paleomagnetism of the silurian volcanism at almaden southern spain the silurian volcanism of the almaden mining district southern spain has been the object of a very detailed paleomagnetic study the 175 cores 25 sites sampled all along the eastern termination of the almaden syncline were subjected to magnetic susceptibility measurements thermomagnetic cycles progressive alternating field and thermal demagnetizations as well as microscopic observations the magnetic mineralogy of these volcanics flows is dominated by magnetite in at least two very distinct forms including small cubic distributed grains and large elongated crystals the observed anisotropy of magnetic susceptibility varies according to the relative abundance of the two magnetite phases showing a complete inversion of the susceptibility tensors this anisotropy has been related to the emplacement of the flows on a gentle slope dipping 10 toward the southwest implying that their volcanic source was located to the northeast the paleomagnetic analysis has revealed two characteristic components of magnetization the first direction d 109 i 11 95 9 pole position at 11 s 69 e dp 5 dm 9 yields a negative fold test and is interpreted as a late paleozoic partial overprint its declination is 46 away of the expected late paleozoic field direction which confirms that rotations are still occurring after the main folding phases of the variscan orogeny the second direction d 62 i 36 95 14 pole position at 8 n 118 e dp 9 dm 13 is prefolding as evident from the significant clustering when applying appropriate tectonic corrections and we interprete it as the original magnetization of silurian age the compilation of all paleozoic paleomagnetic results from spain reveals that the iberian meseta has been moving along with gondwana at high drift rates throughout ordovician and silurian times

the site response of two rock and soil station pairs to strong and weak ground motion abstract the site response to strong and weak ground motion depends largely on the subsurface conditions at the soil site for the two rock soil station pairs studied the first station pair consists of a soft soil site treasure island and a sandstone and shale site yerba buena island these stations recorded strong ground shaking from the loma prieta mainshock and weak ground motion from four aftershocks the range of peak ground acceleration is from approximately 0 00006 to 0 07 g at the rock site compared to the rock site the strong ground motion at the soft soil site is amplified by a factor of about 3 over a frequency range from 0 5 to 2 0 hz the amplification is much higher for weak motion and suggests a dependence on signal amplitude for example near 1 hz the site response shows an increasing amplification as magnitude and the peak velocity at the rock site decreases for events of local magnitude 7 0 4 3 4 1 3 5 and 3 3 the maximum soil site amplifications are 4 12 17 19 and 25 respectively a second station pair consisting of a stiff soil site gilroy 2 and a sandstone site gilroy 1 was also studied with contrasting results these two stations recorded strong ground shaking from the 1979 coyote lake 1984 morgan hill and 1989 loma prieta mainshocks weak ground motion was recorded at these stations after the loma prieta mainshock the range of peak ground acceleration is from 0 006 to 0 43 g at the rock site unlike the results for the soft soil study above the estimated stiff soil site responses are not significantly different for strong and weak motion from 0 5 to 2 0 hz near 0 7 hz the stiff soil site responses range from 2 5 to 4 5 for strong ground shaking from three mainshocks and from 1 5 to 4 0 for weak ground shaking from thirteen aftershocks

paleolatitudes of the tibetan himalaya from primary and secondary magnetizations of jurassic to lower cretaceous sedimentary rocks the tibetan himalaya represents the northernmost continental unit of the indian plate that collided with asia in the cenozoic paleomagnetic studies on the tibetan himalaya can help constrain the dimension and paleogeography of greater india the indian plate lithosphere that subducted and underthrusted below asia after initial collision here we present a paleomagnetic investigation of a jurassic limestones and lower cretaceous volcaniclastic sandstones section of the tibetan himalaya the limestones yielded positive fold test showing a prefolding origin of the isolated remanent magnetizations detailed paleomagnetic analyses rock magnetic tests end member modeling of acquisition curves of isothermal remanent magnetization and petrographic investigation reveal that the magnetic carrier of the jurassic limestones is authigenic mag netite whereas the dominant magnetic carrier of the lower cretaceous volcaniclastic sandstones is detrital magnetite our observations lead us to conclude that the jurassic limestones record a prefolding remagnetiza tion whereas the lower cretaceous volcaniclastic sandstones retain a primary remanence the volcaniclastic sandstones yield an early cretaceous paleolatitude of 55 5 s 52 5 s 58 6 s for the tibetan himalaya suggesting it was part of the indian continent at that time the size of greater india during jurassic time cannot be estimated from these limestones instead a paleolatitude of the tibetan himalaya of 23 8 s 21 8 s 26 1 s during the remagnetization process is suggested it is likely that the remagnetization caused by the oxidation of early diagenetic pyrite to magnetite was induced during 103 83 or 77 67 ma the inferred paleolatitudes at these two time intervals imply very different tectonic consequences for the tibetan himalaya

shrimp u pb zircon geochronology and thermal modeling of multilayer granitoid intrusions implications for the building and thermal evolution of the central system batholith iberian massif spain abstract this work shows the results of a u pb shrimp zircon geochronological study of the central part of the gredos massif spanish central system batholith the studied batholith is composed of several granodiorite and monzogranite tabular bodies around 1 km thick each intruded into partially molten pelitic metasediments granodiorites and monzogranites belonging to three distinct intrusive bodies and samples of anatectic leucogranites have been selected for shrimp u pb zircon geochronology distinct age groups separated by up to 20 ma have been distinguished in each sample important age differences have also been determined among the most representative age groups of the three analyzed granitoid bodies 312 6 2 8 ma for the circo de gredos bt granodiorites floor intrusive layer 306 9 1 5 ma for the barbellido plataforma granitoids top intrusive layer and 303 5 2 8 ma for las pozas crd monzogranites middle intrusive layer these age differences are interpreted in terms of sequential emplacement of the three intrusive bodies contemporary with the late paleozoic d3 deformation phase the anatectic leucogranites are coeval to slightly younger than the adjacent intrusive granodiorites and monzogranites 305 4 1 6 ma for refugio del rey leucogranites and 303 2 ma for migmatitic hornfelses it is suggested that these anatectic magmas were generated in response to the thermal effects of granodiorite intrusions thermal modeling with comsol multiphysics reveals that sequential emplacement was able to keep the thermal conditions of the batholith around the temperature of zircon crystallization in granitic melts around 750 c for several million of years favoring the partial melting of host rocks and the existence of large magma chambers composed of crystal mush prone to be rejuvenated after new intrusions

age of amphibolitic metamorphism in the ophiolitic unit of the morais allochthon portugal implications for early hercynian orogenesis in the iberian massif structurally lower portions of the allochthonous morais complex are comprised of an ophiolitic unit exposed in two contrasting structural units separated by a ductile thrust fault the lower unit is dominated by variably foliated amphibolite whereas the upper unit is largely comprised of metaperidotite and metagabbro most of the ophiolitic unit has recrystallized synkinematically under amphibolite facies metamorphic conditions c 500 c and 5 kbar amphiboles formed during this metamorphism are typically magnesio hornblende an amphibole concentrate from the lower structural unit records a 39 ar 40 ar plateau age of 397 3 4 ma an amphibole concentrate from the upper structural unit displays an internally discordant 39 ar 40 ar age spectrum corresponding to a total gas age of 431 1 27 3 ma gas fractions evolved from each amphibole concentrate at intermediate and high experimental temperatures are characterized by similar apparent k ca ratios and yield well defined 36 ar 40 ar v 39 ar 40 ar isotope correlations which result in similar ages of 384 2 5 2 and 392 4 7 3 ma these are considered to be more reliable than the plateau ages and are interpreted as dating the last cooling through those temperatures required for intracrystalline retention of argon within constituent amphibole grains the associated tectonothermal event represented a first stage of hercynian orogenesis in the iberian massif it was probably related to oceanic closure and associated obduction of granulitic eclogitic massifs which at present structurally overlie the ophiolitic unit

on the coherence of ground motion in the san fernando valley we present an analysis of the coherence of seismic ground motion recorded on alluvial sediments in the san fernando valley california using aftershocks of the 17 january 1994 m w6 7 earthquake recorded at a quasi dense array of portable stations we analyze the coherence of three well recorded magnitude 3 7 to 4 0 events over the frequency range 0 5 to 15 hz and a distance range of 0 5 to 5 3 km all stations are located at sites with broadly similar near site geology characterized by medium to fine grain quaternary alluvial sediments on average relatively high values of coherence are observed for distances up to 3 to 4 km and frequencies up to 2 to 3 hz coherence drops sharply at frequencies near and above 3 hz although average coherence functions are described reasonably well by a log linear relationship with frequency the curves at all distances exhibit a flattening at low frequencies that is not consistent with previous observations of coherence at hardrock sites the distance decay of coherence is also markedly less strong with high coherence values observed over station separations corresponding to multiple wavelengths this may reflect fundamental differences in shallow wave propagation in the two environments with high frequency scattering relatively more dominant in regions of hard rock near surface geology within a sedimentary basin or valley the site response itself generally reflects a resonance phenomenon that may tend to give rise to more uniform ground motions however previous studies have demonstrated the existence of pathological focusing and amplification effects within complex sedimentary basin environments such as the greater los angeles region our results undoubtedly do not quantify the full range of ground motion variability at all sites but rather represent the level of that variability that can be expected and quantified for typical source receiver paths

emplacement of the cuera and picos de europa imbricate system at the core of the iberian armorican arc cantabrian zone north spain new precisions concerning the timing of arc closure the final stages of the variscan orogeny mostly carboniferous in the western european variscan belt involved the development of the iberian armorican arc which is cored by the cantabrian zone nw iberia the cantabrian zone is the foreland of the western european variscan belt and it is interpreted to record the waning stages of the closure of the rheic ocean the distalmost tectonic units within the cantabrian zone the cuera unit and the picos de europa province were the last tectonic units emplaced at the core of the western european variscan belt orocline together they form an imbricate system and associated wedge top basins that are key to understanding the development of the orocline the emplacement of the cuera unit and the picos de europa province occurred in the latest pennsylvanian between moscovian and gzhelian times new detailed mapping together with stratigraphic sedimentologic and biostratigraphic data analysis of syntectonic successions and structural information constrain the timing and evolution of this imbricate system our analysis indicates that the thrust sheets were emplaced roughly perpendicular to previous tectonic units of the cantabrian variscan foreland fold and thrust belt most probably during the oroclinal bending of the sw european variscan belt that formed the iberian armorican arc the n s directed imbricate system was characterized by a shallow dip of the topographic surface 150 15 km and complex structure of the orogenic wedge are thought to result from the progressive increase in the dip of the basal decollement during bending of the underlying gondwana lithosphere and may reflect the closure of the iberian armorican arc

