Iberia Abyssal Plain Research Articles

Hydrothermal minerals, fluid flow and brittle/semi-brittle deformation: A comparison between the Skyttorp-Vattholma Fault Zone (SVFZ), south-central Sweden and the Iberia Abyssal Plain (ODP Leg 173, Hole 1067)

Hydrothermal minerals, fluid flow and brittle/semi-brittle deformation: A comparison between the Skyttorp-Vattholma Fault Zone (SVFZ), south-central Sweden and the Iberia Abyssal Plain (ODP Leg 173, Hole 1067)

Azimuthal seismic anisotropy in a zone of exhumed continental mantle, West Iberia margin

SUMMARY P-wave azimuthal seismic anisotropy of the uppermost mantle has been shown to indicate directions of extension, both recent and fossil, in oceanic and continental settings. We have determined the P-wave anisotropy of a zone of exhumed continental mantle beneath the southern Iberia Abyssal Plain, as a possible indication of the initial direction of separation of Iberia from North America in a region where there are no magnetic lineations or fracture zone lineations adjacent to the margin with which to constrain this direction. Over 10 000 airgun shots were recorded by 11 ocean-bottom seismometers/hydrophones from which 24 900 arrivals that had traversed unaltered/weakly serpentinized upper mantle were picked. Complete 360 ◦ azimuthal coverage was attained between the shots and receivers for ranges up to 55 km. P-wave raypaths through the zone of exhumed continental mantle were ray-traced through a velocity model that incorporated the 3-D relief of the seabed and acoustic basement. Time residuals with respect to this model were fitted using an azimuthally dependent function. Small anisotropy (<7 per cent) with a fast direction of 143 ◦ was determined to exist between 3.1 and 6.7 km below the top of the basement. Although the fast direction suggests that the post-breakup mantle stretching between Iberia and North America was northwest‐southeast, this hypothesis is hard to reconcile with other observations. Instead, we conclude that motion along concave-downward faults during the exhumation of the upper mantle could have been sufficient to change an initially roughly east‐west margin-normal fast direction to the 143 ◦ azimuth observed today. The degree of serpentinization was estimated to decrease from ∼20 to 0 per cent between 3 and 7 km depth into the basement in the zone of exhumed continental mantle. The degree of upper-mantle anisotropy is similar to that observed in other oceanic and continental settings where similar degrees of serpentinization are inferred from mean P-wave velocities.

Gabbroic Pegmatite Intrusions, Iberia Abyssal Plain, ODP Leg 173, Site 1070: Magmatism during a Transition from Non-volcanic Rifting to Sea-floor Spreading

the unroofed mantle grows younger outboard, just as is the case for On the Iberia Abyssal Plain (Ocean Drilling Program Site 1070), normal, volcanic sea floor. gabbroic pegmatites and related rocks (127 ± 4 Ma, U–Pb zircon) intrude upper mantle that was subsequently exposed and serpentinized during Early Cretaceous non-volcanic rifting. The pegmatites include a 3–4 m dike or sill (the ‘main’ pegmatite), numerous dikelets of

Petrochemistry of serpentinized peridotite from the Iberia Abyssal Plain (ODP Leg 173): its character intermediate between suboceanic and sub-continental upper mantle

Abstract Highly serpentinized peridotites derived from the upper mantle beneath a non-volcanic continental margin were sampled from the southern Iberia Abyssal Plain (Ocean Drilling Program Leg 173), and the primary mantle minerals were analysed for major and trace elements to petrologically characterize the upper mantle. The Cr -number (= Cr/(Cr + Al) atomic ratio) of the peridotite spinels varies widely from 0.1 to 0.6. The Na 2 O content of clinopyroxene is rather constant, 0.5–0.8 wt %. The chondrite-normalized rare earth element (REE) patterns have light REE (LREE) depleted convex-upward shapes. The LREE/HREE (heavy REE) ratio in clinopyroxene is constant irrespective of the degree of melt extraction of the sample as measured by the Cr -number of spinel. The trend of the peridotites’ mineral chemistry is different from both the simple melt extraction and the general mantle metasomatic trends. The geochemical character of the Iberia Abyssal Plain peridotite is intermediate between those of abyssal peridotites and peridotite xenoliths from continental regions. These geochemical features, especially for the trace elements in clinopyroxene, are rather similar to those in peridotite xenoliths from arcs. This chemical trend is probably the result of ‘open-system melting’, which involves melting simultaneously with enrichment of trace elements by the influx agent.

