Late Cretaceous–earliest Paleogene deformation in the Longmen Shan fold‐and‐thrust belt, eastern Tibetan Plateau margin: Pre‐Cenozoic thickened crust?

Abstract

This study presents structural and 40Ar/39Ar geochronological data from the southern part of the Longmen Shan fold‐and‐thrust belt that forms the eastern margin of the Tibetan Plateau. Investigations focused on hinterland ductile top‐to‐the‐WNW shear deformation, which has been linked previously to late Cenozoic lower crustal flow. Consistent with previous studies, the sense of deformation is mapped as top‐to‐the‐WNW in the Longmen Shan hinterland. The timing of the deformation is constrained by 40Ar/39Ar geochronological data of recrystallized minerals aligned along the shear foliation as Late Cretaceous–earliest Paleogene, thus predating the inferred late Cenozoic crustal flow. This deformation is contemporaneous with SE verging thrusting and loading along the Longmen Shan front, which formed a coeval ~2–3 km thick foredeep sequence along the southwestern margin of the Sichuan Basin. In the context of the regional geology, this tectonic configuration could result from either extrusion of a crustal wedge or back thrust in a duplex. Compared to other orogens, where similar crustal configurations have been reported, it is speculated that the eastern Tibetan Plateau margin acquired thickened crust and highly elevated topography in Late Cretaceous–earliest Paleogene time.

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

“…The inset shows the 40 Ar/ 39 Ar white mica ages for sample ba14–15 sorted by increasing age. (c) P ‐ T lines obtained from the chemical compositions of white mica analyzed by Tian et al () for samples similar to ba15–12. (e) P ‐map obtained with the barometer of Massonne and Schreyer () for a white mica‐bearing layer in sample ba15–12.…”

“…The WF and BF are thought to be inherited from the Paleozoic, when the South China block represented a passive margin (Burchfiel et al, ; Chen & Wilson, ; Roger et al, ) structured in tilted blocks (Jia et al, ). In the southern LMS, the Wulong and the Xiaoguanzi faults are often considered as the southern prolongation of the WF and BF, respectively (e.g., Cook et al, ; Tian et al, ), while the Shuangshi fault has been proposed to be the continuity of the GF (Lichun et al, ; Figures b–d).…”

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

“…Yet the estimated shortening rates across the margin over geodetic timescales (<3 mm/yr; Shen et al, ) is not consistent with this significant topographic signal. Due to these apparently conflicting observations, the eastern Tibetan margin has been a priority target for investigations related to orogenic processes (Airaghi et al, ; Burchfiel et al, ; Clark & Royden, ; Clark, House, et al, ; Guo et al, ; Hubbard & Shaw, ; Kirby et al, ; Royden et al, ; Robert et al, ; Tian et al, ; ), active continental deformation (Burchfiel et al, ; Densmore et al, ; De Michele et al, ; Feng et al, ; Kirby et al, ; Qi et al, ; Ren et al, ; Shen et al, ; Wang et al, ; Xu et al, ), and interactions between tectonics and surface processes (Ansberque et al, ; Clark, House, et al, ; Godard et al, ; Kirby et al, ; Liu‐Zeng et al, ; Ouimet et al, , ; Parker et al, ; Tian et al, ; Wang et al, ).…”

“…Impingement of the channel flow against the Yangtze craton would imply simultaneous normal and thrust faulting on both sides of the Longmen Shan (Royden et al, ). Yet unlike in the Himalayas, evidence for structures accommodating large‐scale normal‐sense motion has not been unambiguously reported in the Longmen Shan (Tian et al, ). Furthermore, recent seismic studies (e.g., Guo et al, ), which image crustal‐scale structures and faults, question the channel flow model.…”

Constructing the Eastern Margin of the Tibetan Plateau During the Late Triassic

“…Although our data for Late Triassic crustal thickness in the eastern Tibetan Plateau cannot track possible changes of thickness during the Jurassic and Cretaceous, thermochronological studies on magmatic rocks in the SGFB suggest that a large part of the eastern Tibetan Plateau experienced long‐term tectonic quiescence from Jurassic to early Cenozoic (Kirby et al, ; Roger et al, ). The eastern margin may have been further thickened as evidenced by Late Jurassic to Early Cretaceous crustal imbrication (Xue et al, ) and Late Cretaceous deformation (Tian et al, ) documented in the Longmen Shan Fault Belt. Therefore, our results indicate that the construction of the eastern margin of the Tibetan Plateau was initiated during Late Triassic orogenesis.…”