D. C. Rex

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.

Collision tectonics in the NW Himalayas

Summary West Himalayan tectonics involve the collision of microplates between the Indian and Asian Plates. The Kohistan Complex consists largely of tightly folded basic volcanics and sediments generated as Late Jurassic to Late Cretaceous island arcs. These were intruded by post-folding Mid-Cretaceous — Eocene plutonics produced from continued subduction of the Indian Plate after closure of a suture between Kohistan and the Karakorum. The Himalayan structures show major thrust sheets and the Kohistan Arc is essentially a crustal ‘pop-up’ with southward-upright and northward-verging structures developed above a thick ductile decoupling zone (the Indus Suture), which can be traced for >100 km beneath Kohistan on large reentrants. This pop-up formed by a two stage process, closure of the Northern Suture followed by closure of the southern Indus Suture. Granitic rocks of the Kohistan-Ladakh Batholith (dated at ≅ 100-40 Ma) post-date most of the structures related to the Northern Suture but were deformed and carried southwards on shear structures related to the Indus Suture. Post-collisional deformation carried this Kohistan Complex on deep decoupling zones over the Indian Plate on a series of imbricated gneiss sheets, the thrusts climbing up section in the movement direction so that in the far S some override their own molasse debris. Folds above these deep decoupling zones deformed their overlying thrust sheets into large antiforms—i.e. the Nanga Parbat and Hazara Syntaxes. The Nanga Parbat Syntaxis probably formed due to a shear couple near a branch line where one of the main Himalayan thrusts joined the Indus Suture beneath Kohistan. Crustal delamination, to produce the imbricated gneiss sheets, could not account for all the displacement of India into Asia, suggested by palaeomagnetic data. There must also have been lateral displacement as demonstrated by the large oblique-slip shear zone in the Hunza Valley, N of Kohistan.

Rift deflection, migration, and propagation: Linkage of the Ethiopian and Eastern rifts, Africa

The Main Ethiopian and Eastern (Gregory) rifts, sectors of the East African rift system, overlap in a 300-km-wide system of extensional basins that is more than three times the breadth of either rift away from the zone of overlap. The oldest volcanic rocks (Eocene) and possibly the oldest rift basins (Oligocene) of the East African rift system occur in this zone of overlap. The objectives of field, remote sensing, and geochronology (K-Ar and 40 Ar/ 39 Ar) studies in southwestern Ethiopia were to establish a chronology of rifting and volcanism in the zone of overlap, and to correlate stratigraphic sequences with those in the Kenya rift to the south and in the Main Ethiopian rift to the north. Field observations and cross sections show that basins are bounded by steeply dipping faults, stratal dips are <30°, and that extension accommodated by the intrusion of dikes is volumetrically insignificant. Thus, the style of faulting is similar to that elsewhere in East Africa south of the Afar rift. Initial volcanism between ca. 45 and 33 Ma preceded faulting and uplift, except for reactivation of some Mesozoic rift structures near the Sudan-Ethiopia border and in northern Kenya. Extensional basins began to form in late Oligocene time in the Eastern rift, and in early Miocene time in the Main Ethiopian rift. Small degrees of extension and associated volcanism in the broadly rifted zone may have been triggered by extension in the Red Sea, as well as by lithospheric heating above a mantle plume. The anomalous breadth of the zone is a consequence of rift propagation and migration, rather than basin-and-range‐style extension; both the Main Ethiopian rift and Eastern rifts have propagated along north-south lines, and the Eastern rift has migrated ~200 km eastward since late Oligocene time. The distribution of seismicity and Quaternary volcanism suggest that the Eastern and Main Ethiopian rifts are currently linked across a 200-km-wide zone between the Omo and Segen basins.

