Deformation processes in an accretionary prism: a study from the Torlesse terrane of New Zealand

Abstract

The style of deformation observed in rocks of the Torlesse (Pahau) terrane, exposed in the Aorangi Range of the North Island, records accretion of thick trench fill by offscraping at the toe of a growing accretionary prism during the early Cretaceous. The relatively coherent nature of the Aorangi Range rocks enables detailed study of the deformation processes produced by offscraping and subsequently within the prism, and many of the structures observed in these rocks are consistent with those described from both modern accreting margins and other ancient accretionary terranes. Overprinting relationships indicate three phases of folding and multiple faulting events. Early deformation involved large-scale sheath-like folding oblique to the overall trend of the margin, and development of an anastomosing axial-planar cleavage. Folding of the sediments promoted dewatering; the subsequent disruption of strata, by shearing parallel to bedding and low-angle to bedding faulting, records the transition to more brittle responses to the deformation. The most widespread folding phase ( D 2) produced numerous upright, typically isoclinal folds, with local development of an axial-planar S 2 cleavage in macroscopic and some mesascopic fold hinges. The variable plunge of the fold axes to the NNE and SSW within the axial surface indicates progressive rotation of the fold axes after formation. Mesozoic strike-slip faulting most likely produced the open E-W-trending folds and warps of the third folding phase ( D 3), in bedding already rotated to moderate dips. Faults which overprint the Mesozoic deformation, were formed in response to renewed subduction along the eastern coast of the North Island during the Cenozoic.