lithospheric processes and products in the southern canadian cordillera a lithoprobe perspective analyses of lithoprobe and other data from southwestern canada provide new insights on how this portion of the cordillera formed during plate convergence along the western margin of north america crustal rocks are detached from their mantle lithosphere which must have been consumed during subduction detachment occurred at or near the base of the crust beneath the intermontane and or omineca belts probably along the tips of tectonic wedges while the rocks were still outboard of the relatively cool mechanically rigid north american craton during the late cretaceous and early tertiary rotation of detached rocks caught between the north american craton and the oceanic plates accounts for some apparently conflicting results between paleomagnetic data that indicate large northward translation of rocks in the western cordillera and regional geological features that appear to preclude comparable amounts of translation of rocks in the eastern cordillera during the same time interval transpression assoc

tectonic denudation of the shuswap metamorphic terrane of southeastern british columbia large scale over thrusting and regional extension are both known to have been operative within the shuswap metamorphic terrane of the southern omineca belt the monashee core complex of the northern shuswap terrane is interpreted to be a crustal scale duplex that was developed during late mesozoic shortening and thickening of the crust uplift of the monashee complex caused by basement duplexing is estimated to have been in excess of 25 km late cretaceous overthrusting of high grade metamorphic and related plutonic rocks during later stages of uplift and basement duplexing led to weakening of the crust and to gravitationally driven extension of a regional crustal welt shuswap culmination denudation and spreading of this uplifted metamorphic terrane are attributed to displacements across listric normal shear zones and faults localized along the outward dipping flanks of the shuswap culmination total extension across the shuswap culmination at the northern end of the monashee complex is estimated to be no greater than 30 this model of denudation and gravity induced spreading is extrapolated southward into the valhalla and okanagan regions where extension may be far more significant

deep basement seismic reflection profiling of the purcell anticlinorium using a land air gun source seismic reflection profiling in the purcell anticlinorium of southern british columbia canada using a land air gun source has revealed reflections from depths of at least 15 20 km interpretations of these data in conjunction with related geological and geophysical information confirm the allochthonous structure of the purcell anticlinorium and suggest that autochthonous north american basement at 15 20 km depth beneath the anticlinorium has a westward dip of 15 20 the thickness of the crust above the north american basement surface may be accommodated by structural repetition of purcell strata by thrust slices of north american basement rocks or by both the geometry observed on the reflection data implies that the purcell anticlinorium is cored by a crustal scale thrust anticline hanging wall anticline further the data demonstrate that land air guns are effective sources for crustal reflection work

nature and origin of seismic reflection fabric ruby east humboldt metamorphic core complex nevada seismic reflection profiling across exposed upper and middle crustal rocks of the ruby east humboldt metamorphic core complex delineates important characteristics of the crustal structure developed during tertiary extensional deformation the goals of this study were to trace a tertiary extensional shear zone from mylonitic surface outcrops into a seismic section and to characterize the deeper crustal fabric associated with the polyphase deformational history of the complex reflections in the shallow surface correlate with a plastic to brittle shear zone that formed during the tectonic unroofing of the middle crustal rocks constructive interference from strong planar layering in the mylonitic shear zone is considered chiefly responsible for generating the reflections east and southwest dipping reflectors in the seismic section appear to correlate with exposed opposing dipping mylonitic foliation domains the opposing dips of the mylonitic layering may reflect warping of the normal sense shear zone during tectonic exhumation coupled with an overall anastomosing character deeper in the section a heterogeneous reflection character correlates with increases in velocity shown from wide angle measurements in this region and is interpreted as penetrative fabric originating from extensional flow facilitated by broad scale pure shear and localized simple shear lower crustal rocks apparently achieved granulite facies metamorphism during extension a maximum of 6 8 km of mafic material could have been added to the crust during cenozoic extension the moho reflection has such a high amplitude that it may be caused by partially molten rocks interleaved with peridotite reflections from the moho show no significant upwarping of the base of the crust beneath the core complex this suggests that the lower crustal configuration is at least as young as the period of extensional activity that lasted from approximately 40 to 20 ma which was responsible for the exhumation of the metamorphic core complex

