Differential Exhumation Across the Longriba Fault System: Implications for the Eastern Tibetan Plateau

Abstract

The deformation processes at work across the eastern margin of the Tibetan Plateau remain controversial. The interpretation of its tectonic history is often polarized between two deformation models: ductile flow in the lower crust and shortening and crustal thickening accommodated by brittle structures in the upper crust. Many geological investigations on this plateau margin focused on the Longmen Shan, at the western edge of the Sichuan Basin. However, the Longriba fault system (LFS) located 200 km northwest and parallel to the Longmen Shan structures provides an opportunity to understand the role of hinterland faults in eastern Tibet geodynamics. For this reason, we investigate the exhumation history of rocks across the LFS using (U‐Th)/He and fission track ages from apatite and zircon. Results show a significant contrast in cooling histories across the Maoergai fault, the southernmost fault of the LFS. South of the Maoergai fault, the bedrock records a rapid increase in exhumation rate since ~10–15 Ma. In contrast, the area north of the fault has experienced steady cooling since ~25–35 Ma. We attribute this cooling contrast to ~2 km of differential rock uplift across the Maoergai fault, providing the first evidence of activity of the LFS in the Late Cenozoic. Our results indicate that deformation of the eastern Tibetan margin has been partitioned into the LFS and the Longmen Shan over an ~200 km wide block, which should be incorporated in future studies on the region's deformation, and in both above‐mentioned deformation models.

Complex Lithospheric Deformation in Eastern and Northeastern Tibet From Shear Wave Splitting Observations and Its Geodynamic Implications

“…Our SWS observations (Stations s149 to s151; Figures and b) in the Longriba fault vicinity show obvious fault‐parallel FPDs, which is comparable with the relatively high strain rates near the fault (Figure ). The Longriba fault has been an important structure and block boundary within the Songpan‐Ganzi terrane since at least 10–15 Ma (Ansberque et al, ). The fault may contribute to absorb the slip rate of the Kunlun fault and change the crustal kinematics at the eastern Tibetan margin (Gan et al, ; X.‐W.…”

“…GPS measurements (Gan et al, ) show an abrupt change in lateral extrusion velocity crossing the fault. Despite its relatively small scale, the average dextral rate of 5.6 ± 2.0 mm/year and the crustal shorting rate of 0.55 mm/year suggest that the Longriba fault is currently very active (Ansberque et al, ; X.‐W. Xu et al, ).…”

Late Miocene hinterland crustal shortening in the Longmen Shan thrust belt, the eastern margin of the Tibetan Plateau

“…Several studies [Xu and Kamp, 2000;Wilson and Fowler, 2011;Li et al, 2012;Cook et al, 2013;Tian et al, 2013] reported thermochronological data from other parts of the Longmen Shan, which are broadly consistent with the results outlined above. Other studies from adjacent areas reported late Miocene [Clark et al, 2005;Enkelmann et al, 2006;Ouimet et al, 2010;Duvall et al, 2012;Tian et al, 2015], Oligocenemiddle Miocene [Tian et al, 2013[Tian et al, , 2014Shen et al, 2016;Zhang et al, 2016;Ansberque et al, 2018] or Paleocene to Eocene and Miocene [Liu-Zeng et al, 2018] phases of exhumation with spatially variable exhumation rates (Fig. 1b).…”

The Mesozoic Along‐Strike Tectonometamorphic Segmentation of Longmen Shan (Eastern Tibetan Plateau)

“…Both tectonic dynamics can, however, coexist and be consistent with a southeast thrusting during the Early Cretaceous, resulting in the formation of basement‐slice‐imbricated structures (Airaghi et al, ; Xue et al, ; this study). Furthermore, a Cretaceous reactivation is also recorded ~200 km NW of the LMS, in the Longriba fault zone and in the Aba block (Ansberque et al, ; Tan et al, ), suggesting that a large portion of the eastern Tibet remained coupled during Cretaceous exhumation (Ansberque et al, ).…”