intracontinental movements in western gondwanaland a palaeomagnetic test abstract early cretaceous intracontinental movements within africa and or south america to account for misfit problem in the pre drift reconstruction of western gondwana have been tested using palaeomagnetic poles from both continents each continent has been considered as comprising separate subplates according to the boundaries proposed by pindell and dewey 1982 and curie 1983 i e northern africa naf southern africa saf northern south america nsa and southern south america ssa visual and statistical distribution of late permian to jurassic poles from the african subplates indicate two distinct groups before rotating saf relative to naf while after the rotation they become indistinguishable the distribution and sparsity of data from south america limit the study in this continent as the data are statistically indistinguishable before and after rotation of ssa with respect to nsa when the test is extended to the reconstruction of western gondwana the tightest grouping of the poles occurs when an intracontinental movement is assumed within africa this suggests that the misfit between the two continents is probably due to intracontinental deformation within africa and not in south america if confirmed this may have profound implications in the kinematics of the processes related to the opening of the south and central atlantic oceans and also the origin of the west african rift system

paleomagnetism of middle late jurassic sediments from poland and implications for the polarity of the geomagnetic field abstract paleomagnetic data for the middle late jurassic sediments from nine localities of krakow czestochowa upland belonging to stable europe and from three valleys of the lower subtatric nappe belonging to mobile europe are presented the majority of the rocks investigated comprising limestones stromatolites and radiolarites were found to be normally magnetized natural remanent magnetizations nrm of reversed polarity were found in limestones and stromatolites of middle callovian age from the stable region and in one exposure of oxfordian radiolarites from the mobile region along one of the middle callovian sections studied the nrm direction of the specimens changed from r to n the mean paleomagnetic pole positions obtained for the krakow czestochowa upland lat 72 3 n long 150 4 e 95 7 3 and for the lower subtatric nappe lat 71 7 n long 132 2 e 95 4 3 are consistent with the data for stable europe lat 74 7 n long 147 1 e 95 5 0 the paleomagnetic results presented do not reveal any major relative rotations either within the lower subtatric nappe or between this area and the krakow czestochowa upland the reversed directions of nrm observed indicate the existence of a period of reversed field direction in the middle callovian and perhaps also in the oxfordian

new early jurassic paleopole from france and jurassic apparent polar wander abstract a new early jurassic paleopole for stable europe obtained from a doleritic dyke system in brittany france is given at latitude 61 n and longitude 79 e k 36 anda95 10 2 based on seven equivalent sites and more than a hundred samples demagnetized by alternating field and thermal treatments d 41 i 62 k 66 95 7 5 radiometric datings provide ages between 190 and 200 ma this result represents the first indication obtained from european igneous data for a very westerly cusp in the apparent polar wander path around 200 ma the european data also tend to support the hypothesis of a higher latitude track for the jurassic apwp as already proposed from north american data

late cretaceous polar wander of the pacific plate evidence of a rapid true polar wander event we reexamined the late cretaceous early tertiary apparent polar wander path for the pacific plate using 27 paleomagnetic poles from seamounts dated by 40ar 39ar geochronology the path shows little motion from 120 to 90 million years ago ma northward motion from 79 to 39 ma and two groups of poles separated by 16 to 21 degrees with indistinguishable mean ages of 84 2 ma the latter phenomenon may represent a rapid polar wander episode 3 to 10 degrees per million years whose timing is not adequately resolved with existing data similar features in other polar wander paths imply that the event was a rapid shift of the spin axis relative to the mantle true polar wander which may have been related to global changes in plate motion large igneous province eruptions and a shift in magnetic field polarity state

relative displacement between eurasia and north america prior to the formation of oceanic crust in the north atlantic abstract the relative positions of north america and eurasia prior to the opening of the north atlantic can be reconstructed using three independent data sets ocean floor magnetic anomalies and fracture zones the shape of the shelf line at the continental margins of north america and eurasia and the apparent polar wander paths apwps of these two continents together with geological constraints we here examine the internal consistency of these reconstructions discuss the tectonic implications of the inconsistencies and show how all three data sets can be combined to obtain a best fit reconstruction pole in our analysis we find that the combined errors of the new north american and european apwps are smaller for 290 240 ma than at earlier or later times therefore this interval was used rotation poles derived from magnetic anomalies from the north atlantic do not bring the apwps into coincidence a rotation pole located at 88 n 215 e with rotation angle 42 superimposes the two apwps and is reasonably consistent with the geology of the continental margins in the north atlantic the difference between the rotation pole based on apwps and that based on magnetic anomalies probably reflects relative motion of the two cratons between the early triassic and the late cretaceous cratonal germany moving about 800 km to the east relative to north america and norway moving about 500 km northeast relative to greenland this motion is expressed in extension in the north sea and the continental shelves on the other side of the globe in the vicinity of siberia and alaska our reconstruction places cratonal north america farther away from cratonal eurasia than do reconstructions based on north atlantic magnetic anomalies this greater separation is consistent with the presence in the late paleozoic and early mesozoic of a sea way in siberia located in the gap between eurasia and north america the magnitude of this gap is uncertain because of mesozoic deformation in alaska and siberia

paleomagnetism of the lower jurassic copper mountain intrusions and the geotectonics of terrane i british columbia the 198 ma lower jurassic copper mountain intrusions outcrop in the quesnellia subterrane of terrane i in the canadian cordillera thermal demagnetization at 550 10 c defines a single stable remanence component of 25 9 41 2 95 3 6 in 11 reliable sites the resulting pole position of 11 8 e 57 3 n p 2 7 m 4 4 is discordant for the north american craton and reflects 39 6 of clockwise rotation and 9 5 of northward translation combined with previous paleomagnetic and geologic data a consistent model for the geotectonic evolution of quesnellia and terrane i emerges

