Jason R. Ali

When and where did India and Asia collide

Timing of the collision between India and Asia is the key boundary condition in all models for the evolution of the Himalaya-Tibetan orogenic system. Thus it profoundly affects the interpretation of the rates of a multitude of associated geological processes ranging from Tibetan Plateau uplift through continental extrusion across eastern Asia, as well as our understanding of global climate change during the Cenozoic. Although an abrupt slowdown in the rate of convergence between India and Asia around 55 Ma is widely regarded as indicating the beginning of the collision, most of the effects attributed to this major tectonic episode do not occur until more than 20 Ma later. Refined estimates of the relative positions of India and Asia indicate that they were not close enough to one another to have collided at 55 Ma. On the basis of new field evidence from Tibet and a reassessment of published data we suggest that continent-continent collision began around the Eocene/Oligocene boundary (∼34 Ma) and propose an alternative explanation for events at 55 Ma.

Origin and motion history of the Philippine Sea Plate

Abstract The Philippine Sea Plate is the one major plate whose Tertiary motion is poorly constrained and whose origin is problematical. Its southern boundary is the Sorong Fault system which is part of a major left-lateral fault system at the northern margin of the Australian plate. The southern part of the plate in eastern Indonesia has been neglected in most syntheses but includes some of the oldest rocks within the plate which are separated from remnant arcs of the Daito Ridge province of the northern Philippine Sea by the West Philippine Central Basin. The east Indonesian islands of the Halmahera-Waigeo region contain a good Mesozoic and Tertiary stratigraphic record indicating a long arc history for the southern part of the plate. New palaeomagnetic data from these islands define two sub-areas: an area forming part of the Philippine Sea Plate north of the Sorong Fault, and an area within the Sorong Fault system. The area north of the fault records a long-term clockwise rotation history whereas that within the fault zone records local rotations interpreted as due to deformation at the plate edge. Rocks of Philippine Sea Plate origin within both areas record similar latitudinal shifts. The rotation of the area north of the Sorong Fault is considered to represent the motion of the southern part of the Philippine Sea Plate. The new data indicate large Tertiary clockwise rotations similar to earlier suggestions for other parts of the plate but record a discontinuous and more complex motion history than previously suggested. For the southern part of the plate there was 40° rotation with northward translation between 0 and 25 Ma, no significant rotation between 25 and 40 Ma, and there was 50° rotation with southward translation between 40 and 50 Ma. We show that the new palaeomagnetic data form part of a single set with earlier palaeomagnetic data from elsewhere in the plate. The translation history of the southern part of the plate in eastern Indonesia can be reconciled with northward motions recorded elsewhere and can be used to determine rotation poles for the plate (15°N, 160°E for the interval 5–25 Ma, and 10°N, 150°E for the interval 40–50 Ma). Reconstructions based on these poles predict that at ∼ 45 Ma the Palau-Kyushu Ridge had a WNW-ESE orientation which is very different from that postulated by many models used to explain the widespread boninite volcanism in the Izu-Bonin-Marians forearc at this time. The long arc history of the southern part of the plate and the reconstructions based on the rotation poles calculated from the palaeomagnetic data favour an origin for the West Philippine Basin by spreading in a backarc basin.

Aspects of the Tectonic Evolution of China

The subject of this Special Publication is one of the most interesting in global geoscience, the tectonic evolution of China. The assemblage of terranes that underlie this part of the world provides outstanding opportunities to elucidate global processes, and many of the factors that shape the Earths lithosphere are best exemplified by the geology of China and its immediately adjacent areas In addition, there are geological features that are particular and unique to the region. Some have been the focus of recent attention and have attracted international interest because of their global importance. This volume provides accounts of up-to-date research by Chinese and international geological teams on key aspects of the tectonic evolution of China and its surrounding areas. The papers describe the formation of the geological terranes that make up this part of east Asia, place constraints on plate tectonic models for their assembly and provide accounts of unique geological feature of the subcontinent.