Rifting along non-volcanic passive margins: stratigraphic and seismic evidence from the Mesozoic successions of the Alps and western Iberia

Abstract The paper examines aspects of the sedimentology and stratigraphy of rift basins that evolved in deep marine settings near the ocean-continent transition. It focuses on the applicability of a low-angle extensional detachment model developed in the Alps to the West Iberian margin, and on difficulties of objectively identifying syn-rift stratigraphic intervals in both areas. The paper examines evidence obtained from Ocean Drilling Program holes drilled in the Iberia Abyssal Plain. Despite the fact that all the holes were sited above highs in the acoustic basement and so did not penetrate a complete sedimentary record of rifting, they do provide some constraints on the age and mechanism of rifting. We suggest that published identifications of syn-rift intervals in distal basins off West Iberia and in the Southern Alps have not demonstrated, using objective criteria, the occurrence of syn-rift stratigraphic intervals. They have, therefore, probably overestimated the duration of rifting by as much as 20 Ma. The absence of syn-rift related stratal divergence towards fault footwalls may be due to resedimentation of syn-rift sediments towards basin centres, lack of significant hanging-wall rotation along flat detachment faults, or the syn-rift interval being too thin to resolve on seismic data. The syn-rift episode beneath the deep Galicia margin postdates Tithonian-Berriasian shallow-water carbonates, and predates Late Valanginian turbiditic sediments. Drilling results from the Iberia Abyssal Plain suggest a similar age because Tithonian siliciclastic mudrocks are overlain by Berriasian pelagic chalks. It seems likely that in both regions rifting lasted for &lt;5 Ma, probably from late Berriasian to early Valanginian. At Site 1068 in the Iberia Abyssal Plain, the interpretation from seismic reflection data, of a low-angle detachment dipping about 10° west, was confirmed by drilling, which revealed sedimentary and tectonic breccias containing clasts of lower-crustal rocks overlying a fault zone below which occurs serpentinized periodotite showing a downward decrease in deformation. At least 20.5 km of displacement is interpreted to have occurred along this fault, but it is not accompanied by large-amplitude, rift-related topography. This paradox is resolved if the detachment developed as a deepening-downwards, rolling-hinge fault.

The evolution of amphibolites from Site 1067, ODP Leg 173 (Iberia Abyssal Plain): Jurassic rifting to the Pyrenean compression

