origin and metamorphic history of an early cretaceous polybaric granulite terrain fiordland southwest new zealand regionally extensive two pyroxene granulites in fiordland southwest new zealand are products of metamorphism of a suite of anhydrous magmas which crystallized two pyroxenes the granulite protolith igneous charnockitic rock synkinematically intruded metasediment and other orthogneiss in an early cretaceous subduction related magmatic arc and during cooling experienced deformation induced recrystallization to form granoblastic gneiss the granulites occur side by side with coeval rocks of amphibolite facies mineral zoning and textural relationships in both granulites and amphibolite facies rocks provide evidence of two distinct periods of crystallization 1 an early high temperature comparatively low pressure event accompanying magmatic intrusion andalusite sillimanite facies series recorded locally in the country rock followed by 2 high pressure metamorphism under conditions of 650 700 c at 12 13 kbar garnet granulite locally overprinted earlier formed two pyroxene granulite during the latter event the pressure increase 6 kbar between the two events is attributed to crustal thickening by overthrusting and is equivalent to unloading of a 20 km thick slab over rocks already buried at mid crustal depths both events occurred over a 20 m y interval between the time of magmatic emplacement of the granulite protolith and uplift controlled final cooling of the terrain the phanerozoic granulites in fiordland share some petrologic similarities with precambrian granulite terrains suggesting that at least some aspects of the former may serve as a useful model for development of the latter

compositional variation and the origin of deep crustal reflections abstract identifying the sources of crustal reflections is essential for deriving more geological information from deep crustal reflection profiles theoretical and model studies help place constraints on the role of compositional variation in producing deep crustal reflections analysis of laboratory derived velocities and densities of rock types typical for the continental crust indicates that reflection coefficients are generally small but significantly 17 of the possible reflection coefficients have magnitudes between 0 1 and 0 2 comparison between reflection coefficients derived from well logs and reflections observed in associated seismic profiles indicates that constructive interference associated with geological layering is at least as important as the magnitude of the reflection coefficients for producing detectable reflections constructive interference can increase reflection amplitude by two to three times but is limited to a relatively narrow range of layer thickness for a typical 10 40 hz seismic wavelet and typical crustal velocities of about 6 km s constructive interference occurs for layer thickness ranging between about 35 and 80 m layers thinner than 35 m interfere destructively if reflections result from compositional variation seismic models of hypothetical and observed geologic relations provide analogs for interpreting complex reflection patterns observed in deep crustal reflection profiles such models show reflection patterns similar to those observed in the reflection profiles the models indicate that the reflections could originate in the complexly deformed and intruded terranes that are common in the crystalline crust and it may not be necessary to appeal to unobserved phenomena such as special lamellae or fluid filled fractures to explain the reflections

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

cocorp seismic profiling of the appalachian orogen beneath the coastal plain of georgia a southeastward extension onto the coastal plain of an earlier cocorp traverse which confirmed large scale thin skinned thrusting of crystalline rocks of the southern appalachians has provided some of the most spectacular reflections yet seen in crustal seismic data most of the reflectors can be interpreted as either fault surfaces or as metamorphosed strata of late precambrian early paleozoic age they are consistent with the hypothesis that a major detachment extends eastward beneath this part of the orogen although other interpretations with a more complex pattern of detachments or sutures are also possible large scale overthrusting provides a mechanism for incorporating sedimentary rocks into the lower crust and may help to explain many of the layered features on crustal seismic data reflections from deep beneath the coastal plain indicate that the structural configuration of the rocks is complex and that the remains of a collision zone are being observed several east dipping horizons which bear strong similarities to thrust faults in valley and ridge sedimentary rocks are seen in the basement at shallow and mid crustal levels beneath the coastal plain the augusta fault for example displays a reflection which extends at a low angle some 80 km or more southeast of its surface position in conjunction with surface geologic information these new data demonstrate that late paleozoic compressive deformation was pervasive and resulted in lateral movements in the upper crust extending from the valley and ridge to the crystalline rocks beneath the coastal plain a distance of 400 km or more a large antiform cresting at about 2 3 sec or about 6 km below the surface and other structures beneath the coastal plain of georgia deserve further consideration for petroleum exploration although metamorphism may have eliminated petroleum from these rocks refracted arrivals and fault geometries indicate two triassic rift basins beneath coastal plain sedimentary rocks one of which has apparently not been recognized previously

contribution of deep seismic profiling to the knowledge of the lower crust in france and neighbouring areas abstract in france and neighbouring areas the crust is essentially formed of segments of the variscan orogen 380 300 ma and older formations cratonic basins began to form in the permian but extensive subsidence only occurred from the lias in basins such as the paris and aquitaine basins and also along the margin of the alpine tethys ocean the pyrenees and alps range eventually formed during the tertiary as a result of the collision between the european and african continental plates large grabens were also rifted in the rhine and rhone valleys deep seismic profiles show remarkable features common to the crust of the whole area although it underwent such a complicated geological history the most striking feature is the occurrence of conspicuous flat reflections densely distributed in the lower part of the crust below a comparatively transparent upper crust these reflections hereafter called laminations have been found to occur on most profiles within a large area covering western europe although they vary in depth thickness and energy dipping reflections have frequently been observed in the upper crust some of them extend across the lower crust and even penetrate into the upper mantle they may be reasonably interpreted as variscan compressional features by correlation with surface geology and comparison with similar features found in deep seismic profiles from other regions the origin of the flat laminations which generally lie in the lower part of the crust is more controversial this paper attempts to use the well known geological history of the region to place some constraints on the hypotheses which may be advanced to explain the formation of these laminations these features may be explained by the following tectonothermal events 1 1 compression which prevailed during the variscan orogeny may have formed the laminations as a result of delamination and shearing near the bottom of the crust associated with melting intrusion and metamorphism during the different stages of the crustal evolution 2 2 the thermal equilibration of the upper mantle which occurred in the permian at the end of the variscan orogeny may have caused metamorphism and magmatism in the overlying crust 3 3 a regional thermal event which might be associated with subordinate extension occurred during the rifting of the alpine tethys and the initiation of mesozoic cratonic basins in late triassic middle jurassic times it may also have caused metamorphism and magmatism in the lower crust while the first model implies conservation of the crust material the other two suggest that the crust mantle transition has been somewhat uplifted since the end of the variscan orogeny the latter hypothesis is one way to explain the undeniable occurrence of seismic events below the present moho reflection the above three models are not mutually exclusive and all these phenomena may well have affected the crust to some extent however the data at hand are still insufficient to determine what parts tectonic and nontectonic processes have played in the creation of the laminations in the lower crust and the moho further experiments are needed to check these hypotheses

