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Basement faults as a control on crustal architecture and topography at the transiton between Northern Victoria Land and the Wilkes Subglacial Basin (East Antarctica)
No full textMajor terrane bounding and intra-terrane faults have been recognised from extensive geological investigations
within the partially exposed basement rocks of Northern Victoria Land (NVL) in East Antarctica. These major
fault systems were active during the Ross Orogen and are related to several phases of Cambrian to Ordovician age
subduction and crustal accretion along the active paleo-Pacific margin of Gondwana. Here we compile and analyse
enhanced aeromagnetic and gravity anomaly images from NVL to the eastern margin of the Wilkes Subglacial
Basin (WSB) to image the subglacial extent and tectonic architecture of these major fault systems within the base-
ment. Our two-dimensional magnetic and gravity models predict that linear and long-wavelength magnetic lows
and residual Bouguer gravity highs over the central Wilson Terrane reflect several-km thick inverted sedimentary
basins of early Cambrian(?) age. Tectonic inversion occurred primarily along major thrust faults, formed in a dom-
inantly transpressional late stage of the Ross Orogen. Further west, a major fault system flanks the eastern margin
of the Wilkes Subglacial Basin, and connects to the previously interpreted Prince Albert Fault System to the south.
This fault system can now be recognised as lying west of the Exiles Thrust fault system, rather than represent-
ing its southern continuation (e.g. Ferraccioli and Bozzo, 1999, JGR). Relatively thin sheets of mylonitic sheared
granitoids and possible ultramafic lenses are modelled as being associated with the late-Ross (ca 480 Ma) Exiles
Thrust fault system, while significantly larger and thicker batholiths were emplaced along the Prince Albert Fault
System. Recent zircon U–Pb dating over small exposures of gabbro-diorites within the Prince Albert Mountains to
the south lead us to propose that this part of the magmatic arc was emplaced along a major pre-existing fault during
an earlier phase of subduction (>520 Ma or older). This attests to a long-lived and composite magnatic arc system,
which likely migrated in response to changes in the geometry and dynamics of the subduction system, much like
several modern arc systems. Whether the Prince Albert Fault System was indeed a major arc-continent suture in
early Cambrian times, as proposed by Ferraccioli et al., 2002 (GRL), or simply an arc to back-arc, or alternatively
an arc to forearc transition, remains to be more fully understood. Irrespective of possible alternative models for
the original tectonic setting of these faults during the Ross Orogen, we show by combining aeromagnetic inter-
pretation and topographic lineament analyses that these major terrane bounding and intra-terrane basement faults
exerted a key influence both on the tectonic segmentation of the Transantarctic Mountains into discrete Cenozoic
fault-blocks and on the subglacial topography along the eastern margin of the WSB
Potential fields as a tool to characterize the inaccessible areas of the earth: The case of Pine Island–Ellsworth Mountains area, West Antarctica
No full textThe Wilkes subglacial basin eastern margin electrical conductivity anomaly
No full textWe have analyzed the deep conductivity structure at the transition between the Transantarctic Mountains (TAM)
and the eastern margin of the WSB in NVL, by means of the GDS (Geomagnetic Deep Sounding) technique, in
order to constrain the geodynamical interpretation of this antarctic sector.
The TAM form the uplifted flank of the Mesozoic and Cenozoic West Antarctic Rift System.
Structure of the TAM rift flank has been partially investigated with different geophysical approaches.The Wilkes
Subglacial Basin is a broad depression over 400 km wide at the George V Coast and 1200 km long. Geology,
lithospheric structure and tectonics of the Basin are only partially known because the Basin is buried beneath
the East Antarctic Ice Sheet and is located in a remote region which makes geophysical exploration logistically
challenging.
Different authors have proposed contrasting hypothesis regarding the origin of the WSB: it could represent
a region of rifted continental crust, or it may have a flexural origin or might represent an "extended terrane".
Recently aerogeophysical investigations have demonstrated a strong structural control on the margin.
Magnetovariational studies carried out at high geomagnetic latitudes are often hampered by source effects, mainly
due to the closeness to the Polar Electrojet currents systems (PEJ). Its presence, in fact, makes the uniform
magnetic field assumption, on which the magnetovariational methods are based on, often invalid, which outcome
is a bias in the GDS transfer functions and to compromise the reliability of the inverted models.
