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

Research areas:
Year:
1998
Authors:
Journal:
EARTH AND PLANETARY SCIENCE LETTERS
Volume:
157
Number:
1-2
Pages:
105-127
Month:
APR 15
ISSN:
0012-821X
BibTex:
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 gains 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. (C) 1998 Elsevier Science B.V. All
rights reserved.