Mid-crustal dynamics and island-arc accretion in the Arabian Shield: insight from the Earth's natural laboratory

Earth-Science Reviews 49 Ž2000. 77–120 www.elsevier.comrlocaterearscirev

Mid-crustal dynamics and island-arc accretion in the Arabian Shield: insight from the Earth’s natural laboratory R. Weijermars a

a,b,)

, M. Asif Khan

c

Center of Postgraduate Studies, Delft UniÕersity of Technology, P.O. Box 612, 2600 17P Delft, Netherlands b Alboran Media Group, PO BOX 76321, 1070EH Amsterdam, Netherlands c Research Institute, King Fahd UniÕersity of Petroleum and Minerals, Dhahran, Saudi Arabia Received 24 June 1998; accepted 13 August 1999

Abstract The late Proterozoic terrane of the Arabian Shield uniquely exposes the records of magmatic processes and deformation patterns that shaped the Earth over half a billion years ago. This Precambrian to Cambrian terrane is composed of supracrustal rocks underlain and intruded by igneous rocks. The plutonic shedding and accretion of the crust has been studied in detail. Satellite images ŽTM Landsat and SPOT. were processed to enhance both lithologic content and structural patterns, using spectral transformation and spatial filtering techniques. Results obtained by integrating the image interpretation with detailed ground studies are discussed. The supracrustal rocks display complex fold interference patterns, associated with the emplacement of the basic plutons ŽJarshah and Mifsah. and mantled gneiss-domes ŽQardath and Miktaa. which took place coeval with crustal shortening by regional deformation. The intrusion of a younger pluton ŽArafadi. caused the collision of two pre-existing plutons ŽJarshah and Mifsah., involved extensive sideway shedding of supracrustal rocks and formed a major shear zone ŽQurayn.. Finally, a subvolcanic complex ŽMahala. was emplaced, which includes a 2.5-km radius ringdike of several hundred meters thickness. The geologic features of our study area can be explained by a simple model of island arc evolution in six stages: ŽI q II. Pre-tectonic and pre-plutonic, paleogeographic situation Ž0.9 to 0.66 Ga.; ŽIII. Syntectonic, basic plutonism Ž0.65 Ga.; ŽIV. Syntectonic gneiss-domes Ž0.62 Ga.; ŽV. Post-tectonic, acidic plutonism Ž0.59 to 0.54 Ga.; and ŽVI. Post-orogenic volcanism Ž0.54 to 0.5 Ga.. In our scenario, the relative ages of events inferred on the basis of field relationships are compatible with the geochronologic ages of rock units previously dated by radiometric methods only. The accretion rate of the Arabian Shield region is here estimated at 0.1 km3 yry1 or 10% of the total global accretion rate at the present-day. The average Arabian arc addition rate is 33 km3 kmy1 May1 for the late Proterozoic, which is rather close to estimates of modern island-arc accretion rates of 30 km3 kmy1 May1. We conclude that the process and rate of island-arc accretion onto active continental margins of Arabia in the late Precambrian eon evolved in a fashion similar to that of island-arc accretion in the Phanerozoic eon. This conclusion is important because it counters the view that, based on unrealistic assumptions, accretion rates for the late Proterozoic crust of the Arabian craton were as much as 10 times faster than the modern crustal accretion rates. q 2000 Elsevier Science B.V. All rights reserved. Keywords: structural geology; geochronology; field geology

) Corresponding author. Tel.: q31-20-3640-331 or q31-15-2788019; fax: q31-20-3640-145 or q31-15-2781009; e-mail: [email protected] or [email protected]

0012-8252r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 5 2 Ž 9 9 . 0 0 0 5 2 - 5

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1. Introduction The Arabian–Nubian Shield ŽANS. represents a prominent exhibit of continental crust that formed when the Proterozoic eon drew to a close. The ANS has thus provided valuable geologic evidence to erect conceptual models of crustal evolution and continental accretion. For example, much of the ANS is now believed to have accreted from island arcs that piled onto each other at convergent plate margins during the Pan-African orogeny Žca. 550 to 1200 Ma.. The Arabian Shield is regarded as one of the best documented examples of Precambrian plate tectonics and arc accretion Že.g., Stoeser and Camp, 1985; Kroner et al., 1992, and references therein.. ¨

Ophiolites became entrapped in well-defined linear suture zones when the ocean between the island arcs closed ŽKroner, 1985.. In Saudi Arabia, some of ¨ these former island arcs are now juxtaposed in at least six terranes separated by ophiolite sutures ŽFig. 1.. We investigated a 900-km2 outcrop area located within the southern center of the Asir microplate which constitutes the southwestern most terrane distinguished in the Arabian Shield. The Asir terrane constitutes one of the six named microplates ŽStoeser and Camp, 1985; Pallister et al., 1987, 1988; Quick, 1991; Kroner ¨ et al., 1992., each separated by ophiolite sutures Žc.f., Fig. 1.. Ground surveys, in combination with the enhanced satellite imagery, allowed

Fig. 1. Tectonic map of the Arabian Shield outlining terranes and sutures proposed by Stoeser and Camp Ž1985 and others.. Sutures are dashed, ophiolites are black, and faults of the Najd system are indicated by NW trending solid lines. Mainly after Quick Ž1991..

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us to unravel the geologic structure and the geotectonic and magmatic evolution of this complex Precambrian terrane. Our 1:25,000 scale basemaps were more detailed than in any earlier regional studies of this area. A summary of logistic constraints, physiographic conditions, and review of previous mapping work is given in Appendix A. Our research documents the structural geology and the dynamics of the Asir Terrane at the medium-scale Ž30 = 30 km., which, in essence, gives a mid-crustal view of the dynamics of island-arc accretion. One puzzling aspect of the ANS formation is that its accretion rate has been estimated at 310 km3 per km arc length per Ma, assuming that all the crust has accreted through island arcs alone ŽReymer and Schubert, 1984, 1986.. This rate is one order of magnitude faster than the modern rates of continental accretion by the amalgamation of island arcs, which are estimated to accrete at only 30 km3 per km arc length per Ma ŽReymer and Schubert, 1984.. Several explanations have been offered to resolve this apparent difference between modern and late Proterozoic crustal accretion rates. One explanation is that the late Proterozoic formation of the ANS may have occurred not by island-arc accretion alone ŽStein and Goldstein, 1996.. There is a growing body of evidence that crustal accretion of island arcs was preceded by the formation of oceanic plateaus, which could explain the extraordinary fast formation of the ANS ŽStein and Goldstein, 1996.. Isotope studies indeed suggest that the older ANS basalts are tholeitic and that only the younger magmatism is typical of convergent plate margins. However, another explanation suggests that, in addition to the inclusion of exotic terranes, the surface area used in the calculation of crustal accretion rates for the ANS is vastly overestimated and that, in fact, the revised ANS accretion rates are similar to that of modern growth rates for continental crust ŽPallister et al., 1990.. Our own estimate Žsee Section 3.3. of the crustal accretion rate for the ANS also supports the view that earlier estimates suggesting rates faster than the present-day are not sustainable. Our field observations further indicate that over 10 km crustal thickening occurred Žsee Section 3.2. coeval with the isostatic exhumation of mid-crustal levels during the main Pan-African orogenic episode in our region Ž0.65 to 0.54 Ga.. The older, 0.65 Ga

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Pan-African structures evolved in ductile regimes at mid-crustal depth. The younger structures of the later, 0.54 Ga Pan-African episode were formed at shallow depths and include fracture-bound ring dikes. We conclude that the gross rate of crustal thickening in the Proterozoic, estimated here at 10 km per 100 Ma, is consistent with that occurring in modern mountain belts. This provides yet another new argument against the idea of faster island arc accretion rates in the Proterozoic.

2. Asir terrane Our area of study of 900 km2 covers only a tiny portion of the Arabian Shield ŽFig. 1.. However, its geologic features are characteristic and representative for much of the shield area. The supracrustal rocks display complex fold interference patterns and several stages of plutonism can be recognized. Within the Asir microplate, numerous north-trending tectonic belts have been distinguished ŽGreenwood et al., 1982.. These belts are separated by N–S-trending faults, fault zones and shear zones, and the correlation of major rock units across belt boundaries is not clearly established. Our study area is entirely located within the Tayyah tectonic belt, which is bound by the Ablah belt to the west and the Khadra belt to the east ŽFig. 2a and b.. These deformation belts are not bordered by ophiolites and, therefore, do not constitute lithospheric microplates, but rather individual deformation zones of different intensity within the Asir terrane. The rocks of the Tayyah belt are supracrustal rocks of the Bahah group that consist mainly of metamorphosed volcanoclastic and other sedimentary rocks ranging from greenschist to amphibolite facies. Bahah group rocks are among the oldest supracrustal rocks Žtogether with the Baish and Jeddah groups. of the Arabian Shield, ranging in age between 950 and 900 Ma ŽKroner ¨ et al., 1992.. No reliable ages in excess of 1 Ga have been established for the Asir region, according to J.S. Pallister Žpers. commun., February 1999.. The earlier 1150 Ma ‘‘isochron’’ ŽRb–Sr. of Fleck et al. Ž1980. is now considered a pseudo-isochron, as it is based on a collection of rocks that may not be of the same igneous series. For further details on the regional

80 R. Weijermars, M. Asif Khan r Earth-Science ReÕiews 49 (2000) 77–120 Fig. 2. Ža. Enhanced Landsat MSS image of the central Asir region. The detailed image area of Fig. 3 is outlined. Žb. Regional, geotectonic interpretation map of the central Asir region Žcorresponding to the image area of Fig. 2a. according to Greenwood et al. Ž1982.. Key: plain background, Bahah group; pattern, Jiddah group; horizontal rule, Ablah group; diagonal rule, Halaban group; qd, diorite suite; tg, tonalite and granodiorite suite; di, diorite; sh, shonkonite; gb, gabbro; gd, granodiorite and granite suite; OC-w, Ordovician Wajid sandstone; Tb, Tertiary basalt.

