Evolution of magnetic fabrics during incipient deformation of mudrocks (Pyrenees, northern Spain)

Abstract

The anisotropy of magnetic susceptibility (AMS) of Eocene mudrocks from the Southern Pyrenean Foreland Basin documents the progressive development of tectonic fabrics in sediments that mesoscopically show no evidence for deformation. Two end members are observed: (1) a strong oblate fabric, with strong clustering of the minimum axes of susceptibility that is characteristic of depositional and compaction processes; and (2) a prolate magnetic ellipsoid with moderate to strong minimum axes girdle that reflects weak cleavage in the mudrocks. Low-temperature experiments and anhysteretic remanent magnetization reveal that the paramagnetic fraction dominates the AMS signal and thus, represent the preferred orientation of phyllosilicates. Characteristic stages of magnetic fabrics in mudrocks can be established and thus the AMS can be used as an indicator of cleavage development and intensity in mudrocks. The three stages of fabric development (early deformation, pencil structure and weak cleavage) reflect a process of increasingly preferred crystallographic orientation of the phyllosilicates that is evidenced by the change of distribution of the minimum susceptibility axes. This is particularly evident in the eigenvalues, which we plot on a Woodcock diagram that describes both shape and fabric strength, and allows different stages of deformation to be distinguished. Because appreciable depth during deformation (elevated temperatures and high confining pressures) can be ruled out, we propose that the sediments were wet and only partly lithified when the phyllosilicates became progressively reoriented. Thus, the observed fabric is explained as a result of the close interaction of deformation and diagenetic processes. Tectonically induced deformational fabrics formed while diagenesis progressed. Although the study area is a foreland basin, it offers a valuable analogue to active accretionary wedges. The application of AMS in these settings, where sediments are progressively dewatered through the development of fabrics, permits a similar quantification of fabric development and, eventually, of deformation.