associations between burial diagenesis of smectite chemical remagnetization and magnetite authigenesis in the vocontian trough se france results of a paleomagnetic rock magnetic geochemical and petrographic study on jurassic and cretaceous carbonates in the vocontian trough support a hypothesized connection between burial diagenetic alteration of smectite and the widespread occurrence of a chemical remanent magnetization crm carried by magnetite where smectite has altered to other clay minerals limestones are characterized by a prefolding secondary normal polarity magnetization throughout the basin the magnetization is interpreted to be a crm based on low burial depths which cannot cause thermoviscous resetting where significant smectite is still present the crm is absent weakly developed and where the clays show no evidence for burial alteration the units are characterized by a primary magnetization crm intensity also varies with the amount of smectite and burial isothermal anhysteretic and natural remanent magnetization intensities increase where smectite has altered both stratigraphically and geographically this is interpreted to indicate magnetite authigenesis associated with clay diagenesis superparamagnetic magnetite is more dominant in highly altered units based on the results of low temperature experiments all sections away from the alps have 87sr 86sr values that are similar to coeval seawater and stable isotopes of carbon and oxygen show no sign of alteration orogenic type fluids therefore are not a likely agent of remagnetization near the alps the rocks are characterized by an additional reversed polarity component which is interpreted to reflect acquisition of the crm through a reversal a postfolding magnetization is also present there and strontium isotopic ratios are higher than elsewhere in the basin and might indicate some alteration by orogenic type fluids we conclude that burial diagenesis of smectite is the likely cause for the development of the widespread crm in the vocontian trough and that this mechanism might explain widespread chemical remagnetization elsewhere

sedimentology of the bondi main heavy mineral beach placer deposit murray basin southeastern australia abstract drilling and pit wall exposures at the southern end of the bondi main heavy mineral beach placer deposit provided access to the loxton parilla sands which are here divided into five lithofacies the depositional environment of each lithofacies is interpreted as shoaling zone breaker zone surf zone swash zone and backshore dune other units encountered include the duddo limestone loddon river group bookpurnong formation and shepparton formation the interpreted depositional environments for these units are respectively shallow marine fluvial shallow marine shelf and fluvio lacustrine a palaeogeographical model is presented which proposes that the bondi main deposit formed at the southern end of a north trending shoreline this shoreline was truncated to the south by the dundas tableland and is broadly analogous in its geomorphological setting to the modern geographe bay in western australia the deposit therefore appears to have formed in association with a heavy mineral trap at its southern end postdepositional weathering overprints include ferruginous mottling clay eluviation and possibly quartz fracturing and disaggregation the ferruginous mottles formed under spatially and temporally variable redoximorphic conditions with fe being derived from the weathering of fe bearing primary minerals particularly ilmenite postdepositional clays in the loxton parilla sands are interpreted as being derived from the overlying shepparton formation

early cretaceous syn rotational extension in the organya basin new constraints on the palinspastic position of iberia during its rotation article i nfo in this study we interpret the paleo stress pattern in the organya basin southern pyrenees northern spain as inferred from the anisotropy of magnetic susceptibility ams of 39 sites distributed over the entire basin combined with information from other cretaceous iberian basins such analysis adds to constrain kinematic reconstructions of the cretaceous rotation of iberia allied with the opening of the bay of biscay and the northward propagation of the north atlantic the organya basin is an inverted cretaceous basin in the hanging wall of the boixols thrust the lithologies are mainly weakly deformed pelagic and hemi pelagic limestones and marls which recorded the aptian 35 counterclockwise rotation of iberia three types of ams fabrics could be distinguished all representing typical intermediate and tectonic fabrics ew magnetic lineations dominate in the eastern part of the basin and are related to crustal shortening during the pyrenean orogeny this interpretation is consistent with structural cross sections across the basin showing more intense shortening in the east in the central part of the basin approximately ns oriented magnetic lineations are observed interpreted as the original extensional direction during basin foundering so in line with results from previous studies ams can still unveil the original extensional direction in an inverted sedimentary basin something which may be difficult to reconstruct from geological data alone the original extension direction in the organya basin is perpendicular to the boixols thrust bounding the basin to the south correction for the aptian rotation of iberia leads us to infer a ne sw oriented extension direction at the onset of the rotation of iberia this extension direction is inconsistent with current plate kinematic reconstructions of the cretaceous rotation and motion of iberia we therefore suggest that during the opening of the bay of biscay and related iberian rotation iberia was in a much more westerly position than assumed in current models and that the rotation of iberia was followed by dominantly eastward translation with respect to southern france prior to n s convergence and shortening in the pyrenees since the latest cretaceous

what was the paleogene latitude of the lhasa terrane a reassessment of the geochronology and paleomagnetism of linzizong volcanic rocks linzhou basin tibet the paleogene latitude of the lhasa terrane southern tibet can constrain the age of the onset of the india asia collision estimates for this latitude however vary from 5 n to 30 n and thus here we reassess the geochronology and paleomagnetism of paleogene volcanic rocks from the linzizong group in the linzhou basin the lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes we report consistent 40ar 39ar and u pb zircon dates of 52 ma for the upper linzizong and 40ar 39ar dates 51 ma from the lower linzizong are significantly younger than u pb zircon dates 64 63 ma suggesting that the lower linzizong was thermally and or chemically reset paleomagnetic results from 24 sites in lower linzizong confirm a low apparent paleolatitude of 5 n compared to the upper part 20 n and to underlying cretaceous strata 20 n detailed rock magnetic analyses end member modeling of magnetic components and petrography from the lower and upper linzizong indicate widespread secondary hematite in the lower linzizong whereas hematite is rare in upper linzizong volcanic rocks of the lower linzizong have been hydrothermally chemically remagnetized whereas the upper linzizong retains a primary remanence we suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower linzizong we estimate that the paleogene latitude of lhasa terrane was 20 4 n consistent with previous results suggesting that india asia collision likely took place by 52 ma at 20 n

intersections of remanence small circles new tools to improve data processing and interpretation in palaeomagnetism summary intersections of remanence small circles are increasingly used in palaeomagnetism to determine the palaeofield direction from synfolding remanences the approach small circle intersection sci method presupposes that remanence small circles from sites tilted to different directions intersect in a common point which is assumed to represent the palaeofield direction the sci method is not restricted to synfolding remanences it can be applied also to derive the palaeofield direction from pre folding remanences in addition to tilt correction and as a cross check for fold tests however the sci method has specific requirements and does not work in all cases the suitability of data sets and reliability of palaeofield estimates have to be assessed carefully and always against the background of local geology this paper examines the occurrence and distribution of sci as a tool of judgement first a data set is most suitable and a palaeofield estimate by sci most reliable when most of the theoretically possible intersections are realized and in addition clearly grouping in a narrow region second the small circles that greatly determine the result of sci are identified by the highest number of intersections they have with the others these criteria allow a systematic investigation of sci estimates upon stability and dependencies moreover scis can be an indicator of irregular unexpected tectonic settings when the intersections disperse strongly although significant bedding strike differences are given relative rotations or other unexpected displacements are likely to have occurred if so the sci estimate will be invalid and probably biased however also fisher mean calculation and most fold tests will be affected in this case the scis can help to separate the data into subsets valid for fisher statistics and sci methods the other possibility is to use tentative reconstructions to infer the true sequence of tilts and rotations and correct the data adequately

the tectonic history of the nigde kirsehir massif and the taurides since the late mesozoic paleomagnetic evidence for two phase orogenic curvature in central anatolia the nigde kirsehir massif known also as the central anatolian block is bordered by the sutures of the neotethys ocean the massif suffered several deformation phases during and after the consumption of the surrounding oceans and the post collisional events of the continental pieces of anatolia in latest cretaceous to miocene previous paleomagnetic studies on the nigde kirsehir massif and its surroundings displayed either insufficient data or have claimed large rotations and or remagnetization in order to understand the tectonic history of the nigde kirsehir massif and its adjacent blocks we have sampled 147 different sites in the age range of upper jurassic to miocene from the nigde kirsehir massif throughout its w sw and e se boundaries and the central southeastern taurides the results display that except the limestones in central taurides all rocks examined carry a primary magnetization among these an important finding is that rotations between the massif and the central eastern taurides indicate an oroclinal bending with counterclockwise rotation of r 41 1 7 6 in the se and clockwise rotation of r 45 9 9 3 in the central taurides from upper cretaceous rocks with respect to the african reference direction paleomagnetic rotations in the se taurides are compatible with the vergent direction of the thrusts generated from consumption of the intra tauride ocean prior to post collisional convergence between taurides and the massif in the central taurides it has been shown that the clockwise rotation of 45 9 9 3 started in middle eocene because of a remagnetization in upper creataceous limestones the deformation was linked to the final closure of the southern neotethys and the collision between the african and eurasian plates in the nigde kirsehir massif counterclockwise rotation up to 25 5 7 3 is recognised during middle eocene and interpreted in terms of block rotation together with the taurides after the miocene a counterclockwise rotation of 16 8 3 9 along the eastern taurides shows that this area was mostly affected by the westward movement of anatolia despite the nigde kirsehir massif and its sw w area the central taurides which is recognised as stable with counterclockwise rotation less than 10

titanium mobilization by hydrocarbon fluids related to sill intrusion in a sedimentary sequence scotland abstract an occurrence of vein hosted bitumen in carboniferous rocks central scotland exhibits unusual enrichments in titanium zirconium and chromium detected by electron microprobe these elements occur as mineral inclusions of titanium and titanium iron oxides a close association between titanium rich tuffs and a quartz dolerite sill suggests that metals were leached from igneous rocks by a hydrocarbon rich fluid generated by the heat of the intrusion into organic rich shales this occurrence supports other evidence for widespread titanium mobility under low temperature conditions inclusions of nickel arsenide may reflect regionally important mineralization including former ore deposits related to sills and dykes as sills are extensively intruded into petroleum prospective basins offshore they may have a significant influence upon sediment geochemistry

