Eduardo Garzanti

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.

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.

Modern sand from obducted ophiolite belts - (Sultanate of Oman and United Arab Emirates)

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 mélange along the suture zone of Alpine‐type collision orogens also shed ultramafic grains, but their pervasively foliated, interpenetrating to interlocking textures are readily distinguished from the dominantly pseudomorphic mesh textures that characterize the serpentinite grains derived from obducted mantle slabs.

Paleolatitudes of the Tibetan Himalaya from primary and secondary magnetizations of Jurassic to Lower Cretaceous sedimentary rocks

The Tibetan Himalaya represents the northernmost continental unit of the Indian plate that collided with Asia in the Cenozoic. Paleomagnetic studies on the Tibetan Himalaya can help constrain the dimension and paleogeography of Greater India, the Indian plate lithosphere that subducted and underthrusted below Asia after initial collision. Here we present a paleomagnetic investigation of a Jurassic (limestones) and Lower Cretaceous (volcaniclastic sandstones) section of the Tibetan Himalaya. The limestones yielded positive fold test, showing a prefolding origin of the isolated remanent magnetizations. Detailed paleomagnetic analyses , rock magnetic tests, end-member modeling of acquisition curves of isothermal remanent magnetization, and petrographic investigation reveal that the magnetic carrier of the Jurassic limestones is authigenic mag-netite, whereas the dominant magnetic carrier of the Lower Cretaceous volcaniclastic sandstones is detrital magnetite. Our observations lead us to conclude that the Jurassic limestones record a prefolding remagnetiza-tion, whereas the Lower Cretaceous volcaniclastic sandstones retain a primary remanence. The volcaniclastic sandstones yield an Early Cretaceous paleolatitude of 55.5 S [52.5 S, 58.6 S] for the Tibetan Himalaya, suggesting it was part of the Indian continent at that time. The size of Greater India during Jurassic time cannot be estimated from these limestones. Instead, a paleolatitude of the Tibetan Himalaya of 23.8 S [21.8 S, 26.1 S] during the remagnetization process is suggested. It is likely that the remagnetization, caused by the oxidation of early diagenetic pyrite to magnetite, was induced during 103–83 or 77–67 Ma. The inferred paleolatitudes at these two time intervals imply very different tectonic consequences for the Tibetan Himalaya.

Raman spectroscopy as an effective tool for high-resolution heavy-mineral analysis: Examples from major Himalayan and Alpine fluvio-deltaic systems

Raman spectroscopy represents a new way to obtain detailed comprehensive information on heavy-mineral assemblages. In this work are presented several examples from major Alpine (Po River) and Himalayan (Ganga and Brahmaputra Rivers) fluvio-deltaic sands. Our attention was focused on the chemical properties of garnet, which is a widespread mineral in orogenic sediments, easy to be identified, and relatively stable during both equatorial weathering and intrastratal dissolution. Garnet grains were studied in different samples representative of various depositional environments (fluvial bar, fluvial levee, shoreface, beach berm, eolian dune), in order to investigate specifically the hydraulic behaviour of grains with different density in different hydrodynamic conditions. Raman spectra and semi-quantitative analysis of Raman shifts allowed us to rapidly determine the distribution of garnet types in each sample in order to obtain chemical composition, to calculate the density of each garnet, and finally to infer their respective provenance. This manuscript presents one possible application of the MIRAGEM method described by Bersani et al. in this volume. References, data sets and details on the analytical routine are widely explained in the above mentioned work.

