Search

An animated reconstruction shows South Pacific plate kinematics, in the reference frame of West Antarctica, between 55 Ma and the present-day. The ocean floor in the region formed due to seafloor spreading between the Antarctic, Pacific, Phoenix and Nazca plates (a plate formed by fragmentation of the Farallon plate early in Oligocene times). The Pacific-Antarctic Ridge remained fairly stable throughout this time, migrating relatively northwestwards, by various mechanisms, behind the rapidly-moving Pacific plate. The Nazca and Phoenix plates also moved quickly, but relatively towards the cast or southeast, and were subducted in these directions beneath the South American and Antarctic plates. Segments of spreading centres forming at the trailing edges of the Nazca and Phoenix plates periodically collided with these subduction zones, resulting in the total destruction of the Nazca Phoenix spreading centre and the partial destruction of the Nazca-Antarctica spreading centre (the Chile Ridge) and Antarctic-Phoenix Ridge, which ceased to operate shortly before its northeasternmost three segments could collide with the Antarctic margin. Following collision of segments of the Chile Ridge, parts of the Antarctic plate underwent subduction at the Chile Trench. After these collisions, slab windows Should have formed beneath both the South American and Antarctic convergent margins, and the animation shows Occurrences of alkaline volcanism that have been, or can newly be, related to them. Further occurrences of alkali basalts, at the margins of the Powell Basin and, more speculatively, James Ross Island, can be related to the formation of a slab window beneath them following the collision of segments of the South America Antarctica spreading centre in the northwest Weddell Sea. (C) 2007 Elsevier B.V. All rights reserved.

This chapter explores the Middle Miocene to Pliocene terrestrial and marine records of Antarctica and the Southern Ocean. The structure of the chapter makes a clear distinction between terrestrial and marine records as well as proximal (on or around Antarctica) and more distal records (Southern Ocean). Particular geographical regions are identified that reflect the areas for which the majority of palaeoenvironmental and palaeoclimatic information exist. Specifically, the chapter addresses the terrestrial sedimentary and fjordal environments of the Transantarctic Mountains and Lambert Glacier region, the terrestrial fossil record of Antarctic climate, terrestrial environments of West Antarctica, and the marine records of the East Antarctic Ice Sheet (EAIS), the West Antarctic Ice Sheet (WAIS) and the Antarctic Peninsula Ice Sheet (APIS), as well as the marine record of the Southern Ocean. Previous and current studies focusing on modelling Middle Miocene to Pliocene climate, environments and ice sheets are discussed.

The Belgica Trough and the adjacent Belgica Trough Mouth Fan in the southern Bellingshausen Sea (Pacific sector of the Southern Ocean) mark the location of a major outlet for the West Antarctic Ice Sheet during the Late Quaternary. The drainage basin of an ice stream that advanced through Belgica Trough across the shelf during the last glacial period comprised an area exceeding 200,000 km(2) in the West Antarctic hinterland. Previous studies, mainly based on marine-geophysical data from the continental shelf and slope, focused on the bathymetry and seafloor bedforms, and the reconstruction of associated depositional processes and ice-drainage patterns. In contrast, there was only sparse information from seabed sediments recovered by coring. In this paper, we present lithological and clay mineralogical data of 21 sediment cores collected from the shelf and slope of the southern Bellingshausen Sea. Most cores recovered three lithological units, which can be attributed to facies types deposited under glacial, transitional and seasonally open-marine conditions. The clay mineral assemblages document coinciding changes in provenance. The relationship between the clay mineral assemblages in the subglacial and proglacial sediments on the shelf and the glacial diamictons on the slope confirms that a grounded ice stream advanced through Belgica Trough to the shelf break during the past, thereby depositing detritus eroded in the West Antarctic hinterland as soft till on the shelf and as glaciogenic debris flows on the slope. The thinness of the overlying transitional and seasonally open-marine sediments in the cores suggests that this ice advance occurred during the last glacial period. Clay mineralogical, acoustic sub-bottom and seismic data furthermore demonstrate that the palaeo-ice stream probably reworked old sedimentary strata, including older tills, on the shelf and incorporated this debris into its till bed. The geographical heterogeneity of the clay mineral assemblage

