Exposure Of Rocks Research Articles

Tectonic evolution of Juventae Chasma, Mars, and the deformational and depositional structural attributes of the four major light-toned rock exposures therein

Juventae Chasma, a canyon peripheral to the Valles Marineris trough system, is the product of a unique merger of a Martian chaotic terrain depression and an extensional (tectonic) chasm. The chaotic terrain portion is set along a NNE-SSW axis, while a linear trough (the chasm component) is present along a WNW-ESE axis. Within the chasm are four discrete bodies of layered, light-toned rocks of sedimentary origin, Features A–D. The chaotic terrain component formed during the Early Hesperian as a result of expulsion of liquid water along pre-existing planes of weaknesses resulting from a WNW-ENE compression. The chasm component formed in response to NE-SW extension during the Middle to Late Amazonian. The light-toned Features A–D are layered at <2 m scale. A dark-toned material, previously interpreted as interstratified with the light-toned layers, is a secondary accumulation deposited after the light-toned rocks were lithified and exposed at the Martian surface. The bedding configurations within Features A, C, and D indicate eolian sand deposition; whereas a lacustrine or low-energy eolian origin is more fitting for Feature B. Features A–D also record Early Hesperian compressional events in the form of joints. Bedding joints, formed in response to release of an overburden, are also found. In addition, closed, elliptical outcrop patterns of inward dipping layers are also identified; these are either folds (structural basins) or erosional surfaces cutting three-dimensional eolian cross-beds.

Conservation and Sustainable Development of Geoheritage, Geopark, and Geotourism: a Case Study of Cenozoic Successions of Western Kutch, India

Geoheritage is a concept concerned with the preservation of features with importance to earth science, such as landforms, natural exposures of rocks, and sites where geological features can be examined for further study, reference, and conservation for coming generations. Geoheritage, geoconservation, and geotourism studies are gaining interest worldwide because of their scientific, academic, historical, societal, cultural, and esthetic values. Several countries have their government policy to look after the geoheritage sites and conserve with the help of local agencies; however, in India, the concept is still to be applied in a holistic way, and there is a need of such national legislation. Kutch basin as a whole preserves the records of sedimentation for the past ~ 200 Ma. The Cenozoic (last 65 Ma.) sedimentation (~ 900 m) is considered a strato-type section for the shallow-marine sedimentary records in India. Some Cenozoic sections of Kutch are the only remained reference sections of the respective periods on the globe. This sedimentary succession acts as a geological museum for paleontology, stratigraphy, biostratigraphy, and sequence stratigraphy. Such records of past climates and sea-level fluctuations are characterizing the global sea-level history. It also provides an outcrop analog for reservoir rocks of Bombay High oil field. Taking this into account, we propose seven selected sites in the Cenozoic succession to be preserved as geoheritage sites and development of geotourism in the region viz. Matanomadh cliff section, Naredi cliff section, Fulra limestone section, Miocene succession, Paleosol of Sandhan Formation, Kharai River paraconformity section, and Bermoti River section. The advantage of a geopark comprises also in creating new employment opportunities for local youths. Further, it will help enhance the local economy by infrastructure development, health, and educational pursuits to the village-level society. The prime objective of this investigation is to make local people aware about sustainable mining and insatiable thrust for money that threatens ecologically important sites. The geological records are the consequences of millions of years of processes and are considered precious which require special care. If these records once destroyed will be lost forever and cannot be restored artificially, it is our prime responsibility to transfer the knowledge and geoheritage to future generations.

The effect of scanline direction and extent of rock exposure on assessment of geometrical properties of discontinuities in rock mass

In this study, the borehole coring method and discontinuity scanline survey method are used to characterize the rock mass for the engineering design of underground structures at a dam site. Scanline survey is conducted on rock exposures in an exploratory adit. Cores from a borehole drilled near to the exploratory adit are also examined. The purpose of this study is to highlight the merits and demerits and the capacity of each method to characterize the geometrical properties of discontinuities. It is observed in the study area that the discontinuity scanline survey being conducted on large exposures, provides good results in estimating the orientation, trace length, roughness and interconnectivity of discontinuities. However, the direction of scanline introduces bias in results, because discontinuities parallel to the scanline line are not considered. The borehole method provided a good reflection of the bed rock; however, it presents a very rough estimate of geometrical properties of discontinuities. Moreover, estimation of areal extent of discontinuity plane is not covered by both the methods. However, engineering judgement by experienced engineering geologists can help in taking an informed guess.

