late cenozoic transpressional ductile deformation north of the nazca south america antarctica triple junction abstract the southern andes plate boundary zone records a protracted history of bulk transpressional deformation during the cenozoic which has been causally related to either oblique subduction or ridge collision however few structural and chronological studies of regional deformation are available to support one hypothesis or the other we address along and across strike variations in the nature and timing of plate boundary deformation to better understand the cenozoic tectonics of the southern andes two east west structural transects were mapped at puyuhuapi and aysen immediately north of the nazca south america antarctica triple junction at puyuhuapi 44 s north south striking high angle contractional and strike slip ductile shear zones developed from plutons coexist with moderately dipping dextral oblique shear zones in the wallrocks in aysen 45 46 top to the southwest oblique thrusting predominates to the west of the cenozoic magmatic arc whereas dextral strike slip shear zones develop within it new 40ar 39ar data from mylonites and undeformed rocks from the two transects suggest that dextral strike slip oblique slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the orogen similar ages were obtained on both high strain pelitic schists with dextral strike slip kinematics 4 4 0 3 ma laser on muscovite biotite aggregates aysen transect 45 s and on mylonitic plutonic rocks with contractional deformation 3 8 0 2 to 4 2 0 2 ma fine grained recrystallized biotite puyuhuapi transect oblique slip dextral reverse kinematics of uncertain age is documented at the canal costa shear zone 45 s and at the queulat shear zone at 44 s published dates for the undeformed protholiths suggest both shear zones are likely late miocene or pliocene coeval with contractional and strike slip shear zones farther north coeval strike slip oblique slip and contractional deformation on ductile shear zones of the southern andes suggest different degrees of along and across strike deformation partitioning of bulk transpressional deformation the long term dextral transpressional regime appears to be driven by oblique subduction the short term deformation is in turn controlled by ridge collision from 6 ma to present day this is indicated by most deformation ages and by a southward increase in the contractional component of deformation oblique slip to contractional shear zones at both western and eastern margins of the miocene belt of the patagonian batholith define a large scale pop up structure by which deeper levels of the crust have been differentially exhumed since the pliocene at a rate in excess of 1 7 mm year

nitrogen isotope hydrochemistry and denitrification within the chalk aquifer system of north norfolk uk abstract although there has been much debate over the potential health risks of nitrate in drinking water there is a real issue of the costs associated with removing nitrate from drinking water supplies in the chalk aquifer system of north east norfolk modern contaminants of a mainly agricultural origin produce high levels of nitrate 15 mg l no3 n in the unconfined valley areas whereas in confined regions the levels of nitrate are low and commonly below detection limits

balancing crustal thickening in arcs by tectonic vs magmatic means igneous intrusions were emplaced prior to and contemporaneous with horizontal shortening of the crust in the late cretaceous to late eocene magmatic arc in north chile 21 45 22 30 s temporally changing major and trace elements of magmatic rocks from this paleo arc system chronicled gradual crustal thickening prior to and substantial crustal thickening contemporaneously with crustal shortening balanced structural cross sections indicate a minimum of 9 km of arc normal shortening that occurred simultaneously with dextral arc parallel movements accounting for orogen parallel lengthening of 10 km this shortening produced 5 4 km of tectonic crustal thickening and resulted in a minimum of 42 km late eocene andean crustal thickness temporal and spatial geochemical changes diagnostic of crustal thickening indicate that the remainder 2 6 km was accommodated by basaltic underplating at or near the base of the arc crust prior to and during transpression the ratio of tectonic to magmatic crustal thickening is 2 1 whole crustal magmatic addition rates during the 12 m y duration of arc transpression are 35 km 3 per kilometer of model arc length per million years mafic underplating may have thickened the andean crust considerably but most pre neogene crustal thickening was due to discrete episodes of tectonic shortening

chemical heterogeneity in diffusion dominated aquitards 1 high resolution measurements of the three dimensional 3 d variability in pore water electrical conductivity ec were obtained at a clay rich aquitard research site the king site 120 m 70 m area in southern saskatchewan canada using direct push probing technology n 35 boreholes these measurements revealed complex lateral heterogeneity in the subsurface salt distributions which raised questions on the validity of the commonly used approach of transposing chemical data from distributed piezometers onto 1 d depth profiles for interpretation and solute transport modeling two dimensional numerical modeling simulations demonstrated that distributed nonuniform salt inputs have existed at the top of the unoxidized till aquitard for the past 3 5 kyr the source of the nonuniform salt inputs is unclear but may be due to either variations in microtopography 0 5 m or variable penetration depth of surface fractures a suite of generic solutions was developed to estimate the critical depth required to obtain chemical homogeneity in aquitards with nonuniform solute inputs and diffusion as the dominant transport mechanism these solutions are presented for a range of sinusoidal input functions with varying amplitudes magnitudes and wavelengths the solutions indicate ec distributions at the king site should be laterally uniform below a depth of around 20 m bg which is consistent with observed ec values at the site this study shows that aquitards with nonuniform salt inputs at their upper boundary are chemically heterogeneous above some critical depth and thus require careful attention when modeling solute transport processes within them

evidence of cfc degradation in groundwater under pyrite oxidizing conditions summary a detailed local scale monitoring network was used to assess cfc distribution in an unconfined sand aquifer in southwestern ontario where the zone of 1 5 year old groundwater was known with certainty because of prior use of a bromide tracer groundwater 5 years old was confined to an aerobic zone at 5 m depth and had cfc concentrations consistent with modern atmospheric mixing ratios at recharge temperatures of 7 11 c as was observed in the 3 m thick vadose zone at the site at depths below 6 m the groundwater became progressively more reducing however with a denitrifying horizon at 6 7 m depth and a mn and fe reducing zone below 7 m depth in the anaerobic zone 3 h 3 he ratios indicated that groundwater age continued to increase uniformly with depth to a maximum value of 27 years at 13 m depth cfc concentrations however decreased abruptly within the denitrifying zone leading to substantial age overestimation compared to the 3 h 3 he ages noble gas data indicated that the apparent cfc mass loss was not likely the result of gas stripping from possible bubble formation thus cfc degradation was indicated in the anoxic zone the field data are consistent with first order degradation rates of 0 3 yr 1 for cfc 12 0 7 yr 1 for cfc 11 and 1 6 yr 1 for cfc 113 cfc attenuation at this site coincides with a zone where reduced s pyrite is actively oxidized by no 3 and dissolved oxygen do similar behavior has been observed at other sites tesoriero a j liebscher h cox s e 2000 mechanism and rate of denitrification in an agricultural watershed electron and mass balance along groundwater flow path water resour res 36 6 1545 1559 hinsby k hojberg a l engesgaard p jensen k h larsen f plummer l n busenberg e accepted for publication transport and degradation of chlorofluorocarbons cfcs in a pyritic aquifer rabis creek denmark water resour res further demonstrating that the use of cfcs for age dating anaerobic groundwater should be approached with caution particularly if the sediment contains pyrite

pore water extraction from the unsaturated and saturated zones a simple inexpensive method for extracting pore water from unsaturated soils which uses a combination of immiscible fluid displacement suction and mechanical compression is described this method uses a squeezing chamber to contain the unsaturated sediments in aluminum core tubing to prevent exposure of the sediments and pore water to atmospheric o2 and subsequent oxidation reactions geochemical artifacts resulting from high compression squeezing are prevented by maintaining the pressure applied during the squeezing at a minimum comparison of squeezed profiles with field measured profiles of ph eh oxidation reduction potential alkalinity and metal speciation indicates good agreement between these parameters using this method depending on the water content of the sediments a range of water volumes several millilitres to excess of 100 ml can be extracted from a single core section 7 6 cm in diameter by 20 cm long pore water was produced immediately from sediments at near saturated condition

syntectonic anatexis and magmatism in a mid crustal transpressional shear zone an example from the hercynian rocks of the eastern pyrenees abstract hercynian metamorphism and magmatism in the cap de creus peninsula eastern pyrenees occurred contemporaneously with non coaxial deformation in a transpressive regime an example of this has been taken from a migmatite complex in the northern coast the studied area is formed by partially melted sillimanite schists together with two different suites of igneous rocks a calc alkaline magmatic sequence consisting of small granitoid bodies and later peraluminous leucogranites and pegmatites all these rocks occur within a ne sw to e w trending sub vertical high strain zone where a first schistosity is tightly folded and transposed the relations between folds and crenulation cleavage in the metasediments and deformational fabrics in the granitoids and migmatites provide good evidence for the penecontemporaneity of deformational processes magmatism and migmatization furthermore the analysis of cross cutting relationships between different granitoids as well as the observation of their different degrees of deformation verifies that magmatism in this area took place as synkinematic consecutive intrusions from the more basic magmas to the peraluminous acid dykes

aquifer scale controls on the distribution of nitrate and ammonium in ground water near la pine oregon usa summary geochemical and isotopic tools were applied at aquifer transect and subtransect scales to provide a framework for understanding sources transport and fate of dissolved inorganic n in a sandy aquifer near la pine oregon no3 is a common contaminant in shallow ground water in this area whereas high concentrations of nh4 n up to 39 mg l are present in deep ground water n concentrations n cl ratios tracer based apparent ground water ages n isotope data and hydraulic gradients indicate that septic tank effluent is the primary source of no3 n isotope data n cl and n c relations 3h data and hydraulic considerations point to a natural sedimentary organic matter source for the high concentrations of nh4 and are inconsistent with an origin as septic tank n low recharge rates and flow velocities have largely restricted anthropogenic no3 to isolated plumes within several meters of the water table a variety of geochemical and isotopic data indicate that denitrification also affects no3 gradients in the aquifer ground water in the la pine aquifer evolves from oxic to increasingly reduced conditions suboxic conditions are achieved after about 15 30 y of transport below the water table no3 is denitrified near the oxic suboxic boundary denitrification in the la pine aquifer is characterized well at the aquifer scale with a redox boundary approach that inherently captures spatial variability in the distribution of electron donors

