Structure Of Continental Margin Research Articles

Evolution of the central Catalan margin of the Valencia trough (western Mediterranean)

The evolution of the central Catalan margin was conditioned by major fault activity and thermal processes generated by crustal thinning. The continental margin structure is characterised by major NE-SW-striking, southeast-dipping basement faults which have given rise to a graben, half-graben and horst system. These faults acted essentially as normal faults. No major Neogene strike-slip activity has affected the continental margin, although minor episodic variations from the dominant extensional regime to strike-slip tectonics cannot be discounted. Two major structural evolutionary stages took place at the margin, the transition from one stage to the other being gradual: (1) rifting stage (Late Oligocene? to early Burdigalian) related to the opening of the Valencia trough and from which originated both the Barcelona and Vallès-Penedès half-grabens; (2) thermal subsidence stage (late Burdigalian to present) generated by the subsequent crustal cooling and thinning. Fault activity during this stage was essentially concentrated in the major half-graben bounding faults. The sedimentary evolution of the central sector of the Catalan margin was controlled by the tectonic and crustal evolution as well as by the successive sea-level and paleoclimatic changes which affected the region. Drastic changes in the depositional systems took place during the transition from the rifting stage into the thermal subsidence stage. During Aquitanian-Langhian time—characterized by an overall trend of pulsating encroachment of marine conditions over the continental margin—a noticeable diversity of depositional systems (alluvial, lacustrine, coastal evaporitic, fan delta-bay-shelf, carbonate platform systems) developed. From the latest Serravallian up to the present, the depositional framework was less varied due to a general lowering of sea-level and to the final onlapping of most of older structural highs, which are now buried below the Miocene sediments. Two progradational terrigenous shelf-talus systems separated by the Messinian erosive surface developed during the latest Serravallian-Tortonian and Pliocene-Quaternary.

Three‐dimensional seismic imaging of the Costa Rica Accretionary Prism: Field program and migration examples

Seismic reflection techniques are the primary geophysical tool used to examine the structure of continental margins. On convergent plate margins, widely separated seismic reflection profiles often do not image complex structural features because out‐of‐plane reflections and diffractions obscure the seismic images. Yet the structural features are often critical to understanding the tectonic processes. Thus, the resulting geologic interpretations are based on images that may not accurately represent the true subsurface structure. To image the complex geologic structures of an active continental margin, a three‐dimensional (3‐D) seismic survey was conducted off of the Nicoya Peninsula of Costa Rica in April 1987. Over 60,000 shot points of 96‐trace multichannel data were collected in a 9×22 km grid. This detailed survey was located over the accretionary prism and designed to investigate the active tectonic processes and evolution of this continental margin. In this paper we outline the data acquisition program, including the navigation processing critical to successful imaging, and the seismic processing methodology and then show several examples of images improved by 3‐D surveying. The resulting 3‐D seismic data were migrated both as independent lines and as a 3‐D volume. Seismic images were produced as conventional depth and time sections and as cross‐sectional slices in depth and time. We present comparisons of the migration results that show significant improvements of the subsurface image using 3‐D techniques. To illustrate these improvements, we present images of two complex geologic areas: a mud volcano and reflections near the top of the accretionary prism, and the top of the subducting oceanic basement beneath the prism. Comparisons between the original stacked data, the data of the two‐dimensional migration, and the results of the 3‐D migration illustrate the value of 3‐D techniques in studies of active margins. In addition to the advantages of the 3‐D imaging which include better structural delineation and improved signal‐to‐noise ratio in the final image, we show that the ability to view the 3‐D data volume in both section and plan view offers significant interpretational advantages.

Material transfer in an arc‐ridge collision zone

The influence of colliding and subducting ridges on the structural evolution of volcanic island arcs was investigated during Leg 134 of the Ocean Drilling Program (ODP) in the Central New Hebrides Island Arc, Southwest Pacific. Documenting tectonic accretion of colliding ridges onto the forearc of the central New Hebrides Island Arc (Vanuatu) and fragmentation of the island‐arc crust were the primary drilling objectives. Other objectives included studies of compressional dewatering, diagenetic alteration of pore fluids with rocks of an accretionary mass, and the tectonic evolution of a colliding guyot. Ridge‐arc collisions significantly alter the morphology and structure of volcanic island arcs and continental margins; however, the processes of tectonic deformation are poorly understood. Leg 134 documented for the first time material transfer from a ridge to an arc and defined the style of deformation occurring in the forearc area of a ridge‐arc collision zone.

