Geological Field Data Research Articles

Triassic warm subduction in northeastTurkey: Evidence from theAğvanis metamorphic rocks

AbstractWithin theTethyan realm, data for the subduction history of thePermo–TriassicTethys in the form of accretionary complexes are scarce, coming mainly from northwestTurkey andTibet. Herein we present field geological, petrological and geochronological data on aTriassic accretionary complex, the Ağvanis metamorphic rocks, from northeastTurkey. TheAğvanis metamorphic rocks form aSSE–NNWtrending lozenge‐shaped horst, ∼20 km long and ∼6 km across, bounded by the strands of the activeNorthAnatolianFault close to the collision zone between theEasternPontides and theMenderes–TaurusBlock. The rocks consist mainly of greenschist‐ to epidote‐amphibolite‐facies metabasite, phyllite, marble and minor metachert and serpentinite, interpreted as a metamorphic accretionary complex based on the oceanic rock types and ocean island basaltic, mid‐ocean ridge basaltic and island‐arc tholeiitic affinities of the metabasites. This rock assemblage was intruded by stocks and dikes ofEarlyEocene quartz diorite, leucogranodiorite and dacite porphyry. Metamorphic conditions are estimated to be 470–540°Cand ∼0.60–0.90 GPa.Stepwise40Ar/39Ar dating of phengite–muscovite separates sampled outside the contact metamorphic aureoles yielded steadily increasing age spectra with the highest incremental stage corresponding to age values ranging from ∼180 to 209 Ma, suggesting that the metamorphism occurred at ≥ 209 Ma. Thus, theAğvanis metamorphic rocks represent the vestiges of theLateTriassic or slightly older subduction in northeastTurkey. EstimatedP–Tconditions indicate higher temperatures than those predicted by steady state thermal models for average subduction zones, and can best be accounted for by a hot subduction zone, similar to the present‐dayCascadia. Contact metamorphic mineral assemblages around anEarlyEocene quartz diorite stock, on the other hand, suggest that the present‐day erosion level was at depths of ∼14 km during theEarlyEocene, indicative of reburial of the metamorphic rocks. Partial disturbance of white‐micaAr–Ar age spectra was probably caused by the reburial coupled with heat input by igneous activity, which is probably related to thrusting due to the continental collision betweenEasternPontides and theMenderes–TaurusBlock.

Submarine allochthonous salt sheets: Gravity-driven deformation of North African Cretaceous passive margin in Tunisia – Bled Dogra case study and nearby salt structures

We used structural, stratigraphic and sedimentologic data, together with a comparison of nearby structures and a Bouguer gravity map, to evaluate the evolution of the Bled Dogra salt structure (northern Tunisia) during the Cretaceous. Triassic salt sheets are recognized in the northwestern region of the Tunisian Atlas. These salt sheets are the result of Cretaceous thick and/or thin-skinned extension along the south Tethyan margin. The Bled Dogra salt structure is one of these submarine allochthonous salt sheets, which was emplaced during the Early Cretaceous. The geologic framework, during this period, produces conditions for a predominantly gravity-driven deformation: extension has produced space for the salt to rise; vigorous differential sedimentation created differential loading that resulted in the emplacement and extrusion of a large volume of Triassic salt and formation of large submarine salt sheets. Geologic field data suggest an interlayered Triassic salt sheet within Albian sequences. Salt was extruded at the sea floor during the Early–Middle Albian and was initially buried by Middle–Late Albian strata. The Coniacian corresponds to a second transgressive cover onto the salt sheet after the gliding of the first salt cover (Late Albian–Turonian). In addition, this northwest Tunisian area exposes evidences for salt flow and abundant slump features at the base of a northward facing submarine slope, which was probably dominant from the Early Cretaceous to Santonian. Two gravity deformation processes are recognized: gravity gliding and gravity spreading. Acting concurrently, these two processes appear indistinguishable in this geologic context. Like the present-day salt-involved passive margins – such as the northern Gulf of Mexico, the Atlantic margin of Morocco, the Brazilian Santos basin, the Angola margin, Cadiz in western Iberia, and the Red Sea – the North African Cretaceous passive margin in Tunisia provides evidences that deformation in a passive-margin salt basin is predominantly gravity-driven deformation.

