Randall R. Parrish

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.

Timing of India‐Asia collision: Geological, biostratigraphic, and palaeomagnetic constraints

[1] A range of ages have been proposed for the timing of India-Asia collision; the range to some extent reflects different definitions of collision and methods used to date it. In this paper we discuss three approaches that have been used to constrain the time of collision: the time of cessation of marine facies, the time of the first arrival of Asian detritus on the Indian plate, and the determination of the relative positions of India and Asia through time. In the Qumiba sedimentary section located south of the Yarlung Tsangpo suture in Tibet, a previous work has dated marine facies at middle to late Eocene, by far the youngest marine sediments recorded in the region. By contrast, our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50.6–52.8 Ma, in broad agreement with the timing of cessation of marine facies elsewhere throughout the region. Double dating of detrital zircons from this formation, by U-Pb and fission track methods, indicates an Asian contribution to the rocks thus documenting the time of arrival of Asian material onto the Indian plate at this time and hence constraining the time of India-Asia collision. Our reconstruction of the positions of India and Asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early Eocene. We conclude the paper with a discussion on the viability of a recent assertion that collision between India and Asia could not have occurred prior to ∼35 Ma.

Himalayan metamorphic sequence as an orogenic channel: Insight from Bhutan

The Bhutan Himalayas differ from the rest of the Himalayas in two major ways: (i) low-grade sedimentary rocks lie above the Greater Himalayan Sequence as klippen (i.e. erosional remains of the South Tibetan Detachment); and (ii) an out-of-sequence thrust, the Kakhtang thrust, lies structurally above the klippen, and it doubles the exposed thickness of the Greater Himalayan Sequence. Our field observations and geochronological data constrain the main kinematic events in the Bhutan Himalayas. Crystallisation ages of leucogranite dykes deformed by the Main Central Thrust and the South Tibetan Detachment indicate that these two structures operated together between 16 and 22 Ma. The out-of-sequence Kakhtang thrust was active at 10–14 Ma and was concurrent with reactivation of the South Tibetan Detachment. Restoration of the Bhutan Himalayas prior to the out-of-sequence thrusting shows that the Greater Himalayan Sequence was the core of a long, low-viscosity crustal channel extending under the Tibetan plateau. We propose that the gravity-driven southward extrusion of the channel material from underneath the Tibetan plateau caused the inverted metamorphic sequence in the Lesser Himalayan Sequence and in the Greater Himalayan Sequence. This process also led to occurrences of present-day surface rocks that were derived from variable distances down dip, but from similar crustal depths. Such an exhumation pattern can explain the similar peak pressures for the Greater Himalayan Sequence along the length of the Himalayas.

An improved micro-capsule for zircon dissolution in UPb geochronology∗

A new design of Teflon® micro-capsule for mineral dissolution is described which offers advantages over the conventional Krogh vessel for use in zircon UPb geochronology. The TFE Teflon® capsules have friction-fit caps and are intended to reside inside a larger Teflon® pressure vessel. The capsules have a capacity of ∼ 0.35 ml, and are easy to machine, handle and clean. Because the capsules are not in themselves pressure vessels, they do not deform, even at 250°C. Without this pressure differential, they have less tendency to trap undissolved crystals, Pb or U in the walls due to pressure enhanced diffusion or migration. Their capacity to be used at 240–250°C allows for rapid dissolution of any zircon sample in ≤ 30 hr. A number of tests have failed to detect either vapor phase transfer of Pb or U, or radiogenic Pb memory from one sample to the next at the picogram level.

Simultaneous Miocene Extension and Shortening in the Himalayan Orogen

The South Tibetan detachment system separates the high-grade metamorphic core of the Himalayan orogen from its weakly metamorphosed suprastructure. It is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front. Geochronologic data from the Rongbuk Valley, north of Qomolangma (Mount Everest) in southern Tibet, demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago, an interval characterized by large-scale crustal thickening at lower structural levels. These data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of Himalayan tectonics in Miocene time.

