Latest Quaternary slip history of the central Altyn Tagh Fault, NW China, derived from faulted terrace risers.

摘要

The active, left-lateral Altyn Tagh Fault defines the northwestern margin of the Tibetan Plateau (NW China) and is one of several first order strike-slip faults within the Indo-Asian orogen. Based on its length (>1200 km), total offset (360-475 km), initiation age (ca 49 Ma), and depth (upper mantle), it is arguably the most important structure accommodating Indo-Asian convergence north of the Himalaya. To resolve the late Quaternary slip rate along the central (85-90°E) portion of this fault system and also to resolve potential secular variation in slip, I have determined its post-16 ka slip history using morphochronology (the study of dated and displaced landforms). I began by systematically characterizing the uncertainties associated with determining slip rates from laterally faulted terrace risers. One source of previously uncharacterized uncertainty centers on constraining the magnitude of lateral erosion of a displaced terrace riser. I then obtained new morphochronologic data from 8 risers at 4 slip-rate sites: Kelutelage, Tuzidun, Yukuang, and Keke Qiapu. These new data, in combination with a related result from this study at Yuemake (1 riser), tripled the morphochronologic constraints on the ATF faulting history. I analyzed this large volume of data by developing a new Monte Carlo modeling approach for determining (1) a precise average slip rate; and (2) a slip history from which secular variations in slip can be quantified. Results from the modeling yield an average slip rate of 9.1 +/- 1.1 mm/yr from 16.6 +/- 3.9 ka to present, which is the most tightly constrained slip rate reported for the ATF. Furthermore, the analysis reveals a pulse of accelerated strain release in the mid Holocene that is a factor of 3 greater than the average slip rate. This accelerated strain release is interpreted to represent a cluster of 2-6, >Mw 7.2 earthquakes in an 800 yr period and is the first such clustered earthquake series detected using morphochronologic techniques. This result highlights the utility of integration and analyzing morphochronologic datasets to quantitatively resolve temporally uniform and secularly varying late Quaternary fault slip records.