Coupled Zircon-Rutile U-Pb Chronology: LA ICP-MS Dating, Geological Significance and Applications to Sediment Provenance in the Eastern Himalayan-Indo-Burman Region

U-Pb dating by LA ICP-MS is one of the most popular and successful isotopic techniques available to the Earth Sciences to constrain timing and rates of geological processes thanks to its high spatial resolution, good precision (absolute U/Pb age resolution of ca. 2%, 2s), rapidity and relative affordability. The significant and continuous improvement of instrumentation and approaches has opened new fields of applications by extending the range of minerals that can be dated by this method. Following the development and distribution to the community of good quality reference materials in the last decade, rutile U-Pb thermochronology (with a precision only slightly worse than zircon) has become a commonly used method to track cooling of deep-seated rocks. Its sensitivity to mid- to low-crustal temperatures (~450 °C to 650 °C) is ideal to constrain exhumation in active and ancient orogens as well as thermal evolution of slow-cooled terranes. Recrystallization and secondary growth during metamorphism and the presence of grain boundary fluids can also affect the U-Pb isotopic system in rutile. A growing body of research focusing on U-Pb dating of rutile by LA ICP-MS is greatly improving our understanding of the behavior of this mineral with regards to retention of radiogenic Pb. This is key to fully exploit its potential as a tracker of geological processes. The latest developments in this field are reviewed in this contribution. The combined application of U-Pb zircon and rutile chronology in provenance studies, particularly when complemented by lower-T thermochronometry data, allows the isotopic characterization of the sources across a wide range of temperatures. The benefits of applying detrital zircon-rutile U-Pb chronology as a coupled provenance proxy are presented here, with a focus on the Eastern Himalayan-Indo-Burman region, where a growing number of successful studies employs such an approach to help constrain river drainage and basin evolution and to infer feedback relationships between erosion, tectonics and climat

Some asymptotics for Sobolev orthogonal polynomials involving Gegenbauer weights

AbstractWe consider the Sobolev inner product 〈f,g〉=∫−11f(x)g(x)(1−x2)α−12dx+∫f′(x)g′(x)dψ(x),α>−12, where dψ is a measure involving a Gegenbauer weight and with mass points outside the interval (−1,1). We study the asymptotic behaviour of the polynomials which are orthogonal with respect to this inner product. We obtain the asymptotics of the largest zeros of these polynomials via a Mehler–Heine type formula. These results are illustrated with some numerical experiments

Asymptotics for Jacobi–Sobolev orthogonal polynomials associated with non-coherent pairs of measures

AbstractWe consider the Sobolev inner product 〈f,g〉=∫−11f(x)g(x)dψ(α,β)(x)+∫f′(x)g′(x)dψ(x), where dψ(α,β)(x)=(1−x)α(1+x)βdx with α,β>−1, and ψ is a measure involving a rational modification of a Jacobi weight and with a mass point outside the interval (−1,1). We study the asymptotic behaviour of the polynomials which are orthogonal with respect to this inner product on different regions of the complex plane. In fact, we obtain the outer and inner strong asymptotics for these polynomials as well as the Mehler–Heine asymptotics which allow us to obtain the asymptotics of the largest zeros of these polynomials. We also show that in a certain sense the above inner product is also equilibrated

The Brahmaputra tale of tectonics and erosion:early Miocene river capture in the Eastern Himalaya

