t phase spectra seismic moments and tsunamigenesis t phase spectrograms and spectra derived from deep ocean hydrophones for tsunamigenic and non tsunamigenic earthquakes in the pacific reveal the following 1 substantial amounts of t phase energy at high frequencies i e 1 hz to at least 35 hz 2 strong correlations of spectral strengths in the 10 hz to 35 hz frequency range to seismic moments and 3 large spectral strengths at high frequencies for tsunamigenic earthquakes these findings are consistent with the low attenuation of the so und fixing and ranging sofar channel which permits efficient propagation of high frequencies to great distances and with the perception that large earthquakes may generate high frequency signals at the water sediment interface which can best be observed by hydroacoustic sensors in the low noise environment of the deep oceans in addition these studies confirm that 1 source bathymetry can strongly influence the character of t phases 2 islands or seamounts near the source area can act as secondary radiators of energy which add to the complexity and duration of the coda 3 there appears to be no significant dependence of t phase strength on earthquake source mechanism and 4 the strength of the t phase is dependent on the purity of its oceanic travel path the strong correlation of t phase spectral strengths to seismic moments suggests that substantial improvements in the reliability of tsunami warnings may be possible by using t phases as an additional independent single station method for estimating seismic moments finally the high frequencies observed in the t phase may be associated with important alternate mechanisms for tsunamigenesis other than faulting i e slumpage compaction or liquefaction

hydroacoustic events located at the intersection of the atlantis 30 n and kane 23 40 n transform faults with the mid atlantic ridge we investigate the characteristics of t phase events located at the ends of two segments of the mid atlantic ridge our motivations for the study were to understand whether t phase locations represent earthquake epicenters and thus whether accurate geological inferences can be made from their spatial patterns and to understand further the relationship between t phase event characteristics and earthquake properties we examine the characteristics of 158 t phase events with respect to both event location water depth and source to receiver distance the propagation paths of the t phases are also modeled to study the effects of encountering seafloor topography we find that existing models for t phase excitation and propagation cannot explain adequately all of our observations the amplitudes received levels of t phases at the hydrophones show no dependence on event water depths in contrast to current excitation models which predict a decrease in event magnitude with increasing water depth the received levels are observed to decrease with increasing source to receiver distance and events from the two study areas exhibit different trends in relative received levels between hydrophones once attenuation is taken into account our acoustic ray trace model is able to reproduce similar trends in relative amplitudes at the hydrophones based on 1 d topography between the event and each hydrophone but the variances in both the observed data and model are high we observe a pattern of short t phase onset times for shallow water events and long onset times for deep water events where onset time is defined as the time interval between the appearance of the t phase envelope above the ambient noise and its first peak this suggests that the onset time may be a function of several variables including efficiency of energy conversion based on local topography efficiency of propagation based on event water depth and hypocentral depth in the crust the results of this study underscore the complexity of t phase excitation and propagation and argue that current models of t phase excitation and propagation need to be improved to explain the observed characteristics of t phase data

moho signature from wide angle reflections preliminary results of the 1993 trans hudson orogen refraction experiment abstract in 1993 a combined crustal seismic refraction and wide angle reflection experiment was conducted over the northern saskatchewan and manitoba portion of the trans hudson orogen by lithoprobe the program included three seismic lines extending over 1900 km in length the exceptionally high quality data collected during the field experiment included seismic reflections and refraction events from the moho and upper lithospheric mantle the relatively high seismic fold and signal to noise ratio of the field observations made it possible to process the wide angle portion of the data set with standard reflection signal enhancement techniques the moho is revealed as a complex interface with significant structural relief 40 52 km that correlates in first order fashion with crustal geology and structure the moho topography is interpreted as a result of variable ductile reworking and eclogitization of the lower crust

seismic structure of the central metasedimentary belt southern grenville province crust and upper mantle structure interpreted from wide angle seismic data along a 260 km profile across the central metasedimentary belt of the southern grenville province in ontario and new york state shows i relatively high average crustal and uppermost mantle velocities of 6 8 and 8 3 km s respectively ii east dipping reflectors extending to 24 km depth in the central metasedimentary belt iii weak lateral velocity variations beneath 5 km iv a mid crustal boundary at 27 km depth and v a depth to moho of 43 46 km the wide angle model is generally consistent with the vertical incidence reflectivity of an intersecting lithoprobe reflection line the mid crustal boundary correlates with a crustal detachment zone in the lithoprobe data and the depth extent of east dipping wide angle reflectors regional structure and aeromagnetic anomaly trends support the southwest continuity of grenville terranes and their boundaries from the wide angle profile to two reflection lines in lake ontario a zon

layered and laterally constrained 2d inversion of resistivity data in a sedimentary environment quasi layered models often can represent the actual geology more accurately than smooth minimum structure models we present a 2d inversion scheme with lateral constraints and sharp boundaries lci for continuous resistivity data all data and models are inverted as one system producing layered solutions with laterally smooth transitions the models are regularized through lateral constraints that tie interface depths or thicknesses and resistivities of adjacent layers a priori information used to resolve ambiguities and to add for example geological information can be added at any point of the profile and migrates through the lateral constraints to parameters at adjacent sites similarly information from areas with well resolved parameters migrates through the constraints to help resolve areas with poorly constrained parameters the estimated model is complemented by a full sensitivity analysis of the model parameters supporting quantitative evaluation of the inversion result a simple synthetic model proves the need for a quasilayered 2d inversion when compared with a traditional 2d minimum structure inversion a 2d minimum structure inversion produces models with spatially smooth resistivity transitions making identification of layer boundaries difficult a continuous vertical electrical sounding field example from sweden with a depression in the depth to bedrock supports the conclusions drawn from the synthetic example a till layer on top of the bedrock hidden in the traditional inversion result is identified using the 2d lci scheme furthermore the depth to the bedrock surface is easily identified for most of the profile with the 2d lci model which is not the case with the model from the traditional minimumstructure inversion

seismic images of the grenville orogen in ontario in 1990 lithoprobe acquired 240 km of seismic reflection data across parts of the central gneiss belt cgb and the central metasedimentary belt cmb within the western grenville province of southern ontario interpretation of these data in conjunction with geological constraints provided by bedrock mapping supports a model of northwest directed thrusting and crustal shortening for the grenville orogen within the cgb the parry sound shear zone is imaged as a 3 km wide zone of reflections dipping southeastward at 20 25 and soling at depths 7 km in the north and 3 km in the south beneath parry sound domain parry sound domain and the immediately adjacent domains are underlain by a gently southeast dipping reflective zone at 4 5 12 0 km depth interpreted as a detachment surface likely associated with the central britt shear zone this zone may have accommodated northwesterly transport of parry sound southern britt and northwestern rosseau domains over britt domain during grenvillian thrusting wit

a seismic based cross section of the grenville orogen in southern ontario and western quebec a schematic crustal cross section is presented for the southwestern grenville province based on reprocessed lithoprobe near vertical incidence seismic reflection data and compiled seismic refraction wide angle velocity models interpreted with geological constraints the schematic crustal architecture of the southwest grenville province from southeast to northwest comprises allochthonous crustal elements frontenac adirondack belt and composite arc belt that were assembled prior to ca 1160 ma and then deformed and transported northwest over reworked rocks of pre grenvillian laurentia and the laurentian margin primarily between 1120 and 980 ma reworked pre grenvillian laurentia and laurentian margin rocks are interpreted to extend at least 350 km southeast of the grenville front beneath all of the composite arc belt three major structural boundary zones the grenville front and adjacent grenville front tectonic zone the central metasedimentary belt boundary thrust zone and the elzevir frontenac bou

new data on the rock magnetism in the ivrea verbano zone northern italy and its relationships to the magnetic anomalies abstract the ivrea verbano zone southern alps northern italy is regarded as a typical example of a section of the lower continental crust a considerable magnetic anomaly runs along its outcrop area a study has been made of 153 samples collected along three transverse sections the cannobina strona and sessera valleys and from the southernmost outcrops near ivrea to determine the relationships between this anomaly and the magnetic properties of the rocks and to gain an insight into the nature of magnetism of the lower crust most rocks displayed magnetic susceptibility values ranging from 100 to 10 000 10 6 si units serpentinized peridotites from the finero and baldissero massifs the main gabbro and kindred units and a few amphibolites and mafic granulites of the kinzigite series are the only rocks with values 29 000 200 000 10 6 si comparable with those required to produce the observed anomaly in addition the intensity of their remanent magnetization is of the same order of magnitude as the induced magnetization and the directions of remanence are mostly scattered around the direction of the present field their total magnetization is enough to explain the anomaly of the ivrea verbano zone the results of these measurements and the surface geology suggest that a set of distinct slab like bodies formed of serpentinized peridotite and main gabbro can be proposed as a model for interpretation of the ivrea verbano magnetic anomaly amphibolites and mafic granulites can be regarded as responsible for the short wavelength anomalies only the main magnetic minerals are pyrrhotite which is widely distributed but always in low concentrations and magnetite which is confined to a few lithotypes but can reach high concentrations magnetite must thus be held responsible for the anomaly as in other sections of the lower crust investigated so far the reason for its presence in individual lithotypes however is not always the same and no systematic regional correlation with mineralogical and metamorphic characteristics can be established in conclusion the phenomena observed in the ivrea verbano zone suggest that the magnetism of rocks regarded as typical of the lower crust is highly variable and that there is no ready explanation for the high magnetization values postulated in the light of the satellite data

