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The Transantarctic Mountains (TAM) separate the warmer lithosphere of the Cretaceous-Tertiary West Antarctic rift system and the colder and older provinces of East Antarctica. Low velocity zones beneath the TAM imaged in recent seismological studies have been interpreted as warm low-density mantle material, suggesting a strong contribution of thermal support to the uplift of the TAM. We present new Curie Point Depth (CPD) and geothermal heat flow (GHF) maps of the northern TAM and adjacent Wilkes Subglacial Basin (WSB) based exclusively on high resolution magnetic airborne measurements. We find shallow CPD and high GHF beneath the northern TAM, reinforcing the hypothesis of thermal support of the topography of the mountain range. Additionally, this study demonstrates, that limiting spectral analysis to areas with a high density of aeromagnetic measurements increases the resolution of CPD estimates revealing localized shallow CPD and associated high heat flow in the Central Basin of the WSB and the Rennick Graben. Across the study area the CPD ranges from 15 to 35 km and the GHF values range from 30 to 110 mW/m2. The recovered CPD range is compatible with recent Moho depth estimates, as the CPD predominantly lies within the crust, rather than in the magnetite-poor mantle. GHF estimates, based on the CPD estimates, show a good agreement to sparse in situ GHF measurements and the location of active volcanoes. Comparison to existing continent-wide GHF estimates shows strong differences from magnetically-derived heat flow estimates, while seismologically-derived heat flow estimates show the best agreement to our results.

Ice shelves restrict outflow from many of the largest glaciers in Antarctica, thus limiting the Antarctic contribution to sea-level rise. However, past ice-shelf collapse events show they are highly vulnerable to surface and basal melting. Collapse of ice shelves in front of glaciers flowing on retrograde slopes could initiate runaway retreat processes. Difficulty in projecting how quickly these could play out makes dynamic ice loss from Antarctica the largest uncertainty in predicting future sea-level rise. Basal melting can impact structural integrity of ice shelves in several ways. Results from analyses of variations in ice-shelf roughness by Watkins, Bassis, & Thouless (2021; https://doi.org/10.1029/2021GL094743) raise the tantalizing prospect that this may provide a simple quantitative measure of how the structural integrity of an ice shelf has been impacted by basal melting. Applying the method to additional ice shelves would be useful to examine how other factors may contribute to roughness.

A multibarrel seabed drill rig was used for the first time to drill unconsolidated sediments and consolidated sedimentary rocks from an Antarctic shelf with core recoveries between 7% and 76%. We deployed the MARUM-MeBo70 drill device at nine drill sites in the Amundsen Sea Embayment. Three sites were located on the inner shelf of Pine Island Bay from which soft sediments, presumably deposited at high sedimentation rates in isolated small basins, were recovered from drill depths of up to 36 m below seafloor. Six sites were located on the middle shelf of the eastern and western embayment. Drilling at five of these sites recovered consolidated sediments and sedimentary rocks from dipping strata spanning ages from Cretaceous to Miocene. This report describes the initial coring results, the challenges posed by drifting icebergs and sea ice, and technical issues related to deployment of the MeBo70. We also present recommendations for similar future drilling campaigns on polar continental shelves.

Ice shelves are critical features in the debate about West Antarctic ice sheet change and sea level rise, both because they limit ice discharge and because they are sensitive to change in the surrounding ocean. The Pine Island Glacier ice shelf has been thinning rapidly since at least the early 1990s, which has caused its trunk to accelerate and retreat. Although the ice shelf front has remained stable for the past six decades, past periods of ice shelf collapse have been inferred from relict seabed “corrugations” (corrugated ridges), preserved 340 km from the glacier in Pine Island Trough. Here we present high-resolution bathymetry gathered by an autonomous underwater vehicle operating beneath an Antarctic ice shelf, which provides evidence of long-term change in Pine Island Glacier. Corrugations and ploughmarks on a sub-ice shelf ridge that was a former grounding line closely resemble those observed offshore, interpreted previously as the result of iceberg grounding. The same interpretation here would indicate a significantly reduced ice shelf extent within the last 11 kyr, implying Holocene glacier retreat beyond present limits, or a past tidewater glacier regime different from today. The alternative, that corrugations were not formed in open water, would question ice shelf collapse events interpreted from the geological record, revealing detail of another bed-shaping process occurring at glacier margins. We assess hypotheses for corrugation formation and suggest periodic grounding of ice shelf keels during glacier unpinning as a viable origin. This interpretation requires neither loss of the ice shelf nor glacier retreat and is consistent with a “stable” grounding-line configuration throughout the Holocene.

