Brittle Fosen Deformation History of Fault Rocks on Peninsula, Trendelaq

Brittle deformation history of fault rocks on the Fosen Peninsula, Trendelaq, Central Norway ARNE GR0 NLlE, BJ0RN NILSEN & DAVID ROBERTS Grenlle, A., Nilsen, B. & Roberts, D. 1991: Brittle deformation history of fault rocks on the Fosen Peninsula, Tr0ndeJag, Central Norway. Nor.geo/.unders.Bu ll.42 1, 39-57. Late- and post-Caledonian fault deformation of rock units on the Fosen Peninsula follows a time sequence wherein lower crustal ductile deformation was followed by several episodes of brittle fragmentation, some with coeval hydrothermal mineral precipitation. Field evidence, including vari ous minor structures and hydroth ermal parageneses, supported by fission-track and palaeomagne tic dating, indicates the followi ng general history of development: (1) Sinistral ductile shear in Early to Middle Devonian times is manifested both in regional southwest-directed shear and in concen trated strike-slip movement along the Verran Fault. (2) A phase of retrogress ion of the mylonitic rocks can be recognised within and close to the principal displacement zones. (3) Several episodes of brittle faulting followed , some of which were accompanied by major pulses of hydrothermal fluid penetration. Principal brittle faulting events occurred in Late Devonian, Perrno-Triasstc and post- Mid-Jurassic times. A major phase of zeolite fault mineralisation was associated with the post- Mid-Jurassic event. (4) Dip-slip normal faulting along the MTFZ characterised the Late Juras sic/Early Cretaceous period but was directly followed by a component of dextral strike-slip move ment. (5) NW-SE trending extensional joints and subordinate faults in the Outer Fosen district are considered to relate mainly to Tertiary tectonic events and in particular to a stress field created by ridge-push forces generated from the Mid-Atlantic spreading ridge. (6) NW-SE to NNW-SSE minor joints with plumose and fringe structures represent a latest stage of brittle fracturing on the Fosen Peninsula; these mesojoints may have been initiated in post-glacial times. Arne Grentie & David Roberts , Norges geologiske unaersekeise, Postb oks 3006-Lade, N-7002 Trondheim, Norway . Bjorn Ni/sen, /nstitutt {or Geotoq! og Bergteknikk, Norges Tekniske Hogsk o/e, N-7034 Trondheim NTH, Norway. Introduction In an earlier study of the nort heastern part the bedrock there shows abundant brittle defor of the Mare-Trandelaq Fault Zone (MTFZ) mation structures along the main fault zones. (Gabrielsen & Ramberg 1979a, 1979b, Gabriel As the fault rocks developed both below and sen et al. 1984) northwe st of Trondheimsfjord, above the brittle/ductile transition (Sibson 1977) analysis was based largely on an integration and also carry upper crustal zeolite mineralisa of satellite imagery, aerial photographs and tion, a relative timing of events can be estab existing geological maps (Gr0nlie & Roberts lished. 1989). This showed an array of faults and frac The second area is situated on the coast , tures reflecting a geometric arrangement akin near Stokksund, and is dominated by NW-SE to that of a major dextral strike-slip fault zone. trending fractures. Field mapping has shown It was emphasised, however that the MTFZ that these fractures are mostly vertical extensi had existed as a zone of major crustal weak on joints and their consistent occurrence over ness since at least the time of the Caledonian the entire Outer Fosen coastal region would orogeny, and that slip motions have varied seem to indicate that this particular trend is appreciably, from strike-slip to dip-slip, at diffe of more than local significance. rent points in time. Comparison of the fault and fracture pat In the present account we compare and terns in these two areas shows easily notice discuss the nature of brittle fracturing and able disparate trends, which epitomises the fault rock products in two selected areas on contrasting lineament patterns seen in the the Fosen Peninsula, one within and the other Outer and Inner Fosen districts, to the north outside the MTFZ. Verrabotn -Beitstadfjord, and to the south of the Hitra-Snasa Fault, located within the MTFZ, was chosen because respectively (Fig. 1a-c). 40 Arne Grenue, Bj'Hn Nilsen & David Roberts NGU - BULL.4 21. 1991 B [ LEGEND [ill2] MES OZ OIC AND CE NOZOIC SEDIME NTS ~ BElT STADF JOR D & FROAN BASINS. ~ SEDIMENTS OF MIDDLE JURASSIC AGE r::7l DEV ON IAN OLD RED SANDSTO NE L:2:J MOLASSE SEDIMENT S MAINLY EARLY PALAE OZOIC D CALED ONIAN NAPPE RO CKS Ga MAINLY PROTE ROZOIC BASEME TGNEISSES BEDROCK FOLIAnON FAULT FRACT URE . JOINT Fig. 1 (A): Simplilied bedrock geology and tectonic map 01 the Fosen Peninsula. Central Norway , showing the main geologi cal units and laults as well as foliation trends. (b): TM-lineament trends in the crystalline basement 01 the Outer Fosen area. northwest 01 the Hitra-Snasa Fault. Number 01 lineaments = 397.