Problematic plate reconstruction

Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 5 (2012): 676-677, doi:10.1038/ngeo1596.As has been previously proposed, Bronner et al. suggest that opening of the rift between Newfoundland and Iberia involved exhumation of mantle rocks until 112 million years ago, subsequent seafloor spreading, and crustal thickening along the high-amplitude J magnetic anomaly by magma that propagated from the Southeast Newfoundland Ridge area. Conventionally, the anomalous magnetism and basement ridges associated with the J anomaly north of the Newfoundland-Gibraltar Fracture Zone are thought to have formed about 125 million years ago at chron M0 (Fig. 1a), although the crust probably experienced some later magmatic overprinting. The M0 age would make their formation simultaneous with that of the similar J anomaly and basement ridges (the J Anomaly Ridge and Madeira Tore Rise) along the Mid-Atlantic Ridge to the south and place them within a zone of exhumed mantle in the Newfoundland-Iberia rift. In contrast, Bronner et al. propose that the J anomaly and associated basement ridges were formed by later magmatism (about 112 million years ago) that marked the end of mantle exhumation in the rift. We argue here that constraints from plate tectonic reconstructions render this possibility untenable.2013-04-0

Turbidity Currents, Submarine Landslides and the 2006 Pingtung Earthquake off SW Taiwan

Submarine landslides or slumps may generate turbidity currents consisting of mixture of sediment and water. Large and fast-moving turbidity currents can incise and erode continental margins and cause damage to artificial structures such as telecommunication cables on the seafloor. In this study, we report that eleven submarine cables across the Kaoping canyon and Manila trench were broken in sequence from 1500 to 4000 m deep, as a consequence of submarine landslides and turbidity currents associated with the 2006 Pingtung earthquakes offshore SW Taiwan. We have established a full-scale scenario and calculation of the turbidity currents along the Kaoping canyon channel from the middle continental slope to the adjacent deep ocean. Our results show that turbidity current velocities vary downstream ranging from 20 to 3.7 and 5.7 m/s, which demonstrates a positive relationship between turbidity current velocity and bathymetric slope. The violent cable failures happened in this case evidenced the destructive power of the turbidity current to seafloor or underwater facilities that should not be underestimated

Multi-disciplinary investigation of fluid seepage on an unstable margin: The case of the Central Nile deep sea fan

We report on a multidisciplinary study of cold seeps explored in the Central Nile deep-sea fan of the Egyptian margin. Our approach combines in situ seafloor observation, geophysics, sedimentological data, measurement of bottom-water methane anomalies, pore-water and sediment geochemistry, and 230Th/U dating of authigenic carbonates. Two areas were investigated, which correspond to different sedimentary provinces. The lower slope, at ∼ 2100 m water depth, indicates deformation of sediments by gravitational processes, exhibiting slope-parallel elongated ridges and seafloor depressions. In contrast, the middle slope, at ∼ 1650 m water depth, exhibits a series of debris-flow deposits not remobilized by post-depositional gravity processes. Significant differences exist between fluid-escape structures from the two studied areas. At the lower slope, methane anomalies were detected in bottom-waters above the depressions, whereas the adjacent ridges show a frequent coverage of fractured carbonate pavements associated with chemosynthetic vent communities. Carbonate U/Th age dates (∼ 8 kyr BP), pore-water sulphate and solid phase sediment data suggest that seepage activity at those carbonate ridges has decreased over the recent past. In contrast, large (∼ 1 km2) carbonate-paved areas were discovered in the middle slope, with U/Th isotope evidence for ongoing carbonate precipitation during the Late Holocene (since ∼ 5 kyr BP at least). Our results suggest that fluid venting is closely related to sediment deformation in the Central Nile margin. It is proposed that slope instability leads to focused fluid flow in the lower slope and exposure of ‘fossil’ carbonate ridges, whereas pervasive diffuse flow prevails at the unfailed middle slope

Structure across the northeastern margin of Flemish Cap, offshore Newfoundland from Erable multichannel seismic reflection profiles: evidence for a transtensional rifting environment

