Comment on “Sandbox modeling of evolving thrust wedges with different preexisting topographic relief: Implications for the longmen Shan thrust belt, eastern Tibet” by C. Sun et al.

Abstract

Sun et al.'s (2016) sandbox modeling takes preexisting topographic relief into account for faulting activity at the compressional orogenic belt, which promotes better understanding of Longmenshan thrust belt's (LTB) orogenic process and faulting behavior. However, topographic relief in sandbox modeling is contradictory to central LTB's actual topographic relief, and as a result, both the comparison between sandbox modeling and actual tectonics and topography and the conclusion that Xiaoyudong Fault is a tear fault should be reconsidered. Actually, based on Sun et al.'s (2016) sandbox modeling, the Xiaoyudong Fault cannot be treated as a tear fault.

Topographic Loads Modified by Fluvial Incision Impact Fault Activity in the Longmenshan Thrust Belt, Eastern Margin of the Tibetan Plateau

“…Compared with the HKSS, the higher topography in the LMSS causes stronger normal stresses on the BYF (Figure ), which increases the friction along this subsegment (given a uniform coefficient of internal friction in the central LTB; Dahlen et al, ). Since it is not favorable to accommodate the strains solely on the BYF, thrust slips are thus partitioned onto the JGF in the LMSS, resulting in the along‐strike variation of the long‐term activity and coseismic slip on the JGF (Lu et al, ; Tan, Xu, Lee, et al, ; Tan, Xu, & Lu, ). Therefore, the topographic variations in the hanging‐wall block of the BYF play a major role in producing the long‐term fault activity and coseismic slip differences between the LMSS and HKSS.…”

“…The Xiaoyudong fault is an enigmatic secondary structure in the central LTB that marks the boundary between the two subsegments (Figure ; Chang et al, ; Y. Li et al, ; Liu‐Zeng et al, ; Sun et al, ; Tan et al, ; Tan, Xu & Lu, ). The 8‐km‐long fault generally strikes northwest and dips to the southwest, with local, short wavelength fluctuation on the surface trace.…”

“…Previous studies also highlighted the relationships between the topography and fault activity by lateral comparison between the northern, central, and southern segments of the LTB (Gao et al, ; Sun et al, ; H. Zhang et al, ; Figure ). However, the chicken‐or‐egg debate still holds due to the lack of a control group in these studies, because both the landscape and total shortening strains vary, sometimes significantly, among the segments of LTB, which weakens the argument for or against either mechanism (Godard et al, , ; Tan, Xu, Lee, et al, ; Tan, Xu, & Lu, ; E. Wang et al, ). In fact, a large spatial scale tends to reduce the consistency of both the external and internal forces, particularly the latter one, because of the structural and mechanical heterogeneities within the crust.…”

Reply to comment by Tan et al. on “Sandbox modeling of evolving thrust wedges with different preexisting topographic relief: Implications for the Longmen Shan thrust belt, eastern Tibet”

“…In the comment, Tan et al . [] conducted topographic analysis in the central Longmen Shan and provided two along‐strike topographic profiles located in the hanging wall and footwall of Yinxiu‐Beichuan fault separately. They concluded that southwest block of the Xiaoyudong fault is higher than its northeast counterpart.…”

“…The superficial transfer zone could be estimated more than 20 km, corresponding to the 20 km wide aftershock zone of Wenchuan earthquake in the same position [ Chang et al ., ]. However, the strike‐slip deformation represented by such a transfer zone in the Longmen Shan thrust belt would be more overestimated at a smaller length scale, for example, the 50 km scale [ Tan et al ., ], while the Longmen Shan is dominated by compression, not strike‐slip deformation. It is also the foundation of our model design.…”