stacking paleointensity and oxygen isotope data for the last 1 5 myr piso 1500 coupled relative geomagnetic paleointensity rpi and oxygen isotope records are used to construct rpi and oxygen isotope stacks for the last 1 5 myr the coupled correlations are accomplished using the match algorithm lisiecki l e and lisiecki p a 2002 application of dynamic programming to the correlation of paleoclimate records paleoceanography 17 1049 doi 10 1029 2001pa000733 to simultaneously correlate isotope and rpi records the simultaneous match reduces the degree of freedom associated with correlations using rpi or oxygen isotope records alone the overall compatibility of rpi and oxygen isotopes indicates a dominant global but independent component in both signals the local wavelet power spectrum lwps for the rpi stack indicates little significant orbital power although the accompanying oxygen isotope stack has all the lwps characteristics expected of a high resolution oxygen isotope record containing orbital frequencies the piso 1500 stack represents a new stratigraphic template that can be used to correlate among marine sediment records and link them to polar ice cores via variations in cosmogenic nuclide production scaling the stack to values for virtual axial dipole moment vadm indicate maxima at 15 1022 am2 and minima that imply a threshold of 2 5 1022 am2 below which values are associated with either polarity reversals or magnetic excursions

relocation and assessment of seismicity in the iran region summary more than 2000 instrumentally recorded earthquakes occurring in the iran region during the period 1918 2004 have been relocated and reassessed with special attention to focal depth using an advanced technique for 1 d earthquake location a careful review of starting depths association of teleseismic depth phases and the effects of reading errors on these phases are made and when necessary waveforms have been examined to better constrain ehb focal depths uncertainties in ehb epicentres are on the order of 10 15 km in the iran region owing to the earth s lateral heterogeneity and uneven station distribution uncertainties of reviewed ehb focal depth estimates are on the order of 10 km as compared to about 4 km for long period p and sh body waveform inversions nevertheless these ehb depth estimates are sufficiently accurate to resolve robust differences in focal depth distribution throughout the iran region and within their errors show patterns that are in agreement with the smaller number of earthquakes whose depths have been confirmed by body wave modelling or local seismic networks the importance of this result is that future earthquakes with apparently anomalous depths can easily be identified and checked if necessary most earthquakes in the iranian continental lithosphere occur in the upper crust with the crustal shortening produced by continental collision accommodated entirely by thickening and distributed deformation in the zagros mountains nearly all earthquakes are confined to the upper crust depths 20 km and there is no evidence for a seismically active subducted slab dipping ne beneath central iran by contrast in southeastern iran where the arabian seafloor is being subducted beneath the makran coast low level earthquake activity occurs in the upper crust as well as to depths of at least 150 km within a northward dipping subducting slab near the oman line a region transitional between the zagros and the makran seismicity extends to depths of up to 30 45 km in the crust consistent with low angle thrusting of arabian basement beneath central iran in north central iran along the alborz mountain belt seismic activity occurs primarily in the upper crust but with some infrequent events in the lower crust particularly in the western part of the belt the talesh where the south caspian basin underthrusts nw iran earthquakes that occur in a band across the central caspian following the apscheron balkhan sill between azerbaijan and turkmenistan have depths in the range 30 100 km deepening northwards these are thought to be connected with either incipient or remnant northeast subduction of the south caspian basin basement beneath the east west trending apscheron balkhan sill curiously in this region of genuine mantle seismicity there is no evidence for earthquakes shallower than 30 km

seasonal stratification shell flux and oxygen isotope dynamics of left coiling n pachyderma and t quinqueloba in the western subpolar north atlantic blooms this annual population overturning is marked by a 0 7 increase in d 18 o in both species the shell flux of n pachyderma peaks during the spring bloom and in late summer when stratification is close to its minimum and maximum respectively both export periods contribute about equally and account for 95 of the total annual flux shell fluxes of t quinqueloba show only a single broad pulse in summer thus following the seasonal stratification cycle the d 18 oo fn pachyderma reflects temperatures just below the base of the seasonal sml without offset from isotopic equilibrium the d 18 op attern oft quinqueloba shows a nearly identical amplitude and correlates highly with the d 18 oo fn pachyderma therefore t quinqueloba also reflects temperature near the base of the sml but with a positive offset from isotopic equilibrium these offsets contrast with observations elsewhere and suggest a variable offset from equilibrium calcification for both species in the irminger sea the species consistently show a contrast in their flux timings their flux weighted dd 18 o will thus dominantly be determined by seasonal temperature differences at the base of the sml rather than by differences in their depth habitat consequently their sedimentary dd 18 o may be used to infer the seasonal contrast in temperature at the base of the sml citation jonkers l g j a brummer f j c peeters h m van aken and m f de jong 2010 seasonal stratification shell flux and oxygen isotope dynamics of left coiling n pachyderma and t quinqueloba in the western subpolar north atlantic paleoceanography 25 pa2204 doi 10 1029 2009pa001849

u th pb ages of single zircons in archaean supracrustals from nain province labrador canada ion probe u pb dating of igneous zircons from an acid metavolcanite belonging to the nulliak supracrustal association of the saglek area northern labrador suggests an original age of 3776 8 ma 2 interpreted as the deposition age of the volcanite in accordance with the field model this is slightly but significantly older than the previously obtained ion probe u pb age of igneous zircons from the main phase of the uivak gneisses which carries inclusions of the supracrustal association a secondary event of zircon growth recorded in the metavolcanite has a maximum age of 3065 8 ma 2 a metapelite belonging to the weekes amphibolite supracrustal association of the hopedale area south of nain contains rounded low u zircons of 3258 24 ma 2 age occasional textural evidence suggests that some of these zircons are most likely detrital grains of igneous origin in which case 3258 24 ma is a minimum age of the source from which the sediment was derived and a maximum age for the deposition of

why are geomagnetic excursions not always recorded in sediments constraints from post depositional remanent magnetization lock in modelling abstract many geomagnetic excursions have been documented during the brunhes chron however high resolution paleomagnetic studies of sediments often provide evidence of few if any excursions we have investigated the reasons for this observation by modelling the post depositional remanent magnetization pdrm lock in process a cubic lock in function produces more rapid lock in compared to linear and exponential functions proposed in the literature we therefore used a cubic function to model best case scenarios for the quality of the paleomagnetic record when a high frequency geomagnetic input signal is convolved with the sediment lock in function for a wide range of sedimentation rates even for a lock in depth of 10 cm where 95 of the pdrm is locked in 5 cm below the surface mixed layer an input signal containing abundant excursions with 1 kyr duration is smoothed so drastically at low sedimentation rates 1 3 cm kyr that no excursions are recorded excursions are further attenuated by increasing the lock in depth as well as by sampling at discrete stratigraphic intervals the pdrm process acts as a low pass filter for high frequency secular variation signals even at moderate sedimentation rates 10 cm kyr we have also modelled the effects of modulation of sedimentation rate by a climatic forcing function e g insolation even though the average sedimentation rate might be substantial pdrm recording becomes much less reliable in intervals where sedimentation rates are lowest in these cases some excursions are reliably recorded in detail while others may not be recorded at all our modelling suggests that in order to consistently detect the presence of geomagnetic excursions it is ideal to work with sediments that maintain minimum sedimentation rates above 10 cm kyr if the normal spectrum of geomagnetic field behaviour contains abundant excursions failure to document excursions in many apparently high resolution analyses probably results from pdrm lock in within relatively slowly deposited sediments or to unrecognized intervals of slow sedimentation in environments with higher average sedimentation rates

region specific key seismic parameters for earthquakes in northern iran strong motion accelerograms recorded within northern iran are used to examine the propagation characteristics of shear wave including geometric spreading behavior q value j 0 and horizontal to vertical h v ratio these region specific key seismic parameters are estimated from 259 three component records of 22 earth quakes with magnitude ranging from m 4 9 to m 7 4 in northern iran the geometric spreading follows a trilinear behavior with a strong postcritical reflection from the moho the first and second hinges of the trilinear behavior are at 75 and 150 km respectively the associated q value based on the vertical component is q 87 f 1 46 j0 value for vertical and horizontal components are 0 03 and 0 05 respec tively because of lack of station specific site information the h v ratio is considered to be a rough estimation of generic site amplification the obtained region specific parameters are used to estimate the average stress drop based on three stochastic modeling approaches stochastic point source modeling suggests a brune stress drop of 125 bars whereas stochastic finite fault modeling based on static and dynamic corner frequency approaches suggests a stress drop of 68 bars

10be evidence for delayed acquisition of remanent magnetization in marine sediments implication for a new age for the matuyama brunhes boundary abstract fluxes of the meteoric cosmogenic radionuclide 10 be vary with changes in the incoming cosmic rays modulated by geomagnetic field intensity variations the variability in the 10 be flux can be used to synchronize ice cores as well as marine sediments by comparison with the relative paleointensity variations of the geomagnetic field however lock in of the paleomagnetic signal at some depth below the sediment water interface in marine sediments through acquisition of a post depositional remanent magnetization pdrm adds uncertainty to synchronization despite the long history of such studies the magnitude of the pdrm lock in depth remains controversial in this article we present clear evidence for a downward offset of the paleointensity minimum relative to the 10 be flux anomaly at the matuyama brunhes m b geomagnetic polarity boundary which we interpret to result from a 15 cm pdrm lock in depth this lock in depth indicates that up to several tens of thousands years of age offset probably occurs when a paleomagnetic record is used for dating marine sediments and the age of the m b boundary should be revised to ca 10 kyr younger which is consistent with a younger ice core derived age of 770 6 ka 2 this cosmogenic age tuning strategy will contribute to refining paleomagnetic based age models for marine sediments and identifying of lead lag relationships for global abrupt environmental changes

the structure of archean ketilidian crust along the continental shelf of southwestern greenland from a seismic refraction profile the velocity structure of the continental crust on the outer shelf of southwestern greenland is determined from dense wide angle reflection refraction data obtained with large air gun sources and ocean bottom seismometers along a 230 km seismic line this line crosses the geological boundary between the archean block and the ketilidian mobile belt although the data have high noise levels p and s wave arrivals from within the upper intermediate and lower crust and at the moho boundary can be consistently identified and correlated with one dimensional wkbj synthetic seismograms in the archean p and s wave velocities in the upper crust are 6 0 and 3 4 km s while in the intermediate crust they are 6 4 and 3 6 km s these velocities match for the upper crust a quartz feldspar gneiss composition and for the intermediate crust an amphibolitized pyroxene granulite in the ketilidian mobile belt p and s wave velocities are 5 6 and 3 3 km s for the upper crust and 6 3 and 3 6 km s for the intermediate

