error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

geology geochronology and rift basin development in the central sector of the main ethiopia rift based on stratigraphic relationships and k ar dating of volcanic rocks from both of the escarpments flanking plateaus and from the rift floor of the central sector of the main ethiopian rift six major volcanic episodes are recognized in the rift9s development over a time span from the late oligocene to the quaternary using the k ar data correlation of volcanic units from the six periods of activity throughout the study area forms the basis for establishing six time stratigraphic chronozones for the central sector that are related to volcanism in the ethiopian cenozoic volcanic province the oldest basalt and rhyolite flows exposed along the rift margins of the central sector are time correlative to or older than those in river canyons for example blue nile on the adjacent northwest plateau a thinned mesozoic stratigraphic sequence along the guraghe western rift margin suggests that doming may have preceded volcanism and rifting of the cenozoic by late miocene time at least by 8 3 ma and 9 7 ma the eastern and western faulted margins respectively of the rift had formed at guraghe and at agere selam as indicated by containment of flows of that age within the rift wall during eruption a paroxysm of calc alkaline ignimbrite activity produced voluminous flows nearly fully contained within the rift during the pliocene epoch the munesa crystal tuff 3 5 ma a prominent marker tuff exposed on both rift margins is present at depth in a geothermal well beneath the rift floor and indicates a minimum of 2 km of downthrow in the central sector since its eruption structural and stratigraphic relationships in the central sector indicate a two stage rift development this is characterized by an early phase late oligocene or early miocene of a series of alternating opposed half grabens along the rift with alternating polarity such as that in the present gregory and western rifts of east africa and symmetrical rifts that evolved from these grabens in late miocene or early pliocene time thus evolution from alternating half graben to a full symmetrical graben with a medially located neovolcanic zone that is bifurcated to marginal grabens in the northern part of the study area may be a fundamental part of the rifting process the study indicates that there are major petrologic and tectonic differences between the main ethiopian rift and the gregory kenyan rift

volcanic vents rooted on extension fractures and their geodynamic implications in the ethiopian rift abstract most of the extension fractures located in the ethiopian rift are related to rift dynamics and they have consistently similar orientations for hundreds of kilometers the orientation of extension fracture gives the local extension direction which is perpendicular to its horns end segments of an extension fracture we have established clear geometrical relationships between tectonics and volcanism in the main ethiopian rift by interpreting high resolution images obtained from landsat tm and spot satellites and by quantitatively analyzing the geometry of extension fractures elongated vents and linear volcanic clusters applying the relationships obtained we show that extension fractures in the rift served as channels for magma rising to the surface and that extension fractures underlie most of the elongated volcanic vents and linear volcanic clusters the geometry of extension fractures beneath volcanic edifices can be deduced from the shape of elongated vents and pattern of linear volcanic clusters rooted on them utilizing the orientations of extension fractures directly observed and those inferred from elongated volcanic vents and linear clusters we found that the extension direction of the main ethiopian rift is northwest southeast and that the direction has been rotated clockwise for about 20 in the time interval 2 83 to 0 023 ma the recent axis of rift opening is oriented n40 and located closer to the southeastern escarpment giving an asymmetric geometry to the main ethiopian rift mer

quaternary oblique extensional tectonics in the ethiopian rift horn of africa abstract the ethiopian rift extends in a northeasterly direction from southern ethiopia to the afar region it shows a complex fault pattern characterised by the interplay of a n30 e n40 e trending border fault system with the quaternary wonji fault belt which is constituted by right stepping en echelon n s to n20 e trending faults the wonji fault belt affects mainly the rift floor but it also overlaps with some segments of the margins its en echelon arrangement indicates a left lateral component of displacement along the rift trend the general fault pattern of the ethiopian rift as well as mesoscopic fault analyses and structural features of some key areas indicate the occurrence of a roughly e w extension which is compatible with the sinistral shear component of motion along the rift this paper proposes that oblique rifting related to the e w direction of extension has been active since the beginning of the quaternary and that it came after an earlier phase of roughly pure extension orthogonal to the rift trend

analogue modelling of orthogonal and oblique rifting abstract the three dimensional progressive development of orthogonal and oblique rift systems has been simulated using scaled analogue sandbox models the orientation of the zone of rifting where angle between the rift axis and the extension direction was varied systematically from 90 orthogonal rifting to 75 60 and 45 oblique rifting completed models were impregnated and sectioned both vertically and horizontally orthogonal rift models are characterized by long straight rift border faults and intra rift faults perpendicular to the extension direction oblique rift models are characterized by shorter segmented rift border faults and intrarift faults in the oblique models the rift border faults formed en echelon arrays parallel to the inderlying zone of basement stretching whereas the intra rift faults formed at a high angle to the extension vector increased extension in the moderately oblique and oblique rift systems 60 and 45 caused the intra rift faults to rotate towards parallelism with the rift border faults no strike slip or oblique slip transfer faults were found in any experiments rift border faults and intra rift faults in the models are segmented along strike with relay ramp structures formed between overlapping fault tips along the strike of the rift variations in half graben polarities and offsets of depocentres are accomplished by accommodation zones formed by interlocking conjugate fault arrays the experimental fault patterns compare well with the fault architectures found in those modern or relatively young rift systems where well defined basement controlled strike slip or oblique slip transfer faults are absent an accommodation zone model consisting of interlocking conjugate extension faults is proposed for the along strike switching of half graben asymmetries and offsetting of depocentres in rift systems the fault geometries in the analogue models can be used as templates for recognition of fault styles in both orthogonal and in oblique rift environments

