Exhumation mechanisms of metamorphic core complexes

Extensive studies of metamorphic core complexes (MCCs) have highlighted a fundamental problem; the relationship between the mylonitic rocks in the footwall of the ductile low-angle detachment fault at its top, and the brittlely deformed to undeformed hangingwall unit. The project aims to set up a scheme between several possible end-member type cases of exhumation mechanisms of metamorphic core complexes, e.g., classification of different detachment modes (e.g., rolling hinges, initial low-angle detachment) and contribution of pure shear vs. simple shear modes of exhumation. In many cases, upward motion along a detachment (ductile low-angle normal fault) and internal ductile thinning implies gradual exhumation with the youngest exhumation along a rolling hinge at the trailing edge of the MCCs. We want to test also how much simple shear contributes to exhumation within the MCCs and when vertical thinning occurred compared to deformation along the detachment zone at the top of the MCC. Cordilleran-type MCCs are exhumed virtually parallel to the regional extension direction. Such cases are common in post-collisional settings with extension of previously overthickened lithosphere, or as in the case of the Aegean Sea, in a back-arc basin setting, which formed due to the retreat of a subduction zone. The study areas will be the Naxos MCC in the Aegean Sea and the Rechnitz "window" MCC located at the transition between the Eastern Alps and the Pannonian basin. Orogen-type MCCs occur during the late stage of collision in the center of orogens, with the extension direction subparallel to the orogen, and roughly perpendicular to the regional shortening direction. Such MCCs often represent relay/overstep structures along regional strike-slip faults and, commonly, the final structure is governed by the complex superposition of shortening, related updoming and associated thrusting, and strike-slip displacement. The study area will be the Diancang Shan MCC along the Ailao Shan-Red River shear zone. The principal goals of the proposed project are threefold: Study of (1) the microstructures and textures of rock-forming minerals from the top of the detachment to the interior of the MCC to reveal the deformation regime (e.g., pure vs. simple shear), (2) the microfabric formation and to date the exact age of shearing during exhumation of the MCCs, and (3) the exhumation history (e.g. cooling and deformation) of MCCs using thermochronological tools along and across different types of MCCs to constrain the kinematics of the exhumation of rocks. We also want to build up a classification scheme of metamorphic core complexes using observational features and kinematics.

The project Exhumation of Metamorphic Core Complexes (MCC) focused on specific kinematic questions in the three working areas: the Naxos-Paros MCC in the Aegean Sea, the Rechnitz MCC at the easternmost termination of the Eastern Alps and the Diancang MCC along the Red-River-Aialoshan fault within the southeastern Tibet plateau. The main aim was to investigate how the kinematic and thermochronological results may contribute to a better understanding of exhumation and the kinematic development of the MCC. The combination of methodologies have been applied to shear zones bordering the three distinct MCCs to explain the tectonic evolution of the MCCs: (1) detailed structural and microstructural analysis, and EBSD textures of rock-forming minerals (e.g. quartz and calcite) from the top of the detachment to the interior of the three MCCs to reveal the deformation regime (e.g., pure vs. simple shear), (2) microfabric formation of shearing during exhumation of the MCCs (e.g. high- to low- temperature microfabrics), and (3) the geothermal evolution to constrain the kinematics of the exhumation of rocks (e.g. cooling and deformation) by using geochronology (Ar-Ar white mica dating), geothermometry (e.g. amphibolite and chlorite) and barometry (e.g. white mica). Extensive studies of the three MCCs have highlighted the fundamental problem that the relationship between the mylonitic rocks in the footwall of the ductile low-angle detachment fault at its top, and the brittle-deformed to undeformed hanging wall unit and on its rheological behavior of the crust. We also am thinking and writing to build up a classification scheme of metamorphic core complexes using observational features and kinematics. Deformational styles at all scales are dominated by extensional structures similar to those documented in numerous MCCs. In all cases investigated by us, the level of the ductile low-angle normal fault is controlled by the presence of thick successions of calcite-dominated lithologies, which are rheologically weak at low temperatures. One of the most important meanings to occur in these low grade metamorphism rocks is the new recrystallized assemblage lowered the strength of rocks, active representing a matrix-controlled interconnected weak layer rheology. These dominant fabric-forming minerals formed under decreasing temperature during motion through the brittle-ductile transition. Strain partitioning results in preservation of high-temperature microfabrics, minerals and textures with low-grade mylonitic shear zones. As a result, grain size reduction associated with fluids circulating within shear zones leads to rock softening, which results in strain localization weakening rock rheology and the overall thermal structure of the crust.