Magnetic properties of Neogene regional dikes from east Iceland with special reference to magma flow

Abstract: This thesis deals with rock magnetic measurements on Neogene dikes from the eastern fjords of Iceland. A vast amount of dikes generally striking north-north-east occur as swarms in the glacially eroded lava pile. They are considered as the underlying extensions of fissure swarms in active volcanic systems which like the dike swarms converge at central volcanoes. The dike swarms and associated central volcanoes are uncovered by ca. 1500 m of glacial erosion, leaving the upper parts of these igneous units bare. Two research papers have been prepared based on field studies and measurements of rock magnetism on carefully chosen occurrences of Icelandic dikes in order to test contradicting models of dike origin. The manuscripts are included in the thesis.The first is a detail study of a single composite dike, with a core of fine grained quartz-porphyry surrounded by dolerite margins. The core of the dike was sampled in three locations, separated by ~12 km and measured for magnetic remanence and anisotropy of magnetic susceptibility. Magma flow direction in this dike was interpreted using the mirror imbrication of the minor susceptibility axes from each margin of the quartz-porphyry core of the dike. The flow regime in all three locations showed a marked flow component from north to south, supported by field evidence in the form of parabolically aligned enclaves, and shear folds. Statistical procedures of bootstrapping was utilized to define the flow and handle imbrication in both the vertical and horizontal plane thus defining direction and inclination of flow. The main contribution of this paper was to prove that anisotropy of magnetic susceptibility used to infer flow direction may give consistent and reliable results, and to present new ways (to use old techniques) to define magma flow directions.The same technique used to infer flow in paper one has sub-sequentially been applied to a far larger set of mafic dikes, extending north-east from the exhumed Álftafjörður central volcano. The second paper documents that the flow regimes from the mafic dikes showed a predominantly horizontal flow from the central volcano, supporting tectonic models that suggest shallow magma chambers to be the source of the dikes. The thesis further discusses the results from these studies in comparison to other studies on Icelandic dikes where the magma flow is determined by the direction of the major susceptibility axis.