dual polarity magnetizations from the upper devonian mcaras brook formation nova scotia and their implications for the north american apparent polar wander path stearns c and van der voo r 1988 dual polarity magnetizations from the upper devonian mcaras brook formation nova scotia and their implications for the north american apparent polar wander path tectonophysics 156 179 191 thermal demagnetization of 57 samples from four lava flows of the upper devonian mcaras brook formation sampled along the northumberland strait in northern nova scotia has yielded complex paleomagnetic results samples from three of the four lava flows revealed two superimposed sets of magnetic directions d z 33 2 and d i 7 23 both tilt corrected that both appear to be paleozoic in age the corresponding paleopole for the northeasterly direction is similar to previously published devonian poles for north america although the age of this component cannot be constrained by a field stability test the presence of stratigraphically separated dual polarities supports a primary age for the magnetization application of the structural correction strike and dip of the beds 214 36 nw does not significantly change the direction 32 1 in situ vs 33 2 tilt corrected the palcopole for this direction is located at 35 o n 76 o e the additional and more northerly direction can be seen in samples from two lava flows in this set of directions both polarities occur without apparent stratigraphic separation the corresponding paleopoles for this direction 46 o n 116 e in situ 32on 109 e tilt corrected are similar to published permian and devonian paleopoles for north america respectively again the age cannot be constrained by a field test although the in situ pole plots near the permian paleopoles permian magnetizations are not expected to show dual polarities because of the long riaman reversed interval we therefore interpret it to be pre kiaman because the tilt corrected paleopole is near devonian paleopoles we tentatively interpret the northerly component as secondary late devonian but acquired before tilting of the rocks our results show the same longitudinal distribution as that of the previously published devonian paleopoles for north america although some authors have suggested that this spread is due to local tectonic rotation the dispersion in our data cannot be explained by tectonic rotation instead we conclude that the magnetizations of the mcaras brook formation were acquired over a long period of time during the post acadian infilling of a rift basin and that the longitudinal distribution of the paleopoles was caused by the clockwise rotation of the north american plate as a whole during the devonian magnetization acquisition over a long time interval is supported by the presence of dual polarities seen within individual samples finally samples from the second highest lava flow of the mcaras brook formation revealed univectorial magnetic directions d i 269 34 in situ 240 58 tilt corrected that are difficult to explain because the corresponding paleopole is dissimilar to any existing phanerozoic pole for north america because these directions cannot be explained by remagnetization or by tectonic rotation we tentatively ascribe them to a transitional field direction recorded during a reversal

accretion of southern alaska abstract paleomagnetic data from southern alaska indicate that the wrangellia and peninsular terranes collided with central alaska probably by 65 ma ago and certainly no later than 55 ma ago the accretion of these terranes to the mainland was followed by the arrival of the ghost rocks volcanic assemblage at the southern margin of kodiak island poleward movement of these terranes can be explained by rapid motion of the kula oceanic plate mainly from 85 to 43 ma ago according to recent reconstructions derived from the hot spot reference frame after accretion much of southwestern alaska underwent a counterclockwise rotation of about 50 as indicated by paleomagnetic poles from volcanic rocks of late cretaceous and early tertiary age compression between north america and asia during opening of the north atlantic 68 44 ma ago may account for the rotation

paleomagnetism of middle miocene volcanic rocks in the mojave sonora desert region of western arizona and southeastern california paleomagnetic directions have been obtained from 190 early to middle miocene 12 20 ma mafic volcanic flows in 16 mountain ranges in the mojave sonora desert region of western arizona and southeastern california these flows generally postdate early miocene tectonic deformation accommodated by low angle normal faults but predate high angle normal faulting in the region after detailed demagnetization experiments 179 flows yielded characteristic directions interpreted as original thermal remanent magnetizations trm because of the episodic nature of basaltic volcanism in this region the 179 flows yielded only 65 time distinct virtual geomagnetic poles vgps the angular dispersion of the 65 vgps is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation the paleomagnetic pole calculated from the 65 cooling unit vgps is located at 85 5 n 108 9 within a 4 4 circle of 95 confidence this pole is statistically indistinguishable at 95 confidence from reference poles calculated from rocks of similar age in stable north america and from a paleomagnetic pole calculated from rocks of similar age in baja california the coincidence of paleomagnetic poles from the mojave sonora desert region with reference poles from the stable continental interior indicates that 1 significant vertical axis net tectonic rotations have not accompanied post middle miocene high angle normal faulting in this region 2 there has been no detectable post middle miocene latitudinal transport of the region and 3 long term nondipole components of the middle miocene geomagnetic field probably were no larger than those of the recent 0 5 ma geomagnetic field in contrast paleomagnetic data indicate vertical axis rotations of similar age rocks in the transverse ranges the eastern transverse ranges and the mojave block we speculate that a major structural discontinuity in the vicinity of the southeastward projection of the death valley fault zone separates western areas affected by vertical axis rotations from eastern areas that have not experienced such rotations

