structural analysis of a transform fault rift zone junction in north iceland abstract on the north coast of iceland the rift zone in north iceland is shifted about 120 km to the west where it meets with and joins the mid ocean kolbeinsey ridge this shift occurs along the tjornes fracture zone an 80 km wide zone of high seismicity which is an oblique non perpendicular transform fault there are two main seismic lineaments within the tjornes fracture zone one of which continues on land as a 25 km long wnw trending strike slip fault this fault referred to as the husavik fault meets with and joins north trending normal faults of the theistareykir fissure swarm in the axial rift zone the most clear cut of these junctions occurs in a basaltic pahoehoe lava flow of holocene age where the husavik fault joins a large normal fault called gudfinnugja at this junction the husavik fault strikes n55 w whereas gudfinnugja strikes n5 e so that they meet at an angle of 60 the direction of the spreading vector in north iceland is about n73 w which is neither parallel with the strike of the husavik fault nor perpendicular to the strike of the gudfinnugja fault during rifting episodes there is thus a slight opening on the husavik fault as well as a considerable dextral strike slip movement along the gudfinnugja fault consequently in the holocene lava flow there are tension fractures collapse structures and pressure ridges along the husavik fault and pressure ridges and dextral pull apart structures subparallel with the gudfinnugja fault the 60 angle between the husavik strike slip fault and the gudfinnugja normal fault is the same as the angle between the tjornes fracture zone transform fault and the adjacent axial rift zones of north iceland and the kolbeinsey ridge the junction between the faults of husavik and gudfinnugja may thus be viewed as a smaller scale analogy to the junction between this transform fault and the nearby ridge segments using the results of photoelastic and finite element studies a model is provided for the tectonic development of these junctions the model is based on an analogy between two offset cuts mode i fractures loaded in tension and segments of the axial rift zones or parts thereof in the case of the husavik fault the results indicate that the tjornes fracture zone in general and the husavik fault in particular developed along zones of maximum shear stress furthermore the model suggests that as the ridge segments propagate towards a zero underlapping configuration the angle between them and the associated major strike slip faults gradually increases this conclusion is supported by the trends of the main seismic lineaments of the tjornes fracture zone

determination of residual strength parameters of jointed rock masses using the gsi system abstract the geological strength index gsi system proposed in 1995 is now widely used for the estimation of the rock mass strength and the rock mass deformation parameters the gsi system concentrates on the description of two factors rock structure and block surface conditions the guidelines given by the gsi system are for the estimation of the peak strength parameters of jointed rock masses there are no guidelines given by the gsi or by any other system for the estimation of the rock mass residual strength that yield consistent results in this paper a method is proposed to extend the gsi system for the estimation of a rock mass s residual strength it is proposed to adjust the peak gsi to the residual gsir value based on the two major controlling factors in the gsi system the residual block volume v b r and the residual joint condition factor j c r methods to estimate the residual block volume and joint condition factor are presented the proposed method for the estimation of rock mass s residual strength is validated using in situ block shear test data from three large scale cavern construction sites and data from a back analysis of rock slopes the estimated residual strengths calculated using the reduced residual gsir value are found to be in good agreement with field test or back analyzed data

stress measurements in soft rocks the objective of this paper is to demonstrate that accurate and reliable in situ stress measurements can be performed in soft rocks a quick overview of the mechanical behaviour of soft rocks is presented after reviewing stress measurement techniques that have been used in soft rocks the modified doorstopper technique is presented and the advantages it has over other techniques in soft rocks are underlined results from laboratory simulations in controlled conditions show that the technique is reliable and accurate it is then shown through field applications in a potash mine in brazil in an underwater tunnel in shales in canada and in an exploratory drift in molassic rocks of the french alps how the technique yields results that can sometimes be confirmed by field observations

rift jump process in northern iceland since 10 ma from 40ar 39ar geochronology due to the westward american eurasian plate boundary migration the rift zone location in northern iceland imposed by the stable icelandic hotspot has jumped eastward the present day north volcanic zone of iceland is thus shifted about 100 km to the east with respect to the kolbeinsey ridge a miocene paleo rift location was proposed along the hunafloi skagi axis unconformities inside the lava pile on both sides of the present day rift as a result of the new rift formation have been used to date the rift jump but timing is still controversial whether extinction of the paleo rift was synchronous with initiation of the new rift or the two rifts were active during some time period is unknown using the 40ar 39ar radiochronology method 37 dykes were dated along a 350 km profile parallel to the divergent plate motion direction i e ese wnw crossing the present rift the unconformities and the inferred paleo rift our results reveal that the paleo rift axis was not located along the hunafloi skagi axis but 60 km to the east this previously unknown paleo rift is named the skagafjordur paleo rift it remained active until approximately 3 ma the activity of the present north volcanic zone started about 8 8 5 ma intruding 9 9 5 myr rocks located at least 10 km east of the skagafjordur paleo rift axis consequently the two rifts were simultaneously active for 5 5 5 myr during this time period their accretion rates were nearly equal but with asymmetrical opening the largest rates occurring on external flanks these results are coherent in the icelandic context and permit a first reconstruction of the evolution of northern iceland since 10 ma

interaction and linkage of extension fractures and normal faults examples from the rift zone of iceland abstract field and photogeological studies were made of 90 zones of interacting fracture segments along the rift zone of iceland each zone consists of a pair of extension fractures or a pair of normal faults with lengths from metres to kilometres these zones evolve from an underlapping stage through an overlapping stage the most common configuration and finally to a linkage stage of all the zones only 7 are underlapping whereas 93 are overlapping with hook shaped fracture pairs the length width ratios of the overlapping zones have a mean value of 3 5 the preferred geometry of the overlapping zones depends upon the initial configuration of the interacting fractures length overstep and the development of the interaction in the overlapping zones most fracture pairs show moderate shear strike slip components related to local variations in the extension opening directions vertical displacements on normal faults decrease as the overstep and length of overlapping zones increase both in turn are proportional to the total lengths of the faults forming the pair the geometrical and kinematic features of overlapping spreading centres at mid ocean ridges show close similarities to those reported here these similarities indicate that the architecture and evolution of overlapping zones are scale independent

