Peter F. Barker

The opening of Drake Passage

Abstract Marine magnetic anomalies are used to deduce the history of the opening of Drake Passage, the deep-water channel between South America and West Antarctica. Coherent spreading started in Drake Passage at anomaly 8 time (about 29 Ma), after an earlier, chaotic episode. In the early stages of opening, overlap of two shallow ridges at the Shackleton Fracture Zone confined the submarine connection between Pacific and Atlantic to shelf depths, but the cross-section grew rapidly after the ridge ends cleared at about 23.5 Ma. Reflection profiles from Drake Passage are used to support the existence of these ridges at that time. By comparison with other regions around Antarctica it is shown that the final barrier in an otherwise-complete deep circumpolar path lay at the Shackleton Fracture Zone; subsequently, the gap of smallest cross-section lay either always at the Shackleton Fracture Zone or possibly, for part of the time, in the developing East Scotia Sea. It is concluded that the Antarctic Circumpolar Current started at 23.5 ± 2.5 Ma, a time indistinguishable from the Oligocene—Miocene boundary.

The Cenozoic subduction history of the Pacific margin of the Antarctic Peninsula: ridge crest–trench interactions

New magnetic anomaly identifications W of the Shackleton Fracture Zone show 5 spreading sections, separated by fracture zones. In the 2 most southerly, the ridge crest collided with a trench at the margin of the Antarctic Peninsula only 6.5 and 4 Ma ago, the latest of a series of collisions starting at the base of the peninsula SO Ma ago. Following each collision, spreading and subduction both stopped. Opposite the South Shetland trench and actively extending Bransfield Strait, spreading also stopped 4 Ma ago, but before the spreading centre reached the trench. The tendency of the subducting plate to continue sinking probably initiated the opening of the Bransfield Strait. The parallelism between fracture zone orientation on the descending plate and convergence direction, previously thought responsible for the tectonic segmentation of the peninsula, was effective for only 10–30 Ma before collision. Before that, either the fracture zones did not extend to the trench, or the subducting ocean floor formed at the Farallon–Phoenix (Nazca–Aluk) boundary, with fracture zones oblique to the subduction direction. The Aleutian Arc–Kula Ridge model, in which arc magmatism virtually ceases when ocean floor younger than 25–30 Ma occurs beneath the arc, fits the distribution of Antarctic Peninsula radiometric dates, explaining a 50–60 Ma gap between collisions and youngest ages in the S, and possibly the migration of the youngest activity towards the trench. Thus, gaps in the geologic record of arc magmatism need not imply cessation of subduction. The progressive steepening of the peninsular margin towards the 4 Ma collision site suggests tectonic erosion of the fore-arc as the ridge crest approached.

Scotia Sea regional tectonic evolution: implications for mantle flow and palaeocirculation

Abstract The Scotia Sea and surrounding Scotia Arc have evolved over the past 40 Ma, by extension behind an east-migrating subduction zone, at the boundary between the South American (SAM) and Antarctic (ANT) plates. The considerable data set now available (regional geology and geophysics, earthquake seismology, satellite altimetry, global plate analyses) suggest why east-migrating subduction began, what has been the driving force that has sustained it, and what other processes have controlled the mode of back-arc extension in the Scotia Sea. A suite of six reconstructions has been developed, based on this data set. The reconstruction to 40 Ma creates a compact, cuspate continental connection between South America and the Antarctic Peninsula at the subducting Pacific margin, with fragments (now dispersed around the Scotia Arc) occupying positions within it compatible with their known geology. The driving force has been subduction of South American ocean floor, which began as a result of southward migration of the pole of South American–Antarctic plate rotation, and a key modulator of back-arc extension has been collision of ridge crest sections of the South American–Antarctic plate boundary with the east-advancing trench. Cenozoic regional tectonic evolution has two other likely consequences which greatly increase its importance. Firstly, this region saw the tectonic disruption of the final barrier to complete circum-Antarctic deep water flow, that may have had a profound effect on palaeoclimate. Secondly, it is possible that the rapid roll-back of the hinge of subduction is related to shallow eastward flow in the sub-lithospheric mantle. Both of these consequences are explored. The reconstructions show that rapid roll-back of the subduction hinge (averaging 50 mm/a over the last 40 Ma with respect to the South American plate) has been a feature of all of Scotia Sea evolution, and provide a history of motion of several oceanic microplates, most of which are now welded together within the Scotia Sea. This will guide the location of seismometers and/or dredge hauls to test the hypothesis of shallow mantle flow, and help interpret the results. The reconstructions also allow an assessment of the creation of deep-water pathways that would have permitted the development of the present-day Antarctic Circumpolar Current (ACC). An early Miocene onset (within the period 22–17 Ma) seems likely for the ACC, depending on the structure and palaeo-elevation of Davis Bank and Aurora Bank, sections of the North Scotia Ridge. However, the study shows there was a delay (of one or more million years) between initial provision of a deep-water pathway and the major mid-Miocene change in global climate (involving the general level of Antarctic glaciation) that may have been related. If these changes were related, then the delay suggests that other factors, possibly rough elevated ocean floor but also non-tectonic factors (such as atmospheric CO2), were important in determining palaeoclimate.

