mesozoic plate motion history below the northeast pacific ocean from seismic images of the subducted farallon slab the high resolution seismic imaging of subducted oceanic slabs1 2 has become a powerful tool for reconstructing palaeogeography3 the images can now be interpreted quantitatively by comparison with models of the general circulation of the earth s mantle4 here we use a three dimensional spherical computer model of mantle convection5 6 to show that seismic images of the subducted farallon plate provide strong evidence for a mesozoic period of low angle subduction under north america such a period of low angle subduction has been invoked independently to explain rocky mountain uplift far inland from the plate boundary during the laramide orogeny7 the computer simulations also allow us to locate the largely unknown kula farallon spreading plate boundary the location of which is important for inferring the trajectories of suspect terrain across the pacific basin8

50 ma initiation of hawaiian emperor bend records major change in pacific plate motion the hawaiian emperor bend has played a prominent yet controversial role in deciphering past pacific plate motions and the tempo of plate motion change new ages for volcanoes of the central and southern emperor chain define large changes in volcanic migration rate with little associated change in the chain s trend which suggests that the bend did not form by slowing of the hawaiian hot spot initiation of the bend near kimmei seamount about 50 million years ago ma was coincident with realignment of pacific spreading centers and early magmatism in western pacific arcs consistent with formation of the bend by changed pacific plate motion

insect faunal response to environmental changes during the last interglacial in western beringia abstract reconstruction of the last interglacial li environment could help us understand the mammoth faunal extinction and facilitate predictions of the effects of future on ecosystems this work applied the fossil insect method to a number of interglacial sites in the siberian arctic west beringia the main problem of the li in western beringia is uncertain age determinations many interesting sites are undated interpretation of warming signals is also problematic because of the special character of regional pleistocene interglacial insect communities these faunas are different from the modern ones investigation can clearly recognize interglacial faunas and with additional stratigraphic evidence correlate some units to the li insect study helps shed light on the survival of large grazing mammals in western beringia during li 1 steppe tundra areas remained in the forested regions 2 climate change provided sufficient warmth to support the steppe tundra environment in the north early holocene climate change in combination with marine transgression progressively reduced suitable habitats for large mammals until a critical point was reached insect populations were also affected but due to their small size many steppe tundra insect species remained as relicts in isolated patches of steppe tundra

alteration of monazite and zircon and lead migration as geochemical tracers of fluid paleocirculations around the oklo okelobondo and bangombe natural nuclear reaction zones franceville basin gabon abstract large scale light rare earth element lree uranium lead and phosphorus migration has been evidenced in the fa lower proterozoic sandstones of the franceville basin gabon hosting oklo natural nuclear reaction zones rz in relation with extensive accessory mineral alteration by highly saline diagenetic brines 28 7 wt nacl eq to 30 wt cacl2 eq at about 140 c and 1 kbar monazite is the most severely altered accessory mineral in the coarse grained sandstones of the basal fa formation detrital monazite crystals are altered to th oh silicate microcrystalline phase with very low concentrations of u and lree the th la ratio increase from non altered th la 0 27 to altered sandstones th la 1 14 shows that about 76 of the lree was leached this corresponds to the leaching of 2 01 109 metric tons at the scale of the fa formation in the franceville basin similarly the th u increase from monazite th u 18 6 to the th silicate phase th u 88 7 is interpreted as a result of an alteration by oxidizing brines with leaching of u together with lree and p it corresponds to the leaching of 9 06 106 metric tons of uranium this amount of uranium largely exceeds the known uranium reserves from the franceville basin in zircon crystals the cores are generally homogeneous weakly fractured and well preserved as attested by the archean ages 2867 24 and 2865 51 ma obtained by ionic microprobe analysis on zircon of the fa formation respectively from the marginal and central parts of the basin their composition corresponds to the pure end member zr hf sio4 poor in th and u th u 1 at the contrary their rims which present several growth zones with cracks fillings are enriched in ree p th and u with higher th u ratios 5 10 both altered monazite and altered zircon contain galena as numerous inclusions in the outer growth zones and as crack fillings for example in zircon the pb of galena crystals 3 23 wt largely exceeds the amount of pb maximum 0 1 wt that would have been produced in situ by radioactive decay in this mineral nearly all the lead were introduced into altered zones of accessories dissolution of accessory minerals occurred at 2000 ma producing a porous and distorted crystal structure which has allowed a later incorporation of pb galena inclusions in altered zircons located in the vicinity of reactor zones have radiogenic lead compositions altered zircon rims and galena inclusions in altered zircon located far from reactor zones have non radiogenic pb isotopic compositions confirming the external origin of lead pb isotopic evolution models indicate a crystallization age sometime after 1000 ma both for galena located close to and far from u mineralizations and reactor zones which may be synchronous with a regional extension event contemporaneous with intrusion of dolerite dyke swarms between 1000 and 750 ma at the scale of the franceville basin the present study also illustrates the different retention capacities of accessory mineral for elements representing analogs of the radiotoxic nuclides in the relatively extreme natural conditions created by the circulation of moderately hot and chloride rich fluids during the diagenesis of a sedimentary basin