temporal and spatial relationships of thick and thin skinned deformation a case study from the malargue fold and thrust belt southern central andes abstract in this paper we analyse two end member models of temporal and spatial interactions between thick and thin skinned structures in a thrust front with pre existing rift structures in the most commonly accepted model a hinterland to foreland sequence of inversion of pre existing normal faults is proposed as a result the emplacement of shallow thrust sheets in the sedimentary cover occurs before the basement inversion in the foreland in the other model basin inversion occurs early in the deformation history of the external part of a fold and thrust belt as the result of a foreland to hinterland sequence of inversion the malargue fold and thrust belt 34 36 s formed in response to compression of the mesozoic neuquen basin during neogene to pleistocene times integrating detailed structural data from the northern part of this belt with new ar ar dating we propose a revised kinematic model of thick and thin skinned interaction and define the temporal spatial evolution of the belt comparison of the timing of deformation in the thick and thin skinned areas strongly supports the hypothesis that the reactivation of normal faults was coeval with the insertion of shallow detachments and low angle thrusting along the migrating front of the thrust belt and occurred from the foreland to the hinterland detachments occur at several stratigraphic horizons including a deep basement decollement related to the basement involved thrusting and shallow detachments located within the jurassic and cretaceous beds these shallow and deep detachments were coeval producing simultaneous development of thrusts during the complex deformation of the thrust front between 15 and 8 ma

evidence for a 20 tilting of the earth s rotation axis 110 million years ago true polar wander tpw the shift of the earth s rotation axis with respect to the entire globe is most probably due to mass redistribution in the earth s mantle as a result of convection using a new rigorously selected palaeomagnetic database gathering only directions obtained from magmatic rocks we find that tpw has been clearly intermittent over the last 200 ma with two long periods of strict standstill from the present to 80 ma and from approximately 150 to 200 ma a single period of shifting is observed between 80 and about 150 ma ago this period culminates around 110 ma ago in an 20 abrupt tilting during which an angular speed exceeding 5 ma 0 5m yr may have been reached assuming that the time averaged geomagnetic field is axial our results indicate that the changes in the position of the rotation axis and therefore in the inertia tensor of the earth are intermittent we suggest that a major reorganization of the mass distribution in the earth s mantle occurred in the lower cretaceous this event concomitant with plume hyperactivity at the earth s surface and probable drastic changes at the core mantle boundary attested by the inhibition of geomagnetic reversals suggests unmixing of upper and lower mantle by avalanching of upper mantle material down to the core mantle boundary the astonishingly strict stability of the time averaged position of the rotation axis before and after this episode of shifting implies the existence of some steady convection which does not modify the large scale distribution of mass within the mantle given the intermittence of mantle avalanching we suggest that these long periods of stability correspond to the temporary reestablishment of a basically two layered convection system within the mantle

extensional tectonics cretaceous andes northern chile 27 s in the andes of northern chile the sierra fraga puquios region 27 s exposes a complete record of superposed deformation that affected this part of the orogen during the mesozoic and cenozoic a major extensional event which probably occurred between the aptian and cenomanian generated a suite of structures remarkably similar to those observed in the extensional terranes of western north america four allochthons involving paleozoic and mesozoic strata are bounded by low angle normal faults that place younger rocks over older omitting as much as several kilometers of stratigraphic section in one area an extensional chaos is composed of domino and boudined blocks of neocomian limestone floating in a matrix of cataclastically deformed volcanics domino orientation boudin asymmetry sheath folds and mesoscopic sense of shear indicators identify two senses of movement on the detachments the first and less important was top to the northwest and the second major event was top to the northeast the parautochthonous upper paleozoic basement core of the extensional terrane differs from the metamorphic core complexes in that it is not highly metamorphosed or ductilely deformed we attribute this difference to the lack of a prior crustal thickening event in the andes and to the position of the sierra fraga puquios at the margin of the central chile aborted marginal basin the extensional events occurred during a global episode of rapid spreading which was accompanied along the andean margin by a substantial increase in magmatism

paleomagnetism of parts of the late triassic diabase dike system associated with the trans new brunswick aeromagnetic lineament a paleomagnetic study was conducted on dike samples taken from three widely separated localities along the northeasterly trending caraquet dike system new brunswick in order to define their age relationship this dike system is recognized on the basis of a well defined aeromagnetic lineament stable magnetization directions were isolated after alternating field 15 40 mt and thermal 300 550 c treatments of the samples taken from the three localities c 353 56 n 353 50 and t 14 37 the virtual pole derived from dike t 82 e 62 n agrees well with previously determined late triassic early jurassic 200 190 ma poles for the appalachians on the other hand the other two virtual pole positions from dikes c 143 e 78 n and n 133 e 74 n fall in the general area of average paleopoles for the middle late jurassic 180 170 ma age the possibility that the dike at t differs in age from that of the dikes at localities c and n by at least 20 ma is suggested in the absence of relia

plate motions and the geomagnetic field i quaternary and late tertiary summary we have tested the hypothesis that the apparent increase in the northward offset of the axial dipole i e the ratio g gy with age for the last 25 ma wilson mcelhinny is due to the failure to correct for plate motions spherical harmonic analyses were performed on two types of data palaeomagnetic poles from continents islands and seamounts and magnetic inclinations from deep sea cores after returning the sampling sites to their predrift locations in fixed hotspots coordinates the results show that the quadrupole term g maintained a value of about 0 05 gy throughout the last 35 ma and that the axial octupole had a small value of about 0 02 gy for the last 5 ma sea core inclinations analysed separately gave essentially the same results as continental palaeopoles for the last few million years independent data sets for the past 2 ma and for the period 2 6 ma gave nearly identical solutions showing that the persistence of the axial terms is not a phenomenon of the more densely sampled quaternary field alone nor is it an artifice of predominantly northward motion of the plates returning all 0 5 ma data to a 5 ma reconstruction failed to eliminate the quadrupole the non zonal coefficients for the 0 5 ma field are typically an order of magnitude less than the zonal terms with the exception of h and are probably insignificant suggesting that longitudinal drift is generally effective in averaging out these components the present distribution of data is inadequate to determine coefficients of third and higher degree prior to 5 ma but there is evidence that g may have changed little during the last 35 ma in addition there has been very little relative motion between the palaeomagnetic dipole axis as defined by the first degree terms and the axis of the hotspots reference frame

