King's Trough Research Articles

A New Southern North Atlantic Isochron Map: Insights Into the Drift of the Iberian Plate Since the Late Cretaceous

AbstractThis paper presents a new southern North Atlantic plate model from Late Cretaceous to present, with the aim of constraining the kinematics of the Iberian plate during the last 83.5 Myr. This model is presented along with a detailed isochron map generated through the analysis of 3 aeromagnetic tracks and ~400 ship tracks from the National Centers for Environmental Information database. We present a new technique to obtain well‐constrained estimates of the Iberia‐North America plate motions from magnetic anomalies, overcoming the scarcity of large‐offset fracture zones and transform faults. We build an integrated kinematic model for NW Africa, Morocco, Iberia, Europe, and North America, which shows that the deformation is partitioned between Pyrenees and Betic‐Rif orogenic domain during the Late Cretaceous‐Oligocene time interval. In the Eastern Betics domain, the calculated amount of NW Africa‐Iberia convergence is ~80 km between 83.5 and 34 Ma, followed by ~150 km since the Oligocene. The motion of Iberia relative to Europe in the Central Pyrenees is characterized by overall NE directed transpressional motion during the Campanian and the Paleocene, followed by NW directed transpressional movement until the Lutetian and overall NNW directed convergence from Bartonian to Chattian. This motion occurs along the axis of the Bay of Biscay from the Santonian–Campanian boundary to the middle Priabonian, subsequently jumping to King's Trough at Anomaly 17 (36.62 Ma).

Cenozoic deformation of Iberia: A model for intraplate mountain building and basin development based on analogue modeling

Inferences from analogue models support lithospheric folding as the primary response to large‐scale shortening manifested in the present day topography of Iberia. This process was active from the late Oligocene‐early Miocene during the Alpine orogeny and was probably enhanced by reactivation of inherited Variscan faults. The modeling results confirm the dependence of fold wavelength on convergence rate and hence the strength of the layers of the lithosphere such that fold wavelength is longest for fast convergence rates favoring whole lithosphere folding. Folding is associated with the formation of dominantly pop‐up type mountain ranges in the brittle crust and thickening of the ductile layers in the synforms of the buckle folds by flow. The mountain ranges are represented by upper crustal pop‐ups forming the main topographic relief. The wavelengths of the topographic uplifts, both, in model and nature suggest mechanical decoupling between crust and mantle. Moreover, our modeling results suggest that buckling in Iberia took place under rheological conditions where the lithospheric mantle is stronger than the lower crust. The presence of an indenter, inducing oblique shortening in response to the opening of the King's Trough in the north western corner of the Atlantic Iberian margin controls the spacing and obliquity of structures. This leads to the transfer of the deformation from the moving walls towards the inner part of the model, creating oblique structures in both brittle and ductile layers. The effect of the indenter, together with an increase on the convergence rate produced more complex brittle structures. These results show close similarities to observations on the general shape and distribution of mountain ranges and basins in Iberia, including the Spanish Central System and Toledo Mountains.

Anomalous mantle at 45°N Mid‐Atlantic Ridge?

The Mid‐Atlantic Ridge at 45°N has a ridge crest depth close to normal for slow spreading ridges and a crustal thickness of 6 km. However, the free‐air gravity over the region is higher than expected for such axial depths, and erupted basalts are enriched in incompatible trace elements and radiogenic isotopes, which could be taken to indicate the presence of a mantle plume beneath the spreading axis. But the ridge is at normal depths and the crust is of normal thickness. We model free‐air gravity across and parallel to the strike of the ridge. A density structure derived from a cooling model provides a good fit for gravity across the ridge axis. A gravity model along the ridge axis indicates that most of the gravity signal over the Azores and Reykjanes Ridge is related to thicker crust, while the gravity signal over 45°N can be modeled by a density anomaly, which extends along the spreading axis from about 43°N to 50°N and from zero age to 35 m.y. Such a mass excess can be explained by an increase in the garnet content of the mantle within the stability field of garnet peridotite. The enriched basalts occur in the same region as the mass anomaly. We interpret them as originating from cool hotspot mantle. This anomalous mantle may derive from a hotspot that existed at the North America‐Africa‐Eurasia triple junction when it was at 45°N between 59 and 26 m.y. The King's Trough Complex is the remnant of that hotspot. We consider that this cool hotspot mantle still underlies the spreading axis and contributes to crustal construction.