geochronology and tectonic implications of magmatism and metamorphism southern kootenay arc and neighbouring regions southeastern british columbia part i jurassic to mid cretaceous k ar dates and u pb zircon dates define three periods of igneous activity in the southern kootenay arc 1 emplacement of late synkinematic to post kinematic granodioritic plutons in mid jurassic time 170 165 ma accompanying amphibolite facies regional metamorphism 2 emplacement of post kinematic granitic plutons in mid cretaceous time 100 ma and 3 emplacement of small bodies of syenite in eocene time 50 ma in the western part of the area micas from mid jurassic plutons that yield the oldest k ar dates 158 166 ma also yield plateau shaped 40ar 39ar age spectra age spectra for biotites younger than these but older than 125 ma reflect thermal overprinting in southeastern british columbia the kootenay arc marks the transition from the north american rocks of the cordilleran miogeocline to the tectonic collage of allochthonous terranes that have been accreted to it deformation metamorphism and plutonism recorded in rocks of the southern kootenay arc commenced in mid jurassic time as a co

conductive structures in southwestern canada a regional magnetometer array study an array of 33 three component recording magnetometers was operated in june and july 1980 in alberta and british columbia south of the edmonton prince rupert highway this very large array with its stations dispersed through 550 000 km2 and on average 150 km apart had limited resolution and was designed to confirm known conductive structures discover new ones and locate them sufficiently for suitable placement of further arrays with closer spaced stations and therefore higher resolution magnetograms and three sets of fourier transform anomaly maps are presented they show the general attenuation of the vertical component of variation fields west of the rocky mountain belt known from previous work and generally attributed to a conductive layer in the lower crust or upper mantle two prominent local anomalies are shown by variation fields of periods 15 30 min the first indicates induced currents near tete jaune cache west of jasper the highly conductive structure carrying the induced currents ma

mt and reflection an essential combination summary at many localities in the world there have been coincident comprehensive electromagnetic em studies and seismic reflection profiles conducted unfortunately over many more regions the seismic reflection images are interpreted without the constraints afforded by electrical conductivity information this paper is an attempt to convince the reader that a collocated magnetotelluric mt study should in almost every case be made wherever a seismic reflection survey is undertaken examples are shown from six studies in which the em results aided the geological tectonic interpretations of the seismic sections also difficulties with the mt technique are discussed and the interpretations of conducting zones within the lower crust are examined finally a generalised model is proposed for the continental crust that may account for both the reflectivity and conductivity of the zone at the top of the lower crust

lithoprobe seismic reflection structure of the southeastern canadian cordillera initial results nearly 270 km of crustal seismic reflection data obtained by lithoprobe in the southern canadian cordillera provide a geometric link between the rocky mountain foreland thrust and fold belt the purcell anticlinorium and the extensional regime superimposed on the crystalline core zone autochthonous north american basement and its overlying deformed and transported cover can be traced from the thrust and fold belt beneath the rocky mountain trench to 20 km depth beneath the central part of the purcell anticlinorium the purcell anticlinorium is cored by foreshortened proterozoic supracrustal rocks that were carried northeastward on a series of west dipping imbricate thrust faults these faults crop out within and east of the anticlinorium and converge downward with subhorizontal detachments above the autochthonous north american basement beneath the western purcell anticlinorium and kootenay arc reflections associated with the purcell stratigraphy and its underlying crystalline basement terminate at about 20 km depth and may be truncated against the east dipping eocene slocan lake fault zone the slocan lake fault zone is clearly imaged from the surface to about 12 km depth and can probably be followed discontinuously to about 25 km depth a west dipping high amplitude reflection from beneath the valhalla gneiss complex outlines the domal geometry of the complex and is probably related to an east verging compressional shear zone

magnetotelluric observations along the lithoprobe southeastern canadian cordilleran transect as part of the lithoprobe southern cordilleran transect investigations twenty seven wideband magnetotelluric mt responses were obtained along 150 km of the reflection seismic lines the mt profile begins to the east on the rocky mountain trench above autochthonous north american basement traverses across the purcell anticlinorium the kootenay arc the nelson batholith and ends on the east side of the valhalla gneiss complex sites near kootenay lake respond to a major near surface conductivity anomaly that is probably due to either graphitic schists or to an extension of the silver bearing mineralization found further north the short period 1 s data are highly complex and exhibit 3d regional features inversions of the data from the two sites on the valhalla complex yield 1d models with interfaces in electrical conductivity at depths that agree with an increase in the number of reflectors at 9 km the base of layered reflections at 22 km and the mono at 35 km of major significance to our understanding of the current composition and state of the lower continental crust is that although for the valhalla complex there are few reflections below 22 km the mt responses indicate that the zone is conducting this is the only location known to the authors where the lower crust is seismically transparent but electrically conductive

