The Strength of Fracture Zones and their Role in Plate Boundary Evolution
Abstract
The strength of fracture zones, and whether they inhibit or aid changes in oceanic plate motions, is of great importance in understanding plate reorganizations. For example, self-sustaining subduction zones form if convergence during plate reorganizations exceeds ~100 km across fracture zones, but only if they yield at ~10 MPa with low coefficients of friction. Convergence across fracture zones with higher yield strengths requires forces in excess of those likely available from plate tectonics, and typically does not lead to sustained subduction. Many fracture zones are indeed thought to be strong since they preserve bathymetric steps from when they transitioned from transform faults, while far-field differential subsidence leads to elastic flexure of the lithosphere. These evolutionary models fix the bathymetric step at the value determined by the lithospheric age offset. In contrast, we use a two dimensional visco-elastoplastic numerical scheme in which the fracture zone offset is determined dynamically and explore fracture zone offset as a function of yield properties. We show that oceanic lithosphere can be locally weakened at a fracture zone while still preserving long-lived bathymetric steps and flexural responses to differential subsidence, in agreement with observed gravity profiles. Models that best fit the observations typically yield at less than 10 MPa and may have very low coefficients of friction, whereas models with stronger fracture zones over-predict the amplitude of the bathymetric step. Furthermore, local gravity highs above the older plate near the fracture zone cannot typically be fit by the locked fault model and could be the result of partial convective removal of the thermal boundary layer. The extent of convective removal necessary offers a means of constraining the effective temperature dependence of viscosity in oceanic lithosphere. Our analysis of gravity profiles across fracture zones suggests that they are weaknesses in oceanic lithosphere that may, for example, allow localized convergence that can lead to the initiation of subduction.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.T33A..06H
- Keywords:
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- 1236 Rheology of the lithosphere and mantle (8160);
- 8010 Fractures and faults;
- 8120 Dynamics of lithosphere and mantle: general;
- 8159 Rheology: crust and lithosphere