Normal fault interactions, paleoearthquakes and growth in an active rift

Abstract

Fault interactions are an essential feature of all fault systems on timescales of individual earthquakes to millions of years. We examine the role of these interactions in the development of an array of normal faults within the active Taupo Rift, New Zealand. Stratigraphic horizons (0–26 ka) exposed in 30 trenches and laterally extensive topographic surfaces (∼18–340 ka) record displacements during surface-rupturing earthquakes over time intervals of up to 100’s of thousands of years. Complementary changes in displacements, displacement rates and earthquake histories between faults are observed for along-strike displacement profiles and at points on fault traces. Variations of displacement are attributed mainly to fault interactions, and decrease with the aggregation of displacements on progressively more faults and over longer time intervals. Rift-wide displacement rates are, for example, near-constant over timescales of <60 kyr and suggest a level of order which is greater than that of individual fault traces. Each fault is, nevertheless, a vital element of a system that displays a remarkable degree of kinematic coherence which produces, and maintains, a hierarchy of fault size throughout the deformation history. As a consequence, on spatial scales greater than an individual fault trace and over temporal scales more than several earthquake cycles, the behaviour of individual faults can be relatively predictable. Fault interactions are accompanied by changes in fault system geometries consistent with increases in their maturity arising from strain localisation processes, including fault linkage and death.