Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology and fault-slip analysis

Walsh, Emily O.; Hacker, Bradley R.; Gans, Phillip B.; Wong, Martin S.; Andersen, Torgeir B.

doi:10.1016/j.tecto.2013.06.030

Cited by 43 publications

(53 citation statements)

References 80 publications

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“…Most of these dikes are at least weakly deformed, indicating that melt crystallization and deformation of the WGR persisted for a considerable time over a broad area, prior to cooling of the terrane recorded by 40 Ar/ 39 Ar white mica dates. Though there are relatively few zircon dates from Scandian melts compared to mica and titanite dates [ Kylander‐Clark et al ., ; Root et al ., ; Spencer et al ., ; Walsh et al ., ], the difference in zircon dates and white mica chrontours (Figures and ) can constrain the time interval from the onset of amphibolite‐facies conditions to cooling through circa 400°C. However, it is first necessary to estimate the P‐T conditions at which the melts were produced and crystallized.…”

Section: Discussionmentioning

confidence: 99%

“…[]). Other low‐pressure dates from previous studies include muscovite dates (blue chrontours) [ Root et al ., ; Hacker et al ., ; Walsh et al ., ] and inherited (pre‐Scandian) titanite dates (green) [ Spencer et al ., ]. Scandian deformation intensities from Hacker et al .…”

Section: Dating the End Of Uhpm In The Western Gneiss Regionmentioning

confidence: 99%

“…These differences could include factors such as temperature, duration of heating, composition of fluid influx, etc. [Kylander-Clark et al, 2008;Root et al, 2005;Spencer et al, 2013;Walsh et al, 2013], the difference in zircon dates and white mica chrontours (Figures 4 and 8) can constrain the time interval from the onset of [Terry and Robinson, 2003;Walsh and Hacker, 2004;Young et al, 2007]: Nordøyane UHP domain (A); southeast HP area (B); Nordfjord UHP domain (C). Graywacke experimental solidus and 20% melt contour [Auzanneau et al, 2006] indicate conditions in which exhuming crust is likely to melt, i.e., the eclogite-amphibolite facies boundary.…”

Section: 1002/2014tc003582mentioning

confidence: 99%

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Age and significance of felsic dikes from the UHP western gneiss region

Kylander‐Clark

Hacker

2014

Tectonics

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Twenty-one plagioclase-bearing dikes were analyzed to place firmer limits on the end of (ultra)high-pressure (UHP) metamorphism across the Western Gneiss Region (WGR). Nineteen dikes were analyzed with laser ablation split-stream petrochronology to tie the U-Pb dates to zircon rare earth element (REE) chemistry, and a few key samples were analyzed by chemical abrasion thermal ionization mass spectrometry to provide high-precision constraints. All analyzed dikes yielded zircons with REE chemistry consistent with low-pressure crystallization. Approximately half of the dikes yield Precambrian dates; nondeformed dikes of this age support previous interpretations that much of the WGR underwent limited deformation during Caledonian subduction and exhumation. The oldest Caledonian dikes have dates that overlap with the circa 420-400 Ma eclogite dates from the region; this discrepancy indicates that either (1) cryptic structures separate early exhumed material from later exhumed material, (2) some of the dike dates are not low-pressure crystallization ages, or (3) post-406 Ma dates from eclogites are posteclogite-facies retrogression ages.

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Section: Discussionmentioning

confidence: 99%

Section: Dating the End Of Uhpm In The Western Gneiss Regionmentioning

confidence: 99%

Section: 1002/2014tc003582mentioning

confidence: 99%

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Age and significance of felsic dikes from the UHP western gneiss region

Kylander‐Clark

Hacker

2014

Tectonics

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“…The history of Scandian eclogite facies metamorphism in the WGR is fairly well constrained: U–Pb chronology of zircon and Lu/Hf and Sm/Nd geochronology of garnet indicate burial from 425 to 400 Ma (Root et al ., ; Johnston et al ., ; Kylander‐Clark et al ., , ; Peterman et al ., ; Krogh et al ., ; Beckman et al ., ). Cooling and exhumation is recorded by titanite U–Pb and muscovite 40 Ar/ 39 Ar ages from 400 to 380 Ma (Fossen & Dallmeyer, ; Walsh et al ., , ; Spencer et al ., ).…”

Section: Regional Geologymentioning

confidence: 99%

Does continental crust transform during eclogite facies metamorphism?