mesozoic paleomagnetism and northward translation of the baja california peninsula paleomagnetic data from cretaceous igneous and sedimentary rocks of the peninsular ranges and continental borderland of the baja california peninsula yield an average pole position of 83 n 344 e a95 3 4 suggesting that the peninsula as a whole has moved northward 11 and rotated 32 clockwise with respect to the north american craton since cretaceous time only a small portion of this movement is due to the neogene opening of the gulf of california results from stratified rocks giving both positive and negative fold tests are included in the average cretaceous pole the rocks that fail fold tests have been deformed and remagnetized by nearby intrusives that are only slightly younger unconnected directions from these sites closely match corrected directions from sites where the fold tests are positive oceanic rocks were also sampled in the continental borderland from upper triassic and upper jurassic tectonostratigraphic terranes which are apparently allochthonous with respect to the western baja peninsula and each other these oceanic rocks show magnetizations indicating low paleolatitudes although the hemisphere of origin is presently unknown the cretaceous rocks of the baja california peninsula may have originally formed adjacent to the now truncated continental margin along the northern middle america trench northward transport of the peninsula probably occurred by transcurrent faulting associated with oblique subduction of the farallon plate beneath western north america between late cretaceous and late miocene time

paleomagnetism of the oligocene kalamazoo tuff implications for middle tertiary extension in east central nevada the oligocene kalamazoo tuff 35 ma was sampled for paleomagnetic analysis across a 100 km wide zone of highly extended crust in east central nevada to estimate between site vertical axis rotations and thus the relative importance of strike slip faulting to the mechanism of extension subordinate sampling was also undertaken in a younger hornblende dacite intrusion 35 ma and the overlying tuff of north creek 35 ma in general ash flow tuffs cool and are magnetized within a short period of time relative to geomagnetic secular variation and their eutaxitic structures typically indicate the paleohorizontal these attributes of ash flow tuffs in addition to their generally widespread occurrence make them excellent candidates for combined paleomagnetic and structural studies of regional deformation however in this study a number of problems associated with the paleomagnetism of the kalamazoo tuff are documented they include topography related variations of the eutaxitic structures secular variation during slow cooling postblocking rheomorphic flow and local remagnetization of the tuff by younger igneous activity the tilt corrected data with these sources of error reduced or eliminated exhibit a 28 12 clockwise rotation of the schell creek range relative to the kern mountains region this rotation implies differential extension accommodated by strike slip faulting or n s shortening the paleomagnetic results also suggest that large changes in strike of layered units near faults with presumed strike slip movement need not be the result of oroclinal bending but could result from superimposed sets of orthogonal normal faults

remagnetization and northward translation of mesozoic red chert from cedros island and the san benito islands baja california mexico samples for paleomagnetic analysis were collected from red radiolarian ribbon chert in a franciscan like subduction complex western baja terrane exposed along the western margin of the baja california peninsula a 40 m thick section of chert was sampled on cedros island and a 30 m thick and two smaller sections of chert were sampled on the san benito islands to the west a site in pillow basalt underlying the chert also was sampled on cedros island the sections of chert on cedros island and the san benito islands contain radiolaria ranging in age from late triassic to early cretaceous all of these rocks have similar normal polarity direction of magnetization indicating that they were remagnetized long after deposition results from a chert site including an intrastratal fold also indicate that the magnetization is postdepositional a significantly improved grouping 95 confidence level of directions after correction for the structural attitudes of the cedros and san benito chert sections however implies that the remagnetization predates breaking up and incorporation of the chert into the subduction complex studies of modern subduction zones imply that the attitude of the ancient subducting slab including the chert sections was near horizontal dip angles

the relative contribution of accretion shear and extension to cenozoic tectonic rotation in the pacific northwest large cenozoic clockwise rotations defined by paleomagnetic data are an established fact in the pacific northwest and many tectonic models have been proposed to explain them including 1 rotation of accreted oceanic microplates during docking 2 dextral shear between north america and northward moving oceanic plates to the west and 3 microplate rotation in front of an expanding basin and range province stratigraphic onlap relations and local structure indicate that microplate rotation during docking was not a major contributor to the observed rotations coast range structures basin and range extension and paleomagnetic data from middle miocene 15 ma coast range rocks indicate that dextral shear is responsible for at least 40 of the post 15 ma rotation of the coast range and that basin and range extension is responsible for the remainder reconstructions based on extrapolation of this ratio back to 37 and 50 ma are consistent with reconstructions based on paleomagnetic and stratigraphic relations in older rocks and suggest that dextral shear has been a significant contributor to rotation during most of tertiary time changes in the dextral shear rotation rate over the past 50 m y correlate directly with changes in the velocity of the farallon plate parallel to the coast and provide a strong argument for oblique subduction as the driving mechanism continental reconstructions incorporating shear may provide constraints on the rate of extension in the northernmost basin and range region and suggest 17 extension since 15 ma 39 since 37 ma and 72 since 50 ma near latitude 42 n