geomorphic analysis of the sierra cabrera an active pop up in the constrictional domain of conjugate strike slip faults the palomares and polopos fault zones eastern betics se spain the nne ssw sinistral palomares and the conjugate dextral wnw ese striking polopos fault zones terminate in the sierra cabrera antiform in order to test the quaternary activity and topographic relief control in the termination of these fault zones here we present new qualitative and quantitative geomorphic analyses supported by a new structural map of the region the main mountain fronts of the cabrera antiform are formed by the north and south cabrera reverse faults that merge laterally into the palomares and polopos faults respectively these faults produce knickpoints stream deflections complex basin hypsometric curves high slk anomalies and highly eroded basins in their proximity furthermore the drainage network shows an s shaped pattern reflecting progressive anticlockwise rotation related to the sinistral palomares fault zone the estimated uplift rates determined by the integration between mountain front sinuosity index and valley floor width to height ratio are larger than those obtained for strike slip faults in the eastern betics these larger uplift rates with our geomorphic and structural dataset indicate that the topographic relief of the sierra cabrera antiform is controlled by reverse faults that form a pop up structure in the constrictional domain between the larger palomares polopos conjugate strike slip faults existing gps geodetic data suggest that the north and south cabrera reverse faults probably accommodate a large part of africa iberia convergence in the region

remagnetization in lava flows recording pretransitional directions basaltic lava flows associated with the last reversal in three distinct volcanic sequences of the island of la palma are characterized by overlapping components with very high unblocking temperatures a medium high temperature mht component with normal polarity is resistant up to 500 550 c while in most cases the high temperature ht primary reverse component cannot be isolated below 520 c the same situation with opposite polarities is observed also in a lava flow associated with the onset of the upper reunion reversal in the gamarri sequence in ethiopia the presence of both polarities within all these single flows cannot be reconciled with the hypothesis that a full reversal was systematically recorded during their cooling the direction of mht is either close or similar to the direction of the ht component of the overlying flows which indicates that remagnetization occurred shortly after emplacement rock magnetic studies microscopic observations microprobe analyses and remagnetization experiments have been carried out to investigate the origin of mht the rock magnetic parameters do not exhibit specific anomalies the unblocking temperatures are mostly higher than the simulations involving reheating by the overlying flow combined with thermoviscous overprinting caused by slow cooling the most plausible scenario is that baking by the overlying flows was accompanied by low temperature oxidation of titanomagnetite to cation deficient titanomagnetite this description also includes the presence of cracks fractures and other heterogeneities of the lava which induce variability between parallel profiles similarities with characteristics observed at steens mountain could suggest that the hypothesis of rapid geomagnetic changes recorded by a single lava flow should be considered with caution

hydrocarbon and ore bearing basinal fluids a possible link between gold mineralization and hydrocarbon accumulation in the youjiang basin south china the youjiang basin which flanks the southwest edge of the yangtze craton in south china contains many carlin type gold deposits and abundant paleo oil reservoirs the gold deposits and paleo oil reservoirs are restricted to the same tectonic units commonly at the basinal margins and within the intrabasinal isolated platforms and or bioherms the gold deposits are hosted by permian to triassic carbonate and siliciclastic rocks that typically contain high contents of organic carbon paragenetic relationships indicate that most of the deposits exhibit an early stage of barren quartz pyrite stage i a main stage of auriferous quartz arsenian pyrite arsenopyrite marcasite stage ii and a late stage of quartz calcite realgar orpiment native arsenic stibnite cinnabar dolomite stage iii bitumen in the gold deposits is commonly present as a migrated hydrocarbon product in mineralized host rocks particularly close to high grade ores but is absent in barren sedimentary rocks bitumen dispersed in the mineralized rocks is closely associated and or intergrown with the main stage jasperoidal quartz arsenian pyrite and arsenopyrite bitumen occurring in hydrothermal veins and veinlets is paragenetically associated with stages ii and iii mineral assemblages these observations suggest an intimate relationship between bitumen precipitation and gold mineralization in the paleo petroleum reservoirs that typically occur in permian reef limestones bitumen is most commonly observed in open spaces either alone or associated with calcite where bitumen occurs with calcite it is typically concentrated along pore vein centers as well as along the wall of pores and fractures indicating approximately coeval precipitation in the gold deposits aqueous fluid inclusions are dominant in the early stage barren quartz veins stage i with a homogenization temperature range typically of 230 c to 270 c and a salinity range of 2 6 to 7 2 wt nacl eq fluid inclusions in the main and late stage quartz and calcite are dominated by aqueous inclusions as well as hydrocarbon and co2 rich inclusions the presence of abundant hydrocarbon fluid inclusions in the gold deposits provides evidence that at least during main periods of the hydrothermal activity responsible for gold mineralization the ore fluids consisted of an aqueous solution and an immiscible hydrocarbon phase aqueous inclusions in the main stage quartz associated with gold mineralization stage ii typically have a homogenization temperature range of 200 230 c and a modal salinity around 5 3 wt nacl eq homogenization temperatures and salinities of aqueous inclusions in the late stage drusy quartz and calcite stage iii typically range from 120 c to 160 c and from 2 0 to 5 6 wt nacl eq respectively in the paleo oil reservoirs aqueous fluid inclusions with an average homogenization temperature of 80 c are dominant in early diagenetic calcite fluid inclusions in late diagenetic pore and fissure filling calcite associated with bitumen are dominated by liquid c2h6 vapor ch4 ch4 h2o and aqueous inclusions with a typical homogenization temperature range of 90 c to 180 c and a salinity range of 2 8 wt nacl eq it is suggested that the hydrocarbons may have been trapped at relatively low temperatures while the formation of gold deposits could have occurred under a wider and higher range of temperatures the timing of gold mineralization in the youjiang basin is still in dispute and a wide range of ages has been reported for individual deposits among the limited isotopic data the rb sr date of 206 12 ma for au bearing hydrothermal sericite at jinya as well as the re os date of 193 13 ma on auriferous arsenian pyrite and 40ar 39ar date of 194 6 2 ma on vein filling sericite at lannigou may provide the most reliable age constraints on gold mineralization this age range is comparable with the estimated petroleum charging age range of 238 185 ma and the sm nd date of 182 21 ma for the pore and fissure filling calcite associated with bitumen at the shitouzhai paleo oil reservoir corresponding to the late indosinian to early yanshanian orogenies in south china the close association of carlin type gold deposits and paleo oil reservoirs the paragenetic coexistence of bitumens with ore stage minerals the presence of abundant hydrocarbons in the ore fluids and the temporal coincidence of gold mineralization and hydrocarbon accumulation all support a coeval model in which the gold originated migrated and precipitated along with the hydrocarbons in an immiscible gold and hydrocarbon bearing basinal fluid system

