Alex P. Cunningham

Tectonic evolution of the Pacific margin of Antarctica - 1. Late Cretaceous tectonic reconstructions

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 Thurston Island, is assumed to have been within the Bellingshausen plate seem more plausible than ones in which the plate is assumed to have been entirely oceanic.

The depositional pattern and distribution of glacial-interglacial sequences on the Antarctic Peninsula Pacific margin

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.

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.

Geometry and development of glacial continental margin depositional systems in the Bellingshausen Sea

We present multi-channel seismic (MCS) reflection profiles and bathymetry data acquired across a remote and poorly surveyed part of the Antarctic continental margin in the Bellingshausen and Amundsen Seas. This new information has been combined with published data and used to interpret the style of sedimentation on the continental shelf, slope and rise, and to describe sedimentation processes which have been active in this region. Most seismic reflection profiles crossing the continental margin show prograded sequences beneath the outer shelf and upper slope, and we infer that the stratal characteristics of these sequences indicate that grounded ice sheets reached the shelf edge during previous glacial times. Although there are general similarities in stratal geometry on these profiles, in detail, they reveal significant longitudinal variations in sediment input from the shelf to the upper slope. On several profiles, we found evidence of mass wasting of the continental slope in the form of slump and debris flow deposits. At greater depth, turbidity flows, bottom currents and Coriolis force have controlled the further transportation and deposition of sediment, which has resulted in the development of mounds, channels and sediment wave fields. The distribution, and variations in the size and geometry of the mounds reflect sediment input and the relative contribution of these other factors which control sedimentation on the continental rise.

Tectonics and sedimentary environment of the North Scotia Ridge region revealed by side-scan sonar

The North Scotia Ridge is a series of islands and submarine ridges extending 2000 km from Tierra del Fuego to South Georgia in the western South Atlantic. The ridge forms the elevated northern tectonic margin of the Scotia Sea, and accommodates E–W sinistral strike-slip motion at the South American–Scotia plate boundary. Existing studies have shown that the northern flank of the North Scotia Ridge is a large and continuous accretionary prism, formed during presumed mid–late Cenozoic N–S convergence. In this study, we present long-range side-scan sonar (GLORIA) images and seismic reflection profiles which show the structural style of the accretionary prism for the first time. The youngest accreted sediments show a uniform fabric of initial deformation (symmetric–gently asymmetric folds of 1–4 km wavelength), which has been subsequently disrupted at shallower depths by additional shortening and uplift. Between 52º45W and 50º30W, the deformation front is exposed at the sea floor, and the Falkland Trough retains the appearance of an active convergent margin. Elsewhere, however, the deformation front is buried beneath younger, undeformed drift sediments indicating that convergence has ceased. GLORIA sonographs also show geological features consistent with current-control of sedimentation, non- deposition, and erosion beneath the Antarctic Circumpolar Current. In particular, this study describes current-influenced sedimentation in the Falkland Trough, and steep-sided, eroded depressions and diffuse slope-parallel fabric on the elevated Falkland Plateau.

Evidence for westward-flowing Weddell Sea Deep Water in the Falkland Trough, western South Atlantic

The North Scotia Ridge controls the eastward and northward flow of the Antarctic Circumpolar Current (ACC) emerging from Drake Passage. Existing physical oceanographic data in this region are sparse and do not define the flow pattern of Circumpolar Deep Water (CDW) within the ACC, or of Weddell Sea Deep Water (WSDW) heading northward beneath it, in the region of the North Scotia Ridge and Falkland Trough. 3.5-kHz reflection profiles show mudwaves at the surface of a sediment drift along the axis of the eastern Falkland Trough that have a consistent NE-SW alignment and are migrating SE, indicating persistent westward bottom-current flow along the trough axis. Sediment thinning and non-deposition at the southern drift margin indicate intensified westward flow, considered to be Weddell Sea Deep Water from the Malvinas Outer Basin to the east. This flow probably continues to 48°W, but beyond there its fate is unknown. Similar non-deposition along the northern margin of the drift is considered to result from intensified eastward return flow of WSDW, or from CDW. The mudwave geometry appears to extend to at least 400-m depth within the drift, which therefore most probably contains a record of southern-origin bottom water (presently WSDW) extending back for several million years.

Tectonic evolution of the Pacific margin of Antarctica 2. Structure of Late Cretaceous–early Tertiary plate boundaries in the Bellingshausen Sea from seismic reflection and gravity data

[1]xa0Interpretations 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 83° and 93°W, 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 78°W. 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 95°W, 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 plate). Tectonism probably ceased at the BGA at chron 27 (61 Ma), as a result of a general plate reorganization in the South Pacific.

PLIOCENE-HOLOCENE CONTOURITE DEPOSITION UNDER THE ANTARCTIC CIRCUMPOLAR CURRENT, WESTERN FALKLAND TROUGH, SOUTH ATLANTIC OCEAN

The eastward-flowing Antarctic Circumpolar Current (ACC) has influenced sedimentation on the slope and floor of the western Falkland Trough, where the axis of the current is topographically constrained. Deep-water flow (below 3000 m) has produced a symmetrical sediment drift on the trough floor, with non-depositional margins indicating higher current velocities at the base of slope. To the southeast of the Falkland Islands there is a gap in the North Scotia Ridge, north of which the floor of the trough is swept clean of sediment by the ACC. Both echo character mapping and GLORIA side-scan data indicate that currents follow the bathymetric contours along the slope, redistributing sediment and locally eroding furrows. From six cores on the drift and on the northern slope, two styles of contourite deposition have been identified. On the drift, Holocene biogenic sandy contourites overlie Last Glacial Maximum muddy contourites and fine-grained diatomaceous hemipelagites. Sedimentation rates here average 3–4 cm ka−1. The sandy contourites present in four of the cores from the sediment drift are sharply underlain by the finer-grained, diatomaceous hemipelagites. The lack of a coarsening upward sequence, commonly associated with an increase in current velocity may be indicative of high current activity eroding away the finer (negative) sequence. Pliocene and Mid-Pleistocene glaucony-rich sandy contourites containing radiolaria characterise the Falkland Plateau and the floor of the trough near the gap in the North Scotia Ridge. We suggest that the glaucony is derived from a combination of authigenic formation and erosion of locally outcropping Cretaceous and Tertiary strata; this is supported by dinoflagellate analysis. Sedimentation rates in these current-swept areas average < 1 cm ka−1.

Contourite sedimentation in the Falkland Trough, western South Atlantic

Abstract The Falkland Trough is a west-east bathymetric deep that separates the Falkland Plateau from the North Scotia Ridge in the western South Atlantic. It lies in the path of Circumpolar Deep Water flowing within the Antarctic Circumpolar Current (ACC), and Weddell Sea Deep Water flowing beneath the ACC east of Shag Rocks passage. Marine geophysical and sediment core data demonstrate the influence of ambient bottom currents on deposition in this area, and reveal two styles of contourite sedimentation: (1) deposition of glauconite-rich sandy contourites in exposed areas of the Falkland Plateau and Falkland Trough, where vigorous ACC bottom currents control sedimentation, and (2) deposition of biogenic sandy contourites, muddy contourites and hemipelagites (western Falkland Trough), and muddy diatom ooze (eastern Falkland Trough), in the form of two elongate sediment drifts, which have developed in the presence of more sluggish bottom currents. The drift sediments contain a depositional record of bottom current flow through the glacial cycle (southern-origin bottom water flow in the east, and probably ACC flow in the west); analyses of core data from the western Falkland Trough suggest a reduction in bottom current strength during the Last Glacial Maximum at present depths of > 2500 m below sea level.