sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

records of an ancient martian magnetic field in alh84001 although mars does not presently appear to have a global dynamo magnetic field strong crustal fields have recently been detected by the mars global surveyor above surfaces formed 3 or more ga we present magnetic and textural studies of martian meteorite alh84001 demonstrating that 4 ga carbonates containing magnetite and pyrrhotite carry a stable natural remanent magnetization because 40 ar 39 ar thermochronology demonstrates that most alh84001 carbonates have probably been well below the curie point of magnetite since near the time of their formation weiss et al earth planet sci lett 2002 this issue their magnetization originated at 3 9 4 1 ga on mars this magnetization is at least 500 million years myr older than that known in any other planetary rock and its strong intensity suggests that mars had generated a geodynamo and global magnetic field within 450 650 myr of its formation the intensity of this field was roughly within an order of magnitude of that at the surface of the present day earth sufficient for magnetotaxis by the bacteria whose magnetofossils have been reported in alh84001 and possibly for the production of the strong crustal anomalies chromite in alh84001 may retain even older records of martian magnetic fields possibly extending back to near the time of planetary formation

upper proterozoic and phanerozoic pole positions and potassiumargon k ar ages from the east sahara craton summary new palaeopole positions from southern egypt and northern sudan with ages ranging from upper proterozoic 720 ma to uppermost cretaceous are presented and discussed in the context of the apparent polar wander path apwp of africa rockmagnetic studies were also performed to determine the carriers of magnetization and to test the reliability of the remanence data ages pertinent to the apwp were taken from the literature or for most of the localities have been determined by k ar dating on whole rocks or minerals the localities geographic coordinates ages rock types and positions of the palaeomagnetic south

uncertainties about the proterozoic and paleozoic polar wander path of the west african craton and gondwana evidence for successive remagnetization events abstract the importance of remagnetization phenomena in rocks which is without doubt the main cause of uncertainties in paleomagnetic interpretations is illustrated by the study of proterozoic and early paleozoic redbeds from mauritania cratonic west africa except for one formation these units carry multicomponent or secondary magnetizations up to three components are found in a single unit in the absence of geochronological arguments dating of these components relies only on comparisons with previous paleomagnetic data here we critically review the paleomagnetic data set available for the paleozoic of gondwana and the proterozoic of the west african craton using geologic as well as magnetic criteria for the selection of reliable data the selected paleopoles are not regularly arranged in space and time but tend to fall into groups which we interpret as remagnetization periods our interpretation recognizes three or four remagnetization events from middle proterozoic to late paleozoic in addition to the well established late paleozoic overprint paleomagnetic data suggest the existence of a middle paleozoic silurian an early paleozoic late cambrian and maybe a late middle proterozoic remagnetization event these overprints may correspond to generalized geological or climatic events or alternatively be simply related to intervals with low rate of geomagnetic reversals

methane flux and origin in the othrys ophiolite hyperalkaline springs greece abstract the occurrence and origin of methane ch4 generated by serpentinization of ultramafic rocks is of current timely interest in planetary geology astrobiology and energy resource exploration as it may contribute in particular to decipher the source of methane on mars the origin of life and the potential of abiotic hydrocarbon synthesis methane of dominant abiotic origin in serpentinized peridotites on continents ophiolites or igneous intrusions has been documented so far with complete carbon and hydrogen isotope composition in six countries in the philippines turkey oman new zealand japan and italy we report the discovery of two new sites in greece at archani and ekkara located in the othrys ophiolite massif portable sensors based on fourier transform infrared spectrometry ftir and tunable diode laser absorption spectroscopy tdlas allowed to realize that out of 21 ophiolitic springs methane is released only by four hyperalkaline ph from 10 7 to 11 3 and calcium hydroxide ca oh type waters all other 17 springs with ph

source of stable remanence in chromite ores rock magnetic investigations of both the natural chromite ores and their purified chromite concentrate mg 6fe 4cr1 4al 6o4 were carried out these include measurement of natural and saturation remanent magnetism and their stability with respect to alternating field and thermal demagnetisation irm acquisition studies saturation magnetisation versus temperature studies and high field 1000 mt hysteresis characteristics from these studies it was observed that the medium high coercivity component of remanence is associated with the chromite minerals and is carried largely or entirely by them therefore it is concluded that chromites carry permanent magnetisation of palaeomagnetic significance however this result is in sharp contrast to the assumption in the literature that the spinels of the above composition are non magnetic the study has also demonstrated that the magnetic properties of artificial fe cr spinels do not conform to their natural analogues reasons for the discrepancy have been postulated

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

how well do precambrian paleomagnetic data agree with the phanerozoic apparent polar wander path a baltica case study abstract precambrian paleomagnetic data are crucial for various tectonic and geophysical applications the reconstructions of fusion and fission of supercontinents in particular due to many objective reasons the variety of stability tests for these data are abridged in comparison to phanerozoic results and is further hindered by the rather common refusal to consider the resemblance between precambrian and younger poles in this paper we selected precambrian paleomagnetic data with ages of

the silica supersaturated waters of northern evia and eastern central greece the area of north evia and eastern central greece is characterized by strong geomorphological contrast and is built up mainly of consolidated rocks unconsolidated young sediments of pleistocene to holocene age cover the valley and basin flats forming the most productive aquifers in this area however two more types of aquifers can be distinguished within the consolidated rock area the first one is associated with karstified limestones and the second with strongly tectonized ultramafic rocks the schist chert formation with intercalations of shales and cherts seals the ultramafic masses underneath surface and spring waters associated with ultramafic rocks in north evia and eastern central greece were studied two types of water can be distinguished 1 high mg2 and sio2 bicarbonate as the dominant anion ph 7 4 9 2 temperature 9 5 16 3 c low tds total dissolved solutes 459 1037 mg l found both in peridotite and serpentinite areas classified as mg hco3 type 2 high ca2 low mg2 and sio2 hydroxyl ion as the major anion ph 11 2 temperature 28 c very low tds 122 mg l found in peridotite areas classified as ca oh type the studied waters are highly supersaturated with respect to quartz amorphous silica brucite and most low temperature magnesium silicates antigorite sepiolite talc etc these waters show relatively narrow sio2 concentration ranges and a trend parallel to the amorphous silica saturation surface the silica supersaturated waters have the potential to precipitate silica and consequently could affect the people of the local communities that use it as drinking water causing health problems kidney stones copyright 1999 john wiley sons ltd

magnetic and petrologic characterization of synthetic martian basalts and implications for the surface magnetization of mars 1 a suite of synthetic martian basalts is generated with the objective of providing fundamental material properties data for use in modeling and interpretation of mission data we systematically evaluate the effects of major element composition oxygen fugacity fo2 and cooling rate on phase chemistry and magnetic mineralogy grain size and intensity of remanent magnetization the range of experimental compositions and fo2 are chosen to bracket the range expected in the martian crust our results should therefore span the range of possible mineralogies textures and magnetic properties in rapidly cooled mars crustal materials two starting compositions are used for the sample synthesis 1 an fe rich al poor composition patterned after snc basaltic meteorites and 2 a composition based on thermal emission spectrometer tes data with a much lower fe al ratio the resulting magnetic phase in samples generated at the quartz fayalite magnetite qfm buffer is a spinel structured oxide with varying amounts of cr ti mg and al compositional differences depend on bulk composition cooling rate differences in crystallization sequence and the kinetics of silicate mineral nucleation and growth oxide abundance and magnetic intensity are most strongly influenced by fo2 with more subtle composition and cooling rate effects moderately oxidizing qfm conditions result in an intense magnetization 2 3 10 5 am2 kg 1 to 1 4 10 2 am2 kg 1 especially in the meteorite derived basalts however an increase of magnetic grain size into the multidomain range meteorite type and or low unblocking temperatures resulting from increased cr substitution tes type may affect the long term stability of the remanence in qfm samples

