Detailed field study of the Likhu Khola region in east-central Nepal has provided the basis for new lithological, metamorphic, and structural interpretations of the region. Metamorphic mineral assemblages define an inverted metamorphic field gradient from middle greenschist facies rocks (micaceous phyllitic schist) in the structurally lowest levels observed to upper amphibolite facies migmatitic rocks (sillimanite migmatite) in the structurally highest levels observed in this study. Quartz textures and c-axis orientations indicate that deformational temperatures also generally increase upward in the structural section, ranging from ∼490 °C to >650 °C. These temperatures are compatible with observed metamorphic mineral assemblages at lower structural levels, indicating that deformation and metamorphism may have been contemporaneous. All rocks in the mapped area are pervasively deformed and typically record a top-to-the-south sense of shear; no discrete large-scale thrust or normal-sense structures were observed. The deformational temperatures recorded, however, are similar to those observed in the immediate hanging wall of the Main Central thrust, the base of the exhumed metamorphic core. The new observations from the Likhu Khola region are compatible with nearby studies that highlight structural, metamorphic, and temporal discontinuities within the exhumed Himalayan mid-crust.
The Himalaya continue to serve as a modern-day analogue for ancient orogens studied around the globe (e.g., St. Onge et al., 2006; Simony and Carr, 2011; Gervais and Brown, 2011); our understanding of the Himalaya therefore controls the practical application of this analogue. While there has been much detailed work over the past few decades, large portions of the Himalaya have been mapped only at reconnaissance scale. Moreover, most of this reconnaissance mapping took place prior to the development of recent ideas about convergence accommodation processes in the orogen, such as mid-crustal channel flow (e.g., Beaumont et al., 2001, 2004; Godin et al., 2006), and the recognition of cryptic, large-scale thrust-sense structural discontinuities within the metamorphic core (e.g., Carosi et al., 2010; Larson et al., 2010, 2011, 2013; Corrie and Kohn, 2011; Yakymchuk and Godin, 2012; Rubatto et al., 2012; Montomoli et al., 2013).
We stand to benefit from reexamining and reinterpreting the rocks in these undermapped regions within the current understanding of the Himalayan system, and they can serve as internal tests for ideas about the evolution of the Himalaya. Furthermore, documenting the geology in regions that have been mapped only at reconnaissance scale provides the foundation necessary to build more advanced, detailed studies that will in turn provide the data required for further refinement of orogenic models. The development of a geologic map and accompanying detailed descriptions allows for comparison not only to other areas along the Himalaya, which is essential to advancing our understanding of the evolution of the orogen (see summaries by Yin and Harrison, 2000; Hodges, 2000, 2006), but also to other regions worldwide that are compared to the Himalaya. These new data can be incorporated into conceptual models to help elucidate the kinematics, metamorphism, and anatexis in a variety of different orogens around the globe.
This study presents a detailed examination of the tectonostratigraphy, deformation, map-scale metamorphism, and anatexis of the Likhu Khola region in east-central Nepal (Fig. 1). Previous investigations of east-central Nepal by Ishida (1969), Ishida and Ohta (1973), and Schelling (1992) included parts of the present mapped area; these studies, however, focused on regional-scale observations and the generation of reconnaissance-scale maps. While the studies show that the Likhu Khola region is underlain by a metamorphic tectonostratigraphy, they offer strikingly different interpretations about the structural framework of the area and the relationships between different lithotectonic units. Ishida (1969) and Ishida and Ohta (1973) drew multiple discrete faults through the study area, mapped as separating most of the main lithotectonic units, whereas Schelling (1992) mapped only one large-scale thrust fault, the Main Central thrust, through the region. Moreover, a recent examination of the Tama Kosi region, the next major drainage to the west of the Likhu Khola, concluded that neither the interpretation of Ishida (1969) nor Schelling (1992) was applicable there (Larson, 2012), and that the Main Central thrust occurred farther to the south.
This study examines the area along the Likhu Khola river valley from Tholo Priti in the south to Gyajo La in the north and adjacent areas of the Nupche and Khimti Kholas from Gyajo La in the north to Those in the south (Fig. 2). We build on the previous work in the region, examine the area in more detail, and add supporting laboratory microstructural analysis and petrology. This work allows assessment of the interpretations put forward by Ishida (1969), Ishida and Ohta (1973), and Schelling (1992) and comparison to the findings in the nearby Tama Kosi region (Larson, 2012). It also serves as the baseline from which to reinterpret the geology of the Likhu Khola area in the context of current orogenic models.
