34th International
Geological Congress
|
Updated: Jun 02, 2016
8th International Symposium on Deep Seismic Profiling of the Continents and Their Margins
Barcelona, Spain, 20-25 September 1998
from TECTONOPHYSICS Volume 329, Issues 1-4, Pages 1-359 (31 December 2000)
Edited by R. Carbonell, J. Gallart and M. Torné
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)
Evidence for crustal extension and inversion in eastern Tasmania, Australia, during the Neoproterozoic and Early Palaeozoic
Pages 1-21
B. J. Drummond, T. J. Barton, R. J. Korsch, N. Rawlinson, A. N. Yeates, C. D. N. Collins and A. V. Brown
The island of Tasmania in southeast Australia consists of a number of stratotectonic elements. The relationships between these elements are largely obscured by younger cover of the Tasmania Basin, which contains extensive dolerite sills that limit the ability of potential field techniques to map basement. Therefore the development of a robust tectonic model for Tasmania has been inhibited. To assist in the development of a tectonic model, a deep seismic reflection program undertaken offshore around the entire island was designed to map the large-scale structures of Tasmania at depth. The airgun seismic energy was also recorded at a number of seismographs deployed across the island, allowing low resolution 3D tomographic imaging. Short reflection profiles were recorded onshore across structures which could not be imaged by the offshore profiling. This paper focuses on eastern Tasmania. In the seismic sections, the Proterozoic basement in the southeast is mostly featureless, except for large rotated blocks with weakly reflective boundary faults, indicating extension of the Tyennan Element by block faulting. The deposition of the sedimentary succession of the Adamsfield-Jubilee Element was related to this extensional event. In the northeast, a reflective lower crust is interpreted to represent thrust slices of previously highly extended continental crust and possibly fragments of oceanic crust. The Early Palaeozoic sedimentary succession of the Northeast Tasmania Element formed across the inverted margin. The apparently complex geology of eastern Tasmania therefore fits into an extensional model where continental extension eventually led to the formation of very thin continental crust and possibly oceanic crust to the east. The extension was probably related to Late Neoproterozoic extension recorded elsewhere in Australia. The region was subsequently shortened, probably in a northeast-southwest direction, with most shortening accommodated in the seismically reflective, probably oceanic part of the crust, and little or no shortening in the block-faulted, and extended continental crust.
Elsevier online abstract
A crustal transect between Precambrian Australia and the Timor Trough across the Vulcan Sub-basin
Pages 23-38
P. Petkovic, C. D. N. Collins and D. M. Finlayson
Wide-angle seismic data from ocean bottom seismographs, together with gravity and deep marine reflection profiling data along the Vulcan transect in northern Australia, define the crustal-scale features between the Precambrian Australian craton and the Timor Trough. The transect provides an outline of crustal and upper mantle architecture across the major boundary between the Australian and SE Asian plates when linked with earlier deep marine seismic profiling. Near the Australian coast, relatively unaltered Precambrian Kimberley Basin rocks are inferred to extend to the edge of a shallow-water shelf area (Yampi Shelf) with a crustal thickness of 35 km. The crust then thins to 26 km under the outer shelf near the Timor Trough. Over the same distance Palaeozoic/Mesozoic basin sequences are interpreted to thicken to 12-13 km, inferring an attenuation of Precambrian basement rocks from 35 to 13-14 km across the margin (β=2.6).
On the outer shelf, the Vulcan Sub-Basin is a trans-tensional rift within Permo-Triassic platform areas (Ashmore Platform, Londonderry High). Within the lower crust under major bounding faults at the sub-basin/platform margins, there are elevated P-wave velocities to 7 km/s, suggesting emplacement of intrusive, more mafic rocks at depth during basin-forming processes. At mid-crustal levels, near the top of the inferred attenuated Precambrian crustal rocks, there are strong near-vertical-incidence reflections at about 13 km depth that are interpreted to be a detachment or further evidence of intrusive rocks. Additionally, seismic energy reflected at wide angles from within the upper mantle at 38-45 km depth indicates that compositional boundaries/heterogeneities continue at depth.
