Terminations of large strike-slip faults: an alternative model from New Zealand

Abstract

Abstract The 500-km-long strike-slip North Island Fault System (NIFS) intersects and terminates against the Taupo Rift. Both fault systems are active, with strike-slip displacement transferred into the rift without displacing normal faults along the rift margin. Data from displaced landforms, fault-trenching, gravity and seismic-reflection profiles, and aerial photograph analysis suggest that within 150 km of the northern termination of the NIFS, the main faults in the strike-slip fault system bend through 25°, splay into five principal strands and decrease their mean dip. These changes in fault geometry are accompanied by a gradual steepening of the pitch of the slip vectors, and by an anticlockwise swing (up to 50°) in the azimuth of slip on the faults in the NIFS. As a consequence of the bending of the strike-slip faults and the changes in their slip vectors, near their intersection, the slip vectors on the two component fault systems become subparallel to each other and to their mutual line of intersection. This subparallelism facilitates the transfer of displacement from one fault system to the other, accounting for a significant amount of the NE increase of extension along the rift, whilst maintaining the overall coherence of the strike-slip termination. Changes in the slip vectors of the strike-slip faults arise from the superimposition of rift-orthogonal differential extension outside the rift margin, resulting in differential motion of the footwall and hanging-wall blocks of each fault in the NIFS. The combination of rift-orthogonal heterogeneous extension (dip-slip) and strike-slip, results in a steepening of the pitch of the slip vectors on the terminating fault system. Slip vectors on each splay close to their terminations are, therefore, the sum of strike-slip and dip-slip components, with the total angle through which the pitch of the slip vectors steepens being dependent on the relative values of both these two component vectors. In circumstances where interaction of the velocity fields for the intersecting fault systems cannot resolve to a slip vector that is boundary-coherent, either rotation about vertical axes of the terminating fault relative to the through-going fault system may take place to accommodate the termination of the strike-slip fault system, or the rift may be offset by the strike-slip fault system rather than terminating into it. At the termination of the NIFS, an earlier phase of such rotations may have produced the 25° anticlockwise bend in fault strike and contributed up to about one-third of the anticlockwise deflection in slip azimuth. On the terminating strike-slip NIFS, therefore, rotational and non-rotational termination mechanisms have both played a role, but at different times in its evolution, as the thermal structure, the rheology and the thickness of the crust in the rift intersection region have changed.