The Lithoprobe corridor across the Vancouver Island continental margin: the structural and tectonic consequences of subduction

1995 ◽  
Vol 32 (10) ◽  
pp. 1777-1802 ◽  
Author(s):  
R. D. Hyndman
Keyword(s):  
Heat Flow ◽  
Continental Shelf ◽  
Continental Slope ◽  
Continental Margin ◽  
Deep Structure ◽  
Vancouver Island ◽  
Source Zone ◽  
Oceanic Plate ◽  
Tectonic History ◽  
South Central

The deep structure and tectonic history of the continental margin of southwestern Canada have been determined by phase I of the Lithoprobe program across south-central Vancouver Island and associated marine studies across the continental shelf and slope. This article reviews results from the marine portion of the corridor but also presents continuous onshore–offshore data and interpretation. The geophysical data include multichannel seismic reflection, seismic refraction, magnetics, gravity, bathymetry, sea-floor acoustic imagery, heat flow, and seismicity. There has been Ocean Drilling Program (ODP) coring and downhole measurements on the continental slope. The margin structure and Cenozoic tectonic history are dominated by the consequences of subduction. Two narrow terranes, the Eocene volcanic Crescent and the Mesozoic mainly sedimentary Pacific Rim, were emplaced along the coast at the time of north Pacific plate reorganization at 43 Ma. They provide the landward-dipping backstop to a large accretionary sedimentary prism formed from the sediments scraped off the incoming oceanic plate. The prism exhibits active folding, thrusting, and tectonic consolidation, and it provides a model for the formation of ancient fold and thrust belts. An extensive gas hydrate layer was detected beneath the continental slope by a seismic BSR (bottom-simulating reflector) and was penetrated by the ODP drilling. The surface heat flow decreases landward from high values over the young oceanic plate to low values just seaward of the volcanic arc as a consequence of the heat sink provided by the underthrusting oceanic plate. The margin seismicity includes continental crust-events and earthquakes in the downgoing oceanic plate. No earthquakes have been detected on the subduction thrust fault but great thrust events are inferred to occur with an average interval of 600 years. The seismic source zone for such events is restricted to a narrow region beneath the continental shelf because of the high temperatures over the young Juan de Fuca plate.

scholarly journals GEOMORPHOLOGICAL AND SEDIMENTOLOGICAL CHARACTERIZATION OF THE URUGUAYAN CONTINENTAL MARGIN: A REVIEW AND STATE OF ART / CARACTERIZAÇÃO GEOMORFOLÓGICA E SEDIMENTOLÓGICA DA MARGEM CONTINENTAL DO URUGUAI: UMA REVISÃO E ESTADO DA ARTE

2018 ◽  
Vol 3 (4) ◽  
pp. 253-264 ◽  
Author(s):  
Leticia Burone ◽  
Paula Franco-Fraguas ◽  
Michel Mahiques ◽  
Leonardo Ortega
Keyword(s):  
Continental Shelf ◽  
Continental Slope ◽  
Continental Margin ◽  
Global Climate ◽  
Rio De La Plata ◽  
Río De La Plata ◽  
Southwest Atlantic ◽  
La Plata ◽  
Deep Circulation