Detailed information within the basin sediments includes the interpretation of crosscutting velocity patterns within the Mesozoic sequences as an indication of decreasing porosity with depth of burial, and the presence of a low-velocity sequence at 1-2 km depth across the Londonderry High.
Elsevier online abstract
Effective elastic thickness of the northern Australian continental lithosphere subducting beneath the Banda orogen (Indonesia): inelastic failure at the start of continental subduction
Pages 39-60
K. Tandon, J. M. Lorenzo and G. W. O'Brien
Pliocene-Recent continent-island arc collision of the northern Australian continental lithosphere across the Banda orogen from Roti to the Kai Plateau ( 121-137°E longitude) has formed an underfilled foreland basin within the Timor-Tanimbar-Aru Trough. Continental collision on northern Australian lithosphere is most advanced near central Timor Island in terms of shortening and absorbing the forearc basin (Savu Basin) within the accretionary prism. Australian continental lithosphere north of area around central Timor Island is believed to be detached from the oceanic lithosphere. Effective Elastic Thickness (EET) of the northern Australian continental lithosphere from Roti to the Kai Plateau are derived using an elastic half-beam model. Modeled deflection is matched to the seafloor bathymetry and the marine complete 3D Bouguer gravity anomalies. The EET varies from 27 to 75 km across the northern Australian continental lithosphere from Roti to Kai Plateau when the thickness of the elastic half-beam is kept constant. The highest EET values lies near central Timor. From the shelf to beneath the Banda orogen, the EET of the northern Australian continental lithosphere is reduced from 90 to 30 km when the thickness of the elastic half-beam is allowed to vary down dip. Elastic half-beam modeling approximates the Banda orogen as a triangular load and hidden subsurface loads as end-point loads. Wider triangular loads modeling the load contribution from Banda orogen need higher values of EET. Such an observation highlights the role of high EET in thin-skinned collisional tectonics by promoting the support of wider accretionary prisms by parts of foreland basins with higher EET. Variations in EET may result from inelastic yielding in the northern Australian continental lithosphere. Oroclinal bending of the Australian continental lithosphere in the east, from Tanimbar to the Kai Plateau, may create additional yielding and further decrease the EET. Change in EET occurred at the start of continental subduction in the late Miocene-early Pliocene boundary due to change in curvature of the northern Australian lithosphere near the shelf-slope, both in map and cross-sectional view. Evidence for the inelastic yielding of the northern Australian continental lithosphere near the present-day shelf-slope at the continental subduction is found in: (1) the maximum change of EET near shelf-slope in laterally variable EET calculations, and (2) in the almost cessation of normal faulting in late Miocene-early Pliocene seen on seismic reflection data from northern Australian continental shelf-slope. Elastic half-beam models across central Timor do not require end-point loads and may imply that there are no slab pull forces from the oceanic lithosphere that are acting on the leading edge of the subducted northern Australian continental lithosphere.
Elsevier online abstract
Seismic tomography with local earthquakes in Costa Rica
Pages 61-78
V. Sallarès, J. J. Dañobeitia and E. R. Flueh
The Costarican isthmus is located at the western limit of the Caribbean oceanic plateau, where the Cocos plate subducts along the Middle American Trench. This plate shows strong lateral variations in its morphology and structure. The main objective of this study is to investigate the effects of subduction-related magmatism as a function of morphology and structure of the subducting plate, by performing a simultaneous inversion of the 3-D crustal velocity field and hypocenter locations from local earthquakes. For that, we used the traveltimes of P-waves first arrivals from more than 5000 events, recorded at the Costarican seismic networks between 1991 and 1998. In order to prevent data and modeling inaccuracies, we followed an inversion scheme consisting of four steps. (1) Selection of an adequate data subset for tomographic inversion, (2) estimation of the best reference 1-D model, (3) determination of the finest parameterization and (4) evaluation of resolution. The results show that the uppermost levels of the crust (0-6 km) are consistent with geology, since the velocity anomalies reflect the most meaningful geological features observed in the surface. Below these levels (6-20 km deep), we found two different zones separated by a SW-NE seismic alignment. The northern part is characterized by a highly heterogeneous velocity field and is seismically active, while the southern part is much more homogeneous and practically inactive. Moreover, the northern part shows a certain accumulation of low velocity material within the upper mantle. We suggest and illustrate that the structural differences between the northern and southern zones can be a consequence of the differences on the geometry and structure of the subducting slab.