The Himalayan orogen provides a type example on which a number of models of the causes and consequences of crustal deformation are based and it has been suggested that it is the site of a variety of feedbacks between tectonics and erosion. Within the broader orogen, fluvial drainages partly reflect surface uplift, different climatic zones and a response to crustal deformation. In the eastern Himalaya, the unusual drainage configuration of the Yarlung Tsangpo–Brahmaputra River has been interpreted either as antecedent drainage distorted by the India–Asia collision (and as such applied as a passive strain marker of lateral extrusion), latest Neogene tectonically-induced river capture, or glacial damming-induced river diversion events. Here we apply a multi-technique approach to the Neogene paleo-Brahmaputra deposits of the Surma Basin (Bengal Basin, Bangladesh) to test the long-debated occurrence and timing of river capture of the Yarlung Tsangpo by the Brahmaputra River. We provide U–Pb detrital zircon and rutile, isotopic (Sr–Nd and Hf) and petrographic evidence consistent with river capture of the Yarlung Tsangpo by the Brahmaputra River in the Early Miocene. We document influx of Cretaceous–Paleogene zircons in Early Miocene sediments of the paleo-Brahmaputra River that we interpret as first influx of material from the Asian plate (Transhimalayan arc) indicative of Yarlung Tsangpo contribution. Prior to capture, the predominantly Precambrian–Paleozoic zircons indicate that only the Indian plate was drained. Contemporaneous with Transhimalayan influx reflecting the river capture, we record arrival of detrital material affected by Cenozoic metamorphism, as indicated by rutiles and zircons with Cenozoic U–Pb ages and an increase in metamorphic grade of detritus as recorded by petrography. We interpret this as due to a progressively increasing contribution from the erosion of the metamorphosed core of the orogen. Whole rock Sr–Nd isotopic data from the same samples provide further support to this interpretation. River capture may have been caused by a change in relative base level due to uplift of the Tibetan plateau. Assuming such river capture occurred via the Siang River in the Early Miocene, we refute the “tectonic aneurysm” model of tectonic–erosion coupling between river capture and rapid exhumation of the eastern syntaxis, since a time interval of at least 10 Ma between these two events is now demonstrated. This work is also the first to highlight U–Pb dating on detrital rutile as a powerful approach in provenance studies in the Himalaya in combination with zircon U–Pb chronology

Plio-Pleistocene exhumation of the eastern Himalayan syntaxis and its domal ‘pop-up’

The eastern termination of the Himalayan orogen forms a structural syntaxis that is characterised by young (from 10 to < 1 Ma) mineral growth and cooling ages that document Late Miocene to Pleistocene structural, metamorphic, igneous and exhumation events. This region is a steep antiformal and in part domal structure that folds the suture zone between the Indian and Asian plates. It is dissected by the Yarlung Tsangpo, one of the major rivers of the eastern Himalayan–Tibet region, which becomes the Brahmaputra River in the Indian foreland basin before emptying into the Bay of Bengal. Exceptionally high relief and one of the deepest gorges on Earth have developed where the river's tortuous route crosses the Namche Barwa–Gyala Peri massif (> 7 km in elevation) in the core of the syntaxis. Very high erosion rates documented in sediment downstream of the gorge at the foot of the Himalaya contribute ~ 50% of total detritus to the sediment load of the Brahmaputra. The initiation of very high rates of exhumation has been attributed either to the extreme erosive power of a river flowing across a deforming indentor corner and the associated positive feedback, or to the geometry of the Indian plate indentor, with the corner being thrust beneath the Asian plate resulting in buckling which accommodates shortening; both processes may be important. The northern third of the syntaxis corresponds to a steep domal ‘pop-up’ structure bounded by the India–Asia suture on three sides and a thrust zone to the south. Within the dome, Greater Himalaya rocks equilibrated at ~ 800 °C and 25–30 km depth during the Miocene, with these conditions potentially persisting into the latest Miocene and possibly the Pliocene, with modest decompression prior to ~ 4 Ma. This domal ‘pop-up’ corresponds to the area of youngest bedrock ages on a wide variety of thermochronometers and geochronometers. In this paper we review the extensive scientific literature that has focused on the eastern syntaxis and provide new chronological data on its bedrock and erosion products to constrain the age of inception of the very rapid uplift and erosion. We then discuss its cause, with the ultimate aim to reconstruct the exhumation history of the syntaxis and discuss the tectonic context for its genesis. We use zircon and rutile U–Pb, white mica Ar–Ar and zircon fission track dating methods to extract age data from bedrock, Brahmaputra modern sediments (including an extensive compilation of modern detrital chronometry from the eastern Himalaya) and Neogene palaeo-Brahmaputra deposits of the Surma Basin (Bangladesh). Numerical modelling of heat flow and erosion is also used to model the path of rocks from peak metamorphic conditions of ~ 800 °C to < 250 °C. Our new data include U–Pb bedrock rutile ages as young as 1.4 Ma from the Namche Barwa massif and 0.4 Ma from the river downstream of the syntaxis. Combined with existing data, our new data and heat flow modelling show that: i) the detrital age signature of the modern syntaxis is unique within the eastern Himalayan region; ii) the rocks within the domal pop-up were > 575 ± 75 °C only 1–2 Myr ago; iii) the Neogene Surma Basin does not record evidence of the rise and erosion of the domal pop-up until latest Pliocene–Pleistocene time; iv) Pleistocene exhumation of the north-easternmost part of the syntaxis took place at rates of at least 4 km/Myr, with bedrock erosion of 12–21 km during the last 3 Ma; v) the inception of rapid syntaxial exhumation may have started as early as 7 Ma or as late as 3 Ma; and vi) the Yarlung Tsangpo is antecedent and subsequently distorted by the developing antiform. Together our data and modelling demonstrate that the domal pop-up with its exceptional erosion and topographic relief is likely a Pleistocene feature that overprinted earlier structural and metamorphic events typical of Himalayan evolution. Keywords: Eastern Himalayan syntaxis; Namche Barwa; Surma Basin; Yarlung Tsangpo–Brahmaputra; U–Pb rutile dating; Thermal modellin

