evolution of the south central segment of the archean abitibi belt quebec part iii plutonic and metamorphic evolution and geotectonic model textural criteria permit distinction between the pre kenoran and kenoran phases of plutonism and metamorphism the pre kenoran plutons and pre kenoran metamorphic phases are directly related to the volcanic evolution synvolcanic tonalite trondhjemite plutons and swarms of mafic and felsic dykes core central volcanic complexes the volcanic rocks underwent three types of pre kenoran metamorphism namely a pervasive alteration a thermal contact metamorphism that affected narrow aureoles around synvolcanic plutons and a high intensity hydrothermal alteration that affected cross cutting pipes in central volcanic complexes synkinematic kenoran metamorphism resulted in the growth of minerals chlorite actinolite etc parallel to schistosities synkinematic metamorphic grade ranges form the pumpellyite prehnite facies to the amphibolite facies late to post kinematic metamorphic phases resulted in the growth of minerals across schistosities syn to post kinematic plutons are not voluminous in the part o

seismic reflection study of the precambrian crust of central minnesota a cocorp consortium for continental reflection profiling seismic reflection survey across the boundary zone between the late archean superior province granite greenstone terrane and older archean gneiss and granulite terrane of the minnesota river valley shows numerous north to northwest dipping reflection events throughout the crust a particularly prominent and continuous series of such events projects to the surface near the trace of a fault zone that had been previously identified as the boundary between these two terranes within the great lakes tectonic zone the moderate dips of these events about 30 their correlation with presumed faults near the surface their continuity over tens of kilometres and the persistence of these and other reflection zones throughout the crust suggest that these events correspond to fault zones probably thrusts they may have originated during late archean collision between the superior province crust and older continental crust to the south reflections are sparser and more concordant in the gneiss and granulite terrane than in the superior province there are several prominent reflection groups with gentle northerly dips in the middle and upper crust of the granulite and gneiss terrane their distribution appears to correlate with the sources of aeromagnetic and gravity anomalies that are part of a belt that extends west to central south dakota and that apparently reveal the position of the postulated archean suture the axes of deposition and deformation of proterozoic metasedimentary rocks are approximately parallel to and apparently coincide with the boundary between two archean terranes farther east in minnesota wisconsin and michigan but proterozoic penokean deformation has not been distinguished either in the study area or in the seismic sections the minnesota cocorp survey has demonstrated the capability of this seismic reflection method to reveal important but previously unsuspected structures in complexly deformed archean terranes this has provided structural evidence that the separate subprovinces of the north american archean crust might have been assembled by subduction related collision

comment on preorogenic exhumation of the north pyrenean agly massif eastern pyrenees france by a vauchez et al

pericontinental crustal growth of the southwestern abitibi subprovince canada u pb hf and nd isotope evidence the abitibi subprovince is widely regarded as a classic example of neoarchean continental crust formed in an ensimatic setting however sialic crust as old as 2 93 ga occurs in the adjacent wawa subprovince and the two subprovinces are thought to have formed a contiguous block since the initiation of abitibi magmatic activity at 2 75 ga a detailed geochemical u pb geochronological and nd hf isotope study of plutonic and volcanic rocks from the southwestern abitibi provides evidence for the involvement of older crust during magma genesis near the abitibi wawa boundary kapuskasing structural zone inherited zircons dated at ca 2 85 ga occur both in 2747 ma tonalite and 2700 ma granodiorite of the rice lake batholith part of the larger kenogamissi plutonic complex igneous zircons from this batholith display large intrasample variations in 176 hf 177 hf which we attribute to mixing of magmas derived from mantle juvenile crust and older crustal sources nd isotope data for felsic volcanic and plutonic rocks in this region indicate a dominantly juvenile source however two older felsic volcanic units 2739 and 2729 ma show nd isotope evidence of significant crustal contamination post tectonic plutonic units that were emplaced at 2676 and 2662 ma also contain inherited zircons but none are older than 2 76 ga in contrast tonalitic units dated at 2744 and 2715 ma from the round lake plutonic complex located 150 km east of the kapuskasing structural zone lack zircon inheritance and display less intrasample hf isotope variability in zircons our data suggest that the influence of older sialic crust during abitibi volcanic and plutonic activity is restricted both in space and time similar evidence for an older crustal influence occurs elsewhere in the southwestern abitibi near the kapuskasing structural zone the data collectively suggest that an approximately 75 km wide belt of abitibi crust adjacent to this structure was underlain by older wawa crust from 2 75 ga onward this crustal geometry is consistent with development of the abitibi sub province at the edge of a wawa protocraton perhaps as the result of rifting of this older continental block

interaction between co2 rich sulfate solutions and carbonate reservoir rocks from atmospheric to supercritical co2 conditions experiments and modeling abstract a test site for co 2 geological storage is situated in hontomin burgos northern spain with a reservoir rock that is mainly composed of limestone during and after co 2 injection the resulting co 2 rich acid brine gives rise to the dissolution of carbonate minerals calcite and dolomite and gypsum or anhydrite at depth may precipitate since the reservoir brine contains sulfate experiments using columns filled with crushed limestone or dolostone were conducted under different p p co 2 conditions atmospheric 1 10 3 5 bar subcritical 10 10 bar and supercritical 150 34 bar t 25 40 and 60 c and input solution compositions gypsum undersaturated and gypsum equilibrated solutions we evaluated the effect of these parameters on the coupled reactions of calcite dolomite dissolution and gypsum anhydrite precipitation the crunchflow and phreeqc v 3 numerical codes were used to perform reactive transport simulations of the experiments within the range of p p co 2 and t of this study only gypsum precipitation took place no anhydrite was detected and this only occurred when the injected solution was equilibrated with gypsum under the p p co 2 t conditions the volume of precipitated gypsum was smaller than the volume of dissolved carbonate minerals yielding an increase in porosity up to 4 a decrease in t favored limestone dissolution regardless of p co 2 owing to increasing undersaturation with decreasing temperature however gypsum precipitation was favored at high t and under atmospheric p co 2 conditions but not at high t and under 10 bar of p co 2 conditions the increase in limestone dissolution with p co 2 was directly attributed to ph which was more acidic at higher p co 2 limestone dissolution induced late gypsum precipitation long induction time in contrast to dolostone dissolution which promoted rapid gypsum precipitation moreover owing to the slow kinetics of dolomite dissolution with respect to that of calcite both the volume of dissolved mineral and the increase in porosity were larger in the limestone experiments than in the dolostone ones under all p co 2 conditions 10 3 5 and 10 bar by increasing p co 2 carbonate dissolution occurred along the column whereas it was localized in the very inlet under atmospheric conditions this was due to the buffer capacity of the carbonic acid which maintains ph at around 5 and keeps the solution undersaturated with respect to calcite and dolomite along the column 1d reactive transport simulations reproduced the experimental data carbonate dissolution and gypsum precipitation for different p p co 2 t conditions drawing on reaction rate laws in the literature we used the reactive surface area to fit the models to the experimental data the values of the reactive surface area were much smaller than those calculated from the geometric areas

co2 percolation experiment through chlorite zeolite rich sandstone pretty hill formation otway basin australia underground co2 sequestration is highly recommended as an effective means of significantly decreasing co2 concentration in the atmosphere mineral storage is the more secure technology but requires the presence of high concentration of divalent cations in the pore fluid results from co2 percolation experiments through chlorite zeolite rich sandstone samples from the pretty hill formation otway basin australia are presented the dissolution of the laumontite 7 wt and chamosite 7 wt are the potential sources of calcium iron and magnesium required for carbonate precipitation the percolation experiment was setup to reproduce at laboratory scale the in situ temperature and pressure conditions t 95 degrees c and p 10 mpa the fluid injected at constant flow rate is a rock equilibrated brine subsequently enriched in co2 up to partial pressure of 6 mpa we observe feldspars laumontite and chamosite dissolution kaolinite and silica precipitation and a noticeable sink of co2 in the sample which is attributed to the precipitation of both amorphous carbon due to the reduction of co2 and fe rich carbonate permeability decreases of about one order of magnitude due to the localization of the kaolinite precipitation in the main flow paths while porosity increases the high reactivity of this sandstone makes this reservoir a valuable target for co2 mineralization but the associated permeability decrease may limit the injection rate and the spreading of the co2 in the reservoir

