Structural History of the Southwestern Corner of the Kaapvaal Craton and the Adjacent Namaqua Realm: New Observations and a Reappraisal

Home , Boegoeberg Dam, Cape Province, Griekwastad, Griqualand West, Kaapvaal Craton, Kakamas

Precambrian Research, 52 ( 1991 ) 133-166 133 Elsevier Science Publishers B.V., Amsterdam

Structural history of the southwestern corner of the Kaapvaal Craton and the adjacent Namaqua realm: new observations and a reappraisal

Wladyslaw Altermann~ and Ingo W. H/ilbich Department of Geology, University of Stellenbosch, Stellenbosch 7600, Republic of South Africa (Received September 27, 1989; revised and accepted January 28, 1991 )

ABSTRACT

Altermann, W. and Hiilbich, I.W., 1991. Structural history of the southwestern corner of the Kaapvaal Craton and the adjacent Namaqua realm: new observations and a reappraisal. Precambrian Res., 52:133-166.

The rocks along the southwestern margin of the Kaapvaal Craton were deformed up to 7 times during the Early to Middle Proterozoic. The oldest deformation D, is recorded in the N-S-trending Uitkoms cataclasite of pre-Makganyene age ( > 2.24 Ga) on the craton, and interpreted as a bedding-parallel thrust. It is assumed to be a branch rising towards the surface from a blind sole thrust that initiated early N-S-trending F,-folds above it. D2 is represented by mainly N-S but also NE-SW and NW-SE-trending imbricates and recumbent fold zones ranging in size from small gravity slumps to large tectonic decollements in Asbesheuwel BIF and the Koegas Subgroup, and is younger than D1 or equals DI in age. These structures pre-date the Westerberg dyke-sheet intrusion. D 3 south-verging folds and thrusts are the oldest post-Matsap deformations, just less than 2.07-1.88 Ga. D4 are upright to east vergent and N-S-trending folds deforming all previous structures. D4 post-dates the Westerberg dyke-sheet and probably reactivates N-S folds above the earlier sole thrust during renewed E-W compression. Ds, producing the main NW-trending Namaqua structures, is only very feebly developed in the Kheis terrain and absent from the cratonic areas overlain by Olifantshoek and older strata, i.e. NE, E and SE of the Marydale High. Very gentle D6 E-W to ENE-WSW folds produce culminations and depressions in all NW-trending older structures. During D7 the NW-SE-trending Doornberg Lineament, an oblique left-lateral wrench, and smaller N-trending faults such as the Westerberg Fault developed. These and similar, but right-lateral faults are the last movements along the rim of the craton and occurred around 1.0 Ga. Multiple folding and thrusting with riebeckite mobilization happened prior to Namaqua events and resulted inter alia in discernable duplication and thickening of the Transvaal Supergroup along the southwestern margin of the Kaapvaal Craton and at least some 130 km into the craton interior. This complicates stratigraphic correlation as well as true thickness estimates of BIF units in Griqualand West, and affects the model for the environmental evolution of the Ghaap Group. A structural model of thin-skin decoupling at the base of the Transvaal Supergroup and starting in the Middle- Early Proterozoic is proposed.

Introduction Kaapvaal Craton is stated to be "overlain by only slightly deformed Early Proterozoic cover The Transvaal Basin sediments in Griqua- successions" (Stowe, 1986, p. 186). Beukes land West, northern Cape Province are bor- (1983) correlated parts of the Ghaap Group dered by multiply folded orogenic realms of the (Table 1 ) from north to south across Griqua- Namaqua Mobile Belt (Vajner, 1974a) and the land West regardless of possible structural Kheis Tectonic Province (Stowe, 1986). complications. This correlation is based mainly Nevertheless, the southwestern margin of the on borehole data gathered between Koegas (Westerberg) and Kuruman (Whitebank) *Present address: Goethering 27a, D-5804 Stein, FRG. (Fig. 1 ). However, recent work along the

10RMATIONand AGE (Ma) in Possiblecorrelates in Kakamas SEOUEN( EGROUI' SUBGItOt;P UpingtonTerranc (U), Kheis HTIIOI.OGY,and possiblethrust ot (KA), and Upington(U) q'errunes, Subprovince(K1 I) and Kaapvaa[ decollementhm izons - I) in KaapvaalCraton (K) Pos,sibl¢ DEFORMATION EI'ISODE Cralon (K) thrust horizons I)

WILGE- Leerkranz(U) arid Zonderhuis Schist,quarlzite, phyllilc, Jannelscpan(KA) (c. 1350Ma on [35 + D6 and late NamaquaI.I. wrench NI IOUI" (KH), (1330± 100, Button and conglomerate,jaspilile, calc-alkalineamphibolitc) faulting= D 7along Doornberglineament. IIRIF Burger, 1983) pillowba,sahs < 1,35 up to 1,0 Ga.

Groblclshoop(KI I) 0780 Ma SACS,1980) -probably Phyllite, micaceousquilrlzite, Korannaland`sequence, Toeslaan nletamorphicage, (Stowc,pelSOf/U[ chloriteschist and anlphibolile Formation(KA)? comnlunication) D 4 Brittlethru,sts. Post dating Westerbergdyke-sheet, post Mat.sap. al"Z Velwater(Bt ulsand) (KI I) Mainlywhite quartzite Uitdraai?(Kaaien) + Sultanoord÷ Associatedwith N-S trendingand O VOIOP Glen lyon "]. MatsapG,I) Coarse,brown gray pebbly I)agbreek(U) (1800-2 led Ma detrital Ewergentfolds. <2,07 to 1,88 Ga Ellie,s Rus (KI 1) quartzite zircons,Barton & Burger, 1983) Fuller J Z D 3 First post Mat,sap deformation,S to SE verging thrustsand folds late Kheis- llartley(KIt) (1881 --+57, Andesite Kotunna? = 2,2 -2,0 Ga only in north west "3 Arnls/rong,1987) Lucknow(KI 1) Quartzite of study area. © Mupedi(KI1) Shale (140- 2eDm) I) U NCONFORMIT't (.3 vOel. Mooidraai(K) Dolomite,chert, jaspilite,lava ~ WATER I Iotazel (K) A,Sabove, withB1F .~ Ongeluk(K) (2239 + 90/ 92, Armstrong,1987) Andesiticlavas and ruff Low ANGLE U NCONFORMII Y Makganyene Diamictite REGIONAL Ill(ill ANGLE UNCONFORMITY~ Nelani ] Jaspilites,flagstones, quartzite, D 2 Fold-thrustzones in BIF. Pro Matsap, RooinekkeI dolomite,BIF, pro- We,sterbergdyke-sheet (amphibolitised Naragas (K) Mudstone,amphibolitc, age unknown),syn- to post- Asbesheuwels Kwakwas quartzite, juspilite, Subgroup.Equivalent to DI? Dorodale conglomerate,crocidolite Pannetjie Griquatownand Kurumun:2432 D I = Uitkomsthmst,prc-Makganyenr, ASBES ±31 zilcousfrom tuffin AIIochemicalBIF ÷ croc-I) post-Koegas> >2,24 Ga (Ongcluk), EII3.JWH S Kuruman(qrendall et al 199(/) OrthochemicalBIF + croc -11 <2432 of Kutuman BIF

Gamohaan Z< Kogelbcen Alternatingstromatolitic Naute Shale F. (+100m)only in Kippan and chertylimestones and south-western(K) 11 Papkuil dolomites.Thin `shale, mudslouc Leterallygrading south-wcstwm ds Klipfontcin1 Icuwcl (K) and proto-BIFintercalations into shallowwater, carbonateswh h Fairfield towardstop. (±800m) - D intercalatedironstones Beukcs ~. Rcivilo (1983) (K) < Momeville l.okamonna ~ 2557±49oil Intercalatedshales and S(IIMIDIS Boompktas ~(K) cztlhonatcs carbonates. -D (l d e Shales,siltst, quartzites, DR[[ Vryburg J m pzcss) lava(100m) -I) U NCONFI')RM IT'I -5 Zeckoebaatl(K) 2135 + 74/ 71, Parts of VentcrsdotpSupcJgloup Pb/l~b-nletamorphic;2745 .!628 Andesiticlavas, tuffitcs, (K). 2371 ÷ ]02/-106(ArmstJong, Mild foldingand/or tilling? l,~b/Sr (ArmMrong,1987) conglomeratesand qualtzhc~ 1987) NON CONI()RMII'I z MaryduleGioup. Intrudedby Z Archean I)raghocndc[gnmile Metabasitcsquartzites, thin BII' I)cfotreed twice in AichucLitltimes. (2900 Ma Burger & Cocrtzc, 1973) (2990, Sm/Nd,Cornell etul, 19SI~; behare and dut ing Z and Skalkscput granite(26eD Ma Ilarton et ah, 1986). Stcenkoppic inlrusionof 2,6 Ga g~unite Burger& Coerlzc, 1973) and granite Sm/Ndmodel age 3100 (Corncllct al, 1986) =basclnctll h, Steenkoppicgt anite west ol -r Doornberglineanlcnt MarydatcGroup (2600Ma Bmgcr& l'mgieter. >: 1979), (K) t'-' STRUCTURAL HISTORY OF THE SOUTHWESTERN CORNER OF THE KAAPVAAL CRATON 13 5

southwestern margin of the Kaapvaal Craton found as far as Koupoort and Griquatown, up between Prieska and Boegoeberg Dam (Fig. 1 ) to 130 km across the craton boundary. Present reveals a strong multiple tectonic imprint on stratigraphic correlations, especially within the Transvaal cover rocks including possible blind Asbesheuwels Subgroup (Table 1 ), are there- thrusts, stacked thrust-sheets and recumbent fore questioned. folds. Some of these structures have now been Detailed structural descriptions are pre-

LOCALITIES GEOLOGY BOED Bcogoeberg Dam VOLOP COP Copperton LU + GA Ltn~ow + GmC,A~ MP M.~,m~l DAN Danielskuil VW VOELWA~ GRK Griquatown CA ON ONOF~UK& MAKOANYENE KAT Kathu KO KOEGAS KOE Koegas GR+KU GRIOUATOWN& KURUMAN KOU Koupoort CA CAMPBELLRAND MAR Marydale CA + S CAMPBELLRAND and POS Postmasburg SCHMIDTSDRIF ZEEKOEBAART PRI Prieska MARYDALE PRP Prieskapoort BASEMENTG RANITE ~OI Rooinekke Boundary of Kaapvaal SIS Sishen Craton UPI Upington Prominent fold traces USB Uitspanberg

OX~Xr* oo*~ \ KHEIs "t

01 ,f- a \

29°S -,to~ i \ 29°S

~o'*z % CA ~°° R I ~AU_LT NOMENCLATURE: 18" Bf Brakfontein Bk Brakbos Bu Brulsand Do Doornberg Es Eselberg Gk - Griquatown Wb - Westerberg

