Tectonics of the Najd Transcurrent Fault System, Saudi Arabia

J. geol. Soc. London, Vol. 136, 1979, pp. 441-454, 6 figs. Printed in Northern Ireland.

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 secondarystructures 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 periodicallydeveloped 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. metamorphismhas 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 theshield. 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 majorstructures 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 thefaulting. 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 parallel tural level in the Southern Najd was that of ductile to but S of the outcropping Najd System faults indicate a semi-brittledeformation in theearlier stages of the total width for the tectonic zone of 400 km. Interpre- faults’history and of brittledeformation during the tation of Andreason & Petty’s maps gives a composite later events. The absence of widespread metamorph- impression of fault geometry to some depth below the ism during the brittle deformations indicates that the present surface. It is possible that magnetic lineaments current erosion surfacewas at a relatively high structural which do not correspond to mapped faults may be an level at the time and that the older ductile structures expression of shear zones whose movement has been had been brought several km towards surface before accommodated,at the structural levels currently ex- the onset of brittle fracturing. The shallow origin of posed, by displacementson older structures in the the later, brittle structures is confirmed by thepres- Hijaztectonic fabric. The width of thefault zone, ence of Jibalah Group sediments in fault-controlled, indicated by theaeromagnetic maps, suggests that grabenbasins in thebraided zone, similar to those structures throughout the shield, in addition to those described by Kingma (1958) in the Alpine Fault Zone within the belt of outcropping faults, could have been (New Zealand). It appears that perhaps 1 km or less of affected by the Najd deformation. This would account superstructure has been removed from the Najd Belt forthe widespread re-activation of Hijazfaults re- since the beginning of Phanerozoic times.

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40° 420 440 460 440 MO 420 3'eO 40° 3t NAJO FAULT SYSTEM Faults rhown on goological mapo -----___ Linoamentr from ratellito imagery

220 \

KEY

FIG. 1. Outcrop traces of major faults of the Najd Fault System in the Arabian Shield. Inset: Location map showing the Najd Belt prior to Tertiary rifting in the Red Sea region. (Sources: D.G.M.R. published 1 : 100 000, 1 :250 000 geological maps, air photo mosaics and satellite imagery).

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36O 3bO 440 40° 420 460

AEROMAGNETIC LINEAMENTS - NAJD FAULT SYSTEM

KEY

sedimentary & Meta-volcanic rocks)

Scale

16O

__ 440 4p FIG. 2. Aeromagnetic lineaments attributable to structures in the Najd Fault System. Interpreted from disturbance in the magnetic fabric on 1 :500 000 scale maps (Andreason & Petty 1974 a-f).

