STRUCTURAL, PETROLOGIC AND GEOCHRONOLOGIC RESULTS ON THE EVOLUTION OF THE ERTIX THRUST IN THE SOUTH-CENTRAL CHINESE ALTAI

The Ertix fault in the most dominant structure in the Sengor and Natal'in (1996) reconstruction of the Central Asian Orogenic System (CAOS, also the Altaids), which is inferred to have accommodated >2000-km syn-subduction right-slip motion at 510-310 Ma. In the south-central Chinese Altai, the fault is a northeast-dipping southwest-directed thrust. Metamorphic grades change abruptly across the fault, from lower greenschist facies in the footwall to amphibolite facies in the hanging wall. Thermobarometric studies suggest a systematic increase in metamorphic grade upward across the Ertix hanging wall: at 621 ± 26 °C and P = 7.1 ± 0.9 kbar directly above the fault, 659±39°C and 6.4±1.0 kbar ~200 m above the fault, 770±71°C and 9.1±1.2 kbar ~700 km above the fault. Ductile deformation in the Ertix hanging wall postdates ~ 410 Ma and predates ~ 275 Ma as indicated by the U-Pb zircon ages on deformed granitic gneiss and undeformed granodiorite in the hanging wall. Additional constraints on timing of metamorphism and deformation were derived from in-situ Th-Pb dating of monazite inclusions in garnet that displays prograde growth zoning and inclusion trails oblique to the matrix foliation suggesting synkinematic growth. This analysis yields a weighted mean age of 279 ± 9 Ma (MSWD = 8.1). 40Ar/39Ar analysis of muscovite and biotite from a gneiss sample ~700 m north of the fault yields a total gas age of 278 ± 8 Ma and 257 ± 10 Ma, respectively. Multi-domain analysis of a K-feldspar from the Ertix hanging wall suggests that it cooled from ~ 375°C to ~150°C between 275 and 260 Ma. From the above results, we suggest that the Erix fault was a southwest-directed thrust active at 280-250 Ma. This conclusion does not support the strike-slip duplex model of Sengor and Natal'in (1996) which requires the fault to be active during 510-310 Ma as a major right-slip fault along this segment of the CAOS.