Fig. 1. Location of the study area (between Koupoort, Prieskapoort and Boegoeberg Dam) along the southwestern margin of the Kaapvaal Craton and of the structural model of Fig. 24. Geology after Vajner (1974), Stowe ( 1986 ), Beukes and Smit (1987), modified, and after our own observations. 136 W ALTERMANN AND l.W. H)~LBICH

sented elsewhere (Altermann and Hiilbich, radiometric dating of the associated volcanic 1990). Here we use the essence of that infor- rocks. In the Transvaal the Ghaap Group is mation and present it together with new data underlain by the Ventersdorp Supergroup. to discuss these in the light of the objectives of Judging from stratigraphic relationships, the this paper. This is, to compare the deforma- Ventersdorp appears equivalent to Vajner's tional and erosional episodes found in the (1974a) Zeekoebaart Formation in the south- cover rocks of the craton and correlate them western part of the Kaapvaal Craton. How- with published information of the surrounding ever, the Ventersdorp Supergroup dates at 2.7 Kheis, Gordonia and Bushmanland sub-prov- Ga (Armstrong et al., 1986 ), while Armstrong inces. The main new evidence presented is (1987) dated the Zeekoebaart Formation at threefold. (a) The first deformation (D1) on 21a~+71jj-74 Ma by the Pb/Pb method and at the Kaapvaal Craton pre-dates the Makgan- 2745___628 Ma by Rb/Sr. Walraven et al. yene diamictite and is therefore older than the (1982) dated parts of the Zeekoebaart For- Ongeluk lava (2.24 Ga). (b) These cataclas- mation at around 2.2 Ga, which is comparable ites could very well represent the eroded branch to the more reliable age of the Ongeluk Ande- of a blind sole thrust. Both were inactive dur- site Formation above the Ghaap Group. How- ing D2 and D3, but the sole thrust was reacti- ever, the Asbesheuwels Subgroup (Kuruman vated and extended eastwards during D4, pro- IF) has been dated at 2432 __ 31 Ma (Trendall ducing F4 folds in its hanging wall over large et al., 1990) and the Schmidtsdrif Subgroup areas in Griqualand West with craton-ward (lowest Ghaap Group ) has been dated by Jahn convex trends. (c) Evidence of intense folding et al. (1990) at 2557___49 Ma. Therefore, the and thrusting is observed up to the Griqua- Zeekoebaart must be of Archaean age. town Fault. The Ongeluk Andesite Formation of the A wide variety of stratigraphic names exists Postmasburg Group in Griqualand West has in the literature for local units and facies been dated at 2240+ 57 Ma (Walraven et al., changes of the Transvaal Supergroup. 1982) and at 2249+69,J -72 Ma , (Armstrong, 1987). Throughout this paper we stick to the strati- The possibly equivalent Hekpoort lavas from graphic column of Beukes and Smit ( 1987 ) as the Transvaal sequence have an age of expanded to include strata from the subprov- 2224+21 Ma (SACS, 1980). inces around the craton (Table 1, Fig. 1 ). The Hartley Formation which forms the lower part of the Olifantshoek Group, has an age of 2070_+90 Ma (SACS, 1980) or Geological background 1881 +57 (Armstrong, 1987) and the Grob- lershoop Formation lavas which overlie the The Lower Proterozoic I (Vaalian) Trans- Olifantshoek Group (Beukes and Smit, 1987) vaal Supergroup of Beukes ( 1980, 1983), i.e. and outcrop in the Kheis Subprovince (Fig. 1 ) the Ghaap, the Postmasburg and the Oli- have a minimum age of 1780 Ma (SACS, fantshoek Groups (Table 1 ) are described in 1980) thought by C.W. Stowe to be metamor- many papers and unpublished M.Sc. and Ph.D. phic (pers. commun., 1989). Thus, the Oli- theses of which the most important are by fantshoek Group is at least between 1.881 Ga Rogers and Du Toit (1909), Du Toit ( 1945 ), and 1.78 Ga old. The Dagbreek Formation un- Cilliers ( 1961 ), Hanekom (1966), Fockema derlying parts of the Upington Terrain (Fig. 1 ( 1967 ), Vajner ( 1974a, b ) and Beukes ( 1980, and Table 1 ) has a maximum age of 1.8 to 2.1 1983). Ga (U/Pb-detrital zircons--Barton and The age of this sequence is determined by Burger, 1983) STRUCTURAL HISTORY OF THE SOUTHWESTERN CORNER OF THE KAAPVAAL CRATON

TABLE 2: POSSIBLE CORRELATION OF STRUCTURAL EVENTS AS PERCEIVED BY VARIOUS At

VAJNER (1974) In Kheis and POTGIETER (1982) South of THEART (1985) AND STOWE (1986), HARRIs (1988) STOWE (1986) SC Gordonia Subprovinces and 29°30'S and west of 23°E from HUMPHREYS et al. (1988a + b) Kaapvaal Craton between 29°S Kaapvaal Craton in to Namaqua in Copperton area, Namaqua Upington and Kakamas Terranes Kheis Subprovince north of Na and 29°30'S, west of 22°30'E Province (Fig. I) Province (Fig. 1). (Fig. 1) Marydale High (Fig. 1) IX)(

Marydale Group, deformed prior to intrusion of 2,9 and 2,6 Ga : granites

Zeekocbaart Fm. re~ting non- conformably on 2,6 Ga granite. Deformed twice prior to the deposition of the Ghaap Group. NE trending schistosity = D 2 in present paper, N-S trending cleavage = D 4 in present paper.

Gravity folding slumps in BIF by subsidence of"intrageo- syncline" on the craton - Slump folded BIF (Asbesheuwel two episodes. Subgroup)

NFla and NFIb SE and SSW- KD 1 and KD 2 E to SE verging verging isoclines respectively. folds and thrusts. C. 1,75 Ga and < 1,35 Ga Tight "Z"-KF2-foldsdevelop respectively. in zones.

DN horizontal, N-S, open folds, Open NE-folding, vertical upright, no cleavage, in Z,eekoeba- Only remnants recognisable. axial planes in Transvaal KF3 NNE to N-trending folds in art Formation and Griqualand Almost totally transformed by discrete zones. Refolding KT1 Supergroup. Considered pre- West Sequence. (Also found post and KT2 thrusts. Matsap. Matsap by present authors) e.g.- NF 2 Ongeluk - Witwater syncline (Fig.l) F 1 Sharp hinged. Staxial planar FNIa and FNIb Both isoclinal, Uplngton Terrane: NF 2 NW- KF4-open folds at SW-margin of cleavage in Kaaien Group DN + 1. Most intense folding horizontal, co-axial. Produced by trend, NE-verging tight, main Kheis Subprovince c. 1,2-1,1 Ga. considered Archean by Vajner. west of Bn (Fig. 1) NW in south, west vergent back-folding during Namaqua phase. eg. Orange River Synform (Fig. 1). Now Sultanaoord-Dagbreek NNE in N. Orientation changing east-directed thrusting < 1,3 Ga equivalents in domain III (Table around Marydale High. = age of Areachab = Kaaien = Kakamas Terrane: SW-verging 1, Fig. 1) (Stowe 1986) Uitdraai Groups recumbents in a south-westerly zone; reclined, subvertical NF2 isoclines in central zone (1,2 to F 2 Round hinged, upright, 1,t Ga). F l isoclinal. Deforming F 1 and S 1 in ver[ Gordonia Subprovince (conside- DN + 2. NW-striking, upright FN 2 NW-trending, upright and red Archean by Vajner because it deforming SN + 1. Horizontal SE plunging. Main Namaqua Pre- NF 3 thrusts along Bf (Fig. 1) plat axes. Co-axial with FN + 1, no phase, not related to transpression lith( affects Kaaien and Marydale concomitant foliation. during late Namaqua dextral duri Groups). shear (Harris, 1988) F 3 Post Matsap. NNE-plunging DN + 3. NE-striking, upright, FN 3 N to NE-striking, interfe- NF3 ENE-trending, open, (e.g. axes on Kaapvaal Cr. NE- open, zonally developed folds in rence.with FN2 to form domes Graafwater synform. - Harris, F2 plunging in Namaqua Foreland Namaqua Foreland (Fig. 1). and basins wltfi NW or SE 1988) refolding NF 2 into dm~ and Kheis Subprov. (Fig. 1) plunging axes. "recumbent" and steep zones elor

NF4 NW-trending monoclines eF~oI F 4 NW-alignment of structural DN + 4. Doornberg wrench interfering with NF3 antiforms. elements everywhere, associated fault zone, left lateral. G ri with Doornberg Lineament. lffW-trendlng shear-zones c. 1,0 Doornberg Fault Zone. Do Ga

* Note: Coward and Potgieter (1983) gave no chronological order of events across all 5 of thei

pp. 137-140 rED BY VARIOUS AUTHORS AROUND AND ON THE SOUTH-WESTERN KAA VAAL CRATON

BEUKES AND SMIT (1987) COWARD AND POTGIETER PRESENT AUTtI,ORS ['OWE (I 986) SCOTT (1987) Griqualand West Sequence, (1983) Kheis, Gordonia and Kaapvaal Craton up to Matsap thrusts, ~eis Subprovince north of Namaqua Foreland - Prieska- east of Kheis Subprovince Bushmanland Subprovinces, Marydalc ltigh and Doornberg Lineament arydale High (Fig. 1) poort to Copperton (Fig. 1) (Fig. 1) Namaqua Foreland. (Fig. 1) Marydale Group (2,99 Ga) foliated before granite intrusion. Early syntetonic Marydale granite (2,6 Ga) foliated NW-SE Minor folds in Zeekoebaart Mild folding in Zeekoebaart Formation prior to Formation uncomformably erosion and deposition of Griqualand West overlain by Griqualand West Se- quence. Severalyounger clea- Sequence. Older foliation(s) with southerly dips vages destroy older structures in deformed by $4; latter dips steeply WNW with Seekoebaart Format on. down dip lineation. D t Uitkoms Thrust. > >2,24 Ga, <2,5 Ga. Bedding parallel cataclasites in Koegas Subgroup following limbs of early N-S (FI?) folds reactivated during D 4 by post-humous folding of Ongeluk. Probably branch of sole structure following Vryburg shale horizon. Movement to E. dD2 Recumbent fold zones, thrusts, imbricates, ecollements in BIF on all scales. Varying cleavage in BIF-lutites, Movement to SE, E, and NE. Syn- to post-Asbesheuwels Subgroup. pre-Westerberg dyke sheet, pre-Magkanyene. Equivalent to or incorporating D 1? Black Ridge Thrust Zone. Mo- ve.ment tom and SE; ~Merging SE-vergent thrusts and imbrica- w~th basal thrust or Matsap m ...... I and KD 2 E to SE verging study area. Oldest rocks thrust tes ~ ot uttspanoerg, t:o.r , D3 First post-Matsap deformation(s). S to SE s and thrusts. are Matsan • Thrust nost-da_es t Doornberg Lineament. west ot . vergent near horizontal folds and thrusts in Hart ey la~'a (<2 0:7"---'i88 Ga) the latter the Doornberg Shear m ,t "Z"-KF2-foldsdevelop deformed byDa N-S trending' basement granite is equivalent of Boegoeberg Dam area. North dipping axial )nes. folds in Boegoeberg Dam arga thrusts but back-folded. Age: planar cleavage. < 2.07 - 1,88Ga (Fig. 1) as reinterpreted by Post Hartley (< 2,07 - 1,88 Ga) present authors). N-S to NE-SW-trending D4 Upright to E-vergent folds and limited asymmetric SE- verging folds brittle thrusts trending N-S. Folds deform D 3 , NNE to N-trending folds in thrusts. Post humous folding along early N-S fete zones• Refolding KT 1 with varying plunges (refolded by D6, present authors) Local axial Fl-folds. Post dating Westerberg dyke. Steeply KT2 thrusts• planar cleavage dipping NW west dipping cleavage. <2,07 - 1,88 Ga. Some found N of Prleska reactivation of T l - T 2 is possible. -open folds at SW-margin of Doornberg Thrust, back-folded into near vertical attitude with is Subprovince c. 1,2-1,1 Ga. down

NW-trending right-lateral wren- D 7 Doornberg Lineament, a major long-lived ches (or tilted thrusts?) e.g. Cop- left-leteral oblique wrench. Westerberg Fault is F 3 Steep fold axes refolding NW pertou Fault. Drags axial planes a smaller branch. Marydale ltigh lifted up by elongated domes of FI-F2 in of just older NW-folds in scissor-type movement between both. Between Griqualand West Sequence along Prieska-Prieskapoort-Uitspanberg produces rnberg Fault Zone. Bushmanland Subprovince. Doornberg Lineament (Back - ~olding episode?, present deflection and re-orientation by drag and authors ) transpression of F 4 and T 4 in up to 8km wide zone of Lineament +_ 1,0 Ga. across all 5 of their structural zones.

