Insights from Sediment Provenance in the Minbu Sub-Basin

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Basin Research (2014) 1–15, doi: 10.1111/bre.12108 Cenozoic evolution of the central Myanmar drainage system: insights from sediment provenance in the Minbu Sub-Basin Alexis Licht,*,†,‡ Laurie Reisberg,† Christian France-Lanord,† Aung Naing Soe§ and Jean- Jacques Jaeger‡ *Department of Geosciences, University of Arizona, Tucson, AZ, USA †Centre de Recherches Petrologiques et Geochimiques, Universite de Lorraine, Vandoeuvre les Nancy, France ‡Institut de Paleoprimatologie, Paleontologie Humaine: Evolution et Paleoenvironnements, Universitede Poitiers, Poitiers, France §Department of Geology, Defence Services Academy, Pyin Oo Lwin, Myanmar

ABSTRACT Located at the southern edge of the eastern Himalayan syntaxis, the Central Myanmar Basin (CMB) is divided into several Tertiary sub-basins that have been almost continuously filled since the Indo- Asia collision. They are currently drained by the Irrawaddy River, which flows down the eastern Tibetan Plateau and the Sino-Burman Ranges. Tracing sediment provenance from the CMB is thus critical for reconstructing the past denudation of the Himalayan-Tibetan orogen; it is especially relevant since a popular drainage scenario involves the capture of the Tsangpo drainage system in Tibet by a precursor to the Irrawaddy River. Here, we document the provenance of sediment samples from the Minbu Sub-Basin at the southern edge of the CMB, which is traversed by the modern stream of the Irrawaddy River. Samples ranging in age from middle Eocene to Pleistocene were investigated using Nd isotopes, trace element geochemistry and sandstone modal compositions. Our data provide no evidence of a dramatic provenance shift; however, sandstone petrography, trace element ratios and isotopic values display long-term trends indicating a gradual decrease of the volcanic input and its replacement by a dominant supply from the Burmese basement. These trends are interpreted to reflect the progressive denudation of the Andean-type volcanic arc that extended onto the Burmese margin, along the flank of the modern Sino-Burman Ranges, where most of the post-collisional deformation of central Myanmar is located. Though our results do not exclude an ephemeral or diluted contribution from a past Tsangpo-Irrawaddy connection, sedimentation rates suggest that this hypothesis is unlikely before the development of a stable Tsangpo-Brahmaputra River in the Miocene. These results thus suggest that the central Myanmar drainage basin has remained restricted to the Sino-Burman Ranges since the beginning of the India-Asia collision.

INTRODUCTION starting in the late Miocene (Royden et al., 1997). Sedi- ment provenance studies provide a tool for unravelling Understanding the processes and products of erosion of a the respective impacts of capture, deformation and exhu- mountain belt has been shown to be critical for decipher- mation on river geometry and sourcing (Hallet & Molnar, ing the various causes and mechanisms of orogenesis 2001; Clark et al., 2004); they thus help to explore topo- (Garzanti et al., 2007). This is particularly true for the graphic evolution and related denudation in response to Indo-Asian orogen, for which numerous different moun- uplift (Najman, 2006; Clift et al., 2008). In South and tain-building mechanisms have been proposed, each East Asia, provenance studies have mostly focused on the resulting in a speciﬁc timing and extent of erosion, for history of the Red River drainage system as inferred from example lateral extrusion in the Oligo-Miocene (Tappon- sediment in the Hanoi Basin (Clift et al., 2006, 2008; Ho- nier et al., 1986), long-term Cenozoic continental under- ang et al., 2009), on the Ganges River drainage in the thrusting (Zhao & Nelson, 1993) or lower crustal ﬂow Indian Foreland Basin (De Celles et al., 1998), on the Ganges-Brahmaputra river system in the Bengal Basin (Uddin & Lundberg, 1998; Galy et al., 2010; Bracciali Correspondence: Alexis Licht, Department of Geosciences, University of Arizona, Tucson, AZ 85721 USA. E-mail: alicht et al., 2013; Chirouze et al., 2013), or on the Indus River @email.arizona.edu in the Indus fan delta (Clift et al., 2001; Roddaz et al.,

Despite its critical importance, the evolution of the sedimentary supply to the CMB is still a matter of debate (e.g. Licht et al., 2013; Robinson et al., 2014). Much discussion has focused on a provenance scenario inspired by the singular geometry of the Tsangpo River in the eastern Himalayan syntaxis, interpreted to provide evidence for a former connection between Tibet and the Irrawaddy River in central Myanmar (Fig. 1; Brookfield, 1998; Clift et al., 2008). Whereas the Irrawaddy waters flow down the Eastern Tibetan Plateau and the Sino-Burman Ranges on the eastern side of the CMB, the Tsangpo River drains the northern side of the Himalayas and the southern flanks of the Lhasa Terrane, including the Transhimalayan arc area, and flows through the Indus- Tsangpo Suture Zone (Fig. 2a; Najman, 2006); a Tsangpo-Irrawaddy connection would have carried ero- sional debris from these areas to the Central Myanmar Basin (Fig. 2b). Fig. 1. Map of Southeast Asia showing the Bengal Basin and Using U/Pb dating and Hf isotopic measurements of the Central Myanmar Basin (BB and CMB, respectively, yellow detrital zircons from various scattered localities in the shaded areas), the Indo-Burman Ranges (IBR), the Tsangpo- CMB, Liang et al. (2008) and Robinson et al. (2014) sug- Brahmaputra and Irrawaddy Rivers and their possible connec- gest that such a connection existed but was lost in the tion through the eastern Himalayan syntaxis (EHS). early Miocene. However, this hypothesis is difficult to 2011). In contrast, only a few studies have been done on evaluate because Hf isotopes data of potential Burmese the Irrawaddy River in central Myanmar and on sediment parent rocks are quasi-nonexistent (Wang et al., 2014). provenance in the Central Myanmar Basin (CMB). On the basis of U/Pb ages, Hf isotopic data and petro- Located at the transition between the Himalayan orogen graphic analyses of detrital grains along with paleocurrent and the Indochinese margin, the CMB is separated from measurements and Nd-Sr isotopic analyses of bulk sedi- the Bengal Basin by the Indo-Burman Ranges (Fig. 1) ment, several studies in the Indo-Burman Ranges (Allen and includes two troughs of pull-apart sub-basins that et al., 2008; Naing et al., in press), and in the northern have been quasi-continuously filled during the Tertiary (Wang et al., 2014) and southern extensions of the CMB (Pivnik et al., 1998). Tracing the sediment sources of (Licht et al., 2013) have shown that, during the Eocene, these deposits would thus help to reconstruct the denuda- central Myanmar was open to the Indian Ocean and tion and the geomorphologic evolution of the ranges at recorded the local unroofing of an Andean-type cordillera the edge of the Indo-Asian collision zone. that extended along the Burmese margin (Fig. 2c). How-

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Fig. 2. Cenozoic drainage variations in the Bengal Bay since the India-Asia collision, showing the main structural units in the neighbouring area, modern drainage connections (a), and various hypothesized former drainage patterns in the late Oligocene (b) and late Eocene (c; after Licht et al., 2013) with schematic paleogeography based on Hall (2012) and arbitrary distances for shortening amount and strike-slip motion. Approximate location of the study sites is indicated by a yellow star. In blue: river networks, including Tsangpo (Ts), Irrawaddy (Ir) and Ganga (Gg) Rivers. Note the different scenario of past river courses for the Tibetan waters (blue arrows).

