RICERCA ITALIANA     

Structural, exhumational, and erosional evolution of the Himalayan Belt

Università degli Studi di Milano-Bicocca

Abstract

The prototype belt produced by continent-continent collision, the Himalaya represents a unique natural laboratory in which to investigate mountain-building processes. A significant part of current tectonic, magmatic, metamorphic, and sedimentary models are based on studies of the Himalayan Range.
The researchers of all four Units involved in the present Project have long experience on Alpine-type belts in general and on the Himalayas in particular, and possess, in the fields of structural geology, igneous and metamorphic petrology, geochemistry, geochronology and sedimentary geology, the competence required to deal with all of the complex multidisciplinary aspects involved in unraveling the structural and erosional evolution of the Himalayan belt since the initial stages of continental collision. The complementarity of their expertise and experiences makes them a well-integrated research team qualified to study both deep-seated and earth-surface geological processes, and to continue a long-standing tradition of Italian geological studies in the Himalaya begun almost a century ago by Giotto Dainelli and Ardito Desio (www.ighg.it). This tradition was carried on without interruption since the late Seventies in the framework of projects financed by MURST-MIUR and Ev-K2-CNR, on themes including structural geology, igneous and metamorphic petrology and geochronology (Lombardo et al. 1993; Villa et al. 1996 a,b; Rolfo et al. 1997; Carosi et al. 1999; Lombardo & Rolfo 2000; Visonà & Lombardo 2002) as well as the stratigraphy of sedimentary successions (Gaetani & Garzanti 1991; Gaetani 1997; Garzanti 1999) and sedimentary petrology (Garzanti et al. 1987; 1996; 2004; 2005).
The researches, carried out jointly by the four Research Units, will be focused in Sikkim and western Bhutan (areas free from political problems compared to adjacent Nepal). Our principal goals are: 1) the study of complete geological sections across the thrust belt, to be compared with the eastern Nepal transect studied during previous research projects; 2) the analysis of igneous and metamorphic rocks within all different tectonic domains (e.g., Greater and Lesser Himalaya), and the description of their mineralogic, petrologic, and geochemical nature; 3) the dating and reconstruction of the zonation of successive metamorphic events, with specific focus on the earlier stages of attempted continental subduction; 4) the dating and geochemical characterization of various leucogranitic bodies contained within different tectonic domains, in order to establish their origin and to verify the validity of current models for their generation and emplacement; 5) the analysis of petrographic, mineralogical, geochemical, thermochronological, and geochronological modes of detrital sediments supplied by the different tectono-metamorphic units from the headwaters to the mountain front, in order to obtain guidelines for provenance diagnoses of foreland-basin and remnant-ocean-basin deposits.
The Milano-Bicocca Research Unit, beside supervising the geochronological analysis of rock and sediment samples collected jointly in the field, will focus on the study of foreland-basin and remnant-ocean-basin successions from the Arabic Sea to the Gulf of Bengal, in order to reconstruct their main evolutionary steps since collision onset to present times. The large base of petrographic and mineralogical data acquired during previous projects (including some 350 petrographic and mineralogical analyses on sediments of about 180 Himalayan streams) will ideally allow us to make, in collaboration with European and American researchers, precise provenance diagnoses for any stratigraphic succession sampled at the front of the range, and to attempt paleogeographic reconstructions of drainage patterns, at least as far as the Neogene portion of the foreland basin is concerned. <<<

Principal Investigator

Eduardo Garzanti Università degli Studi di MILANO-BICOCCA

Research Objectives

This new Project PRIN 2006 will allow us to begin, in collaboration with European, American, and Australian universities and research centers, key studies aimed at integrating the existing knowledge on the evolution of the Himalayan belt and associated sedimentary basins. New data collected in the field and in the laboratory will help us to test current models and to improve on our present understanding of continental-collision and mountain-building processes. The analysis of Tertiary and Quaternary sedimentary sequences accumulated in foreland and remnant-ocean basins will help us to reconstruct specifically the earlier Paleo-Himalayan evolutionary stage, when the shallower structural levels of the nappe stack were eroded and thus removed forever from the orogen. Because sedimentary successions provide a virtually continuous record of the very same tectono-metamorphic events recognized by the study of rock outcrops, provenance analysis will allow us to constrain their age more accurately and independently by means of stratigraphic, thermochronological, and geochronological techniques.

