Crustal structure of the Newfoundland rifted continental margin from constrained 3-D gravity inversion

The rifting history of the Atlantic continental margin of Newfoundland is very complex and so far has been investigated at the crustal scale primarily with the use of 2-D seismic surveys. While informative, the results generated from these surveys cannot easily be interpreted in a regional sense due to their sparse sampling of the margin. A 3-D gravity inversion of the free air data over the Newfoundland margin allows us to generate a 3-D density anomaly model that can be compared with the seismic results and used to gain insight into regions lacking seismic coverage. Results of the gravity inversion show good correspondence with Moho depths from seismic results. A shallowing of the Moho to 12 km depth is resolved on the shelf at the northern edge of the Grand Banks, in a region poorly sampled by other methods. Comparisons between sediment thickness and crustal thickness show deviations from local isostatic compensation in locations which correlate with faults and rifting trends. Such insights must act as constraints for future palaeoreconstructions of North Atlantic rifting.     

Strain partitioning due to salt: insights from interpretation of a 3D seismic data set in the NW German Basin

We present results from interpretation of a 3D seismic data set, located within the NW German sedimentary basin, as part of the Southern Permian Basin. We focused on the development of faults, the timing of deformation, the amount of displacement during multiphase deformation, strain partitioning, and the interaction between salt movements and faulting. We recognised the central fault zone of the study area to be the Aller-lineament, an important NW-trending fault zone within the superimposed Central European Basin System. From structural and sedimentological interpretations we derived the following evolution: (1) E–W extension during Permian rifting, (2) N–S extension within cover sediments, and E–W transtension affecting both basement and cover, contemporaneously during Late Triassic and Jurassic, (3) regional subsidence of the Lower Saxony Basin during Late Jurassic/Early Cretaceous, (4) N–S compression within cover sediments, and E–W transpression affecting both basement and cover, contemporaneously during Late Cretaceous/Early Tertiary inversion and (5) major subsidence and salt diapir rise during the Cenozoic. We suggest that the heterogeneity in distribution and timing of deformation in the working area was controlled by pre-existing faults and variations in salt thickness, which led to stress perturbations and therefore local strain partitioning. We observed coupling and decoupling between pre- and post-Zechstein salt units: in decoupled areas deformation occurred only within post-salt units, whereas in coupled areas deformation occurred in both post- and pre-salt units, and is characterised by strike-slip faulting.     

3D seismic technology: the geological 'Hubble'

The proliferation of three‐dimensional (3D) seismic technology is one of the most exciting developments in the Earth Sciences over the past century. 3D reflection seismic data provide interpreters with the ability to map structures and stratigraphic features in 3D detail to a resolution of a few tens of metres over thousands of square kilometres. It is a geological ‘Hubble’, whose resolving power has already yielded some fascinating (and surprising) insights and will continue to provide a major stimulus for research into geological processes and products for many decades to come. Academic and other research institutions have a major role to play in the use of this data by exploiting the enormous volume of geological information contained in 3D seismic surveys. This paper reviews some of the recent advances in basin analysis made using the medium of 3D seismic data, focusing on the fields of structural and sedimentary geology, fluid–rock interactions and igneous geology. It is noted that the increased resolution of the 3D seismic method provided the essential catalyst necessary to stimulate novel observations and discover new geological structures such as mud diapir feeders, km‐long gas blow‐out pipes, giant pockmarks and sandstone intrusions, and to capture the spatial variability of diagenetic fronts. The UKs first impact crater was also discovered using 3D seismic data. The potential for future developments in this field of geophysical interpretation is considerable, and we anticipate that new discoveries will be made in many years to come.     

Lithoprobe east: results from reflection profiling of the continental margin: Grand Banks Region.

Summary. The 1985 Lithoprobe East profiles resolve deep crustal structure of the Grand Banks off eastern Canada. Basins are 7 to 20 km deep, and bounded by major faults traceable to Mono depth. The lower crust is reflective along much of the survey, and the top of this reflective layer has a variable depth. Prominent and often surprisingly flat Mono reflections are observed. Puzzling rotated fault blocks are imaged at the continent-ocean transition.     

