Oroclinal origin of the Simao Arc in the Shan-Thai Block inferred from the Cretaceous palaeomagnetic data

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SUMMARY

An active oroclinal bending is discovered in the Shan-Thai Block to the south of the eastern Himalayan syntaxis. To investigate the evolution of the Simao Arc using palaeomagnetic techniques, the Middle Cretaceous red beds of the Nanxin Formation were sampled at the Zhengwan (22.8°N, 100.9°E) and Dadugang (22.4°N, 101.0°E) localities in the southern Simao Basin. Most of the studied samples revealed the presence of characteristic remnant magnetization with unblocking temperatures around 680 °C. A primary nature for this magnetization is interpreted on the basis of a positive fold and reversal test. Tilt-corrected mean directions calculated for Zhengwan and Dadugang localities are characterized by large easterly deflected declination; D= 51.8°, I= 47.9°, ks= 45.0, α95= 6.9°, N= 11 and D= 64.1°, I= 48.1°, ks= 36.0, α95= 7.3°, N= 12, respectively. Steep inclination values at both these localities with respect to those expected are in the range previously reported from the Shan-Thai Block, confirming their southward displacement by 6.2°± 1.7° as part of the Shan-Thai Block. Combination of the present data (two localities) with those previously reported from Simao Basin (seven localities) reveals a positive palaeomagnetic oroclinal test, indicating that the present-day arc-like geometry of the Simao Basin was formed by oroclinal bending. Comparison with recent GPS and structural data suggest that formation of the Simao curvature started after the early Pliocene (after 4 Ma) and continuing until the present. Origin of the Simao Arc is ascribed to southwestward movement of the crustal material across the Ailao Shan-Red River Fault (around the eastern Himalaya syntaxis), which was formed by westward movement of the decollement with progressive eastward deepening of the Lanping-Simao Basin. Decoupling between the upper and the middle–lower crusts is a requisite condition for the arc formation on the continent.

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