Abstract During ODP Leg 173 (April–May 1997) five new sites (1065, 1067–1070) were drilled at the ocean-continent transition (OCT) zone off the West Iberia margin in the Iberia Abyssal Plain. At Site 1067 the 92 m of cored amphibolites were subdivided into three units based on textural criteria. Unit 1 consists of highly foliated and folded amphibolites and acidic gneiss concordant to and folded along with the foliation of the amphibolite. Unit 2, in the middle part of the section, consists of brecciated amphibolites. Unit 3, at the bottom of the hole, is a weakly deformed zone where magmatic textures are observed in the amphibolites and in associated anorthosites. The amphibolites contain tschermakitic to magnesio-hornblende amphibole, plagioclase, zircon, apatite ± titanite ± Fe-oxide ± quartz. The acidic gneiss consists of garnet, plagioclase, alkali-feldspar, quartz, biotite and zircon. Chlorite, sericite and ilmenite occur as secondary phases. The metamorphic evolution of the amphibolite and acidic gneiss started under amphibolitefacies conditions (Stage I: 670 ± 40°C and 7 ± 1 kbar). Further exhumation took place through low-grade amphibolite-facies (Stage II: 550 ± 50°C and 5.5 ± 1 kbar) to green-schist-facies (Stage III: &lt; 500°C and &lt; 3 kbar) conditions contemporaneous with the development of ductile structures. The late metamorphic evolution of the amphibolite ended under ocean-floor conditions. Oxygen isotope ratios and studies of fluid inclusion indicate that a magmatic water-rich fluid in equilibrium with the igneous protolith predominated during Stage I. During Stages II and IIII low-temperature water-rich fluids of metamorphic origin predominated, with a probable contribution of sea water. Apatite fission-track dating indicates that the amphibolites record two thermal excursions below 120°C. The first took place at c. 113–100 Ma and could be related to Cretaceous rifting. The second was brief, and so did not anneal older tracks; it occurred between 75 and 55 Ma and could be related to the Pyrenean orogeny.

The role of detachment faulting in the formation of an ocean-continent transition: insights from the Iberia Abyssal Plain

Abstract The Iberia Abyssal Plain segment of the West Iberia margin was drilled during Ocean Drilling Program Legs 149 and 173 and has been extensively studied geophysically. We present new microstructural investigations and new age data. These, together with observed distribution of upper- and lower-crustal and mantle rocks along the ocean-continent transition suggest the existence of three detachment faults, one of which was previously unrecognized. This information, together with a simple kinematic inversion of the reinterpreted seismic section Lusigal 12, allows discussion of the kinematic evolution of detachment faulting in terms of the temporal sequence of faulting, offset along individual faults, and thinning of the crust during faulting. Our study shows that the detachment structures recognized in the seismic profile became active only during a final stage of rifting when the crust was already considerably thinned to c. 12 km. The total amount of extension accommodated by the detachment faults is of the order of 32.6 km corresponding to a β factor of about two. During rifting, the mode of deformation changed oceanwards. Initial listric faulting led to asymmetric basins, accommodating low amounts of extension, and was followed by a situation in which the footwall was pulled out from underneath a relatively stable hanging wall accommodating high amounts of extension. Deformation along the latter faults resulted in a conveyor-belt type sediment accumulation in which the exhumed footwall rocks were exposed, eroded and redeposited along the same active fault system.

Tethyan flysch-type benthic foraminiferal assemblages in the North Atlantic: cretaceous to Palaeogene deep water agglutinated foraminifers from the Iberia abyssal plain (ODP Leg 173)

Leg 173 recovered in Holes 1067A, 1068A, 1069A, and 1070A a sequence of deep-water claystones and turbidites, that contains in its hemipelagic units unusually well-preserved “flysch-type” agglutinated foraminiferal assemblages ranging from the Turonian to the early Oligocene. The species composition of these deep water agglutinated foraminiferal assemblages shows close relations to that of Tethyan assemblages known from the Campo de Gibraltar flysch, the North African Rifian and Tellian flysch units, the Italian Northern Appennines, the Alps and the Carpathians and exhibits distinct differences to the composition of abyssal agglutinated assemblages from other North— and Central Atlantic ODP-Sites. This biogeographic distribution pattern provides evidence for a widely opened paleo-Gibraltar-Seaway at least until the early Oligocene. The complete Late Cretaceous to Palaeogene succession of Carpathian agglutinated foraminiferal zones is observed and can be directly calibrated to the shipboard calcareous nannoplankton zonation of redeposited calcareous sediments. Due to the low sediment overburden of maximum 720 m the preservation of agglutinated foraminifers is almost pristine with little secondary cementation and allows new insight in the wall structure of diagenetically almost unaltered flysch type agglutinated foraminiferal species.

