Suturing of the Proto- and Paleo-Tethys oceans in the ...directory.umm.ac.id/Data Elmu/jurnal/J-a/Journal of Asian Earth Scie… · Suturing of the Proto- and Paleo-Tethys oceans

Suturing of the Proto- and Paleo-Tethys oceans in the western Kunlun(Xinjiang, China)

F. Matterna,* , W. Schneiderb

aInstitut fur Geologıe, Geophysik und Geoinformatik, Freie Universita¨t Berlin, Matteserstraße 74-100, D-12249 Berlin, GermanybInstitut fur Geowissenschaften, Technische Universita¨t Braunschweig, Pockelsstraße 4, D-38106, Braunschweig, Germany

Received 5 February 1999; accepted 30 September 1999

Abstract

The Proto-Tethys Ocean between the North and South Kunlun began to form during the Sinian. Remnants of this ocean are preserved at theOytag-Kudi suture. The presence of Paleozoic arc batholiths in the northern South Kunlun and their absence in the North Kunlun indicatessouthward subduction of the Proto-Tethys Ocean beneath the South Kunlun. Opposite subduction polarity can be demonstrated for the LatePaleozoic to mid-Mesozoic when the southerly located Paleo-Tethys Ocean was consumed beneath the South Kunlun and generated a LateCarboniferous to mid-Jurassic magmatic arc in the southern South Kunlun. Arc magmatism affected the southern South Kunlun and the largeKara-Kunlun accretionary prism (a suture sensu lato) which formed as a result of Paleo-Tethys’ consumption. The dextral shear sense ofductile faults which are located at the margins of the arc batholiths, and which parallel the South Kunlun/Kara-Kunlun boundary, suggestsoblique plate convergence with a dextral component. Different lines of evidence encourage us to interpret the Proto-Tethys ophiolites of theOytag-Kudi zone as at least partly derived from an oceanic back-arc basin. In contrast, we assume that Paleo-Tethys was a large ocean basinwhich was eliminated directly at the southern margin of the South Kunlun where no oceanic back-arc region existed.q 2000 ElsevierScience Ltd. All rights reserved.

1. Introduction

Knowledge of the geology of the Kunlun was alwayssparse, as it is difficult to access this geographic frontier.Until recently, fundamental geological knowledge of thewestern Kunlun remained obscure to the international geos-cientific community because since 1949 foreigners were notallowed to visit the area (Gaetani et al., 1990). Only as of1988 could non-Chinese workers carry out investigations inthe region again. The Italian expedition team was the firstone to resume foreign research efforts (Gaetani et al., 1990).On the basis of recently collected data by Chinese and otherworkers (e.g. Liu et al., 1988; Gaetani et al., 1990, 1991;Matte et al., 1991; 1996; Pan et al., 1992; Yao and Hsu¨,1994; Mattern et al., 1996), it is now possible to review themain aspects of the geology of the western Kunlun. Ourdescriptions pay special attention to those aspects signifi-cant for the reconstruction of the geodynamic development.For more details on the regional geology the reader isreferred to the quoted literature and the sources therein.

Our main intention is to decipher the pre-Cenozoic platetectonic processes which shaped the western Kunlun.Subduction of oceanic lithosphere plays a key role in thisregard. At the same time we will also address problematicaspects in the understanding of the tectonic history.

The western Kunlun is one of the Earth’s highest moun-tain ranges. It is located south of the Tarim Basin (TaklaMakan Desert) and north of the westernmost part of theTibet Plateau which is referred to as the “Kara-Kunlun”area (Fig. 1). To the northwest, parts of the Kunlun aremorphologically and geologically transitional to the Pamirs.The western Kunlun represents an accretion zone at whichCentral Asia grew during the Phanerozoic. It is tectonostra-tigraphically subdivided into the North Kunlun, i.e. thesouthern margin of the Tarim Block, and the narrowSouth Kunlun, which was the site of two Phanerozoicmagmatic arc-related intrusion cycles. Both tectonic unitsare separated by the ophiolite-bearing Oytag-Kudi suture(Fig. 1). The Kara-Kunlun is a sizeable mid-Phanerozoicaccretionary wedge.

We use the geological time table by Haq and van Eysinga(1987) in correlating radiometric ages with geological timeunits and corresponding stratigraphical time–rock units andin assigning numerical time spans.

Journal of Asian Earth Sciences 18 (2000) 637–650

1367-9120/00/$ - see front matterq 2000 Elsevier Science Ltd. All rights reserved.PII: S1367-9120(00)00011-0

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* Corresponding author.E-mail address:[email protected] (F. Mattern).

2. Sinian rifting of the North Kunlun and creation of theProto-Tethys Ocean

The basement of the North Kunlun is characterized byPrecambrian gneisses and migmatites (Pan et al., 1992;Matte et al., 1996). It is overlain by a mildly metamorphosedSinian (Late Proterozoic) succession of laminated carbo-nates, volcanites, shales, marls and tuffites, well-exposedaround Akaz Pass. Locally, we observed the alternation ofparallel-bedded limestone and dolomite. The laminatedcarbonates indicate a shallow marine depositional environ-ment. According to Pan et al. (1992), the metavolcanites areformer oceanic tholeiites. The thickness of the Siniansuccession may reach 1 km or more. At Akaz Pass, a thick-ness of several hundred meters is exposed. Similar Sinianrocks of great thickness occur in the Tarim Basin (Tian etal., 1989). South of Hotan, the Sinian age of these shallow

marine strata has been determined on the basis of micro-floral and small shelly fossils (Yao and Hsu¨, 1994).

Along with Pan et al. (1992), we interpreted the Siniansuccession as a rift sequence which formed during thefragmentation of a subsiding shallow marine platform(Mattern et al., 1996). Following Sinian rifting of the south-ern margin of the Tarim Block, ocean spreading is assumedto have taken place during the later Sinian and Lower Paleo-zoic resulting in the formation of the Proto-Tethys Oceanwhose remnants are found at the Oytag-Kudi suture (Pan etal., 1992; Mattern et al., 1996). According to Chang et al.(1989), this ocean developed during the Sinian andCambrian. We are unable to say which continental massdrifted away from the southern margin of the TarimBlock, but we do not consider the South Kunlun a likelycandidate because it lacks a similar Sinian succession(Fig. 2).

