the magnetostratigraphy of leg 73 sediments abstract the main objectiveof dsdp leg 73 was to obtain high quality records of major paleooceanographic events in the south atlantic this was achieved by coringsix sites on the african plate the sediments thus recovered span the cenozoic and five of the six sites proved ideally suited for magnetostratigraphic analysis the results presented in this paper and elsewhere in this volume constitute the first opportunity to extend the direct correlation of the magnetostratigraphic and biostratigraphic time scales into the paleogene in deep sea cores the magnetostratigraphic analyses from dsdp leg 73 sediments are presented in this paper the correlation of the magnetostratigraphy to the magnetic polarity time scale provides tight age depth control for the five sites analyzed allowing the accurate calculation of sediment accumulation rates the data presented here represent a remarkable record of the fine scale polarity history of the earth s magnetic field these data place constraints on the interpretation of small scale marine magnetic anomalies which are modelled equally effectively by field intensity fluctuations as polarity reversals at least some of the tiny wiggles correspond to very short polarity units in the magnetostratigraphic record by assuming an axial geocentric dipole the inclination of the time averaged magnetic field recorded in the sediments can be used to calculate the paleolatitude at which the sediments were deposited combining the age and average inclination information available from the magnetostratigraphy we present paleolatitudes versus time for the leg 73 drill sites

the late neogene 87sr 86sr record of lowland himalayan rivers fossil shells and paleosol carbonate from ancestral himalayan river deposits provide a 87 sr 86 sr record of lowland himalayan river water during the late neogene reconstructed 87 sr 86 sr river values increased sharply in the late miocene probably marking the beginning of exhumation of high 87 sr 86 sr metalimestones more in the central than in the western himalayas these results imply that the marine 87 sr 86 sr record may not be a proxy for silicate weathering or consumption of atmospheric co 2 resulting from that weathering

observations and analysis of b type earthquakes explosions and volcanic tremor at pavlof volcano alaska an eight station network of short period seismometers has been operated near pavlof volcano 55 25 n 161 54 w alaska peninsula since 1976 high quality analog to digital and direct digital data were recorded during eruptions in 1980 through 1983 data are analyzed from explosions with distinct air phases b type volcanic earthquakes shallow low frequency events lacking clear s phases and volcanic tremor to determine source and propagation effects between zero and several hundred b type events and explosions per day are recorded with higher numbers during eruptions magnitudes range between 0 5 and 1 3 and b values range between 1 9 0 1 and 2 6 0 1 with no systematic relation to eruptive activity six results are most important 1 data from a recently installed three component station reveal that the highest amplitude waves of the b type events show retrograde elliptical particle motion and normal dispersion and are hence probably rayleigh waves 2 stacking of the events enhances the p wave which has a velocity of 3 1 0 1 km sec 1 3 the explosions and b type events have similar hypocenters as evidenced by signal characteristics and wave arrival times 4 the b type events contain virtually no high frequency energy even when allowing for attenuation effects in particular we estimate the anelastic attenuation coefficient q to have a value of 45 20 for rayleigh waves in the shallow part of the volcano 5 volcanic tremor spectra exhibit evenly spaced narrow peaks which we model as eigenvalues of organ pipe resonance in the magma conduit we estimate the length of the conduit to be a maximum of 1 6 km 6 the volcano shows evidence of extreme lateral variation of velocity and attenuation in its shallow structure

sandstone body architecture and stratigraphic trends in the middle siwalik succession of the jammu area india abstract sedimentological analysis has been carried out on the coarsening upward middle siwalik succession of the jammu area india major sandstone bodies are composed of several storeys and are tens of metres thick continuing along strike for tens of kilometers these sandstone bodies are mostly multistoried representing several channel fill events few thin sandstone bodies are single channel fill events the detailed facies architecture of three of these sandstone bodies is discussed vertical sediment variation over a 1450 m thick succession was studied to document the changes the 550 m thick upper part of the succession has a greater proportion and thicker sandbodies than the underlying 900 m of the succession this is accompanied by coarser grain size and poorer representation of fine grained deposits in the upper part of the succession as compared to lower part in general palaeocurrent trends show a gradual change from ssw in lower part to sse in upper part which may be due to a change in direction of slope the probable explanations for these up section variations are tectonic and climatic change

local earthquake tomography of the jalisco mexico region abstract the states of jalisco colima and michoacan in western mexico overlie the boundary of the subducting rivera and cocos plates presenting an ideal target for seismological inquiry to better understand the resulting mantle flow and regional volcanism the different dips between the two subducting plates are thought to provide a mantle conduit that has contributed to the colima volcanic complex cvc but there is considerable debate on the depth of the rivera plate and width of the resulting conduit with data from the mapping the rivera subduction zone mars and colima deep seismic experiment codex networks two temporary broadband arrays deployed in the region between 2006 and 2008 we inverted for three dimensional p and s wave velocity as well as vp vs structure of the upper 70 km of the crust and mantle in the jalisco region using a newly developed automatic p and s wave picker we increased p picks by 74 and s picks by more than a factor of four compared to a database of manual picks for the 803 earthquakes used in the inversion additional relocated earthquakes extending to the trench are consistent with previous interpretations of the rivera and cocos plate interfaces areas of high vp vs above both subducting slabs suggest the presence of fluids resulting from dehydration of subducted material extensive crustal seismicity occurs near these anomalies a zone of high vp vs is also present under the cvc we also compare the results of different methods for obtaining vp vs a direct inversion for vp vs from s minus p times versus simply dividing the vp model by the vs model we find direct inversions of s minus p times to be more reliable

the occurrence and fission track ages of late neogene and quaternary volcanic sediments siwalik group northern pakistan abstract volcanic sediments now mostly bentonites and bentonitic mudstones occur throughout the late neogene and quaternary siwalik group of northern pakistan a number of these deposits have been dated by the fission track method utilizing zircon phenocrysts from these deposits and provide the chronometric constraints upon which a paleomagnetic stratigraphy is developed for the siwalik group notable in the occurrence of these altered tuff horizons is an apparent mode in their stratigraphic development from approximately 3 0 to 1 5 m y b p which coincides with the period of activity of the dacht e nawar volcanic complex of east central afghanistan fission track ages of certain tuffs for critical areas of northern pakistan are reported herein

seismic structure beneath the rivera subduction zone from finite frequency seismic tomography 1 the subduction zone of western mexico is a unique region on earth where microplate capture and overriding plate disruption are occurring today the young small rivera plate and the adjacent cocos plate are subducting beneath the jalisco block of mexico here we present a p wave tomographic model of the upper mantle to 400 km depth beneath the jalisco block and surrounding regions using teleseismic p waves recorded by the mapping the rivera subduction zone mars and colima volcano deep seismic experiment codex seismic arrays the inversion used 12 188 p wave residuals and finite frequency theory to backproject the 3 d traveltime sensitivity kernels through the model below a depth of 150 km the tomography model shows a clear gap between the rivera and cocos slabs that increases in size with depth the gap between the plates lies beneath the northern part of the colima graben and may be responsible for the location of colima volcano the images indicate that the deep rivera plate is subducting more steeply than does the adjacent cocos plate and also has a more northerly trajection at a depth of about 100 km both the rivera and cocos slabs have increased dips such that the slabs are deeper than 200 km beneath the trans mexican volcanic belt tmvb it is also found that the rivera plate is at roughly 140 km depth beneath the young central jalisco volcanic lineament our images suggest that the rivera plate and westernmost cocos plate have recently rolled back toward the trench this scenario may explain the unusual magmatic activity seen in the tmvb

listric growth faults in the kenya rift valley abstract many of the major faults in the kenya rift valley are curved in section were active over considerable periods and form sets which are related in space and time they can therefore be regarded as systems of listric growth faults the elgeyo fault marks the western limit of rift structures at this latitude and displaces the basement surface by up to about 6 km the kamasia hills are a block rotated above this fault plane movement on the elgeyo fault has been grossly continuous since at least 16 ma ago but deposition of volcanics and sediments has generally kept pace with the growth of the escarpment the kaparaina arch is a rollover anticline on the downthrown side of the saimo fault on the eastern side of the kamasia hills on the eastern side of the rift the block between the bogoria and wasages marmanet faults has shown continued rotation since about 15 ma the pleistocene lavas on the rift floor here show rollover into the bogoria fault and have formed a facing near the top of the escarpment area balancing calculations suggest depths to decollement of 25 km for the elgeyo fault 6 km for the saimo fault and 12 km for the bogoria fault the most direct evidence for the listric nature of the faults is provided by microearthquakes near lake manyara which appear to lie on fault planes connected to surface escarpments

rock magnetic properties and structural development in the core of the hazara kashmir syntaxis ne pakistan paleomagnetic and structural analyses have been conducted on three well exposed sections through a 6 8 km thick pile of molassic red beds murree formation in the lowermost tectonic unit of the hazara kashmir syntaxis micropaleontological age determinations of the lowermost murree sediments indicate late paleocene deposition 55 m y from south to north the sections are situated in the jhelum neelum both in azad kashmir and kaghan northeastern pakistan valleys thermal demagnetization experiments suggest that haematite with high unblocking temperatures carries stable characteristic remanence directions the relationship between finite strain and magnetic fabric was established by mapping deformed reduction spot strain markers and by measuring the anisotropy of magnetic susceptibility ams for the jhelum valley the weakly tectonized murree beds are characterized by flattened ams ellipsoids resulting from diagenetic compaction inclination values suggest that the murree foreland basin started to develop at about 8 n during the early suturing of india and the development of island arcs to the north india has moved northward by at least 2600 km since collision with eurasia in the paleocene declination values suggest 45 of clockwise rotation of the block supporting the murree formation relative to the indian craton for the neelum and kaghan valleys quantitative strain mapping shows a progressive increase of deformation northward nrm directions rotate passively toward the cleavage plane which parallels the foliation of the ams ellipsoids nrm directions and ams ellipsoids are deformed because of superposition of tectonic strain on a primary compactional strain the ams pattern is interpreted in light of this superposition and a regional deformation path from south to north is suggested a tectonic rotation model is proposed which is consistent with the transport directions around the hazara kashmir syntaxis and the rotation of thrust sheets indicated by the nrm data this model relates the convergent transport directions with the mean indentation direction of india into asia

confirmation of a late miocene subchron c4n 2n 1r from the eastern qaidam basin in the ne tibetan plateau

inclination flattening and the geocentric axial dipole hypothesis abstract william gilbert first articulated what has come to be known as the geocentric axial dipole hypothesis the gad hypothesis is the principle on which paleogeographic reconstructions rely to constrain paleolatitude for decades there have been calls for permanent non dipole contributions to the time averaged field recently these have demanded large contributions of the axial octupole which if valid would call into question the general utility of the gad hypothesis in the process of geological recording of the geomagnetic field earth filters distort the directions many processes for example sedimentary inclination flattening and random tilting can lead to a net shallowing of the observed direction therefore inclinations that are shallower than expected from gad can be explained by recording biases northward transport or non dipole geomagnetic fields using paleomagnetic data from the last 5 million years from well constrained lava flow data allows the construction of a statistical geomagnetic field model such a model can predict not only the average expected direction for a given latitude but also the shape of the distribution of directions produced by secular variation the elongation of predicted directions varies as a function of latitude from significantly elongate in the up down direction at the equator to circularly symmetric at the poles sedimentary inclination flattening also works in a predictable manner producing elongations that are stretched side to side and the degree of flattening depending on the inclination of the applied field and a flattening factor f the twin tools of the predicted elongation inclination relationship characteristic of the geomagnetic field for the past 5 million years and the distortion of the directions predicted from sedimentary inclination flattening allows us to find the flattening factor that yields corrected directions with an elongation and average inclination consistent with the statistical field model the method can be tested using sediments deposited in a known field application of the elongation inclination correction method to two magnetostratigraphic data sets from red beds in asia and pakistan brings the inclinations into agreement with those predicted from modern gps measurements and from global paleomagnetic data there appears to be no compelling reason at this time to abandon the geocentric dipole hypothesis which has provided such an excellent working model for so long