Effects of ridge crest-trench interaction on Antarctic-Phoenix Spreading: Forces on a young subducting plate

Precise measurements of spreading rates on marine magnetic profiles collected to the west of the Antarctic Peninsula have enabled some consideration of the forces governing plate motion, since Antarctic—Phoenix motion has been controlled by the local rather than global force balance over the last 35 m.y. The total effective driving force per unit length of trench is calculated to have ranged between 2.6 and 3.6×1012 N/m, which is much less than is commonly thought necessary to support subduction. Conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench. The low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed. Driving forces show a strong correlation with observed spreading rates, which indicates that resistive forces were largely velocity dependent. Fluid migration up the subduction zone may elevate temperatures in and around the shear zone, reducing resistive forces below the levels required by purely conductive models. Changes in convergence rate may affect the depths of both the brittle/ductile deformation boundary and the basalt/eclogite phase change, causing a negative feedback which would appear as a velocity-dependent resistive force. The different driving forces acting on the NE and SW parts of the Phoenix plate, as a consequence of older oceanic lithosphere at the trench in the NE, caused Antarctic–Phoenix spreading to take place about a near pole to the SW since 21 m.y. ago, and ultimately resulted in disruption of the Phoenix plate about 9 m.y. ago. Spreading rates decreased abruptly about 6 m.y. ago, probably because of E-W compression across the long transform faults bounding the Phoenix plate. However, spreading on the last three segments of the Antarctic–Phoenix Ridge continued at least until 4 m.y. ago. Either spreading stopped progressively from SW to NE, or the final stage took place about a very near pole to the SW. A magnetic quiet zone extends up to 95 km from the margin between the Tula Fracture Zone and the North Anvers Fracture Zone, and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision. The absence of a magnetic quiet zone associated with the most recent ridge crest–trench collisions suggests a change in sedimentary regime during the late Miocene. Anomalously fast apparent spreading rates between 23 and 21 m.y. ago are thought to indicate an error in this part of the magnetic reversal time scale.

Ice sheet retreat from the Antarctic Peninsula shelf

Side-scan sonar and sub-bottom acoustic profiler data and sediment cores reveal the processes that controlled sediment transport and deposition on the continental shelf of the Antarctic Peninsula Pacific margin off Anvers Island, during deglaciation over the last 11,000 years or more. Glacial flutes and striations mark the flow of low-profile ice streams draining the interior, across the middle and outer shelf. Most probably, ice sheets were grounded to the continental shelf edge along this margin during the last glacial maximum. Iceberg furrows overwrite the ice sheet record in areas between 500 and 350 m water depth, and reflect calving from a retreating ice shelf front. Cores show open marine sedimentation replacing diamicton deposition close to the grounding line during this retreat, which rapidly cleared the outer and middle shelf shortly before 11,000 years BP (from AMS14C dates on organic carbon). The shallower, scoured and largely sediment-free inner shelf cleared later, probably before 6000 years BP. Open marine sediments on the middle and outer shelf include a pelagic biogenic component and suspended sediment from modern glacier tongues, supplemented by resuspension of older sediment in shallow shelf regions (by currents and by grounded icebergs). Sedimentation is too slow to be able to fill in the concave-up profile of the continental shelf during a full interglacial, confirming the intense glacial-interglacial cyclicity of sedimentation on the continental slope inferred from seismic reflection profiles. The observed rapid deglaciation of the middle and outer shelf supports published numerical model results that the Antarctic Peninsulas narrow interior and broad continental shelf make the ice sheet sensitive to imposed eustatic sea-level change. A low-profile marine-based ice sheet over the continental shelf during glacial maximum would have made a major contribution to that sensitivity, in the early stages of deglaciation. It follows that the Antarctic Peninsula ice sheet, and probably most others, are not so sensitive today.

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.