eocene structural development of the valhalla complex southeastern british columbia the valhalla complex a cordilleran metamorphic core complex is a 100 km by 30 km structural culmination within the omineca belt of southeastern british columbia it comprises sheets of granitic orthogneiss ranging in age from 100 to 59 ma with intervening paragneiss of uncertain age and stratigraphic correlation the complex is roofed by the ductile valkyr shear zone and the ductile brittle slocan lake fault zone the upper plate comprises lower grade metasedimentary rocks intruded by middle jurassic plutons the valkyr shear zone and the slocan lake fault zone deform 62 and 59 ma granitic sheets in their footwalls the easterly directed valkyr shear zone is a 2 to 3 km thick zone of distributed ductile strain which is arched over the complex and is exposed around the periphery on the northern western and southern margins the shear zone was active between 59 and 54 ma under amphibolite facies conditions the juxtaposition of upper and lower plates with different structural and metamorphic histories indicates that the valkyr shear zone is a significant structure with large displacement there is evidence to support an easterly rooting direction consistent with an extensional origin its surface breakaway is suggested to be west of the valhalla complex the slocan lake fault zone on the eastern side of the complex is a gently 30 easterly dipping ductile brittle normal fault which roots to the east it was active between 54 and approximately 45 ma and truncates the valkyr shear zone timing and structural relationships indicate that the valkyr shear zone and the slocan lake fault zone are genetically related movement on the ductile valkyr shear zone arching of the complex and displacement on the slocan lake fault zone occurred as a continuum in early to middle eocene time this paper documents the presence of significant eocene ductile strain in the valhalla complex and suggests that the role of extension in this region is more profound than had been previously recognized

cocorp deep seismic reflection traverse of the interior of the north american cordillera washington and idaho implications for orogenic evolution deep seismic reflection data collected by the consortium for continental reflection profiling along a transect of the interior of the northwestern u s cordillera indicate the presence of crustpenetrating faults these faults developed during an orogenic cycle consisting of jurassic to paleocene accretion related crustal thickening followed by eocene crustal thinning the reflection data also suggest that the moho is a young dynamic feature which developed a relatively smooth flat geometry beneath a complexly deformed crust probably during an eocene episode of crustal extension the profiles reveal prominent west dipping reflection sequences and associated diffractions interpreted as a mesozoic thrust system of crustal dimensions that linked accretion related shortening in the western cordillera with the thrust belt in the eastern cordillera east dipping reflections that crosscut the west dipping reflections in the middle and lower crust and are interpreted as ductile deformation zones that accommodated eocene crustal extension and essentially flat moho reflections at 33 35 km beneath complexly deformed crust these features are consistent with a sequence of 1 late mesozoic crustal imbrication of transitional crust beneath the easternmost accreted terrane which overrode the north american passive continental margin along a jurassic thrust 2 propagation of crust penetrating thrusts to shallower levels in the east where they probably formed the sole of the rocky mountain thrust belt 3 eocene extension that disrupted these thrusts and led to uplift of core complexes along detachments which fed into broad deformation zones in the lower crust and 4 development of a flat moho geometry during cenozoic extension and magmatism these data provide new information about the structure of the entire crust in the cordilleran interior and offer a working model both for evolution of cordilleran core complexes and possibly for other collisional orogens

geometric test for late cretaceous paleogene intracontinental transform faulting in the canadian cordillera the tintina trench northern rocky mountain trench tt nrmt fault zone and the fraser river straight creek fr sc fault zone are separate en echelon concentric small circle fault segments of a composite intracontinental transform fault zone more than 2500 km long that cuts diagonally across the canadian cordillera from the outboard part of a tectonic collage of accreted foreign terranes in the south into the north american preaccretionary continental margin in the north most of the 450 km of right hand slip on the tt nrmt fault zone was transformed southward during the late cretaceous and paleocene into oblique convergence in the southern canadian rockies the remainder probably comprising less than 100 km was transformed southwestward during early and middle eocene time via a zone of distributed shear and east west crustal stretching into right hand slip on the en echelon fr sc fault zone these interpretations based on regional systematic mapping of geologic structures are in conflict with interpretations of paleomagnetic measurements that call for more than 1000 km of post mid cretaceous right hand displacement along the general locus of the tt nrmt fault zone involving foreign terranes and the parts of north america to which they had been accreted this paradox must be resolved

