Wave-Modified Turbidites: Combined-Flow Shoreline and Shelf Deposits, Cambrian, Antarctica

Sandstone tempestite beds in the Starshot Formation, central Transantarctic Mountains, were deposited in a range of shoreline to shelf environments. Detailed sedimentological analysis indicates that these beds were largely deposited by wave-modified turbidity currents. These currents are types of combined flows in which storm-generated waves overprint flows driven by excess-weight forces. The interpretation of the tempestites of the Starshot Formation as wave-dominated turbidites rests on multiple criteria. First, the beds are generally well graded and contain Bouma-like sequences. Like many turbidites, the soles display abundant well-developed flutes. They also contain thick divisions of climbing-ripple lamination. The lamination, however, is dominated by convex-up and sigmoidal foresets, which are geometries identical to those produced experimentally in current-dominated combined flows in clear water. Finally, paleocurrent data support a turbidity-current component of flow. Asymmetric folds in abundant convolute bedding reflect liquefaction and gravity-driven movement and hence their orientations indicate the downslope direction at the time of deposition. The vergence direction of these folds parallels paleocurrent readings of flute marks, combined-flow ripples, and a number of other current-generated features in the Starshot event beds, indicating that the flows were driven down slope by gravity. The wave component of flow in these beds is indicated by the presence of small- to large-scale hummocky cross-stratification and rare small two-dimensional ripples. Wave-modified turbidity currents differ from deep-sea turbidity currents in that they may not be autosuspending and some proportion of the turbulence that maintains these flows comes from storm waves. Such currents are formed in modern shoreline environments by a combination of storm waves and downwelling sediment-laden currents. They may also be formed as a result of oceanic floods, events in which intense sediment-laden fluvial discharge creates a hyperpycnal flow. Event beds in the Starshot Formation may have formed from such a mechanism. Oceanic floods are formed in rivers of small to medium size in areas of high relief, commonly on active margins. The Starshot Formation and the coeval Douglas Conglomerate are clastic units that formed in response to uplift associated with active tectonism. Sedimentological and stratigraphic data suggest that coarse alluvial fans formed directly adjacent to a marine basin. The geomorphic conditions were therefore likely conducive to rapid fluvial discharge events associated with storms. The abundance of current-dominated combined-flow ripples at the tops of many Starshot beds indicates that excess-weight forces were dominant throughout deposition of many of these beds

Proterozoic crustal evolution of central East Antarctica: Age and isotopic evidence from glacial igneous clasts, and links with Australia and Laurentia

Rock clasts entrained in glacial deposits sourced from the continental interior of Antarctica provide an innovative means to determine the age and composition of ice-covered crust. Zircon U-Pb ages from a suite of granitoid clasts collected in glacial catchments draining central East Antarctica through the Transantarctic Mountains show that crust in this region was formed by a series of magmatic events at ∼2.01, 1.88–1.85, ∼1.79, ∼1.57, 1.50–1.41, and 1.20–1.06 Ga. The dominant granitoid populations are ca. 1.85, 1.45 and 1.20–1.06 Ga. None of these igneous ages are known from limited outcrop in the region. In addition to defining a previously unrecognized geologic history, zircon O and Hf isotopic compositions from this suite have: (1) mantle-like δ18O signatures (4.0–4.5‰) and near-chondritic Hf-isotope compositions (εHf ∼ +1.5) for granitoids of ∼2.0 Ga age; (2) mostly crustal δ18O (6.0–8.5‰) and variable Hf-isotope compositions (εHf = −6 to +5) in rocks with ages of ∼1.88–1.85, ∼1.79 and ∼1.57 Ga, in which the ∼1.88–1.79 Ga granitoids require involvement of older crust; (3) mostly juvenile isotopic signatures with low, mantle-like δ18O (∼4–5‰) and radiogenic Hf-isotope signatures (εHf = +6 to +10) in rocks of 1.50–1.41 Ga age, with some showing crustal sources or evidence of alteration; and (4) mixed crustal and mantle δ18O signatures (6.0–7.5‰) and radiogenic Hf isotopes (εHf = +3 to +4) in rocks of ∼1.2 Ga age. Together, these age and isotopic data indicate the presence in cratonic East Antarctica of a large, composite igneous province that formed through a punctuated sequence of relatively juvenile Proterozoic magmatic events. Further, they provide direct support for geological correlation of crust in East Antarctica with both the Gawler Craton of present-day Australia and Proterozoic provinces in western Laurentia. Prominent clast ages of ∼2.0, 1.85, 1.57 and 1.45 Ga, together with sediment source linkages, provide evidence for the temporal and spatial association of these cratonic elements in the Columbia supercontinent. Abundant ∼1.2–1.1 Ga igneous and metamorphic clasts may sample crust underlying the Gamburtsev Subglacial Mountains, indicating the presence of a Mesoproterozoic orogenic belt in the interior of East Antarctica that formed during final assembly of Rodinia.Field and analytical portions of this project were supported by the National Science Foundation (award 0944645)

