Accretionary Prism Rocks of the Torlesse Terrane: Western Aorangi Range – Cape Palliser, New Zealand

Abstract

Mesozoic Torlesse rocks in the western Aorangi Ranges, southernmost North Island, comprise two units: complexly deformed 'basement' greywacke and argillite, with minor associated rocks, interpreted as part of an accretionary prism; and a relatively undeformed sequence of greywacke and argillite named the Whatarangi Formation, interpreted as a trench slope basin deposit. Two broad rock associations are recognised within the basement greywacke. The sedimentary association comprises the volumetrically dominant greywacke and argillite, with minor conglomerate, calcareous argillite, pebbly argillite, and chert. These rocks were deposited by a variety of sediment gravity flows in the trench of an active convergent margin, and have subsequently undergone varying degrees of stratal disruption. Highly disrupted greywacke-argillite sequences occur in fault zones where strain rates were greatest. Melange is a minor constituent of the sedimentary association, and melange units commonly exhibit features which indicate a sedimentary origin, and less often a diapiric origin. The sediments, clasts and blocks were derived from reworking of previously accreted material on the inner trench slope, and also from a volcanic-plutonic source located westward of the trench slope break. The volcanic association rocks comprise tholeiitic and alkalic metabasite, coloured argillite, chert and minor marble. They represent remnants of seamounts and accompanying pelagic sediments which were in the trench at the time of incorporation of the sedimentary association turbidites into the accretionary prism. Observations of seamounts in modern trenches support these conclusions. Radiometric and microfaunal dating techniques indicate that the sedimentary association rocks were deposited and deformed during Latest Jurassic to Early Cretaceous, and they are therefore correctly assigned to Torlesse fossil zone five. The Torlesse basement rocks have been complexly deformed. Overprinting relationships indicate three phases of folding which occurred during offscraping and accretion, and subsequently within the prism. The following order of events is proposed for the deformation: (i) early large-scale folding (F1) with associated cleavage (S1) and disruption of strata, principally by shearing parallel to bedding and low-angle to bedding faulting, during imbrication of the trench sediments; (ii) widespread folding (F2) characterised by locally developed axial planar cleavage (S2), and variable plunges within the axial plane, indicating progressive deformation and rotation of fold axes. Metamorphism of the Torlesse rocks to prehnite-pumpellyite grade most likely accompanied this deformation; (iii) continued faulting at high and low angles to bedding; and (iv) F3 folding superimposed on bedding already rotated to moderate dips, in response to strike-slip faulting. Alkalic dykes which post-date all folding episodes were intruded at approximately 100 Ma, thereby constraining all folding to the Mesozoic. Whatarangi Formation crops out as a down-faulted block in the basement greywacke, and has an age of Albian-Aptian at the top of the formation. The sediments were derived from both accreted Torlesse and volcanic-plutonic sources, and were deposited in an elongate basin as overlapping base-of-slope sediment aprons. The style of deformation, best described by a model of simple shear, is consistent with its having been deposited as a slope basin on top of an actively deforming accretionary prism. Cenozoic deformation of the Torlesse rocks in the western Aorangi Range by dipslip faulting has occurred in response to the growth of the currently active accretionary prism off the east coast of the North Island.