1,721,060 research outputs found
Buckling of the oceanic lithosphere from geophysical data and experiments
No full textTwo major hypothesis have been advanced for the formation of the long wavelength (100-300) km undulations of oceanic basement and overlying sediments developed in the central Indian Ocean: whole layer folding (buckling) and local thickening (inverse boudinage). Using appropriately scaled two layer models of the oceanic lithosphere comprising a brittle layer overlying a ductile lower layer, we show that buckling of the entire brittle layer is likely to be the mode of deformation. However the lithosphere-asthenosphere boundary remains undisturbed. We find a relationship between the thickness of the brittle layer and the wavelength of folding such that the wavelength is 7 times the brittle layer thickness
Modelling rough interfaces on seismic reflection profiles - the application of fractal concepts
No full textThe distortion of reflection continuity and amplitude by overburden structure in seismic reflection images of the sub- surface is easily recognised and modelled when the wavelength of the shallower structure is relatively large. The effects of shorter wavelength structure although giving rise to little reflective response itself, cause significant distortion of the propagating wavefield, particularly when a moderate or strong acoustic impedance contrast is present in the shallow sub-surface. Here we show how short as well as long spatial wavelengths of horizon roughness affect deeper reflection continuity, and develop a new method using fractal interpolation techniques to predict the total roughness of sub-surface horizons from information contained in seismic reflection sections. Fractally complete depth-velocity models are used in forward models, using the finite difference technique, to produce synthetic seismic profiles. The technique is illustrated with data from the Edoras Bank area of the Rockall Plateau, NE Atlantic, where apparently discontinuous reflectors underlying basalt flows are shown to be from continuous sedimentary horizons distorted by overlying rough horizons
The southern margin of the Rockall Plateau: stratigraphy, Tertiary volcanism and plate tectonic evolution
No full textThe geological evolution of the southwest Rockall Plateau in the area of Edoras Bank has been clarified using seismic reflection, gravity and magnetic data. Four principal reflectors are observed within the Tertiary sedimentary sequence: I, Late Miocene; II, latest Early Miocene; III, Late Eocene; IV, earliest Eocene. A period of pronounced sediment drift accumulation marks the interval I–II. Reflector III, the top of a sediment wedge prograding southward from Edoras Bank, marks a change from terrigenous to pelagic sedimentation that is probably related to subsidence of the Rockall Plateau following the separation of Greenland from Eurasia in the earliest Eocene. Reflector IV marks the top of a wedge-shaped seismically transparent layer that also thins southward away from Edoras Bank. On the basis of its seismic attributes and magnetic signature, this layer is interpreted as a volcanic sheet, formed as part of the North Atlantic Tertiary Volcanic Province during rifting of Greenland from Eurasia. The recognition of voluminous volcanic rocks south of Edoras Bank extends the known area of the Tertiary volcanic province several hundred kilometres to the south. Gravity anomaly modelling and continental reconstructions suggest that the region south of Edoras Bank is underlain by thinned continental crust. A four stage geological evolution for this region is indicated, (i) Initial rifting associated with the separation of Labrador from Greenland in the late Cretaceous is characterized by enhanced crustal thinning and subsidence in the region of a rift triple junction. (ii) Passive subsidence and accumulation of late Cretaceous and earliest Tertiary sediments followed the initiation of seafloor spreading in the Labrador Sea. (iii) Blanketing of the area by Palaeocene volcanic rocks masked pre-existing magnetic lineations, providing an explanation for some of the problems in earlier interpretations based mainly on magnetic data. (iv) Post-volcanic sedimentation, continuing to the present day
Structural style of intra-plate deformation, Central Indian Ocean Basin: evidence for the role of fracture zones