A Precambrian odyssey in East Antarctica: more pieces, more tectonic stages and less puzzle?

East Antarctica is the least understood piece of continental crust on Earth. With an extension comparable to the conterminous United States of America, it contains cryptic clues into the origin, evolution and demise of three supercontinents, and it forms the lithospheric cradle for the largest ice sheet remaining on our planet. While rock exposures and provenance studies provide glimpses into up to 3 billion years of its geological history, extensive ice sheet cover and the lack of drilling, restricts our knowledge of Precambrian geology and crustal architecture in its interior. Consequently, many different aspects regarding the geodynamic processes that were responsible for the growth and amalgamation of East Antarctica during the Precambrian still remain elusive and controversial. This adds uncertainty to our knowledge of how East Antarctica linked up with major Precambrian domains of Australia, India, Africa and Laurentia, further hampering our ability to unravel Earth\u2019s early supercontinental cycle, in particular from the assembly and demise of the Nuna supercontinent to its successor Rodinia. To enhance our understanding of parts of the Precambrian evolution of East Antarctica, we present new interpretations derived from the recent ADMAP 2.0 magnetic compilation and satellite magnetic views, combined with the AntGG gravity compilation, and the latest satellite gravity gradient GOCE datasets; we also include selected insights from new aerogeophysical imaging over the Recovery and South Pole regions. We then combine Antarctic geophysical and geological data with global magnetic, gravity and geological, geochronological and paleomagnetic datasets in a plate tectonic reconstruction framework. Our main goal is to develop new interpretations and reconstructions that re-address the key stages of East Antarctic tectonic evolution between ca 1800 and ca 1300 Ma, in particular as part of long-lived and predominantly accretionary phases in Nuna\u2019s supercontinental history. We show that our interpretations provide new views into several key crustal elements in interior East Antarctica, including a proposed Archean ribbon microcontinent, an inverted Paleoproterozoic rift system, and a Paleoproterozoic to Mesoproterozoic continental margin arc, and two inferred Mesoproterozoic intra-oceanic accretionary belts. We suggest that these proposed crustal elements were affected by four major Paleoproterozoic and Mesoproterozoic tectonic stages, which we link with key tectonic and magmatic events recognised in the Gawler Craton, the Mt Isa Province, and the Coompana Block and Madura Province in Australia. Our geophysical reconstructions of East Antarctica and Laurentia also enable tantalising new perspectives into the so called proto-SWEAT hypothesis, which links these two key components of Nuna in Paleoproterozoic to Mesoproterozoic times

Real-space imaging of polar and elastic nano-textures in thin films via inversion of diffraction data

Exploiting the emerging nanoscale periodicities in epitaxial, single-crystal thin films is an exciting direction in quantum materials science: confinement and periodic distortions induce novel properties. The structural motifs of interest are ferroelastic, ferroelectric, multiferroic, and, more recently, topologically protected magnetization and polarization textures. A critical step towards heterostructure engineering is understanding their nanoscale structure, best achieved through real-space imaging. X-ray Bragg coherent diffractive imaging visualizes sub-picometer crystalline displacements with tens of nanometers spatial resolution. Yet, it is limited to objects spatially confined in all three dimensions and requires highly coherent, laser-like x-rays. Here we lift the confinement restriction by developing real-space imaging of periodic lattice distortions: we combine an iterative phase retrieval algorithm with unsupervised machine learning to invert the diffuse scattering in conventional x-ray reciprocal-space mapping into real-space images of polar and elastic textures in thin epitaxial films. We first demonstrate our imaging in PbTiO3/SrTiO3 superlattices to be consistent with published phase-field model calculations. We then visualize strain-induced ferroelastic domains emerging during the metal-insulator transition in Ca2RuO4 thin films. Instead of homogeneously transforming into a low-temperature structure (like in bulk), the strained Mott insulator splits into nanodomains with alternating lattice constants, as confirmed by cryogenic scanning transmission electron microscopy. Our study reveals the type, size, orientation, and crystal displacement field of the nano-textures. The non-destructive imaging of textures promises to improve models for their dynamics and enable advances in quantum materials and microelectronics