three dimensional finite difference modeling of the san andreas fault source parameterization and ground motion levels olsen et al 1995 recently simulated an mw 7 75 earthquake on the san andreas fault predicting long period t 2 5 sec ground velocities of 140 cm sec in the los angeles basin about 60 km from the fault these motions are much larger than estimates derived from empirical relations or other numerical simulations standard area magnitude relations predict that the 170 16 km fault used in the simulations would produce an mw 7 5 earthquake giving a moment of 2 0 1027 dyne cm which is 2 4 times smaller than the moment used by olsen et al 1995 further self similar scaling predicts a rise time of 3 sec for an mw 7 75 event and 2 2 sec for an mw 7 5 event the filtered impulse slip function used by olsen et al 1995 has an effective rise time of 1 6 sec yielding a response that is about 2 times larger than expected for periods less than 5 sec this combination of high seismic moment and short rise time along with the use of a uniform slip distribution leads to the extreme ground motion levels predicted by olsen et al 1995 to quantify the sensitivity of the long period ground motion response to source parameterization we have performed 3d finite difference fd simulations using various combinations of seismic moment source rise time and slip heterogeneity these calculations incorporate the same grid dimensions fault size and bandwidth employed by olsen et al 1995 with a moment of 2 0 1027 dyne cm a rise time of 2 sec and a smoothly heterogeneous slip distribution we simulate peak long period ground velocities of 155 cm sec in the near fault region and 40 cm sec in the los angeles basin these values are much closer to although still higher than empirical predictions a uniform slip distribution produces the largest peak motions both in the near fault region and in the los angeles basin whereas a rough asperity slip distribution noticeably reduces the maximum near fault ground velocities our results indicate that the accurate simulation of long period ground motions requires a realistic source parameterization including appropriate choices of seismic moment and rise time as well as the use of spatial and temporal variations in slip distribution

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

evolution of the rheic ocean the rheic ocean which separated laurussia from gondwana following the closure of iapetus is arguably the most important ocean of the palaeozoic its suture extends from mexico to turkey and its closure produced the climactic variscan alleghanian ouachita orogeny that assembled the supercontinent pangaea following protracted cambrian rifting that represented a continuum from neoproterozoic orogenic processes the rheic ocean opened in the early ordovician with the separation of several neoproterozoic arc terranes from the continental margin of northern gondwana separation occurred along the line of a former neoproterozoic suture following the onset of subduction in the outboard iapetus ocean the timing of rift drift transition and drive for subsequent spreading was likely governed by slab pull accounting for the rapid rate 8 10 cm yr at which the rheic ocean widened during the ordovician the ocean broadened at the expense of iapetus and attained its greatest width 4000 km in the silurian by which time baltica had sutured to laurentia and the neoproterozoic arc terranes had accreted to laurussia closing iapetus in the process closure of the rheic ocean began in the devonian and was facilitated by northward subduction beneath southern baltica and southward subduction beneath northwest gondwana closure was largely complete by the mississippian as gondwana and laurussia sutured to build pangaea north africa colliding with southern europe to create the variscan orogen in the devonian carboniferous and west africa and south america suturing to north america to form the alleghanian and ouachita orogens respectively during the carboniferous permian the rheic ocean consequently plays a dominant role in the basement geology of southern europe in the appalachian ouachita orogeny of north america and in the palaeozoic sedimentary structural and tectonothermal record from middle america to the middle east with its closure the ocean brought about the assembly of pangaea and brought the palaeozoic era to an end

origin of the rheic ocean rifting along a neoproterozoic suture the rheic ocean is widely believed to have formed in the late cambrian early ordovician as a result of the drift of peri gondwanan terranes such as avalonia and carolina from the northern margin of gondwana and to have been consumed in the devonian carboniferous by continent continent collision during the formation of pangea other peri gondwanan terranes e g armorica ossa morena northwest iberia saxo thuringia moldanubia remained along the gondwanan margin at the time of rheic ocean formation differences in the neoproterozoic histories of these peri gondwanan terranes suggest the location of the rheic ocean rift may have been inherited from neoproterozoic lithospheric structures formed by the accretion and dispersal of peri gondwanan terranes along the northern gondwanan margin prior to rheic ocean opening avalonia and carolina have sm nd isotopic characteristics indicative of recycling of a juvenile ca 1 ga source and they were accreted to the northern gondwanan margin prior to voluminous late neoproterozoic arc magmatism in contrast sm nd isotopic characteristics of most other peri gondwanan terranes closely match those of eburnian basement suggesting they reflect recycling of ancient 2 ga west african crust the basements of terranes initially rifted from gondwana to form the rheic ocean were those that had previously accreted during neoproterozoic orogenesis suggesting the rift was located near the suture between the accreted terranes and cratonic northern gondwana opening of the rheic ocean coincided with the onset of subduction beneath the laurentian margin in its predecessor the iapetus ocean suggesting geodynamic linkages between the destruction of the iapetus ocean and the creation of the rheic ocean

orocline timing through joint analysis insights from the ibero armorican arc abstract the timing and kinematics of oroclinal bending in the core of the ibero armorican arc iaa has recently been constrained using paleomagnetic data from the cantabrian zone in northern iberia this study analyzes the joint patterns present in rock units deposited pre syn and post oroclinal bending systematic changes in the orientations of tensional joint sets in superimposed stratigraphic units are interpreted to record the progressive stages of oroclinal bending in the core of the iaa time constrains for joint set development are constrained by the known ages of the bounding unconformities that limit the studied stratigraphic units joint azimuth variability in the pre orocline rocks neoproterozoic to pre upper carboniferous is comparable to the present arc curvature of the orocline about 180 the joints in the syn orocline rocks upper pennsylvanian or stephanian 304 to 299 ma show a lower azimuthal variability that is comparable to about 50 70 of the total curvature seen in pre orocline rocks finally post orocline rocks permian contain joints that have uniform azimuths for each set across the entirety of the present day arc together these spatially and temporally distinct joint sets suggest that rotations in the cantabrian zone took place in the upper pennsylvanian during a ca 10 ma time period which agrees well with previous paleomagnetic arguments the data also provides supporting evidence for oroclinal bending by rotation around vertical axes of an initially linear or nearly linear orogenic belt and finally these data highlight the potential power in using tectonic joint sets for constraining thrust belt kinematics in curved orogenic systems when unconformity bounded stratigraphic sequences are present that are coeval with orocline development

upper neoproterozoic lower cambrian sedimentary successions in the central iberian zone spain sequence stratigraphy petrology and chemostratigraphy implications for other european zones o3 tio2 zr and nb abundances in shales from all the units strongly suggest a gradual compositional change within this sedimentary succession together with the petrological data the chemical results do not reveal any obvious coeval volcanic contribution to the sediments on the basis of the chemical data a comparison is made with other european zones containing detrital sediments composed of reworked crustal components

tectonothermal evolution associated with variscan crustal extension in the tormes gneiss dome nw salamanca iberian massif spain abstract the tectonothermal variscan evolution of the tormes gneiss dome is controlled by a subhorizontal ductile shear zone of crustal scale and extensional characteristics that induced a quick exhumation of the deep parts of the metamorphic complex during crustal thinning the shearing affected a broad band of metamorphic rocks but as the temperature decreased became progressively concentrated in a low grade ductile detachment that separates two distinct units characterized by their lithology and different tectonothermal evolution kinematic indicators in non coaxial fabrics show a displacement of the hanging wall to the southeast i e parallel to the trend of the foldbelt the subhorizontal shearing is related to the extensional collapse of the variscan crust previously thickened during the collision tectonics

broadband modeling of local earthquakes three component broadband waveforms of two small earthquakes near upland california recorded on the pasadena broadband high dynamic range instrument were modeled to obtain useful green s functions for this path and to examine the sensitivity of the synthetic seismograms to perturbations of the crustal model we assumed that the source of each event was both simple and known as determined from the caltech usgs array first motions a trapezoidal time function was chosen to fit the width of the direct s wave generalized rays reflectivity and finite difference techniques were used to compute the synthetic seismograms we found that a simple layer over a half space model is an adequate approximation of the upper crust along this profile in particular the waveforms are controlled by a relatively slow 4 km thick surficial layer 4 5km s 1 2 6 km s 1 over a faster layer 5 9 km s 1 3 5 km s 1 the relative amplitudes of direct and multiple s indicate that the main shock occurred at a depth of 6 km while the aftershock occurred at a depth of 8 to 9 km sensitivity analyses indicate that for distances less than 50 km and for periods longer than 1 sec the synthetic seismograms are not very sensitive to perturbations of the deep crustal structure analysis of upper crustal model perturbations revealed that the surficial layer is between 3 to 5 km thick in addition the contact between this layer and the underlying material can be smoothed with a 2 km wide velocity gradient without adversely affecting the fit to the data two dimensional finite difference calculations show that a ridge structure beneath the recorder acts as a lowpass filter the lower frequency phases are largely unaffected other two dimensional models with ridges between the source and receiver clearly did not fit the data synthetic seismograms computed for the best fitting model were used to estimate a long period moment of 6 2 1022 dyne cm ml 4 6 and 1 1022 dyne cm ml 3 7 with identical triangular source time durations of 0 3 sec assuming the same fault dimension of 0 4 km from standard scaling laws stress drop estimates of 410 and 70 bars are obtained for the two events respectively generally we found that it is possible to reproduce local waveforms at frequencies up to 1 hz without a complete knowledge of fine structural detail resulting green s functions can be useful in studying historic events and in simulations of large events from a given source region

