late paleozoic glacial episodes in gondwanaland reflected in transgressive regressive depositional sequences in euramerica the late paleozoic glaciation of gondwanaland comprised two short episodes in the famennian i and visean ii confined to brazil and adjacent northwest africa and a long episode that started in the namurian iiia of eastern australia and bolivia argentina expanded to cover much of gondwanaland in the stephanian asselian iiib and collapsed in the early sakmarian iiic dropstones in eastern australia indicate that small ice centers lingered to the kazanian across the belt of low latitudes north of gondwanaland short ranging fossils in widespread shallow marine and paralic deposits indicate synchronous deposition of transgressive regressive sequences in different parts of euramerica these sequences correlate with glacial events in gondwanaland at three levels a four major regressions in euramerica in the famennian 1 visean 2 namurian 3 stephanian 4 and the tastubian transgression that preceded the sterlitamakian regression 5 also recorded in gondwanaland correlate with glacial episodes i ii and iiia iiib and iiic b the time interval of cyclothemic deposition in euramerica brigantian or latest visean to sterlitamakian correlates with that of glacial episode iii and c the dominant period of the euramerican cyclothems as estimated from the middle and late pennsylvanian deposits of the mid continent of north america and of the thickest known gondwanaland glacigenic sediment the earliest permian lyons group of western australia is 0 4 ma equivalent in turn to the long orbital eccentricity period of the quaternary ice age and the dominant period of fluctuation of the late miocene antarctic ice cap the three levels of correlations confirm wanless and shepard9s 1936 hypothesis that the late paleozoic cyclothems are controlled largely by sea level fluctuations related to the gondwanaland glaciation

the fold test in palaeomagnetism summary the fold test suggested by mcelhinny is shown to be inappropriate more correctly one should determine whether the mean direction of a group of sites from one limb of a fold may be distinguished statistically from the mean direction of a group of sites from another limb details of the application of this test are given with examples it is shown that the test indicates significance under far less severe folding than does the test suggested by mcelhinny

namurian uplift in australia and south america triggered the main gondwanan glaciation the permo carboniferous glacigenic sediment at the base of the gondwanan basins reflects the culmination1 2 of the glacial episode that started in australia and southern south america in the namurian after two local episodes in northern south america and adjacent africa in the visean and latest devonian3 fig 1 here we show that the start of each episode was associated with orogenic uplift of the bordering regions northern south america and adjacent africa in the latest devonian and visean and southern south america and eastern and central australia in the namurian all areas except southern south america in the namurian were at mid and not high latitudes thus we show that whereas the first order control of glaciation was the configuration of the continents and oceans during the closing of palaeo tethys orogenesis provided a second order control by the accumulation of ice about bordering uplifts during the three glacial episodes

multicomponent magnetizations from the mississippian mauch chunk formation of the central appalachians and their tectonic implications previous paleomagnetic study of mississippian red beds of the mauch chunk formation from the central appalachians reported shallow inclination directions a positive fold test was obtained after thermal demagnetization typically to only 550 c the near equatorial position for north america indicated by these results has been used to support the idea of a circa 15 latitudinal tectonic offset of the acadia displaced terrain in the northern appalachians the present study of 153 samples from 29 sites occupied in the mauch chunk formation of eastern and southern pennsylvania reveals multicomponent magnetizations a characteristic component declination d 161 0 inclination i 27 8 alpha95 a95 7 9 with unblocking temperatures usually concentrated above 650 c and with dual polarity 10 sites normal 13 sites reversed is isolated in 23 sites and passes a fold test at the 99 confidence level it is interpreted as an early acquired hematitic detrital or chemical magnetization of pre alleghanian orogeny and pre kiaman reverse polarity interval age a southern hemisphere paleolatitude of 15 for the sampling area is indicated another component has generally lower unblocking temperatures which however can extend up to 650 c or more and therefore is also likely to be carried by hematite this secondary magnetization d 170 6 i 5 3 a95 5 0 isolated in 25 sites is of uniform reverse polarity and application of fold tests suggests that it is of synfolding origin a near equatorial paleolatitude 3 n at the time of acquisition is obtained consistent with permo carboniferous kiaman directions from north america incomplete removal of the secondary component can explain the previous results from the mauch chunk comparison of the present paleomagnetic results with those from similar age rocks in the canadian maritimes does not support a latitudinal offset of acadia with respect to interior north america in the early carboniferous in fact according to these and other paleomagnetic data thought to be representative of this age the atlantic bordering continents already were near to a pangea configuration the alleghanian and particularly the hercynian orogenies therefore may not be associated with the closure of a large latitudinal ocean later in the carboniferous and permian alleghanian deformation however may have resulted in the partial bending of the pennsylvanian salient on the basis of apparently systematic deviations in paleomagnetic declinations

