T11A-1224 0800h
David Glacier, North Victoria Land, 2003-04 Local Seismicity Monitoring Campaign
Low magnitude seismic activity has been recorded in the North Victoria Land area, Antarctica, by permanent seismographic stations operating in the region (Terra Nova Bay, TNV, Italy; Scott Base, SBA, New Zealand; Vanda, VNDA, USA), as well as by portable broadband arrays positioned in the Transantarctic Mountains. Previous studies of these events have located hypocentres beneath the David Glacier, one of the largest glaciers in the Ross Sea region. During the 2003-2004 austral summer a temporary broadband seismic array was placed in the David Glacier region, with the aim of improving the estimates of the hypocentral and source parameters, in an effort to discriminate between the possible source processes (e.g. brittle ice failure, basal shearing, fault slip). The temporary array involved siting nine portable seismic recorders on the few rock outcrops surrounding the target area, with a total array aperture of approximately 100 km. The array recorded continuously for more than 4 months, from November 2003 to February 2004. Preliminary location work, utilising the double difference technique, indicates that all of the located events lie close to the area known as the David Cauldron, where the bedrock beneath the glacier has a marked change in slope and where there is considerable surface crevassing. The main cluster of events extends spatially less than 5 km, and lies at the downstream side of the Cauldron. All other events lie within 10-15 km of the main event cluster, within the approximate outer boundary of the Cauldron. The tight nature of the event cluster, and its proximity to the Cauldron, suggest that the source process for these events likely involves basal shear or ice failure, rather than fault rupture beneath the glacier.
T11A-1225 0800h
Seismic activity in the Transantarctic Mountains recorded by the TAMSEIS seismic array.
To investigate the links between glaciation and tectonics, we conducted a large-scale seismic deployment in Antarctica that measured local and regional seismicity of both the glaciated terrain of East Antarctica and the non-glaciated Transantarctic Mountains (TAM). The TAM are hypothesized to have formed by rift-flank uplift of the southwestern margin of the West Antarctic Rift System. Active extension of this rift and/or continued uplift of the TAM would likely result in relatively high levels of seismicity along the mountain front. In addition to seismicity from tectonic activity, we suggest that the flow of glaciers, particularly where they accelerate through the TAM, could result in glacier-induced seismicity. We recorded relatively high levels of local seismicity in the TAM. The majority of the seismicity was close to and slightly west of the TAM, beneath the East Antarctic Ice Sheet. We used the double-difference hypocenter location method (Waldhauser and Ellsworth, 2000; Waldhauser 2001) to better image clusters of events. Many of the events are shallow and cluster beneath the David Glacier (which leads to the Drygalski Ice Tongue) and the Darwin Glacier. We suggest that these events are due to fracture at the base of the glaciers, as they steepen towards the coast. We continue to investigate the possibility of surface crevassing and TAM uplift-induced seismicity (along faults which the glaciers have exploited) as the cause of the seismicity.
T11A-1226 0800h
Upper Mantle Structure Beneath the Transantarctic Mountains From Body-Wave Tomography and Receiver Functions Using TAMSEIS Data
The Transantarctic Mountains (TAM) consists of gently tilted fault blocks resulting from vertical crustal movement during the Cenozoic. Paralleling much of the West Antarctic Rift System, the TAM is considered by many to be a classic example of rift flank uplift, however evidence supporting a clear uplift mechanism has yet to be provided. Additionally, the adjacent East Antarctic Craton exhibits anomalously high elevation for a cratonic block, approximately 1 km above sea level, when corrected for glacial loading. To investigate these two unique tectonic features of the Antarctic continent, body-wave tomography and receiver-function stacking are being conducted with broadband seismic data collected by the 2000-2003 Transantarctic Seismic Experiment (TAMSEIS). With these analyses, we can make inferences about the thermal structure of the upper mantle beneath portions of the TAM and East Antarctic Craton. Constraints on the thermal state of the upper mantle beneath these regions may enable us to discriminate between the competing uplift models. The multi-channel cross-correlation method of VanDecar and Crosson (1991) has been used to accurately determine relative P and S wave arrival times and uncertainty estimates for teleseismic events. Travel-time residuals indicate azimuthal variability and that slower velocities are present beneath the Transantarctic Mountains than the East Antarctic Craton. The travel time residuals will be inverted for upper mantle structure. Receiver functions for several hundred teleseismic earthquakes have been computed and are being stacked to image the 410 and 660 km discontinuities. Topography on the discontinuities will be correlated with seismic velocity anomalies to help constrain the depth extent of thermal anomalies in the upper mantle.
T11A-1227 0800h
Lithoepheric Structure And Tectonics Derived From Active And Passive Seismic Studies In The Early-Paleozoic Lutzow-Holm Complex, Eastern Dronning Maud Land, East Antarctica
Lithospheric structure beneath the Lutzow-Holm Complex (LHC), Eastern Dronning Maud Land, East Antarctica, were studied using both the active and passive seismic sources. LHC is considered to be one of the collision zones between East- and West- Gondwana during the formation of a paleo-supercontintnt in Pan-African orogeny. Seismic velocities and reflection structure beneath LHC were obtained by deep explorations on continental ice sheet of Mizuho Plateau in LHC, in 1999-2000 and 2001-2002 austral summers by Japanese Antarctic Expedition. Travel-time analyses for wide-angle reflection and refraction data revealed the Moho depths ranging in 38-42 km from the coast to 200 km inland, with the averaged velocities of upper, middle, lower crust and uppermost mantle, as 6.1, 6.4, 6.5 and 8.0 km/s, respectively. Velocities in the topmost crust have a variation in 5.9-6.2 km/s along the NE-SW direction profile (parallel with coast), which corresponds to metamorphic grade of surface geology from amphibolite to granulite facies. These crustal velocities have almost coincident with those derived from the receiver functions analyses by applying non-linear Genetic Algorithm (GA) waveform inversion for the Broadband-Array data of outcrops along the coast in LHC. Second, reflection section derived from the total 12 explosions revealed several reflections around crust-mantle boundary; which can be traced at 13-14 s of TWT. These Moho reflections consist from several horizontally lying segments partially duplicated architecture in NE-SW profile: on the contrary, fairly flat and smooth signature in NW-SE (perpendicular to the coast). Inner crustal reflectors, moreover, are found around lower crustal depths at 9-10 s of TWT. Layered structure around the Moho can also be supported by spectral ratio analyses for PmP phases against refracted P phases in the near-field records. The above heterogeneous structure from lower crust to crust-mantle boundary may be originated from the ancient tectonic setting, such as Pan-African metamorphism associated with the continent-continent collision between East- and West Gondwana, followed by the breakup of LHC with India and Sri Lanka in mid Mesozoic. In presentation, we also introduce the characteristic waves originated from natural sources recorded on the exploration, such as of deep teleseismic and regional events around Antarctic together with local icequakes. For instance, frequency contents of 2.0 Hz in the waveforms show discordances around the stations at where just above the valley topographic interface between ice sheet and topmost crust.
