Roy A. Livermore

Drake Passage and Cenozoic climate: An open and shut case?

Drake Passage opening has often been viewed as a single, discrete event, possibly associated with abrupt changes in global circulation and climate at or near the Eocene-Oligocene boundary. A new plate tectonic model, based on recent reinterpretations of the opening history of basins in the Scotia Sea, suggests that an effective ocean gateway may have developed even earlier, during the middle Eocene. This is consistent with a growing body of evidence from sediment core proxy data for Eocene changes in Southern Ocean circulation and biological productivity. The period between earliest opening after ∼50 Ma and the latest Eocene was characterized by the evolution of various current pathways across the subsiding continental shelves and intervening deep basins. This shallow opening may have caused important changes in Southern Ocean circulation, contributing to Eocene cooling and the growth of Antarctic ice sheets.

Autopsy on a dead spreading center: The Phoenix Ridge, Drake Passage, Antarctica

New bathymetric and magnetic anomaly data from the Phoenix Ridge, Antarctica, show that extinction of all three remaining segments occurred at the time of magnetic chron C2A (3.3 ± 0.2 Ma), synchronous with a ridge-trench collision south of the Hero Fracture Zone. This implies that the ultimate cause of extinction was a change in plate boundary forces occasioned by this collision. Spreading rates slowed abruptly at the time of chron C4 (7.8 ± 0.3 Ma), probably as a result of extinction of the West Scotia Ridge, which would have led to an increase in slip rate and transpressional stress across the Shackleton Fracture Zone. Spectacular, high-relief ridges flanking the extinct spreading center, mapped for the first time using multibeam swath bathymetry, are interpreted as a consequence of a reduction in spreading rate, involving a temporary magma oversupply immediately prior to extinction.

Shackleton Fracture Zone: No barrier to early circumpolar ocean circulation

The opening of Southern Ocean gateways was critical to the formation of the Antarctic Circumpolar Current and may have led to Cenozoic global cooling and Antarctic glaciation. Drake Passage was probably the final barrier to deep circumpolar ocean currents, but the timing of opening is unclear, because the Shackleton Fracture Zone could have blocked the gateway until the early Miocene. Geophysical and geochemical evidence presented here suggests that the Shackleton Fracture Zone is an oceanic transverse ridge, formed by uplift related to compression across the fracture zone since ca. 8 Ma. Hence, there was formerly (i.e., in the Miocene) no barrier to deep circulation through Drake Passage, and a deep-water connection between the Pacific and Atlantic Oceans was probably established soon after spreading began in Drake Passage during the early Oligocene.

Tectonic evolution of the west Scotia Sea

Joint inversion of isochron and flowline data from the flanks of the extinct West Scotia Ridge spreading center yields five reconstruction rotations for times between the inception of spreading prior to chron C8 (26.5 Ma), and extinction around chron C3A (~6.65.9 Ma). When they are placed in a regional plate circuit, the rotations predict plate motions consistent with known tectonic events at the margins of the Scotia Sea: Oligocene extension in Powell Basin; Miocene convergence in Tierra del Fuego and at the North Scotia Ridge; and Miocene transpression at the Shackleton Fracture Zone. The inversion results are consistent with a spreading history involving only two plates, at rates similar to those between the enclosing South America and Antarctica plates after chron C5C (16.7 Ma), but that were faster beforehand. The spreading rate drop accompanies inception of the East Scotia Ridge back-arc spreading center, which may therefore have assumed the role of the West Scotia Ridge in accommodating eastwards motion of the trench at the eastern boundary of the Scotia Sea. This interpretation is most easily incorporated into a model in which the basins in the central parts of the Scotia Sea had already formed by chron C8 contrary to some widely accepted interpretations and which has significant implications for paleoceanography and paleobiogeography.

Subglacial bedforms reveal complex basal regime in a zone of paleo- ice stream convergence, Amundsen Sea embayment, West Antarctica

The flow of ice streams, which account for most discharge from large ice sheets, is controlled by processes operating at the ice stream bed. Data from modern ice stream beds are difficult to obtain, but where ice advanced onto continental shelves during glacial periods, extensive areas of the former bed can be imaged using modern swath sonar tools. We present new multibeam swath bathymetry data analyzed alongside sparse preexisting data from the Amundsen Sea embayment. The compilation is the most extensive, continuous area of multi-beam data coverage yet obtained on the inner continental shelf of Antarctica. The data reveal streamlined subglacial bedforms that define a zone of paleo–ice stream convergence, but, in contrast to previous models, do not show a simple downflow progression of bedform types along paleo–ice stream troughs. We interpret high spatial variability of bedforms as indicating a complex mechanical and hydrodynamic regime at the former ice stream beds, consistent with observations from some modern ice streams. We conclude that care must be taken when using bedforms to infer paleo–ice stream velocities.