seismicity at the southern east pacific rise from recordings of an ocean bottom seismometer array 1 seismic data recorded by ocean bottom seismometers obss at the southern east pacific rise sepr were analyzed to characterize seismicity at fast spreading mid ocean ridges an automated event detection algorithm based on pattern recognition was developed to identify earthquake generated t phases the waveform cross correlation and least squares method was found to provide more robust estimates of t phase relative travel times than the method of picking t phase maximum amplitude the obss and subsequent equatorial pacific hydrophones recorded earthquakes near an earthquake swarm in 1992 which was located 300 km west of the ridge axis at about 18 s and 116 w the process that generated the 1992 swarm either was reactivated 3 years later during the obs experiment or has been continuously active in the past decade this longterm activity suggests renewable stresses which are consistent with the interpretation that magma injection was the cause of the 1992 swarm in contrast to the swarm characteristics of earthquakes on the west flank of the ridge axis earthquakes on the nazca flank were randomly distributed in time and space this difference in seismicity may result from the difference in the thermal structure of the pacific and nazca plates the few earthquakes located at or near the ridge axis had very small magnitudes reflecting the presence of shallow magma chambers and the thin elastic layer of the sepr this result suggests that acoustic monitoring of earthquakes associated with dike intrusion is difficult at regional scales at the sepr there is no apparent correlation between seismicity and off axis seamounts index terms 3025 marine geology and geophysics marine seismics 0935 3035 marine geology and geophysics midocean ridge processes 4259 oceanography general ocean acoustics 7230 seismology seismicity and seismotectonics 9355 information related to geographic region pacific ocean keywords t phase ocean bottom seismology mid ocean ridges east pacific rise marine seismics seismicity citation shen y seismicity at the southern east pacific rise from recordings of an ocean bottom seismometer array j geophys res 107 b12 2368 doi 10 1029 2001jb001742 2002

a seismic reflection based regional cross section of the southern abitibi greenstone belt seismic reflection profiles from the southern abitibi greenstone belt reveal four first order subdivisions 1 between 0 and 4 5 s the upper crust is weakly reflective with prominent local to laterally extensive reflections 2 between 4 and 9 s the crust is strongly and heterogeneously reflective with laterally continuous reflections 3 from 9 to 13 s the crust is more homogeneously reflective and displays downward decreasing reflectivity 4 below 13 s moho the upper mantle is weakly reflective the upper layer may correspond to subgreenschist greenschist facies supracrustal rocks cut by low angle shear zones and intruded by regional tabular batholiths the middle layer to ductiley deformed amphibolite facies gneisses granitoids and or metasediments and the lower layer to more homogeneously deformed granulite facies rocks north dipping low angle reflections extending beneath both diverse supracrustal assemblages and regional batholiths may represent structural detachments upon

seismic evidence for late proterozoic orogenic structures below the phanerozoic sedimentary cover in the kattegat area sw scandinavia 1 commercial seismic reflection profiles from sw scandinavia image sveconorwegian orogenic structures in the late precambrian basement beneath the thin phanerozoic cover we interpret 1 pronounced west to wnw dipping reflections as sveconorwegian localized ductile shear which overprint 2 weaker east dipping reflections interpreted as gothian or early sveconorwegian compressional structures folded and laminar seismic fabrics may represent f and c type crystalline thrust sheets emplaced during the gothian and sveconorwegian events some of the dipping reflections are observed to sole into 3 an interval of subhorizontal reflections at 2 5 4 5 s two way travel time which we interpret as a fossil ductile to brittle transition that was unroofed during the gothian orogeny 4 nw se striking thrusts cutting the sveconorwegian structures may indicate a shift in the late sveconorwegian stress field they may represent an early stage in the subsequent phanerozoic basin evolution in the danish area

on the mechanism of conversion of seismic waves to and from t waves in the vicinity of island shores high frequency seismic records from the polynesian seismic network are used to investigate in detail the processes of conversion of acoustic t wave energy from and to seismic waves at island shores on the source side we study the seismic to acoustic conversion based on t phases from hawaiian events recorded at polynesian stations located on the coral platter on the receiver side we study the acoustic to seismic conversion based on t phases from marine sources recorded across the polynesian islands in both instances our results underline the importance of steep slopes typically 50 in allowing an efficient conversion between p waves in the island structure and t waves in the water column these slopes can be coral reefs or the heads of young presumably unconsolidated basalt flows under this geometry modeling based on raytracing indicates that the seismic record of the t phase consists of a p wave at distances from the conversion point greater than 9 km at shorter distances s waves and surface waves are generated in the absence of a steep slope only surface waves are present these models can be used to compute precise and predictable travel time station corrections allowing the use of seismometers located inland to exploit the superb detection and location capabilities of t waves

gpr characterization of a lacustrine uxo site we used 135 mhz ground penetrating radar gpr reflection profiles to determine if high explosive he depth bombs were possibly detonated within a small new england lake formerly used as a bombing range as well as the depths and distribution of inert and possibly unexploded ordnance uxo within the subbottom sediments maximum water depth was 8 6 m the primary munitions were unarmed practice bombs but terrestrial fragments from he depth bombs led us to seek stratigraphic obliteration and distortion as evidence of their use the low water conductivity and quartz rich sediments provided more than 8 m of subbottom penetration beneath slope and shelf sediments the high permittivities of the water and subbottom sediments provided good antenna directivity excellent 2d migrations multicentimeter vertical resolution and a unique pulse waveform signature for metal objects our line profiles revealed hundreds of subbottom diffractions which implied an areal distribution of thou sands of suspicious object

signal penetration in the cocorp basin and range colorado plateau survey seismic sections from cocorp s 1982 survey from the eastern basin and range to the colorado plateau of central utah exhibit distinct cutoff times after which reflections are rare to nonexistent in the eastern basin and range this cutoff time is approximately 11 s 33 km but beneath the central colorado plateau it exceeds 15 s 45 km these depths appear to correspond to the base of the crust moho with the lack of reflections from greater depths indicating mantle homogeneity in general absence of deeper reflections may be due either to geologic homogeneity or to lack of signal penetration cocorp line 3 in the colorado plateau basin and range transition zone shows that variations in penetration are significant on line 3 few reflections are evident below the structurally complex sedimentary cover which extends to only 4 s 8 km and virtually none are identifiable later than 7 s 21 km lateral variations in the temporal decay of source generated energy together with estimates of correspondin

removing infragravity wave induced noise from ocean bottom seismographs obs data deployed offshore of taiwan abstract vertical ocean bottom seismograph obs data at frequencies below 0 05 hz are contaminated by noise induced by infragravity waves we constructed the transfer function between pressure and velocity data from obss deployed in taiwan waters to remove the wave pressure induced noise from seismic recordings data were analyzed from five portable broadband obss deployed each for 10 months at water depths from 1740 to 4600 m and from a cabled shallow buried seismograph eos1 installed on the seafloor at 300 m depth removing long period noise from these obs data improves the identification of teleseismic phases such as p s ss p diff and pkikp that are otherwise ambiguous or unidentifiable for eos1 infragravity wave signals completely mask the p and s waveforms in the 10 50 s period band suitable for centroid moment tensor cmt solutions for most of the local events application of the transfer functions to predict and remove wave deformation yielded clean prominent p and s waveforms at these periods and aided in the cmt determination for small events jointly with land stations the relative amplitudes of the wavenumber normalized transfer function for some of the obss are mostly determined by the thickness of the sediment at the obs site

case history a 5 km long waterborne geophysical survey along the po river within the city of turin northwest italy abstractthe geologic investigation of water covered areas is often difficult and inefficient with only the use of traditional surveying techniques i e local drilling and sampling waterborne geophysical surveys can offer a valuable alternative to achieve adequate data coverage in a cost effective way two geophysical waterborne methods were combined in this study for the delineation of the submerged subsurface geology along a 5 km stretch of the po river within the urban area of turin the adopted methods were continuous vertical electrical sounding cves and ground penetrating radar gpr special attention was devoted to the accurate geo referencing of both surveys for combined interpretation gpr results provide a high quality representation of the river bed forms with identification of clear sand dunes in the finer alluvial sediments cves resistivity sections enable a deeper characterization identifying the interface between the shallow alluvial deposits and the deeper low resistivity marls

short pulse radar detection of groundwater in the sagavanirktok river floodplain in early spring short pulse radar operating in the uhf band was used to search for unfrozen water beneath ice blisters and within the gravels of the sagavanirktok river floodplain near prudhoe bay alaska the investigations were carried out in early april 1991 when daily mean air temperatures still were below 20 c the radar pulse spectrum was centered near 400 mhz and the radar time range was set to cover about 10 m depth the flat snow covered surface allowed good antenna ground coupling and the ice and gravels provided a low loss propagation medium that allowed detail of the gravel structure to be seen in the data grids of several traverses were profiled at three sites all within an area of about 0 5 km2 unfrozen water was inferred from the strength and polarization of the radar reflections one site contained a large icing blister beneath which an extensive reservoir was mapped and seen to extend several tens of meters beyond the mound data from two other sites indicated taliks 5 7 m deep near a frozen mound and within the gravels one of the profiles extended 700 m across the floodplain and revealed other high reflectivity targets which were presumed to be water within about 7 m of the surface a deeper sounding confirmed this depth limit it is concluded that taliks exist in the floodplain throughout the winter and are a probable source for the numerous icing mounds seen along most arctic rivers east of prudhoe bay