A 330-km length of the little known continental shelf edge and slope of the Bellingshausen Sea, West Antarctica, is investigated using multibeam swath-bathymetric and sub-bottom profiler evidence. The shelf break is at 650-700 m across the 150-km wide Belgica Trough, and to either side is about 500 m. When full-glacial ice advanced across the shelf to reach the shelf break, it was partitioned into fast- and slow-flowing elements, with an ice stream filling the trough. This had important consequences for the nature and rate of sediment delivery to the adjacent continental slope. Off Belgica Trough, the upper continental slope has convex-outward contours indicating a major sedimentary depocentre of gradient 1-2 degrees. Acoustic profiles and cores from the depocentre show a series of diamictic glacigenic debris flows. The depocentre is interpreted as a trough-mouth fan, built largely by debris delivered from the ice stream. The slope is steeper beyond the trough margins at up to 6 degrees. The main morphological features on the Bellingshausen Sea slope are gully systems and channels. Major canyons and Late Quaternary slides are absent. Most gullies and channels are found on the fan. Gullies are about 15-25 m deep, a few hundred metres wide and some are >25 km long. The largest channel is over 60 km long, about a kilometre wide and 10 to 15 m deep. The channels provide pathways for sediment by-passing of the upper slope and transfer to the continental rise and beyond by turbidity currents. Gullies on the Bellingshausen Sea margin cut through debris flows on the slope. Assuming the debris flows are linked mainly to downslope transport of diamictic debris when ice was at the shelf edge under full-glacial conditions, then those gullies cut into them formed during deglaciation. Belgica Fan is >22,000 km(2) in area and about 60,000 km(3) in volume. It is the largest depocentre identified to date on the continental margin of the West Antarctic Ice Sheet, fed by an interior i

The Amundsen Sea continental shelf is one of the remotest areas of coastal Antarctica, and was relatively unexplored until the 1980s. Over the last two decades, however, oceanographic and geological interest has led to several cruises, one result of which is that there is now sufficient bathymetric data to compile a fairly detailed regional map of the Amundsen continental shelf. We have combined the available multibeam and single beam bathymetry data from this region and created a new regional bathymetric map of the Amundsen Sea continental shelf and slope. After editing the individual data sets we used a natural neighbor algorithm to interpolate between the existing data and create a grid at 5km raster resolution. The most prominent regional feature is a series of separate trough systems along the inner shelf, which are aligned with present glaciers, separated by shallower ridges, and shoaling seaward.

Summary Understanding the past glacial history of regions undergoing potential rapid deglaciation is essential in order to estimate the possible threat of sea level rise. Recently acquired data have given new images of mega-scale glacial lineations on the sea floor of the Amundsen Sea, which provide us a new understanding of the direction of glacial flow on the continental shelf of the Amundsen Sea region. Two adjacent areas of seafloor on the outer shelf of the Amundsen Sea embayment exhibit remarkably different styles of glacial lineations, and allow the interpretation of a divergent glacial trough for the Pine Island Glacier during the last glacial maximum, whereas ice flow from the Abbot Ice Shelf probably converged with that from the Pine Island Glacier to the north of a grounding zone wedge.

The Amundsen Sea embayment of West Antarctica is in a prominent location for a series of tectonic and magmatic events from Paleozoic to Cenozoic times. Seismic, magnetic and gravity data from the embayment and Pine Island Bay (PIB) reveal the crustal thickness and some tectonic features. The Moho is 24-22 km deep on the shelf. NE-SW trending magnetic and gravity anomalies and the thin crust indicate a former rift zone that was active during or in the run-up to breakup between Chatham Rise and West Antarctica before or at 90 Ma. NW-SE trending gravity and magnetic anomalies, following a prolongation of Peacock Sound, indicate the extensional southern boundary to the Bellingshausen Plate which was active between 79 and 61 Ma.

