Oblique collision and evolution of large-scale transcurrent ......tonic mechanisms leading to the present-day pattern of this part of the Neoproterozoic orogenic belt system of Namibia

Precambrian Research 136 (2005) 139–157

Oblique collision and evolution of large-scale transcurrentshear zones in the Kaoko belt, NW Namibia

Jirı Konopaseka,∗, Stephan Kronera, Shawn L. Kittb,Cees W. Passchiera, Alfred Kronera

a Institut fur Geowissenschaften – Tektonophysik, Universit¨at Mainz, 55099 Mainz, Germanyb Geological Survey of Namibia, P.O. Box 2168, Windhoek, Namibia

Received 4 June 2003; accepted 18 October 2004

Abstract

Early structures in the central part of the Kaoko orogenic belt of NW Namibia suggest that the initial stage of collision wasgoverned by underthrusting of the medium-grade Central Kaoko zone below the high-grade Western Kaoko zone, resulting inthe development of an inverted metamorphic gradient. In the Western zone, early structures were overprinted by a second phaseof deformation, which is associated with localization of the transcurrent Puros shear zone along the contact between the Westernand Central zones. During this second phase, extensive partial melting and intrusion of∼550 Ma granitic bodies occurred inthe high-grade Western zone. In the Central zone, the second phase of deformation led to complete overprinting of the earlyfoliation in the zone adjacent to the Puros shear zone, and to the development of kilometre-scale folds in the more distal parts.S ntral zonei continuedd ment ofe observed.T controllingt©

K

vF

sev-ajorandate

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train partitioning into transcurrent deformation along the Puros shear zone and NE–SW oriented shortening in the Ces consistent with a sinistral transpressional regime during the second phase of deformation. Transcurrent deformationuring cooling of the entire belt, giving rise to the localized low-temperature Village Mylonite Zone that separates a seglevated Mesoproterozoic basement from the rest of the Western zone in which only Pan-African ages have so far beenhe data suggest that the boundary between the Western and Central Kaoko zones represents a modified thrust zone

he tectonic evolution of the Pan-African Kaoko belt.2004 Elsevier B.V. All rights reserved.

eywords:Kaoko belt; Namibia; Oblique collision; Pan-African; Puros shear zone; Transpression

∗ Corresponding author. Present address: Czech Geological Sur-ey, Klarov 3, Prague 1, 118 21, Czech Republic.ax: +2 420 221951533.

E-mail address:[email protected] (J. Konopasek).

1. Introduction

Crustal-scale transcurrent shear zones occur ineral tectonic settings around the world. Several mshear zones such as the Alpine Fault in New Zealand the San Andreas Fault in California accommod

301-9268/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
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140 J. Konopasek et al. / Precambrian Research 136 (2005) 139–157

subduction-related plate motion at their tips (e.g.,Lamarche and Lebrun, 2000; Lebrun et al., 2000;Hole et al., 2000; McCrory, 2000). Other transcurrentshear zones accommodate continental collision, suchas the North Anatolian and Tibetan fault systems (e.g.,Bozkurt, 2001; Matte et al., 1996). A less spectacularbut equally important type of transcurrent faulting oc-curs in the upper plate of oblique subduction systemsparallel to the subduction zone and apparently accom-modating the strike-slip component of subduction (e.g.,Scheuber and Gonzalez, 1999). Nearly all these crustal-scale shear zones are only exposed in their upper parts,mainly because the structures are relatively young andlack a dip-slip component so that deeper (mylonitic)equivalents are not exposed. For this reason, the ge-ometry of these shear zones at depth and the way theyinteract with the external parts and thrust belts along-side have largely been ignored.

Several Neoproterozoic mobile belts, such as theMozambique belt of East Africa and the Ribeira andDom Feliciano belts of Brazil contain orogen-parallelcrustal-scale strike-slip fault systems (e.g.,Ring et al.,2002; Egydio-Silva et al., 2002; Frantz and Botelho,2000). Because of the age of these belts, much deeperlevels of the crust are exposed than in modern oro-gens, providing an opportunity to study the deep struc-ture of such belts and the relation with tectonics andmetamorphism in adjacent blocks. The Neoprotero-zoic Kaoko belt in NW Namibia contains a particularlywell-exposed example of a crustal-scale transcurrentse

longc Pan-A elt( arz ichw dW .( dCc okob hicf gingl on-t then de-v

and Dingeldey (1996)suggested contemporaneous de-velopment of these structures and ascribed this phe-nomenon to a transpressional regime as a combinationof pure- and simple-shear deformation.Goscombe etal. (2003a)attributed the fabric development in both theWestern and Central Kaoko zones to an early wrench-dominated transpressional phase, which created the ge-ometry of the belt by progressive sinistral shearing,and interpreted a gradual change in the orientation oflineations towards the foreland and folding of meta-morphic fabrics in the Central zone as a result of aprogressive change from wrench-dominated to moreconvergence-dominated collision. In any case, there is ageneral agreement that the sub-vertical fabric along thePuros shear zone represents a first deformation eventwhich is recognized in the central Kaoko belt, and thusthat the development of this shear zone controlled thestructural and metamorphic architecture of the entireKaoko belt (Durr and Dingeldey, 1996; Goscombe etal., 2003a,b).

We studied the development and localization of thePuros and adjacent shear zones and present a detailedanalysis of the succession of deformation structures de-veloped in the central part of the Kaoko belt. In contrastto previous models, we argue that the tectonic evolu-tion of the area is the result of a progressive deforma-tion involving a distinct early period of SSE-directedthrusting preceding the development of the Puros shearzone. Combining our structural data with descriptionsof metamorphic conditions and Pan-African intrusive,m e tec-t rn oft ofN

2

sentt enicb tero-z ndK ataCK in oft At-l ternA

hear zone, known as the Puros shear zone (Goscombet al., 2003a).

