The post-Caledonian tectono-thermal evolution of Western Norway

En riftet kontinentalmargin viser ofte et karakteristisk landskap som domineres av høye og flate fjell som er separert fra en lavtliggende kyststripe av en tydelig skrent. Mye av vår nåværende forståelse av disse områdene er basert på omfattende seismiske studier og data fra borehull samlet inn fra havbunnen utenskjærs, hvor innsamlingen av data er motivert av kontinentalmargin potensiale som kilde til hydrokarboner. Kunnskapen innhentet på land, derimot, blir ofte begrenset av forløpende komplekse berggrunnstrukturer og mangel på overliggende sedimenter som ville gjort innsikten i tektonisk bevegelse lettere. For å få en god forståelse av en riftet kontinentalmargin må kunnskapen vår om den tektoniske og termiske utviklingen på land forbedres. Vest-Norge er et godt eksempel på en riftet kontinentalmargin. Her har dannelsen og utviklingen siden den Kaledonske fjellkjeden har vært en pågående debatt gjennom mange år. Målet med denne avhandlingen er å utvikle en integrert tektonisk og termisk modell for utviklingen av et feltområde som ligger i de nordlige deler av Vest-Norge. For å få til dette, kombineres detaljerte feltstudier av sprø deformasjonsstrukturer med ulike geokronologiske og termokronologiske metoder. Artikkel I fokuserer på utviklingen av sprø deformasjonsstrukturer gjennom tid og rom for det valgte studieområdet. Ved å kombinere fjernanalyser av lineamenter med et omfattende datasett som består av feltobservasjoner av sprekker og forkastninger, gir denne artikkelen ny innsikt i forståelsen av sprø deformasjon for regionen og hvordan denne deformasjonen er påvirket av forløpende duktile strukturer. I tillegg gjennomføres K-Ar datering av sleppemateriale fra seks ulike forkastninger. Studien påviser fire forskjellige spenningsfelt som dominerte i tiden etter den Kaledonske fjellkjeden: 1) NV-SØ kompresjon i silur som ble etterfulgt av NV-SØ ekstensjon i tidlig til midt devon. 2) Sen devon og tidlig karbon var karakterisert av et dominerende sidelengs spenningsfelt, hvor σ1 roteres fra N-S i den nordlige delen av studieområdet, til NØ-SV i den sørlige delen. 3) Et mindre utpreget spenningsfelt som viser Ø-V ekstensjon som trolig sammenfaller med riftfaser i perm-trias eller i sen jura. 4) K-Ar datering av sleppemateriale viser to perioder med økt forkastningsaktivitet i midtre (123-115 Ma) og sen (86-77 Ma) kritt under VNV-ØSØ transtensjon. Artikkel II beskriver resultater fra de første U-Pb dateringer gjennomført på sprekkefyllende kalsitt i Norge. Disse dateringene gir informasjon om subtile tektoniske hendelser som dekker både høye og lave temperaturer, som blant annet er for kalde til å dateres med K-Ar datering av sleppemateriale. I denne studien beskrives det fire regionale tektoniske hendelser ved å kombinere 15 U-Pb kalsittaldre med informasjon fra sprø strukturell deformasjon tilhørende sprekker og forkastninger, samt stabile isotop analyser: 1) Dalsfjord forkastningen, som er en dal av Nordfjord-Sogn skjærsonen, viser reaktivering i trias-jura og som relateres til riftepisoder utenskjærs. 2) Sprekker parallelle med Møre-Trøndelag forkastningskomplekset dateres til sen kritt (90-80 Ma) og kobles til strekking av litosfæren og normal reaktivering langs forkastningskomplekset. 3) Kalsitt fra sprekker som viser varierende orientering er datert til sen kritt – tidlig paleocen alder (70-60 Ma) og relateres til oppløft av en strukturell dom som følge av ankomst av den proto-Islandske manteldiapiren. 4) Kalsitter fra sprekker som viser NØ-SV strøk er datert til ulike aldere yngre enn 50 Ma. Disse alderne kobles til flere episoder av sprekke-utvidelse etter oppbruddfasen av Nord-Atlanteren, noe som indikerer en langvarig kenozoisk deformasjonshistorie. Artikkel III presenterer lav temperatur termokronologiske data fra et høydeprofil fra indre Nordfjord. Denne studien viser den første termiske modellen som inkluderer flere prøver av et høydeprofil fra regionen. Høyden på profilet er 1841 meter (fjellet Skåla) og inkluderer 12 prøver analysert med apatitt fisjonsspor datering (AFT) og 4 prøver analysert med apatitt (U-Th)/He datering. Prøvene viser økende alder med høyde, hvor AFT prøvene viser aldre fra 159 ± 11 Ma til 256 ± 21 Ma og apatitt (U-Th)/He prøvene viser aldre fra 80 ± 4 Ma til 277 ± 15 Ma. Ved å kombinere flere AFT og (U-Th)/He prøver sammen i en flerprøve termisk modell, begrenser vi modellen og reduserer datastøy sammenlignet med modeller som bare inkluderer en prøve. Når modellen testes for ulike termisk avkjølingshistorier, viser modellen en foretrukket utvikling med rask avkjøling fra den Kaledonske fjellkjeden til øvre nivåer i skorpen (~3 km dypt) i tidlig til midtre trias, etterfulgt av langsom og stødig avkjøling gjennom store deler av mesozoikum til sen kritt, hvor avkjølingen igjen øker til den når dagens temperaturer. Flerprøve-modellen tillater også avkjøling til overflatetemperaturer i sen jura om utviklingen etterfølges av oppvarming i kritt og begravelse av omtrent 1,5-3 km med sedimenter. Denne artikkelen understreker at for indre Nordfjord ble de høytliggende