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Home , Geology of Laos, Khok Kruat Formation, Khorat Group, Nam Phong Formation, Phu Kradung Formation, Sao Khua Formation

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 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 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 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 /Early reactivation event, which affected the Qinling Belt and adjacent foreland basins. The mechanism for regional 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- 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 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 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 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 that accreted to form continental clastic rocks traditionally considered to range Southeast Asia can be traced back to ± from the Triassic to the Cretaceous. The sedimentology times when the Indochina, North and South China Blocks and the stratigraphy of the Khorat Group have been stud- rifted and separated from northeastern Gondwana. In the ied in detail by Heggeman et al. (1992) and Racey et al. Late Palaeozoic and Early Mesozoic this rifting was (1996). Almost all the Khorat Group sediments were followed by separation of other continental slithers, in- transported by large braided river systems that flowed cluding the Qiantang and Sibumasu (also known as Shan from the present-day N to NE. A continental drainage Thai) Blocks (Figs 1 and 2). After drifting across the system is implied with a river system similar in scale to Meso- and Palaeo-Tethys, these various blocks began to the modern large river systems that drain Indochina. collide and accrete to each other in the Late ± Changes in fluvial character, which forms the basis for Triassic times (first the North and South China Blocks, the lithostratigraphic subdivision of the Khorat Group, then the Qiantang and Sibumasu to the South China are probably as a result of a combination of climate change, Block), and in the Cretaceous (Lhasa, and the West tectonics or redirection of fluvial drainage systems but not Burma Blocks to the Qiantang and Sibumasu terrane). a major change in source. Through the Group there is an For the Khorat Plateau Basin, which is situated on the increase in grain size accompanied by a decrease in miner- Indochina terrane, the most important accretion events alogical maturity indicating either a reduction in the time/ relate to the adjacent Sibumasu block to the west and the transport distance or an increase in the rate of erosional South China Block to the northeast (Figs 1 and 2). denudation of the hinterland and south prograding facies

ß 2003 Blackwell Publishing Ltd, Basin Research, 15, 271±285 273 A. Carter and C. S. Bristow

Tarim Ala shan Qaidam North China

Qamdao-Simao Songpan Qinling Belt Qiantang Ganze 35ЊN Belt Qinling foreland Lhasa basins Longmen Mts West Burma ? South China ? 25ЊN ? SIBU- MASU RRFZ Khorat Basin Њ Past location? 15 N Present location Indo- China N 10ЊN

105ЊE 115ЊE 125ЊE

Fig. 2. Simplified map of Southeast Asia to show present-day outcrop of the Khorat Group sediments (Laos and Thailand), Songpan-Ganze Basin and the Qinling Orogenic Belt (China), relative to underlying continental blocks and fragments (modified from Metcalfe, 1999). belts. Metamorphic detritus is present in all formations Sarakham evaporites, which ± until recently ± were in- and points to the underlying composition of the sediment cluded as part of the Khorat Group. source terrane. The youngest formations also contain acid The chronology attributed to formations within the to intermediate volcanic clasts but this change is not linked Khorat Group has been based on a ± age to any noticeable shift in palaeocurrent directions. from the base of the sequence, and the ±Albian ages The Khorat Group was originally divided into six for- from the top of the sequence (Sattayarak et al., 1991). mations by Ward & Bunnag (1964), which form the basis Despite the presence of some remains for the present correlations (Fig. 4). Iwai et al. (1966) (Buffetaut & Ingavat, 1986; Buffetaut & Suteethorn, recognised the Huai Hin Lat Formation at the base of 1991; Buffetaut et al., 1993), the intervening time gap has the Khorat Group, which was assigned a Norian Age by until recently been filled by adjusting the formations to fill Konno & Asama (1973) on the basis of plant . The the time available. This is clearly unsatisfactory and the age lowermost formation in the Khorat Group, the Nam of the Formations has been disputed (Mouret et al., 1993; Phong, lies unconformably over the Huai Hin Lat Forma- Mouret, 1994). Racey et al. (1994) and Carter et al. (1995) tion. The top of the group is marked by the Maha have suggested an Lower Cretaceous age for the Phra

274 ß 2003 Blackwell Publishing Ltd, Basin Research, 15, 271±285 ß 03BakelPbihn Ltd, Publishing Blackwell 2003 Present-day Late Cretaceous (pre-extrusion) Њ Њ Њ Њ 95 E 100 E 105 E 110 E Qinling Belt Longmen- Qinling Belt Longmen- shan shan Foreland basins 30ЊN Sichuan Simao Foreland basins Basin Basin ai Research Basin