Estimation of the deep geothermal potential within the Tertiary Limagne basin (French Massif Central): An integrated 3D geological and thermal approach

The geothermal potential of a deep sedimentary-rock reservoir, in a Tertiary graben, the Limagne d’Allier basin (Massif Central, France), is assessed. The most interesting geothermal target is identified as a thick basal Tertiary sandstone overlying crystalline Paleozoic basement. The total amount of recoverable energy in this clastic aquifer is estimated at over 500PJ (500×1015J) in the modelled area. The most promising zones appear along the north-western edges of the basin, where sediment infill is thickest. The methodology used for estimating geothermal potential starts from geological field data. The first step is to obtain a better understanding of the structure and geometry of the target zone, using various data such as field measurements, and borehole and geophysical data. These data are reinterpreted through the construction of a 3D geological model. Inconsistencies are checked and turned into a coherent 3D interpretation. The second step consists in meshing the geological model into an unstructured 3D finite-element mesh where realistic thermal boundary conditions are applied. The temperature field is computed in a third step. The thermal calculation is achieved by assuming a purely conductive behaviour and through comparison with existing borehole profiles. The computed temperatures fit the measurements in the deepest part of the Limagne d’Allier basin, while the potential role of fluid flow is highlighted in its upper part, either within more permeable formations, or around the boreholes. A fourth, final, step maps the geothermal potential (recoverable energy) in the deepest part of the Tertiary graben, where the total amount of geothermal energy available is calculated. The result of this work provides valuable guidelines for geothermal exploration in the area and our methodology can be replicated elsewhere.

The effect of total grain-size distribution on the dynamics of turbulent volcanic plumes

The impact of explosive volcanic plumes on climate and on air traffic strongly depends on the concentration and grain-size distribution (GSD) of pyroclastic fragments injected into the atmosphere. Accurate and robust modelling of the evolution of GSD during pyroclast transport from the vent to the ash cloud is therefore crucial for the assessment of major volcanic hazards. Analysis of field deposits from various recent Plinian eruptions shows that their total GSD is well described by a power law, as expected from the physics of magma fragmentation, with an exponent (D) ranging from 3.0 to 3.9. By incorporating these measured GSD into the initial conditions of a steady-state 1D model of explosive eruption columns, we show that they have a first-order impact on the dynamical behaviour of explosive eruption columns. Starting from an initial value of D, the model tracks the evolution of GSD in the column and calculates the dynamical consequences of particle sedimentation. The maximum height reached by the column, one of the first-order results relevant to aircraft safety, changes by 30% for mass fluxes of 107 kgs−1 or larger, and by 45–85% for mass fluxes between 105 and 107 kgs−1, depending on exponent D. We compare our predictions to a specially assembled set of geologic field data and remote sensing observations from 10 Plinian eruptions for which maximum column height and mass flux are known independently. The incorporation of realistic power-law GSD in the model greatly improves the predictions, which opens new perspectives for estimation of ash load and GSD in volcanic clouds from near real-time measurements available from satellite payloads. Our results also contribute to the improvement of volcanic source term characterization that is required input for meteorological dispersion models.