The Puros shear zone represents a ca. 400 kmrustal-scale structure that developed during thefrican tectono-metamorphic event in the Kaoko b

Fig. 1). In the central part of the belt, this sheone is developed at the contact of two units, where described byMiller (1983) as the Central anestern zones of the Kaoko belt.Dingeldey et al

1994), Durr and Dingeldey (1996), Stanistreet anharlesworth (2001)andGoscombe et al. (2003a)dis-ussed the structural evolution of the central Kaelt in which a shallow, westward dipping metamorp

oliation and associated west–northwestward plunineation in the eastern part of the Central zone crast with the steep westward inclined foliation andorth–northwest inclined, gently plunging lineationeloped in the area around the Puros shear zone.Durr

etamorphic and cooling ages, we discuss possiblonic mechanisms leading to the present-day pattehis part of the Neoproterozoic orogenic belt systemamibia.

. Geological setting

The Kaoko and Damara belts of Namibia reprewo perpendicular branches of the Damara orogelt system, which developed as a result of Neoprooic (Pan-African) collision between the Congo aalahari Cratons in Africa, and the Rio de la Plraton in South America (Fig. 1; Porada, 1989). Theaoko belt developed on the southwestern marg

he Congo Craton and is today exposed along theantic coast of northwestern Namibia and southwesngola.

J. Konopasek et al. / Precambrian Research 136 (2005) 139–157 141

Fig. 1. Schematic geological map of the northern part of Namibia showing position of the Pan-African Kaoko and Damara belts and of theCongo and Kalahari Cratons. Tectonic units of the Kaoko belt (wz: Western zone; cz: Central zone; ez: Eastern zone; ki: Kamanjab Inlier; ec:Epupa Complex; st: Sesfontein thrust) and of the Damara belt (nz: Northern zone; cz: Central zone; sz: Southern zone) afterMiller (1983).A black square delimits the position ofFig. 2. Cratons indicated in inset: A: Amazon; C: Congo; K: Kalahari; RP: Rio de la Plata; SF: SaoFrancisco; WA: West African. The Dom Feliciano Belt is located east of the Rio de la Plata Craton.

Ortho- and paragneisses of the Congo Craton are ex-posed in the Kamanjab Inlier and in the Epupa Complex(Fig. 1), but most of the craton in Namibia is hidden be-low sedimentary cover (the northern platform ofMiller,1983). Emplacement ages of granitoids in the Ka-manjab Inlier and granitoid gneisses of the EpupaComplex fall into the interval between 1662 and1860 Ma (Burger and Coerze, 1973; Burger et al., 1976;Tegtmeyer and Kroner, 1985), suggesting a complexmagmatic history in this part of the craton. Depositionof the Damara sedimentary sequence at the southwest-ern margin of the Congo craton began at about 756 Ma(Hoffman et al., 1996).

Miller (1983) subdivided the Kaoko belt into threestructurally contrasting tectonic units resulting from

Pan-African deformation and metamorphism (Fig. 2).From east to west these are: (a) The Eastern Kaokozone, representing a low-grade, folded, autochthonoussedimentary cover of the Kamanjab Inlier (Dingeldeyet al., 1994; Goscombe et al., 2003a). To the west, theEastern Kaoko zone is bounded by the low-angle Ses-fontein thrust. (b) The Central Kaoko zone is exposedto the west of the Sesfontein thrust and is interpreted asa fold-and-thrust belt with basement-cored, kilometer-scale steep to recumbent anticlines and thrust sheets(Guj, 1970; Stanistreet and Charlesworth, 2001; Seth etal., 1998; Goscombe et al., 2003a). Franz et al. (1999),Gruner (2000)andGoscombe et al. (2003b)describedBarrovian-type metamorphism in Damaran metased-iments of this zone with the metamorphic grade


Fig. 2. Schematic geological map of the central part of the Kaoko belt (see upper right inset for actual position of the figure). Tectonic zones ofthe belt are delimited afterMiller (1983), references related to ages of granitoid rocks are listed in the text.

increasing towards the west. The western boundaryof the Central Kaoko zone is the Puros shear zone.The Western Kaoko zone extends from the Purosshear zone to the Atlantic coast and experienced high-temperature/low-pressure metamorphism (Franz et al.,1999; Gruner, 2000; Goscombe et al., 2003b) and sinis-tral strike-slip deformation refolding an earlier meta-morphic fabric (Miller, 1983; Durr and Dingeldey,1996; Goscombe et al., 2003a). Dingeldey et al. (1994)suggested a structural and lithological subdivision ofthe Kaoko belt which only partially corresponds to thatof Miller (1983). However, the zones ofMiller (1983)

better highlight differences in thermal and structuralevolution of the Kaoko belt and are therefore used inthis paper.

Because of lack of infrastructure and considerabletopographic relief, the Kaoko belt is best studied inNE–SW trending dry river valleys. Best exposed are thevalleys of the Hoanib, Hoarusib and Gomatum rivers(Fig. 2). Recent geochronological investigations in theHoanib River valley (Franz et al., 1999; Seth et al.,1998) have shown that there is Archaean basement inthe Central zone (zircon ages of 2584–2645 Ma) aswell as 1933–1985 Ma Palaeoproterozoic metagrani-


toids. The easternmost part of the Western zone hasa single basement zircon age of 1507 Ma and a Pan-African monazite age of 554 Ma. In other parts ofthe Western zone, two groups of Pan-African agesof 656–645 Ma (zircon ages) and 552–580 Ma (zir-con and monazite ages) have been found. A similardistribution of ages occurs in a river section farthernorth, the Gomatum–Hoarusib section (Fig. 2). Anage of∼1980 Ma and imprecise ages between 2153and 2384 Ma have been obtained byKroner et al.(2004) for metagranitoids at the western margin ofthe Central zone.Kroner et al. (2004)have reportedtwo groups of basement zircon ages of 1500–1530 Maand 1650–1680 Ma, respectively, for orthogneisses andmigmatites of the easternmost part of the Western zoneand a Pan-African zircon ages between 730 and 550 Mafor granites in the Western zone.

3. The Puros shear zone

Field observation suggests that the Puros shearzone represents an important boundary between thestructural and metamorphic units described above.Sillimanite- and cordierite-bearing low-pressure min-eral assemblages dominate the eastern part of the West-ern zone west of the Puros shear zone, whereas a typi-cal Barrovian succesion of mineral assemblages is de-veloped in metasediments eastward of the Puros shearzone (Gruner, 2000; Goscombe et al., 2003b). A hetero-g nitics dayr uctureo s isd h-t tactb d thise s theh tivity.

