Crustal Spreading Research Articles

Intraplate tectonics in stable Europe as related to plate tectonics in the Alpine system

Western Europe is traversed by the Rhinegraben rift system. The stages of graben formation evolved coincidentally with the culminations of compressional folding in the Alps. Rhinegraben rifting has been controlled by mantle diapirism, but the Alpine orogeny by subduction of lithosphere. Presumably, Alpine subduction forced compensating mantle uplift in the foreland. The Middle Eocene to Oligocene crustal spreading of the Rhinegraben implies a state of stress with a maximum horizontal component parallel to the graben axis (about 20‡). In the same area, the Recent average direction of maximum compressive stress is of about 320‡ (NW), as calculated by in-situ stress measurements, fault-plane solutions of earthquakes and Recent crustal movements. The rotation of the stress components relative to the crust of stable Europe evolved subsequent to counterclockwise rotations of microplates in the Mediterranean. A model is proposed which ascribes these rotations to alterating shear motions of the Afro-Arabian macroplates relative to stable Europe exerting a ball-bearing effect to the intervenient microplates. The postulated motions are in accord with the patterns of inhomogeneous ocean floor spreading east and west of the African plate. The stages of Alpine plate collision had induced a significant readjustment of intraplate stress conditions, and deformation in the cratonic foreland of stable Europe.

Two-dimensionality of magnetic anomalies over Iceland and Reykjanes Ridge

The relationship between the magnetic anomalies over Iceland and those over Reykjanes Ridge is investigated using the data of the 1965 Dominion Observatory survey. A method is developed for determining the two-dimensionality of the anomalies from the component data measured in this survey. This method is based on testing the first and the second derivative of the magnetic potential with respect to the direction of two-dimensionality, using the component data along a single flight line. Testing the first derivative also yields the direction of two-dimensionality. The outcomes of the two tests (based on a single line) are compared with the observed two-dimensionality (established by narrowly spaced earlier surveys) of Reykjanes Ridge, showing good agreement. As the outcomes of the two tests provide complementary information they are combined into a single factor: A. This factor of two-dimensionality is very low for the anomalies over the shelf of Iceland indicating that the anomalies over Iceland cannot be continued directly into those over Reykjanes Ridge. Over Iceland A is generally low. Over the neovolcanic zone in eastern Iceland twodimensionality is associated with long wavelengths that are not present in the spectrum of the anomalies over Reykjanes Ridge. Thus, Reykjanes Ridge-type anomalies are absent with the exception of the central anomaly. This may not be used as evidence against crustal spreading since the kinematic model proposed by Palmason for Iceland has a wide transition zone between rock of opposite polarity. The same model if computed for a mid-ocean ridge has narrow transition zones. The larger width of the transition zone blurs the anomalies related to the reversals of the earth magnetic field.

Global study of seismic wave attenuation in the upper mantle behind island arcs using pPwaves

Observations of striking and consistent differences in the attenuation of pP phases produced by mantle earthquakes and recorded by the World-Wide Standard Seismograph Network (WWSSN) provide data for mapping variations in the attenuation of high-frequency (0.5- to 2-Hz) compressional waves in the wedge of mantle above nearly all of the inclined seismic zones on earth. The data reveal several zones of high attenuation that in nearly all cases correspond to major presently active tectonic features above or near the inclined seismic zones. Zones of high attenuation behind the Tonga, New Hebrides, Mariana, and Japanese island arcs coincide with zones of presently or recently active crustal extension and creation of marginal basins. In western South America a zone of high attenuation underlies most of the broad uplifted terrain of the Andean Altiplano. In contrast to the zones beneath active marginal basins this zone is not related to crustal spreading. A high-attenuation zone exists beneath the Sea of Okhotsk that, although it is near, is offset from the Kuril-Kamchatka arc and thus may not be directly related to that system. This zone may instead be related to possible offshore continuations of the Baykal-Aldan rift system or the postulated plate boundary between Asia and North America. Where they are determined, the high-attenuation zones seem to exist in the upper 200–300 km of the mantle. Wherever data are available, they show that the zones of high compressional wave attenuation correspond to zones of high attenuation for shear waves, low seismic wave velocities, and high heat flow. These properties, in addition to the geological evidence of crustal extension or uplift, suggest that the compressional wave attenuation results from high temperatures and/or partial melting of the upper mantle material. Although all the high-attenuation zones occur near inclined seismic zones that reach depths greater than 300 km, the absence of such zones in the Indonesian, Philippine, Izu-Bonin, and New Britain-Solomon regions shows that lithospheric subduction to great depths is not a sufficient condition for their occurrence. The zones of high attenuation are also not systematically related to the distributions of active volcanos; this condition, together with other data, indicates that large bodies of partially melted material probably do not exist beneath the volcanos of island arcs.