Mesoarchaean-Palaeoproterozoic crustal-scale tectonics of the central Witwatersrand basin - Interpretation from 2D seismic data and 3D geological modelling

The Witwatersrand Basin represents one of the largest exposures of Meso-Neoarchaean rock on Earth and hosts the Vredefort Dome at its geographic centre. The southern half of the basin is covered by thin Palaeozoic to early Mesozoic marginal sequences of the Karoo Supergroup. However, visualisation and analysis of this unexposed portion of the basin can be achieved in a 3D geomodelling environment through the integration of geophysical (2D reflection seismics) and traditional geological data. In this study, the 3D structural architecture around the Vredefort Dome in the Witwatersrand Basin is evaluated to establish strato-tectonic relationships, first-order scale structures, and a new model for the architecture of basement rocks. From the geological modelling, several strato-structural features have been identified in the seismic sections, including a well-developed listric fault system in the southwest, a prominent fold system observed in the Transvaal Supergroup, and a large listric fault in the east that offsets the aforementioned fold system. Integration of these geophysical and geological data (in 3D space) has provided 3D constraints on the volumes for the Witwatersrand Supergroup, Ventersdorp Supergroup and Transvaal Supergroup to the south, southeast and east of the Vredefort Dome.

The Archean Victoria Fjord terrane of northernmost Greenland and geodynamic interpretation of Precambrian crust in and surrounding the Arctic Ocean

In far North Greenland at the head of Victoria Fjord (∼81°30′N), a ∼1000 km2 exposure of Precambrian crystalline basement rocks is a window through the region's extensive latest Neoproterozoic and Paleozoic Arctic Platform sedimentary cover sequence. These basement rocks, named here the Victoria Fjord terrane, are dominated by weakly-foliated granodioritic orthogneisses, with lesser amounts of migmatite. These are intercalated with strips of supracrustal rocks dominated by paragneisses. This poorly-exposed Archean terrane at Greenland's northern tip is succeeded to the south by the extensive east-west trending Paleoproterozoic Inglefield Mobile Belt of juvenile arc rocks.The Victoria Fjord terrane granodiorites have zircon U-Pb ages from 3292 ± 14 Ma to ∼3260 Ma. Reconnaissance U-Pb dating on schlieric migmatite zircons yielded ages of ∼3280, 3400 and 3500 Ma. Paragneiss detrital zircons have ages of mostly ∼2710 Ma, with a minority of 3000–2900 Ma grains. This demonstrates that these sedimentary rocks were not derived from the local basement. Post- Archean magmatic or metamorphic zircon was not detected in any of the samples.Initial εHf of 3290–3260 Ma granodiorite and granite zircons ranges from −4 to +2, and the magmatic protoliths are interpreted as sourced from the melting of somewhat older (3500–3400 Ma) Paleoarchean crust, represented by paleosome in the migmatites. The paragneiss 3000–2900 Ma detrital zircons were derived from juvenile felsic crust of that age. The ∼2710 Ma group show negative correlation of Th/U and initial εHf, such that those with high Th/U up to >2.0 have εHf values of −6 to −10. The ∼2710 zircons were sourced from mafic rocks formed by melting of much older (Paleoarchean?) enriched mantle which was variably contaminated by 3000–2900 Ma crust during its ascent.The Victoria terrane is unique amongst the Archean basement terranes in Greenland because it is dominated by 3500–3260 Ma crust. However, the Arctic basin contains detrital zircons of that age, and rocks of that age also occur in eastern Siberia and northwestern Canada. Implications for high Arctic early Precambrian geodynamics are discussed.