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Fig. 2 Žcontinued..

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geology of the Arabian Shield, several compilations can be consulted ŽGreenwood et al., 1980; Johnson, 1982, 1983; Brown et al., 1989, and references therein.. A brief outline of the geological setting of the Asir terrane is given in Appendix B. Fig. 3 is an enhanced Landsat index image of our 30 by 30 km study area. It includes the outlines of six detailed geologic map sheets, which are discussed in this paper. The six map sheets are named after major geographic landmarks as indicated in Fig. 3. The area displays a complex pattern of supracrustal rocks, repeatedly folded, sheared and shedded by buoyant plutons. After our assessment of the terrane, the geologic units were grouped into six geochronologic groups Žprincipally as inferred from field relationships, and in agreement with radiometric datings available from previous studies.. These groups are in order of decreasing age: ŽI. gneissic basement; ŽII. supracrustal rocks; ŽIII. basic plutons; ŽIV. gneiss domes; ŽV. acid plutons; and ŽVI. a subvolcanic complex. Each of these groups is discussed in detail below at hand of representative areas and illustrated by magnified portions of the index image of Fig. 3, complemented with corresponding geologic structure maps and field photographs. Subsequently, the observed features are incorporated in, and explained by, a geotectonic reconstruction Žsee Section 3.1., also postulated here for the first time. 2.1. Group I: gneissic basement Most probably, the gneisses exposed in the Miktaa area ŽFig. 4. are the oldest rocks of the region. The gneisses are principally composed of quartz, plagioclase, mica, and hornblende, all of which define the gneissosity. Minor amounts of epidote, sphene and apatite are discerned in thin sections. Coarse-grained leucocratic granite–gneiss is interlayered on meter-scale with melanocratic granodiorite layers, better foliated due to a higher mica content and generally less coarse-grained than the simatic gneiss bands. The granodiorite gneiss bands are less resistant to erosion than the coarser-grained granitic gneiss bands. The gneiss complex, previously termed the Khamis Mushayt gneiss ŽSchmidt et al., 1972., also contains meter-scale bands of amphibolite. The gneiss complex comprises abundant concordant and discordant pegmatite veins and dikes, foliated and

tight to isoclinally folded. There are also discordant, non-foliated pegmatites and aplites cross-cutting the gneiss complex, and ptygmatically folded quartz veins, as well as non-foliated microgranite dikes of up to 5 m thick. The gneiss complex hosts the Miktaa granite intrusion, characterized by a whitish albedo in Fig. 3. The Miktaa granite is transected by joints, but its peaks tower as inselbergs above the surrounding plains and provide clear landmarks which can be recognized from tens of kilometers distance ŽFig. 5a and b.. The semi-arid landscape is corrugated and barren from vegetation. The Miktaa granite outcrops in a steeply N-plunging isoclinal anticlinal closure, and is foliated concordantly with the main trend of the regional gneissosity. However, pegmatites emanating from it are folded around the hinge zone ŽFig. 5c.. The pegmatites are concordant with folded slivers of amphibolites, suggesting that the gneiss complex was already foliated before the emplacement and subsequent folding of the gneissosity around the Miktaa granite. The granite comprises quartz, plagioclase and muscovite, little or no K–feldspar, and minor amounts of magnetite. It is transected by foliated, grey-weathered, finer-grained, granitic dikes ŽFig. 5d. and, also, by pseudotachylite veins. The gneiss complex, which is metamorphosed in upper amphibolite facies and incipiently melted in some areas, laterally merges with supracrustal rocks, progressively lower metamorphic toward the west. Within the supracrustal sequence, the metamorphic grade decreases from amphibolite facies toward greenschist facies, with the isograd separating these facies approximately dividing the area of Fig. 3 into an eastern amphibolite and western greenschist facies region. The amphibolite facies rocks may comprise porphyroblasts of several centimeters length: hornblende, Žactinolite, stilpnomelane., sillimanite, garnet, mica, albite, chlorite and biotite. Thin section study further revealed minor amounts of epidote and zoisite. The main foliation within the supracrustal rocks near the gneiss contact is everywhere concordant with the gneissosity of the gneiss complex and gneissic bands are found within the supracrustal rocks of amphibolite facies. Any sharp boundary between supracrustal rocks and gneiss complex cannot be defined, the transition is everywhere gradational.

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Fig. 3. Landsat image of the 30 = 30 km study area in the central Asir region Žwhite albedo, granite pluton of Arafadi in upper left corner provides good regional reference pattern, c.f., center of Fig. 2a.. Bands 2 Žblue., 4 Žgreen. and 7 Žred.. Image width is 30 km. Outlines of the position of the six map sheets that are included in this paper are indicated: Al Miktaa area ŽFig. 4.; Mahala area ŽFig. 6.; Mifsah area ŽFig. 9.; Khamis Mushayt area ŽFig. 11b.; Arafadi area ŽFig. 15.; Shohat area ŽFig. 17..

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The gneissic basement of Khamis Mushayt was once thought to be over 1 Ga old ŽColeman, 1973a., based on the assumption that the Mecca granite, exposed 250 km further north which yielded an early K–Ar age of 1 Ga Žc.f., Brown, 1970., is intruded into similar basement. However, it has now been established that a number of the early K–Ar and Rb–Sr ages are unreliable indicators of igneous ages and modern U–Pb zircon or high-resolution Pb–Sr ages in excess of 950 Ma are not available for this region ŽPallister, personal communication, February 1999.. The Miktaa granite has yielded whole-rock Rb–Sr ages of 664 q 9 Ma ŽFleck et al., 1980. and lead isotopes in zircon of the gneiss samples from localities adjacent to our mapping area have yielded ages of 654 to 714 Ma ŽCooper et al., 1979; Stoeser et al., 1984.. On the basis of foliated granite exposed in anticlines within the supracrustal rocks, similar to that of the Miktaa granite within the gneiss complex, it is assumed that both the supracrustal rocks and the gneissic basement, as well as their collusion, predate the 664 q 9 Ma Rb–Sr age of the Miktaa granite. 2.2. Group II: supracrustal rocks The lithology of the supracrustal rocks is very diverse but the tectonic transposition of these rocks precludes the detailed mapping of individual lithologic units on a regional scale. Locally, we distinguished carbonate-bearing metabasalt, amphibolite, metagabbro, quartzofeldspatic schist, graphite schist, silvergrey micaschist, chert, quartzite, marble, skarn, and black and grey laminated metasediment. Generally, the supracrustal rocks are reddishbrown on the

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false color TM Landsat image of Fig. 3, except for graphite schist, silvergrey micaschist and laminated metasediments, all of which are characterized by grey-black tones on remote sensing images. Careful structural mapping has revealed that the supracrustal rocks are isoclinally folded and refolded ŽFig. 6.. All lithologic bands are transposed and individual lithologic bands commonly only are a few meters or few tens of meters thick. The main foliation commonly subparallels the transposed lithologic bands ŽFig. 7a.. The bedding orientation within the greenschist sequence could be inferred from lithologic contrasts, accentuated by differential erosion, between the schists and intercalations of metabasalt layers and quartzite layers. The angle between the bedding and the cleavage is usually small, to subparallel, which itself implies that the folds in this area are isoclinal. Fold closures ŽFig. 7a to c. were mapped making use of cleavage vergence and minor fold vergence, using standard methods for mapping fold closures at various scales ŽWeijermars, 1982a,b, 1985.. Structural map symbols are included in the geologic map of the Mahala region of Fig. 6. Systematic structural mapping also confirmed the existence of isoclinal folds at scales with wavelengths ranging from several centimeters ŽFig. 7d. to at least 100 m. These folds are parasitic on the regional fold closures. Fig. 8a is an interpretative perspective diagram of the Nabutah synform and the transecting Najd faults. Fig. 8b illustrates the fold interference pattern of Sport City. The entire greenschist unit in the Mahala region is transected by a penetrative schistosity which is consistently dipping eastward. The axial planes of folds within the greenschist are subparallel to the mean

Fig. 4. Geologic map of the Al Miktaa area Žsee index map of Fig. 3 for location.. Al Miktaa town is located on the apex of a doubly plunging antiformal closure of the gneiss complex Žgrey shade.. Formlines trace the main trend of the gneissosity. Strikerdip symbols indicate orientation of gneissic foliation surfaces. Antiformal and synformal closures are indicated by solid arrows. Rock sample locations are encircled numbers, for which GPS coordinates have been determined. Regional metamorphic grade is of amphibolite facies. The gneissic basement complex hosts the folded Al Miktaa granite Žprominently jointed., northerly plunging. The Miktaa granite intrusion is foliated and folded with gneissosity surface axial planar to the fold structure. Anatexis is widespread in the northeast region due to the intrusion of the younger Arafadi granitoid Žsee Section 2.5.2.. Migmatite belt of anatectic supracrustal rock is entrained in the Arafadi pluton. The Arafadi pluton and anatexis area are prominently jointed in directions that are attenuated by the trellis pattern in the wadi system. The area can be accessed by the Khamis Mushayt–Bishah highway Žnorth–south. and offroad by dirt tracks in the wadi system. Wadi Bishah runs off toward the north to northwest.

