34th International
Geological Congress
|
Updated: Jun 02, 2016
6th International Symposium on Deep Seismic Reflection Probing of the Continents and Their Margins
Budapest, Hungary, 12 September - 17 September 1994
from TECTONOPHYSICS Volume 264, Issues 1-4, Pages 1-392 (30 October 1996)
Edited by D.J. White, J. Ansorge, T.J. Bodoky and Z. Hajnal
Abstracts of papers presented for publication in a special issue of Tectonophysics
(Full copies of these papers may be purchased for about US$30 each from Elsevier through their web site http://www.elsevier.com and follow the "journals" link to Tectonophysics)
Seismic image of the Cantabrian Mountains in the western extension of the Pyrenees from integrated ESCIN reflection and refraction data
Pages 1-19
ESCIN Group, J. A. Pulgar, J. Gallart, G. Fernández-Viejo, A. Pérez-Estaún and J. Álvarez-Marrón
Integrated analysis of normal-incidence and large-aperture seismic reflection data collected in 1992 and 1993 within the Spanish ESCIN and complementary projects provide a first complete N---S crustal transect across the Northern Iberian Peninsula and continental margin. Images of the crustal structure of the Cantabrian Mountains and their transition to the Duero basin and to the Cantabrian margin are obtained from: (a) a 65-km-long vertical reflection profile ESCIN-2 on land; (b) a 200-km-long reversed refraction profile; and (c) wide-angle recordings of the marine ESCIN-4 profile.
Consistent results between reflectivity pattern and velocity-depth distribution reveal important lateral variations in the deep structure. The reflective crust imaged in the ESCIN-2 profile changes its attitude from sub-horizontal beneath the Duero basin to north-dipping beyond the Mountain front. Basement thrusts are observed in the upper crust merging into a detachment at 6 s (TWT) and may have triggered the Alpine uplift of the range. The Moho is identified at the bottom of the reflective lower crust and deepens from 12 to 15 s at the northern end of the profile, about 35 km inland. Modelling of the refraction data laterally extends the seismic image and provides evidence for Variscan crustal features beneath the Duero basin. Northwards, the velocity in the lower crust decreases and the Moho, constrained by the wide-angle data from profile ESCIN-2, deepens to about 60 km ending abruptly at the shoreline. The velocity-depth model is constrained along the Asturian platform up to the continental slope, where the crust-mantle boundary is located at 24 km depth. This 'margin Moho' shows a progressive deepening southwards, and extends to the coast where it is found at 30 km depth.
The present seismic data support an important Alpine reworking and thickening of the crust under the Cantabrian Mountains. The onshore/offshore transition is marked by an imbrication of two crusts of very different thicknesses. This signature offers a strong parallelism with the one previously observed further east across the Pyrenees in the ECORS seismic profile.
Elsevier online abstract
Crustal evolution of the Middle Urals based on seismic reflection and refraction data
Pages 21-34
C. Juhlin, J. H. Knapp, S. Kashubin and M. Bliznetsov
Integration of seismic refraction and deep reflection data from the Middle Urals of central Russia provides important new constraints on the structure of the Uralian crust. Re-analysis of the GRANIT refraction profile and comparison with coincident reflection data from the ESRU profile shows a high-velocity (7.6-7.8 km/s) root zone from c. 45 to 55 km with low reflectivity beneath the Urals. We interpret this interval as crustal material, consistent with previous Russian interpretations of this velocity anomaly. Above this crustal root, one of the principle features imaged on the ESRU profile is a thick zone (3-4 s TWT) of relatively strong reflectivity which may characterize the lower crust of the East European platform, but is considerably shallower to the east at 8-12 s TWT (25-37 km) than in the west at 10-14 s TWT (31-43 km). Reprocessing of 20 s records from the R-17 Profile in the West Siberian basin, 315 km northeast of the ESRU profile, reveals a similar pronounced lower-crustal reflectivity between 11-14 s TWT (34-43 km), in the hinterland of the Middle Urals. This lower-crustal reflection fabric may represent a feature developed during collisional orogenesis, or a younger property imparted through post-orogenic extension. Future deep reflection profiling will be critical to address the continuity, and accordingly, the tectonic significance of this lower-crustal reflectivity in the Urals.
