Early deformation fabrics of melange in the Mesozoic Waipapa Terrane, North Island, New Zealand

Abstract

The Waipapa Terrane in northern North Island, New Zealand, is a Mesozoic accretionary complex formed along the Gondwana margin. It contains abundant melange rocks with distinctive characteristics. Precise analyses of their mesoscopic fabrics in Waiheke Island near Auckland have revealed the following sequence of deformation. The earliest phase of deformation of the sandstone/mudstone association, which is the main constituent of this terrane, originated by chaotic mixing of sand and mud due to liquidization of water-saturated, poorly consolidated sediments. The second phase was characterized by hydrofracturing and subsequent forceful injection of ductile mud into rather brittle sand. Local intrusions of sand forming dykes and sills followed these events, as well as intrusions of pelagic/hemipelagic green argillite originally underlying the sandstone/mudstone association. An abundant occurrence of these mixing and multi-stage injection/intrusion fabrics strongly suggest that the Waipapa Terrane around the study area was a site of high pore-fluid pressure. Scaly-foliated melange fabrics with monoclinic symmetry, originating from layer-parallel shearing, were then locally superimposed on the pre-existing melange fabrics. Similar scaly-foliated fabrics also developed in the chert beds originally intercalated between the green argillite and the uppermost part of the oceanic crust. These scaly fabrics might have been related to the regional stacking and juxtaposition of the accreted sediments. The sequence and variation in style of deformation forming the melange fabrics presumably reflected changes in porosity and state of compaction of accreted sediments in a shallow tectonic level.

Deformation processes in the shallow levels of an accretionary prism: The Mesozoic Torlesse Terrane, South Island, New Zealand

“…The mud intrusions or injections have been found in other ancient accretionary complexes, and are regarded as an important structural process during accretion (e.g. Barber et al 1986; Byrne & Fisher 1990; Kano 1995; Ujiie 1997). This analysis provides a good example of multistage intrusions of mudstone.…”