modified frequency wavenumber algorithm for regional seismograms using filon s quadrature modelling of lg waves in eastern north america summary the main focus of this study is to develop an efficient frequency wavenumber code to synthesize high frequency say 10 hz regional seismograms up to a distance of more than 1000 km in a medium consisting of a large number of crustal layers and investigate effects of regional waveguides on lg waves in this newly developed code the frequency wavenumber response is evaluated by using the filon s quadrature at high frequency and the polynomial approximation of bessei s functions implementing trapezoidal integration at low frequency this has reduced the computation time by several folds compared to the time needed for the trapezoidal integration using this algorithm both broad band seismograms at harvard and short period ectn eastern canadian telemetered network seismograms from 1988 november 25 saguenay earthquake were successfully modelled a moment magnitude of 5 9 was used initially an average waveguide was calibrated to successfully excite the regional pnl and snl waves at hrv its near surface structure constrained by matching the recorded frequency content the crust mantle transition zone was constrained by modelling the strength of the broad band snl waves consisting of sn ssmsnsmssms and ssmssms phases relative to the initial pnl waves the model was modified to consist of thin layers with an alternating high and low velocity distribution to match the high frequency ectn lg waves observed from 300 to 600 km away in addition the effect of a near surface irregular receiver structure on the duration of lg waves was also investigated by convolving its response with the regional lg seismograms initial study suggests this method as viable for modelling the entire lg waves including the coda numerical studies suggest that the shapes and peak amplitudes on lg seismograms depend more strongly on the source depth anelasticity and crustal waveguide than do the pnl seismograms

correlation of the nappe stack in the ibero armorican arc across the bay of biscay a joint french spanish project a correlation between allochthonous units exposed in the nw iberian massif and the southern armorican massif is carried out based on lithological associations structural position age and geochemistry of protoliths and tectonometamorphic evolution the units on both sides of the bay of biscay are grouped into upper middle and lower allochthons whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the parautochthon the lower allochthon represents a fragment of the outermost edge of gondwana that underwent continental subduction shortly after the closure of a palaeozoic ocean which in turn is represented by the middle allochthon the latter consists of supra subduction ophiolites and metasedimentary sequences alternating with basic mid ocean ridge basalt morb type volcanics with inheritances suggesting the proximity of a continental domain seafloor spreading began at the cambro ordovician boundary and oceanic crust was still formed during the late devonian covering the lifetime of the rheic ocean which is possibly represented by the middle allochthon the opening of the oceanic domain was related to pulling apart the peri gondwanan continental magmatic arc which is represented by the upper allochthon

site response at treasure and yerba buena islands california a variety of methods are utilized to reinvestigate the physical relationship between the seismic response of treasure island ti and yerba buena island ybi in california these islands are a soil ti and rock ybi site pair separated by 2 km the site pair has been used previously by researchers to identify soil response to earthquake shaking linear regime ground motions mw4 0 mw4 6 and pga 0 014 0 017 g recorded in the ti vertical array indicate a coherent wavefield in the sediments and an incoherence between the rock and sediments our analyses show that the greatest change in the wavefield occurred between the rock and soil layers corresponding to a significant impedance contrast the waveforms change very little as they propagate through the sediments indicating that the site response is a cumulative effect of the entire soil structure and not a result of wave propagation within individual soil layers in order to highlight the complexity of the site response correlation analysis was used to demonstrate that the rock and soil ground motions were not highly coherent between the two sites ybi was therefore shown to be an inappropriate reference site for ti one dimensional 1d vertical wave propagation and inverse techniques were used to differentiate between 1d site response and more complex site behavior both 1d methods vertical wave propagation and inverse transfer functions proved incapable of capturing the site response at ti beyond the initial four seconds of motion finite difference waveform modeling based on a two dimensional velocity structure of the northern san francisco bay was needed to explain the linear site response at ti as horizontally propagating surface waves trapped in the bay sediments a simplified velocity structure for the san francisco bay including a single 100 m basin layer constant shear wave velocity of 400 m s over a 1 5 km s layer of franciscan bedrock was able to trap energy in the basin and produce surface waveform ringing similar to that observed in the ti data due to surface waves propagating in the san francisco bay sediments any 1d model will not fully characterize site response at ti all 1d models will fail to produce the late arriving energy observed in the ground motions

generation and emplacement of shear related highly mobile crustal melts the synkinematic leucogranites from the variscan tormes dome western spain the tormes dome consists of s type granites that intruded into ordovician augen gneisses and neoproterozoic lower cambrian metapelites metagreywackes at different extents of migmatization s type granites are mainly equigranular two mica granites occurring as 1 enclave laden subvertical feeder dykes 2 small external sill like bodies with size and shape relations indicative for self similar pluton growth and 3 as large pluton bodies emplaced at higher levels than the external ones these magmas were highly mobile as it is inferred from the high contents of fluxing components the disintegration and alignment of pelitic xenoliths in feeder dykes and at the bottom of some sill like bodies field relations relate this 311 ma magmatism u pb monazite to the regional shearing of the d3 variscan event partial melting modeling and the relatively high estimated liquidus temperatures indicate biotite dehydration partial melting 800 840 c and 400 650 mpa rather than water fluxed melting implying that there was no partial melting triggered by externally derived fluids in the shear zones instead the subvertical shear zones favored extraction of melts that formed during the regional migmatization event around 320 ma nd isotope variation among the granites might reflect disequilibrium partial melting or different protoliths mass balance and trace element partial melting modeling strongly suggest two kinds of fertile crustal protoliths augen gneisses and metapelites slight compositional variation among the leucogranites does not reflect different extent of protolith melting but is related to a small amount of fractional crystallization 13 for the equigranular granites which is generally more pronounced in shallower batholitic leucogranites than in the small and homogeneous sill like bodies the lower extent of fractional crystallization and the higher pressure emplacement conditions of the sill like bodies support a more restricted movement through the crust than for batholitic leucogranites

three dimensional structure influences on the strong motion wavefield of the 1989 loma prieta earthquake abstract the accurate calculation of ground motions for future large earthquakes depends on detailed knowledge of three dimensional 3d geologic structure and the earthquake source process as well as sufficient computational resources in this article we describe the results of finite difference simulations for the 1989 loma prieta ca earthquake with a 3d seismic velocity model for the san francisco bay region and a heterogeneous slip model of the source additionally we explore the sensitivity of the synthetics to the major geologic structures in the velocity model the san francisco bay region particularly the loma prieta region is a unique area for the study of 3d wave propagation because of the pronounced lateral velocity contrasts across the strike slip faults of the region understanding the effects of such long wavelength structure is doubly important when considering a source located close to such a contrast our simulations show that the lateral velocity contrast across the san andreas fault saf would be expected to substantially affect the propagation of elastic waves radiated from a source in the loma prieta region indeed we find that the refraction of energy by the saf serves to reduce ground motions at stations located along the san francisco peninsula and the quaternary and tertiary alluvial basins of the san francisco bay region are found to amplify and extend the duration of ground motions in santa clara valley livermore valley and san pablo bay we find that the 3d model as currently defined accurately describes the spatial variation of peak ground velocity for frequencies less than 0 5 hz which suggests that this model may be used to estimate ground motions for future earthquake scenarios

burial claystones remagnetization and some consequences for magnetostratigraphy we investigate the broad lines of magnetic mineral formation for non metamorphic claystones more particularly we focus on the formation of magnetite from c 20 day experiments aiming to reproduce burial conditions it is shown that the sole action of temperature from 50 c to 250 c leads to the formation of magnetite the neoformed magnetites carry a chemical remanent magnetization which equates at least the natural remanent magnetization we propose a schematic of burial with three magnetic windows where a chemical remanent magnetization superimposed the natural remanent magnetization these are the greigite window subsurface the magnetite window depth 2 km and the pyrrhotite window depth 6 km the formation of magnetic minerals has profound consequences for the magnetostratigraphy record we propose a conceptual model that shows that the continuous production of magnetite during burial may result in magnetozones that have no relation to the age of the sediment

a comparison of coda and s wave spectral ratios as estimates of site response in the southern san francisco bay area abstract we calculate spectral ratios for s waves and codas to evaluate the amplification of 14 sites in the southern san francisco bay area relative to a nearby bedrock site in the coyote hills our data are seismograms written by loma prieta aftershocks the epicentral distances and azimuths to the stations are effectively the same all the sites in the study are amplified with respect to the reference site at frequencies from 0 5 to 7 hz the shapes of the s wave and coda spectral ratios are similar but the coda ratios are greater than the s wave ratios by as much as a factor of 4 the difference is larger for sites on alluvial and bay mud deposits particularly at frequencies around 1 hz suggesting the presence of waves trapped in the alluvial basin in general the length of the analysis window affects the s wave spectral ratios for alluvial sites longer windows give ratios similar to coda ratios apparently because these windows include more of the phases that contribute to the coda we classify the sites according to their geological characteristics and surficial shear wave velocities for the s wave ratios the differences between the classes showed no systematic trend the softest and hardest soil classes we consider have practically identical s wave amplifications the average coda ratios for the site classes clearly increase as the soil classes include slower and softer materials after correction for differences in reference sites the coda amplifications are very similar to the relative amplifications for these site classes estimated by borcherdt and glassmoyer 1992 from the strong motion recordings of the 1989 loma prieta earthquake

the effects of unconsolidated sediments upon the ground motion during local earthquakes abstract a vertical seismic array at the richmond field station on the margin of san francisco bay california has provided some excellent experimental data showing the effects of unconsolidated sediments on ground motion during local earthquakes the experiments are well controlled in that good estimates of the material properties of the sediments are available with the largest uncertainty being in the attenuation three components of acceleration have been recorded in bedrock below the sediments within the sediments and at the surface of the sediments from two local earthquakes the maximum accelerations at the surface are between 1 5 and 4 3 times those in the bedrock analytical calculations which assume vertically propagating waves and which are suitable for highly attenuating materials explain the major differences between the bedrock records and the surface records for both vertical and horizontal components maximum accelerations duration frequency content general character of the records and response spectra are all in approximate agreement some of the energy arriving late in the records can best be explained by surface waves propagating horizontally in the sediments

