Deep Seismic Reflection Survey Research Articles

2.5D seismic velocity modelling in the south-eastern Romanian Carpathians Orogen and its foreland

The DACIA-PLAN (Danube and Carpathian Integrated Action on Processes in the Lithosphere and Neotectonics) deep seismic reflection survey was performed in August–September 2001, with the objective of obtaining new information on the deep structure of the external Carpathians nappe system and the architecture of the Tertiary/Quaternary basins developed within and adjacent to the Vrancea zone, including the rapidly subsiding Focsani Basin. The DACIA-PLAN profile is about 140 km long, having a roughly WNW–ESE direction, from near the southeast Transylvanian Basin, across the mountainous south-eastern Carpathians and their foreland to near the Danube River. A high resolution 2.5D velocity model of the upper crust along the seismic profile has been determined from a tomographic inversion of the DACIA-PLAN first arrival data. The results show that the data fairly accurately resolve the transition from sediment to crystalline basement beneath the Focsani Basin, where industry seismic data are available for correlation, at depths up to about 10 km. Beneath the external Carpathians nappes, apparent basement (material with velocities above 5.8 km/s) lies at depths as shallow as 3–4 km, which is less than previously surmised on the basis of geological observations. The first arrival travel-time data suggest that there is significant lateral structural heterogeneity on the apparent basement surface in this area, suggesting that the high velocity material may be involved in Carpathian thrusting.

Measurement and prediction of ultralow frequency ocean ambient noise off the eastern U.S. coast.

Ultralow frequency (0.02-2 Hz) acoustic ambient noise was monitored from January to April 1991 at six ocean bottom stations off the eastern U.S. coast. The depths of the stations ranged from about 100 m to 2500 m. The measured spectra are in good agreement with predictions made using Cato's theory [J. Acoust. Soc. Am. 89, 1076-1095 (1991)] for noise generation by surface-wave orbital motion after extending the calculations to incorporate horizontally stratified environments. Contributions from both the linear, single-frequency (virtual monopole) and the nonlinear, double-frequency (dipole) mechanisms are clearly recognizable in the data. The predictions make use of directional wave data obtained from surface buoys deployed during the SWADE experiment and an ocean bottom model derived from compressional wave speed data measured during the EDGE deep seismic reflection survey. The results demonstrate conclusively that nonlinear surface-wave interactions are the dominant mechanism for generating deep-ocean ULF noise in the band 0.2-0.7 Hz.

Crustal evolution along a seismic section across the Grenville Province (western Quebec)

Results of deep seismic reflection survey along a 375 km long transect of the Grenville Province in western Quebec are combined with a review of geological observations and published isotopic ages. The seismic profile offers a remarkably clear image of the crust-mantle boundary and a good definition of the various crustal blocks. Crust about 44 km thick beneath the Grenville Front zone thins abruptly to ca. 36 km southeastward, perhaps the result of extension on southeast-dipping surfaces extending to the Moho. Other zones of relatively thin crust, although less pronounced, occur where Proterozoic crust overlies Archean crust, and beneath the Morin anorthosite complex. The thickest crust is found at the extreme southeast of the transect, east of the Morin anorthosite. From northwest to southeast, three main crustal subdivisions are (1) deformed Archean rocks with southeast-dipping reflectors in the Grenville Front zone, (2) an Archean parautochthon with northwest-dipping reflectors extending to the lower crust, and (3) an overlying three-layer crust interpreted as accreted Proterozoic terranes. The boundary between (2) and (3) is a major, southeast-dipping, crustal-scale ramp (Baskatong ramp) interpreted to have accommodated strain during and after accretion. U-Pb and Pb-Pb ages on detrital zircons show that metasedimentary rocks of the allochthons (Mont-Laurier, Réservoir Cabonga, and Lac Dumoine terranes) range from Archean to as young as 1.21 Ga. A single zone with 1.4 Ga old Sm-Nd model ages appears to lack Archean components and may be considered as a fragment of juvenile Mesoproterozoic crust pinched in a shear zone (Renzy shear zone) that could be raised to the status of terrane (Renzy terrane). In the allochthons, U-Pb ages of metamorphic zircon and monazite cluster around 1.17 Ga (Mont-Laurier and Réservoir Cabonga terrane) and 1.07 Ga (Renzy and Lac Dumoine terrane) and are interpreted to record late and post-accretion crustal reworking, a common feature of the Grenville orogen. A final high-grade metamorphic event (ca. 1.0 Ga) documented only in the parautochthon and the Grenville Front zone records large-scale, piggyback-style thrusting of allochthonous slabs onto the parautochthon. The age of transcurrent displacement following peak metamorphism affecting both the allochthons and the parautochthon decreases northwestward from 1.07 to 1.00 Ga. Dating thus shows that Grenvillian deformation in western Quebec occurred in pulses over an interval of 180 million years, with a tendency to propagate from the inner part of the orogen toward the Grenville Front. Reworked migmatites from the parautochthon cooled from the ca. 1.0 Ga peak of metamorphism through about 450°C (Ar closure in hornblende) at ca. 0.96 Ga with calculated cooling rates of about 6°C per million years, and unroofing rates of 0.33 km per million years. The cooling-unroofing history of the allochthons is not so straightforward, probably due to tectonic disturbances related to allochthon emplacement. Cooling through 450°C occurred between 1.04 and 1.01 Ga, at least 50 million years earlier than cooling in the parautochthon; this contrast agrees with the northwestward propagation of the orogen.

