IGCP Project 474
Images of the Earth's Crust & Upper Mantle
contact us: contact@earthscrust.org.au | about us
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IGCP Project 474
Images of the Earth's Crust & Upper Mantle |
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contact us: contact@earthscrust.org.au | about us
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34th International
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Updated: Jun 02, 2016 10th International Symposium on Deep Seismic Reflection Probing of the Continents and Their Margins
Taupo, New Zealand, 6-10 January 2003from TECTONOPHYSICS Volume 388, Issues 1-4, Pages 1-297 (13 September 2004) 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)
IntroductionPages 1-5 Configuration of subducting Philippine Sea plate and crustal structure in the central Japan regionPages 7-20 A seismic experiment with six explosive sources and 391 seismic stations was conducted in August 2001 in the central Japan region. The crustal velocity structure for the central part of Japan and configuration of the subducting Philippine Sea plate were revealed. A large lateral variation of the thickness of the sedimentary layer was observed, and the P-wave velocity values below the sedimentary layer obtained were 5.3–5.8 km/s. P-wave velocity values for the lower part of upper crust and lower crust were estimated to be 6.0–6.4 and 6.6–6.8 km/s, respectively. The reflected wave from the upper boundary of the subducting Philippine Sea plate was observed on the record sections of several shots. The configuration of the subducting Philippine Sea slab was revealed for depths of 20–35 km. The dip angle of the Philippine Sea plate was estimated to be 26° for a depth range of about 20–26 km. Below this depth, the upper boundary of the subducting Philippine Sea plate is distorted over a depth range of 26–33 km. A large variation of the reflected-wave amplitude with depth along the subducting plate was observed. At a depth of about 20–26 km, the amplitude of the reflected wave is not large, and is explained by the reflected wave at the upper boundary of the subducting oceanic crust. However, the reflected wave from reflection points deeper than 26 km showed a large amplitude that cannot be explained by several reliable velocity models. Some unique seismic structures have to be considered to explain the observed data. Such unique structures will provide important information to know the mechanism of inter-plate earthquakes. Thermo-mechanical structure beneath the young orogenic belt of TaiwanPages 21-31 We investigate the thermo–mechanical properties beneath the young orogenic belt of Taiwan by constructing a shear strength profile from a vertical stratified rheological structure. The stratified rheological structure is estimated based on the recently developed thermal structure and its likely composition. Subduction–collision in the young orogenic belts and the thick accretionary wedge make a significant contribution to the growth of sialic crust in the hinterland. The sialic bulk crust not only results in a low seismic velocity but also produces weak crust in the hinterland. The earthquake depth–frequency distribution in the foreland and hinterland correlates very well with the regimes of the brittle/ductile transition revealed in the strength profile. Our results show that the observed two–layer seismicity in the foreland is due to a moderate geotherm and an intermediate mafic bulk composition; while single–layer seismicity in the hinterland is due to its felsic bulk composition. In the foreland, the mechanically strong crust (MSC) and the mechanically strong lithosphere (MSL) coincide with frequent seismicity. The shallow MSC in the hinterland is consistent with the 20– to 25–km seismicity occurring there. The total lithospheric integrated strength (LIS) in the hinterland is only about half of that in the foreland, suggesting a weak lower crust and lithosphere mantle in the hinterland. The results confirm that the earthquake cutoff depth is a proxy for temperature. The calculated decrease of effective elastic thickness (EET) from the orogenic margin (foreland) to the center (hinterland) is consistent with the results of flexure modeling in most orogenic belts. Due to the weak LIS in the hinterland, crustal thinning and rifting may occur in the future. Our results, thus, suggest that the mechanical structure is also closely related to the composition and is not directly reflected in the thermal structure. Lower crustal fluid distribution in the northeastern Japan arc revealed by high-resolution 3D seismic tomographyPages 33-45 The Ou Backbone Range strikes northwards through the central northeastern