Acquisition and blocking of magnetic fabrics in synsedimentary structures, Eocene Pyrenees, Spain

Abstract

Magnetic fabric analysis can be used as a tool to investigate tectonic processes and deformational events but the manner in which the studied fabrics were acquired and blocked must also be carefully considered. In studies using anisotropy of magnetic susceptibility (AMS), the timing of AMS moment acquisition and its blocking must be identified. The former term refers to the acquisition of a preferred orientation of the AMS ellipsoid while the latter is the last process that can be inferred to modify it. In this paper, the very early diagenetic acquisition of magnetic fabrics is analysed in synsedimentary slump structures within Eocene marls from the Southern Pyrenees. The paramagnetic contribution to susceptibility ranges from 80 to 98 per cent, although iron-oxides and magnetite are present in all of the studied samples. AMS is analysed at two scales: at site scale with synsedimentary deformational structures and at regional scale. Slumps systematically exhibit a magnetic lineation (Kmax) parallel to the slump fold axes and a girdle of Kmin axes normal to the slump lamination at sample scale. The obtained results show that acquisition of magnetic fabrics occurred during pre-lithification stages (pre-slump folding) and that blocking immediately post-dates folding (no modifications of slump AMS fabrics after slump folding occurred). Comparison between magnetic fabrics in slump folds and in the undisturbed series indicates a contemporary synsedimentary acquisition occurring after sedimentation and prior to lithification. © 2011 The Authors Geophysical Journal International © 2011 RAS.