T11A-1228 0800h
Upper Mantle Seismic Anisotropy of the Ross Sea, Trans-Antarctic Mountains, and East Antarctica From SKS Splitting Analysis
The Trans-Antarctic Mountains seismic experiment (TAMSEIS), a two year deployment of 43 broadband seismographs extending from Ross Island to the interior of East Antarctica, offers an excellent opportunity to study the anisotropic fabric of the Antarctic upper mantle, which is largely unconstrained. We analyze SKS and SKKS phases for shear wave splitting using the method of Silver and Chan (1991). To check the robustness of our results, we also utilize the cross-correlation method (Bowman and Ando, 1987) and visually inspect particle motions. The splitting functions are then stacked to obtain the best fit splitting parameters for each station. Results show that the Antarctic lithosphere beneath the Trans-Antarctic Mountains (TAM) and the adjacent East Antarctic craton are characterized by a uniform region of mantle anisotropy, with fast axes oriented at about N35E to N65E and splitting magnitudes of 0.5 - 1.0 seconds. This is consistent with azimuthal variations of Rayleigh wave phase velocities reported by Lawrence et al [2004]. The Rayleigh wave anisotropy is strongest at periods of about 40 seconds, suggesting the anisotropy is strongest in the uppermost mantle. This suggests that it represents upper mantle lattice preferred orientation that is remanent from past deformational episodes, rather than the current upper mantle flow pattern. The continuity of anisotropic directions between the TAM and the East Antarctic craton suggests the fabric is remnant from past deformational episodes, rather than a result of the current TAM uplift. The mapping of upper mantle anisotropic directions offers a possible method for delineating geologic terranes in ice-covered East Antarctica.
http://epsc.wustl.edu/seismology/TAMSEIS/
T11A-1229 0800h
Lithospheric Structure of East Antarctica: Results From the First Year of the SSCUA Broadband Seismic Deployment
Recent geological and geochronological work has changed our concept of the former Gondwana continents surrounding a central Archaean craton, East Antarctica. The revised tectonic framework shows mobile belts and tectonic province boundaries which correlate well with East Antarctica's former neighbours in the supercontinent and trend perpendicular to the present day Antarctic coastline. Constraints are however restricted to those areas where the rock is exposed above the ice. Indirect, geophysical methods are required to map the extent of tectonic provinces in the continental interior and provide information on the deep structure. Following a pilot deployment in 2001/02, a set of 6 remote broadband earthquake recording stations were deployed in the Lambert Glacier region, East Antarctica, from the coast at 65°S deep into the interior to a latitude of 75°S. The aim was to determine the Seismic Structure of the Continent Under Antarctica (SSCUA) using a variety of seismological techniques. The stations were solar-powered and hence shut down during the Antarctic winter to re-commence recording after the return of daylight in the austral spring. After one year, 3 stations were relocated to further test contemporary terrane models of the lithosphere in this region. At the present time (December 2004) recording continues across the deployment with most stations due to be uplifted at the end of the 2004/05 summer season. Results of receiver function inversions for seismic structure beneath the recording stations of the first main deployment are presented. To the west of the Lambert Glacier, the Rayner province extends from Mawson Station to Beaver Lake with a fairly deep Moho at 42 km. To the east, seismic velocity profiles have a different character, showing slower crustal velocities. A province boundary exists between the Reinbolt Hills and the northern Mawson Escarpment where the crust is shallower at 34 km. These results are the first determinations of the seismic structure of the lithosphere in this area of continental East Antarctica
T11A-1230 0800h
Fault Patterns Within the Eastern Terror Rift, Western Ross Sea, Antarctica
The Terror Rift region is an important key to understanding the timing and kinematics of neotectonic rifting between East and West Antarctica. In January/February 2004, a 30-day geophysical cruise (NBP04-01) was conducted in the western Ross Sea with the goal of analyzing faulting associated with the Terror Rift. New, east-west, multichannel seismic reflection profiles (line spacing between 5-15 km) have revealed new details about a prominent fault zone striking generally N-S between 166°E and 167.5°E within the western Ross Sea. The fault zone can be traced along strike for at least 150km. Initial comparison to previous seismic surveys correlates this fault zone with the margin of the "Discovery Graben" and the "Lee Arch" in the eastern part of the Terror Rift. Our new lines show that the faults clearly have normal displacement, having down-to-the-west offset in the east, and down-to-the-east offset in the west. Bedding in the fault zone defines an antiformal structure, and faults change dip and sense of normal displacement across the antiformal hinge zone. On some seismic profiles a prominent unconformity truncates the stratigraphic interval affected by intense faulting, with fewer younger faults cutting the unconformity and reaching the seafloor. On other lines the interval of intense faulting is at the seafloor and the unconformity is absent, possibly indicating more recent glacial erosion. Comparison of the reflectors and unconformities within this fault zone to the dated cores of the Cape Roberts Project should allow us to reconstruct a detailed interpretation of the timing and history of rifting in the region.