The interrelationships of crustal and upper mantle parameter values in the Pacific

The data from some 400‐odd seismic refraction measurements of crustal structure in the Pacific Ocean basin are examined from the standpoint of possible systematic interrelationships of crustal and upper mantle parameter values as a function of crustal age, changes in heat flow alone and in combination with crustal age, changes in crustal spreading rate, changes in free air gravity anomaly values, and possible changes in the petrology of newly formed crust on the rise crests. It is found that there are systematic interrelationships between depth of water, thickness of the crystalline rock crust, mean seismic velocity of the crust, and seismic velocity of the mantle and also that these interrelationships change with crustal age. The age periods that appear to have significance in terms of the duration for a given set of interrelationships are 0–25, 25–36, 36–63, 63–90, and 90–≈110 m.y. Although it is likely that this pattern of changes in the interrelationships of crustal and upper mantle parameter values at intervals of between 20 and 30 m.y. continues for ages greater than 110 m.y., the data available for older age areas free of volcanic and tectonic disturbance are too few to permit the establishment of meaningful results. In the first age zone (0–25 m.y.) all parameter values for the crust and upper mantle vary directly with each other, so that on an overall basis the decrease in heat flow encountered in moving away from the rise crest axis is accompanied by an increase in depth of water, a thickening of the crust, and an increase in mean crustal velocity and mantle velocity. The second age zone (25–36 m.y.) marks a zone of transition in which a new pattern is established, the mean velocity of the crust bearing an inverse relationship to the velocity of the mantle and the thickness of the crust. This zone of transition appears to be related to the rise flank zone of low heat flow. The change from a direct to an inverse relationship between mean crustal velocity and the velocity of the mantle appears to be related to an anhydrous change as a consequence of temperatures building up beneath the Moho in association with an underlying phase transformation boundary in the upper mantle. The parameter values of the crust and upper mantle found beyond the zone of low heat flow appear to represent frozen characteristics resulting from the anhydrous transformation's not being able to reverse because the water released escaped. There is a correlation of crustal and upper mantle parameter values with past crustal spreading rate and also free air anomaly values for crust having an age of >36 m.y. The correlation with spreading rate is related to the time available for modification as the crust passed through zones of crustal change such as that associated with the low heat flow zone. The correlation with gravity is related to the original composition of the crust formed beneath the rise crest spreading center and its modified compositional thickness. All the evidence suggests that the original Moho is a chemical discontinuity and that the original composition and structure of the crust and upper mantle is related to density segregation. Once established, this structure is modified by hydrothermal metamorphism, the gabbro‐garnet granulite‐eclogite transformation, and olivine‐serpentine transformation. These transformations appear to go on concurrently where heat flow exceeds 1.7 µcal/cm² s, but in the zone of low heat flow, each appears to become progressively inactive and terminates. Beyond the zone of low heat flow and for ages of >36 m.y. the only active process is densification and subsidence of the crust as a consequence of continued cooling with increasing distance from the rise crest, the subsidence being in part due to the weight of the attendant increase in depth of water. Otherwise, crustal relations are as they were developed in the zone of low heat flow.

Geomagnetic reversals and crustal spreading rates during the Miocene

Statistical analysis of the geomagnetic time scale suggests that the rate of reversals was anomalously low during the Miocene. To determine whether undetected reversals actually occurred in the Miocene, 14 magnetic profiles from a survey of the northeast Pacific by NOAA are analyzed by signal-averaging techniques. The data suggest that during the time period 7.3 to 22.7 m.y., eight short-wavelength anomalies are present that are not predicted by previously published time scales. Anomaly 5 in particular, originally considered a major polarity epoch, is actually composed of numerous shorter events. Additional data, both from the east Pacific rise and the Indian-Antarctic ridge, corroborate these observations. A statistical study of the spreading rates of all the data indicates apparent accelerations of the order of 30 km/m.y.2 for the Pacific, Indian, and antarctic tectonic plates. These abrupt changes in spreading rate are considered artifacts due to errors in the published time scale, and a new Miocene time scale is derived on the assumption of constant spreading rates for the three plates. The new time scale contains 18 additional reversals and 14 short events ranging from 10,000 yr to 0.1 m.y. in length. The widths of the transition zones from normal to reversely magnetized crust are determined to be 6 km for the northeast Pacific data, which is consistent with a recently proposed two-layer crustal model.