magnetic mineralogy of sandstones from the lunde formation late triassic northern north sea uk origin of the palaeomagnetic signal the minerals responsible for the magnetic properties of the late triassic lunde formation have been examined using a combination of magnetic mineral extraction microscopy and rock magnetic measurements the magnetic mineralogy is controlled by the depositional environment and the effects of diagenesis by a process of differential preservation and modification of the original detrital minerals the magnetic susceptibility and remanence properties of the sediments are strongly influenced by the proportion of silt and clay sized clastic material which contains most of the magnetic minerals paramagnetic minerals both of depositional origin and produced during diagenesis are largely responsible for the magnetic susceptibility a suite of residual ferrimagnetic oxides chromite and mn substituted magnetite is responsible for the remanence properties in grey sandstones because most of the originally deposited magnetic fe ti oxides have been removed by reducing conditions during diagenesis however a small number of reddened horizons preserve a higher proportion of original detrital fe ti oxides ferrimagnetic ilmenite ferrimagnetic chromite and magnetite although authigenic hematite has replaced most of the original mineralogy in the reddened horizons magnetic oxide inclusions in quartz and feldspars are important contributors to the remanence signal particularly in horizons which have a low content of discrete detrital ferrimagnetic grains either through diagenetic dissolution or paucity of the silt and clay fraction within the grey sandstones the triassic palaeomagnetic signal is carried by magnetite as inclusions in detrital silicates with a contribution from detrital ferrimagnetic chromite a model of the effects of diagenesis on the magnetic mineralogy in deeply buried sequences is proposed

widespread chemical remagnetization orogenic fluids or burial diagenesis of clays paleomagnetic rock magnetic and geochemical results from mesozoic carbonates in the vocontian trough in southeast france support a hypothesized acquisition of a chemical remanent magnetization crm during burial diagenesis of smectite and are inconsistent with an orogenic fluid remagnetization mechanism the geographic and vertical distribution of a widespread crm which resides in magnetite is parallel to the degree of burial diagenesis of clays the crm is absent only where there is no evidence for clay diagenesis the crm is poorly developed where significant smectite is still present and is well developed with higher natural and anhysteretic remanent intensities where smectite has completely altered to illite strontium isotope results do not support alteration by orogenic type fluids a commonly invoked mechanism for similar crms the results suggest that the burial diagenesis of clays is a viable remagnetization mechanism for limestones in the vocontian trough

the continuous alteration of ilmenite through pseudorutile to leucoxene ore microscopic examinations and microprobe analyses show the alteration of ilmenite by leaching to be a continuous process proceeding from leached ilmenite through pseudorutile to leucoxene in this process which takes place near the surface and requires the presence of water a temperature of 50 c is not exceeded therefore diffusion processes are of no importance there are continuous phase transitions between ilmenite and its successive products pseudorutile with its iron poor members leached pseudorutile in particular is unstable and decomposes into leucoxene as a result of this and also of leucoxenic changes brought about by recrystallization there are sharp grain boundaries towards other alteration products simulating discontinuous leaching processes ilmenite leucoxene and their intermediate products are well distinguishable from each other by ore microscopic and subordinate also by x ray methods with ore microscopic studies enabling this distinction to be made faster and more easily water is the main agent in the leaching process which keeps going on as long as there is iron present that is able to migrate the continuous alteration of ilmenite can be expressed in terms of two reactions a 3fe2 tio3 fe3 x2 fe 23 x3 13 x ti3o9 in this equation x 0 3 x 0 corresponds to ilmenite x 3 to pseudorutile in the range of x 0 2 1 leached ilmenite occurs reaction a is established by oxidation and leaching of iron b fe23 ti3o9 fe2 y3 ti3o9 3y oh 3y in this equation y 0 2 y 0 corresponds to pseudorutile y 2 to leucoxene the range of y from 0 8 1 4 is identical with leached pseudorutile reaction b can be explained by hydrolization and leaching of iron whereas reaction a includes all phases representing oxides with different fe2 fe3 ratios reaction b comprises all phases with iron occurring in its trivalent state and the o oh ratio increasing as the fe3 content is decreasing

carbonate platform sequence stratigraphy in a rapidly subsiding area the late barremian early aptian of the organya basin spanish pyrenees abstract a detailed sedimentological and stratigraphical study of the late barremian early aptian urgonian carbonate platform of the organya basin spanish pyrenees has identified four depositional sequences the first two sequences consist of transgressive and highstand systems tracts and the last two sequences of lowstand transgressive and highstand systems tracts sequence boundaries at the top of sequences or 1 and or 4 are type 1 boundaries with evidence of exposure and erosion sequence boundaries at the top of sequences or 2 and or 3 are type 2 with no evidence of exposure or erosion sand shoal high energy facies characterise the first sequence or 1 sequences or 2 and or 3 are dominated by lagoonal facies with an important accumulation of lacustrine facies at the base of or 3 a succession of lacustrine deep platform and sand shoal facies characterises sequence or 4 the last sequence also contains the record of an anoxic event oae 1a all these characteristics have been largely influenced by the high subsidence rate of the organya basin 1100 m of compacted limestone deposited in about 5 ma however the main global eustatic changes registered in other contemporaneous basins were also identified

thermoviscous remagnetization in some appalachian limestones experimental evidence shows that blocking temperature relaxation time theory for single domain magnetite can grossly underestimate laboratory thermal demagnetization temperatures of a present day viscous remanence in devonian limestones of new york state thermoviscous remagnetization at moderately elevated burial temperatures during the late paleozoic cannot therefore be readily excluded for the origin of the stable secondary component in these rocks more generally these results emphasize that single domain thermal activation theory cannot be assumed a priori to correctly predict temperature of thermoviscous remanence acquisition in all magnetite bearing rocks the prob leto

remagnetization of carbonate rocks in southern tibet perspectives from rock magnetic and petrographic investigations netherlands organization for scientific research nwo with a rubicon grant 825 15 016 institute for rock magnetism irm at the university of minnesota instruments and facilities program of nsf erc starting grant 306810 nwo vidi 864 11 004

tethys atlantic interaction along the iberia africa plate boundary the betic rif orogenic system abstract initial se dipping slow subduction of the ligurian tethys lithosphere beneath africa from late cretaceous to middle oligocene twisting to a later faster e dipping subduction of the subcrustal lithosphere is proposed as an efficient geodynamic mechanism to structure the arcuate betic rif orogenic system this new subduction related geodynamic scenario is supported by a kinematic model constrained by well dated plate reconstructions tectonic sedimentary and metamorphic data sets the slow initial se dipping subduction of the ligurian tethys realm beneath the malaguide upper plate unit is sufficient to subduct alpujarride and nevado filabride rocks to few tens of kilometers of depth in middle eocene times the shift from se to e dipping subduction during latest oligocene early miocene was possibly caused by both the inherited geometry of the highly segmented ligurian tethys domain and by the fast roll back of the subducted lithospheric slab the early miocene rather synchronous multiple crustal and subcrustal processes comprising the collision along the betic front the exhumation of the hp lt metamorphic complexes the opening of the alboran basin its flooring by hp alpujarride rocks and subsequent ht imprint can be explained by the fast nw and w directed roll back of the ligurian tethys subcrustal lithospheric slab the w retreat of the ligurian tethys lithosphere in middle late miocene times could partly explain the initiation of its lateral tear and consequent subcrustal processes from latest miocene onward the betic rif system evolved under both the northerly push of africa resulting in tightening at crustal and subcrustal levels and by the distinct current dynamics of the steep lithospheric slab the sw directed scape of the rif fold belt is one of the most striking evidences linked to the recent evolution of the squeezed betic rif system between africa and iberia

miocene remagnetisation of carbonate platform and antalya complex units within the isparta angle sw turkey abstract a palaeomagnetic study has been carried out within the mesozoic and tertiary units of the relatively autochthonous carbonate platforms and the allochthonous deep sea volcanics and sediments of the antalya complex exposed around the isparta angle sw turkey the antalya complex is interpreted as a mosaic of carbonate platforms basinal sediments volcanic and ophiolitic rocks which formed within a southerly strand of the neotethyan ocean adjacent to gondwana the results indicate a widespread remagnetisation event negative fold tests show that the remanence at most sites is of secondary origin e g within the cirali lavas the magnetisation is carried by magnetite of presumed authigenic origin the remagnetisation event is believed to have occurred in the early middle miocene burdigalian langhian it was possibly triggered by the migration of orogenic fluids ahead of the advancing lycian nappes during their emplacement onto the carbonate platforms subsequent to remagnetisation a large segment of the isparta angle underwent an anticlockwise rotation of 30 this rotation is attributed to the overall convergence and bending of the hellenic arc and the final stages of emplacement of the lycian nappes during the late miocene in agreement with previous studies previously southerly palaeolatitudes were inferred from late triassic extrusives of the godene zone calbali dag unit the post folding magnetisation identified here within the cirali lavas of the godene zone to the south implies that these low palaeolatitudes result from the inappropriate application of structural tilt corrections the available data cannot be used to substantiate an origin for the antalya units south of the equator in the early mesozoic instead a position close to the northern margin of gondwana is indicated

pyrite chlorite and pyrite biotite relations in sandstones abstract authigenic euhedral pyrite in feldspathic sandstones of the visingso group upper proterozoic southern sweden is observed to be closely associated with authigenic fine crystalline interstitial chlorite and or sand sized chlorite or biotite microprobe analyses revealed that chlorites which are accompanied by relatively large amounts 3 vol of pyrite are characterized by a lower fe fe mg ratio than chlorites with lesser amounts 0 5 vol of associated pyrite the lower fe fe mg ratio of the former chlorites might have resulted from subsequent substitution of fe 2 by mg 2 in an initially fe rich chlorite the released fe 2 would then react with sulfide ions in an anoxic sulfidic environment to form pyrite another alternative is that the chlorites with lower fe fe mg ratio were crystallized from solutions characterized by a higher activity of mg 2 than chlorites with higher fe fe mg ratio iron from the biotite grains became available due to diagenetic illitization kaolinization and or chloritization