Research activities will be carried out jointly by all four Research Units involved in the Project (Pisa, Torino, Padova, Milano-Bicocca), starting from close collaboration and sampling in the field. Also because of the recent deterioration of the political situation in Nepal, our attention will be focused on adjacent regions to the east, in southern and eastern Sikkim to western Bhutan, which are presently safe from political troubles and logistically easily accessible, but for heavily forested lower reliefs along the front of the range. Such a choice is aimed at obtaining new data to be compared with those collected in recent years during the expeditions carried out along the eastern Nepal - southern Tibet geotraverse, which extends from the lower Arun Valley to the Everest-Makalu region and to the ophiolitic Tsangpo suture farther north. We will thus be hopefully able to correlate the principal tectonic structures across a significant part of the central Himalaya. Petrographic, mineralogical, geochemical, thermochronological, and geochronological analysis of modern sediments will provide further clues even on the least exposed tectono-stratigraphic units. In Sikkim (North District and West District) and in north-western Bhutan (Paro, Thimpu, and Punakha regions) we will study in detail the structural framework, the metamorphic zonation, and the intrusion mechanisms of leucogranite bodies. Radiometric techniques (Ar-Ar, Rb-Sr, Sm-Nd, U-Pb) will be widely used in order to reconstruct the succession of main magmatic and metamorphic events and to shed light on the tectono-metamorphic evolution of the central-eastern Himalaya. In particular, the definition of pressure/temperature conditions and of the age of eo-Himalayan continental subduction and high-pressure metamorphism will allow us to compare the tectono-metamorphic evolution in these regions with other parts of the Himalayan belt (including the Kharta Valley in South Tibet, the Suru Valley in northern India, and the Stak Valley in northern Pakistan), and to crytically re-assess the diachroneity of tectonic processes.

Specifically, the Pisa Research Unit will focus on the structural analysis of selected cross-sections, in order to reconstruct the exhumation path and extrusion mechanism of metamorphic units (particularly as the Greater Himalaya is concerned), as well as the geometry, kinematics and age of tectonic deformation along major tectonic lineaments (Main Central Thrust and South Tibetan Detachment System). The structural record of the Himalayan collision will be reconstructed along several transects cutting across the structural grain of the range, from the Main Central Thrust to the Tibetan Zone, in western Bhutan, in Sikkim, and in South Tibet to the east of the Everest region. Specific attention will be dedicated to investigate the role of localized shear zones, recently recognized within the Greater Himalayan units of western Bhutan and which, we believe, has been grossly underestimated in previous studies. On the numerous samples collected during field expeditions, detailed microstructural, petrographic, fluid-inclusion, and geochronological analyses will be carried out.

Aim of the The Torino Research Unit will be the microstructural, mineral chemical, geothermobarometric, and geochronologic investigation of at least two geological traverses between southern Sikkim and northwestern Bhutan, in order to reconstruct the tectono-metamorphic evolution of the central-eastern Himalaya. Microstructural and petrological studies will be carried out on all tectonic units, from the lowermost structural levels of the orogen to the metamorphic complexes of the Main Central Thrust Zone and Greater Himalaya. We will specifically investigate every occurrence of eclogitic relics and leucogranite bodies intruded within the Greater Himalayan Zone, and try and accurately constrain the pressure/temperature/time paths of medium to high-grade metamorphic units in this sector of the Himalayan Range. Petrological studies will be complemented by analyses of primary and secondary fluid inclusions in various lithologies, mostly in syn-extensional leucogranites and in syntectonic veins associated to the Main Central Thrust Zone. Microthermometric analysis of fluid inclusions, combined with micro-Raman spectroscopy, will assist in constraining the thermo-baric conditions of the different tectono-metamorphic events and the composition of the circulating fluid phases.

