deformation of the caledonide lithosphere of northwest scotland abstract an interpretation of the moist seismic reflection profile along the north coast of scotland has shown the pattern of deep crustal structure across the margin of the caledonian orogen with the hebridean cratonic foreland to the northwest it is proposed that the lower crust of the orogen is characterised by a suite of easterly dipping thrusts that divide it into flakes that were imbricated during lateral compression of the lithosphere the effects of concomitant and subsequent uplift and erosion led to the removal of the major part of the upper crust within the orogen so that the present crust and moho are largely the relics of the caledonian lower crust as time progressed the thrust front encroached further into the foreland and cut down to a basal decollement at successively deeper levels this behaviour is explained as a consequence of the response of the brittle ductile transition to changing the temperature regime rock composition and strain rate as the orogeny proceeded palinspastic reconstructions of the whole lithosphere illustrate the process following the compressional phase of the orogeny uplift led to initiation of extension and the reactivation of the thrusts as normal listric faults rotation of basement blocks gave rise to wedge shaped sedimentary basins

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

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

contrasting structural features in the granulite gneiss charnockite granite complex lake baikal u s s r evidence for diverse geotectonic regimes in early proterozoic times abstract evidence for two major geotectonic regimes in a deep level early proterozoic crystalline complex is provided by its structural metamorphic and igneous features the earlier one is characterized by 1 isoclinal to tight folds in gneisses and granulites formed by dynamothermal metamorphism 2 asymmetrical folds and tectonically emplaced bodies of ultramafic material and 3 tight to open upright folds and minor granite bodies these structural metamorphic and igneous features are considered to be the results of collisional tectonism they are more than 100 ma older than other products interpreted as resulting from transcurrent faulting extensional tectonism and which show many similarities with the basin and range province of the u s a here patchy distribution of charnockite is a distinctive feature associated with knuckle shaped asymmetrical folds with moderately dipping axial planes which exert a strong structural control on the process of charnockitization this is related to the passage of co 2 whose source is considered to be crystallizing mantle derived basic magma later upright folds steeply inclined shears and successively emplaced pyroxene bearing granitoids biotite bearing leucogranites and pegmatites are interpreted as features associated with caldera formation the igneous products are considered to be the result of progressive fractionation of basic magma possibly with incorporated melted crustal material some movement on gently dipping faults that cut all other structures is likely to be associated with himalayan rifting rather than with precambrian tectonism and to represent a third geotectonic regime

the influence of depth on reactivation in normal faulting abstract the study examines the influence of depth on the development of steep normal faults above a gently dipping fault which is reactivated in its deeper part and abandoned in its upper part the isotropic and anisotropic mohr coulomb anderson theory is used to determine the ability of any part of a fault to be reactivated by calculating the difference between the stress deviator necessary to induce reactivation and that required to induce fracturing when linear depth dependent properties are assumed it appears that a new normal fault can branch off a gentle pre existing fault only if the friction coefficient of the latter is much less than that used by byerlee and if the ratio cohesion of fault cohesion of intact rock is non negligible for a 30 dip fault the friction coefficient has to be less than 0 35 friction angle less than 20 neither pore fluid pressure nor constant excess fluid pressure affects the above conditions when the cohesion of the surrounding rocks increases with depth steep normal faults can branch off byerlee faults with dips as gentle as 30 a low shear strength of the lowest part of the preexisting fault also increases the chances of reactivation of the deeper part and abandonment of the upper part

evidence of an imbricate crustal thrust belt in the southern british variscides contributions of southwestern approaches traverse swat deep seismic reflection profiling recorded through the english channel and the celtic sea the southwestern approaches traverse swat seismic reflection profiles recorded through the celtic sea and the english channel provide unexpected data concerning notably the deep structure of the irish and south british variscan crust one of the most significant results is the recognition of prominent deep southerly dipping reflectors regarded as large scale foreland directed variscan thrusts which constitute in the southern innermost zones a crustal stacking wedge the resulting overthickened crust may have induced the petrogenesis of the cornubian granitic batholith by an anatectic melting process the roots of this crustal duplex are not imaged on the swat lines because of their subsequent assimilation by the more recent layering of the present day lower crust upward the deep crustal ramps are assumed to flatten out and to join a roof thrust which acts as a major midcrustal decoupling zone not revealed by the swat profiles and which underlies the thick devonian allochthonous high strained units of cornwall northward this shallow northerly verging shear zone is overstepped by the tintagel antithetic back thrusts which are closely related to deeper northward dipping basement faults considered as caledonian features southward the imbricate crustal thrust unit is overridden by the lizard ophiolitic suture which appears as a 8 km thick gently southerly dipping sheared zone crosscuttting the entire crust and rooting deep beneath the present day moho its hanging wall is constituted by the channel cadomian block characterized by an almost seismically featureless upper crust which is not involved in the main variscan thrust stacking northward the imbricate thrust unit is delineated by a moderately southerly dipping ramp which penetrates straight down into the upper crust without any evidence of an intervening flat lying decollement level this frontal ramp emerges along a broadly 110 n trending discontinuous thrust front located above the northern downfaulted margins of the munster ireland and pembroke wales devonian basins the swat deep seismic data relate the overall variscan crustal shortening undergone by the south british domain to an oblique converging process involving a continental shelf pinched between the rigid allochthonous channel block and the northern autochthonous foreland this crustal shortening occurred during a carboniferous intracontinental collisional context subsequent to the closure and the obduction of a small oceanic crust floored basin lizard