late archean convergent margin tectonics and gold mineralization a new look at the norseman wiluna belt western australia on a world scale the most important hydrothermal gold deposits occur either in late archean ca 2700 ma greenstone belts or at late paleozoic to quaternary convergent plate boundaries in the latter environments epithermal and porphyry hosted deposits form during subduction related magmatism in volcanic arcs and deep level meso thermal deposits form during deformation in continental margin orogenic belts the mesothermal deposits are similar to archean gold deposits in greenstone belts such as the norseman wiluna belt in western australia we here suggest that this similarity exists because the norseman wiluna belt is an orogenic belt with a tectonic history similar to that of younger mineralized convergent margins such as the north american cordillera it is most likely that the norseman wiluna belt resulted from the interaction of lithospheric plates during the major period of continental growth and stabilization that occurred in the late archean the distribution of gold mineralization being controlled by convergent margin tectonics

experimental evaluation of mixed fluid reactions between supercritical carbon dioxide and nacl brine relevance to the integrity of a geologic carbon repository abstract the reactive behavior of a mixed fluid supercritical co2 and brine under physical chemical conditions relevant to geologic storage and sequestration in a carbon repository is largely unknown experiments were conducted in a flexible cell hydrothermal apparatus to evaluate fluid rock aquifer plus aquitard reactions that may adversely impact the integrity of the repository a 5 5 molal nacl brine rock system was held at 200 c and 200 bars 20 mpa for 32 days 772 h to approach steady state then injected with co2 and allowed to react for an additional 45 days 1079 h in a separate experiment at 200 c and 200 bars the system was allowed to react for 77 days 1845 h without injection of co2 corroded magnesite and euhedral siderite crystallized in a paragenetic sequence after co2 injection nucleation and growth of siderite on shale suggests the aquitard is a reactive component in the system changes in elemental abundances in the brine following addition of co2 include ph decrease and depletion of sodium due to accelerated growth of analcime a ph increase follows pressure and temperature decrease and loss of saturated co2 from acidic brine silica concentrations and dissolution rates are enhanced and silica precipitation inhibited in the acidic brine geochemical reactions in a carbon repository extend beyond ph decrease and carbonate mineral precipitation rock dominated reaction systems yield to acid dominated and related reactions controlled by mixed fluid equilibria i e a fluid dominated system escape of co2 or migration of brine from the repository into overlying aquifers may cause silica super saturation and increased alkalinity due to mixed fluid phase equilibria these geochemical changes could be monitored in aquifers as indicators of repository integrity return of silica super saturated brine to a rock dominated reaction system buffered to neutral ph conditions may enhance precipitation of quartz chalcedony or amorphous silica in addition to the potential effects beneficial or deleterious that silica super saturation and precipitation may hold for repository performance an understanding of the effects of multi phase equilibrium relationships between supercritical co2 and dissolved silica in aquifer brine systems also raises new questions for a variety of geologic systems multi phase fluid equilibria may for example account for quartz cements in some sedimentary basin sandstones and quartz vein mineralization in some ore districts

high frequency magnetotelluric investigation of crustal structure in north central abitibi quebec canada summary thirty one audiomagnetotelluric amt soundings have been collected along a n s profile across the northern abitibi greenstone belt the recording frequency ranges of 10000 1 hz for 25 stations and 10000 0 001 hz for the other six stations were appropriate for defining the near surface geoelectric structures and for imaging major faults and shear zones down to approximately 10 km along the profile six time domain electromagnetic tem soundings were also collected near the southern end of the profile to provide a correction for static shift extensive static distortion analysis reveals that most of the amt data are not distorted by localized near surface structures however they are affected by static shift in order to remove this static shift a spatial filter was designed and applied to the data effectively reducing static shift as verified by tem soundings at the same sites extensive 2 d inversions of the filtered data were carried out the results suggest the presence of a thin overburden with both conductive and resistive blocks overlying a very resistive basement among the four geological deformation zones crossed by the profile only two zones are imaged as subvertical conductive crustal structures extending to the limit of resolution of amt these correspond to major breaks porcupine destor fault and casa berardi tectonic zone as evidenced by the seismic reflection data collected over the same area

comment on thermal control on the modes of crustal thinning leading to mantle exhumation insights from the cretaceous pyrenean hot paleomargins by clerc and lagabrielle

the north american midcontinent rift beneath lake superior from glimpce seismic reflection profiling the midcontinent rift system is a 1 1 b y old structure extending from kansas through the lake superior region and into southern michigan the rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments for most of its extent it is buried beneath paleozoic rocks but can be traced by its strong gravity and magnetic anomalies the rocks of the rift system are exposed only in the lake superior region and comprise the keweenawan supergroup much of the geology of the keweenawan is beneath lake superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on shore geology seismic reflection surveys by the great lakes international multidisciplinary program on crustal evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail the reflection profiles across the rift reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented they show that in addition to crustal sagging documented by previous investigations normal faulting played a major role in subsidence of the axial region of the rift a sequence of volcanic and sedimentary rocks in places greater than 30 km thick fills the graben thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben near the axis the prerift crust is thinned to about one fourth of its original thickness apparently by a combination of low angle extensional faulting and ductile stretching or distributed shear the sense of asymmetry of the central graben changes along the trend of the rift documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments the location of the accommodation zones is inferred from abrupt disruptions in the bouguer gravity signature of the rift uplift of the central graben occurred when the original graben bounding normal faults were reactivated as high angle reverse faults with throws of 5 km or more in places the midcontinent rift has some striking similarities to some younger passive continental margins we propose that it preserves a record of nearly complete continental separation which had it not been arrested would have created a middle proterozoic ocean basin

archean continental assembly in the southeastern superior province of canada between 1988 and 1993 seismic reflection and refraction surveys were acquired across the medium to high grade opatica plutonic gneiss belt the low grade abitibi greenstone belt and the pontiac metasedimentary belt all of which form part of the late archean superior province shallowly north dipping reflections define a structural style consistent with the northward underthrusting and accretion over about 30 ma of various exotic terranes against a backstop provided by the opatica belt this rapid southward growth of the archean protocraton was driven by at least one north dipping subduction zone as revealed by north dipping reflections that extend to 65 km depth in the upper mantle below the opatica belt in contrast to the mainly orthogneissic opatica and pontiac belts the midcrust of the abitibi belt comprises metasedimentary and igneous rocks plus imbricated units of unknown affinity relict midcrustal accretionary complexes of substantial size whieh are indicative of primary suture zones are interpreted near the northern and southern limits of the abitibi belt an interpreted basal decollement and significantly older ages in the north suggest that the upper crustal greenstone rocks are allochthonous evidence of large scale extension appears to be confined to the southern volcanic zone of the abitibi which developed into a half graben as the original suture zone was reactivated in extension unusually high seismic p wave velocities 7 5 8 2 kms 1 are present in the lower 8 km of the abitibi crust and they correlate well with a downward reduction in seismic reflectivity attributable to late modification of the deepest part of the crust crustal xenolith studies suggest that this process may be linked to early proterozoic magmatism

