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GEOLOGICAL INVESTIGATIONS OF THE REPPARFJORD TECTONIC WINDOW, NORTHERN NORWAY: Backtracking two billion years of geological history by structural analysis, K–Ar and Re–Os geochronology and carbonate chemostratigraphy

Torgersen, Espen
Doctoral thesis
Åpne
Fulltext not available (Låst)
Permanent lenke
http://hdl.handle.net/11250/2360529
Utgivelsesdato
2015
Metadata
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Samlinger
  • Institutt for geovitenskap og petroleum [2175]
Sammendrag
This thesis presents the results of a multidisciplinary geological study of the

Repparfjord Tectonic Window (RTW) in northern Norway. The study was

carried out to refine the understanding of the RTW’s long geological evolution

and was motivated by the urgent need for a modern, well-constrained

geological model for future ore mineral potential assessment in northern

Norway. The structural, tectonic and depositional development of the RTW is

investigated and presented in five self-standing manuscripts. These contribute

also to an improved understanding of several fundamental geological processes

in the field of fault-rock dating, fault mechanics, Re–Os pyrite–chalcopyrite

systematics and Early Paleoproterozoic C isotopic fluctuations and

geodynamics.

The RTW is exposed within the Caledonian Kalak Nappe Complex

along the northwestern passive margin of the Eurasian plate and comprises a c.

8000 m thick Early Paleoproterozoic sedimentary–volcanic succession, which

represents the northwestern-most termination of the Fennoscandian Shield.

This succession is intruded by two Paleoproterozoic Svecofennian magmatic

suites and is, in turn, overlain by a Neoproterozoic sedimentary cover.

In contrast to the intraplate rift-setting inferred for most parts of the

Fennoscandian Shield in the Early Paleoproterozoic, the Repparfjord

sedimentary–volcanic succession is interpreted to have formed in a continental

back–arc domain. The revised stratigraphy of the RTW suggests that arkosic

sandstones and volcaniclastic conglomerates and siltstones of the Saltvann

Group represent the lowermost exposed stratigraphic level. The group’s

lithofacies association is interpreted to reflect deposition in a rapidly subsiding

half-graben basin. C isotope ratios in carbonate rocks constrain the depositional

age of this group to a relatively short time window of no longer than 75 Myr,

and possibly as short as c. 10 Myr, around the termination of the global

Lomagundi–Jatuli isotopic event at c. 2060 Ma. The present-day geometry of

the northwestern RTW is that of a km-scale upright anticline, the Ulverygg

Anticline. Metabasaltic rocks of the Nussir Group rest on the northwestern

limb and the chronologically correlative metabasaltic- to rhyolitic volcanic rocks

of the Holmvann Group on the southeastern limb. Carbonate

chemostratigraphy constraints a post-2060 Ma depositional age of the Porsa

Group, the uppermost exposed group in the Repparfjord succession.

Cu-rich carbonate veins were emplaced (in metabasalts) and structurally

reactivated during episodes of oscillating frictional–viscous deformation that

caused repeated brecciation along the vein margins and mylonitization of the

core. Re–Os pyrite–chalcopyrite geochronology constrains the initial

emplacement of the veins at c. 2069 Ma. K–Ar fault gouge dating indicates

reactivation of the veins during the Silurian Caledonian Orogeny. Structural

analysis shows that fracturing and vein formation took place in a dextral

transpressive deformation corridor during a phase of overall NW–SE

shortening, which represents one of the earliest phases of Paleoproterozoic

contraction ever documented in Fennoscandia. The study demonstrates that

strain and ingress of oxidizing fluids has a significant, yet localized, effect on

the isotopic integrity of the Re–Os pyrite–chalcopyrite system. The isotopic

disturbance of the Re–Os chronometer in calc-ultramylonitic intervals is

interpreted as independent evidence for considerable fluid flow during grainsize

sensitive viscous deformation.

The northwestern-most part of the RTW is characterized by a series of

discrete thrusts that repeat the stratigraphy and form the Porsa Imbricate Stack

(PIS). The tectonic repetition of metabasalts, dolostones and slates within the

PIS was largely controlled by the geometry of inherited Paleoproterozoic kmscale

folds and by localized strain weakening steered by dolomite decarbonation

and metabasalt carbonation reactions. K–Ar fault gouge dating of brittle–

ductile and brittle faults constrains the timing of PIS development to the main

episode of nappe emplacement during the Caledonian orogeny (c. 445–400

Ma), although pre-Caledonian (c. 530 Ma; Finnmarkian?) deformation is also

indicated. Extension and fault reactivation occurred in the Carboniferous (c.

330–300 Ma) and Early Cretaceous (c. 120 Ma). The study has demonstrated

that the common inclined K–Ar "age–grain-size" spectra obtained from the

dating of fault gouges reflect primarily mixing between authigenic and

protolithic mineral phases inherited from either the wall rock or from an earlier

faulting event.
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NTNU
Serie
Doctoral thesis at NTNU;2015:165

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