crustal velocity structure of the omineca belt southeastern canadian cordillera travel time inversion and amplitude modeling of a 350 km lithoprobe seismic refraction wide angle reflection profile determined the velocity structure of the crust and upper mantle along strike in the omineca belt of the canadian cordillera the upper crust to 12 18 km depth has velocities from 5 6 to 6 2 km s 1 and two shear zones the monashee decollement and gwillim creek shear zone are imaged by the wide angle reflections and velocity trends minor velocity differences on either side of the monashee decollement may be related to separate rock origins prominent reflections define the boundaries of a low velocity midcrustal layer from 10 15 km to 20 25 km depth with velocities less than 6 1 km s 1 the low velocities of the midcrust associated with high electrical conductivities and high heat flow may be considered as support for the hypothesis of fluids in the cordilleran crust though other possibilities such as the effect of high temperatures on rock velocities are possible in the lower crust velocities range from 6 4 6 5 km s 1 at the top of the lower crust to 6 6 6 8 km s 1 at its base the moho is very clearly defined by the refraction wide angle reflection data and has a gentle southerly dip crustal thicknesses are 35 37 km a thin crust mantle transition zone of 1 2 km thickness in which velocities vary between 7 6 and 7 7 km s 1 is consistent with coincident reflection data upper mantle velocities range from 7 9 to 8 1 km s 1 with indications from the data of upper mantle layering in comparison with neighboring regions the omineca belt has an anomalously thin crust low crustal velocities and a low velocity upper mantle similar only to the basin and range province the velocity structure may partly mirror the temperature profile which has overprinted the geological signature of the region as measured by the seismic refraction method the characteristics of a thin crust and lithosphere along with low velocities from midcrust to mantle suggests that both the basin and range and the southern canadian cordillera are currently being heated from a source within the mantle

composition and structure of the central aleutian island arc from arc parallel wide angle seismic data new results from wide angle seismic data collected parallel to the central aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry the 3 d ray tracing 2 5 d inversion of this sparse wide angle data set which incorporates independent phase interpretations and new constraints on shallow velocity structure produces a faster and smoother result than a previously published velocity model middle crustal velocities of 6 5 7 3 km s over depths of 10 20 km indicate an andesitic to basaltic composition high lower crustal velocities of 7 3 7 7 km s over depths of 20 35 km are interpreted as ultramafic mafic cumulates and or garnet granulites the total crustal thickness is 35 37 km this result indicates that the aleutian island arc has higher velocities and thus more mafic compositions than average continental crust implying that significant modifications would be required for this arc to be a suitable building block for continental crust lateral variations in average crustal velocity below 10 km roughly correspond to trends in major element geochemistry of primitive mg 0 6 lavas the highest lower crustal velocities and presumably most mafic material are detected in the center of an arc segment between unmak and unalaska islands implying that arc segmentation exerts control over crustal composition

crustal structure and tectonics of the southeastern canadian cordillera the crustal structure of the omineca ob and foreland fb belts of the southeastern canadian cordillera are interpreted from the inversion of seismic refraction reflection travel times and amplitudes and modeling of the bouguer gravity data from a 350 km east west wide angle profile the main features of the resultant velocity and density models include 1 a low average crustal velocity of 6 2 km s 2 variable upper crustal velocities 5 6 6 3 km s within the fb and purcell anticlinorium as compared to farther west in the ob 6 1 6 2 km s 3 a midcrustal 20 km depth 0 4 0 5 km s velocity increase within the ob 4 eastward crustal thickening from 35 to 42 km over 80 km distance beneath the ob fb boundary 5 the slocan lake fault slf dipping east at 15 20 to at least 35 km depth 6 decreased lower crustal velocities 6 7 6 4 km s across the slf from the ob into the fb 7 increased uppermost mantle velocities 7 9 8 0 km s from the ob into the fb and 8 an average crustal density increase of 40 kg m3 from the ob into the fb lower velocities higher conductances and increased densities within the upper crust 0 10 km depth of the purcell anticlinorium appear to be associated with middle and upper proterozoic rift related metasediments and gabbroic dikes 35 55 by volume in contrast to the mainly mesozoic felsic intrusives and metamorphic rocks that dominate farther west in the ob elevated crustal temperatures within the ob a high heat flow province are responsible for low velocity gradients to 20 km depth a more mafic lower crust beneath the ob inferred from higher velocities and the westward decrease in crustal thickness are interpreted as resulting primarily from middle and or late proterozoic extension and rifting of the craton penetration of the slf into the lower crust may be controlled by lateral strength contrasts at the edge of the craton farther west a midcrustal strength contrast a velocity boundary at 20 25 km depth may have acted as a regional detachment zone during compressional and extensional tectonic episodes

