Iberia Abyssal Plain Research Articles

The continent-to-ocean transition in the Iberia Abyssal Plain

Abstract Conceptual models of magma-poor rifting are strongly based on studies of the nature of the basement in the continent-to-ocean transition of the Iberia Abyssal Plain, and suggest that exhumed mantle abuts extended continental crust. Yet, basement has only been sampled at a few sites, and its regional nature and the transition to seafloor spreading inferred from relatively low-resolution geophysical data are inadequately constrained. This uncertainty has led to a debate about the subcontinental or seafloor-spreading origin of exhumed mantle and the rift-related or oceanic nature of magmatic crust causing the magnetic J anomaly. Different interpretations change the locus of break-up by >100 km and lead to debate of the causative processes. We present the tomographic velocity structure along a 360-km-long seismic profile centered at the J anomaly in the Iberia Abyssal Plain. Rather than delineating an excessive outpouring of magma, the J anomaly occurs over subdued basement. Furthermore, its thin crust shows the characteristic layering of oceanic crust and is juxtaposed to exhumed mantle, marking the onset of magma-starved seafloor spreading, which yields the westward limit of an ~160-km-wide continent–ocean transition zone where continental mantle has been unroofed. This zone is profoundly asymmetric with respect to its conjugate margin, suggesting that the majority of mantle exhumation occurs off Iberia. Because the J anomaly is related to the final break-up and emplacement of oceanic crust, it neither represents synrift magmatism nor defines an isochron, and hence it poorly constrains plate tectonic reconstructions.

Morphostructure of the Galicia continental margin and adjacent deep ocean floor: From hyperextended rifted to convergent margin styles

A joint study and mapping of the morphological features and architecture of the Galicia margin based on high-resolution multibeam bathymetry data and multichannel seismic profiles, provides new insights of the margin morphostructure. Tectonic processes are the major control of the margin's morphology, where the imprint of the North Atlantic rifting and opening and later subduction of the Bay of Biscay and compressional events are still well preserved. The West Galicia margin, which is up to 350 km wide, is composed from east to the west of inner, transitional and outer sectors. The inner sector shows a graded upper slope strongly dissected by a system of submarine canyons and valleys that flow into the 170 km long and 40–60 km wide NW-SE Valle Inclán Valley. The transitional sector, which is up to 75 km wide, displays a smooth relief dominated by landslides and giant craters. Finally, the outer sector is characterised by several banks, N-S sub-parallel ridges and elongated valleys that project seaward over 200 km (i.e., Galicia Bank, Half-Graben, Deep Galicia margin and Peridotite Ridge domains). The North Galicia margin is characterised by a system of terraces up to 30 km wide, that form a stepped slope followed by an abrupt lower slope affected by large-scale rotational failures.The Galicia margin constitutes an exceptional area within the Atlantic margins for identifying different morphostructure styles that characterised the main stages of the margin's evolution along the Wilson cycle. The Galicia margin hosts the transition between two end-members of continental margin styles: a western hyperextended rifted margin (being the conjugate with the Newfoundland margin during the Late Jurassic) and a northern convergent margin (now fossilised) associated with the partial subduction of the southern sector of the Bay of Biscay underneath the North Iberian margin, which occurred in Paleocene-Eocene times. Thus, the morphostructure of the West Galicia margin still reflects the detached fault systems and tilted blocks that resulted from the Mesozoic North Atlantic rifting. Furthermore, the Coruña and Jean Charcot seamounts, which are adjacent to the Galicia margin, correspond to spreading ridges formed by the simultaneous opening of North Atlantic Ocean and the Bay of Biscay in the Upper Cretaceous. The rest of the terraced morphology of the North Galicia margin is derived from extensional faults caused by an overriding of the Iberian plate in relation to the subduction of the Bay of Biscay oceanic crust during Cenozoic times. Since the late Neogene, compressional tectonics resulting from the reorganisation of the Eurasia/Iberia and Africa plates is also reflected on the present-day morphology as the following: (i) an elevation of the Biscay Abyssal Plain up to 150 m over the Iberia Abyssal Plain; (ii) deep-seated landslides associated with an uplift of the Galicia Bank; and (iii) an orientation of submarine canyons along NW-SE strike slip faults.

