Tectonics of SE China: New insights from the Lushan massif (Jiangxi Province)

International audienceIn south China the Lushan massif forms a topographic high of the South China Block south of the Qinling-Dabie belt. The Lushan massif consists of two main lithotectonic units separated by a major tectonic contact: a Neoproterozoic (upper Sinian)-Paleozoic unit comprising primarily unmetamorphosed sandstones overlies a Paleoproterozoic unit mainly composed of low-pressure, high-temperature gneisses and micaschists. Both units are cut by Cretaceous granitic intrusions. Three primary tectono-metamorphic and magmatic events are recognized. The eastern part of the Lushan massif is cut by a NNE-SSW trending ductile normal fault (D3 deformation) coeval to the emplacement of a 100-110 Ma leucogranite dated by 40Ar/39Ar laserprobe on biotite and muscovite. D2 deformation is responsible for the formation of a decakilometer-scale NE-SW trending upright anticline characterized by NE-SW stretching and NW-SE shortening. The age of this folding event is defined by a 127±1 (2σ) Ma U/Pb titanite date obtained for a syntectonic granodiorite and 40Ar/39Ar ages of 133 Ma for amphibole. This Cretaceous age also corresponds to the 40Ar/39Ar ages of 126 Ma found on syntectonic muscovites at the base of the Sinian unit. An older deformation event, D1, characterized by a top-to-the-NW extensional decollement of the Sinian-Paleozoic series above Proterozoic metamorphic rocks is related to the Triassic tectonics of the Dabieshan. Lastly, in the lower part of Sinian rocks, the occurrence of kyanite cataclased during D1 documents an older, poorly preserved, late Paleozoic-early Mesozoic tectonometamorphic event (Dx) related to a blind thrust in the continental crust of the South China Block in the southern foreland of the Dabieshan

Evolution of olivine lattice preferred orientation during simple shear in the mantle

Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 272 (2008): 501-512, doi:10.1016/j.epsl.2008.03.063.Understanding the variation of olivine lattice preferred orientation (LPO) as a function of shear strain is important for models that relate seismic anisotropy to the kinematics of deformation. We present results on the evolution of olivine orientation as a function of shear strain in samples from a shear zone in the Josephine Peridotite (southwest Oregon). We find that the LPO in harzburgites re-orients from a pre-existing LPO outside the shear zone to a new LPO with the olivine [100] maximum aligned sub-parallel to the shear direction between 168% and 258% shear strain. The strain at which [100] aligns with the shear plane is slightly higher than that observed in experimental samples, which do not have an initial LPO. While our observations broadly agree with the experimental observations, our results suggest that a pre-existing LPO influences the strain necessary for LPO alignment with the shear direction. In addition, olivine re-alignment appears to be dominated by slip on both (010)[100] and (001)[100], due to the orientation of the pre-existing LPO. Fabric strengths, quantified using both the J- and M- indices, do not increase with increasing shear strain. Unlike experimental observations, our natural samples do not have a secondary LPO peak. The lack of a secondary peak suggests that subgrain rotation recrystallization dominates over grain boundary migration during fabric re-alignment. Harzburgites exhibit girdle patterns among [010] and [001] axes, while a dunite has point maxima. Combined with the observation that harzburgites are finer grained than dunites, we speculate that additional phases (i.e., pyroxenes) limit olivine grain growth and promote grain boundary sliding. Grain boundary sliding may relax the requirement for slip on the hardest olivine system, enhancing activation of the two easiest olivine slip systems, resulting in the [010] and [001] girdle patterns. Overall, our results provide an improved framework for calibration of LPO evolution models.This work was partly supported by NSF grants EAR-0230267 and EAR-0409609. Funding for fieldwork was provided by the WHOI Academic Programs Office as part of a 2003 field class run by P.B.K. and G.H