Seismic reflection fabrics of continental collision and post-orogenic extension in the Middle Urals, central Russia

Deep seismic reflection surveys in the Middle Urals, central Russia, provide the basis for a revised structural and tectonic interpretation of late Paleozoic continental collision and early Mesozoic post-orogenic extension in this orogen. These profiles (ESRU '93 and ESRU '95) form a transect from the former East European platform margin in the west to accreted island-arc and oceanic terranes in the east, crossing the Main Uralian fault (MUF), the principal suture zone in the Urals. The foreland of the Middle Urals is characterized by a fold-and-thrust belt developed above a gently E-dipping detachment at ∼10 km depth, and a deeper, presumably older, and previously unrecognized detachment system of probable Late Proterozoic (Baikalian) age. A large (∼5 km) frontal ramp marks the axis of the regional Kvarkush anticlinorium which dominates the foreland structure. Reflectivity in the middle and lower crust of the East European platform is inferred to be primarily a Precambrian fabric. Based on correlation of the seismic data with surface geology, the MUF is redefined spatially, seismically and structurally as the top of an ∼40°E-dipping reflective package, placing it (1) at the boundary between upper greenschist to amphibolite facies rocks of continental affinity to the west and unmetamorphosed to lower greenschist facies rocks of the Tagil island arc to the east, (2) at the primary topographic break in the Middle Urals, and (3) ∼10 km further east at this latitude than recent interpretations, but consistent with previous Russian maps. At depth, the MUF appears to merge in a listric fashion with a zone of subhorizontal mid-crustal reflectivity (∼15–20 km depth) that truncates a series of large-scale antiforms and associated W-dipping (E-vergent?) shear zones of inferred Paleozoic age across the Uralian hinterland. These relationships suggest that the MUF was reactivated as a normal fault during late- to post-orogenic evolution of the orogen. Based on interpretation of the Moho as the base of lower-crustal reflectivity, crustal thickness varies from ∼48 to 51 km (∼14–15 s) across the ESRU '93 profile, in concert with existing Russian studies showing a thickened crust beneath the Urals. To the east, on the ESRU '95 profile (beneath the Tagil island arc), a strongly W-dipping reflective package in the lower crust suggests abrupt thinning of the crust to ∼45 km (∼13 s), consistent with nearby wide-angle reflection data. At the surface, this crustal thinning coincides with the Triassic Imenovskii graben. We interpret these collective observations as evidence for post-orogenic extension of the Middle Urals in early Mesozoic time, in association with reactivation of the MUF as a breakaway normal fault at the westernmost boundary of the West Siberian basin extensional province. In contrast, results from the URSEIS '95 experiment indicate that the Southern Urals are largely unextended, implying that post-orogenic evolution varied along strike within the orogen.

