thermal effects of normal faulting during rifted basin formation 1 a finite difference model abstract a finite difference model is presented describing the thermal structure of a region undergoing fault controlled extension the model is two dimensional and time dependent we combine temperature changes resulting from conduction advection due to fault movements sediment blanketing and heat production cooling curves can be derived at chosen points in the area this study mainly investigates the effect of the extension rate on the thermal field which turns out to be very important 20 km of extension with extension rates of 0 2 2 0 and 20 mm yr yield maximum deviations in temperatures from the steady slate situation of 2 26 and 84 the sediment blanketing effect can even be reversed by variations in the sedimentation rates fast sedimentation leads to crustal cooling slow sedimentation to crustal heating the turning point value is lithology dependent and corresponds to 2 0 mm yr for shale 2 2 mm yr for clay and 1 4 mm yr for sand a sandstone will cause an increased cooling at both high and low velocities our modelling shows that footwall cooling during or immediately after a rifting event cannot be explained by the downwards movement of the adjacent relatively cold hanging wall consequently in areas where such a cooling is observed other cooling mechanisms must play an important role these can be phenomena like footwall uplift or the fading out of an older thermal anomaly

geological geophysical and thermal characteristics of the salton sea geothermal field california abstract the salton sea geothermal field is the largest water dominated geothermal field in the salton trough in southern california within the trough local zones of extension among active right stepping right lateral strike slip faults allow mantle derived magmas to intrude the sedimentary sequence the intrusions serve as heat sources to drive hydrothermal systems we can characterize the field in detail because we have an extensive geological and geophysical data base the sediments are relatively undeformed and can be divided into three categories as a function of depth 1 low permeability cap rock 2 upper reservoir rocks consisting of sandstones siltstones and shales that were subject to minor alterations and 3 lower reservoir rocks that were extensively altered because of the alteration inter granular porosity and permeability are reduced with depth field permeability is enhanced by renewable fractures i e fractures that can be reactivated by faulting or natural hydraulic fracturing subsequent to being sealed by mineral deposition in the central portion of the field temperature gradients are high near the surface and lower below 700 m surface gradients in this elliptically shaped region are fairly constant and define a thermal cap which does not necessarily correspond to the lithologic cap at the margin of the field a narrow transition region with a low near surface gradient and an increasing gradient at greater depths separates the high temperature resource from areas of normal regional gradient geophysical and geochemical evidence suggest that vertical convective motion in the reservoir beneath the thermal cap is confined to small units and small scale convection is superimposed on large scale lateral flow of pore fluid interpretation of magnetic resistivity and gravity anomalies help to establish the relationship between the inferred heat source the hydrothermal system and the observed alteration patterns a simple hydrothermal model is supported by interpreting the combined geological geophysical and thermal data in the model heat is transferred from an area of intrusion by lateral spreading of hot water in a reservoir beneath an impermeable cap rock

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

evidence for li rich brines and early magmatic fluid rock interactionin the larderello geothermal system abstract the geochemical features of fluids accompanying the first stages of geothermal activity linked to magmatic intrusions have been documented for the larderello geothermal system italy deep drilling has provided samples which preserve evidence of this early geothermal activity four wells san pompeo 2 monteverdi 7 sasso 22 and serrazzano vc 11 penetrated the deeper parts of the larderello system located in a metamorphic basement underlying the tertiary nappe complex which constitutes the main aquifer at larderello the drill holes terminated close to the inferred roof of a granitic complex thought to be responsible for geothermal activity fluid inclusion data were obtained from recrystallized quartz lenses and quartz veins in samples displaying high temperature assemblages plagioclase actinolite biotite tourmaline clinopyroxene andradite wollastonite and also from magmatic quartz in a leucogranite dike the inclusions are mainly secondary in origin oriented in fluid inclusion planes fip related to hydrothermal circulation in the larderello system several generations of high temperature fluids were trapped and include 1 1 h2o co2 dominated vapors displaying variable but significant contents of ch4 and n2 2 2 aqueous vapors containing licl with variable salinity 3 3 aqueous licl brine often oversaturated with respect to halite at room temperature 4 4 complex brine always oversaturated at room temperature with respect to two halite and sylvite or more n 4 salts the presence of licl was confirmed by identification of the salt hydrate licl 5h2o at very low temperature using raman spectroscopy bulk salinities could be roughly estimated at around 30 wt eq licl for the licl brine geometric and chronologic relationships between fip reveal close relationships and mutual contamination between the h2o co2 vapors and licl brine indicating synchronism in their trapping these fluids were generated and trapped at pressures of 100 130 mpa nearly 23 mpa above the estimated present day lithostatic pressure this implies a denudation rate between 0 2 and 0 5 mrn a 1 since the onset of hydrothermal activity compatible with the setting of larderello in a young tortonian collision belt fluid inclusion trapping temperatures 425 650 c show a monotonous increase towards the inferred granite and are around 100 200 c higher than the highest present day temperatures the results are interpreted as recording the interaction between magmatic and contact metamorphic fluids in the early larderello system the h2o co2 vapors resulted from the reheating of the basement metamorphic series often c rich under relatively high temperatures during contact metamorphism lirich fluids expelled from an underlying li rich leucogranite migrated through the metamorphic series and the already cooled granite dikes and experienced local boiling the fluid inclusion data demonstrate the involvement of magmatic fluids during the initial development of this high energy geothermal field

interdune deposits of the namib sand sea abstract the namib sand sea of southwestern africa is dominated by large north south trending linear dunes interdune areas however comprise about 50 of the area of the sand sea and may consist of exposures of the pre dune surface or sediments deposited between the dunes four major types of interdune deposits are identified and described 1 1 coarse poorly sorted aeolian sands 2 2 localized calcareous lacustrine deposits 3 3 silty deposits of ephemeral rivers that have flooded into the sand sea margins 4 4 sabkhas and salt marshes near the coast the preservation potential of these sediments is poor except where they are buried and protected by dunes or stabilised by high groundwater levels in coastal areas

structure chemistry and origins of gypsum crusts in southern tunisia and the central namib desert gypsum crusts are broadly defined as accumulations at or within about 10 m of the land surface from 0 10m to 5 0 m thick containing more than 15 by weight gypsum caso4 2h2o and at least 5 0 by weight more gypsum than the underlying bedrock the deposits are often but not invariably consolidated owing to cementation by gypsum the crusts are found in many of the world s deserts where mean monthly potential evaporation exceeds mean monthly precipitation throughout the year using structural fabric and textural criteria three main types of crust may be distinguished 1 bedded crusts found either at or beneath the land surface which are made up of discrete horizontal strata up to 0 10 m thick each showing a gradation in gypsum crystal size from less than 50 m at the top to more than 0 50 mm at the base 2 subsurface crusts of which there are two forms one made up of large lenticular crystals up to 0 50 m in diameter the desert rose crusts and the other a mesocrystalline form with gypsum crystals up to about 1 0 mm in diameter and 3 surface crusts which are subdivided into columnar powdery and cobble forms all of which are made up of predominantly alabastrine gypsum crystallites less than 50 m in diameter in southern tunisia and the central namib desert bedded crusts are found around ephemeral lakes and lagoons they are characterized by size graded beds gypsum contents of 50 80 by weight and comparatively high concentrations of sodium potassium magnesium and iron they are interpreted as shallow water evaporites which accumulate when saline pools evaporate to dryness desert rose crusts or croutes de nappe generally contain 50 70 by weight gypsum and have higher sodium concentrations than the second subsurface form texturally they are characterized by poikilitic inclusion of clastic material within large lenticular crystals they are interpreted as hydromorphic accretions which precipitate in host sediments at near surface water tables through the evaporation of groundwater the second form of subsurface crust the mesocrystalline often occurs in close association with the various surface forms unlike the hydromorphic crusts they are not restricted to low lying terrain they are characterized by gypsum contents reaching 90 by weight and have a close chemical and textural similarity to columnar surface crusts this mesocrystalline form represents an illuvial accumulation the surface forms excluding the bedded crusts are exhumed examples at various stages of solutional degradation subsurface precipitation of gypsum from meteoric waters containing salts leached from the surface results in displacive gypsum accumulation in the soil zone in southern tunisia the gypsum is derived from sand and dust deflated from evaporitic basins in the central namib salts dissolved in fog water are the most likely source where other salts are present differential leaching may form two tiered crusts calcrete gypsum or gypsum halite if rainfall is sufficient to mobilize the less soluble salt yet insufficient to flush the more soluble gypsum crust genesis is restricted to arid environments and if their susceptibility to post depositional alteration is acknowledged they can provide valuable palaeoclimatic indicators

restricted rifting and its coexistence with compressional structures results from the crop 3 traverse northern apennines italy a geological interpretation of the deep seismic reflection line crop 3 italian program crosta profonda deep crust profile no 3 which crosscuts the northern apennines from the tyhrrenian to the adriatic coast is presented the profile images the lithosphere up to 15 s twt and highlights several peculiarities a the lower crust is bedded by discontinuous sub parallel reflective markers which terminate at 7 s beneath the western side of the profile b in the same area a notable reflection is recognizable at 10 s and is interpreted as the top of the asthenosphere c east plunging shear zones are recognizable throughout the crust by contrast the adriatic outer side of this line shows a reflections deepening westward from 13 to 15 s twt related to the base of the crust 33 38 km depth b the absence of thick bedding of reflective markers within the lower crust and c tectonic structures affecting the basement which are different from those which deform the cover of the northern apennines geological interpretation is based on the eastward migration of ductile shear zones which have been recognized on the seismic line the bending of the outer zone crust is considered to be a consequence of the rifting process with the application of pushing forces against the adriatic lithosphere which cannot escape toward east