T11A-1231 0800h
Franklin Volcanic Field: Characteristics of a Submarine Volcanic Province in the Western Ross Sea, Antarctica
The RVIB N. B. Palmer collected bathymetric and seismic data in the western Ross Sea in January/February 2004 with the specific purpose of mapping neotectonic structural and volcanic features on and beneath the seafloor. The Franklin Volcanic Field (FVF) is an important feature revealed by the bathymetric data. The volcanic field, which is expressed as a regional bathymetric high, is a minimum of 80 km long and up to 40 km wide. As one moves north, the overall trend of the volcanic field appears to curve from N-S to NE. Superimposed on the regional high are the subaerial portion of Franklin Island and clusters of submarine volcanic cones. Franklin Island has been interpreted as the western remnant of a shield volcano and has a single K-Ar date of 4.8+2 Ma. Its submarine morphology is characterized by a steep western flank and an eastern flank that, within the limits of the mapped region, appears to have a shallower slope. South of Franklin Island there is a cluster of cones which lead into the steep southern slope. North of the island, there are at least 7 discrete cones. Five of the volcanic cones in the FVF were dredged, yielding basaltic lava, hyaloclastite and tuff. Abundant glass is preserved in the volcanics, some lavas are olivine-phyric, and lavas locally contain mantle and crustal xenoliths. The seafloor morphology in the northern sector of the field is characterized by lineations at several scales, some hummocky terrain marked by circular to oval depressions, and superimposed iceberg scours. Based on morphology alone, the linear features could reflect volcanic ridges, faults, current or glacial lineations, or may have formed by a combination of these processes. The origin of the seafloor features within the FVF is being determined through integrated analysis of magnetic, gravimetric and seismic data. When genesis is established, regional and local feature trends will be used to interpret the direction of glacial flow as well as maximum and minimum crustal stress directions at the time of formation of the FVF.
T11A-1232 0800h
Neogene-Quaternary Volcanic Alignments in the Transantarctic Mountains and West Antarctic Rift System of Southern Victoria Land, Antarctica
Neogene-Quaternary volcanism in southern Victoria Land, Antarctica, produced the Erebus Volcanic Province, a suite of alkaline volcanic rocks that extend from the Transantarctic Mountains rift-flank uplift to offshore localities within the West Antarctic rift system. We are mapping volcanic vent patterns in the province to detect alignments indicative of stress/strain patterns during rift evolution. In the southern sector of the Erebus Volcanic Province in the Royal Society Range Block of the Transantarctic Mountains, mapping shows that elliptical scoria cones, fissures, dikes, and linear vent arrays define volcanic alignments that have a dominant NNE trend, with subsidiary WNW trends. Age data for the alignments suggest that this pattern persisted from 14.6 to 0.25 Ma. We are currently completing mapping along an east-west transect crossing the rift margin, and results obtained so far within the rift region indicate a similar pattern of alignments. On the northern flank of Mount Morning, a large volcano just to the east of the Royal Society Range, elliptical scoria cones and linear vent arrays define volcanic alignments that have a dominant NE trend, with a subsidiary NNW trend. Available age data suggest that many of these cone alignments may be of Quaternary age. At Brown Peninsula, further east from the rift flank, cone alignments trend NNE and available ages range from 2 to 3 Ma. To the east of Brown Peninsula, cone alignments trend NW at Black Island, but are of uncertain age; age data on Black Island range from 11 to 3.4 Ma. At White Island, the farthest east into the rift, cone alignments trend NNE and available age data suggest volcanism as young as 0.2 Ma. Although some differences in cone alignment trends are apparent between the rift flank and the rift system across our transect, both regions appear to be dominated by NE trending alignments, which implies a WNW to NW minimum horizontal stress (Shmin) direction. This is oblique to the ENE Shmin Cape Roberts in situ contemporary stress direction determined from borehole breakouts and induced core fractures, and to a single volcanic stress alignment of Neogene age within the rift flank in that sector. In both regions the Shmin direction is perpendicular to the interpreted trend of the structural boundary of the rift. The stress directions inferred from volcanic alignments are thus consistent with spatial changes in the Neogene-to-contemporary stress field.
T11A-1233 0800h
Seismic Stratigraphy of the Terror Rift, Southern McMurdo Sound, Antarctica
During the austral summer of 2004 the RVIB Nathanial B Palmer (NBP0401) collected 2000 km of multi-channel and 500 km of single-channel seismic data over the Terror Rift and western Victoria Land Basin, Antarctica (VLB). Here we summarize the results of integrating the data from the VLB margin with other reflection data and with lithological information from Cape Roberts (CRP) and CIROS drillholes, which sampled in CRP-1 to 3 a 1500-m-thick sequence of near-shore glaciomarine strata deposited over 70 cycles between 34 and 17 Ma ago. The stratigraphic framework is based on distinct, regionally traceable horizons that correspond to significant and recognizable changes in glaciomarine sequence geometry and can be interpreted as recording a basin-wide climate and tectonic events. These horizons can be traced from the cores for many kilometres in both coast-parallel and coast-normal directions. The seismic data also reveal a sedimentary succession in the Terror Rift (Late Miocene and younger in age) that reaches up to 2.5 s (at least 3500 m in thickness), which overlies the succession sampled by CRP drillholes.
T11A-1234 0800h
High Resolution Magnetic Anomaly Imaging of Southern McMurdo Sound (Antarctica)
During the 2003-04 austral summer season, a low-altitude, high-resolution airborne magnetic survey was conducted in the area of Southern McMurdo Sound, western Ross Sea, in the framework of the joint Italian-US initiative called GEOIMAG (GEOphysical IMAGing of Antarctic tectonic and volcanic units in the Transantarctic Mountains and Ross Sea area). The survey was flown by helicopter from McMurdo Station, at a constant terrain clearance of 70 m. Profile spacing was 350 m. The resolution of the survey, the accuracy of the data acquisition, and a preliminary digital enhancement of the processed magnetic data have contributed to highlight the short-wavelength component of the regional magnetic anomaly field, resolving volcanic bodies and faults. A broad positive anomaly tapers northward from the volcanic Brown Peninsula. The breadth and low amplitude of the anomaly suggests this may mark relatively thin deposits of volcanic material on or near the seafloor. High-frequency and high-amplitude anomalies are associated with the volcanic Daily Islands at the edge of the Ross Ice Shelf. Anomalies of similar amplitude and size in adjacent areas indicate additional volcanic cones occur on the seafloor and beneath the ice sheet. Low-amplitude, curvilinear anomalies with a convex shape toward McMurdo Sound occur offshore of the outlet of Ferrar Glacier and extending southward parallel to the coastline. The new data are being integrated with a multi-source database (magnetic, digital terrain data, bathymetry, regional structural trends) to obtain an unprecedented view of the magnetic signature of the major tectonic elements in the area. The structural and volcanic framework resulting from this investigation will provide important site survey information for the proposed ANDRILL drill sites in southern McMurdo Sound.
