The Kara-Kunlun accretionary wedge

The thickness of the Devonian molasse may be several hundred meters Matte et al., 1996. Pan et al. 1992 noticed that the Devonian succession exhibits an overall fining-upward trend. The Devonian clastic rocks have been dated by plant fossils reviews by Sengo¨r and Okur- ogullari, 1991; Yao and Hsu¨, 1994. Terrestrial red molasse deposits of Devonian age also occur in the South Kunlun Pan et al., 1992. As shown in Fig. 2, the stratigraphic development of the North and South Kunlun is continuous and similar across the Oytag-Kudi suture as of the Devonian. Thus, the accretion of the South Kunlun against the southern margin of the Tarim Block must have been completed by that time. It appears reasonable to attribute the pre-Devonian stratigraphic gap in both terranes Fig. 2 to uplift and erosion related to regional shortening in the course of accretionary processes. Since shallow marine Ordovician and Silurian strata are preserved in 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 the Late Silurian. Matte et al. 1996 concluded a Silurian colli- sion. They listed a UPb zircon age of 377 Ma and RbSr ages of 392 35 Ma on whole rock, and of 381 4 Ma on biotite on the potassic postkinematic Kudi granite, whose intrusion postdates metamorphism and anatexis in the South Kunlun. This metamorphism and anatexis is indicated by a complex 40 Ar 39 Ar age spectra on K-feldspars of migmatites south 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 South Kunlun grades upward into shallow marine Carboniferous and Permian carbonates Fig. 2; Pan et al., 1992 which are well-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 indicates that the SilurianDevonian Kunlun mountains were already widely denuded when the carbonates accumulated.

6. The Kara-Kunlun accretionary wedge

The Kara-Kunlun area is characterized by the occurrence of thick monotonous and partly metamorphosed succes- sions of clastic marine deposits which exhibit a flyschoid character Fig. 9. Dark shale is by far the predominant sediment type which probably gave the Kara-Kunlun its name “kara” ˆ “black”. Besides shale we found silici- clastic turbidites and tuffites. The turbidite beds exhibit thicknesses mostly between 0.4 and 1.5 m. Bouma inter- vals “a” may be present. The pelitic intervals usually measure 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 seem to be very scarce. In the area of Mazar, fine- to coarse-grained laminated graywackes contain granitoid debris. Fine- to coarse- grained graywackes at the Qitai Pass contain granitoid debris as well, but also subordinate amounts of chert, phyl- lite and quartzite fragments. Accessory constituents are zircon, apatite, muscovite, biotite and chlorite. The compo- sition of metasiltstones from the Xaidulla area corresponds to that of the coarser material from the Qitai Pass. They contain quartz, feldspar, biotite, and muscovite. Tourma- line, apatite, sphene and ore minerals represent accessory minerals. 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, compositional aspects granitoid debris as well as the occurrence of tuffites can be reconciled with an active margin setting and is in support of the accretionary wedge interpretation. Gaetani et al. 1990, 1991, who worked in the area of transition from the Kunlun to the Karakorum, estimated the thickness of these deposits “Bazar Dara Slates” to measure several 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 Cambrian to the Triassic Matte et al., 1992 or from the Precambrian to the Mesozoic Yao and Hsu¨, 1994. The sediments must be older than the Mesozoic plutons which intruded them Fig. 9 and below. Paleozoic to Triassic fossils are known to occur in exotic limestone slabs Yao and Hsu¨, 1994. From the Kara-Kunlun belt Yao and Hsu¨ 1994 reported the occurrence of ophiolite me´langes northwest and south- east of Mazar, containing blocks of serpentinite, gabbro, greenstone, radiolarite, metagraywacke, marble, gneiss F. Mattern, W. Schneider Journal of Asian Earth Sciences 18 2000 637–650 644 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, cleaved matrix 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 indicate ophiolites in the Mazar area, and since Yao and Hsu¨ 1994 were unspecific about the location of the studied ophiolite 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 ophiolite body at Mt. Muztag 7723 m, 700 km eastsoutheast of Hotan — not to be confused with Mt. Muztag, 7282 m, 115 km southsoutheast of Hotan; Pan et al., 1992; Yao and Hsu¨, 1994, their Fig. 8. The presence of ophiolites in rocks of the Kara-Kunlun Liu et al., 1988; Yao and Hsu¨, 1994 is impor- tant because it indicates that internal thrust planes exist in the siliciclastic Kara-Kunlun belt, such as one would expect to occur in an accretionary wedge. The ratio between the amount of suture sediments and basic to ultrabasic oceanic igneous rocks is relatively large in the Kara-Kunlun wedge. In the westernmost part of the Kara-Kunlun, the clastic rocks of the Kara-Kunlun are either unmetamorphosed or display a very low metamorphic grade Gaetani et al., 1990. According to Matte et al. 1996, the clastic rocks are affected by only low-grade metamorphism. At several road- side outcrops between Mazar and Hez Pass and also farther east, for example, at the Qitai Pass, unmetamorphosed strata can be observed. At several places we found evidence of contact metamorphism. These areas include the aureole of the Mazar pluton, west of Mazar, the Dahongliutan area, and a location 40 km eastsoutheast of Shanshilli outcrop along a new road, Mattern et al., 1996. At the latter we found sillimanite-bearing metashales. Ductile faults can be observed in these zones below. We noticed that the strata of the Kara-Kunlun mainly dip to the north and northnortheast, that is inclined towards the South Kunlun. Observations of the preferred dip include those that were made on mountain-scale outcrops, for exam- ple, in the areas around the Hez Pass, Dahongliutan, and the Qitai Pass. Folds exhibit a south or southsouthwest vergence. The observed slaty cleavage is genetically related to the folds Mattern et al., 1996. These aspects are compa- tible with the interpretation of an accretionary wedge which formed south of the South Kunlun in response to northward B-type subduction. As shown below, the Kara-Kunlun was intruded by subduction-related granitoids indicating an active margin environment in which the Kara-Kunlun wedge became the site of arc magmatism. Whereas the stratigraphy and age of the Kara-Kunlun’s rocks are poorly understood, the geodynamic significance of the Kara-Kunlun zone appears to be clear. Hsu¨ 1988 inter- preted the Kara-Kunlun area as an accretionary wedge which formed due to northward subduction of the Paleo- Tethys Ocean beneath the South Kunlun. This view is consistent with the flyschoid facies and composition of the thick clastic sediments and their association with tuffites, ophiolite me´langes and arc granitoids as well as with the implied internal thrust planes and with the dominant dip direction of strata and the observed vergences. The Kara- Kunlun accretionary wedge can be correlated with the Sonpan Ganze Belt or parts of it Bayan Har Group farther east Sengo¨r and Okurogullari, 1991; Matte et al., 1996; Mattern et al., 1996. The Songpan Ganze Belt occupies a position south of the Kunlun, like the Kara-Kunlun accre- tionary wedge, and is also characterized by a great thickness and 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 accretionary wedge south of the eastern Kunlun Leeder et al., 1988.

7. The Uygur Terrane and adjacent sutures