cocorp profiling across the southern oklahoma aulacogen overthrusting of the wichita mountains and compression within the anadarko basin cocorp consortium for continental reflection profiling deep reflection profiles recorded across the wichita mountains and anadarko basin suggest that significant crustal shortening occurred in the final stages of the evolution of the southern oklahoma aulacogen the crystalline rocks of the wichita mountains were thrust in pennsylvanian time northeastward over sedimentary rocks of the anadarko basin along a series of faults with moderate average 30 to 40 and southwesterly dips these faults can be traced possibly as deep as 20 to 24 km listric thrust faults and hanging wall anticlines developed in the sedimentary rocks of the basin these features contrast with conventional interpretations of pennsylvanian structures as the result of predominantly vertical movements along high angle faults and they suggest that pennsylvanian downwarping of the anadarko basin was at least partially due to thrust loading truncations of reflections from cambrian ordovician rocks in the deepest part of the basin suggest normal faulting which would support ideas of an early extensional stage in the aulacogen cycle the distinctive precambrian layering seen on earlier cocorp data recorded south of the wichita mountains cannot be recognized under the anadarko basin and the proterozoic basin containing that layering may have been bounded on its north side by a precambrian fault this inferred fault was probably twice reactivated during formation of the southern oklahoma aulacogen once during late precambrian early cambrian extension and again during pennsylvanian compression the popular view that aulacogens originated from radial rifting of updomed homogeneous continental crust is probably too simplified and a more important constraint on their location and development may be the nature of pre existing lines of weakness

intracrustal complexity in the united states midcontinent preliminary results from cocorp surveys in northeastern kansas unusually clear indications of complex structure in the mid to lower crust is revealed by seismic reflection surveys in northeastern kansas this complexity contrasts markedly with the layer cake simplicity of both the overlying sedimentary cover and most previous crustal models for the central united states seismic sections collected by cocorp consortium for continental reflection profiling as part of a major east west traverse across the neniaha ridge and midcontinent geophysical anomaly indicate that below a thin relatively flat layered paleozoic sedimentary section the deep crust is characterized by numerous dipping and arcuate reflections and diffractions in many places layered and crosscutting these reflections suggest convoluted three dimensional folded faulted and intruded structures specific identification of these deep features may be possible if future surveys can trace them to accessible depths the basement above these reflection complexes contains significantly fewer reflections consistent with but not necessarily diagnostic of the granitic terrane that dominates basement drill hole samples in the region among the events at these shallower basement depths are several east dipping reflections some of which may be major faults travel times corresponding to expected moho depths about 36 km are characterized less by specific reflections than by an apparent decrease in the density and number of reflections while evidence of crustal heterogeneity is common among deep reflection studies the kansas seismic results outlined in this brief report stand out as being unusually clear representations of such

thrusting in the lower crust evidence from the oygarden islands kemp land east antarctica layered orthogneisses of the oygarden islands preserve evidence for four high grade deformation events d 1 to d 4 archaean d 1 and d 2 structures are only patchily preserved due to extensive recrystallization during d 3 and d 4 which represent effects of the c 1000 ma rayner structural episode ductile thrusting at middle to lower crustal levels occurred during d 3 which is separated into two mutually cross cutting phases based on structural geometry the two phases represent changes in finite strain that developed during progressive deformation east directed transport during d 3a developed subhorizontal thrusts that contain co axial east trending f 3a folds and l 3a lineations buckling as a consequence of constriction in thrust duplexes developed upright f 3b folds coaxial to f 3a folds and steeply south dipping d 3b shear zones garnet clinopyroxene and garnet orthopyroxene bearing assemblages in mafic lithologies and garnet sillimanite bearing assemblages in pelitic lithologies reflect d 3 conditions of p 9 kbar and t 800 850 c the well exposed d 3 duplex structures indicate that shortening of the lower crust may be accommodated by extensive strain partitioning to develop contemporary kilometre scale thrust stacking and ductile flow

sea level changes and past vegetation in the punic period 5th 4th century bc archaeological geomorphological and palaeobotanical indicators south sardinia west mediterranean sea abstract the ancient shorelines and main differences in the vegetational landscape of s sardinia during the punic period 5th 4th century bc have been recognized by an interdisciplinary archaeological paleobotanical and geomorphological approach sixteen trading amphoras and one olla of punic manufacture brought to light during underwater exploration in santa gilla lagoon s sardinia w mediterranean provide data on relative sea level changes the infilling mud of each retrieved amphora is representative of the ages lasting for about 200 300 years the macro archaebotanical contents document the use of seeds and fruits of vitis sorbus ficus olea and prunus during punic trading and suggest the occurrence of agro pastoral practices in the surrounding plains or the area behind the lagoon system the palynological analysis documents the presence of holm and cork oak forests a widespread mediterranean forest with juniper and pinus and a developed stagnant coastal system and reveals a climate induced past vegetation change that is likely to have been emphasized by anthropogenic pressure and agriculture activity during the 4th century bc the gps localization of the amphoras shows a distribution according to curved alignments that probably correspond to the shorelines 5th 4th century bc at 1 95 10 m and 1 70 10 m geomorphological data reveal a sea level rise of about 25 cm during a century corresponding to 300 m of shoreline regression towards the alluvial plain with these rapid processes of drowning of the coastal plain the presence of punic settlements along the lagoon borders documents the reorganization of commercial and economic activities this shows the great adaptability of the ancient population to the sea level rise slr and fast flooding processes of the coastline in a lagoonal area vulnerable to subsidence phenomena

