A 1 Ga Eclogite-bearing Hot Migmatitic Nappe in the Sveconorwegian Orogen – a Window into Depth beneath the Frontal Part of a Precambrian Himalayan-Tibetan-type Plateau

English Abstract:

The Eastern Segment of the Sveconorwegian orogen exposes a formerly deeply (35-40 km) buried section of continental crust, corresponding to a setting beneath an orogenic plateau. A 75 x 50 km large eclogite-bearing nappe proves deeper burial (>55 km) of parts of the same crustal block. The eclogite-bearing nappe was exhumed by tectonic extrusion, melt-weakening assisted and foreland-directed translation of eclogitized crust, and stalled at 35–40 km depth within the collisional belt.

The Eastern Segment is semi-continuous with the continental foreland in the east (Baltica) but tectonically bound by a major crustal shear zone, the “Mylonite Zone”, at the contact to the Idefjorden Terrane in the west. Different lines of evidence indicate that this boundary is a deep collisional suture, where the Eastern Segment was down-thrusted “India-style” beneath the frontal part of a Himalayan-Tibetan-type plateau: 1) the eclogite-bearing nappe is located at the boundary; 2) the Eastern Segment and the Idefjorden Terrane both experienced metamorphism ≤ 0.98 Ga, but share neither pre-Sveconorwegian 1.7-1.1 Ga nor Sveconorwegian 1.05-0.99 Ga igneous and tectonic evolutions.

The eclogite-bearing nappe in the Eastern Segment demonstrates that extrusion of melt-weakened crust is a viable exhumation mechanism within hot and deep collision zones. The eclogites are structurally restricted to a regional recumbent fold in which stromatic orthogneiss with pods of amphibolitized eclogite make up the core. High-temperature mylonitic gneiss with remnants of kyanite eclogite compose a basal shear zone, <4 km wide. Heterogeneously sheared and partly migmatized augen gneiss forms a tectonostratigraphic marker in front of and beneath the nappe, and is in turn structurally enveloped by a composite sequence of orthogneisses and metabasites. The entire tectonostratigraphic pile underwent near-pervasive deformation and recrystallization under high-pressure granulite and upper-amphibolite conditions.