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: Dec 09, 2024 11th International Symposium on Deep Seismic Reflection Probing of the Continents and Their Margins
Mont-Tremblant, Quebec, Canada, 26 September - 1 October 2004from TECTONOPHYSICS, Vol. 420, Issues 1-2, Pages 1-344 (26 June 2006) Edited by D. B Snyder, D. W. Eaton and C. A. Hurich 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)
Integration of P- and SH-wave high-resolution seismic reflection and micro-gravity techniques to improve interpretation of shallow subsurface structure: New Madrid seismic zonePages 5-21 Shallow high-resolution seismic reflection surveys have traditionally been restricted to either compressional (P) or horizontally polarized shear (SH) waves in order to produce 2-D images of subsurface structure. The northernmost Mississippi embayment and coincident New Madrid seismic zone (NMSZ) provide an ideal laboratory to study the experimental use of integrating P- and SH-wave seismic profiles, integrated, where practicable, with micro-gravity data. In this area, the relation between "deeper" deformation of Paleozoic bedrock associated with the formation of the Reelfoot rift and NMSZ seismicity and "shallower" deformation of overlying sediments has remained elusive, but could be revealed using integrated P- and SH-wave reflection. Surface expressions of deformation are almost non-existent in this region, which makes seismic reflection surveying the only means of detecting structures that are possibly pertinent to seismic hazard assessment. Since P- and SH-waves respond differently to the rock and fluid properties and travel at dissimilar speeds, the resulting seismic profiles provide complementary views of the subsurface based on different levels of resolution and imaging capability. P-wave profiles acquired in southwestern Illinois and western Kentucky (USA) detect faulting of deep, Paleozoic bedrock and Cretaceous reflectors while coincident SH-wave surveys show that this deformation propagates higher into overlying Tertiary and Quaternary strata. Forward modeling of micro-gravity data acquired along one of the seismic profiles further supports an interpretation of faulting of bedrock and Cretaceous strata. The integration of the two seismic and the micro-gravity methods therefore increases the scope for investigating the relation between the older and younger deformation in an area of critical seismic hazard. Constraints from Moho geometry and crustal thickness on the geodynamic origin of the Vrancea Seismogenic Zone (Romania)Pages 23-36 Reprocessing of industry deep seismic reflection data (Ramnicu Sarat and Braila profiles) from the SE Carpathian foreland of Romania provides important new constraints on geodynamic models for the origin of the intermediate depth Vrancea Seismogenic Zone (VSZ). Mantle (70-200 km) earthquakes of the VSZ are characterized by high magnitudes (greater than 6.5), frequent occurrence rates (approximately 25 years), and confinement in a very narrow (30×70×200 km3) near vertical zone atypical for a Wadati-Benioff plane, located in front of the orogen. These two deep (20 s) seismic reflection profiles (70 km length across the foreland) reveal (1) a high-amplitude, gently east-dipping reflection across most of the section from what we interpret to be the Moho at ˜ 15 s (40-42 km) on the Ramnicu Sarat line to ˜ 16 s (47-48 km) on the Braila line, (2) a thick sedimentary cover increasing in thickness from east (1 s; ˜ 800 m) to west (7.5 s; 14 km), (3) an eastward increase in crustal thickness from 38 km (near VSZ) to ˜ 45 km, (4) seismic and topographic evidence for a newly imaged, possibly seismically active basement fault with a surface offset of 30 m observed on the Ramnicu Sarat line, (5) a lack of notable west-dipping structures in the crust and across the Moho, and (6) variable displacements on Peceneaga-Camena Fault of ˜ 5 km at Moho and ˜ 200 m at the basement-sedimentary cover contact. These observations appear to argue against recent models for west-dipping subduction of oceanic lithosphere at or in the vicinity of the Vrancea Seismogenic Zone given the lack of west-dipping fabrics in the lower crust and across the crust-mantle boundary. Consequently, one possible explanation for the geodynamic origin of VSZ could be partial delamination of the continental lithosphere in an intra-plate setting along a sub-horizontal lithospheric interface in the Carpathian hinterland that likely involves remnant lithospheric coupling between the crust and uppermost mantle in the foreland. Imaging granitic plutons along the IBERSEIS profilePages 37-47 The parameters used for the acquisition of the IBERSEIS deep seismic reflection profile in the SW Iberian Peninsula provide seismic images of the deep crust as well as a high resolution section of the shallow subsurface. A very dense array of sources and receivers allowed high resolution tomographic studies in zones of special interest (granitic plutons). The three dimensional tomographic inversion produced velocity models along a 500 m wide and 1000 m deep strip along the IBERSEIS transect in the areas of La Bazana, La Dehesilla, Feria and Villafranca. In these high resolution velocity models (sampled by 50×50×50m cells), high velocity anomalies indicate the geology and