Predictive mapping of prospectivity for orogenic gold in Uganda

Integration of enhanced regional geo-datasets has facilitated new geological interpretation and modelling of prospectivity for orogenic gold in southwestern Uganda. The geo-datasets include historical geological maps, geological field data, digital terrain models, Landsat TM data and airborne geophysical data. The study area, bordered by the western branch of the East African Rift, covers a range of different aged terranes including the Archaean basement gneisses, Palaeoproterozoic volcano-sedimentary Buganda Toro Belt, Mesoproterozoic clastic sedimentary Karagwe Ankolean Belt and several outliers of undeformed Neoproterozoic sediments.The mineral systems approach to practical exploration targeting requires a framework to link conceptual models of mineralisation with available data. A conceptual model requires good understanding of key processes and their timing within the geodynamic history of an area. The challenge is that processes cannot be mapped, only their results or effects. In this study, a district-scale (1:100,000) investigation is considered appropriate given the scarcity of geological information and the absence of world-renowned gold deposits in southwestern Uganda. At this scale of orogenic gold mineral systems understanding, evidence for the source of gold, active pathways and the physical traps are considered critical. Following the mineral system approach, these processes critical to orogenic gold systems are translated into district-scale mappable proxies using available regional-scale datasets. Tectono-stratigraphic domains, mantle indicators and gold occurrences represent the “source of gold” as a critical process. Zones of hydrothermal alteration were extracted from radiometric data, structures involved in the orogenies and terrane contacts were extracted to represent the active pathway as a critical process and finally the physical throttle is represented by rheological contrasts and geological complexity. Then, the knowledge-driven multi-class index overlay method was used to integrate predictor layers representing those processes in order to model orogenic gold prospectivity into a single map. Weighting of the predictor layers, prior to integration, occurs at the level of the critical process and takes into account the relative importance of the critical process mineralisation, the representativeness of a proxy and the accuracy of the proxy. The resultant prospectivity model shows that 83% of all gold occurrences are delineated within predicted prospective areas covering 30% of the study area. Eight sub-areas, covering 2500km2, have been recommended for follow-up exploration.

Effect of slope failures on river-network pattern: A river piracy case study from the flysch belt of the Outer Western Carpathians

Landslides are important geomorphic agents in various mountainous settings. We document here a case of river piracy from the upper part of the Malá Brodská Valley in the Vsetínské Mts., Czech Republic (Rača Unit of the flysch Magura Group of Nappes, flysch belt of the Outer Western Carpathians) controlled by mass movement processes. Based on the field geological, geomorphological and geophysical data, we found out that the landslide accumulations pushed the more active river of out of two subparallel river channels with different erosion activity westwards and forced intensive lateral erosion towards the recently abandoned valley. Apart from the landslide processes, the presence of the N-striking fault, accentuated by higher flow rates of the eastern channel as a result of its larger catchment area, were the most critical factors of the river piracy. As a consequence of the river piracy, intensive retrograde erosion in the elbow of capture and also within the upper portion of the western catchment occurred. Deposits of two landslide dams document recent minimum erosion rates to be 18.8mm.ky−1 in the western (captured) catchment, and 3.6mm.ky−1 in the eastern catchment respectively. The maximum age of the river piracy is estimated to be of the late Glacial and/or the early Holocene.

New detailed aeromagnetic and geological data of eastern Dronning Maud Land: Implications for refining the tectonic and structural framework of Sør Rondane, East Antarctica

The Sør Rondane Mountains (SRM) in eastern Dronning Maud Land (DML) are located in an area, where two apparent Pan-African (650–520Ma) orogenic mobile belts appear to intersect, the East African-Antarctic Orogen and the Kuunga Orogen. Hence, a better understanding of the tectonic structure of the Sør Rondane region is an important key for unravelling the complex geodynamic evolution of the eastern DML and adjacent regions of East Antarctica during the Late Neoproterozoic/Early Palaeozoic amalgamation of Gondwana. The SRM were recently (2011–2012) aerogeophysically investigated with a 5km flight line spacing, covering a total area of ∼140,000km2. The aeromagnetic data are correlated with ground-based magnetic susceptibility measurements and geological field data and allow to project tectonic terranes and individual structures into ice-covered areas. Magnetic anomalies and basement foliation trends are collinear in areas dominated by simple shear deformation, whereas an area of large-scale refolding correlates with a subdued small-scale broken magnetic anomaly pattern. The latter area can be regarded as a distinct tectonic domain, the central Sør Rondane corridor. It magnetically separates the SRM into an eastern, a central, and a western portion. This subdivision is presumably related to late Pan-African extensional tectonics and suggests that such a tectonic regime may play a larger role than previously assumed. Voluminous late Pan-African granitoids, which are mainly undeformed, correlate with positive magnetic anomalies between +30 and +80nT, while a strong magnetic high (+680nT) near the granitic intrusion at Dufekfjellet is caused by a highly magnetised enigmatic body. The recently discovered prominent magnetic anomaly province of southeastern DML continues into the southern part of the Sør Rondane region, where only a few outcrops are exposed. Findings at these westernmost nunataks of the SRM indicate that the subdued magnetic anomaly pattern of this southeastern DML province is most likely caused by the predominance of metasedimentary rocks of yet unknown age.

