No abstract

[1] The 2008 M w 7.9 Wenchuan earthquake is a result of ongoing India-Tibet collision and reflects the growth of the Longmen Shan fold-and-thrust belt. In this paper, we construct a 3-D structural model of the geometry of the coseismic faults and related structures of the Wenchuan earthquake by integrating geological investigations, relocated aftershocks, and seismic reflection profiles. In the 3-D structural model, the differences between the southern and northern segments of the rupture are highlighted. The structural transition zone between the two segments contains a major geometric segment boundary, reflecting differences in the structural configuration of the thrust ramp and the tectonic evolution of the fault system, which appears to have localized significant damage from Anxian to Beichuan. Within the northern segment, we identify a transverse structure across which the Beichuan fault plunges under the Tangwangzhai syncline. This boundary corresponds to a marked change in the nature of the surface rupture and is illuminated by a microearthquake sequence perpendicular to the Longmen Shan thrust belt. In the southern segment, our investigations confirm that uplift due to active faulting and folding is largely responsible for the areas of steepest topography. On the basis of this association, the southwestern segment of the Longmen Shan, south of the Wenchuan earthquake, is likely active and presents a significant earthquake hazard, despite the lack of historical earthquakes in this region. This study illustrates the importance of building 3-D models to study active faulting and folding, as well as to assess earthquake hazard.

No abstract

This file includes figures, tables, models divided into seven Data Repository (DR) items: 2013); Q4, Holocene. We highlight the uplift of Q2 (middle Pleistocene Ya'an Formation) and its corresponding age, and then translate this uplift to a slip based on the relationship from Shaw et al., (2002) assuming a fault dip of 33-56°. Final, we obtain the range of slip rate for the RFBT to be 0.13-0.39 mm/yr. Figure DR4: Interpretation of industry seismic reflection profile L2 in the Sichuan basin, modified from our previous study by Jia et al., (2010). The large rectangle denotes the locations of Figure 4a, showing the hanging wall deformation associated with the RFBT. The small rectangle denotes the location of shallow seismic reflection profile B, which is shown on Fig. 4b. The RFBT extends into the basin, and forms the structural wedge, with its wedge-tip at about 8-9 km. The hanging wall of the RFBT contains a kink band, which extend from the ramp to the surface. This kink band is formed by the splay fault on the RFBT, thus the growth triangle on the shallow seismic reflection profile B records the activity on the RFBT. The range of possible splay fault dips is 33°-56°, which is constrained using the dips range of the axial surface (upper limit) and the ramp dips (lower limit) on the RFBT. Figure DR5: Interpretation of industry seismic reflection profile E in the Sichuan basin. The large rectangle denotes the locations of shallow seismic reflection profile C, which is shown on Fig. 4c. A splay fault of the RFBT rises from the ramp to the near surface, and displaces geological Triassic and younger layers in the hanging wall. We interpret this fault propagates to the shallow surface, producing the tip folding, and forming two back thrusts, which are shown on shallow section C. PGT, Pengguan thrust fault; RFBT, Range Front blind thrust, J, Jurassic; T, Triassic. Data Repository Item DR6 Data Repository Item DR7

To understand the effects of substantial topographic relief on deformation localization in the seismically active mountains, like the Longmen Shan thrust belt in the eastern Tibet, sandbox experiments were performed based on the framework of the critical taper theory. First, a reference experiment revealed that the critical taper angle was 12° for our experimental materials. Subsequently, different proto wedges (subcritical (6° in taper angle), critical (12°), and supercritical (20°)) were introduced to cover the range of natural topographic relief, and we used two setups: setup A considered only across‐strike topographic relief, whereas setup B investigated along‐strike segmentation of topography, consist of two adjacent proto wedges. In all experiments, thrust wedges grew by in‐sequence accretion of thrust sheets. Setup A revealed an alternating mode of slip partitioning on the accreted thrusts, with large‐displacement thrust and small‐displacement thrust developing in turn. And contrasting wedge evolutions occurred according to whether the proto wedge was subcritical or critical‐supercritical. In setup B, the differential deformation along the strike produced transverse structures such as tear fault and lateral ramp during frontal accretion. The observed tear fault and its associated thrust system resemble the seismogenic fault system of the 2008 Mw7.9 Wenchuan earthquake. Our experimental results could also explain first‐order deformation features observed in the Longmen Shan. Consequently, we conclude that topographic features, including topographic relief across the range and along‐strike segmentation of topography, contribute significantly to the kinematics and deformation localization in such active mountains.

No abstract

The U–Pb geochronology of 687 detrital zircons from the voluminous Upper Neoproterozoic–Ordovician succession in the Wuyishan Fold Belt of South China reveals a common dominant c. 1200–950 Ma group, indicative of an outboard provenance terrane with a Grenville-age province to the southeast during the late Neoproterozoic–Early Palaeozoic. Compared with coeval samples from the Gondwanan and eastern Laurentian margins, our data show a scarcity of distinctive Gondwanan provenances (c. 650–500 Ma) and reveal some Laurentian signatures. These results argue against the peri-Gondwanan setting for South China during the late Neoproterozoic–Ordovician, instead implying a Laurentian affinity.

No abstract