magnetic polarity stratigraphy of siwalik group sediments of karnali river section in western nepal the remanent magnetization of siltstones and sandstones sampled at 476 levels sites throughout a 3560 m thick molasse sequence belonging to the siwalik group 0 2015 m lower 2015 3560 m middle has been studied by stepwise thermal demagnetization this section is exposed along the karnali river in nepal the natural remanent magnetization nrm usually consists of two components a viscous or thermoviscous component of recent field origin and an ancient characteristic component chrm the former component is of normal polarity and resides either in goethite unblocking temperature 150 c resistant to afd up to 150 mt or in maghemite unblocking temperature 150 400 c goethite contributes up to 90 per cent of the total intensity in the finer variegated muddy samples belonging to the lower half of the section maghemite content is significant in the grey mud free lithologies from the upper half of the section the main component unblocked in the high temperature range commonly 610 680 c and believed to reside in haematite presumably of mostly detrital origin represents a characteristic remanence chrm the tilt corrected chrm directions at individual sites show antipodal clusters ratio of normal to reverse polarity sites 0 62 and yield mean inclinations recording significant inclination shallowing a feature well recorded in the siwaliks this chrm is interpreted to represent a largely primary detrital remanence the chrm data from 430 sites yield the karnali river magnetic polarity sequence whose correlation with the geomagnetic polarity timescale cande kent 1995 suggests a depositional age of 16 ma younger than chron c5cn 1n to 5 2 ma around the top of chron c3r for the 3560 m section sampled hence the karnali river exposes the oldest part of the siwalik group in nepal estimates of the sediment accumulation rate sar average to 32 9 cm kyr 1 for the 10 8 myr time span of deposition

deriving palaeomagnetic poles from independently assessed inclination and declination data implications for south american poles since 120 ma summary palaeomagnetic poles for a stable continental block are typically defined from a combination of declination and inclination information from several temporally constrained studies poles from regions that have undergone vertical axis rotation as a consequence of tectonics are excluded due to the absence of declination data these poles however do contain useful information in their inclinations we develop a simple but statistically rigorous technique allowing palaeomagnetic poles to be calculated from a mixture of declination and inclination data drawn from localities in the stable continental block and inclination data from the regions disturbed by vertical axis rotation together this provides a larger data set of high quality palaeomagnetic poles from which to calculate reference poles the technique was used to define palaeomagnetic poles for south america for the late cretaceous cenozoic 120 5 ma period data from stable cratonic south america combined with data from africa rotated into a south america reference frame and the andean margin yield reference poles as well as mean poles for the palaeogene and neogene analysis of the data reveals systematic biases in the data set and in particular the fit of the inclination data is poor for most time periods in many cases this situation is improved if the effect of inclination shallowing due to sedimentary depositional processes and subsequent compaction is removed the best fit poles define an apparent polar wander path for south america that is consistent with the global plate reconstruction parameters use of the new poles in studies of tectonic rotation should allow greater temporal and spatial resolution of vertical axis rotation and offer the ability to identify smaller rotations in the andean margin

evolution of miocene fluvial environments eastern potwar plateau northern pakistan the miocene pliocene siwalik group records changing fluvial environments in the himalayan foreland basin the nagri and dhok pathan formations of this group in the eastern potwar plateau northern pakistan comprise relatively thick tens of metres sandstone bodies and mudstones that contain thinner sandstone bodies metres thick and palaeosols thick sandstone bodies extend for kilometres normal to palaeoflow and are composed of large scale stratasets storeys stacked laterally and vertically adjacent to each other sandstone bodies represent single or superimposed braided channel belts and large scale stratasets represent channel bars and fills channel belts had widths of km bankfull discharges on the order of 103 cumecs and braiding parameter up to about 3 individual channel segments had bankfull widths maximum depths and slopes on the order of 102 m 101 m and 10 4 respectively and sinuosities around 1 1 these rivers are comparable to many of those flowing over the megafans of the modern indo gangetic basin and a similar depositional setting is likely thin sandstone bodies within mudstone sequences extend laterally for on the order of 102 m and have lobe wedge sheet and channel form geometries they represent crevasse splays levees and floodplain channels mudstones are relatively bioturbated disrupted and represent mainly floodbasin and lacustrine deposition mudstones and sandstones are extremely disrupted in places showing evidence of prolonged pedogenesis these mature palaeosols are m thick and extend laterally for km lateral and vertical variations in the nature of their horizons apparently depend mainly on deposition rate the 500 m thick nagri formation has a greater proportion and thicker sandstone bodies than the overlying 700 m thick dhok pathan formation the thick sandstone bodies and their large scale stratasets thicken and coarsen through the nagri formation then thin and fine at the base of the dhok pathan formation compacted deposition rates increase with sandstone proportion 0 53 mm year for nagri 0 24 mm year for dhok pathan and palaeosols are not as well developed where deposition rates are high within both formations there are 100 m scale variations representing on the order of 105 years in the proportion and thickness of thick sandstone bodies and tens of m scale alternations of thick sandstone bodies and mudstone sandstone strata that represent on the order of 104 years formation scale stratal variations extend across the potwar plateau for at least 100 km although they may be diachronous however 100 m and smaller scale variations can only be traced laterally for up to tens of km alluvial architecture models indicate that increases in the proportion and thickness of thick sandstone bodies can be explained by increasing channel belt sizes mainly average deposition rate and avulsion frequency on a megafan comparable in size to modern examples 100 m scale variations in thick sandstone body proportion and thickness could result from regional shifts in the position of major channels possibly associated with fan lobes on a single megafan or with separate megafans however such variations could also be related to local changes in subsidence rate or changes in sediment supply to the megafan system formation scale and 100 m scale stratal variations are probably associated with interelated changes in tectonic uplift sediment supply and basin subsidence increased rates of hinterland uplift sediment supply and basin subsidence recorded by the nagri formation may have resulted in diversion of a relatively large river to the area alternatively changing river sizes and sediment supply rates may be related to climate changes affecting the hinterland possibly linked to tectonic uplift climate during deposition of the siwalik group was monsoonal although the deposits contain no direct evidence for climate change independent evidence indicates global cooling throughout the miocene and the possibility of glacial periods e g around 10 8 ma corresponding to base of nagri formation if the higher himalayas were periodically glaciated a mechanism would exist for varying sediment supply to megafans on time scales of 104 105 years although eustatic sea level changes are related to global climatic change they are not directly related to siwalik stratigraphic changes because the shoreline was many 100 km away during the miocene

paleomagnetism of the middle siwalik formations of northern pakistan and rotation of the salt range decollement abstract sampling for paleomagnetic study has been carried out at seven localities in the potwar plateau region of pakistan in the vicinity of the salt range in all 253 sites consisting of three samples per site were studied the sites were taken from the chinji nagri dhok pathan and upper siwalik formations which span the last 14 m y of earth history the formations have been deformed in the ongoing himalayan orogeny and are exposed today in sections which dip at angles of from 10 to 90 in different directions a characteristic direction of magnetization was isolated at all sections by thermal demagnetization at temperatures of from 550 to 650 c the sediments have been folded within the last 2 m y and because of a positive fold test it can be shown that the characteristic magnetization of these sediments was acquired before folding four of the sections have directions of magnetization which have been rotated in a counter clockwise fashion up to 40 however sections taken along the indus river the north flank of the soan syncline and the south flank of the salt range show small counter clockwise rotations of under 10 these observations are in agreement with previous suggestions that strata in the salt range have been moved to the south and rotated in a counter clockwise fashion the observations reported here indicate that the paleomagnetic poles derived from cambrian and permian formations within the salt range have probably also suffered rotation and should be removed from consideration in gondwanaland reconstruction of the paleozoic

seismicity patterns prior to the 2011 el hierro eruption abstract the first fully monitored volcanic eruption in the canary islands started on 10 october 2011 2 km south of the island of el hierro this submarine eruption was preceded by various precursory signals the most evident of which were the seismic series that started in july 2011 consisting of almost 10 000 low magnitude earthquakes located by manual picking this seismicity revealed a 20 km horizontal migration from the north of the island to the south at depths of between 10 and 17 km the deeper events occurring further south we applied a relative location algorithm hypodd to improve hypocentral locations the resulting distribution is more compact slightly shallower and reveals a clearer difference in depths from north to south tests performed to check the reliability of the results gave maximum uncertainties of 400 m in the relocations furthermore new features were found including the origin of the seismicity in the center of the island and the presence of two alternating seismogenic zones in the north of the island during the first month of activity online material figures of waveforms from cross correlation data statistics stereogram of ray parameters ray tracing tests and table of hypodd relocations

a biostratigraphic zonation of the middle and upper siwaliks of the potwar plateau of northern pakistan abstract two biostratigraphic reference sections are designated for the middle and upper siwalik formations of northern pakistan the stratigraphic ranges of 41 mammalian taxa are established in the reference sections and the two sections are correlated to each other and to the standard magnetic polarity time scale by the magnetic polarity stratigraphy the resulting stratigraphic and magnetostratigraphic framework is used to define four contiguous biostratigraphic interval zones the proposed interval zones are bounded by five important faunal events which are identified in the reference sections the four proposed interval zones and the approximate ages in the reference sections of their lower and upper boundaries are 1 the hipparion s l interval zone 7 4 9 5 m y b p 2 the selenoportax lydekkeri interval zone 5 3 7 4 m y b p 3 the hexaprotodon sivalensis interval zone 5 3 2 9 m y b p and 4 the elephas planifrons interval zone 2 9 1 5 m y b p the paleomagnetic and biostratigraphic evidence indicates that the tatrot beds from which pilgrim developed his tatrot faunal zone are between 2 5 and 3 4 m y b p with a preferred maximum age of 2 9 m y b p the biostratigraphic evidence indicates the sethi nagri locality y311 the primary source of pilgrim s nagri faunal zone is in the hipparion s l interval zone the paleomagnetic evidence suggests an approximate age of 9 m y b p a major faunal event at 9 5 m y b p introduced equids suids and large giraffes from eurasia into the indain subcontinent a second more diffuse faunal event between 5 3 and 2 9 m y b p introduced several african taxa as well as equus and cervids from eurasia

