The crustal structure of the Guayana Shield, Venezuela, from seismic refraction and gravity data

We present results from a seismic refraction experiment on the northern margin of the Guayana Shield performed during June 1998, along nine profiles of up to 320 km length, using the daily blasts of the Cerro Bolívar mines as energy source, as well as from gravimetric measurements. Clear Moho arrivals can be observed on the main E–W profile on the shield, whereas the profiles entering the Oriental Basin to the north are more noisy. The crustal thickness of the shield is unusually high with up to 46 km on the Archean segment in the west and 43 km on the Proterozoic segment in the east. A 20 km thick upper crust with P-wave velocities between 6.0 and 6.3 km/s can be separated from a lower crust with velocities ranging from 6.5 to 7.2 km/s. A lower crustal low velocity zone with a velocity reduction to 6.3 km/s is observed between 25 and 25 km depth. The average crustal velocity is 6.5 km/s. The changes in the Bouguer Anomaly, positive (30 mGal) in the west and negative (−20 mGal) in the east, cannot be explained by the observed seismic crustal features alone. Lateral variations in the crust or in the upper mantle must be responsible for these observations.     

Mekometer measurements in the Imperial Valley, California

A network of about 130 bench marks, mostly about 800 m apart at road intersections and forming a continuous chain of quadrilaterals crossing the Imperial Valley in the vicinity of El Centro, has been measured three times, in 1971, 1973 and 1975. The instrument used was a Mekometer, a superior short-range electronic-distance-measuring (EDM) instrument having a sensitivity of 0.1 mm and a potential accuracy of better than 1 ppm. In 1975 the network was extended, particularly where it crosses the Imperial fault, to about 200 bench marks defining about 500 lines.For various reasons, it has not been possible to achieve standard errors of much less than 3 ppm in the distance measurements. Even so, about one line in three shows significant change at the 95% level in one or more of the three intervals 1971–1973, 1973–1975 and 1971–1975. Adopting stringent consistency criteria, the number of places where significant changes occurred can be reduced to seven. One of these is almost certainly associated with the Heber geothermal anomaly, one with the Imperial fault, and the others with one or other of the faults identified on the ground further to the north. The largest and most consistent changes occur on the Imperial fault, and imply an aseismic slip on it averaging about 8 mm/yr.The data also lend themselves to regional strain analysis. The results are consistent with a linear strain rate of about 0.3 ppm/yr, which is comparable with the rate deduced from large-scale geodetic surveys of the area.     

A three-dimensional crustal model of southwest Germany derived from seismic refraction data

From densely covered seismic refraction data obtained in 1978 (Urach experiment) and 1984 (“Schwarzer Zollern-Wald” experiment) and from seismic reflection data and results from previous refraction investigations, a three-dimensional crustal model of southwest Germany was derived. Travel-time and amplitude information of seismic refraction data were interpreted with two-dimensional forward modeling (ray tracing) to calculate two crustal cross sections in southwest Germany. These results fill a gap in the existing data and enabled the construction of a detailed three-dimensional crustal model.While seismically the upper crust is laterally homogeneous (5.9–6.0 km/s) throughout the area, the middle and lower crust show pronounced lateral variations in thickness, velocity, and reflectivity. The Moho is a flat surface at a relatively shallow depth (25–26 km). We classify the middle and lower crust of southwest Germany into two characteristic crustal types. Type I consists of a mid-crustal low-velocity zone (5.4–5.8 km/s) overlying a thick (> 10 km), high-velocity (6.6–6.8 km/s) lower crust. Type II has no prominent mid-crustal low-velocity zone, and a thin (< 10 km), low-velocity (6.3–6.4 km/s) lower crust. The crustal types correlate with the major geologic units exposed in the area: Type I is present beneath the Black Forest, forming the eastern flank of the Rhinegraben and beneath the Swabian Jura, while Type II is present beneath the intervening Triassic sediments. Beneath the South German Molasse Basin, a low-velocity zone is also present in the upper middle-crust. Seismic reflection investigations have shown that the lower crust in southwest Germany comprises a stack of layers of alternating high- and low-velocities. The lateral variation of the reflectivity of this laminated lower crust has been recognized even on refraction data. We found that high-reflectivity of the lower crust correlates to high average velocity (6.7–6.8 km/s) in the lower crust (Type I). Thus, the average velocity of the lower crust in southwest Germany seems to be an indicator of the intensity of its lamination. The uppermost mantle has a velocity of 8.3 km/s in the area and a strong, positive velocity gradient.     