structure of an island arc wide angle seismic studies in the eastern aleutian islands alaska new seismic wide angle data from the eastern aleutian islands show a mafic composition and a 30 km thick island arc crust traveltimes of p and s refracted arrivals and prominent crustal and mantle reflectors observed to offsets of over 300 km were used to derive velocity models for the eastern aleutian arc between the islands of atka and unimak using a three dimensional finite difference modeling and tomography code we interpret the highest crustal p wave velocities of 7 2 7 4 km s between about 12 and about 22 km depth to the south and north of the main volcanic line as remainders of preexisting oceanic crust into which arc magma is intruded apart from the pieces of oceanic crust the velocities suggest an overall mafic composition for the arc composed mainly of metabasalts diorite and diabase in the upper crust and garnet granulite or amphibolite to hornblendite in the lower crust reflected arrivals from the subducting pacific plate at depths of 45 55 km beneath the fore arc together with pn show a mantle wedge with p wave velocities as low as 7 4 km s increasing with depth to about 8 1 km s with an average of about 7 7 km s a mantle composition that grades from mainly pyroxenite probably ultramafic cumulates near the moho to dunite at greater depths best explains these observed velocities

u series dating of palaeolithic rock art at fuente del trucho aragon spain abstract this paper concerns the dating of the palaeolithic rock art of the fuente del trucho cave we used the u th method to date calcite crusts superimposed to the paintings whenever sub samples of a single crust could be dated we obtained stratigraphically consistent results and the correction for detrital contamination is negligible our results are strictly reliable as minimum ages and suggest that the decoration of this site is of a gravettian or earlier age even though a solutrean chronology cannot be excluded in a few cases these results are upheld by stylistical comparison with spanish and french parietal and mobiliary art

seismic velocities and composition of the canadian crust abstract to investigate variations in crustal structure and composition we analyze approximately 80 000 teleseismic p wave seismograms recorded at 343 broadband stations distributed about the canadian landmass these data are binned by horizontal slowness and simultaneously deconvolved into receiver functions we apply stacking methods to retrieve estimates of the bulk crustal velocity ratio vp vs and thickness h from this receiver function dataset under the assumption of locally 1d structure these crustal parameters are analyzed together with a compilation of velocity and thickness estimates from previous active source experiments and global crustal thickness models to investigate competing models of crustal composition and evolution crustal thicknesses vary between 25 km and 48 km with an area weighted average of 37 km for all stations vp vs estimates range from 1 65 to 1 95 and display a systematic increasing trend from nw to se across the churchill superior and grenville provinces analysis of intra crustal conversions indicates little evidence for a well defined widespread conrad discontinuity although the lower crust appears to be more transparent to teleseismic converted waves than the upper crust this observation runs contrary to that of p reflectivity in many active source studies and may be reconciled through a dominant scale length of heterogeneity that is less than 800 m the vp vs observations and vp constraints from active source studies are compared with laboratory measurements for common crustal rocktypes and found to be consistent with bulk compositions falling between granite gneiss and diorite previously reported systematic variations in crustal thickness and vp vs with age in a global data set are not supported by our data set

seismic properties of rocks from an exposure of extended continental crust new laboratory measurements from the ivrea zone abstract compressional wave velocities for new samples from the ivrea verbano zone ivz and strona ceneri zone scz have been measured to 600 mpa in order to better understand the petrologic controls on velocity and to provide an expanded base for seismological models of the crust as represented by these terrains these data coupled with previous measurements show that preferred orientation of mica controls both the velocity hysteresis with pressure and seismic anisotropy of scz and ivz schists and gneisses ivz paragneisses form the basis of a linear velocity density relationship for metagraywackes of various metamorphic grade this relationship falls close to that expected for mafic rocks and overlaps velocity density data for ivz mafic gneisses one dimensional near vertical reflection models based on these data and recent structural reconstructions of the terrain show that lower crustal reflectivity can be related to high reflection coefficients associated with interlayered sequences of upper amphibolite granulite facies paragneiss mafic gneiss and ultramafic rocks transparent upper crustal levels in the model result from the overall similarity of velocities for schists gneisses and granites from the structurally and lithologically complex scz a mean lower crustal velocity of 7 1 km s is calculated for the part of the ivz composed of interlayered mafic gneiss and paragneiss and for another area dominated by a mafic layered igneous complex the velocity density analysis coupled with this observation emphasize the ambiguity inherent in interpretation of crustal composition from p wave velocity data in the absence of other constraints