Japan arc and is bounded on both sides by the active reverse Uwandaira and Sen'ya faults. We have applied a traveltime inversion method (seismic tomography) with spatial velocity correlation to active and passive seismic data in order to investigate a three-dimensional (3–D) velocity structure. The data set contains 33,993 P– and 18,483 S–wave arrivals from 706 natural sources and 40 blasts, as well as 2803 P–wave traveltime data from 10 explosions detonated during the 1997 controlled source experiment. The traveltime inversion reveals a zone beneath the Ou Backbone Range in which P–wave velocities (VP) are approximately 6–8% lower than the average velocity at equivalent depths. The low VP and a low VP to S–wave velocity (VS) ratio (VP/VS) of about 1.65 suggest the presence of aqueous fluids in the middle crust. Formation and shortening deformation of a back-arc rift basin revealed by deep seismic profiling, central JapanPages 47-58 The northern Fossa Magna (NFM) basin is a Miocene rift system produced in the final stages of the opening of the Sea of Japan. It divides the major structure of Japan into two regions, with north–trending geological structures to the NE of the basin and EW trending structures to the west of the basin. The Itoigawa–Shizuoka Tectonic Line (ISTL) bounds the western part of the northern Fossa Magna and forms an active fault system that displays one of the largest slip rates (4–9 mm/year) in the Japanese islands. Deep seismic reflection and refraction/wide–angle reflection profiling were undertaken in 2002 across the northern part of ISTL in order to delineate structures in the crust, and the deep geometry of the active fault systems. The seismic images are interpreted based on the pattern of reflectors, the surface geology and velocities derived from refraction analysis. The 68–km–long seismic section suggests that the Miocene NFM basin was formed by an east dipping normal fault with a shallow flat segment to 6 km depth and a deeper ramp penetrating to 15 km depth. This low–angle normal fault originated as a comparatively shallow brittle/ductile detachment in a high thermal regime present in the Miocene. The NFM basin was filled by a thick (>6 km) accumulation of sediments. Shortening since the late Neogene is accommodated along NS to NE–SE trending thrust faults that previously accommodated extension and produce fault-related folds on their hanging wall. Based on our balanced geologic cross–section, the total amount of Miocene extension is ca. 42 km and the total amount of late Neogene to Quaternary shortening is ca. 23 km. Upper and middle crustal deformation of an arc–arc collision across Hokkaido, Japan, inferred from seismic refraction/wide-angle reflection experimentsPages 59-73 The Hidaka Collision Zone (HCZ), central Hokkaido, Japan, is a good target for studies of crustal evolution and deformation processes associated with an arc–arc collision. The collision of the Kuril Arc (KA) with the Northeast Japan Arc (NJA), which started in the middle Miocene, is considered to be a controlling factor for the formation of the Hidaka Mountains, the westward obduction of middle/lower crustal rocks of the KA (the Hidaka Metamorphic Belt (HMB)) and the development of the foreland fold–and–thrust belt on the NJA side. The "Hokkaido Transect" project undertaken from 1998 to 2000 was a multidisciplinary effort intended to reveal structural heterogeneity across this collision zone by integrated geophysical/geological research including seismic refraction/reflection surveys and earthquake observations. An E–W trending 227 km–long refraction/wide–angle reflection profile found a complicated structural variation from the KA to the NJA across the HCZ. In the east of the HCZ, the hinterland region is covered with 4–4.5 km thick highly undulated Neogene sedimentary layers, beneath which two eastward dipping reflectors were imaged in a depth range of 10–25 km, probably representing the layer boundaries of the obducting middle/lower crust of the KA. The HMB crops out on the westward extension of these reflectors with relatively high Vp (>6.0 km/s) and Vp/Vs (>1.80) consistent with middle/lower crustal rocks. Beneath these reflectors, more flat and westward dipping reflector sequences are situated at the 25–27 km depth, forming a wedge–like geometry. This distribution pattern indicates that the KA crust has been delaminated into more than two segments under our profile. In the western part of the transect, the structure of the fold–and–thrust belt is characterized by a very thick (5–8 km) sedimentary package with a velocity of 2.5–4.8 km/s. This package exhibits one or two velocity reversals in Paleogene sedimentary layers, probably formed by