geology and tectonic setting of paleoproterozoic granitoid suites in the island harbour bay area makkovik province labrador paleoproterozoic granitoid rocks in the island harbour bay area kaipokok domain makkovik province labrador are divided into four separate suites on the basis of field relations petrology and age the redefined island harbour bay plutonic suite consists of ca 1895 1870 ma dioritic to granitic mainly granodioritic and granitic units the rocks are variably foliated as a result of emplacement under amphibolite facies conditions in a dextral transpressive regime during andean type subduction the dominant mafic mineral is biotite and accessory epidote allanite and titanite are abundant the suite is calc alkalic but with rare earth element patterns similar to those of archean tonalitic trondhjemitic granodioritic suites it is interpreted to have formed deep in an andean type magmatic arc at the margin of the nain continent in contrast the younger hares islands and drunken harbour granites emplaced at ca 1805 and 1790 ma respectively were part of widespread late orogenic magmatic activity i

crustal structure of the labrador sea conjugate margin and implications for the formation of nonvolcanic continental margins wide angle seismic studies have determined the detailed velocity structure along a 350 km long profile across the labrador margin combination of this model with a previously published cross section for the southwestern greenland margin constitutes the first combined conjugate margin study based on seismic velocity structure the results indicate three distinct zones across the labrador margin similar to the structure of the conjugate greenland margin zone 1 represents 27 to 30 km thick continental crust thinning gradually seaward over 100 km distance farther seaward zone 2 is 70 80 km wide characterized by a distinct lower crust 4 5 km thick in which velocity increases with depth from 6 4 to 7 7 km s interpretation for this lower crustal block favors an origin by serpentinized peridotite rather than by magmatic under plating zone 3 represents two layered normal oceanic crust the cross sections from both margins are reconstructed to an early drift stage at chron 27 this demonstrates that the serpentinites in zone 2 are symmetrically distributed between previous identifications of chrons 31 and 33 on both margins zone 1 shows a marked asymmetry with a gradual thinning of continental crust off labrador contrasted with a rapid thinning off greenland the abundant serpentinization of upper mantle peridotite in zone 2 and the asymmetric shape of zone 1 are both probably related to a very slow rate of continental rifting which produced little if any melt

lithospheric structure of the labrador sea from constrained 3 d gravity inversion regional inversions of free air gravity data constrained by bathymetric and sediment thickness information were undertaken over the labrador sea and its margins to generate 3 d density anomaly models to investigate broad scale crustal structural variations across the extinct spreading centre benchmarked against independent seismic moho depth constraints a density anomaly isosurface within the inverted volumes was selected as a moho proxy and regional maps of moho structure were developed inversions using two different sources for depth to basement constraints revealed similar moho structures with a depth to moho of 12 km beneath the labrador sea which deepens to 20 km and greater towards davis strait and beneath the offshore extension of the grenville province density anomaly slices through the models corresponding to seismic lines show good agreement between the inverted moho proxy and the seismic moho with the only exceptions occurring where a high velocity lower crustal zone or underplate has been modelled from wide angle reflection refraction profiling studies the inverted depth to moho estimates were combined with depth to basement constraints to investigate crustal thickness both for the full crust and for individual crustal layers revealing that the crust of the labrador sea is generally 5 10 km thick but thickens to 20 25 km towards davis strait and beneath the offshore extension of the grenville province not taking into account high density underplates or anomalously high density lower crust sediment and crustal thickness variations were investigated to compute stretching factors across the labrador sea and to identify zones which deviate from local isostatic compensation assuming both an initial unstretched crustal thickness of 35 km and using a variable unstretched crustal thickness model much of the labrador sea has experienced 70 90 per cent thinning the derived values suggest that embrittlement of the entire crust and serpentinization of the upper mantle are likely to have occurred for large portions of the central and southern labrador sea inboard of known oceanic crust isostatic considerations reveal sediment excess and deficiency on the labrador and greenland margins respectively possibly reflecting fundamental rheological asymmetry with the greenland margin appearing weaker than the labrador margin a strong gradient from sediment deficiency to excess exclusive to the labrador margin may reveal the southwestern limit of a regional graben bounding listric detachment or of a zone of distributed faults and detachments

the crustal structure of the southern nain and makkovik provinces of labrador derived from refraction seismic data data from a refraction seismic profile parallel to the coast of labrador canada were used to determine the crustal structure across the boundary of the nain and makkovik provinces and to look for evidence for an offshore continuation of the mesoproterozoic nain plutonic suite nps seven seismometers recorded airgun shots along the 283 km long line p and s wave velocity models were developed from forward and inverse modeling of travel times the velocity model distinguishes three distinct zones in the saglek block of the nain province the crust is divided into three layers with p wave velocities between 5 8 and 6 9 km s farther to the south upper crustal velocities increase to 6 3 6 5 km s and the poisson s ratio increases from 0 24 to 0 27 this zone correlates with a gravity low that is interpreted to outline the offshore continuation of the nps the upper crustal velocities are intermediate between anorthositic and granitoid rock samples collected from the nps a lower crustal reflector is li

millennial scale cyclicity in the geodynamo inferred from a dipole tilt reconstruction constraining past changes in the strength and configuration of the geomagnetic field provides a valuable perspective on the processes that govern the geodynamo here we update a modelled dipole tilt reconstruction for the last 9000 years by using palaeomagnetic records that originate from globally well distributed sites some features predicted by earlier dipole models become more distinct due to added data sets we identify a dominant 1350 year cycle in the dipole tilt variations and two preferred states of the dipole axis with north geomagnetic pole longitudes confined to either c 120 degrees west or c 30 degrees east the dipole tilt reconstruction is shown to be consistent with independent geomagnetic field intensity data from western eurasia which show generally higher intensities when the dipole is tilted towards this region our study implies that vadm reconstructions that are constrained by a biased spatial distribution of data can show variations resulting from dipole tilt instead of true dipole moment the preferred states of the modelled dipole axis can be related to the four semi stationary high latitude flux lobes that have been observed at the core mantle boundary in long term time averaged palaeomagnetic field models the tilt episodes towards 30 degrees east in the northern hemisphere are perhaps related to the appearance of a high intensity flux lobe beneath western eurasia the dipole tilt reconstruction is highly correlated to millennial scale variations in the length of day that have been reconstructed from ancient records of eclipses which indicates that the cyclicity may constitute an important component in core flow dynamics c 2011 elsevier b v all rights reserved less

sedimentary records of relative paleointensity of the geomagnetic field theory and practice sediments have proved irresistible targets for attempts at determining the relative variations in the earth s magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution the assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short no dependence on magnetic field was expected nonetheless a number of experiments showed that sedimentary magnetizations varied in accordance with the field albeit not always in a simple linear fashion experiments in which the sediments were stirred in the presence of a field to simulate bioturbation showed a reasonably linear relationship with the applied field and these results spurred the hope that variations in the earth s magnetic field might indeed be recoverable from appropriate sedimentary sequences examination of existing paleointensity data sets allows a few general conclusions to be drawn it appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the earth s magnetic field the dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences moreover when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences there is considerable agreement among the various records when viewed over timescales of hundreds to thousands of years relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another suggesting that they are dominated by nondipole field behavior when viewed over timescales of a few tens of thousands to hundreds of thousands of years however the records show coherence over large distances at least thousands of kilometers and may reflect changes in the dipole field on the basis of a sequence spanning the brunhes and matuyama chrons the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years but these oscillations are not clear in the record prior to about 300 ka thus they are probably not an inherent feature in the geomagnetic field and the correspondence of the period of oscillation to that of obliquity is probably coincidence

site amplifications for generic rock sites seismic shear wave velocity as a function of depth for generic rock sites has been estimated from borehole data and studies of crustal velocities and these velocities have been used to compute frequency dependent amplifications for zero attenuation for use in simulations of strong ground motion we define a genetic rock site as one whose velocity at shallow depths equals the average of those from the rock sites sampled by the borehole data most of the boreholes are in populated areas for that reason the rock sites sampled are of particular engineering significance we consider two genetic rock sites rock corresponding to the bulk of the borehole data and very hard rock such as is found in glaciated regions in large areas of eastern north america or in portions of western north america the amplifications on rock sites can be in excess of 3 5 at high frequencies in contrast to the amplifications of less than 1 2 on very hard rock sites the consideration of unattenuated amplification alone is computationally convenient but what matters for ground motion estimation is the combined effect of amplification and attenuation for reasonable values of the attenuation parameter k 0 the combined effect of attenuation and amplification for rock sites peaks between about 2 and 5 hz with a maximum level of less than 1 8 the combined effect is about a factor of 1 5 at 1 hz and is less than unity for frequencies in the range of 10 to 20 hz depending on t using these amplifications we find provisional values of about ao 70 bars and k 0 0 035 sec for rock sites in western north america by fitting our empirically determined response spectra for an m 6 5 event to simulated values the borehole data yield shear velocities 033o of 618 and 306 rrdsec for rock and soil sites respectively when averaged over the upper 30 m from this we recommend that v30 equals 620 and 310 m sec for applications requiring the average velocity for rock and soil sites in western north america by combining the amplifications for rock sites and the site factors obtained from our analysis of strong motion data we derive amplifications for sites with 17 3o 520 m sec nehrp class c corresponding to a mix of rock and soil sites and v3o 310 and 255 m sec average soil and nehrp class d sites respectively for the average soil site the combined effect of amplification and attenuation exceeds a factor of 2 0 for frequencies between 0 4 and about 4 hz with a peak site effect of 2 4 the peak of the nehrp class d site effect is 2 7

wide angle seismic transect across the torngat orogen northern labrador evidence for a proterozoic crustal root a refraction wide angle reflection seismic transect across the labrador peninsula covers the core zone of the se churchill province the paleoproterozoic torngat orogen and the archean nain province including a portion of the labrador continental margin an airgun array was used as source and 11 ocean bottom seismometers and 16 land stations recorded the shots forward modeling of travel times and amplitudes reveals a deep asymmetric crustal root beneath the torngat orogen with a crustal thickness of 49 km and with p wave velocities of 6 9 7 0 km s the geometry of the velocity model and the root can be best explained by either westward dipping subduction or westward underthrusting of the nain crust gravity modeling suggests a correlation of the crustal root with a gravity low that extends 100 km in an e w direction and 200 km from north to south the preservation of the crustal root is attributed to the lack of postorogenic heating and ductile reworking consistent with the lack of abundant postcollisional magmatism in the se churchill province a discontinuity possibly cutting through the entire crust is interpreted as a zone of major transcurrent shearing associated with the main phase of deformation west of the torngat orogen the crustal thickness in the core zone of the churchill province varies between 35 and 38 km p wave velocities of 5 8 7 0 km s east of the orogen the crystalline crust in the nain province is 38 km thick velocities from 5 8 to 6 9 km s but thins to 9 km in the shelf area of the labrador margin where it is covered with up to 8 km of sediments no high velocity zone was found beneath the thinned continental crust at the margin

