Method for estimating ductile horizontal strain from magnetic fabrics in poorly consolidated clay-rich sediments

Abstract

We propose a simple method based on the normalized eigenvalues of the tensor of anisotropy of magnetic susceptibility (AMS) to estimate horizontal ductile strain in detrital clay-rich sedimentary rocks subjected to layer parallel shortening (LPS). This method is mainly based on two assumptions: (1) The magnetic carriers are contacting clay-mineral platelets, and (2) The platelets respond to LPS by crenulation and formation of a zone axis of unchanged magnetic susceptibility perpendicular to the plane of deformation. We further simplify the problem by considering, as the exact distribution of particles orientations is unknown, that the AMS tensor arises from a single equivalent clay platelet, of which the angle with the bedding plane directly reflects the amount of horizontal strain. The proposed method is applied to data from samples retrieved during Integrated Ocean Drilling Program (IODP) Expeditions 315 and 316 of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) complex drilling project. Sample-wise finite strain values assist AMS data interpretation, which is traditionally based on the AMS ellipsoid shape parameter, and allow comparison with strain estimates obtained by other means. In the Nankai Accretionary Prism, AMS-derived strain values range from 0 to 25% and suggest substantial mechanical compartmentalization with inter-unit strain contrasts larger than 5 percentage points, including the penetrated shallow portion of a megasplay fault. At a local scale, AMS strain also appears to be affected by thrusting, what can be interpreted as a record of a ductile strain component away for the brittle zone or as an ‘interseismic’ ductile strain within otherwise brecciated material.