Neotectonic fault analysis by 2D finite element modeling for studying the Himalayan fold-and-thrust belt in Nepal
Introduction
Thin-skinned fold-and-thrust belts are common in orogenic belts of all ages. These zones of folding and thrusting along the margin of a mountain belt constitute one of the most widely recognized and best understood deformational features of the earth. Mechanics of fold-and-thrust belts and accretionary wedges have been extensively studied during the last two decades (Davis et al., 1983; Dahlen, 1990; Lallemand et al., 1994). According to Chapple (1978), the main characteristics of thin-skinned fold-and-thrust belts are: (1) a thin skinned belt, where the limiting horizon is commonly, but not always, close to the crystalline basement; (2) a basal layer of detachment or décollement, composed of weak rock, often shale or evaporite, dipping towards the hinterland; (3) a wedge-shaped region of deformation, thicker in the rear where the thrust originates; (4) large amounts of shortening and thickening in the rear of the wedge.
This thin-skinned belt is usually composed of a series of thrust sheets with the propagation of deformation from the hinterland towards the foreland, and the development of a surface topography sloping towards the foreland. These typical characteristics can be observed in the Himalayan fold-and-thrust belt, which has developed since ca. 50 Ma as a result of the collision between the Indian and Eurasian plates and the subsequent subduction of India beneath the Himalayas. As the Indian Plate has moved continuously towards the north, the sedimentary prism deposited at the northern tip of the Indian Plate has been detached from the underlying basement to form large south vergent thrust sheets, which are bounded by intra-crustal thrusts (Fig. 1). From north to south these are the Main Central Thrust (MCT), the Main Boundary Thrust (MBT) and the Main Frontal Thrust (MFT).
Quantitative estimation of the tectonic stress field in the fold-and-thrust belt, both on regional and local scales, is an important constraint for understanding faulting and the resulting geodynamics. In this regard, numerical modeling (Willet et al., 1993; Beaumont et al., 1994; Sassi and Faure, 1997; Mikhailov et al., 2002; Chamlagain and Hayashi, 2004) has been applied extensively to explore the style of deformation and the development of thrust systems during the evolution of the fold-and-thrust belt. Compared to other techniques, numerical techniques allow the modeling of the structure and the deformation at full scale and to compute stress and strain values over a long time period, using various constitutive laws. However, computations are lengthy and require knowledge of the rheological parameters, which are usually poorly constrained. Mäkel and Walters (1993) applied finite element modeling to the study of the mechanics of thrust formation in rectangular and wedge-shaped basins overlying a weak substratum. Using an elastic–plastic law with strain softening characteristics, they obtained a thrust in the rectangular block, and a system of a thrusts and a backthrusts in the tapered wedge, for a relatively small amount of shortening (tens of meters for a 20 km block). Vanbrabant et al. (1999) proposed numerical models for the Variscan fold-and-thrust belt of Belgium to obtain an insight into its evolution. They found that ramps have a significant influence on the development of the belt. For the Himalayan region, Wang and Shi (1982) simulated a 1500 km long section from the Gangetic Plain in the south to Kunlun in the north, using a 2D finite element method under plane strain conditions. Using nonlinear viscous rheology, they predicted a thickening rate (2 mm yr−1) for Tibet, with shortening partitioned across the entire orogen. They proposed that the Himalaya is dynamically supported. Singh et al. (1990) also modeled a cross-section extending from the Ganga Basin to the Tibetan Plateau, aiming to simulate earthquake activity using force rather than displacement boundary conditions. They suggested that the stress concentration indicated by frequent shallow earthquakes in the Himalayan and Tibetan regions is due to force acting from the north side, which is associated with a convection current beneath the Tibetan Plateau. This idea is somewhat controversial in the present state of knowledge of earthquake mechanisms in the Himalaya. Recently, Berger et al. (2004) simulated the interseismic deformations in Nepal Himalaya. They pointed out that the brittle–ductile transition and geometry of the Main Himalayan Thrust (MHT) are the main factors that influence present-day deformation in the Nepal Himalaya.