Constraining lithosphere deformation modes during continental breakup for the Iberia–Newfoundland conjugate rifted margins

A kinematic model of lithosphere and asthenosphere deformation has been used to investigate lithosphere stretching and thinning modes during continental rifting leading to breakup and seafloor spreading. The model has been applied to two conjugate profiles across the Iberia–Newfoundland rifted margins and quantitatively calibrated using observed present-day water loaded subsidence and crustal thickness, together with observed mantle exhumation, subsidence and melting generation histories. The kinematic model uses an evolving prescribed flow-field to deform the lithosphere and asthenosphere leading to lithospheric breakup from which continental crustal thinning, lithosphere thermal evolution, decompression melt initiation and subsidence are predicted. We explore the sensitivity of model predictions to extension rate history, deformation migration and buoyancy induced upwelling. The best fit calibrated models of lithosphere deformation evolution for the Iberia–Newfoundland conjugate margins require; (1) an initial broad region of lithosphere deformation with passive upwelling, (2) lateral migration of deformation, (3) an increase in extension rate with time, (4) focussing of the deformation and (5) buoyancy induced upwelling. The model prediction of exhumed mantle at the Iberia–Newfoundland margins, as observed, requires a critical threshold of melting to be exceeded before melt extraction. The preferred calibrated models predict faster extension rates and earlier continental crustal separation and mantle exhumation for the Iberia Abyssal Plain–Flemish Pass conjugate margin profile than for the Galicia Bank–Flemish Cap profile to the north. The predicted N–S differences in the deformation evolution give insights into the 3D evolution of Iberia–Newfoundland margin crustal separation.

The peridotite ridge province in the southern Iberia Abyssal Plain: Seismic constraints revisited

AbstractThe west Iberia margin has become the type example of a magma‐starved margin in which extension during breakup led to the exhumation of mantle rocks apparently over a broad area. Much of our understanding of this process is based on geophysical and drilling data from the southern Iberia Abyssal Plain, where the seaward part of the region of exhumed mantle is composed of a series of margin‐parallel basement ridges. Here we analyze wide‐angle and normal‐incidence seismic data from a series of intersecting profiles across this peridotite ridge province and develop a three‐dimensional picture of its structure. Using these seismic data, we infer a location and age (magnetic anomaly M1; 125–127 Ma) of first‐formed oceanic crust in this region that are consistent with Ocean Drilling Program data and data from the conjugate margin. We infer that the peridotite ridge province is about 70 km wide with strong serpentinization (>75%) at the top of the basement reducing to much lower degrees (< 25%) around 2 km below. We map the geometry of some prominent reflectors on the landward margin of the peridotite ridge province that likely represent detachment faults and show that one of these has a domal structure similar to that of detachment faults at slow to ultraslow spreading mid‐ocean ridges.

Seismic evidence of exhumed mantle rock basement at the Gorringe Bank and the adjacent Horseshoe and Tagus abyssal plains (SW Iberia)

The Gorringe Bank is a gigantic seamount that separates the Horseshoe and Tagus abyssal plains offshore SW Iberia, in a zone that hosts the convergent boundary between the Africa and Eurasia plates. Although the region has been the focus of numerous investigations since the early 1970s, the lack of appropriate geophysical data makes the nature of the basement, and thus the origin of the structures, still debated. In this work, we present combined P-wave seismic velocity and gravity models along a transect that crosses the Gorringe Bank from the Tagus to the Horseshoe abyssal plains. The P-wave velocity structure of the basement is similar in the Tagus and Horseshoe plains. It shows a 2.5–3.0km-thick top layer with a velocity gradient twice stronger than oceanic Layer 2 and an abrupt change to an underlying layer with a five-fold weaker gradient. Velocity and density is lower beneath the Gorringe Bank probably due to enhanced fracturing, that have led to rock disaggregation in the sediment-starved northern flank. In contrast to previous velocity models of this region, there is no evidence of a sharp crust–mantle boundary in any of the record sections. The modelling results indicate that the sediment overlays directly serpentinite rock, exhumed from the mantle with a degree of serpentinization decreasing from a maximum of 70–80% under the top of Gorringe Bank to less than 5% at a depth of ∼20km. We propose that the three domains were originally part of a single serpentine rock band, of nature and possibly origin similar to the Iberia Abyssal Plain ocean–continent transition, which was probably generated during the earliest phase of the North Atlantic opening that followed continental crust breakup (Early Cretaceous). During the Miocene, the NW–SE trending Eurasia–Africa convergence resulted in thrusting of the southeastern segment of the exhumed serpentinite band over the northwestern one, forming the Gorringe Bank. The local deformation associated to plate convergence and uplift could have promoted pervasive rock fracturing of the overriding plate, leading eventually to rock disaggregation in the northern flank of the GB, which could be now a potential source of rock avalanches and tsunamis.

