Amuri Range geology : Torlesse rocks from the Hope Fault to the Clarence River

Worley, Steven R., 1966-

Basement geology of the Arnuri Range, North Canterbury, between the Hope Fault and the Clarence River comprises Torlesse metasediments and associated slivers of melange and seafloor volcanict..., which together are ascribed to the Pahau subterrane (Bishop et al. 1985). Detailed mapping and lithological analysis reveals three distinct Torlesse sandstone- argillite fades on the basis of sand: mud ratios and individual bed thicknesses: Lithofacies I; >5: 1(sandstone: argillite) produced by high velocity grain flow or a high density turbidity current, Lithofacies II; 5 : 1 to 2 : 1 produced by classic turbidity currents, Lithofacies Ill; < 2 : 1 produced by low density turbidity currents. These turbidites accumulated in mid to outer regions of submarine fans at the base of the submarine slope at a convergent continental margin. Fades associations reflect a progradational fan that sloped to the west towards it's sediment source. Continued subduction at the continental margin resulted incorporation of the submarine fan and slivers of oceanic crust into an accretionary prism complex, and eventual regional metamorphism of zeolite to prehnite - pumpellyite grade. Compressive tectonics produced two phases of coaxial deformation (D1 and D2), while melange formation resulted from decoupling of sediments with subduction and related shearing during Dt. Ancient thrust faults that now dip steeply to the east and strike northeast - southwest were formed during initial incorporation into the prism. A change from compressive to strikeslip tectonics resulted in a third phase of deformation (D3) and provoked much of the faulting in the Marlborough area, including the major and active faults in the present area (i.e. Hope Fault, Tinline Fault, Lyford Fault). Modification of drainage patterns of streams and rivers traversing the Hope Fault illustrate substantial recent and continuing dextral transcurrent movement along the fault. Comparisons of sandstone geochemical and petrographic signatures with other studies suggest a Lower to Middle Jurassic age for the Amuri Range rocks, placing them between petrofacies 4 and petrofacies 5 of MacKinnon (1983), and indicate that the source was possibly a Continental Island Arc, contributing a dominant acid plutonic component with evidence of important contemporaneous intermediate to silicic volcanism and substantial reworking of older uplifted Torlesse. Geochemical analyses of pillows and flows from the Green Volcanic Association indicate they have N - MORB affinities and it is suggested that they represent peeling of the uppermost basaltic oceanic crust .· subduction beneath the accretionary prism and eventual underplating beneath the complex as opposed to incorporation at the toe of prism.

109, 6, 2 leaves : ill. (some col.), maps (some col.) ; 30 cm.

1996Worley

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