Environments of formation of lithologic associations in the Torlesse accretionary wedge, Tararua Range, New Zealand

Abstract

The Torlesse Complex in the southeastern Tararua Range, New Zealand, consists of two distinctive lithologic associations: a sedimentary association (95% of outcrop), consisting of sandstone, mudstone, calcareous siltstone, conglomerate, and olistostrome; and a volcanic association (5% of outcrop) which consists of basalt, red and green argillite, chert, and limestone. Rocks of the sedimentary association were deposited by turbidity currents and debris flows in a deep marine environment, as part of a mid-outer submarine fan. Detrital framework modes and whole rock geochemistry indicate that the sediments were eroded from an active, continental-margin magmatic arc that was heavily dissected and shedding sediment of mainly plutonic and metamorphic origin. Rocks of the volcanic association, although volumetrically insignificant, are widely distributed but occur mainly in the western half of the area. Both tholeiitic and alkalic basalts occur, and were erupted in an oceanic environment, both in mid-ocean ridge and intraplate settings. Sedimentary rocks of the volcanic association were also deposited in these environments. Where observed, contacts between the two associations are always faulted, implying postdepositional juxtaposition of the two associations, which is considered to have occurred during the development of a subduction-related accretionary wedge.