Structural architecture and composition of crystalline basement offshore west Norway

Antje Lenhart,Robert L Gawthorpe,Oliver B Duffy,Haakon Fossen,Christopher A.-L Jackson,Rebecca E Bell

doi:10.1130/l668.1

Abstract

Numerous studies have investigated the geodynamic history and lithological composition of the Proterozoic basement, Caledonian nappes, and Devonian extensional basins and shear zones onshore west Norway. However, the offshore continuation of these structures, into the northern North Sea, where they are suspected to have influenced the structural evolution of the North Sea rift, is largely unknown. Existing interpretations of the offshore continuation of Caledonian and Devonian structures are based on simple map-view correlations between changes in offshore fault patterns and pronounced onshore structures, without providing evidence for the presence, nature, and geometry of offshore, basement-hosted structures. By integrating three-dimensional (3-D) seismic, borehole, and onshore geological and petrophysical data, as well as two-dimensional (2-D) forward modeling of gravity and magnetic data, we reveal the structural architecture and composition of the crystalline basement on the Maloy Slope, offshore west Norway. Based on 3-D mapping of intrabasement reflection patterns, we identified three basement units that can be correlated with the Caledonian thrust belt, and the major Devonian Nordfjord-Sogn detachment zone, located only 60 km to the east, onshore mainland Norway. Similar to that observed onshore, offshore crystalline basement of the Proterozoic basement (Western Gneiss Region) and allochthons is folded into large-scale antiforms and synforms. These units are separated by the strongly corrugated Nordfjord-Sogn detachment zone. Our analyses show that different types of crystalline basement can be distinguished by their seismic reflection character, and density and magnetic properties. We speculate that the main causes of the observed intrabasement reflectivity are lithological heterogeneities and strain-induced structures such as shear and fracture zones. Our interpretation of the architecture of crystalline basement offshore west Norway has important implications for the location of the suture zone between Baltica and Laurentia.

Highlights

The discovery of hydrocarbon plays related to fractured and weathered crystalline basement (e.g., Nelson, 2001; Cuong and Warren, 2009; Hartz et al, 2013) has driven interest in the nature and three-dimensional (3-D) geometry of intrabasement structures
We have shown that different basement units (WGR, allochthons, and NSDZ) can be distinguished by their seismic characteristics and physical properties (Table 2; Fig. 8)
Offshore welllog and core data indicate the presence of different kinds of gneisses that are distinguished by their mineralogy and internal deformation fabrics

Summary

INTRODUCTION

The discovery of hydrocarbon plays related to fractured and weathered crystalline basement (e.g., Nelson, 2001; Cuong and Warren, 2009; Hartz et al, 2013) has driven interest in the nature and three-dimensional (3-D) geometry of intrabasement structures. Proterozoic basement (Western Gneiss Region [WGR]), remnants of the Caledonian thrust belt, a major Devonian extensional shear zone, and several intermontane Devonian sedimentary basins are preserved (e.g., Andersen and Andresen, 1994; Milnes et al, 1997; Johnston et al, 2007, Vetti and Fossen, 2012) These basement units are suspected to form the metamorphic, crystalline basement offshore west Norway (e.g., Smethurst, 2000; Reeve et al, 2014), but their offshore continuation and 3-D geometry remain unclear (Færseth et al, 1995; Smethurst, 2000). Nordfjord-Sogn detachment zone Normal fault Wells that reached basement 3D seismic survey

Near Base Turonian

Findings

DISCUSSION

CONCLUSIONS