And Paleo-Tethys Oceans in the Western Kunlun (Xinjiang, China)

Journal of Asian Earth Sciences 18 (2000) 637–650 www.elsevier.nl/locate/jseaes Suturing of the Proto- and Paleo-Tethys oceans in the western Kunlun (Xinjiang, China) F. Matterna,*, W. Schneiderb aInstitut fu¨r Geologıë, Geophysik und Geoinformatik, Freie Universita¨t Berlin, Matteserstraße 74-100, D-12249 Berlin, Germany bInstitut fu¨r 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 the Oytag-Kudi suture. The presence of Paleozoic arc batholiths in the northern South Kunlun and their absence in the North Kunlun indicates southward subduction of the Proto-Tethys Ocean beneath the South Kunlun. Opposite subduction polarity can be demonstrated for the Late Paleozoic to mid-Mesozoic when the southerly located Paleo-Tethys Ocean was consumed beneath the South Kunlun and generated a Late Carboniferous to mid-Jurassic magmatic arc in the southern South Kunlun. Arc magmatism affected the southern South Kunlun and the large Kara-Kunlun accretionary prism (a suture sensu lato) which formed as a result of Paleo-Tethys’ consumption. The dextral shear sense of ductile faults which are located at the margins of the arc batholiths, and which parallel the South Kunlun/Kara-Kunlun boundary, suggests oblique plate convergence with a dextral component. Different lines of evidence encourage us to interpret the Proto-Tethys ophiolites of the Oytag-Kudi zone as at least partly derived from an oceanic back-arc basin. In contrast, we assume that Paleo-Tethys was a large ocean basin which was eliminated directly at the southern margin of the South Kunlun where no oceanic back-arc region existed. ᭧ 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction Our main intention is to decipher the pre-Cenozoic plate tectonic processes which shaped the western Kunlun. Knowledge of the geology of the Kunlun was always Subduction of oceanic lithosphere plays a key role in this sparse, as it is difficult to access this geographic frontier. regard. At the same time we will also address problematic Until recently, fundamental geological knowledge of the aspects in the understanding of the tectonic history. western Kunlun remained obscure to the international geos- The western Kunlun is one of the Earth’s highest moun- cientific community because since 1949 foreigners were not tain ranges. It is located south of the Tarim Basin (Takla allowed to visit the area (Gaetani et al., 1990). Only as of Makan Desert) and north of the westernmost part of the 1988 could non-Chinese workers carry out investigations in Tibet Plateau which is referred to as the “Kara-Kunlun” the region again. The Italian expedition team was the first area (Fig. 1). To the northwest, parts of the Kunlun are one to resume foreign research efforts (Gaetani et al., 1990). morphologically and geologically transitional to the Pamirs. On the basis of recently collected data by Chinese and other The western Kunlun represents an accretion zone at which workers (e.g. Liu et al., 1988; Gaetani et al., 1990, 1991; Central Asia grew during the Phanerozoic. It is tectonostra- Matte et al., 1991; 1996; Pan et al., 1992; Yao and Hsu¨, tigraphically subdivided into the North Kunlun, i.e. the 1994; Mattern et al., 1996), it is now possible to review the southern margin of the Tarim Block, and the narrow main aspects of the geology of the western Kunlun. Our South Kunlun, which was the site of two Phanerozoic descriptions pay special attention to those aspects signifi- magmatic arc-related intrusion cycles. Both tectonic units cant for the reconstruction of the geodynamic development. are separated by the ophiolite-bearing Oytag-Kudi suture For more details on the regional geology the reader is (Fig. 1). The Kara-Kunlun is a sizeable mid-Phanerozoic referred to the quoted literature and the sources therein. accretionary wedge. We use the geological time table by Haq and van Eysinga (1987) in correlating radiometric ages with geological time * Corresponding author. units and corresponding stratigraphical time–rock units and E-mail address: [email protected] (F. Mattern). in assigning numerical time spans. 1367-9120/00/$ - see front matter ᭧ 2000 Elsevier Science Ltd. All rights reserved. PII: S1367-9120(00)00011-0 638 F. Mattern, W. Schneider / Journal of Asian Earth Sciences 18 (2000) 637–650 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 ophiolite units 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 shown since 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 from Yecheng to the area around Tianshuihai and in side valleys. 