ALBERT

Description: The collision of India with Asia had a profound influence on Cenozoic topography, oceanography, climate, and faunal turnover. However, estimates of the time of the initial collision, when Indian continental crust arrived at the Transhimalayan trench, remain highly controversial. Here we use radiolarian and nannofossil biostratigraphy coupled with detrital zircon geochronology to constrain firmly the time when Asian-derived detritus was first deposited onto India in the classical Sangdanlin section of the central Himalaya, which preserves the best Paleocene stratigraphic record of the distal edge of the Indian continental rise. Deep-sea turbidites of quartzarenite composition and Indian provenance are replaced upsection by turbidites of volcano-plutoniclastic composition and Asian provenance. This sharp transition occurs above abyssal cherts yielding radiolaria of Paleogene radiolarian zones (RP) 4–6 and below abyssal cherts containing radiolaria of zone RP6 and calcareous shales with nannofossils of the Paleocene calcareous nannofossil zone (CNP) 7, constraining the age of collision onset to within the middle Paleocene (Selandian). The youngest U-Pb ages yielded by detrital zircons in the oldest Asia-derived turbidites indicate a maximum depositional age of 58.1 ± 0.9 Ma. Collision onset is thus mutually constrained by biostratigraphy and detrital zircon chronostratigraphy as 59 ± 1 Ma. This age is both more accurate and more precise than those previously obtained from the stratigraphic record of the northwestern Himalaya, and suggests that, within the resolution power of current methods, the India-Asia initial collision took place quasi-synchronously in the western and central Himalaya.

Description: 〈span〉This study focused on uppermost Cretaceous sedimentary rocks deposited in the Himalayan region and around the core of peninsular India just before the eruption of the Deccan Traps. Detailed stratigraphic and sedimentological analysis of Late Cretaceous successions in the Himalayan Range together with literature data from the Kirthar fold-and-thrust belt and central to southeastern India document a marked shallowing-upward depositional trend that took place in the Campanian−Maastrichtian before the Deccan magmatic outburst around the Cretaceous-Tertiary boundary. Topographic uplift of the Indian peninsula began in Campanian time and is held responsible for thick sediment accumulation associated with shorter periods of nondeposition in peripheral areas (Himalayan Range, Kirthar fold belt, and Krishna-Godavari Basin) than in the central part of the Deccan Province. Surface uplift preceding Deccan volcanism took place at warm-humid equatorial latitudes, which may have led to an acceleration of silicate weathering, lowered atmospheric 〈span〉p〈/span〉CO〈sub〉2〈/sub〉, and climate cooling starting in the Campanian−Maastrichtian. The radial centrifugal fluvial drainage in India that is still observed today was established at that time.〈/span〉

Description: The composition, morphology and crystal order of clay minerals in silt-sized sediments carried in suspensions from 25 major rivers across tropical southern Africa have been studied by X-ray diffractometry and scanning and transmission electron microscopy. Our goal was to determine the spatial variability of clay-mineral associations in diverse geological settings, and in climatic conditions ranging from humid Angola and Zambia to hyperarid Namibia and the Kalahari. Specific attention was paid to the micromorphology and chemical composition of smectite particles. The relative abundance of smectites, illite/mica, kaolinite and chlorite enabled identification of regions characterized by different physical and chemical processes: (1) negligible chemical weathering is documented in Namibia, where river muds mostly contain illite/mica or smectite derived from Damara metasedimentary or Etendeka volcanic rocks; (2) kaolinite documenting intense weathering, reaches a maximum in the Okavango, Kwando and Upper Zambezi, sourced in subequatorial Angola and Zambia; (3) suspended-load muds in the Limpopo and middle Zambezi catchments display intermediate features, with varied assemblages and smectite compositions reflecting diverse parent lithologies. Clay mineralogy and chemical composition are confirmed as a most effective tool to unravel present and past climatic conditions on a continental scale.

