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The Messinian-Quaternary history of tectonic and climatic control on sedimentation in the eastern Southern Alps, northern Italy, was reconstructed using an integrated petrographic and sedimentological analysis of five sedimentary successions. These units mainly consist of fluvial conglomerates of the Tagliamento sequence, deposited within the eastern Southern Alps since the Upper Miocene ("Messinian Salinity Crisis"). At that time, the closure of marine gateways between the Atlantic Ocean and the Mediterranean Sea caused a drop in sea level, causing exchanged fluvial erosion and widening of the Alpine catchments. Sediment composition in the Upper Miocene to Lower Pliocene units shows an abrupt change in the source areas: from sediments characterized by carbonate rock fragments to detritus rich in low-grade metamorphic grains. In spite of tectonic activity within the eastern Southern Alps, no major modifications in sediment supply took place during the Pliocene-Early Pleistocene time span. In the Middle Pleistocene the first major expansion of the Alpine glaciers triggered a change in drainage patterns and a marked increase in erosion rates. Predominantly climatic control on sedimentation in the Tagliamento basin and the erosion of the Carnic Alps produced sediments rich in quartz, siltstone and metamorphic rock fragments, deposited in the Upper Pleistocene unit and in modern river sediments. © 2011 Elsevier B.V.

The Po Basin, a Pliocene marine gulf between the Alps and the Apennines, was filled progressively from west to east and finally capped by fluvial sediments during the Pleistocene. By similar to 1.25 Ma, a meandering trunk river (paleoDora Baltea) reached westernmost Lombardia (Cilavegna); at similar to 1 Ma, the paleoAdda, entering the plain through the Como Valley, joined the paleoTicino in the Milano area. The coastline was directed NNE/SSW, and the open sea persisted east of similar to 10 degrees E. Metamorphiclastic prodelta sediments of the Alpine trunk river reached central Lombardia (Pianengo) by MIS 36 and eastern Lombardia (Palosco) at MIS 31. Trunk-river delta foresets accumulated rapidly in easternmost Lombardia (Ghedi) during the Jaramillo. Fluvial sedimentation, continuous at Pianengo since MIS 28, eventually reached Ghedi at MIS 22. In this time interval, the embayment secluded between the prograding trunk-river delta and the Southalpine front was progressively filled by Southalpine fan deltas.With the onset of major Alpine glaciations in the late Matuyama, detrital supply increased markedly and fluvial deposits spread all over Lombardia. Because of outward growth of Alpine fans, the paleoDora Baltea was replaced at Cilavegna by the paleoSesia and paleoToce. Milano lay in the paleoAdda braidplain. Southalpine alluvial fans reached as far south as Pianengo. The paleoOglio glacier exited Lake Iseo at Cremignane.In the early Brunhes, accumulation rates markedly dropped in Lombardia; carbonaticlastic paleoPiave turbidites accumulated rapidly at Venezia, capped by the prograding paleoPo delta and finally by fluvial paleoBrenta-Bacchiglione deposits. Subsequently, accumulation rates decreased further, and paleosols developed during stages of prolonged exposure. The paleoTicino continued to flow southwest of Milano, where detritus from the paleoOlona is documented locally. PaleoAdda sediments were deposited at Milano and sedimentaclastic detritus at Trezzo up

Tectonic reconstructions and quantitative models of landscape evolution are increasingly based on detailed analysis of detrital systems. Since the definition of closure temperature in the 1960s, mineral ages of low- temperature geochronometers are traditionally interpreted as the result of cooling induced by erosion, whose rate is a simple, unique function of age patterns. Such an approach can lead to infer paradoxically high erosion rates that conflict with compelling geological evidence from sediment thickness in basins. This indicates that tectonic and landscape models that solely interpret mineral ages as due to cooling during exhumation may not be valid. Here we propose a new approach that takes into account the effects of both crystallization and exhumational cooling on geochronometers, from U–Pb on zircon to fission tracks on apatite. We first model the mechanical erosion of an unroofing magmatic complex and the resulting accumulation and burial of the eroded units in reverse order in the basin. Detrital mineral ages follow a regular pattern downsection. Some mineral ages, such as e.g. U–Pb ages of zircons, cluster around the “magmatic age”, i.e. the crystallization of the magma. Its value is constant along the stratigraphic column in the sedimentary basin; we refer to this behavior as “stationary age peak”. Some other mineral ages, such as e.g. apatite fission-track ages, are often younger than the magmatic age. When they vary smoothly with depth, they define a “moving age peak”, which is the only possible effect of undisturbed cooling during overburden removal, and can therefore be used to calculate an erosion rate. The predictions of our model were tested in detail on the extremely well-studied Bregaglia (Bergell) orogenic pluton in the Alps, and on the sedimentary succession derived from its erosion, the Gonfolite Group. The consistency between predicted and observed age patterns validates the model. Our results resolve a long- standing paradox in quantit

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.

This study investigates the natural processes that control concentration of detrital minerals and consequently chemical elements in river sand. The novelty of our approach consists in the systematic integration of detailed textural, petrographical, mineralogical and chemical data, and in the quantitative description and modeling of relationships among mineralogical and chemical variables for each sample and each grain-size class in each sample. Bed sediment in transit in the largest sedimentary system on Earth chiefly consists of fine-grained lithofeldspathoquartzose sand including rich amphibole-epidote-garnet suites, mixed with minor very-fine-grained-sand to silt subpopulations containing less heavy minerals and representing intermittent suspension. Mineralogical and particularly chemical differences between Ganga and Brahmaputra bedload are orders of magnitude less than both intersample variability associated with selective-entrainment effects and intrasample variability associated with settling-equivalence effects. Any provenance interpretation of mineralogical, chemical, or detrital-geochronology datasets therefore requires quantitative understanding of hydraulically controlled compositional variability. Mineralogical and chemical, intrasample and intersample variability can be deduced with simple equations and numerical solutions. The underlying assumptions on the chemical composition of detrital minerals, as well as the possible pitfalls, uncertainties and approximations involved are discussed. Principal results include calibration of rare REE-bearing ultradense minerals, ill-determined by optical analyses but crucial in both detrital-geochronology and settling-equivalence studies, and assessment of progressively changing concentration for any detrital component with increasing intensity of selective-entrainment effects. Contributions by each mineral group to the chemical budget were inferred with sufficient precision and accuracy. Although complex because o

