Downloaded from gsabulletin.gsapubs.org on 30 July 2009 Geological Society of America Bulletin The Kumaun and Garwhal Lesser Himalaya, India: Part 1. Structure and stratigraphy Julien Célérier, T. Mark Harrison, Andrew Alexander G. Webb and An Yin Geological Society of America Bulletin 2009;121;1262-1280 doi: 10.1130/B26344.1 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Structure and stratigraphy Julien Célérier1†, T. Mark Harrison1,2, Andrew Alexander G. Webb2, and An Yin2 1Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia 2Department and Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567 ABSTRACT tary strata in the Ramgarh Thrust hanging Himalayan Crystallines only represent ~25% wall are correlative with basal units of the of the Himalaya, and its evolution surely does Our understanding of the geologic evolu- Outer Lesser Himalayan Sequence, remov- not represent that of the whole orogen. We con- tion of the Himalaya remains incomplete, ing the necessity that the Ramgarh Thrust ducted an integrated geologic investigation of particularly in regard to structural and hanging wall was allochthonous with respect the Lesser Himalayan Sequence in the Kumaun geochronologic details of the Proterozoic- to its footwall Lesser Himalayan Sequence and Garwhal Himalaya of NW India (78°00′E Paleozoic Lesser Himalayan Sequence. We units. Schistose gneisses with U-Pb ages of and 80°30′E). There the Lesser Himalayan conducted an integrated fi eld mapping, ca. 1850 Ma are older than Lesser Hima layan Sequence is composed of generally low-grade geochronological study, and geochemical Sequence units in the area, suggesting that rocks juxtaposed beneath the Greater Himala- analysis of the Lesser Himalayan Sequence they are Lesser Himalayan Sequence base- yan Crystallines by the Main Central Thrust. strata in the Kumaun and Garwhal Hima- ment. We conclude that the Ramgarh Thrust Our investigations (fi eld mapping, detailed laya of NW India (78°°00′–80°°30′E). Struc- is an out-of-sequence thrust postdating a structural analyses, U-Pb zircon dating, and tural observations reveal a systematic change folding event in its footwall. The earlier pro- geochemistry) clarify aspects of the chrono- in deformation styles from lower to higher posal that Neoproterozoic-Cambrian Lesser stratigraphy of the Kumaun-Garwhal thrust structural levels in the Main Central Thrust Himalayan Sequence strata are the south- belt, indicate basement-involved thrusting, and footwall. In the south, and at lower struc- ern extension of the Tethyan Hima layan document a systematic change in deformation tural levels, the Main Central Thrust foot- Sequence requires that the Tons Thrust, mechanism from lower to upper structural lev- wall is characterized by parallel folding and which separates the distal and proximal els. These results are combined with thermo- sparse development of axial cleavage. Quartz facies of the Lesser Himalayan Sequence, be chronologic and thermometric data in a com- microstructures indicate that deformation a major, south-directed structure with a slip panion study (Célérier et al., 2009) to constrain occurred at temperatures below 350 °°C. In magnitude of >50–100 km. a thermokinematic model for the evolution of contrast, in the north and at higher struc- the region. tural levels close to the Main Central Thrust, INTRODUCTION deformation is characterized by replacement REGIONAL GEOLOGY of original bedding, the development of pene- Following the development of plate tectonic trative cleavage, and schistosity locally. Folds theory in the 1960s, it was recognized that Structural Framework are tight, in places isoclinal and overturned. the Himalayan mountain system represents The corresponding quartz microstructures a unique opportunity to study the response The geology of the Kumaun and Garhwal indicate that deformation occurred above of continental lithosphere to collision (e.g., Himalaya has been studied for over a cen- 350 °°C. Dating of detrital zircons from Dewey and Burke, 1973; LeFort, 1975). Our tury (e.g., Middlemiss, 1885; Holland, 1908; Lesser Himalayan Sequence metasedimen- present knowledge of this spectacular moun- Auden, 1935; Heim and Gansser, 1939; Misra tary units, and igneous zircons from schistose tain range has been largely derived from stud- and Sharma, 1967; Jain, 