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

on the fit of broken ridge and kerguelen plateau abstract we present revised finite rotation parameters for australia and east antarctica in the early tertiary incorporating new magnetic data off of wilkes land antarctica reconstructions based on these and recently published rotation parameters document a relatively stationary position of broken ridge abutted against central kerguelen plateau between chron 34 83 ma and chron 20 43 ma prior to the commencement of seafloor spreading between the two late cretaceous features these reconstructions are significant as they are the first which are consistent with geological and geophysical constraints in not producing an overlap of broken ridge and kerguelen plateau between chron 34 and chron 24 53 ma and not requiring significant extension or strike slip motion within kerguelen plateau we also find that the finite rotations for chrons 20 to 34 outline the area of rough topography inferred from the satellite derived free air gravity for the diamantina zone and labuan basin a consequence of these rotations is that there is considerable overlap of continental crust in the region of tasmania the south tasman rise and east antarctica in the late cretaceous solutions to this problem may imply either post santonian 79 ma extension or transform motion in the bass strait or late cretaceous or cenozoic extension in wilkes land the reconstructions also agree with prior estimates of roughly 150 km of motion between east and west antarctica since chron 20

the younger dryas impact hypothesis a cosmic catastrophe in this paper we review the evidence for the younger dryas impact hypothesis ydih which proposes that at 12 9k cal a bp north america south america europe and the middle east were subjected to some sort of extraterrestrial event this purported event is proposed as a catastrophic process responsible for terminal pleistocene environmental changes onset of yd cooling continent scale wildfires extinction of late pleistocene mammals and demise of the clovis culture in north america the earliest well documented continent scale settlement of the region the basic physics in the ydih is not in accord with the physics of impacts nor the basic laws of physics no yd boundary ydb crater craters or other direct indicators of an impact are known age control is weak to non existent at 26 of the 29 localities claimed to have evidence for the ydih attempts to reproduce the results of physical and geochemical analyses used to support the ydih have failed or show that many indicators are not unique to an impact nor to 12 9k cal a bp the depositional environments of purported indicators at most sites tend to concentrate particulate matter and probably created many ydb zones geomorphic stratigraphic and fire records show no evidence of any sort of catastrophic changes in the environment at or immediately following the ydb late pleistocene extinctions varied in time and across space archeological data provide no indication of population decline demographic collapse or major adaptive shifts at or just after 12 9 ka the data and the hypotheses generated by ydih proponents are contradictory inconsistent and incoherent copyright 2014 john wiley sons ltd

pacific absolute plate motion since 145 ma an assessment of the fixed hot spot hypothesis 1 using the geometry and ages from 12 pacific seamount chains we have determined two new absolute plate motion models that now extend our self consistent and high resolution models with covariance estimates back to 145 ma the wk08 a model maps the full uncertainty in the age progressions into uncertainties in rotation opening angles yielding a relatively smooth plate motion model the wk08 g model relaxes the mapping of age uncertainties in order to better isolate secondary geometry changes seen along many coregistered chains both models have been used to assess the viability of the fixed hot spot hypothesis in the pacific in determining the models we found that only a small group of age samples had to be discarded on the grounds that they were discordant with the dominant trends we were able to connect plate motions for pre and post emperor age intervals by including the ratak gilbert ellice and musicians trails in our analysis however as no active hot spot locations exist for the older chains their inclusion adds additional model parameters both age and geometry misfits increase with age reflecting the observed increase in age uncertainties and the general widening of trails secondary and short lived changes in absolute plate motion mapped in wk08 g appear to correlate with the timing and sense of motion of known pacific rim tectonic events analysis of interchain distances between coeval samples from the hawaii and louisville chains suggests possible discrepancies during the older emperor stage that are compatible with predictions of hot spot drift we computed a new apparent polar wander path for the pacific and found a high degree of correspondence with paleomagnetically derived paths as long as solutions allowing for anomalous skewness were included in the latter our polar wander path suggests that there might have been some true polar wander during the emperor stage complemented by a smaller amount of hot spot drift than otherwise required we show that chain geometries and ages combined with future paleolatitude determinations from additional sites and chains could enable an observation based description of both hot spot and plate motions without relying on predictions of hot spot drift derived from mantle flow calculations