GEOLOGY OF THE LIKHU KHOLA STUDY AREA
The Likhu Khola region is in east-central Nepal between the Kathmandu nappe and the well-studied Everest region (Fig. 1). The geology of the study area consists of exhumed, pervasively ductilely deformed metamorphosed rocks of variable sedimentary and igneous protoliths. Metamorphic grade generally increases from middle greenschist in the structurally lower southern part of the study area to upper amphibolite grade in the structurally higher northern reaches (Figs. 2 and 3). This map pattern defines an inverted metamorphic field gradient consistent with those observed along the length of the Himalaya near the base of the Greater Himalayan sequence (e.g., Mallet, 1874; von Loczy, 1878; Oldham, 1883; Arita, 1983; and many others). The lithologies of the mapped area will be discussed in order from structurally lowest to highest. For simplicity and brevity the dominant mineralogy observed in each unit is presented in Table 1. The mineral percentages listed are visual estimates made during thin section analyses.
The structurally lowest unit exposed in the study area is an augen orthogneiss (Table 1), which crops out just south of the town of Those (Fig. 2). The thickness of this unit is not constrained because it extends outside the mapped area to the southwest. The augen within this unit consist of variably fractured and rotated perthitic feldspar crystals (Fig. 4A). The foliation is defined by micaceous partings of biotite and muscovite with elongate laths that also mark a distinctive, well-developed mineral shape lineation. This lineation is also expressed as quartz rods in the same orientation, plunging shallowly to the north (Fig. 4B). The orthogneiss contains quartz-rich pods or veins that compose as much as 10% of the total rock volume locally (Table 1).
Micaceous Phyllitic Schist with Intercalations of Marble and Calc-Silicate
Overlying the augen orthogneiss is an ∼7500-m-thick unit of micaceous phyllitic schist (Table 1) with intercalations of quartzite, marble, and calc-silicate (Figs. 2 and 3). The foliation is defined by planar muscovite, biotite, and local chlorite that also define a mineral shape lineation that plunges shallowly to the north. Plagioclase in these rocks is partially sericitized and is typically found between semicontinuous bands of biotite (Fig. 4C). Garnet ranges in size from 0.5 to 3 mm in diameter and variably occurs as sigma- and delta-type porphyroblasts (Fig. 4D). Marble to calc-silicate layers (Table 1) intercalated within the phyllitic schist are 10–40 m thick and marked by strongly recessive weathering. They have visible micaceous partings that define the foliation and are affected by a locally developed crenulation cleavage. The calc-silicate rocks typically contain pockets of fine-grained quartz aggregates, perhaps metachert, rimmed by muscovite and surrounded by a coarser grained carbonate-rich fabric (Figs. 4E, 4F). Leucogranite, in the form of strataform layers or boudins, composes 5%–15% of the total rock volume within the metapelitic portion of this unit (Table 1).
A 1500–2300-m-thick unit of graphitic schist occurs within the micaceous phyllitic schist ∼1000 m upward in the structural section from its lowermost contact (Fig. 2; Table 1). The foliation in the graphitic schist is defined by micaceous partings of biotite and muscovite and aligned graphite. No lineation was observed in this unit; however, a poorly developed crenulation cleavage is developed locally (Fig. 5A).
Overlying the micaceous phyllitic schist is an ∼400-m-thick quartzite unit (Fig. 2; Table 1). The foliation in the quartzite is defined by muscovite and sparse biotite and is locally isoclinally folded (Fig. 5B). Muscovite defines an aligned mineral lineation that plunges shallowly to the north. The quartzite is locally interlayered with quartz-dominated leucogranite-bearing micaceous schist (Table 1).