array analyses of smks waves and the stratification of earth s outermost core abstract we perform array analyses of smks waves in order to investigate the vp structure of the earth s outermost core for earthquakes recorded by broadband seismometer networks in the world we measure differential travel times between s3ks and s2ks between s4ks and s3ks and between s5ks and s3ks by array techniques the differential times are well fit by a vp model of the earth s outermost core khomc kaneshima and helffrich 2013 differential slownesses of s4ks and s2ks relative to s2ks are also measured for the highest quality data the measured slownesses with unique sensitivity to the outer core 200 400 km below the cmb are matched by khomc these observations consolidate the evidence for the presence at the top of the outer core of a layer that has a distinctively steeper vp gradient than the bulk of the outer core we invert new smks differential time data set by a tau p method and attempt to refine the vp profile of khomc the essential features of khomc are preserved after the model refinement however the newly estimated layer thickness is nearly 450 km which is thicker than that of khomc the vp anomalies relative to prem for the depths 400 800 km below the cmb are less than 0 03 km s consistent with the degree of agreement between different vp models for the depth range

the red bed controversy revisited shape analysis of colorado plateau units suggests long magnetization times several triassic and earliest jurassic sedimentary units from the colorado plateau region have distributions of virtual geomagnetic poles vgps that are highly elongate along the path of apparent polar wander apw this suggests that the remanent magnetizations measured in these units were acquired over an extended period of time possibly approaching 35 m y and are not precisely coeval with the stratigraphic age of the rock comparison with other paleomagnetic studies shows that the observed elongation is not a general attribute of the age of the rock nor is it related to paleolatitude the rocks that yield elongate vgp distributions are dominantly red to brown mudstones and it is possible that their remanence is dominated by a slowly acquired chemical remanent magnetization as suggested by larson et al j geophys res 87 1982 1081 and other authors however several superficially similar units from the colorado plateau have nearly circular vgp distributions the process by which remanence is acquired in clastic sedimentary rocks merits further study d 2002 elsevier science b v all rights reserved

palaeointensity in the pelagic realm marine sediment data compared with archaeomagnetic and lake sediment records summary four box cores collected from the ontong java plateau during the eurydice expedition have been used to make relative geomagnetic palaeointensity measurements rock magnetic measurements on the sediments show that they are characterized by a uniform magnetic mineralogy and that they are suitable for relative intensity estimates these are obtained by normalizing the nrm by an arm imparted in a low dc bias field the palaeoceanographic event known as the preservation spike is used to establish a crude time scale for the record so that it may be compared with other data from the same region and also with global palaeointensity estimates the marine sediment data are quite similar to australian intensity data from lake sediments and archaeomagnetic sources but as might be expected exhibit some obvious differences from the global record

the siwaliks of pakistan time and faunas in a miocene terrestrial setting magnetostratigraphy provides unparalleled chronologic precision for long terrestrial sedimentary records the siwaliks of the potwar plateau northern pakistan present perhaps the best example of a sequence of well dated faunal horizons spanning most of the neogene in a single terrestrial biogeographic province temporal control on fossil localities tied to multiple paleomagnetic sections is ca 0 1 m y less given special conditions of short magnetozone duration or superpositional relationships new correlations are presented for two long sections in the lower siwaliks showing the power and the limitations of mag netostratigraphic correlation within the siwalik depositional setting correlation of potwar fossil localities to those of an east african section also dated paleomagnetically are established with fossil localities from those two biogeographic provinces shown to be coeval on a scale of 10 5 yr the composite biostratigraphy rendered by the thick well exposed well dated potwar siwalik

possibility of a low p wave velocity layer in the outermost core from global smks waveforms abstract a global data set consisting of 1211 smks m 2 waveforms has been analyzed to investigate the radial seismic velocity structure around the core mantle boundary cmb band passed f 0 02 0 1 hz and stacked waveforms coincide with reflectivity synthetic ones for prem very well whereas those for other global models iasp91 ak135 and sp6 yield disagreements a preliminary waveform modeling for the outermost core with prem as initial structure results in a core surface layer with a p wave velocity of 7 95 km s at the core top and thickness of 90 km based on a 3 d global s wave velocity model smks phases collected in this study mainly pierce slightly high velocity regions at the base of the mantle and the predicted differential travel time residuals for s3ks skks are expected to be negative approximately 0 5 s for the averages however positive s3ks skks residuals of which average is 0 3 s are predominant in the stacked waveforms moreover a waveform modeling for the d structure can result in a 30 km thick layer with a 10 s wave velocity reduction at the mantle bottom ulvz in which waveform fitness for the part of mainly s3ks is improved on the other hand the waveform data is not well explained by the lowermost mantle structures with thickness of several hundred kilometers these suggest that the smks data is still affected by the structure of the lowermost several tens kilometers in the mantle that is not sufficiently modeled by a global tomography although the thin low s wave velocity model is not conclusive the possibility of a low p wave velocity layer in the outermost core is remained because the waveform fitness for the part of s4ks is improved by the combination of the ulvz and a 140 km thick layer with a 0 8 p wave velocity reduction at the core top

constraining outermost core velocity with smks waves smks waves m 2 3 4 seismic waves that travel as s waves in the mantle p waves in the core and reflect m 1 times on the underside of the core mantle boundary are well suited for constraining outermost core vp structure this is due to closeness of the mantle paths and also the shallow depth range these waves travel in the outermost core high quality digitized wwssn and canadian network recordings from a deep focus java sea event which sample the outer core beneath the northern pacific the artic and northwestern north america roughly th of the globe are utilized as an example to show the strength of smks waves to resolve outermost core structure s3ks s2ks and s4ks s3ks differential travel times were measured and compared to those from reflectivity synthetics created from core models of past studies constraints and uncertainties in outer core structure using smks are discussed for the event studied the prem core model with possibly a small vp decrease in the outermost 50 km of the core provides a good fit to the data

a study of the outermost liquid core using differential travel times of the sks skks and s3ks phases abstract differential travel times of sks skks and s3ks seismic phases are analysed to specify the structure of the outermost liquid core which is poorly sampled by the pkp waves currently used for core tomography the data are mainly broadband records at epicentral distances of 83 140 they have been selected to obtain a good global earth coverage skks sks and s3ks skks travel time residuals have been computed with respect to the mean earth model preliminary reference earth model prem and corrected for ellipticity the skks sks residuals plotted as a function of epicentral distance exhibit a general positive slope for distances greater than 105 with mean positive values of approximately 2 s at 120 130 c these features may be accounted for by slight perturbations in prem involving a 2 5 s velocity increase in the d layer at the base of the mantle and a small decrease in the p velocity in the upper 800 km of the liquid core the structure in the uppermost 100 km of the core is mainly constrained by the s3ks data a high velocity gradient followed by a low gradient is obtained immediately beneath the core mantle boundary this may suggest the existence of a thin low velocity layer at the top of the liquid core as a result of a possible accumulation of light elements beneath the core mantle boundary the skks sks residuals exhibit strong coherent regional variations whereas the s3ks skks residual variations remain small this indicates that the lateral heterogeneitics at the core side of the core mantle boundary if any are very small the simultaneous analysis of s3ks sks and skks sks residuals suggests a prevailing contribution of long wavelength heterogeneities in d

oxygen isotope evidence of climate change from pedogenic clay minerals in the himalayan molasse abstract in this study we examine the oxygen isotope ratios of smectite and kaolinite in a stratigraphic sequence of paleosols in the siwalik himalayan molasse on the potwar plateau northern pakistan there is a 3 4 o increase in the smectite 18 o values synchronous with a 3 5 o increase in 18 o values of pedogenic calcite at 8 5 6 5 ma previously observed by quade et al 1989 the covariation of the 18 o values of smectite with calcite gd 18 o values through time strongly suggests a pedogenic origin of smectite and that its 18 o values reflect the oxygen isotope ratio of the soil water present during pedogenesis these data show that the 18 o values of clay minerals in paleosols in aggrading sedimentary sequences are viable paleoclimate indicators kaolinite 18 o values are similar to smectite 18 o values as would be expected for pedogenic kaolinite but there is greater variation in these data which we attribute to incomplete mineral separation the measured permil oxygen isotope fractionation between calcite and smectite within individual paleosols is on average 3 o higher than the equilibrium fractionation this isotopic disequilibrium can be explained by the calcite forming predominantly during the dry winter season and the smectite forming predominantly during the wet summer season the synchronous 3 5 o increase in oxygen isotope ratios of smectite and calcite at 8 5 6 5 ma indicates that this 18 o 16 o increase is not due to diagenesis but rather results from a climate change the most likely causes of the 18 o 16 o increase in smectite and calcite are either increased aridity resulting in increased soil water evaporation thus 18 o enrichment and or a change from more continental to more marine sourced precipitation the tibetan plateau which is suggested to have undergone rapid uplift around 8 ma e g harrison et al 1992 may have acted as an effective orographic barrier to low 18 o precipitation originating from central asia causing higher 18 o values of the precipitation falling on the potwar plateau currently it is impossible to distinguish between these two potential causes of the 18 o 16 o increase at 8 5 6 5 ma but these clay mineral 18 o values support that there was a significant climate change in this region

the stably stratified outermost core revisited the p wave velocity structure of the outermost 200 km of the outer core the e region is analyzed using a suite of differential travel times for sks skks and s waves bottoming below within and above this depth range core penetrating phases with paths beneath alaska exhibit similar sks skks differential travel time patterns to phases sampling the core beneath the central pacific but very different sks s differential travel times these patterns can be explained by a core velocity model in which the outermost 50 to 100 km has 1 2 slower p wave velocities than standard earth models perhaps due to a stably stratified chemical boundary layer combined with 3 lateral variations in s wave velocity in the lowermost mantle

seismic detectability of anomalous structure at the top of the earth s outer core with broadband array analysis of smks phases abstract i have examined precisely the differential travel times and waveforms of smks seismic phases propagating under the southern indian ocean obtained from african broadband seismic arrays the smks phases analyzed in this study travel in the mantle with weak heterogeneity confirmed by a global tomographic study for the distance range of 115 135 the smks differential times were obtained from a vespagram a stack intensity on a time slowness diagram and comparison with the vespagram created from synthetic waveforms with prem gives the travel time residual for each event array pair although the residuals of s3ks s2ks times exhibit apparently a systematic dependence on epicentral distance this is likely due to small scale heterogeneity beneath the oceania where is covered by the smks ray entering points at the cmb waveform modeling was applied to a record section with a small travel time residual that suggests a small effect from the mantle heterogeneity on the data set i found that a low velocity zone in the outermost 50 km in the core rather than prem can explain an additional arrival detected just after the s3ks phase this result is still inconclusive because of the small number of data and non uniqueness of the model and ambiguity due to mantle structure however accumulation of the precise measurement described in this study may help the reduction of uncertainty and trade offs

using repeating volcano tectonic earthquakes to track post eruptive activity in the conduit system at redoubt volcano alaska in the year following the end of the 2009 eruption of redoubt volcano alaska four signifi cant swarms of low frequency lowmagnitude m l 0 1 earthquakes occurred at shallow depths beneath the summit because swarms of low frequency lf earthquakes preceded eruptions in 1989 and 2009 the posteruption swarms caused considerable concern and prompted the alaska volcano observatory to raise the monitoring levels on three occasions none of these swarms led to eruptions however and most observers including us initially concluded that the swarms had been caused by minor stress adjustments in the new lava dome or in the surrounding summit glaciers new observations reveal that the shallow lf swarms were accompanied by 2 families of repeating earthquakes at depths between 3 km and 6 km below sea level where the magma storage region is thought to be these mid crustal volcano tectonic vt type earthquakes were identical to earthquakes recorded during the 2009 redoubt eruption more than 6 months earlier focal mechanisms demonstrate that these earthquakes have thrust mechanisms inconsistent with the strike slip nature of regional faulting based on these observations we conclude that they are generated through processes occurring within the magma storage region the concurrence of the repeating vt earthquakes with the shallow lf swarms indicates that the shallow lf earthquakes were also magmatically driven our results emphasize that even brief episodes of low amplitude earthquake activity such as the lf swarms observed at redoubt following the 2009 eruption can be indicative of magmatic activity perhaps more signifi cant however is the demonstration that the conduit system at redoubt remained active intact and capable of transporting heat and fl uids to the surface months after the eruption was considered over