Petrogenesis of plagiogranites from the Troodos Ophiolite Complex, Cyprus

Small volumes of felsic melt, commonly known as oceanic plagiogranites, appear as melt pockets or dikes within the gabbroic section and the sheeted dikes root zone of the oceanic crust. Plagiogranites from the Troodos Ophiolite Complex on Cyprus are among the best exposures of felsic rocks that are embedded in a complete section of obducted oceanic lithosphere. Nevertheless, their exact petrogenesis is still a matter of debate, largely due to limited high-quality trace element and radiogenic isotope data. Previously proposed models for Troodos plagiogranites have included both low-pressure dehydration melting and fractional crystallisation at deeper levels of the oceanic crust. To evaluate both models, oceanic plagiogranites from the Troodos Ophiolite Complex were analysed in this study for their major and trace elements, and for the first time also for Hf–Nd–Sr isotope compositions. The trace element measurements also include for the first time high-precision measurements of high-field-strength element (HFSE) abundances that now permit to better unravel the petrogenesis of the Troodos plagiogranites and their possible mantle sources. In general, the Troodos plagiogranites exhibit a narrow range of Nb/Ta and Zr/Hf that overlap the compositions of mid-ocean ridge basalts (MORB). In line with earlier studies, three compositional groups can be identified: two groups formed by either fractional crystallisation or combined fractional crystallisation and wall rock assimilation, and one group derived from partial melting of slightly altered oceanic crust. The majority of the Troodos plagiogranites (Main Group) are the product of extensive fractional crystallisation of ambient arc-tholeiitic mafic melts. A second group of plagiogranites (Spilia Group) is generated by fractional crystallisation of boninitic precursor melts and the assimilation of arc-tholeiitic crustal material. Variable HFSE concentrations and diagnostic Hf–Nd isotope signatures that are unique to both suites allow discriminating between the two parental melts and fractionation processes. A small group of plagiogranites (Zoopigi Group) is interpreted to derive from partial melting in the conductive layer of active magma chamber lenses (AML). Elevated Nb/Ta, Zr/Hf, and light rare-earth element (LREE) enrichments in these rocks are in support of this model. Collectively, our data suggest that low-pressure fractional crystallisation (also in combination with assimilation of wall rocks) might be the predominant process controlling the formation of felsic rocks on Cyprus, whereas dehydration melting appears to be less important. If compared to Archean tonalitic–trondhjemitic–granodioritic suites (TTGs), compositions of plagiogranites from Cyprus mirror shallow-level processes in thin oceanic crust, which is illustrated by their narrow, MORB-like range of HFSE ratios and their distinct enrichment in heavy rare-earth elements (HREE) that distinguishes them from the Archean TTGs.

Role of radiation modification in ore pretreatment

It is found that the increased radiation exposure of rocks treated by accelerated electrons changes the rock deformation and strength characteristics: ultimate compression strength as well as moduli of deformation and elasticity. The uniaxial compression strength of granite is 68.33 MPa without treatment and 35.08 MPa at the absorbed radiation dose of 10 kGy. The established possibility of weakening of rocks by accelerated electrons can be used to reduce consumption of electricity during ore pretreatment and to minimize loss of useful components in processing.

Geochemistry of Late Mesozoic volcanic rocks in the central Great Xing’an Range, NE China: petrogenesis and crustal growth in comparison with adjacent areas

