Abstract

Studies of modern convergent margins have highlighted the variety of depositional environments and deformational processes that occur in this tectonic setting and provide analogues by which ancient uplifted sequences can be interpreted. The Whatarangi Formation is an Early Cretaceous trench-slope basin deposit, preserved within accretionary prism rocks of the (Pahau) Torlesse terrane of New Zealand, and is distinguished from the underlying accreted rocks by its younger age, different depositional history, structural style and metamorphic grade, and lack of oceanic volcanic outcrops. Furthermore, the Whatarangi Formation records the types of processes inferred to occur in modern slope basins. Sediments were derived from sources on the western side of the basin and were deposited mainly by gravity flows on channelized base-of-slope aprons. The narrow, elongate shape of the basin resulted in restricted, longitudinal flow of the sandy turbidity currents and deposition of finer sediment farther down the inner trench slope or in the trench. Deformation of the turbidites produced open to tight asymmetric folds that are concentrated along the western margin and lower half of the formation. The folds are best described by a model of simple shear and formed in response to active west-over-east thrusting in the underlying prism. Cleavage was initiated prior to folding in response to overall shortening of the basin, with subsequent development of an axial-planar cleavage in the tightest folds. The compositional similarity between the slope basin strata and accreted sedimentary rocks of the Aorangi Range has important implications for the Early Cretaceous tectonic history of the Torlesse terrane. The sediments were eroded from an uplifted accretionary prism and an acid igneous source, and the deposition of these sediments in upslope basins and the trench suggests that all the Pahau sediments could have been transported from these transversely located sources.

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