The giant chaotic body in the Atlantic Ocean off Gibraltar: new results from a deep seismic reflection survey

The central-eastern Atlantic continental slope and parts of the Horseshoe and Seine Plains, west of the Gibraltar Arc, are occupied by a seismically chaotic body. This paper reports new deep seismic reflection data that allows a first assessment of the three-dimensional geometry of the body, and discussion of its evolution. The seismically chaotic body is huge in area and volume and complex in origin, comprising mainly gravity deposits (debris flows, olistostromes) in the northern sector, and tectonic melanges in the southern, where the body forms part of a south-verging accretionary prism related to Africa-Iberia convergence (submarine extension of the Rifian system of N-Africa). The northern part, up to several tens of thousands of cubic kilometers in volume, results from the superposition of a few endoolistostromes, probably of Miocene age, centripetally discharged onto the Eastern Horseshoe Plain and adjacent slope from structural highs generated during the Late Miocene paroxismal phase of Iberia-Africa convergence. Submarine flows were triggered along regional, continuous slopes up to 100 km in length. Their volume largely exceeds previously reported marine occurrences of gravity flow deposits, the outstanding examples of which occur in intraplate settings (hot spots, old passive margins). At the boundaries (lateral or frontal ramps) of the southern tectonic melange complex, smallscale olistostromes of Pliocene age also occur, recalling the so-called precursory olistostromes of collisional chains. The gravity and tectonic structures occur in a convergent plate setting, with low shortening rates, and their eventual fate will be incorporation, with disruption, in a collisional chain. Their volume and extent appear comparable to those that can be calculated for huge chaotic units in orogens, such as the ‘Argille scagliose' (=Liguride complex) of the Apennines, in which disrupted olistostromes and tectonic melanges coexist.

Migration of the 3-D deep-seismic reflection survey at the KTB location, Oberpfalz, Germany

A near-vertical deep 3-D seismic survey covering an area of approximately 17.85 × 19.1 km has been carried out across the German continental deep borehole (KTB) site. This survey is the first of this kind to investigate complicated crystalline crust. Two major problems were encountered during the post-stack 3-D migration of these data. First, the survey region includes areas with strong lateral velocity gradients, which are difficult to treat with many migration algorithms. Velocities in the sedimentary rocks are ∼ 3000 m/s and in the crystalline rocks ∼ 5500 m/s. Secondly, the data are sparsely sampled (50 × 50 m binning), but include steeply dipping events with true dips up to 60°. To determine the optimum 3-D migration approach three different algorithms have been tested: summation (modified Kirchhoff), finite-difference and phase-shift. Results of the various tests suggest that one-pass 3-D migration algorithms produce noticeably superior results to the much faster two-pass methods. The combination of steep dips and sparsely sampled data was a problem for the finite-difference migration, whereas shallow dipping features were imaged well by this technique. Although the adopted phase-shift migration algorithm could not handle lateral velocity gradients, it produced excellent results for both shallow and steeply dipping structures. Both of these migration algorithms are relatively fast. By comparison, the summation migration algorithm produced overall good results for regions containing steep dips and strong lateral velocity variations, but it requires a lot of computing time.

Ground-Water Salinity Control Assessment Along the Canadian River--An Integrated Geophysical Approach

Increased salinity in Lake Meredith has been attributed by water sampling to a reach of 5.5 mi of the Canadian River immediately below Ute Dam near Logan, New Mexico, as a source of saline inflow. Since Lake Meredith, impounded by Sanford Dam 150 mi downstream from Ute Dam, and the Canadian River are primary water supplies for several municipalities, including the city of Amarillo, Texas, remediation of salinity contamination is needed. Investigations performed in the late 1960s and early 1970s by the Texas Water Quality Board, the Canadian River Municipal Water Authority, and the U.S. Bureau of Reclamation conclude that the saline contamination is the result of naturally occurring artesian aquifers associated with evaporites. Proposals concluded that relieving the artesian head on these aquifers through pumping and disposing of the brine in deep-injection wells would greatly reduce the saline inflow into the river. An exploration program was initiated in November 1993 to determine the character of the brine aquifer(s), lateral extent, porosity/permeability, and also potential deep injection strata for disposal wells for the brine water. Included in this program were exploration drilling, borehole geophysics, surface seismic reflection, and pumping tests. Shallow geophysical reflection surveys in the river channel mapped the brine aquifer and located lateral discontinuities, interpreted as collapse zones providing vertical flow paths from the brine aquifer to the river channel. Deep seismic reflection surveys, tied to existing deep oil wells in the area, show sufficiently thick sedimentary sequences for deep disposal wells. These sedimentary sections of arkosic sands are associated with a deep graben in the vicinity of Logan where brine flows into the Canadian River. Log analysis of existing wells in the area allowed the correlation of lithology to seismically derived structural horizons. Information on a variety of lithologic parameters, including primary and fracture porosity, allowed selection of disposal formations prior to injection testing.

