Kinematics of frictional melts at the base of the world's largest terrestrial landslide: Markagunt gravity slide, southwest Utah, United States

Abstract

The pseudotachylyte of the Markagunt slide in Utah is the best example of gravity slide frictional melt. A key exposure near Panguitch provides opportunities to investigate the kinematics of frictional melts. Here we analyze the anisotropy of magnetic susceptibility (AMS) in eight oriented samples and show that magnetite dominates the magnetic susceptibility (97.7% in generation veins and 87.6% in injection veins) and most likely also the AMS. These rocks have magnetic susceptibility one order of magnitude greater (∼46,106 • 10−6 [SI]) than their host rock (∼1850 • 10−6 [SI]), which shows that magnetite must have formed during the gravity slide. The AMS of these rocks records the laminar flow kinematics of the frictional melt, with a North-South direction and a top-to-South sense of displacement consistent with regional observations. The dominantly planar symmetry of the AMS points to a flow regime dominated by pure shear, towards the end of the gravity slide movement. Our results, obtained on a landslide pseudotachylyte where the emplacement kinematics was already known, fundamentally validate the use and interpretation of AMS in other pseudotachylytes including those formed along seismogenic faults.