F. Mattern, W. Schneider / Journal of Asian Earth Sciences 18 (2000) 637–650638

Fig. 1. Geological elements of the study area (simplified). Note that the Kudi ophiolite thrust unit appears to be much larger in map view than the ophioliteunits of the suture trace. According to Yao and Hsu¨ (1994), ophiolites also occur in the Kara-Kunlun northwest and southeast of Mazar. They are not shownsince their exact location was not indicated by Yao and Hsu¨ (1994). Ophiolite distribution after Liu et al. (1988). Fieldwork was carried out along the road fromYecheng to the area around Tianshuihai and in side valleys.

The alternation of metacarbonates and metatholeiitesindicates that marine conditions persisted during the riftprocess. Therefore, the visited area of the North Kunlunwas probably not located on a rift dome, as this wouldhave likely caused uplift of the platform above sea-level.The Sinian rift sequence of the Akaz Pass area might havebeen located to the side (i.e. “north”, according to presentday directions) of such a dome. This interpretation implies adip-slip type of rifting.

Alternatively, one could argue that rifting was due tostrike-slip motion as this would neither require nor inducea rift dome (Mattern et al., 1998). However, strike-slip rift-ing appears to be an unlikely option because of the abun-dance of volcanogenic strata in the Sinian succession,

considering that strike-slip rifts are low-volcanicity rifts atbest (Mattern et al., 1998).

3. Oytag-Kudi suture and Kudi Ophiolite Complex

The Oytag-Kudi suture which separates the North andSouth Kunlun is marked by an alignment of ophiolite sliceswhich appear narrow in map view (Fig. 1; Liu et al., 1988).The suture is inclined towards the South Kunlun (Pan et al.,1992, their Fig. A-2; Matte et al., 1996, their Fig. 2). Little isknown about the geology of this suture trace (“suture trace”sensu Sengo¨r et al., 1988, their Fig. 21). Yang et al. (1996)reported peridotite, cumulate gabbro and basalt from theKegang ophiolite occurrence (ca. 100 km NW of Kudi).Approximately 80 km east of Kudi, the structure along thesuture trace appears to be complicated, in so far as there is alarge lenticular, possibly fault-bounded rock unit, adjacentto the suture north of the ophiolites, which displays an enechelon rock fabric (Matte et al., 1996). In Figs. 1 and 8 weassign it to the South Kunlun.

Knowledge of the Kudi Ophiolite Complex, whichappears much larger in map view than the ophiolite unitsof the suture trace, is much more detailed. The Kudi Ophio-lite Complex represents an obducted ophiolite unit whichwas thrust from the suture and emplaced on the SouthKunlun (Fig. 3). We studied the northern and eastern partof this complex in the north/south-trending Kudi Valleyalong the road north of the village of Kudi and in thesmall, canyon-like Yishak Valley which trends approxi-mately east/west and leads into the Kudi Valley from thewest (Fig. 3). We investigated the southern part of thecomplex at the northern slope of the Boziwan Valleywhich joins the Kudi Valley from the west at the northernmargin of Kudi (Fig. 3). Except for a sheeted dike complex

F. Mattern, W. Schneider / Journal of Asian Earth Sciences 18 (2000) 637–650 639

Fig. 2. Juxtaposition of the North and South Kunlun’s stratigraphy. Note thesimilar development as of the Devonian.

Fig. 3. Cross section sketch of the obducted Kudi Ophiolite Complex, constructed mainly from the investigations in the Boziwan Valley and Kudi Valley(roadsection). Fault marked with “x” is the one depicted on Fig. 6, ca. 15.5 km north of Kudi at the bridge. Fold vergence close to the Yishak Valley may be due topost-obductional shortening. Minor outcrops of metamorphic rocks are not shown.

we found all main layers of a complete ophiolite section(Fig. 4). Instead of the sheeted dike complex we observeda thick layer of massive basalt. Yang et al. (1996), whoinvestigated the Kudi Ophiolite Complex as well as someophiolites of the suture trace, also noted the absence ofsheeted dikes. A summary of our findings pertaining tothe layering of the complex is shown in Fig. 4.

The Dunite as the lowermost layer, generally exhibits acoarse-grained, structureless fabric. Locally we observedthin parallel laminations of chromite which are intersectedby only a few centimeters thick dikes of pyroxenite. More-over, we found up to 1.5 m thick hornblendite dikes in thedunite. The peridotite above the dunite is partly serpenti-nized. Among the rocks of this layer we distinguished twovarieties — a relatively fine-grained and massive one, andanother that displays layering of pyroxene cumulates.Within this layer also distinct pyroxenite bodies occur.The lithology of the peridotites is dominated by harzburgite(Yang et al., 1996). According to Yang et al. (1996), theperidotites are highly depleted. The next layer consists ofcoarse-grained gabbro.

We can neither describe the transition from the gabbrolayer to the next layer, nor do we know whether a transitioneven exists. The next layer is thick and comprises a gener-ally fine-grained, massive basalt (Fig. 4) above which alayer of pillow basalt accumulated. The diameter of thepillows usually does not exceed 1 m (Fig. 5). The pillowsdisplay glassy rims and amygdales. According to Pan et al.(1992), the pillow basalts of the Oytag-Kudi suture repre-sent mature ocean ridge type tholeiites, but, according toYang et al. (1996), lavas from this suture zone displaygeochemical patterns suggesting a supra-subduction zoneenvironment.

Above the pillow basalts there are lenses of hematiticchert (up to 2 cm thick), shales and tuffites. The followinglayer is represented by massive, amygdaloidal, slightly red

or brown basalt. A debrite with a thickness of several deci-meters is associated with this layer in the Yishak Valley.

At the base of the uppermost layer, lenses of up to 1 dmthick hematitic chert occur. The bulk of the uppermost layer,however, comprises red shales and green tuffites which mayalternate in thin layers of only a few centimeters or formmonotonous successions of several meters or even tens ofmeters. The red shales are frequently intercalated with graygraded layers of several millimeters to centimeters in thick-ness. The grain size of the tuffites ranges from dust to lapilli.Coarse tuffites from the Yishak Valley bear lithic compo-nents of basic volcanites, devitrified glass components,plagioclase phenocrysts and fragments displaying flowfabrics, like deformed vesicles. All of these rock typesappear to be silicified.

