Crystalline Sequence Research Articles

Crustal thickening prior to 43 Ma in the Himalaya: Evidence from lower crust‐derived adakitic magmatism in Dala, eastern Tethyan Himalaya, Tibet

The Himalayan–Tibetan orogen is the world's largest active orogen with the thickest continental crust on Earth. However, the timing of crustal thickening of this orogen remains controversial. In recent years, magmatic rocks with adakitic affinities have been widely used to constrain the crustal thickness. Here, we present zircon U–Pb ages, geochemical and Sr–Nd–Pb–Hf isotopic data for the Dala two‐mica granites (eastern Tethyan Himalaya) that may shed light on this issue. Zircon U–Pb dating shows that the Dala two‐mica granites were emplaced at ca. 43 Ma. The granites display adakitic signatures, including high SiO2, Al2O3 and Sr contents, low Y and Yb contents with high Sr/Y (27−67) and La/Yb (35−99) ratios. High K2O and Th contents, low Nb/U, Ce/Pb, Ti/Eu and Nd/Sm ratios and low MgO, Mg#, Cr and Ni contents suggest that the Dala two‐mica granites were derived from partial melting of a thickened lower crust. Low negative zircon εHf(t) values (−10.4 to −0.5) with old two‐stage Hf model ages (1.1–1.8 Ga), high (87Sr/86Sr)i (0.715861–0.717665) with low εNd(t) (−12.5 to −11.3) and high Pb isotopes [(206Pb/204Pb)i=18.783–18.861, (207Pb/204Pb)i=18.783–18.861 and (208Pb/204Pb)i=39.052–39.285], together with previous literature data, indicate that the magma source for the Eocene granites in the Yardoi area was composed of ancient lower crust consisting mainly of garnet‐amphibolite and probably subordinate metapelites of the High Himalayan Crystalline Sequence. The residual mineral assemblage of garnet + rutile + plagioclase ± amphibole in the source region demonstrates that the crust beneath the Himalaya could have been thickened to ﹥50 km in the Eocene. Combining with previous studies, we propose that the Eocene magmatic rocks in the Tethyan Himalaya (e.g. ca. 43 Ma old Dala two‐mica granites) can be best interpreted as the consequence of breakoff of the Neo‐Tethyan oceanic slab.

Cooperative effect through different bridges in nickel catalysts for polymerization of ethylene

A series of mononuclear (M1 and M2) and dinuclear (C1–C6) Ni α‐diimine catalysts activated by modified methylaluminoxane were used in polymerization of ethylene. Catalyst C2 bearing the optimum bulkiness showed the highest activity (1.6 × 106 g PE (mol Ni)−1 h−1) and the lowest short‐chain branching (32.5/1000 C) in comparison to the dinuclear and mononuclear analogues. Although the mononuclear catalysts M1 and M2 polymerized ethylene to a branched amorphous polymer, the dinuclear catalysts led to different branched semicrystalline polyethylenes. Homogeneity and heterogeneity in the microstructure of the polyethylene samples was observed. Different trends for each catalyst were assigned to syn and anti stereoisomers. In addition, thermal behavior of the samples in the successive self‐nucleation and annealing technique exhibited different orders and intensities from methylene sequences and lamellae thickness in respect of each stereoisomer behavior. Higher selectivity of hexyl branches obtained by catalyst C2 showed a cooperative effect between the centers. The results also revealed that for catalysts C5 and C6, selectivity of methyl branches led to very high endotherms and crystalline sequences with melting temperatures higher than that of 100% crystalline polyethylene indicating ethylene/propylene copolymer analogues. For catalysts C3 and C4, more vinyl end groups were a result of the long distance between the Ni centers. Kinetic profiles of polymerization along with a computational study of the precatalysts and catalysts demonstrated that there is a direct relation between rate constant, energy interval of catalyst and precatalyst, and interaction energy of Et···methyl cationic active center (Et···MCC or π–Comp.). Based on this, narrow energy interval (activation energy) of precatalyst and catalyst leads to fast and higher activation rate (catalyst M2), and strong interaction of ethylene and catalyst leads to high monomer uptake and productivity (catalyst C2). Moreover, theoretical parameters including electron affinity, Mulliken charge on Ni, chemical potential and hardness, and global electrophilicity showed optimum values for C2.