Multibeam swath bathymetry and sub-bottom profiler data are used to establish constraints on the flow and retreat history of a major palaeo-ice stream that carried the combined discharge from the parts of the West Antarctic Ice Sheet now occupied by the Pine Island and Thwaites glacier basins. Sets of highly elongated bedforms show that, at the last glacial maximum, the route of the Pine Island-Thwaites palaeo-ice stream arced north-northeast following a prominent cross-shelf trough. In this area, the grounding line advanced to within similar to 68 km of, and probably reached, the shelf edge. Minimum ice thickness is estimated at 715 m on the outer shelf, and we estimate a minimum ice discharge of similar to 108 km(3) yr(-1) assuming velocities similar to today's Pine Island glacier (similar to 2.5 km yr(-1)). Additional bed forms observed in a trough northwest of Pine Island Bay likely formed via diachronous ice flows across the outer shelf and demonstrate switching ice stream behavior. The "style" of ice retreat is also evident in five grounding zone wedges, which suggest episodic deglaciation characterized by halts in grounding line migration up-trough. Stillstands occurred in association with changes in ice bed gradient, and phases of inferred rapid retreat correlate to higher bed slopes, supporting theoretical studies that show bed geometry as a control on ice margin recession. However, estimates that individual wedges could have formed within several centuries still imply a relatively rapid overall retreat. Our findings show that the ice stream channeled a substantial fraction of West Antarctica's discharge in the past, just as the Pine Island and Thwaites glaciers do today.

Amundsen Sea Embayment: Tectonic and Climatic Evolution; Granada, Spain, 9 September 2009; Geoscientists working on the Amundsen Sea Embayment (ASE) of West Antarctica met at a workshop during the First Antarctic Climate Evolution Symposium to discuss recent advances from, and future priorities for, work in this region. The ASE is the most rapidly changing sector of the West Antarctic Ice Sheet (WAIS) and contains enough ice to raise sea level by 1.2 meters. Ice sheet modeling studies suggest that this sector of the WAIS is potentially unstable. Considerable efforts have been made through several national Antarctic programs to acquire new data on the geological structure, subglacial topography, bathymetry, and glacial history of this remote region. These data are important for establishing boundary conditions for ice sheet modeling, for providing constraints on past ice sheet changes that can be used to test models, and for putting recent changes into a longer-term context.

Decades after the recognition of the West Antarctic Rift System, and in spite of its global importance, the location and nature of the plate boundary it formed at are unknown east of the Byrd Subglacial Basin. Alternative constructions of the circuit of South Pacific plate boundaries suggest the presence of either a transcurrent plate boundary or a continuation of the extensional rift system. We identify George VI Sound, a curved depression separating Alexander Island from Palmer Land, as the easternmost basin of a rift system that terminated at a triple junction with the Antarctic Peninsula subduction zone. The history of the triple junction's third, transform, arm suggests extension started around 33.5-30 Ma. A more speculatively identified basin further west may have formed earlier during the same episode of rifting, starting around 43 Ma. Proposals of earlier Cenozoic relative motion between East and West Antarctica cannot be verified from this region. Citation: Eagles, G., R. D. Larter, K. Gohl, and A. P. M. Vaughan (2009), West Antarctic Rift System in the Antarctic Peninsula, Geophys. Res. Lett., 36, L21305, doi: 10.1029/2009GL040721.

[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.

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.

An animated reconstruction shows South Pacific plate kinematics between 90 and 45 Ma, using the satellite-derived gravity anomaly field, interpolated isochrons and plate rotation parameters from both published and new work on marine geophysical data. The Great South Basin and Bounty Trough, New Zealand, are shown as the earliest Pacific-Antarctic plate boundary that opened before 83 Ma. The earliest true Pacific-Antarctic seafloor formed within the eastern parts of this boundary, but later and farther west, seafloor formed within its Antarctic flank. After 80 Ma, the Bellingshausen plate converged with an oceanic part of the Antarctic plate to its east, while its motion simultaneously caused rifting in continental Antarctica to the south. The Pacific-Bellingshausen spreading center developed a set of long offset transform faults that the Pacific-Antarctic plate boundary inherited around chron C27 when the Bellingshausen plate ceased to move independently as part of a Pacific-wide plate tectonic reorganization event. Southwest of these transforms the Pacific-Antarctic Ridge saw an increase in transform-fault segmentation by similar to58 Ma. One of the long offset Pacific-Bellingshausen transforms, referred to as "V,'' was modified during the C27 reorganization event when a Pacific-Antarctic-Phoenix triple junction initiated on its southern edge. Eastern parts of "V'' started to operate in the Pacific-Phoenix spreading system, lengthening it even more, while its western parts operated in the Pacific-Antarctic system. This complicated feature was by-passed and deactivated by ridge axis propagation to its northwest at similar to47 Ma. We interpret our animation to highlight possible connections between these events.

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.

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