The T‐Reflection and the Deep Crustal Structure of the Vøring Margin, Offshore mid‐Norway

Abstract

Seismic reflection data along volcanic passive margins frequently provide imaging of strong and laterally continuous reflections in the middle and lower crust. We have completed a detailed 2‐D seismic interpretation of the deep crustal structure of the Vøring Margin, offshore mid‐Norway, where high‐quality seismic data allow the identification of high‐amplitude reflections, locally referred to as the T‐Reflection. Using a dense seismic grid, we have mapped the geometry of the T‐Reflection in order to compare it with filtered Bouguer gravity anomalies and seismic refraction data. The T‐Reflection is identified between 7 and 10 s. Sometimes it consists of one single smooth reflection. However, it is frequently associated with a set of rough multiple reflections displaying discontinuous segments with varying geometries, amplitudes, and contact relationships. The T‐Reflection seems to be connected to deep sill networks and is locally identified at the continuation of basement high structures or terminates over fractures and faults. The T‐Reflection presents a low magnetic signal. The spatial correlation between the filtered positive Bouguer gravity anomalies and the deep dome‐shaped reflections indicates that the latter represent a high‐impedance boundary contrast associated with a high‐density and high‐velocity body. In ~50% of the outer Vøring Margin, the depth of the mapped T‐Reflection is found to correspond to the depth of the top of the Lower Crustal Body (LCB), which is characterized by high P wave velocities (>7 km/s). We present a tectonic scenario, where a large part of the deep crustal structure is composed of preserved upper continental crustal blocks and middle to lower crustal lenses of inherited high‐grade metamorphic rocks. Deep intrusions into the faulted crustal blocks are responsible for the rough character of the T‐Reflection, whereas intrusions into the ductile lower crust and detachment faults are likely responsible for its smoother character. Deep magma intrusions can be responsible for regional metamorphic processes leading to an increasing velocity of the lower crust to more than 7 km/s. The result is a heterogeneous LCB that likely represents a complex mixture of pre‐ to syn‐breakup mafic and ultramafic rocks (cumulates and sills) and old metamorphic rocks such as granulites and eclogites. An increasing degree of melting toward the breakup axis is responsible for an increasing proportion of cumulates and sill intrusions in the lower crust.

Cretaceous‐Paleocene Evolution and Crustal Structure of the Northern Vøring Margin (Offshore Mid‐Norway): Results from Integrated Geological and Geophysical Study

“…Notably, the gravity high observed at that level also fits a high‐amplitude reflection observed at 15 km in the deep part of the Grimm High (Figures a, b, and ). The deep reflection coincides spatially with the deep and faulted T ‐Reflection described and defined in the outer Vøring Basin (Abdelmalak et al, ; Gernigon et al, , ). At the level of the Grimm High, the new seismic data revealed tilted fault block system below the volcanics (Figure ), with a structural style similar to that observed in the adjacent North Gjallar Ridge (Gernigon et al, , ; Ren et al, ).…”

“…The gravity anomaly is clearly connected to a prominent high‐amplitude reflection observed in the deep part of the high (Figure ). We have to note here that the observed reflection represents some local deep features and we were not able to establish any direct and spatial connection with the notable and similar deep T ‐Reflection defined and described farther west in the outer part of the Vøring Basin (see Abdelmalak et al, ; Gernigon et al, ). In this study, we named it the UH (Utgard High)‐Reflection to specify the difference from the T ‐Reflection.…”

“…Earlier schemes suggest that LCB in the distal part of the mid‐Norwegian Margin represented mafic crust underplated during continental breakup (e.g., White & McKenzie, ). However, later studies promoted a variety of alternatives on the origin and properties of LCB, ranging from high‐velocity intrusions into the lower crust (White et al, , ), heavily serpentinized/exhumed mantle (Osmundsen & Ebbing, ; Péron‐Pinvidic & Osmundsen, ; Ren et al, ; Reynisson et al, ), and/or a retrograde/high‐grade Caledonian lower crust (Abdelmalak et al, ; Ebbing et al, ; Gernigon et al, ; Mjelde et al, ; Petersen & Schiffer, ).…”

“…While analogue and numerical models deal mostly with homogenous and uniform mode of extension in the rifted margins (Lavier & Manatschal, ), crustal inheritance and late magmatism could explain complex geometries and deformation migration and localization discussed in this paper. The inherited crustal blocks and associated structures/fabrics initiate and control the onset and development of thinning deformations of the continental crust (Abdelmalak et al, ). This “boudinage style of deformation” is similar to what has been previously described and proposed by Lister et al (), Gartrell (), or Clerc et al ().…”

“…The most recent studies have suggested some similarities with the Iberian (magma‐poor) type of passive margins (Lundin & Doré, ; Péron‐Pinvidic & Osmundsen, ; Reynisson et al, ; Rüpke et al, ) implying the presence of relatively wide, serpentinized/exhumed mantle domains in the central, distal, and volcanic parts of the margin. In contrast, other publications consider fundamental differences with magma‐poor margin (Iberian type) highlighting the role of crustal inheritance and rift duration on passive margin development and showing that there are no clear prerequisites and clear evidence to support a broad zone of mantle exhumation/serpentinization within the mid‐Norwegian Margin (Abdelmalak et al, ; Gernigon et al, , , ; Maystrenko et al, ; Nirrengarten et al, ; Petersen & Schiffer, ; Theissen‐Krah et al, ). One of the debate concerns the origin and geological interpretation of the strong crustal reflections underlying mostly Cretaceous highs and basins in the distal and outer domains of the margin.…”