Kazunori Arita

Thrust tectonics, crustal shortening, and the structure of the far‐eastern Nepal Himalaya

Balanced and restored structural sections across the far-eastern Nepal Himalaya have been constructed in order to determine the structure and evolution of the Himalayan orogenic wedge and the amount of tectonic shortening the region has undergone since the initiation of thrusting along the Main Central Thrust (MCT). The far-eastern Nepal Himalaya is comprised of three distinct, thrust-bound tectonic packages; the Higher Himalayan (Crystalline) thrust sheet, the Lesser Himalayan (Metasediment) thrust package, and the Sub-Himalayan imbricate fan. The Higher Himalayan Crystallines, consisting of kyanite- and sillimanite-bearing gneisses intruded by the Miocene (?) Jannu leucogranites, have been thrust over the Lesser Himalayan Metasediments along the MCT for a distance of 140 km to 175 km. The Lesser Himalayan Metasediments are a 12 km thick unit consisting primarily of phyllites, metaquartzites, and mylonitic augen gneisses in which garnet, biotite and chlorite metamorphic zones are exposed in progressively deeper structural levels. The Lesser Himalayan (Metasediment) thrust package is underlain by a decollement, the Main Detachment Fault (MDF), which lies at a calculated depth of between 6 and 10 km underneath the Mahabharat Lekh, and at a calculated depth of 20 to 25 km north of the Tamar Khola Dome. The Tamar Khola Dome overlies a footwall ramp along the MDF where the MDF cuts upsection through the Lesser Himalayan Metasediments. The Lesser Himalayan thrust package probably has an internal structure aproximating a hinterland-dipping duplex, with the MCT and the MDF corresponding to the roof and floor thrusts, respectively. Both the Tamar Khola Thrust, an out-of-sequence breach thrust, and the Main Boundary Thrust (MBT) are splay thrusts off of the MDF. The Sub-Himalaya, consisting of nonmetamorphosed sedimentary rocks, displays an emergent imbricate fan geometry and is underlain by the southern continuation of the MDF which lies at a depth of 5.5 km to 6 km beneath the Siwalik Hills. Folding and thrusting within the Lesser Himalayan thrust package and the Sub-Himalayan imbricate fan have accomodated 45 to 70 km of tectonic shortening. Total north-south shortening across the Higher, Lesser, and Sub-Himalaya of far-eastern Nepal, south of the Tibetan Plateau, has been of the order of 185 km to 245 km and has occurred at an average rate of 7.4 mm to 15.3 mm per year since the initiation of the MCT between 16 and 25 Ma.

Origin of the inverted metamorphism of the Lower Himalayas, central Nepal

Abstract The southern slope of the central Nepal Himalayas is divided geologically by the Main Central Thrust zone into the underlying Midland metasediment zone (Lower Himalayas) in the south and the overlying Himalayan gneiss zone (Higher Himalayas) in the north. The former zone consists of a thick pile of various kinds of metasediments folded into an anticlinorium, and the latter zone consists of high-grade crystalline rocks with a north-dipping homoclinal structure and is further overlain by the Tethys Himalayas, composed of non- or slightly metamorphosed Tethyan sediments. The Main Central Thrust zone shows a characteristic lithology such as calcareous schist, quartzite, phyllitic schist and augen gneiss, and is intensely mylonitized. The mineral composition and paragenesis of the rocks in the present area indicate an inverted metamorphism from the Midland metasediment to the Main Central Thrust zones, that is, the metamorphic grade of the Midland metasediment zone increases upward toward the thrust zone from the chlorite to the biotite zones of the greenschist facies, and the greenschist-amphibolite transitional zone is attained in the thrust zone. The Himalayan gneiss zone is apparently much higher in metamorphic grade than the thrust zone. On the basis of the compositional zoning of garnets and muscovite compositions, however, it is concluded that the Himalayan gneisses suffered a polymetamorphism. The younger metamorphism resulted in a retrograde metamorphism of the Himalayan gneisses together with the above-mentioned inverted metamorphism and an anatectic melting in depth. This was caused by shear heating due to the thrust movement along the Main Central Thrust zone during the Alpine tectonism. The older metamorphism, which seems to be Precambrian in age, was much higher in metamorphic grade than the younger one and formed the pyrope-rich cores of garnets in the Himalayan gneisses. The maximum metamorphic temperature of 550°C, estimated from the mineral paragenesis, in the thrust zone, is compatible with that caused by shear heating due to the thrusting with thrusting velocity of 5–10 cm/yr, maximum stress of 300 bars and thrusting distance of 250 km (Bird, 1978).

