Mesozoic Crustal Thickening of the Longmenshan Belt (NE Tibet, China) by Imbrication of Basement Slices: Insights From Structural Analysis, Petrofabric and Magnetic Fabric Studies, and Gravity Modeling

Abstract

This work first presents field structural analysis, anisotropy of magnetic susceptibility (AMS) measurements, and kinematic and microstructural studies on the Neoproterozoic Pengguan complex located in the middle segment of the Longmenshan thrust belt (LMTB), NE Tibet. These investigations indicate that the Pengguan complex is a heterogeneous unit with a ductilely deformed NW domain and an undeformed SE domain, rather than a single homogeneous body as previously thought. The NW part of the Pengguan complex is constrained by top‐to‐the‐NW shearing along its NW boundary and top‐to‐the‐SE shearing along its SE boundary, where it imbricates and overrides the SE domain. Two orogen‐perpendicular gravity models not only support the imbricated shape of the Pengguan complex but also reveal an imbrication of high‐density material hidden below the Paleozoic rocks on the west of the LMTB. Regionally, this suggests a basement‐slice‐imbricated structure that developed along the margin of the Yangtze Block, as shown by the regional gravity anomaly map, together with the published nearby seismic profile and the distribution of orogen‐parallel Neoproterozoic complexes. Integrating the previously published ages of the NW normal faulting and of the SE directed thrusting, the locally fast exhumation rate, and the lithological characteristics of the sediments in the LMTB front, we interpret the basement‐slice‐imbricated structure as the result of southeastward thrusting of the basement slices during the Late Jurassic‐Early Cretaceous. This architecture makes a significant contribution to the crustal thickening of the LMTB during the Mesozoic, and therefore, the Cenozoic thickening of the Longmenshan belt might be less important than often suggested.

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

“…These observations deviate from the top‐to‐NW ductile movement documented in the western part of the Pengguan massif along the WSZ by some previous study (Xu et al, ; Xue et al, ). 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, ).…”

“…In the southern LMS, only one major ductile deformation phase is observed and is defined by field structures indicating a top‐to‐east (top‐to‐the foreland) shearing associated with greenschist facies metamorphism, in contrast with the Late Cretaceous to Paleogene, top‐to‐west (top‐to‐the hinterland) shearing previously proposed (Tian et al, ). However, these divergent observations can coexist as documented in the Pengguan massif during the Early Cretaceous (Xue et al, ). They are both compatible indeed with a phase of shortening and crustal thickening of the basement and the sedimentary cover.…”

“…An increasing number of studies propose that part of the crustal thickness of the SPG terrane and of the central and southern Tibetan plateau is acquired during the Mesozoic (e.g., Airaghi, Lanari, et al, ; Airaghi, de Sigoyer, et al, ; Billerot et al, ; Kapp et al, , ; de Sigoyer et al, ; Xue et al, ). Metamorphic and structural studies carried out in the Danba area (~ 00 km west of the LMS) and in the central LMS (Airaghi, Lanari, et al, ; Billerot et al, ; Dirks et al, ; Harrowfield & Wilson, ; Weller et al, ; Worley & Wilson, ) as well as 40 Ar/ 39 Ar ages on clay mineral in pseudotachylites of the BF (Zheng et al, ) suggest a compressive phase of deformation during the Late Triassic‐Early Jurassic, driven by the closure of the Paleotethys.…”

Structure and Metamorphism of Markam Gneiss Dome From the Eastern Tibetan Plateau and Its Implications for Crustal Thickening, Metamorphism, and Exhumation

“…200 Ma (Zheng et al, ). A fault of the Longmen Shan Thrust system separates the two regions with different burial depth of the Neoproterozoic basement (Xue et al, ). Although these gneiss domes are cored by different materials and structures, they record coeval deformation, similar P‐T paths and not far from each other; therefore, we proposed they formed by similar mechanism.…”

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.…”