Antarctic geothermal heat flow: future research directions

Antarctic geothermal heat flow (GHF) affects the ice sheet temperature, determining how it slides and internally deforms, as well as the rheological behaviour of the lithosphere. However, GHF remains poorly constrained, with few borehole-derived estimates, and there are large discrepancies in currently available glaciological and geophysical estimates. This SCAR White Paper details current methods, discusses their challenges and limitations, and recommends key future directions in GHF research. We highlight the timely need for a more multidisciplinary and internationally-coordinated approach to tackle this complex problem

Composition and age of the East Antartic Shield in eastern Wilkes Land determined by proxy from Oligocene-Pleistocene glaciomarine sediment and Beacon Supergroup sandstones, Antarctica

The Precambrian East Antarctic Shield played a central role in the tectonic evolution of Rodinia and Gondwana, as well as growth of the East Antarctic Ice Sheet, yet little is known about its ice-covered interior. Glaciogenic deposits of Oligocene-Miocene and Pleistocene age on the Wilkes Land margin include glaciomarine diamictons containing basement rock clasts and fine-grained siliciclastic detritus, which provide proxy samples of the continental basement. Rock clasts obtained by dredge (81% metamorphic, 14% igneous, and 5% sedimentary lithologies) provide petrographic, geochemical, and age information about the glacial source area. Igneous clasts with Ross orogen U-Pb zircon ages (ca. 500 Ma) include a notably old ca. 585 Ma granitoid; they and Ross-age metamorphic rocks give discrete inherited-zircon age populations of 670-780, 900-1300, 1740-2300, and >2700 Ma that reflect basement sources. Paleoproterozoic rock clasts (granitoid, charnockite gneiss, and granulite gneiss) range from ca. 1720 to 1740 Ma. Detrital zircon populations from glacio marine sediments vary with depositional age but show common terrigenous provenance ages of 460-660, 1045-1315, 1545-1815, and 2420-2605 Ma, which overlap those from inherited zircons in the Ross granitoids. Detrital zircon ages from onshore Permian and Triassic terrestrial sedimentary rocks reveal a different provenance and indicate that the glaciogenic deposits do not contain significant secondcycle material from older interior basins. Together, these data suggest that metamorphic rock units with distinctive Neoproterozoic, Paleoproterozoic, and Archean ages dominate East Antarctic Shield basement inland from the eastern Wilkes Land margin, and that Ross-age granitoids either intruded or were derived by partial melting of this composite metamorphic basement

Rapid Access Ice Drill: a new tool for exploration of the deep Antarctic ice sheets and subglacial geology

ABSTRACTA new Rapid Access Ice Drill (RAID) will penetrate the Antarctic ice sheets in order to create borehole observatories and take cores in deep ice, the glacial bed and bedrock below. RAID is a mobile drilling system to make multiple long, narrow boreholes in a single field season in Antarctica. RAID is based on a mineral exploration-type rotary rock-coring system using threaded drill pipe to cut through ice using reverse circulation of a non-freezing fluid for pressure-compensation, maintenance of temperature and removal of ice cuttings. Near the bottom of the ice sheet, a wireline latching assembly will enable rapid coring of ice, the glacial bed and bedrock below. Once complete, boreholes will be kept open with fluid, capped and available for future down-hole measurement of temperature gradient, heat flow, ice chronology and ice deformation. RAID is designed to penetrate up to 3300 m of ice and take cores in <200 hours, allowing completion of a borehole and coring in ~10 d at each site. Together, the rapid drilling capability and mobility of the system, along with ice-penetrating imaging methods, will provide a unique 3-D picture of interior and subglacial features of the Antarctic ice sheets