Michael P. Searle

The closing of Tethys and the tectonics of the Himalaya

Recent geological and geophysical data from southern Tibet allow refinement of models for the closing of southern (Neo-) Tethys and formation of the Himalaya. Shelf sediments of the Indian passive continental margin which pass northward into deep-sea Tethyan sediments of the Indus-Tsangpo suture zone were deposited in the Late Cretaceous. An Andean-type margin with a 2,500-km-long Trans-Himalayan (Kohistan-Ladakh-Gangdese) granitoid batholith formed parallel to the southern margin of the Lhasa block, together with extensive andesites, rhyolites, and ignimbrites (Lingzizong Formation). The southern part of the Lhasa block was uplifted, deformed, and eroded between the Cenomanian and the Eocene. In the western Himalaya, the Kohistan island arc became accreted to the northern plate at this time. The northern part of the Lhasa block was affected by Jurassic metamorphism and plutonism associated with the mid-Jurassic closure of the Bangong-Nujiang suture zone to the north. The timing of collision between the two continental plates (ca. 50-40 Ma) marking the closing of Tethys is shown by (1) the change from marine (flysch-like) to continental (molasse-like) sedimentation in the Indus-Tsangpo suture zone, (2) the end of Gangdese I-type granitoid injection, (3) Eocene S-type anatectic granites and migmatites in the Lhasa block, and (4) the start of compressional tectonics in the Tibetan-Tethys and Indus-Tsangpo suture zone (south-facing folds, south-directed thrusts). After the Eocene closure of Tethys, deformation spread southward across the Tibetan-Tethys zone to the High Himalaya. Deep crustal thrusting, Barrovian metamorphism, migmatization, and generation of Oligocene-Miocene leucogranites were accompanied by south-verging recumbent nappes inverting metamorphic isograds and by south-directed intracontinental shear zones associated with the Main Central thrust. Continued convergence in the late Tertiary resulted in large-scale north-directed backthrusting along the Indus-Tsangpo suture zone. More than 500 km shortening is recorded in the foreland thrust zones of the Indian plate, south of the suture, and > 150 km shortening is recorded across the Indian shelf (Zanskar Range) and the Indus suture in Ladakh. There was also large-scale shortening of the Karakoram and Tibetan microplates north of the suture; as much as 1,000 km shortening occurred in Tibet. The more recent deformation, however, involved the spreading of this thickened crust and the lateral motion of the Tibetan block along major approximately east-west–trending strike-slip fault zones.

Tectonic evolution of the central Annapurna Range, Nepalese Himalayas

The metamorphic core of the Himalayan orogen, or Greater Himalayan sequence, is a northward tapering prism bound at the bottom by a N dipping family of thrust faults (the Main Central thrust system) and at the top by a N dipping family of normal faults (the South Tibetan detachment system). Research in the central Annapurna Range of Nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the Early Miocene. The Main Central thrust system is represented here by a 2- to 3-km-thick zone of high strain that developed during two or more episodes of movement. Most of its displacement was concentrated along the Chomrong thrust, a sharp, late-metamorphic discontinuity that places middle amphibolite facies rocks of the Greater Himalayan sequence on top of lower amphibolite facies rocks of the Lesser Himalayan sequence. The earliest demonstrable movement on this thrust system occurred ∼22.5 Ma; the most recent movement may be as young as Pliocene. The oldest element of the South Tibetan detachment system in this area is the Deorali detachment, which appears to have been active at the same time as the earliest shortening structures of the Main Central thrust system. Fabrics related to the Deorali detachment are disrupted by a previously unrecognized, SW vergent, thrust structure, the Modi Khola shear zone. The effect of this structure, which is constrained to be between 22.5 and 18.5 Ma, was to shorten rock packages that had been extended previously during movement on the Deorali detachment. Transition back to a local extensional regime after 18.5 Ma was marked by development of the Machhupuchhare detachment and related splays. Geologic evidence for rapid, two-way transitions between contraction and extension in the Annapurna Range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle; it also appears to be an important factor in mountain range development.