the apparent polar wander path of the european plate in upper triassic lower jurassic times and the liassic intraplate fracturing of pangaea new palaeomagnetic constraints from nw france and sw germany summary in order to constrain the european plate motions during triassic lower jurassic times new palaeomagnetic data were acquired from various triassic liassic sediments of northeastern france and southwestern germany a carnian palaeopole 49 n 131 e k 193 a95 7 n 4 was obtained for keuper marls with consistent directions of normal and reversed polarities rhaetian sandstones also with both polarities and carnian dolomites with a magnetization of probable secondary origin provide a mean pole 51 n 112 e k 109 a95 6 n 6 more westerly than that given above the lower hettangian sampled in limestones of the xeuilley quarry near nancy shows a n r n polarity sequence with well defined anti parallel directions the mean hettangian pole is 55 n 100 e k 123 a95 11 n 3 induan sandstones from the northern vosges show two distinct overprints one consistent with the ladinian carnian magnetizations already observed in the same region the second consistent with the recalculated mean lower jurassic pole of europe five other sites with different rock types also exhibit late overprints with lower jurassic directions the mean pole calculated from these overprints is 74 n 124 e k 125 a95 6 n 6 the revised apparent polar wander path apwp presented here shows that for the mid permian to the earliest jurassic the palaeopoles of europe stayed around 50 latitude while the longitudes drifted from 185 e to 100 e the previously established upper triassic lower jurassic loop of the apwp is shifted westwards by about 25 the revised apwp involves the following plate motions after the late alleghanian tectonic phase which marked the end of the variscan orogeny central western europe rotated counterclockwise during about 70 myr up to the earliest jurassic the south north drift in latitude by 35 nearly 4000 km remained regular from 320 to 205 ma with a constant rate of 3 4cm yr 1 around 205 ma this motion ceased probably because of a collision somewhere in the north of eurasia then in the lower liassic the plate underwent a rapid counterclockwise rotation with a eulerian pole located in northeastern asia synchronous with fracturing in the atlantic and the neotethys areas it is proposed that the intraplate fracturing of pangaea was initiated by the rapid change of the megacontinent motion at the end of the trias

paleomagnetism and k ar isochron dates of early jurassic basaltic flows and dikes of atlantic canada k ar isochron dates of 193 2 189 3 and 191 2 ma are reported for the shelburne dike of nova scotia the avalon dike of newfoundland and the caraquet dike of new brunswick respectively these major tholeiitic dikes agree in radiometric date with the north mountain basalt of nova scotia and with other tholeiitic flows of the newark supergroup that have been paleontologically assigned to the hettangian however further work is needed to resolve the discrepancy between the triassic jurassic boundary date of 193 6 ma suggested by the newark supergroup flows and the boundary dates of 204 213 ma adopted in recent time scales the north mountain basalt flows at nine sites on the north shore of the bay of fundy are shown to have likely magnetized during initial cooling yielding an hettangian virtual paleopole at 66 n 72 e dp 9 dm 14 new paleomagnetic results are also reported for the caraquet and avalon dikes the virtual paleopoles from hettangian flows and dikes of atlantic canada lie on a

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

palaeomagnetic study of the el quemado complex and marifil formation patagonian jurassic igneous province argentina summary the upper jurassic el quemado complex was sampled at 36 sites from five localities in the cordilleran foothills of southern patagonia between lago argentino and lago posadas sierra colorada and the middle jurassic marifil formation at 12 sites in the somuncura massif near camarones the main lithology was ignimbrite with minor tuff and lava petrographical and sem observation show that the el quemado rocks suffered an intense high temperature alteration which resulted in transformation of most primary ti magnetite in pseudobrookite rutile and minor ti haematite and fe hydrated oxides a similar less pronounced alteration occurred in the marifil rocks 40ar 39ar dating of el quemado was possible for one sample from sierra colorada and yielded an age of 156 5 1 9 ma magnetic mineralogy measurements isothermal remanence hysteresis loop curie balance show that the remanent magnetization is dominated by psd low ti magnetite often associated to a minor high coercive mineral haematite secondary magnetization components are usually absent or weak at el quemado sites strong at marifil they were completely erased by thermal and af demagnetization and a characteristic remanence chrm stable up to temperatures higher than 550 c or peak field values of 100 mt was successfully isolated the virtual geomagnetic pole vgp from the marifil formation 83 s 138 e is in agreement with the literature data for jurassic rocks from stable south america the el quemado vgps fall in two groups the localities to the north of latitude 48 s lago posadas sierra colorada yield a vgp 81 s 172 e close to that of marifil whereas those south of latitude 49 s lago san martin lago argentino show a highly elongated vgp distribution consistent with counter clockwise block rotation about vertical axes these rotations were likely caused by thrust sheets which were rotating counter clockwise at the same time they were advancing towards the foreland the amount of rotation varies according to the location of the sampling sites in the thrust and fold belt

the anari and tapirapua jurassic formations western brazil paleomagnetism geochemistry and geochronology the anari and tapirapua formations are very similar from the point of view of paleomagnetic geochronological and geochemical results they date from around 197 ma and the flows are mainly tholeiitic basalts with a low tio2 and incompatible element content the magnetic carriers in rocks from these two formations were highly oxidized titanomagnetites maghemites and probably titanomaghemites paleomagnetic analysis has shown that the magnetizations are all normal in polarity and virtual geomagnetic poles obtained for both formations are indistinguishable at the 95 confidence level the calculated mean for these poles is 250 3 e 65 5 s n 15 a95 3 6 ksc 1578 this pole is compatible with jurassic poles which have been determined for south america or transposed from africa

