Structure and metamorphism of the Haast Schist and Torlesse Zones between the Alpine Fault and the D'urville Valley, South Nelson (1986)
AuthorsRose, R. V.show all
Within the bends region of the Alpine Fault two structural trends have been identified. These are a D1-D2 “Rangitata” trend and a D3 “Kaikoura” trend. The D1 - D2 trend maintains a northeasterly strike of both bedding and S2 schistosity which is independent of the Alpine Fault, intersecting in such a way as to verge into the Fault from the south at an acute angle. Within the Glenroy section, the relative angle changes such that this trend approaches obliquely from the north. The D3 trend overprints D2 close to the Alpine Fault and is subparallel to the fault. Two metamorphisms (M2 and M3) are identified, M2 being associated with D2 and M3 with D3. S3 schistosity, L3 lineations and F3 folds are associated with strain in, and adjacent to the Alpine Fault Zone. There is no direct evidence that the Haast Schists slip around and past a fixed bend in the Alpine Fault Zone. The double bend of the Alpine Fault is thus fixed with respect to the “Wairau Block” (that area between the Alpine/Wairau and Awatere Faults), and forms the leading edge of a wedge shaped body of Haast Schist and Torlesse Zone metasediments. L3 lineations including quartz rodding, trend and plunge at approximately 060/50° northeast, and are assumed to reflect the transport direction in non-coaxial plane strain within the Alpine Fault Zone. The Haast Schists are being thrust over the Western Province with the vector direction and rate of movement of the rock mass remaining relatively constant at all points around the curve of the Alpine Fault. Likely orientations of the XY plane of strain in the Alpine Fault Zone are discussed. It is concluded that a distinctly separate schistosity will not form unless pre-existing metamorphic layering is suitably oriented with respect to the shear zone. Consequently, transition from M2 (Rangitata Phase) schists to M3 (Alpine Fault generated) schists may be subtle and difficult to determine either in the field or in thin section. Rotation of the D2 structural trend by ductile drag on a macroscopic scale within the Alpine Fault Zone appears to be counterclockwise. This is to be expected if (as is the case in the bends region) the early anisotropy has a more northerly strike than the convergence vector. Equal area projection plots of post-metamorphic brittle shear surfaces cluster strongly, defining dominant shear sets oriented west-northwest to northwest and east-northeast respectively. Those shears which strike northwest exhibit minor movement of sinistral sense, whereas those striking northeast show a dextral movement. No large brittle offsets have been demonstrated, and cumulative offset on these shear systems has not altered the gross structural trends.