Brittle and ductile deformation in a zone of rapid uplift: Central Southern Alps, New Zealand

Schists exposed in the central Southern Alps, New Zealand, 1–5 km east of the Alpine fault zone, have been rapidly uplifted during the late Cenozoic. A regionally consistent sequence of mesoscopic structures discordant to the schistosity is recognized extending backward in time from recent brittle displacements of deglaciated surfaces through structures exhibiting brittle‐ductile transitional behaviour to ductile deformational features. Kinematic analysis of these structures gives consistent principal subhorizontal shortening directions similar to the present day principal horizontal shortening direction, indicating their relation to late Cenozoic uplift of the Southern Alps. Analysis of deformed veins suggests a minimum shortening strain of about 50% perpendicular to the foliation. The veins crosscut upright mesoscopic to macroscopic folds which commonly develop high‐strain zones on their limbs. Within such zones a strong stretching lineation plunges gently SW, approximately perpendicular to the stretching lineation in the mylonites along the Alpine fault. The folds and high‐strain zones do not appear to be related to late Cenozoic uplift but may have originated during an earlier phase of dominantly strike‐slip motion. Fluid inclusion studies give depth‐temperature estimates of around 3 km and 285°C for the development of the brittle structures and 6–8 km at 310°–350°C for the brittle‐ductile transition. The deformed veins show evidence of extensive fluid infiltration during the later phases; data from fluid inclusions and metamorphic assemblages give an estimate of 15–20 km and 400°–450°C for their deformation. These data, combined with uplift rates determined by other studies, allow the construction of a depth‐temperature‐time path which indicates nearly isothermal decompression associated with rapid uplift. A shallow brittle‐ductile transition (6–8 km) is consistent with numerical modeling of thermal and mechanical behavior of the crust during rapid uplift associated with continental collision and with the presence of high heat flow in the area. Because of the small temperature change over a large amount of decompression, pressure may be an important factor in controlling the depth of the brittle‐ductile transition. The marked temperature drop above the brittle‐ductile transition is thought to be enhanced by the influence of convection as an effective cooling mechanism.