Gravity Anomalies of the Antarctic Lithosphere

Gravity anomalies of the Antarctic lithosphere

John G. Weihaupt1, Alan Rice2, and Frans G. Van der Hoeven3 1DEPARTMENT OF GEOLOGY, UNIVERSITY OF COLORADO–DENVER, DENVER, COLORADO 80217, USA 2DEPARTMENT OF EARTH AND PLANETARY SCIENCES, AMERICAN MUSEUM OF NATURAL HISTORY, NEW YORK, NEW YORK 10024, USA 3DEPARTMENT OF GEOPHYSICS, DELFT UNIVERSITY OF TECHNOLOGY, DELFT, THE NETHERLANDS

ABSTRACT

Anomalous free-air gravity signals in and around the Antarctic continent have been reported for some decades. Recent defi nition of the Antarctic gravity fi eld from fi eld-based oversnow traverses and supporting data from Earth-orbiting satellites reveal discrete regions of both negative and positive free-air gravity anomalies. The data from these observations have enabled us to construct a free-air gravity anomaly map of Antarctica. Negative free-air gravity anomalies are found to occur mainly on the Antarctic continent, in particular, in the Wilkes Land, Ross Sea, central continental, and Weddell Sea sectors. Positive free-air gravity anomalies are found to occur mainly in the offshore circum- continental sectors. While each of these regions of anomalies provides excellent opportunities for further investigation, including identifi cation of the causes of the negative and positive free-air gravity anomalies, special attention is given to the negative free-air gravity anomaly sites of the continent proper. Three potential sources of the negative free-air gravity anomalies are identifi ed: the mantle, lithosphere, and crust. Examination of thermally induced density variations in the mantle based upon seismic tomography, and analysis of mantle-related gravity anomaly wavelengths favor a gravity anomaly source other than the mantle. Examination of the subcrustal lithosphere based on upper-mantle thermal structure, the origin of the lithosphere, and crustal infl uences on the underlying lithosphere, including radiogenic heat, implies that the source of the negative free-air gravity anomalies is less likely to be the subcrustal lithosphere and more likely to be located in the Antarctic crust. Examination of possible crustal features that might account for these anomalies leads to a consideration of subglacial topography and specifi c locales of anomalously low rock density.

LITHOSPHERE; v. 2; no. 6; p. 454–461. doi: 10.1130/L116.1

INTRODUCTION mally found in other oceanic regions, nor adjacent to most continental land masses. In recent years geological and geophysical studies of the Antarctic Finally, the distribution of negative free-air gravity anomalies in inte- continent have evolved from ground surveys and fi eld work to airborne rior Antarctica, extending from the Ross Sea to the Weddell Sea, appears surveys, and more recently to satellite remote sensing. In each case, con- to be regionalized to a central axis of the continent. Diffi cult to explain on siderably more information has been introduced, which enables us to inte- the basis of traditional geological structures or Earth’s interior structure, grate earlier data with the more recent data to form a more comprehensive it is the purpose of this paper, fi rst, to provide a useful gravimetric map view of the gravity fi eld of the continental and immediately offshore oce- of the Antarctic continent for use by other investigators and, second, to anic lithospheres. examine the Antarctic gravity spectrum in an effort to explain the likely Figure 1, which we have constructed from fi eld-based gravity obser- source of these gravity anomalies as they may relate to the Antarctic man- vations, and, in particular, CHAllenging Minisatellite Payload System tle, lithosphere, or crust. (CHAMP) global positioning system (GPS), European Improved Grav- ity model of the Earth by New Techniques (EIGEN–1S) satellite data DATA SOURCES AND ANALYTICAL METHODS (Reigber et al., 2002; Barthelmes, 2002), depicts the complete Antarctic free-air gravity anomaly spectrum. Negative free-air gravity anomalies The initial data for the present study were acquired during the Victoria are prominent on the continent, and include anomalies in the Wilkes Land Traverse, a 4 mo, 2400 km seismic and gravity survey of interior East Land and northern Transantarctic Mountain sector, the Ross Sea sec- Antarctica (Weihaupt, 1961; Smith et al., 2011) (Fig. 2). Gravity observa- tor, the continental interior, and the Weddell Sea sector. Some of these tions were made using a Frost Gravimeter, ID no. C-3–65, operated from anomalies are comparable to gravity anomalies in other regions on a 24 V power supply, thermostatically controlled to maintain a constant Earth, but some are surprisingly different. The Ross Sea, for example, 107 °F temperature. The gravimeter was essentially free from drift, show- displays a “paradoxical” gravity fi eld (Karner, 2004; Karner and Watts, ing a variation of 0.1 mGal upon reoccupation of stations. Seismic obser- 1983) of unusually large, absolute magnitude negative free-air gravity vations were made using a Texas Instruments 7000B seismograph with a anomalies uncharacteristic of most other oceanic regions of this kind, frequency range of 5–500 cycles per second (cps). A 24 trace system was and it appears therefore to require a unique explanation. In addition, used in which each trace represented a single 20 cps vertical component the gravity data depict a somewhat radial circum-continental fi eld of seismometer. Four banks of six amplifi ers each were used in conjunction numerous positive free-air gravity anomalies, and comparatively weak with the standard 7000B camera oscillograph, control unit, and dynamo- negative free-air gravity anomalies offshore. All are of lesser absolute tor. Commencing at Scott Base on Ross Island, the traverse crossed the magnitude than the continental interior negative free-air gravity anoma- Ross Ice Shelf, ascended the Skelton Glacier through the Transantarctic lies. The somewhat radial circum-continental fi eld is a pattern not nor- Mountains, onto Victoria Land Plateau. From the head of the Skelton

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Figure 1. Free-air gravity anomaly contour map of the Antarctic continent and offshore oceanic sectors (gravity anomaly values are in mGal; con- Figure 2. Route of the Victoria Land Traverse from the head of the Skelton tour interval is 10 mGal). Letters A through O identify areas of particularly Glacier, northwest across northern Victoria Land to the end point of the strong negative free-air gravity anomalies in the Wilkes Land sector, the French Traverse, at station 531, and east to the USARP Mountains at sta- Ross Sea sector, interior Antarctica, and the Weddell Sea sector. The star tion 559. icon near anomaly A is the site of the Wilkes Land anomaly; its deﬁ nition is not apparent at the EIGEN–1S resolution. The negative free-air gravity anomalies range from approximately −10 mGal to −60 mGal, the greatest absolute negative values distributed in the Wilkes Land–Ross Sea sectors. G = 978.0490 (1 + 0.0052884 sin2 ϕ – 0.0000059 sin2 2ϕ), The lower negative free-air gravity anomaly values are distributed in the Weddell Sea sector, and the lowest absolute magnitude negative free-air gravity anomalies and the largely positive anomalies are distributed in where ϕ is latitude. By assuming ice and subice rock densities of the offshore circum-continental oceanic sector, and range from −20 to 3 3 +20 mGal. (Constructed from CHAllenging Minisatellite Payload System 0.90 g/cm and 2.67 g/cm , respectively, with a resultant density difference 3 [CHAMPS] global positioning system [GPS], European Improved Gravity of 1.77 g/cm , 1.0 mGal is found to correspond to a change of 13.5 m in ice model of the Earth by New Techniques [EIGEN-1S] Satellite data, Geo- thickness on the basis of the gravity effect for a semi-inﬁ nite slab, that is, forschungs Zentrum, Projektbereich 1.3, Potsdam.) (Polar stereographic projection.) Center line axis at the South Pole lies along 180° longitude. 1 = 0.04185 × 1.77 × τ.

