This article appeared in Geophysical Research Letters, v. 25, no.23, p.4385-4388, December 1, 1998

Aeromagnetic Evidence for a Volcanic Caldera(?) Complex Beneath the Divide of the West Antarctic Ice Sheet

John C. Behrendt^1,2, Carol A. Finn², Donald Blankenship³, and Robin E. Bell⁴

¹ Institute of Arctic and Alpine Research, University of Colorado, Boulder.
² U.S. Geological Survey, Denver, Colorado.
³ Institute for Geophysics, University of Texas, Austin.
⁴ Lamont Doherty Earth Observatory, Columbia University, Palisades, New York.

A 1995-96 aeromagnetic survey over part of the Sinuous Ridge (SR) beneath the West Antarctic Ice Sheet (WAIS) divide shows a 70-km diameter circular pattern of 400-1200-nT anomalies suggesting one of the largest volcanic caldera(?) complexes on earth. Radar-ice-sounding (RIS) shows the northern part of this pattern overlies the SR, and extends south over the Bentley Subglacial Trench (BST). Modeled sources of all but one the caldera(?) anomalies are at the base of <1-2-km thick ice and their volcanic edifices have been glacially removed. The exception is a 700-m high, 15-km wide "volcano" producing an 800-nT anomaly over the BST. "Intrusion" of this "volcano" beneath 3 km of ice probably resulted in pillow basalt rather than easily removed hyaloclastite erupted beneath thinner ice. The background area (-300 to -500-nT) surrounding the caldera(?) is possibly caused by a shallow Curie isotherm. We suggest uplift of the SR forced the advance of the WAIS.

The WAIS flows through the vast enigmatic West Antarctic rift system characterized by exposures of bimodal alkaline volcanic rocks [LeMasurier, 1990] and >106km3 of subglacial volcanic rocks interpreted from aeromagnetic data [Behrendt et al., 1994] ranging in age from ~30 Ma to the present. The source of the extensive volcanism is probably a mantle plume [Behrendt et al., 1992] possibly combined with lower lithospheric extension [Behrendt et al., 1996]. The 1800-m high divide of the WAIS, is underlain by the 400-km long SR (Figure 1), which rises to nearly one kilometer above sea level. Jankowski et al.[1983] interpreted the SR to comprise volcanic rocks based on >1000-nT anomalies observed on several aeromagnetic profiles. In 1995-96 we made a 5-km line spaced orthogonal gridded aeromagnetic survey (to the same specifications as our earlier survey over WAIS about 300 km to the south [Blankenship et al., 1993; Behrendt et al., 1994] over the west end of the SR (Figures 1 and 2) combined with RIS, aerogravity and laser surface altimeter measurements using the facility of the U.S. Support Office for Aerogeophysical Research (SOAR) [Blankenship et al., 1997].

Figure 1 (a) Generalized isostatically compensated (after ice removal) bedrock elevation map of part of Antarctica (from Behrendt et al., 1994; modified from Drewry, 1983]. Coarse dots show edge of present grounded ice. Rectangular box in center of figure indicates area of aeromagnetic survey (Figure 2) and high topography of SR which separates the Byrd Subglacial Basin and BST. Cross marks South Pole. The Meridian at 0o (grid north) is at top of the map following usual convention for small-scale maps of Antarctica; in contrast, true north is at top of Figure 2. (b) Bedrock topographic map modified from Drewry [1983] and Jankowski et al.[1983]. Box outlines area of aeromagnetic survey (Figure 2). Circle indicates location of anomaly pattern (Figure 2) inferred as caused by a subglacial caldera(?). The divide of the WAIS approximately overlies the SR.

The prominent positive and negative anomalies (Figure 2) range from 1200 (anomaly Z) to -500 nT (anomaly B), very high considering that the shallowest possible sources lie at a minimum of 1.5 km below the survey aircraft. The magnetic field is dominated by a striking 70-km diameter circular pattern of positive anomalies whose amplitudes range from 400-1200 nT surrounding a low amplitude ~-150-nT central area. We interpret this pattern of anomalies to mark a volcanic caldera(?) complex associated with late Cenozoic volcanism which is probably related to tectonic uplift of the SR. Similar circular anomaly patterns surrounding central "lows" including amplitudes and diameters mark other active or young known calderas such as Yellowstone [Bhattacharyya, et al., 1975; Smith et al., 1994], Jemez, and San Juan [Committee for the Magnetic Map of North America, 1987; USGS, unpublished data] in the tectonically active Rocky Mountains area of the U.S.

