Acquisition of anhysteretic remanence and tensor subtraction from AMS isolates true palaeocurrent grain alignments

Abstract

Abstract Anisotropy of magnetic susceptibility (AMS) is commonly used to detect subtle palaeocurrent directions. A Proterozoic subarkose shows extreme anomalous inverse AMS fabrics that cannot be attributed to the well-known effect of single-domain magnetite. This was confirmed by anisotropy of anhysteretic remanent magnetization (AARM). Further, by subtracting increasing proportions of the AARM tensor from the AMS tensor the paramagnetic/diamagnetic response of the silicate fabric was isolated. This reveals an inverse quartz fabric but analysis of the results shows that two grain alignments are present. A prominent inverse magnetic fabric is due to quartz long axes aligning with current flow revealed by the difference [AMS]-[AARM]. This is due to the avalanche flow of concentrated sand dispersions down the slip-face of cross-stratified units. Dispersive pressure caused by grain interactions and shear within the fluid produces a subhorizontal resultant force that aligns grains by frictional freezing during deposition. In some samples AARM isolates a secondary feeble alignment of magnetite nearly perpendicular to bedding. This is due to suspension rainout and sweeping by the reverse circulation separation eddy that supplies additional material to the top of the sand laminae. It particularly affects fine grains such as magnetite that infiltrate the framework’s pores to produce a secondary population of vertically oriented grains.