Therefore, Glacier, the traverse crossed a major portion of East Antarctica northwest to 71°08′S, 139°11′E, and then east to the USARP (United States Antarc- τ = 1/0.04185 × 1.77 = 13.499 m/mGal, tic Research Program) Mountains at 72°37.8′S, 161°32′E (Fig. 2). Gravity observations were made at 5.0 km intervals, and seismic soundings were where τ is the thickness of the plate per mGal, and the gravitational attrac- made at 15.0 km intervals for subglacial rock surface control. tion is then, Subsequent gravity data were acquired from CHAMP GPS, EIGEN– π τ, 1S low-altitude remote sensing satellite (Reigber et al., 2002). This gz = 2 EIGEN–1S data set, containing fully normalized spherical harmonic coef-

fi cients complete to at least degree/order 100, continuously and precisely where gz is the gravitational attraction in mGal, is the gravitational con- tracked by global positioning system (GPS), provides powerful data to test stant, and is the density difference between 2.67 g/cm3 and 0.90 g/cm3. gravity fi eld models (Klokoenik et al., 2004). This has enabled identifi cation of at least one major negative free-air Reduction and analysis of the gravity data collected in the fi eld (Wei- gravity anomaly in the region of Station 531 (Fig. 2), which represents haupt, 1961, 1976) involved correction of observed gravity, 0.3086 mGal subglacial basin-shaped crustal topography, and is believed also to repre- per meter of elevation, and comparison of the result with theoretical sea- sent low lithospheric density. Reduction and analysis of the EIGEN GPS– level gravity for the latitude of each station using the international formula: 1S data (Reigber et al., 2002), along with those of the fi eld-based gravity

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traverse, have enabled identiﬁ cation of several other major negative free- air gravity anomalies in the Wilkes Land, Ross Sea, central continental, and Weddell Sea regions, as well as the somewhat circum-continental oceanic largely positive free-air gravity anomaly ﬁ eld. The continental and oceanic data, representing 43,200 data points from 60°00′S to 90°00′S latitude and 360°00′ of longitude at half degree intervals, were subse- quently applied to a stereographic projection of Antarctica and its offshore periphery, and free-air gravity anomaly points were used to construct the gravity contour map (Fig. 1) of this segment of the Southern Hemisphere.

PRINCIPAL OBSERVATIONAL RESULTS Figure 3. Free-air gravity anomaly profi le of the Wilkes Land anomaly in the vicinity of the star icon in Figure 1, discovered by the Victoria Land Traverse party. Positive free-air gravity is observed at points a and c, and Distributed across Antarctica, the negative free-air gravity anomalies the total anomaly is 158.3 mGal (from Weihaupt, 1961, 1976). range from −20 mGal to −60 mGal and are accompanied by an apparently random distribution of fewer continental positive free-air gravity anomalies ranging from 0 mGal to +30 mGal, with the exception of a +50 mGal anomaly in central East Antarctica, and another +50 mGal anomaly near the coast of the continent. The circum-continental gravity values range from 0 mGal to +20 mGal in the Atlantic Ocean sector, and from 0 mGal to −20 mGal in the Pacifi c Ocean sector, except for a −30 mGal anomaly offshore in the Indian Ocean sector (Fig. 1). Specifi cally, these include three negative free-air gravity anomalies in the Wilkes Land and northern Transantarctic Mountain sectors (A, B, and G), three large negative free-air gravity anomalies in the Ross Sea sector (C, D, and E), four in the continental interior (F, H, I, and J), and fi ve in the Weddell Sea sector (K, L, M, N, and O) (Weihaupt and Van der Hoeven, 2004; Abbott, 2006; Weihaupt et al., 2006, 2010). A principal result of the Victoria Land Traverse is the discovery of a major negative free-air gravity anomaly in Wilkes Land. Centered at 71°31′S, 140°00′E, this feature is a minimum of 243 km across, with a minimum absolute magnitude of 158.3 mGal (Fig. 3) (Weihaupt, 1961, 1976). The region of the anomaly was traversed twice, confi rming the validity of the data. In contrast to this large negative free-air gravity anomaly, free-air gravity on the Victoria Land Plateau traversed elsewhere by the fi eld party ranges from −42.6 mGal to +23.7 mGal, a range of 66.3 mGal over 700 km. This is equivalent to a rate of change of 0.1 mGal/ km. In contrast, the Wilkes Land anomaly displays a rate of change in gravitational acceleration more than forty times that observed regionally, suggesting a region of the lithosphere out of isostatic equilibrium (Heis- kanen and Vening Meinesz, 1958). Based upon radiosound survey (Steed and Drewry, 1982), this Wilkes Land anomaly profi le is seen to refl ect also the profi le of the subglacial rock topography, although topography does not explain the total magnitude of the anomaly. Some infl uence other than subglacial topography therefore appears to augment this anomaly. As the fi rst major negative free-air gravity anomaly recognized in Ant- arctica, the Wilkes Land anomaly has focused attention on other, more recently observed negative free-air gravity anomalies in Antarctica. Fig- ure 4 lends better defi nition to the sizes, locations, and patterns of distribu- Figure 4. Antarctic free-air gravity anomaly map showing the pattern, tion of the negative free-air gravity anomaly sites (blue), and the positive distribution, and relative locations of negative free-air gravity anomalies free-air gravity anomaly sites (yellow). This map, which we constructed (blue), and the largely positive free-air gravity anomalies (yellow) mainly largely from EIGEN–1S data, reveals at least 15 negative free-air grav- offshore and around the Antarctic continent. ity anomalies distributed across the continent (compare with Fig. 1). Although the resolution from satellite remote sensing is inadequate to provide profi les comparable to the ground-based profi le of the Wilkes Land to counterpart gravity/seismic points in Figure 3, viz., a, b, and c. While anomaly (Fig. 3), we are able to construct profi les of the subglacial surface similar radiosound data are not available for the other EIGEN–1S negative for anomalies A and B based upon radiosound survey (Steed and Drewry, free-air gravity anomalies observed in Antarctica, the profi les in Figures 3 1982). These profi les appear in Figure 5 for comparison with the Wilkes and 5 call attention to their similarity and suggest that the profi les of the Land anomaly (Fig. 3), and are found to be comparable in broad aspect. other negative free-air gravity anomalies may be similar. High and low points, identifi ed as d, c, f, g, h, i, j, and k, while representing While radiosound data reveal important subglacial topographic infor- radiosound subglacial topographic points, may tentatively be compared mation, seismic and gravity data are more useful for identifi cation of

