Subduction, exhumation, and resubduction in the Scandinavian Caledonides: studying the deep levels of a Himalaya-type orogen

点击翻译按钮获取中文摘要

The Scandinavian Caledonides are a collisional orogen comparable to the present-day Himalaya but deeply eroded since their formation in the Paleozoic. Therefore, deep levels of the mountain belt can be directly studied in the field and complemented with surface and geophysical information from present-day mountain belts into a more complete picture of Himalaya-type collisional mountain-building. The Seve Nappe Complex, an assemblage of thrust sheets from the outer part of the Baltoscandian margin, emplaced towards east over Baltica during the collision with Laurentia at ca. 400 Ma and cropping out over a distance of ca. 1000 km in Sweden and Norway, has most recently been established as an ultrahigh-pressure (UHP) metamorphic terrane by thermobarometry and the finding of microdiamond inclusions. UHP metamorphism in the Seve probably occurred in the Late Ordovician (ca. 460 to 450 Ma), earlier than the UHP metamorphism of the structurally deeper Western Gneiss Complex (ca. 420 to 400 Ma). As a working hypothesis, Seve UHP rocks formed in an arc-continent collision, were exhumed by extraction of the forearc lithosphere block, partly resubducted together with the Western Gneiss Complex during Baltica-Laurentia collision, and exhumed again by extraction of the Iapetus oceanic slab. Analogue processes in the Himalaya would encompass the collision of India with the Kohistan-Ladakh arc and UHP metamorphism in Indian margin rocks, their exhumation by extraction of the overlying mantle wedge, and the presently ongoing subduction of Indian crust to UHP depths, not accessible to direct observation. We will test the hypothesis using the following methods: structural and microstructural work on the HP and UHP units of the Seve in order to establish the deformation history of the terrane; Lu-Hf garnet dating of eclogites; U-Pb zircon dating of exhumation-related melts and pre-, syn-, and posttectonic dikes; step-wise retrodeformation of four cross-sections across the Scandinavian Caledonides.