University of Arizona ......CMB comprises two lateral troughs of Cenozoic pull-apart sub-basins, including the Minbu Sub-Basin, where a 15 km thick succession of Cenozoic deposits

Cenozoic evolutionof the central Myanmardrainagesystem: insights fromsediment provenance in theMinbu Sub-BasinAlexis 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 P�etrologiques et G�eochimiques, Universit�e de Lorraine, Vandoeuvre les Nancy,France‡Institut de Pal�eoprimatologie, Pal�eontologie Humaine: Evolution et Pal�eoenvironnements, Universit�e dePoitiers, 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 rele-

vant 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

Understanding the processes and products of erosion of a

mountain belt has been shown to be critical for decipher-

ing the various causes and mechanisms of orogenesis

(Garzanti et al., 2007). This is particularly true for the

Indo-Asian orogen, for which numerous different moun-

tain-building mechanisms have been proposed, each

resulting in a specific timing and extent of erosion, for

example lateral extrusion in the Oligo-Miocene (Tappon-

nier et al., 1986), long-term Cenozoic continental under-

thrusting (Zhao & Nelson, 1993) or lower crustal flow

starting in the late Miocene (Royden et al., 1997). Sedi-ment provenance studies provide a tool for unravelling

the respective impacts of capture, deformation and exhu-

mation on river geometry and sourcing (Hallet & Molnar,

2001; Clark et al., 2004); they thus help to explore topo-

graphic evolution and related denudation in response to

uplift (Najman, 2006; Clift et al., 2008). In South and

East Asia, provenance studies have mostly focused on the

history of the Red River drainage system as inferred from

sediment in the Hanoi Basin (Clift et al., 2006, 2008; Ho-

ang et al., 2009), on the Ganges River drainage in the

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; Braccialiet al., 2013; Chirouze et al., 2013), or on the Indus River

in the Indus fan delta (Clift et al., 2001; Roddaz et al.,

Correspondence: Alexis Licht, Department of Geosciences,University of Arizona, Tucson, AZ 85721 USA. E-mail: [email protected]

© 2014 The AuthorsBasin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists 1

Basin Research (2014) 1–15, doi: 10.1111/bre.12108

2011). In contrast, only a few studies have been done on

the Irrawaddy River in central Myanmar and on sediment

provenance in the Central Myanmar Basin (CMB).

Located at the transition between the Himalayan orogen

and the Indochinese margin, the CMB is separated from

the Bengal Basin by the Indo-Burman Ranges (Fig. 1)

and includes two troughs of pull-apart sub-basins that

have been quasi-continuously filled during the Tertiary

(Pivnik et al., 1998). Tracing the sediment sources of

these deposits would thus help to reconstruct the denuda-

tion and the geomorphologic evolution of the ranges at

the edge of the Indo-Asian collision zone.

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 dis-

cussion 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).

Using U/Pb dating and Hf isotopic measurements of

detrital zircons from various scattered localities in the

CMB, Liang et al. (2008) and Robinson et al. (2014) sug-gest that such a connection existed but was lost in the

early Miocene. However, this hypothesis is difficult to

evaluate because Hf isotopes data of potential Burmese

parent rocks are quasi-nonexistent (Wang et al., 2014).On the basis of U/Pb ages, Hf isotopic data and petro-

graphic analyses of detrital grains along with paleocurrent

measurements and Nd-Sr isotopic analyses of bulk sedi-

ment, several studies in the Indo-Burman Ranges (Allen

et al., 2008; Naing et al., in press), and in the northern

(Wang et al., 2014) and southern extensions of the CMB

(Licht et al., 2013) have shown that, during the Eocene,

central Myanmar was open to the Indian Ocean and

recorded the local unroofing of an Andean-type cordillera

that extended along the Burmese margin (Fig. 2c). How-

Fig. 1. Map of Southeast Asia showing the Bengal Basin and

the Central Myanmar Basin (BB and CMB, respectively, yellow

shaded areas), the Indo-Burman Ranges (IBR), the Tsangpo-

Brahmaputra and Irrawaddy Rivers and their possible connec-

tion through the eastern Himalayan syntaxis (EHS).

(a) (b) (c)

Fig. 2. Cenozoic drainage variations in the Bengal Bay since the India-Asia collision, showing the main structural units in the neigh-

bouring 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 AuthorsBasin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists2

A. Licht et al.

ever, these studies do not provide information concerning

potential Oligo-Miocene river capture.