Isotopic constraints on the age and provenance of the Lesser and Greater Himalayan sequences, Nepalese Himalaya

The Main Central thrust of the central Himalaya juxtaposes the Lesser Himalayan sequence (footwall) with the Greater Himalayan sequence (hanging wall), both of which are regarded as components of the northern passive margin of India. Considerable uncertainty surrounds their respective ages and sedimentary derivation. U-Pb and Sm-Nd isotopic studies of samples from the Langtang area, central Nepal, demonstrate important distinctions between the two rock packages. The Greater Himalayan sequence had a sedimentary provenance that included a major source of 0.8–1.0 Ga zircons, implying a Late Proterozoic age. The source of the Precambrian part of the Lesser Himalayan sequence contained 1.87–2.60 Ga zircons, and its depositional age is probably Middle Proterozoic. Nd isotopic characteristics of the two sequences are different: at 21 Ma, the Greater Himalayan rocks had eNd values between −14.6 and −18.5 while their Lesser Himalayan counterparts had values between −21.4 and −25.9, giving rise to major differences in their model ages. The Lesser and Greater Himalayan sequences at Langtang are interpreted as correlative to the lower and upper parts of the Vindhyan Supergroup, currently exposed south of the Gangetic plain in northern India. On the basis of available age data, the exposed hanging wall of the Main Central thrust in central and eastern Nepal contains no Archean-Early Proterozoic basement. If the fault does root into the basement at depth, it must do so farther north than recently published cross sections indicate. This implies a total displacement on the fault that is substantially greater than the 140–210 km required by tectonic overlap in the eastern Himalayas.

Crustal thickening leading to exhumation of the Himalayan Metamorphic core of central Nepal: Insight from U-Pb Geochronology and 40Ar/39Ar Thermochronology

New and published U-Pb geochronology and 40Ar/39Ar thermochronology from footwall and hanging wall rocks of a segment of the South Tibetan detachment system exposed in the Annapurna area of central Nepal Himalaya bring additional constraints on the timing of metamorphism, crustal thickening, and normal faulting resulting in exhumation of the Himalayan metamorphic core. Early Oligocene crustal thickening led to Eohimalayan kyanite-grade metamorphism between 35 Ma and 32 Ma. The resulting thermal event affected the Early Ordovician augen gneiss (Formation III) and produced kyanite-bearing leucosomes in the upper part of the metamorphic core. This event is linked with underthrusting of the Greater Himalayan metamorphic sequence below the Tethyan sedimentary sequence and the growth of an Oligocene fan structure that has thickened the Tethyan sedimentary sequence to 25 km, thus provoking kyanite-grade melting at deeper structural levels. Early Paleozoic monazite and zircon populations indicate that part of the metamorphism affecting the Himalayan metamorphic core could be pre-Cenozoic. Regional correlations indicate that the Annapurna detachment was active during early Miocene time. A weakly deformed leucogranitic dike intruding into the immediate hanging wall yielded reversely discordant monazite ages between 23 and 22.5 Ma, which suggest that the ductile strain in the Annapurna detachment zone terminated at ca. 22 Ma. On the basis of a 40Ar/39Ar muscovite age, renewed southwest verging deformation (D4) is interpreted to occur at ca. 18 Ma. Rapid exhumation resulting from extensional faulting cooled the entire metamorphic core through the muscovite Ar closure temperature (330°–430°C) between 15 and 13 Ma. Muscovites from the immediate hanging wall of the Annapurna detachment yielded slightly younger ages, between 13 and 11 Ma, testifying to late hydrothermal activity in the Annapurna detachment zone that could be linked with the initiation of brittle faulting associated with the late Neogene Thakkhola graben system.

Provenance and U-Pb geochronology of the Mesoproterozoic Belt Supergroup (northwestern United States): implications for age of deposition and pre-Panthalassa plate reconstructions

