The geology of the Tama Kosi and Rolwaling valley region, East-Central Nepal

Abstract

The Tama Kosi/Rolwaling area of eastcentral Nepal is underlain by the exhumed mid-crustal core of the Himalaya. The geology of the area consists of Greater Himalayan sequence phyllitic schist, paragneiss, and orthogneiss that generally increase in metamorphic grade from biotite ± garnet assemblages to sillimanite-grade migmatite up structural section. All metamorphic rocks are pervasively deformed and commonly record top-to-the-south sense shear. The top of the Greater Himalayan sequence in the mapped area is marked by an undeformed, pegmatitic leucogranite stock. Relationships in adjacent areas constrain the age of the leucogranite and the deformation structures it crosscuts, including the top-to-thesouth sense deformation, to be older than middle Miocene. The lower portion of the exhumed midcrustal package has been subject to late-stage folding during the formation of the Tama Kosi window, a structural culmination that may refl ect out-of-sequence adjustment of the orogenic wedge. The geology of the mapped area appears similar to that observed in the adjacent, better-studied Everest region.

The P–T–t evolution of the exhumed Himalayan metamorphic core in the Likhu Khola region, East Central Nepal

“…() inferred the location of a thrust discontinuity in the middle portion of the study area (Figure b). The inferred structure occurs along the transition from the garnet schist below to kyanite‐bearing paragneiss above (Figure b), which also coincides with a marked increase in the volume of anatexite in the rock up structural section (From et al., ; Larson, ; Larson & Cottle, ; Larson et al., ). A discontinuity at a similar structural level has also been reported in the nearby Nylam region to the west (Wang, Rubatto, et al., ; Wang, Zhang, et al., ; Wang et al., ).…”

“…In contrast to previous studies which mapped several, discrete large‐scale structures across the Likhu Khola region (Ishida, ; Ishida & Ohta, ), From and Larson () reported the evidence of pervasive ductile strain characterized by top‐to‐the‐south sense shear across the entire mapped area. Moreover, following the work of Larson () and Larson et al. () in an adjacent region, From et al.…”

Metamorphism and geochronology of the exhumed Himalayan midcrust, Likhu Khola region, east-central Nepal: Recognition of a tectonometamorphic discontinuity

“…Work in the adjacent Tama Kosi region to the west (Larson, 2012) has outlined a similar tectonostratigraphic sequence to that observed in the Likhu Khola region (From and Larson, 2014) that includes a dominantly metasedimentary package of exhumed midcrustal rocks intercalated at specific structural levels with orthogneiss. Moreover, Larson et al (2013) documented a metamorphic, geochronologic, and deformational discontinuity between stauroliteand kyanite-grade metamorphic rocks in the Tama Kosi valley.…”

“…1B) to investigate the lateral continuity of a documented structural discontinuity within similar rocks in the nearby upper Tama Kosi valley (Larson, 2012;Larson et al, 2013) to the west. Mapping was integrated with thermobarometric estimates and in situ monazite geochronology to provide constraints on the pressure-temperature-time history of the rocks.…”

“…As more detailed, precise, spatially registered, field-based data are contributed to the general Himalayan knowledge base, it is becoming apparent that the proposed "end members" are not mutually exclusive (e.g., Beaumont and Jamieson, 2010;Larson et al, 2010Larson et al, , 2011 www.gsapubs.org | Volume 6 | Number 5 | LITHOSPHERE 2013). Indeed, field-based studies have begun to outline a temporal and spatial continuum between the different "end-member" models (Larson et al, 2010(Larson et al, , 2011(Larson et al, , 2013Larson, 2012;Yakymchuk and Godin, 2012;Larson and Cottle, 2014). These studies describe discontinuities in the exhumed midcrust across which rocks record different deformational, metamorphic, and geochronologic histories.…”