coastal and baja california paleomagnetism reconsidered systematic reappraisal of paleomagnetic data from upper mesozoic and lower cenozoic sedimentary rocks and igneous rocks of late mesozoic age in coastal and baja california indicates that pluton tilt and shallowing of paleomagnetic directions by sedimentary compaction resolve observed discordances with the north american apparent polar wander apw path when the best available reference paleopoles are used for comparison the hypothesis of major tectonic transport in excess of distances inherent in current models for evolution of the san andreas transform system is unnecessary and in conflict with geologic observations fresh insights derived from recent data sets and improved analysis include the following 1 the limited paleomagnetic data recording primary remanence for upper jurassic and lower cretaceous volcanic rocks in coastal california are concordant with the north american apw path 2 the tilt of the peninsular ranges batholith inferred from paleobarometric data eliminates its declination discordance and reduces its inclination discordance to a level where ovals of confidence for observed and expected paleomagnetic directions overlap 3 key cretaceous and paleogene red bed successions on the salinian block of coastal california and in baja california have yielded concordant paleolatitudes 4 the discordances of different apparent paleolatitudes inferred from cretaceous and paleogene marine sedimentary strata lie within a range that can be attributed to compaction shallowing on the basis of both experimental information and observations from elsewhere 5 turbidites are more susceptible to compaction shallowing than shelf and deltaic strata 6 the uniform rotation of paleomagnetic directions inferred previously for peninsular california is not supported by the full array of data now available from multiple localities 7 basement ties of the salinian block in california and of the baja california peninsula to the continental interior preclude any hypothesis for tectonic transport much in excess of san andreas transform slip

plate motions in the north pacific the 43 ma nonevent the hawaiian emperor seamount chain in the north pacific ocean is commonly considered to have been produced by motion of the pacific plate over a hotspot if the hotspot is assumed to have remained fixed in a mantle frame of reference the 60 change in trend between the hawaiian and emperor portions of the chain results from a change in motion direction of the pacific plate relative to the mantle at 43 ma during middle eocene time such a large and abrupt direction change should result in significant plate reorganizations and tectonic events in the pacific and surrounding plates but no evidence for these events can be found in addition in this paper it is shown that there were no changes in relative plate motions between pacific area plates that can be associated with the hawaiian emperor bend this is done by calculating motion of the pacific farallon and kula plates relative to asia and north america using the plate circuit farallon pacific antarctica africa north america asia with no major regional tectonic event at the time of the bend it is suggested that there was no plate reorganization at 43 ma and that the emperor portion of the seamount chain was formed by a nonstationary hotspot

compaction corrected paleomagnetic paleolatitudes for late cretaceous rudists along the cretaceous california margin evidence for less than 1500 km of post late cretaceous offset for baja british columbia the paleolatitudinal distribution of bivalve rudists has important significance for the baja british columbia baja bc hypothesis that western canadian superterranes from british columbia have been displaced 3000 km since cretaceous time rudists are not observed in baja bc sedimentary rocks yet they are common in late cretaceous strata in california and baja california which have the same paleomagnetically determined paleolatitudes approximately 25 n as baja bc rocks of late cretaceous age in order to resolve this contradiction and to delimit more exactly the southern paleolatitudes of baja bc paleomagnetic inclinations corrected for the effects of burial compaction were used to determine the paleolatitudinal distribution of rudists along the california margin compaction corrected paleomagnetic data from the peninsular ranges and salinia terranes indicate that rudists were restricted to paleolatitudes between 34 and 40 n evidence of coastal upwelling in the latest cretaceous marca shale may explain the northern limit of the rudist distribution these data suggest that baja bc was no farther south than 40 n in the late cretaceous thus limiting its post cretaceous displacement to less than 1500 km and that burial compaction has also affected the paleomagnetism of nanaimo group sedimentary rocks from vancouver island this result also helps resolve the conflict between paleomagnetic results which show 1500 km of post late cretaceous offset between the insular coast plutonic complex superterrane and the intermontane superterrane and geologic observations which can allow only tens of kilometers of offset between these terranes in the methow tyaughton basin

