Clare J. Warren

Rapid synconvergent exhumation of Miocene-aged lower orogenic crust in the eastern Himalaya

Rare granulitized eclogites exposed in the eastern Himalaya provide insight into conditions and processes deep within the orogen. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb, Ti, and rare earth element (REE) data from zircons in mafic granulitized eclogites located in the upper structural levels of the Greater Himalayan Sequence in Bhutan show that zircon was crystallized under eclogite-facies metamorphic conditions between 15.3 ± 0.3 and 14.4 ± 0.3 Ma, within a couple million years of the later granulite-facies overprint. In conjunction with pressure estimates of the eclogite- and granulite-facies stages of metamorphism, the age data suggest that initial exhumation occurred at plate-tectonic rates (cm yr−1). These extremely rapid synconvergence exhumation rates during the later stages of the India-Asia collision require a revision of theories for the transportation and exhumation of crustal materials during continental collisions. In contrast to western Himalayan examples, the eastern Himalayan eclogites cannot be tectonically related to steep subduction of India beneath Asia. Instead, they more likely represent fragments from the base of the overthickened Tibetan crust. Based on the zircon age and trace-element data, we hypothesize that the protolith of the mafic granulites was middle Miocene mafic intrusions into the lower crust of southern Tibet, linked to Miocene volcanism in the Lhasa block. We suggest that a transient tectonic event—possibly the indenting of a strong Indian crustal ramp into crust under southern Tibet that had been weakened by partial melting—may have promoted exhumation of the eclogitized lower crust under Tibet. The mafic magmatism and volcanism themselves may have been related to the convective thinning of the lithospheric mantle triggered by a reduction in the India-Eurasia convergence rate during the middle Miocene, which in turn could have facilitated the rapid extrusion of the lower crust over the earlier-exhumed middle crust.

Dating the subduction of the Arabian continental margin beneath the Semail ophiolite, Oman

The presence of high-pressure ( P ) eclogites along the northeast margin of the Arabian continental plate beneath the Semail ophiolite demonstrates that subduction of thinned continental crust to pressures of 20 kbar (∼80 km depth) has occurred. Current debate centers on whether this high- P metamorphism occurred prior to, during, or following ophiolite emplacement, which is known to have started ca. 95–93 Ma. Five concordant U-Pb zircon ages from a garnet-clinopyroxene-phengite-crossite eclogite from As Sifah, northeast Oman, precisely constrain the age of high- P metamorphism at 79.1 ± 0.3 Ma. This age demonstrates that high- P metamorphism occurred during continental subduction 15 m.y. after formation of the ocean floor that subsequently became the Semail ophiolite.

Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya

Geochemical and geochronological analyses provide quantitative evidence about the origin, development and motion along ductile faults, where kinematic structures have been overprinted. The Main Central Thrust is a key structure in the Himalaya that accommodated substantial amounts of the India–Asia convergence. This structure juxtaposes two isotopically distinct rock packages across a zone of ductile deformation. Structural analysis, whole-rock Nd isotopes, and U–Pb zircon geochronology reveal that the hanging wall is characterized by detrital zircon peaks at c. 800–1000 Ma, 1500–1700 Ma and 2300–2500 Ma and an ϵNd(0) signature of −18.3 to −12.1, and is intruded by c. 800 Ma and c. 500–600 Ma granites. In contrast, the footwall has a prominent detrital zircon peak at c. 1800–1900 Ma, with older populations spanning 1900–3600 Ma, and an ϵNd(0) signature of −27.7 to −23.4, intruded by c. 1830 Ma granites. The data reveal a c. 5 km thick zone of tectonic imbrication, where isotopically out-of-sequence packages are interleaved. The rocks became imbricated as the once proximal and distal rocks of the Indian margin were juxtaposed by Cenozoic movement along the Main Central Thrust. Geochronological and isotopic characterization allows for correlation along the Himalayan orogen and could be applied to other cryptic ductile shear zones. Supplementary material: Zircon U–Pb geochronological data, whole-rock Sm–Nd isotopic data, sample locations, photomicrographs of sample thin sections, zircon CL images, and detailed analytical conditions are available at www.geolsoc.org.uk/SUP18704.

The Grenville Orogen explained? Applications and limitations of integrating numerical models with geological and geophysical data

Numerical models offer powerful insights into tectonic processes, especially when their validity can be tested against geological and geophysical observations from natural orogenic belts. Here we explain some of the criteria for success in integrating orogenic models with data, using examples from the Grenville Orogen. Model designs must be simplified by comparison with nature to illuminate the first-order processes that control orogenic evolution, which limits the extent to which model results can reproduce geological observations. For the western Grenville Orogen, observed variations in geological properties are represented by lower crustal blocks with strength decreasing from the exterior to the interior of the model. GO-series models with this design reproduce the first-order crustal architecture of the Georgian Bay and Montreal – Val d’Or Lithoprobe transects. Both constant-convergence and stop-convergence models produce similar geometries, but only stop-convergence models produce normal-sense shear ...

