Doctoral Dissertations

Date of Award

12-1993

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Robert D. Hatcher, Jr.

Committee Members

Steve Driese, Hap McSween, Carol Remenyik

Abstract

The central Scandinavian Caledonides comprise a historic area for the study of thrust and nappe tectonics and metamorphic petrology, and more recently have been an important site for studies of Paleozoic plate tectonics. Reconstruction of the tectonic history of the lapetus Ocean basin between Greenland and Baltica, prior to and during its final closure in the Siluro- Devonian Scandian orogeny, largely depends upon deciphering the tectonic history of the upper allochthon, a composite thrust complex containing elements of an accreted terrane. Although final emplacement of the upper allochthon occurred during the Siluro-Devonian Scandian orogeny, the complex experienced an intricate and still debatable early Paleozoic history. In the central Norwegian Caledonides the upper allochthon comprises the Trondheim nappe complex (TNC). Determination of the nature of internal contacts and timing of assembly of the TNC has been hampered by wide variations in lithology, strain, and metamorphic grade, and scanty age constraints in some rock units. In the Einunnfjellet-Savalen area, southeastern TNC, a polydeformed continental margin quartzite-schist sequence occurs in the core of the Einunnfjellet dome. These rocks, which can be linked to the Baltoscandian miogeocline, were overthrust by the Savalen thrust complex (STC), containing fragments of oceanic crust and associated sediments, then eroded and unconformably succeeded by polymict conglomerate and fine-grained clastic sediment. Regional correlations indicate that tectonostratigraphic relationships similar to the Einunnfjellet-Savalen area are widespread in the TNC and throughout the upper allochthon, and isotopic age dates or faunal control in these areas imply that thrusting of the STC onto the underlying quartzite-schist sequence occurred prior to the Scandian orogeny.

The Einunnfjellet dome is defined by a major NE-SW doubly plunging antiformal axis, and a minor axis that plunges gently NW-SE. A complex interference structure, the dome resulted from several phases of deformation. The first, D1, involved structures present only in rocks stratigraphically below the unconformity. F1 isoclinal, recumbent folds are indicated by a type-2 refold pattern in the core of the dome, and mesoscopic refolded F1 folds occur in pelitic and amphibolitic rocks in the STC. Detailed petrologic study of the rocks in the Einunnfjellet- Savalen area, however, does not reveal firm evidence of metamorphism related to this event.

The Scandian orogenic phase, D2, which produced the NNE-striking antiformal structure of the dome, was a progressive deformation that can be divided into two main phases: a flattening phase (D2a) followed by a phase of pervasive shearing (D2b). The earliest mesoscopic F2a folds are inclined isoclinal folds with an axial planar schistosity, and are overprinted by pervasive D2b deformation that produced tight flexural flow buckle folds in quartzite and other competent rocks, and crenulations in schist. In some rocks two distinct foliations related to these phases can be distinguished, although locally the regional foliation is a composite foliation that formed during a single metamorphic event. Textural zoning in garnet indicates that garnet growth began late during Dža in some samples, although the dominant period of growth was synkinematic with respect to D2b, suggesting that metamorphism peaked after the transition from flattening to a simple shear regime. Temperatures remained high after deformation ceased, however, as indicated by garnet rims that overgrew the crenulated foliation. Garnet-biotite thermometry and amphibole composition indicate a progressive metamorphic increase from greenschist to amphibolite facies across the area. The transition zone between the two facies (i.e., the epidote-amphibolite facies) is characterized by a change from albite to oligoclase and presence of coexisting calcic amphiboles in mafic rocks, and strong compositional zoning of garnet in pelitic rocks. Mineral assemblages and compositions are consistent with medium-pressure regional metamorphism.

Late gentle warping of the Einunnfjellet antiform about a NW-SE axis accentuated the elongate dome structure, and reoriented fold axes and mineral lineations to a NE plunge on the north side of the dome. The thrust contact between the STC and the underlying quartzite- schist sequence is entirely overprinted by Scandian metamorphism-no metamorphic break occurs at the contact and D2 structures occur in both the hangingwall and footwall rocks- consistent with the STC having been emplaced prior to Scandian orogenesis. In contrast, the eastern contact of the STC with the Fundsjø Group (Meraker nappe) is a prograde D2b shear zone that developed at greenschist facies conditions. Kinematic analysis indicates a clockwise (dextral) sense of movement on the shear zone, although the amount of displacement may be relatively minor.

Much of the areal extent of the TNC comprises the central Gula nappe, a thick sequence of metasedimentary rocks that contains subordinate and discontinuous mafic/ultramafic horizons termed the Gula greenstone. Although much work has been done on metavolcanic sequences that border the Gula nappe on the west (Støren and related complexes) and east (Fundsjo/ Hersjo complexes), the tectonic environments of the Gula nappe have remained speculative. Field and geochemical evidence from two metaigneous complexes that occur in the Einunnfjellet-Savalen area, indicates that both complexes formed in a suprasubduction-zone setting. Metabasalts of the Lomsjødalen complex, directly correlative with the Gula greenstone, are primitive arc tholeiites. Overlying metalliferous sediments and possible remnants of black smokers suggest a spreading ridge environment, probably either an opening marginal basin or the earliest stage of island arc development. Mafic rocks of the Bangardsvola complex, previously correlated with either the Fundsjø Group or with rocks equivalent to the structurally lower Seve nappe, have boninitic affinities uniquely characteristic of subduction- related settings. Although isotopic dating is not yet available for the metaigneous rocks of the Einunnfjellet-Savalen area, they are lithologically and geochemically similar to dated Early Ordovician Norwegian ophiolites that formed in a suprasubduction-zone environment. The association of primitive IAT, boninite, and Fe- and Mn-rich metalliferous sediments is strongly analogous to the upper parts of the Troodos ophiolite, Cyprus. Although the tectonic setting of Troodos is still highly debated, Mooreset al. (1984) suggested that an actualistic model for this and other Mideast ophiolites is provided by the Andaman Sea region of the Indian Ocean, where short spreading segments formed above an oblique subduction zone. Some variation of this model could account for the field and geochemical features of the metavolcanic rocks of the southeastern TNC, and other Norwegian ophiolites that have similar characteristics.

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