characteristic reflection patterns in the southeast canadian cordillera northern appalachians and swiss alps abstract there are some striking similarities and some differences between the seismic reflection sections recorded across the fold and thrust belts of the southeast canadian cordillera quebec maine appalachians and swiss alps in the fold and thrust belts of all three mountain ranges seismic reflection surveys have yielded high quality images of 1 nappes thin thrust sheets stacked on top of ancient continental margins 2 ramp anticlines in the hanging walls of faults that have ramp flat or listric geometries 3 back thrusts and back folds that developed during the terminal phases of orogeny and 4 tectonic wedges and regional decollements a principal result of the cordilleran and appalachian deep crustal studies has been the recognition of master decollements along which continental margin strata have been transported long distances whereas a principal result of the swiss alpine deep crustal program has been the identification of the adriatic indenter a crustal scale wedge that caused delamination of the european lithosphere significant crustal roots are observed beneath the fold and thrust belts of the alps southeast canadian cordillera and parts of the southern appalachians but such structures beneath the northern appalachians have probably been removed by post orogenic collapse and or crustal attenuation associated with the mesozoic opening of the atlantic ocean

harmonic distortion on a seismic reflection profile across the quebec appalachians relation to bouguer gravity and implications for crustal structure seismic reflection data obtained across the quebec appalachians using the vibroseis trademark conoco technique were recorded with parameters that allowed harmonic distortion arrivals to interfere with layered reflections the data exhibit reflections from layered miogeoclinal rocks dipping eastward beneath the allochthonous rocks of the orogen the layering appears to terminate beneath the notre dame anticlinorium however as the apparent termination of the layers also occurs at the arrival times of high amplitude noise harmonics it may have no geological significance precambrian grenville crust which probably underlies the layered sediments extends at least as far east as the apparent termination and may extend much farther examination of the bouguer gravity field in relation to the seismic reflection data shows that a major gravity change is due to density differences that occur considerably west of the eastern limit of precambrian grenville crust the gravity thus shows no correlation with sur

structure of the crust and upper mantle beneath the great valley and allegheny plateau of eastern pennsylvania 2 gravity modeling and migration of wide angle reflection data linear inversion of p composites for three different profiles suggests that the lower crust beneath eastern pennsylvania consists of material with high averaged p wave velocities 6 8 7 5 km s compared with 6 4 7 1 km s for most crustal models derived for surrounding areas within the appalachians and the grenville province of the canadian shield interpretations of these velocities in terms of rock type are limited by the low resolving power of the data and by the nonuniqueness in inferring rock type from p wave velocity alone the high averaged p wave velocities could be associated with substantial volumes of mafic rock perhaps emplaced by underplating or with high grade metamorphic assemblages of intermediate composition and sedimentary origin similar to those found in the ivrea zone in northern italy because of the sparseness of the s wave data averaged shear wave velocities are not well enough resolved over most depth ranges to distinguish among the different compositional models computations of gravity anomalies for two dimensional models using various linear velocity density relations however suggest that at least part of the crust beneath the scranton gravity high is mafic moreover if the entire basement section beneath this anomaly consists of mafic material then the gravity studies indicate that the lower crust beneath the great valley must be mafic as well migrated shot gathers show a concentration of reflectors at depths between 35 and 45 km a result that is controlled in part by the limited range of offsets used but that is also consistent with recent observations of a highly reflective lower crust reflections that are migrated to depths within the uppermost mantle could be multiples or primary reflections from upper mantle layering although the data are rather sparse the interpretation of these later arrivals as primary reflections is supported by their small ray parameters which suggests that they are not converted phases and by their large relative amplitudes which suggests that they are not multiples layering within the upper mantle could be in the form of alternating high and low velocity zones perhaps related to emplacement of eclogites or garnet granulites in the uppermost mantle during the process of underplating

deep seismic reflection profiling of the lewisian foreland sum m a r y a number of marine deep seismic reflection profiles cross the hebridean shelf the lewisian foreland to the caledonian orogen they include the birps moist winch and drum profiles several recent commercial deep lines and many old and new conventional commercial exploration profiles basement comprising the upper crust is largely devoid of coherent seismic reflections in contrast the mid crust contains many 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 as a normal fault caused the formation of the sea of the hebrides minch and north lewis basins the moho is defined by a strong band of reflections at a rather uniform 27 km depth the eastward dipping flannan thrust can be mapped into the upper mantle from about is to 45 km depth the moine thrust which carries rocks of the orogen over lewisian foreland dips at 20 25 to the east on moist and is either the western most of a series of easterly dipping reflections thrusts which flatten or terminate at 17 20 km depth or a more easterly thrust which structurally overlies these easterly dipping reflectors in neither case are the easterlydipping reflectors themselves likely to be simply caledonised lewisian foreland