extensional reactivation of an abandoned thrust a bound on shallowing in the brittle regime abstract shallow dip angles 45 suggested by field observations of continental extensional faults are not predicted by classical isotropic mohr coulomb anderson theory earthquake data indicate that normal faults exist in the upper crust with dip angles commonly as shallow as 30 some structural evidence suggests brittle normal faulting with dip angles as shallow as 10 one explanation of the apparent conflict between theory and structural seismic observations is that intrinsically weak shallow dipping pre existing faults are preferentially reactivated any reduction in frictional strength of the pre existing structure below that of surrounding rock increases the likelihood that such structures will be first breaking when extension is initiated enhanced fluid pore pressure on the pre existing fault reduces the effective strength and can further enhance shallow fault reactivation an analytical treatment clarifies the roles of geometry intact pre existing fault strengths and fluid pore pressures frictional strength ratios of 3 or greater could account for extremely shallow normal faults dips 10 20 without consideration of pore pressures in excess of the least principal stress or of principal stress systems rotated away from the gravity vector moderate reduction in friction 3 4 with respect to wall rock can reduce the dip to 30 and can account for shallow normal slip earthquakes

a model for the deep structure of the moine thrust zone seismic refraction and electrical conductivity profiles across northern scotland together with geological and other geophysical data are interpreted in terms of the deep structure of this part of the orthotectonic caledonides our preferred interpretation is that the moine thrust zone extends to the base of the crust and the caledonian front is steep this contrasts with recently proposed ultra thin skinned solutions the results of the recently completed moist deep seismic reflection experiment should inter alia test our hypothesis in order to test the geometrical feasibility of our interpretation balanced sections have been constructed to illustrate the crustal evolution of the region during the caledonian orogeny these sections permit estimates to be made of crustal shortening thickening and erosion our model requires that the n highlands were once overlain by major exotic nappes which by analogy with nw newfoundland may have been ophiolitic the proposed regional structural pattern comprises a duplex of crustal dimensions we suggest that crustal duplex development is an important mechanism in the orogenic deformation of previously thinned and extended passive continental margins

the mohorovicic discontinuity beneath the continental crust an overview of seismic constraints abstract the seismic signature of the moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image seismic data of different types passive source earthquake broad band recordings and controlled source seismic refraction densely recorded wide angle deep seismic reflection and normal incidence reflection using vibroseistm explosives or airguns have contributed to the description of the moho as a relatively complex transition zone of critical importance for the quality and resolution of the seismic image are the acquisition parameters used in the imaging experiments a variety of signatures have been obtained for the moho at different scales generally dependent upon bandwidth of the seismic source this variety prevents the development of a single universally applicable interpretation in this way source frequency content and source and sensor spacing determine the vertical and lateral resolution of the images respectively in most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the moho and its relationship to a petrologic crust mantle transition in regional seismic studies carried out using passive source recordings the moho is a relatively well defined structure with marked lateral continuity the characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area refraction and wide angle studies suggest the moho to be often a relatively sharp velocity contrast whereas the moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity the origin of the moho and its relation to the crust mantle boundary is probably better constrained by careful analysis of its internal details which can be complex and geographically varied unlike the oceanic moho which is formed in a relatively simple well understood process the continental moho can be subject to an extensive variety of tectonic processes making overarching conclusions about the continental moho difficult speaking very broadly 1 in orogenic belts still undergoing compression and active continental volcanic arcs the moho evolves with the mountain belt 2 in collapsed phanerozoic orogenic belts the moho under the collapse structure was formed during the collapse often by a combination of processes 3 in regions having experienced widespread basaltic volcanism the moho can result from underplated basalt and basaltic residuum in precambrian terranes the moho may be as ancient as the formation of the crust in others precambrian tectonic and magmatic processes have reset it we note that seismic reflection data in phanerosoic orogens as well as from precambrian cratonic terranes often show thrust type structures extending as deep as the moho and suggest that even where crust and mantle xenoliths provide similar age of formation dates the crust may be semi allochothonous