microbial crusts as indicators of stratigraphic diastems in the cambrian breche a micmacca atlas mountains of morocco the presence of microbial crusts interrupting high energy clastic sediments in carbonate and mixed carbonate siliciclastic platforms can be considered as distinct indicators of stratigraphic diastems characterized by either very low rates of deposition or non deposition this analysis is based on a microfacies study of the cambrian breche a micmacca atlas mountains morocco which contains some stratigraphically condensed limestones characterized by i disarticulation and fragmentation of skeletons ii abundance of volcanigenic debris iii rarity of muddy fraction iv dominance of a sessile epibenthic shelly fauna dominated by echinoderms chancelloriids and sponge spicules and v a complex diagenetic history due to erosion and reworking of lithified substrates the breche a micmacca limestones are composite event concentration low relief shoal complexes composed of parautochthonous and allochthonous bioclastic assemblages these units can be desribed as hiatal accumulations because of the subdivision of the coquinas by minor discontinuities or hiatal surfaces and the composite multiple event nature of the grainstones packstones the growth of microbial mats can be used as a record of time interrupting background sedimentation patterns characterized by the amalgamation of high energy events the breche a micmacca was deposited in an environment in which some intervals of non deposition or under extremely low sedimentation rates alternated with the succession of sedimentary events the biological response of microbial communities to coat stratigraphic discontinuities can be considered as an integral part of event stratigraphy they enhance the preservation and identification potential of interruption in the background sedimentation on substrates devoid of burrowing activity a distinct event of non deposition is also recognized based on the presence of microbial fabrics at the top of the breche a micmacca its microkarst structures reflect a major fall in the relative sea level of a reduced region of the platform resulting in dissolution and colonization of coelobiontic cavities by microbial mats and branching filaments

transform deformation of tertiary rocks along the tjornes fracture zone north central iceland significant fracture zone deformation has been imparted to tertiary lavas on the peninsula flateyjarskagi located between eyjafjordur and skjalfandi in north central iceland the region lies immediately south of the flatey fault one of a series of wnw trending left stepping faults comprising the right lateral oblique slip husavik flatey fault system hffs the hffs defines the present southern margin of the 70 to 80 km wide tjornes fracture zone tfz lava bedding dikes and faults in the western half of flateyjarskagi display a progressive 0 110 clockwise change of trend and steepening of lava dip angles to 15 45 over an 11 km wide zone nearest the flatey fault accompanying this structural curvature is an increase in fault and dike frequency and amygdule vein mineralization northeast striking faults develop predominant downthrows to the east approaching the tfz accommodating the steep lava dips via repetition of stratigraphic section and normal fault bounded block rotations although the possibility cannot yet be conclusively ruled out that structural trends formed in curved orientations the simplest interpretation suggests that the lava pile was tectonically rotated by heterogeneous simple shear maximum on land shear strain has been crudely estimated at 3 5 estimates of shear displacement suggest that the tfz was active for at least 2 m y sometime prior to 6 7 ma at a strain rate of approximately 5 5 10 14 s 1 antithetic faulting and associated flexural bending may have been the means by which a great part of the proposed simple shear rotations were accomplished wnw extension produced by rotational normal faulting is interpreted to have accommodated systematic variations in shear strain parallel to the direction of shear paleomagnetic and structural evidence suggest that rocks of tertiary age on tjornes peninsula immediately to the east also have been tectonically rotated implying an original shear zone width of at least 25 km

deformation modulus of heavily jointed sheared and blocky greywackes by pressuremeter tests numerical experimental and empirical assessments deformability of rock masses influencing their behavior is an important geomechanical property for the design of rock structures due to the difficulties in determining the deformability of jointed rock masses at the laboratory scale various in situ test methods such as pressuremeter dilatometer plate loading tests etc have been developed although these techniques are currently the best and direct methods they are time consuming and expensive and present operational difficulties in addition the influence of the test volume on deformation modulus depending on the method employed is also important for these reasons empirical equations to indirectly estimate the deformation modulus have also been recommended by several investigators as an alternative approach in this study the geomechanical quality of weak heavily jointed sheared and or blocky greywacke rock masses on which very concentrated civil works are continuing at the southern and southwestern parts of ankara turkey was assessed the deformation modulus was determined by pressuremeter tests the possible effects of variables on the derived deformation modulus from the pressuremeter test were evaluated by numerical methods and the comparisons between the deformation modulus of the greywackes obtained from the pressuremeter tests and their geomechanical quality gsi and rmr were made numerical simulations revealed that the presence of a disturbed annulus around the borehole causes underestimation of the deformation modulus while the effect of length to diameter ratio of the pressuremeter probe on the deformation modulus is minor based on the geo engineering characterization assessments mainly two greywacke rock masses with different geomechanical qualities were identified geotechnical quality of one of these rock masses was verified by the back analysis of two slope failures the empirical equations to indirectly estimate the deformation modulus of the greywackes using their gsi and rmr values yielded high coefficients of correlation

dynamics of volcanic systems in iceland example of tectonism and volcanism at juxtaposed hot spot and mid ocean ridge systems volcanic systems are swarms of tectonic fractures and basalt volcanoes formed as a result of plate pull as the plates are pulled apart associated with the mid ocean ridges and the magma dynamics of the iceland mantle plume most systems are 40 150 km long 5 20 km wide and develop a central volcano they supply magma to all eruptions in iceland data obtained in the last few years have greatly improved our knowledge of their volcanotectonic environment as a result the geometry of the plume is better constrained and the crust previously considered thin 10 km is now modeled as thick 20 40 km depending on the location of the volcanic systems their activity either decreases or increases faulting in the two main seismic zones from this we can infer that emplacement of the feeder dike to the largest historical eruption in iceland that of laki in 1783 increased shear stress in the south iceland seismic zone and almost certainly triggered the largest m 7 1 in 1784 historical earthquake in ic

evolution and environmental conditions of conophyton jacutophyton associations in the atar dolomite upper proterozoic mauritania within the upper proterozoic sedimentary cover of the west african craton well exposed in the atar area mauritania the carbonate rich atar formation exhibits a peculiar development of stromatolites this formation is subdivided into 6 units based on lithology and stromatolite content units 1 3 and 5 are mainly terrigenous shales and silts and contain carbonate layers of different facies laminated and structureless micrite microbially laminated and detrital carbonates stromatolitic layers the environmental conditions were characterized by low energy sporadically increasing and shallow water with rare desiccation features units 2 and 4 contain conophyton jacutophyton and tilemsina unit 6 has a succession of conophyton jacqueti to baicalia these carbonate units were deposited under water in low energy conditions without any allochthonous sediment supply the rhythmic alternation of terrigenous and carbonate units corresponds to rhythmic variations in environmental conditions related to sea level or climatic changes the morphological diversity of stromatolites is not related systematically to changes in the environment some of the morphological variations are controlled by the evolution of the biological community e g the evolution of conophyton shape with time and its eventual disappearance new forms of stromatolite are described baicalia safia conophyton jacqueti jaculophyton sahariensis and tilemsina agdeireia

