The Akato Tagh bend along the Altyn Tagh fault, northwest Tibet 1: Smoothing by vertical-axis rotation and the effect of topographic stresses on bend-flanking faults

To better understand the mechanics of restraining double bends and the strike-slip faults in which they occur, we investigated the relationship between topography and bedrock structure within the Akato Tagh, the largest restraining double bend along the active, left-slip Altyn Tagh fault. The bend comprises a ∼90-km long, east-west striking central fault segment flanked by two N70°E-striking sections that parallel the regional strike of the Altyn Tagh system. The three segments form two inside corners in the southwest and northeast sectors of the uplift where they link. We find that both the topography and bedrock structure of the Akato Tagh restraining bend are strongly asymmetric. The highest and widest parts of the uplift are focused into two topographic nodes, one in each inside corner of the bend. Structural mapping of the western half of the bend suggests the southwest node coincides with a region of anomalously high, bend-perpendicular shortening. We also find that partitioned, transpressional deformation within the Akato Tagh borderlands is absorbed by bend-parallel strike-slip faulting and bend-perpendicular folding, unlike the thrusting reported from many other double bends. Synthesis of these results leads to three implications of general significance. First, we show that focusing of bend-perpendicular shortening into the two inside corners of a restraining double bend may cause both the bend and the fault to undergo vertical-axis rotation, thereby reducing the bend angle and smoothing the trace during progressive deformation. Such vertical-axis rotation may help explain why fault trace complexity is inversely related to total displacement along strike-slip faults. Second, we calculate four independent age estimates for the Akato Tagh bend, all of which are much younger than the Altyn Tagh system. We use these estimates in a companion study to postulate that the Altyn Tagh and similarly multi-stranded strike-slip systems may evolve by net strain hardening. Third, comparison of the Akato Tagh with other restraining double bends highlights systematic differences in the style of borderland faulting and we speculate that these variations result from different states of stress adjacent to the bends. Strike-slip dominated bends such as the Akato Tagh may form where σ_H = σ ₁,σ_v ⁼ σ₂, and σ_h ⁼ σ₃ whereas thrust-dominated bends like the Santa Cruz bend along the San Andreas fault form when σ_h ⁼ σ₁, σ_h ⁼ σ₂, σ _v ⁼ σ₃. This hypothesis predicts that the style of faulting along a restraining double bend can evolve during progressive deformation, and we show that either weakening of borderland faults or growth of restraining bend topography can convert thrust-dominated bends into strike-slip dominated uplifts such as the Akato Tagh.