The world's most extensive, well preserved, earlyArchaean (>3500 Ma) structures occur in the northern end of the 3850--3560Ma Itsaq Gneiss Complex, southern west Greenland, an area which includes the3800--3700 Ma Isua supracrustal belt. These provide a unique opportunityto study early tectonics. The area is dominated by ~3800--3650 Maorthogneisses containing migmatized inclusions of ≥3800 Ma metavolcanic,sedimentary, ultramafic, and gabbroic rocks. The thickest and most extensiveof these inclusions forms the southern side of the Isua supracrustal belt.Also present are panels of less migmatized 3710--3690 Ma volcanic andsedimentary rocks, in the northern side of the Isua supracrustal belt, andprobably also in the Tussaap supracrustal belt in the south of the area. Combinedmapping and geochronology has identified mylonites which were active beforeregional early Archaean folding and 3580--3560 Ma metamorphism. Sensitivehigh-resolution ion microprobe dating of premylonitization, synmylonitization,and postmylonitization granite sheets indicates some mylonites were activebetween 3650 and 3600 Ma. During this period it is proposed that large horizontalmotions and interleaving occurred, results of which included juxtapositionof supracrustal sequences in the Isua supracrustal belt with different depositionalages. The tectonic interleaving was most likely due to horizontal motionsin the vicinity of plate margins. The new combined field observations andgeochronology support hypotheses that some form of plate tectonics at leastcontributed to early Archaean sialic crustal development.