neoproterozoic reef microstructures from the little dal group northwestern canada basinal facies in the neoproterozoic little dal group contain large reefs up to 300 m in height and 8 km in diameter in contrast to the stromatolitic composition typical of earlier proterozoic buildups little dal reefs consist mainly of three different calcareous microfossils a tubule thread microfossil analogous to the calcimicrobe girvanella in paleozoic reefs formed laminar reticulate structures like those in ordovician bioherms a clotted to saccate microfossil comparable morphologically to paleozoic renalcis constructed botryoidal to encrusting masses an enigmatic component of multicellular appearance that has attributes of an early calcified metaphyte exhibits the encrusting sheetlike habit characteristic of middle to late paleozoic algae these reefs represent an intermediate stage between proterozoic stromatolitic buildups and paleozoic metazoan calcimicrobial reefs and biogenic mounds

hydrofracturing stress measurements in the iceland research drilling project drill hole at reydarfjordur iceland two independent suites of hydrofracturing stress measurements were conducted in the top 600 m of the iceland research drilling project deep hole at reydarfjordur east iceland as indicated by the continuously extracted drill core the tested section consists of tertiary subaerial tholeiitic lava flows cut by many basaltic dikes the density of the basalt was used to estimate the vertical stress v in megapascals at depth d in meters v 0 027d in the 33 to 270 m depth range the horizontal principal stresses hmax and hmin as calculated from hydrofracturing results increase gradually with depth but at a lower rate than the vertical stress hmin 2 1 0 016d hmax 3 3 0 016d in the 300 to 550 m depth range a major perturbation occurs with both horizontal stress magnitudes rapidly rising between 300 and 400 m and then uniformly decreasing between 400 and 550 m to a level apparently predictable by the relationships obtained in the top 270 m conditions favoring normal faulting v hmax hmin were established in the 540 to 580 m range and are conjectured to persist at greater depths since the vertical stress gradient is higher than that of hmax and hmin this would be in accord with the predominant fault type in east iceland the relative low horizontal stress values and the small difference between them are supportive of ocean floor spreading theories and contradict near surface high compressive stresses previously determined by hast 1973 in east iceland the large increase in horizontal stress in the range of 300 to 500 m depth is not understood a correlation appears to exist between the zone of almost unintruded thick lava flows 200 500 m and that of high stresses an intuitive model suggesting locked in stresses in this zone is proposed the hydrofracturing determined overall mean direction of hmax is n50 e while a more selective sampling of results yields n40 e the measured stress directions appear to be reasonably compatible with the local structure such as dike direction nne and lava dip wsw significantly hmax direction is subparallel to the n20 e trend of the nearest fissure swarms in the axial rift zone this almost coincidence of directions like the measured stress magnitudes is also in agreement with the model of spreading lithospheric plates being pulled apart

crustal spreading due to dikes and faults in southwest iceland abstract a 30 km long profile extending from 3 million year old pleistocene rocks to the holocene rocks of the rift zone in southwest iceland is dissected by 443 dikes and 156 normal faults nearly all the dikes are basaltic with an average strike of n26 e which is subparallel to the trend of fractures in the nearby rift zone their average dip is 69 and average thickness 1 37 m both values much lower than averages for regional tertiary dike swarms in iceland many dikes are seen to change into sills the great majority of the faults are pure dip slip with an average throw of 10 2 m their average strike n37 e is different from the average strike of the dikes in this area and the fractures in the nearby rift zone their average dip is 75 2 which is somewhat steeper than in the tertiary areas but the dips are less in the interbedded sedimentary layers than in the lava flows growth faults are very rare in the area and most of the faults appear to have completed their growth in less than 10 000 years in subsections crustal dilation due to dikes is 0 4 3 7 that due to faults is 0 6 5 3 and that due to both dikes and faults is 1 04 6 44 in these subsections average fault spacing is one fault per 170 563 m spacing decreases toward the active rift zone it is concluded that in typical fissure swarms of the rift zone the dilation at the surface is almost entirely due to extensional fractures and gaping vertical normal faults in the depth range of a couple of hundred metres to perhaps 1 km the non vertical faults contribute most to the dilation at deeper crustal levels however dilation due to dikes becomes dominant

new information on the probable isotopic age of the late proterozoic glaciation in west africa abstract new rb sr age determinations on smectitic clay fractions from a formation of the late proterozoic glaciation in west africa constrain the occurrence of that major event to between 630 and 595 ma

the iceland plate boundary zone propagating rifts migrating transforms and rift parallel strike slip faults unlike most of the mid atlantic ridge the north america eurasia plate boundary in iceland lies above sea level where magmatic and tectonic processes can be directly investigated in subaerial exposures accordingly geologic processes in iceland have long been recognized as possible analogs for seafloor spreading in the submerged parts of the mid ocean ridge system combining existing and new data from across iceland provides an integrated view of this active mostly subaerial plate boundary the broad iceland plate boundary zone includes segmented rift zones linked by transform fault zones rift propagation and transform fault migration away from the iceland hotspot rearrange the plate boundary configuration resulting in widespread deformation of older crust and reactivation of spreading related structures rift propagation results in block rotations that are accommodated by widespread rift parallel strike slip faulting the geometry and kinematics of faulting in iceland may have implications for spreading processes elsewhere on the mid ocean ridge system where rift propagation and transform migration occur