© 2014 The Authors 2 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cenozoic evolution of the central Myanmar drainage system ever, these studies do not provide information concerning Pegu Yoma Sub-Basin has been inverted in the late Mio- potential Oligo-Miocene river capture. cene and constitutes a local (and recent) topographic high This study focuses on the provenance of sediments in in the CMB (Khin & Myitta, 1999). At the eastern edge the Minbu Sub-Basin of the CMB in southern Myanmar. of central Myanmar, the Sino-Burman Ranges comprise This sub-basin is currently traversed along its entire the Tenasserim highlands, the Shan Plateau and the Yun- length by the Irrawaddy River. Sandstone modal analysis, nan highlands, all of which are local units of the Sibumasu trace element geochemistry and Nd isotopic analysis of Terrane (eastern unit of the Indochina Peninsula; Met- bulk sediment samples, together with data compilations calfe, 2013) and mainly consist of Paleozoic to Cretaceous from the literature, were used to study the sediment prov- metasediments and plutons (Bender, 1983). Between the enance of eleven units, ranging in age from middle Eocene Sino-Burman Ranges and the Central Myanmar Basin, to Pleistocene. Our goal was to understand the evolution the basement of the Burma Terrane crops out as belts of of the sedimentary supply to southern Myanmar and thus metamorphic rocks: the Slate and the Mogok Metamor- to reconstruct the denudation of the neighbouring areas phic Belts in the south and the Gaoligong Belt in the and to test the capture hypothesis. north (Bertrand & Rangin, 2003; Mitchell et al., 2007). These belts are intruded by young batholiths and related volcanic rocks (mostly <150 Ma), including the lavas and OVERVIEW OF BURMESE GEOLOGY plutons of the Wuntho-Popa Arc (Fig. 3b), which is considered as the eastern continuation of the Transhimalayan Most of central Myanmar is composed of the Burma Ter- Arc of Tibet and contains relics of the Andean-type volca- rane, which currently includes the CMB (Fig. 3a). The nic arc of the Indo-Asian subduction zone (Zaw, 1990; CMB comprises two lateral troughs of Cenozoic pull- Mitchell et al., 2012; Ma et al., 2014; Wang et al., 2014). apart sub-basins, including the Minbu Sub-Basin, where At the western edge of Myanmar, the Central Myanmar a 15 km thick succession of Cenozoic deposits is found Basin is separated from the Bengal fan by the Indo- (Pivnik et al., 1998). East of the Minbu Sub-Basin, the Burman Ranges, which form a Cenozoic accretionary

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Fig. 3. (a) Simpliﬁed geological map of central Myanmar and eastern Himalayan syntaxis (after Mitchell et al., 2012; Metcalfe, 2013). Mb: Minbu Sub-Basin; PYb: Pegu-Yoma Sub-Basin; MMB: Mogok Metamorphic Belt; SB: Slate Belt; GB: Gaoligong Belt; SF: Sagaing Fault. (b) Detailed geological map of the sampling area in central Myanmar (red frame in subﬁg. a; after Bender, 1983; Mitchell et al., 2012). Sampling sites are numbered (1–6). (c) Schematic log of the Minbu Sub-Basin (Licht et al., 2013) displaying the localities of sampling for each stratigraphical unit.

