International Review, Vol. 45, 2003, p. 303¡ª¡-328. Copyright © 2003 by V. H. Winston & Son, Inc. All rights reserved.

Multiple Accretionary Orogenesis and Episodic Growth of Continents: Insights from the Western Kunlun Range,

WENJIAO XIAO,1 Key Laboratory of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029,

FANGLIN HAN, Shaanxi Institute of Geology Survey, Xianyang 712000, Shaanxi, China

BRIAN F. WINDLEY, Department of Geology, University of Leicester, Leicester LE1 7RH, United Kingdom

CHAO YUAN, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China

HUI ZHOU, Bureau of Science and Technology, Peking University, Beijing 100871, China

AND JILIANG LI Key Laboratory of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China


The Western Kunlun Range in northwestern records a history of enlargement of the Central Asian crust. In the Early Ordovician a south-dipping intra-oceanic -accretion complex was accreted to the Tarim block, forming an Andean-type margin in the Late Ordovician. This mar- gin grew southward (present-day coordinates) in the Early Silurian, with an accretionary complex on its southern side against which the Kudi gneiss-schist complex was attached. Clastic sediments continued to infill forearc/intra-arc basins on the top of the arc-accretionary complexes during Devo- nian–Carboniferous time. In the Early to Mid-Carboniferous, tectonic and magmatic quiescence is indicated by an absence of volcanic and plutonic rocks in the Kudi area, whereas subduction was still active in the western part of the range in the Gaz area near the present-day Pamir syntaxis. A Late Carboniferous–Permian to Early Mesozoic accretionary arc and forearc developed in front of the Andean-type margin, indicating that north-dipping subduction was renewed in the Late Carbon- iferous. Final collision of the Karakoram–North Qiangtang block to the western Kunlun orogenic collage by the Mid-Jurassic resulted in closure of Paleo-Tethys. The accretion and development of the Western Kunlun orogen contributed huge quantities of material for the crustal growth of Asia.

Introduction tuting the northwestern margin of the , the present-day Western Kunlun is a spec- STRADDLING THE BORDER between the Gondwanan tacular mountain range, extending from the Pamir Karakoram–North Qiangtang Block to the south and syntaxis in the west to the Altyn Tagh fault and the the Cathaysian Eurasian continent to the north, the Eastern Kunlun Range in the east (Fig. 1A). The Western Kunlun Range is part of a Paleozoic–Early Western Kunlun Range is characterized by high ele- Mesozoic orogenic system that extends some 10,000 vations (mostly more than 5,000 m) and local relief. km along the southern margin of . Consti- Western Kunlun geology deserves the attention of the international earth science community, inas- 1Corresponding author; email: [email protected] much as within its territory is exposed the most

0020-6814/03/659/303-26 $10.00 303 304 XIAO ET AL.

FIG. 1. A. Schematic tectonic map of Central Asia showing the tectonic setting of the Western Kunlun Range. B. The tectonic divisions of the West Kunlun orogen and adjacent areas (modified after Şengör and Okurogullari, 1991; Pan, 1996; Xiao et al., 2002b). Abbreviations: EK-Q = Eastern Kunlun–Qaidam. intact examples of Paleozoic ophiolites (i.e., Western Kunlun orogen include: time and patterns emplaced oceanic lithosphere) within the Tibetan of deformation (Xiao et al., 1998; Zhou et al., 1999); Tethyan region, making this an ideal natural labora- origin of granitic plutons (Yuan, 1999; Jiang et al., tory for the study of sea-floor spreading, and argu- 2002); nature of the Paleozoic arcs in the Yixieke ably, for the initiation of oceanic obduction Valley and at Oytag (Fang 1998; Yuan, 1999; Xiao et processes. Also, it provides important information to al., 2002b); ophiolites (Wang et al., 2001, 2002; help resolve long-standing regional controversies— Xiao et al., 2002b); and forearc and associated fold- e.g., concerning forearc accretion (Şengör et al., thrust belts (Pan, 1996; Xiao et al., 1998, 2002b). 1993; Şengör and Natal’in, 1996) vs. backarc Therefore, it is timely to review these data in an up- collapse tectonics (Yao and Hsü, 1994; Hsü et al., to-date synthesis, and in so doing create a new 1995). The Western Kunlun is an accretionary framework for the mountain belt. This paper sum- orogen that developed into a collisional orogen. marizes the broad structures and nature of the West- Accretionary orogenesis has been recognized by the ern Kunlun orogen and presents an interpretation of international community for over a decade (Wakita, the major tectonic events and settings. We empha- 1988; Şengör and Okurogullari, 1991; Şengör et al., size two important accretionary complexes of the 1993; Robertson, 1994; Windley, 1995; Mascle et Western Kunlun, with regional comparisons and al., 1996). Şengör (1992) defined the southern interpretations. In order to achieve that goal, we also boundary of the Western Kunlun as the southern- document thrust structures concerned with regional most limit of accretionary orogenesis of Central deformation in the Paleozoic and Early Mesozoic. Asia. Thus, tectonic investigations of the Western In the final section, we discuss alternative paleo- Kunlun Range will shed light on some of the basic environmental and tectonic interpretations of the problems of mountain belts. Western Kunlun in its regional context, and con- Western Kunlun geology has commonly been clude with our preferred solution. considered just in a wider regional context (Chang et al., 1986; 1989; Searle, 1991; Yin and Harrison, Regional Geology and Main Tectonic Units 2000). At the end of the last century, joint expedi- tions to investigate the tectonics of the Western The Tibetan Plateau is composed of several con- Kunlun were undertaken and many results have tinental and arc , successively accreted appeared (Matte et al., 1996; Mattern et al., 1996; from to Laurasia (Chang et al., 1986; Yang et al., 1996; Yuan, 1999; Mattern and Dewey et al., 1988). The tectonic evolution of the Schneider, 2000). However, reflecting their different Western Kunlun records the earliest history of the data and syntheses, many studies of the tectonic set- Tibetan Plateau (Chang et al., 1986; Dewey et al., ting and evolution of the Western Kunlun orogen 1988; Şengör and Okurogullari, 1991; Matte et al., have been contentious (see Pan, 1996; Xiao et al., 1996; Pan, 1996; Yang et al., 1996; Mattern and 1998, 2002a, 2002b; Yuan, 1999; Zhou et al., Schneider, 2000), as well as more recent exhuma- 1999). Significant advances in understanding the tion and related structures (Arnaud et al., 1993; GROWTH OF CONTINENTS 305