Tectonic development of the northern Taiizaiiian sector of the East African Rift System

The Eastern Branch of the East African Rift System diverges from a single, c. 50 km wide rift in southern Kenya to a c. 200 km wide zone in northern Tanzania, where it is comprised of three distinct rifts with different orientations. The western part of this zone contains two rift branches: the Natron-Man yara-Balangida and Eyasi-Wembere rifts. Each rift contains individual basins that are defined here on the basis of structural and geophysical interpretations. These basins are shallow (<3km) and total extension across the bounding faults is small. New K/Ar age determinations on basalts from the western rift basins show that volcanism and sedimentation began in the area at c. 5 Ma. Major fault escarpments were present by c. 3 Ma and the present-day rift escarpments developed later than c. 1.2 Ma. Pre-rift volcanism produced large shield volcanoes of a basalt-trachyte-phonolite association that now lie on the rift flanks. Volcanism after the main phase of rift faulting produced volatile- and alkali-rich explosive centres which are active today, and have no equivalent in southern Kenya. The change in morphology of the Eastern Branch of the East African Rift System, and the style of volcanism in northern Tanzania, may be the result of the transition from the rifting of Proterozoic Mozambique Belt lithosphere to the rifting of cratonic Archaean lithosphere.

Age of crustal melting, emplacement and exhumation history of the Shivling Leucogranite, Garhwal Himalaya

We report a U–Pb monazite age of 23.0±0.2 Ma for the Shivling leucogranite, a tourmaline+muscovite±biotite leucogranite at the top of the High Himalayan slab in the Garhwal Himalaya, north India. The Shivling–Bhagirathi leucogranite is a viscous near-minimum melt, emplaced as a foliation parallel laccolith via a dyke network not far from its source region. Prograde heating occurred soon after the India–Asia collision at c . 50 Ma up to melting at 23 Ma and high temperatures (>550 °C) were maintained for at least 15 Ma after garnet growth. The leucogranite was emplaced at mid-crustal depths along the footwall of the Jhala fault, a large-scale low-angle normal fault, part of the South Tibetan Detachment system, above kyanite and sillimanite grade gneisses. The geometry of the leucogranite laccolith shows biaxial extension and boudinage both perpendicular (north-northeast–south-southwest) and parallel to the strike (west-northwest–east-southeast) of the mountain range. Unroofing occurred by underthrusting beneath the High Himalayan slab along the Main Central Thrust zone, progressively ‘jacking up’ the leucogranites, removal of material above by low-angle normal faulting, and erosion. Very rapid cooling at rates of 200–350 °C/Ma between 23–21 Ma immediately followed crystallization, as tectonic unroofing and erosion removed 24–28 km of overburden during this time. K–Ar muscovite ages are 22±1.0 Ma and fission track ages of zircons from >5000 m on the North Ridge of Shivling are 14.2±2.1 and 8.8±1.2 Ma and apatites are 3.5±0.79 and 2.61±0.23 Ma. Slow steady state cooling at rates of 20–30 °C/Ma from 20–1 Ma shows that maximum erosion rates and unroofing of the leucogranite occurred during the early Miocene. This timing coincides with initiation of low-angle, north-dipping normal faulting along the South Tibetan Detachment system.

Pressure, temperature and time constraints on Himalayan metamorphism from eastern Kashmir and western Zanskar

Himalayan metamorphism in the Kashmir and Zanskar sector of the High Himalaya resulted from thrust- and fold-related crustal thickening within the Indian plate following the collision of India and Asia at c. 50 Ma. Interbanded metapelites, marbles, calcareous schists, amphibolites and quartzites represent metamorphosed equivalents of the Palaeozoic-Lower Mesozoic continental margin rocks. Granitic rocks include pre-collision K-feldspar megacrystic and biotite granites and post-collision two-mica ± garnet ± tourmaline granites. Average pressure-temperature conditions of equilibration using the self-consistent thermodynamic data-set of Holland & Powell (1990) are presented across a 45 km traverse of the eastern Kashmir-western Zanskar High Himalaya. Kyanite is the stable aluminosilicate phase across 35 km outcrop width in the middle of the slab. Complex microstructures indicate that prograde metamorphism up to kyanite grade and fabric development in the upper structural nappes is early and unrelated to the Main Central thrust. Diachronous metamorphism propagated southwards with the overall structural evolution. Early isograds and thrust-fold structures were carried passively in the hanging-wall of the Main Central thrust. Peak metamorphism, based on 40Ar/39Ar hornblende ages, occurred pre–30.7 ± 2.0 Ma at the top of the slab, and in the middle of the slab was pre-22 ± 1.0 Ma, probably 25-28 Ma. The lower structural levels in Zanskar record peak metamorphic conditions around 700–750 °C and 8 kbar, reflecting depths of burial of 28–30 km. A new younger schistosity, which is not present in the higher structural levels, was developed under kyanite grade conditions. The regional distribution of high temperatures recorded by thermo-barometry does not support the concept of additional heat being supplied by frictional heating along the Main Central thrust or magmatic heat resulting from anatexis. Exhumation of the Himalayan metamorphic rocks was achieved by three major processes: erosion as a result of crustal thickening, uplift of the rocks along the hangingwall of the Main Central thrust above a major frontal ramp in the Kishtwar Window area, and extensional unroofing along the footwall of a large-scale, NE-dipping normal fault at upper crustal levels, probably synchronous with compression at depth.