This study focuses on the provenance of sediments in

the Minbu Sub-Basin of the CMB in southern Myanmar.

This sub-basin is currently traversed along its entire

length by the Irrawaddy River. Sandstone modal analysis,

trace element geochemistry and Nd isotopic analysis of

bulk sediment samples, together with data compilations

from the literature, were used to study the sediment prov-

enance of eleven units, ranging in age from middle Eocene

to Pleistocene. Our goal was to understand the evolution

of the sedimentary supply to southern Myanmar and thus

to reconstruct the denudation of the neighbouring areas

and to test the capture hypothesis.

OVERVIEWOF BURMESEGEOLOGY

Most of central Myanmar is composed of the Burma Ter-

rane, which currently includes the CMB (Fig. 3a). The

CMB comprises two lateral troughs of Cenozoic pull-

apart sub-basins, including the Minbu Sub-Basin, where

a 15 km thick succession of Cenozoic deposits is found

(Pivnik et al., 1998). East of the Minbu Sub-Basin, the

Pegu Yoma Sub-Basin has been inverted in the late Mio-

cene and constitutes a local (and recent) topographic high

in the CMB (Khin & Myitta, 1999). At the eastern edge

of central Myanmar, the Sino-Burman Ranges comprise

the Tenasserim highlands, the Shan Plateau and the Yun-

nan highlands, all of which are local units of the Sibumasu

Terrane (eastern unit of the Indochina Peninsula; Met-

calfe, 2013) and mainly consist of Paleozoic to Cretaceous

metasediments and plutons (Bender, 1983). Between the

Sino-Burman Ranges and the Central Myanmar Basin,

the basement of the Burma Terrane crops out as belts of

metamorphic rocks: the Slate and the Mogok Metamor-

phic Belts in the south and the Gaoligong Belt in the

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

plutons of the Wuntho-Popa Arc (Fig. 3b), which is con-

sidered as the eastern continuation of the Transhimalayan

Arc of Tibet and contains relics of the Andean-type volca-

nic arc of the Indo-Asian subduction zone (Zaw, 1990;

Mitchell et al., 2012; Ma et al., 2014; Wang et al., 2014).At the western edge of Myanmar, the Central Myanmar

Basin is separated from the Bengal fan by the Indo-

Burman Ranges, which form a Cenozoic accretionary

(a) (b) (c)

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) displayingthe localities of sampling for each stratigraphical unit.


Cenozoic evolution of the central Myanmar drainage system

complex produced during the subduction of the Indian

plate beneath the Burma Terrane (Maurin & Rangin,

2009). Paleocurrents and sediment ages indicate that a

first emergence of the Indo-Burman Ranges must have

occurred between the terminal Eocene and the early Mio-

cene (Allen et al., 2008; Licht et al., 2013); seismic data

in the Indo-Burman Ranges indicate much more rapid

uplift since late Miocene time (Maurin & Rangin, 2009).

During the Cenozoic Era, central Myanmar was

dragged by both the northward movement of the Indian

Plate and the right lateral extrusional motion of Indo-

china, resulting in intense strike-slip deformation and ca.30° clockwise rotation relative to China (Richter & Fuller,

1996; Morley, 2009). Until the middle Miocene, strike-

slip deformation was accommodated by the subsidence of

pull-apart basins (Rangin et al., 1999) and caused high

temperature metamorphism and exhumation of the

Burma Terrane basement along the metamorphic belts

that extend along the Sino-Burman Ranges (Searle et al.,2007). Deformation was particularly significant along the

Mogok Metamorphic Belt, where mineral growth ages

indicate high temperature metamorphism from the late

Eocene to the middle Miocene (Bertrand et al., 2001;

Barley et al., 2003; Searle et al., 2007). Since the middle

Miocene, spreading in the Andaman Sea and develop-

ment of the Sagaing Fault along the Sino-Burman Ranges

accommodated the strike-slip motion (Khan & Chakr-

aborty, 2005). Further exhumation of the metamorphic

belts in the late Neogene is attributed to the uplift of the

Sino-Burman Ranges in response to the growth of the

Eastern Tibetan Plateau and Tibetan crustal flow into

Southeast Asia (Rangin et al., 2013); this late event is alsothe possible cause for the late Miocene inversion of the

Pegu Yoma Sub-Basin and for numerous smaller inverted

structures in the Minbu Sub-Basin (Yenangyat & Yen-

angyang Anticlines, Pondaung Ranges; Pivnik et al.,1998). The total amount of dextral, northward displace-

ment of central Myanmar along the Sino-Burman Ranges

is estimated to be from 300 to about 1100 km (Mitchell,

1993; Curray, 2005; Morley, 2009) and varies in accor-

dance with the preferred Asian paleogeographic models

(Replumaz & Tapponnier, 2003; Hall, 2012).