experimental and modeling study of the interaction between a crushed marl caprock and co 2 rich solutions under different pressure and temperature conditions abstract carbon storage is regarded as a potential technology to reduce co 2 emissions this study is concerned with co 2 storage in a reservoir rock limestone which is a deep saline aquifer containing a nacl and sulfate rich groundwater in equilibrium with calcite and gypsum the reservoir rock is overlain by a very low permeability formation marls which acts as a caprock during and after co 2 injection the resulting co 2 rich acid solution gave rise to mineral dissolution and precipitation processes these reactions led to changes in porosity pore structure and permeability of the caprock and also to the alteration of its co 2 seal capacity flow through experiments using columns filled with three crushed samples marly limestone bituminous black shale and marl which constitute the hontomin caprock unit were conducted under different p total p co 2 atmospheric 1 10 3 5 bar subcritical 10 10 bar and supercritical 150 37 bar conditions and t 25 and 60 c using gypsum undersaturated and gypsum equilibrated solutions to evaluate the hontomin caprock reactivity 1d reactive transport modeling of the results enabled us 1 to better understand the overall processes that bring about the large dissolution of calcite minor dissolution of aluminosilicates and the precipitation of gypsum kaolinite zeolites and some aluminum and iron oxides and oxyhydroxides and sulfates and 2 to evaluate their implication for the hydrodynamic properties of the caprock an increase in porosity over the column length which mostly occurred near the inlet was calculated

seafloor hydrothermal fluids ben nevis area abitibi greenstone belt implications for archean 2 7ga seawater properties abstract a central part of the 2 7 ga abitibi greenstone belt between the destor porcupine and cadillac larder lake structures has been subjected to sub greenschist facies metamorphism t the majority of observed fluid inclusions are two phase and liquid dominated with a vapor bubble volume of 10 microthermometry on 58 samples 1100 fluid inclusions and eutectic temperatures between 45 and 30 c indicate the presence of at least one significant divalent cation in the fluids hence salinity is calculated as wt cacl2 equiv the peak of the salinity distribution of inclusions from vesicles and interstitial and drainage cavity quartz falls between 4 and 10 wt cacl2 equiv a small subset of inclusions yielded salinities between 18 and 22 wt cacl2 equiv most homogenization temperatures fall in an interval between 110 and 160 c a geothermal gradient estimated from the data of 10 15 c km is reasonable and consistent with the interpretation of the area as a regional seafloor recharge zone primary porosity in the modern ocean floor does not show the rapid precipitation of quartz that is indicated for the archean however the global occurrence of silica rich banded iron formations and the lack of silica consuming marine organisms in the precambrian indicates that marine geochemistry and sio2 precipitation mechanisms may have been different during the precambrian the total salinity of a fluid is dependent primarily on its total chlorinity hence water rock interactions with ion exchange cannot be the only reason for the higher salinities encountered in this study indications for boiling or high temperature phase separation are not evident suggesting that these options are not a reason for the observed high salinities our data are in agreement with six other data sets from the 3 75 ga isua greenstone belt w greenland the 3 24 ga pilbara region w australia the 3 2 ga barberton greenstone belt south africa 2 the 2 7 ga matagami area abitibi greenstone belt canada and the proterozoic 2 2 ga ongeluk formation south africa the consistency of fluid inclusion data suggests that archean and early proterozoic seawater may have been more saline and potentially stratified compared to modern seawater

plutonism in the southern abitibi subprovince a tectonic and petrogenetic framework studies of plutonic rocks in archean terranes such as the southern abitibi subprovince have been an important component in the development of an accretionary tectonic model for the superior province in the southern abitibi subprovince pretectonic approximately 2 75 2 70 ga low k tholeiitic calc alkaline dioritic to tonalitic plutons e g bourlamaque batholith flavrian pluton are associated with volcanism and are interpreted to be derived from both partial melting of hydrated mantle during oceanic arc formation and fractionation of tholeiitic to calc alkaline basalt andesite pretectonic synvolcanic tholeiitic intrusions e g dundonald sill kamiskotia gabbroic complex 2 72 2 70 ga show in situ fractionation trends and locally have mixed magma textures with contemporaneous calc alkaline granitoids they are believed to be derived from a midocean ridge basalt morb like depleted mantle with little evidence for contamination by an older crustal component pre to syntectonic approximately 2 72 2 69 ga porphyry and tonalite plutons e g timmins porphyries round lake batholith have geochemical characteristics typical of archean tonalite trondhjemite suites considered to be derived from melting of basaltic rocks at high metamorphic grades late to post tectonic 2 69 ga diorite monzonite and syenite plutons e g watabeag batholith lamaque stocks otto stock have large ion lithophile element enriched calc alkaline to sho shonitic compositions these intrusions and associated timiskaming type volcanic rocks are attributed to melting of large ion lithophile element enriched mantle in the late stages of subduction late to post tectonic biotite muscovite granite intrusions contain a significant contribution of evolved crust e g s type characteristics of the preissac lacorne batholith in the southern abitibi subprovince the tectonic setting of mantle and crust derived intrusions is broadly comparable to magmatic processes in phanerozoic arcs and accreted terranes the plutonic rocks record a transition from pre to synaccretionary subduction related plutonic suites to late to postaccretionary suites derived from enriched mantle and crustal sources this framework provides a basis for understanding mineral deposits associated with intrusive rocks of the southern abitibi subprovince which range from ni cu sulfides in layered synvolcanic tholeiitic intrusions e g kamiskotia gabbroic complex ontario to volcanogenic sulfides in early tholeiitic and calc alkaline sequences e g flavrian pluton noranda district quebec to the spatial association of gold with late large ion lithophile element enriched intrusions e g lamaque plugs val d or district quebec

changes in seal capacity of fractured claystone caprocks induced by dissolved and gaseous co2 seepage 1 claystone caprocks are often the ultimate seal for co2 underground storage when residual co2 gas reaches the reservoir top due to buoyancy permeability changes of a fractured claystone due to seepage of co2 enriched brine and water vapor saturated co2 gas are investigated results show that brine flow induces a large porosity increase up to 50 in the vicinity of the fracture due to dissolution of calcite and quartz while permeability remains unchanged conversely cyclic flows of co2 brine and co2 gas increase the fracture aperture abruptly after each gas flow period producing a progressive decrease of the caprock seal capacity aperture increase is controlled by decohesion of the clay framework within a micrometer scale thick layer induced by co2 gas acidification results show that hydraulic aperture increases linearly with duration of the preceding co2 brine flow period emphasizing the kinetic control of the quartz grains dissolution during the brine flow periods

seismic imaging of the geologic framework and structures related to volcanogenic massive sulfide deposits in the archean rouyn noranda district quebec canada two seismic reflection profiles acquired by xstrata canada in the noranda mining camp were reprocessed and interpreted with the objective of providing key information on the geologic contacts and structures associated with volcanogenic massive sulfide vms deposits at depth the amulet and ribago seismic profiles run approximately from east to west and cross the volcanic rocks of the noranda formation which host most of the ore deposits in this camp the seismic data interpretation relies strongly on a detailed three dimensional geologic model built from an extensive number of exploration boreholes available in this area and is further supported by physical rock property measurements from in situ borehole logging data some reflections observed on the amulet and ribago seismic profiles correlate with rhyolite andesite or silicified andesite andesite contacts that host the prospective exhalites of the noranda formation in particular the silicified andesite andesite contact hosting the c contact exhalite is imaged clearly down to a vertical depth of 1 100 m along the ribago profile the processing sequence included dip moveout dmo corrections and poststack migration the seismic data reprocessing allowed the identification of two diffractions that correlate with known sulfide bodies intersected in boreholes located close to the ribago profile one of these diffractions at approximately 1 200 m depth coincides with the main massive sulfide intersection of the subeconomical ribago orebody diorites cause several reflections observed within the flavrian pluton some diorite units also have sufficient acoustic impedance contrast to produce strong reflections when juxtaposed against volcanic rocks however such reflections do not predominate in the amulet and ribago profiles possibly because reflective diorite units are not so significant or have limited lateral continuity in this part of the noranda formation the reconciliation of the detailed three dimensional geologic model and two dimensional seismic data was not necessarily a straightforward task a significant complication results from the inherent limitations of two dimensional seismic imaging techniques in a complex three dimensional geologic environment nevertheless results presented in this paper indicate that seismic methods can image prospective contacts and deep seated massive sulfide mineralization and can be a valuable exploration tool in the noranda mining camp

