Magma transfer in the continental crust is a first order problem that needs to be addressed to understand how magmatic systems operate through time and in space. This is particularly important in the case of Large Igneous Provinces in which large volumes of magma have moved from the source to the surface of the Earth in, geologically speaking, relatively short times (approximately 1 million years). Magnetic fabric techniques, such as the Anisotropy of Magnetic Susceptibility method, have been successful in constraining flow geometry and kinematics. However, numerous aspects of this approach are still ambiguous. For example, no satisfactory explanation accounts for cases where the magnetic foliation is parallel to a dike wall and yet the magnetic lineation, which is supposed to mimic the flow direction, is perpendicular to the dike boundary. This research project addresses these issues by combining several independent methods (image analysis, Electron BackScatter Diffraction, Anisotropy of Anhysteretic Remanent Magnetism) to determine the mineral fabric and magma flow parameters. Six controlled experiments specifically investigate differences in magnetic fabrics between feeder and non-feeder dikes, the possible contribution of single domain magnetite grains to Anisotropy of Magnetic Susceptibility and the importance of layering-forming processes in development of mineral fabric. The natural laboratory targeted for the experiments is the Karoo Large Igneous Province in South Africa, a region with well-mapped, exceptional exposures in addition to available continuous borehole cores. The project will be carried out by a U.S. research team from Southern Illinois University, College of Saint Rose, and University of New Mexico in partnership with South African scientists from Rhodes University and the Council for Geoscience.
This study will provide the theoretical background and validation for the technique Anisotropy of Magnetic Susceptibility that is currently the most often used to determine magma flow direction. It will define the boundaries within which the Anisotropy of Magnetic Susceptibility technique could and should be used. The results of this investigation will enable the broader community to apply this technique to many other geological settings with greater confidence regarding the significance of its results. Although the project focuses on one type of magmatic system (silica-poor magmas), it is very likely that the results will have broader applications to silica-rich systems. This research will considerably impact the igneous petrology and volcanic community because magma transfer is still one of the key questions in these disciplines. The project involves extensive graduate and undergraduate participation in international research and makes efforts to broaden participation of underrepresented groups in the Earth Sciences. The NSF Office of International Science and Engineering (Near East and South Asia Program) and the NSF Earth Sciences Division (Tectonics Program and Geophysics Program) are supporting this research.
Over earth history, there have been several short-lived periods where very large volumes of magmas of mafic composition, originating in the Earth’s mantle, are transferred into and through the crust of the planet, to eventually form thick, laterally extensive sequences of basaltic lava flows (on the earth’s surface) and intrusive igneous rocks (of gabbroic composition) that often have planar geometries. One well-established example of a short-lived period of extensive mafic magmatism is in the Early Jurassic, at about 183 million years ago, when tens of thousands of cubic kilometers of mafic magma was emplaced into crust of what will eventually become part of the South Atlantic bordering continents. In South Africa today, the major magmatic event is represented by the Karoo intrusive series, consisting of a complex network of sills (planar intrusive igneous rock bodies with contacts parallel to surrounding host geologic materials, such as sedimentary rocks), feeder dikes (planar bodies with contacts that are skew to the surrounding host rocks), and more irregular intrusions. Many of the individual sills are tens of meters in thickness, and can be traced over hundreds of kilometers. In addition, the extruded equivalents of these rocks are exposed as a thick sequence of lavas. The Karoo intrusive series is exposed over some 2000 km extent in South Africa and adjacent countries. Our research has concentrated on addressing the question of how very large volumes of magma can be emplaced over very large distances, so quickly. We used the anisotropy of magnetic properties (e.g., magnetic susceptibility and different forms of magnetic remanence) in the Karoo sills as a proxy for magma flow directions. Such magnetic properties are essentially controlled by the distribution of small grains of the iron oxide magnetite in these rocks. We obtained a collection of oriented samples of gabbros from selected, lowermost sills exposed in a sub-circular pattern over 1500 km across. For all sites investigated, these rocks have a very well-defined magnetic susceptibility "fabric", with a sub-horizontal planar "foliation" defined by the maximum and intermediate directions of susceptibility, and a reasonably well-defined magnetic susceptibility lineation, defined by the directions of maximum susceptibility in the rocks. We interpret these well-developed fabrics to have been acquired as a result of magma flow prior to complete solidification and thus that they reveal actual magma transport directions. Most of the sites throughout this sub-circular sampling area yield flow directions that are oriented roughly northwest-southeast, rather than in a radial pattern. We interpret these data to suggest the possibility that the immense volume of magma was fed by a very long crustal scale dike system at depth, with an orientation roughly perpendicular to the flow of the magma. Our studies of the anisotropy of magnetic remanence, which are more tedious in their nature, have yielded more complex results in that they show the pronounced effects of ultra-fine particles of magnetite. The ancient magnetization (paleomagnetism) of the sills has also been investigated. All of the sites yield magnetizations that are statistically indistinguishable in direction and of uniform normal polarity, consistent with the interpretation of others that individual parts of the Karoo igneous system were emplaced rapidly. This work, in conjunction with colleagues at Southern Illinois University and Rhodes University in South Africa, formed the basis for several student research projects, and led to the development of a new technique in rock magnetic investigations, which the PI is currently writing up for publication.