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

geochemistry of holocene cryptotephras from the north iceland shelf md99 2269 intercalibration with radiocarbon and palaeomagnetic chronostratigraphies tephra grain counting morphology and geochemistry in the 150 1000 m size fraction were used to identify 22 individual tephra horizons and one visible tephra layer saksunarvatn from postglacial sediment of the north iceland shelf core md99 2269 66 37 53 n 20 51 16 w water depth 365 m length 2533 cm from the reykjafjardarall basin is constrained at the base by the vedde tephra c 12 000 cal yr bp geochemical analyses and stratigraphical position of the 22 tephra horizons were used to identify the following cryptotephras kol 1372 ad 1372 hekla 1104 ad 1104 snaefellsnes 1 c 1790 cal yr bp hekla 3 c 2970 cal yr bp hekla 4 c 4230 cal yr bp tv 5 c 6860 cal yr bp and suduroy c 8070 cal yr bp a tephra of the same geochemistry as kol 1372 was found near the base of the core and is here named kol3 2269 11 000 cal yr bp eleven additional cryptotephras were geochemically typed to their respective icelandic volcanic centres of origin and their ages constrained four teph

regionally consistent western north america paleomagnetic directions from 15 to 35 ka assessing chronology and uncertainty with paleosecular variation psv stratigraphy abstract new paleomagnetic secular variation psv data from fish lake utah usa along with previously published regional records allow us to build an independently dated western north america psv stack wnam17 from about 35 to 15 ka that quantifies dating and paleomagnetic uncertainties in february 2014 we recovered a composite 11 m long sediment record from fish lake utah that retrieved glacially influenced and post glacial sediments computed tomography ct scans and magnetic susceptibility were used to build a precise depth scale while radiocarbon dating of terrestrial macrofossils and tephrostratigraphy provide tight age control glacially influenced sediments have well defined characteristic remanent magnetizations with all maximum angular deviation values

marine radiocarbon reservoir corrections for the midto late holocene in the eastern subpolar north atlantic the time dependency of the regional difference r in the marine radiocarbon reservoir correction is an important issue for the calibration of marine radiocarbon ages and also provides information about past ocean circulation changes few measurements of holocene reservoir corrections are available for the climatically important north atlantic we determined r for intervals during the past 6100 years before present bp by comparing radiocarbon ages of 31 marine and terrestrial sample pairs from 14 archaeological sites from the coast of ireland scotland and the orkney islands we find that while a time dependency in r for this region cannot be ruled out within the error of the determinations the r appears to have been constant for at least the last 2000 and possibly the last 6000 years this work results in a recommendation for r of 33 93 for the region

estimates of shear wave q and κ0 for unconsolidated and semiconsolidated sediments in eastern north america measured and calculated values of the effective quality factor qef and the site attenuation parameter 0 for unconsolidated and semiconsolidated sediments in eastern north america ena indicate that the latter is strongly dependent on sediment thickness estimates of 0 for national earthquake hazard reduction program nehrp bc site profiles sediment plus hard rock in the mississippi embayment and the atlantic coastal plain were found to increase from about 9 to 31 msec for sedi ment thicknesses ranging from 116 to 600 m stochastic simulations using the 175 m thick hypothetical nehrp bc site profile used to estimate ena ground motions in the nationalseismichazardmapsbytheu s geologicalsurvey usgs indicatethat 0 20 msec provides a smaller estimate of amplification that agrees more closely with the low strain short period site coefficients in the nehrp recommended provisions for seismic regulations for new buildings and other structures nehrpprovisions than the10msecvalueusedbytheusgs alinearregressionofthe 0 estimatescompiledin this study indicates that 0 20 msec corresponds to a relatively thick bc sediment thickness of 460 116 m these same stochastic simulations indicate that the rela tivelyshallowusgssiteprofileprovidesestimatesofamplificationthataresmallerthan thelow strainlong periodsitecoefficientsinthenehrpprovisions thedependenceof bothsiteattenuationandsiteamplificationonsedimentthicknesssuggeststhattheuseof a single reference site condition for hazard mapping might not be appropriate instead theseresultsimplythateitheraregionalsetofreferencesiteprofilesshouldbedeveloped or that a more uniform site condition such as hard rock should be used to define a more stable reference site condition in ena

ion probe u th pb zircon dating of polymetamorphic orthogneisses from northern labrador canada rounded zircons included within igneous zircons from the uivak i gneisses northern labrador have ages of up to 3863 12 ma 2 showing the former existence of rocks that predate the uivak gneisses by more than 100 ma the igneous zircons themselves have experienced varying amounts of early pb loss but age estimates based on maximum 207pb 206pb from three separate gneiss samples agree to within error at 3732 6 ma 2 which is taken as the age of emplacement of the igneous precursor to the dominant component of the uivak gneisses a new generation of zircons was formed during migmatization at ca 3620 ma the volumetrically less important lister gneiss was emplaced at 3235 8 ma 2 in the late archaean different parts of the gneiss complex were subjected to different grades of metamorphic overprinting the zircon chronology of gneisses affected by granulite facies metamorphism suggests a rapid sequence of events with simultaneous recrystallization of old zircons and growth of new u poor zi

holocene paleomagnetic secular variation psv near 80 n northwest spitsbergen svalbard implications for evaluating high arctic sediment chronologies abstract we present a paleomagnetic study of two holocene lake sediment sequences from northwest spitsbergen on svalbard norway core hap 212 from lake hajeren preserves a rare combination of a well defined characteristic remanent magnetization chrm and robust age control defined by 21 terrestrial macrofossil derived radiocarbon dates providing a unique opportunity to assess the stratigraphic potential of high arctic holocene paleomagnetic secular variation psv close to 80 n core amp 112 from hakluytvatnet which is also well dated is a poor magnetic recorder and cannot be used for geomagnetic reconstruction comparison of the hap 212 psv record with poorly dated regional records show similar directional features suggesting that psv can be used for chronological synchronization comparison of the hap 212 psv with well dated records 1800 km to the southwest and southeast show that these records are in phase at millennial timescales within dating and magnetic uncertainties indicating that a consistent geomagnetic signal is recorded at these temporal and spatial scales these findings support the notion that psv when optimally recorded can be used to import chronologies from lower latitudes to the high arctic where low organic content hard water effects lack of datable material and other difficulties often preclude more conventional radiocarbon dating

holocene geocentric dipole tilt model constrained by sedimentary paleomagnetic data paleomagnetic data although undeniably limited have been used to constrain complex time varying geomagnetic field models here we present four different versions of a time varying geocentric dipole model for the past approximately 9000 years based on a limited selection of sedimentary paleomagnetic data with good spatial coverage the robustness of the modeling approach is evaluated using modern magnetic observation station data provided by the world data centre for geomagnetism in edinburgh and we discuss the magnitude of the error introduced by insufficient data coverage the prediction in terms of directional variations is tested against paleomagnetic data from 63 different sedimentary sites and compared to results from recently published spherical harmonic geomagnetic field models we find that the dipole estimate can explain the paleomagnetic data almost as well as the more complicated spherical harmonic models specific areas and records are identified where there are significant differences between the performance of the dipole estimate and the spherical harmonic models our results suggest that many of the differences between the dipole model predictions and the paleomagnetic data can be related to poor paleomagnetic quality and or the lack of accurate and precise time control inherent to some of the records these results agree with valet et al 2008 who suggested that the cals7k 2 model probably underestimates the dipole contribution in order to reduce the misfit between the inversion and the data we conclude that better paleomagnetic data and associated time control are needed to improve global geomagnetic field models less

wide angle reflectivity across the torngat orogen ne canada data from a lithoprobe refraction wide angle reflection seismic transect across the torngat orogen in ne canada were depth migrated to produce a record section similar to near vertical incidence seismic data densely spaced airgun shots and a station spacing between 9 and 25 km were able to produce a low frequency image of the lower crustal reflectivity for most of the transect over the western end of the profile a series of east dipping lower crustal reflectors continue down to the moho which is clearly defined at a depth of 36 to 38 km the lower crustal reflectors can be correlated laterally for up to 80 km this reflection fabric is interpreted as evidence for thick skinned deformation caused by convergence of the nain and superior provinces over the eastern end of the line reflectors in the nain province are less continuous and may indicate steeply dipping faults which have not been modified during formation of the orogen

application of dynamic programming to the correlation of paleoclimate records 1 signal matching is a powerful tool frequently used in paleoclimate research but it is enormously time consuming when performed by hand previously proposed automatic correlation techniques require very good initial fits to find the correct alignment of two records a new technique presented in this paper utilizes dynamic programming to find the globally optimal alignment of two records geological realism is instilled in the solution through the definition of penalty functions for undesirable behavior such as unlikely changes in accumulation rate examples with synthetic and real data demonstrate that the dynamic programming technique produces accurate high resolution results with much less effort than hand tuning or preexisting automated correlation techniques

secular variation of the geomagnetic dipole during the past 2000 years we have constructed a very simple model of a time varying geocentric dipole based on the archeomagnetic records obtained at four widely separated sites on the globe for the past 2 ka the predictions of the model in terms of directional variations have been tested against actual archeomagnetic data from 12 sites distributed over the globe they were also compared with the hongre et al 1998 time varying spherical harmonic model and with the cals7k 2 model by korte et al 2005 we find that the misfits between the predictions of our simple dipole model and the data are equivalent to those of the spherical harmonic models for the european sites and not strikingly larger for the rest of the world many discrepancies can be accounted for by uncertainties inherent to the archeomagnetic records which along with the small number and poor geographical distribution of sites leads us to conclude that the present state of the database does not allow the extraction of secular variations described by terms going beyond degree 2 it appears also that dipole tilt could be responsible for the main part of the secular variation associated with time constants exceeding 102 years in a second step we used the paleointensity records contained in the same database to construct the curve depicting the variations of the true dipole moment the present decrease of the dipole did not begin prior to 1000 years ago and the dipole was actually increasing from 0 until a d 500 the dipole moment of cals7k is lower than the present estimate probably due to large repartition of energy to higher harmonics to minimize the misfit between the inversion and the data the tilt and strength of the dipole can predict the dipole field at any site and were used to derive the contribution of the nondipole field to values of paleointensity at paris during the past 2 ka the results show that the archeomagnetic jerks are associated with various configurations depending on the phase relationship between the nondipolar and dipolar parts of the field

improvements in long core measurement techniques applications in palaeomagnetism and palaeoceanography we report on new developments in the long core measurement techniques using u channels in connection with new small access cryogenic magnetometers with these new instruments u channel samples allow us to measure a wide range of magnetic parameters along entire core length with a spatial resolution and accuracy virtually equivalent to that obtained using discrete cubic samples an additional advantage is that the sediment disturbance when sampling is significantly less when using u channels we present comparisons of results from both continuous and discrete measurements and examine the effectiveness of minimal deconvolution using various techniques examples of continuous measurements of low field susceptibility nrm arm irm hrc from north atlantic deep sea cores show that it is possible to generate logs of combination of parameters related to rock magnetic characteristics of the sediments useful in palaeoceanography in particular the continuous method allows us to rapidly detect core regions with fast changing magnetic parameters useful for correlation between cores and also for understanding changes in palaeo environmental conditions the fast scan also allows us to detect regions of the core where the rock magnetic parameters are suitable or not suitable for palaeomagnetic studies related to changes in the geomagnetic field such as the relative changes in palaeointensity