Volcanism, mass extinction, and carbon isotope fluctuations in the middle permian of China

Middle Permian Extinction A major extinction in the Middle Permian 260 to 270 million years ago preceded the huge end-Permian extinction. Wignall et al. (p. 1179) present a detailed analysis of the Middle Permian event from rocks in southwest China. The extinction coincided with extensive nearby volcanic eruptions. A major drop in carbon isotope values followed the extinction event, implying massive disruption of the carbon cycle. Fossiliferous rocks from southwest China show that a major extinction in the Middle Permian coincided with extensive volcanic eruptions. The 260-million-year-old Emeishan volcanic province of southwest China overlies and is interbedded with Middle Permian carbonates that contain a record of the Guadalupian mass extinction. Sections in the region thus provide an opportunity to directly monitor the relative timing of extinction and volcanism within the same locations. These show that the onset of volcanism was marked by both large phreatomagmatic eruptions and extinctions amongst fusulinacean foraminifers and calcareous algae. The temporal coincidence of these two phenomena supports the idea of a cause-and-effect relationship. The crisis predates the onset of a major negative carbon isotope excursion that points to subsequent severe disturbance of the ocean-atmosphere carbon cycle.

Mammalian biodiversity on Madagascar controlled by ocean currents

Madagascar hosts one of the world’s most unusual, endemic, diverse and threatened concentrations of fauna. To explain its unique, imbalanced biological diversity, G. G. Simpson proposed the ‘sweepstakes hypothesis’, according to which the ancestors of Madagascar’s present-day mammal stock rafted there from Africa. This is an important hypothesis in biogeography and evolutionary theory for how animals colonize new frontiers, but its validity is questioned. Studies suggest that currents were inconsistent with rafting to Madagascar and that land bridges provided the migrants’ passage. Here we show that currents could have transported the animals to the island and highlight evidence inconsistent with the land-bridge hypothesis. Using palaeogeographic reconstructions and palaeo-oceanographic modelling, we find that strong surface currents flowed from northeast Mozambique and Tanzania eastward towards Madagascar during the Palaeogene period, exactly as required by the ‘sweepstakes process’. Subsequently, Madagascar advanced north towards the equatorial gyre and the regional current system evolved into its modern configuration with flows westward from Madagascar to Africa. This may explain why no fully non-aquatic land mammals have colonized Madagascar since the arrival of the rodents and carnivorans during the early-Miocene epoch. One implication is that rafting may be the dominant means of overseas dispersal in the Cenozoic era when palaeocurrent directions are properly considered.

Emeishan Basalts (SW China) and the 'end-Guadalupian' crisis: magnetobiostratigraphic constraints

A magnetostratigraphic investigation of the Permian Emeishan LIP (large igneous province) was carried out on a composite section in Ebian County, close to the type area in Sichuan, SW China. The main succession of twelve flows (175 m thick) carries a normal polarity whilst the one reliable site from the overlying 30 m thick volcanic waning sequence is marked by a reverse polarity. The data enable a correlation to be proposed with an Emeishan Basalt sequence in western Guizhou, c. 400 km to the SE. From our knowledge of the geomagnetic fields reversal behaviour during the Permian, it suggests that the main lava pile along the eastern half of the Emeishan Basalt outcrop belt was erupted within a half to one million years. Using magnetobiostratigraphic data from the adjacent South China platform, the normal polarity magnetozone is correlated with the normal polarity chron associated with the upper part of the Maokou Limestones. Constrained by conodont data, the main Emei Basalts appear to be at least two biozones older (1–1.5 Ma) than the Mid–Late Permian boundary. It is possible, however, that the Emei Basalt waning zone sequence, which represents an explosive volcanic phase, might be coeval with the ‘end-Guadalupian’ biotic crisis. Thus arguments implicating Emei Basalt volcanism as the causal mechanism behind this major global event have to accommodate the new relative-age constraints.

Spatial and temporal arrival patterns of Madagascar's vertebrate fauna explained by distance, ocean currents, and ancestor type

How, when, and from where Madagascars vertebrates arrived on the island is poorly known, and a comprehensive explanation for the distribution of its organisms has yet to emerge. We begin to break that impasse by analyzing vertebrate arrival patterns implied by currently existing taxa. For each of 81 clades, we compiled arrival date, source, and ancestor type (obligate freshwater, terrestrial, facultative swimmer, or volant). We analyzed changes in arrival rates, with and without adjusting for clade extinction. Probability of successful transoceanic dispersal is negatively correlated with distance traveled and influenced by ocean currents and ancestor type. Obligate rafters show a decrease in probability of successful transoceanic dispersal from the Paleocene onward, reaching the lowest levels after the mid-Miocene. This finding is consistent with a paleoceanographic model [Ali JR, Huber M (2010) Nature 463:653–656] that predicts Early Cenozoic surface currents periodically conducive to rafting or swimming from Africa, followed by a reconfiguration to present-day flow 15–20 million years ago that significantly diminished the ability for transoceanic dispersal to Madagascar from the adjacent mainland.