imbrication associated with the collision process. From the horizontal distribution of these velocity reversals and other geophysical/geological data, the rate of crustal shortening in this area is estimated to be greater than 3–4 mm/year, which corresponds to 40–50% of the total convergence rate between the NJA and the Eurasian Plate. This means that the fold–and–thrust belt west of the HCZ is absorbing a large amount of crustal deformation associated with plate interaction across Hokkaido Island. Has the plate boundary shifted from central Hokkaido to the eastern part of the Sea of Japan?Pages 75-84 A NS trending Cenozoic fold-and-thrust belt has developed in the western part of the Hidaka Collision Zone (HCZ), central Hokkaido, Japan. A quantitative estimation of the late Cenozoic convergence rate at the front of the Hidaka thrust system is important in revealing the plate tectonic framework around northern Japan. High-resolution seismic reflection profiling across the active fault-related folds was carried out to ascertain the temporal change in the crustal shortening rate. Overlapping ramp anticlines and growth folds within thrust sheets were examined using balanced cross-sections combined with industry seismic and drilling data. The rate of shortening was examined using a 3.5 Ma horizon and late Quaternary horizons at 115 and 41 ka. These horizons show that the convergence rate of the Hidaka thrust system has not decreased during the last 3.5 Ma. This suggests that the plate boundary between the Eurasian (Amurian) and North American (Okhotsk) plates has not jumped from the central part of Hokkaido to the eastern part of the Sea of Japan since 3.5 Ma and that a significant amount of plate convergence is still being absorbed in the Hidaka Collision Zone. Orogenic structure of the Eastern Alps, Europe, from TRANSALP deep seismic reflection profilingPages 85-102 The TRANSALP Group, comprising of partner institutions from Italy, Austria and Germany, acquired data on a 340 km long deep seismic reflection line crossing the Eastern Alps between Munich and Venice. Although the field work was split into four campaigns, between fall 1998 and summer 2001, the project gathered for the first time a continuous profile across the Alps using consistent field acquisition and data processing parameters. These sections span the orogen itself, at its broadest width, as well as the editor Fred Davey and the two adjacent basins. Vibroseis and explosion data, complementary in their depth penetration and resolution characteristics, were obtained along with wide–angle and teleseismic data. The profile shows a bi–vergent asymmetric structure of the crust beneath the Alpine axis which reaches a maximum thickness of 55 km, and 80–100 km long transcrustal ramps, the southward dipping 'Sub–Tauern–Ramp' and the northward–dipping 'Sub–Dolomites–Ramp'. Strongly reflective patterns of these ramps can be traced towards the north to the Inn Valley and towards the south to the Valsugana thrust belt, both of which show enhanced seismicity in the brittle upper crust. The seismic sections do not reveal any direct evidence for the presence of the Periadriatic Fault system, the presumed equivalent to the Insubric Line in the Western Alps. According to our new evolutionary model, the Sub–Tauern–Ramp is linked at depth with remnants of the subducted Penninic Ocean. The 'crocodile'–type model describes an upper/lower crustal decoupling and wedging of both the European and the Adriatic–African continents. On the nature of mantle heterogeneities and discontinuities: evidence from a very dense wide-angle shot recordPages 103-117 A seismic survey with a receiver spacing of 50 m provided one of the most densely sampled wide–angle seismic reflection images of the lithosphere. This unique data set, recorded by an 18–km–long spread, reveals that at wide–angles the shallow subcrustal mantle features high amplitude reflectivity which contrasts with a lack of reflectivity at latter travel times. This change in the seismic signature is located at approximately 120–150 km depth, which correlates with the depth estimates of the lithosphere–asthenosphere boundary (LAB) of previous DSS studies. This seismic signature can be simulated by two-layer mantle model. Both layers with similar average velocities differ in their degree of heterogeneity. The shallow heterogeneous layer and the deeper and more homogeneous one correlate with the lithosphere and the asthenosphere, respectively. Studies involving surface outcrops of ultramafic massifs and mantle xenoliths infer that the upper mantle is a heterogeneous mixture of ultramafic rocks (lherzolites, harzburgites, pyroxenites, peridotites, dunites, and small amounts of