T. J. Wilson

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth’s orbital geometry control the ice ages, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the ‘warmer-than-present’ early-Pliocene epoch (∼5–3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, ∼40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth’s axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to ∼3 °C warmer than today and atmospheric CO2 concentration was as high as ∼400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt under conditions of elevated CO2.

Transition from back-arc to foreland basin development in the southernmost Andes: Stratigraphic record from the Ultima Esperanza District, Chile

During the Mesozoic evolution of the southernmost Andes, back-arc basin formation in an extensional setting was followed by foreland basin development in a compressional setting. A stratigraphic record of this tectonic transition is described from Jurassic and Cretaceous rock units exposed in the Patagonian fold-thrust belt at 51°S latitude. Initiation of an extensional deep-marine trough in Late Jurassic time is documented by interstratified rhyolites and marine mudstones in the Tobifera Formation. Development of the Early Cretaceous Rocas Verdes back-arc basin is recorded by Zapata Formation submarine-slope deposits, which draped the west-facing, passively subsiding cratonic margin of the basin. The onset of Andean compressional orogenesis and creation of the Magallanes foreland basin is marked by the influx of coarse-grained sandstone turbidites of the Albian-Cenomanian Punta Barrosa Formation. During Late Cretaceous time, the foreland basin configuration consisted of a narrow foredeep trough bounded by a gently sloping foreland ramp on the craton to the east. This geometry reflects greater flexural subsidence of the stretched and thermally weakened passive back-arc basin margin beneath the load of the obducted Rocas Verdes basin floor. Changes in depositional regime and sediment dispersal patterns in the latest Cretaceous, together with an eastward shift in the Magallanes basin depocenter, record cratonward migration of deformation in the Patagonian fold-thrust belt. Loading of the thicker, cratonic lithosphere caused subsidence of the foreland ramp to form a wide flexural basin.

Were aspects of Pan-African deformation linked to Iapetus opening?

The convergence recorded in some Pan-African deformational belts (sensu lato) in South America, Africa, Madagascar, southern India, Sri Lanka, and Antarctica is temporally correlated with opening of the Iapetus ocean. We propose a model in which continent-continent collision and closure of the Adamastor ocean between the Amazon–West African–Rio de La Plata cratons and the Sao Francisco–Congo–Kalahari cratons in the late Neoproterozoic are linked to rifting and orthogonal spreading between Laurentia and the South American cratons. By the Early Cambrian, the cratons in South America and Africa were assembled as West Gondwana. Closure of the Mozambique ocean, which appears to have extended across Antarctica between Lutzow-Holm Bay and the Shackleton Range, resulted in continued convergence between the Congo–Kalahari–Queen Maud Land block and East Gondwana in the Cambrian. Coeval deformation in the Transantarctic Mountains may be related to the obliquity of the Antarctic margin relative to Iapetus spreading directions. Initiation of voluminous arc magmatism along the paleo-Pacific margin of Gondwana in the Early Cambrian is broadly synchronous with the cessation of intra-Gondwana Pan-African deformation, possibly reflecting a change in plate motions at the time of final Gondwana assembly. The new subduction regime along the Gondwana margin in the Early Cambrian may be linked to the closure of the Iapetus ocean basin.

Cenozoic transtension along the Transantarctic Mountains‐West Antarctic rift boundary, southern Victoria Land, Antarctica

Brittle fault arrays mapped along the structural boundary between the Transantarctic Mountains and the West Antarctic rift system are oriented obliquely to the axis of the mountains and offshore rift basins. The north to northwest trending regional rift boundary is thus not controlled by continuous rift border faults. Instead, the rift margin trend must be imposed by inherited lithospheric weaknesses along the ancestral East Antarctic craton margin. Fault kinematic solutions indicate that a dextral transtensional regime characterized the rift boundary in the Cenozoic and that dominantly transcurrent motion occurred during the most recent faulting episode. The Transantarctic Mountains are considered to be a rift-flank uplift, yet no substantial isostatic uplift is expected in a transtensional setting, and the mechanism of large-magnitude Cenozoic uplift of the mountains remains problematical. Regional deformation patterns in Victoria Land and the Ross Sea can be explained by a transtensional model and are not compatible with large-magnitude crustal stretching within the West Antarctic rift system in the Cenozoic. The crustal thinning across the rift system more likely took place in the Mesozoic, when major West Antarctic crustal block motions occurred. The Cenozoic intracontinental deformation can be related to plate interaction resulting from the global Eocene plate reorganization, prior to the final separation between Antarctica and a narrow salient of the southeastern Australian margin. Displacement magnitude was probably minor, and thus early Tertiary east–west Antarctic motion is unlikely to account for discrepancies in global plate motion circuits.

