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The Geology of the Tama Kosi and Rolwaling Valley Region, East-Central Nepal

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The Geology of the Tama Kosi and Rolwaling Valley Region, East-Central Nepal The geology of the Tama Kosi and Rolwaling valley region, East-Central Nepal Kyle P. Larson* Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan, S7N 5E2, Canada ABSTRACT Tama Kosi valley in east-central Nepal (Fig. 1). which were in turn assigned to either the Hima- In order to properly evaluate the evolution of the layan gneiss group or the Midlands metasedi- The Tama Kosi/Rolwaling area of east- Himalaya and understand the processes respon- ment group (Fig. 2). The units of Ishida (1969) central Nepal is underlain by the exhumed sible for its formation, it is critical that all areas and Ishida and Ohta (1973) are quite similar in mid-crustal core of the Himalaya. The geol- along the length of the mountain chain be inves- description to the tectonostratigraphy reported ogy of the area consists of Greater Hima- tigated, at least at a reconnaissance scale. by Schelling (1992) who revisited and expanded layan sequence phyllitic schist, paragneiss, The Tama Kosi valley is situated between the the scope of their early reconnaissance work. and orthogneiss that generally increase in Cho Oyu/Everest/Makalu massifs to the east Schelling (1992) separated the geology of metamorphic grade from biotite ± garnet and the Kathmandu klippe/nappe to the west the lower and middle portion of the Tama Kosi assemblages to sillimanite-grade migmatite (Fig. 1). Recent work in these areas serves to into the more traditional Greater Himalayan up structural section. All metamorphic rocks highlight stark differences between them. In the sequence (Higher Himalayan Crystallines) and are pervasively deformed and commonly Kathmandu region, the extruded midcrustal core Lesser Himalayan sequence lithotectonic assem- record top-to-the-south sense shear. The top is folded and preserved far into the orogenic blages. His “Higher Himalayan Crystallines” of the Greater Himalayan sequence in the foreland in the form of a klippe or nappe (e.g., approximately correspond to Ishida and Ohta’s mapped area is marked by an undeformed, Johnson et al., 2000). Furthermore, there may be (1973) Himalayan gneisses (Fig. 2) and include pegmatitic leucogranite stock. Relation- evidence for a merger of the two major, antithetic a series of sillimanite-bearing para gneiss and ships in adjacent areas constrain the age of fault systems that bound the mid-crustal core orthogneiss units that display varying degrees the leucogranite and the deformation struc- (Webb et al., 2011). In contrast, the geology of of partial melting, granitic intrusion, and mig- tures it crosscuts, including the top-to-the- the Everest region refl ects a deeply eroded, for- matization. The rocks within Schelling’s (1992) south sense deformation, to be older than merly ductily extruded mid-crustal channel and Lesser Himalayan sequence comprise much of middle Miocene. The lower portion of the associated leucogranitic bodies (e.g., Searle et al., Ishida and Ohta’s (1973) Midland metasedimen- exhumed midcrustal package has been sub- 2006; Jessup et al., 2006; Cottle et al., 2009; tary group (Fig. 2). The lithologies mapped by ject to late-stage folding during the forma- Streule et al., 2010). This makes the Tama Kosi Schelling as the Lesser Himalaya include locally tion of the Tama Kosi window, a structural valley area important not only to help complete graphitic-rich, garnet ± staurolite ± kyanite culmination that may refl ect out-of-sequence the geologic map of the Himalaya, but also for schist, and orthogneiss, commonly K-feldspar adjustment of the orogenic wedge. The geol- potential assessments of lateral variation during augen-bearing. ogy of the mapped area appears similar to the evolution of the mountain belt. This prelimi- that observed in the adjacent, better-studied nary study presents the basic lithology, structure, Structure Everest region. and metamorphism recorded in the Tama Kosi area and interprets those fi ndings within the cur- Differences in the structural interpretation INTRODUCTION rent conceptual framework of the orogen. between previous studies of the lower and middle Tama Kosi valley and area are more sig- Much has changed in our understanding of GEOLOGIC SETTING— nifi cant than those for the tectonostratig raphy. orogenesis since the fi rst reconnaissance map- PREVIOUS WORK Ishida (1969) mapped thrust faults between ping of the Himalaya. The ideas put forth to almost every “formation” or “zone” (Fig. 2), explain the varied evolution of the orogen, such Lithology separating them into tectonically bound rock as midcrustal fl ow (e.g., Bird, 1991; Grujic et al., units. Each of these faults is considered to 1996; Beaumont et al., 2001), critical taper (e.g., The geology of the lower and middle por- be an originally north-dipping structure that DeCelles et al., 1998a, 1998b), and plateau col- tions of the Tama (also written as Tamba) Kosi accommodated top-to-the-south sense dis- lapse (e.g., England and Houseman, 1988), have valley (Fig. 1) was fi rst reported on in the late placement. Subsequent folding of some of the all been based on geologic map interpretation. 