Seismic stratigraphy of the Antarctic Peninsula Pacific margin: A record of Pliocene-Pleistocene ice volume and paleoclimate

Multichannel seismic profiles across the Pacific margin of the Antarctic Peninsula show a series of oblique progradational sequences. These sequences exhibit a variety of unusual characteristics that suggest they were produced by the action of ice sheets grounded out to the shelf edge at times of glacial maximum. Reflection events from deeper stratigraphic levels, followed down the continental slope and onto the rise, overlie ocean crust of known age, showing that at least eight such glacial sequences have been deposited within the past 6 m.y. Similar groundings have probably occurred on most Antarctic margins, but the depositional record is particularly well preserved at this margin because of Pliocene-Pleistocene thermal subsidence. Neogene global sea-level fluctuations have been attributed to changes in volume of continental ice sheets. The depositional sequences on the Pacific margin of the Antarctic Peninsula are thought to record West Antarctic ice-sheet fluctuations directly. Further investigation of these sequences would assess the relation between fluctuations in ice volume and the low-latitude record of global sea-level change.

Giant sediment drifts on the continental rise west of the Antarctic Peninsula

Multichannel seismic reflection profiles from the continental rise west of the Antarctic Peninsula between 63° and 69°S show the growth of eight very large mound-shaped sedimentary bodies. MCS profiles and long-range side-scan sonar (GLORIA) images show the sea floor between mounds is traversed by channels originating in a dendritic pattern near the base of the continental slope. The mounds are interpreted as sediment drifts, constructed mainly from the fine-grained components of turbidity currents originating on the continental slope, entrained in a nepheloid layer within the ambient southwesterly bottom currents and redeposited downcurrent.

The Evolution of the Scotia Ridge and Scotia Sea

Marine geophysical surveys over the Scotia Ridge show it to be composed of blocks mainly of continental origin. Major structures found on the blocks are in many cases truncated at block margins and their existence is also inconsistent with the present isolated situation of the blocks. The evidence suggests post-Upper Cretaceous fragmentation of a continuous continental area. Complementary marine geomagnetic studies over the deep water of the Scotia Sea have dated two areas as younger than 22 million years (Ma) and have indicated the direction of spreading in others. A model of present plate motions, based on the magnetic anomalies, explains the active volcanism of the South Sandwich Islands as being caused by consumption of Atlantic crust at the associated trench at a rate of 5.5 cm/year for the past 7 to 8 Ma at least. An Upper Tertiary episode of plate consumption at 5 cm/year at the South Shetland trench, suggested by the magnetic lineations, with a secondary slow extensional widening of Bransfield Strait is used to explain similarly the contemporaneous volcanism of the South Shetland Is. Making the reasonable assumption of a Tertiary formation of the undated parts of the Scotia Sea by spreading in the directions indicated by the magnetic lineations, a tentative reconstruction of the component blocks of the Scotia Ridge is made. The attempt is only partly successful in matching structural patterns across adjacent margins of reconstructed blocks, South Georgia being most obviously wrongly situated. It is suggested that the misfits result from minor errors in the initial assumptions and the modification of structures during fragmentation and drift. South Georgia may have formed on the Atlantic rather than the Pacific side of the compact continental region which is thought to have joined South America and west Antarctica for much of the Mesozoic at least. A Gondwanaland reconstruction is presented which is consistent with the Scotia Ridge reconstruction, in which the Antarctic Peninsula lies alongside the Caird Coast of east Antarctica. Upon break-up of Gondwanaland, the Antarctic Peninsula remained rigidly attached to South America, east Antarctica rotating clockwise to open the Weddell Sea, until early Tertiary times when the Peninsula transferred to east Antarctica which continued rotating clockwise to open the Scotia Sea.

Back-Arc Extension in the Scotia Sea [and Discussion]

The nature of back-arc extension in the East Scotia Sea is re-examined with the use of an enlarged geophysical data set. Well developed oceanic magnetic lineations confirm that the present spreading episode started about 8 Ma ago, that spreading is asymmetric, and that the total rate increased from 50 to 70 m m /a about 1.5 Ma ago. Most of the currently active South Sandwich volcanic island arc lies upon ocean floor only 6-8 M a old and generated at the current spreading ridge. Subsequent extension has not modified the curvature of the arc. East-west magnetic lineations of Miocene age in the Central Scotia Sea and contemporaneous low-K arc tholeiites dredged from the eastern South Scotia Ridge (Discovery Bank) indicate a regime of coupled subduction and back-arc extension preceding that occurring now. A speculative model involving a series of collisions of parts of this earlier Discovery trench with ridge crest sections of the South American-Antarctic plate boundary explains the transformation of this earlier regime into the present, self-contained Sandwich plate regime. The considerable small-scale variability observed in the back-arc region may be seen as an inevitable consequence of the action of the ridge-trench collision mechanism. The entire Scotia Sea could have formed by a similar kind of back-arc extensional modification of the South American-Antarctic plate boundary.