active normal faulting and crustal extension a world wide review of fault plane solutions and focal depths for large normal faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6 15 km on faults dipping between 30 and 60 in the few cases where levelling or seismic data are good enough these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust there is evidence that in some cases as a result of the enormous transitory increase in strain rate during large earthquakes rupture continues into the normally ductile lower crust on surfaces with substantially gentler dips these low angle surfaces may be analogous to some of the detachments seen in metamorphic core complexes of the western usa but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the detachments and the blocks on either side such contrasts however are unlikely to introduce substantial curvature to an originally planar shear zone if the observed spread in active normal fault dips is caused by rotation of the faults and the blocks they bound during extension then a maximum value of 1 7 can be accommodated by seismic activity on a single generation of normal faults with continued extension either a new generation of steeper faults cutting the rotated first faults is likely to form or the deformation will continue aseismically on faults dipping at less than 30 large earthquakes have not been observed to nucleate on very low angle 20 normal faults within the continental crust anywhere in the world such faults can of course move aseismically but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active thus extensional models that require concentrated simple shear on large sub horizontal faults within the brittle upper crust will also require a spatial separation between aseismic very low angle faulting and seismic highangle faulting since seismogenic high angle faults dominate the topography of extending regions upper crustal very low angle faulting if it occurs on a large scale presumably does so in areas that are relatively fiat

north atlantic marine rifting in the early triassic new evidence from east greenland two early triassic marine rifting events have been recognized in new outcrop data from east greenland increasing our understanding of this important tectonic episode in the development of the ne atlantic norwegian greenland sea a detailed ammonoid stratigraphy together with stratigraphic and radiometric evidence show that the first rifting event took place in the early griesbachian this event was largely confined to major basin margin faults the second rifting event which involved splay faulting and intra basinal fault block tilt is dated as late griesbachian deposition in east greenland took place in half grabens dominated by deep water gravity flow sandstones and mudstones as well as shallow marine conglomerates and sandstones similarly recent core data from offshore mid norway show that deposition on the eastern margin of the norwegian greenland sea took place in deep marine settings dominated by gravity flows this suggests that the early triassic marine realm extended from east greenland to the present day norwegian coastline and that the marine rift basin was significantly wider than previously thought

high resolution field measurements and numerical modelling of intra wave sediment suspension on plane beds under shoaling waves abstract intra wave sediment suspension is examined using high resolution field measurements and numerical hydrodynamic and sediment models within 120 mm of a plane seabed under natural asymmetric waves the detailed measurements of suspended sediment concentration at 5 mm vertical resolution and at 4 hz showed two or three entrainment bursts around peak flow under the wave crest and another at flow reversal during the decelerating phase at flow reversal the mixing length was found to be approximately double the value attained at peak flow under the crest to examine the cause of multiple suspension peaks and increased diffusion at flow reversal a numerical side view hydrodynamic model was developed to reproduce near bed wave induced orbital currents predicted currents at the bed and above the wave boundary layer were oppositely directed around flow reversal and this effect became more pronounced with increasing wave asymmetry when the predicted orbital currents and an enhanced eddy diffusivity during periods of oppositely directed flows were applied in a lagrangian numerical sediment transport model unprecedented and extremely close predictions of the measured instantaneous concentrations were obtained the numerical models were simplified to incorporate only the essential parameters and by simulating at short time scales empirical time averaged parameterisations were not required key factors in the sediment model were fall velocities of the full grain size distribution diffusion separation of entrainment from settlement and non constant but vertically uniform eddy diffusivity over the plane bed sediment convection by wave orbital vertical currents was found to have no significant influence on the results

conditions for flow in the continental crust 1 earthquake focal depth distributions and effective elastic thickness variations on the continents suggest that in old cold regions such as shields the lower continental crust can be stronger than the upper mantle and imply that the upper mantle can flow even when the lower crust does not however in other places substantial flow in the lower crust does occur particularly near extensional metamorphic core complexes we suggest that the lower crust only flows when its viscosity is significantly reduced either by heating from igneous intrusions or by the addition of water in both cases the important characteristics of the flow are a consequence of the limited time over which the viscosity reduction occurs the timescale is controlled either by conductive cooling of intrusions or by the separation of melt from its matrix which removes the water

deep structure of the scottish caledonides revealed by the moist reflection profile the moine and outer isles seismic traverse moist is a deep crustal reflection profile shot at sea off the north coast of scotland in 1981 spectacular reflections are observed from the moho and from thrust zones within the caledonian fold belt and foreland deep reflection profiling of the continental crust of the united states under the direction of the cocorp group has amply demonstrated the value of this technique when applied to suitable geological problems see for example refs 1 2 a similar group the british institutes reflection profiling syndicate birps has now been set up in the uk to organize such projects funds are provided by the natural environment research council nerc through the core group based at cambridge university however a preliminary experiment of which moist is the result was organized through the institute of geological sciences igs