extension of the granitic plutons at depth. This directly correlates with the surface outcrops. Moreover, tomographic models provided valuable information for the geometry and characterization of fractured and fresh domains in a rock volume. Furthermore, a piggy back seismic acquisition experiment using additional seismic instrumentation from the University of Paris Sud (40-channel DMT) provided perpendicular, offline recordings of the Vibroseis sources. This additional recording system was deployed perpendicularly to the main IBERSEIS seismic reflection line and provided additional 3D control. Seismic velocity model of the crust and uppermost mantle around the Mirnyi kimberlite field in SiberiaPages 49-73 We present the first detailed seismic velocity models of the crust and uppermost mantle around the Mirnyi kimberlite field in Yakutia, Siberia. We have digitized vintage seismograms that were acquired in 1981 and 1983 by use of Taiga analogue seismographs along two perpendicular seismic profiles. The 370-km long, northwest striking profile I across the kimberlite pipe was covered by 41 seismographs, which recorded seismic signals from 21 chemical shots along the line, including one off-end shot. The perpendicular, 340-km long profile II across profile I ca. 30 km to the south of the Mirnyi kimberlite field was covered by 45 seismographs, which recorded seismic signals from 22 chemical shots, including four off-end shots. Each shot involved detonation of between 1.5 and 6.0 tons of TNT, distributed in individual charges of 100-200 kg in shallow water (<2 m deep). The data is of high quality with high signal/noise ratio to the farthest offsets. We present the results from two-dimensional ray tracing, forward modelling. Both velocity models show normal cratonic structure of the ca. 45-km-thick crust with only slight undulation of the Moho. However, relatively small seismic velocity is detected to 25-km depth in a ca. 60-km wide zone around the kimberlite pipe, surrounded by elevated velocity (*6.3 km/s) in the upper crust. The lower crust has a relatively constant velocity of 6.8-6.9 km/s. It appears relatively unaffected by the presence of the kimberlite field. Extremely large P-wave velocity (>8.7 km/s) of the sub-Moho mantle is interpreted along profile I, except for a 70-km wide zone with a "normal" Pn velocity of 8.1 km/s below the kimberlite. Profile II mainly shows Pn velocities of 8.0-8.2 km/s, with unusually large velocity (>8.5 km/s) in two, ca. 100-km wide zones, at its southwestern end, one zone being close to the kimberlite field. The nature of these exceptionally large, sub-Moho mantle velocities is not yet understood. The difference in velocity in the two profile directions indicates anisotropy, but the effect of unusual rock composition, e.g. from a high concentration of garnet, cannot be excluded. An integrated multi-scale 3D seismic model of the Archaean Yilgarn Craton, AustraliaPages 75-90 The collection of a range of different seismic data types has greatly improved our understanding of the crustal architecture of Australia's Archaean Yilgarn Craton over the last few years. These seismic data include broadband seismic studies, seismic receiver functions, wide-angle recordings and mine-scale to deep seismic reflection transects. Each data set provides information on the three-dimensional (3D) tectonic model of the Yilgarn Craton from the craton scale through to the mine scale. This paper demonstrates that the integration and rationalisation of these different seismic data sets into a multi-scale 3D geological/seismic model, that can be visualised at once in a single software package, and incorporating all available data sets, significantly enhances this understanding. This enhanced understanding occurred because the integrated 3D model allowed easy and accurate comparison of one result against another, and facilitated the integrated questioning and interrogation across scales and seismic method. As a result, there are feedback questions regarding understanding of the individual seismic data sets themselves, as well as the Yilgarn Craton as a whole. The methodology used, including all the data sets in the model range, had to allow for the wide range of data sets, frequencies and seismic modes. At the craton scale, P-wave, S-wave and surface wave variations constrained the 3D lithospheric velocity model, revealing noticeable large-scale velocity variations within and across the craton. An interesting feature of the data, easily identified in 3D, is the presence of a fast S-wave velocity anomaly (>4.8 km.s-1) within the upper mantle. This velocity anomaly dips east and has a series of step-down offsets that coincide approximately with province and terrane boundaries of the Yilgarn Craton. One-dimensional receiver function profiles show variations in their crustal velocity across the craton. These crustal velocity variations are consistent with the larger-scale geological subdivision of the craton, and provide characteristic profiles for provinces and terranes. The receiver function results and the deep seismic reflection data both agree on the depth to the Moho, and both indicate an increase in Moho depth to the east. The 2D seismic refraction results in the south-west of the craton provide crustal thickness