Exhumation of the Caledonian Orogenic Infrastructure in West Norway : Concepts – Structures – Ages – Reactivation
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Domes of high-grade metamorphic rocks are found in recent and ancient orogens alike. They commonly expose parts of the deep interior of mountain belts, the so-called orogenic infrastructure. This thesis explores the controversial topic of infrastructure exhumation in two different ways: First, through reviewing the history of tectonic research leading to the concept of metamorphic core complexes (MCCs). Second, in a case study of basement windows in the deeply eroded Paleozoic Caledonian orogen of West Norway. Including the (ultra)high-pressure Western Gneiss Region, these domal windows record Caledonian continental subduction of the Baltic Shield and controversial exhumation mechanisms. Paper 1 explores the lithological and structural composition of the Baltic Shield in the eastern part of the Øygarden Complex, which represents the westernmost Caledonian basement window. Secondary-ion mass spectrometry (SIMS) U-Pb zircon geochronology constrains two pulses of voluminous Sveconorwegian intrusions into a Telemarkian (1506 ± 5 Ma) granitic basement. Bimodal magmatism at 1041 ± 3 Ma, followed by leucogranite magmatism at 1027-1022 Ma, correlate the Øygarden Complex with the Sirdal Magmatic Belt in South Norway. A low-temperature resetting of metamict grains at ~482 Ma is the only Caledonian record in zircon. The Sveconorwegian intrusions were strongly reworked by Caledonian ductile deformation. Starting at amphibolite facies conditions, retrograde top-to-E shearing involved fluid-induced phyllonitization that localized ductile-to-brittle low-angle shear zones. Structural features of the Øygarden Complex are discussed in the light of MCC exhumation. Paper 2 investigates the southernmost culmination of the eclogite-bearing Western Gneiss Region in the footwall of the Nordfjord-Sogn detachment (Gulen dome). Semi-quantitative mapping of ductile strain along glacier-polished fjords reveals two distinct structural levels. The amphibolite-facies core domain involved fluid-controlled eclogite retrogression and records coaxial E-W stretching in subvertical shear zones and vast extension-perpendicular shortening in upright folds. Detachment mylonites wrap around the core and record amphibolite-facies to semi-brittle, non-coaxial deformation. Our reconstruction constrains dome formation through extension-perpendicular inward flow of solid-state material in the deep crust isostatically compensating upper crustal thinning. Differential folding within solid-state MCCs can resemble the dynamics of migmatite-cored MCCs and exhume deep parts of the crust. Paper 3 provides the first comprehensive study of the Øygarden Complex with a focus on ductile-to-brittle Caledonian structures. The comparison of 23 classified shear zones and a structural dataset comprising >4500 data constrain pervasive E-W stretching with distinct deformation styles at three structural levels. The upper unit shows localized ductile deformation and top-to-E kinematics, in contrast to distributed flow and top-to-W kinematics in the middle unit. The lower unit consists of migmatites, which define a double-dome in the core of the complex. Retrograde shearing affected all levels and localized ductile-to-brittle shear zones through fluid-induced weakening (phyllonitization). We suggest exhumation of a bivergent MCC in two stages. Extension-perpendicular flow of partially molten crust formed the migmatite dome in response to intra-crustal necking. Fluid-induced retrograde weakening feedbacks occurred in a wide zone and facilitated detachment formation and rapid exhumation of previously ductile crust. Paper 4 presents new geochronology from the previously poorly dated Gulen and Øygarden domes and integrates them in an orogen-scale synthesis of infrastructure exhumation. Three of four SIMS U-Pb zircon migmatite samples from the Øygarden Complex date melt crystallization at 405 ± 3 Ma. This result conforms to dated melts in other parts of the Western Gneiss Region, but largely expands the known spatial extent of Devonian melting. Most of the 28 new Ar-Ar white mica and biotite dates from shear zones in the Øygarden and Gulen domes record short-lived MCC exhumation in between 405 and 399 Ma, in accordance with previously published ages. We compiled a database comprising >450 U-Pb zircon, monazite, titanite and rutile as well as Ar-Ar white mica, biotite and hornblende ages. The intermingled spatial distribution of different chronometers suggests that ages cannot be assigned to simple cooling histories. On the other hand, various chronometers robustly constrain the duration of ductile stretching in each segment of the infrastructure, with significant variations between different segments (~10-30 Myr). We combine our new geochronological insights with structural geometries, inferred from large-scale foliation-trace mapping and previous descriptions of the windows. Temperature-pressure-time-deformation discontinuities within the Caledonian infrastructure are explained in a model of segmented transtensional MCC exhumation. Paper 5 combines field observations from onshore West Norway with new 3D seismic data revealing deep structures underneath the northern North Sea rift. We correlate two distinct structural units from the Gulen dome to corresponding seismic facies in the acoustic basement offshore. Our interpretation reveals an offshore dome, which resembles the onshore dome in size, geometry and kilometer-scale, upright folds. Both domes are connected by a >100 km long, shallowly W-dipping, extensional detachment zone with a non-planar, hyperbolic geometry. Devonian collapse formed dome and detachment, which were reactivated during Permian-Triassic North Sea rifting. Brittle reactivation of steep detachment segments resulted in strongly deviating rift fault orientations at the eastern margin of the rift around 61°N. This study highlights large spatial and temporal variations in the evolution of the Caledonian infrastructure, intimately linked to preceding and subsequent tectonic events. During Caledonian collision the infrastructure represented a cold and rigid slab that allowed continental subduction. During Devonian transtensional collapse, in contrast, it represented a variably mobile substrate that could flow in response to pressure gradients. Structural inheritance might have controlled along-strike segmentation of the Caledonian orogen. During North Sea rifting it behaved rigid again, but inherited weak zones were inevitably reactivated. The conceptual focus of this study may be its biggest limitation. On the other hand, it highlights the usefulness of the paradigm of extensional MCCs: It is a paradigm that raises many, testable questions.
Paper I: Wiest, J. D., Jacobs, J., Ksienzyk, A. K., and Fossen, H., 2018, Sveconorwegian vs. Caledonian orogenesis in the eastern Øygarden Complex, SW Norway – Geochronology, structural constraints and tectonic Implications. Precambrian Research, v. 305, p. 1-18. The article is available in the main thesis. The article is also available at: https://doi.org/10.1016/j.precamres.2017.11.020Paper II: Wiest, J. D., Osmundsen, P. T., Jacobs, J., and Fossen, H., 2019, Deep Crustal Flow Within Postorogenic Metamorphic Core Complexes: Insights From the Southern Western Gneiss Region of Norway. Tectonics, v. 38, p. 4267-4289. The article is available at: http://hdl.handle.net/1956/22070Paper III: Wiest, J. D., Fossen, H., and Jacobs, J., Rheological Evolution of Ductile Crust Exhumed in a Metamorphic Core Complex. The article is not available in BORA.Paper IV: Wiest, J. D., Jacobs, J., Fossen, H., Ganerød, M. and Osmundsen, P. T., Melting, flow and post-orogenic exhumation of the Caledonian infrastructure, W Norway. The article is not available in BORA.Paper V: Wiest, J. D., Wrona, T., Bauck, M. S., Fossen, H., Gawthorpe, R.L., Osmundsen, P. T. and Faleide, J. I. From Caledonian Collapse to North Sea Rift – The Extended History of a Metamorphic Core Complex. The article is not available in BORA.
PublisherThe University of Bergen
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