Multiproxy Isotopic and Geochemical Analysis of the Siwalik Sediments in NW India: Implication for the Late Cenozoic Tectonic Evolution of the Himalaya

AbstractProvenance analysis of the Sub‐Himalayan Late Miocene‐Pleistocene foreland basin deposits (Siwaliks) from the Dehradun reentrant area provides a 10‐Myr long record of the denudation history and tectonic evolution of the northwestern Indian Himalaya. We studied Siwalik sediments exposed along the Mohand‐Rao and Haripur‐Khol sections, using detrital zircon U‐Pb geochronology, major and trace elements, and Sr‐Nd isotope geochemistry. Results suggest that the erosion pattern has been relatively stable since the Late Miocene with sediments derived from the Tethyan Himalayan (THS), Greater Himalayan (GHS), and outer‐ (oLHS) and inner‐Lesser Himalayan (iLHS) sequences. Provenance data indicate that erosional unroofing of the Lesser Himalayan Crystalline sequences (LHCS) initiated around 6 Ma, possibly related to out‐of‐sequence movement of the Ramgarh‐Munsiari Thrust. Our data also suggest erosional recycling of older foreland basin deposits into younger Siwaliks since ~5.5 Ma, which may indicate the time of thrust propagation from the Lesser Himalaya into the foreland basin. While the iLHS has been exposed to erosion since at least ~10 Ma, the Siwaliks were dominated by materials derived from the GHS and THS sources. We interpret these results as an indication that tectonic uplift and erosion of the orogenic wedge occurred in response to duplexing of the iLHS and concomitant high topography and rock uplift rate in the Greater and Tethyan Himalaya. Comparing the provenance of the Siwalik sediments with that of the modern Ganga and Yamuna river sediments further indicates that deposition during the Late Cenozoic was most likely accomplished by southward flowing transverse Himalayan rivers, analogous to the modern ones.

Yttrium-zoning in garnet and stability of allanite in metapelites from the Main Central Thrust Zone and adjacent higher Himalayan crystallines along the Alaknanda Valley, NW Himalaya

Yttrium (Y)zoning in garnet and the stability of accessory allanite in metapelites from a greenschist to amphibolite facies inverted metamorphic sequence in the Main Central Thrust Zone (MCTZ) and the overlying Higher Himalayan Crystalline Sequence (HHCS) along the Alaknanda Valley in NW Himalaya have been studied. Garnet porphyroblasts from the garnet grade MCTZ metapelites commonly show chemical zoning with a Y-rich core (~3311 ppm) and Y-poor rim (b.d.). Phase diagram modelling for an MCTZ sample shows that the garnet zoning developed during prograde metamorphism in the P–T ranges of ~5.3–7.3 kbar and ~500–585 °C. Accessory allanite (up to 3.45 wt% Y2O3) occurs as inclusion in the rim of garnet porphyroblasts and also in the matrix in most of the MCTZ samples. The Y depletion in the rim of zoned garnets from the MCTZ has possibly resulted from sequestration of Y by allanite, which became a stable phase after the formation of the garnet core. Alternatively, the Y zoning has developed by strong partitioning of the element into garnet with the result that it is mostly incorporated in a narrow core and there is little Y available towards the later part of garnet growth. High-Y annulus occurs in a relatively inclusion-free zone of garnet porphyroblast from an MCTZ rock which has likely developed due to a decrease in the growth rate of the crystal. A garnet from the HHCS metapelites with localised resorption of the rim exhibits weak Y zoning marked by a slight increase in Y at the rim which is attributed to back-diffusion of the element during resorption. With increasing structural level across the MCTZ, the ΣLREE content of allanite first increases then decreases which is complemented by variation in the Al content of the mineral. This compositional variation of allanite appears to be linked to the modal abundance of garnet in rocks.

Mechanically inferior constituents in spider silk result in mechanically superior fibres by adaptation to harsh hydration conditions: a molecular dynamics study.

