Tectonic reconstructions for paleobathymetry in Drake Passage

A minimum-complexity tectonic reconstruction, based on published and new basin opening models, depicts how the Scotia Sea grew by Cenozoic plate divergence, dismembering a Jurassic sheared margin of Gondwana. Part of the Jurassic–early Cretaceous ocean that accreted to this margin forms the core of the Central Scotia Plate, the arc plate above a trench at the eastern end of the Scotia Sea, which migrated east away from the Antarctic and South American plates. A sequence of extensional basins opened on the western edge of the Central Scotia Plate at 50– 30 Ma, decoupled from the South American Plate to the northwest by slow motion on a long transform fault. Succeeding the basins, seafloor spreading started around 30 Ma on the West Scotia Ridge, which propagated northwards in the 23–17 Ma period and ceased to operate at 6 Ma. The circuits of plate motions inside and out- side the Scotia Arc are joined via rotations that describe Antarctic–Central Scotia plate motion in Powell Basin until 20 Ma, and along the South Scotia Ridge thereafter. The modelled relative motion at the northern edge of the Scotia Sea is thus constrained only by the plate circuit, but nonetheless resembles that known coarsely from the geological record of Tierra del Fuego. A paleobathymetric interpretation of nine time slices in the model shows Drake Passage developing as an intermediate-depth oceanographic gateway at 50–30 Ma, with deep flow possible afterwards. Initially, this deep flow would have been made tortuous by numerous intermedi- ate and shallow barriers. A frontal pattern resembling that in the modern Scotia Sea would have awaited the clearance of significant barriers by continuing seafloor spreading in the Scotia Sea at ~18.5 Ma, at Shag Rocks Passage, and after 10 Ma southeast of South Georgia

The Skytrain plate and tectonic evolution of southwest Gondwana since Jurassic times

Uncertainty about the structure of the Falkland Plateau Basin has long hindered understanding of tectonic evolution in southwest Gondwana. New aeromagnetic data from the basin reveal Jurassic-onset seafloor spreading by motion of a single newly-recognized plate, Skytrain, which also governed continental extension in the Weddell Sea Embayment and possibly further afield in Antarctica. The Skytrain plate resolves a nearly century-old controversy by requiring a South American setting for the Falkland Islands in Gondwana. The Skytrain plate’s later motion provides a unifying context for post-Cambrian wide-angle paleomagnetic rotation, Cretaceous uplift, and post-Permian oblique collision in the Ellsworth Mountains of Antarctica. Further north, the Skytrain plate’s margins built a continuous conjugate ocean to the Weddell Sea in the Falkland Plateau Basin and central Scotia Sea. This ocean rules out venerable correlation-based interpretations for a Pacific margin location and subsequent long-distance translation of the South Georgia microcontinent as the Drake Passage gateway opened

Kinematic and paleobathymetric evolution of the South Atlantic

The opening of the South Atlantic Ocean is one of the most extensively researched problems in plate kinematics. In recent years focus has shifted to the early stages of continental separation. General agreement exists about ocean opening being the result of the diachronous separation of two major plates, having involved a certain degree of intracontinental deformation. However, in order to achieve their best fits, most modern models assign most of this intracontinental deformation to narrow mobile belts between large, independently moving plate-like continental blocks, even though timings and motions along their boundaries are not well known. Aiming to step away from the very large uncertainty introduced by this approach, here we present a model of oceanic growth based on seafloor spreading data (fracture zone traces and magnetic anomaly identifications) as a context within which to interpret intracontinental tectonic motions. Our model results are illustrated by an animated tectonic reconstruction. Spreading started at 138 Ma, with movement along intracontinental accommodation zones leading to the assembly of South America by 123 Ma and Africa by 106 Ma. Our model also provides an explanation for the inception and evolution of the Malvinas plate and its connection with the formation of a LIP south of the Falkland-Agulhas Fracture Zone. Finally, we challenge the view of narrow deformation belts as the sole sites of stress accommodation and discuss the implications of our model in terms of the distribution of intracontinental strain. However, paleobathymetry (depth variations through time) also needs to be considered for a fuller understanding of the ocean’s evolution and development of its petroleum systems. At first order, this is controlled by plate tectonics, which determines changes in the geographical location of the lithosphere, along with thermal subsidence, which controls changes in its vertical level. Thermal subsidence is modelled by applying plate-cooling theory to a high-resolution seafloor age grid derived from the plate kinematic model. Then, this thermal surface is refined to account for other factors that affect bathymetry at smaller scales or amplitudes, both within the ocean and the continent-ocean transition zones. The results are a series of paleobathymetric reconstructions of the South Atlantic, which provide a fuller picture of its evolution from Cretaceous times to present

Spatial patterns in the evolution of Cenozoic dynamic topography and its influence on the Antarctic continent