© 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists 3 A. Licht et al. complex produced during the subduction of the Indian waddy River and the Chindwin River, one of its main plate beneath the Burma Terrane (Maurin & Rangin, tributaries (sites 1–3 on Fig. 3b; detailed maps in Licht 2009). Paleocurrents and sediment ages indicate that a et al., 2013, 2014). Upper Oligocene and lower Miocene first emergence of the Indo-Burman Ranges must have sediments were sampled in the Yenangyat Anticline (site occurred between the terminal Eocene and the early Mio- 4 on Fig. 3b), along the modern stream of the Irrawaddy cene (Allen et al., 2008; Licht et al., 2013); seismic data River. Middle Miocene to Pleistocene sediments were in the Indo-Burman Ranges indicate much more rapid sampled in the Pondaung Ranges (site 3) and along the uplift since late Miocene time (Maurin & Rangin, 2009). Irrawaddy River further South (sites 5 and 6). Potential During the Cenozoic Era, central Myanmar was geographical bias in our results caused by our sampling dragged by both the northward movement of the Indian locations is adressed in the discussion. Plate and the right lateral extrusional motion of Indo- Thin sections of 28 sandstone samples were prepared china, resulting in intense strike-slip deformation and ca. and counted according to the Gazzi-Dickinson method to 30° clockwise rotation relative to China (Richter & Fuller, determine their contents of quartz, feldspar and lithic 1996; Morley, 2009). Until the middle Miocene, strike- grains (Dickinson, 1985); at least 300 grains were counted slip deformation was accommodated by the subsidence of per section. pull-apart basins (Rangin et al., 1999) and caused high Five to eight samples per formation were selected for temperature metamorphism and exhumation of the analysis of the bulk Nd isotopic ratios of their silicate frac- Burma Terrane basement along the metamorphic belts tions and their whole rock trace element contents (62 sam- that extend along the Sino-Burman Ranges (Searle et al., ples in total). Samples from different locations as well as 2007). Deformation was particularly significant along the samples of various lithofacies were chosen. Powdered sed- Mogok Metamorphic Belt, where mineral growth ages iments were analysed for trace elements using a Thermo indicate high temperature metamorphism from the late X7 ICP-MS at the Service d’Analyse des Roches et des Eocene to the middle Miocene (Bertrand et al., 2001; Mineraux (SARM-CRPG, Vandoeuvre-les-Nancy, Barley et al., 2003; Searle et al., 2007). Since the middle France). Precisions are better than 5–10% for nearly all Miocene, spreading in the Andaman Sea and develop- elements reported. Chemical extraction of Nd and Nd ment of the Sagaing Fault along the Sino-Burman Ranges isotopic analyses were performed at the CRPG, according accommodated the strike-slip motion (Khan & Chakr- to the standard procedures of the laboratory (see Licht aborty, 2005). Further exhumation of the metamorphic et al., 2013). Briefly, after decarbonation with HCl and belts in the late Neogene is attributed to the uplift of the dissolution in a mixture of HF, HNO3 and a small amount Sino-Burman Ranges in response to the growth of the of HClO3, Nd was separated using Eichrom TRU-spec Eastern Tibetan Plateau and Tibetan crustal flow into and Ln-spec resins. Nd isotopic compositions were mea- Southeast Asia (Rangin et al., 2013); this late event is also sured using a Neptune Plus MC-ICP-MS. Nd isotopic the possible cause for the late Miocene inversion of the ratios are normalized to 146Nd/144Nd = 0.7219. During Pegu Yoma Sub-Basin and for numerous smaller inverted the period of measurement the JNdi Nd standard gave a structures in the Minbu Sub-Basin (Yenangyat & Yen- mean value of 143Nd/144Nd = 0.512086 0.000011 angyang Anticlines, Pondaung Ranges; Pivnik et al., (2r). To allow comparison with data from other laborato- 1998). The total amount of dextral, northward displace- ries, 0.000029 was added to each 143Nd/144Nd result to ment of central Myanmar along the Sino-Burman Ranges make our data consistent with a value of 0.512115 for the is estimated to be from 300 to about 1100 km (Mitchell, JNdi standard, which is equivalent to a value of 0.511858 1993; Curray, 2005; Morley, 2009) and varies in accor- for the La Jolla Nd standard (Tanaka et al., 2000). Nd dance with the preferred Asian paleogeographic models procedural blanks represented less than 1% of the amount (Replumaz & Tapponnier, 2003; Hall, 2012). of Nd measured in the samples and were thus insignificant. Ages, localities, GPS coordinates and dominant lithologies are given in Table 1. SAMPLING STRATEGYAND METHODS Sediment samples from the Minbu Sub-Basin were col- RESULTS lected in the Pakokku District, central Myanmar, along a ca. 200 km line that roughly follows the modern North- Isotopic results and trace element contents are given in South axis of the Sub-Basin (Fig. 3b). Our dataset covers Table 2, grain-counting results are given in Table 3. eleven geological units, spanning in age from middle Samples already published in Licht et al. (2013) have Eocene to Pleistocene (Fig. 3c). Due to the poor rock been highlighted in both tables (17 isotopic and 5 exposure in the sub-basin, sediments from this entire time petrographic Eocene data). As samples from each forma- span could not be obtained from a single site and were tion were taken from different localities, their relative instead sampled in three main locations. Sampling of stratigraphic positions were often not evident. For this Eocene and lower Oligocene sediments was limited to the reason, values for each formation were grouped together, Pondaung Ranges, near the confluence between the Irra- and no attempt was made to investigate intraformational

© 2014 The Authors 4 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cenozoic evolution of the central Myanmar drainage system

Table 1. Unit, Age, Location (number corresponding to the locality number in Fig. 3b), GPS coordinates and dominant lithology of the sediment samples (data already published in Licht et al., 2013 have been marked with a star *)

Formation Age Sample Name Lithology Sampling site Location

Upper Irrawaddy Plio-Pleistocene BRI01 Clay 5 N20°17047.5″ E095°00027.6″ BRI02 Sand 5 – BR2-01 Sand 5 N20°17035.6″ E095°00017.7″ PLN-01 Sand 6 N20°04058.0″ E094°43006.0″ CHA-02 Clay 5 N20°58043.0″ E094°40048.4″ Lower Irrawaddy Upper Miocene HOM03 Sand 6 N20°05047.8″ E095°08004.1″ HOM-01 Clay 6 – HOM-02 Sand 6 – MON2-02 Clay 5 N20°27016.3″ E094°54042.2″ MON2-03 Sand 5 – Obogon Late middle Miocene OIL06 Clay 5 N20°29056.6″ E094°53020.1″ MON7 Sand 5 N20°26033.2″ E094°54032.7″ MON-03 Sand 5 – MON-05 Sand 5 – MON-06 Clay 5 – Kyaukkok Middle Miocene KYA1 Sand 3 N21°41017.8″ E094°42035.0″ KYA2 Sand 3 – KYA3 Sand 3 – KYA4 Sand 3 – KYA5 Sand 3 – Pyawbwe Upper Miocene PYA1 Clay 4 N21°08028.0″ E094°45043.1″ PYA2 Sand 4 – PYA3 Sand 4 N21°08028.0″ E094°45043.1″ PYA4 Clay 4 – PYA5 Clay 4 N21°07049.0″ E094°46008.1″ Okhmintaung Upper Oligocene OKH1 Clay 4 N21°09022.3″ E094°46047.6″ OKH2 Sand 4 N21°08047.9″ E094°46054.1″ OKH3 Sand 4 – OKH4 Sand 4 – OKH5 Clay 3 N21°09022.3″ E094°46047.6″ Padaung Lower to Upper PAD1 Clay 3 N21°41032.5″, E094°43001.0″ Oligocene PAD2 Clay 3 N21°41035.2″, E094°42045.9″ PAD3 Clay 3 – PAD4 Sand 3 N21°41035.3″, E094°42042.9″ PAD5 Sand 3 N21°41028.1″, E094°42044.6″ Shwezetaw Lower Oligocene SH1 Clay 3 N21°42012.3″, E094°42041.5″ SH2 Sand 3 N21°42007.0″, E094°42044.2″ SH3 Sand 3 N21°42005.2″, E094°42053.1″ SH4 Sand 3 N21°41047.7″, E094°43003.9″ SH5 Sand 3 N21°42012.3″, E094°42041.5″ Yaw Upper Eocene YAW-SA* Sand 3 N21°42020.4″, E094°42041.6″ YS2* Clay 3 N21°42040.5″, E094°42057.0″ YS63* Sand 3 – YAW-A* Clay 3 – YTP (2)* Clay 2 N21°43011.1″, E094°40023.8″ YAW-RE* Clay 2 N21°43032.6″, E094°40039.7″ Pondaung Late middle Eocene TH2* Clay 2 N21°45041.0″, E094°50029.4″ (Bartonian) TH63* Sand 2 – PA2* Clay 2 N21°42031.0″, E094°49021.6″ PA63 (2)* Sand 2 – GA2* Clay 2 N21°44003.0″, E094°43026.3″ GA63* Sand 2 – GAN08* Clay 2 – PK2-21* Sand 2 N21°45016.3″, E094°39010.2″ PK2-06* Sand 2 – YAS-06* Clay 2 N21°44012.5″, E094°38015.3″ YPL-11* Sand 2 N21°45003.8″, E094°37035.3″

