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Tectonics of the Najd Transcurrent Fault System, Saudi Arabia

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J. geol. Soc. London, Vol. 136, 1979, pp. 441-454, 6 figs. Printed in Northern Ireland. Tectonics of the Najd Transcurrent Fault System, Saudi Arabia John McMahon Moore SUMMARY: The Najd Belt is a major transcurrent (strike-slip) fault system of Proterozoic age in the Arabian Shield. The belt is a braided complex of parallel and curved, en echelon faults. Complexarrays of secondary structures including strike-slip, oblique-slip,thrust and normal faults, together withfolds and dyke swarms are associated with some major faults, particularly near their terminations. The secondary structures indicate that compressional and extensional/dilational conditions existed synchronouslyin different parts of the fault zone. The outcroptraces of faults andsyn-tectonic dykes are used to interpret the configuration of principalcompressive stresses during formation of parts of thesecondary fracture systems. Second order deformatio-n wasa series of separate events in a complex episodic faulting history. Comparison with model studies indicates that master faults extended in length in stages and periodically developed arrays of secondary structures. Propagation of the major faults took place along splay trajectories which inter-connected to form a sub-parallel sheeted and braided zone. Interpretation of the aeromagnetic maps indicates that the Najd Belt is broader at depth than the outcropping fault complex, and that more continuous structures underlie arrays of faults at surface. The fault pattern is mechanically explicable in terms of simple shear between rigid blocks beneath the exposed structures. TheArabian Shield is a complex of Proterozoic annual rate of movement of 0.5 cm (Fleck et al. 1976). plutonic,meta-volcanic and meta-sedimentary rocks This is comparablewith the displacement rates of which was produced by multiple episodes of sedimen- severalcurrently active major strike-slip faults. tation, volcanism and intrusive activity, accompanied Brown’s estimate of displacement in the central parts by deformationknown as the Hijaz Orogenic Cycle of the belt is based on correlation of displaced strings (Brown & Coleman 1972). The Hijaz tectonic fabric of basic andultra-basic intrusions (‘ophiolitebelts’), of theshield has a predominantly N-S or NE-SW whose strike directions are parallel to the N-S Hijaz trend which is visible in aeromagnetic maps. Regional tectonic fabric. metamorphism has produced mineral assemblages of Igneous intrusion associated with the Najd tectonics greenschist and amphibolite facies in various parts of hasproduced small plutons and dyke swarms. thevolcano-sedimentary complex. The Hijaz events Radiometric ages obtained from small intrusions indi- culminated in dislocation of the complex by strike-slip cate that the faults were active from late Proterozoic faultingin late Proterozoic and early Phanerozoic to early Phanerozoic times (580-530 Ma) (Fleck et al. times-designated the Najd Faulting (Brown & Jack- 1976).Some of thealkaline and calc-alkaline intru- son1960). sionsand basalt-andesite-rhyolite dykeswarms have TheNajd system of transcurrentstrike-slip faults beendislocated by continuedfault movement after and related secondary structures traverses the shield emplacement. Lavas are intercalated among theclastic NW-SE, displacing the Hijaz metamorphic and igne- sediments of theJibalah (Jubaylah) Group in fault- ous rocks.Najd deformation was mainly brittle but bounded grabens (Hadley 1974b). there isa penetrativetectonic fabric parallel to the Hydrothermal activity was widespread and small ore faultzone in the SE of the shield. The outcropping deposits(mineralized quartz veins) occur in some Najdfault belt is c. 300 kmin width andextends areas (Moore & A1 Shanti, in press). The widespread 1100 km inland from the Red Sea coast, where it was hydrothermaland igneous activityindicate thatthe truncated and locally re-activated by the Tertiary Red faultzone was an area of anomalously high heat Sea rifts. TheNW extensions are probably in the transfer in Proterozoic/Eocambrian, and possibly dur- EasternDesert of Egypt.Block movement in the ing Phanerozoic times. The hydrothermal alteration is basement along the line of the Najd has affected the probablyalso areflection of themechanical impor- Phanerozoiccover strata for more than 100 km SE tance of fluid pressure in the mechanism of faulting at from the edge of the shield. Extrapolated SE, the line this structural level (Phillips 1972). of faulting coincides with structures in the S Yemen This paper is a preliminary review of the geometry coast and in the bed of the Arabian Sea (Brown 1972). of major structures in theNajd Belt and includes a Thismakes a totalpossible length of morethan description