the recognition of reactivation during continental deformation reactivation involves the accommodation of geologically separable displacement events intervals 1 ma along pre existing structures the definition of a significant period of quiescence is central to this phenomenological definition and the duration of the interval chosen represents the resolution limit of reactivation criteria found in most ancient settings in neotectonic environments reactivation can be further defined as the accommodation of displacements along structures that formed prior to the onset of the current tectonic regime this mechanistic definition cannot always be applied to ancient settings due to the uncertainties in constraining relative plate motion vectors four sets of criteria may be used to recognize reactivation in the geological record stratigraphic structural geochronological and neotectonic some structural criteria may not be reliable if used in isolation to identify reactivated structures much of the previously published evidence cited to invoke structural inheritance is equivocal as it uses similarities in trend dip or three dimensional shape of structures numerous fault and shear zone processes can cause significant weakening both synchronously with deformation and in the long term and may be invoked to explain reactivation the collage of fault bounded blocks forming most continents therefore carries a long term architecture of inheritance which can explain much of the observed complexity of continental deformation zones

the moine thrust in the birps data set a second generation of birps profiles combine with older data to form a loose 10 by 40 km spacing grid over the offshore projection of the moine thrust these profiles show clear reflections that arguably represent fault plane reflections from the moine and related caledonian thrust structures in the location suggested by some earlier workers reflectors predominate at two structural levels 8 10 km 2 5 3 5 stwt and 13 16 km 4 0 5 5 stwt depths the deeper set appears to define an antiformal stack of thrusts or duplex with possible continuation to deeper levels by analogy with onshore sections and after subsequent extension is restored this deeper set correlates with the moine thrust onshore the shallower set aligns with normal faults that bound half grabens filled with devonian and permo triassic rocks this set correlates with the naver thrust after similar restoration normal faulting appears to have reactivated some of the thrusts increasing their reflectivity detailed stacking and interval velocity studies indicate that an increase of about 1 2km s 1 occurs across the reflections here associated with the two fault zones the overall reflection pattern is consistent with a complex stack of thrust sheets but lack of resolution prevents a complete structural analysis

evaluation of southern eastern overthrust belt beneath blue ridge piedmont thrust seismic reflection surveys in the southern part of the eastern overthrust reveal that crystalline rocks of the blue ridge and piedmont were thrust westward burying a large segment of sedimentary rocks of the valley and ridge surface and subsurface data suggest that the buried segment includes cambrian and lower ordovician shallow shelf clastic and carbonate rocks overlain by middle ordovician foredeep black shale because eastward increasing regional thermal patterns existed prior to thrusting westward movement of thrust sheets disrupted and telescoped that pattern by placing more thermally mature eastern rocks over less mature western rocks the original thermal pattern can be reconstructed by restoring the thrust sheets to their prethrust position the reconstruction mphasizes that rocks less metamorphosed than greenschist facies are contained in only two thrust sheets all of the saltville and part of the pulaski because subgreenschist rocks of the saltville and pulaski thrust sheets appear to be confined to the subsurface of the blue ridge and westward it is most likely that rocks with commercial gas potential will not be present in the subsurface of the piedmont east of the brevard fault nevertheless the concealed area of the southern eastern overthrust with possible gas potential is about as wide as its exposed part thus the combined surface and subsurface area for future exploration in the southern appalachians is about doubled in size

ambient noise tomography reveals basalt and sub basalt velocity structure beneath the faroe islands north atlantic abstract ambient noise tomography is applied to seismic data recorded by a portable array of seismographs deployed throughout the faroe islands in an effort to illuminate basalt sequences of the north atlantic igneous province as well as underlying sedimentary layers and precambrian basement rayleigh wave empirical green s functions between all station pairs are extracted from the data via cross correlation of long term recordings with phase weighted stacking implemented to boost signal to noise ratio dispersion analysis is applied to extract inter station group travel times in the period range 0 5 15 s followed by inversion for period dependent group velocity maps subsequent inversion for 3 d shear wave velocity reveals the presence of significant lateral heterogeneity up to 25 in the crust main features of the final model include i a near surface low velocity layer interpreted to be the malinstindur formation which comprises subaerial compound lava flows with a weathered upper surface ii a sharp velocity increase at the base of the malinstindur formation which may mark a transition to the underlying beinisvor formation a thick laterally extensive layer of subaerial basalt sheet lobes iii a low velocity layer at 2 5 7 0 km depth beneath the beinisvor formation which is consistent with hyaloclastites of the lopra formation iv an upper basement layer between depths of 5 9 km and characterized by s wave velocities of approximately 3 2 km s consistent with low grade metamorphosed sedimentary rocks v a high velocity basement with s wave velocities in excess of 3 6 km s this likely reflects the presence of a crystalline mid lower crust of archaean continental origin compared to previous interpretations of the geological structure beneath the faroe islands our new results point to a more structurally complex and laterally heterogeneous crust and provide constraints which may help to understand how continental fragments are rifted from the margins of newly forming ocean basins