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

Collision orogens such as the Alps or the Himalayas are generated by plate convergence, culminating in attempted subduction of a thinned continental margin. Massive amounts of metamorphic rocks displaying high-pressure parageneses are produced during such relatively brief tectonic events and then rapidly exhumed to formthe axial backbone of the new orogen. Sediment composition provides a fundamental key to identify past events of continental subduction, although coupled detrital-geochronology techniques are needed to discriminate neometamorphic and paleometamorphic sources of detritus. Within the Austroalpine Cretaceous and Penninic Eocene axial belts of the Alps, we ideally distinguish three structural levels, each characterized by diagnostic detrital fingerprints. The shallow level chiefly consists of offscraped remnant-ocean turbidites and unmetamorphosed continental-margin sediments and mostly produces lithic to quartzolithic sedimentaclastic sands yielding very poor heavy mineral suites including ultrastable minerals. The intermediate level includes low-grade metasediments and polymetamorphic basements and sheds quartzolithic to feldspatholithoquartzose metamorphiclastic sands yielding moderately rich epidote-amphibole suites with chloritoid or garnet. The deep level contains eclogitic remnants of continent-ocean transitions and supplies feldspatholithoquartzose/feldspathoquartzose high-rank metamorphiclastic to lithic ultramaficlastic sands yielding rich to extremely rich suites dominated by garnet, hornblende, or epidote, depending on protoliths (continental vs. oceanic) and pressure/temperature paths during exhumation. Although widely overprinted under greenschist-facies or amphibolite-facies conditions, occurrence of ultradense eclogite in source areas is readily revealed by the heavy mineral concentration (HMC) index, which mirrors the average density of source rocks in the absence of hydraulicsorting effects. Rather than the pressure peak reached at depth

The Pleistocene history of climate control on sedimentation in the Southern Alps-Po Plain system, northern Italy, was reconstructed using an integrated magnetostratigraphic, palynological, and petrographical approach on a 47-m-deep core. The core mainly consists of lacustrine sediments pertaining to the Bagaggera sequence, deposited at the foothills of the Southern Alps during the late Matuyama subchron (0.99-0.78 Ma). At that time, climate worsened globally and locally it caused the progradation of an alluvial fan unit onto the nearby Po Plain, triggering lake formation by damming of a tributary valley. These new data are used in conjunction with data from the literature to highlight and track the effects of climate forcing on sedimentation during the late Matuyama subchron in different orographic and geodynamic settings of the Southern Alps-Po Plain system as part of the greater Alpine area. We found that the episodes of alluvial fan and braidplain progradation observed in the southern foreland of the Alps during the late Matuyama global cooling seem broadly synchronous with the deposition of most of the so-called Günz and Älterer Deckenschotter deposits in the northern forelands of the Alps as well as with the first major waxing of the Alpine valley glaciers, possibly around the Marine Isotope Stage 22 (∼0.87 Ma). © 2009 Elsevier Ltd. All rights reserved.

Quantitative provenance analysis of the Quaternary sediments from the Collina di Torino and the Po alluvial plain was assessed by using statistical techniques. This method, based on high-resolution heavy mineral analysis on modern and ancient sands processed by compositional biplot and linear mixing model, allowed reconstruction of the drainage change of the western Po River and its tributaries in the last 1. Ma. Mineralogical and statistical analyses discriminate three groups in the Quaternary samples. Sediments from the Po alluvial plain are rich in garnet, epidote and minor amphiboles. Detritus from the western slope of the Collina di Torino is characterized by epidote-rich assemblages, actinolite, blue-green hornblende and chloritoid. Heavy minerals from sediments of the northwestern slope include epidote-group minerals, glaucophane and hypersthene. A linear mixing model was used to compare detrital modes from modern and Quaternary detritus, and to identify the best modern analogue for any sample of ancient sedimentary deposit. Statistical analysis indicates that the first mineralogical group compares best with detritus of the modern Po River. Detritus from the second and third group compare best with sediments of the modern Dora Riparia River and Stura di Lanzo River, respectively. These results outline how the distribution of the Middle-Late Pleistocene fluvial deposits of the Collina di Torino is linked with the ancient Alpine watercourses, before the shifting of the Po River on the northern edge of the hill. During the Middle Pleistocene, rivers draining the Piedmont Zone of the Western Alps (PaleoDora Riparia and PaleoStura di Lanzo rivers) ran across the hill, while the PaleoPo River, which drained the Dora Maira Massif (Internal Massif), flowed to the south of the Collina di Torino. At the end of the Pleistocene, the PaleoPo River was diverted to the north to its present-day position in the alluvial plain. This case history shows how quantitative provenan

Fission-track dating on detrital apatites from modern sands of the Po Delta is used for a provenance study of sediments in the Po River basin. Analysed samples show a fission-track grain-age distribution characterized by two prominent peaks at 7.7 Ma and 17 Ma. The youngest peak accounts for 46% of the total population of dated grains. This young component in the grain-age distribution is consistent with bedrock cooling ages observed in the Western Alps between the External Massifs and the Houiller unit, as well as in the Lepontine dome of the Central Alps and in the Miocene foredeep units of the Apennines, that overall represent only 12% of the orogenic source area. Results suggest that most of the sediment load in the last 10(2)-10(5) years was supplied by focused erosion of relatively small areas that experienced short-term erosion rates one order of magnitude higher than in the rest of the.