The Padova Research Unit will study the petrography, geochemistry, and geochronology of granite dikes and stocks which intrude the distinct litho-tectonic units of the Himalayan belt. Field work, aimed at unraveling the relationships between deformation, metamorphism, and granite emplacement, will be carried out jointly with the Pisa and Torino Research Units along transects cutting across the thrust belt from southern Sikkim to western Bhutan. Zircon-, monazite-, or muscovite- bearing granite bodies intruded across major shear zones will be dated with geochronological techniques, in order to constrain the sequence of principal mountain-building events.

The Milano-Bicocca Research Unit will focus its activities on the petrographic, mineralogical, thermochronological, and geochronological study of modern sediments and Cenozoic clastic wedges deposited in the Himalayan foreland basin and on the remnant ocean floor of the Bengal Sea and Arabic Sea. The primary goal of our research in the next two years will be, beside the quantitative description of the overall Ganga drainage basin, both the detailed study of selected transects across the belt in central Nepal (Marsyandi and Kali Gandaki valleys), Sikkim and central-western Bhutan, and specifically of the large deltaic areas that connect the Indus and Ganga-Brahmaputra fluvial systems with the Indus and Bengal turbidite fans. In the Indus and Bengal deltas we will also study, in the framework of international collaborations which will grant us access to Shell and Cairn cores, selected successions in the subsurface, in order to verify both the influence of Quaternary climatic changes on sediment composition and the incidence of diagenesis in progressively older strata.
As far as Cenozoic clastic wedges are concerned, our attention will be focused on pre-, syn-, and post-collisional successions exposed in Bangladesh, western Myanmar (Arakan Yoma) and Andaman Islands. This will allow us to verify the isochrony or heterochrony of the principal tectonic and erosional events recorded by the sedimentary record of the Himalayan foreland and remnant-ocean basins. <<<

Timescale

24 months

National and international background

The prototype belt produced by continental collision, the Himalayan Range is a unique geological laboratory. Unequalled exposure of all kind of igneous and metamorphic rocks (from coesite eclogites produced during the initial stages of continental subduction, to leucogranites generated by subsequent crustal melting, to Pleistocene granulites and migmatites exposed at the core of crustal-scale antiforms at opposite ends of the belt, where uplift rates are unsurpassed on Earth) provide ideal conditions in which to investigate active mountain-building processes. Not only a large number of current orogenic models, including tectonic, magmatic, metamorphic aspects, have been based on studies carried out here, but the Himalaya is also the largest “sediment factory” on our planet. The up to two billion tons of clastic detritus produced each year have, in the course of the Tertiary, buried wide areas of the Indian Ocean floor on both sides of the Indian subcontinent under several km-thick wedges of turbidite sand and mud.

Formed as a consequence of continental collision between India and Asia, which begun at about 55 Ma (Garzanti et al. 1987; Rowley 1996), the Himalayan belt is under every respect a typical product of plate tectonics (Le Fort 1996; Hodges 2000). The rapid northward flight of India towards Eurasia in the Late Cretaceous to Paleocene is well documented by magnetic anomalies recorded in the Indian Ocean (Powell &amp; Conaghan 1973; Patriat &amp; Achache 1984; Klootwijk et al. 1992). In such a period of time, northward subduction of Neotethyan oceanic lithosphere has determined the generation of calc-alkaline magmas beneath the Asian active margin, and the consequent intrusion of granodioritic batholiths and effusion of andesitic lavas all along the Transhimalayan belt (Kohistan, Ladakh, Gangdese, Mishmi; Petterson &amp; Windley 1985; Xu et al. 1985; Weinberg &amp; Dunlap 2000).
The final closure of the Neotethys Ocean took place around the Paleocene/Eocene boundary. The earliest stages of continent-continent collision were characterized by attempted subduction of the strongly thinned outermost portion of the Indian passive margin beneath the Asian active margin, as documented by the Tethys Himalayan sedimentary succession at the surface and by the eclogite-facies metamorphism of the Tso Morari Dome at depth (Guillot et al. 2003). The climax of continental collision took place at the beginning of the Miocene, with large-scale thrusting of huge crystalline nappes along the Main Central Thrust. Accelerated subsidence of the foreland basin took place only during this relatively late stage of continental indentation (De Celles et al. 1998; Najman &amp; Garzanti 2000; White et al. 2002; Najman 2006). Continuing convergence is documented by progressive lateral growth of the thrust belt in the Neogene, with successive accretion first of Lesser Himalayan units along the Main Boundary Thrust at 10 - 12 Ma (Burbank et al. 1996; Szulc et al. in preparation; Najman et al. in preparation), and next of Subhimalayan molasse along the Main Frontal Thrust in the Plio-Quaternary.