prefolding and overprint magnetic signatures in precambrian 2 9 2 7 ga igneous rocks from the pilbara craton and hamersley basin nw australia a total of 446 oriented samples have been collected from an archaean intrusion and the fortescue group volcanics and intrusions in the pilbara and hamersley regions of northwestern australia the paleomagnetism and rock magnetism of these samples have been investigated using susceptibility and hysteresis properties and both alternating field af and thermal demagnetization the most consistent paleomagnetic results were achieved by first applying af demagnetization and then stepwise thermal demagnetization evidence of the effects of lightning are widespread which accounts for the efficacy of this particular demagnetization strategy application of the fold test has resulted in placing significant constraints on the acquisition ages of some of the magnetizations and their corresponding pole positions a prefolding pole position from the millindinna complex is latitude 11 9 s longitude 161 3 e dp 6 8 dm 8 4 and is correlated with a radiometric age of 2 86 0 02 ga folding in the millindinna complex occurred prior to the deposition of the fortescue group new geochronological data indicate that the extrusion of the flood basalts commenced about 2 8 ga this provides a minimum age for the magnetization of the millindinna complex deformation responsible for the formation of the mobile belt separating the pilbara and yilgarn blocks during the ophthalmian orogeny has affected the fortescue group and produced a moderate degree of folding in the southern part of the hamersley basin in the region of millindinna complex outcrop the fortescue group is generally flat lying prefolding pole positions from the fortescue group are for the mount roe basalt latitude 52 4 s longitude 178 0 e dp 6 4 dm 9 1 and for the mount jope volcanics latitude 40 5 s longitude 128 7 e dp 19 9 dm 20 8 the overprint magnetization in the mount jope volcanics yields a pole position latitude 35 0 s longitude 211 5 e dp 2 8 dm 5 5 and is interpreted as having been acquired during folding relating to the formation of the ophthalmian fold belt

paleomagnetism of rocks from burin peninsula newfoundland hypothesis of late paleozoic displacement of acadia criticized the posttectonic st lawrence granite 360 5 ma and associated subvertical dikes have two magnetizations n and h n points north with an intermediate upward inclination declination 005 inclination 50 alpha 95 10 paleopole 12 n 120 e we consider n to have been acquired close to the time of initial cooling the h magnetization points south with low inclination h is also the dominant magnetization in the nearby permo carboniferous spanish room formation h is considered to be an overprint acquired chemically during the kiaman reversed superchron late carboniferous and permian the n magnetization corresponds to a paleolatitude of 31 7 s very different from the near equatorial values expected from the commonly accepted late devonian and earliest carboniferous tournaisian reference paleofields for cratonic north america this is generally consistent with the hypothesis of a southerly displaced acadia proposed by kent and opdyke although the displacement is much greater 30 rather than 15 however we show that all inclinations observed from middle to upper devonian and tournaisian rocks from the craton are not significantly different either from the kiaman paleofield or from the h overprints observed from newfoundland some of these cratonic studies are not very detailed but it is noteworthy that the declination observed in the most comprehensive of them catskill red beds also agrees with the kiaman paleofield and with the h overprints from newfoundland declinations in some other cratonic studies show small departures of only a few degrees with one exception based on limited sampling and two from the colorado plateau which may have been rotated which could be caused by experimental inaccuracies moreover the directions obtained from cratonic studies are 97 reversed as they are in rocks with kiaman magnetizations we suggest therefore that the commonly accepted late devonian to earliest carboniferous reference paleofield for north america is either entirely kiaman in age or has been seriously affected by kiaman overprinting consequently the 30 of southerly displacement which the n magnetization of the st lawrence granite apears to indicate and the 15 invoked by kent and opdyke we consider to be illusory three late precambrian rock units 623 to 606 ma have scattered declinations but similar inclinations yielding paleolatitudes of about 35 the scattered declinations presumable were caused by local pre carboniferous relative rotations there is little or no evidence of kiaman overprinting in precambrian rocks indicating that overprinting is comparatively superficial and is probably related to late paleozoic groundwater circulation