Elsevier online abstract
A traverse of the ionian islands front with coincident normal incidence and wide-angle seismics
Pages 35-49
The STREAMERS-PROFILES group, Alfred Hirn, Maria Sachpazi, Risto Siliqi, John Mc Bride, Fedon Marnelis and Licio Cernobori
Marine vertical reflection profiling with a powerful airgun source, augmented with a few wide-angle seismometer stations on land, has been applied along a 180-km line across the presently active deformation front of the subduction of the Ionian Sea plate beneath the western Hellenides. East of the Ionian islands, there is limited evidence for reflectivity down to 23 km interpreted as the base of a rather thin continental-type crust. Under the western slope of the islands, a major normal-incidence reflector dips eastward, first gently to the west of the islands, then more steeply under them. This reflector may be extrapolated southwestwards beneath a bulge that is thought to represent the modern pre-Apulian front located over the subduction zone. The continuity, signature, and geometry of this reflector suggest that it may act as the lower limit of the western Hellenides where they override the Ionian Sea quasi-oceanic crust, rather than just an intracrustal interface of a pre-Apulian crust. The location of the previous deformation front, the Ionian thrust proposed in previous models, can be constrained by the new seismic data. The new data raise the possibility of a larger area of evaporite mobility than previously considered, insofar as active block motion apparently related to halokinesis is recognized west of the Zakynthos anticline. Diapirism, décollement, and westward-directed over-thrusting in the pre-Apulian crust may have been brought on by Late Pliocene-Quaternary reactivation of regional extension associated with the separation of the Peloponnesus from northern Greece as it was captured by the Anatolian-Aegean rotation and associated also with the fast clockwise rotation of the Ionian islands as they were sheared off the Apulian domain to the southwest by the initiation of the Kefallinia transform.
Elsevier online abstract
Gently north-dipping Median Tectonic Line (MTL) revealed by recent seismic reflection studies, southwest Japan
Pages 51-63
Tanio Ito, Takeshi Ikawa, Satoshi Yamakita and Takuya Maeda
The Median Tectonic Line (MTL), with a length of more 1000 km, is the most significant fault in Japan. It juxtaposes the high-P/T Sambagawa metamorphic rocks against the low-P/T metamorphic rocks of the Ryoke belt. The MTL was probably formed in the Cretaceous with many subsequent reactivations. The western segment of the MTL is still active with an almost pure right-lateral sense of motion. Although a great amount of geological information on the MTL has been accumulated, information about the subsurface, especially the deep-seated structure of the MTL, is still insufficient. It has been generally assumed that the MTL is vertical or steeply dipping at depth because of its straight surface trace and its recent lateral motion. Recently, new geophysical data have suggested that the MTL dips gently northward at depth. We have acquired a complementary set of geophysical profiles (seismic reflection and refraction, gravity and MT) across the MTL in east Shikoku. Our results confirm that the MTL dips northward at about 30 to 40 degrees from the surface to about 5 km depth, where it becomes listric. This fault geometry more reasonably explains the reactivation history of the MTL: the motion has occurred on a listric-type fault in so-called oblique or lateral ramp manner.
Elsevier online abstract
LITHOPROBE reflection studies of Archean and Proterozoic crust in Canada
Pages 65-88
Ron M. Clowes, Andrew J. Calvert, David W. Eaton, Zoltan Hajnal, Jeremy Hall and Gerald M. Ross
LITHOPROBE, Canada's national collaborative earth science research project established to develop a comprehensive understanding of the evolution of the North American continent, is a multidisciplinary program spearheaded by seismic reflection studies. Five recently recorded seismic lines, discussed in this paper, are located in Precambrian regions: the Mesoproterozoic Grenville Province, the Paleoproterozoic Trans-Hudson orogen (THO), the Paleoproterozoic/Archean basement of Alberta, and the Archean Superior Province. Data acquired across the Grenville orogen in eastern Quebec show strong reflectivity throughout the crust; upper crustal reflections can be correlated with exposed structural elements, including extensional shear zones and packages of deformed high-pressure rocks (eclogites). In a marine survey across the Grenville orogen off southeastern Labrador, seismic images show variably dipping reflections and a structural high associated with a major gravity anomaly. Data acquired across central Alberta show crustal-scale thrust stacking and imbrication of the Archean Hearne craton. To the east across the Trans-Hudson orogen, images of similar collisional features are observed. Geochronologic constraints indicate contemporaneity of tectonic activity between the two regions at 1.8 Ga, suggesting that collisional tectonic activity was coeval over a broad crustal region, ca. 1000 km across strike. In the Superior Province, seismic data across a collision zone involving the northern Abitibi greenstone belt and the arc-related Opatica plutonic belt show spectacular crustal reflectivity and dipping reflections that extend 8 s ( 30 km) into the mantle. The latter feature is interpreted as representing a relict 2.69-Ga-old suture associated with subduction, providing the first direct evidence that plate tectonics was active in the late Archean.
These five examples, supported by other LITHOPROBE results, refute a number of generalizations about crustal reflectivity that have been made in the past and illustrate how reflection studies, combined with other geoscience studies, can lead to a better understanding of Precambrian tectonics. Reflectivity persists throughout the crust; there is no general separation into a poorly reflective upper crust and a reflective lower crust. Crustal reflectivity in Archean and Proterozoic regions is as pervasive as that in areas of more recent tectonism. The Precambrian reflection Moho is generally well defined but shows a range of characteristics. Relative ages of reflectors can be discerned and tectonic significance can be attached to characteristic features of the crustal reflectivity.
