Richard W. Hobbs

The deep structure of northern England and the Iapetus Suture zone from BIRPS deep seismic reflection profiles

The NEC deep seismic reflection profile images structures in the lower crust which represent the Iapetus Suture zone beneath southern Scotland and northern England. On the basis of reflectivity characteristics of the lower crust and reflection Moho we recognize four different crustal zones which correspond to four crustal/terrane types: Midland Valley (zone A), a sub-continental subduction complex (zone B), Lake District (zone C) and Midland Platform (zone D). The junctions between each of these zones are interpreted as being tectonic. A and B originate from the northern continental margin of the Iapetus Ocean and are separated from C and D (which are derived from a southern continent) by strong northerly dipping reflections between approximately 15 and 30 km deep in the crust. The structure responsible for these reflections does not displace the reflection Moho. North of the AB/CD junction, the base of zone B (and base of the reflective crust) is marked by a pair of parallel reflectors persistent for a horizontal length of about 55 km. These reflectors are interpreted as the top and bottom of a slice of remnant oceanic crust. At their northerly limit the deepest of the two parallel reflectors transects the Moho to a depth of 6-7 km into the uppermost mantle. We suggest that their termination delimits the northerly extent of a décollement or shear zone at the present continental Moho. The shear zone was active during the late stages of Caledonian collision when mantle, originally from beneath the southern continent, underthrust old and newly created crust of the collision zone.

The crust as a heterogeneous "optical" medium" or "crocodiles in the mist"

Abstract Based on petrophysical data, geologic maps, and a well log, we present statistical descriptions of likely upper-, middle-, and lower-crustal rocks to characterize the fine-scale heterogeneity observed in crustal exposures and inferred from deep-crustal seismic data. The statistical models, developed for granitic and metamorphic upper crust, and for an extended metamorphic lower crust, are used to construct whole-crustal models of seismic velocity heterogenity. We present finite-difference synthetic CMP data from several models which compare favorably with field data. The statistical models also permit classification of the seismic reflection experiment and the crustal heterogeneity according to scattering regime. The “optical”, or scattering properties of importance for classification are the velocity fluctuation intensity, the horizontal and vertical correlation lengths of the medium, the correlation function of the medium, and the velocity population function. For the crustal properties we measured, the bandwidth of a typical deep crustal experiment overlaps from the weak to the strong scattering regime, with implications for crustal seismic data processing and imaging. Notably, deep-crustal signals are likely to have experienced multiple scattering, making common seismic imaging techniques of questionable value. Moreover, the details of the unmigrated CMP stacked section bears little resemblance to the underlying medium.

Ocean temperature and salinity inverted from combined hydrographic and seismic data

Recently seismic reflection methods have been successfully applied to oceanographic issues. Here, we present a new approach, combining XBT and CTD surveys with seismic observations, to visualize long sections with a resolution down to a few meters. The challenge to a full investigation of mixing processes has been the tremendous span of spatial scales ranging from hundreds of kilometers to centimeters. Traditional hydrographic observations could only resolve the large scale effects by measuring temperature and salinity profiles at discrete locations typically several kilometers apart, whereas dedicated localized measurements allowed investigation of the ocean fine structure at the other end of the spatial spectrum. The intermediate scales have in contrast been difficult to observe systematically. Here we present temperature and salinity data inverted from seismic observations that cover the intermediate scales and provide a new approach to image mesoscale processes and allow the investigation of their dynamics at unprecedented resolution.

Some attributes of wavefields scattered from Ivrea-type lower crust

Abstract The Ivrea Zone in northern Italy is a well studied sliver of extended lower continental crust which was brought to the surface in the course of Alpine lithospheric shortening. Based on two 1:25,000 geological maps from the central Ivrea Zone and petrophysical parameters of the corresponding lithologies we have constructed geologically constrained, stochastic seismic models of Ivrea-type lower crust. A stochastic rather than a deterministic approach is applied due to the limited availability of. detailed geological maps and the high degree of small-scale structural and petrophysical complexity of the Ivrea Zone. The primary characteristic of the resulting model is its “layered” self-affine or fractal structure and its bimodal velocity distribution. While we are not suggesting that Ivrea-type structure is universal for extended lower continental crust, synthetic seismograms may help to constrain some pertinent quantitative aspects observed in deep seismic reflection data: 1. (1) Ivrea-type lower crust can explain the igh observed reflection coefficients and bright reflectivity in the lower crust; 2. (2) observed lower-crustal Q -factors can be largely explained by scattering losses and intrinsic Q -factors of reflective lower crust are likely to be larger than 1000, which is incompatible with the presence of free volatiles; 3. (3) in agreement with recent broad band studies of the lower crust, Ivrea-type lower crust does not create any tuning effects; and 4. (4) for Ivrea-type heterogeneity the lateral correlation of the reflected signal is largely independent of the lateral characteristic scale of the scattering structures, which suggests that multiple scattering may be important in the reflective lower crust.

Violent Hypocrisy Governance and the Night-time Economy

The development of alcohol-based night-time economies as part of government-sponsored post-industrial urban regeneration involves two interconnected political and economic processes. The first is the shift to a consumer economy, and the second is the movement within local governance from the provision of services towards a focus upon nurturing economic growth. The violence and disorder that have resulted from the huge expansion in these night-time economies have produced a crisis for state policing that has led, via licensing, to the expansion of commercially relevant control strategies. This paper, based upon extensive empirical research, discusses the hypocrisy that is inherent in the governance of liminal licence.

