Deep Crustal Flow Within Postorogenic Metamorphic Core Complexes: Insights From the Southern Western Gneiss Region of Norway

Abstract

Viscous 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.Plain Language Summary The Earth's crust has different layers with contrasting mechanics.Rocks in the upper crust tend to break, while higher temperatures at depth make rocks flow, although very slowly. This contrast is important when continents collide forming mountain belts but also when plates drift apart and mountain ranges collapse. In SW Norway, hundreds of million years of erosion have exposed rocks that once were deep below a large mountain range (the Caledonides). Today, glacier-polished fjords reveal large dome structures that formed when the Caledonides collapsed. Inside such a dome, we find rocks originating from different levels of the crust. Rocks and structures in the core formed at high pressures and temperatures. Wrapped around, we find rocks that deformed while they cooled down, became more resistant to flow, and finally broke apart. Above the dome, we find remnants of the upper crust, which was broken up by faults, eroded, and deposited in sedimentary basins. We reconstruct how mechanical contrasts between crustal layers brought rocks from the root of the mountain belt in contact with sediments deposited at the surface. Understanding this process is important, because it can entirely transform the crust within a-geologically speaking-short period o...

From Caledonian Collapse to North Sea Rift: The Extended History of a Metamorphic Core Complex

“…We suggest that extension-perpendicular inward flow of deep crustal low-viscosity material formed both domes in response to upper crustal thinning during Devonian transtension (Wiest et al, 2019). The onshore and offshore domes formed in the footwall of the same large-magnitude detachment (Figure 5a), likely related to individual excisement splays (sensu Lister & Davis, 1989).…”

“…The description of the Gulen MCC by Wiest et al (2019) provides the onshore part of this study, complemented by observations from the detachment footwall exposed in the Øygarden Complex (Wiest et al, 2018, 10.1029/2020TC006178 Tectonics 2020). The Gulen MCC is a westward plunging antiform consisting of a high-grade metamorphic core with subvertical foliations flanked by detachment shear zones (Figure 1).…”

“…Both, the Gulen dome and the Øygarden Complex, have abundant exposures (Figure 4) that show a variable relationship of Early Devonian to Mesozoic brittle faults (Fossen et al, 2016;Ksienzyk et al, 2014Ksienzyk et al, , 2016Larsen et al, 2003) and ductile structures mostly formed during Caledonian postorogenic collapse (Wiest et al, 2019(Wiest et al, , 2020. Steep foliation surfaces have been commonly reactivated as brittle slip surfaces (Figure 4a), especially where mica-rich layers are present (Figure 4b).…”

“…Note the gradual transition between the seismic facies and overprinting of the Western Gneiss Region (WGR; BU1) by the detachment (BU2). Seismic data courtesy of CGG.10.1029/2020TC006178 Onshore-offshore map and cross sections of the study area; onshore parts modified fromWiest et al (2019). (a) The offshore map shows the depth-contoured enveloping surface of the MCC.…”

“…Outcrop-scale field examples showing variable relationship between brittle faults and ductile structures described byWiest et al (2019) andWiest et al (2020). (a) Steeply dipping foliation surface reactivated as fault plane coated with chlorite and epidote.…”