lithoprobe reflection studies of archean and proterozoic crust in canada abstract lithoprobe canada s national collaborative earth science research project established to develop a comprehensive understanding of the evolution of the north american continent is a multidisciplinary program spearheaded by seismic reflection studies five recently recorded seismic lines discussed in this paper are located in precambrian regions the mesoproterozoic grenville province the paleoproterozoic trans hudson orogen tho the paleoproterozoic archean basement of alberta and the archean superior province data acquired across the grenville orogen in eastern quebec show strong reflectivity throughout the crust upper crustal reflections can be correlated with exposed structural elements including extensional shear zones and packages of deformed high pressure rocks eclogites in a marine survey across the grenville orogen off southeastern labrador seismic images show variably dipping reflections and a structural high associated with a major gravity anomaly data acquired across central alberta show crustal scale thrust stacking and imbrication of the archean hearne craton to the east across the trans hudson orogen images of similar collisional features are observed geochronologic constraints indicate contemporaneity of tectonic activity between the two regions at 1 8 ga suggesting that collisional tectonic activity was coeval over a broad crustal region ca 1000 km across strike in the superior province seismic data across a collision zone involving the northern abitibi greenstone belt and the arc related opatica plutonic belt show spectacular crustal reflectivity and dipping reflections that extend 8 s 30 km into the mantle the latter feature is interpreted as representing a relict 2 69 ga old suture associated with subduction providing the first direct evidence that plate tectonics was active in the late archean these five examples supported by other lithoprobe results refute a number of generalizations about crustal reflectivity that have been made in the past and illustrate how reflection studies combined with other geoscience studies can lead to a better understanding of precambrian tectonics reflectivity persists throughout the crust there is no general separation into a poorly reflective upper crust and a reflective lower crust crustal reflectivity in archean and proterozoic regions is as pervasive as that in areas of more recent tectonism the precambrian reflection moho is generally well defined but shows a range of characteristics relative ages of reflectors can be discerned and tectonic significance can be attached to characteristic features of the crustal reflectivity

an extensional fault model for the early development of greenstone belts with reference to a portion of the abitibi belt ontario canada abstract detailed geological mapping of a small part of the archaean abitibi greenstone belt northwestern ontario suggests that the mafic and ultra mafic lavas were extruded in a submarine environment in which the volcanism was intimately associated with deformation of the growing volcanic pile the deformation led to the subsidence and progressive tilting of the lavas and to the development of discontinuities a review of selected areas of quaternary volcanism suggests that subsidence of the lava pile during volcanism is not unusual and that activity is frequently also associated with normal faulting in both volcanic and non volcanic extensional regimes deformation is dominated by the development of listric faults we argue that greenstone belts such as the abitibi were formed in extensional regimes and we show that a simple geometric model involving fissure volcanism and synvolcanic rotation of the lavas on listric faults accounts for the major primary features of greenstone belts and for some associated geophysical properties

deep seismic probing of the continents and their margins abstract application of deep seismic methods to studies of the crust and lithospheric mantle receives considerable interest and the methods are constantly refined and new methods are developed which allows the extension of studies to new subjects and regions deep seismic methods are applied to a long range of geoscientific subjects in research on the formation and development of crust and lithosphere in continental and oceanic environments from being an experimental discipline some 30 40 years ago a series of methods can now be applied almost routinely to research on the lithosphere however in many applications the methods are used up to their limits at the present technological state therefore development of methods has high priority in the seismic community this volume provides an overview of recent development of deep seismic techniques and their application to the imaging and probing of the continents and their margins

block and shear zone architecture of the minnesota river valley subprovince implications for late archean accretionary tectonics the minnesota river valley subprovince of the superior province is an archean gneiss terrane composed internally of four crustal blocks bounded by three zones of east northeast trending linear geophysical anomalies two of the block bounding zones are verified regional scale shears the geological nature of the third boundary has not been established potential field geophysical models portray the boundary zones as moderately north dipping surfaces or thin slabs similar in strike and dip to the morris fault segment of the great lakes tectonic zone at the north margin of the subprovince the central two blocks of the subprovince morton and montevideo are predominantly high grade quartzofeldspathic gneiss some as old as 3 6 ga and late tectonic granite the northern and southern blocks benson and jeffers respectively are judged to contain less gneiss than the central blocks and a larger diversity of syntectonic and late tectonic plutons a belt of moderately metamorphosed mafic and ultramafic rocks h

teleseismic studies of the lithosphere below the abitibi grenville lithoprobe transect in the past decade the abitibi grenville lithoprobe transect has been the site of numerous geological and geophysical surveys oriented towards understanding the lithospheric evolution of the southeastern superior and adjoin ing grenville provinces among the different geophysical methods that have been employed earthquake seismology provides the widest range of information on the deep structures of the upper mantle this paper presents a review of studies both complete and ongoing involving teleseismic datasets that were collected in 1994 and 1996 along the transect a complete shear wave splitting analysis has been performed on the 1994 dataset as part of a comparative study on electrical and seismic anisotropies results suggest a correlation between the two anisotropies supported by xenolith data and favour a lithospheric origin for the seismic anisotropy the two anisotropies are believed to represent the fossilized remnants of archean strain fields in the lithospheric roots of the canadian shield preliminary splitting results for the 1996 experiment suggest that the s wave azimuthal anisotropy may be depth dependent and laterally varying ongoing receiver function analysis and traveltime inversion studies provide velocity models of the crust and upper mantle beneath the study area preliminary receiver function results reveal the presence of an s velocity increase at 90 100 km depth which appears to be laterally continuous over 200 km traveltime inversion models indicate the presence of an elongate low velocity anomaly beneath the southern portion of the 1996 array which strikes obliquely to major geological structures at the surface e g grenville front preliminary interpretation relates this anomaly to the same process e g fixed mantle plume continental rifting responsible for the emplacement of the monteregian hills igneous province

diverse metal sources of archaean gold deposits evidence from in situ lead isotope analysis of individual grains of galena and altaite in the ross and kirkland lake deposits abitibi greenstone belt canada lead isotope compositions for individual grains of galena and altaite pbte were determined in situ using a secondary ion mass spectrometer sims galena was collected from the ross deposit and altaite from the kirkland lake kl deposits in the southern abitibi greenstone belt superior province of canada the samples from kl are more radiogenic than those from the ross deposit isotopic compositions vary significantly between different grains in each deposit and form broad linear arrays in 207pb 204pb 206pb 204pb and 208pb 204pb 206pb 204pb diagrams the linear arrays of pb isotope data are attributed to mixing of pb from different sources at least two sources are required for individual deposits one with low u pb and th pb ratios and the other with high ratios lead minerals occurring with au are less radiogenic than those that are not obviously associated with au suggesting that au was supplied from low u pb sources such as sulphides or older ultramafic mafic rocks while most data are consistent with the derivation from local rocks highly radiogenic pb with relatively low 207pb 206pb ratios recorded at kl require post archaean mineralization or derivation of the pb from an unusual crustal source with low the latter interpretation is favored because of the lack of textural evidence and because it is difficult to dissolve and precipitate altaite at low temperatures the presence of a pb reservoir with low is also inferred from the data of archaean banded iron formations and volcanogenic massive sulphide deposits different isotopic patterns of the two deposits suggest different sources of metals in the two deposits while this conclusion does not reject the derivation of fluids from the lower crust or mantle the data are not in accord with conceptual models invoking a common source reservoir for metals the study suggests that fluids which may have a common origin leached metals and other constituents from the upper crustal rocks during their ascent the proposed model different origins for different constituents explains much of the conflicting evidence presented by archaean au deposits including provinciality of mineralogy and relatively uniform fluid inclusion and c isotope data from many au deposits

