Monotis fauna of China and its palaeobiogeographic significance

Abstract

Norian Monotis was discovered in continental shelf, flysch and paraflysch facies of eastern Heilongjiang and Xizang (Tibet). Most of the Monotis-bearing strata in the Mishan-Hulin area of eastern Heilongjiang are thought to be of the Ochotica zone and a part of them may be correlated to the Typica zone of Siberia. The continuous Carnian to Norian sequence in the Qomolangma, Gyirong-Namzho Yumco and Nagqu-Sog areas of Tibet shows that the Monotis sarinaria beds are situated above the Discotropites and Protrachyceras beds and its precise age is confirmed as Norian. Two distinct Monotis faunas, Entomonotis and Monotis, are recognized on the basis of the mutual occurrence and species groups in eastern Heilongjiang and Tibet, China. From the viewpoint of terrane, the occurrence of Monotis fauna would be bound to the margins of the Bureya-Khanka-Jiamusi massif and the southern and northern Tibetan blocks. An accreted complex of the spreading trough type has been newly discovered in the Raohe area near the Usuli river. Fragments of serpentinite surrounded by tectonic melange are exposed in the area. Both serpentinite and tectonic melanges provide definite evidence of a subduction zone between the Nadanhada and the Sikhote Alin terranes. Rapid accumulations of the continental slope and continental shelf sediments are distributed close to the complex. The Monotis-bearing slate and sandstone are developed within the fault-bound depression on the shelf. This shows that the Monotis ochotica fauna occurred in a shelf basin of the Nadanhada-Sikhote Alin terrane. Norian conodonts and Triassic radiolarias from the matrix of the tectonic melanges indicate that the accretion of the Nadanhada-Sikhote Alin terrane started in the late Triassic. The same accreted terrane can be traced to the Japanese islands which were located at a low paleolatitude according to paleomagnetic data (Hattori and Hirooka 1977, 1979). On the basis of the research results of Chinese geologists (Yin et al. 1976, Wang and He 1976, Gu 1980; Wang et al. 1983, Han 1983), three kinds of Monotis-bearing strata in Tibet are recognized: eugeosyncline deposits on the banks of the Yalung Zangbo river (the Gyirong, Namyaixoi and Garbo groups); shelf sea assemblage of the India-Gondwana block (the Qulung formation of the Qomolangma area); peri-continental paraflysch sediments in northern Tibet (a part of the so-called “Shading Slate”) on the bank of the Nujiang. The geologists of the Sino-French cooperative investigation team in the Himalayas studied the High Himalayas (Tibetan side) and came to the conclusion that southern Tibet must have been the northern margin of the India-Gondwana plate during the Mesozoic (Mercier et al. 1980). Recent paleomagnetic data denote that the High Himalayas (south of the Yalung Zangbo river) started to break up during the Triassic and moved northward in the late Triassic, but they were still linked with the Indian plate near the paleoequator (Zhu and Teng 1980). The paleomagnetic data also indicate that northern Tibet (between the Yalung Zangbo river and the Bangong Co-Nujiang) was separated from the Indian plate and drifted to the north of the paleoequator. Silberling (1985) interpreted the data as showing that the Monotis faunas had originally an equator and bipolar provincialism. With a stable load of the Monotis fauna, the terrane or block gave rise to differentiation and provincialism. Therefore, there would be three Monotis realms in the paleo-Pacific Ocean: the Entomonotis realm at the mid-high northern paleolatitude, the Eomonotis realm at the mid-southern paleolatitude and the Monotis realm near the paleoequator.