shear zones at the laxford front nw scotland and their significance in the interpretation of lower crustal structure the lewisian rocks in the scourie laxford region nw scotland show variations in shear strains and associated structures on the southern upper margin of a zone of intense laxfordian reworking the scourian block was thrust a large distance obliquely northwards over the northern laxfordian gneisses as part of a laxfordian episode of crustal thickening this was followed by crustal extension during which time the scourian block moved down to the south granite sheets were intruded along the main shear zone in a dilational region at the base of a rotational extensional block the laxfordian contractional and extensional deformation occurred under amphibolite facies conditions and analogies can be made with middle crustal structures inferred from deep seismic data for example structures produced during late laxfordian crustal spreading can be compared to those seen on the birps offshore scottish data which formed during devonian basin development following caledonian crustal thickening the scourie laxford structures give an idea of fault kinematics at the brittle ductile transition in the middle to lower crust steep faults pass down into more gently dipping ductile shears and then to a zone of anastomosing shears which accommodate the strains and rotations at the base of the rigid upper crust the shears may concentrate along a more planar zone of deformation forming a detachment or decoupling zone below which the extension may be variable depending on the mode of lithospheric stretching

displacement of the moho by the outer isles thrust shown by seismic modelling deep seismic data fig 1 from the caledonides off the north west coast of scotland collected 1981 82 by the british institutions reflection profiling syndicate birps show evidence that caledonian thrusts have been reactivated as mesozoic normal faults1 2 the best example is the outer isles oi thrust which is mapped on land as a compressional fault and is revealed by these data to be overlain offshore by a thick wedge shaped mesozoic rift basin1 known as the north lewis nl basin here we have used synthetic seismic sections to identify distortions of the oi thrust reflection due to the nl basin models which compensate for the effects of this distortion show that the oi thrust has a steeper angle of dip than has been predicted previously assuming no distortion causing the oi thrust to project into a 2 3 km offset of the moho and hence to penetrate the entire crust our data suggesting that shear strains affecting the lower crust may be localized in character have adverse implications for rheological models which predict to the contrary that the lower crust is a region where strain is distributed

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

the relationship between k ar mineral ages mica grainsizes and movement on the moine thrust zone nw highlands scotland muscovite and biotite separated from moine schists close to the moine thrust zone yield a range of k ar ages incompatible with the simple cooling regime previously proposed for the caledonides of northern scotland differences of only 1 ma between muscovite and biotite of the same grainsize contrast with differences of 12 13 ma between the 125 200 m and 250 500 m grainsize fractions this is contrary to the normally accepted dominance of mica type over grainsize effects dynamic recrystallization of the micas during early deformation associated with the moine thrust zone is thought to play a role as important as temperature in determining the k ar ages the conclusions drawn have important implications for the use of uplift and cooling rates derived from the absolute k ar closure temperatures of micas close to thrust zones although the k ar ages from micas with diameters greater than 200 m are not reproducible the data can be used to show that deformation associated with thrusting commenced between c 443 ma and c 435 ma and that the thrust zone was active until at least 422 ma ago

late caledonian extensional reactivation of a ductile thrust in nw scotland evidence is given for ductile extension across a fault zone located on the western seaboard of the northern highlands of scotland the fault zone lies within the metamorphic caledonides and extensional movement occurred during the waning stages of caledonian orogenic activity an earlier phase of westerly directed overthrusting took place on the same fault zone at higher metamorphic temperatures but after the peak of caledonian metamorphism the compression may be broadly coeval with early displacements on the moine thrust zone which probably lies offshore some 10 km to the northwest the extensional reactivation could relate to local crustal instability resulting directly from overthrusting it may have been due to local gravitational collapse following overthickening of the crust alternatively the extensional movement may witness the onset of regional gravitational collapse of the orogen during the late silurian to early devonian reactivation accords with interpretations of the movement histories of similarly eastward dipping offshore faults recognized on seismic reflection profiles

three dimensional mapping of seismic reflections from the crust and upper mantle northwest of scotland abstract a survey area 190 km by 110 km to the northwest of scotland on the uk continental shelf has been the site of intense exploration using the deep seismic reflection technique a closely spaced grid of deep reflection data totalling 1600 line km has been acquired by the birps group between 1981 and 1987 the majority of the data were recorded to 15 s two way travel time 50 km depth profiles with recording times up to 60 s 230 km depth were also acquired the survey includes a two ship synthetic aperture profile the slave line which synthesises a 0 16 km offset cdp profile the main features revealed and consistently imaged across the survey area by this combined dataset are an upper crust containing half graben style basins bounded to the west by planar faults a reflective lower crust and dipping and sub horizontal high amplitude reflections within the mantle three dimensional mapping of the dipping reflectors shows a complex package of reflections which updomes into the lower crust in the central area of the survey disrupting the moho and continuing into the uppermost mantle where it thins to a couplet of reflections in the north and south of the survey area this structure is observed only in the mantle to a depth of 80 km this dataset is unique in deep seismic reflection profiling in terms of the quality of the image returned the great depth to which reflections are imaged and the intensity of surveying of the lower crust and upper mantle the continuity and strength of the reflectors seen on these records show that even at the great depths at which these structures exist the seismic reflection technique is able to image them as high amplitude coherent and highly continuous reflections the reflections are interpreted as shear zones or faults within the lower crust and upper mantle this indicates that in this part of the world at least both the lower crust and upper mantle are able to sustain discrete zones of deformation