4m

ies( rn-m stlyo ani-

toid orthogneisses covered by migmatitized metasedi-ments of unknown age (Figs. 2 and 3). This segment ofexhumed basement is bordered to the southwest by amajor shear zone defined byGoscombe et al. (2003a)as the Village Mylonite Zone (Fig. 3). Farther to thesouthwest, only Pan-African granitoids were observed,and limited mapping has shown only metasedimen-tary country rocks. The Puros shear zone representsthe northeastern limit of the Mesoproterozoic segment,and the Central zone exposed to the east is domi-nated by metasediments with exhumed Palaeoprotero-zoic granitoids in cores of kilometre-scale folds. TheMesoproterozoic segment exposed between the Purosand Village shear zones obviously represents a zoneof greater exhumation of a granitoid basement, the ageof which cannot be compared to the Palaeoproterozoicages recorded in the easterly Central Kaoko zone.

Gruner (2000)and Goscombe et al. (2003b)de-scribed metamorphism along the Gomatum–Hoarusibvalley section. In the Central Kaoko zone, meta-morphic conditions are characterized by a mediumdP/dT (Barrovian type) metamorphic gradient withincreasing grade towards the west (Fig. 2). Forparticular mineral zones metamorphic conditions wereestimated by Gruner (2000) as follows: garnetzone—500± 30◦C/9± 1 kbar; staurolite zone—580± 30◦C/7–8 kbar; kyanite zone—590± 30◦C/6.5–8kbar and kyanite–sillimanite–muscovite zone—650± 20◦C/9± 1.5 kbar. Goscombe et al. (2003b)provided PT data of 534± 47◦C/9± 1.1 kbarf nd6 rto on-d bya als –K-f parz ionsc7o8e eadm ownt theC d at∼i one

eneous network of several metres wide ultramylohear zones trending parallel to the main foliation toepresents the Puros shear zone as a marked strn the satellite images. This array of ultramyloniteeveloped within a∼10 to 15 km zone of steep, hig

emperature metamorphic foliation along the conetween the Central and Western Kaoko zones, anntire zone of subvertical fabric probably representigh-temperature stage of the Puros shear zone ac

. Basement–cover relationships andetamorphism in the Puros area

Detailed mapping and geochronological studKroner et al., 2004) have shown, that the easteost part of the Western Kaoko zone is built mof Mesoproterozoic basement migmatites and gr

or the central part of the Central zone a58± 45◦C/8.5± 1.6 kbar for the western paf the Central zone. In contrast, metamorphic citions in the Western zone are characterized

low dP/dT metamorphic gradient. Petrologictudies have shown the presence of sillimaniteelspar and garnet–cordierite–sillimanite–K-felsones, and estimated metamorphic conditorresponding to 690± 40◦C/4.5± 1 kbar and50± 30◦C/4–5.5 kbar, respectively (Gruner, 2000)r, alternatively, 843± 64◦C/8.1± 1.6 kbar and11± 58◦C/6.2± 0.7 kbar, respectively (Goscombet al., 2003b) are consistent with observed widesprelting. Garnet Sm–Nd geochronology has sh

hat the matrix assemblages in samples from bothentral and Western Kaoko zones have forme576± 15 Ma (Goscombe et al., 2003b). Our prelim-

nary petrological investigations of the Central z

144J.K

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136(2005)139–157

Puros, two reconnaissance mapping profiles in the area NE ofs show the position of cross-sections presented inFigs. 4 and 6,ne labeled PSZ represents approximate position of the axial parte Mylonite Zone.

Fig. 3. Geological map of the area studied. The map is the result of field mapping in the area SW of the village ofPuros, and of the interpretation of satellite images and aerial photographs of the area. Numbered solid black linesolid black rectangles delimit areas with well-preserved D1 structures (D1 domains—see text). Heavy solid black liof the Puros shear zone. The same line labeled VMZ represents approximate position of the axial part of the Villag


metasediments in the area north of the village of Puroshave shown the presence of the kyanite–sillimanite–K-feldspar assemblage and thus confirm the presenceof a medium dP/dT metamorphic gradient. Thekyanite–sillimanite–muscovite zone gradually passesinto the low-pressure sillimanite–K-feldspar zone,which seems to terminate at the Puros shear zone.Our investigation of metasediments of the Westernzone revealed low-pressure metamorphic conditionsdominated by a garnet–sillimanite–biotite–K-feldsparassemblage with rare occurrence of spinel-bearingmetasediments.

5. Structural evolution west of the Puros shearzone

The eastern margin of the Western Kaoko zone(west of the Puros shear zone) is dominated by steepto subvertical metamorphic foliations. However, sev-eral domains close to the village of Puros (furthertermed as the D1 domains; seeFig. 3) show onlya moderately refolded, shallowly inclined metamor-phic foliation, suggesting that the widespread subver-tical fabric is the result of later stages of deform-ation.

5.1. Deformation phase D1

D1 is characterized by the development of a shallow-d -p atym lite,a pre-f s.I db ons.B Sv -g theWi el ldedLa rma-t ideu rva-t ent

migmatites and orthogneisses over their sedimentarycover, but the amount of displacement is difficult toestimate.

5.2. Deformation phase D2

All domains with well-preserved D1 structures wereheterogeneously affected by D2, which resulted in re-folding of the S1 foliation (Fig. 5a) and rare devel-opment of an S2 fabric. Open to close F2 folds arecommon and have subvertical axial planes and foldaxes plunging moderately to the NW, parallel to the L1or newly developed L2 lineation. Domains with pre-served D1 structures are sharply bounded by a pene-trative subvertical S2 foliation, which is dominant inthe entire eastern part of the Western zone, includingthe Puros shear zone itself. Associated L2 lineationsplunge at shallow angles to the NW, parallel to thelineation developed on the S1 foliation planes. Nu-merous close to isoclinal F2 folds preserved in theS2 foliation (Fig. 5b) support the idea of a D2 ori-gin for the subvertical foliation in this area. Numer-ous kinematic indicators such as�-objects, asymmet-ric boudinage and shear bands suggest a sinistral senseof shear during D2. Rarely, high-temperature dextralshear bands have been observed in orthogneisses. Weinterpret these structures as passively rotated D1 shearbands.