Foreland basins are comprehensive stratigraphic archives, from which many details of the tectonic history of associated orogens can potentially be retrieved by quantitative provenance analysis of sedimentary sequences. Because of complex basin‐fill architecture, however, provenance diagnoses often remain ambiguous. Trends in sediment composition cannot be safely interpreted as documenting paleotectonic or paleoclimatic changes whenever paleodrainage shifts represent an equally reasonable alternative. Understanding drainage patterns is thus a fundamental step in correctly unraveling tectonic and climatic events that punctuate the erosional evolution of mountain belts. In this study, we focus on well‐studied Quaternary subsurface successions of the Po Plain to illustrate a quantitative method of paleodrainage analysis. This method, based on high‐resolution petrographic and heavy‐mineral data from modern and Pleistocene sands, allowed us to objectively reconstruct the drainage changes of Po River tributaries triggered by major Alpine glaciations in the past million years.

This article investigates both experimentally and theoretically the compositional changes associated with textural effects and hydraulic sorting during sediment transport and deposition, which cause systematic distortion in quantitative provenance analysis (“environmental bias”). Traditional procedures aimed at eliminating textural noise find limited success. The Gazzi–Dickinson method cannot remove hydrodynamic-related modal variability. Multiple size-window strategies are time-consuming. Narrow or moving size-window strategies represent misleading or impractical short cuts, being less convenient options than simply analysing each sample in bulk. New concepts introduced here unravel the superposed causes of compositional variability in modern sediments. Intrasample modal variability, fundamentally explained by settling-equivalence relationships, can be accurately modelled mathematically. Intersample modal variability, principally resulting from selective entrainment, can be assessed and removed by a simple principle. In absence of provenance changes and environmental bias, the weighted average density of terrigenous grains (SRD index) should be equal, for each sample and each grain-size class of each sample, to the weighted average density of source rocks. By correcting relative abundances of detrital minerals in proportion to their densities, we can restore the appropriate SRD index for any provenance and subprovenance type in each sample or grain-size class. Modal variability is effectively reduced by this procedure, which can be applied confidently to modern sediments deposited by tractive currents in any environment. Good results are obtained even for placer sands and finest classes where heavy-mineral concentration is strongest. Such “SRD correction” also successfully compensates for biased narrow-window modes, thus providing a numerical solution of general validity to the problem of environmental bias in sedimentary petrology. After compensating for settling

The Andaman Islands are part of the Andaman-Nicobar Ridge, an accretionary complex that forms part of the outer-arc ridge of the Sunda subduction zone. The Tertiary rocks exposed on the Andaman Islands preserve a record of the tectonic evolution of the surrounding region, including the evolution and closure of the Tethys Ocean. Some of the Paleogene sediments on Andaman may represent an offscraped part of the early Bengal Fan. Through field and petrographic observations, and use of a number of isotopic tracers, new age and provenance constraints are placed on the key Paleogene formations exposed on South Andaman. A paucity of biostratigraphic data poorly define sediment depositional ages. Constraints on timing of deposition obtained by dating detrital minerals for the Mithakhari Group indicate sedimentation after 60 Ma, possibly younger than 40 Ma. A better constraint is obtained for the Andaman Flysch Formation, which was deposited between 30 and 20 Ma, based on Ar-Ar ages of the youngest detrital muscovites at ca. 30 Ma and thermal history modeling of apatite fission-track and U-Th/He data. The latter record sediment burial and inversion (uplift) at ca. 20 Ma. In terms of sediment sources the Mithakhari Group shows a predominantly arc-derived composition, with a very subordinate contribution from the continental margin to the east of the arc. The Oligocene Andaman Flysch at Corbyn's Cove is dominated by recycled orogenic sources, but it also contains a subordinate arc-derived contribution. It is likely that the sources of the Andaman Flysch included rocks from Myanmar affected by India-Asia collision. Any contribution of material from the nascent Himalayas must have been minor. Nd isotope data discount any major input from cratonic Greater India sources.

This study discusses the laws which govern sediment deposition, and consequently determine size-dependent compositional variability. A theoretical approach is substantiated by robust datasets on major Alpine, Himalayan, and African sedimentary systems. Integrated (bulk-petrography, heavy-mineral, X-ray powder diffraction) multiple-window analyses at 0.25phi to 0.50phi sieve interval of eighty-five fluvial, beach, and eolian-dune samples, ranging from very fine silt to coarse sand, document homologous intrasample compositional trends, revealed by systematic concentration of denser grains in finer-grained fractions (“size–density sorting”). These trends are explained by the settling-equivalence principle, stating that detrital minerals are deposited together if their settling velocity is the same. Settling of silt is chiefly resisted by fluid viscosity, and Stokes' law predicts that size differences between detrital minerals in phi units (“size shifts”) are half the difference between the logarithms of their submerged densities. Settling of pebbles is chiefly resisted by turbulence effects, and the Impact law predicts double size shifts than Stokes' law. Settling of sand is resisted by both viscosity and turbulence, the settling-equivalence formula is complex, and size shifts increase – with increasing settling velocity and grain size – from those predicted by Stokes' law to those predicted by the Impact law. In wind-laid sands, size shifts match those predicted by the Impact law; size–density sorting is thus greater than in water-laid fine sands. New analytical, graphical, and statistical techniques for rigorous settling-equivalence analysis of terrigenous sediments are illustrated. Deviations associated with non-spherical shape, density anomalies, inheritance from source rocks, or mixing of detrital species with contrasting provenance and different size distribution are also tentatively assessed. Such integrated theoretical and experimental approach allows us to mat

