Whitmore Mountains Research Articles

Radial Anisotropy and Sediment Thickness of West and Central Antarctica Estimated From Rayleigh and Love Wave Velocities

AbstractMany recent Antarctic seismic structure studies use Rayleigh wave data and thus determine only the SV structure. Love waves provide greater resolution for shallow structure, and coupled with Rayleigh waves, can constrain radial anisotropy by comparing vertically (VSV) and horizontally (VSH) polarized shear velocities. In this study, we jointly analyze Rayleigh and Love wave phase and group velocities from ambient noise to develop a new radially anisotropic velocity model for West and Central Antarctica with an improved shallow crustal resolution using all broadband data collected in Antarctica over the past 20 years. Group and phase velocity maps for Rayleigh and Love waves are estimated and inverted for shear wave velocity structure using a Monte Carlo method. We determine a new sediment distribution map that reveals a thick sedimentary basin (∼4 km) beneath the Southeastern Ross Embayment. Sediment thicknesses at interior basins such as the Polar Subglacial Basin and Bentley Subglacial Trench are modest (<1.5 km), suggesting that these basins are sediment‐starved. The shallow crust as well as the mid‐to‐lower crust in several places shows strong positive anisotropy (VSH > VSV), likely due to lattice preferred orientation of mica‐bearing rocks. However, large regions of the mid‐to‐lower crust show negative anisotropy, likely due to lattice preferred orientation of plagioclase. The uppermost mantle is characterized by strong positive radial anisotropy (4%–8%) in West Antarctica, with the largest anisotropy beneath the Transantarctic and Whitmore Mountains, likely resulting from horizontal olivine preferred orientation due to tectonic activity.

REE Tetrad Effect and Sr-Nd Isotope Systematics of A-Type Pirrit Hills Granite from West Antarctica

The Pirrit Hills are located in the Ellsworth–Whitmore Mountains of West Antarctica. The Pirrit Hills granite exhibits significant negative Eu anomalies (Eu/Eu* = 0.01~0.25) and a REE tetrad effect indicating intensive magmatic differentiation. Whole-rock Rb-Sr and Sm-Nd geochronologic analysis of the Pirrit Hills granite gave respective ages of 172.8 ± 2.4 Ma with initial 87Sr/86Sr = 0.7065 ± 0.0087 Ma and 169 ± 12 Ma with initial 144Nd/143Nd = 0.512207 ± 0.000017. The isotopic ratio data indicate that the Pirrit Hills granite formed by the remelting of Mesoproterozoic mantle-derived crustal materials. Both chondrite-normalized REE patterns and Sr-Nd isotopic data indicate that the Pirrit Hills granite has geochemical features of chondrite-normalized REE patterns indicating that REE tetrad effects and negative Eu anomalies in the highly fractionated granites were produced from magmatic differentiation under the magmatic-hydrothermal transition system.

Miocene to Pleistocene glacial history of West Antarctica inferred from Nunatak geomorphology and cosmogenic-nuclide measurements on bedrock surfaces

We report geomorphic observations and cosmogenic-nuclide measurements on bedrock surfaces from three isolated nunatak groups in West Antarctica: the Pirrit Hills and Nash Hills, located in the Weddell Sea sector, and the Whitmore Mountains, located on the Ross-Weddell divide. The objectives of this paper are to (i) establish a chronology for landscape development at these sites and (ii) quantify the long-term history of ice-thickness variations in West Antarctica. These nunataks display relic alpine landscapes on which weathered bedrock surfaces are superimposed. In the Pirrit Hills, an erosional trimline is etched into alpine ridges and separates smooth-crested ridges below from serrated ridges above. Below the trimline, geomorphic evidence indicates repeated frozen-based ice cover, while above the trimline evidence for ice cover is entirely absent. There is also no geomorphic evidence for thicker-than-present ice at the Whitmore Mountains. Cosmogenic nuclide concentrations in the oldest bedrock surfaces from the Whitmore Mountains and from above the Pirrit Hills trimline indicate uninterrupted exposure for ∼12 Myr at extraordinarily low erosion rates. This places a lower limit on the timing of the formation of alpine landscapes in West Antarctica, and we hypothesize that this occurred during the relatively warm climates prior to the mid-Miocene cooling. The absence of evidence for thicker ice at the Whitmore Mountains is consistent with the hypothesis that the divide was typically thinner than present during Pleistocene glacial periods due to reduced accumulation. Bedrock surfaces below the trimline have much lower nuclide concentrations and are most easily explained by a scenario of repeated frozen-based ice cover and occasional subglacial plucking, which is consistent with geomorphic observations. Bedrock surfaces near the modern ice level appear to have been covered more than half of the time, while higher elevation surfaces indicate progressively less cover. The trimline and associated geomorphic features are very similar to a prominent trimline in the Ellsworth Mountains, and we conclude that these are, in fact, part of the same feature. The Ellsworth trimline is hypothesized to have formed during the mid-Miocene cooling and the transition from alpine to continental glaciation, and our results are consistent with this hypothesis.