STRUCTURAL HISTORY OF THE SOUTHWESTERN CORNER OF THE KAAPVAALCRATON 141

Review of previous structural work much younger and probably correlatable with DN+ 1 and DN+2 (Potgieter, 1981 ), FN+ 1 The tectonic evolution of the Transvaal Su- and FN+2 (Theart, 1985) and NFI and NF2 pergroup in Griqualand West was discussed by (Stowe, 1986 ) and with the Doornberg thrust Visser ( 1944 ), who recognized up to six thrust episode followed by backfolding (Coward and planes along which slabs of the Transvaal strata Potgieter, 1983 ). Vajner's observation on pre- were pushed eastwards onto the craton. How- Proterozoic deformations in Archaean Mary- ever, Visser's (1944) investigations remained dale granite and metabasites remain valid unaccepted among most of the later workers. (Table 2 ). More recently Beukes and Smit ( 1987 ) pre- Stowe (1983, 1986) described the tectonic sented stratigraphic and structural evidence for frame of the Namaqua Province and the Kheis post-Olifantshoek ( 1.8-2.1 Ga--Barton and and Gordonia Subprovinces off the Kaapvaal Burger, 1983) thrusting along the northwest- Craton (Fig. 1 and Table 2 ). He recognized up ern margin of the Kaapvaal Craton. Their to three main folding events (KFI-KF3) in the Blackridge thrust system (Table 2 and Fig. l ) Kheis Subprovince located north and north- has been traced intermittently over a distance west of our area of investigation. In the younger of 180 km in a N-S direction and they sug- Namaqua Province to the south and west, gested that the low-angle thrusts accompanied Stowe (1986) recognized four folding events by mylonite zones are related to the Koranna- (NF1-NF4). Kheis ( 1.8 to 2.2 Ga) orogeny. According to Stowe (1986), the KFL pro- Vajner (1974a) recognized two pre-Matsap duced E to SE-vergent recumbent isoclines and deformation episodes affecting the Griqua- eastward directed thrusting ~ 1750 Ma ago. land West Sequence. The first is gravity fold- The KF2 event resulted in "Z"-folds and re- ing of BIF by subsidence of the "intrageosync- newed thrusting < 1350 Ma ago and the KF3 line" on the craton, and the second is a post- produced NNE- to N-trending folds in discrete consolidation folding resulting in open fold zones. The NF1 is age equivalent to KFI-KF 2 structures. Post-Matsap F 3 are N- to NE- and resulted in SSW to SE-verging isoclines. plunging structures with E to SE vergency and The NF2-folds are recumbent to isoclinal and F4 produces mainly gentle upright folds with a may be SW- to NE-verging in different locali- northwesterly trend. A final overprint of the ties. Because NF2 refolded and rotated KF~- area was thought to be related to the Doorn- KF3 structures it must be younger than these berg Fault Zone (Fig. l), which is a major ( 1200-1100 Ma). NF3 resulted in ENE-trend- NW-SE-striking tectonic lineament along the ing open folds and NF4 in NW-trending shear southwestern rim of the Kaapvaal Craton. Ac- zones and locally in NW-trending monoclines. cording to Vajner (1974b), the Doornberg The age of NF4 is estimated as ~ 1000 Ma Fault Zone is genetically related to the upright, (Stowe, 1986). northwest-trending folds. It has a dextral, The findings of Theart (1985) and Scott oblique slip inferred to be produced by anti- ( 1987 ) as well as Humphreys et al. ( 1988 ) are clockwise rotation of the craton relative to the based on field work clone in part much earlier Namaqua Mobile Belt. during the eighties. Their results are based on A re-interpretation of Vajner's FL and F2 a high intensity of localized data and an at- (pre-Matsap) in Kaaien rocks (Table 2 ), some tempt was therefore made to include their con- of which are now known as the Uitdraai For- clusions (Table 2). The findings of Coward mation, equivalent to the Sultanaoord and and Potgieter (1983) (Table 2) which sum- Dagbreek Formation ( 1800-2100 Ma, Barton marize and provide new outlooks are treated and Burger, 1983) makes these deformations further below. 142 W. ALTERMANN AND I,W. HALB1CH

Structure of the study area Structural relationships between Prieskapoort, Prieska and Uitspanberg (Fig. 2) The area described in the present paper is outlined in Fig. 1. Apart from covering new Coward and Potgieter (1983) described ground, it includes parts of the Kaapvaal Cra- thrusting of two ages in this area. They suggest ton investigated by Vajner (1974a) and parts an older Kheis orogeny (shown by Stowe, of the Namaqua Foreland mapped by Potgie- 1986, to have two components, KF~ and KF2 ter ( 1981 ) and is located south of the Griqua- in the Kheis Subprovince, Table 2) with town growth fault. The entire Griqualand West thrusting to the SE, that probably occurred Sequence of the Transvaal Supergroup (Table around 1.75 Ga ago (Stowe, 1986). A mini- 1) is present in the region investigated. The mum displacement of 120 km to the south was latter rests with a basal conglomerate on the determined for this episode. According to Zeekoebaart Formation. The cratonic base- them, a younger Namaqua episode has folds ment underlying the Zeekoebaart Formation is and thrusts verging NE. Both thrust systems exposed in the extreme southwest, bordering converge between Boegoeberg and Marydale to the Namaqua Province across the Doornberg form a double system along the Doornberg Li- Lineament (Fig. 1). The presently accepted neament farther south (Fig. 1 ). This was then boundary of the craton is the Brakbos Fault. steepened along NW-trending back-folds that

,'S,- / v~ ,- c,-,-;'..;,", F4

GLENALLEN ~ ~ ~ PRIESKA F4• F 5

~x ~ T2 / ...... :-: ", , F2

~ MARYDALEGRANITE X" ...... 10t /~ ..~d ~F4 ~o "~, - ""T4"). ,"1- ",~0 [~ DOORNBERG MYLONITE ~ Trend of stra 15 ...... F4 65 ~'20_.~ ~..t.I.=~\ ~--~ DOORNBERGFAULT ~17 10 T~. ".

lineation 40*~p ~" ),20 []

BIF @ N...... tical lineation ~20 10 '11~ '- ~. Faults :':'"" Steep 1.1.drag folds K20 i Bedding; .dl-- horizontal. • SETTLEMENT~,; 76~,~, r~ 1"~ DH = De Hock PP = Prieska Poort ...~ Vertical foliation LP = Lemocnpoort ., Fold axis and axial [] Structuralzones along Prieska - ¢ plane dip Prieskapoort profile () 4 8 12km [] I I I I []

Fig. 2. Geological map of the Prieska-Prieskapoort-Uitspanberg area. See Figs. 1 and 7 for location. Dotted lines separate domains mentioned in text. STRUCTURAL HISTORY OF THE SOUTHWESTERN CORNER OF THE KAAPVAAL CRATON 143

Fig. 3. T2-T 4 thrust formed in Kuruman BIF on Glen Allen (Fig. 2 ). The structure cuts upwards in the stratigraphy from right to left (W to E) and the iron-formation is folded with vergency to the E above and below the decoupling plane. Phyllonitization of BIF occurs up to 1 m thick above the thrust plane. Highest part of cliff is 15 m above base. For fabric see Fig. 4A. developed parallel to the craton rim towards succession (Fig. 2) and interpreted these the end of the younger thrust episode. Late Na- structures as back-folded thrusts. maqua right-lateral wrenching seems to have We have found low angle, west dipping T2- occurred at least along the Brakfontein-Cop- T4 thrusts in BIF around the Glen Allen moun- perton fault. Coward and Potgieter ( 1983 ) also tain farther south (Fig. 2) with phyllonites interpreted the mylonitised Doornberg Shear drag-folds and lineations that clearly prove Zone (Fig. 2) as an oblique slip left-lateral thrust movement from W to E (Figs. 3 and shear-thrust belonging to the early phase of SE- 4A). From Prieska to Prieskapoort (Fig. 2) directed movements prior to back-folding. five structural zones can be recognized. In zone Stowe (1983) found several NW-trending 1 at Prieska three fold phases are seen in BIF. mega-shear zones with left- and right-lateral D2 folds are recumbents on a metre scale, zon- movements along the Orange River in the Up- ally developed and parallel to bedding, with ington area (Fig. 1 ). axes plunging into the NW and SE quadrants Potgieter ( 1981 ) showed two thrust planes along bedding planes deformed by D4 and D5 in the Uitspanberg mountain (Fig. 2). Folds (B and C of Fig. 4). F4 are open to isoclinal, above these dislocations verge NE with low upright to asymmetric E-vergent NNE-SSW- plunges but both thrust planes are clearly not trending structures, and F5 are open upright backfolded. Just west and north of this locality NW-trending (A and B of Fig. 5 ). The F4 folds Coward and Potgieter ( 1983 ) reported steeply may even develop into injection structures and dipping faults that duplicate the carbonate-BIF sheared box folds, revealing plastic as well as 144 W. ALTERMANN AND l.W. HALBICH

÷ "/

• THRUST PLANE • F-AXIS o F 2 - AXlS + SLIP LINE ON SS "d~ St.IP LINE ON THRUST Q F 2 - Axis • F 1 - AXIS • F 1 - AXIS +FIBRE GROWTH LINE ~- SLIP LINE ON BEDDING

