Strain localization through the continental lithosphere: toward a new mantle rheology..
Abstract
Based on rheological data, numerous models show that strain localization at the scale of the continental lithosphere, such as narrow rift, requires the occurrence of a stiff and localizing uppermost mantle. The predicted brittle mantle has been usually involved to promote such a lithosphere-scale strain localization. However, both the few mantle earthquakes and the ductile shear zone-like structures beneath extending crust suggest that ductile rheology could also play a major role in this process. Recently, new experimental data and observations of mylonite of naturally deformed peridotites have emphasized an olivine rheology that accounts for the combination of grain boundary sliding (GBS) and dislocation creep at low temperature. According to this rheology, dynamic recrystallization and allied grain size reduction could promote strong weakening and subsequent strain localization in the conditions of the sub-continental mantle. Here, we performed one-dimensional numerical modeling at constant temperature in order to quantify this GBS-related strain localization. Our model shows that intense strain localization occurs between 550 °C and 650 °C, leading to locally increase the strain rate from 10-15 s-1 up to more than 10-13 s-1; at higher temperature, the degree of strain localization decreases progressively until being inexistent at temperature higher than 787 °C; and below 550 °C, the peak of strain rate slightly decreases with decreasing temperature. By extrapolating these results at the scale of the continental lithosphere, we then proposed a new mantle rheology that accounts for a ductile and «localizing» uppermost mantle, i.e., in agreement with models and geophysical observations. In addition, we tested the implications of this «localizing» ductile mantle on extending lithosphere by using a pre-existing two-dimensional numerical model. After a first stage of distributed extension, mantle deformation and olivine grain size reduction promoted strain localization in the uppermost mantle, allowing to initiate a narrow continental rift.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.T32C..02P
- Keywords:
-
- 8120 TECTONOPHYSICS / Dynamics of lithosphere and mantle: general;
- 8162 TECTONOPHYSICS / Rheology: mantle