Abstract
The Cenozoic retreat of the Paratethys Sea, as well as uplift of the Tibet plateau and global cooling, have been considered as the main mechanisms for the onset of the Asian monsoon. However, due to the weak constraints on the time and path of the Paratethys Sea, the relative contribution of each of the three factors to the onset of the Asian monsoon remains debatable. Previous studies on the retreat process of the Paratethys suggested that its east coastline had already withdrawn from the Chinese Tianshan area to the west by the Oligocene. Here, we provide a new perspective on this discussion through the study of the Oligocene Anjihaihe dolomite from the northern Chinese Tianshan. Through the comparisons of carbon, oxygen, magnesium, and strontium isotope compositions between the dolomite beds and their interlayered precursor lacustrine limestone beds, we show that the Anjihaihe dolomites were formed at the end of the Oligocene from dolomitization by a fluid characterized by high δ18O, low δ26Mg, low temperature, relatively poor in carbon and Sr. This fluid was likely derived from seawater from the close-by Paratethys Sea at the time. This discovery suggests a larger Paratethys Sea during the Oligocene than previously thought and may have important implications to understand the evolution of the palaeogeography in the Chinese Tianshan region and the onset of the Asian monsoon.
Highlights
The Cenozoic uplift of the Tibetan plateau and subsequent retreat of the proto-Paratethys Sea, as well as global cooling, are considered to be the major factors responsible for the origin of the Asian monsoons climate, the detailed mechanism and relative contribution of each factor are still not clear[1,2,3,4,5,6,7,8]
Dolomites beds interlayered with beds of siltstone, mudstone or limestone were discovered in the Oligocene Anjihaihe Formation in the Junggar Basin in the north of the Chinese Tianshan Mountains
Given that the siltstone, mudstone and limestone in the Anjihaihe Formation were all lacustrine in origin, the source and inflow pathway of the involved seawater is puzzling – whether it was older seawater that had been buried in deeper sedimentary layers but reactivated in the Oligocene by tectonic activities, or it was concurrent seawater that invaded into the Junggar Basin
Summary
The Cenozoic uplift of the Tibetan plateau and subsequent retreat of the proto-Paratethys Sea, as well as global cooling, are considered to be the major factors responsible for the origin of the Asian monsoons climate, the detailed mechanism and relative contribution of each factor are still not clear[1,2,3,4,5,6,7,8]. Whether the repeated layers of dolomite were produced by a single or repeated incursion of seawater is an important question to address in order to understand the regional tectonic and environmental history In this contribution, we provide new strontium and magnesium isotopic data, together with the published but undiscussed carbon, oxygen, and magnesium isotopic data[16] for the dolomite beds and their interlayered limestone beds, to show the dolomitization was induced by one episodic incursion of seawater, likely from the Paratethys Sea into the Junggar Basin in the Oligocene. 24 Ma, the region started to uplift due to the India-Eurasia plate collision and eventually evolved into the northern Tianshan Mountains in the Cenozoic[19,20,21,22] This regional tectonic event exhumed the Mesozoic to Cenozoic sedimentary strata in this area and formed three rows of foreland thrust-anticline belts in the adjacent southern Junggar Basin (Fig. 1b). Apatite fission-track analysis of the Cenozoic strata in the Manas section, ~40 km east of the Anjihaihe section, indicates that the burial of these Cenozoic sediments was less than ~2 km[28,29]
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