Shisha Pangma Leucogranite, South Tibetan Himalaya: Field Relations, Geochemistry, Age, Origin, and Emplacement

The Shisha Pangma pluton forming most of the Xixabangma (8027 m) massif in south Tibet is one of the 20+ larger leucogranite intrusives that mark the highest structural levels of the Himalayan metamorphic core. The pluton occurs immediately below the Shisha Pangma Detachment, a strand of the South Tibetan Detachment (STD) system, a low angle (30°) north‐dipping normal fault placing Paleozoic black slates atop sillimanite‐grade pelites and calc‐silicate rocks. K‐feldspar augen gneisses containing fibrolite and sillimanite paragneisses along the footwall show strong internal S‐C fabrics indicative of down‐to‐the‐north extension. The Shisha Pangma leucogranite is a heterogeneous, polyphase intrusion with an earlier, foliated biotite‐rich phase and a later, tourmaline + muscovite rich phase typically containing the assemblage: Kfs + Pl + Qtz + Ms + Tur ± Gt ± Bt ± Sil ± Ap. The highly peraluminous granites have high 87Sr/86Sr ratios (0.738‐0.750) typical of pelite‐derived anatectites. Nd‐depleted mantle model ages (from present Nd isotopic data and an assumed crustal 147Sm/144Nd of 0.10 ± 0.02) are 1.5‐2.2 Ga, indicating a substantial early Proterozoic or older crustal residence age for much of the source material. Xenotimes and monazites from a weakly foliated biotite granite immediately beneath the STD (X8) give consistent U‐Pb ages of 20.2 ± 0.2 Ma. Zircon, uraninite, and monazite from the main Shisha Pangma tourmaline + muscovite ± garnet phase (X20) give an U‐Pb age of 17.3 ± 0.2 Ma. A sill complex above the main leucogranite body is aligned parallel to the metamorphic fabric dipping at 10‐30± N, although a few dikes cross‐cut the metamorphic fabric beneath the STD. Nowhere do the leuco‐granites cut the STD, and the age of normal faulting must largely post‐date 17.3 ± 0.2 Ma. Muscovite from the main leucogranite phase has an 40Ar/39Ar plateau age of 16.74 ± 0.22 Ma. Apatite fission track ages for leucogranite samples from 5800‐8000 m range from 12.3 ± 1.9 to 14.8 ± 0.8 Ma (± 2ρ), only slightly younger than the main leucogranite crystallization age. Following crustal melting, steep cooling curves (>90‐180°C/myr) and rapid exhumation rates (∼ 4 mm/yr) from 17‐14 Ma resulted in removal of at least 12 km of overburden, both by erosion and normal faulting. If high erosion and exhumation rates correlate with high topography (and high precipitation) these data suggest that the Himalaya reached their maximum topographic elevation around 17 Ma.

Strain, deformation temperatures and vorticity of flow at the top of the Greater Himalayan Slab, Everest Massif, Tibet

This paper presents quantitative data on strain, deformation temperatures and vorticity of flow at the top of the Greater Himalayan Slab. The data were collected from the Tibetan side of the Everest Massif where two low-angle normal faults bound the upper surface of the Greater Himalayan Slab, the earlier and structurally lower Lhotse Detachment and the later and structurally higher Qomolangma Detachment. Greenschist- to sillimanite-grade quartz-rich metasedimentary rocks exposed in the Rongbuk to North Col region of the Everest Massif are characterized by cross-girdle quartz c-axis fabrics indicating approximate plane strain conditions. Fabric opening angles progressively increase with depth beneath the overlying Lhotse Detachment, and indicate progressively rising deformation temperatures of 525–625 ± 50 °C at depths of 300–600 m beneath the detachment. Deformation temperatures of c. 450 °C are indicated by fabric opening angles in epidote amphibolite-facies mylonites located closer to the overlying detachment. A top down-to-the-north (normal) shear sense is indicated by the asymmetry of microstructures and c-axis fabrics, but the degree of asymmetry is low at distances greater than 400 m beneath the detachment, and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine, indicating that deformation occurred at close to peak metamorphic temperatures. These ‘quenched’ fabrics and microstructures indicate rapid exhumation in agreement with previous isotopic dating studies. Mean kinematic vorticity numbers (Wm) were independently calculated by three analytical methods. Calculated Wm values range between 0.67 and 0.98, and indicate that although a simple shear component is generally dominant, particularly in greenschist-facies mylonites located between the Lhotse and overlying Qomolangma detachments, there is also a major component of pure shear in samples located at 400–600 m beneath the Lhotse Detachment (pure and simple shear make equal contributions at Wk=0.71). Our integrated strain and vorticity data indicate a shortening of 10–30% perpendicular to the upper surface of the Greater Himalayan Slab and confirm that the upper surface of the slab is a ‘stretching fault’ with estimated down-dip stretches of 10–40% (assuming plane strain deformation) measured parallel to the flow plane–transport direction.