The Uruguayan Continental Margin is located in a strategic location of great relevance to understand the dynamics of the regional and global climate and to study the sedimentary processes. The Río de la Plata represents the largest fluvial discharge of the Southwest Atlantic (23.000 m3/s of water and 57.000.000 m3/year of sediment for the South Atlantic Ocean). The sub-surface and deep circulation are characterized by the confluence of water masses with contrasting thermohaline characteristics that determine the formation of well-marked hydrological fronts. In the continental shelf, this characteristic is represented by the Subtropical Shelf Front, as a shallow extension of the Brazil-Malvinas Confluence that occurs in deep water off the shelf. Although the knowledge of the regional circulation is acceptable, there are significant gaps in the knowledge related to the influence of this complex hydrological system on sedimentation processes. The objective of this work is to synthesize the present knowledge about the morpho-sedimentary characteristics of the Uruguayan Continental Margin and the processes involved in its formation. It also aims to discuss some knowledge gaps that are the subject of current research in the region. The knowledge reflects the scientific-technological evolution at a global level that is translated in the increase of recent works on the continental slope. Nowadays, most of the available information is associated with the inner shelf and upper slope. Starting from the basis of the close relationship between bathymetry and sedimentological distribution it is clear that efforts should be concentrated on the better understanding of the interaction between the shelf and the slope. The scientific knowledge regarding sedimentology and geochemistry along the Uruguayan Continental Margin is still, scarce and heterogeneous. However, it indicates complexity in the sedimentary coverture present both in the continental shelf and slope. The Uruguayan Continental Margin can be considered as an in situ laboratory to improve the understanding of the influence of oceanic hydrodynamics on the margin configuration, a key point to achieve better paleoceanographic interpretations. In this sense, due to the complexity of both modern hydrology and morphological configuration of the margin, it would be interesting to understand the relative importance of the control of each environment on surface sedimentation. A better understanding of the processes that link the near bottom circulation and continental slope features is needed. At the same time, this will allow deepening a new field of research in Uruguay, considering the joint and interrelated vision between sedimentary and oceanographic processes. ResumoA Margem Continental do Uruguai está localizada num local estratégico de grande relevância para entender a dinâmica do clima regional e global e para estudar os processos sedimentares. O Río de la Plata representa a maior descarga fluvial do Atlântico Sudoeste (23.000 m3/s de água e 57.000.000 m3/ano de sedimento para o Oceano Atlântico Sul). A circulação de subsuperfície e profunda são caracterizadas pela confluência de massas de água com características termohalinas contrastantes que determinam a formação de frentes hidrográficas bem marcadas. Na plataforma continental, essa característica é representada pela Frente Subtropical da Plataforma, uma extensão rasa da Confluência Brasil-Malvinas que ocorre em águas profundas ao largo da plataforma. Embora seja considerável o conhecimento da circulação regional, existem lacunas significativas no conhecimento da influência deste complexo sistema hidrológico nos processos de sedimentação. O objetivo deste trabalho é sintetizar o conhecimento atual sobre as características morfo-sedimentares da Margem Continental Uruguaia e os processos envolvidos na sua formação. Também pretende discutir algumas lacunas de conhecimento que são objeto de pesquisas atuais na região. O conhecimento reflete a evolução científico-tecnológica a nível global, que se traduz no aumento de trabalhos recentes na vertente continental. Atualmente, a maior parte da informação disponível está associada à plataforma continental interna e ao talude continental superior. Partindo da base da estreita relação entre a batimetria e a distribuição sedimentológica, torna-se claro que os esforços devem ser concentrados na melhor compreensão da interação entre a plataforma e o talude continental. O conhecimento científico sobre sedimentologia e geoquímica ao longo da Margem Continental do Uruguai é ainda escasso e heterogêneo. No entanto, os dados existentes indicam que a cobertura sedimentar presente tanto na plataforma quanto no talude continental é complexa. A Margem Continental do Uruguai pode ser considerada um laboratório in situ para melhorar a compreensão da influência da hidrodinâmica oceânica na configuração da margem, ponto chave para se obterem melhores interpretações paleoceanográficas. Nesse sentido, devido à complexidade da hidrologia moderna e da configuração morfológica da margem, seria interessante entender a importância relativa do controle de cada ambiente sobre a sedimentação superficial. É necessário um melhor entendimento dos processos que ligam as características de circulação próxima do continente à que ocorre no talude continental. Este conhecimento permitirá aprofundar um novo campo de pesquisa no Uruguai, considerando a visão conjunta e inter-relacionada entre processos sedimentares e oceanográficos. 

Seismic reflection constraints on imbrication and underplating of the northern Cascadia convergent margin

1996 ◽  
Vol 33 (9) ◽  
pp. 1294-1307 ◽  
Author(s):  
A. J. Calvert
Keyword(s):  
Continental Shelf ◽  
Seismic Reflection ◽  
Deep Structure ◽  
Vancouver Island ◽  
Cascadia Subduction Zone ◽  
Accretionary Wedge ◽  
Seismic Profiles ◽  
Outer Continental Shelf ◽  
E Region ◽  
Seismic Reflection Profiles