Mesozoic sedimentary covers of the Ligure Piemontese Ocean: from source areas to the architecture of margins

Provenance studies started in the 19th century with the aim of determining the parent rocks of single minerals or mineral varieties. With the advent of the Plate Tectonic theory, classification schemes were proposed, to quantitatively evaluate sandstone composition, which in turn was used to relate sandstone composition to the tectonic setting of the source areas. The traditional provenance studies based on modal analysis of arenites were enhanced by the application of other methods not limited solely to the sand-sized fraction. After the development/improvement of high resolution analytical techniques, the geochemical features of fine-grained siliciclastic rocks have been fruitfully used in provenance studies, along with the petrographic and chemical analysis of coarse-grained deposits. Moreover, the field of provenance analysis has undergone a revolution with the development of high resolution single-crystal isotope-dating techniques, such as SHRIMP U-Pb dating on zircons and laserprobe 40Ar –39Ar dating. As suggested and recommended in recent works dealing with siliciclastic provenance, a multi-disciplinary approach is highly desirable. In this PhD thesis, a multidisciplinary approach is proposed to face a provenance issue, which combines: i) petrography and geochemistry of conglomerate clasts, ii) modal analysis of arenites, iii) geochemistry of major (XRF) and trace element (ICP-MS) of pelites and arenites, iv) Ar-Ar geochronology of detrital white micas. This approach has been applied to sedimentary rocks in order to unravel a geological problem: the reconstruction of the configuration of continental margins of the Ligure-Piemontese oceanic basin (a branch of Western Tethys opened during Jurassic between the Corsica/Europe and Adria plates). The main goals related to this issue are: i) to unravel the composition of the two margins, which in turn can provide further constraints for their configurations prior to their involvement in the collisional events; ii) to discuss the results in order to evaluate the different models existing for the opening history of the Ligure Piemontese Ocean. Remnants of the Ligure Piemontese oceanic basin, today preserved as tectonic units within the Alpine-Apennine belt (originated after the continental collision between Corsica/Europe and Adria plates during Eocene), display well developed Upper Jurassic to Cretaceous sedimentary successions. The investigated units are: the Err-Platta units from the Central Alps, the Balagne Nappe from Corsica, the Internal and External Ligurian units from Northern Apennines. Paleogeographic reconstructions permit the pristine location of these units in the Ligure Piemontese oceanic basin. The studied sedimentary rocks were all fed by the passive margins of the Ligure Piemontese Ocean, making it possible to investigate the nature of the source areas located in the two conjugate margins. The geochemical investigation of pelites permitted to highlight ‘time’-related first-order chemical large-scale features for the sediments of the Ligure Piemontese oceanic basin. A more mafic character is shown by the Radiolarite formation vs the Calpionella Limestone and Palombini Shale formations sampled in the different investigated units (the three formations representing the pelagic post-rift deposits of the Ligure Piemontese oceanic basin). This difference in composition can be explained considering the erosion and distribution inside the oceanic basin of the rough primary morphologies of the basaltic sea-floor at the time of deposition of the Radiolarites. This possibly resulted in a mafic geochemical signature inherited by the fine-grained siliciclastic sediments from the first post-rift oceanic formation (the Radiolarite Formation). This common oceanic basin-scale chemical feature is preserved and still decipherable thanks to the geochemical features, despite the investigated formations belong to tectonic units from various sectors of the Alpine-Apennine chain that experienced different tectonic/metamorphic histories. The composition of the sedimentary covers from the Ligure Piemontese oceanic basin has been then considered under the ‘space’-related perspective, comparing the units to each other. Along a Corsica-Apennines transect, petrographic data from rudites and arenites have been combined with geochemical data from pelites, and the main features of the source areas (i.e. the continental margins) have been highlighted using the differences in composition inferred from these data. The source area of Corsica and Internal Ligurian units, regarded as the Corsica/Europe continental margin, was made up of the upper part of a continental basement plus its carbonate sedimentary cover. The