mean crustal velocity a critical parameter for interpreting crustal structure and crustal growth abstract mean crustal velocity is a critical parameter for genesis of continental crystalline crust because it is a function of mean crustal composition and therefore may be used to resolve continental crustal growth in space and time although the best values of mean crustal velocity are determined from wide angle reflection measurements most studied here necessarily come from vertical averages in crustal refraction determinations the mode of 158 values of mean crustal velocity is 6 3 km s a velocity which corresponds to a mean crustal composition of granodiorite to felsic quartz diorite archean crust may be slightly more mafic mean crustal velocities range from 5 8 to 7 0 km s the lowest values invariably are found in thermally disturbed rift zones and the highest values correspond to velocities in gabbro velocities in island arcs may be as low as 6 0 km s but are typically 6 5 6 9 km s which corresponds to andesitic composition estimates of island arc composition are andesitic if values of mean crustal velocity are not biased this observation suggests that continental crust did not grow simply by addition of island arc material possibilities are that crust formed from fusion of island arcs and was later changed to more felsic composition by addition of material from the mantle or that the late archean episode of major crustal growth did not involve processes similar to younger island arcs some crustal blocks might be changed in composition and thickness by such processes as underplating interthrusting necking and sub crustal erosion specially designed experiments are suggested to determine this parameter so critical for understanding genesis of continental crust

composition of the crust beneath southeastern china derived from an integrated geophysical data set 1 the nonuniqueness in determining the composition of the crust can be reduced by comparing the data for p wave velocity vp s wave velocity vs vp vs ratio mass density and lame impedances to data determined from laboratory measurements of these physical variables on a variety of crustal rock samples the composition of the crust along the tunxi wenzhou transect southeastern china is presented as a model based on a complete set of geophysical data involving seismic p wave and s wave velocity density gravity heat flow and temperature surveys which allow us to place tighter constraints on possible crustal models the yangtze and the cathaysia blocks have a crustal structure characterized by remarkable differences between the upper middle and lower crust integrated geophysics data are here interpreted to indicate 1 an average composition of granite gneiss for the upper crust with presence of mica quartz schist felsic granulite paragranulite and granite granodiorite beneath the yangtze block and basalt down to the upper part of the middle crust just over the jiangshan shaoxing fault followed laterally by granite gneiss beneath the songyang qintiang and wenping depressions 2 granite granodiorite and biotite gneiss for the upper half of the middle crust with presence of mica quartz schist beneath the cathaysia block 3 gabbro and in less proportion basalt for the lower part of the middle crust and 4 amphibolite for the lower crust with presence of mafic garnet granulite just over the jiangshan shaoxing fault and beneath the wenping depression

seismological evidence for variable growth of crust along the izu intraoceanic arc 1 the processes that create continental crust in an intraoceanic arc setting are a matter of debate to address this issue we conducted an active source wide angle seismic study to examine along arc structural variations of the izu intraoceanic arc the data used were acquired over a 550 km long profile along the volcanic front from sagami bay to tori shima the obtained structural model showed the existence of felsic to intermediate composition middle crust with a p wave velocity vp of 6 0 6 5 km s 1 in its upper part and 6 5 6 8 km s 1 in its lower part the thickness of the middle crust varied markedly from 3 to 13 km the underlying lower crust also consisted of two layers vp of 6 8 7 2 km s 1 in the upper part and vp of 7 2 7 6 km s 1 in the lower part the upper of these layers was interpreted to consist of plutonic gabbro and the lower layer was interpreted to be mafic to ultramafic cumulates average crustal velocities calculated from our model showed remarkable lateral variation which correlated well with arc volcanism low average crustal seismic velocities 6 7 km s 1 due to thick middle crust were obtained beneath basaltic volcanoes e g o shima miyake jima hachijo jima aoga shima while higher average velocities 7 1 km s 1 were obtained beneath rhyolitic volcanoes e g nii jima kurose south hachijo caldera myoji knoll and south sumisu caldera we concluded from these observations that continental crust grows predominantly beneath the basaltic volcanoes of the izu arc and that rhyolitic volcanism may be indicative of a more juvenile stage of crustal evolution or remelting of preexisting continental crust or both

crustal thickness and average vp vs ratio variations in southwest yunnan china from teleseismic receiver functions 1 recently many portable broadband three component seismic stations have been deployed in yunnan china we use teleseismic receiver functions to determine the crustal thickness h and the crustal average compressional to shear wave velocity vp vs ratio k variations beneath in southwest yunnan by comparing the synthetics generated from dipping moho models with those from a flat moho model it reveals that for the rays traveling along or around the updip direction the differential arrival times of ps and multiples generated at the dipping moho are comparable with those generated at the flat moho and the result from stacking with these synthetics is a better estimate of model structure for the synthetics traveling along or around the downdip direction the differential arrival times of ps and multiples generated at dipping moho are smaller than those generated at flat moho and make the h k stacking results deviate from the model structure thus the ambiguity induced by dipping moho can be reduced significantly by only stacking with the rays traveling along or around the updip direction applying this technique to 19 stations we obtained that h ranges from 35 42 km and the k ranges from 1 66 1 78 from the central simao basin h increases northward and northeastward k increases significantly northeastward across the ailao shan red river shear zone

a review of ocean acoustic tomography 1987 1990 the techniques and instruments used in acoustic tomography of the oceans are surveyed and results are summarized sections are devoted to theoretical studies and numerical simulations mesoscale mapping vertical slice tomography flow imaging and nonlinear bias series of tomographic experiments analyses to identify global changes and outstanding problems the forward and inverse problems and ocean physics typical data are presented in tables and graphs and a comprehensive bibliography is provided 72 refs