86 R. Weijermars, M. Asif Khan r Earth-Science ReÕiews 49 (2000) 77–120 Fig. 5. Ža and b. Panoramic view of the eastern margin of the Miktaa granite looking due north. Žc. Northern apex of the plunging fold closure in the Miktaa granite outcrop. The plungertrend of the fold hinge is 50r360. Žd. Foliated granitic dikes within the Miktaa granite.

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schistosity. Poles to the axial plane cleavage of the regional folds were plotted and contoured and the mean axial plane has azimuthrdip of 090r65. Because these folds are overturned, all transposed lithology layers consistently dip eastward. Consequently, bedding could relatively easily be recognized on west-sloping hills which cut the bedding at a large angle. East-facing hill-slopes generally differ only little from the dip of the beds, they are almost dip slopes, which makes it more difficult to determine bedding orientations on such slopes. The mean orientation of the fold axes was determined by measuring cleavagerbedding intersectionlineations throughout the area. The fold hinges are subparallel to the intersection lineation. On the basis of the measurements of cleavage and intersection lineations alone, the folds can be classified as steeply SSW-plunging with E-dipping, overturned axial planes. The statistical mean orientation of the intersection lineation based on nearly 100 measurements is 508 due south. Locally, the folds are reclined when the plunge of their axes become perpendicular to the strike of their schistosity. It is concluded from the well-developed axial plane cleavage in the supracrustal rocks that the associated major folds must have developed during regional E–W compression. The folding is also assumed to be coeval with the emplacement of the gabbroic plutons Žsee Section 2.3.. The regional supracrustal antiforms elongated aligned with the Jarshah and Mifsah plutons Žsee group III. are displaced by the E–W-trending Qurayn shear zone of dextral strike–slip, which is discussed in Section 2.5.3 Žgroup V.. Recent lead isotope data for zircons from a dacite clast in a volcanic breccia of the Baish group, the oldest oceanic assemblage of the southwest Arabian Shield, suggest that initial oceanic magmatism did not begin until after 900 Ma ŽKroner et al., 1992.. ¨ Previously, the supracrustal rocks of our region that have been incorporated in the Bahah group were considered to range in age between 1100 and 900 Ma ŽGreenwood et al., 1982.. 2.3. Group III: basic plutons The most prominent examples of basic and intermediate plutonism are provided by the Jarshah and

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Mifsah plutons, prominently shown in the TM Landsat image of Fig. 3. The geologic map of the Mifsah pluton ŽFig. 9., reveals its pseudo-elliptical outline with principal dimensions of 4.5 by 10 km. The main pluton comprises several gabbro bodies separated by diorites. The gabbroic regions are extensively mined at the surface and crushed for use in building aggregate. The Jarshah pluton is predominantly made up of granodiorite. The present groundsurface seems to expose the roof level of the Jarshah pluton, because onliers of supracrustal rocks can be found inside the pluton area. The lighter toned rocks to the northwest of the Jarshah and Mifsah plutons are much younger, acidic rocks of the Arafadi pluton discussed in Section 2.5.2 Žgroup V.. The supracrustal rocks adjacent to these basic plutons have been deformed in a ductile fashion during the emplacement of the Jarshah and Mifsah plutons ŽFig. 10.. The marginal zones of these plutons are foliated concordantly with the foliation of the supracrustal belts enveloping both plutons. The Jarshah and Mifsah plutons are interpreted to have been emplaced coevally, because the supracrustal rocks between them occur in a tight syncline that is likely to have formed during the rise and expansion of both plutons. Careful structural mapping of the supracrustal rocks to the south of the Jarshah and Mifsah plutons has revealed that the N–S-striking axial plane trace of the tight to isoclinal syncline between them continues for at least another 20 km further south. Each of the igneous domes of Jarshah and Mifsah themselves form the core for doubly plunging anticlines in the adjacent supracrustal rocks. However, the northern margin of the mantling supracrustal rocks is largely entrained in, and truncated by, the younger granite of the Arafadi pluton. Map-scale, south plunging antiforms are exposed to the south of both the Jarshah and Mifsah plutons Že.g., Figs. 6 and 9.. The N–S elongation in the map appearance of the Jarshah and Mifsah plutons suggests that they were both emplaced during regional shortening. The ellipticity of the Mifsah pluton indicates the shortening component of strain in the horizontal plane is 0.67 Žthis determination of strain accounts for conversion of dimensional lengths into normalized lengths, e.g., Weijermars, 1997a, Chap. 15.. The present ground surface must have been buried at the time of pluton

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emplacement, sufficiently deep to allow ductile deformation without any disruption of the enveloping supracrustal rocks. Assuming the emplacement occurred during active orogeny, a geothermal gradient of 208 centigrade per kilometer is common, and this would require emplacement below 10 km depth to accommodate the flow of the ductile structures, using creep laws for basic igneous minerals Žfor details, see Weijermars, 1997a, Chap. 8.. Unfortunately, none of the rocks of either Jarshah or Mifsah have been radiometrically dated. However, the smaller Mohra gabbro pluton north of Khamis Mushayt ŽFig. 11a and b. has been inferred to be of similar age as the Jarshah and Mifsah plutons on the basis of field relationships and has been dated by K–Ar isotopes in biotites at 649 q 23 Ma ŽAldrich et al., 1978.. This age differed from the 690 q 22 Ma age given earlier from K–Ar on biotite by Coleman et al. Ž1972., because updated decay constants were used by Aldrich et al. Ž1978.. 2.4. Group IV: gneiss domes 2.4.1. Qardath dome An esthetically rather beautiful structural pattern is exposed in the SE quadrant of our study area directly north of Khamis Mushayt ŽFig. 11a.. The corresponding geologic and integral structure map is given in Fig. 11b Žsee map of Fig. 4 for the adjacent northerly Miktas region.. Two major gneiss domes can be distinguished, the Miktaa and the Qardath domes. The Miktaa gneiss-dome is separated from the southerly adjacent Qardath dome by a narrow overturned syncline. The gneisses exposed inside the Qardath dome are similar to those in the Khamis Mushayt gneiss complex proper. The supracrustal

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rocks wrap around the gneiss domes and their respective gneissosity and schistosity are everywhere concordant. The concordant wrapping of supracrustal rocks is characteristic for gneiss domes in general ŽWeijermars, 1997b, chap. 13.. The Qardath dome has an apophysis or satellite east of the Mohra gabbro ŽFig. 12a.. Pegmatites emanating from the Qardath dome intrude the Mohra gabbro, and the Qardath dome therefore postdates the central gabbro stock of Mohra. The Qardath intrusion, inclusive of its eastern apophysis, is here principally interpreted as a single dome wrapped around the competent Mohra gabbro. The Mohra gabbro may have been continuous with smaller bodies to the southeast ŽFig. 11., but was left-laterally displaced by the ballooning of the eastern appendix of the Qardath gneiss-dome ŽFig. 12a and b.. Inside the Qardath gneiss-body refolded isoclinal folds of gneissosity can be discerned ŽFig. 11b.. Both the Miktaa and Qardath gneiss-domes are interpreted as reactivated basement, which domed close to the melting temperature but not hot enough to ductilely yield the adjacent Mohra gabbro. The basic composition of the Mohra pluton would require higher temperatures for ductile deformation to occur. But the ductile emplacement of the gneiss dome presently exposed must have occurred below 10 km depth. The density of the Khamis Mushayt gneiss is 2.77 q 0.17 g cmy3 ŽColeman, 1973a., therefore making it buoyant below the sequence of heavy supracrustal rocks of predominantly basic composition. The age of the Qardath gneiss dome formation by reactivation of the gneissic basement can be inferred from radiometric dating of pegmatites intruded into the Mohra gabbro. The K–Ar age of muscovite from the pegmatite was published as 615 q 12 Ma ŽAldrich et al., 1978.. This age was based on updated

Fig. 6. Geologic map of the Mahala area Žsee index map of Fig. 3 for location.. The region comprises isoclinally folded greenschist facies rocks. Formlines trace the bedding, which is folded about southerly plunging hinges. Strikerdip symbols indicate orientation of the schistosity. Rock sample locations are encircled numbers, for which GPS coordinates have been determined. The megascopic fold closure in the southwest of the map is the south plunging Nabutah synform ŽFig. 8a.. The folded supracrustal rocks in the NW corner of the map area Žsouth of the Jarshah pluton. are sheared by the Qurayn shear zone ŽFig. 19.. The eastern portion of the map hosts the Mahala subvolcanic complex ŽFig. 22., with the spectacular fold interference pattern of Sport City at its southern margin ŽFig. 8b.. The area can be accessed by tarmac roads from Abha to Balshuhatah, Mahala Road past Sport City, or via the Industrial Road from Khamis Mushayt to the aggregate mining area of Al Mifsah.

90 R. Weijermars, M. Asif Khan r Earth-Science ReÕiews 49 (2000) 77–120 Fig. 7. Ža to c. Composite of megascopic isoclinal folds demonstrating common cleavage-bedding relationship in supracrustal rocks. Cleavage dips eastward, image looks due south. Žd. Isoclinal fold closure of minor fold.

