ABSTRACT: Kinematic indicators in fault gorge: tectonic analog for soft-bedded ice sheets.

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Abstract

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. Crescentric 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 casts produced by glacial abrasion. Kinematic indicators at Lake Pleasant are directly comparable to flutes and drumlins in glaciated terrains. We highlight the glacial sedimentological implications that stem from this recognition with regard to interpreting evidence of ancient cold climates in Earth history and for models of subglacial erosion and deposition. Sichelwannen (crescentric grooves) and ridge-in-groove structures in glaciated terrain are attributed to subglacial abrasion either below debris-rich basal ice or 'streams' of wet, overpressured debris moving below the ice base and forced to flow around obstacles (Gjessing 1956). This interpretation is supported by direct quantitative observations of abrasional processes at the base of modern glaciers (Vivian 1970; Boulton 1974). Recent work, however, has argued that these erosional forms are not the result of glacial abrasion but the product of so-called 'horse-shoe vortices' within turbulent subglacial meltwater flows (Shaw 1994). Our data do not support a meltwater flood hypothesis as a general mechanism for the origin of streamlined features below ice sheets. Kinematic indicators such as those on fault slip surfaces at Lake Pleasant and which are widely reported in the literature, are directly comparable to glacial forms and are clearly unrelated to floods. The movement of diamict (gouge) by low-grade shearing along fault slip planes in tectonically active areas is directly comparable to the generation and transport of diamict below 'soft-bedded' glaciers (deformation till); both facies are tectonically generated cataclastities that display morphologically identical, genetically related, but differently scaled, streamlined bedforms.

Bibliography of Canadian Geomorphology