The structure and origin of the Azores-Biscay Rise, North-east Atlantic Ocean

Summary. The Azores—Biscay Rise is a roughly linear north-east—south-west trending feature rising 1500–3000m above its surroundings, which extends from about 4°N, 1°30'W towards the Azores. Its south-western termination is near 40°30'N, 21°30'W. About halfway along its length the Rise intersects the WNW-trending King's Trough. In 1978 a set of bathymetric, magnetic, gravity, GLORIA and seismic reflection and refraction data were obtained in the vicinity of the Rise. Together with earlier data these observations suggest that: (1) there has been no substantial post-emplacement tectonic activity, with the possible exception of the construction of some volcanic seamounts at the south-western end of the Rise, and (2) the Rise is underlain by a low-velocity (low-density) lower crust and is in isostatic equilibrium.
The Rise can be convincingly shown to be the eastern half of a pair of ridges formed by abnormal crustal generation at the Mid-Atlantic Ridge crest between the times of anomalies 33 and 24 (76–56 Ma ago). The western counterpart of the Rise includes Gauss and Milne seamounts in the Newfoundland Basin.
Magnetic anomaly 31 passes uninterruptedly across the Rise and therefore hypotheses that the northern part of the Rise was the site of a Cenozoic transform fault or subduction zone are not supported by our data. It is speculated that King's Trough was linked to the North Spanish Trough by an early Cenozoic east—west transform fault across the northern Iberia Abyssal Plain. This plate boundary became inactive about the middle of the Oligocene epoch.     

Paleo‐fluid expulsion and contouritic drift formation on the Chatham Rise,New Zealand

The Chatham Rise is located offshore of New Zealand's South Island. Vast areas of the Chatham Rise are covered in circular to elliptical seafloor depressions that appear to be forming through a bathymetrically controlled mechanism, as seafloor depressions 2–5 km in diameter are found in water depths of 800–1100 m. High‐resolution P‐Cable 3D seismic data were acquired in 2013 across one of these depressions. The seafloor depression is interpreted as a mounded contourite. Our data reveal several smaller buried depressions (<20–650 m diameter) beneath the mounded contourite that we interpret as paleo‐pockmarks. These pockmarks are underlain by a complex polygonal fault system that deforms the strata and an unusual conical feature results. We interpret the conical feature as a sediment remobilization structure based on the presence of stratified reflections within the feature, RMS amplitude values and lack of velocity anomaly that would indicate a nonsedimentary origin. The sediment remobilization structure, polygonal faults and paleo‐depressions are the indicators of the past subsurface fluid flow. We hypothesize that the pockmarks provided the necessary topographic roughness for the formation of the mounded contourites thus linking fluid expulsion and the deposition of contouritic drifts.     

An analysis of P-wave amplitudes recorded by seismological stations in the USSR

Summary. Short period P -wave amplitudes at Soviet seismic stations are analysed to determine an amplitude–distance curve in the range 0°–180° from the USSR. Station amplitude terms, corrected for crustal amplification effects, are determined from seismic sources located between 30° and 90°. A relationship between these station terms and regional P_n wave speed and regional heat flow is determined and is found to be in close agreement with similar observations in North America. Station amplitude terms were found to be reasonably uniform across the continental portion of the USSR but the Baikal Rift valley is characterized by very low P -wave amplitudes and is the most significant feature discemible from the P -wave amplitude terms.     

Ocean-bottom seismograph observations on the Charlie—Gibbs fracture zone

Summary. Two networks of four ocean-bottom seismographs were deployed successfully in 1982 September on spreading centre sites in the region of the Charlie–Gibbs fracture zone. Activity was observed beneath both networks and hypocentres for over 100 events have been determined. The observations confirm the existence of seismic activity along a suspected short spreading centre near longitude 31.75°W linking the north and south transform valleys of the fracture zone. A relatively thick (7–9 km) seismogenic zone is seen beneath the axis which is in agreement with earlier microearthquake observations on the Mid-Atlantic Ridge.     

