Geotectonic Provinces Research Articles

Seismic model of the Earth's crust and upper mantle beneath the Fennoscandian Shield

Well-founded concepts of the crust and upper mantle structure of the Fennoscandian Shield were developed. Quantitative evidence for horizontal and vertical lithospheric heterogeneities was obtained. Highly detailed integrated geological and geophysical investigations, supported by superdeep drilling, have been conducted in the eastern part of the shield in recent years. Seismogeological crustal models of individual geotectonic provinces were constructed, which indicate that the structure of the crystalline crust displays a pattern of blocks, arranged hierarchically, with no persistent seismic boundaries across the entire shield. Both low- and high-velocity zones, correlatable with geological bodies, occur locally. The structural plans of contours and the velocities of various deep sections were shown to be inconsistent. We conclude that the upper mantle is laterally complex and heterogeneous, and that there is no asthenosphere in the classical sense of the word.

Multiple Reactivations of the Rennick Graben Fault System (Northern Victoria Land, Antarctica): New Evidence From Paleostress Analysis

AbstractThe dense fault network that separates the tectonic units of northern Victoria Land to the E from the East Antarctic Craton to the W represents a regionally sized, NNW‐SSE trending deformation zone that is also known as the Rennick Graben Fault system (RGF). This long‐lived deformation zone is characterized by great structural complexity, with the superposition and polyphasic reactivation of regional faults; it is known to have been active since the Cambrian‐Ordovician, when it formed as a suture zone between different terranes, up to the recent. Its complete framework and geodynamic evolution are still debated and have only been partially investigated. In this research, we explore the Meso‐Cenozoic tectonic framework of the RGF by investigating the paleostress fields that characterized the last geodynamic events in the area and their associated brittle deformation. We analyzed faults and fractures data collected at 89 sites during several PNRA ItaliAntartide expeditions and combined fault‐slip data inversion with the azimuthal orientation of faults and the spatial distribution of fracture intensities across the RGF. The results from this multi methodological approach confirm the existence of two geotectonic provinces (Bowers Mountains province to the E and Usarp Mountains to the W) characterized by different spatial distributions of brittle deformation, which are significantly more intense in the Bower Mountains domain. Here, the repeated reactivation of the RGF led to the superposition of two recent (Meso‐Cenozoic) major tectonic events, with prevalent strike‐slip kinematics and characterized by fault reactivation with right‐lateral movement overprinting the previous left‐lateral event.

Reconstructed ocean plate stratigraphy sequences from the Permian subduction-accretionary complex in the Qinghai-Tibet Plateau

The Permian Tethys subduction-accretionary complex (SAC) in the Qinghai-Tibet Plateau (QTP) is distributed in six geotectonic provinces, which are known as the Zongwulong-Ganjia, the Western Kunlun, the Eastern Kunlun, the Ganzi-Litang, the Xijinwulan-Jinshajiang-Ailaoshan and the Longmucuo-Shuanghu-Nujiang from north to south. The SAC is a set of mélange generated by subduction of oceanic crust at trenches, which consists of relics from mid-ocean ridge, abyssal plains, oceanic island-seamounts, intra-oceanic arcs and trench turbidites. In this paper, we apply the ocean plate stratigraphy (OPS) theory to reconstruct the OPS sequence of the Permian SAC in the QTP. We divided the Permian SAC in the QTP into two types, i.e., convergent zone and accretionary zone according to their tectonic environments. Six relics from mid-ocean ridge, five relics from abyssal plain, five relics from seamount, two relics from intra-oceanic arc, two relics from fore-arc trench, and seven relics from continental fore-arc trench were recognized and then the OPS sequences from different tectonic environments were reconstructed. The spatial-temporal distributions and evolution of ocean basins during the Permian represented by six SACs are discussed.

Heterogeneous lithospheric structure of the Fennoscandian shield: seismic data

A generalization has been obtained based on the data for deep seismic studies on longitudinal DSS profiles (explosion seismology) and seismological evidence found for local and distant earthquakes (earthquake seismology) in the Fennoscandian Shield over the past 40 years. This provided an opportunity to obtaine quantitative data on the horizontal and vertical heterogeneities of the lithosphere. 3D seismological models were first constructed for the individual parts of the Fennoscandian Shield which show that the crystalline crust structure acquires a block hierarchy pattern, with no persistent seismic boundaries found within the entire shield. The upper crust is characterized by the local occurrence of both waveguides and high-velocity zones correlated with geological bodies. The structural plans of the velocity isolines of various deep sections were found to be inconsistent. It has been proved that major geotectonic provinces have a thick crust and that the Early Proterozoic volcanogenic belts are characterized by elevated velocities and a large crustal thickness. In the upper mantle of the Fennoscandian Shield there were identified the layers with anomalously high velocities. No robust evidence was found for a waveguide, which is sometimes related to the concept of the lithosphere.