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Most of the major faults, seen in plan, have similar with granular cataclastic texture. Shear zones and slip geometry.Each has asinusoidally curved outcrop surfaces range in thickness from a few cm to several trace with NW-SE strike direction in the central part. hundredm, depending on the tectonic style of the Theterminal sections of tracesshow asystematic fault and nature of the host rocks. tendencyto change strike direction towards NNW- There are 2 types of minor structures: those which SSE. Many of the major structures terminate in arrays pre-date or are independentof major faults, and those of curved splays, some of which link separate strands. which are directly related to master structures. Among The total system consists of separate structures, some the most important in the first category are arrays of of which areinter-connected. Numerous uplifted, discontinuous,en echelon shears which occuralong down-faulted and tilted blocks occur in various parts theprojected lines of certainmajor faults. These of the belt, illustrating the vertical movements which structures,known as Riedel shears and conjugate accompaniedthe strike-slip motion onthe master Riedel shears (Tchalenko & Ambraseys 1970) are sets faults. of subordinate wrench faults which form in a zone of Several typesof folds occur in the fault zone, includ- simple shear not marked by a continuousfault (Fig. ing setswith axial traces parallel, transverse and en 6~). echelonto the major faults. Open to close folds The Riedel shears are complementary sets of faults, (Fleuty1964), with sigmoidally curved axial traces, one of which-termed the synthetic set-is oriented at occur in rock septa between major faults (Fig. 3~,B). anacute angle (commonly about 15") tothe main In some cases these may be Hijaz folds which have shear direction, and has the same sense of movement. been re-activated and 'tightened' (Fig. 3~).In others The conjugate Riedel shears, termed the antithetic set, the orientations of the axial traces indicate that they have the opposite senseof movement and are oriented couldbe the products of secondarydeformations at 75-90' to the main shear direction. The synthetic whichcaused thrusting nearby (Fig. 3~).A separate set is usually much better developed and widespread. and perhaps younger set of flexural folds occurs in the Continued movement often causes dislocation of the JibalahGroup strata in fault-boundedgrabens and antithetic shears while the synthetic shears increase in basins. The axial traces of these folds are parallel to length to accommodate displacement until superseded the major faults or en echelon at a very acute angle. by establishment of a master fault parallel to the main These folds appear to be the products of local secon- shear direction. darystresses generated during movement between Inthe Najd Belt it is commonto find onlythe lenticular blocks in the braided zone. Kingma (1958) synthetic Riedel shear set, with sinistral sense of dis- demonstratedthat dip and strike variations along placement,oriented at an acute angle to the master curved fault surfaces can cause significant local varia- faultdirection (master fault strike 140", sinistral tions in compressivestress conditions during move- Riedelset strike 110-130"). Thedextral antithetic ments in a wrench fault system. shears (strike 000-050°) are rarer, shorter than their The Najd Belt is dominated by faults striking NW- complements, and take the form of minor cross-faults SE.Major dextral faults with NE-SW strike which or re-activated N-S Hijaz structures. would be the theoretical complements to the main belt Geophysical evidence locally indicates the presence arerare. It appears that the Najd fault belt is the of continuousstructures below outcropping assemb- product of simple shear which allowed the Nubian and lages of Riedel shears. It appears that in some cases S Arabianshield to move several hundred km SE theminor shears formed at higher structural levels with respect to N Arabia. The ends of many of the above continuous structures (Fig. 3~).In other cases world's currently active transcurrent fault systems ter- Riedel shear formation was a precursor to establish- minate in transform faults and at ridges and trenches ment of a continuous major fault. definingplate boundaries. Unfortunately, neither of Sets of pinnate minor faults adjacent to oneor both the original endsof the Najd Belt is still visible, and its sides of a majorstructure are common in sheeted significance in terms of Proterozoicand Eocambrian complexes.In the Juqjuq sheeted zone (Fig. 3c), globaltectonics remains enigmatic. The Najd move- movement on the master faults has truncated sets of ments, like those on the Great Glen Fault in Scotland, secondaryfractures whichcould haveoriginated as were the final events in a complex orogenic history. Riedel shears. Dislocation of a major fault by its own secondary structures, followed by renewed movement Secondary structures and re-establishment of the continuous master fault is one of thefactors responsible for formation of the Susceptibility toweathering of the faults,associated sheeted or sub-parallel arrays of major faults.Many extensional fractures and dyke swarms make the Najd major faults in the NW Hijaz district have been dislo- one of the most clearly visible complexes of primary cated by related WNW trending secondary shears(Fig. and secondary strike-slip faults in the world. The fault 3B). rocks are commonly obscured by wadi sediments but Minorstructures of thesecond group, related to where visible they are schistose and fissile, commonly establishedmajor faults, are numerous and varied.

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Jibalah Group sediment

‘Ophiolite Belt ’

Master fault (strike-slip) +Secondary fault (strikeslip

W Thrust fault

Normal /obliqueslip fault 4- Antiform +Synform ...... Magnetic lineament

Au-Cu,or Pb-Ag veins

Scale bar : 10 kilometres

10 km C I

- J. McM.M. 1978

FIG.3. Simplified geological maps showing secondary structures in selected parts of the Najd Fault System. A, Faulting and folding in the Nuqra quadrangle. Fault displacement is shown byoffset of the ‘ophiolite belt’. (simplifiedfrom Delfour 1977). B, Faults and fold axial traces in the Wayban-Qal’at as Sawrah-Sahl a1 Matran district, NW Hijaz (simplified from Hadley 1973, 1974a, 1975). C, Second order faults in a pinnate array adjacent to sheeted master faults in the Bi’r Juqjuq quadrangle, S Najd (simplifiedfrom Hadley 1976). D, Primary and secondary faulting in the Ad Dawadami district (simplified from Moore & AI Shanti 1973).

