Pre‐breakup Extension in the Northern North Sea Defined by Complex Strain Partitioning and Heterogeneous Extension Rates

The early stages of continental rifting are accommodated by the growth of upper crustal normal fault systems that are distributed relatively evenly across the rift width. Numerous fault systems define fault arrays, the kinematics of which are poorly understood due to a lack of regional studies drawing on high‐quality subsurface data. Here we investigate the long‐term (~150 Myr) growth of a rift‐related fault array in the East Shetland Basin, northern North Sea, using a regionally extensive subsurface data set comprising 2‐D and 3‐D seismic reflection surveys and 107 boreholes. We show that rift‐related strain during the pre‐Triassic to Middle Triassic was originally distributed across several subbasins. The Middle to Late Triassic saw a decrease in extension rate (~14 m/Myr) as strain localized in the western part of the basin. Early Jurassic strain initially migrated eastward, before becoming more diffuse during the main, Middle‐to‐Late Jurassic rift phase. The highest extension rates (~89 m/Myr) corresponded with the main rift event in the East Shetland Basin, before focusing of strain within the rift axis and ultimate abandonment of the East Shetland Basin in the Early Cretaceous. We also demonstrate marked spatial variations in timing and magnitude of slip along strike of major fault systems during this protracted rift event. Our results imply that strain migration patterns and extension rates during the initial, prebreakup phase of continental rifting may be more complex than previously thought; this reflects temporal and spatial changes in both thermal and mechanical properties of the lithosphere, in addition to varying extension rates. Key Points Regionally extensive subsurface data are used to quantify basin‐wide strain behavior during early stages of continental rifting Variable magnitude and rate of extension‐related strain affect the structural development of upper crustal fault systems and host array Three‐dimensional strain behavior during initial continental rift phases might be more complex than previously assumed