The Himalayan crystalline core zone exposed along the Sutlej Valley (India) is composed of two high‐grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded, as a consequence of the foreland‐directed propagation of crustal deformation in the Indian plate margin. The High Himalayan Crystalline Sequence (HHCS) is composed of amphibolite facies to migmatitic paragneisses, metamorphosed at temperatures up to 750°C at 30 km depth between Eocene and early Miocene. During early Miocene, combined thrusting along the Main Central Thrust (MCT) and extension along the Sangla Detachment induced the rapid exhumation and cooling of the HHCS, whereas exhumation was mainly controlled by erosion since middle Miocene. The Lesser Himalayan Crystalline Sequence (LHCS) is composed of amphibolite facies para‐ and orthogneisses, metamorphosed at temperatures up to 700°C during underthrusting down to 30 km depth beneath the MCT. The LHCS cooled very rapidly since late Miocene, as a consequence of exhumation controlled by thrusting along the Munsiari Thrust and extension in the MCT hanging wall. This renewed phase of tectonic extrusion at the Himalayan front is still active, as indicated by the present‐day regional seismicity, and by hydrothermal circulation linked to elevated near‐surface geothermal gradients in the LHCS. As recently evidenced in the Himalayan syntaxes, active exhumation of deep crustal rocks along the Sutlej Valley is spatially correlated with the high erosional potential of this major trans‐Himalayan river. This correlation supports the emerging view of a positive feedback during continental collision between crustal‐scale tectono‐thermal reworking and efficient erosion along major river systems.

No abstract

International audienceA sharp increase in deposited sediment volume since Pliocene times has been observed worldwide and in particular around the European Alps. This phenomenon has been linked to a rise in denudation rates controlled by an increase of either climatic or tectonic forcing. Observation of in-situ cooling histories for orogens is critical to assess the reality of the inferred increase in denudation rates, and to determine whether this phenomenon is widespread or localized at active tectonic structures. We exploit the unique density of fission-track ages in the Western European Alps to reconstruct cooling isoage surfaces and to estimate exhumation rates on the orogen scale between 13.5 and 2.5 Ma. Our novel technique is based on the association of isoage contours with age–elevation relationships. It uses map-view interpolation, enabling a spatio-temporal analysis of exhumation rates over the entire Western Alps. The resulting exhumation histories reconstructed for eight areas of the Western Alps display strong similarities in timing and rates with orogen-wide average denudation rates inferred from sediment volumes. This consistency validates the use of both techniques for the study of an orogen characterized by strong relief and high recent exhumation rates. We conclude that exhumation rates in the Western Alps have increased more than twofold since Late Miocene times. This increase may have been locally modulated by the distinct response of different tectonic units

The Zermatt-Saas serpentinite complex is an integral member of the Penninic ophiolites of the Central Alps and represents the mantle part of the oceanic lithosphere of the Tethys. Metamorphic textures of the serpentinite preserve the complex mineralogical evolution from primary abyssal peridotite through ocean-floor hydration, subduction-related high-pressure overprint, meso-Alpine greenschist facies metamorphism, and late-stage hydrothermal alteration. The early ocean floor hydration of the spinel harzburgites is still visible in relic pseudomorphic bastite and locally preserved mesh textures. The primary serpentine minerals were completely replaced by antigorite. The stable assemblage in subduction-related mylonitic serpentinites is antigorite-olivine-magnetite ± diopside. The mid-Tertiary greenschist facies overprint is characterized by minor antigorite recrystallization. Textural and mineral composition data of this study prove that the hydrated mineral assemblages remained stable during high-pressure metamorphism of up to 2.5 GPa and 650°C. The Zermatt-Saas serpentinites thus provide a well documented example for the lack of dehydration of a mantle fragment during subduction to 75 km depth.