Geo-tectonic framework of the Himalaya of N Pakistan

In the Karakorum Range there is a structurally complicated Cretaceous are comprising the Kohistan sequence. On its northern side the Northern Suture consists of a mega-mélange and is bounded to the S by tightly folded pillow-bearing volcanics and sediments. To the S the Kohistan Plutonic Belt consists of (southwards): (a) early foliated and late post-tectonic tonalites and diorites, (b) aplites and pegmatites (up to 30% of rock volume), (c) basic dykes up to 10 m thick, (d) the Chilas Complex, a stratiform cumulate body over 300 km long and 8 km thick (chromite-layered dunites, gabbros and norites) with a low pressure granulite-facies mineral fabric of tectonic origin, (e) an amphibolite belt with a complex mixture of other rocks, and (f) the Jijal Complex, a 200 km2 tectonic wedge of high pressure granulites and chromite-layered dunites. Cumulate graded units in the Chilas Complex show that it is folded by an isoclinal anticline (F1). The mid-upper crust of the are is folded by a 50 km half-wavelength F2, syncline. The whole Kohistan sequence with its two phases of isoclinal folds was tilted during Himalayan collision so that the structures are now subvertical. The Southern Suture (Main Mantle Thrust) has a wedge of glaucophane schists. The Indian plate contains a basement of psammites and schists intruded by Cambrian granites and overlain by isoclinally folded and metamorphosed carbonates and shales.

Structure and metamorphism of blueschist–eclogite facies rocks from the northeastern Oman Mountains

The northern part of the Saih Hatat window, Oman, shows high-pressure metamorphic rocks derived from shelf sediments and pre-Premian continental basement, and is atypical of sub-ophiolite metamorphism elsewhere. The high-pressure rocks are divided into structural units originally bounded by foreland-propagating thrusts formed during ophiolite obduction, although now many contacts are backthrusts, normal faults or extensional shear zones. Metamorphic breaks exist across many unit boundaries. The deepest unit (As Sifah) has eclogite-facies assemblages in metabasites and metapelites which record evolution of P–T conditions along a clockwise path culminating al 23±2.5kbar, 540±75°C, contrasting markedly with overlying units (5–10 kbar, 200–500°C), although separated by <l0km on the ground. The dominant penetrative structures in the eclogites predate exhumation, but broad zones in the enveloping and overlying schists show a later, greenschist-facies extensional fabric. Phengites from eclogite-facies schist show discordant 40Ar/39 Ar apparent ages. Our tectonic model relates all the high-P units to a single convergent event in the Late Cretaceous. The As Sifah eclogites were exhumed in two stages: (i) tectonic emplacement against other units at c. 20–25 km depth, and (ii) exhumation of the entire high-P zone by culmination collapse after obduction.

Volcanic rocks beneath the Semail Ophiolite nappe in the northern Oman mountains and their significance in the Mesozoic evolution of Tethys