Overlying the quartzite is a pelitic to semipelitic Quartz+Feldspar+Muscovite+Biotite+Garnet (Qtz+Fsp+Ms+Bt+Grt) gneiss (Table 1) that is ∼8000 m thick (Figs. 2 and 3). There is significant compositional variation within this unit; the dominant lithology is interleaved with lenses of psammopelitic gneiss and of calc-silicate rocks. The foliation in the Qtz+Fsp+Ms+Bt+Grt gneiss is defined by semicontinuous partings of biotite, and a lineation is defined by elongate quartz grains that plunge shallowly to the north. The garnet in this unit is large relative to other units (to 2 × 2 mm) and is spatially associated with leucogranite layers (Fig. 5C). Leucogranite composes 15%–25% of the total rock volume (Table 1) and occurs as a leucosome surrounded by a biotite-rich melanosome. Kyanite occurs in the leucosome and immediate melanosome toward the upper boundary of this unit (Fig. 5D).
Aluminosilicate-Bearing Migmatitic Gneiss
The next structurally higher unit is an aluminosilicate-bearing migmatitic gneiss (Table 1) that is estimated to be between 7000 and ∼10,000 m thick with an upper contact that occurs outside the current map area (Fig. 2). The appearance of kyanite in the residuum, outside of association with leucosome or immediate melanosome (Figs. 6A, 6B), marks the transition into this unit. Well-developed gneissosity is enhanced by the occurrence of stromatic leucosome layers. This kyanite-bearing migmatitic gneiss is characterized by a strongly developed tectonic lineation defined mainly by kyanite and muscovite that plunge shallowly to the north. Approximately 2000 m upward in the structural section, kyanite is lost in favor of sillimanite, which commonly defines a mineral alignment lineation that plunges shallowly to the north. Sillimanite first occurs as discrete millimeter-scale pods of fibrolite associated with biotite (Fig. 6C), whereas farther upward in the structural section sillimanite occurs as microscopic, prismatic needles and stringers of discrete needle aggregates (Figs. 6D, 6E). Sillimanite is also observed intergrown with quartz ± K-feldspar ± muscovite ± magnetite composing ovoid nodules or faserkiesel (Fig. 6F), the long axes of which are parallel to the north-south mineral stretching lineation defined by muscovite and sillimanite. At higher structural levels sillimanite occurs as coarse intergrowths of sillimanite and muscovite that overprint the foliation (Figs. 7A, 7B). The sillimanite-bearing migmatitic gneiss gradually becomes more psammopelitic in nature upward in the structural section, and leucosome lenses and the micaceous melanosome become increasingly segregated.
There are two distinct phases of anatexite that together compose as much as 65% of the total rock volume locally. The stromatic, foliation-parallel leucosome (Fig. 7C; Table 1) is the older of the two phases. This phase may be related to rims of feldspar-rich anatexite found locally in the strain shadows surrounding large garnet grains (Fig. 7D). The younger anatexite phase, however, consists of pegmatitic leucogranite (Fig. 7E; Table 1) that crosscuts both the foliation and the strataform anatexite (Fig. 7F).
An augen orthogneiss unit with distinct 10–15-cm-diameter K-feldspar augen overlies the sillimanite-bearing migmatite. The augen orthogneiss was observed only as float around Gyajo La at the northernmost portion of the study area. It must have been sourced from the peaks adjacent to the pass and therefore must occur at structurally higher levels than the pass. The thickness of this unit is unknown as it continues northeastward out of the study area. In the adjacent Tama Kosi region, it is 1.5–2 km thick (Larson, 2012).