the pangong injection complex indian karakoram a case of pervasive granite flowthrough hot viscous crust in the karakoram range of nw india the tangtse gorge cuts across deep crustal rock sexhumed by transpression along two strands of the dextral karakoram fault crustal leucogranitic magmas pervasively intruded amphibolites and migmatites in the form of sheets that locally coalesced and expanded to form kilometre scale plutons sheets resulted from slow seeping of granite into foliation planes magma wedging rather than from dyking driven by magma buoyancy and possibly syn intrusive tectonic deformation high temperature of the country rocks at the time of intrusion is inferred from similar crystallization ages of the intrusive leucogranite and in situ partial melt in the migmatites and reflected in the ductile structures developed whereas low country rock viscosity inhibited dyking high temperature freed the magma from the constraints of freezing and permitted pervasive intrusion the injection complex exposed along the tangtse gorge rather than representing the final emplacement structures represent transient structures of the granite pathways on their upward journey to build the karakoram batholith which crops out a few kilometres north at structurally shallower levels we suggest that pervasive magma flow is a transitional step between magma segregation at the source and later rise through cold crust and it may be one of several factors controlling whether dyking or diapirism becomes dominant during late ascent

fault assisted vertical pluton growth coastal cordillera north chilean andes abstract extensive tracts of magmatic arc plutonic suites in the mesozoic coastal cordillera of northern chile imply that large volumes of granitic to dioritic magmas were transferred from a lower crustal source to the upper crust space for single intrusions was created by interaction of vertical pluton growth and dip slip reactivation of arc parallel faults during episodic emplacement of subhorizontal compositionally distinct magma pulses cross sectional pluton shapes are broadly tabular in geometry and were controlled by differential subsidence of the pluton floors during fault reactivation and incremental assembly of subhorizontal sheets equal amounts of floor subsidence of the fault footwall and hanging wall during magma accommodation led to symmetrical intrusions whereas differential subsidence caused plutons to grow asymmetrically strongly asymmetrical plutons resulted when only the hanging wall was reactivated to accommodate floor subsidence late post intrusion contractional deformation resulted in open folding of some tabular plutonic units and local inversion of bounding faults which has modified but not obscured the original syn emplacement geometry and fault kinematics

kinematic setting and structural control of arc volcanism abstract the evaluation of the kinematic setting and the structural control of volcanic arcs are important in defining the tectono magmatic processes along convergent plate boundaries however our knowledge is fragmented and the available data highlight different behaviours this study analyzes the kinematic setting the structural control and the volcanic productivity of 16 arcs these arcs are characterized by predominant extensional compressional strike slip or oblique motions there is an overall coupling between the normal vs parallel motions along the arc and those of the underlying slab therefore the higher the trench normal or parallel component of the subduction rate the higher is the amount of arc normal or parallel motion this relation confirms that strain partitioning is in general feasible at many convergent settings involving also the volcanic arc portion the arc normal motion may be characterized by extension or compression as a function of the increase in the trench normal convergence or subduction velocity the lack of an evident relation between the subduction rate or convergence rate and the volcanic productivity of the arc is partly inconsistent with previous studies which highlight a relation between the subduction rate and melt production below arcs this discrepancy mainly suggests that different processes control the generation rise and eruption of magma at different depths varying the intrusive extrusive ratio along arcs the structure of the arc does not control the distribution of the volcanoes however it does control the volcanic output through different processes even though regional or local extension associated with strike slip or compressive structures is the ultimate requisite in any setting in general the higher the amount of extension the higher is the output rate along the arc

dating of fault zone activity by apatite fission track and apatite u th he thermochronometry a case study from the lavanttal fault system eastern alps introductionfaulting in the upper crust may leadto thermal anomalies caused by fric tional heating e g maddock 1983 dalessio et al 2003 and or in ltra tion of hydrothermal uids yamadaet al 2007 heating e ects are ex pected to be localised along faultzones and restricted to zones a fewcentimetres adjacent to the main faultsurface dalessio et al 2003 in this study we attempted to datethermal anomalies within fault zonesby comparison of cooling ages at thetransition from fault related rocks tothe undeformed host rocks for thispurpose we chose the lavanttal faultsystem due to underground excava tion and cored drillings related togeological site investigations for amajor railway project in austria it isone of the best exposed fault zones inthe eastern alps we used a combi nation of ssion track aft and u th he analysis on apatites thesensitivity intervals of these thermo chronometers are often referred to aspartial annealing and retention zones apaz heprz and range from 120to 60 c and 80 to 40 c respectively green et al 1986 wolf et al 1996 assuming holding times over geo logical time scales wagner 1979 ketcham et al 1999 the low temperature sensitivity of these ther mochronometers allows thermalanomalies along surfaces of fault slipor uid pathways to be detected forsuch applications however annealingand retention depend not only onheating temperature but also on heat ing duration laslett et al 1987 murakami et al 2006 a combina tion of forward and inverse modellingon apatite age track length and hedata allows quantitative analysis ofthe cooling behaviour of the hostrocks our new results underline theimportance of using multiple low temperature thermochronometersin elucidating details of the thermalhistory of a fault zone geological outlinethe nnw trending lavanttal faultsystem one of the major strike slipfaults in the eastern alps fig 1 separates the basement complexes ofthe kor and saualm by dextral o setof approximately 10 km reinecker 2000 it is assumed to have beenactive since early miocene times withpeaks in activity between 18 16 and14 12 ma reinecker 2000 as indi cated by the evolution of adjacentsedimentary basins and the regionalstructural framework e g pischingeret al 2008 near its southern termi nation the lavanttal fault o sets theperiadriatic fault by about 20 km despite some models at the scale ofthe eastern alps describing the gen eral evolution of con ning fault zonesduring miocene times e g peressonand decker 1997 frisch et al 1998 2000 neubauer et al 2000 littleremains known about the evolutionof the lavanttal fault system how ever these models include a relation ship between the lavanttal faultsystem and the uplift of the koralmcomplex neubauer and genser 1990 previous thermochronologicalstudies documented that the koralmcomplex was already near the surfaceduring the whole cenozoic period hejl 1997 the timing of its naluplift however is less constrained sample descriptionparts of the lavanttal fault system arecharacterised by cataclasites and faultgouges up to several tens of metresthick thermochronological analyseswere carried out on drill core samples diameter 10 cm taken during thegeological investigations for thekoralm railway tunnel drillingabstract

correlation of nitrate profiles with groundwater and sediment characteristics in a shallow sandy aquifer abstract sediment profiles of a shallow sandy aquifer in an agricultural area of denmark can be characterized by a new term called total reduction capacity trc quantifying the total amount of reduced compounds in the aquifer material ferrous iron reduced sulfur compounds solid organic carbon a distinct increase of trc was observed at and below the oxidation reduction front in this study the front was a zone 30 cm wide oxygen nitrate and sulfate were present above the oxidation reduction front but disappeared at the front detailed porewater profiles indicate that denitrification also takes place above the front supposedly in anaerobic microenvironments not evident in the water samples from the small wells the trc profiles showed that some reduction capacity is still present above the oxidation reduction front and is apparently able to support denitrification

40ar 39ar and rbsr analyses from ductile shear zones from the atacama fault zone northern chile the age of deformation abstract the influence of deformation on the k ar and the rb sr isotope system is investigated it is assumed that due to the diffusion processes involved deformation has a similar effect on isotopic equilibrium as has temperature in order to examine the influence of deformation on the k ar and the rb sr isotope systems two shear zones from the atacama fault zone afz situated in the north chilean coastal cordillera have been investigated the afz which was active as a sinistral strike slip fault during the mesozoic has two sets of shear zones one formed under amphibolite sz1 one under greenschist facies conditions sz2 rb sr and 40ar 39ar age determinations were conducted on samples from cross sections of each set in sz1 the hornblendes and bioties from a weakly deformed sample reveal cooling ages of 153 152 and 150 1 ma respectively biotite from the center of the shear zone of sz1 gave an isochron of 143 9 0 3 ma mswd 0 04 which is interpreted as the age of deformation which produced resetting of the mineral system in sz2 hornblendes yielded 40ar 39ar plateau cooling ages of 138 ma biotites from undeformed samples gave rb sr and 40ar 39ar total degassing ages of 130 1 ma whereas biotite from the mylonitic rocks yielded 126 125 ma which dates the time of deformation sr isotope homogenization occurred in the mylonitic rocks and is most likely a result of deformation the formation of sz1 can be correlated to the araucanian nevadan phase the deformation in sz2 is related to the onset of uplift and cooling of the coastal cordilleran magmatic arc

denitrification in the recharge area and discharge area of a transient agricultural nitrate plume in a glacial outwash sand aquifer minnesota 1 recharge rates of nitrate no3 to groundwater beneath agricultural land commonly are greater than discharge rates of no3 in nearby streams but local controls of no3 distribution in the subsurface generally are poorly known groundwater dating cfc 3h was combined with chemical ions and gases and stable isotope n s and c analyses to resolve the effects of land use changes flow patterns and water aquifer reactions on the distributions of o2 no3 so4 and other constituents in a two dimensional vertical section leading from upland cultivated fields to a riparian wetland and stream in a glacial outwash sand aquifer near princeton minnesota within this section a plume of oxic no3 rich groundwater was present at shallow depths beneath the fields and part of the wetland but terminated before reaching the stream or the wetland surface groundwater dating and hydraulic measurements indicate travel times in the local flow system of 0 to 40 years with stratified recharge beneath the fields downward diversion of the shallow no3 bearing plume by semiconfining organic rich valley filling sediments under the wetland and upward discharge across the valley and stream bottom the concentrations and 15n values of no3 and n2 indicate that the no3 plume section was bounded in three directions by a curvilinear zone of active denitrification that limited its progress however when recalculated to remove the effects of denitrification the data also indicate changes in both the concentrations and 15n values of no3 that was recharged in the past isotope data and mass balance calculations indicate that fes2 and other ferrous fe phases were the major electron donors for denitrification in at least two settings 1 within the glacial fluvial aquifer sediments beneath the recharge and discharge areas and 2 along the bottom of the valley filling sediments in the discharge area combined results indicate that the shape and progress of the oxic no3 plume termination were controlled by a combination of 1 historical and spatial variations in land use practices 2 contrast in groundwater flow patterns between the agricultural recharge area and riparian wetland discharge area and 3 distribution and abundance of electron donors in both the sand aquifer and valley filling sediments the data are consistent with slow migration of redox zones through the aquifer in response to recharging oxic groundwater during holocene time then an order of magnitude increase in the flux of electron acceptors as a result of agricultural no3 contamination in the late twentieth century to which the redox zone configuration still may be adjusting the importance of denitrification for no3 movement through formerly glaciated terrains should depend on the source areas and depositional environments of the glacial sediments as well as geomorphology and recent stream valley sediment history

