University of Arizona ......CMB comprises two lateral troughs of Cenozoic pull-apart sub-basins,...
Transcript of University of Arizona ......CMB comprises two lateral troughs of Cenozoic pull-apart sub-basins,...
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.
© 2014 The AuthorsBasin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists 3
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
© 2014 The AuthorsBasin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists4
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)
© 2014 The AuthorsBasin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists 5
Cenozoic evolution of the central Myanmar drainage system
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 –
© 2014 The AuthorsBasin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists6
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)
© 2014 The AuthorsBasin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists 7
Cenozoic evolution of the central Myanmar drainage system
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
© 2014 The AuthorsBasin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists8
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).
© 2014 The AuthorsBasin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists 9
Cenozoic evolution of the central Myanmar drainage system
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.
© 2014 The AuthorsBasin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists10
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
© 2014 The AuthorsBasin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists 11
Cenozoic evolution of the central Myanmar drainage system
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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© 2014 The AuthorsBasin Research © 2014 John Wiley & Sons Ltd , European Association of Geoscientists & Engineers and International Association of Sedimentologists 15
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