Seismological and Thermochronological Constraints on Brittle–Ductile Transition Temperatures and Exhumation Rates Near the Late-Interseismic Central Alpine Fault, Southern Alps, New Zealand

Abstract:

The Alpine Fault is a transpressive plate-bounding fault delineating the western edge of the Southern Alps orogen. Paleoseismological studies reveal the fault to regularly rupture in large (Mw 7–8) earthquakes and to currently be in the late stages of its typical <300-yr interseismic period, having last ruptured in 1717 CE. The hanging-wall of the central section of the Alpine Fault exhibits high rates of uplift, rainfall, and erosion, and low seismicity rates when compared to adjacent areas. Recent borehole measurements and previous thermal modelling results reveal high temperatures in the crust near the central Alpine Fault. We revisit those observations using a newly developed microseismicity catalog to investigate the effects of exhumation on the depths of earthquakes and the brittle–ductile transition throughout the orogen. The new catalog contains 7,719 relocated earthquakes, which we analyse jointly with published thermochronological data using multiple 1-D thermal models. The estimated exhumation rates vary from 1 to 8 mm/yr, with maximum values observed in the area of highest topography near Aoraki/Mount Cook. We obtain a mean brittle–ductile transition temperature beneath the central Southern Alps of 410–430^oC, which is higher than expected for a quartz-dominated rheology and likely reflects unmodeled effects of laterally varying strain rates or elevated fluid pressures. The pronounced along-strike variation in exhumation rates and thermal structure may affect down-dip and along-strike patterns of coseismic slip in future Alpine Fault earthquakes. We are now undertaking a comprehensive matched-filter analysis using all events in the existing catalog as templates in order to detect additional lower-magnitude seismicity and explore spatial variations in moment release and susceptibility to triggering by regional earthquakes.