[1] Marine geophysical data from the Antarctic continental shelf indicate the former presence of ice streams that drained through bathymetric troughs during the last glacial cycle. Streaming flow is recorded by elongate subglacial bedforms, formed in the upper part of an acoustically transparent sediment layer. Cores from the layer demonstrate that it is a weak and porous subglacial till ("soft till''). Shear within the soft till was concentrated in zones that were 0.1-0.9 m thick. The soft till is a "hybrid'' till, formed by a combination of subglacial sediment deformation and lodgment. The base of the soft till is marked by a strong reflector which represents the top of a denser and stronger "stiff till.'' The form of this reflector ranges from flat and continuous to deeply grooved and irregular, implying the operation of different mechanisms of sediment mobilization and incorporation into the soft till layer. The irregular form of the basal reflector is consistent with grooving, whereas the flat and continuous form is more consistent with sediment mobilization as a subglacial traction carpet related to changes in basal effective stress. Geophysical evidence for large-scale advection of the soft till, combined with evidence for deformation partitioning into relatively thin layers up to 0.9 m thick, indicates that localized shear zones integrated to transport significant volumes of sediment beneath Antarctic paleo-ice streams.

A major glacial trough ("Belgica Trough") on the continental shelf in the southern Bellingshausen Sea acted as an important outlet for ice draining the West Antarctic Ice Sheet during the Last Glacial Maximum (LGM). Mega- scale glacial lineations, drumlins and grounding-zone wedges indicate that Belgica Trough represents the former pathway of a grounded ice stream, which advanced to the shelf break during the LGM and was fed by ice draining through Eltanin Bay and Ronne Entrance. Here we present the preliminary results of sedimentological investigations carried out on 26 sediment cores recovered from the shelf and slope. This unique dataset allows the identification of various facies types that reflect the different phases of grounded ice advance, retreat, and post-glacial onset of seasonal open-water conditions. We will reconstruct the complex processes of erosion, transport and (re-)deposition controlling sedimentation on the margin and the timing of ice-sheet retreat from the shelf.

Marine geoscientists from the British Antarctic Survey BAS) have carried out a wide range of seismic and sonar investigations over the past eighteen years in the sector of the Southern Ocean between 100 °W and 5 "E. Earlier UK seismic investigations in this region were mainly carried out by a research group based at the University of Birmingham, which was absorbed by BAS in mid-1980s.

The onset of independent Bellingshausen plate movement in the South Pacific is estimated at just after chron C33o. This event closely follows a change in the offset sense on the transform arm of the Late Cretaceous Pacific-Phoenix-Antarctic triple junction and accompanies the creation and/or lengthening, by different mechanisms, of long-offsets on the Pacific-Antarctic ridge at the Udintsev, Tharp, Heezen and V transforms, as the triple junction stepped away from the ridge. At around chron C27, the Bellingshausen plate ceased to move independently and the Pacific-Antarctic ridge inherited the set of long offset transforms. Shortly afterwards, the gravity fabric on the flanks of much of the ridge displays evidence for a spreading rate decrease possibly related to transpression on the long-offset transforms. We speculate on a link between these events and regional tectonic changes around chron C27.

