Composition and evolution of the Ancestral South Sandwich Arc: implications for the flow of deep ocean water and mantle through the Drake Passage gateway

The Ancestral South Sandwich Arc (ASSA) has a short life-span of c.20 m.y. (Early Oligocene to Middle-Upper Miocene) before slab retreat and subsequent ‘resurrection’ as the active South Sandwich Island Arc (SSIA). The ASSA is, however, significant because it straddled the eastern margin of the Drake Passage Gateway where it formed a potential barrier to deep ocean water and mantle flow from the Pacific to Atlantic. The ASSA may be divided into three parts, from north to south: the Central Scotia Sea (CSS), the Discovery segment, and the Jane segment. Published age data coupled with new geochemical data (major elements, trace elements, Hf-Nd-Sr-Pb isotopes) from the three ASSA segments place constraints on models for the evolution of the arc and hence gateway development. The CSS segment has two known periods of activity. The older, Oligocene, period produced basic-acid, mostly calc-alkaline rocks, best explained in terms of subduction initiation volcanism of Andean-type (no slab rollback). The younger, Middle-Late Miocene period produced basic-acid, high-K calc-alkaline rocks (lavas and pyroclastic rocks with abundant volcanigenic sediments) which, despite being erupted on oceanic crust, have continental arc characteristics best explained in terms of a large, hot subduction flux most typical of a syn- or post-collision arc setting. Early-Middle Miocene volcanism in the Discovery and Jane arc segments is geochemically quite different, being typically tholeiitic and compositionally similar to many lavas from the active South Sandwich island arc front. There is indirect evidence for Western Pacific-type (slab rollback) subduction initiation in the southern part of the ASSA and for the back-arc basins (the Jane and Scan Basins) to have been active at the time of arc volcanism. Models for the death of the ASSA in the south following a series of ridge-trench collisions, are not positively supported by any geochemical evidence of hot subduction, but cessation of subduction by approach of progressively more buoyant oceanic lithosphere is consistent with both geochemistry and geodynamics. In terms of deep ocean water flow the early stages of spreading at the East Scotia Ridge (starting at 17-15 Ma) may have been important in breaking up the ASSA barrier while the subsequent establishment of a STEP (Subduction-Transform Edge Propagator) fault east of the South Georgia microcontinent (< 11 Ma) led to formation of the South Georgia Passage used by the Antarctic Circumpolar Current today. In terms of mantle flow, the subduction zone and arc root likely acted as a barrier to mantle flow in the CSS arc segment such that the ASSA itself became the Pacific-South Atlantic mantle domain boundary. This was not the case in the Discovery and Jane arc segments, however, because northwards flow of South Atlantic mantle behind the southern part of the ASSA gave an Atlantic provenance to the whole southern ASSA

Bathymetry and geological setting of the Drake Passage (Antarctica)

IX Congreso Geológico de España, Huelva, Septiembre 2016The Drake Passage is an oceanic gateway of about 850 km width located between South America and the Antarctic Peninsula that connects the southeastern Pacific and the southwestern Atlantic oceans and is an important gateway for mantle flow, oceanographic water masses, and migrations of biota. This gateway developed within the framework of geological evolution of the Scotia Arc. As a consequence of this and subsequent submarine processes, this region shows a varied physiography. The new detailed map in the Drake Passage region is mainly founded on a compilation of precise multibeam bathymetric data obtained on cruises between 1992 and 2014, and covers the area between parallels 52ºS and 63ºS and meridians 70ºW and 50ºW. The new map that we present is based in a DTM with 200 m cell resolution of the seafloor in Drake Passage that permits identification of the main seafloor features and the map includes additional useful geological information. This work constitutes an international cooperative effort and is part of IBCSO project (International Bathymetric Chart of the Southern Ocean), under the SCAR umbrella.Instituto Geológico y Minero de España, EspañaBritish Antarctic Survey, Reino UnidoDepartamento de Geodinámica, Universidad de Granada, EspañaInstituto Andaluz de Ciencias de la Tierra, Consejo Superior de Investigaciones Científicas, EspañaInstituto Andaluz de Ciencias de la Tierra, Universidad de Granada, EspañaJet Propulsion Laboratory M/S 300-323, Estados UnidosAlfred Wegener Institute, AlemaniaKorea Polar Research Institute, CoreaDepartamento de Geología y Geoquímica, Universidad Autónoma de Madrid, EspañaLamont-Doherty Earth Observatory, Columbia University, Estados Unido

