Abstract
The Proterozoic mobile belt of South-East Greenland has been regarded as a classic example of amphibolite facies reworking of an Archaean granulite facies gneiss terrain. Its northern boundary has been interpreted as a transcurrent shear zone in which reworking was associated with major basic dyke emplacement. A re-examination of the northern boundary shows it to be a diffuse region more than 50 km wide in which retrogression, unrelated to dykes or shear zones, gradually intensifies southwards. Superimposed on this are discrete belts of retrogression associated with dykes and shear zones. The sense of displacement on the latter is compatible with thrusting of the northern Archaean block southwards over the reworked terrain of the mobile belt.
Highlights
The northern boundary of the Proterozoic mobile belt of South-East Greenland has previously been studied during regional mapping by teams headed by D
Published views of the northern boundary envisage an irregular but continuous, sharply defined roughly east-west shear-zone between Kangertigtivatsiaq and the Inland Ice separating granulite facies Archaean rocks in the north from the amphibolite facies reworked terrain of the mobile belt (Bridgwater & Myers, 1979; Myers, 1984, 1987). This boundary interpretation, based on prominent shearing was proposed, Wallis (1967, p. 5) had described the diffuse, large-scale passage from granulite facies to amphibolite facies terrains, and had categorically stated that "no major structural discontinuity was seen anywhere" in the inland region
Outcrops of rocks affected by Nagssugtoqidian deformation and retrogression were recognized north of the boundary, and augen of unmodified Archaean gneiss were shown occurring within the mobile belt
Summary
The northern boundary of the Proterozoic mobile belt of South-East Greenland has previously been studied during regional mapping by teams headed by D. In regional descriptions of the mobile belt (previously termed the Nagssugtoqidian belt) the northern boundary has been interpreted as being defined by a steeply dipping E-W zone of high deformation in which granulite facies Archaean (2800--3000 Ma) gneisses are retrogressed to amphibolite facies (Bridgwater & Myers, 1979; Myers, 1984, 1987). The shear zones which make up this belt are the earliest (so-called Nag. 1) phase of Proterozoic deformation, dated at 2660 Ma (Pedersen & Bridgwater, 1979). Wager described the zone as being several miles at least in width and with indefinite boundaries, recognizing that the period of shearing and associated retrogression represented a late stage in the formation of the metamorphic complex having taken place at high temperature and pressure This zone is pertinent to later ideas about the northern boundary of the mobile belt. '... ~~,--:.-.,..~.. ,.~ :$,;/_>?~ ~
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