Controls on forearc basin architecture from seismic and sequence stratigraphy of the Upper Cretaceous Great Valley Group,central Sacramento Basin,California

Seismic and sequence stratigraphic analysis of deep-marine forearc basin fill (Great Valley Group) in the central Sacramento Basin, California, reveals eight third-order sequence boundaries within the Cenomanian to mid-Campanian second-order sequences. The third-order sequence boundaries are of two types: Bevelling Type, a relationship between underlying strata and onlapping high-density turbidites; and Entrenching Type, a significantly incised surface marked by deep channels and canyons carved during sediment bypass down-slope. Condensed sections of hemipelagic strata draping bathymetric highs and onlapped by turbidites form a third important type of sequence-bounding element, Onlapped Drapes. Five tectonic and sedimentary processes explain this stratigraphic architecture: (1) subduction-related tectonic tilting and deformation of the basin; (2) avulsion of principal loci of submarine fan sedimentation in response to basin tilting; (3) deep incision and sediment bypass; (4) erosive grading and bevelling of tectonically modified topography by sand-rich, high-density turbidite systems; and (5) background hemipelagic sedimentation. The basin-fill architecture supports a model of subduction-related flexure as the principal driver of forearc subsidence and uplift during the Late Cretaceous. Subduction-related tilting of the forearc and growth of the accretionary wedge largely controlled whether and where the Great Valley turbiditic sediments accumulated in the basin. Deeply incised surfaces of erosion, including submarine canyons and channels, indicate periods of turbidity current bypass to deeper parts of the forearc basin or the trench. Fluctuations in sediment supply likely also played an important role in evolution of basin fill, but effects of eustatic fluctuations were overwhelmed by the impact of basin tectonics and sediment supply and capture. Eventual filling and shoaling of the Great Valley forearc during early Campanian time, coupled with dramatically reduced subsidence, correlate with a change in plate convergence, presumed flat-slab subduction, cessation of Sierran arc volcanism, and onset of Laramide orogeny in the retroarc.     

VRANCEA99—the crustal structure beneath the southeastern Carpathians and the Moesian Platform from a seismic refraction profile in Romania

The VRANCEA99 seismic refraction experiment is part of an international and multidisciplinary project to study the intermediate depth earthquakes of the Eastern Carpathians in Romania. As part of the seismic experiment, a 300-km-long refraction profile was recorded between the cities of Bacau and Bucharest, traversing the Vrancea epicentral region in NNE–SSW direction.

The results deduced using forward and inverse ray trace modelling indicate a multi-layered crust. The sedimentary succession comprises two to four seismic layers of variable thickness and with velocities ranging from 2.0 to 5.8 km/s. The seismic basement coincides with a velocity step up to 5.9 km/s. Velocities in the upper crystalline crust are 5.9–6.2 km/s. An intra-crustal discontinuity at 18–31 km divides the crust into an upper and a lower layer. Velocities within the lower crust are 6.7–7.0 km/s. Strong wide-angle PmP reflections indicate the existence of a first-order Moho at a depth of 30 km near the southern end of the line and 41 km near the centre. Constraints on upper mantle seismic velocities (7.9 km/s) are provided by Pn arrival times from two shot points only. Within the upper mantle a low velocity zone is interpreted. Travel times of a PLP reflection define the bottom of this low velocity layer at a depth of 55 km. The velocity beneath this interface must be at least 8.5 km/s.

