Structural Province Research Articles

GEOLOGY OF THE FRONTEIRA OESTE RIFT, SOUTHERNMOST BRAZIL – A FIELD GUIDE

The Fronteira Oeste Rift located in the state of Rio Grande do Sul, southern Brazil, covers an area of 15,000 km2 and constitutes both a rhombohedral structural province and a paleohot spring field. These major features can be observed during a two-day field trip. A 4x4 vehicle is recommended. The weather is cold (freezing), humid climate during winter and hot (35 °C), varying from humid to dry during summer-. Excellent access to the outcrops is possible through paved and unpaved roads. Outcrops described in this field guide are stratigraphically located above the Botucatu Formation (paleoerg) and below the Serra Geral Group (volcanics) of the Paraná Basin. Sandstones of the Botucatu Formation were quickly filled by rainwater and after covered by a basalt (Mata Olho Flow) and a quartz andesite (Catalán Flow), thus originating the Guarani Paleoaquifer. The third Cordillera Flow (basaltic andesite) occurs in places, eventually covered by the Muralha Flow (basaltic andesite). Two additional basaltic andesites (UR13 and Coxilha Flows) were described in a drill hole. Aquifer heating by raised geothermal gradient led to pre-compaction cementation of sandstones by iron-rich minerals. Eolian quartz-rich sediments were deformed and hydrothermally altered in the composite paleodunes, showing thousands of circular, elliptical and curvilinear structures. Compaction caused upward motion of hot water + liquefied sand leading to the formation of many sand injectites and extrudites. Simultaneous doming of overlying rocks caused both large- and small-scale rhombi in sandstones and volcanic rocks. Post-compaction silicification occurred together with the formation of a myriad of 0.1-1.0 m-large hydrothermal vents at the surface of the paleoerg and also the first three lava flows. Features resulting from these processes can be visited, as described in this Geological Field Guide. A shorter geotrail is also described.

Deepwater fold-thrust belt contraction driven by mixed deformation components

The formation of fold-thrust belts is driven by compressive stresses caused by crustal tectonics, gravity, or a combination of both. The formation mechanism of a single compressive stress component has been thoroughly investigated by laboratory experiments and field observations. It is yet less well established how fold-thrust systems deform under multiple compressional stresses. The formation of the deepwater fold-thrust belt on the northwest Borneo margin has been previously explained by compressive gravity-driven stresses in the south and crustal-driven tectonics in the north. This study focuses on the transition zone between the gravity-driven south and crustal-driven north, and investigates the structural diversity around this transition using 3D seismic-reflection data. Structural interpretation results reveal two structural provinces distinct in deformation styles and structural kinematics within the deepwater fold-thrust belt. Folds and thrusts in each structural province reflect deformation stresses that occurred at different times with different orientation. Structural features in each structural province developed independently and were preserved largely without mixing in the fold-thrust belt, except for an interference zone of ca. 15 km in width. The fold-thrust belt can be interpreted to have formed by a multi-directional compressive stress system and exhibits greater structural heterogeneity than deformation caused by a single-component compressive stress system. Particular deformation characteristics and fault activity in the interference zone reflect a complex disparate compressive stress relationship.

The Tectonic Map and Structural Provinces of the Late Neoproterozoic Egyptian Nubian Shield: Implications for Crustal Growth of the Arabian–Nubian Shield (East African Orogen)