information, an indication of middle and lower crustal compositions, and information regarding the broad-scale architectural framework. At the province- and terrane-scale, the deep seismic reflection data and the mine-scale seismic data provide geometric constraints on crustal architecture, in particular the orientation of the region's fault systems as well as variations in the thickness of the granite-greenstone succession. Integration of the results from wide-angle seismic refraction data coincident with the deep seismic reflection data provided additional constraints on likely upper crustal lithologies. The integrated 3D seismic model implies the dominant geodynamic process involved the development of an orogenic belt that developed with a series of contractional (folding and thrusting) events, separated by equally important extensional events. The seismic reflection data in particular suggests that extensional movement on many shear zones was more common than previously thought. The seismic reflection data suggest that the dominant mineral systems involved deeply sourced fluid flowing up crustal-penetrating shear zones. These deeply sourced fluids were further focussed into sites located above fault-breached domal regions in the upper crust. Constraining models of the tectonic setting of the giant Olympic Dam iron oxide-copper-gold deposit, South Australia, using deep seismic reflection dataPages 91-103 In the Gawler Craton, the completeness of cover concealing the crystalline basement in the region of the giant Olympic Dam Cu-Au deposit has impeded any sufficient understanding of the crustal architecture and tectonic setting of its IOCG mineral-system. To circumvent this problem, deep seismic reflection data were recently acquired from 250 line-km of two intersecting traverses, centered on the Olympic Dam deposit. The data were recorded to 18 s TWT ( 55 km). The crust consists of Neoproterozoic cover, in places more than 5 km thick, over crystalline basement with the Moho at depths of 13-14 s TWT ( 40-42 km). The Olympic Dam deposit lies on the boundary between two distinct pieces of crust, one interpreted as the Archean-Paleoproterozoic core to the craton, the other as a Meso-Neoproterozoic mobile belt. The host to the deposit, a member of the 1590 Ma Hiltaba Suite of granites, is situated above a zone of reduced impedance contrast in the lower crust, which we interpret to be source-region for its 1000°C magma. The crystalline basement is dominated by thrusts. This contrasts with widely held models for the tectonic setting of Olympic Dam, which predict extension associated with heat from the mantle producing the high temperatures required to generate the Hiltaba Suite granites implicated in mineralization. We use the seismic data to test four hypotheses for this heat-source: mantle underplating, a mantle-plume, lithospheric extension, and radioactive heating in the lower crust. We reject the first three hypotheses. The data cannot be used to reject or confirm the fourth hypothesis. Seismic velocity structure of a large mafic intrusion in the crust of central Denmark from project ESTRIDPages 105-122 The origin of regional sedimentary basins is being investigated by the ESTRID project (Explosion Seismic Transects around a Rift In Denmark). This project investigates the mechanisms of the formation of wide, regional basins and their interrelation to previous rifting processes in the Danish-Norwegian Basin in the North Sea region. In May 2004 a 143 km long refraction seismic profile was acquired along the strike direction of a suspected major mafic intrusion in the crust in central Denmark. The data confirms the presence of a body with high seismic velocity (>6.5 km/s) extending from a depth of 10-12 km depth into the lower crust. There is a remarkable Moho relief between 27 and 34 km depth along this new along-strike profile as based on ray-tracing modelling of PmP reflections. The lack of PmP reflections at a zone of very high velocity in the lowest crust (7.3-7.5 km/s) suggests a possible location of a feeder channel to the batholith. The presence of volcanic rocks of Carboniferous-Permian age above the intrusion (mafic batholith) suggests a similar age of the intrusion. An older obliquely crossing profile and two new fan profiles deployed perpendicular to the main ESTRID profile, show that the batholith is about 30-40 km wide. The existence of this large mafic batholith supports the hypothesis that the origin of the Danish-Norwegian Basin is related to cooling and contraction after intrusion of large amounts of mafic melts into the crust during the late Carboniferous and early Permian. The data and interpretations from project ESTRID will form the basis for subsidence modelling. Tentatively, we interpret the formation of the Danish-Norwegian Basin as a thermal subsidence basin, which developed after widespread rifting of the region. Deep seismic investigation across the Barents-Kara region and Novozemelskiy Fold Belt (Arctic Shelf)Pages 123-140 Since 1995 SEVMORGEO has collected wide-angle reflection/refraction profiling (WARRP), multichannel seismic data (MCS) and seismoacoustic profiling, along regional lines 1-AR, 2-AR and 3-AR. These lines cross the whole Barents-Kara Region and Novozemelskiy Fold Belt. As a result, new