ALBERT

Description: An understanding of the glacial history of the Amundsen Sea Embayment (ASE) and Pine Island Bay (PIB) is essential for proposing models on the future development of the West Antarctic Ice Sheet. This requires an understanding of the tectonic history and knowledge of tectonic features such as lineaments, ridges, sills and basins, because basement morphology and inherited erosional features control the flow direction of ice-sheets and the influx of Circum-Polar Deep Water (CDW). This is an attempt to reconstruct the tectonic history with the aim to search for basement features and crustal boundaries which may be correlated to the flow and dynamics of the ice-sheet. 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 PIB reveal the crustal thickness and significant tectonic features. NE-SW trending magnetic and gravity anomalies and the thin crust indicate a former rift zone which was active during or in the run-up to the breakup process 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. It is likely that the prominent Pine Island Trough follows a structural boundary between the crustal blocks of Ellsworth Land and Marie Byrd Land. Data are shown from the ASE and PIB which can be interpreted in context with the reconstruction of the ice advance and retreat history in this area. Differences in the behaviour of the ice-sheet are shown to exist for the western and eastern parts of PIB due to basement structures affecting the inflow of CDW.

Description: The West Antarctic Rift System (WARS) is one of the largest continental rifts globally, but its lateral extent, distribution of local rifts, timing of rifting phases, and mantle processes are still largely enigmatic. It has been presumed that the rift and its crustal extensional processes have widely controlled the history and development of West Antarctic glaciation with an ice sheet of which most is presently based at sub-marine level and which is, therefore, likely to be highly sensitive to ocean warming. While the western domain of the WARS in the Ross Sea has been studied in some detail, only recently have various geophysical and geochemical/thermochronological analyses revealed indications for its eastern extent in the Amundsen Sea and Bellingshausen Sea sectors of the South Pacific realm. The current model, based on these studies and additional data, suggests that the WARS activity included tectonic translateral, transtensional and extensional processes from the Amundsen Sea Embayment to the Bellingshausen Sea region of the southern Antarctic Peninsula. We present the range of existing hypotheses regarding the extent of the eastern WARS as well as published and yet unpublished data that support a conceptual WARS model for the eastern West Antarctica with implications for glacial onset and developments.

Description: Vor 90 Millionen Jahren brachen das Marie Byrd Land und Neuseeland auseinander und bildeten zwei völlig verschiedene, konjugierend liegende, geriftete Kontinentalränder, welche sich in höchstem Maße voneinander unterscheiden. Auf neuseeländischer Seite entstand das submarine Chatham Rise und das Campbell Plateau während sich auf antarktischer Seite ein kontinentales Hochland ausbildete. Zwei Schiffsexpeditionen liefern erstmals geophysikalische Daten vom westantarktischen Kontinentalrand, welche größtenteils aus reflexionsseismischen, gravimetrischen und aeromagnetischen Daten bestehen und einen völlig neuen Einblick in den Krustenaufbau und die tektonisch-geodynamische Entwicklung dieser Region gewähren. Diese Daten liefern sowohl Hinweise auf mehrere Rift- und Driftphasen als auch Spuren magmatischer Ereignisse nach dem Aufbruch. Das Hochland des zentralen Marie Byrd Landes liegt 3 km hoch, ist von einen schmalen Schelf vorgelagert und liegt konjugierend zum Chatham Rise, während das ebenfalls submarine Amundsen Sea Embayment konjugierend zum Campbell Plateau liegt und durch einen bis zu 450 km breiten Schelf gekennzeichnet ist. Die Basementstruktur dieser Region deutet auf multiple Riftphasen und demnach auf eine komplexe tektonische Geschichte hin. Basierend auf Resultaten aus der Reflexions- und Refraktionsseismik liefern verschiedene gravimetrische Modellierungen im Marie Byrd Land eine Kruste von 24-28 km Mächtigkeit welche von einem Mantel abnormal niedriger Dichte unterlegt ist. Diese Resultate sind konsistent mit Ergebnissen bereits publizierter seismologisch-tomographischer Modelle welche in diesem Bereich einen Mantel niedriger seismischer Geschwindigkeit vorhersagen. Dieser Beitrag liefert neue Erkenntnisse welche die Existenz eines Mantel Plumes unter dem Marie Byrd Land forciert, welcher für dessen Hebung verantwortlich gemacht wird, sowie Modelle zur lithosphärischen Entwicklung des westantarktischen Kontinentalrandes.