The S2 foliation is of the same character as S1.The development of a subvertical S2 foliation coin-c o-s aves andsl ilars t thee ed-i east chm einso othm , ex-tb ketsh -s tran-s eadm earz

ipping migmatitic foliation S1. S1 is defined by comositional layering and preferred orientation of plinerals in metasedimentary rocks and amphibond by foliation-parallel oriented leucosomes and

erred orientation of biotite in granitoid migmatiten the orthogneisses the S1 foliation is characterizey the development of quartz and feldspar ribbecause of later refolding, the dip orientation of1aries between SSW and NNE (Fig. 4a). L1 is an agregate or grain lineation and plunges mostly toNW at shallow to moderate angles (Fig. 4a). In local-

ties with strongly folded S1, it is not clear whether thineation observed on the fold limbs represents fo1 or newly developed L2 lineation. Isoclinal F1 foldsre rare and were refolded by later phases of defo

ion. Kinematic indicators are rare and do not provnambiguous sense of movement during D1. Obse

ions in the D1 domains show thrusting of the basem

ided with the formation of foliation-parallel leucomes in all lithologies. Granitoid migmatites htromatitic character, and leucosomes form long bocally connected by melt-filled shear bands. Simtructures are developed in the orthogneisses, buxtent of melting is lower. Strongly anatectic metas

ments usually show lower amounts of biotite, wherhe degree of partial melting is low in biotite-rietasediments. Medium- to very coarse-grained vf decimetre to metre width are developed in betasedimentary and meta-igneous units. Rarely

rusion of melt into the hinge zones of F2 folds haseen observed, forming aligned zones of melt pocighlighting the S2 axial planes (Fig. 5d). These obervations suggest that thermal conditions of theition between D1 and D2 coincide with widesprigmatitization observed W of the Puros sh

one.


Fig. 4. Geological cross-sections over the Western Kaoko zone in the Puros area (seeFig. 3for position of cross-sections). (a) The cross-sectionover the northeastern part of the Western Kaoko zone showing the relationship between the D1 and D2 deformations. The presence of foldedD1 domains (NE part of the cross-section) suggests early flat-lying D1 metamorphic fabric. To the SW, S1 fabric becomes intensively foldedand later completely overprinted by subvertical S2 foliation. (b) The cross-section over the southwestern part of the studied area shows relicsof subvertical S2 fabric in the pre-550 Ma granitoids, and its reworking by D3 deformation. In the 550 Ma old Amspoort-type granite, only onesolid-state fabric (S3) is developed. Progressive steepening of the S3 fabric and stress concentration along the NE edge of the Amspoort-typegranite during D3 is responsible for the development of the Village Mylonite Zone.


5.3. Structural position of∼550Ma Pan-Africanintrusions

Apart from widespread migmatitization, Pan-African high-temperature metamorphism in the studyarea resulted in the intrusion of several granitoid bod-ies. The most prominent is the intrusion of the por-phyritic Amspoort monzogranite dated at∼550 Ma(sample BK19 inSeth et al., 1998and samples NA00/14 and 118/2 inKroner et al., 2004). This mon-zogranite forms a NW–SE elongated sheet-like body(Fig. 3) suggesting emplacement into the S2 fab-ric. However, porphyritic granitoids invaded both theflat-lying S1 and vertical S2 fabrics. This suggeststhat magmatic activity coincided with the progressivedevelopment of S2. Limited ascent of melt pocketsfrom migmatitized basement during D2 suggests thatmigmatitization and the intrusion of large granitoidbodies were contemporaneous processes. This inter-pretation is supported by the fact that the age of a leuco-cratic melt filling neck zones of D2 boudins is the sameas the age of the Amspoort-type intrusion (Kroner etal., 2004).

5.4. D3 fabric and the Village Mylonite Zone

The granitoid gneisses immediately southwest ofthe Amspoort-type intrusion (Figs. 3 and 4b) showtwo crosscutting fabrics. A subvertical and partlymigmatitic planar fabric is overprinted by a moder-as u-a yingd e ofi ub-v t-t e-f ma-t

ab-r ved,a to-g bver-tl isz isi-b d byG e

(Figs. 3 and 4b). The S3 fabric of the Village MyloniteZone developed under low-temperature (greenschist-facies) conditions, which is documented by the brittlebehaviour of feldspars accompanied by ductile defor-mation of quartz. Numerous discrete shear zones ofbrittle–ductile deformation have been observed parallelto the subvertical D2 fabric. In this case, the distinctionbetween S2 and S3 is not based on fabric orientationbut on the temperature of deformation.

An L3 aggregate lineation is associated with S3. Incontrast to L2, L3 is horizontal or inclined only fewdegrees towards WNW or SSE (Fig. 4b). Rotated K-feldspar porphyroclasts in the granite (Fig. 5h), as wellas�- and�-objects in mylonitic orthogneisses consis-tently show a sinistral sense of shear. F3 folds and kink-band folds with fold axes plunging moderately to theSE were observed in metasediments in the core of theVillage Mylonite Zone (Fig. 5c).

6. Structural evolution east of the Puros shearzone

The deformation fabric along the western mar-gin of the Central Kaoko zone (east of the Purosshear zone) is characterized by large-scale antiformsmostly composed of metasediments and cored byPalaeoproterozoic granitoids (Figs. 3 and 6). The south-western limb of a kilometer-scale antiform immedi-ately adjacent to the Puros shear zone shows numerousm eira foli-a ric( hes shearz atici -t eren rm.I to re-f ss uth-w llelt reo tors(s limb.T D1

tely dipping, non-migmatitic foliation (Fig. 4b). Thetrain intensity of the non-migmatitic foliation gradlly increases towards the contact with the underleformed Amspoort-type granitoid body. Becaus

ts orientation and migmatitic nature, relics of a sertical foliation are interpreted to be S2. The crosscuing fabric, labelled S3, developed during amphibolitacies conditions as documented by ductile deforion of feldspars.

