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A spectacular, dense network of cataclastic faults characterizes the Late Cretaceous Ngatuturi Claystone, a massive and mechanically almost isotropic siliceous mudstone. It is part of a Cretaceous to late Oligocene shelf sequence deposited NE of New Zealand that was translated SW in the late Oligocene with the Northland Allochthon in an obduction event associated with southward propagation of a new convergent plate boundary. The allochthon was reactivated in the Miocene, forming the southward-moving substrate of the Waitemata piggyback basin. The cataclasites are submillimeter- to several centimeters–thick black seams that were formed without contemporaneous open tensile fractures, because any fault asperities were immediately ground away. Riedel shear patterns are prominent at all scales, due to multiple reactivation of preexisting fault surfaces. Some fault arrays are so closely spaced that they resemble a cleavage compatible with large-scale folds in the Ngatuturi Claystone. Movement on such faults has allowed formation of structures that appear mesoscopically ductile. More than twenty phases of cross-cutting structures (events E1–E22) are part of the following stages of tectonic development: (I) northeastward thrusting in an accretionary prism; (II) southward transport in the Northland Allochthon; (III) southwestward movement during the main phase of allochthon emplacement; (IV) renewed southward movement of the allochthon; (V) sliding during sedimentation of the Miocene Waitemata Group; and (VI) further intrabasinal thrusting to the south. During the pre-Miocene phases (I–IV), the cataclasites fault network allowed the Ngatuturi Claystone to deform in a macroscopically ductile manner, simultaneously acting as a dynamic aquiclude, thereby facilitating high fluid pressures in the surrounding rocks.

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