Rebecca E. Bell

Strain migration during multiphase extension: Observations from the northern North Sea

Many rifts develop through multiphase extension; it can be difficult, however, to determine how strain is distributed during reactivation because structural and stratigraphic evidence associated with earlier rifting is often deeply buried. Using 2-D and 3-D seismic reflection and borehole data from the northern North Sea, we examine the style, magnitude, and timing of reactivation of a preexisting, Permian-Triassic (Rift Phase 1) fault array during a subsequent period of Middle Jurassic to Early Cretaceous (Rift Phase 2) extension. We show that Rift Phase 2 led to the formation of new N-S striking faults close to the North Viking Graben but did not initially reactivate preexisting Rift Phase 1 structures on the Horda Platform. We suggest that at the beginning of Rift Phase 2, strain was focused in a zone of thermally weakened lithosphere associated with the Middle Jurassic North Sea thermal dome, rather than reactivating extant faults. Diachronous reactivation of the Permian-Triassic fault network eventually occurred, with those faults located closer to the Middle Jurassic to Early Cretaceous rift axis reactivating earlier than those toward the eastern margin. This diachroneity may have been related to flexural down bending as strain became focused within the North Viking Graben, and/or the shifting of the locus of rifting from the North Sea to the proto-North Atlantic. Our study shows that the geometry and evolution of multiphase rifts is not only controlled by the orientation of the underlying fault network but also by the thermal and rheological evolution of the lithosphere and variations in the regional stress field.

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

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.

Basement structure and its influence on the structural configuration of the northern North Sea rift

The northern North Sea rift basin developed on a heterogeneous crust comprising structures inherited from the Caledonian orogeny and Devonian post-orogenic extension. Integrating two-dimensional regional seismic reflection data and information from basement wells we investigate the pre-rift structural configuration in the northern North Sea rift. Three seismic facies have been defined below the base rift surface: 1) relatively low-amplitude and low-frequency reflections, interpreted as pre-Caledonian metasediments, Caledonian nappes and/or Devonian clastic sediments; 2) packages of high-amplitude dipping reflections (>500 ms thick), interpreted as basement shear zones; and 3) medium-amplitude and high frequency reflections interpreted as less sheared crystalline basement of Proterozoic and Paleozoic (Caledonian) origin. Some zones of Seismic Facies 2 can be linked to onshore Devonian shear zones whereas others are restricted to the offshore rift area. Interpreted offshore shear zones dip S, ESE and WNW in contrast to W to NW dipping shear zones onshore W Norway. Our results indicate that Devonian strain and ductile deformation was distributed throughout the Caledonian orogenic belt from central South Norway to the Shetland Platform. Most of the Devonian basins related to this extension are, however, removed by erosion during subsequent exhumation. Basement shear zones reactivated during the rifting and locally control the location and geometry of rift depocenters, e.g. in the Stord and East Shetland basins. Pre-rift structures with present-day dips >15° were reactivated, although some of the basement shear zones are displaced by rift faults regardless of their orientation relative to rift extension direction.

Techniques to determine the kinematics of synsedimentary normal faults and implications for fault growth models

Abstract Normal faults grow via a sympathetic increase in their displacement and length (‘isolated model’) or by rapid establishment of their near-final length prior to significant displacement accumulation (‘constant-length model’). The isolated model has dominated the structural geology literature for >30 years, although some 3D seismic data-based studies support the constant-length model. Because they make different predictions regarding rift development, and earthquake size and recurrence intervals in areas of continental extension, it is critical to test these models with data from natural examples. Here we outline a range of techniques that constrain the kinematics of synsedimentary normal faults and thus test competing fault growth models. We then apply these techniques to three seismically imaged faults, showing that, in general, they grew in accordance with the constant-length model, although periods of relatively minor tip propagation and coeval displacement accumulation, characteristics more consistent with the isolated model, also occurred. We argue that analysis of growth strata represents the best way to test competing fault growth models; most studies utilizing this approach support the constant-length fault model, suggesting it may be more widely applicable than is currently assumed. It is plausible that the very early development of large faults is, however, characterized by the development of faults that, pre-linkage, grow in accordance with the isolated model; we may simply lack the data resolution, especially in the subsurface, to resolve this very early stage of fault growth.