SOUTH CHINA SOUTH

, 25ЊN CHINA 15, 271±285 Khorat

Simao RRFZ sedimentation basin continental and evolution hinterland Linking Basin Basin

20ЊN INDOCHINA

INDOCHINA SOUTH CHINA SEA Khorat Basin 15ЊN

200 km

Cretaceous granites Mesozoic red-beds 10ЊN

Fig. 3. The present-day location of the Khorat sediments and other basins containing Cretaceous continental sediments is a consequence of Tertiary extrusion along major strike-slip faults such as the Red River Fault Zone (RRFZ). Restoration of the basin to a pre-extrusion location places the Khorat sediments much closer to the Qinling foreland. During the Cretaceous the Khorat basin and continental foreland basin sediments of the Qinling belt would probably have extended over a much larger area. 275 A. Carter and C. S. Bristow

TRIASSIC CRETACEOUS . (1996) ALBIAN- - LT. NORIAN- ? NORIAN CENOMANIAN − APTIAN BARREMIAN RHAETIAN APTIAN et al

Hua Hin Lat Fm. Phu Hi sandstone Sam Khaen ???? Phu Phan Sao Khua Khok Kruat Phra Wihan Conglomerate Phu Kradung ? Hiatus? Formation Nam Phong

This study after Racey Dat Fa Shale Maha Sarakham KHORAT GROUP KHORAT . (1996) forms the basis of this study. Pho Hai Volcanics I M o Volcanics et al . (1993) et al Phu Phan Sao Khua Khok Kruat Phra Wihan U. Nam Phong U. Phu Kradung Maha Sarakham Lwr. Phu Kradung Lwr. Nam Phong Mouret KHORAT GROUP Phu Kradung Phu Phan Formation Formation Formation Formation Formation Khok Kruat Phra Wihan Hua Hin Lat map of Thailand KHORAT GROUP Resources Geological Department of Mineral Phu Maha Kradung Phu Phan Sao Khua Formation Sarakham Formation Formation Formation Formation Formation Formation Formation Hua Hin Lat Khok Kruat Phra Wihan Nam Phong Buffetaut & Ingavat (1986) KHORAT GROUP

Hua Hin Lat Fm. Phu Phu Hi Member Kradung Phu Phan Sao Khua Formation Formation Formation Formation sandstone Phra Wihan Nam Phong Sam Khaen Conglomerate Dat Fa Shale

Chonglakmani & KHORAT GROUP Sattayarak (1978) Pho Hai Volcanics I M o Volcanics Lwr Phra Member Member Kradung Member Member Sao Kua Maha Upper Phu Phu Phan Nam Phong Wihan member Ban Na Yo Sarakham PHRA WIHAN FORMATION PHU KRADUNG FM. (=Khok Kruat)

KHORAT GROUP Tr3 Tr2 Lias Gallic Malm Dogger Evolution of the chronostratigraphy of the Khorat Group. The most recent definition of the Group by Racey Neocomian AGE Workman (1977)

CRETACEOUS JURASSIC TRIASSIC Fig. 4.

276 ß 2003 Blackwell Publishing Ltd, Basin Research, 15, 271±285 Linking hinterland evolution and continental basin sedimentation

