Structural and AMS study of some granite plutons emplaced during Mesozoic pervasive continental extension in Eastern China
Résumé
Previous studies have shown that extension of the continental lithosphere can follow different modes in accordance with different mechanical behaviours of rocks: (1) the narrow rift mode that corresponds to a ‘normal', stable, geothermal gradient and a normal thickness of the crust, (2) the core complexes structure development that is related to internal body forces acting during the collapse of a previously thickened crust, and (3) the wide rift mode that develops for a normal thickness of the crust, but an abnormal geothermal gradient. During Mesozoic to early Cenozoic times, a major continental extension event occurred in eastern Asia. This event is marked by the development of intracontinental basins, intrusion of consequent volumes of magmatic rocks and formation of metamorphic core complexes in the upper crust. As seen before, this latter kind of structure is generally diagnostic of post-orogenic collapse of the continental lithosphere; however, no trace of high pressure units of this age, and thus, of crustal thickening, has been documented. This could call into question the different modes of extension as they have been established so far. Others geodynamical models have also been proposed to explain this pervasive continental extension, e.g. Pacific plate paleo-subduction, lithospheric delamination or thermal “erosion” at its base. In that context, the structural study of granite bodies is a key to constrain spatial and time evolution of kinematics during this large-scale extensional event. The first results obtained for the Jiaodong Peninsula show that two types of granite intrusions can be distinguished: older ones present a strong fabric on the edge that is interpreted as a syn-kinematics intrusion below a normal ductile shear zone. In contrast, younger plutons do not present any fabric at a macro-scale and an AMS analysis has thus been performed to characterise the internal micro-fabric of the granite and its spatial variations.