A palaeomagnetic study of the Middle Permian and Middle Triassic limestones from Shan State, Myanmar: Implications for collision of the Sibumasu Terrane and Indochina Terrane

Abstract

We report herein a new palaeomagnetic study on the Middle Permian and Middle Triassic limestones from Shan State of eastern Myanmar, aiming to resolve the spatial location of the Sibumasu Terrane during the collision of tectonic blocks in Southeast Asia and to constrain the timing of convergence between the Sibumasu Terrane and Indochina Terrane. Based on the rock magnetic and palaeomagnetic experiments, three stable characteristic remnant magnetizations are recognized from limestone specimens of Middle Permian and Middle Triassic age. The characteristic Component C of the Middle Permian limestone passed the fold test at 95% confidence level, indicative of a pre‐folding origin. The mean direction of this component is D = 10.7°, I = 17.9° (κ = 130.3, α95 = 10.8°) before and D = 13.7°, I = −25.3° (κ = 1894.9, α95 = 2.8°) after tilt correction, corresponding to a palaeomagnetic pole at 254.1°E, 53.2°N (A95 = 1.7°). This suggests that the Sibumasu Terrane was placed at a palaeolatitude of ~13.0°S during the Middle Permian. The Component C separated from the Middle Triassic samples also passed the fold test with a mean direction of D = 207.3°, I = −8.3° (κ = 12.9, α95 = 6.2°) before tilt correction and D = 205.6°, I = −33.5° (κ = 27.1, α95 = 4.2°) after tilt correction, corresponding to a palaeomagnetic pole at 187.3°E, 65.8°N (dp/dm = 2.7°/4.8°). This implies that the Sibumasu Terrane was located at a palaeolatitude of ~18.3°N during the Middle Triassic. Based upon the detailed palaeomagnetic analysis, this study supports the view that the collision of the Sibumasu Terrane and Indochina Terrance occurred during or before the Middle–Late Triassic times.

New palaeomagnetic results of Permian tectonic blocks in the western segment of the Yarlung Zangbo Suture Zone and their tectonic implications

“…To better understand the palaeoposition of major blocks in the Neo‐Tethys during the Late Permian to Early Triassic, we compiled the published available palaeomagnetic results (Table 2; Cheng et al, 2012, 2013; Chi et al, 2016; Guan et al, 2021; Huang et al, 1992; Ran et al, 2012a, 2012b; Zhao et al, 2020; Zhou et al, 2016, 2019). All palaeomagnetic directions were converted to the site of this study (30.7°N, 80.7°E), and the palaeolatitudes were recalculated (Table 2).…”

“…Due to too few samples, large age uncertainty ($24 Ma), the lack of field test, and polarity reversal, the result fails criteria 1, 2, 4 and 6. For the Sibumasu terrane, the Middle Permian palaeopole of 53.2 N and 254.1 E, (A 95 = 1.7 ) (Zhao et al, 2020) from the Shan State limestone does not have enough samples and lacks of dual polarities, which fails criteria 2 and 6. After recalculation, the above palaeomagnetic poles correspond to the similar palaeolatitudes of the Late Permian 7.7 S of the Indochina terrane and the Middle Permian 5.9 S of the Sibumasu terrane, respectively.…”

“…These lines of evidence support our reconstruction of the Indochina and Sibumasu terranes. Although the credibility of the results of Chi et al (2016) and Zhao et al (2020) are questionable, as the only palaeomagnetic data of the Indochina and Sibumasu terranes in this period, they showed a good match with a series of other pieces of evidence, so we chose their results as reference.…”

Tectonic evolution and geodynamics of the Neo-Tethys Ocean

“…(a) Early Permian: Cimmerian blocks (including Sibumasu, Lhasa, South Qiangtang, Afghan, Iran) rifted from the northern Gondwana, leading to the opening of the Neo-Tethys Ocean (Scotese and Golonka, 1993;Stampfli and Borel, 2002;Angiolini et al, 2003a) diamictite-bearing deposits, containing cold water biota with Gondwana affinity (Ingavat and Douglass, 1981;Stauffer and Peng, 1986;Rao, 1988;Ampaiwan et al, 2009), and it was located on the north margin of Gondwana in the high latitude area of the southern hemisphere (Fang et al, 1989;Huang and Opdyke, 1991). In the Middle Permian, the Sibumasu block had drifted to 13.0°S (Zhao J et al, 2020), indicating that the opening of eastern segment of the Neo-Tethys Ocean may have started from the Early Permian (Figure 2a). The Sibumasu block was located at 15°N-20°N in the Middle-Late Triassic (Zhao J et al, 2015(Zhao J et al, , 2020, and had collided with the Indo-China block, indicating that the Changming-Menglian segment of the Paleo-Tethys ocean had closed in the Late Triassic (Figure 2b).…”

“…In the Middle Permian, the Sibumasu block had drifted to 13.0°S (Zhao J et al, 2020), indicating that the opening of eastern segment of the Neo-Tethys Ocean may have started from the Early Permian (Figure 2a). The Sibumasu block was located at 15°N-20°N in the Middle-Late Triassic (Zhao J et al, 2015(Zhao J et al, , 2020, and had collided with the Indo-China block, indicating that the Changming-Menglian segment of the Paleo-Tethys ocean had closed in the Late Triassic (Figure 2b).…”