Spider silk exhibits mechanical properties such as high strength and toughness that are superior to those of any man-made fibre (Bourzac 2015 Nature519, S4-S6 (doi:10.1038/519S4a)). This high strength and toughness originates from a combination of the crystalline (exhibiting robust strength) and amorphous (exhibiting superb extensibility) regions present in the silk (Asakura et al 2015 Macromolecules48, 2345-2357 (doi:10.1021/acs.macromol.5b00160)). The crystalline regions comprise a mixture of poly-alanine and poly-glycine-alanine. Poly-alanine is expected to be stronger than poly-glycine-alanine, because alanine exhibits greater interactions between the strands than glycine (Tokareva et al 2014 Acta Biomater.10, 1612-1626 (doi:10.1016/j.actbio.2013.08.020)). We connect this characteristic sequence to the interactions observed upon the hydration of spider silk. Like most proteinaceous materials, spider silks become highly brittle upon dehydration, and thus water collection is crucial to maintaining its toughness (Gosline et al 1986 Endeavour10, 37-43 (doi:10.1016/0160-9327(86)90049-9)). We report on the molecular dynamic simulations of spider silk structures with different sequences for the crystalline region of the silk structures, of wild-type (WT), poly-alanine, and poly-glycine-alanine. We reveal that the characteristic sequence of spider silk results in the β-sheets being maintained as the degree of hydration changes and that the high water collection capabilities of WT spider silk sequence prevent the silk from becoming brittle and weak in dry conditions. The characteristic crystalline sequence of spider dragline silk is therefore relevant not for maximizing the interactions between the strands but for adaption to changing hydration conditions to maintain an optimal performance even in harsh conditions.

Early Oligocene partial melting via biotite dehydration melting and prolonged low-pressure–low-temperature metamorphism of the upper High Himalaya Crystalline Sequence in the far east of Nepal

Abstract Early Oligocene partial melting and prolonged low-pressure–low-temperature (low-P/T) metamorphism were investigated in migmatites and orthogneisses from the upper High Himalaya Crystalline Sequence (HHCS) in the far east of Nepal. The migmatites were formed by biotite dehydration melting at c. 800°C from 33 to 25 Ma. Cordierite was only produced at shallow crustal levels at pressures <6 kbar. After Early Oligocene partial melting, the low-P/T metamorphism continued until 17 Ma during exhumation of the cordierite-bearing migmatites. Early Oligocene biotite dehydration melting in the upper HHCS occurred at different times and locations from the Early Miocene muscovite dehydration melting in the underlying HHCS and the metamorphic discontinuity was accompanied by thrusting of the High Himalayan Discontinuity at c. 27–19 Ma. Pervasive partial melting and prolonged low-P/T metamorphism in the upper HHCS is more compatible with a lateral southwards channel flow of the upper HHCS along the High Himalayan Discontinuity, whereas current channel flow models explaining the exhumation of the HHCS as driven only by the coupled activity of the Main Central Thrust and South Tibetan Detachment have faced difficulties in explaining the timing of the low-P/T metamorphism observed in the upper HHCS. Supplementary material: The representative cordierite compositions from the cordierite migmatites, the far east of Nepal are available at https://doi.org/10.6084/m9.figshare.c.4068815

Defect-mediated polarization switching in ferroelectric films for low-power-consuming and ultra-high-density memories