eclogites). Laboratory measurements of physical properties of these mantle rocks indicate that compositional variations alone can account for the wide–angle reflectivity. A temperature increase would homogenize the mixture, decreasing the seismic reflection properties due to melting processes. It is proposed that this would take place below 120–150 km (1200 °C, the LAB). Deep seismic reflection profiling in the Archaean northeastern Yilgarn Craton, Western Australia: implications for crustal architecture and mineral potentialPages 119-133 Deep seismic reflection data across the Archaean Eastern Goldfields Province, northeastern Yilgarn Craton, Western Australia, have provided information on its crustal architecture and on several of its highly mineralised belts. The seismic reflection data allow interpretation of several prominent crustal scale features, including an eastward thickening of the crust, subdivision of the crust into three broad layers, the presence of a prominent east dip to the majority of the reflections and the interpretation of three east–dipping crustal–penetrating shear zones. These east–dipping shear zones are major structures that subdivide the region into four terranes. Major orogenic gold deposits in the Eastern Goldfields Province are spatially associated with these major structures. The Laverton Tectonic Zone, for example, is a highly mineralised corridor that contains several world–class gold deposits plus many smaller deposits. Other non crustal–penetrating structures within the area do not appear to be as well endowed metallogenically as the Laverton structure. The seismic reflection data have also imaged a series of low–angle shear zones within and beneath the granite–greenstone terranes. Where the low–angle shear zones intersect the major crustal–penetrating structures, a wedge shaped geometry is formed. This geometry forms a suitable fluid focusing wedge in which upward to subhorizontal moving fluids are focused and then distributed into the nearby complexly deformed greenstones. Seismic evidence for preservation of the Archean Uchi granite–greenstone belt by crustal–scale extensionPages 135-143 In the westernmost Superior Province of Canada, the east–west alignment of granite–greenstone belts and the adjacent, highly deformed gneiss belts led to the first proposals that plate tectonics existed before 2.5 Ga ago, with the belts thrust against one another by east–west–oriented subduction zones. Here, we present seismic reflection data, which demonstrate that in this region the present juxtaposition of the Uchi granite–greenstone belt and the North Caribou gneiss terrane occurred along a late southeast–dipping extensional shear zone that extends from the surface into the lower crust. The preservation of the Uchi belt and probably the English River metasedimentary belt is directly related to their dropping along extensional shear zones, which limited subsequent erosion. The relative lateral transport of these greenstone rocks implies that they were neither derived from the immediately underlying crust, nor preserved by vertical crustal movements as might occur in the absence of plate tectonics. Extension may have been associated with the emplacement of mantle–derived magmas at 2700 Ma, which has been linked to slab break–off or lithospheric delamination, making the extension approximately coeval with local gold mineralisation. Since crustal–scale faults can facilitate the circulation of gold–bearing fluids, we suggest that greenstone rocks preserved in the hanging walls of syn- to post-accretion extensional shear zones may preferentially host Archean lode-gold deposits. In the westernmost Superior Province, our seismic observations imply that some of the late structures in the well-developed belts defined by surface mapping arose through the collapse of a collage of laterally accreted terranes. Structural fabric of the Central Metasedimentary Belt of southern Ontario, Canada, from deep seismic profilingPages 145-159 The Central Metasedimentary Belt boundary tectonic zone (CMBbtz) is a 10–20–km–wide zone of intense structural deformation within the 1.3–1.0 Ga Grenville orogen of southeastern Canada. The crustal structure of the exposed CMBbtz has been well studied, but its sub-Phanerozoic location and geometry beneath the urban development and nuclear stations of the Toronto region are not well known. A new 75-km Lithoprobe reflection profile acquired close to Toronto provides a clear image of the CMBbtz as a panel of southeast-dipping reflections that extends with moderate dip (<25°) to mid-crustal depth (˜25 km). These dipping reflections truncate and (or) overprint a subhorizontal band of reflectivity at ˜21 km depth. The