Elsevier online abstract
Seismic image of an early Proterozoic rift basin
Pages 89-100
Bernd Milkereit and Jianjun Wu
Three high-resolution vibroseis reflection profiles were acquired across the Paleoproterozoic Huronian Supergroup (2.5-2.2 Ga) of the Canadian Shield, in the vicinity of the Temagami Lake magnetic anomaly, one of the largest positive magnetic anomalies in North America. The Supergroup, formed as a passive margin sequence and composed of volcanic and sedimentary rocks, unconformably overlies Archean rocks of the Superior Province. Quality seismic images, obtained through innovative pseudo-3D seismic processing and constrained by lithological data from deep boreholes, reveal a 20-km-wide early Proterozoic rift basin beneath a thick cover of metasediments, one of the oldest well-preserved rift basins on Earth.
Elsevier online abstract
Coincident vibroseis and dynamite surveys across the western flank of the Trans-Hudson Orogen
Pages 101-109
Sándor Bezdán and Zoltan Hajnal
Coincident dynamite and vibroseis reflection surveys provide comparable seismic signals along a 200-km segment of the Trans-Hudson Orogen Transect. On single-fold field records, signal amplitudes from explosive sources are consistently 40 dB higher than on the corresponding vibroseis records. However, the vibroseis final stack exhibits higher crustal reflectivity due primarily to the difference in data acquisition fold of the two surveys. Within the crust, distinctive seismic patterns, such as dominant westerly dipping reflections on the western end of the profile and a 3-s depression in the Moho beneath a crustal scale culmination, are clearly visible on both sections. The advantages of the dynamite source include deeper signal penetration and mapping of a number of previously unrecognized subcrustal reflections. Moreover, the dynamite data also indicate that diffraction patterns, detected at lower crustal and Moho depths, have large apertures and extend to greater two-way traveltimes than were recorded in the vibroseis data. Thus, proper migration of lower crustal reflections requires recording greater traveltimes than were recorded during the 1991 LITHOPROBE vibroseis acquisition program.
Elsevier online abstract
Moho signature from wide-angle reflections: preliminary results of the 1993 Trans-Hudson Orogen refraction experiment
Pages 111-121
B. Németh, Z. Hajnal and S. B. Lucas
In 1993 a combined crustal seismic refraction and wide-angle reflection experiment was conducted over the northern Saskatchewan and Manitoba portion of the Trans-Hudson Orogen by LITHOPROBE. The program included three seismic lines extending over 1900 km in length. The exceptionally high-quality data collected during the field experiment included seismic reflections and refraction events from the Moho and upper lithospheric mantle. The relatively high seismic fold and signal-to-noise ratio of the field observations made it possible to process the wide-angle portion of the data set with standard reflection signal enhancement techniques. The Moho is revealed as a complex interface with significant structural relief (40-52 km) that correlates in first-order fashion with crustal geology and structure. The Moho topography is interpreted as a result of variable ductile reworking and eclogitization of the lower crust.
Elsevier online abstract
Results of seismic reflection profiling in the Turanian Platform
Pages 123-135
E. R. Sheikh-Zade
Experimental studies of the Turanian Platform, which were carried out for more than 15 years by seismic reflection profiling, outline the main characteristics of its crust. The basins of the Turanian Platform are characterized by substantial changes of the crustal reflection character and a complex structure of the Moho boundary, which is marked primarily by subhorizontal reflectors. Reflectors are clearly concentrated in the upper crust of the Amu-Darya basin, in contrast to the Ustyurt and the Aral Sea basins where the major reflectivity is concentrated in the lower crust. The predominance of subhorizontal reflections is the most general feature of the crust beneath the basins. This picture differs greatly from that of the Central Kazakhstan Massif, where the major part of the crust is highly reflective with abundant inclined reflections and interspersed local 'transparent' zones which may indicate the existence of intrusive bodies. These differences are the result of important tectonic and geological processes. In contrast to the Central Kazakhstan Massif which was formed as the result of Palaeozoic overthrusting of thin-skinned nappes, the crust of the basins consists of monolithic massifs of ancient cratonic fragments (micro-continents) separated by narrow suture zones.