extensional and strike slip tectonics across the monte amiata monte cetona transect northern apennines italy and seismotectonic implications abstract this paper deals with the structural evolution along a transect across the inner northern apennines monte amiata volcano geothermal area monte cetona mainly focussing on the post orogenic middle miocene quaternary structures the tectonic setting of the study area derives from the superimposition of three main faulting events the first and second events developed during the middle late miocene and early middle pliocene respectively and took place in response to extensional tectonics the third faulting event developed in a transcurrent transtensional setting during the quaternary quaternary faults mainly strike ne sw and show left lateral kinematics with a minor dip slip normal component they caused the monte amiata volcanic eruptions 300 190 ka and controlled the widespread pleistocene and present hydrothermal fluids circulation quaternary strike slip faults have been documented in the monte amiata monte cetona area for the first time in this paper correlation with the adjoining areas e g vulsini region identifies the quaternary strike slip faulting recognised in the study area as a feasible source for the seismic activity in the monte amiata region where the origin of diffuse low magnitude seismicity is still under debate and poorly understood

geological structure of a long living geothermal system larderello italy we have examined the data of 600 geothermal wells and re interpreted approximately 500 km seismic profiles through the field of larderello italy we conclude that the two main seismic reflectors present below the geothermal area host two different fluids 1 superheated steam in the upper h horizon reached by drillholes and 2 supercritical fluid in the deeper k horizon reached by few unproductive or damaged wells the superheated steam has the physical and chemical connotation of the geothermal fluid exploited so far at larderello whereas the supercritical fluid represents a potential unconventional deep seated resource still to be assessed the high temperatures existing in correspondence of the k horizon suggest that the silica rich rocks are close to a plastic state and the fluids should remain confined in a medium sealed to the confining rocks unless occasional fluid overpressure and abrupt high strain rates occur

observation and analyses of shear wave splitting at the larderello travale geothermal field italy abstract we measured shear wave splitting sws parameters from a large dataset of local microearthquakes recorded at the larderello travale geothermal field ltgf tuscany italy for that geothermal area seismic anisotropy is distributed in the upper crust following a complex pattern although the overall trend reflects the strike of the normal faults dominating the region measurements at the southern and central part of the ltgf show large up to 90 deviations from the dominant polarization direction this anomalous pattern suggests that besides the extensive dilatancy anisotropy the fast wave polarization direction is also likely affected by the presence of over pressurized geothermal fluids by local rearrangement of the regional stress and by the presence of non vertical cracks we found large differences in normalised delay times between sparse and clustered seismicity while the average anisotropy percentage is on the order of 1 7 a significant amount of our measurements exceeds the 4 5 reaching values as high as 16 the highest anisotropy percentages are associated with earthquakes located at the center and at the se margin of the geothermal area at depths lower than 5 km and in the 5 10 km range respectively this latter occurrence may be interpreted in terms of cracks filled with fluids which given the expected pressure and temperature conditions are likely in supercritical conditions shear wave splitting thus confirms to be a powerful tool for better constraining location and extent of those deep fractured rock portions possibly hosting supercritical fluids that represent the next frontier of geothermal exploitation due to their enhanced heat capacity

recent aeolian origin of surficial gypsum crusts in southern tunisia geomorphological archaeological and remote sensing evidence recent quarring of the surficial gypsum crusts adjacent to djebel sidi bou hellas has revealed sections typically showing a discontinuous surface gravel cover underlain by more than 7 m of microcrystalline gypsum the location elongate shape form in cross section and chemistry of this deposit suggests an aeolian origin whereby aeolian sands have been trapped against a glacis d erosion terrace and subsequently consolidated by meteoric waters one gypsum quarry revealed a midden and the remains of a roman dwelling now buried within the crust a radiocarbon date of organic matter in the midden and a roman coin found within it suggest an age of ad324 345 for the deposit this is the first firm date for a surficial gypsum crust in southern tunisia and the age is surprisingly young previous studies have speculated on phases of crust development between the villefranchian and early holocene but none since remote sensing and field evidence show that gypsiferous sands are currently deflated from the dry parts of the mudflats of chott fedjaj they are subsequently transported in a southwesterly trajectory and trapped against glacis on the southern margins of chott fedjaj forming contemporary analogues of the roman deposit sands that are not trapped form dune fields and sandflats where gypsum crusts appear to be forming today if the source area of gypsum sands has remained constant since roman times then the predominant wind direction has moved 45 to the southwest since then the other possible source of aeolian gypsum for the roman deposit the vast mudflats of chott djerid involves an even greater change in predominant wind direction 1997 john wiley sons ltd

correlation between seismic reflectivity and rheology in extended lithosphere southern tuscany inner northern apennines italy abstract in southern tuscany inner zone of the northern apennines the lithosphere has been affected by extensional deformation since the middle miocene and is characterized by high heat flow 120 mw m 2 both local shallow and regional deep seismic reflection profiles clearly show the occurrence of a sharp reflector k horizon within the upper crust which is probably located at the top of a shear zone acting as the decollement horizon for the latest pliocene quaternary normal faults its depth varies between 3 and 6 km in geothermal areas and 9 and 12 km elsewhere borehole control in the geothermal areas shows that it occurs at a temperature of approximately 450 c the hypothesis that the k horizon represents the top of the brittle ductile transition agrees with two lines of evidence a it approximately follows the same isotherm over the whole region and b its depth coincides with rheological predictions of the brittle ductile transition for the range of high pore fluid pressures and strain rates estimated for southern tuscany it appears therefore that at least under specific conditions very high pore fluid pressures strain concentration the brittle ductile transition may have a seismic signature

multi stage fluid circulation in a hydraulic fracture breccia of the larderello geothermal field italy abstract the deep well mv5a drilled in the western part of the larderello geothermal field crossed a 20 cm thick hydraulic fracture breccia unit at a depth of 1090 m below ground level b g l this breccia occurs in a fine grained triassic metasandstone and consists of angular to subangular clasts of up to some centimeters in size pervasive alteration has affected the breccia clasts and wall rock around the breccia with the formation of mg fe chlorite after such alteration hydrothermal circulation caused the precipitation of two generations of calcite cement then ankerite partially replaced these two calcite generations ankerite also precipitated in late veinlets with chlorite late hydrothermal activity led to the crystallization of albite quartz and finally anhydrite the calcite contains vapor rich inclusions and two populations of liquid rich l1 and l2 inclusions l1 inclusions are characterized by homogenization temperatures between 304 and 361 c and salinities from 7 4 to 11 6 wt nacl equivalent l2 inclusions revealed homogenization temperatures in the range of 189 245 c and salinities from 2 6 to 6 3 wt nacl equivalent the fluids contained in l2 inclusions were probably trapped coevally with some vapor rich inclusions under boiling conditions after the l1 inclusions formed some of the abundant vapor rich inclusions in calcite may also represent early low temperature inclusions affected by decrepitation and or stretching and or leaking during l1 trapping the liquid rich l inclusions trapped at later stages in ankerite albite and anhydrite display respectively homogenization temperature ranges of 189 198 c 132 145 c and 139 171 c and salinities ranging from 1 6 to 1 7 wt nacl equivalent 1 4 to 2 1 wt nacl equivalent and 3 7 to 6 2 wt nacl equivalent the inclusions studied record the evolution over time of the fluids flowing in the breccia level l1 inclusions capture high temperature fluid about 300 to 350 c of high salinity around 10 wt nacl equivalent at above hydrostatic pressures up to about 150 bar the l2 inclusions in calcite and liquid rich inclusions in ankerite and albite represent subsequent hydrothermal fluid evolution toward lower temperatures about 250 to 130 c pressures 45 to a few bar and salinities 6 3 to 1 4 wt nacl equivalent during this stage boiling processes and infiltration of meteoric waters probably occurred finally moderately saline fluids around 5 wt nacl equivalent at a temperature about 160 c close to that of present day in hole measurements was trapped in the anhydrite inclusions the liquids trapped in liquid rich inclusions circulated at 41 000 years maximum age of calcite or later this age represents an upper limit for the development of vapor dominated condition in this part of the geothermal system the fluids circulating at the breccia level were probably meteoric and or connate waters these fluids may have interacted with the anhydrite and carbonate bearing formations present in the larderello area the occurrence of the hot and saline fluids trapped in l1 inclusions at above hydrostatic pressure suggests that similar fluids but with higher pressure 167 bar and temperature 360 c may have been responsible for rock fracturing

mapping the iron oxide content of dune sands namib sand sea namibia using landsat thematic mapper data abstract a linear mixture model was applied to landsat thematic mapper data to map iron oxide content over the northern part of the namib sand sea namibia field samples were collected to calibrate the remotely sensed iron oxide proportions to dithionite extractable fe in mg g 1 the in situ proportions correlating strongly with the remotely sensed estimates r 0 91 the results reveal a region of high fe oxide concentration in the east an area of low fe oxide concentration in the west separated by a mixing zone we interpret this to indicate that there is more than one sand source contributing to the sand sea