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HiJaz {&A I Amat reverse fault

FIG. 4. A, Simplifiedgeological map showing faulting in the Idsas area, prepared from aerial photograph interpretation and from Eijkelboon (1966). B, Stresstrajectory interpretation of faultand syn-tectonic dyke systems. Solid lines: orientation of maximum principal compressive stress (ul);broken lines: minimum principal compressive stress (uJ.

Complicated assemblages of secondary fractures occur and Juqjuq show that while some secondary faults cut around the terminations of major fault strands (Figs. and displace their parent structure, elsewhere the ex- 3~,4~). These arrays are found irregularly throughout tension of a masterfault can pass through its own theNajd Belt but are most noticeable inmassive secondary fracture system. It is a consequence of such country rocks. Chinnery (1966) suggested that dissipa- a complexsequence of developmentthat parts of a tion of anomalousstress concentrations around the singlefault may have different histories as well as terminations of shears could be achieved by propaga- varied amounts of displacement. tion of the master fault along divergent splay faults, or Inaddition to strike-slip movement, minor thrust- bycreation of arrays of secondaryfractures. Both ing, oblique-slip and normal faulting have occurred in these mechanisms are thought to have operated on the some areas. Thrust faults and folds adjacent to terrain faults which traverse the Ad Dawadami district (Fig. containing normal faults can be seen in Fig. 3~.These 3~).The westernmost Ad Dawadami fault apparently unusual assemblages of minor structures define local onceterminated in the map area but subsequently areas of anomalous ‘compression’ or ‘dilation’ within extended NW along a splay trajectory which diverged the faultbelt. The compressionalregimes, in which from its earlierstrike direction. A complicatedas- thrusting and folding accompany second order wrench semblage of secondaryfractures occurs around the faulting, occur on the NE side of the NW terminations termination of themost easterly fault where sets of andon the SW side of SE terminations,and in the arcuate complementary shears can be seen. Secondary ‘overlap’ between en echelon major faults. The com- fracture systems of this type occur at irregular inter- pressional regimes are complemented by ‘dilation’ on vals along major fault lines. theopposite side of themaster fault, marked by The timerelationships observed at Ad Dawadami normal faulting and extension fissure formation. The