crustal extension from exposed sheet intrusions review and method proposal abstract sheet intrusions are probably the most widespread magmatic features on earth they create a space for themselves by dilating the host rock the exposed sheet intrusions allow an estimate of a minimum value of the extension to which the crustal layers and the volcanic edifices are subject the correct evaluation of the contribution of sheet intrusions to the crustal extension is of great importance in various geologic settings the current methods used to evaluate the extension due to exposed sheet intrusions are reviewed in this paper the effect of the dip of sheet intrusions on the evaluation of horizontal crustal extension is analysed if the dip effect is neglected and the intrusions are sub vertical the error due to the dip effect in the extension evaluation is small if the intrusions are shallow dipping the error may be great and varies as a function of the dyke dip for dykes with dip range 45 65 the error due to neglecting the dip effect is 30 to 15 increasing discrepancies of more than 25 are produced on the computed dilation when the intrusions progressively dip less than 50 a simple method based on structural field measurements of exposed sheet intrusions and on trigonometric considerations is presented as an improvement of pre existing methods to estimate the horizontal component of the crustal extension using this method field data of 1154 dykes from different geodynamic contexts are analysed the horizontal component of extension is computed using dyke data from the tertiary massifs of lanzarote and tenerife and from the active stratovolcanoes of monte somma vesuvius and etna since the analysed dykes are steeply dipping the cumulative difference between the extension computed neglecting and taking into account the dip effect remains less than 10 for all the data sets the percentage difference is nearly constant and cumulatively higher for monte somma where the mean dip of the intrusions is smaller in lanzarote 237 dykes emplaced between 15 5 and 3 7 ma give a peak extension of 276 m in direction n135 and a time averaged minimum extension rate of 1 9 10 2 to 2 3 10 2 mm a 1 in tenerife an extension of 562 m is computed in the direction n65 from 563 dykes emplaced from 7 4 to 3 3 ma the time averaged minimum rate of extension for the tertiary massifs of tenerife ranges from 0 14 to 0 25 mm a 1 the peak extension computed for monte somma is 81 7 m in the direction n90 based on 96 exposed sheets very likely most of monte somma sheets intruded within 12 ka giving a time averaged minimum extension rate of 7 mm a 1 from 184 exposed dykes of etna a minimum extension rate of 6 2 mm a 1 is computed peak extension is 212 m in direction n35 the poor resolution of age constraints may affect deeply the reliability of the extension rates however from the examples presented it appears that the extension rates of pre erosional successions from intraplate volcanic islands can be smaller than those of active stratovolcanoes this may suggest a more active dynamics in the stratovolcanoes

characterization of intensely jointed rock masses by means of in situ penetration tests abstract the task of characterizing a highly fractured rock mass correctly is often complex due to errors produced by the scale effect in the majority of laboratory tests furthermore current methods of in situ characterization pose logistical technical or even economic problems while geomechanical classifications are not precise enough for this kind of ground this makes the stability analysis of jointed rock mass slopes a complex undertaking that is generally far from reliable this paper proposes a new method of in situ analysis to characterize this type of highly fractured rock mass accurately applied here to the case of the large scale slopes of an open cast coal mine the method is based on carrying out a novel in situ penetration test to obtain the strength properties of the ground as a whole the results are calibrated by means of numeric simulation further back analysis of the current slopes in the mine was carried out using classical methods to test the reliability of the results obtained using this technique

is cratonic sedimentation consistent with available models an example from the upper proterozoic of the west african craton abstract the atar group part of the upper proterozoic sequence covering the west african craton stable since 2000 ma is characterized by an alternation of extensive carbonate beds and mixed siliciclastic and carbonate facies the carbonate beds comprise essentially columnar stromatolite biostromes and bioherms which reflect sublittoral environments the mixed facies contain a variety of laterally discontinuous facies which imply more variable environmental conditions the settings of the mixed facies are not always clear but they do not contain thick sequences of high energy facies few obvious facies sequences are discernable those that are present are considered to be punctuated aggradational cycles pacs and they always start with biostromes of columnar stromatolites with very few sediments composite sequences are interpreted as due to shallowing upward or increasing energy environments that may be laterally contiguous despite the fact that the contacts are not gradational however much of the stratigraphic sequence cannot be subdivided into cycles and seems to consist of unrelated individual facies bound by sharp boundaries the basin analysis reveals that biostromes of columnar stromatolites start after an instantaneous geological event corresponding to a sea level rise consequently their appearance can be considered as a time line we describe in the atar group and its equivalents three sedimentation trends all of which are interpreted to be of shallowing upward character the atar group appears to have been deposited in an epeiric sea i e an extremely flat ramp there are two contrasting styles of sedimentation 1 after the submergence of the whole area columnar stromatolites built extensive biostromes 2 during the stable phase sediments are deposited in a mosaic of laterally discontinuous facies tidal influence cannot be recognized in the sequence neither can a salinity increase toward the land both common features in published epeiric sea models a cratonic sedimentation area such as this is characterized by its size and flatness only during the stable phase of the cycle does small scale topographic relief lead to deposition of a mosaic of facies the sedimentation is storm and wave dominated

formation of dykes feeder dykes and the intrusion of dykes from magma chambers the paper discussed the formation of dykes and applies the results to iceland it is postulated that dykes follow the pathway of least work and of least tensile strength and thereby intrude the subvertical joints in lava flows it is suggested that dykes form in magma pulses where each dyke is split in two by the next magma pulse and so on in iceland the estimated time between successive magma pulses is of the order of several hundred days statistical considerations indicate that in iceland the probability of seeing dykes end upwards is only 1 2 which agrees with observations it is concluded that many and probably the majority of dykes are non feeders this together with the low probability of finding the connection between feeder and lava flow explains the scarcity of observed feeder dykes it is concluded that overpressure in shallow magma chambers needed to drive magma through crustal fractures in iceland is usually smaller than the tensile strength of the host rock several mpa which thereby is the critical factor in dyke intrusion

fe mn encrusted kamenitza and associated features in the jurassic of monte kumeta sicily subaerial and or submarine dissolution abstract an unusually jagged dissolution surface capped by a thick fe mn crust is well exposed in small quarry cuts of the jurassic of monte kumeta it was formed on a crinoidal limestone substrate of pliensbachian age and is covered by upper bajocian ammonitico rosso type sediments all cross cut by several generations of neptunian dykes this peculiar surface is more or less coeval with hardgrounds fe mn capped dissolution surfaces and associated neptunian dykes described from other localities of the western tethys and currently subject to fierce debates as to their purely submarine or perhaps partly subaerial origin the major goal of this paper is to add new arguments to this debate by revealing the finest details of field relationships at a site particularly adapted to the study of this phenomenon field observations are supported by petrography and to a lesser extent by geochemistry results are as follows i vertical dissolution grooves pointing to dissolution by gravitationally controlled waters were detected on the sides of several micro topographic highs ii extensive intergranular dissolution predating the formation of the fe mn crust was proved in the substrate both on the micro and meso scale iii intense micro bio erosion and local phosphate enrichment were detected immediately underneath the crust iv a toarcian fauna was identified from the hollows of the irregular surface v synsedimentary faults and fractures clearly predating the major fe mn encrusted surface were observed and vi a meso scale synsedimentary growth structure post dating the fe mn crust which controlled the liassic depositional environment of monte kumeta is documented our conclusion is that the studied surface records at least three separate events of dissolution and precipitation sedimentation each having either erased or overprinted the effects of the previous one and therefore not permitting the exact reconstruction of all the details of the complex story to form the irregular surface in addition to a transient phase of subaerial exposure a complex history of bio erosion and submarine dissolution by fluids of widely different chemical composition is proposed to permit the mixing of sea water with fault controlled waters of higher temperature and with groundwaters introduced by deep circulation a scenario of down faulted blocks and an adjoining distant subaerially exposed region is invoked such a region provided the hydraulic drive for the postulated circulation the ultimate cause for the unusual phenomena under scrutiny was the combined effect of tectonics the local manifestation of early liassic rifting in the western tethys and the well known pliensbachian toarcian sea level rise