interaction of a rheomorphic peralkaline ash flow tuff and underlying deposits menengai volcano kenya abstract in the caldera wall of the volcano menengai kenya a late quaternary peralkaline trachyte ash flow tuff is densely welded and shows evidence e g a well defined lineation or rheomorphism the ash flow tuff overlies a variety of volcanic and volcaniclastic deposits with some of which it has intimate and complex relationships interpreted to have formed during post emplacement processes where emplaced over a low density pumice fall deposit the dense rheomorphic ash flow tuff invaded the pumice deposit as tongues and locally the two deposits mixed intimately blocks of welded foliated ash flow tuff were incorporated when cooled and rigid into the underlying pumice deposit indicating mobility of the pumice deposit locally the pumice fall deposit was modified during processes associated with the emplacement of the ash flow tuff to a pumice bearing silt the modifying process apparently involved smashing of fragile pumice lumps during physical agitation of the pumice deposit where emplaced over dense deposits shear at the base of the rheomorphic ash contributed to the development of a gently dipping nearly planar erosional surface a microbreccia buff silt deposit formed along this shear plane consists of material from the overlying ash flow and underlying deposits and was probably rich in gas and mobile during its formation

unravelling an allochthonous subaqueously deposited volcanic epiclastic to subaerial andesitic lava assemblage in hong kong age stratigraphy and provenance studies of the middle jurassic tuen mun formation the middle jurassic tuen mun formation hong kong is a fault bounded block of rare andesitic and related rocks that preserve a snapshot of the developing se china continental arc system during the late mesozoic the forearc depositional setting is considered to have been dominated by an emergent andesitic volcanic massif which shows a transition to a fluvial dominated volcanic plain and then to an offshore marine environment the youngest laser ablation inductively coupled plasma mass spectrometry u pb detrital zircon ages obtained from the three packages define a coherent group n 396 mswd 1 8 suggesting a maximum depositional age of 169 5 0 3 ma the dominance of large euhedral and concentrically zoned zircons reflects a common volcanic provenance hf isotope data e hf t 0 to 11 on the youngest representative zircon grains imply derivation of magmas from dominantly recycled crustal sources the detrital zircon age signatures and hf isotope data show no indication of magmatic interaction with adjacent late palaeozoic and early to middle jurassic sedimentary rocks or middle jurassic to early cretaceous magmatic rocks of hong kong on the basis of the structural zircon age and isotope data the tuen mun formation is interpreted to be an allochthonous block emplaced in the hong kong region during the late middle jurassic supplementary material a full dataset of u pb dating and hf isotope analysis of detrital zircons stratigraphic drillhole logs and a description of the terminology used in this paper are available at https doi org 10 6084 m9 figshare c 3736972

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

palaeomagnetic studies of some palaeozoic sediments southwest sinai egypt abstract thirty nine sites comprising 301 samples of sandstones and shales of cambrian cambro ordovician and upper carboniferous age were collected from the gebel abu durba area southwest sinai they mostly show stable magnetisation against af and or thermal demagnetisation with a single component of remanence both normal and reverse polarities are recorded two of the 14 sites of the cambrian rocks give a palaeopole position at 18 0 n and 341 2 e which is considered as probably primary the remaining cambrian sites give a palaeopole at 29 8 s 56 e with a 95 6 7 the palaeopole calculated from 11 reliable cambro ordovician sites is at 32 2 s 53 0 e with a 95 7 9 the palaeopole from the 9 upper carboniferous sites lies at 25 6 s 64 0 e with a 95 7 1 the obvious similarity of the palaeopoles from most of the cambrian and cambro ordovician rocks to those of the upper carboniferous indicates that all these rocks have a magnetisation of similar age which by comparison with other african palaeomagnetic poles appears to have been acquired during the upper carboniferous five of the cambro ordovician and upper carboniferous sites have normal and reversed polarities and a mean pole of 76 8 n 265 2 e with a 95 15 1 suggesting a tertiary remagnetisation associated with the opening of the gulf of suez

a new zealand late miocene magnetostratigraphy glacioeustatic and biostratigraphic correlations magnetostratigraphy is described for a sequence of predominantly hemipelagic marine mudstone 2200 m thick representing the late miocene and earliest pliocene from the mangapoike river section eastern north island new zealand together with the early pliocene magnetostratigraphy previously described from mangapoike it now provides a 3000 m thick magnetostratigraphic reference section for new zealand ranging in age from 10 5 ma to 3 5 ma thirteen magnetozones are recognised in the 2200 m described here and all but the highest are confidently correlated with the polarity timescale from the base of chron 11 to chron 7 correlation of the highest normal magnetozone to the timescale is ambiguous and two possible interpretations are presented both of which imply a previously unrecognised latest miocene unconformity the inferred unconformity is supported by biostratigraphy it is shown that the integration of biostratigraphy lithostratigraphy and magnetostratigraphy can discern diastems of the order of 104 years magnetostratigraphy and 13c data also indicates a diastem between 6 30 ma to 5 41 ma at the miocene pliocene angular unconformity at mangapoike these two unconformities and a distinct lithological change at the base of the late miocene are synchronous with deep sea benthic 18o records and unconformities expected from postulated eustatic sea level changes a coherent correlation model for new zealand on shore magnetostratigraphic biostratigraphic and isotopic data conflicts with the two magnetostratigraphic interpretations proposed for dsdp site 594 150 km east of new zealand both of which are probably incorrect

magnetic properties of kurokami pumices from mt sakurajima japan both chromite and low ti titanomagnetite are carriers of natural remanent magnetization nrm in kurokami andesitic pumices of mt sakurajima japan thermal demagnetization of the nrm and of a laboratory isothermal remanence indicate unblocking of chromite remanence between 200 and 260 c and of titanomagnetite remanence between 400 and 520 c electron microprobe analyses support the compositions fe1 9cr1 11o4 and fe2 9ti0 1o4 indicated by the respective curie temperatures of 260 c and 520 530 c chromite is thermally unstable and converts to magnetite during thermal demagnetization leading to a self reversal of nrm between 500 and 580 c the chromite contributes about 25 of the nrm intensity of the sample fl 2001 elsevier science b v all rights reserved

emergence of subaqueous depositional environments in advance of a major ignimbrite eruption capel curig volcanic formation ordovician north wales an example of regional volcanotectonic uplift abstract the garth tuff of the ordovician capel curig volcanic formation comprises a laterally extensive sheet of largely welded rhyolitic ash flow tuff that can be traced from a proximal subaerial to a distal marine environment six facies associations are recognized within subjacent sedimentary successions facies are interpreted as deposits of stream dominated alluvial fans ephemeral streams braidplain lower delta plain delta front and shallow marine environments the development of a large scale coarsening and or shallowing upward sequence reflects the amount of differential tectonic uplift between fault bound highlands and depositional basin there is no evidence for volcanicity associated with sedimentation uplift resulted in aggradation and progradation of fan and braidplain deltas a basinward shift in the loci of dominant sedimentation and a 15 km advance in the position of the pre eruption coastline associated tilting of fault blocks influenced the position of fluvial systems on braidplain with channel belts localizing along one margin in the area of greatest subsidence the spatial correlation between pre emplacement geography and location of probable vent for garth tuff implicates upward and or lateral migration of the magma as the most likely mechanism driving these tectonic changes

two examples of subaqueously welded ash flow tuffs the visean of southern vosges france and the upper cretaceous of northern anatolia turkey abstract pyroclastic deposits interpreted as subaqueous ash flow tuff have been recognized within archean to recent marine and lacustrine sequences several authors proposed a high temperature emplacement for some of these tuffs however the subaqueous welding of pyroclastic deposits remains controversial the visean marine volcaniclastic formations of southern vosges france contain several layers of rhyolitic and rhyodacitic ash flow tuff these deposits include from proximal to distal settings breccia lapilli and fine ash tuff the breccia and lapilli tuff are partly welded as indicated by the presence of fiamme fluidal and axiolitic structures the lapilli tuff form idealized sections with a lower coarse and welded unit and an upper bedded and unwelded fine ash tuff sedimentary structures suggest that the fine ash tuff units were deposited by turbidity currents welded breccias interbedded in a thick submarine volcanic complex indicate the close proximity of the volcanic source the lapilli and fine ash tuff are interbedded in a thick marine sequence composed of alternating sandstones and shales presence of a marine stenohaline fauna and sedimentary structures attest to a marine depositional environment below storm wave base in northern anatolia thick massive sequences of rhyodacitic crystal tuff are interbedded with the upper cretaceous marine turbidites of the mudurnu basin some of these tuffs are welded as in southern vosges partial welding is attested by the presence of fiamme and fluidal structures the latter are frequent in the fresh vitric matrix these tuff units contain a high proportion of vitroclasis and were emplaced by ash flows welded tuff units are associated with non welded crystal tuff and contain abundant bioclasts which indicate mixing with water during flowage at the base basaltic breccia beds are associated with micritic beds containing a marine fauna the welded and non welded tuff sequences are interbedded in an alternation of limestones and marls these limestones are rich in pelagic microfossils the evidence above strongly suggest that in both examples tuff beds are partly welded and were emplaced at high temperature by subaqueous ash flows in a permanent marine environment the sources of the pyroclastic material are unknown in both cases we propose that the ash flows were produced during submarine fissure eruptions such eruptions could produce non turbulent flows which were insulated by a steam carapace before deposition and welding the welded ash flow tuff deposits of southern vosges and northern anatolia give strong evidence for existence of subaqueous welding

