Some thermal and mechanical consequences of rapid uplift: an example from the Southern Alps, New Zealand

The thermal evolution of continental crust during active collision is modeled through numerical solutions of the two-dimensional heat conduction equation for a rapidly moving medium. The boundary conditions used in the modeling are derived from geological and geophysical observations from the active collision zone in the South Island of New Zealand. The problem domain over which the solutions are obtained consists of a 40 km horizontal by 25 km vertical spatial plane with a vertical discontinuity at 10 km from the western boundary. To the east of this discontinuity, vertical uplift rates of up to 10 mm/a occur over a timespan of up to 4 Ma. Temperature distributions are calculated at 10 ka intervals over the 4 Ma duration. A two-dimensional high-temperature region is established upon initiation of uplift of the eastern block due to the advective component carrying heat upwards more rapidly than it can be dissipated laterally from the problem domain. Temperatures within the upper 5 km are greater than 400°C after 2.25 Ma with geothermal gradients of up to 200°C/km attained within the upper 3 km. At times greater than 2.5 Ma, the vertical temperature distribution changes little while the anomalously high temperatures spread laterally into the stationary crust.Using rheological equations to describe the brittle behaviour of a water-saturated upper crust and the ductile behaviour of a quartz-dominated lower crust, together with the thermal distribution of the conduction models, the mechanical evolution of a collision zone is investigated. In addition to high crustal temperatures and associated high heat flow, rapid uplift produces a weakening of the crust by raising of the depth of transition from brittle to ductile behaviour. Within the zone of most rapid uplift, the brittle-ductile transition rises from 13 km to less than 5 km after 1.5 Ma of uplift. Further uplift reduces the brittle layer to 3 km thickness and causes lateral spreading of the low-strength zone. The reductions in crustal strength caused by the thermal weakening produce a high-strain zone within the region of maximum uplift which is incapable of sustaining large differential stresses. This causes horizontal and vertical stress transfer and results in shallow seismicity increases in the adjacent crust as well as in intermediate depth seismicity within the high-strength upper mantle.Because the thermal and mechanical anomalies discussed are a function of rapid uplift, all regions of active continental collision may be expected to exhibit similar behaviour. Some mechanical and thermal characteristics of the Himalayan collision zone are briefly examined in light of the numerical modeling.