Towards the NE, a gradual transition of the D3 fic into a subvertical orientation has been obsernd the NE edge of the Amspoort-type intrusionether with adjacent metasediments contain a su

ical solid-state mylonitic S3 fabric (Fig. 5i), paral-el to the S2 fabric of the adjacent migmatites. Thone of intense solid-state deformation is also vle on the satellite image and has been defineoscombe et al. (2003a)as the Village Mylonite Zon

etre- to decimetre-scale tight to isoclinal folds. Thxial planes are parallel to the northwest-dippingtion, suggesting D2 refolding of an earlier D1 fabFigs. 5e and 6). An L2 lineation in this area has tame orientation as that observed W of the Purosone, e.g. shallowly plunging to the NW, and kinemndicators such as asymmetric�-objects show a sinisral sense of movement. Close to isoclinal folds wot observed in the northeastern limb of the antifo

nstead, open to close recumbent folds were seenold a steep foliation (Figs. 5f and 6). Their axial planehow similar orientations as the foliation on the soestern limb of the antiform, and fold axes are para

o the L2 lineation. The granitoid gneiss in the cof the antiform shows numerous kinematic indicashear bands and isoclinal folds,Fig. 5g) with oppositeense of shear to indicators in the southwesternhe most plausible explanation of this feature is a



origin for these indicators in the (probably flat-lying)S1 foliation and their passive rotation during D2.

We interpret the fabric of the antiform as a result ofD2 deformation superimposed on an early shallowlydipping D1 fabric. The southwestern limb of the an-tiform represents an early D1 fabric, which was rotatedand shortened during D2 as indicated by numerous tightto isoclinal F2 folds. In contrast, lack of such folds inthe northeastern limb suggests passive rotation of theS1 foliation with minor folding during D2. The devel-opment of open, recumbent folds is late and probablyattributed to the increasing role of overburden after re-laxation of the D2 deformation in this part of the studyarea.

7. Discussion

7.1. D1 deformation and associated metamorphicconditions

The presence of D1 domains in the area southwest ofthe Puros shear zone is strong evidence that the devel-opment of the subvertical S2 fabric is a secondary fea-ture affecting already existing metamorphic fabric. Thequestion arises, whether the D1 and D2 fabrics havedeveloped contemporaneously during transpressive de-formation or if there is an evidence for two successivestages of their development. The theory of deforma-tion partitioning involving thrusting and strike-slip de-formation during transpression (Teyssier et al., 1995)a zonea reast ora-n e, thep shearz ion.W uch a

ductile conditions would result in the development ofsteep fabric slightly oblique to the major shear zone,whereas more distal, low-temperature parts might de-velop discrete zones of thrusting. We do indeed seethis process, but it is superimposed on already presentmetamorphic foliation. Moreover, in the case of con-temporaneous development of flat-lying foliation andsubvertical transcurrent shear zones, the reverse caseshould be developed in which ductile, flat-lying fabrictransects existing vertical foliation of the transcurrentshear zones. This case has never been observed. Thereis no indication of any high-temperature event betweenthe emplacement of basement orthogneiss protolith(∼1500 Ma) and the Neoproterozoic (∼575–550 Ma)deformation and magmatism in the basement rocks ofthe study area (Goscombe et al., 2003b; Kroner et al.,2004), which would allow interpreting the observedD1 fabric as pre-Neoproterozoic. In our interpretation,the parallelism of L1 and L2 lineations and ubiquitouspresence of relics of pre-D2 metamorphic foliation sug-gests that the D1 and D2 fabrics are part of a single pro-gressive deformation that was separated in two distinctHT–MT deformation phases. Some authors considersuch a two-phase evolution as a common tectonic pro-cess on convergent boundaries (McClellan et al., 2000).

The flat-lying S1 foliation and WNW plung-ing L1 lineation in the D1 domains suggest thatD1 deformation was associated with east–southeast-directed thrusting, oblique with respect to the west-ern flank of the Central Kaoko zone. Local D1t sed-i wella ismi eta )a a-m

F West ft -band one.( abric a foldss ure con thec diment margin oft f the c tral zone.T of mo core of theV ldspar icrographo par po documents

ssumes the development of a transcurrent shearccommodating the simple-shear component, whe

he deformation in the adjacent regions is contempeously accommodated by pure shear. In our casure shear-dominated regions outside the Purosone clearly show folded high-temperature foliate argue that any pure shear deformation under s

ig. 5. (a) Relic migmatitic S1 foliation in orthomigmatites of thehe Western zone, surrounded by subvertical S2 foliation. (c) F3 kinkd) Migmatitic S1 foliation folded during D2. Flat migmatitic S1 fuggest, that the D1–D2 transition occurred at high-temperatontact with the Central zone. (f) Recumbent folds in metasehe Central zone. (g) Isoclinal fold in granodioritic migmatites ohe foliation is subvertical and the fold suggests dextral senseillage Mylonite Zone. The foliation is vertical and aligned K-fef deformed porphyric Amspoort-type granite. Relics of K-feldsolid-state deformation of this granitoid body.

hrusting of basement migmatites over their metamentary cover observed in the D1 domains, ass the development of the inverted metamorph

n the studied area (Gruner, 2000; Goscombel., 2003b) support this interpretation.Gruner (2000nd Goscombe et al. (2003b)estimated peak metorphic pressures of∼9 kbar at ∼650◦C for the

ern zone folded by F2 folds. (b) Isoclinal F2 folds in amphibolites ofolds associated with the deformation on the Village Mylonite Znd alignment of melt pockets in steep axial zones of the F2ditions. (e) Large F2 fold in orthomigmatites of the Western zone ats of the eastern limb of a large-scale antiform at the westernore of a large-scale antiform at the western margin of the Cenvement. (h) Deformed porphyric Amspoort-type granite in thepophyroclasts show sinistral sense of movement. (i) Photom

rphyroclasts and ribbons of recrystallized quartz and feldspars

150J.K

onopaseketal./P

recambrianResearch

136(2005)139–157

Fig. 6. Geological cross-sections over the westernmost part of the Central Kaoko zone in the Puros area (seeFig. 3 for position of cross-sections; the arrows show actual positionof the Okamaruru and Okosume rivers). The fabric is characterized by a large-scale antiform built of metasediments with exhumed granitoid basement in its core. SW limb showsnumerous closed to isoclinal F2 folds suggesting its strong shduring D2 event. Gentle refolding of the steep fabric by recu

ortening during D2. In contrast, lack of these folds in the NE limb suggests only passive rotation of the S1 foliationmbent kink-band folds suggests deformation activity during late D2 to D3.


