Interpretation of Gravity and Magnetic Measurements across the Continental Margin of British Columbia, Canada

A synthesis of Canadian Cordilleran refraction data recorded prior to 1971 with other geophysical data shows major features which correlate well with the regional geological structure. The wavelength of M topography decreases from about 200 km at 54"N to about 110 km at 52"N and culminates in a major lithospheric discontinuity east of Vancouver Island. The seismic data indicate the region of the Fraser River at Quesnel, the region immediately east of and parallel to the Coast Plutonic Complex and possibly the western edge of the Hazelton Mountains are sites of significant changes in lithospheric structure.Lateral variations in the average crustal density are necessary to reconcile both gravity and seismic data. The crust beneath the central, intermontane region is characterized by a mass deficiency, whereas the density of the crust beneath Vancouver Island appears greater than average.Calculation of synthetic record sections shows that the Pn phase may propagate considerably beyond crossover in a layer a few wavelengths thick. A model for the Canadian Cordillera which includes a low velocity layer as little as 8 km below the M appears plausible. Model calculations suggest that the M approximates a discontinuity beneath the Coast Plutonic Complex, but is better modelled as a transition zone beneath the Omineca Crystalline belt.Une synthese des resultats seismiques et d'autres donnes geophysiques de la Cordilliere Canadienne fait ressortir des traits majeurs qui concordent bien avec la structure geologique regionale. La surface de longueur d'ondes M descend de 200 km au 54"N jusqu'a 110 km au 52"N et culmine dans une discordance lithospherique a I'est de I'ile de Vancouver. Les resultats de seismiques indiquent que la region de la riviere Fraser a Quesnel, la region immediatement i I'est et parallele au complexe plutonique &tier, et possiblement la bordure ouest de Monts Hazelton sont les sites de changements signiticatifs dans la structure lithospherique.Des variations laterales de la densite movenne de la croiite sont necessaire pour concilier les donnes seismiques et gravimktriques. La cioiite sous la partie centrale est caractensee par une deficience en masse alors que la densite de la croiite sous I'ile de Vancouver apparait superier a la moyenne.Les calculs a partir des resultats de sections types montrent que la phase Pn peut se propager bien en-dessous de I'idexion dans une couche de quelques longueur d'ondes d'epaisseurs. Un modele de la Cordilliere Canadienne impliquant une couche a faible vitesse sous M apparait plausible. Les modeles calculCs suggerent une discordance comme approximation de M sous le complexe plutonique cBtier mais une zone de transition sous la ceinture cristalline Omineca en rendrait mieux compte.[Traduit par le journal]

Section: Discussionmentioning

confidence: 91%

“…may herald the time when the action at the plate boundary changed from one of predominant subduction to the present day style of rightlateral motion along the Fairweather -Queen Charlotte system and possible minor subduction of the Juan de Fuca plate (Srivastava 1973).…”

Section: Discussionmentioning

confidence: 99%

Structure of the Canadian Cordillera from Seismic Refraction and Other Data

Berry¹,

Forsyth²

1975

“…(b) Sediment thickness Various estimates have been made of the total sediment thickness in the inner basin. They range from that of Srivastava (1973) at 6.5 krn to that of Lynch (1977) at 2 km. Maximum depths of basement traced along profiles 4a and 6a are 4.6 and 4.8 s, respectively.…”

Section: (A) History Of Sedimentationmentioning

confidence: 94%

“…Two previous gravity interpretations (Couch 1969;Srivastava 1973) suggested the existence of a basin or graben with sedimentary thicknesses of between 5 and 7 km underlain by crust of an "intermediate" thickness of between 10 and 15 krn. Gravity surveys carried out from Pacific Geoscience Centre (Earth Physics Branch, Open File 77-2) combined with recent seismic refraction and reflection interpretation (E. E. Davis and R. M. Clowes, manuscript in preparation, 1982) now enable us to reinterpret the crustal structure and to examine some of the sediment structures in the basin in detail.…”

