltflows is located in the northeast.

7. Jibalah basins

The Jibalah group (Delfour, 1970) (alternativespellings: J’balah, Jubaylah) crops out in small, iso-

lated basins adjacent to Najd faults. At least fivebasins are located in the area covered byFig. 2; oth-ers are farther west and northwest (Hadley, 1974).The Jibalah basins are mostly pull-aparts relatedto movements on the Najd faults (Hadley, 1974;Husseini, 1989; Al-Husseini, 2000). The Al Kibdibasin (Figs. 2 and 9) is located between twoleft-stepping strike-slip faults and is a pull-apart thatformed during sinistral shear. The Al Jifn basin de-veloped at a releasing bend on the Halaban-Zarghatfault during a period of dextral shear. The An-taq basin is a half graben that appears to haveformed as a result of normal dip-slip, perhaps duringeast–west-directed extension. No plutons intrude thegroup.

The group is not well dated and may be diachronous.The Al Jifn basin is bracketed between 625± 4 Ma, aU–Pb zircon crystallization age from rocks beneath thebasin, and 576± 5 Ma, a U–Pb zircon crystallizationage from a felsite dike that intrudes the basin (Matsahand Kusky, 2001). Granite beneath the group in theJabal Jibalah type area yields an Rb–Sr whole-rockage of 574± 28 Ma (Calvez et al., 1983). K–Ar agesof 567± 6 and 581± 7 Ma reported byBrown et al.(1989)are probably cooling ages.

The Antaq basin, elongate 40 km north–northwestand 0–5 km wide, contains gently east-dipping Jibalahgroup rocks (Fig. 6C) unconformable on the Suwajterrane on the west and faulted against Halabanophiolite on the east. The rocks include polymictpebble-to-cobble fanglomerate that thickens towardthe eastern bounding fault, sandstone, siltstone, chertylimestone, basalt, andesite and felsic tuff bands. Thefault-bounded Al Jifn basin comprises basal polymictconglomerate and cobble-to-boulder fanglomeratedeposited against the bounding faults, andesite, an-desitic tuff, and basalt, cherty limestone, sandstone,and shale. Rocks in the Al Kibdi basin, which arelocally unconformable on the Siham arc but other-wise fault bounded, include basalt, andesite, polymictconglomerate, sandstone, arkosic arenite, siltstone,shale, and limestone. Limestone in each of thesebasins is thinly bedded and commonly has wavybedding surfaces reflecting particle entrapment byalgal mats (Fig. 6D). Sedimentary structures includeripple marks, graded bedding, and cross-lamination.The purple-brown coloration of many of the Jibalahrocks, the algal-mat carbonates, and the sedimentary


structures indicate deposition in shallow water. Whe-ther the basins were lacustrine or marine is not cer-tain, but the isolated character of each basin andlimited thickness of the carbonate successions suggesta series of lakes along the controlling faults ratherthan a marine incursion.

Dips in the Jibalah group are generally moderate,except where locally steepened by faulting (Matsahand Kusky, 1999) and are evidence of a moderateamount of post-Jibalah deformation in the northeasternshield. Structures include inward dipping homoclines,as in the Al Jifn basin outward dipping homoclines in-clined toward the basin-margin faults, as in the Antaqbasin (Fig. 6C), and broad synclines and anticlines.

8. Discussion and implications

Any tectonic model proposed for the final assem-bly of the northern part of the EAO must accountfor the development and deformation of the post-amalgamation basins seen in the northeastern Ara-bian shield. Tectonic models of Neoproterozoic IIIevents farther west in the EAO envisage (1) uplift andgravity-driven extensional collapse expressed by theexhumation of metamorphic core complexes and thedevelopment of low-angle detachments, and(or) (2)orogen-normal shortening, orogen-parallel extension,and tectonic escape evidenced by thrusting, gneis-sic duplexing, and transcurrent faulting (Blasbandet al., 1997; Blasband et al., 2000; Fowler andOsman, 2001; Fritz et al., 1996, 2000; Greiling, 1997;Greiling et al., 1994, 2000). The geologic settingsand histories of the post-amalgamation basins implyhowever that neither model is fully appropriate forthe northeastern Arabian shield.