Young

Kylander‐Clark

2015

Journal Metamorphic Geology

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Eclogites within exhumed continental collision zones indicate regional burial to depths of at least 60 km, and often more than 100 km in the coesite‐stable, ultra‐high pressure (UHP) eclogite facies. Garnet, omphacitic pyroxene, high‐Si mica, kyanite ± coesite should grow at the expense of low‐P minerals in most felsic compositions, if equilibrium obtained at these conditions. The quartzofeldspathic rocks that comprise the bulk of eclogite facies terranes, however, contain mainly amphibolite facies, plagioclase‐bearing assemblages. To what extent these lower‐P minerals persisted metastably during (U)HP metamorphism, or whether they grew afterwards, reflects closely upon crustal parameters such as density, strength and seismic character. The Nordfjord area in western Norway offers a detailed view into a large crustal section that was subducted into the eclogite facies. The degree of transformation in typical pelite, paragneiss, granitic and granodioritic gneiss was assessed by modelling the equilibrium assemblage, comparing it with existing parageneses in these rocks and using U/Th–Pb zircon geochronology from laser ablation ICPMS to establish the history of mineral growth. U–Pb dates define a period of zircon recrystallization and new growth accompanying burial and metamorphism lasting from 430 to 400 Ma. Eclogite facies mafic rock (~2 vol.% of crust) is the most transformed composition and records the ambient peak conditions. Rare garnet‐bearing pelitic rocks (<10 vol.% of crust) preserve a mostly prograde mineral evolution to near‐peak conditions; REE concentrations in zircon indicate that garnet was present after 425 Ma and feldspar broke down after 410 Ma. Felsic gneiss – by far the most abundant rock type – is dominated by quartz + biotite + feldspar, but minor zoisite/epidote, phengitic white mica, garnet and rutile point to a prograde HP overprint. Relict textures indicate that much of the microstructural framework of plagioclase, K‐feldspar, and perhaps biotite, persisted through at least 25 Ma of burial, and ultimately UHP metamorphism. The signature reaction of the eclogite facies in felsic rocks – jadeite/omphacite growth from plagioclase – cannot be deduced from the presence of pyroxene or its breakdown products. We conclude that prograde dehydration in orthogneiss leads to fluid absent conditions, impeding equilibration beyond ~high‐P amphibolite facies.

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“…400 Ma in the east, and by ca. 385-360 Ma in the west, with the youngest ages preserved in the UHP domains (Root et al, 2005;Walsh et al, 2007Walsh et al, , 2013Steenkamp, 2012).…”

Section: The Western Gneiss Region: a Large And Hot Late-orogenic Uhpmentioning

confidence: 99%

Paradigm lost: Buoyancy thwarted by the strength of the Western Gneiss Region (ultra)high-pressure terrane, Norway

2015

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Subduction and exhumation of ultrahigh-pressure (UHP) metamorphic terranes are typically envisaged as short-lived processes associated with the transition from oceanic subduction to continent-continent collision. Norway's Western Gneiss Region, by comparison, is a giant, late-orogenic UHP terrane that underwent protracted residence at UHP conditions during the Scandian phase of the Caledonian orogeny followed by relatively slow exhumation. Here, we use two-dimensional numerical thermal-mechanical models to explore the tectonics of orogens of this type and the associated controls on the size of their UHP terranes and the duration of UHP metamorphism.The models have four tectonic phases designed to capture the main stages of the Caledonian evolution: oceanic subduction and microcontinent accretion; continental margin subduction; plate quiescence; and postorogenic extension during plate divergence. Contrasting styles of exhumation are explored by varying the strength of the margin crust and investigating melt-induced weakening. The tectonic and metamorphic evolution of the Western Gneiss Region is consistent with a model in which continental margin crust was subducted beneath a thick orogenic wedge where it underwent metamorphism at (U)HP conditions for at least 15 million years (Myr) as subduction ended. The buoyant Baltican crust must have been especially strong in order to have stayed coupled to the underlying lithosphere during this phase, perhaps reflecting its refractory composition and/or a lack of fluids. Subsequent exhumation of the Western Gneiss Region can be explained by orogen-scale extension resulting from minor (~100 km) plate divergence, with removal of the orogenic wedge by combined top-to-thehinterland transport, normal faulting, and erosion. We conclude that large, long-duration UHP terranes are fundamentally different from transient smaller ones. The latter are often explained by the paradigm of buoyant exhumation. This paradigm is incomplete, but both types can be explained by control of the system by the exhumation number (ratio of buoyancy force to basal traction). By implication, the existence of this type of large UHP terrane is a consequence of the high strength of the subducted crust.

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Crustal exhumation of the Western Gneiss Region UHP terrane, Norway: 40Ar/39Ar thermochronology and fault-slip analysis

Cited by 43 publications

References 80 publications

Age and significance of felsic dikes from the UHP western gneiss region

Age and significance of felsic dikes from the UHP western gneiss region

Does continental crust transform during eclogite facies metamorphism?

Paradigm lost: Buoyancy thwarted by the strength of the Western Gneiss Region (ultra)high-pressure terrane, Norway

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