origin of deep crystal reflections seismic profiling across high grade metamorphic terranes in canada abstract in an attempt to better understand the origin of deep crustal reflections lithoprobe has sponsored or co sponsored seismic reflection surveys across tracts of high grade metamorphic rock in the archean superior craton the proterozoic grenville orogen and the phanerozoic cordilleran orogen common to these three diverse terranes are near surface zones of prominent seismic reflectivity that are typically associated with velocity discontinuities at highly strained contacts between gneissic rocks of varying lithology at some locations the reflective layering resulted from transposition and rearrangement of previously layered rocks stratified assemblages sills etc whereas in other regions it was generated by extreme attenuation stretching and ductile flow of weakly layered or irregularly organized rocks it seems likely that compositionally layered gneissic rock is a common source of reflections in the deep crust with reflections originating at lithological boundaries and zones of mylonite

tertiary calc alkaline magmatism associated with lithospheric extension in the pacific northwest the long standing assumption that calc alkaline magmatism requires contemporaneous subduction is critically reviewed two of many widely spread fields of tertiary calc alkaline magmatism in the pacific northwest one eocene and one miocene provide clear examples of the association between calc alkaline magmatism and lithospheric extension we suggest that the calc alkaline magmatism of these fields is better explained by decompression melting of the subcontinental lithospheric crust and mantle enriched by previous subduction events than by contemporaneous eocene subduction this model proposes that extension will cause calc alkaline dominated magmatism in areas of continental crust with a history of previous subduction as along the western margin of north america currently popular models for the tectonic magmatic evolution of the basin and range province in which subduction driven calc alkaline magmatism is followed by extension related bimodal magmatism may need modification much more of this magmatism may result from extension in the pacific northwest we suggest that subduction beneath the idaho batholith ceased at the end of the cretaceous blocked by the accreted terranes to be reactivated in its present position beneath the cascades during the eocene the various isolated eocene volcanic fields spread across the pacific northwest were due to lithospheric extension not to the presence of a subducting plate immediately below

conductive structures under the canadian rocky mountains the paper reports results from an array of 33 three component magnetometers that recorded time varying fields in 1981 over an area of some 56 000 km2 in the canadian cordillera the array was centr

lithoprobe southern vancouver island cenozoic subduction complex imaged by deep seismic reflections reply the lithoprobe seismic reflection project on vancouver island was designed to study the large scale structure of several accreted terranes exposed on the island and to determine the geometry and structural characteristics of the subducting juan de fuca plate in this paper we interpret two lithoprobe profiles from southernmost vancouver island that were shot across three important terrane bounding faults leech river san juan and survey mountain to determine their subsurface geometry and relationship to deeper structures associated with modem subduction the structure beneath the island can be divided into an upper crustal region consisting of several accreted terranes and a deeper region that represents a landward extension of the modern offshore subduction complex in the upper region the survey mountain and leech river faults are imaged as northeast dipping thrusts that separate wrangellia a large mesozoic paleozoic terrane from two smaller accreted terranes the leech river schist mesozoic rock

lithoprobe southern canadian cordilleran transect rocky mountain thrust belt to valhalla gneiss complex summary lithoprobe has acquired nearly 270 km of crustal seismic reflection data across the eastern portion of the southern canadian cordillera these reflection profiles obtained during the fall of 1985 extend from the rocky mountain thrust and fold belt across the rocky mountain trench purcell anticlinorium kootenay arc nelson batholith and valhalla gneiss complex north american basement and its overlying foreshortened miogeoclinal rocks can be traced westward to the kootenay arc the purcell anticlinorium is carried by a series of west dipping thrust faults which emerge east of the anticlinorium and converge downward and merge with a detachment surface above autochthonous north american basement proterozoic supracrustal rocks thickened by folding and thrusting occupy the core of the anticlinorium steeply dipping surface structures of the western purcell anticlinorium and kootenay arc appear to be truncated at 3 4 s 9 12 km by a gently east dipping reflection that may delineate the upper boundary of an allochthonous wedge inserted between the near surface rocks and autochthonous basement below beneath the kootenay arc at a travel time of 9 10 s 27 30 km the north american basement seems to be truncated by the major east dipping slocan lake fault zone which can be traced from its surface exposure at the east edge of the valhalla gneiss complex eastward to near the base of the crust a high amplitude west dipping reflection underlies the valhalla complex and may be related to a major compressional shear zone