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

the structure of mountain fronts commonly the part of a mountain front which is visible at the surface consists of foreland dipping thrust belt rocks elevated above their regional structural position and overlain more or less conformably by molasse several explanations for their geometry are possible 1 major detachments exist within or beneath the molasse resulting in transport of the foreland basin examples of this geometry come from the swiss molasse plain the southern pyrenees and the mackenzie mountains of canada 2 displacement is lost on major backthrusts beneath the frontal monocline examples cited here are the rockies of alberta the sulaiman ranges of pakistan the mackenzies and the andes in peru 3 thrust sheets travelled over an old land surface and syntectonic molasse contemporaneously offlaps the topographic high of the thrust front this phenomenon occurs along the alpine thrust front in haute provence 4 the frontal fold represents deformation above a large scale thrust tip no unequivocal example of tip line strain at this scale has been recorded although this type of deformation may occur in the brooks range of alaska in many areas mountain fronts show a combination of these idealized geometries

seismic refraction shooting on the continental margin west of the outer hebrides northwest scotland seventeen sonobuoy refraction profiles have been shot to determine the nature of the basement and the broad pattern of sedimentation on the continental margin west of the outer hebrides nw scotland under much of the shelf crystalline rocks vp 5 1 km s lie within 100 m of the seafloor the basement being largely an extension of the precambrian lewisian metamorphic complex of western scotland vp vs gives poisson s ratios of 0 26 0 30 for the lewisian values which are significantly higher than in the deep crust under northern britain implying important compositional differences comparisons with ultrasonic velocities in rocks from the scourian 2700 ma and laxfordian 2200 1500 ma belts of the scottish mainland suggest that the lewisian on the inner continental shelf is predominantly laxfordian vp 5 5 km s higher velocity rocks probably scourian with only a moderate degree of laxfordian reworking vp 5 9 km s and cenozoic intrusions occur locally two seismic profiles indicate that the outer continental shelf may be underlain by a zone of dense scourian early laxfordian granulites whose presence possibly influenced the siting of the continental slope the sediments covering the basement are generally thin thicknesses exceeding 1 km are restricted to a fault bounded trough off the isle of lewis and to the outer shelf and continental slope the deposits can be divided into cenozoic 1 7 1 9 km s and mesozoic 3 0 4 4 km s units velocity variations in the latter probably reflecting the abundance of early cenozoic basic intrusions the distribution of the mesozoic is partly controlled by faults which appear to be related to early precambrian shear zones in the basement these highly foliated belts seem to have facilitated stress relief by normal faulting during permo triassic rifting activity the general lack of subsidence of the outer hebridean block is attributed to the buoyancy of granitic material incorporated at an early stage of its history

a model for the moist seismic reflection profile n scotland an interpretation of the moine and outer isles seismic traverse reflection profile along the north coast of scotland is developed here by the construction of a model crustal cross section from which by ray tracing a synthetic seismic section has been prepared for comparison with the observed record section faults are modelled by narrow low velocity zones and the moho by narrow high and low velocity zones the geometry of the faults shown in the model is believed to imply that during the caledonian orogeny crustal thickening was achieved by imbrication of thrust flakes from the se in which the thrust front propagated northwestward into the foreland and at the same time cut down so that the basal decollement deepened relative to the moho eventually into the mantle concomitant erosion and uplift initiated extension and the development of sedimentary basins in half grabens

the influence of pre existing thrust faults on normal fault geometry in nature and in experiments relations between normal faults and pre existing thrust faults are classically described in terms of three basic situations normal faults can cross cut thrust faults they can branch out from thrust faults at depth on a de collement level or they can entirely reactivate thrust planes the mechanical aspects of these types of interaction were studied by analogue modelling in which sand simulates the brittle rocks and silicone putty an interlayered de collement the models underwent compression producing thrust faults with variable dips followed by extension three possible ways of interaction are described here a no interaction occurs in the case of low dip thrust faults 32 1 and normal faults are developed independently displaying a listric geometry b branching at depth on the de collement level occurs when dip of the thrust faults reaches 32 1 in this case the dip of the normal faults whose geometry becomes planar decreases with increasing thrust dip we suggest that this change in dip of normal faults depends upon the rotation of stress tensor axes along the pre existing fault zone where a drop in the friction coefficient is likely to occur c reactivation occurs in brittle material when dip of the pre existing fault exceeds 41 1