seismic line is oblique to the major structural trends; cross-dip analysis shows that the southeast-dipping reflections have a strike and dip of ˜N13°E and 25°, whereas the "subhorizontal" reflections strike and dip at ~N65°E and 20°, respectively. Both of these bands of reflectivity can be correlated to magnetic anomalies in the CMBbtz or its immediate footwall. Magnetic anomalies with similar strike directions are well expressed within a distinct rhomboid-shaped region (106×109 km) in the subsurface of western Lake Ontario, herein named Mississauga domain. Taken together, the seismic and magnetic data are inconsistent with existing models, in which the CMBbtz is extrapolated beneath Lake Ontario along a linear magnetic anomaly. We propose a revised subsurface trace of the CMBbtz along the western edge of the Mississauga domain. Small earthquakes in western Lake Ontario appear to cluster along trends co-linear with ENE magnetic anomalies, suggesting a possible degree of basement tectonic control on local intraplate seismicity. Deep 3-D seismic reflection imaging of Precambrian sills in southwestern Alberta, CanadaPages 161-172 A number of bright reflector sequences within the crystalline basement were imaged on 2-D multichannel crustal seismic reflection profiles collected by Lithoprobe from 1991 to 1995. One such sequence in southern Alberta, the Head-Smashed-In (HSI) reflector, spans a depth range of 6–21 km with an aerial extent of at least 6000 km2. This series of reflectors and similar sequences in central Alberta and northern Saskatchewan have been interpreted as packages of horizontal, sheet-like mafic sills that intruded the Precambrian basement hundreds of millions of years after crustal formation. In 1999 through our request to extend an exploration survey to longer travel times, a partnership group from the Canadian petroleum industry recorded a 9 by 6 km 3-D reflection dataset to 8-s two-way-traveltime (TWTT) at a location southwest of the HSI reflector. Following processing, which included a special procedure to remove strong ground roll that persisted at travel times of interest, we identified a sequence of reflections between 4 and 5 s that we correlate with those associated with the HSI reflector. However, the reflections are not as bright or as continuous as those on the 2-D profiles. Based on these results and the characteristics of the sills on the Lithoprobe data, we infer that we imaged the feathering ends of the sill intrusions. Such sills represent a net addition of crustal material and may provide insight into the ancient stress field during emplacement and clues as to how cratons achieve their thickness and stability. Although the survey area is small, this study represents the first attempt in North America, and only the second in the world, to examine upper-middle crustal structure to depths of approximately 20 km using industry-style 3-D seismic reflection data. The results indicate that industry 3-D data, with extended recording times, could provide useful information for studying the crystalline crust below sedimentary basins. Seismic studies of the Brasília fold belt at the western border of the São Francisco Craton, Central Brazil, using receiver function, surface-wave dispersion and teleseismic tomographyPages 173-185 The Tocantins Province in Central Brazil is composed of a series of SSW–NNE trending terranes of mainly Proterozoic ages, which stabilized in the Neoproterozoic in the final collision between the Amazon and São Francisco cratons. No previous information on crustal seismic properties was available for this region. Several broadband stations were used to study the regional patterns of crustal and upper mantle structure, extending the results of a recent E–W seismic refraction profile. Receiver functions and surface wave dispersion showed a thin crust (33–37 km) in the Neoproterozoic Magmatic Arc terrane. High average crustal Vp/Vs ratios (1.74–1.76) were consistently observed in this unit. The foreland domain of the Brasília foldbelt, on the other hand, is characterized by thicker crust (42–43 km). Low Vp/Vs ratios (1.70–1.72) were observed in the low–grade foreland fold and thrust zone of the Brasília belt adjacent to the São Francisco craton. Teleseismic P–wave tomography shows that the lithospheric upper mantle has lower velocities beneath the Magmatic Arc and Goiás Massif compared with the foreland zone of the belt and São Francisco craton. The variations in crustal thickness and upper mantle velocities observed with the broadband stations correlate well with the measurements along the seismic refraction profile. The integration of all seismic observations and gravity data indicates a strong lithospheric