outer core compositional layering and constraints on core liquid transport properties a variety of studies of the earth s outer core report wave speeds near the top of the core slightly lower than reference models for core properties one interpretation of the slower wavespeed profile is that it could represent a change in the core s light element concentration with depth in the core i explore the consequences of this idea by interpreting the velocity profile as arising from diffusion gradients imposed in the outer core by various mechanisms in order to estimate relative diffusion rates for light elements in liquid iron i also examine theories for transport properties of high pressure metallic liquids that are based on hard sphere models from the seismic wavespeed profile an effective diffusivity may be obtained which ranges from 0 1 to 10 10 7 m2s 1 depending on the particular boundary condition or initial condition chosen the upper bound of the range is higher than expected from high pressure experiments and models of diffusivity in liquid metals for all elements except h the lower bound is within the uncertainty of theoretical predictions and experimental determinations given the range of expected outer core temperatures if diffusion involves low z elements plausible agreement arises from a class of models that represent diffusion out of a compositionally different layer existing from the time of the formation of the earth if the wavespeed profile in the core is diffusive in nature the data suggest that it is an original feature of the core

geomagnetic reversals and excursions their paleomagnetic record and implications for the geodynamo since the earliest reports of paleomagnetic evidence in support of the existence of geomagnetic field reversals much attention has been focused on understanding the relevant core mechanism given the far reaching implications such an understanding would have on the whole of geomagnetism past investigations of this phenomenon have involved experimentalists and theoreticians alike in particular during the previous quadrennium 1975 1978 significant contributions were gained from each of various approaches namely experimental studies of paleomagnetic records of polarity transitions statistical studies involving the frequency of occurrence of reversals as well as the time averaged full polarity states and theoretical studies concerning the dynamo continued progress along many fronts has been made during the last four years see the recent review by hoffman 1982a such is the subject of this report

stratification of the top of the core due to chemical interactions with the mantle 1 chemical interactions between the core and the mantle have been proposed as a mechanism to transfer o and si to the core adding light elements to the top of the core creates a stratified layer which grows by chemical diffusion into the underlying convecting region we develop a physical model to describe the evolution of the layer and estimate physical properties to aid in its detection the interface between the stratified layer and the convecting interior is defined by the onset of double diffusive instabilities oscillatory instabilities arise from the interplay of a stable compositional stratification and an unstable thermal stratification due to a superadiabatic heat flow at the core mantle boundary double diffusive convection in the region of neutral stratification establishes the base of the layer and sets the rate of entrainment of excess light element into the interior of the core growth of the layer by diffusion is interrupted by nucleation of the inner core which segregates light elements into the convecting part of the core for calculations using o as the sole light element we find that the base of the stratified layer retreats toward the core mantle boundary as the radius of the inner core increases representative but uncertain model parameters yield a present day layer thickness of 60 to 70 km we also calculate the anomaly in p wave velocity relative to the value for a well mixed core the velocity anomaly decreases almost linearly across the layer with a peak value of 2 at the core mantle boundary this anomaly should be large enough to detect in seismic observations although the sign of the anomaly is opposite to estimates obtained in previous seismic studies we suggest possible explanations for this discrepancy and speculate about the implications for the structure of the earth s interior

mapping active magma chambers by b values beneath the off ito volcano japan the b value of the frequency magnitude relation and thus the mean magnitude in the off ito volcanic area is not uniform based on detailed b value tomography using about 10 000 events in an area of 10 km radius and in the upper 15 km of the crust a high b value anomaly b 15 below 7 km depth and with a radius of about 2 km located below the coast southeast of ito contrasts with lower values of typically b 0 7 north of it and at shallower depths on the basis of surface deformations a tensile crack dislocation source was located above 7 km depth thus we surmise that the magma chamber in the off ito area was located below 7 km depth the correlation of a high b value anomaly with this model supports our hypothesis that active magma chambers may be mapped by high b value anomalies the magma body may be described as consisting of two parts a lower well established chamber below 11 km depth and a shallower part 11 7 km depth the b value in the volume surrounding the magma chamber increased from 0 7 during the 1980s to 1 5 during the earthquake swarms of the 1990s this may reflect a trend of increasing crack density due to the intrusion activity

geodesy constraints on the interior structure and composition of mars abstract knowledge of the interior structure of mars is of fundamental importance to the understanding of its past and present state as well as its future evolution the most prominent interior structure properties are the state of the core solid or liquid its radius and its composition in terms of light elements the thickness of the mantle its composition the presence of a lower mantle and the density of the crust in the absence of seismic sounding only geodesy data allow reliably constraining the deep interior of mars those data are the mass moment of inertia and tides they are related to mars composition to its internal mass distribution and to its deformational response to principally the tidal forcing of the sun here we use the most recent estimates of the moment of inertia and tidal love number k2 in order to infer knowledge about the interior structure of the mars we have built precise models of the interior structure of mars that are parameterized by the crust density and thickness the volume fractions of upper mantle mineral phases the bulk mantle iron concentration and the size and the sulfur concentration of the core from the bulk mantle iron concentration and from the volume fractions of the upper mantle mineral phases the depth dependent mineralogy is deduced by using experimentally determined phase diagrams the thermoelastic properties at each depth inside the mantle are calculated by using equations of state since it is difficult to determine the temperature inside the mantle of mars we here use two end member temperature profiles that have been deduced from studies dedicated to the thermal evolution of mars we calculate the pressure and temperature dependent thermoelastic properties of the core constituents by using equations state and recent data about reference thermoelastic properties of liquid iron liquid iron sulfur and solid iron to determine the size of a possible inner core we use recent data on the melting temperature of iron sulfur within our model assumptions the geodesy data imply that mars has no solid inner core and that the liquid core contains a large fraction of sulfur the absence of a solid inner is in agreement with the absence of a global magnetic field we estimate the radius of the core to be 1794 65 km and its core sulfur concentration to be 16 2 wt we also show that it is possible for mars to have a thin layer of perovskite at the bottom of the mantle if it has a hot mantle temperature moreover a chondritic fe si ratio is shown to be consistent with the geodesy data although significantly different value are also possible our results demonstrate that geodesy data alone even if a mantle temperature is assumed can almost not constrain the mineralogy of the mantle and the crust in order to obtain stronger constraints on the mantle mineralogy bulk properties like a fixed fe si ratio have to be assumed

fidelity and time resolution of the magnetostratigraphic record in siwalik sediments high resolution study of a complete polarity transition and evidence for cryptochrons in a miocene fluviatile section summary the surai khola section in southwest nepal a 5000 m continuously exposed record of fluvial sedimentation since middle miocene was revisited for high resolution magnetostratigraphy in sequences with expected cryptochrons and reversals of the geomagnetic field polarity intervals with durations of a few tens of thousands of years are recorded as zones of stable palaeomagnetic directions polarity transitions are recorded as zones with complex demagnetization behaviour of specimens in the sedimentary column almost antiparallel palaeoremanence directions residing in diverent haematite phases in the same specimens could generally not be separated properly by thermal demagnetization divering demagnetization paths for neighbouring specimens during a reversal suggest that measured transitional directions are not true geomagnetic field directions but rather are generated by the superposition of variable amounts of at least two almost antiparallel components of magnetization accompanying studies of recent river sand deposits demonstrate that these sediments acquire a true depositional remanent magnetization drm with considerable inclination errors and scattered directions for individual specimens

quantitative filling model for continental extensional basins with applications to early mesozoic rifts of eastern north america in many half graben strata progressively onlap the hanging wall block of the basins indicating that both the basins and their depositional surface areas were growing in size through time based on these constraints we have constructed a quantitative model for the stratigraphic evolution of extensional basins with the simplifying assumptions of constant volume input of sediments and water per unit time as well as a uniform subsidence rate and a fixed outlet level the model predicts 1 a transition from fluvial to lacustrine deposition 2 systematically decreasing accumulation rates in lacustrine strata and 3 a rapid increase in lake depth after the onset of lacustrine deposition followed by a systematic decrease when parameterized for the early mesozoic basins of eastern north america the model s predictions match trends observed in late triassic age rocks significant deviations from the model s predictions occur in early jurassic age strata in which markedly higher accumulation rates and greater lake depths point to an increased extension rate that led to increased asymmetry in these half graben the model makes it possible to extract from the sedimentary record those events in the history of an extensional basin that are due solely to the filling of a basin growing in size through time and those that are due to changes in tectonics climate or sediment and water budgets

the origin of volcano tectonic earthquake swarms volcano tectonic vt seismicity is commonly recorded prior to and during eruptions vt seismicity may reflect stresses induced by dike propagation as indicated by propagating hypocenters and fault plane solutions fps reflecting regional stresses or stresses induced by dike inflation indicated by randomly distributed hypocenters and fps with pressure axes rotated 90 to regional maximum compression although numerical models of dike induced stresses indicate that both regimes should occur during dike emplacement this has not yet been observed according to published seismic data instead 90 rotated fps are observed in some cases while propagating hypocenters mark dike formation in other cases we suggest that differences in the seismic expression of upper crustal magma migration may result from differences in the regional tectonic setting and in the nature of magma wall rock interactions ultimately it may be possible to use information contained in vt seismicity to forecast changes in a volcano s behavior by establishing the characteristic stress field response for a given volcano or through a deeper understanding of the complex relationships between vt seismicity local crustal stresses and the physical mechanisms of magma migration

a paleomagnetic investigation of the minturn formation colorado a study in establishing the timing of remanence acquisition abstract one hundred sixty five oriented hand samples were collected from 55 sites in the minturn formation located between wellsville and howard colorado in an attempt to locate the base of the permo carboniferous reversed superchron pcrs in western north america after the samples were demagnetized using thermal and alternating field cleaning techniques the normal polarity magnetizations which marks the base of the permo carboniferous reversed superchron pcrs were not found in the minturn formation reversed polarities were consistent throughout the section studied a conglomerate test was conducted on syndepositional rip up clasts taken from the formation the test indicates that a post depositional secondary component of magnetization has been acquired by the clasts and remains stable up to temperatures of 610 640 c however the high temperature components for three of the four test sites show random directions indicating that a primary remanent magnetization was acquired shortly after deposition the conglomerate test demonstrates that a secondary permian direction can be successfully isolated from a primary pennsylvanian one revealed at blocking temperatures above 650 c in this study the two components were isolated using step wise thermal demagnetization the results of the study indicate that the base of the pcr superchron lies stratigraphically below the minturn formation probably in early atokan or morrowan age sediments

