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Shear zone and nappe formation by thermal softening, related stress and temperature evolution, and application to the Alps
Author(s) -
Schmalholz S. M.,
Duretz T.
Publication year - 2015
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/jmg.12137
Subject(s) - nappe , geology , shear zone , shear (geology) , lithosphere , shear stress , crust , critical resolved shear stress , differential stress , tectonics , petrology , shear rate , geophysics , seismology , materials science , deformation (meteorology) , composite material , viscosity , oceanography
We present the results of two‐dimensional thermo‐mechanical numerical simulations of the formation of kilometre‐scale shear zones and tectonic nappes which are generated by thermal softening during horizontal shortening. This model is thermo‐mechanically coupled and conserves energy by converting mechanical work into heat. The results show that in homogeneous material, the temperature increase in shear zones caused by viscous heating can be ~100 °C. The characteristic width of the thermally perturbed part of the deforming zone is three times larger than the width of the corresponding shear zone because of diffusive heating of the wall rock. Therefore, temperature variations initiated by shear heating may be difficult to identify based on petrological inferences of field data. During lithospheric shortening with a bulk rate of 2.5 × 10 −15 s −1 , a nappe forms in the crust at a depth of ~45 km. Peak temperatures in the nappe reach ~600 °C (~100 °C increase due to shear heating). Significant shear heating occurs only locally and is not active continuously in time along the entire crustal shear zone. Peak values of the maximal principal stress locally reach 2.5 GPa in the nappe. Peak values of temperature and maximal principal stress increase locally to ~770 °C (~300 °C increase due to shear heating) and ~3.8 GPa, respectively, if the applied bulk shortening rate is 5 × 10 −15 s −1 and if higher effective viscosities for the crust are applied. In the model lithosphere, high differential stresses (>500 MPa) and significant stress deviations from the lithostatic pressure (>50%) occur only locally and are transient. The highest average effective viscosity of the model lithosphere is ~2 × 10 22 Pa s, which is in agreement with the estimates for the continental lithosphere. A characteristic feature of the presented dynamic nappe model is a significant stress decrease and cooling within a hundred thousand to 2 million years. These rates of decompression and cooling are discussed in the context of microstructural and metamorphic observations. The results also show that differential stresses in mature shear zones can be several hundred MPa smaller than at the onset of shear zone formation, which suggests that estimates based on microstructural analysis of rocks in ductile shear zones provide minimum values. The presented model could explain the formation of coherent Alpine nappes with inferred high‐pressure (1.5–2.5 GPa) and ultrahigh‐pressure (>3 GPa) conditions in 40–60 km depth at the base of an orogenic wedge.