The intermediate-wavelength magnetic anomaly maps of the North Atlantic Ocean derived from satellite and shipborne data

SUMMARY Two intermediate-wavelength magnetic anomaly maps of the North Atlantic Ocean are derived based on satellite and marine magnetic observations. The satellite map is produced from the covariant features of POGO and Magsat dawn and Magsat dusk magnetic anomalies. The marine map is compiled using shipborne data collected since 1965. The two maps are in excellent agreement over well-defined anomalies both in location and amplitude, suggesting that these anomalies are real and reflect magnetization contrasts in the oceanic lithosphere. Both maps show the intermediate-wavelength seafloor-spreading anomalies of the Cretaceous quiet zone of the central North Atlantic Ocean as well as the negative seafloor-spreading anomaly of the Labrador Sea caused by dominantly reversed polarity periods. The Iceland hotspot, Alpha Ridge, Azores High, King's Trough and the Rockall microcontinent have positive anomalies. There is no consistent magnetic signature associated with the ocean-continent boundary, although the deep sedimentary basin of Nova Scotia is delineated by a well-defined negative anomaly.

King's Trough: reactivated pseudo-fault of a propagating rift

SUMMARY A compilation of magnetic data in the western North Atlantic shows that seafloor spreading magnetic anomalies 26 to 18 are dislocated across a (pseudo-) fault with a direction mirroring that of King's Trough. It is interpreted that this feature and King's Trough were formed by northward propagation of the rift, initiated by a hotspot that created the Azores-Biscay Rise. The rift propagation stopped when King's Trough became part of the plate boundary between Eurasia and Iberia at chron 17. Most of the present-day structure of King's Trough was created by extensional motion across this plate boundary from chron 17 to 9/6c.

Motion of Iberia since the Late Jurassic: Results from detailed aeromagnetic measurements in the Newfoundland Basin

A detailed aeromagnetic survey carried out in the Newfoundland Basin shows well developed seafloor spreading anomalies 24 to 34. Comparison of these anomalies with the corresponding anomalies in the Northeast Atlantic suggests asymmetric spreading from anomalies 31 to 34, with slower spreading in the Newfoundland Basin. Anomaly M0 is a very weak anomaly in the Newfoundland Basin compared to south of the Newfoundland Fracture Zone where it forms a prominent low within the large amplitude “J” anomaly. A similar behaviour of this anomaly is observed off Iberia. In the Newfoundland Basin it does not continue as far as the Flemish Cap but terminates in the vicinity of the Newfoundland Seamounts. The position of this anomaly as obtained here differs from previous identifications. The shapes of magnetic lineations in the Newfoundland Basin are significantly different from the corresponding lineations off Iberia. This has been interpreted as arising from shifts in the plate boundary between Africa and Eurasia during the time when Iberia was moving as part of the African plate. By combining the present data with other detailed survey data to the north we have been able to derive a plate kinematic solution for Iberia which shows that from the middle Cretaceous to the Late Eocene Iberia moved as part of the African plate and then as an independent plate until the Late Oligocene. Since then it has been moving as part of the Eurasian plate. During these times the boundary between Eurasia and Africa jumped successively from the Bay of Biscay accretion axis to the King's Trough-North Spanish Trough lineament to the Azores-Gibraltar Fracture Zone. The kinematic solution for Iberia so derived, from chron M0 to the present, not only explains the formation of some prominent bathymetric features in the oceanic regions, such as King's Trough, but equally well the formation of geological features on land, such as the Pyrenees. The difficulties in deriving a kinematic solution for Iberia for times earlier than chron M0 are discussed and a speculative position of Iberia at the time of its initial separation from the Grand Banks of Newfoundland is proposed. Furthermore, with the availability of a well-constrained model for the motion of Iberia, it should now be possible to relate more accurately the relative motions among Eurasia, Iberia and Africa to the history of the Mediterranean region.