westernmost (kyanite–sillimanite) zone of the Cen-tral Kaoko zone. Metamorphic conditions in the east-ern part of the Western Kaoko zone (sillimanite–K-feldspar zone) were estimated at∼690◦C at∼4.5 kbar(Gruner, 2000), or even 750–900◦C at 7–10 kbar(Goscombe et al., 2003b) which are consistentwith observed melting during D1–D2. The pres-ence of a garnet–sillimanite–biotite–K-feldspar as-semblage in the folded S1 foliation confirms thathigh-temperature/low-pressure conditions existed atpeak conditions during D1. Evidence for an ear-lier medium- or high-pressure history of the WesternKaoko zone has been provided byGoscombe et al.(2003b)who described inclusions of kyanite from gar-nets in the low-pressure assemblages. The low-pressurere-equilibration of the westernmost part of the Cen-tral Kaoko zone suggests the onset of equilibration ofmetamorphic conditions between the high-temperatureWestern zone and the medium-temperature Centralzone, probably during transition between D1 andD2. In any case, high-temperature metamorphism andmigmatitization during D1 in the Western Kaoko zoneand medium-pressure/medium-temperature metamor-phic conditions in the western part of the Central Kaokozone (Gruner, 2000; Goscombe et al., 2003b) suggest,that D1 thrusting resulted in the development of a tec-tonically induced inverted metamorphic gradient in thecentral Kaoko belt.

7.2. D2 and the development of the Puros shearz

entds ingL ata terna rma-t inga -D1e tingt en-t liq-u or-m oft g oft ved.S e hot

migmatitic Western Kaoko zone and colder CentralKaoko zone led to the development of a large-scaletranscurrent sinistral shear zone—the Puros shear zone.The high intensity of deformation along this shear zoneis demonstrated by an almost complete reworking ofthe D1 fabric to the southwest, and by strong short-ening and deformation of the southwestern limb of thekilometer-scale antiform at the southwestern margin ofthe Central Kaoko zone (Figs. 4a and 6).

Extensive migmatitization reached a peak duringthe transition from D1 to D2. Emplacement of por-phyritic granitoids into both the S1 and S2 fabrics coin-cided with this transition. The Amspoort-type graniteintruded mostly into the S2 subvertical fabric, which issupported by field observation and by marked elonga-tion of this granitoid body parallel to the D2 fabric of theWestern Kaoko zone. Similar ages for the Amspoort-type granite and leucocratic melt in migmatites (Kroneret al., 2004) is strong evidence that granite emplace-ment coincided with peak metamorphic conditions insurrounding lithologies.

7.3. D3 and the evolution of the Village MyloniteZone

Solid-state deformation of the Pan-AfricanAmspoort-type granite (Fig. 5i), as well as medium-to low-temperature deformation of surroundingmetasediments and basement granitoids suggeststhat the D3 phase played an important role duringd oner ationa typei rdingt t oft itht chA theP zonef ireu oreo ntiree ver,c iono ento at,a theV e

one

Refolding of S1 in the D1 domains and subsequevelopment of a steep to sub-vertical S2 foliation is as-ociated with the development of a shallowly plung2 lineation (Figs. 4a and 6). These data suggest thll rock types along the contact between the Wesnd Central Kaoko zones underwent strong defo

ion associated with NNE–SSW oriented shortennd WNW–ESE stretching. We interpret the postvolution in the Puros area as follows: The contrashermal state between the colliding Western and Cral Kaoko zones, as well as a high degree of obity during thrusting, led to concentration of defation in the rheologically weaker, migmatitic unit

he Western Kaoko zone after maximum thickeninhe downgoing Central Kaoko zone had been achietress concentration along the contact between th

ecreasing temperatures. The Village Mylonite Zepresents a zone of concentrated D3 deformlong the northeastern edge of the Amspoort-

ntrusion and the adjacent metasediments. Regahe localization of deformation, the developmenhe Village Mylonite Zone is strongly coupled whe earlier evolution of the D2 fabric. Biotite-rimspoort-type granite (elongated parallel withuros shear zone) acted as a rheologicaly weak

or strain localization during cooling of the entnit. High-temperature D2 deformation was mr less homogeneously distributed over the eastern margin of the Western Kaoko zone. Howeooling of the entire unit enhanced localizatf deformation and resulted in the developmf the Village Mylonite Zone. We assume thfter cooling of the unit (e.g., during the D3),illage Mylonite Zone took over the role of th



Puros shear zone and accommodated most of the D3strain.

7.4. Tectonic evolution of the Puros area

In previous studies, the structure of the centralKaoko belt has been interpreted as a result of trans-pressive deformation with strain partitioning into tran-scurrent and overthrusting components (Durr andDingeldey, 1996; Durr et al., 1995; Goscombe etal., 2003a). In this view, both the subvertical high-temperature fabric along the Puros shear zone and themore shallow, medium-temperature fabric of the Cen-tral Kaoko zone developed at the same time. The obser-vations of the above authors lead them to conclude thatthe structure of the central Kaoko belt represents a halfpop-up structure generated during transpression (Durrand Dingeldey, 1996; Durr et al., 1995; Goscombe etal., 2003a). However, our data document two sepa-rate HT–MT deformation phases responsible for theformation of the present-day structural pattern of theCentral and Western zones, accompanied by localizedbrittle–ductile LT deformation.