mechanisms of continental growth in extensional arcs an example from the andean plate boundary zone during the late triassic jurassic and early cretaceous an extensional magmatic arc was formed in the andean margin of northern chile plutons emplaced at ramps within a hinterland propagating extensional duplex were fed by dikes that transferred magma through the lower crust from a reservoir in the mantle phases of volcanism separated phases of plutonism and our model demonstrates that plutonism was favored only when upper plate extensional fault systems were active when they were not dikes cut across inactive faults and magma rose directly to the surface during volcanic phases of arc growth

nitrate reduction in an unconfined sandy aquifer water chemistry reduction processes and geochemical modeling nitrate distribution and reduction processes were investigated in an unconfined sandy aquifer of quaternary age ground water chemistry was studied in a series of eight multilevel samplers along a flow line deriving water from both arable and forested land results show that plumes of nitrate contaminated groundwater emanate from the agricultural areas and spread through the aquifer the aquifer can be subdivided into an upper 10 to 15 m thick oxic zone that contains o2 and no3 and a lower anoxic zone characterized by fe2 rich waters the redox boundary is very sharp which suggests that reduction processes of o2 and no3 occur at rates that are fast compared to the rate of downward water transport nitrate contaminated groundwater contains total contents of dissolved ions that are two to four times higher than in groundwater derived from the forested area the persistence of the high content of total dissolved ions in the no3 free anoxic zone indicates the downward migration of contaminants and that active nitrate reduction is taking place nitrate is apparently reduced to n2 because both nitrite and ammonia are absent or found at very low concentrations possible electron donors in the reduced zone of the aquifer are organic matter present as reworked brown coal fragments from the underlying miocene and small amounts of pyrite at an average concentration of 3 6 mmol kg electron balances across the redoxcline based on concentrations of o2 no3 so42 and total inorganic carbon tic indicate that pyrite is by far the dominant electron donor even though organic matter is much more abundant groundwater transport and chemical reactions were modeled using the code phreeqm which combines a chemical equilibrium model with a one dimensional mixing cell transport model only the vertical component of the water transport was modeled since in contrast to rates along flow lines the vertical rates are close to constant as required by the one dimensional model average vertical transport rates of water in the saturated zone were obtained by tritium dating the modeling process is a two step procedure first the sediment column is initialized with natural water containing only oxygen as electron acceptor and subsequently agricultural waters containing both oxygen and nitrate are fed into the column the nitrate concentration of agricultural waters entering the saturated zone varies with time and an input function was therefore constructed by linear mixing of natural waters and agricultural waters this input function was fed into the column initialized with natural water and the model run forward in time to the year 1988 where field data are available comparison with field data shows that the variation in groundwater chemistry is well described by the model when reduction of oxygen and reduction of nitrate by pyrite oxidation are the only redox reactions occurring finally predictions are made for the distribution of water chemistry in the year 2003 downward progression of the redoxcline is accelerated by a factor of five due to nitrate pollution of the aquifer but absolute rates remain small of the order of a few centimeters per year the controlling factor for nitrate migration through the aquifer once it has reached the anoxic zone is the concentration and distribution of pyrite in the sediments

attenuation of nitrate in aquitard sediments of southern ontario to examine nitrate persistence detailed geochemical profiling using core squeezed water and piezometer samples was carried out at five sites in southern ontario where groundwater is moving downward in silt rich aquitard sediments at rates of 16 to more than 20 cm year 1 elevated levels of no3 n 5 50 mg 1 1 that occur in the shallow groundwater as a result of agricultural activity were found to be consistently attenuated generally to very low levels 0 05 mg 1 1 n at the redoxcline the horizon marking the boundary between the surficial weathered brown sediments and the underlying unweathered grey sediments tritium dating suggests that groundwater at the redoxcline depths 3 5 m was recharged between 1970 and 1980 thus the n03 depletion appears to result from biodegradation reactions since no major landuse changes have occurred during this period the close association of the nitrate depletion zones with the redoxcline where in particular sediment sulphur contents increase abruptly and where also porewater so42 levels increase suggests that the dominant attenuation reaction is autotrophic denitrification using reduced sulphur compounds present in the unweathered sediment as the electron donor mass balance calculations suggest that the increase in the downward rate of migration of the redoxcline owing to added sulphur consumption from no3 contamination is only about 1 mm year 1 at these sites review of the literature indicates that most silt and clay rich sediments have s contents in the same range or higher than those investigated here thus in most cases where aquifers are overlain by several metres or more of unweathered confining sediments it is likely that a high degree of protection is afforded from surficial no3 contamination

structural setting of the candelaria fe oxide cu au deposit chilean andes 27 30 s the candelaria fe oxide cu au ore deposit lies within the thermal aureole of the lower cretaceous magmatic arc plutonic suite in the candelaria punta del cobre district atacama region northern chile 27 30 s economic mineralization is present within a synplutonic ductile shear zone candelaria shear zone containing foliated and potassically altered volcanic and volcaniclastic rocks of the punta del cobre formation in the candelaria shear zone there was a complex interplay in space and time between ductile and brittle deformation during mineralization at high structural levels brittle moderately to steeply dipping extensional faults curve downward into gently dipping ductilely deformed rocks in the shear zone textural and structural characteristics of the ore and the deformed rocks indicate that the dilation of the sulfide bearing vein systems occurred at the same time as ductile deformation and implies that ductile brittle cycling occurred in a cooling hydrothermal system in the thermal aureole of the arc plutonic rocks the la brea and the san gregorio plutonic complexes are exposed in the plutonic arc immediately adjacent to the candelaria orebody the intrusions are interpreted to be flat lying tabular bodies emplaced by roof uplift floor depression mechanisms during regional extensional deformation an 40 ar 39 ar age of 119 6 1 2 ma and five k ar ages between 123 and 117 ma were obtained for the la brea pluton and an 40 ar 39 ar age of 111 5 0 4 ma was obtained for the san gregorio plutonic complex 40 ar 39 ar ages of 111 0 1 4 and 110 7 1 6 ma obtained from syntectonic biotite from the candelaria shear zone are interpreted to be the age of the mineralization these are indistinguishable from the age of the san gregorio plutonic complex and therefore heat from this pluton could have promoted ductile deformation during emplacement of the orebody this result together with known isotopic similarities between candelaria ore and the plutonic rocks implies that the san gregorio plutonic complex was the most probable source of the mineralizing fluids at candelaria although the critical intrusive unit may not be exposed the ore deposit is located where a roof uplift fold in the side wall of the san gregorio plutonic complex provided a dilatational site which may have drawn hydrothermal fluids out of the adjacent fractionating magmas into a structural trap in the punta del cobre formation below the carbonate rocks of the chanarcillo group

on the mechanism of crustal block rotations in the central andes comment in this paper beck 1998 reviews the database of palaeomagnetically observed rotations in the andean margin of south america in particular with respect to the well known change in sense of rotation linked to the arica deflection bolivian orocline in the orogen the paper argues firstly for the latitudinal distribution of rotations supporting an interpretation that distributed shear combined with oroclinal bending is the best hypothesis to explain the observed pattern of rotations and secondly that this rotation deformation is time dependent while we essentially agree with the former conclusion we would define it more tightly in that the over arching control on the pattern of rotations is a combination of differential shortening as opposed to oroclinal bending along the orogen coupled with distributed shear across the orogen our main concern with this paper is that it fails to provide a convincing demonstration that there is a time dependent element to the magnitude of rotations primarily we will argue that it is far from clear from the palaeomagnetic data presented that there is a case for a continuous rate of rotation applicable to the southern limb of the orocline at least not withstanding this we would also argue that geologically the deformation cannot and should not be described by a continuous deformation rate over time as postulated we also make specific points about aspects of the paper dealing with the atacama fault zone and its relation to the observed rotations

sulphur cycling in the drinking water catchment area of torgau mockritz germany insights from hydrochemical and stable isotope investigations the hydrochemical composition of groundwater and the isotopic composition of sulphur compounds in sediments 3 4 s of sulphide inorganic sulphate and groundwater 3 4 s 1 8 o of dissolved sulphate have been investigated to reveal the reasons for elevated concentrations of dissolved groundwater sulphate in the drinking water catchment area of torgau mockritz germany the three most important anthropogenic sources of sulphate identified in this study are historic atmospheric sulphate deposition predominantly of anthropogenic origin inorganic fertilization and dissolution of gypsum from waste dumps owing to their overlapping isotopic range no clear differentiation between the individual anthropogenic sources is possible sulphate from the oxidation of sedimentary sulphides however was recognized by its negative sulphur isotope signature 3 4 s 5 the mobilization of sulphate from different soil sulphur species is closely connected with variable isotopic fractionations resulting in a wide isotopic variation range for the sulphate entering the saturated zone generally sulphur and oxygen isotope ratios of the dissolved groundwater sulphate range from 19 to 37 vienna canon diablo troilite and from 2 to 19 vienna standard mean ocean water respectively sulphate from the majority of groundwater samples can be assigned to anthropogenic sources sulphate from sulphide oxidation is especially present in samples from the upper sampling level and the groundwater surface sulphate concentrations above 700 mg l 1 are mostly caused by the oxidation of sulphide sulphate that occurs in low and moderate concentrations dominantly originates from anthropogenic sources 3 4 s distribution patterns were used to locate the sources for the elevated sulphate concentrations in the raw water it was shown that the sulphate is not mobilized in the immediate vicinity of the production wells rather it originates in an area 1 5 km west of the intake the main mobilization mechanism there is the oxidation of reduced sedimentary sulphur species beyond its local significance this study presents a general example how changes in the sulphur cycle indicate major anthropogenically induced alterations of the hydrochemical situation in catchment areas

