Rapid slip along the central Altyn Tagh Fault: Morphochronologic evidence from Cherchen He and Sulamu Tagh

Abstract

1] To better constrain the ongoing rates of deformation in northern Tibet, the ages of fluvial and glacial geomorphic markers left-laterally displaced by the Altyn Tagh Fault have been determined by radiocarbon and 10 Be-26 Al cosmic ray exposure dating. Two sites were investigated: Cherchen He and Sulamu Tagh, both near Tura ($37.6°N, 86.6°E). The sites are geomorphologically distinct with Cherchen He dominated by fluvial processes and the Sulamu Tagh by glacial action. Nine offsets ranging from 166 to 3660 m with ages between 6 and 113 ka yield an average slip rate of 26.9 ± 6.9 mm/yr. Landscape evolution appears to have been modulated by climate change and is temporally consistent with the d 18 O record from the Guliya ice cap in the West Kunlun; the features of interest were all formed by glacial and fluvial processes subsequent to marine isotope stage 5e, with the youngest features having formed during the early Holocene Optimum. This ''near-field,'' morphochronological slip rate is averaged over many earthquake cycles and is hence little affected by interseismic strain. It is kinematically consistent with other, somewhat lower, geomorphic slip rate measurements to the east. The average rate, and lower bounds obtained from alternate interpretational models, 18.4 mm/yr, cannot be reconciled with the most rece geodetic measurements ($7 mm/yr), suggesting that interseismic strain and interactions with adjacent faults may lead to disparate geologic and geodetic rate estimates. This late Pleistocene-Holocene, morphochronologic rate would imply that, at this longitude, the Altyn Tagh Fault, on the north edge of Tibet, might absorb almost as much of India's convergence relative to Siberia as the Himalayan Main Frontal Thrust does on the southern edge of the plateau.

Crustal–lithospheric structure and continental extrusion of Tibet

“…However, many Quaternary-derived estimates from cosmogenic and radiocarbon dating of offset terraces have resulted in estimates 2-3 times greater (e.g. Meriaux et al 2004). Furthermore, many of these latter observations have been subsequently reinterpreted (e.g.…”

“…Gold et al 2009) constrained by cosmogenic exposure dating of displaced surface features indicate that the Altyn Tagh Fault has an average millennial slip of 20-30 mm a À1 (e.g. Meriaux et al 2004). This high rate contrasts with many of the longer-term geological or decadal rates that indicate a slip of 5-10 mm a À1 .…”

“…Consequently, previous cosmogenic age data of displaced fluvial risers would carry such large errors that the derived slip rate will inevitably be within error of geodetic slip estimates for the Altyn Tagh Fault. Rapid slip has been suggested to occur at 20-34 mm a À1 (Meriaux et al 2004) and 14-20 mm a À1 (Meriaux et al 2005) from such riser offset dating at 878E and 948E. By applying newly derived geomorphological indices to the Cherchen He sites of Meriaux et al (2005), Cowgill (2007) suggested a revised slip rate of 9.4 AE 2.3 mm a À1 (Table 3), one-third of the original interpretation and within error of those derived from GPS and palaeoseismic studies for this region.…”

Irregular earthquake cycle along the southern Tianshan front, Aksu area, China

“…To account for possible different exposure histories, we performed multiple samplings on each surface [e.g., Van der Woerd et al, 1998, 2002Lasserre et al, 2002;Mériaux et al, 2004]. We sampled large embedded cobbles by breaking pieces from their top surfaces.…”

“…These exposure ages are in agreement with the relative height of the surfaces, their degree of weathering, and the height of the fault scarp uplifting them. Sample ages on the terraces tend to group into well-defined clusters whereas the moraine shows more scattered ages, as observed in other fluvioglacial settings [e.g., Owen et al, 2003;Mériaux et al, 2004]. To reduce age scattering, we defined outliers having a potential complex exposure history.…”