Oo \ / ++

\\\ '7 V "4-

E r • F- AXES " ~EDDING • DRAG FOLD AXIS 7 AXIALPLANE AND CLEAVAGE • '111RIJST -I- SLIP LINE ON THRUST • THRUST

v CI EAVA(;E El ]NTERSECTI(INLINEATIDN q- SLIP LINE (IN THRUST * BEDDING

Fig. 4. Structural fabric from various localities: (A) Glen Allen on T2-T4 thrust; ( B ) Prieska town; (C) highwayoutcrops SW of Prieska town; (D) zone 2 of Fig. 2; (E) zone 3 of Fig. 2. elastic behaviour. D2-fold zones preferably de- ways dips westerly (Fig. 4D ). In zone 3, strike velop in riebeckite lutite-rich thinly bedded stays NW-SE but the vergency of large folds units, whereas the thicker chert and sideritic with a low plunge is clearly to the SW and rie- chert layers ride on these dislocation zones. beckite lutite foliation dips at 80 ° to the NE Movement above the decollements is to the (Fig. 4E ). The rotated thrusts have steeper dips east. The style and orientation of these D2, D4 to the SW (Figs. 4E, 5D). This is the zone of and D5 structures is very similar to that found back-folding of Coward and Potgieter ( 1983 ). in BIF of the entire study area. Next we find a 2 km wide zone 4, in which a In zone 2 (Fig. 2), NW-SE-trending folds near-vertical NW-SE-trending foliation and and thrusts (C and D of Fig. 5 ) verge NE and near-vertical lineation of shear fold closures have quartz and asbestos slip fibre lineation appear as transformed structures. Then fol- zones up to 25 cm thick, trending across as well lows zone 5 with mylonites of Schmidtsdrif as parallel to the strike of thrusts. The axial quartzite and Marydale granite (2.6 GamPot- plane foliation in riebeckite lutite bands al- gieter, 1982). We prefer to call zone 4 the STRUCTURAL HISTORY OF THE SOUTHWESTERN CORNER OF THE KAAPVAALCRATON 14 5

A_ ~~7 ---F-" c__

T F T F N ~ 40m S

N 0I 5mi S

Fig. 5. Field sketches of specific structures. (A) Structural relationships at Prieska town. For fabric see Fig. 4. B. (B) Highway outcrops SW of Prieska. 1=4 upright fold deforms F2 thrusts and recumbents. For fabric see Fig. 4C. Looking south. (C) Fold and thrust behavior in zone 2 (Fig. 2) looking SW. Note folds are verging NE. For fabric see Fig. 4D. (D) "Thrust" with synthetic foliation in zone 3 (Fig. 2), rotated to SW and with slip movement nearly parallel to strike. For fabric see Fig. 4E.

Doornberg Fault Zone and zone 5 the Doom- and "Z" inferred by Potgieter ( 1981 ), the for- berg Mylonite. Together they form the Doom- mer with deformed older lineations, on the berg Lineament. Coward and Potgieter inter- transition between quartzite-mylonite and pret zone 4 as vertically back folded thrusts and granite-mylonite. This combination suggests zone 5 as a left-lateral oblique shear-thrust sheath folds but is no proof of their existence. (oblique wrench). In our opinion they both No closed sheaths could be followed out or seen have a very similar origin. This is apparent in in outcrop in the Doornberg Lineament. Some both cases from the relationship of shear folds large S-shaped folds in zone 3, plunge steeply to an older mylonite lineation. (Fig. 2). The simplest interpretation of these S-shaped asymmetric meso shear folds in structural relationships together with the anti- mylonitized Schmidtsdrif quartzite (Fig. 6A) clockwise drag of D4 structures in zones 2 and bordering the Doornberg Fault Zone have a 3 and with what looks like "back-folding" in near vertical axis with a penetrative b-linea- zone 3, is a first order left-lateral oblique tion parallel to the shear fold axes (Fig. 6B). wrench. To the west of this structure, move- They bear a set of older deformed penetrative ment was obliquely up to the SE at first, with mineral lineations orientated along a plane closer to horizontal wrenching later on. Both (Fig. 6B ). No foliation occurs along this plane. phases were associated with compression The mylonite lineation is therefore definitely across the shear zone. The first probably pro- older than the b-lineation. Its plunge on the duced more displacement than the second. The long, undisturbed limb of shear folds varies latter was more compressive, however, with from moderate to steep in a northwesterly di- near-vertical extension along shear fold axes. rection. "S" macro-folds have been mapped This is clearly displayed by strings of horizon- 146 W. ALTERMANN AND l.W. H,~LBICH

Fig. 6. Mylonitized Schmidtsdrif quartzite. (A) S-shaped shear folds looking SW with near-vertical axes (parallel to pencil). Older mylonitic lineation is parallel to hammer handle. (B) Closer look at hinge of S-shaped shear fold with b- lineation parallel to hinge and older deformed mylonitic lineation lying in a plane parallel to hammer handle. Looking NW. STRUCTURAL HISTORY OF THE SOUTHWESTERN CORNER OF THE KAAPVAAL CRATON 147 tally oriented tension gashes in competent chert of metres at a time and occur at a frequency of (quartz) bands of vertically plunging shear about one per several tens of metres. These folds in BIF of zone 4, the Doornberg Fault thrusts may separate units that differ in Zone. lithology. Contrary to Coward and Potgieter's ( 1983 ) ( 3 ) Recumbent folds are found in sheet-like explanation of back-folding of earlier thrusts, zones, which can be traced between nearby which must have tilted many kilometres if not outcrops. They are subparallel to bedding. Dis- some tens of kilometres of crust between the crete shear planes have developed within these Copperton and Doornberg Faults (Fig. 1 ) and zones parallel to axial planes and at their base. which now simulates sinistral shear, we think The vergency and direction of folds always in- that the bulk of the evidence is in favour of dicate a movement of the hanging wall to- oblique left-lateral wrenching (Potgieter, wards the craton (Fig. 4B). From 0.5 to sev- 1981 ) along the Doornberg Lineament. This is eral tens of metres of strata may be deformed responsible for the juxtaposition of Archaean in this way. They may be encased above and granites against the Early Proterozoic Camp- below by an envelope of asymmetric shear bell Group, and most satisfactorily explains a folds. gradual syntaxial change in strike of the early (4) Bedding-parallel as well as obliquely formed D4 thrusts and folds on the craton into cross-cutting zones of totally phyllonitised BIF the northwesterly direction with re-activation contain subzones of chaotic folds. They may of some deflected thrusts that now bear strike- grade laterally into jaspilitic breccias or de- slip lineations in zones 2 and 3 (Fig. 2 ). velop a very distinct "augen"-structure up to several tens of metres thick in the Orange River Structures in the remaining study area area. The structures of types (3) and (4) above Four different types of D2 thrust were found can be traced for kilometres and are com- in the Asbesheuwels Subgroup (Table 1 ) and monly separated by 50 to 100 m of strata. In fully described by Altermann and H~ilbich drill cores they appear as a profusion of small (1990). folds associated with an axial planar cleavage :They are very common and briefly summa- over considerable core lengths, whereas the rized here in order of increasing size. Their re- mylonitic meso-augen-structure might easily be corded distribution and relationship to other mistaken for primary "pillow and billow struc- structural elements are shown in Fig. 7. tures" (Beukes, 1980) that also occur in the ( 1 ) Discrete slip planes that gradually cross BIF. Types (1) and (2) thrusts occur inter- from one bedding plane to the next can be fol- mittently in outcrops near the base of the As- lowed for several metres and occur at frequen- besheuwels Subgroup between Griquatown and cies of about one per metre thickness of strata Kuruman (Fig. 1 ). in some sequences. The first three types have been found as far (2) Mylonitic, ferruginous carbonate and north as Griquatown, some 130 km northeast chert breccias and lenticular jaspilites and and east of the craton edge (Fig. 1 ), with quartz veins, centimetres to decimetres thick movement directed NE, E and SE. occur subparallel to bedding, with shear phe- Directly southwest of the Griquatown Fault nomena clearly indicating the sense of move- erosion-valley NW-trending thrusts dipping ment which is always towards the craton. These SW have stacks of NE-verging folds overlying dislocations are commonly accompanied by quartz and crocidolite slip fibre dislocation higher-angle synthetic splay faults. They can be zones alternating with zones of tension gashes followed in outcrop for up to many hundreds (Figs. 8A and 9A). Z oo

22o30iv! LEGEND o lO}Cm

P ...... ] Town

• Settlement >~>>>>>>>>>>>>>>>>~ ,>>>,>>>> > ,,.>>>>>>~ • Trig. Beacons ) .>~>>>>>>~ > >,.>>>>>>> Main roads • >~,>>>>>:~> :UFig 13 vvvvvvvvvvv, ...... ~vvvvvvvvvvv' up • Thrust (folded) ~v vvvvvvvvvvvvvv • Uitkomsvvvvvv' with plungeof vvvvvvvvvvvvvv' vvvvvvvvvvvvvv' v~vvvvv~vvv~vv y,K vvvvvvvvvvvv ~l:goega'~VVVVv~vvvvv ':.~.. vvvvvvvvvvvv _ ~N'.<.~vvvvvvvvvvvv ~[ w2- hor~hol~I/0 F 2 plunging(lO °)

SEDIMENTS D /F 2 folded byF 4 NAMAQUA / MATEMAORPHI C Doornberg Lineament MorFdd/e :7.73 "/, BELT

:X F2 .... horizontalwith MATSAPFm. ! total variationin directionand in shallow dip of axial plane

.29°J0 D ONGELUKFm. i / horizontal, F 2 axial plane Kransfontein KOEGAS horizontal SUBGROUP I

O SBESME~E~i F 2 horizontaland dipping SUBGROUP / axial plane (20 ° )

SUBGROUP F 4 horizontal,axial plane vertical ~ra:svaa- Jonanneskur~ ~ SCI~IDTSDRIFT SUBGROUP ', - se:zmen:s ; • m ~rieska • " F 6 horizontal,axial plane vertical K .... :\ area z.... ~ > Z Preferrentially orientated Z ' a~e 2r a:~// mineralgrowth in thrust "~-"-"~- / /E \ "I £-:,-1 BASEME.T Z zone; qz=quartz,am=amphibole

>: Fig. 7. Simplifiedgeological map of the studyarea. Structuralinformation is summedup diagrammaticallyfor variouslocalities. N-S-trending r" F4 mega-foldsdominate the outcroptrends away from the DoornbergLineament. For locationsee Fig. 1. STRUCTURAL HISTORY OF THE SOUTHWESTERN CORNER OF THE KAAPVAALCRATON 149

./ \°-. "7

A B

* BEDDING • FOLD AXES ,~ AXIAL PLANE CLEAVAGE + FIBRE AND SLIP LINEATION tn JOINTS ACROSS FIBRE • THRUST PLANE

Fig. 8. Structural fabric between Koupoort and Griquatown. Refer to Fig. I. (A) From BIF just southwest of the Griqua- town Fault erosion-valley. For explanation see text. (B) From BIF 10-20 km north of Koupoort.

