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

Abstract

The exhumed Himalayan midcrustal core exposed in the Likhu Khola region of east-central Nepal includes upper-greenschist-to upperamphibolite-grade metamorphic rocks that record pervasive, top-to-the-south sense deformation. Metamorphic temperature estimates are within error across the mapped area ranging from 772 ± 37 °C in the structurally lower, southern part of the study area to 853 ± 58 °C in the structurally higher, northern area. Estimated metamorphic pressures are relatively constant at lower structural levels, but they decrease from 11.8 ± 1.4 kbar to a minimum of 6.5 ± 1.3 kbar up structural section. The decrease in pressures coincides with an abrupt change in pressure estimates up structural section that is interpreted to mark a tectonometamorphic discontinuity that separates two domains with distinct structural, thermal, and metamorphic histories. In situ laser-ablation split-stream monazite U-Th/Pb and rare earth element (REE) petrochronology outlines dates ranging from ca. 27.8 Ma to 15.1 Ma in the hanging wall of the interpreted discontinuity; monazite REE data indicate the spread in ages is the result of a protracted metamorphic history and late-stage anatexis. Metamorphic and petrochronologic data from the Likhu Khola are consistent with a kinematic model in which material structurally above the discontinuity was metamorphosed in the deep hinterland of the orogen and was subsequently incorporated into the foreland of the orogen. The transition from hinterland-to foreland-style processes was marked by a shift to discrete deformation and the development of the discontinuity. Movement across the discontinuity is interpreted to have driven metamorphism and deformation of the rocks structurally below at or after 15 Ma. Discontinuities similar to that identified in this study are being identified and described across the orogen, indicating they are important, orogenwide features.

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

“…Reconnaissance scale mapping that included the Likhu Khola region was carried out in the 1960s (Ishida, ) and followed up in the 1990s (Schelling, ). More recent detailed geological mapping and initial metamorphic characterization of the region was reported in two companion studies (From & Larson, ; From et al., ). These works show that the rocks of the study area are dominantly metasedimentary schists and quartzites with subordinate graphitic and calc‐silicate layers to migmatitic paragneiss, and subordinate orthogneiss (From & Larson, ).…”

“…Buick, Hermann, Williams, Gibson, & Rubatto, ; Hermann & Rubatto, ; Rubatto, Hermann, & Buick, ) and monazite petrochronology (e.g. Cottle, Searle, Horstwood, & Waters, ; Larson et al., ) with published monazite geochronology results (From et al., ) as a complete package to elucidate the P–T–t evolution of the exhumed mid‐crustal rocks exposed across the region. The results of this study confirm the existence of the proposed discontinuity, and provide new insight into the cryptic mid‐crustal structures within the exhumed HMC.…”

“…Study area for this project is outlined by the black box. (b) Simplified geological map and geological cross‐section of the study area outlining the metamorphic zones and the inferred discontinuity (after From & Larson, ; From et al., )…”

“…During the last two decades, disagreement over the models of development of the Himalayan metamorphic core (HMC), a package of pervasively deformed and metamorphosed rocks that record mid‐crustal deformation and metamorphism during the Cenozoic evolution along the Himalaya (Cottle et al., ; From, Larson, & Cottle, ), has led to the proliferation of numerous orogenic models explaining the various data sets collected. For example, end member critical‐taper wedge models (Hodges, Parrish, & Searle, ; Kohn, ; Robinson, DeCelles, & Copeland, ), channel flow models (Beaumont & Jamieson, ; Beaumont, Jamieson, Nguyen, & Lee, ; Beaumont, Jamieson, Nguyen, & Medvedev, ; Godin, Grujic, Law, & Searle, ; Grujic, Hollister, & Parrish, ; Grujic et al., ; Hodges, , ; Jamieson, Beaumont, Medvedev, & Nguyen, ); tectonic wedging models (Webb, Schmitt, He, & Weigand, ; Webb, Yin, Harrison, Célérier, & Burgess, ) and “hybrid models” (Cottle et al., ; Larson, Ambrose, Webb, Cottle, & Shrestha, ; Larson, Gervais, & Kellett, ) have all attempted to explain the kinematics and structural geometries of the HMC.…”

“…This field‐based, integrated study examines a previously inferred discontinuity across a well‐exposed portion of the HMC in the Likhu Khola region of east central Nepal (Figure b; From et al., ), combining phase equilibria modelling (e.g. Guilmette, Indares, & Hébert, ; Indares, White, & Powell, ; Larson et al., ; Vance & Mahar, ), high‐precision Lu–Hf garnet geochronology (e.g.…”