paleomagnetic evidence for neogene clockwise tectonic rotations in the central mojave desert california paleomagnetic directions from early miocene volcanic flows indicate that a large part of the central mojave desert has been tectonically rotated stratigraphic relations and isotopic ages suggest that two separate clockwise rotations have occurred an early miocene rotation of 50 15 6 is coincident in time and space with opening of the daggett extensional terrane a post early miocene rotation of 30 to 50 may have affected the alvord mountains and the immediate vicinity

polar wander and paleomagnetic reference pole controversies the purpose of this paper is to review the progress made over the past four years in studies of phanerozoic and precambrian polar wander with an emphasis on the contributions made by u s scientists we perceive a general theme embracing the notion that the accuracy of cratonic reference poles and therefore of apparent polar wander apw paths is not as good as was generally believed four years ago a renewed interest in research concerning cratonic reference poles is largely the result of paleomagnetic studies in orogenic belts namely the western cordillera and appalachians of north america the andes of south america and elsewhere which seek to document the existence or non existence of displacements of suspect terranes i e rotation and or translation with respect to their presently associated cratons some of the results of this research are reviewed in this volume by hillhouse and mcwilliams an important realization of which paleomagnetists have long been aware but which was demonstrated recently by a number of sobering examples for both the paleozoic and mesozoic apw paths for north america is that the reliability of concordance discordance parameters that define terrane displacement is dependent not only on the precision and accuracy of the individual paleomagnetic study but also on the precision and accuracy of the appropriate cratonic reference pole

the southern kahiltna terrane implications for the tectonic evolution of southwestern alaska regionally extensive and highly deformed jurassic to cretaceous basinal turbidite deposits occur along most of the inboard boundary of the talkeetna superterrane in southern alaska and western canada the southern kahiltna terrane of southwestern alaska consists largely of such basinal deposits regional and detailed studies in and adjacent to this terrane suggest several new conclusions regarding the origin of the basin and its role in the tectonic evolution of southwestern alaska stratigraphic sedimentologic compositional and structural evidence suggests that the upper jurassic to lower cretaceous strata of the southern kahiltna terrane were derived from and deposited on rocks of the adjacent talkeetna superterrane deposition of these clastic rocks apparently postdated arc magmatism in the talkeetna superterrane suggesting deposition in a basin developed on the suture between the talkeetna superterrane and north america if true this observation implies that collision of the talkeetna superterrane with north america began prior to latest jurassic time probably at a more southerly location the superterrane has since been translated northward probably by coast parallel strike slip displacement to its present location where it was structurally juxtaposed with middle to upper creteceous rocks of the kuskokwim group a magmatic overprint indicates that the talkeetna superterrane was in approximately its present location by latest cretaceous to paleocene time

dual polarity early carboniferous remagnetization of the fisset brook formation cape breton island nova scotia summary red siltstones and volcanic flows of the fisset brook formation of cape breton island nova scotia were folded or tilted in two phases one of late tournaisian and the other of mid namurian age upon thermal demagnetization both rock types yield three components of magnetization herein denoted as l i and h the l component has low unblocking temperatures and a direction conforming to that of the present day geomagnetic field l is inferred to be of recent origin the intermediate i component carried by haematite is of synfolding origin directions could be obtained through vector subtraction for the demagnetization interval of 300 550 c and yield a mean of d i 160 38 k 90 8 ag5 s lo after 39 per cent of the tilt correction is applied the h component has even higher unblocking temperatures which overlap to a large degree with those of the i component but analysis of intersecting great circles appears to yield a best fit direction for h which is nearly antipodal to that of the i component h is also synfolding and yields a mean direction of d i 342 38 k 120 9 ag5 6 9 after 60 per cent of tilt correction both components are interpreted as secondary chemical remanent magnetizations of late tournaisian to early namurian age a comparison of all available carboniferous results from the craton and the northern appalachians indicates that palaeolatitudes for nova scotia changed from about 20 s to about 10 s in the interval between the early and late namurian and that they changed again from about 10 s to 0 between the late namurian and the stephanian declinations show good agreement between newfoundland nova scotia new brunswick and the craton with the exception of the meguma terrain which underwent a counterclockwise rotation with respect to the mainland in the late carboniferous as noted previously by spariosu kent keppie 1984