Wihan Formation, previously attributed to the Middle The detrital zircon study of Carter & Moss (1999) Jurassic. Age assignment has since been confirmed by provided useful insight into the age structure of the Khorat more detailed palynological evidence (Racey et al., 1996) sediments but the data are incomplete as the Phu Kradung and new vertebrate remains from the Sao Khua and Khok Formation was omitted because it was assumed at the time Kruat Formations (Buffetaut & Suteethorn, 1999). As a that the Nam Phong was the lowermost unit in the Khorat result, the chronostratigraphy of the Khorat Group has Group. In order to complete this geochronological data- been adjusted to begin in the Late Jurassic (Fig. 4). base we have collected a representative sample from the A consequence of a chronostratigraphic realignment is Phu Kradung Formation (located at 101.24.478N; the creation of an apparent major temporal gap between 15.51.048E) for combined U-Pb and FT analysis. The the Nam Phong Formation (Norian to Rhaetian) and the same analytical techniques and conditions were used as overlying Phu Kradung Formation (Late Jurassic/Early described by Carter & Moss (1999). Raw data are available Cretaceous), which suggests that the Nam Phong Forma- on request. tion does not belong to the Khorat Group sensu stricto. However, because the Phu Kradung Formation lacks suit- ably age-diagnostic material the Late Jurassic/Early Cret- Fission track results aceous age for this formation is inferred on the basis of a Sample T99/5 selected for analysis contained abundant well-constrained Berriasian±Barremian age for the overly- zircon, but proved difficult to analyse because the average ing Phra Wihan Formation. The assigned age is not un- grain size (< 150 mm) was small reflecting the finer- reasonable as most outcrops, including the type locality at grained nature of the Phu Kradung Formation. Only Khao Phu Kradung, are confined to the uppermost parts 20 grains could be analysed (despite making several grain of the formation where it is clearly conformable with the mounts), because most grains had very high track densities overlying Phra Wihan Formation. This also fits better with and/or small counting areas. This number of FT grain a petrography that shows that the Nam Phong Formation ages is not adequate for obtaining a full measure of the has closer affinity with the underlying Late Triassic Huai range of possible provenance types, but the data are suit- Hin Lat Formation, and radial palaeocurrent directions able for detecting the principal source ages. that flow towards the basin centre (in contrast to the The FT results are given in Table 1. Significant disper- overlying formations, which consistently show flow from sion among the single-grain age data indicates a mixed age N to NE). For the rest of this paper the Khorat Group is population, which mixture modelling based on the ap- considered as a mostly Late Jurassic/Early Cretaceous proach of Sambridge & Compston (1994) reveals as bi- succession with the Upper Jurassic to Lower Cretaceous modal. The two age modes, occurring at 141 + 17 and Phu Kradung Formation as the lowermost unit. 210 + 24 Ma (+ 2s), are comparable with age modes detected in the overlying formations in the earlier study (Table 2) (Carter & Moss, 1999). DETRITAL ZIRCON GEOCHRONOLOGY U-Pb results Tectonic displacement of the Khorat Plateau Basin has disrupted the normal geographical relationship that helps Fifty-two detrital zircon grains were analysed and the data tie a particular basin to its source area. Sediment petrog- are displayed on concordia plots in Fig. 5. Data quality is raphy and geochemistry in the Khorat Group are not generally good, with few grains displaying systematic uniquely diagnostic of any specific source within South- evidence of Pb loss. The zircons are generally small east Asia and hence geochronological evidence contained (< 150 mm) reflecting the fine-grained nature of the within the individual sediment grains may represent the Phu Kradung Formation. Grains are predominantly best source of information to locate the source region. translucent and colourless, but have a wide range of

Table 1. Fission track (FT) data for the Phu Kradung Formation.

Age Dosimeter Spontaneous Induced dispersion Sample No. of Central Component ages + 2s (Ma) No. grains rdNdrsnsriniw2 RE% Age + 1s(Ma) (percentage abundance)

T99/5 20 0.417 2888 20.19 2379 2.343 276 0 33.0 222 + 22 141 + 17 210 + 24 (36%) (64%)

Notes: (i) Track densities are (Â106 tr cmÀ2) numbers of tracks counted (N ) shown in brackets; (ii) analyses by external detector method by using 0.5 for the 4p/2p geometry correction factor; (iii) ages calculated using dosimeter glass CN-2; analyst Carter zCN2 ˆ 127 + 5; calibrated by multiple analyses of IUGS zircon age standards (Hurford, 1990); (iv) Pw2 is probability for obtaining w2 value for n degrees of freedom, where n ˆ number of crystals ± 1; (v) Central age is a modal age, weighted for various precisions of individual crystals; (vi) Age modes calculated using the approach of Sambridge & Compston (1994).

ß 2003 Blackwell Publishing Ltd, Basin Research, 15, 271±285 277 A. Carter and C. S. Bristow

Table 2. Principal zircon U-Pb and Fission track (FT) ages (+ 2s) for the Khorat Group. Data are from Carter & Moss (1999) and this study. Minor U-Pb modes are given in parentheses.