Farther south (Figs. 1 and 10) N-trending clined western limb of the Uitkoms syncline thrusts are lined with quartz and have overly- (Figs. 7 and 13-16) in the Koegas Subgroup. ing folded zones with internal dislocations sev- Movement indicators are very scarce. Some eral tens of metres thick (Figs. 8B, 9B, 9C, dragfolds may belong to the FI-F2 or F4 fold 9D). Thrust planes at the base of the folded phases. Microscopically, eye-structures and stacks clearly separate lithologically differing connecting bands are totally recrystallised by units (Figs. 9B(i~, 9D and 10). From Spioen- quartz and calcite, both heavily strained after- kop to Koupoort three thrust and thrust-fold wards. The entire tectonite is a recrystallised horizons are recorded (Figs. l0 and 11 ). Sev- impure marble of which parts are still seen in eral thrusts of type (3) and (4) occur between situ. Large amounts of fluids moved along this Orange View and Klein Noute in Fig. 7. Near zone as is revealed by irregularly shaped quartz- Nauga and Geelbeksdam at the Orange River, or carbonate-rich volumes of rock with near- several-centimetres long riebeckite, aegirine vertical boundaries. and acmite crystals grow with preferred orien- The overlying mixtite bears rare clasts of ca- tation in thrust zones across recumbent E-vet- taclasite but itself is neither recrystallised nor gent folds and west-dipping C-type shears. An sheared or cleaved. There can be no doubt that acmite-aegirine-riebeckite-quartz-carbon- this event of disruption and fluid penetration ate-magnetite paragenesis exists in these dy- occurred prior to the deposition of a porous namometamorphic zones. heterogeneous rock like the Makganyene mix- The oldest deformation recorded so far on tite immediately above. This Uitkoms cata- the craton in Griqualand West seems to be a clasite is interpreted as a splay thrust diverging zone of cataclasites that parallel the steeply in- from a sole in depth. This blind D l thrust is 150 W. ALTERMANN AND 1.W. HALBICH

A(i) A(ii)

t_~ q 0 lm

L I 0 50 cm

B(i) t i ~

, -- b

1 a Thrustplanes with mylonites /~ / /d b Slip lineations on mylonite

~ ~ ~--~ ~-, ==a

C °°m

W E a lmm bands, undisturbed c Thrust planes b Highly contorted = c d lcm bands, lenticular E 6 5()0m W

15m

E [ l w 0 lOOm

Fig. 9. Field sketches of folds and thrusts between Koupoort and Griquatown. Refer to Fig. 1. (A) Bordering the Griqua- town Fault erosion-valley to the SW: (i) slip lineation zones and transverse tension gashes mark thrust zones, looking NW; (ii) stacks of thrusts and decapitated drag folds; looking SW. (B) Elandsfontein, 20 km north of Koupoort. Fold and thrust piles at the base of a lithologic unit in Griquatown BIF. For fabric see Fig. 8B. (C) Doornlaagte and Spioenkop 5-l0 km east and north of Koupoort. Fold and thrust relationship to lithology and bedding in lower Griquatown BIF. For location see Fig. 10. For fabric see Fig. 8B. STRUCTURAL HISTORY OF THE SOUTHWESTERN CORNER OF THE KAAPVAAL CRATON 151

Fig. 10. Geological map of Doornlaagte-Koupoort area. After du Plooy (1986), amended, and the authors' own work. Various mappable units of mainly allochemical banded iron-formation overly each other from bottom to top in the order A, B, C, D. The general dip is between 5 and 10 degrees to the west and has been omitted, except where it deviates significantly from these limits. One larger anticline and numerous fold zones with asymmetric to recumbent east-verging folds are indicated schematically. Thrusts shown were mapped. Koupoort (KpT) thrust, and Spioenkop (SpT) thrust are cutting upwards through the lithology to the east. Most bedding plane thrusts as inferred from fold zones and correlation with data from borehole Sp 1, are not shown on the map. See also Fig. 11 for an interpretation of the structure. Kp = Koupoort farm house. between 2500 and 2240 Ma in age, and possi- and verging east in leg AB, of the profile in Fig. bly initiated N-S-trending F l-folds in its hang- 18. This is the ideal stratigraphic horizon for a ing wall from Kuruman to Prieska (Fig. 1 ). sole thrust to develop relative to all younger The eastward convex shape of these Fl axial units. Thus the Uitkoms cataclasite with its traces around the Maremane double-plunging evidence for fluid involvement may be closely anticline is seen as part of this development. linked to a sole thrust. Re-activation of these structures may have oc- The fabric of bedding and lineations (Fig. curred during D4 N-S folding. This is also sug- 19 ) from the Koegas-Westerberg area (Fig. 7 ) gested by the two unconformities separating the reveals that N-S-trending folds plunging at low Koegas, Makganyene and Ongeluk units (Fig. angles dominate. These are mainly F4 folds, 12). The Ongeluk seems to have undergone many of which are E-vergent (Figs. 19A and posthumous folding along older trends. Ag- 19C ). But some interference of F4 with F2 re- gressive fluids have moved along only one cumbents and F6 (Table 2) occurs producing other dislocation plane in the study area. It is fold plunges at larger angles and in many direc- a T2 thrust separating carbonates verging W tions (Figs. 19B and 19D). and facing E from Schmidtsdrif shales facing In this same area, 200 m thick decollement 152 W. ALTERMANNAND l.W. HALBICH

Spioenkop 300m 1 .... " I -.'" - "" -" ," - -','[,7"" ~[f 7" 300 m 200t ," ,'1,," " . r-.;-" " - 200

12 ° 19 ° 35 ° 44 ° 55 °

w E V = 4xH

Fig. 11. Interpreted cross-section through the map, Fig. 10. Lithology as for Fig. 10. Unit A was apparently intersected at the bottom of hole Sp 1. This and the interpretation that A (and not any other horizon) overlies carbonates at the eastern slopes of Spioenkop, determines the reconstruction of the lower half of the figure. Thicknesses of units. Campbell Rand carbonates (E) and Schmidtsdrif shales+quartzite (F) are estimates. The interpretation is based on the surface and the drill-hole data, assuming that the lithological correlation across the valley to Spioenkop is correct. This correlation is neatly corroborated by the presence of a well-developed fold zone that consistently occurs near the base of unit C. The reconstruction assumes considerable differential bedding thrust slip (shown schematically by the slip-vector triangle) as is evident on meso- to macro-scales from Fig. 9B, (i) and (ii). This affects an earlier thrust cutting down the lithology to the east, and placing unit C on A at Spioenkop. A logical deflection of Sp l is assumed because in doing so it brings intersected horizons into very good alignment with surface data and because of observed dip angles relative to core. Note that the details shown here, differ from those that can be shown in Fig. 24.

R W E -'F o__.11 o .-

I I I 0 300m

Fig. 12. Sketch profile through the western flank of the Libertas syncline. For location see Fig. 7. Two erosional episodes are in evidence. The earliest pre-Makganyene erosion occurred during or after considerable tilting of Koegas beds to form a high-angle unconformity. The later, pre-Ongeluk erosion produced a low-angle unconformity with the Makganyene. Note that dips in the Koegas Subgroup become steeper to the east as the axial plane of the syncline is approached. K= Koegas Subgroup, M= Makganyene mixtite, O= Ongeluk lava, R = river. zones with recumbent D2 fold stacks exist in Seekoebaard (Fig. 17 ). The intervening sync- BIF (Altermann and H~ilbich, 1990). lines were thrust-out during D2 and the struc- The largest folds that developed during D2 tures were co-axially refolded by F4 producing are the originally north-plunging, now reclined type (3) (Ramsay, 1967) interference pat- north-plunging mega-anticlines with Schmits- terns. F2 and F4 both plunge north to day in drif and Campbell Rand cores found on Bo this area. STRUCTURALHISTORY OF THE SOUTHWESTERN CORNER OF THE KAAPVAAL CRATON 153

A A Profiles in Fig. 14 I q

ALLUVIUM

ONGELUK FORMATION

MAKGANYENE MIXTITE

KOEGAS SUBGROUP

Thrust

0 500m

Elevations in metres above see level

Fig. 13. Map of the Nuwevlei-Uitkoms area. For location see Fig. 7.

W m £ • d -.

A~~~ m A

a c b

\. n d __ n B ,,'.- "'"

'~,'u' a b c " ~ d -

C h...i'~-~.,', .', j '"."" -- --, J c

" 1i ..~v e

c ::?'!?.:i:'i"a

= I I I ~OITI

Fig. 14. Three short sections across the western limb of the asymmetric F4 Uitkoms syncline. For location see Fig. 13. The bedding-parallel cataclasite in Koegas beds predates later F4 folding and the angular unconformity on which the Makgan- yene mixtite was deposited, a= Koegas Subgroup; b = totally recrystallised carbonate and/or siliceous cataclasite partly with an augen structure in the Koegas Subgroup; c = Koegas Subgroup limestone with chert lenses (parent rock of b); d = Makganyene mixtite; e = Ongeluk lava; h-j = basal conglomerate of Makganyene; i has a black carbonaceous matrix containing angular fragments of cataclasite; k=contorted laminae of magnetic chert and limestone (varvite?); m = carbonate clasts floating in mixtite; n = lenses of sandstone floating in mixtite. 154 W. ALTERMANN AND I.W. HALBICH

Fig. 15. Outcrop of the Uitkoms cataclasite zone on Uitkoms. Dark = carbonate recrystallisation. White = siliceous recrystallisation. Note irregular, lenticular form and near vertical attitude. Looking south. Difference in elevation is 15 m.

Fig. 16. Uitkoms cataclasite--detail showing lenticular structure. Scale is in centimetres.