Iberian plate kinematics: a jumping plate boundary between Eurasia and Africa

THE rotation of Iberia and its relation to the formation of the Pyrenees has been difficult to decipher because of the lack of detailed sea-floor spreading data, although several models have been proposed1–7. Here we use detailed aeromagnetic measurements from the sea floor offshore of the Grand Banks of Newfoundland to show that Iberia moved as part of the African plate from late Cretaceous to mid-Eocene time, with a plate boundary extending westward from the Bay of Biscay. When motion along this boundary ceased, a boundary linking extension in the King's Trough to compression along the Pyrenees came into existence. Finally, since the late Oligocene, Iberia has been part of the Eurasian plate, with the boundary between Eurasia and Africa situated along the Azores–Gibraltar fracture zone.

A note on the isostatic compensation and origin of King's Trough

Summary. The spectral analysis of a 550 km long ship-board gravity and topography profile, across the central part of King's Trough in the north-eastern Atlantic Ocean, shows that the topography is regionally compensated on lithosphere older than 20Myr. This feature, with its enigmatic trend oblique to that of the mid-Atlantic ridge, therefore dates from no earlier than the late Oligocene and was formed by intra-plate activity. Such a constraint to its age agrees with the recent analysis of geological samples taken on the northern flank of the trough close to the position of the profile. It is not consistent with previous suggestions that King's Trough was created either by the rotations of the Iberian plate to form the Pyrenees or by a readjustment in the strike of the mid-Atlantic ridge. Furthermore, this analysis shows no evidence for either thin crust or high density mantle under the trough. There is no structural evidence to suggest that it formed as part of a compressional boundary.

A note on the isostatic compensation and origin of King's Trough

Summary. The depth and seismic source parameters of the three largest reservoir-induced earthquakes associated with the impoundment of Lake Kariba, Africa, were determined, using a moment tensor formalism of the generalized inverse technique. The events, characterized by mb > 5.5, consisted of the foreshock and main event (0640 and 0901 GMT, 1963 September 23) and the principal aftershock (0703 GMT, 1963 September 25). From the analysis of analogue rift valley source crustal models, a three-layer over a half-space model were selected for the analysis of the Lake Kariba events. The epicentral proximity and the comparable source parameters of the three large events suggest a common source fault with an approximate strike of S 9°W ± 4°, dip of 62°± 10°, and rake of 266°± 4°. The body wave modelling indicates that the Lake Kariba events occurred at depths of less than 10 km. The inferred source parameters, complemented by the structural maps derived from the photo-interpretation of Landsat imagery of the region, indicate a normal fault that uniquely transects the reservoir's deep Sanyati Basin in a north-south heading. The active fault seems to correlate with the location of the Umi River's submerged river channel. The confirmation of shallow depths and the determination of source parameters establishes a basis for addressing the actual triggering mechanisms of these earthquakes.

The geology and formation of King's Trough, northeast Atlantic Ocean

King's Trough comprises a 450 km long chain of roughly parallel basins and flanking ridges situated 700 km northeast of the Azores. Its formation has been variously described as: a short-lived plate boundary; a compressional boundary; or a re-orientated former bend in the Mid-Atlantic Ridge. Recent studies of seismic and gravity data concluded that the flanking ridges result from hot-spot activity which began around 55 m.y. B.P., while the trough could have been produced by contemporaneous or later rifting or shearing. The geological sampling described in this paper was planned to resolve the mode of formation of the trough. A long-range sidescan sonar survey of the region was used as control for new bathymetric and geologic maps of the area and for transponder-navigated dredging and coring operations on the flanks of the trough. The samples recovered show that basalt volcanism dated at over 52 m.y.B.P. was followed by the deposition of a chalk sequence until Miocene times. Trachyte volcanism at 32.2 m.y.B.P. was followed by a period of tuff and ash volcanism near sea level in the Late Oligocene and Miocene. A middle Miocene chalk sequence is at present exposed on the trough walls. The detailed morphology and geological sequence at King's Trough agrees best with a hypothesis which involves uplift by around 2 km of an aseismic ridge (“hot-spot trace”) at about 32 m.y. ago which was followed by intra-plate rifting at around 16–20 m.y.B.P. A number of interpretations are possible for the amounts of vertical displacement during this rifting. We favour total vertical fault movements of the order of 2.5 to 3.5 km.

