Abstract

Detailed studies of 11 segmented normal faults from five grabens in the Needles District of Canyonlands National Park, Utah, demonstrate key processes in the growth, linkage, and evolution of grabens. Field observations and stereophotogrammetry reveal a ubiquitous asymmetry in cross-sectional geometry, based on distinct map patterns of graben-bounding faults, rollover anticlines with attendant joint dilation, footwall uplift with joint closure, and spoon-shaped graben floors. Master and antithetic faults across Devils Lane and several other grabens are defined quantitatively by the displacement distributions along the faults. Two-dimensional shape parameters that characterize the displacement profiles indicate that inelastic processes such as changes in fault frictional strength influence strain accumulation along the faults. The degree of graben asymmetry increases systematically with distance from the Colorado River; greater symmetry is associated with locally greater age and/or strain of grabens nearer the river. Scatter in plots of maximum displacement vs. fault length was attributed previously to linkage of fault segments alone but is here shown to correlate additionally with distance from the Colorado River and, therefore, to spatial strain gradients within the graben array. Extensional strain across the fault array, accumulating at rates of perhaps 1.5 to 2 cm/yr or 10−14 to 10−13 s−1, is accommodated at depth by salt flow and formation of reactive salt diapirs beneath the grabens and was probably initiated when the Colorado River had cut sufficiently deep into the section for active salt diapirism to commence.

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