THE INFLUENCE OF FAULT GROWTH ON DRAINAGE DEVELOPMENT IN THE CANYONLANDS GRABENS, UTAH

In regions influenced by normal faulting, the landscape is determined by how fluvial systems interact with growing fault arrays. We examine how the growth of a complex fault array in the Canyonlands Grabens, Utah has dramatically altered late Quaternary-Holocene fluvial systems in the area. We analysed an ephemeral stream that formerly flowed SE-NW from the Abajo Mountains to the Colorado River, that has been disrupted from its original course by the growth and linkage of a simple two segment normal fault system. The displacement profile of the fault is characterised by linear tip gradients, with zero displacement at the tips increasing uniformly to a maximum of 32m near the centre of the fault. The presence of 11m of post breaching displacement suggests that the fault accrued a significant amount of displacement for its new length, in order to attain an almost ideal profile. The drainage incises into the footwall of the fault near its southern tip and enters the graben via a waterfall that is 7m high. The waterfall separates two graded sections of the stream profile suggesting that displacement accumulation on the fault was rapid, so that the rate of incision of the stream was unable to keep up with the rapid increase in displacement. The height of the waterfall corresponds to the post-breaching displacement of the fault. Upon breaching, the fault is "under-displaced" with respect to the increase in length and hence there is a rapid increase in displacement to regain the initial displacement-length ratio. Thus our study suggests that hard linkage can lead to an abrupt increase in rates of surface uplift that is sufficient to lead to the stream being defeated and diverted around the growing structure. The conceptual model developed in the relatively simple two segment fault system will be used to reconstruct the linkage evolution of a complex four segment fault that has a series of windgaps and waterfalls along its length.