We present the results from processing and interpreting nine multichannel seismic reflection lines collected during the 1992 Erable experiment over the northeastern margin of Flemish Cap offshore Newfoundland. These lines, combined into five cross-sections, provide increased seismic coverage over this lightly probed section of the margin and reveal tectonically significant along-strike variations in the degree and compartmentalization of crustal thinning. Similar to the southeastern margins of Flemish Cap and the Grand Banks, a transitional zone of exhumed serpentinized mantle is interpreted between thinned continental and oceanic crust. The 25 km wide transitional zone bears similarities to the 120 km wide transitional zone interpreted as exhumed serpentinized mantle on the conjugate Irish Atlantic margin but the significant width difference is suggestive of an asymmetric conjugate pair. A 40–50 km wide zone of inferred strike-slip shearing is interpreted and observed to extend along most of the northeastern margin of Flemish Cap. Individual shear zones (SZs) may represent extensions of SZs and normal faults within the Orphan Basin providing further evidence for the rotation and displacement of Flemish Cap out of Orphan Basin. The asymmetry between the Flemish Cap and Irish conjugate pairs is likely due in large part to the rotation and displacement of Flemish Cap which resulted in the Flemish Cap margin displaying features of both a strike-slip margin and an extensional margin

Structure of the Northern Bay of Bengal offshore Bangladesh: Evidences from new multi-channel seismic data

New multi-channel seismic data were acquired in the northern part of the Bay of Bengal and at the northernmost termination of the 90°E Indian Ridge offshore Bangladesh. This survey was coupled with a seismic refraction experiment indicating this offshore basin is here floored by a thinned (15 km thick) continental crust, injected by Mesozoic volcanism. This attenuated continental crust is interpreted as formed during Gondwana super-continent fragmentation during a syn-rift period. The dominant tectonic pattern is marked by NE-SW trending tilted blocks filled by syn-rift sediments clearly identified on seismic profiles. The uppermost part of this continental crust (3–4 km thick) shows a complex assemblage of dipping reflectors and west-facing tilted blocks injected by volcanic build-ups. The lower crustal sequence (11–12 km thick) does not reveal significant reflectors. This syn-rift fabric is attributed to the Mesozoic up to the Early Cretaceous by correlation with published seismic data along the eastern coast of India. Opposite normal faults vergency on the Indian and Burma sides indicate an asymmetrical rifting (simple shear) creating a wide COT on the Burma side and a short COT on the opposite Indian side, a geometry typical of continental crust stretching. This crustal fabric is overlain disconformably by a thin reflector attributed to the Late Cretaceous-Paleocene pelagic sequence deposited during India/Bay of Bengal drift phase, before the Cenozoic-India Asia collision marked by thick clastic sedimentation associated with the Ganges delta southward progradation. Below this delta, this Mesozoic rift closes axially and is affected by the incipient Late Miocene shortening of the Shillong Plateau. The NE-SW fabric of this attenuated crust might be traced southward to 15°N, close to magnetic chron 34, where steady state spreading of the Central Indian Ocean occurred

Plate kinematic implications of Atlantic equatorial fracture zone trends

We present a plate kinematic evolution of the South Atlantic which is based largely on the determination of the equatorial fracture zone trends between the African and South American continental margins. Four main opening phases are dated by oceanic magnetic anomalies, notably MO, A34, and A13, and are correlated with volcanism and tectonic events on land around the South Atlantic Ocean. The Ceara and Sierra Leone rises are probably of oceanic origin and were created 80 m.y. ago or later in their present-day positions with respect to South America and Africa

Thermal Regime of the Northern Bay of Biscay Continental Margin in the Vicinity of DSDP Sites 400 to 402

Ten conventional oceanic surface heat flow measurements were made over the northern margin of the Bay of Biscay in the vicinity of the DSDP Sites 400 to 402 during the R/V Suroit - SU 01 (December 1975) and R/v Jean Charcot - CH 66 (February 1976) cruises of CNEXO. These measurements, complemented by the heat flow determination made at Site 402 during Leg 48 of the Deep Sea Drilling Project (Erickson et al; this volume) provide information on the thermal regime of the margin.The main observation is that the regional heat flux over the margin is substantially lower than over the adjacent Western European continental area.We suggest in this report that the observed heat flux contrast provides constraints on the debatable nature of the crustal thinning processes under the margin. [NOT CONTROLLED OCR

Dominant Structural Trends on the Western Continental Margin of Iberia: Implications on Initial Rifting

Preparatory to IPOD site surveys for Leg 47B, a provisional bathymetric map was established (three sheets, scaled 1:500,000) for the area West of the Iberian peninsula. This report presents a new version of this bathymetric map (scaled 1:2,400,000 at 41°N) which incorporates data obtained since 1974. [NOT CONTROLLED OCR