Elsevier online abstract
Deep seismic images and the tectonic framework of early rifting in the Otway Basin, Australian southern margin
Pages 137-152
D. M. Finlayson, D. W. Johnstone, A. J. Owen and K. D. Wake-Dyster
The Otway Basin is one of a number of basins formed along Australia's southern continental margin during the Late Jurassic-Early Cretaceous rifting of Australia from Antarctica. It lies on a transitional area of extended lithosphere between the margin to the west, where continental separation took place close to the Australian craton, and the region to the east where separation failed to develop through Bass Strait, but instead switched to the south of Tasmania. Seven AGSO regional deep seismic profiles recorded during 1992 across onshore parts of the basin provide images of synrift basin bounding faults that dip predominantly towards the continent, basin sequences in half-graben rift segments at two-way times greater than 4 s, and mid-crustal detachments with ramp and flat geometry at mid-crustal levels. When combined with industry seismic data, trend variations on faults indicate that extensional strain direction varied along the rift system, probably locally controlled by pre-existing Palaeozoic geology but consistent with general north-south lithospheric extension. Basement highs between some of the rift segments are interpreted as accommodation (or transfer) zones. Multiple basin-bounding faults in some places mirror features within basement and may indicate a component of local strike-slip during early rifting. The syn-rift segments form part of the first-stage Early Cretaceous failed rift system in the Otway-Bass Strait region. Crustal thickness is interpreted to be 31 km (10.3-10.5 s TWT) near the northern basin margin, thinning to about 25 km (9 s TWT, 40% reduction in thickness of Palaeozoic crust) seaward of the Tartwaup fault zone. This fault zone, and the Timboon fault system farther east, are interpreted to be headwall faults (landward limit) of a Late Cretaceous second-stage rifting system along a lower plate margin which ultimately separated Australia from Antarctica.
Elsevier online abstract
Seismic structure of the northern continental margin of Spain from ESCIN deep seismic profiles
Pages 153-174
J. Alvarez-Marrón, A. Pérez-Estaún, J. J. Danñobeitia, J. A. Pulgar, J. R. MartínezCatalán, A. Marcos, F. Bastida, P. AyarzaArribas, J. Aller, A. Gallart et al.
By the end of the Carboniferous, the crust of the continental shelf in northwestern Spain was made up of deeply rooted structures related to the Variscan collision. From Permian to Triassic times the tectonic setting had changed to mainly extensional and the northern Iberian continental margin underwent rifting during Late Jurassic-Early Cretaceous times, along with sea-floor spreading and the opening of the Bay of Biscay until the Late Cretaceous. Subsequently, the northern Iberian margin was active during the north-south convergence of Eurasia and Iberia in the Tertiary.
A multichannel seismic experiment, consisting of two profiles, one north-south (ESCIN-4) crossing the platform margin offshore Asturias, and another (ESCIN-3) crossing the platform margin to the northwest of Galicia, was designed to study the structure of the northern Iberian margin. The ESCIN-4 stacked section reveals inverted structures in the upper crust within the Le Danois Basin. North of the steep continental slope, ESCIN-4 shows a thick sedimentary package from 6 to 9.5 s, two-way travel time (TWT). Within this latter package, a 40-km-long, north-tapering wedge of inclined, mainly south-dipping reflections is thought to represent a buried, Alpine-age accretionary prism. In the north western part of the ESCIN-3 (ESCIN-3-1) stacked section, horizontal reflections from 6.5 to 8.5 s correspond to an undisturbed package of sediments lying above oceanic-type basement. In this part of the line, a few kilometres long, strong horizontal reflection at 11.2 s within the basement may represent an oceanic Moho reflection. Also, a band of reflections dips gently towards the southeast, from the base of the gently dipping continental slope. The part of ESCIN-3 line that runs parallel to the NW-Galicia coast (ESCIN-3-2), is characterized by bright, continuous lower crustal reflections from 8 to 10 s. Beneath the lower crustal reflectivity, a band of strong reflections dips gently toward the southwest from 10 to 13.5 s. The part of ESCIN-3 that parallels the northern margin (ESCIN 3-3), shows good reflectivity in all levels. Upper crustal reflections image the sedimentary fill of probable Mesozoic to recent basins. Mid-crustal reflectivity is characterized by dipping reflections until 8 s that are probably related to compressional Variscan features. The lower crustal level shows 'layered' reflections between 8 and 12 s. Dipping reflections are found below the continental Moho.
Elsevier online abstract
Crustal image of the Ionian basin and its Calabrian margins
Pages 175-189
Streamers/Profiles Working Groups, L. Cernobori, A. Hirn, J. H. McBride, R. Nicolich, L. Petronio and M. Romanelli
Previous seismic investigation of the crustal structure in the Ionian basin has been limited to shallow penetration seismics of the 1970's, characterized by inadequate source power and low fold. Earlier OBS and ESP seismic refraction experiments have not been able to firmly resolve one of the principal scientific problems for this region which is whether the Ionian basin is floored by oceanic crust or by highly attenuated continental crust. A second elusive problem is the nature of the transition of the boundaries between the Ionian basin and its margins.