Geologic interpretation of the seismic data suggests that the Neogene tectonic convergence of the Eastern Carpathians resulted in thin-skinned shortening of the sedimentary cover and in thick-skinned shortening in the crystalline crust. On the autochthonous cover of the Moesian platform several blocks can be recognised which are characterised by different lithological compositions. This could indicate a pre-structuring of the platform at Mesozoic and/or Palaeozoic times with a probable active involvement of the Intramoesian and the Capidava–Ovidiu faults. Especially the Intramoesian fault is clearly recognisable on the refraction line. No clear indications of the important Trotus fault in the north of the profile could be found. In the central part of the seismic line a thinned lower crust and the low velocity zone in the uppermost mantle point to the possibility of crustal delamination and partial melting in the upper mantle.     

Comparison of crustal velocity profiles determined by seismic refraction and teleseismic methods

Recently, two diverse seismic techniques were applied independently to the study of the crustal structure of the Cumberland Plateau, eastern Tennessee. One involved a reinterpretation of a refraction experiment performed in 1965 by the U.S. Geological Survey, consisting of two 400 km long, reversed refraction lines. The other entailed the inversion of broadband teleseismic P waveforms recorded at a single three-component broadband station, RSCP, located at the intersection of the two refraction profiles. A comparison of the two sets of velocity profiles revealed many similarities and some significant differences. Both sets of velocity models consist of three major crustal layers: (1) an upper crust (V_p = 6.1–6.4 km/s) down to about 17 km, (2) a mid-crust (V_p = 6.7–6.9 km/s) between 17 and 40 km depth, (3) a lower crust (V_p = 7.2–7.4 km/s) from 40 to 51 km depth. The refraction models have linear transition zones up to 11 km thick at the base of each layer, whereas the teleseismic models have more irregular transition zones at the base of the mid- and lower crust. The differences in the results of these studies are attributed to the differing frequency bandwidths of the data sets; the predominant sensitivity of the teleseismic data to shear velocities, compared to compressional velocities for the refraction data; and the different analysis procedures involved in each method. Nevertheless, the similarities indicate that the teleseismic waveform method with broadband data is capable of retreiving comparable crustal information as the Cumberland Plateau refraction survey. In addition, it provides the kind of complementary information required to constrain the composition of the continental lower crust and uppermost mantle.     

The structure of the upper crust in the Alps-Apennines boundary region deduced from refraction seismic data

Analysis of data gathered during the 1983 European Geotraverse southern segment (EGT-S '83) experiments in the region extending from the Emilia-Liguria Apennines to the western Alpine Arc together with data from seismic profiles in the northwestern Apennines accumulated within the framework of the Alps-Apennines Orogene Study Group indicate new details on the structure of the upper crust east and west of the Alps-Apennines boundary.The main results of this analysis centre on two areas. In the Piedmont Tertiary Basin we could determine the depocenter configurations of the 6–7 km thick terrigenous sequence and differentiate the tectonic units in the Piedmont (Alpine) and the Ligurian (Apennine) domains within the basement. In the other area, the Insubric domain underneath the Ligurian nappes of the northern Apennines, we found indications of tectonic doubling within the terrigenous-carbonate sequence in which thrusting attenuates towards the underlying basement, detected at a depth of 12–15 km. In addition, we found that, on a line from the Emilia Apennines to the Monferrato Hills, displacement of the Ligurian nappes over the Insubric domain diminishes to nearly one-third its original extent.     

Isotopic geochemistry of burial-metamorphosed volcanogenic sediments,Great Valley sequence,northern California