Sedimentary and tectonic evolution of the southern Qiangtang basin: Implications for the Lhasa‐Qiangtang collision timing

The Mesozoic stratigraphic record of the southern Qiangtang basin in central Tibet records the evolution and closure of the Bangong-Nujiang ocean to the south. The Jurassic succession includes Toarcian-Aalenian shallow-marine limestones (Quse Formation), Aalenian-Bajocian feldspatho-litho-quartzose to feldspatho-quartzo-lithic sandstones (shallow-marine Sewa Formation and deep-sea Gaaco Formation) and Bathonian outer platform to shoal limestones (Buqu Formation). This succession is truncated by an angular unconformity, overlain by upper Bathonian to lower Callovian fan-delta conglomerates and litho-quartzose to quartzo-lithic sandstones (Biluoco Formation) and Callovian shoal to outer-platform limestones (Suowa Formation). Sandstone petrography coupled with detrital-zircon U-Pb and Hf isotope analysis indicate that the Sewa and Gaaco formations contain intermediate to felsic volcanic detritus and youngest detrital zircons (183-170 Ma) with eHf(t) ranging widely from +13 to -25, pointing to continental-arc provenance from igneous rocks with mixed mantle and continental-crust contributions. An arc-trench system thus developed towards the end of the Early Jurassic, with the southern Qiangtang basin representing the forearc basin. Above the angular unconformity, the Biluoco Formation documents a change to dominant sedimentary detritus including old detrital zircons (mainly > 500 Ma ages in the lower part of the unit) with age spectra similar to those from Paleozoic strata in the central Qiangtang area. A major tectonic event with intense folding and thrusting thus took place in late Bathonian time (166 ± 1 Ma), when the Qiangtang block collided with another microcontinental block possibly the Lhasa block.

Early cretaceous topographic growth of the Lhasaplano, Tibetan plateau: Constraints from the Damxung conglomerate

Constraining the timing of early topographic growth on the Tibetan plateau is critical for any models of India-Asia collision, Himalayan orogeny and subsequent plateau development in the Cenozoic. Stratigraphic, sedimentological and provenance analysis of the Lower Cretaceous red-beds of the Damxung Conglomerate provide new key information to reconstruct the paleogeography and the tectonic evolution of the Lhasa terrane at the time. The over 700-m-thick Damxung Conglomerate documents distal alluvial fan to braidplain sedimentation passing upward to proximal alluvial fan sedimentation. Deposition began near sea level, as documented by limestone beds occurring at the base of the unit. Zircon U–Pb dating of interbedded tuff layers constrain deposition age at ca. 111xa0Ma. Abundance of volcanic clasts, Cretaceous U–Pb ages and Hf isotopes of detrital zircons yielding mainly negative eHf(t) values together with paleocurrent data indicate an active volcanic source located in the North Lhasa subterrane. Pre-Mesozoic-aged zircon, recycled quartz and (meta) sedimentary rock fragments increase up-section, indicating progressive erosional exhumation of the Paleozoic sedimentary/metasedimentary basement. The Damxung Conglomerate thus records a significant uplift and unroofing stage in the source region, implying initial topographic growth on the Lhasa terrane at early Albian time. Early Cretaceous topographic growth on the Lhasa terrane is supported by the stratigraphic record in the Linzhou basin, the Xigaze forearc basin and the southern Nima basin. In contrast, marine strata in the central-western Lhasa terrane lasted until the early Cenomanian (ca. 96xa0Ma), indicating diachronous marine regression on the Lhasa terrane from east to west.