No full textThe structural style of the intraplate deformation developed in the Central Indian Ocean Basin was investigated in an area (7S°E - 82°E, 0°S - 6°S) to the west of the Afanasy Nikitin seamount using an integrated data set of seismic reflection profiles from Edinburgh University and Lamont-Doherty Geological Observatory. The study area contains two fracture zones, which strike ~ 005°E to 010°E, with oceanic lithosphere (age range ~ 65-78 Ma B.P.) younging westwards across them. No evidence for recent fault activity in the oceanic basement along the fracture zones could be detected in this area, although the close association between intraplate earthquakes and fracture zones elsewhere suggests reactivation of the fracture zones at upper mantle depths in a left lateral strike-slip sense. A statistical study was carried out into the first and second orders of deformation, long wavelength basement undulations and high-angle reverse faults respectively, and the relationships between them. The orientations of the axes of the undulations vary from 065°E to 085°E while the high-angle faults strike consistently at 090°E to 100°E. The results of this analysis suggest that the high-angle faults are the result of the reactivation of two sets of pre-existing spreading-centre normal faults, one set originally facing towards the spreading centre and the other facing away. Furthermore, although the long wavelength undulations are not fault generated, the high-angIe faults have modified the basement topography causing the accentuation of some of the basement highs. The observation that the undulations are not fault-generated is consistent with them being of flexural origin (including buckling of the crust and/or lithosphere). Basement undulations are clearly discontinuous across fracture zones and the facing direction of faulting is also offset. This discontinuity, the orientation of the axes of the undulations, the presence of other strong oblique basement trends, and information from regional earthquake studies suggest that the deformation resulted from not only ~ N-S compression as a result of the continental collision between India and Asia, but also left lateral strike-slip along fracture zones caused by the difference in resistance to plate motion between the continental collision to the north and subduction at the Sunda Arc to the northeast
Sediment velocities and deep structure from wide-angle reflection data around Leg 116 sites
No full textThe reduction of six wide-angle reflection profiles shot within the two fault blocks visited by Ocean Drilling Program (ODP) Leg 116 in combination with the ODP sonic logs has produced a velocity-depth structure for this area. The sediment velocity increases from 1.6-1.7 km/s in the near surface to 3.4-3.5 km/s immediately above basement with a velocity gradient of 0.75/s. A depth converted seismic reflection profile suggests that the pre-deformational basement surface was similar to the abyssal hill topography developed in the Pacific Ocean. A velocity for the top of oceanic layer 2 of 4.1 km/s was identified as layer 2A. Assuming a velocity gradient of 0.7/s, an estimate of layer 2 thickness was obtained of 1.5 km. It is possible to interpret residual depth anomalies in terms of a layer 3 that may be thinner than for normal oceanic crust
Analysis of a strike-slip fault network using high resolution multibeam bathymetry, offshore NW Devon U.K.
No full textImaging of the sea floor offshore from Hartland Point (north Devon, U.K.), using high resolution multibeam bathymetry, reveals a strike-slip fault network. This consists of NE-trending left-lateral faults and NW-trending right-lateral faults that cut folded and steeply dipping strata (~ 60°). Faults were accurately mapped using the multibeam imagery, and lateral separations of marker beds measured along fault traces. These data are used to examine the spatial arrangement, fault displacement, and strain distribution within the network at different displacement cut-offs.At high displacement cut-offs, the fault network is dominated by a few long isolated right-lateral fault segments that bound fault blocks, but at lower displacement cut-offs shorter left-lateral and right-lateral fault segments make up fault tips and infill fault blocks. The majority (70%) of fault trace-length is taken up by small fault segments that have < 10 m displacement whereas 84% of strain is localized onto large fault segments with > 10 m displacement. The topology and relative connectivity of the network is analysed in terms of a system of fault branches between tips (I-nodes) or intersections (X or Y-nodes), the relative proportions of which reflect the connectivity of the network. Although the kinematic behaviour of the fault network is controlled by large fault segments, connectivity is very dependent on the small fault segments.A comparison with a similar, nearby, strike-slip fault network at Westward Ho! (north Devon) shows many similarities and indicates that fault networks are better connected with increasing strain and that the network becomes better connected when strain is localized within damage zones rather than on individual faults
Scaling of fault displacements and implications for the estimation of sub-seismic strain