A joint inversion of receiver function and Rayleigh wave phase velocity dispersion data to estimate crustal structure in West Antarctica

SUMMARY We determine crustal shear wave velocity structure and crustal thickness at recently deployed seismic stations across West Antarctica, using a joint inversion of receiver functions and fundamental mode Rayleigh wave phase velocity dispersion. The stations are from both the UK Antarctic Network (UKANET) and Polar Earth Observing Network/Antarctic Network (POLENET/ANET). The former include, for the first time, four stations along the spine of the Antarctic Peninsula, three in the Ellsworth Land and five stations in the vicinity of the Pine Island Rift. Within the West Antarctic Rift System (WARS) we model a crustal thickness range of 18–28 km, and show that the thinnest crust (∼18 km) is in the vicinity of the Byrd Subglacial Basin and Bentley Subglacial Trench. In these regions we also find the highest ratio of fast (Vs = 4.0–4.3 km s–1, likely mafic) lower crust to felsic/intermediate upper crust. The thickest mafic lower crust we model is in Ellsworth Land, a critical area for constraining the eastern limits of the WARS. Although we find thinner crust in this region (∼30 km) than in the neighbouring Antarctic Peninsula and Haag-Ellsworth Whitmore block (HEW), the Ellsworth Land crust has not undergone as much extension as the central WARS. This suggests that the WARS does not link with the Weddell Sea Rift System through Ellsworth Land, and instead has progressed during its formation towards the Bellingshausen and Amundsen Sea Embayments. We also find that the thin WARS crust extends towards the Pine Island Rift, suggesting that the boundary between the WARS and the Thurston Island block lies in this region, ∼200 km north of its previously accepted position. The thickest crust (38–40 km) we model in this study is in the Ellsworth Mountain section of the HEW block. We find thinner crust (30–33 km) in the Whitmore Mountains and Haag Nunatak sectors of the HEW, consistent with the composite nature of the block. In the Antarctic Peninsula we find a crustal thickness range of 30–38 km and a likely dominantly felsic/intermediate crustal composition. By forward modelling high frequency receiver functions we also assess if any thick, low velocity subglacial sediment accumulations are present, and find a 0.1–0.8-km-thick layer at 10 stations within the WARS, Thurston Island and Ellsworth Land. We suggest that these units of subglacial sediment could provide a source region for the soft basal till layers found beneath numerous outlet glaciers, and may act to accelerate ice flow.

Thickness of the divide and flank of the West Antarctic Ice Sheet through the last deglaciation

Abstract. We report cosmogenic-nuclide measurements from two isolated groups of nunataks in West Antarctica: the Pirrit Hills, located midway between the grounding line and the divide in the Weddell Sea sector, and the Whitmore Mountains, located along the Ross–Weddell divide. At the Pirrit Hills, evidence of glacial-stage ice cover extends ∼320 m above the present ice surface. Subsequent thinning mostly occurred after ∼14 kyr BP, and modern ice levels were established some time after ∼4 kyr BP. We infer that, like at other flank sites, these changes were primarily controlled by the position of the grounding line downstream. At the Whitmore Mountains, cosmogenic 14C concentrations in bedrock surfaces demonstrate that ice there was no more than ∼190 m thicker than present during the past ∼30 kyr. Combined with other constraints from West Antarctica, the 14C data imply that the divide was thicker than present for a period of less than ∼8 kyr within the past ∼15 kyr. These results are consistent with the hypothesis that the divide initially thickened due to the deglacial rise in snowfall and subsequently thinned in response to retreat of the ice-sheet margin. We use these data to evaluate several recently published ice-sheet models at the Pirrit Hills and Whitmore Mountains. Most of the models we consider do not match the observed timing and/or magnitude of thickness change at these sites. However, one model performs relatively well at both sites, which may, in part, be due to the fact that it was calibrated with geological observations of ice-thickness change from other sites in Antarctica.