T11A-1235 0800h
Subglacial Geology of the Southern Transantarctic Mountains, Antarctica, From Airborne Radar Sounding
In light of the upcoming International Polar Year (IPY), the value of airborne radar sounding to infer subglacial geology from high resolution geophysical surveys of East Antarctica must be stressed. The potential benefit of such surveys, especially when combined with existing geological data, is demonstrated, in part, by bedrock characterization of the southern Transantarctic Mountains (TAM) between the Scott and Reedy Glaciers and from Ice Stream A to the South Pole along $150\deg$W. Absolute bedrock elevation maps derived from ice sheet surface elevation and thickness reveal four distinct regional subglacial morphologies including 1) a polar basin and plateau region with low relief features and thick (3km) ice cover; 2) an area of well-preserved U-shaped valleys possibly formed by alpine glaciation, 3) the TAM massif, the 30-50 km wide region of maximum uplift that includes subglacial fault blocks and the subaerial part of the TAM; and 4) the TAM front, a normal fault zone that forms the northern terminus of the range by down-dropping the TAM from $>$3000 m to sea level over approximately 50 km. The southern TAM have a southward tilted fault - block structure and are bounded by normal faults on both the north and south sides of the massif. Faults are oriented primarily subparallel to the TAM and to the West Antarctic Rift System fabric in the Interior Ross Embayment. Faults oriented obliquely to the TAM break the area of maximum uplift into three blocks. Ongoing studies of the TAM front focus on discriminating between erosional and faulted topography through more detailed analysis of radar sounding profiles. Adaptation of seismic migration techniques to these data enhances our ability to measure, identify, and map formational contacts and faults indicative of Cenozoic (or older) tectonic processes.
http://www.ig.utexas.edu/research/projects/tam/PPT
T11A-1236 0800h
Magnetic Depth Estimates and Their Potential for Constraining Crustal Composition and Heat Flow in Antarctica
Crustal composition, temperature, and heat flow, key parameters in ice sheet and tectonic models, are difficult to measure in largely ice-covered Antarctica. Aeromagnetic data have been used on other continents to determine depths to the bottom of magnetic sources from which heat flow has been calculated. However, these depths can also reflect compositional variations, and the consistency of results from various depth determination methods has not been tested. Present spectral magnetic depth determination methods require 1) large window sizes (roughly 10 times the depth to the bottom of the magnetic layer), 2) statistical random or fractal behavior of sources, and 3) datasets free of intermediate to long-wavelength problems stemming from datum shifts or warping of surveys. Depths to both the top (Spector and Grant, 1970, as revised by Fedi et al., 1998) and bottom (Bhattachharyya and Leu, 1975; Shuey et al., 1977; Connard et al., 1983; Okubo et al., 1985; and Blakely, 1988) of magnetic units were calculated. Cross-checking the results by modeling the spectral slopes and locations of peaks ensures reasonable results. Consistency of results from the different methods leads to confidence in the derived estimates. Application of these methods helps determine depths to the top and bottom of magnetic units from several regions in east and west Antarctica using single surveys flown in short periods. Assuming the spectra are obtained from sufficiently large window sizes, comparison of the aeromagnetically-determined layer thicknesses and positions from those computed with other data yield information on crustal composition, and possibly, depth to the Curie isotherm and in turn, heat flow. For example, preliminary results show that a primary magnetic layer thins from ~15-20 km in inferred Precambrian crystalline shield beneath the polar plateau to ~7 km in reworked and juvenile crust of the central Transantarctic Mountains. If the 7 km depth represents the Curie isotherm, heat flow values of 100-200 mW/m2 would be expected, depending on the magnetic petrology of rocks. The lack of Cenozoic volcanic rocks in the region suggest that the heat flow is not this high; implying that the magnetic sources observed spectrally lie well above the Curie isotherm and that the thickness measurements reflect changes in crustal composition.
T11A-1237 0800h
Aeromagnetic and Gravity Data Reveal Crustal Structure and Tectonic History of the Central Transantarctic Mountains Region
Near complete coverage of the East Antarctic shield by ice hampers geological study of the crustal architecture important for understanding global tectonic and climate history. Limited exposures in the central Transantarctic Mountains, however, show that Archean and Proterozoic rocks of the shield as well as Neoproterozoic-lower Paleozoic sedimentary successions were deformed during oblique convergence associated with Gondwana amalgamation. Subsequently, the area was overprinted by Jurassic magmatism and Cenozoic uplift. To extend the known geology of the region to ice-covered areas, we conducted a draped aeromagnetic survey flown by helicopters over the Transantarctic Mountains and by fixed-wing aircraft over the adjacent polar plateau. We flew $>$32,000 line km covering an area of nearly 60,000 $km^{2}$ at an average altitude of 600 m (average line spacing 2.5 km over most areas and 1.25 km over basement rocks exposed in the Miller and Geologists ranges). Additional lines flown to true north, south and west extended preliminary coverage and tied with existing surveys. Broad, moderate amplitude magnetic highs and lows over the ice-covered plateau resemble those inferred for Precambrian shield provinces to the north, suggesting a similar origin. Additionally, seismic tomographic models and a ground-based gravity profile show that the region is underlain by thick, cold lithosphere. Exposed high-grade metamorphic rocks, representing lower crust exhumed from $\sim$25-30 km depth during the Ross orogeny, show variable magnetic anomalies, with pronounced central highs and a linear, SE-trending corrugated fabric that correlates with ductile shear structures and regional folds. The magnetic highs correlate with retrogressed mafic eclogites in exposed layered gneiss and are in line with trends of high-amplitude magnetic highs and lows over the adjacent polar plateau. The parallelism of these anomaly trends with those farther to the west suggests that rock types exposed in the surveyed ranges extend beneath the polar plateau. Outboard lower Paleozoic siliciclastic rocks show uniformly quiet magnetic character, whereas Jurassic Ferrar sills and inferred dike swarms yield magnetic highs. Distinctive magnetic lows are associated with exposed Ross-age granitic plutons and indicate several large bodies hidden beneath the ice. A prominent thrust system exposed north of Nimrod Glacier, which places Neoproterozoic-Cambrian platform rocks upon Cambrian-Ordovician molasse deposits, can be traced magnetically several 10's of km to the south, including places previously mapped erroneously as an unconformity boundary.