control of rift obliquity on the evolution and segmentation of the main ethiopian rift the main ethiopian rift has evolved in two stages with successive activation of differently oriented fault systems analogue modelling of the earth s lithosphere demonstrates that such a rift evolution requires neither magma weakening nor a change in plate kinematics the main ethiopian rift is an active rift in the break up stage and it marks the incipient boundary between the nubia and somalia plates1 rifting started with the activation of large boundary faults and diffuse volcanism followed by focused magmatism and faulting in the rift floor along step like segments oblique to the rift axis that now act as a protoridge for future seafloor spreading2 this concentration of volcano tectonic activity has been thought to be either magma assisted2 3 or controlled by a change in rift kinematics with a late oblique rifting phase that would have caused the development of the step like fault segments that focused magma upwelling4 5 geodetic6 7 seismic8 and stress field9 data confirm current oblique rifting kinematics but plate kinematics models do not predict a change in nubia somalia motion in the past 11 million years10 here i use lithospheric scale analogue models of oblique rifting to analyse the development of the main ethiopian rift i find that neither magma weakening nor a change in plate kinematics are required to simulate a two phase evolution with successive activation of differently oriented fault systems i conclude that rift evolution and segmentation are controlled by rift obliquity independent of magmatic processes

correction to apparent and true polar wander and the geometry of the geomagnetic field over the last 200 myr 1 in the paper apparent and true polar wander and the geometry of the geomagnetic field over the last 200 myr by jean besse and vincent courtillot journal of geophysical research 107 b11 2300 doi 10 1029 2000jb000050 2002 the data in the top part of table 4 were incorrect the correct table 4 is given here journal of geophysical research vol 108 no b10 2469 doi 10 1029 2003jb002684 2003

hydrocarbon biomarkers in the topla mežica zinc lead deposits northern karavanke drau range slovenia paleoenvironment at the site of ore formation the mississippi valley type zinc and lead deposits at topla 250 150 metric tons t of ore grading 10 wt zn and 3 3 wt pb and me ica 19 million metric tons mt of ore grading 5 3 wt pb and 2 7 wt zn occur within the middle to upper triassic platform carbonate rocks of the northern karavanke drau range geotectonic units of the eastern alps slovenia the ore and host rocks of these deposits have been investigated by a combination of inorganic and organic geochemical methods to determine major trace and rare earth element ree concentrations hydrocarbon distribution and stable isotope ratios of carbonates kerogen extractable organic matter and individual hydrocarbons these data combined with sedimentological evidence provide insight into the paleoenvironmental conditions at the site of ore formation the carbonate isotope composition the ree patterns and the distribution of hydrocarbon biomarkers normal alkanes and steranes suggest a marine depositional environment at topla a relatively high concentration of redox sensitive trace elements v mo u in the host dolostones and ree patterns parallel to that of the north american shale composite suggest that sediments were deposited in a reducing environment anoxic conditions enhanced the preservation of organic matter and resulted in relatively higher total organic carbon contents up to 0 4 wt the isotopic composition of the kerogen 13 c kerogen 29 4 to 25 0 15 n kerogen 13 6 to 6 8 suggests that marine algae and or bacteria were the main source of organic carbon with a very minor contribution from detrital continental plants and a varying degree of alteration extractable organic matter from topla ore is generally depleted in 13 c compared to the associated kerogen which is consistent with an indigenous source of the bitumens the mineralization correlates with 15 n kerogen values around 0 per mil 13c depleted kerogen 13c enriched n heptadecane and relatively high concentrations of bacterial hydrocarbon biomarkers indicating a high cyanobacterial biomass at the site of ore formation abundant dissimilatory sulfate reducing bacteria feeding on the cyanobacterial remains led to accumulation of biogenic h 2 s in the pore water of the sediments this biogenic h 2 s was mainly incorporated into sedimentary organic matter and diagenetic pyrite higher bacterial activity at the ore site also is indicated by specific concentration ratios of hydrocarbons which are roughly correlated with total pb plus zn contents this correlation is consistent with mixing of hydrothermal metal rich fluids and local bacteriogenic sulfide sulfur the new geochemical data provide supporting evidence that topla is a low temperature mississippi valley type deposit formed in an anoxic supratidal saline to hypersaline environment a laminated cyanobacterial mat with abundant sulfate reducing bacteria was the main site of sulfate reduction