Isotopic and mineralogic data from an 8500-m thick section of the Great Valley sequence, northern California, indicate that changes in the δ¹⁸O values of authigenic minerals resulted from the conversion of smectite to a 10 Å clay-mineral as temperature increased with burial in the Jurassic- Cretaceous outer-arc basin. The clay-mineral assemblage in mudstone is characterized by a proportional increase of the 10 Å clay-mineral with increasing stratigraphic depth, and by a depletion in the δ¹⁸O value of the mixed-layer smectite/10 Å clay-mineral with descending stratigraphic position from +21.9 to + 15.5%. SMOW. Modeling of the oxygen isotopic data from authigenic phases, based on equilibrium fractionation during clay-mineral diagenesis, indicates that δ¹⁸O values of calcite in mudstones and of calcite cements in sandstone precipitated along a temperature gradient of about 25°C/km during maximum burial to about 6–7 km. δD values of the mixed-layer smectite/10 Å clay-mineral range between ?69 to ?44%. SMOW. Using temperatures calculated from the oxygen isotopic data, the deuterium and oxygen isotopic data indicate that the smectite underwent late-stage dehydration and probably buffered the composition of formation waters from sea water values to isotopic compositions of δ¹⁸O ≈ +8%. SMOW and δD ≈ ?25%. SMOW. The δ¹³C values of calcite from mudstone and sandstone imply that crystallization of authigenic calcite was linked to organic diagenesis during which dissolved HCO^t-₃ was continuously enriched in ¹³C as temperature increased with burial. At the base of the sequence and immediately overlying the ophiolitic basement rocks, several hundred meters of strata were altered by more oxygen-depleted (δ¹⁸O ? +4 to +5%.) hydrothermal fluids emanating from the ophiolitic rocks, probably at maximum burial depth.     

The crustal structure of the western Po plain: reconstruction from integrated geological and seismic data

A reflection/refraction seismic experiment performed in 1991 in the western Po plain gave basic data to constrain the interpretation of the crustal structures across the Alps/Apennines junction zones. Two different seismic domains, north and south of the western supposed prosecution of the Villalvernia-Varzi line, are evidenced from the interpretation of the data. The boundary between the two domains is characterized by strong lateral variations from southern high to northern low velocity layers. The northward abrupt deepening of the refractor/reflector basement is followed at depth by a similar deepening of the crust/mantle boundary. The geological interpretation evidences domains with coherent and independent evolution at surface level juxtaposed along oblique discontinuities cutting across the crust. A peculiar feature is the presence of both crust and mantle north-verging wedges into the crustal structure and the overthrust at depth of the 'alpine' metamorphic crust onto the 'apenninic' nappes (Monferrato region).     

Deep seismic refraction and gravity crustal model and tectonic deformation in Tocantins Province, Central Brazil

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.     

Submarine fan facies of the Upper Cretaceous Great Valley sequence,northern and central California

The Upper Cretaceous part of the Great Valley Sequence provides a unique opportunity to study deep-marine sedimentation within an arc-trench gap. Facies analysis delineates submarine fan facies similar to those described from other ancient basins. Fan models and facies of Mutti and Ricci-Lucchi allow reconstruction of the following depositional environments: basin plain, outer fan, midfan, inner fan, and slope. Basin plain deposits are characterized by hemipelagic mudstone with randomly interbedded thin sandstone beds exhibiting distal turbidite characteristics. Outer fan deposits are characterized by regularly interbedded sandstone and mudstone, and commonly exhibit thickening-upward (negative) cycles that constitute depositional lobes. The sandstone occurs as proximal to distal turbidites without channeling. Midfan deposits are characterized by the predominance of coarse-grained, thick, channelized sandstone beds that commonly are amalgamated. Thinning-upward (positive) cycles and braided channelization also are common. Inner fan deposits are characterized by major channel-fill complexes (conglomerate, pebbly sandstone, and pebbly mudstone) enclosed in mudstone and siltstone. Positive cycles occur within these channel-fill complexes. Much of the fine-grained material consists of levee (overbank) deposits that are characterized by rhythmically interbedded thin mudstone and irregular sandstone beds with climbing and starved ripples. Slope deposits are characterized by mudstone with little interbedded sandstone; slumping and contortion of bedding is common. Progressions of fan facies associations can be described as retrogradational and progradational suites that correspond, respectively, to onlapping and offlapping relations in the basin. The paleoenvironments, fan facies associations, and tectonic setting of the Late Cretaceous fore-arc basin are similar to those of modern arc—trench systems.     

