Yadong Zheng

Great Jurassic thrust sheets in Beishan (North Mountains)—Gobi areas of China and southern Mongolia

Abstract Jurassic thrust sheets with minimum displacements of 120–180 km have been discovered within the ‘Hercynian-Indosinian’ orogenic belt of the Beishan of China and south Gobi area. The thrusts strike E-W, extend over 1200 km in length, and carried Meso-Proterozoic massive dolomitic limestones over strata ranging from NeoProterozoic (Cryogenian and Terminal Proterozoic) to Lower-Middle Jurassic. Slip-linear plots based on kinematic indicators, such as slickenlines and groove lineations, fiber lineations and ‘drag folds’ adjacent to the fault surface, vergence of folds and imbricated thrusts in the upper plate, indicate northward movement in the Beishan area to the west and southward movement in the South Gobi area to the east. The two major thrust faults, the Beishan thrust and South Gobi thrust, are presumably separated by a major tear fault, the Ruo Shui fault. The major thrust faults were later deformed into a series of E-W antiforms and synforms and the sheets are separated, due to erosion, into a number of klippen mainly located on synforms of the major faults. The Yagan metamorphic core complex, which is a result of an extensional event that postdates the thrust event, yields an 40 Ar- 39 Ar plateau age of 155.1 +- 10 Ma, and Rb Sr isochron age of 153 +- 6.2 Ma. The thrust sheets formed during the late Middle Jurassic, long after the closure of any oceans in the study area previously reported for this region, and are ascribed to a phase of intracontinental deformation. The closing of the Jurassic Tethys or retroarc deformation behind an active continental margin at the southern edge of Asia, prior to the Tethyan collision, or/and the closing of Mongolo-Okhotsk oceans might be responsible for this event.

Geochronological evidence for existence of South Mongolian microcontinent——A zircon U-Pb age of grantoid gneisses from the Yagan-Onch Hayrhan metamorphic core complex

A zircon U-Pb age of (916±16) Ma is measured for grantoid gneisses from the Yagan-Onch Hayrhan metamorphic core complex and represents the crystallization age of the grantoid magma. This age provides evidence for the existence of the South Mongolian microcontinent, which is consistent with the analysis of the regional geology.

Sliding-thrusting tectonics caused by thermal uplift in the Yunmeng Mountains, Beijing, China☆

Abstract A ductile shear zone analogous to a normal fault with a low dip-angle occurs on the southeastern flank of the Yunmeng Anticlinorium north of Beijing in Huairou County. The southeast-dipping Shuiyu ductile shear zone is composed of mylonitic granitic gneiss and has an average width of about 1 km. The structures in the area show a symmetrical pattern. This pattern may be generated by a process of sliding and thrusting caused by thermal uplift. An alternative possible model for the area is that for the North American Cordilleran regions.

Polar Mohr constructions for strain analysis in general shear zones

Abstract The polar Mohr diagram is a useful tool for strain analysis in general shear zones. Polar Mohr diagrams for general shear zones can be constructed with the following measured data: (a) the stretches of two line markers which were originally perpendicular to each other; (b) the ratio of their stretches and the principal directions; (c) the stretches in the shear direction and in the principal direction; and (d) the stretches in any two directions. With these diagrams, the angle between eigenvectors, v , can be obtained, and the kinematic vorticity number W k in the shear zone and the ratio of pure shear rate e to simple shear rate γ during deformation can be computed with the formulae W k = cos v and W k = cos[cot(2e/γ)].

Syn-deformation P-T paths of Xiaoqinling metamorphic core complex

TheP-T paths of the Xiaoqinling metamorphic core complex (XMCC) have been investigated with the Gibbs method by researching the compositional changing of zoned epidotes which formed during syn-deformation metamorphism. TheseP-T paths indicate that the XMCC had experienced the following thermodynamics processes: firstly, near isobaric falling slightly in temperature in lower crust; secondly, fast decompression and rising in temperature during extensional uplifting to middle crust level; and finally, isobaric falling in temperature in middle crust. The upwelling and emplacement of the deep magma may be a major factor during the uplifting processes of the metamorphic core complex.

Interpretation of the experimental data provided by Gómez-Rivas and Griera (2012) in terms of the MEM-criterion

The systematical experiments carried out by Gómez-Rivas and Griera (2012) demonstrate that the ductile shear zones initiate at ∼55° to σ1 just as predicted by the MEM-criterion. However, the data are explained in terms of dilatancy, which requires many prerequisites and implies that the ±55° angle is only valid for the used material. In contrast, the ∼55° predicted by the MEM-criterion is material independent, which makes it widely applicable to explaining the development of ductile shear zones in nature.

On the material invariant of 110°-conjugate angle of shear bands—Reply to Gomez-Rivas and Griera (2015)

After their experimental data were re-explained in terms of the maximum-effective-moment (MEM) criterion, Gomez-Rivas and Griera (2015) challenge the validity of the MEM-Criterion in terms of shear fractures, which have mixed up with shear fractures and shear bands. The two features are similar in appearance but different in deformation mechanism (s). The MEM-criterion proves that ±55° to σ1 are the maximum effective moment directions and the shear bands that formed by material-line (beddings or fabrics) rotation mechanism have a constant conjugate angle of 110°. Theoretically, the 55° or 110° is a material-invariant, and practically, a statistic-invariant or preferred direction with average deviation of ~10°. By this angle, shear bands can be easily recognized from shear fractures with conjugate angle never over 90°. The High-strain deformation in the lozenges usually predates the surrounding shear bands. Two stress states can not coexisted simultaneously in the same place and the resolving σ1′ normal to the related shear zone represents 0–100% deformation partitioning, depending on the original kinematic vorticity of the shear zones.