Neotectonic studies (Nakata, 1989; Nakata et al., 1990; Mugnier et al., 1994) in the Himalaya have revealed various recent movements (e.g. normal faulting, strike–slip faulting and thrust faulting) along active faults close to the MBT and within the Lesser Himalaya. These movements have been attributed to recent changes in the tectonic regime in the Himalaya. It has also been found that the apparent slip of normal faults close to the intra-crustal thrusts is down to the north, indicating extension. This is not consistent with seismic events of a compressive nature. In this paper, we attempt to clarify the cause of normal faulting in such compressional regimes by applying a 2D plane strain, elastic finite element modeling technique. We further explore the effect of a mid-crustal ramp on the neotectonic stress field in the Nepal Himalaya. Finally, we will compare overall results of the modeling with the record of microseismicity and active faulting in the region to gain a better understanding of the neotectonics of the Nepal Himalaya.
Section snippets
Tectonic setting of the Himalayan fold-and-thrust belt
As we mentioned in the previous section, the regional structural geology of the fold-and-thrust belt of Nepal Himalaya to the south of South Tibetan Detachment System (STDS) is controlled mainly by three major thrust systems, e.g. from north to south the MCT, MBT and MFT (Fig. 1). These thrust faults, with a N–S transport direction, are generally inferred to be splay thrusts of the MHT, which marks the underthrusting of the Indian Plate. Most of the cross-sections across the Himalaya suggest a
Active faults in the Nepal Himalaya
The Himalaya is one of the most neotectonically active mountain belts in the world. The active faults, in and around this belt are direct indicators of recent crustal movement due to the collision between the Indian and Eurasian plates. In this section, therefore, active faults and their tectonic significance are reviewed, in order to clarify the neotectonics of the Himalayan fold-and-thrust belt. In the Nepal Himalaya (Fig. 2) active faults, are distributed along the major tectonic elements,
Microseismicity
Present-day deformation in the Himalayan fold-and-thrust belt is characterized by large earthquakes (e.g. 1905 Kangra earthquake; 1934 Bihar–Nepal earthquake) of which the magnitude in Mw is ca. 8. These earthquakes appear to be due to ruptures along the MHT. During these events segments of the MHT, 200–300 km along strike and 60–100 km down-dip, are affected by coseismic displacements. The locations of the ruptured areas indicate that there is a gap along the mountain range between the Kangra
Simulation of stress and faulting
The neotectonic stress distribution and the resulting faults can be computed using 2D finite element method adopting a simple modeling approach. In this study, we adopt elastic rheology under plane strain conditions. Gravitational force is incorporated in all models. We consider that the MHT is a weak zone, so that it is allocated a Young's modulus two orders less than that of the other layers, to simulate a weak fault zone subject to slip. This treatment gives a similar effect to the dual-node
Results
Stress state is one of the good proxies for understanding deformation in the crust and depends entirely on the tectonic boundary conditions and the rheology of the rock layers. These constraints also maintain for the Himalayan fold-and-thrust belt, where the state of stress is governed by geometry, tectonic boundary conditions and the convergence rate of the Indian Plate. The MHT is the main structural discontinuity in the Himalaya, its geometry has a significant influence on the changes in the
Modeling assumptions
We have presented a series of 2D finite element models to gain a better understanding of the neotectonics of the fold-and-thrust belt of the Nepal Himalaya using three different structural cross-sections, which are representative of the present-day geometry of the Nepal Himalaya. We have imposed reasonable boundary conditions consistent with the present-day plate kinematics of the region. Further, a convergence displacement has been applied, instead of the forces or stress, because latter are
Conclusions
The neotectonic stress field and resulting faults have been numerically modeled using 2D finite element method. Our models are based on present-day cross-sections of eastern, central and western Nepal Himalaya with different geometries of the MHT as revealed by structural and geophysical data. The results presented in this study are first-order estimates. The results are compared with records of active faults and seismicity in the fold-and-thrust belt of the Nepal Himalaya. Comparing the
Acknowledgements
DC is indebted to the Ministry of Education, Culture, Sports, Science and Technology (Monbukagakusho) Japan, for financial support to carry out this research. We are grateful to Dr François Jouanne, who permitted us to use his figure. We thankfully acknowledge constructive comments and corrections from Dr A.J. Barber, University of London and two anonymous reviewers. We are equally thankful to guest editors (Profs. K. Arita, A. Yin, H. Okada and Dr S. Singh) and the editorial board for their
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