geological data about the deep structure of the Earth's crust have become available. Four main tectono-stratigraphic units are distinguished in the section of the Earth's crust: (1) a sedimentary cover; (2) the Upper Proterozoic (mainly Riphean for the Barents Plate) and Riphean-Paleozoic (the South-Kara Syneclise) deformed and folded complexes; (3) the upper crystalline crust (granite-gneissic metamorphic Archean-Proterozoic complex); (4) the lower crust (basalt complex). The Barents-Kara Region is characterized by moderately thinned continental and subcontinental crust with an average thickness of 37-39 km. On islands and areas of uplifts with ancient massifs, the thickness of the crust (38-42 km) approaches the typical crust for a continental platform. In the Novozemelskiy Fold Belt the thickness of the crust reaches 40-42 km. Rift-related grabens are characterized by significant crustal thinning with thicknesses of 33-36 km. Several grabens are revealed: the Riphean Graben on the Kola-Kanin Monocline, the Lower Paleozoic West-Kola Graben, the Devonian Demidovskiy Aulacogen, the Upper Paleozoic Malyginskiy Graben in the Barents Region and Upper Paleozoic-Triassic Noyabr'skiy and the Chekinskiy grabens in the Kara Region. Data concerning the deep structure lead us to conclude that mainly destructive processes contributed to the dynamics of the forming of the Barents-Kara Region. Coupled basin evolution and late-stage metamorphic core complex exhumation in the southern Basin and Range Province, southeastern ArizonaPages 141-160 Records of lithospheric extension and mountain-range uplift are most continuously contained within syntectonic sedimentary rocks in basins adjacent to large structural culminations. In southeastern Arizona, metamorphic core complexes form mountain ranges with the highest elevations in the region, and supposedly much less extended terranes lie at lower elevations. Adjacent to the Santa Catalina-Rincon metamorphic core complex, within the Tucson Basin, stratigraphic-sequence geometries evident in a large suite of 2-D seismic reflection data suggest a two-phase basin-evolution model controlled by the emplacement and subsequent uplift of the core complex. In its earliest stage, Phase I of basin formation was characterized by extensive faults forming relatively small-scale proto-basins, which coalesced with the larger basin-bounding detachment fault system. Synextensional sedimentation within the enlarging basin is evidenced by sediment-growth packages, derived from adjacent footwall material, fanning into brittle hanging-wall faults. During this phase, volcanism was widespread, and growth packages contain interbedded sediments and volcanic products but, paradoxically, no mylonitic clasts from the adjacent metamorphic core complex. Phase II of basin evolution begins after a significant tectonic hiatus and consists of a symmetric deepening of the central basin with the introduction of mylonitic clasts in the basin fill. This is coupled with the activation of a series of high-angle normal faults ringing the core complex. These observations suggest a two-phase model for metamorphic core complex evolution, with an initial stage of isostatic core complex emplacement during detachment faulting that resulted in little topographic expression. This was followed, after a significant tectonic hiatus, by late-stage exhumation and flexural uplift of the Santa Catalina-Rincon metamorphic core complex through younger high-angle faulting. Moreover, the geometry of upper basin fill units suggests an extremely low effective elastic thickness in the region and that flexural uplift of the core complex induced asymmetric transfer of ductile mid-crustal rocks from beneath the subsiding Tucson Basin to the uplifting mountain range. Reprocessing and enhanced interpretation of the initial COCORP Southern Appalachians traversePages 161-174 Reprocessing of the 1978-1980 COCORP Southern Appalachian seismic reflection data has produced improved images of structures related to the emplacement of the Blue Ridge-Inner Piedmont allochthon. The results enhance and extend the interpretation presented previously that the Blue Ridge and Inner Piedmont are allochthonous above a shallow, and shallow dipping, detachment that can be followed from outcrop at the Blue Ridge/Valley and Ridge transition to at least beneath the Carolina terrane. The continuity of reflections in the new images supports the interpretation that the southern Appalachian detachment is not rooted on the east side of the Inner Piedmont, but rather projects as a low-angle detachment (or zone of decoupling) to beneath the Coastal Plain. An implication of this geometry is that terranes, such as the Carolina terrane, between autochthonous North America and the Alleghanian suture beneath the Coastal Plain are detached, thin flakes. Local thickening of the Cascadia forearc crust and the origin of seismic reflectors in the uppermost mantlePages 175-188 Seismic reflection profiles from three different surveys of the Cascadia forearc are interpreted using P wave velocities and relocated hypocentres, which were both derived from the first arrival travel time inversion of wide-angle seismic data and local earthquakes. The subduction decollement, which is characterized beneath the continental