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Fig. 8. Ža. Interpretation perspective diagram of the Nabutah isoclinal synform transected by the consistently left-lateral Najd strike–slip faults. The Nabutah fold plunges about 408 southeasterly. For location, see map of Fig. 6 and its caption. Žb. Interpretative perspective diagram of the refolded folds northeast of Sport City, located at the southeast corner of the geologic map of Fig. 6.

decay constants, and therefore differed from the earlier 607 q 12 Ma age of Coleman et al. Ž1972. for

the same pegmatite. However, most K–Ar dates in the ANS are reset by metamorphic events, and should

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Fig. 10. Interpretation perspective diagram of the Mifsah and Jarshah plutons and the isoclinally folded supracrustal rocks adjacent to and in between them. The Arafadi intrusion front is halted by the basic plutons of Jarshah and Mifsah.

be used with caution ŽJ.S. Pallister, pers. commun. February 1999.. 2.4.2. Janfoor fold interference pattern Rather spectacular refolding of the isoclinal folds in the supracrustal rocks of the Janfoor region is exposed between the Mifsah pluton and the Khamis Mushayt gneiss complex ŽFig. 13a. due to local E–W shortening caused by the emplacement of the Arafadi pluton. Fig. 13b is a structural formline map of the refolded folds of Janfoor. The southern part of the map shows an example of a single basin that would classify as a type 1 interference pattern according to the classification of Ramsay Ž1967.. The

northern part of the Janfoor region hosts type 3 interference patterns. Such interference patterns are commonly observed in outcrop scale in polyphase deformed tectonites, but are less commonly seen on such a regional scale. The fact that the transposed layers remained coherent throughout the formation of these fold interference patterns indicates that the rocks at the time of deformation resided at crustal depths which favored pervasive ductile creep mechanisms. A similarly spectacular pattern of regional-scale refolded folds from the Moroccan Meseta is included here for comparison ŽFig. 13c.. Semicircular inliers of Precambrian basement are covered by gently

Fig. 9. Geologic map of the Mifsah area Žsee index map of Fig. 3 for location.. The Mifsah pluton is cored by gabbro pods Žmarked ‘‘bbb’’. which laterally grade into granodiorite. The Jarshah pluton is covered by a thin slab of folded supracrustal rocks and cored by gabbro Ž‘‘bbb’’.. Transposed bedding, main foliation, and fold hinges inside the supracrustal rocks are outlined by solid formlines. Strikerdip symbols indicate orientation of the schistosity. Rock sample locations are encircled numbers, for which GPS coordinates have been determined. The Mifsah and Jarshah plutons both are surrounded by concordant belts of supracrustal rocks that are folded about north plunging hinges in the east of the map region. The supracrustal rocks include a large proportion of mafic basalt intercalations which are not affected by the anatexis that occurred during the northern Arafadi intrusion. The migmatite belt inside the Arafadi is continuous with that in the easterly adjoining map sheet ŽFig. 4.. The Mifsah pluton is cut by Wadi Mifsah running westward toward Wadi Bishah proper. The area can be accessed from the south by the Industrial Road, which begins NW of Khamis Mushayt and ends in the gabbro aggregate mines Žopen quarries. of the Mifsah pluton.

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folded sedimentary rocks of Paleozoic age ŽWeijermars, 1993, and references therein.. The Precam-

brian inliers are structural highs and between them occur the Paleozoic basin onliers Žalso termed out-

Fig. 11. Ža. Detail of Landsat image of the Qardath mantled gneiss dome and the central gabbro pluton of Mohra. Wadi Bishah is seen to the south of the Qardath gneiss dome. Image width is 7 km. The geology is mapped in Žb.. Žb. Geologic map of the Khamis Mushayt area Žsee index map of Fig. 3.. Formlines trace the gneissosity in the Qardath gneiss dome and transposed bedding in the surrounding supracrustal rocks. Strikerdip symbols indicate orientation of gneissic foliation surfaces Žin the gneiss domes. and schistosity Žin the supracrustals.. Antiformal and synformal closures are indicated by solid arrows. Rock sample locations are encircled numbers, for which GPS coordinates have been determined. The ring dike of the Mahala subvolcanic complex is exposed at the west margin of the map. The Hijlah pluton occupies the region west of Khamis Mushayt. The city of Khamis Mushayt is located in the southeast corner of this map sheet. The area is accessed through the Industrial Road and by the Khamis Mushayt–Bishah highway.

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Fig. 11 Žcontinued..

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Fig. 12. Ža and b. Interpretative perspective diagram of the Qardath gneiss dome and the central Mohra gabbro pluton; Ža. present situation, Žb. appearance of the gneiss dome if the emplacement were not obstructed by the existence of the older Mohra gabbro.

liers., which clearly show evidence of WNW–ESE shortening. The resulting outcrop pattern is an interference of an E–W-trending regional synclinal on which are superposed a series of much shorter wavelength NE–SW-trending anticlinals and synclinals ŽFig. 13d..

2.5. Group V: acid plutons

2.5.1. Hijlah pluton The geochronologic rock units of group V comprise two major acidic plutons, the Hijlah and Arafadi plutons, composed of granites, granodiorites and relatively small volumes of gabbro. The Hijlah, exposed westerly and beneath the town of Khamis Mushayt ŽFig. 11b., is principally composed of granodiorite, but also includes granite. This pluton is

commonly not foliated, and pegmatite veins intruded earlier into the supracrustal rocks by the Qardath dome have been folded near the contact with the Hijlah pluton but these folds developed no axial planar fabric ŽFig. 14a and b.. The older foliation is refolded together with the pegmatite. In contrast, other pegmatites in the supracrustal rocks have been folded isoclinally during the older, regional deformation and have well-developed axial planar fabric ŽFig. 14c and d.. The supracrustal layers are partly discordant to the pluton boundary and they are invaded by schistosity parallel pegmatites emanating from the Hijlah pluton. Earlier radiometric datings on samples inferred to be taken from Hijlah exposures range from Rb–Sr whole rock ages of 626 q 17 Ma ŽFleck et al., 1980. to K–Ar muscoviterbiotite ages of 567 q 7 Ma ŽFleck et al., 1976; recalculated by Fleck in Gettings and Stoeser, 1981.. The younger age may be a reset, thus indicating a late metamor-

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phic age rather than an igneous age ŽPallister, personal communication, February 1999..

2.5.2. Arafadi pluton The Arafadi intrusion occurs in prominent light tone on the false color TM Landsat image of Fig. 3. The geologic map of the Arafadi region proper is given in Fig. 15. The Arafadi intrusion is zoned, comprising a central gabbro in the upper central area southward grading into diorite, granodiorite and granite. The contact between the Arafadi pluton and the supracrustal host rock is largely fault bound. The pre-intrusion foliation in the supracrustal rocks is draped around the Arafadi pluton and forms a topographic ridge that surrounds the shallow topographic depression occupied by the Arafadi pluton proper. The supracrustal rocks along the western margin have been deformed into gentle to open folds of the main foliation and transposed bedding ŽFig. 16a and b.. The supracrustal rocks within a several hundred meter wide zone near the pluton contact are invaded by a suite of concordant and discordant dikes of granite, pegmatite and aplite, all originating from the main pluton. The main pluton itself is internally non-foliated, but the marginal zone and the suite of early dikes just mentioned are. The foliation in the marginal zone and the formation of minor folds in some of the adjacent dikes are interpreted to be synplutonic, and formed due to inflation of a rectangular region now occupied by the Arafadi pluton. Another important aspect of the Arafadi intrusion is the anatexis of the supracrustal wall rock. The map of Fig. 15 also outlines remains of entrained supracrustal layers that can be discerned inside the Arafadi pluton on the Landsat image of Fig. 3. Signs of anatexis are abundant in the field, wall rock is locally abruptly terminated along foliation strike.

2.5.3. Qurayn shear zone The emplacement of the Arafadi pluton has also caused a major, right-lateral strike–slip shear zone along its southern margin ŽFig. 17.. This E–W-trending Qurayn shear zone is marked by the deflection of the schistosity in the supracrustal rocks, schistosity

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parallel microgranitic dikes, as well as the displacement of axial plane traces of regional anticlines and synclines. The shear zone is about 1 km wide, caused up to 3 km dextral displacement of the axial plane traces of the Mifsah and Jarshah antiforms, and the intermediate synformal trace. The shear zone was principally accommodated by horizontal and oblique slip over the steep cleavage surfaces in the supracrustal rocks and is considered semi-ductile. Late tectonic kinking of the foliation in the supracrustal rocks occurs along minor faults within the Qurayn shear zone ŽFig. 18a and b.. The rightlateral displacement of the Qurayn shear zone also caused near-collision of the pre-existing Jarshah and Mifsah plutons, and a mylonitic zone along the eastern margin of the Jarshah pluton, and an eastward convex dent in the N–S-striking supracrustal belt in the eastern margin of the Mifsah antiform ŽFig. 19.. Ages ranging between 539 and 587 Ma are suggested for the Arafadi pluton by K–Ar dating of biotite, muscovite and hornblende in rhyolitic and granitic dikes in the adjacent supracrustal rocks ŽFleck et al., 1976, recalculated by Fleck in Gettings and Stoeser, 1981.. This implies that Arafadi’s emplacement may have occurred at the Early Paleozoic or Infracambrian. Regional shortening had ceased during this emplacement of the Arafadi pluton and downward erosion of the overlying groundsurface to form the Cambrian peneplain underlying the future Wajid sandstone may have commenced already.