In this paper we describe and interpret new deep seismic reflection and wide-angle data collected in the western Ionian Sea and the Calabria region of Italy. One of the principal features of our multichannel reflection data beneath the Ionian basin is a band of 'layered' high-amplitude reflections near the base of the crust. This band shows a quasi-monochromatic (ca. 8 to 10 Hz) frequency and a traveltime thickness of 1 to 1.5 s. These images contrast with the well known reflection patterns of Mesozoic oceanic crust investigated in the Atlantic Ocean. There is evidence that the low-frequency band dips down towards the edge of the Malta Escarpment (ME), where landward-dipping reflectors separate continental and intermediate type crust in the central tract of the ME. The increased traveltimes of the lower-crustal reflectors and Moho, from the basin towards the southern and eastern margins of southern Calabria, could be partially due to the velocity pull-down effect of the sedimentary pile of the arc, although a true dip of 15 to 18%, over 60 km distance, can be substantiated. Moreover, the reflecting band maintains its reflectivity and thickness until its abrupt termination beneath the Ionian continuation of the Calabrian compressional crustal structures. The coincident acquisition of wide-angle seismic data and marine reflection seismic data provided a landward extension of the survey which will complement existing geologic information on the deep framework of the Ionian basin and its Calabrian margin.
Elsevier online abstract
Crustal-scale extension in the central Pannonian basin
Pages 191-204
Z. Hajnal, B. Reilkoff, K. Posgay, E. Hegedus, E. Takacs, I. Asudeh, St. Mueller, J. Ansorge and R. DeIaco
Within the Hungarian segment of the Panonian basin, a 100-km-long seismic reflection profile images the sedimentary fill of two major depressions and their associated crustal signatures. These deep ( 7000 m) subbasins are asymmetric half grabens, bounded along their western margins by low-angle crust-penetrating subparallel listric faults. The hanging walls are intensely fractured and comprise pre-Tertiary basement and upper-crustal rocks. The footwalls are formed by uplifted and ductilely deformed middle to lower crust. The crust is relatively thin (25-27 km) and decoupled by well-defined detachment faults. The Moho has a gentle northeasterly dip and a spatially variable seismic signature. The seismic images imply that the present crustal setting of the area was developed contemporaneously through the northeastward extension of the Pannonian basin.
Elsevier online abstract
Coincident normal-incidence and wide-angle reflections from the Moho: evidence for crustal seismic anisotropy
Pages 205-217
K. A. Jones, M. R. Warner, R. P. Ll. Morgan, J. V. Morgan, P. J. Barton and C. E. Price
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%.
Elsevier online abstract
Shear wave anisotropy of laminated lower crust at the Urach geothermal anomaly
Pages 219-233
Wolfgang Rabbel and Ewald Lüschen
Deep seismic reflection studies have shown that 'lamellae' are a widespread reflectivity pattern of the lower crust of the central European Variscan belt. This pattern has been interpreted, inter alia, as alternating subhorizontal layering of mafic and felsic rocks implying a tectonic process of structural and textural ordering. Consequently, laminated lower crust should be elastically anisotropic. The specific type of anisotropy should provide some insight into the mineral composition and the preferred orientation of minerals in the lower crust.
We have investigated this problem in the area of the Urach geothermal anomaly (South Germany) where a 'classical' example of lower-crust lamellae is found. A restricted range of subsurface points was probed in a controlled-source expanding spread seismic experiment with two orthogonal azimuths of observation up to 90-km source-geophone offset. Both P- and S-waves were recorded with 3-component geophones at 80-140 m geophone spacing. Based on polarization analysis and traveltime interpretation the following results were obtained: (1) S-wave splitting is observed only for SMS arrivals (not for shallower reflections) implying that the lower crust is anisotropic; (2) the type of anisotropy is quasihexagonal (transversely isotropic) implying that there is no preferred mineral orientation within the horizontal plane; (3) the coefficient of S-wave anisotropy [(Vmax - Vmin)/Vmin] is estimated at 6-13% for SV-type waves; the SH-wave velocity shows only small variation with offset; (4) the observed relation between direction and velocity of S-wave propagation can be explained by mafic rocks containing a high amount of orthopyroxene minerals horizontally aligned in the pure shear stress regime.
Elsevier online abstract
Waveform inversion of deep seismic reflection data: the polarity of mantle reflections
Pages 235-247
C. E. Price, J. V. Morgan, M. R. Warner, P. J. Barton, K. A. Jones and R. P. Ll. Morgan
The Flannan and W-reflectors are two prominent mantle features observed on seismic reflection data off the northwest coast of Scotland. They are the brightest, most laterally extensive, intra-mantle reflectors identified on a deep seismic dataset anywhere in the world. Despite extensive study, their physical origin is still the subject of speculation.