shelf by a reflection of 0.5 s duration, can be traced landward into a large duplex structure in the lower forearc crust near southern Vancouver Island. Beneath Vancouver Island, the roof thrust of the duplex is revealed by a 5-12 km thick zone, identified previously as the E reflectors, and the floor thrust is defined by a short duration reflection from a <2-km-thick interface at the top of the subducting plate. We show that another zone of reflectors exists east of Vancouver Island that is approximately 8 km thick, and identified as the D reflectors. These overlie the E reflectors; together the two zones define the landward part of the duplex. The combined zones reach depths as great as 50 km. The duplex structure extends for more than 120 km perpendicular to the margin, has an along-strike extent of 80 km, and at depths between 30 km and 50 km the duplex structure correlates with a region of anomalously deep seismicity, where velocities are less than 7000 m.s-1. We suggest that these relatively low velocities indicate the presence of either crustal rocks from the oceanic plate that have been underplated to the continent or crustal rocks from the forearc that have been transported downward by subduction erosion. The absence of seismicity from within the E reflectors implies that they are significantly weaker than the overlying crust, and the reflectors may be a zone of active ductile shear. In contrast, seismicity in parts of the D reflectors can be interpreted to mean that ductile shearing no longer occurs in the landward part of the duplex. Merging of the D and E reflectors at 42-46 km depth creates reflectivity in the uppermost mantle with a vertical thickness of at least 15 km. We suggest that pervasive reflectivity in the upper mantle elsewhere beneath Puget Sound and the Strait of Georgia arises from similar shear zones. Non-transparent uppermost mantle in the island-arc region of JapanPages 189-204 In Japan, the crust and uppermost mantle seismic character is yet unimaged although many refraction surveys have been recorded. The longest seismic profiles are analyzed. A remarkable feature, a long-duration coda wave after the PmP wave (reflected wave at the Moho boundary), is observed on the record sections. Several possible models are considered to explain the long-duration coda wave. The model with many scatterers located in the uppermost mantle explains the observed data well while the undulating Moho and continuous layering models do not account for some aspects of the observed data. The scatterer distributed uppermost mantle is not consistent with that of continental region which is often characterized as transparent. We estimate the scattering coefficient of the uppermost mantle and crust using simulations. The scattering coefficients obtained for upper crust, lower crust, and uppermost mantle are 0.01, 0.02, and 0.025, respectively. The scattering coefficient of the uppermost mantle is slightly larger than that of lower crust, which is characterized as being reflective. The many scatterers in the uppermost mantle might be related to magmatism in Japan. This will be one of the important observations for understanding formation processes of the Moho boundary and uppermost mantle in the island-arc environment. Moho depth variation beneath southwestern Japan revealed from the velocity structure based on receiver function inversionPages 205-221 The Philippine Sea plate is subducting under the Eurasian plate beneath the Chugoku-Shikoku region, southwestern Japan. We have constructed depth contours for the continental and oceanic Mohos derived from the velocity structure based on receiver function inversion. Receiver functions were calculated using teleseismic waveforms recorded by the high-density seismograph network in southwestern Japan. In order to determine crustal velocity structure, we first improved the linearized time-domain receiver function inversion method. The continental Moho is relatively shallow ( 30 km) at the coastline of the Sea of Japan and at the Seto Inland Sea, and becomes deeper - greater than 40 km - round 35°N and 133.8°E. Near the Seto Inland Sea, a low-velocity layer of thickness 10 km lies under the continental Moho. This low-velocity layer corresponds to the subducting oceanic crust of the Philippine Sea plate. The oceanic Moho continues to descend from south to northwest and exhibits complicated ridge and valley features. The oceanic Moho runs around 25 km beneath the Pacific coast and 45 km beneath the Seto Inland Sea, and it extends to at least to 34.5 N. The depth variation of the Moho discontinuities is in good qualitative agreement with the concept of isostasy. From the configurations of both the continental and oceanic Mohos, we demonstrate that the continental lower crust and the subducting oceanic crust overlap beneath the southern and central part of Shikoku and that a mantle wedge may exist beneath the western and eastern part of Shikoku. The southern edge of the overlapping region coincides with the downdip limit of the slip area of a megathrust earthquake. Crustal thickness and VP/VS variations in the Grenville orogen (Ontario, Canada) from analysis of teleseismic receiver functionsPages 223-238 We have developed a simple semblance-weighted stacking technique to estimate