A structural analysis of the Main Central Thrust zone, Langtang National Park, central Nepal Himalaya A. M. MACF ARLANE* 1 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, K. V. HODGES J Massachusetts 02139 D. LUX Department of Geological Sciences, University of Maine at Orono, Orono, Maine 04469 ABSTRACT within the MCT zone may have initiated as orogen has experienced multiple regional meta- the MCT zone was transported over a ramp morphic events (see Hodges and others, 1988, The Main Central Thrust (MCT) is one of in the MBT. and Pêcher, 1989, for reviews), but the latest the most tectonically significant structures in and most widespread in the central Himalaya the Himalayan orogen. Detailed geologic INTRODUCTION was a medium- to low-pressure amphibolite- mapping and structural analysis of the MCT facies episode that included anatectic melting re- in the Langtang National Park region of cen- The Main Central Thrust (MCT) is a major sponsible for leucogranite plutons like the tral Nepal reveals that this segment of the north-dipping fault zone in the Himalayan oro- Manaslu granite of central Nepal (Le Fort, fault zone experienced multiple episodes of gen that is estimated to have accommodated at 1975). Reliable U-Pb zircon and monazite crys- south-directed movement, under both brittle least 100 km of crustal shortening in Tertiary tallization ages for the leucogranites in the cen- and ductile conditions, during the Tertiary time (Gansser, 1966; Brunei, 1975; Pêcher, tral Himalaya range from late Oligocene to period. Early (mid-Miocene) movement re- 1978). One of the most controversial issues as- Miocene (Schârer, 1984; Schârer and others, sulted in the development of mylonitic fabrics sociated with the MCT is its temporal relation- 1986; Deniel and others, 1987; Copeland and synchronous with amphibolite-facies meta- ship with regional metamorphism. In the Garh- others, 1988; Parrish, 1992). On the basis of morphism. The mean orientation of the wal Himalaya of India (Fig. 1), the MCT geochronologic data for the latest metamorphic dominant mylonitic foliation is N28°W, corresponds to a sharp discontinuity between event from the Everest region, Hubbard and 38°NE. An associated mineral/stretching lin- middle to upper amphibolite-facies rocks to the Harrison (1989) established that some move- eation plunges 40° to N40°E. Kinematic indi- north and greenschist-facies units to the south ment on the MCT occurred at approximately 21 cators suggest hanging-wall movement to the (Heim and Gansser, 1939; Gansser, 1964; Val- Ma. In contrast, if it is assumed that most of the southwest relative to the footwall along the diya, 1979; Valdiya, 1980; Hodges and Silver- movement on the MCT was post-metamorphic north-dipping fault. It is not possible to con- berg, 1988). In the Manaslu region of central (Gansser, 1964; Valdiya, 1979; Valdiya, 1980; strain the magnitude of high-temperature dis- Nepal (Fig. 1 ), the MCT has been described as a Brunei, 1986; Brunei and Kienast, 1986; Schell- placement on the MCT at the longitude of ductile shear zone across which there is no sig- ing, 1987), then the implication is that this Langtang. Late-stage structures in the MCT nificant metamorphic break (Bordet, 1961; Le faulting occurred more recently than mid- zone at Langtang include a series of imbri- Fort, 1975; Bouchez and Pêcher, 1976; Pêcher, Miocene time. cate, brittle thrust faults that separate differ- 1977). More-recent studies have produced evi- There are three possible explanations for dis- ent lithostratigraphic units and correspond to dence for syn-metamorphic thrusting in the agreements among Himalayan geologists about metamorphic discontinuities. We interpret Everest region of eastern Nepal (Hubbard, the relative ages of metamorphism and thrusting this fault system as a duplex structure. Mus- 1988; Hubbard, 1989; Fig. 1), post-metamor- 40 39 on the MCT. First, it is possible that the MCT covite Ar/ Ar cooling ages from the MCT phic thrusting in the Makalu region of eastern has become a generic term for any fault zone zone range from 8.9-6.9 Ma. Because the Nepal (Brunei and Kienast, 1986; Fig. 1), and that marks that physiographic break between nominal closure temperature of Ar diffusion diachronous movement spanning regional meta- the higher Himalaya and the lower Himalaya, in muscovite (approximately 62S K) is higher morphic events in the Rowaling region of east- such that the MCT in one region may have a than the apparent temperature conditions central Nepal (Schelling, 1987; Fig. 1) and the different age than the MCT in another region under which late brittle deformation oc- Annapurna region of central Nepal (Caby and (Hodges and others, 1988; Hodges and others, curred, we suggest that brittle deformation others, 1983; Fig. 1). 