SAMPLING STRATEGYANDMETHODS

Sediment samples from the Minbu Sub-Basin were col-

lected in the Pakokku District, central Myanmar, along a

ca. 200 km line that roughly follows the modern North-

South axis of the Sub-Basin (Fig. 3b). Our dataset covers

eleven geological units, spanning in age from middle

Eocene to Pleistocene (Fig. 3c). Due to the poor rock

exposure in the sub-basin, sediments from this entire time

span could not be obtained from a single site and were

instead sampled in three main locations. Sampling of

Eocene and lower Oligocene sediments was limited to the

Pondaung Ranges, near the confluence between the Irra-

waddy River and the Chindwin River, one of its main

tributaries (sites 1–3 on Fig. 3b; detailed maps in Licht

et al., 2013, 2014). Upper Oligocene and lower Miocene

sediments were sampled in the Yenangyat Anticline (site

4 on Fig. 3b), along the modern stream of the Irrawaddy

River. Middle Miocene to Pleistocene sediments were

sampled in the Pondaung Ranges (site 3) and along the

Irrawaddy River further South (sites 5 and 6). Potential

geographical bias in our results caused by our sampling

locations is adressed in the discussion.

Thin sections of 28 sandstone samples were prepared

and counted according to the Gazzi-Dickinson method to

determine their contents of quartz, feldspar and lithic

grains (Dickinson, 1985); at least 300 grains were counted

per section.

Five to eight samples per formation were selected for

analysis of the bulk Nd isotopic ratios of their silicate frac-

tions and their whole rock trace element contents (62 sam-

ples in total). Samples from different locations as well as

samples of various lithofacies were chosen. Powdered sed-

iments were analysed for trace elements using a Thermo

X7 ICP-MS at the Service d’Analyse des Roches et des

Min�eraux (SARM-CRPG, Vandoeuvre-les-Nancy,

France). Precisions are better than 5–10% for nearly all

elements reported. Chemical extraction of Nd and Nd

isotopic analyses were performed at the CRPG, according

to the standard procedures of the laboratory (see Licht

et al., 2013). Briefly, after decarbonation with HCl and

dissolution in a mixture of HF, HNO3 and a small amount

of HClO3, Nd was separated using Eichrom TRU-spec

and Ln-spec resins. Nd isotopic compositions were mea-

sured using a Neptune Plus MC-ICP-MS. Nd isotopic

ratios are normalized to 146Nd/144Nd = 0.7219. During

the period of measurement the JNdi Nd standard gave a

mean value of 143Nd/144Nd = 0.512086 � 0.000011

(2r). To allow comparison with data from other laborato-

ries, 0.000029 was added to each 143Nd/144Nd result to

make our data consistent with a value of 0.512115 for the

JNdi standard, which is equivalent to a value of 0.511858

for the La Jolla Nd standard (Tanaka et al., 2000). Nd

procedural blanks represented less than 1% of the amount

of Nd measured in the samples and were thus insignifi-

cant. Ages, localities, GPS coordinates and dominant

lithologies are given in Table 1.

RESULTS

Isotopic results and trace element contents are given in

Table 2, grain-counting results are given in Table 3.

Samples already published in Licht et al. (2013) have

been highlighted in both tables (17 isotopic and 5

petrographic Eocene data). As samples from each forma-

tion were taken from different localities, their relative

stratigraphic positions were often not evident. For this

reason, values for each formation were grouped together,

and no attempt was made to investigate intraformational


A. Licht et al.

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

Oligocene

PAD1 Clay 3 N21°41032.5″, E094°43001.0″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

(Bartonian)

TH2* Clay 2 N21°45041.0″, E094°50029.4″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)



variations. Therefore temporal changes can only be exam-

ined at time scales similar to or greater than those of the

formations (i.e., 2 to 5 Myr).