Elsevier online abstract
The continental margin off Oregon from seismic investigations
Pages 79-97
M. Gerdom, A. M. Trehu, E. R. Flueh and D. Klaeschen
In April and May 1996, a geophysical study of the Cascadia continental margin off Oregon and Washington was carried out aboard the German RV Sonne as a cooperative experiment between GEOMAR, the USGS and COAS. Offshore central Oregon, which is the subject of this study, the experiment involved the collection of wide-angle refraction and reflection data along three profiles across the continental margin using ocean-bottom seismometers (OBS) and hydrophones (OBH) as well as land recorders. Two-dimensional modelling of the travel times provides a detailed velocity structure beneath these profiles. The subducting oceanic crust of the Juan de Fuca plate can be traced from the trench to its position some 10 km landward of the coastline. At the coastline, the Moho has a depth of 30 km. The dip of the plate changes from 1.5° westward of the trench to about 6.5° below the accretionary complex and to about 16° further eastward below the coast. The backstop forming western edge of the Siletz terrane, an oceanic plateau that was accreted to North America about 50 Ma ago, is well defined by the observations. It is located about 60 km to the east of the deformation front and has a seaward dip of 40°. At its seaward edge, the base of the Siletz terrane seems to be in contact with the subducting oceanic crust implying that sediments are unlikely to be subducted to greater depths. The upper oceanic crust is thinner to the east of this contact than to the west. At depths greater than 18 km, the top of the oceanic crust is the origin of pre-critical reflections observable in several land recordings and in the data of one ocean bottom instrument. These reflections are most likely caused by fluids that are released from the oceanic crust by metamorphic facies transition.
Elsevier online abstract
Seismic tomography of the crust and lithospheric mantle in the Betic Cordillera and Alboran Sea
Pages 99-119
E. Gurría and J. Mezcua
The Alboran Domain lies between the South of Spain and the North of Morocco and contains a Miocene extensional basin which formed during continuous compression between the African and Eurasian plates. We have performed a 1-D and 3-D passive velocity tomography study of the structure of the crust and lithospheric mantle of the Betic Cordillera and Alboran Sea region. An important drawback encountered in most passive seismic tomography studies is that the quality of the data set used is inhomogeneous and it is often impossible to measure the error. This occurs as the data set needs to be very large which implies using several years' worth of data from different seismic networks with different station noise levels and phase travel time picking accuracy. We present a simple method to quantitatively determine the errors in the travel time data typically used in passive seismic tomography studies. We use this method to determine the standard error of our data set and present the tomography results with quantitatively determined error bars. An additional advantage of performing the error analysis is that the data can be weighted in terms of its standard error, and hence the contribution of noisy data can be reduced in the inversion, resulting in substantially enhanced tomography images.
The main structural results obtained in the 1-D inversion of the data are: (1) the average velocity of the crust in the Betic Cordillera is Vp=6.0±0.05 km/s and Vs=3.5±0.05 km/s, indicating an average Vp/Vs ratio of 1.71 for the crust. (2) The average Moho depth in the Betic Cordillera is less than 36 km. (3) The average crustal velocity in the Alboran Sea is Vp=6.4±0.1 km/s and Vs=3.4±0.2 km/s, indicating an average Vp/Vs ratio of 1.88 for the crust. (4) The average depth of the Moho in the Alboran Sea is greater than 20 km.
