Abstract
AbstractExtensional systems evolve through different stages due to changes in the rheological state of the lithosphere. It is crucial to distinguish ductile structures formed before and during rifting, as both cases have important but contrasting bearings on the structural evolution. To address this issue, we present the illustrative ductile‐to‐brittle structural history of a metamorphic core complex (MCC) onshore and offshore western Norway. Combining geological field mapping with newly acquired 3‐D seismic reflection data, we correlate two distinct onshore basement units (BU1 and BU2) to corresponding offshore basement seismic facies (SF1 and SF2). Our interpretation reveals two 40 km wide domes (one onshore and one offshore), which both show characteristic kilometer‐scale, westward plunging upright folds. The gneiss domes fill antiformal culminations in the footwall of a >100 km long, shallowly west dipping, extensional detachment. Overlying Caledonian nappes and Devonian supradetachment basins occupy saddles of the hyperbolic detachment surface. Devonian collapse of the Caledonian orogen formed dome and detachment geometries. During North Sea rifting, brittle reactivation of the MCC resulted in complex fault patterns deviating from N‐S strike dominant at the eastern margin of the rift. Around 61°N, only minor N‐S faults (<100 m throw) cut through the core of the MCC. Major rift faults (≤5 km throw), on the other hand, reactivated the detachment and follow the steep flanks of the MCC. This highlights that inherited ductile structures can locally alter the orientation of brittle faults formed during rifting.
Highlights
The rheological state of the lithosphere largely determines the mode of continental extension (Brun et al, 2018; Brune et al, 2017; Buck, 1991; Whitney et al, 2013)
During North Sea rifting, brittle reactivation of the metamorphic core complex (MCC) resulted in complex fault patterns deviating from N‐S strike dominant at the eastern margin of the rift
Brittle Reactivation of Ductile Structures Both, the Gulen dome and the Øygarden Complex, have abundant exposures (Figure 4) that show a variable relationship of Early Devonian to Mesozoic brittle faults (Fossen et al, 2016; Ksienzyk et al, 2014, 2016; Larsen et al, 2003) and ductile structures mostly formed during Caledonian postorogenic collapse (Wiest et al, 2019, 2020)
Summary
The rheological state of the lithosphere largely determines the mode of continental extension (Brun et al, 2018; Brune et al, 2017; Buck, 1991; Whitney et al, 2013). Other hand, can provide important insights for the influence of structural inheritance on early rift phases (Fazlikhani et al, 2017; Fossen et al, 2016; Phillips et al, 2019). Both field and seismic studies have strengths and weaknesses when it comes to the study of ductile and brittle structures. State‐of‐the‐art 3‐D seismic reflection data, on the other hand, reveal three‐dimensional structural geometries over large areas and to great depths Since both methods complement each other, this study combines onshore and offshore observations to address the progressive evolution of a metamorphic core complex (MCC) from orogen collapse to rifting. The onshore‐offshore correlation allows us to investigate (1) three‐dimensional aspects of dome and detachment formation and (2) the role of the inherited MCC during rifting
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