The deep structure of the east Oman continental margin: preliminary results and interpretation

We describe preliminary results of a coincident normal incidence and wide-angle Seismic experiment across the east Oman continental margin just north of the Masirah Island ophiolite. This margin is affected by tectonic deformation and may be the site of margin-parallel shear in a zone parallel to the Owen Fracture Zone. The reflection profile is used to define upper crustal structure and shows a deep offshore basin dammed oceanwards by a ridge, interpreted here as a rotated fault block. Wide-angle data were collected using ten digital ocean-bottom Seismometers and 110 explosive shots. Preliminary raytracing of a crustal model based on the gravity model of Whitmarsh (1979) shows rapid changes in crustal thickness across the margin. A steep landward dipping reflector, probably the Moho, lies beneath the continental slope. The orientation of this reflector is exactly opposite to the direction of tectonic fabric predicted by a simple overthrust model of ophiolite emplacement from the ocean basin to the east.

Continental Margin Stratigraphy and Structure Offshore Vietnam, South China Sea Between 11ø and 16øN: ABSTRACT

Continental Margin Stratigraphy and Structure Offshore Vietnam, South China Sea Between 11ø and 16øN: ABSTRACT

Dipping reflector styles in the NE Atlantic Ocean

Summary The geometries and structure of volcanic rifted continental margins, that is, those with an ocean-continent transition masked by thick sequences of inclined tholeiitic basaltic flows, are examined. Small- and large-scale differences in sequence seismostratigraphy along strike, down-dip, and between conjugate configurations are discussed. Detailed appraisals of deep drilling results and seismic data for the Vøring Plateau and western Rockall Plateau are used to contrast margin architecture and demonstrate difficulties in interpolating data between sites of dipping reflector formation.

Hatton Bank (northwest U.K.) continental margin structure

Summary. The continent-ocean transition near Hatton Bank was studied using a dense grid of single-ship and two-ship multichannel seismic (mcs) profiles. Extensive oceanward dipping reflectors in a sequence of igneous rocks are developed in the upper crust across the entire margin. At the landward (shallowest) end the dipping reflectors overlie continental crust, while at the seaward end they are formed above oceanic crust. Beneath the central and lower part of the margin is a mid-crustal layer approximately 5 km thick that could be either stretched and thinned continental crust or maybe newly formed igneous crust generated at the same time as the dipping reflector sequence. Beneath this mid-crustal layer and above a well defined seismic Moho which rises from 27 km (continental end) to 15 km (oceanic end) across the margin, the present lower crust comprises a 10-15 km thick lens of material with a seismic velocity of 7.3 to 7.4 kds. We interpret the present lower crustal lens as underplated igneous rocks left after extraction of the extruded basaltic lavas. A considerable quantity of new material has been added to the crust under the rifted margin. The present Moho is a new boundary formed during creation of the margin and cannot, therefore, be used to determine the amount of thinning.

On the tectonic events correlation in the northwestern Pacific Ocean and Asian continental margin

We consider the correlation of Cretaceous-Cenozoic tectonic events in the oceanic and continental parts of the northwestern Pacific. The first results are based presumably on the data about Sikhote Alin, Sakhalin, Japan, the Japan Sea and Okhotsk Sea, the Kuril-Kamchatka arc and trench, and the northwestern part of the Pacific Ocean. The subjects of discussion are the structural rebuildings at the age boundaries of 100–110 and 25–30 Ma, connected with the fundamental changes of the northwestern Pacific tectonic plan and replacing of the continent-ocean boundary to the east. When correlating the tectonic events we discovered not only the coincidence in time of the major structural rebuildings but also the affinity of the stress fields and counter movements of the ocean and continental margin structures, with some signs of compression on their boundary. These structural peculiarities, the data about the source regions in the sedimentation process, the preliminary comparison of paleolatitude data allow us to question that the relative movements of the Pacific and Asian structures may possibly be limited. Inside the Pacific plate we found some signs of the considerable dislocations and the layering of the lithosphere. For every tectonic stage we discuss the succession of tectonic events on the ocean-continent boundary which result in the continental crust formation. The connection of major moments of the structural rebuildings with the reorganisation of the lithospheric plates boundary is emphasized.