Understanding the offshore flood basalt sequence using onshore volcanic facies analogues: an example from the Faroe–Shetland basin

Flood basalts in associated volcanic rifted margins, such as the North Atlantic Igneous Province, have a significant component of lavas which are preserved in the present day in an offshore setting. A close inspection of the internal facies architecture of flood basalts onshore provides a framework to interpret the offshore sequences imaged by remote techniques such as reflection seismology. A geological interpretation of the offshore lava sequences in the Faroe–Shetland Basin, using constraints from onshore analogues such as the Faroe Islands, allows for the identification of a series of lava sequences which have characteristic properties so that they can be grouped. These are tabular simple flows, compound-braided flows, and sub-aqueously deposited hyaloclastite facies. The succession of volcanic rocks calculated in this study has a maximum thickness in excess of 6800 m. Down to the top of the sub-volcanic sediments, the offshore volcanic succession has a thickness of about 2700 m where it can be clearly identified across much of the area, with a further 2700 m or more of volcanic rock estimated from the combined gravity and seismic modelling to the north and west of the region. A large palaeo-waterbody is identified on the basis of a hyaloclastite front/apron consisting of a series of clinoforms prograding towards the eastern part of the basin. This body was > 500 m deep, must have been present at the onset of volcanism into this region, and parts of the water body would have been present during the continued stages of volcanism as indicated by the distribution of the hyaloclastite apron.

Crustal structure of the central and southern North Sea from BIRPS deep seismic reflection profiling

Through the generous gift of shiptime on M/V Mobil Search in June 1987, BIRPS (British Institutions Reflection Profiling Syndicate) was able to acquire nearly 1500 km of deep seismic profiles recorded to 16s TWT in a regional survey across the central and southern North Sea. Reflective lower crust beneath the southern North Sea basin gives way laterally to a zone of mid-crustal reflections within the London–Brabant platform. These mid-crustal reflectors are interpreted as due to crustal shearing, although whether related to stretching during basin formation or to some earlier event remains enigmatic. Sub-crustal, SW-dipping reflections are observed at the edge of the London–Brabant platform. Across the mid-North Sea High, N–S profiles show distinctive patterns of lower-crustal reflectivity that indicate lateral variability in structure and character on a scale of 30–50 km. These are interpreted as crustal units relating to the Iapetus suture developed during the Caledonian orogeny. They may have been modified, however, during Carboniferous basin development, or subsequently. The absence of any distinctive sub-crustal reflectors associated with the Iapetus Suture, where a Caledonian subduction zone should have existed, tends to suggest that sub-crustal reflections observed elsewhere around Britian are not subduction related but are more likely to be from younger faults or shear zones relating to lithosphere extension and basin formation. Onshore wide-angle recording in northern England of the marine seismic source produced good quality data. The high resolution obtained has revealed details of mid-crust (PcP) and Moho (PmP) reflections, from around 18 and 30 km depth respectively, not observed before in this area. Both reflectors show events of laterally variable reflectivity that appear as bands of discontinuous reflection segments.

Estimating mixing rates from seismic images of oceanic structure

An improved understanding of the spatial distribution of diapycnal mixing in the oceans is the key to elucidating how meridional overturning circulation is closed. The challenge is to develop techniques which can be used to determine the variation of diapycnal mixing as a function of space and time throughout the oceanic volume. One promising approach exploits seismic reflection imaging of thermohaline structure. We have applied spectral analysis techniques to fine-structure undulations observed on a seismic transect close to the Subantarctic Front in the South Atlantic Ocean. 91 horizontal spectra were fitted using a linear combination of a Garrett-Munk tow spectrum for internal waves and a Batchelor model for turbulence. The fit between theory and observation is excellent and enables us to deduce the spatial variability and context of diapycnal mixing rates, which range from 10?5 to 10?3.5m2s?1.

Seismic attenuation of Atlantic margin basalts: observations and modeling

Paleogene basalts are present over much of the northeastern Atlantic European margin. In regions containing significant thicknesses of layered basalt flows, conducting seismic imaging within and beneath the volcanic section has proven difficult, largely because the basalts severely attenuate and scatter seismic energy. We use data from a vertical seismic profile (VSP) from well 164/07-1 that penetrated 1.2 km of basalt in the northern Rockall Trough west of Britain to measure the seismic attenuation caused by the in-situ basalts. The effective quality factor Qeff of the basalt layer is found from the VSP to be 15–35, which is considerably lower (more attenuative) than the intrinsic attenuation measured on basalt samples in the laboratory. We then run synthetic seismogram models to investigate the likely cause of the attenuation. Full waveform 1D modeling of stacked sequences of lava flows based on rock properties from the same well indicates that much of the seismic attenuation observed from the VSP can b...

Estimating movement of reflectors in the water column using seismic oceanography

The observation of spatial and temporal dynamics of the ocean is fundamental to understand global and regional aspects of water mixing. Physical oceanography has traditionally observed ocean structures with in situ measurements, often limited in temporal and/or spatial resolution. In exploration seismology a set of techniques has been developed over the last decades to image and characterize the physical properties of sub-seafloor structures by inversion methods at high horizontal resolution. The two different fields have made contact in seismic oceanography where the well developed methods of marine reflection seismology have been applied to the dynamic ocean. However, one aspect, so far ignored in seismic oceanography, is the dynamical, temporally varying nature of water structures. Here we show that it is possible to estimate temporal variations of reflectors in water structures as an inversion parameter. The new dynamic property reflector movement velocity gives an additional parameter to characterize ocean water dynamics.