Zheng Yadong

Geometric and temporal evolution of an extensional detachment fault, Hohhot metamorphic core complex, Inner Mongolia, China

The Early Cretaceous Hohhot metamorphic core complex and its master Hohhot detachment fault are ;400 km west of Beijing in the Daqing Shan (Mountains) of Inner Mongolia. The complex developed across the east-trending Yinshan fold-and-thrust belt within ,4 m.y. following cessation of thrusting ca. 125 Ma (see note added in proof in main text). Postcontractional extension was initiated within a mid-crustal zone of mylonitic and ductile shear that was in part controlled by Carboniferous(?) strata sandwiched between its Proterozoic and Archean crystalline basement and an overlying thrust sheet of similar crystalline rocks. The Hohhot detachment fault appears to have rooted into deep, kinematically active levels of the mid-crustal shear zone. Higher, inactive levels of the mylonitic section were transected by the fault and carried upward in its footwall. Geometries of the footwall mylonitic rocks indicate localized ramp-flat geometries of the fault within and across them. The crosscut top of the mylonitic sequence defines a mylonitic front that departs from the gently south dipping detachment fault and dips northward into its footwall. Early Cretaceous extension was widespread elsewhere in northern China, and was particularly pronounced in the Yunmeng Shan core complex north of Beijing. The gravitational collapse of orogenically thickened crust acting in concert with localized centers of deep-seated plutonism appear to have led to the development of isolated metamorphic core complexes within a broad region of more distributed extensional deformation.

The enigmatic Yinshan fold-and-thrust belt of northern China: New views on its intraplate contractional styles

The east- to east-northeast–trending Yinshan belt lies within North China, extending westward at least 1100 km from Chinas eastern coast to Inner Mongolia. This intraplate Jurassic-Cretaceous belt underwent contractional and normal faulting, folding, and contemporaneous terrestrial sedimentation and magmatism. Current views on its contractional deformational style favor relatively limited “thick-skinned” faulting of Archean basement and cover units. These views are challenged, however, by recent discoveries in the eastern part of the belt of south-directed ductile nappe formation and large-displacement (>40–45 km) “thin-skinned” northward thrust faulting, both involving Archean and younger rock units. Collision of the Siberian and North China plates upon closure of a Jurassic and Early Cretaceous Mongolo-Okhotsk ocean more than 800–1100 km to the north may have been responsible for Yinshan north-south contraction. Some patterns of contraction, e.g., Jurassic-Cretaceous ductile nappe formation, appear to have been influenced by a superposed magmatic regime related to westward subduction of a Pacific basin plate beneath the North China plate.

Strain and shear types of the Louzidian ductile shear zone in southern Chifeng, Inner Mongolia, China

The Louzidian ductile shear zone at the south of Chifeng strikes NE-SW and dips SE at low-medium-angles. This ductile shear zone is mainly composed of granitic mylonite, which grades structurally upward into a chloritized zone, a microbreccia zone, a brittle fault and a gouge zone. All these zones share similar planar attitudes, but contain different linear attitudes and kinematic indicators. Finite strain measurements were performed on feldspar porphyroclasts using the Fry method. These measurements yield Fulin indexes of 1.25–3.30, Lode’s parameters of −0.535–−0.112 and strain parameters of 0.41–0.75 for the protomylonite, respectively. These data are plotted within the apparent constrictional field in Fulin and Hossack diagrams. In contrast, for the mylonite, corresponding parameters are 0.99–1.43, −0.176–−0.004 and 0.63–0.82, respectively, and located in the apparent constrictional field close to the plane strain. The mean kinematic vorticity numbers of the protomylonite and mylonite by using three methods of polar Mohr circle, porphyroclast hyperbolic and oblique foliation, are in the range of 0.67–0.95, suggesting that the ductile shearing is accommodated by general shearing that is dominated by simple shear. Combination of the finite strain and kinematic vorticity indicates that shear type was lengthening shear and resulted in L-tectonite at the initial stage of deformation and the shear type gradually changed into lengthening-thinning shear and produced L-S-tectonite with the uplifting of the shear zone and accumulating of strain. These kinds of shear types only produce a/ab strain facies, so the lineation in the ductile shear zone could not deflect 90° in the progressively deformation.

Deformation Localization-A Review on the Maximum-Effective-Moment （MEM） Criterion

The essential difference in the formation of conjugate shear zones in brittle and ductile deformation is that the intersection angle between brittle conjugate faults in the contractional quadrants is acute (usually ∼60°) whereas the angle between conjugate ductile shear zones is obtuse (usually 110°). The Mohr-Coulomb failure criterion, an experimentally validated empirical relationship, is commonly applied for interpreting the stress directions based on the orientation of the brittle shear fractures. However, the Mohr-Coulomb failure criterion fails to explain the formation of the low-angle normal fault, high-angle reverse fault, and the conjugate strike-slip fault with an obtuse angle in the σ1 direction. Although it is ten years since the Maximum-Effective-Moment (MEM) criterion was first proposed, and increasingly solid evidence in support of it has been obtained from both observed examples in nature and laboratory experiments, it is not yet a commonly accepted model to use to interpret these anti-Mohr-Coulomb features that are widely observed in the natural world. The deformational behavior of rock depends on its intrinsic mechanical properties and external factors such as applied stresses, strain rates, and temperature conditions related to crustal depths. The occurrence of conjugate shear features with obtuse angles of ∼110° in the contractional direction on different scales and at different crustal levels are consistent with the prediction of the MEM criterion, therefore ∼110° is a reliable indicator for deformation localization that occurred at medium-low strain rates at any crustal levels. Since the strain-rate is variable through time in nature, brittle, ductile, and plastic features may appear within the same rock.

The characteristics and KAr age of clay minerals in the fault gouges in the Yunmeng Mountains, Beijing, China

Abstract There is an abundance of gouges in the sliding-thrusting fault zones in the Yunmeng Mountains, Beijing, China. The 15 samples of gouge clay minerals were examined and found to consist mainly of illite, montmorillonite and kaolinite; half of them have an illite content over 80%. The illite has a good form which is quite similar to the illite of hydrothermal origin. The illite belongs to K-bearing phyllosilicates and can be dated with the KAr age method. Two clusters of KAr model ages are 120 and 95 Ma, which represent the ages of two main faulting events. This inference is basically consistent with the evolution of geological events in the area.

Mechanism of inversion metallogeny of quartz vein type gold deposits in the Xiaoqinling region

There are a lot of important gold-bearing quartz veins in the Xiaoqinling metamorphic core complex. The quartz veins are strictly controlled by shear zones. Stress analysis indicates that the metallizing process of the Xiaoqinling gold deposits of quartz vein type can be divided into two stages. At the first stage, the shear zones were formed in an extensional environment, and the temperature was higher and thr buried depth was greater; at the second stage, the brittle thrusts were superimposed in ductile shear zones after the tectonic stress field turnrd form the extension to compression. The ore-bearing fluids were concentrated in these weak positions, and the gold deposits of quartz vein type were developed.