Geochemistry of Ocean Floor and Fore-arc Serpentinites: Constraints on the Ultramafic Input to Subduction Zones

We provide new insights into the geochemistry of serpentinites from mid-ocean ridges (Mid-Atlantic Ridge and Hess Deep), passive margins (Iberia Abyssal Plain and Newfoundland) and fore-arcs (Mariana and Guatemala) based on bulk-rock and in situ mineral major and trace element compositional data collected on drill cores from the Deep Sea Drilling Project and Ocean Drilling Program. These data are important for constraining the serpentinite-hosted trace element inventory of subduction zones. Bulk serpentinites show up to several orders of magnitude enrichments in Cl, B, Sr, U, Sb, Pb, Rb, Cs and Li relative to elements of similar compatibility during mantle melting, which correspond to the highest primitive mantle-normalized B/Nb, B/Th, U/Th, Sb/Ce, Sr/Nd and Li/Y among subducted lithologies of the oceanic lithosphere (serpentinites, sediments and altered igneous oceanic crust). Among the elements showing relative enrichment, Cl and B are by far the most abundant with bulk concentrations mostly above 1000 µg g−1 and 30 µg g−1, respectively. All other trace elements showing relative enrichments are generally present in low concentrations (µg g−1 level), except Sr in carbonate-bearing serpentinites (thousands of µg g−1). In situ data indicate that concentrations of Cl, B, Sr, U, Sb, Rb and Cs are, and that of Li can be, increased by serpentinization. These elements are largely hosted in serpentine (lizardite and chrysotile, but not antigorite). Aragonite precipitation leads to significant enrichments in Sr, U and B, whereas calcite is important only as an Sr host. Commonly observed brucite is trace element-poor. The overall enrichment patterns are comparable among serpentinites from mid-ocean ridges, passive margins and fore-arcs, whereas the extents of enrichments are often specific to the geodynamic setting. Variability in relative trace element enrichments within a specific setting (and locality) can be several orders of magnitude. Mid-ocean ridge serpentinites often show pronounced bulk-rock U enrichment in addition to ubiquitous Cl, B and Sr enrichment. They also exhibit positive Eu anomalies on chondrite-normalized rare earth element plots. Passive margin serpentinites tend to have higher overall incompatible trace element contents than mid-ocean ridge and fore-arc serpentinites and show the highest B enrichment among all the studied serpentinites. Fore-arc serpentinites are characterized by low overall trace element contents and show the lowest Cl, but the highest Rb, Cs and Sr enrichments. Based on our data, subducted dehydrating serpentinites are likely to release fluids with high B/Nb, B/Th, U/Th, Sb/Ce and Sr/Nd, rendering them one of the potential sources of some of the characteristic trace element fingerprints of arc magmas (e.g. high B/Nb, high Sr/Nd, high Sb/Ce). However, although serpentinites are a substantial part of global subduction zone chemical cycling, owing to their low overall trace element contents (except for B and Cl) their geochemical imprint on arc magma sources (apart from addition of H2O, B and Cl) can be masked considerably by the trace element signal from subducted crustal components.

Seismic constraints on the three-dimensional geometry of low-angle intracrustal reflectors in the Southern Iberia Abyssal Plain

Several lines of evidence suggest that simple shear rifting of the continental crust, in the form of low-angle detachment faulting, occurred during the final stages of continental breakup between West Iberia and the Grand Banks. The primary evidence for such faulting is the occurrence of low-angle, high amplitude reflectors within the basement adjacent to the ocean– continent transition zone. Here we present a series of intersecting, depth migrated seismic reflection profiles that image one such reflector, the H-reflector, located on the southern edge of Galicia Bank. ‘H’ lies beneath several boreholes drilled during ODP Legs 149 and 173, in a region where the oceanward extent of extended continental crust steps at least 150 km westward from its location in the southern Iberia Abyssal Plain to its location off the relatively shallow Galicia Bank. In our profiles ‘H’ appears to define a surface that extends over a region of at least 200 km2 and that dips down ?19? to the north, towards Galicia Bank. The profiles show that a close affinity exists between ‘H’ and the most seaward continental crust. Based on geophysical data and ODP drilling results, we infer that the basement above ‘H’ is composed of continental crust deformed by extensional faults into a series of wedge-shaped blocks and thin slivers. These basement wedges have a complex 3-D geometry. ‘H’ rises to the basement surface on a number of the seismic profiles and appears to define locally the oceanward extent of continental fault blocks.

Compressional structures on the West Iberia rifted margin: controls on their distribution

Abstract The West Iberia margin is a magma-poor rifted margin that resulted from Jurassic to Cretaceous polyphase rifting leading to the opening of the North Atlantic Ocean. The Mesozoic rift structures were overprinted by two compressive tectonic events during Eocene and Miocene times resulting from collision between Iberia, Europe and Africa. The effects of these compressive tectonic events are expressed by faults and folds within the post-rift sedimentary sequence. We mapped and studied these Cenozoic deformation structures throughout the Southern Iberia Abyssal Plain (40°–41°N, 11°–13°W) on the basis of an extensive dataset of time migrated seismic profiles acquired by various academic institutions. Acoustic basement has also been analysed on the basis of its seismic aspect, in order to test potential relationships with the distribution of the post-rift sedimentary deformation. Our observations lead to three major conclusions concerning the deformation affecting the post-rift sediments in the Southern Iberia Abyssal Plain: (1) the deformation occurs within the zone of exhumed continental mantle and not at its transition to continental or oceanic crust; (2) it is localized within a zone overlying basement with well-defined seismic characteristics; and (3) it is closely related to the major topographic features observed in the ocean–continent transition. The localization of the deformation within the zone of exhumed continental mantle and not at its boundaries to the adjacent oceanic and continental crust suggests that the limits between the different types of crust are transitional rather than sharp. Our results show that the zone of exhumed continental mantle represents the weakest zone within the margin that is preferentially deformed during initial convergence. At higher convergence rates, this zone may coincide with the location of a future subduction.

