Guo Jinghui

Southward propagation of the Karakoram fault system, southwest Tibet: Timing and magnitude of slip

The net slip on the southern portion of the Karakoram fault system in southwest Tibet is estimated by restoring a piercing line defined by two key surfaces in the South Kailas thrust system, a regional counter thrust along the Indus-Yalu suture. Assuming that the thrust system is planar across the Karakoram fault, we calculate 66 ± 5.5 km of normal right slip. Documentation of the South Kailas thrust active at 13 Ma implies that the Karakoram fault in southwest Tibet did not initiate until after the cessation of motion on the thrust. However, field investigations of the central portion of the Karakoram fault system document the fault to have been active at 17 Ma and to have accumulated a maximum of 150 km of right slip. We suggest that these along-strike variations in the magnitude of slip and timing constraints are best explained by southward propagation of the Karakoram fault system. This is inconsistent with major right-lateral slip on the fault system, which was used in support of extrusion models for Tibet.

Structural evolution of the Gurla Mandhata detachment system, southwest Tibet: Implications for the eastward extent of the Karakoram fault system

Field mapping and geochronologic and thermobarometric analyses of the Gurla Mandhata area, in southwest Tibet, reveal major middle to late Miocene, east-west extension along a normal-fault system, termed the Gurla Mandhata detachment system. The maximum fault slip occurs along a pair of low-angle normal faults that have caused significant tectonic denudation of the Tethyan Sedimentary Sequence, resulting in juxtaposition of weakly metamorphosed Paleozoic rocks and Tertiary sedimentary rocks in the hanging wall over amphibolite-facies mylonitic schist, marble, gneisses, and variably deformed leucogranite bodies in the footwall. The footwall of the detachment fault system records a late Miocene intrusive event, in part contemporaneous with top-to-the-west ductile normal shearing. The consistency of the mean shear direction within the mylonitic footwall rocks and its correlation with structurally higher brittle normal faults suggest that they represent an evolving low-angle normal-fault system. 4 0 Ar/ 3 9 Ar data from muscovite and biotite from the footwall rocks indicate that it cooled below 400 °C by ca. 9 Ma. Consideration of the original depth and dip angle of the detachment fault prior to exhumation of the footwall yields total slip estimates between 66 and 35 km across the Gurla Mandhata detachment system. The slip estimates and timing constraints on the Gurla Mandhata detachment system are comparable to those estimated on the right-slip Karakoram fault system, to which it is interpreted to be kinematically linked. Moreover, the mean shear-sense direction on both the Karakoram fault and the Gurla Mandhata detachment system overlap along the intersection line between the mean orientations of the faults, which further supports a kinematic association. If valid, this interpretation extends previous results that the Karakoram fault extends to mid-crustal depths.

Timing of the granulite facies metamorphism in the Sanggan area, North China craton: zircon U-Pb geochronology

Zircon U-Pb ages are reported for three samples of intrusive rocks in Khondalite series in the Sanggan area, North China craton. The age of meta-granite is dated as 2005∓9 Ma, implying that the sedimentary sequences in Khondalites series formed before 2.0Ga. The age of 1921 ∓ 1Ma for the meta-diorite constrain the age of granulite facies metamorphism younger than this date. The age of 1892 ∓ 10 Ma for garnet granite is obtained, but the granite crystallization age seems a little younger than the date considering the morphology of zircons. On the basis of these dates and of a concise review of previous age data, it is inferred that the Khondalite series was subjected to granulite facies metamorphism at about 1.87Ga together with tonalitic granulites and HP basic granulites in the Sanggan area.Zircon U-Pb ages are reported for three samples of intrusive rocks in Khondalite series in the Sanggan area, North China craton. The age of meta-granite is dated as 2005±9 Ma, implying that the sedimentary sequences in Khondalites series formed before 2.0Ga. The age of 1921±1Ma for the meta-diorite constrain the age of granulite facies metamorphism younger than this date. The age of 1892±10 Ma for garnet granite is obtained, but the granite crystallization age seems a little younger than the date considering the morphology of zircons. On the basis of these dates and of a concise review of previous age data, it is inferred that the Khondalite series was subjected to granulite facies metamorphism at about 1.87Ga together with tonalitic granulites and HP basic granulites in the Sanggan area.

SHRIMP zircon U-Pb dating in Jingshan "migmatitic granite", Bengbu and its geological significance

The petrographic characteristics of Jingshan “migmatitic granite” and the occurrence of the magmatic zircons indicate that the granite was formed by normal crystallization of felsic melts. All zircons in the granite have inherited cores and fine-scale oscillatory zoning rims of magmatic origin. It is realized that the granite was formed at 160.2±1.3 Ma through dating magmatic zircons. The generation of the granitic magma could be related to the lithospheric mantle and/or lower crust delamination after the ultrahigh pressure metamorphism (UHPM) in Triassic. Most inherited zircons yield the ages of 217.1±6.6 Ma, which is consistent with the peak UHPM in the Dabie-Sulu orogenic belt. Some of the inherited zircons (433–722 Ma) constitute a discordia line with the upper intercept age of 850+85/−68 Ma and a lower intercept age of 261+100/−140 Ma. These ages imply that the granite could be derived from the partial melting of the crustal materials of the South China Block that was intensively superimposed by the UHPM. The UHPM could be the reason for the major Pb loss at −220 Ma.

Exhumation of the Dabie UHP terrane, China

Exhumation of deeply buried ultrahigh-pressure (UHP) terranes remains a puzzle in understanding lithospheric dynamic process. New evidence that constrains the exhumation process of the Dabie UHP terrane includes the following. (1) Structural data record the architecture of the Dabie UHP terrane. Three slices are recognized—i.e., from top to bottom, the Susong blueschist slice, the Huangzhen-Huangweihe (HH) HP slice, and the Jinheqiao-Shuanghe-Bixiling (JSB) UHP slice. These slices have been stacked and domed. (2) Geophysical data depict two offsets in the Moho. Offset-I separates the Dabie UHP terrane from the North China craton, and has served as a subduction channel, as well as an exhumation channel. Offset-II, separating the Dabie UHP terrane from the foreland belt, developed due to geodynamic regime reorganization. (3) Geochronological data reveal that the HP slice was exhumed earlier that the UHP slice. A northward-younging exhumation polarity is implied, with younger exhumation age closer to the exhumation channel. These pieces of evidence point to a multistage exhumation process. The Yangtze craton collided with and subducted beneath the North China craton at the very beginning of Triassic time. By 230 Ma, the Susong blueschist slice and the southern end of the HH HP slice had arrived at a middle-crustal level, while its northern part was still at mantle depth. Such processes continued until the JSB UHP slice was exhumed to mid-crustal levels at 210 Ma. After geodynamic regime reorganization during 210—180 Ma, continuing compression of the Yangtze craton and underthrusting along offset-II in the Moho caused the dome structure, and eventually brought the UHP rocks to the Earths surface.