direct p wave anisotropy measurements at homestake mine implications for wave propagation in continental crust we measured anisotropic seismic properties of schists of the homestake formation located at a depth of 1478 m in the sanford underground research facility surf in the black hills of south dakota usa we deployed a 24 element linear array of three component geophones in an area in the homestake mine called 19 ledge an airless jackhammer source was used to shoot two profiles 1 a walkaway survey to appraise any distance dependence and 2 a fan shot profile to measure variations with azimuth slowness estimates from the fan shot profile show a statistically significant deviation with azimuth with the expected 180 variation with azimuth we measured p wave particle motion deviations from data rotated to ray coordinates using three methods 1 a conventional principal component method 2 a novel grid search method that maximized longitudinal motion over a range of search angles and 3 the multiwavelet method the multiwavelet results were computed in two frequency bands of 200 600 and 100 300 hz results were binned by azimuth and averaged with a robust estimation method with error bars estimated by a bootstrap method the particle motion results show large statistically significant variations with azimuth with a 180 cyclicity we modelled the azimuthal variations in compressional wave speed and angular deviation from purely longitudinal particle motion of p waves using an elastic tensor method to appraise the relative importance of crystalline fabric relative to fracturing parallel to foliation the model used bulk averages of crystal fabric measured for an analogous schist sample from southeast vermont rotated to the homestake formation foliation directions supplied by surf from old mine records we found with average crustal crack densities crack induced anisotropy had only a small effect on the observables we found strong agreement in the traveltime data the observed amplitudes of deviations of p particle motion showed significantly larger variation than the model predictions and a 20 phase shift in azimuth we attribute the inadequacies of the model fit to the particle motion data to inadequacies in the analogue rock and or near receiver distortions from smaller scale heterogeneity we discuss the surprising variability of signals recorded in this experimental data we show clear examples of unexplained resonances and unexpected variations on a scale much smaller than a wavelength that has broad implications for wave propagation in real rocks

geophysical investigations of the geology and structure of the pangushan tieshanlong tungsten ore field south jiangxi china evidence for site selection of the 2000 m nanling scientific drilling project sp nlsd 2 abstract the pangushan tieshanlong tungsten ore field located in the center of southern jiangxi province eastern nanling region contains many granitic plutons both exposed and concealed and hosts numerous tungsten polymetallic ore deposits despite many previous geological studies the deep geology and structure of the field are poorly known because few geophysical measurements have been undertaken in this study we carried out a variety of geophysical measurements including gravity magnetic field detection magnetotelluric sounding mt and high resolution seismic reflection to better outline the nature of the ore field and to aid in site selection for the 2000 m nanling scientific drilling project sp nlsd 2 to be carried out by the chinese sinoprobe program four granitic intrusives the tangcun pangushan huangsha and bai e plutons crop out independently at the surface but were found to be parts of a large batholith at depth the tangcun composite pluton which at the surface consists chiefly of caledonian granite and minor early yanshanian intrusive material was found to be chiefly early yanshanian granite concealed beneath the lower part of the caledonian granite the bai e pluton initially decreases in size with depth and then expands as it merges with the major batholith the pangushan and huangsha plutons are domes on the inferred batholith four granitic magma channels and a considerable number of faults have been identified due to its favorable geological conditions the top of the pangushan pluton is predicted to be the best candidate for ore prospecting it is basically a granite cupola with many cross cutting and parallel faults in addition it is characterized by a high gravity anomaly a high magnetic anomaly and locally a low electrical resistivity anomaly

crustal thickness in the central andes from teleseismically recorded depth phase precursors summary although crustal thickness is a critical constraint for geodynamic models of andean orogenesis relatively few measurements exist from local seismic studies in this study we use reflections from the underside of the moho as illuminated by intermediate depth earthquakes to provide new estimates of crustal thickness for the central andes 16 34 s these reflected signals pmp are identified as precursors to the depth phase pp when recorded at teleseismic distances 35 85 although relatively small in amplitude the pmp phase is often clear even on single seismograms less obvious pmp phases were enhanced by stacking traces from arrays this method was most effective for events of m w 6 and depth 100 km crustal thickness determined in this study ranges from 59 to 70 km in southern peru from 49 to 80 km across the puna altiplano and from 50 to 60 km above the pampean flat slab the lack of such phases for certain events may be evidence of heterogeneity at the moho and some precursors appear to correspond to intracrustal magma and lithospheric mantle discontinuities the results obtained using the pmp technique combined with those from other studies show the crustal thicknesses under the central andes are more variable than expected from purely isostatic considerations we infer that one or a combination of geodynamic processes involving lower crustal flow lower crustal delamination and mantle crustal lithospheric coupling can explain the highly variable topography on the moho beneath the central andes

cocorp and the continental crust this is a speculative paper on the continental crust a major frontier of modern earth science seismic reflection profiling data from the cocorp project coupled with other kinds of geophysical and geological observations suggest some fresh perspectives on the crust for example there is evidence for great differences within the basement at a single site and from one cocorp site to another a simple layered model is surely not an adequate representation of the crust in most areas there is however the puzzling and rather common occurrence of limited zones of parallel though not always flat layered features at various depths in certain parts of the crust are the corresponding rocks metasedimentary or igneous or both there are transparent zones from which little or no coherent reflected seismic energy is detected are the corresponding rocks intrusions or highly deformed metasediments can degree of deformation suggest whether rocks were deformed in the hanging wall or footwall of a thrust there is evidence for large scale thrusting and related features that emphasizes the great importance of the wilson cycle to continental tectonics and that raises the possibility that water and rocks of sedimentary origin are trapped deep in the crust by the thrusting process and remain there for long periods with a variety of possible rheological and petrological consequences in some places the modern moho and adjacent rocks may correspond to parts of former oceanic or continental margin sections that have been emplaced at depth as a result of thrusting the widely held concept of the moho as a consistent boundary that is everywhere similar and that always separates the same kinds of rocks may be a major barrier to further progress in understanding the continental crust

a review of geomagnetic variation measurements in the eastern united states implications for continental tectonics geomagnetic variation data for all available stations in eastern north america are presented here in the form of induction arrow maps at periods of 6 20 and 64 min the region is subdivided on th

crustal laminations in deep seismic profiles in france and neighbouring areas summary remarkable crustal features appear on the ecors profiles carried out in northern france and the bay of biscay as well as on the swat profiles shot in the western channel and the celtic sea the most striking one is the occurrence of flat laminations in the lower crust dipping events and laminations are also present in the upper and lower crust especially in the swat profiles they can readily be related to tectonic events variscan in age some of them identified in the field the flat laminations in the lower crust are at first interpreted as resulting from delamination shearing magmatism and metamorphism at the crust mantle transition during the variscan orogeny this interpretation raises some difficulty concerning the space and time correlation of the laminations with the variscan orogeny they seem to have been emplaced after the permian triassic infilling of the plymouth bay basin and before the early cretaceous opening of the bay of biscay an early to middle jurassic age is suggested a period when large cratonic basins were formed without noticeable extension heat flow increase and magmatism are proposed as a second hypothesis for the formation of the lower crust laminations choosing between orogenic and non orogenic causes of these laminations will require further deep seismic profiles together with good velocity determination