On the basis of the unconformities exposingamphibolite- and locally granulite-grade metamorphicrocks at the base of the Murdama and Bani Ghayygroups, and Hibshi formation; east-trending Neopro-terozoic III dike swarms; and extensional-fault controlon the Bani Ghayy, Jurdhawiyah, and Hibshi them-selves, it is clear that the region underwent periodicuplift and extension and potentially underwent gravitycollapse. However, the shallow-marine environmentsof the Murdama and Bani Ghayy rocks imply that sea-ways penetrated the orogen by 670–650 Ma and thatlarge parts of the northeastern Arabian shield were

at or near sea level during most of the Neoprotero-zoic III. As a consequence, it is considered unlikelythat gravity would have been a significant source ofstress in the region and that gravitational collapse isan invalid model for the northeastern Arabian shield.Unconformities at the bases of the Jurdhawiyah, Hi-bshi, and Jibalah basins, unroofed epizonal graniteplutons (Agar, 1986b; Cole, 1988), and 39Ar/40Ardating of mineral closing temperatures that definerapid cooling (Al-Saleh et al., 1998) indicate thatpost-Murdama/post-Bani Ghayy uplifts also occurred,but nowhere is there evidence of significant uplift. Onthe contrary, it appears that large parts of the regionhad subsided to sea level within 10–15 million yearsof peak orogeny and probably remained at low eleva-tions throughout the remainder of the NeoproterozoicIII ( Cole, 1988).

A dynamic implication of the post-amalgamationbasins is that the stress field in the northeastern Ara-bian shield changed orientation with time. Creationof the Bani Ghayy grabens implying broadly east–west extension at about 650 Ma was followed soonafter by Murdama and Bani Ghayy group foldingand basin closures suggesting east–west shortening.Subsequent northerly directed extension resulting indevelopment of the fault-controlled Jurdhawiyah andHibshi basins was followed by north–south shorteningcausing Jurdhawiyah and Hibshi folding, thrusting,and basin closures. After a 30–40 million year depo-sitional hiatus, transcurrent and dip-slip movementson the Najd faults created the Jibalah-group basins,followed again by shortening, folding, and basinclosures.

Most workers accept that Neoproterozoic III eventsin the EAO as a whole were driven by the global con-vergence of East and West Gondwana (Abdelsalamand Stern, 1996; Stern, 1994). Broadly east–westshortening associated with such convergence wouldaccount for folding in the Murdama and Bani Ghayygroups. It would also account for strike-slip move-ments on the northwest- and locally northeast-trendingNajd faults that cut the Murdama and Bani Ghayybasins and, as a corollary, the north–south-directedextension that controlled Jurdhawiyah and Hibshibasin formation.

General east–west convergence of Gondwana mayalso account for creation of the Murdama and BaniGhayy basins themselves. The Murdama basin is


located in the core of the northern EAO, as much as400 km from the eastern orogen margin proposed byJohnson and Stewart (1995)and ∼600 km from thewestern margin in Egypt (after closing up the RedSea to remove the effect of Tertiary spreading). Fromthe perspective of the orogen as a whole, the Mur-dama has the character of an intraorogenic collisionalbasin (Graham et al., 1993), but from the local per-spective of the Halaban suture it may be a forelandbasin. Foreland basins result from flexure caused byloading of the lithosphere during overthrusting by anadvancing orogenic front. As shown byBradley andKidd (1991)loading results in subsidence in the innerpart of the flexed lithosphere closest to the advanc-ing orogenic front and extension and normal faultingin the convex outer part of the flexed lithosphere.It is conceivable that this mechanism accounts forthe origin of the Murdama and Bani Ghayy basins,whereby subsidence in the inner part of a flexed Afiflithosphere close to the obducting Halaban ophiolitecontrolled Murdama deposition, and concurrent ex-tension and normal faulting farther west controlledBani Ghayy deposition.

Acknowledgements

This paper is a development of a talk given as part ofa Special Session on the “Evolution of the East Africanand related orogens and the assembly of Gondwana”at the GSA Annual Meeting, 2000, and I thank theconveners, T.M. Kusky, R.D. Tucker, and R.J. Stern,for arranging the session and giving me the impetusto describe these features of the Arabian shield. Fieldwork was done as part of a project of small-scale geo-logical mapping by the Saudi Geological Survey, andDr. M.A. Tawfiq, President of the Survey, is thankedfor encouragement to do the work and permission topublish. Comments and suggestions by Mark Gordonand one anonymous reviewer on behalf of PrecambrianResearch are gratefully acknowledged and helped tomake the paper more succinct and focused.

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Post-amalgamation basins of the NE Arabian shield and ...rjstern/egypt/PDFs/General/JohnsonPCR03.pdf · Gondwana supercontinent that resulted from East and West Gondwana convergence