Simulation of Subsea Gas Hydrate Exploitation

The recovery of methane from gas hydrate layers that have been detected in several subsea sediments and permafrost regions around the world is a promising perspective to overcome future shortages in natural gas supply. Being aware that conventional natural gas resources are limited, research is going on to develop required and sustainable technologies for the production of natural gas from such new sources since the early 1990s.In recent years, intensive research has focused on the capture and storage of CO2 from combustion processes to reduce climate impact. While different man-made or natural reservoirs like deep aquifers, exhausted oil and gas deposits or other geological formations are considered to store gaseous or liquid CO2, the storage of CO2 as hydrate in former methane hydrate deposits is another promising alternative. Due to beneficial stability conditions, methane recovery may be well combined with CO2 storage in the form of hydrates. Regarding technological implementation many problems have to be overcome.Within the scope of the German research project »SUGAR« different technological approaches for the optimized exploitation of gas hydrate deposits are evaluated and compared by means of dynamic system simulations and analysis. Detailed mathematical models for the most relevant chemical and physical processes are developed. The basic mechanisms of gas hydrate formation/ dissociation and heat and mass transport in porous media are considered and implemented into simulation programs.Simulations based on geological field data have been carried out. The effects occurring during gas production and CO2 storage within a hydrate deposit are identified and described for various scenarios. The behavior of relevant process parameters such as pressure, temperature and phase saturations is discussed for different strategies.Simulation results for different scenarios, e. g. simple depressurization and CO2 injection, reveal that both the production of natural gas and the CO2 storage are possible with acceptable rates. In case of simultaneous CO2 injection an increased production rate can be expected. The results strongly depend on deposit conditions, especially multiphase flow rates are dominated by permeability terms – in other words the reservoir permeability controls production and injection rates. Furthermore, the heat transport within heterogeneous layered deposits leads to enhanced hydrate decomposition and thus higher production rates.

Long-range hazard assessment of volcanic ash dispersal for a Plinian eruptive scenario at Popocatépetl volcano (Mexico): implications for civil aviation safety

Popocatepetl is one of Mexico’s most active volcanoes threatening a densely populated area that includes Mexico City with more than 20 million inhabitants. The destructive potential of this volcano is demonstrated by its Late Pleistocene–Holocene eruptive activity, which has been characterized by recurrent Plinian eruptions of large magnitude, the last two of which destroyed human settlements in pre-Hispanic times. Popocatepetl’s reawakening in 1994 produced a crisis that culminated with the evacuation of two villages on the northeastern flank of the volcano. Shortly after, a monitoring system and a civil protection contingency plan based on a hazard zone map were implemented. The current volcanic hazards map considers the potential occurrence of different volcanic phenomena, including pyroclastic density currents and lahars. However, no quantitative assessment of the tephra hazard, especially related to atmospheric dispersal, has been performed. The presence of airborne volcanic ash at low and jet-cruise atmospheric levels compromises the safety of aircraft operations and forces re-routing of aircraft to prevent encounters with volcanic ash clouds. Given the high number of important airports in the surroundings of Popocatepetl volcano and considering the potential threat posed to civil aviation in Mexico and adjacent regions in case of a Plinian eruption, a hazard assessment for tephra dispersal is required. In this work, we present the first probabilistic tephra dispersal hazard assessment for Popocatepetl volcano. We compute probabilistic hazard maps for critical thresholds of airborne ash concentrations at different flight levels, corresponding to the situation defined in Europe during 2010, and still under discussion. Tephra dispersal mode is performed using the FALL3D numerical model. Probabilistic hazard maps are built for a Plinian eruptive scenario defined on the basis of geological field data for the “Ochre Pumice” Plinian eruption (4965 14C yr BP). FALL3D model input eruptive parameters are constrained through an inversion method carried out with the semi-analytical HAZMAP model and are varied by sampling them using probability density functions. We analyze the influence of seasonal variations on ash dispersal and estimate the average persistence of critical ash concentrations at relevant locations and airports. This study assesses the impact that a Plinian eruption similar to the Ochre Pumice eruption would have on the main airports of Mexico and adjacent areas. The hazard maps presented here can support long-term planning that would help minimize the impacts of such an eruption on civil aviation.