We present a scheme to determine the polarity of these mantle reflectors, and constrain their upper structure using near-normal-incidence seismic reflection data. The technique exploits the convolutional model of the earth; we use a deterministic source-signature deconvolution to invert the data. We have explored the parameterization of the inversion by testing real and synthetic data. We find that it is critical to the legitimacy of the reflectivity model that many traces are stacked prior to the inversion and that the data have a good signal-to-noise ratio. Furthermore, an accurate estimate of the effective source wavelet is a fundamental requirement for obtaining a valid reflectivity model; in particular we find the deconvolution results are most sensitive to the precise value of the water depth and reflection coefficient used in estimating the sea-bed multiple train.
In the case of the Flannan-reflector, the inversion shows unequivocally that it has a positive polarity. Modelling the W-reflector is less straightforward as a result of reduced signal-to-noise ratio. None-the-less, the inversion suggests a positive polarity for the W-reflector, in agreement with observations of post-critical reflections seen on wide-angle seismic data. The near-normal-incidence polarity measurements support the suggestion that both the Flannan and W-reflectors represent an eclogitic slab, presumably a relict oceanic subduction zone, preserved within the continental lithospheric mantle.
Elsevier online abstract
Seismic velocity, heterogeneity, and the composition of the lower crust
Pages 249-259
John Brittan and Mike Warner
Many wide-angle seismic experiments attempt to constrain lower crustal seismic velocity and thus composition. Deep near-normal-incidence seismic data recorded during the past decade suggest a widely varying and highly complex pattern of lower crustal heterogeneity. Synthetic seismic modelling indicates that velocities measured from wide-angle experiments may depend significantly upon the geometrical arrangement and scale of the heterogeneities. Heterogeneities with a correlation length considerably large than the seismic wavelength will tend to produce observed velocities biased towards high values. Heterogeneities much smaller than the seismic wavelength can lead to a velocity bias of either sense depending upon their arrangement. Seismic velocities greater than 7.3 km/s measured near the base of the crust in underplated and intruded passive continental margins suggest compositions that are markedly more mafic than mantle melting calculations predict. Our seismic modelling suggests that the mismatch between modelled and observed velocities at passive margins may be attributable to large-scale (correlation length greater than seismic wavelength) heterogeneities.
Elsevier online abstract
General features of the uppermost mantle stratification from long-range seismic profiles
Pages 261-278
N. I. Pavlenkova
Long-range seismic studies made by Russian institutions during the last two decades reached depths of 700-800 km on the continent (Peace Nuclear Explosions were used as a source) and 100 km in the ocean (ocean bottom stations recording 500-kg explosions in water). Comparison of the wave-fields of these and other seismic profiles in Russia, Western Europe and the South Atlantic shows that the classical lithosphere-asthenosphere model is too simple to explain the observed data. A stratified medium with alternating weak and rigid layers is a more realistic model of the upper mantle. One of the layers is globally observed at a depth of 80 to 100 km. It is located in the thermal lithosphere beneath old platforms, at the bottom of the lithosphere under active tectonic areas and inside the asthenosphere below mid-oceanic ridges. Relatively high velocities (8.4 to 8.5 km/s) are typical for this boundary both in the old platforms and in the high heat flow oceanic areas. A change of structural pattern where the block structure of the uppermost mantle is transformed into a horizontally homogeneous one is typical at that boundary level. Such features suggest that the boundary separates the brittle and more ductile upper mantle layers and may be considered as the bottom of the global mechanical lithosphere.
Elsevier online abstract
Faults and folds, fact and fiction
Pages 279-293
R. Meissner
After reviewing the microscopic and macroscopic texture of fault zones, the localisation of rupture and creep is described, and the reduced strength of fault zones is investigated. Simple strength and viscosity models for the whole lithosphere play a major role for the geometry of fault zones. Compressional faults (thrusts) show ramp- and flat structures, often soling in a (weak) detachment zone, extensional faults prefer a listric shape, often turning subhorizontal and being invisible in the seismic lamellae of the ductile lower crust which according to the models is also a broad zone of detachment and decoupling. Faults in the lower crust are rare and restricted to strain hardened fault rocks or intrinsically rigid rocks like oceanic mafic-ultramafic rocks of suture zones. Transient faults in the lower crust are created by rupture processes of large earthquakes. Some deep faults, moderately dipping or flat, are again observed in the (rather rigid) uppermost mantle. The observation of folding inside the crust seems to be connected with an intermediate viscosity range and with a broad (vertical) transition zone where rigid reactions and ductile processes are mixed.
Various methods of detecting faults by seismic studies are critically reviewed, and some sequence of reliability is suggested. Impedance contrasts, polarity, dip, thickness, and multiple fault strings have to be deciphered by wavefront modelling. Origin and survival of faults are considered to be a consequence of tectonic and thermal evolution under specific stress systems.