a paleomagnetic approach toward refining holocene radiocarbon based chronologies paleoceanographic records from the north iceland md99 2269 and east greenland md99 2322 margins 1 we report the intercalibration of paleomagnetic secular variation psv and radiocarbon dates of two expanded postglacial sediment cores from geographically proximal but oceanographically and sedimentologically contrasting settings the objective is to improve relative correlation and chronology over what can be achieved with either method alone core md99 2269 was taken from the hunafloaall trough on the north iceland shelf core md99 2322 was collected from the kangerlussuaq trough on the east greenland margin both cores are well dated with 27 and 20 accelerator mass spectrometry 14c dates for cores 2269 and 2322 respectively paleomagnetic measurements made on u channel samples document a strong stable single component magnetization the temporal similarities of paleomagnetic inclination and declination records are shown using each core s independent calibrated radiocarbon age model comparison of the psv records reveals that the relative correlation between the two cores could be further improved starting in the depth domain tie points initially based on calibrated 14c dates are either adjusted or added to maximize psv correlations radiocarbon dates from both cores are then combined on a common depth scale resulting from the psv correlation support for the correlation comes from the consistent interweaving of dates correct alignment of the saksunarvatn tephra and the improved correlation of paleoceanographic proxy data percent carbonate these results demonstrate that psv correlation used in conjunction with 14c dates can improve relative correlation and also regional chronologies by allowing dates from various stratigraphic sequences to be combined into a single higher dating density age to depth model

petrogenesis of a mid proterozoic anorthosite mangerite charnockite granite amcg complex isotopic and chemical evidence from the nain plutonic suite the nain plutonic suite nps about 1 30 ga underlies some 19 000 km intruding the boundary between middle to late archean rocks in the east nain province and late archean and early proterozoic rocks in the west churchill province nd isotopic compositions in most igneous units of the nps reflect their geographic position relative to the inferred boundary between the nain and churchill provinces rare exceptions are local intrusions of olivine bearing basic rocks with highest epsilon nd 1 3 ga near 3 0 present in both eastern and western sectors of the nps anorthositic rocks have variable and significant contributions of crustal nd but notably lower i sr than granitoid rocks ferrodiorites are similar isotopically to the anorthositic group and have chemical and mineral compositions more akin to the anorthositic group than the granitoids aluminous orthopyroxene megacrysts in nps anorthositic rocks indicate crystallization pressures of 6 2 to 11 0 kbar consistent with lower cru

structural evolution of the makkovik province labrador canada tectonic processes during 200 myr at a paleoproterozoic active margin in the makkovik province the structures and plutonic rocks of the reworked southern edge of the archean nain craton parautochthon and an adjacent juvenile domain form a spatially concentrated record of 200 ma at a paleoproterozoic active margin d1 thin and thick skinned structures formed during terrane collision before ca 1896 ma were nearly erased by d2 transpressive structures present across the entire parautochthon that had been thermally softened by emplacement of arc plutons at ca 1895 1870 ma d3 transpressive inversion of a back arc rift and d4 strike slip shearing formed structures at the oceanward margin of the parautochthon and were responses to events in the active arc that was now widely separated from the parautochthon by accreted terranes the tectonic isolation of the parautochthon ended by the time of d5 ca 1740 1710 ma and d6 with the reappearance there of granite and low grade shear zones structural and plutonic style varied at each stage primarily controlled by the crustal level attained

anatomy and orogenic history of a paleoproterozoic accretionary belt the makkovik province labrador canada the makkovik province is a segment of a paleoproterozoic accretionary belt the makkovik ketilidian orogen that developed on the southern margin of laurentia at 1 9 1 7 ga in contrast to coeval laurentian orogenic belts that mainly resulted from collision of archean plates makkovikian ketilidian orogenesis was dominated by active margin processes including continental margin arc plutonism and juvenile terrane accretion both of which were accompanied by regional transpression in the makkovik province earliest deformation and amphibolite facies metamorphism of paleoproterozoic rift drift assemblages post hill and moran lake groups and the archean foreland nain province occurred at 1 9 ga in response to accretion of a paleoproterozoic island arc following this collision cratonward dipping subduction was established resulting in the formation of the 1895 1870 ma island harbour bay plutonic suite a calc alkaline magmatic arc built on reworked archean crust crust formation continued between ca 1

holocene tephra from iceland and alaska in se greenland shelf sediments the record of icelandic volcanic events in holocene marine sediments off se greenland provides evidence for the frequency and timing of atmospheric tephra plume dispersal from iceland towards greenland geochemistry of tephra abundance peaks from two se greenland shelf cores md99 2322 and jm96 1215 2gc are compared with core md99 2269 north iceland shelf to evaluate the dispersal direction of icelandic eruptions glass shard counts 106 1000 m in md99 2322 revealed 16 distinct cryptotephra peaks geochemical analyses of eight cryptotephra peaks in md99 2322 and two in jm96 1215 indicate sources in the volcanic systems of iceland and alaska a tephra layer matching in geochemistry and stratigraphy to the c 3600 bp eruption of the aniakchak volcano in the aleutian islands was identified in jm96 1215 2gc the settlement tephra ad 871 2 and hekla b h b were identified in md99 2322 a new marker horizon katla eg 6 73 was found in both se greenland cores three basaltic peaks between 9 9 and 10 4 cal kyr bp exhibit major element geochemistry indistinguishable from the c 10 2 kyr saksunarvatn tephra these layers represent 3 out of seven westward and northward dispersed grimsvotn layers found on the se greenland shelf and the north iceland shelf between 9 9 and 10 4 cal kyr bp

natural remanent magnetization acquisition in bioturbated sediment general theory and implications for relative paleointensity reconstructions we present a general theory for the acquisition of natural remanent magnetizations nrm in sediment under the influence of a magnetic torques b randomizing torques and c torques resulting from interaction forces dynamic equilibrium between a and b in the water column and at the sediment water interface generates a detrital remanent magnetization drm while much stronger randomizing torques may be provided by bioturbation inside the mixed layer these generate a so called mixed remanent magnetization mrm which is stabilized by mechanical interaction forces during the time required to cross the surface mixed layer drm is lost and mrm is acquired at a rate that depends on bioturbation intensity both processes are governed by a mrm lock in function the final nrm intensity is controlled mainly by a single parameter that is defined as the product of rotational diffusion and mixed layer thickness divided by sedimentation rate this parameter defines three regimes 1 slow mixing 10 with mrm acquisition and full drm randomization and 3 intermediate mixing because the acquisition efficiency of drm is larger than that of mrm nrm intensity is particularly sensitive to in case of mixed regimes generating variable nrm acquisition efficiencies this model explains 1 lock in delays that can be matched with empirical reconstructions from paleomagnetic records 2 the existence of small lock in depths that lead to drm preservation 3 specific nrm acquisition efficiencies of magnetofossil rich sediments and 4 some relative paleointensity artifacts

three dimensional structure of the torngat orogen ne canada from active seismic tomography the crustal velocity structure and the moho depth of the proterozoic torngat orogen ne canada is determined by active seismic tomography using travel times of crustal turning rays and moho reflections the orogen developed during oblique convergence of the archean superior and nain provinces which trapped an interior belt of archean crust core zone between them with the torngat orogen evolving between the core zone and the nain province beneath the central orogen a crustal root is found with a preserved depth of 52 km and a width of 100 km to the north the root shallows to 44 km and narrows to a width of 45 km the root correlates with a set of major late orogenic shear zones that accommodated oblique convergence of the superior and nain provinces this correlation suggests that the transpressional shearing focused strain in the region of the root and contributed to the crustal thickening absence of postorogenic magmatic activity prevented reworking or thermal relaxation of the root the lack of late magmatism is probably related to the depleted and refractory nature of the archean lithosphere underlying the orogen upper crustal velocities are lowest in the core zone 5 7 km s at the surface and are compatible with laboratory measurements carried out on gneissic rock samples from that area higher velocities in the nain province 5 9 km s correlate with felsic gneiss and anorthosite rock samples a high velocity region immediately to the north of the crustal root is associated with a moho uplift 34 km this is explained by extension along the ungava transform fault and possibly in hudson strait at 55 ma when rifting in the labrador sea was transferred into baffin bay

wide angle seismic imaging of a mesoproterozoic anorthosite complex the nain plutonic suite in labrador canada the mesoproterozoic nain plutonic suite nps of labrador canada one of the largest anorogenic plutonic terranes was studied by a refraction wide angle seismic experiment four ocean bottom seismometers and 18 land stations were deployed along a 330 km profile and recorded air gun shots from the easternmost 160 km with the nps located in the center of the line at the suture of the nain and churchill provinces p and s wave velocity models were developed by forward modeling of travel times and amplitudes upper and middle crustal p wave velocities outside and beneath the nps range from 5 9 to 6 5 km s lower crustal p wave velocities range from 6 55 to 7 0 km s within the anorthositic rocks velocities are as high as 6 8 km s and reflections define the base of the nps to be 8 km deep in the se churchill province and 11 km in the nain province a variation that may be the result of lateral density changes within the country rocks or the anorthosites the total crustal thickness is 39 km west of the nps but is only 32 34 km beneath the nps some 5 km less than nain province crust distal from the nps the inferred crustal thinning is possibly related to anatexis of the lowermost crust by a thermal plume that generated the plutonism the poisson s ratios are 0 275 within the anorthosite plutons 0 27 in the upper and middle crust and 0 285 in the lower crust these values are some 0 03 higher than in the archean nain crust distal to the nps indicating a higher plagioclase content at all crustal levels as result of the plutonism we postulate that a crustal root similar to the root observed farther north in the torngat orogen was completely removed by anatexis and the silicic and basic magmas probably ascended to midcrustal levels along preexisting zones of weakness at the nain churchill boundary

relative paleointensity and environmental magnetism since 1 2 ma at iodp site u1305 eirik drift nw atlantic abstract integrated ocean drilling program iodp expedition 303 to the north atlantic in 2004 recovered rapidly deposited deep sea sediments at iodp site u1305 on eirik drift located south of greenland at 3460 m water depth along the path of the western boundary under current wbuc about 200 m of sediment was sampled with u channels from the composite section providing a continuous record of paleomagnetic field directions and relative paleointensity rpi variations covering the past 1 2 myr the age model based on an oxygen isotope record is consistent with the fit of the rpi record to a calibrated template and indicates higher sedimentation rates during interglacials relative to glacial epochs magnetite grain size and concentration proxies indicate higher concentrations of magnetite with larger grain sizes during interglacials enhanced interglacial deposition can be attributed to a combination of elevated entrainment of terrigenous detritus into the wbuc due to glacial retreat on continents flanking the upstream path of the wbuc east greenland and iceland and of increased bottom current wbuc vigor leading to elevated transport and deposition at the site during interglacials this pattern is opposite to observations of flow of the antarctic circum polar current acc in the south indian ocean which suggests an inter hemispheric antiphase in marine circulation at the milankovitch scale with strong circulation in the deep north atlantic when the acc is weak and vice versa

