geometric changes of plate boundaries along part of the northern dead sea transform geochronologic and paleomagnetic evidence the korazim block an elevated pressure ridge along with the faults bounding the sea of galilee and hula pull apart basins are internal structures of the dead sea transform plate boundary k ar dates and paleomagnetic data from basalt flows and sedimentary strata are utilized to determine the stratigraphy and the deformation of the region the integration of the two methods together with structural data yield high resolution age determination of the stratigraphic sequence and provides insight into the timing and style of deformation the korazim block is covered in its southern and central areas by pliocene basalt flows the cover basalt 5 1 3 6 ma and ruman basalt 2 9 2 2 ma formations and in its northern area by basalts yarda basalt 0 9 0 8 ma and sediments 3 0 2 3 and 1 6 0 9 ma of plio pleistocene age the magnetic polarity of a total of 29 lava flows and intercalated sedimentary strata is mixed the mean magnetic declination of the 29 sites shows an anomalous direction which yields 11 4 4 0 counterclockwise rotation with respect to the expected field the mean inclination of 46 0 4 1 does not significantly deviate from the expected field the anomalous declination is interpreted as the consequence of faulting and block rotation accommodating internal deformation this deformation did not begin before late pliocene to early pleistocene probably not before 0 9 ma and may still be active simultaneous right lateral slip of 50 175 m with a small reverse component of 30 90 m along nw trending faults and 11 counterclockwise rotation created internal deformation of the korazim pressure ridge this deformation is the consequence of left lateral slip along the almagor strand of the dead sea transform subparallel to the plate slip vector thus a compression component was formed which presumably led to the internal deformation of the korazim block the recent deformation of the korazim block in relation to the age of adjacent structures and to the age of the dead sea transform permits temporal resolution of geometric changes which must typify transform boundaries such as the dead sea transform internal structures within transforms may change along with the transform internal geometry and become inactive while new structural features may develop

dating crystalline groundmass separates of altered cretaceous seamount basalts by the 40ar 39ar incremental heating technique abstract alteration of submarine basalts compromises geochronology using conventional k ar and 40 ar 39 ar techniques to help overcome these problems we re evaluated the potential of groundmass dating techniques incremental heating on acid leached groundmass samples following an overnight bakeout at 200 c and using high resolution heating schedules eliminated most of the low temperature alteration effects in the submarine basalts studied more than 75 of the groundmass analyses n 32 display accurate age plateaus consisting of 30 70 of the total amount of 39 ar k released more than 50 of the analyses have plateau ages concordant with their total fusion ages implying minor or proportional loss of radiogenic 40 ar and 39 ar k overall we could show a high degree of coherence between ages of groundmass separates and comagmatic phenocrysts this suggests that the dating of aphyric basalts which previously has proven problematic can be accomplished with increasing confidence as well adding these rock types to the list of datable submarine basalts significantly enhances our ability to understand the eruptive history of linear submarine volcanic chains

numerical analysis of the mobility of the palos verdes debris avalanche california and its implication for the generation of tsunamis abstract analysis of morphology failure and post failure stages of the palos verdes debris avalanche reveals that it may have triggered a significant tsunami wave our analysis of the failure itself indicates that the slope is stable under aseismic conditions but that a major earthquake with a magnitude around 7 could have triggered the slide a post failure analysis considering the debris avalanche as a bi linear flow shows that peak velocities of up to 45 m s could have been reached and that the initial movement involved a mass of rock less than 10 km wide 1 km long and about 50 80 m thick initial wave height estimates vary from 10 to 50 m tsunami waves propagating to the local shoreline would be significantly smaller such a range demonstrates our lack of proper knowledge of the transition from failure to post failure behavior related to mass movements further investigations and analyses of terrestrial and submarine evidence are required for a proper hazard assessment related to tsunami generation in the los angeles area