magnetic polarity stratigraphy and ages of siwalik group rocks of the potwar plateau pakistan abstract six magnetic polarity sections have been established over the potwar plateau region of pakistan including the major stratotypes of the siwalik group in all six sections the dominant feature of the magnetic polarity stratigraphy is a long normal polarity zone which is contained within the nagri formation this conspicuous normal polarity zone has been radiometrically dated at 9 5 0 6 m y which identifies it as magnetic chron 9 radiometric dates from the upper siwalik formation have also been used to identify the chron 2 3 boundary in two of the sections the magnetic polarity stratigraphy of three of the sections has been correlated securely with the accepted magnetic polarity time scale so that the ages of the local stratigraphy are indexed accordingly based exclusively on data from stratotype sections the chinji nagri and dhok pathan formations have nominal age ranges of 10 1 13 1 7 9 10 1 and 5 1 7 9 m y age fluctuations on the order of 105 years may be anticipated for these formational boundaries within radius of some 20 km of the designated stratotype mean sediment accumulation rates during the chinji nagri and dhok pathan interval range from 13 to 52 cm 103 yr essentially linear sediment accumulations are locally maintained over time intervals of several million years the chinji nagri lithofacies boundary marks a transition from slow to faster sediment accumulation over much of the potwar plateau indicating a fundamental sedimentary tectonic change at this time

conditions and threshold for magma transfer in the layered upper crust insights from experimental models 1 magma transfer i e dike propagation is partly controlled by young s modulus elasticity contrasts ratio upper layer to lower layer modulus in the host rock here we try to better constrain the elasticity contrasts controlling the propagation velocity of dikes and their arrest we simulate dike propagation in layered elastic media with different elasticity contrasts salted gelatin and water represent host rock and magma respectively for common density ratios between magma and host rock 1 1 velocity variations are observed and a critical threshold in the elasticity contrast between layers results in the young s modulus ratio of 2 1 0 6 naturally occurring elasticity contrasts can be much higher than this experimental threshold suggesting that dike arrest due to heterogeneous elastic host rock properties is more frequent than expected examples of recently deflected or stalled dikes inside volcanoes and the common presence of high velocity bodies below volcanoes suggest that better defining elasticity contrasts below volcanoes helps in forecasting eruptions

semi automatic delimitation of volcanic edifice boundaries validation and application to the cinder cones of the tancitaro nueva italia region michoacan guanajuato volcanic field mexico abstract the shape and size of monogenetic volcanoes are the result of complex evolutions involving the interaction of eruptive activity structural setting and degradational processes morphological studies of cinder cones aim to evaluate volcanic hazard on the earth and to decipher the origins of various structures on extraterrestrial planets efforts have been dedicated so far to the characterization of the cinder cone morphology in a systematic and comparable manner however manual delimitation is time consuming and influenced by the user subjectivity but on the other hand automatic boundary delimitation of volcanic terrains can be affected by irregular topography in this work the semi automatic delimitation of volcanic edifice boundaries proposed by grosse et al 2009 for stratovolcanoes was tested for the first time over monogenetic cinder cones the method based on the integration of the dem derived slope and curvature maps is applied here to the tancitaro nueva italia region of the michoacan guanajuato volcanic field mexico where 309 plio quaternary cinder cones are located the semiautomatic extraction allowed identification of 137 of the 309 cinder cones of the tancitaro nueva italia region recognized by means of the manual extraction this value corresponds to the 44 3 of the total number of cinder cones analysis on vent alignments allowed us to identify ne sw vent alignments and cone elongations consistent with a ne sw max and a nw se min constructing a vent intensity map based on computing the number of vents within a radius r centred on each vent of the data set and choosing r 5 km four vent intensity maxima were derived one is positioned in the nw with respect to the volcano tancitaro one in the ne one to the s and another vent cluster located at the se boundary of the studied area the spacing of centroid of each cluster 24 km can be related to the thickness of the crust 9 10 km overlying the magma reservoir

magnetic stratigraphy of cenozoic terrestrial sediments and mammalian dispersal the existing literature on the correlation of the magnetic polarity timescale to vertebrate mammalian chronology for the cenozoic is reviewed for all continents except the soviet union the magnetostratigraphic data from north america is the most complete followed by the indian sub continent ten criteria for rating magnetostratigraphic studies are established and the papers reviewed are rated from 1 10 with 10 being the maximum score the timing of mammalian dispersal is examined against the background of information from magnetostratigraphic studies allied with radiometric dating north american land mammal ages are partly defined on immigrant taxa and the boundaries of the nalma are based on dispersal events it has been suggested that dispersal and evolutionary episodes observed in siwalik sediments were correlative to sea level change deduced from delta 18 o data from benthic foraminifera to test this possibility in the general case dispersal events into north america were determined by us

magnetic polarity stratigraphy of siwalik group sediments of tinau khola section in west central nepal revisited summary remanent magnetization of sandstones sampled at 127 levels sites throughout a 1710 m thick siwaliks molasse sequence exposed along the tinau khola river in nepal has been studied using thermal demagnetization the magnetic remanence consists of a secondary low temperature component of normal polarity unblocked mostly below 400 500 c close to the present day field and a high temperature characteristic remanence chrm unblocked mostly between 600 and 685 c represented by both normal and reverse polarities demagnetization behaviour and isothermal remanence acquisition indicate that the secondary component resides on goethite and maghemite magnetite whereas specular haematite carries chrm anisotropy of magnetic susceptibility data reveals primary depositional magnetic fabric as judged by oblate ellipsoids and subvertical tilt corrected minimum susceptibility axes a magnetic polarity sequence established using tilt corrected chrm directions from 124 levels reveals more details not found in the polarity stratigraphy worked out by munthe et al 1983 comparison of the sequence with a standard polarity time scale harland et al 1989 suggests a depositional time range between c 5 9 ma older than chron 3r and 11 ma younger than chron 5r 2 for the section considered according to new data the horizon of sivapithecus punjabiecus fells close to the reversal boundary at c 8 54 ma the lower age limit of the normal polarity chron 4ar 1 hence the hominoid should be regarded as of c 8 5 ma rather than 9 0 9 5 ma as suggested by munthe et al 1983

a reinterpretation of seismicity associated with the january 1983 dike intrusion at kilauea volcano hawaii in january 1983 a dike intrusion fissure eruption generated a swarm of 375 magnitude 1 to 3 earthquakes along a 16 km segment of kilauea s middle east rift zone we searched the hawaiian volcano observatory catalog for multiplets of similar events from this region from 1980 through 1985 and obtained precise relative locations by waveform cross correlation over 150 of the intrusion earthquakes could be grouped into 14 multiplets of five or more events with sufficient similarity for accurate relocation some multiplets were active for only a few minutes during the downrift migration phase of the seismic swarm consistent with generation near the propagating dike tip while others were active for several days the two multiplets nearest the origin of the seismic swarm include events from the preceding days and months most multiplets span only 50 to 100 m following relocation are located at about 3 to 4 km depth and appear to deepen downrift the catalog depths of those earthquakes in multiplets and those not in multiplets are similar suggesting that most of the recorded seismicity may have come from a very limited depth interval despite the fact that the dike breached the surface by analogy with a mechanical model used to explain a similar clustering of background seismicity in the upper east rift in 1991 we infer that the earthquakes are generated in regions of high stress concentration immediately above kilauea s deforming deep rift body this conclusion is consistent with the depth of the top of the deep rift body inferred from geodetic data and with numerical calculations suggesting that a significant ambient differential stress is required for dikes to produce earthquakes larger than magnitude 1

the structure and rate of late miocene expansion of c4 plants evidence from lateral variation in stable isotopes in paleosols of the siwalik group northern pakistan this study uses stable isotope variation within individual mio pliocene paleosols to investigate subkilometer scale phytogeography of late miocene vegetation change in southeast asia between ca 8 1 and 5 ma a time interval that coincides with dramatic global vegetation change we examine trends through time in the distribution of low latitude grasses c4 plants and forest c3 plants on indo gangetic floodplains using carbon 13c and oxygen isotopic 18o values in buried soil carbonates in siwalik series sediments exposed in the rohtas anticline north central pakistan revised high resolution magnetostratigraphy and a new 40ar 39ar date provide improved age control for the 2020 m rohtas section carbon isotope results capture lateral variability of c3 versus c4 plants at five stratigraphic levels r11 8 0 ma r15 6 74 6 78 ma r23 5 78 ma r29 4 8 4 9 ma and upper boundary tuff ubt 2 4 ma using detailed sampling of paleosols traceable laterally over hundreds of meters paleosols and the contained isotopic results can be assigned to three different depositional contexts within the fluvial sediments channel fill crevassesplay and floodplain environments 13c results show that near the beginning 8 0 ma and after 4 0 ma the period of major ecological change vegetation was homogeneously c3 or c4 respectively regardless of paleo landscape position in the intervening period there is a wide range of values overall with c4 grasses first invading the drier portions of the system floodplain surfaces and c3 plants persisting in moister settings such as topographically lower channel swales although abrupt on a geologic timescale changes in abundance of c4 plants are modest 2 per 100 000 yr compared to rates of vegetation turnover in response to glacial and interglacial climate changes in the quaternary earlier research documented a sharply defined c3 to c4 transition in pakistan between 8 1 and 5 0 ma based on vertical sampling but this higher resolution study reveals a more gradual transition between 8 0 and 4 5 ma in which c3 and c4 plants occupied different subenvironments of the siwalik alluvial plain 18o values as well as 13c values of soil carbonate increase up section at rohtas similar to isotope trends in other paleosol records from the region spatially however there is no correlation between 13c and 18o values at most stratigraphic levels this implies that the changes in soil hydrology brought about by the shift from forest to grassland i e an increase in average soil evaporation did not produce the shift through time in 18o values we interpret the trend toward heavier soil carbonate 18o values as a response to changes in external climatic factors such as a net decrease in rainfall over the past 9 ma

a new jurassic recent apparent polar wander path for south america and a review of central andean tectonic models abstract the jurassic to recent palaeomagnetic data from the stable cratonic areas of south america and africa are reviewed and evaluated from the combined data set seven mean reference pole positions are calculated for the early middle and late jurassic early and late cretaceous palaeogene and neogene most of these new palaeopoles are well defined and indicate a complex course of apparent polar wander for the south american plate comparison of the palaeomagnetic data from the andean margin with the reference poles indicates vertical axis rotations defining a broad pattern of anticlockwise rotations north of the arica deflection at approximately 19 s and clockwise rotations to the south further analysis indicates however that the distribution and magnitude of rotations is more complicated than this broad division tectonic models proposed to explain the pattern of rotations and the possible origins of the arica deflection are discussed it is suggested that the rotations probably result from localised in situ rotations controlled by strike slip fault systems or thrust sheets rather than large scale rotation mechanisms the broad rotation pattern may result from the influence of pre existing structures in the margin or variations in the flexural rigidity of the brazilian shield behind the deformation zone

molecular and isotopic records of c4 grassland expansion in the late miocene abstract siwalik paleosol and bengal fan sediment samples were analyzed for the abundance and isotopic composition ofn alkanes in order to test for molecular evidence of the expansion of c4 grasslands on the indian subcontinent the carbon isotopic compositions of high molecular weight alkanes in both the ancient soils and sediments record a shift from low 13c values ca 30 to higher values ca 22 prior to 6 ma this shift is similar in magnitude to that recorded by paleosol carbonate and fossil teeth and is consistent with a relatively rapid transition from dominantly c3 vegetation to an ecosystem dominated by c4 plants typical of semi arid grasslands the n alkane values from our paleosol samples indicate that the isotopic change began as early as 9 ma reflecting either a growing contribution of c4 plants to a dominantly c3 biomass or a decrease in water availability to c3 plants molecular and isotopic analyses of other compounds including n alcohols and low molecular weightn alkanes indicate paleosol organic matter contains contributions from a mixture of sources including vascular plants algae and or cyanobacteria and microorganisms a range of inputs is likewise reflected in the isotopic composition of the total organic carbon from these samples in addition then alkanes from two samples show little evidence for pedegenic inputs and we suggest the compounds were derived instead from the paleosol s parent materials we suggest the record of vegetation in ancient terrestrial ecosystems is better reconstructed using isotopic signatures of molecular markers rather than bulk organic carbon this approach provides a means of expanding the spatial and temporal records of c4 plant biomass which will help to resolve possible tectonic climatic or biological controls on the rise of this important component of the terrestrial biosphere