1971; Rupke, 1974; gneisses throughout the Lesser Himalayan ies of the high-grade Greater Himalayan Crys- Valdiya, 1980a; Valdiya, 1995; Srivastava and Sequence allow refi nements to the strati- tallines in the core of the orogen. The Greater Mitra, 1994; Richards et al., 2005) (Figs. 1 graphic framework. First, unlike what has Himalayan Crystallines preserve a wealth of and 2). It was in this area that Heim and Gans- been observed in western Nepal immediately pressure-temperature conditions and exhuma- ser (1939) fi rst established their classic Hima- west of our study area, U-Pb zircon dating tion paths under which the mountain range has layan division: the Subhimalayan Sequence, the suggests the absence of Paleoproterozoic been developed (see Yin and Harrison, 2000). Lesser Himalayan Sequence, the Greater Hima- strata in the Ramgarh Thrust hanging wall. For this reason, kinematic and dynamic models layan Crystallines, and the Tethyan Himalayan This suggests that the thrust cuts upsection of the Himalaya have been largely developed Sequence that are stacked from south to north laterally along strike. Second, U-Pb detrital based on our current knowledge of the Greater by the north-dipping Main Frontal Thrust, Main zircon dating suggests that metasedimen- Himalayan Crystallines (see reviews by Yin Boundary Thrust, Main Central Thrust, and later [2006] and Kohn [2008]). A major problem recognized South Tibet Detachment (also see †E-mail: juliencelerier@groundfl oorgraphics.com with the above approach is that the Greater LeFort, 1996) (Fig. 2). GSA Bulletin; September/October 2009; v. 121; no. 9/10; p. 1262–1280; doi: 10.1130/B26344.1; 13 fi gures; 1 table; Data Repository item 2008254. 1262 For permission to copy, contact [email protected] © 2009 Geological Society of America America Bulletin,September/October2009 Geological Societyof 1263 Downloaded from Figure 1 (to left). Digital elevation model of the western Himalaya, Tibetan plateau, and Tarim basin. Inset on bottom right shows the 31o30’ position and physiography of the Kumaun and Garwhal Himalaya. Himachal gsabulletin.gsapubs.org Pradesh TIBET (CHINA) andstratigraphy andGarwhalLesserHimalaya:Structure The Kumaun Bhagi Garwhal Figure 2 (below). Simplifi ed geologic map of the Himalaya. The posi- Tons tion of the study area within the Himalaya relative to the Kunlun- rathi Rampur regions (location of Richards et al. [2005] and Miller et al. [2000] studies) shown. Location of Ulleri gneiss sample also indicated Alaknanda (sections 4 and 5). on30July2009 Ganges Kumaun Uttar Kali NEPAL 50 km Pradesh Indus R. Bangong- Cz, Cenozoic foreland basin LHS, Lesser Himalayan Sequence and age 100°E Bangong- Nujiang sediments equivalents in the Shillong Plateau GHC, Greater Himalayan Sequence and Nujiang Cz suture Late Jurassic to early Tertiary 32°NN age-equivalent rocks in the Shillong Plateau Gangdese batholith (150–50 Ma) suture THS, Tethyan Himalayan Sequence MFT, Main Frontal Thrust MBT, Main Boundary Thrust Sutlej R. Mt. Kailas MCT, Main Central Thrust STD, South Tibet Detachment Mekong R. Jinsha suture Yalong R. Lhasa Terrane MFT Ga NW India ng South 30°N 76°E7 Himalaya de 78°E Indus- se B China Tsangpo ath Block Bhagirathi R. olith B B suture elt Namche Barwa Yangtze (7782 m) 80°E YaluTsangpo E.E STD . AAruAr 40°N ruu R. iri n.n Tarim basin Subans R. HimHHi THS m A STD ma aa.. NepalN Manas R. Hima.H ma.a. Arun MCT Tibetan plateau MCT Sikkimi GHC W. Arun. Hima. 82°E A I n Hima.i H d us im -T Bhutanan a sa 30°N 84°E n Salween R. Kunlun-Rampur area la gpo LHS ya suture MBT Himalayaa tic n ge n a O G rog en 86°E Pl . ain a R Arun R. Teesta R. tr o Region investigated in this study Kali pu d 90°E Brahma Indo-Burma-In Gandaki R. 88°E AndamanIndian Peninsular Highlands Shillong Plateau terrane Ulleri gneiss sample location 0 100 200 km 20°N Jamuna R. 92°E929 94°EE 70°E 80°E 90°E 100°E Downloaded from gsabulletin.gsapubs.org on 30 July 2009 Célérier et al. The Main Central Thrust, as defi ned by Heim and Garwhal Himalaya, the Lesser Himalayan tacts between crystalline bodies (possibly base- and Gansser (1939) in Kumaun, has been cor- Sequence can be further divided into the Inner ment) and sedimentary rocks of the Lesser related across the orogen (see review by Yin (older) and Outer (younger) Lesser Himalayan Himalayan Sequence.