the tyrrhenian apennines system structural setting and seismotectogenesis abstract the available geological geophysical and seismological data of the tyrrhenian apennines system are briefly reviewed and synthesized in a number of sketch maps integrating and comparing these data a large scale simple shear kinematic model is proposed for the tyrrhenian apennines system that is here interpreted as an oblique asymmetric passive continental margin developed during the progressive counterclockwise rotation of the adriatic foreland from the late miocene to the present in the frame of this working hypothesis the thinning and stretching of the tyrrhenian crust were accommodated by progressive eastward motion of extensional allochthonous slices bounded by low angle west dipping oblique shear zones rotating above a common detachment that penetrates into the mantle lithosphere at any given time the most external eastward moving slice is bounded by a transtensional shear zone at the top and by a transpressional one at the bottom the displacement within the hanging wall of the transtensional shear zone is accommodated by high angle normal faults and en echelon grabens the displacement within the hanging wall of the transpressional shear zone is instead accommodated by co axial en echelon sets of folds thrusts and high angle strike slip faults thus the development of the late miocene pliocene pleistocene contractional strain field of the apennines of italy might not be a consequence of complex subduction processes but subordinate to the deformation processes that lead to the progressive opening of the tyrrhenian sea the seismicity of the crustal apennines of italy is analyzed in the frame of the above history of progressive deformation and a seismotectogenetic model is outlined

crustal structures in the geothermal areas of southern tuscany italy insights from the crop 18 deep seismic reflection lines abstract two deep seismic reflection lines named crop 18a and 18b and roughly nnw sse oriented were acquired to investigate the crust between the larderello and the mt amiata geothermal areas both located in southern tuscany since the early middle miocene southern tuscany has been affected by extensional tectonics and since the pliocene by widespread magmatism presently extension is demonstrated by ne dipping normal faults and brittle shear zones which affect the crust down to the brittle ductile transition in the larderello area these shear zones are seismically evidenced by ne dipping weak reflections and by the loss of the k horizon reflectivity a regional mid crustal high amplitude reflector located at the top of the brittle ductile transition the crop 18a and 18b lines were reprocessed down to 10 s twt down to the crust mantle boundary their reflectivity results are closely related to the direction of the survey a clear decrease in reflectivity occurs when the survey plane intersects the crustal shear zones along strike nevertheless both lines display segments with low reflectivity and homogeneous acoustic impedance which indicate intrusive magmatic bodies at different crustal levels a wide partially crystallised granitoid may be sited at about 7 8 km depth underneath the larderello geothermal field in the lower part of the crust the crop 18a and 18b transects show group of bright reflections mainly related to the occurrence of pressurised fluids and or lithological differences the base of the crust is located at about 20 km depth

fluid geochemical transect in the northern apennines central northern italy fluid genesis and migration and tectonic implications chemical and isotopic characteristics of natural gas and thermal water discharges from the western back arc tyrrhenian sea across the apennine thrust belt to the po valley and adriatic coast foredeep basins in the northern apennines central northern italy reveal a large scale fluid motion in the upper crust both vertically and horizontally on the basis of gas compositions two different domains of rising fluids have been distinguished 1 co2 rich he poor 3he 4he high domain in the western peri tyrrhenian extensional sector 2 ch4 rich he rich 3he 4he low domain in the eastern adriatic compressional sector such gases rising from various depths are crossed by a huge lateral n2 rich water flow in the peri tyrrhenian sector of ca so4 hco3 meteoric derived waters that move in a regional wide aquifer hosted in a quite thick mesozoic carbonate series morphologically the co2 vents consist of mud basins with high gas rate emission where the rising fluids move upwards through diatremes on the other hand ch4 emissions seep out from typical mud volcanoes with a reduced gas flow rate where the fluid motion is likely related to saline diapir extrusions and the methane is mostly carried to the surface by the associated mud the two rising mechanisms described locate southwest and northeast of the apennine watershed respectively from a seismic point of view the ch4 domain in the thrust belt and foredeep areas is characterized by a large number of earthquakes indirectly pointing to a different rheological behavior of the terrains with respect to the more internal peri tyrrhenian area owing to the quite high thermal gradient of the latter the boundary of the brittle ductile crust in the peri tyrrhenian sector can be located at a 10 km shallow depth although the presence of several post orogenic basins would suggest widespread extensional tectonics in all areas west of the apennine watershed those located in the easternmost part display gas vents with typical crustal 3he 4he ratios as this ratio is very sensitive to deep fluids rising from the mantle we hypothesize that such basins at the foot of the apennines are not due to tensive stress as suggested by their morphological shape they are piggy back thrust top basins developed and evolved in a still acting compressive tectonic regime

a threshold in soil formation at earth s arid hyperarid transition the soils of the atacama desert in northern chile have long been known to contain large quantities of unusual salts yet the processes that form these soils are not yet fully understood we examined the morphology and geochemistry of soils on post miocene fans and stream terraces along a south to north 27 to 24 s rainfall transect that spans the arid to hyperarid transition 21 to 2 mm rain y 1 landform ages are 2 my based on cosmogenic radionuclide concentrations in surface boulders and ar isotopes in interbedded volcanic ash deposits near the driest site indicate a maximum age of 2 1 my a chemical mass balance analysis that explicitly accounts for atmospheric additions was used to quantify net changes in mass and volume as a function of rainfall in the arid 21 mm rain y 1 soil total mass loss to weathering of silicate alluvium and dust 1030 kg m 2 is offset by net addition of salts 170 kg m 2 the most hyperarid soil has accumulated 830 kg m 2 of atmospheric salts including 260 kg sulfate m 2 and 90 kg chloride m 2 resulting in unusually high volumetric expansion 120 for a soil of this age the composition of both airborne particles and atmospheric deposition in passive traps indicates that the geochemistry of the driest soil reflects accumulated atmospheric influxes coupled with limited in soil chemical transformation and loss long term rates of atmospheric solute addition were derived from the ion inventories in the driest soil divided by the landform age and compared to measured contemporary rates with decreasing rainfall the soil salt inventories increase and the retained salts are both more soluble and present at shallower depths all soils generally exhibit vertical variation in their chemistry suggesting slow and stochastic downward water movement and greater climate variability over the past 2 my than is reflected in recent 100 y rainfall averages the geochemistry of these soils shows that the transition from arid to hyperarid rainfall levels marks a fundamental geochemical threshold in wetter soils the rate and character of chemical weathering results in net mass loss and associated volumetric collapse after 105 to 106 years while continuous accumulation of atmospheric solutes in hyperarid soils over similar timescales results in dramatic volumetric expansion the specific geochemistry of hyperarid soils is a function of atmospheric sources and is expected to vary accordingly at other hyperarid sites this work identifies key processes in hyperarid soil formation that are likely to be independent of location and suggests that analogous processes may occur on mars

aeolian landscapes in central australia gypsiferous and quartz dune environments from lake amadeus there are two different dune systems in central australia regional quartz dunefields and transverse gypsiferous dunes associated with playa lakes these two systems especially gypsiferous dunes at lake amadeus the largest playa in central australia provide a sedimentary geomorphological and environmental history of the region during the late quaternary the gypsifierous dunes consist of a surficial gypcrete overlying an aeolian sediment sequence below a mixture of gypsum sand and quartz sand no clay pellets have been found in the dune sequence in significant contrast to the gypsiferous clay dunes in other parts of australia three possible models of the environmental controls of gypsiferous dune formation are discussed the most plausible one suggests simultaneous gypsum precipitation and deflation sandsized gypsum was precipitated in a groundwater seepage zone around the playa margin during seasonally high water tables and these crystals were deflated onto land during dry intervals forming the marginal gypsiferous dunes these processes require conditions of high regional water table strong climatic seasonality and probably a windier and overall wetter climate at least two separate gypsiferous duneforming episodes can be recognized the age of formation of the younger one has been dated by thermoluminescence at 44 54 ka the gypcrete crust capping the dunes is characterized by intergrown microcrystalline gypsum crystals showing evidence of leaching dissolution and recrystallization it is interpreted as a pedogenic product formed during a stable period after accumulation of the gypsiferous dune after the construction of the younger gypsiferous dune there was a major episode of activation of regional quartz dunefields which formed thick quartz sand mantles overlying gypsiferous dunes on both playa margins and the dune islands within the playa an equivalent aeolian sand layer was deposited within the playa soil structures in this unit indicate that the sand sheet over the playa was later colonized by vegetation activation of the regional dunefields suggests a major period of dry climate which although not dated may correlate with the last glacial maximum identified as a period of maximum aridity from 25 to 18 ka at other sites in australia

geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy abstract serri g innocenti f and manetti p 1993 geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 117 147 the neogene quaternary magmatism of the northern apenninic arc took place in four phases separated in space and time which become progressively younger from west to east phase i 14 ma phase ii 7 3 6 0 ma phase iii 5 1 2 2 ma phase iv 1 3 0 1 ma this magmatism is the result of the activation of three physically separate sources 1 the adriatic continental crust extracted from the mantle in the late proterozoic 2 a strongly refractory recently k enriched harzburgitic mantle located in the mechanical boundary layer mbl of the lithosphere and 3 a recently metasomatized cpx rich mantle compositionally variable from iherzolite to wehrlite clinopyroxenite interpreted as an ephemerally k enriched asthenosphere the adriatic continental crust is the dominant source of the acid plutonic and volcanic rocks of the tuscan region the acid magmatism is mostly found inside an ellipsoidal area about 150 300 km centred on giglio island here defined as the tuscan crustal dome within this area mantle derived magmas unaffected by important crustal contamination processes and mixing with crustal anatectic melts have so far not been found pure crustal magmas are rare but are represented for example by some of the san vincenzo and roccastrada rhyolites virtually all the tuscan acid centres show evidence of mixing with potassic mantle derived magmas major and trace elements as well as 87 sr 86 sr and 143 nd 144 nd data on primitive rocks mg 65 reveal two groups of mantle derived magmas these define two distinct mantle enrichment trends both essentially due to the additions of k rich components which metasomatized separate compositionally diverse upper mantle sectors in both cases the most remarkable mineralogical effect of these enrichment processes is the production of variable amount of phlogopite through reaction between fluids and or melts with the mantle the rocks of group i ol hy and q normative lamproites ultrapotassic high mg latites ultrapotassic shoshonites and shoshonites saturated trend are considered to be derived by partial melting at low pressure 87 sr 86 sr 0 717 ce sr 0 3 and k 2 o na 2 o 6 7 and low ratios of 143 nd 144 nd 0 5121 0 5120 and ba la 87 sr 86 sr 0 712 and k 2 o na 2 o 8 9 values and low 143 nd 144 nd ba la 0 10 ratios these constraints do not allow to exclude a complete identity between the k rich components which metasomatized the mantle sources of the saturated and undersaturated trend magmas the geochemical and isotopic features of the components that metasomatized the mantle sources of the northern apenninic arc magmatism can be explained by a geodynamic process which causes a large amount of crustal materials to be incorporated within the upper mantle we propose that the delamination and subduction of the adriatic continental lithosphere related to the still ongoing northern apennine continental collision provide a viable mechanism to explain the genesis and eastward discontinuous migration of the magmatism in central italy the subduction of delaminated lithospheric mantle with lower crustal slivers would have exposed uppermost mantle adriatic mbl and crustal units previously imbricated in the apennine chain to the heating advected by the upwelling of a recently and ephemerally k enriched asthenospheric mantle wedge and by the underplating of magmas derived from it we consider that the diapiric uprising of a hot crustally contaminated asthenosphere occurs in the wake left above the sinking of the adriatic delaminated subducting continental lithosphere the delamination subduction process of the adriatic lithosphere has probably started in the early middle miocene but earlier than 15 14 ma ago as indicated by the age and petrologic characteristics of the first magmatic episode sisco lamproite of the northern apennine orogenesis

mapping capable faulting hazard in a moderate seismicity high heat flow environment the tuscia province southern tuscany northern latium italy abstract this study is focused on potential evidence of capable faulting affecting southern tuscany northern latium a region of high heat flow and moderate to low seismicity here we refer to this region as tuscia the latin name for the land of the etruscans the review of the evidence for active tectonics in this area has allowed us to identify several potentially active capable faults mapped using a combination of analysis of digital terrain models with remote sensing and local geological information derived from bibliographic data and different scale maps active and capable faulting is certainly more evident in the eastern sector of tuscia chianti cetona ridge pienza amiata and bolsena caldera however even in the western tuscia e g colline metallifere and tyrrhenian coast capable faulting may be hypothesized along numerous structural elements the characterization of active faulting potential in terms of geometry slip types and rates in such a high heat flow environment beyond its usefulness in improving the assessment of fault displacement and earthquake hazard also helps to improve knowledge of the stress field and structural setting of the many geothermal fields in the area this allows new useful insights into geothermal exploitation and hazard e g induced triggered seismicity moreover this new knowledge of active tectonics and capable faults of the hinterland sector of the northern apennines may also provide useful insights into the siting processes of highly hazardous and strategic structures and more generally in land use planning and management of infrastructures especially during emergencies

crustal models of the geothermal areas of southern tuscany italy abstract geological and geophysical data gathered in the geothermal areas of southern tuscany located in the inner part of the northern apennines indicate the presence of granite intrusions the most relevant data are 1 regional asthenospheric ascent to a depth of 40 50 km 2 granitic dykes as old as 3 8 2 9 ma and a widespread contact aureole found in several deep geothermal wells 3 very high heat flow values interpreted as the result of convective circulation of hot fluids above shallow magmatic bodies 4 the presence of low density bodies inferred from low gravity values often correlated with positive heat flow anomalies 5 the occurrence of a highly reflective horizon named k in a depth range of 3 12 km in the geothermal areas as shown by seismic reflection data 6 a 30 40 km wide low velocity body below the k horizon whose existence is supported by local earthquake tomography and teleseismic studies 7 the presence of a conductive body which correlates well with the anomalous tomographic and teleseismic body as revealed by magnetotelluric surveys the goal of the paper is to test different alternative crustal models by using these data the models are 1 granite intrusion 2 geopressurized fluids below impervious layers 3 contact metamorphic aureole 4 mylonite layers at the upper lower crust transition and 5 some combination of the four the occurrence of shallow intrusive bodies still partially molten and associated with saline brines is considered the most tenable model

upper crustal structure of the larderello geothermal field as a feature of post collisional extensional tectonics southern tuscany italy abstract reflection seismic profiles crossing the larderello geothermal field permit the distinction of three different seismic facies units in the upper continental crust the upper 3 4 km of the crust is characterized by low reflectivity its base corresponding to a reflector referred to as the k horizon and recognized in many other areas of southern tuscany beneath this reflector a seismic facies of 0 8 1 0 twt thickness is characterized by strong convergent and divergent reflections forming a lozange shaped geometry the lowermost seismic facies is characterized by almost horizontal reflections based on the distribution of the hypocentral microearthquakes temperature data recorded in deep boreholes and the geometry of normal faults the k horizon is explained here as a kinematically active rheological boundary which separates a brittle upper part from a ductile lower part according to this hypothesis the k horizon and the lozange reflections could originate from a mylonite band located at the contact between two geological bodies characterized by different rheological behavior it is inferred that this horizon acts as a decoupling plane in the upper continental crust it developed as a consequence of the extensional tectonics and the high heat flow which characterize southern tuscany

recent extreme slope failures in glacial environments effects of thermal perturbation abstract this paper describes recent exceptional slope failures in high mountain glacial environments the 2002 kolka karmadon rock ice avalanche in the caucasus a series of ice rock avalanches on iliamna volcano alaska the 2005 mt steller rock ice avalanche in alaska and ice and rock avalanches at monte rosa italy in 2005 and 2007 deposit volumes range from 10 6 to 10 8 m 3 and include rock ice and snow here we focus on thermal aspects of these failures reflecting the involvement of glacier ice and permafrost at all sites suggesting that thermal perturbations likely contributed to the slope failures we use surface and troposphere air temperatures near surface rock temperatures satellite thermal data and recent 2d and 3d thermal modeling studies to document thermal conditions at the landslide sites we distinguish between thermal perturbations of volcanic geothermal and climatic origin and thermal perturbations related to glacier permafrost interaction the data and analysis support the view that recent current and future climatic change increases the likelihood of large slope failures in steep glacierized and permafrost terrain however some important aspects of these settings such as the geology and tectonic environment remain poorly understood making the identification of future sites of large slope instabilities difficult in view of the potentially large natural disasters that can be caused by such slope failures improved data and understanding are needed

helium isotopes in paleofluids and present day fluids of the larderello geothermal field constraints on the heat source 1 the he isotope composition of paleofluids entrapped in fluid inclusions of hydrothermal minerals is compared with the present day fluid composition of the larderello geothermal field almost constant values of 3he 4he m 3he 4he air r ra over time indicate that no important changes have occurred in the deep source of gases at least during the last 3 8 million years on a regional scale a correlation has been found between the r ra spatial distribution heat flow and bouguer gravity anomaly high values of r ra and heat flow and low bouguer anomaly values indicate that the larderello field is an area of preferential escape for mantle derived fluids a positive correlation has also been found between the r ra spatial distribution and a major seismic reflector named the k horizon a deep magma source refilled by periodic gas input from the mantle is the most likely source of 3he enriched fluids and the anomalously high heat flow the nearly constant value of r ra clearly indicates that input of fresh mantle material has occurred up to recent times clear evidence of mixing between mantle and crustal fluids indicates that the high r ra is the lower limit of the actual mantle value which is suggested to be similar to the subcontinental european mantle the decrease of r ra over time in the peripheral part of the larderello field indicates that important changes in the feeding fracture system and or cooling rate have occurred in these areas

extension uplift of rocks and cooling ages in thinned crustal provinces the larderello geothermal area inner northern apennines italy in extensional tectonic settings vertical movements of the earth s surface are largely controlled by rates of strain extensional strain rates within the larderello geothermal area located in inner parts of the northern apennines have been evaluated together with uplift rates and 40 ar 39 ar cooling ages results indicate that extensional strain rates have been relatively low 5 8 10 17 s 1 since the middle pliocene differential uplift rates are estimated to have been high c 0 2 mm year strain and uplift rates within the larderello region are consistent with the regional tectonic framework the low strain rate rapid uplifting and high heat flow within southern tuscany can be explained by an eastward dipping lithospheric shear maintained since at least middle pliocene times

anomalous 17o compositions in massive sulphate deposits on the earth the variation of 18o that results from nearly all physical biological and chemical processes on the earth is approximately twice as large as the variation of 17o this so called mass dependent fractionation is well documented in terrestrial minerals1 2 evidence for mass independent fractionation 17o 17o 0 52 18o where deviation from this tight relationship occurs has so far been found only in meteoritic material and a few terrestrial atmospheric substances3 in the rock record it is thought that oxygen isotopes have followed a mass dependent relationship for at least the past 3 7 billion years ref 4 and no exception to this has been encountered for terrestrial solids5 here however we report oxygen isotope values of two massive sulphate mineral deposits which formed in surface environments on the earth but show large isotopic anomalies 17o up to 4 6 these massive sulphate deposits are gypcretes from the central namib desert and the sulphate bearing miocene volcanic ash beds in north america the source of this isotope anomaly might be related to sulphur oxidation reactions in the atmosphere and therefore enable tracing of such oxidation these findings also support the possibility of a chemical origin of variable isotope anomalies on other planets such as mars6