Using U‐Th‐Pb petrochronology to determine rates of ductile thrusting: Time windows into the Main Central Thrust, Sikkim Himalaya

Quantitative constraints on the rates of tectonic processes underpin our understanding of the mechanisms that form mountains. In the Sikkim Himalaya, late structural doming has revealed time-transgressive evidence of metamorphism and thrusting that permit calculation of the minimum rate of movement on a major ductile fault zone, the Main Central Thrust (MCT), by a novel methodology. U-Th-Pb monazite ages, compositions, and metamorphic pressure-temperature determinations from rocks directly beneath the MCT reveal that samples from ~50 km along the transport direction of the thrust experienced similar prograde, peak, and retrograde metamorphic conditions at different times. In the southern, frontal edge of the thrust zone, the rocks were buried to conditions of ~550°C and 0.8 GPa between ~21 and 18 Ma along the prograde path. Peak metamorphic conditions of ~650°C and 0.8–1.0 GPa were subsequently reached as this footwall material was underplated to the hanging wall at ~17–14 Ma. This same process occurred at analogous metamorphic conditions between ~18–16 Ma and 14.5–13 Ma in the midsection of the thrust zone and between ~13 Ma and 12 Ma in the northern, rear edge of the thrust zone. Northward younging muscovite 40Ar/39Ar ages are consistently ~4 Ma younger than the youngest monazite ages for equivalent samples. By combining the geochronological data with the >50 km minimum distance separating samples along the transport axis, a minimum average thrusting rate of 10 ± 3 mm yr−1 can be calculated. This provides a minimum constraint on the amount of Miocene India-Asia convergence that was accommodated along the MCT.

Hydrochemical associations and depth profiles of arsenic and fluoride in Quaternary loess aquifers of northern Argentina

Abstract Arsenic and fluoride in groundwater from Quaternary loess deposits in Argentina pose major health concerns. Common sources for arsenic and fluoride have been suggested but the processes of mobilization are disputed, and distributions in groundwater are largely unresolved at a sample density >1/50 km2. At Los Pereyras in Tucuman Province, northern Argentina, we have evaluated distributions and hydrochemical associations of arsenic and fluoride with a sample density of 0.75 per km2 over an area of 75 km2, to a depth of 230 m. Groundwater in the loess is oxic and alkaline. Fluoride is restricted to the upper 20 m of the Quaternary loess, where it reaches 8.3 mg/l. Arsenic has a vertical layering consistent with that of fluoride, ranging from 20 to 760 μg/l in the upper 20 m and 58-163 μg/l below this. There are two sources of arsenic, one unrelated to the fluoride source. Positive correlations between arsenic and fluoride with pH, but not with alkalinity, support desorption from iron oxyhydroxides as the likely mechanism of release to groundwater for arsenic and fluoride, rather than the weathering of silicate minerals. Stratigraphic and/or palaeohydrological controls may explain the observed depth distributions within the loess aquifer.

Subduction zone polarity in the Oman Mountains: implications for ophiolite emplacement

Abstract Two end-member models have been proposed to account for the structure and metamorphism of rocks beneath the Semail ophiolite in the Oman mountains. Model A involves a single, continuous NE-directed subduction away from the continental margin during the late Cretaceous. The ophiolite and underlying thrust sheets of distal to proximal oceanic sediments were emplaced a minimum of 250 km SW onto the continental margin. Subduction of Triassic-Jurassic oceanic basalts to c. 10 kbar (c. 39 km depth) led to the accretion of amphibolite-facies rocks to the base of the ophiolite. Thrusting propagated towards the continental margin and ended with subduction of the thinned continental crust to c. 20 kbar (c. 78 km depth), choking the subduction zone. Buoyancy forces caused the rapid exhumation of eclogites, blueschists and carpholite-grade HP rocks along the NE margin of the continental plate. During the later phase of foreland-propagating thin-skinned thrusting in the SW, NE-facing backfolding and backthrusting occurred in the hinterland, with the final exhumation of the HP rocks. Model B follows recent suggestions that a nascent SW-dipping subduction zone, dipping beneath the continental margin, existed between 130 and 95 Ma, prior to formation and emplacement of the ophiolite. A major NE-facing fold-nappe structure in the pre-Permian basement rocks of Saih Hatat is interpreted as reflecting subduction beneath the margin. Two high-pressure metamorphic events have been suggested, the first predating ophiolite emplacement, the second caused by ophiolite loading. This model is untenable, being based on a misinterpretation of the NE-facing structures in northern Saih Hatat, and on some dubious older 40Ar/39Ar cooling ages from the eclogite-facies rocks of As Sifah. We conclude that all structures in northern Oman and all the reliable geochronology point to a single emplacement-obduction event lasting from Cenomanian-Turonian time (c. 95 Ma) when amphibolites were accreted along the metamorphic sole of the ophiolite, to Campanian time, when the continental margin was subducted to the NE producing blueschists and eclogites, to the final thin-skinned emplacement of all thrust sheets, which ended before the Late Maastrichtian, at c. 68 Ma.