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dilational areas offer the most mechanically favourable Theneed for secondary stress dissipation around loci for dyke intrusion during the faulting events (syn- active faults in the Najd Belt was particularly strong in tectonically) and, subsequently, for hydrothermal vein parts of thebraided zone where fault surfaces were emplacement.Somewhat similar phenomena as- non-planar. It was achieved by extension in length of sociated with faultingat Owens Valley (USA) have themain fault along a splay fracture, creation of a been described by Pakiser (1960). secondaryfracture complex, re-activation of older Thejuxtaposition of compressionaland dilational structures, or by vertical movement or tilting of fault conditions on opposite sides of a major fault means blocks in the braided zone. thatnormal faulting and dyke intrusion could take place adjacent to part of afault plane while, simultaneously,thrusting and folding could occur on the Fault mechanics opposite side. The NW Hijaz area contains excellent examples of the tectonic phenomena (Fig. 3~)and the The mechanistic terminology used in this paper is from Idsas district illustrates dyke intrusion adjacent to fault Wilcox et al. (1973) andLajtai (1969). Thelatter terminations (Fig. 4~).Time relationships between states 'Primary or first order state of stress refers to dyke intrusion and secondary faulting in the AI Amar the condition under which faultingis initiated. It is the district(Fig. 3~)were investigated and described by regional stress field in nature and the applied stress in Moore & AI Shanti (1973). Detailed study in that area the laboratory. Second order state of stress refers to demonstrated that intrusion of dykes associated with the re-adjusted stress field which develops during and the secondary fault system took place while adjacent afterfaulting and results in the formation of second faults were subjected to tectonic stresses. The config- orderfractures.' Chinnery (1966) consideredstress uration of manydykes in theIdsas district (dyke conditionsafter movement on amaster fault, but swarm 2, Fig. 4~)indicated that they too may have experimental evidence shows that many arrays of en beenintruded into extensional and shear fractures echelon minor structures (e.g. Riedel shears) can form duringthe tectonic events. Elsewhere (e.g. dyke earlierthan the master fault (Wilcoxet al. 1973; swarm 1, Fig. 4~),the distribution of dykes is not Tanner 1962). Itappears from fieldevidence that consistentwith the Najd stress field. These dykes some of theNajd secondary minor structures are cannot be explained in terms of syntectonic intrusion explicable in terms of theconditions proposed by into extensional fissures in the u1-a2 principal plane. Chinnerybut others conform to those discussed by The alternative explanations for this discrepancy are: Lajtai.Lajtai stated that fractures in anen echelon thatswarm 1 consists of dykesemplaced before or shear array are the product of anomalous conditions after the Najd events, or that they may be syntectonic which do not bear a direct relationship to the regional intrusions into older joint fractures. The second condi- stress field. Chinnery's models and the interpretation tion could result from magma fluid pressure finding it of field evidence presented below, support this. mechanically easier to dilateexisting fissures normal to The outcrop traces of strike-slip faults, syn-tectonic u1 than to create new fractures normal to u3. dykesand extensional fissures, together with their Systems of secondary fractures can usually be attri- sense of displacement, can be used to construct dia- buted to a particular master fault. Although fractures grams showing the orientation of principal stress axes maylocally dislocate the major structure, renewed (a,-maximum, U,-mimimum) within the stress system movement on themaster fault generally restores its which relatedto the faulting. Stress trajectory dia- continuity.Secondary faulting in theNajd Belt is grams, drawn by means of second order shear outcrop composite of several generations of structures. Normal trace interpretation, give animpression of the stress and reverse faulting with associated folding as separate field around the master structure as it was during the eventsfrom secondary strike-slip movements can be secondaryfracturing. This construction, made on seen in Fig. 3~. geological maps, requires that the land surface should Differing amounts of displacement on various parts coincide approximately with a principal plane contain- of fault surfaces produce problems of accommodation, ingtwo of the principal stress axes. The best results particularly in terminal areas. Some stress dissipation are achieved when the construction is carried out on a is achieved by re-activation of olderstructures. The regional scale, for an area of subdued topography. The Idsas area provides a good example of this, where the requirements are met regionally in the Najd Belt but NW terminations of two Najd faults mergewith the A1 local thrusting, oblique-slip and normal faulting show AmarFault (AI Shanti & Mitchell 1976), anolder thatdepartures from plane stress conditions are not structurewhich has been offset by Najdmovements uncommon. (Fig. 4~).The AI AmarFault was re-activated as a Regional scale stress trajectory construction shows high angle reverse fault to accommodate stresses simi- thatthe stress fieldcontaining the main faults was lar to those which generated splay and other secon- relativelyregular with the maximum principal com- daryfractures at the SE terminations of theNajd pressive stress (al)oriented approximately E-W. The structures. main fault zone is oriented at 35-45" to the maximum