large scale motion between pacific and atlantic hotspots studies of true polar wander tpw the rotation of the solid earth with respect to the spin axis1 have suggested that there has been 10 15 of relative motion over the past 130 myr refs 2 4 in such studies the orientation of the spin axis is recovered from continental palaeomagnetic poles corrected for relative plate motions and compared with a deep mantle reference frame defined by hotspot locations but deducing relative plate motions becomes increasingly difficult for older mesozoic time periods hindering tests of tpw on timescales comparable to those of large scale mantle convection moreover the assumption of hotspot fixity is controversial5 6 we examine here a more direct approach7 8 using palaeolatitudes derived from pacific guyots contrary to predictions from tpw models these data suggest only minor latitudinal shifts of pacific hotspots during the cretaceous period instead of tpw relative motion between the atlantic and pacific hotspot groups9 is required at a velocity of approximately 30 mm yr 1 more than 50 larger than previously proposed5

geophysics of the pitman fracture zone and pacific antarctic plate motions during the cenozoic multibeam bathymetry and magnetometer data from the pitman fracture zone fz permit construction of a plate motion history for the south pacific over the past 65 million years reconstructions show that motion between the antarctic and bellingshausen plates was smaller than previously hypothesized and ended earlier at chron c27 61 million years ago the fixed hot spot hypothesis and published paleomagnetic data require additional motion elsewhere during the early tertiary either between east antarctica and west antarctica or between the north and south pacific a plate reorganization at chron c27 initiated the pitman fz and may have been responsible for the other right stepping fracture zones along the ridge an abrupt 8 clockwise rotation in the abyssal hill fabric along the pitman flowline near the young end of chron c3a 5 9 million years ago dates the major change in pacific antarctic relative motion in the late neogene

radiometric ages for basement rocks from the emperor seamounts odp leg 197 1 the hawaiian emperor seamount chain is the type example of an age progressive hot spot generated intraplate volcanic lineament however our current knowledge of the age distribution within this province is based largely on radiometric ages determined several decades ago improvements in instrumentation sample preparation methods and new material obtained by recent drilling warrant a reexamination of the age relations among the older hawaiian volcanoes we report new age determinations 40ar 39ar incremental heating method on whole rocks and feldspar separates from detroit sites 1203 and 1204 nintoku site 1205 and koko site 1206 seamounts ocean drilling program odp leg 197 and meiji seamount deep sea drilling project dsdp leg 19 site 192 plateaus in incremental heating age spectra for site 1203 lava flows give a mean age of 75 8 0 6 2 ma which is consistent with the normal magnetic polarity directions observed and biostratigraphic age assignments site 1204 lavas produced discordant spectra indicating ar loss by reheating and k mobilization six plateau ages from lava flows at site 1205 give a mean age of 55 6 0 2 ma corresponding to chron 24r drilling at site 1206 intersected a n r n magnetic polarity sequence of lava flows from which six plateau ages give a mean age of 49 1 0 2 ma corresponding to the chron 21n 22r 22n sequence plateau ages from two feldspar separates and one lava from dsdp site 192 range from 34 to 41 ma significantly younger than the cretaceous age of overlying sediments which we relate to postcrystallization k mobilization combined with new dating results from suiko seamount dsdp site 433 and volcanoes near the prominent bend in the lineament sharp and clague 2002 the overall trend is increasing volcano age from south to north along the emperor seamounts consistent with the hot spot model however there appear to be important departures from the earlier modeled simple linear age progression which we relate to changes in pacific plate motion and the rate of southward motion of the hawaiian hot spot

late pleistocene and holocene small and large mammal faunas from the northern urals data from the mammal fauna of the north urals during the late pleistocene and holocene are synthesised analysis of differentiation the degree of small and large mammal faunas during this time has been undertaken only differences of mammal species composition were significant between the late pleistocene and holocene complexes and within these complexes the distinction between faunas was insignificant the transition from the late pleistocene to the holocene complex small mammal faunas occurred in the middle late valdai due to expansion of the forest species in large mammal faunas the process was recorded later in dryas 3 early holocene because of the extinction of some species and others that changed their areas of occupation

