Interpretation of anisotropy of magnetic susceptibility fabric of ignimbrites in terms of kinematic and sedimentological mechanisms: An Anatolian case-study

Abstract

A study of the Neogene low-aspect-ratio Kizilkaya ignimbrite (in the calc-alkaline Central Anatolian Volcanic Province, Turkey) has been carried out using the anisotropy of magnetic susceptibility (AMS) technique on 46 sites. IRM, hysteresis loops and Curie temperature analysis indicate that magnetite is the main carrier of the magnetic signal in the ignimbrite. The shape of the AMS ellipsoids varies widely from site to site and the anisotropy is commonly low: we review the data processing used in the literature on the AMS of ignimbrites to calculate principal directions or to evaluate the quality of the results, and we develop various simple techniques (data filtering and contouring procedures) in order to more accurately define the axis or symmetry of the AMS fabric. There is no simple relationship between the shape of the AMS ellipsoids and the shape of the petrofabric ellipsoids. Similarly, there is no direct relationship between magnetic and kinematic axes. Our results show that the source of the measured magnetic signal in these rocks is a complex result of various contributions including shape anisotropy of free and fixed magnetic crystals (shape AMS), crystallization of magnetic grains on all surfaces and discontinuities during cooling of the ignimbrite (distribution anisotropy), and alteration by hydrothermal and meteoric water. Particular attention is paid to the interpretation of the AMS stereoplots departing from the “standard” fabric of sedimentary rocks formed under unidirectional flow. In order to explain these “non-standard” AMS fabrics, we discuss their possible origin: (1) in analytical and mineralogical terms (instrumental artifact, pollution by xenoclasts, mineralogical inversion of the susceptibility axes, secondary mimetic fabric); and (2) in terms of sedimentological mechanisms (rheology, depositional processes). In the present state of knowledge, we conclude that, due to the physico-chemical complexity of the magnetic source, the AMS signal should be used and interpreted with care when attempting to decipher the sedimentological or rheological processes occurring in moving pyroclastic flows. On the other hand, AMS has it greatest potential when used in conjunction with other techniques, and when a large number of sampling sites and specimens is used.