the directionality of acoustic t phase signals from small magnitude submarine earthquakes acoustic transients radiated from small magnitude undersea earthquakes were studied in an experiment carried out using a towed horizontal line array operating in the south fiji basin the transient signals consisted of p s and t phases with the t phase signal from each earthquake lasting up to 7min the directionality of the t phase signal was determined by processing the array hydrophone data with a conventional beamformer the weakest part of the signal arrived first on a direct bearing between the earthquake source and the array however subsequent stronger components arrived from different directions farther south of the source in a region where a number of seamounts and ridges rose within the sound channel a simple model based on ray path travel times for elastic wave travel in the earth and acoustic wave propagation in the water suggests that the later components of the t phase signal were radiated into the water by downslope propagation from the seamounts and ridges the initial weaker compon

applications of seismic pattern recognition and gravity inversion techniques to obtain enhanced subsurface images of the earth s crust under the central metasedimentary belt grenville province ontario summary project lithoprobe s abitibi grenville transect seismic reflection lines 32 and 33 traverse the exposed central metasedimentary belt cmb located in the grenville province of the precambrian shield of canada in southern ontario these seismic lines image a zone with a protracted deformational history spanning more than 300 myr detailed examination of the commercially processed stacked sections reveals a number of significant deficiencies in some important areas the image quality in these zones of reduced coherency needs to be enhanced to examine specific features and their relation to the surface geology examination of near vertical seismic data from lines 32 and 33 revealed that the signal to noise ratio was not improved by stacking due to misalignment of signals even after static normal moveout corrections and residual static corrections the presumed reason is that reflected seismic energy following long ray paths in heterogeneous media suffers from relative advances and delays in its propagation and hence arrives at slightly different times at the receivers tending to be poorly aligned relative to its theoretical traveltime curves a pattern recognition pr method for signal enhancement followed by energy stacking in moving time windows was used in this study to improve the images in spite of misalignments reprocessing has refined the geometry of the reflection profiles the objective of this paper is to use enhanced images of the seismic reflection data obtained by using a pr approach together with gravity data using 2 5 d forward and 3 d inversion routines to give an improved model of subsurface structure in the vicinity of lines 32 and 33 line 32 is dominated by southeast dipping reflectors soling into the lower crust the listric geometry of the strong reflection packages of the cmb boundary thrust zone is interpreted to represent a crustal scale ramp flat geometry that accommodated northwest directed tectonic transport of the cmb this interpretation is also supported by the gravity data

petrology and age of the a type mulock granite batholith northern grenville province ontario abstract the mulock granite batholith is exposed over 530 km2 in the northern part of the grenville province 25 km north of north bay ontario the batholith has a u pb zircon emplacement age of 1244 4 3 ma two regional tectonic events affected the batholith 1 moderate grade regional metamorphism and 2 shearing and mylonitization related to the grenville front tectonic zone which slices across the northern third of the batholith a suite of fine to coarse grained pink gneissic granites containing accessory fluorite and with colour indices between 1 and 27 dominate the batholith and are cut by abundant aplite dikes anorthosite dikes cut the granites and aplite in the central part of the batholith the granites were originally alkali feldspar granite and syenogranite and the coarser grained varieties were originally hypersolvus the suite is mainly metaluminous to marginally peraluminous peralkaline varieties are rare most of the suite is subalkalic but subordinate alkalic granites enriched in biotite and ferrohastingsite are early phases the suite shows all the major and trace element compositions and elemental ratios diagnostic of a type granite suites metamorphism altered the primary distribution of some of the trace elements particulary rb and f the granites crystallized under relatively low pressures

seismic images of eclogites crustal scale extension and moho relief in the eastern grenville province quebec seismic images from a 250 km lithoprobe reflection profile in the interior of the eastern grenville province provide important new constraints on the crustal architecture of this part of the orogen prominent upper crustal reflections can be correlated with exposures of high pressure metamorphic rocks in the manicouagan shear belt providing the first direct evidence for eclogite reflectivity in the grenville province the eclogites are cut by major late grenvillian normal faults that penetrate the deep crust and preserve evidence of extensional collapse of the overthickened orogen north to south crustal thinning indicated by a change in moho reflection time from 16 to 13 s correlates well with regional bouguer gravity trends and is accompanied by a dramatic increase in the reflectivity of the lower crust these features underscore the significance of recently recognized along strike variations in tectonic style within the grenville province and point to the internal complexity that characterizes the root zones of collisional orogens

southern abitibi greenstone belt archean crustal structure from seismic reflection profiles lithoprobe seismic reflection profiles from the 2 75 2 68 ga southern abitibi greenstone belt reveal subhorizontal reflection zones of regional extent in the upper 15 km of crust that contrast with the steep dips of rock units and structures at the surface the shallowest 6 12 km regional subhorizontal reflection zone is interpreted to be the base of the abitibi greenstone belt supracrustal assemblage which may be an intrusive and or tectonic contact shallowly dipping subparallel reflectors in the upper crust are interpreted as low angle faults consistent with out of sequence stratigraphy revealed by u pb geochronology truncations of reflectors suggest that regional steeply dipping gold mineralized fault zones penetrate to depths of 15 km the profiles are consistent with the kapuskasing crustal model of the central superior province wherein greenstone belts are underlain by middle crustal felsic intrusions and gneiss and a lower crustal zone of mafic gneiss and intrusions the profiles support tectonic models of deformation of the abitibi greenstone belt that invoke either folding and thrusting followed by regional steep faulting or folding and thrusting linked to the steep faults

crustal and lithospheric scale structures of the precambrian superior grenville margin abstract the lithosphere beneath the margin of the archean superior and proterozoic grenville provinces was investigated with a northwest southeast oriented 650 km long profile of 40 magnetotelluric stations dominant geoelectric strike azimuths of n45 e and n85 e were defined for the crust and the lithospheric mantle respectively a 2 d isotropic resistivity model derived using the crustal strike images resistive laurentian margin rocks dipping southeast to the base of the crust bounded to the northwest by the grenville front and to the southeast by the central metasedimentary belt boundary zone the observation is in contrast to conductive mid to lower crust elsewhere in the region a 2 d isotropic resistivity model determined using the lithospheric mantle strike azimuth reveals an extremely resistive region in the upper 100 km of the mantle lithosphere of the northern grenville province the geometry of this body which includes a well defined base and southeast dip suggests that it is superior lithosphere a sub vertical conductor located approximately 50 km along strike from the mesozoic kirkland lake and cobalt kimberlite fields is interpreted to be due to re fertilization of an older mantle scar the resistivity model includes a horizontal conductor at 160 km depth beneath the southern superior province that is possibly the resistivity signature of the lithospheric asthenospheric boundary

messinian carbonate rich beds of the tertiary piedmont basin nw italy microbially mediated products straddling the onset of the salinity crisis abstract the seven messinian microbial carbonate rich layers cropping out in the pollenzo section tertiary piedmont basin nw italy are interbedded with a precession related cyclic succession composed of euxinic shale marl couplets and straddle the onset of the messinian salinity crisis msc a sharp change of sedimentological compositional and geochemical characteristics was observed from the onset of the msc onward suggesting strong differences in the mechanisms responsible for carbonate precipitation pre msc beds are mainly composed of dolomite and are interpreted as the product of early diagenesis formed by bacterial sulphate reduction in the shallow subsurface along the sulphate methane interface dolomite precipitation was temporarily enhanced by an upward flux of methane rich fluids possibly sourced by gas hydrate destabilisation conversely from the onset of the msc onward carbonate beds are thinly laminated and show abundant filaments interpreted as beggiatoa like bacteria while calcite and aragonite are the dominant authigenic phases these beds are interpreted as microbialites resulting from the lithification of chemotrophic bacterial mats dominated by sulphide oxidizing bacteria the carbonate necessary for their preservation was provided by bacterial sulphate reduction these microbialites that appear as the deeper water counterpart of bottom grown selenite layers deposited in the marginal part of the basin formed on an anoxic sea bottom under a density stratified water column the high sulphate concentration in the pore waters related to the presence of concentrated brines and to the regeneration of sulphate by sulphide oxidizing bacteria is considered as the driving factor controlling the mineralogical change from dolomite to calcite and aragonite finally the sharp shift towards negative 18 o values of carbonates observed from the onset of the msc onward was probably related to fractionation processes operated by sulphate reducing bacterial communities in the pore waters this suggests that care must be taken when dealing with microbial carbonates in the interpretation of the oxygen isotope values in terms of fluctuating salinity condition of the water mass in conclusion this study suggests that the onset of the msc had a strong impact on microbial populations governing carbonate precipitation in a sector of the basin dominated by depositional conditions not suitable for gypsum precipitation

t wave generation and propagation a comparison between data and spectral element modeling t waves are underwater acoustic waves generated by earthquakes modeling of their generation and propagation is a challenging problem using a spectral element code specfem2d this paper presents the first realistic simulations of t waves taking into account major aspects of this phenomenon the radiation pattern of the source the propagation of seismic waves in the crust the seismic to acoustic conversion on a non planar seafloor and the propagation of acoustic waves in the water column the simulated signals are compared with data from the mid atlantic ridge recorded by an array of hydrophones the crust water interface is defined by the seafloor bathymetry different combinations of water sound speed profiles and sub seafloor seismic velocities and frequency content of the source are tested the relative amplitudes main arrival times and durations of simulated t phases are in good agreement with the observed data differences in the spectrograms and early arrivals are likely due to too simplistic source signals and environmental model these examples demonstrate the abilities of the specfem2d code for modeling earthquake generated t waves