Geologic evolution of the neoproterozoic Zambezi orogenic belt in Zambia

The Neoproterozoic Zambezi belt links with the Mozambique belt, Lufilian arc, and the inland branch of the Damara belt within the regional Pan-African tectonic framework of southern Africa. The belt contains a thick supracrustal sequence deposited on older sialic basement and penetratively deformed with it during Neoproterozoic (Pan-African) orogenesis. In Zambia, where the entire width of the orogen is exposed, local bimodal volcanic rocks at the base of the sequence are overlain by psammites and pelites, which in turn are succeeded by extensive carbonate and calc-silicate rocks. Abundant scapolite in metamorphic assemblages within the belt is taken as evidence for the original presence of evaporites. The nature of the rock types and the inferred stratigraphic sequence are consistent with deposition in an intracontinental rift basin invaded by marine waters. Available isotopic age brackets for the timing of supracrusta deposition show that the basin developed between 880 nad 820 Ma. Main-phase deformation in the belt involved both transcurrent shearing and south- to southwest-vergent thrusting and was associated with predominantly amphibolite-facies regional metamorphism. Mineral assemblages throughout much of the belt in Zambia, together with limited thermobarometric data, indicate typical Barrovian-type intermediate P/T conditions during metamorphism. Eclogites and other high-pressure metamorphic assemblages in parts of the belt, however, provide evidence that significant crustal thickening occurred, presumably in relation to thrusting. Reworked basement and syntectonic granite were subjected to extensive mylonitization related to strike-slip and oblique, reverse-slip shearing. The major orogenic event is dated at c. 820 Ma, based on an igneous age for a sheet-like, syntectonic batholith injected into a transcurrent shear zone within the central part of the belt. Pan-African orogenesis along the Zambezi-Lufilian-Damara trend was diachronous and records closure of intracratonic basins in the Zambezi belt and Lufilian arc, with evidence for the involvement of oceanic lithosphere present only in the Damara belt.

Polar research: Six priorities for Antarctic science

Antarctica. The word conjures up images of mountains draped with glaciers, ferocious seas dotted with icebergs and iconic species found nowhere else. The continent includes about one-tenth of the planets land surface, nearly 90% of Earths ice and about 70% of its fresh water. Its encircling ocean supports Patagonian toothfish and krill fisheries, and is crucial for regulating climate and the uptake of carbon dioxide by sea water.

UPb zircon ages from the Hook granite massif and Mwembeshi dislocation: constraints on Pan-African deformation, plutonism, and transcurrent shearing in Central Zambia

The Hook granite massif in the inner part of the Lufilian arc in central Zambia generally has been interpreted as granitized Archean or Paleoproterozoic sialic basement partly remobilized during Pan-African orogenesis. Reconnaissance field studies and UPb zircon geochronology reveal no evidence for exposed basement within the massif, which instead is shown to be a large, syn- to post-tectonic, composite batholith intrusive into upper Katangan (Kundelungu) strata. Syntectonic parts of the massif contain a single, moderate- to high-temperature, solid-state foliation that is continuous with the regionally developed S1 fabric in the adjacent lower-grade Kundelungu metasedimentary rocks. Two separate phases of syntectonic granite yield UPb zircon upper intercept ages of 559±18 and 566±5 Ma. Both samples show normal, linear discordance patterns, and zircon residues from acid leaching plot nearer concordia. An undeformed rhyolite dike intruding a Katangan pendant in the central part of the massif shows more complex zircon isotopic systematics, but a nearly concordant fraction has an age of 538±1.5 Ma. Post-tectonic granite yields an upper intercept age of 533±3 Ma. These ages constrain Kundelungu deposition in central Zambia as pre-570 Ma and show that regional deformation and voluminous syntectonic granite plutonism occurred in the inner part of the Lufilian arc at 560–570 Ma, with the major tectonic activity ending by 530–540 Ma. The Mwembeshi dislocation, a regionally significant Pan-African transcurrent shear zone, runs along the southern margin of the Hook massif. Syntectonic rhyolite intruded in the dislocation yields an upper intercept age of 551±19 Ma, showing that transcurrent shearing occurred in the same time frame as batholith emplacement, probably within an overall transpressive regime. Deformation and batholith emplacement in the inner part of the Lufilian arc were synchronous with a major pulse of syn- to post-tectonic granite plutonism in the Damara belt of Namibia, supporting a direct link between these regions during Pan-African orogenesis. In contrast, tectonothermal activity in central Zambia is unrelated to main-phase orogenesis in the Zambezi belt to the east, dated at ∼820 Ma. Models that propose a direct kinematic link between deformation in the Lufilian arc, shearing along the Mwembeshi dislocation, and main-phase orogenesis in the Zambezi belt are not supported by the present age data.

