Kinematics of the Høybakken detachment zone and the Møre–Trøndelag Fault Complex, central Norway

Abstract

The tectonic disintegration of the Caledonian orogen through combined extension, contraction and strike-slip was characterized by spatial and temporal strain partitioning through a period of at least 30 Ma. Early to Mid-Devonian exhumation of the Central Norway basement window was associated with retrograde, top-to-the-SW extensional shearing in the Høybakken detachment zone, sinistral shearing along the Møre-Trøndelag Fault Complex, and formation of extension-parallel folds. Progressive exhumation led to increasing strain localization and to the transition from ductile to brittle deformation. In the interval between c. 370 and 320 Ma, the Høybakken detachment fault cut previously folded detachment mylonites, capturing mylonites in its hanging wall. 40 Ar/ 39 Ar mica and K-feldspar ages indicate a Late Devonian or younger age for the uppermost parts of the adjacent 'Old Red' basin. Gentle folding of this stratigraphic level attests to the continuation of shortening and orogen-oblique extension into Late Devonian-Carboniferous time. Shortening was intensified along strands of the Møre-Trøndelag Fault Complex, as shown by mutually cross-cutting reverse and strike-slip faults. 'Flower structures' may be particularly common in constrictional strain systems where strike-slip faults develop parallel to the principal elongation trend, but normal to the principal axis of shortening.

Styles of extension offshore mid‐Norway and implications for mechanisms of crustal thinning at passive margins

“…It could be argued that the Slørebotn detachment reactivated an existing structural grain in the basement, such as mylonites inherited from Devono‐Carboniferous postorogenic collapse. Indeed, the relationship between the Slørebotn detachment and strands of the MTFC resemble the relationship between the MTFC and the Devono‐Carboniferous Høybakken detachment ∼120 km to the northeast [ Osmundsen et al , 2006]. It is important to note, however, that the extension direction changed ∼90° from the Devonian to the Cretaceous, and that basin flank detachments such as the Bremstein Fault Complex clearly cuts the Paleozoic configuration of structures and basins [e.g., Osmundsen et al , 2002].…”

Crustal‐Scale Fault Interaction at Rifted Margins and the Formation of Domain‐Bounding Breakaway Complexes: Insights From Offshore Norway

“…For the Mid‐Norwegian margin, this structural boundary is located onshore and corresponds to an array of (commonly reactivated) extensional shear zones and faults with normal and oblique displacements that roughly follow the line along which the crust becomes ~38–40 km thick (Mosar, ; Olesen et al, ; Redfield & Osmundsen, ). Extensional structures mapped between this area and the coast comprise mainly shear zones and faults related to Devonian “orogenic collapse” (e.g., Braathen et al, , ; Osmundsen et al, , ; Séranne, ), some with several up to tens of kilometers of displacement, that dissect and displace the Caledonian nappe pile and bound gneiss‐cored extensional culminations in their footwalls (Osmundsen et al, ). The brittle detachments in southwest Norway are associated with a regional, top to the west extensional shear zone up to several kilometers thick (Andersen & Jamtveit, ), and it has been suggested that they were connected with the Høybakken Detachment of Mid Norway along a NE‐SW trending shear zone segment that later became the locus of the Møre‐Trøndelag fault Complex (Osmundsen et al, ; Séranne, ) (Figure ).…”

“…Extensional structures mapped between this area and the coast comprise mainly shear zones and faults related to Devonian “orogenic collapse” (e.g., Braathen et al, , ; Osmundsen et al, , ; Séranne, ), some with several up to tens of kilometers of displacement, that dissect and displace the Caledonian nappe pile and bound gneiss‐cored extensional culminations in their footwalls (Osmundsen et al, ). The brittle detachments in southwest Norway are associated with a regional, top to the west extensional shear zone up to several kilometers thick (Andersen & Jamtveit, ), and it has been suggested that they were connected with the Høybakken Detachment of Mid Norway along a NE‐SW trending shear zone segment that later became the locus of the Møre‐Trøndelag fault Complex (Osmundsen et al, ; Séranne, ) (Figure ). Extensional shear zones in Mid Norway north of the Høybakken Detachment appear to have accommodated less displacement than the latter and are separated by relay zones expressed as deflections in the Caledonian nappes (Osmundsen et al, , ).…”

Metamorphosed cumulate gabbros from the Støren Group of the Upper Allochthon, northern Western Gneiss Region, Norway: petrology and metamorphic record

“…13B, C), during which deep, hot rocks were emplaced against the cooler Upper Allochthon. This episode continued into the period of orogen-parallel extension superimposed on that detachment (Terry & Robinson, 2003;Robinson et al, 2004Robinson et al, , 2014Osmundsen et al, 2006). The last part of this episode is confidently dated in the northern Western Gneiss Region at ~395 Ma, or late Emsian, in the Early Devonian Krogh et al, 2011).…”