In our interpretation, the structures associated withD1 deformation indicate oblique thrusting as a mech-anism responsible for crustal thickening of the CentralKaoko zone (Fig. 7a). There are several examples fromhigh- to low-grade terrains, which document the earlydevelopment of a thrust-related fabric and its subse-quent reworking by localized large-scale transcurrents sia eta st,a aysa or-p ntsw

et al., 2001), and the change in tectonic regime towardsthe development of localized (at any scale) transcurrentshear zones seems to be the result of increased resis-tance of the down-going plate due to buoyancy forces.In the case of the central Kaoko belt, thickening is docu-mented by Barrovian-type medium-pressure metamor-phism in metasediments of the Central zone. Estimatedpeak pressure metamorphic conditions of∼9 kbar at∼650◦C (Gruner, 2000; Goscombe et al., 2003b) sug-gest that the western part of the Central zone must havebeen buried to depths of∼35 km. A metamorphic gra-dient with decreasing conditions towards the east indi-cates preservation of a palaeogradient developed dur-ing oblique underthrusting of the Central Kaoko zonebelow the Western Kaoko zone.

A collisional scenario requires at least the samedepth of burial for the downgoing Central zone andoverriding Western zone prior to its exhumation bythrusting. Kyanite inclusions in garnets from the LP as-semblage of the Western zone (Goscombe et al., 2003b)suggest an earlier MP–HP history for this unit, but lackof a corresponding mineral assemblage does not al-low any PT estimates. Metasediments in the D1 do-mains only show sillimanite-bearing low-pressure as-semblages in the matrix, and meta-igneous lithologiesshow the formation of foliation-parallel leucosomes.The obvious contrast between low-pressure assem-blages developed during D1 oblique thrusting in theoverriding unit and preservation of medium-pressureassemblages in the underlying metasediments suggestt ne)w thant her-m u-i un-d ablyl gh-

F nts res lt. (a) Thed oblique aoko zone( l leuco evelopmento ntral K maximump que thr WSW–ENEs accom uros shearz zones. c bodies intos gime o decreasingt nes of Zone is ther of the

hear zones (e.g.Vassallo and Wilson, 2002; Carond Palmeri, 2002; Little et al., 2002; McClellanl., 2000). Especially in the middle and lower crusignificant period of underthrusting probably pl

n important role in the development of a metamhic fabric during oblique collision of crustal segmeith initially different rheological profiles (Thompson

ig. 7. Block diagrams showing three major deformation eveevelopment of flat-lying D1 fabric suggests an early period ofupper plate) already show the development of foliation-parallef inverted metamorphic gradient in metasediments of the Ceossible thickening of the Central zone has been attained, oblihortening associated with strong folding of all lithologies andone developes along the contact of the Central and Westernubvertical D2 fabric suggest that this change in deformation reemperature conditions and concentrates into narrow shear zoesult of concentration of deformation at the northeastern edge

hat the overriding unit (i.e., the Western Kaoko zoas subjected to a higher geothermal gradient

he underlying Central Kaoko zone. Elevated tal conditions in this unit as well as influx of fl

ds released from progressively metamorphosederlying metasediments of the Central zone prob

ed to complete transformation of potential early hi

ponsible for the present-day geology of the central Kaoko bethrusting. During this event, basement rocks of the Western K

somes. Northwestward underthrusting is responsible for the daoko zone (western margin of the Congo Craton). (b) After

usting changes into sinistral transpression. This is justified bypanied by strong NNW–SSE stretching. At this stage the P

Enhanced partial melting and intrusion of dykes and magmaticcurs at peak-temperature conditions. (c) D3 is associated withbrittle-ductile character. The low-temperature Village MyloniteAmspoort-type intrusion.


to medium-pressure assemblages and enhanced earlymigmatitization in the D1 domains.

The development of a subvertical fabric along thecontact between the Central and Western Kaoko zonesrepresents a separate phase of deformation overprint-ing an early flat-lying fabric (Fig. 7b). Oblique thrust-ing ceased when metasediments of the western mar-gin of the Central zone were buried to depths of30–40 km, as inferred from recorded peak pressures.At this stage the overriding Western unit was alreadypartially molten and thus weaker than the underly-ing medium-temperature unit. The resulting rheologi-cal contrast and increasing buoyancy in the downgoingcrustal segment inhibited further underthrusting of theCentral zone under the Western zone, and the regimeof D1 oblique southeastward thrusting changed into D2sinistral transcurrent movement. Since the beginning ofD2, most deformation was of strike-slip character andconcentrated in the weak Western zone, as documentedby the almost complete transformation of the early D1fabric. In the Central Kaoko zone, the intensity of theD2 overprint decreases rapidly with increasing distancefrom the Western zone, and approximately 20 km fromthe contact with the Western zone, the early metamor-phic fabric was only refolded by large-scale folds sug-gesting pure shear-dominated deformation. The com-bination of strike-slip deformation along the contactof the Western and Central zone and pure shear defor-mation in more distal parts is consistent with a sinistraltranspressional regime (Sanderson and Marchini, 1984;T gt theP silli-m canb ationi , andt ap-p ne(

7e

l Sp s tot stst n oft res-

sion. The angle between the strike of the Puros shearzone and the L1 lineation preserved in the D1 domainsapproximates the initial obliquity of convergence be-tween the Western and Central Kaoko zones, reachingvalues <25◦ in the Puros area. As the lineation showsalmost identical orientation during D1–D3, we may as-sume that the angle of convergence did not change sub-stantially in time.Thompson et al. (1997)have shownthat in transpressive zones, an angle of convergence<30◦ leads to a substantial increase in temperature dueto a low exhumation rate. In regions with a hot initialgeotherm such a tectonic environment may result in along period of high-temperature metamorphism associ-ated with extensive melting. This is in good agreementwith the high degree of melting in rocks of the easternpart of the Western Kaoko zone observed during theD2 deformation.

