Deciphering viscous flow of frictional melts with the mini-AMS method

Abstract

The anisotropy of magnetic susceptibility (AMS) is widely used to analyze magmatic flow in intrusive igneous bodies including plutons, sills and dikes. This method, owing its success to the rapid nature of measurements, provides a proxy for the orientation of markers with shape anisotropy that flow and align in a viscous medium. AMS specimens typically are 25 mm diameter right cylinders or 20 mm on-aside cubes, representing a volume deemed statistically representative. Here, we present new AMS results, based on significantly smaller cubic specimens, which are 3.5 mm on a side, hence~ 250 times volumetrically smaller than conventional specimens. We show that, in the case of frictional melts, which inherently have an extremely small grain size, this small volume is in most cases sufficient to characterize the pseudotachylyte fabric, particularly when magnetite is present. Further, we demonstrate that the mini-AMS method provides new opportunities to investigate the details of frictional melt flow in these coseismic miniature melt bodies. This new method offers significant potential to investigate frictional melt flow in pseudotachylyte veins including contributions to the lubrication of faults at shallow to moderate depths.

Earthquakes in the Mantle? Insights From Rock Magnetism of Pseudotachylytes

“…A postseismic origin for magnetite can be ruled out for three main reasons: (i) magnetite grains are elongated parallel to the viscous flow fabric of the pseudotachylyte and carry an AMS fabric that is kinematically consistent with coseismic slip, at least in the case of Cima di Gratera (Ferré et al, ); (ii) in contrast to other pseudotachylytes (Ferré, Friedman, et al, and Ferré, Geissman, et al, ), the ultramafic pseudotachylyte veins that we sampled, away from fractures, do not display any color variation from margin to center, a characteristic demonstrating the lack of alteration/serpentinization in our samples; and (iii) the magnetite grains observed in thin section in our samples are not spatially associated with any hydrous phases, which precludes a serpentinization origin (Figure ).…”

Faulting Processes Unveiled by Magnetic Properties of Fault Rocks

“…AMS analysis of pseudotachylyte fabric on such 3.5 mm cubes enabled determining the focal mechanisms of ancient earthquakes (Ferré et al, 2015(Ferré et al, , 2016. Despite sample size issues for very small samples where a cautious and careful interpretation is appropriate, AMS studies on small samples are nonetheless a promising new avenue to unveil detailed geological features (Almqvist et al, 2020;Ferré et al, 2015Ferré et al, , 2016Zhu et al, 2017). Berndt et al (2016) demonstrated that samples with a thermoremanent magnetic moment larger than 10 −11 Am 2 irrespective of their sizes (ranging from the centimeter scale to tens of nm scale) contain enough magnetic particles to be accurate magnetic recorders.…”

Orogen‐parallel deformation of the Himalayan midcrust: Insights from structural and magnetic fabric analyses of the Greater Himalayan Sequence, Annapurna‐Dhaulagiri Himalaya, central Nepal

“…AMS fabrics controlled by SPO and/or CPO fabrics may correlate with 3‐D strain, such that principal susceptibility axes ( K 1 ≥ K 2 ≥ K 3 ) and finite strain axes ( X ≥ Y ≥ Z ) are parallel [ Borradaile , ; Borradaile and Jackson , ; Kruckenberg et al ., , ; Ferré et al ., , ]. Correlations may also exist between P ′ and finite strain magnitude [e.g., Benn , ; Tripathy , ] and between T and strain geometry [e.g., Sidman et al ., ].…”