reservoir property changes during co2 brine flow through experiments in tight sandstone implications for co2 enhanced oil recovery in the triassic chang 7 member tight sandstone ordos basin china abstract the effects of supercritical co2 scco2 on reservoir property changes of the chang 7 member tight sandstones of the yanchang formation ordos basin china were investigated using co2 brine flow through experiments in conjunction with detailed rock and fluid characterization pre and post experiment using x ray diffraction mineral analysis 3d computed tomography scanning high resolution scanning electron microscopy and inductively coupled plasma atomic emission spectrophotometry an in situ 3d model was developed to define porosity mineral evolution experiments on three contrasting samples show that minerals such as potassium k feldspar albite and calcite are variably dissolved during co2 brine mineral interactions the dissolution migration and re precipitation of clay minerals are common and are characterized by the dissolution of chlorite and the re precipitation of kaolinite calcium montmorillonite and sodium montmorillonite the original chlorite kaolinite framework of the host rocks was destroyed precipitated and re migrated clay particles e g kaolinites and smectites that were released by the dissolution accumulated in pore throats resulting in changes to the pore system and in the physical properties of the host rocks physical property change pathways vary among the studied samples with porosity being reduced by 4 5 and 12 8 or increased by 13 7 by the end of the experiments permeability reduction was 33 3 81 2 and is positively correlated to host rock clay mineral content the influence of the type and content of clay minerals on porosity requires further evaluation the findings of the study have implications for the large scale injection of co2 into reservoirs during enhanced oil recovery programs and for carbon capture utilization and storage programs

archean wrench fault tectonics and structural evolution of the blake river group abitibi belt quebec structural analysis of the blake river group volcanic rocks in the rouyn noranda region shows the formations to be distributed largely in z shapes resulting from the interference of two early fold systems oriented west northwest east southeast and east west respectively the first of these two fold systems is probably related to the shortening associated with left lateral movement along the two major fractures in the region namely the porcupine destor and larder lake cadillac faults the second system appears to be the result of north south compression perpendicular to the two major fractures the two major faults the first system of early folding the normal and reverse faults and the minor dextral and sinistral strike slip faults that have been observed in the blake river group rocks can all be integrated into one tectonic system that of wrench fault tectonics the orientations of the principal structures recognized in the abitibi belt major shear zones folding and faulting suggest that the d

deep structure of an archaean greenstone terrane more than half of the continental crust was formed before about 2 500 million years ago1 yet there is still considerable uncertainty regarding the structure and evolution of the ancient archaean cratons2 7 modern analogues proposed for the setting of archaean greenstone belts include aborted intra cratonic rifts continental margins and various oceanic settings2 7 alternatively archaean tectonics may have been quite different from the plate tectonics we see today2 4 6 the abitibi belt in canada is the largest archaean greenstone ternane3 and has been a touchstone for many models because its surface geology is relatively well known8 10 it contains significant gold and base metal deposits10 and it is less deformed and has suffered less erosion than most archaean ter ranes here we present deep seismic reflection data from part of the abitibi belt which reveal extensive subhorizontal and gently dipping reflection zones truncated by steeply inclined discontinuities which project to major fault zones at the surface our preliminary model of the abitibi belt comprises a mostly juvenile magmatic crust similar to that exposed in the nearby kapuskasing uplift11 12 dissected and imbricated by deep penetrating discontinuities associated with thrust and strike slipe faulting in view of the similarity of greenstone belts worldwide2 7 13 our seismic sections provide new constraints on the general processes that have affected archaean metavolcanic terranes

geologic evolution of the late archean abitibi greenstone belt of canada the southern volcanic zone svz of the late archean abitibi belt of the superior province of canada is dominated by komatiitic to tholeiitic volcanic plateaus and large bimodal mafic felsic volcanic centers these volcanic rocks were erupted between about 2717 and 2700 ma in a series of rift basins that formed as a result of wrench fault tectonics they overlie and juxtapose a volcano plutonic assemblage characterized in the northern abitibi belt the age of the assemblage is about 2720 ma or older and it comprises basaltic to andesitic and dacitic subaqueous massive volcanics cored by comagmatic sills and layered anorthositic complexes and overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 2 ma a tectonic model is proposed in which the svz formed in a series of rift basins that dissected an earlier formed volcanic arc comparisons are made with rift environments that have been postulated for phanerozoic areas such as the hokuroko basin of japan the taupo volcanic zone of new zealand and the sumatra and nicaragua arcs

the geometrical relationship between the stretching lineation and the movement direction of shear zones abstract the geometrical relationship between foliations and lineations in shear zones is considered quantitatively assuming that the internal structures of shear zones have monoclinic symmetry the common practice of directly correlating the pitch or plunge of the stretching lineation with the obliquity of the shear direction is challenged it is the orthogonal projection of the stretching lineation on the shear zone boundary that is parallel to the shear direction a shear zone is parallel to a plane containing both the pole to the symmetry plane and the strike of the shear zone and the shear direction is parallel to the intersection of the symmetry plane and the shear zone boundary the symmetry plane is determined by the stretching lineation and the poles to the s foliation the implications of this study for the interpretation of shear zones are discussed with reference to an actual example

stable isotope geochemistry of the archean val d or canada orogenic gold vein field a systematic study of the auriferous quartz veins of the val d or vein field abitibi quebec canada demonstrates that the c o s isotope composition of silicate carbonate borate oxide tungstate and sulphide minerals have a range in composition comparable to that previously determined for the whole superior province the oxygen isotope composition of quartz from early quartz carbonate auriferous veins ranges from 9 4 to 14 4 whereas later quartz tourmaline carbonate veins have 18oquartz values ranging from 9 2 to 13 8 quartz carbonate veins have carbonate 18o 6 9 12 5 13c 6 2 1 9 and pyrite 34s 1 2 and 1 9 isotope compositions comparable to those of quartz tourmaline carbonate veins 18o 7 9 11 7 13c 8 0 2 4 34s 0 6 6 0 18oquartz values in quartz tourmaline carbonate veins have a variance comparable to analytical uncertainty at the scale of one locality irrespective of the type of structure the texture of the quartz or its position along strike across strike or down dip a vein in contrast the oxygen isotope composition of quartz in quartz tourmaline carbonate veins displays a regional distribution with higher 18o values in the south central part of the vein field near the cadillac tectonic zone and which 18o values decrease regularly towards the north another zone of high 18o values in the northeast corner of the region and along the trace of the senneville fault is separated by a valley of lower 18o values from the higher values near the cadillac tectonic zone oxygen isotope isopleths cut across lithological contacts and tectonic structures this regional pattern in quartz tourmaline carbonate veins is interpreted to be a product of reaction with country rocks and mixing between 1 a deep seated hydrothermal fluid of metamorphic origin with minimum 18o 8 5 13c 0 6 and 34s 0 4 and 2 a supracrustal fluid most likely archean seawater with a long history of water rock exchange and with maximum 18o 3 9 13 c 5 6 and 34s 5 0

heat flow and structure of the lithosphere in the eastern canadian shield twenty two new determinations of heat flow and radiogenic heat production in the superior and grenville provinces of the canadian shield are presented the new data and previously published measurements strongly constrain the thermal structure of the eastern canadian shield in the abitibi greenstone belt heat flow gradually increases from 29 mw m 2 near the grenville front to 44 mw m 2 east of the kapuskasing uplift this heat flow variation is interpreted in terms of crustal thickening and increased thickness of a tonalitic layer with average heat production of about 1 1 w m 3 this interpretation based on estimated heat production of major rock types in the region is consistent with crustal models derived from recent seismic reflection and refraction studies it also leads to an estimate of about 12 mw m 2 for the mantle heat flow beneath the area the average heat flow in the grenville province 41 10 mw m 2 is the same as that of the superior province this similarity and the lack of significant variation of heat flow across the grenville front indicate that the crust on both sides of the front has similar heat production and thus composition in the western part of the grenville province heat flow reaches high values in the vicinity of the boundary between the allochtonous polycyclic and monocyclic belts where enriched granitic plutons are found in the crystalline terranes in the central part of the grenville province heat flow and heat production are related to each other the parameters of the linear heat flow heat production relationship qr 30 2 in mw m 2 and d 7 1 1 7 km are close to those of the much younger appalachian province implying that the higher appalachian heat flow is due solely to higher heat production in the upper crust the data provide no evidence for variation of mantle heat flow between the superior grenville and appalachian provinces whose tectonic ages range between 2700 and 400 ma the small value of the mantle heat flow about 12 mw m 2 implies that the depth to the 450 c isotherm which controls the effective elastic thickness of the lithosphere is very sensitive to crustal heat production