T11A-1238 0800h
Evidence That Early to Middle Miocene ice Streams From West Antarctica cut Into Southeastern Ross Sea Continental Shelf
The extent of the West Antarctic ice sheet during mid-Cenozoic time is controversial and important to climate models. High-resolution multichannel seismic reflection data were acquired using the RVIB Palmer along the edge of the Ross Ice Shelf across the Eastern Basin of Ross Sea, in an area where calving of the ice shelf has exposed seafloor that has not been accessible to marine geophysics in several decades. A sub-basin in the far southeast corner of Ross Sea contains a succession of sediment-filled troughs, each capped by an unconformity. These troughs range between 5 and 20 km across, and are 100 to 150 m-deep. They are cut into a sequence correlated to slightly predate ~24 Ma. The shallowest of these unconformities (named "Red" here) can be tentatively projected across the northern plunge of a basement ridge located north of Roosevelt Island (named "Roosevelt Ridge" here), and is a regional unconformity in the Eastern Basin. Reflections just below Red can be correlated to DSDP 272, where they are dated at ~14 Ma. Red is flat and level in the south, adjacent to the ice shelf edge. Older sequence boundaries beneath Red merge with it across a 70 km extent between the deep Eastern Basin and Roosevelt Ridge. Much of the late-Early Miocene and early-Middle Miocene section is missing and appears to be removed by erosion associated with the Red unconformity. There is no evidence for broad glacial troughs that predate Red west of Roosevelt Ridge. If our correlations are correct, the succession of glacial troughs must be Middle Miocene and older, and we cannot rule out the oldest being Late Oligocene without additional data. Because we do not believe that a glacier or grounded ice sheet could erode a thick interval of sedimentary rocks and still produce a smooth and level unconformity, our preferred hypothesis is that Red is a wavecut surface that has since subsided to its present 700 m depth. The fill of one 20 km-wide trough is exposed at the seafloor and accessible to Shaldrill coring to test our hypothesis. The Ross Ice Shelf is advancing over the surveyed troughs and future drilling from the ice shelf will also be possible. Finally, it may be possible to correlate the fill of the troughs to DSDP stratigraphy with acquisition of carefully-positioned seismic reflection profiles. These troughs are consistent with erosion by ice streams or glaciers fed from an ice cap over at least part of western Marie Byrd Land. An Early Miocene ice cap contrasts with the Late Miocene age for the earliest widely accepted time for grounded ice in West Antarctica.
T11A-1239 0800h
Isostatic Rebound due to Glacial Incision within the Transantarctic Mountains
In temperate climates about 25% of peak elevations in mountain ranges can be created by isostatic rebound as a response to incisional erosion. Significantly more relief generation and peak uplift is, however, possible for glacial erosion in a polar climate. Here we us a 3D flexure study to show that up to 2000 m or 50% of peak elevation in the central Transantarctic Mountains (TAM) is due to isostatic rebound as a response to glacial incision. Maximum rebound is predicted for the Beardmore-Nimrod region. Here peak elevations are about 800 m higher than elsewhere along the range and we show that this elevation difference can be ascribed to the rebound response to deep incisional erosion by the Nimrod and Beardmore glaciers. Comparable global localities for relief on the scale of the central TAM are only seen in the Himalayas where fluvial erosion has cut gorges of similar magnitude. But such strong relief in the Transantarctic Mountains is possible because of the special conditions afforded by a polar climate and adjacent ice sheet. In particular, the combination of freezing conditions at high elevations, which acts to preserve the peaks, and wet based glaciers at lower elevations produce optimal conditions for enhanced glacial incision. Based on our knowledge of the glacial history of the TAM, the likely time for creating relief, and hence rebound, is probably mid-Miocene when the East Antarctic ice cap became fully developed. As glacial incision is an easily quantified, negative-load, we use this loading to test different rheological models for the central, Transantarctic Mountain front. What is clear from the minimal disturbance of seismic stratigraphy in the adjacent Ross Embayment, is that if any shear stresses are transmitted across the front they are relatively minor and restricted to be within 40 km of the front. Finally, because isostatic rebound results in permanent peak uplift this mechanism provides an explanation of why the Transantarctic Mountains are one of the higher and more long-lived continental rift-margins on Earth.
T11A-1240 0800h
Geologic framework for the East Antarctic Ice Sheet: Geophysical Constraints
Recent geophysical studies of the West Antarctic Ice Sheet have demonstrated that regional geology plays a dominant role in controlling the dynamics of ice sheet motion. However, understanding of the geologic setting for the East Antarctic Ice Sheet remains limited. Recently acquired geophysical data (ice penetrating radar, seismic, gravity, magnetic, surface elevation) in the region of the Wilkes subglacial basin provides new insights into the sub-ice conditions, allowing us to better address the influence of the subglacial geology on ice flow in East Antarctica. In particular we look at the ice dynamics associated with sedimentary basins, as determined by seismic and gravity data, as well as the role of water-saturated sediments as inferred by radar amplitude analysis. In addition, we couple these constraints on subglacial conditions with observations of large-scale englacial structures to better characterize ice flow in the region.