The Late Neoproterozoic Egyptian Nubian Shield (ENS) has attracted increasing attention since the establishment of the Egyptian Geological Survey and Mining Authority (EGSMA (1896)), which conducted the first mapping. In the last three decades, rapid improvements in analytical techniques, along with field-oriented studies, have made it possible for the interpretation of the ENS as an integral part of the juvenile Arabian–Nubian Shield (ANS) as a portion of the East African Orogen (EAO). However, a consistent tectonic map of the ENS does not exist. Presentation of such a map is the main objective of the present work where Landsat-based lithological discrimination is combined with the systematic structural investigation and careful reassessment of previously published geological maps. Our interpretation of this map indicates that in accordance with previous work, the basement units of the Eastern Desert (ED) can be divided into three structural domains: Northern, Central, and South-Eastern Desert provinces. The proposed provinces are ascribed to three different plate tectonic far-field boundary conditions. The earliest magmatic, metamorphic, and tectonic history was set off by the approximate north–south convergence of the Gabgaba-Gebeit, Jiddah-Asir terranes, and Eastern Desert-Midyan terranes along the Yanbo-Onib-Sol-Hamid-Gerf-Allaqi-Heiani (YOSHGAH) suture between c. 800 and 620 Ma. The second event between c. 640 and 580 Ma was correlated with the Nabitah Orogeny when the Ad Dawadimi-Ar Rayn terranes in the eastern Arabian Shield accreted to the earlier consolidated arc terranes (Afif and Tathlith terranes). During this period, east–west convergence between northwestern Saudi Arabia and the Central Eastern Desert (CED) in Egypt caused a pronounced strike-slip deformation associated with the displacement along the wider Najd Fault system. The third orogenic phase, which was younger than c. 580 Ma, was controlled by the evolution and retreat of the Cadomian Arc. The northern portions of the ENS and the northern Arabian Shield experienced extension whereas extensive post-orogenic magmatism was related to mantle delamination and associated crustal thinning.

Magnetostratigraphy of the Upper Cretaceous Nenjiang Formation in the Songliao Basin, northeast China: Implications for age constraints on terminating the Cretaceous Normal Superchron

We developed an integrated chronology for the non-marine Upper Cretaceous Nenjiang Formation based on high-resolution magnetostratigraphic results and previously published secondary ion mass spectrometry (SIMS) U–Pb zircon analyses of the eastern borehole of the Cretaceous Continental Scientific Drilling (CCSD-SK-II) borehole and two outcrop sections located in different structural provinces of the Songliao Basin, China. Detailed rock magnetic results demonstrated that pseudo-single-domain magnetite and single-domain greigite coexisted in the lacustrine black shales of the Nenjiang Formation, with the latter dominating remanence carriers. The reliable primary remanent magnetizations were isolated, which passed a class A positive reversal test and positive bootstrap reversal test, after correction for inclination shallowing, yielding a high-quality paleopole of 79.6°N/208.4°E, A95 = 2.3°. The borehole sequence and outcrop sections were stratigraphically correlated by combining lithostratigraphy, biostratigraphy, and SIMS U–Pb zircon geochronology. The correlation of the recognized magnetic polarity sequences to the geomagnetic polarity timescale suggests that the Nenjiang Formation from the CCSD-SK-II borehole sequence and the two outcrop sections span from very late chron C34n to very early chron C33r. Furthermore, the age of the Cretaceous Normal Superchron (CNS) termination can be constrained to 82.7 ± 0.6 Ma based on magnetostratigraphy, radiometric dating, and the perfect/typical averaged sediment accumulation rate for Member 1 of the Nenjiang Formation of the borehole sequence. The estimated age obtained in this study accurately represents the age at CNS termination.

Structural architecture and tectonic evolution of the Shabrawet Syrian Arc inverted zone at the Northern termination of the Gulf of Suez rift, Egypt

ABSTRACT The present study offers a novel opportunity to characterize the structural style and tectonic evolution of the Mesozoic Shabrawet inverted structure zone and its interaction with the northward propagation of the Gulf of Suez rift, Egypt. Based on the integration of fieldwork, detailed surface mapping and subsurface imaging by borehole data and seismic interpretation, the geology of Shabrawet area is characterized into the northern, central, and southern structural subareas. The central subarea has exposures of Cretaceous rocks whereas the northern subarea has the Jurassic/Cretaceous subcrop capped by the Oligo/Miocene rocks with remarkable stratigraphic hiatus probably due to differential erosion or non-deposition of the Lower Tertiary-Cretaceous sediments. The southern subarea represents a SW-dipped gulf-parallel block of Middle/Late Eocene and Oligo/Miocene sediments. The structural elements of the Shabrawet area are represented by a group of plunging and double plunging folds of NE-ENE-, E-W, WNW- trends that are dissected by NE-reverse faults, ENE-trending transpressional faults, N-S sinistral faults, E-W to WNW – trending dextral faults and NW-trending normal faults. Structural analysis, fault slip data, and the constructed subsurface seismic profiles and thickness maps reveal the tectonic history of the area through a Late Cretaceous-Eocene transpressional inversion of E to ENE-extensional Jurassic border faults. This ENE-inverted Syrian arc structure zone played as structural high with non-deposition of Eocene sediments and differential erosion of Cretaceous sediments. During the Oligo-Miocene NE-extension, this high-oblique inverted zone terminates the northward propagation of gulf parallel faults and SW- block rotation of the northern structural province of the Gulf of Suez rift at the southern subarea and the central Bitter Lake. Further westward, this ENE-inverted Shabrawet-Abu Sultan intrabasinal Jurrassic half-graben of N- dipping might be structucturally correlatable with the NE- inverted structural belt of Gebel Maghara in Northern Sinai.