tectonic and metallogenic implications of regional seismic profiles in the timmins mining camp four regional seismic reflection profiles totalling 153 line km were acquired near timmins ontario within the southern abitibi greenstone belt five additional high resolution lines targeted specific metallogenically important features such as the porcupine destor deformation zone interpretation of these profiles individually and in a composite north south transect reveals a number of prominent bands of reflectors within the upper 15 km of the crust that define a series of folds or antiformal stacks of thrust nappes structures and stratigraphy mapped at the surface confirm structural culminations in these locations at depths greater than 10 km the reflectors have generally shallower dips implying broad folding major ore deposits such as those at the kidd creek hollinger mcintyre and dome mines are located on the northern steeply dipping limbs of these antiformal stacks implying that the fold structures focused post tectonic mineralizing fluids within the upper crust into the near surface the porcupine destor deformation zone is proximal to the large gold deposits near timmins and is revealed by the new seismic data to be a composite of early fold structures and late transpressive fault arrays

an archean megacaldera complex the blake river group abitibi greenstone belt abstract the archean blake river group of the southern abitibi greenstone belt is defined as a subaqueous megacaldera compelling evidence include 1 a radial and concentric organization of synvolcanic mafic to intermediate dykes 2 an overall dome geometry defined by the volcanic strata 3 a peripheral distribution of subaqueous volcaniclastic units 4 a zonal distribution of carbonate alteration and 5 a distinct annular synvolcanic inner and outer ring fault pattern three caldera forming events have been identified 1 the early misema caldera 2 the new senator caldera and 3 the well known noranda caldera the multi vent brg mafic volcanic complex developed on a monotonous sequence of tholeiitic basalts forming a submarine plain and experienced a first major collapse that created the giant misema caldera 80 km in diameter an endogenic dyke swarm intruded the synvolcanic fractures and an underlying magma chamber developed major volcaniclastic units were generated by local volcanic centers and summit calderas formed along the outer and inner ring faults this fault system was used as a conduit for co2 rich hydrothermal activity renewed volcanic activity was associated with a resurgent central dome inside the misema caldera this second collapse event created the 35 km by 14 km nw trending new senator caldera this caldera was produced by multi step sagging after the underlying magmatic chamber migrated to the se and formed the flavrian powel plutons the final collapse resulted in the formation of the noranda caldera that generated a well developed 070 trending fracture pattern associated with several vms deposits the multiple caldera setting provides an effective model to explain the presence of vms mineralization along the synvolcanic fractures associated with the three collapse episodes however the importance of the new senator caldera fracture pattern is emphasized because of its role in the positioning of the giant horne mine deposit

archean crust as revealed in the kapuskasing uplift superior province canada in the central superior province of the canadian shield a 120 km wide transition from the low grade michipicoten greenstone belt to the high grade kapuskasing structural zone represents an oblique section through some 20 km of archean crust uplifted along a northwest dipping thrust fault the restored vertical section through the upper and middle crust consists of three megalayers with undulating boundaries 1 0 to 25 km a high grade heterogeneous gneissic assemblage in part older than the upper supracrustal succession

preorogenic exhumation of the north pyrenean agly massif eastern pyrenees france 1 the pyrenees north of the north pyrenean fault display a complex structure involving a succession of peridotite massifs basement massifs and mid cretaceous to late cretaceous basins located in a narrow domain which was affected by a mid cretaceous preorogenic high temperature low pressure metamorphism the late cretaceous basins were interpreted either as pull apart basins formed during transcurrent motion of iberia relative to eurasia or as remnants of a larger extensional basin recent models support that peridotite massifs result from the exhumation of the mantle during this preorogenic event the northern boundary of the agly basement massif shows evidence of ductile deformation of the basal formations of the agly sedimentary cover macroscopic and microscopic kinematics indicators consistent with asymmetry of crystallographic fabrics suggest normal sense of shear and thus suggest detachment at least partial of the mesozoic cover from its basement triassic to early cretaceous limestones are mylonitic and consistently shows a foliation a ns to ne trending lineation shear criteria suggesting top to the north shearing and locally boudinage at the microscopic scale mylonites are characterized by a very fine grain size frequently 10 m they contain larger partially recrystallized calcite parent grains and undeformed quartz grains with calcite fringes crystallized in pressure shadows in these mylonites calcite systematically shows a weak but well defined crystallographic preferred orientations suggesting ht dislocation creep combined to diffusion creep and possibly grain boundary sliding in the finest fraction of the mylonites paleotemperatures determined using raman spectrometry suggest synkinematic temperature conditions in the range 340 390 c in good agreement with observed microstructures and calcite cpo the mylonitic fabric in mesozoic limestones is folded by ew trending pyrenean folds north of the agly basement massif attesting that this fabric formed before the pyrenean orogeny these data consistently support preorogenic extension under medium temperature conditions of the northern agly massif and likely of the massif itself since simultaneously 100 ma a mid cretaceous basin opened south of the basement massif we suggest a model of preorogenic exhumation of the agly massif in response to the regional extension associated to the rotation of iberia this model may explain the exhumation of the north pyrenean massifs during a single preorogenic event that allowed the opening of extensional basins and the exhumation of the lithospheric mantle all these structures being subsequently reworked during the pyrenean orogeny

archaean cratonic roots mantle shear zones and deep electrical anisotropy the extent to which the mantle participated in the growth and stabilization of ancient cratons is central to our understanding of the evolution of the continents1 the detection of seismic anisotropy beneath precambrian north america for example has been interpreted as showing that strain induced orientation of mantle minerals in subcontinental lithospheric mantle can preserve a record of ancient episodes of deformation2 here we present mag netotelluric measurements from the superior province of the canadian shield which reveal pronounced electrical anisotropy in the upper 100 km of the underlying mantle we argue that this anisotropy is best explained by conducting graphite films oriented within fractures or on grain boundaries and associated with metasomatism of the mantle roots of major archaean shear zones which transect the entire superior province the uppermost mantle beneath the canadian shield has therefore remained fixed to the crust and isolated from significant tectonic reworking since the late archaean

geochemical oxygen and neodymium isotope compositions of metasediments from the abitibi greenstone belt and pontiac subprovince canada evidence for ancient crust and archean terrane juxtaposition abstract the abitibi greenstone belt agb and pontiac subprovince ps in the southwestern superior province are adjacent greenstone plutonic and metasedimentary dominated terranes respectively separated by a major fault zone metasediments from these two contrasting terranes are compared in terms of major and trace element and o and nd isotope compositions and detrital zircon ages the following two compositional populations of metasediments are present in the low grade abitibi southern volcanic zone 1 1 a mafic element enriched population mep characterized by flat depleted ree patterns enhanced mg cr co ni and sc low incompatible element contents and minor or absent normalized negative troughs at nb ta and ti and 2 2 a low mafic element population lmep featuring lree enriched patterns enhanced rb cs ba th and u contents and pronounced normalized negative troughs at nb ta and ti these geochemical features are interpreted to indicate that the mep sediments were derived from an ultramafic and mafic dominated oceanic provenance whereas the lmep sediments represent mixtures of mafic and felsic arc source rocks the ps metasediments are essentially indistinguishable from abitibi lmep on the basis of majorelement and transition metal abundances suggesting comparable types of source rocks and degrees of maturity but are distinct in terms of some trace elements and o isotope compositions the pontiac metasediments are depleted in 18o and enriched in cs ba pb th u nb ta hf zr and total ree and also have higher ratios of rb k cs rb ba rb ta nb th la and ba la relative to the abitibi lmep two subtypes of ree patterns have been identified in ps metasediments both have moderately fractionated distributions with la yb n 3 9 17 and 7 8 13 6 respectively but the first has minor eu anomalies eu eu 0 89 1 16 whereas the second features pronounced negative anomalies eu eu 0 52 0 81 coupled with high concentrations of incompatible elements such as k rb th u nb and ta the first subtype is interpreted to be derived from provenances of mixed mafic and felsic volcanic rocks whereas the eu depleted type has geochemical features that are typical of post archean sediments or archean k rich granites and volcanic equivalents abitibi metasediments have nd 2 7 ga values of 0 71 to 1 83 corresponding to model tchur 2 82 2 55 ga with the majority around 2 65 ga close to the depositional age of 2 7 ga in contrast ps metasediments are characterized by nd 2 7 ga 0 95 to 3 51 corresponding to t dhur 2 7 to 3 1 ga consistent with a detrital zircon age spectrum of 2 69 ga to 3 1 ga pontiac subprovince and abitibi metasediments have contrasting provenances suggesting their separate initial geodynamic settings and later tectonic juxtaposition major and trace elements o and nd isotopic compositions and detrital zircon ages collectively suggest that some of the ps metasediments were derived mainly from juvenile rocks whereas others require various degrees of contribution from older 3 0 3 5 ga evolved crust mixed with juvenile 2 7 ga rocks older components were derived either directly from erosion of an ancient continental arc or from contemporaneous volcanic rocks which may contain recycled older components if the ps represents in part an accretionary prism to the abitibi southern volcanic zone then oblique subduction may have been involved which allowed the pontiac subprovince to juxtapose or sample exotic source rocks