Description: 〈span〉Constraining the growth of the Tibetan Plateau in time and space is critical for testing geodynamic models and climatic changes at the regional and global scale. The Lhasa block is a key region for unraveling the early history of the Tibetan Plateau. Distinct from the underlying shallow-marine limestones, the Jingzhushan and Daxiong formations consist of conglomerate and sandstone deposited in alluvial-fan and braided-river systems. Both units were deposited at ca. 92 Ma, as constrained by interbedded tuff layers, detrital zircons, and micropaleontological data. Provenance and paleocurrent analyses indicate that both units were derived from the same elevated source area located in the central-northern Lhasa block. These two parallel belts of coeval conglomerates record a major change in paleogeography of the source region from a shallow seaway to a continental highland, implying initial topographic growth of an area over 160,000 km〈sup〉2〈/sup〉, named here the Northern Lhasaplano. The early Late Cretaceous topographic growth of the Northern Lhasaplano was associated with the demise of Tethyan seaways, thrust-belt development, and crustal thickening. The same paleogeographic and paleotectonic changes were recorded earlier in the Northern Lhasaplano than in the Southern Lhasaplano, indicating progressive topographic growth from north to south across the Bangong-Nujiang suture and Lhasa block during the Cretaceous. Similar to the Central Andean Plateau, the Northern Lhasaplano developed by plate convergence above the oceanic Neo-Tethyan subduction zone before the onset of the India-Asia collision.〈/span〉

Description: 〈p〉This paper shows how heavy minerals and single-grain varietal studies can be conducted on silt sediments to obtain quantitative data as efficiently as for sand-sized sediments. The analytical workflows include heavy mineral separation using a wide grain-size window analysed through integrated optical analysis, RAMAN , QemScan, and zircon geochronology. Upper Raman-aided heavy mineral analysis on Jurassic-Cretaceous silt-sized sediments from the Mandawa basin in Tanzania reveals garnet and apatite to be the most common minerals together with zircon, tourmaline and subordinate rutile. Secondary diagnostic phases are titanite, staurolite and epidote. Multivariate statistics highlights the association among durable minerals (zircon, tourmaline and rutile) while garnet and apatite plot alone reflecting independence between the group of variables with garnet increasing in Jurassic samples. Raman data for garnet end-member analysis document different associations between Jurassic and Cretaceous samples. U-Pb dating of detrital zircon and their statistical integration with other datasets provide further insights on changes in the drainage systems. Early and late Pan-African terranes of the Mozambique belt and those of the Irumide Belt acted as main source of sediment during the Jurassic. Cretaceous sediments record a broadening of the drainage system reaching as far as the Usagran-Ubendian Belt and the Tanzanian Archean Craton.〈/p〉

Description: The Indus River, one of Asia’s premier rivers, drains the western Tibetan Plateau and the Nanga Parbat syntaxis. These two areas juxtapose some of the lowest and highest topographic relief and commensurate denudation rates in the Himalaya-Tibet orogen, respectively, yet the spatial pattern of denudation rates upstream of the syntaxis remains largely unclear, as does the way in which major rivers drive headward incision into the Tibetan Plateau. We report a new inventory of 10 Be-based basinwide denudation rates from 33 tributaries flanking the Indus River along a 320 km reach across the western Tibetan Plateau margin. We find that denudation rates of up to 110 mm k.y. –1 in the Ladakh and Zanskar Ranges systematically decrease eastward to 10 mm k.y. –1 toward the Tibetan Plateau. Independent results from bulk petrographic and heavy mineral analyses support this denudation gradient. Assuming that incision along the Indus exerts the base-level control on tributary denudation rates, our data show a systematic eastward decrease of landscape downwearing, reaching its minimum on the Tibetan Plateau. In contrast, denudation rates increase rapidly 150–200 km downstream of a distinct knickpoint that marks the Tibetan Plateau margin in the Indus River longitudinal profile. We infer that any vigorous headward incision and any accompanying erosional waves into the interior of the plateau mostly concerned reaches well below this plateau margin. Moreover, reported long-term (〉10 6 yr) exhumation rates from low-temperature chronometry of 0.1–0.75 mm yr –1 consistently exceed our 10 Be-derived denudation rates. With averaging time scales of 10 3 –10 4 yr for our denudation data, we report postglacial rates of downwearing in a tectonically idle landscape. To counterbalance this apparent mismatch, denudation rates must have been higher in the Quaternary during glacial-interglacial intervals.

