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

Abstract

[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 . In the past the lithosphere strength beneath Tien Shan was ∼10 12 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.Citation: Bagdassarov, N., V. Batalev, and V. Egorova (2011), State of lithosphere beneath Tien Shan from petrology and e...

Influence of temperature, pressure, and oxygen fugacity on the electrical conductivity of dry eclogite, and geophysical implications

“…When considering our new results for the influence of pressure on the electrical conductivity of eclogite, we compared our data with those from previous studies [e.g., Laštovičková and Parchomenko , ; Čermák and Laštovičková , ; Bagdassarov et al ., ], as shown in Figure . It is clear that our results plot in a higher position than other data, and some discrepancies exist between the earlier data and ours.…”

“…Bagdassarov et al . [] used a piston‐cylinder high‐pressure apparatus to investigate the electrical conductivity of the At‐Bashi eclogite (from the lithosphere of the Tien Shan) at 2.5 GPa, 623–1423 K, and with a Mo + MoO 2 oxygen buffer. However, all these earlier studies of the electrical conductivity of eclogite were performed at a defined pressure, and the effects of pressure and oxygen fugacity on the electrical conductivity of eclogite remain unclear.…”

“…It would be usual for eclogite to exist at a depth of ∼100 km in the deep interior of the Earth, and the typical eclogite would be composed of approximately equal volume fractions of garnet and omphacite [ Zheng et al ., ; Tang et al ., ]. Therefore, there have been many studies on the electrical conductivity of natural eclogite [e.g., Laštovičková and Parchomenko , ; Čermák and Laštovičková , ; Bagdassarov et al ., ]. Measurements of the dielectric constant and the electrical conductivity for both direct and single‐frequency alternating currents in eight natural eclogites from the Bohemian Massif were conducted by Laštovičková and Parchomenko [] under pressures up to 2.0 GPa, temperatures of 473–1173 K, and for different mineralogical compositions.…”

Basin evolution in a folding lithosphere: Altai–Sayan and Tien Shan belts in Central Asia

“…The Moho depth progressively deepens northwards under the Tien Shan from 45-50 to 60 km, with a 16-25 km thick upper brittle part and a 30-35 km ductile lower part . In combination to crustal shortening, Bagdassarov et al (2011) used xenoliths to show that the Tien Shan lithosphere thickened by an average of 25± 5 km since the onset of shortening, 20-30 Ma ago. Combining electric resistivity patterns with gravity field, Bragin et al (2001) and Tychkov et al (2008) indicate the presence of a Moho 5-7 km deeper beneath Lake Issyk-Kul than under the neighboring areas.…”

Lateral change of sources for the Cretaceous-Paleogene magmatism of the Tian Shan

“…The interpretation of a magnetotelluric sounding profile (Bagdasarov et al, 2011;Batalev et al, 2011) relies on the supposed thrusting of the folded struc tures of the Tian Shan over the lithosphere of the Tarim block. Two low resistivity zones were distin guished; one of them was identified as a paleosubduc tion zone extending north of the Naryn basin beneath the Kyrgyz Range, and the other, as a collision struc ture above the lithosphere of the Tarim block (such structures are traced over 200 km south of the Naryn basin).…”