Elsevier online abstract
Vertical seismic profile results from the Kola Superdeep Borehole, Russia
Pages 295-307
B. J. Carr, S. B. Smithson, N. Kareav, A. Ronin, V. Garipov, Y. Kristofferson, P. Digranes, D. Smythe and C. Gillen
Multi-offset vertical seismic profiles (VSPs) from the Kola Superdeep Borehole (SG-3), as part of a larger seismic study of the Kola region conducted during the spring of 1992, sample the dipping Pechenga complex from 2175 m to 6000 m and contribute to the understanding of reflectivity in crystalline and Precambrian environments. From the surface to 6000 m, the SG-3 borehole penetrates interlayered Proterozoic metavolcanic and metasedimentary units and a mylonitic shear zone ranging from greenschist to amphibolite metamorphic grade, respectively. The Kola VSPs display a 6% velocity decrease which coincides to a mylonitic shear zone located between 4500 m and 5100 m within the SG-3 borehole. Seismic interfaces are identified by mode-converted energy (PS, and SP transmissions and reflections) in addition to primary seismic phases. The VSP shear wave energy is generated at or near the source by vertical vibrators. P-wave and S-wave reflections are generally detected from the same reflecting horizons, but increases in relative S-wave and SP reflection amplitudes originate at 1900 m, 3800 m, 4500 m, and 5100 m depths. These depths coincide with zones of elevated Vp/Vs and may support the presence of free pore fluid which is reported from initial drilling. For the Proterozoic lithologies sampled by the VSP, reflection events result from five mylonitic shear zones and three lithologic contrasts.
Elsevier online abstract
Nature of seismic reflections and velocities from VSP-experiments and borehole measurements at the KTB deep drilling site in southeast Germany
Pages 309-326
Ewald Lüschen, Kurt Bram, Walter Söllner and Stephan Sobolev
The 1989 VSP program at the KTB pilot hole was complemented in 1992 by a standard VSP in the KTB super-deep borehole from 3000 to 6013 m. P-wave velocities oscillate around 5.8 km/s in the upper 3150 m in accordance with sonic log velocities and correlate with paragneisses which prevail in this depth range. At about 3150 m depth, velocities increase to 6.4 km/s correlating with metabasites which dominate in the depth range 3150 to 7500 m. Laboratory measurements and petrophysical modelling provide evidence that the intrinsic velocities were reduced by 5-10% by fracture density and porosity at all depth ranges. Subvertical dip (50-70°) in structures and textures prevail, causing about 10% S-wave anisotropy, indicated by direct observations of S-wave splitting. Pronounced P-wave reflections, accompanied by P- to S-wave conversions and a lack of S-wave reflections, occur in the lower depth range only (3000-6000 m) and correlate with fluid-filled fracture systems. Lithological contrasts (gneiss-amphibolite) play a minor role in generating reflections. The most prominent reflecting elements known from surface profiling and 3D-surveys (i.e. the 'Franconian Lineament' reflector at about 7 km, and P-wave reflections at 8.3 km with notably absent associated S-wave reflections) coincide with pronounced anomalies observed in the logging data indicating the presence of major fracture zones.
Elsevier online abstract
KTB - The structure of a Variscan terrane boundary: seismic investigation - drilling - models
Pages 327-339
G. Hirschmann
On October 12, 1994, the Hauptbohrung (main hole) of the German Continental Deep Drilling Program (KTB) in the Oberpfalz/NE Bavaria reached its final depth of 9101 m. The drilling results provide a test of the validity of the initial structural concept for the KTB location in the boundary zone between the Saxothuringian and Moldanubian terranes of the Variscan orogen. This paper outlines some of the geological results as well as additional information from seismic investigations, and discusses briefly alternative structural models. The interpretation of local and regional seismic structures leads, in connection with the drilled profile, to the conclusion that the ZEV (Zone Erbendorf-Vohenstrauss, with the KTB drill-site) is derived from and linked with the 'Erbendorf Body' and equivalents of the Mariánské Lázn Complex and/or the Bohemicum in the middle and lower crust. Instead of a rootless klippe resting on the Saxothuringian-Moldanubian suture, the ZEV appears to be a part of the suture itself. The present position of the ZEV is tied to the junction of the ENE-WSW suture with the NNW-SSE Oberpfalz Block as a part of a large wrench system, and developed, probably, by a two-stage transpressive exhumation process prior to and during the Variscan collision.