synfolding and prefolding magnetizations in the upper devonian catskill formation of eastern pennsylvania implications for the tectonic history of acadia the upper devonian catskill formation was sampled for paleomagnetic study in east central pennsylvania 41on 76ow in one area the dominant component of magnetization se is revealed over a broad spectrum of demagnetization temperatures ranging to at least 660oc a conventional fold test is positive at the 99 confidence level however statistical analysis of dispersion with incremental bedding tilt correction shows a significant peak in the precision parameter after about 3 4 unfolding the magnetization is therefore secondary with a mean direction of d 166 6 o i 1 8 o the

paleomagnetism of upper devonian reefal limestones canning basin western australia two conflicting models have been proposed for the middle paleozoic position of gondwana with respect to euramerica a pole from morocco msissi norite that may be of late devonian age suggests that a broad ocean 3 000 km separated the northern continents from gondwana whereas results from mauritania gneiguira supergroup indicate that this ocean had closed by devonian time detailed thermal and alternating field demagnetization experiments have been carried out on red and gray limestones from seven sites 89 samples in an essentially undeformed late devonian reef complex northern canning basin western australia after minor tilt correction the site mean characteristic directions average 42 7 28 9 declination inclination with k 61 6 and 95 7 8 these results position northwestern australia at 15 s latitude in the late devonian a paleolatitude consistent with the presence of a well developed reefal sequence an early age is inferred for these magnetizations on the basis of the occurrence of mixed polarities and the fact that no post devonian directions for australia resemble the observed declinations and inclinations if the smith and hallam reconstruction for gondwana is used the paleomagnetic pole calculated for the late devonian of western australia agrees with the msissi norite result from morocco and suggests that an ocean still separated gondwana from euramerica in the late devonian the pole measured from the gneiguira supergroup in mauritania may be based on remagnetizations of late paleozoic age the directions determined from canning basin limestones are similar in inclination to but differ by 40 in declination from paleomagnetic results from the upper devonian comerong volcanics lachlan fold belt southeastern australia canning basin samples are from the stable craton and comerong volcanics samples are from the tasman fold belt the discrepancy between the two australian results may be best explained by tectonic rotation of the comerong volcanics locality

grain scale deformation and the fold test evaluation of syn folding remagnetization the paleomagnetic fold test examines for pre syn or post folding re magnetization generally passive limb rotation is used to restore folded units to their original position and the position of the fisherian maximum deter mines the relationship between folding and magnetization in a more complete analysis the reorientation of material lines in different fold models is used however the magnetic vec tor should not be considered a material line as it represents the sum of vectors in individual magnetic carriers and does not cross grain boundaries therefore the role of grain scale deformation mechanisms was examined using two end mem bers rigid body rotation spin and homogeneous strain from this study it is concluded that 1 the fisherian maximum will in general occur before complete unfolding 2 the sign of the inclination may change during unfolding and consequently cross over unfolding is not representati ve of syn folding remagnetization 3 statistical analysis will not necessarily produce significant results a number of recommendations are given which include stepwise unfol ding and unplunging should be carried out routinely local rather than regional fold tests should be performed both limbs and the hinge of a single folded layer should be exa mined in order to determine the active deformation mecha

palaeomagnetism of australian hematite ore bodies i the middleback ranges of south australia summary the results of a palaeomagnetic study on high grade hematite ore bodies of south australia are reported the two major ore bodies iron monarch and iron prince gave consistent directions of magnetization with either steep to moderate negative and positive inclinations the positive and negative groups of iron monarch are not antiparallel because of the presumably long time interval represented by the sampling this is interpreted as being due to polar wandering with a superimposed reversal of the earth s field the remanent magnetization is carried by hematite and is extremely stable against alternating field and thermal demagnetization the intensity of the nrm is very weak compared to the trm which suggests it to be a crm in origin lack of a detailed polar wandering curve for the pre cambrian of australia prevents precise palaeomagnetic dating of the ores but the results clearly suggest a pre cambrian age the magnetic anisotropy of the ores is low and the distribution of the principal susceptibility axes reflects in part the structure of the host rock the iron duke orebody did not yield a reliable palaeomagnetic result