paleomagnetism of middle late triassic plutons in southern maine fang wu and van der voo r 1988 paleomagnetism of middle late triassic plutons in southern maine tecionophysics 156 51 58 a paleomagnetic study of two small syenitic to alkali granitic plutons abbott and agamenticus southern maine yields paleopoles at 48o n 92 e and 48o n 99 o e with k ar ages of 221 f 8 and 228 5 ma respectively a third pluton the litchfield was much more weathered and did not contain stable magnetizations dual polarity magnetizations a dike contact test and excellent statistical parameters obtained after detailed thermal and alternating field demagnetization support our claim that the magnetizations and paleopoles are representative of the middle to late triassic paleomagnetic field however the two paleopoles fall about 10 to the southwest of previously published triassic apparent polar wander path segments for cratonic north america unless this small deviation can be ascribed to post triassic tilt of the plutons this requires a modification of the polar wander path to bring the poles into agreement a tilt correction of some 15 20 needs to be applied about a strike which is perpendicular to the area s geologic trends and the main coast line which appears to be highly unlikely given the stabilized tectonic setting of this area in post triassic time a small but significant revision of the north american cratonic apparent polar wander path may therefore be suggested by these results but this needs further corroboration the more southerly paleolatitudes indicated by these triassic results for north america would imply lesser displacements than previously proposed on the basis of triassic paleomagnetism for exotic terranes in the western cordillera

paleomagnetic evidence for poleward transport of upper jurassic rocks in the decatur terrane san juan islands washington the san juan islands are underlain by a west vergent late cretaceous thrust and nappe system part of a larger thrust belt exposed in the northwest cascades of washington and the southwestern coast mountains of british columbia published paleomagnetic data from cretaceous plutons for example the coast plutonic complex intruding the thrust belt suggest that it has been displaced northward by as much as 2400 km since 90 100 ma this study focuses on the preintrusive travel history of the decatur terrane one of the units involved in the san juan thrusting the decatur terrane is of special interest because it may be correlative with the coast range ophiolite cro and great valley sequence of the southern california coast ranges we report new paleomagnetic data from the james island formation an upper jurassic preintrusive turbidite unit and from the obstruction formation a terrigenous sandstone unit deposited on the decatur terrane in the late cretaceous nine sites in the obstruction formation yield a single remanence direction grand mean d 90 7 i 67 9 that is clearly a postfolding overprint possibly of late cretaceous age the six site mean remanence directions from the james island formation cluster well only after restoring the beds to horizontal by a scheme that takes account of their complex structure this positive fold test and the presence of two polarities provide evidence that the remanence is primary the mean paleolatitude corresponding to the corrected remanence direction from the james island formation is 0 9 2 the data thus corroborate equatorial paleolatitudes derived from parts of the cro by others and suggest that the decatur terrane and cro represent fragments of a larger terrane originally located near the jurassic equator unlike the cro however the decatur terrane was still near the equator at 145 ma if the plutons intruding the san juan north cascades thrust belt formed 2400 km to the south then the paleomagnetism of the decatur terrane suggests it moved 20 farther poleward in the interval 145 ma to 97 ma than did an equivalent site on north america a much smaller amount of pre late cretaceous motion is allowed by error limits on the paleolatitudes but more is required if tilting is partly responsible for the shallow paleomagnetism of the cretaceous plutons

paleomagnetism of the methow region north central washington structural application of paleomagnetic data in a complexly deformed variably remagnetized terrane jurassic through paleocene rocks of the methow pasayten belt were studied in order to use paleomagnetic directions to resolve the question of cretaceous northward transport in the end circumstanc

paleomagnetic results from the shasta bally plutonic belt in the klamath mountains province northern california available paleomagnetic data show approximately 100 of clockwise rotation for permian and triassic strata of the eastern klamath terrane jurassic strata of this terrane are rotated approximately 60 clockwise which is comparable to rotations reported for jurassic plutons that occur elsewhere in the klamath mountains province paleomagnetic data obtained during the present study from the shasta bally belt of cretaceous plutons indicate 25 7 13 6 of clockwise rotation for the province since early cretaceous time 136 ma the waning stages of rotation at the time of emplacement of the shasta bally belt plutons which was closely followed by deposition of basal strata lower cretaceous of the great valley sequence probably represents completion of accretion of the province to cratonic north america

discordant paleomagnetic poles from the canadian coast plutonic complex regional tilt rather than large scale displacement stratigraphic petrologic and isotopic data indicate that parts of the coast plutonic complex and the north cascade range have been tilted northeast side up by angles of approximately 30 degree about north northwest trending axes these tilts can account for discordant paleomagnetic directions observed in mid cretaceous plutons from these regions without large scale displacement relative to north america

