Quaternary temporal stability of a regional strike-slip and rift fault intersection

Abstract

Intersecting strike-slip and normal fault systems form either two or three plate configurations. In circumstances where they intersect to form a triple junction, internal block deformation produces a quasi-stable 3-D configuration permitting maintenance of both the regional geometry and kinematics of the intersection. This paper examines the temporal stability of such a triple junction in New Zealand over a range of timescales, from individual earthquakes to millions of years, and discusses the factors that impact its stability. Using seismic-reflection, gravity, drill-hole and outcrop data, the accumulation of vertical displacements is examined, both on individual faults and across the intersection zone between the strike-slip North Island Fault System (NIFS) and the Taupo Rift. Fault-throws and throw rates for the NIFS and Taupo Rift suggest that despite a three-fold increase in throw rates in both fault systems at about 0.3 Ma, which may have modified the orientation of the slip vectors and therefore the contemporary kinematics of the triple junction, the intersection of the two fault systems has functioned coherently for the last 0.6–1.5 Myr accumulating vertical displacements interdependently since the early stages of rifting. The apparent arrest of a large magnitude earthquake on the rift-bounding fault adjacent to the intersection, suggests that the short-term dynamic behaviour of the system is relatively complex compared to the more coherent long-term kinematics.