sedimentary record generated by its erosion is in fact represented by turbidite deposits derived from low-grade metamorphic rocks, granites, acidic volcanic rocks and carbonate platform rocks. The geochemical data relative to the fine-grained siliciclastic fraction of these deposits indicate a composition resulting from the mixing of felsic and mafic sources, with the felsic component being prominent. On the other hand, the source area of the External Ligurian units (the Adria margin) was represented by a complete lithospheric section, from the upper mantle to the siliceous/carbonate sedimentary cover. The erosion of this margin produced turbidite deposits derived from mantle rocks, low- to high- grade metamorphic rocks, granites, acidic volcanic rocks, carbonatic platform rocks and pelagic sedimentary rocks. The geochemical data relative to the fine-grained siliciclastic fraction of these deposits indicate an ultramafic source standing out from the mafic-felsic components also contributing to the sediment. This local second-order chemical feature overprints the first-order time-related one, recognized on a basin scale. Petrographic and geochemical data of rudites and arenites from the Alpine transect, where the Err-Platta nappes represent the Ocean-Continent (Adria) Transition, do not show contrasting evidence with this overall asymmetric compositional picture, and in some cases point to that direction (e.g.: rare occurrence in these deposits of material from possible lower crust/mantle sources). On the whole, the ‘space’-related differences in composition between the Corsica/Europe and the Adria continental margins are thought to reflect different configurations of the two margins that in turn provide further constraints to the understanding of the rifting process. Data collected in this work lend support to an asymmetric opening mechanism for the Ligure-Piemontese oceanic basin, with the Adria margin acting as the lower plate during the rifting processes (west-dipping detachment fault). Concerning this point, the multidisciplinary study of sediments can be considered a valuable tool in paleotectonic reconstructions, especially in those cases in which the regional-scale features of the investigated geological framework are lost due to major successive tectonic events. 40Ar –39Ar geochronological data on detrital white micas from the Corsica/Europe and Adria margin-sourced deposits have been presented in this thesis. The age-distribution of the two arenite samples from Corsica and Internal Ligurian units well match the known-from the literature picture defined by the Variscan ages found in the crystalline rocks from the Corsica/Europe basement. The age-distribution of the sample from External Ligurian units partially overlaps the oldest Variscan white mica ages known from the literature for the Adria basement. Hence, this sample gives an original contribution to the geochronological characterization of the Adria continental basement. The geochronological data collected along a Corsica-Northern Apennines transect indicate that: i) the detrital micas occurring in the Mesozoic sedimentary covers of the Ligure Piemontese Ocean were sourced by crystalline igneous and variably metamorphosed basement rocks of Variscan age, and ii) detrital white mica geochronology in provenance studies is a useful tool for the characterization of source areas. After having investigated the compositional features of the sediments from the Ligure Piemontese Ocean, with the aim of characterize their origin, (i.e. the source areas), an attempt is made to explore their fate, i.e. what kind of modifications they undergo when involved in a subduction setting. This is a noteworthy issue, since variably metamorphosed sediments and ocean floor can give a geochemical contribution to the mantle-wedge (and related arc-magmas) in terms of released fluids/elements. The composition of basalts and siliciclastic sediments from the Ligure Piemontese oceanic basin are compared to the corresponding metabasalt/calc-schist from tectonic units metamorphosed under blue schist and eclogite facies conditions. The almost unaffected chemical patterns of the metamorphic equivalent both of the basalt and sediment lend support to those works from the literature suggesting the occurrence of the processes responsible for the mobilization of elements at "deeper" levels than eclogite facies along the subducting slab. In summary, a multidisciplinary approach is definitely to recommend when a provenance study of sediments is pursued. The combined analysis of the different grain size fractions of sedimentary rocks by means of various methods has in fact the potential to provide a comprehensive picture of the case study, being pelites, arenites and rudites able to supply different kinds of geological meaningful informations

