Rifting of Collapsed Orogens: Successive Incision of Continental Crust in the Proximal Margin Offshore Norway

Abstract

Structures related to orogenic "collapse" may provide an important template for reactivation during later rifting. In the proximal rifted margin offshore Norway, the tectono-sedimentary template related to collapse of the Caledonian mountain belt became widely eroded but inherited metamorphic core complexes and detachment faults evolved further in Late Paleozoic and Earliest Mesozoic time and became associated with very significant crustal thinning. The earliest rifting mode was dominated by structures that mimic the Devonian post-Caledonian template and comprised a suite of Late Paleozoic to Early Triassic metamorphic core complexes, detachment faults, and supradetachment basins. The deep structure indicates necking of brittle over remnant ductile crust, and we document how the main decoupling level utilized by the earliest rifting structures were cut by a suite of steeper, more deeply incising set of seaward-and landward-dipping Triassic faults. The latter exploited deep shear zones related to the previous intracrustal necking, enabling changes in polarity over short distances. The change from a core complex mode of extension to one involving more deeply incising faults took place prior to deposition of the Late Triassic-Early Jurassic interrift succession and pre-dated the main phase of lithospheric-scale necking associated with Jurassic-Cretaceous extension. The latter phase of large-magnitude faulting commenced with reactivation of seaward-dipping, deeply incised faults inherited from early rifting as well as structures related to the phase of Devonian post-orogenic transtension. The current study demonstrates that great structural complexity and significant magnitudes of extension may be present in proximal margins, in particular those developed along collapsed orogens. Plain Language Summary The extensional "collapse" of orogens leaves behind a structural template which becomes reused and developed further in subsequent stages of rifting. Offshore Mid-Norway, it was the earliest phase of rifting that became most profoundly affected by this inherited structural template. A system of detachment zones and faults, similar in style but younger in age than the orogenic collapse structures, became responsible for very significant crustal thinning in what was to become the margin's proximal domain. A suite of Late Paleozoic into Early Triassic basins became associated with this stage of deformation. This early configuration became cut by more deeply rooted normal faults in the Early Triassic, prior to thermal relaxation and deposition of a Late Triassic and Early Jurassic "interrift sequence." In Late Jurassic times, a suite of seaward-dipping large magnitude faults reactivated the more steeply dipping Triassic faults, heralding what was to become a necking of the entire crust. The significance of these observations lies in the documentation of the complex structural evolution and very significant crustal thinning that may accompany formation of the proximal parts of margins that develop on collapsed orogens.

Rheological inheritance controls the formation of segmented rifted margins in cratonic lithosphere

“…The Scotian margin, for instance, shows major changes in the crustal architecture and the magmatic budget during breakup, with a magma-poor segment in the north and a magma-rich segment in the south 48 . Whereas the Norwegian margin, on the other hand, appears to be affected by a strong structural segmentation from the earliest phases of extension preserved in the proximal domain 49 , 50 to segmentation along the necking and hyperextension-exhumation domains 51 . Unlike the Labrador Sea, which opened perpendicular to Precambrian terrains, these margins initiated on, and parallel to, Palaeozoic Appalachian and Caledonian terrains, respectively, with a substantially more complex structural inheritance.…”

Repeated brittle reactivations of a pre-existing plastic shear zone: combined K–Ar and 40Ar–39Ar geochronology of the long-lived (>700 Ma) Himdalen–Ørje Deformation Zone, SE Norway

“…Viola et al (2011) observed that an early high-grade fabric associated with top-to-the SE compression is partly reworked by later lower-grade (greenschist facies) top-tothe-W extensional shearing. The Ørje Shear Zone has been traced southwards into the Grann zone (Hageskov, 1980), which is a local equivalent to the Dalsland boundary thrust (Berthelsen, 1980;Gorbatschev, 1988;Park et al 1991) and is accordingly regarded as a part of the Dalsland boundary thrust and the Lerdal zone system (Berthelsen, 1980;Park et al 1991). The latter is described as an imbricate stack of shear zones dipping 20-40°to the W and with a top-to-the-E displacement (Park et al 1991).…”

Potential secondary events caused by early Holocene paleoearthquakes in Fennoscandia – a climate-related review

“…Fig. 3 shows an earthquake cluster (Byrkjeland et al, 2000) in the area where the North Sea N-S-trending Mesozoic fault system interacts with the older, Devonian and Mesozoic, NE-SW Møre-Trøndelag Fault Complex (MTFC) system (Doré et al, 1997;Gabrielsen et al 1999;Olsen et al, 2007;NPD, 2020;Osmundsen et al, 2021), whereas the eastern branches of the MTFC are mostly aseismic at present. It is likely that this region experienced tectonic deformation during deglaciation, not least since some deformation is still ongoing (Fig.…”