Geological Belts Research Articles

Gravity and magnetism of southern Africa in relation to Craton structures and belts

The general tectonic structural architecture of southern Africa is an ensemble comprised of Kalahari Craton, a vast range of mosaics of the best-preserved geological Belts, and exposed crustal blocks. In the region, demarcation of geological boundaries, tracing of magnetic and gravitational bodies, and detrending lineaments are essential to understanding the structural limits. The study presents the prevalent gravitational and magnetic investigation of major geological Belts, enhancing edges of Cratons, and evaluation of surface invariants to improve the geological understanding and evaluate the correlation of structures with the general structural tectonic framework. The utilized filter methods in the gravity maps are the Directional derivatives along x, y, and z-directions, the Modulus tensor, the Total horizontal derivative, and the Tilt angle techniques.The phase-based filters used in the magnetic section include the Total horizontal derivative, Analytical signal, Tilt angle method, Tilt of total horizontal derivative, Theta method, Horizontal tilt angle method, Enhanced tilt filter, and Enhanced total derivative of the tilt angle. The Directional derivatives of the gravity field and the Modulus of the gravity gradient tensor demarcate the boundaries of geological structures. The Total horizontal derivative method gives an immediate and easy-to-read image of the linear structures and fault systems. The signal cluster on the Analytical signal and Tilt angle maps delineate the boundaries of causative geological sources. The Theta map is comparable with the enhanced total derivative of the tilt angle. The Tilt of the total horizontal derivative, Theta map, and Enhanced tilt filter accentuate the traces of magnetic bodies, including the Cratons. The study finds regional lineaments surrounding the Cratons and concentrated along the geological Belts. The approach of using joint elegant filters is effective and increases the certainty of the interpretation.

Uncovering Hidden Microbial Diversity in Nitrate/Iodide Deposits (NIDs) in the Domeyko District, Atacama Desert, Chile

Unique worldwide, nitrate/iodine deposits (NIDs) are located along a 700 km geological belt in the Atacama Desert, Chile. They serve as the primary source of mineral ores for the extraction of iodine, sodium, and potassium nitrates. NIDs have been relatively underexplored from a biological perspective. To address this, we collected sixteen soil samples from abandoned mines in Oficinas Pissis and Savona for chemical, mineralogical, and metagenomic analyses. The soils primarily consisted of halite and darapskite, with only one sample being predominantly composed of thenardite. Deliquescence and water activity measurements yielded values ranging from 0.02% to 0.40% and 0.47 to 0.62, respectively. To investigate the presence, identification, relative abundance, and diversity of microbial life in NID soils, we employed MiSeq high-throughput sequencing and bioinformatic tools. The dominant phyla observed were Firmicutes and Proteobacteria, with Actinobacteria and Cyanobacteria being predominant in two soil samples. Furthermore, we detected nitrate/perchlorate-reducing bacterial activity in enriched cultures from the soil samples. This study sheds light on the resilience of microbial life in the Atacama Desert NIDs, providing compelling evidence for its existence and offering insight into factors that could facilitate it within this unique environment.

Cretaceous to early Paleogene sediment provenance transition from continental to magmatic arc systems in the Northwestern Pacific Region

Unraveling the Paleo-Kuril Arc's origins is key to understanding northwest Pacific tectonics. The Paleo-Kuril Arc is viewed as an intraoceanic arc from trench subduction between the Izanagi and Pacific Plates. Alternatively, several studies suggest the Paleo-Kuril Arc as a continental magmatic arc, hypothesizing the existence of a mid-oceanic ridge and Paleogene subduction, placing the Paleo-Kuril Arc near the Okhotsk Block's southern edge. This study clarifies these hypotheses, previously clouded by limited geochronological data on deposits in the Paleo-Kuril Arc. We conducted U–Pb dating to examine the origins of detrital zircons from the Cretaceous–Paleogene Tokoro and Nemuro Belts of the Paleo-Kuril Arc. Cluster analysis, merging new and existing data, identified two unique detrital zircon age clusters. The abundance of Precambrian zircons in Cretaceous–Paleocene Paleo-Kuril Arc sandstones (Type 1 Cluster) suggests a continental magmatic origin, supporting the ridge subduction model. However, an early Eocene shift to a consistent local volcanic source (Type 2 Cluster) highlights a significant provenance change. This geochronological evidence, indicating a separation from continental sources, calls for further research to decode the simultaneous development of sediment sources in different geological belts, potentially tied to the ridge subduction event.