og flate fjellpartiene sannsynligvis dannet i kenozoikum og representerer ikke et mesozoisk peneplan. Artikkel IV presenterer et nytt regionalt datasett av lav temperatur termokronologiske data fra studieområdet. Prøver som ble samlet fra hele området resulterte i 29 AFT analyser og 45 apatitt (U-Th)/He analyser av enslige korn. Hverken AFT eller (U-Th)/He aldre viser en korrelasjon mellom alder og høydemeter over havet på prøvelokalitetene. AFT aldere varierer fra 323 ± 27 til 140 ± 4 Ma og (U-Th)/He aldere varierer fra 228 ± 12 til 57 ± 3 Ma. Regionalt sett viser prøver yngre aldre innerst i fjorder og langs Mørekysten, mens de eldre prøvene finnes i nærheten av Hornelen devonbasseng ved havnivå. Prøvene viser stedvis stor spredning i alder over korte avstander, noe som her relateres til sprang langs reaktiverte forkastninger ved Nordfjord-Sogn skjærsonen. Regionen viser en kompleks utvikling i tid og rom, hvor 1) regionen ved indre Nordfjord viser sakte avkjøling gjennom mesozoikum og økt avkjøling fra sen kritt-kenozoikum frem til i dag, som sammenfaller med funn fra artikkel III. 2) Regionen ved Sognefjorden viser liknende avkjølingstrend som indre Nordfjord, men med mindre ned-til-vest normal forkastningsreaktivering i trias/jura langs sprø forkastninger ved kysten som er en del av Nordfjord-Sogn skjærsonen. 3) Regionen rundt Hornelen viser store variasjoner i alder over korte avstander, som relateres til forkastningsaktivitet i sen jura-kritt langs forkastninger med Ø-V strøk. Termiske modeller viser at prøvene ble avkjølt til øvre skorpe (<8 km) fra sen karbon-perm og tillater avkjøling til overflatetemperaturer i sen Jura etterfulgt av oppvarming i kritt. Vi viser at lokale sedimentære bassenger muligens kan ha blitt dannet i hengblokken på disse forkastningsstrukturene i kritt. 4) Møreregionen nord for Nordfjord viser avkjøling til øvre skorpe (<8 km) i jura som kobles til heving av fotvegg under reaktivering i jura av Møre-Trøndelag forkastningskompleks under rifting utenskjærs. Denne studien viser at Vest-Norge har vært gjennom en kompleks tektonisk og termisk utvikling i tid og rom siden den Kaledoniske fjellkjeden for omtrent 400 millioner år siden. Ved å bruke nye tilgjengelige metoder innen termokronologi og geokronologi, og ved å kombinere disse med veletablerte metoder, konkluderer vi blant annet med å vise 1) fremtredende system av strøkforkastninger som ikke tidligere er blitt beskrevet, 2) flere subtile perioder med tektonisk aktivitet i kenozoikum, 3) en robust flerprøve termisk modell som danner et sterkt grunnlag for regionale termiske tolkninger, og 4) at avkjøling langs en riftet kontinentalmargin ikke er kontinuerlig og homogen, men varierer langs strøk og er kontrollert av langvarige forkastningssystem.The onshore areas of rifted margins are often characterized by high elevation and low-relief landscapes separated from a low laying coastal strip by a distinct escarpment. A lot of our current understanding from these areas is today based on extensive seismic studies and well data information from the offshore realm motivated by the margins high potential as a source for hydrocarbons. The knowledge gain from the onshore regions has often been hampered by the complex basement structural architecture and the often lack of overlaying sedimentary records. In order to fully understand the evolution of a rifted margin, however, our understanding of the tectonic and thermal evolution of onshore areas needs to be improved. The margin of Western Norway is one example of a rifted margin where its formation and development since the Caledonian orogeny has been a long lived debate. The aim of the thesis is to develop a integrated tectonic and thermal model for the evolution of the field area located in the northern part of Western Norway by combining detailed field work on brittle structures, with various geo- and thermochronological methods. Paper I of this thesis focuses on the brittle structural temporal and spatial evolution of the study region. By combining an extensive field dataset of fracture and fault information with remote sensing analysis and K-Ar fault gouge dating of six faults, the paper gives new insight into the brittle architecture of the region and how it is influenced by ductile precursor structures. Following the Caledonian orogeny, four major stress fields were resolved: 1) NW-SE Silurian compression was followed by Early to Mid-Devonian NW-SE extension. 2) The late Devonian to early Carboniferous was characterized by a dominant strike-slip stress field, with σ1 rotating from N-S to NE-SW from the northern to the southern study area. 3) A minor E-W extensional stress field was possibly related to the Permian-Triassic or the Late Jurassic offshore rift phases. 