contrast between the Goiás Massif and the foreland domain of the Brasília belt, whereas little variation was found across the foldbelt/craton surface boundary. These results support the hypothesis that the Brasília foreland domain and the São Francisco craton were part of a larger São Francisco-Congo continental plate in the final collision with the Amazon plate. Deep seismic refraction and gravity crustal model and tectonic deformation in Tocantins Province, Central BrazilPages 187-199 Interpretation of seismic refraction data in the central sector of Tocantins Province, Central Brazil, has produced a seismic crustal model with well-defined upper, intermediate, and lower crust layers having smooth velocity gradient in each layer. The depths to Moho vary from 32 to 43 km, and mean crustal P velocity varies from 6.3 km/s, beneath Goiás magmatic arc on the western side, to 6.4 km/s, below Goiás massif in the central portion and the foreland fold-and-thrust belt on the eastern side. The behaviour of the lower crust layer allows an improved understanding of regional gravimetric features of the central and northern sectors of Tocantins Province and suggests subduction of the Amazon plate in Central Brazil. In the southeastern sector, the refraction experiment resulted in the detection of a thinner crust (38 km) below Brasília fold belt and a thicker crust (41 km) below Paraná basin and São Francisco craton (42 km). The upper crust beneath Paraná Basin is around 20 km thick, whereas it is less than 10 km thick below the craton. These results bring new insights into the geological history of the central and southeastern sectors of Tocantins Province. Gravimetric measurements in the central sector of Tocantins Province delineate a high and a low anomaly separated by a steep gradient with a NE direction. The axis of the gradient seems to bend still further to NE in the northern sector of that province, whereas the gravimetric high continues northwards, defining a separation between them. This suggests that those features belong to different tectonic processes that occurred during Tocantins Province orogenesis. The gravimetric model, which incorporates seismically resolved structure beneath Tocantins Province, better matches the observed gravimetric data. Although tectonic movements have only been monitored with high–precision GPS for short time interval (1999–2001), the results suggest observable deformations. The main seismicity of Central Brazil, the Goiás–Tocantins seismic belt, seems to be spatially associated with the large gravimetric high anomaly and with the observed tectonic deformation. A method for avoiding artifacts in the migration of deep seismic reflection dataPages 201-212 Conventional wave-equation-based migration of deep seismic reflection data can produce severe artifacts, which appear as broad circular arcs or "smiles", due to the existence of apparent truncations of reflections on the stack section arising from poor signal penetration, changes in orientation of the acquisition profile, and the existence of strong overlying lateral velocity variations. These artifacts limit the interpretation of deep seismic profiles, because they obscure weak reflections and reflection truncations that may, e.g., indicate the presence of subsurface faults. Here I present a new migration algorithm, in which each sample of the stack is migrated to a short linear segment whose position and dip are determined by its original position on the stack, an estimate of the local apparent dip at that point, and a user-specified migration velocity. No subjective interpretation of reflections on the stack section is required, and the algorithm produces no arc-like migration artifacts. The degree of lateral smearing can be easily controlled, allowing reflection truncations to be revealed. In practice, the algorithm is most effectively applied to data that have been coherency-filtered to remove low amplitude noise, which would otherwise be preserved. The effects of out-of-plane seismic energy on reflections in crustal-scale 2D seismic sectionsPages 213-224 Crustal–scale seismic surveys mostly collect data along single profiles, and the data processing has an underlying assumption that the data have imaged two–dimension (2D) structure striking at right angles to the seismic profile. However, even small amounts of out–of–plane topography on a reflector can result in reflections that do not map the reflector shape accurately. Out–of–plane energy will migrate within the plane of the section to an apparent depth (represented as two-way-time, TWT) that is greater than the depth of the reflection point out of the plane of the section. It will fall within the plane of the section at