Lahar hazard assessment in the southern drainage system of Cotopaxi volcano, Ecuador: Results from multiscale lahar simulations

The ice-capped Cotopaxi volcano is known worldwide for the large-scale, catastrophic lahars that have occurred in connection with historical explosive eruptions. The most recent large-scale lahar event occurred in 1877 when scoria flows partially melted ice and snow of the summit glacier, generating debris flows that severely impacted all the river valleys originating from the volcano. The 1877 lahars have been considered in the recent years as a maximum expected event to define the hazard associated to lahar generation at Cotopaxi. Conversely, recent field-based studies have shown that such debris flows have occurred several times during the last 800years of activity at Cotopaxi, and that the scale of lahars has been variable, including events much larger than that of 1877. Despite a rapid retreat of the summit ice cap over the past century, in fact, there are no data clearly suggesting that future events will be smaller than those observed in the deposits of the last 800years of activity. In addition, geological field data prove that the lahar triggering mechanism also has to be considered as a key input parameter and, under appropriate eruptive mechanisms, a hazard scenario of a lahar with a volume 3-times larger than the 1877 event is likely. In order to analyze the impact scenarios in the southern drainage system of the volcano, simulations of inundation areas were performed with a semi-empirical model (LAHARZ), using input parameters including variable water volume. Results indicate that a lahar 3-times larger than the 1877 event would invade much wider areas than those flooded by the 1877 lahars along the southern valley system, eventually impacting highly-urbanized areas such as the city of Latacunga.

Finite-Fault Simulation of Broadband Strong Ground Motion from the 2010 Mw 7.0 Haiti Earthquake

Abstract Because of the socioeconomic environment in the small island nation of Haiti, no seismic recording stations were operating in the source region at the time of the 2010 M w 7.0 earthquake. The lack of strong motion instruments has hindered the estimation of the amplitude, duration, and frequency content of the ground shaking caused by the seismic event. In this study, synthetic broadband strong ground motions are generated in the vicinity of the 2010 Haiti earthquake. The low‐frequency components of the synthetic motions are simulated using a slip model compatible with seismological observations, geologic field data, and space geodetic measurements. The computations are carried out using the discrete wavenumber representation method and the generalized transmission and reflection coefficient technique. The high‐frequency ground‐motion components are generated using the stochastic modeling approach and the specific barrier model. The two independently derived ground‐motion components are subsequently combined using matched filtering at a crossover frequency of 1 Hz to generate broadband ground‐motion time histories and response spectra for the city of Port‐au‐Prince and other sites in the vicinity of the earthquake. Finally, the synthetic strong ground motions are compared against peak ground‐motion estimates inferred from the U.S. Geologic Survey ShakeMap of peak ground accelerations, ground‐motion prediction equations, and observed structural damage. The synthetic time histories and response spectra generated in this study should be seen as plausible, but not necessarily unique, ground motions that capture the primary features of the 2010 Haiti earthquake.

Characteristics of Lead and Zinc Polymetallic Deposit and Wall Rock Alteration Zoning in Luziyuan, Yunnan

The lead and zinc polymetallic deposit in LuziYuan is an ultra-large type skarn deposit. The ore body originated from the interlayer crushed zone between the second and thrid section of stratum marble and slate (schist) interbedding of upper Cambrian series Shahechang group, the wall rock alteration is intensice in the diggings, and the alteration zoning is obvious. Through field geological investigation and data analysis, based on the petrography of alterated rocks and combined with catalog information of drilling, the wall rock alteration in the diggings of LuziYuan is divided into four alteration zones,which are in sequence: carbonate-quartz-chloritization zone; carbonate-quartz-skarn lithification zone; skarn-lead zinc ore mineralization zone; and skarn-pyritization zone. The forming of ore body is related to alterations such as skarn lithification, silicification, marmarization, magnetite, etc.