fe and mn levels regulated by agricultural activities in alluvial groundwaters underneath a flooded paddy field abstract iron and mn concentrations in fresh groundwaters of alluvial aquifers are generally high in reducing conditions reflecting low so4 concentrations the mass balance and isotopic approaches of this study demonstrate that reduction of so4 supplied from agricultural activities such as fertilization and irrigation is important in lowering fe and mn levels in alluvial groundwaters underneath a paddy field this study was performed to investigate the processes regulating fe and mn levels in groundwaters of a point bar area which has been intensively used for flood cultivation four multilevel groundwater samplers were installed to examine the relationship between geology and the vertical changes in water chemistry the results show that fe and mn levels are regulated by the presence of no3 at shallow depths and by so4 reduction at the greater depths isotopic and mass balance analyses revealed that no3 and so4 in groundwater are mostly supplied from the paddy field suggesting that the fe and mn rich zone of the study area is confined by the agricultural activities for this reason the geologic conditions controlling the infiltration of agrochemicals are also important for the occurrence of fe mn rich groundwaters in the paddy field area

atmospheric sulfur deposition 1950 1985 inferred from sulfate in groundwater groundwater sulfate that occurs at concentrations of 6 27 mg l in a shallow water table sand aquifer at a forested site 100 km east of sudbury ontario is used to deduce the trend in atmospheric sulfur deposition between 1950 and 1985 the simple geologic nature of the aquifer made feasible this trend reconstruction the age of groundwater that entered the aquifer during the past 60 years was determined with unusual accuracy 3 years from tritium analyses and flow modeling sulfur isotopes and mass balances indicate that the sulfate in the groundwater zone is of atmospheric origin and is not derived from solid phase material in the sediments evapotranspiration enriches atmospheric sulfate by a factor of about three as it enters the aquifer adsorption of sulfate is not significant groundwater sulfate indicates a decrease of 37 in sulfur deposition at the site between a maximum in the 1960s and lower values observed in the 1980s we attributed most of the decrease bout 80 to lower so2 emissions from the sudbury smelters these emissions are now about one third of the 1960s levels further major decreases in sulfur deposition at this site can be achieved only if emissions from sources distant from sudbury are lowered as well because sudbury now contributes less than 25 of total sulfur deposition

origin and distribution of sulfate in a fractured till in southern alberta canada glacial till in a 22 km sup 2 area of the interior plains region of southern alberta canada consists of an upper brown gypsiferous weathered zone up to 20 m thick and a lower discontinuous grey nonweathered zone up to 25 m thick although the total sulfur content of the two till zones is the same 0 34 the mean total sulfate content of the weathered till 4 0 mg g sup 1 exceeds that the nonweathered till 0 8 mg g sup 1 similarities suggest that all so sub 4 sup 2 in the nonweathered till and 20 of the sulfate in the weathered till was derived from sulfate rich bedrock fragments incorporated in the till during deposition laboratory experiments and delta sup 18 o and delta sup 34 s analyses of total so sub 4 sup 2 and the sulfur forms in the two zones show that the majority of the sulfate in the weathered till was derived from the oxidation of organic s at some time after the till was deposited mass balance and flux calculations indicate that this oxidation occurred under partially saturated conditions groundwater flow simulations suggest that the water table was much lower and that a thick partially saturated zone was possible during the drier altithermal period 11 000more to 3000 years b p the following chemical processes account for the major ions that are found in the groundwater but were generated when the partially saturated zone existed the oxidation of reduced s produced so sub 4 sup 2 and h sup the h sup reacted with carbonate minerals under elevated p sub co2 with ca sup 2 loss by exchange for na sup and gypsum precipitation volume shrinkage in the weathered till resulting from these geochemical processes may have enhanced the development of fractures and caused the associated increased hydraulic conductivity less

iron oxide copper gold deposits an andean view iron oxide copper gold iocg deposits defined primarily by their elevated magnetite and or hematite contents constitute a broad ill defined clan related to a variety of tectono magmatic settings the youngest and therefore most readily understandable iocg belt is located in the coastal cordillera of northern chile and southern peru where it is part of a volcano plutonic arc of jurassic through early cretaceous age the arc is characterised by voluminous tholeiitic to calc alkaline plutonic complexes of gabbro through granodiorite composition and primitive mantle derived parentage major arc parallel fault systems developed in response to extension and transtension induced by subduction roll back at the retreating convergent margin the arc crust was attenuated and subjected to high heat flow iocg deposits share the arc with massive magnetite deposits the copper deficient end members of the iocg clan as well as with manto type copper and small porphyry copper deposits to create a distinctive metallogenic signature

mineralogy geochemistry and age constraints on the zn pb skarn deposit of maria cristina quebrada galena northern chile the maria cristina zn pb skarn deposit hosted by carbonate rocks of the lower cretaceous chanarcillo group in northern chile is described to provide a basis of comparison to adjacent 2 km barite deposits and regional pb zn and ba deposits with mississippi valley type affinities strongly retrograded garnet andradite to ad30gr70 diopsidic pyroxene and epidote skarn and barite bearing massive sulfides sphalerite pyrite galena marcasite magnetite occur at the contact of potassically altered diorite porphyry of mid cretaceous age the skarn mineral compositions similar to those of cu skarns may reflect emplacement in a high f o2 context sulfur isotope values of sulfides 8 8 to 7 1 and barite 14 0 26 1 indicate disequilibrium conditions and different sulfur sources including magmatic sulfur lead isotope ratios of galena and strontium isotope ratios of barite indicate a similar metal reservoir for the different ore deposit types constituted by the intercalated lower cretaceous volcanic rocks or mid cretaceous intrusive rocks 40ar 39ar dating of hydrothermal k feldspar suggests an age between 94 and 90 ma for the mineralization

granitoid pluton formation by spreading of continental crust the wiley glacier complex northwest palmer land antarctica abstract the emplacement mechanism geometry and isotope geochemistry of plutons of the wiley glacier complex suggest that new continental crust grew by multiple injection of tonalitic dykes during dextral transtension in the antarctic peninsula magmatic arc in early cretaceous times the suggested mechanism is analogous to basalt dyke injection during sea floor spreading during normal dextral shear the burns bluff pluton a sheeted moderately east dipping syn magmatically sheared tonalite granodiorite intruded syn magmatically sheared quartz diorite of the creswick gap pluton and 140 5 ma hornblende gabbro u pb dating of zircon and ar ar dating of hornblende and biotite suggest that both granite s l plutons were emplaced between 145 and 140 ma but that extensional shearing was active from the time of emplacement until ca 127 ma the burns bluff pluton is chilled at its margin and grades through mylonitised porphyritic tonalite granodiorite sheets and tonalite granodiorite sheets with minor chilling to a kilometre scale body of coarse grained hypidiomorphic tonalite granodiorite co magmatic microdiorite forms dykes and abundant synplutonic mafic enclaves these dykes opened as echelon veins during episodic dextral shear and were deformed to trains of enclaves during continued normal dextral shear pluton marginal porphyritic and hypidiomorphic tonalite granodiorite forms large fault hosted sheets emplaced progressively under extension with minor dextral shear kinematic indicators from pluton marginal granite s l dykes suggest that early in pluton accretion intrusive sheets cooled rapidly with simple shear prior to full crystallisation changing to ductile simple shear during cooling kinematic indicators towards the pluton core suggest that as the pluton grew and cooled more slowly emplacement switched from sheeting to in situ inflation with simple shear distributed across a broad zone prior to full crystallisation of magma cross cutting relationships with the coeval syn extensional creswick gap pluton suggest that the burns bluff pluton was emplaced in a steeper second generation shear structure like those in normal fault systems this suggests that the wiley glacier complex was emplaced above the base of the brittle ductile transition zone 15 18 km depth the burns bluff pluton has nd and sr isotope values that range from mantle dominated nd141 3 8 87 sr 86 sr 141 0 70468 to more crustally influenced nd141 1 7 87 sr 86 sr 141 0 70652 this range probably represents different degrees of mixing between mantle derived magma and lower crustal partial melts generated in the garnet stability zone 40 km depth addition of new crustal material by mafic underplating at the base of the crust and by redistribution of granitic s l and mafic modified underplated magma to mid crustal levels along extensional shear zones as the arc spread were the primary mechanisms of crustal growth

modeling the thermal evolution of fault controlled magma emplacement models implications for the solidification of granitoid plutons a two dimensional finite diaerence model is used to simulate the conductive thermal regime attend ing construction and maintenance of a continental magma chamber by intrusion of granite dikes into grano diorite host rocks displaced at various spreading rates final intrusion shapes include tabular square and vertical rectangular bodies emplaced in the shallow crust 5 15 km and tabular bodies emplaced in the middle crust 15 20 km fed by dikes with widths of between 20 and 100 m the formation of a steady state chamber is defined as the point at which the ambient temperatures surpass the intrusion solidus forestalling the solidifi cation of subsequently intruded material for spreading rates 10 mm year 1 construction of a steady state magma chamber in the shallow crust took 260 ka rectangular 360 ka square and 1 ma tabular whereas in the mid crust a steady state was reached in less than 30 ka tabular at faster spreading rates 25 and 50 mm year 1 ambient temperatures pass the solidus isotherm forming a steady state reservoir within 55 ka depending on intrusion depth and size for 10 25 mm year 1 spreading rates sheeted dikes make up from 10 to 100 of the intrusion the thermal modeling supports the following conclusions a episodic magma emplacement into a fault con trolled setting is a thermally viable means of constructing a steady state chamber at moderate to fast spreading rates only if the duration of faulting and intrusion are long enough to elevate ambient temperatures above the intrusion solidus b isotherms will migrate outward during successive intrusion before converging back on the center of the intrusion after chamber construction c the margins of most intrusions formed by this scen ario should contain sheeted dikes d the solidus isotherm and thus the solidification front that it tracks will become progressively curviplanar during the construction of the magma chamber and will not represent the in itial shape of the intrusions i e sheets e the steady state chamber will be smaller than the total intrusion dimensions and f magmatic fabrics will form diachronously and not always parallel to sheet margins as they track the migrating solidification front because it is unlikely that most large intrusions formed instan taneously the eaects of continued addition of heat on the migration of solidification fronts may have signifi cant implications for magmatic processes in many emplacement scenarios 1998 elsevier science ltd all rights reserved