preserved and modified mid archean crustal blocks in dharwar craton seismological evidence abstract we report significant lateral variability in shear wave velocity and moho depth in the archean crust beneath the dharwar craton using earthquake waveform data recorded over 50 broadband seismographs the craton is a continuously exposed archean continental fragment divided into the west dharwar craton wdc of age 2 7 3 36 ga and the east dharwar craton edc of age dominantly 2 5 ga the craton progressively transitions into the southern granulite terrain sgt with age of metamorphism around 2 6 ga the inversion and modeling of receiver function data reveal significant variation of moho depth viz 38 54 km in the wdc and 40 46 km in the sgt and 32 38 km in the edc the average shear wave velocity vs of crust beneath the wdc is 3 85 km s as compared to 3 6 km s in the edc we infer highly variable thickness 16 30 km of mafic cumulate vs 4 0 km s and vp 7 0 km s beneath the wdc in contrast with a thin one 50 km as compared to felsic to intermediate composition for the edc crust with almost flat moho down to 38 km these results suggest preserved mafic crustal root beneath the middle archean terrain in the wdc that has remained inert since then on the other hand felsic to intermediate composition of crust with a nearly flat moho beneath the late archean edc could be a consequence of regional delamination of lower crust the preserved distinct moho topography across the archean terrains suggests it as compositional boundary considering the surface exposure of 15 20 km crust based on p t condition in the granulite segment of the wdc we speculate a himalaya like crustal thickness 50 70 km beneath the middle archean crust pointing toward a plate tectonic like scenario at 3 0 ga

seismic velocities from the kohistan volcanic arc northern pakistan the rocks bounded by the main mantle thrust and the northern suture in northern pakistan constitute an exhumed section through a cretaceous volcanic arc samples have been collected from all the principal lithological groups of the arc and p and s wave seismic velocities have been measured in the laboratory with the prime objective of comparing the velocities with those determined by seismic refraction experiments on modern volcanic arcs velocities were measured at up to 0 7 gpa and consideration has been given to the effect of confining pressure pore pressure and temperature and to the deformation and metamorphism involved in the himalayan collision the reconstructed velocity section through the arc shows a distinct upper crust comprising granitic dioritic intrusions metasediments and volcanics with a p wave velocity of 6 2 6 4 km s 1 depending on the parameters used beneath the lower crust of amphibolitcs and pyroxene granulite has a velocity mainly about 6 4 6 7 kms 1 though the garnet granulitcs extend to 7 8 km s 1 the more mature arc with a higher proportion of granitic rocks would show a slightly lower upper crustal velocity than the younger arc velocity inversions might be expected with a thermal gradient of as little as 10 15 c km 1 depending on the pore pressure in general the proposed velocity structure is comparable with that of volcanic arcs and with many other sections of the continental crust

evolution from fore arc oceanic crust to island arc crust a seismic study along the izu bonin fore arc 1 petrological studies have suggested that oceanic crust is formed during the initial stage of subduction however there is little geophysical evidence for the formation of oceanic crust in the fore arc region we conducted an active source seismic survey in the fore arc region of the izu bonin intraoceanic arc to examine processes of crustal formation associated with the initiation of subduction we used seismic refraction tomography to obtain a detailed seismic velocity model and diffraction stack migration of picked reflection travel times to derive reflectivity images these data show a remarkably thin crust 10 km thick along the northern half of the bonin ridge and abrupt crustal thickening to 20 km toward the south of the ridge comparison of velocity depth profiles of the thin fore arc crust of the bonin ridge with those of typical oceanic crusts showed them to be seismologically identical boninitic magmatism is evident in the area of thin crust and tholeiitic calcalkaline andesitic volcanism in the area of thick crust high precision dating of the volcanic rocks showed that the thin fore arc oceanic crust was created soon after initiation of subduction 48 45 ma and that the nonoceanic thick crust was created with tholeiitic calcalkaline andesitic magmatism after the boninitic magmatism was ceased our seismological image strongly supports the view that fore arc oceanic crust was formed by fore arc spreading during the initial stage of subduction along the izu bonin intraoceanic arc