magnetostratigraphy in pursuit of missing links since the last quadrennial report magneto stratigraphy has continued to flourish the primary issues have been 1 refinements to the geomagnetic reversal time scale grts including recalibrations correlation of biostratigraphic data to the crts and revision of the terminology 2 use of magnetostratigraphic data for estimating rates of such processes as sediment accumulation evolution and faunal migration and 3 magnetic mineralogy and the origin of remanence for convenience we have divided the publications into studies of continental and marine sediments and highlight the most exciting issues within each of these sub headings in the first section of this review we will concentrate on the development of the geomagnetic reversal time scale grts with particular emphasis on refinements from the last few years we then turn our attention to applications of magnetostratigraphy in marine and continental sequences

precise seismic wave velocity atop earth s core no evidence for outer core stratification abstract earth s outer core is composed of liquid fe and ni alloyed with a 10 fraction of light elements such as o s or si secular cooling and compositional buoyancy drive vigorous convection that sustains the geodynamo but critical details of light element composition and thermal regime remain uncertain seismic velocities can provide important observational constraints on these parameters but global reference models such as preliminary reference earth model prem iasp91 and ak135 exhibit significant discrepancies in the outermost 200 km of the core here we apply an empirical transfer function method to obtain precise arrival times for smks waves a whispering gallery mode that propagates near the underside of the core mantle boundary models that fit our data are all characterized by seismic velocities and depth gradients in the outermost 200 km of the core that correspond best with prem this similarity to prem which has a smooth velocity profile that satisfies the adiabatic adams and williamson equation argues against the presence of an anomalous layer of light material near the top of the core as suggested in some previous studies a new model ae09 is proposed as a slight modification to prem for use as a reference model of the outermost core

isochronous fluvial systems in miocene deposits of northern pakistan a palacomagnetic isochron dated at about 8 1 myr bp and detailed lithostratigraph of a 40 m interval exposed along strike for 40 km establish the depositional patterns of two contemporaneous interfingering fluvial systems in the upper part of the meddle siwalik sequence the two systems referred to as the buff and blue grey differ in unit shape lithofacies bedding sequence palaeocurrent direction and sand composition interfingering occurs along the south west north east strike of the outcrops with the palaeodrainage directions of the two systems generally perpendicular to this line the axis of the blue grey system which deposited widespread sheet sands and silts lay toward the south west end of the study area the more complex axis of the buff system which deposited shoe string sand bodies and lage volumes of silt and clay lay toward the north east the source area for both systems was the rising himalyan belt to the north and noth east of the study area at maximum extent the blue grey system occupied a channel belt at least 25 km wide channel belt widths and depths for the buff system are 1 3 km and 3 7 m respectively current directions averge 94 for blue grey sands and 136 for buff sands blur grey sands contain 20 more rock fragments and are otherwise less mature than buff sands the buff system shows a verticla pattern of avulsion palaeosol formation and floodplain aggradation which we attribute to autocyclic processes of parallel rivers the blue grey system shows phases of erosion accompaniced laterally by plaeosol formation folowed by valley fill and overfowing of interfluve surfaces theis pattern may be caused by allocyclic presses affecting the source area we interpret the blue grey system as a major drainage from the interior himalayas perhaps the ancestral indus and the buff system as a complex of smaller drinages along the mountain front which were probably ributaries to the bluegrey syste vertebrate fossils including hominoid primates from the area are almost exclusively associated with lithofacies of the buff system and this probably refects both taphonomic and palaeoecological differences between the two systems

fluvial and lacustrine palaeoenvironments of the miocene siwalik group khaur area northern pakistan the miocene siwalik group upsection the chinji nagri and dhok pathan formations in northern pakistan records fluvial and lacustrine environments within the himalayan foreland basin thick 5 m to tens of metres sandstones are composed of channel bar and fill deposits of low sinuousity 1 08 1 19 single channel meandering and braided rivers which formed large low gradient sediment fans or megafans river flow was dominantly toward the south east and likely perennial palaeohydraulic reconstructions indicate that chinji and dhok pathan rivers were small relative to nagri rivers bankfull channel depths of chinji and dhok pathan rivers were generally 15 m and up to 33 m for nagri rivers widths of channel segments including single channels of meandering rivers and individual channels around braid bars were 320 710 m for chinji rivers 320 1050 m for nagri rivers and 270 340 m for dhok pathan rivers mean channel bed slopes were on the order of 0 000056 0 00011 bankfull discharges of channel segments for chinji and dhok pathan rivers were generally 700 800 m3s 1 with full river discharges possibly up to 2400 m3s 1 bankfull discharges of channel segments for nagri rivers were generally 1800 3500 m3s 1 with discharges of some larger channel segments possibly on the order of 9000 32 000 m3s 1 full river discharges of some of the largest nagri braided rivers may have been twice these values thin decimetres to a few metres sandstones represent deposits of levees crevasse channels and splays floodplain channels and large sheet floods laminated mudstones represent floodplain and lacustrine deposits lakes were both perennial and short lived and likely less than 10 m deep with maximum fetches on the order of a few tens of kilometres trace fossils and body fossils within all facies indicate the former existence of terrestrial vertebrates molluscs bivalves and gastropods arthropods including insects worms aquatic fauna e g fish turtles crocodiles trees bushes grasses and aquatic flora palaeoenvironmental reconstructions are consistent with previous palaeoclimatic interpretations of monsoonal conditions

taphonomy of a middle miocene autochthonous forest assemblage ngorora formation central kenya abstract a 12 6 million year old forest assemblage of leaves twigs and rare fruit is preserved in the ngorora formation of central kenya the assemblage is autochtonous preserved in airfall tuff and represents well the diversity heterogeneity and richness of the original forest the herbaceous component not usually preserved in paleobotanical assemblages is present nearest neighbor analysis and mapping of the vertical distribution of plant organs indicates that leaves and twigs are preserved near the parent plant and that there is no apparent size weight or species bias in deposition of these remains leaves occur isolated in clusters or attached to branches and fell directly to the ground as a result of ashfall most falling very soon after ashfall commenced the absence of upright trunks and large stems is probably due to post depositional weathering which may also be responsible for the absence of a leaf litter layer however leaf litter may have been absent at the time of ash deposition

two long geological records of continental ecosystems abstract the early paleogene continental sequence of northwestern wyoming and south central montana usa and the neogene siwalik sequence of northern pakistan are exceptionally long fossiliferous and well studied in terms of geology paleontology mammalian evolution paleoecology and paleoclimatology each record spans about 15 myr of alluvial deposition in a foreland basin the fluvial systems differed in size drainage of floodplains and change in alluvial architecture through time both sequences preserved abundant paleosols which are a source of paleoclimatic signals in stable isotopes from soil carbonates fossil collections from both records are dominated by vertebrate remains and especially by mammals the paleogene sequence also preserved abundant floral remains patterns of vertebrate preservation differed markedly between the two sequences placing inherent limitations on paleoecological reconstruction and on the scope of evolutionary studies of mammalian lincages in this introductory paper we summarize the major similarities and differences in geologic setting chronology and time resolution and mammalian faunal composition of these two long sequences finally we present a brief overview of the organization of papers in this special issue

major changes in east asian climate in the mid pliocene triggered by the uplift of the tibetan plateau or global cooling since the mid pliocene east asian climates have experienced significant changes one view suggests that significant uplift of the tibetan plateau during this period could have been responsible for these dramatic changes in the strength of the east asian monsoon and for asian interior acidification while some other authors attribute these changes to the ongoing global cooling and rapid growth of the arctic ice sheet up to the present which factor dominates the major changes of east asian climate in the mid pliocene is still a contentious issue this study presents an analysis of several climate proxies including grain size cao na2o mgo tio2 ratio na ka ratio and dust accumulation rates of the xifeng red clay sequence in the eastern chinese loess plateau and the xihe pliocene loess soil sequence in west qinling they reveal that aridity in the continental interior and winter monsoon circulation both intensified whereas the east asian summer monsoon showed a weakening rather than intensifying trend since the mid pliocene these changes are also supported by the other multi proxy records from various regions in east asia previous numerical modeling studies have demonstrated that uplift of the tibetan plateau would have simultaneously enhanced continental scale summer and winter monsoon strength as well as central asian aridity the mid pliocene climate changes in east asia are therefore unlikely to be a response to plateau uplift on the contrary our recent modeling results give support to the view that ongoing cooling could have intensified both the aridity of the interior and the strength of the winter monsoon but weakened the summer monsoon in east asia c 2012 elsevier ltd all rights reserved

development of asian monsoon revealed by marked ecological shift during the latest miocene in northern pakistan carbon isotopes from soil carbonate1 4 and soil organic matter5 6 yield palaeoecological information because the carbon in the soil carbonate forms in isotopic equilibrium with local soil co2 refs 1 7 the isotopic composition of which is in turn determined by local plant cover siwalik group sediments in northern pakistan contain a well exposed palaeosol record spanning the past 18 myr here we report on stable carbon isotope results from associated pedogenic carbonate which indicate a dramatic ecological shift from to dominated floodplain biomass beginning 7 4 7 0 myr ago the earlier c3 floodplain biomasses were probably mainly composed of trees and shrubs whereas c4 grasslands dominated in the plio pleistocene oxygen isotopes also exhibit a shift in the latest miocene probably corresponding to a major climate change which may have induced the forest to grassland transition this dramatic ecological shift in the latest miocene may mark the inception or a marked strengthening of the asian monsoon system

precursory earthquakes of the 1943 eruption of paricutin volcano michoacan mexico abstract paricutin volcano is a monogenetic volcano whose birth and growth were observed by modern volcanological techniques at the time of its birth in 1943 the seismic activity in central mexico was mainly recorded by the wiechert seismographs at the tacubaya seismic station in mexico city about 320 km east of the volcano area in this paper we aim to find any characteristics of precursory earthquakes of the monogenetic eruption though there are limits in the available information such as imprecise location of hypocenters and lack of earthquake data with magnitudes under 3 0 the available data show that the first precursory earthquake occurred on january 7 1943 with a magnitude of 4 4 subsequently 21 earthquakes ranging from 3 2 to 4 5 in magnitude occurred before the outbreak of the eruption on february 20 the s p durations of the precursory earthquakes do not show any systematic changes within the observational errors the hypocenters were rather shallow and did not migrate the precursory earthquakes had a characteristic tectonic signature which was retained through the whole period of activity however the spectra of the p waves of the paricutin earthquakes show minor differences from those of tectonic earthquakes this fact helped in the identification of paricutin earthquakes except for the first shock the maximum earthquake magnitudes show an increasing tendency with time towards the outbreak the total seismic energy released by the precursory earthquakes amounted to 2 10 19 ergs considering that statistically there is a threshold of cumulative seismic energy release 10 17 18 ergs by precursory earthquakes in polygenetic volcanoes erupting after long quiescence the above cumulative energy is exceptionally large this suggests that a monogenetic volcano may need much more energy to clear the way of magma passage to the earth surface than a polygenetic one the magma ascent before the outbreak of paricutin volcano is interpretable by a model of magma filled crack formation proposed by weertman based on seismic data and other field observations