A model of hydrothermal circulation in fault zones at mid-ocean ridge crests

Summary. A model of hydrothermal circulation close to mid-ocean ridge crests has been developed. The flow is modelled as a transient open-loop thermosyphon, restricted to a single fracture parallel to the spreading centre. Circulation is to a depth of 500–1000 m, and heat exchange occurs at the fault walls. Conduction is the sole heat transfer mechanism within the rock.
The aim is to model the conditions under which deposition of ore deposits, such as those in the Cyprus ophiolites, could occur.
Hot springs, with the characteristics of the black smokers at 21°N on the East Pacific Rise, are obtained from the model, but only when the vents are at the unrealistically large spacing of 10 km apart, and then water at over 350°C is vented for a maximum of only 175 yr. The model demonstrates that heat transfer through the rock by conduction alone is too slow to maintain the power output of a group of black smokers. The skin depth of the temperature field limits the total heat available to the system, but even with-out this constraint insufficient high-grade heat would be available from the volume of rock within the likely field of influence of a vent.
It would take about 2500 yr to accumulate a 3 million ton ore deposit from 1 vent or 250 yr from a group of 10 vents. The rate of heat extraction and the duration of the vents must both be at least an order of magnitude greater than those obtained from this model, thus requiring a much greater source of heat than in the rocks alone. The only source likely to be great enough is the magma chamber itself.     

A Maastrichtian palaeomagnetic pole for the Pacific plate from a skewness analysis of marine magnetic anomaly 32

The asymmetry (skewness) of marine magnetic anomaly 32 (72.1–73.3  Ma) on the Pacific plate has been analysed in order to estimate a new palaeomagnetic pole. Apparent effective remanent inclinations of the seafloor magnetization were calculated from skewness estimates of 108 crossings of anomaly 32 distributed over the entire Pacific plate and spanning a great-circle distance of ~12  000  km. The data were inverted to obtain a palaeomagnetic pole at 72.1°N, 26.8°E with a 95 per cent confidence ellipse having a 4.0° major semi-axis oriented 98° clockwise of north and a 1.8° minor semi-axis; the anomalous skewness is 14.2° ± 3.7°. The possible dependence of anomalous skewness on spreading rate was investigated with two empirical models and found to have a negligible effect on our palaeopole analysis over the range of relevant spreading half-rates, ~25 to ~90  mm  yr⁻¹ . The new pole is consistent with the northward motion for the Pacific plate indicated by coeval palaeocolatitude and palaeoequatorial data, but differs significantly from, and lies to the northeast of, coeval seamount poles. We attribute the difference to unmodelled errors in the seamount poles, mainly in the declinations. Comparison with the northward motion inferred from dated volcanoes along the Hawaiian–Emperor seamount chain indicates 13° of southward motion of the Hawaiian hotspot since 73  Ma. When the pole is reconstructed with the Pacific plate relative to the Pacific hotspots, it differs by 14°–18° from the position of the pole relative to the Indo–Atlantic hotspots. This has several possible explanations including bias in one or more of the palaeomagnetic poles, motion between the Pacific and Indo–Atlantic hotspots, and errors in plate reconstructions relative to the hotspots.     

A seismic study along the East Greenland margin from 72°N to 77°N

Around 4370 km of new seismic reflection data, collected along the East Greenland margin between 71°30'N and 77°N in 2003, provide a first detailed view of the sediment distribution and tectonic features along the East Greenland margin. After processing and converting the data to depth, we correlated ODP-Site 913 stratigraphy into the new seismic network. Unit GB-2 shows the greatest glacial sediment deposits beneath the East Greenland continental shelf. This unit is characterized by the beginning of prograding sequences and has, according to our stratigraphic correlation, a Middle Miocene age. It might have been caused by rapid changes in sea level and/or glacial erosion by an early ice sheet or glaciers along the coast. A basement high, presumably a 360 km long basement structure at 77°N–74°54'N, prevents continuous sediment transport from the shelf into the deep sea area in times before 15 Myr. The origin of this prominent structure remains speculative since no rock sample from this structure is available. Seaward dipping reflectors at the eastern flank of this structure strongly support that it is a volcanic construction and is most likely emplaced on continental or transitional crust. The compilation of sediment thickness provide an insight into the regional sediment distribution in the Greenland Basin. An average sediment thickness of 1 km is observed. The north bordering Boreas Basin has a sediment thickness of 1.8 km close to the Greenland fracture zone (GFZ).     

