KINEMATIC INDICATORS IN FAULT GOUGE - TECTONIC ANALOG FOR SOFT-BEDDEDICE SHEETS

Polished slip planes resulting from Basin and Range detachment faultin g of mid-Tertiary volcaniclastics are well exposed at Lake Pleasant, A rizona, 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 t hose found in glaciated terrains. Slip planes show 'rat-tail' ridges a nd grooves ('ridge-in-groove structures'), 'nail head' striations and 'wear tracks'. A prominent morphological element consists of flute rid ges, 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. C rescentic scours around the stoss side of such boulders record enhance d 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 do not support a recent hypothesis arguing that glacial striations, fl utes and drumlins are the product of turbulent subglacial meltwater. I nstead, diamict produced alone fault planes (gouge) by low-grade shear ing is directly comparable to diamict facies (deformation till) formed by pervasive shear at the base of 'soft-bedded' ice sheets. Both faci es are tectonically generated cataclastites produced in low-grade shea r zones and display genetically related, but differently scaled, strea mlined bedforms. (C) 1998 Elsevier Science B.V. All rights reserved.