source of the climate signal recorded by magnetic susceptibility variations in indian ocean sediments the origins of magnetic susceptibility variations in deep sea sediments from the indian ocean ocean drilling program odp leg 117 have been examined to identify the specific connections between climate shifts and sediment magnetic properties in a previous study the magnetic susceptibility variations in odp hole 722b were identified as an outstanding proxy paleoceanographic record despite evidence of postdepositional loss of magnetic minerals through reductive diagenesis this paradox is addressed by assessing the contributions of ferrimagnetic detrital iron oxides bacterial magnetosomes and paramagnetic detrital fe silicate minerals to the magnetic susceptibility signal in addition to detailed magnetic analyses mineralogical morphological and grain size data have been obtained from representative magnetic extracts for hole 722b we find that magnetic responses to climate change result from 1 fluctuations in the volume magnetic susceptibility which is primarily controlled by carbonate dilution 2 a ferrimagnetic signal which is restricted to the upper 7 meters below sea floor mbsf and which largely reflects source area aridity and 3 a paramagnetic susceptibility record below 7 mbsf which is coincident in frequency with variations in lithogenic grain size

evidence of widespread cretaceous remagnetisation in the iberian range and its relation with the rotation of iberia a palaeomagnetic investigation has been carried out at 13 sites of jurassic age in the iberian range northern spain two components of remanent magnetisation have been found at each site a primary high temperature component shows an average counterclockwise rotation with respect to the north of 33 2o clockwise about a vertical axis corresponding to the absolute rotation of the iberian plate since the jurassic a secondary low temperature component shows a systematic declination difference of 16 4o with respect to the primary component this indicates that a rotation of iberia must have occurred between the two acquisition times comparison of the magnetisation directions with previous palaeomagnetic data and with sea floor spreading data constrains the age of the remagnetisation between 95 and 125 ma the remagnetisation may be associated with the extensional phases in the iberian basin in the early cretaceous barremian early albian or late cretaceous cenomanian a principal characteristic of the remagnetisation is its widespread character in the iberian range 1998 elsevier science b v all rights reserved

nano crystal formation of tio2 polymorphs brookite and anatase due to organic inorganic rock fluid interactions the occurrence of the titania polymorphs brookite and anatase as nano crystals in organic matter rich sediments of differing age and thermal maturity has been investigated by means of a multidisciplinary analytical approach fib tem organic geochemistry and petrography it was the aim of the study to analyze the formation mechanisms fate and behavior of the titania nano crystals as a result of organic inorganic rock fluid interactions brookite nano crystals have been detected in immature mediterranean sapropels of quaternary age but anatase also occurs in deeper and older black shales furongian alum shale sweden devonian to carboniferous bakken shale williston basin usa whereas anatase prevails as single crystals brookite nano crystals often are agglomerated single brookite nano crystals from posidonia shale lower jurassic northern germany have increasing crystal diameters with increasing thermal maturity exclusively anatase nano crystals both as single crystals or as agglomerates have been detected at oil water contacts in oilfields and along fractures with fluid flow enriched in dissolved organic carbon titania nano crystal precipitation growth and agglomeration takes place in the pore water of micro environments at low to high temperatures and where low ph is coupled to the occurrence of dissolved organic components low sedimentation rates preserving a critical geochemical environment or higher temperatures seem major controls for the precipitation of anatase and its tendency not to agglomerate

evaluation of sediment provenance using magnetic mineral inclusions in clastic silicates comparison with heavy mineral analysis magnetic fe oxide inclusions within framework clastic grains in sediments provide an indication of the provenance of the enclosing host silicate particles magnetic mineral inclusion characterisation is performed using a variety of magnetic properties that are related to magnetic mineral abundance magnetic grain size domain state oxidation state and magnetic grain interaction the magnetic methodology extends that based on a conventional set of environmental magnetic measurements using a variety of recent sediments transported from known rock sources within the uk and eire it is demonstrated using discriminant function analysis that the magnetic properties of the fe oxide inclusions provide a clear distinction of primary provenance this is both at the large scale in terms of various gneiss schist and granite sources and also at a smaller scale when considering subdivisions of these sources using case studies from the marine upper jurassic piper formation and non marine triassic sediments otter bank sandstone and foula sandstone formations it is shown that provenance differences clearly expressed by the results of heavy mineral analysis are also sensitively displayed by the magnetic mineral inclusion data the provenance classification is based around use of hierarchical cluster analysis and multidimensional scaling simple statistical tools are developed to determine a least noisy subset of magnetic parameters which are most suitable for stratigraphic provenance discrimination the triassic case study has indicated the magnetic mineral inclusion technique is most sensitive when using the sediment fractions larger than 150 m

new paleomagnetic mesozoic data from the algarve portugal fast rotation of iberia between the hauterivian and the aptian abstract we sampled nine sites of jurassic and cretaceous limestones in the algarve region portugal our study yields a paleopole for the portlandian valanginian 256 58 a95 5 and a paleopole for the aptian albian 232 75 a95 8 these results allow better constraint of the relative rotation of iberia with respect to europe this rotation of 22 14 is a rather fast event occurring during the barremian between 132 and 124 ma confinement of iberia between africa and eurasia would explain the rapid rotation of this microplate the iberian plate was probably attached to europe until 132 ma then detached from europe and acted as an independent plate during the time of fast rotation preceding the oceanic spreading in the bay of biscay

synfolding magnetization detection testing and geological applications summary a correct method for determining the direction of a synfolding remanence is described and a new test which in a number of cases can discriminate between a true synfolding remanence and a remanence that is the sum of pre and post folding components is suggested it is shown that it is possible to reveal the folding evolution of a local object under an assumption that the acquisition of the synfolding remanence was synchronous over this object

ferrimagnetic cr and mn spinels in sediments residual magnetic minerals after diagenetic dissolution ferrimagnetic cr fe and mn fe spinels are the dominant remanence carrying phases in the late triassic lunde formation these have largely been identified by magnetic mineral extraction from the sediments combined with microscopy the extensive removal of most of the originally deposited magnetic fe ti oxides during diagenesis has left the cr fe and mn fe spinels as a mineral residue which is resistant to dissolution the rock magnetic signature of these minerals is not sufficiently known to be able to identify these minerals on the basis of their magnetic properties alone

the direction correction tilt test an all purpose tilt fold test for paleomagnetic studies abstract the tilt or fold test in paleomagnetism is used to infer whether paleomagnetic remanence was acquired before or after tectonic tilting while several tilt test formulations have been proposed none fully satisfies the requirements of statistical validity applicability to all bedding geometries and ease of use this paper introduces the direction correction dc test which examines the relationship between the paleomagnetic site mean directions and their corresponding bedding tilt corrections the dc test is similar to mcfadden s correlation test because both test whether or not the site directions contain information about the bedding tilts however the dc test gives greater weight to sites with greater counter bedding the dc test is similar to watson s numerical tilt tests because they determine the degree of untilting which gives optimal concentration of site directions however the dc test uses analytical rather than numerical methods graphical output aids the researcher in recognizing problem sites using both real and simulated data the dc test is demonstrated to be more discriminating than other tilt test formulations for all bedding geometries the simulations show that the power of the tilt test is inversely proportional to the 95 confidence interval 95 of the overall mean as a rule of thumb paleomagnetists should attempt to sample sufficient sites to obtain an 95 less than 1 6 of the bedding attitude difference

paleomagnetism of the central iberian curve s putative hinge too many oroclines in the iberian variscides abstract the variscan mountain belt in iberia defines a large s shape with the cantabrian orocline in the north and the central iberian curve an alleged orocline belt of opposite curvature to the south the cantabrian orocline is kinematically well constrained but the geometry and kinematics of the central iberian curve are still controversial here we investigate the kinematics of the central iberian curve which plays an important role in the amalgamation of pangea since it may have accommodated much of the post collisional deformation we have performed a paleomagnetic study on carboniferous granitoids and cambrian limestones within the hinge of the curve our paleomagnetic and rock magnetic results show a primary magnetization in the granitoids and a widespread carboniferous remagnetization of the limestones syn kinematic granitoids show ca 70 counter clockwise rotations consistent with the southern limb of the cantabrian orocline post kinematic granitoids and cambrian limestones show consistent inclinations but very scattered declinations suggesting that they were magnetized coevally to and after the 70 rotation our results show no differential rotations between northern southern limb and the hinge zone therefore we discard a late carboniferous oroclinal origin for the central iberian curve

remagnetization of lower cretaceous limestones from the southern pyrenees and relation to the iberian plate geodynamic evolution paleomagnetic study 34 sites of thick cretaceous 5000 m marine carbonate strata from the organya basin ob in the southern pyrenees reveals a uniformly normal polarity characteristic remanence with varying declination d berriasian barremian d 295 o aptian albian d 348 o upper cretaceous d 354 o that resides in magnetite and predates late cretaceous tertiary folding the lack of reverse polarity magnetizations in the berriasian barremian series together with their distinct hysteresis properties that are typical of remagnetized carbonates elsewhere are taken as evidence for a secondary overprint in the lower part of the succession a hypothesized age of remagnetization near the barremian aptian boundary fits with a major tectosedimentary event in the basin that changed deposition from platform to basinal conditions and increased subsidence during the aptian the angular difference 53 o between declination from the remagnetized strata and the mean for aptian albian strata indicates that a counterclockwise rotation occurred between the two acquisition times part of this rotation is interpreted to be local and due to slip variation along strike of a normal fault that bounded the basin and tilted the pre aptian series this cretaceous extensive phase is framed in the rift evolution of the northern margin of the iberian plate the inferred remagnetization age within the ob falls within the range of previously inferred widespread cretaceous remagnetization events affecting the iberian plate in the ob remagnetization is speculated to have taken place through chemical burial diagenetic origin rather than simple thermoviscous resetting