Overlapping spreading centres: 3-D inversion of the magnetic field at 9°03'N on the East Pacific Rise

Summary. Overlapping spreading centres (OSCs) represent a new type of plate boundary interaction in which en échelon rise segments overlap significantly and are not joined by a transform fault.
A three-dimensional Fourier inversion of the magnetic field was performed on an overlapping spreading centre to remove the effects of topography and ridge orientation. A magnetic high exists at the tip of one of the two ridge segments. Forward modelling suggests that the anomalous magnetic field cannot be attributed to the effects of topography alone. The inversion reveals the existence of a magnetization high at the tip of the eastern spreading centre. Maximum magnetization values are consistent with ones obtained in other high amplitude zones in the Pacific as well as with the measured magnetization of samples dredged in the same areas. We suggest that the magnetization high over the eastern ridge tip of the 9°03'N OSC is associated with highly evolved basalts enriched in iron and titanium. Such enrichment may be caused by enhanced crystal fractionation within an axial magma chamber which is intermittent and occasionally freezes as the eastern spreading axis propagates into older lithosphere.     

Crustal Structure and Origin of Peake and Freen Deeps, NE Atlantic

Summary Peake and Freen Deeps are elongate structures some 30 nautical miles long by 7 miles wide situated near 43° N 20° W on the lower flanks of the Mid-Atlantic Ridge. Seismic reflection records show that underneath about 400 fm of layered sediment the bedrock lies at a depth greater than 3600 fm in Peake Deep and 3300 fm in Freen Deep; the surrounding seafloor is at about 2100 fm. Freen Deep is the eastern end of King's Trough, a flat floored feature some 400 fms deeper than the adjacent seafloor. The Trough extends 220 miles west-north-westwards towards the crest of the Mid-Atlantic Ridge. The area is aseismic and heat flow is normal; there is no displacement of the crest of the mid-ocean ridge on the projected line of King's Trough. Gravity and magnetic surveys have been made. With minor exceptions, magnetic anomalies are not due to bodies elongated parallel with the structure, which, therefore, cannot be a volcanic collapse caldera. Seismic refraction results in the Peake-Freen area show that the crust is not thinned under the deeps although the Moho may be depressed by 2 km. Bouguer anomalies also suggest that the Moho is flat and does not rise to compensate the deeps. Models consistent with gravity and seismic information suggest there is a dense block in the upper mantle under the area. Since no reason to ascribe the origin of the structure to tear faulting has yet been acquired, it is interpreted in terms of over thrusting perpendicular to the deeps, followed by inversion of the lower part of the thickened basaltic crust to eclogite, and its subsequent sinking into the mantle.     

The significance of the pdaeomagnetism of Jurassic—Cretaceous rocks from South America: predrift movements, hairpins and magnetostratigraphy

Summary. In this paper we show that: (1) The positions of the Cretaceous palaeomagnetic poles (PP) for South America and Africa exhibit elongated distributions that are due to rapid movement of these continents from the south pole.
(2) The positions of the Middle—late Jurassic virtual geomagnetic poles for South America exhibit an elongated distribution along the meridians 20–200° E; it is suggested that this is due to a rapid shift of South America in Middle—late Jurassic time.
(3) The late early—early late Cretaceous sections of the apparent polar wandering paths for South America and Africa are consistent with South Atlantic seafloor spreading data.
On the basis of the comparison of the reliable late Palaeozoic—late Cretaceous PPs for South America and Africa, taking into account the restrictions established by geological, palaeontological and seafloor spreading data, it is suggested that minor movements could have occurred within Western Gondwana in middle—late Jurassic time along a narrow zone which later became the South Atlantic divergent boundary.
Four 'hairpins' are defined in the late Palaeozoic—late Cretaceous section of the apparent polar wandering path for South America; the two youngest of these can be correlated with the origin of the South Atlantic Ocean basin and the onset of the Andean Orogeny, respectively.
The magnetostratigraphy for the Serra Geral lava flow sequence suggests that some of these flows were poured out rapidly without significant interruption.     

