Origin and significance of intra-basement seismic reflections offshore western Norway

Abstract

We use 3D seismic data to image a series of enigmatic, SW-dipping reflection packets within pre-Mesozoic crystalline basement offshore western Norway. Based on their low-angle dip and complex reflection wave-train our preferred interpretation is that the reflection packets are the seismic expression of mylonitic zones generated by nappe emplacement during the Caledonian orogeny. Late Jurassic faults truncate and offset these reflection packets by several hundred metres, suggesting that these faults did not exploit pre-existing basement weaknesses. Our observations suggest that older basement fabrics may not always play a significant role in determining the geometry of later fault systems. Supplementary materials: Details of the time-depth relationships from wells that have been used to depth-convert interpretations (S1), a supplementary figure showing an uninterpreted version of the seismic profile presented in Fig. 2 (S2) and a throw v. distance plot for fault F1 at the structural levels of the basement reflection and intra-basement reflection 1 are available at www.geolsoc.org.uk/SUP18683 .

Middle to Late Devonian–Carboniferous collapse basins on the Finnmark Platform and in the southwesternmost Nordkapp basin, SW Barents Sea

“…However, kilometer-thick layers bearing strong basement fabrics such as widespread, gently dipping foliation or pronounced mylonitic fabric commonly found along large shear zones may turn out to be resolvable on the seismic scale (see Sect. 4.1.2.;Fountain et al, 1984;Reeve et al, 2013;Phillips et al, 2016;Fazlikhani et al, 2017). For instance, we observed a severalkilometer-thick, curved, shallow-dipping layer that is characterized by moderate-amplitude reflections, which are parallel to the layer's upper and lower boundaries (see "Sørøya-Ingøya shear zone" reflections in Fig.…”

Terrane Boundary Reactivation, Barriers to Lateral Fault Propagation and Reactivated Fabrics: Rifting Across the Median Batholith Zone, Great South Basin, New Zealand

“…Based on its geometry and seismic reflection character, we interpret the fault‐parallel reflection package in the footwall of the Terrane Boundary Fault as the seismic expression of a crustal‐scale shear zone (here termed the Terrane Boundary Shear Zone). The reflection package resembles the characteristic seismic expression of shear zones, which arises through constructive interference between highly strained mylonite zones and intervening relatively undeformed material (Carreras, ; Jones & Nur, ; Phillips et al, ; Reeve et al, ; Rennie et al, ). Elsewhere, similar reflection packages have been confidently linked to onshore shear zones (Bird et al, ; Fazlikhani et al, ; Fossen & Hurich, ; Freeman et al, ; Lenhart et al, ; Phillips et al, ; Wang et al, ) or those encountered in boreholes (Hedin et al, ).…”

“…In addition, preexisting structures and fabrics at outcrop scale may be exploited by and control the geometry of later faults and fractures (Chattopadhyay & Chakra, 2013;De Paola et al, 2005;Dichiarante et al, 2016;Duffy et al, 2015;Kirkpatrick et al, 2013;Morley, 2010;Morley et al, 2004;Mortimer et al, 2016;Paton & Underhill, 2004;Phillips et al, 2017). Equally, preexisting structures do not always influence rift physiography; some may remain passive during subsequent tectonic events, while certain structures may only be selectively reactivated (e.g., Reeve et al, 2013;Roberts & Holdsworth, 1999). Numerous examples of interactions between preexisting structures and rift-related faults have been documented in rift systems worldwide; however, we often lack an in-depth understanding of the mechanics of these interactions, as well as how different scales of preexisting structure may influence different aspects of rift geometry and development.…”

Geology and regional significance of the Sarnoo Hills, eastern rift margin of the Barmer Basin, NW India

“…The involvement of pre-existing structures that are inherent in the crust during extension has been investigated extensively through combined experimental, seismic and outcrop studies (Lezzar et al, 2002;McClay et al, 2002;Younes & McClay, 2002;Morley et al, 2004;Bellahsen & Daniel, 2005;Bellahsen et al, 2006;Henza et al, 2010Henza et al, , 2011Giba et al, 2012;Chattopadhyay & Chakra, 2013;Reeve et al, 2014;Whipp et al, 2014). However, few detailed studies exist that characterise the effect that structures inherent in the crust can have on an evolving extensional fault network based on outcrop-data alone (e.g.…”