magma intrusion near volcan tancitaro evidence from seismic analysis abstract between may and june 2006 an earthquake swarm occurred near volcan tancitaro in mexico which was recorded by a temporary seismic deployment known as the mars network we located 1000 events from this seismic swarm previous earthquake swarms in the area were reported in the years 1997 1999 and 2000 we relocate and analyze the evolution and properties of the 2006 earthquake swarm employing a waveform cross correlation based phase repicking technique hypocenters from 911 events were located and divided into eighteen families having a correlation coefficient at or above 0 75 90 of the earthquakes provide at least sixteen phase picks we used the single event location code hypo71 and the p wave velocity model used by the jalisco seismic and accelerometer network to improve hypocenters based on the correlation adjusted phase arrival times we relocated 121 earthquakes which show clearly two clusters between 9 10 km and 3 4 km depth respectively the average location error estimates are

improving seismic resolution of outermost core structure by multichannel analysis and deconvolution of broadband smks phases the presence of a thin 12km layer atop the outermost core has important implications for the geodynamic and geomagnetic nature of the earth but detecting such a layer is difficult smks seismic phases which traverse the mantle as an s wave and reflect m 1timesfromtheundersideofthecore mantleboundary cmb offerameanstotestproposedmodelssincetheyareextremely sensitive to the velocity structure of the outermost core to improve seismic resolution in this region we have developed and validated a modified vespagram slant stack technique that makes use of phase independent amplitude envelope traces confidence limits on arrival time and slowness estimates are obtained by bootstrap resampling in order to mitigate the effects of waveform complexity and facilitate comparison between observed and modeled waveforms we have used a simple deconvolution method similar to techniques used in exploration seismology we have applied these methods to high quality broadband recordings of three deep focus earthquakes recorded by seismic arrays in canada some of our measurements have turning depths 190km in the outer core a region of sparse coverage in previous global compilations of smks measurements based on the analysis of travel times our data show that the p wave velocity of the outermost core is prem like until the outer 150km or less where it becomes much slower than prem and most standard earth models additionally amplitudes of s3ks s4ks phases relative to skksac are significantlylargerthanthosepredictedbystandardmodels usingreflectivitysyntheticseismogramsweshowthatnopreviousearth

venus a thick basal magma ocean may exist today basal magma oceans develop in earth and venus after accretion as their mantles solidify from the middle outwards fractional crystallization of the basal mantle is buffered by the core and radiogen

structure and evolution of the northern potwar deformed zone pakistan the northern potwar deformed zone npdz is part of the active foreland fold and thrust belt of the salt range and potwar plateau in northern pakistan about 500 km of seismic reflection profiles are integrated with surface geologic and drilling data to examine the deformation style and structure of the npdz with particular emphasis on history of deformation of the dhurnal oil field the seismic lines suggest that the overall structure of the eastern npdz is a duplex structure developed beneath a passive roof thrust the roof thrust is generated from a tipline in the miocene murree formation and the sole thrust is initiated from the same eocambrian evaporite zone that extends 80 km southward beneath the soan syncline and salt range the dhurnal oil field structure is a pop up at the southern margin of the npdz and developed beneath the passive roof thrust the passive roof thrust crops out just north of dhurnal on the steep northern limb of the soan syncline an overstep passive roof thrust sakhwal fault is interpreted west of dhurnal this fault developed due to

a new calibration point for the late miocene section of the geomagnetic polarity time scale 40ar 39ar dating of lava flows from akaroa volcano new zealand recent dating of geomagnetic field reversals over the past 2 ma by both the radiometric and astrochronologic techniques indicates that the k ar derived ages for these reversals are too young we report herein on 40ar 39ar incremental heating study of two geomagnetic field reversals trapped in lavas on akaroa volcano new zealand that demonstrate that the k ar derived ages for field reversals at 10 9 ma are too young by about 7 the new age for the termination of chron 5 a critical tie point on the geomagnetic polarity time scale for late miocene time is 9 67 0 11 ma

cretaceous palaeomagnetic results from hainan island in south china supporting the extrusion model of southeast asia abstract we report the cretaceous palaeomagnetic results from hainan island south china in hainan island we collected the early cretaceous redbeds of the lumuwan formation at eleven sites we also describe the tectonic kinematics for and around hainan island since the cretaceous deduced from our and previous palaeomagnetic results the palaeolatitude of hainan island is 25 9 n 3 4 3 2 implying that hainan island was situated about 7 north from the present position during the cretaceous the palaeopole of hainan island latitude 77 7 n longitude 162 1 e k 65 6 and a95 4 4 suggests 4 0 5 8 counterclockwise rotation and 14 1 5 5 southward translation relative to the suspected coherent part of the south china block scb since the cretaceous the rotation and translation of similar sense 18 8 7 4 and 7 8 6 9 respectively are detected in the existing palaeomagnetic result from the xinlong formation in guangxi which is situated approximately 400 km north northwest from hainan island the southward translation of both areas seems to have been due to the southeastward extrusion of dissected zones within the southwestern part of the scb in a similar pattern to the indochina block which had resulted from the indentation of india into asia this sw part seems to have slightly rotated counterclockwise because its extrusion was probably smaller in scale than the indochina block and therefore it was dragged out by the indochina block this hypothesis is supported by the existence of a northwest southeast trending fault system parallel to the red river fault

paleomagnetism of miocene east african rift sediments and the calibration of the geomagnetic reversal time scale paleomagnetic stratigraphy and k ar age determinations are reported for the type section of the middle miocene ngorora formation found in the kenya rift valley the magnetostratigraphy is well correlated to the geomagnetic reversal time scale grts and spans from the lower part of chron c5 9 to chron c5ab r 14 k ar dates were determined for euhedral sanidines handpicked from seven tuff horizons within the ngorora formation and the underlying turn phonolite flow these dates can therefore be tied directly to the grts the eight dates fall into three discrete groups averaging 12 5 0 22 ma mean and standard deviation of results from four tuffs 11 6 0 06 ma mean and standard deviation from three tuffs and 10 16 0 38 average of three analyses from one tuff we interpret the age groups as resulting from three successive eruptive episodes the stratigraphic positions of which are well constrained in spite of episodic supply at the eruptive source sediment accumulation is continuous at the resolution of the grts this suggests that accumulation is controlled by basin subsidence rather than sediment supply sanidine dates support an age for the older boundary of marine magnetic anomaly 5 of about 10 ma as opposed to 11 12 ma suggested by the most recent results from icelandic basaltic lava flows

on the thermo chemical origin of the stratified region at the top of the earth s core abstract i developed a combined model of the thermal and chemical evolution of the earth s core and investigated its influence on a thermochemically stable region beneath the core mantle boundary cmb the chemical effects of the growing stable region are caused by the equilibrium chemical reaction between silicate and the metallic core the thermal effects can be characterized by the growth of the sub isentropic shell which may have a rapid growth rate compared to that of the chemically stable region when the present day cmb heat flow was varied the origin of the stable region changed from chemical to thermochemical to purely thermal because the rapid growth of the sub isentropic shell can replace the chemically stable region physically reasonable values of the present day cmb heat flow that can maintain the geodynamo action over 4 billion years should be between 8 and 11 tw to constrain the thickness of the thermochemically stable region beneath the cmb the chemical diffusivity is important and should be o 10 8 m 2 s to obtain a thickness of the thermochemically stable region beneath the cmb consistent with that inferred from geomagnetic secular variations 140 km however the strength of the stable region found in this study is too high to be consistent with the constraint on the stability of the stable region inferred from geomagnetic secular variations

test of climate leaf physiognomy regression models their application to two miocene floras from kenya and 40ar 39ar dating of the late miocene kapturo site abstract to test multiple regression models developed to predict seven variables of climate from leaf margin and size characters leaf character data were collected from modern woodland and rainforest plant communities in uganda these localities were chosen for their structural similarity to two fossil sites for which climate values were desired the test indicated that for these communities the model accurately predicts mean annual temperature however the multiple regression models overestimate seasonality of temperature and mean annual and seasonal precipitation amounts for the two modern communities reasons for the models inaccuracies may include no african analog in the non african database on which the model is based too much intraspecific variability in leaf size an important character in precipitation models and relationships between leaf form and climate that may not be well defined by multiple regression mean annual temperatures were reconstructed for two miocene paleobotanical localities from the tugen hills of the eastern rift valley kenya using leaf form data primarily margin characters the results indicate that mean annual temperatures at kabarsero 12 6 ma and kapturo 6 7 7 2 ma were essentially equivalent to temperatures at those localities today and differed little from one another the kapturo paleofloral assemblage is a deciduous woodland interpreted as indicating less available or more seasonal moisture than the kabarsero paleofloral assemblage a wet forest with west african floral affinities laser fusion 40 ar 39 ar dating of volcanic materials at kapturo provides the first control of this paleoflora bracketed by a 7 2 ma trachyte at the base of the sedimentary sequence and an age of 6 7 ma on a reworked tuff overlying the fossiliferous horizon the chronologic data places the paleofloral assemblage in stratigraphic context relative to other tugen hills units containing paleofloras and abundant vertebrate remains including hominoids

dike emplacement near paricutin volcano mexico in 2006 a major seismic swarm occurred near paricutin volcano between the end of may and early july 2006 more than 700 earthquakes with magnitude ml exceeding 2 4 were located paricutin located in the michoacan guanajuato volcanic field in western mexico is well known as the site of the 1943 eruption in which a new 400 m cinder cone was constructed in what had been farmland the 2006 swarm exhibits all of the characteristics typically associated with swarms of volcanic origins the earthquake rate showed the typical ramp up and ramp down over the course of several days magnitudes were evenly distributed in time with a notably high b value of 2 45 the earthquake locations cluster around a northeast striking trend extending approximately 6 km over the first two weeks hypocenters migrated steadily a few hundred meters per day rising from 9 to 5 km depth and moving northeast about 5 km on approximately june 7 the ascent of hypocenters stalled for the next three weeks hypocenters held their depth while migrating laterally back to the southwest focal mechanisms during the first part of the swarm reflected the increased stress caused by dike inflation following june 7 the stress orientation changed and became more consistent with the inflation of horizontal sill like structures though only limited information is available from the seismic swarm preceding the 1943 eruption several features including the swarm duration and magnitude relationships were comparable to those of the 2006 episode the strong indicators of a magmatic origin to the 2006 swarm suggest that at this location there are few if any traditional seismic discriminants that could be used to distinguish which seismic swarms and dike emplacement events might culminate in eruption