lithoprobe crustal reflection structure of the southern canadian cordillera 2 coast mountains transect the lithoprobe seismic reflection transect across the southern coast mountains of the canadian cordillera images fundamental crustal structures presumably related to collision of the intermontane and insular composite terranes and deep levels in the upper plate of the offshore cascadia subduction belt the eastern part of the coast mountains are characterized by east dipping upper crustal reflectors that project to exposed faults and east dipping lower crustal reflectors they are truncated by subhorizontal to west dipping middle and upper crustal reflectors these geometric relationships are interpreted to have formed during an early phase of primarily west directed contraction that created the east dipping structures of the upper and lower crust and a later phase of east directed shortening caused by wedging of the intermontane belt into the lower and middle crust of the tectonic stack subsequently the coast belt may have been displaced eastward on contractional faults that ascend from the lower crust beneath the intermontane belt and surface in the omineca and foreland belts extensional faults bounding the east flank of the coast mountains and west flank of the central nicola horst in the intermontane belt flatten into the middle and lower crust of the intervening region and geometrically outline crustal boudinage within the western coast mountains east dipping reflections spanning the middle crust to upper mantle are traced updip to vancouver island and the underlying cascadia subduction zone the c reflector on vancouver island is believed to separate wrangellia from underlying accreted terranes and is correlated to the mainland where it forms the upper boundary of a reflective lower crustal wedge that flattens into the moho if the moho is not a young feature then some accreted material appears to have wedged into the continental framework above the crust mantle boundary possibly causing shortening in the overlying crust and creating midcrustal ramps observed on the reflection data the structurally lower e reflections interpreted as shear zones originate at the subduction contact offshore and project landward into sub moho reflections within the upper plate on the mainland the region between the e reflector and the descending oceanic plate is interpreted to be subducted lower continental crust and mantle

structure and tectonic development of the southern rocky mountain trench the rocky mountain trench is one of the youngest most prominent and most enigmatic structures of the canadian cordillera approximately 650 km of seismic reflection data providing regional three dimensional coverage over an area of 10 000 km include six crossings of the rocky mountain trench between 49 n and 50 15 n prominent reflections from mid proterozoic moyie sills outline thrust and fold structures of a late jurassic to early cretaceous fault system that was cut by the rocky mountain trench fault in the tertiary the near basement reflections outline a 10 km high west facing basement ramp the hinge line of which spatially coincides with the rocky mountain trench in this area this ramp is part of a mid proterozoic margin upon which the belt purcell supergroup was deposited and is preserved beneath the trench during mesozoic contraction the basal detachment of the foreland belt closely followed the craton cover contact across the basement ramp thrusting ceased and extension was initiated when a culmination of thick basinal strata was juxtaposed with the basement ramp in the eocene miocene the basement ramp and the culmination above it focused stress reactivating the basal detachment and causing extensional faulting in the southern rocky mountain trench the rocky mountain trench fault may be linked via the basal detachment to the flathead fault on the east and the eocene extensional faults that flank the omineca belt on the west

seismic imaging of the alpine fault near inchbonnie new zealand 1 the transpressive alpine fault is the boundary between the pacific and australian plates across the south island of new zealand earthquakes on the alpine fault and related structures pose a serious risk to many urban centers including the city of christchurch although it is a major feature on satellite images the alpine fault is a difficult target for surface studies along much of its length it mostly traverses densely forested and mountainous terrain and where it occurs in the lowlands it is usually covered by recent sediments to investigate the alpine fault at a rare accessible location inchbonnie we have acquired high resolution seismic reflection data along five 380 1200 m long lines images produced from these data reveal a glacially overdeepened valley containing a thick sequence of diverse glacigenic sediments that have been disrupted by three en echelon strands of the principal alpine fault and several secondary fault strands based on their seismic facies the sedimentary sequence is interpreted to comprise basal lacustrine beds overlain successively by alluvial colluvial deposits that possibly include the remnants of large landslides deltaic fan units and braided river gravels whereas the principal alpine fault strands disrupt the entire post glacial sedimentary section and likely offset basement at depths up to 400 m most of the secondary faults either merge with the principal fault strands at shallow depths or are surficial features limited to the sedimentary section

