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Linking Hinterland Evolution and Continental Basin Sedimentation by Using Detrital Zircon Thermochronology: a Study of the Khorat Plateau Basin, Eastern Thailand A

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Linking Hinterland Evolution and Continental Basin Sedimentation by Using Detrital Zircon Thermochronology: a Study of the Khorat Plateau Basin, Eastern Thailand A Basin Research (2003) 15, 271±285 Linking hinterland evolution and continental basin sedimentation by using detrital zircon thermochronology: a study of the Khorat Plateau Basin, eastern Thailand A. Carter and C. S. Bristow Research School of Earth Sciences, Birkbeck College and University College London, Gower Street, London, WC1E 6BT, UK ABSTRACT The effectiveness of detrital zircon thermochronology as a means of linking hinterland evolution and continental basin sedimentation studies is assessed by using Mesozoic continental sediments from the poorly understood Khorat Plateau Basin in eastern Thailand. New uranium lead (U-Pb) and fission-track (FT) zircon data from the Phu Kradung Formation identify age modes at 141 + 17 and 210 + 24 Ma (FT) and 2456 + 4, 2001 + 4, 251 + 3, and 168 + 2 Ma (U-Pb), which are closely similar to data from the overlying formations. The FT data record post-metamorphic cooling, whereas the U-Pb data record zircon growth events in the hinterland. Comparison is made between detrital zircon U-Pb data from ancient and modern sources across Southeast Asia. The inherent stability of the zircon U-Pb system means that 250 Myr of post-orogenic sedimentary recycling fails to change the regional zircon U-Pb age signature and this precludes use of the U-Pb approach alone for providing unique provenance information. Although the U-Pb zircon results are consistent with (but not uniquely diagnostic of ) the Qinling Orogenic Belt as the original source terrane for the Khorat Plateau Basin sediments, the zircon FT cooling data are more useful as they provide the key temporal link between basin and hinterland. The youngest zircon FT modes from the Khorat sequence range between 114 + 6 (Phra Wihan Formation) and 141 + 17 Ma (Phu Kradung Formation) that correspond to a Late Jurassic/Early Cretaceous reactivation event, which affected the Qinling Belt and adjacent foreland basins. The mechanism for regional Early Cretaceous erosion is identified as Cretaceous collision between the Lhasa Block and Eurasia. Thus, the Khorat Plateau Basin sediments might have originated from a reactivation event that affected a mature hinterland and not an active orogenic belt as postulated in previous models. INTRODUCTION which unambiguously tie basin sediments to a specific location or terrane; alternative approaches such as detrital Understanding the origin of ancient terrestrial depos- geochronology are required. Definition of the hinterland itional systems can be problematic. A common issue is and transport route to the depocentre provides essential poor biostratigraphic control, which hinders correlation constraints on the mechanism of basin formation. between sedimentation and regional geodynamic events. The Khorat Plateau Basin, located in eastern Thailand, For some basins, postdepositional tectonic displacement covers about 180 000 km2 (Fig. 1) and provides a good has resulted in uncertain geographical relationships, so example of how problems associated with interpretation the original source regions are missing or unclear. In of continental clastic sequences can result in a diverse such cases, sediment provenance indicators may provide range of basin models. An early model assumed the Khorat the only evidence of hinterland composition. However, sediments represented molasse from the Indosinian Or- because a source terrane may lie within an area of bro- ogeny (Hahn, 1976; Bunopas & Vella, 1978; Hutchison, adly uniform geology, methods such as sediment petrog- 1989). But there are considerable problems with this inter- raphy or sediment geochemistry lack unique signatures, pretation because sandstone petrography and palaeocur- rent measurements are not consistent with proximal Correspondence: A. Carter, Research School of Earth Sciences, derivation from a young orogenic belt in central Thailand. Birkbeck College and University College London, Gower Street, An alternative model suggested that the Khorat Group London, WC1E 6BT, UK. E-mail: [email protected] accumulated in a thermal sag basin that formed after the ß 2003 Blackwell Publishing Ltd 271 A. Carter and C. S. Bristow LAOS VIETNAM Vientiane SIBUMASU Basin (SHAN-THAI) TERRANE Phu Phan anticline BURMA Sukhothai fold belt Loei-Phetchabun fold belt Khorat Plateau Basin THAILAND INDOCHINA TERRANE Bangkok J Andaman Sea CAMBODIA 200 km GULF OF THAILAND Khorat sediment palaeocurrents in the Cretaceous Fig. 1. Location map of the Khorat Plateau Basin region with terrane boundaries and associated fold belts. The principal Khorat Group sediment palaeocurrent directions are also