Stacking of F2-folds and thrusts, tectonic du- (Table 1 ) tectonically overlie Koegas, Mak- plication of Koegas and BIF strata as well as ganyene and Ongeluk strata that were thrust refolding of F2 by F4 is also seen in other areas and sheared previously. The Matsap klippe has (Figs. 20 and 21 ). a basal T 3 thrust breccia consisting of very T4 thrusts displacing T2 and T 3 are shown in small angular fragments of Koegas and BIF Fig. 17. The relationship of T 3 to Te structures derivation in a ferruginous matrix. is also clear from Figs. 17 and 22. Matsap beds T3 thrusts and S- to SE-verging horizontal F 3 STRUCTURAL HISTORY OF THE SOUTHWESTERN CORNER OF THE KAAPVAALCRATON 15 5 folds have developed in the Matsap beds as de- youngest tectonic feature in the area under in- scribed by Vajner (1974b) for the area around vestigation. Altermann and H~ilbich (1990) Boegoeberg Dam (Fig. 7). These structures discuss the possibility that this feature which very rapidly die out or are absent in older strata is a splay-fault off the Doornberg Lineament southwards and it seems that the Volop Group (Fig. 1 ), has rejuvenated previously refolded is tectonically decoupled from the older strata older thrust planes. This happens in the south by a T 3 sole thrust south of Boegoeberg Dam. of Fig. 17. Farther north on this figure, older Several klippes are outcropping to the south D2-recumbents and T2-thrusts are refolded by (Figs. 1 and 7), one of which underlies Groot N-S-trending F4 and the Westerberg Fault Witberg (Fig. 17 ). The Fz folds in Asbesheu- merges with T2 and T4 structures. wels Subgroup on Bo Seekoebaard (Fig. 17) The Schmidtsdrif and Naute shales and have north-plunging axes and form type (3) phyllites each have only one N-S-trending interference (Ramsay, 1967) with F4 as out- steeply west dipping axial plane $2 cleavage lined above. One would expect type (2) inter- (Fig. 17 ). This structure cannot be genetically ference patterns if they were refolded F 3 linked to the Westerberg Fault because in both structures. cases the cleavage is developed with equal in- In the area between Seekoebaard 1 and Boe- tensity and attitude next to and several kilo- goeberg Dam west of the Orange River (Figs. metres away from it. 7 and 17), Zeekoebaart, Schmidtsdrif and The age of the Uitkoms cataclasites (Dl) Matsap strata all display two cleavages in fine- relative to the oldest deformations (D2) seen grained rocks. The younger, non-penetrative in the BIF of the Orange River of Fig. 17 is not and spaced $6 cleavage trends E-W and is ap- certain. D2 and DI may be of very similar age, proximately vertical everywhere. On Seekoe- but the intensely recrystallised nature of the baard 1 and 2 it crenulates an older $2 that is Uitkoms zone in thin section, and without a axial planar to remnants of large reclined NE- prominent lineation suggests that it may have trending F2 folds in Schmidtsdrifmeta-quartz- a different origin, possibly as a splay thrust ris- ires. They plunge to the NW or N and are tran- ing from a sole in Schmidtsdrif shales. sected by thick bedding-parallel quartzite my- Discussion lonites (Figs. 23A and 23B). Farther north, grey Matsap conglomerates and quartzites tec- Age of deformations tonically overlie the Zeekoebaart meta-lava with a different style of D 3 folds, thrusts and The absolute age of deformation events de- cleavages (Fig. 23C). scribed above can only be roughly compre- F4 N-S-trending folds refolded the Matsap hended from dated volcanic rocks which often and all older cover rocks. The intensity of these have overlapping error margins of hundreds of folds of which the Orange River syncline (Fig. Ma, probably in part because they were dy- 1 ) and the synclines in the Ongeluk Formation namically metamorphosed at least once. Seven (Figs. 1 and 7) are examples, decreases from events are recorded (Table 2) W to E. Those in Ongeluk and older rocks seem to be posthumous folds following older F~ Dt synclines. The Westerberg dextral oblique-slip fault It is now certain that the deformations on the (WF--Fig. 17), described by Vajner (1974b) craton predate the deposition of the Makgan- as the Doringberg Fault, is probably the yene diamictite and the extrusion of the On- 156 W. ALTERMANN AND l.W. H,~LB1CH

Geolooical Mao of the Bo Seekoebaard. Lelikstad. Pyowater area Sources: Own mapping, photo interpretation and re-interpretation of the map of Vsjner (1974)

ALLUVIUM S 8A Seekoebaard Antiform

WESTERBERG DYKE H B S Hardeberg Synform basic intrusion

MATSAP FORMATION Wl F Witberg Fault conglomerates, grits and sandstones

ONGELUK FORMATION F Westerberg Fault basic lava W UNCONFORMITY

MAKGANYENE DIAMICTITE O / • / /k S .... y spot; farm h .... ; trig. b .....

UNCONFORMITY KOEGAS SUBGROUP /~ Farm boundaries siltstones, shales and limestones

I ASBESHEUWELS SUBGROUP j Geological boundary known - I orthochemical and allochemical BIF /~_~ extrapolated

NAUTE FORMATION ..'~'-~o.-"~" Form lines - photo interpretation shale with chert bands

Axial planes: Number referes to CAMPBELLRAND SUBGROUP relative age. carbonates and rare tuffs Open arrows: General plunge of axis and dip of axial plane. Closed arrows: Specific plunge.

mainly shales and phyllites ~I with chert bands ~ / ~--~9~--~,/ knownStrikebed;andoverturned dip of ...... k .... ;

SCHMIDTSDRIF SUBGROUP T~ Thrust and thrust-fold plane (T), mainly quartzites fault (F), - observed, dip direction indicated UNCONFORMITY ~ ZEEKOEB/~RTFORMATION Thrust and thrust-fold plane (T), ~F~--T~ fault (F), - extrapolated, re- basic lava interpreted or inferred STRUCTURAL HISTORY OF THE SOUTHWESTERN CORNER OF THE KAAPVAALCRATON 157

~koebaard__~\ r~ I / '

Seekoeb / 2 I'l ~' 0 / .!'::":'.":':':':r,

t 9t,.>, i // S 7]', '1 ,'" t'~' ,/,2. '~ iO:

Seekoebaard // //

~: .-- , / -/.:<:"." - _ ...... :..:._.:;',:.

r~ ~, A ,I

/ l~j /I

4 ,~i p o o r ~ out c tops \

/" I~ ,likstad ~ ~ 2 V -- k ~ V Groot • -~ x< r v /~/'l

A I..i

,\ ~ ~ U~g.t

LI.J o o ' \ Ln

0 (~1 ('4

0 2km

29015 , "~ l/ r r2 1 r2

Fig. 17. Geological map of the Bo Seekoebaard-Lelikstad-Pypwater area after Vajner (1974) as amended, and the authors' own mapping. For location see Fig. 7. For description see text. 158 W. ALTERMANN AND I.W. HALBICH

A 8 [ O E F (3 H I I I I I I I F

W orange River Westerberg Fault E , ...... I se Berg

m a.s.1.

F / r/ ~ I- 1184 106491 ~.-~ ~;/ ¢ ,./ \r., i . /.C,~ ' /r r~ r ^,r I- 1064

[] ..... ] schmidtsdrif ~ ...... ~ ...... ~ ~ [] ZeekoebaartFormation Quartzite Campbellrand Subgroup Koegas Subgroup Westerberg dyke - (sheet}

Fig. 18. Sections A-H from Fig. 17. Note the style of deformation of F2 (recumbent north-plunging folds) and thrusts refolded by upright to E-vergent F4. Late Westerberg faulting transects all older structures. geluk Andesite Formation with an age of relation of D2 with D~ is possible but uncertain 2240+_57 Ma (Walraven et al., 1982) or and it is preferable to keep them separated 2239+_ 90 Ma (Armstrong, 1987), but postdate (Tables 1 and 2). It is also possible that some the deposition of the Koegas Subgroup (2400- of the earliest slump folds classed as D2 here 2300 Ma?) The D~- episode is therefore older are earlier than D~ and that some of the latest than the Kheis-Koranna orogeny (2.2-2.0 Ga; D2 cataclasites in BIF only just pre-date the de- Barton et al., 1986 ), and thus the first orogenic position of the Makganyene diamictite. event of post-Archaean but pre-Ongeluk age so D2 movements are therefore younger than far known in the region under investigation. the age of deposition of the Kuruman BIF (2432 _+ 31 Ma, Trendall et al., 1990), but ap- D2 parently older than the Ongeluk lava (2239 The slump folds, decollements, mega re- Ma). They also pre-date the intrusion of the cumbents, recumbent fold stacks and thrusts metamorphosed Westerberg dyke-sheet (Fig. that are considered to belong to this episode, 17). Certainly D~, but parts of Dz as well, are seem to represent an extended period of move- older than the earliest deformations (Stowe, ments that started just after sedimentation of 1986; KF, ) recorded from the Kheis Subprov- the Kuruman BIF, and in the present study area ince (Table 2 and Fig. 1 ), thought to have hap- finally affected the entire Ghaap Group (Ta- pened about 1750 Ma ago. The movement di- ble 1 ). D2 structures have not been observed rections and vergencies of KF~, KF2, D~ and in Makganyene or Ongeluk strata. However, a Dr, however, are comparable. We conclude that set of shear planes displacing D2 thrust brec- already in Early Proterozoic times several cias also affects the Makganyene Formation easterly (cratonward) directed pulses de- (Fig. 22 ), but not the Matsao Group. The cor- formed pre- and post- Makganyene strata. A A I=

tt ....tt : : :.:.:.:::.::::::::::::::::::::::::::::::::::::: ::::::=.::: :.. <

r, 11

....~:~,. ~:~:~ ~:~:~

:::.:::: .¢ ::::~tzz::~ rl

" iiii'L,ii::.. rl _Z

1- 11 B= 22.6~ per 2.0~ Of hatispheres n = 62

"3 l:

ii '-=:':::::

::::::

I: 9.8~ per 1.0~ of hemispheres n = 256 ~= 14.8~ per 1.0~ of hemispherew n = llS

:ig. 19.Structural fabric of the area Koegas-Westerberg.(A) Measuredfold axes of meso (F2-F4). (B) Measured foldaxes of refoldedF2 olds only. (C) r~ssto beddingof large F4 folds. (D) ~tssto beddingof largerF~ folds. 160 w. ALTERMANN AND I.W. H,~LBICH

• / . - *.: - . ~- ' :]:{" • :'.::'.\":j/~':~:::::': vvvvv vvv / • ...... ~',, . ..:-:... i: ::i::!~!<:i.:.:.::.!?.:;:.:7...-::~......

: ° "~.~ ...... ~,~ "::!i' • ..:.2.XS:;::"':?) ~:i7:..'.:>: vvv' ,v ...... ¢ ..:;:.. :;7....>:::..:..:::::...... vvv v WESTERBERG ...... , • :i:~:-' .:.v.v'.v.:.'....:::.'::.'-,.'.:/.!:'."~\.'::!:."~f'::::::: :: :.':.. vvv~ v" v, DYKE-SHEET ...... A :'::. • :::v?: :-.. : :::..,'::: :: :..,vv .... Koegas Mt ..... ::7 . .:L.';'Z::.:::'-..~:..'~-'.'.:'i:':"'"t" $ # ~vvl ONGELUK .-.: :...v...,..:..~:...:.....:::. ... , If..V..,ll FORMATION (\ . % " . 7 c. . {:{'{" .!{!if{.":::'.]-<:.::i" ":' vw ...... ~ • •...... :: .::: ...... '.":.:- vvl r,-T1 MAKGANYENE • " .'" .:.:.v.... "'"-:'::"~.i...-!:i...j~~i,~..':-" '. v .~ -- MIXTITE

~ "~/ils ' ~i,il~__'I \''''" .. "' ' .".!:" :.;i~' "..!!i~:5[i~?.[(~ 'i:i!ii :: ::i~::ii~:,{~ii,.Fi::%-::.."i/~!:':i'71:~{~i!:i:! ...... ~~~;ii{" 'i~::i!-

. <. " ~ :" :~ ".'.: :.:3~: : .:,'::.:..':.:: vvv.~ SUBGROUP

~1~ t F ': ": "'" "'': ~: ": :':"::':": ::':': ..... :'~:::':::~::::.~:::".~i:~: '.':.':'::'" :" ::":::#.:.:f~v l~ ; ::~;. ; ;.;; :t!~i-{C;:"]}}!~ :;;!i:;];:;i}i;:!;{)7[{~:{!{{i;i!i:(i.!i? ~'::1!~;:"~.;i{!Ci:i:!'. W v Wv O 500m

:[::!: ((%":}:i2 ~,~C? ? :~]:!:i!::i!S~ii~::(~i::!ii'}~!/i~}?;i;i:!5:;:.[[[:iii~!'}[:ii;[;];:...~:]~!.~i~:~ Elevations in metres

Fig. 20. The geology southeast of Koegas showing difference in style and intensity of D2 as controlled by lithology. For location see Fig. 7.