Thompson et al. (1997)have shown that the angleof convergence <30◦ will result in a long time span be-tween high-temperature and low-temperature events.In the case of the Kaoko belt, identical conclusions canbe deduced from geochronological data.Ahrendt et al.(1983)published K–Ar cooling ages for muscovite andbiotite from the Central and Western Kaoko zone. Inthe area strongly affected by D2–D3 deformation, agesof 527–530 (±11) Ma suggest a∼20–25 Ma time spanbetween high-grade metamorphism and melting (ca.550 Ma—see above) and cooling of this part of the belt.Such time interval is consistent with that modelled byThompson et al. (1997)for collisional zones with ana no ans-p inceh est-e gesi er-vS rma-t gena pro-d ta

7

l oft eo-l ko

ikoff and Teyssier, 1994). Intense deformation alonhe contact with the Western zone gave rise touros shear zone. The replacement of kyanite byanite at the western margin of the Central zonee seen as a direct consequence of D2 deform

mposed on the medium-pressure metasedimentshe extent of an intense D2 overprint can thus beroximated by the width of the first sillimanite zoFig. 2).

.5. The influence of D2 and D3 on the thermalvolution of the Western Kaoko zone

L2 stretching lineations developed on subvertica2lanes are consistently plunging at shallow angle

he northwest (Fig. 4a) and their orientation suggehat there is a consistent component of exhumatiohe Western Kaoko zone during D2 sinistral transp

ngle of convergence of 30◦. An identical orientatiof D2 and D3 structures suggests that sinistral trression was the controlling tectonic mechanism sigh-temperature metamorphic conditions in the Wrn Kaoko zone have been attained. K–Ar cooling a

n the vicinity of the Sesfontein thrust fall in an intal between 442 and 479 Ma (Ahrendt et al., 1983).uch young ages may suggest that the latest defo

ion was transferred to the marginal parts of the orofter cooling of the Western and Cental zones anduced the low-angle Sesfontein thrust (Goscombe el., 2003a).

.6. Tectonic setting of the Kaoko belt

It is problematic to infer any large-scale modeectonic setting of the Kaoko belt just from the gogical information, since most of the original Kao


belt is missing due to opening of the Atlantic ocean.As stated byDurr and Dingeldey (1996), the lack ofophiolites or subduction-related high-pressure rockscasts doubt on the idea of subduction-related collisionin the Kaoko belt. The lack of eclogites in the Kaokobelt may, however, be the result of long-lasting high-temperature metamorphism completely overprintingany earlier high-pressure assemblage. An important in-formation for the interpretation of the geologic historyof the Kaoko belt is the onset of rift-related felsic vol-canism along the thinned southern margin of the CongoCraton, which was dated byHoffman et al. (1996)at∼756 Ma. However, this age may only apply to theDamara belt, and it remains uncertain whether or notthe onset of rifting at the southwestern margin of theCongo Craton occurred at the same time.da Silva etal. (1999)and Hartmann et al. (2002)reported zir-con ages of 762± 8 and∼780 Ma for granitoids in theDom Feliciano belt (South American counterpart of theKaoko belt), which are similar to those ofHoffman etal. (1996).

Calc-alcaline batoliths in the Dom Feliciano beltwere dated at 620–590 Ma and interpreted as the deeppart of a continental arc (Basei et al., 2000), whereasother authors relate magmatic rocks of this age tothe syn-collisional peak of regional metamorphism(Babinski et al., 1997; da Silva et al., 1999; Hartmann etal., 2000). Two ages corresponding to early high-grademetamorphism at 656± 8 Ma and 645± 3.5 Ma wereobtained from the Western Kaoko zone bySeth et al.( ta latedt ismi herei eent they .(t okob

ofp est-e f thec udys Cen-t tingo be-l ra-

dient, and this suggests that the contact between thesetwo units represents a major tectonic boundary control-ling the tectonic evolution of the Pan-African Kaokobelt.

8. Conclusions

Our study has shown that, prior to the developmentof the large-scale transcurrent Puros shear zone, a pe-riod of oblique underthrusting led to the developmentof an inverted metamorphic gradient in the central partof the Kaoko belt. An early period of high-temperaturesinistral transpression was responsible for almost com-plete reworking of an earlier, underthrusting-relatedfabric in the eastern part of the Western Kaoko zoneand at the westernmost margin of the Central Kaokozone. In the Western Kaoko zone, continuous sinis-tral transcurrent movements are documented by thedevelopment of the low-temperature Village MyloniteZone, which is accompanied by a heterogeneous ar-ray of small-scale, low-temperature shear zones inmigmatites of the eastern part of the Western Kaokozone. Consistent kinematics and orientation of thestretching lineation in the high-temperature migmatitesalong the Puros shear zone and in low-temperature my-lonites of the Village Mylonite Zone suggest that theVillage Mylonite Zone took over the role of the Purosshear zone during cooling of the central part of theKaoko belt. In our interpretation, the Puros shear zoner oned in oft

A

forh out-p salN 0-0 ion.J ro-v e-s ofF edf K.W

1998)and byFranz et al. (1999), respectively.Franz el. (1999)suggested that these ages may still be re

o the rifting period and granulite-facies metamorphn the lower crust. The above data suggest, that ts a time-span of about 140–170 million years betwhe onset of rifting and magmatic arc formation. Allounger ages of∼580–550 Ma obtained bySeth et al1998), Franz et al. (1999)andKroner et al. (2004)mayhen correspond to collisional processes in the Kaelt.

Very little is known about the tectonic positionre-580 Ma magmatism in the coastal part of the Wrn Kaoko zone and about the tectonic evolution ooastal part of the Kaoko belt in general. Our field sthows that the boundary between the Western andral Kaoko zones is a modified zone of underthrusf the southwestern margin of the Congo Craton

ow a unit with a completely different geothermal g

epresents a modified major Pan-African thrust zeveloped along strongly reworked western marg

he Congo Craton.

cknowledgements

The authors are indebted to Charlie Hoffmannis support and helpful discussions. This is anut of a Marie Curie Fellowship project (propoo. MCFI-2000-01852; contract No. HPMF-CT-2001101) granted to J.K. by the European Commiss.K., S.K. and A.K. acknowledge travel grants pided by the German Ministry of Education and Rearch (BMBF) through the International Officeorschungszentrum Julich. This paper has benefit

rom constructive reviews by T. Blenkinsop, M.atkeys and one anonymous reviewer.


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Oblique collision and evolution of large-scale transcurrent ......tonic mechanisms leading to the present-day pattern of this part of the Neoproterozoic orogenic belt system of Namibia