Crustal transition between continental and oceanic domains along the North Iberian margin from wide angle seismic and gravity data (original) (raw)
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Subduction-related structures in the North Iberian Margin
Journal of Geophysical Research: Solid Earth, 1997
The oblique convergence of Eurasia and Iberia since the Early Cretaceous, caused the formation of the Pyrenean intracontinental collisional orogen in the east, and progressed to continent-ocean collision with subduction of the Bay of Biscay oceanic crust beneath the North Iberian Margin in the west. Two deep multichannel seismic profiles (IAM-12 and ESCIN-4), integrated with gravity modeling and other geological and geophysical data, provide the crustalscale architecture of this margin and its tectonic evolution during the convergence. The North Iberian Margin is modeled with a south or south-southeast dipping oceanic crust beneath the outer part of the continental shelf. Mesozoic basins on the shelf were inverted during the Tertiary, and compressional activity continued until recent times in the ESCIN-4 section, while a shallower, probably Neogene age basin is subjected to active recent erosion in the IAM-12 section. In the oceanic areas, a marginal trough deepens and widens toward the east as a result of the regional east dip of the oceanic basement. The accretionary prism increases in size from west to east (18-56 km), and its internal structure and morphology varies along strike. The prism is buried by postconvergence sediments in both sections and in the iAM-12 section appears to have been active at least during Lutetian to Burdigalian times. The crustal-scale structure of the North Iberian Margin is that of an arrested subduction zone in which a remnant oceanic basin was being consumed near two continental plates that collided obliquely. 1 a). In the east, continent-continent collision resulted in northward Underthrusting of the Iberian crust beneath Europe [ Choukroune and ECORS Team, 1989], and construction of the Pyrenean orogen during Tertiary times. In contrast, the western prolongation of the margin evolved as a continent-ocean collision with thrusting of Iberia over the Bay of Biscay oceanic crust during Paleocene-Eocene times [Le Pichon et al., 1971; Boillot et al., 1979], and possibly late Eocene to early Miocene times [Roest and Srivastava, 1991 ]. The amount of convergence along the margin appears to be small, with only 125-150 km of shortening calculated for the Pyrenean section [Muhoz, 1992; Verges et al., 1995]. The amount of underthrusting of the southern Bay of Biscay varies from a maximum of 120 km [e.g., Le Pichon and Sibuet, 1971; Grimaud et al., 1982] to a minimum of 40 km [$rivastava et al., 1990b]. While the crustal-scale structure of the collisional zone is well known in the Pyrenees [Choukroune and ECORS Team, 1989; Roure et al., 1989; Mufioz, 1992], it is still largely unknown in the Bay of Biscay. Recent studies in the Cantabrian Mountains indicate that Tertiary compression resulted in thrusting and uplift of the Variscan basement over the northern edge of the Duero foreland basin [Alonso et al., 1996], and a crustal root exists beneath the uplifted Variscan basement [Pulgar et al., 1996; Fernandez-Viejo, 1997]. This paper presents the crustal architecture of the North Iberian Margin based on two parallel, N-S trending, multichannel deep reflection seismic profiles of the
The western Iberia Margin: a geophysical and geological overview
1996
This paper presents a general overview of the geology and geophysics of western Iberia, and in particular of the western Portuguese Margin. The links between the onshore and offshore geology and geophysics are especially emphasized. The west Iberia Margin is an example of a nonvolcanic rifted margin. The Variscan basement exposed on land in Iberia exhibits strikeslip faults and other structural trends, which had an important effect on the development, in time and space, of subsequent rifting of the continental margin and even perhaps influences the present-day offshore seismicity. The margin has had a long tectonic and magmatic history from the Late Triassic until the present day. Rifting first began in the Late Triassic; after about 70 Ma, continental separation began in the Tagus Abyssal Plain. Continental breakup then appears to have progressively migrated northwards, eventually reaching the Galicia Bank segment of the margin about 112 Ma. Although there is onshore evidence of magmatism throughout the period from the Late Triassic until 130 Ma and even later, this was sporadic and of insignificant volume. Important onshore rift basins were formed during this period. Offshore, the record is complex and fragmentary. An ocean/ continent transition, over 150 km wide, lies beyond the shelf edge and is marked on its western side by a peridotite ridge and thin oceanic crust characterized by seafloor spreading anomalies. Rifted fault blocks are recognized within the ocean/continent transition along the whole margin. Mostly, they merge westwards into a transitional zone where the basement often has low relief of unknown origin, and linear magnetic anomalies parallel the seafloor spreading anomalies. There is indirect geophysical evidence that this zone is underlain by intrusions in the lower crust. The most plausible, but not the only, explanation seems to be that this part of the ocean/continent transition consists of fragments of magmatically disrupted and intruded thinned continental crust. The margin also underwent important postrift compression in Eocene and Miocene time, as demonstrated by folding and nondeposition or erosion of abyssal plain sediments. The Eocene deformation is clearly visible off Galicia but the Miocene deformation is dominant in the rest of the margin, where it may have overprinted the former compressional episode. The margin is still seismically active in its southern part at the present day, mainly because of its proximity to the Azores-Gibraltar plate boundary.