syn deformational remanent magnetization of the mount eclipse sandstone central australia summary the palaeomagnetism of the latest devonian early carboniferous mount eclipse sandstone in central australia reveals samples from 16 out of 31 sites having a consistent hightemperature component of natural remanent magnetization site mean directions from the two limbs of the most detailed studied syncline attain their best grouping after 75 per cent of unfolding and further unfolding causes a cross over feature mineralogical studies using microscope and electron microprobe show that this remanance is most likely to be carried by secondary haematite formed by alteration of fe rich minerals during the midcarboniferous deformation four different models are examined to reconstruct the palaeohorizontal for the syn deformational remanence the best estimate of the palaeomagnetic south pole position is lat 33 8 s and long 121 2 e dp 19 2 drn 19 7 close to australia this result indicates a rapid polanvard movement of eastern gondwanaland during the first half of the carboniferous we also suggest that the widespread alice springs orogeny related overprints in central australia were acquired during the beginning of the main phase of deformation in the mid carboniferous

linearity spectrum analysis of multi component magnetizations and its application to some igneous rocks from south eastern australia summary a method that enables the objective resolution of almost parallel multi component magnetizations is described and demonstrated a feature distinguishing this method from others is its simultaneous analysis of demagnetization data from a group of specimens rather than the analysis of data from one specimen at a time the only prerequisite is that the specimens are derived from a homogeneous source thus for a formation carrying a simple single component magnetization all specimens from the formation may be simultaneously reduced for a more complicated two component magnetization it is shown that only specimens from a particular site can be considered homogeneous and for a complex three component system each sample often requires undivided attention thus the workload is proportionally increased to achieve analyses of comparable reliability from data of variable quality new pole positions from mesozoic intrusions of the sydney basin nsw are from the marsden park breccia pipe 48 s 127 e a95 6 the st marys breccia pipe 46 s 150 e a95 8 the prospect dolerite 60 s 142 e a95 13 and 53 s 180 e a95 6 and from the dundas breccia pipe 58 s 162 e a95 36 and 31 s 195 e a95 16 the last two formations possess multi component magnetizations these pole positions are consistent with previous results from south eastern australia

palaeomagnetism and age of mid palaeozoic ring complexes in niger west africa and tectonic implications summary detailed af and thermal demagnetization has been performed on 185 oriented samples from 36 sites in a series of basic to acid subvolcanic ring complexes in central and southern niger published rb sr whole rock and mineral isochrons from acid members of the suite range north to south from 480 ma to 320 ma two distinct vectors have been isolated by linear regression of demagnetization data 1 a due north and negative inclination rarely reversed component is present in basic rocks from 12 sites in four distinct northern complexes published rb sr ages date from 480 ma to 420 ma this component is carried by single domain magnetite occasionally also by titanhaematite and is considered to be primary acquired during the terminal stages of this igneous episode 40ar 39ar dating of biotite from three samples used in the palaeomagnetic study yield 435 myr ages 2 a due north and positive inclination rarely reversed direction is present as a second component in some of the basic suite sites and is the predominant component in most sites in syenitic to granodioritic rocks primary fe ti oxides are rare in these acid igneous rocks and the feldspars within them are usually pervasively altered this component carried primarily by haematite is believed to be either a modern day vrm or a secondary tcrm acquired during hydro thermal activity associated with widespread quaternary volcanism in the southern air the primary component gives a mean south pole at 43 4s 08 6e n 12 k 50 95 6 2 located southwest of cape town this position requires a rapid southward excursion of the gondwana a p w path in early silurian time but it is generally consistent with most of the other silurian devonian palaeomagnetic data from gondwana these however are from the lachlan fold belt in southeast australia whose relation to the australian craton in early palaeozoic time is not clear integrating all stratigraphic palaeoclimatic tectonic and other palaeomagnetic data for early and middle palaeozoic time suggests that the proto atlantic closed during the silurian period if a late devonian early carboniferous gondwana pole in central africa is valid the ocean may have opened again before the final closure and reassembly of gondwana in the late carboniferous