Crustal‐scale shortening and extension across the Grenville Province of western Québec

A deep seismic reflection survey shot in 1993 crosses almost the full width of the Grenville Province in western Québec. The seismic transect provides a very clear image of the crust‐mantle boundary and the most precise definition to date of the various Grenvillian terranes. The crust is around 44 km thick beneath the Grenville Front but thins rapidly to 36 km some 60 km to the southeast; it is also notable that the greatest crustal thickness of 50 km occurs at the southeast end of the transect, far from the inferred location of the main Grenvillian collision. The Grenville Front zone, in which NW directed thrusting at about 1 Ga was followed by SE directed extension, is defined by discontinuous, SE dipping reflections, which extend down to the Moho. To the southeast, the overlying migmatitic Archean parautochthon, almost half of the transect, is characterised instead by NW dipping reflectors extending into the lower crust. These reflectors are in turn truncated to the southeast by a 12‐km‐thick zone of intense SE dipping reflections, the Baskatong crustal ramp. The base of the allochthonous terranes (Allochthon Boundary Thrust) is likely located at this ramp, which flattens out at around 30‐km depth into the base of the relatively transparent intermediate crust. A highly reflective upper crustal deck corresponding to rocks of the Mont‐Laurier terrane was thrust over the Baskatong crustal ramp and is represented further to the northwest by the klippelike Cabonga allochthon. The synformal, transparent Morin anorthosite‐charnockite complex belongs to the same upper crustal level. Ramp anticlines and the overriding basal thrust of the upper allochthons demonstrate the NW directed propagation of tectonic transport during the Grenvillian orogeny, involving deformation and displacement along the Baskatong ramp with a relay into the Grenville Front zone. Postaccretional extension appears to have been primarily accommodated along the latter two crustal discontinuities thinning the crust immediately south of the Grenville Front and affecting the crust up to 350 km away from the front.

A deep seismic reflection survey across the Betic Chain (southern Spain): first results

Two land seismic reflection profiles across the Betic Chain have imaged the deep structure of the crust belonging to two different crustal domains. To the north, one profile samples the crust of the Variscan Iberian Massif that underlies the sedimentary cover of both the Guadalquivir foreland basin and the South Iberian crustal domain. The upper crust is non-reflective along the profile, but the lower crust is reflective in the northern half of the profile between 7 and 12 s two-way travel time. In this segment of the profile the Mono is also well imaged. The quality of the data diminishes significantly as the profile enters the Neogene Guadix-Baza basin. A second profile crossing the Alpine metamorphic complexes of the Betics reveals a reflective lower crust and a conspicuous reflection in the upper crust that is continuous for about 10 km. At 11 s, a distinct reflection has been interpreted as the reflection Moho. Deeper reflections are also seen in the central part of the profile in a segment about 10 km long. Comparison with available refraction and wide-angle reflection data shows some differences in both the crustal configuration and in the depth to the Moho.

High-resolution images of the lower crust: deep seismic reflections from 15 to 180 Hz