1960s (Ishida, 1969). This early work outlined faults has since modifi ed their dip direction There are still some areas along the mountain the basic lithologic framework of the area and and, paired with erosion, favored the develop- belt, however, that have not been mapped. One how it is related to the adjacent Everest/Makalu ment of tectonic windows (Ishida, 1969). In of those areas is the uppermost portion of the region. Ishida subdivided the area into a series contrast, Schelling (1992) considered the con- of tectonic units termed “formations” (Ishida, tacts between most lithotectonic units to be *[email protected]. 1969) or “zones” (Ishida and Ohta, 1973), gradational, at least in the Higher Himalayan Geosphere; April 2012; v. 8; no. 2; p. 507–517; doi:10.1130/GES00711.1; 8 fi gures. For permission to copy, contact [email protected] 507 © 2012 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/2/507/3343632/507.pdf by guest on 29 September 2021 Larson 70°E 80°E 90°E 40°N Sub-himalaya Lesser Himalaya Miocene Granites MFT=Main Frontal thrust Tethyan Himalaya MBT=Main Boundary thrust 30°N RT=Ramgarh thrust Greater Himalaya MCT=Main Central thrust Lower Greater/ STDS=South Tibetan detachment system NEPAL Upper Lesser MHT=Main Himalayan thrust 20°N (A) Figure 1. Regional-scale geologic 80° map of Nepal after McQuarrie et al. (2008). The present study 30° MCT Tama Kosi area is outlined by a thin black INDIA 88° S map-area line in B. 28° RT MCT MFT Pokhara NEPAL (B) MBT KathmanduKathmandu MCT 28° 0 100 MFT 80° km INDIA Crystallines . He mapped a major thrust disconti- Ishida (1969) and Ishida Schelling (1992) nuity, the Main Central thrust, at the base of the and Ohta (1973) Higher Himalayan Crystallines that juxtaposed H IMALAYAN them on top of the Lesser Himalayan sequence Rolwaling-Khumbu (Fig. 2). While Schelling (1992) considered his granites Main Central thrust to be the major structural Rolwaling-Khumbu Khumbu GNEISSES discontinuity in the region, he also recognized IMALYAN Khumbu formation thrust intense shearing of his mapped Lesser Hima- H paragneiss layan sequence rocks below the structure. He RYSTALLINES Rolwaling-Khumbu Solo C considered these rocks, which include all units IGHER Solo formation thrust H Main migmatite below the Main Central thrust outlined in Fig- Central ure 2, to be part of the Lesser Himalayan Shear thrust Junbesi paragneiss M Jiri formation Jiri IDLAND Zone (Schelling, 1992). He notes that it is akin Khare phyllite thrust to the Zone des Ecailles of Bordet (1961), the GROUP Melung-Salleri augen METASEDIMENT “MCT zone” of Arita (1983), and the “Nappes Melung augen gneiss Midland gneiss thrust Inferieurs” of Brunel (1986) and Brunel and IMALAYAN Kienast (1986). The rocks that comprise this H Dolakha phyllite Dolakha formation zone are characterized by mylonitic deforma- SERIES Suri Dhoban augen Tam(b)a Kosi window tion structures (Schelling, 1992) and record ESSER inverse metamorphism with low-grade rocks at L gneiss formation low structural levels and higher-grade rocks at higher structural levels (Ishida and Ohta, 1973; Figure 2. Lithotectonic correlations between previous studies that have examined a portion Schelling, 1992). of the present study area. TECTONOSTRATIGRAPHY OF THE UPPER TAMA KOSI AND Kosi river from Dolhaka in the south to the of the Everest region (e.g., Jessup et al., 2006; ROLWALING VALLEYS Nepal-Tibet border and the river’s headwaters Goscombe et al., 2006) and the Kathmandu in the north (previous studies did not include klippe/nappe (e.g., Johnson et al., 2000; Webb The present study builds on the previous areas north of Lamabagar; Fig. 3). This map- et al., 2011), which is signifi cantly different work by Schelling (1992) and Ishida and Ohta ping was also extended into the tributary Rol- from the interpretations of either Ishida (1969) (1973) and extends into areas not yet reported waling and Khare valleys (Fig. 3). These new or Schelling (1992). on geologically. This study presents the results observations allow for direct comparison of All rocks in the study area have been meta- of detailed geologic mapping along the Tama the Tama Kosi region to the present knowledge morphosed; in general, metamorphic grade 508 Geosphere, April 2012 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/2/507/3343632/507.pdf by guest on 29 September 2021 Geology
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