The statherian taphrogenesis of the South American Platform

Records of Statherian intracontinental ruptural tectonics, magmatism and sedimentation events are recognized all over the South American Platform as an immediate tectonic response to the formation of a large continental domain during Rhyacian-Orosirian orogenic collage. In this paper, we present a preliminary synthesis of the variety of tectonic-sedimentary and magmatic occurrences of these taphrogenic processes along the Statherian Period, which are recorded in all cratons, as well as in the basement of most Brasiliano orogenic belts. The Statherian rock units are better preserved over the cratons, and within the Brasiliano structural provinces they are reduced by erosional processes and discretely overprinted by Tonian and Cryogenian structures, and strongly by the Ediacaran-Brasiliano Orogeny. Clastic sediments are dominantly mature, primary structures clearly pointing to the environmental setting. Anorogenic bimodal magmatism, represented by volcanic and plutonic rocks is common, where felsic rocks are largely dominant. Their geochemical signature indicates within-plate settings and dike swarms are common within the cratons. The magmatism was very intense locally, suggesting LIP occurrences, although in need of additional geophysical and petrological support. The taphrogenic and magmatic events took place within Orosirian and Rhyacian basement, in which main tectonic-magmatic events ceased just before the Late Statherian, occasionally extending into the Calymmian.

Integrated assessment and prospectivity mapping of geothermal resources for EGS in Brazil

Studies in Brazil related to geothermal resources have been carried out mainly by the National Observatory. Based on the results vast areas of Brazil have been delimited with a surface heat flow of over 60 mW/m2 and an excess temperature above 100 °C at depths between 3 and 6 km. The nature of the heat source and the mode of heat dissipation in these areas are largely unknown. Geothermal resources that might be associated with these areas can occur in hot sedimentary aquifers (HSA) and units of hot dry rocks (HDR) that are hosted, respectively, in intracratonic sedimentary basins and high heat producing granites (HHPG). The focus of our study is on finding resources of high enthalpy (above 150 °C) in HHPG, which regardless of their hydrological properties (porosity, permeability) could be considered as potential targets for enhanced geothermal systems (EGS). The structural provinces of Tocantins and Borborema were chosen as pilot regions based on both the indication of HDR and the wealth of open access georeferenced information. A comprehensive database was compiled and processed under GIS that includes geology, faults, soil typology, radiogenic heat, heat flow and geothermal gradient from wells, thermal spring locations, airborne gamma-ray spectrometry and magnetic surveys, terrestrial gravimetric surveys, and earthquake locations. In parallel, various critical factors were defined primarily to identify and characterize the most promising HHPG under insulating sedimentary rocks for EGS. These critical factors were grouped into three main categories labelled as: ‘thermal source’, ‘thermal insulation’ and ‘active pathways’. In the absence of exploratory data on existing EGS in Brazil these critical factors were used in this study along three separate work streams to develop a knowledge-driven model based on fuzzy logic for improving the knowledge on the prospectivity of geothermal resources. The developed model was further tested for its validation. The selected approach is expected to be applicable to mapping areas with EGS potential in other regions with similar geology and open access georeferenced databases.