Metamorphic grade increases upward in the structural section from approximately middle greenschist facies rocks at lower structural positions in the southern part of the mapped area to upper amphibolite facies migmatitic rocks at the highest structural position in the northern part of the mapped area. The distribution of indicator minerals is consistent with an increase in recorded metamorphic conditions northward, or upward in the structural section (Fig. 3). This outlines an inverted metamorphic field gradient. Similar gradients have been long recognized along the length of the Himalaya (e.g., Mallet, 1874; von Loczy, 1878; Oldham, 1883; Arita, 1983; and many others) and have been variably attributed to shear heating (e.g., Hubbard, 1996), a hot iron effect (e.g., LeFort, 1975), or tectonic juxtaposition (e.g., Jamieson et al., 1996; Larson et al., 2010). Middle greenschist facies rocks have a typical assemblage of biotite + muscovite ± chlorite, as identified near the town of Tholo Priti (Fig. 2). Garnet is first observed as part of this assemblage immediately above the graphitic unit in this map area; however, previous mapping in the region indicates that the first appearance of garnet may occur farther downward in the structural section to the south of the study area (Ishida, 1969). A few kilometers above the graphitic schist (Fig. 2) chlorite is no longer observed and the typical metamorphic assemblage here includes biotite + muscovite + garnet. Staurolite is not observed in the map area, although it does occur in a thin portion of the exhumed metamorphic core in adjacent areas (Larson, 2012). It may not have been observed in the study region due to a bulk rock composition not conducive to its growth or due to a lack of outcrop exposure. Kyanite occurs in a narrow range of exposure, first appearing within strataform leucosome and then as part of the residuum (see preceding). The first occurrence of kyanite in the leucosome was mapped just south of the village of Kangematar where the Likhu Khola meets the Nupche Khola (Fig. 2). Sillimanite first appears in association with anatexite ∼1900 m to the north of the first occurrence of kyanite along the Likhu Khola and then as part of the residuum in immediately adjacent rocks.
Kyanite and sillimanite are observed to coexist in at least one rock specimen. There is no evidence to indicate that sillimanite is forming directly from kyanite, and therefore two separate reactions are interpreted to have taken place simultaneously in different domains of the rock (e.g., Carmichael, 1969). Sillimanite remains a constituent of the metamorphic assemblage at the most northern and structurally highest point reached in the study area, Gyajo La (Fig. 2). A minor amount of coarse prismatic muscovite (1%–4% of the mineral assemblage) is noted in the rocks at the structurally highest point in the study area. The coarse prismatic muscovite + sillimanite intergrowth texture preserved at that structural level (Figs. 7A, 7B) indicates that it is the product of retrograde reactions involving K-feldspar (e.g., Spear et al., 1999) and that rocks at the structural highest positions reported in this study reached the second sillimanite isograd during prograde metamorphism.
Sillimanite + quartz ± muscovite ± K-feldspar ± magnetite nodule intergrowths, or faserkiesel, are observed at high structural levels in the study area. The formation of a faserkiesel texture has been attributed to preferential composition of metasedimentary rocks, although their origin is still considered enigmatic: Tippett (1984) suggested that the presence of faserkiesel indicates the former presence of a stable K-feldspar + sillimanite assemblage that has been subjected to retrograde metamorphism involving paired ionic equilibria. The preferential alignment of the faserkiesel with the regional foliation indicates that the formation of these intergrowth nodules must have been pretectonic to syntectonic, consistent with their formation early in the protracted metamorphic history of the mid-crust. If the faserkiesel formed from retrograde metamorphism, it would imply that the current metamorphic assemblage, which contains both muscovite and sillimanite, does not represent peak metamorphic conditions. This is consistent with textural observations of sillimanite and muscovite intergrowths at the highest structural levels that are indicative of retrograde formation through the breakdown of K-feldspar.
FIELD AND MICROSTRUCTURAL OBSERVATIONS
Asymmetric secondary foliation planes including S and C′ fabrics were identified in many of the quartz- and mica-rich units across the southern part of the mapped area. The geometry of these secondary foliation planes consistently indicates a top-to-the-south sense of shear (Figs. 8A, 8B). That shear sense is confirmed by sigma-type clasts; geometries also indicate the same shear sense (Figs. 8C, 8D). Similar deformational textures recording synthetic shear are observed at the microstructural scale (Figs. 9A–9D), supporting the existence of pervasive top-to-the-south shear sense throughout the study area at a variety of scales.
While small-scale isoclinal folding (1 cm to 1 m) is recorded in the quartzite unit near Bandar (Fig. 2), regional-scale open folding (100 m to >1 km) occurs in the southern portions of the mapped area structurally below the quartzite layer (Fig. 3). This folding is most evident in the rock units just east of Dolu (Fig. 2), where the foliations on the north and south sides of a ridge dip in opposite directions (Fig. 3). This open folding postdates the development of the pervasive foliation it affects and the associated top-to-the south deformation in these units. The crenulation cleavage observed locally in some of the lower units may be related to the development of this fold geometry. Similar large-scale folding has also been mapped in the adjacent Tama Kosi valley to the west, where a structural window has been interpreted to reflect interference folding of separate regional events with perpendicular vertical fold axial planes (Ishida and Ohta, 1973), though a variety of other factors could have influenced the development of such a structure (Long et al., 2011a).
