Giovanni Vezzoli

Timing of India‐Asia collision: Geological, biostratigraphic, and palaeomagnetic constraints

[1] 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.

Orogenic Belts and Orogenic Sediment Provenance

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 interpret petrofacies from arc‐related, foreland‐basin, foredeep, and remnant‐ocean clastic wedges.

Loess plateau storage of northeastern Tibetan plateau-derived Yellow River sediment

Marine accumulations of terrigenous sediment are widely assumed to accurately record climatic- and tectonic-controlled mountain denudation and play an important role in understanding late Cenozoic mountain uplift and global cooling. Underpinning this is the assumption that the majority of sediment eroded from hinterland orogenic belts is transported to and ultimately stored in marine basins with little lag between erosion and deposition. Here we use a detailed and multi-technique sedimentary provenance dataset from the Yellow River to show that substantial amounts of sediment eroded from Northeast Tibet and carried by the rivers upper reach are stored in the Chinese Loess Plateau and the western Mu Us desert. This finding revises our understanding of the origin of the Chinese Loess Plateau and provides a potential solution for mismatches between late Cenozoic terrestrial sedimentation and marine geochemistry records, as well as between global CO2 and erosion records.

Sand petrology and focused erosion in collision orogens: the Brahmaputra case

Abstract 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 ∼4% of total basin area, contributes 35±6% to the total Brahmaputra sediment flux, and ∼20% 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-mineral studies of fluvial sands provide a basis for calculating sediment budgets, for tracing patterns of erosion in mountain belts, and for better understanding the complex dynamic feedback between surface processes and crustal-scale tectonics.

Provenance of the Tertiary sedimentary rocks of the Indo-Burman Ranges, Burma (Myanmar): Burman arc or Himalayan-derived?

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.

Petrology of Rifted‐Margin Sand (Red Sea and Gulf of Aden, Yemen)

The Red Sea–Gulf of Aden rift system, displaying a complete record of magmatic activity and characterized by arid climate and negligible anthropic modifications, provides an ideal natural laboratory for studies aimed at defining actualistic references for both volcanic and nonvolcanic rifted‐margin provenances. Rifted‐margin sands are derived in various proportions from volcanic to plutonic rocks emplaced before, during, or after the climax of tectonic extension (volcanic rifted‐margin provenance) and from prerift sedimentary successions and underlying crystalline basements progressively unroofed during uplift of rift blocks (rift‐shoulder provenance). Volcaniclastic rifted‐margin sands are feldspatholithic, as are those shed by Pacific‐type magmatic arcs, but are characterized by bimodal (basalt/rhyolite) lithics, abundant granophyre grains, and low plagioclase/total feldspar (P/F) ratios due to supply from synrift hypersolvus alkali granites, representing the upper levels of rift‐generated juvenile crust. Augite dominates among dense minerals; detritus from postrift alkali‐basalt fields includes olivine and, locally, enstatite and spinel. Sedimentary detritus from undissected rift shoulders consists of recycled quartz and carbonate sedimentary lithics; dense mineral assemblages include largely rounded to subrounded, recycled durable grains, zircon, and rutile being concentrated locally due to their higher density. Arkosic sands from basement rocks exposed on dissected rift shoulders display remarkably consistent compositions, with excess quartz with respect to “ideal arkose”; hornblende‐rich assemblages from amphibolite‐facies gneiss terranes contrast with epidote‐dominated assemblages from greenschist‐facies arc terranes. Diagnostic signatures and compositional trends recorded by modern Yemen sands may help in interpreting provenance of ancient rift‐related sandstone suites.

Weathering and Relative Durability of Detrital Minerals in Equatorial Climate: Sand Petrology and Geochemistry in the East African Rift