ABSTRACTThe central Great Xing’an Range contains widespread exposures of Mesozoic igneous rocks, yet the origin of its Late Mesozoic volcanic rocks has long been controversial. They are composed of andesite, trachyte, trachyandesite, trachydacite, rhyolite, and minor basaltic andesite. The intermediate rocks are enriched in large ion lithophiles and light rare earth elements (LILEs and LREEs) and are depleted in high field strength elements (HFSEs) such as Nb, Ta, and Ti, with positive εHf(t) values (+6.2 to +13.4) and Hf single-stage model ages (TDM1 = 556–227 Ma). This indicates that they were derived from the partial melting of a mantle wedge that was modified by previously subducted slab-derived fluids was likely introduced with limited crustal contamination. The felsic rocks show positive εHf(t) values (+4.3 to +10.2) and Hf two-stage model ages (TDM2 = 910–544 Ma), with more enrichment of LILEs and LREEs and more depletion of HFSEs and negative Eu anomalies. This indicates that felsic rocks were mainly derived from partial melting of Neoproterozoic accreted quartz-amphibolite facies mafic crust. We summarise the geochronological and Hf-Nd isotope data for Mesozoic igneous rocks within the central Great Xing’an Range (CGXR), the northern Xing’an block (NXB), and the Erguna block (EB), with the aim of constraining the geochronological framework for Mesozoic volcanism and comparing the growth of continental crust. The evolution of Late Mesozoic magmatism in the three areas reached its climax ~125 Ma ago. Our data, together with regional isotope evidence, suggest that crustal accretion of the CGXR occurred during the Neoproterozoic–Phanerozoic. However, that of the EB occurred during the Meso- to Neoproterozoic. The EB exhibits more lateral and vertical heterogeneities and ancient continental crust than the CGXR and NXB in the northern magma source region.

Structural controls on the location and distribution of CO2 emission at a natural CO2 spring in Daylesford, Australia

Secure storage of CO2 is imperative for carbon capture and storage technology, and relies on a thorough understanding of the mechanisms of CO2 retention and leakage. Observations at CO2 seeps around the world find that geological structures at a local and regional scale control the location, distribution and style of CO2 emission. Bedrock-hosted natural CO2 seepage is found in the Daylesford region in Victoria, Australia, where many natural springs contain high concentrations of dissolved CO2. Within a few meters of the natural Tipperary Mineral Spring, small CO2 bubble streams are emitted from bedrock into an ephemeral creek. We examine the relationship between structures in the exposed adjacent outcropping rocks and characteristics of CO2 gas leakage in the stream, including CO2 flux and the distribution of gas emissions. We find that degassing is clustered within ˜1 m of a shale-sandstone geological contact. CO2 emission points are localised along bedding and fracture planes, and concentrated where these features intersect. The bubble streams were intermittent, which posed difficulties in quantifying total emitted CO2. Counterintuitively, the number of bubble streams and CO2 flux was greatest from shale dominated rather than the sandstone dominated features, which forms the regional aquifer. Shallow processes must be increasing the shale permeability, thus influencing the CO2 flow pathway and emission locations. CO2 seepage is not limited to the pool; leakage was detected in subaerial rock exposures, at the intersection of bedding and orthogonal fractures.These insights show the range of spatial scales of the geological features that control CO2 flow. Microscale features and near surface processes can have significant effect on the style and location and rates of CO2 leakage. The intermittency of the bubble streams highlights challenges around characterising and monitoring CO2 stores where seepage is spatially and temporally variable. CCS monitoring programmes must therefore be informed by understanding of shallow crustal processes and not simply the processes and pathways governing CO2 fluid flow at depth. Understanding how the CO2 fluids leaked by deep pathways might be affected by shallow processes will inform the design of appropriate monitoring tools and monitoring locations.

Climate change effects on landscape and environment in glacierized Alpine areas: retreating glaciers and enlarging forelands in the Bernina group (Italy) in the period 1954–2007

ABSTRACTWe analysed the recent involution of glaciers in the Bernina group (Italy), which are shrinking thus permitting a rapid enlargement of the forelands. We delimited glacier outlines upon aerial photographs (1954 and 1981 stereo pairs analysed through an optical system) and orthophotos (2003 and 2007 digital imagines directly managed via GIS software). All the obtained data were overlapped and compared. The estimated glacier area change during 1954–2007 was −36.5 ± 2.4% (−16.2 ± 0.4 km2). The changes sped up more recently; in fact, during 1981–1954 (27 years) the variation was −0.206 km2/y, against −0.387 km2/y during 1981–2003 (22 years), and −0.535 km2/y during 2007–2003 (4 years). In the 1954–2007 period, the forelands experienced a continuous increase (+14.7 km2). Moreover, the analysis of the colour orthophotos allowed observations of: (i) changes affecting shape and geometry of glaciers (growing rock outcrops, tongue separations, increasing supraglacial debris and collapse structures) and (ii) main features of glacier forelands (bare rock exposures, debris and sediments and, in the latter case, occurrence of vegetation colonizing such pristine areas). Glacier forelands resulted also subjected to the action of melting water, debris transport, and periglacial processes, with consequences on landscape and geoheritage.