Point-counting results are plotted on Q-F-L and Lm-

Lv-Ls diagrams (Fig. 4). The evolution of the grain pro-

portions shows a regular shift from lithic volcaniclastic

detritus in the middle Eocene to lithoquartzose metamor-

phiclastic/sedimentaclastic orogenic detritus in the Mio-

Pliocene. Carbonate lithic fragments were detected in

insignificant proportions in all the samples (<1%).

Middle Eocene sediments (sites 1 and 2) display eNd

values ranging from �7.8 to +1 over the 43–37 Ma per-

iod, (average �3.4, n = 16), while those of upper Eocene

– Oligocene sediments (sites 2, 3 and 4) vary from �9 to

�2.4 over the 37–22 Ma period (average �5.9, n = 21).

Taken together, the Eocene – Oligocene samples show a

gradual shift of eNd from near zero to moderately nega-

tive values (Fig. 5). The eNd values of Mio-Pliocene sed-

iments (sites 3, 5 and 6) range from �13.7 to �5.9 over

the last 22 Ma (average �8.1, n = 25), and in contrast

with the Eocene – Oligocene samples show no systematic

temporal variations resolvable at our sampling density.

The average eNd value of each individual unit within this

time interval is stable at about �8 to �9 (with the excep-

tion of the Obogon unit: average�6.4, n = 5).

We focus the discussion of the trace element results on

the Zr/TiO2 and La/V ratios, which are sensitive to the

mafic character of the detritus and the sedimentary cycle:

low ratios indicate mafic contributions and high ratios

represent mature, recycled material (e.g. Dingle & Lav-

elle, 1998; Zhang, 2004). The Zr/TiO2 and La/V ratios

exhibit similar temporal trends among the Burmese sam-

ples (Fig. 5), increasing from low values in middle Eocene

times to high values in the upper Miocene-Pliocene inter-

val (from 0.1 to 0.5 and from 200 to 300, respectively).

INTERPRETATION

Middle Eocene, upper Eocene – Oligocene and Mio-Plio-

cene sediments present contrasting characteristics reflect-

ing different provenances (Table 4). Two main Burmese

geographic provinces, with distinctive geological features,

are considered as potential local sources for the sediment

deposited in the Minbu Sub-Basin: The Sino-Burman

Ranges (including also the Wuntho-Popa Arc and the

metamorphic belt rocks that crop out along the ranges)

and the Indo-Burman Ranges (Figs 2 and 3) since the

Oligocene (Allen et al., 2008; Licht et al., 2013). Any

Neogene input from the Indus-Tsangpo Suture Zone is

likely to display similar features to the modern Tsangpo

River load before its connection to the Brahmaputra in

the Siang Gorges, with eNd around -11 (Singh & France-

Lanord, 2002) resulting from the mixed contribution of

sedimentary, metamorphic and carbonate rock fragments

from the Himalayan, Transhimalayan and Tibetan areas.

Considering the probable late Paleogene exhumation of

the Himalayan Ranges (Najman et al., 2008), older inputsare expected to be more depleted in Himalayan sourced

sediment and thus to be dominated by Transhimalayan

and Tibetan rock fragments, with higher eNd (from �10

to +8) and lower metamorphic lithic content (Table 4).

Middle Eocenesediments

Middle Eocene samples are rich in volcanic rock frag-

ments and display similar petrographic and isotopic fea-

tures to Paleogene flysch sediments of the Indo-Burman

Ranges, mainly constituted of volcaniclastic sandstones

with mildly negative eNd values (�7 to �4; Allen et al.,2008). Considering that the Indo-Burman Ranges were

not emerged at that time, these results indicate a similar

sediment source for both regions (Allen et al., 2008; Lichtet al., 2013). The presence of occasional positive eNd val-

ues indicates a contribution from magmatic rocks of the

Paleogene Andean-type arc that extended along the Asian

margin, located north and east of the Minbu Sub-Basin

(Ji et al., 2009; Mitchell et al., 2012; Ma et al., 2014).The northern (Transhimalayan) and eastern (Wuntho-

Popa Arc) sections of this volcanic arc cannot be easily

differentiated on the basis of Nd isotopes. Transhimalay-

an provenance could have been achieved if the morpho-

logical setting was similar to modern Bengal geography,

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-

tern, which parallels the convergence zone, is

nevertheless unlikely during the Eocene before the emer-

gence of the Indo-Burman Ranges. Licht et al. (2013)show that the orientation of Eocene delta systems,

inferred through paleocurrent analysis, indicates a source

area located to the east, that is, on the Burmese margin.