the giles county virginia seismic zone a well defined seismic zone recently detected in virginia has an orientation that is not related to the surrounding geologic structures the orientation of the zone appears to be related to features below the appalachian overthrust belt a damaging earthquake that is important in evaluating seismic hazard in the southeastern united states may have occurred in the zone in 1897

acadian and alleghenian remagnetization of the jim pond formation central western maine northern appalachians lombard a d van der pluijm b a and van der voo r 1991 acadian and alleghenian remagnetization of the jim pond formation central western maine northern appalachians tectonophysics 186 279 291 samples were collected from ten sites of the late cambrian early ordovician jim pond formation for paleomagnetic study stepwise thermal demagnetization reveals three separable components of magnetization component i is typically removed by 350ac it is subparallel to the present day field 354o 76 vs 342 72o at the site location 45 3on 289 4 e and is considered to be a recent partial overprint component ii without tilt correction is a south southeasterly and shallow direction mean 165 o o k 31 4 a s5 8 6 o that is removed over an intermediate temperature range 350 600 c component iii without tilt correction is a northeasterly and shallow upward direction mean lo 24o k 21 5 ass 7 3 and is removed over the highest temperature range 480 to 69ooc though not statistically significant for components ii and iii the precision parameter k decreases and the as5 increases when tilt correction is applied suggesting that both are post folding magnetizations component ii without tilt correction has a corresponding paleomagnetic pole located at 43on 130 e dp dm 4 3 8 6 o which falls near the late carboniferous segment of the laurentian apparent polar wander path apwp component iii without tilt correction has a corresponding pole located at 32 n 98 e dp dm 4 7o 7 8 which falls near the lower middle devonian segment of the apwp we conclude that the jim pond formation has undergone two paleozoic remagnetization events one in the early to middle devonian and a second one in the late paleozoic the ages of these remagnetizations coincide with the timing of major erogenic activity in the area i e the acadian and alleghenian respectively the remagnetization event associated with the acadian pulse can be recognized in other paleomagnetic investigations in the northern appalachians

reprocessing and reinterpretation of cocorp southern appalachian profiles abstract reprocessing of cocorp southern appalachian data was focused on basic seismic evidence for continuation of sediments beneath a master decollement the most important evidence is a nearly continuous series of subhorizontal reflections extending from the valley and ridge province into the piedmont province continuity of subhorizontal reflections becomes tenuous in the inner piedmont careful reprocessing has yielded evidence for termination of strong reflections beneath the allochthon and the beginning of a relatively weak and complex series of events termination of sedimentary rocks beneath the piedmont is interpreted from true amplitude seismic data a zone of detachment continues southeast of the sediment termination as far as the master decollement root zone research on stacking velocities has indicated that complex velocity structures could create apparent low stacking velocities this phenomenon may occur in the charlotte belt of georgia bouguer gravity can be modeled as a former craton of normal density with an accreted margin of very slightly higher density variation in crustal thickness also contributes to the bouguer gravity gradient no continuous large scale overthrust is needed southeast of the interpreted master decollement root zone located beneath the kings mountain belt and charlotte belt

results of recent cocorp profiling in the southeastern united states summary recently acquired cocorp profiles in the southeastern united states show that 1 reflections associated with the appalachian detachment are prominent beneath the inner piedmont of western georgia but do not extend further southeast beneath the pine mountain belt 2 the brunswick magnetic low is associated with a broad zone of crustal penetrating dipping reflections that probably marks the alleghanian suture in the southeastern u s 3 the south georgia basin is a composite feature consisting of several half graben locally containing 5 km of triassic e jurassic basin fill these basins occur within the interior of the alleghanian orogen but are not specifically associated with alleghanian suture 4 across strike crustal thickness variation and distribution and character of lower crust and moho reflections in the southern appalachians is grossly similar to that observed in other parts of the appalachiadhercynian orogenic belt global comparisons suggest that these regional variations are a consequence of post collisional extensional tectonics rather than a primary palaeozoic or older feature of the orogenic belt

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

continental tectonics selected topics cause and effect relations between plate boundary processes and orogeny were investigated using new and refined plate models for the mesozoic and cenozoic terrane accretion and large components of horizontal shear were shown to be fundamental aspects of orogen evolution deep crustal thrust systems with large lateral transport and vertical stacking of slices of crystalline basement were documented in the cordillera and appalachians new geomechanical models of crystalline thrust sheets and foreland thrust belts emphasized the dominance of tectonic rather than gravitational stresses lithospheric thickening was shown to cause buoyancy forces that control the lateral spread of deformation outward from the interior of an orogen and may modify regional stresses in the vicinity of topographic fronts resulting in extension within the upper part of the lithosphere coeval with shortening at depth