The role of syn-rift magmatism in the rift-to-drift evolution of the West Iberia continental margin: geophysical observations

Abstract The presence of a well-defined ocean-continent transition (OCT) and the absence of large volumes of extrusive or intrusive rocks on the West Iberia margin make it a good place to investigate how the largely amagmatic rifting and break-up of continental lithosphere evolves into oceanic crust produced by magmatic sea-floor spreading. In the southern Iberia Abyssal Plain there is a broad OCT with a characteristic seismic and magnetic character, distinct from both thinned continental crust and normal oceanic crust, which supports the notion that it consists predominantly of exhumed and serpentinized mantle. Interpretations of magnetic and seismic data indicate that on average only small amounts of syn-rift melt exist within the OCT. Isolated, probably margin-parallel, intrusive melt bodies are scattered within the eastern part of the OCT well beneath the top of acoustic basement. Within the western part of the OCT, closer to unambiguous sea-floor spreading magnetic anomalies, such bodies were later(?) emplaced at higher levels and more closely together in the basement until eventually sea-floor spreading began. The evidence does not support the hypothesis that ultraslow sea-floor spreading can explain the magnetic anomalies observed within the wider parts of the West Iberia OCT, where the OCT evolution is best resolved.

Petrology and geochemistry of exhumed peridotites and gabbros at non-volcanic margins: ODP Leg 173 West Iberia ocena-continent transition zone

Abstract Ultramafic and mafic rocks recovered at Holes 1068A and 1070A were drilled during Leg 173 of the Ocean Drilling Program (ODP) in the ocean-continent transition zone of the Iberia Abyssal Plain. Peridotites show contrasting petrographic characteristics. Hole 1068A peridotites are fine grained and show a well-defined high-temperature foliation marked by elongated pyroxene as well as aligned spinels. Hole 1068A peridotites are strongly serpentinized. Hole 1070A peridotites are coarse grained and show little evidence of high-temperature foliation. The degree of serpentinization is lower and relicts of silicate minerals are preserved. In both sets of recovered material, spinels show a wide range of composition and suggest a complex magmatic evolution. Gabbros dykes, which are found only in Hole 1070A, are very coarse grained and are locally sheared and/or crushed. Magmatic amphiboles are kaersutites and Ti-rich tschermakites that are partially replaced by hornblende and actinolite, and are associated with plagioclase of intermediate composition. Peridotites and pyroxenite have low TiO 2 , Al 2 O 3 and CaO contents in carbonate-free samples. Ni and Cr contents fall into the upper-mantle array. On the other hand, gabbros have relatively high TiO 2 and V contents reflecting modal ilmenite, and suggesting that they are relatively differentiated. This paper presents the very first geochemical data on platinum group elements (PGE) of peridotites and gabbros from passive margins. Peridotites and gabbros show low PGE (25.83 ppb and 1.44 ppb), Pd (2.75 ppb and 0.15 ppb), Pd/Ir ratios (1.45 and 1.3) and mafic index. Pyroxenite has the highest PGE (27.97 ppb), Pd/Ir (19.87) and Pt/Ir (10.25). Interelemental correlation and observation of PGE-bearing sulphide phases suggest that the PGE are hosted by single sulphide phases. From a PGE point of view, extraction of magmas involved very PGE-depleted liquids similar to gabbroic veins cutting the peridotites at Hole 1070A. Partial melting is interpreted as occurring just before oceanic accretion. Geochemical attributes suggest that the peridotites belong to the Ronda and Beni Bousera peridotitic depleted end-member clan. Thus they are believed to be of subcontinental origin. Deformation and retrograde metamorphism of peridotites and gabbros are consistent with exhumation in a rift environment post-dating the 120 Ma magmatic stage.