depths less than, equal to or greater than the intersection of the reflector with the plane of the section, depending on both the amount of out–of–plane topographic relief on the reflector, and the offset of the topographic relief from the plane of the section. Reflectors that are a single surface can therefore be manifested in the seismic section as a band of several reflections, rather than a single reflection. More complex reflectors, such as shear zones that have a finite thickness because they are made up of several to many anastomosing layers of altered and anisotropic rock embedded in protolith, will appear as laterally short reflections within a laterally continuous reflection band. Other examples of such reflectors would be the Moho in some places, and rock with compositional layering. With increasing out–of–plane topographic relief on the reflector, the top of the reflection band for both single– and multi–layer reflectors will be a poor indicator of the top of the reflector in the Earth. The bottom of the reflection band will always be a poor indicator of the bottom of the reflector. Because out–of–plane energy can arrive at TWTs that are different from those of the reflector in the plane of the section, out-of-plane energy has the potential to interfere constructively or destructively with the in–plane energy. In synthetic data calculated for a simple model assuming one layer and topographic relief of 250 m over wavelengths of 4–5 km, similar to that imaged in a real sub-horizontal detachment, amplitudes ranged up to 2.6 times the expected amplitude for the layer. A model with anastomosing layers built to resemble a thick shear zone rather than a discrete fault surface allowed tuning between layers. The effects of out-of-plane energy when combined with the effects of tuning caused amplitudes up to 3.1 times those expected. Larger amplitudes could be achieved if a suitable model was contrived. The results indicate that care must be taken when calculating impedance contrasts using real data. The highest amplitude reflections are likely to yield overestimates of the true impedance contrast. Imaging low-velocity anomalies with the aid of seismic tomographyPages 225-238 Theoretical considerations (Snell's law) suggest that low-velocity fanomalies are undersampled and therefore should be poorly resolved by inversion schemes based on ray-tracing methods. A synthetic study considering a 40×20 m low-velocity anomaly (300 m/s) placed at the center of a 400×50 m block with gradient background velocity model (from 3000 m/s at the surface to 4000 m/s at the base) indicates that the low ray density in ray-tracing coverage diagrams of tomographic inversion studies can be used as evidence for the existence of low-velocity anomalies. Combined normal incidence seismic reflection images and the velocity models obtained by tomographic inversions of first-arrival travel times form an efficient scheme to resolve low-velocity anomalies such as fracture zones. Furthermore, the velocity models derived from tomographic inversions are used in a wave equation datuming algorithm to account for statics caused by a strongly laterally variable shallow surface (weathering) layer and provide seismic reflection images of fracture zones (low-velocity anomaly) within a granitic pluton. Low-frequency 3-D seismic survey of upper crustal magmatic intrusions in the northeastern Pannonian basin of HungaryPages 239-252 A specially designed 700-km2 grid survey, deploying 1000 regularly distributed low-frequency seismic recording systems, successfully investigated one of the most complex geologic environments of the Pannonian basin. The wide-angle signals penetrated through over 1000 m of multi-phase igneous lithology and recognized, for the first time, the underlying enigmatic Permian to Early Triassic basement rocks. Tomographic inversion of the first arrival grid data resulted in determination of an accurate three-dimensional (3-D) velocity field, to a depth of 4 km. The anomalous changes of the spatial velocity data outline the regional extent of the Late Miocene magmatic intrusions, which are covered by over 2000 m of Mid-Miocene to Pleistocene clastics. Complex relationship was found between the surface potential data and the intrusive bodies. This multi-faceted geophysical data analysis established a functional technique for mapping a subsurface with intricate acoustic and structural complexity. The northern Walker Lane refraction experiment: Pn arrivals and the northern Sierra Nevada rootPages 253-269 In May 2002, we collected a new crustal refraction profile from Battle Mountain, Nevada across western Nevada, the Reno area, Lake Tahoe, and the northern Sierra Nevada Mountains