Exhumed mantle‐forming transitional crust in the Newfoundland‐Iberia rift and associated magnetic anomalies

Transitional zones located between Iberia and North America formed during continental rifting and mostly consist of exhumed mantle. In this study we show that ages of exhumed mantle at Ocean Drilling Program (ODP) sites 1068 and 1070 in the Iberia Abyssal Plain and site 1277 in the Newfoundland Basin are similar to ages determined from magnetic lineations created by serpentinization during mantle exhumation. On the basis of paleomagnetic and geological data and a comparison with a fossil ocean‐continent transition in the Alps, we envisage a first episode of mantle serpentinization during which a strong component of magnetization was acquired followed by a second episode occurring at the contact with cold seawater, and which only affects the upper tens of meters of the exhumed mantle. The inversion of magnetic data (Euler deconvolution) shows that magnetic sources are N‐S trending horizontal cylindrical bodies located within the highly serpentinized upper crust. Therefore the serpentinization process is able to produce magnetic lineations formed in a similar way to those formed by seafloor spreading. Within transition zones, sequences of magnetic anomalies can provide information concerning the timing of the emplacement of crust, but not on its nature (oceanic versus exhumed mantle). This discovery enables us to date the exhumation of mantle rocks in transition zones and allows kinematic reconstructions of the final stages of continental rifting. During rifting the deep distal continental margins and the adjacent transitional zones in the Newfoundland‐Iberia rift system were formed by ultraslow extension from early Berriasian to late Valanginian–early Hauterivian and by slow extension from early Hauterivian to the late Aptian–early Albian boundary. Therefore transitional zones share many similarities with slow and ultraslow spreading midoceanic ridges.

Characterizing the Galicia Bank‐Southern Iberia Abyssal Plain rifted margin segment boundary using multichannel seismic and ocean bottom seismometer data

We present multichannel seismic reflection and ocean bottom seismometer reflection/refraction data from ISE‐9, a margin‐parallel, north–south oriented profile ∼200 km west of the Portuguese coast. ISE‐9 images the boundary between two distinct segments of the Iberia nonvolcanic rifted margin: Galicia Bank (GB) and the Southern Iberia Abyssal Plain (SIAP). The bathymetric contrast between GB (2 km depth) and SIAP (4–5 km depth) spans only 25 km. The crustal thickness transition, however, spans 137 km, from 13–18 km thick beneath GB to <2 km thick beneath SIAP. We define this crustal thickness transition as the segment boundary. Crustal structure along the segment boundary, tilted blocks bounded by normal faults, is surprisingly similar to crustal structure observed along orthogonal, east–west profiles of the Iberia margin. The apparent north–south extension is similar in magnitude to previously calculated east–west extension, implying an overall northeast–southwest extension. However, paleoreconstructions and rift basin orientations constrain lithospheric extension to a nearly east–west direction. We speculate that north–south extension is limited to the crust and is caused by large‐scale mass wasting sometime between the Tithonian and Valanginian. This rotational slump spans the 137‐km‐wide modern segment boundary, emplacing GB continental crust directly onto exhumed, serpentinized mantle of SIAP. Palinspastic reconstruction restores the southern edge of the blocks to coincide within <10 km of a steep Moho transition and a near‐vertical fault which extends through the slump and offsets Moho. We interpret this location as the preslump segment boundary that accommodated transform motion between the two rifted margin segments.

Seismic velocity structure of the rifted margin of the eastern Grand Banks of Newfoundland, Canada

