Tectono-stratigraphic evolution and crustal architecture of the Orphan Basin during North Atlantic rifting

The Orphan Basin is located in the deep offshore of the Newfoundland margin, and it is bounded by the continental shelf to the west, the Grand Banks to the south, and the continental blocks of Orphan Knoll and Flemish Cap to the east. The Orphan Basin formed in Mesozoic time during the opening of the North Atlantic Ocean between eastern Canada and western Iberia–Europe. This work, based on well data and regional seismic reflection profiles across the basin, indicates that the continental crust was affected by several extensional episodes between the Jurassic and the Early Cretaceous, separated by events of uplift and erosion. The preserved tectono-stratigraphic sequences in the basin reveal that deformation initiated in the eastern part of the Orphan Basin in the Jurassic and spread towards the west in the Early Cretaceous, resulting in numerous rift structures filled with a Jurassic–Lower Cretaceous syn-rift succession and overlain by thick Upper Cretaceous to Cenozoic post-rift sediments. The seismic data show an extremely thinned crust (4–16 km thick) underneath the eastern and western parts of the Orphan Basin, forming two sub-basins separated by a wide structural high with a relatively thick crust (17 km thick). Quantifying the crustal architecture in the basin highlights the large discrepancy between brittle extension localized in the upper crust and the overall crustal thinning. This suggests that continental deformation in the Orphan Basin involved, in addition to the documented Jurassic and Early Cretaceous rifting, an earlier brittle rift phase which is unidentifiable in seismic data and a depth-dependent thinning of the crust driven by localized lower crust ductile flow

Tying seismic data to geologic information from core data: an example from ODP Leg 177

The integration of seismic data with core data provides ground-truth toa structural interpretation of seismic data.The most important difficulty that arises in an integration effort isthe correct translation between the different scales of the core dataand the seismic data.In the absence of check-shots, detailed knowledge of the velocitystructure at the drilling locations is required, either from downholelogging measurements, velocity analysis of the seismic data, or directmeasurements on core samples.Three of the seven drill-sites during ODP (Ocean Drilling Program) Leg177 in the south-eastern Atlantic were located on the Agulhas Ridge andconnected through eight seismic profiles.Synthetic seismograms created from velocity and density measurements onselected core samples generally show a good agreement with real seismicdata with respect to amplitude and waveform, whereas timing of theevents is troublesome.The use of velocity profiles with inaccurate sections along cores, afalse depth scale due to recovery problems, and inaccuracies in thepositioning during both seismic and coring operations are the mainshortcomings of this method.The main reflectors identified on seismic data correspond to hiatuses orperiods of reduced sedimentation rates, and correlate well with densityvariations.In this way the cored data provide a calibration tool for the overallgeological interpretation of the seismic sections

The importance of crustal structure in explaining the observed uncertainties in ground motion estimation

In this short article, the possible reduction in the standard deviation of empirical ground motion estimation equations through the modelling of the effect of crustal structure is assessed through the use of ground-motion simulations. Simulations are computed for different source-to-site distances, focal depths, focal mechanisms and for crustal models of the Pyrenees, the western Alps and the upper Rhine Graben. Through the method of equivalent hypocentral distance introduced by Douglas et al. [(2004) Bull Earthquake Eng 2(1): 75-99] to model the effect of crustal structure in empirical equations, the scatter associated with such equations derived using these simulated data could be reduced to zero if real-to-equivalent hypocentral distance mapping functions were derived for every combination of mechanism, depth and crustal structure present in the simulated dataset. This is, obviously, impractical. The relative importance of each parameter in affecting the decay of ground motions is assessed here. It is found that variation in focal depth is generally more important than the effect of crustal structure when deriving the real-to-equivalent hypocentral distance mapping functions. In addition, mechanism and magnitude do not have an important impact on the decay rate