Ørsted verifies regional magnetic anomalies of the Antarctic lithosphere

Authors


Abstract

[1] Initial magnetic measurements from the Ørsted satellite reveal lithospheric anomalies over the Antarctic that are similar to those obtained by Magsat. Accordingly, lithospheric anomalies can be extracted from the Ørsted data, despite the much greater operational altitude of Ørsted (650–865 km) relative to Magsat (350–550 km). Furthermore, these correspondences confirm the lithospheric origins for the resulting small-amplitude anomalies in the satellite data. In studies of the Antarctic lithosphere, the Magsat data particularly were limited by the large relative uncertainties of their lithospheric components. These uncertainties occurred because the short nearly seven-month mission more than 20 years ago collected data over austral high summer and early fall when the contaminating large-amplitude external field effects were at a maximum. Therefore, the recent and more numerous Ørsted measurements greatly facilitate our efforts to separate effectively the core, lithospheric, and external field components for enhanced studies of the Antarctic lithosphere.

1. Introduction

[2] Since February 23, 1999, Denmark's first satellite, Ørsted, has been providing high-precision vector and scalar geomagnetic field measurements over nearly polar orbits at altitudes between roughly 650–865 km [Neubert et al., 2001]. The Ørsted measurements augment previous data obtained by POGO (Polar Orbiting Geophysical Observatory) and Magsat [e.g., Langel and Hinze, 1998]. The POGO scalar magnetic field data were collected by a series of missions flown in 1967–1971 at altitudes ranging between 400–1100 km to map the core magnetic field and its variation. These data, however, also revealed small magnetic anomalies that appeared to reflect regional lithospheric features. Magsat was launched on October 30, 1979 to map these small anomalies in greater detail over a period of nearly seven months at altitudes between 352–561 km. While both scalar and vector magnetic data were gathered, attitude errors significantly corrupted the vector measurements. Hence, in this study we compare only the scalar total field anomalies from Magsat and Ørsted for their lithospheric components.

[3] Magsat recorded lithospheric anomalies ranging typically between ±20 nT while the Ørsted signals vary around ±3 nT. Errors for the scalar anomaly values are <3 nT for Magsat [Langel and Hinze, 1998] and <0.3 nT for Ørsted [Neubert et al., 2001]. Lithospheric anomaly errors predominantly result from errors in modeling for the core and external field contributions [Alsdorf et al., 1994]. These errors are especially problematic over the poles due to the ubiquitous presence of highly dynamic external fields produced by the auroral electrojets, field-aligned currents and large-scale ring currents [e.g., Langel and Hinze, 1998]. The problem of extracting lithospheric components is particularly critical in the Antarctic Magsat data that were obtained during austral summer and fall when south polar external field activity was at a maximum.

2. Data Processing

[4] The core and external fields cannot be modeled with sufficient accuracy at present to extract effective lithospheric anomaly estimates from satellite magnetometer data [Alsdorf et al., 1994; von Frese et al., 1999b]. However, these polar magnetic fields can be separated effectively by spectral correlation theory [von Frese et al., 1997a] that exploits the coherent or static properties of lithospheric anomalies and the core field relative to the dynamic features of the external fields [Alsdorf et al., 1994; von Frese et al., 1999b]. Figure 1a gives the Magsat lithospheric anomalies estimated from this method by Alsdorf et al. [1994] superposed on the Antarctic topography surface free of snow, ice and seawater from von Frese et al. [1999a].

Figure 1.

Scalar total magnetic anomalies for the Antarctic south of 55°S from (a) Magsat at 430-km altitude with 1-nT contour interval, and (b) Ørsted at 700-km altitude with 0.5-nT contour interval. Data gaps out to about 87°S occur because both satellite missions were not completely polar orbiting. Annotations for correlative anomaly features are given in Table 1.

[5] Except for some relatively minor modifications, essentially the same procedure was used to reduce the Ørsted data for the lithospheric anomaly estimates shown in Figure 1b. The Ørsted anomalies were derived at 700-km altitude from Antarctic orbital data collected from June to August, 1999 and 2000 at altitudes below 800 km. Removing the preliminary Ørsted main field model [Olsen et al., 2000] yielded unacceptably large residuals for estimating the lithospheric components. A better approach, borrowed from our initial Magsat studies before the comprehensive main field model of Langel and Estes [1985] became available, was to fit polynomials up to degree three to each Ørsted track for residuals with amplitudes in the range of a few nT. We also implemented the spectral quadrant swapping method of Kim et al. [1998] to minimize track-line or corrugation noise in the Ørsted anomalies.

3. Interpretation

[6] In general, Figures 1a and 1b suggest remarkable correspondences between the Magsat and Ørsted anomalies. It is impossible in this short report to present detailed lithospheric models for these anomalies, however, we will briefly discuss their tectonic significance. For better clarification, the correlative anomaly features are alphabetically labeled in the two maps and summarized in Table 1. They are generally consistent with the large-scale lithospheric features of the Antarctic.