Figure 2. (a) Shaded aeromagnetic map. Navigation accuracy by GPS is higher than 1 m (as in Blankenship et al, 1993; Behrendt et al. 1994]. Contour interval: 50 nT between 400 and -400 nT. Profile Y is shown in Figure 3. The circular anomaly pattern is interpreted as caused by volcanic rocks associated with an inferred caldera complex. Anomalies A, C (Figure 4), X, and Z were modeled to have very highly magnetized sources at the base of the ice sheet. The high subice topography of the SR approximately underlies the north half of the map. Models fit to anomaly B require either a reversed remanent magnetization, or more likely indicate a broad background magnetic "low" (the blue area) interpreted as a shallow Curie isotherm, contrasted to the high amplitude shallow source positive anomalies whose sources lie at the base of the ice sheet. (b) Terrace map calculated for the survey in (a) using method of Cordell and McCafferty [1989] after "reducing to pole." Boundaries indicated by small "+" symbols define edges of sources of magnetic anomalies. This map is a first approximation of a 3 dimensional magnetic model. Colors boundaries are in nT but could be converted to magnetizations or susceptibilities by defining bases of sources.

Almost certainly the shallow subglacial sources of very high-amplitude anomalies which comprise the circular structure (Figure 2) are volcanic whether or not they mark the edge of a caldera. However, an alternate interpretation of a meteorite impact structure in a volcanic terrane cannot be ruled out.

Although we show the Jankowski et al. [1983] map (Figure 1), we use a number of our 1995-96 RIS profiles (e.g. Figure 3) to constrain our interpretations. Profile Y, which crosses the caldera anomaly pattern from the BST at the south to the SR at the north, shows no obvious correlation of the magnetic field with topography with the exception of the source of anomaly C (Figures 3 and 4). The circular pattern is obviously not directly correlated with the SR as confirmed by examination of a number of orthogonal RIS profiles crossing the circular magnetic anomaly pattern including the peaks of the high amplitude anomalies A, X, and Z in Figure 2. We calculated magnetic models for several of the anomalies defining the circular pattern (Figure 2), constrained by their steep gradients and the RIS data, which showed their sources to be at the base of the ice sheet overlying the SR and the deeper bedrock of the BST to the south.

Figure 3. Aeromagnetic and RIS profile Y (Figure 2). Approximate bedrock elevation. Flight elevation was about 200 m above ice surface. Models fit to anomaly C (Figure 4), anomaly A and several other high amplitude anomalies comprising the circular caldera complex (Figure 2) require highly magnetic volcanic sources at the base of the ice sheet as measured by RIS data. Note high vertical exaggeration (VE).

Figure 4 shows the magnetic model for anomaly C (Figures 2 and 3) fit to this bedrock topography. The high magnetization required implies a very high remanent magnetization in the present field direction for the interpreted volcanic edifice which is the source of the anomaly. However, considering measured susceptibilities and remanent magnetizations discussed [Behrendt et al., 1996] for late Cenozoic volcanic rocks in the McMurdo area, a Q>10 is implied for our calculated models, which is well within the reported range. Similar very high magnetizations were required to fit anomaly A, X, Z (Figures 2 and 3) and other high amplitude anomalies of the circular pattern interpreted as caldera complex (Figure 2).

Figure 4. Model (2.5d, 3 and 10 km strike length to east and west respectively) fit to magnetic profile over anomaly C (Figures 2-3). Bedrock profile as in Figure 3. Indicated apparent susceptibility contrasts are in S.I. units. The single central body (which is not totally "pinched out") has a constant .59 which implies a high remanent normal magnetization in the present field direction. An apparent very magnetic subglacial (young?) volcanic edifice is inferred which has not been glacially removed [Behrendt et al., 1995], in contrast to models for anomaly A, X, Z, and other high amplitude anomalies comprising the caldera pattern (Figure 2). There could easily be a masked, less magnetic subvolcanic intrusion (plug or feeder dike) beneath the interpreted volcanic edifice. The source is likely volcanic flows "intruded" into the thick ice rather than relatively nonmagnetic hyaloclastite.

Figure 2 shows significant -500-nT and -300-nT anomalies to the SW and NE of the circular feature respectively. However, calculated magnetic models (not shown) can be interpreted to show that if the -300-nT to -500-nT anomalous area were the general background field, possibly caused by a shallow Curie isotherm beneath the caldera as is the case in the Yellowstone area [Bhattacharyya, et al., 1975; Smith et al., 1994], all of the anomalies could be explained by positive magnetizations (induced or remanent) in the present field direction. Were this the case, the magnetic low in the center of the circular caldera ring (which consists of low amplitude shallow source positive anomalies; e.g. Figures 2-3) would be underlain by normal magnetization having a shallower depth to the Curie isotherm resulting in a lower "regional" amplitude (~-150 nT) beneath the center of the circular pattern than beneath the sources of the high amplitude anomalies on the periphery. Therefore, any interpretation of reversed magnetization for the prominent negative anomalies must be viewed cautiously.