We report the results of an unconventional, broad-band deep-seismic reflection survey undertaken on land in southwest England. Previous marine seismic lines in this area have shown strong layered lower-crustal reflectivity. The broad-band survey shows near-normal-incidence P-wave reflections from the lower continental crust and from the Moho with frequencies that range from 15 to 180 Hz. High-frequency lower-crustal S-wave reflections were also observed. These high frequencies were obtained by placing both sources and receivers in boreholes up to 2 km deep, drilled within a large homogeneous granite pluton. Recording down-hole avoids loss of high frequency energy during transmission through heterogeneous and highly attenuative near-surface layers. The high frequencies were generated using explosive sources, and were recorded on sub-surface accelerometers and hydrophones, and on a conventional array of surface geophones. Seismic attenuation within the granite is low; the quality factor, Q, lies above 2000. These new, high-frequency observations show that the lower crust does not preferentially reflect particular frequencies. The data are most readily explained as resulting from the combination of a frequency-independent Q in the upper crust with a white reflectivity response in the lower crust. The data are not consistent with crustal models which are highly tuned to reflect preferentially the frequencies employed in conventional deep seismic reflection surveys. The data require relatively sharp boundaries within the lower crust; significant variation in physical properties must occur over vertical distances of not more than about 10 m. Sharp boundaries are most easily explained if the reflections result from lithological contrasts.

Mini-Sosie High Resolution Seismic Reflection Profiles Along the Bootheel Lineament in the New Madrid Seismic Zone

Abstract Results of geological and geophysical research conducted in the New Madrid seismic zone since the early 1970’s indicate that much of the seismicity of the area is associated with late Precambrian age rift-related geological structures that have been reactivated by contemporary stresses. Deep seismic reflection surveys have been used to detect and delineate deeply buried geological structures thought to be associated with the seismicity. Satellite imagery and aerial photographs have recently been used to detect a linear feature named the Bootheel lineament inferred to be the surface expression of one of the faults responsible for the 1811–1812 earthquakes. To assess the seismogenic potential of these deep structures and linear features, high resolution seismic reflection and geomorphic studies are required. In July and August, 1990, Mini-Sosie high resolution reflection surveys were conducted in the New Madrid seismic zone. A total of 23 line-kilometers of high resolution reflection data were collected at nine locations. Specific targets for the new surveys include several locations on the Bootheel lineament in the New Madrid area, its northern projection near Sikeston, Missouri, and its southern projection near Blytheville, Arkansas at locations related to the Blytheville arch. A location several kilometers south of Charleston, Missouri, was also selected. Data presented in this paper consist of 7 line-kilometers recorded at locations on or close to the Bootheel lineament near New Madrid, Missouri, Hayti, Missouri, and Blytheville, Arkansas. Numerous small-offset faults, channels and other structures in Tertiary, Cretaceous and Paleozoic age rocks have been interpreted from the Mini-Sosie seismic sections. These structures, although generally not major features themselves, may be associated with deep seated rift-related reactivated structures. Many of the small-offset faults appear to deform or offset Quaternary age sediments. The spatial correlation of the observed faulting with sandblows and lineaments identified from aerial photographs, suggests the possibility that the observed faulting, sandblows, and linear features may be genetically related. If this is the case, then, because the origin of the sandblows has generally been attributed to the 1811–1812 seismic activity, the observed faulting may have been active at that time. It is not possible to directly link a single correlatable seismic signature with the Bootheel lineament, and thus we cannot state unequivocally that the lineament is continuous from Blytheville, Arkansas to New Madrid, Missouri. However, each seismic line has imaged similar small-offset faulting and gentle folding. If the faults and deformation observed are directly caused by reactivated deep structures associated with the Bootheel lineament, then, due to its great length, the total of which is yet undefined, this structure may be a source zone for major earthquakes, and therefore requires further investigations. The possibility exists, however, that the small scale faulting and deformation are ubiquitous throughout the New Madrid seismic zone. Additional high resolution seismic data are required to resolve this question.

Predictions and observations of seafloor noise generated by sea surface orbital motion.

A model is developed for the prediction of the seismo-acoustic noise spectrum in the microseism peak region (0.1 to 0.7 Hz). The model uses a theory developed by Cato [J. Acoust. Soc. Am. 89, 1096–1112 (1991)] for an infinite depth ocean in which the surface orbital motion caused by gravity waves may produce acoustic waves at twice the gravity wave frequency. Using directional wave spectra as inputs, acoustic source levels are computed and incorporated into a more realistic environment consisting of a horizontally stratified ocean with an elastic bottom. Noise predictions are made using directional wave spectra obtained from the SWADE surface buoys moored off the coast of Virginia and the SAFARI sound propagation code, with a bottom model derived using wave speeds measured in the EDGE deep seismic reflection survey. The predictions are analyzed for noise level variations with frequency, wave height, wind direction, and receiver depth. These predictions are compared to noise measurements made in ECONOMEX using near-bottom receivers located close to the surface buoys. Good agreement is found between the predictions and observations under a variety of environmental conditions. [Work supported by ONR.]