The ratio between the amount of suture sediments andbasic to ultrabasic oceanic igneous rocks is relativelysmall in the Kudi ophiolite complex. According to mapanalyses (e.g. Liu et al., 1988), this also holds true for thesuture trace.

Age information pertaining to the Oytag-Kudi ophiolitesis controversial. Moreover, radiometric literature informa-tion is often unspecific about the applied method, investi-gated minerals, sample location, type of age and so forth.Rb–Sr isochron dating of basaltic rocks by Jiang et al.(1992, as quoted by Yang et al., 1996) yielded an age of360 Ma. According to the Institute of Geology and MineralResources, Urumqi, Xinjiang (correspondence), radiolariancherts of the Kudi ophiolites date as Carboniferous toPermian. Wang (1983) reported an age of “earlier than860.5 Ma” for an ultrabasic rock. He also reported the ageof a quartz diorite which intruded volcanic rocks of the Kudi


Fig. 4. Layering of the Kudi Ophiolite Complex. The total thicknessmeasures more than 2 km (Wang, 1983). According to Yang et al.(1996), arc-typical lavas are a part of the complex.

Fig. 5. Roadside outcrop of pillow lava belonging to the Kudi OphioliteComplex, ca. 8 km north of Kudi.

Ophiolite Complex to be 517 Ma. Pan et al. (1992)mentioned a Rb–Sr isochron age of 816 Ma for a pegmatiticamphibolitic dike which intruded an ultramafic rock, and aK–Ar whole rock age of 517 Ma for a diorite which intrudedvolcanites north of Kudi. We wonder whether Pan et al.(1992) were referring to Wang’s (1983) data and whetherthe different numbers “860” of Wang (1983) and “816” ofPan et al. (1992) resulted from a translation mistake or mix-up in one of the two English texts as both numbers arephonetically very similar. According to Pan et al. (1992),this diorite was also dated as 458 Ma (Rb–Sr isochron) and480 Ma (40Ar/ 39Ar). They also reported ages pertaining to agranite which intruded the volcanites of 384 Ma (40Ar/ 39Ar)and 423 Ma (Rb–Sr isochron). Pan et al. (1992) also listed amodel age for pillow lavas, not located in the Kudi area, of900–600 Ma. Xu et al. (1992) determined the followingages for a granodiorite which intruded the pillow lava:474 Ma (40Ar/ 39Ar plateau age on hornblende), 44924Ma (40Ar/39Ar–39Ar/ 36Ar isochron age on biotite), and 458Ma (U/Pb concordant age on zircon). The same age numbers(partly different methods) were also published by Matte etal. (1996) for a schistose granodiorite south of Akaz Passwhich intruded mafic rocks. Its location is apparently indi-cated on their Fig. 2 with the age information “gd4, 460Ma”.

We had the opportunity to investigate the zone betweenthe southernmost exposures of this pluton and the northern-

most part of the Kudi Ophiolite Complex, at the bridgeapproximately 15.5 km north of Kudi. Microscopic kine-matic evidence shows that the granodiorite was ductilelysheared in a strike-slip mode along the northwest/southeastdirections (Mattern et al., 1996). The main mylonitic folia-tion of the granodiorite is conspicuous in the field (Fig. 6).We also observed steeply dipping brittle faults (Mattern etal., 1996, p. 708–709), trending east/west (Fig. 7). Thecontact between the Kudi Ophiolite Complex and the plutonis covered by a 300 m wide zone of unconsolidated sedi-ments (Fig. 6). The northernmost outcrop of the KudiOphiolite Complex also exhibits signs of brittle fracturing.The massive basalt was intruded by a mafic dike. We did notfind any evidence for an intrusive pluton/ophiolite contact.Instead we interpret the contact relation between the plutonand the Kudi Ophiolite Complex as a faulted one (Fig. 3).Although we saw many outcrops of the Kudi OphioliteComplex in the three valleys, we found no evidence ofgranitoid intrusions into the complex. All contacts betweengranitoids and the ophiolites were covered by unconsoli-dated sediment. Within the ophiolite complex, we also didnot observe any aplite or silicic pegmatite dikes which couldbe associated with the granitoid intrusions. The literatureprovides neither specific phenomenological nor formal(location) details on the intrusive contact relations betweengranitoids and ophiolites.

Acknowledging the controversial age data, Pan et al.


Fig. 6. Southern margin of the 460 Ma granodiorite, 15.5 km north of Kudi. In the foreground on the right side, the pluton displays a mylonitic foliation(015/85), marked by the arrow on the lower right. The view is exactly parallel to the foliation towards 2858. Note that in the projected extension of the foregroundmylonitic zone, the mountain crest of the background, displays a saddle (upper arrow). Note the more or less horizontally stratified unconsolidated riverdeposits in the south and the loess in the center covering the contact between the pluton and the more southerly located Kudi Ophiolite Complex.

(1992) and Mattern et al. (1996) considered the ophiolites ofthe Oytag-Kudi suture to have formed in the Proto-Tethysbetween the Sinian and Early Paleozoic. Further timeconstraints on the formation of these rocks is provided bythe age of subduction-related magmatites (see below) whichformed during subduction of the Proto-Tethys Ocean sinceoceanic lithosphere must first be created before it can besubducted and since ocean-spreading can be coeval withsubduction.

4. Paleozoic subduction of Proto-Tethys beneath theSouth Kunlun

The basement of the South Kunlun is characterized by theoccurrence of gneisses, amphibolites and migmatitic gneisses.The gneisses and amphibolites were intruded by basic and

acidic dikes which experienced amphibolite facies conditions(Pan et al., 1992). Protoliths of the migmatites south of Kudiseem to be of Proterozoic age (Matte et al., 1996).40Ar/39Arage spectra on K-feldspars suggested to Matte et al. (1996) aminimum age of 380–350 Ma for metamorphism.