\u3csup\u3e40\u3c/sup\u3eAr–\u3csup\u3e39\u3c/sup\u3eAr dating of volcanogenic products from the AND-2A core (ANDRILL Southern McMurdo Sound Project, Antarctica): Correlations with the Erebus Volcanic Province and implications for the age model of the core

The AND-2A drillcore (Antarctic Drilling Program—ANDRILL) was successfully completed in late 2007 on the Antarctic continental margin (Southern McMurdo Sound, Ross Sea) with the aim of tracking ice proximal to shallow marine environmental fluctuations and to document the 20-Ma evolution of the Erebus Volcanic Province. Lava clasts and tephra layers from the AND-2A drillcore were investigated from a petrographic and stratigraphic point of view and analyzed by the 40Ar–39Ar laser technique in order to constrain the age model of the core and to gain information on the style and nature of sediment deposition in the Victoria Land Basin since Early Miocene. Ten out of 17 samples yielded statistically robust 40Ar–39Ar ages, indicating that the AND-2A drillcore recovered ≤230 m of Middle Miocene (~128–358 m below sea floor, ~11.5–16.0 Ma) and \u3e780 m of Early Miocene (~358–1093 m below sea floor, ~16.0–20.1 Ma). Results also highlight a nearly continuous stratigraphic record from at least 358 m below sea floor down hole, characterized by a mean sedimentation rate of ~19 cm/ka, possible oscillations of no more than a few hundreds of ka and a break within ~17.5–18.1 Ma. Comparison with available data from volcanic deposits on land, suggests that volcanic rocks within the AND-2A core were supplied from the south, possibly with source areas closer to the drill site for the upper core levels, and from 358 m below sea floor down hole, with the “proto-Mount Morning” as the main source

The upper lithostratigraphic unit of ANDRILL AND-2A core (Southern McMurdo Sound, Antarctica): Local Pleistocene volcanic sources, paleoenvironmental implications and subsidence in the southern Victoria Land Basin

We report results from the study of the uppermost 37 m of the Southern McMurdo Sound (SMS) AND-2A drill core, corresponding to the lithostratigraphic unit 1 (LSU 1), the most volcanogenic unit within the core. We present data on the age, composition, volcanological and depositional features of the volcanic sedimentary and tephra deposits of LSU 1 and discuss their source, mechanisms of emplacement and environment of deposition. Sedimentary features and compositional data indicate shallow water sedimentation for the whole of LSU 1. Most of LSU 1 deposits are a mixture of near primary volcanic material with minor exotic clasts derived from the Paleozoic crystalline basement rocks. Among volcanic materials, glassy particles are the most abundant. They were produced by mildly explosive basaltic eruptions occurring in subaerial and subaqueous environments. The Dailey Islands group, 13 km south-southwest of the SMS drill-site, has been identified as a possible source for the volcanics on the basis of similarity in composition and age. 40Ar–39Ar laser step-heating analyses on a lava sample from Juergens Island yields an age of 775 ± 22 ka. Yet because of the minimal reworking features of vitriclasts, preservation of fragile structures in volcaniclastic sediments and evidence for volcanic seamounts to the north of the Dailey Islands, it is likely that some of the material originated also from vents close to the drill-site. Evidence for local volcanic sources and for deposition of sediments in a shallow marine environment provides indications about the local paleogeography and implications for the subsidence history of the southern Victoria Land Basin from Pleistocene to Recent