the deep structure of the grenville front a new perspective from western quebec the grenville front is a major tectonic boundary exposed on the canadian shield the front is defined as the northwestern limit of grenvillian deformation on the basis of geochronological and metamorphic data this boundary is also evident in some geophysical data sets the lithoprobe abitibi grenville transect crosses the grenville front near lac temiscamingue in western quebec a new 114 km long deep seismic reflection line shows a crustal structure quite different from that seen on previous surveys across the grenville front the archean foreland pontiac subprovince has a pattern of reflectivity similar to that seen in most of the superior province this pattern continues for some 30 km south of the surface exposure of the grenville front there is no evidence for a band of dipping reflectors truncating the horizontal pontiac reflectors in fact the leading edge of the grenville province is difficult to identify on the seismic section the moho is well defined and reveals that the crust thins under

mechanisms linking active rock glaciers and impounded surface water formation in high mountain areas rock glaciers are slowly flowing mixtures of debris and ice occurring in mountains they can represent a reservoir of water and melting ice inside them can affect surface water hydrochemistry investigating the interactions between rock glaciers and water bodies is therefore necessary to better understand these mechanisms with this goal we elucidate the hydrology and structural setting of a rock glacier marginal pond system providing new insights into the mechanisms linking active rock glaciers and impounded surface waters this was achieved through the integration of waterborne geophysical techniques ground penetrating radar electrical resistivity tomography and self potentials and heat tracing results of these surveys showed that rock glacier advance has progressively filled the valley depression where the pond is located creating a dam that could have modified the level of impounded water a sub surface hydrological window connecting the rock glacier to the pond was also detected where an inflow of cold and mineralised underground waters from the rock glacier was observed here greater water contribution from the rock glacier occurred following intense precipitation events during the ice free season with concomitant increasing electrical conductivity values the outflowing dynamic of the pond is dominated by a sub surface seepage where a minor fault zone in bedrock was found characterised by altered and highly fractured rocks the applied approach is evaluated here as a suitable technique for investigating logistically complex hydrological settings which could be possibly transferred to wider scales of investigation

combining shallow and deep geophysical information the yehuin chepelmut fault zone in the magallanes fold and thrust belt tierra del fuego argentina the outer fuegian fold and thrust belt is the deformed sector of the austral magallanes basin which underwent a contractional phase since the late cretaceous and a strike slip phase during the neogene the area hosts two neogene basins yehuin and chepelmut which have been mainly developed by the influence of wrench deformation and were subsequently modified by glacial activity of the ewan and fuego glacier lobes of the fagnano palaeoglacier geophysical surveys using electrical resistivity tomography onland shallow seismic surveys and magnetometry have been combined with outcrop analysis and deep seismic lines to recognize both shallow and intermediate deep structures related to the lacustrine basins four sets of faults with different deformation history were involved in the basin genesis comparison with the deseado fault zone indicates similarities of the structural features between the two zones the stress field interpreted as responsible for the origin of the studied basins is analogous to the one in the deseado valley located in the western tierra del fuego therefore a transcurrent fault zone here named yehuin chepelmut fault zone is proposed as part of the diffuse boundary between south america and scotia plates

use of electrical imaging and distributed temperature sensing methods to characterize surface water groundwater exchange regulating uranium transport at the hanford 300 area washington 1 we explored the use of continuous waterborne electrical imaging cwei in conjunction with fiber optic distributed temperature sensor fo dts monitoring to improve the conceptual model for uranium transport within the columbia river corridor at the hanford 300 area washington we first inverted resistivity and induced polarization cwei data sets for distributions of electrical resistivity and polarizability from which the spatial complexity of the primary hydrogeologic units was reconstructed variations in the depth to the interface between the overlying coarse grained high permeability hanford formation and the underlying finer grained less permeable ringold formation an important contact that limits vertical migration of contaminants were resolved along 3 km of the river corridor centered on the 300 area polarizability images were translated into lithologic images using established relationships between polarizability and surface area normalized to pore volume spor the fo dts data recorded along 1 5 km of cable with a 1 m spatial resolution and 5 min sampling interval revealed subreaches showing 1 temperature anomalies relatively warm in winter and cool in summer and 2 a strong correlation between temperature and river stage negative in winter and positive in summer both indicative of reaches of enhanced surface water groundwater exchange the fo dts data sets confirm the hydrologic significance of the variability identified in the cwei and reveal a pattern of highly focused exchange concentrated at springs where the hanford formation is thickest our findings illustrate how the combination of cwei and fo dts technologies can characterize surface water groundwater exchange in a complex coupled river aquifer system

archean and proterozoic crustal evolution evidence from crustal seismology seismic velocity models for archean and proterozoic provinces throughout the world are analyzed the thickness of the crust in archean provinces is generally found to be about 35 km except at collisional boundaries whereas proterozoic crust has a significantly greater thickness of about 45 km and has a substantially thicker high velocity 7 0 km s layer at the base we consider two models that may explain these differences the first model attributes the difference to a change in the composition of the upper mantle the higher temperatures in the archean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted lithosphere unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation in the second model convection in the hot archean mantle is considered to have been too turbulent to sustain stable long lived subduction zones by the proterozoic the mantle had cooled sufficiently for substantial island and continental arcs to be constructed and the high velocity basal layer was formed by basaltic underplating

crustal velocity structure of the superior and grenville provinces of the southeastern canadian shield the 1992 lithoprobe abitibi grenville seismic refraction experiment was conducted using four profiles across the grenville and superior provinces of the southeastern canadian shield delay time analysis and tomographic inversion of the data set demonstrate significant lateral and vertical variations in crustal velocities from one terrane to another with the largest velocity values occurring underneath the central gneiss and the central metasedimentary belts south of the grenville front the grenville front tectonic zone is imaged as a southeast dipping region of anomalous velocity gradients extending to the moho the velocity anomaly maps suggest an archean crust may extend horizontally 140 km beneath the northern grenville province near surface velocity anomalies correlate well with the known geology the most prominent of these is the sudbury structure which is well mapped as a low velocity basinal structure the tomography images also suggest underthrusting of the pontiac and quetico subprovinces

applications of geophysical inversions in mineral exploration increases in computing power and advances in mathematical optimization theory have combined to produce a new generation of algorithms that can invert geophysical data to recover 1 d 2 d or 3 d images of the earth s physical properties these images may indicate mineralization directly or delineate the associated structures in both cases they are valuable aids to mineral exploration and they provide information that previously had to be distilled from data maps alone in this short article we illustrate both the practicability of inverting geophysical data and the important even decisive information that it provides applications are in time domain electromagnetics dc resistivity and induced polarization gravity and magnetics

transect across the northwestern grenville orogen georgian bay ontario polystage convergence and extension in the lower orogenic crust the grenville orogenic cycle between 1190 and 980 ma involved accretion of magmatic arcs and or continental terranes to the laurentian craton a transect across the western central gneiss belt georgian bay ontario which crosses the boundary between parautochthonous and allochthonous units at an inferred orogenic depth of 20 30 km offers some insights on the thermal and mechanical behavior of the lower crust during the development of the grenville orogen prior to grenvillian metamorphism this part of laurentia consisted largely of meso proterozoic 1450 ma granitoid orthogneisses granulites and subordinate mafic and supracrustal rocks grenvillian convergence along the transect began with transport of the previously deformed and metamorphosed 1160 ma parry sound domain over the craton sometime between 1120 ma and 1080 ma this stage of transport was followed by out of sequence thrusting and further convergence along successively deeper foreland propagating ductile thrust zones a major episode of extension at 1020 ma resulted in southeast directed transport of allochthonous rocks along the midcrustal shawanaga shear zone the final stage of convergence involved deformation and metamorphism in the grenville front tectonic zone at 1000 980 ma peak metamorphism along most of the transect at 1065 1045 ma followed initial transport of allochthonous rocks over the craton by 15 35 m y regional cooling which postdated peak metamorphism by 70 m y was probably delayed by the combined effects of late stage extension and convergence transport of allochthons at least 100 km over the craton was accomplished along a weak migmatitic decollement further propagation of the orogen into the craton followed partial melting and weakening of parautochthonous rocks below this decollement extensional deformation was associated with distributed ductile flow the formation of regional transverse folds with axes parallel to the stretching direction and reactivation of the allochthon parautochthon thrust boundary as an extensional decollement the extensional lower crustal flow was likely the primary cause of the subhorizontal attitude of many structures and seismic reflectors in this part of the central gneiss belt

