254 Earth and Planetary Science Letters, 41 (1978) 254-264 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [31 FLOW AND EMPLACEMENT DIRECTION DETERMINED FOR SELECTED BASALTIC BODIES USING MAGNETIC SUSCEPTIBILITY ANISOTROPY MEASUREMENTS BROOKS B. ELLWOOD Department of Geology, University of Georgia, Athens, GA 30602 (U.S.A.) Received July 31, 1978 Low-field anisotropy of magnetic susceptibility (AMS) has been determined for a total of 248 basaltic specimens taken from cross sections between the cooling interfaces of 6 subaerial lavas, 6 deep-sea lavas, and 6 intrusives (5 dikes and 1 sill). Statistically significant AMS clusters are exhibited by all the dikes examined and, based upon these clus- ters, derivation of emplacement direction becomes possible. Two lavas are observed to have statistically significant AMS clusters which can be used for flow direction determinations. The methods of emplacement and flow direction analysis are discussed as well as the statistics used. It is concluded that most of the dikes examined have low angle emplacement directions. A classification scheme for AMS data distributions is presented. The AMS analysis shows that intrusives and deep-sea lavas can be distinguished from subaerial lavas approximately 80% of the time by the random AMS ellipsoid orientations exhibited in subaerial lavas. Contrasts in the fluid prop- erties, degassing, wall effects with subsequent distortion of the fluid, and grain interaction during the extrusion of subaerial lavas can be expected to distort magnetic grain alignment. Further effects such as convection and secondary processes contribute to yield the random distributions observed for most of these bodies. 1. Introduction Major portions of the Earth's crust are composed of thick sequences of basalt. The source of individual units in such sequences may be difficult to trace due to similarities with other units and to local faulting and regional folding. Determination of lava flow direc- tion, mode of occurrence, and dike emplacement char- acteristics are just a few of the problems which are potentially solvable using magnetic measurements. One such technique involves measurement of the anisotropy ,~f magnetic susceptibility. Magnetic susceptibility for rock specimens is defined by: Ji = K,Hj (1) where ~ is a magnetization induced by a field, if/, and K~-J (in volume sI units) is a tensor of the second rank (magnitude ellipse of Nye [1 ]). The anisotropy of mag- netic susceptibility (AMS) is conventionally expressed as a triaxial ellipsoid whose major (Ka), intermediate (Kb) , and minor (Kc), axes represent the correspond- ing susceptibility directions and magnitudes. In unal- tered basaltic rock specimens, the ellipsoid shape represents the preferred alignment of ferrimagnetic crystals resulting from either crystal growth or realign- ment in a stress field, or flow during emplacement of a magma which has already partially crystallized. This paper examines the orientations of AMS ellip. soids for samples taken between the cooling faces of individual igneous units, to compare intrusive and extrusive AMS distributions, and to see if these distri- butions can provide estimates of flow or emplace- ment directions. 2. Previous work 2.1. Anisotropy of magnetic susceptibility Graham [2] has suggested that AMS measurements might be useful as petrofabric indicators in rocks. Khan [3] has examined basaltic materials using AMS and microscopic analyses and has shown, by direct examina-