The 3-D inversion provides new structural information of the Alboran basin over the depth range 24-60 km. For the depth range 24-31 km we image the centre and north western margin of the basin and observe velocities of the order of 7.2-7.5±0.45 km/s, which could correspond to lower crust material or else anomalous low velocity mantle material. For the depth range 31-40 km we image the centre and the southern margin of the basin and observe P phase velocities of 8.0-8.2±0.55 km/s which we interpret as normal mantle material. Below this layer, in the depth range 40-60 km we image the centre and northern margin of the basin and observe low P velocities (Vp=7.8±0.55 km/s). At present the preferred model used to explain the formation of the Alboran Sea Basin makes use of the process of Lithospheric Delamination. The low velocity layer observed in the mantle starting at approximately 40 km depth could be interpreted to be hot mantle material or the asthenosphere, however, the normal mantle velocities observed in the 31-40 km layer above it do not support replacement of the lithosphere by hot asthenospheric material in the lower crust or immediately beneath the crust. If this is so, then delamination must have occurred within the lithospheric mantle at an approximate depth of 40 km, or else not occurred at all.
Elsevier online abstract
Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution
Pages 121-139
R. Nicolich, M. Laigle, A. Hirn, L. Cernobori and J. Gallart
Crustal imaging could be achieved on normal-incidence reflection profiles offshore eastern Sicily by using industrial-grade reflection seismic with improved marine sources. Thick recent sediments, a reflective pile including the Mesozoic deposits, a transparent upper crust, and a band of low frequency reflections attributed to the lower crust, are imaged in the seismic sections. The structure of the crust and its thickness show features inherited from the Mesozoic evolution as a passive margin, by which Ionian basin crust was formed around the Hyblean continental promontory of Africa to constitute the southern plate in the later convergence with Europe. The seismic images are also marked by the lithospheric deformation due to the Neogene overriding of the northern part of this paleomargin by the Calabro-Peloritan block of European continental crust. This transpressive motion may have been guided along the northern part of this paleomargin where the seismic profiles evidence a hinge line between the northward upslope of the Moho of that old passive margin and its downslope to the present slab under the Tyrrhenian Sea. Etna volcano is located at the intersection of this mantle upwarp by a zone of active sea-bottom normal-faults, which cut across the formerly constructed compressional belt. The onset of its volcanic activity is roughly coeval with that of the cessation of interplate thrusting and could hence be related to a change of the coupling of the Ionian slab. This slab is now probably disconnected from the overriding plate and rolled back in front of the expanding hot Tyrrhenian asthenospheric dome with the mobilisation of a viscous mantle material at depth. An active lithospheric fault is here imaged which cuts over more than 100 km into the Ionian basin. The fault runs from the Tyrrhenian margin in the SSE direction of Etna updip of the southwestern lateral edge of this slab, leaving north-eastward the extruded Calabrian block and on its south-westward edge the uplifted crust and mantle structures of Etna. Along it, the crust, including the Mesozoic and deeper layers, has sagged vertically in the segment in front of the slab, with a finite throw across the fault increasing from the basin towards Etna.
Elsevier online abstract
Seismic structure and the active Hellenic subduction in the Ionian islands
Pages 141-156
C. Clément, A. Hirn, P. Charvis, M. Sachpazi and F. Marnelis
In the region of the Ionian Islands of western Greece, the active margin of the Hellenic domain passes from oceanic subduction in the south to continental collision in the north, linked by the right-lateral Cephalonia transform fault. A slightly landward dipping interface revealed at 13 km depth by a single previous line in the channel between Cephalonia and Zante has been suggested as the interplate subduction boundary. New marine multichannel reflection profiles and OBS refraction and wide-angle reflection data confirm the reflector as a regional feature. These data evidence its extension to the south, where large, low-angle thrust earthquakes occur offshore to Zante. The new profiles establish a coincidence between the focal depths of these large subduction events and the imaged bright reflective level, confirming its tentative interpretation as the interplate boundary, which generally appears with a positive reflection polarity. In this context, the Ionian Islands outcrop corresponds to a shallowing of the interplate boudary from south to north. In the south, offshore Zante, the interplate boundary comprises a stratified zone that may be considered as the sedimentary cover of the Ionian Basin oceanic-like crust, which forms the lower plate here. The shallower position and single-cycle reflection character of the interplate further north suggest that the lower plate could there be the Apulian paleomargin to that basin.