Zhang et al. (1992) distinguished two arc granitoid beltsin the South Kunlun, an older, Paleozoic one in the north,and a younger, Late Paleozoic to Mesozoic one in the south.In this section we concentrate on the northern belt which isdiscontinuous at the surface, and relatively small comparedto the southern belt (Fig. 8). Considering the geochemicalcharacteristics of the granitoids and their geological setting,which includes the presence of parallel-trending suturezones, as well as the long, linear, orogen-parallel extent ofthe belts, Zhang et al. (1992) suggested that their genesis isclosely related to subduction. Geochemical data (Pan et al.,1992) support this interpretation. Hsu¨ (1988) had already


Fig. 7. Roadside outcrop of a brittle, steeply south-dipping, east/west-trending fault in the 460 Ma granodiorite; ca. 15.5 km north of Kudi, directly at the firstbridge north of Kudi.

attributed the presence of batholiths to a former active platemargin. The two granites from the northwest-trending partof the western Kunlun dated by Fan and Wang (1990) as 445Ma (K/Ar) and 480.43 5 Ma (U/Pb) belong to this belt.These ages indicate Ordovician subduction. It cannot beruled out that subduction occurred also during the Cambrianand Silurian. Southward subduction is indicated by thenorthern position of this older arc granitoid belt within theSouth Kunlun and its occurrence south of the Oytag-Kudisuture. The interpretation of an Early Paleozoic episode ofsubduction is difficult to reconcile with coeval granitoidintrusions into oceanic lithosphere, unless some ophioliteobduction occurred relatively early, or unless the upperplate also had an oceanic back-arc region north of theSouth Kunlun, whose associated oceanic arc was firstintruded by more or less M-type(?) arc granitoids (“M-type” sensu Pitcher, 1982) and then obducted(?).

5. Mid-Paleozoic suturing of Proto-Tethys

In the Kongur Shan area (Mt. Kongur, 7719 m, 200 km

westnorthwest of Yecheng), at the transition between thePamirs and the Kunlun, a slightly metamorphosed succes-sion of shallow marine Ordovician fossil-bearing limestonesand arc-related volcanites as well as Ordovician and Silurianclastic deposits occurs above the Sinian strata and below upto 1350 m thick Upper Devonian terrestrial clastic red beds(Yao and Hsu¨, 1994, p. 79 and their Fig. 6). A pre-Devonianstratigraphic gap also exists in the western North Kunlunwhere there is no stratigraphic record between the Sinian riftsequence and unconformably overlying Upper Devonianterrestrial red molasse deposits.

The Upper Devonian molasse consists of fluvial and allu-vial fan arkoses and conglomerates as well as volcanogenicstrata. At a roadside outcrop, approximately 10 km east-northeast of the Akaz Pass, these clastic rocks are compo-sitionally and texturally immature. Lithic components wereeroded from nearby granitoids, rhyolites, acidic to inter-mediate altered pyroclastic rocks and basic volcanites. Atthe same outcrop we observed an angular unconformitywithin these sediments with an angle close to the angle ofrepose. Only a larger angle would justify the interpretationof a tectonic cause for the formation of this unconformity.


Fig. 8. Map sketch of granitoids of the study area mainly after Matte et al. (1988) supplemented by data from Liu et al. (1988) for the South Kunlun. Note thatthe northern granitoid belt is much smaller than the southern one. Mazar pluton according to Liu et al. (1988), Gaetani et al. (1991), and own observations. Mapdepictions of the Kudi pluton differ greatly (compare, for example, Liu et al., 1988 and Matte et al., 1996). The anatectic laccoliths after Matte et al. (1996).

The thickness of the Devonian molasse may be severalhundred meters (Matte et al., 1996). Pan et al. (1992)noticed that the Devonian succession exhibits an overallfining-upward trend. The Devonian clastic rocks havebeen dated by plant fossils (reviews by Sengo¨r and Okur-ogullari, 1991; Yao and Hsu¨, 1994).

Terrestrial red molasse deposits of Devonian age alsooccur in the South Kunlun (Pan et al., 1992). As shown inFig. 2, the stratigraphic development of the North and SouthKunlun is continuous and similar across the Oytag-Kudisuture as of the Devonian. Thus, the accretion of theSouth Kunlun against the southern margin of the TarimBlock must have been completed by that time. It appearsreasonable to attribute the pre-Devonian stratigraphic gap inboth terranes (Fig. 2) to uplift and erosion related to regionalshortening in the course of accretionary processes. Sinceshallow marine Ordovician and Silurian strata are preservedin the Kongur Shan area we assume that significant short-ening and uplift started after deposition of the Silurian clas-tic sediments. According to Yao and Hsu¨ (1994, their Fig.6), uppermost Silurian to mid-Devonian sediments are miss-ing in the Kongur Shan area. This indicates to us that sutur-ing, shortening, and mountain building started during theLate Silurian. Matte et al. (1996) concluded a Silurian colli-sion. They listed a U/Pb zircon age of 377 Ma and Rb/Srages of 392 35 Ma on whole rock, and of 381 4 Ma onbiotite on the potassic postkinematic Kudi granite, whoseintrusion postdates metamorphism and anatexis in the SouthKunlun. This metamorphism and anatexis is indicated by acomplex40Ar/39Ar age spectra on K-feldspars of migmatitessouth of Kudi, suggesting a minmum age of 380–350 Ma,due to Silurian collision (Matte et al. 1996).

The Devonian molasse of both the North and SouthKunlun grades upward into shallow marine Carboniferousand Permian carbonates (Fig. 2; Pan et al., 1992) which arewell-dated by fusulinids, bivalves, brachiopods and corals(De Terra, 1932 as quoted in Matte et al., 1996; BGMR,1993). The carbonates may be 1 km thick (Matte et al.,1996). The fact that these carbonates are marine indicatesthat the Silurian/Devonian Kunlun mountains were alreadywidely denuded when the carbonates accumulated.

6. The Kara-Kunlun accretionary wedge

The Kara-Kunlun area is characterized by the occurrenceof thick monotonous and partly metamorphosed succes-sions of clastic marine deposits which exhibit a flyschoidcharacter (Fig. 9). Dark shale is by far the predominantsediment type which probably gave the Kara-Kunlun itsname (“kara”� “black”). Besides shale we found silici-clastic turbidites and tuffites. The turbidite beds exhibitthicknesses mostly between 0.4 and 1.5 m. Bouma inter-vals “a” may be present. The pelitic intervals usuallymeasure only a few centimeters. Only at one locality(wadi 3 km west of Mazar) we observed a 30 m thick

succession of distal allodapic limestones. Carbonates seemto be very scarce.