Updated mapping of terrestrial heat flow in Brazil

An updated geothermal database is presented by providing a new mapping of terrestrial heat flow over the entire Brazilian territory. New estimates of this geothermal parameter were obtained using an indirect method based on groundwater silica content (silica geothermometry) measured in 4949 wells. The Brazilian Geothermal Database of the Geothermal Laboratory from the National Observatory currently comprises geothermal data obtained directly from temperature-depth profiles mostly in hydrocarbon wells and from several other records in groundwater wells using indirect temperature estimates. The procedure of indirect temperature estimates used here assumes that the amount of silica dissolved in groundwaters depends on the in-situ temperatures of the host rocks or geothermal reservoirs. Following this initial calculation procedure, the heat flow, the depth of the aquifer and the temperature at the reservoir bottom were calculated using a numerical relationship that was specifically modified and adapted to the Brazilian geological context. Measurements of silica content in groundwater wells were obtained from the Groundwater Information System of the Geological Survey of Brazil. When available in the database, the lithological profiles of these wells were additionally used to calculate weighted average thermal conductivity by considering the documented lithotypes and their respective thicknesses. For wells without lithological information, their thermal conductivity was reassessed by including information from the nearest documented well lithotypes and that belong to the same geological context. The results indicate that regions encompassing a system of fractured aquifers, with a mean silica content of 40 ppm, show the highest heat flow values of the order of 70 mWm2 on average. Regions with karstic aquifers, characterized by a mean silica content of 29 ppm, display most of the lowest heat flow values ≤40mWm2. Regions with porous aquifers of mean silica content between 30 and 33 ppm present a range of intermediate heat flow values between 50 and 60 mWm2. Taking into account the different associated lithological types, the Brazilian structural provinces display values of thermal conductivity ranging between 2.1 and 2.7 WK.m. The refinement in mapping terrestrial heat flow in Brazil indicates various thermal anomalies at regional scale that can be further studied for prospecting geothermal resources of low-to-high temperature. These regional thermal anomalies are essentially found in the NW region of the Paraná Basin, N of the Tocantins Province (western portion), south-central Tocantins Province (eastern portion), north-central part of the Sáo Francisco Craton and the NE region of the Borborema Province.

Deciphering salt tectonic deformation patterns in the Eastern Mediterranean: Insights from the Messinian evaporite in the Herodotus Basin

Salt-bearing basins are commonly deformed in response to gravity mechanisms (gliding and spreading) and regional tectonics. However, these processes often overlap both in time and space that challenges the understanding of the main causes for salt flow and overburden deformation. In this study, salt-related deformation is investigated at the eastern part of the Herodotus Basin in the Eastern Mediterranean. Deformation in this area is associated with the flow of Messinian salt (evaporite) that was deposited during the Messinian Salinity Crisis (MSC). The MSC sequence is relatively young and is characterized by a thin overburden, and as such provides a unique opportunity to explore key controls of salt-related deformation. High-quality 2D seismic reflection data from the broader area of the carbonate platform of the Eratosthenes Continental Block, which constitutes the natural boundary between the Herodotus Basin to the west and the Levantine Basin to the east, reveal significant thickness variations of the MSC sequence and the overlying Plio-Quaternary clastic sediments. Five distinct salt-tectonic structural provinces characterize the study area that depicts different salt-related deformation styles. Furthermore, the morphology of the Eratosthenes Continental Block plays a critical role in controlling salt flow. Finally, the compressional thickening of the Messinian salt driven by the combination of gravity gliding and spreading results in the absence of a bathymetric step on the seafloor at the western side of the Eratosthenes Continental Block.

Mapping the basement architecture using magnetotelluric data across a coastal part of the Borborema structural province, Ceará – Brazil

The Borborema Province (BP) constitutes part of the west Gondwana supercontinent formed by several tectonic collisions during Neoproterozoic, widely known as Brasiliano - Pan-African orogeny. It consists of a complex tectonic mosaic composed mainly by Archean to Paleoproterozoic migmatite-gneiss basement complexes, Proterozoic metasedimentary belts, a large volume of Neoproterozoic granitic intrusions, and Phanerozoic covers that represent ca. 25 % of the total area of BP. This work presents the results from 21 Magnetotelluric (MT), and electromagnetic time-domain (TDEM) soundings interpretations carried out along a 120 km profile in a coastal area of the northeast Borborema Province covered by Phanerozoic sedimentary rocks and sediments. The final geoelectric model allows delineating the architectural geometry of the subsurface and variations in physical properties of different crustal blocks bounded by major electrical discontinuities. The interpretation was based on a 2D and 3D model from the inversion of the MT data. The 2D model has a visually satisfactory agreement with the region's geological framework and a reasonable resolution of the upper crust compared to the 3D model. The geoelectric section allows delineating the variation in physical properties of different upper crust blocks bounded by major electrical discontinuities. The result enables the characterization of 1) the uppermost structures correlated to the Mesozoic to mid-Cenozoic sedimentary formations; 2) covered lithotectonic framework represented by lithotypes and main shear zones related to Ceará Central and Rio Grande do Norte crustal tectonic domains; 3) distinctive anomalies corresponding to the Senador Pompeu, Orós, and Jaguaribe Neoproterozoic main shear zones. Our interpretations agree with the results and interpretation presented by previous MT works in the Borborema Province, which reveals the existence of an important conductive zone, interpreted as a suture zone, spatially related to the Senador Pompeu shear zone and in smaller proportion to the Orós shear zone. The results presented here are a starting point to further MT long-period investigation to understand this complex part of the Borborema Province.