Anomalous melt production after continental break-up in the southern Iberia Abyssal Plain

Abstract Recent geophysical work and Ocean Drilling Program drilling in the southern Iberia Abyssal Plain have indicated that, in a transition zone up to 170 km wide between thinned continental crust and oceanic crust, the basement consists of serpentinized peridotite mantle with sparse mafic intrusive or extrusive rocks. There is no evidence for the addition of significant magmatic material to the stretched continental crust landward of this zone during the last phase of rifting, whereas seaward of this zone, where the half-spreading rate is about 10 mm a −1 , the crust rapidly reaches a thickness of c. 6 km, which is normal for Atlantic oceanic crust. Models of melt generation during pure shear, finite-duration continental rifting can successfully reproduce the observed absence of significant syn-rift magmatism on, within and beneath the thinned continental crust if the rifting episode is longer than 10–20 Ma. However, for normal mantle potential temperatures, such models predict significant melt generation in the transition zone seaward of the thinned continental crust even for rift durations longer than 20 Ma. Restricted melting beneath the transition zone might be explained partly by lateral heat loss to the adjacent continental lithosphere, by anomalously low mantle potential temperatures at break-up time, or by depth-dependent stretching such that the observed infinite stretching factor for the crust is not representative of the lithosphere as a whole. An additional mechanism for restricted melt production involves a transitional state between the end of continental extension and the onset of steady-state sea-floor spreading, during which mantle upwelling is less focused than at normal oceanic spreading centres.

Geodynamic models of continental extension and the formation of non-volcanic rifted continental margins

Abstract Finite-element models of continental rifting show that formation of non-volcanic rifted margins may be the result of extension of a rheologically homogeneous crust. In such circumstances lithosphere necking does not become well developed until late in the rift history, delaying the onset of decompression melting in the asthenosphere until the last 10% of the rifting episode. This result is robust over a broad range of mantle temperatures, margin geometries, and extension rates. A cool mantle is not required, so the models are able to account for the production of oceanic crust at the end of amagmatic rifting episodes. The duration of the syn-rift melting episode is most sensitive to changes in extension rate, with higher extension rates leading to shorter periods of melt production. The duration of the rifting episode is controlled by extension rate and initial crustal thickness, and the geometry of the margin after continental break-up is controlled by initial crustal thickness and the distribution of pre-existing rheological heterogeneity in the crust. The model results are generally compatible with the dimensions and extension rates of rifted continental margins across the globe, and provide a particularly good fit to the evolution of the Iberia Abyssal Plain margin.

Palaeomagnetic and rock magnetic results from serpentinized peridotites beneath the Iberia Abyssal Plain

Abstract A palaeomagnetic and rock magnetic study has been performed on serpentinized peridotite rocks recovered at Ocean Drilling Program (ODP) Leg 149 and 173 sites, off the west coast of Portugal (Sites 897, 899 and 1070). Stable components of magnetization are revealed after detailed thermal and alternating field demagnetization. Results from a series of rock magnetic measurements with advanced instrumentation all corroborate the demagnetization behaviour and show that (titano)magnetites and maghemite are present in the peridotites. At Leg 149 and 173 sites, the inclinations of characteristic magnetization in the ‘fresher’ lower part of the serpentinized peridotite section show a predominantly reversed polarity in a depth zone of c. 25 m, which is compatible with a reversed event (probably correlated with marine Anomaly M0 at c. 121 Ma) before the Cretaceous Long Normal Superchron (83–120 Ma) and is incompatible with a Holocene field direction. In contrast, the inclinations of samples from the more ‘altered’ upper part are predominantly normal. In view of the polarity of magnetization identified from the overlying Cretaceous and Tertiary sediments and the newest interpretation of the drilling results for the Iberia margin, our results suggest that the Iberian peridotites were emplaced during Aptian-Barremian time and recorded the middle Cretaceous geomagnetic field during Anomaly M0 time ( c. 121 Ma). Our data are consistent with new radiometric dates for the Iberian peridotites at Leg 173 drill sites.