constraints in 3d gravity inversion a three dimensional 3d inversion program is developed to interpret gravity data using a selection of constraints this selection includes minimum distance flatness smoothness and compactness constraints which can be combined using a lagrangian formulation a multigrid technique is also implemented to resolve separately large and short gravity wavelengths the subsurface in the survey area is divided into rectangular prismatic blocks and the problem is solved by calculating the model parameters i e the densities of each block weights are given to each block depending on depth a priori information on density and the density range allowed for the region under investigation the present computer code is tested on modelled data for a dipping dike and multiple bodies results combining different constraints and a weight depending on depth are shown for the dipping dike the advantages and behaviour of each method are compared in the 3d reconstruction recovery of geometry depth size and density distribution of the original model is dependent on the set of constraints used from experimentation the best combination of constraints for multiple bodies seems to be flatness and a minimum volume for the multiple bodies the inversion method is tested on real gravity data from the rouyn noranda quebec mining camp the 3d inversion model for the first 10 km is in agreement with the known major lithological contacts at the surface it enables the determination of the geometry of plutons and intrusive rocks at depth

a summary of electromagnetic studies on the abitibi grenville transect electromagnetic surveys on the abitibi grenville lithoprobe transect have elucidated a number of conductivity signatures that can be genetically linked to precambrian tectonic processes some major fault zones are moderately conductive possibly signalling graphite deposition from a mantle co2 flux along crust penetrating fault systems however conductive graphitic metasedimentary rocks characteristic of foreland basins are apparently absent from the transect area a weak inverse correlation between metamorphic grade and electrical conductivity was observed by following rock units across the grenville front into high grade equivalents within the parautochthonous belt a uniformly conductive mid crustal layer extends across the grenville front apparently without change in character the existence of this ubiquitous mid crustal conductor has been interpreted to mean that electrical conductivity is controlled by the present day pressure temperature and fluid saturation of the lower crust independent o

seismic delineation of upthrust archaean crust in kapuskasing northern ontario the kapuskasing structure fig 1 is a 500 km long high grade metamorphic zone which cuts obliquely across the predominantly east west structural grain of the superior province of the canadian shield results from surface geological mapping and gravity profile analysis suggest that the structure is a partial cross section of archaean crust exposed by thrust faulting along the ivanhoe lake cataclastic zone during the early proterozoic1 2 similar exposures of the crust have been observed elsewhere3 4 but no seismic investigation has been conducted over a contiguous exposed cross section of archaean crust here we report results from a seismic refraction profile across the kapuskasing structural zone carried out to investigate the deep structure of this greenstone gneiss belt modelling of travel times and amplitudes yields a high velocity anomaly of 6 6 6 7 km s 1 extending from 20 km depth to close to the surface under the kapuskasing zone with a suggested dip of 15 2 to the west this anomaly is in agreement with a structure comprising upthrusting middle to lower crust

an archean fold thrust belt in the northwestern abitibi greenstone belt structural and seismic evidence an integration of structural field data and lithoprobe seismic reflection line 28 in the northwestern abitibi greenstone belt agb reveals a crustal scale south to southwest vergent thrusting event that developed in sequence above a shallowly 15 north dipping sole thrust at a mid crustal level seismic reflector geometry above this decollement suggests a mid crust 6 20 km depth dominated by low angle thrusts with smooth trajectory ramps and culmination folds or antiformal stacks similar to the structural style of neighbouring high grade plutonic gneissic opatica and sedimentary pontiac subprovinces in contrast low to high angle east west trending thrusts at the upper crust greenstone belt level 6 9 km depth are interpreted to be listric they occur in two fault systems the chicobi and taibi that resemble imbricate fan systems the contrasting structural geometry of the upper and mid crust is interpreted as variations in level through the thrust stack and resembles paleozoic mountain

the use of magnetotellurics for mineral exploration an experiment in the chibougamau region of quebec ten magnetotelluric mt soundings were recorded in the range 300 hz to 200 s along an 11 km profile across a long well defined east west sub vertical conductor in the vicinity of chibougamau this conductor had been recognized by the airborne transient electromagnetic technique and confirmed by other ground electromagnetic methods the primary purpose of the mt survey was to further understand and constrain the structure of the conductor especially its extent at depth this study is the first to demonstrate the utility of the mt method for mineral exploration in abitibi subprovince the subsurface of the survey area can be roughly divided into a resistive northern section a conductive central section containing the conductive sheet and a resistive southern boundary although some data are distorted by static shifts or three dimensional effects there is evidence that the data from the central conductive section are relatively free of static distortions therefore taking into account existing geolo

origin and early growth rate of continents abstract data suggest that early continents formed over mantle sinks and grew rapidly between 2 7 and 3 0 ga ago nd isotopic results indicate that archean tonalites which are largely responsible for early continental growth are derived chiefly from ancient depleted mantle sources trace element and oxygen isotope data however indicate enriched sources for such tonalites these constraints can be explained if the mantle is fractionated into depleted dmr and enriched emr reservoirs during early degassing of the earth 4 4 ga blocks of emr are recycled back into the mantle and the emr dmr mixtures serve as sources for archean basalts these basalts are enriched in incompatible elements by deep sea alteration metasomatism in sinks or by other mantle processes and then undergo wet partial melting to produce tonalite magmas enrichment must occur not more than 150 ma prior to tonalite production basalt plateaus accumulate at sinks and as cooling continues geotherms at sinks pass into the eclogite stability field dense eclogite roots form on the basalt plateaus which pulls them into the mantle between 2 7 and 3 0 ga ago resulting in rapid tonalite production and thus in rapid continental growth

tectonic evolution of the northern volcanic zone abitibi belt quebec the archean abitibi subprovince has been divided formally into a northern volcanic zone nvz including the entire northern part of the subprovince and a southern volcanic zone svz on the basis of distinct volcano sedimentary successions related plutonic suites and precise u pb age determinations the nvz has been further formally subdivided into i a monocyclic volcanic segment mvs composed of an extensive subaqueous basalt plain with scattered felsic volcanic complexes 2730 2725 ma interstratified with or overlain by linear volcaniclastic sedimentary basins and ii a polycyclic volcanic segment pvs comprising a second mafic felsic volcanic cycle 2722 2711 ma and a sedimentary assemblage with local shoshonitic volcanic rocks a sequence of deformational events d1 d6 over a period of 25 ma in the nvz is consistent with a major compressional event north south shortening was first accommodated by near vertical east trending folds and with continued deformation was concentrated along ma