pre aged soil organic carbon as a major component of the yellow river suspended load regional significance and global relevance abstract large rivers connect the continents and the oceans and corresponding material fluxes have a global impact on marine biogeochemistry the yellow river transports vast quantities of suspended sediments to the ocean yet the nature of the particulate organic carbon poc carried by this system is not well known the focus of this study is to characterize the sources composition and age of suspended poc collected near the terminus of this river system focusing on the abundance and carbon isotopic composition 13c and 14c of specific biomarkers the concentrations of vascular plant wax lipids long chain c24 n alkanes n fatty acids and poc co varied with total suspended solid tss concentrations indicating that both were controlled by the overall terrestrial sediment flux poc exhibited relatively uniform 13c values 23 8 to 24 2 and old radiocarbon ages 4000 4640 yr however different biomarkers exhibited a wide range of 14c ages short chain c16 c18 fatty acid 14c ages were variable but generally the youngest organic components from 502 yr to modern suggesting they reflect recently biosynthesized material lignin phenol 14c ages were also variable and relatively young 1070 yr to modern suggesting rapid export of carbon from terrestrial primary production in contrast long chain plant wax lipids display relatively uniform and significantly older 14c ages 1500 1800 yr likely reflecting inputs of pre aged mineral associated soil oc from the yellow river drainage basin even carbon numbered n alkanes yielded the oldest 14c ages up to 26 000 yr revealing the presence of fossil petrogenic oc two isotopic mass balance approaches were explored to quantitively apportion different oc sources in yellow river suspended sediments results indicate that the dominant component of poc 53 57 is substantially pre aged 1510 1770 yr and likely sourced from the extensive loess paleosol deposits outcropping within the drainage basin of the remaining poc between 10 and 31 is fossil in origin 26 000 yr resulting from the physical erosion of ancient sedimentary rock and input of fossil fuel residues from anthropogenic activity and 16 33 is modern carbon derived from terrestrial and aquatic productivity these findings have implications both regarding the provenance and vintage of organic matter signatures emanating from the yellow river basin and similar catchments containing extensive paleosol sequences as well as for the reactivity and fate of this poc upon supply to adjacent marginal seas

the seismic reflection moho beneath the united kingdom and adjacent areas abstract onshore and offshore deep seismic reflection data have been integrated to produce two way travel time and depth maps of the seismic reflection moho beneath the uk moho travel times are mostly in the range 9 5 11 5 s showing only weak correlation with specific upper crustal structural features but decreasing generally northwestward towards the atlantic margin moho depths show a much stronger correlation with upper crustal structure than do travel times there is pronounced crustal thinning moho depths 32 km are found generally beneath basement massifs particularly onshore short wavelength topographic features on the moho are quite rare and are generally restricted to ancient zones of continental collision and or subduction some of these ancient collision and or subduction zones are spatially associated with a highly reflective moho this is particularly evident where low angle shear zones in the lower crust or uppermost mantle converge with or intersect the moho it is suggested that development of a moho detachment surface can tectonically smooth the moho and thereby markedly increase its reflectivity a tendency in certain areas for the moho to remain at roughly constant two way travel time though not constant depth beneath both sedimentary basins and intervening basement massifs is interpreted as indicating that structural features on the scale of several tens of kilometres are in regional isostatic equilibrium conversely travel time perturbations beneath smaller features 10 km across indicate that these are at least partially uncompensated the uk crust is therefore weak but not sufficiently weak to attain local airy isostasy basement massifs west of scotland show systematically smaller moho two way travel times than similar structures farther south within the constraints of regional isostasy this can be interpreted in two main ways greater rigidity and strength of lewisian rocks within the west scottish massifs manifest as higher seismic velocity relative to density could be a cause of the observed travel time differences alternatively low density material beneath the crust could buoy up the western scottish massifs if this buoyancy were the consequence of igneous underplating then any underplated material would have to lie beneath the seismic reflection moho

the thrust and shear zones of the moine thrust zone and the nw scottish caledonides the caledonian thrust belts of nw scotland exhibit many features typical of foreland fold and thrust belts the major structures propagated westwards towards the foreland the thin carapace of cambro ordovician sediments being accreted in imbricate and duplex zones beneath the major thrust sheets on a broad scale the rules of thrust geometry are obeyed but many local anomalies are found particularly in the assynt region in assynt several large fault bounded gravity spreading structures surge zones have been identified extensional faults have been traced into strike slip and then contractional faults there are zones of extensional and contractional imbricate faults and extensional and contractional ductile flow similar structures have been found along the length of the moine thrust belt suggesting that gravity spreading was an important mechanism during the later stages of movement within the main caledonian fold belt there are several major tectonic slides which are recognized by a local increase in intensity of deformation the majority are nw verging and were originally gently dipping in the highlands se of the great glen there are early major se verging recumbent folds associated with considerable crustal thickening reflected in high pressure 12kbar metamorphism many of the major slides may have formed by spreading away from this thickened crust the caledonian deformation and metamorphism occurred later in the moines and moine thrust zone than in the dalradian and grampian moines to the se of the great glen this may reflect different levels of deformation alternatively it may argue for a large strike slip displacement on the great glen fault juxtaposing crustal blocks which had experienced different ages of caledonian deformation and metamorphism the late movement on the moine thrust involved spreading from an area of thickened caledonian crust formed either by the stair step trajectory of the basal thrust to the moines and or by thickening due to intrusion of large masses of late caledonian granites

