Abstract
AbstractThe 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.
Highlights
Continental rifting is accommodated by the growth of upper-crustal normal faults
Cowie et al, 2005; Triassic Ninian and Alwyn North fields, Tomasso et al, 2008). Because they focus on relatively small areas and/or for only a relatively short part of the much longer rift episode, these studies can only show how strain accumulates during the development of individual fault systems; the longer-term (~150 Myr) dynamics of the larger host fault array remains unknown
Our results suggest that relative changes in extension rate play an important role in the basin-wide strain behaviour we observe in the East Shetland Basin
Summary
This supporting information contains supplemental material for 3.3 Fault array analyses and references, and includes a detailed data availability statement. The extension factor is calculated along each transect line per time period to constrain the relative strain distribution across the fault array during rifting. Similar to the expansion indices and backstripped throw values, we calculate fault slip rate along each major fault for every time period, with the exception of the pre-Triassic units as the age of these is unconstrained (Units 1 and 2) (Figure 11). Displacement backstripping involves the sequential subtractions of displacements on successively older horizons; where the displacement is calculated using the throw and heave of a faulted horizon (e.g., Childs et al, 1993; Ten Veen & Kleinspehn, 2000; Walsh et al., 2002; Taylor et al, 2004, 2008; Bell et al, 2014; Jackson et al, 2017). Borehole data: Diskos: NO blocks 31, 32, and NDR: UK quadrants 210, 211, 1, 2, and 3
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