crustal thickness and average VP/VS ratio using teleseismic receiver functions. We have applied our method to data from 32 broadband seismograph stations that cover a 700×400 km2 region of the Grenville orogen, a 1.2-0.98 Ga Himalayan-scale collisional belt in eastern North America. Our seismograph network partly overlaps with LITHOPROBE and other crustal refraction surveys. In 8 out of 9 cases where a crustal-refraction profile passes within 30 km of a seismograph station, the two independent crustal thickness estimates agree to within 7%. Our regional crustal-thickness model, constructed using both teleseismic and refraction observations, ranges between 34.0 and 52.4 km. Crustal-thickness trends show a strong correlation with geological belts, but do not correlate with surface topography and are far in excess of relief required to maintain local isostatic equilibrium. The thickest crust (52.4±1.7 km) was found at a station located within the 1.1 Ga mid-continent (failed) rift. The Central Gneiss Belt, which contains rocks exhumed from deep levels of the crust, is characterized by VP/VS ranging from 1.78 to 1.85. In other parts of the Grenville orogen, VP/VS is found to be generally less than 1.80. The thinnest crust (34.5-37.0 km) occurs northeast of the 0.7 Ga Ottawa-Bonnechere graben and correlates with areas of high intraplate seismicity. Crustal structure of mainland China from deep seismic sounding dataPages 239-252 Since 1958, about ninety seismic refraction/wide angle reflection profiles, with a cumulative length of more than sixty thousand kilometers, have been completed in mainland China. We summarize the results in the form of (1) a new contour map of crustal thickness, (2) fourteen representative crustal seismic velocity-depth columns for various tectonic units, and, (3) a Pn velocity map. We found a north-south-trending belt with a strong lateral gradient in crustal thickness in central China. This belt divides China into an eastern region, with a crustal thickness of 30-45 km, and a western region, with a thickness of 45-75 km. The crust in these two regions has experienced different evolutionary processes, and currently lies within distinct tectonic stress fields. Our compilation finds that there is a high-velocity (7.1-7.4 km/s) layer in the lower crust of the stable Tarim basin and Ordos plateau. However, in young orogenic belts, including parts of eastern China, the Tianshan and the Tibetan plateau, this layer is often absent. One exception is southern Tibet, where the presence of a high-velocity layer is related to the northward injection of the cold Indian plate. This high-velocity layer is absent in northern Tibet. In orogenic belts, there usually is a low-velocity layer (LVL) in the crust, but in stable regions this layer seldom exists. The Pn velocities in eastern China generally range from 7.9 to 8.1 km/s and tend to be isotropic. Pn velocities in western China are more variable, ranging from 7.7 to 8.2 km/s, and may display azimuthal anisotropy. Crustal structure of the northeastern margin of the Tibetan plateau from the Songpan-Ganzi terrane to the Ordos basinPages 253-266 The 1000-km-long Darlag-Lanzhou-Jingbian seismic refraction profile is located in the NE margin of the Tibetan plateau. This profile crosses the northern Songpan-Ganzi terrane, the Qinling-Qilian fold system, the Haiyuan arcuate tectonic region, and the stable Ordos basin. The P-wave and S-wave velocity structure and Poisson's ratios reveal many significant characteristics in the profile. The crustal thickness increases from northeast to southwest. The average crustal thickness observed increases from 42 km in the Ordos basin to 63 km in the Songpan-Ganzi terrane. The crust becomes obviously thicker south of the Haiyuan fault and beneath the West-Qinlin Shan. The crustal velocities have significant variations along the profile. The average P-wave velocities for the crystalline crust vary between 6.3 and 6.4 km/s. Beneath the Songpan-Ganzi terrane, West-Qinling Shan, and Haiyuan arcuate tectonic region P-wave velocities of 6.3 km/s are 0.15 km/s lower than the worldwide average of 6.45 km/s. North of the Kunlun fault, with exclusion of the Haiyuan arcuate tectonic region, the average P-wave velocity is 6.4 km/s and only 0.5 km/s lower than the worldwide average. A combination of the P-wave velocity and Poisson's ratio suggests that the crust is dominantly felsic in composition with an intermediate composition at the base. A mafic lower crust is absent in the NE margin of the Tibetan plateau from the Songpan-Ganzi terrane to the Ordos basin. There are low velocity zones in the West-Qinling Shan and the Haiyuan arcuate tectonic region. The low velocity zones have low S-wave velocities and high Poisson's ratios, so it is possible these zones are due to partial melting. The crust is divided into two layers, the upper and the lower crust, with crustal thickening mainly in the lower crust as the NE Tibetan plateau is approached. The results in the study show that the thickness of the lower crust increases from 22 to 38 km as the crustal thickness increases from 42 km in the Ordos basin to 63 km in the Songpan-Ganzi terrane south of the Kunlun fault. Both the Conrad discontinuity and Moho in the West-Qinling Shan and in the Haiyuan arcuate