Volcanic rocks, locally up to 700 m thick, occur within a complex imbricate zone beneath the Upper Cretaceous Semail Ophiolite nappe in the northern Oman mountains. Although the rocks were considerably disrupted during late Cretaceous thrusting and nappe emplacement, intact sequences occur and show that there is a lower unit of alkaline lavas and pyroclastics, including ankaramites, alkali basalts and trachytes, and an upper one of predominantly tholeiitic pillow lavas. The latter contain interbedded cherts and limestones and well as large blocks of Permian and Triassic ‘exotic’ limestones. In places the volcanics are intruded by minor intrusives of alkaline peridotite and gabbro. Geochemical studies, particularly of the ‘immobile’ elements, show that the lower volcanics and the intrusives are strongly alkaline with high Ti, P, Zr and Nb contents and steep LREE enriched rare earth patterns. They are typical ‘within-plate’ alkaline magmas characteristic of continental rift zones and some ocean islands. Within the tholeiitic basalt lava pile two types are recognized: a predominant relatively trace element enriched ‘transitional’ type which probably formed in a within-plate oceanic setting, and a ‘depleted’ type with the geochemical characteristics of some island arc tholeiites. K-Ar ages of biotite separates from the alkaline lavas give Triassic ages (230-200 Ma) while one of the alkali periodotite sills gives an Upper Cretaceous age (92 Ma). The bulk of the tholeiites are late Triassic as they are interbedded with or enclose the Upper Triassic ‘exotics’. The Triassic volcanics are interpreted as having formed during continental rifting and then as marginal ocean islands during the early stages of ocean basin development. However, preliminary radiolarian ages from interbedded cherts in the uppermost lavas are Cretaceous. The presence of these island-arc type tholeiites is taken as evidence that subduction off the Oman continental margin began prior to ophiolite emplacement in the late Cretaceous.

Hobbs Coast Cenozoic volcanism: Implications for the West Antarctic rift system

Abstract Basaltic lavas were erupted from a 40-km-long lineament near the Hobbs Coast of Marie Byrd Land, Antarctica, over the period from 11.7 m.y. to 2.3 m.y. ago. The lavas from the southernmost locality, Coleman Nunatak, are virtually constant in major, trace element and isotopic composition over this entire age span. Their high FeO-low Al203 character indicates melting of garnet peridotite at about 140 km depth. There is no evidence for the involvement of ancient continental lithosphere or MORB asthenosphere in the magmatism. Isotopically, the lavas show the highest 206 Pb/ 204Pb ratios (up to 20.7) of any of the Cenozoic volcanism associated with the West Antarctic rift system (WARS). This HIMU isotopic signature is also clear in the trace element patterns, which closely mimic end-member HIMU basalts from the oceanic islands of Tubuai and Mangaia. From the other localities along the Hobbs Lineament, the earliest volcanism, which is coeval with that at Coleman Nunatak, is of shallower derivation (∼ 110 km), and isotopically like the oceanic FOZO end-member (206Pb/204Pb ∼19.5). The trace-element patterns are similar to those at Coleman, but less enriched in the most incompatible elements by a factor of two. Modeling of the trace element data is consistent with a uniform mantle source composition, depleted in major elements, but hydrous and mildly enriched in the incompatible and LREE. Inversion for the bulk distribution coefficients of the source mantle reveals a spidergram with a marked negative Ti anomaly and marked positive anomalies for K, Sr, Zr and HE From this modeling, the extent of melting at Coleman is inferred to be ∼ 1.6%, as compared to ∼ 3.2% during the earliest volcanism elsewhere on the lineament. With time, the volcanism from these other localities progresses to greater depth, becomes more HIMU in character, and lower in extent of melting (i.e., approaches the character of basalts from the Coleman locality). The FOZO component is prevalent as a mixing end-member in WARS volcanism from numerous other Marie Byrd Land (MBL) and Northern Victoria Land (NVL) localities. It is also the main constituent of the three nearby oceanic plumes (Balleny, Scott, Peter I islands). The HIMU component is at best a minor constituent of these oceanic plumes, but is present at several other MBL and NVL localities, as well as in pieces of Zealandia which were adjacent to this coast of Antarctica prior to fragmentation of Gondwana. We propose that this HIMU mantle source was emplaced under Gondwana lithosphere prior to breakup, as a large weak plume head, with little or no accompanying volcanism. This ‘fossil-plume’ proto-lithosphere is now being sampled during WARS extension. Likely mechanisms for the volcanism relate either to small-scale convection associated with strong basal topography of the lithosphere (such as that recorded by the Hobbs Lineament volcanism), or to emplacement of a new plume, which may in part be driving the extension.