Table 1. The Alphabetical Identifiers, Affiliated Geological/Geographical Features, and Relative Anomaly Polarities in Parentheses are Listed Below for the Correlative ØRsted and Magsat Anomalies of the Antarctic in Figure 1
LabelAnomaly featuresLabelAnomaly features
(A)Queen Maud Land (−)(K)Thurston Microplate (+)
(B)Gamburtsev Mountains (−)(L)Marie Byrd Land Microplate and Byrd Subglacial Basin (+)
(C)Northwest Pensacola Basin (+)
(D)Southeast Pensacola Basin (+)(M)Maud Rise (+)
(E)Wikes Land (+)(N)Southern Crozet Plateau (+)
(F)Prince Charles Mountains (+)(O)Southern Kerguelen Plateau (+)
(G)Enderby Land (+)(P)Pacific-Atlantic Ridge (+)
(H)Continental margin ocean basins (−)(Q)Southeastern Pacific Basin Maxima (+)
(I)Filchner and Ellsworth Microplates (−)(R)Southeastern Pacific Basin Minima (−)
(J)Antarctic Peninsula Microplate (+)(S)Transantarctic Mountains and Ross Sea Margin (−)

[7] Over East Antarctica, for example, prominent anomaly minima mark Queen Maud Land (A) and the Gamburtsev Subglacial Mountains (B). The Pensacola Basin is bordered by regional anomaly maxima on the northwest (C) and its opposite end (D) between the Gamburtsev and Transantarctic Mountains.

[8] Continent-ocean edge effect anomalies [e.g., Bradley and Frey, 1991] may be reflected along the eastern margin of Antarctica by several maxima extending from Wilkes Land (E) up to the Prince Charles Mountains (F) and Enderby Land (G) with seemingly complementary flanking oceanic basin minima (H). Quantitative magnetic modeling of available crustal thickness data [von Frese et al., 1999a] suggests the edge effect anomalies can be accommodated by 2-A/m crust that abruptly thins from about 35 km beneath the continent to roughly 12 km under the oceans. However, to model the oceanic minima (H) fully, an additional contrast in crustal magnetization of about −1 A/m is required.

[9] These satellite anomalies also facilitate extrapolating tectonic information into East Antarctica from better-studied components of Gondwana [Frey et al., 1983; Galdeano, 1983; von Frese et al., 1986] and earlier supercontinents [von Frese et al., 1997b]. A particularly striking example is the Wilkes Land anomaly maximum (E) that shows a Gondwana correlation with comparable satellite magnetic anomalies overlying Archean-Proterozoic cratonic blocks in south central and western Australia [von Frese et al., 1986]. Another prominent example involves maxima (C and G) between the southern margin of the Weddell Sea and Enderby Land that show an apparent late Precambrian association with the east-west band of satellite magnetic maxima over the U.S. mid-continent [von Frese et al., 1997b]. The U.S. anomalies, observed by both Magsat and Ørsted missions [Purucker et al., 2002], have been related to the distribution of Middle Proterozoic granite-rhyolite rocks inferred from limited deep drilling of the mid-continent [ Starich et al., 1985; von Frese et al., 1997b].

[10] West Antarctica is recognized as a series of microplates related to the circum-Pacific Mobile Belt that also appear to be well marked by regional magnetic anomalies [von Frese et al., 1999b]. For example, the region of the Filchner and Ellsworth Microplates is overlain by a prominent anomaly minimum (I), whereas magnetic maxima delineate the Antarctic Peninsula (J) and Thurston (K) Microplates. In addition, a magnetic maximum (L) overlies the region of the Marie Byrd Land Microplate and Byrd Subglacial Basin. These anomaly maxima also appear to be complemented by ocean basin minima (H) much like those along the margin of East Antarctica.

[11] In the off shore areas, prominent maxima overlie the Maud Rise (M), the southern Crozet (N) and Kerguelen (O) Plateaux, and major portions of the Pacific-Atlantic Ridge (P). These maxima may reflect strongly magnetized, possibly serpentinized and thickened oceanic crust. Additional correlative marine anomalies include the maxima (Q) in the southeastern Pacific Basin, as well as a number of minima (R) extending around the western margin of the study region.

[12] Discrepancies between these two data sets involve mostly distorted anomaly patterns rather than a total lack of correlative anomaly features. A good example is the Pacific-Atlantic Ridge that reflects maxima (P) mapped more prominently in the Ørsted than the Magsat data. The minima (S) over the Transantarctic Mountains, on the other hand, extend further westwards across the Ross Ice Shelf and Sea in the Ørsted data than in the Magsat data. Clearly, a major advantage of the Ørsted mission is that additional austral winter cycles of observations will be obtained to further limit the uncertainties in these anomalies for lithospheric analysis.

4. Conclusions

[13] In general, the Ørsted mission confirms the veracity of satellite magnetometer observations for studies of the Antarctic lithosphere. According to our Ørsted experiences at least, a remarkably decreased level of non-lithospheric noise is observed in the Ørsted data relative to the lower altitude Magsat data due in part to the longer duration of the Ørsted mission. The resulting increased volume of Ørsted data facilitates improving the signal-to-noise ratio of lithospheric anomalies that grows as the square root of the number of data points.

Acknowledgments

[14] We are grateful for the support of the National Aeronautics and Space Administration (NASA) under grant NAG5-7645 and the Ørsted Project Office and the Ørsted Science Data Centre at the Danish Meteorological Institute. The Ørsted Project is funded by the Danish Ministry of Transport, Ministry of Research and Information Technology, and Ministry of Trade and Industry. Additional support was provided by the European Space Agency (ESA), Centre Nationale d'Etudes Spatiales (CNES), Deutsche Agentur für Raumfahrtangelegenheiten (DARA), and the Ohio Supercomputer Center.

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