uncertainty in vs30 based site response abstract methods that account for site response range in complexity from simple linear categorical adjustment factors to sophisticated nonlinear constitutive models seismic hazard analysis usually relies on ground motion prediction equations gmpes within this framework site response is modeled statistically with simplified site parameters that include the time averaged shear wave velocity to 30 m v s 30 and basin depth parameters because v s 30 is not known in most locations it must be interpolated or inferred through secondary information such as geology or topography in this article we analyze a subset of stations for which v s 30 has been measured to address effects of v s 30 proxies on the uncertainty in the ground motions as modeled by gmpes the stations we analyze also include multiple recordings which allow us to compute the repeatable site effects or empirical amplification factors eafs from the ground motions although all methods exhibit similar bias the proxy methods only reduce the ground motion standard deviations at long periods when compared to gmpes without a site term whereas measured v s 30 values reduce the standard deviations at all periods the standard deviation of the ground motions are much lower when the eafs are used indicating that future refinements of the site term in gmpes have the potential to substantially reduce the overall uncertainty in the prediction of ground motions by gmpes

paleomagnetism of the central iberian curve s putative hinge too many oroclines in the iberian variscides abstract the variscan mountain belt in iberia defines a large s shape with the cantabrian orocline in the north and the central iberian curve an alleged orocline belt of opposite curvature to the south the cantabrian orocline is kinematically well constrained but the geometry and kinematics of the central iberian curve are still controversial here we investigate the kinematics of the central iberian curve which plays an important role in the amalgamation of pangea since it may have accommodated much of the post collisional deformation we have performed a paleomagnetic study on carboniferous granitoids and cambrian limestones within the hinge of the curve our paleomagnetic and rock magnetic results show a primary magnetization in the granitoids and a widespread carboniferous remagnetization of the limestones syn kinematic granitoids show ca 70 counter clockwise rotations consistent with the southern limb of the cantabrian orocline post kinematic granitoids and cambrian limestones show consistent inclinations but very scattered declinations suggesting that they were magnetized coevally to and after the 70 rotation our results show no differential rotations between northern southern limb and the hinge zone therefore we discard a late carboniferous oroclinal origin for the central iberian curve

seismic signature of a phreatic explosion hydrofracturing damage at karthala volcano grande comore island indian ocean karthala volcano is a basaltic shield volcano with an active hydrothermal system that forms the southern two thirds of the grande comore island off the east coat of africa northwest of madagascar since the start of volcano monitoring by the local volcano observatory in 1988 the july 11th 1991 phreatic eruption was the first volcanic event seismically recorded on this volcano and a rare example of a monitored basaltic shield from 1991 to 1995 the vt locations 0 5 ml 4 3 show a crack shaped pattern 3 km long 1 km wide within the summit caldera extending at depth from 2 km to 2 km relative to sea level this n s elongated pattern coincides with the direction of the regional maximum horizontal stress as deduced from regional focal mechanism solutions this brittle signature of the damage associated with the 1991 phreatic eruption is a typical pattern of the seismicity induced by controlled fluid injections such as those applied at geothermal fields in oil and gas recovery or for stress measurements it suggests the 1991 phreatic eruption was driven by hydraulic fracturing induced by forced fluid flow we propose that the extremely high lp and vt seismicity rates relative to other effusive volcanoes during the climax of the 1991 phreatic explosion are due to the activation of the whole hydrothermal system as roughly sized by the distribution of vt hypocenters the seismicity rate in 1995 was still higher than the pre eruption seismicity rate and disagrees with the time pattern of thermo elastic stress readjustment induced by single magma intrusions at basaltic volcanoes we propose that it corresponds to the still ongoing relaxation of pressure heterogeneity within the hydrothermal system as suggested by the few lp events that still occurred in 1995

are the oroclines of the variscan belt related to late variscan strike slip tectonics terra nova 23 241 247 2011 abstract early variscan structures and magnetic anomalies delineate four oroclines along the variscan belt the arcs bend the lithostratigraphic zones of the autochthon and the allochthonous terranes which include ophiolites of the rheic suture and units of gondwanan derivation subducted on both sides of the rheic ocean a transcurrent model dominated by right lateral motion explains the genesis and evolution of the oroclines and a simple plate tectonics scenario links late variscan dextral strike slip movements in the variscides to a broad intracontinental shear zone kinematically equivalent to a transform fault terra nova 00 1 7 2011

the loma prieta earthquake ground motion and damage in oakland treasure island and san francisco abstract the basis of this study is the acceleration velocity and displacement wave forms of the loma prieta earthquake 18 october 1989 m 7 0 at two rock sites in san francisco a rock site on yerba buena island an artificial fill site on treasure island and three sites in oakland underlain by thick sections of poorly consolidated pleistocene sediments the waveforms at the three rock sites display a strong coherence as do the three sedimentary sites in oakland the duration of strong motion at the rock sites is very brief suggestive of an unusually short source duration for an earthquake of this size while the records in oakland show strong amplification effects due to site geology the s wave group at treasure island is phase coherent with the oakland records but at somewhat diminished amplitudes until the steps in acceleration at approximately 15 sec apparently signaling the onset of liquefaction all seven records clearly show shear wave first motion opposite to that expected for the mainshock radiation pattern and peak amplitudes greater than expected for sites at these distances 95 3 km from an earthquake of this magnitude while the association between these ground motion records and related damage patterns in nearby areas has been easily and eagerly accepted by seismological and engineering observers of them we have had some difficulty in making such relationships quantitative or even just clear the three oakland records from sites that form a nearly equilateral triangle about the cypress street viaduct collapse are dominated by a long period resonance 1 1 2 sec period far removed from the natural frequency of the structure to transverse motion 2 5 hz or from high frequency amplification bands observed in aftershock studies a spectral ratio arbiter of this discrepancy confuses it further the failure of the east bay crossing of the san francisco oakland bay bridge cannot be attributed to relative displacements of the abutments in oakland and yerba buena island but the motions of the bay bridge causing failure remain unknown the steps in acceleration at treasure island present unusual strong motion accelerogram processing problems and modeling suggests that the velocity and displacement waveforms are contaminated by a spurious response of the filtering operations to the acceleration steps a variety of coincidences suggests that the treasure island accelerogram is the most likely strong motion surrogate for the filled areas of the marina district for which no mainshock records are available but the relative contributions of bad ground poor construction and truly strong ground motion to damage in the marina district will never by known in any quantitative way the principal lesson of all of this is that until a concerted effort is mounted to instrument ground and structures that are likely to fail during earthquakes our understanding of the very complex relationships between strong ground motion and earthquake damage will in general remain rudimentary imprecise and vague

untangling inconsistent magnetic polarity records through an integrated rock magnetic analysis a case study on neogene sections in east timor inconsistent polarity patterns in sediments are a common problem in magnetostratigraphic and paleomagnetic research multiple magnetic mineral generations result in such remanence haystacks here we test whether end member modeling of isothermal remanent magnetization acquisition curves as a basis for an integrated rock magnetic and microscopic analysis is capable of isolating original magnetic polarity patterns uppermost miocene pliocene deep marine siliciclastics and limestones in east timor originally sampled to constrain the uplift history of the young timor orogeny serve as case study an apparently straightforward polarity record was obtained that however proved impossible to reconcile with the associated biostratigraphy our analysis distinguished two magnetic end members for each section which result from various greigite suites and a detrital magnetite suite the latter yields largely viscous remanence signals and is deemed unsuited the greigite suites are late diagenetic in the cailaco river section and early diagenetic thus reliable in the viqueque type section by selecting reliable sample levels based on a quality index a revised polarity pattern of the latter section is obtained consistent with the biostratigraphy and unequivocally correlatable to the geomagnetic polarity time scale although the cailaco river section lacks a reliable magnetostratigraphy it does record a significant postremagnetization tectonic rotation our results shows that the application of well designed rock magnetic research based on the end member model and integrated with microscopy and paleomagnetic data can unravel complex and seemingly inconsistent polarity patterns we recommend this approach to assess the veracity of the polarity of strata with complex magnetic mineralogy

consistency of ground motion estimates made using system identification this article isolates the systematic effects of the soil profile on earthquake ground motions recorded at vertical seismic arrays an empirical green s function is calculated for a soil interval between two sensors in a vertical array the estimation technique used here falls into the category of site response estimates but differs from the standard spectral ratio methods in that an extended weiner filter arma model becomes the impulse response function from one point to another in the soil profile this method allows quantification of the accuracy of predicted ground motions at a given site with normalized mean square prediction errors generally under 10 indicating effective and consistent estimates the models are shown to reproduce site ground motions for inputs that differ over a wide range of peak ground acceleration pga hypocentral locations and over numerous occurrences we demonstrate consistency of site ground motion estimates at four sites that have undergone multiple strong shakings chiba japan garner valley california lotung taiwan and port island japan consistency of site response over a wide range of shaking levels indicates an effective range of linear soil behavior for pga up to 0 33 g at chiba 0 1 g at gvda and 0 21 g at lotung at port island consistency was evident below 16 m at a surficial pga of 0 54 g but the surface layer liquefied results are presented as predicted time series and in the frequency domain with calculated variance results from garner valley and chiba indicate that the location and magnitude of significant impedance contrasts within the soil profile control response this article concentrates on levels of shaking that do not excite the soils into phase transitions our results indicate that strong nonlinear behavior liquefaction controls site response but more minor and localized softening of the soil may not alter the site response enough to deter prediction within 10