high resolution 40ar 39ar dating of the oldest oceanic basement basalts in the western pacific basin 1 we report new 40ar 39ar ages for the oldest pacific oceanic floor at ocean drilling program site 801c in the pigafetta basin and site 1149d close to the izu bonin subduction zone in the nadezhda basin these ages were determined by applying high resolution incremental heating experiments including 15 30 heating steps to better resolve the primary argon signal from interfering alteration signatures in these low potassium ocean crust basalts combined with previous results from pringle 1992 for site 801b and 801c we arrive at a multistage history for the formation of the pigafetta ocean crust the oldest part of the pacific plate was formed at the spreading ridges at 167 4 1 4 3 4 ma n 2 2 internal absolute error offering an important calibration point on the geological reversal timescale grts since it represents the old end of the mesozoic magnetic anomalies this mid ocean ridge basalt sequence however is overlain by more tholeiites and alkali basalts that were formed 7 3 1 5 myr later around 160 1 0 6 ma n 7 2 internal error the older age group is confirmed independently by radiolarian ages ranging from late bajocian to middle bathonian 167 173 ma bartolini and larson 2001 and by profound differences in the structural characteristics of this basement section pockalny and larson 2003 thin layers comprising hydrothermal deposits separate these sequences which in addition to the difference in isotopic age show distinct major and trace element compositions this indicates that key volcanic and hydrothermal activity took place 400 600 km away from the spreading ridges on the basis of a jurassic 66 km myr half spreading rate in the pacific it remains unclear if these processes were active continuously after the initial formation of the pacific oceanic crust but all our observations seem to point to an episodic history site 1149d gives another important calibration point on the grts of 127 0 1 5 3 6 ma n 1 2 internal absolute error for anomaly m12 that is slightly younger when compared to current timescale compilations 134 2 2 1 ma gradstein et al 1995 this might suggest that the dated basalt from site 1149d does not represent the age of the ocean crust formed at its ridge axis it may also be part of the early cretaceous intraplate events that have produced dolerite sills in the pacific crust at sites 800 and 802 around 114 126 ma

asteroid 3628 božněmcova covered with angrite like basalts a detailed analysis of the reflectance spectrum of asteroid 3628 bounimcova previously identified as a possible ordinary chondrite parent body indicates that its surface consists of an assemblage dominated by clinopyroxene and plagioclase feldspar the clinopyroxene is fe 2 bearing likely in the range fs 10 20 with 90 of the fe 2 being present in the m1 crystallographic site spectral type a the clinopyroxene plagioclase feldspar ratio is between 2 and 3 55 75 clinopyroxene 20 33 plagioclase feldspar if olivine is present the clinopyroxene olivine ratio is 3 20 olivine the derived mineralogy of bounimcova is most similar but not identical to the known angrite meteorites the data suggest that bounimcova formed by melting and differentiation of an oxidized chondritic precursor and probably represents an unsampled angrite like body

