The Esk Head melange in the Esk Head/Okuku area, North Canterbury

The Esk Head Mélange is a zone of chaotically mixed rocks roughly 12 km wide, trending for at least 60 km north-northwest across North Canterbury. It is characterized on all scales by a "block and matrix" structure and consists of blocks of sandstone/mudstone, conglomerate, metavolcanics, chert and limestone within a dark muddy matrix. The zone demonstrates gradational boundaries with Torlesse subterranes to the southwest and northeast. Within the Esk Head/Okuku study area, the mélange is a structurally coherent zone, steeply overturned to the southwest, with a systematic fabric dominated by pervasive shearing and modified by latter flattening. The zone has acted as a structural marker which records post mélange Mesozoic deformation dominated by steeply plunging, gentle to open flexures and sinistral faults. Fragmentation within the mélange was accomplished by boudinage and shear-related processes. Several families of minor folds are directly related to shearing and asymmetric folding suggests a southwest over northeast sense of movement. Early faulting and related shear fractures readjust the melange and result in further mixing. Low grade metamorphism, i.e. zeolite facies, was ongoing during deformation throughout the zone and margins. Sandstone/mudstone sequences within the melange represent highly dismembered elements of original submarine fans. Volcanics, cherts, and limestones comprise a "volcanogenic" association and represent scraps of an original basement. The mélange also incorporates "olistostromal components" up to 1 km in thickness, dismembered to varying degrees and directly related to mélange formation. The Esk Head Mélange is regarded as a tectonic mélange created in the upper levels of a shallow westward dipping subduction zone during the Early Cretaceous and possibly Jurassic. Emplacement of the zone may be related to accretionary underplating. However a model of near vertical emplacement by subduction-driven upward movement on the landward margin of the active accretionary wedge is considered more likely.