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major structure were a local disturbance. It is important to appreciate that trajectory constructions represent very short periods of time during which perhaps only part of the composite fault system formed. Fig. 5 showsthe trajectories of secondary principalcompressive stresses constructed from geological maps of the Juqjuq and Ad Dawadami areas. These show stress axis orientation interpreted from particular groups of secondary fractures. In the Juqjuq district, majorfaults, coinciding with theboundaries of the sheetedcomplex, mark discontinuities in the pattern of stresstrajectories. The secondary fracture system appears to be the truncated remains of Riedel shears (Fig. 64) which have been cut by the sheeted master faults. The stress trajectory construction illustrates the pattern of stressas it was during secondary fault formation and movement. In some cases, e.g. Juqjuq (Fig. 5~)thetrajectory construction illustrates the dislocatedremains of the probablycontinuous stress field within which Riedel shear arrays formed prior to truncation by establishment of the sub-parallel master faults. The sub-parallel master faults apparently continued to move in response to the regional stress field oriented E-W (approximately 40” to their strike direction). The AdDawadami trajectories (Fig. 5~)arean interpretation of the secondary fracture array around FIG. 5. Stresstrajectory diagrams showing the the termination of the most easterly fault in the area; orientation of principal stresses deduced from the but detailed maps reveal another, older and less well faulting maps in Figs. 3c, D. developed fracture system adjacent to the most south- A, Juqjuq; B, AdDawadami. Solid lines: trajec- erly of the 3 major faults (Moore & AI Shanti 1973). tories of maximum (al);broken lines: trajectories Fig. alsoshows the localizednature of the stress of minimum principal compressive stress 513 (a3). field disturbancearound the termination of a major fault. Ad Dawadami is perhaps the most informative principalcompressive stress axis and the systemap- second order fracture complex in the shield because of pears to have been subjected to stress approaching the itsmechanically isotropic host rocks and the precise uniaxial compressive condition. time relationships which can be interpreted from study Thestress configuration around the Najd faults in of structuresrelated to the 3 majorfaults which the Idsas district is shown in Fig. 413.Note the asym- traverse the area. metricaldisturbance of stressaxis orientations adjacent to the SE fault terminations. The array of splays Discussion and other minor faults adjacent to the Najd structures has been intruded by a dyke swarm. The minor fault Interpretation of aeromagnetic maps indicates that the arrays are different on each side of the master struc- NajdFault Belt is wider atdepth than shown on turesbut their geometry compares closelywith that geological maps, and that the major structures below predicted on theoretical grounds (Chinnery 1966). surface are better defined than the complex arrays of The stresstrajectory diagrams (Figs. 4B, 5) have faults seen in outcrop. The width of the sub-surface been constructed by interpretation of secondary fault faultzone suggests that the Najd movements could geometryand represent conditions during second havebeen responsible For there-activation of older order faulting.Continuing fault development has in structures noted in many areas of the shield. somecases subsequently dislocated secondary faults, The tectonic style of Najd primary and secondary and it has only been possible to make a partial recon- structures is similar tothat produced experimentally struction of the secondary stress field(Fig. 5~).The by Wilcox et al. (1973) and Tanner (1962) in a brittle relationship between the regional stress field and that andsemi-brittle ‘cover’overlying a ‘basement’ com- relatedto secondary faulting is ambiguous in such prising rigid blocks which can move laterally relative cases,but elsewhere (Fig. 513) there appears to have to each other. The well-definedlinear structures re- been a regularregional stress field withinwhich vealedby the aeromagnetic maps may be the upper secondaryconditions around the termination of a parts of the ‘basement’ fracture system.

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FIG. 6. Generalized models (in plan). A, The geometry of Riedel shears in relation to the shear direction which created them. S, synthetic; A, antithetic conjugate Riedel shears. B, Generalizedplan showing areas of convergence(anomalous compression (c)),characterized by thrusting, oblique-slip, secondary strike-slip faulting and folding; and divergence betweenthe master faults, (dilation/extension (D)) characterized by normal faulting and dyke intrusion, in relation to major faults. C, Trajectories representing the orientation of maximum and minimum principal compressive stresses around the termination of a fault, after movement has occurred. Approximately one eighth of total fault length is shown (After Chinnery 1966, based on models representing a fault under uniaxial compressive stress in plane stress condition). D, Secondary structures associated with major faults in the Najd System. c: compressional minor structures; D: dilation or extensional minor structures; R: synthetic Riedel shears. Keyas Fig. 3.