The first swath bathymetry and side-scan sonar imagery from the South Sandwich forearc reveals detailed seafloor morphology, tectonic fabric, and sedimentary features in an area where plate convergence is approximately normal to the trench. Simultaneously collected seismic reflection, gravity, and magnetic data provide information about the structure and composition of the forearc crust. Interpretation of these data together with the marine magnetic record of seafloor spreading in the east Scotia Sea back arc basin since 15 Ma has enabled quantitative estimation of sediment subduction and subduction erosion rates. Any accretionary prism present is constrained to be very small, extending 6 km or less from the trench. The contrast between this result and a time-integrated estimate of potential accretionary prism volume suggests that >95% of the sediment which has entered the trench since 15 Ma has been subducted. The position of the South Sandwich island arc in relation to back arc magnetic lineations implies that the arc has migrated ∼70 km westward relative to the Sandwich plate since 15 Ma. Taking account of published observations concerning the typical geometric evolution of arc-trench systems, the average rate of forearc slope retreat during this interval is inferred to be 3.1–4.7 km Myr−1. Gravity modeling suggests that the forearc crust averages ≤10 km in thickness within 110 km of the trench. For 10-km-thick forearc crust the rate of slope retreat implies an average rate of subduction erosion of forearc crust of 31–47 km3 km−1 Myr−1. At the present convergence rate of 74 km Myr−1 this erosion rate requires a 420–635 m thick layer of subducting material derived from the forearc crust. A time-integrated estimate of the potential volume of forearc-derived trench fill sediments subducted since 15 Ma implies that frontal erosion can only account for <40% of total subduction erosion. Therefore basal erosion must significantly exceed frontal erosion. The new data als

We present new Late Cretaceous tectonic reconstructions of the Pacific margin of Antarctica based on constraints from marine magnetic data and regional free-air gravity fields. Results from interpretation of new seismic reflection and gravity profiles collected in the Bellingshausen Sea are also incorporated in the reconstructions. The reconstructions show regional constraints on tectonic evolution of the Bellingshausen and Amundsen Seas following the breakup between New Zealand and West Antarctica. The breakup began at c. 90 Ma with the separation of Chatham Rise, probably accompanied by the opening of the Bounty Trough. Campbell Plateau separated from West Antarctica later, during chron 33r (83.0-79.1 Ma). A free-air gravity lineation northeast of Chatham Rise represents the trace of a triple junction that formed as a result of fragmentation of the Phoenix plate a few million years before Chatham Rise separated from West Antarctica. Remnants of the western fragment, the Charcot plate, are preserved in the Bellingshausen Sea. Subduction of the Charcot plate stopped before 83 Ma, and part of it became coupled to the Antarctic Peninsula across the stalled subduction zone. Subsequent convergence at the western margin of this captured ocean floor produced the structures that are the main cause of the Bellingshausen gravity anomaly. Part of a spreading ridge at the western boundary of the Phoenix plate (initially Charcot-Phoenix, evolving into Marie Byrd Land-Phoenix, and eventually Bellingshausen-Phoenix (BEL-PHO)) probably subducted obliquely beneath the southern Antarctic Peninsula during the Late Cretaceous. All of the Phoenix plate ocean floor subducted at the Antarctic Peninsula margin during the Late Cretaceous was probably <14 Myr old when it reached the trench. Several observations suggest that independent Bellingshausen plate motion began near the end of chron 33n (73.6 Ma). Reconstructions in which part of the West Antarctic continental margin, including Thur