tectonic region are laminated interfaces, implying intense tectonic activity. The arcuate faults and large earthquakes in the Haiyuan arcuate tectonic region are the result of interaction between the Tibetan plateau and the Sino-Korean and Gobi Ala Shan platforms. Upper-mantle velocity structure of the lower Great Lakes regionPages 267-281 The lithospheric root beneath North America contains a prominent indentation beneath the lower Great Lakes region that is approximately aligned with the track of the New England seamounts. By combining data from the recently installed POLARIS network in southern Ontario, Canada with data acquired in 1996 during the Abitibi-Grenville teleseismic experiment, we have performed a tomographic inversion using 4543 P-wave traveltimes from 213 events (5.0=mb=6.6), and 1860 S-wave traveltimes from 98 events (5.0=mb=6.6), to obtain high-resolution images of the upper mantle beneath the lower Great Lakes. Two salient features of the 3-D models are: 1) a patchy, NNW-trending low-velocity region, and 2) a linear, NE-striking high-velocity anomaly. S-wave images show that the low-velocity anomaly changes from an arcuate feature at 400-km depth, to a NW-striking linear feature at 100-km depth beneath the Neoproterozoic Ottawa-Bonnechere graben. The linear high-velocity anomaly extends to at least 300-km depth and strikes parallel to surface geological belts and the Laurentian continental margin. We interpret the high-velocity anomaly as a possible relict slab associated with ca. 1.35-1.3 Ga subduction beneath the Composite Arc Belt, whereas the low-velocity anomaly is interpreted as a zone of alteration and metasomatism associated with the ascent of magmas that produced the Late Cretaceous Monteregian plutons. Our data support an interpretation of these plutons as melts generated by the passage of North America across a mantle plume, rather than a far-field response to opening of the North Atlantic. Lithological interpretation of crustal composition in the Fennoscandian Shield with seismic velocity dataPages 283-299 In this study, we report the results of an investigation of lithological interpretation of the crust in the central Fennoscandian Shield (in Finland) using seismic wide-angle velocity models and laboratory measurements on P- and S-wave velocities of different rock types. The velocities adopted from wide-angle velocity models were compared with laboratory velocities of different rock types corrected for the crustal PT conditions in the study area. The wide-angle velocity models indicate that the P-wave velocity does not only increase step-wise at boundaries of major crustal layers, but there is also gradual increase of velocity within the layers. On the other hand, the laboratory measurements of velocities indicate that no single rock type is able to provide the gradual downward increasing trends. Thus, there must be gradual vertical changes in rock composition. The downward increase of velocities indicates that the composition of the crust becomes gradually more mafic with increasing depth. We have calculated vertical velocity profiles for a range of possible crustal lithological compositions. The Finnish crustal velocity profiles require a more mafic composition than an average global continental model would suggest. For instance, on the SVEKA'81 transect, the calculated models suggest that the crustal velocity profiles can be simulated with rock type mixtures where the upper crust consists of felsic gneisses and granitic-granodioritic rocks with a minor contribution of amphibolite and diabase. In the middle crust, the amphibolite proportion increases. The lower crust consists of tonalitic gneiss, mafic garnet granulite, hornblendite, pyroxenite and minor mafic eclogite. Assuming that these rock types are present in sufficiently extensive and thick layers, they would also have sufficiently high acoustic reflection coefficients for generating the generally well-developed reflectivity in the crust in the central part of the shield. Density profiles calculated from the lithological models suggest that there is practically no density contrast at Moho in areas of the high-velocity lower crust. Comparison of reflectors from FIRE-1 and FIRE-3 transects and the velocity model from SVEKA'81 wide-angle transect indicated that the reflectors correlate with velocity layering, but the three-dimensional structures of the crust complicate such comparisons. Lessons from a joint interpretation of vibroseis wide-angle and near-vertical reflection data in the northeastern Yilgarn, Western AustraliaPages 301-316 A wide-angle reflection seismic experiment was carried out in the Eastern Goldfields granite-greenstone terrane of the Archaean Yilgarn Craton during 2001. This was the first time in Australia that wide-angle data were collected using a vibrator source and with a high density of observations. Unlike other wide-angle surveys carried out in other parts of the world, our survey used both a smaller number of sweeps, and shorter sweeps. We recorded three sweeps (each with its own frequency range) at each vibration point. The experiment demonstrated that the sum of three 12 s sweeps using 3 large vibrators provides enough energy to record signal at offsets up to up to 60-70 km. A comparison of individual shot