new 40ar 39ar age progression for the louisville hot spot trail and implications for inter hot spot motion 1 in this study we present 42 new 40ar 39ar incremental heating age determinations that contribute to an updated age progression for the louisville seamount trail louisville is the south pacific counterpart to the hawaiian emperor seamount trail both trails representing intraplate volcanism over the same time interval 80 ma to present and being examples of primary hot spot lineaments our data provide evidence for an age progressive trend from 71 to 21 ma assuming fixed hot spots this makes possible a direct comparison to the hawaiian emperor age progression and the most recent absolute plate motion apm model wk08g of wessel and kroenke 2008 we observe that for the louisville seamount trail the measured ages are systematically older relative to both the wk08g model predictions and hawaiian seamount ages with offsets ranging up to 6 myr taking into account the uncertainty about the duration of eruption and magmatic succession at individual louisville volcanoes these age offsets should be considered minimum estimates as our sampling probably tended to recover the youngest lava flows these large deviations point to either a contribution of inter hot spot motion between the louisville and hawaiian hot spots or to a more easterly location of the louisville hot spot than the one inferred in the wk08g model both scenarios are investigated in this paper whereby the more eastern hot spot location 52 0 s 134 5 w versus 52 4 s 137 2 w reduces the average age offset but still results in a relatively large maximum offset of 3 7 myr when comparing the new ages to the apm models s04p s04g by steinberger et al 2004 that attempt to compensate for the motion of hot spots in the pacific hawaii or globally hawaii louisville reunion and walvis the measured and predicted ages are more in agreement showing only a maximum offset of 2 3 myr with respect to the s04g model at face value these more advanced apm models which consider both plate and hot spot motions therefore provide a better fit to the new louisville age data the fit is particularly good for seamounts younger than 50 ma a period for which there is little predicted motion for the louisville hot spot and little inter hot spot motion with hawaii however discrepancies in the louisville age distance record prior to 50 ma indicate there is an extra source of inter hot spot motion between louisville and the other pacific hot spots that was not corrected for in the global s04g model finally based on six new 40ar 39ar age dates the 169 w bend in the louisville seamount trail seems to have formed at least 3 myr before the formation of the hawaiian emperor bend the timing of the most acute parts of both bends thus appears to be asynchronous which would require other processes e g plume motions than a global plate motion change between 50 and 47 ma to explain these two observations

block rotation by strike slip faulting structural and paleomagnetic evidence geometric analysis shows that motion of sets of strike slip faults should cause block rotation otherwise the distorted fault domain would not fit with its surrounding the sense of the block rotation depends on the sense of fault slip and the spacing and orientation of the faults all of which can be obtained from structural data the predictions of the geometric model can then be independently tested by paleomagnetic measurements the model was tested in northern israel structural data reveal several domains of contemporaneous right and left lateral strike slip faults paleomagnetic measurements show that in a domain of nnw trending left lateral faults blocks rotated 23 3 8 2 clockwise and in a domain of right lateral faults blocks rotated 22 4 9 0 anticlockwise these results are in agreement with the prediction of the geometric analysis of the structure the combined results of the heterogenous deformation of the area resembles a pure shear which allows n s extension by about 1 3 in other domains rotations of 34 6 9 1 and 53 1 11 0 were found in their original orientation the left and right lateral faults intersected at angles of 60 70 enclosing the principal axis of shortening in accordance with theories of brittle failure as a result of block rotations this angle is now larger and may reach 110 the results demonstrate large block rotations in different senses in domains of strike slip faults the independent structural and paleomagnetic data can be interpreted within the framework of a simple geometric model northern israel is considered to be a good model for this efficient mechanism of intraplate deformation which is expected to occur in similar tectonic settings elsewhere

long term geometry of the geomagnetic field for the last five million years an updated secular variation database lee 1983 assembled more than two thousand paleomagnetic directional data from lava flows in 65 sampling sites with ages spanning the last 5 million years constable 1992 recently suggested that the virtual geomagnetic poles vgp derived from this data base have been preferentially located within two antipodal bands of longitude we have reanalyzed and updated the lee data base yielding a total of 3179 data from 86 distinct sites neither the total data set nor various subsets of the data unambiguously show any large and clear maxima in the longitude distribution of vgps the shape of the common site longitude distribution is far more striking with a pronounced and robust minimum close to the common site longitude and secondary maxima about 120 o away from it the former first order feature is equivalent to the far sided effect discovered by wilson 1970 and corresponds to a persistent axial quadrupole amounting to about 5 of the axial dipole as previously found by several authors the latter second order feature could correspond to the vgp biasing effect described by egbert 1992 although its observed amplitude appears larger than predicted part of this effect an asymmetry in the maxima may also correspond to wilson s 1971 right handed effect given only a small axial quadrupolar component and the very uneven actual site distribution the expected longitude distribution of vgps can be calculated this is found to be in good agreement with the paleomagnetic observations i e rather flat with a moderate maximum near 120oe this study emphasizes problems related to the unsatisfactory distribution of sites the amplitude and significance of other terms beyond the dominant axial dipole and significant axial quadrupole contribution remain to be assessed