a geophysical study of the earth s crust in central virginia implications for appalachian crustal structure a regional seismic reflection line i 64 across the virginia piedmont has provided a stacked section suitable for an integrated interpretation of geophysical data in the region a highly reflective upper crust an allochthonous blue ridge province underlying thrust sheets including the blue ridge master decollement and a basal decollement at a depth of about 9 km 3 s are confirmed on the seismic data immediately east of the blue ridge province appalachian structures plunge to as much as 12 km 4 s depth the evington group hardware terrane and chopawamsic metavolcanic rocks carolina terrane crop out in the piedmont province and numerous eastward dipping reflections originate from these rocks in the subsurface these eastward dipping reflectors overlie a gently west dipping 10 15 highly reflective zone that varies in depth from 1 5 s 4 5 km beneath the goochland terrane to 4 s 12 km beneath the rocks of the evington group some of the overlying eastward dipping reflections apparently root in this zone the zone may include decollement surfaces along which the overlying rocks were transported relatively few reflections originate from within autochthonous grenville basement at the western end of the profile the goochland granulite terrane is interpreted to be a westward thrust nappe structure that has overridden a portion of the chopawamsic metavolcanic rocks a broad zone of east dipping 20 45 reflections bounds the goochland terrane on the east these reflections may originate from deformation zones and continue to moho depths they appear to be correlative with similar events seen on other appalachian lines the pervasiveness of the zone of east dipping events on other seismic reflection lines and the continuity of the adjacent piedmont gravity high suggest continuity of crustal features along the length of the appalachians a major conclusion of this study is that crustal thinning is responsible for the main components of the gravity field in virginia that is the appalachian gravity gradient and the piedmont gravity high the crust thins from about 52 km beneath the appalachian mountains to about 35 km beneath richmond virginia and then rethickens by up to 10 km beneath the zone of east dipping reflections mylonites east of richmond the i 64 seismic data also contain a sequence of reflections at about 9 12 s indicative of lower crustal layering the base of this zone of reflections coincides almost exactly with the mohorovicic discontinuity interpreted from earlier refraction work the layering extends about 70 km west from richmond virginia and is interpreted as a lower crustal transition zone that is believed to persist across most of virginia

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

the extension of grenville basement beneath the northern appalachians results from the quebec maine seismic reflection and refraction surveys from 1983 to 1984 the united states geological survey and the geological survey of canada undertook a series of seismic experiments to determine the crustal structure of the appalachian mountain belt in maine and southeast quebec these experiments included four deep crustal seismic reflection profiles perpendicular to the strike of regional geological trends and several high resolution refraction wide angle reflection profiles both parallel and perpendicular to strike here the data recorded in quebec and the northwest part of maine are interpreted in conjunction with existing reflection profiles from quebec synthetic seismogram calculations in laterally varying media are used to model the refraction wide angle reflection data while reprocessing migration and attribute plotting are used in the interpretation of the reflection data a major zone of reflections extending over a distance exceeding 200 km can be traced from shallow depths beneath the st lawrence lowlands southeast to about 25 km depth beneath the southeast edge of the chain lakes massif this zone of reflections is interpreted as a master decollement separating autochthonous grenville basement from overlying allochthonous rocks of the appalachian orogen including foreshortened miogeoclinal rocks the remnants of one or more magmatic arcs the connecticut valley gaspe synclinorium and the chain lakes massif in the vicinity of the chain lakes massif extension has caused grenville basement to be thinned by as much as 40 reflections from the decollement show strong layering and evidence for basement faulting associated with the guadeloupe and other faults normal incidence and wide angle reflections are observed to be almost continuous beneath the baie verte brompton line and connecticut valley gaspe synclinorium indicating that the baie verte brompton line is confined to the upper crust the connecticut valley gaspe synclinorium is characterized by p wave velocities of about 5 3 km s 1 and is shown to be a shallow structure with a maximum depth of about 3 km rock velocities immediately below the connecticut valley gaspe synclinorium are typical of chain lakes material 6 1 km s 1 a new model for the tectonic development of the region is proposed in which all pre silurian units between the baie verte brompton line and the southeastern edge of the chain lakes massif are allochthonous and confined to the upper crust we infer that the chain lakes massif volcanics of the ascot weedon formation and associated deeper crustal rocks were thrust over grenvillian basement as a composite unit and may now underlie parts of the connecticut valley gaspe synclinorium

evolution of gravity anomalies across collisional mountain belts clues to the amount of continental convergence and underthrusting a series of density models illustrates the gross form of free air and bouguer gravity anomalies anticipated during ocean basin closure and consequent development of collisional orogens when compared to gravity anomalies observed across some mountain belts the hypothetical anomalies provide a clue to the degree of under thrusting of crust associated with one lithospheric plate beneath crust of the opposing plate margin the results of the study suggest very early stage collision in the ouachita mountains of arkansas and the sulaiman range of pakistan with thin transitional or oceanic crust still intact on the lower plate in contrast the himalaya of pakistan represent a much more advanced stage of collision where continental crust may have underthrust the mountains for 600 km