3d viscoelastic wave propagation in the upper borrego valley california constrained by borehole and surface data we have simulated 2 hz wave propagation in a three dimensional model of the upper borrego valley southern california for a m 4 9 earthquake with epicenter 5 km north of the valley a 4th order staggered grid finite difference method was used to calculate viscoelastic ground motion in a basin model 9 km by 5 km by 0 4 km consisting of heterogeneous sediments surrounded by bedrock we simulated the earthquake as a double couple point source and computed the ground motions in the valley separately for the parts of the source incident from below and from the north the earthquake was recorded by a surface array as well as a deep downhole array 0 238 m depth in the center of the valley all equipped with digital three component seismic instruments the simulation reproduces the overall pattern of ground motions at basin and borehole sites and shows a good correlation of observed to synthetic waveforms in particular the 3d simulation reproduces the recorded peak motions cumulative kinetic energies and fourier spectral amplitudes within a factor of 2 for most components at the individual sites the correlation between data and simulation allows us to identify the secondary arrivals in the records as love and rayleigh waves generated at the edges of the valley and the troughs of the basin the peak velocities for the waves incident into the valley from below are generally more than an order of magnitude larger than those for the waves incident from the north the success of the prediction requires the inclusion of anelastic attenuation in the simulation with q values for p and s waves in the alluvium of about 30 we also used a profile of the 3d model and the soil parameters at the deep borehole to examine the ability of 2 5d and 1d model approximations to predict the data the maximum peak velocities and total cumulative kinetic energies are reproduced at the recording sites within a factor of 2 for both 2 5d and 1d model approximations but are underpredicted by up to an order of magnitude at some depths for individual components in particular the 2 5d and 1d simulations tend to underpredict the duration

permian clockwise rotations of the ebro and corso sardinian blocks during iberian armorican oroclinal bending preliminary paleomagnetic data from the catalan coastal range ne spain abstract paleomagnetic investigations of early permian and triassic magmatic rocks from catalan coastal ranges provide three magnetic directions which are interpreted as primary magnetizations and partly as overprints the succession of magnetic directions is interpreted in terms of a succession of large scale clockwise rotations which bring the whole assembly of pyrenees catalonia corsica sardinia and maures esterel massifs from the southeast of the massif central to the west during permian this major movement is associated with clockwise rotation of northern limb of the iberian orocline during latest carboniferous and early permian 305 290 ma subsequently permian large scale dextral transtensional extensional shearing operated along aquitanian shear belt between the massif central in the northeast and the iberian variscan massifs in the southwest the latest phase of the rotation in the late permian early triassic is associated with alkaline magmatism probably linked to neo tethys opening activity in western europe

the contorted new england orogen eastern australia new evidence from u pb geochronology of early permian granitoids 1 a series of sharp bends oroclines are recognized in the paleozoic to early mesozoic new england orogen of eastern australia the exact geometry and origin of these bends is obscured by voluminous magmatism and is still debated here we present zircon u pb ages that confirm the lateral continuation of early permian 296 288 ma granitoids and shed new light on the oroclinal structure orogenic curvature is defined by the alignment of early permian granitoids parallel to the structural grain of the orogen as well as the curved geometry of sub vertical deformation fabrics forearc basin terranes and serpentinite outcrops alternative geometrical interpretations may involve two bends texas and coffs harbour oroclines three bends manning orocline or even four bends nambucca orocline we argue that the model involving four bends is most consistent with available data although further kinematic constraints are required to confirm the existence of the manning and nambucca oroclines a subsequent phase of younger magmatism 260 ma cuts across the curved structural grain providing a minimum age constraint for orocline development assuming a structure of four oroclines we suggest a tentative tectonic model that involves an early stage of subduction curvature during slab rollback at 300 285 ma followed by bending associated with dextral transpression a final tightening of the curved structures was possibly obtained by e w shortening during the late permian to triassic 265 230 ma hunter bowen orogeny

kinematic constraints on buckling a lithospheric scale orocline along the northern margin of gondwana a geologic synthesis abstract the paleozoic variscan orogeny was a large scale collisional event involving amalgamation of multiple continents and micro continents existing data suggests oroclinal buckling of an originally near linear convergent margin during the last stages of variscan deformation in the late paleozoic closure of the rheic ocean resulted in e w shortening present day coordinates in the carboniferous producing a near linear n s trending east verging belt subsequent n s shortening near the carb permian boundary resulted in oroclinal buckling this late stage orogenic event remains an enigmatic part of final pangea amalgamation the present day arc curvature of the variscan has inspired many tectonic models with little agreement between them while there is general consensus that two separate phases of deformation occurred various models consider that curvature was caused by dextral transpression around a gondwana indentor strike slip wrench tectonics or a change in tectonic transport direction due to changing stress fields more recent models explain the curvature as an orocline with potentially two opposite facing bends caused by secondary rotations deciphering the kinematic history of curved orogens is difficult and requires establishment of two deformation phases an initial compressive phase that forms a relatively linear belt and a second phase that causes vertical axis rotation of the orogenic limbs historically the most robust technique to accurately quantify vertical axis rotation in curved orogens is paleomagnetic analysis but recently other types of data including fracture geochemical petrologic paleo current and calcite twin data have been used to corroborate secondary buckling a review of existing and new variscan data from iberia is presented that argues for secondary buckling of an originally linear orogenic system together these data constrain oroclinal buckling of the cantabrian orocline to have occurred in about 10 ma during the latest carboniferous which agrees well with recent geodynamical models and structural data that relate oroclinal buckling with lithospheric delamination in the variscan

zircon thermometry and u pb ion microprobe dating of the gabbros and associated migmatites of the variscan toledo anatectic complex central iberia in the central iberian zone there are several large thermal domes in which small bodies of ultramafic mafic and intermediate rocks appear intimately associated with crustal granites and migmatites the closest spatial association between the ultramafic mafic and intermediate rocks and migmatites is in the toledo anatectic complex where field relationships suggest that these rocks are coeval and have an age close to 340 ma this and the recent discovery in the neighbouring ossa morena zone of a large mid crustal seismic reflector interpreted as a 335 350 ma mafic sill reinforce the hypothesis that heat for crustal melting was supplied from early variscan mantle magmas emplaced in the middle crust however precise ion microprobe u pb zircon dating and ti in zircon thermometry in toledo do not support this idea whereas the mean age of four mafic bodies is 307 2 ma the migmatites are c 25 ma older the migmatites hosting ultramafic mafic and intermediate bodies have the same age and ti in zircon temperatures as migmatites far from any mafic intrusion these data reveal that ultramafic mafic and intermediate magmas are late variscan they were emplaced in already cooling anatectic zones once the extensional collapse was initiated and their thermal impact on the mid crustal variscan anatexis of central iberia was negligible

pennsylvania salient of the appalachians a two azimuth transport model based on new compilations of piedmont data new compilations of geometry and tectonic transport in the pennsylvania salient suggest that these features continue uninterrupted from the piedmont through the valley and ridge provinces these data are the basis for a new two azimuth transport model for origin of the curvature edge geometry of an eocambrian craton and a thickened stratigraphy acted as templates for alleghanian regional decollement first as reading prong n35 w motion followed by n68 w blue ridge motion both displacements involved some drag rotation and overprinting in a central zone this model avoids the problem of minimal tangential stretching explains the anthracite basins as trailing edge graben and basin structures against the autochthonous pocono plateau and produces the nittany juniata culmination as an overprinted pileup of duplexes in the central zone the model of detachment followed by changing azimuth of motion of a megadecollement sheet may apply to other salients throughout the world

finite difference seismograms for sh waves the accuracy of the finite difference method for generating synthetic seismograms of sh wave propagation in cylindrically symmetric media is discussed the finite difference method has the advantage that arbitrary density and velocity fields in the medium may be specified a point source is generated by a simple transformation of a line source the accuracy of the finite difference seismograms in flat and dipping layered media is confirmed by comparison with the generalized ray method a source radiation pattern is inserted by introducing a near field which has permanent displacement near the source strong motion synthetics are constructed with this new method for the 1968 borrego mountain earthquake as recorded at el centro good fits to the data are achieved using the laterally varying model determined by a detailed refraction survey and the source parameters determined by teleseismic waveform modeling shallow faulting is no longer necessary to explain the long period surface wave development

diachronous post orogenic magmatism within a developing orocline in iberia european variscides 1 u pb zircon crystallization ages of 52 late variscan granitoid intrusions from nw iberia 19 from new data 33 from previous studies constrain the lithospheric evolution of this realm of the variscan belt of western europe and allow assessment of the relationship between oroclinal development and magmatism in late carboniferous early permian times the u pb ages in conjunction with a range of geological observations are consistent with the following sequence of events i oroclinal bending starts at 310 305 ma producing lithospheric thinning and asthenospheric upwelling in the outer arc of the orocline accompanied by production of mantle and lower crustal melts ii between 305 and 300 ma melting continues under the outer arc of the orocline central iberian zone of the iberian variscan belt and mid crustal melting is initiated coevally the lithospheric root beneath the inner arc of the orocline thickened due to progressive arc closure iii between 300 and 292 ma foundering of the lithospheric root followed by melting in the lithospheric mantle and the lower crust beneath the inner arc due to upwelling of asthenospheric mantle iv cooling of the lithosphere between 292 and 286 ma resulting in a drastic attenuation of lower crustal high temperature melting by 285 ma the thermal engine generated by orocline driven lithospheric thinning delamination had cooled down beyond its capability to produce significant amounts of mantle or crustal melts the model proposed explains the genesis of voluminous amounts of granitoid magmas in post orogenic conditions and suggests that oroclines and similar post orogenic granitoids common constituents of numerous orogenic belts may be similarly related elsewhere