Mapping Crustal Shear Wave Velocity Structure and Radial Anisotropy Beneath West Antarctica Using Seismic Ambient Noise

AbstractUsing 8‐ to 25‐s‐period Rayleigh and Love wave phase velocity dispersion data extracted from seismic ambient noise, we (i) model the 3‐D shear wave velocity structure of the West Antarctic crust and (ii) map variations in crustal radial anisotropy. Enhanced regional resolution is offered by the UK Antarctic Seismic Network. In the West Antarctic Rift System (WARS), a ridge of crust ∼26–30 km thick extending south from Marie Byrd Land separates domains of more extended crust (∼22 km thick) in the Ross and Amundsen Sea Embayments, suggesting along‐strike variability in the Cenozoic evolution of the WARS. The southern margin of the WARS is defined along the southern Transantarctic Mountains and Haag‐Ellsworth Whitmore Mountains (HEW) block by a sharp crustal thickness gradient. Crust ∼35–40 km is modeled beneath the Haag Nunataks‐Ellsworth Mountains, decreasing to ∼30–32 km thick beneath the Whitmore Mountains, reflecting distinct structural domains within the composite HEW block. Our analysis suggests that the lower crust and potentially the middle crust is positively radially anisotropic (VSH>VSV) across West Antarctica. The strongest anisotropic signature is observed in the HEW block, emphasizing its unique provenance among West Antarctica's crustal units, and conceivably reflects a ∼13‐km‐thick metasedimentary succession atop Precambrian metamorphic basement. Positive radial anisotropy in the WARS crust is consistent with observations in extensional settings and likely reflects the lattice‐preferred orientation of minerals such as mica and amphibole by extensional deformation. Our observations support a contention that anisotropy may be ubiquitous in the continental crust.

Jurassic high heat production granites associated with the Weddell Sea rift system, Antarctica

The distribution of heat flow in Antarctic continental crust is critical to understanding continental tectonics, ice sheet growth and subglacial hydrology. We identify a group of High Heat Production granites, intruded into upper crustal Palaeozoic metasedimentary sequences, which may contribute to locally high heat flow beneath the West Antarctic Ice Sheet. Four of the granite plutons are exposed above ice sheet level at Pagano Nunatak, Pirrit Hills, Nash Hills and Whitmore Mountains. A new UPb zircon age from Pirrit Hills of 178.0±3.5Ma confirms earlier RbSr and UPb dating and that the granites were emplaced approximately coincident with the first stage of Gondwana break-up and the developing Weddell rift, and ~5m.y. after eruption of the Karoo-Ferrar large igneous province. Aerogeophysical data indicate that the plutons are distributed unevenly over 40,000km2 with one intruded into the transtensional Pagano Shear Zone, while the others were emplaced within the more stable Ellsworth-Whitmore mountains continental block. The granites are weakly peraluminous A-types and have Th and U abundances up to 60.7 and 28.6ppm respectively. Measured heat production of the granite samples is 2.96–9.06μW/m3 (mean 5.35W/m3), significantly higher than average upper continental crust and contemporaneous silicic rocks in the Antarctic Peninsula. Heat flow associated with the granite intrusions is predicted to be in the range 70–95mW/m2 depending on the thickness of the high heat production granite layer and the regional heat flow value. Analysis of detrital zircon compositions and ages indicates that the high Th and U abundances are related to enrichment of the lower-mid crust that dates back to 200–299Ma at the time of the formation of the Gondwanide fold belt and its post-orogenic collapse and extension.

Widespread Antarctic glaciation during the Late Eocene

Marine sedimentary rocks drilled on the southeastern margin of the South Orkney microcontinent in Antarctica (Ocean Drilling Program Leg 113 Site 696) were deposited between ∼36.5 Ma to 33.6 Ma, across the Eocene–Oligocene climate transition. The recovered rocks contain abundant grains exhibiting mechanical features diagnostic of iceberg-rafted debris. Sand provenance based on a multi-proxy approach that included petrographic analysis of over 275,000 grains, detrital zircon geochronology and apatite thermochronometry rule out local sources (Antarctic Peninsula or the South Orkney Islands) for the material. Instead the ice-transported grains show a clear provenance from the southern Weddell Sea region, extending from the Ellsworth–Whitmore Mountains of West Antarctica to the coastal region of Dronning Maud Land in East Antarctica. This study provides the first evidence for a continuity of widespread glacier calving along the coastline of the southern Weddell Sea embayment at least 2.5 million yrs before the prominent oxygen isotope event at 34–33.5 Ma that is considered to mark the onset of widespread glaciation of the Antarctic continent.