Description: Studies of the sedimentary architecture and characteristics of the Antarctic continental margin provide clues about past ice sheet advance-retreat cycles and help improve constraints for paleo-ice dynamic models since early glacial periods. A first seismostratigraphic analysis of the Amundsen Sea Embayment shelf and slope of West Antarctica reveals insights into the structural architecture of the continental margin and shows stages of sediment deposition, erosion and transport reflecting the history from pre-glacial times to early glaciation and to the late Pleistocene glacial-interglacial cycles. The shelf geometry consists of a large pre- and syn-rift basin in the middle shelf region between basement cropping out on the inner shelf and buried basement ridge and highs on the outer shelf. A subordinate basin within the large basin on the mid-shelf may be associated with motion along an early West Antarctic Rift System branch. At least 4 km of pre-glacial strata have been eroded from the present inner shelf and coastal hinterland by glacial processes. Six major sedimentary units (ASS-1 to ASS-6) separated by five major erosional unconformities (ASS-u1 to ASS-u5) are distinguished from bottom to top. Unconformity ASS-u4 results from a major truncational event by glacial advance to the middle and outer shelf, which was followed by several episodes of glacial advance and retreat as observed from smaller-scale truncational unconformities within the units above ASS-u4. Some of the eroded sediments were deposited as a progradional wedge that extends the outer shelf by 25 to 65 km oceanward of the pre-glacial shelf-break. We compare the observed seismic characteristics with those of other Antarctic shelf sequences and assign an Early Cretaceous age to bottom sedimentary unit ASS-1, a Late Cretaceous to Oligocene age to unit ASS-2, an Early to Mid-Miocene age to unit ASS-3, a Mid-Miocene age to unit ASS-4, a Late Miocene to Early Pliocene age to unit ASS-5, and a Pliocene to Pleistocene age to the top unit ASS-6. Buried grounding zone wedges in the upper part of unit ASS-5 on the outer shelf suggest pronounced warming phases and ice sheet retreats during the early Pliocene as observed for the Ross Sea shelf and predicted by paleo-ice sheet models. Our data also reveal that on the middle and outer shelf the flow-path of the Pine Island-Thwaites paleo-ice stream system has remained stationary in the central Pine Island Trough since the earliest glacial advances, which is different from the Ross Sea shelf where glacial troughs shifted more dynamically. This study and its stratigraphic constraints will serve as a basis for future drilling operations required for an improved understanding of processes and mechanisms leading to change in the West Antarctic Ice Sheet, such as the contemporary thinning and grounding line retreat in the Amundsen Sea drainage sector.

Description: The Amundsen Sea Embayment of West Antarctica is a centrepiece in understanding the history of the New Zealand – Antarctica breakup. This region plays a key role in plate kinematic reconstruction of the southern Pacific from the collision of the Hikurangi Plateau with the Gondwana subduction margin to the evolution of the West Antarctic Rift System. During two RV Polarstern cruises in 2006 and 2010, a large geophysical dataset was collected consisting of seismic refraction and reflection profiles, shipborne gravity and helicopter magnetic measurements. The data provide constraints on the crustal architecture, the structural evolution and the tectonic block formation during and after the Cretaceous continental breakup. We present two continental rise-to-shelf P-wave velocity models which were derived from forward travel-time modelling of ocean bottom hydrophone recordings which provide an insight into the crustal and upper mantle architecture beneath the Amundsen Sea Embayment for the first time. The sedimentary sequences and the basement were constrained by seismic reflection data. A 2-D density-depth model supports and complements the P-wave modelling. Observed P-wave velocities show 10 to 14 km thick crust of the continental rise and up to 28 km thick crust beneath the middle and inner shelf. The crust of the continental rise is characterized by a small gradient in thickness. Including horst and graben structures this can be associated with wide-mode rifting. A high velocity zone with velocities ranging between 7.1 and 7.6 km/s indicate magmatic underplating of variable thickness along the entire transect. We classify this margin as one of volcanic type rather than magma poor because of the high-velocity zone and seaward dipping reflectors observed from the seismic reflection data. We discuss the possibility of a serpentinized upper mantle caused by seawater penetration at the Marie Byrd Seamounts. The crustal structure, distinct zones in potential field anomalies indicate several phases of fully developed and failed rift systems and a possible branch of the West Antarctic Rift System in the Amundsen Sea Embayment.

Description: The Amundsen Sea Embayment of West Antarctica represents a key component in the tectonic history of Antarctic-New Zealand continental breakup. The region played a major role in the plate-kinematic development of the southern Pacific from the inferred collision of the Hikurangi Plateau with the Gondwana subduction margin at approximately 110-100 Ma to the evolution of the West Antarctic Rift System. However, little is known about the crustal architecture and the tectonic processes creating the embayment. During two RV Polarstern expeditions in 2006 and 2010 a large geophysical dataset was collected consisting of seismic-refraction and reflection data, ship-borne gravity and helicopter-borne magnetic measurements. Two P-wave velocity-depth models based on forward travel-time modelling of nine ocean bottom hydrophone recordings provide an insight into the lithospheric structure beneath the Amundsen Sea Embayment. Seismic-reflection data image the sedimentary architecture and the top-of-basement. The seismic data provide constraints for 2-D gravity modelling, which supports and complements P-wave modelling. Our final model shows 10 - 14 km thick stretched continental crust at the continental rise that thickens to as much as 28 km beneath the inner shelf. The homogenous crustal architecture of the continental rise, including horst and graben structures are interpreted as indicating that wide-mode rifting affected the entire region. We observe a high-velocity layer of variable thickness beneath the margin and related it, contrary to other “normal volcanic type margins”, to a proposed magma flow along the base of the crust from beneath eastern Marie Byrd Land – West Antarctica to the Marie Byrd Seamount province. Furthermore, we discuss the possibility of upper mantle serpentinization by seawater penetration at the Marie Byrd Seamount province. Hints of seaward-dipping reflectors indicate some degree of volcanism in the area after break-up. A set of gravity anomaly data indicate several phases of fully developed and failed rift systems, including a possible branch of the West Antarctic Rift System in the Amundsen Sea Embayment.