Penny J. Barton

The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary

The Fall of the Dinosaurs According to the fossil record, the rule of dinosaurs came to an abrupt end ∼65 million years ago, when all nonavian dinosaurs and flying reptiles disappeared. Several possible mechanisms have been suggested for this mass extinction, including a large asteroid impact and major flood volcanism. Schulte et al. (p. 1214) review how the occurrence and global distribution of a global iridium-rich deposit and impact ejecta support the hypothesis that a single asteroid impact at Chicxulub, Mexico, triggered the extinction event. Such an impact would have instantly caused devastating shock waves, a large heat pulse, and tsunamis around the globe. Moreover, the release of high quantities of dust, debris, and gases would have resulted in a prolonged cooling of Earths surface, low light levels, and ocean acidification that would have decimated primary producers including phytoplankton and algae, as well as those species reliant upon them. The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.

Evidence from three-dimensional seismic reflectivity images for enhanced melt supply beneath mid-ocean-ridge discontinuities

Quantifying the melt distribution and crustal structure across ridge-axis discontinuities is essential for understanding the relationship between magmatic, tectonic and petrologic segmentation of mid-ocean-ridge spreading centres. The geometry and continuity of magma bodies beneath features such as overlapping spreading centres can strongly influence the composition of erupted lavas and may give insight into the underlying pattern of mantle flow. Here we present three-dimensional images of seismic reflectivity beneath a mid-ocean ridge to investigate the nature of melt distribution across a ridge-axis discontinuity. Reflectivity slices through the 9° 03′ N overlapping spreading centre on East Pacific Rise suggest that it has a robust magma supply, with melt bodies underlying both limbs and ponding of melt beneath large areas of the overlap basin. The geometry of melt distribution beneath this offset is inconsistent with large-scale, crustal redistribution of melt away from centres of upwelling. The complex distribution of melt seems instead to be caused by a combination of vertical melt transport from the underlying mantle and subsequent focusing of melt beneath a magma freezing boundary in the mid-crust.

SEISMIC REFLECTIONS FROM THE MANTLE REPRESENT RELICT SUBDUCTION ZONES WITHIN THE CONTINENTAL LITHOSPHERE

In many areas of the world, the continental lithospheric upper mantle contains bright, continuous, regionally extensive, seismic reflectors. Despite the increasingly common observation of such mantle reflectors on deep seismic reflection profiles, their significance and geologic origin remain obscure. We report results from a series of seismic experiments acquired across two of the brightest of these reflectors and provide new constraints upon the composition and thickness of the reflecting region. These new seismic data reveal regionally extensive dipping and subhorizonal slabs of high-velocity (>8.4 km/s), high-density (>3500 kg/m 3 ) material, several kilometres (>2 km) thick, entrained within otherwise unremarkable upper mantle. The geometry, physical properties, and geologic setting of these mantle reflectors suggest that they represent fragments of eclogitic oceanic crust—a relict of pre-Caledonian oceanic subduction now preserved within the lower continental lithosphere. Such relict subduction zones appear to be widespread within the continental lithosphere and to exert an important influence upon the location and style of subsequent continental deformation.

Seismic reflection images of the Moho underlying melt sills at the East Pacific Rise

The determination of melt distribution in the crust and the nature of the crust–mantle boundary (the ‘Moho’) is fundamental to the understanding of crustal accretion processes at oceanic spreading centres. Upper-crustal magma chambers have been imaged beneath fast- and intermediate-spreading centres but it has been difficult to image structures beneath these magma sills. Using three-dimensional seismic reflection images, here we report the presence of Moho reflections beneath a crustal magma chamber at the 9° 03′ N overlapping spreading centre, East Pacific Rise. Our observations highlight the formation of the Moho at zero-aged crust. Over a distance of less than 7 km along the ridge crest, a rapid increase in two-way travel time of seismic waves between the magma chamber and Moho reflections is observed, which we suggest is due to a melt anomaly in the lower crust. The amplitude versus offset variation of reflections from the magma chamber shows a coincident region of higher melt fraction overlying this anomalous region, supporting the conclusion of additional melt at depth.

Elastic full waveform inversion of multicomponent ocean-bottom cable seismic data: Application to Alba Field, U. K. North Sea

Elastic full waveform inversion of multichannel seismic data represents a data-driven form of analysis leading to direct quantification of the subsurface elastic parameters in the depth domain. Previous studies have focused on marine streamer data using acoustic or elastic inversion schemes for the inversion of P-wave data. In this paper, P- and S-wave velocities are inverted for using wide-angle multicomponent ocean-bottom cable (OBC) seismic data. Inversion is undertaken using a two-dimensional elastic algorithm operating in the time domain, which allows accurate modeling and inversion of the full elastic wavefield, including P- and mode-converted PS-waves and their respective amplitude variation with offset (AVO) responses. Results are presented from the application of this technique to an OBC seismic data set from the Alba Field, North Sea. After building an initial velocity model and extracting a seismic wavelet, the data are inverted instages. In the first stage, the intermediate wavelength P-wave v...

