Paleomagnetic Study of Central Asia: The Tertiary Low-Paleolatitude Dilemma and Closure of the Mongol-Okhotsk Ocean

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Zhao0003512Two important problems obstruct understanding of the tectonics of central Asia. The first is the consistent pattern of disturbingly low paleolatitudes derived from a large number of high-quality paleomagnetic studies of Tertiary rocks from sites that reach all the way from eastern China to Kyrgyzstan, 3,000 km to the west. The difference between these paleolatitudes and those expected from paleomagnetic reference poles for Eurasia averages 18o in western China and Kyrgystan, implying northward displacements relative to Siberia in the Tertiary that are much larger than geological estimates of shortening across intervening mountain belts. Many researchers in Asian tectonics have recognized this problem. A significant clue to its solution may lie in the fact that none of the Eurasian reference poles for Tertiary time is from rocks anywhere near Siberia, suggesting that they may not be appropriate for calculating convergence with Siberia. This question must be answered before it will be possible to evaluate how much of the discrepancy is tectonic, geomagnetic, and rock magnetic. The second problem is the history of closure of the Mongol-Okhotsk Ocean. This large ocean used to separate the Mongolia-North China composite block from Siberia. The geologic evidence concerning this subject is interpreted in a wide variety of ways, resulting in controversy over the timing and geometry of closure and when and to which margin accretion of terranes occurred.