Elsevier online abstract
Seismic images at the convergence zone from south of Cyprus to the Syrian coast, eastern Mediterranean
Pages 157-170
N. Vidal, D. Klaeschen, A. Kopf, C. Docherty, R. Von Huene and V. A. Krasheninnikov
Multichannel seismic profiles from cruise 5 of the R/V Akademik Nikolaj Strakhov provide the first deep seismic reflection images and extensive coverage south and east of Cyprus. Five NW-SE-trending seismic lines cross an area of active continental collision. Main tectonic structures are the Eratosthenes Seamount collision zone, the Hecateaus Rise and the Latakia-Larnaca Ridge systems. The Levantine Basin extends to the south all over the area. The data required careful processing due to the low-fold coverage, the reverberatory character of the signal and the strong multiple energy. The seismic results clearly image the thick sedimentary sequence of the Levantine Basin. This basin is observed to terminate abruptly at the junction with the Hecateaus Rise south of Cyprus. To the east and north, the deformation appears to be partitioned along two separated structures next to the Latakia and Larnaca Ridges. They correspond to major oblique fault systems.
Elsevier online abstract
Structure of the Makran subduction zone from wide-angle and reflection seismic data
Pages 171-191
C. Kopp, J. Fruehn, E. R. Flueh, C. Reichert, N. Kukowski, J. Bialas and D. Klaeschen
Makran is one of the largest accretionary wedges on the globe, formed by the convergence between the Eurasian and the Arabian Plates. It is characterised by an extremely high sediment input of 7 km and a shallow subduction angle. We present seismic velocity models from four wide-angle seismic lines that image the wedge sediments and the subducted oceanic crust. Three strike-lines show complex but rather one-dimensional structures, where the observed seismic phases have been studied with the reflectivity method. A 160 km long dip line was surveyed coincident with a MCS line collected by Cambridge University in 1986. Prestack depth migration of this data with the new wide-angle velocities shows improved images compared to earlier results.
Interpretation of the wide-angle and the MCS data indicates that a décollement is developed along a bright reflector within the turbiditic sequence. More than 3 km of sediment bypasses the first accretionary ridges (underthrusting) and is transported to greater depth. This might help to explain the sparse earthquake activity associated with subduction here. In the strike lines, low-velocity layers occur landward of the deformation front, in zones where active thrusting or underplating takes place. The oceanic crust is characterised by a strong velocity contrast between layer II and layer III.
Elsevier online abstract
Crustal signature of Late Archaean tectonic episodes in the Yilgarn craton, Western Australia: evidence from deep seismic sounding
Pages 193-221
B. J. Drummond, B. R. Goleby and C. P. Swager
Deformation in the greenstone supracrustal rocks of the Eastern Goldfields Province of the Archaean Yilgarn Craton in Western Australia is delaminated from the underlying basement along a regional detachment surface presently at 3-7 km depth. This might suggest that the history of crustal deformation cannot be inferred with any certainty from that of the greenstones. However, seismic images of the crust below the greenstones show structures that can be interpreted in terms of a series of tectonic events similar to those within the greenstones. The upper crust (below and to the west of the greenstones) is largely unreflective, with interpreted west-dipping reverse faults. The middle crust is reflective, with reflector geometry implying thickening by west directed thrust stacking, and the lower crust has a fabric indicative of ductile deformation. These reflection fabrics imply crustal shortening, probably during the Late Archaean regional D2 ENE-WSW shortening event. They were subsequently overprinted and disrupted by structures consistent with regional NNW-SSE strike slip D3 faulting, and probably younger, more localised D4 faulting. The seismic images of the crust therefore show that the crust suffered tectonic events in which both the order and direction of deformation are similar to those of the greenstones. This is evidence that the whole crust deformed when the greenstones deformed. However, the scale and style of deformation vary with depth through the crust, and include thrusting and probably folding in the upper crust, thrust stacking in the middle crust, and ductile deformation in the lower crust. The length scale (wavelength) of structures in the middle and lower crust is greater than that in the greenstones.