We present a compressional seismic velocity profile of the crust of the eastern margin of the Grand Banks of Newfoundland, Canada. This velocity model was obtained by a tomographic inversion of wide‐angle data recorded on a linear array of 24 ocean bottom seismometers (OBSs). At the landward side, we imaged a crustal thickness of 27 km in Flemish Pass and beneath Beothuk Knoll, which is thinner than the 35‐km‐thick crust of the central Grand Banks. We therefore assume that the eastern rim of the Grand Banks stretched uniformly by 25%. Farther seaward, the continental crust tapers rapidly beneath the continental slope to ∼6 km thickness. In the distal margin we find a 60‐km‐wide zone with seismic velocities between 5.0 and 6.5 km s−1 that thins to the southeast from 6 to 2 km, which we interpret as highly extended continental crust. Contrary to other seismic studies of the margins of the Grand Banks, we find seismic velocities of 8 km s−1 and higher beneath this thin crustal layer in the continent‐ocean transition. We conclude that mantle was locally emplaced at shallow levels without significant hydration from seawater or serpentinized mantle was removed along a décollement in the final stages of continental rifting. The outer edge of highly extended continental crust borders a 25‐km‐wide zone where seismic velocities increase gradually from 6.3 km s−1 just below the top of acoustic basement to 7.7 km s−1 at 5 km below basement. We interpret this area as a relatively narrow zone of exhumed and serpentinized continental mantle. Seaward, we imaged a thin and laterally heterogeneous layer with a seismic velocity that increases sharply from 5.0 km s−1 in basement ridges to 7.0 km s−1 at its base, overlying mantle velocities between 7.8 and 8.2 km s−1. We interpret this area as unroofed mantle and very thin oceanic crust that formed at an incipient, magma‐starved, ultraslow spreading ridge. A comparison of the conjugate rifted margins of the eastern Grand Banks and the Iberia Abyssal Plain show that they exhibit a similar seaward progression from continental crust to mantle to oceanic crust. This indicates that before continental breakup, rifting exhumed progressively deeper sections of the continental lithosphere on both conjugate margins. A comparison between the continent‐ocean transition of the Grand Banks and Flemish Cap shows that the final phase of continental rifting and the formation of the first oceanic crust required more time at the Grand Banks margin than at the southeastern margin of Flemish Cap.

Crustal structure across the Grand Banks-Newfoundland Basin Continental Margin - I. Results from a seismic refraction profile

SUMMARY A P-wave velocity model along a 565-km-long profile across the Grand Banks‐Newfoundland Basin rifted margin is presented. Continental crust ∼36 km thick beneath the Grand Banks is divided into upper (5.8‐6.25 km s −1 ), middle (6.3‐6.53 km s −1 ) and lower crust (6.77‐ 6.9 km s −1 ), consistent with velocity structure of Avalon zone Appalachian crust. Syn-rift sediment sequences 6‐7 km thick occur in two primary layers within the Jeanne d’Arc and the Carson basins (∼ 3k m s −1 in upper layer; ∼ 5k m s −1 in lower layer). Abrupt crustal thinning (Moho dip ∼35 ◦ ) beneath the Carson basin and more gradual thinning seaward forms a 170-km-wide zone of rifted continental crust. Within this zone, lower and middle continental crust thin preferentially seawards until they are completely removed, while very thin (<3 km) upper crust continues ∼60 km farther seawards. Adjacent to the continental crust, high-velocity gradients (0.5‐1.5 s −1 ) define an 80-km-wide zone of transitional basement that can be interpreted as exhumed, serpentinized mantle or anomalously thin oceanic crust, based on its velocity model alone. We prefer the exhumed-mantle interpretation after considering the non-reflective character of the basement and the low amplitude of associated magnetic anomalies, which are atypical of oceanic crust. Beneath both the transitional basement and thin (<6 km) continental crust, a 200-km-wide zone with reduced mantle velocities (7.6‐7.9 km s −1 ) is observed, which is interpreted as partially (<10 per cent) serpentinized mantle. Seawards of the transitional basement, 2- to 6-km-thick crust with layer 2 (4.5‐ 6.3 km s −1 ) and layer 3 (6.3‐7.2 km s −1 ) velocities is interpreted as oceanic crust. Comparison of our crustal model with profile IAM-9 across the Iberia Abyssal Plain on the conjugate Iberia margin suggests asymmetrical continental breakup in which a wider zone of extended continental crust has been left on the Newfoundland side.

Crustal structure across the Grand Banks-Newfoundland Basin Continental Margin - II. Results from a seismic reflection profile

SUMMARY A P-wave velocity model along a 565-km-long profile across the Grand Banks‐Newfoundland Basin rifted margin is presented. Continental crust ∼36 km thick beneath the Grand Banks is divided into upper (5.8‐6.25 km s −1 ), middle (6.3‐6.53 km s −1 ) and lower crust (6.77‐ 6.9 km s −1 ), consistent with velocity structure of Avalon zone Appalachian crust. Syn-rift sediment sequences 6‐7 km thick occur in two primary layers within the Jeanne d’Arc and the Carson basins (∼ 3k m s −1 in upper layer; ∼ 5k m s −1 in lower layer). Abrupt crustal thinning (Moho dip ∼35 ◦ ) beneath the Carson basin and more gradual thinning seaward forms a 170-km-wide zone of rifted continental crust. Within this zone, lower and middle continental crust thin preferentially seawards until they are completely removed, while very thin (<3 km) upper crust continues ∼60 km farther seawards. Adjacent to the continental crust, high-velocity gradients (0.5‐1.5 s −1 ) define an 80-km-wide zone of transitional basement that can be interpreted as exhumed, serpentinized mantle or anomalously thin oceanic crust, based on its velocity model alone. We prefer the exhumed-mantle interpretation after considering the non-reflective character of the basement and the low amplitude of associated magnetic anomalies, which are atypical of oceanic crust. Beneath both the transitional basement and thin (<6 km) continental crust, a 200-km-wide zone with reduced mantle velocities (7.6‐7.9 km s −1 ) is observed, which is interpreted as partially (<10 per cent) serpentinized mantle. Seawards of the transitional basement, 2- to 6-km-thick crust with layer 2 (4.5‐ 6.3 km s −1 ) and layer 3 (6.3‐7.2 km s −1 ) velocities is interpreted as oceanic crust. Comparison of our crustal model with profile IAM-9 across the Iberia Abyssal Plain on the conjugate Iberia margin suggests asymmetrical continental breakup in which a wider zone of extended continental crust has been left on the Newfoundland side.