computer synthesis of geomagnetic palaeosecular variations computer models designed to synthesize palaeosecular variations of the geomagnetic field cast doubt on some widely accepted palaeomagnetic dogmas westward or eastward drift cannot be uniquely deduced from the sense of rotation of the geomagnetic vector the use of unit vectors in statistical analyses leads to systematic errors which render investigation of the global symmetry of the palaeofield more difficult than previously supposed at least some geomagnetic excursions may be regarded as large amplitude secular variations

palaeosecular variation over the past 5 myr based on a new generalized database summary a generalized database of global palaeomagnetic data from 3719 lava flows and thin dykes of age 0 5 ma has been constructed for use with a relational database management system the database includes all data whose virtual geomagnetic poles vgp lie within 45 of the spin axis and can be used for studies of palaeosecular variation and for geomagnetic field modelling because many of these data were collected and processed more than 15 20 years ago each result has been characterized according to the demagnetization procedures carried out analysis of these data in terms of the latitude variation of the angular dispersion of vgps palaeosecular variation from lavas strongly suggests that careful data selection is required and that many of the older studies may need to be redone using more modern methods differences between the angular dispersions for separate normal and reverse polarity data sets confirm that many older studies have not been adequately cleaned magnetically therefore the use of the database for geomagnetic field modelling should be carried out with some caution using a vgp cut off angle that varies with latitude the best data set consists of 2636 results that show a smooth increase of vgp angular dispersion with latitude model g for palaeosecular variation which is based on modelling of the antisymmetric dipole and symmetric quadrupole dynamo families provides a good fit to these results

high resolution 40 ar 39 ar study of mount avital northern golan reconstructing the interaction between volcanism and a drainage system and their impact on eruptive styles we present a high resolution 40ar 39ar data set of a volcanic complex at mt avital northern golan which experienced a transition from strombolian to phreatomagmatic activity previous studies attributed this transition to a change in the drainage basin of a nearby stream due to the damming by a lava flow which resulted in flooding of the eruption site in this study we determined the age of different volcanic phases and events in the history of mt avital as well as that of the damming flow and examined the cause of the transition in light of the new 40ar 39ar ages the 40ar 39ar results show that the history of mt avital includes two main phases of volcanic activity an early phase at ca 800 600 ka and a late phase at 120 95 ka most of the activity of the late phase was continuous within the 40ar 39ar error including the transition to phreatomagmatic explosions which occurred sometime between 115 107 ka probably between 115 113 ka the age of the damming flow is 115 6 3 1 ka which suggests that the volcanic activity immediately reacted to the change in the drainage basin dynamics the activity culminated with lava flow eruptions the latest at 100 4 ka either due to the establishment of the tuff ring levees which prevented access of water to the eruption site or due to the migration of activity to the northern part and then to the southern part of the complex

paleomagnetism of the southwestern u s a recorded by 0 5 ma igneous rocks 1 the issue of permanent nondipole contributions to the time averaged field lies at the very heart of paleomagnetism and the study of the ancient geomagnetic field in this paper we focus on paleomagnetic directional results from igneous rocks of the southwestern u s a in the age range 0 5 ma and investigate both the time averaged field and its variability about the mean value several decades of work in the southwestern united states have resulted in the publication of paleomagnetic data from over 800 individual paleomagnetic sites as part of a new investigation of the san francisco volcanics we collected paleomagnetic samples from 47 lava flows many of which have been previously dated the new data combined with published data are highly scattered contributions to the scatter were considered and we find that removal of data sets from tectonically active areas and judicious selection according to fisher s 1953 precision parameter results in an axially symmetric data distribution with normal and reverse modes that are indistinguishable from antipodal monte carlo simulations suggest that a minimum of 5 samples per site are needed to estimate the precision parameter sufficiently accurately to allow its use as a determinant of data quality numerical simulations from statistical paleosecular variation models indicate the need for several hundred paleomagnetic sites to get an accurate determination of the average field direction and are also used to investigate the directional bias that results from averaging unit vectors rather than using the full field vector average directions for the southwestern u s a show small deviations from a geocentric axial dipole field but these cannot be considered statistically significant virtual geomagnetic pole vgp dispersions are consistent with those from globally distributed observations analyzed by mcelhinny and mcfadden 1997 however a systematic investigation of the effect of imposing a cutoff on vgps with large deviations from the geographic axis indicates that while it may reduce bias in calculating the average direction such a procedure can result in severe underestimates of the variance in the geomagnetic field a more satisfactory solution would be to use an unbiased technique for joint estimation of the mean direction and variance of the field distribution