Brunel et al., 1994; Kao et al., 2001; Rolland and minor volcanics. This relationship led Xiao et al. Pêcher, 2001; Tapponnier et al., 2001; Li et al., (2002b) to suggest that the -marble slices are 2002). Pan (1990, 1996) subdivided the geology of fragments of a volcanic incorporated into the Kunlun Range into the North Kunlun, South an early Paleozoic accretionary prism. Therefore, we Kunlun, and terranes separated by the distinguish a Tarim rift-passive margin sequence of Kudi and Mazar- sutures, respectively mainly Precambrian age (Pan, 1996) in the north, (Fig. 2). Collision of these three blocks resulted in and an accretionary complex of mainly early Paleo- the closure of intervening oceans and consequently zoic age farther south. significant orogenesis in the early Paleozoic and Early Mesozoic, respectively (Pan, 1996; Matte et Akaz suture al., 1996; Mattern et al., 1996). However, this divi- The Akaz fault (Fig. 3) is an important tectonic sion was recently revised (Zhou et al., 1999; Xiao et boundary against which the Tarim passive margin al., 2002a, 2000b; Yuan et al., 2002a). We describe (North Kunlun ) in the north was juxtaposed below the main tectonic units from north to south. against the Kunlun active arc in the south during late to early Paleozoic time. Although no Tarim marginal sequences ophiolite has been found along this fault, we term it The northernmost belt near Akaz Daban (or Akaz the Akaz suture (Xiao et al., 2002b). Pass, Fig. 3) mainly comprises the of the The Akaz suture is not the “Kudi suture” of Tarim block (Unit I in Fig. 3), which is composed of Matte et al. (1996). In their geological map of the Proterozoic gneiss, schist, migmatite, stromatolite- Western Kunlun, Matte et al. (1996) marked the bearing limestone, clastics, and chert, overlain by Kudi ophiolite on the Akaz fault, mainly in the Buzi- Sinian conglomerates, tillites, clastics, and carbon- wan Valley, which is more than 30 km south of the ates (Fig. 4). The Halasitan Group (the Bashikagong Akaz fault. Also, there could not have been north- Group of Li et al., 1995) is representative of the ward subduction under the Tarim block in the early basement rocks and consists of schist, augen gneiss Paleozoic, because there is no Andean arc to indi- (Fig. 4A), amphibolitic gneiss, and migmatite inter- cate it. Accordingly, south-dipping subduction and calated with quartzite and marble. These rocks were subsequent collision along the Akaz suture took intruded by a 2.2 Ga granite (Xu et al., 1994; Pan, place before emplacement of the 128 km arc 1996). The amphibolitic gneisses show complicated 3granitic complex in the Early Ordovician (Xiao et fold styles (Figs. 4B and 4C). There are two groups al., 2002b). Tectono-stratigraphic analysis suggests of marbles: the first is a thin-bedded, ca. 600 m northward thrusting during the Silurian (e.g. Matte thick marble with a very steep dip, but no compli- et al., 1996; Xiao et al., 2002b). cated folds (Fig. 4D); the second is a strongly folded marble intercalated with quartzite. Yuan et al “128 km” arc granitic pluton (2002b) regarded the former as a probable Precam- The “128 km pluton” (named after a signpost on brian rift sediment that rests unconformably on the -Tibet road) is located about 20 km gneisses of the Tarim block (Fig. 5). A basic dike north of Kudi (Fig. 3). It is in fault contact with vol- that intrudes the gneisses yields a 40Ar-39Ar age of canic rocks of the Yishake Group on its southern 1800 Ma (Zhou, in prep.). The second marble, which margin (Mattern and Schneider, 2000; Xiao et al., is in thrust contact with the former marble, contains 2002b), is crosscut by a younger biotite monzogran- Cambrian–Ordovician crinoids (Zhang et al., 1997; ite on its western side, and to the east it is truncated Xiao et al., 2002b). On the southern slope of Akaz by the Halastan fault. The pluton has a granodioritic Daban, Late Ordovician–Silurian radiolaria occur in composition (Yuan, 1999), is deformed, has a horn- chert in complexly imbricated basaltic greenschists blende-biotite lineation, and its foliation direction is (Zhou, 1998) (Fig. 3). The basaltic protolith of the parallel with NW/SE-trending shears in the wall greenschist (Pan, 1996) has a within-plate trace-ele- rocks (Mattern et al., 1996; Yuan, 1999). ment signature (Yuan, 1999), and intercalated mar- The pluton is enriched in Al2O3 (15.7–18.4 bles suggest a shallow-water passive/continental wt%), Sr (470–864 ppm), and other large-ion margin environment (Pan, 1996). These rocks are lithophile elements (LILE), but relatively depleted overlain unconformably by and Permo- in high-field-strength elements (HFSE), with Carboniferous marine clastics and carbonates and LREE-enriched patterns and low to medium Permo-Triassic terrestrial clastics, carbonates, and europium anomalies (σEu = ~0.7), showing typical 306 XIAO ET AL. Figures 3, 10, and 13; also shown is the also shown 13; and 10, 3, Figures d after Xiao et al., 2002a, 2002b). Boxes indicate the areas of some post-Jurassic sedimentary units are not shown. . 2. Generalized tectonic map of the West Kunlun orogen (modifie . 2. Generalized tectonic map of the West IG F cross-section line of Figure 14. To enhance legibility, cross-section line of Figure 14. To GROWTH OF CONTINENTS 307

FIG. 3. Tectonic facies map of the Akaz-Kudi area (modified after Xiao et al., 2002b). characteristics of I-type, volcanic-arc granitoids. not generated in an oceanic arc far from a continental Although its relatively high Al2O3 and Sr, and low margin. Rather, major-element compositions have a MgO contents resemble those of typical adakites, its calc-alkaline signature; thus Li et al. (1995) and relatively high Yb (1.92–2.88 ppm) and Y (19.4– Zhang et al. (1996) concluded that this is a subduc- 34.0 ppm) contents and low Sr/Y (24.2–37.0) and Zr/ tion-related granite. However, it has a syn-tectonic Sm (7.3–21) ratios do not support a subduction ori- fabric (Li et al., 1995; Mattern and Schneider, 2000), gin. Nd-Sr isotope data (εNd = 0.66 to –1.09; initial and thus it is more likely it was generated in a south- 87Sr/86Sr = 0.7075–0.7091) show that the source of ward-accreting, active continental margin. the pluton contains both crustal- and mantle-derived The granite has a U/Pb age of 460 ± 2.4–2.5 Ma materials (Yuan et al., 2002b), suggesting that it was (Xu et al., 1996; Sobel and Arnaud, 1999), a 471± 5 308 XIAO ET AL.

FIG. 4. A. Augen gneiss showing top-to-SE asymmetric kinematics, looking northwest. B. Rootless folds in amphib- olite gneiss, looking northwest. C. Asymmetric s-type fold showing sinistral strike-slip faulting, looking southwest. D. Steep bed of marble, looking northeast. All occur in an unnamed E-W–trending valley roughly perpendicular to the highway on the southern slope of Akaz Pass.

Ma concordant U-Pb zircon age (Yuan, 1999), and a (Figs. 3, 6, and 7). Our fieldwork and geochemical U/Pb age of 490.9± 2.8 Ma (Yuan, 1999). It is not investigations indicate that this “Kudi ophiolite” is clear why the U-Pb zircon ages are so different; we actually an arc-ophiolite (Xiao et al., 2002b; Yuan et tentatively suggest that the granite was emplaced at al., 2002b) composed of Yixieke arc volcanics, Yix- ca. 471 ± 5 Ma and contains inherited zircon grains shak intra-arc/forearc sediments, and Buziwan of 491 ± 3 Ma, because the 471 ± 5 Ma zircon age is ophiolitic ultrabasic rocks. close to the 40Ar/39Ar age of amphibole from the Yixieke arc volcanics. As shown in Fig. 7A, the same pluton: 475.5± 8.8 Ma (plateau) and 471 ± 43 Yixieke arc consists of basaltic and andesitic Ma (concordia). The “128 km” granitic pluton rep- lavas. The lithology includes fine- and coarse- resents the main component of an early Paleozoic grained basaltic pillow lava (Figs. 8 and 9A) inter- magmatic arc (Pan, 1996; Matte et al., 1996; Sobel bedded with red chert, amygdaloidal andesite, and Arnaud, 1999; Yuan, 1999). andesitic basalt and tuff, and welded andesitic breccia and agglomerate in the Yixieke Valley and Yixieke arc along the Xinjiang-Tibet Highway road section The previously termed “Kudi Ophiolite Suite” (Wang, 1995; Pan, 1996). The andesites mostly (Matte et al., 1996; Pan, 1996; Mattern and overlie (see Fig. 7A), which are character- Schneider, 2000) mainly consists of the Yishak ized by imbricated pillowed and massive lavas, Group in the Yixieke Valley and of ultrabasic rocks intercalated with cherts and turbidites. Figure 8 in the Buziwan and unnamed valleys south of Kudi shows pillows in a 30 m high cliff, about 15 km GROWTH OF CONTINENTS 309