The velocity structure of the Cariaco sedimentary basin,northeastern Venezuela,from refraction seismic data and possible relationship to earthquake hazard

The main damage from the July 9, 1997, Cariaco earthquake (Ms=6.8) was concentrated in the town of Cariaco and surrounding villages, which are located in the western part of the Cariaco sedimentary basin, close to the Gulf of Cariaco. Casanay, located at the eastern end of the sedimentary basin, suffered considerably less damage. The El Pilar fault, a right-lateral strike-slip fault that generated the earthquake, runs parallel to the southern border of the valley and crosses both towns. The determination of the velocity structure of the basin is the main objective of this study. Seismic refraction data were recorded along three lines, one of them along-strike and two perpendicular to the valley axis in the northern and southern bedrocks. Beneath Cariaco, approximately 1 km thick Quaternary sediments with seismic velocities of 1.9–2.1 km/s and bedrock velocities of more than 4 km/s were observed. The thickness of the Quaternary sediments varies within the basin, and Pleistocene sediments outcrop beneath Casanay. The increased thickness of the unconsolidated, water-saturated Quaternary sediments, together with the difference in the quality of buildings prior to the earthquake, probably is responsible for the damage pattern of the Cariaco earthquake.     

3-D upper crustal tomographic structure across the Vrancea seismic zone, Romania

The VRANCEA99 and VRANCEA2001 seismic refraction experiments are part of a multidisciplinary project to study the Eastern Carpathians in Romania. The objectives of these studies are intended to disclose a more detailed picture of the crustal and upper mantle structures above the seismically active Vrancea region. In this paper we provide additional constraints for the upper crustal structures of the area. The 1999 campaign consisted of a 320-km-long N–S profile and a 70-km-long E–W profile. The intersecting 2001 profile extended in E–W direction from the Hungarian border to the Black Sea. In order to enhance the model resolution, first arrival data from local crustal earthquakes were also included.This configuration allowed for the first time to derive a 3-D velocity model for the upper crust of the Romanian Carpathian Orogen, within a 115×235 km wide region, centred over the Vrancea seismic zone. The 3-D model reveals lateral velocity variations, which were not visible on the in-line interpretations. It allows us to distinguish between foreland platform areas, foreland basins and the Carpathian Orogen. Clear velocity differences between the foreland basins south and southeast of the Eastern Carpathians and the Focsani Basin further north indicate different pre-Miocene sedimentary compositions and geological evolutions of these foreland platforms. The involved Moesian and Scythian platforms are separated by the Trotus Fault system, which is observed as a velocity discontinuity. An upper crustal high-velocity zone, above the northern Vrancea seismic zone, could also be identified. This high-velocity zone is explained by a Middle Pliocene to Pleistocene E–W oriented out-of-sequence thrust of the crystalline basement, below the decollement of the flysch nappes.     

The determination of joint directions by seismic refraction measurements at the Avedat plateau (Israel)

A seismic refraction experiment was conducted in an area at a site where joints had been mapped in detail in order to determine whether velocity anisotropy could be correlated with joint directions. The purpose was to assess the possibility of using seismic refraction techniques for mapping joint directions in alluvium-covered areas.The results of the experiment show that a velocity anisotropy can be detected by simple seismic refraction measurements, and that the anisotropy can be correlated with the dominant joints. It is believed that the technique is applicable to the determination of joint directions in alluvium-covered areas.     

Geophysical interpretation of satellite laser ranging measurements of crustal movement in California

As determined by satellite laser ranging the rate of contraction of a 900 km baseline between sites located near Quincy in northern California and San Diego in southern California is about 61–65 mm/yr with a formal uncertainty of about 10 mm/yr (Christodoulidis et al., 1985). The measured changes in baseline length are a manifestation of the relative motion between the North America and Pacific tectonic plates. This long baseline result is compared to measurements made by more conventional means on shorter baselines. Additional information based on seismiscity, geology, and theoretical modelling is also analyzed. Deformation lying within a few tens of kilometers about the major faults in southern California accounts for most, but not all, of the observed motion. Further motion is attributable to a broader-scale deformation in southern California. Data suggesting crustal movements north of the Garlock fault, in and near the southern Sierra Nevada and local motion at an observatory are also critically reviewed. The best estimates of overall motion indicated by ground observations lie between 40 and 60 mm/yr. This lies within one or two standard deviations of that deduced from satellite ranging but the possibility of some unresolved deficit cannot be entirely dismissed. The long time scale RM2 plate tectonic model of Minster and Jordan (1978) predicts a contraction between 47 and 53 mm/yr depending on the extension rate of the Basin and Range. Thus the ground based observations, SLR results, and RM2 rates differ at about the 10 mm/yr level but are not inconsistent with one another within the data and model uncertainties.     