Quartz texture analysis can potentially provide important information on the deformation, recovery, and thermal histories of a specimen (e.g., Hirth and Tullis, 1992; Stipp et al., 2002; Long et al., 2011b). While the temperature attached to a set of textural observations may not be accurate in an absolute sense, the relative temperatures across the mapped area can provide insight into deformational processes and timing. The results of our analyses are summarized in Table 2.
Quartz at the structurally lowest levels of the mapped area has a variably developed core and mantle texture (Fig. 10A), minor internal subgrain development, and local pinning structures (Fig. 10B). These characteristics are consistent with recovery through the operation of dislocation creep and dynamic recrystallization through subgrain rotation and minor grain boundary migration, respectively, indicative of regime 2–3 of Hirth and Tullis (1992) or at the transition between SGR (subgrain rotation) and GBM I (grain boundary migration) of Stipp et al. (2002). Subgrain rotation is not observed upward in the structural section at higher metamorphic grades (Table 2). A transition from irregular grain boundaries through interlobate and eventually to amoeboid boundaries (Figs. 10C, 10D) at higher structural levels reflects the increasing importance of grain boundary migration recrystallization in quartz upward in the structural section (to the north). This is further supported by the existence of window structures between biotite laths (Fig. 10E). Moreover, at the highest structural levels in the mapped area, chessboard quartz texture is observed (Fig. 10F); this distinctive internal subgrain texture is thought to develop at temperatures near 650 °C (e.g., Law et al., 2004) and to reflect activation of dominant slip along the prism [c] direction in quartz grains (see overview by Law et al., 2013).
QUARTZ LATTICE PREFERRED ORIENTATION (LPO) ANALYSES
Quartz lattice preferred orientation (LPO) analysis techniques have been employed successfully throughout the Himalaya (e.g., Bouchez and Pêcher, 1976; Grasemann et al., 1999; Law et al., 2004; Larson and Godin, 2009; Larson et al., 2010; Langille et al., 2010; Yakymchuk and Godin, 2012) to provide constraints on the detailed deformational history of the orogen. The LPO of quartz-rich rocks can provide information about the dominant slip planes that were active during dynamic recrystallization (e.g., Schmid and Casey, 1986) and verify the sense of shear (e.g., Bouchez, 1978; Law, 1991). Moreover, assuming that the slip system is dominantly controlled by temperature as opposed to strain rate (Lister et al., 1978) or hydrolytic weakening (Mainprice and Nicolas, 1989), quartz c-axis fabrics can be used to estimate the temperature during deformation (Kruhl, 1998; Morgan and Law, 2004). The opening angle of quartz c-axis girdles has been shown to increases linearly with deformation temperature to ∼650 °C (Kruhl, 1998). At higher temperatures the linearity of the relationship between opening angle and temperature is strongly dependent on the hydration of the system and activation of prism [c] slip (Morgan and Law, 2004). The opening angle thermometer is based on the correlation of opening angles with empirical temperature data and assigned an estimated error of ±50 °C (Kruhl, 1998).
Quartz LPOs in this study were measured using an automated fabric analyzer fabricated by Russell-Head Instruments (e.g., Wilson et al., 2007, 2009; Peternell et al., 2010, 2011). Similar instruments have been shown to yield quartz c-axis data equivalent to analyses carried out using electron backscatter diffraction systems (Peternell et al., 2010). All thin sections analyzed were cut perpendicular to the dominant foliation and parallel to lineation. Therefore, an asymmetric pattern would indicate that the lineation is an elongation lineation. The plane of projection for all equal area, lower hemisphere stereonets shown in Figure 11 is set up looking west with a sinistral sense of shear representing a top-to-the-south sense of shear in the field. The specimen foliation is vertically along the east-west axis while the lineation is horizontal along the same axis.
Quartz LPO Results
Specimens were chosen for LPO analysis based on their quartz content, apparent strain, and structural position. As discussed in the following, not all specimens analyzed yielded a discernable pattern.
