Introduction

For more than a quarter of a century, the spectacular grabens of Canyonlands National Park, Utah, have provided planetologists with a fundamental analog for understanding what planetary grabens should look like and – more importantly – what may be implied about the depth variation of mechanical properties and horizontal extensional strain.

The seminal work on Canyonlands grabens was done by George McGill and coworkers in support of their investigations of the origin and kinematic significance of lunar and Martian straight rilles (McGill, 1971; McGill and Stromquist, 1975, 1979; Stromquist, 1976; Wise, 1976). McGill and Stromquist (1979) hoped to invert graben widths, assessed on an aerial or orbital image, for the depth of faulting (i.e., fault intersection depth). By equating this depth with stratigraphic layer thickness and assuming a symmetric graben geometry and plausible values of fault dip angles, grabens provided ready and seemingly reliable probes of the near-surface planetary stratigraphy and strain. Interestingly, the analog modeling of brittle-layer extension over a ductile (quasiplastic) substrate, appropriate to Canyonlands stratigraphy (McGill and Stromquist, 1975, 1979), anticipated the key role of faulting in triggering and mobilizing salt or shale diapirism at depth (Jackson and Vendeville, 1994; Jackson, 1995). Other observations and inferences made in the 1970s, including flexure of rock layers at ramps near graben terminations and incremental growth of fault slip (McGill and Stromquist, 1979), anticipated these fundamentally important ideas by at least a decade (Sibson, 1989; Peacock and Sanderson, 1991; Cowie and Scholz, 1992).