Tectonic evaluation of the Indochina Block during Jurassic-Cretaceous from palaeomagnetic results of Mesozoic redbeds in central and southern Lao PDR

Highlights

• We presented the palaeomagnetic result from the Khorat redbeds from Lao PDR, Indochina.

• We presented the positive fold tests for these redbeds of Upper and Lower Cretaceous ages.

• We presented the inconclusive fold tests for these redbeds of Jurassic age.

• Our palaeomagnetic results support the extrusion model for Indochina.

• Plate reconstruction of Indochina is provided.

Abstract

Rock magnetic and palaeomagnetic studies were performed on Mesozoic redbeds collected from the central and southern Laos, the northeastern and the eastern parts of the Khorat Plateau on the Indochina Block. Totally 606 samples from 56 sites were sampled and standard palaeomagnetic experiments were made on them. Positive fold tests are demonstrated for redbeds of Lower and Upper Cretaceous, while insignificant fold test is resulted for Lower Jurassic redbeds. The remanence carrying minerals defined from thermomagnetic measurement, AF and Thermal demagnetizations and back-field IRM measurements are both magnetite and hematite. The positive fold test argues that the remanent magnetization of magnetite or titanomagnetite and hematite in the redbeds is the primary and occurred before folding. The mean palaeomagnetic poles for Lower Jurassic, Lower Cretaceous, and Upper Cretaceous are defined at Plat./Plon. = 56.0°N/178.5°E (A₉₅ = 2.6°), 63. 3°N/170.2°E (A₉₅ = 6.9°), and 67.0°N/180.8°E (A₉₅ = 4.9°), respectively. Our palaeomagnetic results indicate a latitudinal translations (clockwise rotations) of the Indochina Block with respect to the South China Block of −10.8 ± 8.8° (16.4 ± 9.0°); −11.1 ± 6.2° (17.8 ± 6.8°); and −5.3 ± 4.7° (13.3 ± 5.0°), for Lower Jurassic, Lower Cretaceous, and Upper Cretaceous, respectively. These results indicate a latitudinal movement of the Indochina Block of about 5–11° (translation of about 750–1700 km in the southeastward direction along the Red River Fault) and clockwise rotation of 13–18° with respect to the South China Block. The estimated palaeoposition of the Khorat Plateau at ca. 21–26°N during Jurassic to Cretaceous argues for a close relation to the Sichuan Basin in the southwest of South China Block. These results confirm that the central part of the Indochina Block has acted like a rigid plate since Jurassic time and the results also support an earlier extrusion model for Indochina.

Introduction

Intercontinental collisions and deformations have taken place in a variety of modes in the Asia continent. The deformations of Southeast Asia have been modeled in a number of laboratory and numerical experiments during the last two decades (e.g. Tapponnier et al., 1982, Cohen and Morgan, 1986, Houseman and England, 1993, Royden et al., 2008). An extrusion tectonic model has been proposed to explain the tectonic of Southeast Asia after the Pre-Tertiary India-Asia collision which occurred in the Lower Eocene (ca. 50 Ma) (e.g. Molnar and Tapponnier, 1975, England and Houseman, 1986, Beck et al., 1995, Rowley, 1996, Tong et al., 2008) or in the Oligocene (ca. 35 Ma) (Aitchison et al., 2007). The southeastward movement of Indochina along the Ailao Shan-Red River (ASRR) shear zone is partly supported by geological evidences (Leloup et al., 1995, Leloup et al., 2001, Wang et al., 1998, Wang et al., 2000, Wang et al., 2001, Gilley et al., 2003, Replumaz and Tapponnier, 2003, Replumaz et al., 2004, Ali et al., 2010) and geochronological data (Lacassin et al., 1997, Gilley et al., 2003). Palaeomagnetic studies have been performed in South China and Indochina blocks in attempts to reconstruct tectonic events in these regions (Zhu et al., 1988, Kent et al., 1986, Gilder et al., 1993, Gilder et al., 1999, Morinaga and Liu, 2004, Li et al., 2005, Bai et al., 1998, Yokoyama et al., 2001, Enkin et al., 1992, Tanaka et al., 2008, Takemoto et al., 2009, Otofuji et al., 2010, Otofuji et al., 2012, Cung and Geissman, 2013). On the basis of reliable palaeomagnetic data from the Indochina Block a consecutive translation and rotation of Indochina during Mesozoic has been suggested (Yang, 1992, Yang and Besse, 1993, Bhongsuwan and Elming, 2000, Charusiri et al., 2006, Takemoto et al., 2005, Takemoto et al., 2009, Tanaka et al., 2008, Otofuji et al., 2010, Otofuji et al., 2012, Cung and Geissman, 2013). However, such translation and rotation interpreted from paleomagnetic data seems to have varied within the block, i.e. no significant rotation has been indicated for the parts, northwest and south of Vietnam when comparing with the Eurasia Apparent Polar Wander Path (APWP) (Takemoto et al., 2005, Otofuji et al., 2012, Cung and Geissman, 2013), while palaeomagnetic results from the Khorat Plateau Basin in the central part of the Indochina Block indicate a clockwise rotation and latitudinal translation relative to the South China Block (Yang and Besse, 1993, Bhongsuwan and Elming, 2000, Charusiri et al., 2006, Takemoto et al., 2009). These differences may be the results of complex internal deformation of the Indochina Block and parts of the Sundaland caused by the Indian–Asia collision. In order to test the coherency of the Indochina Block and to constrain the number of tectonic models, palaeomagnetic data from other parts of the blocks are necessary.

In this paper, we are going to provide new palaeomagnetic results and propose a tectonic model for the Indochina Block. This model is based on interpretation of palaeomagnetic data from Jurassic to Cretaceous redbeds in Laos. Anisotropy of magnetic susceptibility (AMS) data is also provided in order to determine eventual deformation of the studied rocks that may have affected the remanence directions.