Late Quaternary and present‐day rates of slip along the Altyn Tagh Fault, northern margin of the Tibetan Plateau

Abstract

Both Global Positioning System (GPS) measurements and studies of Late Quaternary faulting are consistent with a slip rate of ∼10 mm/yr along the central segment of the Altyn Tagh Fault and a systematic decrease in that rate toward the eastern end of the fault. Dates of terraces above and below laterally offset terrace risers yield bounds on Quaternary slip rates that range from those that agree with GPS measurements to values as much as three times faster. We argue that offset terrace risers that are protected by topography upstream of them are more closely dated by the age of the upper terrace than by that of the lower terrace. In some cases, valleys upstream of the fault have been incised into bedrock, and few if any terrace risers can be seen within the valleys. Such streams debouch onto alluviated floodplains or fans that become incised, presumably during climate changes, to create terrace risers. The terrace risers are then displaced so that they lie downslope from bedrock ridges on the upstream side of the fault, and thus the risers become protected from further incision. In such cases, dates of upper terraces should more closely approximate the ages of the risers than those of lower terraces. Such dates yield slip rates of ∼10 mm/yr in the central segment of the fault and decreasing rates eastward. Although we cannot with certainty rule out the higher slip rates along the Altyn Tagh Fault, our analysis does show that viable interpretations consistent with GPS measurements are more likely, at least along some segments of the fault. Not only do these rates support the view that the Tibetan Plateau deforms internally by slip on a distributed network of faults in the shallow brittle crust, and hence behaves as a continuum at depth, but the gradual decrease toward the east also shows that the Altyn Tagh Fault does not separate two effectively rigid lithospheric plates. Correspondingly, the relatively low slip rate and the eastward decrease in slip rate suggest that the Altyn Tagh Fault does not transfer a significant portion of the convergence between India and Asia into northeastward extrusion of the Tibetan Plateau. Thus, large‐scale extrusion of crustal material in India's path into Eurasia seems to be limited largely to the confines of the Tibetan Plateau.

Interseismic Deformation of the Altyn Tagh Fault Determined by Interferometric Synthetic Aperture Radar (InSAR) Measurements

“…In fact, contemporary instrumental recordings also show a low level of seismicity along the ATF. Paleoseismo logical studies suggest recurrence intervals of major earthquakes in the range of 700˘400 and 1100˘300 years for offsets of 4 and 7 m, respectively, consistent with a slip rate of 10 mm/year [79,81,82].…”

“…Only two great historical earthquakes have occurred along the ATF, both in 1924 west of 85˝E [79,80]. In fact, contemporary instrumental recordings also show a low level of seismicity along the ATF.…”

Crustal–lithospheric structure and continental extrusion of Tibet

“…East of 948E, the slip rate decays to ,5 mm a À1 by 998E. Zhang et al (2007) stated that this gradual decrease in slip rate to the east indicates that the Altyn Tagh Fault does not separate two effectively rigid lithospheric plates. Therefore, the overall relatively low slip rate and eastward decrease suggest that a significant proportion of the India-Asia convergence is not transferred into northeastward extrusion of the Tibetan Plateau .…”

“…Yue et al 2003;Zhang et al 2007). 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 2005;Gold et al 2009); (3) modern decadal geodetic measurements of interseismic strain from GPS and InSAR (e.g. Wright et al 2004;Zhang et al 2007). In addition to these three observation-derived slip-rate estimates, attempts have been made to model the deformation of Asia and infer slip rates from these, although many are constrained by GPS observations or Quaternary faulting (e.g.…”