Lithospheric Mantle Evolution during Continental Break-Up: The West Iberia Non-Volcanic Passive Margin

Ultramafic (lherzolites, metasomatized peridotites, harzburgites, websterites and clinopyroxenites) and mafic igneous (basalts, dolerites, diorites and gabbros) rocks exposed at the sea-floor along the West Iberia continental margin represent a rare opportunity to study the transition zone between continental and oceanic lithosphere. The igneous rocks are enriched in LREE, unlike North Atlantic MORB. A correlation between their 143Nd/144Nd isotopic composition and Ce/Yb ratio suggests that they originate from mixing between partial melts of a depleted mantle source similar to DMM and of an enriched mantle source which may reside within the continental lithosphere. Clinopyroxenes and amphiboles in the ultramafic rocks are LREE depleted and have flat HREE patterns with concentrations higher than those of abyssal peridotites. Clinopyroxenes in the harzburgites are less LREE depleted but have lower HREE concentrations. The clinopyroxenes in the Galicia Bank (GB) lherzolites have radiogenic Nd (143Nd/144Nd ranging from 0·512937 to 0·513402) and unradiogenic Sr (87Sr/86Sr ranging from 0·702100 to 0·702311) isotopic ratios similar to, or higher than, DMM (Depleted MORB Mantle) whereas the clinopyroxenes in the Iberia Abyssal Plain websterites have low-Nd isotopic compositions (143Nd/144Nd ranging from 0·512283 to 0·512553) with high-Sr isotopic ratios (87Sr/86Sr ranging from 0·704170 to 0·705919). Amphiboles in Galicia Bank lherzolites and diorites have Nd–Sr isotopic compositions (143Nd/144Nd from 0·512804 to 0·512938 and 87Sr/86Sr from 0·703243 to 0·703887) intermediate between those of the clinopyroxenes from the Galicia Bank and the Iberia Abyssal Plain, but similar to the clinopyroxenes in the 5100 Hill harzburgite (143Nd/144Nd = 0·512865 and 87Sr/86Sr = 0·703591) and to the igneous rocks (143Nd/144Nd ranging from 0·512729 to 0·513121 and 87Sr/86Sr ranging from 0·702255 to 0·705109). The major and trace element compositions of cpx in the Galicia Bank spinel lherzolites provide evidence for large-scale refertilization of the lithospheric upper mantle by MORB-like tholeiitic melts. The associated harzburgites did not undergo partial melting during the rifting stage, but, in earlier times, probably during, or even before, the Hercynian orogeny. Iberia Abyssal Plain websterites are interpreted as high-pressure cumulates formed in the mantle. Their high Sm/Nd ratios (from 0·43 to 0·67) coupled with very low-Nd isotopic compositions are best explained by a two-stage history: formation of the cumulates from the percolation of enriched melts long before the rifting, followed by low-degree partial melting of the pyroxenites, accounting for their LREE depletion. This last event probably occurs during the rifting episode, 122 Myr ago. The isotopic heterogeneities observed in the ultramafic rocks of the Iberia margin were already present at the time of the rifting event. They reflect a long and complex history of depletion and enrichment events in an old part of the mantle, and provide strong arguments for a sub-continental origin of this part of the upper mantle.

Constraining the rate and extent of mantle serpentinization from seismic and petrological data: implications for chemosynthesis and tectonic processes