SHRIMP U-Pb component ages

FT component Phanerozic Archean Formationages (Ma) (Ma) (Ma)

Khok Kruat 135 + 9 166 + 2 (737 + 8) 2500 + 14 203 + 14 299 + 5 (876 + 10) 1799 + 8 Sao Khua 188 + 7 170 + 1 (770 + 6) 2535 + 15 254 + 1 1832 + 7 445 + 2 Phra Wihan 114 + 6 161 + 2 (889 + 6) 2450 + 16 175 + 10 242 + 2 (1152 + 11) 433 + 3 1813 + 7 Phu Kradung 141 + 17 168 + 2 (794 + 11) 2456 + 4 210 + 24 251 + 3 2001 + 4 (2643 + 6) (330 + 4) (458 + 8) morphologies. Small rounded zircons are most likely poly- against sample depositional age in order to show the time cyclic, but overall there is no systematic relationship be- difference between the zircon source lithology cooling tween grain morphology, colour and age, although the through its closure temperature and entering the Khorat majority of euhedral grains have the youngest ages. This depositional system. Such plots can only be interpreted suggests that the euhedral grains are the first-cycle zircon when it is possible to distinguish between ages that record derived directly from a crystalline basement, whereas the cooling and those that record zircon formation as in the smaller rounded grains were probably derived from a case of volcanic grains. This will be the case when a zircon source that retains a record of several cycles of burial and FT age is identical with error of a corresponding U-Pb age. erosion. The principal detrital source age modes were Figure 7 plots the FT age modes against the nearest U-Pb extracted from the mixed age population by using the age mode for each of the Khorat Group units. Data that approach of Sambridge & Compston (1994), and the log plot on the one-to-one line correspond to samples that likelihood for defining the optimum number of age modes have identical U-Pb and FT age modes indicative of a (Table 2). volcanic source. As no data plots on this line all of the FT ages must record cooling because of exhumation. INTERPRETATION The lag plot in Fig. 6 shows that the oldest zircon FT source ages are between 50 and 90 Myr older than the Fission track sample depositional ages, consistent with their derivation from a slowly cooled hinterland. If cooling is assumed to be The relationship between detrital zircon FT cooling ages linked to an environment where heat flow and geothermal and sample depositional age can provide valuable insight gradients have global average values (30 8C/km), this into a source regions geodynamic setting (e.g. Garver et al., would equate to denudation rates broadly in the range 1999). For convergent orogens it is possible to distinguish 50±150 m/Myr. Such moderate rates are not consistent between the construction, steady-state and decay stages. with either construction or steady-state phases of an oro- According to Bernet et al. (2001) each phase will give a gen. Instead it could be argued that these ages reflect a characteristic pattern of detrital zircon cooling ages with decaying orogen, and all but one sample shows an upsec- respect to the sample depositional age. Thus zircon sourced tion increase in age, although analytical uncertainties over- from a newly formed orogen associated with growing top- lap. Alternatively, these ages may represent reworked ography and high exhumation rates will show upsection sediments the zircon having been effectively held in decrease in the time difference between zircon FT cooling long-term storage. The younger detrital age modes have ages and sedimentdepositional age. This lag time (Garver & very short lag times, between 0 and 20 Myrs, indicative of Brandon, 1994) will become more uniform in orogens ap- faster exhumation. proaching steady-state, while cessation of convergence and a decaying orogen would produce zircon FT ages that show an upsection increase in lag time representing a gradual U-Pb data decline in exhumation rates as topography is reduced. Figure 6 plots the zircon FT ages, together with the Table 2 contains the U-Pb results from the Phu Kradung results from Carter & Moss (1999) summarized in Table 2, Formation together with previously published data from

278 ß 2003 Blackwell Publishing Ltd, Basin Research, 15, 271±285 Linking hinterland evolution and continental basin sedimentation

A - all grains 0.55 2600 0.50 2400 0.45 2200 0.40 2000 0.35 1800 206 Pb 0.30 1600 238 U 0.25 1400 1200 0.20 1000 0.15 800

0.10 B. 0.05

0.00 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 207Pb/ 235 U B. 0.20

0.16 900

800

0.12 700 206 Pb 600 238 U 500 0.08 400

300 0.04 200

100

0.00 0.0 0.4 0.8 1.2 1.6 207Pb/ 235 U

Fig. 5. Concordia plots of detrital zircon U-Pb ages. Plot B shows the Phanerozoic zircons in more detail. Parallelograms define 2s error.