NW SE E

A B C I I I

ma.m.s+]. m a.m.s.1. Koegas Mountain .i097 !066 1"2 Liebertas Syncline -1066 1036 F "I036 '1006 '975 • ~ ~I~ ,945 914 ~//v ,914 8841 "Iy" F 2 / '884

~BANDED IRON FORMATION (KURUMAN) MAKGANYENE MIXTITE ~DYKE ~ THRUST

ONGELUK ANDESITE 0 o ~ FORMATION 500 m I I

Fig. 21. Section A-B-C of Fig. 20. Note the difference in style of deformation between iron tbrmation in Koegas Moun- tain and Koegas sediments to the east. Lithological duplication occurs along the eastern flank of Koegas Mountain, and the dips of T2-thrusts vary across the mountain because of asymmetric E-vergent F4 anticlinal refolding with a steep eastern limb.

long time-span could have separated the ear- 03 lier from the later episodes because of the size of the pre-Makganyene high-angle The thrust below the Groot Witberg tectonic unconformity. outlier of Matsap rocks could have happened STRUCTURALHISTORY OF THE SOUTHWESTERNCORNER OF THE KAAPVAALCRATON 161

S W Groot Witberg Syncline N E

K-~ .~.~:.._~.~_<-~."'~.. q ~" ~ . .. ~ ~ .... " ". " ~. "

"" r M

lkm

.- / • I ,

Fig. 22. Panoramic sketch of Groot Witberg Mountain looking north from near the dwelling on farm Grasgat (Fig. 17 ). The tectonic contact KDLM at the base of the Matsap (Q) is folded by F4, as are the Matsap beds themselves, to form the Groot Witberg syncline (Fig. 17 ). A = Ongeluk amygdaloidal lava. B = Koegas Subgroup of north striking siltstones and tectonically intercalated B|F, isoclinally folded and east-vergent. C = Makganyene mixtite. Both B and C are affected by a set of post D ~and D2 shear planes striking NE and absent from Ongeluk and Matsap. D = lenses of thrust fault breccia along contact to Matsap sediments. This breccia truncates both over and underlying beds. E=bedding-parallel D,-D2 thrust breccia in Koegas. R= scree. Time sequence: ( 1 ) folding and thrusting of Koegas Subgroup (B) during D,-D2; (2) erosion and deposition of Makganyene (C); (3) shearing of B and C; (4) erosion and deposition of Ongeluk (A) (2240 Ma ); ( 5 ) erosion and deposition of Matsap (Q); (6) emplacement of Matsap allochthone (T3); (7) N-S (F4) folding of Matsap and T4 thrusting.

v ~ v

50bm , 0 lkm Fig. 23. Structural relationships in rocks west of the Orange River in Fig. 17. Note that $6 has a constant attitude and is the youngest structure in all these outcrops. F 3 are not developed in strata older than Matsap beds and F2 are not developed in Matsap strata. The decoupling thrust plane between Matsap and older strata is not exposed in this area. (A) On Seekoebaard 1, in Vryburg quartzites (Q) and Zeekoebaart lava ( V). M= lenses of marble; rn=thick quartzite mylonite. (B) On Seekoebaard 2, lenses of metabasites (MB) and metaquartzites (MQ) drift in sheared amygdaloidal meta-lava (AML). For (A) and (B), $2 is equivalent to Vajners ( 1974, pp. 104-105, fig. 10b) main schistosity in the Zeekoebaart Formation of pre-Matsap age. (C) Synclines of Matsap quartzites and conglomerates, overlying older meta-lavas north of Seekoebaard 2. 162 W. ALTERMANNAND l.W. HALBICH late in the Kheis-Koranna orogeny (2.2-2.0 ton terrain (Fig. 1 and Table 2 ) are feebly de- Ga, Barton and Burger, 1983) but is more veloped in the Kheis Subprovince (Stowe, likely to be of post-Hartley lava age (2.07 to 1986 ) and do not occur on the Marydale High 1.88 Ga). KFI and NFla (Table 2) from the and east thereof probably because of effective Kheis and Gordonia subprovinces are compa- shielding of a proto-Marydale High. rable or somewhat younger in age. A somewhat younger KF2 and NFIb (Table 2) re- 06 corded by Stowe (1986) from these areas These very open E-W to ENE-WSW folds respectively have not been detected elsewhere produce type ( 1 ) interference patterns (Ram- around and on the craton. They must have had say, 1967) with and F5 (Figs. 1 and 7). All a restricted distribution due to specialized, lo- F4 the other subprovinces underwent a very sim- cal circumstances. Beukes and Smit (1987) ilar late Namaqua folding (e.g. Graafwater suggested that the Blackridge Thrust Fault sys- synform, Harris, 1988, Table 2). It is esti- tem is of Kheis-Koranna age. However, from mated to be 1.1 Ga old (Stowe, 1986). their stratigraphic table it is evident that this system duplicates the Mapedi Formation and the conformable strata of the Olifantshoek D7 Group, including the Hartley lavas, above it. The Doornberg Lineament is an oblique Consequently, the Blackridge Thrust system is wrench fault with a transpression component of post-Matsap age or younger than the transporting up in the SW, along the NW-SE 2070+90 Ma (1881_+57 Ma, Armstrong, Doornberg Fault Zone, and the Westerberg 1987) of Hartley lava. Therefore, the Black- Fault is a very similar but right-lateral splay ridge fault system of Beukes and Smit (1987) fault of lesser effect. The age is considered to is probably D3-related (Table 2). The thrust be just less than 1.0 Ga (Stowe, 1986). below Groot Witberg described above seems to belong to this episode. Structural model

D4 An attempt has been made to combine the These N-S-trending upright and asymmet- new evidence for mid-Early Proterozoic re- ric E-vergent folds and associated minor brit- gional thrusting described here with well know tle thrusts post-date the intrusion of the Wes- regional fold structures of the F4, N-S-trend- terberg dyke-sheet (e.g. the Witberg oblique ing set, by assuming a sole thrust at the shale fault, Fig. 17 ), and are associated with a steeply horizon of the Schmidtsdrif Subgroup (Fig. west-dipping cleavage. They deform the Groot 24). From 15 km east of the Westerberg Fault Witberg klippe and the D3-thrust and folded the principle of balanced cross-sections Matsap strata around Boegoeberg Dam. They (Dahlstrom, 1969; Woodward et al., 1989 ) has also develop the most prominent N-trending been applied for the construction. It appears mega-anticlines and synclines in the Griqua- that not more than about 25% of cumulative land West sequence (Figs. 1, 7 and 17). The horizontal shortening has occurred by thrust- age of D4 structures is less than 2.07 to 1.88 ing and homogeneous deformation (cleavage Ga. Some re-activation of D I-D2-structures development, especially in riebeckite lutites may have occurred during this episode. during ramp climb) over about 100 km of section. D5 The westernmost part underwent considerable ductile deformation at amphibolite facies D5 structures comparable to the main NF2 metamorphism. The diagram does not show Namaqua phase (Stowe, 1986) in the Uping- the full extent of possible overriding of the IrulpanFault Westerberg Fault Ongeluk-WitwaterSyncline I BH Westerberg I Fault projected I I

(900m), UitkomsT. I BH Koupoort i Spioenkop I (500m) ~m f , T I ; IKtupoort T. [m ~ ..... i- .'. I , ' , ~Spi°enk°p T. 2 ~~e ----'Z~.~.~.....surface ! ~i~i~!.,,.... ~~ ~ " -i:";-:!.....-.:..-.:::'-"" .--.-.::"~"""~ ~--~~" 1

' >',7;~,,'7.7...... -. -.,-

/:H~4:1

N 0 20 40 60 80km E I I I I l

~ MATSAPFORMATION ] CAMPBELLRAND SUBGROUP • Unconformity

~ MAKGANYENEand/or ONGELUK FORMATION ~ SCHMIDTSDR1FSUBGROUP Thrustplanes of D3,4,5. All other thrusts have D 1 above unconformity I------I or D 2 age. Possiblyreactivated during D 4. KOEGASSUBGROUP ~ ZEEKOEB~RT~OR~TION

~ Above:GRIQUATOWN BIF ~ BASEMENTGRANITE Intensityof cleavageshows degree of Below: KURUMANBIF tectonicthickening during ramp climb

?ig. 24. Geologicalsection through the Early Proterozoic coverrocks of the southwestern KaapvaalCraton. See Fig. 1 for location. The figure s schematicin so far that all thrusts have not yet been mapped, and many more may exist than shown here. Note the tectonic duplicationthat nust occur in the 900 m Westerberg borehole.The reason for the 500 m Koupoort hole not penetratingthe BIF is obvious, but see also Fig. I 1. :or further descriptionrefer to text. 164 W. ALTERMANN AND l.W. HALBICH

Zeekoebaart Formation and Marydale Gran- from the present craton margin (Figs. 1 and ite onto the craton because of the younger T3- 24). Stowe (1986) found several phases of thrusts coming from the north and which thin-skinned thrusting that developed from probably follow a major post-Postmasburg pre- 1.75 Ga onwards in Middle Proterozoic rocks Olifantshoek erosion surface in this area. overlying the Kaapvaal Craton in the Kheis Figure 24 implies that the Early Proterozoic Subprovince. Because of competency differ- DI-D2 deformations ( > 2240 Ma and < 2432 ences, still older thrusts and folds have devel- Ma) have come from root zones on the Mary- oped mainly in the Asbesheuwels but also in dale High and beyond it to the west, where the the Koegas Subgroups and the apparently sim- Namaqua Province (deformed much later at ple lithological sequence (Beukes, 1980, 1983 ) high-grade metamorphism), is found today. actually represents a tectonically thickened and The many macro-structures and the riebeckite partly duplicated pile (Fig. 24). Stratigraphic mobilization along thrust zones of course can thickness of various units that have so far been not be depicted on this scale. However, rie- recorded from the western Kaapvaal Craton beckite mobilization is borne out by the fol- are therefore definitely overestimated, which lowing facts: has certainly led to some stratigraphic and en- (i) Only riebeckite lutites developed a pen- vironmental misinterpretations in the past. The etrative cleavage in folds in and above dislo- treatment of this effect is beyond the scope of cation planes. this paper. (ii) Selective solution along these cleavage If the Griquatown Fault (Fig. 1 ) is viewed planes supplies the necessary material to be in the light of tectonic complications reported transported into riebeckite halos that accom- here, it seems likely that further thickening of pany many of the fold-thrust zones. the stratigraphic column to the south of it was (iii) Riebeckite lutites are also physically the influenced by thrusting and decollement, pos- most mobile stratigraphic units, injected into sibly following an older growth fault. and along thrust planes (Altermann and H~ilbich, 1990). Acknowledgements (iv) Commercial quantities of crocidolite grow as secondary fibre across dilating tension We greatly appreciate the hospitality of the fissures parallel to bedding planes. They con- farmers in the region, but especially of D.J. centrate in fold zones that are associated with Oosthuyzen and his family. Mr. S.D. Roos thrusts and decollements. drew our attention to important outcrops and (v) South of the Griquatown Fault, quartz was always interested in the progress of our is mobilized together with riebeckite, and ti- work. GEFCO permitted us to stay at Koegas- gers eye appears as a pseudomorphic replace- brug and the CSIR provided funds to under- ment of crocidolite along slip- and transverse- take this research. We acknowledge fruitful fibre. North of the Griquatown Fault, croci- discussions with C.W. Stowe. The help and ad- dolite is not regularly replaced by SIO2. vice of colleagues and friends at the Depart- ment of Geology, University of Stellenbosch is Conclusion sincerely appreciated. Mrs. S. Smit very pa- tiently prepared the various versions of this Substantial evidence exists that duplication manuscript until completion. of Transvaal strata by complex thrust-faulting References and -folding occurred all over the southwestern Kaapvaal Craton in Griqualand West. The Altermann, W. and H~ilbich, I.W., 1990. Thrusting, fold- thrust planes can be found at least 130 km away ing, and stratigraphy of the Ghaap Group along the STRUCTURAL HISTORY OF THE SOUTHWESTERN CORNER OF THE KAAPVAAL CRATON 165