PALM KERNEL SHELL AS AN ALTERNATIVE AGGREGATE ON HIGH PERFORMANCE CONCRETE CONCRETE

Coarse aggregates commonly used in concrete are coarse natural aggregates, which are broken stones or gravel. Continuous rock exploration can cause environmental damage or even more severe ecosystem damage. Therefore it is necessary to substitute an alternative aggregate. Indonesia has the second largest oil palm plantation (Elaeis guineensis Jacq) in the world after Malaysia. Plantations are renewable resources, so palm oil is also potential to be used as an alternative to diesel fuel. Palm kernel shells are palm oil industry wastes which are generally underutilized. Palm kernel shells can be used as an alternative to coarse aggregate, because oil palm shells have the advantage of being hard, tough and good durability due to the high content of lignin and silica dioxide (SiO2), such as hard wood, but low cellulose content so it is not easy rot. The strength of the palm oil shell is quite good. In addition, the aggregate gradation also fulfills the requirements without a breakdown process, which has a thickness of 2-4 mm and a maximum width of 15 mm. The volume of oil palm shells is + 600 kg/m3, so it will produce significant light weight concrete. The concrete studied was concrete with a coarse aggregate of tenera palm kernel shells, with fine aggregates of natural sand, and a Portland Composite Cement (PCC), but the PCC content was reduced and replaced by FA which varied from 0%, 5%, 10 %, 15%, 20 and 25%. Concrete also added superplasticizer (SP). SP is used to reduce water use, because the shell absorbs water. SP levels also vary, namely 0%, 1%, and 1.2%. The weight of the volume of concrete with various levels of fly ash and SP is 1700-1800 kg/m3, so it can be classified as lightweight concrete. Increased FA levels will increase compressive strength, but only up to 10%, after which the strength decreases. Compressive strength of specimens with SP 0% and FA 10% is 17.92 MPa, for SP levels of 1% and FA 10% is 22.15 MPa, while for SP levels of 1.2% and FA 10% is 19.35 MPa . So that it can be concluded that the palm shell as bio-material (renewable resources) can be used as a substitute for natural coarse aggregates. The optimum fly ash level is 10%, and to reduce water use SP 1%. The use of oil palm shells as a substitute for gravel means reducing the waste of the palm oil industry, while reducing rock exposures. In addition, in Indonesia there are many areas where there are no rock sources while oil palm plantations are quite extensive.

Fracture Fillings and Implication of Fluid Activities in Volcanic Rocks: Dixi Area in Kelameili Gas Field, Junggar Basin, Northwestern China

The Carboniferous Batamayineishan Formation of the Kelameili Gas Field is a specific weathered crust-related volcanic reservoir that has a significant production rate in the Junggar Basin, Northwestern China, attributed to debatable processes of fluid evolution. The results suggest that various types of fluids occurring in volcanic rocks lead to the filling of quartz and calcite in fractures and their associated alteration haloes. The silica that formed quartz veins was mainly derived from deep hydrothermal fluids, while the carbon dioxide that formed calcite veins originated from sources characterized by mixing and alteration of deep hydrothermal and hydrocarbon fluids. Siliceous hydrothermal fluids rich in sulphur dioxide and other volatile components were driven by a pressure gradient and buoyancy, and circulated both laterally and vertically along the fractures, forming quartz veins and tension fractures under different temperature conditions. Moreover, changes in salinity, pressure, and carbon dioxide of deep fluids, varying from acidic to weakly alkaline, resulted in earlier calcite precipitation in contraction fractures and weathered fractures. Tectonic uplift resulted in the long-term exposure of volcanic rocks, where fresh water mixed with the partially alkaline fluid escaping the basin to form calcite cements, thus retaining the characteristics of a seepage environment in the weathered fractures. Structural fractures occurred due to tectonic movements during the burial period. Filling and leakage of hydrocarbons caused pore fluids to convert from acidic to alkaline, precipitating late sparry calcite in dissolution fractures. Late hydrothermal fluid metasomatism, brought about by infiltration into the permeable zone, caused partial dissolution of local calcite along cleavage cracks.