modeling three dimensional site response effects in the marina district basin san francisco california abstract loma prieta aftershock data recorded in the marina district of san francisco clearly show a large amplification in ground motion and a much longer duration of shaking relative to the response recorded at nearby rock sites for frequencies between 1 and 10 hz some of the amplification can be explained by the difference in impedance between the marina sites and the rock sites however 1d models underpredict the amplification in the range 1 to 3 hz and cannot account for the increased durations the available geologic information within the marina district indicates lateral variations in sediment thickness and composition our previous experience with modeling wave propagation in these types of basin environments has shown that these structures can greatly amplify and prolong the duration of the signals observed at sites within the basin in this study the seismic response of the marina basin is modeled with a 3d finite difference wave field simulation technique results from the numerical simulations at the basin sites indicate that the large amplitude arrivals following the direct s wave are surface waves traveling at relatively slow apparent velocities in general the surface waves contain the strongest recorded motions and produce the long duration of shaking observed at these sites in addition spectral ground motions from the 3d basin models do fairly well in reproducing the observed spectral amplification near 1 hz that is characteristic of the basin sites the simulation results suggest that focusing effects due to the geometry of the basin structure itself have a significant impact in determining the resonant periods at these basin sites

one or two oroclines in the variscan orogen of iberia implications for pangea amalgamation the supercontinent pangea formed in the late carboniferous as a result of the gondwana laurussia collision producing the strongly sinuous variscan alleghanian orogen iberia is interpreted to comprise two variscan bends forming an s shaped orogenic belt the cantabrian orocline to the north and the central iberian bend to the south coeval formation of both oroclines however requires significant north south shortening in present day coordinates during pangea s amalgamation in contrast to the cantabrian orocline neither the kinematics nor geometry of the central iberian bend is well constrained we provide paleomagnetic data from the southern limb of the central iberian bend showing 60 counterclockwise vertical axis rotation during the late carboniferous to early permian comparable to that determined for the southern limb of the cantabrian orocline this result is incompatible with the hypothesized s shaped bend in the iberian variscides we argue that central iberia if really bent must have acquired its curvature before the cantabrian orocline the curvature being an inherited structure we propose a new mechanism of pangea formation compatible with the geology geochronology and paleomagnetism in which a clockwise rotation of gondwana produces the necessary change in the stress field to form the late variscan cantabrian orocline

a non cylindrical model for the northwest iberian allochthonous terranes and their equivalents in the hercynian belt of western europe abstract a case is made for a non cylindrical development of the european hercynian belt far travelled allochthonous terranes in the belt consist of a mixture of units including catazonal units ophiolites and units derived from the continental margin of gondwana they often depict a metamorphic history of intermediate to high pressure type and caledonian to eo hercynian age and are thought to have been put together in the early stages of the orogeny through a process partly obductive directed onto the continental margin of gondwana considerations upon the disposition of the terranes their relationships with the paleogeography of the gondwana realm and the structures in them suggest that their emplacement took place in an oblique fashion at a small angle to the margin of the continent the proposed allochthon could be related to the development of an island arc or a wedge like subplate in the southern part of the baltica plate later convergence between gondwana and laurussia involving transcurrent mostly dextral movements combined with tectonic escape phenomena would account for the present complexity of the belt and for the narrow strip the allochthonous terranes delineate in westernmost europe

polarization and coherence of 5 to 30 hz seismic wave fields at a hard rock site and their relevance to velocity heterogeneities in the crust abstract except for its very onset the p wave of earthquakes and chemical explosions observed at two narrow aperture arrays on hard rock sites in the adirondack mountains have a nearly random polarization the amount of energy on the vertical radial and transverse components is about equal over the frequency range 5 to 30 hz for the entire seismogram the spatial coherence of the seismograms is approximately exp cf x where c is in the range 0 4 to 0 7 km 1 hz 1 f is frequency and x is the distance between array elements vertical radial and transverse components were quite coherent over the aperture of the array indicating that the transverse motion of the compressional wave is a property of relatively large 10 6 m 3 volumes of rock and not just an anomaly caused by a malfunctioning instrument poor instrument rock coupling or out crop scale effects the spatial coherence is approximately independent of component epicentral azimuth and range and whether p or s wave coda is being considered at least for propagation distances between 5 and 170 km these results imply a strongly and three dimensionally heterogeneous crust with near receiver scattering in the uppermost crust controlling the coherence properties of the waves

granite emplacement in orogenic compressional conditions the la alberca bejar granitic area spanish central system variscan iberian belt abstract the la alberca bejar granitic area comprises a set of intrusions of upper carboniferous to lower permian age located in the hinterland of the variscan belt of western spain gravity data anisotropy of magnetic susceptibility and microstructural studies carried out in the granitoid and its country rocks indicate that the granitoids of this region have tabular and inverted drop shapes and show sub horizontal magmatic fabrics and sw ne lineations the granitoid bodies were intruded episodically with single intrusive pulses recorded by deformation in the country rocks all intrusive events took place during d 3 compressive stage in the core of large d 1 anticlines and are not related to structures in the country rock a syn orogenic emplacement model for granite intrusion in such conditions is proposed the proposed model implies the activity of coeval emplacement mechanisms and room generation

what is a reference site many methods for estimating site response compare ground motions at sites of interest to a nearby rock site that is considered a reference motion the critical assumption in these methods is that the surface rock site record reference is equivalent to the input motion at the base of the soil layers data collected in this study show that surface rock sites can have a site response of their own which could lead to an underestimation of the seismic hazard when these sites are used as reference sites data were collected from local and regional earthquakes on digital recorders both at the surface and in boreholes at two rock sites and one basin site in the san jacinto mountains southern california the two rock sites keenwild and pinon flat are located on granitic bedrock of the southern california peninsular ranges batholith the basin site garner valley is an ancestral lake bed with watersaturated sediments on top of a section of decomposed granite which overlies the competent bedrock ground motion is recorded simultaneously at the surface and in the bedrock at all three sites when the surface rock sites are used as the reference site i e the surface rock motion is used as the input to the basin the computed amplification underestimates the actual amplification at the basin site for frequencies above 2 to 5 hz this underestimation by a factor of 2 to 4 depending on frequency and site results from the rock sites having a site response of their own above the 2 to 5 hz frequencies the near surface weathering and cracking of the bedrock affects the recorded ground motions at frequencies of engineering interest even at sites that appear to be located on competent crystalline rock the bedrock borehole ground motion can be used as the reference motion but the effect of the downgoing wave field and the resulting destructive interference must be considered this destructive interference may produce pseudo resonances in the spectral amplification estimates if one is careful the bedrock borehole ground motion can be considered a good reference site for seismic hazard analysis even at distances as large as 20 km from the soil site

impediments to predicting site response seismic property estimation and modeling simplifications abstract we compare estimates of the empirical transfer function etf to the plane sh wave theoretical transfer function ttf within a laterally constant medium for invasive and noninvasive estimates of the seismic shear wave slownesses at 13 kiban kyoshin network stations throughout japan the difference between the etf and either of the ttfs is substantially larger than the difference between the two ttfs computed from different estimates of the seismic properties we show that the plane sh wave ttf through a laterally homogeneous medium at vertical incidence inadequately models observed amplifications at most sites for both slowness estimates obtained via downhole measurements and the spectral analysis of surface waves strategies to improve the predictions can be separated into two broad categories improving the measurement of soil properties and improving the theory that maps the 1d soil profile onto spectral amplification using an example site where the 1d plane sh wave formulation poorly predicts the etf we find a more satisfactory fit to the etf by modeling the full wavefield and incorporating spatially correlated variability of the seismic properties we conclude that our ability to model the observed site response transfer function is limited largely by the assumptions of the theoretical formulation rather than the uncertainty of the soil property estimates

diachronous variscan tectonothermal activity in the nw iberian massif evidence from 40ar 39ar dating of regional fabrics abstract multigrain concentrates of hornblende and muscovite together with whole rock slate phyllite samples have been dated 27 analyses using 40ar 39ar incremental release methods along a systematic traverse across the various lithotectonic structural elements which comprise northwestern sectors of the variscan iberian massif hornblende concentrates from amphibolites in the allochthonous ordenes complex yield plateau isotope correlation ages of 425 ma and 377 ma muscovite concentrates and whole rock slate phyllite from this and the cabo ortegal complex yield plateau ages which range from 367 ma to 295 ma analyses of similar material from the relative autochthon yield plateau ages between 359 ma and 275 ma muscovite concentrates from three late to post kinematic granitic stocks yield plateau ages between 309 ma and 274 ma at least seven of the 40ar 39ar analyses from metamorphic rocks record variable thermal rejuvenation of intracrystalline argon systems associated with emplacement of proximal granitic stocks the remaining analyses may be used to constrain the local age of various variscan tectonothermal events the oldest fabric ages are recorded in allochthonous units whereas the youngest fabric ages occur along the boundary between internal and external zones middle devonian ages are recorded in the allochthon and suggest a chronological continuity with deformational events in the relative autochthon where variscan deformation initiated in the upper devonian and diachronously prograded eastward the first deformational events recorded in the limit within the internal and external zones occured ca 20 25 ma later lower namurian variscan deformation systematically prograded diachronously eastward across the orogen as new crustal material was added along the front of the developing orogenic wedge however the entire orogen remained tectonically active with different structural features forming at different times and at different places an average propagation rate of ca 5 km m y is suggested by consideration of a 20 25 ma difference in correlative fabric ages and present separations

insight into liquefaction by system identification this paper introduces system parameter modelling commonly known as system identification to the estimation of dynamic soil properties these methods are optimized for short and non stationary signals the wildlife site in california subject to two large earthquakes elmore ranch and superstition hills on 24 november 1987 was used in an example analysis as the associated data are the only publicly available record of buried and surface motions during the second larger earthquake the site soils liquefied for the first time both stationary and recursive system parameter modelling methods were applied to the unique input output data from the two earthquakes the results show weakening of the soils system as pore pressure increases estimates of natural frequency damping ratio and participation factor are given cet article presente la modelisation par parametre systeme usuellement connue comme systeme d identification afin d estimer les proprietes dynamiques des sols ces methodes sont optimisees