relationship between zn pb ore and oil accumulation processes example of the bou grine deposit tunisia abstract the bahloul formation of cenomanian turonian age combines qualities of a potential source rock for oil and of a rich lead zinc mineralization in the bou grine area on the border of the lorbeus massif diapir tunisian atlas analysis of 46 core samples from 7 boreholes drilled within or outside the mine area revealed the richness in organic matter of the bahloul formation 4 5 toc its low maturity t max 423 c and marine planktonic origin hi 600 mg hc g 1 toc abnormally high values of the oil production index up to about 30 indicate the presence of migrated oil in the mine and in samples from the northeast boreholes this information is confirmed by other observations high relative amounts of hydrocarbons in the bitumen extracts high pr ph ratios and high amounts of light naphthenic hydrocarbons the abundance of the latter components observed in chromatograms where n alkanes are nearly absent may indicate the development of a biodegradation process during migration however this biological activity may have exerted the greatest intensity at the expense of indigenous organic constituents in the mine area where considerable amounts of hopanes and steranes were observed all these data supported by geological observations allow us to propose the following genetic model for the bou grine deposit the bahloul formation which associates high amounts of organic matter zinc and lead on a regional scale zn pb 0 5 2 and a high porosity could have served as a source for the migrated oil and for the metals necessary to the mineralization as well as a conduit for the oil and metal bearing brines an intense biological activity developed at the expense of the organic content of the bahloul formation and of the sulfate from the adjacent triassic diapir may have produced the hydrogen sulfide responsible for ore deposition

successive orthogonal and oblique extension episodes in a rift zone laboratory experiments with application to the ethiopian rift small scale modeling was performed to examine the effects of the superposition of two successive extensional phases from orthogonal to oblique type 1 and from oblique to orthogonal type 2 in both the type 1 and type 2 models faults produced during the first stage strongly control fault development during the second stage in type 1 models the oblique faults developed during the second oblique phase are confined within a first phase graben whereas in type 2 models the oblique faults produced during the first phase continue to develop during orthogonal extension and connect with each other to give sigmoidal fault blocks type 1 models are compared with the structural setting of the ethiopian rift the evolution of the rift is related to a recent extensional event whose principal direction of stretching trends at around 50 to preexisting major normal faults type 1 laboratory models are fairly comparable to the northern sector of the ethiopian rift referred to here as mer they account for both the development of the en echelon oblique faults of the wonji fault belt and the sinistral shear gradient running parallel to the eastern border of the mer which formed during an oblique rifting extension the statistical analysis of the whole ethiopian rift fault pattern by reference to the experimental data allows the determination of a n100 n110 mean direction of stretching

evolution pattern and partitioning of deformation during oblique continental rifting inferences from lithospheric scale centrifuge models oblique rifting is investigated through centrifuge experiments that reproduce extension of a continental lithosphere containing a preexisting weakness zone during extension this weakness localizes deformation and different rift obliquity is obtained by varying its trend with respect to the stretching direction model results show that deformation is mostly controlled by the obliquity angle a defined as the angle between the orthogonal to the rift trend and the extension direction for low obliquity alpha 45 degrees no boundary faults form and the extensional deformation affects the rift depression since early stages of extension dominance of the strike slip motion over extension leads to the development of oblique slip and nearly pure strike slip faults oblique to both the rift trend and the orthogonal to the extension direction with no strain partitioning between the margins and the rift floor these results suggest that oblique reactivation of preexisting weaknesses plays a major role in controlling rift evolution architecture and strain partitioning findings that have a significant relevance for natural oblique rifts

northwest to north northwest extension direction in the ethiopian rift deduced from the orientation of extension structures and fault slip analysis northeast to north northeast trends of eruptive centers some of which formed at the termination of faults and en echelon extension fractures the largest one of which is 0 1 km wide up to 1 km long and at least 50 m deep exist between lat 7 n and 9 n in the ethiopian rift on systeme probatoire d9observation de la terre spot imagery the extension fractures form scars in the rift floor border faults of the rift are generally oriented northeast to north northeast two fault systems observed on land sat multi spectral scanner mss cross the rift the bilate weyira fault with a north northwest strike and the wonji fault belt with a north northeast trend north northwest striking faults cutting across the rift are interpreted to be transfer faults that cut the rift into rhomboid shaped basins extension fractures and rows of eruptive centers collectively referred to as extension structures are used in the paper to infer the direction of plate motion in the ethiopian rift northwest to north northwest extension of the rift is deduced by assuming that the direction of opening of extension structures is also the orientation of the minimum principal stress 3 fault slip inversion on faults that cut post miocene volcanic rocks confirms this interpretation

three dimensional seismic imaging of a protoridge axis in the main ethiopian rift models of continental breakup remain uncertain because of a lack of knowledge of strain accommodation immediately before breakup our new three dimensional seismic velocity model from the main ethiopian rift clearly images mid crustal intrusions in this active transitional rift setting supporting breakup models based on dike intrusion and magma supply the most striking features of our velocity model are anomalously fast elongate bodies velocity v p 6 5 6 8 km s extending along the rift axis interpreted as cooled mafic intrusions these 20 km wide and 50 km long bodies are separated and laterally offset from one another in a right stepping en echelon pattern approximately mimicking surface segmentation of quaternary volcanic centers our crustal velocity model combined with results from geologic studies indicates that below a depth of 7 km extension is controlled by magmatic intrusion in a ductile middle to lower crust whereas normal faulting and dike intrusion in a narrow zone in the center of the rift valley control extension in the brittle upper crust this zone is inferred to be the protoridge axis for future seafloor spreading