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in density variations, may also account for variations in the gravity data. Tomographic studies, normally focused on variations in seismic veloci- ties, allow density variations to be inferred as well. On these bases, we examine the strengths and weaknesses of the possible explanations for the Antarctic negative free-air gravity anomalies related to mantle, lithosphere, and subglacial crust. At least three aspects of the Antarctic gravity ﬁ eld must be considered in providing an explanation for the Antarctic free-air gravity anomalies. These include (1) identifying possible alternative sources of these anomalies, (2) explaining the unusually large magnitudes of the negative free-air gravity anomalies, and (3) providing a rationale for the offshore largely positive and relatively low-absolute-magnitude negative free-air gravity anomalies around the continent.

Gravitational Effects of the Antarctic Mantle

Variations in Earth’s mantle caused by composition, density, and thermal structure have the potential to create variations in gravity at Earth’s surface, and therefore the potential to explain the Antarctic free-air gravity anomaly fi elds. Earth’s broad gravity spectrum, for example, can be Figure 5. Subglacial topographic profi les in the vicinities of negative explained in part by a combination of viscous mantle fl ow and random free-air gravity anomalies shown in Figure 1 (from radiosound data). (A) density variations in the mantle (Richards, 1984; Steinberger and Holme, Anomaly A in Figure 1. (B) Anomaly B in Figure 1. 2002). Similarly, seismic tomography (Vogt et al., 1998; Zhao, 2001) reveals that viscous mantle fl ow, in the form of mantle convection driven primarily by lateral differences in temperature and density (Anderson and Dziewonski, 1984; Van der Hist et al., 1991), accounts for much if not all possible mantle (Guillou-Frottier et al., 1996; Mareschal et al., 1999; of the mantle structure. Treating Earth’s mantle as a highly viscous fl uid, Ritsema et al., 2009; Verhoeven et al., 2009), lithospheric (Nyblade and then, yields an excellent approximation for the behavior of the upper man- Pollack, 1993), and crustal (Plescia, 1999) mass and density variations. tle, especially when whole mantle fl ow is assumed (Richards and Enge- bretson, 1992). Substantial density anomalies in such a fl uid may thus DISCUSSION drive a fl ow component both regionally and in the entire mantle (Richards and Hager, 1984; Richards and Engebretson, 1992), components which The major interior positive free-air gravity anomaly (+50 mGal) in have the potential to generate gravity anomalies at Earth’s surface. Such East Antarctica (Fig. 1) exhibits a maximum gradient of 0.22 mGal/ km variations appear, therefore, to offer an explanation for the shapes, sizes, and is associated with subglacial topographic high ground (Lythe and and regional distributions of the negative and positive Antarctic free-air Vaughan, 2000), demonstrating the importance of subglacial topography gravity anomaly fi elds. in accounting for free-air gravity anomalies. Steep negative free-air grav- However, lower-mantle structural features are normally revealed as ity gradients also occur with the negative gravity anomalies in the Ross broad, large-scale variations (Richards, 1984), which are resolved into Sea, and again at anomalies A and B (Fig. 1) in Wilkes Land. These sites long-wavelength gravity anomalies representing features of the order represent negative free-air gravity gradients of, for example, 0.351 mGal/ of 2000–3000 km, i.e., much larger than those in the present study. The km, and 0.272 mGal/km, respectively. These gravity gradients exceed Antarctic continental interior anomalies as well as the circum-continental most gravity gradients elsewhere on Earth, although similar gradients radial pattern of anomalies appear unlikely, therefore, to be related to occur over quite different and well-understood structures such as deep-sea lower-mantle effects. On a more continental or regional scale, massive trenches and tectonically active mountains. Such features, like those of xenoliths or subducted slabs of contrasting densities in the mantle are the Mariana Trench and the Himalayas, bear little geological similarity capable of producing gravity anomalies with wavelengths closer to the to geological features in Antarctica, or to the shapes and distributions of sizes of the Antarctic free-air gravity anomalies. Massive xenoliths (Rud- Antarctica’s negative free-air gravity anomalies. Similarly, the gradient of nick and Fountain, 1995) and subducted slabs in the mantle (White et the Wilkes Land negative free-air gravity anomaly, south of and adjacent al., 1999; Bina and Navrotsky, 2000; Levin et al., 2002; Bellahsen et al., to A in Figure 1 (star icon), exhibits an unusually steep maximum gradient 2005; Shito et al., 2009; Krien and Fleitout, 2008) remain possible expla- of 4.71 mGal/km. nations for gravity anomalies in Antarctica; the shapes and locations of The origin of the anomalies, functions of mass and distance variations, such features tend, however, to be quite different from most of those of the may be related to compositional, structural, or geologic mass or density Antarctic gravity fi eld. differences of the mantle, subcrustal lithosphere, or crust (Weihaupt and Considering the Antarctic radial circum-continental gravity anomaly Van der Hoeven, 2004). Gravity signals from all of these sources may, fi eld, it is interesting to note the radial chains of elongate positive grav- of course, be somewhat obscured by the overlying continental ice sheet. ity anomalies around the passive continental margins of the Arctic Ocean Mass variations may also result from density differences caused by tem- Basin, for which there is “no simple unifying explanation” (Vogt et al., perature differences in the mantle, lithosphere, or crust, and may thus 1998). These Arctic Ocean positive gravity anomalies may be related to refl ect radiogenic variations and heat-fl ow variations in these structural postglacial rebound or to rising thermal convection currents in the mantle, units. Heat-fl ow variations, manifested in gravity anomalies (Anderson which tend to generate positive (rather than negative) anomalies due to and Dziewonski, 1984) and thermally driven mass movements refl ected accompanying elevation increase at the surface (Anderson and Dziewonski,