sources of sulphur in gypsiferous sediments and crusts and pathways of gypsum redistribution in southern tunisia southern tunisia contains one of the most extensive gypsum accumulations in africa comprising triassic cretaceous eocene and mio pliocene marine evaporites spring deposits playa sediments aeolian sands and gypsum crusts sulphur isotope analysis delta 34 s of bedrock samples groundwater playa brines playa sediments and gypsiferous crusts provides insight into the sources of gypsum in the region and sheds light on the processes that lead to gypsum crust formation results suggest that recycling of marine gypsum is the most likely source of the sulphate in the groundwater playa sediments and crusts the low ps values found in eocene and mio pliocene samples suggest that this recycling has been going on for millions of years though bedrock appears to be the ultimate source of the gypsum in the crusts transport of this sulphate to playas concentration therein and subsequent dispersal across the landscape by aeolian processes provides the most likely pathway for surticial gypsum crust formation comparison of these results with those from australia chile and namibia suggests that although the source of the sulphur varies from region to region the processes of surficial crust formation appear to be similar copyright c 2004 john wiley sons ltd

tertiary extensional tectonics in tuscany northern apennines italy abstract two extensional events have been recognized in the tertiary evolution of tuscany the first event occurred in the period between the late oligocene and late tortonian and caused a stretching of 120 or greater the second event occurred between the late tortonian and quaternary periods and caused a stretching not greater than 10 the first event is characterized by the exhumation of middle crust rocks and therefore by the development of core complexes the most significant structures of the first event are low angle normal faults which characterize the upper plate of the core complexes and ductile shear zones which characterize the lower plate the low angle normal faults tend to die out in the late triassic evaporite level which constitutes the main detachment of the core complexes the main structures of the second extensional event are high angle normal faults which dissect all the previous tectonic features these faults border tectonic depressions which were the site of clastic sedimentation from late tortonian to quaternary time these normal faults may tend to flatten in a seismic lozenge shaped band explained as an active extensional shear zone which is located at the brittle ductile boundary a similar explanation is given for the detachment located in the late triassic evaporite level during the first extensional event while the first extensional event is in the framework of the northern apennine post collisional evolution the second extensional event because it affects an already stretched crust could be attributed to the southern tyrrhenian opening which began during late tortonian

paleohydrological indicators in playas and salt lakes with examples from canada australia and africa abstract the sedimentary records of playas and salt lakes provide some of the best evidence for hydrological change in a region these records are comprised of siliciclastic sediments and chemically precipitated minerals that typically vary temporally and spatially in some cases the interpretation is straightforward with mineralogical sequences related to changing brine concentrations and hydrology as well as to climate in other cases it is difficult to identify the controlling factors which include climate groundwater geological setting and basin morphology in addition diagenesis commonly affects the sedimentary record models of lake level fluctuations are given for basins in the canadian prairies and southeastern australia the lake manitoba model is based mainly on secondary changes associated with pedogenesis that alters the deeper water sediment during low water stages smaller canadian lakes in more arid regions display a larger variety of diagnostic paleohydrological indicators such as specific carbonate and salt mineralogy nature of bedding grain size and secondary alterations decreasing water levels in shallow lakes tend to show a progression from clastics to carbonates to salts and then back to clastics as groundwater levels fall below the lake floor similar parameters are used in australian lakes the hydrological model for the lake tyrrell playa in victoria whose present ionic composition is similar to seawater shows clastics being deposited when average lake levels are high followed by deposition of primary carbonates gypsum and halite once seasonal drying on the lake floor begins secondary gypsum grows within already deposited sediment and clay pelletization in the capillary fringe of surface sediment leads to basin deflation and the accumulation of adjacent eolian lunettes continued water level decline leads to pedogenesis subsequent lake level rises in all of these models may initiate sedimentation in any part of the hydrological cycle and a complete cycle in the sedimentary record appears to be uncommon

evolution of a fault controlled fissure ridge type travertine deposit in the western anatolia extensional province the cukurbag fissure ridge pamukkale turkey in recent decades various interpretations have been proposed to explain the evolution of fissure ridge type travertine deposits in this paper we discuss the relationships between fissure ridges and brittle structures affecting their substratum through a detailed analysis of an inactive fissure ridge near cukurbag located in the pamukkale geothermal area western turkey the cukurbag fissure ridge can be taken as a model as it offers an opportunity to examine its internal structure on the walls of a roman quarry in addition this ridge has been studied by several researchers who have discussed the processes promoting the fissure ridge evolution the cukurbag fissure ridge is composed of irregularly alternating travertine laminated facies bedded travertine crosscut into rather large lithons by subvertical crystalline veins banded travertine the relationships between bedded and banded travertine indicate that the banded veins are diachronous and migrated through time suggesting a progressive fault zone enlargement in the footwall such a fault zone was characterized by polycyclic activity with normal to transtensional kinematics and was active during the latest quaternary we demonstrate that formation of banded veins is coeval with bedded travertine deposition and strictly depends on fault activity therefore highlighting the fundamental role of travertine fissure ridges in reconstructing palaeotectonic activity in a region

growth of a pleistocene giant carbonate vein and nearby thermogene travertine deposits at semproniano southern tuscany italy estimate of co2 leakage abstract a giant carbonate vein 50 m thick fissure ridge travertines and nearby travertine plateaus in the semproniano area mt amiata geothermal field southern tuscany italy are investigated through a multidisciplinary approach including field and laboratory geochemical analyses u th geochronology c nd o and sr isotope systematics ree abundances and fluid inclusion microthermometry the main aim of this work is to understand 1 modes and rates for the growth of the giant vein and nearby travertine deposits within a quaternary volcano tectonic domain 2 implications in terms of the co 2 leakage and 3 possible relationships with quaternary paleoclimate and hydrological oscillations results show that the giant vein was the inner portion of a large fissure ridge travertine and grew asymmetrically and ataxially through repeated shallow fluid injections between 650 and 85 ka with growth rates in the 10 2 10 3 mm a order the giant vein developed mainly during warm humid interglacial periods partially overlapping with the growth of nearby travertine plateaus estimated values of co 2 leakage connected with the vein precipitation are between about 5 10 6 and 3 10 7 mol a 1 km 2 approximately representing one millionth of the present global co 2 leakage from volcanic areas temperature estimates obtained from o isotopes and fluid inclusion microthermometry indicate epithermal conditions 90 50 c for the circulating fluid during the giant vein growth with only slight evidence of cooling with time geochemical and isotope data document that the travertine deposits formed mainly during pleistocene warm humid periods within a tectonically controlled convective fluid circuit fed by meteoric infiltration and maintained by the regional geothermal anomaly hosted by the carbonate reservoir of the mt amiata field

heat flow heat transfer and lithosphere rheology in geothermal areas features and examples abstract surface heat flow measurements over active geothermal systems indicate strongly positive thermal anomalies whereas in normal geothermal settings the surface heat flow is usually below 100 120 mw m 2 in active geothermal areas heat flow values as high as several watts per meter squared can be found systematic interpretation of heat flow patterns sheds light on heat transfer mechanisms at depth on different lateral depth and time scales borehole temperature profiles in active geothermal areas show various signs of subsurface fluid movement depending on position in the active system the heat transfer regime is dominated by heat advection mainly free convection the onset of free convection depends on various factors such as permeability temperature gradient and fluid properties the features of heat transfer are different for single or two phase flow characteristic heat flow and heat transfer features in active geothermal systems are demonstrated by examples from iceland italy new zealand and the usa two main factors affect the rheology of the lithosphere in active geothermal areas steep temperature gradients and high pore fluid pressures combined with lithology and structure these factors result in a rheological zonation with important consequences both for geodynamic processes and for the exploitation of geothermal energy as a consequence of anomalously high temperature the mechanical lithosphere is thin and its total strength can be reduced by almost one order of magnitude with respect to the average strength of continental lithosphere of comparable age and thickness the top of the brittle ductile transition is located within the upper crust at depths less than 10 km acts as the root zone of listric normal faults in extensional environments and at least in some cases is visible on seismic reflection lines these structural and rheological features are well illustrated in the larderello geothermal field in tuscany

influences of eolian and pedogenic processes on the origin and evolution of desert pavements well developed desert pavements are present above eolian deposits that mantle flows of the cima volcanic field located in the mojave desert california soil stratigraphic data and geochemical data demonstrate that eolian and pedogenic processes play major roles in the evolution of these pavements eolian dust 1 accelerates mechanical fragmentation of flow rock providing the source material for pavements and 2 accumulates slowly below basaltic colluvium in flow depressions eventually promoting development of cumulate soils below the evolving stone pavement an increase in dust flux during the holocene has raised ancient pleistocene pavements as much as 20 cm above the former land surface the results of our studies demonstrate for the first time that most desert pavements do not form by deflation by overland flow or by upward migration of stones through a slowly formed clayey argillic horizon desert pavements are born and maintained at the surface