2.6. Group VI: subÕolcanic complex A final episode of granitic to granodioritic plutonism is represented by the subvolcanic complex of Mahala, with northerly extension into the Hijlah granite. The Landsat map and corresponding geologic structure map are given in Fig. 20a and b, respectively. The Mahala granite forms a large ring dike complex surrounding a central area of 2.5 km across, which suffered subsidence as it contains many onliers of supracrustal roof rock. The granitic ring dike is mostly discordant with the surrounding host rock and locally the granite of the ringdike forms inselbergs that dominate the landscape ŽFig. 21.. The

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western section of the ring dike of the Mahala granite provides a prominent landmark with one major conical shaped granite mount rising above the surrounding landscape ŽFig. 21.. The ring dike is transected by numerous sytematic tension joints, which are commonly filled with quartz veins. Numerous satellite dikes emanate from the Mahala

ringdike and extend radially outward into the host rock for up to several kilometers. At least one such dike strikes N–S discordantly into the Hijlah pluton, thereby confirming that the Mahala subvolcanic complex represents the youngest magmatic event in our study area. The Mahala subvolcanic complex was emplaced into shallow crust as can be inferred

Fig. 13. Ža. Detail of Landsat image, illustrating the Janfoor fold interference pattern that occurs within the supracrustal belt at the eastward termination of the Qurayn shear zone. Image width is 6 km. Žb. Structural map of the area of Ža., illustrating a synorogenic folded interference pattern that gave rise to a structural basin between the Miktaa granite and the Tanah gabbro pluton. Žc. For comparison, regional basin due to fold-interference pattern in Silurian rocks of the Moroccan Meseta as seen on multispectral Landsat image. Žd. Structural map of the area of Žc., emphasizing structural basin of Silurian layers between synorogenic basement domes. The fold interference patterns in Ža,b and c,d. were both created by the rise of regional basement domes coeval with horizontal orogenic shortening.

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Fig. 13 Žcontinued..

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100 R. Weijermars, M. Asif Khan r Earth-Science ReÕiews 49 (2000) 77–120 Fig. 14. Ža and b. Superposed folds in pegmatite formed without axial plane fabric during emplacement of the Hijlah pluton. Žc and d. Older folds in pegmatite vein with well-defined axial plane fabric formed during regional deformation.

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from the brittle features Ždikes intruding into radial tension fractures. associated with its emplacement. Fig. 22 is an idealized view of this subvolcanic complex when still overlain by the Žnow eroded. volcanic superstructure. The Mahala ring-dike is intruded by meter-thick basic dikes exposed in a few locations. Although gabbro intrusions occur at Tanah and in a smaller body directly to the west of Mahala, these are in turn intruded by Mahala granite and therefore must be older than it. The source of the younger basic dikes that intrude the Mahala granite is unknown, but they could be of Tertiary age and related to the Cenozoic opening of the Red Sea. The entire subvolcanic complex of Mahala, which contains minor amounts of magnetite, shows up as a positive anomaly on the regional aeromagnetic map ŽFig. 23.. The age of the Mahala subvolcanic complex is poorly constrained as no radiometric dating is available. It certainly predates the Tertiary extension that led to the opening of the Red Sea, emplacement of basic plutons, and associated flood basalts. The predominantly granitic composition of the Mahala complex suggests that it was emplaced during a late stage of continental accretion. The emplacement would represent a final episode of Precambrian or early Paleozoic magmatism in our area of study. At this moment a younger age cannot be precluded, but seems unlikely. The near surface position of the Mahala complex, the ring dike and its central subsidence are all characteristic for a subvolcanic complex. The extrusives that must have surfaced from this complex have been eroded away, but remnants possibly still await discovery as igneous intercalations Žpyroclastic tuffs. within the basal Wajid sandstone of the Paleozoic cover sequence.

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3.1.1. Groups I q II: Pre-tectonic and pre-plutonic, paleogeographic situation (0.9 to 0.66 Ga) Basalts and pelites interlayer on the floor of an ocean adjacent to a gneissic craton rifted earlier to create the neighbouring oceanic plate. 3.1.2. Group III: syntectonic, basic plutonism (0.65 Ga) Subduction sets on after initiating closure of the ocean. The cratonic margin near the subduction zone deforms and slabs of the upper oceanic suite Žcommonly known as seismic layer 1. are obducted onto the craton. Simatic melt from the root zone of the thickened continental margin mingles with gabbroic melts generated along the friction zone at the surface of the subducting oceanic slab. This leads to the emplacement of zoned plutons of gabbro and granodiorite ŽJarshah and Mifsah.. These plutons are emplaced at mid-crustal depths into anticlinal hinges of supracrustal rocks that are isoclinally folded during the emplacement. Granites from earlier late orogenic phases are also folded ŽMiktaa.. 3.1.3. Group IV: syntectonic, gneiss-domes (0.62 Ga) After subduction of the oceanic slab, its descending portion detaches and sinks into the mantle. Basic plutonism ceases. This final stage of tectonism in the collisional orogen is characterised by continental buoyant gneiss domes at mid-crustal depth ŽQardath..

3.1. Geotectonic reconstruction

3.1.4. Group V: post-tectonic, acidic plutonism (0.59 to 0.54 Ga) Acidic to intermediate plutons are fed by buoyant partial melts originating from the deep root zone of the mature orogen now formed at the aborted continental margin. The plutons are emplaced without significant background stress and the deformation is confined to deformation in discrete faults, shear zones, and dike intrusions near balooning plutons ŽHijlah and Arafadi..

The geologic observations reported above and the six geochronologic groups distinguished above can be incorporated and explained by a relatively simple geotectonic model ŽFig. 24..

3.1.5. Group VI: post-orogenic Õolcanism (0.54 to 0.5 Ga) Buoyant simatic magma continues to leak from the partially molten orogenic root. Some of this melt

3. Discussion

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3.2. Rheologic regimes

Fig. 16. Ža. Mesoscopic folds due to the emplacement of the Arafadi pluton. Žb. Open minor folds developed in the supracrustal belt adjacent to the Arafadi pluton.

may reach shallow crustal levels and may even surface through vents marked by andesitic volcanism underlain by subvolcanic complexes ŽMahala.. In summary, the entire orogenic cycle, which formed our section of the Tayyah belt, lasted approximately 100 Ma Ži.e., 0.65 to 0.54 Ga..

The structures seen in the crustal level presently exposed in the central Asir region includes features resulting from extremely ductile Žfolds. as well as brittle Žfaults. deformational features. The occurrence of juxtaposed folds and faults indicates a range of rheologic behavior that can partly be attributed to the fact that the different structures, now exposed at the surface by progressive uplift and erosion, must have formed at different depths. This section investigates how the paleo-rheology of the mapped structures can be explained by crustal strength profiles. Strain hardening rocks show a time-dependent rheology which hardens in the course of their deformation. The rheologic behavior of crustal rocks can be summarized in so-called crustal or lithospheric strength profiles ŽFig. 25.. Crustal strength profiles illustrate, approximately, which portions of the crust behave in brittle and ductile fashions, respectively. This type of diagram was first used by Goetze and Evans in 1978, using Byerlee’s law for the brittle regime and the flow law for solid-state creep for the ductile regime. The shallow sections of the hypotethical crust in Fig. 25 has a strength as determined by Byerlee’s law Žfor details, see Weijermars, 1997a, Chap. 8.. The frictional resistance is very sensitive to pore water-pressure, and differences in pore pressure have been incorporated in shallower parts of the crustal strength profile of Fig. 25. The assumption of a pre-fractured crust is essential to such diagrams, in accordance with Byerlee’s experimental conditions. The deeper, ductile section of the crustal strength profile assumes that crustal rock, commonly taken to be principally quartzitic for modeling purposes, flows by ductile creep with a temperature-dependent

Fig. 15. Geologic map of the Arafadi area Žsee index map of Fig. 3.. The Arafadi pluton is cored by a gabbro pod Ž‘‘bbb’’. in the north of the map area. The margins of the Arafadi pluton are fault-bound and accompanied by extensive anatexis. The Arafadi pluton is also surrounded by marginal synformal closures. Remnants of supracrustal rock units that became detached from supracrustal belt can be discerned as entrainments within the plutonic body. The rectangular-shaped intrusion of Arafadi is engulfing the two older, oval-shaped gabbroic plutons of Jarshah and Mifsah as seen on the easterly adjoining map area of Fig. 9. The emplacement of the Arafadi pluton was accompanied by the formation of the Qurayn shear zone in the supracrustal rocks near its southern margin, as seen in the southern, adjoining map area of Fig. 17 and cartooned in Fig. 19. Strikerdip symbols indicate the main schistosity. Rock sample locations are encircled numbers, for which GPS coordinates have been determined. The region is accessed by Abha-Taif highway and Maiwan road to the west of the Arafadi pluton as well as via the outlined wadis. Access to the central area of Arafadi is handicapped by a restricted military area.