geomagnetic secular variation in the cretaceous normal superchron and in the jurassic abstract it is now widely thought that geomagnetic polarity reversals occur spontaneously as a result of normal dynamo action rather than being externally triggered if this is the case then it may well be that periods of time in which the geomagnetic reversal frequency was dramatically different were characterised by different styles of secular variation two such periods were the cretaceous normal superchron cns 84 125 ma when the field was dominantly of a single polarity for 40 myr and the jurassic period 145 200 ma when reversals occurred at an average rate of as much as 4 6 myr 1 here we analyse a database of new and published palaeomagnetic directions from lavas emplaced during these periods in order to obtain first order descriptions of the palaeosecular variation psv during these times we then compare these records with one another and with that produced for the period 0 5 ma with average reversal frequency of 4 0 myr 1 our results are more equivocal than those obtained in a previous similar study mcfadden p l merrill r t mcelhinny m w lee s h 1991 reversals of the earths magnetic field and temporal variations of the dynamo families journal of geophysical research solid earth and planets 96 3923 3933 we demonstrate that this is probably a result of the previous study being affected by an artefact of their correction for within site scatter the usefulness of our jurassic record is severely limited by the restricted palaeolatitudinal span of the available data however our record for the cns is sufficient to allow us to conclude that it was likely that secular variation then was different from that in the 0 5 ma period this supports the hypothesis of a link between psv and reversal frequency and therefore endorses psv analysis as a first order tool for determining geomagnetic stability in the past

paleomagnetism and paleosecular variations from the plio pleistocene golan heights volcanic plateau israel author s behar nicole shaar ron tauxe lisa asefaw hanna ebert yael heimann ariel koppers anthony ap ron hagai

osl dating of a pleistocene maar birket ram the golan heights abstract direct dating of maars and their phreatomagmatic deposits is difficult due to the dominance of lithic host rock fragments and glassy particles of the juvenile magma in this paper we demonstrate that optically stimulated luminescence osl dating can be successfully used for age determination of phreatomagmatic deposits we studied the tuff deposit of birket ram a basanitic maar located at the northern edge of the golan heights on the western arabian plate the maar is underlain by a thick section of pleistocene basalts and currently hosts a small lake it is filled by approximately 90 m of lacustrine sediments with radiocarbon ages extrapolated to 108 ka at the base osl was applied to quartz grains extracted from tuffs and paleosols in order to set the time frame of the phreatomagmatism at the site a maximum age constraint of 179 13 ka was determined for a paleosol that underlies the maar ejecta quartz grains from two layers in the tuff section yielded a direct age of 129 6 ka for the phreatomagmatic eruption a younger age of 104 7 ka which was determined for a tuff layer underlying a basaltic flow was attributed to thermal resetting during the lava emplacement this was confirmed by an 40 ar 39 ar age of 101 3 ka determined on the overlying basalt the internal consistency of the osl ages and the agreement with the 40 ar 39 ar age determination as well as with previous estimates demonstrates the potential of osl for maar dating

