Kinematic indicators in fault gouge: tectonic analog for soft-bedded ice sheets

Polished slip planes resulting from Basin and Range detachment faulting of mid-Tertiary volcaniclastics are well exposed at Lake Pleasant, Arizona, U.S.A. Slip planes show erosional features formed by abrasion below moving sheets of gouge (coarse-grained diamict) during repeated faulting and shearing. Tectonically generated forms are identical to those found in glaciated terrains. Slip planes show ‘rat-tail’ ridges and grooves (‘ridge-in-groove structures’), ‘nail head’ striations and ‘wear tracks’. A prominent morphological element consists of flute ridges, up to 30 cm wide and at least 7 m long, formed of gouge preserved in the lee of boulders projecting as obstacles above slip surfaces. Crescentic scours around the stoss side of such boulders record enhanced abrasion by ‘streams’ of gouge debris. Clasts are striated and show shapes comparable to ‘flat iron’ clasts produced by glacial abrasion. Kinematic indicators at Lake Pleasant are directly comparable in form to flutes and drumlins in glaciated terrains. The data presented here not support a recent hypothesis arguing that glacial striations, flutes and drumlins are the product of turbulent subglacial meltwater. Instead, diamict produced along fault planes (gouge) by low-grade shearing is directly comparable to diamict facies (deformation till) formed by pervasive shear at the base of ‘soft-bedded’ ice sheets. Both facies are tectonically generated cataclastites produced in low-grade shear zones and display genetically related, but differently scaled, streamlined bedforms.