Synchronous palynological changes in early pleistocene sediments off new jersey and Iberia, and a possible paleoceanographic explanation

Palynomorphs record the establishment of modern conditions in the subtropical North Atlantic during the early Pleistocene. Prior to ‐1.4 Ma, muds on both the New Jersey shelf and the Iberia Abyssal Plain contained relatively few terrestrial palynomorphs, and had a dinocyst flora rich in Operculodinium israelianum and other dinocyst taxa recording warmer surface waters than at present (e.g., Tectatodinium pellitum, Lingulodinium machaerophorum, and Polysphaeridium zoharyi). Over a span of ‐250 ka, this palynological assemblage was succeeded by one rich in pollen and with a dinocyst flora similar to the “modern”; flora, i.e. rich in Operculodinium centrocarpum, Bitectatodinium tepikiense, Spiniferites spp. (predominantly S. ramosus), and Brigantedinium spp. (predominantly B. simplex). The synchronous palynological changes in such different geological settings in the eastern and western subtropical North Atlantic are attributed to global climatic deterioration and the expansion of ice sheets in the northern hemisphere. Climatic cooling increased the velocity of the Gulf Stream and other surface currents in the subtropical gyre, causing the gyre to contract and pull away from the continents. Glacioeustatically lowered sea levels also exposed a large percentage of the continental shelf areas, and together with other bathymetric highs like the Charleston Bump, deflected the gyre boundary currents (like the Gulf Stream) offshore. Consequently, surface waters of polar origin were able to penetrate between the warm waters of the gyre and the North American continent north of Cape Hatteras in the west, and Iberia in the east, around 1.4 Ma. This paleoceanographic change increased the area of “neritic”; sedimentation at mid latitudes in the North Atlantic, allowing greater terrestrial influx beyond the shelfbreak.

Tectonic implications of exposure of lower continental crust beneath the Iberia Abyssal Plain, Northeast Atlantic Ocean: Geophysical evidence

We present new seismic data from a basement high beneath the southern Iberia Abyssal Plain that appears, from the latest Ocean Drilling Program (ODP) results and indirectly from various seismic observations, to consist of lower continental crustal material. This is a unique opportunity to use seismic profiles and other geophysical evidence to investigate the tectonic process which led to the exhumation of these rocks. We infer that the lower crust was exhumed at the seafloor and then uplifted toward the end of rifting. Our results lead to the following new observations: low‐angle detachment faults, previously reported along the east‐west Lusigal‐12 profile across the southern Iberia Abyssal Plain, penetrate both the 3–10 km thick continental crust and, unusually, the uppermost mantle. Processing of new multichannel seismic reflection profiles across the basement high on which ODP Sites 900, 1067, and 1068 are located reveals that the high is bounded by a previously unsuspected steep, landward dipping normal fault on its east flank. Basement cores from the above ODP sites also reveal that the high is not capped by the previously predicted early syn‐rift sediment but by rocks from the lower continental crust. These unexpected observations are incorporated into a new tectonic model of the development of this part of the west Iberia margin which is also consistent with other geophysical observations. A novel feature of this model is the proposal that the seaward edge of the continental crust is thinned to 3–6 km by a currently poorly understood process. The model implies three stages of deformation: (1) lithospheric extension, principally by symmetric pure shear, which leads to a ∼10 km thick crust in which the lower crust is largely absent over the axial zone and the crust‐mantle boundary forms a shear zone, (2) further thinning and then dissection of the most distal continental crust by seaward dipping, low‐angle normal faults, (3) inception of a high‐angle, landward dipping normal fault that offsets the tectonized crust‐mantle boundary and uplifts the lower crust to the crest of the basement high on which ODP Sites 900, 1067, and 1068 were drilled.