On the construction of geomagnetic timescales from non-prejudicial treatment of magnetic anomaly data from multiple ridges

When marine magnetic-anomaly data are used to construct geomagnetic polarity timescales, the usual assumption of a smooth spreading-rate function at one seafloor spreading ridge forces much more erratic rate functions at other ridges. To eliminate this problem, we propose a formalism for the timescale problem that penalizes non-smooth spreading behaviour equally for all ridges. Specifically, we establish a non-linear Lagrange multiplier optimization problem for finding the timescale that (1) agrees with known chron ages and with anomaly-interval distance data from multiple ridges and (2) allows the rate functions for each ridge to be as nearly constant as possible, according to a cumulative penalty function. The method is applied to a synthetic data set reconstructed from the timescale and rate functions for seven ridges, derived by Cande & Kent (1992) under the assumption of smooth spreading in the South Atlantic. We find that only modest changes in the timescale (less than 5 per cent for each reversal) are needed if no one ridge is singled out for the preferential assumption of smoothness. Future implementation of this non-prejudicial treatment of spreading-rate data from multiple ridges to large anomaly-distance data sets should lead to the next incremental improvement to the pre-Quaternary geomagnetic polarity timescale, as well as allow a more accurate assessment of global and local changes in seafloor spreading rates over time.     

The Darwin Rise demise: the western Pacific guyot heights trace the trans-Pacific Mendocino fracture zone

The Darwin Rise has been proposed so many times and in so many forms and places that the time has come to make a more comprehensive examination of the region. Lying on the NW Pacific Plate between the Geisha Guyots, the Mid-Pacific Mountains, the equator, and the trenches, the region is roughly bounded by magnetic anomaly M20 (147 Ma). It was subjected to a massive outpouring of lava about 105 to 120 Ma, which created the guyots and seamounts in that region. Guyots are excellent tools for studying events of long ago because they eroded in the same lowstand in the Cretaceous and guyot relief, therefore, is a surrogate for paleo-sealevel. The relief is derived by subtracting the break depth of the summit plateau of a guyot from the regional depth. Guyot relief would necessarily be less in the center than to the periphery if the feature formed on a pre-existing rise, as has been postulated. The existence of a paleo-Darwin Rise would give concentric contours for the region in question. Of the sixty guyots used in this study, thirty-seven of these guyots were surveyed using SASS multibeam in the Marcus-Wake seamount group. Twenty-three guyots were surveyed using random track single-beam sonar surveys. An entirely different scenario is shown. Data revealed a major fracture passing through the area coevally or after the guyots formed. Because the depths to the summit are not the same now, vertical tectonics occurred after subaerial erosion. This means the fracture formed during and after the erosion (roughly 105 Ma) and influenced the normal sequence of events in guyot formation. Depending on how one deciphers trends through the Hess Rise morass, SASS bathymetry shows a continuation of the Surveyor/Mendocino fracture zone swarm inside the M20 region to the NE of these data. The fracture swarm continues to the western Pacific trench system. Based on this information, if the Darwin Rise ever existed, it had to have done so elsewhere.     

The late Maastrichtian palaeomagnetic pole of the Pacific Plate

Summary. The Pacific plate's late Maastrichtian (∼ 69 Ma) palaeomagnetic pole, which constrains the northward motion of the Pacific plate during the Cainozoic and latest Cretaceous, was studied. A recently proposed method for obtaining oceanic plate palaeomagnetic poles by combining dissimilar data was extended to accept, as input, the relative amplitudes of magnetic lineations with different azimuths or widely separated sites or both. Combining late Maastrichtian palaeomagnetic data-the relative amplitudes and skewness of magnetic lineations, palaeolatitudes from a palaeomagnetic study of basalt and sediment in vertical cores, a pole from the inversion of the magnetic anomaly over a seamount, and present locations of equatorial sediment facies—yielded a best fit pole of 71°N, 9°E and a 95 per cent confidence ellipse with the major semiaxis of 6° striking 91° clockwise from north and the minor semiaxis of 2° striking 1° clockwise from north. This best fit pole, when compared to the pole expected if the hotspots have been fixed with respect to the spin axis, demonstrates that the hotspots in the Pacific Ocean have shifted ∼ 10° south with respect to the spin axis during the Cainozoic. This best fit pole, when compared to the best fit Campanian pole of the Pacific plate, demonstrates that the pole wandered rapidly, 1.1° Ma^-1, with respect to the Pacific plate during the latest Cretaceous.     