the formation of pangea abstract the making of pangea is the result of large scale amalgamation of continents and micro continents which started at the end of the neoproterozoic with the formation of gondwana as pieces were added to gondwana on its south american antarctica and australia side ribbon like micro continents were detached from its african and south chinese side cadomia in the late neoproterozoic avalonia and hunia in the ordovician galatia in the devonian and cimmeria in the permian cadomia was re accreted to gondwana but the other ribbon continents were accreted to baltica north china laurussia or laurasia finding the origin of these numerous terranes is a major geological challenge recently a global plate tectonic model was developed together with a large geological geodynamic database at the lausanne university covering the last 600 ma of the earth s history special attention was given to the placing of gondwana derived terranes in their original position using all possible constraints we propose here a solution for the variscan terranes another paper deals with the altaids the galatian super terrane was detached from gondwana in the devonian during the opening of paleotethys and was quickly separated into four sub terranes that started to by pass each other the leading terranes collided at the end of the devonian with the hanseatic terrane detached from laurussia in the carboniferous gondwana started to impinge onto the amalgamated terranes creating the variscan chain and the pangean super continent east of spain paleotethys remained opened until the triassic subducting northward under laurasia roll back of the paleotethyan slab triggered the collapse of most of the european variscan orogen which was replaced by series of permian rifts some of them becoming oceanized back arc basins during the triassic major force changes at the pangean plate limits at the end of the triassic provoked its break up through the opening of the proto caribbean central atlantic alpine tethys oceanic seaways

seismic waveform modeling in the los angeles basin the proximity of several recent earthquakes to the los angeles sedimentary basin provides an opportunity to isolate the effects of the basin on wave propagation the 4 october 1987 aftershock m l 5 3 of the whittier narrows sequence and the 28 june 1991 sierra madre mainshock m l 5 8 are on a similar azimuth to stations overlying the deepest part of the basin a distinctive feature of records from basin stations recording the 4 october after shock is the large amplitude of multiple s ss etc the multiples have up to twice the amplitude of the direct s phase on the tangential component at such a short range less than 25 km a horizontal seismic velocity gradient is needed to turn rays rapidly enough for large amplitude multiples to form a forward modeling approach is employed using finite difference numerical techniques that produce double couple point source solutions a model based on a recent geologic cross section constructed for the east edge of the los angeles basin generates more phases than are seen in the seismic records simpler models based on dipping layers with low shear velocities in the top few layers fit the data better the seismic velocity depth and dip of the layers are varied to fit the timing between the direct p the direct s and the first s multiple the timing and amplitude of the direct and first multiple s pulses are well modeled though the phase of the first multiple does not match the data including a steeply dipping west edge in the basin model has little effect on the synthetic waveforms except at distances near that basin edge the amplitude of ss is greatest in the deepest part of the basin where it is two times larger than direct s the coda duration increases from 8 sec to more than 20 sec from the ne to the sw the sierra madre mainshock occurred about 25 km to the ne of the whittier narrows sequence the model for whittier narrows was extended to this distance with a shallow basin between whittier and the sierra madre hypocenter to simulate the san gabriel sedimentary basin phases generated by the edge of the deep basin continue to dominate the synthetic waveforms but this model generates a lengthy coda this study shows that specific phases with frequencies up to 1 hz that have traveled through deep sedimentary basins can be explained by two dimensional seismic velocity models

earthquake site response estimation a weak motion case study using weak motion recordings of aftershocks of the 1989 loma prieta earthquake recorded in oakland california near the failed nimitz freeway two methods have been applied to estimate the site response of an alluvium site and three mud over alluvium sites the first estimate is the traditional spectral ratio and the second utilizes the cross spectrum recordings obtained at a nearby bedrock site are used as estimates of the sediment site input motions while the two site response estimates produce similar peaks and troughs there is an approximate factor of 2 difference in amplitudes this discrepancy is evidence that there is a much greater level of noise than would be expected from the pre event ambient noise we interpret this as signal generated noise produced by scattering from heterogeneities which causes the true sediment site input to differ significantly from the bedrock site recording given this level of noise the cross spectrum estimate suffers a severe downward bias by a factor of 2 in this study and should probably not be used when the input motion is estimated from a bedrock site recording the spectral ratio estimates are relatively unbiased but the level of noise introduces a large degree of uncertainty therefore inferences about site response from individual spectral ratios should probably be avoided on the other hand ensemble averages of the estimates significantly reduce the scatter to reveal resonances that agree quite well in frequency and overall shape with those of one dimensional models whose parameters were determined independently a discrepancy of higher observed amplitudes than predicted by theory remains unexplained but most likely results from the effects of boundary layer topography which are not accounted for by the simple one dimensional models

the late variscan ht lp metamorphic event in nw and central iberia relationships to crustal thickening extension orocline development and crustal evolution

ground motion amplification in the marina district abstract following the 18 october 1989 loma prieta earthquake triggered seismographs were deployed in and around the marina district to investigate site amplification in the 3 weeks after the mainshock 16 aftershocks were recorded by two or more of these stations two additional stations deployed in march 1990 recorded three earthquakes that occurred in contra costa county by recasting the method of spectral ratios into a generalized inverse problem we combine the shear wave spectra from these 19 aftershocks to estimate the relative site amplification as a function of frequency all five stations located in the marina district are amplified by factors of 6 to 10 at 1 hz and 2 to 4 at 3 hz relative to a station located on franciscan sandstone at fort mason irrespective of whether they are sited on artificial fill or beach sand two stations located on dune sand just outside the marina district are amplified by factors of 2 to 4 for frequencies above 1 hz four stations located on franciscan sandstone in pacific heights nob hill rincon hill and diamond heights show little relative amplification conditional estimates of the mainshock spectra can be determined at stations that only recorded aftershocks if they can be linked to stations that recorded the mainshock these estimates are predicated on the assumption that the ground behaved linearly in the mainshock an assumption that is clearly violated for those sites where there was ground failure using this method of extrapolation for a site in the marina district that showed no evidence of ground failure yields mainshock spectra that are slightly greater than the mainshock spectra from the outer harbor wharf in oakland and near the pacific park plaza in emeryville and that significantly exceed the mainshock spectra from all the free field accelerograms located in san francisco

syn collisional extensional collapse parallel to the orogenic trend in a domain of steep tectonics the salamanca detachment zone central iberian zone spain the area south of the town of salamanca is a zone of the spanish variscan belt characterized by a first compressional event which gave rise to steep structures followed by the development of an extensional ductile shear zone more than 4 km thick that is described as the salamanca detachment zone the strain associated to the detachment is analyzed using quartz pebbles in conglomerates comparing the measured strain values with theoretical strain paths leads to the conclusion that the deformation was approximately of plane strain type quartz c axis fabrics and kinematic criteria indicate that simple shear was a very important component of the deformation though coaxial components were probably involved the extensional character of the detachment is indicated by the geometry of the metamorphic zones and the metamorphic evolution taken together the data indicate that the extensional event was related to gravitational collapse induced by the thickening of the continental crust and that it was syn collisional the movement of the hangingwall unit was to the southeast parallel to the trend of the fold belt and the translation was of the order of one to a few dozens of km though important this is not exceedingly large and allowed the preservation of low grade metamorphic conditions in the hangingwall preventing it from being affected by extensive brittle tectonic processes

dating of lithospheric buckling 40ar 39ar ages of syn orocline strike slip shear zones in northwestern iberia abstract orogenic curvature is a ubiquitous feature of mountain belts and the plate tectonic and geodynamic setting responsible for the development of curved orogens is a subject of debate in order to distinguish between different models of orocline formation it is necessary to tightly constrain the absolute timing of oroclinal development however determining the absolute isotopic timing of oroclinal bending is challenging the most direct method available to constrain the time interval during which an orocline formed is to obtain absolute age constraints on fabrics generated within syn orocline strike slip shear zones that accommodated rotations around a vertical axis during the development of crustal or lithospheric scale orogenic curvature here we present a data set of 40ar 39ar ages obtained from five shear zones some of which display curved traces parallel to the cantabrian orocline structural grain in nw iberia and are interpreted to have been generated coevally to it the 40ar 39ar ages were obtained by laser rastering induced step heating on single muscovite crystals that grew synkinematically during shearing all five samples yielded ages that cluster tightly at 308 3 ma providing direct evidence for the age of oroclinal bending this age is consistent with the age constraints obtained from paleomagnetic and structural data that place the onset of oroclinal bending in moscovian times our 40ar 39ar age determinations therefore provide a reliable absolute constraint on the age of buckling of the variscan orogen around a vertical axis and provide further evidence that oroclinal bending is unrelated to variscan convergence collision or the subsequent extensional collapse of the mountain belt

the central iberian arc an orocline centered in the iberian massif and some implications for the variscan belt an arcuate structure comparable in size with the ibero armorican arc is delineated by variscan folds and magnetic anomalies in the central iberian zone of the iberian massif called the central iberian arc its sense of curvature is opposite to that of the ibero armorican arc and its core is occupied by the galicia tras os montes zone of nw iberia which includes the rheic suture other zones of the iberian massif are bent by the arc but the ossa morena and south portuguese zones are not involved the arc formed during the late carboniferous at final stages of thermal relaxation and collapse and an origin related with right lateral ductile transpression at the scale of the variscan belt is proposed the central iberian arc explains the width of the central iberian zone clarifies the position of the allochthonous terranes of nw iberia and opens new perspectives for correlations with the rest of the variscan belt in particular with the armorican massif whose central zone represents the continuation of the southwest branch of the arc detached by strike slip tectonics