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1984; Thomas et al., 2009). Thus, at least one other radial gravity anomaly in crustal geology. The Antarctic negative free-air gravity anomalies pattern exists in an oceanic region, this one also in a high latitude and appear, then, to be more likely the result of lithospheric or crustal control polar region, offshore of the continents around the Arctic Ocean basin. than of mantle infl uence. These circumstances tend to favor isostatic upper-mantle effects as the explanation for this offshore Antarctic circum-continental radial pattern. Gravitational Effects of the Subcrustal Lithosphere Such an explanation for the circum-continental radial fi eld is believed to have merit also because the Antarctic continent and the continents around In contrast to gravity signals from the mantle, gravity signals from the Arctic Ocean basin both have experienced glacial depression and post- Earth’s lithosphere and crust are normally of greater absolute magnitude glacial rebound, as the Arctic continues to do today. While revealing little due to the shorter distance of the generating source from observing instru- of the negative free-air gravity anomalies across interior Antarctica, the ments, and due to distinctive geological domains that possess particular radially elongated anomalies in the Arctic Ocean Basin and around the structural or density differences (Lucas et al., 1996; Leclair et al., 1997; Antarctic continent imply the infl uence of upper-mantle fl ow (Anderson Ashton et al., 1999). Such signals also derive from associated lithospheric and Dziewonski, 1984; Vogt et al., 1998), especially as related to shallow heat-fl ow domains (Mareschal et al., 1999; Rolandone et al., 2002). sublithospheric mantle heat fl ow and convection (Nyblade and Pollack, Treating the lithosphere in terms of viscous rheology, assuming a free 1993; Rolandone et al., 2002). upper boundary (Richards and Hager, 1984), and treating differences of On the other hand, regarding the negative free-air gravity anomalies average heat fl ow as functions of variations in lithospheric thickness and of the continental interior, the arguments that detract from a lower-mantle composition (Nyblade and Pollack, 1993; Jaupart et al., 1998), it is pos- explanation of the negative free-air gravity anomaly fi eld also apply to the sible to broadly approximate lithospheric thermal conditions. For stable mid- and upper mantle. That is, the magnitudes and wavelengths of anom- cratonic and shield regions, the maximum possible lithospheric thickness alies generated in the upper mantle are components of, and resemble, in which such variations can be detected by gravity sensing is taken to be those of the lower mantle (Richards, 1984). Additionally, regarding crustal 200–250 km (Jaupart et al., 1998), or at the upper limit, 200–350 km (Jau- temperatures and mantle heat fl ow beneath the Trans-Hudson orogen of part and Mareschal, 1999). Yet, because of lateral diffusion, such approxi- the Canadian continental shield, for example, Rolandone et al. (2002) mations are of limited use for heat-fl ow purposes, and variations of heat arrived at a mantle heat fl ow in the range of 11–16 mW m2, permitting supply at the base of the lithosphere are not detectable at Earth’s surface only relatively weak convective and density-induced gravity anomalies. over distances of a few hundred kilometers (Rolandone et al., 2002). Fur- Furthermore, only relatively weak convection is evident in Earth’s upper thermore, there is little evidence for signifi cant differences in heat sup- mantle, as revealed by seismic tomography (Anderson and Dziewonski, ply from the mantle to the lithosphere throughout Precambrian shields, 1984; Van der Hist et al., 1991; Zhao 2001), as well as in the upper thermal such as that of North America (Rolandone et al., 2002), due at least in boundary (Richards and Hager, 1984; Montelli et al., 2004). So, while the part to the relative lack of thermal structure in the upper mantle (Bred- moderate temperature differences at the base of the upper-mantle layer are dam, 2002; Foulger and Natland, 2003). For these reasons, we conclude a function of the underlying mantle, the temperature and density structures that the Antarctic subcrustal lithosphere also is not likely to be the main at the top of the upper mantle are more a function of lithospheric control signal source for the negative free-air gravity anomalies in interior Ant- (Ritzwoller et al., 2001). Also, while the temperature at the Moho, 1073 arctica. Further, as Rolandone et al. (2002) noted, heat-fl ow variations at K (Rolandone et al., 2002), is signifi cantly above the threshold value for Earth’s surface can only be due to the presence of different types of crust. melting, radiogenically undepleted upper-crustal rocks remain below the We would modify this to include also the presence of crustal density and melting temperature, and therefore dictate a lower bound on mantle heat topographic variations. For these reasons, variations in Earth’s crust are fl ow. This, in turn, limits density and heat-fl ow variations in the upper potentially more likely to be the cause of the free-air gravity anomalies mantle (Rolandone et al., 2002). Finally, heat-fl ow determinations and of interior Antarctica. We tentatively conclude that while neither mid- nor petrology suggest that there are only very modest temperature anomalies, upper-mantle conditions, nor subcrustal lithospheric conditions, are the even in the shallow mantle (Breddam, 2002). While a variety of rela- likely sources of the interior negative free-air gravity anomaly fi eld, sub- tively simple and complex models have been applied to the distribution glacial topographic or crustal density variations are the more likely source of density anomalies in the mantle, the one that most nearly approximates of the anomalies. Earth’s observed gravity fi eld is that of Steinberger and Holme (2002). This model prescribes randomly dispersed anomalies that may be applied Gravitational Effects of the Subglacial Crust to continental and larger than continental-scale domains, unlike the pattern of negative free-air gravity anomalies revealed in interior Antarctica. Geologic variations in the crust of East Antarctica now become central Because conditions remain largely unknown in the convecting mantle to interpreting the signifi cance of the observed interior Antarctic nega- below the continents (Rümpker et al., 2003), and because we are not yet tive free-air gravity anomaly fi eld, and while much is now known of the near a quantitative knowledge of the mantle’s density anomalies or heat- Transantarctic and other mountain ranges, and of the geologic periphery fl ow conditions (Steinberger and Holme, 2002), the heat fl ow at the Moho of the continent, relatively little is known of the geology, heat-fl ow, and can be taken as something of a constant for large scale geologic provinces density variations of the interior beneath the continental ice sheet. None- (Rolandone et al., 2002). Additionally, mantle heat-fl ow estimates, sig- theless, by direct survey (Fitzsimons, 2000; Studinger et al., 2004), and by nifi cantly affected by residual errors (Guillou et al., 1994; Jaupart et al., extrapolation from studies on other continents, continental East Antarc- 1998), tend to be small relative to those of the crust, while in the upper tica is considered to represent an ancient continental shield not unlike the mantle layer features tend to be dominated instead by surface tectonic shields of North America and Fennoscandia. Both of these have been well effects (Dziewonski and Anderson, 1984). On the basis of these consider- investigated, and both have many geological and geophysical properties ations, we believe that the mantle is unlikely to be the source of Antarc- in common. We believe it is reasonable and even necessary, therefore, to tica’s interior negative free-air gravity anomaly fi eld. Variation in Moho consider the East Antarctic continental crust as representing a generally temperature and heat fl ow is due mainly to differences in lithospheric comparable continental shield and, as Rolandone et al. (2002) reminded radiogenic heat production (Rolandone et al., 2002), and thus differences us, to use the same arguments, in this case for East Antarctica, as those