Geological and geophysical expression of a primary salt weld: An example from the Santos Basin, Brazil

Primary salt welds form at the base of minibasins in response to complete evacuation of autochthonous salt. Analytical and numerical models suggest it is difficult to completely remove salt from a weld by viscous flow alone, which is especially true in multilayered evaporites, within which flow is likely heterogeneous due to lithologically controlled viscosity variations. Welds are important in the hydrocarbon industry because they may provide a hydrodynamic seal and trap hydrocarbons, or may allow transmission of fluids from source to reservoir rocks. Few papers document the subsurface expression of welds, principally because they have not been penetrated by wells or because the associated data are proprietary. We use 3D seismic and borehole data from the Santos Basin, offshore Brazil to characterize the geological and geophysical expression of a primary weld associated with flow of Aptian salt. The seismic data that we evaluated suggested that, locally, presalt and postsalt rocks are in contact at the base of an Upper Cretaceous minibasin, implying that several apparent welds, separated by low-relief salt pillows, are present. However, borehole data indicated that 22 m of anhydrite, carbonate, and sandstone are present in one of the welds, indicating that this and other welds may be incomplete. We find that seismic data may be unable to discriminate between a complete and incomplete weld, and we suggested that, during the subsurface analysis of welds, the term apparent weld is used until borehole data unequivocally proves the absence of salt. Furthermore, we speculate that preferential expulsion of halite and potash salt from the autochthonous layer during viscous flow and welding resulted in the formation of an incomplete weld, which, when compared with the initial autochthonous layer, is volumetrically enriched in nonevaporite lithologies and relatively viscous evaporite lithologies (anhydrite). The composition and stratigraphy of the autochthonous layer may thus dictate weld thickness and seal potential.

The stratigraphic record of prebreakup geodynamics: Evidence from the Barrow Delta, offshore Northwest Australia

The structural and stratigraphic evolution of rift basins and passive margins has been widely studied, with many analyses demonstrating that delta systems can provide important records of postrift geodynamic processes. However, the apparent lack of ancient synbreakup delta systems and the paucity of seismic imaging across continent-ocean boundaries mean that the transition from continental rifting to oceanic spreading remains poorly understood. The Early Cretaceous Barrow Group of the North Carnarvon Basin, offshore NW Australia, was a major deltaic system that formed during the latter stages of continental rifting and represents a rich sedimentary archive, documenting uplift, subsidence, and erosion of the margin. We use a regional database of 2-D and 3-D seismic and well data to constrain the internal architecture of the Barrow Group. Our results highlight three major depocenters: the Exmouth and Barrow subbasins and southern Exmouth Plateau. Overcompaction of pre-Cretaceous sedimentary rocks in the South Carnarvon Basin, and pervasive reworking of Permian and Triassic palynomorphs in the offshore Barrow Group, suggests that the onshore South Carnarvon Basin originally contained a thicker sedimentary succession, which was uplifted and eroded prior to breakup. Backstripping of sedimentary successions encountered in wells in the Exmouth Plateau depocenter indicates that anomalously rapid tectonic subsidence (≤0.24 mm yr−1) accommodated Barrow Group deposition, despite evidence for minimal, contemporaneous upper crustal extension. Our results suggest that classic models of uniform extension cannot account for the observations of uplift and subsidence in the North Carnarvon Basin and may indicate a period of depth-dependent extension or dynamic topography preceding breakup.

Determining the three-dimensional geometry of a dike swarm and its impact on later rift geometry using seismic reflection data

Dike swarm emplacement accommodates extension during rifting and large igneous province (LIP) formation, with ancient dike swarms serving to localize strain during later tectonic events. Deciphering three-dimensional (3-D) dike swarm geometry is critical to accurately calculating magma volumes and magma-assisted crustal extension, allowing syn-emplacement mantle and tectonic processes to be interrogated. It is also important for quantifying the influence of ancient dike swarms on post-emplacement faulting. However, the essentially 2-D nature of Earth’s surface, combined with the difficulties in imaging subvertical dikes in seismic reflection data and the relatively low resolution of geophysical data in areas of active diking, means our understanding of dike swarm geometry at depth is limited. We examine an ∼25-km-wide, >100-km-long, west-southwest–trending dike swarm imaged, due to post-emplacement rotation to shallower dips, in high-quality 2-D and 3-D seismic reflection data offshore southern Norway. Tuned reflection packages correspond to thin (<75 m thick), closely spaced dikes. These data provide a unique opportunity to image and map an ancient dike swarm at variable structural levels. Crosscutting relationships indicate emplacement occurred in the Late Carboniferous–Early Permian, and was linked to the formation of the ca. 300 Ma Skagerrak-centered LIP. Dike swarm width increases with depth, suggesting that magma volume and crustal extension calculations based on surface exposures are dependent on the level of erosion. During the Mesozoic, rift-related faults localized above and exploited mechanical anisotropies within the dike swarm. We demonstrate that seismic reflection data are a powerful tool in understanding dike swarm geometry and the control of dikes on subsequent faulting.