biomagnetostratigraphy of the vikinghogda formation svalbard arctic norway and the geomagnetic polarity timescale for the lower triassic a bio magnetostratigraphy for the lower triassic is constructed using the ammonoid biostratigraphy from arctic boreal successions combined thermal and alternating field demagnetisation determines the triassic magnetic field polarity in 86 of specimens with 36 showing linear trajectory line fits and the remainder great circle trends towards the characteristic magnetisation mean pole directions for the deltadalen 50 159 dp dm 3 9 5 1 lusitaniadalen 56 163 dp dm 4 4 5 4 and vendomdalen 57 143 dp dm 4 4 5 4 members fall close to the european lower triassic apparent polar wander path mean directions for two of these member means pass the reversal test the remanence is predominantly carried by magnetite the polarity stratigraphy when integrated with the ammonoid and meager conodont data is similar to that determined from successions in the sverdrup basin canada the permian triassic boundary post dates a pronounced palynofloral turnover and pre dates a short duration reverse magnetozone lt1n 1r in the correlated shangsi section in s china lt1n 1r occurs after the fad of h parvus but in the arctic is within the otoceras boreale zone the late griesbachian to early smithian is mostly reverse polarity with three normal polarity intervals overlain by mid and late smithian normal polarity the spathian contains four reverse polarity intervals the oldest one within the early spathian with the remainder in the late spathian transition into the anisian is within the uppermost reverse magnetozone a feature documented in other sections of this age the polarity pattern is correlated to other marine sections indicating the robustness of the bio magnetostratigraphic composite and its utility in calibrating lower triassic time

comment on assessing discrepancies between previous plate kinematic models of mesozoic iberia and their constraints by barnett moore et al in their recent paper barnett moore et al 2016 reflect on current models of iberian plate motion in the jurassic and cretaceous as well as ongoing debates on the reliability of the various types of kinematic data that form independent constraints on iberia s motion relative to eurasia they question the validity of various marine geophysical seismic tomographic geological and paleomagnetic data sets from the bay of biscay central atlantic ocean and iberia for kinematic reconstruction of iberia and conclude that neither models invoking aptian albian transtension nor compression are consistent with currently available data an important element in their analysis is that they discard the large paleomagnetic data set from the jurassic and cretaceous from iberia based on perceived limitations of that data set in addition they argue that seismic tomographic images exclude a scenario of subduction in the aptian albian in the pyrenees and based on this question the validity of current plate reconstructions their constraints and geodynamic scenarios which are in support of this scenario e g vissers et al 2016 we welcome the discussion raised by barnett moore et al 2016 on the reliability and usefulness of paleomagnetic data as independent constraint for iberia s plate motion in the mesozoic taking these paleomagnetic data at face value vissers et al 2016 recently showed that these are consistent with an 40 counterclockwise rotation of iberia in the aptian requiring up to 500 km of aptian convergence across the pyrenees that is through subduction in this comment we aim to critically assess whether and how the concerns on the quality of paleomagnetic data raised by barnett moore et al 2016 may allow for an alternative explanation particularly one with a mesozoic rotation of iberia that is small enough so as to not requiring subduction we also reassess whether seismic tomographic images indeed refute subduction scenarios using 8 s wave and p wave tomographic models including those used in barnett moore et al 2016

magnetic carriers and remanence mechanisms in magnetite poor sediments of pleistocene age southern north sea margin pleistocene sedimentary sequences in the east anglian region of britain record both major and minor climatic oscillations and the impact of isostatic and eustatic variations intensively studied in terms of their lithology and biostratigraphy the sequences have been difficult to place in an absolute timeframe dating and correlation by magnetostratigraphy has been attempted over a num ber of years however these sediments are difficult to date by palaeomagnetic means because they are poor in detrital magnetite are subject to post depositional deformation and diagenesis and have unknown rates of sedimentation determining whether their natural remanence magnetisation nrm directions are reliable thus requires information on the mode and timing of remanence acquisition here we apply palaeomagnetic rock magnetic and mineralogical analyses to identify the nrm carriers in these sediments and hence their palaeomagnetic reliability within oxidised fluvial sediments the kesgrave formation the magnetic carriers appear to be relict magnetic minerals fer rian ilmenites chromites haematite and goethite which sometimes carry a reliable primary deposi tional remanence drm but often an overprinting viscous time varying remanence vrm within some reduced marine and intertidal sediments within the crag basin the iron sulphide greigite has been found to carry a reliable syn depositional chemical remanence crm in all the sediments magnetic inclusions within silicates are abundant are significant for the mineral magnetic signal but contribute little to any recoverable palaeomagnetic information copyright 2005 john wiley sons ltd

the role of detrital composition and climate on the diagenetic evolution of continental molasses evidence from the cambro ordovician guaritas sequence southern brazil abstract the cambro ordovician molassic guaritas sequence camaqua basin southern brazil comprises alluvial fan and braided fluvial sandstones and conglomerates with intercalated aeolian and lacustrine deltaic deposits and andesitic lava flows the sediments display a complex detrital composition derived from plutonic gneissic acidic volcanic and low grade metamorphic source rocks this detrital assemblage was strongly modified by semi arid continental near surface diagenesis early cementation by hematite smectite quartz and calcite and the relatively limited burial prevented strong compaction and preserved some primary macroporosity in most of the sandstones whereas the absence of early cements and or abundance of ductile grains promoted substantial porosity destruction by compaction and the inhibition of further diagenetic modifications the diagenetic dissolution and replacement of volcanic rock fragments and detrital feldspars by clays and albite changed the original framework composition as well as the tectonic provenance classification of the sandstones detailed quantitative petrographic study allowed the reconstruction of the original detrital compositions and the distinction of six different pathways of diagenetic evolution of the sandstones attesting to the efficiency of this method for diagenetic modelling and provenance analyses of ancient sandstones

heavy mineral characteristics of the neogene quaternary sediments from the southern part of the blagoevgrad basin sw bulgaria abstract the blagoevgrad basin a graben filled with neogene quaternary continental sedimentary successions is one of the most prospective placer gold bearing regions in southwestern bulgaria the quaternary sediments have been well explored and economically prospective areas have been identified previous studies indicated very limited occurrences of placer gold in the neogene sediments the genesis of which is connected with resedimentation of paleogene sediments from the adjacent bobov dol basin to the north from these previous studies a detailed allo and lithostratigraphic scheme and paleogeographic models were developed the present study aimed to evaluate the plausibility of the existing stratigraphic schemes and paleogeographic models for the area through detailed study of the heavy mineral fraction of the neogene and part of the quaternary sediments from the southern part of the blagoevgrad basin additional objectives were to determine the mode of occurrence and the stratigraphic position of the placer gold in the neogene sediments and to investigate possible source areas special attention was paid to the mineral composition and the structure of the opaque detrital fe ti oxide minerals and the typologic features of the zircons the results obtained could be used as a provenance indicator and for determination of the nature of the source area and the processes that operated therein a very important finding is the discovery of placer gold in an additional three neogene units the morphologic features of the gold grains indicate short transport or proximity to the primary source this interpretation is inconsistent with existing hypotheses on the origin and presence of the gold in the sediments of the basin the characteristics of the magnetic heavy mineral fraction and the morphologic features of zircon grains in auriferous samples indicate the diorites and gabbro diorites from the strouma diorite group which outcrop along the eastern edge of the graben as a possible source the stratigraphic distribution of the heavy minerals fully supports the plausibility of the existing detailed allo and lithostratigraphic subdivision and the paleogeographic model for the development of the southern part of the basin also the results provide support for the soundness of an allostratigraphic approach for the study and exploration of placer deposits in basins similar to the blagoevgrad

pyrenean orogeny and plate kinematics 1 the evolution of the pyrenees a mountain range between iberia and eurasia has remained the subject of many debates between geologists and geophysicists for a long time by combining the identification of seafloor spreading anomalies a33o to m0 in the bay of biscay with those in the north atlantic we have derived a position of a mean pole of rotation for the entire opening of the bay of biscay four hundred kilometers of shortening took place between the iberian and eurasian plates in the pyrenean domain during the opening of the bay of biscay from chrons m0 to a33o time 118 to 80 ma the deep seismic etude continentale et oceanique par reflexion et refraction sismique ecors profile shot across the pyrenees and teleseismic data show the presence of two distinct slabs which dip to the north the southern slab is linked to the subduction of the neo tethys ocean which was created from late jurassic to early aptian simultaneously elongated back arc basins formed along the future pyrenean domain this slab was active from at least 118 ma early aptian to 100 ma late albian the northern slab active since 85 ma is linked to the subduction of the lower continental crust located south of the pyrenean domain in the upper crust normal faults as well as the north pyrenean fault became reverse faults and former back arc basins were inverted giving rise to the uplift of the pyrenees as a double asymmetrical wedge

remagnetizations as a tool to analyze the tectonic history of inverted sedimentary basins a case study from the basque cantabrian basin north spain 1 in this work we use syntectonic remagnetization data to determine the synrift geometry of inverted basins this methodology analyzes remagnetization directions in order to obtain the tilting of beds at the moment of the acquisition of the overprint filtering the subsequent compressional deformation we collected samples from jurassic and lower cretaceous limestones and red beds cabuerniga and pas groups in the cabuerniga basin one of the most important mesozoic basins developed near the iberia europe mesozoic plate boundary in northern spain that later inverted during the uplift of the pyrenees these deposits represent the synrift sequence linked to a widespread extensional stage in the iberian basins during the mesozoic all sampled rocks show a normal polarity remagnetization fold and conglomerate tests indicate that remagnetization postdates the main extensional stage of basin formation late jurassic barremian and was acquired before the subsequent compressional deformation the analysis of these overprinted directions allows the basin geometry of the studied area to be restored at the moment of acquisition of the remagnetization this approach represents a powerful tool to study basinal evolution and uplift of sedimentary basins to be tested in other inverted basins with syn extensional related secondary overprints

diachronous pervasive remagnetization in northern iberian basins during cretaceous rotation and extension article i nfo northern iberia exposes a series of mesozoic sedimentary basins whose final formation history is tied to a rotation episode of iberia during the aptian related to the opening of the bay of biscay many of these basins experienced widespread remagnetization previously loosely tied to a mid cretaceous thermal event here we make use of the improved apparent polar wander path apwp of iberia to narrowly constrain the age of their remagnetization to show that several basins experienced a regional diachronous remagnetization we apply the small circle intersection sci method to reconstruct the paleomagnetic field direction during remagnetization in the organya basin in the southern pyrenees spain where the age of remagnetization is stratigraphically well dated to around the barremian aptian boundary and where the paleomagnetic direction during remagnetization has been well established the resulting direction of d i 316 8 5 7 54 8 3 3 where the uncertainty in declination is approximated by 95 cos i and the error in inclination by 95 correspondscloselytoarecently improvedapwpofiberia providingsupportfortheapplicabilityof thismethod applicationofthescimethodallowsustoshowthattheolderportionoftheremagnetizedbedswasalreadytilted