Syncontractional deposition of the Cretaceous Newark Canyon Formation, Diamond Mountains, Nevada: Implications for strain partitioning within the U.S. Cordillera

Abstract The timing of deformation and deposition within syntectonic basins provides critical information for understanding the evolution of strain in mountain belts. In the U.S. Cordillera, contractional deformation was partitioned between the Sevier thrust belt in Utah and several structural provinces in the hinterland in Nevada. One hinterland province, the Central Nevada thrust belt (CNTB), accommodated up to ∼15 km of shortening; however, in most places, this deformation can only be bracketed between Permian and Eocene. Cretaceous deposits of the Newark Canyon Formation (NCF), which are sparsely exposed along the length of the CNTB, offer the opportunity to constrain deformation timing. Here, we present mapping and U-Pb zircon geochronology from the NCF in the Diamond Mountains, which demonstrate deposition of the NCF during proximal CNTB deformation. Deposition of the basal NCF member was under way no earlier than ca. 114 Ma, a tuff in the middle part of the section was deposited at ca. 103 Ma, and the youngest member was deposited no earlier than ca. 99 Ma. Intraformational angular unconformities and abrupt along- and across-strike thickness changes indicate that NCF deposition was related to growth of an east-vergent fault-propagation fold. Clast compositions define unroofing of upper Paleozoic sedimentary rocks, which we interpret as the progressive erosion of an anticline ∼10 km to the west. CNTB deformation was contemporaneous with shortening in the Sevier thrust belt, which defines middle Cretaceous strain partitioning between frontal and interior components of the Cordillera. Strain partitioning may have been promoted by renewed underthrusting during a period of high-flux magmatism.

New geochronological constraints for the Upper Cretaceous Nenjiang Formation in the Songliao Basin, NE China

The Nenjiang Formation in the Songliao Basin, northeastern China, is of great significance because it records a series of geological, geodynamical and paleoenvironmental events, such as lake transgression events, lake anoxic events, sea water incursion events, Cretaceous Normal Superchron, and important stratigraphic boundaries. Here we report a chronology for the non-marine Upper Cretaceous Nenjiang Formation based on secondary ion mass spectrometry (SIMS) U–Pb zircon analyses from the east borehole of the Cretaceous Continental Scientific Drilling and two outcrop sections, which are located in different structural provinces of the Songliao Basin. Stratigraphic correlation between the borehole sequence and the exposed outcrop sections is achieved by combining lithostratigraphy, biostratigraphy, and SIMS U–Pb zircon geochronology. Two bentonite layers are recognized in the Nenjiang Formation: the older occurs at the formational contact between the Yaojia and Nenjiang Formations; the younger occurs between Members 1 and 2 of the Nenjiang Formation. SIMS U–Pb zircon dating yields ages of 85.1–85.2 Ma and 83.0–83.3 Ma, respectively, representing ages of the boundaries of the Yaojia/Nenjiang Formations and Members 1/2 of the Nenjiang Formation, as well as ages of lake transgression events and associated lake anoxic events, and two of the sea water incursion events. Furthermore, the age of Cretaceous Normal Superchron termination can be estimated at 82.5–82.8 Ma.