T11A-1241 0800h
An Aerogeophysical Transect South of the Prince Charles Mountains, East Antarctica
Interpretations of the existing magnetic data in the Lambert region raise questions about the nature of the southern Lambert Rift system. Preliminary interpretations of this data have allowed distinction of major geological blocks and magnetic trends, which can be correlated with the known geology of the Southern Prince Charles Mountains (PCMs). The objective of this investigation was to extend the magnetic data to the south and also acquire ice penetrating radar and gravity data to assist in understanding the Lambert / Amery rift system. This will provide a better understanding of the formation and breakup of Gondwana. Ice radar data will also contribute to ice mass balance analysis of this major ice drainage area in Antarctica. This research presents the methodology, data and preliminary interpretations of the airborne data acquired as part of the Prince Charles Mountains Expedition of Germany and Australia 2002/03 (PCMEGA). The area of study covered part of the southern PCMs and the adjacent polar ice cap, from approximately 72.5 to 77.5 South and 62 to 72 East. Within a month of field work more than 20000 km of survey lines at 5km line spacing and 25 km tie-line spacing was acquired over an area of approximately 81000 square kilometers. Transit flights between the base camp at Mt. Cresswell and the survey grid were utilized to collect ice radar data over the major ice tributaries feeding the Lambert Glacier. Increasing surface elevation towards the southern region of the survey grid made it necessary to divide the survey into three separate flying elevations of 2160 m, 2760 m and 3360 m (GPS height). The aeromagnetic image is characterized by long wavelength magnetic anomalies which display different trends. The southern region of the image is dominated by alternating high-low northeast-trending magnetic anomalies. These anomalies have been truncated by a northwest trending feature suggesting separation of major blocks by crustal scale faults / shear zones. The gravity data presented here is simply free-air gravity. The gravity image displays long wavelength anomalies which are contrasted by sharp gradients. North to north-north-east trending features are truncated in the northern region of the grid by a north-west trending gravitational low. The high gravitational response in the south-east region of the grid is interpreted as the `foothills' of the Gamburtsev Mountains. Detailed maps of sub-ice topography were produced that allow a 3-dimensional ice thickness correction of the gravity data and the production of crustal thickness maps. The PCMEGA airborne survey provided new elevation and ice thickness data of the upper Lambert glacier basin extending to 78 South. This data can be used to assess the state of balance by computed balance fluxes based on an improved DEM and fluxes derived from ice thickness and velocity. Maps illustrating the sub-ice topography are one of the major results of the PCMEGA project and the basis for a range of geological and glaciological interpretations.
T11A-1242 0800h
Passive Margin Uplift in Antarctica Possibly Controlling the Global Climate Change
The basement uplifts on the continental margin in Antarctica is important for the development of ice sheet and its movement. If there was no uplift in Antarctica, a large amount of ice sheet might flow into the sea, and the present amount of ice sheet might not be maintained. Since the amount of ice on the land and that flowing into the sea are very important factors controlling the Earth_fs climate, the passive margin uplift in Antarctica may partly play an important role in the global climate change. Therefore, to reveal the mechanism of uplift of the passive margin in Antarctica relates to the study on the development of ice sheet and the global climate change. Although the lack of geological data makes it difficult to expect the importance of the passive margin uplift in Antarctica for the Earth_fs climate, we propose a possibility that the passive margin uplift in Antarctica partly controls the climate of the Earth. In order to investigate the hypothesis we first have to examine the origin of the elevated mountain chains on the margin in Antarctica. This study is a first step in the investigations of the linkage between the passive margin uplift in Antarctica and global climate change. In this study we mainly discuss the mechanism of Gondwana breakup and its associated marginal uplift. Based on a two-dimensional thermo-mechanical finite element model, we examine the response of the continental lithosphere to extensional tectonic force, and the dependence of the magnitude of the uplift on the initial lithospheric structure is evaluated. We also estimate the initial lithospheric structure leading to the continental breakup due to the horizontal tectonic forces, and discuss the mechanism of Gondwana breakup. Our numerical results indicate that larger amplitude of the uplift can be obtained for the stronger lithosphere, in which the uplift is an intrinsic consequence of the necking process and is supported by the flexural strength. It is also found that there is a critical strain rate leading to the continental breakup and it seems difficult to obtain the Gondwana breakup by the currently accepted magnitude of the tectonic force for the initial rheological condition inferred beneath the East Antarctica. The breakup may require the dynamic effects of rising plume. However, the dynamic interaction between the convective asthenosphere and strong lithosphere has never clarified well yet, and the passive margin uplift should be affected by the thermal buoyancy and subsequent cooling. Our future studies to deepen the understanding of the mechanism of Gondwana breakup and passive margin uplift will address these important issues.
T11A-1243 0800h
Provenance of Glaciomarine Sediments in the Circum-Antarctic Oceans: Implications for Geological History of Bedrock Sources and Sediment Dispersal
The Antarctic ice sheet covers a large fraction of the Antarctic continent thereby preventing a detailed study of the underlying basement rocks. Although the main geological divisions of Antarctica are reasonably well understood, more information is needed on the bedrock geology in order to better constrain the sources of sediments originating from Antarctica, and secondly, to characterize the dispersal of sediments entering the circum-polar oceans. Here we present the radiogenic (Nd and Sr) isotope composition of the $<$63 um terrigenous sediment fraction as well as $^{40}$Ar/$^{39}$Ar ages of individual hornblende grains taken from the $>$150 um fractions of core-top samples from a series of deep-sea cores that are located around the perimeter of the Antarctic continent (latitude $\sim$65$\deg$°S). The sediments are glaciogenic in origin and likely derive from the glacial erosion by ice streams that are fed by tributaries that drain the deep interior of the continent. The sediment samples should thus represent the variety of bedrock lithologies underlying the different sectors of the ice sheet. The results show a geographic variability of Nd and Sr isotope compositions that reflects the large age-range of basement domains of Antarctica. For instance, samples with radiogenic Nd and unradiogenic Sr near the Antarctic Peninsula are concordant with the recent volcanics and Mesozoic rocks of this region ($\epsilon$Nd: -1 to -3; $^{87}$Sr/$^{86}$Sr: 0.7068-0.7078), while samples with highly unradiogenic Nd and radiogenic Sr around the East Antarctic region reflect the presence of basement rocks with Archean and Proterozoic heritage (ex.: Amery Ice Shelf sector, $\epsilon$Nd: -17 to -21; $^{87}$Sr/$^{86}$Sr: 0.7046-0.7077). Samples from West Antarctica and the Antarctic Peninsula yield hornblende $^{40}$Ar/$^{39}$Ar ages $<$200 Ma, in agreement with the surficial bedrock geology of this sector and the Nd and Sr isotope data. In contrast, the $^{40}$Ar/$^{39}$Ar hornblende ages from samples of the eastern sector have a dominant population of grains with ages of $\sim$~500 Ma. We interpret this observation to imply that an important episode of metamorphism, approximately the age of the Ross Orogen, affected a large fraction of the terrains of East Antarctica. These results provide new information on the sediment sources around Antarctica, which will aid in the identification of the sectors of the Antarctic ice sheet that contribute to ice rafted detritus in the marine environment of the circumpolar region. Additionally, the discovery of a very broad zone of 500 Ma metamorphism indicates the power of sedimentary provenance studies for better understanding the geology of this continent.
