Microstructural evolution within a partitioned midcrustal transpression zone, northeast Greenland Caledonides

The thrust‐related Imbricate Zone and the sinistral strike‐slip Storstrømmen Shear Zone (SSZ) are two major orogen‐parallel ductile shear zones that mark the western limit of intense Caledonian deformation in northeast Greenland and transect the orogenic hinterland, respectively. Detailed microstructural analysis of associated mylonitic fault rocks from Dronning Louise Land and Hertugen af Orléans Land has been carried out to provide insights into the evolution of these structures and to constrain the relative timing of movement. Amphibolite‐facies mylonites were developed across the entire ca. 8‐km‐wide SSZ and are currently exposed across an area ca. 3–4 km wide within the center of the shear zone. In contrast, retrograde greenschist‐facies mylonites are concentrated within a ca. 2‐ to 4‐km‐wide belt along the western margin of the shear zone. This metamorphic pattern is interpreted to reflect progressive localization of strain associated with exhumation and syntectonic cooling. The ca. 15‐km‐wide Imbricate Zone consists of numerous east‐dipping ductile thrust sheets. At low structural levels, mylonitic fabrics indicate that thrust‐related deformation occurred at middle‐greenschist facies. Toward higher levels, however, microstructures suggest that the metamorphic grade progressively increases. We propose that ductile thrusting across the Imbricate Zone was synchronous with uplift and exhumation of the orogenic hinterland and SSZ, accounting for the observed metamorphic variations. This implies that both structures were active broadly contemporaneously during regional partitioned transpression, an inference supported by scattered 40Ar/39Ar and recent U‐Pb zircon ages. The microstructural observations therefore support models that involve sinistrally oblique Silurian‐Devonian collision between Laurentia and Baltica.