The influence of base-salt relief, rift topography and regional events on salt tectonics offshore Morocco

Abstract This study integrates borehole-calibrated 2D and 3D seismic interpretation with numerical models to provide a regional analysis of the complex salt tectonics offshore central Morocco. We investigate the mechanisms controlling along-margin structural variations, the effects of thick-skinned shortening and the sequential evolution of allochthonous sheets. Additionally, we analyse how base-salt relief generated complex flow kinematics and alternation of extensional and contractional domains by causing flux variations at both autochthonous and allochthonous salt levels. The area is divided into three structural provinces, with the central, Essaouira segment, having greater downdip translation, structural complexity and volume of allochthonous salt, which suggests the salt was originally thicker and better connected across multiple syn-rift structures. The southern segment, Agadir, is dominated by up-right squeezed diapirs formed by early load-driven rise and late shortening; limited salt and overburden translation and no allochthonous sheets. The northern segment, Safi, is narrower and has a smaller number of salt structures that were affected by abrupt translation due to steep detachment gradient. Late, oblique thick-skinned shortening generated contractional structures in the entire salt basin, which are most prominently developed in Essaouira due to a favourably-oriented NW-SE syn-rift structure, the Talfeney Accommodation Zone. Allochthonous salt sheets formed during four phases from the Albian, Late Cretaceous, Paleocene to Oligo-Miocene by two main mechanisms: contraction and basinward salt expulsion. This work improves understanding of the structural configuration along the Moroccan margin guiding the identification of potential sub-salt plays and contributing to a better comprehension of salt-related deformation along rifted passive margins worldwide.

A Review of the Geology of the Rio Del Rey Basin, Offshore Cameroon

A review of the tectonic evolution, stratigraphic and structural framework, petroleum systems and hydrocarbon potential of the offshore portion of Rio Del Rey (RDR) Basin in Cameroon is presented in this paper. The RDR basin is a passive margin basin present in the Gulf of Guinea and it is located southeast of Niger Delta over an area of about 7,000 km2 offshore. Its stratigraphic components, like those of Niger Delta, comprise of a diachronous succession of prodelta shales (Akata Formation), overlain by delta front sands and shales (Agbada Formation), in turn, overlain by fluvial sands (Benin Formation). The age of this basin ranges from Eocene to Recent and it consists of four structural provinces namely; the growth fault province in the north, the Cretaceous Onshore province in the southeast, the Shale Ridge province in the southwest, and the Delta Toe-thrust in the south-central region. The RDR Basin features three stages of tectonic development: Pre-rift phase (Late Proterozoic to Late Jurassic); Syn-rift phase (Late Jurassic to Early Cretaceous) and Post-rift phase (Late Cretaceous to Holocene). It presently stands out as the major contributor of the petroleum in the country.

Space-based observations of crustal deflections for drought characterization in Brazil

Widespread environmental impacts of frequent drought episodes in Brazil have resulted in several drought-related diagnostics studies. However, the potential of many “opportunistic sensors”, such as the Global Positioning System (GPS), has not yet been considered in hydrological hazard monitoring in Brazil. In this study, the response of the Earth's crust to Brazil's 2012–2015 drought event in different structural provinces is analyzed by comparing GPS-observed vertical crustal deformations (VCDs) with the terrestrial water storage (TWS) derived from the Gravity Recovery and Climate Experiment (GRACE). The results indicate that there is no spatial correlation between annual amplitudes of the TWS and VCDs in different structural provinces apart from the purely elastic response of the crust to TWS dynamics, at almost all the 39 GPS stations that were analyzed. However, approximately 15% of the monitoring stations show that VCD leads TWS with a phase lag of 2–4 months. Errors associated with VCD and TWS are within the accepted range for space geodetic techniques (i.e., GPS and GRACE) and despite the need for further investigation, the phase lead seems to be associated with rainfall, which impacts the TWS through the hydrographs. Overall, the GPS-based drought index (DIVCD) reflects the water depletion in many regions of Brazil, which agrees with the GRACE-based DITWS in terms of the Spearman correlation coefficient (ranging from 0.4 to 0.9) in the Amazon, Tocantins, La Plata, and São Francisco river basins. This agreement confirms the drought persistence during the study period and that DIVCD can be used to monitor hydrological droughts. In regions in which DITWS sufficiently agrees with DIVCD (48% of the sites), near real-time drought monitoring is feasible. This could be useful in the optimization of models for the forward prediction of drought events in other regions worldwide, where GPS vertical displacements strongly correlate with hydrological GRACE signals.