The lithic volcaniclastic sediments are therefore better-

explained by local supply from the unroofing of the

Wuntho-Popa Arc (Wang et al., 2014). Negative eNd val-

ues and low-grade metamorphic material observed by

Licht et al. (2013) highlight a minor contribution from a

secondary source in the Burmese substratum.

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 –


A. Licht et al.

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 *)

Formation

Sample

Name

Ti

(%)

La

(ppm)

V

(ppm)

Zr

(ppm)

Nd

(ppm)

Sm

(ppm) 143Nd/144Nd

Standard

error (2r) εΝd

Upper

Irrawaddy

BRI01 0.776 33.9 105.1 203.9 30.6 6.2 0.512335 0.000006 �5.92

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

Irrawaddy

HOM03 0.841 33.6 118.0 214.6 30.2 6.0 0.512218 0.000009 �8.19

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

(continued)



Upper Eocene –Oligocene sediments

Upper Eocene – Oligocene sediments display eNd values

intermediate between those of Mio-Pliocene and middle

Eocene sediments. The occurrence of mildly negative

eNd in several Oligocene samples, notably those of the

Shwezetaw Formation (ca. 31–30 Ma), could reflect

minor input from the rising Indo-Burman Ranges, which

would deliver reworked, volcanic-sourced sediment

(Fig. 2b). Nevertheless, this early unroofing of the Indo-

Burman Ranges must have been limited, given the low

Oligocene sediment accumulation rate in central Myan-

mar (ca. 5 km3 kyr�1, Fig. 5; M�etivier et al., 1999).

Sandstones display increasing quartz grain abundance,

and follow a clear unroofing trend that indicates the

progressive denudation of the volcanic areas in the drain-

age basin. This interpretation is compatible with the ris-

ing Zr/TiO2 and La/V ratios during this time period,

representing an increase in quartz, metamorphic and sedi-

mentary rock fragments interpreted to be caused by the

progressive loss of a mafic source. These results therefore

suggest that after advanced unroofing of the Wuntho-

Popa Arc, river incision cut into metamorphic and recy-

cled sedimentary rocks of the basement of the Southeast

Asian terranes. However, an ephemeral input from the

Tibetan region after the probable uplift of the Indo-

Burman Ranges at 31–30 Ma cannot be excluded because

themixed sources of the lower Oligocene Burmese samples

display similar geochemical and petrographic features to

those inferred for the Tsangpo River precursor (Table 4).

Table 2 (continued)

Formation

Sample

Name

Ti

(%)

La

(ppm)

V

(ppm)

Zr

(ppm)

Nd

(ppm)

Sm

(ppm) 143Nd/144Nd

Standard

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


A. Licht et al.

Mio-Pliocenesediments

The small proportion of lithic material in the Mio-Plio-

cene sediments contrasts with what is observed in the Pal-

eogene samples; it also contrasts with the loads of the

modern rivers of the Tsangpo drainage basin, which are

commonly enriched in sedimentary and carbonate lithic

fragments (Garzanti et al., 2004; Fig. 4). The paucity of

lithic fragments also indicates that the Mio-Pliocene

deposits did not form from tributaries flowing down the

neighbouring, newly uplifted Indo-Burman Ranges,

which yield sediments enriched in volcanic rock frag-

ments with mildly negative eNd values (�7 to �4; Allen

et al., 2008). Mio-Pliocene units display, like the Quater-

nary Irrawaddy sediment load, more abundant orogenic

detritus enriched in quartz, metamorphic and sedimentary

rock fragments, stable average eNd values and an eNd

range that is similar to the range of the Irrawaddy load

(�11 to �8; Colin et al., 2006; Allen et al., 2008). Thesedata indicate that sources have remained relatively

unchanged over the last 22 Ma at the study sites and sug-

gest a prominent, stable provenance area located in the

Sino-Burman Ranges, where the modern Irrawaddy River

is sourced.

DISCUSSION

Localgeographic variationsorabroadtemporal trend?

The variation of sediment provenance identified in the

different geological units can be interpreted as reflecting

either long-term temporal variation of the sediment sup-

ply in the Minbu Sub-Basin, or local geographical varia-

tions caused by the relatively scattered character of our

sampling localities. This question is particularly relevant

for the sediment deposited after the Oligocene uplift of

the Indo-Burman Ranges and the development of a south-

ward oriented drainage system in central Myanmar,

merging water supplies from several distant sources.