In the area of Mazar, fine- to coarse-grained laminatedgraywackes contain granitoid debris. Fine- to coarse-grained graywackes at the Qitai Pass contain granitoiddebris as well, but also subordinate amounts of chert, phyl-lite and quartzite fragments. Accessory constituents arezircon, apatite, muscovite, biotite and chlorite. The compo-sition of metasiltstones from the Xaidulla area correspondsto that of the coarser material from the Qitai Pass. Theycontain quartz, feldspar, biotite, and muscovite. Tourma-line, apatite, sphene and ore minerals represent accessoryminerals. The main constituents in metatuffites are quartz,feldspar and actinolite.

The facies character of these clastic deposits (e.g. silici-clastic turbidites, allodapic limestone beds), compositionalaspects (granitoid debris) as well as the occurrence oftuffites can be reconciled with an active margin settingand is in support of the accretionary wedge interpretation.

Gaetani et al. (1990, 1991), who worked in the area oftransition from the Kunlun to the Karakorum, estimated thethickness of these deposits (“Bazar Dara Slates”) to measureseveral thousand meters, and, according to Matte et al.(1992), as much as 6 km or more. The age of the unfossili-ferous Kara-Kunlun sediments could range from the Ordo-vician to the Triassic (Pan et al., 1992) or from the Cambrianto the Triassic (Matte et al., 1992) or from the Precambrianto the Mesozoic (Yao and Hsu¨, 1994). The sediments mustbe older than the Mesozoic plutons which intruded them(Fig. 9 and below). Paleozoic to Triassic fossils areknown to occur in exotic limestone slabs (Yao and Hsu¨,1994).

From the Kara-Kunlun belt Yao and Hsu¨ (1994) reportedthe occurrence of ophiolite me´langes northwest and south-east of Mazar, containing blocks of serpentinite, gabbro,greenstone, radiolarite, metagraywacke, marble, gneiss


Fig. 9. Juxtaposition of the South Kunlun and Kara-Kunlun stratigraphy.Note the similar development as of the Mesozoic.

and volcanites, embedded in a pervasively sheared, cleavedmatrix of sericite-quartz schist, chlorite-sericite schist, two-mica schist and phyllite. Since the maps by Liu et al. (1988),BGMR (1993), and Matte et al. (1996) do not indicateophiolites in the Mazar area, and since Yao and Hsu¨(1994) were unspecific about the location of the studiedophiolite outcrops, we are unable to show them in Fig. 1.Another ophiolite unit occurs east of Dahongliutan (Fig. 1).Also farther east, outside the study area, there is an ophiolitebody at Mt. Muztag (7723 m, 700 km eastsoutheast of Hotan— not to be confused with Mt. Muztag, 7282 m, 115 kmsouthsoutheast of Hotan; Pan et al., 1992; Yao and Hsu¨,1994, their Fig. 8). The presence of ophiolites in rocks of theKara-Kunlun (Liu et al., 1988; Yao and Hsu¨, 1994) is impor-tant because it indicates that internal thrust planes exist in thesiliciclastic Kara-Kunlun belt, such as one would expect tooccur in an accretionary wedge. The ratio between the amountof suture sediments and basic to ultrabasic oceanic igneousrocks is relatively large in the Kara-Kunlun wedge.

In the westernmost part of the Kara-Kunlun, the clasticrocks of the Kara-Kunlun are either unmetamorphosed ordisplay a very low metamorphic grade (Gaetani et al., 1990).According to Matte et al. (1996), the clastic rocks areaffected by only low-grade metamorphism. At several road-side outcrops between Mazar and Hez Pass and also farthereast, for example, at the Qitai Pass, unmetamorphosed stratacan be observed. At several places we found evidence ofcontact metamorphism. These areas include the aureole ofthe Mazar pluton, west of Mazar, the Dahongliutan area, anda location 40 km eastsoutheast of Shanshilli (outcrop alonga new road, Mattern et al., 1996). At the latter we foundsillimanite-bearing metashales. Ductile faults can beobserved in these zones (below).

We noticed that the strata of the Kara-Kunlun mainly dipto the north and northnortheast, that is inclined towards theSouth Kunlun. Observations of the preferred dip includethose that were made on mountain-scale outcrops, for exam-ple, in the areas around the Hez Pass, Dahongliutan, and theQitai Pass. Folds exhibit a south or southsouthwestvergence. The observed slaty cleavage is genetically relatedto the folds (Mattern et al., 1996). These aspects are compa-tible with the interpretation of an accretionary wedge whichformed south of the South Kunlun in response to northwardB-type subduction. As shown below, the Kara-Kunlun wasintruded by subduction-related granitoids indicating anactive margin environment in which the Kara-Kunlunwedge became the site of arc magmatism.

Whereas the stratigraphy and age of the Kara-Kunlun’srocks are poorly understood, the geodynamic significance ofthe Kara-Kunlun zone appears to be clear. Hsu¨ (1988) inter-preted the Kara-Kunlun area as an accretionary wedgewhich formed due to northward subduction of the Paleo-Tethys Ocean beneath the South Kunlun. This view isconsistent with the flyschoid facies and composition of thethick clastic sediments and their association with tuffites,ophiolite melanges and arc granitoids as well as with the

implied internal thrust planes and with the dominant dipdirection of strata and the observed vergences. The Kara-Kunlun accretionary wedge can be correlated with theSonpan Ganze Belt or parts of it (Bayan Har Group) farthereast (Sengo¨r and Okurogullari, 1991; Matte et al., 1996;Mattern et al., 1996). The Songpan Ganze Belt occupies aposition south of the Kunlun, like the Kara-Kunlun accre-tionary wedge, and is also characterized by a great thicknessand by a dominance of fine-grained siliciclastic deposits(Leeder et al., 1988; Coward et al., 1988; Nie et al.,1994). It has also been interpreted as an accretionarywedge south of the eastern Kunlun (Leeder et al., 1988).

7. The Uygur Terrane and adjacent sutures

The Uygur Terrane located south of the Kara-Kunlunaccretionary wedge (Fig. 1) is the southernmost unit weinspected. Although the Uygur Terrane is a poorly under-stood element of the regional tectonic collage there iscertainty that its rock facies contrasts sharply with that ofthe Kara-Kunlun accretionary wedge. No stratigraphic rela-tionship exists between these two units. We observed athrust contact between the two terranes in the Tianshuihaiarea (Mattern et al., 1996). The Uygur Terrane consists of afossiliferous and generally well-dated Paleozoic to Triassicshallow marine succession (BGMR, 1993). The successionmeasures hundreds of meters in thickness and contains asignificant amount of carbonates. Because we could onlyinspect a few of the formations within the terrane wedecided not to depict its stratigraphic development.