measurement of the degree of smoothing in sediment paleomagnetic secular variation records an example from late quaternary deep sea sediments of the bermuda rise western north atlantic ocean abstract replicate paleomagnetic secular variation psv records for the last 15 000 yr have been recovered from two deep sea sediment gravity cores on the bermuda rise western north atlantic ocean the records have been correlated using several different parameters and dated by correlation to a nearby piston core which has a high resolution 14 c chronology the psv records are systematically subdued with respect to the expected local magnetic field variability during the holocene when sedimentation rates were about 10 cm kyr during the late pleistocene when sedimentation rates were a factor of 2 3 higher the psv variability was significantly larger we attribute this to smoothing of the psv records by a depositional post depositional remanence drm pdrm acquisition process with a 10 20 cm lock in interval the degree of psv smoothing was evaluated by comparing the vgp angular dispersion and vector spectral content of the bermuda rise cores with high resolution psv records from north america and great britain these results suggest a 50 reduction in the drm pdrm recording of vector variability during the holocene on the bermuda rise with most of the psv reduction occurring for features of less than 2000 yr in duration we can reproduce the subdued psv pattern with a mathematical model that simulates the site drm pdrm remanence acquisition process finally the replicate gravity cores document that psv records may be reproducible on a local scale even though they only retain a low pass filtered record of true magnetic field variability

geophysical characteristics of the continental crust along the lithoprobe eastern canadian shield onshore offshore transect ecsoot a review the eastern canadian shield onshore offshore transect ecsoot of the lithoprobe program included 1200 km of normal incidence seismic profiles and seven wide angle seismic profiles across archean and proterozoic rocks of labrador northern quebec and the surrounding marine areas archean crust is 33 44 km thick p wave velocity increases downwards from 6 0 to 6 9 km s there is moderate crustal reflectivity but the reflection moho is unclear archean crust that stabilized in the proterozoic is similar except for greater reflectivity and a well defined moho proterozoic crust has similar or greater thickness variable lower crustal velocities and strong crustal reflectivity geodynamic processes of paleoproterozoic growth of the canadian shield are similar to those observed in modern collisional orogens the suturing of the archean core zone and superior provinces involved whole crustal shearing top to west in the core zone linked to thin skinned deformation in the new quebec orogen the torngat orog

inversion for crustal structure using reflections from the passcal ouachita experiment an interface inversion has been tested and applied to wide angle reflection data from the 1986 passcal ouachita experiment an interface corrugation with a relief of 10 km and a width of 30 km was successfully imaged in a test of the interface inversion velocity depth curves derived for shot points 14 19 using the tail sum method are similar to an average one dimensional velocity model from forward modeling using travel time correlations from southern profiles 14 19 the similarity of these velocity depth curves positioned over the southern half of the passcal experiment suggests that crustal structure in this region is approximately laterally homogeneous lower crustal velocities are not well resolved due to the recording on most profiles of only small segments of the travel time triplications a two dimensional velocity structure was derived by using the average one dimensional velocities for the deeper crustal layers and formally inverting for depth to interfaces the final inversion model is found to be consistent with previous refraction interpretations south of the ouachita orogenic trend and concurrent interpretations of the passcal data set inversion results for the central and southern portion of the passcal profile indicate a depth of 10 12 km for a midcrustal layer which thins southward from approximately 10 km to about 4 km a lower crustal layer with an average thickness of 12 km and a moho depth of approximately 29 5 km are also determined interface depths are in agreement with a normal moveout stack of the passcal data set in particular the shallowing of the moho to a depth of 30 km over the northern 50 km of the profile matches previous interpretations of the data set and has been interpreted here and in previous studies as the location of the paleozoic continental margin geophysical studies of the modern atlantic continental passive margin provide the simplest comparison to the crustal structure derived here the lower crustal layer found south of the shallowing of the moho to 30 km beneath the passcal profile is analogous in thickness and position to rift stage lower crust however other tectonic interpretations are possible given the uncertainties in velocity and lithology relationships

crustal structures of grenville makkovik and southern nain provinces along the lithoprobe ecsoot transect regional seismic refraction and gravity models and their tectonic implications crustal structures of the eastern grenville makkovik and southern nain provinces are determined using seismic reflection refraction and gravity data along the lithoprobe eastern canadian shield onshore offshore transect ecsoot within the grenville province the velocity model contains a 5 km thick upper crust and a variable thickness middle to lower crust the total crustal thickness varies from 25 to 43 km with the thickest crust in the south and thinnest crust in the north a high velocity lower crustal wedge is coincident with a strong band of northward dipping reflectors the two dimensional velocity structure is compatible with modelling of a 60 mgal gravity high over the hawke river terrane in the makkovik province the thickness of upper crustal velocities increases to 17 km the velocity decrease in the upper to middle crust from the grenville province to the makkovik province is similar to that of refraction models across the grenville front in ontario and quebec it is possibly related t

interpretation of crustal seismic velocities in the san gabriel mojave region southern california the increased concern over seismic hazards in the los angeles region has created interest in the crustal structure and mechanical properties associated with faults like the sierra madre and san andreas this study presents the seismic properties of major lithologies found in the vicinity of the san andreas fault zone in order to correlate with the los angeles region seismic experiment lars e results scientists from the larse group have recently acquired high quality seismic data along three transects thirty nine rock samples were collected along the larse line i which trends ne sw across the central mojave desert and the san gabriel mountains these rock samples represent the following lithologies precambrian gneisses san gabriel anorthosite mesozoic granitic intrusives pelona schist mendenhall granulite gneisses and san andreas fault zone cataclasites the seismic properties were determined by measuring compressional and shear wave velocities as a function of confining pressure average compressional and shear wave velocities at 150 mpa show a wide range of values the mendenhall gneiss and the san gabriel anorthosite complex have high velocities 6 5 km s and 6 4 km s respectively the pelona schist the mojave granitic intrusives and the san andreas cataclasitic rocks have average velocities of 5 8 km s 5 2 km s and 5 1 km s respectively average densities range from 2320 kg m3 for fault zone cataclasites to 3100 kg m3 for a mendenhall granulite gneiss gneisses and schists show significant seismic anisotropy 5 20 due to preferred mineral orientation while most other rocks are nearly isotropic poisson s ratios range from a high of 0 29 in the san gabriel anorthosite to a low of 0 22 in the san gabriel gneiss comparison of the measured seismic properties with the larse velocity model allows interpretation of crustal structure in terms of rock compositions the upper crust southwest of the san andreas fault contains granitic intrusives gneisses and schists while under the mojave desert the upper crust is dominated by gneisses high velocities under the san gabriel mountains represent either lower crustal granulites or anorthosite the velocity decrease beneath the san andreas fault zone originates from minor amounts of cataclasite or the presence of low velocity lithologies such as pelona schist 1998 elsevier science b v all rights reserved

simulation of strong ground motion in northern iran using the specific barrier model summary in this study based upon the calibrated specific barrier model sbm against the latest available strong motion data ground motion prediction equations for soil and rock sites in northern iran are developed the sbm may provide the most complete simple and self consistent description of the faulting process which is applicable in both near fault and far field regions consequently the sbm may provide consistent ground motion simulations over the entire necessary frequency range and for all distances of engineering interests to determine source parameters in this study we used 163 three component records of 32 earthquakes with magnitude ranging from mw 4 9 to 7 4 in northern iran in the database records with hypocentral distances less than 200 km are chosen and only earthquakes whose moment magnitude estimates are available have been used furthermore using the best available information recording sites are classified into two main geologic categories rock and soil because of the lack of site amplification information in the most regions of the world including iran we used the h v ratio method for estimating the site amplification moreover the kappa factor that shows diminishing the high frequency amplitude is determined in this study two data sets are considered for determining the source parameters g and l and the h v ratio and the kappa factor only s wave part of signals is used in each analysis regression analysis is performed using random effects method that considers both interseismic event to event and coseismic within event variabilities to effectively deal with the problem of weighting observations from different earthquakes the residuals are controlled against available northern iranian strong ground motion data to verify that the model predictions are unbiased and that there are no significant residual trends with magnitude and distance at first it is assumed that no sign of self similarity breakdown is observed between the source radius and its seismic moment after controlling the results the modified sbm should be used as some deviations have been observed to verify the robustness of the results the number of observations is changed by removing various randomly selected data sets from the original database which results in unchanged results of the model stochastic simulations are then implemented to predict peak ground motion and response spectra parameters the stochastic sbm predictions are in relatively good agreement with other available attenuation relationships proposed for iran europe and middle east it has been shown that the proposed sbm of this study provides unbiased ground motion estimates over the entire frequency range of most engineering applications it provides a reliable and physically realistic yet computationally efficient way to model strong ground motions

cosmogenic signature of geomagnetic reversals and excursions from the reunion event to the matuyama brunhes transition 0 7 2 14 ma interval long term variations of the geomagnetic dipole moment gdm during periods of stable polarity and in transitional states reversals and excursions provide key information for understanding the geodynamo regime following several studies dealing with the brunhes chron and the matuyama brunhes transition this study presents a new authigenic 10 be 9 be ratio be ratio record obtained from the md97 2143 core western pacific ocean this new be ratio series yields a record of gdm variations covering the early brunhes and mid to late matuyama time period i e 700 2140 ka independently from the relative paleointensity rpi record obtained from the same core that can be compared with available rpi records and stacks stratigraphic offsets measured between the be ratio peaks and the corresponding rpi minima reach 2 to 14 cm they can be assigned to post detrital remanent magnetization pdrm effects leading to magnetization locking in delays varying from 2 to 12 ka in the studied core 10 be overproduction episodes triggered by geomagnetic dipole moment lows gdl linked to polarity reversals and excursions confirm the global control exerted by the gdm on cosmogenic radionuclides production a dipole moment reconstruction derived from the beryllium 10 bedimo was compiled and calibrated using absolute paleointensity data this independent record complements the available paleomagnetic rpi records permitting 1 to overcome the pdrm lock in offsets induced below the mixing layer 2 to confront and increase the robustness and precision of gdm reconstructions and 3 to better constrain the chronology of geomagnetic field instabilities during the mid to late matuyama chron our new 10 be derived inventory is fully compatible with the gdl series linked to geomagnetic polarity reversals and events matuyama brunhes transition jaramillo and olduvai subchron boundaries cobb mountain reunion and it strengthens the occurrence of several excursions kamikatsura santa rosa punaruu bjorn gilsa gardar that were until now reported from only sparse locations

