Two‐Stage Cooling and Exhumation of Deeply Subducted Continents

Abstract

The burial and exhumation of continental crust during collisional orogeny exert a strong control on the dynamics of mountain belts and plateaus. Constraining the rates and style of exhumation of deeply buried crust has proven difficult due to complexities in the local geology and thermochronometric methods typically used. To advance this field, we applied trace-element and U-Pb laser ablation inductively coupled plasma mass spectrometry analyses to rutile from eclogite and amphibolite samples from the Western Gneiss Complex of Norway-an archetypal continental (ultra)high-pressure (UHP) terrane. Peak temperature and timing of midcrustal cooling were constrained for samples collected along a subduction-and exhumation-parallel transect, using Zr-in-rutile thermometry and U-Pb rutile geochronology, respectively. Peak temperatures decrease from 830°C in the UHP domain to 730°C at the UHP-HP transition, remain constant at 730°C across most of the terrane, and decrease to 620°C at the eclogite-out boundary. U-Pb results show that most of the terrane cooled through 500°C at 380-375 Ma except for the lowest grade region, where cooling occurred approximately 20 million years earlier. The results indicate that exhumation was a two stage process, involving (1) flexural rebound and slab flattening at depth combined with foreland-directed extrusion, followed by (2) synchronous cooling below 500°C across the, by then, largely flat-lying Western Gneiss Complex. The latter implies and requires relatively homogeneous mass removal across a large area, consistent with erosion of an overlying orogenic plateau. The Caledonides were at near-equatorial latitudes at the time. A Caledonian paleo-plateau thus may represent a so far unrecognized factor in Devonian and Carboniferous atmospheric circulation and climate forcing.

Sveconorwegian vs. Caledonian orogenesis in the eastern Øygarden Complex, SW Norway – Geochronology, structural constraints and tectonic implications

“…Yet, it remains controversial to what extent melting initiated at ultrahigh-pressure conditions (e.g., Kohn et al, 2015) and whether pressure estimates reflect other processes than burial only (e.g., Vrijmoed et al, 2009). Amphibolite-facies reworking and exhumation of the WGR are dated by U-Pb monazite (410-390 Ma;Hacker et al, 2015;Holder et al, 2015), U-Pb titanite (405-385 Ma;Kylander-Clark et al, 2008;Spencer et al, 2013), U-Pb rutile (400-375 Ma;Butler et al, 2018;Cutts et al, 2019), and Ar-Ar white mica ages (405-375 Ma;Chauvet & Dallmeyer, 1992;Walsh et al, 2007;Young et al, 2011;Walsh et al, 2013). These ages show regional trends with younger ages toward the NW and imply progressive SE to NW unroofing of the WGR between 400 and 375 Ma.…”

“…Exhumation models of the WGR commonly involve two stages. The first stage is seen either as postorogenic eduction of the continental slab , gravity-driven ductile rebound of the orogenic root Milnes & Koyi, 2000), or flexural rebound and flattening of the slab (Cutts et al, 2019). A second phase comprising transtensional collapse of the overthickened crust and the formation of large-magnitude detachments is widely accepted (e.g., Butler et al, 2015;.…”

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

“…Yet, it remains controversial to what extent melting initiated at ultrahigh‐pressure conditions (e.g., Kohn et al, ) and whether pressure estimates reflect other processes than burial only (e.g., Vrijmoed et al, ). Amphibolite‐facies reworking and exhumation of the WGR are dated by U‐Pb monazite (410–390 Ma; Hacker et al, ; Holder et al, ), U‐Pb titanite (405–385 Ma; Kylander‐Clark et al, ; Spencer et al, ), U‐Pb rutile (400–375 Ma; Butler et al, ; Cutts et al, ), and Ar‐Ar white mica ages (405–375 Ma; Chauvet & Dallmeyer, ; Walsh et al, ; Young et al, ; Walsh et al, ). These ages show regional trends with younger ages toward the NW and imply progressive SE to NW unroofing of the WGR between 400 and 375 Ma.…”

“…Exhumation models of the WGR commonly involve two stages. The first stage is seen either as postorogenic eduction of the continental slab (Andersen et al, ; Fossen, ), gravity‐driven ductile rebound of the orogenic root (Milnes et al, ; Milnes & Koyi, ), or flexural rebound and flattening of the slab (Cutts et al, ). A second phase comprising transtensional collapse of the overthickened crust and the formation of large‐magnitude detachments is widely accepted (e.g., Butler et al, ; Fossen, ; Krabbendam & Dewey, ).…”

A diachronous record of metamorphism in metapelites of the Western Gneiss Region, Norway

“…DesOrmeau et al, 2015; Kylander‐Clark et al, 2007; Root et al, 2005; Terry et al, 2000a). It provides the unique opportunity to investigate the timescales of deep continental burial and exhumation, as well as the structural processes associated with UHP metamorphism (Butler et al, 2018; Cutts et al, 2019; Roberts, 2002).…”