The Agulhas Ridge, South Atlantic: the peculiar structure of a fracture zone

The Agulhas Ridge is a prominent topographic feature that parallels the Agulhas-Falkland Fracture Zone (AFFZ). Seismic reflection and wide angle/refraction data have led to the classification of this feature as a transverse ridge. Changes in spreading rate and direction associated with ridge jumps, combined with asymmetric spreading within the Agulhas Basin, modified the stress field across the fracture zone. Moreover, passing the Agulhas Ridges location between 80 Ma and 69 Ma, the Bouvet and Shona Hotspots may have supplied excess material to this part of the AFFZ thus altering the ridges structure.The low crustal velocities and overthickened crust of the northern Agulhas Ridge segment indicate a possible continental affinity that suggests it may be formed by a small continental sliver, which was severed off the Maurice Ewing Bank during the opening of the South Atlantic.In early Oligocene times the Agulhas Ridge was tectono-magmatically reactivated, as documented by the presence of basement highs disturbing and disrupting the sedimentary column in the Cape Basin. We consider the Discovery Hotspot, which distributes plume material southwards across the AAFZ, as a source for the magmatic material

Seismic wide-angle study of accreted Proterozoic crust in southeastern Wyoming

A seismic wide-angle xperiment was conducted in southeastern Wyoming, USA to investigate the seismic character of a postulated Proterozoic magmatic arc south of the suture (Cheyenne Belt) to the Archean Wyoming Province. Recordings from vibrator and dynamite sources with offsets between 34 and 126 km reveal no evidence for Moho reflections. The large-offset recordings contain multicyclic bands of reflective phases from the middle to lower crust. The data were transformed into the intercept ime-ray parameter (~--p) domain to estimate local depth bounds. A subsequent 1D inversion using high-amplitude ~'-p arrivals shows that the reflective part of the crust ranges from the depths of 25 to 40 km. This part of the crust exhibits velocities increasing from about 6.5 to 7.5 km/s. Reflectivity modeling shows that the lower crust might consist of a zone of alternating low- and high-velocity layers with average velocity increasing. The average lower crustal velocity of about 6.9 km/s suggests a predomi-nantly mafic composition with interlayered intermediate to felsic components generating impedance contrasts that cause observable amplitudes from reflections at large offsets but not at clearly pre-critical and near-vertical distances. Our model is consistent with observations of interlayered sequences of gabbroic to ultramafic rocks with more felsic anorthositic and charnockitic rocks in the exposed lower crust of magmatic arc complexes. The lack of wide-angle Moho reflections might be explained by a gradational compositional boundary, or a transitional phase change from granulite to eclogite facies. 1

The crustal structure and tectonic development of the continental margin of the Amundsen Sea Embayment, West Antarctica: implications from geophysical data

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

Workshop - Amundsen Sea Embayment Tectonic and Glacial History - Programme and Abstracts

Overall Objective: Review existing data and identify priorities for future geoscience research (terrestrial, marine and airborne) in the Amundsen Sea embayment (ASE) region required to develop a better understanding of the past, present and future behaviour of this sector of the West Antarctic Ice Sheet (WAIS). Background: The ASE is the most rapidly changing sector of the WAIS and contains enough ice to raise global sea level by 1.2 m. Over the past few years considerable efforts have been made to acquire new data to improve knowledge of the geological structure, subglacial topography, continental shelf bathymetry and glacial history of this remote region. In this workshop we aim to review the current state of knowledge on the tectonic and glacial evolution of the Amundsen Sea embayment. Particular emphasis will be placed on work that will improve boundary conditions for ice sheet models (e.g. subglacial topography, shelf bathymetry, palaeotopography, heat flow and substrate types) and provide palaeo-data against which model outputs can be compared. There will also be a focus on plans and targets for future scientific drilling that will reveal the history of this sector of the WAIS and its sensitivity to major climate changes