Such a simple model for the evolution of the Himalayan belt, already introduced in its essential features about forty years ago (Gansser 1964), represents a paradigm universally accepted today (Le Fort 1996; Hodges 2000). It is widely agreed that the structure of the Himalayan thrust belt is as a first approximation relatively simple, and basically cylindrical. The general acceptance of a simple structural model has allowed the formulation in recent years of advanced models for the tectono-metamorphic and morpho-structural evolution of the Himalaya (e.g., Grujic et al. 1996; Beaumont et al. 2001). Such a simple subdivision in only three fundamental tectonic domains (Tethys Himalaya, High Himalaya, Lesser Himalaya), recognizable with similar features all along the Himalaya and separated by major, more or less continuous tectonic lineaments such as the Zanskar Normal Fault-South Tibetan Detachment System (Burg et al. 1984; Herren 1987; Burchfiel et al. 1992; Carosi et al. 1999), the Main Central Thrust, and the Main Boundary Thrust (Gansser 1964; 1980), has been elegantly confirmed by geochemical evidence, with the discovery that isotopic characteristics and Nd model ages are sharply distinct for Greater Himalayan and Lesser Himalayan units (Parrish &amp; Hodges 1996; Whittington et al. 1999).

But general agreement on a simple tectonic model does not necessarily represent only a positive fact. Experience tells us that simplicity is a value greatly appreciated and eagerly looked for by our limited mind, which instinctively shuns the risk of feeling lost when confronted by the complexity of natural phenomena. As a consequence, scientific observation may be unconsciously constrained to rigid familiar tracks, thus loosing sensibility for geological and structural differences which characterize different portions of the Himalayan belt.
Moreover, we are not willing to forget that oversimplification, pushed to the limit of superficiality, matches quite well the need felt more and more urgently particularly by researchers of Anglo-saxon countries (who live in a very competitive milieu and are therefore forced to produce, often within overshort deadlines, glamorous scientific results). Such an approach is obviously also welcomed by modellers, who are forced not to consider in their calculations each and every complication that typically characterizes all complex natural systems.

Our four Research Units thus intend to follow in the future, coherently with our experience in the past, a quite different analytical approach, moving from a wide base of facts carefully observed in the field and tested in the laboratory (Gansser 1990). Experience teaches us in fact that models yet currently fashionable with the scientific community are not necessarily adequate nor last for long time spans. This chiefly because the tectonic structure of the Himalaya appears to be more complex than commonly assumed, and because large parts - particularly the southern (i.e., Lesser Himalaya) and eastern ones (e.g., Sikkim, Bhutan, Arunachal Pradesh) - are still known only at reconnaissance level.

Least understood are the initial stages of continental collision, particularly as regards tectonic, topographic, and sedimentary evolution. No satisfactory explanation has yet been provided to the surprising fact that a subsident foreland basin did not form until the close of the Oligocene, some 30 Ma after the onset of continental collision (DeCelles et al. 1998; 2004; Najman &amp; Garzanti 2000; Najman et al., 2001; 2003; 2006). As the structure of the thrust belt is concerned, it has long been recognized that major tectonic lineaments such as the Main Central Thrust and Main Boundary Thrust cannot be traced everywhere unambiguously. In the north-western Himalaya, for instance, they tend to loose their identity (e.g.,Greco &amp; Spencer 1993; Fontan et al. 2000), whereas the tectono-stratigraphic position of some major tectonic unit is still unclear (e.g., Chamba Nappe of Thakur 1998; Steck 2003). Also in the central Himalaya the location of the Main Central Thrust has been long and lively debated (Upreti 1999; Searle, Law &amp; Godin 2006), and the origin of some tectonic units remains uncertain (e.g., Kathmandu Nappe; Upreti &amp; Le Fort 1999). In Bhutan, the reported widespread occurrence of crystalline units ascribed to the High Himalaya, overlain in several localities by weakly metamorphosed successions of Tethyan affinity (Gansser 1983), may in fact conceal a more complex tectono-stratigraphy, and a greater areal distribution of Lesser Himalayan crystalline units.
These cautionary lines intend to suggest that an approach strongly focused on the application of advanced technologies and aimed at rapidly achieving sensational results may lead to partly misleading interpretations if not coupled by patient attention to the analysis of geological objects in their structural context and in their full lateral extent.