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applied to other similar geological provinces. This includes making simi- that from a radiosound survey of the region bounded by 67°00′S–75°00′S lar estimates of, for example, geological composition, structure, crustal and 105°00′E–155°00′E (Steed and Drewry, 1982). These data, we have radiogenic heat production, and heat fl ow as those known or estimated seen, reveal a topographic lowland in the lower reaches of the Wilkes for continental shields such as North America and Fennoscandia (Jõeleht Subglacial Basin inland from George V Coast (Fig. 6). Extending well and Kukkonen, 1998; Jokinen and Kukkonen, 1999; Russell and Kopy- below sea level, this lowland feature in Wilkes Land, presently occupied lova, 1999). We therefore proceed on the assumption that East Antarctica by coastward-fl owing continental ice, which has modifi ed the subglacial is broadly geologically and geophysically comparable to these continental topography, confi rms the presence of complex subglacial topography. shields, and we next examine the crust as the possible source of the inte- The unusually large absolute magnitude of the negative free-air gravity rior Antarctic negative free-air gravity anomaly fi eld. anomaly in this area suggests that an additional geologic component is Given the geologic constraints on the mantle and subcrustal litho- required to explain the anomaly, such as low crustal density. The gravity sphere discussed previously, observed relatively long-wavelength heat- decrease may therefore be related to the presence of a sedimentary basin fl ow variations can only be attributed to differences in the composition or of a substantial thickness of glacial drift. On the basis of a sedimen- and structure of the crust (White et al., 1999), including differences in tary or glacial drift density of 2.20 g/cm3, and a density contrast between the major radiogenic heat generators, namely uranium and thorium (Bur- rock and sedimentary or glacial material of 0.47 g/cm3, the thickness (h) wash and Cumming, 1976; Bingen et al., 1996; Bea and Montero, 1999; of lower-density materials required to account for this anomaly may be Bizzarro et al., 2003). Current knowledge of heat production in the deep approximated by transposing g = 0.04185/h, i.e., crust is summarized by other investigations (Rudnick and Fountain, 1995; Jõeleht and Kukkonen, 1998), but it is important to note that few thermal h = 101.9 mGal/[0.04185 × (2.67 – 2.20)] = 5180 m. conductivity measurements have been made in the 700–1200 K range of relevance to the lower continental crust. Beyond that, there generally is no A sedimentary or glacial drift thickness of 5180 m, however, is con- signifi cant heat-fl ow change across crustal geologic contacts (Mareschal sidered to be an unreasonably great thickness for low-density materials et al., 2000a), and determination of reliable average heat production on a such as these, and is therefore regarded not to be the source of the negative large scale, for purposes of density-related heat-fl ow studies, is diffi cult free-air gravity anomaly in Wilkes Land. Subglacial topographic and sub- (Jaupart, 1983). Because the surface heat-fl ow fi eld does not display a glacial rock density variations therefore appear to be the only remaining systematic spatial pattern on regional or whole-continent scales (Decker et explanations. Both requirements could be satisfi ed by the presence of an al., 1980; Jaupart and Mareschal, 1999), no continentwide systematic density-related heat-fl ow trend (nor related gravity signal) is normally observ- able (Mareschal et al., 2000b). Furthermore, removing the radiogenic heat contribution of the crust from broad heat-fl ow measurements is not possible without a geologic knowledge of the lower crust (Jaupart, 1983; Mareschal et al., 1999; Rolandone et al., 2002), and we have seen that relatively little is known of the geologic structure of Antarctica beneath the continental ice sheet. So, while radiogenic heat production is the major component of crustal heat fl ow, as well as a large component of the heat supplied to the lithosphere (Rudnick and Fountain, 1995), little is known of heat-related subglacial crustal geologic structure and density variations. On the other hand, geological structure, density, and topographic variations may be revealed by gravity and magnetic signals emanating from beneath the ice sheet. Earth’s continental crust is heterogeneous, often on a scale of some ten kilometers on average (Fountain and Salisbury, 1981). Such heterogene- ity in geologic structure, composition, density, and topography translates into variations in gravity signals. In continental shields such as those of North America and Fennoscandia, structural, density, and topographic differences are well known (Jokinen and Kukkonen, 1999; Russell and Kopylova, 1999; Jaupart and Mareschal, 1999; Rolandone et al., 2002), as are their consequent geophysical properties. The strong correlation among surface geology, radiogenic heat production, heat fl ow, and gravity is therefore a function mainly of these geologic variations in the upper crust (Pandit et al., 1998; Zelt and Ellis, 1999; Rolandone et al., 2002). Among the earliest evidence for variation in the geologic structure and density beneath the East Antarctic continental ice sheet and consequent gravity signals is the discovery of the Wilkes Land anomaly. We have seen that these data (Weihaupt, 1961, 1976), when combined with gravity data collected from Rouillon (1960), reveal a total negative free-air gravity anomaly in the region of 158.3 mGal. Representing a geological feature Figure 6. Radiosound data map of the complex subglacial topography in at least 243 km wide, this discovery supports the belief that the negative Wilkes Land, in the region of free-air gravity anomaly A in Figure 1 (con- free-air gravity fi eld in this area is a consequence of subglacial geological tour interval is 250 m). Notable features are the lowland features (white structure and, perhaps, geological density variations. Similarly, among the and blue), and the subglacial topography modifi ed by the coastward earliest evidence for variation in the topography beneath the ice sheet is movement of the continental ice sheet (after Steed and Drewry, 1982).