Ferroelectric poly(vinylidene fluoride) (PVDF) based polymers are attracting tremendous interest because of their potential applications in flexible non-volatile memories. Current research suggests that the existence of a large hysteresis loop results in high-voltage operation and low writing/erasing speed, which is originated from uniform chain packing and coherent ferroelectric sequences in the crystalline regions. Here, we demonstrate the novel approach to understand the defect-mediated switching mechanisms in ferroelectric polymers by PFM-probe based technology. The single-point and linear polarization reversal is well controlled by defect-mediated ferroelectric phases that determine activation energy, switching rate, and the thermal stability. By the regulation of VDF/TrFE ratio and varying processing conditions, the coherent ferroelectric phase with all-trans sequence in the P(VDF-TrFE) has been disrupted by defects. Specially, the crystallites formed at high temperature in the copolymer P(VDF-TrFE) 50:50 mol% exhibit less ordered ferroelectric crystalline sequences, thus attaining the excellent features of the low operation voltage of about 4 V with a switching time of 100 ms, ultra-high memory density of 43.9 Gbit∙in−2 (by a 7 V, 1 ms pulse) and a high usage temperature of 60 °C. Compared with P(VDF-TrFE) 68:32 mol%, normal ferroelectric, it saves approximate 50% ratio of energy cost, and realizes four times higher resolution. Understanding and controlling defect functionality in ferroelectric materials is as critical for realizing their reliable applications in ferroelectric memories.

Molecular tectonics: control of crystalline sequences

Isostructural and almost isometric crystals that differ in their colour may be welded in solution and under mild conditions into crystalline materials with imposed sequences.

Age of crustal melting in Higher Himalayan Crystalline Sequence (Darjeeling, Eastern Himalaya): Constraints from SHRIMP U–Pb geochronology

The paper presents the petrological and geochronological studies of migmatite from an important section of Higher Himalayan Crystalline Sequence of Eastern Himalaya. Migmatites collected from the area characteristically contain biotite‐garnet‐sillimanite‐plagioclase‐K‐feldspar‐quartz as an important mineral assemblage, which has experienced extensive partial melting through dehydration melting reaction involving biotite. In this study, P–T evolution of these migmatites has been constrained through the use of multiequilibrium thermobarometry program winTWQ, conventional thermobarmetry, and pseudosection modelling in the MnNCKFMASHTO model system using Perple_X software. The unification of these three calculations demonstrates that the migmatite experienced peak pressure and temperature at 7.2 ± 0.5 kbar and 775 ± 20 °C, respectively. SHRIMP U–Pb chronological results yield the timing of crustal melting (21 Ma) in the migmatite.

Nanoporous Crystalline Templates from Double-Crystalline Block Copolymers by Control of Interactive Confinement

Single, double, and coincident crystallizations under hard or soft confinement are all carried out using a single type of syndiotactic poly(p-methylstyrene)-block-poly(l-lactide) (sPPMS–PLLA) block copolymers. The single crystallization of sPPMS matrix can lead to the disordered arrangement of hexagonally packed PLLA cylinders under soft confinement. In contrast, the lamellar nanostructure remained unchanged regardless of the PLLA crystallization under hard or soft confinement. Crystallization-induced morphological transitions from the confined monosized lamella to the metastable dual-sized lamella and finally to the breakout morphology are evident by transmission electron microscopy and small-angle X-ray scattering. The dual-sized lamella is attributed to the thermodynamically and kinetically controlled nanocrystallite growth templating along the ordered microphase separation. Despite crystalline sequences, the double-crystallized morphologies are determined by the first-crystallized event even though th...

Mantle inputs to Himalayan anatexis: Insights from petrogenesis of the Miocene Langkazi leucogranite and its dioritic enclaves

Oligocene and Miocene Himalayan anatexis is generally thought to have been induced by intracrustal heating or processes without the involvement of mantle-derived heat and materials, suggesting that the Himalayan leucogranites are typical examples of purely crustal melts. This study focuses on a Miocene leucogranite at Langkazi within the Himalayan orogen, an intrusion that contains a large number of dioritic enclaves. These enclaves have typical igneous textures, contain acicular apatites, and have back-veining structures, quenched margins, and crystallization ages identical to the hosting two-mica granites, indicating that these enclaves are magmatic. Although the enclaves are evolved, the most primitive samples contain high concentrations of MgO (up to 4.3wt.%), Cr (up to 159ppm), and Ni (up to 102ppm), are strongly enriched in large-ion lithophile elements, are depleted in high-field-strength elements, have negative εNd(t) values (–8.6 to –6.1), and have relatively high 87Sr/86Sr(i) values (0.7085–0.7137), suggesting that they were derived from a relatively enriched region of the lithospheric mantle source. Whole-rock geochemical data indicate that the hosting Langkazi leucogranite formed from magmas generated by the partial melting of metapelite material within the High Himalaya crystalline sequence, and these magmas subsequently mixed with mantle-derived melts that are now represented by the Langkazi dioritic enclaves. This indicates that mantle-derived material played an important role in the generation of the Langkazi intrusions. The whole-rock geochemical compositions of samples from the study area also indicate that the primary melts that formed the Langkazi enclaves were significantly contaminated by the relatively juvenile Himalayan lower crustal material, suggesting in turn that these mantle-derived magmas underwent MASH (crustal melting, melt assimilation, magma storage, and homogenization) processes at the base of crust, introducing heat to the lower crust. This mantle-derived heat may have induced partial melting of the Himalayan lower crust, forming adakite-like magmas. Although the small volumes of these dioritic enclaves suggest that this heat may have assisted the genesis of the north Himalayan granites and the High Himalayan leucogranites, whereas in situ crustal radiogenic heating, shear heating, and decompression melting dominated the melting processes in this region.