the structure of the monte amiata volcano geothermal area northern apennines italy neogene quaternary compression versus extension the tectonic evolution of the mt amiata volcano geothermal area is under discussion some authors state that this region as well as the hinterland of the northern apennines were affected by compression from the cretaceous to the quaternary in contrast most authors believe that extension drove the tectonic evolution of the northern apennines from the early miocene to the quaternary field data seismic analyses and borehole logs have been integrated in order to better define the structural features of the continental crust in the mt amiata geothermal area in this paper i propose the hypothesis that the structure of the crust in the mt amiata volcano geothermal area derives from two main geological processes 1 contractional tectonics related to the stacking of the northern apennines cretaceous early miocene 2 subsequent extensional collapse of the hinterland of the mountain chain and related opening of the northern tyrrhenian sea early miocene quaternary compressional and extensional structures characterise the mt amiata region although extensional structures dominate its geological framework in particular the extension produced a middle late miocene boudinage of the previously stacked tectonic units b pliocene quaternary normal faulting which favoured the emplacement of a magmatic body in the middle upper crust and c the eruption of the mt amiata volcano which gave rise to an acid and intermediate volcanic complex 0 3 0 19 ma the extension produced the space necessary to accommodate the middle late miocene marine and continental sediments pliocene and quaternary normal and transtensional faults dissected the previous structures and influenced the early middle pliocene marine sedimentation within the structural depressions neighbouring the mt amiata volcano the magmatic body was emplaced at depth about 6 7 km during the pliocene extension and produced the eruption of the mt amiata volcano during the late pleistocene this gave rise to local uplift presently reaching about 3 000 m as well as a negative bouguer anomaly 16 mgal both centred on the mt amiata area the crustal dome shows a good correspondence with the convex shape of the regional seismic marker known as the k horizon which corresponds to the 450 c isotherm and the areas with greatest heat flow this is probably a consequence of the above cited magmatic body presently in the process of solidification a late pleistocene eruption occurred along a crustal fissure striking n50 mt amiata fault which crosscuts the crustal dome hydrothermal circulation proven by the occurrence of thermal springs and gas vents mainly co2 and h2s mainly occurs along the mt amiata fault both in the northeastern ans southwestern sides of the volcano

rapid salt weathering in the coastal namib desert implications for landscape development data on the rate and style of breakdown on naturally occurring rock types are important to improving geomorphological understanding of landscape development in arid environments two years exposure of rock blocks cut from an english limestone bath stone as well as locally relevant namibian rocks karibib marble and damara granite within four distinctive microenvironments in and around a coastal salt pan environment reveals high rates of breakdown in the bath stone blocks observable breakdown in the marble blocks and weight gains and strength increases in the granite blocks breakdown is associated with high conductivities and large amounts of halite and gypsum within the surface sediments frequency of wetting and drying appears to be a major control on the resultant breakdown rates these findings can be compared with recent data from further inland in the namib desert which suggest very slow rates of breakdown of similar blocks of damara granite and karibib marble even where salts are readily available high rates of rock breakdown within coastal salt pans appear to contribute to dust production and the effective flattening of relief along the coastal strip

studying travertines for neotectonics investigations middle late pleistocene syn tectonic travertine deposition at serre di rapolano northern apennines italy middle late pleistocene tectonic activity has been inferred through studies on travertine deposits exposed in a tract of the hinterland northern apennines a detailed study on the relationships between tectonics and travertine deposition coupled with 230th 234u age determination of travertines at cava oliviera quarry located close to serre di rapolano village southern tuscany northern apennines allowed us to recognise pleistocene faults whose activity has been referred to 157 24 ka at least travertine deposition was tectonically controlled by wsw ene striking oblique and normal faults associated to a main fault named as the violante fault this structure dissected a regional normal fault known as the rapolano fault early middle pliocene in age which bounded the eastern side of the pliocene siena basin and gave rise to space accommodation for clayey and sandy marine sediments hydrothermal circulation and related travertine deposition was favoured by the damaging enhancement due to the fault fault intersection tectonic activity has been also documented by deformation recorded by travertines which suggest a main tectonic event between 64 5 and 40 5 ka the tectonic activity described for the study area agrees with the quaternary tectonic evolution documented in the surrounding areas e g mt amiata and mt vulsini as well as the tyrrhenian margin of the central apennines indicating that a widespread tectonic activity affected the inner part of the apennines until the latest quaternary

oil and gas wells and their integrity implications for shale and unconventional resource exploitation data from around the world australia austria bahrain brazil canada the netherlands poland the uk and the usa show that more than four million onshore hydrocarbon wells have been drilled globally here we assess all the reliable datasets 25 on well barrier and integrity failure in the published literature and online these datasets include production injection idle and abandoned wells both onshore and offshore exploiting both conventional and unconventional reservoirs the datasets vary considerably in terms of the number of wells examined their age and their designs therefore the percentage of wells that have had some form of well barrier or integrity failure is highly variable 1 9 75 of the 8030 wells targeting the marcellus shale inspected in pennsylvania between 2005 and 2013 6 3 of these have been reported to the authorities for infringements related to well barrier or integrity failure in a separate study of 3533 pennsylvanian wells monitored between 2008 and 2011 there were 85 examples of cement or casing failures 4 blowouts and 2 examples of gas venting in the uk 2152 hydrocarbon wells were drilled onshore between 1902 and 2013 mainly targeting conventional reservoirs uk regulations like those of other jurisdictions include reclamation of the well site after well abandonment as such there is no visible evidence of 65 2 of these well sites on the land surface today and monitoring is not carried out the ownership of up to 53 of wells in the uk is unclear we estimate that between 50 and 100 are orphaned of 143 active uk wells that were producing at the end of 2000 one has evidence of a well integrity failure

anaerobic methane oxidation in low organic content methane seep sediments abstract sulfate dependent anaerobic oxidation of methane aom is the key sedimentary microbial process limiting methane emissions from marine sediments and methane seeps in this study we investigate how the presence of low organic content sediment influences the capacity and efficiency of aom at bullseye vent a gas hydrate bearing cold seep offshore of vancouver island canada the upper 8 m of sediment contains

highly extended terrains lateral segmentation of the substratum and basin development the middle late miocene radicondoli basin inner northern apennines italy 1 this paper deals with the evolution of sedimentary basins not delimited by normal faults with a substratum characterized by an upward concave shape and with infilling sediments synclinally deformed we describe the middle late miocene radicondoli basin representing an example of such bowl shaped basins its tectonic origin is controversial being related both to compression and extension these opposite interpretations bear significant consequences on the geodynamic context in which the inner northern apennines developed during the middle late miocene the results of our structural studies carried out in the substratum and infilling sediments indicate that the radicondoli basin is an example of a hanging wall basin developed in an extensional setting extensional tectonics determined the lateral segmentation of the substratum competent levels i e tuscan nappe and verrucano group and the consequent collapse of overlying less competent levels i e the ligurian units with the formation of a bowl shaped tectonic depression here the syntectonic sediments serravallian late messinian are deformed in a large syncline characterized by minor gravity driven folds with vergences toward the depocenter and traces of their axial planes parallel to the basin margins this paper highlights the role of the competence contrast during the postcollisional tectonic evolution and the influence of substratum lateral segmentation for the accommodation of syntectonic sediments

episodic hydrothermal alteration recorded by microscale oxygen isotope analysis of white mica in the larderello travale geothermal field italy abstract microscale oxygen isotope analysis 18o 16o of minerals can identify distinct events of fluid rock interaction this method is however still limited to a few major and accessory minerals of which most are anhydrous minerals we present the systematic study of oxygen isotope distribution in white mica by secondary ion mass spectrometry sims texturally and chemically distinct white mica populations in granitic and contact metamorphic rocks from the larderello travale geothermal field ltgf italy record stages of magmatic crystallization metamorphic hydrothermal replacement and fluid rock interaction the large range in intra and inter grain white mica 18o values between 1 and 14 reflects varying protoliths and degrees of fluid mineral interaction at variable temperatures 180 450 c present day temperatures p d t this variability reflects the large scale circulation of both 1 magmatic syn intrusive to early contact metamorphic hydrothermal fluids with high 18o values and 2 meteoric fluids with 18o values of 7 during a post intrusive late hydrothermal stage metasedimentary rocks from the upper reservoir contain distinct white mica populations occurring in close proximity m scale including detrital grains 18o 12 14 high na low mg partially altered white mica 18o 8 9 and late hydrothermal white mica 1 6 low na mid mg the late hydrothermal white mica has similar 18o values to other secondary minerals and is in equilibrium with meteoric dominated fluids with a 18o of 6 to 0 5 which circulated in the late hydrothermal stage downhole towards the lower reservoir white mica from two contact metamorphic micaschist samples shows either 1 homogeneous 18o values of ca 9 likely due to recrystallization in the contact metamorphic hydrothermal stage t ca 600 c or 2 a large spread in 18o from 2 to 12 within and across grains of variable texture and chemistry in the host rock and a cross cutting quartz white mica vein ca 300 c present day temperature hereafter p d t this contrasting 18o signature of white mica is also recorded in granite cored at up to 4 6 km depth the carboli granite contains white mica with a homogeneous magmatic 18o of 10 0 6 whereas older granite samples from radicondoli have magmatic to hydrothermal white micas that vary in 18o from 4 to 10 a pronounced intra grain 18o variability of up to 6 occurs in white mica domains with higher fe mg ti halos around inclusions of chloritized biotite as a result of interaction with dominantly meteoric fluids that infiltrated to depths of at least 4 6 km in the porto azzurro granite on elba italy altered white mica has 18o values of 2 6 down from 10 in unaltered grains the distribution of oxygen isotope ratios in white mica is thus firstly a result of pervasive versus selective fluid alteration at depth sample and grain scale secondly the actual preservation of these m scale variabilities indicates that volume diffusion is not detectable at microscale at p d t at or below 350 c where most of the heterogeneous white mica is found selective sample and grain scale fluid penetration occurs episodically and anisotropically on micro and megascale along faults fractures and cleavages producing lower 18o white mica at various times in zones of higher secondary permeability and active hydrothermal fluid circulation