Mesozoic Syn- and Postrifting Evolution of the Central Apennines, Italy: The Role of Triassic Evaporites

AbstractJurassic–Cretaceous syn- and postrift successions from the central Apennines were backstripped to gain information on the Mesozoic evolution of the passive margin of the Adriatic Plate. Early Jurassic rifting led to the development of a horst-and-graben paleogeography (the Latium-Abruzzi Carbonate Platform and the Sabina-Umbria-Marche Pelagic Basin). Subsidence curves were built for both carbonate platform and pelagic-basin domains from original and literature stratigraphic data. The paleodepositional depths of the deepwater sediments were reconstructed from field geology data, including new paleontological data. It is proposed that after the deposition of lower Hettangian shallow-water carbonates, an abrupt increase in paleowater depth, to 600–1000 m, occurred during the late Hettangian–Sinemurian synrift stage. The postrift stage was characterized by basin filling, with decreasing paleowater depths during the Jurassic, and by a new deepening during the Cretaceous. Our backstripping curves show, ...

Application of ASTER remote sensing data to geological mapping of basement domains in arid regions: a case study from the Central Anti-Atlas, Iguerda inlier, Morocco

Satellite remote sensing is shown to provide critical support for geological and structural mapping in semiarid and arid areas. In this work, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data were used to clarify the geological framework of the Precambrian basement of the Iguerda Proterozoic inlier in the Moroccan Central Anti-Atlas. In this study, the interpretation of the processed digital data has been ground truthed with geological field data collected during a reconnaissance-mapping program in the Central Anti-Atlas. The Iguerda inlier offers a deeply eroded Precambrian massif dominated by a Paleoproterozoic basement composed of supracrustal metasedimentary units intruded by various Eburnian granitoids. Impressive mafic dyke swarms mainly of Proterozoic age crosscut this basement. Eburnian basement rocks are unconformably overlain by Lower Ediacaran volcanosedimentary rocks of the Ouarzazate Group and Upper Ediacaran–Lower Cambrian carbonates. The applied ASTER analyses are particularly effective in the lithological differentiation and discrimination of geological units of the Iguerda inlier. The spectral information divergence (SID) classification algorithm coupled with spectral angle mapper and maximum likelihood classification effectively discriminates between metamorphic rocks, granitoid bodies, and carbonate cover. SID classification improves geologic map accuracy with respect to the spatial distribution of plutonic bodies and metamorphic units. In addition, Paleoproterozoic granitoids have been well discriminated into separate distinct suites of porphyritic granites, granodiorites, and peraluminous leucogranite suites. This discrimination was initially identified via remote sensing analysis and later ground truthed in the field. This methodology enhances geological mapping and illustrates the potential of ASTER data to serve as a vital tool in detailed geologic mapping and exploration of well-exposed basement of arid regions, such as the Proterozoic of the Anti-Atlas Mountains of Morocco.

Discussion on the article ‘Coastal and inland karst morphologies driven by sea level stands: a GIS based method for their evaluation' by Canora F, Fidelibus D and Spilotro G

ABSTRACTComments are presented on the article by Canora et al. (2012) dealing with karst morphologies driven by sea level stands in the Murge plateau of Apulia, southern Italy. Our comments start from cave levels, that are considered in the cited article as a proof of sea level stands. We argue that the presence of sub‐horizontal passages in cave systems is not a sufficient condition for correlating them with hypothetical past sea level stands. Such a correlation must be based upon identification of speleogenetic features within the karst systems, and/or geological field data. The problems encountered when using cave surveys for scientific research, and their low reliability (especially in the case of old surveys) are then treated, since they represent a crucial point in the paper object of this discussion. Eventually, we present some final consideration on cave levels and terraces, and on the specific case study, pointing out once again to the need in including geological field data to correctly find a correspondance between flat landforms and sea level fluctuations. Our main conclusion is that field data and information on speleogenesis of the underground karst landforms cannot be disregarded in a study that claims to deal with the influence of sea‐level changes on caves. Copyright © 2013 John Wiley & Sons, Ltd.