Elsevier online abstract
Stochastic analysis of sonic logs from the upper crystalline crust: methodology
Pages 341-356
K. Holliger, A. G. Green and C. Juhlin
To relate local fluctuations observed in sonic logs to small-scale velocity fabric along boreholes, both filtering effects and noise introduced by the logging procedure must be taken into account. Sonic log velocities are represented as a time series consisting of a large-scale deterministic and a small-scale stochastic component. The deterministic trend, approximated by a low-order polynomial best-fit, contains information on the average velocity structure, whereas the small-scale stochastic variations consist of noise plus in situ velocity variations convolved with the logging system response. The velocity fluctuations of the sonic data considered here are zero-mean and have quasi-Gaussian probability density functions. Therefore, they are well characterised by their second statistical moment, i.e. their autocovariance function. Tests on synthetic data indicate that the autocovariance function corresponding to this data model may be used to extract information on the second-order statistics of the in situ velocity variations along the borehole and to constrain the level of white noise in sonic logs. Ignoring the presence of filtering effects and noise in sonic logs may result in seriously flawed estimates of the second-order statistics of the actual velocity structure. Assuming a von Kármán autocovariance function for the in situ velocity variations, this model provides a good match to the autocovariance functions of sonic log data from the Siljan Ring (Sweden) and Sudbury areas (Canada). Although differing significantly in their noise content, these two data sets yield similar results for the small-scale velocity structure, which is modelled as a bandlimited self-affine time series. For the Siljan Ring borehole we found a close relation between small-scale variations of the borehole diameter as determined from caliper logs and the level of uncorrelated noise present in the sonic log data.
Elsevier online abstract
Relative-amplitude preserving processing for crustal seismic reflection data: an example from western Canada
Pages 357-370
David W. S. Eaton and Jianjun Wu
Most crustal-scale seismic reflection profiles are processed using methods that preserve reflection geometries but not true-relative amplitudes, and therefore do not provide direct constraints on the magnitude of impedance contrasts at major crustal boundaries, the effects of attenuation in the crust or the maximum time of signal penetration. Here we describe a new relative-amplitude preserving (RAP) processing scheme tailored for high-fold, land-based acquisition systems that can be used to provide some of these constraints. Our processing philosophy employs a statistical treatment of amplitudes, and is based on the following assumptions: (1) temporal decay of recorded amplitudes is proportional to the product of zero-offset geometrical spreading in a layered earth and attenuation based on a constant-Q model; and, (2) spatial variations in recorded amplitudes are surface-consistent. We apply this methodology to a profile from western Canada, where well logs from oil and gas exploration are available to calibrate observed seismic reflections. Using a spectral-ratio method tailored for noisy data, we obtain an estimate of 450 for apparent Q in the sub-Phanerozoic crust. Signal penetration is frequency-dependent, and is estimated to decrease from 20 to 13 s over the frequency range of 10 to 40 Hz. Application of our processing methodology leads to a stack section in which lower-crustal reflectivity is much more prominent than reflectivity in the middle crust, compared with conventional processing of the same data which yields a stack section characterized by conspicuous mid-crustal reflections. The P-wave reflection coefficient for the Moho is estimated to be about 0.1.
Elsevier online abstract
Wave-equation datuming for improving deep crustal seismic images
Pages 371-379
Steven P. Larkin and Alan Levander
Reflection quality in deep crustal seismic sections is often poor due to surface waves, irregularities in the surface layer and variations in topography. In an effort to remove the effects of the variable near-surface layer and topography, we have used wave-equation datuming to downward continue shot gathers to the base of the surface layer, and then redatumed upward to a flat surface using the basement velocity as the replacement velocity. We discuss briefly the theory behind the method, present a simple synthetic example of a datumed shot gather, and conclude with an example of a datumed shot gather from the RISC crustal seismic experiment in southeastern California.
Elsevier online abstract
Pre-stack migration and true-amplitude processing of DEKORP near-normal incidence and wide-angle reflection measurements
Pages 381-392
M. Simon, H. Gebrande and M. Bopp
An alternative to CMP-processing is presented which seems to be more appropriate to crustal seismics. It has been developed for wide-angle seismics but it can also be applied to near-normal incidence data. The essentials of this processing sequence are shot domain processing, retention of true amplitudes and pre-stack migration.
True-amplitude processing of land data requires very careful editing. In addition, the ground coupling of individual sources and receivers must be taken into account by applying surface-consistent amplitude corrections. Subsequent pre-stack migration is very time-consuming. Although the isochron migration applied is based on kinematic principles, the amplitude distribution may be regarded qualitatively as an image of the reflectivity. The method can resolve steeply dipping reflectors and works with physically meaningful (dip-independent) migration velocities. The migration velocities are computed prior to migration by ray tracing using a macro velocity model. Robustness of the method can be increased by forming envelopes after single shot migration and stacking them to obtain a final image.
When applied to DEKORP-data from near the German Continental Deep Drilling site (KTB), the Franconian Lineament fault zone (dipping at 55°) has been well imaged, in addition to many other reflectors throughout the crust. Furthermore, areas with high reflectivity can be distinguished from areas with low reflectivity due to the true-amplitude processing.
Elsevier online abstract
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