New structural and U–Pb zircon data from Anafi crystalline basement (Cyclades, Greece): constraints on the evolution of a Late Cretaceous magmatic arc in the Internal Hellenides

The Asterousia Crystalline Complex consists of Late Cretaceous amphibolite facies metamorphic rocks and associated granitoids, which can be found in exposures on Crete and the Cyclades (Greece). It is attributed to the Uppermost Unit and therefore to the Pelagonian domain of the Internal Hellenides. The tectonometamorphic evolution of this unit is still a matter of debate. We present new structural and petrological data of Asterousia-type rocks and greenschist facies metamorphic rocks from the island of Anafi in the southern Aegean Sea as well as U–Pb zircon ages of granitoids from Anafi. The crystalline sequence of Anafi rests on top of Eocene flysch and comprises from bottom to top: (a) Anafi Greenschist; (b) Anafi Amphibolite Group (orthoamphibolite with intercalations of metasedimentary rocks at the base); and (c) Chalepa Group (amphibolite facies metasediments with slices of serpentinite and granitoids). LA-ICP-MS and ID-TIMS 206Pb/238U zircon ages of granodiorite from the Chalepa Group reveal several similar zircon populations suggesting continuous emplacement of granitoids inside a magmatic arc from ca. 72.5 to 79 Ma. The minimum emplacement age of granodioritic magma, deduced from the 206Pb/238U median age of the youngest zircon population, is 72.6 +0.1/−0.2 Ma. Deformation (micro)fabrics of granodiorite result from low strain obtained at T > 600 °C. This along with the U–Pb ages and published K–Ar ages indicates intrusion of the plutonic rocks at deep structural levels followed by very slow cooling. Monzogranitic dykes cutting through granodiorite in north-eastern Anafi are undeformed and yielded a 206Pb/238U median age of 69.9 +0.7/−0.7 Ma. Based on the new and published data, the following implications for the tectonometamorphic evolution on Anafi can be made: (1) obduction and accretion of mantle slices (serpentinite) to the Asterousia-type rocks were prior to amphibolite facies metamorphism; (2) intrusion of granitoids during the middle to late Campanian within a magmatic-arc setting and coeval shift of the magmatic arc towards the south; (3) Maastrichtian intrusion of dykes; (4) Palaeocene greenschist facies metamorphism and coeval ductile top-to-the SE thrusting of the Anafi Amphibolite Group on top of the Anafi Greenschist; (5) post-Eocene brittle top-to-the SE thrusting of the Anafi Greenschist and the Anafi Amphibolite Group on top of flysch sediments; (6) clockwise rotation of the system and (Early) Oligocene brittle top-to-the SE thrusting of the Chalepa Group on top of all units mentioned above; and (7) ongoing clockwise rotation of the Aegean block during the Oligocene to Early Miocene and change in stress field (NE–SW compression).

Search for conserved amino acid residues of the α-crystallin proteins of vertebrates.

