Rapid locking of tectonic magnetic fabrics in weakly deformed mudrocks

Abstract

The anisotropy of magnetic susceptibility (AMS) has provided important insights into early deformation conditions in compressive settings through characterization of tectonic fabrics in mudrocks that appear otherwise undeformed. The validity of these insights relies on the assumption that tectonic fabrics are rapidly locked shortly after sediment deposition. However, the time lag between deposition and the tectonic overprint has yet to be quantified to verify this assumption. Here we present AMS data from Late Holocene sediments recovered in two cores from Lake Issyk-Kul in the Kyrgyz Tien Shan fold-and-thrust belt. These sediments, in which magnetic fabrics reflect the preferred orientation of phyllosillicates, have typical tectonic magnetic fabrics with varying degrees of tectonic overprint likely controlled by small-scale active faults. The mean orientation of the susceptibility maxima parallels neighbouring active thrust faults and is perpendicular to the geodetically-derived local shortening direction. The age model for one of the studied cores, which is based on 7 calibrated accelerator mass spectrometer radiocarbon dates, indicates that sediments as young as 25. yr record this clear tectonic fabric, which is locked within sediments older than 1180. cal. yr BP. Our demonstration of rapid locking of tectonic fabrics in weakly deformed mudrocks shortly after deposition provides the required validation of their reliability for tectonic studies, and opens new opportunities for providing quantitative insights into the relationship between magnetic ellipsoids, shortening rates, and stress directions in compressive settings.