improving epicentral and magnitude estimation of earthquakes from t phases by considering the excitation function a standard technique for locating events with t phases is to pick the peak energy of t phases as the arrival time then proceed as if it was an unscattered phase originating at the epicenter the peak energy arrival time however can shift to different parts of the wave train due to incoherent scattering we show that a 50 reduction in variance relative to picks of peak arrival times can be achieved by fitting an assumed functional shape to the log of the entire envelope of the t phase we test the stability of this approach by comparing relative event locations based on this method with those determined by cross correlating waveforms of rayleigh and love surface waves recorded teleseismically using a swarm of earthquakes at the northern end of the easter microplate as an example relative event locations show that there is no systematic bias in t phase locations the t phase location and detection can be extended to much smaller events than are detectable by surface waves we estimate t phase maximum amplitudes of about 50 events within the swarm from the amplitude of the fitted functions rather than directly from the seismograms twenty four of these events are large enough to estimate their magnitudes by surface wave analysis the empirical analysis shows the log of t phase maximum amplitude and surface wave magnitude ms exhibit a relatively uniform linear relationship with much less scatter than in previously published studies of t phase amplitude versus magnitude the reduction in scatter is due to both the stability of the functional estimate of amplitude and the use of a swarm of earthquakes with similar depth mechanism and source receiver geometry the observed t phase coda for shallow earthquakes can be synthesized using a simple model of multiple reverberation seafloor scattering the results show that scattering of energy at a rough seafloor from multiple reverberations in the water column is significant in t phase generation even where the water depths are relatively uniform

high resolution geophysical characterization of shallow water wetlands we describe a procedure for rapid characterization of shallow water contaminated wetlands terrain conductivity tc vertical magnetic gradiometry and surface waterchemistry data were obtained from a shallow draft paddleboat operable in as little as 0 3 m of water measurements were taken every 2 s with data acquisition rates exceeding 10 kmofline12 000datapointsper8 hrfieldday we applied this procedure to an urban wetland that is affected by point and nonpoint sources of pollution we used a one dimensional laterally constrained inversion algorithm toinverttheapparent conductivitydatasetobtainedfromthe tcsurveyandtocreateapseudo 2dimageofsedimentconductivity the continuously recorded surface water depth and conductivity values were input as a priori information in theinversion weusedsoilchemistrydeterminedfor28sediment samples collected from the site as well as lithologic logs from across the wetland to constrain interpretation of thegeophysicaldata theinvertedsedimentconductivitydescribes a pattern of contamination probably attributable to leachates from adjacent landfills and or to saltwater ingress from a partial tidal connection that is not obvious in the surface water data magnetic gradiometry values and the inphasecomponentofanem31responsebothreflectprimarily thedistributionofjunkmetalassociatedwithalegacyofillegal dumping historic aerial photographs suggest that this distribution reflects land use history and defines the maximum previous extent of an adjacent landfill and a pattern of dumpingcorrelatedwithhistoricroadways

t waves excited by s waves and oscillated within the ocean above the southeastern taiwan forearc the generation processes of t waves were investigated from seismograms produced by a local intermediate depth earthquake and recorded at one short period station in taiwan the first group of t phases arrived 44 seconds after the s phases while the following 2 groups consistently had the same travel time difference of about 88 seconds each analyses of the ray paths and travel times of these phases show that the t waves were converted from the s waves and oscillated within the ocean above the forearc in the southeastern taiwan area further comparisons of these results with those of previous studies suggest that the generation of t waves is probably dependent on the incidence angles of the seismic waves the t waves were converted from s waves when the incidence angles of the seismic waves were near vertical whereas they were generated from p waves when those angles were near horizontal

coincident seismic reflection refraction studies of the continental lithosphere a global review nearly 50 coincident seismic reflection refraction studies to depths of at least the moho provide an improved understanding of the continental lithosphere some conclusions include the following 1 a transparent upper crust a common observation on vertical reflection profiles cannot generally be correlated with velocity gradients or low velocity zones rather a commonly transparent upper crust may be explained by short wavelength steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source generated noise 2 the reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity 6 6 7 3 km s and to consist of laminated high and low velocity layers with typical thicknesses of 100 200 m 3 landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high and low velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin 4 the crust mantle boundary may differ sufficiently when imaged with vertical incidence and wide angle data to justify the retention for the present of the concept of separate reflection and refraction mohos while there is good evidence that these features are coincident within measurement uncertainties in most regions recently recorded data from the basin and range admit the possibility for noncoincidence in that area 5 upper mantle reflections which cannot be migrated into the lower crust remain rare despite isolated unequivocal examples thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust the wide angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come 6 there appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes 7 no truly three dimensional coincident experiment i e including three dimensional migration has been conducted but some three dimensional data have been collected using both methods measurements of attenuation poisson s ratio and anisotropy within the crust using coincident data sets remain frontiers

ductile thrusting at mid crustal level southwestern grenville province the northwestern boundary zone of the central metasedimentary belt grenville province in the haliburton area ontario is a stack of alternating tonalitic and syenitic crystalline thrust sheets transported toward the northwest on out of sequence upper amphibolite facies ductile thrust zones during the grenvillian orogeny at 1060 ma approximately 100 ma after the initiation of thrusting in the underlying central gneiss belt kinematics of the deformation are complex predominant northwestward thrusting was at least partly coeval with subordinate northeastward thrusting late synmetamorphic extensional shears cut both thrusts and thrust sheets minor late thrusting on discrete ductile shear zones postdates the extensional structures belts of mechanically weak pelite appear to have localised the thrust sheets highly mobile marble behaved as a relatively low viscosity fluid during transport able to intrude and erode more competent wall rock

modal scattering a key to understanding oceanic t waves the excitation mechanism of oceanic t waves has been a puzzle for almost fifty years with refraction from a sloping seafloor and seafloor scattering as two of the most commonly invoked mechanisms by representing the earthquake source field as a normal mode sum it can be seen that both mechanisms are very closely related strict modal orthogonality prohibits the existence of t waves in a laterally homogeneous semi infinite half space or radially symmetric sphere as energy cannot be transferred from one mode to another in an homogeneous medium deterministic non planar bathymetry random boundary roughness upper crustal heterogeneity or a combination of these provides a physical mechanism to break the strict orthogonality we show that modal scattering from the rough seabottom in the epicentral region converts energy from the directly excited ocean crustal water column modes to the propagating acoustic modes comprising the oceanic t wave submarine earthquake fault orientation also appears to be reflected in the t wave excitation

new tectonic divisions of the grenville province southeast canadian shield on the basis of geological geophysical and geochronological data the grenville province has been divided into three first order longitudinal belts the parautochthonous belt pb allochthonous polycyclic belt apb and allochthonous monocyclic belt amb these are set apart by three first order tectonic boundaries the grenville front gf allochthon boundary thrust abt and monocyclic belt boundary zone mbbz the belts are subdivided into terranes based on internal lithological character the gf separates the archean to proterozoic foreland northwest of the orogen from reworked equivalents to the southeast continuous at the scale of the orogen its main characteristic is that of a crustal scale contraction fault the pb although less clearly identified along the length of the orogen in most places represents upgraded and tectonically reworked rocks of the adjacent foreland the boundary between the pb and the apb to the southeast the abt is most clearly delineated in the eastern half of the province it is the locus of major crustal delamination along which high grade mostly middle proterozoic polycyclic terranes were tectonically transported northwest toward and onto the pb the amb comprises two separate areas underlain by the wakeham supergroup and what is currently known as the grenville supergroup respectively its basal contact the mbbz is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks this first order zonation implies a tectonic polarity to the grenville province superimposed on which are second order features evident from contrasting tectonic styles and radiometric ages these characteristics are consistent with a diachronous or oblique collisional model for the grenville orogen

crustal structure of the grenville front and adjacent terranes under the auspices of the great lakes international multidisciplinary program on crustal evolution approximately 320 km of deep seismic reflection data were collected in lake huron along a profile that extends east from the manitoulin terrane across the grenville front to the interior of the grenville orogen the manitoulin terrane is characterized by a series of gently east dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer imaged by strong reflections at the western end of a spectacular band of moderately east dipping reflections the grenville front clearly truncates manitoulin terrane structures to the west these data are interpreted in terms of a speculative two stage model involving 1 creation of a major decollement during northward collision of an allochthonous terrane with the southern superior cratonic margin 1 83 1 89 ga penokean orogeny and 2 northwest directed stacking of microterranes at the southeastern margin of the craton followed by crust penetrating ductile imbrication under high pressure high temperature conditions leading to the ramping of deeply buried rocks to the near surface 1 0 1 3 ga grenvillian orogeny

seismic images of a grenvillian terrane boundary a series of gently dipping reflection zones extending to mid crustal depths is recorded by seismic data from lakes ontario and erie these prominent reflection zones define a broad complex of southeast dipping ductile thrust faults in the interior of the grenville orogen one major reflection zone provides the first image of a proposed grenvillian suture the listric boundary zone between allochthonous terranes of the central gneiss and central metasedimentary belts curvilinear bands of reflections that may represent ramp folds and ramp anticlines that originally formed in a deep crustal scale duplex abut several faults vertical stacking of some curvilinear features suggests coeval or later out of sequence faulting of imbricated and folded thrust sheets grenvillian structure reflections are overlain by a thin wedge shaped package of shallow dipping reflections that probably originates from sediments deposited in a local half graben developed during a period of post grenville extension this is the first seismic evidence for such extension in this region which could have occurred during terminal collapse of the grenville orogen or could have marked the beginning of pre appalachian continental rifting