precipitation and dissolution of gibbsite during weathering of andesitic boulders in humid tropical west java indonesia abstract several weathered andesite boulders sampled in typic humitropepts from the cigudeg area west java indonesia were studied using micromorphological mineralogical xrd tg dta sem edxra and chemical techniques because all data of the boulders are similar only the results of the largest boulder are presented in this paper three zones are distinguished the rim of the boulder core a transition layer and outer layers in the rim of the core weathering attacks the glass matrix and the anorthite rich parts of the plagioclases even before sufficient iron is liberated from pyroxenes goethite is deposited in their cracks as a first step to the formation of a boxwork in the transition layer mono and divalent cations have been lost but al shows an absolute accumulation as gibbsite in plagioclase pseudomorphs and pores it is presumed that the al originates from the more internal parts of the core towards the outer layer gibbsite gets completely removed supposedly as the result of a lowering of ph and the plagioclase pseudomorphs become filled with illuvial kaolinitic clay illuviation of clay and the import of iron from the surrounding soil material put limits to the use of isovolumetric calculations the goethite boxworks after pyroxenes reflect the original pattern of fractures rather than cleavage patterns

cretaceous paleomagnetism of the methow pasayten belt washington detailed demagnetization experiments isolated a characteristic remanent magnetization in ten stable sites from the upper cretaceous winthrop and midnight peak formations in the methow pasayten belt of north central washington this remanence agrees best between opposite limbs of a fold the goat peak syncline when corrected for 46 of tilt this is consistent with magnetization acquired during deformation synfolding magnetization may have been facilitated by a thermo chemical event associated with synkinematic intrusions along the axis of folding the mean direction d 12 0 i 61 1 alpha 95 4 8 is highly discordant with respect to the expected direction for north central washington this discordance points to about 1 400 km of poleward transport and 48 of clockwise rotation between 93 and 45 48 ma

pre and post folding magnetizations from the early devonian snowy river volcanics and buchan caves limestone victoria summary multicomponent magnetizations of the snowy river volcanics have been resolved into pre folding dec 340 1 inc 59 2 and post folding dec 10 5 inc 72 3 elements the former is attributed to initial cooling of the volcanics while the latter is probably related to the waning stages of the deformational episode which was presumably accompanied by cooling we argue that the pole position from the pre folding magnetizations lat 74 3 s long 222 7 e represents the australian palaeopole for the early devonian a reassessment of other australian pole positions determined from rocks of about this age suggests that remagnetization during the carboniferous may be common interpretation of the pole path prior to the devonian is equivocal and awaits further study the post folding magnetizations from the volcanics are similar in stability and direction to those from the buchan caves limestone dec 27 8 inc 75 6 which appears to be completely remagnetized the age of this remagnetizing event appears to be mid carboniferous if our interpretation of the pole path for this time is correct it is noted that a much unappreciated attribute of the fold test lies with identifying not just the relative age of magnetization but also the palaeohorizontal only when the palaeohorizontal is established can the calculation of a palaeopole be justified

paleomagnetic study of thrust sheet rotation during foreland impingement in the wyoming idaho overthrust belt upper jurassic and lower cretaceous strata in the darby and absaroka thrust sheets were sampled in order to paleomagnetically evaluate thrust sheet rotation in the wyoming idaho overthrust belt no significant rotations were found lack of rotation in these thrust sheets suggests that previously reported rotation of the prospect thrust sheet was not transmitted to the older darby and absaroka thrust sheets indicating that the prospect thrust sheet only experienced local rotations in response to collision with the teton gros ventre uplift the upper jurassic stump formation possesses a magnetization that passes the fold test and gives a paleomagnetic pole at 64 n 170 e k 18 a95 15 in good agreement with other published upper jurassic poles for cratonic north america samples from all lower cretaceous sites are believed to possess a synfolding remagnetization of chemical origin

folding strain and graham s fold test in palaeomagnetic investigations summary graham s fold test is an important test of stability of magnet ization and is widely used in palaeomagnetic studies the change in dispersion of palaeomagnetic directions achieved by returning layers to their original orientation can give an indication of the relationship between the time of deformation and of magnetization acquisition however mcelhinny s criteria for application of the test are regarded by mcfadden jones to be inappro priate additionally simple rotation of directions about the strike of the layer may not be a sufficient procedure the rock type and style and amount of deformation may often appreciably affect the orientation of a vector or magnetic direction during deformation hence detailed observations are necessary to provide the basis of the steps used in applying the fold test apparent failure of the test may not indicate the absence of pre deformation stable magnetization in a rock although graham s fold test is important in palaeomagnetic and related tectonic investigations of deformed rocks application of the test may be very complex