A knowledge of Himalayan erosion history is critical to understanding crustal deformation processes, and the proposed link between the orogen's erosion and changes in both global climate and ocean geochemistry. The most commonly quoted age of India–Asia collision is ~ 50 Ma, yet the record of Paleogene Himalayan erosion is scant — either absent or of low age resolution. We apply biostratigraphic, petrographic, geochemical, isotopic and seismic techniques to Paleogene rocks of the Bengal Basin, Bangladesh, of previously disputed age and provenance. Our data show that the first major input of sands into the basin, in the > 1 km thick deltaic Barail Formation, occurred at 38 Ma. Our biostratigraphic and isotopic mineral ages date the Barail Formation as spanning late Eocene to early Miocene and the provenance data are consistent with its derivation from the Himalaya, but inconsistent with Indian cratonic or Burman margin sources. Detrital mineral lag times show that exhumation of the orogen was rapid by 38 Ma. The identification of sediments shed from the rapidly exhuming southern flanks of the eastern–central Himalaya at 38 Ma, provides a well dated accessible sediment record 17 Myr older than the previously described 21 Ma sediments, in the foreland basin in Nepal. Discovery of Himalayan detritus in the Bengal Basin from 38 Ma: 1) resolves the puzzling discrepancy between the lack of erosional evidence for Paleogene crustal thickening that is recorded in the hinterland; 2) invalidates those previously proposed evidences of diachronous collision which were based on the tenet that Himalayan-derived sediments were deposited earlier in the west than the east; 3) enables models of Himalayan exhumation (e.g. by mid crustal channel flow) to be revised to reflect vigorous erosion and rapid exhumation by 38 Ma, and 4) provides evidence that rapid erosion in the Himalaya was coincident with the marked rise in marine 87Sr/86Sr values since ~ 40 Ma. Whether 38 Ma represents the

The Indo-Burman Ranges in western Myanmar extend along the Sunda Arc subduction zone and may be divided into a western portion of Neogene sedimentary rocks and an eastern portion of Palaeogene sedimentary rocks, separated by the Kaladan Fault. Both Himalayan and Burman sources have been proposed for these sediments. Our thermochronological analyses on detrital grains, isotopic analyses on bulk rock, and petrographic and heavy mineral data indicate that the Palaeogene Indo-Burman Ranges contain a significant component of arc-derived material, interpreted as derived from the Burmese portion of the Mesozoic–Tertiary arc to the east. And older crustal component is also identifiable, which may have been sourced from the Himalaya or the Burmese margin. By contrast, the Neogene Indo-Burman Ranges show dominant derivation from the Himalaya. A minor arc-derived component may have been sourced from the Trans-Himalaya, or recycled from the arc-derived Paleogene Indo-Burman Ranges.

By selecting a limited number of variables ( westward vs. eastward subduction polarity; oceanic vs. continental origin of downgoing and overriding plates), we identify eight end-member scenarios of plate convergence and orogeny. These are characterized by five different types of composite orogenic prisms uplifted above subduction zones to become sources of terrigenous sediments (Indo-Burman-type subduction complexes, Apennine-type thin-skinned orogens, Oman-type obduction orogens, Andean-type cordilleras, and Alpine-type collision orogens). Each type of composite orogen is envisaged here as the tectonic assembly of subparallel geological domains consisting of genetically associated rock complexes. Five types of such elongated orogenic domains are identified as the primary building blocks of composite orogens: magmatic arcs, obducted or accreted ophiolites, neometamorphic axial belts, accreted paleomargin remnants, and accreted orogenic clastic wedges. Detailed provenance studies on modern convergent-margin settings from the Mediterranean Sea to the Indian Ocean show that erosion of each single orogenic domain produces peculiar detrital modes, heavy-mineral assemblages, and unroofing trends that can be predicted and modeled. Five corresponding primary types of sediment provenances ( magmatic arc, ophiolite, axial belt, continental block, and clastic wedge provenances) are thus identified, which reproduce, redefine, or integrate provenance types and variants originally recognized by W. R. Dickinson and C. A. Suczek in 1979. These five primary provenances may be variously recombined in order to describe the full complexities of mixed detrital signatures produced by erosion of different types of composite orogenic prisms. Our provenance model represents a flexible and valuable conceptual tool to predict the evolutionary trends of detrital modes and heavy-mineral assemblages produced by uplift and progressive erosional unroofing of various types of orogenic belts and to

We use petrographic and mineralogical data on modern sediments to investigate erosion patterns in the Marsyandi basin of the central Himalaya, a privileged natural laboratory in which a series of multidisciplinary geomorphological, sedimentological, geochemical and geochronological studies have been recently carried out to unravel the interrelationships between tectonic, climatic and sedimentary processes in high-relief orogenic belts. Although relative erosion patterns are effectively constrained by analyses of replicate samples along six successive tracts of the Marsyandi River, uncertainties are caused by potential compositional variation between the monsoon and post-monsoon season. Estimates of erosion rates are significantly affected by poor knowledge of total sediment flux through the basin. Our results support focused erosion of the southern, tectonically-lower part of the Greater Himalaya in the hangingwall of the MCT Zone, where the summer monsoon reaches its peak intensity (up to 5 m/a), and sediment yields and erosion rates reach 14,100 ± 3400 t/km2 and 5.1 ± 1.2 mm/a. Erosion rates sharply decrease southward in low-relief Lesser Himalayan units (1.6 ± 0.6 mm/a), and also progressively decrease northwards in the high-altitude, tectonically-upper part of the Greater Himalaya, where rainfall decreases rapidly to b 2 m/a. Even areas of extreme topography such as the Manaslu Granite are characterized by relatively low erosion rates (2.4 ± 0.9 mm/a), because precipitations become too scarce to feed significant ice flux and glacial activity. Monsoonal rainfall decreases further to b 0.5 m/a in the Tethys Himalayan zone farther north, where erosion rates are ∼ 1 mm/a. Coupling between erosion and peak monsoonal rainfall along the southern front of the Greater Himalaya is consistent with both channel-flow models of tectonic extrusion and tectonic uplift above a mid-crustal ramp. Altitude and relief are not the principal factors controlling erosion, and the centra