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

depositional chronology and fabric of siwalik group sediments in central nepal from magnetostratigraphy and magnetic anisotropy abstract magnetostratigraphic research undertaken within the past 15 years in the siwaliks distributed along 400 km of the sub himalaya in central nepal has proved that the sediments possess highly reliable hematite based primary detrital remanent magnetization suitable to determine depositional chronology in order to bring out the polarity sequences in a common chronological frame all available data are newly correlated to the latest global magnetic polarity time scale of cande and kent s c cande d v kent 1995 revised calibration of the geomagnetic polarity timescale for the late cretaceous and cenozoic journal of geophysical research 100 6093 6095 chronological data presented are referred in relation to the diverse lithological nomenclature to the formations whose ages are not constrained by isotopic or paleontologic ages the age of the sections dated by magnetostratigraphy ranges between 14 and

high pressure melting experiments on fe si alloys and implications for silicon as a light element in the core abstract we carried out melting experiments on fe si alloys to 127 gpa in a laser heated diamond anvil cell dac on the basis of textural and chemical characterizations of samples recovered from a dac a change in eutectic liquid composition in the fe fesi binary system was examined with increasing pressure the chemical compositions of coexisting liquid and solid phases were quantitatively determined with field emission type electron microprobes the results demonstrate that silicon content in the eutectic liquid decreases with increasing pressure to less than 1 5 0 1 wt si at 127 gpa if silicon is a single light element in the core 4 5 to 12 wt si is required in the outer core in order to account for its density deficit from pure iron however such a liquid core whose composition is on the si rich side of the eutectic point crystallizes less dense solid cscl b2 type phase at the inner core boundary icb our data also show that the difference in silicon concentration between coexisting solid and liquid is too small to account for the observed density contrast across the icb these indicate that silicon cannot be the sole light element in the core previous geochemical and cosmochemical arguments however strongly require 6 wt si in the core it is possible that the earth s core originally included 6 wt si but then became depleted in silicon by crystallizing sio2 or mgsio3

structure of the michoacan guanajuato volcanic field mexico abstract the distribution and the alignment of volcanoes and fault scarps provide new insights into the structural framework of the michoacan guanajuato volcanic field a total of 1894 volcanoes occur over an area of 34 500 km 2 contour maps of the density of volcanoes and average volcano height indicate a shift from polygenetic to monogenetic volcanism from north to south in the field volcano edifice height has a bimodal density distribution which can be described by two populations with mean heights of 95 m and 510 m plotting the distribution of these two populations on separate contour maps reveals a n35e trend in monogenetic volcanoes and a n60w trend in polygenetic volcanoes fault scarps cut tertiary volcanic rocks and older quaternary volcanoes but are overprinted by volcanoes and lava flows in the central part of the field fault scarps define two fault zones oriented n60e in the northeastern part of the area and n60w in the northwest part of the area e w trending faults are common and alignments of volcanoes often parallel these faults regional trends in volcano distribution appear to be related to plate convergence the n60w trend of the polygenetic volcanoes is colinear to the western trans mexican volcanic arc parallel to the middle america trench and parallel to a major fault zone and represents an extinct volcanic arc the n35e band of monogenetic volcanoes may reflect the segmentation of the subducted plate

on the relation between ground surface deformation and seismicity during the 2012 2014 successive magmatic intrusions at el hierro island abstract six different magmatic intrusions were detected around el hierro island in the two years that followed the end of the 2011 2012 submarine eruption each intrusion lasted between few days to three weeks and produced intense seismic swarms and rapid ground deformation we performed a hypodd relocation of 6000 earthquakes and inverted the gps data in order to obtain the location of the magma source of each intrusion each episode presents a spatial gap between seismicity and magma source of 3 8 km with the earthquakes located always deeper than the deformation sources we propose a magma plumbing system consisting on a deep structure injecting magma to a more ductile shallower location beneath el hierro crust while the seismicity is associated with the deeper structure the ascent and accumulation of magma at shallower level deforms the crust aseismically the mechanism of most of these episodes consists of an initial injection of magma producing most of the ground deformation and high b values of the seismicity indicating fluid fractures during the first days and finishes with high magnitude earthquakes and low b values indicating an overpressure of the injection process there is a correlation between the seismic and geodetic moment ratio and the direction of propagation of each intrusion towards one of the volcanic rifts of the island suggesting the possible existence of a deep structure beneath the island related with to the triaxial origin of the island this work presents important advances in the knowledge of monogenetic magmatic intrusions and specifically in those occurred in el hierro island between 2011 and 2014 with important implications for future volcano monitoring in the canary islands

melting experiments on the fe c binary system up to 255 gpa constraints on the carbon content in the earth s core abstract phase relations including the eutectic liquid composition in the fe c binary system remain unclear under the core pressure range which makes estimating the carbon budget in the earth s core difficult to explore this issue we have conducted melting and subsolidus experiments on fe c alloys in a diamond anvil cell up to 255 gpa textural and compositional characterizations of quenched samples show that carbon concentration in the eutectic liquid slightly decreases with increasing pressure and is about 3 wt at the inner core boundary icb pressure the solubility of carbon in solid fe is found to be almost constant at 1 0 wt in situ x ray diffraction data indicate that fe forms eutectic melting with fe3c to 203 gpa and with fe7c3 at 255 gpa previous studies on liquid fe c alloys suggested that the density of the outer core is explained by liquid fe containing 1 8 to 4 2 wt c if the liquid core includes

the age of the inner core abstract the energy conservation law when applied to the earth s core and integrated between the onset of the crystallization of the inner core and the present time gives an equation for the age of the inner core in this equation all the terms can be expressed theoretically and given values and uncertainties of all relevant physical parameters the age of the inner core can be obtained as a function of the heat flux at the core mantle boundary and the concentrations in radioactive elements it is found that in absence of radioactive elements in the core the age of the inner core cannot exceed 2 5 ga and is most likely around 1 ga in addition to have an inner core as old as the earth concentrations in radioactive elements needed in the core are too high to be acceptable on geochemical grounds

melting experiments on fe fe3s system to 254 gpa abstract melting experiments were performed on the fe fe 3 s system at high pressures between 34 and 254 gpa in a laser heated diamond anvil cell dac using starting materials of fine grained homogeneous mixtures of fe and fes 5 7 0 3 wt s coexisted with both solid fe 3 s and fe containing 3 9 0 4 wt s at 254 gpa and 3550 k the eutectic liquid at inner core boundary icb pressure includes less sulfur than is required to account for the density deficit of the outer core 10 wt s furthermore the difference in sulfur concentration between coexisting liquid and solid is not sufficient to account for the observed density jump across the icb these indicate that sulfur cannot be a predominant light element in the core

southeast indian ridge between the rodriguez triple junction and the amsterdam and saint paul islands detailed kinematics for the past 20 m y the southeast indian ridge has the fastest spreading rates of the three mid oceanic ridge systems of the indian ocean and has recorded the movements of antarctica relative to australia and india since the late cretaceous new bathymetric and magnetic data have been collected by the r v marion dufresne 1983 and the r v jean charcot 1984 on the western part of this ridge between the rodriguez triple junction 25 5 s 70 0 e and the amsterdam and saint paul islands 38 s 78 e these data bring additional information on the seafloor magnetic pattern produced by the southeast indian ridge during the past 20 m y a new tectonic chart is proposed for the area around the amsterdam and saint paul islands we have mapped 17 isochrons ranging from anomalies 6 to 1 20 5 0 7 ma based on the compilation of all the data available in this area 25 cruises their distribution clearly shows asymmetric features reconstructions at short time intervals show that stage poles of rotation describe oscillatory movements along a direction parallel to the southeast indian ridge axis observed changes in spreading rates and the stability of the spreading directions since the miocene support this result

the late cenozoic chronologic and stratigraphic development of the kashmir intermontane basin northwestern himalaya abstract the intermontane basin of kashmir developed within the thrust faulted southern margin of the himalayan range detailed lithostratigraphies magnetic polarity stratigraphies and fission track dates on enclosed volcanic ashes were determined at four separate localities in order to develop a chronology of the late cenozoic evolution of the kashmir basin the results indicate that sedimentation had commenced by about 4 m y ago since then over 1300 m of sediments have aggraded at inferred average rates varying from 16 to 64 cm per 1000 yr lacustrine and deltaic sediments dominate the kashmiri sequences and appear to respond sensitively to tectonic events along the basin margins conglomerates shed from the faulted basin margins at about 1 7 2 1 2 7 and 3 0 3 5 m y ago punctuate the predominantly low energy fluviolacustrine depositional record paleocurrent analyses indicate a switch from northeasterly derived conglomeratic facies to southwesterly derived ones about 1 7 m y ago this transition reflects enhanced activity along the main boundary thrust complex to the southwest and an apparent diminution of faulting along the northeastern margin of the basin in the pir panjal range 1400 3000 m of uplift at inferred rates of up to 10 mm yr 1 have terminated widespread sedimentation within the kashmir basin since the middle pleistocene

fault structure and mechanics of the hayward fault california from double difference earthquake locations 1 the relationship between small magnitude seismicity and large scale crustal faulting along the hayward fault california is investigated using a double difference dd earthquake location algorithm we used the dd method to determine high resolution hypocenter locations of the seismicity that occurred between 1967 and 1998 the dd technique incorporates catalog travel time data and relative p and s wave arrival time measurements from waveform cross correlation to solve for the hypocentral separation between events the relocated seismicity reveals a narrow near vertical fault zone at most locations this zone follows the hayward fault along its northern half and then diverges from it to the east near san leandro forming the mission trend the relocated seismicity is consistent with the idea that slip from the calaveras fault is transferred over the mission trend onto the northern hayward fault the mission trend is not clearly associated with any mapped active fault as it continues to the south and joins the calaveras fault at calaveras reservoir in some locations discrete structures adjacent to the main trace are seen features that were previously hidden in the uncertainty of the network locations the fine structure of the seismicity suggests that the fault surface on the northern hayward fault is curved or that the events occur on several substructures near san leandro where the more westerly striking trend of the mission seismicity intersects with the surface trace of the aseismic southern hayward fault the seismicity remains diffuse after relocation with strong variation in focal mechanisms between adjacent events indicating a highly fractured zone of deformation the seismicity is highly organized in space especially on the northern hayward fault where it forms horizontal slip parallel streaks of hypocenters of only a few tens of meters width bounded by areas almost absent of seismic activity during the interval from 1984 to 1998 when digital waveforms are available we find that fewer than 6 5 of the earthquakes can be classified as repeating earthquakes events that rupture the same fault patch more than one time these most commonly are located in the shallow creeping part of the fault or within the streaks at greater depth the slow repeat rate of 2 3 times within the 15 year observation period for events with magnitudes around m 1 5 is indicative of a low slip rate or a high stress drop the absence of microearthquakes over large contiguous areas of the northern hayward fault plane in the depth interval from 5 to 10 km and the concentrations of seismicity at these depths suggest that the aseismic regions are either locked or retarded and are storing strain energy for release in future large magnitude earthquakes

a time framework based on magnetostratigraphy for the siwalik sediments of the khaur area northern pakistan the magnetostratigraphy of eleven new sections in the khaur area of northern pakistan is presented all specimens have been subjected to thermal demagnetization the sections taken in adjacent ravines incising the siwalik formations nearly penpendicular to strike form a temporal framework in which to interpret the biological and lithological information a composite section is constructed from several long sections and is correlated to the magnetic polarity time scale chron 6 to chron 15 providing absolute age information for the biostratigraphic zonations of barry et al 1982 detailed sections along the strike of major stratigraphic marker beds document an isochronous horizon and form the basis for paleoenvironmental reconstructions the preservation of a polarity transition in fluvial sediments suggests continuity of sedimentation on a time scale of 103 104 years