paleomagnetism geochronology and possible tectonic rotation of the middle miocene barstow formation mojave desert southern california the barstow formation of the central mojave desert in the mud hills consists of 965 m of epiclastic and volcanic sediments deposited in an extensional rift this widespread unit is of importance because its mammalian faunas characterize the barstovian land mammal age previous isotopic ages on interbedded tuffs have been used to calibrate middle miocene time and it was deposited in a tectonically active region volcanic ash samples were collected from the barstow formation for isotopic dating using conventional 40 k 40 ar and 40 ar 39 ar single crystal laser fusion methods multiple analyses on minerals from 5 superposed tuffs yielded the following mean 40 k 40 ar ages rak tuff at 16 3 0 3 m y oreodont tuff at 15 9 0 2 m y dated tuff at 14 8 0 2 m y hemicyon tuff at 14 0 0 1 m y and lapilli tuff at 13 4 0 2 m y similar mean ages were also obtained on the following tuffs using single crystal 40 ar 39 ar laser fusion methods oreodont tuff at 15 9 0 06 m y dated tuff at 14 8 0 06 m y and hemicyon tuff at 14 0 0 09 m y in order to develop a local magnetostratigraphy 272 paleomagnetic sites were collected from 7 important sections within the mud hills saturation isothermal remanence experiments suggested the presence of either hematite or magnetite as carriers of remanence after either stepwise alternating field or thermal demagnetization characteristic directions were usually isolated between 30 and 50 mt or 300 and 500 c positive reversal and fold tests indicate a stable remanence acquired prior to folding there are 21 magnetic polarity zones interpreted from the continuous part of the barstow formation in the study area by using the 5 isotopic ages determined from interbedded tuffs the local composite polarity pattern is correlated to the top of chron c5d through the bottom of c5ab on the magnetic polarity time scale or an age range from 17 6 to 13 4 m y the boundary between hemingfordian and barstovian land mammal ages herein occurs 30 m below the oreodont tuff 15 8 0 2 m y within c5b and is taken at about 16 0 m y ago the barstow formational mean declination is 347 1 inc 47 5 95 6 51 d 7 97 suggesting negligible or possibly slight counterclockwise rotation r 4 1 r 10 5 for this part of the mojave block during the past 18 m y a 40 ar 39 ar mean age of 19 3 0 02 m y for the red tuff which is near the local base of the barstow formation and unconformably underlying our composite section constrains a minimum age for the beginning of local basin filling with principally epiclastic sediments

palaeomagnetism of the cordillera de la costa northern chile evidence for local forearc rotation abstract palaeomagnetic studies of lower jurassic to lower cretaceous volcanic plutonic and sedimentary rocks from the cordillera de la costa of northern chile indicate that there has been no significant latitudinal movement of northern chile since early cretaceous times a palaeomagnetic pole for the cordillera de la costa 63 s 187 e 95 7 7 was derived from combining the results of three studied units the la negra formation jurassic volcanics jurassic lower cretaceous granodioritic plutons and the e1 way formation hauterivian marine carbonates together with the coloso formation lower cretaceous red beds comparison with a cretaceous reference pole for the stable shield area of south america 87 s 235 e 95 2 4 indicates that 24 7 of clockwise rotation has taken place possible mechanisms for the rotation of the andean forearc in this area have been examined in relation to the kinetics of the subducting nazca and over riding south american plates the cordillera de la costa has been subjected to both compressional and extensional tectonics since the lower cretaceous major eastwardly directed thrusting in mid cretaceous and early tertiary times possibly associated with a period of oblique subduction of the nazca plate contributed to the accretion of the cordillera to the main forearc rotation may have occurred during this process in oligo miocene and later tertiary times extensional faulting possibly associated with roll back of the nazca plate led to the reactivation of the atacama fault zone and the formation of oblique extension faults in the cordillera de la costa structural rotation associated with this phase of extension could equally explain the observed palaeomagnetic results

geomagnetic field intensity in early jurassic investigation of the newark supergroup eastern north america abstract a collection of 220 samples from early jurassic newark supergroup volcanic units watchung talcott holyoke hampden and deerfield basalts and intrusive units manitook and palisades sills was studied with the aim of testing the previously suggested occurrence of a low average dipole moment of the earth s magnetic field some 200 ma ago after viscosity tests alternating field demagnetization of all samples and thermal variation studies of saturation magnetization of 74 selected samples we find that only 8 of the entire collection was a priori suitable for paleointensity experiments using the original thellier and thellier method we have obtained 17 reliable paleointensity values of average quality from the palisades sill and the holyoke basalt although these two units recorded substantially different field directions they provide similar estimates of the virtual dipole moment 9 4 1 2 and 11 3 2 2 10 22 a m 2 respectively these values are within the range of secular variation of the recent geomagnetic field