cretaceous slab break off in the pyrenees iberian plate kinematics in paleomagnetic and mantle reference frames the pyrenees at the iberia europe collision zone contain sediments showing albian cenomanian high temperature metamorphism and coeval alkaline magmatic rocks stemming from different views on jurassic cretaceous iberian microplate kinematics two schools of thought exist on the trigger of this thermal pulse one invoking hyperextension of the iberian and eurasian margins the other suggesting slab break off competing scenarios for mesozoic iberian motion compatible with pyrenean geology comprise 1 transtensional eastward motion of iberia versus eurasia or 2 strike slip motion followed by orthogonal extension both favoring hyperextension related heating and 3 scissor style opening of the bay of biscay coupled with subduction in the pyrenean realm favoring the slab break off hypothesis we test these kinematic scenarios for iberia against a newly compiled paleomagnetic dataset and conclude that the scissor type scenario is the only one consistent with a well defined 35 counterclockwise rotation of iberia during the early aptian we proceed to show that when taking absolute plate motions into account aptian oceanic subduction in the pyrenees followed by late aptian early albian slab break off should leave a slab remnant in the present day mid mantle below nw africa mantle tomography shows the reggane anomaly that matches the predicted position and dimension of such a slab remnant between 1900 and 1500 km depth below southern algeria mantle tomography is therefore consistent with the scissor type opening of the bay of biscay coupled with subduction in the pyrenean realm slab break off may thus explain high temperature metamorphism and alkaline magmatism during the albian cenomanian in the pyrenees whereas hyperextension that exhumed pyrenean mantle bodies occurred much earlier in the jurassic

authigenesis of titania minerals and the mobility of ti new evidence from pro deltaic sandstones cretaceous scotian basin canada titania tio2 minerals are minor diagenetic phases in many sandstone shale successions but the conditions of ti mobility and titania authigenesis are poorly known here new modes of occurrence of authigenic titania minerals filling pores and replacing phytodetritus are documented from lower cretaceous prodeltaic sandstones in the offshore scotian basin of southeastern canada textural relationships suggest that titania minerals formed at the same time as kaolinite most commonly below lowstand sequence boundaries related to flux of meteoric water such authigenic titania is most abundant in thin bedded sandstones interbedded with thick prodeltaic mudstone in which fine grained detrital ti rich minerals are concentrated textures in titania that has replaced uncompacted phytodetritus suggest microbial mediation in near surface eodiagenesis eogenetic titania is enriched in phosphorus 0 6 p2o5 and may co occur with authigenic fluorapatite and crandallite crystals such titania resulted from prolonged flow of meteoric water that dissolved p from apatite and feldspar thereby enhancing ti solubility in basins with supply of immature sediment from granitic terranes and important diagenesis by meteoric water the assumption that ti is an immobile element and that rutile or other titania minerals are exclusively of detrital origin may not be valid probable mesodiagenetic titania has low p content suggesting that ti mobility under these conditions was facilitated by some other agent probably organic acids the results of this study illustrate that even in cases where titania minerals are volumetrically insignificant their microtextures and trace element chemistry can provide important insights into the character of fluid flow during diagenesis of siliciclastic rocks

diagenetic chloritization of feldspars in sandstones abstract detrital microcline and albite are commonly replaced diagenetically by chlorite in sandstones of the visingso group upper proterozoic southern sweden authigenic quartz and in some cases albite and illite are associated with the chlorite the sem examinations suggest that chloritization of feldspar has usually been preceded by dissolution of the replaced feldspar grain fe 2 and mg2 which are needed for the chloritization have partly been derived from the detrital fe mg silicates mainly biotite fe oxide pigment and fe ti oxide grains whereas si4 and al3 are supplied mostly by the altered feldspars the k released from microcline and na from albite were mostly incorporated in authigenic illite and albite respectively

microstructural trace element and geochronological characterization of tio 2 polymorphs and implications for mineral exploration abstract the geochemistry of rutile tio2 has recently found its use in mineral exploration with some studies reporting anomalous concentrations of fe w v sn and sb in rutile associated with mineralized ore systems however the use of rutile as a prospecting tool is likely to be complicated by the systematic changes in trace element composition with tio2 polymorph type anatase brookite and rutile here we present tio2 trace element and u pb geochronological data from the mineralized and barren portions of the palaeoproterozoic moorarie supersuite and capricorn orogen western australia with a focus on the minnie creek molybdenum prospect in the northern part of the gascoyne province the barren samples contain all three tio2 polymorphs anatase brookite and rutile textures suggest anatase and brookite may have formed during low t metamorphism either through replacement of previous rutile grains or titaniferous minerals rutile grains from barren samples yield variable u pb ages ca 3 0 2 2 ga as well as variable textures and chemical compositions suggesting detrital origins thus most likely representing metasedimentary units intruded by the moorarie supersuite rutile grains from the minnie creek prospect yield palaeoproterozoic ca 1 77 1 75 ga u pb cooling ages and nb ta concentrations of up to 17 wt that along with inclusions of manganocolumbite oscillatory and patchy zonation of nb and fe suggest a magmatic origin the commonly used pathfinder elements for gold and base metal mineralisation fe cr v w sn and sb are shown to be systematically lower in anatase and brookite thus yielding false negatives if polymorph type is not identified during reconnaissance studies for this reason a ternary diagram was constructed based on the systematic changes in chemistry of tio2 polymorphs to provide a relatively fast and easy chemical discrimination of polymorphs in large volumes of reconnaissance data furthermore it is shown that high al concentrations are characteristic of brookite and to a lesser degree anatase but not rutile in addition sn nb ta and w concentrations in rutile may be more sensitive to igneous processes and may be used to track processes occurring in strongly fractionated granitic magmas such as pegmatites and associated deposits

can a primary remanence be retrieved from partially remagnetized eocence volcanic rocks in the nanmulin basin southern tibet to date the india asia collision paleomagnetic dating of the india asia collision hinges on determining the paleogene latitude of the lhasa terrane southern tibet reported latitudes range from 5 n to 30 n however leading to contrasting paleogeographic interpretations here we report new data from the eocene linzizong volcanic rocks in the nanmulin basin which previously yielded data suggesting a low paleolatitude 10 n new zircon u pb dates indicate an age of 52 ma negative fold tests however demonstrate that the isolated characteristic remanent magnetizations with notably varying inclinations are not primary rock magnetic analyses end member modeling of isothermal remanent magnetization acquisition curves and petrographic observations are consistent with variable degrees of posttilting remagnetization due to low temperature alteration of primary magmatic titanomagnetite and the formation of secondary pigmentary hematite that unblock simultaneously previously reported paleomagnetic data from the nanmulin basin implying low paleolatitude should thus not be used to estimate the time and latitude of the india asia collision we show that the paleomagnetic inclinations vary linearly with the contribution of secondary hematite to saturation isothermal remanent magnetization we tentatively propose a new method to recover a primary remanence with inclination of 38 1 35 7 40 5 95 significance and a secondary remanence with inclination of 42 9 41 5 44 4 95 significance the paleolatitude defined by the modeled primary remanence 21 n 19 8 n 23 1 n is consistent with the regional compilation of published results from pristine volcanic rocks and sedimentary rocks of the upper linzizong group corrected for inclination shallowing the start of the tibetan himalaya asia collision was situated at 20 n and took place by 50 ma

a reply to the comment on assessing discrepancies between previous plate kinematic models of mesozoic iberia and their constraints by barnett moore et al we welcome the comments of van hinsbergen et al 2017 on the recent efforts of barnett moore et al 2016 specifically van hinsbergen et al 2017 raise concerns about two of the major conclusions made by barnett moore et al 2016 firstly barnett moore et al 2016 choose to negate the cretaceous iberian paleomagnetic database as a viable plate kinematic constraint on the plate motions of mesozoic iberia this conclusion criticized by van hinsbergen et al 2017 was based on citing the previous efforts of neres et al 2012 2013 which exposed several shortcomings elaborated on below within this dataset secondly van hinsbergen et al 2017 criticize barnett moore et al 2016 for dismissing mantle tomographic interpretations in support of a preserved cretaceous pyrenean subducted slab beneath northern africa below we have addressed each of these major criticisms from van hinsbergen et al 2017 in a two section layout similar to their comment above

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

impact of pore fluid salinity on the mechanical behavior of unsaturated bentonite sand mixture high salinity of local groundwater and the thermal gradient present within the deep geological repository environment make it necessary to investigate the effect of the pore fluid salinity on the hydro mechanical behavior engineered barriers at different degrees of saturation this study examines the impact of cacl2 salt solution at two concentrations i e 100 and 250 g l on the compressibility and shear strength behavior of the bentonite sand buffer bsb within a range of saturations both as compacted and desiccated bsb specimens were studied desiccated specimens were prepared in a constant suction environment using the vapor equilibrium technique and high pressure cells were used to carry out the triaxial shear experiments experimental results were compared with the available data from the literature for bsb materials prepared with distilled water in order to get a better understanding of the impact of the salinity the comparison shows that the existence of salt in the specimens has notable effect on the total suction levels compressibility and shear strength of the material

diagenetic magnetite carries ancient yet secondary remanence in some paleozoic sedimentary carbonates many sedimentary carbonate rocks carry stable magnetizations that can be shown to reside in magnetite when such magnetizations are observed it is often argued or demonstrated that the magnetite was incorporated into the sediment during deposition however paleomagnetic and rock magnetic studies in conjunction with analyses of magnetic extracts from the helderberg and bonneterre carbonates united states indicate that the magnetite present in these rocks is most likely of diagenetic i e postdepositional origin