Geochemistry and Petrography of Western Tethys Cretaceous sedimentary covers (Corsica and Northern Apennines): from source areas to configuration of margins

Provenance studies most commonly apply the classical approach based on petrographic modal analysis of arenites. In this this is combined to both petrographic study on conglomerate clasts and to geochemical investigation on major and trace elements of pelites. This work is aimed to: i) understand the nature of the source areas of the Mesozoic sedimentary covers of the Ligure-Piemontese oceanic basin, a branch of Western Tethys; these sedimentary successions are preserved as tectonic units in the Alpine-Apennine belt (Balagne Nappe in Alpine Corsica and Internal and External Ligurian units in Northern Apennines); ii) provide further constraints to the understanding of the Jurassic rifting process. Petrographic and geochemical results indicate significant differences between Corsica and Internal Ligurian units against External Ligurian units. Petrography of pebbles from rudites and lithic fragments from arenites shows that Corsica and Internal Ligurian units contain a debris of low-grade metamorphic rocks and carbonate platform rock fragments, while the External Ligurian units are represented by low-, medium- and high-grade metamorphic rocks; mm-sized Cr-spinel probably derived from a mantle-rock source; additionally, both carbonate platform and pelagic siliceous/carbonatic rock fragments have been found. Geochemical data on pelites show that samples from External Ligurian units suggest a more mafic/ultramafic character: they are enriched in elements such as Cr, Co, Ni (abundant in mafic-ultramafic rocks), and the Th/Sc/Cr/V/Ni relationships show a systematic shift towards ultramafic contribution. On the whole, petrographic and chemical data collected in this paper indicate that the source for sediments of Corsica and Internal Ligurian units were similar, and made up by the upper part of a continental basement and its carbonatic sedimentary cover (the Europe continental margin). On the other hand, the sedimentary cover of the External Ligurian units was supplied by a source area where a complete lithospheric section was exposed, from the upper mantle up to the deep sea sedimentary cover (the Adria continental margin). These findings are useful to unravel the processes related to the opening mechanisms of the Ligure-Piemontese oceanic basin: among the different rifting models existing, our data support an asymmetric mechanism dominated by a west-dipping detachment fault, with the Adria margin acting as the lower plate

Evolving strain partitioning in the Eastern Himalaya:the growth of the Shillong Plateau

The Shillong Plateau is the only raised topography (up to 2000 m elevation) in the Himalayan foreland. It is proposed to have had a major influence on strain partitioning and thus tectonics in the Eastern Himalaya. Additionally, its position on the trajectory of the summer monsoon means it has influenced the regional climate, with reduced erosion rates proposed over geological timescales in its lee. The timing of surface uplift of the plateau has been difficult to determine. Exhumation rates have been calculated over geological timescales, but these seem at variance with estimates based upon extrapolating the present day velocity field measured with GPS, and it has thus been suggested that exhumation and surface uplift are decoupled. We determine the timing of surface uplift using the sedimentary record in the adjacent Surma Basin to the south, which records the transition from a passive margin with southward thickening sedimentary packages to a flexural basin with north-thickening strata, due to loading by the uplifting plateau. Our method involves using all available 2D seismic data for the basin, coupled to well tie information, to produce isochore maps and construct a simple model of the subsidence of the Surma basin in order to assess the timing and magnitude of flexural loading by the Shillong Plateau. We conclude that the major period of flexural loading occurred from the deposition of the Tipam Formation (3.5–∼2 Ma) onwards, which is likely to represent the timing of significant topographic growth of the Shillong Plateau. Our isochore maps and seismic sections also allow us to constrain the timing of thinning over the north–south trending anticlines of the adjacent basin-bounding Indo–Burman Ranges, as occurring over this same time interval. The combined effect of the uplift of the Shillong Plateau and the westward encroachment of the Indo–Burman Ranges to this region served to sever the palaeo-Brahmaputra drainage connection between Himalayan source and Surma Basin sink, at the end of Tipam Formation times (∼2 Ma)