Earth-Science Reviews 76 (2006) 1–131 www.elsevier.com/locate/earscirev Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation An Yin * Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, United States Received 26 August 2003; accepted 2 May 2005 Available online 8 February 2006 Abstract Despite a long research history over the past 150 years, the geometry, kinematics, and dynamic evolution of the Himalayan orogen remain poorly understood. This is mainly due to continued emphasis on the two-dimensionality of the Himalayan orogenic architecture and extrapolation of geologic relationships from a few well-studied but small areas to the rest of the orogen. Confusion and misconception are also widespread in the Himalayan literature in terms of the geographic, stratigraphic, and structural divisions. To clarify these issues and to provide a new platform for those who are interested in studying the geologic development of this spectacular mountain belt, I systematically review the essential observations relevant to the along-strike variation of the Himalayan geologic framework and its role in Cenozoic Himalayan exhumation, metamorphism and foreland sedimentation. A main focus of my synthesis is to elucidate the emplacement history of the high-grade Greater Himalayan Crystalline Complex (GHC) that occupies the core of the orogen. Because the north-dipping Main Central Thrust (MCT) above and South Tibet Detachment (STD) below bound the GHC in most parts of the Himalaya, it is critical to determine the relationship between them in map and cross-section views. The exposed map pattern in the central Himalaya (i.e., Nepal) indicates that the MCT has a flat-ramp geometry. The thrust flat in the south carries a 2–15-km-thick slab of the GHC over the Lesser Himalayan Sequence (LHS) and creates a large hanging-wall fault-bend fold continuing N100 km south of the MCT ramp zone. In the western Himalayan orogen at the longitude ~778E, the MCT exhibits a major lateral ramp (the Mandi ramp). West of this ramp, the MCT places the low-grade Tethyan Himalayan Sequence (THS) over the low-grade LHS, whereas east of the ramp, the MCT places the high-grade GHC over the low-grade LHS. This along-strike change in stratigraphic juxtaposition and metamorphic grade across the MCT indicates a westward decrease in its slip magnitude, possibly a result of a westward decrease in total crustal shortening along the Himalayan orogen. Everywhere exposed, the STD follows roughly the same stratigraphic horizon at the base of the THS, exhibiting a long (N100 km) hanging-wall flat. This relationship suggests that the STD may have initiated along a preexisting lithologic contact or the subhorizontal brittle–ductile transition zone in the middle crust. Although the STD has the THS in its hanging wall everywhere in the Himalayan orogen, no THS footwall cutoffs have been identified. This has made slip estimates of the STD exceedingly * Tel.: +1 310 825 8752; fax: +1 310 825 2779. E-mailaddress:[email protected]. 0012-8252/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2005.05.004 2 A. Yin / Earth-Science Reviews 76 (2006) 1–131 difficult. The southernmost trace of the STD either merges with the MCT (e.g., in Zanskar) or lies within 1–2 km of the MCT frontal trace (e.g., in Bhutan), suggesting that the MCT may join the STD in their up-dip directions to the south. This geometry, largely neglected by the existing models, has important implications for the deformation, exhumation, and sedimentation history of the entire Himalayan orogen. D 2005 Elsevier B.V. All rights reserved. Keywords: Himalayan orogen; Main Central Thrust; South Tibet Detachment; passive-roof fault; active-roof fault; erosional exhumation 1. Introduction entirety. This may be attributed to the following factors: bDistinguishing between myth and science is sub- (1) Himalayan research is long (over 150 yr, e.g., tle, for both seek to understand the things around us. Hooker, 1854; Godwin-Austen, 1864; Mallet, The characteristic style of mythic thinking is to 1875; La Touche, 1883; Pilgrim, 1906; Auden, place special emphasis on a selective conjecture, 1935; Lahiri, 1941) and rich, making its litera- based typically on the initial observation or recog- ture nearly intractable. The problem is com- nition of a phenomenon, which is thereafter given pounded by the accelerated rate of publication privileged status over alternate interpretations.Q on more and more specialized subjects on Hi- William. R. Dickinson malayan geology in the past two decades. (2) The terminology of the Himalayan geology is The Himalaya is a classic example of an orogen- often confusing. Its physiographic division is ic system created by continent–continent collision commonly equated to structural and stratigraphic (e.g., Dewey and Bird, 1970; Dewey and Burke, divisions. Names of the same stratigraphic units 1973). Its youthfulness and spectacular exposure and structures vary