growth dynamics of neoproterozoic calcimicrobial reefs mackenzie mountains northwest canada abstract neoproterozoic buildups of the little dal group grew in a deep water epicratonic basin these kilometer scale reefs display aggradational and progradational geometries comparable to those described from phanerozoic reefs four phases of reef growth correspond to four regional shale to carbonate packages in laterally equivalent off reef strata the lower shaly part of each package is interpreted to reflect transgressive to highstand deposition the upper carbonate part of each package reflects carbonate precipitation in the water column as a result of postulated basin restriction and increased salinity during sea level lowstand reefs nucleated at the beginning of the first major transgressive event reefs typically aggraded during transgressive intervals but could also prograde or contract likely depending on the rate of relative sea level rise versus reef growth rate reefs prograded during regressions probably owing to reduction of accommodation space they shed talus at lowstand in response to increased erosion and or progradation over unstable substrates reef growth ended with return of shallow water conditions the overall architecture of aggradation and progradation demonstrates that the neoproterozoic calcimicrobial reef ecosystem was capable of responding to environmental changes in the same way as the more extensively studied ecologically complex and faunally diverse buildups of the phanerozoi

extension direction re orientation in the oceanic rift of iceland and comparison with continental rifts abstract in this study we report the results of a structural analysis carried out on fault systems that form the active rift segments of two areas of iceland namely 1 the northern part of the eastern rift segment northern volcanic zone nvz and 2 the western rift segment that include the western volcanic zone thingvellir area and the reykjanes peninsula in the n s trending nvz mainly krafla volcanic area the obtained extension directions trend around ese at thingvellir the extension directions trend ca nw se and are perpendicular to the main ne sw trending faults in the ene wsw trending tectono magmatic segments of the reykjanes peninsula our data show a marked difference with the gps data and spreading directions the extension directions trend roughly orthogonal to the strike of the boundary faults in all the investigated sectors extension shows an overall southward increase in divergence from the regional directions indicated by gps vectors this behavior has been compared with the continental main ethiopian rift of east africa where structural studies show a re orientation of the extension direction between the rift margins and the internal sectors resulting in a divergence between local and regional extension directions aimed at investigating the causes of these differences we performed a new series of analogue models by varying different parameters such as rift obliquity and width our findings suggest that rift width exerts a primary control on the boundary axis variation of the extension direction in particular significant differences in fault orientation and local direction of extension between the margins and the rift axis are observed in wider rift valleys e g continental rift of ethiopia conversely fault orientation and local extension direction are much more homogeneous in narrower rift valleys e g rifts of iceland where a variations in the direction of extension from the margins to the rift axis are not observed

proterozoic microbial reef complexes and associated hydrothermal mineralizations in the banfora cliffs burkina faso abstract the proterozoic guena souroukoundinga formation of the mopti arm gourma aulacogen southerm taoudeni basin consists of a shale dominated succession up to 200 m thick with scattered microbial reef complexes quarry exposures of the tiara reef complex allow reconstruction of a transect across back reef peritidal laminites reef margin and peri reef ooidal shoals and fore reef slope strata microbial carbonate productivity nucleated on isolated palaeohighs during transgression whereas its end was controlled by two tectonically induced drowning pulses that led to the successive record of onlapping kerogenous limestones and pelagic shales reef carbonates are crosscut by fractures and fissures occluded by hydrothermal mineralizations which are related to the rifting activity of the gourma aulacogen the tiara reef complex is similar to other proterozoic reefs in being composed nearly entirely of stromatolites although calcimicrobial filamentous and thromboid textures are locally abundant which contrast with their scarcity or absence in coeval stable platform microbial reefs of the northern taoudeni basin

locking depth and slip rate of the husavik flatey fault north iceland derived from continuous gps data 2006 2010 summary located at the northern shore of iceland the tjornes fracture zone tfz is a 120 km offset in the mid atlantic ridge that connects the offshore kolbeinsey ridge to the on land northern volcanic zone this transform zone is seismically one of the most active areas in iceland exposing the population to a significant risk however the kinematics of the mostly offshore area with its complex tectonics have not been adequately resolved and the seismic potential of the two main transform structures within the tfz the grimsey oblique rift gor and the husavik flatey fault hff in particular is not well known in summer 2006 we expanded the number of continuous gps cgps stations in the area from 4 to 14 the resulting gps velocities after four years of data collection show that the tfz accommodates the full plate motion as it is predicted by the morvel plate motion model in addition envisat interferograms reveal a transient uplift signal at the nearby theistareykir central volcano with a maximum line of sight uplift of 3 cm between summers of 2007 and 2008 we use a combination of an interseismic backslip and a mogi model in a homogeneous elastic half space to describe the kinematics within the tfz with a non linear optimization approach we fit the gps observations and estimate the key model parameters and their uncertainties which are among others the locking depth the partition of the transform motion between the two transform structures within the tfz and the slip rate on the hff we find a shallow locking depth of 6 3 1 7 1 2 km and transform motion that is accommodated 34 3 per cent by the hff and 66 3 per cent by the gor resulting in a slip velocity of 6 6 0 6 mm yr 1 for the hff assuming steady accumulation since the last two large m6 5 earthquakes in 1872 the seismic potential of the fault is equivalent to a mw6 8 0 1 event