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Fig. 18. Ža and b. Examples of the mesoscopic kink zones that are abundant in the Qurayn shear zone.

strength. The depth of the brittle–ductile transition ŽBDT. in Fig. 25 is determined by the intersection of the brittle and ductile strength envelopes. The maximum stress possible in the crustal sections of Fig. 25 occurs at the depth of BDT. Larger stresses cannot occur within the rock, because such stresses would be relieved, either by ductile creep or by brittle faulting, depending upon the depth of deformation. The dominant deformation mechanism Žrequiring the

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least deviatoric stress for activation. at shallow depths is brittle faulting Žfrictional plastic slip.. This is true simply because the deviatoric stress cannot attain the magnitude required to activate flow by crystalline creep, as such stress is first relieved by frictional slip. Reversely, the dominant deformation mechanism in the deeper crust is ductile creep, because the deviatoric stresses required there are lower than those required to activate fault movement. The BDT depth at any moment in geologic time is determined by several major factors: Ž1. geothermal gradient; Ž2. pore water pressure; Ž3. deformation rate; and Ž4. deformation regime. These factors can be explained as follows. A shallow geothermal gradient Ži.e., colder rock. leads to an increase in the BDT depth. A general rule is that doubling of the gradient halves the BDT depth ŽFig. 25.. An increase in pore water-pressure may also slightly increase the BDT depth, because it increases the slope of the brittle strength envelope ŽFig. 25.. Increase in the strain-rate also brings the creep curve to deeper levels and deepens the BDT depth, because it lowers the point of intersection of the britle and ductile strength envelopes. The tectonic deformation regime affects the BDT depth as follows: BDT depth under extensional deformation is deeper than that of deformation involving shortening, because the frictional faulting envelope for extension is steeper than that for shortening ŽFig. 25.. The depth of the BDT will be different for different lithologies, because the position of the ductilecreep curve is somewhat shifting for each particular rock type. If the crustal portion of the lithosphere is taken to be principally composed of quartz, then the BDT lies at about 15 km depth, and coincides with the separation between the upper and lower crust Ždepending upon the local geothermal gradient.. The plot in Fig. 25 further assumes a constant strain-rate

Fig. 17. Geologic map of the Shohat area Žsee index map of Fig. 3.. The southern margin of the Arafadi pluton and the adjacent Qurayn shear zone occur in the upper part of the map. The Red Sea escarpment impinges on the northwest corner of the map. The major wadis are wadi Tahal and Wadi Al Hitr, which conflue to run off eastward toward Wadi Bishah. The region is entirely composed of isoclinally folded and transposed supracrustal rocks of greenschist facies. Strikerdip symbols indicate the main schistosity. Faults are dashed and fold hinges are traced by solid arrows. Rock sample locations are encircled numbers, for which GPS coordinates have been determined. Samples 6 and 11 are located on a prominent granitic dike Žblack outline, NNW–SSE-trending.. The area can be accessed by the Abha-Taif highway.

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Fig. 19. Interpretative perspective diagram of the Qurayn shear zone. The prominent reverse fault, the Khamis–Mushayt Fault, upthrew the NE block coeval with the intrusion of the Arafadi granitoid.

of 10y1 4 sy1 . An increase in pore pressure increases the slope of the brittle strength envelope and, therefore, brings the BDT to deeper levels. An increased geothermal gradient brings the ductile creep curves to shallower levels and, therefore, also, the BDT. In contrast, an increase in strain-rate brings the ductile creep curve to deeper levels and, therefore, would increase the depth of BDT. This theoreticalrexperimental knowledge of rock rheology and its effect on the BDT depth can be applied to the structures in the Asir terrane studied here. For example, the ductile deformation belt of supracrustal rocks concordantly foliated about the Jarshah and Mifsah plutons suggests that their emplacement depth lay at about 10 to 15 km at 0.65 Ga. Similarly, supracrustal rocks are neatly wrapped around the mantled gneiss-dome of Qardath Ž0.62 Ga.. Both magmatic events are syntectonic and the

plutons are elongated in N–S direction due to E–W shortening coeval with their rise in the crust. However, the Arafadi and Hijlah plutons are discordantly cutting through the foliation of the host rock and are partly fault bound. Although some folds were locally formed in the supracrustal margins of these plutons ŽFig. 14a to d, Fig. 16a and b., many dikes forced the earlier foliation surfaces apart to intrude along them ŽFig. 26.. The Qurayn shear zone is semi-ductile and includes kink zones ŽFig. 18a and b. rather than ductile flow folds. The brittle–ductile structures associated with the post-tectonic, acidic plutons of Arafadi and Hijlah Ž0.59 to 0.54 Ga. suggests that the present level of observation at the surface shows a crustal slice that is now much shallower than it was during the syntectonic plutonism Ž0.65 to 0.62 Ga.. The trend is toward the formation of progressively more brittle

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structures and was completed by the emplacement of the Mahala subvolcanic complex Ž0.54 to 0.5 Ga.. In conclusion, the structures formed between 0.65 and 0.5 Ga range from purely ductile Ž0.65 Ga. to purely brittle Ž0.5 Ga.. The principal explanation suggested here is that progressive uplift of the region led to strain hardening and cooling of crustal sections which were previously deep enough to allow ductile deformation ŽFig. 27.. The depth of the crustal slice now exposed at the surface is estimated to range from 10 to 15 km at 0.65 Ga to about 3 km overburden at 0.5 Ga. The 3 km overburden at 0.5 Ga may have included the early Precambrian Wajid sandstone that was deposited onto an already eroded basement. In other words, the emplacement of the Mahala subvolcanic complex is poorly constrained and may be late Precambrian or early Phanerozic. Our analysis of the paleo-rheology and crustal exhumation suggests that over 10 km crustal thickening occurred coeval with the isostatic exhumation of mid-crustal levels during the main Pan-African orogenic episode in our region Ž0.65 to 0.54 Ga.. The older, 0.65 Ga structures evolved in ductile regimes at mid-crustal depth. The younger structures of the later, 0.54 Ga episode were formed at shallow depths and include fracture-bound ring dikes. We conclude that the gross rate of crustal thickening in the Proterozoic, estimated here at 10 km per 100 Ma, is consistent with that occurring in modern mountain belts. This provides a new argument against the idea of faster island-arc accretion rates in the Proterozoic.

3.3. Crustal accretion rates The accretion rate for the ANS was first estimated at 310 km3 per km arc length per Ma ŽReymer and Schubert, 1984, 1986.. This accretion rate is 10 times faster than the 30 km3 kmy1 May1 , quoted for the average Mesozoic–Cenozoic arc addition rate ŽReymer and Schubert, 1984. and has been subject of considerable debate Že.g., Stein and Goldstein, 1996.. However, the earlier estimates of anomalously fast accretion rates for the ANS is in disagreement with our own estimate of crustal growth rates for the Arabian Shield alone. The Arabian Shield occupies 610,000 km2 , about two thirds of the total

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surface area of the Arabian Peninsula. The crustal volume for an initial average crustal thickness of 50 km Ž38 km remaining and 12 km to account for erosion. is 30.5 = 10 6 km3. The time span available for accretion is about 300 Ma, which implies an accretion rate of 0.1 km3 May1 . This annual accretion rate for the Arabian Shield alone is one tenth of the present-day rate of 1 km3 May1 for all arc accretion sites globally combined Žand time-averaged.. Consequently, we see no abnormal fast accretion rates recorded in the Arabian Shield. What is more, our estimate for the arc-addition rate is 33 km3 kmy1 May1 , remarkably similar to the 30 km3 kmy1 May1 quoted as the average Mesozoic– Cenozoic arc addition rate ŽReymer and Schubert, 1984.. Our estimate uses an overall length for the ophiolite-hosting suture zones of the Arabian Shield of about 3000 km: 400 km for the Yanbu suture, 400 km for the B’ir Umq suture, 1500 km for the Nabitah suture, 500 km for the Urd suture, and 200 km for the Al Amar suture. In fact, veteran students of the Arabian Shield have stated before that the crustal accretion rate of the Arabian Shield is not anomalously high as compared to modern accretion rates ŽPallister et al., 1988, 1990.. Late Precambrian accretion rates of the Arabian Shield have been estimated at 0.1 km3 kmy1 yry1 by all field workers of the Arabian Shield ŽPallister et al., 1988, 1990, this study.. As noted by Pallister et al. Ž1990., one major cause for the high accretion rates postulated for the ANS by Reymer and Schubert Ž1984; 1986. is that the area for the ANS craton was assumed to be 6 = 10 6 km2 , which is much larger than the exposed shield area of the ANS. In their estimate the ANS craton is taken to stretch beneath the Phanerozoic platform cover from the Nile to the Zagros Mountains. The area used may well be correct, but it seems too speculative to assume that the time of accretion for this enlarged craton is only 300 Ma. Recent isotopic work on rocks from the exposed margins of the ANS in Egypt and Yemen suggests that even the exposed rocks of the ANS may include marginal zones of up to 3 Ga old ŽSultan et al., 1990; Windley et al., 1996.. Much of the rocks beneath the cover of the ANS craton may turn out to be much older than the Pan-African deformation episode indeed.

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The earlier estimates of the crustal accretion rates for the ANS ŽReymer and Schubert, 1984, 1986., assumed a craton area that is 10 times larger than the exposed shield surface, in which the majority of rocks accreted in only 300 Ma. Not surprisingly, this approach results in an apparent crustal accretion rate that is 10 times faster than observed for the Mesozoic and Cenozoic arc accretion rates. It is well possible that the rocks for the enlarged ANS craton of 6 = 10 6 km2 took close to 3 Ga to accrete, 10 times longer than assumed in the ‘‘fast estimate’’ of Reymer and Schubert Ž1984; 1986.. If this holds true, then the ANS accretion rate is similar to present-day accretion rates of continental crust. In any case, our own estimate of accretion rates of 33 km3 kmy1 May1 for the exposed section of the Arabian Shield, using its established representative age range of 300 Ma, is comparable to modern accretion rates. This result may also help to understand why all the geologic features of our study area in the Asir terrane can be explained by a simple, actualistic model of island-arc accretion ŽFig. 24..

detailed fold structure of the supracrustal rocks of the central Asir region. Finally, our geotectonic scenario is the first attempt to explain the detailed plutonic and tectonic evolution of this area. In our scenario, the relative ages of events inferred on the basis of field relationships are compatible with the geochronologic sequence of rock units previously dated by radiometric methods only. According to our best knowledge the following tectonic and plutonic features have been named first here: Miktaa, Jarshah, Mifsah, Mohra, Tanah, Hijlah, Arafadi plutons; Qar-

4. Conclusions Detailed field surveys in combination with enhanced satellite imagery allowed us to unravel the geologic structure, and geotectonic and magmatic evolution of a 900-km2 area of the Asir terrane. The 1:250,000 scale and 1:100,000 scale reconnaissance geologic maps prepared by the US Geologic Survey ŽColeman, 1973a,b; Greenwood, 1985., consulted during our own mapping campaign as a starting point Žsee Appendix A., could be substantially improved. The various plutons were not distinguished in relative age groups in any previous study. The Jarshah pluton was not discovered before and had been mapped previously as part of an undifferentiated supracrustal unit instead. The gabbro intrusions of Mohra and Mifsah, interpreted as lopoliths exposed in gabbroic synforms by Coleman Ž1973b., are instead plutonic stocks in their own right. The occurrence of marble and skarn in the supracrustal sequence of the map area has not been reported before. No attempts were made, until our study, to map the

Fig. 20. Ža. Detail of Landsat image over the Mahala subvolcanic complex. The Mahala ring dike is highlighted by white albedo. Wadi Bishah is visible in the upper right corner. The square outlines of Sport City are located near the southwest margin of the image onto black toned silvergrey schist. Image width is 6 km. Žb. Geologic map of the Mahala subvolcanic complex imaged in Fig. 20a. Supracrustal unit is white with solid formlines outlining transposed bedding, main foliation and fold hinges. Further marked are gabbro Ž‘‘bbb’’. and pegmatite unit Ž‘‘ppp’’. and Hijlah granite ŽH.. Locations of numbered rock samples are encircled.