gathers from near-vertical data and receiver gathers from wide-angle data demonstrated higher reflectivity in near-vertical data. This may be due to differences in the frequency bands of the recording equipment. The after stack section obtained from dense wide-angle data is different from that obtained from conventional near-vertical reflection data. The conventional reflection section provides higher quality image of the crust compared to the wide-angle section. This could be explained by the low-fold in wide-angle data and differences in the acquisition and processing methodology. The wide-angle survey, which was coincident with a regional vibroseis seismic reflection transect, was focused on the Leonora-Laverton region. The survey was designed to supplement the deep seismic reflection studies with velocity information. This also created an opportunity to compare velocity model derived from wide-angle reflection seismic data with a structural image obtained from the deep common mid-point seismic reflection data, and thus refine our geological understanding of the area. A high velocity body reaching a maximum thickness of 2 km was identified exclusively from the seismic velocity model derived from wide-angle study. This body is interpreted as mafic rocks within the Archaean Granite-Greenstone Belt. The joint interpretation also shows that structural boundaries do not always follow lithological boundaries in our study area. The combination of wide-angle reflection and near-vertical reflection data has facilitated a more complete geological interpretation of the seismic data. Effects of trench-zone scattering on receiver functions over a subduction zone: A 3-D finite-difference modelling studyPages 317-332 As Morozov [Morozov, I. B. (2004). Crustal scattering and some artefacts in receiver function images. Bull. Seismol. Soc. Am., 94 (4), 1492-1499.] suggested, for a teleseismic array targeting subducting crust in a zone of active subduction, scattering from the strong horizontal velocity heterogeneity beneath the trench zone itself produces subhorizontally-propagating waves that should be observed as coherent dipping events in receiver functions (RF). Due to similar RF delay times and moveouts, these events could be difficult to distinguish from backscattered P- and S-wave modes. To further verify this suggestion, we performed a full-waveform, 3-D visco-elastic finite-difference modelling of teleseismic wave propagation within a simplified model of a subduction zone. The synthetics show strong scattering from the area beneath the trench, dominated by the mantle and crustal P-waves propagating at 6.2-8.1 km/s and slower. These scattered waves occupy the same time and moveout intervals as the backscattered converted modes, and also have similar amplitudes. Although their amplitude decay characters are different, the uncertainty in the knowledge of the velocity and density structure of the subduction zone could make distinguishing between these modes difficult. However, under minimal assumptions, recent observations of receiver function amplitudes decreasing away from the trench support the interpretation of (sub-) trench-zone scattering. Modeling sideswipe in 2D oceanic seismic surveys from sonar data: Application to the Mariana arcPages 333-343 In two-dimensional (2D) marine seismic-reflection surveys, out-of-plane rough seafloor bathymetry can cause multiple ocean-bottom reflections that complicate the interpretation of shallow reflections. Although migration corrects for the in-plane position of reflectors, it cannot resolve the inherent ambiguity in their out-of-plane positions. We show how swath bathymetry, routinely collected in many such surveys, can be used to model out-of-plane seafloor reflections and prevent their misinterpretation as subsurface geology. We use both raw and gridded multi-beam bathymetry data to build images that represent seafloor reflections in migrated seismic data. Comparison of these images to the seismic sections reveals whether suspicious features are out-of-plane water bottom reflections or subsurface reflections. Multi-channel seismic surveys across the Marianas intra-oceanic arc system provide examples where rough seafloor topography produced reflections that were initially misinterpreted. We use our seafloor reflection modeling (SRM) approach to help distinguish a possible landslide from a volcanic cone, to help distinguish real from apparent fault-plane reflections bounding a sediment-filled basin, and to verify that a possible magma chamber reflection results from sub-surface structure, not seafloor sideswipe. Although still limited in its representation of crustal heterogeneity, 3-D modelling suggests that scattering from near-Moho crustal structures plays a key role in the formation of teleseismic wavefields. Recognition of scattered noise in teleseismic records could help to constrain major crustal structures, particularly those with strong horizontal velocity contrasts at near-Moho depths, such as crustal sutures, subduction fault zones, and mountain roots. Matching of the observed arrivals with wavefield synthetics could help constrain the locations and parameters of such structures and also help substantiate the interpretations. |
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