Specimen 083 was collected from a quartzite near the structurally lowest part of the study area immediately above the basal augen orthogneiss (Fig. 2). The quartz c-axis in this specimen yields an asymmetric single girdle LPO pattern that appears to be dominated by rhomb <a> and prism <a> slip (Fig. 11A). The asymmetry of the pattern is consistent with a top-to-the-south sense of shear (Fig. 11A). The fabric development and asymmetry confirm that the lineation observed and used to orient the thin section represents a tectonic transport direction.
Specimen 032 was collected from quartz-rich schist (Fig. 2). It yields a poorly defined type-II cross-girdle pattern with contributions of slip on the basal <a>, prism <a>, and rhomb <a> crystal planes (Fig. 11B). There is no discernable asymmetry. The LPO fabric appears to define an opening angle of ∼63° (Fig. 11B); if valid, this would correspond to a deformation temperature of ∼475 ± 50 °C (Law et al., 2004). That temperature is indistinguishable from quartz recrystallization texture-derived estimates following the criteria of Stipp et al. (2002) for the same unit of 500–550 °C (Table 2).
Specimen 033 was collected from a quartzite unit in the micaceous phyllitic schist (Fig. 2). It yields a well-defined asymmetric single girdle pattern similar to that of specimen 083 (Fig. 11C) with slip dominated by glide along the rhomb <a> and prism <a> crystal planes. The fabric asymmetry is indicative of top-to-the-south shear.
Specimen 039 is quartz-rich schist collected structurally above specimen 033 (Fig. 2). It does not yield a well-defined LPO pattern; however, it does have concentrations of quartz c-axes that plot near the lineation direction (Fig. 11D). This may represent the activation of prism [c] slip in quartz (Schmid and Casey, 1986) which generally initiates near temperatures of ∼650 °C (Law et al., 2004). That is consistent with quartz recrystallization texture-derived temperature estimates following the criteria of Stipp et al. (2002) of 550–700 °C for the same unit (Table 2). Well-developed chessboard quartz textures, which have been linked to combined prism [c] and basal <a> slip (Blumenfeld et al., 1986; Mainprice et al., 1986; Stipp et al., 2002) become prominent farther upward in the structural section in the overlying migmatitic gneiss unit.
With the exception of specimen 039, all specimens analyzed from positions structurally higher than specimen 033 yield disorganized LPO patterns. This may reflect deformation that occurred at temperatures above those that could be recorded by quartz, that subsequent deformation overprinted and destroyed any quartz LPOs that were originally imparted, that no LPO was developed, or some combination thereof.
The new, detailed geologic data and paired microstructural observations and analyses presented in this study can be compared with previous, reconnaissance-scale interpretations for the mapped area. Ishida (1969) and Ishida and Ohta (1973) mapped the rocks in this area as several formations separated by discrete thrust faults (Fig. 1B); however, no such structures, or equivalent localized high-strain zones, were observed during this study. The entire mapped area records pervasive ductile strain at both the outcrop and microscopic scale characterized by top-to-the-south sense shear. Moreover, the general north-northeast–south-southwest trend of lineations in the study area (Fig. 2), confirmed through quartz LPO analyses to be related to the tectonic transport direction, indicates top-to-the-south and southwest displacement. Comparison to the interpretation of Schelling (1992), who mapped the Main Central thrust through the middle of the mapped area, requires an exploration of quartz textures.
Quartz recrystallization textures record temperatures ranging from ∼490 to 700 °C (Table 2) across the mapped area and define a general increase in temperatures upward in the structural section (to the north). This general trend is also observed in quartz LPO patterns that indicate an increase in deformation temperatures upward in the structural section. A temperature of ∼475 ± 50 °C is indicated by specimen 032 (Fig. 11B), while the structurally higher specimen 039 (Fig. 11D) shows evidence of prism [c] slip and deformation temperatures of ∼650 °C. The increasing deformation and quartz texture–related temperatures generally mimic the observed inverted metamorphic field gradient. This may be indicative of a contemporary relationship between deformation and metamorphism; however, quantitative information on metamorphic conditions is required to test this.