FIG. 5. Photograph showing the thrust boundary between gneiss and marble in the same unnamed E-W–trending valley as in Fig. 4, looking east. north of the village of Kudi on the Xinjiang-Tibet tent with an arc geochemical signature (Fang, 1998; Highway. The steeply dipping pillow lavas are in Wang, 1998). thrust contact with massive basalts to the south and Evidence for the age of the Yixieke arc is pro- north. vided by the following. A maximum age is indicated The tholeiitic basalts have initial 143Nd/144Nd by the Cambrian Akaz seamount, the presence of and 87Sr/86Sr isotopic ratios ranging from 0.5122 to which implies the existence of a trench adjacent to

0.5123 (εNd = +5.8 – +8.0) and from 0.7037 to the arc. The 470–460 Ma Andean-type, “128 km” 0.7050, respectively. In a diagram of Sr-Nd isotopic granodiorite (Fig. 3) provides a maximum, Early variations, data points fall in the overlapping back- Ordovician age for the , and a precise age arc basin, arc, and OIB fields (Yuan, 1999). This of the formation of the active continental margin arc. distribution, based on major- and trace-element Late Ordovician (ca. 450 Ma) radiolaria in forearc data, supports the idea that these lavas show a turbidites (Zhou et al., 1998; Xiao et al., 2002b) strong arc signature due to possible mixing of the immediately overlying the arc lavas indicate that different proportions of three components: fertile volcanism had ceased by this time. OIB, depleted sub-arc mantle, and fluids from a In the section north of the Yishak turbidites (see subducting slab that resulted in formation of the vol- Fig. 3), Figure 6 shows that the rocks dip shallowly canic rocks in an incipient oceanic-arc setting to the south, and that basalts in the north are over- (Yuan, 1999). lain southward by andesites and turbidites. In con- Recently discovered boninitic rocks (Yuan, trast, in the section south of the turbidites, the lavas 1999; Xiao et al., 2002b) are characterized by high dip steeply northward, and basalts are overlain by

Al2O3/TiO2 values >20, low TiO2 and Al2O3 values, andesites. From these relations, Li et al. (1995) and high SiO2 and Na2O values (Yuan, 1999). The envisaged the limbs of a major syncline. However, samples show LREE-enriched patterns [(La/Yb)N = the contacts between many volcanic and turbidite 1.5–2.0], but do not exhibit the U-shaped REE pat- units are commonly thrusts (Fig. 7); thus we con- terns common in boninites (Yuan, 1999). The εNd clude that thrusting has tectonically imbricated the values are lower than +3.0. Although not typical of lava-turbidite stratigraphy. boninites, these rocks are akin to evolved boninites Yishak intra-arc/forearc basin. Ophiolite-derived of the Mariana forearc (Yuan, 1999; Xiao et al., debris flows form a 1500 m thick turbidite succes- 2002b). This idea needs to be confirmed by future sion, the majority of which lies tectonically above investigations, but the preliminarily data are consis- the arc volcanic rocks (Figs. 3, 6A, and 7). The 310 XIAO ET AL.

succession includes tuffaceous sandstone, andesitic sandstone, and radiolarian chert (Wang, 1983; Pan, 1996). Fang (1998) described sandy contourites and well-preserved radiolaria in siltstone and mudstone. Late Ordovician–Silurian radiolaria occur in lower- most beds of the turbidite immediately above the volcanic rocks (Fang, 1998; Zhou, 1998); these provide a minimum age for the arc volcanism. Petrochemical data of the turbidites suggest that they are an arc-related basin-fill (Fang, 1998; Wang, 1998; Yuan, 1999; Xiao et al., 2002b). In the Yixieke Valley, the volcanic and sedimentary rocks mostly dip to the south and have been thrust north-

in Figure 3 excepttext indicated. See where in Figure 3 ward toward the Akaz accretionary prism; in the absence of any evidence of subsequent overturning, we infer that the subduction polarity was to the south. The fact that the basalts are mostly in the north, andesites farther south, and felsic lavas in the far south confirms this polarity. Therefore, we conclude that they were intra-arc/forearc basins (Fang, 1998; Xiao et al., 2002b). The youngest turbidites, which lie on a thrust above the Late Ordovician–Silurian turbidites (Figs. 6 and 7A), contain radiolaria of Late Devonian– Early Carboniferous age (Fang, 1998; Zhou, 1998).

Accordingly, we place them in a D3-C1 forearc basin 2002b). Symbols are the same as those used same Symbols are the 2002b). (Figs. 3 and 7) whose tectonic setting is very similar to the present-day Great Valley of western (Dickinson, 1995). Mattern and Schneider (2000) reported Carboniferous–Permian radiolarian cherts in the turbidites; this implies that forearc deposition may have continued to the Permian. Con- sidering the age range of the turbidites, the arc was covered not only by intra-arc/forearc basins, but also very likely by post-collisional clastic debris. Buziwan ultrabasic complex in Figure 3 (modified afteretal., Xiao in Figure 3 (modified A major ultrabasic slab, about 3 km thick, is located northwest of Kudi, and in the Buziwan Valley and its surrounding area (Deng, 1995; Li et al., 1995). Stratigraphically from bottom to top it consists of sheared serpentinite on a basal thrust, layered harzburgite-dunite, layered dunite with chromitite, and harzburgite with fine-grained gab- bro (Fig. 7B). No plagiogranite dikes are known, and no isotopic data are available to date these rocks precisely. Although shown on the geological map as a 15 km long, continuous, SE-trending lens, it has been highly intruded and dismembered by an Early

. 6. Cross-section A-B along the line shown the line 6. Cross-section along A-B . Mesozoic granite that yielded consistent zircon ages IG F of 212 ± 1.9 Ma for granodiorite and 214 ± 1.0 Ma

for discussion. for discussion. for monzogranite (Yuan, 1999). Within the granite GROWTH OF CONTINENTS 311

