Brittle-ductile transition zone case study (Zagros-Iran)

A three-dimensional lithosphere model with horizontal dimensions of 1500 km x 600 km and a depth extent of 70 km is constructed from available geophysical data to find out strength (as the maximum deviatoric stress the lithosphere supports at a given depth) in Arabia-Eurasia plate boundary zone. The constructed model is first used in the calculation of thermal initial condition and then strength envelopes were calculated using the obtained geotherm and rheological flow laws. Numerical models are run for initial temperature distributions representing cold and hot lithosphere with two different rheology (wet and dry). The obtained mechanical structures were analyzed and compared with other geophysical data; e.g., seismicity-depth and isotherm-depth distributions. Model results for wet and dry rheology indicate that the depth of the brittle-ductile transition is about 8 km and 11 km for hot geotherm and 10.5 km to 14 km and 21-28 for cold geotherm. The results are in good agreement with focal depth in the Zagros that most earthquakes occur in 815 km depth i.e. the long-term strength of the continental lithosphere resided only in its upper part, which was contained wholly within the crust. The ILSs (integrated lithospheric strengths) values for a wet crustal rheology have high range of 1.4-10 x 10(12) N m(-1), which is mainly the result of a low heat-flow (cold geotherm) and 0.1 x 10(12) N m(-1) for wet rheology and hot geotherm. (C) 2012 Elsevier Ltd. All rights reserved.