Natural radioactivity in soils of the state of Rio de Janeiro (Brazil): Radiological characterization and relationships to geological formation, soil types and soil properties

Located in the south-western part of Brazil, the state of Rio de Janeiro is geotectonically contained within a complex structural province that resulted in the amalgamation of the Western Gondwana Paleocontinent. To undertake an extensive radiological characterization of this complex geological province and investigate the influence of bedrock, soil type and soil chemical-physical characteristics on natural radionuclide levels in soils, 259 surface soil samples were collected that encompassed the main soil types and geological formations throughout the state. Gamma spectrometry analysis of the samples resulted in median values of 114 Bq.kg−1for 40K, 32 Bq.kg−1 for 226Ra and 74 Bq.kg−1 for 228Ra. The median value for 226Ra was similar to the world median value for soils, the 40K value was well below the worldwide value, and that for 228Ra exceeded the world median value. The intense weathering caused by the high rainfall rates and high temperatures may be responsible for the low levels of 40K in the soils, of which the strongly acidic and clayey soils are markedly K-depleted. A soil from a high-grade metamorphic rock (granulite) presented the lowest 226Ra (18 Bq.kg−1) content, whereas the highest levels for 226Ra (92 Bq.kg−1) and 228Ra (139 Bq.kg-1) were observed in a young soil enriched in primary minerals (Leptsol). A lowland soil (Gleysol) showed the highest median of 40K (301 Bq.kg−1). Strongly acidic soils tended to present high amounts of 226Ra, and sandy soils tended to contain low levels of 228Ra. The external radiation dose indicates that the state has a background radiation level within the natural range.

Crustal structure of the Amazonian Craton and adjacent provinces in Brazil

The study of the crust using receiver functions can provide valuable geological information, such as average crustal composition, its formation dynamics and the tectonic evolution of a region, as well as serve as an initial reference for the generation of seismic wave velocity models to improve earthquake location. To fill in gaps in information on the crust of the Amazonian Craton and adjacent provinces in Brazil, we used receiver functions and H-k stacking to estimate crustal thicknesses and the VP/VS ratios. The results indicate that the crust of the study region is predominantly felsic, with an average VP/VS around 1.73 and an average thickness of 38.2 km, with a range of 27.4–48.6 km. Minimum curvature interpolation of the crustal thickness values has made it possible to delimitate of the Amazonian Craton, which corresponds to the area with an average thickness equal to or greater than 39 km. In addition, it was possible to identify its potential cratonic blocks, as well as the Paranapanema Block of Paraná Basin. The geometry of the craton, defined by its crustal thickness, is corroborated by the distribution of natural seismicity that accompanies its edges. These are related to suture zones between the Amazonian, São Francisco/Congo and Paranapanema paleocontinents. The sedimentary basins that have undergone rifting processes have a thinner crust, usually less than 37 km thick. Due to the great variability of the results, it was not possible to determine a characteristic value of crustal thickness or VP/VS ratio for each structural province located in the study region.

A window into the Cambrian basement and early Carboniferous sedimentation of the Hathern Shelf: the British Geological Survey borehole at Ticknall, South Derbyshire, UK

The Ticknall Borehole was drilled in 1995 to a depth of 209 m in order to investigate the succession on the Hathern Shelf, a fault-bounded structural province that lies on the southern margin of the Widmerpool Half-graben, part of a major early Carboniferous rift system. The borehole demonstrated a ‘basement’ of Upper Cambrian cleaved mudrocks, correlated with the Stockingford Shale Group. Unconformably overlying this is a basal Carboniferous unit, the Calke Abbey Sandstone Formation, of probable Visean (?Holkerian) age, comprising 82.34 m of fluvial sandstones and conglomerates, with interbedded red-grey palaeosols exhibiting highly distinctive ‘pseudogley’ fabrics indicative of emergent episodes. The unit may have been deposited in a localized, possibly fault-controlled basin and was in part sourced from the Precambrian volcanic terrain of Charnwood Forest. As rifting and subsidence proceeded, the encroachment of nearshore/peritidal environments is indicated at the top of the formation by interbedded calcilutites that have yielded marine faunas. Fully marine conditions were established during deposition of the overlying Peak Limestone Group, comprising the early Asbian Cloud Hill Dolostone Formation succeeded by the Ticknall Limestone Formation, of Brigantian age. Comparisons between the Ticknall Borehole and Peak Limestone strata exposed in quarries farther east, around Breedon, show major changes in water depths over a distance of only 4.5 km. Such variations can be reconciled with seismostratigraphical studies in the adjacent Widmerpool Half-graben, which show that sedimentation on the Hathern Shelf was in part controlled by movements along nearby rift-bounding faults.