(continued)

Table 1 (continued)

Formation Age Sample Name Lithology Sampling site Location

Tabyin Middle Eocene TAB1 Clay 1 N21°54007.6″ E094°33047.2″ TAB2 Clay 1 – TAB3 Sand 1 – TAB4 Sand 1 – TAB5 Clay 1 – variations. Therefore temporal changes can only be exam- Neogene input from the Indus-Tsangpo Suture Zone is ined at time scales similar to or greater than those of the likely to display similar features to the modern Tsangpo formations (i.e., 2 to 5 Myr). River load before its connection to the Brahmaputra in Point-counting results are plotted on Q-F-L and Lm- the Siang Gorges, with eNd around -11 (Singh & France- Lv-Ls diagrams (Fig. 4). The evolution of the grain pro- Lanord, 2002) resulting from the mixed contribution of portions shows a regular shift from lithic volcaniclastic sedimentary, metamorphic and carbonate rock fragments detritus in the middle Eocene to lithoquartzose metamor- from the Himalayan, Transhimalayan and Tibetan areas. phiclastic/sedimentaclastic orogenic detritus in the Mio- Considering the probable late Paleogene exhumation of Pliocene. Carbonate lithic fragments were detected in the Himalayan Ranges (Najman et al., 2008), older inputs insignificant proportions in all the samples (<1%). are expected to be more depleted in Himalayan sourced Middle Eocene sediments (sites 1 and 2) display eNd sediment and thus to be dominated by Transhimalayan values ranging from 7.8 to +1 over the 43–37 Ma per- and Tibetan rock fragments, with higher eNd (from 10 iod, (average 3.4, n = 16), while those of upper Eocene to +8) and lower metamorphic lithic content (Table 4). – Oligocene sediments (sites 2, 3 and 4) vary from 9to – = 2.4 over the 37 22 Ma period (average 5.9, n 21). Middle Eocene sediments Taken together, the Eocene – Oligocene samples show a gradual shift of eNd from near zero to moderately nega- Middle Eocene samples are rich in volcanic rock frag- tive values (Fig. 5). The eNd values of Mio-Pliocene sed- ments and display similar petrographic and isotopic fea- iments (sites 3, 5 and 6) range from 13.7 to 5.9 over tures to Paleogene flysch sediments of the Indo-Burman the last 22 Ma (average 8.1, n = 25), and in contrast Ranges, mainly constituted of volcaniclastic sandstones with the Eocene – Oligocene samples show no systematic with mildly negative eNd values (7to4; Allen et al., temporal variations resolvable at our sampling density. 2008). Considering that the Indo-Burman Ranges were The average eNd value of each individual unit within this not emerged at that time, these results indicate a similar time interval is stable at about 8to9 (with the excep- sediment source for both regions (Allen et al., 2008; Licht tion of the Obogon unit: average 6.4, n = 5). et al., 2013). The presence of occasional positive eNd val- We focus the discussion of the trace element results on ues indicates a contribution from magmatic rocks of the the Zr/TiO2 and La/V ratios, which are sensitive to the Paleogene Andean-type arc that extended along the Asian mafic character of the detritus and the sedimentary cycle: margin, located north and east of the Minbu Sub-Basin low ratios indicate mafic contributions and high ratios (Ji et al., 2009; Mitchell et al., 2012; Ma et al., 2014). represent mature, recycled material (e.g. Dingle & Lav- The northern (Transhimalayan) and eastern (Wuntho- elle, 1998; Zhang, 2004). The Zr/TiO2 and La/V ratios Popa Arc) sections of this volcanic arc cannot be easily exhibit similar temporal trends among the Burmese sam- differentiated on the basis of Nd isotopes. Transhimalay- ples (Fig. 5), increasing from low values in middle Eocene an provenance could have been achieved if the morpho- times to high values in the upper Miocene-Pliocene inter- logical setting was similar to modern Bengal geography, val (from 0.1 to 0.5 and from 200 to 300, respectively). with emerged Indo-Burman Ranges channeling water southeastwards from Tibet and the Eastern Tibetan Pla- teau into central Myanmar (Fig. 2b). Such a drainage pat- INTERPRETATION tern, which parallels the convergence zone, is nevertheless unlikely during the Eocene before the emer- Middle Eocene, upper Eocene – Oligocene and Mio-Plio- gence of the Indo-Burman Ranges. Licht et al. (2013) cene sediments present contrasting characteristics reflect- show that the orientation of Eocene delta systems, ing different provenances (Table 4). Two main Burmese inferred through paleocurrent analysis, indicates a source geographic provinces, with distinctive geological features, area located to the east, that is, on the Burmese margin. are considered as potential local sources for the sediment The lithic volcaniclastic sediments are therefore better- deposited in the Minbu Sub-Basin: The Sino-Burman explained by local supply from the unroofing of the Ranges (including also the Wuntho-Popa Arc and the Wuntho-Popa Arc (Wang et al., 2014). Negative eNd val- metamorphic belt rocks that crop out along the ranges) ues and low-grade metamorphic material observed by and the Indo-Burman Ranges (Figs 2 and 3) since the Licht et al. (2013) highlight a minor contribution from a Oligocene (Allen et al., 2008; Licht et al., 2013). Any secondary source in the Burmese substratum.

© 2014 The Authors 6 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cenozoic evolution of the central Myanmar drainage system

Table 2. Ti, La, V, Zr, Nd and Sm concentrations and detailed isotopic results of the samples (data already published in Licht et al., 2013 have been marked with a star *)

Sample Ti La V Zr Nd Sm Standard Formation Name (%) (ppm) (ppm) (ppm) (ppm) (ppm) 143Nd/144Nd error (2r) εΝd