Experimentsindicate that major faults form in structure and tilted blocks in the manner discussed by stagesby shear crack propagation. In some cases Lensen(1958) and Emmons (1969). These vertical deformationbegan with establishment of Riedel movements in thebraided zone have created the shears (Fig. 6~)which subsequently merged into con- basins in which Jibalah Group clastic sediments and tinuous faults. Movement on a shear zone containing volcanics were deposited. an array of Riedel shears changes the dihedral angle There are 2 types of minor structures in the Najd between the conjugate sets and modifies the angular Belt. The first type formed independently of the major relationships between the minor structures and major faults, e.g. Riedelshears. The second are results of sheardirection. In theirexperiments, Wilcox et al. disturbance of theorientation of theregional stress (1973) recorded an anticlockwise rotation of 3 and 15" field andthe value of lithostatic confining stress for the synthetic and antithetic Riedel shears respec- by strike-slip movement on established master struc- tively, in the course of one experiment on a sinistral tures. Arrays of second order faults characterize areas shear zone. Dextral displacements, including reactiva- inwhich thestress acting normal to the major fault tion of faultswith NW-NE strikedirections can surface (a,)is unusually low for the fault system as a therefore be explained in terms of the stresses which whole(Lajtai 1969). Variations in strikedirection cause movement on rotated antithetic Riedel shears. among strike-slip faults can create these conditions. Strike-slipon intersecting faults with differing dip Experimental evidence shows that variations in the angles and curved shears has created horst and graben angle between the regional shear direction and major

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faultstrike (known as the angle of convergenceor approximatelyparallel to the main shear direc- divergence, Wilcox et al. 1973), causes significant vari- tion. The main faults may cut and dislocate both ations in the style of secondary structures. Structures sets of Riedel shears. characteristic of convergenceor divergence are best 4. More arrays of secondary faults develop around developed adjacent to the arcuate terminal sections of theterminations and in tectonicslices between Najdfaults. Curvature on some faults is suchas to major faults. create angles of convergence of as much as 30" relative 5. Continuedpropagation of themajor faults to the shear direction for the belt as a whole. Com- creates the braided zone in which vertical move- pressional minor structures characterise areas of con- ments produce tilting and horst and graben struc- vergence, and extensional/dilational phenomena those ture among the lenticular fault blocks. of divergence (c and D in Fig. 6~). 6. Repetition of 1-5, and re-establishment of mas- Commonly, there are differences between the orien- ter faults earlier dislocated by their own secon- tation of the secondary stressfield and that responsible dary faulting, creating sheeted zones. formovement on the major faults. Secondary stress fields affect the orientation of regional principal stress TheNajd Belt displays many of thephenomena axes for some distance from the major structure, pro- commonto major strike-slip fault systems in many ducingminor structures in areas of severaltens of parts of the world, e.g. the Transverse Ranges in the km'. SanAndreas Fault System. The Arabian Shield is Fig. 6c showsthe asymmetrical configuration of unique only in the clarity with which the plan view of principalstresses adjacent to the termination of a thefaults and associated fractures can be observed. This paper is a summary of preliminary compilations fault,based on theoreticalconsiderations (Chinnery 1966). It demonstrates how different patterns of sec- but much more information remains to be studied, and ondary fractures can form on opposite sidesof a major new data is accumulatingas D.G.M.R. sponsored fault. Fig. 6~ is a summary in diagrammatic form of mapping programmes progress. the minor structures associated with the Najd Zone. Interpretation of field observations and the model ACKNOWLEDGMENTS.The Directorate General of Mineral experimentsreferred to above, indicate that the se- Resources of the Kingdom of Saudi Arabia is acknowledged quence of development of secondary structures in the forkindly making available published maps and open-file Najd Belt may have been as follows: reports of the D.G.M.R., U.S.G.S. and B.R.G.M. missions, together with aerial photograph mosaics. Compilation of Fig. 1. Riedel shear formation dominated by the synthe- 1 would not have been possible without the maps of numer- tic set. This probably occurred simultaneously on ous D.G.M.R., U.S.G.S. and B.R.G.M.geologists, whom I several incipient shear zones in various parts of gratefully acknowledge collectively. I thank Drs Don Hadley the proto-Najd belt, accompanied by initiation of and Ralph Roberts for their assistance in supplying many of foldin;: and local extension fissure formation. the maps from which several of my diagrams were compiled 2. Rotation of the Riedel shear sets and fold axial and for their helpful comments; colleagues from the Institute for Applied Geology, Jedda for their help in arranging the traces,accompanied by dislocation of thean- fieldwork which complemented this study; and MS R. Ur- titheticconjugate Riedel shears. The traces of quhart and Dr G. Thomas, Imperial College, for their assis- antithetic shears may become sigmoidally curved tance in getting the paper to press. by angular rotations of up to 15". Note: A complete list of the maps published by D.G.M.R. 3. Establishmentand propagation of majorfaults from which Fig. 1 was compiled is available on request. Ref e1mences

