Figure 1.
Geological map showing the curvature structures in the Songpan-Ganzi fold-belt.
Detailed structural map of the region marked with a green box is presented in Fig. 2.
Figure 2.
Geologic and structure map of the Jiaochang area in the Songpan-Ganzi fold-belt.
All stereonets are lower-hemisphere equal area projection, showing the S1 foliation. The solid circles and triangles in the stereonets stand for F1 hinge and L1 intersection lineation, respectively.
Figure 3.
Field photographs and photomicrographs from the southern region of the Jiaochang structure.
a–b) D1 E–W striking, parallel folds of centimeter to meters scale in the Middle Triassic strata. c) metamorphic polyharmonic -folds (S0//S1) in the Middle Triassic strata. d) Sub-parallel multiphase foliations (S1 and S2) and the second lineation (F2). The stereonet shows NE∼NEE trending F2 lineation. e) Rotated helicitic structure (porphyroblast) of biotite in lepidoblastic quartz-mica schist. The quartz inclusions (Si) rotated about 75° with respect to matrix minerals (Se) during growth, indicating progressive deformation and metamorphism. f) Grain Boundary Migration (GBM) recrystallization in lepidoblastic to equigranular mica-quartz schist. The micas (pervasive metamorphic foliation S2) pin some grain boundaries in quartz minerals and were later crenulated to form crenulation cleavage (S3).
Figure 4.
Field photographs and photomicrographs from the central region of the Jiaochang structure.
a) A-axis elongated lineation (L2) in upper Paleozoic marble. The stereonet shows the westward plunging lineation (elongated calcite) with ∼276°∠36°(trend and plunge). b) Shearing echelon lens with nearly E–W trending B-axis in Middle Triassic marble. c) Sub-parallel bedding veins formed during northward thrusting to accommodate asymmetric fold. Many of these veins are asymmetrically shortened and sheared to form pinch-and-swell structures and are potential kinematic indicators. d) Dragged recumbent fold in Upper Paleozoic with E–W-strike and horizontal plunge. e) Sub-parallel multiphase foliation of the first foliation (S1) and later crenulation cleavage (S3) in Devonian. The stereonet shows the NNW-ward pole of foliations. f) Growth and deformation twins in calcite with deforming and tapering edges, indicating deformation temperatures of ca.300∼400°C.
Figure 5.
Field photographs and photomicrographs from the northern region of the Jiaochang structure.
a) Fault-related fold with E-W-strike and horizontal plunge is result of northward thrusting. b) Conjugated joint. The rose diagram of restored joints shows the latest N–S compression. c) Widespread parallel-layer shearing veins, showing southward thrust and deformation. d) Sub-parallel bedding veins with high-angle plunge, indicating sinistral strike-slip in western limb of the Jiaochang. e) Subgrain Rotation (SGR) recrystallization in equigranular mica-quartz schist, indicating medium deformation temperatures of ca. 400∼500°C. f) Sub-symmetric eye structure (porphyroblast) of biotite in lepidoblastic mica-quartz schist, showing no or little sense of the shear. Biotite grain shows crenulation cleavage and later fracture.
Figure 6.
Geological map of the Jiaochang structure showing the major structures and locations with kenematic data.
The azimuth of ellipsoid long-axis orientation of the three-dimensional finite strain (bold arrows) indicates the maximume stretching direction and also provides local transport direction. All stereonets are lower-hemisphere equal area projection.
Figure 7.
Flinn diagram with contours of Flinn K-values and natural octahedral strain magnitude, εs, from the Jiaochang structure.
Note that most samples cluster around K = 5, between εs = 0.8 to εs = 1.0.
Table 1.
Three-dimensional strain data in the Jiaochang structure.
Figure 8.
Photomicrographs of metamorphic minerals and of multiphase foliation in the Jiaochang area.
a) Biotite porphyroblasts in lepidoblastic quartz-mica schist, showing pervasive schistosity (S2) and crenulation cleavage (S1). Note that the quartz inclusion (S0) is weakly oriented with no or little elongation, which is different than the early deformation (D1). The inclusion-poor zones in biotite porphyroblast represent the differentiated limbs of microfolds, showing that the crenulation cleavage in the early matrix has been destroyed since the porphyroblast formed. Biotite porphyroblast shows undulose extinction and is wrapped by deflected matrix mineral in contrast to none-deflection of wrapping mineral (post-D2). b) Multiphase growth of biotite, pyrite and chlorite porphyroblasts. Pre-D2 porphyroblast is wrapped by deflected matrix mineral in contrast to post-D2 porphyroblast. c) Multiphase growth of biotite, garnet and chlorite porphyroblasts. Post-D2 chlorite without strain shadow and deflection of wrapping mineral superposes early undulose extinction biotite (pre-D2). The elongated inclusion (S1) is not continuous with matrix minerals (S2, metamorphic foliation of muscovite and biotite) outside of the porphyroblast. Note that the garnet was superposed by late brittle fracture (conjugated joint). d) Multiphase growth of biotite, pyrite and chlorite porphyroblasts. S2 shows clear deviation around the metamorphic crystal (syn-D2 biotite and pyrite, the latter has local strain-shadow) in which the inclusion is continuous with matrix minerals. There is difference between the pre-D2 and post-D2 chlorite minerals, post-D2 chlorite without strain-shadow cuts across early pervasive foliation (S2). Symbols: (bi) = biotite, (ch) = chlorite, (gt) = garnet, (pyr) = pyrite, (mus) = muscovite.
Figure 9.
Schematic representation of pre-, inter-, syn, and post-tectonic prophyroblast and mineral growth, in which the D1,D2 and D3 correspond to the multiphase deformations of foliation.
Pyrite is divided into pre- to syn-D1 and pre- to syn-D2. Biotite is divided into syn- to post D1, and syn-D2. Chlorite and garnet are pre- to post-D2, and pre- to syn-D2, respectively. Furthermore, muscovite is divided into three growth phases, syn-D1, and syn-D2 and syn-D3.
Figure 10.
Diagram of strain pattern and mechanism of formation of orogen curvature.
a) Oroclinal bending, strain ellipse remains the same but varies in orientation without undergoing tangential extension (after Ries and Shackleton [26]); b) Simple shear along boundary, strike-slip shearing along one limb of orogen curvature causes stretch and deformation increase toward the boundary fault (after Marshak [7]); c) Differential transport, oroclinal bending and tangential extension resulted from differential transport to the foreland. It could accommodate axis rotation and strain decrease toward the foreland (after Marshak [7]); d) Finite strain in Jiaochang, showing tangential extension and strain increase toward the apex.