Elsevier online abstract
Constraints on crustal composition beneath a metamorphic core complex: results from 3-component wide-angle seismic data along the eastern flank of the Ruby Mountains, Nevada
Pages 223-250
P. Satarugsa and R. A. Johnson
Metamorphic core complexes expose rocks deformed at deep upper to middle crustal levels during extreme crustal extension. However, mechanisms of crustal extension and exhumation of core complexes remain to be fully understood. Detailed study of crustal velocity structure and inferences about the composition of the crust beneath core complexes (and nearby) provide useful constraints on core-complex evolution. P- and S-wave velocity structures determined from seismic experiments along the eastern flank of the Ruby Mountains metamorphic core complex, Nevada, show that the crust can be divided into three main layers corresponding to the upper, middle and lower crust. We interpreted crustal composition by integrating results of P-wave velocities (Vp), S-wave velocities (Vs), Poisson's ratios (s), seismic anisotropy, and reflection character with published geologic maps of the area. Near-surface estimates of Vp, Vs, s, and anisotropy of 1.90-4.8 km/s, 1.01-2.75 km/s, 0.25-0.33, and 0.6-2.5%, respectively, are consistent with surface exposures of unconsolidated to consolidated sedimentary rocks, limestone, dolomite, siltstone, sandstone, porous sandstone, conglomerate, and weathered granite. Results from analysis of reflection responses, Vp, Vs, s, and anisotropy also indicate that: (1) upper crustal rocks most likely consist of metaquartzite, schist, granite gneiss, and granite-granodiorite with Vp of 5.80-6.25 km/s, Vs of 3.20-3.72 km/s, s of 0.22-0.25, and anisotropy of 0.6-2.5%; (2) possible middle crustal rocks are paragranulite, felsic granulite, felsic amphibolite gneiss, granite-granodiorite, and mica-quartz schist with Vp of 6.35-6.45 km/s, Vs of 3.70-3.75 km/s, and s of 0.24; and (3) lower crustal rocks most likely consist of granulite- rather than amphibolite-facies rocks with Vp of 6.60-6.80 km/s, Vs of 3.85-3.92 km/s, s of 0.24-0.25, and anisotropy of <3%. Our principal conclusions are: (1) significant addition of gabbroic rocks (underplating) is unlikely in the lower crust; (2) lower crustal rocks were stretched into sub-horizontal geometries with aligned minerals during extension, creating seismic lamellae in the lower crust; (3) present-day seismic velocities of highly extended core complex crust and normally extended Basin and Range crust are similar; and (4) orientations of fast shear waves near the surface and in the upper crust are sub-parallel to the regional maximum horizontal compressive stress in the Nevada part of the Basin and Range province.
Elsevier online abstract
Statistical approaches to interpretation of seismic reflection data
Pages 251-267
C. A. Hurich and A. Kocurko
Statistical approaches offer a potentially powerful tool for interpretation of the complex seismic reflection patterns typically recorded from the crystalline crust. These methods focus on characterization of the complexity, scaling characteristics and spatial variability of the wave field with the ultimate aim of recovering information on the geometric variation of the geology. In this paper we examine the potential and limitations of statistical approaches, focusing on the issues of quality of the statistical estimator, how noise and migration affect spatial statistics and the potential for both qualitative and quantitative approaches to statistics-based interpretation. The limited spatial bandwidth of typical seismic data presently inhibits complete quantitative recovery of the spatial properties of the geology. Migration, which should increase the spatial bandwidth, has to date, not proven particularly effective on complex wave fields. Mapping of relative variation in the spatial properties of the wave field reveals subtle differences that are directly associated with the geometric character of the geology. Such mapping allows identification and mapping of variations in the geologic fabric at scales significantly larger than the seismic wavelength and objective access to scaling information that has previously been unavailable.
