Elsevier

Gondwana Research

Volume 16, Issues 3–4, December 2009, Pages 697-715
Gondwana Research

A new kinematic evolutionary model for the growth of a duplex — an example from the Rangit duplex, Sikkim Himalaya, India

https://doi.org/10.1016/j.gr.2009.07.006Get rights and content

Abstract

The Lesser Himalayan duplex (LHD) is a prominent structure through much of the Lesser Himalayan fold–thrust belt. In the Darjeeling - Sikkim Himalaya a component of the LHD is exposed in the Rangit window as the Rangit duplex (RD). The RD consists of ten horses of the upper Lesser Himalayan Sequence (Gondwana, Buxa, Upper Daling). The duplex varies from hinterland-dipping in the north, through an antiformal stack in the middle to foreland-dipping in the south. The Ramgarh thrust (RT) is the roof thrust and, based on a balanced cross-section, the Main Himalayan Sole thrust is the floor thrust at a depth of ~ 10 km and with a dip of ~ 3.5° N.

Retrodeformation suggests that the RD initiated as a foreland-dipping duplex with the Early Ramgarh thrust as the roof thrust and the RT as the floor thrust. The RT became the roof thrust during continued duplexing by a combination of footwall imbrication and concurrent RT reactivation. This kinematic history best explains the large translation of the overlying MCT sheets. The restoration suggests that RD shortening is ~ 125 km, and the original Gondwana basin extended ~ 142 km northward of its present northernmost exposures within the window.

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

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