A new kinematic evolutionary model for the growth of a duplex — an example from the Rangit duplex, Sikkim Himalaya, India
Introduction
Thrust duplexes are an integral part of fold–thrust belts (FTBs) and have been described from most major orogens (e.g. Dahlstrom, 1970, Elliott and Johnson, 1980, Boyer and Elliott, 1982, Butler, 1982, Coward, 1984, Diegel, 1986, Mitra, 1986, Fermor and Price, 1987, Geiser, 1988, Srivastava and Mitra, 1994, DeCelles and Mitra, 1995, DeCelles et al., 1998, McQuarrie and DeCelles, 2001, McQuarrie et al., 2008). They are generally found in the internal portions of FTBs where a longer deformation history and the consequent higher connectivity between faults can lead to the formation of duplexes (Boyer and Elliott, 1982) in both crystalline basement rocks and sedimentary cover rocks. They accommodate a large fraction of the total shortening in most FTBs, and provide an efficient mechanism for transferring slip upward from the basal decollement into the FTB wedge and for transporting roof thrust sheets over long distances. In addition, continued reactivation of hinterland duplexes may provide the necessary thickening in the back of an orogenic wedge to maintain critical taper and allow continued thrusting onto the foreland (DeCelles and Mitra, 1995). Because of their critical role in various aspects of FTB shortening, understanding the kinematic evolution of duplexes can provide many clues to evaluating the growth of an orogen as a whole.
In the Himalayan FTB (Fig. 1), the growth of the Lesser Himalayan duplex (LHD) plays a prominent role in the overall evolution of the FTB. The LHD has been described all along the length of the Himalayan arc, from Kumaon (Srivastava and Mitra, 1994), through Nepal (DeCelles et al., 1998) and Sikkim (Bhattacharyya et al., 2006, Bhattacharyya et al., 2008), to Bhutan (McQuarrie et al., 2008). In most places along this belt the LHD accommodates a significant fraction of the total shortening of the Himalayan FTB, ranging from less than ~ 25% of the total minimum shortening in the western Himalaya (Kumaon and Nepal) to as much as ~ 50% of the total minimum shortening in the eastern Himalaya (Sikkim and Bhutan) (Mitra et al., in press). In addition, the roof thrust of the LHD has translated Greater Himalayan crystalline thrust sheets southward over long distances resulting in the formation of the Greater Himalayan klippen, which are exposed in the Lesser Himalaya; examples include the Almora klippe in Kumaon (Srivastava and Mitra, 1994), the Dadeldhura klippe in western Nepal (DeCelles et al., 1998), and the Darjeeling klippe in the eastern Himalaya. Clearly, deciphering the kinematics of the LHD is critical to understanding the overall evolution of the Himalayan FTB at different locations along the Himalayan arc.
In the Darjeeling - Sikkim Himalaya (DSH; Fig. 1, Fig. 2) the Lesser Himalayan Sequence, made up of a 6–8 km thick sequence of Proterozoic - Paleozoic Daling, Buxa and Gondwana rocks, is repeated several times along horses forming the LHD. Part of this repetition is exposed in the Rangit window (Fig. 2) (Ghosh, 1956, Raina, 1976, Gangopadhyay and Ray, 1980) as the Rangit duplex (Fig. 3; Bhattacharyya et al., 2006). Unlike the Kumaon, Nepal and Bhutan Himalaya where the LHD generally has an overall hinterland-dipping geometry (Srivastava and Mitra, 1994, DeCelles et al., 1998, McQuarrie et al., 2008), the geometry of the LHD in the DSH is significantly different and more complicated.
Existing kinematic models for the evolution of duplexes are inadequate for explaining the kinematics of the complex geometry observed in the Rangit duplex. The generally accepted Boyer-style duplexing model with hinterland-to-foreland progression of thrusting (Boyer and Elliott, 1982), or a model calling on duplexing by forming connecting splays joining two preexisting thrusts (Mitra and Sussman, 1997), both result in simpler duplex geometries. A model incorporating hinterland-to-foreland progression together with imbricate reactivation (Boyer, 1992) provides a basis for evaluating a kinematic history for the Rangit duplex, but the Rangit duplex differs in detail because it never develops out-of-sequence imbricates in the hanging wall of the roof thrust. In this paper we describe in detail the geometry of the Rangit duplex and propose a new kinematic model to explain its structural evolution. Retrodeformation along this kinematic path yields a well constrained palinspastic restoration of the Buxa–Gondwana basin that defines the original northern extent of this basin in Peninsular India. We also discuss the implication of the Rangit duplex for the evolution of the DSH as a whole.