dating degassed groundwater with 3h 3he the production of gases in groundwater under contaminated locations by geochemical and biological processes is not uncommon degassing of these gases from groundwater and repartitioning of noble gases between water and gas phase distorts groundwater dating by 3h 3he we observed noble gas concentrations below atmospheric equilibrium in 20 out of 34 groundwater samples from agriculturally polluted sandy areas in the netherlands from the absence of nitrate in degassed samples we conclude that denitrification causes degassing the 22ne 20ne ratios show that degassing had attained solubility equilibrium and had not caused isotopic fractionation by diffusion to correct for the loss of tritiogenic 3he due to degassing we present a single step equilibrium degassing model we use the total dissolved gas pressure at the monitoring screen to estimate the depth and timing of degassing which is essential to estimate travel times from degassed samples by propagating the uncertainties in the underlying measurements and assumptions through the travel time calculations we found a travel time uncertainty of 3 years a we therefore conclude that 3h 3he dating can produce valuable information on groundwater flow even at sites with strong degassing copyright 2007 by the american geophysical union

denitrification in marine shales in northeastern colorado parts of the south platte river alluvial aquifer in northeastern colorado are underlain by the pierre shale a marine deposit of late cretaceous age that is 1000 m thick ground water in the aquifer is contaminated with no3 and the shale contains abundant potential electron donors for denitrification in the forms of organic carbon and sulfide minerals nested piezometers were sampled pore water was squeezed from cores of shale and an injection test was conducted to determine if denitrification in the shale was a sink for alluvial no3 and to measure denitrification rates in the shale measured values of no3 n2 nh4 15n no3 15n n2 and 15n nh4 in the alluvial and shale pore water indicated that denitrification in the shale was a sink for alluvial no3 chemical gradients reaction rate constants and hydraulic head data indicated that denitrification in the shale was limited by the slow rate of no3 transport possibly by diffusion into the shale the apparent in situ first order rate constant for denitrification in the shale based on diffusion calculations was of the order of 0 04 0 4 yr 1 whereas the potential rate constant in the shale based on injection tests was of the order of 60 yr 1 chemical data and mass balance calculations indicate that organic carbon was the primary electron donor for denitrification in the shale during the injection test and ferrous iron was a minor electron donor in the process flux calculations for the conditions encountered at the site indicate that denitrification in the shale could remove only a small fraction of the annual agricultural no3 input to the alluvial aquifer however the relatively large potential first order rate constant for denitrification in the shale indicated that the percentage of no3 uptake by the shale could be considerably larger in areas where no3 advection

fault zone development and strain partitioning in an extensional strike slip duplex a case study from the mesozoic atacama fault system northern chile abstract upper crustal strike slip duplexes provide an excellent opportunity to address the fundamental question of fault zone development and strain partitioning in an evolving system detailed field mapping of the mesozoic atacama fault system in the coastal cordillera of northern chile documents the progressive development of second and third order faults forming a duplex at a dilational jog between two overstepping master faults the sinistral strike slip nnw striking jorgillo and bolfin faults these are constituted by a meter wide core of foliated s c ultracataclasite and cataclasite flanked by a damage zone of protocataclasite splay faults and veins lateral separation of markers along master faults is on the order of a few kilometers second order nw striking oblique slip subsidiary fault zones do not show foliated ultracataclasite lateral sinistral separations are in the range of 10 to 200 m with a relatively minor normal dip slip component in turn third order east west striking normal faults exhibit centimetric displacement oblique slip sinistral normal fault zones located at the southern termination of the bolfin fault form a well developed imbricate fan structure they exhibit a relatively simple architecture of extensional and extensional shear fractures bound by low displacement shear fractures kinematic analysis of fault slip data from mesoscopic faults within the duplex area document that the nw striking and the ew striking faults accommodate transtension and extension respectively examination of master and subsidiary faults of the duplex indicates a strong correlation between total displacement and internal fault structure faults started from arrays of en echelon extensional extensional shear fractures that then coalesced into throughgoing strike slip faults further displacement leads to the formation of discrete bands of cataclasite and ultracataclasite that take up a significant part of the total displacement we interpret that the duplex formed by progressive linkage of horsetail like structures at the southern tip of the bolfin fault that joined splay faults coming from the jorgillo and coloso faults the geometry and kinematics of faults is compared with that observed in analog models to gain an insight into the kinematic processes leading to complex strike slip fault zones in the upper crust

magmatic belts in accretionary margins a key for tectonic evolution the tonalite belt of north victoria land east antarctica occurring in the early palaeozoic palaeomargin of the east antarctica craton the tonalite belt of north victoria land is a narrow linear magmatic belt up to 200 km long made up of strongly foliated tonalitic and granodioritic intrusions they were synkinematically emplaced into the lanterman murchison shear zone a major tectonic structure across which different tectono metamorphic terranes were accreted during the late cambrian ross orogeny the deformational and kinematic features of the intrusions indicate that strain and displacement during terrane accretion were partitioned into oblique thrust high angle thrust and strike slip shear zones from this it is deduced that a transpressional regime prevailed during early palaeozoic deformation and terrane accretion this and other magmatic belts which exploited major tectonic structures in collisional margins such as the coast plutonic complex of nw america are basic keys for understanding deformational processes and kinematic regimes during terrane accretion moreover the occurrence of such types of magmatic belts might be regarded as a reliable indication of large scale strike slip motion between accreting terranes in an oblique convergent setting

mesozoic fault systems deformation and fault block rotation in the andean forearc a crustal scale strike slip duplex in the coastal cordillera of northern chile abstract in this paper we discuss the evolution and tectonic significance of the mesozoic trench parallel fault systems which affected the coastal cordillera and their relation to magmatism and crustal rotation the oldest extensional fault system separates basement from rift related late triassic and younger sedimentary units this system i subsequently developed into a wider extensional fault system which acted as the locus of magma ascent and emplacement of the coastal batholith during much of the jurassic to earliest cretaceous period this extensional fault system defined the forearc sliver during this period and was the consequence of a retreating subduction boundary during the early cretaceous c 132 125 ma the kinematics of this fault system changed to transtension ii and accommodated a major component of left lateral strike slip motion the principal fault being the atacama fault zone along which plutons continued to be emplaced the final phase of pluton emplacement within the coastal cordillera appears to be c 106 ma after which this magmatic arc and fault system was abandoned an late cretaceous arc and fault system iii developed some 20 ma later and located some 50 km to the east in what is now the central valley of northern chile this paper seeks to show that the coastal cordillera was deformed as a whole by this late cretaceous fault system iii which formed a crustal scale left lateral transpressional duplex during this deformation the thermally weakened crust was dissected into a series of large scale blocks bounded by nw trending left lateral strike slip faults which merge into a nne ssw fault zone which forms the eastern boundary to the duplex we term this eastern boundary zone the central valley fault zone cvfz and this together with the nw trending faults defines the duplex system which we refer to as a whole as the coastal cordillera fault system ccfs iii we have traced the ccfs duplex between 25 s and 29 s and suspect that it continues northward the timing of the deformation is constrained to be post 106 ma the age of coastal cordillera arc abandonment and pre tertiary based on the deformation and pluton emplacement in and along the central valley fault zone palaeomagnetic data from the fault bound blocks within the ccfs duplex indicate 35 45 of post early cretaceous clockwise rotation with no substantial latitudinal motion we suggest that the observed fault kinematics of the ccfs are consistent with this crustal scale duplex model where rotations would have occurred in response to left lateral transpression

mesoscale pervasive felsic magma migration alternatives to dyking abstract this paper reviews the literature on dyking as a mechanism of felsic magma extraction from a source and transport to shallower crustal levels and review the recent literature suggesting a range of alternative mechanisms of magma migration in hot crustal zones which produce mesoscale pervasive granite sheet intrusions recent papers have strongly favoured dyking as the main mechanism controlling magma migration however the initiation of dykes from a felsic magma source is fraught with difficulties even when magma is immediately available for transportation as in magma chambers within a partially molten source magma may reside in a range of structures with a wide range of shapes sizes and degrees of connectivity whereas the growth of individual dykes within a partially molten zone and the self propagation of large dykes into subsolidus crust have both been studied in some detail little attention has been given to the crucial intermediate step of the growth of a dyke network capable of producing wide crustal scale dykes the rarity of granite dyke swarms suggests that if dyking is the preferred mechanism of magma transport felsic magma sources produce only few major transporting dykes during their lifetime alternatively dyking is not an important mechanism the parameters controlling the volume of the catchment drained by one such dyke as well as other basic geometrical parameters controlling the structure of the dyke network within the source are unknown the ability of dyking to drain a partially molten source depends crucially on these variables and particularly on the horizontal permeability of the source the slow velocity of viscous felsic magmas traveling in rock pores implies that magma drained during dyking is mostly that previously extracted from the pores and resident in irregular magma bodies or dyke networks the observation that large volumes of buoyant magma are commonly present in migmatite zones and that dyking in these zones plays a secondary role suggest that dyking is inefficient and is able to extract only a fraction of the total melt available in the source in support of this conclusion recent detailed studies of exhumed hot crustal zones have revealed a range of alternative migration mechanisms characterized by mesoscale pervasive magma flow outcrop scale as opposed to porous flow pervasive migration gives rise to magma sheets preferentially emplaced parallel to high permeability zones such as foliation or bedding planes apart from local dyking three alternative mechanisms have been proposed to account for pervasive migration of magmas namely tectonic pumping magma wedging into low viscosity rocks and volatile driven intrusion because of their unfocused pervasive character these mechanisms are restricted to hot crust where magmas are not exposed to rapid freezing a model is proposed whereby heat advected with the magma pushes crustal geotherms upwards allowing pervasive magma migration to shallower depths