Jurassic magmatism in Dronning Maud Land: synthesis of results of the MAMOG project

The Jurassic Karoo large igneous province (LIP) of Antarctica, and its conjugate margin in southern Africa, is critical for investigating important questions about the relationship of basaltic LIPs to mantle plumes. Detailed aerogeophysical, structural, anisotropy of magnetic susceptibility (AMS), geochronological and geochemical investigations completed under the British Antarctic Survey’s MAMOG project have provided some of the answers. Across most of the area, magma volumes were small compared to those in southern Africa. Jurassic dikes intruding the Archean craton are sparse and the Jutulstraumen trough, a Jurassic rift, is interpreted, from aerogeophysical data, as largely amagmatic. The largest volumes of magma were emplaced along the margin of the craton and close to the Africa-Antarctica rift. Although dikes were emplaced by both vertical and horizontal flow, overwhelmingly magmas in Dronning Maud Land were locally derived, and not emplaced laterally from distant sources. Basaltic magmatism was protracted in Dronning Maud Land (several dike emplacement episodes between ~206 and 175 Ma), and the small magma volumes resulted in highly diverse magma compositions, including picrites and ferropicrites interpreted to have been derived from hot mantle in a mantle plume. The protracted magmatism before the locally ~177 Ma flood lava eruptions, and evidence for a radiating dike swarm, favor a model of mantle plume incubation for 20-30 million years before flood lava eruption

A revised geochemical grouping of Gondwana LIP: distinctive sources and processes at the Weddell and Limpopo triple junctions

The magma types belonging to the Gondwana large igneous province can be divided into two categories based on, respectively, their primitive mantle-like and fractionated Sm/Yb and Sr/Zr values, different Sr and Nd isotopic trends, and geographic affinity to the Weddell and Limpopo triple junctions. In the new grouping, the Ferrar magmas, the Karoo Central Area magmas, and the Kirwanveggen-Sembberget magmas from Dronning Maud Land are viewed as three major magmatic lineages generated at the Weddell triple junction. These Weddell group magmas were produced by voluminous low-pressure partial melting of possibly subduction-modified upper mantle and tended to be laterally transported over long distances. The Limpopo group magmas include the Karoo high-Ti magma types and various low- Ti types from Lebombo, Vestfjella, and Heimefrontfjella. They represent magmas that were produced at high pressure from an eclogite-bearing mantle source below the Kaapvaal craton. The distribution of the Limpopo group magmas was mainly confined within the rift valleys of the Limpopo triple junction. The chemically distinctive magma types probably reflect heterogeneities in the Weddell and Limpopo magma sources and lithospheric level differentiation within Archean (Limpopo group) and younger lithospheric terranes

Iceberg ploughmarks and associated sediment ridges on the southern Weddell Sea margin

Ploughing by deep keels of floating icebergs is a common feature of high-latitude continental margins. Icebergs that have calved from glaciers or ice sheets produce a range of seafloor signatures, including linear to curvilinear grooves, rounded pits and ploughmarks terminating in sediment ridges. The dimensions and patterns of iceberg ploughmarks vary with iceberg size, water depth, local current, tide and wind conditions, seafloor sediment and past glacial history. The outer shelf and upper continental slope of the southern Weddell Sea is extensively ploughed by icebergs, and three different types of iceberg ploughmarks are recognized. The first signature is small grounding pits which occur over a wide area of the upper slope to water depths of c. 720 m (white arrow in Fig. 1a). The grounding pits have a mean depth of 8 m and a mean diameter of 280 m. The pits appear to be distributed randomly, with the highest intensity occurring in water depths of 410–670 m

Submarine gullies on the southern Weddell Sea slope, Antarctica

Submarine gullies are small-scale, confined channels on the order of tens of metres depth that form one of the most common morphological features of high-latitude continental slopes. Gully morphology varies in width, incision depth, length, sinuosity, branching order, shelf-incision, cross-sectional shape and gully spacing, with six distinct gully signatures recognized on high-latitude continental slopes (Gales et al. 2013a, b). Here we analyse the morphology of slope gullies off Halley and Filchner troughs in the southern Weddell Sea (Fig. 1a–f)