To the south, the Uygur Terrane is separated from the south-erly located Karakorum/Qiangtang Terrane by the “cryptic”Longmu Co suture (Fig. 1) of Baud (1989), which is alsoreferred to as the “Taaxi-Qiaoertianshan-Hongshanhanusuture” (Pan et al., 1992). Since we did not reach this suture,the following information is taken from the literature.Although most of this suture has been covered by post-accre-tionary deposits (Pan et al., 1992) ranging from the mid-Juras-sic to the Quaternary (Liu et al., 1988; BGMR, 1993), Pan et al.(1992) identified this zone as a suture because it delineatesblocks of marked geological and paleontological differences.Ophiolites do not occur in the suture segment south of thewestern Kunlun but are known from this suture farther east(Pan et al., 1992). This zone also represents the northernboundary for the distribution of marine Jurassic strata (Panetal., 1992).There isa mid-Jurassic sutureoverlap assemblagewhose age is documented by a brachiopod fauna (BGMR,1993), indicating that this segment of the Paleo-Tethys waseliminated at the latest by the mid-Jurassic.

8. Late Paleozoic to Early Mesozoic subduction of thePaleo-Tethys Ocean beneath the South Kunlun

In the western Kunlun there is no rift sequence preservedwhich could help to determine when the Paleo-Tethys


Ocean formed to the south of the South Kunlun. However,there are arc magmatites whose ages indicate at least whenmelts developed in response to subduction of Paleo-Tethyslithosphere. The onset of subduction must have predated arcmagma generation, and ocean spreading must havepredated, and may have been coeval with, subduction.From a Kunlun perspective one must conclude that oceanspreading of the Paleo-Tethys must have started before theLate Carboniferous.

Upper Carboniferous and Permian intermediate and basicvolcanites intercalated with the well-dated Upper Paleozoicshallow marine carbonates of the South Kunlun (Fig. 9) areof the calc-alkaline arc type (Pan et al., 1992). They repre-sent the earliest evidence for a new subduction cycle.Further evidence is provided by Late Paleozoic to EarlyMesozoic orogen-parallel arc batholith belts (Fig. 8). Asmentioned above, Zhang et al. (1992) distinguished anolder Paleozoic arc granitoid belt in the north and a youngerLate Paleozoic to Mesozoic one in the south of the SouthKunlun. In this chapter we are concerned with the southernbelt which is significantly larger than the northern one (Fig.8). The finding by Zhang et al. (1992) is based on radio-metric and geochemical evidence as well as on considera-tions of the geological setting.

An altered granodiorite at Xaidulla yielded a Rb/Srisochron age of 267 Ma on biotite (Xu et al. 1992). Froma granite located 22 km northeast of Mazar an40Ar/ 39Ar–39Ar/ 36Ar isochron age of 211.8 10.8 Ma was publishedby Xu et al. (1992). For the same pluton an40Ar/39Ar wholerock age of 211 8 Ma and an40Ar/39Ar minimum age of180^ 10 Ma on K-feldspars was reported (sources in Matteet al., 1996). A red porphyry volcanite located close to thegranite yielded a Rb/Sr whole rock age of 180^ 10 Ma(source in Matte et al., 1996). According to Pan et al.(1992), the plutonic rocks of the South Kunlun belong tothe calc-alkaline series and owe their genesis to subductionand related arc magmatism.

The Kara-Kunlun accretionary wedge was also affectedby this magmatic arc activity (Matte et al., 1996). Mesozoicarc batholiths intruded the Kara-Kunlun sediments, display-ing a trend parallel to the orogen. Gaetani et al. (1991)obtained a K/Ar biotite cooling age of 171.55.4 Ma forthe Mazar Pluton located west of Mazar. Xu et al. (1992)determined an40Ar/39Ar plateau age of 184 Ma on biotiteand a low intercept age on zircon of 199.31 2.2/22.5 forthe Mazar monzogranite. Xu et al. (1992) also determinedan 40Ar/ 39Ar plateau age of 187 Ma for a granite whichbelongs to the batholith south of the road between Shanshilliand Kengxiwar. They also published a lower intercept ageon zircon from a granite near Kengxiwar of 192 Ma. More-over, they dated a monzogranite (located 30 km southeast ofKengxiwar?) 215 Ma (40Ar/ 39Ar, biotite) and 196.3 Ma(40Ar/ 39Ar plateau age, biotite). Matte et al. (1996) carriedout 40Ar/ 39Ar measurements on muscovites and biotitesfrom granites along the Karakax Valley (area south of Shan-shilli and Kengxiwar and at Dahongliutan) yielding plateau

ages of 190 8 Ma and 177 3 Ma. The Kara-Kunlunaccretionary wedge or part of it must have already formedbefore the intrusion of those granitoids with the UpperTriassic ages.

Matte et al. (1996) and Mattern et al. (1996) concludedthat the magmatites of the southern South Kunlun and theKara-Kunlun formed as a result of the subduction of Paleo-Tethys at a north-dipping Benioff plane beneath the SouthKunlun. Apparently, arc magmatism started to affect theSouth Kunlun already during the Late Carboniferous (volca-nites) and the Kara-Kunlun only as of the Late Triassic. Inboth areas, arc magmatism seems to have ceased during themid-Jurassic. In Kara-Kunlun, arc activity could haveslightly outlasted that in South Kunlun. Mattern et al.(1996) hypothesized that with the growth of the Kara-Kunlun accretionary wedge, the trench might have movedoceanwards and with it the magmatic front, so that the Kara-Kunlun became the site of arc granitoid intrusion.

The subduction polarity is indicated by the geometrywithin the Kunlun/Kara-Kunlun subduction accretion zonewith the early magmatic arc in the northerly located SouthKunlun and the Kara-Kunlun accretionary wedge to thesouth. The observed vergences in the Kara-Kunlun wedgelend further evidence for a north-dipping Benioff plane.

