Devonian subduction and syncollisional exhumation of continental crust in Lofoten, Norway

Froitzheim, Nikolaus; Miladinova, Irena; Janák, Marian; Kullerud, Kåre; Ravna, Erling J. Krogh; Majka, Jarosław; Fonseca, Raúl O. C.; Münker, Carsten; Nagel, T.

doi:10.1130/g37545.1

Cited by 28 publications

(25 citation statements)

References 22 publications

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“…Hence, like in the Bergen Arcs case, amphibolite‐ and eclogite‐facies mineral assemblages appear to form synchronously, and the occurrence of eclogite is confined to rheologically weak shear zones. This is again consistent with high‐P metamorphism due to weakening induced pressure perturbations at “normal” lower crustal depths and not to subduction to extreme depths (~90 km) as proposed by Froitzheim et al ().…”

Section: The Lofoten Areasupporting

confidence: 90%

“…Eclogites or retrograded eclogites have been described from several localities at Flakstadøya in Lofoten (Froitzheim et al, 2016;Kullerud et al, 2001;Markl & Bucher, 1997;Steltenpohl et al, 2006). They invariably occur in shear zones embedded in feldspar-rich host rocks.…”

Section: Eclogites In Shear Zones At Flakstadøyamentioning

confidence: 99%

“…They invariably occur in shear zones embedded in feldspar-rich host rocks. Conditions of formation were initially estimated at 1.1-1.5 GPa pressure and a temperature of 650-700°C (Kullerud et al, 2001;Markl & Bucher, 1997), but a recent study suggests pressures approaching 2.8 GPa (Froitzheim et al, 2016).…”

Section: Eclogites In Shear Zones At Flakstadøyamentioning

confidence: 99%

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The Effects of Earthquakes and Fluids on the Metamorphism of the Lower Continental Crust

et al. 2019

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Rock rheology and density have first‐order effects on the lithosphere's response to plate tectonic forces at plate boundaries. Changes in these rock properties are controlled by metamorphic transformation processes that are critically dependent on the presence of fluids. At the onset of a continental collision, the lower crust is in most cases dry and strong. However, if exposed to internally produced or externally supplied fluids, the thickened crust will react and be converted into a mechanically weaker lithology by fluid‐driven metamorphic reactions. Fluid introduction is often associated with deep crustal earthquakes. Microstructural evidence, suggest that in strong highly stressed rocks, seismic slip may be initiated by brittle deformation and that wall‐rock damage caused by dynamic ruptures plays a very important role in allowing fluids to enter into contact with dry and highly reactive lower crustal rocks. The resulting metamorphism produces weaker rocks which subsequently deform by viscous creep. Volumes of weak rocks contained in a highly stressed environment of strong rocks may experience significant excursions toward higher pressure without any associated burial. Slow and highly localized creep processes in a velocity strengthening regime may produce mylonitic shear zones along faults initially characterized by earthquake‐generated frictional melting and wall rock damage. However, stress pulses from earthquakes in the shallower brittle regime may kick start new episodes of seismic slip at velocity weakening conditions. These processes indicate that the evolution of the lower crust during continental collisions is controlled by the transient interplay between brittle deformation, fluid‐rock interactions, and creep flow.

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Section: The Lofoten Areasupporting

confidence: 90%

Section: Eclogites In Shear Zones At Flakstadøyamentioning

confidence: 99%

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The Effects of Earthquakes and Fluids on the Metamorphism of the Lower Continental Crust

et al. 2019

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“…This is in contrast with anorthositic and granulitic 15 Baltica crust in the Western Gneiss Region (WGR) and Lofoten further south in Norway. These crustal segments were subducted to depths >100 km with little internal deformation (Engvik et al, 2000;Hacker et al, 2010;Froitzheim et al, 2016).…”

Section: Discussion: Nappe Stacking In Continental Collision Zonesmentioning

confidence: 99%

An anticlockwise metamorphic P-T path and nappe stacking in the Reisa Nappe Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision

Faber¹,

Stünitz²,

Gasser³

et al. 2018

Preprint

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This study investigates the Caledonian metamorphic and tectonic evolution in northern Norway, examining the structure and tectonostratigraphy of the Reisa Nappe Complex (RNC; from bottom to top, Vaddas, Kåfjord and Nordmannvik 15 nappes). Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and P-T conditions of deformation and metamorphism that formed the nappes and facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P-T path attributed to the effects of early Silurian heating followed by thrusting. An early Caledonian S1 foliation in the Nordmannvik Nappe records kyanite-grade partial melting at ~760 -790 °C and ~9.4 -11 kbar. Leucosomes formed at 439 ± 2 Ma (U-Pb zircon) in fold 20 axial planes in the Nordmannvik Nappe indicate that compressional deformation initiated while the rocks were still partially molten. This stage was followed by pervasive solid-state shearing as the rocks cooled and solidified, forming the S2 foliation at 680 -730 °C and 9.5 -10.9 kbar. Multistage titanite growth in the Nordmannvik Nappe records this extended metamorphism between 444 and 427 Ma. In the underlying Kåfjord Nappe, garnet cores record lower P-T (590 -610 °C and 5.5-6.8 kbar) but a similar geothermal gradient as the S1 migmatitic event in the Nordmannvik Nappe, indicating formation at a higher 25 relative position in the crust. S2 shearing in the Kåfjord Nappe occurred at 580-605 °C and 9.2-10.1 kbar, indicating a considerable pressure increase during nappe stacking. Gabbro intruded in the Vaddas Nappe at 439  1 Ma, synchronously with migmatization in the Nordmannvik Nappe. In the Vaddas Nappe S2 shearing occurred at 630-640 ºC and 11.7-13 kbar.Titanite growth along the lower RNC boundary records S2-shearing at 432 ± 6 Ma. It emerges that early Silurian heating (~440 Ma), probably resulting from large-scale magma underplating, initiated partial melting that weakened the lower crust, which 30 facilitated dismembering of the crust into individual nappe units. This tectonic style contrasts subduction of mechanically strong continental crust to great depths.Solid Earth Discuss., https://doi.Figure 1: (a) Simplified map of the Scandinavian Caledonides and their inferred palaeotectonic origin, modified from Gee et al. (2010). LN = Lyngsfjellet Nappe, WGR = Western Gneiss Region, NNC = Narvik Nappe Complex, KNC = Kalak Nappe Complex, RNC = Reisa Nappe Complex., TNC = Trondheim Nappe Complex. (b) Map showing the extent of the Reisa Nappe Complex in northern Norway, based on Zwaan (1988) and own correlations. The Vaddas, Kåfjord and Nordmannvik nappes are shown in 5colour. The study area is denoted by the black box (Fig. 2).

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“…A weighted mean temperature yields 620-625 (± 15)°C using the P-dependent Tomkins calibration [P at 27 kbar (Froitzheim et al, 2016)] and around 550 (± 12)°C using the Watson calibration (Table 2). The calculated T for both samples using the P-dependent…”

Section: Peak T Of Fluidsmentioning

confidence: 99%

Slow cooling versus episodic fluid injections: Deciphering the Caledonian orogeny in Vestvågøy, Lofoten islands, Norway

Fournier

Lee

Camacho

2019

Journal Metamorphic Geology

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The determination of the thermal (temperature-time) histories of highpressure metamorphic terranes has been commonly based on the concepts of slow cooling and closure temperatures. In this paper, we find that this approach cannot reconcile a geochronological data set obtained from the amphibolite-facies allochthonous Leknes Group of the Lofoten islands, Norway, which reveals an extremely complex thermal history. Using detailed results from several different geochronometers such as 40 Ar/ 39 Ar, Rb-Sr, and U-Pb, we show that a model invoking multiple, short-lived thermal pulses related to hot-fluid Accepted ArticleThis article is protected by copyright. All rights reserved. infiltration channelized by shear zones can reconcile this complicated data set. This model suggests that hot fluids infiltrated throughout basement shear zones and affected the overlying cold allochthon, partially resetting U/Pb titanite and rutile ages, crystallizing new zircon, and produced identical 40 Ar/ 39 Ar and Rb/Sr ages in muscovite, biotite, and amphibole in various rocks throughout the region. This paper shows the enormous potential of coupling laser Ar-spot data with thermal modelling to identify and constrain the duration of short-lived events.An optimal P-T-t history has been derived by modelling the age data from a previously dated large muscovite crystal (Hames & Andresen, 1996) and using Zr-in-rutile thermometry which is consistent with all geochronological data and geological constraints from the basement zones and allochthon cover. This tectonothermal model history suggests that there has been three episodic hot-fluid and 40 Ar-free infiltration events, resulting in the total resetting of Ar ages during the Scandian (425 Ma) for 1 Ma at 650°C, and two reheating events at 415 Ma for 400 ka at 650°C and at 365 Ma for 50 ka at 600°C, which are modelled as thermal spikes above an ambient temperature of 300°C. Independent confirmation of these parameters was provided by Pb-diffusion modelling in rutile and titanite. The model suggest that the amphibolite-facies rocks of the Leknes Group probably remained cold before being exhumed for at least 60 Ma (425-365 Ma) and successfully explains the presence of different minerals that crystallised or were totally/partially reset in the allochthon and in the basement. The migration of hot fluids for short period of times within conduits extending through the basement and allochthon rock units is likely associated with episodic seismic activity.

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Devonian subduction and syncollisional exhumation of continental crust in Lofoten, Norway

Cited by 28 publications

References 22 publications

The Effects of Earthquakes and Fluids on the Metamorphism of the Lower Continental Crust

The Effects of Earthquakes and Fluids on the Metamorphism of the Lower Continental Crust

An anticlockwise metamorphic P-T path and nappe stacking in the Reisa Nappe Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision

Slow cooling versus episodic fluid injections: Deciphering the Caledonian orogeny in Vestvågøy, Lofoten islands, Norway

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