La marge meridionale de la Mer d'Alboran: Caracteres structuro-sedimentaires et evolution recente

High-resolution seismic reflection profiles (2000 km) were collected across the southern continental margin and major structural trends of the Alboran Sea. Seismic units are dated on the basis of well-known marine Neogene onshore sections and offshore dredged samples. A better assessment of margin structuration and Neogene chronology is made. A conformable succession of Pliocene-Quaternary and older series provides evidence for the non-evaporitic nature of the Upper Miocene, similar to the facies from the onshore Neogene Boudinar and Melilla basins. It also suggests that Alboran Ridge is in part sedimentary rather than only volcanic. This Ridge appears as an “en echelon” anticlinal complex. Conformable Upper Miocene and Lower Pliocene-Quaternary strata are folded and faulted. Primarily important manifestation of the movements is dated as Pliocene, but the main vertical relative movements are of Quaternary to recent age as emphasized by well-developed divergent strata lying above. Submarine volcanism, probably Miocene in age, often controls the morphology and structure of the eastern Moroccan continental margin and adjacent south Alboran Basin. The latter one is characterised by a thick sedimentary accumulation; it is tectonically bounded, except on the southwestern part seaward of the Bokkoyas cliffs. Vertical movements occur during much of Plio-Quaternary time, prevailing during the Quaternary. The structure and related morphology of the study area are mainly the result of neotectonic events.

Structure of the North American Atlantic Continental Margin

The use of multichannel seismic-reflection profiles to study oceanic areas has advanced our understanding of deep crustal structure and the history of its development. Off eastern North America, where the structure of the continental margin is essentially constructional, seismic profiles have approximated geologic cross sections up to 10–15 km below the sea floor and revealed major structural and stratigraphic features that have regional hydrocarbon potential. These features include (a) a block-faulted basement hinge zone; (b) a deep, broad, rifted basement filled with clastic sediment and salt; and (c) a buried paleoshelf-edge complex that has many forms. The mapping of seismo-stratigraphic units over the continental shelf, slope, and rise has shown that the margin's developmental state included infilling of a rifted margin, buildup of a carbonate platform, and construction of an onlapping continental-rise wedge that was accompanied by eroison of the slope. Correlation of seismic stratigraphy with well-l...

Geophysical controls on sedimentation within the African Rift Systems

Summary Gravity and seismological data are used, together with unpublished oil company data, to investigate the lithospheric structures associated with the East, and the West and Central African Rift Systems. Although these two rift systems are the result of extensional tectonics, they appear to be tectonically very different, with uplift and volcanism in East Africa, and subsidence and sedimentation in West and Central Africa. These differences can be explained in terms of the amounts of crustal extension (and thinning) that these rift systems have undergone and the isostatic response of these crustal structures during, and subsequent to, the extensional phase. In West and Central Africa, gravity modelling indicates crust extensions in the order of 70 km, while in East Africa extension across the Kenya Rift could be as little as 10 km. The implications of these lithospheric models for the formation of sedimentary basins are discussed, as well as how the rift structures relate to the crustal structure of continental margins.

Seismic stratigraphy and structure of the east Canadian continental margin between 41 and 52°N

The seismic stratigraphy of the eastern Grand Banks continental margin is examined, and a five-fold division of the sedimentary sequence overlying basement is proposed. Oceanic basement of Cretaceous age underlies the eastern part of the study area; to the west, continental basement ranging in age from Late Precambrian to ?Jurassic underlies the Grand Banks. The sediment units, ranging in age from Early Cretaceous to Recent, have been dated by extrapolation of both commercial and DSDP drilling results from the Grand Banks and from the formerly conjugate Iberian margin. Identification of oceanic magnetic anomalies in the Newfoundland Basin agrees with the proposed age of the two oldest, Early Cretaceous units.

