New insights into the Cenozoic lateral extrusion of crustal blocks on the southeastern edge of Tibetan Plateau: Evidence from paleomagnetic results from Paleogene sedimentary strata of the Baoshan Terrane

Abstract

In order to characterize the lateral extrusion of crustal material during the Cenozoic, we conducted a paleomagnetic study of the Eocene‐Oligocene Zhushan Formation and Paleocene Muguahe Formation in the central part of the Baoshan Terrane (BST), on the southeastern edge of Tibetan Plateau. A primary magnetic component and a remagentized component with the Miocene acquisition age were isolated from the Muguahe Formation and Zhushan Formation, respectively. These data indicate that the BST did not commence clockwise rotation in the period of the Paleocene and the Oligocene and that the BST experienced ~80° clockwise rotation relative to East Asia, since the Miocene. Combining with the documented history of crustal boundary strike‐slip faults on the southeastern edge of the Tibetan Plateau shows that the BST and Tengchong Terrane (TCT) first underwent latitudinal crustal shortening in the Oligocene, and the eastward extension was the main type of motion of the crustal material during this period, which induced the initial strike‐slip movement of their boundary faults. Since the Miocene, the main form of crustal motion of BST, TCT, and Simao Terrane was gradually transformed into clockwise rotation, which possibly indicates that the fold and thrusting fault system‐induced crustal shortening and thickening in the southeastern edge of the Tibetan Plateau have reached extremity, and the southeastern part of the Tibetan Plateau has already been uplifted to the similar elevation with today's elevation in the Miocene.

Paleomagnetic Evidence for 25–15 Ma Crust Fragmentation of North Indochina (23–26°N): Consequence of Collision With Greater India NE Corner?

“…The composite IRM results show that most of the IRM is unblocked along the high and intermediate‐coercivity (2.7–0.12 T) axes and is fully unblocked at 680 °C (Figure S1 in the supporting information). Thus, all paleomagnetic data show the almost exclusive occurrence of hematite with variable grain size as remanence carrier, consistently with previous evidence from red beds sampled in the same region (Funahara et al, ; Huang & Opdyke, , ; Li, Yang, et al, ; Otofuji et al, , ; Sato et al, ; Tanaka et al, ; Tong et al, ; Wang et al, ).…”

“…The internal deformation of the north Indochina “blocks” has been proposed in several papers, besides the seminal work by Wang & Burchfiel, ; Wang et al, ; Tanaka et al, ; Otofuji et al, ; Tsuchiyama et al, ). Other studies considered rotation variability to represent simply data scatter compared to megablock rigid rotations (Funahara et al, ; Huang & Opdyke, , ; Li, Advokaat, et al, ; Li et al, ; Otofuji et al, ; Tong et al, ). Yoshioka et al (), Kondo et al (), and Gao et al () proposed that local rotations are due to oroclinal bending of already formed thrust‐fold structures, a pattern that is not substantiated by our data.…”

“…We make clear that the HT components were calculated by connecting only two demagnetization points (640 and 680 °C); thus, we admit that the HT component directions of our data set may be slight biased. However, several studies of red beds from Indochina have showed that the HT component is usually defined at a temperature range varying between 600–680 °C and 640–680 °C, never found to be higher than 670–690 °C (Chen et al, ; Huang & Opdyke, ; Kondo et al, ; Sato et al, ; Tanaka et al, ; Tong et al, , , ; Wang, Yang, et al, ; Yoshioka et al, ). Figure shows that—considering samples where a HT occurs—remanence directions change slightly from 600 to 640 °C, testifying the final unblocking of the MT components.…”

“…The Chuandian domain yields a rather clear pattern of 15–20° CW rotations for Cretaceous‐Oligocene red beds (Funahara et al, ; Gao et al, ; Li et al, , ; Otofuji et al, ; Tong et al, ; Wang, Yang, et al, ; Yang, Sun, et al, ; Yoshioka et al, ; Zhu et al, ). Jurassic‐Oligocene red beds from the Baoshan, Lanping, and northern Simao domains provide highly variable CW rotations, in few cases exceeding 90° (Chen et al, ; Funahara et al, ; Gao et al, ; Huang & Opdyke, , ; Kondo et al, ; Li, Yang, et al, ; Li et al, ; Sato et al, , , ; Tanaka et al, ; Tong et al, , ; Yang, Yin, et al, ; Yang, Sun, et al, ). In the Tengchong domain, both CW and counterclockwise (CCW) rotations have been documented (Kornfeld et al, , ).…”

“…Most of the published paleomagnetic studies (Funahara et al, ; Huang & Opdyke, , ; Li, Advokaat, et al, ; Li et al, ; Otofuji et al, ; Tong et al, ) associate the CW rotations to the eastward drifting microplate model put forward by Tapponnier et al (, ), identifying each shear zone‐bounded domain with a rigid rotating block. However, a major problem of space would arise if one assumes that the N‐S elongated Baoshan and Lanping domains rotated CW by some 80°, questioning the internal block rigidity and appropriate delineation of a tectonic block.…”