Microstructures and Rheology of Harzburgites from Dongqiao, Northern Tibet

The peridotite section of the Dongqiao ophiolite is one of the few peridotite bodies exposed in Tibet, which may play an important role in revealing the rheological properties of the upper mantle in this area. In this paper, the microstructural and rheological characteristics of harzburgites from Dongqiao are investigated. Microstructural analysis indicates that harzburgites exhibit porphyroclastic texture and undulose extinction and kink bands are common in olivine and orthopyroxene. The oxidation decoration technique is used to examine the dislocation microstructure in olivine. The results show olivines are abundant in dislocation substructures, including free dislocation; dislocation wall, which can be subdivided further into widely and closely spaced types; and dislocation bow and net, indicating that the deformation of olivine may be dominated by dislocation creep. Following the microstructural analysis, two deformation stages are recognized. The first is deformation due to slow upper mantle flow, which results in a wide dislocation wall. The second is deformation related to the emplacement of ophiolite, which generates a closely spaced dislocation wall. The average flow stress calculated is 39.3 MPa (for the wide dislocation wall) and 113.9 MPa (for the closely spaced dislocation wall) respectively. Based on Chopra and Paterson's olivine flow law, the creep rate estimates of the upper mantle in this area are 1.13×10^-12-2.95×10^-11 s^-1 and effective viscosity estimates are 4.44×10¹⁷-1.16×10¹⁹ Pa°s. All these provide fundamental constraints on the rheological properties of the upper mantle in the Dongqiao area.