Section: Gravitymentioning

confidence: 98%

The Winona Basin: structure and tectonics

Davis¹,

Riddihough²

1982

A compilation of published and new geophysical data from the Winona Basin off northern Vancouver Island has allowed a detailed interpretation of the sedimentary and tectonic history of the region to be made. The basin is forming as a result of the asymmetric subsidence of a recently isolated lithospheric block that is slowly converging with the continental margin. The crust beneath the basin is young (1 -5 Ma, increasing in age from southeast to northwest) and of normal oceanic thickness. It is virtually non-magnetic, however, probably because of its having been rapidly buried1 by turbidite sedimentation. Subsidence of the basin and uplift of the Paul Revere Ridge began in the Early Pleistocene (ca. 1 ..8 Ma) and, since that time, up to 8 km of turbidite sediments has accumulated in the basin. The nature of the fanning of the deposits suggests that the basin has been kept full throughout its history; the minimum average supply rate necessary to ac:complish this is about 70 x 106Mg year-'. This Pleistocene average is considerably greater than the present discharge rates of any of the major rivers in the area. Subsidence, indicated by the large gravity anomaly over the basin (-130 x lo-' m s-2 (-130 mGal)) and by the tilting of sediment layers at depth, and convergence, indicated by folding of sediments throughout the blasin fill, appear to be continuing at the present time. From the timing of various events associated with the formation of the b,asin, we conclude that the recent reorganization of spreading and the recent relocation of the Pacific-Explorer-America triple jlunction have occurred in response to the demands of local small plate motions that are controlled by the interaction of the small1 plates with the continental margin.Une compilation de donnees nouvelles ou dejh publiks de geophysique sur le Bassin Winona au nord de 1'Ile Vancouver a permis de formuler une interpretation trapnt 1'Bvolution ddimentaire et tectonique de la region. Le bassin s'est form6 grLe h une subsidence asymetrique d'un bloc lithospherique recemment isole lequel converge lentement vers la marge continentale. La crohte terrestre sous le bassin est jeune (1-5 Ma, l'hge croissant du sud-estvers le nord-ouest) et elleest d'une Bpaisseur oceanique normale. Elle semble etre non-magnetique, neanmoins, probablement parce: qu'elle fut rapidement recouverte par des turbidites. La subsidence du bassin et le soul&vement de la Paul Revere Ridge ont comnnence h se manifester dhs le debut du PlBistoc&ne (ca.1,8 Ma) et depuis ce temps il s'est accumul6 dans le bassin jusqu'h 8 km de turbidites. La nature des c6nes de dejection des depots sugghre que le bassin fut constamment bien rempli dwant toute son evolution historique; la quantitk minimum moyenne de s6diments apportes au bassin pour le maintenir comble est environ 70 x 1 0 6~g annee-'. Cette valeur moyenne durant le Pleistoc&ne depasse consid6rablement les taux de dtcharge actuellemen~t observes dans n'importe laquelle des principales rivi&res de la dgion. La subsidence est revel& par une ...

“…The northtrending linear 'high' near the western edge of the map (marked as A5 in Fig. 3) corresponds with the Anomaly 5 as identified by Srivastava (1973) using the Heirtzler et al (1968) time scale. Other smaller anomalies are tentatively correlatable.…”

Section: Regions Of Distinctive Magnetic Charactermentioning

confidence: 95%

Large scale magnetic anomalies over western Canada and the Arctic: a discussion

Coles¹,

Haines²,

Hannaford³

1976

A conroured map of vertical magnetic held residuajs (relative to the IGRF) over western Canada and adjacent Arctic regions has been produced by amalgamating new data with those from previous surveys. The measurements were made at altitudes between 3.5 and 5.5 km above sea level. The map shows the form of the magnetic field within the waveband 30 lo 5000 km. A magnetic feature of several thousand LiIomerres wavelength dominates the map. and is probably due in major part to sources in the earth's core. Superimposed on t h i~ are several groups of anomalies which contain wavelengths of the order of a thousand kilometres. The patterns of the rhort wavelen@h anomalies provide a broad view of major structures and indicate several regimes of di~tinctive evolutionary development. Enhancement of viscous magnetization at elevated temperatures may account for the concentration of in~enqe anomalies observed near the western edge of the craton.Une carte de contours residuels du champ magnetique vertical (par rapport au IGRF) du Canada de I'ouest et les regions contigues de I'Arctique a ete fabriquee par I'amalgamation des nouvelles donnees avec celles des leves anterieurs. Les mesures ont t t e faites a des altitudes entre 3.5 et 5.5 km au-dessus du niveau de la mer. La carte montre la forme du champ magnetique pourles longueurs d'onde de 30 b 5000 km. Une anomalie magndtique avec les longueurs d'onde de plusieurs milk kitomitres domine la carte, et probablement est due. en grande partie. i des sources dans le nayau terrestre. Rusieurs groupes d'anomalies de longueur d'onde de I'ordre de rnille kilornttres sont superposis sur I'anomalic dominante. LRs motifs des anomalies des longueurs d'onde courtes donnent une vue d'ensemhle des sauctures rnajeures e t indiquent quclqu~s rkgirnes de diveloppemenl ivolutiannaire distinctif, L'augmentation du mapnetisme visqueux i des ternpkralures Clevies peut expliquer les anomalies prononckes qu'on a observe pres du bord ouest du craton.