[Formula: see text]-crystallin is the major eye lens protein and a member of the small heat-shock protein (sHsp) family. [Formula: see text]-crystallins have been shown to support lens clarity by preventing the aggregation of lens proteins. We performed the bioinformatics analysis of [Formula: see text]-crystallin sequences from vertebrates to find conserved amino acid residues as the three-dimensional (3D) structure of [Formula: see text]-crystallin is not identified yet. We are the first who demonstrated that the N-terminal region is conservative along with the central domain for vertebrate organisms. We have found that there is correlation between the conserved and structured regions. Moreover, amyloidogenic regions also correspond to the structured regions. We analyzed the amino acid composition of [Formula: see text]-crystallin A and B chains. Analyzing the occurrence of each individual amino acid residue, we have found that such amino acid residues as leucine, serine, lysine, proline, phenylalanine, histidine, isoleucine, glutamic acid, and valine change their content simultaneously in A and B chains in different classes of vertebrates. Aromatic amino acids occur more often in [Formula: see text]-crystallins from vertebrates than on the average in proteins among 17 animal proteomes. We obtained that the identity between A and B chains in the mammalian group is 0.35, which is lower than the published 0.60.

A P–T pseudosection modelling approach to understand metamorphic evolution of the Main Central Thrust Zone in the Alaknanda valley, NW Himalaya

The Main Central Thrust Zone (MCTZ) in the Alaknanda valley, NW Himalaya, affected the Lesser Himalayan Crystalline Sequence and has a gradual transition to the structurally overlying Higher Himalayan Crystalline Sequence (HHCS). This boundary is defined on the basis of the following petrographic features in pelitic rocks at the base of the HHCS: (i) first appearance of microscopic needles of kyanite upon moving up-section along the transect; (ii) garnet porphyroblasts with random inclusion pattern and non-rotational growth history, which contrast with the garnet porphyroblasts in MCTZ rocks with spiral inclusion trails; and (iii) chemically homogeneous garnet porphyroblasts as opposed to the growth-zoned garnets in MCTZ rocks. Pseudosection modelling and garnet isopleth thermobarometry of pelitic rocks yield peak metamorphic conditions of 6.3–7.5 kbar and 550–582 °C in the MCTZ, and 8.0–10.0 kbar and 610–650 °C in the basal part of the HHCS. The results indicate continuity in the P–T field gradient across the contact between the MCTZ and HHCS. The MCTZ shows an inverted metamorphic sequence from biotite to garnet zones. Metamorphism in the basal part of the HHCS is in the kyanite zone, which is continuous with the inverted metamorphic sequence. Both P and T increase up-section, peak in the lower HHCS and then decrease higher up in the HHCS unit. The observations are consistent with predictions of a recently proposed thermomechanical model in which temperature in the shear zone rises due to viscous heating and pressure rises as a result of weakening of the rocks.

Oligocene HP metamorphism and anatexis of the Higher Himalayan Crystalline Sequence in Yadong region, east-central Himalaya

The Higher Himalayan Crystalline Sequence (HHCS) provides an excellent natural laboratory to study continental subduction, crustal melting and tectonic evolution of orogenic belt generated through the collision of India with Eurasia. Our petrological study and phase equilibrium modeling reveal that the pelitic migmatites in the HHCS of Yadong region, east-central Himalaya, preserve an early mineral assemblage garnet, kyanite, biotite, quartz, plagioclase, K-feldspar, rutile and ilmenite, and a late sillimanite- and/or cordierite-bearing assemblage, and underwent the high pressure (HP) and high temperature (HT) granulite-facies metamorphism and associated partial melting under P–T conditions of ca. 12kbar and 825–845°C, followed by nearly isothermal decompression and isobaric cooling. The anatexis of the migmatites occurred dominantly through dehydration-melting of both muscovite and biotite during the prograde metamorphism. The melt produced in the peak metamorphic conditions is about 20 to 30vol.% of the rocks, and a significant amount of melt has been extracted from the source leading to the formation of Himalayan leucogranites. The zircon U–Pb dating data shows that the migmatites probably witnessed a prolonged melting episode that began at ca. 30Ma and lasted to ca. 20Ma. These results show that the thickening lower crust of the Himalayan orogen experienced long-lived and continued HP and HT metamorphism and pervasive anatexis, supporting the models on channel flow.