soil amplification with a strong impedance contrast boston massachusetts abstract in this study we evaluate the effect of strong sediment bedrock impedance contrasts on soil amplification in boston massachusetts for typical sites along the charles and mystic rivers these sites can be characterized by artificial fill overlying marine sediments overlying glacial till and bedrock where the depth to bedrock ranges from 20 to 80 m the marine sediments generally consist of organic silts sand and boston blue clay we chose these sites because they represent typical foundation conditions in the city of boston and the soil conditions are similar to other high impedance contrast environments the sediment bedrock interface in this region results in an impedance ratio on the order of ten which in turn results in a significant amplification of the ground motion using stratigraphic information derived from numerous boreholes across the region paired with geologic and geomorphologic constraints we develop a depth to bedrock model for the greater boston region using shear wave velocity profiles from 30 locations we develop average velocity profiles for sites mapped as artificial fill glaciofluvial deposits and bedrock by pairing the depth to bedrock model with the surficial geology and the average shear wave velocity profiles we can predict soil amplification in boston we compare linear and equivalent linear site response predictions for a soil layer of varying thickness over bedrock and assess the effects of varying the bedrock shear wave velocity vsb and quality factor q in a moderate seismicity region like boston many earthquakes will result in ground motions that can be modeled with linear site response methods we also assess the effect of bedrock depth on soil amplification for a generic soil profile in artificial fill using both linear and equivalent linear site response models finally we assess the accuracy of the model results by comparing the predicted linear site response and observed site response at the northeastern university neu vertical seismometer array during the 2011 m 5 8 mineral virginia earthquake site response at the neu vertical array results in amplification on the order of 10 times at a period between 0 7 0 8 s the results from this study provide evidence that the mean short period and mean intermediate period amplification used in design codes i e from the fa and fv site coefficients may underpredict soil amplification in strong impedance contrast environments such as boston

timing and structural evolution in the limb of an orocline the pisuerga carrion unit southern limb of the cantabrian orocline nw spain abstract oroclines are the largest scale folds on earth and the process of oroclinal formation is a key topic in tectonics however most studies of oroclines have focused on the hinge areas where the changes in strike and therefore the orocline shape are most obvious in this paper we investigate the deformation mechanisms the timing and the structural and tectonic evolution of the pisuerga carrion unit situated on the southern limb of the cantabrian orocline at the nw of the iberian peninsula the cantabrian orocline located in the variscan belt of western europe has been recently defined as a secondary orocline constraining kinematics and deformation timing our study in the pisuerga carrion unit reveals that an out of sequence thrust system developed and reactivated existing structures by a flexural slip mechanism that was diachronous with respect to oroclinal formation joint analysis of unconformity bounded rock sequences provide a late moscovian age for oroclinal initiation ca 308 ma at least locally additionally comparing those joint sets found in different series we quantify a minimum of 40 counterclockwise vertical axis rotation for the pisuerga carrion unit during the late pennsylvanian

site effects in unstable rock slopes dynamic behavior of the randa instability switzerland we evaluate the dynamic response of the large around five million cubic meters unstable rock slope at randa switzerland based on ground motion recordings during small earthquakes seismic measurements revealed strong polarization of the wavefield in the direction of maximum slope displacement with spectral amplification factors 5 within the unstable area results further highlighted resonant frequencies of both the large unstable rock mass and individual blocks within it block vibration was confirmed by phase analysis of in situ displacement measurements from two bounding tension fractures during a small earthquake numerical simulation using a discontinuous elastic model showed that the presence of steeply dipping compliant fractures is crucial for recreating the observed site response such fractures are common in rock slope instabilities and are here shown to contribute to polarization and amplification of seismic shaking site effects that are rarely considered for hard rock slopes and which may influence the potential for earthquake triggered failure

oroclines of the variscan orogen of iberia paleocurrent analysis and paleogeographic implications abstract coupled structural and paleomagnetic analyses have shown that the northern iberian bend of the variscan orogen referred to as the cantabrian orocline developed by vertical axis rotation of an originally linear orogen however palinspastic restoration of the orocline has proven difficult owing to 1 an unusually great orogenic width of over 700 km and 2 exposure of shallow water strata of the gondwanan margin in the northern and southern portions of the orogen we present paleocurrent data from lower ordovician shallow marine clastic sedimentary rocks across the variscan of northern and central iberia collected to constrain palinspastic restoration of the orogen paleocurrent data were collected from over 50 sites and include cross bed foresets ripple crests and casts as well as rare ball and pillow structures syn sedimentary slump folds and incised channels paleocurrent directions fan around the cantabrian orocline are consistently oriented at a high angle to structural strike and yield a consistent offshore direction outward from the oroclinal core similarly changes in structural strike and paleocurrent direction across central iberia imply the presence of a second more southerly orocline the central iberian orocline that is continuous with but convex in the opposite direction of the cantabrian orocline together the cantabrian and central iberian oroclines define an s shaped pair of continental scale buckle folds palinspastic restoration of the oroclines yields a linear continental margin 1500 km long characterized by consistent offshore paleoflow to the west defining a westerly oceanic domain presumably the rheic ocean and an easterly landward direction presumably gondwana recognition of the southern orocline explains the unusual width of the orogen the geometry of aeromagnetic anomalies attributable to variscan rocks and is consistent with available structural data paleomagnetic declination data and the distribution of correlative paleozoic and older rock sequences including shallow water strata of the gondwanan margin the s geometry of the oroclines is similar to the z geometry of the alaskan oroclines of the north america cordillera suggesting that as in the cordillera the iberian oroclines may record a significant margin parallel translation event

the effect of shallow san francisco bay sediments on waveforms recorded during the mw 4 6 bolinas california earthquake to investigate the effect of the shallow low velocity sediments on the seismic wave field in the northern san francisco bay we modeled tangential component displacement seismograms recorded during the 18 august 1999 m w 4 6 bolinas california earthquake the modeling indicates that the velocity structure of pleistocene horizons in the san francisco bay is important for simulations of weak ground motions for bay area earthquakes models including the pleistocene sediments generate the 1 sec period surface waves observed at several stations modeling of treasure and yerba buena island records requires structures approximately an order of magnitude higher in spatial resolution than the current 3d velocity models for the region this pair of sites located only 2 km apart in the bay records a sixfold difference in peak ground acceleration during the bolinas earthquake three transects are forward modeled using 1d frequency wavenumber integration and 2d finite difference methods generally the ground motions are characterized by a direct shear wave s a midcrustal reflection s 1 a near receiver multiple s 2 and surface waves the direct s arrival at all six stations requires a faster model than gil7 the model routinely used to estimate earthquake source parameters using the berkeley digital seismic network in addition the timing of s 1 indicates the possibility of a dipping midcrustal interface s 2 can be matched with a single strong impedance contrast at 3 km depth a thin 200 m surface layer of weathered rock and sediments simulates the surface waves that follow s 2 at the richmond field station site however the surface waves at treasure island and the berkeley sites are longer in duration and higher amplitude than at richmond and require 2d structure a simple shallow uniform basin model for the san francisco bay consisting of stiff sediments shear wave velocity 400 m sec thickness 100 m over weathered rock 1 5 km sec of the franciscan assemblage produces surface waves in the 0 02 2 hz passband at treasure island and the berkeley sites manuscript received 27 july 2001

distinct sources for syntectonic variscan granitoids insights from the aguiar da beira region central portugal abstract the variscan syntectonic granitoid plutons from the aguiar da beira region central portugal were emplaced into metasediments of late proterozoic early cambrian age during the last variscan ductile tectonic event d 3 which is related to dextral and sinistral shearing the older intrusion with a u pb id tims zircon age of 321 8 2 0 ma consists of porphyritic biotite granodiorite granite with transitional i s type geochemical signature relatively low 87 sr 86 sr 322 ratios 0 7070 0 7074 end 322 values of 3 9 to 4 6 and whole rock and zircon 18 o values of 10 6 and 8 0 respectively by contrast the younger intrusion is an s type muscovite biotite leucogranite emplaced at 317 0 1 1 ma showing more radiogenic 87 sr 86 sr 317 0 7104 0 7146 lower end 317 values of 7 7 to 8 7 and higher 18 o wr 11 3 and 18 o zr 9 5 the combined isotopic and geochemical evidence supports a lower crustal origin for the biotite granodiorite granite involving the anatexis of lower crustal metaigneous protoliths and possible hybridization with mantle derived magmas a shallower origin at mid crustal levels from pure crustal derivation through moderate degrees of partial melting of proterozoic cambrian metasediments is proposed instead for the muscovite biotite leucogranite

gustav steinmann medal 2011 a tribute to a m celâl şengor a m cel l engor receives the gustav steinmann medal 2010 for his fundamental contributions to the tectonic evolution of asia and to the history of geology cel l engor is one of the foremost experts of the geology of asia an area he has investigated over decades the central theme of his research is the reconstruction of the plate movements from the palaeozoic to the present day that led to the existing collage of continental fragments island arcs and relics of the different branches of the palaeozoic and mesozoic tethyan oceans or in his own words the question how many wives did okeanos have by his research engor has revived argand s tectonique de l asie indeed his work covers almost the entire area from the sutures of south western china across tibet to iran and turkey in particular the discovery of cimmerian continental fragments and the reconstruction of the accretion of gondwanian microplates to asia was a milestone other important publications treat in an exemplary fashion fundamental geodynamic processes such as the formation of accretionary wedges and melanges formation of rift basins and the role of intracontinental epirogenetic movements ali mehmet cel l engor was born in 1955 in istanbul where he also went to school after finishing high school in 1973 he went to munich and berlin for a year learning the german language at the goethe institut from there he moved to the university of houston where he spent 2 years from 1976 to 1982 he studied at the state university of new york at albany then a hotbed of the new global tectonics obtaining his b sc m sc and ph d degrees his doctoral dissertation under the supervision of john f dewey brought him to the high alps where he investigated the tectonics of the albula zone a complex area within the austroalpine nappe system in eastern switzerland in the geology institute of the swiss federal institute of technology he made many life long friends last but not least rudolf trumpy from 1981 to 1986 he was lecturer and then reader of geology at the istanbul technical university since 1992 he is professor of geology at this institution cel l lives true scholarship with great passion he is a brilliant historian of science and a true homme de lettre once i told him probably slightly irritated by his encyclopaedic explanations cel l you are a man of the nineteenth century he replied you could not have paid me a greater compliment indeed all his work is rooted in the tradition of our science and is committed to the historical heritage of geology in the widest sense with the help of his prodigious memory and his wide cultural approach he is rendering the observations of the authors of the nineteenth and early twentieth century a living source of information extracting the elements relevant for a d bernoulli geologisch palaontologisches institut der universitat basel bernoullistrasse 32 4056 basel switzerland e mail daniel bernoulli unibas ch