the southeastern churchill province synthesis of a paleoproterozoic transpressional orogen the paleoproterozoic southeastern churchill province secp is located in the northeastern canadian shield of labrador and quebec the secp formed through the oblique collisions of the archean nain and superior cratons with a third intervening archean block the core zone the belt has a tripartite structure comprising the torngat orogen to formed by nain craton core zone collision in the east the core zone in the centre and the new quebec orogen nqo formed by superior craton core zone collision in the west the secp thus records transpressional development on the flanks of the superior and nain cratons as they indented northwards to form the larger trans hudson nagssugtoqidian orogenic belt to the north principal stages of tectonic development were 1 2 2 2 1 ga crustal rifting of nain and superior cratons 2 ca 1 9 ga subduction under eastern nain craton 3 ca 1 87 1 85 ga collision of nain craton and core zone to form the to 4 1 845 1 820 ga sinistral transpression in the to and subduction under the western core zone and 5 1 82 1 77 ga collision of superior craton and core zone to form the nqo in association with dextral transpression crustal scale cross sections of the secp have been developed from reflection and refraction seismic data the western part of the nqo is dominantly west vergent and associated with an imbricate thick skinned thrust stack that ramps from the base of the crust the core zone is characterized by a 35 40 km thick crust and pervasive east dipping fabrics related to westerly thrusting the to is a narrow doubly vergent belt associated with a 48 km thick crust that forms a crustal root with a moho relief of 12 km the root is interpreted to result from attempted subduction of the core zone under the nain craton possibly as a result of mid crustal wedging by the nain craton the to was the site of intense convergence that resulted in excision of juvenile crust possibly including tectonic removal of the axial magmatic arc as a result the middle to lower levels of the secp consist largely of refractory archean lithosphere this may account for the lack of widespread post collisional plutonism in the secp and the preservation of the to root

crustal structure across the nain makkovik boundary on the continental shelf off labrador from seismic refraction data a detailed seismic refraction profile was shot along the continental shelf off labrador across the boundary between the archean nain province to the north and the proterozoic makkovik orogenic zone to the south a large air gun source was used with five ocean bottom seismometers as receivers the data were analysed by forward modelling of traveltimes and amplitudes and provided a well determined seismic velocity structure of the crust along the profile within the nain province thin postrift sediments are underlain by crust with a p wave velocity of 6 1 km s which increases with depth and reaches 6 6 km s at about 8 km moho is at around 28 km and there is no evidence for a high velocity 7 km s lower crust the p and s wave velocity structure is consistent with a gneissic composition for the archean upper crust and with granulites becoming gradually more mafic with depth for the intermediate and lower crust in the makkovik zone the sediments are thicker and a basement layer of p wave velocity

the 2004 may 28 baladeh earthquake mw 6 2 in the alborz iran overthrusting the south caspian basin margin partitioning of oblique convergence and the seismic hazard of tehran we use teleseismic waveform analysis and locally recorded aftershock data to investigate the source processes of the 2004 baladeh earthquake which is the only substantial earthquake to have occurred in the central alborz mountains of iran in the modern instrumental era the earthquake involved slip at 10 30 km depth with a south dipping aftershock zone also restricted to the range 10 30 km which is unusually deep for iran these observations are consistent with co seismic slip on a south dipping thrust that projects to the surface at the sharp topographic front on the north side of the alborz this line is often called the khazar fault and is assumed to be a south dipping thrust which bounds the north side of the alborz range and the south side of the south caspian basin though its actual structure and significance are not well understood the lack of shallower aftershocks may be due to the thick pile of saturated overpressured sediments in the south caspian basin that are being overthrust by the alborz a well determined earthquake slip vector in a direction different from the overall shortening direction across the range determined by gps confirms a spatial separation partitioning of left lateral strike slip and thrust faulting in the alborz these strike slip and thrust fault systems do not intersect within the seismogenic layer on the north side though they may do so on the south the earthquake affected the capital tehran and reveals a seismic threat posed by earthquakes north of the alborz located on south dipping thrusts as well as by earthquakes on the south side of the range closer to the city

crustal structure at the se greenland margin from wide angle and normal incidence seismic data abstract results from a seismic refraction and reflection line along the southeast coast of greenland give information on both the precambrian structures on the greenland continent and on the effects of the tertiary breakup of the north atlantic three seismic stations on the greenland coast recorded the airgun shots from a 279 km reflection seismic line approximately 20 km offshore the maximum offset recorded was 313 km the wide angle data show crustal thickness variation from 39 km in the south to 49 km in the north with an 8 to 17 km thick high velocity 7 5 km s layer at the base of the crust interpreted as underplating related to the opening of the north atlantic in the tertiary the boundary between the early proterozoic ketilidian orogen in the south and the archaean block to the north show little variation in seismic velocities and the reflection pattern suggests that the archaean underlies the ketilidian at depth we see no evidence that the julianehab batholith at the boundary between rocks of the ketilidian orogen and the north atlantic block is a deep structure

global comparisons of earthquake source spectra a key question in earthquake seismology is whether earthquake sources in different tectonic regions are functionally equivalent in terms of radiated ground motion this article compares the apparent earthquake source radiation for six different regions japan mexico turkey california british columbia b c western canada and eastern north america ena the source path and site effects were consistently separated to the extent possible to allow interregional comparisons of apparent source radiation large fourier spectra databases were created for both horizontal and vertical components for earthquakes with magnitude between 2 5 and 7 5 regional parameters characterizing source path and site properties including crustal and near surface site amplifications were investigated to isolate apparent source spectra based on the vertical component spectra at a reference distance of 1 km a new magnitude scale m 1 was defined from the spectral level at 1 hz the magnitude scale m 1 is a natural complement to existing magnitude scales that measure the low frequency and high frequency spectral amplitudes moment magnitude m and high frequency m hf respectively in all regions the apparent source spectra for small to moderate earthquakes m 6 show good agreement with the single corner brune 1970 1971 point source model 2 spectrum suggesting a stress parameter of about 100 bars 10 mpa a two corner source model better matches the spectra for large magnitude earthquakes m 6 especially at intermediate frequencies for small to moderate events the two source models are nearly equal apparent source spectra from small to moderate earthquakes e g m 1 3 5 6 5 suggest a general similarity between the different tectonic regions studied although minor discrepancies appear in some cases there are no noticeable regional characteristics no evident depth effects were found on the apparent source spectra manuscript received 1 april 2001

calibration of the specific barrier model to earthquakes of different tectonic regions the specific barrier model proposed and developed by papageorgiou and aki 1983a b 1985 provides the most complete yet parsimonious self consis tent description of the faulting process it applies both in the near fault and in the far field region thus allowing for consistent ground motion simulations over the entire frequency range and for all distances of engineering interest the model has been implemented in the stochastic method and calibrated with extended databases of response spectral amplitudes from earthquakes of intraplate regions mainly east ern north america events interplate regions and regions of tectonic extension spu dich et al 1999 database the ensemble average value of a key parameter of the specific barrier model the local stress drop dr l i s 161 bars for interplate earth quakes 114 bars for extensional regime earthquakes and 180 bars for intraplate earthquakes the high frequency source spectral levels of interplate and extensional regime earthquakes deviate significantly from self similar scaling the deviation is most likely caused by the effective source area and or irregularities in the rupture kinematics we account for their overall effects through a high frequency source complexity factor f in the source spectrum of the specific barrier model as a result inter and intraplate source spectra show similar high frequency levels at moderate magnitudes but intraplate earthquakes have higher spectral levels at the larger mag nitudes the interplate soil residuals show clear signs of nonlinear site response whereas only slight signs of such nonlinearity are observed for the extensional data set the regional models calibrated in this study are in reasonably good agreement with other regional attenuation relationships and provide a reliable and physically realistic yet computationally efficient way to model strong ground motions with implications for seismic hazard and risk analysis online material strong motion station and event station pair information

coupling of palaeoceanographic shifts and changes in marine reservoir ages off north iceland through the last millennium abstract our study of spatial and temporal marine reservoir age changes on the north icelandic shelf is based on two high resolution 1000 year sedimentary records from images core sites md99 2273 and md99 2275 located at 650 m and 470 water depths respectively the sedimentary and fossil record is extremely sensitive to past oceanographic and climatic changes in this region it has been demonstrated that the position of the polar front across the shelf has been very dynamic through the last millennium and our results show that deviations in the marine reservoir ages can be used as a palaeoceanographic tool for tracing changes in the water masses the presence of historically and terrestrially dated tephra markers from icelandic volcanoes makes it possible to construct a reliable tephrochronological age depth model for each of the marine sediment cores a comparison of these age models with age models based on calibrated ams radiocarbon age determinations of marine molluscs displays major deviations through the last 1000 years the discrepancy between the two types of age models shows that there was an average reservoir age of about 650 years at md99 2273 and 590 years at site md99 2275 with considerable changes through time this is suggested to be related to a generally high but fluctuating inflow of apparently old polar waters to the area a comparison with the distribution of two benthic foraminiferal indicator species in the cores one for polar and arctic water masses islandiella norcrossi and one for atlantic water cassidulina neoteretis as well as with different proxies for sea ice cover in the area shows that there was an increase in the penetration of cold polar and arctic water masses after ad 1300 i e during the little ice age at both sites resulting in an increase in the reservoir ages relatively high reservoir ages throughout the last millennium at the deepest core site md99 2273 can be attributed to a continuously high polar water influence at the sea floor the location of this site to the west of the kolbeinsey ridge which has acted as a submarine barrier throughout the holocene may have contributed to a relatively high influence of cold denmark strait overflow water in this area

amphibole and mica 40ar 39ar ages from the kaipokok and aillik domains makkovik province labrador towards a characterization of back arc processes in the paleoproterozoic we report amphibole and mica 40ar 39ar ages from the makkovik province amphibole ages from metamorphic rocks decrease towards the interior of the province indicating a first order pattern of monotonic cooling with progressive migration of the province into a more distal back arc location the amphibole data in combination with muscovite ages reveal a second order pattern consisting of four stages corresponding to changing spatial and temporal configurations of plutonism and deformation 1 the western kaipokok domain cooled through muscovite closure by 1810 ma long after the cessation of arc magmatism 2 the kaipokok bay shear zone bounding the kaipokok and aillik domains cooled through amphibole closure during 1805 1780 ma synchronous with emplacement of syn tectonic granitoid plutons 3 between 1740 and 1700 ma greenschist facies shearing occurred along the boundary between the kaipokok domain and nain province synchronous with a type plutonism and localized shearing in the western kaipoko

crustal structure of the grenville province in southeastern labrador from refraction seismic data evidence for a high velocity lower crustal wedge the crustal structure of the eastern grenville and makkovik provinces was determined using two onshore offshore refraction seismic lines of the lithoprobe eastern canadian shield onshore offshore transect ecsoot a gravity high in the hawke river terrane correlates with increased p wave velocities in the upper 30 km of the crust 6 2 6 7 km s in the upper and middle crust and 6 9 7 1 km s below which we interpret as structure inherited from the labradorian orogen velocities in the adjacent groswater bay terrane are 6 0 6 55 km s in the upper and middle crust and 6 6 6 95 km s in the lower crust the entire grenville crust is underlain by a 15 20 km thick high velocity lower crustal hvlc wedge consisting of an upper layer 7 1 7 4 km s and a lower layer 7 6 7 8 km s the hvlc wedge is interpreted as an underplated layer formed during iapetan rifting this interpretation is based on the correlation with the 615 ma long range dykes onshore and the eastward termination of the wedge at the cartwright