FIG. 7. A. Stratigraphic column of the Yixieke arc and forearc basin turbidites (after Xiao et al., 2002b). Not to scale. B. Stratigraphic column of the Akaz-Kudi area, showing mainly Buziwan ophiolite thrust over the Kudi gneiss complex and intruded by a 214 Ma arc granite (after Xiao et al., 2002b). Not to scale. are two main lenses of fine-grained gabbro 30 m and lenses up to several hundred meters long and 10–20 100 m across, plus a 100 m wide body of dunite, m wide within the marbles and metasediments of the which contains a 20-30 m wide lens of chromitite. A gneiss-schist complex (Pan, 1996; Shen et al., similar slab of grain size–layered dunite (ca. 1.5 km 1996). Their southward imbrication within the Kudi wide and 3 km long) occurs on the eastern side of the gneisses, as well as the similar lithology and close granite. The dunite contains layers of clino- proximity to the Buziwan ultrabasic rocks near the pyroxenite, and is traversed by discordant veins of village of Kudi, may indicate that they were part of clinopyroxenite, olivine-orthopyroxene (Figs. 9B– the Buziwan ophiolite. Therefore their southward 9D), and asbestos. On its eastern side, the dunite imbrication may indicate an early obduction phase, overlies the gneisses on a thrust that dips moderately but this notion needs further geochemical and geo- to steeply northwest and on which there are 50 meters chronological confirmation. of mylonite (25 m of sheared serpentinite and 25 m of The Kudi arc-ophiolite is mainly characterized augen gneiss) (Figs. 6 and 7B); the ultrabasic rocks by harzburgite-depleted mantle overlain by IAT- or were clearly thrust southeastward over the Kudi OIB-type basalts with boninitic and calc-alkaline gneiss-schist complex. The Kudi gneisses below the lavas, which are in turn covered by arc/ophiolite– eastern side of the dunite dip consistently to the derived debris. This sequence is similar to the northwest. Following Matte et al. (1996), Pan (1996), suprasubduction zone ophiolite tectonic facies of Mattern and Schneider (2000) and Wang et al. (2001), Robertson (1994), although no sheeted dikes are we interpret these ultrabasic rocks as the lowermost present. Recently, Wang et al. (2001, 2002) sug- unit of the Kudi ophiolite. A strong down-dip linea- gested the whole Kudi ophiolite is a remnant of an tion (with a plunge of 70° NW) on the foliation of the arc-related backarc basin. As the 470–460 Ma “128 mylonitic gneiss at the contact suggests that the ophi- km” granitic rocks postdate the arc-subduction sys- olite was obducted to the southeast (Fig. 2). tem, the former are part of a magmatic arc that Minor meta-peridotites and meta-harzburgites, intruded the arc volcanics. We suggest that the located in unnamed valleys south of Kudi, occur as stratigraphy can be best interpreted as a substrate of 312 XIAO ET AL.

FIG. 8. Photo-mosaic of pillow lavas on the Xinjiang-Tibet Highway north of Kudi village, looking west. Geologist and motor vehicle shown for scale.

FIG. 9. Photographs of ultrabasic and basic rocks in the Kudi area. A. Pillow lava in the Yixieke Valley. B, C, and D are in the Buziwan Valley. B. Cumulate peridotite. C and D. Dunite intruded by pyroxenite veins.

Buziwan ocean floor (ophiolitic ultrabasic-basic Kudi gneiss-schist complex rocks) overlain by an Upper Cambrian–Lower The Kudi gneiss-schist complex, or Kudi Ordovician intra-oceanic, suprasubduction-arc terrane, extends as the main ridge of the western units, in turn overlain by Upper Ordovician–Sil- Kunlun Range. It is composed of biotite/horn- urian and Upper Devonian–Lower Carboniferous blende–bearing gneiss and migmatite that contain turbiditic intra-arc/forearc basinal strata. minor lenses of schist, marble, phyllite, and quartz- GROWTH OF CONTINENTS 313

ite. Where hornblende-bearing, the gneiss includes about 7–10 km wide: in the north, mica-quartz many lenses of amphibolite and several generations schist with amphibolite and marble; in the center, of amphibolite (metamorphosed diabase) dikes. The quartz-feldspar gneiss; and in the south, quartz- last generation consists of subhorizontal dikes mica schist. Kinematic indicators, such as asym- highly discordant to the steep gneisses, this highly metric folds, oriented stretching lineations, large- angular relationship demonstrating that there has scale recumbent folds, and rotational porphyro- been no shearing after emplacement of the dikes. blasts display a top-to-the-south sense of ductile Some biotite paragneisses contain rare lenses of thrusting (Zhou et al., 1999; Xiao et al., 2002b). The marble and quartzite (with or without garnet). The preferred orientation of the stretching lineation is rocks are deformed into km-scale monoclinal folds, 340–350° NW (Zhou et al., 1999; Xiao et al., the axes of which plunge moderately to steeply 2002b), which is parallel to the fold axes (Xiao et NNW; they generally strike NE and dip to the NW al., 2002b). Dislocation structures of quartz in the (Fig. 2). This gneiss-schist complex was thought to gneisses show that ductile deformation of the Kudi be Proterozoic in age (Pan, 1990; 1996; XBGMR, megashear zone occurred under a steady-state rhe- 1993; Ding et al., 1996; Hu et al., 2001). ology, and that differential stress took place between However, a U-Pb zircon age from the Kudi gneiss 105 and 119 Ma (Zhou et al., 1999). shows an upper intercept of 1251 ± 231Ma, and a The Kudi gneiss-schist complex is an accreted lower intercept of 533 ± 21Ma. Although Hu et al. Precambrian continental fragment (Xiao et al., (2001) reported a 2045 Ma zircon age from gneisses 2002b; Yuan et al., 2002b). This is in good agree- near the village of Kudi, no detailed data were pre- ment with the fact that granites that have intruded sented in their paper. There is urgent need for zircon the gneisses yield Nd model ages of 1.1–1.5 Ga dating of the metamorphic rocks in the Kudi area. (Yuan, 1999). The gneisses are imbricated with an Recently, 40Ar/39Ar dates on lineated hornblende ophiolitic slice south of Kudi, and have been over- (452 ± 5 Ma) and biotite (428 ± 2 Ma) in the gneiss- thrust by the Buziwan ultrabasic rocks. These rocks, schist complex suggest that Late Ordovician–Early structural relations, and isotopic ages are not Silurian ductile deformation affected the gneisses present in the North Kunlun and Akaz units, nor in (Matte et al., 1996; Zhou, 1998). The North Kudi the Kudi arc-ophiolite. Therefore, the Kudi gneiss- granite (Matte et al., 1996; Mattern and Schneider, schist complex can be regarded as a separate ter- 2000), which has a U/Pb zircon age of 380.0 ± 1.9– rane tectonically distinct from the geological units to 0.7 Ma (Xu et al., 1996) and a U/Pb zircon age of the north. 404.0 ± 3.1 Ma (Yuan, 1999) intrudes the Kudi gneiss-schist complex (Zhou, 1998). On their south- Qimanyute ophiolite ern side, the gneisses are overlain unconformably A Cambrian ophiolitic mélange (Figs. 10 and 11) by well-bedded metapelites (Xiao et al., 2002b). is present farther east on the Altyn Tagh fault, where Neither ductile deformation nor high-grade meta- the eastern and western Kunlun Ranges join in morphism has been reported in the bedded metased- southern Yutian County of the Xinjiang Uygur iments; thus we suggest that the unconformity Autonomous Region (see Fig. 1B for location). Its between the bedded metasediments and the main characteristics are the following. The gneisses could be Paleozoic. However, the precise Qimanyute ophiolite (Han et al., 2002) occurs in ca. age requires further geochronology. 2000 m wide, NE-SW–trending thrust slices imbri- Anorthitic anorthosite rocks, imbricated in the cated with deep-sea sediments, high-grade meta- Kudi gneiss-schist complex near the village of morphic rocks, and metasediments. We describe two Kudi(Zhou et al., 2001), have been interpreted as a sections through this ophiolite (Figs. 11A and 11B). typical product of island arcs or active continental Section A of Figure 11, located along the Tieke- margins rather than oceanic islands and mid-oce- saiyi River southeast of the village of Qimanyute, is anic ridges (Beard, 1986; Beard and Borgia, 1989; typical of the ophiolite. The south-dipping ophiolitic Wilson, 1989). Accordingly, we infer that there was rocks are structurally imbricated between garnet an Andean-type margin in the Western Kunlun biotite amphibole plagioclase gneiss and amphi- Range. bolite of the Milan Group to the north, and biotite Within the Kudi gneiss-schist complex, a major plagioclase gneiss and dolomitic marble of the shear zone at Kudi village is ca. 27 km wide, and Alamasi Group to the south. From south to north, the strikes roughly E-W. It consists of three belts, each section shows diabase, massive basalt, thin-bedded 314 XIAO ET AL.