Review on geology and mineralization of North Korea (I: Precambrian)

AbstractIn this review, we introduce the geology and mineralization of the northern part of the Korean Peninsula based on recently published data in North Korea. Geological characteristics and events are described in order of geological period and the representative mineral deposits are presented according to the geologic period to show the relationship between the geology and mineralization. In particular, we rearrange the stratigraphy of the Precambrian basement and reset the timing of intrusion and mineralization based mainly on the reported zircon U–Pb age data. The Precambrian geotectonic provinces of North Korea contain continental cores and massifs, active continental margins, and rift zones. The Precambrian strata between the eastern North China Craton (NCC) and North Korea are newly correlated considering rearranged stratigraphy in this review. The Precambrian strata of North Korea are characterized by the depletion of the Paleoproterozoic to Mesoproterozoic strata compared to the NCC from 1.8 to 1.5 Ga. It may indicate that the northern part of the Korean Peninsula was the passive continental margin environment during this period. The common occurrences of bimodal igneous suites (mafic‐felsic and alkaline‐subalkaline) are also characteristic of North Korea. The bimodal magmatism is well explained by the progressive tectonic evolution from oceanic crust‐continent subduction (2.0–2.2 Ga) and island arc–continent collision (~1.9 Ga) to post collisional extension (1.82–1.87 Ga) in the NCC including North Korea. The Precambrian geological similarities between Northeastern China and North Korea would have resulted in the same or similar mineralization. For example, Algoma type banded iron formations of the Fe deposits of the Anshan–Benxi area in China and Musan Fe deposit in North Korea were formed during the Neoarchean and Paleoproterozoic. Sedimentary exhalative type Pb–Zn and metasedimentary type magnesite deposits were formed within the Jiao‐Liao‐Ji belt extending from the Liaoning Rift Zone of China (Pb–Zn deposits in the Qingchenzi orefield and magnesite deposits in Dashiqiao magnesite belt) to the Macheollyeong Rift Zone of North Korea (Geomdeok Pb–Zn deposit and Daeheung–Ryongyang magnesite deposits) during Paleoproterozoic. The geotectonic, geological and metallogenic similarities between Northeastern China and North Korea indicate that the Precambrian continental massifs of North Korea might be evolved in accordance with the NCC evolution. This review is expected to be helpful for understanding the geotectonics, geology, and related mineralization of the northern part of Korean Peninsula and correlating with those of the NCC.

Regional variations of sulfur isotope compositions for metallic deposits in South Korea

AbstractSouthern Korean peninsula comprises five major geotectonic provinces, throughout which various metallic deposits are distributed. We reviewed sulfur isotope data (n = 1,574) of sulfide minerals collected from previous works for 177 metallic deposits in the provinces to interpret the sulfur isotope characteristics of each province, comprising different wall rocks and geologic settings. The averaged δ34S values of each metallic deposit associated with Precambrian metamorphic rocks and Jurassic granitoids in the Gyeonggi massif and Yeongnam massif range from −7.1 to +10.3‰ (av. +4.5‰) and from −3.6 to +7.8‰ (av. +3.5‰), respectively. The Taebaeksan basin produced the highest δ34S value among the five, −0.4 to +13.2‰ (av. +6.1‰). This was influenced by sulfate sulfur derived from marine carbonate host rock. The Okcheon metamorphic belt, comprising metasedimentary and metavolcanics rocks, shows an isotope range from +1.9 to +8.3‰ (av. +5.7‰). The sulfur isotope distribution of the Gyeongsang basin with a range from −1.2 to +11.7‰ (av. +5.2‰) can be divided into two zones: higher δ34S values from the inner zone related to the volcanic rocks and magnetite‐series Cretaceous granitoids, and lower δ34S values from the outer zone related to the organic‐rich sedimentary rocks. Sulfur isotope variations of metallic deposits in each geotectonic province were mainly influenced by igneous sulfur and inherent wall rock sulfur sources, 32S‐enriched sedimentary sulfur (e.g., Precambrian metasedimentary rocks and biogenic sulfur‐rich sedimentary rocks), and 34S‐enriched seawater sulfur (e.g., carbonates and acid to intermediate volcanic rocks). These wall rocks also contributed to the changes in δ34S values for granitoid rocks and metallic deposits by time, ore genetic type, and ore species in South Korea.

Differential Phanerozoic evolution of cratonic and non-cratonic lithosphere from a thermochronological perspective: São Francisco Craton and marginal orogens (Brazil)

The São Francisco Craton (SFC) and its marginal Araçuaí and Brasília orogens exhibit a significant diversity in their lithospheric architecture. These orogens were shaped during the Neoproterozoic–Cambrian amalgamation of West Gondwana. The rigid cratonic lithosphere of the SFC and the relatively weak lithosphere of the Araçuaí Orogen were disrupted during the Cretaceous opening of the South Atlantic Ocean, whereas the Brasília Orogen remained in the continental hinterland. In earlier research, the thermal effects of the Phanerozoic reactivations in the shallow crust of the Araçuaí Orogen have been revealed by low-temperature thermochronology, mainly by apatite fission track (AFT) analysis. However, analyses from the continental interior are scarce. Here we present new AFT data from forty-three samples from the Brasília Orogen, the SFC and the Araçuaí Orogen, far from the passive margin of the Atlantic coast (~150 to 800 km). Three main periods of basement exhumation were identified: (i) Paleozoic, recorded both by samples from the SFC and Brasília Orogen; (ii) Early Cretaceous to Cenomanian, recorded by samples from the Araçuaí Orogen; and (iii) Late Cretaceous to Paleocene, inferred in samples from all domains. We compare the differential exhumation pattern of the different geotectonic provinces with their lithospheric strengths. We suggest that the SFC likely concentrated the Meso-Cenozoic reactivations in narrow weak zones while the Araçuaí Orogen displayed a far-reaching Meso-Cenozoic deformation. The Brasília Orogen seems to be an example of a stronger orogenic lithosphere, inhibiting reworking, confirmed by our new AFT data. Understanding the role of the lithosphere rigidity may be decisive to comprehend the processes of differential denudation and the tectonic–morphological evolution over Phanerozoic events.

Introduction to this special section: Southeast Asia

This special section on Southeast Asia features geophysical topics that cover several of the magnificent geotectonic provinces of the region. Southeast Asia is the site of the world's largest archipelago, which features more than 20,000 islands extending east to west more than 3500 miles. The extent of the offshore regions of the archipelago is many times greater than its land area. The Sunda Shelf, with its numerous Tertiary basins in the western part of the archipelago, contains areas of India, Myanmar, Thailand, Laos, Cambodia, Vietnam, Malaysia, Singapore, the Philippines, Brunei, West Indonesia, and their offshore regions that extend from the Andaman Sea to the Makassar Strait. By contrast, eastern Sunda, with its pre-Tertiary basins, embraces the islands along and north of the Banda Arc, from Sulawesi to western Papua in Indonesia and Timor-Leste and surrounding seas ( Figure 1 ). The most distinguishing tectonic features of the archipelago are related to the collision of the Indo-Australian Plate with the Sunda Shelf and the areas east of it. Numerous volcanoes and earthquake epicenters trace an extensive arc of collision-related subduction zones, which makes this one of the most tectonically active regions in the world. Back-arc and other basins within the stable parts of the Sunda Shelf are the sites of significant hydrocarbon accumulation, primarily within the territorial boundaries of Indonesia and Malaysia.

Paleoproterozoic (~2.0 Ga) volcano‐plutonism in the southeastern region of the Amazon Craton: Petrological aspects and geotectonic implications

Detailed studies on the volcano‐plutonic associations of the Amazon Craton (AC) have been conducted since the 1980s. Since then, several issues concerning the definition of the lithostratigraphic units and their role in the AC evolution have been improved. Recently, a volcano‐plutonic association of around 2.0 Ga was described in the south‐central part of the AC, named as Jarinã Formation (volcanic rocks) and Vila Rica Intrusive Suite (granitoids). The present work deals with the unexpected occurrence of this volcano‐plutonic association in the southeastern region of the Amazon Craton. In the study area (Santaninha region), the rocks of the Jarinã Formation comprise two volcanic successions with distinct petrographic and compositional features. The amount and shape of the phenocrysts and crystal fragments allow the individualization of effusive and pyroclastic rocks. The geochemical character indicates that the volcanic rocks belong to two distinct successions as follows: intermediate to acid magnesian I‐type and acid ferroan A‐type. The I‐type succession shows high MgO, TiO2, P2O5, and Sr content and low K2O, Zr, Nb, Ta, Y, and heavy rare earth element and consists of undeformed andesitic to dacitic ignimbrites and subordinate effusive rhyodacites. The A‐type succession present high K2O, FeOt, Zr, Nb, Ta, Y, and heavy rare earth element content and low TiO2, P2O5, and Sr and comprises undeformed rhyolitic ignimbrites and subordinate effusive rhyolites. The granitoids of the Vila Rica Intrusive Suite crosscut the volcanic rocks of the Jarinã Formation and include two phaneritic facies as follows: coarse‐ to medium‐grained granite and medium‐grained granite. The MgO, TiO2, P2O5, Sr, K2O, Zr, Nb, Ta, Y, and rare earth element content demonstrate that the medium‐grained granite facies present geochemical characteristic of I‐type granites. Laser‐ablation inductively coupled plasma mass spectrometry U–Pb zircon data indicate crystallization ages of 2,009 ± 9 Ma for the I‐type dacitic ignimbrite, 1,994 ± 16 Ma for the A‐type rhyolitic ignimbrite, and 1,995 ± 14 Ma for the I‐type medium‐grained granite. These ages agree with the previous data of the Jarinã Formation in the south‐central region of the Amazon Craton and of the Vilca Rica Intrusive Suite in the southeastern of the Amazon Craton. The understanding of the geological context of these units carry on meaningful implications to the contextualization of the ~2.0 Ga volcano‐plutonic event in the AC and, consequently, to the compartimentalization of its geotectonic provinces.

Geomorphic response of the river basin drainage in seismically active regions of India

Over the last few decades, the Indian subcontinent is gradually drifting towards northeast at a rate of 10 mm/year, which is an indication that the areas are actively deforming. Such a gradual rate of drift is expected to bring modifications and changes in the topography, which can be quantified with the help of geomorphic parameters. The current study examines the geomorphological changes in the seismically active areas of different geotectonic provinces of India, i.e., Northeast India and Bhuj region. The geomorphological changes have been estimated with the help of remote sensing and geographical information system modeling by calculating the geomorphic parameters. A total of 39 drainage basins were extracted from both the study areas using Aster GDEM data of 30 m resolution. Geomorphic indices such as stream order, stream length, bifurcation ratio, form factor, asymmetry factor and hypsometric integral were calculated for each drainage basin of the study regions. The geomorphic parameters reveal that the study areas are dendritic in character, even though having different geological formations, both study regions express similar lithological characteristics. With no strong control of structural elements, both study areas seem to be characterized by relatively high permeability and with relatively gentle slope. The extent of the carving of topography reveals that the drainage development and morphologic evolution of both areas have practically reached Late Mature and Old stage of terrain development. Even though the two study areas represent different seismotectonic provinces, it is observed that geomorphologically they are gradually approaching the stage of peneplanation.

Lead isotope mapping of galena from base metal deposits in the southern Korean Peninsula

The purpose of this study is to investigate the differences in sources of discrete four zones that are represented by the lead isotope map of metal mines using galena samples. The lead isotope map has been constructed based on 119 galena samples from 38 metal mines in the southern Korean Peninsula, together with previously published data, using spatial and zoning distribution analysis. The spatial pattern of 206Pb/204Pb ratio shows relatively dense distribution with highest value at the northeastern region, while 207Pb/204Pb and 208Pb/204Pb ratios are the lowest at the southeastern region. These results indicate possible subdivision of the lead isotope maps based on geotectonic subdivisions and geographical locations of the Korean Peninsula. This subdivision has been checked by linear discriminant analysis (LDA). The range of lead isotopic composition for zone 1 has fairly homogeneous values (206Pb/204Pb = 18.156–18.591, 207Pb/204Pb = 15.482–15.644, and 208Pb/204Pb = 37.870–38.768), and are geographically similar to the Gyeongsang Basin. Lead isotopic compositions of zone 2 show the highest ratios (206Pb/204Pb = 18.625–20.483, 207Pb/204Pb = 15.708–16.068, 208Pb/204Pb = 37.734–40.463) in the southern Korean Peninsula, that are geographically corresponding to the Taebaeksan Basin and the northeastern Yeongnam Massif. Relatively lower values (206Pb/204Pb = 17.149–17.817, 207Pb /204Pb = 15.497–15.731, 208Pb /204Pb = 37.847–39.773) along the western Gyeonggi Massif are grouped as zone 4, while the rest of the areas, including the Okcheon metamorphic belt, most of the Gyeonggi Massif, and the Yeongnam Massif, might represent zone 3. The results clearly indicate the possible differences in the source reservoirs that are similar to the geotectonic provinces of the Korean Peninsula.

Airborne geophysical characterization of geotectonic relationships in the southern Ribeira Belt, Luís Alves Craton, and northern Dom Feliciano Belt, Brazilian Shield

ABSTRACTIn many Precambrian provinces, the understanding about the tectonic history is restrained by the limited exposure of rocks so that aerogeophysical data can provide information below the surface cover of sediments, soil, and water to build a tectonic model of the region. However, the tectonic evolution of cratons and shear zones is very complex due to scarce aerogeophysical and geological data. The integrated observation of geology, aerogammaspectrometry, and aeromagnetometry of the Brazilian Shield, states of Santa Catarina, Paraná, and southern São Paulo, is enhanced by a 2011 high-resolution aerogeophysical survey. The Ponta Grossa and Florianópolis dike swarms show two sets of linear high magnetic signal. Peralkaline–carbonatitic complexes are observed in all images, e.g. Jacupiranga, Tunas, and Anitápolis. Three main geotectonic provinces are apparent in the shield. The Dom Feliciano Belt has two domains in terms of radioelements K, Th, and U. This belt includes granitic rocks of the Florianópolis Batholith, Brusque Group schists, and Itajaí Basin of sedimentary and volcanic rocks. The Luís Alves Craton has mostly the Santa Catarina Granulitic Complex and other gneisses. The craton has patches of high emission rates and extends to the northeastern extreme of the studied area with a high-contrast magnetic signal in most of the unit. The Ribeira Belt also has high in gammaspectrometric images, although less homogeneous than the Dom Feliciano Belt. This belt contains granitic complexes, gneisses, and schists and has a low magnetic signal with patches of high analytical signal amplitude. Shear zones delimit different geotectonic units, but are also significant in the Ribeira Belt. The Luís Alves Craton is the most complex unit, as seen in ternary K + eTh + eU images, requiring the future discrimination of small features. This novel study of the Brazilian Shield based on new geophysical data supports the classification of the proposed main geotectonic units.

Crustal structure of the western Indian shield: Model based on regional gravity and magnetic data

Regional surface gravity data and global satellite magnetic data have been utilized to generate a preliminary model of the crustal structure along a southwest–northeast profile (Gadra–Fatehpur) through western Rajasthan. The study area represents the western part of the Indian continental landmass which has undergone several major episodes of repeated subduction/collision, plume traces and rifting from Archaean to recent times. The temporal and spatial relationship between the various geotectonic provinces is quite complex, thereby limiting the emergence of a suitable crustal structure model for this region. Exposures of the Malani Igneous Suite (MIS), a product of bimodal volcanism (∼780 Ma), and considered to be the third largest felsic magmatic province of the world, is evident along the profile and also to the southwest of the study area. The easternmost part of the profile is close to the DAFB (Delhi Aravalli Fold Belt), a Proterozoic orogenic belt.This study probes the geometry of the different crustal units in terms of density and susceptibility variations in order to decipher the imprints of the major tectonic processes the region has undergone. In order to decipher the crustal geometry of the Gadra–Fatehpur profile, two NW–SE gravity and magnetic profile vertical sections (A–A′ in the south and B–B′ in the north) are modelled on the basis of the constraints provided from previous seismic models. The crustal model of the Gadra–Fatehpur profile is composed of alluvium, Tertiary sediments, MIS, Marwar Supergroup, low-density layers (LDLs) and the middle–lower crustal layers, with a distinct change in configuration from the southwest to northeast. The Moho dips from SW to NE, the MIS in the SW gives way to the thick pile of the Marwar Supergroup to the NE. The evolution of MIS has been suggested to have occurred as a consequence of delamination of the upper mantle. LDLs are incorporated in Gadra–Fatehpur model. In the SW, LDL (2550 kg/m3) lies below the MIS in the NE, another LDL (2604 kg/m3) is depicted below the mid-crustal layer.

Hydrogeology of Brahmaputra Basin, India

The Ganga-Brahmaputra river system together forms one of the largest deltas in the world comprising some 59570 sq km. The waterpower resources of the Brahmaputra have been presumed to be the fourth biggest in the world being 19.83 x 103 m3s1. The entire lower portion of the Brahmaputra consists of a vast network of distributary channels, which are dry in the cold season but are inundated during monsoon. The catchment area of the entire river is about 580,000 sq km, out of which 195,000 sq km lies in India. The maximum discharge as measured at Pandu in 1962 was of the order of 72800 m3 s-1 while the minimum was 1750 m3 s-1 in 1968. The drainage pattern in the valley is of antecedent type while the yazoo drainage pattern is most significant over the composite flood plain to the south of the Brahmaputra.
 The Brahmaputra valley is covered by Recent alluvium throughout its stretch except a few isolated sedimentary hills in the upper Assam, inselbergs/bornhardt of gneissic hills in the Darrang, Kamrup and Goalpara districts and a few inlying patches of Older Alluvium in the Darrang and Goalpara districts.
 The basin is very unstable. The present configuration of the basin is the result of uplift and subsidence of the Precambrian crystalline landmasses. Four geotectonic provinces can be delineated in the N-E India through which the Brahmaputra flows. These are bounded by major tectonic lineaments such as the basement E-W trending Dauki fault, a NE-SW trending structural feature of imbricate thrusts known as 'belt of Schuppen' and the NW-SE trending Mishmi thrust.
 Hydrogeologically, the Brahmaputra basin can be divided into two distinct categories, viz(a) dissected alluvial plain and (b) the inselberg zone. The first category is rep resented in the flood plain extending from the south of Sub-Himalayan piedmont fan zone in the north to right upto the main rock promontory of Garo Hills and Shillong Plateau. The inselberg zone is characterized by fractured, jointed and weathered ancient crystalline rocks with interhill narrow valley plains, consisting of thin to occasionally thick piles of assorted sediments.
 From the subsurface lithological data, two broad groups of aquifers are identified. These are i) shallow water table and ii) deeper water table or confined ones, separated by a system of aquicludes. The shallow aquifer materials, in general, consist of white to greyish white, fine grained micaceous sand and the thickness ranges from 1.2 to 10.3 m. The sand and clay ratio varies from 1: 2.5 to 1:26. The bedrock occurs at depth ranges of 30.4 to 39.5 m. The materials of the deeper aquifers comprise grey to greyish white, fine to medium grained sand. The sand and clay ratio varies from 1:2 to 1:7. The effective size of the aquifer materials varies from 0.125 to 0.062 mm with uniformity co-efficient around 4.00, porosity 38 to 42%, co-efficient of permeability 304 to 390 galls per day/0.3m2. The ground water is mildly alkaline with pH value 6.5 to 8.5, chloride 10 to 40 ppm, bi-carbonate 50 to 350 ppm, iron content ranges from a fraction of a ppm to 50 ppm. Total dissolved solids are low, hardness as CaCo3 50 to 300 ppm, specific conductance at 25 °C 150 to 650 mhos/cm. The yield from shallow aquifers is 1440 litres to 33750 litres/hour and for deeper aquifers ~ 1700 litres/hour at a drawdown of 13.41 m, specific capacity 21 litres/minute. The temperatures of ground water are 23°-25° C during winter, 24°-26° C during pre-monsoon and 27°- 28° C during peak monsoon. The general hydraulic gradient in the north bank is 1:800 whereas in the south bank it is 1: 300-400 The Tertiary sediments yield a range of water from 200 to 300 l.p.m whereas the yield from the Older Alluvium is 500 to 700 1.p.m. The estimated transmissibility and co-efficient of storage is of the order of ~ 800 1.p.m/ m and 8.2 x 10-3 respectively. Depths to water levels range from 5.3 to 10m below land surface (b.l.s). In the Younger or Newer Alluvium, ground water occurs both under water table and confined conditions. Depths to water levels vary from ground level to 10 m b.l.s. Depth to water ranges from 6 m b.l.s. to 2 m above land surface. The yield of the deep tubewells ranges from 2 to 4 kl/minute for a drawdown of 3 m to 6 m. The transmissibility of the aquifers varies from 69 to 1600 l.p.m/m and the storage co-fficient is of the order of 3.52 x 10-2.

Regional geology and geophysics of the eastern Gulf of Guinea (Niger Delta to Rio Muni)

There are many key aspects to the geology and geophysics of the petroliferous basin systems of the eastern Gulf of Guinea, which extends from the Niger Delta to North Gabon. This article provides a background to the regional geology and focuses on the structural geology and the structural styles associated with the key tectonic episodes responsible for hydrocarbon trapping. This work results from several regional studies carried out by Exploration Consultants, based on public domain seismic and well data combined with the results of new analytical work. The region has been divided into six geotectonic provinces (Figure 1):

An overview of the sedimentary geology of the Bengal Basin in relation to the regional tectonic framework and basin-fill history

The Bengal Basin in the northeastern part of Indian subcontinent, between the Indian Shield and Indo-Burman Ranges, comprises three geo-tectonic provinces: (1) The Stable Shelf; (2) The Central Deep Basin (extending from the Sylhet Trough in the northeast towards the Hatia Trough in the south); and (3) The Chittagong–Tripura Fold Belt. Due to location of the basin at the juncture of three interacting plates, viz., the Indian, Burma and Tibetan (Eurasian) Plates, the basin-fill history of these geo-tectonic provinces varied considerably. Precambrian metasediments and Permian–Carboniferous rocks have been encountered only in drill holes in the stable shelf province. After Precambrian peneplanation of the Indian Shield, sedimentation in the Bengal Basin started in isolated graben-controlled basins on the basement. With the breakup of Gondwanaland in the Jurassic and Cretaceous, and northward movement of the Indian Plate, the basin started downwarping in the Early Cretaceous and sedimentation started on the stable shelf and deep basin; and since then sedimentation has been continuous for most of the basin. Subsidence of the basin can be attributed to differential adjustments of the crust, collision with the various elements of south Asia, and uplift of the eastern Himalayas and the Indo-Burman Ranges. Movements along several well-established faults were initiated following the breakup of Gondwanaland and during downwarping in the Cretaceous. By Eocene, because of a major marine transgression, the stable shelf came under a carbonate regime, whereas the deep basinal area was dominated by deep-water sedimentation. A major switch in sedimentation pattern over the Bengal Basin occurred during the Middle Eocene to Early Miocene as a result of collision of India with the Burma and Tibetan Blocks. The influx of clastic sediment into the basin from the Himalayas to the north and the Indo-Burman Ranges to the east rapidly increased at this time; and this was followed by an increase in the rate of subsidence of the basin. At this stage, deep marine sedimentation dominated in the deep basinal part, while deep to shallow marine conditions prevailed in the eastern part of the basin. By Middle Miocene, with continuing collision events between the plates and uplift in the Himalayas and Indo-Burman Ranges, a huge influx of clastic sediments came into the basin from the northeast and east. Throughout the Miocene, the depositional settings continued to vary from deep marine in the basin to shallow and coastal marine in the marginal parts of the basin. From Pliocene onwards, large amounts of sediment were filling the Bengal Basin from the west and northwest; and major delta building processes continued to develop the present-day delta morphology. Since the Cretaceous, architecture of the Bengal Basin has been changing due to the collision pattern and movements of the major plates in the region. However, three notable changes in basin configuration can be recognized that occurred during Early Eocene, Middle Miocene and Plio-Pleistocene times, when both the paleogeographic settings and source areas changed. The present basin configuration with the Ganges–Brahmaputra delta system on the north and the Bengal Deep Sea Fan on the south was established during the later part of Pliocene and Pleistocene; and delta progradation since then has been strongly affected by orogeny in the eastern Himalayas. Pleistocene glacial activities in the north accompanied sea level changes in the Bay of Bengal.

Sedimentation and basin-fill history of the Neogene clastic succession exposed in the southeastern fold belt of the Bengal Basin, Bangladesh: a high-resolution sequence stratigraphic approach

The Tertiary basin-fill history of the Bengal Basin suffers from oversimplification. The interpretation of the sedimentary history of the basin should be consistent with the evolution of its three geo-tectonic provinces, namely, western, northeastern and eastern. Each province has its own basin generation and sediment-fill history related mainly to the Indo-Burmese and subordinately to the Indo-Tibetan plate convergence. This paper is mainly concerned with facies and facies sequence analysis of the Neogene clastic succession within the subduction-related active margin setting (oblique convergence) in the southeastern fold belt of the Bengal Basin. Detailed fieldwork was carried out in the Sitapahar anticline of the Rangamati area and the Mirinja anticline of the Lama area. The study shows that the exposed Neogene succession represents an overall basinward progradation from deep marine through shallow marine to continental–fluvial environments. Based on regionally correlatable erosion surfaces the entire succession (3000+ m thick) has been grouped into three composite sequences C, B and A, from oldest to youngest. Composite sequence C begins with deep-water base-of-slope clastics overlain by thick slope mud that passes upward into shallow marine and nearshore clastics. Composite sequence B characteristically depicts tide-dominated open-marine to coastal depositional systems with evidence of cyclic marine regression and transgression. Repetitive occurrence of incised channel, tidal inlet, tidal ridge/shoal, tidal flat and other tidal deposits is separated by shelfal mudstone. Most of the sandbodies contain a full spectrum of tide-generated structures (e.g. herringbone cross-bedding, bundle structure, mud couplet, bipolar cross-lamination with reactivation surfaces, ‘tidal’ bedding). Storm activities appear to have played a subordinate role in the mid and inner shelf region. Rizocorallium, Rosselia, Planolites and Zoophycos are the dominant ichnofacies within the shelfal mudstone. This paralic sedimentation of Neogene succession in the study area can serve as a good point of reference for tide-dominated regressive shelf depositional systems. The top of the composite sequence B is marked by a pronounced erosion surface indicating the final phase of marine regression followed by the gradual establishment of continental–fluvial depositional systems represented by composite sequence A. In this composite sequence, stacked channel bars of low-sinuosity braided rivers gradually pass upsequence into high-sinuosity meandering river deposits. A sequence stratigraphic approach has been adopted to interpret the basin-fill history with respect to relative sea-level changes; and to subdivide the rock record into several sequences and units (systems tracts and parasequences) based on identified bounding discontinuities, such as transgressive erosion surface (TES), regressive erosion surface (RES), marine flooding surface (MFS), and incised valley floor (IVF). This approach provides new insight for both exploration and exploitation strategy for hydrocarbon plays that may prove vital to the oil companies engaged in exploration activities in the Bengal Basin. It is strongly recommended here that the traditional lithostratigraphic classification of this part of the basin, which is based on the Assam stratigraphy, be abandoned or at least revised. A tentative allostratigraphic scheme is presented, and it is suggested that to formalize the scheme further study, both surface and subsurface, is needed.

ABSTRACT: Evolution and Exploration Implications of the Colombian Caribbean Accretionary Prism

Development of the Northern Colombian basins has been strongly influenced by the interaction of Caribbean and South American tectonic plates. As a result, three major geotectonic features or provinces are recognized, the Lower Magdalena Basin (LMB), the San Jacinto Fault Belt (SJFB) which represents the first accretionary event (Accretionary Wedge -1, AW-1), and the Sinu Fault Belt (SFB) formed by two additional stages of accretion, i.e. accretionary wedges 2 and 3 (AW-2 and AW-3).

Magmatic flare-up at the Carboniferous/Permian boundary in the NE German Basin revealed by SHRIMP zircon ages

SHRIMP ages of 12 volcanic samples indicate intense magmatic activity to have occurred at the Carboniferous/Permian boundary throughout much of the NE German Basin. Rhyolitic crystal-rich samples have been taken from quarries in the Halle Volcanic Complex (HVC) and in the Flechtingen Ignimbrite, and from drill cores of the Kotzen, Mirow, Friedland and Penkun areas. Ten samples yielded 206Pb/ 238U ages between 302 and 297 Ma (±3 Ma, respectively) which indicate that the magmatic activity took place concentrated in a relatively short time span throughout much of the NE German Basin. Two HVC samples have ages of 307 and 294 Ma. This remarkably synchronous magmatic activity occurred during the initial phase of the basin development. Considering published volume estimates (48,000 km 3), extrusion rates during this time period probably were in the order of 0.01 km 3 a −1, implying magma production rates of 0.1 km 3 a −1. The calc-alkaline SiO 2-rich volcanic rocks originated from magmas that presumably formed during anatexis and subordinate magma mixing in an intra-continental transtensional setting. The basaltic magma which must have provided the thermal input into the lower crust probably formed during decompressional melting of lithospheric mantle fertilised by previous magmatic processes. The dated volcanic rocks occur in three geotectonic provinces, namely the (a) Mid German Crystalline Rise, which forms the northern part of the Internal Variscides, the (b) External Variscides and the Variscan foreland which is considered as (c) Eastern Avalonia. Many of the old zircons found in the HVC samples reflect the magmatic activity of the Mid German Crystalline Rise (325–400 Ma). However, Cadomian (500–650 Ma) and older Gondwanian elements (1674–2373 Ma) are also present. Although the Flechtingen and Kotzen areas are located within the External Variscides, it seems more likely that the middle to lower crust, that experienced anatexis, formed part of the Eastern Avalonia Plate overridden by the Variscan Front. Thus the old zircons found in Flechtingen (350 and 538 Ma) and Kotzen (345 Ma), but certainly those present in the Penkun drill cores (1483 Ma) represent Eastern Avalonia tectonic history. The anatectic magmas in the Friedland area probably formed partly from Baltic crust thrusted below Eastern Avalonia during the Caledonian Orogeny. Among the old zircons from the Friedland samples, in addition to Proterozoic ages (1456 Ma), we found testimony of Caledonian (443 Ma) and of clearly post-Caledonian (387 Ma) magmatism.

Petrographic temper provinces of prehistoric pottery in Oceania

The mineralogical compositions and petrological character of non-calcareous mineral sand tempers in prehistoric potsherds from Pacific islands are governed by the geographic distribution of geotectonic provinces controlled by patterns of plate tectonics. As sands from different islands are not mingled by sedimentary dispersal systems, each temper sand is a faithful derivative record of parent bedrock exposed on the island of origin. Tempers are dominantly beach and stream sands, but also include dune sand, colluvial debris, reworked volcanic ash, broken rock, and broken pottery (grog). From textural relations with clay pastes, most tempers were manually added to clays collected separately, but naturally tempered clay bodies occur locally. Calcareous temper sands derived from reef detritus are widely distributed, but ancient potters commonly preferred non-calcareous sands for temper. Consequently, beach placer sand tempers rich in diagnostic heavy minerals are typical of many temper suites. Distinctive temper classes include oceanic basalt, andesitic arc, dissected orogen, and tectonic highland tempers characteristic of different geologic settings where contrasting bedrock terranes are exposed. Most Oceanian sherd suites contain exclusively indigenous tempers derived from local island bedrock, but widely distributed occurrences of geologically exotic tempers document limited pottery transfer over varying distances at multiple sites.