paleomagnetism of the red lake greenstone belt northwestern ontario possible evidence for the timing of gold mineralization we report paleomagnetic results from 1 largely unaltered batholiths and 2 sheared and altered units in the archean red lake greenstone belt a major gold producing region in the uchi subprovince of northwestern ontario group 1 includes the hammell lake killala baird trout lake and little vermilion lake batholiths which were intruded between 2730 and 2700 ma ago u pb zircon group 2 includes the howey diorite the dome stock and dikes and andesites from the dickenson mine these units from the mineralized deformation zones dzs between the batholiths were sheared and hydrothermally altered in the same event that caused gold mineralization in supracrustal units the batholiths carry a stable reversed remanence rlr withd 159 i 74 k 20 95 16 n 4 units 14 sites rlr has high coercivities unblocking temperatures and remanence intensities and appears to be a primary thermal remanence dating from 2700 ma the altered units from the gold bearing dzs carry a different remanence rlg withd 180 i 57 k 23 95 3 n 3 units 6 sites rlg also has relatively high coercivities and unblocking temperatures but its magnetization intensities are 1 2 orders of magnitude lower than those of rlr opaque minerals and silicates observed in thin section are highly altered rlg is therefore believed to be a chemical or thermochemical remanence acquired 2580 ma ago during the late stages of cooling and gold mineralization seeing as rlg characterizes the shear zone rocks but is absent from unaltered rocks outside the dzs it could be used as a rapid prospecting tool in delineating areas of alteration and possible mineralization the paleopoles for rlr 2700 ma and rlg 2580 ma link with those of the shelley lake granite 2580 ma 40ar 39ar hornblende and biotite and the matachewan hearst dikes 2450 ma u pb zircon and baddeleyite the sense of motion on the late archean early proterozoic apparent polar wander path for laurentia as defined by this sequence of paleopoles is opposite to that previously accepted

marine and human activity influences on the groundwater quality of southern korinthos area greece in this paper the groundwater quality of the southern part of korinthos region north east peloponnese is discussed the geology is characterized by a thick sequence of neogene marls alternating with sandstones overlain by superficial quaternary deposits the latter consist of a mixture of loose materials such as conglomerates marly sandstones sands and clay to silty sands the area is crossed by a fault system parallel to the coastline and the quaternary sediments have formed extended tyrrhenian marine terraces two aquifers have been identified in the area the first is unconfined and occurs within the quaternary sediments whereas the other is a deep confined aquifer occurring within the underlying neogene marl series analysis of hydrochemical evolution over the past 30 years has indicated significant deterioration of quality owing to seawater intrusion and nitrate pollution the various sources of pollution have rendered to a large extent shallow groundwater unsuitable not only for potable water supply but also for irrigation purposes however this is not the case for the deeper confined aquifer statistical analysis was used to explore the evolution of salinization during the years 1968 and 1998 in view of the alarming conditions caused by the documented groundwater quality deterioration the need for integrated water resources management is stressed to maintain the socio economic growth of the region studied copyright 2003 john wiley sons ltd

large tectonic rotation of part of new zealand in the last 5 ma abstract detailed paleomagnetic data from the wairoa syncline a middle miocene to the present forearc basin on the east coast of the north island new zealand show that the rate of clockwise rotation for the last 5 ma has been 7 8 ma of which less than 1 5 ma can be explained by apparent polar wander due to motion of the australian or pacific plates this rotation is similar to a present day rate of 7 ma determined from geodetic data between 5 and 20 ma ago the rate of tectonic rotation is poorly determined and may be between 0 and 2 ma the change in the rate of rotation of the wairoa syncline around 5 ma is probably related to a markedly different tectonic style in the new zealand region within the last 5 ma associated with a change in position of the euler poles of rotation for the pacific australian plates

fossil bacterial magnetite in deep sea sediments from the south atlantic ocean the magnetostratigraphy of deep sea sediment cores has proven to be a powerful and high resolution method of dating and comparing sedimentary records in contrast to this widespread application relatively few attempts have been made to analyse and identify the mineralogy grain size and origin of the carrier of the remanent magnetization1 2 it is widely assumed that dominant ferrimagnetic minerals in deep sea sediments are of lithogenic origin that is that they derive from oceanic or continental detritus volcanic ashfalls micrometeorites hydrothermal precipitation or chemical mineralization processes a very different view was taken by kirschvink and lowenstam3 who suggested that fossil remnants of magnetite mineralizing organisms in particular magnetotactic bacteria might play an important part in the magnetization of sediments deposited in certain detritus poor aquatic systems we have used rock magnetic diagnostic methods to characterize the magnetic phases in deep sea sediments from the south atlantic these phases were then extracted and studied with the electron microscope our results indicate that fossil bacterial magnetite is the main carrier of the natural remanent magnetization in sediments ranging in age from quarternary to eocene different types of bacterial magnetosomes were detected most of them occurring in chains of identical particles and nearly all of them falling into the theoretically determined4 size range of single domain magnetitite

paleomagnetism of late neoproterozoic african dike swarms from the south eastern desert and the paleo neoproterozoic dataset from egypt paleomagnetic analysis has been carried out on late neoproterozoic dike swarms from two areas along the red sea coast 13 dikes 87 samples close to the um rus gold mine and eight dikes 59 samples at sadi salem rock magnetic experiments indicate that the main carrier of magnetization of these dikes is titanomagnetite and or magnetite most samples display a component of magnetization ca that is in agreement with previously published paleo late neoproterozoic poles from other dikes and from one ore complex but they cluster around the cenozoic 0 60 ma portion of torsvik s gondwana apparent polar wander path apwp when plotted in southern african coordinates we suggest two causes of this behavior remagnetization related to the opening of the red sea or later emplacement of the dikes along pre existing paleo late neoproterozoic fractures will have to be tested with new reliable paleomagnetic data on carefully dated rocks a second component cb seen in many of our samples but which is difficult to estimate is in agreement with late neoproterozoic granite poles and a dike supposed to be triassic in age however all these poles plot on the jurassic cretaceous portion 80 200 ma of torsvik s path these results as well as the cambro carboniferous sediments from sinai and the late neoproterozoic dokhan volcanic formation that cluster around the late paleozoic portion 280 300 ma of torsvik s apwp are tentatively interpreted as global viscous remagnetizations acquired respectively during the cretaceous normal and the kiaman reverse superchrons