deep structure of the foreland to the caledonian orogen nw scotland results of the birps winch profile the winch marine deep seismic reflection profile crosses the hebridean shelf the proterozoic foreland to the caledonian orogen west of scotland the data quality is very good the upper crust is largely devoid of coherent seismic reflections although this may in part be due to acquisition techniques being inappropriate for this problem in contrast the middle and lower crust 10 25 km depth exhibits good reflections the mid crust contains reflectors which may be relics of early palaeozoic caledonian or earlier grenvillian eastward dipping thrust zones which pass into an acoustically strongly layered lower crust the outer isles thrust is mapped from the surface to the mid crust and tied into its land outcrop on north lewis reactivation of this thrust offshore in a normal sense in mesozoic times caused the formation of the minch and north lewis basins the mono is defined by a strong band of reflections at about 27 km depth which correlates well with results from the hmse explosion seismic survey moho depth is apparently rather uniform in the area of the foreland crossed by winch winch and hmse results together suggest a mean crustal velocity for the hebridean shelf of 6 4 0 1 km s 1 the eastward dipping flannan thrust can be mapped into the upper mantle on three lines from about 15 to 45 km depth well into the upper mantle neither the flannan thrust nor the outer isles thrust appear to pass straight through the reflective lower crust suggesting that the lower crust is a region of high strain the outer hebrides is a positive block probably formed as an isostatic response to mesozoic normal faulting which reactivated the outer isles thrust

moist and the continuity of crustal reflector geometry along the caledonian appalachian orogen new aspects of the deep structure of the scottish caledonides are revealed by the moine and outer isles deep seismic reflection traverse moist the caledonian foreland is underlain by an easterly dipping strongly reflecting surface cutting through the moho and traceable to more than 45 km depth thrusts within the foreland basement and caledonian orogen have been reactivated as normal faults bounding half grabens filled with sediments of late palaeozoic to mesozoic age the moine thrust which carries rocks of the orogen over lewisian foreland dips at 20 25 to the e on moist and is either a the westernmost of a series of easterly dipping reflections thrusts which flatten or terminate at 17 20 km depth or b a thrust lying further e which structurally overlies these easterly dipping reflectors by comparison with easterly dipping reflections on cocorp lines in the northern and southern appalachians the latter interpretation is preferred implying that many of the easterly dipping reflections on moist correspond to off shelf metasedimentary rocks with slivers of basement stacked and imbricated against the lewisian basement edge regional seismic refraction and conductivity data from northern scotland suggest that regions of the caledonian orogen have lower mid crustal velocity gradients and higher conductivities than the foreland which might be explained by this interpretation of the easterly dipping reflections despite similarities in crustal reflector geometries there are important differences between the moist and cocorp lines including 1 a remarkably continuous relatively horizontal moho seen on moist data at about 25 km depth and 2 the sedimentary basins offshore from northern scotland which have formed by reactivation under crustal extension of the easterly dipping thrusts furthermore it is probable that the easterly dipping reflectors in these areas of the moist and cocorp lines were formed at different times in the early palaeozoic

extensional tectonics in the honshu fore arc japan integrated results of dsdp legs 57 87 and reprocessed multichannel seismic reflection profiles

a structural analysis of the moine thrust zone between loch eriboll and foinaven nw scotland abstract the northern part of the moine thrust zone as exposed around the valley of srath beag sutherland was developed by thrusts propagating in the tectonic transport direction deformation on any particular thrust surface evolved from dominantly ductile to dominantly brittle with time the foreland has been progressively accreted onto the overriding moine thrust sheet by duplex formation a process which has continuously folded the roof thrust and the rocks above its hanging wall fold culminations and depression can be related to lateral ramps which may give the rocks above the hanging wall a complex history of extensional and compressional strains normal to the transport direction folds within the thrust zone are laterally independent because they are controlled by short lived variations in deformation style on an evolving thrust footwall topography therefore there may be no correlation between structures across or along the thrust zone this variation limits the construction of balanced cross sections as structure cannot be projected onto particular section lines

a comparison of the moho interpreted from gravity data and from deep seismic reflection data in the northern north sea summary we present the results of gravity modelling of the nsdp 84 deep seismic reflection profiles in the northern north sea the moho depths inferred from forward modelling of marine bouguer gravity anomalies from independent inversion of free air gravity anomalies derived from satellite altimetry and from deep seismic reflection profiling are in good mutual agreement this agreement suggests that the moho discontinuity interpreted from seismic reflection data corresponds to a major jump in density and therefore is likely to represent not only the geophysical but also the petrological boundary between the continental crust and the upper mantle in spite of the significant lateral heterogeneity of the crystalline basement below the northern north sea area as documented by seismic reflection profiling the average density contrast between the basement and the upper mantle is interpreted to show rather little variation this is not true for the deepest part of the viking graben where the nature of the lower crust and upper mantle may have been changed by magmatic processes accompanying the graben formation a relationship between the degree of crustal extension and the volume of magmatic intrusion into the lower crust is tentatively supported by our results