Section snippets
Regional geology
The Himalayan orogen is considered to have formed at the northern margin of East Gondwanaland (Valdiya, 1997, Goscombe et al., 2006, Yoshida and Upreti, 2006). The growth of the Himalayan orogen is related to the collision of the Indian lithosphere against the Eurasian lithosphere that started with initial impingement at ~ 52 Ma (Rowley, 1996) and is continuing to the present, and has been in the focus for several studies including the characterization of collisional orogeny as well as the plate
Map patterns
The Rangit window extends for ~ 15 km in the N–S direction from Rangitnagar to Jorethang and ~ 13 km in the E–W direction from Gelling to Bhanzyang and lies directly north of the Darjeeling klippe (Fig. 2, Fig. 3). Erosion through the folded Ramgarh thrust (RT) has exposed the footwall upper LHS rocks, thereby forming the Rangit window. The Ramgarh thrust (RT), the bounding fault of the Rangit window, can be traced around the entire window. The fault is best exposed along the northeastern margin
Kinematics of the Rangit duplex
Proper restoration of the Rangit duplex cross section requires a viable kinematic model that can suggest a suitable retrodeformation path (McNaught and Mitra, 1996). In this section we propose four different kinematic models for the evolution of the Rangit duplex and discuss their viablilties for the evolution of the duplex. The first three kinematic models are based on the Boyer and Elliott (1982) model of hinterland-to-foreland progression of thrusting while the fourth one incorporates
Discussion
The geometry of the Rangit duplex is significantly different from the Lesser Himalayan duplex (LHD) in other parts of the Himalaya along strike where the duplex geometry is generally quite simple. In the Kumaon Himalaya, the LHD has a hinterland dipping geometry (Srivastava and Mitra, 1994), while in the Nepal Himalaya, the LHD has a dominantly hinterland dipping geometry along with a component of antiformal stack in the south (DeCelles et al., 1998, DeCelles et al., 2001, Robinson et al., 2006
Conclusions
In the DSH, both the MCT 1 and 2 sheets are translated much farther southward than anywhere else along the length of the Himalayan FTB and are exposed in the Darjeeling and Labha klippen. The Rangit window lying almost due north of the Darjeeling klippe exposes the Rangit duplex which provides critical insights into the evolution of the DSH. The upper LHS in the DSH is repeated along at least nine horses forming the Rangit duplex whose roof thrust is the Ramgarh thrust and whose floor thrust is
Acknowledgements
This work was supported by NSF grant EAR - 0439999 to G. Mitra and grants from the Geological Society of America and the Nuria Pequera Fellowship of the University of Rochester to K. Bhattacharyya. We thank the reviewers, M. Mukul, N. McQuarrie and the associate editor, S. Kwon, for their detailed and thoughtful reviews that greatly improved the quality of this paper. K.B. gratefully acknowledges the help of M. Mukul and A. Matin in introducing her to the Darjeeling – Sikkim Himalaya. We thank
References (63)
The strain and textural history of thin-skinned tectonic zones: examples from the Assynt region of the Moine thrust zone, NW Scotland
Journal of Structural Geology
(1984)Topologic constraints from imbricate thrust networks — examples from the Mountain City window, Tennessee
Journal of Structural Geology
(1986)The Raniganj coal basin: an example of an India Gondwana rift
Sedimentary Geology
(2002)- et al.
Crustal architecture of the Himalayan metamorphic front in eastern Nepal
Gondwana Research
(2006) Modifying the normalized Fry method for aggregates of non-elliptical grains
Journal of Structural Geology
(1994)- et al.
The use of finite strain data in constructing a retrodeformable cross-section of the Meade thrust sheet, southeastern Idaho
Journal of Structural Geology
(1996) - et al.
Preliminary stratigraphy and structural architecture of Bhutan: implications for the along strike architecture of the Himalayan system
Earth and Planetary Science Letters
(2008) - et al.
Energy balance and deformation mechanisms of duplexes
Journal of Structural Geology
(1986) - et al.
Structural evolution of connecting splay duplexes and their implications for critical taper: an example based on geometry and kinematics of the Canyon Range culmination, Sevier Belt, central Utah
Journal of Structural Geology
(1997) - et al.
Reconsidering Himalayan anticlines
Geomorphology
(2006)
The geometry and kinematics of the Main Boundary Thrust and related neotectonics in the Darjiling Himalayan fold-and-thrust belt, West Bengal, India
Journal of Structural Geology
Tectonic and polymetamorphic history of the Lesser Himalaya in central Nepal
Journal of Asian Earth Sciences
Lateral variations in geometry of thrust planes and its significance, as studied in the Shumar allochthon, Lesser Himalayas, eastern Bhutan
Tectonophysics
Age of initial collision between India and Asia: a review of stratigraphic data
Earth and Planetary Science Letters
CO2 windows from mantle to atmosphere: models on ultrahigh-temperature metamorphism and speculations on the link with melting of snowball Earth
Gondwana Research
Anatomy of a Cambrian suture in Gondwana: Pacific-type orogeny in southern India?
Gondwana Research
Himalaya, the northern frontier of East Gondwanaland
Gondwana Research
Neoproterozoic India within East Gondwana: constraints from recent geochronologic data from Himalaya
Gondwana Research
Structure and stratigraphy of the Darjeeling frontal zone, eastern Himalaya
The Cenozoic foreland basin and tectonics of the Eastern Sub Himalaya: problems and prospects
Himalayan Geology
Geology of the Darjeeling–Sikkim Himalaya
The geometry and implications of a foreland dipping duplex, the Rangit Duplex, Darjeeling–Sikkim Himalayas, India
Geological Society of America Abstract with Programs
The geometry and kinematics of the Darjeeling – Sikkim Himalaya, India
Geological Society of America Abstracts with Programs
Les elements structuraux de l'Himalaya de l'Arun et la region de l'Everest
Comptes Rendus de l'Académie des Sciences
Geometric evidence for synchronous thrusting in the southern Alberta and northwest Montana thrust belt
Thrust systems
American Association of Petroleum Geologists Bulletin
Exhumation during crustal folding in the Namche-Barwa syntaxis
Terra Nova
The terminology of structures in thrust belts
Journal of Structural Geology
Late Miocene movement within the Himalayan Main Central Thrust shear zone, Sikkim, north-east India
Journal of Metamorphic Geology,
Structural geology of the eastern margin of the Canadian Rocky Mountains
Bulletin of Canadian Petroleum Geology,
History of the Sevier orogenic wedge in terms of critical taper models, northeast Utah and southwest Wyoming
Geological Society America Bulletin
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