Microstructural evolution within a partitioned midcrustaltranspression zone, northeast Greenland Caledonides
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The thrust-related Imbricate Zone and thesinistral strike-slip Storstrømmen Shear Zone (SSZ)are two major orogen-parallel ductile shear zones thatmark the western limit of intense Caledoniandeformation in northeast Greenland and transect theorogenic hinterland, respectively. Detailed micro-structural analysis of associated mylonitic faultrocks from Dronning Louise Land and Hertugen afOrléans Land has been carried out to provide insightsinto the evolution of these structures and to constrainthe relative timing of movement. Amphibolite-faciesmylonites 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 areconcentrated within a ca. 2- to 4-km-wide belt alongthe western margin of the shear zone. This meta-morphic pattern is interpreted to reflect progressivelocalization of strain associated with exhumation andsyntectonic cooling. The ca. 15-km-wide ImbricateZone consists of numerous east-dipping ductile thrustsheets. At low structural levels, mylonitic fabricsindicate that thrust-related deformation occurred atmiddle-greenschist facies. Toward higher levels, how-ever, microstructures suggest that the metamorphicgrade progressively increases. We propose that ductilethrusting across the Imbricate Zone was synchronouswith uplift and exhumation of the orogenic hinterlandand SSZ, accounting for the observed metamorphicvariations. This implies that both structures wereactive broadly contemporaneously during regionalpartitioned transpression, an inference supported byscattered 40Ar/39Ar and recent U-Pb zircon ages. Themicrostructural observations therefore support modelsthat involve sinistrally oblique Silurian-Devonian col-lision between Laurentia and Baltica.
|Number of pages||20|
|Publication status||Published - 2007|