ARCHEAN CYCLES OF HORIZONTAL AND VERTICAL TECTONICS IN THE ISUA REGION, SOUTHERN WEST GREENLAND: RESULTS FROM INTEGRATED MAPPING AND U-PB TITANITE GEOCHRONOLOGY

The northeast end of the Færingehavn terrane, in southern West Greenland, experienced both “horizontal” (recumbent folding and thrust/detachment faults) and “vertical” (doming and crustal inflation) tectonics from 3.8 to 2.6 Ga. The Færingehavn terrane, comprising the oldest rocks in Greenland, includes the intensively studied ~3.8 Ga volcano-sedimentary rocks of the Isua greenstone belt (IGB). Because the gneiss units in contact with the IGB experienced comparatively less intense metasomatism and deformation during the Archean, a study of the gneiss complements those focused on the IGB well. We mapped the west side of the central (enclosed) gneiss at 1:8 000 and extracted titanite for U-Pb TIMS dating. The present form of the IGB, a reclined isocline, obscures structural relationships between the IGB and surrounding gneiss units. However, our work shows that this isocline is the result of tightening and overturning of the west limb of a dome. Titanite from shear zones involved in this tightening indicate three generations of motion, at ~3180, 2600, and 2060 Ma. The 2600 Ma event may relate to 2.65-.26 Ga tectonism further south reflecting intercalation of the Færingehavn and Tre Brødre terranes, and accretion of these between the Akia and Tasiusarsuaq terranes. In the pre-3.2 Ga dome, the central ~3.7 Ga gneiss underlay the IGB, which underlay the external ~3.8 Ga gneiss. The doming occurred after 3.65-3.60 intrusion and intercalation of these units, and before the ~3.5 Ga intrusion of the Tarssartôq dolerite dikes. The 3.8-3.6 Ga tectonics of the region— shearing and isoclinal folding within the IGB, intercalation of the three major elements—were thus horizontal. The regional tectonics thus define two (early- and middle-to-late-) Archean cycles of sub-horizontal (crustal-scale nappe) tectonism, intrusion and doming, the last probably resulting from deep-seated diapirism.