impact of coating development on the hydraulic and transport properties in argillaceous limestone fracture 1 results are reported for an acidic water flow through experiment conducted in a fractured argillaceous limestone sample 73 carbonates the change in fracture geometry and related parameters is reported for six data sets obtained from synchrotron x ray microtomography experiments high resolution three dimensional images of the sample allowed quantification of the changes in fracture morphology at a spatial resolution of 6 m mineral mass loss and permeability changes in the sample were also determined several physico chemical phenomena were identified during the experiment initial smooth fracture surfaces evolved rapidly toward rough surfaces with uneven clay coverage due to the preferential dissolution of carbonate minerals compared to clay minerals whose dissolution rate is about 106 slower a microporous clay coating progressively developed at the fluid rock interface during heterogeneous dissolution of the fracture while the global dissolution rate of the fracture walls exponentially decreased the increase in surface roughness and the presumed reorganization of clays caused a progressive reduction in permeability during the last flow through stage a large decrease in sample permeability was attributed to the large removal of clay particles this process was responsible for a dramatic collapse of the fracture walls near the sample inlet and led to the development of preferential flow pathways the development of the clay coating also acted as a barrier to flow and mass transfer between calcite grains and bulk solution and affected transport processes within the fracture

reply to comment by p olivier on thermal control on the modes of crustal thinning leading to mantle exhumation insight from the cretaceous pyrenean hot paleomargins we thank philippe olivier for the great attention he paid in reading our work and for the discussion he initiated p olivier has been alarmed by our proposition that some pyrenean granulites might have been re granulitized during the cretaceous rifting event in a rigorous scientific approach this hypothesis has to be envisioned however in the paper we proposed two alternative scenarios one involving a cretaceous re granulitization and another one implying a succession of metamorphic events with a first paleozoic high temperature ht metamorphism event responsible for the granulitization followed by a second ht metamorphism event during the cretaceous our main point here was to put in relation the strikingly similar fields of metamorphisms responsible for the granulitization of the crystalline basement and for the ht metamorphism of the prerift and synrift mesozoic sedimentary cover in other words with temperatures up to 600 c in the metasedimentary cover we would expect temperatures at least as high as 600 c in the basement if not it would mean that the continental crustal basement was already withdrawn from the metamorphic domain at that point this scenario corresponds in fact to the concept of lateral extraction of the continental crust that we retained from our field constraints and that we defend here

an experimental study of co2 brine rock interaction at in situ pressure temperature reservoir conditions abstract a detailed investigation of co 2 brine rock interactions through a core flooding laboratory experiment was carried out under simulated reservoir conditions 100 c and 24 mpa changes in the ionic chemistry of the outlet solution combined with core scanning electron microscopy sem and bulk rock x ray diffraction xrd analysis of the core pre and post experiment reveal new insights into co 2 brine rock interactions minerals such as potassium k feldspar albite calcite and ankerite are variably dissolved after the experiments calcite is the mineral most affected by dissolution followed by ankerite whereas dissolution of feldspar minerals is minimal small amounts of kaolinite and solid phases were generated as a result of the experiment the solid phases are mainly comprised of c o na cl al and si and are presumed to be the transitional products in the formation of carbonate minerals the very low fluid penetration rate in the experiment resulted in significantly reduced contact area between the acidic co 2 fluid and minerals in the cores resulting in low rates of feldspar dissolution core permeability decreased substantially throughout the experiment although core porosity remained unchanged the permeability reduction is the result of precipitation of new mineral phases e g kaolinite and solid phases and potentially also the presence of clay particles released by the dissolution of carbonate cement which have then been transported in the fluid flow path and accumulated at pore throats the results provide new insights into co 2 trapping mechanisms in depleted oil and gas reservoirs and into the potential formation damage that may result from massive injections of co 2 into reservoirs during enhanced oil recovery programs

seismic reflection profiles across deep continental crust exposed in the kapuskasing uplift structure seismic imaging of precambrian cratons holds the promise of deep structural mapping and interpretation of fundamental crustal construction processes deep structures may be identified by tracing reflections to the surface1 particularly in exposed crustal cross sections2 elevated along deeply penetrating faults but many such cross sections have structural orientations too steep for successful reflection imaging like the ivrea zone3 or occur in plate boundary settings not representative of continental interiors by contrast the archaean greenstone terrain exposed in the kapus kasing uplift of canada4 5 has gently dipping structures and an intracratonic setting within the superior province thus providing an opportunity to examine the third dimension of continental crust down to 25 30 km palaeodepth6 here we present new seismic reflection data which although generally supportive of structural models based on surface geology4 gravity modeling5 and seismic refraction studies7 8 also indicate that the kapuskasing structure is a relatively thin thrust sheet containing seismically reflective sequences of high grade rock which may be analogues of seismi cally reflective in situ lower crust

accretion of japanese island arcs and implications for the origin of archean greenstone belts the present day region of arc arc collision between the izu bonin arc and mainland japan honshu arc in the western pacific called the izu collision zone or icz provides a useful kinematic model for the development of many archean greenstone belts the compositional assemblage thermal structure and structural style of the crust in the icz are very similar to those inferred for many archean greenstone belts in both cases thick young and flaky arc crust with a high heat flux is in collision with another crustal block the icz has resulted in the imbrication of very thick slices of crust and associated sediments juxtaposed against older accretionary prisms many archean greenstone belts show similar geological features and we suggest that the incremental imbrication of the crust was a common tectonic style in some archean convergent margins

thermotectonic evolution of the north pyrenean agly massif during early cretaceous hyperextension using multi mineral u pb thermochronometry

u pb zircon geochronology in the southwestern abitibi greenstone belt superior province the abitibi belt is the largest continuous greenstone belt in the superior province of the canadian shield it comprises several composite komatiitic tholeiitic calc alkalic and sedimentary sequences that are folded transected by major faults and intruded by various generations of plutonic rocks precise u pb geochronology has been carried out in the belt for the past decade to solve chronostratigraphic and metallogenetic problems this paper presents new zircon ages and reassesses previously published ones now refined by the addition of abraded and concordant zircon analyses volcanic and subvolcanic units of the timmins area yield the following ages 2727 1 5 ma for a tuff of the upper deloro formation 2703 1 5 ma for a tuff of the upper tisdale formation and 2698 4 ma for the krist fragmental assigned to the top of the tisdale formation the age of a dunite intrusive into the upper deloro formation is revised at 2707 3 ma whereas the aquarius diorite east of timmins yields a poorly define

timing and characteristics of the archean subaqueous blake river megacaldera complex abitibi greenstone belt canada abstract the archean blake river group of the abitibi greenstone belt represents a megacaldera complex which evolved over 8 11 m y from approximately 2704 to 2696 ma the early misema caldera developed from a series of amalgamated shield volcano complexes identified by the remnants of mafic dyke and sill systems these remnants are found in the jevis clericy montsabrais renault clifford tannahill and colnet regions where summit calderas are delineated by circular ring dyke structures the secondary 330 trending new senator caldera formed within the envelope of the misema caldera and exhibits a box work graben type structure finally the felsic dominated 070 striking noranda caldera well known for its vms endowment represents the final collapse of the megacaldera complex deformation has overprinted the calderas but structural patterns can be used in order to reconstruct the pre existing volcanic architecture structures such as mafic ring dyke complexes and rhyolitic dome flow complexes have nucleated fold geometries and synvolcanic fractures were reactivated within zones of ductile deformation precise u pb geochronological analyses were conducted in selected areas with specific emphasis on the misema and new senator calderas felsic volcanism progressed throughout the evolution of the megacaldera complex and intermediate to felsic units have been dated to limit this evolution the misema caldera formed between 2704 and 2702 ma via the amalgamation of shield volcanoes that were probably active prior to 2704 ma the new senator caldera was generated between 2702 and 2700 ma during paroxysmal felsic volcanism followed by the collapse of the noranda caldera culminating between 2700 2696 ma the misema caldera was generated by a gravitational stress field consistent with the formation of ring and radial dyke architecture whereas the se trending new senator caldera is more compatible with a sw trending principal compression direction related to oblique convergence in the abitibi belt the noranda caldera is interpreted as a ne rift structure that formed in the final stages of megacaldera evolution