The major features of the crustal structure in north-eastern Venezuela from deep wide-angle seismic observations and gravity modelling

Venezuela is located on the plate boundary zone between the South American continent and the Caribbean plate. A relative movement of 2 cm/year is accommodated by a system of strike–slip faults running from the Andes to the Gulf of Paria. The Interior Range, a moderate-height mountain range, separates the Oriental Basin from the Caribbean. To the south, predominantly Precambrian rocks are outcropping in the Guayana Shield south of the Orinoco River. Results of deep wide-angle seismic measurements for the region were obtained during field campaigns in 1998 (ECOGUAY) for the Guayana Shield and in 2001 (ECCO) for the Oriental Basin. The total crustal thickness decreases from 45 km beneath the Guayana Shield, to 39 km at the Orinoco River, and 36 km close to El Tigre, in the center of the Oriental Basin. The average crustal velocity decreases in the same sense from 6.5 to 5.95 km/s. Detailed information was obtained on the velocity distribution within the Oriental Basin. Velocities are as low as 2.2 km/s for the uppermost 2 km, 4.5 km/s down to 4 km in depth, and a maximum depth of 13 km was derived for material with seismic velocities up to 5.9 km/s, interpreted as the base of the sedimentary basin. A gravimetric model confirms the structures derived from the seismic data. Discrete increases in sedimentary thickness along the basin may be associated to extension processes during the passive margin phase in the Cretaceous, or during earlier extension phases.     

地球深部探测的先锋: 深地震反射方法的发展与应用

深地震反射剖面早已被国际上证实是一种地球深部探测的先锋技术,它利用比石油地震勘探更长的接收排列、更大的激发能量,探测上至地表,下达上地幔的精细结构和构造特征,现已被国内外越来越多地应用于大陆及海洋地壳与岩石圈上地幔探测上。深地震反射剖面技术在揭示地球深部动力学过程,论述大地构造演化,确定盆山耦合关系,推测成矿成藏条件,分析地震灾害等方面取得了丰硕成果。系统地梳理和概括了国内外陆地地区深地震反射剖面技术的研究现状以及典型应用案例。在此基础上,从深地震反射野外数据采集、数据资料处理、剖面地质解译以及多方法联合探测4个方面,对地震反射剖面技术未来发展方向进行了展望。研究认为：在野外采集方面,研发不同激发、接收组合类型的采集技术、提升深地震反射数据采集质量,减少环境破坏,降低经济成本;在数据处理中,继续探究提高深地震反射剖面的信噪比与分辨率,定量监控深地震反射数据处理过程中振幅保真度的相对变化;在资料解译时,将深入挖掘深地震反射剖面的叠前、叠后潜在信息,降低单纯依靠深地震反射振幅解译的非唯一性;在综合研究上,从多学科、多角度、多尺度、多方法上相互补充印证,降低剖面的多解性,提高成果的准确性和可靠度。

     

Upper and middle crustal deformation of an arc–arc collision across Hokkaido, Japan, inferred from seismic refraction/wide-angle reflection experiments

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.     

Crustal structure of the Hawaiian Archipelago, northern Melanesia, and the Central Pacific Basin by seismic refraction methods

The crustal structure of the Hawaiian Archipelago, northern Melanesia, and parts of the Central Pacific Basin have been studied by seismic refraction methods. The systematic variation found in crustal thickness in the Hawaiian Islands is explainable by a hypothesis of differential subsidence. The crustal structure of northern Melanesia points to tensional forces in an east-west direction and compressional forces in a north-south direction. In the Central Pacific Basin, a 7.4 km/sec layer in the lower crust seems to be present over a wide area.