A review on out-of-sequence deformation in the Himalaya

Abstract Out-of-sequence deformation in the Himalaya has been caused mainly by thrusting. Out-of-sequence thrusts, usually north- to NE-dipping foreshear planes, occur inside the Sub-Himalaya (SH), Lesser Himalaya (LH) and Greater Himalayan Crystalline (GHC) sequences. Where absolute dates are available, the youngest slip within the SH occurred near the Janauri Anticline (India) at c. AD 1400–1460. The Munsiari Thrust (India) activated within the LH at c. 1–2 Ma and the Main Central Thrust zone in the Marsyandi valley (Nepal) in the GHC was formed during the Holocene ( c. 0.3 ka). Except for the Riasi Thrust (Kashmir, India), the Paonta Thrust (Himachal Pradesh, India) in the Siwalik and the Tons Thrust (Garhwal region, India) within the Main Central Thrust zone, crustal shortening related to out-of-sequence thrusting in the Himalaya has been insignificant. The major litho-/stratigraphic contacts within the SH and the GHC at some places acted as out-of-sequence thrusts. Out-of-sequence thrusts in the SH have been detected mainly based on geomorphological observations. However, more quantitative geochronological studies have detected out-of-sequence thrusting from c. 22 Ma up to Holocene age in the GHC based on age jumps, especially within the Main Central Thrust zone. Crustal channel flow (specifically for the GHC) and/or the critical taper model with or without erosion can be used to explain the Himalayan out-of-sequence thrusts.

Lens defects in Astyanax mexicanus Cavefish: evolution of crystallins and a role for alphaA-crystallin.

The fish Astyanax mexicanus presents, within the same species, populations of river-dwelling surface fish (SF) and blind cave-living fish. In cavefish (CF), the eyes develop almost normally during embryogenesis. But 40 h after fertilization, the lens enters apoptosis, triggering the progressive degeneration of the entire eye. Before apoptosis, the CF lens expresses early differentiation factors correctly. Here, we searched for possible late differentiation defects that would be causal in CF lens degeneration. We reasoned that crystallins, the major lens structural proteins, could be defective or misregulated. We surveyed the CF and SF transcriptomes and uncovered 14 Astyanax crystallins from the beta, gamma, lambda, mu, and zeta families. These proteins are less polymorphic and accumulate more fixed mutations, some at highly conserved positions, in CF than in SF, suggesting relaxed selection at these loci in CF. In situ hybridizations and qPCR show that crybb1c, crybgx, crygm5 are expressed at much lower levels or are not expressed in the CF lens. For the best crystallin candidates, we tested a potential causal role in CF lens apoptosis. Crybgx, crybb1c (not expressed in CF from very early on), and cryaa (previously shown to be faintly expressed in CF) failed to induce any defect when knocked-down in zebrafish embryos. However, the anti-apoptotic cryaa protected lens cells from apoptosis when reexpressed by transgenesis in CF, suggesting a cell-autonomous effect of cryaa on lens cell survival. Altogether, these data suggest that crystallin sequence evolution and expression defects may contribute to the loss of eyes in CF.

P–T conditions of metabasites within regional metapelites in far-eastern Nepal Himalaya and its tectonic meaning

Metabasites exposed in far-eastern Nepal provide an important insight into the metamorphic evolution of the Himalayan orogen independent from data obtained on metapelites. The P–T conditions and formation process of mafic granulite intercalated within Early Oligocene migmatites and two amphibolites surrounded by Early Miocene metapelites were inferred from pseudosection modeling and conventional geothermobarometry combined with the occurrences of field and microstructures. A mafic granulite in the Higher Himalaya Crystalline Sequence (HHCS) yields P–T conditions of 6.5–8 kbar, 730–750 °C. The similar peak P–T condition and retrograde path with low P/T gradient of mafic granulite and surrounding migmatite indicate that both rocks were simultaneously metamorphosed and exhumed together along the tectonic discontinuities in the HHCS. In contrast, the P–T conditions (2–5 kbar, 500–600 °C) of highly-deformed amphibolite block above the Main Central Thrust (MCT) records significantly lower pressure than garnet-mica gneisses in the country rock, suggesting that the amphibolite block derived from upper unit of the MCT zone and became tectonically mixed with the gneisses of hanging wall near the surface. An amphibolite lense below the MCT preserves the prograde P–T conditions (6–7.5 kbar, 550–590 °C) of Early Miocene syn-tectonic metamorphism that occurred in the MCT zone. This study indicates the top-to-the south movement of the MCT zone results in the tectonic assembly of rocks with different P–T–t conditions near the MCT.

