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The North-East Greenland Caledonides record a complex history of crustal thickening and extension during the Paleozoic collision of Baltica with Laurentia. We divide the southern portion of the orogen (70°N–76°N) into three plates separated by low-angle fault systems that are interpreted as extensional detachments superimposed on, and perhaps coeval with, the thrust geometry of the orogen. From structurally lowest to highest, the plates include amphibolite-facies Archean to Paleoproterozoic orthogneiss and lesser paragneiss that retain relics of Devonian high-pressure metamorphism, migmatitic Mesoproterozoic metasedimentary rocks with Silurian leucogranites and lesser orthogneiss at amphibolite-facies conditions, and low-grade Neoproterozoic to Ordovician sedimentary rocks. Individual detachments are characterized by superposition of cataclastic features on mylonitic fabrics, and they record progressive deformation that accommodated exhumation. The extensional faults define two detachment systems that evolved at different crustal levels during two episodes of movement. The upper detachment system, which separates the upper and middle plates, exhumed the midcrustal rocks after ca. 420 Ma. Extension was contemporaneous with crustal thickening and closely followed leucogranite emplacement. The structure may be analogous to the South Tibetan detachment system in the present-day Himalayas. Continental Old Red Sandstone deposition began in the Eifelian, closely following high-pressure metamorphism in the lower plate at ca. 405 Ma. The lower detachment was probably active at some depth below the evolving Devonian basins. The lower detachment system brought lower-plate metamorphic rocks to shallower crustal levels after 400 Ma, excising the overlying extensional system. This second period of extension was similar in timing and style to extension in the Scandinavian Caledonides. Displacement on the younger detachments, which exhumed lower-plate rocks, was broadly syncollisional, as indicated by the overlap in age with ultrahigh-pressure metamorphism in the north at 365–350 Ma, and it may have been synchronous with young thrusts that emplaced high-pressure lower-plate rocks over the foreland and with strike-slip faults in the hinterland. Conversion to extension, accommodated by high-angle brittle faulting in the Carboniferous (after 345 Ma), may mark the final transition to plate divergence that ultimately led to continental rifting.

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