deep crustal break back stacking and slow exhumation of the continental footwall beneath a thrusted marginal basin grenville orogen canada abstract granulite to upper amphibolite facies ductile thrusting in the central gneiss belt grenville orogen ontario represents the tectonic shortening of a continental scale footwall beneath the thrust emplaced central metasedimentary belt during the closure of a postulated back arc basin ca 1 19 1 18 ga break back stacking in the footwall occurred at mid to deep crustal depths ca 35 km within tectonically thickened ca 70 km continental crust and culminated with renewed thrusting at the base of the overlying central metasedimentary belt ca 1 08 1 05 ga the individual mylonite belts which constitute the ductile thrust zones and the scale of penetrative deformation of the intervening crystalline thrust sheets are comparable with the largest known examples of high grade thrust belts elsewhere they reflect the large scale thermal and theological boundary conditions of the deformation flow within individual thrust zones may reflect local boundary conditions such as the rheological behaviour of older thrust sheets and the geometry of interfaces within the thrust stack restoration of the thickness of erosionally removed crustal overburden by break back thrusting may retard the rates of exhumation and cooling of a mid to deep crustal thrust stack

seismic reflection geometry of the columbia river fault zone and east margin of the shuswap metamorphic complex in the canadian cordillera vibroseis conoco incorporated seismic reflection data recorded near the transition between the cordilleran fold and thrust belt and the east margin of the shuswap metamorphic complex of the southern canadian cordillera outline an east tapering wedge geometry in the upper crust the wedge comprises part of the shuswap metamorphic complex and may be the result of either a foreland dipping duplex or tectonic wedging the geometric wedge separates the overlying miogeoclinal rocks of the selkirk fan structure from the crystalline basement rocks below and delineates the eastern limit of the monashee complex brittle motion on the columbia river fault zone produced an east dipping listric normal fault which may have offset the upper portion of the monashee complex the columbia river fault zone is imaged over the entire 15 km of the profile and can be traced to a depth of at least 10 km reflections from the lower crust are generally subhorizontal and discontinuous and they may be a manifestation of crustal extension the crust mantle transition is probably represented as reflections with travel times of 11 0 12 0 s

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

large magnitude crustal seismic sources in el salvador and deterministic hazard scenarios abstract during the last years several crustal seismic sources have been proposed in el salvador however the actual destructive potential of these proposals has not been revealed yet here we present several seismic scenarios related with the main crustal faults in the country we have characterized the corresponding hazard scenarios in a deterministic way estimating the peak ground accelerations pga including local site effects in addition we present a list of 29 sources that can be considered potentially dangerous with their main features when they are available and four seismic scenarios that we consider to be more probable we have resorted to the february 13th 2001 mw 6 6 destructive earthquake the only earthquake occurred in a known crustal source in el salvador and recorded by the strong motion network in order to test the methodological approach and the reliability of the pga results predicted by two available ground motion prediction equations gmpes including the site effect the consistence between the values given by the models and the ones recorded during the event provides reliability to our results this approach allowed us to conclude that faults within the volcanic arc can produce accelerations up to 1 g in the most conservator case mw 7 0 while accelerations up to 0 6 g are usual results related with specific faults within the salvadoran volcanic arc besides the reported damage distribution of the 8th june 1917 earthquakes and its similarity with one of the proposed scenarios allowed us to infer the possible source of this event the guaycume fault

the moho in the northern basin and range province nevada along the cocorp 40 n seismic reflection transect cocorp seismic reflection profiles across nevada at about 40 n image a prominent essentially continuous band of reflectors at a two way traveltime of 9 to 11 s the approximate correspondence of this reflection time with estimates of the two way traveltime to the moho in this area provided by seismic refraction data suggests that the prominent reflections are from the moho the relief on these reflectors the reflection moho beneath nevada across a latitudinal transect of