tectonics of the acadian orogeny in new england and adjacent canada paleogeographic analysis of post taconian rocks in new england and adjacent parts of canada has revealed the existence of two volcanic arcs which shut off at the time of the acadian orogeny one was built on arc basement previously accreted to north america during the taconic orogeny the other on precambrian continental basement of avalonia in the intervening merrimack fredericton trough metamorphosed and poly deformed turbidites and black shales record deep water conditions in silurian following mckerrow and ziegler 1971 this tract is interpreted as the site of an ocean which closed in siluro devonian by simultaneous subduction beneath both continental margins in the molucca sea in indonesia a comparable arc arc collision is in an early stage of development moore et al 1982 suggested that an accretionary prism built against one arc is overthrusting its counterpart which developed on the other side of the ocean an identical geometry with an avalonian accretionary prism overriding the conve

gravity flexure and the deep structure of the grenville front eastern quebec and labrador the grenville front in eastern quebec and southern labrador is characterized by a large negative bouguer gravity anomaly with flanking highs forward flexural modelling of this anomaly provides constraints on the deep structure of the eastern grenville the grenville front is assumed to mark the surface trace of a boundary between less dense crust to the northwest and more dense crust to the southeast with constant dip all the way to the moho an end load is applied to the northwestern plate to simulate the effect of orogenic and or subsurface loads but is then removed underneath a continuous plate to simulate the transient nature of the load this simple model yields good fits to the gravity anomaly provided the effective elastic thickness of the lower plate is less than about 30 km and the crustal boundary dips at about 20 the deep structure of the eastern grenville front may therefore be dominated by a major shallow dipping straight thrust the applied end loads for the best fitting models are o

tectonic implications of subthrust structures revealed by seismic profiling of appalachian ouachita orogenic belt a growing body of deep seismic reflection profiles across the appalachian ouachita orogenic belt reveals subthrust structures which have been previously interpreted as products of continental rifting of later thrusting or of some combination of rifting and thrusting the subthrust features can be placed into three general categories 1 small normal faults which offset lower plate shelf sequences and basement in the foreland and have been attributed to thrust loading perhaps reactivating earlier rifting faults 2 moderately dipping seismic events which lie entirely below lower plate shelf sediments in the foreland interpreted in part as strata deposited during the earlier rifting and 3 hinterland dipping reflection sequences beneath more interior portions of the mountain belt previously interpreted as thrust faults which imbricate sediments and basement related to the earlier rifted margin these interpretations are evaluated through a comparison of reflection profiles across the orogenic belt to previously interpreted seismic lines across modern extensional and compressional settings the comparisons suggest that although some normal fault offsets of lower plate shelf sequences may be original or reactivated rifting faults the dipping events entirely beneath the shelf sequence are more diagnostic of the rifting process these dipping sequences are therefore consistent with previous interpretations that large portions of the earlier rifted margin are preserved in an undeformed state beneath thrust sheets in the appalachian foreland the comparisons also suggest an alternative to interpretations of the hinterland dipping sequences as thrust related in that the sequences are similar in seismic appearance to that of volcanic wedges commonly observed in the narrow zone separating continental from oceanic basement on modern passive margins the implication of this new interpretation is that the actual continent ocean boundary related to the earlier rifted margin may be preserved on the lower thrust plate beneath interior portions of the appalachians and ouachitas

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

seismic velocities in the shallow crust of western new england and northern new york the ontario new york new england refraction study o nynex provides a set of high quality data for a tomographic study of an intraplate continental region with low seismic activity this study focuses on the region crossed by the united states part of the o nynex profile tomographic inversion of the travel time data is used to construct a three dimensional model of the p and s wave seismic velocity distribution within the shallow crust of this region velocity structure is resolved in the top 15 km of the crust with the most detailed picture obtained for the depth range between 1 and 5 km the upper crust of the proterozoic grenville province is found to have a p wave velocity 0 2 to 0 3 km sec higher than the paleozoic appalachians the area of anorthositic intrusions within the adirondack mountains is characterized by p wave velocities 0 3 km sec higher and s wave velocities 0 1 km sec higher than the average values for the grenville province this high velocity anomaly extends to the depth over 5 km possibly as deep as 16 km the velocity structure of central vermont is found to be closer to that of the grenville province than to the rest of the appalachians central new hampshire shows relatively small lateral variation of seismic velocity no strong correlation is seen between velocity anomalies and intrusive bodies of the white mountains magma series