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Fig. 20 Žcontinued..

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Fig. 21. Spectacular Inselberg of jointed, conical granite mound in the southwest margin of the Mahala ringdike. Abundant microgranitic dikes Žone to several meters thick. radially emanate from the Mahala ringdike for several kilometers into the surrounding supracrustal sequence.

dath gneiss dome, Nabutah synform, Janfoor and Sport City fold interference patterns, Water-melon basin, Mahala subvolcanic complex, and the Qurayn shear zone. The field relationships and the geologic inventory of our map area resulted in the distinction of six age groups of tectono-magmatic units as characteristic terrane elements. The evolution of these terrane elements occurred during an orogenic cycle of nearly 100 Ma, i.e., between 650 and 540 Ma ago. The structural variety of the terrane elements is further used to explain the paleo-rheology on the basis of crustal depth at the time the corresponding structures were formed. Ductile structures, which were originally formed below the BDT, have been progressively exhumed and were juxtaposed and overprinted by brittle structures near the end of the orogenic cycle. A comprehensive summary of the erosional excavation of paleo-rheologic relicts is given in Fig. 27. Finally, we considered earlier estimates of crustal accretion rates for the ANS. There are two basic

views presently circulating in geoscience literature: One group of geochemistsrgeophysicists propagates the view that ANS accretion rates were 10 times faster than estimates of modern rates for island arc accretion ŽReymer and Schubert, 1984, 1986; Stein and Goldstein, 1996.. The other group of mainly field geologist and isotope geochronologists, with vast experience in the ANS, contends that there is little evidence to support the view that ANS accretion rates were faster than today’s ŽPallister et al., 1988, 1990.. Our own understanding of the ANS terrane is that extraordinary fast accretion rates can only be concluded if large parts of the Arabian craton, now obscured from direct study by Phanerozoic Platform cover, are assumed to be less than 1 Ga old and took only 300 Ma to accrete. This assumption now seems untenable. Recent isotope work has already revealed that even some ANS rocks in the margins of the exposed shield region are, in fact, up to 3 Ga old ŽWindley et al., 1996.. These facts lead us to the conclusion that previous suggestions of crustal growth rates in the Precam-

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second author. Expenses for reproduction of the color imagery and costs of the professional draftwork were covered by a special grant from the ‘‘Dr. Schurmannfonds’’ Foundation, the Netherlands. Without the support of this foundation, publication of this work would not have been possible.

Appendix A. Logistics and previous investigations

Fig. 22. Schematic representation of Mahala subvolcanic complex and its now eroded volcanic superstructure, inspired by Cloos Ž1936..

brian for ANS accretion are not supported by the ANS’ geologic record.

Acknowledgements The city of Abha is the seat of the Prince of Asir, and his excellency Khaled bin Faisal Al-Saud, son of the late King Faisal, kindly directed instructions to the Ministry of Education of the Asir Province in order to secure suitable logistics. Our high-resolution mapping of the region was further sponsored by King Fahd University of Petroleum and Minerals, partly in combination with summerschool instruction on geologic mapping Ž1994, 1995 and 1998.. The views expressed here are principally that of the first author; image-processing was duly completed by the

Field work in Saudi Arabia is subject to several special circumstances. Firstly, there are travel restrictions and other hurdles related to the policy of national security. The geologic mapping of the Arabian Shield is overseen by the Saudi Arabian Ministry of Petroleum and Mineral Resources. Independent geologic investigations are generally impeded by travel restrictions within the kingdom. Foreign investigators can normally not obtain entry visas, as such visas are normally issued for commercial business purposes only. Consequently, field work in Saudi Arabia is constrained by regulations concerning budget controls and national security. These hurdles can be overcome if such fieldwork is officially supported. In our case, we owe thanks to a long list of individuals and institutions, which all contributed to complete many of the administrative requirements. This undertaking would have been impossible without their help. Secondly, perhaps in part due to the above restrictions, only few parts of the Arabian Shield have been studied in detail. Most of the Arabian Shield has been mapped only at reconnaissance level, using maps at scales of 1:1,000,000 ŽJohnson, 1982, 1983; Brown et al., 1989., 1:500,000 Ž1950s, 1960s., 1:100,000 Ž1970s. and 1:250,000 Ž1980s.. Much of the mapping work in Saudi Arabia has been done under bilateral agreement with the US Geologic Survey and the French Bureau de Recherches geologiques et Minieres, and under contract with several other organizations ŽRiofinex, Seltrust Eng., British Steel and Minatome.. The first comprehensive data on the geology of the Arabian Shield became available in a landmark effort conceived in 1953 under joint sponsorship of Aramco Ži.e., the Saudi Arabian oil company operating on the adjacent

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Fig. 23. Total intensity aeromagnetic map covering the study area. The gammas are relative to an arbitrary datum. Granitoid plutons tend to coincide with magnetic highs Ž) 6200 gammas., and supracrustal rocks are less magnetically susceptible Ž- 6,200 gammas.. Magnetic data from Andreasen and Petty Ž1974..

platform. and the United States Geologic Survey Mission. This mapping effort covered the Arabian Peninsula in a series of 21 geologic and geographic maps on a scale of 1:500,000 each. The mapping used high-altitude photographic basemaps on a scale of 1:60,000. The first map was published in 1956, and the last in 1964. This mapping work ultimately was compiled into a single 1:2,000,000 scale geologic map of the Arabian Peninsula as a whole Ž1963, MG-I-207A B-2, reprinted as AP-4.B-2. and a 1:1,000,000 geologic map for the shield area of Saudi Arabia Žnot until Brown et al., 1989.. Most of the shield is now also covered by 1:100,000 scale maps Ž1970s. and 1:250,000 scale maps Ž1980s., and

a lithofacies map has been compiled at a scale of 1:1,000,000 ŽJohnson, 1982, 1983.. The recent policy of the Saudi government to develop and liberalize ore extraction from the shield has led to some very detailed studies, principally of gold mineralization zones occupying relatively small areas Že.g., 1:15,200 maps by Johnson and Offield, 1994.. Thirdly, the degree of exposure and variety of bed rock is breathtaking, making field investigations in this area a rewarding experience. The geologic mapping of Precambrian terranes, marred in some countries by the lack of adequate exposure, commonly involves a large degree of interpretation to establish a coherent map pattern. Consequently, the recon-

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Fig. 25. General crustal strength profile for wet quartzite. The depth of BDT migrates downward if the geothermal gradient is colder Žshown are examples of 15 and 30 Krkm.. Lowering in pore pressure also strengthens the resistance to frictional sliding over faults at shallow depths Žfor details, see Weijermars, 1997a..

great detail and leaves little room for alternative interpretations. Our field work on the ground was further facilitated by excellent contrast in the tone and texture of the rocks on state-of-the-art satellite images ŽTM Landsat and SPOT.. The combined data enabled us to reconstruct the geotectonic history of this Precambrian terrane and it may serve as a

Fig. 24. Synoptic diagram of the various stages in the development of the central Asir terrane discussed in this paper.

struction of the geotectonic history of such areas is handicapped by uncertainties about the accuracy of the inferred geologic boundaries and only few cross-cutting relationships are actually exposed. The extremely good exposure enables the reconstruction of the crustal evolution of the Arabian Shield in

Fig. 26. Satellite intrusion from Arafadi pluton onto the marginal supracrustal zone.

