Abstracts presented at fall AGU in 1999 & 2000






Structure and Earthquake Geology of the Active Altyn Tagh Fault from Lake Wu Zun Xiao Er (38.5N, 90.0E) to Suerkuli Valley (38.8N, 91.5E).


Zachary Washburn (480-965-4053; zack.washburn@asu.edu)
Arizona State University
Geology Dept
Tempe, AZ USA 85257-1404
Ramon Arrowsmith
Zhong YuQiao


The sinistral Altyn Tagh fault (ATF) is one of the key structures in Asian collisional tectonics. To answer questions about the earthquake geology of the ATF, we conducted fault trace mapping and paleoseismic investigations between Wu Zun Xiao Er and Suerkuli Valley. The results of this work document the fault trace structure, offset distribution, min. rupture length, and also provide numerical age constraints for the last 2 earthquakes. We also estimate relative geomorphic ages of surface rupture and min. magnitude of recent events.
We identified 3 geometric segments along the 164-km-long reach that was mapped. From east to west they are 1) the > 87-km-long Suerkuli Valley segment, 2) the 35-km Snowy Mountain segment (38.61N, 90.6E to 38.56N, 60.45E), and 3) the > 42-km Wu Zun Xiao Er segment. The Suerkuli segment ruptures Holocene alluvium and is characterized by a 065ˇ-072ˇ striking, 30-1000 m wide, linear fault zone. The fault zone is dominantly strike-slip, but has active normal faults on its periphery. In contrast to the linear Suerkuli segment, the Snowy Mtn. segment has a highly variable strike (065ˇ-110ˇ) and ruptures through mountainous topography. The 20-150 m wide, active fault zone consists of 1-5 km long, discontinuous strike-slip faults. The active strike-slip trace has a larger, S. side down, dip-slip component than that of Suerkuli. Geomorphic features, such as strongly developed wineglass valleys and triangular facets, also support a significant normal component of slip along this segment. From east to west, the strike of the Wu Zun Xiao Er segment progressively bends from 090ˇ to 050ˇ and then resumes its characteristic 070ˇstrike as it leaves the fractured bedrock of the Snowy Mtn. range and enters Holocene alluvium. In bedrock, the active trace is expressed as 20-100 m long, right stepping, en echelon strike-slip faults. Dilation between en echelon faults is common, but slip vector measurements show that this segment has a lower component of dip slip than the other two segments.
Air photo interpretation combined with field mapping show the Suerkuli segment has continuous surface rupture for > 130km with min. offset values of 2.0 to 11.2 m. The double-stranded mole track has a fresh morphology that is continuous over most of the segment length and implies that the last event probably occurred in the past few hundred years. Seven paleoseismic excavations reveal at least 3 events in Holocene units and confirm that peripheral normal faults are synchronous with the main strike-slip rupture. The surface rupture of the Snowy Mtn. segment appears to be older than that of the Suerkuli segment because the moletrack is geomorphically less distinctive. Minimum offsets of 3.1 to 5.7 m are notably lower than those seen on the Suerkuli segment. Combined field work from the previous year show the Wu Zun Xiao Er segment has a surface rupture length > 79 km. Offsets for this event range from 2.7 to 5.1 m and the moletrack appears to be geomorhically older than Suerkuli, but younger than Snowy Mtn. An excavation exposed lower N-dipping fractured silts that have been faulted twice while the overlying gravel and S-dipping silts are only broken by fractures that cut to within a few cm of the surface. The 1215 to 1295 cal AD 14C date on pieces of buried shrubs above the lower silts and below the gravels provides a min. age for the penultimate event and a max age for the last earthquake. In summary 2 separate events probably ruptured 129 km of the ATF in the field area, within the last 700 years. The long rupture lengths and > 5 m offsets indicate a min. of M6.8 for these events.





Earthquake Geology of the Central Altyn Tagh Fault from Lake Wuzhunxiao (38.4N, 89.9E) to Lapequan (39.1N, 92.5E)


Zachary Washburn (480-965-4053; zack.washburn@asu.edu)
Arizona State University
Department of Geological Sciences
Tempe, AZ USA 85257-1404
Ramon Arrowsmith
Guillaume Dupont-Nivet
Wang Xiao Feng


We conducted fault trace mapping and paleoseismic investigations to reconstruct the earthquake record of the central Altyn Tagh fault (ATF) for the last 3 ka. This work contributes another data set for great strike-slip faults (in addition to the San Andreas and N. Anatolian faults) and the results have implications for which mechanism, lateral extrusion or crustal thickening dominates within the Tibetan Plateau.
In 1999, we identified three geometric segments bounded by left steps along the central ATF and determined that the last earthquake produced 2-7 m offsets. Radiocarbon and infrared stimulated luminescence dates and trench logs of disrupted sediments indicate that the central ATF produced two earthquakes (M7.1 - 7.6) in the last 3000 years, implying a slip rate of 3-13 mm/year. In the following section we use along fault distance to explain the location of segments and important features. The western end of the field area corresponds to kilometer 0 (km 0). In 1999 we determined that the Wuzhunxiao segment extends from km 0-65, the Pingding segment from km 65-85, and the Xorxoli segment from km 85 to > km 147.
This summer, we extended our field work from km 147 to km 253 and found that surface rupture terminated around km 240 and offsets associated with this rupture decreased from 6 to 2 m between km 190 and km 230. This rupture terminates in a straight portion of the fault and is not associated with a geometric boundary. Air photo interpretation combined with field truthing up to km 253 reveal that a 20 degree bend in strike, (080ˇ to 100ˇat km 300) and the development of a second major fault strand (km 304) probably act as geometric boundary for the eastern end of the Xorxoli segment. The lack of a geometric boundary until ~km 300, the similar magnitude of 4-6 m offsets on the new reach (km 147-253) as compared to the 5-7m offsets identified along the Xorxoli segment in 1999 (km 85-147), and the similar geomorphic age of scarps suggest that the most recent event (MRE) in Xorxoli Valley ruptured from km 85 to ~km 240. A Xorxoli MRE with a surface rupture length of 155 km corresponds to a M7.6 earthquake (Wells and Coppersmith, 1994). Excavations along the new reach show strong evidence for the last two and moderate evidence for a third event. A 33m long x 2.5m deep x 2m wide excavation on the edge of a playa revealed the last 3 events and numerous strand lines containing abundant organic matter. The results from the past 2 years of study are consistent with the geodetically determined slip rates for the central ATF (10 mm/yr) and in general favor the model for crustal thickening of Tibet with a diminished role for strain localization along the ATF.