Characterizing carbonate facies using high-definition frequency decomposition: Case study from North West Australia

AbstractCarbonate facies identification is difficult using conventional seismic attributes due to subtle lithologic changes that cannot be easily recognized. Therefore, there is a need to develop new methodologies to study their evolution and their associated sedimentary processes, which will eventually lead to better prediction for reservoir-quality rocks. New insights into the Cenozoic carbonates in North West Australia have been captured with the application of a high-definition seismic attribute workflow. The workflow starts with conditioning of the seismic volume using structurally oriented noise attenuation filters to remove any random and coherent noise from the input data. It also benefits from a high-definition frequency decomposition that matches the original seismic resolution without smearing interfaces using a “matching pursuit” algorithm. A color blend of multigeometric attributes, such as semblance and conformance, has also been used in the workflow to define edges and discontinuities prese...

High-angle, not low-angle, normal faults dominate early rift extension in the Corinth Rift, central Greece

Low-angle normal faults (LANFs) accommodate extension during late-stage rifting and breakup, but what is more difficult to explain is the existence of LANFs in less-stretched continental rifts. A critical example is the <5 Ma Corinth Rift, central Greece, where microseismicity, the geometry of exposed fault planes, and deep seismically imaged faults have been used to argue for the presence of <30°-dipping normal faults. However, new and reinterpreted data call into question whether LANFs have been influential in controlling the observed rift geometry, which involves (1) exposed steep fault planes, (2) significant uplift of the southern rift margin, (3) time-averaged (tens of thousands to hundreds of thousands of years) uplift-to-subsidence ratios across south coast faults of 1:1–1:2, and (4) north margin subsidence. We test whether slip on a mature LANF can reproduce the long-term (tens of thousands of years) geometry and morphology of the Corinth Rift using a finite-element method, to model the uplift and subsidence fields associated with proposed fault geometries. Models involving LANFs at depth produce very minor coseismic uplift of the south margin, and post-seismic relaxation results in net subsidence. In contrast, models involving steep planar faults to the brittle-ductile transition produce displacement fields involving an uplifted south margin with uplift-to-subsidence ratios of ~1:2–3, compatible with geological observations. We therefore propose that LANFs cannot have controlled the geometry of the Corinth Rift over time scales of tens of thousands of years. We suggest that although LANFs may become important in the transition to breakup, in areas that have undergone mild stretching, do not have significant magmatic activity, and do not have optimally oriented preexisting low-angle structures, high-angle faulting would be the dominant strain accommodation mechanism in the upper crust during early rifting.

Rapid spatiotemporal variations in rift structure during development of the Corinth Rift, central Greece: Rapid Changes in Rift Structure, Corinth

The Corinth Rift, central Greece, enables analysis of early rift development as it is young (<5 Ma) and highly active and its full history is recorded at high resolution by sedimentary systems. A complete compilation of marine geophysical data, complemented by onshore data, is used to develop a high-resolution chronostratigraphy and detailed fault history for the offshore Corinth Rift, integrating interpretations and reconciling previous discrepancies. Rift migration and localization of deformation have been significant within the rift since inception. Over the last circa 2 Myr the rift transitioned from a spatially complex rift to a uniform asymmetric rift, but this transition did not occur synchronously along strike. Isochore maps at circa 100 kyr intervals illustrate a change in fault polarity within the short interval circa 620–340 ka, characterized by progressive transfer of activity from major south dipping faults to north dipping faults and southward migration of discrete depocenters at ~30 m/kyr. Since circa 340 ka there has been localization and linkage of the dominant north dipping border fault system along the southern rift margin, demonstrated by lateral growth of discrete depocenters at ~40 m/kyr. A single central depocenter formed by circa 130 ka, indicating full fault linkage. These results indicate that rift localization is progressive (not instantaneous) and can be synchronous once a rift border fault system is established. This study illustrates that development processes within young rifts occur at 100 kyr timescales, including rapid changes in rift symmetry and growth and linkage of major rift faults.