Upper BRI01 0.776 33.9 105.1 203.9 30.6 6.2 0.512335 0.000006 5.92 Irrawaddy BRI02 0.342 33.9 41.5 105.9 24.6 4.4 0.512204 0.000008 8.47 BR2-01 0.122 13.1 18.5 61.8 10.7 1.9 0.511933 0.000014 13.75 PLN-01 0.562 23.0 76.7 148.2 21.5 4.2 0.512149 0.000012 9.54 CHA-02 0.735 25.8 110.9 199.6 25.4 5.0 0.512293 0.000010 6.73 Lower HOM03 0.841 33.6 118.0 214.6 30.2 6.0 0.512218 0.000009 8.19 Irrawaddy HOM-01 0.807 28.5 88.4 214.8 26.6 5.3 0.512089 0.000008 10.71 HOM-02 0.757 45.7 101.0 225.9 38.6 6.9 0.512247 0.000008 7.63 MON2-02 0.634 30.2 78.1 201.9 28.7 6.1 0.512052 0.000010 11.43 MON2-03 0.507 20.8 61.5 135.5 19.8 3.9 0.512233 0.000008 7.90 Obogon OIL06 0.838 32.3 134.8 155.9 27.2 5.1 0.512302 0.000007 6.56 MON7 0.431 35.4 71.8 106.3 26.2 4.9 0.512409 0.000010 4.47 MON-03 0.382 12.0 66.7 75.3 11.2 2.7 0.512281 0.000016 6.96 MON-05 0.29 23.4 49.2 60.1 16.6 3.2 0.512236 0.000022 7.84 MON-06 0.818 75.0 114.5 162.7 64.1 12.4 0.512321 0.000006 6.18 Kyaukkok KYA1 0.31 24.2 59.5 55.2 18.9 3.5 0.512229 0.000005 7.97 KYA2 0.331 38.3 54.4 128.3 26.9 5.0 0.512167 0.000009 9.18 KYA3 0.286 19.0 56.5 67.0 14.5 2.7 0.512341 0.000008 5.79 KYA4 0.336 31.1 54.2 101.3 22.9 4.4 0.512258 0.000007 7.41 KYA5 0.265 20.0 28.4 116.9 16.1 3.0 0.512153 0.000012 9.46 Pyawbwe PYA1 0.823 30.1 126.6 132.2 26.6 5.4 0.512175 0.000003 9.03 PYA2 0.29 20.0 47.6 75.1 17.3 3.6 0.512263 0.000006 7.32 PYA3 0.392 21.4 60.8 111.9 18.6 3.9 0.512290 0.000025 6.78 PYA4 0.731 27.5 130.2 134.0 24.5 5.0 0.512261 0.000020 7.36 PYA5 0.735 29.7 133.0 120.0 25.4 5.0 0.512141 0.000012 9.70 Okhmintaung OKH1 0.754 62.6 116.0 188.4 289.7 70.9 0.512458 0.000012 3.51 OKH2 0.526 32.5 67.1 189.4 24.0 4.3 0.512242 0.000006 7.73 OKH3 0.286 16.7 48.2 69.7 14.4 2.7 0.512302 0.000014 6.55 OKH4 0.349 15.4 53.5 76.0 21.7 5.4 0.512352 0.000006 5.58 OKH5 0.762 53.4 95.5 193.8 101.4 22.8 0.512237 0.000008 7.83 Padaung PAD1 0.789 31.4 120.0 212.3 27.6 5.5 0.512183 0.000006 8.88 PAD2 0.752 30.9 116.2 159.3 27.3 5.6 0.512183 0.000005 8.87 PAD3 0.418 18.0 60.6 112.2 15.9 3.2 0.512309 0.000008 6.42 PAD4 0.399 18.2 57.8 110.8 15.7 3.1 0.512318 0.000008 6.24 PAD5 0.56 23.3 81.7 160.8 20.1 3.9 0.512233 0.000007 7.89 Shwezetaw SH1 0.74 26.5 122.1 180.4 24.3 5.0 0.512394 0.000006 4.76 SH2 0.593 23.1 97.0 162.5 21.3 4.2 0.512484 0.000007 3.01 SH3 0.511 20.5 83.1 134.3 19.2 3.7 0.512515 0.000009 2.39 SH4 0.568 35.7 91.1 146.1 31.2 6.4 0.512445 0.000007 3.76 SH5 0.403 17.8 65.7 101.3 16.2 3.2 0.512487 0.000013 2.94 Yaw YAW-SA* 0.569 29.7 86.6 170.4 35.8 8.4 0.512254 0.000003 7.49 YS2* 0.73 27.5 111.9 138.3 24.3 4.9 0.512511 0.000004 2.48 YS63* 0.405 17.3 51.2 130.2 16.3 3.6 0.512371 0.000020 5.20 YAW-A* 0.744 24.2 125.9 124.0 24.7 5.3 0.512268 0.000013 7.22 YTP (2)* 0.754 21.6 105.3 206.3 23.1 5.1 0.512363 0.000009 5.36 YAW-RE* 0.728 23.5 125.5 129.0 22.9 4.7 0.512294 0.000007 6.71 Pondaung TH2* 0.6 17.4 172.2 82.4 17.5 3.6 0.512263 0.000021 7.32 TH63* 0.741 28.9 107.5 157.5 26.1 5.3 0.512237 0.000003 7.82 PA2* 1.043 13.8 177.0 142.5 15.6 3.9 0.512651 0.000007 0.26 PA63 (2)* 1.076 13.1 115.2 131.8 11.8 2.7 0.512442 0.000005 3.83 GA2* 0.656 38.0 105.1 143.9 39.9 7.3 0.512438 0.000004 3.90 GA63* 0.699 20.4 179.8 204.1 21.9 4.6 0.512342 0.000005 5.78 GAN08* 0.699 26.7 112.3 133.4 22.1 4.2 0.512319 0.000005 6.23 PK2-21* 0.645 17.2 78.1 138.5 19.4 4.1 0.512627 0.000006 2.28 PK2-06* 0.593 19.8 95.8 112.0 20.5 4.3 0.512521 0.000005 0.21 YAS-06* 0.723 21.6 93.1 141.0 21.5 4.6 0.512240 0.000004 7.76 YPL-11* 0.787 41.7 109.5 179.4 47.3 9.6 0.512553 0.000006 1.66

Table 2 (continued)

Sample Ti La V Zr Nd Sm Standard Formation Name (%) (ppm) (ppm) (ppm) (ppm) (ppm) 143Nd/144Nd error (2r) εΝd

Tabyin TAB1 0.731 13.6 138.8 125.8 14.4 3.3 0.512462 0.000008 3.43 TAB2 0.707 18.6 130.4 118.8 18.0 3.9 0.512429 0.000007 4.08 TAB3 0.793 10.9 125.0 137.2 12.1 2.7 0.512651 0.000009 0.26 TAB4 0.599 10.8 92.3 99.5 14.7 3.6 0.512693 0.000013 1.07 TAB5 0.71 17.7 131.2 112.0 14.8 3.1 0.512508 0.000008 2.54

Table 3. Point-counting results of the sandstone samples (data already published in Licht et al., 2013 have been marked with a star *). Q, quartz; F, feldspar; L, lithic fragments (Lm, metamorphic; Ls, sedimentary; Lv, volcanic)

