Résumé
We have established a new survey mode GPS network across the Altyn Tagh Fault in the northern central Tibetan Plateau. This network consists of 19 sites between 34 degrees N and 39 degrees N and 85 degrees E and 86 degrees E. From 2009 to 2011, the network has been surveyed for 2 to 3 times. Each site has been surveyed for at least 48 hours with Trimble NetRS receivers and zephyr geodetic antennas. By combining the nearest Continuous GPS sites (KIT3, TASH SELE, POL2, URUM, GUAO, ULAB, IRKT, BJFS, XIAN, KUNM, LHAS, HYDE, IISC), we process the observing data with GAMIT/GLOBK software to obtain the velocity field of the network. From the velocity field, we find that the interseismic strain along the approximately 400-km length profile is restricted predominately within a very narrow zone with width of about 40-50 km across the Altyn Tagh Fault, thus the crustal blocks on both sides of the fault experience a minimum deformation. We use a screw dislocation model to mimic the velocity field by assuming that the Altyn Tagh Fault is distributed as a simple vertical plane. The preliminary results suggest that for the central segment of the fault at about 86 degrees E, the left-lateral slip rate of the Altyn Tagh Fault is of about 8-9 mm/yr. This is significantly lower than the earlier estimates of the slip rate based on offset of geological markers located 15 km east of the profile, but in agreement with the recently revised geological slip rate of this Altyn Tagh fault segment. Associated with the predicted slip rate, the locking depth of the Altyn Tagh Fault is shallow with a value of about 8-10 km. By correlating with available geodetic results around this large-scale active strike-slip fault, our findings suggest that the slip rate of the Altyn Tagh Fault on its central segment ( approximately 500-km long) is quite uniform with a value of about 8-9 mm/yr. The predicted slip-rate pattern, together with the localized strain distribution across the fault and the very shallow locking depth of the fault as revealed by the new GPS network highlights that crustal deformation on north side of the Tibetan Plateau could be described as thin rigid blocks divided by active strike-slip fault systems.