Details

Autor(en) / Beteiligte

• Giuntoli, Francesco
• Menegon, Luca
• Warren, Clare J.
• Darling, James
• Anderson, Mark W.

Titel

Protracted Shearing at Midcrustal Conditions During Large‐Scale Thrusting in the Scandinavian Caledonides

Ist Teil von

• Tectonics (Washington, D.C.), 2020-09, Vol.39 (9), p.n/a

Ort / Verlag

Washington: Blackwell Publishing Ltd

Erscheinungsjahr

2020

Link zum Volltext

Quelle

Wiley Online Library Journals Frontfile Complete

Beschreibungen/Notizen

• During continental collision, large tracts of crust are mobilized along major shear zones. The metamorphic conditions at which these zones operate, the duration of thrusting, and the deformation processes that facilitated hundreds of km of tectonic transport are still unclear. In the Scandinavian Caledonides, the Lower Seve Nappe displays a main mylonitic foliation with thickness of ~1 km. This foliation is overprinted by a brittle‐to‐ductile deformation pattern localized in C‐ and C′‐type shear bands proximal to the tectonic contact with the underlying Särv Nappe. Thermobarometry of amphibolites and micaschists suggests a first high‐pressure stage at 400–500°C and 1–1.3 GPa recorded in mineral relics. The main mylonitic foliation developed under epidote amphibolite facies conditions along the retrograde path from 600°C and 1 GPa to 500°C and 0.5 GPa. Age dating of synkinematic titanite grains in the amphibolites indicates that this mylonitic fabric formed at around 417 ± 9 Ma but older ages spanning 460–430 Ma could represent earlier stages of mylonitization. The shear bands developed at lower metamorphic conditions of 300–400°C and ~0.3 GPa. In the micaschists, the recrystallized grain size of quartz decreases toward the shear bands. Monomineralic quartz layers are eventually dismembered to form polyphase aggregates deforming by dominant grain size sensitive creep accompanied by slip in muscovite and chlorite. Plagioclase zoning truncations suggest that the shear bands originated by fracturing followed by ductile deformation. The results suggest protracted and long‐lasting shearing under amphibolite to greenschist facies conditions during the juxtaposition, stacking, and exhumation of the Lower Seve Nappe. Key Points In the Lower Seve Nappe 1 km‐thick mylonitic foliation formed at amphibolite facies conditions between ~460 and ~417 Ma Toward the base of the nappe the foliation is overprinted by a brittle‐to‐ductile fabric of greenschist facies conditions (~417 to 400 Ma) These fabrics formed due to protracted and long‐lasting shearing during the exhumation and assembly of the Seve Nappe Complex