A pilot bioavailable strontium isotope baseline map of Southern British Columbia, Canada

Strontium isotopes are used for provenience and mobility studies in archaeology, ecology, and forensic studies, and rely on accurate baseline maps that are used to compare and interpret human and animal strontium ratios. Here, we present a bioavailable 87Sr/86Sr map, also called an isoscape, for southern British Columbia derived from modern plant samples’ 87Sr/86Sr ratios. We sampled 67 medium root depth plants over a 900 km transect from the southern BC coast to inland BC to capture the natural 87Sr/86Sr ratios of plants along the four major geological belts in British Columbia. Non-parametric Kruskal–Wallis and pairwise Wilcox tests were used to examine whether the geological belts had statistically significant mean differences. It was found that the province could be effectively divided into east and west, with the Coastal–Intermontane and Omineca–Foreland regions having statistically different means from each other. 87Sr/86Sr ratios had statistically significant relationships with salt deposition, volcanic deposition, and mean age of the underlying lithology. Generally, 87Sr/86Sr ratios increased with distance from the coast as the atmospheric input of radiogenic strontium from marine-derived rainwater decreased and the input of radiogenic strontium isotopes from the underlying geology of the Rockies in the far east of the province increased.

Zinc on the edge—isotopic and geophysical evidence that cratonic edges control world-class shale-hosted zinc-lead deposits

The North Australian Zinc Belt is the largest zinc-lead province in the world, containing three of the ten largest known individual deposits (HYC, Hilton-George Fisher, and Mount Isa). The Northern Cordillera in North America is the second largest zinc-lead province, containing a further two of the world’s top ten deposits (Red Dog and Howards Pass). Despite this world-class endowment, exploration in both mineral provinces during the past 2 decades has not been particularly successful, yielding only two significant discoveries (Teena, Australia, and Boundary, Canada). One of the most important aspects of exploration is to choose mineral provinces and districts within geological belts that have the greatest potential for discovery. Here, we present results from these two zinc belts that highlight previously unused datasets for area selection and targeting. Lead isotope mapping using analyses of mineralized material has identified gradients in μ (238U/204Pb) that coincide closely with many major deposits. Locations of these deposits also coincide with a gradient in the depth of the lithosphere-asthenosphere boundary determined from calibrated surface wave tomography models converted to temperature. Furthermore, gradients in upward-continued gravity anomalies and a step in Moho depth correspond to a pre-existing major crustal boundary in both zinc belts. A spatial association of deposits with a linear mid- to lower-crustal resistivity anomaly from magnetotelluric data is also observed in the North Australian Zinc Belt. The change from thicker to thinner lithosphere is interpreted to localize prospective basins for zinc-lead mineralization and to control the gradient in lead isotope and geophysical data. These data, when combined with data indicative of paleoenvironment and changes in plate motion at the time of mineralization, provide new exploration criteria that can be used to identify prospective mineralized basins and define the most favorable parts of these basins.

Key driving factors of selenium-enriched soil in the low-Se geological belt: A case study in Red Beds of Sichuan Basin, China

Selenium (Se) is an essential micronutrient for humans given its varying health benefits. It is generally recognized that China has a wide belt of low-Se soil stretching from the northeast to southwest. Nevertheless, there are Se-enriched areas distributed in the low-Se belt of China. However, the quantificational relationships among soil properties, topographic characteristics, parent materials, land use and soil Se content in those Se-enriched soils remain to be elucidated. Similarly, the key driving factors of the Se-enriched soil in the low-Se geological belt need to be documented. These aims could be an useful basis for evaluating the health of the soil ecosystem (in terms of Se toxicity or deficiency) and the potential intake of Se by humans from soils to food crops and animal products. To solve the above questions, Jiangjin district, Chongqing City, as an area in low Se red beds, was selected, and 156 topsoil samples were collected to explore the relationships between Se content in topsoil under various land use type and soil properties (pH, organic matter (OM), major elements content in topsoil) and topographic characteristics such as elevation (H), slope (SL), and topographic wetness index (TWI) and geological condition (stratum). Geostatistics, principal component analysis (PCA) and Geodetector were used to analyze the controlling factors of Se distribution in topsoil. The results show that in the analyzed topsoil (1) Se contents vary from 0.039 to 1.110 mg/kg, with a mean of 0.315 mg/kg, and higher than the background value of Chinese soil (0.290 mg/kg). 82.3% are classified as having moderate Se levels (Se > 0.175 mg/kg). (2) The Se contents in northeast of Jiangjin district are higher (Se > 0.315 mg/kg) than the midwest (Se < 0.315 mg/kg). (3) The key controlling factor of the soil Se distribution is OM. In addition, the soil weathering and leaching process and pedogenic rock are the secondary factors controlling Se distribution. Together, these findings reveal that Se-rich soil of low Se belt tends to be distributed in regions with higher OM, stronger leaching, and carbonate parent materials. These observations are beneficial to explore the Se-rich soil resources in low Se region in China.

