Deep Crustal Flow Within Postorogenic Metamorphic Core Complexes: Insights From the Southern Western Gneiss Region of Norway. Issue 12 (17th December 2019)

• Record Type: Journal Article
• Authors: Wiest, J. D.; Osmundsen, P. T.; Jacobs, J.; Fossen, H.
• Is Part Of: Tectonics. Volume 38:Issue 12(2019)
• Page Start: 4267
• Page End: 4289
• Publication Date: 2019-12-17
• Subjects: Post‐orogenic collapse -- Transtension -- Eclogite retrogression -- Continental core complexes -- Exhumation of high‐pressure rocks; Geology, Structural -- Periodicals; 551.8
• DOI: 10.1029/2019TC005708
• Languages: English
• ISSNs: 0278-7407

Abstract

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 of time. Key Points: A deeply eroded and particularly well‐exposed transtensional core complex reveals mechanisms of solid‐state viscous flow in the deep crust Vertical metamorphic variations and lateral strain gradients lead to differential folding of distinct crustal levels Solid‐state flow mechanisms are similar to anatectic crust and can contribute to postorogenic exhumation of (ultra‐)high‐pressure rocks