AL SHANTI,A. M. S. & MITCHELL,A. G. H. 1976. Precam- Map GM-11, Directorate General of Mineral Resources, briansubduction and platecollision in the AIAmar- Jiddah. Idsas region, Arabian Shield, Kingdom of Saudi Arabia. -&- 1974d. Total intensity aeromagnetic map of the Tectonophysics, 31, T41-7. southernHijaz quadrangle, Kingdom of SaudiArabia. ANDREASON,G. E. & PE~,A.J. 1974a. Total intensity Geologic Map GM-12, Directorate General of Mineral aeromagnetic map of the northern Hijaz quadrangle and Resources, Jiddah. part of theWadi as Sirhan quadrangle, Kingdom of ~ & -1974e. Total intensity aeromagnetic map of the Saudi Arabia, Geologic Map GM-9, Directorate General southernNajd quadrangle and part of the southern of Mineral Resources, Jiddah. Tuwayq quadrangle, Kingdom of Saudi Arabia. Geologic -& - 1974b. Total intensity aeromagnetic map of the Map GM-13, Directorate General of Mineral Resources, northern Hijaz quadrangle,Kingdom of Saudi Arabia. Jiddah. Geologic Map GM-10, Directorate General of Mineral -&- 1974f. Total intensity aeromagnetic map of the Resources, Jiddah. Tihamatash Shan quadrangle and part of theAsir -&- 1974c. Total intensity aeromagnetic map of the quadrangle,Kingdom of Saudi Arabia. GeologicMap Wadi ar Rimahquadrangle and part of the northern GM-14, Directorate General of MineralResources, Tuwayq quadrangle, Kingdom of Saudi Arabia. Geologic Jiddah.