lithological and structural controls on the genesis of the candelaria punta del cobre iron oxide copper gold district northern chile abstract the candelaria punta del cobre district is the largest iron oxide cu and au iocg district in the chilean iocg belt with more than 13 mt of contained copper candelaria is the most important deposit in the district which includes seven other smaller producers carola punta del cobre mantos de cobre candelaria norte granate alcaparrosa atacama kozan las pintadas the district which is characterized by an early cretaceous volcanic sedimentary arc sequence 135 132 ma punta del cobre formation overlain by the marine sedimentary chanarcillo group formed in an extensional back arc basin 132 130 ma the copiapo batholith which occupies the western side of the district was emplaced between 118 and 110 ma during the change from extensional to transpressional tectonics mineralization is hosted predominately in the upper part of the lower andesite member and the overlying volcano sedimentary and dacite members all within the punta del cobre formation mineralization is hosted by fault zones breccias and specific lithologies north northwest faults are the dominant host for vertically extensive orebodies stratigraphically controlled mineralization forms extensive stratabound ore bodies mantos textural evidence suggest that the hydrothermal system evolved and advanced upwards over time the earliest event was dominated by magnetite actinolite in stratigraphically controlled mantos and extensive zones of disseminated magnetite actinolite below candelaria which were subsequently overprinted by chalcopyrite dominant mineralization with magnetite actinolite biotite k feldspar alteration in addition to magnetite iron oxides include widespread mushketovite and hematite in the upper part of some deposits geochronological data suggest that the main phase of mineralization occurred between 122 ma and 115 ma u pb in zircon overlapping in age with the two major early phases of the copiapo batholith there is no field evidence to indicate that exposed phases of the batholith were the source of mineralizing fluids alteration and mineralization in the earliest phase of the batholith la brea occurs in structures and is minor compared to mineralization in the punta del cobre formation later phases of the batholith cut the main stage of mineralization the extent of iocg mineralization in the candelaria punta del cobre district reflects a combination of factors including changing regional kinematics and related fault architecture multiple intrusive events fluid access to permeable and reactive lithologies vertical changes in redox conditions and the potential role of multiple fluids

characterisation of the input signal to aquifers in the french basque country emphasis on parameters influencing the chemical and isotopic composition of recharge waters summary a hydrochemical and isotopic survey of rainwater and groundwater has been carried out during almost two years on the ursuya mount in the northern basque country southwestern france with the aim of enhancing the understanding of the behaviour of this aquifer and more peculiarly the recharge mode of groundwater the input signal of this aquifer is defined thanks to 112 rainwater samples the computed meteoric water line d 7 3 18 o 5 1 r 0 96 is consistent with that defined in the european iaea wmo network stations the weighted mean deuterium excess about 9 7 is really close to the value obtained for atlantic precipitations and clearly demonstrates an oceanic major origin the computations conducted on the chemical dataset show that the rainwater composition is controlled by four major factors 1 a mixed source of anthropogenic pollution and crustal material 2 a marine source 3 an urban source 4 an acid source further the quantification of neutralizing potentials clearly revealed below cloud processes in which crustal and anthropogenic components are responsible for the neutralization of anions besides the major atlantic origin of the recharge water the chemical and isotopic content of the samples coupled with the corresponding air mass back trajectories revealed four major geographical origins of the components 1 northwestern part of atlantic ocean and 2 southwestern part of atlantic ocean the oceanic influence in airmasses from these origins is highlighted by the stable isotopic content weighted mean close to the atlantic ocean signature and by the chemical concentrations dominated by sea salt elements 3 northern europe with a continental influence shown by a light depletion on isotopic signal besides purely oceanic origin and a higher concentration of crustal and anthropogenic components 4 southeastern area southeastern europe northern africa and mediterranean sea with an isotopic signature consistent with the mediterranean one and a chemical composition clearly influenced by the industrial activities of this region the dataset is confronted with the chemical and isotopic characteristics of groundwater in order to assess the recharge water signature of the aquifer of the ursuya mount the groundwater isotopic signal is close to the defined lmwl showing actual and local recharge nevertheless a slightly difference is observed between the mean value of stable isotopic content in groundwater and in rainwater the slightly depleted values of groundwater besides rainwater can be attributed to a preferential winter autumn recharge an altitudinal gradient about 0 11 18 o 100 m is defined and both water rock interactions and physicochemical processes fingerprint on the groundwater composition are characterised

aquitard characteristics of clay rich till deposits in east anglia eastern england summary an extensive area of quaternary glacial deposits lowestoft till of elsterian anglian age overlies the regionally important cretaceous chalk aquifer in east anglia in eastern england the glacial deposits act as an aquitard potentially affording protection from surface derived mainly agricultural contaminants however there has been little previous research on the physical and chemical characteristics of the glacial tills and contained pore waters in east anglia to demonstrate this benefit hence this study presents the results from the drilling of two boreholes in northern and southern east anglia at morley and cowlinge respectively and the construction of a high pressure squeezing rig to obtain pore water for major and minor ion stable isotope 18 o 2 h and dissolved organic carbon analysis special features of the mechanical squeezing rig included a high diameter to length ratio of the squeezer dual seepage faces and a unique pore water collection system designed to eliminate the risk of alteration of in situ pore water redox characteristics the hydrochemistry of the pore waters is found to be controlled by i incongruent carbonate dissolution given the high proportion of chalk clasts contained in the till ii cation exchange in the unweathered clay rich till and iii pyrite oxidation associated with the mineralogy of both the chalk and clay material content that comprises the till matrix the clay material is sourced from upper jurassic clays oxford and kimmeridge clay formations found to the west of the region these clays are also considered to be the source of organic material contributing relatively high concentrations of dissolved organic carbon above 2 mg l found in till pore waters below the soil zone concentrations of dissolved inorganic constituents increase with depth with a high total dissolved solids tds content in excess of 1500 mg l measured in the unweathered till below a depth of 5 m below ground level with the highest concentrations maximum of 3738 mg l associated with low permeability clay rich till the stable isotope composition of the pore waters with mean values for 18 o and 2 h of 7 01 and 51 7 at cowlinge and 6 44 and 49 9 at morley respectively are similar to local meteoric water and indicate that groundwater recharge of the tills has occurred during the holocene in the last 10 000 years overall the physical and chemical characteristics of the lowestoft till suggest only limited groundwater recharge of the order of 10 mm a or less in interfluve areas where the till deposits are greater than 15 m thick although higher rates are expected where the till becomes more sand rich for example at valley margins and also northwards in east anglia where the lowestoft till is influenced by the component of anglian ice that advanced from the north containing sand material derived from the floor of the north sea basin

distribution source and evolution of nitrate in a glacial till of southern alberta canada this paper addresses the distribution origin and controls upon nitrate in a 30 km2 area of the interior great plains region of southern alberta canada high concentrations of nitrate 100 mg l 1 no 3 n occurred in several isolated enclaves below the water table in brown weathered till nitrate concentrations of over 300 mg l 1 n were encountered in groundwater samples collected from these enclaves low nitrate concentrations 1 1 mg l 1 no 3 n were also encountered in the weathered till upgradient and downgradient of the nitrate enclaves groundwater samples collected from the underlying grey nonweathered till and bedrock had no 3 n concentrations of 1 1 mg l 1 through the application of geochemical no 3 n and nh 4 n studies environmental isotope studies tritium microbial analyses nitrifiers and laboratory experiments it was shown that the high nitrates found in the weathered till are the result of the oxidation of ammonium present within the tills it is postulated that this oxidation occurred during the holocene epoch when water tables were much lower than present day levels 5 18 m and 2 m below ground respectively through the use of eh measurements the enumeration of denitrifying bacteria and laboratory experiments the potential for denitrification was shown to exist below the present day water table in the weathered till as well as in the nonweathered till and bedrock isotopic data showed that less denitrification may be occurring within the nitrate enclaves than in adjacent downgradient areas

reactivity of organic matter in aquifer sediments geological and geochemical controls reduction rates in aquifers are commonly carbon limited but little is known about the molecular composition and degradability of sedimentary organic matter som in aquifer sediments the composition source and degradation status of som in aquifer sediments of fluvio glacial pleistocene and shallow marine pliocene origin were determined using flash pyrolysis gas chromatography mass spectrometry incubation experiments 106 d were used to assess the reactivity of som towards molecular oxygen a dominance of lignin derived components and long chain odd over even predominant alkanes indicate that terrestrial higher land plants were the main source of som even in the shallow marine sediments while bacterial lipid derived hopanoids and iso and anteiso c15 and c17 fatty acids indicate a minor contribution of microbial biomass no compositional difference was observed between som present in the fine 63 m and coarse fraction 63 2000 m a significant part of som was not present as low molecular weight compounds but was macromolecularly bound for the fluvio glacial sediments a relatively higher abundance of resistant macro molecular compounds was in agreement with stronger signs of aerobic lignin alkane and hopanoid oxidation the more degraded status of som in the fluvio glacial sediments was consistent with their significantly lower som mineralization 2 6 during incubation as compared with the shallow marine sediments 9 14 the reactivity towards oxygen of som was controlled by the extent of past aerobic oxidation not the age of som but the extent of oxygen exposure during syn and postdepositional conditions seems most important in affecting the degradation status of som in aquifer sediments and thus their ability to reduce oxidants copyright 2004 elsevier ltd