AbstractWe used seismic velocity as a proxy for serpentinization of the mantle, which occurred beneath thinned but laterally continuous continental crust during continental break up, prior to opening of the Atlantic Ocean. The serpentinized sub‐continental mantle is now exhumed, beneath the Iberia Abyssal Plain and was accessed by scientific drilling on Ocean Drilling Program legs 149 and 173. Chromatographic modelling of kinetically limited transport of the serpentinization front yields a front displacement of 2197 ± 89 m, a time‐integrated fluid flux of 1098 ± 45 m3 m−2 and a Damköhler number of 6.0 ± 0.2. Whether either surface reaction or chemical transport limit the rate of reaction, we calculate timescales for serpentinization of approximately 105–106 years. This yields time‐average fluid flux rates for H2O, entering and reacting with the mantle, of 60–600 mol m−2 a−1 and for CH4, produced as a by‐product of oxidation of Fe++ to magnetite and exiting the mantle, of 0.55–5.5 mol m−2 a−1. This equates to a CH4‐flux of 0.18–1.8 Tg a−1 for coeval serpentinization of the mantle that was exhumed west of Iberia. This represents 0.03–0.3% of the present‐day annual CH4‐flux from all sources and a higher fraction of pre‐anthropogenic (lower) CH4 levels. CH4 released by serpentinization at or beneath the seafloor could provide substrate for biological chemosynthesis and/or promote gas‐hydrate formation. Finally, noting its volumetric extent and rapidity (&lt;106 years), we interpret serpentinization to be a reckonable component of tectonic processes, contributing both diapiric and expansional forces and helping to ‘lubricate’ extensional processes. Given its anisotropic permeability, actively deforming serpentinite might impede melt migration which may be of interest, given the apparent lack of melt in some rifted margins.

MANTLE EXHUMATION PROCESSES IN THE OCEAN CONTINENT TRANSITION OF MAGMA-POOR RIFTED MARGINS

MANTLE EXHUMATION PROCESSES IN THE OCEAN CONTINENT TRANSITION OF MAGMA-POOR RIFTED MARGINS

A wide ocean-continent transition along the south-west Australian margin: first results of the MARGAU/MD110 cruise

Syn-rift exhumation of mantle rocks in a continental breakup zone was highlighted along the present-day west Iberian passive margin [e.g. Boillot et al., 1988, 1995; Whitmarsh et al., 1995, 2001; Beslier et al., 1996; Brun and Beslier, 1996; Boillot and Coulon, 1998; Krawczyk et al., 1996; Girardeau et al., 1998] and along the fossil Tethyan margins [e.g. Froitzheim and Manatschal, 1996; Manatschal and Bernoulli, 1996; Marroni et al., 1998; Muntener et al., 2000; Desmurs et al., 2001]. Along the west Iberian margin, serpentinized peridotite and scarce gabbro and basalt lay directly under the sediments, over a 30 to 130 km-wide transition between the thinned continental crust and the first oceanic crust [Girardeau et al., 1988, 1998; Kornprobst and Tabit, 1988; Boillot et al., 1989; Beslier et al., 1990, 1996; Cornen et al., 1999]. The formation of a wide ocean-continent transition (OCT), mostly controlled by tectonics and associated with an exhumation of deep lithospheric levels, would be an essential stage of continental breakup and a characteristic of magma-poor passive margins. The southwest Australian margin provides an opportunity to test and to generalize the models proposed for the west Iberian margin, as both margins present many analogies. The south Australian margin formed during the Gondwana breakup in the Mesozoic, along a NW-SE oblique extension direction [Willcox and Stagg, 1990]. From north to south, the continental slope is bounded by (1) a magnetic quiet zone (MQZ) where the nature of the basement is ambiguous [Talwani et al., 1979; Tikku and Cande, 1999; Sayers et al., 2001], (2) a zone where the basement shows a rough topography associated with poorly expressed magnetic anomalies [Cande and Mutter, 1982; Veevers et al., 1990; Tikku and Cande, 1999; Sayers et al., 2001], and which is the eastward prolongation of the Diamantina Zone, and (3) an Eocene oceanic domain. The continental breakup zone is believed to be located near or at the southern edge of the MQZ [Cande and Mutter, 1982; Veevers et al., 1990; Sayers et al., 2001]. Breakup is dated at 125 Ma [Stagg and Willcox, 1992], 95 ± 5 Ma [Veevers, 1986] or at 83 Ma [Sayers et al., 2001], and followed by ultra-slow seafloor spreading until the Eocene (43 Ma), and fast spreading afterwards [Weissel and Hayes, 1972; Cande and Mutter, 1982; Veevers et al., 1990; Tikku and Cande, 1999]. The western end of the margin (fig. 1) is starved and bounded in the OCT by basement ridges where peridotite, gabbro and basalt were previously dredged [Nicholls et al., 1981]. Altimetry data [Sandwell and Smith, 1997] show that some of these ridges are continuous over 1500 km along the OCT of the south Australian margin and of the conjugate Antarctic margin. The objectives of the MARGAU/MD110 cruise (May-June 1998; [Royer et al., 1998]; fig. 2) were to define the morpho-structure and the nature and evolution of the basement in the SW Australian OCT. An area of 180 000 km2 was explored with swath bathymetry. Gravimetric data (11382 km) were simultaneously recorded whereas few single channel seismic (1353 km) and magnetic (5387 km) data were obtained due to technical difficulties. Crystalline basement rocks, made of varied and locally well-preserved lithologies, were dredged at 11 sites located on structural highs.

