Pro(:. Indian Acad. Sci. (Earth Planet. Sci.), Vol. 98, No. 1, April 1989, pp. 71-89. Printed in India. Active tectonics of the Himalaya JAMES F NI Departmeht of Physics, New Mexico State University, Las Cruces, NM 88003, USA Al~ract. The Himalayan mountains are a product of the collision between India and Eurasia which began in the Eocene. In the early stage of continental collision the development of a suture zone between two colliding plates took place. The continued convergence is accommodated along the suture zone and in the back-arc region. Further convergence results in intracrustal megathrust within the leading edge of the advancing Indian plate. In the Himalaya this stage is characterized .by the intense uplift of the High Himalaya, the development of the Tibetan Plateau and the breaking-up of the central and eastern Asian continent. Although numerous models for the evolution of the Himalaya have been proposed, the available geological and geophysical data are consistent with an underthrusting model in which the Indian continental lithosphere underthrusts beneath the Himalaya and southern Tibet. Reflection profiles across the entire Himalaya and Tibet are needed to prove the existence of such underthrusting. Geodetic surveys across the High Himalaya are needed to determine the present state of the MCT as well as the rate of uplift and shortening within the Himalaya. Paleoseismicity studies are necessary to resolve the temporal and spatial patterns of major earthquake faulting along the segmented Himalayan mountains. Keywocds. Himalayan mountains; continental collision; earthquakes; active tectonics; tectonic setting; Main boundary thrust; Tethyan slab. 1. Introduction The Himalaya is the largest mountain chain in the world. Accordingly, understanding the mechanics responsible for its formation and for its support must be two of the most important geological problems. Moreover, study of the active tectonics of the Himalayan mountain belt is important because such a region contains information that is unavailable in other, older mountain belts, but necessary in order to understand how mountain building takes place. Early episodes in orogenic research (last quarter of the 18th and first half of the 19th centuries) were characterized by the interpretation that mountain belts are a result of vertical uplift. It was not until the late 19th century that the role of horizontal movements became more widely appreciated. The development of geosynclinal theory in the early 20th century to explain the prehistory of mountain belts and the subsequent horizontal shortening in orogeny as expressed by fold and nappe structures delayed a more accurate understanding of mountain building. As the concept of plate tectonics was developed during the 1960s, numerous models for the evolution of the Himalaya were proposed. The past decade has included a transition from a focus on testing of the Himalayan collision models to research based more on observation. The models developed in the 1970s and refined in the 1980s have provided the direction of investigation as deeper observations have been made possible by geophysical techniques. 71 72 James F Ni This review focuses on the deep crustal and uppermost mantle structures of the Himalaya, because these features reflect the mechanical processes that result in mountain building. The review also will discuss recent work on the Pakistan Himalaya. 2. Collision models Early ideas about the architecture of the Himalaya assumed that the Himalayan Mountains rest on a rigid substratum that is similar to the rocks beneath the Indian Shield. However, geodetic measurements (plumb line) of Pratt (1855) showed that the observed deflection was much smaller than expected for a rigid model. Shortly following this experiment the concept of isostasy was born and a floating Himalayan range with a "crustal root" became the dominant geological thinking for the early twentieth century. With the improvement of gravity meters, numerous studies have shown that gravity measurements deviate from local isostatic equilibriuml This discovery suggested that the Himalaya is not completely supported by the buoyancy force; rather it is held up, at least in part, by an upward force. Searching for the origin of this force is essential to understanding the dynamics of mountain building. The concept of underthrusting of one continental lithosphere beneath another as the cause of mountain building and plateau uplift was first proposed by Argand (1924). With the advent of the new global tectonics (e.g. Isacks et al 1968) a number of different mechanisms for the uplift of the Himalaya and Tibet have been developed. Dewey and Bird (1970) suggested that the Himalaya was formed solely by the collision of an Atlantic-type continental margin with the Asian continent bordered by a marginal trench. In their model they assumed that the buoyancy of the continental lithosphere prevents further sulSduction and they overlooked the possible significance of intracontinentai