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Structural and tectonic development of the Indo-Burma Ranges 1!
By 2!
C. K. Morley
1
, Tin Tin Naing
2
, M. Searle
2
, S. A. Robinson
2
3!
1 = Department of Geological Sciences, Chiang Mai University, 239 Huaykaew Road, 4!
Chiang Mai, 50200, Thailand. 5!
2 = Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 6!
3AN, UK. 7!
Abstract 8!
9!
The Indo-Burma Ranges form an enigmatic mountain belt, with fragments of evidence for an 10!
early accretionary history (Jurassic Jade belt HP-LT metamorphism; Early Cretaceous 11!
ophiolites; highly deformed Triassic turbidites (Pane Chaung Formation, PCF); Kanpetlet 12!
Schists). It remains uncertain whether this early history involved collision of a 13!
microcontinent (Mt. Victoria Land, MVL), unconformably sealed by Aptian-Cenomanian 14!
limestones, or can be explained entirely as an accretionary-type ophiolite on the western 15!
margin of the West Burma Terrane (WBT). Complex deformation in the deepwater Triassic, 16!
Jurassic, Late Cretaceous, and Paleogene deepwater sequences is replaced in the Late 17!
Eocene-Early Oligocene by molasse deposition. These events mark closure of the Neo-Tethys 18!
ocean between India and the IBR/WBT, and the onset of major dextral translation (>2000 19!
km, 40 Ma-Recent), between the coupled India/IBR/WBT region and Sundaland. In the Late 20!
Miocene-Recent major transpressional deformation affected the IBR and Central Basin of the 21!
WBT. The late deformation events, sedimentary depocentres, and impinging thick crustal 22!
regions of the eastern Himalayas and Shillong Plateau, have all affected the overall shape 23!
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(wedge taper) of the modern IBR, with the wedge and retro-wedge behaving anomalously 24!
compared with typical accretionary prisms. All tectonic models proposed for the IBR/WBT 25!
have weaknesses or ambiguities, and there is considerable scope for future research to resolve 26!
the many outstanding, tectonic, metamorphic, structural, and sedimentary issues. These are 27!
important tasks because the IBR is a key region for understanding the development of 28!
northern Gondwana, the Himalayan orogeny, and SE Asia, as well as providing insights into 29!
the complex development of highly oblique collisional margins. 30!
31!
1. Introduction 32!
The Indo-Burma Ranges (IBR), and the adjacent Central Basin and Wuntho-Popa 33!
Arc, represent a Mesozoic accretionary-forearc basin-arc complex (referred to here as the 34!
West Burma Terrane) related to subduction of various stages of the Tethys ocean, analogous 35!
to the Makran accretionary complex some 3000 km to the west (see reviews in Rangin et al., 36!
2013; Rangin, 2017; Burg, 2018). Parts of both complexes are affected by transpressional 37!
Cenozoic post-accretionary phase tectonics at the eastern and western margins of the India 38!
Plate. As India converged with Eurasia, accretionary prism complexes contiguous with the 39!
Makran and Indo-Burma Ranges became incorporated into the Himalayan Orogen. The 40!
resulting the Indus-Yarlung Suture Zone (IYSZ), comprises 177-150 Ma and 130-80 Ma 41!
ophiolites (Hebert et al., 2012), serpentinite and sedimentary matrix mélanges, and trench and 42!
wedge top basins (Ding et al., 2005; DeCelles et al., 2014; Li et al., 2015). The mountain 43!
belts of the Kirthar, Brahui and Sulaiman ranges in Pakistan lie oblique (N-S to NE-SW) to 44!
the E-W Makran and Himalayan trends, on the west side of the India Plate. These ranges are 45!
equivalent to the NW-SE to N-S trending Indo-Burma Ranges on the east side, in 46!
accommodating the lateral motion of India moving northwards relative to Eurasia. SE Asia 47!
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(Sundaland) and the Afghan Block/Central Iran areas of Eurasia formed southerly continental 48!
protrusions on the eastern and western flanks of the Indian Continent respectively (e.g. 49!
Rangin et al., 2013; Burg, 2018). In the oblique position on the eastern margin, the West 50!
Burma Terrane linked with India as it moved northwards, and underwent considerable strike-51!
slip translation (Rangin et al., 2013; Rangin, 2017, 2018). However, the details of how 52!
deformation evolved within the Indo-Burma Ranges, how much dextral translation has 53!
affected the region, and the tectonic context and timing of emplacement of the fragments of 54!
oceanic crust all remain controversial. Like the Makran (Burg, 2018) and IYSZ (e.g. Hebert 55!
et al., 2012), the Indo-Burma Ranges contain a very important record of the Tethys 56!
subduction history that can be used to test and refine Mesozoic-Cenozoic plate 57!
reconstructions. The Eocene sedimentary record of the forearc in the Central Basin is very 58!
significant for understanding the palaeoclimate history of the region, including development 59!
of the monsoon (e.g. Licht et al., 2018). For both tectonic reconstructions and palaeoclimate 60!
history a much better understanding of the development of the IBR and West Burma Terrane 61!
is needed. 62!