Delamination‐wedge structure beneath the Hidaka Collision Zone, central Hokkaido, Japan inferred from seismic reflection profiling

In the Hidaka Collision Zone of Hokkaido, northern Japan, the Kuril island arc collides with the northeast Japan arc. In order to better understand the collision process, new high resolution information about lithospheric structure was obtained by means of a series of seismic reflection surveys. These seismic profiles reveal distinct zones of seismic lamination in the lower crust of the Kuril arc. The upper portion of the lower crust is characterized by numerous east-dipping reflectors. In contrast, west-dipping reflectors dominate the lower part of the lower crust. From this reflector configuration, the lower crust of the Kuril arc is interpreted to be delaminated by the collision. The geometry of delamination is consistent with other geophysical data, as well as the peak metamorphic grades exposed in the Hidaka mountains. As earthquakes indicate that delamination here is ongoing, the Hidaka Collision Zone represents an active model for continental growth by arc accretion with delamination of lower arc crust.

Tectonic and polymetamorphic history of the Lesser Himalaya in central Nepal

Abstract The Lesser Himalaya in central Nepal consists of Precambrian to early Paleozoic, low- to medium-grade metamorphic rocks of the Nawakot Complex, unconformably overlain by the Upper Carboniferous to Lower Miocene Tansen Group. It is divided tectonically into a Parautochthon, two thrust sheets (Thrust sheets I and II), and a wide shear zone (Main Central Thrust zone) from south to north by the Bari Gad–Kali Gandaki Fault, the Phalebas Thrust and the Lower Main Central Thrust, respectively. The Lesser Himalaya is overthrust by the Higher Himalaya along the Upper Main Central Thrust (UMCT). The Lesser Himalaya forms a foreland-propagating duplex structure, each tectonic unit being a horse bounded by imbricate faults. The UMCT and the Main Boundary Thrust are the roof and floor thrusts, respectively. The duplex is cut-off by an out-of-sequence fault. At least five phases of deformation (D 1 –D 5 ) are recognized in the Lesser Himalaya, two of which (D 1 and D 2 ) belong to the pre-Himalayan (pre-Tertiary) orogeny. Petrographic, microprobe and illite crystallinity data show polymetamorphic evolution of the Lesser and Higher Himalayas in central Nepal. The Lesser Himalaya suffered a pre-Himalayan (probably early Paleozoic) anchizonal prograde metamorphism (M 0 ) and a Neohimalayan (syn- to post-UMCT) diagenetic to garnet grade prograde inverted metamorphism (M 2 ). The Higher Himalaya suffered an Eohimalayan (pre or early-UMCT) kyanite-grade prograde metamorphism (M 1 ) which was, in turn, overprinted by Neohimalayan (syn-UMCT) retrograde metamorphism (M 2 ). The isograd inversion from garnet zone in the Lesser Himalaya to kyanite zone in the Higher Himalaya is only apparent due to post-metamorphic thrusting along the UMCT. Both the Lesser and Higher Himalayas have undergone late-stage retrogression (M 3 ) during exhumation.

