Relay-ramp forms and normal-fault linkages, Canyonlands National Park, Utah

Abstract

Fieldwork within a series of mesoscale grabens in southeast Utah has revealed a particularly well-exposed system of interlinked extensional faults. A series of down-faulted grabens are developed within a 460-m-thick brittle layer of upper Paleozoic sandstone and shale, which overlies a ductile layer with a high gypsum content. All the major grabens consist of two or more overlapping elements, which are composed of fault segments. These segments may be hard-linked (fault surfaces are joined) or soft-linked (fault surfaces are isolated, but linked by ductile strain of the rock volume between them) in map view. Relay structures are defined as zones connecting the footwalls and hanging walls of overlapping fault segments representing soft linkage of fault segments. In the Canyonlands grabens, the transfer of displacement between soft-linked fault segments is characterized by well-de-fined, dipping relay ramps, commonly rotated and extended to accommodate the ductile strain between the overlapping fault segments. Relay ramps develop as ephemeral structures, eventually becoming breached by hard linkage of the fault segments. Breakdown of ramps by breaching is part of the process of fault growth by segment linkage. Within the Canyonlands graben system, four orders of segmentation are present. This may be a consequence of different scales of mechanical heterogeneity, but evidence from the Canyonlands grabens and elsewhere points to a fundamental process of discontinuous fault growth by segment linkage.