Author Posting. © The Authors, 2004. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 229 (2005): 287-302, doi:10.1016/j.epsl.2004.11.008., The Indus River has been progressively transformed in the last decades into a tightly-regulated system of dams and channels, to produce food and energy for the rapidly growing population of Pakistan. Nevertheless, Indus River sands as far as the delta largely retain their distinct feldspar- and amphibole-rich composition, which is unique with respect to all other major rivers draining the Alpine-Himalayan belt except for the Brahmaputra. Both the Indus and Brahmaputra Rivers flow for half of their course along the India-Asia suture zone, and receive major contributions from both Asian active-margin batholiths and upper-amphibolite-facies domes rapidly exhumed at the Western and Eastern Himalayan syntaxes. Composition of Indus sands changes repeatedly and markedly in Ladakh and Baltistan, indicating overwhelming sediment flux from each successive tributary as the syntaxis is approached. Provenance estimates based on our integrated petrographic-mineralogical dataset indicate that active-margin units (Karakorum and Transhimalayan arcs) provide ~81% of the 250±50 106 t of sediments reaching the Tarbela reservoir each year. Partitioning of such flux among tributaries and among source units allows us to tentatively assess sediment yields from major sub-catchments. Extreme yields and erosion rates are calculated for both the Karakorum Belt (up to 12,500±4700 t/km2 yr and 4.5±1.7 mm/yr for the Braldu catchment) and Nanga Parbat Massif (8100±3500 t/km2 yr and 3.0±1.3 mm/yr). These values approach denudation rates currently estimated for South Karakorum and Nanga Parbat crustal-scale antiforms, and highlight the major influence that rapid tectonic uplift and focused glacial and fluvial erosion of young metamorphic massifs around the Western Himalayan Syntaxis have on sediment budgets of the Indus system. Detailed information on bulk petrography and heavy minerals of modern Indus sands not only represents an effective independent method to constrain denudation rates obtained f, Financial support by FIRB 2002 and PRIN 2003 to E.Garzanti.

Bedrock fission-track analysis, high-resolution petrography and heavy mineral analyses of sediments are used to investigate the relationships between erosion and tectonics in the Western Alps. Along the Aosta Valley cross-section, exhumation rates based on fission-track data are higher in the fault-bounded western block than in the eastern block (0.4-1.5 vs. 0.1-0.3 mm yr(-1)). Erosion rates based on the analysis of bed-load in the Dora Baltea drainage display the same pattern and have similar magnitudes in the relative sub-basins (0.4-0.7 vs. 0.04-0.08 mm yr(-1)). Results highlight that climate, relief and lithology are not the controlling factors of erosion in the Western Alps. The main driving force behind erosion is instead tectonics that causes the differential upward motion of crustal blocks.

All tectonostratigraphic levels of the continental crust, from carbonate platforms in the Dolomites to granulites in the Ivrea Zone, are exposed along strike in the world-famous Southern Alps cross section. In this extraordinary natural laboratory, we studied the composition of modern stream sands shed by each distinct type of source rock in order to improve on existing models of sediment provenance and specifically to reconstruct the step-by-step changes of detrital signatures ideally produced during unroofing of a continental block. Detritus from progressively deeper crustal levels of the Southern Alps changes systematically from lithic sedimentaclastic (unmetamorphosed cover sequences) to lithoquartzose, quartzolithic, and quartzofeldspathic metamorphiclastic (greenschist facies to amphibolite facies basement units) and finally to feldspathoquartzose and feldspathic composition (granulite facies basement units). Six progressively richer types of heavy mineral assemblages (ZTR-brookite, garnet-chloritoid, garnet-staurolite, blue/green hornblende-kyanite, green/brown hornblende-fibrolitic sillimanite, and garnet-brown hornblende-prismatic sillimanite) faithfully reflect the increasing grade of metamorphic source rocks. Hypersthene-brown hornblende and olivine-enstatite assemblages characterize detritus from lower crustal gabbros and mantle peridotites. In alpine settings undergoing rapid erosion and minor chemical weathering, the mineralogy of detrital sediments faithfully mirrors the mineralogy of their parent rocks. Therefore, our database can be used to integrate direct measurements of physical and chemical properties of exposed bedrock and to reconstruct an ideally complete mineralogy and density profile from the base to the top of the South Alpine crust. This shows a progressive upward increase in quartz and muscovite at the expense of plagioclase and heavy minerals and an effective density stratification from lower crustal (3.10-3.00 g/cm(3)) to middle crusta

Detrital modes of modem Nile sands, together with estimates of sediment volumes trapped in Sudanese reservoirs, allow us to calculate sediment loads of major tributaries (Blue Nile, White Nile, Atbara) and erosion rates in the Nile catchment. A tridimensional array of high-resolution bulk-petrography and heavy-mineral data was obtained on both levee (suspended load) and bar (bedload) deposits, analysed separately for each grain-size subclass at 0.5 Phi intervals. From available information on sediments stored in the Roseires, Khashm el Girba and Lake Nasser reservoirs between 1964 and 1990, the total Nile load is reassessed at 230 20 10(6) t/a, an estimate two to four times higher than figures reported so far, on which previous estimates of sediment yields and erosion rates were based. Of such huge amount of detritus, 82110 10 6 t/a are contributed by River Atbara, which carries more volcanic rock fragments, brown augite and olivine from basaltic rocks, and 140 20 10(6) t/a by the Blue Nile, which carries more K-feldspar and hornblende from amphibolite-facies basement rocks. The additional <= 10(7) t/a of almost purely quartzose sediments supplied by the rest of the Nile catchment, corresponding to insignificant average yields and erosion rates, represent the stable residue which survived extreme subequatorial weathering in southern Sudan swamps (White Nile, Bahr ez Zeraf, and Sobat sands) or fluvial and eolian recycling of ancient quartzarenites in hyperarid climates (Nubian sands). Sediment production is thus markedly focused on Ethiopian rift highlands, where rainfall is concentrated in a single July-August peak. High average yields and erosion rates (800 150 t/km(2) a, 0.29 0.05 mm/a) partly reflect anthropically-accelerated erosion caused by deforestation and intensive land use, and cannot be extrapolated far in the past. Erosion patterns may have changed repeatedly during Quaternary climatic oscillations, and possibly also in the longer term during the multist