arctic siberia refuge of the mammoth fauna in the holocene abstract global climate change at the end of pleistocene led to extinction in the huge territories of northern eurasia of the typical representatives of the mammoth fauna mammoth woolly rhinoceros wild horse bison musk ox and cave lion undoubtedly the mammoth fauna underwent pressure from upper paleolithic humans whose hunting activity could also have played a role in decreasing the number of mammoths and other representatives of megafauna formerly it was supposed that the megafauna of the mammoth complex had become extinct by the beginning of the holocene nevertheless the latest data indicate that extinction of the mammoth fauna was significantly delayed in the north of eastern siberia in the 1990s some radiocarbon dates established that mammoths existed in the holocene on wrangel island from 7700 until 3700 ybp radiocarbon data show that wild horses inhabited the north of eastern siberia 4600 2000 ybp muskoxen lived here about 3000 ybp some bison remains from eastern siberia belong to the holocene the following circumstances could promote the survival of representatives of mammoth fauna cool and dry climate in this region promotes the maintenance of steppe associations the habitats of those mammals late paleolithic and mesolithic settlements are not found in the arctic zone of eastern siberia from taimyr peninsula to the lower yana river they are very rare in basins of the indigirka and kolyma rivers the small number of stone age hunting tribes in the northern part of eastern siberia was probably another factor that contributed to the survival of some mammoth fauna representatives

paleogene history of the kula plate offshore evidence and onshore implications paleocene to middle eocene magnetic anomalies were mapped over oceanic crust that accreted at the kula pacific spreading center and is now obliquely entering the western aleutian trench between 179 e and 168 e the strike of anomalies and the pattern of abyssal hills and fracture zones changed abruptly during 56 55 ma when north south spreading veered to northwest southeast 310 130 kula pacific spreading ceased in 43 ma a 75 km long section of the fossil kula rift axis has avoided subduction although it now intersects the trench axis almost orthogonally near 171 5 e a narrow remnant of the former kula plate northwest of this fossil spreading center is bounded by a fossil kula pacific transform with a high transverse ridge alongside a sediment filled transform valley anomalies on this remnant show that eocene kula pacific spreading was highly asymmetric 2 1 the 56 55 ma change in kula plate rotation inferred from the change in spreading direction coincided with birth of the aleutian subduction zone and was probably a consequence of the resulting change in slab pull stresses on the oceanic lithosphere the change in direction of kula north american motion is a plausible explanation for the detachment of continental terranes from the pacific northwest and their migration around the gulf of alaska and for the early eocene demise of alaska range are volcanism the cessation of kula pacific spreading coincides with a major change in pacific plate rotation and the subsequent direction of convergence of the pacific plate with the aleutian arc was similar to the 55 43 ma direction of kula plate convergence

paleomagnetism of 122 ma plutons in new england and the mid cretaceous paleomagnetic field in north america true polar wander or large scale differential mantle motion a paleomagnetic study of cretaceous white mountains plutonic complexes in new hampshire and vermont yields high unblocking temperature dual polarity magnetizations in different types of igneous rocks the resulting pole position for three plutons 71 9 n 187 4 e a95 6 9 age 122 5 ma agrees with previously published mid cretaceous poles for north america which together give a mid cretaceous standstill reference pole slightly revised from globerman and irving 1988 at 71 2 n 194 1 e a95 3 7 n 5 studies we argue on the basis of the wide geographic distribution of these studies the variety in tectonic settings and rock types positive reversal tests and an overall reversal pattern consistent with geomagnetic polarity time scales that this mean pole represents the north american mid cretaceous reference field for nominally 36 m y 124 to 88 ma the standstill pole limits to within 4 the motion of the north american plate relative to the earth s spin axis during the same mid cretaceous interval the new england hotspot track 124 ma monteregian hills 122 5 ma cretaceous white mountains and 103 to 84 ma new england seamounts requires 11 4 of north poleward motion of north america in direct conflict with the paleomagnetic standstill a similar 13 discrepancy is independently demonstrated between the spin axis and the tristan da cunha hotspot track on the african plate during the mid cretaceous interval the hotspot spin axis discrepancies ended by 90 ma when it is shown that both atlantic hotspots agree with north american and african dipole paleolatitudes and present day locations nondipole fields are an unlikely explanation of the uniform motion of these two widely separated hotspots with respect to the spin axis leaving as possible interpretations true polar wander and large scale but differential mantle motion the southerly motion of the mid cretaceous louisville hotspot relative to the spin axis is ostensively at odds with what would be predicted under the true polar wander interpretation and points to differential mantle kinematics the motions of the three widely separated mid cretaceous hotspots with respect to the spin axis may be related to the recently proposed increase in global oceanic lithosphere production rates which gave rise to the mid cretaceous superplume