(c): TM-Iineament trends 01 the Inner Fosen area. southeast 01 the Hitra-Snasa Fault. Number 01 lineaments = 87. The numbers show locations lor measurement 01 Iracture poputations (cl. rose diagrams in Fig. 3, 1-64). Offshore fault interpretations Irom B0e (pers.comm.1989) (Frohavet). B0e & Bjerkli (19B9) (Beitstadljord) and B0e & Sturt (1991) (Asaney area). NGU · BULL. 421. 1991 Brittle deformation history offault rocks 41 Geological setting In the field, most map-scale fractures show The bedrock of the Fosen Peninsula, north up as vertical joints with walls typically 5-10m of the prominent Verran Fault (Fig.1a) is domi apart. The fractures are generally remarkably nated by heterogeneous, Proterozoic mig straight for several kilometres along strike matitic banded gneisses (Wolff 1976, Gee et (Fig.2a), and splays or en echelon patterns al. 1985, Solli 1990). The gneisses are tecto are not common. The proven faults, the Har nostratigraphically overlain by amphibolite bak, Stokkey and Bj0rnahovud faults, show facies psammites, schists and amphibolites, evidence of normal dip-slip movement. Myloni and higher up by low-grade metasediments tic and brecciated rocks testifying to a prolon and greenstones . These 'cover' rocks repre ged history of faulting have been found only sent slices of the Lower, Middle and Upper along the Stokkey fault. On account of the Allochthons of the Caledonide orogen (Gee commonly homogeneous nature of the gneis et aI.1985). The Proterozoic gneissic rocks are sic lithology, faults could only be defined by also strongly banded and Caledonised and detailed bedrock mapping and even then the are considered to form part of the Lower AI amount of throw on individual faults is uncert lochthon. South of the Verran Fault, the bed ain. In the absence of fault rock products it rock consists mainly of Caledonian nappe is generally not possible to distinguish betwe rocks of the Upper Allochthon (Thorsnes & en master joints and faults due to a lack of Grenlie 1990). marker horizons. On 0 rland and in the Aseney area (Fig.1a), Depending on lithology, mesoscop ic joints Late Silurian to Middle Devonian, Old Red are in places bordered by thin, red, hydrother Sandstone molasse sediments are present mal alteration haloes, generally extending 0.5 (Siedlecka 1975, B0e & Sturt 1991). In the 1cm on either side of the central joint. This is Verran area the geology of Beitstadfjord provi a conspicuous feature of the Harbak Peninsu des evidence bearing on the age of fault la (Fig.2a) where thin, NW-SE trending veins movements along the MTFZ. Fragments of transect the granitic migmatite gneiss. The coal and pebbles of sideritic ironstone with overlying garnet amphibolite is dissected by plant fossils found along the shores of this ubiquitous thin joints of the same trend, but fjord are believed to have been derived from without the red alteration. the fjord bottom (Horn 1931, Oftedahl 1975) and are of Middle Jurassic age. A recent shal low-seismic profiling survey has largely con Regional significance of the NW firmed Oftedahl's original Beitstadfjord inter pretation and has also provided additional in SE trend formation on the Late to post-Mesozoic fault Fracture populat ions have been measured on pattern (B0e & Bjerkli 1989). This shows that outcrop at 64 different localities on the Fosen the Jurassic rocks are preserved in a half Peninsula, providing good geographical cover graben structure whose main (Verran) fault age of the study area (Fig.3, 1-64). A total of strikes NE-SW close to the northern Beitstad ca. 2,400 joint orientations were measured. fjord shoreline. The Fosdalen Fault trend, The strik es and dips of ea. 40 randomly recor NNE-SSW, is also an important element of the ded joints were measured at each locality. The Beitstadfjord structural framework (B0e & vast majority of dips are in the 80-90° range, Bjerkli 1989). and joint orientations are thus conveniently illustrated by rose diagrams. The NW-SE to NNW-SSE fracture trend is a dominant one in approximately 40 of the measured sites, and Stokksund area this trend is also most pronounced north of The coastal district of Stokksund and Skjera the MTFZ.However, the nature of fracturing fjorden is dominated by a set of vertical frac on this scale is much more heterogeneo us tures trending N400W (Figs.2a & b). Bedrock than that shown by the map-scale features, mapping (Gr0nlie & M6 11 er 1988, Fossen et the meso-fractures being more influenced by al. 1988) has shown that the t.andsat-TM and local stress field variations. photolineaments of this area are, in fact, main The average length of NW-SE trending linea ly extensional joints and faults, the former ments north of the Hitra-Snasa Fault (HSF) being dominant. between 0 rland and Namsfjorden, as measu- 42 Arne Grentie, Bjem Nilsen & David Roberts GU - BULl. 421, 1991 lfGE liC - ,. ~ A." T " ~ s.,.. .. -- F(l. IAY , ! I \ . Fig. 2 (a): Simplified geological map of the Stokk sund area (Grentie & M611er 1988, Fossen et al.

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