FIG. 10. Simplified geological map of the Qimanyute area, Yutian County, Xinjiang (modified after Han et al., 2002). See Figure 2 for location. The section lines A and B of Figure 11 are marked. Abbreviations: K = Cretaceous; J1-2 = Lower to Middle Jurassic; AG = Alajiaoyi Group. ChK = Kaqiang Group. limestone, amygdaloidal basalt, basaltic tuff inter- that there are SSZ-type, MORB, and OIB compo- calated with purple and dark grey cherts, strongly nents (Han et al., 2002). This is reminiscent of the foliated granitic rocks, granitic mylonite, pillow Kudi ophiolite-arc complex, described above. The basalt, gabbro, and olivine pyroxenite. Archean–Early Proterozoic Milan Group and the Section B of Figure 11, located southeast of the Neoproterozoic Alajiaoyi Group (Han et al., 2002), village of Alajiaoyi(Fig. 10), is characterized by which are tectonically located north of the thick basalts imbricated with chert, siliceous lime- Qimanyute ophiolite, are the marginal sequences of stone, quartz diabase, basaltic tuff, gabbro, and the Tarim block (XBGMR, 1993). The southerly cumulate gabbro. Limestone and siliceous lime- located units, the Jixianian (Late Proterozoic) Ala- stone also occur as blocks in basaltic tuffs. The masi Group, can be correlated well with the Kudi section includes dolomitic marble and felsic gneiss gneiss-schist complex and the Yixieke arc volca- of the Alamasi Group. The southern boundary is a nics, which were also previously regarded as Jixian- south-dipping ductile thrust along which coarse- ian (XGBMR, 1993; Li et al., 1995; Pan, 1996). A grained quartzite of the Alamasi Group overlies the U-Pb zircon age of 526 ± 1.0 Ma from fine-grained ophiolitic rocks, whereas the northern boundary is gabbro (Han et al., 2002) of the Qimanyute ophiolite marked by a south-dipping ductile thrust along suggests that in the late Middle Cambrian, an ocean which meta-feldspar/quartz sandstone of the (the Qimanyute Ocean) formed part of Proto-Tethys Alajiaoyi Group is structurally overlain by the to the south of the southern Tarim margin. ophiolitic rocks. A granodiorite of unknown age intrudes the basalts. Sailiyak arc South-dipping imbricated thrusts indicate north- The southern part of the South Kunlun terrane ward vergence in both sections (Han et al., 2002). is occupied by the Sailiyak magmatic arc (Fig. 1), Petrochemistry of the Qimanyute ophiolite shows represented by granodiorite, monzogranite, tonalite, GROWTH OF CONTINENTS 315

FIG. 11. A. Cross-section showing imbrication of the Qimanyute ophiolite and gneisses, southeast of the village of Qimanyute (modified after Han et al., 2002). B. Another section of the Qimanyute ophiolite and gneisses, SE of Alajiaoyi village (modified after Han et al., 2002). quartz monzodiorite, andesite, tuff, agglomerate, consists of slate, phyllite, greenschist, sandstone, and pyroclastic rocks with the trace-element signa- siltstone, limestone, volcanic rocks, and tuff ture of a (Pan, 1996; Yuan, 1999). A (Gaetani et al., 1990). They have been described as gneissic granite yields a biotite Ar-Ar age of 211 Ordovician, Silurian, Devonian, Carboniferous, and Ma, indicating that the arc was formed in the Late Permian shallow-water shelf clastic and carbonate Triassic (Pan, 1996; Xu et al., 1996). Pan (1996) rocks and Triassic deep-water clastic turbidites reported Upper Carboniferous (Pennsylvanian) to (XBGMR, 1993; Pan, 1996). Permian calc-alkaline arc-type volcanic rocks; their However, the different lithologies are not inter- presence means that magmatism related to north- bedded, but tectonically intermixed within a ward subduction could have started as early as Late mélange (Hsü, 1988; Du et al., 1990; Yao and Hsü, Carboniferous (Pennsylvanian) (Graham et al., 1994; Hsü et al., 1995; Mattern and Schneider, 1993). Photographs of the arc volcanics are pre- 2000; Xiao et al., 2002a). Blocks as long as several sented in Figures 12 and 13. Magmatic rocks within hundred meters composed of sandstone, arenite, the Bazarand Qitai accretionary wedges (Fig. 2) limestone, and volcanic rocks occur in a matrix of mainly consist of granitic rocks, the majority of meta-siltstone. Some blocks contain Carboniferous which yield a Rb/Sr biotite isochron age of 190 Ma and Permian fossils (Pan, 1996), and carbonate (Zhang and Xie, 1989), suggesting that by the Late rocks ranging from Ordovician to Permian in age Triassic to Early Jurassic, the locus of magmatism occur as exotic blocks or olistostromes (Jiang et al., had migrated southward. 1992; Pan, 1996; Yin and Bian, 1995). The meta- volcanic rocks include diabase, basalt, spilite, and Mazar subduction-accretionary complex andesitic porphyry. In some 150 m sized blocks, pil- Bazar subduction wedge. The Mazar subduction- low and amygdaloidal basalts with a trace-element accretionary complex is situated south of the Sail- geochemistry indicating an oceanic-island tholeiitic iyak magmatic arc (Fig. 2) (Xiao et al., 2002a). The origin (Ding et al., 1996), are imbricated with pelitic northern part of the Mazar complex, which we term and arenaceous quartz-mica schists interpreted as the Bazar accretionary wedges (Xiao et al., 2002a), Triassic deep-sea sediments. Yao and Hsü (1994) 316 XIAO ET AL.

FIG. 12. Photograph looking northeast, showing Permian arc volcanics and associated granitic rocks thrust southward over steeply dipping Lower to Middle Jurassic sediments. Bold white lines are thrusts.