linking the late cretaceous to paleogene pacific plate and the atlantic bordering continents using plate circuits and paleomagnetic data 1 late cretaceous to paleogene paleomagnetic data from the pacific plate the emperor seamounts can be compared with data from the atlantic bordering continents through the use of plate circuit reconstructions here we summarize the uncertainties in all data sets and present formal tests we report agreement between late cretaceous pacific paleomagnetic data and predictions based on the estimates of non pacific pole positions from synthetic apparent polar wander paths this congruency points to the veracity of the plate circuits and the accuracy of the paleomagnetic estimates in contrast to the agreement seen for the late cretaceous small discrepancies are observed in the comparisons of the pacific paleogene data and predictions from synthetic apparent polar wander paths such a disparity in a younger time interval is unexpected given the agreement of the late cretaceous data the possibility that minor temporally variable nondipole field components contribute to the discrepancy cannot be completely discounted however an alternative and more straightforward explanation is suggested by further comparisons of the mean non pacific paleomagnetic data and the highest quality poles that contribute to the means in particular we note that 1 the pacific paleogene data are in full agreement with coeval poles from north america meeting strict reliability criteria and 2 the non pacific paleogene poles of synthetic apparent polar wander paths are dominated by results from the north atlantic igneous province naip but taken as a whole the naip data fail a paleomagnetic reversal test hence minor discrepancies between paleocene paleomagnetic data from the pacific and atlantic hemispheres may point to limitations of the latter which incorporate a relatively large number of older lower quality data these findings call for renewed data collections utilizing comprehensive rock magnetic and paleomagnetic demagnetization procedures to improve resolution of paleocene non pacific data the uncertainties of the pacific paleolatitude data and the non pacific reference poles are larger than the differences related to the use of the two alternative plate circuits through east to west antarctica and through australia to the lord howe rise that link the atlantic and pacific hemispheres while no preference can thus be given to either plate circuit their overall consistency with paleomagnetic data suggests that they can be used to investigate long term motion of the pacific plate since late cretaceous times

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

uncertainties and implications of the late cretaceous and tertiary position of north america relative to the farallon kula and pacific plates we present updated global plate reconstructions and calculated uncertainties of the pacific kula and farallon vancouver plates relative to north america for selected times since 68 ma improved magnetic data from the indian ocean decrease the uncertainties in the global plate circuit approach these uncertainties are now considerably smaller than those inherent in equivalent reconstructions based on the assumption of fixed hotspots major differences between these results and those of others are due to our use of more detailed africa north america reconstructions separate vancouver and farallon plate reconstructions and the assumption of a rigid antarctica plate during cenozoic time the uncertainties in the relative positions of the pacific and north america plates since the time of anomaly 7 26 ma range up to 100 km in position or from 1 to 3 m y in time if the mendocino triple junction initiated at about 28 5 ma its position would have been at 31 3 n 130 km relative to fixed north america unacceptable overlap of oceanic crust of the pacific plate with continental crust of western north america in the anomaly 10 30 ma reconstruction is a minimum of 340 200 km along an azimuth of n60 e and may be accounted for by basin and range extension pacific north america displacement in the past 20 ma is found to be considerably less than that calculated by fixed hotspot reconstructions farallon vancouver north america convergence velocity decreased greatly between the times of anomalies 24 and 21 56 to 50 ma prior to the 43 ma age of the hawaiian emperor bend and the often quoted 40 ma end of the laramide orogeny a change in direction of farallon north america convergence occurred sometime between 50 and 42 ma and also may not correlate with the time of the hawaiian emperor bend the lack of data from subducted parts of the farallon and kula plates permits many possibilities regarding the position of the kula farallon ridge the age of subducted crust or the position of oceanic plateaus during the laramide orogeny leaving open the question of the relationship between plate tectonic scenarios and tectonic style during laramide time displacements of points on the various oceanic plates along the west coast of an arbitrarily fixed north america during the interval between anomalies 30 31 and 18 68 to 42 ma are found to be pacific plate 1700 200 km northward farallon plate 3200 400 km northeastward vancouver plate 3000 400 km northeastward kula plate if attached to the pacific plate after a24 time 2500 400 km northward

sea ice expansion in the bering sea during the neoglacial evidence from archaeozoology the neoglacial was a period of cold that lasted more than 2000 years during the mid holocene from approximately 4700 to 2500 years ago although proxy data from a number of sources document the re