a unified view of craton evolution motivated by recent deep seismic reflection and refraction results summary an attempt is made to integrate recent continental seismic reflection refraction and geologic observations into a unified scheme of craton evolution 1 nascent continental crust of basaltic bulk composition is produced in island arcs as with oceanic crust moho in this setting lies within the magmatic edifice and corresponds to a downward transition from dominantly mafic cumulate rocks above to dominantly ultramafic cumulates below the petrologic crust mantle boundary lying 25 per cent deeper in the lithosphere has no seismic expression and corresponds to a depositional magmatic contact between cumulate ultramafic rocks above and residual mantle ultramafic tectonite below both contacts are presumably extensively disrupted by magmatic injection 2 island arcs are amalgamated into continents at collision zones delamination of the lower mafic ultramafic portion of the crust along with the mantle lithosphere during hard collisions acts as a mechanical refining process that pushes the bulk crust toward intermediate composition 3 subsequent extensional collapse of the overthickened crust together with thermal re equilibration of the lithosphere results in youthful stable continental crust which is 30 km thick with its top surface awash at sea level and has the bulk composition of andesite this crust characteristically exhibits extensional features half grabens in the upper portion and is prominently laminated in the lower portion due to the injection of modest amounts of basaltic magma in the form of sills during delamination the moho and petrologic base of the crust coincide in this setting and can correspond to either a ductile high strain zone or intrusive contact separating mafic intermediate composition material above from fertile mantle lherzolite below 4 subsequent cratonization is the cumulative effect of episodic injection of the crust by mafic magma at intervals of hundreds of myr as manifest by the ubiquitous overlapping dike swarms of the precambrian shields this results in net long term thickening of the crust by underplating shifts the bulk crust back toward mafic composition periodically produces a new deeper moho and petrologic crust mantle boundary that do not coincide and through repeated dike injection disrupts pre existing laminated reflectors the first magmatic injection event sweats out the light melting fraction in the pre existing andesitic crust producing voluminous granitic magmatism typified by the great proterozoic anorogenic granite rhyolite provinces subsequent injection events into refractory crust produce uplift with attendant erosional denudation of supracrustal sequences the cumulative result is shield type crust in which intermediate crustal levels are exposed over wide areas crustal thickness commonly exceeds 40 km average crustal velocity is somewhat higher than in younger stable crust and the crust is commonly complexly reflective diffractive throughout

seismic imaging of a possible paleoarc in the izu bonin intraoceanic arc and its implications for arc evolution processes crustal evolution processes in intraoceanic arcs including crustal accretion and rifting have been long discussed to examine crustal evolution in the izu bonin intraoceanic island arc we conducted an active source wide angle seismic study along a north south profile 500 km long within a possible paleoarc in the rear arc i e the nishi shichito ridge about 150 km west of the present day volcanic front in this study the seismic velocity and reflectivity images are obtained using the wide angle seismic data for the seismic velocity imaging we applied refraction tomography in which 93 535 picks were used the overall root mean square rms misfit calculated from the initial model of the refraction tomography was 483 1 ms and those calculated from the final model were reduced to 66 7 ms the resultant seismic image shows marked variations of crustal thickness along the seismic profile thin crust 10 15 km thick in the northern part three discrete thick crustal segments 20 25 km thick in the central part and a moderately thick crust 15 km thick in the southern part these variations are mainly attributed to thickness variations of the middle crust having seismic velocity of 6 0 6 8 km s this variation of crustal thickness does not correlate with seafloor topography which is characterized by post miocene across arc seamount chains it does correlate well with crustal variations observed along the present day volcanic front of the izu bonin arc these findings suggest that the magmatic activity that created the across arc seamount chains had little effect on the rear arc crust and that the main part of the rear arc crust was created before the rear arc separated from the volcanic front by correlating the structural variations along the rear arc i e the variation of the average seismic velocity as well as the thickness of the middle crust and those along the present day volcanic front we found that the direction of rifting to separate the rear arc paleoarc from the present day volcanic front was north northeast

new seismological constraints on growth of continental crust in the izu bonin intra oceanic arc the process by which continental crust has formed is not well understood though such crust mostly forms at convergent plate margins today it is thus imperative to study modern intra oceanic arcs such as those common in the western pacific ocean new seismic studies along the representative izu bonin intra oceanic arc provide unique along strike images of arc crust and uppermost mantle to complement earlier cross arc lithospheric profiles these reveal two scales 1000 10 km scale of variations one at the scale of the izu versus bonin thick versus thin arc crust the other at the intervolcano 50 km scale these images show that 1 the bulk composition of the izu bonin arc crust is more mafic than typical continental crust 2 the middle crust with seismic velocities similar to continental crust is predominantly beneath basaltic arc volcanoes 3 the bulk composition beneath basaltic volcanoes changes little at thick and thin arc segments and 4 a process to return lower crustal components to the mantle such as delamination is required for an arc crust to evolve into continental crust continued thickening of the izu bonin crust accompanied by delamination of lowermost crust can yield velocity structure of typical continental crust

a relationship between crustal thickness and mean crustal velocity abstract variable parameters of the continental crust are crustal age heat flow reflectivity thickness and velocity while heat flow and reflectivity decrease crustal thickness increases with age for stable crustal regions we present a correlation between crustal thickness and mean crustal velocity the evaluation is based on more than 270 data points it is shown that mean crustal velocity is higher in thicker crusts an attempt is made to relate this observation to crustal growth by accretion from below