The Indus River has been progressively transformed in the last decades into a tightly regulated system of dams and channels, to produce food and energy for the rapidly growing population of Pakistan. Nevertheless, Indus River sands as far as the delta largely retain their distinct feldspar- and amphibole-rich composition, which is unique with respect to all other major rivers draining the Alpine-Himalayan belt except for the Brahmaputra. Both the Indus and Brahmaputra Rivers flow for half of their course along the India-Asia suture zone, and receive major contributions from both Asian active-margin batholiths and upper-amphibolite-facies domes rapidly exhumed at the Western and Eastern Himalayan syntaxes. Composition of Indus sands changes repeatedly and markedly in Ladakh and Baltistan, indicating overwhelming sediment flux from each successive tributary as the syntaxis is approached. Provenance estimates based on our integrated petrographic-mineralogical data set indicate that active-margin units (Karakorum and Transhimalayan arcs) provide similar to81% of the 250 +/- 50 10(6) t of sediments reaching the Tarbela reservoir each year. Partitioning of such flux among tributaries and among source units allows us to tentatively assess sediment yields from major subcatchments. Extreme yields and erosion rates are calculated for both the Karakorum Belt (up to 12,500 +/- 4700 t/km(2) year and 4.5 +/- 1.7 mm/year for the Braldu catchment) and Nanga Parbat Massif (8100 +/- 3500 t/km(2) year and 3.0 +/- 1.3 mm/year). These values approach denudation rates currently estimated for South Karakorum and Nanga Parbat crustal-scale antiforms, and highlight the major influence that rapid tectonic uplift and focused glacial and fluvial erosion of young metamorphic massifs around the Western Himalayan Syntaxis have on sediment budgets of the Indus system. Detailed information on bulk petrography and heavy minerals of modem Indus sands not only represents an effective independent metho

Fission-track dating on detrital apatites from modern sands of the Po delta is employed for a provenance study of sediments in the Po River drainage. Analysed samples show a fission-track grain-age distribution characterized by a well-resolved peak at 10 Ma, beside two other peaks at 19 and 40 Ma. The youngest peak is significantly larger than the older peaks, and accounts for 63% of the total population of dated grains. This young component in the grain-age distribution is consistent with the bedrock cooling ages observed in the Western Alps between the External Massifs and the Houiller unit, as well as in the Lepontine dome of the Central Alps and in the Miocene foredeep units of the Apennines. These potential source areas represent only 10% of the total orogenic source area of the Po drainage. The above results suggest that most of the sediment delivered to the Po delta is supplied by small portions of the drainage that represent in many cases (Lepontine dome and External Massifs) fast-exhuming portions of the belt bounded by major faults.

The Burdigalian to Serravallian turbiditic succession of the upper Gonfolite Lombarda Group exposed in the Brianza region has been sampled to analyze the petrographic composition and the detrital apatite fission-track record. The ultimate goal of this study is to define constraints for the provenance of the clastic sediments of the Gonfolite Lombarda and for the exhumation processes of the Central Alps during the Miocene. The petrographic analysis is still in progress, but preliminary results indicate that the Serravallian Gonfolite sediments include boulders from the Periadriatic Triangia (Sondrio) pluton that was emplaced at ca. 30 Ma. The fission-track-age distribution of the detrital apatites from the Burdigalian to Serravallian succession exhibits two age components: a approximately 30 Ma age component recorded by the largest fraction of grains in all samples, and a 53-106 Ma component shown only by two samples. The Burdigalian to Langhian samples include some detrital apatite ages close to the depositional age, suggesting possible partial annealing. Although the maximum burial conditions of the analyzed succession are not known, we tentatively interpret the fission-track-age distribution as not significantly affected by partial annealing. The approximately 30 Ma component is interpreted as related to the exhumation mainly of the Periadriatic intrusive rocks and of the adjacent Austroalpine units, but also of the portions of the Southalpine basement overprinted by the low-temperature Cenozoic thermal signature. The 53-106 Ma component is interpreted as related to the unroofing of the Variscan basement and the Permian volcanic rocks of the Southern Alps recording the Mesozoic thermotectonic signature. We conclude that the basement rocks of the Southern Alps were subject to erosion at least since the Early Miocene. Indeed, detrital apatites from the Gonfolite succession reveal a Southalpine provenance, but the dominant signal during the Early to Middle Miocene is

The Indus River has been progressively transformed in the last decades into a tightly-regulated system of dams and channels, to produce food and energy for the rapidly growing population of Pakistan. Nevertheless, Indus River sands as far as the delta largely retain their distinct feldspar- and amphibole-rich composition, which is unique with respect to all other major rivers draining the Alpine-Himalayan belt except for the Brahmaputra. Both the Indus and Brahmaputra Rivers flow for half of their course along the India-Asia suture zone, and receive major contributions from both Asian active-margin batholiths and upper-amphibolite-facies domes rapidly exhumed at the Western and Eastern Himalayan syntaxes. Composition of Indus sands changes repeatedly and markedly in Ladakh and Baltistan, indicating overwhelming sediment flux from each successive tributary as the syntaxis is approached. Provenance estimates based on our integrated petrographic-mineralogical dataset indicate that active-margin units (Karakorum and Transhimalayan arcs) provide ~81% of the 250±50 106 t of sediments reaching the Tarbela reservoir each year. Partitioning of such flux among tributaries and among source units allows us to tentatively assess sediment yields from major sub-catchments. Extreme yields and erosion rates are calculated for both the Karakorum Belt (up to 12,500±4700 t/km2 yr and 4.5±1.7 mm/yr for the Braldu catchment) and Nanga Parbat Massif (8100±3500 t/km2 yr and 3.0±1.3 mm/yr). These values approach denudation rates currently estimated for South Karakorum and Nanga Parbat crustal-scale antiforms, and highlight the major influence that rapid tectonic uplift and focused glacial and fluvial erosion of young metamorphic massifs around the Western Himalayan Syntaxis have on sediment budgets of the Indus system. Detailed information on bulk petrography and heavy minerals of modern Indus sands not only represents an effective independent method to constrain denudation rates obtained f