shown, based on data from Howlett (1993) and Heggeman (1994). Late Triassic extension linked to collapse of overthickened Various models for the Khorat Plateau Basin highlight crust produced by the Indosinian Orogeny (Cooper et al., how little is really known about basin geodynamic setting 1989). More recently the Khorat was interpreted as a and palaeogeographic location. In order to resolve these foreland basin associated with flexural subsidence in key aspects of the Khorat Basin evolution the basin needs front of a Jurassic Orogenic Belt, which helps us explain to be tied to a specific source location, but this has been the broad lateral extent and relatively uniform thickness proved difficult by using conventional sedimentological (Lovatt-Smith et al., 1996). and petrographic approaches. Study of the Khorat Basin 272 ß 2003 Blackwell Publishing Ltd, Basin Research, 15, 271±285 Linking hinterland evolution and continental basin sedimentation sediment provenance based on detrital zircon fission-track Sibumasu rifted from Gondwana during the Early Per- (FT) and U-Pb thermochronology (Carter & Moss, 1999) mian (Metcalfe, 1999), drifted and collided with Indochina has provided some important constraints regarding the (including the South China terrane) during the Triassic. Khorat sediment source age patterns, but the dataset is The exact timing for initial collision is not well defined but incomplete and has not been used in order to identify the in Vietnam the initial contact may be as early as 245 Ma location of the sediment hinterland. A primary objective of (Carter et al., 2001) and in Thailand, folding and thrusting this study is to build on this earlier work by completing FT occurred in the Sukhothai Fold Belt in the Late Triassic and U-Pb zircon dating of the Khorat succession and (Fig. 1). Late syn-postkinematic granites in northeastern linking the results to recent palaeogeographical recon- Thailand place an uppermost age limit of 200 Ma on the structions (Metcalfe, 1999) to enable evaluation of possible final stages of collision (Singharajwarapan & Berry, 2000). basin models. This Triassic collision event between the Sibumasu and Geochronology has much to offer sedimentary proven- Indochina blocks, which appears to have lasted 45 Ma, is ance studies and is an ideal method for understanding the Indosinian Orogeny that was originally presumed to sedimentary routing systems and establishing temporal have produced the Khorat Plateau Basin sediments (Hahn, relationships between source evolution and sedimentation 1976; Bunopas & Vella, 1978; Hutchison, 1989). in adjacent basins. Detrital geochronology provides Although the Indosinian Orogeny was regionally im- two fundamental types of source information: (i) mineral portant, there are problems connecting orogenesis to the formation ages (high-temperature methods) and (ii) Khorat Basin sedimentation. One difficulty stems from post-metamorphic cooling histories (low-temperature sediment palaeocurrent directions. Approximately 800 methods) (e.g. Morton et al., 1996; Carter, 1999; Garver palaeocurrent measurements (Heggeman, 1994; Howlett, et al., 1999; Carter & Bristow, 2000; Najman et al., 2001). 1993) indicate flow from the N and NE rather than from It is unclear which approach most benefits the establish- the SW as would be predicted if the Sibumasu±Indochina ment of a temporal relationship between an evolving collision zone were the source. Although palaeocurrents source and basin sedimentation. Thus, in addition to indicate a source region to the present-day NE (Fig. 1), evaluating the Khorat Basin models, this study also con- minor block rotation occurred in the Tertiary. Palaeomag- siders the wider issue of effectiveness of detrital zircon FT netic studies suggest that rotation was not large, limited to and U-Pb methods for understanding ancient terrestrial a maximum 10±158 clockwise rotation of the Khorat depositional systems. Plateau relative to South China (Yang & Besse, 1993). In addition, the Tertiary extrusion has displaced the Indochina±Sibumasu suture by left-lateral expulsion. Al- though the amount of displacement has been subject to REGIONAL GEOLOGY debate, most estimates fall between 500 and 1300 km (e.g. When the first Khorat Basin models were conceived know- Leloup et al., 1995; Sato et al., 1999). Restoration to within ledge of the Asian accretion history and Mesozoic palaeo- this range indicates a pre-extrusion location within South geography was limited (e.g. Bunopas & Vella, 1978; China close to the Sichuan Foreland Basin (Fig. 3). Sengor, 1984). The International Geological Correlation Program (IGCP) Project 321: Gondwana Dispersion and KHORAT SEDIMENTOLOGY AND Asian Accretion (Metcalfe, 1999) has significantly ad- STRATIGRAPHY vanced understanding and established a more robust tem- poral framework for the Asian accretion history. The The Mesozoic Khorat Group in Thailand is composed of origin of the various tectonic blocks
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