strain partitioning of deformation mechanisms in limestones examining the relationship of strain and anisotropy of magnetic susceptibility ams abstract in order to investigate the relationship between rock strain and anisotropy of magnetic susceptibility ams strain partitioning and ams analysis was conducted at 35 sites from two stratigraphically adjacent paleozoic limestone units in the patterson creek and wills mountain anticlines in the central appalachian orogen of west virginia in addition anisotropy of anhysteretic remanent magnetization aarm was conducted on selected samples to examine the role of preferentially oriented magnetite on the ams fabric strain is partitioned into bed normal shortening due to compaction solution strain 35 0 shortening bed parallel shortening due to tectonic solution strain 13 3 shortening calcite twinning strain 5 8 shortening and grain boundary sliding 26 7 shortening the ams fabrics in the rocks were found to be a result of a complex interaction between rock lithology deformation mechanisms and strain magnitude although all the rocks have experienced the same deformation conditions six different ams fabrics are exhibited each of the different ams fabrics is a composite fabric resulting from the overprinting of three components 1 an inherent primary depositional ams fabric that is attributed to preferentially oriented phyllosilicates in the rock matrix 2 a diagenetic and or compaction ams fabric formed during burial that is due to preferentially oriented phyllosilicates in solution structures and in the rock matrix and 3 a tectonic ams fabric that was imparted on the rocks by layer parallel shortening deformation prior to folding and is also attributed to preferentially oriented phyllosilicates in solution structures and in the rock matrix as well as twinning of ferroan calcite

the rotation of iberia during the aptian and the opening of the bay of biscay abstract the cretaceous rotation of the iberian plate in a geodynamical active setting between africa and europe is contemporaneous with the opening of the northern atlantic ocean and provides fundamental temporal and kinematical constraints on the evolution of the bay of biscay and the pyrenees here we report new paleomagnetic data from 50 sites 1109 drilled cores from marine sediments of the cretaceous organya basin in the southern pyrenees to constrain the timing and magnitude of the rotation of iberia berriasian to barremian directions in exclusively limestone lithology are remagnetised in agreement with previous organya basin studies the aptian to cenomanian marls and limestones in the organya basin however provide primary magnetisations asserted by rock magnetic experiments and positive fold tests our results constrain the rotation of iberia to be finished just before the albian paleomagnetic data cannot well constrain the onset of the rotation but sea floor anomaly studies restrict the onset of the rotation to anomaly m0 or the barremian aptian boundary this implies that the entire rotation of iberia happened in one single phase and is confined to the aptian this is in contrast to earlier studies that suggested a two phase rotation of iberia but we argue that the large errors in published ages are not in disagreement with our results in summary we constrain the 35 rotation of iberia to the aptian period which implies a rotation rate of 2 7 5 1 myr depending on the controversial duration of the aptian

comment on gonzalez acebron et al criteria for the recognition of localization and timing of multiple events of hydrothermal alteration in sandstones illustrated by petrographic fluid inclusion and isotopic analysis of the tera group northern spain int j earth sciences 2011 100 1811 1826 the paper by gonzalez acebron et al 2011 deals with both regional and methodological problems about the timing of thermal events in one of the best known and controversial basins in the iberian microplate the late jurassic early cretaceous cameros basin the aforementioned paper contains invaluable data about the thermal evolution of the basin and it is an important contribution to the understanding of the post sedimentary pre inversion processes in this strongly subsiding basin it will undoubtedly add new evidence to the knowledge of hydrothermal processes in general and particularly to the paleothermal evolution of the basin however in our opinion there are some problems involving important matters within the paper namely 1 the interpretation of the data obtained which are too constrained within a univocal line of thinking without considering other simpler and more viable options that in the discussed paper are aprioristically rejected 2 the second problem deals with referencing to previous papers and acknowledgement of previous works by other authors we must say that this aspect does not refer to this paper alone but it is recently becoming a commonplace for many scientific papers in this field on the side of the references to previous works it must be stated that

the diffuse plate boundary of nubia and iberia in the western mediterranean crustal deformation evidence for viscous coupling and fragmented lithosphere a spatially dense gnss based crustal velocity field for the iberian peninsula and northern africa allows us to provide new insights into two main tectonic processes currently occurring in this area in particular we provide for the first time clear evidence for a large scale clockwise rotation of the iberian peninsula with respect to stable eurasia euler pole component n42 612 w1 833 clockwise rotation rate of 0 07 deg myr we favor the interpretation that this pattern reflects the quasi continuous straining of the ductile lithosphere in some sectors of south and western iberia in response to viscous coupling of the nw nubia and iberian plate boundary in the gulf of cadiz we furnish evidence for a fragmentation of the western mediterranean basin into independent crustal tectonic blocks which are delimited by inherited lithospheric shear structures among these blocks an oceanic like western algerian one is currently transferring a significant fraction of the nubia eurasia convergence rate into the eastern betics se iberia and likely causing the eastward motion of the baleares promontory these processes can be mainly explained by spatially variable lithospheric plate forces imposed along the nubia eurasia convergence boundary

alteration of detrital fe ti oxides in sedimentary rocks scanning electron microscope study has revealed that in proterozoic sandstones shales and carbonate rocks of the visingso group in southern sweden detrital fe ti oxides have undergone alteration by dissolution and or replacement mainly by titanium oxides hematite reddish rocks or pyrite greenish sandstones shales and carbonate rocks the titanium oxides occur as either poorly crystalline masses cryptocrystalline aggregates or discrete euhedral crystals of anatase brookite and rutile that attain a variety of crystal habits microprobe analyses have shown that the alteration of ilmenite occurs through several intermediate phases each successively enriched in titanium and depleted in iron to an almost pure form of tio 2 some of the iron and titanium which is released during alteration of the fe ti oxides is incorporated in associated clay minerals

end member modelling as an aid to diagnose remagnetization a brief review remagnetization of a palaeomagnetic signal is difficult to recognize independently of directional information the situation becomes more complex when remagnetized rocks pass palaeomagnetic field tests for example when the remagnetization of a rock sequence has occurred before folding it is evident that palaeogeographic reconstructions are seriously flawed when actu ally remagnetized rocks are not identified as such here we discuss the merits and pitfalls of so called end member modelling of acquisition curves of the isothermal remanent magnetization irm to recognize remagnetized strata the technique requires no a priori information about the irm acquisition curves the algorithm unmixes a set of irm acquisition curves into a number of invariant curves termed end members and calculates the mixing proportions of the end members for each sample since primary natural remanent magnetization nrm and remag netized nrm are acquired by different processes their signatures can be recognized from subtle differences in the magnetic properties we illustrate the potential of the approach by three case studies one from spain and two from turkey in which the magnetic properties of remagnetized and non remagnetized rocks are evaluated the natural remanent magnetization nrm of a rock is described asremagnetized when itrepresents apalaeomagnetically younger age thanthe sediment age of the rock formation under study remagneti zation interferes with palaeogeographic reconstruc tions asisincreasinglybeingappreciated the late palaeozoic remagnetization event was firmly rec ognized in the appalachians in the usa in the early 1980s e g mccabe et al 1983 see the review of van der voo torsvik 2012 while the implica tionsoflatepalaeozoicremagnetizationineuropean

new paleomagnetic results from cretaceous sediments near lisboa portugal and implications for the rotation of iberia abstract paleomagnetic study of outer platform sediments of late jurassic early cretaceous age from portugal lisboa area leads to characteristic remanent magnetization directions for the hauterivian barremian d 317 i 58 and the aptian d 344 i 47 the absolute rotation of iberia between 125 and 110 ma is consequently of about 27 in an anticlockwise sense a later rotation of about 14 follows the first one in the same sense from 110 ma the total absolute rotation of iberia from the hauterivian age determined by paleomagnetism is thus about 41 the comparison of declinations measured with paleodeclinations calculated in lisboa from the eurasian and african poles of similar age indicates that the main phase of relative rotation of iberia with respect to eurasia occurs during the first stage of relative motion i e hauterivian to aptian the tectonic history of the bay of biscay is in good agreement with these two stages of rotation the total relative rotation with respect to europe is 34 this value is consistent with results previously established by van der voo 2 3 25 but we have established that the main phase of rotation 30 occurs in the hauterivian aptian interval the movement of iberia is clearly linked to the movement of africa in direction and age but the amount of rotation of iberia during the early cretaceous is much larger and implies that iberia was not a real part of the african plate

multidisciplinary approach to an extensional syncline model for the mesozoic cameros basin n spain abstract in this paper we explore the possibility of reconstructing the initial geometry of inverted basins using combined geophysical and geological methods seismic reflection profiles paleomagnetism analogue modelling and 3d geometrical analysis these techniques allow for some unknowns in the geometry and evolution of extensional sedimentary basins mainly the normal drag reverse drag geometrical problem to be solved the example chosen for checking these techniques is the cameros basin northern iberian chain formed during the late jurassic early cretaceous and inverted during the tertiary compressional stage according to the reconstruction proposed the cameros basin can be characterised as a lens shaped syncline basin with maximum thickness of deposits of about 8000 m in its depocenter gradually diminishing toward the north and south the extensional geometry of the cameros basin is defined by 1 a large scale growth syncline over the major basement fault with southward dipping beds in the northern basin margin forming a southward opening sedimentary wedge with upward decreasing dip values large scale normal drag geometry 2 roll over anticlines at the contact between the pre rift series against the normal fault limiting the basin smaller scale reverse drag geometry the combined syncline anticline syn extensional structures were characterised by means of paleomagnetic analysis of a remagnetization acquired at the time of extension analogue models indicate that the development of syncline basin geometries in extensional settings as the one interpreted for the cameros basin is closely related to the presence of a detachment level in the pre rift sequence above the main basement faults