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

some comparisons of the structure and evolution of the southern appalachian ouachita orogen and portions of the trans european suture zone region abstract many aspects of the tectonic history of the appalachian ouachita orogen and the trans european suture region are similar in a tectonic sense they are geographically linked because at the end of the paleozoic one could travel from texas to poland following a continuous orogenic belt much of each of these orogenic belts is however buried by younger strata and thus geophysical studies are a key element of efforts to better understand them many more geophysical data are needed to provide a comprehensive picture of the deep structure but in several areas new models of the lithospheric structure are emerging in this paper we review some of these models and introduce two new ones for the southern appalachians in addition we explore the implications of these models for the evolution of these orogenic belts drawing attention to similarities and differences in their structure and evolution the appalachian and caledonian orogens represent temporally parallel development along irregular late proterozoic continental margins but the crustal structure is different because of different accretionary elements and complications in the geometry of the original margins lateral transport via thrusting can confuse efforts to tie geologic structures identified on the surface with deep structures for example in the appalachian ouachita orogen the thrust related transport distance for major features such as the blue ridge piedmont sheet and the benton uplift is at least 200 km on a crustal scale deformation in the appalachian ouachita orogen varies greatly in style and intensity in the ouachita orogen the late proterozoic continental margin seems to be preserved while in the northern appalachians crustal scale deformation is intense along the us appalachians the major crustal structure variation is the thin dense crust of avalonia abutting the relatively thick crust of laurentia along the ouachita orogen several large crustal blocks can be identified that are unlike anything seen along the appalachians

a new map of crustal terranes in the united states from cocorp deep seismic reflection profiling a new map has been constructed to summarize major systematic variations in seismic reflection character observed by over 10000 km of cocorp profiling in the us among the seismic characteristics represented on this map are crustal scale detachments which can be traced as relatively low angle reflectors over large areas deeply penetrating dipping reflection zones some of which mark major shear zones perhaps even crustal sutures moho reflectivity which appears to vary from craton to phanerozoic periphery seismic transparency which can sometimes be interpreted as geologic homogeneity seismic layering prominent in the lower crust in many areas and now known to dominate the upper basement in other regions and unusually strong reflections or bright spots at mid crustal levels that may mark deep crustal fluids

reflectivity of the crystalline crust hypotheses and tests summary the nature of reflectors within the crystalline basement remains the subject of inference except where reflections have been traced directly to outcrop geological models of basement reflectors need to be developed which incorporate geophysical constraints obtained from measurements on seismograms but most geological information still comes from speculative interpretations of seismic experiments run in different regimes pronounced lower crustal reflectivity detected worldwide is ascribed in various geological hypotheses to primary lithologic layering to ductile strain banding or to trapped fluids a birps deep crustal profile across the atlantic continental margin suggests that the observed reflectivity is not related in any simple way to the amount of extensional strain undergone study of worldwide crustal profiles shows that exposed high pressure terranes are not as reflective as in situ lower crust at high pressure suggesting either that these granulite terranes are not representative of the lower crust or that physical properties possibly the presence of fluids or thermally controlled ductile strain banding are more likely responsible for observed reflectivity than are simple lithologic boundaries the argument for the importance of physical properties in causing observed lower crustal reflectivity is strengthened by an observed negative correlation between depth to the reflective lower crust and regional surface heat flow 1 geophysical constraints the notion that geometric effects thin layer interference and focussing in the absence of otherwise significant impedance contrasts are responsible for much observed reflectivity is common in the literature because many reflections are observed on deep crustal profiles velocity and density cannot increase monotonically with depth while remaining within bounds set by refraction experiments implying the existence of a funtion of velocity with depth that is oscillatory while the concept thus developed of the existence of many thin layers in the lower crust is justly important in geologic models of the lower crust seismic interference from such laminae cannot be the only explanation of observed crustal reflectivity for normal incidence no structure embedded in a constant impedance medium gives a stronger reflection than a quarter wavelength layer which produces an amplitude twice that due to a simple interface complex structures can increase the total energy reflected but the energy is spread over longer times and the peak reflected amplitude is not increased observed reflection amplitudes from the lower crust vary by factors u2 so that the presence or absence of thin layer interference cannot be the only effect causing lower crustal

miogeoclines and suspect terranes of the caledonian appalachian orogen tectonic patterns in the north atlantic region the caledonian appalachian orogen was formed by the closing of a paleozoic iapetus ocean the continental margins of iapetus are identified in the deformed early paleozoic miogeoclines of the caledonian appalachian orogen ophiolitic vestiges of iapetus its oceanic plateaus microcontinents and volcanic arcs are caledonian appalachian suspect terranes these were assembled in interior parts of the orogen and locally they were emplaced structurally upon the adjacent miogeoclines the modern north atlantic ocean opened along an axis that traversed the paleozoic orogen longitudinally its opening dispersed the elements of the paleozoic orogen and led to the present arrangement of disjunct paleozoic miogeoclines and suspect terranes throughout the north atlantic borderlands the western or north american margin of iapetus is represented by the miogeoclines along the west flank of the north american appalachians and caledonides of east greenland a small north american miogeoclinal segment occurs in the britis