The Grenvillian Namaqua fold belt adjacent to the western Kaapvaal Craton: 2. Archaean Craton and supercontinent connections

The Namaqua sector of the Namaqua-Natal Province consists of large tectonostratigraphic terranes, some of which includes Paleoproterozoic fragments inherited from the supercontinent Columbia. Most of the terranes became part of Rodinia during the Mesoproterozoic Namaqua Orogeny (1.3–1.0Ga, Grenvillian). The terranes in the western foreland of the Kaapvaal Craton (Kheis Subprovince) form a thin-skin fold and thrust belt (5–15km depth to sole thrust). Westerly directed thrusting towards the hinterland is reminiscent of thick-skin tectonics, associated with the exhumation of high grade facies terranes, such as the granulitic and migmatitic Grünau Terrane. Upper amphibolite Upington Terrane was thrusted from the west onto thrust sheets contiguous to Kaapvaal Craton, in turn over-ridden near Prieska by the Grünau Terrane. Severity of the ductile deformation is illustrated by regional shear zones, ubiquitous recumbent isoclinal folds and allochthonous mega sheath folds in the Grünau Terrane. Vergence reversal of fold and thrust structures along a narrow (10–20km) northwesterly trending zone (Vergence Inversion Zone, VIZ), represents a crustal scale pop-up structure coincident with the Namaqua Front, a paired low-high gravity anomaly linked to a mega-scale anticline-syncline pair caused by deep-seated thrusting of the Columbian Moho.Late Columbian extension (1.6–1.3Ga, diachronous with early Rodinian events) produced parautochthonous basins with volcano-sedimentary sequences, deformed during the Namaqua Orogeny. Early to mid-Columbian (2.3–1.9Ga) rocks form basement to the mobile belt, with Steinkopf Terrane as type outcrop area (probably time-equivalent with the Grünau and Pofadder Terranes, Sperrgebiet Domain, Rehoboth structural Province and part of the Congo Craton). The western boundary of the Kaapvaal Craton (and composite Kalahari Craton) exhibits structural wedge edges of Archaean Kaapvaal basement and Ventersdorp-Vaalian basins. Columbia-Kaapvaal Detachment (COLKAD), a décollement between Kaapvaal and Columbia cratons (footwall) and Olifantshoek and other Namaqua terranes (hanging wall) extends for >600km westwards.Namaqua crust was thickened (20–25km) by both structural stacking and sheeted intrusions (Namaqua tectogenesis) with clockwise PTt-paths in the west-central Namaqua sector (Olifantshoek, Grünau, Pofadder, Bladgrond Terranes). In the western part (Okiep/Garies Terrane), crust was dominantly thickened by massive sheetlike granitoid intrusions resulting in granulite grade and anti-clockwise PTt-paths. COLKAD-rooted inter/intra-terrane thrusts are subhorizontal except where rotated by subvertical (discrete) shear zones (e.g. Tantalite Valley and Brakbos-Doringberg-Dabep). Crustal thickening and granulite-uplift stimulated temperature rise with anatexis-palingenesis of early Columbian/supracrustal rocks. Kilometer-scale granitoid bodies, derived from predominantly Columbian crust (according to isotopic source rock models) that are emplaced along shear zones, caused a rise in the brittle to ductile transition zone (i.e. mid-crustal conditions) and low-pressure granulite facies.

Structural provinces of the Ross Ice Shelf, Antarctica

ABSTRACTThe surface of the Ross Ice Shelf (RIS) is textured by flow stripes, crevasses and other features related to ice flow and deformation. Here, moderate resolution optical satellite images are used to map and classify regions of the RIS characterized by different surface textures. Because the textures arise from ice deformation, the map is used to identify structural provinces with common deformation history. We classify four province types: regions associated with large outlet glaciers, shear zones, extension downstream of obstacles and suture zones between provinces with different upstream sources. Adjacent provinces with contrasting histories are in some locations deforming at different rates, suggesting that our province map is also an ice fabric map. Structural provinces have more complicated shapes in the part of the ice shelf fed by West Antarctic ice streams than in the part fed by outlet glaciers from the Transantarctic Mountains. The map may be used to infer past variations in stress conditions and flow events that cannot be inferred from flow traces alone.