THE TECPAN REGIONAL FAULT: EVIDENCE FOR MAJOR NE LINEAMENTS

Con base en una revisión de algunos trabajos publicados acerca de estudios sobre la margen continental en el sur de México, se encontró una posible asociación de rasgos morfotectónicos tanto del subfondo marino como del continente. Son doce los rasgos geológicos locales aparentemente independientes que cuando se correlacionan en un panorama regional sugieren la existencia de la Falla Regional de Tecpan que aquí se propone. Esta falla sería un miembro representativo de un sistema de fallas conjugadas del sistema de fallas o rift asociado a la Faja Volcánica Mexicana y del cual muchos otros han sospechado. La falla interpretativa va desde el talud interior de la Trinchera Mesoamericana hasta la Faja Volcánica Mexicana con un rumbo general N 17° E .

Crustal structure of rifted continental margins: Geological constraints from the proterozoic rocks of the Canadian shield

Rifts associated with possible Proterozoic continental margins and embryonic ocean basins are characterized by an association of tholeiitic volcanics that have chemical characteristics of MORB's, flysch-like sedimentary rocks and extensive gabbro sill swarms comagmatitic with the volcanic rocks. These rifts are similar in stratigraphy and crustal structure to modern rifts such as the Gulf of California. The similarity of these Proterozoic rifts to modern rifts indicate that the process(es) of rifting on the Earth may have been persistent for the past 2.0 Ga. A variety of other Proterozoic rifts are also present in the Canadian Shield, and workers in the Precambrian must examine each rift on an individual basis in order to determine the tectonic setting of the rift being studied.

Structure and dynamics of plate boundaries in the Indian Ocean based on gravity anomalies

The Indian Ocean has been the site of numerous geophysical investigations which commenced during the International Indian Ocean Expedition. In this paper gravity surveys are reviewed which have been undertaken since this international effort to elucidate the structure and evolution of the most complex of the three major oceans. Particular attention is paid to the interrelationship between the plate boundaries and the gravity field. Some selected boundaries are discussed in the framework of global plate tectonics. These areas comprise the Red Sea, Gulf of Aden and the Indonesian deep sea trench. In addition the continental margin structure south of India is included which provides some further constraints on the location of the initial break-up of Gondwanaland and consequently bears on pre-drift reconstructions of India and Antarctica.

Marine geophysical study of the Comorin Ridge, North Central Indian Basin

Marine geophysical data are used to interpret the continental margin structure south of India and west of Sri Lanka in the northern Central Indian Basin. Free‐air and isostatic anomaly maps are presented. They show the prominent Indian Ocean gravity low with values more negative than −100 mgal south‐west of Sri Lanka. A steep gradient in the free‐air and isostatic anomaly forms a NNW‐SSE trending line extending south and striking parallel to the southwest coast of India. This line marks the eastern boundary of a topographic and basement high here named the ‘Comorin Ridge’ which is also clearly seen on a sediment isopach map. The ridge seems to act as a barrier to the passage of Ganges Cone sediments to the west. A comparison of the eastern (landward) edge of the Comorin Ridge with similar structures of other passive continental margins such as those off southern South Africa (Agulhas Fracture zone), off the Falkland Plateau and off western Norway is made. It is suggested that the eastern edge of the Comorin Ridge may mark a significant structural crustal boundary.

Late Tertiary Structure and Stratigraphy of North Sinai Continental Margin

New seismic data provide information on the structural development and late Tertiary stratigraphy of the continental slope and rise off northern Sinai. The upper continental slope is characterized by a marginal plateau composed of a series of platforms or steps. The lower continental slope is smooth, except for a low ridge paralleling the western part. Numerous diapiric structures along the continental margin north of the Sinai Peninsula emerge from an evaporitic layer of late Tertiary age. The diapirs usually are aligned along west-northwest-trending faults. A salt ridge 90 km long produces a submarine ridge in the lower continental rise. Two main fault trends have been mapped. In the western part of the continental margin they trend west-northwest and, in the eastern part, northeast. These trends parallel the continental slopes of northern Sinai and southern Israel, respectively. The structural grain of the Sinai margin appears to be controlled by two main tectonic elements: (1) rejuvenated basement faults of the continent-ocean transition zone and (2) salt diapirism due to loading of Messinian evaporites with Nile-derived upper Tertiary clastics.