Elsevier online abstract
Deep seismic reflection evidence for ancient subduction and collision zones within the continental lithosphere of northwestern Europe
Pages 269-300
N. Balling
Deep seismic profiling experiments in the region of NW Europe (including BABEL in the Gulf of Bothnia and the Baltic Sea, Mobil Search in the Skagerrak and MONA LISA in the North Sea) have demonstrated the existence of seismic reflectors in the mantle lithosphere beneath the Baltic Shield, the Tornquist Zone and the North Sea basins. Different sets of reflectors are observed, notably dipping and sub-horizontal. Dipping, distinct reflectivity, which may be followed from Moho/Moho offsets into the deeper parts of the continental lithosphere, is of special interest because of its tectonic and geodynamic significance.
Such reflectivity, observed in several places, dipping 15-35° and covering a depth range of 30-90 km, constrained by surface geological information and radiometric age data, is interpreted to represent fossil, ancient subduction and collison zones. Subduction slabs with remnant oceanic basaltic crust transformed into eclogite is assumed, in particular, to generate deep seismic reflectivity. Deep seismic evidence is presented for subduction, crustal accretion and collision processes with inferred ages from 1.9 to 1.1 Ga from the main structural provinces within the Baltic Shield including Svecofennian, Transscandinavian Igneous Belt, Gothian and Sveconorwegian.
Along the southwestern border of Baltica (in the southeastern North Sea) south-dipping crustal and sub-crustal reflectivity is observed down to a depth of about 90 km, close to the lithosphere-asthenosphere boundary. These structures are interpreted to reveal a lithosphere-scale Caledonian (ca. 440 Ma) suture zone resulting from the closure of the Tornquist Sea/Thor Ocean and the amalgamation of Baltica and Eastern Avalonia.
These results demonstrate that deep structures within the continental lithosphere, originating from early crust-forming plate tectonic processes, may survive for a very long time and form seismic marker reflectivity of great value in geotectonic interpretation and reconstructions. Furthermore, the depth of dipping reflectivity from ancient structures, such as subduction slabs, significantly contributes information about the thickness of the coherent lithosphere.
The seismic observations and our interpretations support plate tectonic and structural models, suggesting crustal growth and amalgamation of tectonic units in the Baltic Shield and along its southwestern margin generally from the northeast (in present-day orientation) towards the southwest and west, likely to result in regional deep structural and tectonic age zonations.
Elsevier online abstract
Seismic results at Kola and KTB deep scientific boreholes: velocities, reflections, fluids, and crustal composition
Pages 301-317
S. B. Smithson, F. Wenzel, Y. V. Ganchin and I. B. Morozov
Seismic studies from the Kola and KTB scientific boreholes are used to determine the origins of crustal seismic reflections and to study the effect of fluids, which are encountered through the entire depth range, on seismic wave propagation. Crustal seismic reflections are caused by compositional layering, shear zones, anisotropy and fluid-filled faults and fractures and all of these factors may contribute to one reflection group. Full wavefield recording and analysis, including S-waves and converted waves, may be used to distinguish the mixed origins of reflections. Sonic log velocities are systematically lower than VSP interval velocities because of the effect of drilling damage immediately around the borehole. A striking feature of both boreholes is the presence of brines in fractures and micro-cracks to TDs of 9 and 12 km yet the upper crust generally remains resistive, presumably due to low connectivity. Fluids enhance reflectivity in some zones and lower P-wave seismic velocity by about 0.2 km/s in the upper crust. Thus estimates of crustal composition based on seismic velocity are too felsic so that average upper crustal composition is more like felsic tonalite than granodiorite. In the Kola borehole, brines coexist at 12 km depth and 190°C; in the KTB borehole at 9 km depth, brines coexist with country rock at 265°C, and the depth at which brines disappear is unknown so that presence and effect of fluids in the deep crust should be re-evaluated.
