A late orogenic extensional origin for the infracrustal gneiss domes of the East Greenland Caledonides (72-74°N)

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• 作者：Andresen ; Arild ; Hartz ; Ebbe H. ; Vold ; Jon
• 关键词：extension ; P-T estimates ; collapse ; exhumation ; East Greenland ; Caledonides
• 刊名：Tectonophysics
• 出版年：1998
• 期刊代码：82_00401951
• 类别：ear
• 出版时间：February 28, 1998
• 卷：285
• 期：3-4
• 页码：353-369
• 文件大小：1.61 M

摘要

The Caledonides in the Fjord Region of East Greenland are made of a N-S-trending basement dome of Archaean to Mesoproterozoic migmatites and gneisses, bordered to the east and to the west by Neoproterozoic (Eleonore Bay Supergroup) metasediments. Emplacement of younger rocks on older ones, abrupt break in metamorphic grade, and kinematic indicators, demonstrate that the east-dipping high-strain zone along the eastern margin of the dome is an extensional detachment (the Fjord Region Detachment) and not a west-directed thrust. The detachment zone cuts across contractional structures in the hanging wall. Amphibolite-grade mylonites, up to 1-2 km thick, developed during the initial stage of movement on the Fjord Region Detachment. P-T estimates from less deformed lenses or layers within the mylonite zone indicate that the mylonite protolith originated from a depth of more than 25 km (> 7 kbar). Subsequent deformation occurred under progressively lower P-T conditions. A 10-50-m-thick brecciated, ultracataclasite zone marks the upper part of the high-strain zone. Stratigraphical evidences and isotopic data (⁴⁰Ar/³⁹Ar) on lamprophyres suggest that extension started in early Late Silurian times (ca. 420 Ma) and continued into the Upper Devonian. The Middle to Upper Devonian molasse basin is an upper crustal response to the extensional movement on the Fjord Region Detachment. The structural and metamorphic relationships observed along and across the Fjord Region Detachment in central East Greenland is comparable to the situation encountered below the Neoproterozoic metasediments in the Ardencaple Fjord region to the north (75-76°N). In the light of this similarity we suggest that the Fjord Region Detachment (or some other extensional shear zone) continues northward to the Ardencaple Fjord region. If so, it seems reasonable to relate exhumation of the eclogitic basement terrain around Dove Bugt to a late Caledonian extensional event. Such a scenario is comparable to existing models for the Western Gneiss Region in the Scandinavian Caledonides, where formation of the Devonian basins is related to orogenic extensional collapse and exhumation of coesite-bearing eclogites. Neither a high-strain zone comparable to the Fjord Region Detachment nor a metamorphic or structural break was observed along the eastern boundary of the gneiss dome. Instead a gradual downward increase in metamorphic grade is observed in the Neoproterozoic metasediments (Peterman Bjerg Group) reaching high amphibolite grade, with local migmatization, at the contact.