No full textFault displacement populations have been shown to follow a power-law scaling relationship characterized by an exponent D. This relationship can be used to make predictions of the sub-seismic fault population from data derived from seismic surveys. Although fault populations exist in three dimensions the use of section data is recommended. D-values derived from sections can be applied directly to several problems, and are also related to the D-value for the fault set in higher dimensions. Accurate determination of D requires proper consideration of the scale range and sample size limitations of available data. The most common technique of using a cumulative frequency graph often leads to an upwards bias. An iterative correction procedure is proposed. Discrete frequency methods avoid this bias, but as a standard linear interval graph has other associated problems, a log-interval graph method is preferred. Simulations of these methods, applied to random computer generated samples from power-law distributions, have been made to examine the accuracy of D-values derived from typical data. Equations to estimate the confidence intervals for these D-values have been derived from a synthesis of the results. The application of the techniques is shown using fault data measured on seismic sections from the Southern North Sea and the Inner Moray Firth. Where local differences in D are shown to be significant, there is usually a marked change in structural style. Fault data are used to make improved estimates of crustal extension (B) by extrapolating the derived powerlaw relationship. A value of 13 = 1.20 is calculated for the Inner Moray Firth. Applications predicting the intersection of horizontal wells with 'large' sub-seismic faults and quality control of fault interpretation on seismic sections are also described
Mechanical control of oceanic plate boundary geometry
No full textWe present a global analysis of oceanic plate boundary geometry based on the mechanics of relative plate motion at mid-ocean ridges and transform faults. If the observed geometry formed by the first-order segmentation of oceanic plate boundaries represents a state of mechanical equilibrium, we find the relative strength of spreading ridges, and their bounding transform faults to be fundamental to its organisation. A consideration of power dissipation along adjacent lengths of spreading ridge and transform fault leads to a simple relationship between spreading obliquity and relative strength. Increased spreading rate is found to decrease the strength of spreading segments relative to transform faults. Proximity to an active hotspot reduces the relative strength of spreading ridges
Reflection coefficient calculation from marine high-resolution seismic reflection (Chirp) data
No full textChirp sub-bottom profilers produce high-resolution images of the near-surface. An attribute of the sea-bed reflection in chirp data are fluctuations in polarity between adjacent traces. Two models are proposed and presented to explain this: the first incorporates changes in an acoustic impedance gradient at the sea bed; the second uses changes in the thickness of the uppermost sediment layer. Mixing of adjacent traces produces a consistent polarity for the sea-bed reflector. Reflection coefficients are calculated, using amplitude information derived from single-traces, and polarity information from trace mixing, with application to a marine archaeological case study. The reflection coefficient calculated for the top of a buried 18th century wooden wreck is -0.26.<br/
Localized vs distributed deformation associated with the linkage history of an active normal fault, Whakatane Graben, New Zealand
Full text linkThe deformation associated with an active normal fault is investigated at a high temporal resolution (c. 104 yr). The Rangitaiki Fault (Whakatane Graben, New Zealand) and its adjacent faults accommodated an overall extension of ?0.83% oriented at ?N324°E over the past 17 kyr. This is consistent along strike, but the pattern of faulting that accommodates this strain defines two different spatial domains. To the SW, one domain is characterized by a few large faults, with >80% of strain localized onto geometrically and kinematically linked segments of the main fault. This produces marked heterogeneity in the spatial distribution of strain across the graben. In contrast, to the NE, a domain of distributed faulting is characterized by numerous small faults contributing to the overall deformation, with only ?35% of strain localized onto the Rangitaiki Fault. The transition from distributed to localized deformation is attributed to an increase in linkage maturity of the Rangitaiki Fault. Progressive strain localization has been ongoing within the network over the last 17 kyr, with localization of fault activity increasing by ?12%, indicating this process occurs over kyr time periods that only reflect a few earthquake events
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