Precise U-Pb zircon ages and geochemistry of Jurassic granites, Ellsworth-Whitmore terrane, central Antarctica

The Ellsworth-Whitmore Mountain terrane of central Antarctica was part of the early Paleozoic amalgamation of Gondwana, including a 13,000 m section of Cambrian−Permian sediments in the Ellsworth Mountains deposited on Grenville-age crust. The Jurassic breakup of Gondwana involved a regional, bimodal magmatic event during which the Ellsworth-Whitmore terrane was intruded by intraplate granites before translation of the terrane to its present location in central Antarctica. Five widely separated granitic plutons in the Ellsworth-Whitmore terrane were analyzed for their whole-rock geochemistry (X-ray fluorescence), Sr, Nd, and Pb isotopic compositions, and U-Pb zircon ages to investigate the origins of the terrane magmas and their relationships to mafic magmatism of the 183 Ma Karoo-Ferrar large igneous province (LIP). We report high-precision (±0.1 m.y.) isotope dilution−thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon ages from granitic rocks from the Whitmore Mountains (208.0 Ma), Nash Hills (177.4−177.3 Ma), Linck Nunatak (175.3 Ma), Pagano Nunatak (174.8 Ma), and the Pirrit Hills (174.3−173.9 Ma), and U-Pb sensitive high-resolution ion microprobe (SHRIMP) ages from the Whitmore Mountains (200 ± 5 Ma), Linck Nunatak (180 ± 4 Ma), Pagano Nunatak (174 ± 4 Ma), and the Pirrit Hills (168 ± 4 Ma). We then compared these results with existing K-Ar ages and Nd model ages, and used initial Sr, Nd, and Pb isotope ratios, combined with xenocrystic zircon U-Pb inheritance, to infer characteristics of the source(s) of the parent magmas. We conclude that the Jurassic plutons were not derived exclusively from crustal melts, but rather they are hybridized magmas composed of convecting mantle, subcontinental lithospheric mantle, and lower continental crustal contributions. The mantle contributions to the granites share isotopic similarities to the sources of other Jurassic LIP mafic magmas, including radiogenic 87Sr/86Sr (0.706−0.708), unradiogenic 143Nd/144Nd (eNd < −5), and Pb isotopes consistent with a low-µ source (where μ = 238U/204Pb). Isotopes and zircon xenocrysts point toward a crustal end member of predominantly Proterozoic provenance (0.5−1.0 Ga; Grenville crust), extending the trends illustrated by Ferrar mafic intrusive rocks, but contrasting with the inferred Archean crustal and/or lithospheric mantle contributions to some basalts of the Karoo sector of the LIP. The Ellsworth-Whitmore terrane granites are the result of mafic rocks underplating the hydrous crust, causing crustal melting, hybridization, and fractionation to produce granitic magmas that were eventually emplaced as post-Ferrar, within-plate melts at higher crustal levels as the Ellsworth-Whitmore terrane rifted off Gondwana (47°S) before migrating to its current position (82°S) in central Antarctica.

A seismic transect across West Antarctica: Evidence for mantle thermal anomalies beneath the Bentley Subglacial Trench and the Marie Byrd Land Dome

AbstractWest Antarctica consists of several tectonically diverse terranes, including the West Antarctic Rift System, a topographic low region of extended continental crust. In contrast, the adjacent Marie Byrd Land and Ellsworth‐Whitmore mountains crustal blocks are on average over 1 km higher, with the former dominated by polygenetic shield and stratovolcanoes protruding through the West Antarctic ice sheet and the latter having a Precambrian basement. The upper mantle structure of these regions is important for inferring the geologic history and tectonic processes, as well as the influence of the solid earth on ice sheet dynamics. Yet this structure is poorly constrained due to a lack of seismological data. As part of the Polar Earth Observing Network, 13 temporary broadband seismic stations were deployed from January 2010 to January 2012 that extended from the Whitmore Mountains, across the West Antarctic Rift System, and into Marie Byrd Land with a mean station spacing of ~90 km. Relative P and S wave travel time residuals were obtained from these stations as well as five other nearby stations by cross correlation. The relative residuals, corrected for both ice and crustal structure using previously published receiver function models of crustal velocity, were inverted to image the relative P and S wave velocity structure of the West Antarctic upper mantle. Some of the fastest relative P and S wave velocities are observed beneath the Ellsworth‐Whitmore mountains crustal block and extend to the southern flank of the Bentley Subglacial Trench. However, the velocities in this region are not fast enough to be compatible with a Precambrian lithospheric root, suggesting some combination of thermal, chemical, and structural modification of the lithosphere. The West Antarctic Rift System consists largely of relative fast uppermost mantle seismic velocities consistent with Late Cretaceous/early Cenozoic extension that at present likely has negligible rift related heat flow. In contrast, the Bentley Subglacial Trench, a narrow deep basin within the West Antarctic Rift System, has relative P and S wave velocities in the uppermost mantle that are ~1% and ~2% slower, respectively, and suggest a thermal anomaly of ~75 K. Models for the thermal evolution of a rift basin suggest that such a thermal anomaly is consistent with Neogene extension within the Bentley Subglacial Trench and may, at least in part, account for elevated heat flow reported at the nearby West Antarctic Ice Sheet Divide Ice Core and at Subglacial Lake Whillans. The slowest relative P and S wave velocity anomaly is observed extending to at least 200 km depth beneath the Executive Committee Range in Marie Byrd Land, which is consistent with warm possibly plume‐related, upper mantle. The imaged low‐velocity anomaly and inferred thermal perturbation (~150 K) are sufficient to support isostatically the anomalous long‐wavelength topography of Marie Byrd Land, relative to the adjacent West Antarctic Rift System.