palaeomagnetic and rock magnetic study of a deeply exposed continental section in the charnockite belt of southern india implications to crustal magnetisation and palaeoproterozoic continental nucleii abstract this study has investigated palaeomagnetism and magnetic properties of a tract of continental basement in south india metamorphosed to granulite facies close to the archaean proterozoic boundary 2600 ma at former depths of 20 35 km the metamorphism is characterised by extensive development of charnockite produced by flushing of co2 rich fluids through older crust rock magnetic studies thermomagnetic high and low temperature susceptibility hysteresis and irm show that very low ti metamorphic magnetite is the ubiquitous ferromagnet in all rock types it has predominantly multidomain characteristics but is of high magnetic stability probably due to exsolution precipitation and microfracture filling producing anisotropic grain shapes magnetic susceptibility shows little increase with temperature falling to zero at 550 c implying that induced magnetisation drops to a weak paramagnetic effect near this isotherm susceptibilities and intensities of magnetisation increase from amphibolite to granulite facies and by an order of magnitude within charnockites in the lower dehydrated crust induced magnetisations are able to explain 50 of observed magnetic anomalies of deep origin intensities of magnetisation and qn values are high especially in charnockite where metasomatism has resulted in new magnetite formation and it is concluded this facies is able to explain observed magnetic anomalies provided that viscous remnant magnetisation is enhanced at depth to contribute the remaining magnetisation the magnitude of anisotropy of magnetic susceptibility ams is also increased by charnockite formation as are degrees of lineation and foliation however directions of ams in these anhydrous rocks are essentially random and reflect the static nature of the charnockite metasomatism no appreciable phanerozoic overprinting is evident in this terrane and the dominant feature of the palaeomagnetic record is a west to north shallow swathe of component directions with a minor antiparallel sequence between means summarised by a1 d i 280 8 a2 d i 319 11 and a3 d i 1 10 this swathe is interpreted as an uplift related cooling acquisition following charnockite formation at 2600 ma and a remanence age of 2600 2400 ma is implied by the absence of most of the swathe in early palaeoproterozoic and younger dyke swarms cutting the region a second group of component directions b mean d i 238 87 is similar to magnetisations found in dykes linked to an age of 2370 ma and interpreted as somewhat younger than the a swathe we argue that the entire magnetic record was acquired before final basement uplift prior to 2000 1800 ma collective late archaean early proterozoic palaeopoles from india define a low to high latitude movement of the shield at 2600 2300 ma which is replicated in contemporaneous results from the other ancient gondwana nucleii of southern africa and australia the cratonic nucleii of these three shields have the oldest laterally extensive supracrustal covers and comprise the core of the protocontinent of ur the 2900 2200 ma palaeopoles are consistent with close proximities and subsequent movements between them were apparently limited because continental proximities but not configurations were similar within the palaeozoic supercontinent of gondwana

pleistocene unconformity bounded shelf sequences wanganui basin new zealand correlated with global isotope record abstract eleven successive unconformity bound sequences in the castlecliff section wanganui basin each typically composed of a lower variably conglomeratic 0 1 2 m thick shell bed and an upper 5 20 m thick barren siltstone are identified in the stratotype of the local new zealand middle pleistocene castlecliffian stage based on magnetostratigraphy the section is correlated with the published benthonic oxygen isotope curve for dsdp core 552a from the north atlantic each of the sequences accumulated during one of the odd numbered interglacial isotope stages in the interval 31 to 11 inclusive the unconformities developed during intervening glacial ages when the shelf was exposed through glacio eustatic lowering the correlation provides a geochronologic framework for assessment of the eustatic contribution to the generation of the sequences in a situation where the paleoshorelines and related shelf sequences are preserved and for which an independent approximation of the sea level history is available in the form of an oxygen isotope curve

magnetostratigraphic chronology of late miocene to early pliocene biostratigraphic and oceanographic events in new zealand successions of uplifted shallow marine sediments in new zealand were among the first to provide evidence of latest miocene climatic deterioration and were also among the first onshore successions where late miocene to early pliocene magnetostratigraphic records were established a revised chronology of late miocene to early pliocene events has been determined from new magnetostratiigraphic results from two important successions at blind river and upton brook south island new zealand lat 41 45 s long 174 05 e magnetostratigraphic results from upton brook are the fast from strata containing the kapitean index mollusks and provide the first reliable estimate of the age of the tongaporutuan kapitean boundary a coherent chronology from blind river and upton brook indicates that two synchronous low sea level and cool paleoclimatic events occurred in the late miocene kapitean stage this inference is supported from other well known coeval successions in the new zealand region the revised new zealand chronology supports recent determinations from the mediterranean that indicate a two stage messinian salinity crisis and a 5 2 ma age for the miocene pliocene boundary data from new zealand the mediterranean and ocean cores around the world suggest that these latest miocene events are related to glacio eustatic sea level fluctuations a 6 05 ma age for the first occurrence datum of globorotalia conomiozea indicates that significant diachroneity exists in the placement of this datum between the southwest pacific and the mediterranean confirming the observation that g conomiazea is unsuitable for correlation between ocean basins

orogenesis vs diagenesis can we use organic rich shales to interpret the tectonic evolution of a depositional basin abstract black shales from the southern appalachian basin and the southwest welsh basin have anomalous u pb and nd model ages suggesting syn and post depositional resetting of the sm nd and u pb isotopic systems this alteration to the primary detrital signature of these two shale sequences is indicative of black shale diagenetic depositional processes that obscure paleo environmental and provenance information recorded prior to and during deposition the trace element and isotopic signatures of these two shale sequences reveal a syn post depositional history that is de coupled from the coeval orogenic history of the region making it difficult to reconstruct the tectonic and oceanographic conditions present at the time of deposition both the ordovivian welsh basin and the devonian appalachian basin sequences host ree and u bearing diagenetic phosphate minerals that play a critical role in the whole rock ree and u budgets in the welsh basin shales early diagenetic apatite and a later monazite phase dominate the ree budget and cause the redistribution of ree early in the basin s history ca 460 ma this redistribution is recorded by the sm nd system 450 90 ma and the nd model ages that are anomalously old by as much as 20 t dm 2 0 ga this early history is complicated by a permo triassic fluid event affecting the whole rock u budget and resetting the u pb isotopic system at 193 45 ma the appalachian basin sequence appears to have a much less complicated history yet still records a significant disturbance in both the sm nd isotopic system 392 76 ma and the pb isotopic system 340 50 ma at about the time of deposition ca 365 ma these two sequences suggest a pattern of diagenetic disturbance common to black shales these processes are unique to black shales and must be considered when interpreting provenance and paleo environmental information from the black shale sequences although these rocks are susceptible to alteration the alteration may provide extensive information on the post depositional history of the basin while still retaining some primary depositional information if black shale processes are considered during the interpretation of isotopic and trace element signatures from organic rich shales it may be possible to recover an extensive basin history

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

tectonic controls of ordovician arc and marginal basin volcanism in wales ensialic destructive plate margin volcanism persisted in wales from late tremadoc to caradoc times late tremadoc uplift and erosion in nw wales is attributable to onset of subduction and development of a frontal ridge system fore arc south east of this in the vicinity of the harlech dome and in sw wales more localized uplift was associated with the development of arc volcanoes at a volcanic front in north wales the arc volcanism occurred in a regime of e w crustal extension and was centred on a deep crustal discontinuity during arenig times the arc terrane was transgressed subsequently marine conditions with local and temporary emergence persisted through arenig to caradoc episodes of marginal basin volcanism while there was widespread and persistent subsidence in wales during development of the marginal basin the volcanism occurred mainly along complex and relatively narrow grabens which were sites of repeated channelling of magmas and pronounced subsidence the grabens mark the sites of deep and steep crustal discontinuities fractures one of which controlled the earlier arc volcanism changing sites of volcanism in wales can now be seen as due to transfer of active extension from one fracture to another there is evidence that the fractures were present by tremadoc times and they almost certainly formed before development of the cambrian marine rift basin they are most likely to have been strike slip faults or shear zones and their pattern in wales indicates that they may represent part of a complicated deep crustal strike slip duplex system the volcanically active grabens of the marginal basin are considered to reflect upward propagation of splay faults flower structure which channelled magmas through the cover during extension across the fractures the fractures and associated grabens strongly influenced the location and orientation of zones of relatively high strain that developed during late caledonian compression and transpression in north wales the volcanically active grabens trended broadly n s oblique to the exposed ne sw basement tectonic grain and dominantly e w extension seems to have persisted throughout the ordovician volcanism while there is considerable uncertainty concerning ordovician plate tectonic configurations and displacements the extension is best explained by invoking a n s convergence vector between the oceanic and over riding microcontinental plates with resultant sinistral shear stress in the latter and sinistral transtensional displacements across the arc and marginal basin the model highlights the suspect nature of the relationship between the welsh ordovician terrane and others to the north

