Abstract - Faults are rarely single surface features and usually consist of kinematically related fault segments, on one or more scales. Segmentation of faults is always accommodated by the formation of relay zones across which displacement is transferred between overlapping fault segments. Although previous studies have shown that increasing displacement often causes relay breaching and segment linkage, kinematic, as opposed to geometric, constraints on the stability and evolution of relay zones are sparse. In this talk we consider whether relay zones are geometrically stable or whether they change their map view shape as they accrue displacement. Specifically we investigate whether relay zones establish their shape early in their development and maintain the same overlapping segment geometry until intervening relay ramps become too steep and breaching occurs or whether the tips of overlapping fault segments continue to propagate changing the shape of the relay zone until breaching? The evolution of relays is addressed by examining the Pliocene-recent evolution of relays along a normal fault in the northern Taranaki Basin from high quality 3D seismic data. This area is characterised by sedimentation rates which exceed fault displacement rates, a condition which permits displacement backstripping of this syn-sedimentary growth fault and detailed reconstruction of the growth of relay zones.
The Taranaki Basin is situated offshore south-west of the Northern Island of New Zealand. The Basin has a multiphase deformation history, with extension during the Late Cretaceous to Paleocene, followed by contraction in the Miocene and then, in the northern basin, by Pliocene to Recent backarc extension driven by subduction of the Pacific plate along the Hikurangi margin. We have analysed a fault along the western margin of the northern part of the Taranaki Basin, which has a maximum, Pliocene to Pleistocene, throw of about 2km. Underlying Late Cretaceous horizons have larger displacements (ca 2.5km) reflecting the fault’s existence during the first extensional deformation phase. Reactivation of this approximately NNE-SSW striking fault was accompanied by upward propagation and the formation of new fault segments which are rotated up to 20° in a clockwise direction. This segmentation is attributed to a rotation in extension direction from the earlier to the later phase of extension, with the formation of individual segments and associated relay ramps within a ca 3km wide zone. Examination of displacement profiles highlights the transfer of displacement between segments, whilst sequence thickening and associated displacement changes indicate that the boundaries of relay zones were stable with no tip migration during ramp rotation. Upward decreases of displacement through the growth sequence, reflecting the displacement accumulation through time, are accompanied by a progressive increase in the structural integrity of relay ramps and a decrease in segment linkage. Kinematic analysis favours a model in which relays are formed rapidly and retain a geometrically stable shape in map view during progressive ramp rotation, which ultimately leads to eventual relay breaching.
Abstract of talk given to:
Tectonic Studies Group Annual Meeting, La Roche-en-Ardenne, Belgium, January 2008.