Data from the aforementioned campaigns have been then processed under the ISEE project (Ice Sheet Electromag-
netic Experiment), aimed at evaluate and mitigate the bias effect of the PEJ on geomagnetic an magnetotelluric
transfer functions at high geomagnetic latitudes, by means of suitable processing algorithms, developed upon a
statistical analysis study on PEJ effects (Rizzello et al. 2013).
Recent results allowed for a new processing of a wide dataset acquired during three different international
Antarctic campaigns supported by the Italian Antarctic Project: the BACKTAM, WIBEM and WISE expeditions.
The qualitative analysis of the induction arrows, in the period range 20-170 s, reveals an approximately 2D
regional electrical conductivity pattern with a clear differentiation between the three Terrains crossed by the GDS
transect we have re-analized: the Robertson Bay, the Bowers and the Wilson Terrain. Bi-dimensional conductivity
models, jointly with magnetic and gravimetric profiles, suggest a differentiation of the investigated area in three
crustal sectors separated by the Daniels Range and the Bowers Mts., in close relation with main known structural
lineaments; to the West, a deep conductivity anomaly is associated with the transition to the Wilkes Subglagial
Basin. We deem that such anomaly, together with the magnetic and gravimetric signatures, is compatible with an
extensional regime in the eastern margin of the WS
Arc Boudinage, Basin Inversion and Obduction in an Evolving Subduction System of East Antarctica
No full textThe paleo-Pacific margin of Gondwana experienced protracted subduction and accretionary tectonics starting in late Neoproterozic-early Cambrian times. Northern Victoria Land (NVL), in East Antarctica, preserves a cryptic record of these active margin processes. Most models indicate that NVL contains three main terranes, namely the Robertson Bay, Bowers and Wilson terranes. Significant debate centres, however, on whether these are far travelled terranes with respect to the East Antarctic Craton, and on the tectonic and magmatic processes that affected its active margin and were ultimately responsible for the formation of the Ross Orogen.
Here we interpret new aeromagnetic, aerogravity and land-gravity compilations that enable us to trace the extent of major subglacial faults in the basement of NVL, examine crustal architecture, and propose a new evolutionary model for the active margin of the craton. Prominent aeromagnetic anomalies at the edge of the Wilkes Subglacial Basin delineate the extent of an early-stage magmatic arc (ca 530 Ma?). This arc may have accreted as an exotic element onto the former Neoproterozoic rifted margin of East Antarctica or (perhaps more likely) developed in situ upon a pre-existing suture.
Remnants of magnetic arc basement are also identified ca 150 km further to the east within the Wilson Terrane (WT). We propose that these were originally adjacent arc segments and that transtension triggered significant arc boudinage separating these segments. Transtension may have created accommodation space for the development of thick Cambrian sedimentary basins, which are marked by regional magnetic lows with an en-echelon geometry. Basin inversion likely occurred in a later traspressional stage of the Ross-Delamerian Orogen (ca. 490-460 Ma) that triggered the development of a major pop-up structure within the WT. Several buried thrusts of the pop-up can be traced in the aeromagnetic images and a prominent residual gravity high delineates its high-grade metamorphic core. High amplitude magnetic and gravity anomalies also delineate buried oceanic basement of the northern Bowers Terrane. Oceanic basement was likely uplifted and obducted onto to the margin either because of oblique subduction or because of docking of a microcontinent inferred to underlie part of the Robertson Bay Terrane
Imaging the Mt. Melbourne Volcanic Field (Northern Victoria Land, Antarctica): a Hamiltonian Monte Carlo approach applied to high-resolution aeromagnetic data
No full textThe Mt. Melbourne Volcanic Complex (MMVC) is located in Northern Victoria Land (Antarctica) along the western flank of the West Antarctic Rift System, at the boundary with the Transantarctic
Mountains. It is constituted by two main volcanic areas, i.e. the Mt. Melbourne Edifice (MME) and the Cape Washington Shield (CWS), and some other minor centres.
To date, the inner structure of this volcanic complex is still poorly known, being the direct geological information on site confined to either glacial erratics or a few rock outcrops not hidden
by the ice sheet. Consequently, even the temporal building up and evolution of the MMVC as well as its primary magmatic source are still under investigation (debated).
Recently, we attempted to define the geological structure of the MMVC by means of digital enhancement and forward modeling performed on a high-resolution aeromagnetic dataset
(Ghirotto et al. 2020, EGU). Coupling both information derived from past geological/geophysical studies and unpublished magnetic susceptibility measurements from rock samples collected in
the field, we proposed two models to explain the chronological evolution of the MME and CWS. These models involve either i) major magmatic events occurred in periods of both normal and
reverse magnetic polarity or ii) only magmatic flows with normal polarity.