Moving reactive interfaces and fractal carbonate replacement patterns in serpentinites: evidence from the southern Iberia Abyssal Plain

AbstractSerpentinized ultramafic rocks recovered from beneath the southern Iberia Abyssal Plain (Ocean Drilling Programme Leg 173) provide the first record of fractal carbonate replacement patterns in a serpentinite. The patterns are expressed as microscopic branching aggregates (clusters) of aragonite disseminated throughout the serpentinites. Aragonite growth was the final mineralization event. The aragonite diminishes rapidly in quantity from an essential to a trace component of the serpentinite over a distance of ∼40 m from a normal fault. Decreasing abundance of aragonite away from the normal fault links the growth of the carbonate to the multistage hydrothermal mineralization associated with the fault.Aragonite clusters are concentrated in picrolite, where they are interwoven with colloid-sized chrysotile, and show fractal growth habits. Areas adjacent to the clusters are sites of Mg enrichment of the serpentine medium relative to aragonite-free picrolite. It is interpreted that the aragonite clusters result from incursions of reactive seawater solutions through fine-scale pore structures in and around the fault in response to pressure gradients emanating from active tectonism. Cluster growth is interpreted to be a percolation phenomenon and provides a novel source of information on the nature of fine-scale reactive fluid flow, pore-space connectivity, and carbonate replacement processes in serpentinites.

138–121 Ma asthenospheric magmatism prior to continental break-up in the North Atlantic and geodynamic implications

Along the Galicia and Gorringe banks and in the Iberia Abyssal Plain of the North Atlantic, unroofed sub-continental mantle fills the gap between ‘true’ oceanic crust and the continental crust margin. These lithospheric peridotites are intruded by gabbros and dolerites, and locally covered by basalts. Primary magmatic zircons extracted from gabbros and meta-gabbros of the two banks were dated by the U–Pb chronometer, and initial hafnium isotope signatures ( ϵHf i) were determined on the same grains. For Mt. Gettysburg at Gorringe, gabbro emplacement ages of 137.5±0.5 (2σ) Ma and 135.7±0.8 Ma are obtained, and corresponding ϵHf i lie at +20.5±0.3 (2σ) and +19.5±0.4, substantiating magma formation from severely LILE-depleted mantle domains. Gabbro zircons from Mt. Ormonde at Gorringe yield a much younger age of 77.1±0.4 Ma and the Hf isotopes document an intermediately LILE-depleted mantle source having a ϵHf i of +7.6±0.4. Given its age and Hf signature, emplacement of this rock can be ascribed to the alkaline magmatic event that also affected the Iberian Continent in Upper Cretaceous time. Concerning the Galicia section, zircons from a meta-gabbro yield an emplacement age of 121.7±0.4 Ma and a ϵHf i of +14.0±0.2, and a ϵHf i of +14.6±0.2 is obtained for zircons from a previously dated meta-gabbro of identical age. These results indicate magma extraction from mantle reservoirs that are slightly less LILE-depleted than those sampled by the about 20 Myr older Gorringe gabbros. The data demonstrate that magmatism occurring prior to complete separation of Europe from America was essentially of asthenospheric origin. Both the 138–135 Ma ages for the Gorringe gabbros and 122 Ma ages for the Galicia gabbros are at least 5 Myr older than the oldest sediments on Gorringe, and the break-up unconformity at the Galicia Bank, respectively. Magma source signatures of the syn-rift gabbros are in agreement with values expected for differently depleted Cretaceous MORB-type mantle reservoirs, and the age-difference for magmatism between the Gorringe and Galicia banks suggests a rate of 4.4±0.3 cm/yr for northward progression of continental rifting. Based on the new results, a structural model for Iberia–America rifting is discussed putting forward the idea that magma emplacement produces a level of weakness and decollement between the rifting crust and its underlying lithospheric mantle.