COINCIDENT NORMAL-INCIDENCE AND WIDE-ANGLE REFLECTIONS FROM THE MOHO : EVIDENCE FOR CRUSTAL SEISMIC ANISOTROPY

Abstract We explore the consequences of interpreting wide-angle crustal seismic data assuming isotropic models when the real crust may be anisotropic. We have used a simple anisotropic model to generate synthetic travel-times and have inverted these assuming an isotropic layered model for the crust. We show that neglecting the effects of anisotropy can produce estimates of crustal thickness and crustal velocity structure that are significantly in error. For realistic levels of anisotropy (∼10%) the error in crustal thickness can be several kilometres, and the error in average velocity can be ∼0.5 km/s. We show that isotropic inversion of anisotropic data can lead to an apparent mismatch between the position of the Moho inferred from normal-incidence and wide-angle data. As an example of such a mismatch we show data acquired over the continental crust offshore the north of Scotland. The dataset we have modelled includes a wide-angle expanding-spread profile, a conventional ocean-bottom seismometer profile, a high-resolution wide-angle onshore-offshore experiment, a synthetic-aperture 16-km offset CDP profile, and a conventional deep reflection profile. These data show unusually sharp, bright and continuous reflections from the Moho at all offsets. The normal-incidence reflection Moho and the Moho modelled from the wide-angle data both show the same lateral structure; however, they are offset one from the other in normal-incidence two-way travel-time. This mismatch is considerably larger than the maximum error expected in the wide-angle model, and is much greater than the accuracy with which the Moho can be picked on the reflection data. If we assume that this mismatch is caused by crustal-scale seismic anisotropy, then the synthetic results indicate that anisotropy in the crust north of Scotland is about 7%.

The deep structure of the east Oman continental margin: preliminary results and interpretation

Abstract We describe preliminary results of a coincident normal incidence and wide-angle Seismic experiment across the east Oman continental margin just north of the Masirah Island ophiolite. This margin is affected by tectonic deformation and may be the site of margin-parallel shear in a zone parallel to the Owen Fracture Zone. The reflection profile is used to define upper crustal structure and shows a deep offshore basin dammed oceanwards by a ridge, interpreted here as a rotated fault block. Wide-angle data were collected using ten digital ocean-bottom Seismometers and 110 explosive shots. Preliminary raytracing of a crustal model based on the gravity model of Whitmarsh (1979) shows rapid changes in crustal thickness across the margin. A steep landward dipping reflector, probably the Moho, lies beneath the continental slope. The orientation of this reflector is exactly opposite to the direction of tectonic fabric predicted by a simple overthrust model of ophiolite emplacement from the ocean basin to the east.

The polarity of deep seismic reflections from the lithospheric mantle: Evidence for a relict subduction zone

Abstract BIRPS offshore deep seismic reflection profiles to the north of Scotland have revealed two bright continuous reflectors in the continental lithospheric upper mantle, the dipping Flannan and sub-horizontal W reflectors. The polarity of reflections from the Flannan has been determined from normal-incidence reflection data, using the far-field source wavelet as a starting model. The effective far-field wavelet was calculated by adding a receiver ghost and the effects of recording filters, attenuation during transmission and sea bottom multiples to the source wavelet. Reflection polarity was determined in a blind test by comparing the modelled wavelet with stacks of the Flannan reflection. In eight out of ten stacks, the reflection was picked as positive polarity, two out of ten were unknown. To verify this result, the test was repeated for Moho reflections; in this case nine out of ten stacks were picked as positive. The modelling also demonstrated that the Flannan reflection is apparently from a simple interface; complex interlayering is not required to explain the waveforms, and they are not consistent with reflections from a single thin layer. Seismic data acquired at wide-angle across the W mantle reflector show sub-crustal high-amplitude arrivals, which can only be explained as post-critical reflections. Only a high-velocity eclogitic layer, contained within normal mantle, with a sharp upper boundary and a diffuse base can explain all our observations. We suggest that the Flannan reflector represents the top of a relict oceanic and eclogitic component of a pre-Caledonian subduction zone within the lithospheric mantle.

Strategies For Elastic Full Waveform Inversion of Time-lapse Ocean Bottom Cable (OBC) Seismic Data

Time-lapse seismic data are frequently used to characterize reservoir changes during hydrocarbon production. Typically, comparing amplitude differences between processed data sets can provide an illustration of the change in impedance corresponding to fluid substitution. In order to determine a quantitative change in elastic properties (Pand S-wave velocities), full waveform inversion (FWI) can be used. In this study, elastic full waveform inversion in the time domain is applied to synthetic data to recover the velocity change. Two kinds of inversion strategies are presented: (1) conventional inversion approaches where the inversion is performed for each vintage and the results are compared, and (2) residual time-lapse waveform inversion (RTWI) where the residual time-lapse data are inverted. We first present the theoretical basis of RTWI and then test this method on synthetic data. Results from inversion of elastic permanent ocean bottom cable (OBC) data are used to show the successful recovery of the change in velocities and to demonstrate the differences between these approaches.

A three-dimensional study of a crustal low velocity region beneath the 9°03′N overlapping spreading center

Overlapping spreading centers (OSCs) play a key role in models of magma distribution at fast spreading ridges. To investigate the relationship between ridge-axis discontinuities and magma supply, we conducted a three-dimensional seismic reflection and tomography experiment at the 9°03′N OSC along the East Pacific Rise. Tomographic analysis imaged a broad mid-crustal low velocity zone (LVZ) beneath parts of the overlapper and the associated overlap basin, demonstrating that it is magmatically robust. The complementary datasets reveal a complex storage and tapping of melt: the LVZ and melt sill at either end of the overlap basin are not simply centered beneath the rise crest but are skewed inwards. The subsequent focussing of the LVZ and sill beneath the axis of the eastern limb appears to be due to melt migration toward the tip. The OSC western limb is less magmatically robust and may be in the process of dying.