moho beneath the north sea compared on normal incidence and wide angle seismic records the mohorovicic discontinuity the moho is properly defined as the step in the velocity of horizontally travelling p waves from about 7 km s 1 to about 8 1 km s 1 its detailed structure varies from place to place but it is universally interpreted as the transition from crust to mantle normal incidence reflections originating at about moho depth have been reported sporadi cally during the past 20 yr during the past 10 yr continuous seismic reflection profiles have been obtained to this depth both on the continents and in the deep sea and at the same time striking improvements have been made in the methods available for interpretation of refraction experiments here we report a comparison between the moho determined by a long range refraction experiment across the north sea and the results of a short normal incidence reflection profile shot along the same line fig 1 we believe this to be the first time that such a direct comparison has been made

superimposed variscan caledonian and proterozoic features inferred from deep seismic profiles recorded between southern ireland southwestern britain and western france abstract from southern ireland to the northern bay of biscay subduction was active from the proterozoic to the late palaeozoic they led to the cadomian caledonian and variscan collisions features characterizing these three orogenies have been crossed by the winch swat wam and bay of biscay deep seismic profiles recorded by the birps and ecors groups in the celtic sea the variscan front is characterized by a s dipping ramp rising directly from the middle crust with much less northward flattening than shown by the front in the northern france and west germany profiles south of the variscan front other s dipping reflections occur above a possible deep detachment the lizard ophiolitic suture marks s dipping events traversing the entire crust and the present moho in the english channel several sets of prominent n dipping reflections occurring in the celtic sea and south irish sea are interpreted as caledonian features running parallel to the caledonian iapetus suture and the pattern of folds and faults recognized in wales and southeastern ireland proterozoic s dipping subduction is recorded by geological data in northern wales and the english channel related crustal structures correspond to deep seismic events cutting across the lower crust and the moho and show the same apparent dip as the variscan features parallelism between dips and or strikes of structural features with different ages suggests a tectonic heritage the crust thus appears to be characterized by n and s dipping features which cut across large parts of it and may be assigned to variscan caledonian or candomian thrusts or shear zones the lower part of the crust shows a band of high amplitude subhorizontal reflections and a flat moho reflection which seem to have overprinted the above mentioned dipping features in a later period the lack of evidence of any shallow detachment in the caledonian and variscan belts together with the presence of dipping crustal thrust like features suggest that the region may have reached the very latest collisional stage the end of both of these orogenies

earthquake imprints on a lacustrine deltaic system the kurk delta along the east anatolian fault turkey deltas contain sedimentary records that are not only indicative of water level changes but also particularly sensitive to earthquake shaking typically resulting in soft sediment deformation structures the kurk lacustrine delta lies at the south western extremity of lake hazar in eastern turkey and is adjacent to the seismogenic east anatolian fault which has generated earthquakes of magnitude 7 this paper reevaluates water level changes and earthquake shaking that have affected the kurk delta combining geophysical data seismic reflection profiles and side scan sonar remote sensing images historical data onland outcrops and offshore coring the history of water level changes provides a temporal framework for the depositional record in addition to the common soft sediment deformation documented previously onland outcrops reveal a record of deformation fracturing tilt and clastic dykes linked to large earthquake induced liquefactions and lateral spreading the recurrent liquefaction structures can be used to obtain a palaeoseismological record five event horizons were identified that could be linked to historical earthquakes occurring in the last 1000 years along the east anatolian fault sedimentary cores sampling the most recent subaqueous sedimentation revealed the occurrence of another type of earthquake indicator based on radionuclide dating 137cs and 210pb two major sedimentary events were attributed to the ad 1874 1875 east anatolian fault earthquake sequence their sedimentological characteristics were determined by x ray imagery x ray diffraction loss on ignition grain size distribution and geophysical measurements the events are interpreted to be hyperpycnal deposits linked to post seismic sediment reworking of earthquake triggered landslides this article is protected by copyright all rights reserved

parameterized convection and the thermal evolution of the earth abstract thermal convection is an efficient means of extracting heat from the interior of the earth heat lost from the earth s interior comes from the decay of radioactive isotopes and the cooling of the earth to determine the magnitude of each contribution detailed studies of the convective mechanism must be carried out one approach to this problem is to parameterize a nondimensional mantle temperature in terms of the mantle rayleigh number the time variation of the radioactive heat production and the transient cooling of the entire earth are included also included is the differentiation of radioactive isotopes into the continents important conclusions are 1 1 that between 25 and 30 of the surface heat flow can be attributed to the cooling of the earth this gives a mean concentration of uranium in the mantle close to 30 ppb 2 2 that two scales of mantle convection are occurring the first associated with convection of the surface plates and the second a turbulent convection beneath the surface plates 3 3 that the mean temperature of the mantle in archean times was 100 150 k hotter than at present but that the mean thickness of the lithosphere was about one half the present value 4 4 that 60 to 80 of the area of the continents was formed prior to 3 b y b p

the seismic structure of thinned continental crust in the northern bay of biscay summary the stretching and thinning of the continental crust which occurs during the formation of passive continental margins may cause important changes in the velocity structure of such crust further crust attenuated to a few kilometres thickness can be found underlying oceanic water depths this paper poses the question of whether thinned continental crust can be distinguished seismically from normal oceanic crust of about the same thickness a single seismic refraction line shot over thinned continental crust as part of the north biscay margin transect in 1979 was studied in detail tau p inversion suggested that there are differences between oceanic and continental crust in the lower crustal structure this was confirmed when synthetic seismograms were calculated the thinned continental crust p 7 0 exhibits a two gradient structure in the non sedimentary crust with velocities between 5 9 and 7 4 km s l an upper 0 8 s l layer overlies a 0 4 s l layer no layer comparable to oceanic layer 3 was detected the uppermost mantle also contains a low velocity zone

orogen scale decollements the continental lithosphere responds to stress by deforming as a generally layered medium deep seismic reflection data coupled with a variety of ancillary geological and geophysical data are interpreted to provide images of fault zones that tend to form moderately dipping ramp structures in mechanically rigid layers and flat detachments in mechanically weak layers this geometry is similar to ramp and flat structures observed at smaller scales in thrust and fold belts and leads to the interpretation that most orogens are underlain by orogen scale decollements in a manner that is analogous to so called thin skin deformation in sedimentary rocks decollements may occur within the crust for example near the base of a sedimentary section or in the middle crust near the moho in the subcrustal lithosphere or in the asthenosphere even intracratonic basement cored thick skin uplifts that occasionally occur in foreland regions such as the wyoming province are probably large crustal scale versions of ramp flat features observed in supracrustal rocks and are thus likely caused by the same fundamental tectonic processes