A more classical approach, traditionally peculiar of Alpine geologists, has been followed by leaner groups of Swiss, Austrian, French, or Italian researchers, who produced admittedly less widely-cited publications, but generally quite accurate and very useful for a deeper level of understanding of geological evolution and tectonic structure. A manifest demonstration of this is the striking contrast between the increasing number of high-profile scientific publications dedicated to the Himalaya and the scarcity of high-quality, up-to-date geological maps. General maps of the Himalaya date back to the Sixties, Seventies and early Eighties (Gansser 1964; 1980; Fuchs 1980; Burg et al. 1984), whereas detailed larger-scale maps are limited to a few areas. Major exceptions are the accurate maps and cross-sections drawn by French researchers since the late Sixties in central and eastern Nepal (Bordet 1961; Bordet et al. 1971; Colchen et al. 1986) and the excellent works carried out by the Lausanne group, who in the last twenty years have focused their efforts along the Lahul-Ladakh geological traverse (Steck et al. 1998; Wyss et al. 1999; Steck 2003).

Following a similar approach, which is coherent with the experience gained in the last decades by all four Research Units in the study of both Alpine and Himalayan mountain belts, we will concentrate much of our efforts in the field and laboratory analysis of geological traverses in Sikkim and western Bhutan. The new results will be compared with the numerous data recently obtained along the Eastern Nepal transect (Carosi et al. 1999; Lombardo &amp; Rolfo 2000; Visonà &amp; Lombardo 2002; Borghi et al. 2003; Pertusati et al. in preparation; Rolfo et al. in preparation), and will allow a deeper understanding particularly of early-stage high-pressure metamorphic events and of later-stage generation and emplacement of leucogranite magmas.

By building on the significant results obtained in recent years, the Project PRIN 2006 will make a systematic use of advanced geochemical, thermochronological, and geochronological techniques, which will provide key data to unraveling the problems related to the recognition of protoliths and to the superposition of successive metamorphic and magmatic events. As a strong and original point of our new Project, we will study both the rocks exposed in the Himalayan belt and the foreland-basin successions derived from its erosion. The latter not only record the very same tectono-metamorphic events, and therefore allow their independent reconstrution and dating by using stratigraphic techniques, but also preserve the history of shallower structural levels, which have long been eroded and therefore are not documented anymore in outcrop. The analysis of sedimentary successions will shed new light specifically on the early stages of mountain building. As far as the Neogene evolution is concerned, the tentative reconstruction of paleodrainage changes in time will provide key information to constrain the tectonic and erosive evolution of source areas. To this goal we will follow a strongly actualistic approach, based on detailed knowledge of the relationships between the geology of source areas and sediment composition, largely achieved by the work done in recent years (Garzanti et al. 2004; 2005).

The Project PRIN 2006 will continue the long-standing tradition of Italian geological research in the Himalayas, which began nearly a century ago with the pionieering expeditions of Giotto Dainelli and Ardito Desio, and was continued actively in the last twenty years, particularly as concerns the study of high-pressure metamorphic events (Villa et al. 1996; Lombardo et al. 1998; 2000; Lombardo &amp; Rolfo 2000), of Miocene leucogranitic intrusions (Visonà &amp; Lombardo 2002), and of the pre-collisional to post-collisional sedimentary record (Gaetani &amp; Garzanti 1991; Garzanti et al. 1996; 2004; 2005). <<<