Adaptive unstructured grid finite element simulation of two-dimensional magnetotelluric fields for arbitrary surface and seafloor topography

We present an adaptive unstructured triangular grid finite element approach for effectively simulating plane-wave diffusive electromagnetic fields in 2-D conductivity structures.
The most striking advantage of irregular grids is their potential to incorporate arbitrary geometries including surface and seafloor topography. Adaptive mesh refinement strategies using an a posteriori error estimator yield most efficient numerical solutions since meshes are only refined where required.
We demonstrate the robustness of this approach by comparison with analytical solutions and previously published numerical simulations. Maximum errors may systematically be reduced to, for example, 0.8 per cent for the apparent resistivity and 0.2° in the phase.
An additional accuracy study of the thickness of the air layer in E-polarization suggests to keep a minimum thickness depending on lateral conductivity contrasts within the earth.
Furthermore, we point out the new quality and flexibility of our simulation technique by addressing two marine magnetotelluric applications. In the first case, we discuss topographic effects associated with a synthetic sinusoidal sea bottom model and in the second case, we show a close-to-reality scenario using real bathymetry data from the East Pacific Rise at 17°S.     

Localized velocity anomalies in the lower mantle

Summary. Two localized regions of velocity heterogeneity in the lower mantle with scale lengths of 1000–2000 km and 2 per cent velocity contrasts are detected and isolated through comparison of S, ScS, P and PcP travel times and amplitudes from deep earthquakes in Peru, Bolivia, Argentina and the Sea of Okhotsk. Comparison of the relative patterns of ScS-S differential travel times and S travel-time residuals across North American WWSSN and CSN stations for the different source regions provides baselines for interpreting which phases have anomalous times. A region of low S and P velocities is located beneath Northern Brazil and Venezuela at depths of 1700–2700 km. This region produces S -wave delays of up to 4 s for signals from deep Argentine events recorded at eastern North American stations. The localized nature of the anomaly is indicated by the narrow bounds in azimuth (15°) and take-off angle (13°) of the arrivals affected by it. The long period S -waves encountering this anomaly generally show 30–100 per cent amplitude enhancement, while the short-period amplitudes show no obvious effect. The second anomaly is a high-velocity region beneath the Caribbean originally detected by Jordan and Lynn, who used travel times from deep Peruvian events. The data from Argentine and Bolivian events presented here constrain the location of the anomaly quite well, and indicate a possible short- and long-period S -wave amplitude diminution associated with it. When the travel-time data are corrected for the estimated effects of these two anomalies, a systematic regional variation in ScS-S station residuals is apparent between stations east of and west of the Rocky Mountains. One possible explanation of this is a long wavelength lateral variation in the shear velocity structure of the lower mantle at depths greater than 2000 km beneath North America.     

Reflected-diffracted waves in fracture zone models

Summary. The paper presents the results of modelling of diffracted and reflected-diffracted waves in fracture zones. The Berryhill method was used and the calculations were made for a profile perpendicular to the diffracting edge. Several homogeneous models of the Earth's crust, characterized by different values of crustal thickness, velocity and horizontal distance between shot point and diffracting edge were considered. A dependence of the relative amplitude of diffracted waves on the location of the diffracting edge is given. The pattern of the seismic wavefield depends upon the dimensions of the fracture zone. Amplitude curves of reflected-diffracted waves are presented for a series of models of fracture zones. The possibility of applying the amplitudes of reflected-diffracted wave trains to the interpretation of the structure of fracture zones in the Earth's crust is andysed for different types of fracture zones.     