Contact-style magmatic sulphide mineralisation in the Labrador Trough, northern Quebec, Canada: implications for regional prospectivity

The Labrador Trough in northern Quebec is currently the focus of ongoing exploration for magmatic Ni-Cu-platinum group element (PGE) sulphide ores. This geological belt hosts voluminous basaltic sills and lavas of the Montagnais Sill Complex, which are locally emplaced among sulphidic metasedimentary country rocks. The recently discovered Idefix PGE-Cu prospect represents a stack of gabbroic sills that host stratiform patchy disseminated to net-textured sulphides (0.2–0.4 g/t PGE+Au) over a thickness of ∼20 m, for up to 7 km. In addition, globular sulphides occur at the base of the sill, adjacent to the metasedimentary floor rocks. Whole-rock and PGE geochemistry indicates that the sills share a common source and that the extracted magma underwent significant fractionation before emplacement in the upper crust. To develop the PGE-enriched ores, sulphide melt saturation was attained before final emplacement, peaking at R factors of ∼10 000. Globular sulphides entrained along the base of the sill ingested crustally derived arsenic and were ultimately preserved in the advancing chilled margin.

Gravity Modeling of the Au–U Mineralized Crust at the North-Central Cameroon Illustrating Crustal Permeability

The north-central Cameroon is located at the northern edge of Archean Congo–Sao Francisco craton and at the east of the West African craton. This area includes various Neoproterozoic to Holocene geological litho-structural units. It corresponds to a juvenile crust resulting from the collision of the above cratons. Geological features associated with the Poli–Ounianga–Kebir heavy gravity line trending NE–SW crosscuts the study area and hosts several deposits, chiefly auriferous and uraniferous ore deposits. Uraniferous bodies are described at the SW and NE sides of the Benue Trough and are related to hydrothermal processes and structural events in Neoproterozoic granitoids of Poli (Cameroon) and Zabili (Chad). Gold ore deposits are found around faulted zones mostly in the Tchollire shear zone. Gravity data derived from the Earth Gravitational Model EGM2008, combining terrestrial, maritime and satellite-derived data, were used to model the crust of the north-central Cameroon. The geophysical filtering enabled to highlight that the most fractured areas with the deepest faults are situated on the positive anomalies of the mineralized volcano-sedimentary belts of Poli and Bibemi. The Benue Trough that crosscuts the volcano-sedimentary belts exposes few structural features due to the weak gravity signal of Cretaceous sedimentary cover. The radially averaged power spectrum analysis brought out the density contrast depths of some geological belts and revealed the Moho discontinuity of the study area at about 24 km depth. This Moho depth is in accordance with the assumption of a thin crust and a mantle upwelling in the study area proposed by previous works. Thus, the presence of geological formations of mantle, crustal and mixed crustal-mantle origin such as mafic rocks as well as calc-alkaline to alkaline granitoid outcropping in the study area may be the consequence of the proximity of the upper mantle to the surface. Moreover, fractures highlighted in this work brought out the crustal permeability in the study area illustrating migratory passages for mantle formations found at the surface. Based on the gravity field analysis, the proposed models of basement in this study provide geological architecture highlighting the presence of more or less dense rocks and hydrothermal fluids migration along faults as key elements for ore deposits formation.