Carter & Moss (1999). Results show that the Phu Kradung Two lesser modes occur between 770±890 and Formation has the same distribution of U-Pb source ages as 1150±1350 Ma. These age clusters characterise the principal the overlying formations. Importantly, the Phu Kradung, zircon formation events, which may relate directly to the like the overlying formations, contains a Late Jurassic formation of a lithology within the source terrane, or to zircon source. This evidence together with the youngest recycling of pre-existing material. Given that the Khorat FT source component (141 + 17 Ma) is consistent with a petrography indicates derivation from a predominantly Late Jurassic/Early Cretaceous depositional age. metamorphic terrane it is reasonable to assume that many Five pervasive zircon formation events are recogni- of these zircon ages will relate to older sources, although this sed within the Khorat Group detrital zircon U-Pb data at may not be apparent from the zircon structure and growth 2450±2550, 1800±2000, 430±470, 240±260 and 160±170 Ma. history.

ß 2003 Blackwell Publishing Ltd, Basin Research, 15, 271±285 279 A. Carter and C. S. Bristow

Are detrital zircon U-Pb ages suitable source from sediment associated with the Triassic Qinling Oro- indicators? genic Belt in central China. The first results are from a study of the Triassic marine sediments from the Songpan If zircon U-Pb ages relate to older sources as a conse- Ganze Basin (Bruguier et al., 1997) deposited during the quence of a polycyclic history, it is questionable whether early stages of convergence between the North and South such data are useful for discerning sediment provenance. China Blocks (Fig. 2). Zircon ages form five principal age In order to test whether detrital zircon U-Pb age signa- groups and these are shown in Fig. 8 together with results tures change with an evolving orogen we examined results from detrital zircon U-Pb data collected from modern sediment in four major river systems (the Mekong, the Irrawady, the Salween and the Red River) (Bodet & 100 SchaÈrer, 2000), draining an area between the Tibetan Youngest Oldest age age modes modes Plateau, South China and Indochina. 110 The results (Fig. 8) show that the principal Khorat Basin zircon formation ages are present in the Triassic 120 Songpan-Ganze Basin and modern Southeast Asian river depositional age +/ sediments. Present-day exposure of the Archean and Proterozoic basement in Southeast Asia is localised and 130 therefore most of the old detrital zircon ages detected

10 m.y. 20 m.y. 50 m.y.60 m.y.70 m.y.80 m.y.90 m.y. in the river sediments are probably recycled from the 140 0 m.y. Depositional age (Ma) − Phanerozoic rocks. Consequently, most of the Archean and Proterozoic ages must represent recycled grains, 150 which explains the similarity of principal zircon source 100 120 140 160 180 200 220 240 ages in the Triassic Songpan-Ganze Basin, Cretaceous Fission-track detrital age modes (Ma) Khorat Plateau Basin and modern Southeast Asian river Fig. 6. Plot of time difference between the Khorat Formation sediments. The inherent stability of the zircon U-Pb depositional age (+ 10 Ma) and their constituent detrital zircon system means that 250 Myr of postorogenic sedimentary fission track (FT) cooling ages. Data from this study and Carter & recycling fails to change the regional zircon U-Pb age Moss (1999). signature.

300

200

Volcanism-related zircon

Phu Kradung Fm. U-Pb Zircon age modes (Ma) Phra Wihan Fm. Sao Khua Fm. Phu Phan Fm. 100 100 150 200 250 300 FT Zircon age modes (Ma)

Fig. 7. Plot of paired zircon U-Pb and fission track (FT) age modes. Data that plot on the one-to-one line indicate volcanic zircon ages.

280 ß 2003 Blackwell Publishing Ltd, Basin Research, 15, 271±285 Linking hinterland evolution and continental basin sedimentation