waterborne gpr survey for estimating bottom sediment variability a survey on the po river turin italy we conducted an integrated geophysical survey on a stretchoftheriverpoinordertocheckthegprabilitytodiscriminate the variability of riverbed sediments through an analysis of the bottom reflection amplitudes we conducted continuousprofileswitha200 mhzgprsystemandahandheld broadband em sensor aconductivity meter and a tdr provided punctual measurements of water conductivity permittivity andtemperature theprocessingandinterpretation ofthegem 2andgprdatawereenhancedbyreciprocalresults and by integration with the punctual measurements of the em properties of the water we used a processing flow thatimprovedtheradargramimagesandpreservedtheamplitude ratios among the different profiles and the frequency content at the bottom reflection signal we derived the water attenuationcoefficientbothfromthepunctualmeasurements using the maxwell formulas and from the interpretation of the gpr data finding an optimal matching between the two values the gpr measurements provided maps of the bathymetry and of the bottom reflection amplitude the high reflectivity of the riverbed derived from the gpr interpretation agreed with the results of the direct sampling campaign that followed the geophysical survey the variability of the bottom reflection amplitudesmap whichwasnotconfirmed by the direct sampling could also have been caused by scatteringphenomenaduetotheriverbedclastswhicharedimensionallycomparabletothewavelengthoftheradarpulse

geologic transect across the grenville orogen of ontario and new york revised cross sections of the western grenville province incorporate new geologic results and reprocessed seismic reflection data the geology is presented in terms of three tectonic elements 1 pre grenvillian laurentia and its margin with ca 1740 and 1450 ma continental arc plutons and associated supracrustal rocks 2 composite arc belt of allochthonous 1300 1250 ma volcanic arcs and sedimentary rocks and 3 frontenac adirondack belt characterized by supracrustal and granitoid rocks and anorthosites of uncertain affinity that may represent a distinctive part of the composite arc belt or an offshore micro continent rocks of the composite arc and frontenac adirondack belts were amalgamated with each other by ca 1160 ma were then thrust over laurentia during ca 1080 1035 ma and ca 1010 980 ma phases of convergence and were dissected and exhumed by 1040 ma normal faults penetrative deformation was restricted to that part of the pre grenvillian laurentian margin that lies to the southeast of the grenville front and parts of the accreted composite arc and frontenac adirondack belts the laurentian rocks in the grenville province are bounded to the northwest and southeast by southeast dipping ductile thrust and or normal shear zones the composite arc and frontenac adirondack belts to the southeast are bounded by ductile and brittle ductile thrust and or normal faults that separate domains with contrasting cooling histories despite a long pre grenvillian tectonic and plutonic history the present crustal architecture and much of the seismic reflectivity were acquired during 1080 980 ma phases of compression and extension

moho reflectivity and seismic signal penetration abstract analysis of amplitude decay and frequency change is applied to determine limits of seismic signal penetration on deep seismic reflection data collected by lithoprobe and the consortium for continental reflection profiling cocorp changes in spectral frequency content are efficiently monitored with averaged instantaneous frequency and bandwidth results from montana show pronounced increases in average frequency and slight drops in amplitude associated with a die out of reflections at the moho results from quebec show amplitude and frequency change occurring through the moho on lines with and without moho reflections these contrasting observations confirm both adequate signal penetration and that the differences in moho reflectivity are representative of geologic differences

age of deformation within the central metasedimentary belt boundary thrust zone southwest grenville orogen constraints on the collision of the mid proterozoic elzevir terrane u pb zircon and titanite geochronology on syntectonic intrusions from the ontario central metasedimentary belt boundary thrust zone has constrained the timing of two major periods of northwest directed ductile thrusting on this crustal scale shear zone initiation of deep seated uppermost amphibolite facies deformation is constrained to a period before 1190 ma over 100 ma earlier than previous estimates major reimbrication of the shear zone took place ca 1080 1060 ma and was more pronounced in the southwestern segment the initiation of deformation within the boundary thrust zone coincides with the emplacement of the hanging wall elzevir terrane of the central metasedimentary belt onto the mid proterozoic laurentian margin the structural nature and temporal and kinematic consistency of the reimbrication phase along the 200 km exposed length of the shear zone is thought to be due to within plate reactivation of the existing rheologically weak structure a result of continent continent collision ca

seismic images of a tectonic subdivision of the grenville orogen beneath lakes ontario and erie new seismic data from marine air gun and vibroseis profiles in lake ontario and lake erie provide images of subhorizontal phanerozoic sediments underlain by a remarkable series of easterly dipping reflections that extends from the crystalline basement to the lower crust these reflections are interpreted as structural features of crustal scale subdivisions within the grenville orogen broadly deformed imbricated and overlapping thrust sheets within the western central metasedimentary belt are succeeded to the west by a complex zone of easterly dipping apparent thrust faults that are interpreted as a southwest subsurface extension of the boundary zone between the central metasedimentary belt and the central gneiss belt the interpreted central metasedimentary belt boundary zone has a characteristic magnetic anomaly that provides a link from the adjacent ends of lakes ontario and erie to structures exposed 150 km to the north less reflective west dipping events are interpreted as structures within the

signal enhancement using pattern recognition techniques with application to near vertical crustal seismic reflection experiments most near vertical crustal seismic reflection experiments are conducted over regions where in the past the crust has undergone some form of tectonic deformation the deep structures differ from the near surface sedimentary structures in that they are generally not layered seismic energy after travelling through long ray paths in such media arrives at slightly different times at the receivers and hence tends to be poorly aligned along its respective theoretical travel time curves conventional seismic processing routines which attempt to increase the signal to noise ratio s n by some stacking technique can in many cases actually destroy the poorly aligned signals such that the resultant subsurface images are not very satisfactory our study focuses on a signal enhancement technique based on a pattern recognition approach which is applied in the prestack processing stream in an effort to increase the s n ratio the discrimination between signal and noise is based on attributes such as the frequency amplitude waveshape and lateral continuity of the signal the method was first tested on synthetic data and then applied to real data acquired over the canadian shield signal enhancement using a pattern recognition approach as outlined in this paper dramatically improved the subsurface images over those obtained by more conventional stacking methods

a three dimensional perspective on the evolution of archaean crust lithoprobe seismic reflection images in the southwestern superior province in 1990 1991 the lithoprobe project completed 450 km of seismic reflection profiles across the late archaean crust of the southwestern superior province the results define a broad three fold division of crust upper crust in the abitibi greenstone belt is non reflective and is a 6 8 km veneer of volcanic and plutonic supracrustal rocks whereas in the sediment gneiss dominated pontiac subprovince upper crust comprises shallow northwest dipping turbidite sequences mid crust in both the abitibi and the pontiac subprovinces is interpreted as imbricate sequences of metasedimentary and metaplutonic rocks lower crust in both subprovinces has a horizontal layer parallel strycture which may represent interleaved mafic intermediate gneisses the seismic signature of the northern abitibi greenstone belt may be represented in an exposed 25 km crustal section in the kapuskasing stuctural zone preliminary tectonic models based on the seismic data are consistent with a plate tectonic scenario involving oblique subduction and imbrication of sedimentary plutonic and volcanic sequences the northern abitibi supracrustal sequences either represent an allochthon or overlie an allochthonous underthrust metasedimentary and plutonic sequence which may be equivalent to a metasedimentary subprovince such as the pontiac or quetico seismic velocities have yet to be defined however crustal thicknesses are relatively constant at 35 40 km the thinnest crust is adjacent to the grenville front where moho is very well defined

integrated geophysical modelling of terranes and other structural features along the western canadian margin an integrated geophysical data set has been used to develop structural models across the continental margin west of vancouver island canada a modern accretionary complex underlies the continental slope and shelf and rests against and below the allochthonous crescent and pacific rim terranes these terranes in turn abut against the pre tertiary wrangellia terrane that constitutes most of the island gravity and magnetic anomaly data constrained by seismic reflection seismic refraction and other data were interpreted to determine the offshore positions of these terranes and related features iterative 2 5 dimensional forward models of anomaly profiles were stepped laterally along the margin to extend areal coverage over a 70 km wide swath oriented normal to the tectonic features an average model was then developed to represent this part of the margin the pacific rim terrane appears to be continuous and close to the coastline along the length of vancouver island consistent with emplacement by strike