evaluation of multiple emplacement mechanisms the huichizi granite pluton qinling orogenic belt central china three end member granite emplacement mechanisms host rock ductile shortening hrds host rock rigid displacement hrrd and magmatic absorption ma are identified in this study of the huichizi pluton the internal fabrics are mainly flattening typesona 0 4 0 9u and strain is highoesa 1 0 0 6u near the boundaries and lowoesa 0 7 0 30u toward the central parts of the pluton in the 1 5 km wide contact aureole extension e is measured in syn emplacement deformed veins and the calculated average shortening normal to the sides and tips of the pluton are 1 14 km and 0 74 km respectively from this we conclude that the hrds produced only about 20 space for the present volume of the pluton the regional hrrd derived from the uplift by about 8 km of the qinling group in a flower structure contributed 19 032 space ms including stoping and material transfer processes yielded another 37 space of this stoping contributed a minimum of 1 space estimated from the surface area of 2 8 km 2 covered by xenoliths transfer processes provided about 36 of the needed space deduced from the average 64 36 ratio of hybridization of mantle derived to crustal qinling group derived magma the three mechanisms may act nearly simultaneously as a multiple emplacement mechanism this situation may apply to many other plutons as well 2000 elsevier science ltd all rights reserved

palaeomagnetism applied to magnetic anomaly interpretation a new twist to the search for mineralisation in northern chile the chilean iron belt is an important location for fe and cu fe au deposits and includes the recently developed candelaria deposit which is located some 20 km south of the city of copiapo in northern chile this mine is now a major cu producer in chile but its discovery in the late 1980s was relatively fortuitous the exploration programme included a ground magnetic survey from which lessons can be learnt in the search for further such deposits palaeomagnetic studies throughout the northern part of the chilean iron belt indicate major crustal rotation of the region probably related to oblique convergence and transpression at the andean margin the application of palaeomagnetic techniques to a magnetite apatite deposit mina fresia indicates that magnetite rich ores in this area are capable of maintaining a significant remanence component which will contribute to their magnetic anomaly as with the volcanics intrusives and sediments of the region this remanence is clearly rotated clockwise using 2 5d and 3d magnetic modelling it is demonstrated that the magnetic anomaly associated with the candelaria deposit is also dominated by a remanence component which is significantly rotated but of reversed polarity recognition of clockwise rotated remanence dominated anomalies should provide a new key to the search for deposits in this part of chile and elsewhere an example of an unexplored anomaly showing this key feature of a clockwise rotated remanence component dominating over the induced component is presented modelling of this anomaly indicates a pair of sources which are either ten times as big or ten times as magnetic as the candelaria deposit it is suggested that the low cost of palaeomagnetic study of an exploration target should be a prerequisite to magnetic anomaly interpretation of targets in tectonic areas where vertical axis crustal rotations may be a significant element of the overall deformation pattern

tectonics of the jurassic early cretaceous magmatic arc of the north chilean coastal cordillera 22 26 s a story of crustal deformation along a convergent plate boundary the tectonic evolution of a continental magmatic arc that was active in the north chilean coastal cordillera in jurassic early cretaceous times is described in order to show the relationship between arc deformation and plate convergence during stage i circa 195 155 ma a variety of structures formed at deep to shallow crustal levels indicating sinistral arc parallel strike slip movements from deep crustal levels a sequence of structures is described starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions bolfin complex the high grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike slip and thrust kinematics with a nw se directed maximum horizontal shortening i e parallel to the probable late jurassic vector of plate convergence a kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds thrusts and high angle normal faults which formed during the same time interval as the discrete shear zones during stage ii 160 150 ma strong arc normal extension is revealed by brittle low angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels during stage iii 155 147 ma two reversals in the stress regime took place indicated by two generations of dikes an older one trending ne sw and a younger one trending nw se sinistral strike slip movements also prevailed during stage iv until 125 ma when the atacama fault zone originated as a sinistral trench linked strike slip fault the tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates the structures of stages i and iv suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations however decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages ii and iii

on the mechanism of crustal block rotations in the central andes abstract the results of 62 paleomagnetic studies of jurassic and younger rock units from the andes of argentina bolivia chile and peru are examined for indications of the timing and mechanism of crustal block rotations rotations and their 95 confidence intervals are recalculated in pole space using a new south american reference apw path as observed by many investigators the arica deflection the change in topographic and tectonic trends located near arica northernmost chile divides the data into a counterclockwise rotated set north of the deflection and a clockwise rotated set to the south rotations range from 75 counterclockwise to nearly 50 clockwise and show substantial differences between neighboring crustal blocks south of arica there is a weak indication of a decrease in amount of rotation with distance from the plate margin but no such correlation is observed north of arica rotation increases with age both north and south of arica for rock units of 90 ma for older rock units no correlation between age and amount of rotation exists unlike the western margin of north america no significant margin parallel displacements accompany the rotations several models for rotation are examined it is concluded that a combination of late cenozoic oroclinal bending and post mid cretaceous local block rotations in response to shear driven sub crustal ductile flow can explain most of the paleomagnetic and geologic observations

a geomorphological approach to determining the neogene to recent tectonic deformation in the coastal cordillera of northern chile atacama the large 10000 km2 and local scale 300 m sedimentary succession was deposited east of the afs the succession fills previously deep paleovalleys and it consists of gravel so called atacama gravels which passes laterally into fine grained playa related deposits near the afs we interpret the deposition of this succession as a result of a blocking closure of the valley flowing from the precordillera due to the activity on afs a pedimentation episode followed sediment deposition and is locally strongly re incised by the main modern day river valleys draining the precordillera incision may result from either regional uplift of the forearc and or from more localized activity on the afs furthermore recent quaternary tectonic activity on the afs has been observed which is consistent with a localized relative uplift of the crustal block west of the afs

the jurassic succession in west central argentina stratal patterns sequences and paleogeographic evolution abstract jurassic rocks in west central argentina are predominantly marine and marginal marine siliciclastics associated with prominent but volumetrically subordinate carbonates and evaporites facies developments were ruled by paleogeographic persistence within the southern infratropical latitude belt within 40 50 s and by siliciclastic derivation from the patagonian hinterland to the southwest minor volcanic and volcaniclastic supply arrived from the west out of a magmatic belt related to circum pacific convergence timing of the main marine flooding events and general correspondence with the high rank stratal packaging recorded in western north america suggest that global eustasy was also a factor in controlling the local stratigraphic record early jurassic sedimentation occurred within a series of semi isolated depocentres linked to fault bounded triassic troughs the sinemurian toarcian deposits record depocentre expansion and coalescence these trends were coeval with progressively more widespread marine invasions from the northwest and west leading to an elongate marine seaway which connected central patagonia with the pacific domain during the aalenian bajocian the region was subject to a more subdued tectonism and the foreland side of the basin became fringed by an extensive clastic embankment bathonian and early callovian were times when coarse clastics prograded into the basin while the marine embayment shrank as a result of stepwise forced regressions during late callovian to oxfordian globally rising sea level the depocentre witnessed the appearance of cosmopolitan invertebrates and a stratal pattern of basin widening and depositional underbalance that promoted cratonward onlap and inception of widespread carbonate deposition ooidal coralline carbonate development was terminated after a relatively sudden messinian style event that desiccated a large tract of the andean basin and favored massive precipitation of anhydrite in the course of the kimmeridgian the evaporite basin was largely flooded by siliciclastics and turned into a broad and featureless mudflat salina complex linked to a widespread erg and to an ephemeral drainage system by the tithonian at a period of peak oceanic stand marine connection was reinstated and recorded as the most widespread jurassic transgression across the neuquen aconcagua embayment shelfal deposition consisted of molluscs and ooid dominated carbonate terraces that grew in pulses tuned to eustatic fluctuations like in other prolific petroleum provinces around the world the tithonian basinal strata involve widespread euxinic deposits featuring unusually high organic content

timing kinematics and displacement of the taltal fault system northern chile implications for the cretaceous tectonic evolution of the andean margin

biogeochemistry and natural attenuation of nitrate in groundwater at an explosives test facility an interdisciplinary study was conducted to characterize the distribution and fate of no3 in groundwater at lawrence livermore national laboratory llnl site 300 a high explosives test facility in the semi arid altamont hills of california site 300 groundwater contains no3 concentrations ranging from 200 mg no3 l several lines of evidence strongly suggest that denitrification is naturally attenuating no3 in the confined o2 depleted region of the bedrock aquifer under study tnbs2 a both no3 and dissolved o2 do concentrations in groundwater decrease dramatically as groundwater flows from unconfined to confined aquifer conditions b stable isotope signatures i e 15n and 18o of groundwater no3 indicate a trend of isotopic enrichment that is characteristic of denitrification and c dissolved n2 gas the product of denitrification was highly elevated in no3 depleted groundwater in the confined region of the tnbs2 aquifer long term no3 concentrations were relatively high and constant in recharge area monitoring wells typically 70 100 mg no3 l and relatively low and constant in the downgradient confined region typically 0 1 3 mg no3 l suggesting a balance between rates of no3 loading and removal by denitrification chemolithoautotrophic denitrification with pyrite as the electron donor is plausible in the tnbs2 aquifer based on the low dissolved organic c concentrations 1 5 mg l that could not support heterotrophic denitrification the common occurrence of disseminated pyrite in the aquifer and the trend of increasing so2 4 as groundwater flows from aerobic unconfined to anoxic confined aquifer conditions nitrate sources were investigated by experimentally determining the 15n and 18o signatures of no3 from three potential anthropogenic sources of no3 at site 300 ba no3 2 mock explosive hno3 and photolysis of the explosive rdx hexahydro 1 3 5 trinitro 1 3 5 triazine the isotopic signatures of these potential no3 sources were markedly different than those of no3 in tnbs2 groundwater samples suggesting that other sources must contribute significantly to the no3 loading at site 300 in particular no3 and no2 resulting from rdx photolysis reflected dramatically depleted 15n ca 7 4 and 18o ca 25 7 values

