Earth Planets Space, Vol. 56 (No. 12), pp. 1111-1120, 2004
Klaus Regenauer-Lieb1,2, Bruce Hobbs2, and Alison Ord2
1Johannes Gutenberg-UniversitNat Mainz, Geophysics and Geodynamics, 55099 Mainz, Germany
2CSIRO Exploration and Mining Perth (ARRC), PO Box 1130, Bentley WA 6102, Australia
(Received June 15, 2004; Revised November 21, 2004; Accepted December 4, 2004)
We investigate a novel fully coupled thermal-mechanical numerical model of the crust in order to trace the physics of interaction of its brittle and ductile layers. In a unified approach these layers develop in a natural transition as a function of the state variables pressure, deviatoric stress, temperature and strain-rate. We find that the main storage of elastic energy lies in the domain where brittle and ductile strain-rates overlap so that shear zones are attracted to this zone of maximum energy dissipation. This dissipation appears as a local heat source (shear heating). The brittle-ductile transition zone evolves through extreme weakening by thermo-mechanical feedback. The physics of the weakening process relies on repeated breaching of a critical energy flux threshold for feedback within this sub-horizontal brittle-ductile flow layer, thus developing unstable slipping events at postand pre-seismic strain-rates. The width- and the temperature domain of the feedback layer is controlled by the activation enthalpy Q of the material. For olivine rheology (Q ~ 500 kJ/mol) the layer can be extremely thin <500 m and adheres to the 875 K isotherm. For quartz (Q ~ 135 kJ/mol) the width of the feedback layer fans out into multiple interacting ductile faults covering a temperature domain of 450-600 K. The weakening by thermalmechanical feedback entirely controls the location and rejuvenation of upper crustal shear zones propagating from the detachment upwards in the form of listric faults. Within the detachment shear layer we identify an astonishing rich dynamics featuring distinct individual creep bursts. We argue that the rich ductile dynamics holds the key to earthquakes in the brittle field.
Key words: Quartz rheology, geodynamic modelling, brittle-ductile transition, crustal extension.