Salinia revisited: a crystalline nappe sequence lying above the Nacimiento fault and dispersed along the San Andreas fault system, central California

Salinia revisited: a crystalline nappe sequence lying above the Nacimiento fault and dispersed along the San Andreas fault system, central California

Post-collisional, K-rich mafic magmatism in south Tibet: constraints on Indian slab-to-wedge transport processes and plateau uplift

Post-collisional (23–8 Ma), potassium-rich (including ultrapotassic and potassic) mafic magmatic rocks occur within the north–south-trending Xuruco lake–Dangre Yongcuo lake (XDY) rift in the Lhasa terrane of the southern Tibetan Plateau, forming an approximately 130-km-long semi-continuous magmatic belt. They include both extrusive and intrusive facies. Major and trace element and Sr–Nd–Pb isotopic data are presented for all of the known exposures within the XDY rift. The potassium-rich, mafic igneous rocks are characterized by high MgO (5.9–10.8 wt.%), K2O (4.81–10.68 wt.%), Ba (1,782–5,618 ppm) and Th (81.3–327.4 ppm) contents, and relatively high SiO2 (52.76–58.32 wt.%) and Al2O3 (11.10–13.67 wt.%). Initial Sr isotopic compositions are extremely radiogenic (0.712600–0.736157), combined with low (206Pb/204Pb) i (18.28–18.96) and (143Nd/144Nd) i (0.511781–0.512046). Chondrite-normalized rare earth element patterns display relatively weak negative Eu anomalies. Primitive mantle-normalized incompatible trace element patterns exhibit strong enrichments in large ion lithophile elements relative to high-field-strength elements and display strongly negative Ta–Nb–Ti anomalies. The combined major and trace element and Sr–Nd–Pb isotopic characteristics of the K-rich igneous rocks suggest that the primitive magmas were produced by 1–10 % partial melting of an asthenospheric mantle source enriched by both fluids and partial melts derived from Indian passive continental margin sediments subducted into the shallow mantle as a consequence of the northward underthrusting of the Indian continental lithosphere beneath Tibet since the India–Asia collision at ~55 Ma. The best-fit model results indicate that a melt with trace element characteristics similar to those of the K-rich rocks could be generated by 8–10 % partial melting of a metasomatized mantle source in the south and 1–2 % melting in the north of the XDY rift. Trace element and Sr–Nd–Pb isotopic modeling indicate that the proportion of fluid derived from the subducted sediments, for which we use as a proxy the Higher Himalayan Crystalline Sequence (HHCS), in the mantle source region increases from north (rear-arc) to south (front-arc), ranging from 0 to 5 %, respectively. Correspondingly, the proportion of the melt derived from the subducted HHCS in the source increases from north (2 %) to south (15 %). The increasing proportion of the fluid and melt component in the mantle source from north to south, together with a southward decreasing trend in the age of the K-rich magmatism within the XDY rift, is inferred to reflect rollback of the subducted Indian lithospheric mantle slab during the period 25–8 Ma. Slab rollback may be linked to a decreasing convergence rate between India and Asia. As a consequence of slab rollback at 25 Ma beneath the Lhasa terrane, its geodynamic setting was transformed from a convergent (55–25 Ma) to an extensional (25–8 Ma) regime. The occurrence of K-rich magmatism during the period 25–8 Ma is a consequence of the decompression melting of an enriched mantle source, which may signal the onset of extension in the southern Tibetan Plateau and provide a petrological record of the extension process.