40 ar 39 ar age limit for an acheulian site in israel the lake of berekhat ram fills a caldera which cuts the pleistocene basalt flows that cover the golan heights1 in israel fig 1 in the north west wall of the crater a sequence of two basalt flows is exposed separated by a reddish brown palaeosol complex 2 m thick scattered through this palaeosol and also found in prominent concentration in one layer near the base are lithic artefacts of an upper acheulian industry we have now obtained a 40ar 39ar date of 233 3 kyr for the overlying flow while the underlying flow does not give a well defined plateau during step heating but gives a range of apparent ages from 290 to 780 kyr with an integrated age of 470 kyr

strength of the geomagnetic field in the cretaceous normal superchron new data from submarine basaltic glass of the troodos ophiolite 1 we present here new paleointensity data from 39 sampling sites collected from the quenched margins of pillow lavas and dikes exposed within the troodos ophiolite similar to 92 ma formed during the cretaceous normal superchron cns a period of approximately 40 million years when the geomagnetic field reversed extremely infrequently if at all monte carlo simulations suggest that a minimum of 25 estimates are necessary for a reasonably robust estimate for the average field strength our data suggest a dipole strength equivalent to the present field or nearly twice the post cns average the mean and standard deviation of the dipole moment 81 43 zam 2 z 10 21 from the 57 data points compiled here agree remarkably well with those predicted from the long paleointensity record derived from dsdp site 522 the new data set for the cns suggests a picture of a strong and stable field during the period of time when it stopped reversing moreover the similarity of the cns data with the present geomagnetic field suggests that it is presently in a state of unusual polarity stability

asynchronous bends in pacific seamount trails a case for extensional volcanism the gilbert ridge and tokelau seamounts are the only seamount trails in the pacific ocean with a sharp 60 bend similar to the hawaii emperor bend heb these two bends should be coeval with the 47 million year old heb if they were formed by stationary hot spots and assuming pacific plate motion only new 40ar 39ar ages indicate that the bends in the gilbert ridge and tokelau seamount trail were formed much earlier than the heb at 67 and 57 million years ago respectively such asynchronous bends cannot be reconciled with the stationary hot spot paradigm possibly suggesting hot spot motion or magmatism caused by short term local lithospheric extension

recent investigations of the 0 5 ma geomagnetic field recorded by lava flows we present a synthesis of 0 5 ma paleomagnetic directional data collected from 17 different locations under the collaborative time averaged geomagnetic field initiative tafi when combined with regional compilations from the northwest united states the southwest united states japan new zealand hawaii mexico south pacific and the indian ocean a data set of over 2000 sites with high quality stable polarity and declination and inclination measurements is obtained this is a more than sevenfold increase over similar quality data in the existing paleosecular variation of recent lavas psvrl data set and has greatly improved spatial sampling the new data set spans 78 s to 53 n and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time averaged field taf and paleosecular variation psv for the brunhes and matuyama chrons and for the 0 5 ma interval combined the brunhes and matuyama chrons exhibit different taf geometries notably smaller departures from a geocentric axial dipole field during the brunhes consistent with higher dipole strength observed from paleointensity data geographical variations in psv are also different for the brunhes and matuyama given the high quality of our data set polarity asymmetries in psv and the taf cannot be attributed to viscous overprints but suggest different underlying field behavior perhaps related to the influence of long lived core mantle boundary conditions on core flow psv as measured by dispersion of virtual geomagnetic poles shows less latitudinal variation than predicted by current statistical psv models or by previous data sets in particular the brunhes data reported here are compatible with a wide range of models from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude discriminating among such models could be helped by increased numbers of low latitude data and new high northern latitude sites tests with other data sets and with simulations indicate that some of the latitudinal signature previously observed in vgp dispersion can be attributed to the inclusion of low quality insufficiently cleaned data with too few samples per site our matuyama data show a stronger dependence of dispersion on latitude than the brunhes data the taf is examined using the variation of inclination anomaly with latitude best fit two parameter models have axial quadrupole contributions of 2 4 of the axial dipole term and axial octupole contributions of 1 5 approximately 2 of the octupole signature is likely the result of bias incurred by averaging unit vectors