Abstract
AbstractViscous crustal flow can exhume once deeply buried rocks in postorogenic metamorphic core complexes (MCCs). While migmatite domes record the flow dynamics of anatectic crust, the mechanics and kinematics of solid‐state flow in the deep crust are poorly constrained. To address this issue, we studied a deeply eroded and particularly well‐exposed MCC in the southern Western Gneiss Region of Norway. The Gulen MCC formed during Devonian transtensional collapse of the Caledonian orogeny in the footwall of the Nordfjord‐Sogn detachment zone. We developed a semiquantitative mapping scheme for ductile strain to constrain micro‐ to megascale processes, which brought eclogite‐bearing crust from the orogenic root into direct contact with Devonian supradetachment basins. The Gulen MCC comprises different structural levels with distinct metamorphic evolutions. In the high‐grade core, amphibolite‐facies structures record fluid‐controlled eclogite retrogression and coaxial flow involving vast extension‐perpendicular shortening. Detachment mylonites formed during ductile‐to‐brittle noncoaxial deformation and wrap around the core. We present a sequential 3‐D reconstruction of MCC formation. In the detachment zone, the combined effects of simple shearing, incision/excision, and erosion thinned the upper crust. Internal necking of the ductile crust was compensated by extension‐perpendicular shortening within the deep crust and resulted in differential folding of distinct crustal levels. We identify this differential folding as the main mechanism that can redistribute material within solid‐state MCCs. Our interpretation suggests a continuum of processes from migmatite‐cored to solid‐state MCCs and has implications for postorogenic exhumation of (ultra‐)high‐pressure rocks.
Highlights
Postorogenic metamorphic core complexes (MCCs) show that the crust reequilibrates after orogeny and an entirely new crustal template may be created (e.g., Brun et al, 2017 ; Buck, 1991 ; Coney, 1980 ; Osmundsen et al, 2005 ; Platt et al, 2015 ; Vanderhaeghe & Teyssier, 2001 ; Whitney et al, 2013)
We provide a detailed look inside the anatomy of a well‐exposed postorogenic MCC in order to understand the viscous flow of the crust during transtension
Systematic variations in fabric orientation, ductile strain characteristics, as well as distinct metamorphic evolutions suggest that the detachment mylonites of the Nordfjord‐Sogn detachment zone (NSDZ) and the shear zones in the core represent endmembers of shear zones that formed at different structural levels of the MCC (Figure 7)
Summary
Postorogenic metamorphic core complexes (MCCs) show that the crust reequilibrates after orogeny and an entirely new crustal template may be created (e.g., Brun et al, 2017 ; Buck, 1991 ; Coney, 1980 ; Osmundsen et al, 2005 ; Platt et al, 2015 ; Vanderhaeghe & Teyssier, 2001 ; Whitney et al, 2013). The Western Gneiss Region (WGR) of the SW Scandinavian Caledonides (Figure 1) is an excellent place to do this It represents the deeply eroded core of a Silurian continent‐continent collision orogen where large parts of the crust were turned around during Devonian postorogenic transtension (e.g., Krabbendam & Dewey, 1998). We provide a detailed look inside the anatomy of a well‐exposed postorogenic MCC in order to understand the viscous flow of the crust during transtension. We approach this issue from the micro‐ to the megascale to constrain the timing, quantity, and kinematics of ductile strain as well as the metamorphic conditions and deformation processes
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