Sourced in the M. Bianco, the highest mountain of the Alps, the Dora Baltea River cuts across the arc of the Western Alps, drains all major Alpine tectonic units from the Helvetic domain to the South alpine domain, and finally joins the Po River. Petrographic composition of clastic detritus carried by the Dora Baltea River and its tributaries faithfully mirrors the complex geology of the Western Alps. High-resolution modal analysis of Dora Baltea sands allowed us to distinguish seven end-member metamorphoclastic provenances. River sands from the M. Bianco External Massif are dominated by quartz, feldspars, and hornblende. Detritus from basement and cover rocks of the Brianconnais zone includes low-grade metasedimentary lithic fragments, epidote, chloritoid, and garnet. Sands from the Internal Penninic Massifs include quartz and subordinate feldspars, garnet, and epidote. Abundant metabasite and serpentine-schist lithic grains, along with epidote, actinolite, and few pyroxenes, characterize detritus from the calcschists and meta-ophiolites of the oceanic Piemonte zone. Sands from the Austroalpine zone include quartz, abundant metamorphic lithic fragments, garnet, and locally significant sillimanite (Dent Blanche kinzigites). The relative contribution of each end member to the Dora Baltea sediment flux was assessed with forward modelling of end-member mixing. Our estimates, calculated from the entire petrographic and mineralogical data set, indicate that the main sources of bed load in the Dora Baltea mountain basin are the granitoid to gneissic rocks exposed in the M. Bianco Massif and in the Austroalpine zone. In order to make allowance for the contribution of a particular bedrock constituent in a compound source area to the derivative sand, the original concept of sand generation index (SGI) was modified to assess the relative contribution of the seven end-member sources of detritus. The highest total-basin SGI indices are calculated for the M. Bianco Massif (SGI=1

The high-relief and tectonically active Himalayan range, characterized by markedly varying climate but relatively homogeneous geology along strike, is a unique natural laboratory in which to investigate several of the factors controlling the composition of orogenic sediments. Coupling of surface and tectonic processes is most evident in the eastern Namche Barwa syntaxis, where the Tsangpo^Siang^Brahmaputra River, draining a large elevated area in south Tibet, plunges down the deepest gorge on Earth. Here composition of river sands changes drastically from lithic to quartzofeldspathic. After confluence with the Lohit River, draining the Transhimalayan-equivalent Mishmi arc batholiths, sediment composition remains remarkably constant across Assam, indicating subordinate contributions from Himalayan tributaries. Independent calculations based on petrographical, mineralogical, and geochemical data indicate that the syntaxis, representing only V4% of total basin area, contributes 35 þ 6% to the total Brahmaputra sediment flux, and V20% of total detritus reaching the Bay of Bengal. Such huge anomalies in erosion patterns have major effects on composition of orogenic sediments, which are recorded as far as the Bengal Fan. In the Brahmaputra basin, in spite of very fast erosion and detrital evacuation, chemical weathering is not negligible. Sand-sized carbonate grains are dissolved partially in mountain reaches and completely in monsoon-drenched Assam plains, where clinopyroxenes are selectively altered. Plagioclase, instead, is preferentially weathered only in detritus from the Shillong Plateau, which is markedly enriched in microcline. Most difficult to assess is the effect of hydraulic sorting in Bangladesh, where quartz, garnet and epidote tend to be sequestered in the bedload and trapped on the coastal plain, whereas cleavable feldspars and amphiboles are concentrated in the suspended load and eventually deposited in the deep sea. High-resolution petrographic and dense

The Alps are perhaps the best studied thrust belt formed during continent-continent convergence (collision orogen). Shallow, intermediate, and deep structural levels can be distinguished within the Alpine thrust stack, each shedding sediments with distinct petrographic and mineralogical signatures. In modern first-cycle sands carried by Alpine rivers, bulk composition, rank of metamorphic lithic grains (metamorphic index [MI]), and dense mineral assemblages all faithfully mirror the tectono-metamorphic history of exposed basement and cover units. The shallow structural level, widely preserved in the Ligurian Alps, consists of remnant ocean turbidites, shedding sedimentary to very low rank metasedimentary detritus (MI < 180). The few dense minerals include recycled ultrastables and garnet. The intermediate structural level, widely exposed in the Western Alps from the Simplon Fault to the Sestri-Voltaggio Zone, consists of high-pressure nappes retrogressed to greenschist facies. Detritus includes medium-rank metasedimentary lithic grains from cover units, quartz and feldspars from continental basements, or metabasite and foliated serpentine-schist lithic grains from subducted ophiolites. Dense minerals are epidote dominated and include high-pressure minerals (e.g., carpholite, Mg-chloritoid, glaucophane); Austroalpine and Penninic massifs supply abundant garnet. The deep structural level, unroofed in the Lepontine Dome, consists of amphibolite facies granitoid gneisses shedding very high rank quartzofeldspathic detritus (MI > 380). Dense minerals are hornblende dominated. The hornblende/epidote ratio increases, and the color of hornblende grains (hornblende-color index [HCI]) changes from dominantly blue green to green and green brown, from the periphery (HCI < 10) to the core (HCI 25-50) of the Lepontine Dome. Before reaching foreland basins on both sides of doubly vergent collision orogens, detritus from axial nappes mixes with detritus from basement and cover units

