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The Dayingzi detachment fault system in Liaodong Peninsula and its regional tectonic significance [查看] SHENLiang、LIUJunLai、HULing、JIMo、GUANHuiMei、GregoryADAVIS

Large scale lithosphere thinning is an important characteristic of the destruction of the North China Craton (NCC) during the late Mesozoic. A series of extensional structures were developed under extensional setting, among which is the Dayingzi detachment fault system (DFS). The DFS is constituted by three parts, volcano-sedimentary basins at the hanging wall, the Dayingzi-Huanghuadian detachment fault zone, and Paleoproterozoic metamorphic rock series and Mesozoic plutons at the footwall.In the section across the detachment fault zone, there is a sequence of tectonites including fault gouge, microbreccia, cataclastic-mylonites, mylonites, and gneissic biotite monzonite granite. Microstructural characteristics of tectonites and electron backscatter diffraction (EBSD) patterns of quartz indicate that the rocks from the footwall experienced a process from upper greenschist facies to lower greenschist facies. SHRIMP and LA-ICP MS U-Pb dating of zircons from the volcanic rocks in the basins, the tectonic evolution of the DFS is summarized as follows: 1) regional extension started at 135.0±1.2 Ma ago, when the detachment fault cut through the middle crust. Faulting induced the upwelling of magma and eruption of volcanic rocks and deformed a series of medium-acid volcanic rocks; 2) after 135.0±1.2 Ma, a large scale detachment faulting was active cross-cutting the mid-upper crust. The western margin of Jurassic and Triassic granite was ductilly and brittly sheared; besides,the Cretaceous volcano-sedimentary rocks were tilted when the master fault approached the surface; 3) at around 127±1 Ma,the detachment fault stopped its activity and was intruded by the unsheared Cretaceous granite near Chaoyang. Comparison with the Liaonan metamorphic core complex (MCC) and other extensional structures in Liaodong Peninsula led to a general trend of including three zones in the Peninsula: MCC zone, detachment fault systems (DFS) zone, and half graben zone. MCC commonly cuts through the mid-lower crust, DFS through the mid-upper crust, and half graben through the upper crust.Therefore, development of the extensional structures in Liaodong Peninsula indicates that they are the results of crustal extension and thinning at different crustal levels. They may provide a deep insight into the dynamic mechanism, history of destruction and lithosphere thinning of the North China Craton (NCC).

U-Pb SHRIMP zircon geochronology and T-t-d history of the Kampa Dome, southern Tibet [查看] M.C.Quigley、Y.Liangjun、Gregory、Corvino、M.Sandiford、C.J.L.Wilson、L.Xiaohan

Structural, petrographic and geochronologic studies of the Kampa Dome provide insights into the tectonothermal evolution of orogenic crust exposed in the North Himalayan gneiss domes of southern Tibet. U–Pb ion microprobe dating of zircons from granite gneiss exposed at the deepest levels within the dome yields concordia 206Pb/238U age populations of 506±3 Ma and 527±6 Ma, with no evidence of new zircon growth during Himalayan orogenesis. However, the granite contains penetrative deformation fabrics that are also preserved in the overlying Paleozoic strata, implying that the Kampa granite is a Cambrian pluton that was strongly deformed and metamorphosed during Himalayan orogenesis.Zircons from deformed leucogranite sills that cross-cut Paleozoic metasedimentary rocks yield concordant Cambrian ages from oscillatory zoned cores and discordant ages ranging from ca. 491–32 Ma in metamict grains. Since these leucogranites clearly post-date the metasedimentary rocks they intrude, the zircons are interpreted as xenocrysts that are probably derived from the Kampa granite. The Kampa Dome formed via a series of progressive orogenic events including regional ~N–S contraction and related crustal thickening (D1), predominately top-to-N ductile shearing and crustal extension (D2), top-to-N brittle–ductile faulting and related folding on the north limb of the dome, localized top-to-S faulting on the southern limb of the dome, and crustal doming (D3), and continued N–S contraction, E–W extension and doming (D4). Structural and geochronologic variability amongst adjacent North Himalayan gneiss domes may reflect changes in the magnitude of crustal exhumation along the North Himalayan antiform, possibly relating to differences in the mid-crustal geometry of the exhuming fault systems.

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