crustal structure of the southeastern grenville province northern new york state and eastern ontario the grenville province exposes an oblique cross section through mid lower crustal lithologies that were pervasively deformed and subjected to regional thermal overprinting during the grenvillian orogeny 1 1 ga the southeastern grenville province is divided into two subterranes by the carthage colton mylonite zone a 110 km long lineament characterized by intense ductile shear and igneous intrusion which separates the amphibolite facies metasediments of the central metasedimentary belt to the west from the granulite facies metaplutonics of the central granulite terrane to the east the recognition of distinct lithotectonic domains separated by zones of intense ductile shear in the grenville province raises questions concerning the deep structure of these subterranes and in particular the means by which the mid lower crustal rocks exposed in the grenville province were emplaced seismic refraction wide angle reflection data were acquired to investigate the deep structural interrelationships within the southeastern grenville province a travel time inversion for velocity and interface depth was applied to the seismic data together with constraints from amplitude modeling to produce a seismic velocity model of the crust in the southeastern grenville province in the central metasedimentary belt the upper crust is characterized by velocities in the range 6 3 6 4 km s and a poisson s ratio of 0 26 0 01 which are attributed to quartzofeldspathic rocks farther east in the central granulite terrane upper crustal velocities of 6 55 km s and a poisson s ratio of 0 28 0 01 are associated with the marcy anorthosite the seismic homogeneity of the upper crust in the region of the carthage colton mylonite zone suggests that this boundary is a shallow feature limited to the upper 2 3 km of the crust the deep crustal structure of the southeastern grenville province is characterized by two discrete and laterally discontinuous seismic interfaces in the central metasedimentary belt the top of the lower crust is delineated by an eastward dipping interface at 24 28 km depth in the central granulite terrane prominent en echelon reflections referred to as the tahawus complex form a gently arched dome at 17 22 km depth interpretation of the tahawus complex as a zone of layered mafic cumulates is supported by its high velocity 7 1 km s and poisson s ratio 0 27 0 02 the lower crust is characterized by a velocity of 7 0 7 2 km s and an anomalously high poisson s ratio of 0 30 0 02 which are representative of pyroxene garnet granulites in contrast velocities of 6 8 7 0 km s are modeled beneath the central granulite terrane and appear to signify a lateral change in composition the moho lies at 44 45 km depth and is characterized by pronounced en echelon reflection segments suggesting compositional interlayering around the crust mantle boundary the velocity of the upper mantle is 8 0 8 2 km s an anomalous upper mantle layer with a reversed velocity of 8 6 km s dips eastward from 50 to 60 km depth beneath the southeastern grenville province our results indicate that remnants of magmatic intrusions that mobilized and thickened the crust during the grenvillian orogeny are preserved in the mid lower crust as a layered cumulate body tahawus complex and in the upper mantle as an eclogitic lens possibly delaminated from the overthickened crust during uplift of the southeastern grenville province

coupled seismoacoustic modes on the seafloor 1 wave to wave coupling arises when an acoustic pulse selects a rayleigh mode of the same speed and both travel together swapping energy across an interface ewing et al 1957 a distinctive signal is observed at the hawaii 2 observatory for purely oceanic paths from earthquakes on the blanco and mendocino fracture zones off the coast of north america the signal appears to be a composite of undispersed higher rayleigh modes propagating along the ocean floor both in the sediments and in the water the new coupled modes are identified by their frequency composition and their phase and group velocities seismoacoustic coupling at the seafloor is conditioned on a the presence of a low velocity interface at the ocean floor and b the wavelength of the rayleigh component being shorter than the depth of the water layer

crustal scale shortening and extension across the grenville province of western quebec a deep seismic reflection survey shot in 1993 crosses almost the full width of the grenville province in western quebec the seismic transect provides a very clear image of the crust mantle boundary and the most precise definition to date of the various grenvillian terranes the crust is around 44 km thick beneath the grenville front but thins rapidly to 36 km some 60 km to the southeast it is also notable that the greatest crustal thickness of 50 km occurs at the southeast end of the transect far from the inferred location of the main grenvillian collision the grenville front zone in which nw directed thrusting at about 1 ga was followed by se directed extension is defined by discontinuous se dipping reflections which extend down to the moho to the southeast the overlying migmatitic archean parautochthon almost half of the transect is characterised instead by nw dipping reflectors extending into the lower crust these reflectors are in turn truncated to the southeast by a 12 km thick zone of intense se dipping reflections the baskatong crustal ramp the base of the allochthonous terranes allochthon boundary thrust is likely located at this ramp which flattens out at around 30 km depth into the base of the relatively transparent intermediate crust a highly reflective upper crustal deck corresponding to rocks of the mont laurier terrane was thrust over the baskatong crustal ramp and is represented further to the northwest by the klippelike cabonga allochthon the synformal transparent morin anorthosite charnockite complex belongs to the same upper crustal level ramp anticlines and the overriding basal thrust of the upper allochthons demonstrate the nw directed propagation of tectonic transport during the grenvillian orogeny involving deformation and displacement along the baskatong ramp with a relay into the grenville front zone postaccretional extension appears to have been primarily accommodated along the latter two crustal discontinuities thinning the crust immediately south of the grenville front and affecting the crust up to 350 km away from the front

hydroacoustic ray theory based modeling of t wave propagation in the deep ocean basin offshore eastern taiwan t waves are conventionally defined as seismically generated acoustic energy propagating horizontally over long distances within the minimum sound speed layer in the ocean sofar axis minimum however t waves have also been observed by ocean bottom seismometers in ocean basins at depths greater than the sofar axis minimum previously non geometrical processes such as local scattering at rough seafloor and water sediment interface coupling have been proposed as possible mechanisms for deep seafloor detection of t waves here we employ a new t wave modeling approach based on hydroacoustic ray theory to demonstrate that seismoacoustic energy can propagate to reach deep seafloor previously considered as shadow zone of acoustic propagation our new hydroacoustic simulations explain well the observations of t waves on ocean bottom seismometers at deep ocean basins east of taiwan and shed new light on the mechanism for deep ocean t wave propagation

archaean subduction inferred from seismic images of a mantle suture in the superior province plate tectonics provides the basis for the interpretation of most current terrestrial tectonic activity and is widely accepted as having been active over much of the earth s history1 yet the timing of initiation of this process is subject to debate2 9 so far the earliest seismic evidence for plate tectonics has come from a fossil mantle suture in the svecofennian orogen 1 89 gyr ago 10 and from inferred plate convergence subduction and accretion in the trans hudson orogen 1 91 1 79 gyr ago 11 as yet seismic data from archaean areas have been able to demonstrate only the importance of compression in the construction of the continental crust12 15 here we present seismic data from a collision zone in the superior province of canada involving the abitibi granite greenstone subprovince and the plutonic arc related opatica belt we interpret dipping seismic reflections that extend 30km into the mantle as representing a relict 2 69 gyr old suture associated with subduction although crustal structure lithospheric thicknesses and convergence rates may have differed from those seen today these seismic data provide direct evidence that plate tectonics was active in late archaean times 800 myr earlier than indicated by previous seismic reflection surveys

resistivity imaging of controlled source audiofrequency magnetotelluric data the controlled source audiofrequency magnetotelluric csamt method is an electromagnetic em method where a transmitter a grounded electric bipole or a current loop is placed far away from the receiver sites if the transmitter is located at distances greater than 3 5 skin depths the plane wave approximation is valid and the techniques used for natural source mt interpretation can be applied goldstein and strangway 1975 sandberg and hohmann 1982 the csamt method can be employed in a detailed survey by closely spacing a number of receiver sites along a traverse the borehole csamt technique is proposed in west and ward 1988 to enhance the ability of the surface csamt method to detect subsurface inhomogeneities in these cases two dimensional 2 d smooth model inversion rodi et al 1984 sasaki 1989 degroot hedlin and constable 1990 would be particularly useful for deriving a resistivity model from the far field data and for presenting the survey results in the form of an image

electrical resistivity in support of geological mapping along the panama canal abstract dredging and widening of the panama canal is currently being conducted to allow larger vessels to transit to and from the americas asia and europe dredging efficiency relies heavily on knowledge of the types and volumes of sediments and rocks beneath the waterway to ensure the right equipment is used for their removal to aid this process a waterborne streaming electrical resistivity survey was conducted along the entire length of the canal to provide information on its geology within the confines of the canal a total of 663 line kilometers of electrical resistivity data were acquired using the dipole dipole array the support of the survey data for dredging activities was realized by calibrating and qualitatively correlating the resistivity data with information obtained from nearby logged boreholes and geological maps the continuity of specific strata was determined in the resistivity sections by evaluating the continuity of similar ranges of resistivity values between boreholes it was evident that differing geological units and successions can have similar ranges of resistivity values for example quaternary sandy and gravelly alluvial fill from the former river channel of the chagres river had similar resistivity ranges generally from 40 to 250 m to those characteristic of late miocene basalt dikes from 100 to 400 m but for quite different reasons similarly competent marine based sedimentary rocks of the caimito formation were similar in resistivity values ranging from 0 7 to 10 m to sandstone conglomerate of the bohio formation consequently it would be difficult to use the resistivity data alone to extrapolate more complex geotechnical parameters such as the hardness or strength of the substrate a necessary component for such analyses requires detailed objective information regarding the specific context from which the geotechnical parameters were derived if these data from cored boreholes and detailed geological surveys are taken into account however then waterborne streaming resistivity surveying can be a powerful tool in this case it provided inexpensive and highly resolved quantitative information on the potential volume of loose suctionable material along the gamboa sub reach which could enable large cost savings to be made on a major engineering project involving modification of one of the most important navigable waterways in the world

kinematical and rheological evolution of a crustal scale ductile thrust zone central metasedimentary belt grenville orogen ontario reply the central metasedimentary belt boundary thrust zone is a 10 km thick 200 km long stack of crystalline thrust sheets enclosed by an anastomosing network of ductile thrust zones formed at mid

magnetic electrical and gpr waterborne surveys of moraine deposits beneath a lake a case history from turin italy bathymetry and bottom sediment types of inland water basins provide meaningful information to estimate water reserves and possible connections between surface and groundwater waterborne geophysical surveys can be used to obtain several independent physical parameters to study the sediments we explored the possibilities of retrieving information on both shallow and deep geological structures beneath a morainic lake by means of waterborne nonseismic methods in this respect we discuss simultaneous magnetic electrical and ground penetrating radar gpr waterborne surveys on the candia morainic lake in northerly turin italy we used waterborne gpr to obtain information on the bottom sediment and the bathymetry needed to constrain the magnetic and electrical inversions we obtained a map of the total magnetic field tmf over the lake from which we computed a 2d constrained compact magnetic inversion for selected profiles along with a laterally constrained inversion for one electrical profile the magn