metagabbros of the gangdese arc root south tibet implications for the growth of continental crust abstract the gangdese batholith of the southern tibet is regarded as a principal component of the pre himalayan collision andean type continental margin however the gangdese arc root remains poorly understood because most of previous studies concentrated on granitoids from the middle and upper crust of the arc in this paper we report new petrochemical and geochronological characteristics of metagabbros occurred as a part of the late cretaceous lilong batholith in the eastern gangdese arc the gabbros with protolith ages of 82 95 ma underwent slightly late 68 77 ma granulite facies metamorphism and nearly coeval anatexis typical of lower arc crusts the metagabbros and diorites of the lilong batholith show geochemical features of juvenile crust in continental magmatic arcs and an original magmatic differentiation trend with gabbros formed as the cumulates whereas diorites as the crystalline products of evolved magmas the lilong batholith forms an arc crustal section with a possible thickness of 20 30 km and may have an overall andesitic composition providing a support for the andesite model of continental crust growth

seismic studies of crustal structure during the past four years seismology has continued its role as the key technique for determination of the structure of the continental and oceanic crust by and large however the field and analysis methods used bear little resemblance to those of ten years ago a continued push to increase the number of recording channels has led to the routine use of ocean bottom seismometers sonobuoys and multichannel streamers in oceanic studies and to the use of large sets of portable instruments and multichannel seismic reflection strings on land in studies using earthquakes as sources local and regional arrays provide a capability for obtaining geologically useful two and three dimensional information about the crust

sub crustal lvz below dharwar craton india an evidence for mantle metasomatism and tectonothermal activity in the archean crust abstract improved knowledge of seismic structure of the continental crust provides deep insights into the mechanism of the crustal growth since the archean times reprocessing of deep seismic sounding data acquired over western dharwar craton india reveals presence of i a high velocity magmatic layer 7 30 km s at the base of the crust ii a higher pn velocity 8 35 km s at a deeper depth of 45 km than in an usual archean crust and iii an unusually low velocity layer 7 40 km s at a depth range of 48 60 km in the upper mantle such shallow subcrustal lithospheric lvzs are rarely found in a stable archean craton which is associated with low surface heat flow and devoid of recent tectonic activities considering various geological tectonic and geothermal scenarios occurrence of such a low velocity zone has been ascribed to chemical alteration caused by mantle metasomatism rather than mafic partial melts or high temperature sub solidus the derived velocity structure and distribution of volcanic rocks of different ages in the studied area indicate multiple crust mantle thermal interaction in the archean crust

basin forming processes and the deep structure of the campos basin offshore brazil abstract this paper discusses a worldwide dataset of deep seismic reflection profiles that help the understanding of crustal behaviour and the genetic processes that are involved in the formation of sedimentary basins the events observed in seismic reflection data at lower crustal depths correspond to impedance contrasts associated with one or a combination of the following factors shear zones thin layers of intrusive rocks underplated material gneissic banding fluids and heterogeneities in the crust and mantle several types of sedimentary basins have been recognized with the aid of deep seismic reflection data basins formed along active master faults basins formed along inactive faults basins with no fault control basins formed by low angle detachment faults and basins formed by pervasive pure shear or an approximately pure shear where the lower crust has been locally extended by a different amount than the upper crust the mechanisms of basin forming processes recognized in deep seismic profiles are a simple shear along low angle detachment faults extending to the lower crust and maybe the upper mantle b thinning of the lower crust and mantle by ductile processes convective mechanisms or flow in the mantle and the upper crust by faulting resulting in an overall pure shear and c thinning of the lower crust associated with phase changes in the mantle or non conservation of mass two end member models of basin formation involving lithospheric stretching may be recognized in the south atlantic simple shear stretching where only the upper crust is involved in the process and mantle uplift is offset relative to the basin and pure shear stretching where uplift of the mantle vertically balances the basin fill the first mechanism has been suggested for the onshore tucano basin the data obtained for the offshore campos basin suggests regional lithospheric stretching and crustal thinning with moho uplift compensating for sediment accumulation in the basin depocentre complex relationships between sediment accumulation and crustal thinning are obtained in the western limit of the campos basin