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

the role of true polar wander on the jurassic palaeoclimate from the late carboniferous until the middle jurassic continents were assembled in a quasi rigid supercontinent called pangea the first palaeomagnetic data of south america indicated that the continent remained stationary in similar present day latitudes during most of the mesozoic and even the palaeozoic however new palaeomagnetic data suggest that such a scenario is not likely at least for the jurassic in order to test the stationary versus the dynamic continent model we studied the jurassic apparent polar wander paths of the major continents that is eurasia africa and north america that all in all show the same shape and chronology of the tracks with respect to those from south america we thus present a master path that could be useful for the jurassic pangea one of the most remarkable features observed in the path is the change in pole positions at 197 ma early jurassic which denotes the cessation of the counter clockwise rotation of pangea and commencement of a clockwise rotation that brought about changes in palaeolatitude and orientation until the end of the early jurassic 185 ma here we analyse a number of phenomena that could have triggered the polar shift between 197 and 185 ma and conclude that true polar wander is the most likely in order to do this we used morgan s tectonophysics 94 123 139 1983 grid of hotspots and performed absolute palaeogeographical reconstructions of pangea for the late triassic and jurassic the palaeolatitudes changes that we observe from our palaeomagnetic data are very well sustained by diverse palaeoclimatic proxies derived from geological and palaeoecological data at this time of both the southern and northern hemispheres

the significance of the pdaeomagnetism of jurassic cretaceous rocks from south america predrift movements hairpins and magnetostratigraphy summary in this paper we show that 1 the positions of the cretaceous palaeomagnetic poles pp for south america and africa exhibit elongated distributions that are due to rapid movement of these continents from the south pole 2 the positions of the middle late jurassic virtual geomagnetic poles for south america exhibit an elongated distribution along the meridians 20 200 e it is suggested that this is due to a rapid shift of south america in middle late jurassic time 3 the late early early late cretaceous sections of the apparent polar wandering paths for south america and africa are consistent with south atlantic seafloor spreading data on the basis of the comparison of the reliable late palaeozoic late cretaceous pps for south america and africa taking into account the restrictions established by geological palaeontological and seafloor spreading data it is suggested that minor movements could have occurred within western gondwana in middle late jurassic time along a narrow zone which later became the south atlantic divergent boundary four hairpins are defined in the late palaeozoic late cretaceous section of the apparent polar wandering path for south america the two youngest of these can be correlated with the origin of the south atlantic ocean basin and the onset of the andean orogeny respectively the magnetostratigraphy for the serra geral lava flow sequence suggests that some of these flows were poured out rapidly without significant interruption

a paleomagnetic test of the proposed mojave sonora megashear in northwestern mexico abstract three upper triassic lower jurassic formations in southwestern united states and northwestern mexico have been investigated paleomagnetically as a means of testing for displacement along the proposed mojave sonora megashear luning formation of west central nevada and sil nakya formation of southern arizona lie north of the megashear whereas in northwestern sonora antimonio formation lies south of the megashear previously published results from the nazas formation of presumed lower jurassic age from north central mexico were incorporated into the interpretations these four units compose two sets of correlative formations on opposite sides of the proposed megashear restoration of 800 km of left lateral displacement along the megashear about the pole of rotation determined by anderson and schmidt 1983 improves the grouping of the two sets of paleomagnetic poles noticeably with the improvement having statistical significance at the 95 confidence level we feel that the data support but do not prove the postulated displacement along the megashear all four poles are displaced from irving s 1979 smoothed apparent polar wander path for cratonic north america in the intervals 190 and 200 ma the displacement needed to bring all of these poles into coincidence is a pure rotation of about 8 counterclockwise about the sampling area two probable cretaceous poles were obtained from the sand wells formation of southern arizona and from unnamed volcanics in northwest sonora these poles are essentially identical and suggest that displacement along the megashear had ceased by the cretaceous data from the early jurassic mulberry wash and pitoikam formations in southern arizona appear to have been remagnetized in a later cretaceous event and were not used in this study

remagnetizations and thrusting in the idaho wyoming overthrust belt a study on the relationship between deformation and paleomagnetism was carried out in the frontal segment of the idaho wyoming overthrust belt samples were taken from the jurassic twin creek unit of the most easterly thrust the prospect thrust for paleomagnetic study detailed analysis of the paleomagnetic data showed a synfolding remagnetization with a paleopole latitude longitude 83 n 286 e located on the tertiary segment of the north american apparent polar wander path apwp the age of the paleopole roughly coincides with the age of thrusting on the prospect thrust as determined by synorogenic deposits the magnetization was found to reside in magnetite spherules previous data from the absaroka and darby thrust sheets that lie to the west of the prospect thrust were reevaluated and they also indicate a synfolding age for the remagnetization the corresponding paleopole 64 n 162 e lies on the late mesozoic segment of the north american apwp for these more interior thrust sheets the apparent age of the remagnetization is older than that for the exterior prospect thrust sheet but again roughly corresponds with the inferred age of thrusting for the sampled thrusts we conclude that remagnetization in the idaho wyoming overthrust belt is temporally related to deformation and we suggest that authigenic magnetite growth occurred during discrete fluid pulses that were concurrent with thrusting and folding events