Controls on sand ramp formation in southern Namibia

Sand ramps have the potential to provide rich palaeoenvironmental information in dryland regions wherein proxy records are typically scarce. However, current knowledge of the geomorphic controls and processes of sand ramp formation is limited. This study provides a data-rich examination of the key factors controlling sand ramp formation. The location and morphology of 75 sand ramps in southern Namibia are examined. The sediments and chronologies of 10 sand ramps are studied in detail using 51 OSL dates and 83 grain-size and LOI samples. Heavy mineral assemblages are used to determine the provenance of 10 samples and OSL sensitivity is used to explore geomorphic processes of 8 samples. n nSand ramp morphology can be grouped into one of four classes of increasing size and complexity and is closely linked to the available accommodation space. Heavy mineral assemblages indicate local sediment sources and all 75 studied sand ramps are within 4u2009km of a large ephemeral river channel or within 5.5u2009km of a dune field. Therefore, accommodation space and sediment supply are identified as the primary controls of sand ramp formation. Sedimentology and OSL sensitivity suggest a complex interplay of aeolian, fluvial and colluvial processes contribute to sand ramp formation with large variability observed between ramps. Three of the ten dated sand ramps have been present in the Namibian landscape for >100u2009ka. Eight sand ramps show episodic deposition between >75-12u2009ka and five show evidence of surface reworking over the past 2u2009ka. Environmental sensitivity is likely to be linked to the size and availability of the accommodation space. Therefore, individual sand ramps are expected to reflect local environmental conditions, recording when an abundant sediment supply coincided with available accommodation space, whilst a regional analysis of multiple sand ramps with chronometric data offers the potential to identify larger scale palaeoenvironmental controls of sediment supply.

Partitioning sediment flux by provenance and tracing erosion patterns in Taiwan

We critically evaluate the potential and limitations of an alternative way to calculate erosion rates based on petrographic and mineralogical fingerprints of fluvial sediments coupled with gauged sediment fluxes. Our approach allows us to apportion sediment loads to different lithological units, and consequently to discriminate erosion rates in different tectonic domains within each catchment. Our provenance data on modern Taiwanese sands indicate focused erosion in the Backbone Range and Tananao Complex of the retrowedge. Lower rates are inferred for the northern part of the island characterized by tectonic extension and for the western foothills in the prowedge. The principal factor of uncertainty affecting our estimates is the inevitably inaccurate evaluation of total sediment load, because only the suspended flux was measured. Another is the assumption that suspended load and bed load are derived from the same sources in fixed proportions. Additional errors are caused by the insufficiently precise definition of lithologically similar compositional end-members and by the temporal variability of sediment composition at the outlet of each catchment related to the spatial variability of erosional processes and triggering agents such as earthquakes, typhoons, and landslides. To evaluate the robustness of our findings, we applied a morphometric technique based on the stream-power model. The results obtained are broadly consistent, with local discrepancies ascribed to poorly constrained assumptions and choices of scaling parameters. Our local erosion estimates are consistent with GPS uplift rates measured on a decadal timescale and generally higher than basin-wide results inferred from cosmogenic-nuclide and thermochronology data.

High Throughput Petrochronology and Sedimentary Provenance Analysis by Automated Phase Mapping and LAICPMS

The first step in most geochronological studies is to extract dateable minerals from the host rock, which is time consuming, removes textural context, and increases the chance for sample cross-contamination. We here present a new method to rapidly perform in-situ analyses by coupling a fast Scanning Electron Microscope (SEM) with Energy Dispersive X-ray Spectrometer (EDS) to a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LAICPMS) instrument. Given a polished hand specimen, a petrographic thin section, or a grain mount, Automated Phase Mapping (APM) by SEM/EDS produces chemical and mineralogical maps from which the X-Y coordinates of the datable minerals are extracted. These coordinates are subsequently passed on to the laser ablation system for isotopic analysis. We apply the APM+LAICPMS method to three igneous, metamorphic and sedimentary case studies. In the first case study, a polished slab of granite from Guernsey was scanned for zircon, producing a 609u2009±u20098 Ma weighted mean age. The second case study investigates a paragneiss from an ultra-high pressure terrane in the North Qaidam terrane (Qinghai, China). 107 small (25 μm) metamorphic zircons were analysed by LAICPMS to confirm a 419u2009±u20094 Ma age of peak metamorphism. The third and final case study uses APM+LAICPMS to generate a large provenance dataset and trace the provenance of 25 modern sediments from Angola, documenting longshore drift of Orange River sediments over a distance of 1500 km. These examples demonstrate that APM+LAICPMS is an efficient and cost effective way to improve the quantity and quality of geochronological data.