Formation Sample Q F L Lv Ls Lm

Upper Irrawaddy CHA-03 61 6 33 18 47 35 BR2-01 76 8 16 36 30 34 Lower Irrawaddy HOM-03 47 16 37 14 36 50 MON2-03 71 3 26 6 43 51 MON2-01 60 10 30 12 40 48 Obogon MON-05 64 3 33 11 37 52 MON-03 45 9 46 42 35 23 Kyaukkok KYA5 60 11 29 13 29 58 KYA2 64 7 29 3 55 42 Pyawbwe PYA2 40 21 39 32 39 29 PYA3 23 30 47 17 38 45 Okhmintaung OKH1 30 20 50 26 42 32 OKH2 48 13 39 19 46 35 OKH3 25 28 47 32 38 30 Padaung PAD5 30 11 59 23 51 26 PAD4 37 14 49 20 53 27 Shwezetaw SH1 36 16 48 31 45 24 SH3 26 15 59 32 45 23 SH4 44 11 45 26 45 29 Yaw YAW-S* 23 16 61 45 26 29 Y-SAND 12 11 77 50 33 17 YSABLE 39 15 46 29 50 21 Pondaung PK1-03* 16 13 71 41 21 38 PK2-06* 15 18 67 42 26 32 PK2-17* 13 17 70 46 25 29 PK2-21* 19 16 65 49 27 24 Tabyin TAB1 11 4 85 46 38 16 TAB3 20 12 68 41 39 20

– progressive denudation of the volcanic areas in the drain- Upper Eocene Oligocene sediments age basin. This interpretation is compatible with the ris- Upper Eocene – Oligocene sediments display eNd values ing Zr/TiO2 and La/V ratios during this time period, intermediate between those of Mio-Pliocene and middle representing an increase in quartz, metamorphic and sedi- Eocene sediments. The occurrence of mildly negative mentary rock fragments interpreted to be caused by the eNd in several Oligocene samples, notably those of the progressive loss of a mafic source. These results therefore Shwezetaw Formation (ca. 31–30 Ma), could reflect suggest that after advanced unroofing of the Wuntho- minor input from the rising Indo-Burman Ranges, which Popa Arc, river incision cut into metamorphic and recy- would deliver reworked, volcanic-sourced sediment cled sedimentary rocks of the basement of the Southeast (Fig. 2b). Nevertheless, this early unroofing of the Indo- Asian terranes. However, an ephemeral input from the Burman Ranges must have been limited, given the low Tibetan region after the probable uplift of the Indo- Oligocene sediment accumulation rate in central Myan- Burman Ranges at 31–30 Ma cannot be excluded because mar (ca. 5km3 kyr1, Fig. 5; Metivier et al., 1999). the mixed sources of the lower Oligocene Burmese samples Sandstones display increasing quartz grain abundance, display similar geochemical and petrographic features to and follow a clear unroofing trend that indicates the those inferred for the Tsangpo River precursor (Table 4).

© 2014 The Authors 8 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cenozoic evolution of the central Myanmar drainage system

Fig. 4. Q-F-L and Lv-Ls-Lm plots of Burmese sandstone samples, following the classiﬁcation of Dickinson (1985). Q, quartz; F, feldspar; L, lithic fragments (Lm, metamorphic; Ls, sedimentary; Lv, volcanic). QFL values for the Irrawaddy River and the rivers of the modern Tsangpo drainage basin from Garzanti et al. (2004, 2013).

fluvial systems of the CMB, or instead to local tributaries Mio-Pliocene sediments flowing down the Indo-Burman or the Sino-Burman The small proportion of lithic material in the Mio-Plio- Ranges. cene sediments contrasts with what is observed in the Pal- Our localities in the Pondaung Ranges (sites 1, 2 and 3) eogene samples; it also contrasts with the loads of the are located close to the confluence between the Irrawaddy modern rivers of the Tsangpo drainage basin, which are and the Chindwin Rivers. The Chindwin River currently commonly enriched in sedimentary and carbonate lithic flows down the Indo-Burman Ranges and may have pro- fragments (Garzanti et al., 2004; Fig. 4). The paucity of vided an additional local supply of volcanic reworked lithic fragments also indicates that the Mio-Pliocene clasts in the sampling localities 1, 2 and 3 (Fig. 3). deposits did not form from tributaries flowing down the However, its existence necessarily post-dates the Oligo- neighbouring, newly uplifted Indo-Burman Ranges, cene emergence of the Indo-Burman Ranges and its con- which yield sediments enriched in volcanic rock frag- tribution to the Eocene supply in the Minbu Sub-Basin ments with mildly negative eNd values (7to4; Allen is thus unlikely. Increased volcanic lithic content (Fig. 4) et al., 2008). Mio-Pliocene units display, like the Quater- and eNd values (Fig. 5) in the lower Oligocene (ca. 31– nary Irrawaddy sediment load, more abundant orogenic 30 Ma) Shwezetaw Formation are here suggested to detritus enriched in quartz, metamorphic and sedimentary reflect a short-term volcaniclastic input following the first rock fragments, stable average eNd values and an eNd emergence of the Indo-Burman Ranges; samples from range that is similar to the range of the Irrawaddy load the only post Shwezetaw units in the Pondaung Ranges (11 to 8; Colin et al., 2006; Allen et al., 2008). These (namely the Padaung and Kyaukkok Formations, see data indicate that sources have remained relatively Fig. 3c) do not show any significant volcanic input. Neo- unchanged over the last 22 Ma at the study sites and sug- gene samples from other localities also do not display an gest a prominent, stable provenance area located in the Indo-Burman Range fingerprint. Thus, we argue that a Sino-Burman Ranges, where the modern Irrawaddy River local contribution from the Chindwin River or any other is sourced. past tributary flowing down the Indo-Burman Ranges was insignificant in our samples, except probably for the lower Oligocene Shwezetaw Formation that may reflect DISCUSSION the first emergence episode of the Ranges, as discussed above. Local geographic variations or a broad Finally, we consider whether the sampled outcrops cor- temporal trend? respond to sediment deposited by the main stems of past The variation of sediment provenance identified in the CMB drainage systems, or just by small tributaries flow- different geological units can be interpreted as reflecting ing down the closest highs of the Sino-Burman Ranges, either long-term temporal variation of the sediment sup- on the east side of the CMB. The latter hypothesis implies ply in the Minbu Sub-Basin, or local geographical varia- that these tributaries would have flowed westward from tions caused by the relatively scattered character of our the Shan Plateau to the Minbu Sub-Basin through the sampling localities. This question is particularly relevant neighbouring Pegu Yoma Basin, located between the two for the sediment deposited after the Oligocene uplift of regions (Fig. 3b). Their waters would have then exited in the Indo-Burman Ranges and the development of a south- the main stem of the ancient drainage system, located ward oriented drainage system in central Myanmar, throughout this time to the west of the sampling localities. merging water supplies from several distant sources. However, paleocurrents and stratigraphic architecture of Standard geomorphological observations and our geo- the Miocene deposits in the Pegu Yoma Basin indicate chemical results can help us to distinguish whether our southward directed fluvial systems (Khin & Myitta, 1999) sampling localities correspond to the main stem of past and contradict this hypothesis.