the relationship between bitumens and mineralization in the south pennine orefield central england like many examples of mississippi valley type mineralization the south pennine orefield is well known for the occurrence of hydrocarbons within hydrothermal mineral veins bitumens from this area along with oils from the east midlands have been characterized using standard organic geochemical techniques and compared with potential source rocks lower namurian mudstones from the widmerpool gulf and the gainsborough and goyt troughs were rejected as possible sources for the bitumens and oils on the grounds of lack of sufficient thermal maturity and or essential biomarker correlation lower namurian mudstones with type ii kerogen from the edale gulf are proposed to offer a likely hydrocarbon source for the area with a minor more localized contribution from organic rich type ii mudstone partings within the dinantian limestones simple geochemical models consistent with recent sequence stratigraphic interpretation suggest a late carboniferous age for hydrocarbon generation where geothermal gradients were slightly higher c 50 c km 1 than the present day microthermometric analysis of fluid inclusions from fluorite and calcite allowed three fluid types to be distinguished one na ca cl bearing fluid was discerned to be dominant throughout the orefield being of low temperature uncorrected t h 70 110 c and high salinity c 20equiv wt nacl a second high salinity na ca cl fluid uncorrected t h 70 120 c c 24equiv wt nacl distinguished from the first by varying na ca contents was observed in the westernmost extremities of the orefield a third nacl bearing fluid of higher temperature t h 100 145 c and lower salinity 4 wt nacl was detected in the northernmost part of the orefield hydrocarbon bearing inclusions were not observed in any of the three fluids in this study sulphur isotopic analyses performed on a selection of sulphides sulphates and bitumens to gauge the role of organic matter in sulphate reduction reactions proved inconclusive the mineralization is thought to have occurred as several events that in the north is thought to be derived from the dewatering of the edale gulf towards the end of the carboniferous whilst other mineralization events occurred from the early permian onwards with the ore fluids having more distal sources predominantly from the east with a minor input from the west the main phases of hydrocarbon generation and mineralization appear to be unrelated

the tectonic and geomagnetic significance of paleomagnetic observations from volcanic rocks from central afar africa deformation throughout afar over the past 2 myr has been characterized by widespread and intense crustal fragmentation that results from inhomogeneous extension across the region in eastern afar this situation has evolved to localized extension associated with the westward propagation of the gulf of aden gulf of tadjurah seafloor spreading system into the asal ghoubbet rift during the gradual process of rift propagation and localization crustal blocks in eastern afar sustained clockwise rotations of 11 to better understand the processes of rift propagation and localization and how they affect the rest of afar we have collected and analyzed over 400 oriented paleomagnetic samples from 67 lava flows from central and southern afar unlike eastern afar the mean paleomagnetic direction from central afar indicates that vertical axis rotations are statistically insignificant 3 6 4 4 though small clockwise rotations 8 are permitted thus propagation and localization in central afar have not had the same influence in causing crustal block rotations or perhaps more likely have not reached the same stage of evolution as seen in eastern afar in addition several of the lava flows record intriguing geomagnetic field behavior associated with polarity transitions excursions or large secular variation events interestingly the transitional or anomalous virtual geomagnetic poles vgps tend to cluster in two nearly antipodal regions one in the northern pacific ocean and the other in the southwest indian ocean one lava flow has recorded both of the antipodal transitional components with the two components residing in magnetic minerals with unblocking temperatures above and below 500 c respectively reheating and partial remagnetization by the overlying flow cannot explain either of the transitional directions because both differ significantly from that of the reversely magnetized overlying flow the high temperature component gives a vgp in the northern pacific whereas the lower temperature component gives a nearly antipodal vgp south of cape town south africa hence the configuration of the geomagnetic field appears to have jumped nearly instantaneously from a northern hemisphere transitional state to a southern hemisphere one during this normal to reverse polarity transition

role of organic matter and sulphate reducing bacteria for metal sulphide precipitation in the bahloul formation at the bou grine zn pb deposit tunisia abstract the origin of the bou grine zn pb deposit of tunisia was investigated during a geochemical study of the cretaceous bahloul formation the inorganic and organic chemical compositions of the bituminous marls and limestones as well as the light stable isotope ratios of organic carbon and carbonates were measured closely spaced samples were collected along two profiles at different distances from the enrichments in zn and pb which are best developed near the bou grine diapir the organic geochemical data suggest sulphide precipitation at the bou grine deposit by sulphate reducing bacteria srb srb utilized hydrocarbons derived both from the bahloul sediments and from external source rocks production of s ii by the bacteria was most probably limited by the availability of sulphate although sulphate was available to the bacteria in high concentrations in close proximity to the triassic salt dome ground waters evolved to hypersaline basinal brines by dissolution of salt and anhydrite at the contact of the diapir these brines would have been capable of leaching metals from the sedimentary sequences adjacent to the salt dome the triassic series has often been advocated as the source of the metals and the cretaceous sediments themselves are carriers of metallic elements ore deposition apparently resulted when metal bearing solutions that migrated upward into the roof zone of the diapir mixed with s ii bearing solutions those formed when srb oxidized the high amounts of hydrocarbons within the bahloul sediments and in the brines using so 4 2 from sulphate rich waters in the anhydrite caprock as an oxidant

structural and geochronological implications of the fentale volcanics at a nascent passive margin of the main ethiopian rift constraints from magnetostratigraphy study at the kereyou lodge ethiopia abstract we sampled twenty four cooling units for magnetostratigraphic study from a 110 m thick early pleistocene volcanic section near kereyou lodge in the main ethiopian rift seven samples were used from each site for analysis of remanent magnetization using thermal and alternating field demagnetization techniques the natural remanent magnetization nrm direction comprises two vector components in most samples the first low stability component is isolated generally by heating to 100 c 250 c or by demagnetization using an alternating field intensity of 10 15 mt after these steps most magnetization directions define straight lines directed towards the origin and record the primary nrm or chrm characteristic remanent magnetization magnetization decay curve analyses show the possible magnetic minerals to be titanomagnetite magnetite titanohematite and hematite paleomagnetic site mean directional analyses revealed that upper 18 flows are of reversed polarity and the bottom 4 flows are normal using available age information the normal interval is correlated to the olduvai subchron on the geomagnetic polarity time scale gpts of cande kent 1995 and the flows at the polarity transition are dated to be 1 77 ma the overall declination and inclination differences calculated d 4 8 4 4 i 0 8 4 2 are consistent with the 7 counterclockwise reported kidane et al 2009 site mean inclinations show a systematic change with flow position inclinations get progressively shallower with depth for the top 17 volcanic flows and progressively steeper for the bottom 4 flows consistent with progressive backward and rift ward block tilting mechanisms respectively