comparable with the lithological age structure of the Qinl- Sample Principal zircon source ages (Ma) ing Orogenic Belt: the Proterozoic zircon ages coincide 250450 800 1800 2500 with the Luliang crustal growth event (between 1800 and 2000 Ma) and the Archean zircons (2500 Ma) are similar in Modern Rivers age to some of the crystalline basement exposed in the northern and southern parts of the Qinling Belt. However, Songpan given the earlier observations concerning the stability of Basin zircon U-Pb age signatures in Southeast Asia, we cannot accept these ages as unambiguous evidence for a direct Khorat (first-cycle) link to the Qinling source terrane. For the Basin Phanerozoic zircon ages recycling is less of an issue as a 0 500 1000 1500 2000 2500 3000 result of the smaller residence time, but the regional nature of zircon formation events still prevents correlation with a Age (Ma) unique source area. For example, the zircons, Fig. 8. Plot to show the stability of zircon U-Pb age signatures although consistent with I-type granites in the Caledonian between sediments from the Khorat Plateau Basin, Triassic Belts of the Qinling Orogen and eastern China, are also Songpan Ganze Basin (Bruguier et al., 1997) and modern found in Vietnam, part of the Indochina Block. Similarly, Southeast Asian river sediments (Bodet & SchaÈrer, 2000). Width the Triassic±Jurassic ages linked to the Indosinian mag- of lines denotes the spread in ages for each mode. matism occur throughout South China, Indochina and Sibumasu. Within the U-Pb dataset the only ages that provide a specific indication of source area are between DISCUSSION 750 and 850 Ma. This time interval is associated with a The Khorat Plateau Basin models were based on the Tri- crustal growth event within South China linked to breakup assic Indosinian Orogeny because early sedimentation of the Rodinia/Palaeopangaea supercontinent between (originally incorporating the Nam Phong Formation into 750 and 850 Ma (Li et al., 1995). Zircons of this age, the Khorat Group) took place within extensional half- which are present in some Khorat units, are therefore graben structures formed after the orogeny (e.g. Cooper possibly diagnostic of the South China Block. et al., 1989). We now know that the Nam Phong Formation Although the U-Pb data point to a source within the does not belong to the Khorat Group and this removes the South China Block, the inherent stability of the U-Pb apparent temporal link to the Indosinian event. A revised system, zircon recycling and the regional nature of zircon Khorat chronostratigraphy based on a (mostly) Cretaceous formation events prevent the Khorat sediments being history fits much better within the regional geology linked to a more specific source area. The FT data are because continuous sedimentation throughout the Juras- potentially more useful in that they specifically describe sic was difficult to reconcile with the occurrence of the young cooling history of the source terrain. Two marine near-shore sediments in western Thailand, Laos, groups of cooling ages are present. A dominant group Cambodia and southern Vietnam. An absence of sedimen- with ages 50±90 Myr older than the Khorat sediments tation across the Khorat Plateau Basin during the Jurassic (Fig. 6) implies a slow cooling source, whereas the much makes sense because marine sedimentation in nearby areas younger secondary age mode (Early Cretaceous) is diag- ended by the Bajocian and no Jurassic sediments occur in nostic of more rapid cooling rates normally associated with these areas after this time. Given that global sea levels were tectonically active regions. Both of the FT age modes generally rising in the Middle to Late Jurassic, it seems contain the same zircon U-Pb ages and therefore it is probable that the region was affected by a gentle uplift, unlikely that they represent very different source areas; which may account for truncation of the Nam Phong beds the Khorat sandstone petrography and palaeocurrents are in the southwestern part of the Khorat Plateau Basin. also consistent with a single-source area. Thus, consideration of the regional geology and Khorat Two types of hinterland setting can explain the FT stratigraphy has established that there is no direct relation- cooling data based on whether the primary group of the ship between the Indosinian Orogeny (Sibumasu± Triassic zircon ages represents first-cycle or reworked Indochina collision) and the deposition of the Khorat material: Group sediments, thereby discounting basin models tied . The Khorat sediments represent first-cycle material, so to this event. Furthermore, restoration of the basin to a that hinterland erosion must have occurred during the pre-Tertiary location suggests that the Khorat Plateau area Late Jurassic/Early Cretaceous (constrained by the sec- was originally located in southern China, possibly close to ondary age mode) and therefore the main group of the the Sichuan Foreland Basin which is part of the Qinling Triassic cooling ages (primary age mode) must describe a Orogenic Belt formed by collision between the North and mature hinterland (postorogenic decay?) that was sub- South China/Indochina terranes in the Triassic. Is this jected to the Late Jurassic/Early Cretaceous reactivation. collision belt the Khorat source area? The detrital zircon . The primary zircon age group might have been rapidly formation (U-Pb) events recognised in the sediments eroded during the Triassic (Orogenic belt?) and held in at 2450±2550, 1800±2000, 430±70 and 240±260 Ma are storage (foreland basin sediment?) before a second phase of