Standard geomorphological observations and our geo-

chemical results can help us to distinguish whether our

sampling localities correspond to the main stem of past

fluvial systems of the CMB, or instead to local tributaries

flowing down the Indo-Burman or the Sino-Burman

Ranges.

Our localities in the Pondaung Ranges (sites 1, 2 and 3)

are located close to the confluence between the Irrawaddy

and the Chindwin Rivers. The Chindwin River currently

flows down the Indo-Burman Ranges and may have pro-

vided an additional local supply of volcanic reworked

clasts in the sampling localities 1, 2 and 3 (Fig. 3).

However, its existence necessarily post-dates the Oligo-

cene emergence of the Indo-Burman Ranges and its con-

tribution to the Eocene supply in the Minbu Sub-Basin

is thus unlikely. Increased volcanic lithic content (Fig. 4)

and eNd values (Fig. 5) in the lower Oligocene (ca. 31–30 Ma) Shwezetaw Formation are here suggested to

reflect a short-term volcaniclastic input following the first

emergence of the Indo-Burman Ranges; samples from

the only post Shwezetaw units in the Pondaung Ranges

(namely the Padaung and Kyaukkok Formations, see

Fig. 3c) do not show any significant volcanic input. Neo-

gene samples from other localities also do not display an

Indo-Burman Range fingerprint. Thus, we argue that a

local contribution from the Chindwin River or any other

past tributary flowing down the Indo-Burman Ranges

was insignificant in our samples, except probably for the

lower Oligocene Shwezetaw Formation that may reflect

the first emergence episode of the Ranges, as discussed

above.

Finally, we consider whether the sampled outcrops cor-

respond to sediment deposited by the main stems of past

CMB drainage systems, or just by small tributaries flow-

ing down the closest highs of the Sino-Burman Ranges,

on the east side of the CMB. The latter hypothesis implies

that these tributaries would have flowed westward from

the Shan Plateau to the Minbu Sub-Basin through the

neighbouring Pegu Yoma Basin, located between the two

regions (Fig. 3b). Their waters would have then exited in

the main stem of the ancient drainage system, located

throughout this time to the west of the sampling localities.

However, paleocurrents and stratigraphic architecture of

the Miocene deposits in the Pegu Yoma Basin indicate

southward directed fluvial systems (Khin & Myitta, 1999)

and contradict this hypothesis.

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).



Thus, we argue that our different sites are exempt of

significant local bias in past sedimentary supply. Sampled

sediments were first deposited by westward directed del-

taic systems before the uplift of the Indo-Burman Ranges

in the Oligocene (Licht et al., 2013), then by southward

directed fluvio-deltaic systems, precursors to the modern

Irrawaddy River. The variation of sediment provenance

identified between the different units is therefore inter-

preted as reflecting the temporal evolution of the mean

sedimentary supply in the Minbu Sub-Basin.

Implications for the evolutionof thesedimentarysupply

Our results show a long-term decrease of the volcanic

input and an increasing input from basement rocks into

the Minbu Sub-Basin until the early Miocene. This

gradual change is interpreted as corresponding to the

progressive denudation of the Wuntho-Popa Arc, located

along the Sino-Burman Ranges (Fig. 2). The emergence

of the Indo-Burman Ranges and the shift from westward

directed deltaic systems to southward directed fluvio-

deltaic systems, which our data suggest occurred around

30–31 Ma, is thus coeval with a long-term, somewhat

erratic exhumation of the Sino-Burman Ranges. These

data do not disallow the possibility that Tibetan and

Himalayan sourced sediments may have contributed to

sedimentary supply of the Minbu Sub-Basin between the

emergence of the Indo-Burman Ranges and the early Mio-

cene, because Oligocene Burmese samples display similar

geochemical and petrographic features to those inferred

for the Tsangpo River precursor. However, an ephemeral

capture of Tsangpo waters would significantly extend the

Burmese drainage basin and increase the sedimentary sup-

ply in central Myanmar. This scenario is incompatible

with the evolution of Burmese sedimentation rates that

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, fromM�etivier 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 inthe 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 repre-

sented (values in Table 4). IBR: Indo-Burman Ranges; SBR: Sino-Burman Ranges; ETP: Eastern Tibetan Plateau.


A. Licht et al.

reached their lowest level in the Oligocene (Fig. 5; M�eti-vier et al., 1999).