The Mesoproterozoic Belt-Purcell Supergroup is a thick succession of siliciclastic and carbonate sedimentary rocks in the northwestern United States and adjacent Canada. Recent stratigraphic studies provide strong evidence for an enclosed basin setting with a tectonically active western margin. The age of the western (present coordinates) landmass is unknown, but has been inferred to be different from North American basement that borders the Belt basin on the basis of whole rock Sm-Nd studies. We report U-Pb isotopic analyses of individual detrital zircon, monazite, and xenotime grains separated from westerly derived clastic units in the Belt Supergroup as well as Sm-Nd isotopic data for the same grains where appropriate. These data provide new constraints on the age and isotopic character of the western landmass. Most U-Pb data are concordant or slightly discordant; exceptions are noted below. The Revett Formation (Ravalli Group, lower Belt Supergroup) contains grains with concordant ages of 1590–1600 Ma and a grain with a discordant207Pb/206Pb age of 1780 Ma: the ca. 1600 Ma age is very unusual in North America and effectively dates “Belt Island”, long postulated to be the Revett source in what are now terranes of southeast Washington and northeast Oregon that were accreted in the Mesozoic. Detrital zircons from the Missoula Group (Bonner and Mount Shields formations; upper Belt Supergroup) form a distinct group at 1670–1859 Ma with two ca. 2.6 Ga grains. U-Pb ages from monazite range from 1642 to 1786 Ma, and together with Nd isotopic data (TDM model ages of 2.05–3.36 Ga) suggest variable interaction with previously differentiated crust. The Buffalo Hump Formation in northeast Washington may correlate with the Bonner Formation or may lie unconformably on the Belt Supergroup; it contains detrital zircons of ca. 1840 Ma and, surprisingly young, 1070–1244 Ma (several grains). Monazites have U-Pb ages of 1753–1774 Ma and Nd signatures similar to the Bonner Formation data. Several major conclusions can be drawn from these data. The ages of 1590–1600 Ma have no known possible source within western North America (although 1576 Ma augen gneiss is present in the Priest River Complex northwest of the Belt basin), and the 1642–1786 Ma ages are uncommon to rare in Precambrian rocks to the immediate south and east; together these data support previous Sm-Nd whole rock and stratigraphic conclusions of a dominantly western source for the Belt basin. The ages in the Buffalo Hump Formation can be interpreted in two ways. Either the Buffalo Hump correlates with the Bonner Formation and therefore the top of the Belt Supergroup is substantially younger (< 1070 Ma) than previously inferred (1250 Ma) and may overlap with Grenville-age tectonic activity. Alternatively, the Buffalo Hump Formation may represent a new unit that is younger than the Belt. Irrespective of this issue, these data help to constrain proposed pre-Paleozoic plate reconstructions; from the 1070–1244, 1590–1600 and 1642–1786 Ma source ages and related Nd data, we suggest that the basement blocks of south-central Australia (Gawler, Musgrave, Willyama, Arunta and possibly Mount Isa) were joined to the western side of Laurentia, adjacent to the Belt basin, prior to Neoproterozoic formation of the Panthalassa Ocean.

Plate velocity exhumation of ultrahigh-pressure eclogites in the Pakistan Himalaya

U-Pb ages of zircon and allanite from the coesite-bearing ultrahigh-pressure (UHP) units in the Kaghan Valley, northern Pakistan, demonstrate that peak UHP metamorphism along the northern margin of the Indian plate occurred at 46.4 ± 0.1 Ma at peak pressure-temperature conditions of >27.5 kbar (>100 km) and 720–770 °C. Much lower pressure retrogressive growth of titanite took place between 46.4 and ca. 44 Ma, indicating that the eclogites were exhumed to 35 km depth at or before 44 Ma, implying very rapid exhumation rates within the mantle of ∼30–80 mm/yr or more, comparable to rapid plate velocities. Once entrained in the base of the crust, the eclogites followed a slower cooling history from 45 Ma, similar to the amphibolite facies gneisses of the Pakistan Himalaya.

Two episodes of monazite crystallization during metamorphism and crustal melting in the Everest region of the Nepalese Himalaya

New monazite U-Pb geochronological data from the Everest region suggest that ∼20–25 m.y. elapsed between the initial India-Asia collision and kyanite-sillimanite–grade metamorphism. Our results indicate a two-phase metamorphic history, with peak Barrovian metamorphism at 32.2 ± 0.4 Ma and a later high-temperature, low-pressure event (620 °C, 4 kbar) at 22.7 ± 0.2 Ma. Emplacement and crystallization of the Everest granite subsequently occurred at 20.5–21.3 Ma. The monazite crystallization ages that differ by 10 m.y. are recorded in two structurally adjacent rocks of different lithology, which have the same postcollisional pressure-temperature history. Scanning electron microscopy reveals that the younger monazite is elaborately shaped and grew in close association with apatite at grain boundaries and triple junctions, suggesting that growth was stimulated by a change in the fluid regime. The older monazite is euhedral, is not associated with apatite, and is commonly armored within silicate minerals. During the low-pressure metamorphic event, the armoring protected the older monazites, and a lack of excess apatite in this sample prevented new growth. Textural relationships suggest that apatite is one of the necessary monazite-producing reactants, and spots within monazite that are rich in Ca, Fe, Al, and Si suggest that allanite acted as a preexisting rare earth element host. We propose a simplified reaction for monazite crystallization based on this evidence.