Olistostrome and its implications to geological disaster on coastal area with special reference to the Bantimala tectonic complex, Pangkep Regency South Sulawesi Province

Olistostrome is clastic sedimentary rock, its formation related to tectonic activity along the plate subduction edge on the front region of the accretion prism. Its formation is a submarine landslide with a mudflow sediment mass. The material components can be mixed from various continental and oceanic sources. As in the complex area of Bantimala tectonics, olistostrome shows typical properties of ugly sorting, floating rock blocks in the sandy matrix, tectonite textures, and constituent rocks generally in the form of metamorphic rocks of subduction and plate collision. The criterion shows the submarine sludge deposits formed in the subduction tectonic area of the marine trough environment where there is an increase in slope due to subduction accretion. This study aims to examine the characteristics of olistostrome as a landslide deposit in the subduction zone about the correlation of disaster in convergent tectonic areas in the present. In the area of Bantimala, the exposure of olistostrome rock is large enough with large dimensions with a mega-landslide. This can happen by earthquakes or critical slope in the past. This phenomenon can be a reference to disaster present-day in subduction areas, as in the island arc of Indonesia, which is a collision area of three plates very vulnerable to this disaster. A submarine landslide can cause disaster to infrastructure, telecommunication cable, gas hydrate dissolution, tsunami disaster and environmental destruction of marine benthos.

Overview of Spirit Microscopic Imager Results

AbstractThis paper provides an overview of Mars Exploration Rover Spirit Microscopic Imager (MI) operations and the calibration, processing, and analysis of MI data. The focus of this overview is on the last five Earth years (2005–2010) of Spirit's mission in Gusev crater, supplementing the previous overview of the first 450 sols of the Spirit MI investigation. Updates to radiometric calibration using in‐flight data and improvements in high‐level processing are summarized. Released data products are described, and a table of MI observations, including target/feature names and associated data sets, is appended. The MI observed natural and disturbed exposures of rocks and soils as well as magnets and other rover hardware. These hand‐lens‐scale observations have provided key constraints on interpretations of the formation and geologic history of features, rocks, and soils examined by Spirit. MI images complement observations by other Spirit instruments, and together show that impact and volcanic processes have dominated the origin and evolution of the rocks in Gusev crater, with aqueous activity indicated by the presence of silica‐rich rocks and sulfate‐rich soils. The textures of some of the silica‐rich rocks are similar to terrestrial hot spring deposits, and observations of subsurface cemented layers indicate recent aqueous mobilization of sulfates in places. Wind action has recently modified soils and abraded many of the rocks imaged by the MI, as observed at other Mars landing sites.

Transdimensional ambient noise tomography of Bass Strait, southeast Australia, reveals the sedimentary basin and deep crustal structure beneath a failed continental rift