thrusts. Powell and Conaghan (1973) pointed out that the geological history of the Himalaya does not agree with the simple continent- continent collision model of Dewey and Bird (1970), because the collision itself did not directly produce the present-day Himalaya. Alternatively, Powell and Conaghan (1973, 19.75) and Powell (1979) proposed an "evolutionary" model (figure 1) invoking two phases of orogeny. In their model, the first phase is related to active Mesozoic- early Tertiary subduction zones along the present-day Indus-Tsangpo suture, analogous to the North American Cordilleras. This phase ended in the Eocene (e.g. Molnar and Tapponnier 1975; Tapponnier and Molnar 1977) when India collided with Asia. The Indus-Tsangpo suture is a direct result of this collision. The second phase involves formation of intracontinental fractures within the northern Indian margin during the Middle Tertiary and underthrusting of the Indian continent along this fracture from Miocene time to the present. Since 1973, increasing amounts of data on the Himalaya and Tibet have become available and, consequently, many plate tectonic models have been proposed for the evolution of the Himalaya (e.g. LeFort 1975; Molnar et al 1977; Bird 1978; Seeber et al 1981; Chen and Molnar 1981; Barazangi and Ni 1982; Molnar and Chen 1982; Ni and Barazangi 1984; Molnar 1984). Among these models, much of the debate about the tectonics of the Himalaya is concerned with the nature of the Main Boundary Thrust (MBT) and Main Central Thrust (MCT). According to LeFort (1975), the Himalaya was formed by the imbrication of slices of India's northern margin. In his evolutionary model (figure 1), the MBT and MCT are assumed to be similar, but successive, Active tectonics of the Himalaya 73 MBT MCT A. La Fort 1975 o ~ o I~t~,..-"~.~ ---" ~ 'B,OCKI Himalayan I, Schuppen ~ I q Tethyan , ~ Indus -'i Zone Himalaya Suture M__ MCT MCT CCA I~1" --~ _ - B. Powall 1979 o II..... -'~~-__.~INDIAN ~ "~"'s.IE,~" iHOl^._- J ~o 50~ 50 I ~ Sedimentary -'-~1 Wedge Tethyan Himalaya C. Seeber tlt Armbruster MBT MCT 1981 0-~' 0 50 - - 50 0, Burg &Chen 0 0 1984 i i I Late [] Molasse [] Cretaceous 9 Ophiolite N~Metamorphic Plutons ~,~ Foliation Figure 1. Comparison of tectonic models of the Himalaya. A. After La Fort (1975). The Himalaya was formed by imbrication of slices of India's northern margin. B. After Powell (1979), The MCT and MBT are fundamentally different structures in this model. C. After Seeber and Anabruster (1981). All the elements in this model are active simultaneously and subduction proceeds without fundamental changes in the structure. This model was named the "steady-state" model. D. After Burg and Chen (1984). In their model, decoupling and thrusting, which separate lower and upper crustal layers, occur contemporaneously with each other. tectonic thrusts, The southern thrust zone (i.e. the MBT) becomes the new boundary of the continental convergent zone, while the older thrust zone (i.e. the MCT) becomes a less active or a dormant feature. In contrast, Seeber and Armbruster (1981) proposed a "steady-state" model 74 James F Ni (figure I) for the evolution of the Himalaya. This model requires that the MBT and MCT are contemporaneous features. Central to this model is a detachment (decollement) surface underlying the entire Himalaya. The detachment represents the upper surface of the underthrusting Indian plate. Seeber et al (1981) also postulated the presence of a Basement Thrust (BT) where the northerly and relatively steeply dipping MCT merges with the shallow dipping detachment surface at depth (see figure 11 of Seeber et al 1981). The BT zone represents the front of the overriding crystalline basement (analogous to a bulldozer blade), and it is along this front where the sedimentary rocks above the Indian Shield are scraped off. A more complex model for the Himalaya has been proposed by Him et al (1984a) and Burg and Chen (1984) (figure I). Their model is mostly based on the interpretation of recent reflection data in which a variation in the level of the Moho has been suggested. In their model, decoupling and thrusting, which separate lower and upper crustal layers, occur contemporaneously with eac.h other. However, polarities of thrusting are opposite to each other (figure 1). 3. Tectonic setting The hypotheses of the evolution of the Himalaya discussed above depend largely on understanding the evolution and structure of major tectonic features and their behaviour at depth. The following is a brief summary of the main structural characteristics of these features (figure 2). 3.1 lndus-Tsanopo suture The Indus-Tsangpo suture (e.g. Gansser 1964, 1980; LeFort 1975) marks the northern limit of the Indian subcontinent following the late Cretaceous-early Tertiary closure of the Neo-Tethys. The suture is made up of imbricated melanges of flysch sediments, radiolarite, pillow iavas, voicanics, and basic and ultrabasic rocks that are cut by steep thrust faults (e.g. Stocklin 1980; Allegre et al 1984).