Elsevier online abstract
The results of deep seismic investigations on geotraverse in the Barents Sea from Kola peninsula to Franz-Joseph Land
Pages 319-331
T. S. Sakoulina, A. N. Telegin, I. M. Tikhonova, M. L. Verba, Y. I. Matveev, A. A. Vinnick, A. V. Kopylova and L. G. Dvornikov
In 1995 geophysical investigations in the southern part of the AP-1 geotraverse across the whole Barents Sea Plate were completed. The entire profile links the super-deep SG-3 well on the Kola peninsular (town of Zapolarny) and the base Hayes-1 well on Franz-Joseph Land. The extent of the investigated part of the profile was 700 km. The integrated seismic experiment included offshore and onshore wide-angle reflection/refraction observations (referred hereinafter as Deep Seismic Sounding - DSS) with 3-component ocean bottom seismographs (OBS) and air guns and multi-channel seismic reflection study (MCS) at different offsets ranges. Sufficiently dense deployment of OBS (5-20 km) and a 250 m shooting interval provided reliable phase correlation and wave identification. Intensive reflections from the Moho have been recorded practically continuously along the profile. The DSS data processing built a mid point dynamic reflection Moho image and produced a general depth section. MCS data defined boundaries in the sedimentary layer and the basement surface in the southern part of the profile. The lowermost observed boundary, the Moho, occurs at depths between 35 and 40 km. The thickness of sedimentary cover varies from zero in the southern part of the profile to 14-16 km in its northern part. Results of geological interpretation confirm the continental crust in the studied part of the Barents Sea Shelf Plate.
Elsevier online abstract
Lithospheric boundaries and upper mantle heterogeneity beneath Russian Eurasia: evidence from the DSS profile QUARTZ
Pages 333-344
E. A. Morozova, I. B. Morozov, S. B. Smithson and L. Solodilov
A combined interpretation of seismic data from the 3850-km-long Russian seismic profile QUARTZ using chemical and nuclear explosions reveals complex structure of the lithosphere related to the regional tectonics. Our analysis is focused on the structure of the uppermost mantle and based on 2D ray-tracing and inversion of the data from all 51 shots along the profile, on 2D attenuation measurements, and on the results of an analysis of the long-range Pn phase. Three groups of reflectors within the lithosphere are associated with regional- to global boundaries previously recognized in stable continental environments. One almost continuous upper mantle boundary occurs at 65 to 80-km depth and another with an approximately 40-km-thick low-velocity zone (LVZ) occurs at 120-140 km depth. The shallow upper mantle blocks and the extensive interfaces indicate strong upper mantle heterogeneity imaged by this unique profile. The thickness of the lithosphere varies from 220 km under the Baltic Shield to about 120-130 km under the tectonically active Himalayan Altay-Sayan fold belt. Within the lower part of the lithosphere, a zone of lower velocity, increased attenuation, and varying thickness might be related to hydration and partial melting. Large structural differences related to tectonic ages and histories are characteristic for the entire lithosphere and extend 200-400 km into the asthenosphere. Analysis of QUARTZ data gives confident indication of seismic heterogeneity from crustal to regional mantle scales indicating strong involvement of the mantle in tectonic processes.
Elsevier online abstract
Resolution properties and 3-D reconstruction from multi-azimuth wide-angle data in the Baltic region
Pages 345-359
A. Sanina, O. Yu. Riznichenko, V. G. Markin, A. L. Ushakov and D. B. Snyder
Recent development of geotomography methods and wider use of seismic array observations have improved conditions for studying the 3-D velocity structure of the earth. The solution to this problem has a number of specific features, which ranges from the typical irregular geometry of the observation sites and seismic sources to the possibilities for seismic tomography at different scales (global, regional and local). The international BABEL survey provided one such spatial array with irregular geometry, many sources and a relatively small number of observation points. The seismic ray coverage is not uniformly dense and sufficient enough for reconstructing the media within the framework of tomography approach. This article presents results of a 3-D velocity reconstruction using data from shots along BABEL lines 1, 6 and 7, recorded by all available land stations. Estimation of the accuracy of reconstruction is made on the basis of mathematical modeling, model parametrization and starting model selection. Modeled velocities of 7.45-7.55 km/s between 46 and 57 km are transitional between those traditionally assigned to lower crust and mantle and are interpreted to represent high grade metamorphic crustal rocks or intermixing of crust and mantle at a seismically small scale (100 m).
Elsevier online abstract
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