microfossil populations in the context of synsedimentary micrite deposition and acicular carbonate precipitation mesoproterozoic gaoyuzhuang formation china abstract silicified stratiform stromatolites of the mesoproterozoic gaoyuzhuang formation 1 4 1 5 ga china contain well preserved microfossils the cherts also harbor varied synsedimentary precipitates and void filling cements replaced by early diagenetic silica minerals these precipitates disclose microenvironments characterized by supersaturated solutions in protected shallow depressions within an intertidal setting the precipitates provided surfaces for selective microbial settlement and rigid sediment matrix for microbial growth together with early silicification the frequent precipitation events contributed to preservation of microfossils by reducing sediment compaction and shearing fossiliferous cherts display fine flat and wavy lamination characterized by an alternation of highly silicified thin organic rich layers with thick sediment rich layers organic rich layers are dominated either by coccoid or by filamentous microfossils whereas sediment rich layers contain abundant synsedimentary precipitates within which the microfossils are preserved in their growth position four dominant microfossils coccostratus dispergens n gen et sp eoentophysalis belcherensis eoschizothrix composita and siphonophycus inornatum occur contiguously through several tens of laminae and are identified as main frame building biological components of gaoyuzhuang stromatolites community composition microbial density distribution orientation and developmental patterns of the frame building microfossils are closely correlated with the changing depositional events of gaoyuzhuang cherts contrasting conditions of sedimentary kinetics with those of sedimentary stasis each assemblage of frame building microfossils responded to sedimentation with different mechanisms to escape burial high sedimentation rates correlate with scattered colonies of coccoids and with loose webs of predominantly upright filaments low sedimentation rates correlate with dense laterally connected colonies of coccoids and with a change in filament orientation from vertical to horizontal in multi trichomous microfossil eoschizothrix composita low sedimentation rates are also accompanied with an increase in number of trichomes per filament the observed morphological variability of the frame building microfossils is explained by microbial development reproduction and behavior by interactions between sedimentological and biological controls

influence of rift jump and excess loading on the structural evolution of northern iceland 1 new structural data combined with published structural and geochronological data allow reconstruction of the structural evolution that followed the last rift jump across northern iceland tertiary lava flows erupted along the skagafjordur paleo rift have been down bent under the weight of and in the direction of plio pleistocene lava flows emitted from the northern volcanic zone and the central part of iceland this down bending process involved development of local flexure zones and a flexural extension along the resulting monoclines this structural reorganization explains the existence of the hunafloi skagi synform without need for a paleo rift axis along it in agreement with previous radiometric dating the large amount of plio pleistocene lava flows erupted in central iceland may have been enhanced by ice cap loading

distributed deformation in an oceanic transform system applying statistical tools to structural and paleomagnetic data near the husavik flatey fault northern iceland

seismic evidence of a ridge parallel strike slip fault off the transform system in the gulf of california 1 a left lateral strike slip fault perpendicular to the major transform fault that occurred at the extended continental crust in the middle of the gulf of california produced a remarkably active main shock and aftershock seismic sequence that began on 1 september 2007 in contrast to observations from oceanic transform fault systems this event induced unusually active seismicity by means of an inversion procedure we obtained the moment tensor solutions of all the events greater than mw 4 4 and we observed multiple types of faulting some very similar to the main event and others that included a tensional cracking mechanism with a significant nondouble couple component static stress change for optimally oriented left lateral faults predicts the seismic activity this event has implications for the californian peninsular geology especially for the late miocene fault systems that were optimally reoriented to promote active faulting

seafloor spreading on the amsterdam st paul hotspot plateau the amsterdam st paul asp platform on the intermediate rate southeast indian ridge seir is the only oceanic hotspot plateau outside the atlantic ocean containing an active mid ocean ridge spreading axis because the asp hotspot is small and remotely located it has been relatively unstudied and the ridge axis location in many places near the asp plateau was previously unknown or ambiguous we mapped the seir out to 1 ma crust jaramillo anomaly both on and near the asp platform we located the spreading center to within a few kilometers based on side scan sonar reflectivity recent off platform magnetic anomalies and lineated abyssal hill topography are consistent with a simple spreading history off platform full spreading rates increase from 63 km myr on segment h to the north of the platform to 65 5 km myr on segment k to the south in contrast inversions of seafloor magnetization based on uniform and variable thickness magnetic source layers reflect a complex on platform tectonic history with ridge jumps off axis volcanism and propagating rifts on one section of the asp plateau the spreading location has stabilized and is beginning to rift the plateau apart generating symmetric magnetic anomalies and lineated topography for the last several hundred thousand years the larger more stable spreading segments of the asp platform are aligned with major volcanic edifices suggesting that along axis magma flow away from plume fed centers is an important influence on spreading geometry many complex tectonic features observed on the asp plateau such as ridge jumps en echelon oblique spreading centers and transforms oblique to the spreading direction are comparable to features observed on iceland the similarities suggest that moderate crustal thickening at an intermediate rate spreading center may have similar effects to pronounced thickening at a slow rate spreading center

deterministic stability and sensitivity analyses of slopes in jointed rock masses abstract the paper describes a deterministic stability analysis together with a sensitivity analysis of slopes in heavily jointed rock mass with planar circular or combined modes of failure this approach is based on the modified sarma method of analysis in which the strength parameters are derived from the rock mass classification rmr system and applied to the hoek and brown rock failure criterion the results of the analysis are presented as a plot of factor of safety f against acceleration k which shows the effect of vibrations blasting earthquake etc and acceleration on the factor of safety of slopes the paper also considers the influence of various strength parameters on the factor of safety and the acceleration k application of this technique is illustrated with the aid of an example of a slope in an iron ore mine

pedogenetic and diagenetic fabrics in the upper proterozoic sarnyere formation gourma mali abstract several diagenetic fabrics are described in the uppermost member of the sarnyere formation an upper proterozoic dolomite 700 600 ma micronodular micrite micro sparite showing nodulisation processes orbicular crusts and oncoids are thought to be due to the pedogenetic alteration of an algal porostromata boundstone further evolution in a vadose zone creates a network of cavities which are later filled by cave stromatolites micropopcorn micrite and multilayered cements a rhythmical repetition of facies evidences a polyphased pedogenetic history leading to a 30 m thick highly complex section such numerous and diversified structures are rarely preserved especially in proterozoic rocks but each of them have equivalents in recent examples of caliche soil profiles

dikes minor faults and mineral veins associated with a transform fault in north iceland abstract the tjornes fracture zone is a transform fault connecting the axial rift zone in north iceland with that of the kolbeinsey ridge off the north coast of iceland on the north coast of the flatcyjarskagi peninsula there is a 3 5 km wide zone of intense deformation which we interpret as the main on land part of this transform fault this transform zone strikes n64 w and is characterized by unusually steeply dipping lavas 25 44 wnw extensive secondary mineralization numerous slickensided surfaces normal faults 1 20 m throw dikes that strike parallel to the zone and occasional 3 20 m wide subzones of crushed rocks striking parallel to the transform fault in a 17 km long continuous coastal profile south of the transform zone there is a 30 clockwise change from south to north in the lava strike the dike strike also changes clockwise north along this profile but only by 4 this indicates that the clockwise change in the lava strike is original and not due to subsequent tectonic rotation when the axial rift zones of north iceland and the kolbeinsey ridge are modelled as single offset mode i cracks finiteelement results show that the tjornes fracture zone developed between these mode i cracks in a region of maximum shear stress the underlap between these axial rift zones has apparently decreased during the past several million years the model predicts that as this underlap decreased the angle between the husavik flatey fault and that of the associated axial rift zones increased and the activity of the husavik flatey fault decreased both these predictions are borne out by the available data