broadening of seismogram envelopes in the randomly inhomogeneous lithosphere based on the parabolic approximation southeastern honshu japan analyzing horizontal component seismograms of small earthquakes with intermediate hypocentral distances in southeastern honshu japan we found that time widths of seismogram envelopes around direct s waves are much longer than source duration times estimated from their magnitudes the time lags of the maximum amplitude and the half maximum after the peak were measured from the onset of the direct s wave arrival from band pass filtered seismograms from 2 to 32 hz even though there is considerable scatter both the time lags are found to increase with increasing hypocentral distance up to 305 km we hypothesize that pulse shape broadens and the maximum amplitude is reduced after propagating through the random structure of the lithosphere the parabolic approximation theoretically predicts that the long wavelength component of velocity inhomogeneities compared with the wavelength of seismic waves produces diffraction fluctuations and makes seismogram envelopes broaden with increasing travel distance in the saturated regime supposing a gaussian autocorrelation function for the randomness and an empirical frequency dependent attenuation we propose a formula for the temporal change in the power spectral density of seismic waves applying this formula to the observed data we statistically evaluated the scale of random inhomogeneities the mean square fractional velocity fluctuation was estimated to be 10 3 times the correlation distance a in kilometers

adirondack appalachian crustal structure the cocorp northeast traverse reflection surveys from northeastern new york and central vermont the first elements of a cocorp seismic traverse of grenville and appalachian terranes in the northeastern united states indicate that an unusual zone of layered reflections characterizes the lower crust beneath the eastern adirondacks and that grenville age basement of the green mountains may actually be allochthonous perhaps thrust over paleozoic shelf rocks the top of the adirondack deep layered sequence is found at about 20 km beneath the marcy meta anorthosite massif it appears to dip about 10 northwestward and wedge out against more horizontal reflections at about 28 km this intracrustal sequence may correspond with a previously inferred increase in electrical conductivity possible interpretations range from its being a layered igneous complex to its representing a tectonically buried metasedimentary assemblage although graphitic schists and iron or sulfide segregations in such an assemblage might contribute to its electrical characteristics the problematical issue of water in the lower crust if and how it got there and remained is again raised if not resolved other intrabasement features include major east and west dipping reflections possibly corresponding to faults arched reflections that may represent folds a well defined lens shaped zone virtually devoid of reflections and possible moho reflections unequivocal sub moho reflections are absent superimposed at shallower levels on the eastern part of the traverse are appalachian structures reflections down to about 4 5 km beneath the taconic mountains are interpreted to represent paleozoic shelf sediments of the middlebury synclinorium underlying the taconic allochthons possibly correlative although less well defined reflections appear to extend in disrupted fashion beneath the precambrian rocks of the green mountains suggesting a major basement overthrust the inferred decollement for this overthrust steepens rapidly beneath the east flank of the green mountains east dipping reflections at the east side of the green mountains may represent another major thrust zone involving basement and overlying metasedimentary slices the basement overthrusting suggested by this study is similar in certain respects to the style if not the extent of that demonstrated by previous cocorp surveys in the southern appalachians

cocorp seismic reflection profiling in the ouachita mountains of western arkansas geometry and geologic interpretation cocorp seismic reflection profiling in the ouachita mountains of western arkansas indicates that 1 carboniferous foreland basin deposits within the arkoma basin thicken dramatically toward the south reaching an aggregate thickness in excess of 12 km 4 5 s two way travel time beneath the southern edge of the frontal thrust zone 2 evidence for northward directed low angle thrusting within this clastic sequence is prominent with a probable decollement surface lying at or near the contact with underlying early paleozoic shelf carbonates 3 beneath the benton uplift a sequence of discontinuous events defines a broad antiform cresting at approximately 7 km 2 8 s this structure appears to mimic the anticlinorial shape of the benton uplift indicated by the surface outcrop pattern 4 beneath the southern ouachitas south dipping stratified events are observed to depths in excess of 14 km 5 0 s 5 deeper in the section a prominent gently north dipping reflection occurs at approximately 22 km depth 7 6 s beneath the northern coastal plain southern ouachitas extrapolation of data along strike in the ouachita belt and consideration of large scale structure observed in other collisional orogenic belts suggest that the reflection events observed beneath the benton uplift represent early paleozoic shelf carbonates correlative with those that floor the arkoma basin to the north this interpretation requires that the early paleozoic deep water sediments exposed in the core of the ouachitas be allochthonous and also implies significant basement uplift beneath the core zone to the south the prominent package of layered reflections occurring beneath the southern ouachitas and northern coastal plain indicates that a significant portion of the crust in that region is composed of imbricate sedimentary and or metasedimentary strata at relatively shallow levels these strata are correlative with the carboniferous flysch cropping out in the southern ouachitas and at somewhat deeper levels they are correlative with the early to middle paleozoic deep water sediments exposed on the benton uplift

constraints from deep crustal structure on long term landform development of the british isles and eastern united states abstract it is assumed that some major landforms not easily explained by surface geology could be controlled by deep crustal properties the concept was first applied in the british isles and then to the u s atlantic provinces both regions were affected by similar orogenies in late precambrian paleozoic times and presently belong to the passive margins of the north atlantic ocean in both areas the pattern of rifting was greatly influenced by preexisting tectonic fabric of the continental crust in the british isles pre mesozoic structures are cut obliquely by the atlantic margin while in the u s border the present margin is nearly parallel to the paleozoic sutures the british isles were affected by differential block tectonics during the tertiary examples are given of regional scale landforms linked with crustal hinges recent data confirm the great heterogeneity of the crust beneath the british isles each crustal unit has its own isostatic response according to the state of stress and its own dynamic equilibrium between rock resistance erosion processes and uplift rate in the atlantic provinces of the united states the regional physiographic framework is simpler but allochthonous terranes thrust westward over the grenville basement and shelf sediments have buried some major discontinuities in the deep crust reverse faults associated with the nw se compressional stressfield are numerous but cannot explain regional scale landforms like the blue ridge escarpment above the piedmont rock control is not the direct cause of the escarpment the blue ridge from reaches its highest relief where discontinuities in the deep crust interfere with surficial geological features ocean to continent creep processes are suggested to explain the sustained uplift of the appalachian highlands and the relatively stable position of the mountainous front since middle mesozoic times

