Microstructural evolution and flow of rocks in a zone of active transpressional deformation near the Alpine Fault, New Zealand

R. J. Holcombe (University of Queensland, Queensland 4072, Australia; e-mail: rodh@earthsciences.uq.edu.au )

[Abstract for SGTSG Conference Halls Gap, Victoria, February 1999]

The Alpine Schist adjacent to the active Alpine Fault in New Zealand consists, in the Franz Josef and Fox glaciers area, of a monotonous sequence of graphitic psammopelitic schist, with minor competent psammitic and metavolcanic units. Within ~10 km of the fault, metamorphic grades are upper greenschist to lower amphibolite facies, with metamorphic zones sub-parallel to the fault and apparently attenuated. This generally east-tilted sequence of mid-crustal rocks has been ramped to the surface in the past ~3 m.y. in the hangingwall of the oblique-slip Alpine Fault.

Within about 1.5 km of the present surface trace of the fault, the youngest fabrics in the schist are protomylonitic to mylonitic, with fabrics that broadly reflect the known kinematics of the SE-dipping fault (dextral with a slight reverse component). Grain shape fabrics, poorly developed stretching lineation, and abundance of conjugate shear bands reflect a strongly transpressional flow component that has developed oblate finite strains. Further away from the fault, this youngest ductile fabric is no longer pervasive and is replaced by multiple and composite overprinting fabrics that strike obliquely to the fault. Zones of strong folding (F3) of an older schistosity (S2) separate a 2-3 km-wide zone of very planar, strongly lineated, high strain fabrics. Fabric orientations vary systematically away from the Alpine Fault; dihedral angles between the fault and the dominant foliation, as well as the southwesterly pitch of the prominent lineation within the foliation, increase with increasing distance from the fault.

Abundant coarse porphyroblasts of biotite and garnet, with well-preserved internal foliations, occur in the graphitic schist and have grown syn-kinematically with F3 folds and subsequent fabrics. Both porphyroblast species are coarse enough that individual porphyroblasts generally overgrow several graphitic laminae, and excellent preservation allows each internal lamination to be correlated with its external parent. Thus, we have a fine-scale displacement gauge in these rocks which we have used (together with strain shadow shape data from garnet and ilmenite porphyroblasts) to estimate post-growth stretch components of the finite strain. Both of the main porphyroblast species show evidence of rotation associated with the shortening across the layers; in biotite this is associated with extensive bookshelf faulting, and in both there is an element of shear asymmetry to strain shadows. Estimates of the vorticity of the flow leading to these fabrics has been obtained from widespread oblique quartz grain-shape fabrics, and semi-quantitatively from both the orientation distribution of micro-boudinage fractures in garnet porphyroblasts and the orientation distribution of rotated biotite laths (see Holcombe, this volume). The latter technique has also yielded a quantitative assessment of the shear strain component.

The strain data show that both the folded and planar zones have resulted in only a modest bulk shortening (minimum strain ratios of ~1.4 : 1.4 : 0.5 since peak mineral growth). The planar zones are simply attenuated macroscopic F3 limbs in which the strongly lineated fabrics are largely inherited from the earlier D2 deformation. The hinges of the F3 folds are almost perfectly colinear with the intense inherited lineation in the folded older fabric. In contrast to the strong LS characteristics of the inherited fabric, our data show that the finite strain associated with the folding and subsequent deformation is very oblate, with a slight down-dip maximum stretch.

We attribute the F3 fabric to be the result of late Cenozoic shortening of a pre-existing LS-tectonite, with the inherited D2 fabric probably being Mesozoic in age. East of the Alpine Fault, the schists are inferred to be delaminated along a decollement in the mid-crust, before being translated through the 40-50° dipping footwall ramp of the Alpine Fault. The macroscopic F3 crenulation fabric was probably imprinted on the rocks as they converged obliquely against the toe of the ramp. Alternatively, late Cenozoic deformation at the ramp may have reinforced a pre-existing, near-vertical crenulation fabric. The flow regime producing shortening across the F3 limbs was characterized by sub-simple shear. We estimate a kinematic vorticity number of ~0.2, with a bulk shear strain of about 0.6 (this vorticity number estimate is blurred by 3-D considerations). The shear sense is consistently east-block-down (normal relative to the current steeply SE-dipping orientation of foliation). This shear sense is independent of position on the F3 folds and apparently overprints those structures, resulting in widespread oblique quartz grain-shape fabrics of uniform shear sense. As the schists were transported up the ramp towards the surface, bulk shear of the strongly anisotropic rocks was partitioned into rotation of schist packets, oblate coaxial stretching of packets, and normal-shear between them.

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