3D Visual Geology-Modeling in Wannian Mine of Fengfeng Coalfield

Based on the analysis of regional geology and hydrogeological conditions of Wannian mine of Fengfeng coalfield, according to field's borehole data, geological data and hydrogeological data, we use GMS software to make the three-dimensional visualization strata modeling in Wannian mine of Fengfeng coalfield. Through cutting geological model built in different directions or at different levels and rotating to different directions and different angles, the complex underground geological environment is displayed in three-dimension, which helps geologists to analyze the geological conditions and solve various geological problems.

Prospectivity mapping of Pb–Zn SEDEX mineralization using remote-sensing data in the Behabad area, Central Iran

Based on the characteristics of sedimentary exhalative (SEDEX) Pb–Zn deposits in the Central Iran Structural Zone, the following regional-scale geological criteria appear to be useful for prospectivity mapping of Pb–Zn mineralization in the study area: (1) presence and/or proximity to jarosite-, alunite-, and illite-bearing shales as the host rock; (2) presence of alteration iron oxide minerals; (3) proximity to – faults/lineaments; and (4) proximity to monzodioritic intrusive bodies with chlorite alteration minerals as the heat source. Spectral feature fitting (SFF) was applied to ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data to map the alteration minerals using the reference spectra from the United States Geological Survey (USGS) library. X-ray diffraction analyses of samples taken from the shale and monzodioritic intrusive bodies were used to define the mineral content of these rocks. The geological map and field data were used to assess the alteration maps generated from the SFF method. Landsat ETM+ (Enhanced Thematic Mapper Plus) data were used to interpret faults/lineaments in the study area. Directional filters applied on the Landsat ETM+ 7-4-1 (red–green–blue (RGB)) colour composite image produced better images for visual interpretation of faults and lineaments. A fuzzy logic approach was used as a geographical information system-based geodata integration technique to predict the occurrence of SEDEX Pb–Zn deposits in the study area. The best predictive map generated from integration of input data defines 10% of the study area as having potential for Pb–Zn mineralization.

Combining Quantitative Textural and Geochemical Studies to Understand the Solidification Processes of a Granite Porphyry: Shanggusi, East Qinling, China

Igneous rock textures reflect the cooling history of the parental magma. Combined with chemical data, they can provide physical and chemical information about the evolution of a magma body. The petrographic textures and chemical compositions of 21 coarse- and fine-grained granite samples along an ∼250 m horizontal outcrop of the Shanggusi granite porphyry are presented in this case study. The coarse-grained granite porphyry is an early intrusion, and the fine-grained granite dykes, mostly intruded into the granite porphyry, are later intrusions. The studied samples have nearly homogeneous major element bulk-rock and mineral compositions, but show large variations in their trace element compositions and textural characteristics. The trace element data suggest the influence of hydrous fluids (possibly enriched in CO2, F, and Cl) in the evolution of the plutonic body. Textural analysis of the coarse-grained granite porphyry indicates that the crystal size distribution (CSD) slopes, intercepts and total numbers of groundmass decrease from the center to the margin of the intrusion in contrast to the maximum diameter of the crystals (Lmax) (average length of the four largest quartz crystals for each sample); however, most fine-grained samples and the groundmass of the coarse-grained samples show concave-down CSDs, indicating textural coarsening. Quartz CSDs in the coarse-grained samples are kinked, with a steep-sloped log–linear section representing small crystals (<1 mm) and a shallow-sloped log–linear section representing large crystals (>1 mm). These two crystal populations are interpreted as resulting from a shift in cooling regime. The straight CSDs of two fine-grained samples may be due to a different cooling history. In general, the spatial variation of the CSD patterns can be attributed to various degrees of overgrowth and mechanical compaction. The quartz phenocrysts in several coarse-grained samples exhibit a high degree of alignment, which may be the result of magmatic flow. By integrating the field geology, geochemistry and quantitative textural data from the horizontal profile of the Shanggusi granite porphyry, it is suggested that hydrous fluids at the top of the intrusion not only controlled the fractionation of elements but also affected its cooling history. Fluid migration-controlled undercooling can explain the solidification processes in the Shanggusi intrusion, and may also be prevalent in other fluid-rich shallow intrusions. Quantitative integration of textural and geochemical data for igneous rocks can contribute to our understanding of the relationships between physical and chemical processes in a magma system, and provide relatively comprehensive insights into the petrogenesis of granites.