crustal structure of the celtic sea and western approaches from gravity data and deep seismic profiles constraints on the formation of continental basins a free air anomaly map of the celtic sea and the western approaches is presented including all available marine gravimetric data and bouguer anomalies on land n60 features are predominant in the area they correspond to the n60 so called caledonian grain which played a major role in the variscan orogeny and in the formation of the permo triassic sedimentary basins the fundamental new point which comes out from this gravimetric map is the existence of a closely spaced pattern of n105 features which we regard as late carboniferous transpressional structures in the eastern pan of the celtic sea thrusts are in the same n105 direction west of 6 w longitude however folds and thrusts trend preferentially in a n60 direction following the older grain there the n105 lineaments may be regarded as dextral shear structures with a possible compressive component the hercynian shortening of the celtic sea domain located between the variscan front and the lizard suture resulted from the displacement along the n160 direction of the two adjacent rigid domains north of the variscan front the n105 and n135 features were possibly active in more recent times then both the n60 and n105 trends controlled the formation of permo triassic basins the n105 directions being transfer faults two dimensional gravity modelling of the swat profiles in the celtic sea shows that the base of the reflective lower crust may be associated with the moho discontinuity everywhere in the celtic sea except below the saint george s channel basin the lower crust does not change significantly in depth and thickness beneath the permo triassic basins none of the available models simply satisfies such conditions they all require at least a modification of the geometry of the lower crust after the formation of the celtic sea basins nevertheless if the simple shear model can be ruled out mainly from structural arguments the uniform stretching and to a lesser degree the thermomechanical models seem to be more relevant models however two additional conditions are necessary 1 the preservation of crustal seismic features formed before the creation of basins must be explained and 2 a significant portion of the volume of the stretched lower crust about 40 of this volume for the north celtic sea basin must be added below or perhaps partly above the lower crust by lateral flow of the crust or at the expense of the upper mantle

seismic reflection imaging of the subducting juan de fuca plate mckenzie and parker s early suggestion1 that the juan de fuca plate is underthrusting north america has since been confirmed by numerous studies evidence for plate convergence along this junction includes a sediment filled trench2 7 intense compressional deformation of the continental slope and shelf sediments2 7 seafloor magnetic anomalies that extend beneath the continental slope2 4 inland andesitic volcanoes8 paired low high heat flow and gravity anomalies9 10 active onshore deformation11 12 onshore crustal earthquakes with compressional axes parallel to the direction of plate convergence13 sinistral strike slip earthquakes along the nootka fault fig 1 14 and a well defined benioff wadati zone6 15 16 here we report the results of a seismic reflection survey that has delineated two slabs of oceanic lithosphere underlying vancouver island one that is currently being subducted and one that is underplated these findings lead us to speculate that successive underplating of oceanic lithosphere may be an important process in the evolution and growth of continents

large scale thin skin tectonics the lithosphere is to a first approximation a layered system generally pressure temperature and composition are strongly dependent on depth and vary relatively little laterally these parameters can affect greatly the response to stress of rocks in the crust and upper mantle and are responsible for the stratification of many characteristics such as seismic response rheology and kinematics in the lithosphere asthemosphere system e g chen and molnar 1983 on a smaller scale lithology which often is also horizontally stratified plays a major role in determining strength e g engelder and geiser 1980 pore pressure can also greatly affect strength hubbert and rubey 1959 suppe 1981

western fiordland orthogneiss early cretaceous arc magmatism and granulite facies metamorphism new zealand u pb isotopic analyses of zircons from a distinctive suite of previously undated granulite facies metaplutonic rocks here termed the western fiordland orthogneiss wfo in fiordland southwest new zealand indicate synkinematic magmatic emplacement between 120 and 130 ma ago these rocks were previously interpreted as possibly being of precambrian age initial pb and sr ratios are consistent with arc subduction related magmagenesis with little or no involvement of ancient continental crust subsequent high pressure 12 kb metamorphism of the wfo may reflect a major collision event involving crustal thickening by overthrusting of a 15 km thick sequence metamorphism ceased 116 ma ago based on206pb 238u ages of zircon from a retrogressed granulite u pb isotopic analysis of apatite along with previously published rb sr mineral ages indicate that final uplift and cooling to 300 400 c was largely completed by 90 ma the average uplift rate during this period is inferred to have been in excess of 1 mm yr unmetamorphosed gabbronorites of the darran complex in eastern fiordland inferred by some investigators to be the granulite protolith yield concordant u pb zircon ages of 137 1 ma u pb ages of apatite and previously published k ar mineral ages indicate that these rocks experienced a rapid and simple cooling history lasting only a few million years the high grade wfo and unmetamorphosed darran complex are now separated by a profound structural break however the ages and similarities in initial pb and sr isotopic ratios suggest that both suites are products of the same early cretaceous cycle of subduction related magmatism the timing of early cretaceous magmatism and metamorphism collision and resultant crustal thickening and subsequent great uplift and erosion in fiordland has important implications for terrane accretion and the evolution of relative plate motions along the new zealand segment of the gondwana margin

seismic and tectono metamorphic characters of the lower continental crust in phanerozoic areas a consequence of post thickening extension deep seismic profiles of phanerozoic continental crust commonly show a highly reflective lower crust rheological considerations suggest that the seismic fabric of the lower crust can be attributed to the tectonic transposition of various petrological heterogeneities in the main flow plane the variscan provinces of western europe have been affected during phanerozoic times by several extensional and compressional events the geometrical relationships between seismic and geological structures indicate that the layering of the lower crust was acquired during the late carboniferous to permian when the thickened variscan crust was affected by gravitational collapse petrological and geochronological analyses of deep crustal rocks xenoliths and exposed sections indicate that the lower crust has recorded a major high t medium p granulite facies metamorphism during the late variscan extension whereas on the surface upper carboniferous to permian basins were being deposited a similar scenario characterizes other phanerozoic orogenic belts in the caledonian provinces of the british islands the lower crust is seismically reflective it has undergone medium p granulitic metamorphism during the deposition of devonian sedimentary basins at the end of the caledonian orogeny in the same way collapse of the mesozoic belt in the western part of north america is responsible during the cenozoic for pervasive crustal extension whose consequence is a seismic layering of the lower crust accompanied by a low p granulite grade metamorphic event while in the mid and upper crusts low angle ductile normal faults give rise to the basin and range province therefore it is proposed that there is a genetic relationship between 1 post thickening crustal extension 2 low to medium p granulite facies metamorphism of the lower crust and 3 seismic layering of the lower crust

