Midcrustal Shear Zones Formation Deduced From Field Observations and A 2d Thermo-mechanical Model
Abstract
The formation of low-angle extensional shear zones is often observed at the brittle- ductile transition of the continental crust. Field observations and microstructural data for an exhumed shear zone (East Tenda Shear Zone, Corsica, France) are first re- ported to assess potential deformation mechanisms responsible for localization. The fracture of coarse feldspar grains and the progressive transformation of feldspar into mica are shown to be coeval with the formation of shear zones. Feldspar having a flow stress larger than mica, this coupling of fracture and reaction-softening is the proposed destabilizing factor. This hypothesis is tested by combining these two mechanisms in a rheological model and showing that localization occurs in the lower crust under sim- ple shear. Three phases are considered quartz, mica and feldspar, each deforming by dislocation creep at a common strain rate. A Mohr-Coulomb criterion is used to mark the onset of feldspar-to-mica reaction. Variations in the velocity of the upper crust by a factor of 6 are sufficient to trigger this reaction, at a depth close to 15 km, and inducing a drastic increase in the strain rate by two orders of magnitude, which marks the onset of strain localization. The periodicity and the orientation (dip angle) is then discussed with 2D finite-element results. It is shown that the combination of 80% simple shear (due to the rigid gliding of the upper crust over the lower crust) and 20% pure shear results in the formation of a~10-15 dip extensional shear zones. This orientation is in agreement with the spatial distribution of micro-earthquake clusters below regions of active extension.
- Publication:
-
EGS General Assembly Conference Abstracts
- Pub Date:
- 2002
- Bibcode:
- 2002EGSGA..27.6245G