neogene tilting of crustal panels near wrangell alaska a late oligocene miocene igneous complex south and west of wrangell alaska contains mafic dikes that yield a discordant paleomagnetic direction inclination i 70 4 declination d 39 3 95 4 8 n 72 sites combined with local and regional geobarometric metamorphic and structural observations the discordant paleomagnetic direction indicates east side up tilt by 16 about a tilt axis with azimuth 8 neogene tilt of crustal blocks in the insular superterrane accounts for much of the paleomagnetic discordance in cretaceous plutons without the coastwise translation of 1000 km as suggested by the baja british columbia hypothesis

late cretaceous genesis of the kula plate abstract we propose that the pacific kula ridge began spreading approximately 85 m y b p during late cretaceous time extrapolation of the great magnetic bight backwards in time results in an implausible ridge configuration this implies that plate velocity vectors for the pacific kula and farallon plates were not constant during this interval evidence for splitting of the kula plate from the pacific plate along the chinook trough is the relationship between the north striking amlia and adak fracture zones the chinook and emperor troughs and the magnetic lineations south of the aleutian trench if this hypothesis is correct it will require that mesozoic reconstructions of the pacific basin and their relation to cenozoic reconstructions be re examined a previously unrecognized mesozoic plate may be required we propose calling this the izanagi plate

absolute inclination values from deep sea sediments a reexamination of the cretaceous pacific record new data are used to examine anomalously shallow paleomagnetic inclinations derived from previous analyses of cretaceous dsdp sediments of the central western pacific ocean while viscous remanent magnetizations vrms are shown to produce a similar effect the bias toward shallow values is still present with thoroughly demagnetized data superimposed upon this bias is a trend towards maximum flattening at an expected inclination of approximately 55 degree with the flattening falling to zero at the equator and pole flattening due to sediment compaction or inclination error is proposed to explain these observations

paleomagnetism of the late cretaceous early tertiary north central montana alkalic province paleomagnetic results have been obtained from the late cretaceous early tertiary igneous complexes of the north central montana alkalic province data from 94 sites in eocene volcanic and intrusive rocks give a paleomagnetic pole located at 82 0 n 170 2 e a95 3 5 k 18 6 while 36 sites in paleocene intrusions yield a paleomagnetic pole at 81 8 n 181 4 e a95 5 4 k 20 2 these poles differ by only 1 6 and are not significantly different statistically the 130 site virtual geomagnetic poles show no significant elongation and suggest no significant apparent polar wander apw during the period of magnetization of the igneous centers postmagnetization structural complications in these rocks are minimal the presence of a single predominant polarity in these intrusive complexes reinforces the radiometric age data that suggest that igneous activity within individual centers was of short duration the northcentral montana data together with other early tertiary cretaceous and mid tertiary paleomagnetic results require modification of our earlier apw chronology diehl et al 1980 it now appears that apw relative to north america since the early cretaceous consists of a polar still stand during much of the cretaceous 120 75 m y b p a period of rapid movement in latest cretaceous time 75 65 m y b p and a period of slow polar movement thereafter the onset of this period of rapid apw correlates well with a major change in plate motions at 80 m y and the beginning of the laramide orogeny

franciscan complex calera limestones accreted remnants of farallon plate oceanic plateaus the calera limestone part of the franciscan complex of northern california may have formed in a palaeoenvironment similar to hess and shatsky rises of the present north west pacific1 we report here new palaeomagnetic results palaeontological data and recent plate motion models that reinforce this assertion the calera limestone may have formed on farallon plate plateaus north of the pacific farallon spreading centre as a counterpart to hess or shatsky rises in one model2 the plateaus were formed by hotspots close to the farallon pacific ridge axis on accretion to north america plateau dissection in the late cretaceous to eocene 50 70 myr could explain the occurrence of large volumes of pillow basalt and exotic blocks of limestone in the franciscan complex partial subduction of the plateaus could have contributed to laramide 70 40 myr compressional events3