FIG. 13. Photo-mosaic showing complex folds and thrusts in the Qitai forearc flysch of mainly Triassic age, looking northeast. Also shown is a telephone line pole that is some 8 meters high and about 2 km distant from the foot of the cliff. See Figure 2 for location. and Hsü et al. (1995) reported the occurrence of an Ulugh Muztagh that could be the eastern extension ophiolite mélange near Mazar, containing blocks of of this belt of ophiolites. The matrix is slate, phyl- serpentinite, gabbro, greenstone, radiolarite, meta- lite, and quartz-mica schist, which has a pervasive graywacke, marble, gneiss, and volcanic rocks, cleavage (Zhang et al., 1997), duplexes, and early embedded in a pervasively sheared, cleaved matrix ductile and late brittle structures (Li et al., 1995; of sericite-quartz schist, chlorite schist, two-mica Pan, 1996; Xiao et al., 1998). The matrix has mainly schist and phyllite (Mattern and Schneider, 2000). greenschist-facies to low-amphibolite-facies assem- Mattern and Schneider (2000) also reported an ophi- blages (Li et al., 1995), but locally it is more highly olite east of Dahongliutan. Molnar et al. (1987) metamorphosed to amphibolite-facies schist or reported a Permian–Triassic ophiolite farther east at gneiss (Li et al., 1995; Matte et al., 1996; Pan, GROWTH OF CONTINENTS 317

1996). Calc-alkaline arc-volcanic rocks near Hez element data show that the rocks have an island-arc (Fig. 2), associated with Triassic fossils (Eamonpho- or active-margin affinity (Zhang et al., 1997). The tis sp., Costinorella sp., and Mudispirifena sp.), structural relationships between the Qitai basins occur in the Bazar complex (Li et al., 1995), which and the adjacent, underlying accretionary prism also contains fossils as old as Silurian (Yao and Hsü, suggest that the sediments occupied a forearc posi- 1994; Li et al., 1995). The presence of ophiolitic tion in late Paleozoic (at least Carboniferous) to Late slices, basalt, siliciclastic turbidite, and granitic Triassic time (Xiao et al., 2002a). Provenance anal- rocks (Li et al., 1995) is consistent with an arc- ysis of sediments in the main basin centered at Qitai trench setting (Mattern and Schneider, 2000). demonstrates two sources, one from the accretionary Heweitai . This is the Paleo- prism and arc rocks to the north and the other from zoic–Upper Triassic, southern part of the Mazar the accretionary-prism rocks to the south (Zhang et subduction-accretionary complex, located south of al., 1997). the Qiertianshan-Hongshanhu fault and north of the Prominent NE-dipping thrusts separate large- Konggashankou-Qojir suture (Fig. 3). It is deformed scale nappes and thrust belts, and duplex structures and metamorphosed, but less so than the Bazar com- are common at various scales. Folds and thrusts plex. The Heweitan is a mélange, the matrix of show decreasing deformation intensity from north- which is slate or phyllite with a greenschist-facies east to southwest; this polarity is similar to that of mineral assemblage of sericite + chlorite + quartz the southwestward-decreasing deformation of the (Li et al., 1995). Blocks are composed of graywacke, underlying accretionary prism (Xiao et al., 1998). tuff, arenite, turbidite, tuffaceous sandstone, radi- Some forearc basins in the world are filled by flat- olarite, limestone, and siltstone, intercalated with lying, undeformed sediments deposited unconform- calc-alkaline volcanic rocks (Li et al., 1995; Pan, ably on an accretionary prism (e.g., ; see 1996). The limestones occur as blocks in the tuf- Dickinson, 1995). The fact that the Qitai forearc faceous and siliceous sediments; some are pelagic basin has been highly thrusted and imbricated limestones (Gaetani et al., 1990) yielding mainly suggests that it was in an advanced stage of being Permian fossils (Gaetani et al., 1990; Pan, 1996). incorporated into its underlying accretionary prism. Thick sequences of gray sandstone and litharenite The entire subduction-accretion complex was are intercalated with calc-alkaline amygdaloidal intruded by subduction-related granitoids, indicat- basalt and diabase (Pan, 1996). Folds with a slaty ing an active margin environment (Mattern and cleavage in the whole area south of the Altyn Tagh Schneider, 2000; Xiao et al., 2002a). Granitic fault (Mazar-Kangxiwar fault) exhibit a S or SSW plutons, distributed along both sides of the E-W– vergence (Matte et al., 1996; Mattern et al., 1996; striking, Mazar-Kangxiwar deep fault, include dior- Mattern and Schneider, 2000). ite, tonalite, granodiorite, monzonitic granite, and Qitai forearc basins. These forearc basins extend K-feldspar granite. Petrochemical data show that along strike for some 300 km between the Sailiyak the plutons belong to a volcanic arc, and 40Ar-39Ar magmatic arc (southern Eurasia margin) to the north and U-Pb data indicate a Late Triassic–Early Juras- and the Karakoram-Qiangtang suture to the south. sic age (Zhang et al., 1996). They are mainly filled by weakly metamorphosed Although Hsü (1988), Yao and Hsü (1994), and and thrust-intercalated turbidite, slate, and carbon- Hsü et al. (1995) suggested that rocks south of the ate sediments (Zhang et al., 1997). The deposits in Mazar-Kangxiwar fault and north of the Qiaoertians- one major basin are situated structurally above the han-Hongshanhu fault belong to the Mazar accre- Bazar complex (which crops out to the north and tionary wedge (Mattern and Schneider, 2000), many south), and those of another major basin are struc- other researchers interpreted the geology south of turally above the Heweitan part of the accretionary the Qiaoertianshan-Hongshanhu fault in terms of prism (Fig. 2). Small basins in between occupy different blocks (Matte et al., 1996; Mattern et al., pockets above the prism. These are mainly accre- 1996). Xiao et al. (2002a) re-examined the area and tionary forearc basins, while the southern basins proposed that a huge subduction-accretion complex may also include trench or trench-slope sediments is located south of the Sailiyak magmatic arc and because of the proximity to the suture. Sedimento- north of the Konggashankou-Qojir suture, which we logic analysis indicates that the turbidites were briefly discuss in the following section. As already deep-sea sediments deposited in a middle- to outer- mentioned by previous workers, the huge Mazar fan system (Zhang et al., 1997). Major- and trace- subduction-accretion complex can be correlated 318 XIAO ET AL.