south-western margin of the Kaapvaal Craton. S. Afr. Cornell, D.H. and Barton, E.S., 1979. Age of the Mary- J. Geol., 93: 556-616. dale Group banded iron formation by the PB-PB Armstrong, R.A., 1987. Geochronological Studies on Ar- method. Abstr., 18th Geol. Congr., Geol. Soc. S. Afr., chean and Proterozoic Formations of the Foreland of pp. 111-114. the Namaqua Front and Possible Correlations on the Cornell, D.H., Hawkesworth, C.J., Van Calsteren, P. and Kaapvaal Craton. Unpubl. Ph.D. thesis, University Scott, W.D., 1986. Sm-Nd study of Precambrian crus- Witwatersrand, 274 pp. tal development in the Prieska-Copperton region, Cape Armstrong, R.A., Compston, W., Retief, E.A. and Welke, Province. Trans. Geol. Soc. S. Afr., 89:17-28. H.J., 1986. Ages and isotopic evolution of the Venters- Coward, M.P. and Potgieter, R., 1983. Thrust zones and dorp volcanics. Geocongress '86, Extended Abstracts, shear zones of the margin of the Namaqua and Kheis Geol. Soc. S. Afr. Johannesburg, pp. 89-92. mobile belts, southern Africa. Precambrian Res., 21: Barton, E.S. and Burger, A.J., 1983. Reconnaissance iso- 39-54. topic investigations in the Namaqua Mobile Belt and Dahlstrom, C.D.A., 1969. Balanced cross sections. Can. implications for Proterozoic crustal evolution--Up- J. Earth Sci., 6: 743-757. ington Geotraverse. Spec. Publ. Geol. Soc. S. Afr., 10: Du Plooy, C.L.W., 1986. Die Stratigrafie en Struktuur van 173-191. die Asbesheuwel Formasie tussen Griekwastad en Barton, E.S., Armstrong, R.A., Cornell, D.H. and Welke, Niekerkshoop met Implikasies vir die Krokidoliet H.J., 1986. Feasibility of total-rock Pb-Pb dating of Genese. Unpublished M.Sc. thesis, University of Stel- metamorphosed banded iron formation; the Marydale lenbosch, 120 pp. Group, southern Africa. Chem. Geol., 59:255-271. Du Toit, A.L., 1945. The origin of asbestos deposits of Beukes, N.J., 1980. Lithofacies and stratigraphy of the South Africa. Trans. Geol. Soc. S. Afr., 1: 71-75. Kuruman and Griquatown Iron Formations, North- Fockema, P.D., 1967. Crocidolite and Associated Rocks ern Cape Province, South Africa. Trans. Geol. Soc. S. of the Kuruman Area in the Northern Cape. Unpub- Afr., 83: 69-86. lished Ph.D. thesis, Univ. Witwatersrand, Johannes- Beukes, N.J., 1983. Paleoenvironmental setting of iron burg, 189 pp. formations in the depositional basin of the Transvaal Hanekom, H.J., 1966. The Crocidolite Deposits of the Supergroup, South Africa. In: A.F. Trendail and R.C. Northern Cape Province. Unpublished D.Sc. thesis, Morris (Editors), Iron- Formation: Facts and Prob- Univ. Pretoria, 213 pp. lems. Developments in Precambrian Geology 6. Else- Harris, R.W., 1988. Examination of dextral transpression vier, Amsterdam, pp. 131-210. as a model for the development of thrusts and late folds Beukes, N.J. and Smit, C.A., 1987. New evidence for thrust in eastern Namaqualand. S. Aft. J. Geol., 91: 324-336. faulting in Griqualand West, South Africa: implica- Humphreys, H.C., van Schalkwyk, L. and Scott, W.D., tion for stratigraphy and the age of red beds. S. Afr. J. 1986. A geological and structural map of the Areachab Geol., 90: 378-394. Group succession at Prieska Copper Mines, northwest Burger, A.J. and Coetzee, F.J., 1973. Radiometric age Cape Province. S. Aft. J. Geol., 91 (3): 373-380. measurements on rocks from Southern Africa to the Humphreys, H.C., van Bever Donker, J.M., Scott, W.D. end of 1971. Bull. Geol. Surv. S. Afr., 58: 1-46. and van Schalkwyk, L., 1988. The early deformational Burger, A.J. and Potgieter, G.J.A., 1979. Note on U-Pb history of the eastern Namaqua Province: new evi- zircon ages from granitic rocks near Prieskapoort, dence from Prieska Copper Mines. S. Afr. J. Geol., 91 northwest Cape. Trans. Geol. Soc. S. Afr., 82: 271-273. (2): 174-183. Cilliers, J.J. le R., 1961. The Nature and Origin of Rocks Jahn, B., Bertrand-Sarfati, J., Morin, N. and Mace, J., of the Lower Griquatown Stage and the Associated 1990. Direct dating of stromatolitic carbonates from Deposits of Amphibole Asbestos in the Northern Cape, the Schmidtsdrif Formation (Transvaal Dolomite), with Special Reference to the Koegas-Prieska Area. South Africa, with implications on the age of the Ven- Unpublished D.Sc. thesis, Univ. Pretoria, South Af- tersdorp Supergroup. Geology, 18:1211-1214. rica, 168 pp. Moen, H.G.F., 1976. A Geological Investigation of an Area Clifford, T.N., 1970. The structural framework of Africa. North of Groblershoop, Northern Cape Province with In: T.N. Clifford and I.G. Gass (Editors), African Special Reference to the Wilgenhoutdrift Formation. Magmatism and Tectonics. Olivier and Boyd, Lon- Unpubl. Rep., Geol. Surv. South Africa. don, pp. 1-26. Moody, J.D. and Hill, M.J., 1956. Wrench-fault tectonics. Cornell, D.H., 1975. Petrology of the Marydale Metabas- Geol. Soc. Am. Bull., 67: 1207-1246. ites. Unpublished Ph.D. thesis, Cambridge Univer- Onstott, T.C., Hargraves, R.B. and Joubert, P., 1986. sity, Cambridge. Constraints on the tectonic evolution of the Namaqua Cornell, D.H., 1977. A post-Transvaal age for the Mary- Province III: Reconnaissance Paleomagnetic and 4°Ar/ dale Formation, Kheis Group, southern Africa. Earth 39Ar results from the Namaqua Province and Kheis Planet. Sci. Lett., 37:117-123. Belt. Trans. Geol. Soc. S. Afr., 89: 143-170. 166 W ALTERMANN AND I.W. H,~LBICH

Potgieter, G.J.A., 1981. Die Litostratigrafie en Struktu- Evolution of Southern Africa. Springer Verlag, New rele Aspekte van die Suidwestelike Rand van die Ka- York, N.Y., 523 pp. apvaal-Kraton, Wes van Prieska, Noord Kaapland. Theart, N.F.J., 1985. Copperton-Areachap Cu-Zn Mi- Unpublished Ph.D. thesis, University of the Orange neralization. Ph.D. thesis (unpublished), University Free State, 242 pp. of Stellenbosch, Stellenbosch, 329 pp. Ramsay, J.G., 1967. Folding and Fracturing of Rocks. Trendall, A.F., Compston, W., Williams, I.S., Armstrong, McGraw-Hill, New York, N.Y., 568 pp. R.A., Arndt, N.T., McNaughton, N.J., Nelson, D.R., Rogers, A.W. and Du Toit, A.L., 1909. Report on Geol- Barley, M.E., Beukes, N.J., Laeter, J.R.de, Retief, E.A. ogy of Parts of Prieska, Hay, Bristown, Carnavon, and and Thorne, A.M., 1990. Precise zircon U-Pb chro- Victoria West. Annu. Rep. 1908, Geol. Comm. Cape nological comparison of the volcano-sedimentary se- Colony. Longmans Green and Co., London, 491 pp. quences of the Kaapvaal and Pilbara cratons between SACS--South African Committee for Stratigraphy, 1980. about 3.1 and 2.4 Ga. 3rd Int. Archean Symposium, Perth, 1990, Extended Abstracts, pp. 81-83. Stratigraphy of South Africa, Part 1 (Comp. L.E. Vajner, V., 1974a. The Tectonic Development of the Na- Kent). Lithostratigraphy of the Republic of South Af- maqua Mobile Belt and its Foreland in Parts of the rica, South West Africa/Namibia, and the Republics Northern Cape. PRU Bull. 14, University of Cape of Bophuthatswana, Transkei and Venda. Handbook Town, 201 pp. Geol. Surv. S. Afr., 8, 690 pp. Vajner, V., 1974b. The Doringberg Fault and its relation Scott, W.D., 1987. A Lithostratigraphic and Metamor- to the post-Waterberg deformation. Trans. Geol. Soc. phic Profile Across the Kaapvaal-Namaqua Boundary S. Afr., 77: 295-300. North-west Cape Province. Unpublished M.Sc. thesis, Visser, D.J.L., 1944. Stratigraphic features and tectonics University of Stellenbosch, Stellenbosch, 215 pp. of portions of Bechuanaland and Griqualand West. Stowe, C.W., 1983. The Upington geotraverse and its im- Trans. Geol. Soc. S. Afr., 47:197-254. plication for craton margin tectonics. In: B.J.V. Botha Walraven, F., Burger, A.J. and Allsopp, H.L., 1982. Sum- (Editor), Namaqualand Metamorphic Complex. Spec. mary of age determinations carried out during the pe- Publ., Geol. Soc. S. Afr., 10: 147-172. riod April 1980 to March 1981. Ann. Geol. Surv. S. Stowe, C.W., 1986. Synthesis and interpretation of struc- Afr., 16: 107-114. tures along the north-eastern boundary of the Nama- Woodward, N.B., Boyer, S.E. and Suppe, J., 1989. Bal- qua tectonic province, South Africa. Trans. Geol. Soc. anced Geological Cross-Sections: An Essential Tech- S. Afr., 89: 185-189. nique in Geological Research and Exploration. Short Tankard, A.J., Jackson, M.P.A., Eriksson, K.A., Hobday, course in geology, Am. Geophys. Union, Washington, D.K., Hunter, D.R. and Minter, W.E.L., 1982. Crustal D.C., 132 pp.