ß 2003 Blackwell Publishing Ltd, Basin Research, 15, 271±285 281 A. Carter and C. S. Bristow erosion in the Late Jurassic/Early Cretaceous recycled the 120 and 130 Ma (Arne et al., 1997). The cause of this grains. regional deformation event is not related to a late stage in Sedimentation trends within the Khorat Group show a the convergence history of the North±South China colli- general increase in grain size through time accompanied by sion but instead is attributed to the collision of the Lhasa a decrease in mineralogical maturity consistent with in- terrane with Eurasia (Zhang, 2000). creased denudation of the hinterland and support the Late Inversion, thrusting and reactivation of faults associated Jurassic/Early Cretaceous cooling as the dominant control with the collision of the Lhasa Block (Fig. 2) produced a on denudation of the sediment hinterland. The Phanero- regional unconformity and widespread rapid erosion of the zoic U-Pb data provide additional evidence. Table 3, newly uplifted terranes. In the Longmen Shan this can be which summarises the Phanerozoic U-Pb age data, shows seen as rapid influx of fanglomerates into the western prominent age clusters at 161±170 Ma (Middle Jurassic), Sichuan Basin (Arne et al., 1997). In the southern Qian- 242±261 Ma (Permo-Triassic), 330 Ma (Early Carbon- tang Block (adjacent to the northernmost part of Sibu- iferous) and 435±465 Ma (Late Ordovician±Silurian). masu), the Mangang Formation contains an 800-m-thick The two youngest component populations are present in sequence of coarse continental sediments (Zhang, 2000). significant quantities in most samples and are likely to be Adjacent to the Khorat Plateau in the Vientiane Basin and granitic crystallisation ages rather than volcanism-related Pak-Lay fold-belt Khorat sediments rest unconformably because identical ages are not seen in the FT data (Fig. 7). on imbricated marine Jurassic volcani-clastic sediments This interpretation is supported by the offset between (Stokes et al., 1996). Figure 9 illustrates the regional extent matched FT and U-Pb ages diagnostic of postemplace- of this Late Jurassic/Early Cretaceous deformation event ment slow cooling. Thus, the FT and U-Pb data are and resultant fluvio-clastic sedimentation. Based on this consistent with a slowly exhumed 250 Ma granitic/plu- evidence, we conclude that the Khorat sediment source tonic complex that reached shallow crustal levels by the was the Qinling Orogenic Belt of central China and some Early to Middle Jurassic, evidence that lends support to a component of recycled inverted foreland basin sediments, mature orogenic hinterland subjected to later (Late Juras- both of which experienced the Late Jurassic/Early Cret- sic/Early Cretaceous) reactivation. Given these important aceous deformation. The Khorat sediment was not directly new constraints on source region evolution, can we now sourced from an active orogenic belt as postulated in explain the causes of the Khorat Basin sedimentation? previous basin models. The sandstone petrography, palaeocurrents and U-Pb zircon data are consistent with a hinterland located within CONCLUSIONS the Triassic Qinling Orogenic Belt created by convergence of North and South China terranes. The main growth Detrital zircon thermochronology using FT and U-Pb phase of this collision occurred in the Middle to Late methods has been applied in order to improve understand- Triassic (240±205 Ma) (Meng & Zhang, 1999, 2000), ing of the Khorat Plateau Basin sediments. Results from which significantly predates the Khorat Group sedimen- this and previous published studies show a similarity of tation (by 100 Myrs). Denudation of the Qinling Belt zircon U-Pb formation age signatures in the regional supplied detritus to the adjacent foreland basin (Sichuan Mesozoic basins and modern Southeast Asian rivers that Basin), which developed through crustal loading on the suggest relative stability of the regional signature for at northern edge of the South China Block aided by south least 250 Myrs. This discovery precludes the U-Pb ap- propagating thrusts. Here the Jurassic sediments of the proach for providing unique provenance information in Suining and Penglaizhen Formations were deformed in the Khorat Basin and has important implications the Late Jurassic and unconformably overlain by the Early regarding the suitability of detrital zircon U-Pb studies Cretaceous fluvial sediments of the Chengqiannyan performed elsewhere. Instead, this study highlights how Group. A similar pattern is also recognised in the Long- more useful provenance information can be obtained from men Shan along the western margin of the Sichuan Basin detrital zircon FT data, especially when combined with where argon and FT studies have identified a major ductile U-Pb data from the same samples. Zircon FT data provide deformation event during the Early Cretaceous between key temporal information that can be used in order to link

Table 3. The Phanerozoic detrital zircon U-Pb ages for the principal clastic sedimentary units in the Khorat Group. Data are from Table 2.