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

permo triassic magnetostratigraphy in china northern tarim abstract the upper boundary of the permo carboniferous reversed polarity superchron has been identified in a palaeomagnetic study of the permo triassic of the northern part of the tarim basin china this boundary serves as an important marker horizon for correlation with other permo triassic sequences both in china and world wide a permo triassic palaeomagnetic pole for the tarim block is estimated to be at 71 8 n 187 6 e comparison with similar age poles from the adjacent blocks of china and asia suggests that the tarim was widely separated from the sino korean block in permo triassic times but was not yet sutured to kazakhstan

organic geochemistry and mineralogy i characterisation of organic matter associated with metal deposits abstract as geochemical appraisals of mineral regions of commercial prospectivity evolve the organic matter associated with metal rich ores has attracted greater attention petroleum basin and modern seafloor hydrothermal vent studies have suggested that organic matter can have a significant influence on the behaviour of mineralising fluids there have been many isolated reports of certain organic compositional or morphological e g pyrobitumen features showing an apparent relationships with hydrothermal fluids or minerals raising expectations that organic based parameters might be useful to mineral exploration however the understanding of organic inorganic relationships in earth systems is far from complete for example the detailed mechanics of the interaction of organics with hydrothermal fluids over geological time remain largely undefined organic geochemistry studies have traditionally involved the measurement and interpretation of the hydrocarbon composition of sedimentary rocks here we review the types of aliphatic hydrocarbons aromatic hydrocarbons and metalloporphyrins often detected from organic geochemical investigations in mineral rich regions such molecular data can be particularly diagnostic of biochemical sources and the palaeo environments at the time mineral associated organic matter was deposited sub surface trends of hydrocarbon alteration may also reflect major biogeochemical processes such as thermal maturity and biodegradation organic geochemistry data can also occasionally provide information about the nature e g origin composition temperatures and migration pathways of hydrothermal fluids and can make a contribution to holistic ore genesis models the well preserved organic matter associated with the economic here s your chance pb zn ag mine paleoproterozoic barney creek formation mcarthur basin australia and the transition metal rich early permian kupferschiefer formation germany poland have attracted significant attention a more detailed summary of the organic character of these deposits is provided to highlight the contribution organic geochemistry can make to understanding mineralisation processes most organic geochemical studies of highly mineralised regions however have not adequately addressed the significance of organic matter to mineralisation a slightly different analytical focus than traditionally used for exploration appraisal of petroleum hydrocarbons may be required to properly evaluate the significance of organic species to the mobilisation transport and deposition of ore metals the characterisation and subsequent thermodynamic modeling of organic substances and complexes within metalliferous hydrothermal systems will contribute to a better understanding of the nature and role of organic inorganic fluids or other affiliated organics in ore systems

new age constraints on the timing of volcanism and tectonism in the northern main ethiopian rift southern afar transition zone ethiopia abstract forty new k ar and 40 ar 39 ar isotopic ages from the northern main ethiopian rift mer southern afar transition zone provide insights into the volcano tectonic evolution of this portion of the east african rift system the earliest evidence of volcanic activity in this region is manifest as 24 23 ma pre rift flood basalts transition zone flood basalt activity renewed at approximately 10 ma and preceded the initiation of modern rift margin development bimodal basalt rhyolite volcanism in the southern afar rift floor began at approximately 7 ma and continued into recent times in contrast post subsidence volcanic activity in the northern mer is dominated by mio pliocene silicic products from centers now covered by quaternary volcanic and sedimentary lithologies unlike other parts of the mer mio pliocene silicic volcanism in the mer afar transition zone is closely associated with fissural basaltic products the presence of pliocene age ignimbrites on the plateaus bounding the northern mer whose sources are found in the present rift indicates that subsidence of this region was gradual and that it attained its present physiography with steep escarpments only in the plio pleistocene large 7 5 ma silicic centers along the southern afar and northeastern mer margins apparently formed along an e w oriented regional structural feature parallel to the already established southern escarpment of the afar the addis ababa rift embayment and the growth of 4 5 3 ma silicic centers in the addis ababa area are attributed to the formation of a major cross rift structure and its intersection with the same regional e w structural trend this study illustrates the episodic nature of rift development and volcanic activity in the mer afar transition zone and the link between this activity and regional structural and tectonic features

age and origin of the well preserved organic matter in internal sediments from the silesian cracow lead zinc deposits southern poland the molecular and petrographic characteristics of organic matter in internal sediments from the mississippi valley type lead zinc deposits in southern poland reveal immature and well preserved organic matter huminite reflectance values of organic matter in all internal sediments are low with a mean value of 0 29 corresponding to a lignite range of coalification most organic compounds in the organic matter e g cadalene retene simonellite perylene and sitosterol and its transformation products stigmastanol and stigmasta 3 5 dien one as well as lignin degradation compound products e g benzoic acid vanillin 4 benzaldehyde benzenedicarboxilic acids and hydroxybenzoic acids are of terrestrial origin monosaccharides with dominant and glucose were identified as possible remnants of cellulose degradation products suggesting an excellent state of organic matter preservation given that monosaccharides are preserved only under conditions of limited oxygen after sedimentation resulting from the rapid accumulation of internal sediments in meteoric paleokarst cavities followed by insignificant diagenesis petrologic and palynological data on internal sediments clearly indicate a middle triassic age for organic matter and for the development of a triassic meteoric karst system immediately after anisian carbonate sedimentation