tectonic and magmatic development of the salinian coast ridge belt california 1 we present new field structural petrographic and geochronologic data on a rare midcrustal 25 km exposure of a cordilleran arc the coast ridge belt located in the santa lucia mountains of central california the study area is composed primarily of a deformed suite of upper amphibolite to granulite facies rocks the sur series which is dominated by metaigneous tonalites diorites and gabbros with subordinate metasedimentary quartzite and marble inherited zircons in magmatic rocks suggest that the provenance of framework rocks is drawn heavily from miogeoclinal formations and that sedimentation occurred in the late paleozoic or later minor magmatism in the coast ridge belt began in the early or middle cretaceous but magmatic activity was most intense during a short period time from 93 to 81 ma based on u pb zircon ages of a felsic gneiss and two less deformed diorites the time period 93 81 ma also brackets a period of extensive thickening and high temperature ductile deformation while a thrusting cause for ductile deformation cannot be ruled out we favor the hypothesis that the exposed rocks correspond to a zone of return flow of supracrustal rocks locally displaced by granitoid plutons in the shallower crust magmatism ended throughout salinia between81and76ma coincidentwiththeattainmentof peak pressure and temperature conditions of 0 75 gpa and 800 c exhumation followed immediately bringing the coast ridge belt to the surface within 8 my at a rate of at least 2 3 mm yr exhumation was coincident with an episode of extensional collapse that has been documented elsewhere in the southern california arc during the early laramide orogeny and that may be related to underthrusting of the forearc at that time index terms 8102 tectonophysics continental contractional orogenic belts 8035 structural geology pluton emplacement 3640 mineralogy and petrology igneous petrology 8030 structural geology microstructures 8124 tectonophysics earth s interior composition and state 1212

structure and composition of the aleutian island arc and implications for continental crustal growth we present results of a seismic reflection and refraction investigation of the aleutian island arc designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust the aleutian arc has the requisite thickness 30 km to build continental crust but it differs strongly from continental crust in its composition and reflectivity structure seismic velocities and the compositions of erupted lavas suggest that the aleutian crust has a mafic bulk composition in contrast to the andesitic bulk composition of continents the silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the aleutian intraoceanic arc therefore if island arcs form a significant source of continental crust the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin the pervasive deformation intracrustal melting and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc continent collision or through subsequent establishment of a continental arc the volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two

crustal growth in south eastern australia evidence from lower crustal eclogitic and granulitic xenoliths mafic and ultramafic xenoliths in a basaltic cone at the anakies in south eastern australia are geochemically equivalent to continental basaltic magmas and cumulates the xenolith microstructures range from recognizably meta igneous for intrusive rocks to granoblastic for garnet pyroxenites contact relationships between different rock types within some xenoliths suggest a complex petrogenesis of multiple intrusive metamorphic and metasomatic events at the crust mantle boundary during the evolution of south eastern australia unaltered spinel lher zolite typical of the uppermost eastern australian mantle is interleaved with or veined by the metamorphosed intrusive rocks of basaltic composition geothermobarometry calculations by a variety of methods show a concordance of equilibration temperatures ranging from 880 c to 980 c and pressures of 12 to 18 kbar 1200 1800 mpa these physical conditions span the gabbro to granulite to eclogite transition boundaries the water vapour pressure during equilibration is estimated to be about 0 5 of the load pressure using amphibole breakdown data large fluid inclusions of pure co2 are abundant in the mineral phases in the xenoliths and it is suggested that flux of co2 from the mantle has been an important heat source and fluid medium during metamorphism of the mafic and ultramafic protoliths at the lower crust upper mantle boundary the calculated pressures and temperatures suggest that the south eastern australian crust has sustained a high geothermal gradient in addition the nature of the mineral assemblages and the contact relationships of granulitic rock with spinel lherzolite characteristic of mantle material suggest that the moho is not a discrete feature in this region but is represented by a transition zone approximately 20 km thick these inferences are in agreement with geophysical data including seismic heat flow and electrical resistivity data determined for south eastern australia underplating at the crust mantle boundary by continental basaltic magmas may be an important alternative or additional mechanism to the conventional andesite model for crustal accretion

crustal velocity structure of the omineca and intermontane belts southeastern canadian cordillera we provide information on the physical properties across two important morphogeological regions of the southeastern canadian cordillera the omineca and intermontane belts and tie together two previously interpreted wide angle seismic lines which have separately probed each zone along strike the data analyzed here line 7 of lithoprobe s 1990 southern cordillera refraction experiment in southern british columbia is along a 410 km profile from the fraser river to radium hot springs near the rocky mountain trench iterative forward and inverse modeling of the seismic travel times constrained by seismic amplitude characteristics and earlier cross line interpretations defines a thin crust with relatively low velocities the upper crust has an average thickness of 15 3 km and an average p wave velocity of less than 6 1 km s 1 with velocities at the bottom reaching 6 2 6 3 km s 1 it is further characterized by velocity gradients which decrease markedly with depth and by low velocity trends which correlate with the traces of crustal scale shear zones a high velocity region at the west end of the profile is coincident with a major uplift structure the midcrust has an average thickness of 9 2 km with velocities below 6 1 km s 1 except for the extreme west end of the profile this defines a midcrustal low velocity zone lvz in the omineca belt and partly in the intermontane belt the lower crust from 24 km to 34 km depth has velocities averaging 6 6 6 7 km s 1 higher velocities in the deep crust may mark the westward extent of north american cratonic rocks at the okanagan valley the upper mantle velocity has an average 8 0 km 5 increasing to 8 2 km s 1 at the maximum depth of ray penetration at 48 km the model presented here compliments well the earlier interpretations of vertical incidence seismic reflection profiles the velocity structure of the crust especially the lvz closely correlates with the regional isotherms while there seems to be no well defined velocity boundary between the intermontane and omineca belts there are distinct velocity differences between the intermontane and neighboring coast plutonic complex to the west the low velocity characteristics of the omineca and much of the intermontane belts may be a consequence of temperature control of the physical parameters of the crust the lithosphere in the southeastern canadian cordillera is clearly anomalous with respect to much of northwest north america and in terms of crustal thickness and velocity structure it is similar only to the basin and range province of the united states