Zircon U-Pb age

Formation Ma (+ 2s)

Khok Kruat Formation 166 + 2 299 + 1 299 + 1na Sao Khua Formation 170 + 2 254 + 1 na 445 + 2 Phra Wihan Formation 161 + 2 242 + 2 na 433 + 3 Phu Kradung Formation 168 + 2 251 + 1 330 + 4 458 + 8

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i.9. Fig. Jurassic Cretaceous hootairpi orlto oso h einlntr fteEryCeaeu otnna eietto cossuhr hn,Lo n Tha and Laos China, southern across sedimentation continental Cretaceous Early the of nature regional the show to correlation Chronostratigraphic Late Early Late Qiantang Mankuanghe South Block Hutoushi Mangang Jinxing Fm. Fm. Fm. Fm. Sandstones Sandstone/conglomerate Orogen Qinling South thrusting Tertiary erosion Folding & Foreland (Chengqian- Qinling Penglaizhen Foreland Sichuan Group) Tertiary erosion -nyan Basin Fm. Western v Thalat Fm. Khorat Laos Group v Set Fm. Tertiary erosion v v Nam Ban v (imbricated) Volcani-clastic sandstone Shales andsiltstone Phu Kradung missing Khok Kruat Khorat Basin Khorat Group Hiatus / section Tertiary erosion Fm. Fm. ?

iland. Fluvial sandstones Fluvial 150- 140- 130- 120- 100- 170- 160- 110- 180- 80- 90-

Ma ikn itradeouinadcnietlbsnsedimentation basin continental and evolution hinterland Linking A. Carter and C. S. Bristow source region evolution with basin sedimentation but for vertebrate palaeontology. International Symposium on Biostra- robust interpretation, FT data require U-Pb results from tigraphy of Mainland Southeast Asia: Facies and Paleontology, the same samples. Chiang Mai, Thailand. pp. 51±62. For the Khorat Plateau Basin sediments the combined Bunopas,S.&Vella, P. (1978) Late Palaeozoic and Mesozoic FT and U-Pb approach to zircon dating has identified the structural evolution of northern Thailand: A plate tectonic model. In: Proceedings of the Third Regional Conference on sediment source region as a mature orogenic hinterland Geology and Mineral Resources of Southeast Asia, Bangkok dominated by 250 Ma granitic/plutonic rocks that ex- (Ed. by P. Nutalaya), pp. 133±140. humed slowly and reached shallow crustal levels by the Carter, A. (1999) Present Status and future avenues of source Early to Middle Jurassic. This orogenic belt was then region discrimination and characterisation using fission-track exposed to an Early Cretaceous event that rejuvenated analysis. Sedim. Geol., 124, 31±45. erosion to create a period of enhanced sediment supply to Carter,A.&Bristow, C.S. (2000) Detrital zircon geochron- adjacent basins. Restoration of the Khorat Plateau Basin to ology: Enhancing the quality of sedimentary source informa- a pretectonic displacement location together with evidence tion through improved methodology and combined U-Pb and from sediment petrography, palaeocurrent directions, de- fission track techniques. Basin Res., 12, 47±57. trital zircon geochronology and Qinling foreland basin Carter, A., Bristow,C.&Hurford, A.J. (1995) The applica- stratigraphy support the Qinling Orogenic Belt as the tion of FT analysis to the dating of barren sequences: Examples from red beds in Scotland and Thailand. In: Non- original source terrane for the Khorat Basin sediment but Biostratigraphical Methods of Dating and Correlation (Ed. by in a mature rather than active orogenic setting. This dis- R.E. Dunay & E.A. Hailwood), Geol. Soc. Spec. Publ., London, counts previous basin models that require linkage to an 89, 57±68. active orogenic belt. The mechanism for regional Early Carter,A.&Moss, S.J. (1999) Combined detrital-zircon Cretaceous erosion is identified as Cretaceous collision fission-track and U-Pb dating: A new approach to understand- between the Lhasa Block and Eurasia. ing hinterland evolution. Geology, 27, 235±238. Carter, A., Roques, D., Kinny,P.&Bristow, C.S. (2001) Understanding Mesozoic Accretion in SE Asia: Significance ACKNOWLEDGEMENTS of Triassic thermotectonism in Vietnam. Geology, 29, 211±214. This work was funded by the London Southeast Asia Chonglakmani,C.&Sattayarak, S. (1978) Stratigraphy of the Research Group. 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