integrated depositional model for the cenomanian turonian organic rich strata in north africa abstract during the late cenomanian early turonian c t oceanic anoxic event oae2 organic rich strata was deposited in rift shelf basins and slopes across north africa and in deep sea basins of the adjacent oceans based on a review of published and unpublished information this paper documents the distribution and organic richness of c t strata across the whole region within a palaeogeographic framework and systematically analyses the conditions and processes which controlled their deposition previously the c t in north africa has been most intensively studied in southern morocco tarfaya and tunisia only little data is availabe for other parts of north africa namely algeria libya and egypt because distribution of c t corg strata there becomes more patchy a general decrease in peak organic richness and black shale thickness occurs from west to east partly as a result of upwelling along the moroccan atlantic coast and the absence of upwelling in the eastern mediterranean area furthermore in the confined central atlantic the oxygen minimum zone omz in many places reached down to the deep sea floor 3 4 km while the lower limit of the omz along the north african tethys was much shallower and underlain by oxic water masses as documented by high resolution biostratigraphic and chemostratigraphic data c t black shale deposition in most areas outside the upwelling zone are restricted to a strong eustatic latest cenomanian transgressive phase triggered by this sea level rise the omz impinged onto the north african continental shelf and the margins of intrashelf basins which mostly formed during the early cretaceous as halfgrabens important units containing c t organic rich strata in the region are the atlantic tarfaya black shales morocco western sahara black shales and phtanites in the moroccan and algerian atlas the bahloul fm in the se constantine basin and in northern and central tunisia the etel fm in the sirte basin the al hilal fm in cyrenaica libya the abu roash fm in the abu gharadig and fayium basins western desert egypt the daliyya fm along the ne sinai palestine isreal coast and the shueib fm in west central jordan

biodegradation of hydrocarbons and biogeochemical sulfur cycling in the salt dome environment inferences from sulfur isotope and organic geochemical investigations of the bahloul formation at the bou grine zn pb ore deposit tunisia abstract combined organic geochemical and stable isotope s analyses of samples from the cretaceous bahloul formation tunisia provide insight to oil accumulation processes biogeochemical alteration of hydrocarbons microbial sulfate reduction and mineral deposition at the flanks of the triassic jebel lorbeus diapir forming the bou grine zn pb deposit the sulfur isotopic composition of the metal sulfides correlates with the degree of biodegradation of hydrocarbons with the base metal content and with the proportion of aromatics in the organic extracts the 34 s values are interpreted to reflect bacterial sulfate reduction in a more or less closed system rather than a thermogenic contribution the extent of h 2 s production by the activity of the sulfate reducing bacteria probably was limited by the availability of sulfate which in turn was governed by the permeability of the respective sedimentary sequence and by the distance to the anhydrite cap rock evidence is provided that biodegradation of hydrocarbons and microbial sulfate reduction contribute to the formation of the high grade mineralization inside the bahloul formation at the contact with the salt dome cap rock the metals probably were derived through leaching of deeper sedimentary sequences by hot hypersaline basinal brines evolved by dissolution of salt at the flanks of the diapirs these hot metalliferous brines are proposed to migrate up around the diapir finally mixing with near surface sulfate rich brines in the roof zone when the fluids came in contact with the organic rich sediments of the bahloul formation the dissolved sulfate was reduced by the sulfate reducing bacteria hydrocarbons generated or accumulated in the bahloul formation were utilized by sulfate reducers the occurrence of high amounts of native sulfur in high grade ore samples suggest that the production rate of h 2 s by bacterial sulfate reduction exceeded its consumption by metal sulfide precipitation the supply of dissolved metals probably was the limiting reaction the trend of 34 s of native sulfur s 0 towards lower values to the top of the highly mineralized bahloul profile provides evidence for a biogenic origin of s 0 by sulfide oxidizers the variation in isotopic fractionation between native sulfur and sulfide sulfur in the ore minerals reflects the kinetic isotope fractionation factor connected with metabolic processes and the respective reaction rate during biogenic oxidation of sulfur

oxygen and hydrogen isotopic composition of clay minerals of the bahloul formation in the region of the bou grine zinc lead ore deposit tunisia evidence for fluid rock interaction in the vicinity of salt dome cap rock abstract the siliciclastic bahloul formation of tunisia is the host for zn pb deposits a detailed study of 18 o 16 o and d h ratios of kaolinite quartz and calcite was done to elucidate the conditions and mechanism of deposition of the diapir related bou grine zn pb ores in the formation d h and 18 o 16 o ratios of the minerals of the host rock have been affected by the mineralizing process the magnitudes of the isotopic shift in 18 o 16 o of kaolinite as well as the intensity of base metal mineralization seem to be governed by the permeability of the bahloul strata due to different water rock ratios in contrast the variability in d h ratios of kaolinite shows no correlation with the estimated permeability of the respective sedimentary section published microthermometric fluid inclusions data and organic geochemical maturation parameters both argue for a maximum temperature between 80 and 120 c during ore formation and host rock alteration the 18 o 16 o ratios of the clay minerals of samples taken over a range of distance from the ore body suggest alteration by hot saline 18 o enriched brines these could have evolved at the flanks of the diapir and would have been capable of remobilizing metals from deeper sedimentary sequences providing the metals now found in the bou grine deposit the 18 o value of the mineralizing fluid is estimated to have been between 8 and 18 the d value was between 30 and 55 the high 18 o values of these basinal brines provide evidence for a control of the isotopic composition of the fluids by 18 o rich carbonate minerals of the post triassic limestones hydrogen isotopic data argue for a meteoric origin of the brines the variability of the d values might be explained by a contribution of evaporated meteoric waters entering the groundwater system and mixing of these waters with the hot saline brines in the contact zone of the diapir