Exhumed upper mantle within the ocean‐continent transition on the northern West Iberia margin: Evidence from prestack depth migration and total tectonic subsidence analyses

The West Iberia continental margin contains an enigmatic north‐south ridge of serpentinized peridotite located within the ocean‐continent transition. We used prestack depth migration and total tectonic subsidence analysis to constrain the basement type adjacent to the ridge in order to learn more about its emplacement. Prestack depth migration was performed on segments of the nine east‐west lines from the Iberia Seismic Experiment (ISE 97), and total tectonic subsidence analyses were performed on the depth sections. The seismic data and subsidence analyses are consistent with exhumed upper mantle occurring immediately landward of the ridge, suggesting that the ridge is located within a broad zone of exhumed mantle that has been serpentinized. The subsidence analyses also provide minimum bounds on the thickness of the serpentinized layer, which extends from the zone of exhumed mantle landward under thinned continental crust. Exhumed upper mantle has previously been identified in the wider ocean‐continent transition to the south in the southern Iberia Abyssal Plain, and we conclude that similar but narrower exposures occur to the north in the Galicia Bank area. While upper mantle appears to have been tectonically exhumed along all the ISE 97 lines, it does not always form a distinct basement ridge. There is no well‐defined ridge on three seismic profiles in the middle of the ISE 97 survey. Where it is well‐developed, the peridotite ridge parallels a deeply‐penetrating, west‐dipping normal fault, and the three middle lines appear to represent a transfer zone where this normal fault is not well‐developed.

The hydrothermal plumbing of a serpentinite-hosted detachment: evidence from the West Iberia non-volcanic rifted continental margin

This study documents the critical role of structurally-induced fluid flow during the evolution of the footwall succession to a major low-angle normal (detachment) fault, drilled by the Ocean Drilling Program leg 173, Site 1068 beneath the Southern Iberia Abyssal Plain. The fault zone comprises (carbonate-altered, rodingitized, and albitized) metabasite-rich sedimentary breccias and serpentinized mantle peridotites. The brittle infrastructure of the detachment consists of mineralized high dilation breccias, and meshes of mineralized extensional and shear veins, that root into chlorite and serpentine cataclasite, and gouge. The fault rocks are underlain by cohesive serpentinite that shows kernel textures, indicative of volume expansion accompanying serpentinization of peridotite. The texture is disrupted and offset by small-scale fractures and faults. The distribution of serpentine polytypes, carbonates, Fe–Ni alloys, sulfides, oxides, and other silicate phases, varies across the fault zone in patterns consistent with mineralization, and replacement, from solutions derived from two end member components: seawater, and CH 4-bearing calcium-hydroxide enriched hydrothermal solutions. The latter form when heated seawater reacts with peridotite to form serpentinite at low water to rock ratios. Serpentine mineral chemistries indicate that fracture-controlled serpentinite recrystallization and replacement occurred at various fO 2, aSiO 2 and Ca 2+ conditions. In places this also involved mild prograde thermal events. The serpentinite also hosts tochilinite–valleriite group minerals and aragonite, both are interpreted as indicators of sea water incursions into the upper reaches of the detachment. To account for the evidence of coeval hydrothermal mineralization and displacements across the detachment we relate hydrothermal discharge to the buffering of high pore fluid pressures by fault slip. Localized sources of high fluid pressures at depth are attributed to serpentinization of peridotite around the fault that promotes changes to solution mass density, exothermic reactions and swelling pressures. Sealing of the fault between the serpentinization front and the top of the detachment results from hydrothermal mineralization, alteration, and serpentine gouges. Hydrothermal discharges from the detachment accompanying shear failure allow for variable mixing between the hydrothermal solutions and seawater, and post-slip convective draw down of seawater into the detachment. It is suggested that the latter may have been limited in duration by ongoing mineralization leading to the restoration of the fault seal. Concomitant serpentinization around the detachment at depth provides scope for cyclic hydrothermal discharges and fault slip.

Tectonic nature and seismic response of top‐basement detachment faults in magma‐poor rifted margins

Top‐basement detachment faults (TBDF) represent exhumed segments of downward‐concave detachments accommodating tens of kilometers of offset. TBDFs are key structures of magma‐poor rifted margins, but are difficult to identify in seismic profiles, because they coincide with top‐basement reflections. We have constructed realistic seismic models of an Alpine example, which were used to simulate full marine seismic surveys and to identify the seismic fingerprint of TBDFs in CMP‐stacked and time‐migrated sections. Minor extensional allochthons, which locally cap TBDFs, give rise to pronounced diffractions, which are in stark contrast to continuous reflections from uncapped segments of TBDFs. In time‐migrated synthetic sections we find a continuous and constant amplitude reflection alternating with a laterally discontinuous reflection of variable amplitude. These diagnostics were also identified in the Lusigal 12 seismic profile imaging the Hobby High detachment, a TBDF in the Iberia Abyssal Plain. Using these diagnostics, small extensional allochthons and high‐angle faults cutting the Hobby High detachment are observed in Lusigal 12 and the entire sedimentary cover can be shown to be postrift.