4) K-Ar fault gouge dating revealed two periods of fault activity in the mid (123-115 Ma) and the late (86-77 Ma) Cretaceous under a WNW-ESE transtensional stress field. Paper II provides the first ever U-Pb dating of fracture filling calcite from Norway. These data provide information about subtle tectonic events covering both high and low temperature domains, which generally are too cold to be dated by K-Ar fault gouge dating. Based on 15 U-Pb calcite ages, related brittle structural information of the fractures and faults, and stable isotope analysis, four regional tectonic events were deciphered: 1) A Triassic-Jurassic reactivation of the Dalsfjord fault, a fault strand of the Nordfjord-Sogn detachment zone, broadly relates to offshore rift episodes. 2) Late Cretaceous ages (90-80 Ma) of fractures parallel to the Møre-Trøndelag fault complex relate to lithospheric stretching and normal fault reactivation along the fault complex. 3) Late Cretaceous-early Paleocene ages (70-60 Ma) are detected on fractures and faults with various orientations and are related to a domal exhumation following the arrival of the proto-Icelandic mantle plume. 4) Ages younger than 50 Ma, all from fractures and faults with a NE-SW strike, are related to several episodes of fracture dilation during the post-breakup period, indicating a long-lived Cenozoic deformation history. Paper III presents low-temperature thermochronological data from an elevation transect located in the inner Nordfjord. Here, we show the first multi-sample thermal history model of an elevation transect from the region. The elevation transect reaches 1841 masl, includes 12 samples analysed by Apatite Fission Track (AFT) dating and 4 samples analysed by (U-Th)/He dating, and shows increasing ages with elevation; AFT samples yield ages from 159 ± 11 Ma to 256 ± 21 Ma and apatite (U-Th)/He samples yield ages from 80 ± 4 Ma to 277 ± 15 Ma. By combining the AFT and (U-Th)/He data into a multi-sample thermal history model, we increase the constraints on the model and reduce data noise compared to single-sample models. Testing the model for various thermal cooling histories, the model shows a preferred evolution with fast cooling following the Caledonian orogeny to upper crustal levels (~3 km depth) in the Early-Middle Triassic, slow and steady cooling throughout the Mesozoic until the Late Cretaceous, where the cooling again increases until present day surface temperatures. The multi-sample model also allows for cooling to surface temperatures in the Late Jurassic if followed by Cretaceous reheating and reburial by 1.5-3 km of sediments. This paper highlights that for the inner Nordfjord, at least, the high-elevation low-relief surfaces most likely formed during the Cenozoic and do not represent a simply uplifted Mesozoic peneplain. Paper IV provides a new regional dataset of low-temperature thermochronological data from the study area. Samples collected across the region resulted in 29 AFT analyses and 45 single grain apatite (U-Th)/He analyses. Neither the AFT nor the (U-Th)/He samples show a correlation of age and elevation. AFT ages vary from 323 ± 27 to 140 ± 4 Ma and the (U-Th)/He analysis vary from 228 ± 12 to 57 ± 3 Ma. Regionally, the ages are youngest in the inner fjords and along the onshore Møre margin, whereas the oldest ages are found close to the Hornelen Devonian basin at sea level. The ages show large offset over short distances, related to fault offset along reactivated fault strands of the Nordfjord-Sogn detachment zone. The region shows a complex spatial and temporal evolution, where 1) the inner Nordfjord subregion shows slow cooling throughout the Mesozoic and increased cooling by the Late Cretaceous-Cenozoic until present, corresponding to the findings from paper III. 2) The Sognefjord subregion shows a similar general cooling history as the inner Nordfjord, with minor offsets along brittle faults, such as along brittle fault strands of the Nordfjord-Sogn detachment zone, indicating down-to-the-west normal fault reactivation in the Triassic/Jurassic. 3) In the Hornelen basin subregion, large age variations over short distances are related to Late Jurassic-Cretaceous fault offset along steep E-W striking faults. The thermal models reveal that the samples cooled to the upper crust (<8 km) by the Carboniferous-Permian and allow for cooling to surface temperatures in the Late Jurassic followed by Cretaceous reheating. We suggest that local Cretaceous sedimentary basins could have formed in the hanging block of these fault structures. 4) The region north of the Nordfjord shows cooling to the upper crust (<8 km) during the Jurassic and we relate this to footwall uplift from fault reactivation of the Møre-Trøndelag fault complex during Jurassic rifting. This study shows that Western Norway has undergone a complex tectono-thermal spatial and temporal evolution since the Caledonian orogeny. In conclusion, by using newly available methods within thermochronology and geochronology, and combining them with established methods, we here 1) describe prominent strike-slip fault systems not been properly described earlier, 2) detect several pulses of tectonic activity in the Cenozoic, 3) produce robust multi-sample thermal models forming a strong basis for regional thermal interpretations, and 4) highlight that cooling along a rifted margin is not continuous and homogenous but vary along strike, controlled by long lived detachment and fault systems.Doktorgradsavhandlin