One of our goals during fieldwork was to find and kine-matically analyze ductile shear zones within arc batholithsor within their contact aureoles. The kinematic dataobtained by Mattern et al. (1996) are interpreted here asclues as to the kind of plate convergence. We reason thatarc magmatism is concurrent with subduction, and thatdeformation observed within the “still hot” arc rocks andadjacent country rocks should, therefore, have recordedinformation of the tectonic regime controlled by the kindof plate convergence. We found ductile faults in the vicinityof, and with a trend parallel to, the South Kunlun/Kara-Kunlun boundary at Xaidulla in the southern South Kunlunand 40 km eastsoutheast of Shanshilli in the northern Kara-Kunlun (outcrop along a new road, Mattern et al., 1996).The distance between both locations amounts to 55 km.Mylonites of these faults were sampled and analyzed. Inboth cases we found microscopic evidence for dextralstrike-slip motion (Mattern et al., 1996).

At the first location, the heat required for ductile shearingwas provided by the previously mentioned altered Xaidullagranodiorite dated 267 Ma (Xu et al., 1992). According tothe map by Matte et al. (1996), this granodiorite belongs to abatholith from which the also mentioned granite located22 km northeast of Mazar was dated 211^ 10.8 Ma(40Ar/ 39Ar–39Ar/36Ar isochron) by Xu et al. (1992). Forthe same granite the above listed ages of 211^ 8 Ma(40Ar/ 39Ar whole rock) and 180 10 Ma (40Ar/39Ar mini-mum age) were determined (sources in Matte et al., 1996). Ithas to be noted that the granite of the Mazar area is locatedat a distance of 80 km west of Xaidulla. The ages range fromthe Upper Permian to the uppermost Triassic to the mid-Jurassic (minimum age). We are inclined to interpret these


data as an indication that ductile deformation conditionshave existed at Xaidulla during the Triassic and duringmid-Jurassic cooling.

The granitoid which provided the heat for mylonitizationof contact-metamorphic sillimanite-bearing shales at thesecond location belongs to the batholith south of the roadbetween Shanshilli and Kengxiwar, from which Xu et al.(1992) determined the above mentioned40Ar/ 39Ar plateauage of 187 Ma and the lower intercept age on zircon of 192Ma. These are Early Jurassic ages. We assume that ductiledeformation conditions may have existed during the EarlyJurassic and possibly lasted to the mid-Jurassic during thecooling process.

If the finding of dextral ductile faults does indeed reflect

the kinematic relation between the overriding and the down-going plate, plate convergence should have been of dextralobliquity during the Early Mesozoic.

9. Mesozoic suturing of Paleo-Tethys

The latest shallow marine deposits of the western Kunlunare Upper Permian in age (Liu et al., 1988). The presence ofthese deposits indicates that uplift due to accretion/colli-sion-related shortening postdates their deposition. There isa well-documented angular unconformity with a significantstratigraphic gap between the Upper Paleozoic rocks and theterrestrial Mesozoic molasse in the western Kunlun (Liu etal., 1988; Pan et al., 1992; Yao and Hsu¨, 1994). This gap andthe onset of molasse deposition indicate that uplift hadaffected the region. Early uplift could be attributed to effectsrelated to subduction. Later uplift was more likely due toshortening in the course of complex accretionary/collisionalprocesses coeval with subduction.

At the southern boundary of the South Kunlun we foundone place where Upper Triassic red molasse overliesPermian shallow marine carbonates and volcanic strata ata (now overturned) angular unconformity (wadi north of theroad at road mark 266, 26 road km east of Mazar, elevation4000 m, Mattern et al., 1996). The Triassic successionconsists of terrestrial conglomerates, sandstones and shales.The conglomerates and sandstones are alluvial fan depositsof low compositional and textural maturity. They containreworked Carboniferous and Permian material (limestoneand dolomite fragments) and clasts of chert, shale and quart-zite. At this location, as well as on the southern slope of theSaliyak Pass (at an elevation of 4650 m, also at the southernboundary of the South Kunlun), we observed a fining-upward trend within the Upper Triassic succession.However, we did not see more than 70 m of exposed Trias-sic strata. At both places, the red Triassic shales grade intoslightly green Jurassic shales. In the Kongur Shan area,where there is also a stratigraphic gap between the Permianand Triassic, the terrestrial Triassic succession is 850 mthick (Yao and Hsu¨, 1994). The Triassic rocks are datedin the eastern Kunlun by numerous kinds of fossils (Yangand Long, 1990).

In a wadi north of the road at road mark 272 (32 road kmeast of Mazar, elevation 4100 m), the Triassic is missingabove the Permo-Carboniferous shallow marine carbonates.Deposition of the terrestrial clastic molasse started hereduring the Early Jurassic and continued to the mid-Jurassic(Liu et al., 1988). The Jurassic succession containsconglomerates, sandstones and shales. We observed coalseams which were also described for the Jurassic of theNorth Kunlun by Pan et al. (1992). The depositional envir-onment of the Jurassic may have included lakes andswamps. In contrast to the Triassic strata, the Jurassicdeposits lack red color and contain a significant amount ofgranitoid debris. The lack of red color is also helpful in


Fig. 10. Plate tectonic development of the western Kunlun and Kara-Kunlun. Note that only minor amounts of subduction-related melts reachedthe northern South Kunlun during consumption of Proto- Tethys. Arcmagmatism in the South Kunlun and Kara-Kunlun ended during the mid-Jurassic. Also note the age of overlap assemblages at terrane boundaries.

distinguishing these Jurassic strata from Cretaceous andsome Cenozoic terrestrial deposits. The presence of grani-toid debris shows that the magmatic arc was already erodedto its granitoid level during the Lower Jurassic. The lowcompositional and textural maturity of the Jurassic depositsindicates a close provenance. The Jurassic sediments arerich in plant fossils. Some of them have been listed byBGMR (1993). However, the listed fossil leaves are notvery conclusive as to the age of the succession. In theoutcrop zone in which we worked the Jurassic sedimentsare associated with a red volcanite which has been dated180^ 10 Ma (Rb/Sr whole rock age, source in Matte et al.,1996). In the wadi at road mark 272, the Lower Jurassicrocks overlap the boundary between the South Kunlun andthe Kara-Kunlun and exhibit only weak deformation.

