[1] A global Earth Magnetic Anomaly Grid (EMAG2) has been compiled from satellite, ship, and airborne magnetic measurements. EMAG2 is a significant update of our previous candidate grid for the World Digital Magnetic Anomaly Map. The resolution has been improved from 3 arc min to 2 arc min, and the altitude has been reduced from 5 km to 4 km above the geoid. Additional grid and track line data have been included, both over land and the oceans. Wherever available, the original shipborne and airborne data were used instead of precompiled oceanic magnetic grids. Interpolation between sparse track lines in the oceans was improved by directional gridding and extrapolation, based on an oceanic crustal age model. The longest wavelengths (>330 km) were replaced with the latest CHAMP satellite magnetic field model MF6. EMAG2 is available at http://geomag.org/models/EMAG2 and for permanent archive at http://earthref.org/ cgi-bin/er.cgi?s=erda.cgi?n=970.

Magnetic and gravity derivatives can be likened to seismic attributes in that they can help define/estimate the physical properties of the source structure causing the anomaly. This contribution looks at the tilt derivative, first reported in 1994 and more recently used to derive the local wavenumber (1997). We will show that the combination of the tilt derivative and its total horizontal derivative are highly suitable for mapping shallow basement structure and mineral exploration targets and that they have distinct advantages over many conventional derivatives. We provide the simple theory behind the derivatives, use a range of simple 2D models to illustrate their response, and apply them to mapping a mineral target in Namibia.Tilt derivative (TDR). The physical properties of a magnetic source structure that can be determined from a grid of magnetic data include its shape (plan), location of its edges, depth to top edges, dip, and rock susceptibility contrast. This contribution will focus on the first two-i.e. shape and edge detection. The problems to be overcome in data enhancement are to identify and map (a) subtle anomalies attenuated in the dynamic range due to the presence of high amplitude magnetic anomalies, (b) the continuity of individual bodies where there are lateral changes in susceptibility and/or depth of burial, and (c) the edges of structures by adequately accounting for the nature of the rock magnetization. Rock magnetization is a vector quantity that can consist of both remanent and geomagnetically induced components. The presence of the remanent component can adversely affect the shape of the magnetic field response and result in spurious derivatives, if one has assumed that only the induced component is present. Fortunately, basement rocks are usually dominated by the induced component, whereas mineralized zones often host long-lived remanent components. The following theory and 2D model examples show how these three problems can be successfully overcome, thus generating maps that can provide more reliable descriptions of source body parameters.

[1] Joint inversion of isochron and flow line data from the flanks of the extinct West Scotia Ridge spreading center yields five reconstruction rotations for times between the inception of spreading prior to chron C8 (26.5 Ma), and extinction around chron C3A (6.6-5.9 Ma). When they are placed in a regional plate circuit, the rotations predict plate motions consistent with known tectonic events at the margins of the Scotia Sea: Oligocene extension in Powell Basin; Miocene convergence in Tierra del Fuego and at the North Scotia Ridge; and Miocene transpression at the Shackleton Fracture Zone. The inversion results are consistent with a spreading history involving only two plates, at rates similar to those between the enclosing South America and Antarctica plates after chron C5C (16.7 Ma), but that were faster beforehand. The spreading rate drop accompanies inception of the East Scotia Ridge back-arc spreading center, which may therefore have assumed the role of the West Scotia Ridge in accommodating eastward motion of the trench at the eastern boundary of the Scotia Sea. This interpretation is most easily incorporated into a model in which the basins in the central parts of the Scotia Sea had already formed by chron C8, contrary to some widely accepted interpretations, and which has significant implications for paleoceanography and paleobiogeography.

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