This article investigates how, where, and to what extent the mineralogical and chemical composition of sand-sized sediments is modified by extreme weathering in modern equatorial settings, with the ultimate goal of learning to read climate from the sedimentary record. To single out the weathering effect, we studied the compositional trends of fluvial sands along the western branch of the East African Rift between 5°S and 5°N. The relative durability of different detrital components, as well as potential hydraulic-sorting and grain-size effects, were assessed by comparing samples with similar provenances in different climatic and environmental conditions or of different size classes within the same sample. Sands of equatorial central Africa at the headwaters of the Congo and Nile basins display the full spectrum of petrologic suites characterizing rift-shoulder and volcanic rift provenances. Unlike in arid Arabia, quartzose sands are not restricted to areas where detritus is recycled from prerift sedimentary covers. In a hot humid climate, weathering can effectively obliterate the fingerprint of parent rock lithology and produce a nearly pure quartz residue even where midcrustal basement rocks are being actively uplifted and widely unroofed. In such settings garnet is destroyed faster than hornblende, and zircon faster than quartz. Weathering control on detrital modes is minor only in the rain shadow of the highest mountains or volcanoes, where amphibole-dominated quartzofelicdspathic metamorphiclastic sands (Rwenzori Province) or clinopyroxene-dominated feldspatholithic volcaniclastic sands (Virunga Province) are generated. Our detailed study of the Kagera basin emphasizes the importance of weathering in soils at the source rather than of progressive maturation in temporary storage sites during stepwise transport and shows that the transformation of diverse parent rocks into a quartzose “white sand” may be completed in one sedimentary cycle in hydromorphic soils of subequatorial lowlands. Micas and heavy minerals, which are less effectively diluted by recycling than main framework components, offer the best key to identify the original source-rock imprint. The different behavior of chemical indexes such as the CIA (a truer indicator of weathering) and the WIP (markedly affected by quartz dilution) helps us to distinguish strongly weathered first-cycle versus polycyclic quartz sands.

The Continental Crust as a Source of Sand (Southern Alps Cross Section, Northern Italy)

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/cm3) to middle crustal (∼2.85 g/cm3) and upper crustal levels (2.75–2.70 g/cm3). Sharp jumps in average rock density observed across major structural discontinuities (e.g., Cossato‐Mergozzo‐Brissago Line) mirror the fact that the South Alpine basement is the extensional stack of several slivers that followed distinct metamorphic and exhumation paths during the multistage tectonic evolution linking the Carboniferous Variscan Orogeny with the Jurassic opening of the Alpine Tethys.

Provenance of Passive-Margin Sand (Southern Africa)

This study investigates the petrographic, mineralogical, geochronological, and geochemical signatures of river sands across southern Africa. We single out the several factors that control sand generation, including weathering and recycling, and monitor the compositional changes caused by chemical and physical processes during fluvial transport from cratonic sources to passive-margin sinks. Passive-margin sands have two first-cycle sources. Quartz and feldspars with amphibole, epidote, garnet, staurolite, and kyanite are derived from crystalline basements exposed at the core of ancient orogens and cratonic blocks (dissected continental block provenance). Volcanic rock fragments, plagioclase, and clinopyroxene are derived from flood basalts erupted during the initial phases of rifting (volcanic rift provenance). First-cycle detritus mixes invariably with quartzose detritus recycled from ancient sedimentary successions (undissected continental block provenance) or recent siliciclastic deposits (e.g., Kalahari dune sands; recycled clastic provenance). U-Pb ages of detrital zircons mirror the orogenic events that affected southern Africa since the Archean. Damara (0.5–0.6 Ga) and Namaqua (1 Ga) age peaks are prominent throughout Namibia, from the Orange mouth to the Namib and Skeleton Coast Ergs, and also characterize Kalahari dunes and sands of the Congo, Okavango, and Zambezi Rivers. Instead, sharp old peaks at 2.1 Ga and 2.6 Ga characterize Limpopo and Olifants sands, matching the age of the Bushveld intrusion and the final assembly of the Zimbabwe and Kaapvaal Cratons, respectively; discordant ages indicate Pb loss during the Pan-African event. Chemical indices confirm that weathering is minor throughout the tropical belt from South Africa and Zimbabwe to Namibia and coastal Angola but major for quartzose sands of the Congo, Okavango, and upper Zambezi Rivers, largely produced in humid subequatorial regions. Recycling of quartzose sediments is extensive in all of these catchments. From Congo to Mozambique, along the >5000-km Atlantic and Indian Ocean rifted margins, polycyclic detritus reaches commonly 50% and locally up to 100%, in line with the estimated incidence of recycling worldwide. Quantitative information provided by provenance studies of modern sands helps us to better understand the relationships between sediment composition and plate-tectonic setting and to upgrade the overly simplified and often misleading current provenance models. This is a necessary step if we want to decipher the stratigraphic record of ancient passive margins and reconstruct their paleotectonic and paleoclimatic history with greater accuracy.

Collision‐Orogen Provenance (Western Alps): Detrital Signatures and Unroofing Trends

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 ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape