Stable Continental Areas Research Articles

High mantle helium flux unveils active mantle melting beneath the cathaysia block, south China

Helium (He) in hydrological systems is used to constrain the deep structure and improve interpretations of geophysical techniques beneath the Cathaysia Block (CB), of which the Cenozoic tectonic and geodynamic processes are controversial. The air-corrected 3He/4He ratio of both geothermal and non-geothermal waters in the CB range from 0.10 to 6.41Ra (Ra is the atmospheric 3He/4He ratio), displaying an increasing trend from inland to coastal area, aligning with thinning crust and younger magmatic activities. Mantle-derived He fluxes in the CB vary from 0.11 to 33.41 × 1010 atoms m−2s−1, surpassing those in stable continental areas by up to three orders of magnitude. Due to the absence of active volcanic surface manifestations and identifiable crustal magma chambers, the mantle-derived volatiles are possibly transported from the mantle through the faults with advective flow rates ranging from 1.26 to 154 mm y−1. Two distinct modes of mantle He releases lead to differences in mantle He fluxes between the interior and coastal CB. Seismic activity enhances permeability in the interior CB, leading to the leakage of mantle He. In the coastal CB, high mantle He fluxes with characteristics of volcanic degassing imply degassing from the partial mantle melting. The presence of high 3He/4He ratios (up to 6.41 Ra) and regional thermal anomaly provide evidence for an ongoing process of crustal underplating by mantle melting. Combined with the underplate layer revealed by geophysical results, this implies the continuous compensation mechanism involving mantle influx to counter extension-induced crustal thinning since the Mesozoic–Cenozoic in the coastal CB.

Seismic Azimuthal Anisotropy Beneath a Fast Moving Ancient Continent: Constraints From Shear Wave Splitting Analysis in Australia

AbstractSeismic azimuthal anisotropy beneath Australia is investigated using splitting of the teleseismic PKS, SKKS, and SKS phases to delineate asthenospheric flow and lithospheric deformation beneath one of the oldest and fast‐moving continents on Earth. In total 511 pairs of high‐quality splitting parameters were observed at 116 seismic stations. Unlike other stable continental areas in Africa, East Asia, and North America, where spatially consistent splitting parameters dominate, the fast orientations and splitting times observed in Australia show a complex pattern, with a slightly smaller than normal average splitting time of 0.85 ± 0.33 s. On the North Australian Craton, the fast orientations are mostly N‐S, which is parallel to the absolute plate motion (APM) direction in the hotspot frame. Those observed in the South Australian Craton are mostly NE‐SW and E‐W, which are perpendicular to the maximum lithospheric horizontal shortening direction. In east Australia, the observed azimuthal anisotropy can be attributed to either APM induced simple shear or lithospheric fabric parallel to the strike of the orogenic belts. The observed spatial variations of the seismic azimuthal anisotropy, when combined with results from depth estimation utilizing the spatial coherency of the splitting parameters and seismic tomography studies, suggest that the azimuthal anisotropy in Australia can mostly be related to simple shear in the rheologically transition layer between the lithosphere and asthenosphere. Non‐APM parallel anisotropy is attributable to modulations of the mantle flow system by undulations of the bottom of the lithosphere, with a spatially variable degree of contribution from lithospheric fabric.

Active Degassing of Deeply Sourced Fluids in Central Europe: New Evidences From a Geochemical Study in Serbia

AbstractWe report on the results of an extensive geochemical survey of fluids released in the Vardar zone (central‐western Serbia), a mega‐suture zone at the boundary between Eurasia and Africa plates. Thirty‐one bubbling gas samples are investigated for their chemical and isotopic compositions (He, C, Ar) and cluster into three distinct groups (CO2‐dominated, N2‐dominated, and CH4‐dominated) based on the dominant gas species. The measured He isotope ratios range from 0.08 to 1.19 Ra (where Ra is the atmospheric ratio), and reveal for the first time the presence of a minor (<20%) but detectable regional mantle‐derived component in Serbia. δ13C values range from −20.2‰ to −0.1‰ (versus PDB), with the more negative compositions observed in N2‐dominated samples. The carbon‐helium relationship indicates that these negative δ13C compositions could be due to isotopic fractionation processes during CO2 dissolution into groundwater. In contrast, CO2‐rich samples reflect mixing between crustal and mantle‐derived CO2. Our estimated mantle‐derived He flux (9.0 × 109 atoms m−2 s−1) is up to 2 orders of magnitude higher than the typical fluxes in stable continental areas, suggesting a structural/tectonic setting favoring the migration of deep‐mantle fluids through the crust.

Seismicity and seismic hazard assessment in West Africa

No in-depth seismic hazard study for West Africa (WA) has ever been conducted, as the regional earthquake catalogues are incomplete. Such lack of comprehensive seismic hazard study for the region has negatively affected the planning and development of critical infrastructure and disaster risk management. Therefore, this study aims to bridge the knowledge gap by applying modern techniques to updating the existing catalogues and assessing the seismic hazards for the region. We updated the current earthquake catalogue for WA using information from the International Seismological Centre, published information, and data from seismic stations in the region. The seismotectonic setting of WA is considered stable continental crust despite several recorded earthquakes occurring in the region, the largest being the June 22, 1939 earthquake of magnitude M6.8 with epicentre in Ghana. However, WA has also been classified as a region of shallow crustal seismicity. Therefore, we investigated both these research schools of thought and compared their results. For each scenario, three different ground-motion models (GMMs) were applied and combined to produce each hazard map using logic tree formalism with equal weights. The region was divided into five seismic source zones for computation of the earthquake recurrence parameters, with the same parameters computed for the entire WA region. The computed Gutenberg–Richter b-value, activity rates λ, and regional maximum possible magnitudes mmax for the five zones ranged from 0.84 to 1.0, 0.3–2.1, and 5.2–7.0, respectively. The calculated b-value, λ, and mmax for the entire region were 0.77, 4.1, and 7.2, respectively. The estimated b-value of 0.77 falls within the generally accepted range for tectonic seismicity. The seismic hazard predicted by GMMs for stable continental areas was higher than that predicted for shallow crustal seismicity in the investigated region. Therefore, our results confirmed that WA is characterised by stable continental crust. The highest hazard levels were observed in parts of Ghana, Togo, Guinea-Bissau, Guinea, and the Cameroon Volcanic Line region (CVL), ranging between 0.02 g and 0.03 g.

Seismic risk in Ghana: efforts and challenges

In 1939, Ghana experienced its severest earthquake with a local magnitude of 6.5. It resulted in the death of 17 people and caused significant damage to property. Since then, there have been reported occurrences of earthquakes, though the region is generally considered a stable continental area with few active tectonic features. This seismic region is generally characterized by a major active fault line which stretches from the McCarthy Hill area in Accra westwards towards Kasoa-Nyanyanu in the Central Region and eastwards towards the Akwapim Ridge all the way to the Volta Region. The epicenter of the 22 June 1939 earthquake is located 25 km off the coast of Accra near Kasoa-Nyanyanu in the west. An earthquake occurrence and observation survey was conducted in March 2019, following three earthquakes recorded in March and December 2018 and March 2019 with local magnitude ranging from 3.0 to 4.8. The events were mostly felt by residents in the southern part of the country, who were consequently thrown into a state of panic. The main objective of the survey was to assess the perception, experiences, and adaptation strategies to seismic events of randomly sampled residents of the area. Questionnaires were administered during the survey in the Awutu Senya East and Weija-Gbawe Municipalities. It was found that though the majority of residents are aware that they live in an earthquake prone area, they have put little to no measures in place to mitigate the seismic risk.

Geodetic evidence for a buoyant mantle plume beneath the Eifel volcanic area, NW Europe

SUMMARY The volcanism of the Eifel volcanic field (EVF), in west-central Germany, is often considered an example of hotspot volcanism given its geochemical signature and the putative mantle plume imaged underneath. EVF's setting in a stable continental area provides a rare natural laboratory to image surface deformation and test the hypothesis of there being a thermally buoyant plume. Here we use Global Positioning System (GPS) data to robustly image vertical land motion (VLM) and horizontal strain rates over most of intraplate Europe. We find a spatially coherent positive VLM anomaly over an area much larger than the EVF and with a maximum uplift of ∼1 mm yr−1 at the EVF (when corrected for glacial isostatic adjustment). This rate is considerably higher than averaged over the Late-Quaternary. Over the same area that uplifts, we find significant horizontal extension surrounded by a radial pattern of shortening, a superposition that strongly suggests a common dynamic cause. Besides the Eifel, no other area in NW Europe shows significant positive VLM coupled with extensional strain rates, except for the much broader region of glacial isostatic adjustment. We refer to this 3-D deformation anomaly as the Eifel Anomaly. We also find an extensional strain rate anomaly near the Massif Central volcanic field surrounded by radial shortening, but we do not detect a significant positive VLM signal there. The fact that the Eifel Anomaly is located above the Eifel plume suggests that the plume causes the anomaly. Indeed, we show that buoyancy forces induced by the plume at the bottom of the lithosphere can explain this remarkable surface deformation. Plume-induced deformation can also explain the relatively high rate of regional seismicity, particularly along the Lower Rhine Embayment.

Quaternary development history of coral reefs from West Indian islands: a review

The Western Indian Ocean results from a complex geological history, implying both plate motion since about 180 million years and volcanic hot-spot activity since about 68 million years. This region exhibits a myriad of islands different in nature and origin, including continental remnants (Madagascar, the granitic Seychelles), high volcanic islands (Reunion, Mauritius, Rodrigues, the Comoros), high- and low-lying carbonate reef platforms (the carbonate Seychelles, the French Scattered Island Group, Agalega Islands) and a few submerged reef banks (Geyser, Nazareth, Saya de Malha). This region contains all known types of reefs—fringing, barrier, atoll, platform and bank. The data about reef development history over the Quaternary were gained mainly from two distinct sources: boreholes and emerged outcrops. Reef drilling indicates that modern reefs started regionally to grow from about 10,000–9600 years BP. Rates of vertical reef accretion ranges from 0.2 to about 10 mm/year according to exposure and shape of coral builders. The sea-level curves, reconstructed from vertical reef-accretion and lagoon-deposition curves, express a rapid rise in sea level at rates averaging 6 mm/year between 10,000 and 7500–7000 years BP, with a stabilization at the present position around 2500–3000 years BP. There are significant variations in the composition and distribution of dominant reef-building coral assemblages and facies (framework versus detritus) according to exposure to wave energy. Frameworks composed of robust coral branching assemblages—mainly Acropora robusta, A. humilis groups; Isopora palifera—dominate in higher energy settings, while those with domal and tabular branching forms—mainly Porites, various faviids, Acropora hyacinthus group—are best represented in lower-energy to protected environments. Detritus-reefs are best developed in sheltered sites. The recent contribution of some dominant coral species in modern WIO reefs seem to have resulted from their delayed arrival in the region as controlled by the regional prevailing surface circulation—South Equatorial Current and Agulhas Current—and/or by the competency period of their larvae. Coral-based, geochemical records suggest that, during the mid Holocene, the seasonal variability of sea surface temperatures was lower than today. The reconstruction of the WIO Pleistocene reef history is chiefly based on a limited number of exposed reef remains deposited during the last interglacial MIS 5.5 stage (129,000–125,000 years BP), at present culminating at elevations ranging from − 20 to + 9 m with respect to present mean sea level. Such differences in elevation can be explained by a differential tectonic behaviour of reef-bearing substrates, from stable continental areas to subsiding volcanic islands. The existence of older reef generations (Plio-Pleistocene) was identified from coral-rich inclusions trapped into pyroclastites of Comoro volcanoes.

Methods for Assessing the Seismic Hazard of Stable Continental Areas Using Combined Paleoseismological and Geophysical Data

Assessing the seismic hazard of stable continental areas, where modern seismic activity is insignificant, is a complex task that needs to be solved in order to ensure the safety of people and buildings in these territories. Traditional methods, including seismotectonic research, seismological monitoring, identification of seismic source zones (SSZs), and subsequent hazard assessment, are ineffective in such areas due to the lack of representative earthquake statistics and low informativeness of the data. In the seismic zoning of such areas (especially for solving seismic safety problems for critical objects, such as nuclear power plants), one cannot limit oneself to modern seismicity data: it is necessary to take into account rare seismic events, i.e., earthquakes with a large recurrence period. The joint study of active faults, earthquakes of historical record, and paleoearthquakes can significantly extend the time range of a strong-earthquake catalog and make it possible to identify new, previously unidentified SSZs. To assess the seismic hazard of territories in stable continental regions, the authors of this article propose using paleoseismological data together with the results of seismological and geophysical studies, which makes it possible to combine paleoearthquake and modern seismicity data. seismic observations are analyzed to obtain additional information on the stress–strain state of the study area and determine the earthquake focal mechanisms of identified SSZs. The results of geophysical studies make it possible to judge the existence of ancient but still “nonconsolidated” faults in SSZs, which may, in tandem with other research methods, indirectly confirm their correct identification as active. In the article, this approach is used to assess the seismic hazard of Karelia and the Kola Peninsula. The seismic impacts parameters of events with a long recurrence period are evaluated for individual objects and populated areas of this region.

Outgassing of Mantle Volatiles in Compressional Tectonic Regime Away From Volcanism: The Role of Continental Delamination

AbstractIn this study we discuss the occurrence of mantle‐derived heat and volatiles (i.e., helium and CO2) feeding hydrothermal systems in a seismically active margin between two convergent plates (African and European) without any signals of volcanism. The helium (He) isotopes clearly indicate a mantle‐derived component in the outgassing volatiles. The estimated mantle‐derived He fluxes are up to two to three orders of magnitude greater than those in a stable continental area. Such high He fluxes cannot be provided by a long‐lasting diffusion, thereby implying a more efficient transport (i.e., advective transport through faults). He data coupled to heat‐He relationship suggest the occurrence of active degassing of magmatic intrusions in this area of continental collisional. Geophysical data indicate the presence of a hot mantle wedge below the outgassing of mantle volatiles and a system of faults cutting the continental crust down to the hot mantle wedge. Here we discuss the hot mantle wedge and possible associated magmatic intrusions as the source of the mantle‐derived volatiles outgassing in the region. We also assessed the output of mantle‐derived CO2 from the investigated hydrothermal basins. The possible occurrence of magma at depth as well as the geometry of the thick‐skinned deformed wedge unambiguously indicates delamination processes that are related to continental subduction. Hence, we show that delamination processes can really produce magma at depth without evidences of volcanism at the surface. Finally, we have also provided the fault systems that work as a network of pathways and actively sustain the advective transfer of the mantle fluids toward the surface.

Geochemical and isotopic characteristics of Zangalline marble, Ranya area, Qandil Series, northeastern Iraqi Kurdistan Region: implications for genesis

The Zangalline marble of Paleogene age outcrops in Qandil Thrust Blocks, which are situated in NE Iraq close to the Iranian borders about (16) km from Haj Umaran town, NE Iraq within the thrust front and elevation ranging between (600–1568) m. The marble has been quarried for a long time and has been used as building stone and dimension stone. Chemical characteristics of these marbles are used to determine their provenance. The samples were subjected to a series of analytical techniques, including X-ray fluorescence and C-O isotope analysis. The Zangalline marbles have variable content of Si and Al from almost silicate rich rocks to pure marble and may have been deposited in shallow structural basins. The Chemical Index of Alteration and the Chemical Index of Weathering have the same percentage range values of 12.32–72.74% and 12.33–73.78% with an average value of 27.80 and 27.99 indicating that a low degree of weathering of the source materials must have taken place. The relatively high contents of K2O and Na2O suggest that the source rocks may have been exposed to very week physical and chemical weathering. The tectonic discrimination diagram suggests that the marble of Zangalline area is quartz-rich sediments and derived from plate interiors or stable continental areas and deposited in passive continental margins. The average carbon and oxygen isotope ratios of Zangalline marbles are 2.289% and 25.16%, respectively indicated that the deposit was a typical marine carbonate source and belong to similar geological units which the source may be Aqra Bakhma, Qamchuuqa Formations and/or Sarmord Formations, recrystallized at low temperatures in the open system coupled with either CO2 out gassing or fluid/rock interactions and the heat energy was most likely derived from nearby magmatic fluid/rocks.

High-pressure experimental verification of rutile-ilmenite oxybarometer: Implications for the redox state of the subduction zone

The more oxidized mantle peridotites above subducting slabs than stable continental areas have been attributed to the infiltration of some oxidizing fluids released from the subducting slabs. However, knowledge for the redox states of the slabs itself is very limited. Until now, few oxybarometers can be directly used to constrain the redox states of the subducting slabs. The rutile-ilmenite oxybarometer was proposed and successfully applied to constrain the oxygen fugacity of mantle assemblages. However, its application to rocks equilibrated at crustal P-T conditions has been hampered by some uncertainties in an early solid solution model of ilmenite. With a newly-released solid solution model for the ilmenite, we have conducted high-P experiments (at 3 and 5 GPa, and 900–1300°C) to test the accuracy of this oxybarometer. The experiments were performed with their oxygen fugacities controlled by the CCO buffer (i.e., C+O2=CO2). We demonstrated that the oxygen fugacities calculated for our high-P experimental products by using the rutile-ilmenite oxybarometer were in excellent agreement with the f O2 dictated by the CCO buffer, suggesting a wide applicability of this oxybarometer to crust rocks. As examples, the rutile-ilmenite oxybarometer has been used to constrain the oxygen fugacities of some metamorphic rocks such as eclogite, granulite and amphibolite usually observed from the subduction zones.

Towards a Semi-Kinematic Datum for Egypt

Since 1990, positioning technology has undergone a dramatic improvement in terms of accuracy and accessibility. Prior to 1990, national geodetic datums were considered to be static with fixed coordinates assigned to the datum origin by convention. Datum coordinates were dynamic only as a consequence of re-observation and re-adjustment, or localized disturbance. This strategy has been suitable for terrestrial surveys within stable continental areas. The accessibility of Precise Point Positioning “PPP” is now widespread with the provision of free online processing facilities by various geodetic agencies. Such ready accessibility to a global datum has serious implication for this technology with static geocentric datums. The Geocentric Datum of Egypt (ITRF1994 Epoch 1996 based on GPS Observation Campaign 1996), for example, is now offset by -42.0 to +32.0cm from the ITRF2008 Epoch 2015 due to the inexorable tectonic movements of the Nubian plate since the datum realization in 1996. Unless the local geocentric datums are kept in step with global terrestrial reference frames, discrepancies between the two will increase in magnitude over time and will become discernable, even using PPP. The PPP users nowadays are unaware of this datum offset and incorrectly assume that a coordinated datum monument is in error if there is found to be a disagreement greater than the level of accuracy of the positioning technique. In this study, to avoid the discrepancies between the applied datum and On-Line GNSS Services, a semi-kinematic datum and related deformation mode are proposed for Egypt. The result also confirmed that the PMM model was giving good performance for the other parts out of Egypt. The evaluation study shows the best performances for the Egyptian deformation model over the ITRF2008 PMM.

Zircon provenance in meta-sandstones of the São Roque Domain: Implications for the Proterozoic evolution of the Ribeira Belt, SE Brazil

Detrital zircons from quartzites of the São Roque Domain were subject of studies of crystal growth and stratigraphy, using in situ LA-ICPMS trace element geochemistry and U–Pb dating to deduce their provenance and contribute to understand the tectonic evolution of the Proterozoic metasedimentary successions of southeast Brazil. Trace element chemistry of most detrital cores indicates derivation from plagioclase-rich felsic rocks (e.g., tonalites to granodiorites); however, a small proportion may derive from gabbroic sources, as indicated by a combination of features (strong positive Ce anomaly, high LuN/SmN, low U/Yb; concave-down shape of the intermediate REE patterns). Thin metamorphic overgrowths are chemically distinct (e.g., enriched in several trace elements, especially the LREE), and were dated at 584±47Ma, reflecting the Ediacaran metamorphism and granite magmatism that affected the São Roque Domain. The detrital zircon U–Pb ages for five quartzite samples from different stratigraphic positions within the São Roque Domain are restricted to the Paleoproterozoic and Archean (∼1730–3440Ma). A striking feature of the dataset is a main peak at ∼2.2Ga which is characteristic of all samples; the age and abundance of Archean peaks allow distinguish two groups of samples: Jaraguá and Japi, with 41–43% Archean zircons and a peak at 2.6–2.7Ga; and Voturuna, Pirucaia and Serra da Viúva, with 16–21% Archean zircons and a peak close to the Archean-Paleoproterozoic boundary (2.4–2.5Ga). A limited contribution of late Paleoproterozoic (Statherian) sources places an upper limit for the deposition of the sedimentary successions that is consistent with the estimated age of the metaconglomerate-metarkose rift sequence of the São Roque Group (Boturuna Formation), based on U–Pb dating of interlayered meta-trachydacites (1.75Ga). Our data demonstrate that the different sedimentary successions of the São Roque Domain have an essentially continental (cratonic) provenance which is very similar to those of other sequences in the Ribeira Belt (Açungui and possibly also Embu Domain). The ∼2.2Ga age of the volumetrically most important sources, also indicated by the age of granite pebbles in metaconglomerates, is a diagnostic signature, consistent with the age of regional basement (e.g., orthogneisses from few basement nuclei in the Açungui Domain). Similarities in provenance and tectonic evolution of the metasedimentary sequences are consistent with the interpretation that the Ribeira Belt is part of the reworked border of the São Francisco Paleoplate, displaced southwestward as a result of significant dextral orogen-parallel tectonics during the Neoproterozoic collision, and may imply that a suture zone between this paleoplate and another stable continental area to the SW lies below the Phanerozoic sediments of the Paraná Basin.

Crustal thickness map of Brazil: Data compilation and main features

We present a crustal thickness map of Brazil and adjacent areas based on a compilation of data published in the literature as well as new measurements. We used crustal thicknesses mainly derived from seismic datasets such as deep seismic refraction experiments, receiver function analyses, and surface-wave dispersion velocities. Crustal thicknesses derived from modelling gravity anomalies commonly depend on assumptions, such as constant density contrast across the Moho interface, which are not always easily verifiable and were considered only along the continental shelf to fill large gaps in the seismic data. Our compilation shows that the crust in the stable continental area onshore has an average thickness of 39 ± 5 km (1-σ deviation) and that no clear difference can be observed between low altitude, intracratonic sedimentary basins, NeoProterozoic foldbelts (except for the Borborema Province), and cratonic areas. The thinnest crust is found in the Borborema Province of NE Brazil (30–35 km) and along a narrow belt within Tocantins Province (∼35 km), roughly parallel to the Eastern border of the Amazon craton, while the thickest crust is found in the Amazon and São Francisco cratons (41 ± 4 km), and the Paraná Basin (42 ± 4 km). Both the Ponta Grossa and the Rio Grande Arches are areas of thinned crust, and the western border of the Brazilian platform, near the sub-Andean region, seems to be characterized by a crustal thickness of less than 40 km. Although sparse in data coverage, we expect the resulting crustal thickness map to be useful for future studies of isostasy, dynamic topography, and crustal evolution of the country.

Lithospheric layering in the North American craton

How cratons-extremely stable continental areas of the Earth's crust-formed and remained largely unchanged for more than 2,500 million years is much debated. Recent studies of seismic-wave receiver function data have detected a structural boundary under continental cratons at depths too shallow to be consistent with the lithosphere-asthenosphere boundary, as inferred from seismic tomography and other geophysical studies. Here we show that changes in the direction of azimuthal anisotropy with depth reveal the presence of two distinct lithospheric layers throughout the stable part of the North American continent. The top layer is thick ( approximately 150 km) under the Archaean core and tapers out on the surrounding Palaeozoic borders. Its thickness variations follow those of a highly depleted layer inferred from thermo-barometric analysis of xenoliths. The lithosphere-asthenosphere boundary is relatively flat (ranging from 180 to 240 km in depth), in agreement with the presence of a thermal conductive root that subsequently formed around the depleted chemical layer. Our findings tie together seismological, geochemical and geodynamical studies of the cratonic lithosphere in North America. They also suggest that the horizon detected in receiver function studies probably corresponds to the sharp mid-lithospheric boundary rather than to the more gradual lithosphere-asthenosphere boundary.

Intraplate earthquake swarm in Belo Jardim, NE Brazil: reactivation of a major Neoproterozoic shear zone (Pernambuco Lineament)

SUMMARY Intraplate earthquakes in stable continental areas have been explained basically by reactivation of pre-existing zones of weakness, stress concentration, or both. Zones of weakness are usually identified as sites of the last major orogeny, provinces of recent alkaline intrusions, or stretched crust in ancient rifts. However, it is difficult to identify specific zones of weakness and intraplate fault zones are not always easily correlated with known geological features. Although Northeastern Brazil is one of the most seismically active areas in the country (magnitudes 5 roughly every 5 yr), with hypocentral depths shallower than ∼10 km and seismic zones as long as 30–40 km, no clear relationship with the known surface geology can be usually established with confidence, and a clear identification of zones of weakness has not yet been possible. Here we present the first clear case of seismic activity occurring as reactivation of an old structure in Brazil: the Pernambuco Lineament, a major Neoproterozoic shear zone. The 2004 earthquake swarm of Belo Jardim (magnitudes up to 3.1) and the recurrent activities in the nearby towns of Sao Caetano and Caruaru (magnitudes up to 4.0 and 3.8), show that the Pernambuco Lineament is a weak zone. A local seismic network showed that the Belo Jardim swarm of 2004 November occurred by normal faulting on a North dipping, E–W oriented fault plane in close agreement with the E–W trending structures within the Pernambuco Lineament. The Belo Jardim activity was concentrated in a 1.5 km (E–W) by 2 km (downdip) fault area, and average depth of 4.5 km. The nearby Caruaru activity occurs as both strike-slip and normal faulting, also consistent with local structures of the Pernambuco Lineament. The focal mechanisms of Belo Jardim, Caruaru and S. Caetano, indicate E–W compressional and N–S extensional principal stresses. The NS extension of this stress field is larger than that predicted by numerical models such as those of Coblentz & Richardson and we propose that additional factors such as flexural stresses from the nearby Sergipe-Alagoas marginal basin could also affect the current stress field in the Pernambuco Lineament.

Generation of heat flow map over a part of the Cambay Basin, India using NOAA-AVHRR data

Heat flow measurements carried out in the Cambay Basin, India indicate high heat flow values (1.8–2.2 μcal cm−2 s−1/77–92 mW m−2) compared to the normal heat flow values of approximately (1.4 μcal cm−2 s−1/58.6 mW m−2) for stable continental shield areas. This makes the region interesting from a tectonic as well as a geothermic point of view. The surface heat flow can be computed by multiplying the geothermal gradient by the rock conductivity. The paper develops a methodology to determine heat flow using a remote sensing technique. The same is then measured at a few points in-situ for validation and the methodology is then extended to a larger (potential oil basin) area using NOAA thermal infrared (IR) data. The comparison shows encouraging results at match points that may be useful for prospective hydrocarbon resources. The heat flow estimation technique using the remote sensing approach is, therefore, highly important and can be utilized as an alternative to conventional heat-flow estimation.

Geochemistry of shales from the Paleoproterozoic to Neoproterozoic Vindhyan Supergroup: Implications on provenance, tectonics and paleoweathering

Major, trace and rare earth element (REE) concentrations were measured for 88 shale samples from the Paleoproterozoic to Neoproterozoic Vindhyan Supergroup in central India. The major and trace element data of the Vindhyan shales are compared with North American Shale Composite, upper crust, mid-oceanic ridge basalts and many other shale standards of Archean, Proterozoic and Phanerozoic age. REE concentrations of the Vindhyan shales are comparable to the upper continental crust. Various discriminant diagrams were used to reveal the inferred provenance, tectonics and paleoweathering conditions. A predominantly granitic source rock is inferred for the Lower Vindhyan Shales, whereas partial contribution from basaltic source is evident during deposition of the Upper Vindhyan Shales. Existing paleocurrent data and the present geochemical investigations together suggest Archean–Paleoproterozoic basement rocks, exposed beyond the southern basin margin of the Vindhyan outcrops as the possible source rocks. The deposition of the Vindhyan shales took place in an intracratonic basin where sediments were primarily derived from stable continental areas. Moderate climatic conditions are inferred during the deposition of the shales. The Upper Vindhyan Shales, however, reflect more intense weathering compared to the Lower Vindhyan Shales, which may be attributed to relative tectonic stability and more humid conditions during deposition of Upper Vindhyan Shales.

Controls of stable continental lithospheric thickness: the role of basal drag

The lithosphere is the earth's strong outer layer that remains a coherent unit for long times during plate motions. The motions lead to shearing of its hot and ductile lower part. The base of the lithosphere is placed where the shearing is large enough to effectively decouple the coherent plates from the underlying mantle. Here the depth and temperature where this occurs under stable continental areas were modeled by using the experimentally determined creep properties of olivine (which depend on temperature, pressure and fluid content) for different geotherms and different magnitudes of the basal drag. It is found that the shearing increases rapidly downward in a ca. 20–25 km thick decoupling zone where the bottom of the lithospherecan can be placed. This zone is deeper for cooler geotherms and for smaller a basal drag, while the presence of fluids makes it shallower. In dry rock this zone is at the mantle adiabat or cooler when the basal drag is > 0.5 MPa. Under Archean cratons whose temperature is represented by xenolith geotherms the decoupling zone is 180–240 km deep. Higher crustal heat generation than in cratons (common in post-Archean terranes) by itself causes the decoupling zone to be a few tens of kilometers shallower, and it will be even shallower if the mantle heat flow is larger than under cratons. The base of the lithosphere need not follow an isotherm. As the lithospheric thickness depends on several factors it will change when any of these changes, which is expected to have happened many times in earth history. To persist the lithosphere should escape being sheared off laterally, regardless of its buoyancy.

Selected trace and minor elements in Asunción soft rocks

Geochemical processes in soft immature rocks of the Asuncion area are studied through X-ray fluorescense analysis of some trace and minor elements used as provenance indicators. Good correlation between light rare earth elements and other refractories were found and that the analytical data suggest inter alia an overall stable continental area setting.