structural development of the western metamorphic belt adjacent to the coast plutonic complex southeastern alaska evidence from holkham bay the coast plutonic complex in southeastern alaska and british columbia is bordered on the west by low to medium grade metamorphic rocks of the western metamorphic belt this belt includes intensely deformed rocks underlying a prominent topographic low the coast range megalineament structural data from the northern part of the belt near holkham bay southeastern alaska constrain the shear sense during polyphase deformation tectonite fabrics within the western metamorphic belt record at least three deformational events variation in foliation development fold geometry and metamorphism indicate that deformation was heterogeneously partitioned four tectonic zones can be defined the peninsular coast range megalineament coherent and straight zones the earliest recognized deformation resulted in folding that transposed bedding into a tectonite layering s1 throughout the western metamorphic belt these structures are inferred to have developed during late cretaceous crustal thickening that resulted in moderate to high pressure metamorphism later deformation probably early tertiary resulted in abundant mesoscopic and macroscopic f2 folds of s1 and development of shear bands in the coast range megalineament zone the vergence and variation in plunge of f2 folds are compatible with subvertical east side up displacement across the coast range megalineament zone uplift of the coast plutonic complex shear bands indicate right lateral strike slip motion occurred during the waning stages of metamorphism probably following development of f2 folds brittle faults parallel to the coast range megalineament contain subhorizontal slickensides suggesting additional strike slip movement

neogene clockwise tectonic rotation of the eastern transverse ranges california suggested by paleomagnetic vectors results from a paleomagnetic study of neogene volcanic rocks situated within and around the pinto basin region of the eastern transverse ranges suggest that the ranges have undergone a clockwise tectonic rotation of 41 4 7 7 since late miocene time both normal and reversed polarities are observed and inclination data indicate no significant flattening the rotation was accompanied by progressive sinistral slip on the several east west trending faults which bound and transect the eastern transverse ranges a computation using the observed amount of cumulative left slip on the east west trending faults of the eastern transverse ranges 50 km and the spacing between the pinto mountain and salton creek faults which bound the region 70 km indicates that 35 of clockwise rotation is required which roughly agrees with the paleomagnetic data removal of the eastern transverse ranges rotation in a reconstruction shows that the distributed dextral shear across the mojave desert region to the north must have been on the order of 100 km since late miocene time from the results of this study we infer that crustal rotation of the eastern transverse ranges began later and is of less magnitude than that of the western transverse ranges

relative motions between eurasia and north america in the bering sea region knowledge of the relative motion of the north american and eurasian plates during the late mesozoic and cenozoic provides insight into the observed timing and style of deformational events in the bering sea region periods of strong convergence from 70 to 50 ma maastrichtian to paleocene are correlated with compressional deformation between the chukotsk peninsula and northern alaska and movement along the denali fault the convergence may also be the cause of a previously proposed counterclockwise rotation of western alaska transform motion between these plates from 50 to 38 ma middle to late eocene correlates with subsidence of the bering shelf creation of a series of pull apart basins along the bering margin and cessation of calc alkaline volcanism in western alaska slight compressive convergence from 37 ma to the present may be responsible for the anticlinal deformation of basin filling sediments in the anadyr and khatyrka basins

paleomagnetism and the motion of large and small plates the focus of paleomagnetic research during past decades has varied greatly the first generation in paleomagnetism might be defined as beginning with the pioneering studies aimed at discovering whether the geomagnetic field has been a reliable reference frame for measuring and timing continental displacements an enormous effort followed in the form of individual paleomagnetic studies of cratonic rocks culminating with the construction of apw apparent polar wander paths for the continents which could be used to test paleogeographic reconstructions coupled with parallel research in marine geophysics and geology the refined apwp which result from this effort have helped to develop a reasonable first order history of cenozoic mesozoic paleogeography for the major plates

southern hemisphere origin of the cretaceous laytonville limestone of california new paleomagnetic paleontologic and stratigraphic data from outcrops of the laytonville limestone 101 to 88 million years old support a southern hemisphere origin a paleomagnetic megaconglomerate test is statistically significant and suggests magnetization at 14 5 south predating late cretaceous to eocene 70 to 50 million years ago accretion rapid kula plate movement or the existence and demise of a now vanished oceanic plate or both are required to accommodate the greater than 50 of poleward displacement implied by the paleomagnetic data this rapid motion brings into question the validity of a speed limit for absolute plate velocity based on present day plate motions