mineralogy and magnetism of fe cr spinel series minerals from podiform chromitites and dunites from tąpadla sudetic ophiolite sw poland and their relationship to palaeomagnetic results of the dunites summary this paper presents the mineralogy and magnetic properties of two varieties of chromitite sampled in the same exposure of a serpentinite massif regarded to be a fragment of the sudetic ophiolite the varieties differ mineralogically and magnetically one of them labelled taa and altered to a low degree according to scanning electron microscope sem and microprobe results comprises an unaltered al cr spinel core some secondary chromite and less abundant cr magnetite it has a high magnetic susceptibility and natural remanence nrm and its dominating magnetic phase is ferrichromite with t b t c of about 530 c occurring in grains in the single domain sd pseudo single domains psds and superparamagnetic sp states the second variety labelled tab is highly altered and comprises apart from al spinel two kinds of secondary chromites that do not differ much from the primary al spinel and no cr magnetites this chromitite variety does not reveal one well defined t b t c but a series ranging between 200 and 450 c their grain sizes correspond to the sd sp domain states the same pattern was observed in chromitite taa after annealing in air at a temperature of 700 c dunites associated with the chromitites contain apart from magnetite chromite grains the majority of specimens reveal magnetic characteristics similar to the taa like variety but in some specimens tab like characteristics were also observed a standard palaeomagnetic study performed earlier on the dunites showed that their nrm has three components lower devonian carried by magnetite permian and tertiary recent the results presented here lead us to conclude that the latter two are carried by ferrichromites similar to those found in the taa and tab chromitites

a palaeomagnetic study of charnockites from madras block southern granulite terrain india abstract the south indian craton is composed of low grade and high grade metamorphic rocks across different tectonic blocks between the moyar bhavani and palghat cauvery shear zones and an elongated belt of eastern margin of the peninsular shield the madras block north of the moyar bhavani shear zone which evolved throughout the precambrian period mainly consists of high grade metamorphic rocks in order to constrain the evolution of the charnockitic region of the pallavaram area in the madras block we have undertaken palaeomagnetic investigation at 12 sites chrm directions in 61 oriented block samples were investigated by alternating field af and thermal demagnetization titanomagnetite in cation deficient cd and multi domain md states is the remanence carrier the samples exhibit a chrm with reverse magnetization of d m 148 1 i m 48 6 k 22 2 95 9 0 and a palaeomagnetic pole at 37 5 n 295 6 e dp dm 7 8 11 8 this pole plots at a late archaean location on the indian apparent polar wander path apwp suggesting an age of magnetization in the pallavaram charnockites as 2600 ma the nearby st thomas mount charnockites indicate a period of emplacement at 1650 ma mesoproterozoic thus the results of madras block granulites also reveal crustal evolution similar to those in the eastern ghats belt with identical palaeopoles from both the areas

a conceptual hydrogeological model of ophiolitic aquifers serpentinised peridotite the test example of mt prinzera northern italy the main aim of this study is the experimental analysis of the hydrogeological behaviour of the mt prinzera ultramafic massif in the northern apennines italy the analyzed multidisciplinary database has been acquired through i geologic and structural survey ii geomorphologic survey iii hydrogeological monitoring iv physico chemical analyses v isotopic analyses the ultramafic medium is made of several lithological units tectonically overlapped between them a low permeability discontinuous unit has been identified this unit behaves as an aquitard and causes a perched groundwater to temporary flow within the upper medium close to the surface this perched groundwater flows out along several structurally controlled depressions and then several high altitude temporary springs can be observed during recharge together with several perennial basal i e low altitude springs caused by the compartmentalisation of the system due to high angle tectonic discontinuities

spinel in martian meteorite sau 008 implications for martian magnetism shergotty nakhla chassigny snc meteorites provide the only available samples of martian material the stable permanent magnetization of snc meteorites has been traditionally attributed to magnetite fe3o4 or pyrrhotite fe7s8 on the basis of rock magnetic microscopic and electron microprobe analyses on rock chips and mineral separates we suggest that a new material fe cr ti spinel is responsible for the stable paleomagnetic record of martian meteorite sau 008 it is possible that sau 008 acquired a primary remanence of thermal origin from the martian crustal field however this proposition requires further testing because the effect of shock events on fe cr ti spinel is unknown d 2004 elsevier b v all rights reserved

ordovician intrusions of the strumble head mynydd preseli region wales lateral extensions of the fishguard volcanic complex a major doleritic and gabbroic sill complex with minor microtonalitic components cuts ordovician strata to the east of fishguard their age has long been equivocal new geochemical data combined with petrographic analyses indicate that the intrusions represent an eastward extension of the ordovician llanvirn fishguard volcanic complex lavas and high level intrusions of the strumble head region in addition similar doleritic intrusions crop out to the west of strumble head as far as traeth llyfn the data presented here confirm earlier conclusions that the fvc has tholeiitic affinities and that compositional variations largely result from low pressure fractional crystallization the petrogenetic relationships of the microtonalites are however as yet equivocal incompatible element discriminant diagrams indicate that the parental basic magmas were derived by melting of a slightly depleted mantle source similar to n type morb to which had been added a minor supra subduction zone lil element component the data are consistent with emplacement of the magmas in a marginal basin environment

magnetite diagenesis in marine sediments from the oregon continental margin the magnetochemistry of sediments from the oregon continental margin is examined to determine the effects of iron sulfur diagenesis on the paleomagnetic record magnetic mineral dissolution and transformation into iron sulfides are a common feature in these suboxic to anoxic lutites these processes are evidenced in rapid decreases in natural remanent magnetization intensities and stabilities systematic changes in other rock magnetic properties and increases in solid phase sulfur concentrations with depth hysteresis measurements are used to evaluate downcore changes in magnetite concentration and grain size magnetite abundances decrease downcore from initial values of about 0 1 and nominal grain diameters lie within a narrow pseudosingle domain range of 0 08 to 0 6 m a first order surface area reaction model da dt ka is proposed to explain the magnetite dissolution mechanism where a is the total magnetite surface area and k is the rate constant the solution of this equation predicts that the surface area and concentration decrease exponentially and the concentration in addition depends on grain size application of this model in two cores where grain size varies with depth successfully explains the downcore profiles of both concentration and surface area despite extensive magnetite destruction magnetic directions in such sediments appear to reliably record long wavelength trends of the geomagnetic field

transmission electron microscope and rock magnetic investigations of remanence carriers in a precambrian metadolerite a precambrian metadolerite dyke has two distinct types of remanence carriers those with medium high coercivities unblocking fields of 20 120 mt and those with low coercivities unblocking fields of 15 mt optical examination reveals numerous submicron probably opaque inclusions in the plagioclase feldspar and also large opaque grains consisting of coarse oxidation exsolution intergrowths of magnetite and ilmenite all opaque phases have been examined using transmission electron microscopy together with microanalysis and electron diffraction the submicron inclusions in the plagioclase are titanomagnetites 0 x 0 14 with a size range between about 0 01 and 0 5 m and axial ratios between 1 equidimensional and about 0 3 many of these inclusions fall in the single domain field but some are probably pseudo single domain the large opaque grains contain almost pure magnetite and ilmenite and show no fine scale exsolution the magnetite regions of the intergrowths are of multidomain size and reveal multidomain structure under lorentz electron microscopy there are also some primary ilmenites containing very fine exsolved haematite and there are very fine plates of ilmenite and very elongate needles of magnetite within the augite experiments on artificial samples containing very carefully prepared separates of plagioclase and large opaque grains show that the pure plagioclase acquires a remanence with unblocking fields of 20 140 mt and blocking temperatures of 390 590 c and the large opaque grains acquire a remanence with unblocking fields of less than 15 mt but a wide range of blocking temperatures up to about 570 c it is concluded that the medium high coercivity component of remanence in the rock is carried largely or possibly entirely by the submicron magnetites within the plagioclase and that the low coercivity component is carried largely or entirely by the multidomain magnetites in the large opaques the contribution of the magnetite needles in the augite is uncertain as the rock does not contain any detectable component of remanence with the extremely high coercivities expected from their very elongate shape

alternating field demagnetization of rocks and the problem of gyromagnetic remanence abstract alternating field a f demagnetization has proved to be a very reliable technique for separating the magnetization components of rock samples the method is subject to errors caused by either imperfection of the technique or by intrinsic properties of a rock recently stephenson 1 2 introduced the term gyroremanent magnetization grm for a disturbing remanent magnetization that can be acquired by magnetic material during tumbling or stationary a f demagnetization the implications for the routine a f demagnetization of anisotropic rock samples seemed to be very serious here however a method is presented on how to avoid the effect of grm on results obtained from stationary a f demagnetization