An interpretation of the deep structure of the continental shelf offshore southern Vancouver Island, subject to constraints from other geophysical data, is derived by combining seismic reflection profiles shot in 1989 with those from an earlier 1985 survey. Accretionary wedge sediments, which extend landward beneath the volcanic Crescent terrane, comprise two primary units, both of which have shortened through duplex formation. The maximum thickness of the Crescent terrane, 6–8 km, occurs just seaward of its contact with the inboard, largely metasedimentary Pacific Rim terrane. The E region of reflectivity, first detected dipping landward beneath Vancouver Island, is regionally extensive, being observed on all the seismic profiles. The E reflectivity thins seaward and splits into two or more strands that probably link into major faults within the accreted sedimentary wedge. Reflections from the interplate décollement beneath the outer continental shelf separate from the downgoing plate, continue into the deepest level of the E reflectivity, and are interpreted to represent a single décollement surface above which imbrication of accreted units occurred. It is proposed that at the southern end of Vancouver Island the E reflections represent mainly underthrust sediments above a former subduction décollement, both of which were incorporated into the overlying continent when the subduction thrust stepped down into the descending oceanic plate. This change in depth of the subduction thrust underplated one or more mafic units to the continent. The reflection from the top of the subducting Juan de Fuca plate appears to be around 5 km shallower farther north along the margin, indicating that the underplated region could be confined to the embayment in the Cascadia subduction zone.

The northern Cascadia subduction zone at Vancouver Island: seismic structure and tectonic history

1990 ◽  
Vol 27 (3) ◽  
pp. 313-329 ◽  
Author(s):  
R. D. Hyndman ◽  
C. J. Yorath ◽  
R. M. Clowes ◽  
E. E. Davis
Keyword(s):  
Continental Shelf ◽  
Subduction Zone ◽  
Deep Ocean ◽  
Vancouver Island ◽  
Ocean Basin ◽  
Cascadia Subduction Zone ◽  
Accretionary Wedge ◽  
Seismic Structure ◽  
Tectonic History ◽  
Wide Range

The structure and Tertiary tectonic history of the northern Cascadia subduction zone have been delineated by a series of new multichannel seismic lines acquired across the continental shelf to the deep sea, combined with adjacent land multichannel seismic data and results from a wide range of other geophysical and geological studies. The top of the downgoing oceanic crust is imaged for a remarkable distance downdip from the deep ocean basin to a depth of 40 km beneath Vancouver Island. The reflection depths are in good agreement with seismic refraction models and Benioff–Wadati seismicity. Two broad reflective bands imaged as dipping gently landward at depths of about 15 and 30 km on the land lines merge to a single reflector band offshore. They may represent underplated oceanic material or, alternatively, they may not be structural but may be zones of contrasting physical properties, perhaps representing trapped fluid. Two narrow terranes, the Mesozoic marine sedimentary Pacific Rim Terrane and the Eocene marine volcanic Crescent Terrane, have been thrust beneath, and accreted to, the margin in the Eocene, about 42 Ma, near the start of the present phase of subduction. They provide a landward-dipping backstop to the large sediment wedge accreted since that time. The deformation front is characterized by mainly landward-dipping thrust faults that cut close to basement. This result and the mass balance of the incoming sediment compared with that present in the accreted wedge suggest that there is little subduction of sediment into the mantle. The Tofino Basin sediments, up to 4 km in thickness, have been deposited on the continental shelf over the accreted terranes and the developing accretionary wedge.

Seismic-profiler investigation of the continental margin northeast of Newfoundland

1968 ◽  
Vol 5 (5) ◽  
pp. 1187-1198 ◽  
Author(s):  
A. C. Grant
Keyword(s):  
Continental Shelf ◽  
Continental Slope ◽  
Continental Margin ◽  
Bottom Topography ◽  
Mass Movement ◽  
Unconsolidated Sediments ◽  
Continental Rise ◽  
Structural Disturbance ◽  
Topographic Relief ◽  
Structural Significance