birps deep seismic reflection studies of the british caledonides the western isles north channel winch traverse fig 1 an extension of the successful moist profile1 was recorded in 1982 at sea along the west coast of britain for birps british institutions reflection profiling syndicate by the geophysical company of norway geco the purpose of the winch traverse was to study crustal structure of the caledonian foreland to cross the caledonian orogen and to establish the three dimensional geometry of mantle reflectors originally seen on moist we describe here the data first emphasizing british caledonian structures and then discussing features of the deep crust of wider significance the data are of very good quality and contain clear moho and upper mantle reflections the lower crust is surprisingly reflective deeply penetrative thrusts are observed and there is firm evidence that several of the mesozoic basins round the shores of the united kingdom were formed by reactivation of older features

thrust loads and foreland basin evolution cretaceous western united states two dimensional modeling of loading during the formation of the idaho wyoming thrust belt shows that regional isostatic compensation by flexure of an elastic lithosphere is sufficient to control the formation of a foreland basin the flexural rigidity of the lithosphere is inferred to have been approximately 1023 nm 1030 dyne cm on the basis of palinspastic comparison of predicted downwarping due to the thrust plate loads to the shape of the sedimentary wedge on the west side of the cretaceous western interior seaway erosion of part of the uplifted thrust plates redistributed the load depositing it farther to the east thereby causing subsidence over a much wider area than could have been accomplished only by the loading by thrust plates paleotopography after major cretaceous thrust events was calculated the resulting mountainous terrain gentle alluvial plain and flat sea floor correspond well with the topography of the modern foreland thrust belt and basin system in the andes of south america and to paleogeographic reconstructions in the western united states thrust belt topography is controlled by the subsurface geometry of the thrust faults particularly the positions of ramp zones and by isostatic subsidence

deep crustal extensional faulting in the ivrea zone of northern italy abstract the ivrea zone in northern italy consists of amphibolite and granulite facies metasedimentary metabasic and ultrabasic rocks and is thought to represent a section through the deep continental crust detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones which after the effects of later low temperature faulting and folding have been removed appears to accommodate extension of the sequence the shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks and locally show evidence for development under prograde conditions these observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting in this model the effects of shear heating and displacement of cool hanging wall rocks against hotter footwall rocks are calculated these observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust especially when basic rocks are the dominant components of the rock pile a restored section through the extended lower crust as observed in the ivrea zone is presented and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed

seismic profiling the continental lithosphere deep seismic reflection profiling is successfully being used worldwide to study the structure of the continental crust1 7 the moist8 profile recorded by birps british institutions reflection profiling syndicate was the first to show that the structure of the upper mantle could also be imaged given a powerful source and long recording times birps has recently acquired an ultra deep seismic reflection line drum deep reflections from the upper mantle the drum line was designed specifically to investigate the reflectivity of the lower continental lithosphere and was therefore recorded to a two way time twt of 30 s giving a depth of penetration into the lithosphere of 110 km this profile has now revealed strong reflections which are unequivocally from the lower lithosphere well below the base of the crust and which are as far as we are aware the deepest and most continuous structures imaged in the upper mantle by multi channel seismic reflection profiling

exposed cross sections through the continental crust implications for crustal structure petrology and evolution abstract the continental crust is exposed in cross section at numerous sites on the earth s surface these exposures which appear to have formed by obduction along great faults during continental collision may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle from an examination of five complexes which meet these criteria it is concluded that the most prominent layering in the crust is not compositional but metamorphic the lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks respectively post metamorphic granitic intrusions are common at intermediate to shallow levels position of discontinuities in refraction velocity where present commonly correspond to changes in composition or metamorphic grade with depth the continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by cocorp field observations coupled with geochemical data indicate a complex evolution of the lower crust which can include anatexis multiple deformation polymetamorphism and reworking of older crustal material the complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments

fine structure of the continental reflection moho comparisons of deep seismic reflection profiles from a variety of tectonic regions that span a large fraction of the earth s history lead to a threefold categorization of reflection patterns near the crust mantle transition type i patterns have no distinct reflections including no reflection moho at all type ii patterns have one or more subhorizontal reflections and type iii patterns have reflections that project from the crust into the mantle subdivisions within each of these are based upon the geometric relationships between the reflection moho and reflections immediately above and in some cases below hence type i examples may have essentially no reflectivity near the crust mantle transition or they may be characterized by crustal reflectivity that fades downward type ii examples may have a single prominent reflection or they may have several subhorizontal reflections type ii patterns may also have crustal reflections that are either listric downward or that appear to be truncated at or near the base of reflectivity type iii examples may have a single prominent reflection that projects below the moho as delineated by seismic refraction data or they may have a series of subparallel dipping reflections more than one type of reflection moho may be visible along a single profile sometimes changing within distances of only a few tens of kilometers this diversity of both geometry and character leads to the conclusion that the variable appearance of the reflection moho is a consequence of variations in the thermal and structural evolution of the crust mantle transition at many scales from a few tens to many hundreds of kilometers

crustal structure of southwestern montana and east central idaho results of a reversed seismic refraction line on october 20 1982 we collected seismic refraction data on a 250 km line segment between butte montana and challis idaho these data are from two open pit mine blasts one at anaconda minerals companys southeast berkely pit and the other at cyprus mines corporations thompson creek mine eight of our ten recorders wrote clear records of both blasts with measured travel times accurate to 0 2 seconds we supplemented these data with data from permanent stations in the butte area the pg crustal velocity is 5 8 km s to the southwest and 6 0 km s to the northeast the pn mantle velocity is 7 9 km s to the northeast and 8 1 km s to the southwest total travel times are equal the s wave velocity in the crust is about 3 5 km s the symmetry of the travel time results with their intersection within one kilometer of the midpoint between the pit blasts strongly suggests the refraction line overlies a horizontal moho or parallels the strike line of a dipping moho surface these data indicate the crust in southwestern montana and east central idaho is approximately 33 km thick contrasting tectonic processes compression and thickening during the laramide orogeny with extension and thinning since the paleocene resulted in a crust with slightly less than normal thickness