of secondorder and minor structures in 2000 km, dimensions similar to those of many of the selected parts of the system. There is also a discussion world’s major transcurrent fault systems, including the of timerelationships, mechanisms of formation, and San Andreas (USA) and Alpine Fault (New Zealand). mechanical associations between structures of the first The estimated displacement of 240 km (Brown 1972) and second order and syn-tectonic dyke swarms. which accumulated during 50 Ma, corresponds to an 0016-7649/79/0700-0441$02.00 @ 1979 The Geological Society 4 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/4/441/4885826/gsjgs.136.4.0441.pdf by guest on 30 September 2021 442 John McMahon Moore The major faults portedin many parts of theshield (Schmidt et al. 1973). Unfortunately, aeromagnetic data is available TheNajd Fault System consists of paralleland en forthe shieldarea only andno interpretation of echelon master faults, the largest of which are more basementstructure beneath the areas overlain by than300 km long. Many of thefaults have curved Phanerozoic cover has been possible in this study. outcrop traces and intersect or join to form braided Deviatoricstress has been accommodated in- zones. homogeneously in the fault zone by simple shear on The major fractures are susceptible to weathering, established fault surfaces and by brittle failure as new formingwadi valleys which show clearly in aerial shearand extensional fractures formed. The larger photographs and satellite images. Fig. 1 is a compila- faultshave strike-slip displacements of tensof km, tion map prepared at 1: 2 000 000 scale by interpre- generallywith a sinistral sense. Themost important tation of satelliteimagery (ERTS and LANDSAT) areseveral hundred km long but the majority are and aerial photograph mosaics (scale 1 : 100 000). Ad- 50 km or less in length with displacements of a few ditional information was provided by geological maps km. Maximum offset is generally in the central part of published by the Directorate General of Mineral Re- afault trace and decreases laterally to zero at the sources (GM and M1 series). Unfortunately, published terminations.In the Nuqra area there is adisplace- maps (scale 1:lOO 000) cover only a small part of the ment of at least 40 km across a belt 60-70 km wide fault zone, but provide useful information for detailed (Delfour1977). Individual faults at Nuqra have dis- studies of selected second order fault systems. Thefirst placements up to 25 km. of a new series of 1: 250 000 maps (Delfour 1977) is Menard (1962) suggested that there may be a rela- betterfor regional interpretation. Much additional tionship between total length and offset displacement data is contained in numerousunpublished open-file on large wrench faults. Although this is not valid for reports of theDirectorate General of MineralRe- major structures which terminate in transform faults, sourcesprepared by theU.S.G.S and B.R.G.M. individual fractures in the Najd Belt whose termina- geological mission teams. tions are known, appear to have maximum displace- Althoughincomplete, the compilation map illus- ments (in the central part of the fault trace) propor- trates the general form of the faulting. It shows the tional to outcrop length. belt to be composed of many separate strands, without Parallelism between the regional penetrative fabric a unifying master structure. The outcropping zone is (schistosity and lithological banding) and major faults c. 300 km wide andis dominated by several very impor- in thesouthern Najd (Hadley 1976), indicates that tant faults. part of the fault zone developed a penetrative schistos- Linear structures parallel to the fault zone (Fig. 2) ity prior to the widespread brittle deformations. The are visibleon aeromagnetic maps as disturbances in earlyductile deformation caused transposition of the N-S or NE-SW‘Hijaz’ magnetic fabric of the lithologicalbanding to an orientation sub-parallel to shield (Andreason & Petty 1974a-f). These magneti- the major faults, together with widespread boudinage. cally defined lineaments commonly underlie areas with Thisdeformation has left many minor fold hinges disjointedand complex outcropping arrays of faults isolated in the transposition fabric. The ductile defor- and are commonly seen on 1 : 100 000 and 1: 250 000 mationmust have occurred at depths of several km scalemaps; the Nuqra area shown in Fig. 3~ is a and was accompanied by greenschistfacies regional typicalexample. Many of themagnetic lineaments metamorphism. Field evidence shows that the ductile defined by disturbances in the Hijaz tectonic/magnetic deformation was followed by brittle failure during the fabric coincide with major outcropping faults but the main faulting episodes. The currently exposed struc- presence of magnetically-defined dislocations
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