Abstract
Integrated field data, microstructural and three-dimensional strain analyses are used to document coaxial N-S shortening and southward increase in deformation intensity and metamorphism at the Jiaochang structure. Two episodes of deformation (D1,D2) with localized post-D2 deformation have been identified in the area. The first deformation (D1) episode is defined by a main axial-plane of parallel folds observable on a micro- to kilometer-scale, while the second episode of deformation (D2) is defined by micro-scale metamorphic folds, associated with E–W oriented stretching lineation. These processes are the result of Indosinian tectonism (Late Triassic to Early Jurassic) characterized by nearly coaxial N-S compression and deformation. This is indicated by E–W trending, sub-parallel to parallel foliation (S1, e.g. axial-plane of folds, and S2, i.e. axial-plane of metamorphic folds, crenulation cleavage) and lineation (L1, e.g. axis of folds, and L2, i.e. stretching lineation, axis of metamorphic folds and B-axis of echelon lens). Most of the porphyroblasts and minerals (e.g. pyrite, biotite) show two growth phases with localized growth in the third phase (muscovite). The progressive D1–D2 structure is widespread in the south of the Jiaochang area, but only D1 structure crops out at the north. The strain intensity (γ), compression ratios (c%) and octahedral strain intensity (εs) are similar across the Jiaochang structure (i.e., γ ≈ 1.8, c ≈ 27%, εs = 0.9), showing a broad range of Flinn values (K = 0.77 to 7.57). The long-axis orientations are roughly symmetric between two limbs of the structure. Therefore, we suggest that the architecture of the Jiaochang structure has been controlled by coaxial N-S shortening and deformation (D1–D2) during the Indosinian tectonic epoch, with insignificant post-D2 deformation.
Highlights
The oroclinal bending, which is one of the most striking features in the Songpan-Ganzi fold belt, shows an obvious change in the orientation of structure from northwest direction in the west, to EW direction and northeast direction farther to the east near the border with Longmenshan (Fig. 1)
We present new observations based on field data, microstructure and three-dimensional strain analyses from the Jiaochang structure at the north Songpan-Ganzi fold-belt
The localized post-D2 mineral growth indicates that post-D2 deformation had insignificant impact on tectonic architecture of the Jiaochang area. It suggests that the progressive D1–D2 deformation predominantly accounts for the Jiaochang structure
Summary
The oroclinal bending, which is one of the most striking features in the Songpan-Ganzi fold belt, shows an obvious change in the orientation of structure from northwest direction in the west, to EW direction and northeast direction farther to the east near the border with Longmenshan (Fig. 1) It includes Zoige, Baima, Jiaochang, Xiaojin, Yajiang, and Muli structures in the SongpanGanzi area. Most of the authors recognized the significance of sinistral transpression along Maowen-Wenchuan fault [2,4,5], Xu et al [6] argued that the N-S and E-W deformation took place along different boundaries They suggested that the deformation is result of a closure of Qiangtang, North China and South China Blocks, being the dominant processes forming the curvature structures. It is dissected by the duplex thrusts, which allows roughly subdivide the Jiaochang structure into three parts, which from the south to the north are: the apex, the wedge and the limbs
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