palaeomagnetic study of archaean banded hematite jasper rocks from the singhbhum orissa craton india abstract the banded hematite jasper formation within the iron ore supergroup of the singhbhum craton in eastern india comprises fine alternating layers of jasper and specularite it was deposited at 3000 ma and deformed by a mobile episode at 2700 ma hematite pigment 10 m is chiefly responsible for red to brown rhythmic bands in the hematite jasper facies although thermomagnetic study also shows that minor amounts 1 2 of magnetite are present palaeomagnetic study identifies a dual polarity remanence resident in hematite d i 283 60 95 12 which predates deformation studies of the fabric of magnetic susceptibility and rock magnetic results suggest a diagenetic origin for this magnetisation with the hematite formed from oxidation of primary magnetite the palaeopole 32 e 24 n dp dm 14 18 records the earliest post metamorphic magnetisation event in the orissa craton a minimum apparent polar wander motion of the orissa singhbhum craton of through 80 is identified during late archaean times 2900 2600 ma

a shallow marine volcaniclastic facies model an example from sedimentary rocks bounding the subaqueously welded ordovician garth tuff north wales u k abstract volcaniclastic sedimentary rocks bounding the ordovician garth tuff in north wales were deposited in a shallow marine setting adjacent to a magmatic arc the volcaniclastic sedimentary rocks are primarily of angular to euhedral quartz angular euhedral feldspar and volcanic rock fragments these grains are of a pyroclastic origin but have been reworked to various degrees sedimentary rock fragments rounded quartz muscovite biotite iron rich chlorite and various clay minerals occur in lesser amounts the sedimentary rocks can be divided into a proximal offshore to foreshore facies association poffa and an offshore to lower shoreface facies association olsfa the sand dominated poffa exposed near capel curig is characterized by large wave ripples hummocky cross stratification co sets of trough cross beds reactivation surfaces plane beds and graded turbidite layers in suspension deposited mudstone water depths calculated from bedding plane exposure of large wave ripples and determined by the presence of hummocky cross stratification and by facies association vary from a few meters to around 30 m 45 50 m theoretical maximum in a 45 m thick stratigraphic section beneath the tuff eight kilometers to the southeast the garth tuff is bounded by a thick sequence of suspension deposited laminated mudstone with thin graded beds of siltstone and fine grained sandstone this facies association olsfa represents the introduction of material by distal turbidites and by pelagic sedimentation the olsfa was deposited in water depths well below storm wave base possibly in excess of 200 m no evidence of storm waves or surface generated currents occur until over 40 m above the tuff this study documents marine sedimentary rocks deposited in water depths ranging from foreshore to offshore as the bounding facies of the garth tuff it is thus reasonable to conclude that the garth tuff was emplaced and welded in water depths greater than the thickness of the tuff in the area of this study

late precambrian rifting and crustal evolution in the northeastern desert of egypt basement exposures along the northwestern flanks of the gulf of suez record the rapid formation of continental crust during the interval 670 550 ma a variety of field petrographic petrologic and isotopic considerations indicate that this episode of crust formation took place in an extensional tectonic setting analogous to that of the late paleozoic oslo rift of norway crustal evolution in this region thus contrasts with other regions of the afro arabian shield where the crust appears to have been formed by convergent margin accretionary processes and collisional tectonics this imposes new constraints on our understanding of late precambrian crustal evolution

palaeomagnetic location of the jaramillo subchron and the matuyama brunhes transition in the castlecliffian stratotype section wanganui basin new zealand abstract magnetostratigraphy is described for the lower part of the castlecliff section in wanganui basin new zealand based on analysis of multiple samples from 37 horizons the castlecliff section is the stratotype of the middle pleistocene castlecliffian stage and comprises eleven unconformity bound sequences which accumulated at inner shelf depths during interglacials a range of rock magnetic experiments undertaken on representative lithologies within the section show that the bulk of the remanence in all specimens is carried by ferrimagnetic mineral close to magnetite in composition in the stable single domain or pseudo single domain size range however at some sites post depositional chemical changes appear to have produced small amounts of another ferrimagnetic mineral chiefly within the superparamagnetic single domain size range resulting in hard secondary components of magnetization nevertheless at most sites the primary component of remanence was clearly identified by stepwise demagnetization four magnetozones are identified and these can be correlated to the magnetic polarity time scale using existing biostratigraphic constraints the section sampled spans the interval from below the jaramillo subchron to above the matuyama brunhes boundary the matuyama brunhes transition extends over 10 5 m of section within the upper westmere siltstone virtual geomagnetic pole positions describe an equatorial excursion in an otherwise far sided transition path these represent the first published data on the occurrence of the matuyama brunhes boundary on land in new zealand

fluidization of wet sediments during the emplacement and cooling of various igneous bodies during the emplacement and cooling of igneous bodies fluidization of wet sediment by water vapour may occur by heating or by pressure relief during the opening of fractures momentary fluidization causes sediment reconstitution and local transport continuous fluidization can result in substantial sediment displacement theoretical considerations show that fluidization due to heating is not likely to occur at depths where pressure is much greater than 312 bars studies of the emplacement of sills of andesite ayrshire fluid rhyolite ramsey island and viscous rhyolite moelwyn hills recognize a wide range of phenomena attributable to wet sediment fluidization and in places modify existing interpretations the transgressive bases of the subaqueously welded ash flow tuffs of the capel curig volcanic formation are consistent with emplacement attended by fluidization of the subjacent sediment evidently fluidization is a means by which large volumes of wet sediment can be replaced by igneous material with minimal disturbance of the remaining host also fluidization can result in exceptionally complex contact relationships between sedimentary host and intrusive rocks and can cause fractures formed during cooling to be thoroughly pervaded by sediment fluidization is commonly associated with the formation of peperites

chemical remanent magnetism related to the dead sea rift evidence from precambrian igneous rocks of mount timna southern israel a paleomagnetic and mineralogical study of shallow intrusive basement rocks on mount timna arabo nubian massif in sinai shows that although all the igneous rocks are of late precambrian age a remanent magnetic direction similar to the subrecent field miocene to present is identified in samples of quartz monzodiorite monzodiorite dikes of various composition and altered gabbro the average direction of these rock units is d i 359 41 95 4 and the pole is at 83 6 n 223 2 e the subrecent direction appears both as an overprint direction and as the sole stable vector in dolerite rhyolite and andesite dikes the magnetic mineral assemblage of these rocks includes secondary minerals such as hematite and goethite which formed by oxidation and hydration of the original magnetite and ti magnetite the subrecent direction is interpreted to have been acquired as a chemical remanent magnetization crm by hydrothermal activity and circulation of thermal brines through fractures related to the adjacent dead sea transform the hydrothermal activity occurred before uplift and erosion exposed the basement rocks i e in the middle miocene during the early stages of activity of the dead sea rift the north trending declinations indicate that mount timna has not rotated about a vertical axis after acquisition of the crm a conclusion that confirms a previous structural analysis an absence of reversals implies that the duration of crm acquisition was probably less than 1 m y the maximum length of normal polarities since the oligocene other field directions were found in an alkali granite one site 343 14 95 22 in a dolerite dike one site 318 0 95 2 and in a gabbro seven sites 303 56 95 6 the field direction in the granite is similar to that for early cretaceous a time of magmatic activity in timna northwest trending declinations and shallow inclinations found in several samples of the dikes are carried by unaltered parts of the rocks these directions are interpreted as late precambrian early cambrian in age indicating a near equatorial location of the region at that time

magnetostratigraphic lithostratigraphic and tephrostratigraphic constraints on lower and middle pleistocene sea level changes wanganui basin new zealand wanganui basin north island new zealand contains a complex sedimentary record of lower and middle pleistocene sea level changes palaeomagnetic results allow identification of the matuyama brunhes transition the jaramillo subchron and the cobb mountain subchron correlations of rhyolitic tuff horizons across the basin are consistent with magnetostratigraphic correlations between sections isothermal plateau fission track itpft ages of 1 05 0 05 ma and 1 63 0 15 ma on two tuffs potaka pumice and pakihikura pumice respectively are consistent with the interpreted magnetostratigraphy and also with the astronomically tuned timescale of odp site 677 magnetostratigraphy and itpft ages allow correlation of sedimentary cycles at wanganui with odd numbered oxygen isotope stages 17 31 in deep sea cores the stratotype section for the new zealand castlecliffian stage is shown to be incomplete relative to other studied sections in the basin below stage 31 the character of the cyclothems changes at wanganui from marine dominated to a greater representation of non marine and estuarine strata this change may be partly related to a change in amplitude and frequency of climatic cycles identified in oxygen isotope records but the primary cause is likely to be increased basinal uplift

