FOLD-FAULT RELATIONSHIPS IN LOW-ANGLE DETACHMENT SYSTEMS

A close kinematic and timing relationship between low-angle extensional faulting and upright to moderately inclined folding with fold axes parallel 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 the oldest mylonitic foliation of the footwall. These folds are transected by younger mylonitic foliation(s) and cataclastic detachment faults which are themselves folded about similar axes. In general, the amplitude decreases and the wavelength increases for progressively younger fold structures, consistent with a history of progressive folding during faulting. The interplay between folding and faulting in the footwall reflects a component of shortening perpendicular to fold axial surfaces during extension parallel to fold axes. This strain pattern can develop in regions with a transcurrent component or during lateral extrusion and does not necessarily require overall crustal thinning.