a magnetotectonic study of the hercynian montagne noire france a paleomagnetic study of eleven sites of devonian limestones sampled along the flanks and hinge of a polyphase hercynian recumbent fold in the montagne noire reveals well defined directions of characteristic remanent magnetization chrm the directions have good within site precision but a large between site scatter which fails to be reduced by a classical tilt correction this is readily understood if widespread remagnetization occurred between the two major hercynian phases of folding a and b which are known to have shaped the montagne noire this remagnetization appears to be an instance of a continent wide chemical event that wiped out much of the magnetic memory of devonian and lower carboniferous rocks in laurasia baltica and northern gondwana possibly due to groundwater circulation following the major hercynian tectonic phases comparison of observed and predicted hercynian magnetic directions allows one to determine through a fairly simple geometric construction the tectonic elements of the later phase b it is found that different rotations affected the flanks and hinge zone of the folds generated by phase a thus demonstrating how a pre existing structure can control the development of deformation in a later tectonic phase the effects of the non cylindrical phase b can next be removed to reveal the broad tectonic features of phase a deformations the reversed kiaman polarity of the chrm allows a determination of the age of phase b which is found to be about 300 310 ma and of the age of the remagnetization itself at about 310 320 ma the search for the age of remagnetization included paleomagnetic measurements at two lower permian sites from the nearby lodeve basin these provide new data for the permian pole of europe and point out the limitations of some earlier studies due to insufficient magnetic cleaning finally the principal shortening directions of the two phases can be determined from the magnetic data and are found to have rotated counterclockwise by 30 between phase a and phase b this feature already observed in the collision of india against eurasia may be related to the progressive suturing of former subduction zones during the hercynian collision of gondwana against laurasia this study demonstrates the potential of remagnetizations for the solution of rather complex tectonic problems we propose the term magnetotectonics for this rapidly expanding field of studies in which the methods of paleomagnetism and structural analysis are closely associated

palaeomagnetism of palaeozoic rocks from the cabo de penas asturia spain summary ordovician volcanic samples of the cabo de penas area yielded a primary remanent magnetization d 202 i 78 95 6 which is partially or entirely overprinted by a late palaeozoic remagnetization late devonian early carboniferous sedimentary samples reveal a pre or syntectonic carboniferous remanent magnetization similar to the secondary one of the volcanic samples a soft post tectonic component appears also in few samples for which a late carboniferous age is inferred comparison of the carboniferous remanent magnetizations and structural parameters suggests that in a late stage of the deformation the sites have undergone minor rotations around a vertical axis a strike correction is therefore proposed to take such an effect into account the average of the corrected mean site directions d 178 i 19 95 9 is considered to be a good approximation of the early or middle carboniferous palaeomagnetic direction in asturia

paleomagnetism of the hervey group central new south wales and its tectonic implications paleomagnetic work on the hervey group in southeast australia reveals a predeformational magnetization as suggested by the retention of a predeformational compaction produced magnetic fabric and the effect of deformation on both the magnetic fabric and the direction of the magnetic remanence in cleaved rocks the formation mean direction d 29 2 i 19 3 with 95 15 5 gives a latest devonian early carboniferous palaeomagnetic pole at 54 4 s 24 1 e with dp 8 4 dm 16 2 comparing this pole with existing poles from australia and north america it is suggested that 1 the lachlan fold belt of southeast australia has been part of cratonic australia since at least the late devonian and consequently the late palaeozoic palaeomagnetic data from the lfb may be applied to the whole of gondwanaland 2 cratonic australia and the new england fold belt were all in an equatorial position during the early carboniferous 3 there was probably a rapid anticlockwise rotation of gondwanaland during the late devonian around an axis close to australia 4 a v shaped ocean existed between laurussia and the northern margin of western gondwanaland during the late devonian early carboniferous the appalachian hercynian orogenic belt is probably the result of the closure of this ocean

the tectonic implications of solundian upper devonian magnetization of the devonian rocks of kvamshesten western norway the allochthonous old red sandstone of kvamshesten western norway records polyphase orogenic deformation and palaeomagnetic results from both the devonian sediments and mylonites associated with the basal thrust define a syn to post tectonic magnetization withd 218 i 3 and 95 9 7 the corresponding pole position lat 21 s long 324 e suggests a late devonian early carboniferous magnetization age solundian and probably dates the time of thrust movements