Since the early Miocene, our data highlight a stable

source for the sediment in the Minbu Sub-Basin similar

to the modern Irrawaddy load provenance. A minor sup-

ply from the Indus-Tsangpo Suture Zone, diluted by

these proximal sources, can once again not be excluded,

but would contradict the increasing evidence of a stable

Tsangpo-Brahmaputra connection since the early Mio-

cene (Uddin & Lundberg, 1998; Galy et al., 2010; Bracci-ali et al., 2013). These results thus argue for the stabilityof the sediment provenance in the Minbu Sub-Basin. The

lack of any significant input from the Indo-Burman

Ranges, characterized by mildly negative eNd values, in

the Burmese Mio-Pliocene units and in the modern Irra-

waddy load (Allen et al., 2008) contrasts with the minor,

yet significant Indo-Burman input recorded in the Mio-

Pliocene deposits of the Bengal Basin (Uddin & Lundberg,

1998; Najman et al., 2012). This difference can be

explained by the asymmetric precipitation pattern on the

two sides of the Indo-Burman Ranges (Koons, 1995): on

the Bengal, windboard side of the ranges, monsoonal pre-

cipitation can reach 10 times or more the amount of rain-

fall over the outboard, Burmese side (where annual

rainfall is commonly <800 mm). Most of the rainfall in

the Irrawaddy drainage basin occurs over the Sino-Bur-

man Ranges, where denudation is much more important

(Stamp, 1940).

CONCLUSION

Trace element geochemistry, Nd isotopes, and sandstone

modal compositions of middle Eocene to Quaternary sedi-

ment samples from central Myanmar provide no evidence

of a dramatic provenance shift but highlight a gradual

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

& Shan Plateau series

Ultramafic rocks, low and high grade

metamorphic rocks, S-Type granitoids,

Precambrian to Mesozoic metasediments

�13 to�3

Wuntho-Popa arc Volcanic rocks, I-type granitoids 0 to +8-Indo-Burman Ranges (Inner wedge) Mainly volcanic sediment �7 to�4

Central Tibet domain-Lhasa Terrane basement S-Type granitoids, Palaeozoic to

Mesozoic metasediments

�10 to 0

-Transhimalayan arc & Indus-

Tsangpo Suture Zone

Volcanic rocks, I-type granitoids,

ophiolites

+1 to +8

Himalayas-Tethyan Sedimentary Series

and Higher Himalaya

Medium - high grade metamorphic rocks,

Precambrian to Eocene sediment

�19 to�5 (av.�15)

-Lesser Himalaya Low grade metamorphic rocks,

Palaeoproterozoic sediment

�27 to�21

River loads Sandstone Petrography Bulk rock eNd

Modern Irrawaddy River Litho-feldspatho-quartzose with

metamorphic>sedimentary>volcanic lithics�11 to�8

Modern Tsangpo River & tributaries Quartzo-lithic to Feldspatho-litho-quartzose

with metamorphic>volcanic>sedimentary lithics

�11

Hypothetical “old” Tsangpo River Quartzo-lithic volcaniclastic/ophioliticlastic >�11

Minbu Sub-Basin sediments Sandstone Petrography Bulk rock eNd

Middle Eocene rocks

(Tabyin & Pondaung Fm)

Feldspatho-quartzo-lithic with

volcanic>sedimentary

& metamorphic lithics

�7 to +1

Upper Eocene – Oligocene rocks(Yaw, Shwezetaw,

Padaung & Okhmintaung Fm)

Feldspatho-quartzo-lithic

with sedimentary>volcanic & metamorphic lithics

�9 to�2

Mio-Pliocene rocks

(Pegu Group & Irrawaddy Fm)

Feldspatho-litho-quartzose with

metamorphic>sedimentary>volcanic lithics�13 to�6



decrease of the volcanic input from the local, Burmese

Wuntho-Popa Arc, continuing until the early Miocene.