A fossil, serpentinization‐related hydrothermal vent, Ocean Drilling Program Leg 173, Site 1068 (Iberia Abyssal Plain): Some aspects of mineral and fluid chemistry

The basement at Site 1068, Ocean Drilling Program (ODP) Leg 173 (serpentinized peridotite in fault contact with overlying amphibolite‐clast‐dominated sedimentary and tectonic breccias) is host to a hydrothermal system rooted in serpentinization reactions occurring at depth. The serpentinite grades downward from cataclasites at the fault, through brecciated, recrystallized, tochilinite‐bearing serpentinite, to awaruite‐bearing massive, mesh‐textured serpentinite. Andradite is common throughout and is a major sink for iron. The breccias are similarly zoned, from tectonized rocks near the fault upward into sedimentary breccias. Mg‐silicate vein assemblages and rodingitized amphibolite clasts near the fault give way to calcite veins and nonpervasive albite‐chlorite alteration upsection. Marcasite (± pyrrhotite at the fault) is the sulfide phase and occurs only in the tectonic breccias. Fe oxides are magnetite near the fault and hematite and ferric oxyhydroxides upsection. The zonation reflects mixing of seawater with a fluid whose composition (low fO2, fS2 Si, CO2, high Ca, Fe, Ca/Mg, pH) is controlled by serpentinization reactions. The deepest serpentinites have strongly reduced mineral assemblages that are unusual in a totally serpentinized peridotite. This probably reflects equilibration with a fluid derived from ongoing serpentinization at depth. The upper serpentinites, on through the mineral sequences seen in the breccias reflect increasing input from seawater upsection. Increased fO2 and fS2 stabilizes increasingly S‐ and O‐rich assemblages. Calcite (and ferric oxide) precipitation decreases pH, stabilizing marcasite. Relative to mid‐ocean ridge hydrothermal systems, fluids in serpentinite‐hosted hydrothermal systems are poor in S and rich in Mg and are unlikely to host large sulfide ore deposits.

Late Jurassic sedimentation and deformation in the west Iberia continental margin: insights from FMS data, ODP Leg 173

In the Iberia Abyssal Plain, Ocean Drilling Program (ODP) Site 1065 drilled the top of a continental tilted block, penetrating a Middle to Late Jurassic probable pre-rift sedimentary unit. Borehole images of this unit were obtained by the Formation MicroScanner (FMS) logging tool, thus allowing the identification and 3D measurement of two block tilting phases. The last one is an eastward 15° tilt, probably related to the main Early Cretaceous rifting phase, and consistent with the tilted block image on seismic lines. The first tilt, previously unknown, is Late Jurassic in age. The block tilting is 15° to the southeast, and is not associated with any noticeable change in the depositional environment. Both tilting episodes induced sedimentary instabilities, leading to listric faults and slumps. FMS images also showed that the sediments originated from the north (Galicia Bank) during Late Jurassic times.

The relative timing of serpentinisation and mantle exhumation at the ocean–continent transition, Iberia: constraints from oxygen isotopes

Legs 173 and 149 of the Ocean Drilling Program profiled a zone of exhumed mantle peridotite at the ocean–continent transition (OCT) beneath the Iberia Abyssal Plain. The zone of exhumed peridotite appears to be tens of kilometers wide and is situated between blocks of continental crust and the first products of ocean accretion. Exhumed peridotite is 95–100% serpentinised to probable depths of 2–3 km. Down core oxygen isotope profiles of serpentinised peridotite at Sites 1068 and 1070 (Leg 173) show evidence for two fluid infiltration events. The earlier event involved pervasive infiltration of comparatively warm (>175°C) sea water and accompanied serpentinisation. The later event involved structurally focused infiltration of comparatively cool (<50–150°C) sea water and accompanied active mantle exhumation. We therefore conclude that the uppermost mantle was serpentinised before it was exhumed at the Iberian OCT. Implicit to this conclusion is that a sizeable region of serpentinised mantle existed directly beneath thinned but intact continental crust. Serpentinite has comparatively low density, low frictional strength and low permeability. The presence of such a ‘soft’ layer may have localised deformation and consequently promoted detachment-style exhumation of the uppermost mantle. The low permeability of a serpentinite ‘cap’ layer might help to explain the lack of observed melt at the Iberian OCT.