Abstract
AbstractDuring 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.
Highlights
Thrusts in mountain belts localize much of the tectonic transport associated with crustal shortening during mountain building
We describe the evolution of metamorphic fabrics in a 1 km‐long crustal section provided by the “Collisional Orogeny in the Scandinavian Caledonides (COSC‐1; IGSN ICDP5054EHW1001)” drill core (Lorenz et al, 2015; see section 2)
A multianalysis approach including petrographic and microstructural analyses, thermodynamic modeling, electron backscattered diffraction (EBSD) analyses, and age dating has allowed the reconstruction of the first pressure‐temperature‐time‐ deformation path for the COSC‐1 drill core (Lower Seve Nappe) in the Scandinavian Caledonides
Summary
Thrusts in mountain belts localize much of the tectonic transport associated with crustal shortening during mountain building They may be responsible for the juxtaposition of units characterized by remarkably contrasting tectonometamorphic histories (Bender et al, 2018; Gee, 1975; Giuntoli & Engi, 2016; Jolivet et al, 1998; Searle et al, 2008; Zwart, 1975). As they frequently form via several stages of deformation, shear zones often preserve multiple generations of overprinting mineral fabrics and relics, reflecting the evolution from higher to lower metamorphic grades (e.g., Papapavlou et al, 2018). A robust constrain on the metamorphic conditions, deformation mechanisms, and timescale of shear zones activity is needed to better understand their role during orogenesis
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