geochronology of sedimentary and metasedimentary precambrian rocks of the west african craton abstract this contribution summarizes current knowledge of the geochronology of the upper proterozoic sedimentary rocks covering the west african craton this was done by using direct dating methods correlations between the northern edge of the tindouf basin and the northern and southern part of the taoudeni basin as well as the volta basin are proposed an important depositional lack over almost the entire craton is confirmed between 1 7 ga which represents the time of the latest eburnean activities or cooling effects and about 1 0 ga which is the start of the general upper proterozoic sedimentation this lack is remarkable as the other cratons from gondwana continent are covered by middle proterozoic sediments the upper proterozoic sedimentation on the west african craton may be divided into a basal mainly sandstone period ca 1000 900 ma a middle mainly limestone period ca 900 750 ma and an upper mainly siltstone period ca 750 650 ma tectonic volcanic and thermal activities in connection with the pan african orogeny are recorded only around the craton they induced either sedimentation lacks in morocco or sedimentation excesses in hoggar unsolved problems such as the precise stratigraphic position of the uppermost proterozoic tillitic episode and the correlation within the moroccan anti atlas are also raised

extensional tectonics in the vicinity of a transform fault in north iceland at the junction between an oceanic transform fault and the adjacent ridge segments there is normally a curved fabric the trollaskagi peninsula is located just south of the junction between the kolbeinsey ridge and the tjornes fracture zone and offers a rare opportunity for studying the three dimensional tectonics of the associated curved fabric for the purpose of elucidating its infrastructure we measured the attitudes of tens of lava flows and several hundred dykes faults and mineral veins in 28 profiles in the northern half of the trollaskagi peninsula at sea level the general dip of the lava pile is 6 14 but in the northernmost part of the peninsula the dip is 10 36 with an average of 22 most dykes are basaltic and strike north or nne they are exceptional as regards thickness frequency of multiple dykes and crustal dilation the average thickness 5 9 m and a crustal dilation of 28 in a 4 5 km long profile are the highest figures reported from regional dyke swarms in iceland most faults are normal faults the curved fabric is limited to the northernmost part of the trollaskagi peninsula and consists of two main sets of fractures one set includes north and nne striking faults mineral veins and dykes the other set consists of nw striking oblique faults mineral veins and tilted lava flows a boundary element model indicates that the abnormal lava dip and the nw striking oblique extensional structures in the northernmost part of the peninsula are attributable to the stress field associated with the junction between the kolbeinsey ridge and the tjornes fracture zone

stress fields associated with oceanic transform faults abstract although their formation is commonly attributed to shear stresses oceanic transform faults contain many structural elements indicating extension across them examples include transform parallel tension fractures normal faults grabens dykes and small scale spreading centres at the junctions between transform faults and adjacent ridge segments there are commonly sets of oblique fractures a detailed field study of oblique fractures in the vicinity of a junction between a mid ocean ridge and an oceanic fracture zone in north iceland shows that they are mostly tension fractures and normal faults this paper presents the results of a boundary element study of the stress field associated with the junction between an oceanic transform fault and its ridge segments the results show that uniaxial tensile loading plate pull parallel with the transform fault tends to lock it but also gives rise to shear stress concentration in a zone that coincides with the fault this shear stress is primarily responsible for the strike slip faulting and associated earthquakes in the transform fault to generate the transform parallel fracture zone graben normal faults tensile fractures dykes and spreading centres however requires tensile loading parallel with the ridge axis the results suggest that biaxial ridge parallel and ridge perpendicular tensile loading generates a stress field that 1 unlocks and makes slip easier on the transform faults and 2 explains the oblique fractures at the ridge transform junctions

effect of tensile stress concentration around magma chambers on intrusion and extrusion frequencies what controls the intrusion and extrusion frequencies associated with volcanoes is still poorly understood i propose that for volcanoes at divergent plate boundaries these frequencies may be largely determined by the tensile stress concentration around the magma chambers that feed them this stress concentration is mainly a function of the applied tensile stress associated with spreading and the aspect height width ratios of the chambers high spreading rates and or aspect ratios lead to high rates of tensile stress concentration around the chambers and to an increase in their intrusion frequencies it is found that for chambers at the same depth in a volcanic zone the one of the highest aspect ratio will normally intrude magma most frequently also if the chambers are of equal aspect ratios the one at the greatest depth will intrude magma most frequently because the extrusion frequency of a volcano is a fraction of its intrusion frequency the extrusion frequency may also be largely determined by the rate of tensile stress concentration around the magma chamber that feeds the volcano these results are applied to the divergent plate boundary in iceland where many of the volcanoes appear to be fed by double chambers that is shallow chambers fed by deep seated chambers it is found that except when the aspect ratio of the deep seated chamber is much less than that of the shallow chamber the intrusion frequency of the shallow chamber is normally largely controlled by that of the deep seated chamber it is concluded that whereas the short term i e 103 yrs extrusion frequencies of volcanoes at the plate boundary in iceland may be similar to the dike intrusion frequencies of the source chambers the long term i e 104 yrs extrusion frequencies may be about ten times lower than the intrusion frequencies

platform to basin facies evolution the carbonates of late proterozoic vendian gourma west africa abstract during the vendian uppermost proterozoic the gourma basin was an east west intracratonic feature that opened eastward into an ocean the southern edge of the gourma basin preserves facies belts representing land to deep basin environments shoreward facies consist of tidal belts of microbially laminated dolomites or porostromata dolomites interstratified with clastic carbonates of local origin landward supratidal facies consist of finely laminated sedimentary rocks with sizeable desiccation features but no evaporites the upper part of this unit is affected by a periodic emersion resulting in a complex succession of paleosol profiles offshore of the tidal facies are lagoon facies rocks that were protected from the open sea by a carbonate buildup composed of a framework of stromat lite bioherms associated with detrital dolomites indicating a high energy environment slope facies consist of a thick succession of ribbon limestones with some graded beds synsedimentary slumping and faulting of the ribbon limestones indicate a significant depositional slope allochthonous deposits are sporadically interstratified with the ribbon limestones and consist of muds with small grains derived from the platform deposits polygenic breccias conglomerates composed only of ribbon limestone intraclasts and oolitic calcarenites shelf facies rocks are pervasively dolomitized but original textures are well preserved including fine grained structures the ribbon limestones on the other hand are not dolomitized except for dolomitic layers caused by pressure solution the distribution of facies in gourma basin carbonates corresponds well with ideal models of wilson 1975 comparisons with present day facies distributions are less satisfactory especially considering the dominance of cyanobacteria over all environments for which there is no modern day counterpart comparisons with the modern environment however suggest a subtropical semi arid climate during deposition of the gourma strata