The brittle evolution of Western Norway – A space-time model based on fault mineralizations, K–Ar fault gouge dating and paleostress analysis

Basement fracture and fault patterns on passive continental margins control the onshore landscape and offshore distribution of sediment packages and fluid pathways. In this study, we decipher the spatial-temporal evolution of brittle faults and fractures in the northern section of the passive margin of Western Norway by combining field observations of fault mineralizations and K–Ar fault gouge dating with different paleostress approaches, resulting in the following model: (1) High-T fault mineralizations indicate Silurian NW-SE compression followed by NW-SE extension in the Early to Mid-Devonian. (2) Epidote, chlorite and quartz fault mineralizations indicate a dominant strike-slip stress field in the Late Devonian to early Carboniferous. (3) E-W extensional stress fields which could be related to Permo-Triassic or Late Jurassic rifting are not prominent in our data set. (4) K–Ar fault gouge ages indicate two extensive faulting events under a WNW-ESE transtensional stress regime with related precipitation of zeolite and calcite in the mid (123-115 Ma) and late (86-77 Ma) Cretaceous. Our results show that the brittle architecture of the study area is dominated by reactivation of ductile precursors and newly formed strike-slip faults, which is different from the dip-slip dominated brittle architecture of the southern section of the West Norway margin.publishedVersio

Constraining the tectonic evolution of rifted continental margins by U–Pb calcite dating

We employ U–Pb calcite dating of structurally-controlled fracture fills within crystalline Caledonian basement in western Norway to reveal subtle large-scale tectonic events that affected this rifted continental margin. The ages (15 in total) fall into four distinct groups with ages mainly ranging from latest Cretaceous to Pleistocene. (1) The three oldest (Triassic-Jurassic) ages refine the complex faulting history of a reactivated fault strand originated from the Caledonian collapse and broadly correlate with known rifting events offshore. (2) Two ages of ca. 90–80 Ma relate to lithospheric stretching and normal fault reactivation of a major ENE-WSW trending late Caledonian shear zone. (3) We correlate five ages between ca. 70 and 60 Ma with far-field effects and dynamic uplift related to the proto-Iceland mantle plume, the effect and extent of which is highly debated. (4) The five youngest ages (< 50 Ma) from distinct NE–SW trending faults are interpreted to represent several episodes of post-breakup fracture dilation, indicating a long-lived Cenozoic deformation history. Our new U–Pb data combined with structural and isotopic data show that much larger tracts of the uplifted continental margin of western Norway have been affected by far-field tectonic stresses than previously anticipated, with deformation continuing into the late Cenozoic.publishedVersio

Tracing the Sveconorwegian orogen into the Caledonides of West Norway: Geochronological and isotopic studies on magmatism and migmatization

The Sveconorwegian orogen represents a branch of Grenville-age (~1250–950 Ma) orogenic belts that formed during the construction of the supercontinent Rodinia. This study traces the Sveconorwegian records from its type-area in the Baltic Shield of South Norway into basement windows underneath Caledonian nappes, by combining zircon U–Pb geochronology and Hf–O isotopes. Samples along a N-S trending transect reveal multiple magmatic episodes during Gothian (ca. 1650 Ma), Telemarkian (ca. 1500 Ma) and Sveconorwegian (1050–1020 Ma vs. 980–930 Ma) orogenesis as well as Sveconorwegian migmatization (1050–950 Ma). Newly documented 1050–1020 Ma magmatism and migmatization extend the Sirdal Magmatic Belt to a 300 km-long, NNW-SSE trending crustal domain, with the northern boundary corresponding to the gradual transition from Telemarkian to Gothian crust. These Precambrian crustal heterogeneities largely controlled the development of Caledonian shear zones. The ca. 1050–1040 Ma granitic and mafic magmas show similar isotopic signatures with slightly negative or positive εHf(t) and moderate δ18O values (6–7‰), which indicates that crustal reworking was more dominant than juvenile inputs during their genesis. The generation of leucosomes and leucogranites at ca. 1030–1020 Ma, which have a more evolved Hf isotopic composition, probably reflects an even higher degree of remelting of older crust. The Hf–O isotopic patterns show that Sveconorwegian magmas differ from typical arc magmas by lower involvement of sedimentary components and juvenile material. This makes the 1050–930 Ma magmatism incompatible with a long-term subduction setting. The ca. 1650–1500 Ma samples, in contrast, generally have juvenile Hf isotopic compositions associated with varying δ18O values of 4.5–9‰, consistent with subduction-accretion processes involving significant sedimentary recycling. This accretionary margin was most likely transformed into the Sveconorwegian orogen through collisional interactions of Baltica, Laurentia and Amazonia in the context of Rodinia amalgamation.publishedVersio

Lithological and structural analysis of the Rødberget-Rørvika-Varpneset transect, Mid Norwegian Caledonides - Testing tectonostratigraphic correlations and structural models

