T14A-01 INVITED 16:00h
Aerogeophysical mapping: a powerful tool to explore Antarctica
Within the framework of the upcoming International Polar Year several science opportunities for Antarctic geology and geophysics exist. Of particular interest are key issues in solid earth science that can only be addressed by a coordinated effort between several nations such as the first-order geophysical mapping of tectonic and geologic structures in the interior of East Antarctica. Given the extensive ice cover, airborne geophysical data is the best and most cost-effective method to characterize broad areas of sub-ice basement and expand our knowledge of Antarctica. A number of basic problems can be addressed with a long-range research aircraft that are unique to the Antarctic continent and the polar regions. For example, the interplay between geologic processes, ice sheet dynamics and climate change is of importance for the global environment and affects long-term sea level changes. The onset of glaciation in East Antarctica is only poorly understood. Preliminary ice sheet and climate models suggest that the elevated topography of the Gamburtsev Subglacial Mountains and the episodic uplift of the Transantarctic Mountains play important roles in the evolution of the ice sheet, and thus, climate. Subglacial lakes and their potential for harboring life have great potential for geologic and geophysical research. Of particular interest for geologists and geophysicists are the tectonic framework of subglacial lakes and the nature of sediments at the lake bottom.
T14A-02 INVITED 16:15h
Thermal structure of the upper mantle beneath Antarctica with implications for heat flux and visco-elastic rebound
Although substantial international efforts have and will continue to improve the number and distribution of broad-band seismic stations from both permanent and temporary deployments across Antarctica, the horizontal and vertical resolution of upper mantle structures remains poor beneath most of Antarctica relative to other continents. Short of a revolutionary deployment of seismometers across the continent, resolution is likely to remain less than ideal into the foreseeable future. To produce higher quality lithospheric models, therefore, requires introducing better a priori constraints into the inversion largely from what has been learned from other, better instrumented continents. We present the results of an iterative inversion of surface wave dispersion data for upper mantle temperature structure in which heat flow is applied as an explicit constraint. The heat flow values are extrapolated from other continents based upon similarities in lithospheric structure revealed in the first iteration of the inversion. The result yields estimates of the probabilistic distribution of surface heat flux across Antarctica, as well as bounds on the mantle component of heat flux and lithospheric thickness across the continent. The inferred variations in surface heat flux may have a profound effect on ice stream and ice sheet dynamics. We also present results from numerical simulations that illustrate how the estimated upper mantle temperature variations (and associated changes in lithospheric thickness) are likely to affect the nature and magnitude of visco-elastic rebound. These 3-D visco-elastic effects may prove to be important to understand the growth and decay of ice sheets over long time scales.
T14A-03 16:30h
Constraints on Late Cretaceous and Cenozoic Extension in the Ross Sea from the Southwest Pacific Plate Circuit
Magnetic anomalies and fracture zone trends north of the Ross Sea associated with Australia-Antarctic, Pacific-Antarctic and Lord Howe Rise-Antarctic sea floor spreading place strong constraints on Late Cretaceous and Cenozoic motion between East and West Antarctica. There appear to be two episodes of extension in the Western Ross Sea in this time period. The younger episode, starting about 45 Ma and ending around 25 Ma, is well constrained by differences in spreading rates on the Southeast Indian Ridge (SEIR) on either side of the Balleny FZ and by magnetic anomalies straddling the Adare Trough (Cande et al., 2000). These data document about 150 km of ENE-WSW directed extension across the Adare Basin just north of the Ross Sea. An earlier (80 to 55 Ma) episode of extension is not well constrained in large part because the spreading rate between Australia and East Antarctica was very slow and identifications of magnetic anomalies older than anomaly 24 on the SEIR are problematical. However, there are useful constraints imposed by fitting magnetic anomalies and fracture zone traces from north of the Iselin Bank, from southwest of the Campbell Plateau and from the South Tasman Sea. These features form a network of tectonic constraints that have to be fit by any proposed model of East - West Antarctic motion. A reconstruction of the conjugate splays of the Emerald FZ shows that the Iselin Trough, a fossil rift structure northeast of the Iselin Bank, formed by a local ccw rotation of 12 degrees about 72.6° S, 183° E of the Iselin Bank between anomalies 27 and 24, thus constraining East-West Antarctic extension in this period to the corridor west of the Iselin Bank. These features also enable us to test the plate circuit formed by closing the Campbell Plateau and Challenger Plateau to its pre-rift (50 Ma) configuration and closing the Tasman Sea, SEIR and Pacific-Antarctic ridge. Rotations for the SEIR which treat the older Australia-Antarctic magnetic anomalies as isochrons (Tikku and Cande, 1999) are not appropriate for this test since they produce a large overlap of the South Tasman Rise and Tasmania with Northern Victoria Land. The SEIR rotations of Royer and Rollett (1997), which do not fit the Australia-Antarctic anomalies very well, produce a reasonable closure of the Iselin Bank with Northern Victoria Land and predict about 100 km of additional (pre-55 Ma) extension in the northern Ross Sea.
