State of lithosphere beneath Tien Shan from petrology and electrical conductivity of xenoliths

[1] The shortening of Tien Shan and the evolution of its lithosphere have been evaluated from P-T geothermobarometry of xenoliths and from comparison of their electrical conductivity with conductivities obtained from the inversion of magnetotelluric (MT) data. Spinel lherzolite and granulite xenoliths found in basaltic outcrop Ortosuu represent upper mantle and crust beneath southern Tien Shan. The spinel lherzolite xenoliths correspond to the lithospheric mantle close to the crust-mantle boundary. The studied mantle xenoliths record two types of the upper mantle processes: a low-degree partial melting (about 7–10%) and a cryptic metasomatism. The granulite xenoliths are fragments of the lower crust captured from differing depths. The temperature and pressure estimates of the garnet granulite xenoliths indicate that they were derived from near the crust-mantle boundary. P-T equilibration conditions of mafic granulites and spinel lherzolites infer that the paleogeotherm corresponded to the heat flux 80–85 mW m−2 about 70–66 Myr ago. The present-day heat flux in the region is about 55–60 mW m−2. The position of the Moho discontinuity 70–66 Myr ago was at a depth of 30–35 km, and the present depth of the Moho boundary is 55–60 km. The observed seismic P wave velocities above and below the present-day Moho boundary are 7.3 and 7.9 km s−1, respectively. Elastic P velocities measured on xenoliths samples in laboratory and extrapolated to PT conditions of the Moho discontinuity (1.8 GPa and ∼750°C) are 6.8 km s−1 for the mafic granulite and 8.0 km s−1 for the spinel lherzolite. The electrical conductivity of xenoliths has been measured at 0.8–1.0 GPa and in the range between 500°C and 850°C for mafic granulites and at 1.0–1.5 GPa and from 500°C to 1050°C for spinel lherzolites. The contrast in conductivities between mafic granulite and spinel lherzolite samples under P-T conditions corresponding to the geotherm with the heat flux ∼60 mW m−2 agrees well with the contrast of MT electrical conductivity below and above the present-day Moho boundary at a depth of 55–60 km. Thus, the thickening of the Tien Shan lithosphere is about 25 ± 5 km. Before the shortening of Tien Shan about 20–30 Myr ago the lithosphere was rather hot, with a temperature of 500°C at a 15 km depth and of 850°C at the Moho boundary. In comparison to Tien Shan, the lithosphere beneath the neighboring Tarim Basin was rather cold, 350°C at the depth of 15 km and 500°C at the Moho boundary. These temperature differences were the main factors that caused the mechanical weakening of the Tien Shan crust and upper mantle. The modern strain rates of the Tien Shan shortening are 10−14 × 3 to 10−15 s−1. The total strength of the lithosphere beneath Tien Shan at present is 8–4 × 1012 N m−1. In the past the lithosphere strength beneath Tien Shan was ∼1012 N m−1, which was more than 1000 times weaker in comparison with the lithospheric strength of Tarim Basin at the beginning of the continental compression in the region.