subaqueous pyroclastic flows and ignimbrites an assessment an assessment of the literature on subaqueous pyroclastic flows and their deposits shows that the term pyroclastic flow is frequently used loosely to describe primary hot gas rich pyroclastic flows mass flows which resulted from the transformation of gassupported flows into water supported ones and secondary mass flows carrying redeposited pyroclastic debris based on subaerial pyroclastic flows the term pyroclastic flow should be restricted to demonstrably hot gas rich mass flows of pyroclastic debris using this definition very few examples of subaqueous pyroclastic deposits with evidence for hot emplacement and of having been wholly submerged have been described in the majority of these cases the evidence for a hot state of emplacement and for the subaqueous nature of the host depositional environment is inadequate the only unequivocal cases of hot pyroclastic flow deposits with adequate supporting evidence are the ordovician nearshore shallow marine ignimbrites of ireland and wales and miocene ignimbrites of southwest japan resulting from the passage of subaerially erupted pyroclastic flows into shallow water other possible examples are near vent dense clast deposits in the donzurobo formation of japan possible submarine intra caldera ponded ignimbrite successions in california and wales and near vent pumiceous deposits of ramsay island wales all other purported cases are either clearly the result of water supported mass flow transportation and deposition debris avalanches debris flows turbidity currents or lack adequate supporting evidence regarding the heat state or the palaeoenvironment only the shallow marine ignimbrites of ireland and wales show adequate evidence of welding but even these could have been nearly wholly exposed above sea level when welding occurred we conclude that when pyroclastic flows enter water they are generally disrupted explosively and or ingest water and transform into water supported mass flows and we suggest the various scenarios in which this occurs there is no evidence to suggest that welding in wholly subaqueous environments is common

submarine silicic volcanism and associated sedimentary and tectonic processes ramsey island sw wales lower ordovician marine strata formed in an ensialic marginal basin environment record early episodes of distant explosive silicic volcanism and a later volcanotectonic episode which was a precursor to the development of a wide variety of near to source submarine silicic volcanic phenomena the early episodes are represented by well bedded turbiditic tuffs these and older strata at least 770 m thick in normal stratigraphical succession are allochthonous and large scale wet sediment sliding with reworking of strata in cohesive debris flows reflects major disturbance of the sea floor during the emplacement of silicic magmas volcanotectonic uplift along a n s fault exceeded 1 km and led to the emergence and erosion of a rhyolitic volcanic island this was followed closely by submergence and in water depths perhaps of about 500 m there accumulated welded and non welded rhyolitic ash flow tuffs rhyolitic turbiditic tuffs rhyolite lavas and a wide variety of debris flow deposits shallow intrusions of 9low9 viscosity rhyolite were emplaced contemporaneously the more or less simultaneous emplacement of rhyolitic tuff with both 9low9 viscosity and high viscosity rhyolites is attributed to variable water content related perhaps to volatile fluxing in a magma body the location of the near to source extrusive rocks and consanguineous intrusions along the contemporary n s fault is considered to reflect channelling of magma initially along a deep crustal fracture

palaeomagnetism of sukinda chromites and their geological implications summary thirty eight oriented samples were collected from seven sites over the precambrian sukinda chromite ore between 85 43 e 21 01 n and 85 50 e 21 03 n which is in the form of a plunging syncline on isolating the primary component of magnetization and affecting the necessary tectonic correction four sites a b c f were found to possess a reverse magnetic polarity d 210 i 22 with k 122 and 95 6 and one site e was found to bear a normal polarity d 49 i 16 with k 474 and 95 2 the remaining two sites d and g were rejected the former because of scatter in directions and the latter due to overprinting by a dominant secondary magnetization induced by lateritization on comparison of vgps evaluated for these chromite deposits with those of other precambrian rocks almost of a similar age it is seen that these chromites are probably older than the gwalior traps whose age is 1830 200 ma from the rock magnetic and microscopic examination it is deduced that the massive chromites of the sukinda valley have not been affected magnetically to any great extent by serpentinization though their host rocks are serpentinized ultramafics consistent directions of remanent magnetism point to the fact that these deposits crystallized in fairly stable conditions characteristic of the stratiform type of chromite deposits therefore leading us to infer the sukinda chromites to be of that type these palaeomagnetic data when subjected to the fold test show the primary remanence to pre date the tectonic disturbance and hence prove these chromites to have consolidated prior to the regional tectonic activity of the area

diagenetic formation of ferrimagnetic iron sulphide minerals in rapidly deposited marine sediments south island new zealand abstract detailed magnetostratigraphic studies of late neogene siliciclastic sediments of the awatere group south island new zealand 41 45 s 174 05 e have revealed a wide range of palaeomagnetic behaviour examination of rock magnetic properties was undertaken using conventional palaeomagnetic techniques and thermomagnetic x ray diffraction and electron microprobe analyses these analyses indicate that the ferrimagnetic iron sulphide minerals greigite and pyrrhotite are responsible for a stable and intense magnetic remanence in fine grained sediments whereas titanomagnetite is the only remanence bearing mineral identified in coarser grained sediments which are less strongly and less stably magnetised than the fine grained sediments detrital titanomagnetite grains are likely to have undergone dissolution during early diagenesis as a result of iron sulphide formation which occurs commonly in rapidly deposited anoxic sediments that support active sulphate reduction and h 2 s formation preservation of greigite and pyrrhotite is inferred to result from the arrest of the pyritisation process probably due to the low permeability of the fine grained sediments and consumption of available h 2 s before full reaction to pyrite occurred relative palaeomagnetic instability and weak remanence intensities in coarser grained sediments is likely to be due to low titanomagnetite concentrations resulting from titanomagnetite dissolution

paleomagnetism of mesozoic kimberlites in southern africa and the cretaceous apparent polar wander curve for africa including those previously published good paleomagnetic data have been obtained from 48 of 51 sites in 28 separate kimberlites radiometric dates by one or more methods are available for 17 of these bodies and range from 81 to 145 ma with maxima around 90 and 120 ma these ages correlate with geochemical contrasts distinguishing what have been designated group i younger and ii older kimberlites smith et al 1985b stable magnetization appears to be associated with magnetite developed deuterically after emplacement but in most bodies secular variation has not been completely averaged in the remanence acquired the distribution of vgps calculated is distinctly but not sequentially streaked yet correlates broadly with the older and younger petrochemical types mean north poles calculated on this basis yield kp1 n 14 81 100 ma 64 1 n 226 1 e a95 5 2 and kp2 n 10 113 145 ma 47 6 n 269 9 e a95 9 7 with the exception of one undated pipe meltonwold which was not included in the averages and the melilititic male fissure at swartruggens in group ii average all the kimberlites sampled are normally magnetized including these new data 10 of 17 published african cretaceous poles meet acceptability criteria specified and define an apparent polar wander apw curve from 130 to 75 ma the curve appears to consist of two segments trending latitudinally before 100 ma and meridionally thereafter the latter trend is evident even in the distribution of the individual kp1 poles and may be related to the true polar wander speculated by gordon and livermore 1987 to have occurred in that interval

rock magnetism of lower middle pleistocene marine sediments wanganui basin new zealand rock magnetic studies were carried out in order to identify the remanence carrying minerals and the domain state of magnetic grains in lower middle pleistocene marine sediments from the wanganui basin southwestern north island new zealand isothermal remanent magnetization irm acquisition and stepwise thermal demagnetization of a three axis composite irm suggest that titanomagnetite or titanomaghaemite is the dominant remanence carrier in these sediments other rock magnetic data suggest that these grains occur in a range of sizes in the superparamagnetic to pseudo single domain range which helps account for the wide range of observed palaeomagnetic behaviour isolated occurrences of ferrimagnetic iron sulphides cannot be ruled out