To gain further insights into the geological structure and the geodynamic evolution of the MMVC in relation to the two proposed models, we develop here a Hamiltonian Monte Carlo (HMC)
algorithm (Fichtner et al. 2018) based on the probabilistic approach to inverse problems. To date, this methodology has never been applied to aeromagnetic data for geological studies. In detail,
the above proposed models provide some soft a priori information from which to start exploring potential solutions. The parameterization of the volcanic area is defined in terms of 2-D polygonal
bodies, representing e.g. magmatic lava flows, where the unknown parameters are represented by both the position of the vertices and/or the magnetization (induced and/or remnant), resulting in a
non-linear forward model. The HMC algorithm requires the computation of gradients of the posterior probability density (PPD), i.e., derivatives of the objective functional with respect to the
position of vertices of the bodies and magnetization, in order to better move the inversion process toward high-probability areas in the model space manifold. We implement such calculations using automatic differentiation, a tool which is very accurate and fast compared to other approaches such as finite difference. The result of the inversion is then a collection of models representing the
PPD, from which statistical analysis can provide measures of uncertainty and plausible geological scenarios.
In this study we present some preliminary results of applying the above-mentioned methodology, which finally could help unravel the framework of the MMVC
Magnetic and gravity imaging of terrane boundaries in northern Victoria Land, between the Lanterman Range and the Ross Sea (Antarctica)
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Analysing aeromagnetic, airborne gravity and radar data to unveil variable basal boundary conditions for the East Antarctic Ice Sheet in the Wilkes Subglacial Basin
No full textThe Wilkes Subglacial Basin (WSB) extends for ca 1,400 km from George V Land into the interior of East Antarc-
tica and hosts several major glaciers that drain a large sector of the East Antarctic Ice Sheet (EAIS). The region
is of major significance for assessing the long-term stability of the EAIS, as it lies well below sea level and its
bedrock deepens inland. This makes it potentially more prone to marine ice sheet instability, much like areas of the
West Antarctic Ice Sheet (WAIS) that are presently experiencing significant mass loss. This sector of the EAIS has
also become a focus of current research within IODP Leg 318 that aims to better comprehend the initial stages of
glaciation and the history and stability of the EAIS since the Eocene-Oligocene boundary. Understanding geologi-
cal boundary conditions onshore is important to assess their influence on ice sheet dynamics and long-term stability
and interpret the paleo-ice sheet record. Early geophysical models inferred the existence of a major extensional
sedimentary basin beneath the WSB. This could in principle be similar to some areas of the WAIS, where subglacial
sediments deposited within rift basins or forming thin marine sedimentary drapes have been inferred to exert a key
influence on both the onset and maintenance of fast-glacial flow. However, later geophysical models indicated that
the WSB contains little or no sediment, is not rift-related, and formed in response to Cenozoic flexural uplift of the
Transantarctic Mountains (TAM). A major joint Italian-UK aerogeophysical exploration campaign over parts of
the WSB is super-seeding all these earlier geophysical views of the basin (Ferraccioli et al., 2009, Tectonophysics).
Precambrian and Paleozoic basement faults can now be recognised as exerting fundamental controls on the loca-
tion of both the topographic margins of the basin and it sub-basins; ii) the crust underlying the basin is thinner
compared to the TAM (Jordan et al., 2013, Tectonophysics), but is unlikely to be strongly affected by Cretaceous
or Cenozoic-age rifting, in contrast to the WAIS, which is largely underlain by the West Antarctic Rift System;
iii) its bedrock is composed of rocks of different ages and composition, including Proterozoic basement, Neopro-
terozoic and Cambrian sediments intruded by Cambrian arc rocks, and cover rocks formed primarily by Beacon
sediments intruded by Jurassic Ferrar sills (e.g. Cook et al., 2013 Nature Geoscience). Within the framework of
the collaborative Italian-US-UK BABOC project a new international initiative has been launched to analyse and
model variable geological boundary conditions in the WSB using geophysical data. A large amount of new ICE-
CAP aerogeophysical observations have been acquired over four campaigns over the region since the International
Polar Year, in particular over the southern part of the basin, and some profiles over the northern coastal margin of
the basin. We will present an initial interpretation of the potential field signatures and radar data over the northern
and central parts of the basin to help establish tectonic and lithological controls on the subglacial topography and
different EAIS flow regimes within the WSB
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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