1989). Second, the MCT may be the same struc- was a latest Miocene-Pliocene phenomenon. Inconsistencies in the tectonic histories of dif- ture in all areas, but the age and/or southward Another major Himalayan fault, the Main ferent parts of the orogen, such as Garhwal and extent of regional metamorphism may vary Boundary Thrust (MBT), was developing to central Nepal, have led to disagreements about along the range. Third, the relationship between the south of Langtang at approximately the the age of the MCT. If it is assumed that move- faulting and metamorphism may be difficult to same time. We speculate that brittle faulting ments on the MCT are syn-metamorphic, then it establish with certainty in some areas due to follows that geochronologic measurements of poor exposure. In many parts of the central the age of metamorphism in the hanging wall Himalaya, the MCT lies in thickly wooded or *Present address: Center for Materials Research in Archaeology and Ethnology, Massachusetts Institute provide constraints on the absolute age of fault- extensively cultivated terrain where outcrops are of Technology, Cambridge, Massachusetts 02139. ing. The metamorphic core of the Himalayan relatively sparse. This is not the case in the Lang- Geological Society of America Bulletin, v. 104, p. 1389-1402, 14 figs., 6 tables, November 1992. 1389 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/11/1389/3381265/i0016-7606-104-11-1389.pdf by guest on 02 October 2021 MACFARLANE AND OTHERS ^SJ \ Garhwal N \ T region • 30°-j 100 km Annapurna'y region Langtang National Manaslu\^ Park (Fig. 2) region V Everest ' Kathmandu 1 Everest Tibetan Sedimentary Series Greater Himalayan Sequence ; : ; |Lesser Himalayan Sequence [subhimalayan Sequence 85° 88° Figure 1. General geologic map of Nepal. Location map of south Asia is in box on top right. The Main Central Thrust (MCT) is the barbed thrust contact between the Greater Himalayan sequence and the Lesser Himalayan sequence. The Main Boundary Thrust (MBT) is the barbed thrust contact between the Lesser Himalayan sequence and the Subhimalayan sequence. tang National Park region of central Nepal (Fig. sediments of Cambro-Ordovician to Eocene (?) Himalayan sequence and the Lesser Himalayan 1), where extensive ridgetop exposures and a age. The base of the Tibetan Sedimentary Se- sequence is the Main Central Thrust. As mapped remarkable recent roadcut provide an unusual quence is marked by a family of Miocene, post- by various workers (for example, Gansser, 1964; opportunity for a detailed study of the MCT. metamorphic normal faults collectively referred Le Fort, 1975; Brunei, 1975), the MCT zone Our investigation shows that the MCT in the to as the South Tibetan detachment system ranges in thickness from <50 m to > 10 km and Langtang area had a complex movement history (Burchfiel and others, 1992; Burg and others, dips moderately northward. The MCT zone is involving at least two distinct periods of defor- 1984; Burchfiel and Roy den, 1985; Herren, characterized by well-developed mylonitic fab- mation, with initial thrusting during Miocene 1987). The underlying Greater Himalayan se- rics that indicate hanging-wall movement to regional metamorphism followed by a major quence consists primarily of amphibolite-facies the south relative to the footwall (Pêcher, 1978; displacement after approximately 9-7 Ma. gneisses and schists of Precambrian to Cambrian Brunei, 1986). The Main Boundary Thrust (?) age that are locally migmatitic. Large leu- forms the lower contact of the Lesser Himalayan TECTONIC SETTING cogranite plutons, commonly containing tour- sequence. It is an active north-dipping structure maline, occur near the top of the Greater that carries the Lesser Himalayan sequence over From western India to eastern Nepal, the core Himalayan sequence. The Greater Himalayan unmetamorphosed molasse units of the Sub- of the Himalaya is composed of three major sequence rocks overlie the Lesser Himalayan himalayan sequence (Molnar, 1984). tectonostratigraphic units separated by north- sequence (the "Midlands Formations" of Le Langtang National Park is located approxi- dipping structural discontinuities (Fig. 1). North- Fort, 1975), a package of upper Precambrian (?) mately 50 km north of Kathmandu, at approxi- ernmost and structurally highest is the Tibetan to lower Eocene (?), greenschist- to lower- mately 28°10'N and 85°25'E. Figure 2 is a Sedimentary Sequence, dominated by a series of amphibolite-facies, metasedimentary and meta- geologic map of part of this area, based on shallow-marine continental and miogeoclinal volcanic rocks. The contact between the Greater 1:25,000 mapping conducted in 1989 and 1990 1390 Geological Society of America Bulletin, November 1992 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/11/1389/3381265/i0016-7606-104-11-1389.pdf by guest on 02 October 2021 MAIN CENTRAL THRUST, CENTRAL NEPAL HIMALAYA presence of syn- to post-kinematic porphyro- blasts in these rocks. Quantitative thermoba- rometry provided estimates of the conditions of M2 in Lesser Himalayan sequence rocks of 767-787 K and 626-665 MPa; in lower MCT zone rocks, 700-785 K and 510-634 MPa; in upper MCT rocks, 794-849 K and 522-899 MPa (Macfarlane, 1992).