Geological Belts, Plate Boundaries, and Mineral Deposits in Myanmar

Geological Belts, Plate Boundaries, and Mineral Deposits in Myanmar

Functional microbial diversity dynamics in common effluent treatment plants of South Gujarat and hydrocarbon degradation

Common effluent treatment plants (CETPs) of South Gujarat region, India, process wastewater generated by more than 2500 industries because of the nonfeasibility of processing at the individual industrial unit. This study assessed functional microbial diversity in wastewater samples of CETPs over a geological belt using Ecoplate®, isolation of the most abundant bacteria, and screening for hydrocarbon degradation. The high evenness (EPielou) values (0.9) in almost all samples indicated a highly even community structure. Principal component analysis of carbon source utilization showed a cluster of all inlet samples except E1 and another cluster of all outlet samples; aeration tank community samples were dispersed. In spite of the high richness found in microbial communities, 60 morphologically similar organisms were observed and isolated; 46 out of them were subjected to amplified ribosomal DNA restriction analysis with MboI, HaeIII, and TaqI enzyme, followed by UPGMA clustering. In screening the most abundant bacteria from each cluster, one of the cultures showed a high potential for hydrocarbon degradation and was identified as Pseudomonas citronellolis by 16S rDNA sequencing. Because of its highly adapted inherent nature, this bacterium may help augment the conventional procedure in wastewater treatment and efficiently decrease the organic load.

Seismic Dynamic Model Tests on Tunnel Lining Passing through the Fault

The seismic behaviors of tunnel lining passing through the fault are studied by means of large-scale shaking table tests under the load of Wolong-seismic wave. The model tunnel is installed into a model box with a length of 3.65 m, width of 1.5 m and height of 1.8 m. The sizes of model and prototype tunnel are designed by using the similar principles with a scale factor of 1:30. As the result shows, under the seismic force load, the tunnel lining has a delay of responding the seismic force when it crosses the faults geological belt. The time is 0.085seconds. Model box amplified table acceleration, the farther the distance from shaking table is, the more obvious amplification is. The seismic force load is horizontal, but the resistance applied by the faults geological belt is weak, resulting relative motion, collision, extrusion etc. so, the tunnel lining appears cracks. It is in accord with investigation conclusions of Wenchuan earthquake. Article also aims at some problems in the experiment such as: boundary effect, model box stiffness etc, and gives some suggestions.

Comment on “Integrated Mineralogical Characterization of Banded Iron Ores of Orissa and Its Implications on Beneficiation” by Mohanty &amp;lt;i&amp;gt;et al&amp;lt;/i&amp;gt;. (2012), Journal of Minerals and Materials Characterization and Engineering, 11, 1133-1142*

Mohanty et al. (2012) have wrongly analyzed M?ssbauer spectra obtained from various iron ore samples. We criticized their approach suggesting suitable methodology, and suggested to refer earlier work carried out by Nayak et al. (2001, 2004) on representative samples from the same geological belt.

Geology of the Hida Gaien Belt in the upper Kuzuryu‐gawa River Area in Ono City, Fukui Prefecture, Central Japan

AbstractGeological, paleontological, and geochronological studies of the Hida Gaien Belt were carried out in the upper Kuzuryu‐gawa River area, northern central Japan. The Hida Gaien Belt lies between the Hida and Mino belts of Southwest Japan and is one of the most complex geologic belts in Japan. The geology of the following units in the study area, mostly bounded by longitudinal, high‐angle faults, was particularly reexamined and described: the Ise metamorphic rocks, the Fujikuradani, Tomedoro, Oguradani, Motodo, Ootani, and Konogidani Formations, and the Tetori Group. Among them, the Tomedoro and Konogidani Formations are both composed mainly of greenstone, and were conventionally coupled together as ‘the Tomedoro schalstein member’ or ‘the Konogidani Formation’. However, the conformable relationship between the Tomedoro Formation and overlying Middle Permian Oguradani Formation, and the K–Ar and 40Ar–39Ar ages of 75–69 Ma (Late Cretaceous) from the basalt lava of the Konogidani Formation reveal that they are separate formations with different ages. The Oguradani Formation, consisting of limestone, shale, and sandstone with Middle Permian Boreal‐Tethyan mixed brachiopod fauna, is correlated with the Moribu Formation in the Takayama area of the Hida Gaien Belt, and with the Middle Formation of the Maizuru Group in the Maizuru Belt. The Tomedoro Formation below the Oguradani Formation, in turn, is correlated with the Lower Formation of the Maizuru Belt. The new Late Cretaceous age data from the Konogidani Formation and presence of latest Cretaceous, post‐tectonic volcanic rocks in the study area finally indicate that the fault‐bound structure of the Hida Gaien Belt between the Hida and Mino belts was formed in a very short period in Late Cretaceous age.

Rare-earth elemental analysis of banded iron-formations by instrumental neutron activation analysis

Representative banded iron-formations (BIFs) from various locations of the eastern Indian geological belt were investigated by instrumental neutron activation analysis (INAA). After pre-concentration, irradiation was carried out using a neutron flux of 5.1·1016 m−2·s−1, 1.0·1015 m−2·s−1 and 3.7·1015 m−2s−1, with thermal, epi-thermal and fast neutrons, respectively. The activities in these samples were measured by a HPGe detector. Ten rare-earth elements, such as La, Ce, Nd, Sm, Eu, Tb, Ho, Tm, Yb and Lu, have been qualitatively identified and quantitatively estimated in these samples. The present investigation is an example of employing a pre-concentration method for high iron-containing ores prior to neutron activation analysis.

Crustal thickness and VP/ VS variations in the Grenville orogen (Ontario, Canada) from analysis of teleseismic receiver functions

We have developed a simple semblance-weighted stacking technique to estimate crustal thickness and average V P/ V S ratio using teleseismic receiver functions. We have applied our method to data from 32 broadband seismograph stations that cover a 700 × 400 km 2 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 L ithoprobe 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 V P/ V S ranging from 1.78 to 1.85. In other parts of the Grenville orogen, V P/ V S 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.

Upper-mantle velocity structure of the lower Great Lakes region

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 ≤ m b ≤ 6.6), and 1860 S-wave traveltimes from 98 events (5.0 ≤ m b ≤ 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.

The Great Alaskan Terrane Wreck: reconciliation of paleomagnetic and geological data in the northern Cordillera

Paleomagnetic studies place much of the Cordilleran orogen ≥3000 km south, relative to autochthonous North America at 85 Ma. This conflicts with geological studies which have been interpreted to indicate minor displacement. The inability to reconcile these interpretations has precluded understanding the tectonic development of the Cordillera, and has cast doubt on global paleogeographic reconstructions which rely upon paleomagnetic studies. I demonstrate that Alaska and Yukon are divisible into a series of geological belts, including the northerly east–west-trending Arctic Alaska, the central southwest-trending Ruby, the southerly east-trending Dillinger and the southeast-trending Yukon–Tanana belts. Each belt is characterized by four regularly arranged rock sequences; (1) a Paleozoic continental margin strata, (2) a Devonian–Mississippian arc assemblage, (3) an ophiolite, and (4) an Early to mid-Cretaceous arc. These correlations greatly simplify the terrane nomenclature of the northern Cordillera and provide a basis for a new palinspastic reconstruction of the Cordillera. Continuity of the belts can be demonstrated through connecting oroclines, and facing a structural vergence directions vary in a consistent and predictable fashion indicating that the belts previously formed part of a linear ribbon continent. This ribbon continent, referred to here as SAYBIA, originally extended south from eastern Siberia and was ∼8000 km long. Northward translation in response to coupling with the Kula plate was accommodated by buckling (oroclinal orogeny) of SAYBIA, much like a derailing train, with scissored ‘cars’ forming much of Alaska. Oroclinal orogeny makes consistent interpretations of available geological and paleomagnetic data in the Cordillera, provides a means for the rapid construction of continent, and may have been an important process involved in the construction of ancient continental nuclei.

Terrestrial Gamma Radiation in Kochi Prefecture, Japan.

Absorbed dose rates in air (dose rate) from terrestrial gamma radiations obtained using a scintillation survey meter ranged from 13.8 to 187 nGy/h depending on the geological features in Kochi Prefecture. Mean dose rates of four geological belts were found in the following order; Alluvial Deposits < Sambagawa Belt = Chichibu Belt < Shimanto Belt. The mean dose rate of the prefecture was about 60 nGy/h, which was 20% higher than that of Japan as a whole. The mean effective dose rate per person in this prefecture was about 0.37 mSv/y, which was the same as the Japanese mean. The ratio of the mean dose rates of 40K, nuclides of the thorium series and the uranium series in the soil determined using a germanium detector were 2 : 2 : 1. The relationship between 40K, 208Tl and 214Bi was positive, and therefore 40K, nuclides of the thorium series and the uranium series were considered to have the same behavior in the soil. 137Cs in the soil ranged from 1.4 to 150 Bq/kg with a mean of 24.5 ± 23.1 (S.D.) Bq/kg, suggesting that we continue to be under the influence of nuclear tests in the atmosphere.