Interpretations of multichannel seismic (MCS) reflection and potential field data suggest that some prominent gravity anomalies in the Bellingshausen Sea are associated with plate boundaries that were active during the Late Cretaceous and early Tertiary. Between 83degrees and 93degreesW, a belt of negative anomalies extends along the West Antarctic continental slope, which we term the continental slope gravity anomaly (CSGA). MCS profiles show that the CSGA coincides with an acoustically opaque structural high imaged beneath the lower slope. We interpret this structure as the upper part of an accretionary prism which formed during southward subduction of the Phoenix and Charcot plates, before Chatham Rise separated from West Antarctica. MCS profiles crossing the same margin to the northeast show no evidence of an extensive buried accretionary prism, but instead reveal an abrupt northeastward steepening of the continental slope near 78degreesW. We attribute this change in tectonic style, at least in part, to subduction erosion resulting from subduction of rough oceanic basement which formed at the Antarctic-Phoenix ridge after an abrupt decrease in spreading rate at chron 23r (52 Ma). Near 95degreesW, the Bellingshausen gravity anomaly (BGA) consists of a prominent low-high gravity couple which crosses the West Antarctic continental shelf, slope, and rise. The BGA corresponds to a buried asymmetric basement trough, where Cretaceous oceanic basement dips beneath more elevated basement to the east. The trough probably formed after subduction of Charcot plate ocean floor stalled at the nearby Antarctic Peninsula margin, near the end of the Cretaceous Normal Superchron. Ocean floor to the east of the BGA became attached to the Antarctic Peninsula, and the BGA trough subsequently accommodated a small amount of convergent motion between the Antarctic Peninsula and the ocean floor to the west (initially part of the Marie Byrd Land plate and later part of the Bellingshausen

On the basis of multichannel seismic reflection, magnetics and bathymetry data, a Pleistocene-Holocene (<0.1Ma) volcanic seamount has been identified at a location very close to the shelf edge off the west coast of the Antarctic Peninsula. The seamount overlies the subducted trace of a transform fracture zone which divides segments of the margin along which collision took place 3.1 and 6.0 Ma ago. New trace element and isotopic data from dredged samples demonstrate that the seamount appears to be predominantly formed of relatively primitive undersaturated alkali basalt. The basalts have geochemical signatures that are indistinguishable from ocean island basalts and some continental alkali basalts, (e.g. Th/Ta 1.0–1.5, Rb/Nb<0.5, 87Sr/86Sr 0.70265–0.7028, 143Nd/144Nd approximately 0.5129) and are totally lacking in any evidence for interaction with subduction-enriched mantle. Geochemical similarities with other, more trench distal, postsubduction alkalic basalts along the Antarctic Peninsula are striking, strongly implying that all the postsubduction basalts were derived from a chemically similar asthenospheric source region. The basalts were most likely to have been generated as a result of the formation of a slab window beneath the Antarctic Peninsula following ridge crest-trench collision. Subduction component-free subslab asthenosphere upwelled into the incipient void left by the continued sinking of the leading plate following collision, and decompressional melting resulted. This type of trench-proximal volcanism following ridge crest-trench collision differs from that in other locations where calc-alkaline volcanism persisted or ophiolite obduction occurred.

The outer shelf on the Antarctic Peninsula Pacific margin south of 63°30′S is underlain by Pliocene-Pleistocene prograding sequences which have been produced mainly by the action of ice sheets grounded out to the shelf edge at times of glacial maximum. Most sediment in these sequences has probably been transported to the margin in a deforming basal till, which implies deposition on a broad front: a “line source”. A representative prograding sequence mapped across an extensive network of multichannel seismic reflection lines has an elongate depocentre on the upper palaeoslope, which is consistent with the grounded ice sheet model. However, it is likely that each sequence recognized on existing multichannel seismic data represents several ice advances. Depth-to-surface maps reveal a broad variation along the margin in the amount of progradation, reflecting differences in sediment supply. The pattern of progradation and the bathymetry of the outer shelf suggest that the main depocentre in the area studied was fed by an ice stream at times of glacial maximum. Seismic lines across the margin farther to the southwest indicate the existence of other depocentres. Several broad depositional lobes have probably coalesced to form the extensive outer shelf. The present continental slope is smooth and steep, and is not cut by major canyons. A downslope change in seismic facies and scouring on the uppermost rise probably reflect downslope transitions from slumps to debris flows to turbidity currents. These processes are likely to be most active at times of glacial maximum. Deep drilling data indicate that the rise sediments consist mainly of terrigenous turbidite and ice-rafted detritus. A marked upward change in seismic facies on the continental rise indicates a change to a higher energy sedimentary regime and appears to correlate with the start of glacial progradation on the shelf.