T14A-04 16:45h
Evolution Of The West Antarctic Rift System And the Importance of Crustal Heat Production
Two distinct stages of extension are recognized in the West Antarctic Rift system (WARS). During the first stage in the Late Cretaceous through middle Paleogene Periods extension was broadly distributed throughout most of the Ross Sea region. Later, during the Late Paleogene and younger, the style of extension changed and was focused primarily in the Terror Rift, near the boundary with the East Antarctic craton. We have developed a finite element model to study the processes and conditions responsible for this two-stage evolution of rifting. Model results consistent with the geologic history of the WARS indicate that the transition from a period of broadly distributed extension to a later period of strongly focused rifting can evolve naturally without requiring a change in either the regional stress regime or thermal state. No change in plate motion directions or rates or changes in the mantle thermal state (impingement of a plume) are required. The initial stage of modeled diffuse extension throughout West Antarctica results from a prescribed uniformly weak West Antarctic lithosphere (thinner, hotter) versus a prescribed stronger East Antarctic lithosphere (thicker, colder). The transition from diffuse to focused extension under constant regional stress and thermal conditions occurs only under a limited set of initial thermal conditions. Simulations that have an initial West Antarctic thermal structure with significant heat from the crust result in a lithosphere that strengthens as it thins. This strengthening is due to the cooling of the upper mantle as the thickness of the crust is reduced (thus the total heat generated in the crust is reduced). As a subset of this class of simulations, instances in which the initial East Antarctic crust generates moderately high amounts of heat result in a focusing of extension near the boundary between East and West Antarctica. This focusing is due to the relative weakness of the upper mantle near the East/West Antarctic boundary due to heat conducted from the warm East Antarctic crust. Thus, crustal heat production can play an important role in controlling the deformational evolution of extensional systems.
T14A-05 17:00h
Do the Volcanic Rocks Erupted in the West Antarctic Rift System Beneath the West Antarctic Ice Sheet (WAIS), Interpreted From Aeromagnetic Surveys, Define a Large Igneous Province?
Aeromagnetic and radar ice sounding surveys over the West Antarctic Ice Sheet (WAIS) made in the 1990s allow a recalculation of the estimate of the volume of late Cenozoic volcanic magmatic rocks erupted in the West Antarctic rift system. In the late 1950s and early 1960 widely spaced aeromagnetic profiles acquired over the WAIS area revealed numerous high amplitude ($>$100 nT) short-wavelength anomalies interpreted as evidence of volcanic rocks beneath the ice. By the late 1970s a 50-100-km-spaced grid of aeromagnetic and radar ice sounding profiles over the WAIS combined with the earlier survey flights allowed a fairly accurate delineation of the area covered by the high amplitude anomalies with sources inferred at the base of the ice. The Central West Antarctica (CWA) aerogeophysical survey of the 1990s consists of 5-km-spaced orthogonal aeromagnetic, radar ice sounding and aerogravity profiles. This survey, particularly the aeromagnetic data, defined the character in detail of the volcanic centers defining the late Cenozoic magmatic activity interpreted as associated with the West Antarctic rift system. In a 1994 paper, using a preliminary part of the total CWA survey we estimated $>$500,000 square km areal extent of the high amplitude, shallow source volcanic anomalies corresponding to a volume of $>$one million cubic km magmatic rocks beneath the ice. We interpreted this as evidence of a large igneous province. Now that the CWA survey is complete, we are better able to use the accurately estimated extent of the volume of volcanic centers (subglacial volcanic intrusions) in the CWA area and indirectly (using the widely spaced profiles in the WAIS area surrounding the CWA survey) to infer the extent of the late Cenozoic volcanism throughout the WAIS. During the same period, geologists have better dated volcanic exposures throughout the West Antarctic rift area, which appears to extend the period of volcanism from present to as far back as 48 Ma. We present these revised estimates, which indicate that the interpretation of a large igneous province would be correct if the bulk of the volcanic eruptions beneath the WAIS occurred during a shorter period of time than the apparent maximum age range of 48 m.y. We are unable to estimate a mode age of the volcanic activity, because the great volume of the magmatic rocks beneath the WAIS has not been sampled.