Arc magmatism is coeval with Late Triassic to mid-Juras-sic molasse deposition. It is also coeval with the formationof the terrane boundary overlap assemblages (Fig. 10). Byslightly modifying the model of Mattern et al. (1996) wesuggest a model which implies successive accretion of tworelatively small units, the Kara-Kunlun accretionary wedgeand the Uygur Terrane, and, finally, the Karakorum/Qiangtang microcontinent, all belonging to the realm ofthe Paleo-Tethys Ocean (Fig. 10). The accretion of theseterranes was accompanied by successive oceanward shiftsof new, short-lived, north-directed subduction zones andsuccessive slab break-offs. The broken-off slabs continuedto descend and generated arc melts (Fig. 10). This modelaccounts for the regional uplift and arc magmatism concur-rent with accretionary/collisional processes and formationof molasse-type overlap assemblages.

The youngest known pre-Cretaceous shallow marinedeposits of the Uygur Terrane are Triassic. It is generallyassumed that the flyschoid rocks of the Kara-Kunlun accre-tion complex do not contain post-Triassic sediments (Liu etal., 1988; Pan et al., 1992; BGMR, 1993; Matte et al., 1996).The youngest age of exotic limestone slabs within the accre-tionary wedge is Triassic (Liu et al., 1998; Yao and Hsu¨,1994). Considering these aspects we suggest that the smallUygur Terrane collided with the Kara-Kunlun accretionarywedge during the Late Triassic and put an end to wedge-type accretion and marine sedimentation, due to shorteningand related uplift. This interpretation is supported by theonset of terrestrial molasse deposition during the LateTriassic in the Kunlun and during the Early Jurassic in theKara-Kunlun (e.g. above mentioned boundary overlapassemblage).

As indicated by the different pre-Jurassic development ofthe blocks north and south of the Longmu Co suture and bythe mid-Jurassic suture overlap assemblage, the Karakorum/Qiangtang microcontinent must have been added to theAsian tectonic collage during the Early Jurassic. Accordingto Dewey et al. (1988), the Qiangtang Terrane accreted tothe Songpan-Ganze unit in the eastern part of the TibetanPlateau during the Late Triassic or earliest Jurassic. Oneaspect, which one might consider problematic, needs to be

discussed and that is how a marine mid-Jurassic suture over-lap assemblage was able to accumulate above a suture thatformed only during the Early Jurassic. Possible solutionsare: 1. Accretion was “gentle” and did not result in a signif-icant uplift (e.g. due to the shapes of the accretion zone andof the microcontinent). 2. A time span of 30 million yearselapsed between accretion (Pliensbachian?) and formationof the overlap assemblage (Callovian?) allowing for denu-dation of a moderately uplifted region. 3. The suture zonewas affected by extension (e.g. orogenic collapse or trans-tension). 4. A combination of the above mentioned aspects.

10. Conclusions

The comparison of both the northerly located Oytag-Kudiophiolite zone and the younger South Kunlun suture revealstriking differences. In contrast to the Kara-Kunlun accre-tionary wedge (a suture zone s.l.) only minor amounts ofsediment are associated with the Oytag-Kudi ophiolite zone.The ratio between the amount of suture sediments and basicto ultrabasic oceanic igneous rocks is relatively large in theKara-Kunlun wedge and relatively small in the Oytag-Kudisuture and Kudi complex. The fact that relatively largeamounts of basic and ultrabasic oceanic material werepreserved and obducted at the Oytag-Kudi suture may indi-cate that relatively young, hot, buoyant and, therefore,“easy-to-obduct” oceanic lithosphere was consumed at thenorthern margin of the South Kunlun (compare Dewey,1976). It may also indicate that the ocean basin was rela-tively small (if young!). Since oceanic back-arc basins meetsuch characteristics we are inclined to assume that such abasin was subducted and partly obducted at the Oytag-Kudisuture (Fig. 10). It is intriguing that all major elements of acomplete ophiolite section are represented at the Oytag-Kudi suture, except for the sheeted dike complex. Instead,we observed thick massive basalt (Fig. 4). These circum-stances might also be in line with the back-arc basin inter-pretation taking into account that sheeted dike complexesmost convincingly illustrate spreading (Moores, 1982) andthat spreading of back-arc basins may be diffuse (Marsaglia,1995).

Considering the long time interval from the formation ofthe Sinian passive margin to the Silurian/Devonian end ofsubduction, it appears likely that a large ocean basin existedbetween the North and South Kunlun. If so, this oceaniclithosphere was subducted at the Oytag-Kudi suture or atthe assumed intraoceanic subduction zone which caused theinferred marginal basin to form. Such a marginal basin wasdepicted by Pan et al. (1992) with a south-dipping Benioffplane for the Early Paleozoic. In this regard it should bementioned again that Yang et al. (1996) interpreted lavasof the Oytag-Kudi ophiolite zone on geochemical groundsas having formed in a suprasubduction zone environment.

In line with our interpretation of the Proto-Tethys’subduction setting is the relatively modest amount of arc


granitoids generated during subduction. The Northern SouthKunlun was located in a back-arc and, therefore, relativelydistant to the arc, so that only minor amounts of melts wereable to reach the South Kunlun (Fig. 10).

The formation of the large Kara-Kunlun accretionarywedge required sufficient plate coupling to strip off thickclastic successions from the downgoing slab and to stackthem. However, the degree of plate coupling did not allowfor incorporating significant amounts of basic and ultrabasicrock slices from the downgoing slab into the accretionarywedge. Considering the small amount of preserved oceanicbasement rocks we conclude that the oceanic lithospherewas relatively old and cold and, therefore, easy to subduct.This implies a relatively large ocean.

The interpretation of Paleo-Tethys as a large ocean basinis also supported by the great volume of arc granitoidswhich intruded the immediate arc represented by the south-ern South Kunlun and Kara-Kunlun during the eliminationof Paleo-Tethys (Fig. 1). Moreover, the long time span ofarc magmatism from the Late Carboniferous to the mid-Jurassic points to the likelihood of a relatively large ocean.

The older accretionary processes which took place at thenorthern margin of the South Kunlun (Proto-Tethys realm)are less understood than those along the South Kunlun’ssouthern margin (Paleo-Tethys realm). We hesitate toaccept the young age data on the Oytag-Kudi ophiolitesbecause they are difficult to reconcile with the arc granitoidage spectrum of the northern South Kunlun, the Devonianage of the northern molasse, and, of course, with the olderages of the Oytag-Kudi ophiolites.

Acknowledgements

We thank Mark B. Allen and Brian Windley for theirconstructive comments on the manuscript. We also thankthe VW Foundation for its generous support of our fieldinvestigations in the Kunlun and Kara-Kunlun.

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