denitrification in a major limestone aquifer abstract the chalk aquifer of eastern central england is seriously affected by nitrates leached from agricultural land and like many similar aquifers its future as a potable resource must depend considerably on the nature and rates of denitrification the regional distribution of nitrate supports a viewpoint based on thermodynamic criteria that denitrification is actively occurring and that the problem will be short lived more detailed considerations based on major ion and environmental isotope data however indicate that denitrification is not significant and that the apparent lowering of nitrate concentration from 10 to 1 no 3 n in the apparent direction of flow is due primarily to mixing between waters of different origins presence of reduced nitrogen species is possible evidence of denitrification in some older waters 4000 yr b p it is concluded that denitrification can not be relied upon to reduce elevated concentrations of nitrate in modern recharge waters

delineation of spatial redox zones using discriminant analysis and geochemical modelling in arsenic affected alluvial aquifers this study characterized the redox conditions in arsenic affected groundwater aquifers of the lanyang plain taiwan discriminant analysis was adopted to delineate three redox zones oxidative transitional and reductive zones in different aquifers and yielded 92 3 correctness on groundwater quality data arsenic is mainly distributed in the reductive zone and arsenic distribution in the shallow aquifer is mainly affected by surface activities according to phreeqc modelling results possible mechanisms for arsenic release to groundwater in lanyang plain are explored arsenic released to groundwater in the oxidative zone zone 1 is primarily caused by the oxidations of arsenic bearing pyrite minerals and arsenate is the predominant species while the reductive dissolution of fe oxides are responsible for the high arsenic concentration found in the transitional and reductive zones zones 2 and 3 arsenite is the predominant species the reduction potential of groundwater rises as the depths and zones increase some sulphates may be reduced to form sulphide ions which then react with arsenic to form arseno sulphide deposits such as realgar orpiment and then slightly lower groundwater arsenic concentrations a conceptual diagram which summarized the possible release processes of arsenic in different redox zones along groundwater flow in lanyang plain is postulated arsenic bearing pyrite and arsenopyrite feass are oxidized as they are exposed to the infiltrated oxygenated rainwater releasing soluble arsenate fe ii and so42 into zone 1 the dissolution of arsenic rich fe oxides due to the onset of reducing conditions in zones 2 and 3 is responsible for the mobility of arsenic and likely to be the primary mechanism of arsenic release to groundwater in the lanyang plain copyright 2007 john wiley sons ltd

groundwater nitrate pollution in the recharge zone of a regional quaternary flow system wielkopolska region poland the present study examines the behaviour of nitrate in the recharge zone of a regional quaternary flow system the presence of contaminated groundwater with high nitrate content in the shallow part of the flow system was documented tritium analyses confirmed that these contaminants can migrate downward a high downward gradient exists in the study area extending into the regions of groundwater extraction in the unconfined part of the flow system which is the most vulnerable to pollution a high concentration of nitrate was found to occur at great depth however denitrification processes limit nitrate migration as a result in the deeper parts of the flow system in regions under confined conditions an absence of nitrate was observed and a higher sulphate concentration and total hardness were evident the denitrification was also confirmed by the existence of a high gaseous n2 concentration it was documented that denitrification occurs in both the confined and unconfined parts of the flow system but the potential for denitrification is higher in the confined parts leading to the disappearance of nitrate in deep aquifers autotrophic denitrification supported by sulphide compounds was indicated as the dominant denitrification process

magnetic fabrics and compositional evidence for the construction of the caleu pluton by multiple injections coastal range of central chile the caleu pluton which is located in the coastal range of central chile consists of three n s elongated lithologic zones which define an across pluton compositional variation characterized by a westward increase in sio2 content the gabbro diorite gdz tonalite tz and granodiorite gz zones a total of 547 cores were drilled at 68 sites within the different zones to carry out magnetic fabric and paleomagnetic studies the highest values of magnetic susceptibility and remanent magnetization are found in the gdz 0 04 0 1 si whereas the lowest values are observed in the gz 0 01 0 02 si this is consistent with the significant decrease in magnetite content from the gdz to the gz the dips of the magnetic foliation planes are steep within gdz and tz and shallower dips are observed in gz magnetic foliations along some borders of the pluton are parallel to the corresponding contacts the parallelism observed between the magnetic foliations and the contacts between zones suggests that these zones have recorded the same mechanism of emplacement at the magmatic state the peculiar foliation pattern observed in the central parts of tz and gz is attributed to a lobe like structure that was acquired during the late stages of emplacement and inflation the distinct compositional characteristics of these three zones were not acquired in situ rather they were emplaced successively from mafic to felsic from a subjacent stratified sill like magma reservoir which was filled after tilting of the host volcanic succession the rapid upward transport of these melts of different compositions up to a shallow depth is attested by both the equilibration pressure of late magmatic hornblende 200 mpa and the pressure of early magmatic epidote crystallization 500 600 mpa we conclude that space was created by lateral flow propagation in which the intrusion of later pulses pushed earlier ones away from the feeder system followed by roof lifting and possibly floor depression

stress fields recorded on large scale strike slip fault systems effects on the tectonic evolution of crustal slivers during oblique subduction abstract in continental margins large scale strike slip fault systems resulted from oblique subduction commonly exhibit a complex pattern of faulting where major faults define the inland boundary of tectonic slivers that can be detached from the margin in turn subsidiary faults bound and define internal tectonic blocks within the sliver which are expected to rotate translate and or internally disrupt in order to accommodate the internal deformation the geometrical and spatial arrangement of faults and tectonic blocks thus determines the evolution of the sliver given a particular stress field regime the paposo segment of the atacama fault system in northern chile displays a series of brittle faults whose orientations are hierarchically arranged low order faults splay off higher order faults forming riedel type and strike slip duplexes geometries at several scales the master 1st and highest order paposo fault defines the inland boundary of a tectonic sliver whereas subsidiary faults bound and disrupt internal tectonic blocks by using newly collected brittle fault slip data we estimated the orientations and regimes of the stress fields that acted upon the entire sliver the different fault orders and the tectonic blocks results indicate that an overall transtensional with nw compressional and ne tensional principal axes strike slip regime affected the sliver and triggered the development of left lateral strike slip structures an incomplete split of the stress field imposed by the subduction process resulted in the generation of a nested pattern of r type faults as well as in a combined strike slip normal faulting disruption of the tectonic blocks within the sliver

batholith emplacement at mid crustal levels and its exhumation within an obliquely convergent margin abstract emplacement of the central part of the coast mountains batholith of northern coastal british columbia occurred within a regime characterized by oblique convergence between the farallon kula and north american plates we use new structural kinematic and u pb isotopic data to show that the locations geometry and mechanisms of pluton emplacement within this batholith were controlled by displacements within a network of normal faults and transtensional shear zones these data also show that the most active period of pluton emplacement from 67 to 51 ma coincided with a change in style of deformation within the batholith prior to 67 ma plutons were emplaced within an arc dominated by regional scale contractional shear zones in contrast emplacement of 67 51 ma plutons occurred in an arc increasingly dominated by normal faults with arc parallel to oblique displacement and by sinistral transtensional shear zones we have identified and mapped the structure of three plutonic complexes composed of 67 to 51 ma plutons the khyex sill complex arden lake plutonic complex and quottoon plutonic complex shear zone controlled emplacement of plutons within the batholith accounts for the widely different orientations and structural features that characterize plutons within these three complexes during and after this latest cretaceous paleogene period of intense plutonic activity and accompanying deformation the deep roots of the batholith were rapidly unroofed by ductile normal faulting prior to 50 ma

tectonomagmatism in continental arcs evidence from the sark arc complex abstract the island of sark channel islands uk exposes syntectonic plutons and country rock gneisses within a precambrian cadomian continental arc this sark arc complex records sequential pulses of magmatism over a period of 7 ma ca 616 609 ma the earliest intrusion ca 616 ma was a composite sill that shows an ultramafic base overlain by a magma mingled net vein complex subsequently deformed at near solidus temperatures into the amphibolitic and tonalitic tintageu banded gneisses the deformation was synchronous with d 2 deformation of the paragneissic envelope with both intrusion and country rock showing flat top to the south ls fabrics later plutonism injected three homogeneous quartz diorite granodiorite sheets the creux moulin pluton 150 250 m ca 614 ma the little sark pluton 700 m 611 ma and the northern pluton 500 m 609 ma similar but thinner sheets in the south derrible hogsback dixcart and west port es saies brecqhou are interpreted as offshoots from the creux moulin pluton and little sark pluton respectively all these plutons show the same ls fabric seen in the older gneisses with rare magmatic fabrics and common solid state fabrics recording syntectonic crystallisation and cooling the cooling rate increased rapidly with decreasing crystallisation age 9 ma for the oldest intrusion to cool to lower amphibolite conditions 7 8 ma for the creux moulin pluton 5 6 ma for the little sark pluton and 2 the initiation of the d 2 event is suggested to have been a response to the intrusion of the tintageu magma which promoted a rapid increase in strain rate 10 14 s 1 that focussed extensional deformation into the sark area the increased rates of extension allowed ingress of the subsequent quartz diorite granodiorite sheets although strain rate slowly declined as the whole complex cooled during exhumation the regional architecture of syntectonic cadomian arc complexes includes flat lying sark type and steep guernsey type domains produced synchronously in shear zone networks induced by oblique subduction a pattern seen in other continental arcs such as that running from alaska to california

petrogenesis of migmatites in maine usa possible source of peraluminous leucogranite in plutons high t low p facies series conditions during deformation within a devonian crustal scale shear zone system defined by kilometer scale straight belts of apparent flattening strain that anastomose around lozenges of apparent constrictional strain at upper amphibolite facies grade metapelites are partially melted the onset of which is introduction