FOLD-FAULT RELATIONSHIPS IN LOW-ANGLE DETACHMENT SYSTEMS

A close kinematic and timing relationship between low-angle extensiona l faulting and upright to moderately inclined folding with fold axes p arallel to the extension direction is established for two well-exposed Neogene detachment systems, in the Simplon region of the Alps and in the Death Valley region of California. Fold amplitude is largest in th e oldest mylonitic foliation of the footwall. These folds are transect ed by younger mylonitic foliation(s) and cataclastic detachment faults which are themselves folded about similar axes. In general, the ampli tude decreases and the wavelength increases for progressively younger fold structures, consistent with a history of progressive folding duri ng faulting. The interplay between folding and faulting in the footwal l reflects a component of shortening perpendicular to fold axial surfa ces during extension parallel to fold axes. This strain pattern can de velop in regions with a transcurrent component or during lateral extru sion and does not necessarily require overall crustal thinning.