Fig. 5. Mean lithology (yellow: sandstone; grey: mudstone) in the Minbu Sub-Basin since middle Eocene time; eNd, Zr/TiO2 and La/V ratios of Minbu Sub-Basin sediments (coloured bars: individual data; coloured shades: envelopes spanning the data ranges; black dashed bars: average value per formation). Note that within each formation, individual results are presented as coloured vertical bars, as it was not usually possible to determine relative stratigraphic order of samples collected in different localities; Solid Phase Accumu- lation Rate in central Myanmar with standard error (SPAR, from Metivier et al., 1999; note that sediment accumulation times have been modiﬁed after the recent redating of the base of the Irrawaddy Formation at 10 Ma by Jaeger et al., 2011), and uplift events in the surrounding area (changing thickness of gray bars indicating supposed changes in uplift intensity; after Morley, 2009; Maurin & Rangin, 2009). The isotopic range of the Asian basement rocks and of the Wuntho-Popa and Transhimalayan (WT) rocks is also represented (values in Table 4). IBR: Indo-Burman Ranges; SBR: Sino-Burman Ranges; ETP: Eastern Tibetan Plateau.

Thus, we argue that our different sites are exempt of progressive denudation of the Wuntho-Popa Arc, located significant local bias in past sedimentary supply. Sampled along the Sino-Burman Ranges (Fig. 2). The emergence sediments were first deposited by westward directed del- of the Indo-Burman Ranges and the shift from westward taic systems before the uplift of the Indo-Burman Ranges directed deltaic systems to southward directed fluvio- in the Oligocene (Licht et al., 2013), then by southward deltaic systems, which our data suggest occurred around directed fluvio-deltaic systems, precursors to the modern 30–31 Ma, is thus coeval with a long-term, somewhat Irrawaddy River. The variation of sediment provenance erratic exhumation of the Sino-Burman Ranges. These identified between the different units is therefore inter- data do not disallow the possibility that Tibetan and preted as reflecting the temporal evolution of the mean Himalayan sourced sediments may have contributed to sedimentary supply in the Minbu Sub-Basin. sedimentary supply of the Minbu Sub-Basin between the emergence of the Indo-Burman Ranges and the early Mio- Implications for the evolution of the cene, because Oligocene Burmese samples display similar sedimentary supply geochemical and petrographic features to those inferred for the Tsangpo River precursor. However, an ephemeral Our results show a long-term decrease of the volcanic capture of Tsangpo waters would significantly extend the input and an increasing input from basement rocks into Burmese drainage basin and increase the sedimentary sup- the Minbu Sub-Basin until the early Miocene. This ply in central Myanmar. This scenario is incompatible gradual change is interpreted as corresponding to the with the evolution of Burmese sedimentation rates that

© 2014 The Authors 10 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cenozoic evolution of the central Myanmar drainage system

Table 4. Synthesis of the different drainage provinces surrounding central Myanmar, with their petrologic and Nd isotopic proper- ties, and data from the Minbu Sub-Basin. Source compilations from Singh & France-Lanord (2002), Najman (2006), and Najman et al. (2012) for the Tibetan domain and Tsangpo River, from Colin et al. (2006), Allen et al. (2008), Mitchell et al. (2012), Garzanti et al. (2013) and Licht et al. (2013) for the Burmese domain and Irrawaddy River

Provenance domains Petrography Bulk rock eNd

Myanmar domain -Sino-Burman Ranges: Burma Terrane basement Ultramaﬁc rocks, low and high grade 13 to 3 & Shan Plateau series metamorphic rocks, S-Type granitoids, Precambrian to Mesozoic metasediments Wuntho-Popa arc Volcanic rocks, I-type granitoids 0 to +8 -Indo-Burman Ranges (Inner wedge) Mainly volcanic sediment 7to4 Central Tibet domain -Lhasa Terrane basement S-Type granitoids, Palaeozoic to 10 to 0 Mesozoic metasediments -Transhimalayan arc & Indus- Volcanic rocks, I-type granitoids, +1to+8 Tsangpo Suture Zone ophiolites Himalayas -Tethyan Sedimentary Series Medium - high grade metamorphic rocks, 19 to 5 (av. 15) and Higher Himalaya Precambrian to Eocene sediment -Lesser Himalaya Low grade metamorphic rocks, 27 to 21 Palaeoproterozoic sediment River loads Sandstone Petrography Bulk rock eNd

Modern Irrawaddy River Litho-feldspatho-quartzose with 11 to 8 metamorphic>sedimentary>volcanic lithics Modern Tsangpo River & tributaries Quartzo-lithic to Feldspatho-litho-quartzose 11 with metamorphic>volcanic>sedimentary lithics Hypothetical “old” Tsangpo River Quartzo-lithic volcaniclastic/ophioliticlastic >11 Minbu Sub-Basin sediments Sandstone Petrography Bulk rock eNd

Middle Eocene rocks Feldspatho-quartzo-lithic with 7to+1 (Tabyin & Pondaung Fm) volcanic>sedimentary & metamorphic lithics Upper Eocene – Oligocene rocks Feldspatho-quartzo-lithic 9to2 (Yaw, Shwezetaw, with sedimentary> Padaung & Okhmintaung Fm) volcanic & metamorphic lithics Mio-Pliocene rocks Feldspatho-litho-quartzose with 13 to 6 (Pegu Group & Irrawaddy Fm) metamorphic>sedimentary>volcanic lithics reached their lowest level in the Oligocene (Fig. 5; Meti- 1998; Najman et al., 2012). This difference can be vier et al., 1999). explained by the asymmetric precipitation pattern on the Since the early Miocene, our data highlight a stable two sides of the Indo-Burman Ranges (Koons, 1995): on source for the sediment in the Minbu Sub-Basin similar the Bengal, windboard side of the ranges, monsoonal pre- to the modern Irrawaddy load provenance. A minor sup- cipitation can reach 10 times or more the amount of rain- ply from the Indus-Tsangpo Suture Zone, diluted by fall over the outboard, Burmese side (where annual these proximal sources, can once again not be excluded, rainfall is commonly <800 mm). Most of the rainfall in but would contradict the increasing evidence of a stable the Irrawaddy drainage basin occurs over the Sino-Bur- Tsangpo-Brahmaputra connection since the early Mio- man Ranges, where denudation is much more important cene (Uddin & Lundberg, 1998; Galy et al., 2010; Bracci- (Stamp, 1940). ali et al., 2013). These results thus argue for the stability of the sediment provenance in the Minbu Sub-Basin. The lack of any signiﬁcant input from the Indo-Burman CONCLUSION Ranges, characterized by mildly negative eNd values, in the Burmese Mio-Pliocene units and in the modern Irra- Trace element geochemistry, Nd isotopes, and sandstone waddy load (Allen et al., 2008) contrasts with the minor, modal compositions of middle Eocene to Quaternary sedi- yet signiﬁcant Indo-Burman input recorded in the Mio- ment samples from central Myanmar provide no evidence Pliocene deposits of the Bengal Basin (Uddin & Lundberg, of a dramatic provenance shift but highlight a gradual