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BROWN,G. F. 1972. Tectonic map of the Arabian Peninsula. - 1976. Geology of the Bi'r Juqjuq quadrangle,(sheet Map AP-2, Directorate General of Mineral Resources, 21/43D), Kingdom of Saudi Arabia. Geologic Map GM- Jiddah. 26, Directorate General of Mineral Resources, Jiddah. - & COLEMAN,R. G. 1972. Tectonicframework of the KINGMA,J. T. 1958. Possible origin of piercement structures, Arabian Peninsula.24th Int. geol. Congr. Montreal, 3, local unconformities and secondary basins in the Eastern 300-5. Geosyncline, New Zealand. N.Z. J. Geol.Geophys. 1, -& JACKSON,R. 0. 1960. The Arabian Shield. 2lst hr. 269-74. geol. Congr. Copenhagen, 9, 69-77. LAJTAI, E. Z. 1969. Mechanics of second order faults and CHINNERY,M. A. 1966. Secondaryfaulting: I, Theoretical tension gashes. Bull. geol. Soc. Am. 80, 2253-72. aspects. Can. J. Earth Sci. 3, 163-74. LENSEN,G. J. 1958. A method of graben and horst forma- DELFOUR,J. 1977. Geology of the Nuqra quadrangle 35E, tion. J. Geol. Chicago. 66, 579-87. Kingdom of Saudi Arabia. Geologic Map GM-28 (scale MENARD, H. W.1962. Correlation between length and 1 :250 000), Directorate General of Mineral Resources, offsc~on very large wrench faults. J. Geophys. Res. 67, Jiddah. 409h-8. EIJKELBOOM,G. 1966. Geology of the Idsas-Wadi Jifr reg- MOORE,J. McM. & AL SHANTI,A. M. S. 1973. The useof ion. B.R.G.M. open-file rept. 66 Jed A15, Directorate stress trajectory analysis in the elucidation of part of the General of Mineral Resources, Jiddah. NajdFault System, Saudi Arabia. Proc. Geol. Assoc. EMMONS,R. C. 1969. Strike-slip rupture patterns insand London, 84, 383-403. models. Tectonophysics, 7, 71-87. - & - in press. Structureand mineralization in the FLECK,R. J.,COLEMAN, R. G., CORNWALL,H. R.,GREEN- NajdFault System, Saudi Arabia. In: Eoolution and WOOD, W. R., HADLEY,D. G., PRINZ, W. C., RATE, V. Mineralization of the Arabian-Nubian Shield, 2. Perga- C. & SCHMIDT,D. L. 1976. Geochronology of the Ara- mon, Oxford, 17-28. bian Shield, Western Saudi Arabia: K-Ar results. Bull. PAKISER, L. C. 1960. Transcurrent faulting and volcanism in geol. Soc. Am. 87, 9-21. Owens Valley, California. Bull. geol. Soc. Am. 71, 153- FLEUTY,M. J. 1964. The description of folds. Proc. Geol. 60. Assoc. London, 75, 461-92. ~HULPS, W. J. 1972. Hydraulic fracturing and mineraliza- HADLEY,D. G. 1973. Geology of the Sahl al Matran quad- tion. J. geol. Soc. London, 128, 337-61. rangle, (sheet 26/38C) Northwestern Hijaz, Kingdom of SCHMIDT,D. L., HADLEY, D. G., GREENWOOD,W. R., GON- Saudi Arabia. Geologic Map GM-6, Directorate General ZALEZ, L., COLEMAN,R. G. & BROWN,G. F. 1973. of Mineral Resources, Jiddah. Stratigraphy and tectonism of the southern part of the -1974a. Geologic Map of the Wayban quadrangle, (sheet Precambrianshield of Saudi Arabia. Miner.Resources 25/38b), Kingdom of Saudi Arabia. Geologic Map GM- Bull. 8, Directorate General of MineralResources, 7, Directorate General of Mineral Resources, Jiddah. Jiddah. - 19746. The taphrogeosynclinal Jubaylah Group in the TANNER,W. F. 1962. Surfacestructural patterns obtained Mashad area, northwesternHijaz, Kingdom of Saudi from strike-slip models. J. Geol. Chicago. 70, 101-7. Arabia. Mitrer. Resources Bull. 10, Directorate General TCHALENKO,J. S. & AMBRASEYS,N. N. 1970. Structural of Mineral Resources, Jiddah, 18pp. analysis of the Dasht-e-Bayaz(Iran) earthquake frac- - 1975. Geology of theQal'at as Sawrah quadrangle, tures. Bull. geol. Soc. Am. 81, 41-60. (sheet 26/38D), Kingdomof Saudi Arabia. Geologic WILCOX, R. E., HARDING,T. P. & SEELY,D. R. 1973. Basic Map GM-24, Directorate General of Mineral Resources, wrenchtectonics. Bull. Am.Assoc. Petrol. Geol. 57, Jiddah. 74-96.

Received 29 September 1978; read 7 February 1979; revised typescript received 15 March 1979. J. MCMAHONMOORE, Mining Geology Section, Department of Geology, Royal School of Mines, Imperial College, London SW7 2BP.

Discussion

The PRESIDENT (Professor P. Allen) asked Mr. Moore aeromagnetic maps indicates that the outcropping if he had been able to determine the form of the faults are underlain by more continuous structures up master faults in the vertical dimension. Could they to several hundreds of km in length. Extensional struc- have extended further (e.g. at a higher level) than seen tures arevery common at high structural levels but at the present depth of erosion? If this were so, might give waydownward to shear fractures. Vertical it provide an alternative explanation of the structures geometry around the terminations of major faults is now seen at the termination of the master faults? complex, and many arrays of secondary faults are not In reply, the AUTHORstated that: The subdued vertical and indicate that principal axes in the secon- topography of the outcropping Najd Fault System dary stress field were commonly not oriented makes direct observation of fault geometry in the orthogonally with respect to the masterfault or the vertical dimension very difficult. Interpretation of present land surface.

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