地下深部構造探査

Deep reflection profilings of the continental crust have been revealing major structures in various types of tectonic settings in the world. Most of these surveys in the 1980's have been carried out by using the Vibroseis technique, which have clarified the detailed characteristics of the crust and the upper mantle. With regard to the Vibroseis method, phase and force control techniques based on the concept of ground force were introduced to produce a stable source signature. Diversity stack which improves signal to noise ratio and migration techniques which map dipping reflectors precisely have helped to improve data quality. Transects across genetic mountain belts produce several types of seismic fabrics which reflect the different geological settings. In particular, the reflective lower crust (RLC) is commonly found in the Phanerozoic crust, while the upper crust and the mantle are relatively transparent. The Moho reflections are found to have relatively continuity and flat orientation beneath complex crustal blocks which have undergone major orogenic deformations. Some of the mid-crustal bright spots are interpreted to be intrusive molten magma bodies. Several scientific drilling programs have been planned to obtain direct information of the continental crust, which are supplemented by deep seismic reflection profilings. Transects across several geological provinces will reveal not only local characteristics but also regional tectonics. Deep seismic reflection surveys in Japan have just commenced, and they are expected to reveal the deep crustal structure which is considered to be more complex than that in other areas.

Main results of the ECORS Pyrenees profile

The 250 km long, deep Seismic reflection survey across the Pyrenees from one foreland (Ebro Basin) to the other (Aquitaine Basin) was designed to study the structure of the deformed Mesozoic boundary between the Iberian and European Plates. The results are: 1.(1) the entire crust shows well-defined reflectors with a general fan-shape geometry;2.(2) a deep layered domain is evident above the Moho discontinuity;3.(3) near the surface in the external domains, reflectors accurately define the geometry of major thrusts and structures affecting the Mesozoic and Cenozoic cover of the Pyrenees;4.(4) the middle crust shows two major sets of Seismic events interpreted as Hercynian structures.Using these results, a balanced structural section of the Pyrenean belt is proposed. The mode of thickening of the crust during the Alpine orogeny is briefly discussed; only the Iberian crust was significantly thickened by a southward stacking of slabs.

Reflections from mantle fault zones around the British Isles

Research Article| June 01, 1990 Reflections from mantle fault zones around the British Isles Catherine A. Flack; Catherine A. Flack 1British Institutions Reflection Profiling Syndicate (BIRPS), Bullard Laboratories, Madingley Road, Cambridge CB3 0EZ, England Search for other works by this author on: GSW Google Scholar Simon L. Klemperer; Simon L. Klemperer 1British Institutions Reflection Profiling Syndicate (BIRPS), Bullard Laboratories, Madingley Road, Cambridge CB3 0EZ, England Search for other works by this author on: GSW Google Scholar Susan E. McGeary; Susan E. McGeary 1British Institutions Reflection Profiling Syndicate (BIRPS), Bullard Laboratories, Madingley Road, Cambridge CB3 0EZ, England Search for other works by this author on: GSW Google Scholar David B. Snyder; David B. Snyder 1British Institutions Reflection Profiling Syndicate (BIRPS), Bullard Laboratories, Madingley Road, Cambridge CB3 0EZ, England Search for other works by this author on: GSW Google Scholar Mike R. Warner Mike R. Warner 1British Institutions Reflection Profiling Syndicate (BIRPS), Bullard Laboratories, Madingley Road, Cambridge CB3 0EZ, England Search for other works by this author on: GSW Google Scholar Geology (1990) 18 (6): 528–532. https://doi.org/10.1130/0091-7613(1990)018<0528:RFMFZA>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Catherine A. Flack, Simon L. Klemperer, Susan E. McGeary, David B. Snyder, Mike R. Warner; Reflections from mantle fault zones around the British Isles. Geology 1990;; 18 (6): 528–532. doi: https://doi.org/10.1130/0091-7613(1990)018<0528:RFMFZA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract British Institutions Reflection Profiling Syndicate (BIRPS) deep seismic reflection surveys show that the upper mantle around the British Isles is reflective, exhibiting discrete, isolated reflecting zones within an otherwise seismically transparent mantle. There are 20 known occurrences of reflectors within the upper mantle across the continental shelf of the British Isles, many delimited by intersecting or closely spaced profiles. Most of the mantle reflectors have similar reflection characteristics: they dip 10° to 30° and are thin (∼0.5 s [∼2 km]) and discrete. The geologic history of the continental shelf around the British Isles (an area 1000 x 1000 km) is complex, and the tectonic settings of these reflectors vary. Each mantle reflector is associated with a major zone of deformation in the crust. This association, the dips of the reflectors, and their high reflectivity all indicate a structural origin. The BIRPS mantle reflectors include probable early Paleozoic thrusts and Mesozoic normal faults. In contrast to BIRPS data, deep reflection profiles in other areas of the world, in a variety of geologic settings, have only rarely image reflectors within the upper mantle This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Correlative Geophysical Data in the Australian Region for Use in the MAGSAT Project