Stratigraphy and Structure of Atlantic Continental Margin in Bahama Platform Area: ABSTRACT

Approximately 2,100 km of 24-fold multichannel seismic reflection data reveal the subsurface geology of a large part of the continental margin in the Bahamas region. Discordance between the westward-dipping prebreakup sediments and the eastward-sloping basement along the edge of the Blake Plateau is interpreted as an effect of a splinter of continental margin derived from the African plate by a spreading-center jump in Middle Jurassic. Early rifting centered under the main part of the Blake Plateau became inactive, as a spreading-center jump shifted the active rift to east of the present Blake Escarpment along the Blake Spur magnetic anomaly. In the northern Florida Straits the data reveal that the prebreakup unconformity, underlain by Triassic-Upper Jurassic(?) arkosic volcaniclastics, extends from southern Florida to the western Bahama Banks. These volcaniclastics are associated with the intermediate nature rift-crust formed just prior to and during breakup of the North American and African continental plates. Back-reef platform deposits of limestones, dolomites, and evaporites of Late Jurassic to Albian age extend from the Blake-Bahama Escarpment under Florida. These covered what once was a huge megabank extending over a wider area than the present smaller isolated Bahama Banks. The formation of the modern Florida Straits and Bahama channels occurred in the Cenomanian transgression when the overall area deepened. Only on the present Bahama Banks and Florida platform did shallow-water carbonate deposition persist to maintain the End_Page 1670------------------------------ shallow-bank environments. Evidence of recurring scour by current erosion is found in the Florida Straits. Erosional events apparently occurred in middle Cenomanian, middle Paleocene, early-middle Eocene, and middle Oligocene, which are times of lower eustatic sea level. This evidence of Florida current scour indicates that the current was present as far back as the Cenomanian. A carbonate bank margin and reef complex has been present along the Bahama Escarpment since Middle Jurassic. Apparently these organic buildups seeded on structural relief on oceanic basement created during the spreading-center jump along the Blake Spur anomaly. The bank margin apparently has retreated at least 15 km below the Au unconformity. Active faulting at least through the Late Cretaceous and perhaps into the Tertiary, occurred along the Great Abaco fracture zone. These relatively young tectonic events, along with the post-Albian faults in Providence Channel, indicate interactions between the Atlantic and Caribbean plates and extensions of faulting far to the northeast of Cuba and the greater Antilles. End_of_Article - Last_Page 1671------------

Continental margin and lower slope structures of the Middle America Trench near Acapulco (Mexico)

The continental margin of southern Mexico north of the Tehuantepec Ridge and south of the Rivera Fracture Zone is a narrow crustal transition zone. The margin lacks a well-developed forearc basin and the active volcanic arc is subparallel to the trench and not clearly related in a normal fashion to the Benioff Zone. A geological survey of a small part of the margin 100 km south of Acapulco reveals significant variations in structure over short distances. Multi-channel seismic reflection data, sonobuoy refraction data and dredging indicate that the shelf and upper slope are underlain by continental crust. Geometry derived from the reflection data suggests that the continental crust is less than 10 km thick at its seaward termination. Migrated reflection data reveal structures on the lower slope and within trench axis turbidite fill which are interpreted as folds and thrust faults. Structures and landward dipping reflections suggest that the base of the slope is an accreting zone with trench turbidite fill being uplifted and emplaced. Both reflection and refraction data suggest the trench inner wall contains both slope deposits and uplifted trench deposits. There is no evidence of significant amounts of oceanic crust within the slope. Volume calculations reveal that some of the sedimentary rocks are being consumed along with the oceanic crust.

Seismic models of inner parts of the Euro-Asian continent and its margins

Analogies are drawn between continental and continental margin structures on the basis of seismic data on the crustal structure of Eurasia and its Atlantic margins. Crustal thinning from the inner parts of the continent to its margins is observed to be a general feature common to the formation of deep midland depressions and sedimentary basins of shelf zones. The latter are characterized by crustal thinning and its assimilation. These phenomena cannot be explained solely be sea-floor spreading effects in the process of active rifting and formation of oceanic crust. It appears that the main role in the formation of the margins in played by processes of mantle erosion in connection with heating at continental margins and with the migration of mantle material to the lower part of the crust.