with the extensive Songpan-Ganze Belt or parts of it indicators in the Tianshuihai region that are diag- (Bayan Har Group) farther east (Molnar et al., 1987; nostic of subduction or sutures (e.g., no ophiolites or Şengör and Okurogullari, 1991; Matte et al., 1996; arcs). For the marine/non-marine Jurassic boundary, Mattern et al., 1996; Pan, 1996; Mattern and we suggest that the accretion complex was uplifted Schneider, 2000). and part of the toe of the accretionary prism or the accretionary forearc basins developed in a marine Konggashankou-Qogir suture environment. Therefore, we interpret this fault to be Gondwana cool-water fauna are present to the a blind thrust, situated in the foreland of the Tian- south of the Konggashankou-Qogir fault; to the north shuihai thrust belt. After the Mesozoic orogenesis, are Cathaysian warm-water fauna (Li et al., 1995). Cenozoic strike-slip faulting reworked it (Yao and Thus, a suture is inferred along the Konggashankou- Hsü, 1994), and it appears as a block structure in a Qogir fault separating the Cathaysian blocks to the current geophysical profile (Pan, 1996; Wittlinger et north and the Gondwana blocks to the south; it can al., 1996). be correlated with the Jinsha suture to the east (Li et al., 1995). Therefore, we interpret the Heweitan rocks to represent a subduction complex near the Evolutionary Tectonic Model trench of a north-dipping subduction zone, in which Late Cambrian to Early Ordovician the turbiditic sediments are in basins that overlie the accretionary-prism rocks near or in the trench After the Late Proterozoic, the southern margin (Xiao et al., 2002a). of the Tarim block (North Kunlun terrane) was prob- For the geology south of the Altyn Tagh fault, the ably a passive continental margin, but the ocean Qiaoertianshan-Hongshanhu fault has been (Proto-Tethys: Kudi Ocean/Qimanyute Ocean/Kun- assigned an important tectonic and paleogeographic lun Ocean) to the south (Pan, 1996) did not lie significance (Şengör and Okurogullari, 1991; Pan, between the Kunlun and Tarim blocks, as previously 1996; Mattern and Schneider, 2000). Situated along thought (Pan, 1996), because there is no evidence the SSW margin of the Tianshuihai region, the Qoer- for the existence of the Kunlun block at that time. tianshan-Hongshanhu suture is mainly a thrust fault South-dipping subduction of this oceanic litho- that remains subsurface beneath a cover of Jurassic sphere generated the Buziwan ophiolite in the Late to Cretaceous or Quaternary sediments (Pan, 1996). Cambrian to early Ordovician (Fig. 16A). An intra- Pan (1996) proposed that this fault is a suture, oceanic arc (Yixieke arc) was constructed upon the because the deformational styles are different on its Buziwan ultrabasic rocks in the early Ordovician as two sides, and it is the approximate boundary sepa- subduction continued (Fig. 16A). Geochemical data rating marine and non-marine Jurassic sediments. suggest that a plume component may be involved in However, based on the observations of tectonic the genesis of the lower tholeiitic basalts of the zonation and development of many multi-duplex Yixieke Valley volcanic complex (Yuan, 1999). structures and complicated folds (Xiao et al., 1998; Geochemical evidence also indicates that the slab- 2002a), we conclude that the Tianshuihai region derived component gradually replaced the plume- comprises a foreland fold-and-thrust belt with SW derived component to generate the boninite-like vergence, which represents a tectonic transition volcanic rocks of the Yixieke Valley volcanic com- zone between the complexly deformed northern plex, which represent the incipient stage of the Mazar-Kangxiwar area and the relatively unde- island arc. Possible backarc basins were formed formed to slightly deformed foreland. The horizontal along this arc edifice (Wang et al., 2001; 2002). A shortening and the vertical thickening of the strati- seamount capped by limestone (Akaz seamount) was graphic pile were accommodated by folding and incorporated into the accretionary prism north of the low-angle thrusting. Nevertheless, from our field arc in Cambrian–Early Ordovician time. The inter- observations, the fact that structural styles vary from vening ocean between the oceanic arc and the Tarim duplex structures in the NNE to imbricate fans in block was consumed, and the arc was emplaced the central part, to Jura-type folds in the SW (Figs. northward onto the North Kunlun terrane, giving 14 and 15), is compatible with the SW tectonic ver- rise to the Akaz suture in the Early to Mid-Ordovi- gence, reflecting a decrease in deformational inten- cian, followed by northward thrusting of the Akaz sity from NE to SW. Also, to our knowledge, there prism over the Tarim block (Fig. 16B). This arc-con- are no well-documented Phanerozoic geological tinent collision occurred in the Ordovician rather GROWTH OF CONTINENTS 319 mbols areFigure similar to those used in 2 Mazar accretionary prism. The location is shown in Figure 2. Sy . 14. Structural cross-section showing structural styles in the IG F (modified after Xiao et al., 2002a). 320 XIAO ET AL.

FIG. 15. Field photograph showing imbricate thrusts of the Mazar accretionary prism near Heweitan, looking north- east. Telephone line poles shown are ca. 8 meters high and 300 m distant from the foot of the cliff. See Figure 2 for location. than in the Silurian or Devonian, as previously tion of the Buziwan ultrabasic rocks southward over thought. the Kudi terrane (F1, Fig. 16C). This deformation caused high-grade metamorphism and ductile Early Ordovician to Middle Ordovician shearing within the Kudi gneiss, creating a major North-dipping subduction beneath the southern shear zone (Zhou et al., 1999; Xiao et al., 2002b). margin of the accreted Yixieke arc may have started The convergent deformation may also have propa- at ca. 490 Ma, in what had become an Andean-type gated further northward to the older Akaz suture, continental margin (Fig. 16B). It persisted for some causing north-vergent thrusting and folding (Matte 20 m.y. (478–460 Ma), as documented by U/Pb dat- et al., 1996). ing of the “128 km” granodiorite (Xu et al., 1996; Sobel and Arnaud, 1999; Yuan, 1999) and associ- Early Devonian to Carboniferous ated volcanic rocks in the Yixieke arc. The west From the Early to Middle Devonian (404–380 Datong granodiorite, within the same magmatic belt Ma), the North Kudi granite intruded the northern as the “128 km” granodiorite, has a 478 Ma U-Pb border of the Kudi terrane, which underwent minor zircon age (Jiang et al., 2002). extension (Fig. 16D), enabling the intrusion of lam- prophyre dikes dated by 40Ar/39Ar at 405 ± 3Ma Middle Ordovician to Early Silurian (Zhou, 1998). Ophiolite obduction (F1, Fig. 16C) A high-grade gneiss-schist complex, the Kudi may have initiated partial melting in the Kudi terrane, moved northward and at ca. 452 Ma was schists, a situation similar to anatexis of meta-sedi- accreted to the southern side of the Yixieke arc (Fig. ments during late Caledonian ophiolite obduction in 16C). It is not clear whether the Kudi terrane is a west Norway (Skjerlie et al., 2000). This could subduction-related, high-grade metamorphic ter- explain why trace-element data of the North Kudi rane incorporated into the subduction accretionary granite (Fig. 16D) indicate derivation by partial wedge south of the Yixieke arc (Xiao et al., 2002b) melting of sedimentary material (Yuan, 1999). or a metamorphosed accretionary complex that may The youngest turbidites in the Yixieke forearc have formed by ridge subduction (Şengör and basin (Fig. 16D) were deposited in Late Devonian– Okurogullari, 1991). The impingement process of Early Carboniferous time. A lack of magmatic activ- the Kudi terrane lasted about 24 m.y. (452–428 Ma; ity from 350 Ma to 280 Ma (Yuan, 1999) was proba- Zhou, 1998). This convergence gave rise to south- bly related to cessation of subduction; this would be ward thrusting of the Yixieke forearc during the consistent with paleomagnetic data indicating that Ordovician–Early Silurian, which then led to obduc- the Tarim block was moving northward as an GROWTH OF CONTINENTS 321

