Active tectonics of the Beichuan and Pengguan faults at the eastern margin of the Tibetan Plateau

Abstract

The steep, high‐relief eastern margin of the Tibetan Plateau has undergone rapid Cenozoic cooling and denudation yet shows little evidence for large‐magnitude shortening or accommodation generation in the foreland basin. We address this paradox by using a variety of geomorphic observations to place constraints on the kinematics and slip rates of several large faults that parallel the plateau margin. The Beichuan and Pengguan faults are active, dominantly dextral‐slip structures that can be traced continuously for up to 200 km along the plateau margin. Both faults offset fluvial fill terraces that yield inheritance‐corrected, cosmogenic 10Be exposure ages of <15 kyr, indicating latest Pleistocene activity. The Pengguan fault appears to have been active in the Holocene at two sites along strike. Latest Quaternary apparent throw rates on both faults are variable along strike but are typically <1 mm yr−1. Rates of strike‐slip displacement are likely to be several times higher, probably ∼1–10 mm yr−1 but remain poorly constrained. Late Quaternary folding and dextral strike‐slip has also occurred along the western margin of the Sichuan Basin, particularly associated with the present‐day mountain front. These observations support models for the formation and maintenance of the eastern plateau margin that do not involve major upper crustal shortening. They also suggest that activity on the margin‐parallel faults in eastern Tibet may represent a significant seismic hazard to the densely populated Sichuan Basin.

Structural Setting of the 2008 Mw 7.9 Wenchuan, China, Earthquake

“…We resolve no significant shortening on the Wenchuan fault, based on the mapped geometries and the fault and its cutoff relationships. The activity of this fault is unclear: Burchfiel et al (1995) identified active dextral slip, while Densmore et al (2007) found no unambiguous evidence for Quaternary displacement in their field studies.…”

“…Exhumation rates imply that the present relief of the mountain belt has largely formed since the Miocene (Kirby et al, 2002). However, geodetic studies (Chen et al, 2000;Shen et al, 2005;Meade, 2007), as well as geologic and geomorphic observations (Densmore et al, 2007), generally agreed prior to the Wenchuan earthquake that east-west shortening across the range and within the basin was very limited (< 3 mm=yr). These studies also indicate an important component of strike-slip deformation, at least since 10-15 ka.…”

“…Prior to the Wenchuan earthquake, interpretations of satellite imagery suggested active thrusting and strike slip on faults in the Longmen Shan (Tapponnier and Molnar, 1977), and geomorphic observations of offset terraces identified slip (dominantly dextral) on the Pengguan fault within the last 1000 yr, and of offset terraces and other landforms on the Beichuan fault within the last 12-13 ka (Densmore et al, 2007). However, very low seismicity along the range front suggested that the faults in the range were relatively inactive, although several large historical earthquakes (> M7) ruptured the Min Jiang and Huya faults to the north of the Longmen Shan (Chen et al, 1994).…”

Lushan M S7.0 earthquake: A blind reserve-fault event

“…Among these, the long-term vertical slip rate of the BeichuanYingxiu fault, the middle segment of the mountain-central fault, is 1-2 mm/a, and the inferred vertical slip rate of the Longmenshan thrust belt as a whole is as high as 4-6 mm/a [11][12][13]. The NW-trending crustal shortening rate across the Longmenshan thrust belt is inferred by geological study to be 10 mm/a [14], and the balance geological cross-section shows that its total crustal shortening amount is as high as 40%-60% [15].…”

“…The Lushan earthquake occurred along the southern segment of the Longmenshan thrust belt, where there are NE-striking and NW-dipping imbricate reverse faults, namely, from northwest to southeast, the Gengda-Longdong, YanjingWulong, Shuangshi-Dachuan and Dayi faults ( Figure 2). Similar to the tectonic origin of the Wenchuan earthquake [3,26], the Lushan earthquake is considered to be originated from an abrupt slip on a low angle reverse fault of the southern segment of the Longmenshan thrust belt between the Bayan Har Block and South China Block, where local crustal shortening has been dominated as a result of the Bayan Har Block moving southeastward and colliding with the South China Block [3, [10][11][12][13][14][15][16]. Due to the fact that the epicenter of the Lushan earthquake was located at the site where the Coulomb stress increment was calculated by occurrence of the Wenchuan earthquake [5,6], we thus consider that this Coulomb stress increment may have potentially hastened the occurrence of the Lushan earthquake on a critically stressed reverse fault.…”