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 M_w7.9 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.

This article presents the results of a finite-thickness, three-dimensional (3-D) kinematic model for the fault-related deformation above an active diapir. Based on field examples, the models are (1) circular in map view; (2) offset by radially distributed normal faults; (3) faulted or smooth at the top of the salt; (4) relatively unstrained on the flanks; and (5) highly deformed in the central graben, if present. Field examples suggest that active diapirs may initiate with either two or three master faults crossing near the dome crest. Subsequent evolution depends mainly on whether the master faults cut the salt or detach on the salt top. If the top of the salt is faulted, no minor faults are required except in the central graben. If the top of the salt is unfaulted, additional faulting occurs in the flank grabens. In models with three master faults, very different structural patterns appear in cross sections cut parallel and perpendicular to any of the master faults. The domes are asymmetrical in the fault-perpendicular cross sections, but are symmetrical in the fault-parallel cross sections. The initial fault pattern may influence the piercement efficiency. An active dome with faults offsetting the top of the salt grows more rapidly than a dome with a smooth top and may develop into a passive diapir more rapidly. The model map patterns are used to reinterpret the 3-D structure of the Clay Creek dome, Texas, and the Reitbrook dome, Germany. The model predictions are confirmed by the cross sections, and small, but potentially significant, refinements are proposed.

The organic‐rich lower Silurian shale of the Longmaxi Formation in the Sichuan Basin is the most important target for shale‐gas exploration in China. Most Paleozoic rocks of the Sichuan Basin have experienced extraordinarily pervasive remagnetizations. To test a hypothesized connection between hydrocarbon generation and remagnetization and contribute to shale‐gas exploration in the region, we undertook an integrated magnetic, geochemical, and petrographic study of 160 specimens from the shale. The results suggest that the shale contains a reliable remanent magnetization (Dec = 41.4°, Inc = 40.8°, and α₉₅ = 6.8°). The magnetization predates tilting, and the paleopole plots close to the Late Triassic segment of the south China apparent polar wander path. The rock magnetic data and scanning electron microscope (SEM) observations confirm that framboidal magnetites carry the bulk of the magnetization, which suggest a Late Triassic chemical remanent magnetization in the shale. ⁸⁷Sr/⁸⁶Sr and magnetic analyses indicate that the amount of magnetite was unaffected by fluid alterations around the veins but is strongly covariant with the amount of total organic matter. Moreover, SEM observations reveal possible evidence of the replacement of pyrite framboids by magnetite, probably in the presence of organic acids. These analyses, therefore, suggest that the remagnetization was caused by organic maturation rather than orogenic fluids and that the maturation occurred in the Late Triassic. This timing of organic maturation has been validated by independent modeling studies and provides important constraints on the complex thermal history of the Longmaxi Shale as well as contributing to shale‐gas exploration efforts.