Description: Our new stratigraphic, sedimentological, and micropaleontological analysis, integrated with basalt geochemistry, sandstone petrography, and detrital-zircon U-Pb and Hf isotope data, suggests the revision of current models for the geological evolution of the Asian active margin during the Cretaceous. The Xigaze forearc basin began to form in the late Early Cretaceous, south of the Gangdese arc, during the initial subduction of the Neotethyan oceanic lithosphere under the Lhasa terrane. Well-preserved stratigraphic successions document the classical upward-shallowing pattern of the forearc-basin strata and elucidate the origin of the associated oceanic magmatic rocks. The normal mid-ocean-ridge basalt (N-MORB) geochemical signature and stratigraphic contact with the overlying abyssal cherts (Chongdui Formation) indicate that the Xigaze Ophiolite formed by forearc spreading and represents the basement of the forearc sedimentary sequence. Volcaniclastic sedimentation began with thick turbiditic sandstones and interbedded shales in the late Albian–Santonian (Ngamring Formation) followed by shelfal, deltaic, and fluvial strata (Padana Formation), with final filling of the basin by the Campanian age. Forearc sandstones do not show the classical trend from feldspatho-lithic volcaniclastic to quartzo-feldspathic plutoniclastic compositions, indicating limited unroofing of the Gangdese arc prior to collision. U-Pb age spectra of detrital zircons are unimodal with a 107 Ma peak in the lower Ngamring Formation (104–99 Ma), bimodal with a subordinate additional peak at 157 Ma in the middle Ngamring Formation (99–88 Ma), and multimodal with more abundant pre-Mesozoic ages in the upper Ngamring and Padana Formations (88–76 Ma). These three petrofacies with distinct provenances document the progressive erosional evolution of the Gangdese arc, with uplift of the central Lhasa terrane and expanding river catchments to include the central Lhasa terrane during the Late Cretaceous.

Description: A bstract :  Taiwan is an example of an orogen involving a doubly vergent critical wedge. Critical-wedge theory predicts asymmetrical tectonic development of the pro- and retro- sides of an orogen. Whereas the consequences for exhumation, peak metamorphism, and erosion have been extensively investigated, the implications for sandstone petrography preserved on either side of an orogen have not been adequately documented. Plio-Pleistocene sandstones from the Western Foothills of Taiwan were deposited in a rapidly subsiding foreland basin recording the collision between a volcanic arc and the Asian passive margin. New data on the evolution of sedimentary petrography and clay minerals preserved in Miocene to Pleistocene sandstone of the western pro-foreland basin record the history of Taiwan's orogenesis. Sandstone petrography of the eastern basin on the retro-side of Taiwan's orogenic wedge had been studied previously. Comparison of the provenance record from the western and eastern basins illustrates the opposing signature of unroofing and recycling, held to be characteristic of an asymmetric orogenic wedge. Sandstone petrography can be used in ancient sedimentary basins to complement other indications of the polarity of subduction.

Description: 〈p〉This chapter summarizes the available stratigraphic, petrographical and mineralogical evidence from sediments and sedimentary rocks on the evolution of the Himalayan belt and its associated foreland basin. The use of compositional signatures of modern sediments to unravel provenance changes and palaeodrainage evolution through time is hampered by a poor match with detrital modes of ancient strata markedly affected by selective chemical dissolution of unstable minerals during diagenesis. Only semi-quantitative diagnoses can thus be attempted. Volcanic detritus derived from Transhimalayan arcs since India–Asia collision onset at 〈i〉c.〈/i〉 60 Ma was deposited onto the Indian lower plate throughout the Protohimalayan stage, with the exception of the Tansen region of Nepal that is characterized by quartz-arenites yielding orogen-derived zircon grains. During the Eohimalayan stage, begun in the late Eocene when most sedimentation ceased in the Tethys Himalayan domain, low-rank metasedimentary detritus was overwhelming in the central foreland basin, where a widespread unconformity developed spanning locally as much as 20 myr. Volcanic detritus from Transhimalayan arcs remained significant in northern Pakistan. Arrival of higher-rank metamorphic detritus since the earliest Miocene, and the successive occurrence of garnet, staurolite, kyanite and finally sillimanite, characterized the Neohimalayan stage, when repeated compositional changes in the foreland-basin succession document the stepwise propagation of crustal deformation across the Indian Plate margin and widening of the thrust belt with exhumation of progressively more external tectonic units. The correspondence in time between the activity of major thrusts and petrofacies changes indicates a promising approach to accurately reconstruct the geological evolution of the coupled orogen–basin system. Conversely, a poor conceptual framework and the general reliance on ad hoc mechanisms to explain phenomena unpredicted by simplified models represent major factors limiting the robustness of palaeotectonic interpretations. Improved knowledge requires taking into full account the dynamic role played by still poorly understood subduction processes – rather than exclusively the effect of passive loading – as well as the role played by the presence of inherited structures on the downgoing Indian Plate, which control both lateral variability of orogenic deformation and the location of depocentres in the foreland basin.〈/p〉