New Gravity Map of the Western Galicia Margin:The Spanish Exclusive Economic Zone Project

Since 1995, the most intensive mapping of the seafloor off the Spanish coast has been carried out in the framework of the Spanish Exclusive Economic Zone Project (ZEEE).The main objectives of this project are to obtain improved multibeam bathymetric cartography of the areas off Spanish coastlines, and to perform a geophysical survey,well-suited with a 10-knot navigation velocity (some techniques requires lower navigation velocity). The geophysical survey includes gravity, geomagnetism, and low-penetration seismic techniques in order to infer the geological structure of the seafloor. Other oceanographic variables such as current, surface salinity, and temperature profiles, can be recorded without compromising this systematic survey effort. The ZEEE Project has carried out its survey activities for one month every year.Data acquisition is achieved aboard the Spanish R/V Hesperides. Until 1997, surveying efforts concentrated on the Balearic Sea and Valencia Gulf, both in the western Mediterranean Sea. Between 1998 and 2000, the ZEEE Project investigations were conducted offshore the Canary Archipelago. Since 2001, the third phase of the program has been focused on the West Galicia Margin in the northeastern Atlantic Ocean. Survey results on the West Galicia Margin area are of interest for two key reasons. First, there is great scientific interest in the improvement of the knowledge of this non-volcanic rifting margin, since this margin offers good conditions for the study of the processes that take place in this type of geological context,because it is sediment-starved. Second, the obtained results also have major socioeconomic repercussions because they can prove significant to defining the expansion of the Spanish shelf,beyond Spain’s Economic Exclusive Zone distance of 200 nautical miles. All of the gravity data acquired to date on this area have been stored as a database, with the aim of preparing gravity anomaly maps on a scale 1:200,000.The database and gravity anomaly charts from the ZEEE Project will provide the most coherent and complete gravity perspective available for this area. This article describes the efforts and accomplishments of the project to date

Fault-controlled hydration of the upper mantle during continental rifting

Water and carbon are transferred from the ocean to the mantle in a process that alters mantle peridotite to create serpentinite and supports diverse ecosystems1. Serpentinized mantle rocks are found beneath the sea floor at slow- to ultraslow-spreading mid-ocean ridges1 and are thought to be present at about half the world’s rifted margins2, 3. Serpentinite is also inferred to exist in the downgoing plate at subduction zones4, where it may trigger arc magmatism or hydrate the deep Earth. Water is thought to reach the mantle via active faults3, 4. Here we show that serpentinization at the rifted continental margin offshore from western Spain was probably initiated when the whole crust cooled to become brittle and deformation was focused along large normal faults. We use seismic tomography to image the three-dimensional distribution of serpentinization in the mantle and find that the local volume of serpentinite beneath thinned, brittle crust is related to the amount of displacement along each fault. This implies that sea water reaches the mantle only when the faults are active. We estimate the fluid flux along the faults and find it is comparable to that inferred for mid-ocean ridge hydrothermal systems. We conclude that brittle processes in the crust may ultimately control the global flux of sea water into the Earth

Geometry of extensional faults developed at slow-spreading centres from seismic reflection data in the Central Atlantic (Canary Basin)

We present depth images, from portions of profiles that are close to flow-lines, of Cretaceous oceanic crust in the eastern Central Atlantic. Compared with post-stack time migrations, the images illustrate the improvement resulting from the application of pre-stack depth migration. The images document the scale and geometry of normal faulting in oceanic crust formed over 25 Myr at a half-spreading rate of less than 10 mm yr−1, and the variation in extensional style with position within the spreading segment. Away from major fault zones (FZs), most faults are subplanar, dip more than 35°, are associated with moderate basement relief (0.2–1 km relief) and may penetrate to deep crustal levels. These faults could be related to the lifting of the lithosphere out of the median valley to the flanking mountains. Also observed away from FZs are gently dipping to subhorizontal reflections in the upper crust, which resemble detachment faults. In contrast, the inside corner crust is more rugged, with basement highs rising up to 2 km above the intervening basins. This larger-scale topography is associated with a different style of faulting: the depth images reveal gently dipping (<35°) faults that are rooted in the deep crust and that project to the ridgeward flank of the dome-shaped large basement highs (1–2 km vertical relief). The faults seem to continue as the ridge-facing flank of these highs and some may extend over the crest of the high to breakaways beyond. In this case, the domal highs that form the exhumed footwall to the faults can be described as oceanic core complexes. These controlling faults are up to 20 km long and have a heave of ∼10 km, sufficient to have accommodated up to 50 per cent extension and to have exhumed deep crustal and perhaps even mantle rocks. We suggest that similar faults can explain the structure and lithologies found at megamullion structures (oceanic core complexes) at inside corners near the present-day spreading ridge