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Fig. 27. Strain hardening of the Asir crust occurs by progressive denudation and uplift. This progressive hardening is illustrated here using several chronologic stages in the evolution of the Asir terrane Žsee text..

template for understanding the development of other, less well exposed, Precambrian areas. This article is the first full documentation of our findings which were earlier presented at the second annual meeting of the Saudi Society for Earth Sciences ŽWeijermars et al., 1994.. All the earlier published geologic maps and accompanying descriptions were carefully studied before the onset of our field campaigns. The relevant geochronic datings, aeromagnetics and tectonic belts distinguished are discussed in the main text of this paper. The earlier mapping work is summarized in this appendix. Investigations of the Arabian Shield are coordinated by the Directorate General of Petroleum and Mineral Affairs, which is governed by the Ministry of Petroleum and Mineral Resources. It was incepted in 1954, and was reformed into the Directorate General of Mineral Resources ŽDGMR. in 1962, and once again reformed into the Deputy Ministry of Mineral Resources ŽDMMR. during the 1980s. DMMR-work has been recorded in Bulletins,

Geoscience Maps, Industrial Mineral Resources Maps, Technical Letters, Records, Open-File Reports and Confidential Reports totalling about 4000 items, archived in the DMMR Technical Library. All records, except for Confidential Reports, are listed in the Annotated Bibliographies, published by the DMMR every 5 years. Perhaps the most important publication is the DMMR Bulletin 29, which lists the index maps and principal publications of the DMMR. The present study area lies within the 1:250,000 scale geologic map of the Abha quadrangle prepared by the US Geologic Survey under the agreement with the Saudi Arabian Ministry of Petroleum and Mineral Resources ŽGreenwood, 1985.. Greenwood’s 1985-map was partly compiled from six earlier 1:100,000 scale reconnaissance geologic maps. The 1:100,000 scale maps in turn were the first attempts to achieve more detailed coverage than provided by the 1:500,000 scale geologic map of the Asir quadrangle by Brown and Jackson Ž1959.. Our area occupies the central part of the Abha quadrangle which is

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overlapped by the two 1:100,000 scale reconnaissance maps of Coleman Ž1973a; b.. The geologic interpretations of Coleman Ž1973a; b. were based on fieldwork using for basemaps mosaics of aerial photographs of 1956 by Aero Service Cooperation, USA. Coleman carried out fieldwork by helicopter and 4-wheel drive vehicle traverses between January and May 1971. This information is relevant because at the time of Coleman’s mapping the network of tarmac roads and dirt tracks was extremely poor as compared to the present-day transport infrastructure. In our surveys, any location could be reached by 4-wheel drive vehicle within a couple of kilometers, and was complemented by hiking tours to cover the ground in detail. Our field campaigns not only benefitted from the much improved road network, but also from the advent of detailed satellite imaging techniques. In particular, the false colour Thematic Mapper Landsat images visualized remarkable continuity of many supracrustal structures. We processed our own SPOT and Landsat images at scales of 1:25,000 and 1:75,000, respectively. Earlier, a 1:250,000 panchromatic Landsat image map of the Abha quadrangle had been prepared by the US Geologic Survey for the Ministry of Petroleum and Mineral Resources, Saudi Arabia ŽAnonymous, 1980.. This panchromatic satellite map was again published as an annotated 1:250,000 scale geographic basemap by Faulkender Ž1984.. A 1:500,000 scale false color enhanced Landsat MSS image of the entire Asir quadrangle has been published as Technical Record USGS-TR04-3 SAŽIR.-624 Žundated.. An area adjacent to our area was geologically mapped on a 1:250,000 scale Landsat image ŽBlodget et al., 1979.. The usefulness of TM Landsat images has been demonstrated earlier for a small area north of Khamis Mushayt ŽQari, 1989.. The same area, which displays a complex fold interference pattern, has been previously described in two Master theses by students of King Abdulaziz University, Jeddah ŽAmlas, 1983; Qari, 1985.. Unfortunately, copies of these theses were not available for our inspection. Field campaigns were conducted in the summers of 1994, 1995 and 1998, each of 1 month duration. The general mode of operation was as follows. The field party was flown into Abha airport, and ground transportation in the area was by 4-wheel drive

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vehicles. The 1994 campaign used as a basemap 1:75,000 scale Thematic Mapper enhanced Landsat images in false colours. The 1995 and 1998 surveys were more detailed and were based on 1:25,000 panchromatic SPOT-images Ž141–313.. The scale of resolution of the TM-images was about 30 m and that of SPOT neared 10 m. A portable Magellan Global Positioning System was used to facilitate accurate orientation in the field. The device uses at least three satellites to specify the longitude and latitude of a location with a claimed accuracy of 1 s. Our experience is that the error margin is somewhat larger, closer to 10 s, or a distance of about 200 m on the ground. Positioning and orientation in the field further relied on a commercially available 1:85,000 scale Farsi road map for the southern half of the study area. Detailed 1:25,000 scale topographic maps exist, but are classified and were not available to us. Near Abha, rainfall occurs all-year round. The greatest rainfall is generally in April, and rainfalls of 50 mm in 24 h is not unusual. The annual rate is up to 600 mm, the largest of Saudi Arabia, but is superceded by annual evaporation rates of up to 2000 mm. Nonetheless, the area near Jebel Sawdah supports a relatively large stand of trees, the only natural ‘‘forest’’ of the kingdom. The region enjoys a moderate climate with average diurnal air temperatures of 24 and 13 centigrade in July and August, respectively. The Asir region is increasingly popular as a summer resort for residents who wish to escape from the intense summer heat in major cities like Jeddah and Riyadh. The moderate climate also enabled us to conduct fieldwork in the summer months.

Appendix B. Geological setting of the Asir terrane Our study area is located in the Asir Highlands of Saudi Arabia and covers a rectangular area of 30 by 30 km ŽFig. 28.. Nearby physiographic landmarks are provided by the Red Sea escarpment, the 3207 m high Jebel Sawda, and the two major cities of Abha and Khamis Mushayt. The Serrat, a high plateau with an average elevation of about 2 km above sea level, is separated from the 50 to 100 km wide Tihama, or coastal plain, by the spectacular Red Sea

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Fig. 28. Topographic map of the Red Sea escarpment. The location of our 30 = 30 km study area in the Asir Highlands is outlined. Major geographical locations mentioned in the text are indicated.

escarpment ŽFig. 29.. Both the Serrat and Tihama occur along the entire length of the Red Sea coast of the Arabian Peninsula. This morphology is basically attributed to a thermal swell which accompanied the opening of the Red Sea about 30 Ma ago ŽGirdler, 1991; Makris and Rihm, 1991; Sultan et al., 1992.. The principal normal faults which shaped the Red Sea rift zone do not coincide with the Red Sea escarpment as is sometimes asserted in the literature.

The Red Sea escarpment results from back-cutting by progressive recession of a faultscarp, which itself remains closer to the modern margin of the Red Sea ŽColeman et al., 1978.. The escarpment presently defines the water division ŽFig. 30a.. All drainage to the west of the escarpment flows through initially steep sided canyons ŽFig. 30b., which widen when nearing the Tihama plains before reaching the Red Sea. This system of canyons progressively cuts head-

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Fig. 29. Idealized section across the Red Sea escarpment and adjacent wall rock.

Fig. 30. Ža. Oblique aerial view of the Red Sea escarpment between Abha and Jebel Sawdah, looking SE-ward. The escarpment defines the water division and all precipitation on the Asir plateau runs off due northeast, away from the Red Sea, toward the deserts of the Arabian interior. Žb. View from the Asir Highlands down a canyon near the hanging villages of Habala. The canyon transports sediments and perennial water to the Red Sea. Žc. View of a tributary of Wadi Bishah near Khamis Mushayt on the Serrat Plateau.

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Fig. 31. Ža. The Lower Paleozoic Wajid sandstone, nonconformably overlying the Precambrian basement at Habala. Žb and c. Cross-bedding in Wajid sandstone exposures at Habala and Jebel Sawdah, respectively.

wards, migrating the escarpment eastward at a timeaveraged rate which is estimated here at about 3 mm per year. A good small-scale analog visualizing the migration of the escarpment is provided by the eroding and avalanching edge of an oversteepened river embankment. The plateau of the Serrat, also referred to as the Asir Highlands in our study region, has itself a low relief, rarely exceeding 200 m. The plateau slopes gently northeastward and the drainage on the plateau area is through shallow, meandering wadis ŽFig. 30c.. Wadi Bishah, the largest and most important drainage system of the plateau, flows northeastward for hundreds of kilometers ŽFig. 28. to end up in the desert plains of the Najd region and Rub-al-Khali. The pattern of the flow tributaries is formed by N–S wadis which follow the main foliation of the bedrock,

and E–W and NW–SE flow tributaries follow the trend of the Najd faults. The modern surface of the Asir plateau approximately coincides with a peneplain of late Precam-

Fig. 32. Flood basalt succession of As Sarat.

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brian times. This can be inferred from the flat nonconformity between the Precambrian basement and erosional remnants of a once continuous Wajid sandstone ŽFig. 31a.. These outliers of the Lower Paleozoic Wajid formation cover several tens of square kilometers near Jebel Sawdah and similarly crop out near the escarpment of Halaban. The Wajid, preserved in outliers up to 100 m thick, consists of a basal conglomerate of a few meters thick, followed by cross-bedded, red sandstones ŽFig. 31b and c.. The sequence displays the typical facies of a braided river system, which deposited sediments onto a continental plain. The infracambrian continental margin is likely to have been located far away from the area under study, possibly near the location of the modern margin of the Arabian Shield with the Indian Ocean. The entire Phanerozoic cover sequence, which once must have overlain the Wajid sandstone, had largely disappeared during Tertiary times. This can be concluded unequivocally, because of the emplacement of extensive flood basalts directly onto Precambrian basement, now exposed in the As Sarat mountains, as early as 29 Ma ago ŽOverstreet et al., 1977.. The exposed basalt sequence in the As Sarat ŽFig. 32. must have been much more extensive itself in the past, but erosion has denudated the Asir Highlands since their thermal uplift began about 30 Ma ago. The upper section of the Precambrian crust may also have been removed during the formation of the Infracambrian peneplain on which the Cambrian– Ordovician Wajid sandstone was laid down. Because the same peneplain level is approximately exposed at present, what is seen at the surface is a shallow section of uplifted and eroded Precambrian crust.

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