Fundamental challenges to understanding the IBR include: historically highly limited 63!
access by roads and trails; limited exposures in high relief terrain covered by jungle; highly 64!
complex structure; very extensive, highly monotonous flysch units with a wide age-range 65!
(Triassic-Palaeogene) and limited biostratigraphic control (e.g. Brunnschweiler (1966), 66!
Bannert et al. (2011); and access to areas restricted by political unrest. While these issues still 67!
exist, progress with accessibility to some areas has occurred. Road building has created some 68!
new outcrop sections, and some excellent river sections exist. U-Pb dating of zircons and 69!
other dating methods have advanced our understanding of the timing of tectonic, igneous, and 70!
metamorphic events and stratigraphy (Table 1, see review in Zhang, J. et al., 2018 and Licht 71!
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et al., 2018). Geochemical analysis of ‘ophiolites’ has better identified their tectonic setting 72!
(Table 1). 73!
The geological context of tectonic events in the IBR remain open to multiple 74!
interpretations. In this paper, we review the evidence for key structural relationships, and 75!
timing of events, and additionally provide new structural observations that we have made 76!
from fieldwork over two field seasons in the Kanpetlet-Mindat area, and the Kaylemyo area. 77!
We have also added analysis of satellite and Google Earth data. The aim is to provide an 78!
updated overview of the structural development of the core and Inner Belt of the IBR. While 79!
we cannot resolve all the questions that we pose, a review of all the data is important at this 80!
time in order understand what the data presently suggests, and to focus new research to 81!
address key gaps or uncertainties in our understanding. Key basic questions, for which there 82!
are a diversity of unresolved opinions in the literature, include: how the structural styles 83!
evolved with time, the timing of ophiolite emplacement, how the ophiolites in Myanmar 84!
relate to those in the Naga-Manipur region of India, how the events in the IBR relate to plate 85!
tectonic models of the region, and how the accretionary prism was modified by oblique 86!
collision. 87!
88!
2. Geological Background 89!
The IBR (Fig. 1) comprise a thick sequence of Mesozoic and Cenozoic flysch 90!
deposits associated with several large and numerous small ‘ophiolite’ fragments. The ranges 91!
initially formed in an accretionary prism setting, then evolved to a sub-aerial fold-and-thrust 92!
belt during highly oblique collision between Sundaland and the India Plate, their general 93!
characteristics have been described in a number of publications (e.g. Brunnschweiler, 1966, 94!
1974; Maurin and Rangin, 2009; Bannert et al., 2011; Mitchell, 1993, 2017; Mitchell et al., 95!
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2010; Rangin et al., 2013; Rangin, 2017; 2018). The ranges are also known from the adjacent 96!
areas of Bangladesh (e.g. Gani and Alam, 1999; Steckler et al., 2016b) and India (e.g. Ghose 97!
and Singh, 1981; Singh and Ghose, 1982; Ghose et al., 2014; Fig. 1). Traditionally the IBR 98!
has been divided into an outer Western Belt, and an inner Eastern Belt (e.g. United Nations, 99!
1979; Mitchell et al., 2010). The identification of this division is not facile everywhere in the 100!
ranges, and is clearest in the Chin Hills, where the Kheng Fault (Fig. 1) divides the two belts. 101!
Maurin and Rangin (2009) use Outer Belt for the detached fold and thrust system developed 102!
primarily in Neogene sediments, Inner Belt for the folded and thrusted region of 103!
predominantly Late Cretaceous-Palaeogene section, and Core for the most easterly and 104!
tectonically complex zone (Fig. 1). The Core marks a broad suture zone of Tethys ocean and 105!
related back-arc basin-derived rock units comprising oceanic crust-related units (including 106!
large and small bodies of peridotite and serpentinite, pillow lavas, radiolarian chert, 107!
mélange), a poorly dated flysch unit that appears to be primarily of Late Triassic age (Pane 108!
Chaung Formation), and metamorphic units that include fragments of the metamorphic sole 109!
to large ultrabasic bodies (e.g. Webula Bula area, Zhang et al., 2017). A larger region of 110!
predominantly low-grade metamorphic rocks, called the Kanpetlet Schists, crops out in the 111!
Southern Chin Hills area (Fig. 2). These schists are thought to be partly or entirely 112!
metamorphosed equivalents of the Pane Chaung Formation (United Nations, 1979a; Maurin 113!
and Rangin, 2009; Bannert et al., 2011). 114!
The thick Mesozoic-Cenozoic flysch deposits of the IBR are typically poorly 115!
fossiliferous, and difficult to differentiate, particularly in the Inner (eastern) Belt 116!
(Brunnschweiler, 1966; Bannert et al., 2011). Since mass transport complexes of highly 117!
variable dimensions are common, reworking of older fossiliferous strata (e.g. radiolarian 118!
cherts, foraminiferal limestones) into younger deposits frequently occurs (Brunnschweiler, 119!