The metamorphism and deformational characteristics observed in the study area are consistent with those observed elsewhere along the Himalaya within the exhumed metamorphic core of the orogen (e.g., Jessup et al., 2006; Goscombe et al., 2006; Larson and Godin, 2009; Larson, 2012). Furthermore, the general increase in quartz texture and LPO temperatures upward in the structural section across the study area is consistent with the trend of quartz-derived temperatures in the Sutlej region of northwest India (Law et al., 2013). The Sutlej rocks are also mapped as part of the exhumed Himalayan metamorphic core and are interpreted to occur in the immediate hanging wall of the Main Central thrust (Law et al., 2013), which marks its base. This differs from the interpretation of Schelling (1992) who mapped the Main Central thrust through the Likhu Khola region just above the structural level of the phyllitic schist unit (Figs. 2, 3, and 12). No strain gradient or other structural break was noted at that structural level during our study. The Main Central thrust is therefore interpreted to occur to the south of the mapped area (Fig. 12). Such a position is consistent with the interpreted location of the structure in the adjacent Tama Kosi region (Larson, 2012) and the Searle et al. (2008) definition of the Main Central thrust. Searle et al. (2008) mapped the structure as the lower structural boundary of the exhumed metamorphic core along the strain gradient that occurs at the base of the pervasively deformed metamorphic rocks, commonly coinciding with the base of inverted metamorphism, that record Cenozoic metamorphism, cooling, and deformation.
Previously researchers suggested a link between the inverted metamorphic field gradient and the downward migration of the Main Central thrust with time (e.g., Catlos et al., 2001; Larson et al., 2010). The apparent relationship between deformation temperatures and the inverted metamorphic field gradient across the southern, structurally lower portion of the mapped area indicates that deformation and metamorphism developed coevally and is consistent with that interpretation. This is further supported by new data from the nearby Tama Kosi valley that demonstrate that rocks equivalent to the lower portion of the Likhu Khola region were concurrently metamorphosed and deformed as part of an evolving foreland orogenic wedge (Larson et al., 2013). Furthermore, the rocks mapped at higher structural levels in the Likhu Khola area are continuous with, and similar to, those described in the adjacent Tama Kosi region (see Larson, 2012), where the pressure-temperature-time paths in the migmatitic, kyanite- and sillimanite-grade upper portion are consistent with predicted values from models of lateral mid-crustal flow (Larson et al., 2013). The new lithotectonic, metamorphic, and strain data and observations from this study are consistent with recent findings in the Tama Kosi region and other areas along the length of the orogen such as far west Nepal (Yakymchuk and Godin, 2012), west-central Nepal (Larson et al., 2010, 2011), and Sikkim (Rubatto et al., 2012) that support an evolution of the Himalayan metamorphic core that includes components of spatially and temporally distinct, yet compatible, lateral mid-crustal flow and wedge taper deformation.
This study presents the first detailed geologic map from the Likhu Khola region of east-central Nepal with integrated lithologic, petrographic, structural, and metamorphic descriptions. These data are the critical building blocks for future research in the region. Specifically, in this study we noted the following.
All rocks in the Likhu Khola study area are pervasively strained, recording consistently top-to-the-south shear sense indicators.
Deformational temperatures as constrained by quartz recrystallization textures (e.g., Stipp et al., 2002) and LPO patterns (e.g., Kruhl, 1998) increase upward in the structural section from ∼490 °C to 700 °C. This apparent quartz temperature gradient parallels the observed inverted metamorphic mineral assemblage, indicating a contemporaneous relationship between deformation and metamorphism in the lower portion of the mapped area.
All of the rocks in the mapped area are interpreted to belong to the exhumed metamorphic core of the Himalaya in the hanging wall of the Main Central thrust.
Lithologies, metamorphism, and deformation in the Likhu Khola are consistent with those observed in the adjacent Tama Kosi valley, where they are interpreted to reflect a protracted history recording spatial and temporal changes in structural style as the Himalaya developed.
This study was funded by a University of Saskatchewan Faculty Start-up Grant and a Natural Sciences and Engineering Research Council of Canada Discovery Grant to Larson. Logistical support in Nepal was provided by Teke, Pradap, Pemba, Manoj, Buddhiman, Manggalsine, Lal Bahadar, Man Bahador, and Rajesh Tamang. The manuscript was improved by two anonymous reviews and the editorial direction of T. Wawrzyniec.