Continuous seismic profiler recordings over the continental shelf northeast of Newfoundland show an accumulation of unconsolidated sediments ranging in thickness from 30 to 150 m, unconformably over-lying bedrock that dips gently eastward toward the continental slope. The distribution of these unconsolidated sediments indicates that the banks that compose the 'shelf-edge rise' are not primarily morainal material, but rather they are areas of positive relief on the bedrock surface. Anomalous strike-dip relationships associated with these bank areas suggest that they may also have structural significance. The underlying bedrock layers are truncated by the continental slope, and there is reversal in the dip of these layers from eastward to westward beneath the slope. The axial zone of the resulting fold apparently coincides very nearly with the axis of a large magnetic anomaly that extends along the continental margin in this region. A Tertiary to post-Tertiary age is suggested for the fold, as well as the structural disturbance associated with the topographic relief on the continental shelf. Within the continental rise an area of irregular bottom topography is considered to be due to the accumulation of large masses of sediment by slumping. Subbottom reflectors at depths beyond the continental rise show crumpling, which is assumed to be further evidence of mass-movement due to gravity. Superficially, the continental margin northeast of Newfoundland appears similar to parts of the continental margin to the south, on the flank of the Appalachian structural complex.

Marine geology of the atlantic continental margin of europe

1969 ◽  
Vol 264 (1148) ◽  
pp. 31-75 ◽  
Keyword(s):  
Continental Shelf ◽  
Continental Slope ◽  
Continental Margin ◽  
Western Europe ◽  
Seismic Refraction ◽  
Continental Drift ◽  
Marine Geology ◽  
North East ◽  
The North ◽  
Episodic Evolution

A geological reconnaissance has been made of the continental slope of western Europe between the Faeroe Isles and Lisbon by means of a 60 kj reflexion profiler, supplemented by Boomer profiles and extensive Asdic (side-looking sonar) coverage of the continental shelf. A tentative interpretation of these profiles has been made by references to available rock samples and seismic refraction data. The first-formed continental slope deposits of the north-east Atlantic appear to be Cretaceous in age. Massive erosion (particularly of the continental shelf) preceded the Tertiary phase of downwarping which allowed upbuilding and outbuilding of the continental margin and was followed by a latest-Tertiary and Quaternary phase of important faulting, canyon erosion, slumping and deposition. This episodic evolution of the European continental margin is thought to indicate two phases of continental drift. In contrast to the ground west of the British Isles and France the steep and narrow northern and western continental margins of Iberia show only a little upbuilding and only local outbuilding. The work was done from aboard R.R.S. Discovery II or R.R.S. Discovery (with the exception of a Sparker profile obtained by H.M.T.S. Alert ), the powerful profiler being used in 1965, the Asdic on many cruises between 1958 and 1967.

Late Pleistocene chronology and paleoclimate of Vancouver Island determined from cave deposits

1981 ◽  
Vol 18 (11) ◽  
pp. 1643-1652 ◽  
Author(s):  
M. Gascoyne ◽  
D. C. Ford ◽  
H. P. Schwarcz
Keyword(s):  
Late Pleistocene ◽  
Growth Period ◽  
Vancouver Island ◽  
Isotopic Equilibrium ◽  
South Central ◽  
Cave Deposits ◽  
Short Period ◽  
Thermal Buffering

Speleothems from four caves in south-central Vancouver Island have been analysed by the 230Th/234U method. Only speleothems from one cave, Cascade Cave, near Port Alberni, contained sufficient uranium for them to be dated. Twenty-seven ages determined for seven speleothems were found to cluster in two periods: 67–28 ka, corresponding to the Olympia interstadial, and <23–10 ka (or <15 ka if corrected for detrital thorium contamination), corresponding to the Fraser (late Wisconsin) deglaciation and Holocene periods. Two speleothems were found to be deposited in isotopic equilibrium with their seepage waters. Profiles of variations in δ18O of the calcite (δ18Oc) of each of the two deposits show a decrease of 1.3‰ over the growth period, 64–28 ka. At all times, δ18Oc was less than δ18O of modern calcite in the cave. Using modern cave temperature and the variation of δ18O of seawater over the dated period, the profiles of δ18Oc are interpreted in terms of a paleotemperature record for the Olympia interstadial in Vancouver Island. The results show a gradual cooling from 4 °C at 64 ka, to 0 °C between 35 and 28 ka. These results are consistent with conditions necessary for speleothem growth and with published work on surficial Wisconsin deposits in the area. No distinct, short-period warming or cooling events are seen in the record, probably due to thermal buffering by the adjacent ocean.

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