Crustal structure of the ocean-continent transition at Flemish Cap : seismic refraction results

Author Posting. © American Geophysical Union, 2003. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 108, B11 (2003): 2531, doi:10.1029/2003JB002434.We conducted a seismic refraction experiment across Flemish Cap and into the deep basin east of Newfoundland, Canada, and developed a velocity model for the crust and mantle from forward and inverse modeling of data from 25 ocean bottom seismometers and dense air gun shots. The continental crust at Flemish Cap is 30 km thick and is divided into three layers with P wave velocities of 6.0–6.7 km/s. Across the southeast Flemish Cap margin, the continental crust thins over a 90-km-wide zone to only 1.2 km. The ocean-continent boundary is near the base of Flemish Cap and is marked by a fault between thinned continental crust and 3-km-thick crust with velocities of 4.7–7.0 km/s interpreted as crust from magma-starved oceanic accretion. This thin crust continues seaward for 55 km and thins locally to ~1.5 km. Below a sediment cover (1.9–3.1 km/s), oceanic layer 2 (4.7–4.9 km/s) is ~1.5 km thick, while layer 3 (6.9 km/s) seems to disappear in the thinnest segment of the oceanic crust. At the seawardmost end of the line the crust thickens to ~6 km. Mantle with velocities of 7.6–8.0 km/s underlies both the thin continental and thin oceanic crust in an 80-km-wide zone. A gradual downward increase to normal mantle velocities is interpreted to reflect decreasing degree of serpentinization with depth. Normal mantle velocities of 8.0 km/s are observed ~6 km below basement. There are major differences compared to the conjugate Galicia Bank margin, which has a wide zone of extended continental crust, more faulting, and prominent detachment faults. Crust formed by seafloor spreading appears symmetric, however, with 30-km-wide zones of oceanic crust accreted on both margins beginning about 4.5 m.y. before formation of magnetic anomaly M0 (~118 Ma).This work was supported by National Science Foundation grant OCE 9819053, the Danish Research Foundation (Danmarks Grundforskningsfond), and the Natural Science and Engineering Research Council of Canada. B. Tucholke also acknowledges support by the Henry Bryant Bigelow Chair in Oceanography at Woods Hole Oceanographic Institution

Seismic images and magnetic signature of the Late Jurassic to Early Cretaceous Africa-Eurasia plate boundary off SW Iberia

Over the last two decades numerous studies have investigated the structure of the west Iberia continental margin, a non-volcanic margin characterized by a broad continent–ocean transition (COT). However, the nature and structure of the crust of the segment of the margin off SW Iberia is still poorly understood, because of sparse geophysical and geological data coverage. Here we present a 275-km-long multichannel seismic reflection (MCS) profile, line AR01, acquired in E–W direction across the Horseshoe Abyssal Plain, to partially fill the gap of information along the SW Iberia margin. Line AR01 runs across the inferred plate boundary between the Iberian and the African plates during the opening of the Central Atlantic ocean. The boundary separates crust formed during or soon after continental rifting of the SW Iberian margin from normal seafloor spreading oceanic crust of the Central Atlantic ocean. Line AR01 has been processed and pre-stack depth migrated to show the tectonic structure of the crust across the palaeo plate boundary. This boundary is characterized by a 30–40-km-wide zone of large basements highs related to landward-dipping reflections, which penetrate to depths of 13–15 km, and it marks a change in the character of the basement structure and relief from east to west. In this study, we have used pre-stack depth migrated images, the velocity model of line AR01 and magnetic data available in the area to show that the change in basement structure occurs across the fossil plate boundary, separating African oceanic crust of the M series (M21–M16) to the west from the transitional crust of the Iberian margin to the east

Mechanisms of extension at nonvolcanic margins: Evidence from the Galicia interior basin, west of Iberia

We have studied a nonvolcanic margin, the West Iberia margin, to understand how the mechanisms of thinning evolve with increasing extension. We present a coincident prestack depth‐migrated seismic section and a wide‐angle profile across a Mesozoic abandoned rift, the Galicia Interior Basin (GIB). The data show that the basin is asymmetric, with major faults dipping to the east. The velocity structure at both basin flanks is different, suggesting that the basin formed along a Paleozoic terrain boundary. The ratios of upper to lower crustal thickness and tectonic structure are used to infer the mechanisms of extension. At the rift flanks (stretching factor, β ≤ 2) the ratio is fairly constant, indicating that stretching of upper and lower crust was uniform. Toward the center of the basin (β ∼ 3.5–5.5), fault‐block size decreases as the crust thins and faults reach progressively deeper crustal levels, indicating a switch from ductile to brittle behavior of the lower crust. At β ≥ 3.5, faults exhume lower crustal rocks to shallow levels, creating an excess of lower crust within their footwalls. We infer that initially, extension occurred by large‐scale uniform pure shear but as extension increased, it switched to simple shear along deep penetrating faults as most of the crust was brittle. The predominant brittle deformation might have driven small‐scale flow (≤40 km) of the deepest crust to accommodate fault offsets, resulting in a smooth Moho topography. The GIB might provide a type example of nonvolcanic rifting of cold and thin crust

Tectonosedimentary evolution of the deep Iberia-Newfoundland margins: Evidence for a complex breakup history

Most of the conceptual ideas concerning sedimentary architecture and tectonic evolution of deep rifted margins are based on either intracontinental rift basins or proximal margins, both of which underwent only small amounts of crustal thinning. In this paper, we investigate the tectonosedimentary and morphotectonic evolution related to continental breakup of the highly extended, deep Iberia-Newfoundland margins. Our results show that continental breakup is a complex process distributed in time and space. On the basis of mapping of dated seismic units and borehole data we are able to identify two major phases of extension. During a first phase, dated as Tithonian to Barremian (145–128 Ma), deformation is related to exhumation of mantle rocks; basins become younger oceanward, and fault geometry changes from upward to downward concave resulting in complex sedimentary structures and basin geometries. A second phase, dated as latest Aptian (112 Ma), overprints previously exhumed mantle and accreted juvenile oceanic crust over more than 200 km leading to the formation of basement highs. The observed complex breakup history challenges classical concepts of rifting and leads to new interpretations for the tectonosedimentary evolution of deep rifted margins.<br/

Architecture and tectonic evolution of non-volcanic margins: Present-day Galicia and ancient Adria.

International audienc