rock magnetic and magnetic fabric variations across three thin alkaline dykes from sunnhordland western norway influence of initial mineralogy and secondary chemical alterations summary profiles of natural remanent magnetization nrm rock magnetic properties and anisotropy of magnetic susceptibility ams across three thin 26 to 109 cm alkaline dykes from sunnhordland western norway are presented the magnetic remanence is carried by pseudo single domain psd lowtemperature oxidized titanomagnetite with circa 6 per cent of cations substituted by aluminium systematic variations in magnetic properties across the dykes are caused by differences in titanomagnetite concentration as well as grain size lowtemperature alteration and laboratory oxidation by heating in air to a maximum temperature of 600 c do not homogenize the magnetic properties across the dykes fine grained regions near dyke margins carry the most stable remanent magnetization but are also most amenable to secondary chemical remanent magnetization crm resetting between dyke variations in magnetic properties are larger than within dyke variations a stable primary remanence is found in two of the dykes while the third is totally reset by a soft viscous remanent magnetization vrm despite similar optical grain sizes magnetic mineralogy and oxidation state the difference is attributed to a lower titanium content in the latter dyke preventing effective partitioning of magnetic grains during low temperature alteration laboratory heating affects anisotropy variations across the dykes resulting in more consistent axis orientations within each dyke and symmetric edge to edge variations in the degree of anisotropy ams directions are different from those in the country rock and vary systematically across the thinnest dyke before heating

jurassic and cretaceous apparent polar wander relative to south america some tectonic implications late mesozoic apparent polar wander apw with respect to stable south america has been difficult to define it appears that this observation has a simple explanation very little apw occurred and none at all during a 50 m y period 165 to 115 ma it is possible to compute a stillstand pole for this interval 88 8 s 72 4 e k 244 7 a95 4 9 from five reliable studies comprising a total of 523 sites a second stillstand of much shorter duration 70 80 ma is defined by four poles 52 sites this is located at 80 8 s 346 7 e k 457 7 a95 4 3 apw of 9 2 occurred during the interval 115 80 ma a comparable stillstand is found in the north american record e g van fossen and kent 1992 this has a similar degree of scatter k 421 and spans at least 36 m y 124 to 88 ma thus north america and south america both were rotating due westward during a large part of the cretaceous this has some important implications for global tectonics

low viscosity channels and the stability of long wavelength convection abstract mantle convection simulations with a low viscosity channel akin to the earth s asthenosphere are characterized by long wavelength flow structure boundary layer theory predicts that as the viscosity of the channel decreases the wavelength that maximizes heat transfer increases as a pattern selection criterion this analysis is not complete it provides no mechanism to relate the optimal heat transfer wavelength to the wavelength that is realized in nature we present numerical simulation suites for bottom and internally heated end members to demonstrate that the cell wavelengths that maximize heat transfer are also the most stable this does not rule out the possibility of multiple wavelengths being realizable but it does imply that wavelengths near the stability peak will be preferred and for the configurations we explore the stability peak corresponds to the energetically most efficient flow configuration

an early jurassic deep water stromatolitic bioherm related to possible methane seepage los molles formation neuquen argentina an assemblage of microbial stromatolites and worm tubes makes up a limestone bioherm 15 m thick and 60 m in diameter enclosed in marls and dark shales of the early jurassic los molles formation neuquen basin argentina the los molles formation is a basinal unit consisting primarily of shales silts and sandstone turbidites deposited after the first general marine transgression in the basin below storm wave base in relatively deep water poorly oxygenated conditions the bioherm is composed of stacked stromatolites 1 4 m in diameter and 1 2 m thick they initially grew horizontally tabular stromatolites or vertically columnar stromatolites and later became domal equidimensional to asymmetric structures they often display vertical and overturned flanks each stromatolite consists of micritic laminae alternating with early botryoidal cements which make up the bulk of the structure calcareous worm tubes are embedded in peloidal micrite associated with the stromatolites the growth of the assemblage was uninterrupted by terrigenous input the diagenetic sequence indicates evolution from marine phreatic to burial environments isotopic values for micrite worm tubes early botryoidal cements and spar cements yield very negative 13c values down to 33 and negative 16o values down to 11 these depleted values indicate precipitation in very anomalous conditions in the jurassic ocean under the influence of methane rich fluids these characteristics suggest that the stromatolitic bioherm formed in relation to methane seeps in the seafloor of the neuquen basin it is suggested that the structure grew within the photic zone although a chemosynthetic component cannot be excluded minimum depositional depth has been estimated at 50 m and could be up to 100 m the vent fluids would have migrated through shallowly buried early rift faults which bound underlying depocenters infilled with continental clastic and volcanic rocks the origin of these fluids could be related to magmatic activity which resulted in accumulation of the syn rift volcanics alternatively the structure could indicate seepage related to the presence of hydrocarbon accumulations in the syn rift succession during the jurassic the importance of which is being gradually acknowledged in the petroleum system of the neuquen basin

palaeomagnetic study of mesozoic basalts from scania southernmost sweden summary the mesozoic basalt necks of central scania southernmost sweden yield dual polarity palaeomagnetic directions decl 33 incl 66 n 15 and decl 225 incl 73 n 6 the combined mean with the negative directions reversed is decl 35 incl 68 n 21 the corresponding palaeopole is situated at lat 69 n 103 e this pole corresponds with an early jurassic pole for europe and with early jurassic poles for north america after closing of the atlantic the early jurassic age is in contrast with k ar datings ranging between 171 and 81 ma but is in agreement with datings based on microfossils found in tuffaceous layers connected with the basalts