This gradual decrease is coeval with an intense period of

deformation and exhumation of the metamorphic belts

that extend along the Sino-Burman Ranges (Barley et al.,2003; Searle et al., 2007), where both the relics of the

Wuntho-Popa Arc and the Burma Terrane basement are

currently exposed. We thus propose that the deposits in

the Minbu Sub-Basin, and more generally in the CMB,

have been supplied by the denudation of these belts, first

in response to strike-slip deformation along the Sino-Bur-

man Ranges until the emplacement of the Sagaing fault in

the middle Miocene (Bertrand et al., 2001) and then in

response to the uplift of the Sino-Burman Ranges follow-

ing the growth of the Eastern Tibetan Plateau (Rangin

et al., 2013).Our data show that central Myanmar experienced a

major drainage reorganization, likely dated in the early

Oligocene, with a shift from West-to-East oriented to

North-to-South oriented river systems. However, this

drainage reorganization seems not to have significantly

impacted the locus of the main sedimentary sources. The

gradual decrease of volcaniclastic input during the Oligo-

cene shows that the sedimentary supply from the newly

uplifted Indo-Burman Ranges (rich in volcaniclastic

rocks) has always been exceeded by the sedimentary sup-

ply from the Sino-Burman Ranges, likely due to higher

rainfall and exhumation in the East. However, we do not

exclude that this major drainage reorganization may have

been recorded by a significant but ephemeral (< a few Ma)

volcaniclastic input from the Indo-Burman Ranges that

was not identified in this study due to the resolution limits

of our stratigraphic sampling.

Our proxies do not exclude an ephemeral input in the

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

rates and with evidence of a Tsangpo-Brahmaputra con-

nection since the Miocene. Our study also does not for-

mally rule out an ephemeral Tibetan supply into the

nearby Pegu Yoma Sub-Basin located to the east of the

Minbu Sub-Basin, prior its inversion in the late Miocene;

however, it is difficult to imagine how such a Tibetan-

sourced system would have avoided merging with the

river systems flowing down the Sino-Burman Ranges and

into the Minbu Sub-Basin.

Therefore, these observations suggest that the central

Myanmar drainage basin remained closed and did not

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

the direct connection of the Burmese drainage basin to

the proto-Bengal Bay. The stable nature of Myanmar

drainage, despite the rapidly-evolving tectonic history of

the eastern Himalayan syntaxis, suggests that the role of

drainage reorganization in explaining the pattern of east-

ern Tibetan river courses may have been overestimated

(Clark et al., 2004). Instead, the tight loop of the Tsangpo

in the eastern Himalayan syntaxis may reflect deformed

long-lived relics of precollisional river courses, thus

emphasizing the importance of horizontal, large-scale

shearing in the processes that built the eastern Himalayan

syntaxis (Hallet & Molnar, 2001). These conclusions are

radically different from those of Liang et al. (2008) andRobinson et al. (2014) who argued for a former Tsangpo-

Irrawaddy connection after identifying hafnium isotopic

values typical of Transhimalayan batholiths (eHf >5) inmiddle Eocene to early Miocene Burmese detrital zircons.

However, the latter authors failed to recognize a potential

Wuntho-Popa arc provenance because published eHf data

from central Myanmar volcanic rocks are nonexistent.

The few published Sr isotopic ratios and eNd values from

the Wuntho-Popa volcanic rocks in central Myanmar

indicate that the Transhimalayan and Wuntho-Popa arcs

had similar isotopic values and shared a similar origin,

suggesting that they also displayed similar eHf values

(Mitchell et al., 2012; Wang et al., 2014).

The paucity of sediment from Tibet in the proto-Ben-

gal Bay before the Miocene epoch is noteworthy (Najman

et al., 2008), but does not require the absence of Tibetan-sourced drainage exiting in the Bengal Bay, because sedi-

ment from Tibet may have exited into an independent

delta fan located north of the modern Bengal fan (as seen

in the western Himalayan syntaxis; e.g. Roddaz et al.,2011), and later subducted below the Indo-Burman

Ranges (Fig. 2b; Uddin & Lundberg, 1998). Pre-Miocene

drainage along the Indus-Tsangpo Suture zone may also

have exited westward, into the western Himalayan syn-

taxis (Wang et al., 2013); the Indus-Tsangpo Suture Zonemay also have been internally drained (deposits of the

Kailas Formation along the suture; e.g. Carrapa et al.,2014) and only later connected to the Bengal Fan.

ACKNOWLEDGEMENTS

This work has been supported by the ANR-09-BLAN-

0238-02 Program, the CNRS UMR 7262, the University

of Poitiers, the Ecole Polytechnique, and the Ministry of

Culture of the Republic of the Union of Myanmar. We

thank Catherine Zimmermann, Christiane Parmentier

and Aimeryc Schumacher for their technical support. We

thank the many colleagues of the Franco-Burmese pale-

ontological team who helped us in the field. P. Huyghe,

E. Garzanti, and P. van der Beek are gratefully thanked

for fruitful discussions and comments.

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