DYNAMICS and ACTIVE PROCESSES - International Lithosphere ...

gneissic) rocks occurred on the northern part of the WGR. The MTFC as discrete sinistral
ENE-WSW steep ductile shear zones (Robinson, 1995) formed probably during the Scandian
Orogeny towards the end of the Caledonian Cycle, since it slices through several of Scandian
thrust sheets (Grønlie and Roberts 1989). The fault complex has been repeatedly reactivated
in the brittle domain from Devonian time to the Cenozoic (?) and exhibits a great variety of
fault rocks, exposed mostly on the Fosen Peninsula (i.e. Verran and Hitra-Snåsa faults, Watts
2001 and references therein). These comprise mylonites overprinted by recrystallised breccias
with quartz and epidote veining, phrenite-matrixed cataclasites, cut by zeolite/calcite veining
associated with brecciation, pseudotachylites and zeolite-calcite slickenfibres. The complexity
and variety of fault rocks reflects the longevity of the MTFC. The different fault rocks reveal
the strength evolution of the fault complex and suggest significant weakening of some of its
segments by repeated reactivation (see discussion in Pascal & Gabrielsen 2001). This applies
to some of the primary segments of the present-day MTFC. Some segments of the southern
onshore MTFC do not display such a cortege of fault rocks and exhibit, in turn, a restricted
variety of fault rocks and associated mineralogy (Saintot & Pascal 2010). While epidote
which might be the marker of the Devonian deformation, is widespread along the northern
onshore segments of the MTFC, the southern segments are laumontite-rich and hence, are
believed to represent the latest propagation (presumably Late Jurassic) and widening of the
MTFC.
Structural mapping in combination with analytical dating techniques have revealed three main
phases of activity along the MTFC (Grønlie & Roberts 1989, Séranne 1992, Sherlock et al.
2004): (1) Early Devonian sinistral strike-slip characterised by semi-ductile deformation, (2)
Early Permian sinistral transtension and (3) Late Jurassic (to Early Cretaceous?) normal dipslip
to dextral strike-slip. Normal dip-slip reactivation of the MTFC is assumed to have
occurred in Cenozoic times (Redfield et al. 2005) but to date no clear evidence has been found
yet. The three first phases of fault activity reflect, respectively (Gabrielsen et al. 1999): (1) the
collapse of the Caledonian mountain chain, (2) widespread Permian rifting and (3) Late
Jurassic rifting of the northern North Sea and the Mid-Norwegian margin. The MTFC appears
to have exerted a strong influence on the evolution of the offshore basins, in particular in
controlling their structural and depositional styles though time (Osmundsen et al. 2006). The
fourth phase of activity of the fault complex was linked to the postulated uplift of the
Norwegian mountains while the offshore basins were subsiding in Cenozoic times (Redfield
et al. 2005). The MTFC appears to have strongly controlled the evolution of the landscape
onshore, including the creation of preferential pathways for Quaternary glaciers. The MTFC
is still seismically active today (Olesen et al. 2004). By means of numerical modelling, Pascal
& Gabrielsen (2001) suggested that it acts as a very weak zone, resulting in disparate stress
patterns to the north and south. Recent observations of stress-induced features and in-situ
stress measurements support the modelling results (Roberts & Myrvang 2004).
Little is known about the deep structure (e.g. dip directions of the faults), the links with the
offshore fault segments and the precise kinematics and segmentation of the whole MTFC. The
aims of the ongoing “MTFC integrated” project are (1) to reveal the deep structure of the
MTFC using geophysical methods, (2) to investigate its offshore prolongation and (3) to study
its kinematics and structural evolution. We use a large panel of geophysical methods,
including gravimetry, magnetic profiling, shallow EM methods and seismic
reflection/refraction profiles constrained by petrophysical sampling and structural
observations. Our geophysical observations and experiments show that the MTFC onshore
bounds a major horst structure similar to the ones evidenced by long-range seismic profiling
offshore (Lundberg et al. 2009). Analysis of regional magnetic and gravity analysis allows for
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