paleomagnetism and rock magnetism of early silurian dunn point volcanics avalon zone nova scotia abstract paleomagnetic results have been obtained from the subaerial volcanics andesites rhyolites ignimbrites from the dunn point formation exposed in the antigonish highlands of the avalon zone nova scotia an early silurian age of 430 15 ma is assigned to these volcanic rocks the age is in agreement with the fossil evidence from conformably overlying sedimentary rocks the volcanic units were tightly folded and metamorphosed to sub greenschist facies during one major and one secondary phase of deformation probably associated with the mid late devonian acadian orogeny this paleomagnetic study is based on 45 samples collected from nine sites the site mean directions of the natural remanent magnetization nrm make a large deviation from the present earth s field with nrm intensities ranging from 10 2 to 10 am 1 the most commonly observed remanence direction a d 153 i 28 obtained from both alternating fields 30 75 mt and thermal 500 560 c treatments yields a corresponding direction a d 0 i 63 after correction for geological tilt a large scatter in direction up to 60 is observed in a and the fold test is not significantly positive a second and less common remanence component b d 146 i 54 which presumably postdates folding is also observed this component is mainly isolated by thermal treatment in the 540 630 c range room temperature isothermal experiments and optical examination of polished sections indicate the presence of magnetite and hematite as the carriers of both a and b directions the corresponding reversed paleomagnetic poles fall at a 163 e 52 n and b 146 e 4 n in close agreement with cratonic north american upper jurassic and late devonian poles respectively in the mid late devonian time interval the avalon zone was situated slightly to the south but close to the north american eastern margin

campanian maastrichtian stage boundary in southern california resolution and implications for large scale depositional patterns detailed paleomagnetic study of upper cretaceous predominantly deep sea fan deposits from the point loma formation san diego california situated within the allochthonous peninsular ranges terrane has produced a magnetic polarity stratigraphy with one reversed magnetozone foraminiferal and megafossil primarily molluscan occurrences demonstrate that these strata are late campanian to early maastrichtian in age and that the reversed magnetozone is almost certainly basal maastrichtian chron 32r recognition of this chron provides a direct correlation of the north pacific province biostratigraphic zonations to the standard cretaceous stages in europe the identification of chron 32r indicates that for the available biostratigraphic zonations the pacific slope megafossil campanian maastrichtian stage boundary shows the best accord with the standard european stage boundary improved age relations for these strata suggest that their large scale depositional patterns were strongly influenced by both local active plate margin tectonics and global eustatic factors during the late cretaceous

north american apparent polar wander plate motion and left oblique convergence late jurassic early cretaceous orogenic consequences the north american apparent polar wander apw path indicates an episode of unusually rapid absolute northward motion of western north america between 150 and 135 ma during this time the northward component of absolute motion of points along the washington oregon california coast was in excess of 150 km m y and perhaps as high as 230 km m y we believe that such high absolute northward velocity for north america probably ensures that relative motions of oceanic plates and terranes influenced by them were to the south at this time the inception of rapid northward motion and left oblique convergence was abrupt and should be recorded in the geology of the western cordillera it is tempting to correlate this period of unusual pacific basin north american interaction with the nevadan orogeny in the klamath mountains as well as with left lateral strike slip structures such as the pine nut fault and bear mountains fault zone significant differences exist between north american plate motion recorded by the late jurassic cretaceous apw path and that predicted by a fixed hotspot model we believe that this discrepancy reflects uncertainty associated with pre late cretaceous hotspot tracks and poorly constrained relative plate motions during the cretaceous normal polarity superchron

spatial and temporal variability of the chemistry of the shallow groundwater in the alluvial fan area of the luanhe river north china groundwater in the alluvial fan area of the luanhe river north china was monitored every 2 months for a year subsequently the temporal and spatial variability in the hydrochemical characteristics of the water was investigated using hierarchical cluster analysis hca and a trilinear piper diagram it is found that the spatial variability in the groundwater composition from different monitoring wells has obvious differences even if the distance between many of the wells is small also the difference in the ion concentration was found to be up to fivefold for the same monitored well research has indicated that the groundwater s chemical characteristics are mainly controlled by soil type the amount of precipitation the luanhe river the groundwater and human factors the thickness and continuous distribution of clay influences the velocity and ion exchange rate of the groundwater flow this results in the amount of total dissolved solids in the groundwater increasing from upstream to downstream in the flood season june july and august ion concentration gradually decreases as a result of the increased recharging of the groundwater due to precipitation classification using hca indicates that the samples can be clustered into different groups according to correlation coefficients also the chemical composition of the water from monitoring wells near the luanhe river is approximately consistent with that from the luanhe river this implies that the samples have some compositions resulted from the mixing of groundwater and river water the piper diagram indicates that ca2 mg2 and hco3 ions are the predominant ones present i e the type of groundwater is of the ca mg hco3 type