Lithological, rheological, and fluid infiltration control on 40Ar/39Ar ages in polydeformed rocks from the West Cycladic detachment system, Greece

In situ ultraviolet (UV) laser-ablation 40 Ar/ 39 Ar dating, microstructural analysis, and stable O, H, and C isotope analyses were performed on white mica−bearing calcite− and quartz−mica schists of the West Cycladic detachment system footwall in order to resolve outstanding uncertainties about the timing of deformation and the role of rock rheology on 40 Ar/ 39 Ar dating systematics. In both quartz-rich and calcite-rich samples, deformed and chemically zoned white micas form two chemical populations: (1) a high component of Al-celadonite in undeformed portions of grains (high-pressure remnants), and (2) enrichment in muscovite in deformed portions (low-pressure neocrystallization). Micas in the quartz-rich rocks record higher internal strain, illustrated by elongated, sheared grains and boudinaged mica-fish structures. In this lithology, quartz formed a load-bearing framework that transferred strain to the muscovite packets and facilitated the formation of mica-fish structures. Recrystallization was promoted by coeval fluid infiltration, supported by stable isotope analyses and indented boundaries on bulging quartz grains. In rocks containing calcite-muscovite aggregates, the calcite formed an interconnected weak layer, with strain being accommodated by dislocation creep. In these rocks, micas were only partially neocrystallized. Prismatic white micas, largely unaffected by boudinage or kinking, yielded 40 Ar/ 39 Ar ages that are up to 10 m.y. older than deformed (kinked or sheared) portions of the same grains. Overall, the ages attest to strong lithological control on deformation- and fluid-controlled white mica neocrystallization. The oldest, undeformed grain ages in the calcite-rich rocks are consistent with the timing of Eocene metamorphism, with the deformed grain ages interpreted as representing the transition to lower-pressure conditions during nascent extension. Completely neocrystallized grains in the quartz-rich rocks are interpreted as defining the minimum age of Miocene ductile extension along the detachment system. The new data show the power of combining in situ laser-ablation 40 Ar/ 39 Ar dating, microstructural analysis, mineral chemistry, and stable isotope data for unraveling the timing and time scales of complex deformation histories.

The geology and tectonics of central Bhutan

Lithotectonic mapping, metamorphic observations and U–Pb zircon ages underpin a substantial revision of central Bhutan geology, notably a more extensive and continuous outcrop of the Tethyan Sedimentary Series (TSS) than previously mapped. Metamorphic grade in the TSS increases downward towards a basal north-vergent tectonic contact with the underlying Greater Himalayan Series (GHS), interpreted as a southward continuation of the South Tibetan Detachment (STD). Miocene (c. 17–20 Ma) leucogranite sheets are associated with the STD in this region but appear to diminish southwards. Two leucogranite dykes that cross-cut TSS structures yield ages of 17.8 ± 0.2 and 17.9 ± 0.5 Ma. A 500 ± 4 Ma (U–Pb zircon) metamorphosed ash bed in the Pele La Group within the psammite-dominated lower TSS yields the first direct isotopic age for the TSS in the eastern Himalaya, confirming existing age constraints from detrital zircon and fossil studies. A continuation of the Paro metasedimentary unit underlying the GHS was mapped near Wangdue Phodrang. Our observations, notably the exposure of a wholly ductile STD so far south and the significance of large nappe-like structures in the TSS, prompt a major revision to the geological map of the Bhutan Himalaya and require a reassessment of tectonic interpretations of the Bhutan Himalaya. Supplementary materials: Zircon U–Pb geochronological data, sample locations and descriptions, features of analysed zircons, sample processing method and detailed analytical conditions are available at http://www.geolsoc.org.uk/SUP18876.

Anomalously old biotite 40Ar/39Ar ages in the NW Himalaya

Biotite 40 Ar/ 39 Ar ages older than corresponding muscovite 40 Ar/ 39 Ar ages, contrary to the diffusion properties of these minerals, are common in the Himalaya and other metamorphic regions. In these cases, biotite 40 Ar/ 39 Ar ages are commonly dismissed as “too old” on account of “excess Ar.” We present 32 step-heating 40 Ar/ 39 Ar ages from 17 samples from central Himachal Pradesh Himalaya, India. In almost all cases, the biotite ages are older than predicted from cooling histories. We document host-rock lithology and chemical composition, mica microstructures, biotite chemical composition, and chlorite and muscovite components of biotite separates to demonstrate that these factors do not offer an explanation for the anomalously old biotite 40 Ar/ 39 Ar ages. We discuss possible mechanisms that may account for extraneous Ar (inherited or excess Ar) in these samples. The most likely cause for “too-old” biotite is excess Ar, i.e., 40 Ar that is separated from its parent K. We suggest that this contamination resulted from one or several of the following mechanisms: (1) 40 Ar was released during Cenozoic prograde metamorphism; (2) 40 Ar transport was restricted due to a temporarily dry intergranular medium; (3) 40 Ar was released from melt into a hydrous fluid phase during melt crystallization. Samples from the Main Central Thrust shear zone may be affected by a different mechanism of excess-Ar accumulation, possibly linked to later-stage fluid circulation within the shear zone and chloritization. Different Ar diffusivities and/or solubilities in biotite and muscovite may explain why biotite is more commonly affected by excess Ar than muscovite.