The Transantarctic Mountains Northern Network (TAMNNET): Deployment and Performance of a Seismic Array in Antarctica

Over the past ∼17 years, significant technological developments have advanced seismic investigations of the polar regions. In Antarctica, studies using data from a number of large-scale deployments, such as the Transantarctic Mountains Seismic Experiment (TAMSEIS), the Gamburtsev Antarctic Mountains Seismic Experiment (GAMSEIS), and the Polar Earth Observing Network (POLENET), have greatly improved our knowledge of the continent’s tectonic evolution (Fig. 1). However, these studies have also illustrated that there is still a great deal more to be learned. Figure 1. (a) Subglacial bedrock topography from the BEDMAP2 model (Fretwell et al. , 2013). Shapes indicate stations from various seismic deployments across the continent (black stars, Antarctic Network of Unattended Broadband Seismometers [ANUBIS]; gray triangles, Transantarctic Mountains Seismic Experiment [TAMSEIS]; gray squares, Gamburtsev Antarctic Mountains Seismic Experiment [GAMSEIS]; gray circles, Polar Earth Observing Network [POLENET]; white circles and squares, Transantarctic Mountains Northern Network [TAMNNET]). The polygon highlights the region shown in (b). Key geographic features are labeled: TAMs, Transantarctic Mountains; WSB, Wilkes subglacial basin; VL, Victoria Land; RI, Ross Island; RSE, Ross Sea embayment; WARS, West Antarctic rift system; GSM, Gamburtsev Subglacial Mountains; DML, Dronning Maud Land; AP, Antarctic Peninsula; and EWM, Ellsworth–Whitmore Mountains. (b) Regional map focused on the TAMNNET deployment, with station names indicated. Additional geographic features not labeled in (a) include TR, Terror Rift; TNB, Terra Nova Bay; and MM, Mt. Melbourne. Stations denoted by circles are those powered with absorbed glass mat (AGM) batteries and two lampshade-style solar panel systems, whereas stations denoted by squares have one lampshade-style solar panel, one AGM battery, and 15 lithium battery packs. See text for further details. One of the goals of TAMSEIS was to investigate the origin of the Transantarctic Mountains (TAMs), the transcontinental mountain range that separates East and West Antarctica (Fig. 1; Robinson and Splettstoesser, 1986). The TAMs lack any evidence for a …

Central Antarctic provenance of Permian sandstones in Dronning Maud Land and the Karoo Basin: Integration of U–Pb and TDM ages and host-rock affinity from detrital zircons

In conjugate SE Africa and Antarctica, Early Permian sandstones of the Swartrant Formation of the Ellisras Basin, Vryheid Formation of the Karoo Basin, and Amelang Plateau Formation of Dronning Maud Land (DML) were deposited after Gondwanan glaciation on a westward paleoslope. We analysed detrital zircons for U–Pb ages by a laser ablation microprobe-inductively coupled plasma mass spectrometer (LAM-ICPMS) and attached age significance only to clusters of three or more overlapping analyses. We analysed Hf-isotope compositions by a multi-collector spectrometer (LAM-MC-ICPMS) and trace elements by electron microprobe (EMP) and ICPMS. These analyses indicate the rock type and source (whether crustal or juvenile mantle) of the host magma, and a “crustal” model age ( T DM C). The integrated analysis gives a more distinctive, and more easily interpreted, picture of crustal evolution in the provenance area than age data alone. Zircons from the Ellisras Basin are aged 2700–2540 Ma with minor populations about 2815 Ma and 2040 Ma, which correspond with the ages of the upslope parts of the proximal Kaapvaal Craton and Limpopo Belt. Mafic rock is the dominant host rock, and it reflects the Archean granite–greenstone terrane of the Kaapvaal Craton. The three Karoo Basin samples and the two DML samples have zircons with these common properties: (1) 1160–880 Ma, host magma mafic granitoid (< 65% SiO 2) derived from juvenile depleted mantle sources ( ε Hf positive) at 1.65 Ga and 1.35 Ga, with T DM C of 2.0–0.9 Ga; (2) 760 to 480 Ma, host magma granitoid and low-heavy rare earth element rock (?alkaline rock–carbonatite), derived from mixed crustal and juvenile depleted mantle sources ( ε Hf positive and negative) at 1.50 Ga and 1.35 Ga, with T DM C of 2.0–0.9 Ga. Together with similar detrital zircons in Triassic sandstone of SE Australia, these properties reflect those in upslope central Antarctica, indicating a provenance of ∼ 1000 Ma (Grenville) cratons embedded in 700–500 Ma (Pan-Gondwanaland) fold belts. Detrital zircons in Cambrian sediments of the Ellsworth–Whitmore Mountains block and Cambrian metasediments of the Welch Mountains with comparable properties suggest that the central Antarctic provenance operated also in the ∼ 500 Ma Cambrian.