SUBDUCTION INFLUENCE ON MAGMA SUPPLY AT THE EAST SCOTIA RIDGE

Despite a spreading rate of 65–70 km Ma−1, the East Scotia Ridge has, along most of its length, a form typically associated with slower rates of sea floor spreading. This may be a consequence of cooler than normal mantle upwelling, which could be a feature of back-arc spreading. At the northern end of the ridge, recently acquired sonar data show a complex, rapidly evolving pattern of extension within 100 km of the South Sandwich Trench. New ridge segments appear to be nucleating at or near the boundary between the South American and Scotia Sea plates and propagating southwards, supplanting older segments. The most prominent of these, north of 56°30′S, has been propagating at a rate of approximately 60 km Ma−1 for at least 1 Ma, and displays a morphology unique on this plate boundary. A 40 km long axial high exists at the centre of this segment, forming one of the shallowest sections of the East Scotia Ridge. Beneath it, seismic reflection profiles reveal an axial magma chamber, or AMC, reflector, similar to those observed beneath the East Pacific Rise and Valu Fa Ridge. Simple calculations indicate the existence here of a narrow (<1 km wide) body of melt at a depth of approximately 3 km beneath the sea floor. From the topographic and seismic data, we deduce that a localised mantle melting anomaly lies beneath this segment. Rates of spreading in the east Scotia Sea show little variation along axis. Hence, the changes in melt supply are related to the unique tectonic setting, in which the South American plate is tearing to the east, perhaps allowing mantle flow around the end of the subducting slab. Volatiles released from the torn plate edge and entrained in the flow are a potential cause of the anomalous melting observed. A southward mantle flow may have existed beneath the axis of the East Scotia Ridge throughout its history.

Hydrothermal plumes above the East Scotia Ridge: an isolated high-latitude back-arc spreading centre

We have identified first evidence for the presence of submarine hydrothermal activity along the East Scotia Ridge an isolated back-arc spreading centre located at 55–60°S in the Atlantic sector of the Southern Ocean. Using a combination of in situ optical light-scattering sensor data, and total dissolvable Mn concentrations, we demonstrate the existence of hydrothermal plumes overlying two segments of this ∼500 km ridge-crest; both segments exhibit anomalous topography and at least one segment is also underlain by an axial magma chamber seismic reflector. Future investigation of the fauna that inhabit these remote hydrothermal environments may provide an important ‘missing link’ between the distinct biogeographical provinces delimited from previous investigation of northern Atlantic versus eastern Pacific vent-sites.

Opening history of Powell Basin, Antarctic Peninsula

The opening of Powell Basin was part of the regional response to N55°W relative plate motion of South America away from Antarctica, which led to the formation of Drake Passage during the Eocene and Oligocene. Restoration of microplates around the basin using gridded magnetic anomalies from its margins illustrates the pre-break-up continuity of the Pacific Margin Anomaly magnetic high associated with a Mesozoic arc-batholith. Newly compiled magnetic anomaly data over the Powell Basin show subdued linear seafloor spreading type anomalies. These are used, together with marginal and regional geology, to constrain the opening history of the basin. Magnetic reversal modelling suggests that slow spreading in Powell Basin probably occurred between 29.7 Ma and 21.8 Ma, following rifting of Mesozoic continental crust with associated break-up volcanism. A simple, two-phase model for the rotation of the South Orkney Microcontinent away from the Antarctic Peninsula accounts for the pattern of magnetic reversals recorded in Powell Basin, and for the structure of its margins.

Enhanced magma supply at the southern East Scotia Ridge: evidence for mantle flow around the subducting slab?

Abstract Bathymetric and seismic data show that the southernmost segment of the East Scotia Ridge (segment E9) is anomalous in its curved plan form, and in the presence of a large axial volcanic ridge (AVR). Spreading commenced only within the past 1 million years on this segment, which appears to have propagated both northward and southward. The presence of a caldera at the summit of the AVR indicates that a shallow magma chamber of limited extent existed recently beneath the ridge. Eruption from this magma chamber, probably within the past 0.1 million years, may have led to the formation of the AVR itself. Careful examination of seismic reflection profiles suggests that small pockets of magma may still exist beneath the AVR. Magmatism on segment E9 may be enhanced as a result of the flow of shallow mantle around the southern end of the subducting slab beneath the South Sandwich Arc in a manner similar to that proposed for the northern end. This mantle probably carries a slightly enriched ‘hotspot’ signature, and is affected by volatiles released from the edge of the slab, both tending to increase the supply of magma to the back-arc spreading centre.

Reconstruction and break-out model for the Falkland Islands within Gondwana

The Falkland Islands are one segment of the Permo-Triassic Gondwanian Fold Belt that was displaced during the fragmentation of Gondwana. Palaeomagnetic, structural and palaeocurrent data, reviewed in this paper, provide convincing evidence that the Falkland Islands rotated from an original position off southeast Africa to their present position off South America during break-up. The rotation mechanism and trajectory are less certain but are an essential component of any plausible Gondwana break-up model. The Falkland Islands possess two roughly orthogonal structural grains. D1 structures form a southerly verging fold belt that correlates with the main folding in the eastern part of the Cape Fold Belt following relocation of the islands. D1 folds were overprinted by the Early Mesozoic D2, northeast-southwest trending, Hornby Mountains Anticline producing localised kilometrescale Type I and III fold interference patterns. It is suggested here that the D2 structures may represent a break-out structure related to a dextral transtensional shear couple that may have existed between East and West Gondwana during the initial stages of break-up. Clockwise rotation of the Falkland Islands Block (FIB) could have taken place along a series of east-west faults (e.g. the Gastre Fault Zone) in Patagonia as southern South America moved towards the Pacific during Middle Jurassic times. Contemporaneous Pacific-ward motion of southern South America during rotation of the FIB would have avoided collision with the Falkland Island Block as it docked. On a geological timescale, the break-out of the FIB and associated movements of other Gondwana fragments were rapid events, which appear to correlate with the major magmatic pulse at ca 183 Ma, related to a mantle plume beneath Africa and Antarctica. If this is correct, then doming above a large mantle plume in the South Atlantic region may have helped formation and rotation of Gondwana microplates, with rotation occurring above a viscously deforming, hotter-than-normal, substratum in a transtensional setting.