from place to place across make the orogen ideal for studying diverse geologic international borders or even within the same processes related to mountain building. Its potential country. as a guide to decipher the feedback processes be- (3) There has been a proliferation of kinematic, tween lithospheric deformation and atmospheric cir- thermal, and dynamic models for the develop- culation has motivated intense research in recent ment of the Himalayan orogen in the past two years on the history of the Himalayan–Tibetan oro- decades. Debate is intense and consensus gen, its role in global climate change, and its inter- changes rapidly. This has made it especially action with erosion (e.g., Harrison et al., 1992, difficult to evaluate the validity of each model 1998a; Molnar et al., 1993; Royden et al., 1997; or apply them to other mountain belts. Ramstein et al., 1997; Tapponnier et al., 2001; Beaumont et al., 2001; Yin et al., 2002). Many The classic reviews of the Himalayan geology by workers have also used the Himalayan knowledge Wadia (1953), Gansser (1964) and LeFort (1975) laid to infer the evolution of other mountain belts: the the foundation for productive geologic research in the Altai system in central Asia (Yang et al., 1992; Qu next several decades to follow. However, their reviews and Zhang, 1994), the Trans-Hudson orogen and are largely out of date in light of new observations. Canadian Cordillera in North America (e.g., Nabe- Although several recent reviews on the Himalayan lek et al., 2001; Norlander et al., 2002), the Cale- geology could potentially overcome the problem, donides in Greenland (e.g., McClelland and Gilotti, they cover only selected segments of the Himalayan 2003), and the East African–Antarctic orogen in orogen. For example, reviews by LeFort (1996), Africa and Antarctica (e.g., Jacobs and Thomas, Hodges (2000), Johnson (2002), DeCelles et al. 2004). Despite the broad interests, it has become (2002), and Avouac (2003) emphasize the central Hi- increasingly daunting for both a beginner and an malayan orogen, while syntheses by Searle et al. experienced Himalayan geologist to comprehend the (1992), Thakur (1992), Steck (2003), and DiPietro intricate complexity of the Himalayan geology in its and Pogue (2004) focus exclusively on the western A. Yin / Earth-Science Reviews 76 (2006) 1–131 3 Himalayan orogen. Similarly, geologic summaries by marks the northern limit of the Indo-Gangetic depres- Acharyya (1980), Singh and Chowdhary (1990), and sion (Fig. 1A). Immediately to the west of the Himala- Kumar (1997) only cover the geology of the eastern yan range are the Hindu Kush Mountains, to the east the Himalayan orogen. The lack of an updated overview of Indo-Burma Ranges (commonly known as the Rongk- the entire Himalayan orogen makes it difficult to assess lang Range), and to the north the Karakorum Moun- how the Himalayan deformation has responded to the tains and the Gangdese Shan (also known as the well-understood plate boundary conditions and its im- Transhimalaya in Heim and Gansser, 1939) (Fig. 1A). pact on the overall Indo-Asian collision zone and evo- The southern political boundary of Tibet (i.e., Xizang lution of great rivers in Asia (e.g., Patriat and Achache, in Chinese) follows approximately the crest of the 1984; Dewey et al., 1989; Le Pichon et al., 1992; Himalayan range. The difference in political and geo- Brookfield, 1998; Hallet and Molnar, 2001; Clark et graphic divisions has led to different naming of the al., 2004). same structures in the Himalayan range (e.g., the North The present review intends to introduce the Hima- Himalayan Normal Fault versus South Tibet Detach- layan geology in its entirety. In order to separate obser- ment System; Burg et al., 1984a; Burchfiel et al., 1992). vations from interpretations, I approach the synthesis in The Himalayan orogen is defined by the Indus– the following order. First, I define the basic terminol- Tsangpo suture in the north, the left-slip Chaman ogy commonly used in Himalayan literature and dis- fault in the west, the right-slip Sagaing fault in the cuss their pitfalls in limiting our ability to understand east, and the Main Frontal Thrust (MFT) in the south the complexity of the geology. Second, I provide a (Fig. 2A) (LeFort, 1975). Because the MFT links systematic overview of the Himalayan structural transpressional systems in the Indo-Burma Ranges framework, metamorphic conditions, exhumation his- (=Rongklang Range) in the east (e.g., Guzman-Spe- tory, and foreland sedimentation. Third, prominent ziale and Ni, 1996) and the Kirthar-Sulaiman thrust tectonic hypotheses and quantitative physical models salients in the west (e.g., Schelling, 1999) (Fig. 2A), the for the evolution of the Himalayan orogen are outlined Himalayan orogen defined above extends all the way and their predictions are evaluated in light of the avail- from the Himalayan range to the Arabian Sea and the able data.