Bedrock displacements in Greenland manifest ice mass variations, climate cycles and climate change

The Greenland GPS Network (GNET) uses the Global Positioning System (GPS) to measure the displacement of bedrock exposed near the margins of the Greenland ice sheet. The entire network is uplifting in response to past and present-day changes in ice mass. Crustal displacement is largely accounted for by an annual oscillation superimposed on a sustained trend. The oscillation is driven by earth’s elastic response to seasonal variations in ice mass and air mass (i.e., atmospheric pressure). Observed vertical velocities are higher and often much higher than predicted rates of postglacial rebound (PGR), implying that uplift is usually dominated by the solid earth’s instantaneous elastic response to contemporary losses in ice mass rather than PGR. Superimposed on longer-term trends, an anomalous ‘pulse’ of uplift accumulated at many GNET stations during an approximate six-month period in 2010. This anomalous uplift is spatially correlated with the 2010 melting day anomaly.

Geodetic measurements of vertical crustal velocity in West Antarctica and the implications for ice mass balance

We present preliminary geodetic estimates for vertical bedrock velocity at twelve survey GPS stations in the West Antarctic GPS Network, an additional survey station in the northern Antarctic Peninsula, and eleven continuous GPS stations distributed across the continent. The spatial pattern of these velocities is not consistent with any postglacial rebound (PGR) model known to us. Four leading PGR models appear to be overpredicting uplift rates in the Transantarctic Mountains and West Antarctica and underpredicting them in the peninsula north of 65°. This discrepancy cannot be explained in terms of an elastic response to modern ice loss (except, perhaps, in part of the peninsula). Therefore, our initial geodetic results suggest that most GRACE ice mass rate estimates, which are critically dependent on a PGR correction, are systematically biased and are overpredicting ice loss for the continent as a whole.

Large-scale rhyolite peperites (Jurassic, southern Chile)

Development of a Jurassic volcano-tectonic rift basin in the southern Andes created a setting in which thick, rhyolitic volcaniclastic sequences accumulated in submarine environments and were penetrated by hypabyssal intrusions during or shortly after deposition. In the Ultima Esperanza District of southern Chile, extensive masses of peperite were produced when rhyolite magma underwent quenching, disruption, and commingling with wet, unconsolidated sediments during intrusion at shallow levels beneath the sea floor. The peperite forms discordant intrusive masses with volumes of up to several cubic kilometers, in which large, widely spaced, coherent rhyolite feeder pods are surrounded by, and grade into closely packed and dispersed peperite. Closely packed peperite consists of tightly fitting clasts separated by sediment-filled fractures. In dispersed peperite, the sediment forms a matrix surrounding large masses of fractured rhyolite and smaller more widely separated rhyolite clasts; evidence of in situ quench fragmentation is well preserved on both outcrop and thin-section scales. Thin sections show that clast margins and, in some cases, entire small clasts underwent cooling-contraction granulation, releasing shards of quenched rhyolite and fragments of phenocrysts into the adjacent sediment. Interaction between magma and wet sediment was non-explosive and involved fluidization of the host sediments, creating space for the intruding magma and causing pervasive injection of highly mobile sediment along thermal contraction cracks in quench-fragmented rhyolite. The ability of the magma to undergo complex intermixing with large volumes of sediment, with widespread preservation of in situ fragmentation textures, is interpreted to reflect a relatively low magma viscosity, presumably caused by retention of volatiles in the magma at the ambient pressures involved. Beds of redeposited peperite within the rift-basin fill indicate that some of the intrusive peperite masses reached the sea floor, undergoing slumping and mass flow. The peperites were thus an important local source of coarse-grained debris during the evolution of the basin.