© 2014 The Authors Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists 11 A. Licht et al. decrease of the volcanic input from the local, Burmese in the eastern Himalayan syntaxis may reflect deformed Wuntho-Popa Arc, continuing until the early Miocene. long-lived relics of precollisional river courses, thus This gradual decrease is coeval with an intense period of emphasizing the importance of horizontal, large-scale deformation and exhumation of the metamorphic belts shearing in the processes that built the eastern Himalayan that extend along the Sino-Burman Ranges (Barley et al., syntaxis (Hallet & Molnar, 2001). These conclusions are 2003; Searle et al., 2007), where both the relics of the radically different from those of Liang et al. (2008) and Wuntho-Popa Arc and the Burma Terrane basement are Robinson et al. (2014) who argued for a former Tsangpo- currently exposed. We thus propose that the deposits in Irrawaddy connection after identifying hafnium isotopic the Minbu Sub-Basin, and more generally in the CMB, values typical of Transhimalayan batholiths (eHf >5) in have been supplied by the denudation of these belts, first middle Eocene to early Miocene Burmese detrital zircons. in response to strike-slip deformation along the Sino-Bur- However, the latter authors failed to recognize a potential man Ranges until the emplacement of the Sagaing fault in Wuntho-Popa arc provenance because published eHf data the middle Miocene (Bertrand et al., 2001) and then in from central Myanmar volcanic rocks are nonexistent. response to the uplift of the Sino-Burman Ranges follow- The few published Sr isotopic ratios and eNd values from ing the growth of the Eastern Tibetan Plateau (Rangin the Wuntho-Popa volcanic rocks in central Myanmar et al., 2013). indicate that the Transhimalayan and Wuntho-Popa arcs Our data show that central Myanmar experienced a had similar isotopic values and shared a similar origin, major drainage reorganization, likely dated in the early suggesting that they also displayed similar eHf values Oligocene, with a shift from West-to-East oriented to (Mitchell et al., 2012; Wang et al., 2014). North-to-South oriented river systems. However, this The paucity of sediment from Tibet in the proto-Ben- drainage reorganization seems not to have significantly gal Bay before the Miocene epoch is noteworthy (Najman impacted the locus of the main sedimentary sources. The et al., 2008), but does not require the absence of Tibetan- gradual decrease of volcaniclastic input during the Oligo- sourced drainage exiting in the Bengal Bay, because sedi- cene shows that the sedimentary supply from the newly ment from Tibet may have exited into an independent uplifted Indo-Burman Ranges (rich in volcaniclastic delta fan located north of the modern Bengal fan (as seen rocks) has always been exceeded by the sedimentary sup- in the western Himalayan syntaxis; e.g. Roddaz et al., ply from the Sino-Burman Ranges, likely due to higher 2011), and later subducted below the Indo-Burman rainfall and exhumation in the East. However, we do not Ranges (Fig. 2b; Uddin & Lundberg, 1998). Pre-Miocene exclude that this major drainage reorganization may have drainage along the Indus-Tsangpo Suture zone may also been recorded by a significant but ephemeral (< a few Ma) have exited westward, into the western Himalayan syn- volcaniclastic input from the Indo-Burman Ranges that taxis (Wang et al., 2013); the Indus-Tsangpo Suture Zone was not identified in this study due to the resolution limits may also have been internally drained (deposits of the of our stratigraphic sampling. Kailas Formation along the suture; e.g. Carrapa et al., Our proxies do not exclude an ephemeral input in the 2014) and only later connected to the Bengal Fan. Minbu Sub-Basin from the Tibetan region in the late Oli- gocene, or an extremely diluted input in the Neogene, but both hypotheses are at odds with Burmese sedimentation ACKNOWLEDGEMENTS rates and with evidence of a Tsangpo-Brahmaputra connection since the Miocene. Our study also does not for- This work has been supported by the ANR-09-BLAN- mally rule out an ephemeral Tibetan supply into the 0238-02 Program, the CNRS UMR 7262, the University nearby Pegu Yoma Sub-Basin located to the east of the of Poitiers, the Ecole Polytechnique, and the Ministry of Minbu Sub-Basin, prior its inversion in the late Miocene; Culture of the Republic of the Union of Myanmar. We however, it is difficult to imagine how such a Tibetan- thank Catherine Zimmermann, Christiane Parmentier sourced system would have avoided merging with the and Aimeryc Schumacher for their technical support. We river systems flowing down the Sino-Burman Ranges and thank the many colleagues of the Franco-Burmese pale- into the Minbu Sub-Basin. ontological team who helped us in the field. P. Huyghe, Therefore, these observations suggest that the central E. Garzanti, and P. van der Beek are gratefully thanked Myanmar drainage basin remained closed and did not for fruitful discussions and comments. experience any major capture reorganization since the beginning of the India-Asia collision. The Oligocene rise of the Indo-Burman Ranges is likely to have terminated REFERENCES the direct connection of the Burmese drainage basin to the proto-Bengal Bay. The stable nature of Myanmar ALLEN, R., NAJMAN, Y., CARTER, A., BARFOD, D., BICKLE, M.J., drainage, despite the rapidly-evolving tectonic history of CHAPMAN, H.J., GARZANTI, E., VEZZOLI, G., ANDO,S.&PAR- the eastern Himalayan syntaxis, suggests that the role of RISH, R.R. (2008) Provenance of the Tertiary sedimentary drainage reorganization in explaining the pattern of east- rocks of the Indo-Burman Ranges, Burma (Myanmar): bur- ern Tibetan river courses may have been overestimated man arc or Himalayan-derived? J. Geol. Soc. London, 165, – (Clark et al., 2004). 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© 2014 The Authors 12 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cenozoic evolution of the central Myanmar drainage system

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© 2014 The Authors 14 Basin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists Cenozoic evolution of the central Myanmar drainage system

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