Combined U-Pb geochronology and Hf isotope geochemistry of detrital zircons from early Paleozoic sedimentary rocks, Ellsworth-Whitmore Mountains block, Antarctica

Research Article| March 01, 2007 Combined U-Pb geochronology and Hf isotope geochemistry of detrital zircons from early Paleozoic sedimentary rocks, Ellsworth-Whitmore Mountains block, Antarctica M.J. Flowerdew; M.J. Flowerdew 1Natural Environmental Research Council (NERC) Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar I.L. Millar; I.L. Millar 1Natural Environmental Research Council (NERC) Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar M.L. Curtis; M.L. Curtis 2British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK Search for other works by this author on: GSW Google Scholar A.P.M. Vaughan; A.P.M. Vaughan 2British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK Search for other works by this author on: GSW Google Scholar M.S.A. Horstwood; M.S.A. Horstwood 3Natural Environmental Research Council (NERC) Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar M.J. Whitehouse; M.J. Whitehouse 4Swedish Museum of Natural History, Box 50007, SE-104 05, Stockholm, Sweden Search for other works by this author on: GSW Google Scholar C.M. Fanning C.M. Fanning 5Research School of Earth Sciences, Australian National University, Mills Roads, Canberra ACT 0200, Australia Search for other works by this author on: GSW Google Scholar GSA Bulletin (2007) 119 (3-4): 275–288. https://doi.org/10.1130/B25891.1 Article history received: 25 Aug 2005 rev-recd: 21 Jun 2006 accepted: 11 Sep 2006 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation M.J. Flowerdew, I.L. Millar, M.L. Curtis, A.P.M. Vaughan, M.S.A. Horstwood, M.J. Whitehouse, C.M. Fanning; Combined U-Pb geochronology and Hf isotope geochemistry of detrital zircons from early Paleozoic sedimentary rocks, Ellsworth-Whitmore Mountains block, Antarctica. GSA Bulletin 2007;; 119 (3-4): 275–288. doi: https://doi.org/10.1130/B25891.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract U-Pb detrital zircon geochronology from the upper Cambrian to Devonian part of the Ellsworth Mountains succession, Antarctica, yields dominant late Mesoproterozoic and late Neoproterozoic–Cambrian age populations that are consistent with a provenance from within Gondwana. Hf isotope compositions reveal a source predominantly within west Gondwana and identify a change in provenance up-stratigraphy that coincides with the change of sedimentation setting from active rift to passive margin, which has been independently determined by stratigraphic, structural, and geochemical arguments. For the Late Cambrian Frasier Ridge Formation, late Mesoproterozoic grains have positive ϵHf values, suggesting derivation from juvenile crust, and late Neoproterozoic–Cambrian grains have ϵHf values greater than −5, consistent with remelting of similar juvenile late Mesoproterozoic crust during the Pan African–Ross orogenies. Provenance during rifting was from proximal sources from within west Gondwana, most likely, southernmost Africa and basement to the Ellsworth-Whit-more Mountains block. At higher stratigraphic levels where deposition occurred along a passive margin, in the early Ordovician Mount Twiss Member and middle Devonian Mount Wyatt Earp Formation, late Neo-proterozoic–Cambrian grains have ϵHf values less than −5; this means that early Meso-proterozoic–Archean crust was remelted to generate these zircons. Provenance was from a more expansive source region within west Gondwana, and probably included the Kaapvaal and Congo cratons of south and west Africa. Isolated outcrops of sedimentary rock of uncertain age at Mount Woollard and the Whitmore Mountains have detrital zircon signatures similar to the Frasier Ridge Formation, suggesting correlation with these Late Cambrian deposits. Sedimentary rock from the Stewart Hills contains some late Mesoproterozoic grains with lower ϵHf values than the previously mentioned samples. This suggests that the Stewart Hills sample has a provenance from within east Gondwana and was possibly deposited on the East Antarctic craton prior to the Ross orogeny and is not part of the displaced Ellsworth-Whitmore Mountains crustal block. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Basin evolution during the transition from continental rifting to subduction: Evidence from the lithofacies and modal petrology of the Jurassic Latady Group, Antarctic Peninsula