a 17 000 yr paleomagnetic secular variation record from the southeast alaskan margin regional and global correlations high resolution sedimentary records on two cores from the gulf of alaska margin allow development of a 17 400 yr reconstruction of paleomagnetic secular variation psv general agreement between the two records on their independent chronologies confirms that local psv is recorded demonstrating that such archives notwithstanding complexities due to variable sedimentary regimes deposition rates and diagenetic conditions provide meaningful information on past changes of the geomagnetic field comparisons with other independently dated sedimentary paleomagnetic records from the ne pacific indicate largely coherent inclination records that in combination create a ne pacific sedimentary inclination anomaly stack nepsias capturing the common signal over an area spanning 30 longitude and latitude from alaska through oregon to hawaii comparisons of nepsias with high quality declination records from the northern north atlantic nna show that negative shallow inclination anomalies in nepsias are associated with eastward nna declinations while positive steep inclination anomalies in nepsias are associated with westward nna declinations comparison of these directional records to regional geomagnetic intensities over the past 3000 yrs in north america and back nearly 8000 yrs in the euro mediterranean region are consistent with a driving mechanism of oscillations in the relative strength of the north american and euro mediterranean flux lobes the persistence of these dynamics through the holocene implicates a long lived organizing structure likely imposed on the geomagnetic field by the lower mantle and or inner core these observations underscore a fundamental connection between directional psv in the north pacific with that of the north atlantic supporting the potential for long distance correlation of directional psv as a chronostratigraphic tool

correlation chart of the proterozoic assembly of the northeastern canadian greenland shield chart 1 correlation chart of the proterozoic assembly of the northeastern canadian greenland shield including a location map and reference list

proterozoic orogens of the northeastern canadian shield new information from the lithoprobe ecsoot crustal reflection seismic survey as part of the eastern canadian shield onshore offshore transect ecsoot lithoprobe acquired 1250 km of deep seismic reflection data along the coast of labrador and across ungava bay to image evidence of proterozoic crustal accretion to the archean nuclei of the nain and superior provinces of the canadian shield the relatively pristine archean crust of the nain province has low reflectivity and generally lacks systematic reflector orientations reworking of archean crust on the margins of the makkovik province has little effect on this weak signature in contrast the archean crust in the eastern churchill rae province appears to have been overprinted by a strongly developed whole crustal easterly dipping reflection fabric interpreted to result from proterozoic collision of the nain and superior provinces in the paired new quebec and torngat orogens juvenile proterozoic crust in the makkovik and grenville provinces also shows strong whole crustal dipping reflection fabrics interpretable as outw

an empirical study of earthquake source spectra for california earthquakes we generalize source path and site effects for california earthquakes as functions of magnitude and distance based on regression analysis of 1000 fourier acceleration spectra from 43 california earthquakes in the moment magnitude range from 4 4 to 7 4 recorded at rupture distances from 1 to 200 km empirically derived source spectra for california earthquakes are generally inconsistent with the spectral shape implied by a brune omega squared point source model this is manifested by magnitude and frequency dependence of the brune model parameters for ex ample the brune stress parameter that best matches the data at high frequencies decreases from a value of about 120 bars at m5 5 to a value near 50 bars at m7 5 at frequencies below 1 hz though source spectra have much lower spectral ampli tudes than predicted by the brune point source model for these values of stress this discrepancy grows with increasing magnitude finite fault simulations indicate that this is a consequence of the breakdown of the validity of the point source approxi mation for large ruptures a stochastic finite fault model in which the fault is dis cretized as a number of subfaults each of which is represented by a brune omega squared point source correctly matches the observed spectral shapes and amplitudes the spectral decay parameter kappa representing average near surface attenuation of high frequency motion at rock sites increases with increasing magnitude from values near 0 035 sec at m5 5 to 0 050 sec at m7 5 magnitude dependence of kappa may be interpreted as evidence of nonlinearity for typical california sites subjected to strong ground motion comparisons of our empirical source spectra for california to corresponding spec tra for eastern north america suggest that the spectral amplitudes are similar in the two regions for low frequency motions f 2 hz for m5 5 f 0 5 hz for m7 5 for equivalent crustal conditions the eastern events appear to have enhanced high frequency near source amplitudes relative to the california events this is particularly pronounced for large magnitude earthquakes

depth of post depositional remanence acquisition in deep sea sediments a case study of the brunhes matuyama reversal and oxygen isotopic stage 19 1 although post depositional magnetization pdrm in deep sea sediments appears to be acquired during the earliest stages of sediment compaction the natural variability of the pdrm lock in depth in deep sea sediments is poorly understood and as yet unquantified here we consider variations in the relative stratigraphic positions of oxygen isotopic interglacial stage 19 1 and the brunhes matuyama b m chronozone reversal for eight deep sea sediment cores results from a similar study of the displacement between a widespread microtektite layer and the b m boundary are also included 1 the pdrm lock in depth and the temporal relationships between the b m and stage 19 1 datums can be determined from the offsets between the paleomagnetic and the isotopic and microtektite stratigraphies for cores with sedimentation rates greater than 1 cm kyr a the depth offset between the paleomagnetic and isotopic datums is a linear function of sedimentation rate a simple model r 2 0 77 demonstrates that 1 pdrm acquisition occurs 16 cm below the sediment surface and 2 the b m reversal occurs 6 kyr 4 2 kyr after the stage 19 1 datum and the stage 19 1 datum occurs 9 kyr 4 3 kyr after the deposition of the australasian microtektite strewnfield this example argues against simple geophysical models linking geomagnetic field reversals to climate change or impact events the b m boundary is anomalously deep 30 50 cm in very low accumulation rate sediments 1 cm kyr 1 and this may reflect the unusual physical properties of these sediments a review of the geotechnical literature suggests that very low accumulation rate sediments have sufficient time to develop enhanced interparticle rigidity structural strength which inhibits early compaction and hence pdrm acquisition

millennial scale instability in the geomagnetic field prior to the matuyama brunhes reversal changes in the earth s magnetic field have global significance that reach from the outer core extending out to the uppermost atmosphere paleomagnetic records derived from sedimentary and volcanic sequences provide important insights into the geodynamo processes that govern the largest geomagnetic changes polarity reversals but dating uncertainties have hindered progress in this understanding here we report a paleomagnetic record from multiple lava flows on tahiti that bracket the matuyama brunhes m b polarity reversal 771 thousand years ago our high precision 40 ar 39 ar ages constrain several rapid and short lived changes in field orientation up to 33 000 years prior to the m b reversal these changes are similar to ones identified in other less well dated lava flows in maui chile and la palma that occurred during an extended period of reduced field strength recorded in sediments we use a simple stochastic model to show that these rapid polarity changes are highly attenuated in sediment records with low sedimentation rates this prolonged 33 000 year period of reduced field strength and increased geomagnetic instability supports models that show frequent centennial to millennial scale polarity changes in the presence of a strongly weakened dipole field

deciphering records of geomagnetic reversals polarity reversals of the geomagnetic field are a major feature of the earth s dynamo questions remain regarding the dynamical processes that give rise to reversals and the properties of the geomagnetic field during a polarity transition a large number of paleomagnetic reversal records have been acquired during the past 50 years in order to better constrain the structure and geometry of the transitional field in addition over the past two decades numerical dynamo simulations have also provided insights into the reversal mechanism yet despite the large paleomagnetic database controversial interpretations of records of the transitional field persist they result from two characteristics inherent to all reversals both of which are detrimental to an ambiguous analysis on the one hand the reversal process is rapid and requires adequate temporal resolution on the other hand weak field intensities during a reversal can affect the fidelity of magnetic recording in sedimentary records this paper is aimed at reviewing critically the main reversal features derived from paleomagnetic records and at analyzing some of these features in light of numerical simulations we discuss in detail the fidelity of the signal extracted from paleomagnetic records and pay special attention to their resolution with respect to the timing and mechanisms involved in the magnetization process records from marine sediments dominate the database they give rise to transitional field models that often lead to overinterpret the data consequently we attempt to separate robust results and their subsequent interpretations from those that do not stand on a strong observational footing finally we discuss new avenues that should favor progress to better characterize and understand transitional field behavior

synchronizing holocene lacustrine and marine sediment records using paleomagnetic secular variation high sediment accumulation rates in lacustrine and shallow marine archives around iceland offer the potential to compare high resolution paleoclimatic reconstructions from terrestrial and marine archives however direct comparisons are hampered by difficulties in stratigraphic correlation and in deriving accurate age models for lacustrine archives icelandic paleomagnetic secular variation psv has the potential to synchronize these records here we compare holocene psv from a well dated marine core on the north iceland shelf with psv from two lacustrine archives with comparable sediment accumulation rates hvt03 1a a glacier dominated lake and hak03 1b in a nonglacial catchment geochemically characterized tephra layers combined with unique high amplitude structures in the psv records provide secure tie points every 200 yr once the records are synchronized the chronology from the marine core can be reliably transferred to the two lacustrine records the resultant lacustrine age models reveal large changes in sediment accumulation rate at submillennial scales that escape detection in conventional age models with independent dates every 1 k y sediment accumulation rate changes occur at similar times in both lakes despite very different catchment properties low and regular accumulation rates during the holocene thermal maximum suggest regionally stable vegetated catchments followed by a stepped landscape destabilization during the transition into neoglaciation culminating with maximum sedimentation rates during the little ice age psv allows synchronization between multiple records from nearby marine and lacustrine archives providing improved age models and a means of assessing leads and lags between marine and terrestrial environments

the role of inherited lithospheric heterogeneities in defining the crustal architecture of rifted margins and the magmatic budget during continental breakup during the final stage of continental rifting stretching localizes in the future distal domain where lithospheric necking occurs resulting in continental breakup in magma poor margins the lithospheric necking is accompanied by crustal hyperextension serpentinization and exhumation of mantle lithosphere in the continent ocean transition domain cot in magma rich margins the necking is accomplished by the emplacement of large amounts of volcanics in the cot in the form of seaward dipping wedges of flood basalts sdrs this study examines the factors controlling the final crustal architecture observed in rifted margins and the magmatic budget during continental breakup using observations from the labrador sea the latter shows magma rich breakup with sdrs documented in the north and magma poor breakup with a wide domain of exhumed serpentinized mantle recorded in the south the pre rift strength of the lithosphere defined by the inherited thermal structure composition and thickness of the lithospheric layers controls the structural evolution during rifting while variations in the magmatic budget associated with breakup are controlled primarily by the interaction between the pre rift inheritance the timing and the degree of mantle melting in relation to lithospheric thinning and mantle hydration

late archean evolution of the nain province nain labrador imprint of a collision archean gneisses of the nain province in the nain area labrador comprise narrow septa between intrusions of the mesoproterozoic nain plutonic suite this region occupies a critical position between the northern saglek and southern hopedale blocks of the nain province which experienced distinct thermotectonic histories until the late archean field and geochronological evidence are indicative of a strong late archean thermotectonic overprint that is absent from most of the remainder of the nain province archean gneisses were intruded by granites and mafic dykes at 2578 3 and respectively and subsequently subjected to amphibolite to granulite facies regional metamorphism and ductile shearing at ca 2550 ma granite veins and dykes related to the nain plutonic suite were emplaced at ca 1310 ma the archean events are interpreted to represent the juxtapositioning and final docking of saglek and hopedale blocks to form a single stable cratonic mass during the late archean corroborative evidenc

