IMPLICATIONS OF UNIFORM SLIP RATE ALONG THE CENTRAL ALTYN TAGH FAULT AT MILLENNIAL, CENTENNIAL, AND DECADAL TIME-SCALES ON THE CLASSICAL DEBATE BETWEEN MICROPLATE VS. CONTINUUM VIEWS OF CONTINENTAL DEFORMATION

The active, left-slip Altyn Tagh Fault extends for >1300 km along the NW margin of the Tibetan Plateau and is the most important structure accommodating Indo-Asian convergence north of the Himalayas. In contrast to previous work using faulted landforms [1], our data from the Yuemake site (88.51°E, 38.19°N) show that the central Altyn Tagh Fault only slipped at 13 to 9 mm/yr over the last 4 to 6 thousand years. At this site, a fluvial terrace riser is displaced 56 ± 5 m. The age of this landform is bracketed by 14C dates from terraces at the top (5658 to 5889 yr calBP) and bottom (4068 to 3830 yr calBP) of the riser. This is the first tightly bracketed millennial rate from the central Altyn Tagh Fault, and it refutes the previously reported fast rates, the underlying data for which have already been show to permit similar rates of ~10 mm/yr [2, 3]. While a uniform millennial to decadal slip rate along the Altyn Tagh Fault is compatible with analyses of GPS data suggesting that short-term deformation is block-like, but with more blocks than in classic “microplate” models of continental deformation, geologic data and discrepancies in the block geometries between different models indicate that not all present-day block bounding faults are equal. Some are major faults with lengths, slip rates, ages, and total displacements that are a significant fraction of those variables for the whole collision, while others are minor structures [4]. This suggests that there are two types of block boundaries, some are long-lived domain boundaries whereas others are transient block boundaries within the domain interiors. While at any given point in time decadal deformation is block-like, frequent kinematic reorganization of the active fault network within the domain interiors causes these minor faults to turn on and off over time. Thus, the present-day block field observed with GPS is unlikely to provide an accurate reflection of continental deformation at Myr time scales because many of the present block boundaries are likely short-lived. These minor faults should show strong temporal variations in slip rate, whereas the domain boundaries likely remain similar over time.

[1] Mériaux et al. 2004 JGR, 109 10/1029/2003jb002558; [2] Cowgill 2007 EPSL, 254, p239; [3] Zhang et al. 2007 in press, Tectonics, 10.1029/2006TC002014; [4] Tapponnier et al. 2001, Science, 294 p1671.