The lack of obvious correlation of topography with the circular caldera magnetic anomaly pattern (Figures 2 and 3 and other RIS profiles crossing the anomaly complex) suggests that the sources of these anomalies modeled to lie at the ice bedrock interface are older than the present topography of the SR. The obvious exception of the source of anomaly C suggests that the caldera complex may have been active in the recent geologic past because the source of this anomaly has not been glacially removed as is generally the case for subglacially erupted volcanic edifices over volcanic centers marked by magnetic anomalies over the ice of the West Antarctic rift system [Behrendt et al., 1995].

Pillow lavas erupted beneath thick ice

However, we propose an alternate (or modified) explanation for the source of anomaly C. Gudmundsson et al. [1997] reported observations of a hyaloclastite ridge from the 1996 subglacial eruption beneath the 500-750-m thick ice of Vatnaj¶kull, Iceland. This is the general case as suggested [Behrendt et al., 1995] for eruptions beneath the WAIS such as the nonmagnetic [as discussed by Behrendt et al., 1994] active [Blankenship et al., 1993] volcanic peak beneath ~1800-1900-m thick ice in the CASERTZ area about 300 km south of anomaly C (Figure 2). The very magnetic ridge which is the source of anomaly C (Figures 3 and 4) more likely consists of erosion resistent pillow lavas which were erupted beneath the very thick ice of the BST. If the ice at the time of eruption had about the same thickness as at present (i.e.,~3 km), we suggest that the high pressure prevented degassing of volatiles and absence of hyaloclastite and volcanic glass as observed beneath much thinner ice in active subglacial volcanism in Iceland. Gudmundsson [1997] noted the tripartite (i.e. pillow basalts at the base, hyaloclastite beneath thinner ice, and subaerial flows at the top) character of volcanic eruption through ice sheets in Icelandic exposures. Probably the shallower sources of anomalies defining the circular caldera ring (Figure 2) were erupted under <2 km (or even <1 km, e.g. at anomaly A, 12 km to the east of the profile in Figures 2 and 3) of ice beneath the divide of the WAIS; the former volcanic edifices (since removed) likely consisted of hyaloclastite. The exposed peaks of >3-km high volcanoes in the West Antarctic rift system (e.g. many in Figure 1) are capped by lava flows which were probably erupted subaerially [e.g. McIntosh and Gamble, 1991], which protected these peaks from glacial erosion [Behrendt et al., 1995].

Circular magnetic pattern suggests younger age of basement

Previously published interpretations of closely spaced aeromagnetic surveys over other areas (e.g. the Ross embayment and WAIS area about 300 km to the south) of the West Antarctic rift system showed a linear rift fabric [Behrendt et al., 1996], probably related to initial extensional structures associated with separation of New Zealand and the Campbell Plateau from Antarctica in Cretaceous time The late Cenozoic volcanism apparently followed the older rift trends resulting in the linear rift fabric of their associated magnetic anomalies. In contrast, the magnetic survey over the SR (Figure 2) shows a pronounced circular character and no apparent linearity to the distribution of magnetic anomalies. Our magnetic observations, combined with the nearly 3-km structural relief of the SR suggest a different (younger?) age for the basement terrane in this area. Obviously, large offset seismic measurements are needed to define crustal and upper mantle structure here.

Figures 1 and 3 show that the bedrock topography in the SR area would rise well above sea level by isostatic rebound were the ice removed (compare Figure 1a with 1b). Drewry, [1983] showed 0.5-1.0 km of isostatic rebound in the BST- SR-Byrd Subglacial Basin area assuming ice removal. If this high topography was elevated in late Cenozoic time associat- ed with volcanic and tectonic activity in the West Antarctic rift system, it may have forced the advance (or readvance following an earlier deglaciation) of the WAIS, by providing a nucleus for glaciation at this now subaerial divide between the Byrd subglacial Basin and BST.

We thank the field operations team and the personnel of Kenn Borek Air Ltd. for their long term technical support. Ron Sweeney compiled the aeromagnetic survey. Discussions with Magnus Gudmundsson, John Smellie, Ian Skilling, Wesley LeMasurier, and Lisa Morgan, were particularly helpful. The work was supported by the National Science Foundation and the U.S. Geological Survey. Several reviewers made constructive criticisms and suggestions, which were quite helpful completing the paper.

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J.C. Behrendt, INSTAAR, University of Colorado, Boulder, CO 80309-0450. (e-mail: behrendj@stripe.colorado.edu). Behrendt also at U.S. Geological Survey, MS 964, Federal Center, Denver CO 80225.

C. A. Finn, U.S. Geological Survey, MS 964, Federal Center, Denver CO 80225.

R. E. Bell, Lamont-Doherty Earth Observatory, Palisades, NY 10964.

D. D. Blankenship, Institute for Geophysics, University of Texas, 8701 North Mopac Boulevard, Austin, TX 78759.