crustal structure across the costa rican volcanic arc 1 island arcs are proposed to be essential building blocks for the crustal growth of continents however island arcs and continents are fundamentally different in bulk composition mafic and felsic respectively the substrate upon which arcs are built oceanic crust versus large igneous province may have a strong influence on crustal genesis we present results from an across arc wide angle seismic survey of the costa rican volcanic front which test the hypothesis that juvenile continental crust is actively forming at this location travel time tomography constrains velocities in the upper arc to a depth of 15 km where average velocities are 6 5 km s the upper 5 km of crust is constrained by velocities between 4 0 and 5 5 km s which likely represent sediments volcaniclastics flows and small intrusions between 5 and 15 km depth velocities increase slowly from 5 5 to 6 5 km s crustal thickness and lower crustal velocities are roughly constrained by reflections from an inferred crust mantle transition zone crustal thickness beneath the volcanic front in costa rica is 40 km with best fit average lower crustal velocities between 6 8 and 7 1 km s overall velocities across the arc in central costa rica are at the high velocity extreme of bulk continental crust velocities and are lower than modern island arc velocities suggesting that continental compositions are created at this location these data suggest that preexisting thick crust of the caribbean large igneous province has a measurable effect on bulk composition this thickened arc crust may be a density filter for mafic material and thereby support differentiation toward continental compositions

crustal structure of the southern alps beneath the intersection with the european geotraverse abstract newly digitized and amplitude controlled record sections from the 1977 southern alps refraction campaign permitted a reinterpretation of the crustal structure in the area between western lombardy and the giudicaria fault the resulting model exhibits considerable lateral heterogeneity in the west below 7 5 km of sediments of the lombardy basin the crust reaches a depth of only 31 km whereas it thickens towards the more mountainous area in the east reaching a depth of 46 km below the adamello massif although the signal character of the corresponding reflections is somewhat erratic the data are satisfied best by models with a low velocity zone in the upper crust an additional small velocity discontinuity from 6 2 to 6 4 km s was found in the middle crust at around 20 km earlier interpretations based on travel times alone included a layer with a velocity of about 7 km s at this depth this high velocity layer was then interpreted as lower crustal material of the adriatic african plate which had been overthrust onto the european plate during the alpine orogeny thus explaining the uplift of the southern alps however this model of crustal doubling is questionable because such a mid crustal high velocity layer is not in agreement with the amplitude data the relatively thin crystalline part of the crust under the lombardy basin is interpreted in accordance with geological evidence as a relic of a late hercynian rifting event

laboratory measurement of p wave velocity in crustal and upper mantle xenoliths from ichino megata ne japan ultrabasic hydrous lower crust beneath the ne honshu arc abstract ultrasonic laboratory measurements of p wave velocity vp were carried out up to 1 0 gpa in a temperature range of 25 400 c for crustal and mantle xenoliths of ichino megata northeast japan the rocks used in the present study cover a nearly entire range of lithological variation of the ichino megata xenoliths and are considered as representative rock samples of the lower crust and upper mantle of the back arc side of the northeast ne honshu arc the vp values measured at 25 c and 1 0 gpa are 6 7 7 2 km s for the hornblende gabbros 38 6 46 9 wt sio2 7 2 km s for the hornblende pyroxene gabbro 43 8 wt sio2 6 9 7 3 km s for the amphibolites 36 1 44 3 wt sio2 8 0 8 1 km s for the spinel lherzolites 46 2 47 2 wt sio2 and 6 30 km s for the biotite granite 72 1 wt sio2 combining the present data with the vp profile of the ne honshu arc iwasaki t kato w moriya t hasemi a umino n okada t miyashita k mizogami t takeda t sekine s matsushima t tashiro k miyamachi h 2001 extensional structure in northern honshu arc as inferred from seismic refraction wide angle reflection profiling geophys res lett 28 12 2329 2332 we infer that the 15 km thick lower crust of the ne honshu arc is composed of amphibolite and or hornblende pyroxene gabbro with ultrabasic composition the present study suggests that the vp range of the lower crustal layer 6 6 7 0 km s in the ne honshu arc which is significantly lower than that obtained from various seismic measurements e g the northern izu bonin mariana arc 7 1 7 3 km s is due to the thick hydrous lower crustal layer where hornblende plagioclase and magnetite are dominant