The structural geometry, metamorphic and magmatic evolution of the Everest massif, High Himalaya of Nepal–South Tibet

This paper presents a new geological map together with cross-sections and lateral sections of the Everest massif. We combine field relations, structural geology, petrology, thermobarometry and geochronology to interpret the tectonic evolution of the Everest Himalaya. Lithospheric convergence of India and Asia since collision at c. 50 Ma. resulted in horizontal shortening, crustal thickening and regional metamorphism in the Himalaya and beneath southern Tibet. High temperatures (>620 °C) during sillimanite grade metamorphism were maintained for 15 million years from 32 to 16.9 ± 0.5 Ma along the top of the Greater Himalayan slab. This implies that crustal thickening must also have been active during this time, which in turn suggests high topography during the Oligocene–early Miocene. Two low-angle normal faults cut the Everest massif at the top of the Greater Himalayan slab. The earlier, lower Lhotse detachment bounds the upper limit of massive leucogranite sills and sillimanite–cordierite gneisses, and has been locally folded. Ductile motion along the top of the Greater Himalayan slab was active from 18 to 16.9 Ma. The upper Qomolangma detachment is exposed in the summit pyramid of Everest and dips north at angles of less than 15°. Brittle faulting along the Qomolangma detachment, which cuts all leucogranites in the footwall, was post-16 Ma. Footwall sillimanite gneisses and leucogranites are exposed along the Kharta valley up to 57 km north of the Qomolangma detachment exposure near the summit of Everest. The amount of extrusion of footwall gneisses and leucogranites must have been around 200 km southwards, from an origin at shallow levels (12–18 km depth) beneath Tibet, supporting models of ductile extrusion of the Greater Himalayan slab. The Everest–Lhotse–Nuptse massif contains a massive ballooning sill of garnet + muscovite + tourmaline leucogranite up to 3000 m thick, which reaches 7800 m on the Kangshung face of Everest and on the south face of Nuptse, and is mainly responsible for the extreme altitude of both mountains. The middle crust beneath southern Tibet is inferred to be a weak, ductile-deforming zone of high heat and low friction separating a brittle deforming upper crust above from a strong (?granulite facies) lower crust with a rheologically strong upper mantle. Field evidence, thermobarometry and U–Pb geochronological data from the Everest Himalaya support the general shear extrusive flow of a mid-crustal channel from beneath the Tibetan plateau. The ending of high temperature metamorphism in the Himalaya and of ductile shearing along both the Main Central Thrust and the South Tibetan Detachment normal faults roughly coincides with initiation of strike-slip faulting and east–west extension in south Tibet (<18 Ma).