2. Sinian rifting of the North Kunlun and creation of the marine strata has been determined on the basis of micro- Proto-Tethys Ocean floral and small shelly fossils (Yao and Hsu¨, 1994). Along with Pan et al. (1992), we interpreted the Sinian The basement of the North Kunlun is characterized by succession as a rift sequence which formed during the Precambrian gneisses and migmatites (Pan et al., 1992; fragmentation of a subsiding shallow marine platform Matte et al., 1996). It is overlain by a mildly metamorphosed (Mattern et al., 1996). Following Sinian rifting of the south- Sinian (Late Proterozoic) succession of laminated carbo- ern margin of the Tarim Block, ocean spreading is assumed nates, volcanites, shales, marls and tuffites, well-exposed to have taken place during the later Sinian and Lower Paleo- around Akaz Pass. Locally, we observed the alternation of zoic resulting in the formation of the Proto-Tethys Ocean parallel-bedded limestone and dolomite. The laminated whose remnants are found at the Oytag-Kudi suture (Pan et carbonates indicate a shallow marine depositional environ- al., 1992; Mattern et al., 1996). According to Chang et al. ment. According to Pan et al. (1992), the metavolcanites are (1989), this ocean developed during the Sinian and former oceanic tholeiites. The thickness of the Sinian Cambrian. We are unable to say which continental mass succession may reach 1 km or more. At Akaz Pass, a thick- drifted away from the southern margin of the Tarim ness of several hundred meters is exposed. Similar Sinian Block, but we do not consider the South Kunlun a likely rocks of great thickness occur in the Tarim Basin (Tian et candidate because it lacks a similar Sinian succession al., 1989). South of Hotan, the Sinian age of these shallow (Fig. 2). F. Mattern, W. Schneider / Journal of Asian Earth Sciences 18 (2000) 637–650 639 considering that strike-slip rifts are low-volcanicity rifts at best (Mattern et al., 1998). 3. Oytag-Kudi suture and Kudi Ophiolite Complex The Oytag-Kudi suture which separates the North and South Kunlun is marked by an alignment of ophiolite slices which 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 is known 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 the Kegang ophiolite occurrence (ca. 100 km NW of Kudi). Approximately 80 km east of Kudi, the structure along the suture trace appears to be complicated, in so far as there is a large lenticular, possibly fault-bounded rock unit, adjacent Fig. 2. Juxtaposition of the North and South Kunlun’s stratigraphy. Note the to the suture north of the ophiolites, which displays an en similar development as of the Devonian. ećhelon rock fabric (Matte et al., 1996). In Figs. 1 and 8 we assign it to the South Kunlun. The alternation of metacarbonates and metatholeiites Knowledge of the Kudi Ophiolite Complex, which indicates that marine conditions persisted during the rift appears much larger in map view than the ophiolite units process. Therefore, the visited area of the North Kunlun of the suture trace, is much more detailed. The Kudi Ophio- was probably not located on a rift dome, as this would lite Complex represents an obducted ophiolite unit which have likely caused uplift of the platform above sea-level. was thrust from the suture and emplaced on the South The Sinian rift sequence of the Akaz Pass area might have Kunlun (Fig. 3). We studied the northern and eastern part been located to the side (i.e. “north”, according to present of this complex in the north/south-trending Kudi Valley day directions) of such a dome. This interpretation implies a along the road north of the village of Kudi and in the dip-slip type of rifting. small, canyon-like Yishak Valley which trends approxi- Alternatively, one could argue that rifting was due to mately east/west and leads into the Kudi Valley from the strike-slip motion as this would neither require nor induce west (Fig. 3). We investigated the southern part of the a rift dome (Mattern et al., 1998). However, strike-slip rift- complex at the northern slope of the Boziwan Valley ing appears to be an unlikely option because of the abun- which joins the Kudi Valley from the west at the northern dance of volcanogenic strata in the Sinian succession, margin of Kudi (Fig. 3). Except for a sheeted dike complex Fig.

Load more