reply to a comment on early jurassic palaeomagnetic study of lower jurassic marine strata from the neuquen basin argentina a new jurassic apparent polar wander path for south america fil iglesia llanos maria paula consejo nacional de investigaciones cientificas y tecnicas oficina de coordinacion administrativa ciudad universitaria instituto de geociencias basicas aplicadas y ambientales de buenos aires universidad de buenos aires facultad de ciencias exactas y naturales instituto de geociencias basicas aplicadas y ambientales de buenos aires argentina

the picton and varty lake ultramafic dikes jurassic magmatism in the st lawrence platform near belleville ontario the picton and varty lake ultramafic dikes that intrude ordovician limestone near belleville ontario consist of olivine or serpentinized pseudomorphs and ti phlogopite phenocrysts set in a groundmass of spinel perovskite serpentine calcite apatite and chlorite phlogopite exhibits a complex chemical variation and late stage ba enrichment up to 6 8 wt bao spinels are chemically zoned and commonly exhibit an atoll texture in which a magnesian aluminous chromite core is separated by a silicate zone from late stage magnesian aluminous titanomagnetite chemically similar to kimberlitic spinel atoll spinels from the picton dike also have a zone of titanian pleonaste surrounding a central grain of magnesian aluminous chromite at the centre of the atoll structures both dikes are extremely silica undersaturated with high k na and mg mg fe2 ratios petrographic and chemical features of the picton and varty lake dikes preclude simple classification as both dikes have aspects indicative of kimberl

palaeomagnetism and magnetic fabric in the freetown complex sierra leone summary about six separately orientated cores were collected at each of 14 sites distributed throughout the arcuate west dipping 6 km thick freetown layered igneous complex alternating field and thermal demagnetization both isolate a stable component of remanent magnetism which corresponds to a palaeomagnetic south pole from 13 sites nine reverse four normal polarity at 82 9 s 32 7 e a 95 5 6 this is indistinguishable from that reported in 1971 based on alternating field demagnetization of cores from 10 orientated hand samples the diverence between the freetown pole age 193 3 ma and other mid jurassic poles from west africa could be due to its greater age the diverence between the whole west african jurassic pole group and the karoo pole from southern africa however suggests moderate 10 d i verential rotation of west africa relative to the kaapvaal craton a prevalent magnetic foliation fabric coincides generally with the petrological layering as might be expected but a ubiquitous magnetic lineation is predominantly down dip this is compatible with a down dip pyroxene lineation reported to be present in some field outcrops and interpreted in terms of late stage deformation during the slow crystallization and cooling of the large igneous body however a fold test shows that the igneous layering had already achieved its present attitude before the complex cooled to 570 c the maximum blocking temperature of the characteristic remanence

lower jurassic palaeomagnetic results from yorkshire england and their implications palaeomagnetic study of middle liassic siltstones shows a stable magnetization with a mean direction of d 12 3 i 64 6 n 60 k 26 95 3 9 corresponding to a palaeomagnetic pole at 79 8 n 125 6 e similar to that for southern germany and confirming predictions based on palaeogeographic reconstructions using north american data sideritic concretions of lower liassic age show a higher magnetic stability with a mean direction of d 12 6 i 61 4 n 125 k 50 95 1 8 which is not significantly different from the siltstones this confirms the sedimentological evidence that suggests that such concretions grew very shortly after deposition i e within the liassic and suggests that similar concretions of other ages could thus be used for palaeomagnetic studies although the liassic palaeomagnetic pole 76 9 n 134 7 e based on this work appears valid it is still not possible to evaluate a sensible mesozoic polar wandering curve for the north atlantic bordering continents

paleomagnetism of the lesotho basalt southern africa paleomagnetic study of four sections from middle jurassic karoo lava located in lesotho and south africa yielded a new paleomagnetic pole for africa with coordinates 71 6 n 273 5 e a95 3 7 k 33 n 47 paleosecular variation estimated as the angular standard deviation of vgps distribution is 14 1 2 4 1 8 which is considerably lower than expected for this time interval and latitudinal band preliminary paleointensity results indicate a low field intensity during the acquisition of the magnetization this new result tends to support a possible correlation between secular variation and dipole intensity when the lower to middle jurassic paleomagnetic data set for africa is considered a difference of 13 9 8 7 of arc is found to exist between the mean northwest and south african poles furthermore while the northwest african pole is in very good agreement with the lower to middle jurassic north american pole whichever set of rotation parameters is used to close the north atlantic ocean the south african pole falls some 12 15 of arc apart as intraplate deformation models developed to achieve tighter fit of west gondwana prior to the opening of the south atlantic fail to account fully for the difference between northwest and south african poles the pre cretaceous apparent polar wander path for africa has to be considered with caution

reconstructions of the continents around the north atlantic at about the 60th parallel abstract late carboniferous early tertiary apparent polar wander apw paths 300 40 ma for north america and europe have been tested in various reconstructions these paths demonstrate that the 500 fathom bullard et al fit is excellent from late carboniferous to late triassic times but the continental configuration in northern pangea changed systematically between the late triassic ca 214 ma and the mid jurassic ca 170 ma due to pre drift extension best fit north atlantic reconstructions minimize differences in the late carboniferous early jurassic and late cretaceous tertiary segments of the apw paths but an enigmatic difference exists in the paths for most of the jurassic whereas for the early cretaceous the data from europe are nearly non existent greenland s position is problematic in a bullard et al fit because of a late triassic early jurassic regime of compression 300 km that would be inherently required for the norwegian shelf and the barents sea but which is geologically not defensible we suggest a radically new fit for greenland in between europe and north america in the early mesozoic this fit keeps greenland locked to europe for the late paleozoic early mesozoic and maintains a reconstruction that better complies with the offshore geological history of the norwegian shelf and the barents sea pre drift a24 extension amounted to approximately 450 km on the mid norwegian shelf but with peak extension in the late cretaceous

