Determination of Stress Orientation in Sabzevar Ophiolite Zone in (khorasan Razavi Province, Iran)

Sabzevar ophiolite zone (SOZ) is in north of Central Iran that separated Kop-e-Dagh Sedimentary basin (in north) from the Central Iran plateau (in South). The evolution and replace time of this ophiolite complex has been respectively in upper Cretaceous and Middle Campanian to Upper Mastrishtian. This ophiolite belt contains a large amount of ultramafic rocks, small masses of Gabbro and thick sequence of submarine basaltic lavas. This study surveys the brittle structures that developed during the mountain building process to decipher the history of polyphase deformation. Analytic inversion techniques enabled us to determine and separate different brittle tectonic regimes in terms of stress tensors. Results indicate that oldest generation of stress on the area after the adoption of the ophiolite (Middle Campanian to Late Mastrishtian), is the tensile stress regime. Average direction of σ1 stress axis in this regime is 251/81. This tension is Because of subsidence of ophiolites after the adoption (located) on ground surface. Middle generation of stress has been strike slip regime and average direction of σ2 stress axis in this regime is 323/62. Finally, the newest generation of stress is compressional regime with an average direction of σ3 stress axis 308/81. In this regime σ1 direction is 183/05 that has caused re activity of main pre-existing structures with N121 direction. Our reconstruction of stress fields suggests an anticlockwise reorientation of the horizontal s1 axis since the located of ophiolites on the ground and a significant change in vertical stress from σ1 to σ3 since the late stage of thrusting. Transition from tension to compression has been strike slip movement. Ratio of elliptical stress in any of the regimes, respectively, from old to new is 0.63, 0.75 and 0.62 that due to overcome of shear regimes, average form of stress elliptical in area, is intermediate between the prolate and oblate forms.