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

superimposed w and ag pb zn cu au mineralization and deep prospecting insight from a geophysical investigation of the yinkeng orefield south china abstract the yinkeng orefield is located in the eastern nanling region south china the nanling region hosts numerous w sn dominant polymetallic deposits related to the widely distributed mesozoic granites making it one of the most important w sn polymetallic metallogenic belts in the world in this study we utilized multiple geophysical methods including gravity magnetic audio frequency magnetotelluric sounding amt and high resolution reflection seismic techniques to detect the underground distribution of granites strata and structures our aims are to understand better the ore controlling mechanisms for the large scale w ag au pb zn aggregation to establish a metallogenic model for this type of mineralization and to supplement a 3000 m scientific drilling program in the nanling region for the chinese sinoprobe program project sp nlsd 1 in the yinkeng orefield an ore controlling ne trending nappe structure is composed of several thrust faults at shallow depths f1 and f2 which merge into a single major fault at depth the pre sinian sedimentary rocks of the hanging wall of these faults have much higher velocities densities and wave impedance than the carboniferous permian sedimentary rocks of the footwall the slip surface of the nappe which appears to have low resistivity extends downward to a two way travel time of more than 2 5 s equivalent to a depth of more than 4000 m thrusting was from se to nw and the fault flattens at depth the nappe structure may have provided channels for magmas and mineralized hydrothermal fluids granites in the yinkeng area which are spatially associated with the w ag pb zn orebodies generally show high resistivity low polarizability low seismic velocity and medium magnetism and their geophysical characteristics have revealed their distribution at depth specifically the gaoshanjiao granodioritic pluton closely related to precious metal mineralization is characterized by low density and high magnetism allowing us to identify a large buried part of the pluton se of the exposed body the jiangbei granitic pluton which typically hosts w mineralization has lower magnetism that reveals a ne trending extension about 2 2 2 8 km wide at a depth of 16 6 km one completely concealed granite the yinkeng pluton lies below the nappe structure it is geophysically similar to the jiangbei granitic pluton but has an obviously higher magnetic signature the high magnetism may be due to stratiform polymetallic bodies which could represent a good exploration target for porphyry skarn type w cu polymetallic deposits drilling site for sp nlsd 1 combined with surface mapping the geophysical data collected in this study allow us to propose a deep exploration model that can serve as a guide for w polymetallic ore prospecting in the nanling and adjacent regions

seismic reflection profiles across deep continental crust exposed in the kapuskasing uplift structure seismic imaging of precambrian cratons holds the promise of deep structural mapping and interpretation of fundamental crustal construction processes deep structures may be identified by tracing reflections to the surface1 particularly in exposed crustal cross sections2 elevated along deeply penetrating faults but many such cross sections have structural orientations too steep for successful reflection imaging like the ivrea zone3 or occur in plate boundary settings not representative of continental interiors by contrast the archaean greenstone terrain exposed in the kapus kasing uplift of canada4 5 has gently dipping structures and an intracratonic setting within the superior province thus providing an opportunity to examine the third dimension of continental crust down to 25 30 km palaeodepth6 here we present new seismic reflection data which although generally supportive of structural models based on surface geology4 gravity modeling5 and seismic refraction studies7 8 also indicate that the kapuskasing structure is a relatively thin thrust sheet containing seismically reflective sequences of high grade rock which may be analogues of seismi cally reflective in situ lower crust

proterozoic basin in the southern midcontinent of the united states revealed by cocorp deep seismic reflection profiling cocorp deep crustal seismic profiles in southwestern oklahoma show strong persistent continuous and undeformed layering in the basement over an area probably very much greater than 2 500 km 2 such layering is very unusual judging by cocorp experience with basement rocks elsewhere in the united states the data can be interpreted as representing a proterozoic basin filled with clastic sedimentary and felsic volcanic rocks 7 to 10 km thick whose base lies 10 to 13 km deep these rocks are believed on the basis of sparse evidence from regional geology to have been deposited or extruded about 1 200 to 1 400 m y ago and some of them may now be metamorphosed this basin lies on the south side of the wichita mountains under the paleozoic hardeman basin and is similar in depth to the paleozoic anadarko basin north of the mountains the deep basement layering is truncated on the south side of the wichita mountains probably by precambrian faults in conjunction with granitic intrusions pennsylvanian compression probably reactivated these precambrian trends extensive precambrian basin deposits in this area were unexpected on the basis of evidence from sparse well control and reports of other layered basement reflections elsewhere in the southern midcontinent suggest that precambrian basins may be an important feature of this region simple models for the evolution of southwestern oklahoma as an aulacogen must be reformulated in the light of these new data

reanalysis of the cocorp deep seismic reflection profile across the san andreas fault parkfield california in 1977 cocorp recorded a short seismic reflection test line across the san andreas fault through parkfield california where the salinian crustal block is juxtaposed against a contrasting terrane of franciscan and overlying material although analysis of this seismic line has been limited by crooked geometry lack of cross lines complex terrain possible recording noise and perhaps most of all its short length gross crustal features are discernible after extensive reprocessing and reanalysis the salinian block southwest of the san andreas consists of a maximum of 2 km of relatively underformed sediments overlying a reflective and inhomogeneous basement which exhibits deep subhorizontal layering at 8 to 10 km depth density and amplitude of reflections markedly decay perhaps corresponding to the base of a salinian granitic crust which overlies franciscan or other contrasting material northeast of the san andreas subhorizontal reflections suggest that franciscan material may extend as deeply as 13 km below which is a nonreflective zone which might represent relict oceanic crust reflections from moho depths differ across the san andreas fault beneath the salinian block is a thick zone of reflections beginning at about 22 km depth while less layered and weaker reflections seem to define the moho discontinuity at 29 km beneath the franciscan terrane the san andreas fault zone appears to be traceable as a distinct feature on the reflection section throughout the entire crust forming a near vertical zone about 3 km wide in the seismogenic upper crust down to 15 km the fault zone is marked by sharp structural truncations and short reflection or diffraction segments at depth it seems to be distinguished by a nonreflective column this vertical change in reflection character may correspond to the transition from a brittle to a ductile deformation regime

lhasa block and bordering sutures a continuation of a 500 km moho traverse through tibet a north south wide angle fan profile through the yarlung zangbo suture across the lhasa block and through the bangong nujiang suture shows several significant and sharp changes in crustal thickness both sutures appear as vertical 20 km steps in the moho and may have been the loci for eastward strike slip motion of the tibetan lithosphere