Our knowledge of dynamic topography in Antarctica remains in an infancy stage compared to other continents. We assess the space-time variability in dynamic topography in Antarctica by analysing grids of global dynamic topography in the Cenozoic (and late Cretaceous) based on the tomographic model S40RTS. Our model reveals that the Gamburtsev Province and Dronning Maud Land, two of the major nucleation sites for the East Antarctic Ice Sheet (EAIS) were ~500 m higher 60 Ma ago. The increased elevation may have facilitated ephemeral ice cap development in the early Cenozoic. Between ca 25 and 50 Ma the northern Wilkes Subglacial Basin was ca 200 m higher than today and a major increase in regional elevation (>600 m) occurred over the last 20-15 Ma over the northern and southern Victoria Land in the Transantarctic Mountains (TAM). The most prominent signal is observed over the Ross Sea Rift (RSR) where predicted Neogene dynamic topography exceeds 1,000 m. The flow of warm mantle from the West Antarctic Rift System (WARS)may have driven these dynamic topography effects over the TAM and RSR. However, we found that these effects are comparatively less significant over the Marie Byrd Land Dome and the interior of the WARS. If these contrasting dynamic topography effects are included, then the predicted elevations of the Ross Sea Embayment ca 20 Ma ago are more similar to the interior of the WARS, with significant implications for the early development of the West Antarctic Ice Sheet

Plate kinematics of the Rocas Verdes Basin and Patagonian orocline

The processes of orocline formation are a topic of debate in geosciences. The Patagonian orocline has been a case in point for over a century. Large anomalous paleomagnetic pole rotations show that the orocline started to form at the same time as mid-Cretaceous closure of the Rocas Verdes Basin, today known from ophiolitic and basin fill remnants in the Patagonian and Fuegian Andes. Some studies therefore present bending of the Andes and closure of the basin as shared consequences of rotation of a small plate that was driven by subduction-related forces at the Pacific margin of Gondwana. An alternative view of the orocline is as a product of Cretaceous to Paleogene-aged sinistral oblique convergence at the plate-boundary scale. Geological data from Tierra del Fuego have been interpreted in support of both views. Here, I test these suggestions by comparing the Rocas Verdes Basin's tectonostratigraphy to predictions of a plate kinematic model for fragmentation of the western interior of Gondwana. The model is sufficient to explain the known history of basin opening to a width of ~ 100–300 km during the period 152–141 Ma and later closure in oblique plate convergence. As this convergence occurred by motion around a distant Euler pole, it could not have produced the Patagonian orocline by rotation of a lithospheric plate on its Pacific flank. The large anomalous paleomagnetic rotations of Tierra del Fuego, instead, are likely to have occurred within the crust by rotation and deformation of regional strike-slip faults and the intervening rocks to accommodate oblique convergence of the South American and Antarctic plates between Albian and Paleocene times

Airborne Platforms Help Answer Questions in Polar Geosciences

The polar regions, with their continental ice sheets and partly ice covered oceans, play a crucial role in the Earth system. They are critical to understanding and predicting climate evolution and global sea level change. Airborne platforms offer the most amenable and powerful means of surveying these regions

Plates, plumes and geological time: are we wrong about plume-push?

In setting out to better understand what drives plate movements, not only did Lucia Perez-Diaz, Graeme Eagles and Karin Sigloch cast doubt on the theory of plume-push – they also unearthed a potential error in the calibration of our geological timescal

Plate convergence west of Patagonia and the Antarctic Peninsula since 61 Ma

A new plate kinematic model portrays plate motions immediately west and south of Drake Passage in the southeast Pacific Ocean. Overall intermediate-to-slow rate spreading generated oceanic lithosphere as the Phoenix plate diverged from the Antarctic plate. The model shows a history of Phoenix plate motion that is interpretable as having been affected by a northeast-increasing gradient in the slab pull force since chron 18 (39 Ma), during which time newer, less dense lithosphere was subducting in the southwest than in the northeast. The model allows first calculations of Phoenix–Farallon (Nazca) plate motion parameters in the south Pacific plate circuit. Using these parameters, it is possible to show that the simplest assumptions about the ridge's segmentation, length and migration are consistent with existing suggestions of its location from consideration of slab window-related volcanism at sites in South America around 50 and 20 Ma. The parameters thus define ridge locations that can be used to define which plates were subducting beneath South America and the Magallanes and Antarctic plates, and when. We consider the relationships between the plate convergence rate, obliquity and the history of magmatism on the Antarctic Peninsula and at the North Patagonian batholith, showing that magmatic pulses can be related to accelerations in the plate convergence rate. Between these settings, Phoenix–South American plate motion was almost parallel to the Fuegian trench. Here, magmatism in Paleocene to early Miocene times must be related to the presence of a slab subducted beneath the region by the less oblique collision further north. Later magmatism can be related to migration of the Phoenix–Farallon ridge and Phoenix–Farallon– Antarctic triple junction into the area south of the Fuegian margin, which brought it into slow low-obliquity convergence with first Farallon and then Antarctic plate lithosphere

West Antarctic Rift System in the Antarctic Peninsula

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