Alexander L. Densmore1, Philip A. Allen1, Sanjeev Gupta2
(1) Department of Geology, Trinity College, Dublin, Ireland
(2) Imperial College, London, United Kingdom
ABSTRACT: Sediment flux from normal fault blocks and relay zones
Relay zones between en echelon segments of normal fault systems are considered to be important sediment conduits to half graben within evolving continental rift settings. We present the results of a series of numerical experiments of landscape development near relay zones to examine how the temporal evolution of these structures governs sediment dispersal patterns.
Experiments with solitary normal fault segments suggest that sediment flux to the hangingwall is greatest where the fault displacement rate is highest. The steep topographic gradient set up by high displacement rates drives rapid footwall denudation by bedrock channel incision and bedrock landsliding. Catchment stream power and potential sediment flux are dictated by competition for drainage area.
Within an isolated, en echelon relay zone, our experiments suggest that sediment flux to the hangingwall is relatively low, due to small tectonically-induced slopes. Little sediment accumulation takes place within the zone because of low subsidence rates relative to the hangingwall basins on either side. Breaching of the zone to form a hard-linked relay can increase the local sediment flux by providing a local sediment source and by imposing much higher topographic slopes.
Because large catchment systems generating high sediment fluxes do not develop in our simulated relay zones, it is suggested that observations of high sediment fluxes at relay zones requires alternative explanations. The presence of long-lived sediment conduits at the relays may instead be related to pre-rift drainage systems preferentially utilizing these topographic lows. The assumption that relay zones coincide with volumetrically important coarse clastic basinal deposits may not always be justified.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado