| dc.description.abstract | 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. | nb_NO |