Debate is ongoing as to which tectonic model is most consistent with the known geology of southeast Australia, formerly part of the eastern margin of Gondwana. In particular, numerous tectonic models have been proposed to explain the enigmatic geological relationship between Tasmania and the mainland, which is separated by Bass Strait. One of the primary reasons for the lack of certainty is the limited exposure of basement rocks, which are masked by the sea and thick Mesozoic–Cenozoic sedimentary and volcanic cover sequences. We use ambient noise tomography recorded across Bass Strait to generate a new shear wave velocity model in order to investigate crustal structure. Fundamental mode Rayleigh wave phase velocity dispersion data extracted from long-term cross-correlation of ambient noise data are inverted using a transdimensional, hierarchical, Bayesian inversion scheme to produce phase velocity maps in the period range 2–30 s. Subsequent inversion for depth-dependent shear wave velocity structure across a dense grid of points allows a composite 3-D shear wave velocity model to be produced. Benefits of the transdimensional scheme include a data-driven parametrization that allows the number and distribution of velocity unknowns to vary, and the data noise to also be treated as an unknown in the inversion. The new shear wave velocity model clearly reveals the primary sedimentary basins in Bass Strait as slow shear velocity zones which extend down to 14 km in depth. These failed rift basins, which formed during the early stages of Australia–Antarctica break-up, appear to be overlying thinned crust, where high velocities of 3.8–4.0 km s−1 occur at depths greater than 20 km. Along the northern margin of Bass Strait, our new model is consistent with major tectonic boundaries mapped at the surface. In particular, we identify an east dipping velocity transition zone in the vicinity of the Moyston Fault, a major tectonic boundary between the Lachlan and Delamerian orogens, which are part of the Phanerozoic accretionary terrane that makes up eastern Australia. A pronounced lineament of high shear wave velocities (∼3.7–3.8 km s−1) in the lower crust of our new model may represent the signature of relict intrusive magmatism from failed rifting in the early stages of Australia–Antarctica break-up along a crustal scale discontinuity in the Selwyn Block microcontinent which joins Tasmania and Victoria.

Broadband reflectance, emittance spectroscopy and self-potential geophysical survey for targeting gold sulphide lode deposit in Bhukia, Rajasthan, India

In this study, we have analyzed the comparative potential of reflectance and emittance bands of advanced spaceborne thermal emission and reflection radiometer (ASTER) sensor for detecting surface exposures of Proterozoic rocks and gossan capping of gold mineralization. In this regard, few new image analysis products are derived. These are matched filtering factor-modified ratio (MFR) image and the inverse of a difference image of the ratio and matched filtering map (IDRM). These image products are used with the conventional ratio images, relative band depth images and index-based principal component (PC) images of reflectance and emittance bands of ASTER for delineating metasediments of the study area. We have found that reflectance band products are superior for mapping metasediments and gossan capping than the emissivity products, which are only suitable to enhance quartzite. Self-potential (SP) geophysical survey is carried out around the identified gossan site as SP is sensitive to sulphide mineralization. Results of the SP survey indicated presence of sulphide enrichment underneath the gossan.

Reappraisal of the Sumé Complex: geochemistry and geochronology of metaigneous rocks and implications for Paleoproteorozoic subduction-accretion events in the Borborema Province, NE Brazil

The Alto Moxoto Terrane represents the largest exposure of Paleoproterozoic rocks in the central Borborema Province. Several medium- to high-grade metamorphic units are described in the terrane, however geochronological data are still scarce, and the main periods of crust buildup and recycling are poorly known. In this study, we investigate the nature and age of metagranitic and metamafic-ultramafic rocks in the type area of the Sume Complex. U-Pb zircon data from a metasyenogranite yields a concordant age of 1.97 Ga, which is in contrast with the previously published age of the complex (c. 640 Ma). In addition, Sm-Nd isotope signature indicates Paleoproterozoic to Mesoarchean sources that coupled with negative eNd(t) values suggest reworking of a preexisting crust. Whole-rock geochemical data indicate that metamafic rocks are similar to those already described in the terrane, corresponding to island-arc tholeiites, whereas the metagranitic rocks share similarities with magmas that were generated in an evolved stage of an orogenic setting. The obtained results are coherent with a long-lived Paleoproterozoic accretionary-collisional event (c. 2.1 – 1.9 Ga) described in the Borborema Province, which is correlative with Paleoproterozoic (c. 2.2 – 2.0 Ga) basement inliers/terranes from Pan-African Fold belts.