Structure and early evolution of the Arabian Sea and East Somali Basin

The Laxmi Ridge is a large-scale basement high buried beneath the sediments of the Indus Fan. The location of the ocean–continent transition (OCT) on this margin has previously been proposed at either the southern edge of the Laxmi Ridge or beyond it towards the India–Pakistan shelf. The former explains the margin-parallel Laxmi Basin as thinned continental crust, the latter as a failed rift of earlier seafloor spreading. To examine the structure of this margin, a reassessment of marine magnetic data has detailed seafloor-spreading magnetic anomalies prior to anomaly 24 in both the Arabian and East Somali basins. The previously identified anomaly 28 is not interpreted as a seafloor-spreading anomaly but as a magnetized basement feature adjacent to, and merging with, the ridge—the Laxmi Spur. New gravity models across the Laxmi Ridge and adjacent margin using ship and satellite data corroborate the existence of underplated crust beneath the Laxmi Ridge and Basin and the location of the OCT at the southern edge of the Ridge. The results are not compatible with the existence of a pre-anomaly 28 phase of seafloor spreading, although large-scale intrusions may be the origin of some of the basement features in the Laxmi Basin. The models also identify the Laxmi Spur as a low-density feature with a natural remanent magnetization (NRM) compatible with serpentinization. The Laxmi Ridge is mapped to the southeast, where it appears to terminate at a point coinciding with the appearance of E–W magnetic lineations and gravity anomalies at 15.5°N. Thereafter it becomes indistinct. This is interpreted as necessary in the reconstruction to the Mascarene Plateau to avoid continental overlap.     

Thickening model of the continental lithosphere

Summary. The thickening plate theory proposed by Yoshii and Parker & Oldenburg for the oceanic lithosphere is extended to include the continental lithosphere. The theory is based on the assumption that the lithosphere—asthenosphere boundary is a solidus and that as a result solidification of the top of the asthenosphere is occurring. Observational data imply that the relationship between the plate thickness and basement age for the North American continent is y = 1.7 √ t + (50 ± 10), where y (km) is the plate thickness and t (Myr) is the basement age.
The theory is tested against changes with basement age of the observed surface heat-flow and seismic estimate of plate thickness. The following conclusions are inferred:
(1) The changes both of the observed heat flow and plate thickness with basement age are explained by this theory.
(2) The surface erosion and vertical distribution of radiogenic heat sources are important factors in controlling the thickening process of the continental lithosphere.
(3) The equality of the average surface heat-flow over the oceans and over the continents is a consequence of a faster release of latent heat at the lithosphere—asthenosphere boundary under the oceans, instead of a higher heat production in the continental crust.     

Detection of the 11 yr sunspot cycle signal in Earth rotation

Summary. A two-channel MESA or maximum entropy spectrum analysis (Morf et al. 1978) between Δ T °= ET – UT (Morrison 1973) and sunspot numbers spanning 1832–1975 yields the following results over a bandwidth 2–20yr: (1) The spectra of Δ T ° and sunspot numbers are both dominated by a narrow band signal at 11.0 yr; (2) On average the coherency over the continuum is 0.14 while at 11.0yr it peaks at 0.83; (3) The 11-yr sunspot cycle signal in length of day (lod) has an amplitude of 0.16 ms, and in time lags that in sunspot numbers by 3.4yr. Estimates obtained from segmenting the series yield extremal values which grossly bound the above estimates: The narrow band signal has period range (10.5–11.4yr) coherency (0.8–1.0), amplitude (0.06–0.31 ms), and time lag (3.0–3.Syr). In addition, a two channel analysis of sunspot numbers with a new Δ T ⁿ series from 1861–1978 (Morrison 1979) and an earlier segment of Δ T °, as well as a single-channel analysis of Δ T ° and Δ T ⁿ individually, further support the conclusion that the solar sunspot cycle in Earth rotation has been detected. These experimental results have implications in astronomy, solar physics, meteorology and climatology.     

Geothermal measurements in Palaeogene, Cretaceous and Permo-Carboniferous sediments in northern Bohemia

Summary. Heat flow studies in northern Bohemia have yielded values in the range 60–80 mWm⁻². These data are compatible with the results of earlier heat-flow studies in this area, which forms a large sedimentary basin and represents a zone of relatively weakened Earth's crust in the northeastern sector of the Bohemian Massif.