a seismogenic zone in the deep crust indicated by pseudotachylytes and ultramylonites in granulite facies rocks of calabria southern italy pseudotachylyte veins frequently associated with mylonites and ultramylonites occur within migmatitic paragneisses metamonzodiorites as well as felsic and mafic granulites at the base of the section of the hercynian lower crust exposed in calabria southern italy the crustal section is tectonically superposed on lower grade units ultramylonites and pseudotachylytes are particularly well developed in migmatitic paragneisses whereas sparse fault related pseudotachylytes and thin mylonite ultramylonite bands occur in granulite facies rocks the presence of sillimanite and clinopyroxene in ultramylonites and mylonites indicates that relatively high temperature conditions preceded the formation of pseudotachylytes we have analysed pseudotachylytes from different rock types to ascertain their deep crustal origin and to better understand the relationships between brittle and ductile processes during deformation of the deeper crust different protoliths were selected to test how lithology controls pseudotachylyte composition and textures in migmatites and felsic granulites euhedral or cauliflower shaped garnets directly crystallized from pseudotachylyte melts of near andesitic composition this indicates that pseudotachylytes originated at deep crustal conditions 0 75 gpa in mafic protoliths quenched needle to feather shaped high alumina orthopyroxene occurs in contact with newly crystallized plagioclase the pyroxene crystallizes in garnet free and garnet bearing veins the simultaneous growth of orthopyroxene and plagioclase as well as almandine suggests lower crustal origin with pressures in excess of 0 85 gpa the existence of melts of different composition in the same vein indicates the stepwise non equilibrium conditions of frictional melting melt formed and intruded into pre existing anisotropies in mafic granulites brittle faulting is localized in a previously formed thin high temperature mylonite bands migmatitic gneisses are deformed into ultramylonite domains characterized by s c fabric small grain size and fluids lowered the effective stress on the c planes favouring a seismic event and the consequent melt generation microstructures and ductile deformation of pseudotachylytes suggest continuous ductile flow punctuated by episodes of high strain rate leading to seismic events and melting

palaeomagnetic studies of fold development and propagation in the pembrokeshire old red sandstone abstract the geometry of two folds in lower devonian old red sandstone in pembrokeshire deformed in the hercynian has been defined at intermediate stages during their evolution by this preliminary palaeomagnetic study this is possible because remagnetization appears to have occurred in discrete episodes during deformation and the shape can be reconstructed by restoring the fold limbs to an angle at which the remagnetization directions coincide the plunge of the folds is shown to vary considerably during evolution one fold having an early easterly plunge which changed to the present westerly plunge the other fold having an early plunge which was more steeply to the west than its present attitude clockwise rotation of the whole of southern pembrokeshire by 40 towards the end of or since the hercynian deformation is indicated by the palaeomagnetic data and a structural model for this is proposed

prefolding and premegakinking magnetizations from the devonian comerong volcanics new south wales australia and their bearing on the gondwana pole path palaeomagnetic results from the late middle to early late devonian comerong volcanics in the budawang synclinorium lachlan fold belt lfb new south wales australia satisfy a fold test and support the hypothesis that foliated parts of the lfb have been folded by megakinks during the terminal mid carboniferous orogenic phase the preferred pole position is at 76 9 s 330 7 e a95 7 2 allowing calibration of the australian pole path at around 370 ma ago an alterative pole position at 71 3 s 283 1 e same a95 takes into account a possible rotation of the megakinked terrane by 15 clockwise with respect to the australian craton although there are still problems unravelling previously published results from silurian rocks in the lfb both because of unreliability and uncertainty as to where the lfb crustal units lay in relation to the australian craton there is no geological reason to suspect any large relative movement between the lfb and the craton since the middle devonian and thus the comerong volcanics pole position is representative of the late devonian for australia the new pole requires reassessment of many other poles that have previously been accepted as representing the siluro devonian for australia and gondwanaland we present reasons for believing that the magnetization or remagnetization of many rock units e g mereenie sandstone and mulga downs group australia and the gneigura supergroup africa was later than sedimentation probably during widespread early carboniferous deformation another rock unit msissi norite africa could be of early carboniferous rather than late devonian age