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impact crater, which would possess topographic basin and low rock den- Norway: Geochimica et Cosmochimica Acta, v. 60, p. 1341–1354, doi: 10.1016/0016 -7037(96)00006-3. sity (from impact brecciation) characteristics. Bizzarro, M., Baker, J.A., Haack, H., Ulfbeck, D., and Rosing, M., 2003, Early history of Earth’s crust-mantle system inferred from hafnium isotopes in chondrites: Nature, v. 421, CONCLUSION p. 931–933, doi: 10.1038/nature01421. Breddam, K., 2002, Petrology suggests modest shallow mantle temperature anomalies: Journal of Petroleum Geology, v. 43, p. 345. We began by stating that at least three aspects of the Antarctic gravity Burwash, R.A., and Cumming, G.L., 1976, Uranium and thorium in the Precambrian base- fi eld must be addressed in attempting to provide an adequate explanation ment of western Canada: I. Abundance and distribution: Canadian Journal of Earth Sciences, v. 13, p. 284–293. for the presence of the prominent free-air gravity anomalies in Antarc- Decker, E.G., Baker, K.R., Bucher, G.J., and Heasler, H.P., 1980, Preliminary heat fl ow and tica. Such an explanation must (1) identify possible sources of the anoma- radioactive studies in Wyoming: Journal of Geophysical Research, v. 85, p. 311–321, lies, (2) explain the unusually large magnitudes of the negative free-air doi: 10.1029/JB085iB01p00311. Dziewonski, A.M., and Anderson, D.L., 1984, Seismic tomography of the Earth’s interior: gravity anomalies, and (3) provide a rationale for the offshore positive American Scientist, v. 72, p. 483–494. free-air gravity anomalies. Each of these now has been addressed, and Fitzsimons, I.C.W., 2000, Grenville-age basement provinces in East Antarctica: Evidence for three separate collisional orogens: Geology, v. 28, p. 879–882, doi: 10.1130/0091 reasonable answers provided, and we conclude that the interior Antarctic -7613(2000)28<879:GBPIEA>2.0.CO;2. negative free-air gravity anomaly fi eld is a function not of the Antarctic Foulger, G.R., and Natland, J.H., 2003, Is ‘hotspot’ volcanism a consequence of plate tecton- mantle, but of the lithosphere, particularly the upper lithosphere, namely ics?: Science, v. 300, p. 921–922, doi: 10.1126/science.1083376. Fountain, D.M., and Salisbury, M.H., 1981, Exposed cross-sections through the continental the Antarctic crust. Topographic variations and crustal density variations, crust: Implications for crustal structure, petrology, and evolution: Earth and Planetary other than those due to a sedimentary basin or glacial detritus, are ade- Science Letters, v. 56, p. 263–277, doi: 10.1016/0012-821X(81)90133-3. quate to account for the geographic size and the magnitude of the negative Guillou, L., Mareschal, J.-C., Jaupart, C., Gariépy, C., Bienfait, G., and Lapointe, R., 1994, Heat fl ow, gravity and structure of the Abitibi belt, Superior Province, Canada: Implica- free-air gravity anomaly in Wilkes Land. We believe, also, that compara- tions for mantle heat fl ow: Earth and Planetary Science Letters, v. 122, p. 103–123, doi: ble explanations are likely to account for the other negative free-air grav- 10.1016/0012-821X(94)90054-X. Guillou-Frottier, L., Jaupart, J.C., Mareschal, J.- C., Gariépy, C., Bienfait, G., Cheng, Z., and ity anomalies in interior Antarctica. The radial circum-continental free-air Lapointe, R., 1996, High heat fl ow in the Trans-Hudson orogen, central Canadian Shield: gravity anomalies offshore of the Antarctic continent, on the other hand, Geophysical Research Letters, v. 23, p. 3027–3030, doi: 10.1029/96GL02895. are regarded to be the result of depression and rebound of the oceanic Heiskanen, W.A., and Vening Meinesz, F.A., 1958, The Earth and its Gravity Field, in Shrock, R.R., Chapter 7: New York, McGraw-Hill, p. 187–221. lithosphere resulting from glacial and postglacial variations in the mass of Jaupart, C., 1983, Horizontal heat transfer due to radioactivity contrasts: Causes and conse- the Antarctic continental ice sheet. quences of the linear heat fl ow–heat production relationship: Geophysical Journal of These answers are regarded as a preliminary step in defi ning and the Royal Astronomical Society, v. 75, p. 411–435. Jaupart, C., and Mareschal, J.C., 1999, The thermal structure of continental roots: Litho- understanding the Antarctic free-air gravity anomaly fi eld. We therefore sphere, v. 48, p. 93–114. recommend further detailed study of each of the major free-air gravity Jaupart, C., Mareschal, J.C., Gillou-Frottier, L., and Davaille, A., 1998, Heat fl ow and thickness of the lithosphere in the Canadian Shield: Journal of Geophysical Research, v. 103, anomaly sites. p. 15,269–15,286, doi: 10.1029/98JB01395. Jõeleht, A., and Kukkonen, I.T., 1998, Thermal properties of granulite facies rocks in the Pre- ACKNOWLEDGMENTS cambrian basement of Finland and Estonia: Tectonophysics, v. 291, p. 195–203, doi: 10.1016/S0040-1951(98)00040-7. Jokinen, J., and Kukkonen, I.T., 1999, Inverse simulation of the lithospheric thermal regime We wish to thank Geoforschungs Zentrum, Projektbereich 1.3, Potsdam, using the Monte Carlo method: Tectonophysics, v. 306, p. 