T14A-06 17:15h
Neotectonic Structure of Terror Rift, Western Ross Sea, Antarctica: Initial Interpretations of New Geophysical Data
The Terror Rift in the western Ross Sea is a prominent neotectonic element of the West Antarctic Rift, yet insufficient data have been available to define the geometries of faults and volcanic bodies or to constrain rift timing in any detail. The RVIB Nathaniel B. Palmer completed a geophysical cruise in the western Ross Sea in early 2004 in order to better understand the history and kinematics of the Terror Rift. Over 7000 km of gravity, magnetic and multibeam bathymetric data were obtained, and extensive new multichannel (2000 km) and single-channel (500 km) seismic reflection profiles were acquired. In addition to the geophysical data, dredging and sampling of submarine and exposed volcanic vents were completed. Limited access to the westernmost Ross Sea due to extensive ice cover precluded complete profiling across the Terror Rift. A significant improvement in line spacing over the eastern Terror Rift, however, allows us to define the eastern limit of the rift basin, and to map fault geometries, displacements and along strike variations in border and intrabasinal fault systems. Volcanic features were mapped by both bathymetric and geophysical data. New mapping of the volcanic field surrounding Franklin Island revealed clusters of submarine volcanic cones and connecting volcanic ridges. The field as a whole has a north to northeast elongation. A separate series of prominent volcanic edifices define a NNW trend extending northward from Beaufort Island and is associated with a zone of intense faulting. Another large submarine volcanic complex is located east of Beaufort Island. In McMurdo Sound, volcanic ridges radiate from the western margin of Ross Island. Successful dredging of seven submarine cones yielded basaltic rocks containing abundant glassy material, as well as mantle and crustal xenoliths. Faults were imaged in the western part of McMurdo Sound and prominent unconformities related to downward flexure in response to Ross Island volcano mass loads were imaged in the eastern Sound. These new seismic profiles will allow us to derive a regional flexural history for this part of the basin. Intriguing glacial erosional and depositional features were clearly imaged by the bathymetric data and provide us with a means to constrain relative timing of glaciation, volcanism and tectonism in the region.
T14A-07 17:30h
Neotectonics of the McMurdo Sound Region, Antarctica Interpreted From Multibeam Data
The NBP0401 cruise of {\it RVIB Nathaniel B. Palmer} (Jan-Feb 2004) surveyed the Terror Rift and western Victoria Land Basin (VLB), Antarctica. The very large iceberg B-15, split latitudinally at the end of October 2003 and then split longitudinally in mid-December. These large bergs restricted ice movement out of the VLB and left about half of our proposed study area covered in very thick multi-year ice where multibeam and seismic data collection was impossible. Significant multibeam data were collected in McMurdo Sound between $77\deg$S and $77\deg$40'S from $164\deg$40'W to Ross Island. Approximately 3000 square km of contiguous coverage revealed numerous features indicating significant neotectonism. Lineated submarine volcanic cones and dykes radiating away from Mt. Erebus are common in the eastern side of McMurdo Sound. The western side consists of a shelf and slope with significant glacial deposition cut by submarine channels. The channel draining the Taylor Valley/ Ferrar Glacier is a narrow, steep-walled feature, less than 200 meters wide and 60 meters deep in spots that trends due north, parallel to the shelf before turning northeast into the basin. The unusual direction of the channel is possibly controlled by resistant volcanic features or by older faulting. Transantarctic Mountain parallel faulting at a depth of 500 m is superimposed on the shelf structure. The very linear faulting shows notching by a number of very small incisions perhaps indicating that the uplift is recent. In the deeper portion of the Sound, within the well-defined `flexural moat' surrounding Ross Island, more sinuous submarine channels are found to a depth of 800-850 m, indicating northward directed flow into the basin. In a 15 by 40 km area near $76\deg$S, $176\deg$E, eight nearly circular features up to 5-6 km in diameter and 100+ m high were found. Although the circular shape is suggestive of a magmatic origin, there is no magnetic signature, suggesting that these features are glacial in origin. The features are asymmetrical with the southeastern side being quite steep and the northwestern side gradually sloping to the northwest. The closest analogue for these features is drumlins albeit these are very large. The new bathymetric data will help us to constrain the relative timing of faulting, volcanism, flexural subsidence and glacial erosion/deposition cycles that have shaped the McMurdo Sound region.
T14A-08 17:45h
Comprehensive geophysical study of the Transantarctic Mountains
We use teleseismic receiver function and surface wave phase velocities to model the seismic velocity structure of the crust and upper mantle between the Ross Sea and Vostok Subglacial Highlands. The West Antarctic Rift System (WARS) has a thinner crust (~20 km) and slower seismic mantle velocities than East Antarctica (EA). Attenuation of shear body waves is also higher in the WARS, which suggests the presence of a thermal anomaly. The transition between EA and the WARS occurs beneath the Transantarctic Mountains (TAMs), ~100 km from the coast. Within EA the crust is remarkably uniform in thickness (~35 km) for a lateral distance greater than 1400 km. We calculated theoretical gravity from density models that are based on the seismic results. The observed gravity is consistent with ~1 percent denser mantle material under EA than in the WARS. This density increase is consistent with temperature variations that would cause a 2.5-5 percent velocity increase. The flexural model of ten Brink et al., [1997] adequately accounts for the otherwise uncompensated topography. The buoyant thermal and erosional loads are sufficient to cause the observed uplift. As predicted by Strudinger et al., [2003], a crustal root is present, causing some isostatic support.