to Auburn, CA. Mine blasts and earthquakes were recorded by 199 Texan instruments extending across this more than 450–km–long transect. The use of large mine blasts and the ultra-portable Texan recorders kept the field costs of this profile to less than US$10,000. The seismic sources at the eastern end were mining blasts at Barrick's GoldStrike mine. The GoldStrike mine produced several ripple-fired blasts using 8000–44,000 kg of ANFO each, a daily occurrence. First arrivals from the larger GoldStrike blasts are obvious to distances of 300 km in the raw records. First arrivals from a quarry blast west of the survey near Watsonville, CA, located by the Northern California Seismic Network with a magnitude of 2.2, can be picked across the recording array to distances of 600 km. The Watsonville blast provides a western source, nearly reversing the GoldStrike blasts. A small earthquake near Bridgeport, CA. also produced pickable P-wave arrivals across the transect, providing fan-shot data. Arrivals from M5 events in the Mariana and Kuril Islands also appear in the records. This refraction survey observes an unexpectedly deep crustal root under the northern Sierra Nevada range, over 50 km in thickness and possibly centered west of the topographic crest. Pn delays of 4–6 s support this interpretation. At Battle Mountain, Nevada, we observe anomalously thin crust over a limited region perhaps only 150 km wide, with a Moho depth of 19–23 km. Pn crossover distances of less than 80 km support this anomaly, which is surrounded by observations of more normal, 30–km–thick crust. A 10–km–thick and high–velocity lower–crustal "pillow" is an alternative hypothesis, but unlikely due to the lack of volcanics west of Battle Mountain. Large mine and quarry blasts prove very effective crustal refraction sources when recorded with a dense receiver array, even over distances exceeding 600 km. New elastic synthetic seismogram modeling suggests that Pn can be strong as a first arrival, easing the modeling and interpretation of crustal refraction data. Fast eikonal computations of first–arrival time can match pickable Pn arrival times. Must magmatic intrusion in the lower crust produce reflectivity?Pages 271-297 The Færoe–Iceland Ridge (FIR) provides a laboratory in which to investigate the reflectivity and velocity structure of thick crust generated above a mantle plume in order to constrain models of underplating and the origins of lower-crustal layering in an environment dominated by young igneous processes. Over 600 km of common midpoint (cmp) data were collected along and across the FIR using a large airgun array with a 240-channel streamer. The interpretation of these data has been integrated with a velocity model of the crust and upper mantle along the FIR obtained from wide-angle seismic arrivals into ocean bottom and land seismometers. Due to the intermediate water depths and the presence of basalt near the water bottom, specialized processing steps were required for the cmp data. A wave equation-based multiple attenuation scheme was applied to the prestack data, which used a forward model of the multiple series to predict and attenuate multiple energy. Array simulations were applied in the shot and receiver domains in order to minimize spatial aliasing and reduce low apparent-velocity noise. Most of the sections over the central (oceanic) portion of the FIR show no pronounced reflectivity, although occasional Moho and/or lower-crustal reflections are observed. We believe that the poor reflectivity results largely from a lack of physical property contrasts rather than being an effect of acquisition or processing, although we also conclude that residual energy from strong multiple reflection remains in the final sections. Amplitude decay and reflection strength vary along the FIR, but there is good signal-to-noise ratio to travel times of at least 9 s (i.e., into the lower crust), implying that the reduced reflectivity beneath the main part of the FIR is not an artifact of signal penetration loss. We conclude that the addition of melt to the lower crust along the trace of the plume apparently did not produce strong physical property contrasts in the lower crust, where little reflectivity is apparent. Perhaps this was because the entire crust was hot at the time of formation. In contrast, igneous intrusion into preexisting continental crust (at the Færoe Islands end of the FIR) and into older igneous crust (at the Iceland end of the FIR) produces significant lower-crustal reflectivity. Strong lower-crustal reflectivity elsewhere beneath the northwestern European continental margins may have a similar intrusive origin. |
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