a revised kinematic model for the relative motion between pacific oceanic plates and north america since the late cretaceous 1 the convergence between the pacific farallon and kula oceanic plates and the north american plate has provided a driving force for tectonic processes that have shaped the western margin of north america yet reconstructions of plate motion in the pacific basin have traditionally relied on the assumption of fixed hot spots in the last decade substantial cretaceous to paleogene motion between the pacific and atlantic hot spots has been recognized providing motivation to reevaluate the history of relative motion between pacific oceanic plates and north america by means of plate circuit reconstructions this paper presents new kinematic models based on two alternative plate circuits linking the pacific plate to north america since late cretaceous time when compared to models relying on fixity of pacific and indo atlantic hot spots our reconstructions suggest that the pacific kula and pacific farallon ridges were 600 1000 km closer to the western margin of north america and hence that the farallon and kula plates were smaller in late cretaceous to middle eocene time these findings cast significant doubt on the viability of a configuration of the farallon kula ridge that juxtaposes the kula plate with the southern part of the paleo north american margin during this time interval the results of our new reconstructions also suggest more oblique convergence between the farallon or kula plate and north america from the late cretaceous to the middle eocene related to the combination of dominantly northwest motion of the pacific plate and steady southwest motion of north america this provides an efficient mechanism for the coast parallel translation of some accreted terranes of north america it may also explain the structure of geological units in western north america such as the central belt of the franciscan complex which show a dominant coast parallel fabric formed during an interval coeval with the episode of oblique convergence

pleistocene graminoid dominated ecosystems in the arctic abstract we review evidence obtained from analyses of multiple proxies floristics mammal remains paleoinsects pollen macrofossils plant cuticles phytoliths stable isotopes and modeling that elucidate the composition and character of the graminoid dominated ecosystems of the pleistocene arctic the past thirty years have seen a renewed interest in this now extinct biome sometimes referred to as tundra steppe steppe tundra in north american sources while many questions remain converging evidence from many new terrestrial records and proxies coupled with better understanding of paleoclimate dynamics point to the predominance of xeric and cold adapted grassland as the key former vegetation type in the arctic confirming earlier conjectures completed in the 1960s 1980s a variety of still existing species of grasses and forbs played key roles in the species assemblages of the time but their mixtures were not analogous to the tundras of today local mosaics based on topography proximity to the ice sheets and coasts soil heterogeneity animal disturbance and fire regimes were undoubtedly present however inadequate coverage of terrestrial proxies exist to resolve this spatial heterogeneity these past ecosystems were maintained by a combination of dry and cold climate and grazing pressure disturbance by large e g mammoth and horse and small e g ground squirrels mammals some recent studies from eastern beringia alaska suggest that more progress will be possible when analyses of many proxies are combined at local scales

propagation as a mechanism of reorientation of the juan de fuca ridge we present a revised model of the tectonic evolution of the juan de fuca ridge by propagating rifting the new model has three different relative rotation poles covering the time intervals 17 0 8 5 ma 8 5 5 0 ma and 5 0 ma to the present the rotation pole shifts at 8 5 and 5 0 ma imply clockwise shifts in the direction of relative motion of 10 to 15 at each of these shifts the pattern of propagation reorganizes and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges the model containing a total of seven propagation sequences shows excellent agreement with the isochrons inferred from the magnetic anomaly data except in areas complicated by the separate explorer and gorda plates the agreement between model and data near the explorer plate breaks down abruptly at an age of about 5 ma indicating that the probable cause of the rotation pole shift at that time was the separation of the explorer plate from the juan de fuca plate

uncertainties in reconstructions of the pacific farallon vancouver and kula plates and constraints on the rigidity of the pacific and farallon and vancouver plates between 72 and 35 ma to study the rigidity of the pacific and farallon plates between 72 and 35 ma we analyzed the magnetic anomalies and fracture zones in the pacific ocean between longitudes 125 w 180 and latitudes 43 s 60 n a set of consistent rotations indicates that the pacific and farallon plates between the pioneer and agassiz fracture zones were rigid plates between 72 and 35 ma thus the location of a missing tertiary boundary within the pacific or antarctic plates is restricted to be south of latitude 43 s the farallon plate broke into two plates at the time of the change in the pacific farallon spreading direction north of the pioneer and mendocino fracture zones sometime between 59 and 49 ma the northern fragment termed the vancouver plate was subducted beneath most of north america we present rotations and uncertainties for the histories of motion of the pacific and kula plates from anomaly 32 to anomaly 25 of the pacific and vancouver plates from anomaly 21 to anomaly 13 and of the pacific and farallon plates from anomaly 32 to anomaly 13

paleomagnetic tests of pacific plate reconstructions and implications for motion between hotspots the plate motion circuit through the south pacific and antarctica is shown to fail paleomagnetic tests of consistency these failures imply that reconstructions of pacific basin plates relative to surrounding plates inferred from this circuit are systematically in error and that estimates of motion between hotspots inferred from this circuit are probably too large therefore the motions between hotspots remain poorly known and may be much smaller than previously estimated