T11A-1244 0800h
Ice shelf drill sites proposed to study Pre-Late Oligocene climate and tectonic history, Coulman High, Southwestern Ross Sea, Antarctica.
New geophysical data were collected in front of the Ross Ice Shelf using the RVIB Nathaniel B. Palmer in January of 2003. The primary goal was to collect detailed grids of seismic data to select drill sites to investigate the climate and tectonic history of the Ross Sea region. The survey sites were located where large sections of the ice shelf have broken off, exposing previously inaccessible seafloor. A site survey at the C-19 iceberg calving site, located in 800-900 m of water adjacent to Ross Island and 120 km NE from McMurdo Station, was conducted under the premise that the ice sheet, advancing north at ~1 km/year, will in time cover the survey thereby allowing drilling into the seabed from the ice sheet. We propose an E-W transect of drill sites along the ice shelf front designed to target section where pre-Late Oligocene strata dips east allowing successively deeper stratigraphic sampling with a series of holes across strike. Cores from here may record the transition from warm climate in Eocene time to the cooler Oligocene and will test our hypothesis that extension between East and West Antarctica is recorded in sediments in this sector of the Ross Sea. Rifting in Cretaceous time resulted in widespread extension of the Ross Sea amounting to several hundred kilometers. Adare Trough seafloor spreading in Eocene-Oligocene time adjacent to the Ross Sea continental shelf resulted in about 180 km of spreading, and may project into the western Ross Sea. Syn-rift sediments of these ages may be present. The C-19 site on Coulman High is characterized by N-S trending basement half grabens filled with syn-rift sediments of unknown age truncated by an angular unconformity, and overlain by undeformed Late Oligocene and younger strata that we correlated to DSDP and Cape Roberts drill sites. We have divided the pre-Oligocene units into an upper (Early Oligocene?-Eocene?; ~ 680-1000 m below sea floor (bsf)) unit and lower (Eocene?-Late Cretaceous?; >1000 m bsf) unit, separated by an unconformity, all resting on top of acoustic basement at ~1400m bsf. We suggest that the pre-Oligocene strata are syn-rift units deposited and deformed during both Late Cretaceous and Early Eocene extension.
T11A-1245 0800h
Multidisciplinary Surveys for the Crustal Structure of the Lützow-Holm Complex, Enderby Land, East Antarctica: SEAL-2000, -2002
Lithospheric evolution of the East Antarctic shield is one of the keystones for understanding continental growth process during the Earth's evolution. Architecture of the East Antarctic craton and the surrounding areas would be characterized by a comparison with lithospheric structure of the other Precambrian terrains by deep seismic surveys. A geoscience program named "Structure and Evolution of the East Antarctic Lithosphere (SEAL)" is carrying out since 1997 austral summer as part of the Japanese Antarctic Research Expedition (JARE). Several geological and geophysical surveys were conducted including a deep seismic refraction / wide-angle reflection and gravimetric surveys in the Lützow-Holm Complex (LHC), Western Enderby Land, East Antarctica. The LHC had been experienced a high-grade metamorphism during the Pan-African orogenic event, where is considered to be the collision zones in the last stage of the formation of Gondwana. In the austral summer in 2000, and 2002, two big seismic surveys were carried out on the continental ice sheet of the Mizuho Plateau in the LHC by SEAL program. In both surveys, more than 170 geophones were installed on the plateau totally 190 km in length. A total of 8,300 kg dynamite charges at the fourteen seismic shot points on the ice sheet gave enough information concerning the deep structure of a continental margin of the LHC. The gravity measurements were conducted by a SCINTREX (CG-3M) gravity meter at about 1 km-interval along the seismic survey lines. These surveys had revealed that the Moho depth was more than 40 km with the velocities of the surface layer, the middle crust, the lower crust and the mantle, about 6.2, 6.4, 6.7 and 7.9 km/s. Moreover, the clear reflected waves from the lower crust and the Moho were observed on all the record sections. In this presentation, we review the subsurface structure of the LHC by several geophysical approaches, such as the seismic first arrival and wide-angle analyses, gravity measurements, GPS positioning and also the radio-echo soundings to detect the precise bedrock elevation around the seismic traverse routes. Then the multidisciplinary crustal model of the LHC will be presented including geological evidence to estimate the evolution process around the coastal outcrops.