Structural Correlation of the Southern Transcaucasus (Georgia)-Eastern Pontides (Turkey)

Abstract The eastern Pontides (northeastern Turkey) and Transcaucasus (Georgia) belong to the same geological belt representing an active margin of the Eurasian continent. According to palaeotectonic-palaeogeographic reconstructions, based on regional geological, palaeomagnetic, palaeobiogeographical and petrological data, the eastern Pontides and the major part of the Transcaucasus, situated to the north of the North Anatolian-Lesser Caucasian ophiolitic suture, comprise island arc, forearc, back and interarc basins. The eastern Pontide segment of the belt consists of three structural units which, from north to south, are the northern, central and southern units. The northern unit, the southeastern Black Sea coast-Adjara-Trialeti Unit, represents a juvenile back arc basin formed during the Late Cretaceous (pre-Maastrichtian). This unit separates the southern and northern Transcaucasus zones. The central Artvin-Bolnisi Unit is also known as the northern part of the southern Transcaucasus and is characterized by Hercynian basement, unconformably overlying the Upper Carboniferous-Lower Permian molasse and Upper Jurassic-Cretaceous arc association. The southern unit is the imbricated Bayburt-Karabakh Unit and is known as the southern part of the southern Transcaucasus. This unit has a similar basement to the Artvin-Bolnisi Unit and also includes a chaotic assemblage; it unconformably overlies the Upper Jurassic-Cretaceous forearc association. The eastern Pontide system is interpreted as the product of interference between a spreading ridge and subduction zone during Late Jurassic-Cretaceous times. The North Anatolian-Lesser Caucasus Suture, comprising ophiolites, mélanges and an ensimatic arc association, separates the overlying system from the Anatolian-Iranian Platform in the south. Maastrichtian-Lower Eocene cover rocks in the region unconformably overlie all the other units. Middle Eocene rifting resulted in the formation of new basins, some of which closed during an Oligocene-Early Miocene regression. Others, such as the Black Sea and Caspian Basins, have survived to the present day as relict basins.

Collision structure in the upper crust beneath the southwestern foot of the Hidaka Mountains, Hokkaido, Japan as derived from explosion seismic observations

The P-wave velocity structure of the upper crust beneath a profile ranging from Niikappu to Samani in the southwestern foot of the Hidaka Mountains, Hokkaido, Japan was obtained through analysis of refraction and wide-angle reflection data. The mountains are characterized by high seismicity and a large gravity anomaly. The present profile crosses the source region of the 1982 Urakawa-oki earthquake ( M s 6.8). The length of the profile is 66 km striking northwest and southeast. Along the profile, 64 vertical geophones were set up and 5 shot points were chosen. For each shot, a 400–600 kg charge of dynamite was detonated. The studied area is composed of four major geological belts: Neogene sedimentary rocks, the Kamuikotan belt, the Yezo Group, and the Hidaka belt. The measurement line crosses these geological trend at an oblique angle. The structure obtained is characterized by remarkable velocity variations in the lateral direction and reflects the surface geological characteristics. A thin, high-velocity layer (HVL) was found between low-velocity materials in the central part of the profile, beneath the Kamuikotan Metamorphic Belt, at a depth ranging from 0.5 to 6 km, overthrusting toward the west on the low-velocity materials consisting of Neogene sedimentary rocks, and forming gentle folds. Outlines of the velocity structure of the Hidaka Mountains yielded by other studies have shown a large-scale overthrust structure associated with the collision of the Outer Kurile and the Outer Northern Honshu Arcs. The shallow velocity structure inferred by the present study showed a similar (although small scale) overthrust structure. The obtained structure shows that the composite tectonic force, comprising westward movement of the Outer Kurile Arc and northward movement of the Outer Northern Honshu Arc, plays an important role in the evolution of the tectonic features of the crust and upper mantle in a wide depth range beneath the Hidaka Mountains.