rotated paleomagnetic poles from the sassamansville syncline newark basin southeastern pennsylvania a paleomagnetic fold test was conducted at eight sites collected from the sassamansville syncline which folds the norian age passaic formation along the border fault of the newark basin in southeastern pennsylvania thermal demagnetization isolated two components of magnetization a synfolding age intermediate unblocking temperature normal polarit b component with intermediate inclinations and northwesterly declinations and a prefolding age high unblocking temperature dual polarity c component with shallow inclinations and northwesterly southeasterly declinations the paleomagnetic poles calculated from these magnetizations are both rotated 20 6 counterclockwise from contemporary paleopoles isolated from other parts of the newark basin a paleomagnetic study of the sassamansville diabase kodama and mowery 1993 indicates a 15 5 counterclockwise rotation so the 20 rotation of the passaic formation remanence at the sassamansville syncline is interpreted to be due to a 15 block rotation of the sassamansville area the remainder although not statistically distinguishable may be due to grain scale strain based on rock and magnetic fabric measurements anisotropy of magnetic susceptibility low temperature high temperature susceptibility ratios and x ray pole figure goniometry results suggest that a tectonic fabric developed in the phyllosilicates which has vertical minimum axes and maximum extensional axes parallel to the fold axis could have caused the small 5 grain scale rotation of the b and c component magnetizations fry analysis of quartz grains however does not record a significant finite strain in the rocks and may remain the best method to identify if a large grain scale rotation of remanence has occurred if local block rotations also affected some of the sites collected from the nearby jacksonwald syncline the effect on the b and c component paleopoles reported by witte and kent 1991 and witte et al 1991 from the newark basin is small enough that the discrepancy between newark basin paleopoles and coeval poles from the southwestern united states is not appreciably reduced when the 20 rotations are removed from the sassamansville paleomagnetic data the b component magnetization yields a high latitude paleomagnetic pole which supports the newark b remagnetization pole of witte and kent 1991 and the moat volcanics paleopole of van fossen and kent 1990 the c component magnetization yields a pole consistent with the norian age pole from the passaic formation reported by witte et al 1989

paleomagnetism of the chinle and kayenta formations new mexico and arizona paleomagnetic data were obtained from 22 sites 6 10 samples site in the upper shale member of the chinle formation 43 sites in the owl rock member of the chinle formation and 35 sites in the kayenta formation thermal demagnetization and data analyses indicate that within site dispersion is an important criterion for selecting sites which retain a high unblocking temperature characteristic remanent magnetization chrm site mean directions define at least four antipodal polarity zones within each member formation suggesting the chrm was acquired soon after deposition fifteen site mean virtual geomagnetic poles vgps from the upper shale member of the chinle formation yield an early norian paleomagnetic pole position of 57 4 n 87 8 e k 60 a95 5 0 eighteen site mean vgps from the owl rock member of the chinle formation yield a middle norian paleomagnetic pole position of 56 5 n 66 4 e k 183 a95 2 6 twenty three site mean vgps from the kayenta formation yield a pliensbachian pole position of 59 0 n 66 6 e k 155 a95 2 4 combined with paleomagnetic poles from the moenave formation and the shinarump member of the chinle formation these data record 30 m y of north american apparent polar wander apw within a regional stratigraphic succession during the camian and norian stages of the late triassic chinle poles progress westward during the hettangian through pliensbachian stages of the early jurassic the pattern of apw changed to an eastward progression even after correction for 4 clockwise rotation of the colorado plateau a sharp comer in the apw path j1 cusp is resolved near the pole from the hettangian sinemurian 200 ma moenave formation 59 4 n 59 2 e amongst other implications the sharp change in the apw path at the j1 cusp implies an abrupt change from counterclockwise rotation of pangea prior to 200 ma to clockwise rotation thereafter

pleistocene deposits at stoke goldington in the valley of the great ouse uk at stoke goldington in the valley of the great ouse in buckinghamshire a river terrace at a height of about 7 m above the floodplain is underlain by fluvial sediments representing climatic fluctuations in the late middle pleistocene near the base of the succession at a level only 1 m above the modern floodplain a fossil assemblage including pollen plant macrofossils molluscs insects and ostracods provides evidence for the local development of herb rich grassland under temperate climatic conditions the fossil record amino acid racemisation ratios and uranium disequilibrium dating all suggest deposition of this material during oxygen isotope stage 7 the deposits containing the temperate assemblage are immediately overlain by typical cold climate gravels of the great ouse these have been subjected to a later cut and fill episode with the fill accumulating in cool climatic conditions the cut and fill episode was succeeded by aggradation forming the overlying terrace surface amino acid racemisation ratios indicate that the fill was emplaced and the terrace surface created during or after oxygen isotope stage 5 1996 john wiley sons ltd

magnetostratigraphy of the toarcian stratotype sections at thouars and airvault deux sevres france abstract magnetostratigraphic study of the toarcian type sections of thouars and airvault deux sevres france has yielded two reliable magnetic polarity sequences most samples were treated by mixed cleaning thermal demagnetization 250 300 or 350 c and subsequent alternating field demagnetization polarity intervals are easily identified and correlate well between the two sections using the biostratigraphic data provided by the detailed standard ammonite zonation of the toarcian stage the polarity sequence extends from ammonite horizon v pseudoserpentinum horizon serpentinus zone to horizon xxv subcompta horizon aalensis zone it shows 5 reversed and 5 normal polarity magnetozones