Tectonic setting, origin, and obduction of the Oman ophiolite

The Semail ophiolite in the Oman Mountains is the world9s largest and best preserved thrust sheet of oceanic crust and upper mantle (>10 000 km 2 , ∼550 km long, ∼150 km wide); it was emplaced onto the Arabian continental margin during Late Cretaceous time. The ophiolite originated 96–94 Ma at a spreading center above a northeast-dipping subduction zone associated with initiation of immature island-arc tholeiitic lavas (Lasail arc) at the highest levels of the ophiolite. Simultaneous underthrusting of Triassic (and Jurassic[?]) mid-oceanic-ridge basalt and alkalic volcanic rocks beneath >12 km of upper mantle depleted harzburgites produced garnet + clinopyroxene amphibolites formed at temperatures of ∼850 °C, dated as 95–93 Ma. Subduction cannot have been initiated at a mid-oceanic ridge, otherwise the protolith of the amphibolites in the metamorphic sole would be the same age and composition as the ophiolite volcanic rocks above. In the northern part of the Oman Mountains in the Bani Hamid area, United Arab Emirates, ∼870 m of granulite facies rocks (enstatite + spinel ± diopside quartzites, garnet + diopside + wollastonite calc-silicate marbles, clinopyroxene-bearing amphibolites) were formed at temperatures similar to those of the garnet + diopside amphibolites of the Oman sole, 800–850 °C, but at slightly higher pressures, as much as 9 kbar. They are interpreted as deeper level metamorphosed continental margin sedimentary rocks exhumed by out-of-sequence thrusting placing granulites over mantle sequence harzburgites during the later stages of obduction. Subduction of the Arabian continental crust beneath the obducting Semail ophiolite to ∼78–90 km depth has been proven by thermobarometry of the As Sifah eclogites (to 20–23 kbar) in the eastern sector. In the United Arab Emirates the subducted continental crust began to partially melt, producing unusual biotite ± muscovite ± garnet ± tourmaline ± cordierite ± andalusite–bearing granites that intrude the uppermost mantle sequence harzburgites and lowermost crustal sequence cumulate gabbros of the ophiolite. We suggest that the entire leading (northeast) edge of the Arabian plate was subducted beneath the ophiolite during the final stages of obduction leading to eclogitization of the crustal rocks. Higher temperatures and pressures in the United Arab Emirates sector, possibly due to a thicker or double-thickness ophiolite section, led to blueschist, amphibolite, and granulite facies conditions in the metamorphic sole, and crustal melting in the subophiolite basement produced leucocratic granites that intruded up as dikes through the obducted ophiolite. A model for ophiolite obduction is presented, which accounts for all the structural and metamorphic conditions reported from the Oman Mountains.

Channel flow, ductile extrusion and exhumation in continental collision zones: an introduction

Abstract The channel flow model aims to explain features common to metamorphic hinterlands of some collisional orogens, notably along the Himalaya-Tibet system. Channel flow describes a protracted flow of a weak, viscous crustal layer between relatively rigid yet deformable bounding crustal slabs. Once a critical low viscosity is attained (due to partial melting), the weak layer flows laterally due to a horizontal gradient in lithostatic pressure. In the Himalaya-Tibet system, this lithostatic pressure gradient is created by the high crustal thicknesses beneath the Tibetan Plateau and ‘normal’ crustal thickness in the foreland. Focused denudation can result in exhumation of the channel material within a narrow, nearly symmetric zone. If channel flow is operating at the same time as focused denudation, this can result in extrusion of the mid-crust between an upper normal-sense boundary and a lower thrust-sense boundary. The bounding shear zones of the extruding channel may have opposite shear sense; the sole shear zone is always a thrust, while the roof shear zone may display normal or thrust sense, depending on the relative velocity between the upper crust and the underlying extruding material. This introductory chapter addresses the historical, theoretical, geological and modelling aspects of channel flow, emphasizing its applicability to the Himalaya-Tibet orogen. Critical tests for channel flow in the Himalaya, and possible applications to other orogenic belts, are also presented.

The South Tibetan Detachment and the Manaslu Leucogranite : A Structural Reinterpretation and Restoration of the Annapurna-Manaslu Himalaya, Nepal

The South Tibetan Detachment (STD) System comprises both ductile shear zones and brittle low‐angle extensional faults bounding the upper (northern) margin of the high‐grade metamorphic and anatectic rocks of the Greater Himalayan Sequence (GHS). Along the Himalayan chain from Zanskar in the west to Bhutan in the east, leucogranites are restricted to the footwall of the STD and rarely, if ever, intrude across the fault into unmetamorphosed sedimentary rocks of the Tethyan zone. The Manaslu leucogranite (24–19 Ma) was previously thought to be an exception, intruding up as far as the Triassic sediments. We have mapped a newly discovered 350–400‐m‐thick shear zone of high‐strain mylonites along the upper Nar Valley and Pangre glacier (Phu \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Structure of the North Indian continental margin in the Ladakh–Zanskar Himalayas: implications for the timing of obduction of the Spontang ophiolite, India–Asia collision and deformation events in the Himalaya

\mathrm{Detachment}\,=\mathrm{STD}\,