U-Pb SHRIMP dating of basement rocks of the Iriri-Xingu domain, Central Amazonian province, Amazonian craton, Brazil

Abstract The Iriri-Xingu domain, located in the central part of the Amazonian craton, consists of extensive occurrences of Paleoproterozoic volcanic rocks and granites with published ages of ca. 1990 to 1840 Ma, which show a strong crustal contribution for their magmas. Exposures of basement rocks are small and rare. Samples from two areas were dated in this work by U-Pb SHRIMP in zircon. In the northern Maribel area, a high-grade pelitic paragneiss presented an age of 2160 ± 8 Ma and the leucosome of a migmatitic orthogneiss, probably a diatexite, crystallized at 2149 ± 20 Ma. These Rhyacian ages suggest that it is part of the Bacaja domain, related to the Trans-Amazonian cycle. The Morro Grande area occurs in the central part of Iriri-Xingu domain, where a high-grade muscovite gneiss with a protolith of 2120-2180 Ma is dated at 1982 ± 7 Ma, and the leucosome of a migmatitic orthogneiss is crystallized at 1979 ± 8 Ma. These are the first records of high-grade metamorphism at ca. 1980 Ma in the Central Brazil shield and may be related to the generation of this Orosirian felsic magmatism, which is widespread throughout the nearby Ventuari-Tapajos province.

Satellite-Derived Regional Apparent Thermal Inertia and Gravity for Mapping Different Rock Types in Parts of Banswara, Rajasthan

In this study, utility of satellite derived Apparent Thermal Inertia (ATI) and residual gravity data have been analyzed for delineating the major rock types of Aravalli Supergroup. Density is a common contributing factor responsible for ATI and gravity variations of rock types. In addition to density, thermal inertia is also controlled by specific heat and thermal conductivity. In the present study, MODIS (Moderate resolution imaging spectrometer) day and consecutive night time land surface temperature (LST), albedo data have been processed to derive ATI image for the study area. On the other hand, Bouguer gravity generated from EIGEN6C4 gravity model data is processed to derive the residual gravity over the study area. It has been observed that ATI values of different rock types are influenced by the variations in weathering pattern and forest cover developed over the rock in addition to the thermophysical parameters of rocks. Forest cover and agricultural activity are responsible for shifting the ATI values to higher side for any rocks due to evapotranspiration; which is responsible for reducing day and night land surface temperature difference. ATI pixels( collected from large rock pavements) of different rocks can however, be used to discriminate them from each other. Conjugate analysis of residual gravity and ATI has been proved useful to discriminate different rock types of spatially well distributed rock exposures. It is found that ATI and gravity values are well correlated for the spatially large surface exposures of rocks. In regional scale, low frequency changes of ATI broadly matches with the changes observed on the corresponding spatial profiles of residual gravity with some localized variations. This is due to the fact that shallow surface gravity anomaly is cumulative effect of vertical and horizontal variations of density in contrast to ATI which is more sensitive to near surface density, conductivity and specific heat variations of rocks. Moreover, high frequency variations imprinted on regional ATI spatial profiles are due to the cumulative effect of complex thermophysical properties of surface elements (weathering residuum, surface moisture variations) density variation and intrinsic thermophysical properties of rocks measured at regional scale. High frequency changes of ATI do not correspond well with residual gravity variation.

Crustal structure of the northern Harrat Rahat volcanic field (Saudi Arabia) from gravity and aeromagnetic data

New gravity data reveal a prominent negative anomaly along the main vent axis of the northern Harrat Rahat volcanic field in Saudi Arabia. The low continues north of the volcanic field onto exposures of Proterozoic rocks, indicating that the low is caused not only by the volcanic field (and possibly also underlying Cenozoic sediments), but also the underlying Proterozoic basement. An inversion of the gravity field guided by analysis of aeromagnetic data indicates (1) a broad depression of the basement surface that is deeper along the main vent axis in the eastern part of the harrat and in the southwest part of the volcanic field and (2) less dense basement beneath the vent axis. Low densities within the basement most likely arise from lithologic variations in the basement, predating Cenozoic volcanism, although our analysis does not rule out small volumes of partial melt and higher temperatures or extensive fracturing at depth.