modern and palaeozoic iron ooids a similar volcanic origin abstract the discovery of an unconsolidated deposit of iron ooids of a kind not previously described and in the vicinity of a volcanic island in indonesia has shed new light on the genesis of many ancient iron oolites the discovery shows that iron ooid formation is geologically a rapid process the ooids form by chemical precipitation of cryptocrystalline iron oxyhydroxides on available grains on the seafloor from seawater enriched with fe al and si the enrichment can be a result of hydrothermal fluids volcanic ash falls into shallow basins or rapid weathering of fresh volcanic rocks ordovician iron ooids from northern europe are strikingly similar to the modern ooids in chemical composition ree distributions and internal structures the differences in mineralogy can be explained as diagenetic transformation reactions under different environments a model for the formation of the ordovician iron ooids in northern europe from volcaniclastic material is presented which could be applicable to other ancient oolites in the world

a new k ar age from uppermost tremadoc rocks of north wales new conventional k ar dating of virtually unaltered pargasite phenocrysts extracted from a metabasalt of the uppermost tremadocian rhobell volcanic group cropping out near dolgellau gwynedd north wales yields an age of 508 11 ma this result provides a valuable new time scale calibration point although incompatible with some other versions of the radiometric time scale for the lower palaeozoic which have indicated ages of less than 508 11 ma for the tremadoc arenig boundary

dissolution and pyritization of magnetite in anoxie marine sediments concentrations of magnetite were determined with depth in sediments of varying h2s content taken from long island sound foam nwc and sachem sites and from a site in the rapidly depositing subaqueous portion of the mississippi delta for the three long island sound sites dissolution of magnetite during burial is clearly demonstrated and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide by combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide we find that magnetite dissolution is well described by the following rate law dcmagdt 1 1 10 5c0 5bcmagamag where cmag is the concentration of magnetite grams per gram of sediment cs is the concentration of dissolved sulfide mm and amag is the specific surface area of magnetite in the sediment cm2 per gram of magnetite this equation means that for a typical range of magnetite surface areas and dissolved sulfide concentrations the half life of magnetite in anoxic marine sediments ranges from about 50 to 1000 years sem observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite this only occurs if magnetite is in contact with high concentrations of h2s 1 mm for relatively long periods of time several hundred years with low concentrations of h2s 1 mm magnetite dissolution occurs without identifiable pyrite replacement we suggest that this feature of magnetite diagenesis may prove to be a useful paleosulfide indicator

cenozoic to cretaceous paleomagnetic dataset from egypt new data review and global analysis abstract different phases of igneous activity took place in egypt during the mesozoic and the cenozoic and oriented samples were collected from three cenozoic localities baharya oasis in the western desert abu had in the eastern desert and quseir along the red sea coast and four cretaceous localities toshki abu simbel south of aswan and shalaten abu shihat along the red sea coast rock magnetic properties of the samples indicate magnetite and titanomagnetite as the main carrier of the remanent magnetization following stepwise demagnetization characteristic remanent directions were identified only for 62 of the samples a fairly low rate for that type of samples and 8 new paleomagnetic poles were calculated all our cenozoic poles fall clearly off master polar wander paths proposed for south africa therefore all paleomagnetic results previously published for egypt were compiled from cretaceous to quaternary the published poles largely overlap blurring the egyptian apparent polar wander path a new analysis at the site level was then carried out only poles having a kappa larger than 50 were selected and new pole positions were calculated by area and by epoch when at least 3 sites were available even though the selection drastically reduced the number of considered poles it allows definition of a reliable cenozoic apparent polar wander trend for egypt that differs from the south african master polar wander path by about 10 15 if the cretaceous igneous poles are in good agreement with the rest of the african data the sedimentary poles plot close to the cenozoic portion of the south african master polar wander path a discrepancy that could be related either to inclination flattening and or error on age and or remagnetization in the cenozoic

distributed deformation due to coupling across a subduction thrust mechanism of young tectonic rotation within the south wanganui basin new zealand paleomagnetic data from pliocene sedimentary strata in the south wanganui basin new zealand document clockwise tectonic rotation west of and increasing toward the main zone of deformation axial tectonic belt associated with the australia pacific plate boundary zone in the north island of new zealand measured rotation with respect to the australian plate increases from 1 4 3 9 80 km west of the axial tectonic belt to 6 5 2 0 and 13 2 3 1 at 45 km and 20 km west of the axial tectonic belt respectively these measurements indicate that 1 deformation associated with plate convergence occurs across a wide region up to 270 km west of the hikurangi trench and 2 in the past 2 5 m y at least 30 km of transcurrent motion and 10 km of shortening at the plate boundary have been accommodated within the south wanganui basin by distributed shear west of the plate boundary zone proper the data support a model suggesting that the wanganui basin is a lithospheric flexure basin driven by high friction at the plate interface this strain in the south wanganui basin and inferred high friction across the plate interface at the basin9s eastern margin have occurred since 2 5 ma

magnetic mineral diagenesis in suboxic sediments at bettis site w n ne pacific ocean the effects of iron diagenesis on paleomagnetism and rock magnetism were examined in suboxic sediments from bettis site w n in the ne pacific ocean as part of the oxidative decomposition of organic matter ultrafine grained authigenic magnetites are produced immediately above the iron reduction zone this zone is commonly observed in deep sea sediments as a prominent color change from brown to greenish grey the magnetite authigenesis is evidenced by systematic increases in natural magnetic intensities and stabilities and by comparable shifts in other rock magnetic properties the magnetites are composed of relatively pure fe3o4 and fall within a restricted size range of 0 06 0 12 m which is nominally single domain some of these finest grained magnetites are rapidly destroyed during burial upon entering the zone of iron reduction leaving a relatively coarser population magnetite dissolution appears to continue with depth according to a first order surface area reaction process when at steady state iron reduction is independent of sulfide formation in these sediments because solid sulfur profiles show no increase at depth and most of the sulfur occurs as barite despite a well constrained carbonate stratigraphy comparisons of paleomagnetic directions between nearby cores show little agreement suggesting that the directions are composed of a complex mixture of remanences acquired at different depths subsurface magnetic peaks may represent paleoredox markers preserved by non steady state redox conditions caused by climatically induced variations in organic matter flux and sedimentation rates

stable isotope evidence for near surface low temperature formation of mg hydro carbonates in highly altered greek mesozoic serpentinites authigenic magnesite hydromagnesite and huntite associated with intensely altered and serpentinized ophiolitic rocks in attica mainland greece occur predominantly as veinlets and nodules within a totally weathered former serpentinite groundmass carbonate 18 o values are consistent with post geothermal fluid temperatures between 25 and 70 c but mostly between 25 and 30 c from a dominantly meteoric sourced groundwater indicating near surface low temperature conditions despite the proximity of a volcanic centre with strong co 2 flux 75 of the carbon isotope data imply little or no incorporation of this co 2 into the authigenic mg hydro carbonates indeed many 13 c values are more negative than soil zone calcrete values and in this setting mg hydro carbonate 13 c values below 6 vpdb probably indicate disequilibrium effects in alkaline groundwaters geothermal fluids and groundwaters were mainly routed through structural conduits some of the low temperature hydromagnesite subsequently dehydrated to magnesite under near surface conditions and huntite is probably a diagenetic transformation of hydromagnesite forming close to the volcanic centre where fluid mg ca ratios were low the isotopic signatures are distinct from previously published balkan east mediterranean magnesite data arrays but are consistent with many other ultramafic associated magnesium carbonates worldwide their association with likely fluid compositions provides important context for mg hydro carbonate formation as geothermal conditions cool to near surface temperatures

stability of remanence and paleomagnetic studies of some chromite ores from barramiya and allawi occurrences eastern desert egypt abstract rock magnetic investigations were carried out on natural precambrian chromite ore at two occurrences in the eastern desert egypt a medium high coercivity component of remanence of reversed polarity can be defined the mean direction of the five sites studied is d 198 i 44 with k 55 and 95 10 ore microscopic and magnetic examinations indicate the absence of magnetite and it is shown that the chromite mineral ferrimagnetic phase could be the carriers of the high stable nrm the result of paleopole position of these precambrian chromites are presented and discussed in the context of its age and the apparent polar wander path apwp of africa