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 1500-3000 m above its surroundings, which extends from about 45'N, 1530'W towards the Azores. Its south-western termination is near 4030'N, 2130'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 (76-56Ma ago). The western counterpart of the Rise includes Gauss and Milne seamounts in the Newfoundland Basin. Magnetic anomaly 3 1 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.

Western extension of the Iberian-European plate boundary during the Early Cenozoic (Pyrenean) convergence: A new model

A dense seismic net-work allows of drawing the western prolongation of the northern boundary of the Iberian plate. This early Cenozoic deformed boundary is disrupted by two prominent NW-SE (transform ?) faults. A kinematic reconstruction, with a NW Late Cretaceous-Lower Eocene motion of the Iberian plate, is proposed. The Azores-Biscay Rise is interpreted as a short-lived subduction zone, whereas King's Trough could be a transform fault.

The nature and petrogenesis of intra-oceanic plate alkaline eruptive and plutonic rocks: King's Trough, Northeast Atlantic

Basalts, diorites, and gabbros dredged from the side of King's Trough — a mid-plate fracture on the northeast Atlantic sea floor — are alkaline in character based on major- and trace-element chemistry and mineralogy. The variation in bulk chemical composition and in mineral paragenesis and composition suggest differentiation in a large magma chamber beneath a seamount volcano. Fractional crystallization took place in a hydrous magma and is characterized by early formation of a Ti-rich biotite as rims on olivine. Thus, pyroxene compositions are unusual in having relatively low molar Ti/Al ratios, and increasing differentiation is marked by progressively decreasing ratios of K to Na.

The structure of King's Trough, Northeast Atlantic, from bathymetric, seismic and gravity studies

Summary. We present new seismic and gravity data from the linear chain of deeps and flanking ridges known collectively as King's Trough, and combine them to produce crustal models of the western end of the complex. These models show that there is an abnormally low-velocity , low-density crust under the trough itself, and that the whole King's Trough feature is situated in a region of slightly thicker than normal oceanic crust. The flanks and basins of King's Trough are not in local isostatic equilibrium, although the feature now appears to be inactive. We believe these data indicate a history of extension and of hot-spot activity at King's Trough, and that the feature was formed either as a slow-spreading arm of an R-R-R triple junction associated with a Mid-Atlantic Ridge hot-spot, or by subsequent rifting of a preexisting hot-spot trace.

Geophysical studies of the Atlantic sea floor near 40°N, 24°W, and its relation to King's Trough and the Azores

A detailed survey of a 1°-square of sea floor 100 miles northeast of the Azores has revealed the presence of scarps over 30 km long striking generally 120°, transverse to the regional topographic and magnetic lineations. They are not associated with a major fracture zone. Sedimentation in the area appears to have been by a mixture of pelagic and turbidity-current processes, with a Miocene increase in sedimentation rate probably indicating the subaerial emergence of the Azores Archipelago. Magnetic anomalies were identified along a number of profiles between the survey area and the Mid-Atlantic Ridge crest, and indicate a short period of increased spreading rate from about 12-3 m.y. ago.

Western extension of boundary between European and Iberian plates during the Pyrenean orogeny

The kinematics of the Eocene episode of compression between the Iberian and European plates is examined. It is proposed that the boundary between the two plates extended west of the Pyrenees, along the Spanish marginal trench to end west of King's trough at a triple point junction. The pole of relative rotation computed within this hypothesis lies near 59°N; 62°W and the resulting shortening along the Pyrenees is about 50 km along 45°E.

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.