paleomagnetic and geochronological identification of the reunion subchron in ethiopian afar this paper reports the paleomagnetic and geochronological analysis of a suite of samples from a basaltic fault related scarp of pliocene age in the central part of the afar depression ethiopia paleomagnetic work uncovers a clear characteristic remanent magnetization carried by titano magnetites with high unblocking temperatures close to the curie temperature of pure magnetite the characteristic direction based on 27 non transitional flows is d 8 1 o i 10 1 o 5 4 1 o this is consistent with earlier determinations of the clockwise tectonic rotation due to rift propagation and overlap of the block to which the section belongs magnetic stratigraphy consists of a succession from bottom to top of 16 reversed 7 normal and 4 reversed flows one flow at the upper normal to reverse transition demonstrates complex behavior upon thermal and alternating field af demagnetization which is due to remagnetization by the overlying flow k ar dating of five samples provides consistent determinations averaging 2 07 0 05 ma all ages agree with this average given their rather large individual uncertainties from 0 04 to 0 08 ma at the lc level comparison with recent reference geomagnetic polarity timescales gpts indicates that the normal subchron recorded in the gamarri section must be linked with one of the r6union event s the normal subchron and an earlier episode of large secular variation could provide the best volcanic record of a double r6union event although uncertainties in ages are too large to allow unequivocal inferences our age determination for the r6union event is in better agreement with earlier determinations of mcdougall and watkins 1973 than with a more recent value proposed by baksi et al 1993 and used in the gpts

paleointensity across the reunion event in ethiopia abstract paleointensity experiments have been conducted on a volcanic series of 33 flows which recorded the geomagnetic reunion event successive determinations of absolute paleointensity from 20 flows indicate that the reunion event would actually be characterized by two successive periods of low field intensity with a total duration not in excess of 30 kyr the first period has a mean paleointensity of 12 t and shows significant deviations from initial reverse polarity but no signs of fully normal polarity it is followed by field recovery then a decrease prior to the second directional episode the second period has a mean paleointensity of 17 t and is associated with full normal directions and thus likely a dominantly dipolar configuration these results are compatible with a simple earthlike magnetic field model with a large decrease in dipole field strength which yields local emergence of relatively stronger non dipole components field characteristics from this configuration would be different at different locations and could explain the variability of the paleomagnetic records collected around the world we propose that the absence of field recovery following the event is responsible for not leading to a durable polarity interval

organic geochemical indicators of biological sulphate reduction in early diagenetic znpb mineralization the bois madame deposit gard france abstract at bois madame gard france zn pb s mineralization zn pb 4 occurs in dolomitized kimmeridgian carbonates very near layers which are rich in organic matter of planktonic origin previous geological and geochemical studies have suggested early syn or diagenetic ore deposition controlled by sulphate reducing bacteria the metals probably having originated from regionally mineralized triassic or liassic formations rock eval pyrolysis data complemented by biomarkers show that the organic matter reached only the organic diagenesis catagenesis transition owing to burial pb deposit as in many pb zn s deposits the hydrocarbons almost certainly autochthonous show no indication of relatively late alteration by biodegradation or water washing the presence of hopanes c 27 35 with high abundance in the c 27 32 range indicate that intense bacterial activity occurred during early diagenesis under su ic anoxic conditions this interpretation is reinforced by the presence of n alkanes in the range n c 22 to n c 28 dominated by compounds with an even number of carbon atoms which may have derived from the reduction of bacterial fatty acids except for these even number n alkanes the organic geochemical evidence for sulphate reducing bacteria during early diagenesis must remain equivocal

early biodegradation of ligneous organic materials and its relation to ore deposition in the treves zn pb ore body gard france abstract previous petrographical and geochemical studies on the organic content of mineralized and non mineralized carbonate series in the treves zn pb stratiform deposit led to the conclusion that ore deposition occurred during the diagenetic history of the sediments in a mixed fresh hydrothermal system gauthier and disnar 1984 gauthier et al 1985a these circulations of fluids are probably responsible for the dolomitization of the original limestones and for the deposition of the ore through microbial activity gauthier 1984 gauthier el al 1985b the occurrence of this activity is supported by the formation of two kinds of organic components which are attributable to this uncommon bio alteration process alphatic rich bitumen like products and aromatic polyethers the chemical composition of these two types of compounds suggests that they were produced from ligneous debris by sulfate reducing bacteria other indications of the occurrence of a biological activity have been found in the nature and relative abundance of microbial amino compounds however these constituents are thought to have been introduced only recently into the sediments whereas other evidence supports the idea that ore deposition occurred before diagenesis

processus d alteration thermique physico chimique et biologique de constituants organiques et genese d une mineralisation sulfuree le gite zn pb de la florida cantabria espagne the la florida lead zinc deposit cantabria spain which is hosted by urgonian carbonates is located on the border of the basco cantabric lower cretaceous basin the stratiform orebody which is composed of sphalerite galena and barite in iron dolostones occurs in an envelope of dolostone organic matter studies have been carried out on samples from this deposit to understand its genesis and later evolution the results of this work reveal the immatury of the autochthonous organic matter despite a slight paleothermal anomaly centred on the deposit various alteration processes have affected the hydrocarbons associated with autochthonous organic matter and or allochthonous bitumens namely geochromatographic fractionation water washing and biodegradation together with previous geological work the results of this study are consistent with epigenetic and or diagenetic emplacement of the mineralization the proposed genetic scheme involves hydrothermal fluids that would have acquired at least so