Emeishan large igneous province (SW China) and the mantle-plume up-doming hypothesis

Abstract: The Middle Permian (c. 262 Ma) Emeishan Basalt Formation of SW China is a commonly cited example of a large igneous province (LIP) that formed as a result of a deep-mantle plume impinging on the base of the lithosphere and generating large regional-scale up-doming prior to volcanism. Recently, however, this assertion has been challenged on the grounds that some lava flows close to the centre of the LIP were erupted in a submarine setting. Here we analyse all the available biolithostratigraphical, petrological, geochemical and volcanological information and show that this supports the idea that the terrain was generated by a plume that originated in the mantle. Emeishan basalt magma melted from a hot mantle source with trace element contents that are similar to the source of Icelandic and intraplate ocean island basalts. However, the amount and lateral extent of uplift is significantly less than is predicted by conventional deep-mantle plume models. We conclude that large-scale doming is not a diagnostic feature of mantle plumes; surface topography can be greatly influenced by the type of lower mantle plume (thermal or thermochemical), how it passes through the transition zone, and how it interacts with the lithosphere.

Paleomagnetism of Borneo

Abstract The paleomagnetism of Borneo remains controversial, although the preponderance of results, both from the island itself and from the surrounding regions, suggest that counterclockwise (CCW) rotation has taken place. CCW rotations are seen in minor intrusions in Sarawak, Sabah and Kalimantan, which increase systematically with the age of the intrusion to a maximum value of 51.8°±3.7°. The rotation can be no older than 25 Ma, which is the age of the intrusion showing the maximum rotation. The rotation appears to have neared completion by 10 Ma. Similar CCW rotations are seen in sites from Peninsular Malaysia through Borneo to Sulawesi, the Celebes Sea and Palawan in the Philippines, but the ages of these rotations are, for the most part, unknown. In Mesozoic rocks in Kalimantan and Sarawak, a stronger declination rotation of nearly 90° CCW is recorded at seven sites, including sites which pass fold and reversal tests. This strong rotation is no older than youngest Cretaceous, and although seen over a wide region in Borneo, it is not seen in Peninsular Malaysia, nor in the Celebes Sea or Palawan, where only the weaker CCW rotation is seen. The widespread occurrence of this strong rotation in Western Borneo suggests that it is essentially a rigid plate, or microplate rotation, and not a series of local rotations caused by distributed shear in limited deformation zones. The rotation of Borneo appears to be a consequence of convergence between the Australian and Eurasian plates, which is accommodated by subduction along the northwest margin of Borneo.

Facies analysis and sea-level change at the Guadalupian–Lopingian Global Stratotype (Laibin, South China), and its bearing on the end-Guadalupian mass extinction

Abstract: The Guadalupian–Lopingian boundary stratotype at Penglaitan, and the nearby Tieqiao section, near Laibin, South China, record a series of major environmental changes within the Jiangnan Basin during a Mid-Permian biotic crisis. The sequence-stratigraphic, petrographic and palaeontological record of these sections has been studied and the associated strontium isotopic fluctuations have been assayed. Mass extinction of fusulinid foraminifers is most clearly associated in time with onset of volcanism and a relative sea-level fall that led to the establishment of mid-ramp conditions (Laibin Limestone) in settings that were previously dominated by radiolarian mudstones. The regression also coincides with a low point of 87Sr/86Sr ratios. The lowstand deposits contain mafic scoriaceous grains that record pyroclastic volcanism probably centred in the Emeishan flood basalt province 800 km to the west of Laibin. Thus, unusually violent eruptions associated with flood basalts in this province may have contributed to the environmental stresses responsible for the extinction event. Subsequent environmental changes included transgression, spread of dysoxic waters, indicated by populations of small pyrite framboids, and a major negative C-isotope excursion. All these phenomena have been previously related to the end-Guadalupian extinction but they in fact post-date the crisis because a post-extinction fauna of foraminifers is encountered at this time.