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

rapid warming and ostracods mass extinction at the lower toarcian jurassic of central spain abstract here we present the results of the study of two lower toarcian carbonate sections located in the iberian range of central spain analyses of stable isotope on belemnite calcite allowed calculation of seawater palaeotemperature variations which were compared with the stratigraphical distribution of ostracods these organisms are particularly sensitive to ratios of temperature and salinity variations and hence are good indicators of climate changes from a cooling interval with seawater temperatures of 13 2 c recorded at the pliensbachian toarcian transition seawater temperature began to rise in the lowermost toarcian tenuicostatum zone reaching average temperatures between 14 6 c and 16 3 c during the time of deposition of this zone coinciding with this seawater warming up to 85 of the ostracods species progressively disappeared during a period of approximately 300 kyr marking the extinction interval the extinction boundary located around the tenuicostatum serpentinum zonal boundary coincides with a marked increase in temperature in the serpentinum zone on which average seawater temperatures of 22 c have been calculated warming continued through part of the middle toarcian bifrons zone reaching average temperatures of 24 7 c readjustment of the ostracod population allowed recovery of these faunas in the upper serpentinum zone although the extinction of a major ostracod group the healdioids was also recorded the correlation between mass extinction and warming infers a causal relationship comparison of the results with the records of stable isotopes in belemnites and in bulk carbonates as well as toc and facies analysis suggests that the anoxia linked to the early toarcian oceanic anoxic event was not the main responsible for the ostracod mass extinction

plate motions and the geomagnetic field ii jurassic to tertiary summary the analysis of the time averaged geomagnetic field is extended back to 200ma palaeomagnetic poles from the major plates have been carefully selected from recent compilations of reliable results for each region these were returned with their corresponding sampling sites to their locations at the estimated dates of magnetization in a fixed hotspots framework the corrected results were then grouped into 20ma windows at intervals of 10ma representing the past 100ma and 40ma windows at 30ma intervals for the more sparse 100 200ma data global means and fisher statistics were calculated for each window having included the axial quadrupole in the calculation the value of this coefficient which gave the maximum value for the fisher precision parameter tightest grouping of poles was taken as representative of each interval the results indicate that a small axial quadrupole of the same sign as the axial dipole may have persisted throughout the cenozoic this is equivalent to a northward offset axial dipole field far sided effect during the late cretaceous this component appears to have changed sign with respect to the dipole negative values seem to have obtained throughout the cretaceous long normal polarity interval corresponding to a southward offset dipole near sided effect the data distribution is inadequate for the resolution of the quadrupole at earlier times and zero values cannot be discounted little relative motion is implied between the hotspots and the geomagnetic axis for the past 90 ma the global mean polar path curving around the predicted fixed hotspots pole at a distance of typically 5 latitude with little sign of rapid tertiary polar wander as implied by studies of pacific data alone between 100 and 200 ma however there is a clear difference between the two reference frames amounting to 17 19 in the jurassic this may reflect motion of the mantle relative to the geomagnetic axis but may also include errors due to inaccurate determination of hotspot tracks and inter hotspot motion

bivalve provinces in the proto atlantic and along the southern margin of the tethys in the jurassic abstract using multivariate methods the distribution pattern of jurassic bivalve genera and subgenera within the proto atlantic and along the southern margin of the tethys is analysed based on an extensive literature survey and examination of field collections four bivalve provinces are recognised within the proto atlantic 1 the boreal bivalve province in the north 2 the northern transitional province 3 the southern transitional province and 4 the mediterranean bivalve province the former two provinces are assigned to the boreal faunal realm the latter two to the tethyan faunal realm the southwestern margin of the tethys belongs to the mediterranean bivalve province the southeastern margin to the ethiopian bivalve province the latter is only poorly defined by bivalve genera subgenera in particular its northern boundary is blurred with the arabian region occupying an intermediate position between the mediterranean and the ethiopian bivalve provinces province boundaries based on bivalves do not always coincide with those based on ammonites the reason for this is probably different modes of life ammonites as a rule being active swimmers and bivalves being substrate related and subjected to passive dispersal during the larval stage diversity is not suitable to characterize bivalve provinces due to differences in outcrop size preservation potential and sampling history and due to differences in the degree of environmental heterogeneity within and between regions migration patterns of jurassic bivalves either reflect changes in the climatic condition e g the northward spread of tethyan taxa in the late early jurassic and middle jurassic in the course of amelioration of the climate or prevailing current patterns the origin of faunal provinciality is thought to be complex and a result of the interplay of several factors the importance of which varied throughout the jurassic

mid cretaceous polar standstill of south america motion of the atlantic hotspots and the birth of the andean cordillera article i nfo a paleomagnetic study on mid cretaceous rocks from the san bernardo foldbelt patagonia yields high unblocking temperature and high coercivity magnetizations the results indicate absence of relative vertical axis rotations during development of the foldbelt with the associated pole position being highly concordant with coeval poles from brazil and patagonia taken together mid cretaceous poles derived from studies in widely distributed localities provide supportive evidence that south america was essentially motionless with respect to the paleomagnetic axis from ca 125 to at least 100 ma the paleolatitudes of south america are not consistent with the occurrence of mid cretaceous true polar wander suggesting that the previously observed discrepancy between the paleomagnetic and the fixed indo atlantic hotspot reference frames be related to motion of the atlantic hotspots in agreement with this the discrepancy is diminished by half when the cretaceous poles of the americas are observed in a moving hotpots reference frame with the residual offset being comparable to that seen for younger time intervals the south american paleopoles and the moving hotspot framework provide a kinematic scenario that allows relating the extensional tectonics in the early stages of andean evolution with episodic divergence between the trench and the continental interior likewise the beginning of contractional events correlates with model predicted westward acceleration of south america in the late cretaceous suggesting that the continent episodically overrode the andean trench by those times we argue that this change in andean tectonic regime is associated to major plate reorganization at ca 95 ma