293–310, doi: 10.1016/S0040- Germany, for providing pertinent gravity data in support of this study, par- 1951(99)00062-1. Karner, G.D., 2004, Quantitative basin analysis: Innovative solutions for complex explora- ticularly F. Barthelmes, P. Schwintzer, and Ch. Reigber. We wish to thank tion problems: Lamont-Doherty Earth Observatory, p. 1–3. also J. Wyckoff for assistance in preparing our free-air gravity contour Karner, G.D., and Watts, A.B., 1983, Gravity anomalies and fl exure of the lithosphere at moun- map (Fig. 1), as well as colleagues who accompanied us in the fi eld: C. tain ranges: Journal of Geophysical Research, v. 88, p. 10,449–10,477, doi: 10.1029/ JB088iB12p10449. Lorius, W.M. Smith, A. Stuart, A. Heine, A. Taylor, L. Roberts, T. Bald- Klokoenik, J., Kostelecky, J., Wagner, C.A., Schwintzer, P., Fo´´rste, Ch., and Scharroo, R., 2004, win, and W. Jackman. Finally, we also thank the National Science Founda- Evaluation of the accuracy of the EIGEN-1S and -2 CHAMP-derived gravity fi eld models by satellite crossover altimetry: Journal of Geodesy, doi: 10.1007/s00190-004-0403-0. tion for support of the original fi eld investigations, and the University of Krien, Y., and Fleitout, L., 2008, Gravity above subduction zones and forces controlling plate Colorado–Denver, the American Museum of Natural History, and Delft motions: Journal of Geophysical Research, v. 113, p. B09407, doi: 10.1029/2007JB005270. University of Technology for resource and administrative support. Leclair, A.D., Lucas, S.B., Broome, H.J., Viljoen, D.W., and Weber, W., 1997, Regional map- ping of Precambrian basement beneath Phanerozoic cover in southeastern Trans-Hud- son orogen, Manitoba and Saskatchewan: Canadian Journal of Earth Sciences, v. 34, REFERENCES CITED p. 618–634, doi: 10.1139/e17-049. Levin, V., Shapiro, N., Park, J., and Ritzoller, M., 2002, Seismic evidence for catastrophic slab Abbott, D., 2006, Space impact clue in Antarctica, in Lunar and Space Science Conference loss beneath Kamchatka: Nature, v. 418, p. 763–767, doi: 10.1038/nature00973. Houston, Texas. Lucas, S.B., Stern, R.A., Syme, E.C., Reilly, B.A., and Thomas, D.J., 1996, Intraoceanic tec- Anderson, D.L., and Dziewonski, A.M., 1984, Seismic tomography: Scientifi c American, tonics and the development of continental crust: 1.92–1.84 Ga evolution of the Flin v. 251, no. 4, p. 60–68, doi: 10.1038/scientifi camerican1084-60. Flon belt (Canada): Geological Society of America Bulletin, v. 108, p. 602–629, doi: Ashton, K.E., Heaman, L.M., Lewry, J.F., Hartlaub, R.P., and Shi, R., 1999, Age and origin of the 10.1130/0016-7606(1996)108<0602:ITATDO>2.3.CO;2. Jan Lake Complex: A glimpse at the buried Archean craton of the Trans-Hudson orogen: Lythe, M.B., and Vaughan, D.G., 2000, BEDMAP-Consortium, BEDMAP Bed Topography of Canadian Journal of Earth Sciences, v. 36, p. 185–208, doi: 10.1139/cjes-36-2-185. the Antarctic: Cambridge, UK, British Antarctic Survey. Barthelmes, F., 2002, GeoForschungs Zentrum Projektbereich 1.3: Potsdam, Germany, Geo- Mareschal, J.C., Jaupart, C., Cheng, L.Z., Rolandone, F., Gariépy, C., Bienfait, G., Guillou- Forschungs Zentrum. Frottier, L., and Lapointe, R., 1999, Heat fl ow in the Trans-Hudson orogen of the Cana- Bea, F., and Montero, P., 1999, Behavior of accessory phases and redistribution of Zr, Th, dian Shield: Implications for Proterozoic continental growth: Journal of Geophysical and U during metamorphism and partial melting of metapelites in the lower crust: Research, v. 104, p. 29,007–29,024, doi: 10.1029/1998JB900209. An example from the Kinzingite Formation of Ivrea-Verbano, NW Italy: Geochimica et Mareschal, J.C., Jaupart, C., Gariépy, C., Cheng, L.Z., Guillou-Frottier, L., Bienfait, G., and Cosmochimica Acta, v. 63, p. 1133–1153, doi: 10.1016/S0016-7037(98)00292-0. Lapointe, R., 2000a, Heat fl ow and deep thermal structure near the edge of the Canadian Bellahsen, N., Faccenna, C., and Funiciello, F., 2005, Dynamics of subduction and plate Shield: Canadian Journal of Earth Sciences, v. 37, p. 399–414, doi: 10.1139/cjes-37-2-3-399. motion in laboratory experiments: Insights into the “plate tectonics” behavior of the Mareschal, J.C., Poirier, A., Rolandone, F., Bienfait, G., Gariépy, C., Lapointe, R., and Jau- Earth: Journal of Geophysical Research, v. 110, p. B01401, doi: 10.1029/2004JB002999. parat, C., 2000b, Low mantle heat fl ow at the edge of the North American conti- Bina, C.R., and Navrotsky, A., 2000, Possible presence of high-pressure ice in cold subduct- nent, Voisey Bay, Labrador: Geophysical Research Letters, v. 27, p. 823–826, doi: ing slabs: Nature, v. 408, p. 844–847, doi: 10.1038/35048555. 10.1029/1999GL011069. Bingen, B., Demaiffe, D., and Hertogen, J., 1996, Redistribution of rare elements, thorium, Montelli, R., Nolet, G., Dahlen, F.A., Masters, G., Engdahl, E.R., and Hung, S.-H., 2004, Finite- and uranium over accessory minerals in the course of amphibolite to granulite facies frequency tomography reveals a variety of plumes in the mantle: Science, v. 303, metamorphism: The role of apatite and monazite in orthogenesis from southwestern p. 338–343, doi: 10.1126/science.1092485.