proterozoic craton to basin crustal transition in western canada and its influence on the evolution of the cordillera deep seismic reflection data have imaged a crustal scale west facing ramp or ramp system in the subsurface of western canada in northwestern canada the ramp is within proterozoic crust east of the cordillera and is unconformably overlain by paleozoic sedimentary rocks indicating that it was formed during the proterozoic in this region similar structures are visible within the cordillera in southern canada and the northwestern united states along a south projection of the ramp observed in the north in the monashee complex of british columbia and in the priest river complex in northern washington reflections are visible that dip westward from the surface to near the base of the crust and are structurally discordant with underlying more horizontal reflections thus outlining footwall ramps we propose that the proterozoic crustal ramp in the north and the footwall ramps in the cordillera probably coincide with a proterozoic crustal transition from thick craton on the east to thin crust on the west thi

stratigraphy and deformational history of the scrip nappe monashee mountains british columbia the scrip nappe a large recumbent anticline that occupies the northern selkirk and northern monashee mountains has an inverted lower limb some 50 km in length across strike and comprises stratigraphic divisions of the hadrynian horsethief creek group which can be traced southward with decreasing metamorphic grade through the selkirk mountains to the northern purcell mountains the scrip nappe has a southwesterly vergence and it formed that way during the first folding phase of the mesozoic columbian orogeny metamorphism no greater than biotite zone accompanied that first deformation the nappe was subsequently refolded into tight northeast verging folds metamorphism rose to upper amphibolite facies late in the second deformation phase after the metamorphic climax northeast verging buckle folds and associated crenulation cleavage formed locally during a third folding episode the entire nappe complex was then carried northeastward on the purcell thrust over the folds and thrusts of the western r

cordilleria a newly defined canadian microcontinent palaeomagnetic data and geological field evidence indicate that most of the western canadian cordillera lay 1 500 km south of its present position in the early cretaceous we suggest that this region comprised a microcontinent termed cordilleria which moved on the kula plate and collided with the craton 100 myr ago to form first the mackenzie mountains and then the rockies

imbricated terranes of the cariboo gold belt with correlations and implications for tectonics in southeastern british columbia the cariboo gold belt of east central british columbia is divided into four fault bounded sequences of distinct stratigraphy they are from east to west the cariboo continental shelf sediments barkerville continental shelf sediments and intercalated volcanics slide mountain rift related submarine pillow basalt chert and diorite and quesnel island arc sediments and subaqueous volcanics terranes each is separated from others by thrust faults grit phyllite limestone and volcanics of the barkerville terrane may be correlative with the eagle bay formation near adams lake and the lardeau group near kootenay lake barkerville terrane may be part of a more regional rock package selkirk terrane which is defined to include kootenay terrane badshot formation and horsethief creek and hamill groups selkirk terrane is i separated everywhere by a low angle fault from the overlying age equivalent but stratigraphically and structurally different cariboo terrane and ii separated by a system of f

dynamics and coastal effects of tsunamis generated by asteroids impacting the black sea two dimensional 2d and one dimensional 1d shallow water models are used to evaluate the seashore effects of the tsunami generated by an asteroid impacting the western regions of the black sea about 208 km from the coast the tsunami s speed and the effects on the coastal regions depend on many factors among which the most important is asteroid size the tsunami generated by a 250 m asteroid reaches the nearest land location in 24 min and needs about 2 h to arrive at all black sea coast the run up value is about 7 m high on the turkish and crimean coasts in the western black sea regions the wave height is two or three times smaller the run up values strongly depend on bathymetry and topography peculiarities the run in value in case of the tsunami generated by a 1 000 m asteroid is up to eight times larger than in case of a 250 m impactor depending on location the results reported herein are upper limit values in case of the 250 m asteroid the real wave amplitude may be up to two times smaller the uncertainty factor decreases in case of larger asteroids ways of diminishing the social consequences are briefly discussed

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

identification and interpretation of azimuthally varying crustal reflectivity with an example from the southern canadian cordillera a new approach to seismic interpretation seismic skeletonization is used to analyze crustal reflection data by identifying numbers of reflection events their lengths their energies and their dips with respect to azimuths of recorded profiles application of the method to crustal reflection data in the southern canadian cordillera demonstrates strong directional dependence of crustal reflectivity with persistent trends at 025 040 205 220 085 095 265 275 115 125 295 305 and 140 160 320 340 these trends are visible in both the upper and lower crust and may be related to regional structural trends further applications of the technique particularly in different tectonic regions may allow categorization of seismic patterns and cataloguing of signatures for complex reflection data in a manner similar to seismic stratigraphy and structural patterns in stratified rocks