Mesozoic to Cenozoic sequence stratigraphy and sea-level changes in the Northern Himalayas, Southern Tibet, China

Abstract

The marine Mesozoic and Cenozoic developed continuously in the northern Himalayan region of southern Tibet. From the Lower Triassic to the upper Eocene, 73 sequences have been identified, with an average duration of 2.9 Ma; these can in turn be grouped into 24 super-sequences and 6 super-sequence sets. Most of the sequences can be correlated with those distinguished by HAQ et al., but some of them do show their own distinctive features owing to the regional tectonic effects, especially at the higher ranks. During the Mesozoic and Cenozoic, several large sea-level falls occurred in the eastern Neo-Tethys, which resulted in a number of hiatuses in the strata, various exposed surfaces, and disconformities. Among the recognized sea-level falls, the most important ones include those at the ages of 255 Ma, 177 Ma, 107 Ma, 68 Ma and 50 Ma. Those at 239 Ma, 215 Ma, 157 Ma, 138 Ma and 80 Ma are also significant. Study shows that the third-order sequences and sea-level cycles probably reflect mainly global sea-level fluctuations, while the higher rank cycles seem more closely related to the basin evolution of the Neo-Tethys. Tectonic movements apparently exerted a great influence not only on the relative changes in the sea level, but also on the characters of the sequences. Based on the study, six major periods are suggested for the tectonic evolution of the eastern Neo-Tethys and the plates, i. e. the Pangea Period (Pre-Triassic), Continental Rifting Period (Triassic to Early Jurassic), Inter-Continental Sea Period (Middle Jurassic), Continental Divergence Period (Late Jurassic to Early Cretaceous), Continental Convergence Period (Late Cretaceous) and the Continental Collision Period (Palaeogene). These major periods can be further subdivided into eight stages according to the basin evolution. In each of the periods and stages, sequences and their boundaries show clear characters related to the tectonic background. The present study indicates that the initial breakup of the Pangea along the Indus-Yarlong may have taken place around 239 Ma. The late Bathonian to early Callovian seems to have been a critical time in the evolution of the Neo-Tethys, with the turning point around 158 Ma. The blocks split from the northern margin of the Gondwana Continent did not obviously drift away from the Indian Plate until the Callovian. The oceanic crust subduction in the Neo-Tethys may have started at 113 Ma, while the contraction of the ocean probably began at 107 Ma. The initial contact of the Indian plate with the Eurasian plate may have taken place around 80 Ma, with strong uplift and thrusting in late Paleocene.