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

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

composition of the crust in the grenville and appalachian provinces of north america inferred from v p v s ratios we use the ratios between p and s wave velocities vp vs derived from seismic refraction data to infer the composition of the crust in the grenville and the appalachian provinces of north america the crust exhibits vp vs increasing with depth from 1 64 to 1 84 there is a clear distinction between the grenville province average vp vs 1 81 and the appalachian province average vp vs 1 73 which persists at all depths the boundary between these provinces is east dipping extending for 100 km east of the champlain thrust in the appalachian province the increase in vp vs ratios with depth from 1 67 to 1 74 0 02 may reflect a normal decrease of silica content in the continental crust in the grenville province beneath the central granulite terrane an anomalous vp vs ratio of 1 82 0 02 is observed extending to a depth of 10 km this correlates with the abundance of ca plagioclase in the marcy anorthosite at greater depth 15 20 km where seismic lamination and high electrical conductivity is observed vp vs is 1 84 0 02 and correlates with the tahawus complex a layered mafic intrusion within the 25 km thick lower crust of the grenville province the vp vs is 1 84 0 02 and p velocity is 7 0 0 1 km s which are typical for plagioclase bearing rocks gabbro norite the high vp vs ratio in the grenville province has not been reported in crust of any other age since the grenville province contains 75 of the world s known anorthosites high vp vs ratio is related to high plagioclase we suggest that the composition of the grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid proterozoic

a tectonic window in central newfoundland geological evidence that the appalachian dunnage zone may be allochthonous in the through hill area of central newfoundland mafic to ultramafic complexes which preserve varying amounts of ophiolite stratigraphy mark the trace of a major fault zone that outcrops in a roughly elliptical pattern the major northeast trending axis is about 60 km and the shorter axis is about 30 km in length the most complete ophiolite stratigraphy is preserved in the coy pond and pipestone pond complexes which have steep dips and face east and west respectively outwards from the centre of the ellipse the ophiolitic rocks are bounded on the outside by rocks typical of the dunnage zone of central newfoundland principally lower to middle ordovician volcanic and volcaniclastic rocks interpreted to be of island arc affinity davidsville victoria lake and baie d espoir groups cold spring pond formation and upper ordovician to silurian clastic sediments botwood group the volcano sedimentary sequences are interpreted to have been deposited on the ophiolite conformably but the contacts are n

the distribution of igneous rocks beneath mont megantic the easternmost monteregian as revealed by gravity at the surface mont megantic the easternmost and most imposing of the monteregian hills is composed of 77 5 leucogranite and syenite and 22 5 gabbro nonetheless the intrusion is characterized by a major positive bouguer anomaly of 20 mgal a computed three dimensional gravity adumbration is consonant with the intrusion having the form of a vertical prismatic column with a flat base at a depth of 10 km the base appears to coincide with the unconformity atop precambrian basement rocks gabbro composes 96 of the adumbration leucogranite and syenite make up the remainder and are confined to an uppermost layer no more than about 1 km thick the subvolcanic texture of the granite and presence of miarolitic cavities at megantic show the shallow level of the intrusion the present surface perhaps having been no deeper than 1 or 2 km this is the same as the depth of emplacement elucidated at mont royal at the opposite end of the monteregian chain which in turn points to uniform uplift and erosion sin

40 ar 39 ar age limit for an acheulian site in israel the lake of berekhat ram fills a caldera which cuts the pleistocene basalt flows that cover the golan heights1 in israel fig 1 in the north west wall of the crater a sequence of two basalt flows is exposed separated by a reddish brown palaeosol complex 2 m thick scattered through this palaeosol and also found in prominent concentration in one layer near the base are lithic artefacts of an upper acheulian industry we have now obtained a 40ar 39ar date of 233 3 kyr for the overlying flow while the underlying flow does not give a well defined plateau during step heating but gives a range of apparent ages from 290 to 780 kyr with an integrated age of 470 kyr