The Jurassic Latady Basin (southern Antarctic Peninsula) developed in a broad rift zone associated with the early stages of Gondwana extension. Early Jurassic sedimentation (∼185 Ma) occurred in small, isolated terrestrial to lacustrine rift basins in the present-day northwest and west and became shallow marine by the early Middle Jurassic. Quantitative modal analysis reveals a high proportion of mature, quartzose sandstone derived from cratonic and quartzose recycled-orogen provenances, most likely in the direction of the Ellsworth–Whitmore Mountains in the Gondwana interior. Sandstones with a more volcanolithic provenance probably represent an influx of sands from a Permian volcanic source in West Antarctica. The Early Jurassic Latady sequence contains abundant volcanic quartz and rhyodacite grains, locally derived from the nearby ignimbrites of the rift-related Mount Poster Formation (∼185 Ma). Between the Middle and Late Jurassic (?160–150 Ma), there was a dramatic change throughout the Latady Basin to higher-energy conditions with marked lateral facies variations. Sandstones contain abundant fresh volcanic detritus and plot in the transitional arc field. Their source was a nearby, active continental margin arc, but there is no outcrop of arc material on the Antarctic Peninsula from this time. A possible source area is preserved on the Thurston Island block to the southwest. However, some fluvial systems still had access to areas of uplifted metamorphic/plutonic basement and quartzose, cratonic sources. Evidence of mixing of fluvial systems from different provenances and the lack of mixing of other fluvial systems suggest a complex topography of variably uplifted fault blocks with fluvial systems constrained in narrow valleys. The change from continental rift- to arc-related sources illustrates the shift from plume- (continental provenances) to continental margin arc-dominated tectonics. Thermal relaxation in the Late Jurassic led to the final phase of deposition in anoxic, deep-water conditions in a sediment-starved marine basin stretching from Ellsworth northward into southern South America.

A window on West Antarctic crustal boundaries: the junction between the Antarctic Peninsula, the Filchner Block, and Weddell Sea oceanic lithosphere

A new airborne magnetic survey of the southeastern Antarctic Peninsula and adjacent Weddell Sea embayment (WSE) region suggests a continuity of geological structure between the eastern Antarctic Peninsula and the attenuated continental crust of the Filchner Block. This has implications for the reconstructed position of the Ellsworth–Whitmore Mountains block in Gondwana, which is currently uncertain. Palaeomagnetic data indicate that it has migrated from a Palaeozoic position between South Africa and Coats Land to its current position as a microplate embedded in central West Antarctica. The most obvious route for migration is between the Antarctic Peninsula and the Weddell Sea embayment. Evidence that geological structures are continuous across the boundary places constraints on the timing and pathway of migration. Magnetic textures suggest the presence of shallow features extending from the Beaumont Glacier Zone (BGZ) in the west for at least 200 km into the Weddell Sea embayment. These data suggest that the Eastern Domain of the Antarctic Peninsula and the stretched continental crust of the Filchner Block share a common recent, probably post-Early Jurassic, history. However, examination of deep anomalies indicates differences in the magnetic characteristics of the two blocks. The boundary may mark either the edge of extended continental crust, or a discontinuity between two, once separated, blocks. This discontinuity, or pre-Late Jurassic Antarctic Peninsula terrane boundaries to the west, may have allowed the passage of the Ellsworth–Whitmore Mountains block to its present location.

The role of crustal and mantle sources in the genesis of granitoids of the Antarctic Peninsula and adjacent crustal blocks

Abstract: Magmatic rocks from the Antarctic Peninsula show marked variations in isotope composition, which reflect changes in the geodynamic evolution of the peninsula through time. Most Antarctic Peninsula granitoids formed as a result of subduction: they fall on well‐defined trends on plots of ϵ Nd, 207 Pb/ 204 Pb and δ 18 O against 87 Sr/ 86 Sr i , between a component derived from subduction‐modified mantle or juvenile basaltic underplate ( ϵ Ndi&gt;6, 207 Pb/ 204 Pb=15.61, δ 18 O=5.5‰, 87 Sr/ 86 Sr&lt;0.704) and an end‐member interpreted as a melt of Proterozoic lower crust ( ϵ Nd=−7, 207 Pb/ 204 Pb=15.67, δ 18 O=10‰, 87 Sr/ 86 Sr=0.709). A small group of granitoids, emplaced before or during Gondwana break‐up, plot on distinct trends towards high 87 Sr/ 86 Sr i compositions, reflecting mixing between melts derived from Proterozoic lower crust and melts of middle–upper crustal rocks ( ϵ Ndi=−9, 207 Pb/ 204 Pb=15.64, δ 18 O=10‰, 87 Sr/ 86 Sr=0.726), with little or no input of new material derived from the mantle or from juvenile basaltic underplate. These granitoids are thought to have formed as a result of crustal attenuation during the initial rifting phase of Gondwana break‐up. Similar trends are shown by data from granitoids of the adjacent crustal blocks of West Antarctica. The isotope data suggest that an enriched Ferrar/Karoo‐type lithosphere was not involved in the genesis of granitoids of the Antarctic Peninsula or of the Ellsworth–Whitmore Mountains crustal block.

