Abstract
AbstractIn regions experiencing multiple phases of extension, rift‐related strain can vary along and across the basin during and between each phase, and the location of maximum extension can differ between the rift phase. Despite having a general understanding of multiphase rift kinematics, it remains unclear why the rift axis migrates between extension episodes. The role pre‐existing structures play in influencing fault and basin geometries during later rifting events is also poorly understood. We study the Stord Basin, northern North Sea, a location characterised by strain migration between two rift episodes. To reveal and quantify the rift kinematics, we interpreted a dense grid of 2D seismic reflection profiles, produced time‐structure and isochore (thickness) maps, collected quantitative fault kinematic data and calculated the amount of extension (β‐factor). Our results show that the locations of basin‐bounding fault systems were controlled by pre‐existing crustal‐scale shear zones. Within the basin, Permo‐Triassic Rift Phase 1 (RP1) faults mainly developed orthogonal to the E‐W extension direction. Rift faults control the locus of syn‐RP1 deposition, whilst during the inter‐rift stage, areas of clastic wedge progradation are more important in controlling sediment thickness trends. The calculated amount of RP1 extension (β‐factor) for the Stord Basin is up to β = 1.55 (±10%, 55% extension). During the subsequent Middle Jurassic‐Early Cretaceous Rift Phase 2 (RP2), however, strain localised to the west along the present axis of the South Viking Graben, with the Stord Basin being almost completely abandoned. Rift axis migration during RP2 is interpreted to be related to changes in lithospheric strength profile, possibly related to the ultraslow extension (<1 mm/year during RP1), the long period of tectonic quiescence (ca. 50 myr) between RP1 and RP2 and possible underplating. Our results highlight the very heterogeneous nature of temporal and lateral strain migration during and between extension phases within a single rift basin.
Highlights
In multiphase rifts the location of maximum extension often differs between rift phases
We present a kinematic analysis of basin-bounding faults (Øygarden Fault System, ØFS and Utsira East Fault, UEF) and four significant intra-basin faults (F1-F4) in the Stord Basin are presented below
The Rift Phase 2 (RP2) extension in the north is ca. 66% more than that in the south. These results show that the Stord basin mainly developed during Rift Phase 1 (RP1), with ca. 95% of total extension occurring during this time, and that the area experienced a phase of tectonic quiescence during RP2
Summary
In multiphase rifts the location of maximum extension (the rift axis) often differs between rift phases. The early syn-RP1 depocentres are bound by several rift-related faults distributed across the Stord Basin (Figure 4a). The main fault activity during early post-RP1 occurs along F1 in the north and basin bounding ØFS and UEF faults. Faults that were active during syn-RP2 are located at the eastern (ØFS3-5 and F3) and western (UEF2-5) basin margins (Figure 7a). The time-thickness map of syn-RP2 (Figure 7a) shows that the Stord Basin was tectonically relatively quiescent during RP2, with only minor fault activity occurring during the very earliest stages of rifting (Figures 7a and 8). F1 is about 70 km long and it was only active during RP1 and early post-rift 1 as evidenced by time-thickness maps, during which time it accumulated a maximum throw of c. These results show that the Stord basin mainly developed during RP1, with ca. 95% of total extension (accumulated during RP1, RP2 and inter-rift period) occurring during this time, and that the area experienced a phase of tectonic quiescence during RP2
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