a kinematic model for the collision and complete suturing between gondwanaland and laurussia in the carboniferous maps of middle to late carboniferous strike slip faults in western europe northern africa and eastern north america have been compiled from geological gravity and magnetic onshore and offshore data a large majority of these faults can be interpreted in terms of a logical syn to post collisional pattern superimposed on the initial suture between the african i e gondwana and north american european laurussian continental margins the pre collision positions of the major continents are derived from paleomagnetic data and when combined with the general distribution of the strike slip faults lead to a kinematic model of the complex middle late carboniferous collision in this model the west african precambrian craton acted as a rigid promontory or die with respect to the partially rigid plastic zones between it and the large north american craton these partially rigid plastic zones comprise the external appalachian and canadian maritime margins the mauritanide and hercynian margins of north

upb and rbsr geochronology and geological evolution of the harts range ruby mine area of the arunta inlier central australia abstract supracrustal and meta igneous rock units from the ruby mine area of the harts range eastern arunta inlier central australia have been dated by the zircon u pb and rb sr total rock tr and mica methods a well defined zircon discordia for a weakly deformed specimen of the bruna granite gneiss yields an age of emplacement of 1748 4 5 ma thereby constraining the minimum age of the irindina supracrustal assemblage metapelitic gneiss within the supracrustals and a meta igneous ultramafic boudin from the associated harts range meta igneous complex yield highly discordant zircon data revealing a strong early palaeozoic overprint rb sr tr data from anorthositic gneisses associated with the ultramafic boudin are highly disturbed also apparently during the lower palaeozoic however rb sr model age calculations and the zircon u pb data suggest a maximum age of about 2000 ma for the supracrustal and meta igneous rocks and argue for new proterozoic crust formation zircon u pb data from a deformed pegmatite emplaced in the meta igneous complex yield an emplacement age of 520 4 5 ma further pointing to lower palaeozoic magmatism and deformation correlations of u content and calculated 206 pb 238 u age for the ultramafic boudin zircons suggest that new growth of low u zircons occurred during retrogression associated with this event the sr isotope systematics of the anorthositic gneisses can also be interpreted in terms of introduction of palaeozoic sr our data suggest lower palaeozoic possibly delamerian tectonothermal activity to be more important in the evolution of the harts range area than previously recognised on the other hand rb sr mica ages for deformed and undeformed pegmatites and tr isochrons for the latter show that pervasive tectonothernal activity had ceased by about 315 ma and that regional cooling occurred between about 345 and 325 ma local shear zone biotite resetting may have persisted to about 300 ma consistent with the previously recognised alice springs orogeny possible dilational pb loss in the bruna zircons occurred at about 103 ma

strain reorientation of hematite and its bearing upon remanent magnetization using a computer based nume rical model we suggest that passive reorientation of hematite planar particles induced by strain perturb significantly the remanent magnetization carried by the hematite the evolution of its direction and dispersion in a progressively incre asing strain can be correlated with the evolution of passive 1ines in real rocks the amount of reorientation can be appro ched by measuring one of the thr ee follo wing tensors strain tensor orientation tensor of some hematite cristallographic axes or magnetic susceptibility tensor from examples taken in the maritime alps and in brittany we emphasize the possible correlations between these tensors orien tation of principal axes and axial ratios fina1ly we use our best estimate of strain to correct the paleomagnetic results by the strain removal technique we have already used in the maritime aips cogne and perroud 1985 when applied to the preliminary paleom gnetic results ob tained from ordovician deformed redbeds of brittany it allows to recover a conclu sive primary magnetization direction although the characteristic magnetizations

australian palaeozoic palaeomagnetism and tectonics ii a revised apparent polar wander path and palaeogeography abstract new palaeomagnetic data from mid to late palaeozoic rocks in australia have enabled us to revise the palaeozoic apparent polar wander path apwp this modified australian apwp is supported by data from other parts of gondwanaland the palaeomagnetic poles indicate that during the early and mid palaeozoic australia underwent rapid rotation first clockwise during the mid ordovician to the early silurian then counterclockwise from the mid silurian until the end of the devonian while it remained at low to equatorial latitudes this was succeeded by a rapid southward movement during mid carboniferous times the implications of the palaeomagnetic data for the tectonic relationship between the lachlan fold belt lfb and cratonic australia are consistent with the tectonic evidence that the lfb has been in place since the mid devonian