natural experimental charges an ion microprobe study of trace element distribution coefficients in glass rich hornblendite and clinopyroxenite xenoliths abstract igneous glasses found in hornblende and clinopyroxene rich xenoliths represent former melt pools and contain important information about magmatic processes in mafic alkaline magmas rare glass bearing hornblendite and clinopyroxenite xenoliths entrained in intraplate alkali basalts from germany and israel contain up to 11 interstitial glass surrounded by euhedral crystals of mafic minerals hornblende clinopyroxene phlogopite these xenoliths represent natural experimental charges of alkaline magma quenched by rapid eruption whilst crystallising mafic mineral phases bulk rock compositions of the xenoliths are basanitic considered as representing the starting compositions of the natural experiment whereas the interstitial glasses are chemically more evolved highly alkaline foidites as a result of closed system crystallisation of mafic minerals ion probe trace element data for glass and mineral phases hornblende clinopyroxene phlogopite and apatite yield in situ distribution coefficients kds between magma and minerals the results generally confirm distribution coefficients derived from experimental data and from melt bearing peridotite xenoliths but tend to be slightly higher perhaps because of the evolved nature of the melt disruption of these xenoliths which must have been semi consolidated when entrained may help to explain the origin of clinopyroxene amphibole and phlogopite megacrysts that are commonly observed in alkali basalts

geometry and style of post obduction thrusting in a paleozoic orogen the taconic frontal thrust system the taconic frontal thrust system associated with the later stages of the medial ordovician taconic orogeny a continental margin volcanic arc collision defines the present western boundary of the taconic allochthon in eastern new york and west central vermont the frontal thrust emplaces regionally folded and foliated taconic rocks over deformed but essentially unmetamorphosed flysch the frontal thrust is temporally and kinematically related to the champlain and orwell faults to the north that emplace variably deformed sections of early paleozoic shelf rocks over partially coeval sections of the early paleozoic platform major movement on these faults occurred after initial obduction of the allochthon onto the early paleozoic north american continental shelf and prior to being unconformably overlapped by the early devonian this fault system is considered to be part of the final phase of a continuum of thrusts that evolved during the taconic orogeny and therefore records only the final emplacement

the effect of lateral deformation on heat flow heat production relations deformation that takes place following the fractionation of heat producing elements can cause the relationship of surface heat flow and heat production to deviate from a simple linear trend lateral transport by low angle normal faulting or thrust faulting may place the near surface rocks above rocks with different thermal properties even for a simple model the relationships can be complex but some constraints can be placed on the vertical distribution of heat generation and layer thickness measurements at many sites in a province could produce an overall linear trend of surface flux and heat production departures from linearity may be found at sites that were disturbed transported following the latest cycle of radioelement fractionation the amount of departure from linearity is a function of the thickness of the transported layer and the thermal properties of underlying layers

genesis of clinopyroxene amphibole xenoliths from birket ram trace element and petrologic constraints the xenolith suite of birket ram is composed of a mafic granulite series and an ultramafic series of clinopyroxene amphibole ca nodules the petrographic characteristics of the ca nodules show that they were formed as crystal fractionates from a basaltic melt the trace element data on the nodules show that the xenoliths could have been crystal fractionates from an evolved golan basalt magma the trace element data require that more than one parent magma was responsible for the formation of the ca nodule series in particular the lack of coherence between the mineralogy of the nodules and the trace element abundances plus the dispersion of the trace element data argue for the involvement of several parent magmas rare contact relationships between the granulites and the ca nodules indicate that at least some of the ca nodules were formed within the lower crust the pressure of formation is estimated to have been in the range 550 800 mpa based on geophysical models for the crustal structure in the region

more seismic evidence on the location of grenville basement beneath the appalachians of quebec maine summary high quality multichannel seismic 133 fold and refraction wide angle reflection 1 to 3 km receiver spacing and 30 to 60 km shot spacing data have been collected across the northern appalachians in quebec and maine an integrated interpretation of the seismic data from the southeastern quebec western maine region provides strong evidence that the rocks of the predominantly oceanic dunnage zone are allochthonous having been thrust westwards over precambrian grenville basement during and subsequent to the closing of the iapetus ocean

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

geological setting of gold western wabigoon subprovince canadian shield exploration targets in mixed volcanic successions the archean volcanic rocks in the cameron rowan lakes area may be divided into three distinct successions based on field mapping petrographic studies and lithogeochemical characteristics the lowermost rowan lake volcanics are tholeiitic pillowed basalts these rocks are unconformably overlain by the cameron lake volcanics a mixed succession of tholeiitic massive and pillowed basalts and intermediate to felsic volcaniclastic rocks the brooks lake volcanics consist of tholeiitic basalts and represent the youngest volcanic rocks at the top of the preserved succession most of the gold concentrations in the cameron rowan lakes area are confined to the mixed cameron lake volcanics the majority of these including the cameron lake deposit occur within shear zones near lithologic contacts in the eagle wabigoon and manitou lakes areas there are similar stratigraphic subdivisions of the supracrustal rocks and many of the gold concentrations also occur in deformation zones within the mixed volcanic successio