The chemical and structural alteration of metamict zircon crystals from a 619 ±17 (2σ) Ma old, posttectonic granite in the southern part of the Eastern Desert, Egypt was studied. The crystals show simple oscillatory growth zones with metamictization–induced fractures, which provided pathways for fluid infiltration. Electron and ion microprobe analyses reveal that metamict, i.e. U and Th–rich, areas are heavily enriched in Ca, Al, Fe, Mn, LREE, and a water species, and have lost Zr and Si as well as radiogenic Pb. These chemical changes are the result of an intensive reaction with a low–temperature (120—200°C) aqueous solution. The chemical reactions probably occurred within the amorphous regions of the metamict network. During the zircon–fluid interactions the metamict structure was partially recovered, as demonstrated by micro-Raman and -infrared measurements. A threshold degree of metamictization, as defined empirically by an α–decay dose, Dc, was necessary for zircons to undergo hydrothermal alteration. It is proposed that Dc marks the first percolation point, where the amorphous domains start to form percolating clusters in the metamict network and where bulk chemical diffusion is believed to increase dramatically. The time of the hydrothermal alteration is determined by a lower intercept age of a U-Pb SHRIMP discordia of 17.9 (2σ) Ma, which is in good agreement with an apatite fission track age of 22.2 (2σ) Ma. The hydrothermal alteration event occurred contemporaneously with the main rifting phase of the Red Sea and widespread low- temperature mineralizations along the Red Sea coast.

A zircon fission track-annealing model is calculated on the basis of annealing experiments from the literature with induced tracks in α-decay event damage-free zircon samples. Empirically derived parallel and fanning equations for this "zero-damage" model yield an excellent fit to the data, with the fanning model providing slightly better statistical parameters. A comparison between annealing models with fanning iso-annealing lines but different α-decay event damage densities reveals that annealing temperatures and closure temperatures for the estimated partial annealing zone are highest for the zero-damage model.Compilations of existing geologic constraints on the zircon partial-annealing zone on one hand and the zircon closure temperature on the other show that these constraints do not or only partly overlap with curves of proposed models for the zircon partial-annealing zone and closure temperature. This finding is consistent with the fact that the annealing behavior of zircon from long-duration temperature evolutions is increasingly influenced by the accumulated α-decay event damage. Zircon samples of young age or low U content show a behavior closest to the predictions of the zero-damage model, and are in the predicted range of published models with low α-decay event damage density. For thermal events of more than 10 myr duration, however, constraints from field studies show marked differences from proposed partial-annealing zone boundaries of the zero-or low-damage models.The applicability of the zero-damage model is threefold.(1) It predicts correct closure temperatures in the case of very rapid cooling across the partial annealing zone where basically no α-decay event damage is accumulated. (2) It predicts an uppermost boundary for complete annealing of a mixture of zircon components of different age, as found in sedimentary samples, and in this case may be used as a thermometer. (3) It represents an important reference for the establishment of a more comprehensive model of zircon fission-track annealing that also includes the influence of α-decay event damage. For such a model, two different equations are discussed. However, additional detailed experimental and field data are needed for a more robust annealing model that includes the influence of α-decay event-damage annealing.

Subscribeto subscribe to Geology www.gsapubs.org/subscriptions/index.ac.dtl click Permission requestto contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click official positions of the Society. citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect presentation of diverse opinions and positions by scientists worldwide, regardless of their race, includes a reference to the article's full citation. GSA provides this and other forums for the the abstracts only of their articles on their own or their organization's Web site providing the posting to further education and science. This file may not be posted to any Web site, but authors may post works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent their employment. Individual scientists are hereby granted permission, without fees or further

The evolution of the Central Alpine deformation front (Subalpine Molasse) and its undeformed foreland is recently debated because of their role for deciphering the late orogenic evolution of the Alps. Its latest exhumation history is poorly understood due to the lack of late Miocene to Pliocene sediments. We constrain the late Miocene to Pliocene history of this transitional zone with apatite fission track and (U‐Th)/He data. We used laser ablation inductively coupled mass spectrometry for apatite fission track dating and compare this method with previously published and unpublished external detector method fission track data. Two investigated sections across tectonic slices show that the Subalpine Molasse was tectonically active after the onset of folding of the Jura Mountains. This is much younger than hitherto assumed. Thrusting occurred at 10, 8, 6–5 Ma and potentially thereafter. This is contemporaneous with reported exhumation of the External Crystalline Massifs in the central Alps. The Jura Mountains and the Subalpine Molasse used the same detachments as the External Crystalline Massifs and are therefore kinematically coupled. Estimates on the amount of shortening and thrust displacement corroborate this idea. We argue that the tectonic signal is related to active shortening during the late stage of orogenesis.