Effective elastic thickness of South America and its implications for intracontinental deformation

The flexural rigidity or effective elastic thickness of the lithosphere, Te, primarily depends on its thermal gradient and composition. Consequently, maps of the lateral variability of Te in continents reflect their lithospheric structure. We present here a new Te map of South America generated using a compilation of satellite-derived (GRACE and CHAMP missions) and terrestrial gravity data (including EGM96 and SAGP), and a multitaper Bouguer coherence technique. Our Te maps correlate remarkably well with other proxies for lithospheric structure: areas with high Te have, in general, high lithospheric mantle shear wave velocity and low heat flow and vice versa. In this paper we focus on the Te of the stable platform. We find that old cratonic nuclei (mainly Archean and Early/Middle Proterozoic) have, in general, high Te (&gt;70 km), while the younger Patagonian Phanerozoic terrane has much lower Te (20-30 km), suggesting that Te is related to terrane age as has already been noted in Europe. Within cratonic South America, Te variations are observed at regional scale: relatively lower Te occurs at sites that have been repeatedly reactivated throughout geological history as major sutures, rift zones, and sites of hot spot magmatism. Today, these low Te areas are surrounded by large cratonic nuclei. They concentrate most of the intracontinental seismicity and exhibit relatively high surface heat flow and low seismic velocity at 100 km depth. This implies that intracontinental deformation focuses within relatively thin, hot, and hence weak lithosphere, that cratonic interiors are strong enough to inhibit tectonism, and that the differences in lithospheric rigidity, structure, and composition between stable cratons and sites of intracontinental deformation are not transient, and may have been maintained, in some cases, for at least 500 m.y. Copyright 2007 by the American Geophysical Union

On the recovery of effective elastic thickness using spectral methods: Examples from synthetic data and from the Fennoscandian Shield

There is considerable controversy regarding the long-term strength of continents (Te). While some authors obtain both low and high Te estimates from the Bouguer coherence and suggest that both crust and mantle contribute to lithospheric strength, others obtain estimates of only &lt;25 km using the free-air admittance and suggest that the mantle is weak. At the root of this controversy is how accurately Te can be recovered from coherence and admittance. We investigate this question by using synthetic topography and gravity anomaly data for which Te is known. We show that the discrepancies stem from comparison of theoretical curves to multitaper power spectral estimates of free-air admittance. We reformulate the admittance method and show that it can recover synthetic Te estimates similar to those recovered using coherence. In light of these results, we estimate Te in Fennoscandia and obtain similar results using both techniques. Te is 20-40 km in the Caledonides, 40-60 km in the Swedish Svecofennides, 40-60 km in the Kola peninsula, and 70-100 km in southern Karelia and Svecofennian central Finland. Independent rheological modeling, using a xenolith-controlled geotherm, predicts similar high Te in central Finland. Because Te exceeds crustal thickness in this area, the mantle must contribute significantly to the total strength. Te in Fennoscandia increases with tectonic age, seismic lithosphere thickness, and decreasing heat flow, and low Te correlates with frequent seismicity. However, in Proterozoic and Archean lithosphere the relationship of Te to age is ambiguous, suggesting that compositional variations may influence the strength of continents. Copyright 2004 by the American Geophysical Union

The rift to drift transition at non-volcanic margins: insights from numerical modeling

“Non-volcanic” rifted margins exhibit very little evidence of synrift magmatism, even where the continental crust has been thinned to such an extent that the mantle has been exhumed across a transitional zone (up to ∼100 km wide), called the continent–ocean transition (COT). Using dynamical models of rifting, we explore how extension velocity, mantle composition and potential temperature influence the nature and extent of the COT and compare our results to observations at the West Iberia margin (WIM) and the ancient margins of the Liguria-Piemonte Ocean (LP) now exposed in the Alps. We find a first order relationship between extension velocity and the amagmatic exposure of mantle at the COT. For very slow half extension velocities, ( 10% prior to rifting or that its potential temperature was ∼50 °C lower than normal (≤ 1250 °C)