The present work shows how sediment composition can be used to constrain bed load sediment budgets in mountain catchments. A database of high-resolution petrographic and dense-mineral analyses of detritus carried by the Dora Baltea River and its tributaries allowed us to partition bed-load sediment budgets, obtained from data available from a public authority, between two different sets of sediment sources (major tributaries and major geological units). Relative and absolute budgets of sediment bed-load thus obtained are more robust than those derived from traditional empirical methods alone. Total detrital flux in the Dora Baltea drainage basin is 1,000,000 ton/year (bed-load 590,000 ton/year, suspended load 410,000 ton/year), with denudation rates of 0.12 mm/year (data from Italian public authority-Progetto di piano stralcio per l'assetto idrogeologico (PAI). Parma, Supplemento Straordinario della Gazzetta Ufficiale 166). More than 75% of the Dora Baltea total bed-load (453,000 9000 ton/year) is derived from tributaries (Dora di Ferret, Dora di Veny, Savara, Grand Eyvia, Buthier, Lys) sourced from the highest granitoid peaks (>4000 m a.s.l.: M. Bianco, M. Rosa, M. Cervino, Gran Paradiso massifs), which drain 36% of the whole catchment. The M. Bianco Massif, with its granitoid lithology, extreme relief and heavily glaciated drainage basins, occupies only 3% of total basin area, but it produces 294,000 +/- 6000 tons of arkosic sands per year, which represents 50% of total Dora Baltea bed-load. The Dora di Veny (2% of total Dora Baltea mountain catchment) exhibits the highest denudation rate of the whole Dora Baltea mountain basin (0.73 mm/year). The values calculated in this study compare well with measured modem river loads of pardy glaciated m:id-latitude mountain regions. (C) 2004 Elsevier B.V. All rights reserved.

An operational classification of metamorphic grains in sands and sandstones is proposed with the aim of enhancing data re- producibility among operators and the potential of high-resolution bulk petrography in provenance studies. For each of four protolith compositions (metapelite, metapsammite/metafelsite, metacarbonate, metabasite), six archetype grains displaying increasing degree of re- crystallization and foliation development are illustrated. Such a clas- sification grid is specifically devised as a subsidiary tool for point counting with the Gazzi–Dickinson method. Traditional QFR param- eters can also be easily recalculated from the data set obtained, thus meeting all possible needs (Decker and Helmold 1985; Suttner and Basu 1985). An experiment shows that usage of visual-comparison standards effectively minimizes operator variation and allows retrieval of crucial information during point counting in a reproducible way. A petrogenetic grid is presented as a subsidiary tool for classifying grains that include index minerals and to help correlation with dense-mineral data. The ‘‘metamorphic index’’ (MI) is introduced as an estimator of average metamorphic grade of source rocks. Our classification, an ex- tension of concepts used first in the study of arc–continent collision in Taiwan (Dorsey 1988) and successfully expanded to interpret the evo- lution of continent–continent collision in the Himalayas (Najman and Garzanti 2000; White et al. 2002), proves to be fruitful in provenance analysis of foreland-basin sediments shed from Alpine-type, thick- skinned collision orogens, particularly when integrated with dense- mineral, geochemical, and geochronological data.

Quantitative modal analysis of sandstones represents a useful stratigraphic tool for correlation, revealing major heterochroneities in the largely unfossiliferous fluvial to coastal terrigenous succession of the Austroalpine Domain. Besides its petrostratigraphic significance, sandstone composition provides a wealth of useful information for paleogeographic interpretation. A fundamental step to interpret modal data correctly is the assessment of various factors which control sandstone mineralogy. This case history shows how the relative importance of tectonic evolution of source areas versus effects of sedimentary processes and grain size may drastically change even within single stratigraphic intervals in the same stratigraphic succession. In various Austroalpine nappes, Permian to Anisian siliciclastic wedges represent the base of the post-Hercynian sedimentary cover. Even though each thrust-sheet is characterized by a notably different paleotectonic and sedimentary history, and in spite of lateral facies changes, coeval terrigenous packages maintain a common petrographic signature ("petrologic intervals"). The mineralogy of the metamorphiclastic "Verrucano" exposed in the Livigno area records an unroofing sequence within a single sedimentologically monotonous unit; largely epimetamorphic detritus is progressively replaced upwards by detritus from deeper-seated basement rocks as documented by superposition of distinct petrofacies. Mineralogy of the overlying volcaniclastic "Buntsandstein" and Val Pila Formation, which record a long-term fining-upward trend during transgression in the first part of the Triassic, testifies the overwhelming importance of sedimentary differentiation and grain size effects. Various grain size classes are in fact characterized by markedly different modal compositions, even though the Gazzi-Dickinson method was followed scrupulously. No methodological approach is thus effectively able to minimize grain size control on composition.

Oman and the United Arab Emirates host the largest ophiolites on Earth and are characterized by arid tropical climate and desert conditions. They thus provide an unexcelled natural laboratory in which to study sediments derived from allochthonous oceanic lithosphere and to define an actualistic petrogenetic model for obduction-orogen provenance. Spectacularly exposed in SE Arabia are two distinct obducted ophiolite sequences-Sama'il and Masirah-that contrast widely in formation setting, spreading rate, detachment level, thermal state during emplacement, and relief. Masirah beach sand chiefly consists of basaltic, diabase to metadiabase, plagioclase, gabbroic, and dunite grains from progressively deeper-seated levels of the multilayered oceanic crust. Dense minerals include diopsidic clinopyroxene and hornblende from gabbros, epidote and tremolite/actinolite from altered dikes, and augite from basaltic lavas. In contrast, sand from the Sama'il ophiolite includes cellular serpentinite grains and enstatite from residual mantle harzburgites, with subordinate olivine and minor chrome spinel. Diopside, hornblende, and hypersthene are provided by gabbroic rocks. Supply from upper-crustal levels is subordinate. Mafic to ultramafic detritus shed by obduction orogens virtually lacks quartz, K-feldspar, and metasedimentary rock fragments, thus contrasting radically with detritus from continental-collision orogens. Only when and where erosion bites beneath the oceanic nappe may polycrystalline quartz to metapelite and metafelsite grains be supplied by the metamorphic sole (developed at the base of young ophiolites) or by subducted continental-margin rocks. Outer- to inner-continental-margin successions exposed in tectonic windows provide additional chert, shale to slate, limestone, dolostone, and quartzose sandstone grains. Subducted ophiolites or blueschist melange along the suture zone of Alpine-type collision orogens also shed ultramafic grains, but their pervasively foliate