Airborne gravity and precise positioning for geologic applications

Airborne gravimetry has become an important geophysical tool primarily because of advancements in methodology and instrumentation made in the past decade. Airborne gravity is especially useful when measured in conjunction with other geophysical data, such as magnetics, radar, and laser altimetry. The aerogeophysical survey over the West Antarctic ice sheet described in this paper is one such interdisciplinary study. This paper outlines in detail the instrumentation, survey and data processing methodology employed to perform airborne gravimetry from the multi‐instrumented Twin Otter aircraft. Precise positioning from carrier‐phase Global Positioning System (GPS) observations are combined with measurements of acceleration made by the gravity meter in the aircraft to obtain the free‐air gravity anomaly measurement at aircraft altitude. GPS data are processed using the Kinematic and Rapid Static (KARS) software program, and aircraft vertical acceleration and corrections for gravity data reduction are calculated from the GPS position solution. Accuracies for the free‐air anomaly are determined from crossover analysis after significant editing (2.98 mGal rms) and from a repeat track (1.39 mGal rms). The aerogeophysical survey covered a 300,000 km2 region in West Antarctica over the course of five field seasons. The gravity data from the West Antarctic survey reveal the major geologic structures of the West Antarctic rift system, including the Whitmore Mountains, the Byrd Subglacial Basin, the Sinuous Ridge, the Ross Embayment, and Siple Dome. These measurements, in conjunction with magnetics and ice‐penetrating radar, provide the information required to reveal the tectonic fabric and history of this important region.

Characteristics of Ice Flow in Marie Byrd Land, Antarctica

AbstractExtensive radio echo-sounding has mapped the part of West Antarctica between Byrd Station, the Whitmore Mountains, the Transantarctic Mountains, and the Ross Ice Shelf. The ice sheet in this area is dominated by five major sub-parallel ice streams (A–E), which are up to 100 km wide and extend inland from the grounding line of the Ross Ice Shelf for about 400 km. Their positions have been determined by crevassing seen on radio echo-sounding records, trimetrogon photographs, and Landsat imagery. The ice streams are characterized by their flat transverse cross-sections, while the intervening ice sheet exhibits domes and ridges. Ice flow lines are defined from the ice-surface contour pattern and the trend of the ice streams. It is apparent from this work that the flow line passing through Byrd Station joins ice stream D.The bedrock of the area is relatively smooth near the Ross Ice Shelf, becoming rougher near Byrd Station and especially so near the Whitmore Mountains. Bedrock troughs, which control the positions of the ice streams, are believed to have a tectonic origin.In this paper the role of the ice streams in the glaciological regime of West Antarctica is investigated from radio-echo data and estimates of balance velocity, basal shear stress, and basal temperatures.

Characteristics of Ice Flow in Marie Byrd Land, Antarctica

AbstractExtensive radio echo-sounding has mapped the part of West Antarctica between Byrd Station, the Whitmore Mountains, the Transantarctic Mountains, and the Ross Ice Shelf. The ice sheet in this area is dominated by five major sub-parallel ice streams (A–E), which are up to 100 km wide and extend inland from the grounding line of the Ross Ice Shelf for about 400 km. Their positions have been determined by crevassing seen on radio echo-sounding records, trimetrogon photographs, and Landsat imagery. The ice streams are characterized by their flat transverse cross-sections, while the intervening ice sheet exhibits domes and ridges. Ice flow lines are defined from the ice-surface contour pattern and the trend of the ice streams. It is apparent from this work that the flow line passing through Byrd Station joins ice stream D.The bedrock of the area is relatively smooth near the Ross Ice Shelf, becoming rougher near Byrd Station and especially so near the Whitmore Mountains. Bedrock troughs, which control the positions of the ice streams, are believed to have a tectonic origin.In this paper the role of the ice streams in the glaciological regime of West Antarctica is investigated from radio-echo data and estimates of balance velocity, basal shear stress, and basal temperatures.