Sinian rifting of the North Kunlun and creation of the Proto-Tethys Ocean

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

The basement of the North Kunlun is characterized by Precambrian gneisses and migmatites Pan et al., 1992; Matte et al., 1996. It is overlain by a mildly metamorphosed Sinian Late Proterozoic succession of laminated carbo- nates, volcanites, shales, marls and tuffites, well-exposed around Akaz Pass. Locally, we observed the alternation of parallel-bedded limestone and dolomite. The laminated carbonates indicate a shallow marine depositional environ- ment. According to Pan et al. 1992, the metavolcanites are former oceanic tholeiites. The thickness of the Sinian succession may reach 1 km or more. At Akaz Pass, a thick- ness of several hundred meters is exposed. Similar Sinian rocks of great thickness occur in the Tarim Basin Tian et al., 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 Sinian succession as a rift sequence which formed during the fragmentation 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 assumed to have taken place during the later Sinian and Lower Paleo- zoic resulting in the formation of the Proto-Tethys Ocean whose remnants are found at the Oytag-Kudi suture Pan et al., 1992; Mattern et al., 1996. According to Chang et al. 1989, this ocean developed during the Sinian and Cambrian. We are unable to say which continental mass drifted away from the southern margin of the Tarim Block, but we do not consider the South Kunlun a likely candidate because it lacks a similar Sinian succession Fig. 2. F. Mattern, W. Schneider Journal of Asian Earth Sciences 18 2000 637–650 638 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. The alternation of metacarbonates and metatholeiites indicates that marine conditions persisted during the rift process. Therefore, the visited area of the North Kunlun was probably not located on a rift dome, as this would have likely caused uplift of the platform above sea-level. The Sinian rift sequence of the Akaz Pass area might have been located to the side i.e. “north”, according to present day directions of such a dome. This interpretation implies a dip-slip type of rifting. Alternatively, one could argue that rifting was due to strike-slip motion as this would neither require nor induce a 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 at best Mattern et al., 1998.

3. Oytag-Kudi suture and Kudi Ophiolite Complex