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

laboratory wave velocity measurements on lower crustal xenoliths from calcutteroo south australia abstract seismological and petrologic studies have suggested the presence of mafic granulite eclogite and amphibolite as the major components of the lower crust in southeastern australia elastic wave velocity measurements conducted to 400 mpa confining pressure of selected nodules from one kimberlite pipe in south australia range after corrections for estimated in situ crustal pressures and temperatures from v p 6 9 7 7 km s 1 and v s 3 9 4 3 km s 1 these values fall between previous measurements for more silicic crustal granulites and mafic eclogites and coincide with the published velocities determined by seismic studies for southeastern australia it is possible that these types of compositions may be widespread components of the lower continental crust and may require revision of currently accepted models of crustal evolution

orogeny and metamorphism a model for deformation and pressure temperature time paths with applications to the central and southern appalachians a model is presented for deformation uplift and exhumation in convergent orogens based on the following premises 1 foreland basin subsidence monitors the distribution in time and space of overthrust loads added to an orogen 2 the presence of an inherited rifted margin plays an important role in the subsequent history of an orogen by forming a basement ramp or buttress that must be surmounted in order for thrusts to advance on the craton on the seaward side of this transition up to 20 km of overthrusts can be accumulated without creating topography 3 critical shape theory can be applied to orogens as a whole allowing predictions to be made concerning dynamic balance between mass addition and loss uplift exhumation and growth of topography and 4 pressure temperature time p t t data can be used to reconstruct particle paths within the deforming orogen three states of overthrust orogens can be defined based on the relative rates of mass loss and mass addition as monitored by the foreland rapid exhumation of metamorphic terranes is most likely in the steady state where erosion balances accretion a constructive state where mass addition is greater than mass loss results in crustal thickening and initially little exhumation a destructive state where mass loss outweighs mass addition results in slow exhumation or subsidence of metamorphic rocks in the orogen depending on whether mass is removed by erosion or by extension p t t data on peak metamorphism pluton emplacement uplift exhumation and cooling can be used in conjunction with data from the foreland to distinguish among these various states the topography and strength of the inherited rifted margin play an important role in localizing zones of exhumation and uplift within the orogen application of this model to the central and southern appalachians suggests that during the constructive phases of the taconian and acadian orogenies overthrust loads accumulated mainly on the seaward side of the ocean continent transition as marked by the bouguer gravity gradient between the acadian and alleghanian orogenies the appalachian orogen was probably in a steady state with uplift and exhumation of rocks in the orogen but little subsidence of the foreland or advance of thrusts toward the craton the constructive stage of the alleghanian orogeny achieved the critical topography necessary to drive crystalline thrust sheets over the craton this resulted in widespread uplift and westward transport of older metamorphic rocks post orogenic unroofing of the southern appalachians is compatible with the predictions of flexural models the post orogenic unroofing history of the central appalachians and ouachitas probably reflects mesozoic extension in these areas

aftershock sequences of the 1982 miramichi new brunswick earthquakes the mb 5 7 miramichi new brunswick earthquake of 9 january 1982 was located by comparison to the aftershock distribution in central new brunswick canada at 47 00 n 66 60 w with a focal depth of 7 km and an uncertainty of 3 km on each of the spatial coordinates the main shock was followed by a long and complex aftershock sequence with four events m 4 5 and more than 800 events m 1 0 until 30 june 1982 the aftershocks were studied by three temporary field networks set up in the epicentral areas from 10 to 20 january 2 to 7 april and 17 to 23 june 1982 and by the permanent canadian seismograph networks the aftershock activity was concentrated in a volume approximately 6 km ns 6 km ew 8 km deep and arranged in a north trending conjugate v pattern the miramichi earthquakes were predominatly thrust earthquakes occurring on north trending mid to high angle faults in the upper 8 km of the crust in the presence of an east west compressive horizontal regional stress field detailed field surveys have not yet been able to associate the earthquakes with any known preexisting faults the main shock had a moment of 2 2 0 7 1024 dyne cm and a stress drop of 35 to 70 bars the rupture started at 7 km depth on a plane striking 195 dipping 50 down to the west and propagated up to the east with a rake angle of 120 aftershocks deeper than 3 5 km had composite mechanisms very similar to the main shock s but shallower events took place on more steeply dipping planes up to 78 dip the rupture planes of the main shock and its principal aftershocks may have been similarly curved the magnitude frequency relationship b 0 8 of the aftershocks was comparable to the regional one the aftershock activity decayed as t 0 8 and had the highest rate of activity following the principal aftershock on 11 january that aftershock mb 5 4 appears to have ruptured the east dipping plane conjugate to the main shock s plane two other principal aftershocks mb 5 1 on 9 january and mb 5 0 on 31 march were associated with the main shock rupture plane while the fourth large event mb 4 7 on 16 june occurred 30 km west of the other activity and was seemingly unrelated however all the earthquakes were apparently associated with the same granitic terrain the miramichi earthquakes did not produce any significant surface faulting one small 25 mm surface displacement was found but it was identified as a secondary stress induced movement along a preexisting joint