pervasive palaeogene remagnetization of the central taurides fold and thrust belt southern turkey and implications for rotations in the isparta angle the turkish anatolide tauride block rifted away from the northern margin of gondwana in the triassic which gave way to the opening of the southern neo tethys by the late palaeocene to eocene it collided with the southern eurasian margin leading to the closure of the northern neo tethys ocean to determine the position of the anatolide tauride block with respect to the african and eurasian margin we carried out a palaeomagnetic study in the central taurides belt which constitutes the eastern limb of the isparta angle the sampled sections comprise carboniferous to palaeocene rocks mainly limestones our data suggest that all sampled rocks are remagnetized during the late palaeocene to eocene phase of folding and thrusting event related to the collision of the anatolide tauride block with eurasia to further test the possibility of remagnetization we use a novel end member modelling approach on 174 acquired isothermal remanent magnetization irm curves we argue that the preferred three end member model confirms the proposed remagnetization of the rocks comparing our data to the post eocene declination pattern in the central tauride belt we conclude that our clockwise rotations are in agreement with data from other studies after combining our results with previously published data from the isparta angle that includes our study area we have reasons to cast doubt on the spatial and temporal extent of an earlier reported early to middle miocene remagnetization event we argue that the earlier reported remagnetized directions from triassic rocks in tilt corrected coordinates from the southwestern antalya nappes western taurides are in good agreement with other studies from the area that show a primary origin of their characteristic remanent magnetization this implies that we document a clockwise rotation for the southwestern antalya nappes since the triassic that is remarkably similar to the post eocene 40 rotation of the central taurides for the previously published results that are clearly remagnetized we argue that their remagnetization has occurred in the palaeocene to eocene

widespread cretaceous inversion event in northern spain evidence from subsurface and palaeomagnetic data abstract in this paper we present geological evidence of a mid cretaceous inversion event in the polientes basin northern iberian plate based on geological data analysis of seismic profiles across the basin indicates that this tectonic event resulted in the uplift of the basin depocentre and the formation of an upright gentle anticline before cenomanian times the presence of a pre alpine syntectonic remagnetization in the jurassic and lower cretaceous rocks of the polientes basin allows us to define the geometry of the basin at the time between this early inversion event and the main alpine compressional period the early inversion in the polientes basin is consistent with a transpressional tectonic setting contemporaneous with the left lateral strike slip movement of iberia with respect to europe this study shows the successful use of two approaches to unravel the geodynamic scenario of the northern iberian plate during the cretaceous which was later obliterated during the tertiary pyrenean orogeny

reconciling cretaceous paleomagnetic and marine magnetic data for iberia new iberian paleomagnetic poles 1 the cretaceous paleogeography and the kinematic evolution of the iberian plate are poorly constrained especially problematic is to reconcile iberian paleomagnetic data with paleomagnetic data of the neighboring plates and with euler poles derived from seafloor magnetic anomalies the first limitation arises from the cretaceous normal polarity superchron where paleogeographic reconstruction using marine magnetic anomalies is handicapped the second arises from the paucity of reliable paleomagnetic poles with satisfactory statistical criteria and age in order to address these shortcomings and provide new high quality paleomagnetic poles for iberia we conducted a detailed rock magnetic and paleomagnetic study of two cretaceous magmatic sills the paco de ilhas pi and foz da fonte ff sills from the lusitanian basin portugal recently dated at about 88 and 94 ma respectively our results show that the magnetic mineralogy of the sills is primary i e acquired during magma cooling and essentially represented by titanomagnetite the corresponding paleomagnetic poles match the synthetic apwp from the african plate at 80 and 100 ma on the basis of a rigorous selection of iberian cretaceous poles we then calculated mean paleomagnetic poles for different time intervals and found that iberian paleomagnetic data fit well the global apwp between 70 and 120 ma but move far away from the apwp at pre rift times our approach shows that new and better constrained paleomagnetic poles can aide in solving part of the contradiction between iberian and african apwps

evidence of a cretaceous remagnetization in the cameros basin north spain implications for basin geometry abstract the paleomagnetic results obtained in 23 sites from red beds of the northern border of the inverted cameros basin northern iberian peninsula prove a cretaceous widespread remagnetization paleomagnetic and rock magnetic analyses indicate that the natural remanent magnetization nrm is dominated by a stable and always normal polarity component carried by haematite two conglomerate test and five fold test indicate that this component is a syn tectonic overprint dated between albian and santonian times most probably albian contemporary with a low grade metamorphism remagnetization was acquired before compressional deformation and post dates the main extensional stage of basin formation berriasian albian incremental fold tests provide best clustering solutions showing dispersed results and far from the expected direction an alternative procedure to obtain fold test solution was applied considering asymmetric limb rotation mean directions obtained from these asymmetric solutions are consistent with the expected direction and yield a statistical value for significant grouping at the 95 confidence level in all fold tests performed this reconstruction allows to determine the tilting of beds at the moment of acquisition of magnetization these original dips were used to reconstruct the original geometry of the northern basin border later modified during the tertiary inversion stage this extensional geometry can be characterised as a large scale syn sedimentary normal fault drag with a minor roll over anticline the procedure developed in this paper should be tested in other inverted basins that also have undergone burial remagnetizations

paleomagnetic dating of burial diagenesis in mississippian carbonates utah 1 the objective of this study is to test models for the origin of widespread secondary magnetizations in the mississippian deseret limestone the delle phosphatic member of the deseret limestone is a source rock for hydrocarbons and modeling studies indicate that it entered the oil window in the early cretaceous during the sevier orogeny paleomagnetic and rock magnetic results from the deseret limestone and the stratigraphically equivalent chainman shale in central and western utah indicate that the units contain two ancient magnetizations residing in magnetite burial temperatures are too low for the magnetizations to be thermoviscous in origin and they are interpreted to be chemical remanent magnetizations crms fold tests from western utah indicate the presence of a prefolding triassic to jurassic crm geochemical 87sr 86sr 13c and 18o and petrographic analyses suggest that externally derived fluids did not alter these rocks this crm was acquired at the beginning of the oil window and is interpreted to be the result of burial diagenesis of organic matter a second younger crm in western central utah is apparently postfolding and is probably late cretaceous to early tertiary in age on the basis of the thermal modeling the timing overlaps with the oil window these results are consistent with a connection between organic matter maturation and remagnetization modeling of the smectite to illite transformation in the deseret limestone suggests a mean age prior to acquisition of both crms although the range for illitization overlaps with the triassic to jurassic crm the results of this study support the hypothesis that pervasive crms can be related to burial diagenetic processes in addition paleomagnetism can be used to determine the timing of such processes which can benefit hydrocarbon exploration efforts

magnetic properties of pelagic marine carbonates abstract pelagic carbonates are deposited far from continents usually at water depths of 3000 6000 m at rates below 10 cm kyr and are a globally important sediment type recent advances with recognition of widespread preservation of biogenic magnetite the inorganic remains of magnetotactic bacteria have fundamentally changed our understanding of the magnetic properties of pelagic carbonates we review evidence for the magnetic minerals typically preserved in pelagic carbonates the effects of magnetic mineral diagenesis on paleomagnetic and environmental magnetic records of pelagic carbonates and what magnetic properties can tell us about the open ocean environments in which pelagic carbonates are deposited we also discuss briefly late diagenetic remagnetisations recorded by some carbonates despite recent advances in our knowledge of these phenomena much remains undiscovered we are only at early stages of understanding how biogenic magnetite gives rise to paleomagnetic signals in sediments and whether it carries a poorly understood biogeochemical remanent magnetisation recently developed techniques have potential for testing how different magnetotactic bacterial species which produce different magnetite morphologies respond to changing nutrient and oxygenation conditions future work needs to test whether it is possible to develop proxies for ancient nutrient conditions from well calibrated modern magnetotactic bacterial occurrences a tantalizing link between giant magnetofossils and paleogene hyperthermal events needs to be tested much remains to be learned about the relationship between climate and the organisms that biomineralised these large and novel magnetite morphologies rather than being a well worn subject that has been studied for over 60 years the magnetic properties of pelagic carbonates hold many secrets that await discovery

widespread cretaceous secondary magnetization in the high atlas morocco a common origin for the cretaceous remagnetizations in the western tethys in this paper we present the first palaeomagnetic data 51 sites for mesozoic lower middle jurassic sediments of the moroccan central high atlas and address the study of a widespread remagnetization the remagnetization is characterized by a very stable component with systematic normal polarity carried by magnetite the relationship between the magnetic properties and location within the basin suggests that the acquisition of the remagnetization is controlled by basin geometry fold tests indicate that the overprint acquisition is syn folding in some structures but clearly predates the tertiary compressional stage using the small circle intersection method we have calculated the remagnetization direction d 336 4 i 29 2 comparison with the global apparent polar wander path indicates that the remagnetization was acquired during the late cretaceous probably cenomanian considering both the basinal confinement of remagnetization and the connection with other remagnetization events in the western tethys we propose a scenario explaining widespread remagnetizations in the region concerning both basin scale conditions mainly related to sediment thickness and a regional scale thermal event acting as catalyst of remagnetizations in those sedimentary basins that satisfy the basin scale conditions

syntectonic remagnetization in the southern methow block resolving large displacements in the southern canadian cordillera 1 the upper cretaceous ventura member of the goat wall unit in the southern methow block of southern british columbia and northern washington state holds a syntectonic magnetization eight new sites from manning park in british columbia give a mean direction of d 27 5 i 60 1 k 304 7 95 3 2 after optimal partial tilt correction of five groups of bedded sites from farther south in the basin reported by bazard et al 1990 four have a syntectonic remanence with a direction similar to what we observe the exception is one group which has optimal concentration of remanence directions on 100 untilting and an abherent direction which must be rejected combining the accepted sites the optimal differential syntilting direction is d 11 8 i 61 5 k 39 3 95 3 4 n 47 giving a mean pole of 79 8 n 359 2 e k 19 5 and a95 4 8 the age of the remagnetization is constrained to be between 88 and 80 ma compared to cratonic north america this result indicates that the southern methow block was displaced from the south by 1800 500 km meaning it lay south of the sierra nevada subduction zone but well north of other paleomagnetically constrained cretaceous rock units from the insular superterrane including correlative strata of the mount tatlow area in the northern methow block among several possibilities to reconcile this discrepancy the most plausible has the whole methow block translated coherently but with the southern methow block strata remagnetized during transit