gravitational spreading of mountain ridges coeval with late weichselian deglaciation impact on glacial landscapes in trollaskagi northern iceland abstract during the late weichselian deglaciation the coastal mountains of northern central iceland have experienced significant paraglacial readjustment processes in the form of conspicuous rock slope failures and deep seated gravitational slope deformation dsgsd local topographic slopes and ridges were deeply reshaped by these large scale paraglacial processes located on the eastern side of skagafjor ur one of the largest fjords of northern iceland the oslandshli arfjoll and hnjukar ridges 65 49n 19 14w exhibit geomorphic evidence of spectacular dsgds several series of dsgsd induced landforms such as crestal graben and troughs initiated by ridge top splitting were investigated over a 30 km 2 area on the basis of geomorphological mapping we recognized i a ridge top splitting event mainly controlled by glacial debuttressing induced by a minimal 300 m lowering of the glacier surface in the deildardalur valley ii a rapid late weichselian deglaciation of the deildardalur valley spanning a few thousands of years ice free stage probably reached around 14 000 years cal bp iii ridge top splitting having an influence on large scale glacial patterns by guiding and facilitating glacial erosion along ridge top grabens resulting in accelerated trough widening based on these interpretations we propose an evolutionary sequence of both the kinematic stages of the dsgsd and the late weichselian deglaciation at the valley scale this work provides new insights into i the patterns of the late weichselian deglaciation in the skagafjor ur area especially in tributary valleys of the fjord ii the timing of large scale paraglacial ridge top deformations in relation to the post lgm deglaciation and iii the influence of paraglacial dsgsd features on large scale glacial erosional patterns

facies distribution of the lower cambrian cryptic microbial and epibenthic archaeocyathan microbial communities western anti atlas morocco marine microbial communities recorded in the moroccan anti atlas were unaffected across the neoproterozoic cambrian transition a stromatolite dominated consortium was replaced at the beginning of the atdabanian ca 20 myr after the neoproterozoic cambrian boundary by shelly metazoan and thromboid consortia which contain the oldest biostratigraphically significant fossils of the moroccan cambrian the associated collapse of microbial mat stromatolitic growth appears to coincide with a change from pre atdabanian shallow water restricted conditions into atdabanian deeper open sea conditions it is postulated that this environmental change led to an episode of improved water circulation over carbonate platform interiors promoting shelly metazoan immigration into the region the tiout amouslek lithostratigraphic contact in the early atdabanian marks the end of an episodically unstable seafloor as suggested by the abundance of slumping and sliding structures and synsedimentary microfaults and cracks recorded in the underlying tiout member concurrent with the transition is the occurrence of a network of cryptic fissures and cavities that provided habitats for a coelobiontic chemosynthetic heterotrophic microbial community composed of stromatolitic crusts renalcis epiphyton girvanella intergrowths and kundatia thalli in the overlying amouslek formation archaeocyathan thromboid reefs were constrained by substrate stability water depth and subsidence rate four reef geometries are distinguished i patch reefs surrounded by shales ii bioherms in which flank beds intercalate laterally with carbonate and shale inter reef sediments iii biostromes or low relief structures formed as a result of lateral accretion of patch reefs and iv kalyptrate complexes that nucleated because of a marked tendency for aggregation and in which patch reefs and bioherms occur stacked together bounded by clay marl silt seams

paleoenvironments and growth of early neoproterozoic calcimicrobial reefs platformal little dal group northwestern canada abstract although the calcimicrobial reefs of the basinal little dal group 780 ma are well described little is known about related shallow water facies to the southeast study of these platformal carbonates shows that they represent inner mid and shallow outer ramp zones of a homoclinal storm influenced ramp seven depositional sequences are recognized and correlated with reef bearing strata in the little dal basin and in the shaler supergroup of victoria island reef distribution is distinctly depth partitioned across the little dal ramp inner and mid ramp carbonates contain meter scale mounds patch reefs and biostromes composed of fine grained stromatolites including tungussia baicalia and jurusania these reefs are strikingly similar to those reported from coeval shallow water strata in the shaler supergroup of victoria island and the bitter springs formation of australia multi storey stromatolitic biostrome bioherm complexes greater than 100 m in height seem to be restricted to the mid ramp zone of the little dal ramp and probably reflect high rates of subsidence in the mackenzie basin calcimicrobial reefs occur only in the deepest water strata of the study area associated with outer ramp sediments these reefs contain calcified cyanobacterial filaments in varying degrees of preservation and are petrographically indistinguishable from the kilometre scale basinal calcimicrobial reefs to the northwest the association of calcimicrobial reefs with deep low energy environments suggests initial establishment of reef building calcimicrobes in deep water benthic environments

neoproterozoic garnet glaucophanites and eclogites new insights for subduction metamorphism of the gourma fold and thrust belt eastern mali

interpretation of satellite magnetic anomalies over the west african craton abstract satellite magnetic anomaly maps of west africa display persistent anomalies that are spatially related to the geological structure of the west african craton the reguibat and man shields to the north and south of the taoudeni basin have positive magnetization contrasts whereas the basin which was disrupted by rifting and volcanism in the late proterozoic and mesozoic has a negative magnetization contrast a forward model of the craton is developed based on the surface geology and including aspects of the evolution of deep seated thermal petrological and magnetization contrasts anomalies calculated from the model correspond fairly well to the most reliable observed anomalies the anomaly associated with the basin is due partly to the anomalies arising from the shields and partly to intra basin deep seated demagnetization resulting from mesozoic thermal and metasomatic demagnetization another similar demagnetized zone probably contributes to the reguibat shield anomaly the man shield anomaly results primarily from the archean segment of this shield the largest magnetization contrast required for a 70 km thick shield slab is 0 3 a m 1