Magnetic data available for use in correlation with MAGSAT data in the Australian region include observatory recordings, first-order and third-order vector regional magnetic surveys, long airborne scalar profiles, and approximately 75% coverage of the continent with detailed aeromagnetic mapping of total intensity. Reconnaissance gravity coverage is virtually complete, and deep seismic sounding refraction and reflection surveys have been carried out at widely scattered locations. There are scattered data from geothermal investigations. Magnetic and gravity coverage is also available over much of the surrounding marine area, and in selected areas in Antarctica. It is planned to use this material in a comprehensive study of the broader lithospheric features of the area.

COCORP Seismic Profiling, The Eastern Overthrust, and Continental Evolution: ABSTRACT

COCORP deep seismic reflection surveys in New England, Georgia, and Arkansas confirm that crustal scale overthrusting is an important aspect of Appalachian-Ouachita orogenic evolution. The southeastern Appalachian traverse, by tracing early Paleozoic shelf sedimentary rocks (or their metamorphic equivalents) eastward beneath the Blue Ridge and Piedmont, defined a master decollement above which crystalline rocks were transported 124 mi (200 km) or more. The Arkansas traverse likewise suggests that the southern part of the Ouachitas are allochthonous, with the deep water facies of the Benton uplift thrust over relicts of an early Paleozoic shelf. The southern Appalachian and Ouachita results are consistent with the tectonic burial of continental margins beneath crystalline hrust sheets during collisional orogeny. The New England surveys also testify to the importance of basement -involved thrusting, with indications that the the Green Mountain Precambrian massif is an allochthonous sliver and the crust beneath New Hampshire a complex of imbricate thrust slices of metasedimentary and igneous material. While debate continues regarding interpretational details, these results provide clear support for the hypothesis that thin-skinned style overthrusting on a crustal scale is an important, if not the important, mode of continental accretion. It has been speculated that sediments and water emplaced in the lower crust by such megathrusts may explain certain types of magmas, high conductivity layers, and low velocity zones in the crust. A layered complex discovered in the lower crust beneath the eastern Adirondacks by COCORP profiling may represent just such a relict of underthrust sedimentary material, perhaps buried during the Grenville orogeny and thus representing the predecessor to Appalachian accretion. End_of_Article - Last_Page 1708------------

Deep seismic reflection survey across the Variscan Front of southern England

In 1979, the Institute of Geological Sciences commissioned two lines of seismic reflection profile, a technique which has proved successful1 for studying crystalline basement. The survey was sited to cross the concealed Variscan Front in southern England, with recording time extended to 12 s two-way time (TWT). Similar profiles in adjacent parts of Europe have suggested that the Variscan Front is characterized by major thrust zones extending to considerable depths2,3. Our profiles, reported here, show good primary reflections to 6 s TWT and give new detail on the nature of Variscan deformation in southern England. Zones of differing reflection character are evident; a northern zone of low northerly apparent dips, a central zone of poor reflector quality, and a southern zone of steeper dips to the SSE. The southern zone is presumed to represent deformed Variscan basement, including several minor thrust planes and a proposed major thrust zone, analogous to similar structures in Europe. Impersistent reflections between 9 and 10 s TWT could possibly relate to the Moho.