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

widespread neogene remagnetization in jurassic limestones of the south iberian palaeomargin western betics gibraltar arc abstract palaeomagnetic investigations in upper jurassic carbonate rocks from the western subbetics betic cordillera southern spain testify to a widespread neogene remagnetization progressive thermal and alternating field demagnetization analyses reveal the presence of two stable components of the natural remanent magnetization nrm the nrm is dominated by a pervasive neogene overprint which always shows normal polarity and maximum unblocking temperatures of 450 c throughout the whole region studied the primary jurassic component however can also be isolated in many outcrops this component has low intensity values and maximum unblocking temperatures of about 550 c showing both normal and reversed polarities palaeomagnetic and rock magnetic analyses indicate that both primary jurassic and secondary neogene components are carried by magnetite the incremental fold test performed in all sampling localities demonstrates that the remagnetization occurred during some stage of the neogene deformation of the subbetics being pre syn or post folding in the various folds studied

mineral exploration and basement mapping in areas of deep transported cover using indicator heavy minerals and paleoredox fronts yilgarn craton western australia abstract glacial sediments have been thoroughly integrated into mineral exploration protocols in the northern hemisphere e g canada and fennoscandia but have received less attention in australia in western australia wa permo carboniferous diamictites buried by younger cenozoic sediments cover scattered areas that have potential to host gold and nickel mineralization in the ne of the yilgarn craton a systematic stratigraphic mineralogical and geochemical study was undertaken to determine whether diamictites in the agnew lawlers gold province have properties that may help target concealed mineralization at the agnew lawlers district rocks intersected in 31 drill holes ne of the waroonga gold mine were interpreted in terms of lithological textural and paleolandscape mineralogical and geochemical variations to select the best sampling media for mineral exploration and provenance assessment the paleotopographic variation asymmetry of the depositional basin polymictic composition matrix supported mixed angular and rounded and poorly sorted texture of the diamictite clasts indicate mechanical dispersal from proximal and distal heterogeneous source rocks ferromagnesian minerals chromite cr magnetite magnetite and ni cu fe sulfides are abundant in the diamictite close to mafic ultramafic source rocks monazite apatite zircon and clasts derived from pyrite bearing quartz veins increase in abundance in the diamictite close to the waroonga shear zone that separates granitoid gneiss terrain from the scotty creek metasediments ilmenite is extensively distributed in diamictites throughout the study area and its mn content can be used to differentiate between felsic and mafic source rocks alteration of ilmenite to titanite monazite to apatite and thorite and replacement and fracture filling of pentlandite to monazite suggest felsic intrusive related hydrothermal alteration of the source rocks prior to mechanical weathering late paleozoic tropical weathering associated with oscillation of water table and icehouse to greenhouse climatic shift has created paleoredox fronts in the permo carboniferous sequence below the redox fronts diamictites are unweathered rich in detrital sulfide and opaque oxide minerals and can be used in provenance studies and tracing mineralization these characteristics of unweathered diamictite are the optimal sampling medium target for provenance studies and mineral exploration these are cemented mainly by calcareous cement where they overlie mafic ultramafic rocks and by pyrite cement where they overlie the scotty creek metasediments above the redox fronts diamictites are variably weathered to ferruginous and bleached kaolinitic saprolites which are stable under oxidizing circum neutral conditions ferruginous diamictites are rich in recycled redox sensitive elements derived from oxidation of ferromagnesian sulfide and opaque oxide heavy minerals these weathering features can be used to identify weathered diamictite for sampling for hydromorphic metal dispersion studies

testing iberian kinematics at jurassic cretaceous times 1 paleogeographic reconstructions of iberia at the mesozoic are still a matter of debate the incompatibility between kinematic models and paleomagnetic data older than 120 ma is a major problem for which no cause has yet been determined here we use a new method to investigate the origin of this misfit we solve the inverse problem of finding the euler poles that fit paleomagnetic poles with the global apparent polar wander path gapwp and then test their implications on iberian reconstructions we show that iberian poles from the early cretaceous mean poles for 123 and 130 ma are incompatible with the gapwp bringing into question their validity contrarily late jurassic data mean pole at 151 ma are compatible with the gapwp and thus can be considered reliable based on these results we propose a new magnetic reconstruction of iberia and surrounding plates at 150 ma m22 anomaly this work provides new constraints for the kinematic evolution of iberia during jurassic cretaceous however the development of a detailed and consensual model for the kinematic evolution of iberia is dependent on the acquisition of new high quality paleomagnetic data and a reevaluation of seafloor magnetic anomalies

origin of orogenic remagnetizations in mississippian carbonates sawtooth range montana 1 paleomagnetic results are presented from mississippian madison group carbonates in the sawtooth range northwestern montana samples were collected from sites along two east west trending transects perpendicular to the thrust faults in the sun river canyon and in the north and south forks of the teton river and from three folds the madison group contains a widespread characteristic remanent magnetization chrm that resides in magnetite with southerly declinations and moderately steep up inclinations tilt test results suggest that the chrm is pretilting in the thrust sheets and teton anticline but syntilting in the clary coulee and swift dam folds the chrms all have the same characteristics and were probably caused by the same remagnetization event yet the tilt test results are different one explanation involves the difference in fold types between the teton anticline fault bend fold geometry and the clary coulee and swift dam folds fault propagation fold geometries the deformation that produced the two geometries could have caused variations in strain stress which may have altered an original pretilting into a syntilting chrm a mean paleopole for the three pretilting tilt test results 67 2 n 177 9 e a95 13 1 suggests remanence acquisition in the late jurassic early tertiary the chrm is interpreted as a chemical remanent magnetization crm geochemical studies indicate that the mississippian carbonates were altered by evolved fluids with radiogenic 87sr 86sr values and petrographic analysis indicates that hydrocarbons migrated through the carbonates the crm is interpreted to be related to alteration by one of these fluids

unconventional rutile deposits in the quebec appalachians product of hypogene enrichment during low grade metamorphism unconventional rutile deposits occur in heavy mineral paleoplacers of the pinnacle formation in the humber zone of the quebec appalachians detrital hemo ilmenite represents 65 percent of the heavy mineral assemblage and is the main titanium mineral detrital rutile is also present in trace amounts postdepositional and metamorphic alteration processes increased the titanium content of the hemo ilmenite grains by leaching elements particularly fe new minerals created by these processes include pseudorutile anatase a tio 2 polymorph and neorutile early metamorphic magnetite enclosed detrital grains and preserved evidence of premetamorphic weathering or diagenetic processes by isolating the grains from metamorphic fluids thus preventing further leaching detrital grains that were not protected by this magnetite were metamorphically leached to the purer tio 2 mineral phase of anatase grains of anatase locally recrystallized to neorutile during a late orogenic back thrusting event in the silurian this is supported by 18 o water in equilibrium with metamorphic magnetite and by the fact that neorutile is limited to the eastern part of a synformal keel near the back thrust fault and is rarely observed farther west where anatase is predominant rutile formed under greenschist metamorphic conditions from primary hemo ilmenite detrital grains via intermediate pseudorutile and anatase phases this is the first known report of titanium mineral enrichment occurring under low pressure and low temperature metamorphic conditions our results could have significant implications for titanium exploration because they demonstrate that paleoplacer source regions are not restricted to eclogite and granulite facies terrains

magnetic evidence for slow seafloor spreading during the formation of the newfoundland and iberian margins there is considerable debate concerning the nature and origin of the thin crust within the ocean continent transition oct zones of many passive non volcanic continental margins located between thinned continental and true oceanic crust this crust is usually found to be underlain by upper mantle material of 7 2 7 4 km s velocity at shallow depths 1 2 km it has been proposed that such crustal material could have originated either by exhumation of upper mantle material during rifting of continents or by slow seafloor spreading one of the examples of occurrence of such a crust are the conjugate margins of newfoundland and iberia here we present an interpretation of magnetic data from these regions to show that their oct zones are underlain by crustal material formed by slow seafloor spreading 6 7 mm yr soon after iberia separated from the grand banks of newfoundland in the late jurassic similarities in the magnetic anomalies and velocity distributions from these regions with those from the sohm abyssal plain a region lying immediately south of the newfoundland basin and formed by seafloor spreading at a similar rate of spreading give further support to such an interpretation the idea that these regions were formed by unroofing of upper mantle during rifting of iberia from newfoundland may be likely but the presence of weak magnetic anomalies in these regions which bear all the characteristics of seafloor spreading anomalies makes it difficult to ignore the possibility that these regions could be underlain by oceanic crust formed during slow seafloor spreading the similarities in velocity structure and the presence of small amplitude magnetic anomalies both across this pair of conjugate margins of the north atlantic and that of the labrador sea suggest that this oct velocity structure may be the norm rather than the exception across those passive non volcanic margins where the initial seafloor spreading was slow furthermore the existence of similar velocity distributions along a few active spreading centers raises the possibility of formation of similar crust across slow spreading ridges fl 2000 elsevier science b v all rights reserved

archean cherts derived from chemical biogenic and clastic sedimentation in a shallow restricted basin examples from the gorge creek group in the pilbara block the 3ae0 ga chert sequence of the gorge creek group is exposed at ord ranges about 50 km east of port hedland in the pilbara block the chert sequence examined in this study is 15 m thick and consists of oxide rich laminated chert grey chert silicified clastic rock carbonaceous black chert and carbonate rich laminated chert although the cherts have undergone postdepositional silica enrichment such as cementation and metasomatic silicification primary precipitation of silica at the site of deposition is indicated by abundant microstructures mosaic and spherulitic structures other primary to early diagenetic components were carbonates sulphates gypsum and anhydrite and organic matter although these mineral associations on the whole correspond to those of modern marine evaporites they are different from modern equivalents with respect to abundant precipitation of amorphous silica and presumed primary precipitation of iron carbonate siderite this feature is a possible manifestation of peculiar physicochemical conditions in the water mass from which the chemical sediments were precipitated compared with modern ocean waters the concentrations of fe and si were significantly higher and the ph value might have been lower these conditions could be obtained by contributions of fe and si enriched hydrothermal solutions and continental run off to the site of deposition grey cherts contain detrital quartz and altered fe ti oxides and were formed in a period of input of terrigenous detrital materials they are characterized by higher concentrations of tio2 a l 2 o 3 cr ni zn rb and zr compared with the other types of chert and by very low 4 al2o3 tio2 values these features are attributed to the supply of terrigenous detrital materials that contain abundant fe ti oxides ilmenite and titanomagnetite and fine tio2 particles such detrital materials might have been formed by extensive chemical alteration of source rocks and residual enrichment of ti relative to al