Magnetic and structural study of the Eaux-Chaudes intrusion: understanding the Variscan deformation in the Western Axial Zone (Pyrenees)

The present study provides new magnetic and microstructural data for the Eaux-Chaudes granodioritic massif (Western Axial Zone, Pyrenees) and contributes to the understanding of its geometry, internal structure and emplacement mechanism. Moreover, the geological cross-sections and field data allow to reconstruct the evolution of the whole area from Variscan to Alpine times and to integrate the emplacement of the igneous body in the context of the Variscan orogeny. The Eaux-Chaudes pluton (301 ± 9 Ma) is mainly composed by granodiorite, describing a normal compositional zoning and an approximately concentric arrangement that is consistent with the zonation of the low-field magnetic susceptibility. Magnetic foliation is subhorizontal in the inner part of the intrusion and becomes parallel to the petrographical contacts along pluton margins, roughly describing the geometry of the intrusion. Magnetic lineations are dominantly subhorizontal, with E–W to ENE–WSW directional maximum. The general parallelism between Variscan structures of the host rock and the geometry and magmatic fabric of the intrusion reveals a late syn-Variscan emplacement. The tectonic regime registered during magma emplacement is in agreement with an N–S shortening and an E–W stretching direction, consistent with the transpressive regime deduced for other Pyrenean intrusions. Alpine overprint produced a slight tilting in the southern part of the intrusion, but it can be considered that the original Variscan structure is basically unchanged.

The structure of the upper mantle beneath the Delamerian and Lachlan orogens from simultaneous inversion of multiple teleseismic datasets

Distant earthquake data recorded by seven sub-arrays of the ongoing WOMBAT rolling seismic array deployment in southeast Australia are combined for the first time to constrain 3-D variations in upper mantle P-wavespeed via teleseismic tomography. The seven arrays comprise a total of 276 short period recorders spaced at intervals of approximately 50 km, thus allowing unprecedented resolution of the upper mantle over a large region. In the mantle lithosphere immediately below the crust (~ 50 km depth), dominant variations in velocity tend to strike east–west, and share little resemblance to Palaeozoic boundaries in the shallow crust inferred from surface geology and potential field data. A broad region of elevated wavespeed beneath northern Victoria may represent the signature of underplated igneous rocks associated with detachment faulting during the break-up of Australia and Antarctica. A distinct low velocity anomaly in southern Victoria appears to correlate well with the Quaternary Newer Volcanic Provinces. Towards the base of the mantle lithosphere, the dominant structural trend becomes north–south, and five distinct velocity zones become apparent. Of particular note is a transition from higher wavespeed in the west to lower wavespeed in the east beneath the Stawell Zone, implying that the Proterozoic lithosphere of the Delamerian Orogen protrudes eastward beneath the Western subprovince of the Lachlan Orogen. This transition zone extends northwards from southern Victoria into central New South Wales (the northward limit of the arrays), and is one of the dominant features of the model. Further east, there is a transition from lower to higher wavespeeds in the vicinity of the boundary between the Western and Central subprovinces of the Lachlan Orogen, which has several plausible explanations, including the existence of a Proterozoic continental fragment beneath the Wagga–Omeo Zone. The presence of elevated wavespeeds beneath the Melbourne Zone in Victoria, although not well constrained due to limited data coverage, provides some support to the Selwyn Block model, which proposes a northward extension beneath Bass Strait of the Proterozoic core of Tasmania.