Kinematic history of the Main Central Thrust zone in the Langtang area, Nepal

Abstract This paper describes for the first time evidence for a northeastward brittle extensional movement of the Main Central Thrust (MCT) zone overprinting the southward ductile thrust movement. The MCT zone is the major tectonic boundary between the Lesser Himalayan metasedimentary sequence (LHS) and Higher Himalayan crystalline sequence (HHS) and contributes to the kinematic and tectonic evidence of continent–continent collision in the Neogene. The Langtang area, 50 km north of Kathmandu, is a matter of interest in considering the movement of the Kathmandu Nappe because the general trend of foliation of the HHS and the underlying MCT zone is turned from WNW–ESE to N–S or NNE–SSW. The major tectono-lithostratigraphic units of the MCT zone in the study area are divided into three: the lower and middle units included in the LHS and the upper unit even included in the HHS. Characteristic mesoscopic quartz lenses are developed in the schists of the Lower unit forming asymmetric boudins. This asymmetry indicates an extensional (top-to-the-NE) shear sense. However, a thrust sense of top-to-the-SW is also preserved in microscopic ductile shear bands and mica fish. The extensional shear movement in the Lower unit took place after the major thrust movement in the MCT zone as a negative inversion. The Middle unit contains Ulleri-type augen gneiss (Syabru Bensi augen gneiss) which is widely distributed in the MCT zone in the other parts of the Nepalese Himalaya. Asymmetric clast-tails (or clast-pressure shadows) and asymmetric boudins observed in the Syabru Bensi augen gneiss give a dextral-thrust oblique sense of shear. This dextral component is regarded as movement of a N–S-trending lateral ramp during the overthrusting of the Kathmandu Nappe. The Upper unit above the MCT is commonly made up of kyanite-bearing medium pressure type amphibolite-facies pelitic gneiss. Asymmetric deformational features such as mica fish and shear bands in the pelitic gneisses commonly show a thrust (top-to-the-W) sense of movement. After the emplacement of the Kathmandu Nappe (∼9–6 Ma), brittle extensional shear occurred only in the crystalline schists of the Lower unit presumably associated with layer-parallel slip. This extensional movement in the Lower unit is probably caused by the change in strike of foliations, e.g., from a general trend of E–W in the other areas to N–S as seen in the study area. This is because N–S contraction during continued continent–continent collision produced E–W extension normal to the contraction axis (N–S), thus the relative east-directed movement on normal faults took place only in the N–S trending domain of the MCT zone. Another possibility for the extensional movement is squeezing of the upper part of the LHS between the MCT above and an out-of-sequence thrust or the Main Detachment Fault below.

1.74 Ga crustal melting after rifting at the northern Indian margin: investigation of mylonitic orthogneisses in the Kathmandu area, central Nepal

ABSTRACT Mylonitic orthogneisses in the Kathmandu area, central Nepal have been investigated using whole-rock and mineral chemistry, Rb-Sr isotopes, and zircon U-Pb age dating. Zircon REE patterns determined from orthogneisses are characterized by enriched HREE patterns and the prominent Eu anomalies, consistent with a magmatic origin. The U-Pb zircon age dating and Ti-in-zircon thermometry revealed crystallization took place ca. 1.74 Ga at temperatures of 705–765℃; typical of felsic magmatism in the crust. Whole-rock data from most orthogneisses in this study and from similar rocks in previous studies span the ‘syn-collisional’ and ‘post-collisional’ fields on various tectonic discrimination diagrams, while some data also plot in rift-related magmatism fields. The peraluminous compositions, very high Sr isotopic ratios (0.865–3.585) and high Th and U concentrations for all orthogneisses in this study indicate that mylonitic orthogneisses are largely of S-type crustal origins. The new data presented herein, combined with that of previous studies, outline at least two Palaeoproterozoic magmatic episodes: 1) ca. 1.92–1.90 Ga rift-related magmatism derived from mantle melting and 2) 1.84–1.74 Ga crustal melting, resulting from burial of the Indian basement during thermal subsidence after rifting. This two-stage Palaeoproterozoic magmatism in Nepal occurred along the northern passive margin of the Indian basement during and/or after the breakup of the Columbia supercontinent.