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Nyblade, A.A., and Pollack, H.N., 1993, A global analysis of heat fl ow from Precambrian ter- Steed, R.H.N., and Drewry, D.J., 1982, Radio echosounding investigations of Wilkes Land, rains: Implications for the thermal structure of Archean and Proterozoic lithosphere: Antarctica, in Craddock, C., ed., Antarctic Geoscience: Chicago, University of Chicago Journal of Geophysical Research, v. 98, p. 12,207–12,218, doi: 10.1029/93JB00521. Press, p. 969–975. Pandit, G.I., Hajnal, Z., Stauffer, M.R., Lewry, J., and Aston, K.E., 1998, Seismic images of Steinberger, B., and Holme, R., 2002, An explanation for the shape of Earth’s gravity spec- the crust in the central Trans-Hudson orogen of Saskatchewan: Tectonophysics, v. 290, trum based on viscous mantle fl ow models: Geophysical Research Letters, v. 29, 2019, p. 211–219, doi: 10.1016/S0040-1951(98)00009-2. doi: 10.1029/2002GL015476. Plescia, J.B., 1999, Gravity signature of the Teague Ring impact structure, Western Australia, Studinger, M., Bell, R.E., Buck, W.R., Karner, G.D., and Blankenship, D.D., 2004, Sub-ice geol- in Dressler, B.O., and Sharpton, V.L., eds., Large Meteorite Impacts and Planetary Evo- ogy inland of the Transantarctic Mountains in light of new aerogeophysical data: Earth lution II: Geological Society of America Special Paper 339, p. 165–176, doi: 10.1130/0- and Planetary Science Letters, v. 220, p. 391–408, doi: 10.1016/S0012-821X(04)00066-4. 8137-2339-6.165. Thomas, S.J., Gowan, E.J., Ikuko, W., and Kelin, W., 2009, Viscosity of the asthenosphere Reigber, C., Balmino, G., Schwintzer, P., Biancale, R., Bode, A., Lemoine, J.-M., König, R., from glacial isostatic adjustment and subduction dynamics at the northern Cascadia Loyer, S., Neumayer, H., Marty, J.-C., Barthelmes, F., Persosanz, F., and Zhu, S.U., 2002, A subduction zone, British Columbia, Canada: Journal of Geophysical Research, v. 114, high quality global gravity fi eld model from CHAMP GPS tracking data and accelerom- BO4405, doi: 10.1029/2008JB006077. etry (EIGEN-IS): Geophysical Research Letters, v. 29, 1692, doi: 10.1029/2002GL015064. Van der Hist, R., Engdahl, R., Spakman, W., and Nolet, G., 1991, Tomographic imaging of Richards, M.A., 1984, Geoid anomalies in a dynamic earth: Journal of Geophysical Research, subducted lithosphere below northwest Pacifi c island arcs: Nature, v. 353, p. 37–43, v. 89, p. 5987–6002, doi: 10.1029/JB089iB07p05987. doi: 10.1038/353037a0. Richards, M.A., and Engebretson, D.C., 1992, Large-scale mantle convection and the history Verhoeven, O., Mocquet, A., Vacher, P., Rivoldini, A., Menvielle, M., Arrial, P.-A., Choblet, G., of subduction: Nature, v. 355, p. 437–440, doi: 10.1038/355437a0. Tarits, P., Dehant, V., and Van Hoolst, T., 2009, Constraints on thermal state and compo- Richards, M.A., and Hager, B.H., 1984, Long-wavelength geoid anomalies and mantle con- sition of the Earth’s lower mantle from electromagnetic impedances and seismic data: vection: Eos (Transactions, American Geophysical Union), v. 66, p. 65. Journal of Geophysical Research, v. 114, p. B03302, doi: 10.1029/2008JB005678. Ritsema, J., Cupillard, P., Tauzin, B., Xu, W., Stixrude, L., and Lithgow-Bertelloni, C., 2009, Vogt, P., Jung, W.-Y., and Brozena, J., 1998, Arctic margin gravity highs remain puzzling: Eos Joint mineral physics and seismic wave traveltime analysis of upper mantle tempera- (Transactions, American Geophysical Union), v. 79, no. 49, p. 601, doi: 10.1029/98EO00433. ture: Geology, v. 37, p. 363–366, doi: 10.1130/G25428A.1. Weihaupt, J.G., 1961, Geophysical Studies in Victoria Land, Antarctica: Madison, University Ritzwoller, M.H., Shapiro, N.M., Levshin, A.L., and Leahy, G.M., 2001, Crustal and upper of Wisconsin, Geophysical and Polar Research Center Research Report 1, 123 p. mantle structure beneath Antarctic and surrounding oceans: Journal of Geophysical Weihaupt, J.G., 1976, The Wilkes Land anomaly: Evidence for a possible hypervelocity Research, v. 106, p. 30,645–30,670, doi: 10.1029/2001JB000179. impact crater: Journal of Geophysical Research, v. 81, p. 5651–5663, doi: 10.1029/ Rolandone, F., Jaupart, C., Mareschal, J.C., Gariépy, C., Bienfait, G., Carbonne, C., and JB081i032p05651. Lapointe, R., 2002, Surface heat fl ow, crustal temperatures and mantle heat fl ow in the Weihaupt, J.G., and Van der Hoeven, F.G., 2004, The Antarctic gravity spectrum: Mantle, lith- Proterozoic Trans-Hudson orogen, Canadian Shield: Journal of Geophysical Research, osphere and crustal considerations, in Proceedings of the 32nd International Geologi- v. 107, p. 2341, doi: 10.1029/2001JB000698. cal Congress: Florence, Italy, International Geological Congress, p. 159. Rouillon, G., 1960, Anomalies de la pesanteur et profi l de la calotte glaciaire antarctique en Weihaupt, J.G., Rice, A., and Van der Hoeven, F.G., 2006, Multiple meteoroid impacts in Ant- Terre Adélie: Comptes Rendus Hebdomadaires des Séances de l’Académie des Sci- arctica: Eos (Transactions, American Geophysical Union), v. 87, Fall Meeting supple- ences, v. 251, p. 762–764. ment, abs. P51A-1181. Rudnick, R.G., and Fountain, D.M., 1995, Nature and composition of the continental crust: Weihaupt, J.G., Rice, A., and Van der Hoeven, F.G., 2010, Multiple meteoroid impacts in Ant- A lower crustal perspective: Reviews of Geophysics, v. 33, p. 267–309, doi: 10.1029/ arctica and the mid-Brunes Event/MIS 11 stage: If there is a connection, what are the 95RG01302. implications for humanity?, in American Geophysical Union Meeting of the Americas, Rümpker, G., Ryberg, T., Bock, G., and Desert Seismology Group, 2003, Boundary-layer man- Foz do Iguaçu, Brazil: Washington, D.C., American Geophysical Union, p. 123. tle fl ow under the Dead Sea transform fault inferred from seismic anisotropy: Nature, White, D.J., Jones, A.G., Lucas, S.B., and Hajnal, Z., 1999, Tectonic evolution of the Superior v. 425, p. 497–501, doi: 10.1038/nature01982. boundary zone from coincident seismic refl ection and magnetotelluric profi les: Tecton- Russell, J.K., and Kopylova, G., 1999, A steady-state conductive geotherm for the north ics, v. 18, p. 430–451, doi: 10.1029/1999TC900002. central Slave, Canada: Inversion of petrological data from the Jericho kimberlite pipe: Zelt, B.C., and Ellis, R.M., 1999, Receiver-function studies in the Trans-Hudson orogen, Saskatche- Journal of Geophysical Research, v. 104, p. 7089–7101, doi: 10.1029/1999JB900012. wan: Canadian Journal of Earth Sciences, v. 36, p. 585–603, doi: 10.1139/cjes-36-4-585. Shito, A., Shiobara, H., Sugioka, H., Ito, A., Takei, Y., Kawakatsu, H., and Kanazawa, T., 2009, Zhao, D., 2001, Seismic structure and origin of hotspots and mantle plumes: Earth and Plan- Physical properties of subducted slab and surrounding mantle in the Izu-Bonin sub- etary Science Letters, v. 192, p. 251–265, doi: 10.1016/S0012-821X(01)00465-4. duction zone based on broadband ocean bottom seismometer data: Journal of Geo- MANUSCRIPT RECEIVED 18 JUNE 2010 physical Research, v. 114, p. BO3308, doi: 10.1029/2007JB005568. REVISED MANUSCRIPT RECEIVED 21 AUGUST 2010 Smith, W.S., Stuart, A.W., Van der Hoeven, F.G., and Weihaupt, J.G., 2011, Long Cold Journey: MANUSCRIPT ACCEPTED 27 AUGUST 2010 The Story of the Victoria Land Traverse 1959–1960: Las Vegas, Nevada, Von Raesfeld, Ltd. (in press). Printed in the USA

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