isolated plate in the Devonian to Late Carboniferous consumption of the intervening Paleotethys oceanic (Yin and Nie, 1996) with the Kunlun Ocean (Proto- lithosphere. Tethys) to its south and the Tienshan Ocean to its north (Yao and Hsü, 1994; Windley et al., 1990; Late Triassic–Jurassic Heubeck, 2001). The northward drift of the Tarim In the Late Triassic–Early Jurassic, the Paleot- block decreased convergence with the western Kun- ethys Ocean between the Kunlun and Karakoram- lun Range, which, being submerged, was overlain Qiangtang blocks closed, and collision of these ter- by a thick pile of Upper Devonian to Lower Carbon- ranes caused termination of the Mazar accretionary iferous marine deposits (XBGMR, 1993). process by the mid-Jurassic (Figs. 16F and 17). A While the Kudi area was tectonically quiescent, remnant basin existed between the western Kunlun subduction took place in the Oytag area. This gave accretionary complex and the Karakoram-Qiang- tang block during the Jurassic, in which marine car- rise to the Oytag arc and the Gaz forearc (Xiao et al., bonate-clastic sediments accumulated (Pan, 1996). 2002b). This basin was compressed and uplifted by continu- Permian to Early Triassic ous northward docking of Gondwanan blocks in the Late Jurassic–Early Cretaceous (Zhang et al., 1997; The northward drift of the Tarim block appar- Xiao et al., 1998). ently slowed after the Late Carboniferous, due to its accretion to the Laurasian plate (Li, 1990; Windley et al., 1990; Allen et al., 1993). Correspondingly, Discussion the subduction of Paleo-Tethys was activated and Subduction polarities began to generate arc magmatism (Ding et al., 1996; Xiao et al., 2002a). During Carboniferous to Early The earliest subduction along the Akaz suture Permian time, the Paleotethys Ocean was subducted zone is characterized by an accretionary complex in northward beneath the Kunlun arc, which gave rise which the Akaz seamount was involved. The sub- to the Sailiyak magmatic arc and the Mazar subduc- duction polarity was most likely to the south, tion-accretion complex (Ding et al., 1996; Xiao et because the lithological strata and other geological al., 2002a). In the Early Permian to Late Triassic, bodies are imbricated by north-directed thrusts (Matte et al., 1996; Pan, 1996; Xiao et al., 2002b). north-dipping subduction beneath the southern Although Cenozoic strike-slip faulting may have margin of the accreted Kudi terrane produced arc- reworked them as transpressional thrusts, when we related volcanic rocks and granitic intrusions along correlate the geology further east with the the southern margin of the Kunlun Range (Xiao et Qimanyute geology, we can be certain that the Late al., 2002a). Lower Permian volcanic rocks at Cambrian to Earliest Ordovician was Xiananqiao represent a magmatic arc formed as a subducted to the south, because all the thrusts in the result of continuing subduction beneath the Kunlun Qimanyute sections indicating thrusting to the Andean-type margin (Fig. 3). The magmatic arc north, and Ordovician granitic rocks stitch the developed on the southward-growing Bazar accre- thrusts (Fig. 10). Therefore, we also propose there tionary prism, and the Qitai forearc basin was was southward subduction in the Akaz area. As formed. The tectonic collage was characterized by mentioned above, there could not have been north- the Sailiyak magmatic arc, the Mazar accretionary ward subduction under the Tarim block in the early prism, the Heweitan subduction complex, and the Paleozoic, because there is no Andean arc tectoni- Qitai forearc basin, forming in aggregate a subduc- cally north of the “128 km” pluton. If there were tion-accretion orogen (Fig. 16E). Arc-related mag- northward offshore subduction in the early-mid matism continued in the Early Mesozoic, as Paleozoic, that could account for the Yixieke mag- demonstrated by granites in the Mazar-Kangxiwar matic arc, and the incoming movement of the Kudi area, which have isotopic ages in the range 231–190 gneiss-schist terrane, but it could only explain the Ma (K-Ar, Ar/Ar dates; Zhang and Xie, 1989; Zhang northward collision of the arc against the Tarim et al., 1996) and at 214 Ma (zircon age; Yuan, 1999). block by backarc collapse. In a comparable environ- Geochemical data suggest that these are arc-colli- ment today, the Japan Sea is beginning to close by sional granites (Zhang et al., 1996; Pan, 1996). thrusting over the Japanese arc. In other words, it is Continuation of calc-alkaline volcanic activity in unlikely that northward subduction would cause the this setting well into Late Triassic time indicates backarc to close northward over the Tarim block; 322 XIAO ET AL.

FIG. 16. Sequential diagram showing the Paleozoic–Early Mesozoic tectonic evolution of the western Kunlun Range. A. Late Cambrian to earliest Ordovician. B. Earliest Ordovician to Middle Ordovician; C. Middle Ordovician to Early Silurian. D. Devonian to Carboniferous. E. Permian to Triassic. F. Accretion complex collided with the northern active margin of the Karakoram-Qiangtang block in the Late Triassic. See text for discussion.

northward thrusting of the Akaz implies southward the southward obduction of the Buziwan ultrabasic subduction. Southward subduction from the early to rocks over that gneissic terrane. mid-Paleozoic (i.e., Matte et al., 1996) could The late Paleozoic to early Mesozoic subduction account for the growth of the arc, and the northward system along the southern margin of the accreted accretion and thrusting of the arc and Akaz accre- South Kunlun terrane (now south of the Mazar- tionary prism against/over the Tarim block, but it Kangxiwar fault) has an unambiguous, north- could not account for the northward drift and incom- dipping subduction polarity. This is the result of the ing of the Kudi gneiss-schist terrane. Therefore, N-S sectional distribution of the Sailiyak magmatic there must have been northward subduction from arc and the Mazar subduction-accretion complex– the Ordovician to account for that incoming and for Qitai forearc basins (Xiao et al., 2002a), as well as GROWTH OF CONTINENTS 323

Fig. 16. Continued other observations, such as southward thrust-and- Multiple accretionary orogeny fold vergence (Matte et al., 1996; Pan, 1996; Mat- The ages of the ophiolites, mélanges, and associ- tern and Schneider, 2000). ated subduction-accretion complexes young roughly 324 XIAO ET AL.

FIG. 17. A. Structural cross-section showing Paleozoic rocks thrust over Jurassic–Cretaceous sediments, south of Hecakou. B. Structural cross-section showing Paleozoic rocks thrust over Triassic and Cretaceous sediments, south of Tianshuihai. C. Structural cross-section showing folds and thrusts in Cretaceous sediments, south of Tianshuihai. southward from the early (e.g., Pan, 1996) to late erous–Early Jurassic time, upon which a later mag- Paleozoic in the South Kunlun terrane, and to the matic arc system was constructed. Finally, by the Triassic in the Mazar-Qitai forearc area (e.g., Jiang Mid-Jurassic, this complicated subduction-accre- et al., 1992, Pan, 1996). Furthermore, the ages of all tion collage collided with the northward-traveling the plutons also roughly young southwards from Karakoram–North Qiangtang block, terminating the 475–700 Ma (Xu et al., 1994) and 517 Ma Paleo-Tethys Ocean in the northern Tibetan Plateau (XBGMR, 1993) north of Kudi (Pan, 1996), to 428 (Heubeck, 2001). Ma at Kudi (Zhou, 1998), to late Paleozoic–Triassic Şengör (1992) suggested that the Western Kun- in the Mazar-Kangxiwar area (Zhang et al., 1996), lun is a Turkic-type orogen (actually a Japanese- and to Triassic and Jurassic near Dahongliutan and type), which is characterized by a trench that Qitai (Yin and Bian, 1995). retreats toward the ocean, and arc magmatism This tectonic scenario suggests that the earlier that moves toward the accretionary wedge. The subduction-accretion system grew southward (sea- accretionary wedges with their arc-plutonic and ward migration), during which subduction polarity volcanic rocks were later thrust upon the passive changed from southward to northward. After a short continental margin of the continent block on the magmatic quiescence, subduction-related - other side of the arc after ocean closure. In the West- tism resumed along the southern margin of the ern Kunlun, the superimposition of a later arc on an already stitched accretionary complex in Carbonif- earlier one is well predicted by the model of Şengör GROWTH OF CONTINENTS 325 and Okurogullari (1991). Hsü’s model (Hsü, 1988; REFERENCES Yao and Hsü, 1994; Hsü et al., 1995) also stressed the important role played by forearc accretionary Aitchison, J. C., Badengzhu, Davis, A. M., Liu, J., Luo, H., Malpas, J. G., McDerimid, I. R. C., Wu, H., Ziabrev, wedges. But the Kudi ophiolitic rocks (Buziwan S. V., and Zhou, M. 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