Flow behaviour in the intra-caldera setting: an AMS study of the large (>1290 km3) Permian Ora ignimbrite
Madelaine A. W. Willcock, Massimo Mattei, Pavlína Hasalová, Guido Giordano, Ray A. F. Cas, Corrado Morelli, 2015. "Flow behaviour in the intra-caldera setting: an AMS study of the large (>1290 km3) Permian Ora ignimbrite", The Use of Palaeomagnetism and Rock Magnetism to Understand Volcanic Processes, M. H. Ort, M. Porreca, J. W. Geissman
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Anisotropy of magnetic susceptibility (AMS) data reveal heterogeneous pyroclastic flow processes and variable flow directions within the intra-caldera setting of the Permian rhyolitic welded Ora ignimbrite. Magnetic fabric is primary, orientated during the pyroclastic flow emplacement, and is controlled by paramagnetic and ferromagnetic mineral phases. The ignimbrite has typically weak mean magnetic susceptibilities (1.32–21.8×10−4 SI) but with a large spread and low anisotropy degrees (1.003–1.023), which vary in different parts of the caldera. The intra-caldera magnetic fabric provides significant information on the dynamics of the intra-caldera setting, relating to changing vertical and lateral flow emplacement processes. AMS shape ellipsoids range from oblate to prolate; these are interpreted to reflect the heterogeneous nature of the flow resulting from the influence of underlying topography, constraints of the caldera walls, primary welding and post-emplacement mineral growth. We have identified different depositional units and possible eruptive source regions, indicating that more than one source fissure vent was active during eruption within this caldera system. The lateral variations demonstrate a meandering of flow pulses. The caldera margin acts as an obstacle in preventing and rebuffing certain flows from scaling the caldera margin.
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This volume provides a synopsis of current research on volcanic processes, as gained through the use of palaeomagnetic and rock magnetic techniques. Thermoremanent magnetization information provides a powerful means of deciphering thermal processes in volcanic deposits, including estimating the emplacement temperature of pyroclastic deposits, which allows us to understand better the rates of cooling during eruption and transport. Anisotropy of magnetic susceptibility and anisotropy of remanence are used primarily to investigate rock fabrics and to quantify flow dynamics in dykes, lava flows, and pyroclastic deposits, as well as identify vent locations. Rock-magnetic characteristics allow correlation of volcanic deposits, but also provide means to date volcanic deposits and to understand better their cooling history. Because lava flows are typically good recorders of past magnetic fields, data from them allow understanding of changes in geomagnetic field directions and intensity, providing clues on the origin of Earth’s magnetic field.