Remnants of the Caledonian orogen are visible through tectonostratigraphic and structural fingerprints in Central Norway, and the detailed study of key localities is important for the understanding of the overall evolution of this orogeny. In this study, a lithological and structural analysis of the Rødberget-Rørvika-Varpneset transect on Fosen peninsula, South-Trøndelag, is conducted. In earlier studies, several tectonostratigraphic hypotheses were proposed for this area: 1) the central amphibolites belong to the Støren Nappe, whereas the surrounding mica schists belong to the Gula Nappe, 2) Seve Nappe Complex, Gula and Støren nappes are represented in the area, and 3) the entire area can be correlated to the Seve Nappe Complex. In addition, it was proposed that the central amphibolites represent a larger synformal structure. With the help of lithological mapping, geochemical analysis of aplites and amphibolites, and U-Pb zircon dating, these earlier proposed tectonostratigraphic correlations and structural models are tested in this study. The study area is divided into four units: the Rødberget, Trongen, Rørvika and Varpneset units. The Rødberget unit represents Baltoscandian basement, whereas the Trongen, Rørvika and Varpneset units are correlated with the Seve Nappe Complex based on the following lines of evidence: 1) lithological similarities to Seve Nappe Complex rocks in Central Norway and Sweden, 2) penetrative at least amphibolite-facies metamorphism, 3) U-Pb zircon dating of an intermediate layer within the central amphibolites yielding an age of 435.4±3.3 Ma, interpreted to be the result of decompressional melting of amphibolites related to exhumation of the Seve Nappe Complex, and 4) the presence and geochemistry of aplites, which have intruded both mica schists and amphibolites, resembling similar intrusive rocks in the Seve Nappe Complex, which were emplaced at about 430 Ma. In addition, through a detailed structural analysis of foliations, lineations, fold axes and fold vergences it can be shown that the central Rørvika unit is not lying within a synform, but rather represents a wedge-shape structure where the vergence of the smaller folds reflects the movement of the lithologies in the area. Two large-scale models are proposed for the ductile structural evolution of the entire area: 1) an obstacle and buckling model, and 2) a rotation model. Both models are related to the evolution of antiformal basement windows and the Møre-Trøndelag Fault Complex (MTFC). Stress inversion of brittle structures shows that the study area has been affected by a NW-SE extensional event, reactivating the MTFC in Late Paleozoic to Late Mesozoic/Early Cenozoic. These results imply that the area of interest has experienced a different structural evolution than earlier proposed, with a different and more complex metamorphic and structural history. This has implications for tectonostratigraphic correlations across Trondheimsfjorden

Tracing the Sveconorwegian orogen into the Caledonides of West Norway: Geochronological and isotopic studies on magmatism and migmatization

The Sveconorwegian orogen represents a branch of Grenville-age (~1250–950 Ma) orogenic belts that formed during the construction of the supercontinent Rodinia. This study traces the Sveconorwegian records from its type-area in the Baltic Shield of South Norway into basement windows underneath Caledonian nappes, by combining zircon U–Pb geochronology and Hf–O isotopes. Samples along a N-S trending transect reveal multiple magmatic episodes during Gothian (ca. 1650 Ma), Telemarkian (ca. 1500 Ma) and Sveconorwegian (1050–1020 Ma vs. 980–930 Ma) orogenesis as well as Sveconorwegian migmatization (1050–950 Ma). Newly documented 1050–1020 Ma magmatism and migmatization extend the Sirdal Magmatic Belt to a 300 km-long, NNW-SSE trending crustal domain, with the northern boundary corresponding to the gradual transition from Telemarkian to Gothian crust. These Precambrian crustal heterogeneities largely controlled the development of Caledonian shear zones. The ca. 1050–1040 Ma granitic and mafic magmas show similar isotopic signatures with slightly negative or positive εHf(t) and moderate δ18O values (6–7‰), which indicates that crustal reworking was more dominant than juvenile inputs during their genesis. The generation of leucosomes and leucogranites at ca. 1030–1020 Ma, which have a more evolved Hf isotopic composition, probably reflects an even higher degree of remelting of older crust. The Hf–O isotopic patterns show that Sveconorwegian magmas differ from typical arc magmas by lower involvement of sedimentary components and juvenile material. This makes the 1050–930 Ma magmatism incompatible with a long-term subduction setting. The ca. 1650–1500 Ma samples, in contrast, generally have juvenile Hf isotopic compositions associated with varying δ18O values of 4.5–9‰, consistent with subduction-accretion processes involving significant sedimentary recycling. This accretionary margin was most likely transformed into the Sveconorwegian orogen through collisional interactions of Baltica, Laurentia and Amazonia in the context of Rodinia amalgamation