quaternary magnetic and oxygen isotope stratigraphy in diatom rich sediments of the southern gardar drift iodp site u1304 north atlantic abstract the sediment sequence from integrated ocean drilling program iodp site u1304 53 03 40 n 33 31 78 w water depth 3024 m on the southern gardar drift north atlantic ocean covers the last 1 8 myr with a mean sedimentation rate of 15 cm kyr at site u1304 paleomagnetic directional and relative paleointensity rpi records have been generated for the last 1 5 myr and benthic oxygen isotope data for the last 1 myr the age model for site u1304 was established by matching 18o and rpi data to calibrated reference records prominent intervals of diatom mats at site u1304 are associated with the latter stages of interglacial marine isotope stages mis 9 11 13 15 21 27 and 35 and with the weak glacial of mis 14 the deposition of diatom mats and associated diluted magnetic mineral concentrations leads to weak magnetizations and susceptibilities in these intervals comparison of histograms of paleomagnetic directions and rpi from diatom rich sediments and from surrounding silty clays indicates that although results from diatom rich sediments are more scattered the occurrence of diatom mats does not appreciably distort the paleomagnetic directional and rpi records site u1304 sediments record the matuyama brunhes boundary 772 ka the jaramillo subchron 993 1071 ka and the cobb mountain subchron 1193 1219 ka as well as three apparent geomagnetic excursions in the matuyama chron interpreted as the kamikatsura santa rosa punaruu and gardar excursions at 888 ka 1124 ka and 1463 ka respectively the site u1304 rpi record can be correlated with the piso 1500 rpi stack and with other high resolution rpi records from the north atlantic ocean wavelet analyses performed on the site u1304 rpi record and a new high resolution north atlantic paleointensity stack for the last 1500 kyr hinapis 1500 comprising ocean drilling program odp sites 983 984 and iodp sites u1306 and u1304 did not reveal significant orbital power

holocene break up and reestablishment of the petermann ice tongue northwest greenland abstract over the last decade two major calving events of the petermann ice tongue in northwest greenland have led to speculation on its future stability and contribution to further greenland ice sheet mass loss however it has been unclear if these events are anomalous or typical within the context of limited historical observations we extend the historical record of the floating ice tongue using the stratigraphy of petermann fjord sediments to provide a longer term perspective computed tomography ct scans x ray fluorescence xrf scans ice rafted debris ird counts and the magnetic properties of specific particle size fractions constrain changes in depositional processes and sediment sources at our core sites allowing for reconstructions of past behavior of the petermann ice tongue radiocarbon dating of foraminifera 210pb and paleomagnetic secular variation psv provide age control and help to address uncertainties in radiocarbon reservoir ages a floating ice tongue in petermann fjord formed in late glacial time as petermann glacier retreated from an advanced grounded position this paleo ice tongue broke up during the early holocene when high northern latitude summer insolation was higher than present after gradual regrowth of the ice tongue associated with regional cooling the ice tongue reached its historical extent only within the last millennium little or no ice tongue was present for nearly 5000 years during the middle holocene when decadal mean regional temperatures are estimated to be 0 8 2 9 c higher than preindustrial 1750 ce and seasonal sea ice in the lincoln sea was reduced this pre historical behavior shows that recent anthropogenic warming may already be in the range of ice tongue instability and future projected warming increases the risk of ice tongue break up by the mid 21st century

a test for the presence of covariance between time uncertain series of data with application to the dongge cave speleothem and atmospheric radiocarbon records 1 statistical measures of the relationships between time series are generally altered by the presence of errors in timing i e when applied to time uncertain series for example the covariance sampled between two time series which in truth covary will generally be decreased by errors in timing most previous work on this subject has sought to maximize some goodness of fit between time uncertain series either heuristically or through more quantitative methods however there is a danger that unrelated records can be made to appear to covary by time adjustment here we propose a statistical test for the presence of covariance between time uncertain series wherein the probability of obtaining a maximum covariance from randomly realized time uncertain series is assessed using the theory of order statistics the results of this analytical method provide insight into the influence of timing errors upon covariance and are shown to be consistent with results derived from a monte carlo procedure we apply this methodology to evaluate the covariance between a time uncertain stalagmite record and atmospheric radiocarbon during the holocene and find contradictory to previous interpretation that there is insignificant covariance between the two at the 95 confidence level

the influence of high latitude flux lobes on the holocene paleomagnetic record of iodp site u1305 and the northern north atlantic 1 paleomagnetic analysis and radiocarbon dating of an expanded holocene deep sea sediment sequence recovered by integrated ocean drilling program iodp expedition 303 from labrador sea site u1305 lat 57 28 5 n long 48 31 8 w water depth 3459 m provides insights into mechanisms that drive both paleomagnetic secular variation psv and magnetization acquisition in deep sea sediments seventeen radiocarbon dates on planktonic foraminifera define postglacial ca 8 ka sedimentation rates as ranging from 35 to 90 cm kyr alternating field af demagnetization of u channel samples show that these homogeneous sediments preserve a strong stable and consistently well defined component magnetization normalized remanence records pass reliability criteria for relative paleointensity rpi estimates assuming that the age of magnetization is most accurately defined by well dated psv records with the highest sedimentation rates allows us to estimate and correct for temporal offsets at site u1305 interpreted to result from postdepositional remanence acquisition at a depth of 20 cm comparisons indicate that the northern north atlantic psv and rpi records are more consistent with european than north american records and the evolution of virtual geomagnetic poles vgp are temporally and longitudinally similar to global reconstructions though with much larger latitudinal variation the largest deviations from a geocentric axial dipole gad are observed during times of the highest intensities in contrast to the usual assumption these observations are consistent with the idea that psv in the north atlantic and elsewhere during the holocene results from temporal oscillations of high latitude flux concentrations at a few recurrent locations

modeling wide angle seismic data for crustal structure southeastern grenville province a modeling methodology for obtaining two dimensional 2 d crustal structure from wide angle seismic data is applied to data from the southeastern grenville province pre modeling steps include 1 assignment of arrival pick uncertainties for appropriate data fitting and weighting using an empirical relationship based on signal to noise ratio 2 using a modified form of travel time reciprocity to avoid unreasonable levels of model heterogeneity and 3 identifying data unsuitable for 2 d modeling the goal of the travel time inversion amplitude modeling approach is to obtain a minimum structure and minimum parameter model that takes into account both horizontal and vertical variations in the resolution of typical wide angle data each step of a layer stripping procedure involves a series of inversions in which a one dimensional or simple starting model is improved with additional velocity and or interface nodes until a satisfactory trade off between travel time fit parameter resolution and complete ray coverage of all source receiver pairs is achieved using zero vertical velocity gradient layers and head waves during preliminary first arrival inversion can 1 decrease the number of intermediate models 2 allow greater lateral heterogeneity to be imaged and 3 simplify incorporation of amplitude modeling constraints into the final model using amplitude distance curves allows quantitative modeling of the relative amplitude and offset variations of phases discrepancies between observed and calculated reflection amplitudes are used to infer fundamental non step like velocity changes at layer boundaries later arrivals due to unresolved velocity anomalies are modeled using reflecting segments that float within the model without an associated velocity structure these reflectors provide a spatial image like that obtained from vertical incidence reflection data as opposed to a velocity image the model of grenville crustal structure is more detailed than a model obtained from a previous interpretation of the data and includes elements analogous to those imaged in nearby deep reflection data a crustal scale zone of wide angle reflectors with an average easterly apparent dip of 13 defines a major grenvillian terrane boundary

wide angle seismic imaging of pristine archean crust in the nain province labrador a detailed refraction wide angle reflection seismic experiment was carried out in northern labrador to determine the velocity structure of relatively unaltered archean crust in the nain province six 3 component land seismometers were used to record an airgun source along a profile parallel to the coast forward modeling of traveltimes and amplitudes yields a p and s wave velocity model that shows two crustal blocks separated by a fault magnetic data suggest but do not prove that the fault is the offshore continuation of the handy fault a southwards thickening of the lower crust across the fault indicates that a transcurrent component might have been associated with the faulting the total crustal thickness is 33 km to the north and 38 km to the south of the fault the presence of pms reflections imply a sharp transition at the moho upper crustal velocities of 5 8 6 3 km s and poisson s ratios of 0 20 and 0 24 north and south of the fault respectively are consistent with a gneissic composition

a first order correction to minimize environmental influence in sedimentary records of relative paleointensity of the geomagnetic field we propose a simple method to minimize the residual environmental influence which may exist in paleointensity records obtained from sediments it consists of adding or subtracting a fraction of the normalizer arm or irm to the normalized intensity record so that the corrected record exhibits fluctuations uncorrelated with those of the normalizer this method was tested with paleointensity records obtained from two cores taken at midlatitude in the south indian ocean cores eastward of the kerguelen plateau for each core correction increases the correlation between the three normalized intensity records nrm nrm arm and nrm irm it also reduces the coherence between paleointensity records and bulk magnetic parameters used as normalizers although it has to be used with caution this correction may improve paleointensity records by reducing nongeomagnetic influence it can also be used to detect intervals where paleointensity may be unreliable i e intervals where large differences between uncorrected and corrected records are observed before and after correction

marine isotope stage mis 8 millennial variability stratigraphically identical to mis 3 the marine isotope stage mis 3 stratigraphy is highly robust and was reproduced during another period mis 8 6 global ice volume was similar during mis 8 6 to mis 3 60 to 90 m sea level equivalent but the milankovitch insolation forcing was different implying that earth s predisposition to millennial internal variability is controlled by the configuration of the major ice sheets the involvement of additional factors cannot be ruled out but by identifying several such periods using new deep ice cores from dome concordia and dome fuji antarctica as well as the marine record we may isolate the factors predisposing earth to these highly significant modes of climate variability

environmental factors as complicating influences in the recovery of quantitative geomagnetic field paleointensity estimates from sediments we have recently recovered replicate records of the earth s magnetic field relative paleointensity for 12 71 000 years bp from marine sediments of the western north atlantic ocean our records are remarkably similar to two other recently published relative paleointensity records from marine and lacustrine sediments in europe this suggests to us that even over this wide region sediments may serve as reasonable correlatable recorders of fluctuations in the strength of the earth s magnetic field if this is true then similar records from around the globe could eventually provide valuable information about the deep earth processes which create the field we note however that our intensity record is significantly correlated with the down core ratio of magnetic susceptibility to anhysteretic remanent magnetism arm this ratio is a measure of the relative grain size of magnetite the primary magnetic mineral in these sediments and as such is a sediment magnetic property which is controlled only by the local depositional environment and indirectly by global climate we are concerned that the european records may be similarly biased by climatic or other environmental factors possibly synchronous with ours we caution that extreme care must be taken to understand and remove any such magnetic influences from sediment relative paleointensity records before they are used as quantitative estimators of the past intensity of the earth s magnetic field