T11A-1246 0800h
Crustal Magnetization Model of Maud Rise in the Southwest Indian Ocean
We modeled the crustal magnetization for the Maud Rise in the southwest Indian Ocean off the coast of East Antarctica using magnetic observations from the Oersted satellite and near-surface surveys complied by the Antarctic Digital Magnetic Anomaly Project (ADMAP). A new inversion modeling scheme of the multi-altitude anomaly fields suggests that the magnetic effects due to crustal thickness variations and remanence involving the normal polarity Cretaceous Quiet Zone (KQZ) become increasingly dominant with altitude. The magnetic crustal thickness effects were modeled in the Oersted data using crustal thickness variations derived from satellite altitude gravity data. Remanent magnetization modeling of the residual Oersted and near-surface magnetic anomalies supports extending the KQZ eastwards to the Astrid Ridge. The remaining near-surface anomalies involve crustal features with relatively high frequency effects that are strongly attenuated at satellite altitudes. The crustal modeling can be extended by the satellite magnetic anomalies across the Indian Ocean Ridge for insight on the crustal properties of the conjugate Agulhas Plateau. The modeling supports the Jurassic reconstruction of Gondwana when the African Limpopo-Zambezi and East Antarctic Princess Astrid coasts were connected as part of a relatively demagnetized crustal block
T11A-1247 0800h
Nd-Hf isotope systematics around Antarctica
The motivation to study Nd-Hf systematics around Antarctica is twofold. First of all, a wide range of bedrock ages and lithologies are found around the perimeter of the Antarctic continent, ranging from Archean basement terrains to recent volcanics. However, in Nd-Hf isotope space Antarctica remains one of the poorly known regions of the world's continents. A first order characterization of Nd and Hf isotopes in the detrital fraction of marine sediments will fill this gap and will put additional constraints on Antarctic bedrock geology and isotope systematics. Preliminary data indicate a range of Nd isotopes from -1 to -19 and a range of Hf isotopes from -2 to -20. These values reflect the variable Antarctic provenance and follow local geology. The lowest values are found in the Indian Ocean sector close to Archean outcrops and the highest values are observed in the Pacific sector close to young bedrock ages in the region between the Ross Sea and the Antarctic Peninsula. In Nd-Hf isotope space the new data scatter around the terrestrial correlation line. Two samples from the Indian sector display more radiogenic Hf isotopes for a given Nd isotopic composition, pointing towards potentially different Nd-Hf isotope systematics for the sediments with old provenance. Second, detrital inputs from Antarctica get dispersed into the Antarctic Circumpolar Current (ACC). The ACC is the connection between the Pacific, Atlantic, and Indian Oceans and distributes deep waters formed in the North Atlantic as well as exports deep water generated at sites around Antarctica. It is however not known whether there exists significant transfer of dissolved Nd and Hf from the Antarctic continent to the ACC and ultimately to the world's ocean. An improved understanding of the composition of Southern Ocean water masses and their dependence on inputs from the Antarctic continent and deep water sources on the Antarctic shelf is an important precondition to unravel past global circulation patterns and material inputs to the ocean. We will present new Hf isotope data for ferromanganese nodules from the ACC, which will be compared with existing Nd isotope data. Preliminary results indicate a small range in Hf isotopes for the Pacific sector of the ACC (3.9 to 4.5), which is almost indistinguishable within the analytical error and matches with what would have been predicted from Nd-Hf isotope systematics in seawater.
T11A-1248 0800h
Jun Jaegyu Volcano: A Recently Discovered Alkali Basalt Volcano in Antarctic Sound, Antarctica
Jun Jaegyu is a young volcanic construct discovered in May 2004 by researchers aboard the National Science Foundation (NSF) vessel {\it Laurence M. Gould} (LMG04-04). The volcano is located on the Antarctic continental shelf in Antarctic Sound, approximately 9 km due north of the easternmost point of Andersson Island. Swath bathymetry (NBP01-07) indicates that the volcano stands 700 meters above the seafloor, yet remains 275 meters short of the ocean surface. The seamount lies along a northwest-southeast oriented fault scarp and contains at least 1.5 km$^{3}$ of volcanic rock. Video recording of the volcano's surface revealed regions nearly devoid of submarine life. These areas are associated with a thermal anomaly of up to $0.052\deg$C higher than the surrounding ocean water. A rock dredge collected ~13 kg of material, over 80% of which was fresh volcanic rock; the remainder was glacial IRD. These observations, along with reports by mariners of discolored water in this region of Antarctic Sound, suggest that the volcano has been recently active. The basalt samples are generally angular, glassy and vesicular. Preliminary petrographic observations indicate that plagioclase, olivine, and clinopyroxene are all present as phenocryst phases, and that small ($<$1cm) rounded xenoliths are common. A comprehensive study of the volcano's petrography and whole-rock chemistry is currently underway. Jun Jaegyu is the northernmost volcanic center of the James Ross Island Volcanic Group (JRIVG), and the only center in this region of the Antarctic Peninsula with evidence of recent activity. It lies along the boundary between the Late Cenozoic JRIVG and the Upper Paleozoic rocks of the Trinity Peninsula Formation. While the tectonic setting of the region is complex, volcanism appears to be associated with active faults related to within-plate extension.
T11A-1249 0800h
A Marine Geophysical Study of the Wilkes Land Margin, Antarctica
During the Austral summers of 2000-01 and 2001-02, Geoscience Australia (GA) acquired $\sim$10,000 line km of seismic reflection, magnetic and gravity data across the Wilkes Land margin (WLM), East Antarctica, from 100-150E. Previous studies indicate that the WLM formed during the Late Cretaceous rifting of Australia and Antarctica. Deep-penetrating, 36-fold, seismic reflection data provide constraints on sediment and crustal structure as deep as the lower crust. The seaward extent of stretched continental crust and extetnded pre- and syn-rift sediments can be mapped off the central and east WLM. A region of massively stretched continental crust, subsided to abyssal plain depths, is imaged off Terre Ad\`{e}lie (137-140E). This block is interpreted to represent a sunken marginal plateau that was once conjugate to the Otway basin and the South Tasman Rise (STR). Identification of isochrons older than anomaly 21n over oceanic crust on the WLM is equivocal. Slow seafloor spreading at the Southeast Indian Ridge following rifting (Cande & Mutter, 1982) and widely-spaced data contribute to the difficulties in identifying the earliest isochrons. Anomaly 34y has previously been interpreted as the oldest isochron on the WLM; however, these seismic reflection data indicate the presence of stretched continental crust beneath this magnetic anomaly. Process-oriented gravity modelling has been completed for 12 transects across the Wilkes Land margin. A zone of weakened (as evidenced by low effective elastic thickness, Te) crust occurs at the continent-ocean transition (COT). The margin as a whole (including stretched continental crust), however, exhibits high strength (Te $\sim$30 km) relative to other passive rift margins. Loading of the Wilkes Land margin occurred almost entirely in the Tertiary, and specifically during an interval of polythermal glaciation between 34-9 Ma. High, modelled Te values demonstrate that the Wilkes Land margin flexurally supports the massive sediment load and that the lithosphere has remained strong despite rifting, or has regained strength since rifting. Evidence from seismic, magnetic and gravity data indicates massive crustal thinning and a wide, extended rift-zone, characterised by interpreted exhumed mantle peridotites. In-situ, fertile mantle peridotites recovered over 400 km from the shelf-break off east Wilkes Land support this interpretation of the geophysical data.