Some considerations on the interpretation of seabed images based on commercial 3D seismic in the Faroe-Shetland Channel

Commercial three‐dimensional (3D) seismic surveys now cover much of the continental slope and basin floor areas of the Faroe‐Shetland Channel. A mosaic of the seabed picks derived from these data sets and enhancement with visualisation techniques has resulted in detailed relief images of the seabed that testify to the action of a number of sedimentary processes such as glaciation, downslope and alongslope processes. The wealth of detail in these images is remarkable and extremely valuable for the identification and interpretation of seabed features. However, the level of detail can seduce the interpreter into treating the image purely as an aerial photograph. The interpreter needs to understand the limitations and artefacts inherent in such images to use them appropriately. This paper will present the major artefacts observed in the images and how certain aspects of 3D seismic survey acquisition and processing have contributed to their presence. The vertical and horizontal resolution of the images will also be discussed. Although primarily focused on seabed imagery these comments are equally pertinent to the application of 3D seismic surveys for shallower objectives than for which they were primarily designed.     

Depositional systems in multiphase rifts: seismic case study from the Lofoten margin,Norway

The evolution of depositional systems in multiphase rifts is influenced by the selective reactivation of faults between subsequent rift phases. The Middle Jurassic to Palaeocene tectonic history of the Lofoten margin, a segment of the North Atlantic rift system, is characterised by three distinct rift phases separated by long (>20 Myr) inter‐rift periods. The initial rift phase comprised a distinct fault initiation and linkage stage, whereas the later rift phases were characterised by selective reactivation of previously linked through‐going faults which resulted in immediate rift climax. Using 2‐D and 3‐D seismic reflection data in conjunction with shallow core data we present a 100 Myr record of shallow to deep marine depositional environments that includes deltaic clinoform packages, slope aprons and turbidite fans. The rapid re‐establishment of major faults during the later rift phases impacts on drainage systems and sediment supply. Firstly, the immediate localisation of strain and accumulation of displacement on few faults results in pronounced footwall uplift and possible fault block rotation along those faults, which makes it more likely for any antecedent fault‐transverse depositional systems to become reversed. Secondly, any antecedent axially‐sourced depositional systems that are inherited from the foregoing rift phase(s) are likely to be sustained after reactivation because such axial systems have already been directed around fault tips. Hence, the immediate localisation of strain through selective reactivation in the later rift phases restricts fault‐transverse sediment supply more than axial sediment supply, which is likely to be a key aspect of the tectono‐sedimentary evolution of multiphase rifts.     

Lithology-derived structure classification from the joint interpretation of magnetotelluric and seismic models

Magnetotelluric and seismic methods provide complementary information about the resistivity and velocity structure of the subsurface on similar scales and resolutions. No global relation, however, exists between these parameters, and correlations are often valid for only a limited target area. Independently derived inverse models from these methods can be combined using a classification approach to map geologic structure. The method employed is based solely on the statistical correlation of physical properties in a joint parameter space and is independent of theoretical or empirical relations linking electrical and seismic parameters. Regions of high correlation (classes) between resistivity and velocity can in turn be mapped back and re-examined in depth section. The spatial distribution of these classes, and the boundaries between them, provide structural information not evident in the individual models. This method is applied to a 10 km long profile crossing the Dead Sea Transform in Jordan. Several prominent classes are identified with specific lithologies in accordance with local geology. An abrupt change in lithology across the fault, together with vertical uplift of the basement suggest the fault is sub-vertical within the upper crust.     

Ccte d'Ivoire-Ghana margin: seismic imaging of passive rifted crust adjacent to a transform continental margin

During May 1990 and January-February 1991, an extensive geophysical data set was collected over the Côte d'Ivoire-Ghana continental margin, located along the equatorial coast of West Africa. The Ghana margin is a transform continental margin running subparallel to the Romanche Fracture Zone and its associated marginal ridge—the Côte d'Ivoire-Ghana Ridge. From this data set, an explosive refraction line running ∼ 150 km, ENE-WSW between 3°55'N, 3°21'W and 4°23'N, 2°4'W, has been modelled together with wide-angle airgun profiles, and seismic reflection and gravity data. This study is centred on the Côte d'Ivoire Basin located just to the north of the Côte d'Ivoire-Ghana Ridge, where bathymetric data suggest that a component of normal rifting occurred, rather than the transform motion observed along the majority of the equatorial West African margin.
Traveltime and amplitude modelling of the ocean-bottom seismometer data shows that the continental Moho beneath the margin rises in an oceanward direction, from ∼ 24 km below sea level to ∼ 17 km. In the centre of the line where the crust thins most rapidly, there exists a region of anomalously high velocity at the base of the crust, reaching some 8 km in thickness. This higher-velocity region is thought to represent an area of localized underplating related to rifting. Modelling of marine gravity data, collected coincident with the seismic line, has been used to test the best-fitting seismic model. This modelling has shown that the observed free-air anomaly is dominated by the effects of crustal thickness, and that a region of higher density is required at the base of the crust to fit the observed data. This higher-density region is consistent in size and location with the high velocities required to fit the seismic data.