Geophysical Journal International, 1990
The western continental margin of the Iberian peninsula has the characteristics of a rifted non-volcanic margin with half-graben and tilted fault blocks seen in several places. The ocean-continent boundary (OCB) is therefore expected to be where thinned continental crust and oceanic crust are juxtaposed, as at many similar margins worldwide. It is particularly useful to locate the OCB off western Iberia in order to constrain the pre-rift fit of North America to Iberia and, by implication, the shape of the proto-Bay of Biscay. The fit is only marginally constrained by sea-floor spreading magnetic anomalies because anomaly 34 is believed to be far to the west of the OCB and it is even possible that all older oceanic crust was created during the Cretaceous constant polarity interval. The best way to distinguish oceanic crust from thinned continental crust appears to be the crustal seismic velocity structure. Therefore in 1986 a series of seismic refraction lines was shot parallel to, and normal to, the continental margin. These lines enabled us to bracket the location of the OCB. A further constraint on the location was obtained by modelling an east-west magnetic profile which included the enigmatic J-anomaly. This anomaly can be explained as either just pre-anomaly MO or as part of the Cretaceous constant polarity interval, depending on whether spreading began about 127 or after 118 Myr ago, respectively. The evidence favours the former explanation. Lastly the depth to acoustic basement was contoured from a compilation of seismic reflection profiles. This indicated a new fracture zone at 41"15'N which offsets the OCB. A few key reflection profiles also suggest that the OCB can be identified by an abrupt landward step-down in acoustic basement. We conclude that the OCB in the eastern Iberia Abyssal Plain lies between 12'10' and 12'30'W and has a trend just east of north. This westerly location is consistent with recent estimates of the location of the OCB off the Grand Banks but brings into question the proposed location at about 11"W of the OCB in the Tagus Abyssal Plain.
Tectonics, 2018
The magma‐poor rifted continental margin of Galicia has an extremely complex structure. Its formation involved several rifting episodes that occurred ultimately during the early Cretaceous near a ridge triple junction, which produced a change in the orientation of the main structures in its transition to the north Iberia margin. In addition, there is a superimposed partial tectonic inversion along its northwest and northern border which developed from the Late Cretaceous to at least Oligocene times. The present study integrates a large volume of new geophysical information (mainly marine gravity data and 2‐D seismic reflection profiles) to provide insights on the formation of this rift system and on the development of its later inversion. The combined interpretation and modeling of this data enable the presentation of a new crustal and structural domain map for the whole Galicia margin. This includes the rift domains related to the extreme thinning of the crust and the lithospheric ...
Journal of Geophysical Research: Solid Earth, 2014
We present a new classification of geological domains at the Africa-Eurasia plate boundary off SW Iberia, together with a regional geodynamic reconstruction spanning from the Mesozoic extension to the Neogene-to-present-day convergence. It is based on seismic velocity and density models along a new transect running from the Horseshoe to the Seine abyssal plains, which is combined with previously available geophysical models from the region. The basement velocity structure at the Seine Abyssal Plain indicates the presence of a highly heterogeneous, thin oceanic crust with local high-velocity anomalies possibly representing zones related to the presence of ultramafic rocks. The integration of this model with previous ones reveals the presence of three oceanic domains offshore SW Iberia: (1) the Seine Abyssal Plain domain, generated during the first stages of slow seafloor spreading in the NE Central Atlantic (Early Jurassic); (2) the Gulf of Cadiz domain, made of oceanic crust generated in the Alpine-Tethys spreading system between Iberia and Africa, which was coeval with the formation of the Seine Abyssal Plain domain and lasted up to the North Atlantic continental breakup (Late Jurassic); and (3) the Gorringe Bank domain, made of exhumed mantle rocks, which formed during the first stages of North Atlantic opening. Our models suggest that the Seine Abyssal Plain and Gulf of Cadiz domains are separated by the Lineament South strike-slip fault, whereas the Gulf of Cadiz and Gorringe Bank domains appear to be limited by a deep thrust fault located at the center of the Horseshoe Abyssal Plain.
2006
DSDP Site 398 is located on the West Iberian passive continental margin, south of Galicia Bank, 20 km to the south of Vigo Seamount (Figure 1). The kinematic evolution of the Iberian plate is closely controlled by the relative motions of the African, European, and North American plates. A study based on magnetic data and land data was undertaken to better define the relative motion of Iberia with respect to these main plates (Sibuet et al., in preparation). Trapped between these plates, microcontinents such as Galicia Bank and Flemish Cap (located between Iberia and North America) assumed their own relative motions with respect to Iberia and North America during the early evolution of the North Atlantic. Consequently, the initial oceanic circulation between small basins corresponding to the Bay of Biscay and the North Atlantic west of Iberia and Europe is closely controlled by relative positions and movements of these microcontinents. Site 398 paleoenvironment data, therefore, are o...