deep seismic profile across a proterozoic collision zone surprises at depth collisions between continents generate the world s most striking mountains the deep structure of these collision zones inferred from seismic reflection profiles1 3 can be equally dramatic if less well understood improved knowledge of the structure of ancient collisional belts now known to have developed through plate tectonic processes4 5 can provide important new insights into the mechanisms of crustal growth although many such belts have been partly destroyed by severe post collisional deformation6 7 or magmatism8 some have been preserved in the stable interiors of continents9 one example is the early proterozoic trans hudson orogen9 10 in western canada here we present seismic reflection images across the entire orogen which reveal a broadly symmetric structure with reflections linked to island arc rocks dipping beneath both bounding archaean cratons the observed reflection geometries imply a doubling of crustal thickness during the collision along crust penetrating faults the presence of a significant amount of archaean crust at depth the extent of which could not have been determined from surface mapping suggests the need for caution in inferring crustal growth rate from the surface area of juvenile crust11 12

observations of polarized seismoacoustic t waves at and beneath the seafloor in the abyssal pacific ocean combined seismic and hydrophone observations show that the traditional t wave propagates as a seismoacoustic polarized interface wave ti coupled to the seafloor seismoacoustic ti waves propagating at the sound speed of water are routinely observed over megameter distances at the deep 4979m seafloor hawaii 2 observatory h2o between hawaii and california even though the seafloor site is within a shadow zone for acoustic wave propagation ti has also been observed on seismometers 225 km ssw of oahu at the osn1 site at the seafloor and within an odp borehole into the basalt basement analyses of timing apparent velocity energy and polarization of these interface waves are presented at low frequency 5hz ti propagates dominantly in the sediments and is consistent with higher mode rayleigh waves at higher frequencies the observed ti waves dominantly propagate acoustically with characteristics suggesting local scattering the observation of ti from an earthquake in guatemala at osn1 whose path i

seismic reflection images of high angle faults and linked detachments in the trans hudson orogen postcollisional 1 8 1 7 ga intracontinental deformation in the trans hudson orogen canada produced a series of orogen parallel high angle faults and folds in seismic reflection profiles the faults are imaged by subvertical zones of diffractions and truncated reflections that extend to 4 8 s 12 24 km the folded and faulted upper part of the crust is underlain by laterally coherent shallow dipping reflections that are locally bounded by discrete highly reflective zones these zones are interpreted as detachments shear zones and can be traced from the upper to lower crust where some of them appear to pass into laterally continuous reflections that define the moho two distinct regimes of postcollisional crustal deformation are inferred from the seismic images high angle faulting and lateral block extrusion in the upper crust and low angle ductile shearing in the mid lower crust the surface geology indicates that the faults resulted in southwest orogen parallel extrusion of the orogen s internal zone relative to the bounding archean hearne and superior cratons faulting was concurrent with the development of upright folds with trends that are subparallel to the extrusion direction the seismic images suggest that the high angle fold fault structures are kinematically linked to low angle detachments represented by laterally coherent highly reflective zones the detachment shear zones are inferred to have a top to the southwest sense of shear associated with a subhorizontal northeast southwest extension direction parallel to those observed for 1 83 1 80 ga collisional shear zones exposed in major postcollisional fold culminations long lived orogen parallel extension is interpreted as a consequence of the boundary conditions imposed by the northeast trend of both the superior and hearne margins

new geophysical constraints on a failed subduction initiation the structure and potential evolution of the gagua ridge and huatung basin we modeled the velocity structure of the huatung basin and gagua ridge using offshore wide angle seismic data along four e w transects these transects are accompanied by several multichannel seismic reflection mcs profiles that highlight the shallow deformation in this area east of taiwan although it is agreed that the gagua ridge was the product of a transient compressional episode in the past relatively few data have been collected that reveal the deeper structure resulting from this enigmatic process the velocity models show evidence for normal to thin oceanic crustal thicknesses in the huatung basin and west philippine basin moho reflections from the associated mcs profiles confirm the thickness observed in the velocity models the velocity models indicate significant crustal thickening associated with the gagua ridge to 12 18 km along its entire length most importantly the two central velocity models also show a significant asymmetry in the crustal thickening suggesting a westward underthrusting of 20 km of wpb oceanic crust beneath that of the huatung basin this geometry is extremely unexpected given interpretations that indicate the huatung basin could be significantly older than the west philippine basin early cretaceous versus eocene our observations along with recent geophysical data concerning the age of the huatung basin indicate that the gagua ridge was the result of a failed subduction event during the miocene that may have existed simultaneously and for a short time competed with the manila subduction zone to the west in accommodating convergence between the eurasia and philippine sea plates in this scenario the present day gagua ridge represents a snapshot of a failed subduction initiation preserved in the geologic record

seismic reflection profiles across the mine series in the noranda camp of the abitibi belt eastern canada the abitibi grenville lithoprobe project completed a regional line 21 and a high resolution line 21 1 seismic survey in the noranda central volcanic complex of the blake river group abitibi quebec line 21 provides a regional framework in which the archean crust is divided into three layers two of which are discussed here the uppermost layer which corresponds to the blake river group is the least reflective and lies above 4 s 12 km and the mid crustal layer which is composed of a complex pattern of generally east northeast dipping reflectors and lies between 4 and 8 s within the regional data the mine series of the central volcanic complex is imaged as a semitransparent series of reflectors overlying a highly reflective east facing structure interpreted as the subvolcanic flavrian pluton the high resolution data line 21 1 were collected in the vicinity of the ansil mine the seismic images in this region can be controlled by surface geology and extensive drill hole data and the projec

crustal structure of the cascadia subduction zone southwestern british columbia from potential field and seismic studies the northern cascadia subduction zone is a region of convergence between the oceanic explorer and northern juan de fuca plates and the continental north american plate potential field and new seismic reflection data coupled with previous seismic results and geology enable derivation of a series of density magnetic susceptibility cross sections and a block density model from the ocean basin to the volcanic arc from 2 5 and 3 dimensional interpretations the lateral extent and thickness of the accreted wedge vary significantly along the zone the narrow metasedimentary pacific rim terrane lies immediately west of wrangellia and extends the length of vancouver island consistent with its emplacement by strike slip faulting following the accretion of wrangellia the ophiolitic crescent terrane is a narrow slice lying seaward of the pacific rim terrane but not extending northward beyond the juan de fuca plate in this region the crescent terrane was emplaced in a strike slip or obliquely convergent style

carbonate stromatolites from a messinian hypersaline setting in the caltanissetta basin sicily petrographic evidence of microbial activity and related stable isotope and rare earth element signatures lower messinian stromatolites of the calcare di base formation at sutera in sicily record periods of low sea level strong evaporation and elevated salinity thought to be associated with the onset of the messinian salinity crisis overlying aragonitic limestones were precipitated in normal to slightly evaporative conditions occasionally influenced by an influx of meteoric water evidence of bacterial involvement in carbonate formation is recorded in three dolomite rich stromatolite beds in the lower portion of the section that contain low domes with irregular crinkly millimetre scale lamination and small fenestrae the dominant microfabrics are i peloidal and clotted dolomicrite with calcite filled fenestrae ii dolomicrite with bacterium like filaments and pores partially filled by calcite or black amorphous matter and iii micrite in which fenestrae alternate with dark thin wispy micrite the filaments resemble beggiatoa like sulphur bacteria under scanning electron microscopy the filaments consist of spherical aggregates of dolomite interpreted to result from calcification of bacterial microcolonies the dolomite crystals are commonly arranged as rounded grains that appear to be incorporated or absorbed into developing crystal faces biofilm like remains occur in voids between the filaments the dolomite consistently shows negative d 13 c values down to 11ae3 and very positive d 18 o mean value 7ae9 that suggest formation as primary precipitate with a substantial contribution of organic co2 very negative d 13 c values down to 31ae6 of early diagenetic calcite associated with the dolomite suggest contribution of co2 originating by anaerobic methane oxidation the shale normalized rare earth element patterns of sutera stromatolites show features similar to those in present day microbial mats with enrichment in light rare earth elements and m type tetrad effects enrichment around pr coupled to a decline around nd and a peak around sm and eu taken together the petrography and geochemistry of the sutera stromatolites provide diverse and compelling evidence for microbial influence on carbonate precipitation

crustal velocity models for the archean abitibi greenstone belt from seismic refraction data we present velocity models for two seismic wide angle refraction profiles across the archean abitibi greenstone belt and the pontiac subprovince the seismic profiles are 210 and 220 km long traveltime inversion and amplitude forward modelling were used to obtain two dimensional velocity structure and interface geometry the main features of the velocity models include 1 three crustal layers 2 variable velocities 5 6 6 4 km s in the upper crust 0 12 km with the higher velocities generally associated with mafic metavolcanics and the lower velocities with metasediments and granitic plutons 3 a relatively uniform middle crust 12 30 km with velocities ranging from 6 4 to 6 6 km s 4 a velocity increase of 0 3 km s across the middle crust lower crust boundary 5 a lower crust 30 40 km with velocities increasing from 6 9 km s at the top to 7 3 km s at the base 6 an average upper mantle velocity of 8 15 km s 7 depth to moho of about 40 km in the north central abitibi belt decreasi