origin and diagenetic evolution of gypsum and microbialitic carbonates in the late sag of the namibe basin sw angola abstract ephemeral evaporitic conditions developed within the uppermost part of the transgressive late sag sequence in the namibe basin sw angola leading to the formation of extensive centimetre to metre thick sulphate bearing deposits and correlative microbialitic carbonates rich in pseudomorphs after evaporite crystals the onshore pre salt beds examined in this study are located up to 25 m underneath the major mid aptian evaporitic succession which is typified at the outcrop by gypsiferous bambata formation and in the subsurface by the halite dominated loeme formation carbonate evaporite cycles mostly occur at the top of metre thick regressive parasequences which progressively onlap and overstep landward the former faulted rift topography or fill major pre salt palaeo valleys the sulphate beds are made up of alabastrine gypsum associated with embedded botryoidal nodules dissolution related gypsum breccia and are cross cut by thin satin spar gypsum veins nodular and fine grained fabrics are interpreted as being diagenetic gypsum deposits resulting from the dissolution and recrystallisation of former depositional subaqueous sulphates whereas gypsum veins and breccia result from telogenetic processes the carbonates display a broader diversity of facies characterised by rapid lateral variations along strike thin dolomitic and calcitic bacterial mediated filamentous microbialitic boundstones enclose a broad variety of evaporite pseudomorphs and can pass laterally over a few metres into sulphate beds dissolution related depositional breccias are also common and indicate early dissolution of former evaporite layers embedded within the microbialites sulphate and carbonate units are interpreted as being concomitantly deposited along a tide dominated coastal supra to intertidal sabkha and constitute high frequency hypersaline precursor events prior to the accumulation of the giant saline mid aptian bambata and loeme formations petrographic and geochemical analyses reveal successive dissolution recrystallisation and cementation phases that occurred during burial uplift and exhumation implying a complex diagenetic evolution of both gypsum and carbonates influenced by pore fluids of diverse composition which distinctly varied from meso to telogenetic domains

extensional shear zones as imaged by reflection seismic lines the larderello geothermal field central italy the larderello geothermal field is located in the inner northern apennines in an area which has been subject to extension since the early miocene the latest extensional episode pliocene present has resulted in the formation of nw trending nedipping listric normal faults whose geometry is controlled down to f3 km by borehole data in this paper we integrate a new interpretation of seismic reflection lines with existing seismic field and borehole data to analyse the relations among listric normal faults the top of the brittle ductile transition and the migration of geothermal fluids in accordance with previous interpretations we consider the strong reflector k horizon marking the top of the reflective mid lower crust and located at a depth of 3 5 km in the geothermal area to represent the top of the brittle ductile transition its reflectivity most probably derives from the presence of overpressured fluids we identify three main nw trending nedipping extensional brittle shear zones showing listric geometry and soling out in the vicinity of the k horizon the latter appears to be dislocated in correspondence of the soling out of the shear zones these shear zones because of the associated intense fracturing represent the most natural channels of upward migration of geothermal fluids from the magmatic sources located below the k horizon we suggest that these two conclusions that listric normal faults root at or near the brittle ductile transition and that they act as preferential upward migration paths for magmatic fluids may be of general validity for geothermal fields located in extensional settings d 2002 elsevier science b v all rights reserved

migration of geothermal fluids in extensional terrains the ore deposits of the boccheggiano montieri area southern tuscany italy an integrated study based on fluid inclusion 18o composition and structural analyses was carried out on a pliocene fossil hydrothermal system located to the south of the present active larderello geothermal field in the boccheggiano montieri area the study area is typified by mineralized cataclastic levels related to late oligocene early miocene thrust surfaces and to the following two generations of normal faults of miocene and pliocene ages respectively within the damage zone of the pliocene boccheggiano fault the mineralization is mainly made up of quartz and pyrite quartz pb zn sulfides or quartz pb zn sulfides fluorite carbonates assemblages occur instead in the older cataclastic levels two generations of liquid rich fluid inclusions were recognized in quartz and fluorite the first one with homogenization temperatures ranging between 172 and 331 c and salinity between 0 0 and 8 8 wt naclequiv records the early stage of hydrothermal activity the second generation of fluid inclusions documents a later stage with homogenization temperature from 124 to 288 c and salinity from 0 2 to 1 9 wt naclequiv fluid inclusions analyses also indicate that mixing of fluid with distinct salinities and or temperatures was a widespread process during the early stage and that fluid temperatures decreased moving from the boccheggiano fault toward the more distal and older cataclastic levels the 18o values of water in equilibrium with hydrothermal quartz which range from 5 7 to 0 1 are related to the circulation of meteoric water mixed with saline water that leached the evaporite level and enriched in 18o through water rock interaction and or with magmatically derived fluids results indicate that the damage zone of the pliocene boccheggiano fault represented the main channel for the flow of meteoric water which was heated at depth then mixed with high salinity fluids and finally ascend to infiltrate along the older cataclastic levels our results based on fluid inclusions oxygen isotopic compositions and structural analyses indicate that a single fluid flow path run through the damage zone of the boccheggiano fault and the older cataclasites which were thus hydraulically connected

low angle normal fault lanf zone architecture and permeability features in bedded carbonate from inner northern apennines rapolano terme central italy abstract fault zones have the capacity to be hydraulic conduits within upper crustal levels allowing migration of large volume of fluids through shallow and deeper geological environments low angle normal faults lanfs crosscutting carbonate rocks produce damaged volumes that may have a relevant role in channelling or hosting geothermal fluids therefore deserving of investigation to better predict mining targets deformation along lanfs zones dissecting carbonate successions produces permeable volumes presently exploited in the larderello and monte amiata geothermal areas italy in this paper the architectural and permeability features of an exhumed lanf zone exposed in the northern apennines rapolano terme central italy affecting cretaceous bedded limestone are presented such a fault was not affected by circulation of geothermal fluids but its features could reveal much on the potential impact on fluids migration in the active geothermal areas therefore resulting an intriguing analogue the study lanf zone consists of faults which enucleated at depth 4 km during its earlier stage of evolution dissolution seams often arranged in s c fabric characterised the whole damage zone dissolution seams developed under very low grade metamorphism t 100 150 c as indicated by illite crystallinty analyses fault zone architecture and permeability features changed during the fault growth and exhumation permeability heterogeneity and anisotropy characterised the lanf zone during its development if geofluids circulated within the fault zone it could be an effective barrier during its earlier evolution being accompanied by dissolution seams on the contrary it could play as combined barriers conduits during its later evolution progressively at shallower levels being characterised by intersecting fault planes which define pipe like conduits parallel to the direction of the tectonic transport such a configuration could have the capacity to impact on fluids migration for long paths across the upper crustal levels this adds significant information for predicting deeper geothermal circuits and the location of potential exploitable reservoirs in active tectonic environments

exhumation of deep crustal metamorphic rocks and crustal extension in arc and back arc regions we adress the problem of exhumation of metamorphic rocks in arc and back arc positions based on observations of the aegean and tyrrhenian back arc regions in both regions back arc extension has been active for 25 ma at least while convergence and crustal thickening was going on at the thrust front compressional structures are reworked in the back arc domain as extensional flat lying detachments which allow unroofing of the metamorphic pile metamorphic rocks suffer first a dynamic burial during nappe stacking and then fast exhumation when they reach the extensional region we describe a continuous process of burial and exhumation that is contemporaneous with the migration of the thrust front toward external zones the front of extension follows behind at approximately one hundred kilometers at the convergence front continuous subduction of a cold oceanic slab or continuous accretion of continental units keeps a cool p t gradient and hp lt metamorphic rocks are formed at depth the overall cool environment leads to a deep brittle ductile transition and thus to a resistant crust which can sustain its large thickness while the thrust front propagates forward parts of the accretionary wedge which were formed earlier enter the back arc domain which can be already under extension and which is warmer because of a greater distance to the cold slab and the presence of the volcanic arc the brittle ductile transition rises in the thick crust which then becomes unstable and subsequently collapses the exhumation history of metamorphic rocks below detachments is two fold 1 a constantely thickening crust is unroofed by flat lying detachments which rework early thrusts this first period is characterized by a good preservation of hp lt parageneses 2 the second period is when rocks are in the zone of crustal scale extension of the back arc region deep crustal metamorphics are exhumed below flat lying detachements which affect the whole crust

