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Metamorphic core complexes (MCC) are formations of deep basement metamorphic rocks (lower plate) exhumed through an upper plate of younger sedimentary and igneous supracrustal deposits. Though the mechanism for MCC formation is poorly understood, they are thought to occur in regions of broad lithospheric extension and may represent several different stages of orogenic stresses. These paleostresses, both regional and local, can be recorded by twinning of syntectonic calcite. Prior to this study, the calcite strain-gage technique has never been applied to MCCs. The Liaonan metamorphic core complex (LMCC), located on the southern end of the Liaodong Peninsula in Liaoning Province, Northeast China, is part of a suite of similar MCC structures across the North China tectonic block. It is a Cretaceous-aged Cordilleran-style complex, characterized by a WNW-dipping master detachment fault on its western side, a mylonitic detachment zone within an Archean lower plate, Cretaceous synextensional granodioritic intrusions throughout the structure, and an upper plate of Neoproterozoic and Paleozoic sedimentary rocks, capped by a Cretaceous supradetachment basin. Foliation orientations and kinematic indicators within the mylonitic shear zone suggest WNW-ESE extension with some local variation. Twinned calcite grains in synextensional veins and fault gouge (n = 575) of the LMCC preserve subhorizontal MCC-axis-parallel-shortening (with some variability to local detachment fault orientation) and vertical extension. Strain magnitudes tend to be greater in the detachment samples and smaller in the upper plate and syntectonic supradetachment basin samples. A lack of strain overprint is represented by small amounts of negative expected values in the dataset (NEVs). The differential stress responsible for twinning is about -375 bars throughout the entire sample suite indicating shallow calcite deformation. Vertical extension preserved in our twin results suggests MCC emplacement was strongly influenced by diapiric magmatism. This is likely characteristic of rapid late-stage MCC exhumation, where relative regional extension is still perpendicular to a NNE-SSW shortening direction but a strong buoyancy component recorded by extensional strain ultimately drives the lower plate to its final position at the surface.
Avdievitch, Nikita N., "Geodynamic Evolution of the Cretaceous Liaonan Metamorphic Core Complex, Northeast China: New Insights from Calcite Twin Paleostress Analysis" (2013). Geology Honors Projects. 13.
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