paleomagnetism of eocene plutonic rocks matanuska valley alaska paleomagnetic analysis of 88 samples from 14 sites in four eocene sills in the matanuska valley south central alaska yields a paleomagnetic pole at lat 72 9 n long 281 1 e precision parameter k 15 4 the paleolatitude of this result suggests that this part of the alaskan crust was at or near its present position relative to north america in the middle tertiary this pole also indicates that the region has been rotated clockwise by approximately 50 these results can be explained by local block rotations of fault bounded blocks in response to regional right lateral shear along both the castle mountain and border ranges faults

an analogue model study of em induction in the japan korea china region abstract electromagnetic induction in the coastal region of japan korea china is studied using a scaled laboratory analogue model the model includes a simulation of the known ocean bathymetry the complex coastlines and the deep conductive pacific trench model measurements of the electromagnetic field components are carried out for simulated periods of 15 180 min for two mutually perpendicular polarizations of a uniform horizontal inducing source field in phase and quadrature bx by and bz model measurements are examined in detail for a series of traverses over japan the korean peninsula and the coastal region of china and the u s s r large anomalous in phase model magnetic fields are observed over the korea japan strait for e polarization and over the bohai tsugaru and la perouse straits for b polarization owing to current channelling the significant responses observed at short periods over the peninsulas in the shallow coastal areas decrease with increasing period large gradients in the in phase b2 field are observed over japan for e polarization for both short and long periods owing to the effects of induced currents in the surrounding oceans thus induction arrow responses over all regions of japan are expected to show the dominant effects of the ocean a comparison of analogue model and field site induction arrows over the kii peninsula of japan indicates that the ocean effects alone cannot fully account for the observed responses the differences in the model and field results can be attributed to lateral inhomogeneities in the conductivity structure under japan

some paleomagnetic constraints on the tectonic evolution of the coastal cordillera of central chile paleomagnetic samples collected from 19 sites in radioisotopically dated jurassic mafic dykes and plutons in the coastal cordillera of chile between latitude 32 15 and 33 30 s yield paleomagnetic poles calculated by averaging the virtual geomagnetic poles from approximately coeval sites at latitude 85 9 s longitude 36 7 e n 8 a95 11 9 for sites dated at close to 170 ma and at latitude 80 3 s longitude 207 8 e n 10 a95 9 5 for sites dated at close to 145 ma these paleomagnetic poles from the coast of chile coincide with poles from coeval sequences within the south american craton some of the igneous rocks that constitute these sampling sites crosscut a high grade metamorphic complex that is distinct from other segments of the coastal cordillera in age structural style and metamorphic grade this complex cannot be correlated with metamorphic rocks anywhere else in central western south america and could be considered a potentially exotic allochthonous terrane if this terrane has been added to the western south american margin then accretion occurred prior to 170 ma paleomagnetic results presented here indicate that no major rotation or large scale latitudinal translation of crustal blocks has taken place in this part of central chile since middle jurassic time

paleozoic paleogeography of north america gondwana and intervening displaced terranes comparisons of paleomagnetism with paleoclimatology and biogeographical patterns new paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the silurian and devonian in this paper the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data the data from the three fields of paleomagnetism paleoclimatology and biogeography are generally in excellent agreement and an internally consistent paleogeographic evolutionary picture of the interactions between north america gondwana and intervening displaced terranes is emerging during the interval of the ordovician silurian and devonian north america stayed in equatorial paleoposition while rotating counter clockwise the northwest african part of gondwana was in high southerly latitudes during the late ordovician and was fringed by peri gondwanide terranes such as southern europe armorica and avalonian basement blocks now found in eastern newfoundland nova scotia the boston basin the appalachian piedmont and northern florida subsequently gondwana and the peri gondwanide terranes displayed rapid drift with respect to the pole this drift translates into the following pattern of movement for northwest africa during the latest ordovician early silurian this area moved rapidly northward from polar to subtropical latitudes followed by equally rapid southward motion from subtropical to intermediate about 50 s paleolatitudes during the late silurian middle devonian it is likely that significant east to west motion accompanied the latter shift in paleolatitudes with the caledonian acadian orogeny the result of silurian to early devonian convergence and collision between gondwana and north america this collision sandwiched several of the intervening displaced terranes between gondwana and north america subsequent to this collision gondwana was separated in the late devonian by a medium width ocean from north america and the avalonian and southern european blocks which were left behind adjacent to north america this new ocean closed during the carboniferous and the resulting convergence and collision were the cause of the hercynian alleghanian orogenic belt problems remaining for future research besides the further gathering of reliable paleopoles involve the uncertain pre devonian position of the southern british isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid late ordovician silurian apparent polar wander for gondwana