fluid flow within the damage zone of the boccheggiano extensional fault larderello travale geothermal field central italy structures alteration and implications for hydrothermal mineralization in extensional settings the neogene extensional province of southern tuscany in central italy provides an outstanding example of fossil and active structurally controlled fluid flow and epithermal ore mineralization associated with post orogenic silicic magmatism characterization of the hydrodynamic regime leading to the genesis of the polysulphide deposit known as filone di boccheggiano hosted within the damage zone of the boccheggiano fault is a key target to assess modes of fossil hydrothermal fluid circulation in the region and more generally to provide inferences on fault controlled hydrothermal fluid flow in extensional settings we provide a detailed description of the fault zone architecture and alteration mineralization associated with the boccheggiano ore deposit and report the results of fluid inclusion and stable oxygen isotope studies this investigation shows that the boccheggiano ore consists of an adularia illite type epithermal deposit and that sulphide ore deposition was controlled by channelling of hydrothermal fluids of dominantly meteoric origin within the highly anisotropic permeability structure of the boccheggiano fault the low permeability structure of the fault core compartmentalized the fluid outflow preventing substantial cross fault flow with focused fluid flow occurring at the hangingwall of the fault controlled by fracture permeability fluid inclusion characteristics indicate that ore minerals were deposited between 280 and 350 c in the upper levels of the brittle extending crust lithostatic pressure in the order of 0 1 gpa abundant vapour rich inclusions in ore stage quartz are consistent with fluid immiscibility and boiling and quartz ore vein textures suggest that mineralization in the boccheggiano ore deposit occurred during cyclic fluid flow in a deformation regime regulated by transient and fluctuating fluid pressure conditions results from this study i predict a strongly anisotropic permeability structure of the fault damage zone during crustal extension and ii indicate the rate of secondary structural permeability creation and maintenance by active deformation in the hangingwall of extensional faults as the major factor leading to effective hydraulic transmissivity in extensional terranes these features intimately link ore grade mineralization in extensional settings to telescoping of hydrothermal flow along the hangingwall block s of major extensional fault zones

deep crustal structure of the rhine graben from dekorp ecors seismic reflection data a summary abstract results of two deep reflection seismic lines crossing the northern and southern part of the rhine graben which were recorded in 1988 by the french ecors and the german dekorp teams indicate that the asymmetry of this rift persists to the base of the crust the brittle ductile transition zone approximately coincides with the top of the lower crust ductile shear zones appear to affect the crust mantle boundary upper crustal extension by faulting accounts for a divergence of the graben flanks by 5 7 km whereas the crustal configuration of the rhine graben suggest an extension value of some 17 km

evaluating the geogenic co2 flux from geothermal areas by analysing quaternary travertine masses new data from western central italy and review of previous co2 flux data abstract quantification of carbon fluxes between solid earth and its atmosphere is necessary to understand the global geological carbon cycle some of the main co2 contributors are metamorphism and magmatic mantle degassing co2 is discharged from active and quiescent volcanoes fault zones geothermal systems and co2 rich groundwater here a new method for the estimation of the geogenic flux of co2 from tectonically active regions based on the volume composition and age of travertine deposits is proposed the method is applied to the travertine deposits of western central italy where travertine deposition is driven by degassing of co2 charged groundwater results show that the study areas are characterized since middle pleistocene by diffuse co2 degassing processes with time averaged co2 fluxes ranging between 1 24 0 12 106 mol y 1 km 2 and 1 38 0 42 106 mol y 1 km 2 these values are of the same order of magnitude of carbon dioxide fluxes measured by different methods in western central italy and are higher than the global baseline co2 flux from high heat flow regions the review of the available 234u 230th and 14c data shows that the co2 degassing processes that affects western central italy nowadays were already active at least 350 ka ago proving that this region is a globally relevant case for the study of earth degassing considering the widespread occurrence of travertine deposits in tectonically active areas worldwide the proposed approach can be used as a reliable tool to estimate the co2 flux in different geodynamic settings within system earth

the north tyrrhenian northern apennines post collisional system constraints for a geodynamic model the north tyrrhenian northern apennines post collisional system ntas has been analysed on the basis of marine reflection profiles and bibliographic sources with the aim of providing an up to date geotectonic and evolutive framework consistent with structural data the continental rifting began to affect the most internal western side of the system in the middle upper miocene while the most external eastern zones were touched by rifting only in pleistocene times the birth and development of extensive asymmetric basins took place with a progressive speeding up of rifting towards the external sectors a diachronous regionally extended and eastward rejuvenating unconformity recognizable within the neogene quaternary basins marks the passage from the syn rift to the post rift tectonic regime while the first is everywhere dominated by extension the second one produces vertical movements reflection terminations related to this unconformity are interpreted in terms of vertical tectonic component three zones that experienced different post rift vertical movements are thus recognized in the ntas subsidence in the north tyrrhenian deep water region tectonic stability or alternation of moderate subsidence and uplift in the continental shelf and western tuscany strong uplift in eastern tuscany the updated structural data demonstrate that the ntas upper crust is crosscut by both tyrrhenian dipping and apennine dipping low angle detachment planes above which the upper crustal blocks rotated and experienced extensional transport along two opposite directions to the west and to the east respectively the coexistence of opposite dipping crustal surfaces bears two fundamental implications for the ntas geodynamics first it stresses the importance of the anti apennine transfer faults since they separate crustal blocks of opposite extension polarity secondly it testifies that the extension tectonics not only re utilized former crustal thrust planes but also gave origin to new significant e and ne dipping detachment planes plan and crossview geometries of faults together with considerations on the role of the metamorphic core complexes point to an interpretation of the ntas structure that is more consistent with the anastomosing shear model rather than the simple shear or the delamination ones

heat flow deep temperatures and extensional structures in the larderello geothermal field italy constraints on geothermal fluid flow the larderello geothermal field is located in the inner northern apennines southern tuscany an area which has been affected by extensional tectonics since the early middle miocene the structure of the larderello field is characterised by nw trending ne dipping pliocene to present normal faults their geometry down to depths of 4 5 km is constrained by field borehole and reflection seismic data an association between extensional structures and heat flow maxima up to 1000 mw m2 is recognisable from detailed surface heat flow mapping in order to investigate the relationships among extensional structures and heat flow subsurface isotherms were traced subject to borehole control along variously oriented geological cross sections the isotherms show vertical displacements associated with the recent normal faults and related deformation zones which reach the brittle ductile transition estimates of the relative importance of convective and conductive components of heat flow suggest that fluid circulation is particularly important in correspondence with the normal faults accounting for the correlation between isotherm perturbations and extensional structures in this view extensional shear zones are interpreted as the main structural pathways for the flow of hot geothermal fluids

structural control of geothermal reservoirs in extensional tectonic settings an example from the upper rhine graben abstract in extensional tectonic settings major structural elements such as graben boundary faults are typically oriented subparallel to the maximum horizontal stress component shmax they are often structurally accompanied by transfer zones that trend subparallel to the extension direction in the upper rhine graben such transfer faults are typically characterized by strike slip or oblique slip kinematics a major re orientation of the regional stress field by up to 90 of the upper rhine graben in the early miocene led to the present day normal and strike slip faulting regimes in the north and south of the upper rhine graben respectively and a transition zone in between consequently conditions for fault frictional failure changed significantly moreover it has been observed during tracer and stimulation experiments that such transfer faults may be of major importance for the hydraulic field of geothermal reservoirs under the present stress condition especially when located between production and injection well in this context we have investigated slip and dilation tendencies ts and td of major structural elements at reservoir scale for two representative geothermal sites bruchsal germany and riehen switzerland located close to the eastern main boundary fault of the upper rhine graben we have evaluated the quality and uncertainty range of both tendencies with respect to potential variation in shmax orientation despite significant differences in orientation of the structures and the stress regimes the resulting variation of ts and td reveal major similarities concerning the reactivation potential of both the graben parallel structures and the transfer faults the conditions of criticality for tensile failure and non criticality for shear failure suggest that transfer faults are most likely naturally permeable structures with low stimulation potential this is in agreement with the absence of both immediate tracer recovery and seismicity in the studied geothermal sites

tectonic control on travertine and calcareous tufa deposition in a low temperature geothermal system sarteano central italy this paper presents an integrated study directed toward an understanding of the tectonic control on the hydrothermal circulation and resulting deposition of middle late pleistocene travertine in a low temperature geothermal field of southern tuscany located in the sarteano area central italy the study area is characterized by thermal springs c 25 c and related continental carbonate deposits structurally controlled by sw ne trending oblique to strike slip faults dissecting previous normal faults sedimentological stratigraphical data and 230 th 234 u radiometric age determination for the travertine indicate that continental carbonates were deposited in a tectonically controlled perched spring system from 268 22 ka at least to the present the hydrothermal system is and was characterized by meteoric waters with a minor contribution from fluids of deeper origin meteoric waters infiltrated to depth through tectonically damaged zones of mesozoic and cenozoic carbonate rocks and overheated as a result of localized anomalous geothermal gradient then fluids enriched in bicarbonate sulphate moved up along highly fractured rock masses at the intersection between strike slip faults and normal faults this paper highlights the use of continental carbonates in the reconstruction of palaeohydrothermal systems in terms of the location of the main conduits for hydrothermal fluid flow age of faulting and physical properties of hydrothermal fluids

the role of playas in pedogenic gypsum crust formation in the central namib desert a theoretical model the formation of namibia s extensive pedogenic gypsum crusts caso4 2h2o is interpreted in a new light it is suggested that gypsum primarily precipitates at isolated points of evaporitic concentration such as inland playas and that deflation of evaporitic rich gypsum dust from these playas contributes to the formation of pedogenic gypsum duricrusts on the coastal gravel plains of the namib desert surrounding these playas this study establishes the nature extent and distribution of playas in the central namib desert and provides evidence for playa gypsum deflation and gravel plain deposition remote sensing shows the distribution of playas captures ongoing deflation and provides evidence of gypsum deflation it is proposed that following primary marine aerosol deposition both inland playas and coastal sabkhas generate gypsum which through the process of playa deflation and gravel plain redeposition contributes to the extensive pedogenic crusts found in the namib desert region copyright 2001 john wiley sons ltd