Model for the Late Cenozoic Tectonic History of the Mojave Desert, California, and for Its Relation to Adjacent Regions

The Mojave Desert region is crossed by young northwest-striking right-slip faults. Restoration of their displacement indicates that the over-all shape of this region has been considerably distorted. The assumption that the region was always in contact with the Sierra Nevada forms the basis for the following conclusions: (1) The slices between the faults, and the faults themselves, were rotated counterclockwise, possibly by as much as 30°; the precise magnitude and timing of the rotation can be checked by paleomagnetic studies. (2) Because of this deformation, the part of the San Andreas fault adjacent to the Mojave Desert was bent; originally the fault was much straighter. (3) The major deformation of the Mojave Desert was a west-trending left shear that was supplemented by the left slip on the Garlock fault. A similar deformation, without rotation of blocks, was accomplished in the eastern Transverse Ranges by left slip on approximately west-striking faults. This crustal shearing resulted from the position of the Mojave Desert between the regions of crustal spreading in the Great Basin and those in the continental borderland and the Salton Trough. The shearing of the Mojave Desert accommodated lateral variations of crustal spreading. These deformations distorted the shape of the margin of the North American plate during Neogene and Quaternary time. They mainly reflect movement normal to the plate boundary, whereas much larger movement occurred contemporaneously parallel to the plate boundary.

Neogene Carpathian Arc: A continental arc displaying the features of an ‘island arc’

The Carpathians consist of a series of structural zones closely comparable to the following zones of island arc regions: ocean floor, trench, folded arc, magmatic arc, retroarc basin, and continental margin. The K2O and SiO2 ratio of the magmatic rocks suggests the dipping of a Benioff plane along which the floor with a crust of oceanic character was subduced and is replaced by the modern Carpathian marginal trench. Melting of part of the plate led to the formation of the Carpathian magmatic arc and to the crustal spreading of some retroarc basins (Pannonian basin, Transylvania basin). The Transylvania basin may be interpreted as an interarc basin according to Karig's hypothesis regarding the formation of the retroarc basins.

Mars: Troughed terrain

An assemblage of huge parallel troughs, individually up to 200 km wide and hundreds of kilometers long, forms a belt 500 km wide extending for 2700 km E 15°S through the Tithonius Lacus-Coprates region of equatorial Mars. Some troughs are closed depressions more than 3 km deep, and the maximum trough depth may approach 6 km. These troughs, heading just southeast of the huge Tharsis volcanic ridge, are transitional westward into a festoon of U-shaped hollows and eastward into chaotic and fretted terrain. Parallel linear chains of rimless pits and shallow graben on the adjacent cratered upland suggest structural control of trough development by fractures in the Martian crust. Trough walls show scarring by slides, slumps, and U-shaped avalanche chutes. Some are dissected into narrow sharp anastomosing ridges reminiscent of badland topography. Dendritic tributaries extend 150 km into the bordering upland. These tributaries and the mass movements causing recession of trough walls are attributed to a sapping process possibly involving the evaporation or melting of exposed ground ice. Trough floors range from chaotically rough to rolling and subdued. They are not smoothly graded. In one instance layered materials, possibly 2 km thick, compose a dissected trough floor tableland. Most troughs may be partly filled by such deposits. The major problem of trough genesis involves disposal of about 2 × 10^6 km^3 of material. Running water, solution, deflation, and ground ice deterioration all appear to have significant limitations as the sole or principal agent of trough formation. Subsidence caused by magma withdrawal to supply the extensive nearby volcanic fields or the spreading of crustal plates are both quantitatively adequate. Crustal spreading would have significant implications concerning the conditions and the behavior of the Martian interior, and the lack of any obvious subduction zones would imply planetary expansion. In our ignorance concerning Mars both magma withdrawal and crustal spreading merit continued consideration in respect to trough formation, with some favor to the former because of the obvious large-scale Martian volcanism. Ground ice may have played a significant subsidiary role through sapping to produce extensive recession of trough walls.

Origin of Offsets of the Mid-Atlantic Ridge in Fracture Zones

The offsets of the axis of the Mid-Atlantic Ridge in equatorial fracture zones are characterized by the existence of transverse ridges running adjacent and parallel to the east-west trenches marking each of the fracture zones. These transverse ridges are made essentially of blocks of ultramafic rocks, probably emplaced at the offsets as a result of upward solid protrusion from the upper mantle into the east-west crustal fractures. The ultramafic blocks appear to be older than the adjacent sea floor, and to behave as nonspreading blocks plastered between spreading subplates; they may be coupled with a stagnant zone postulated to exist in the upper mantle beneath the spreading axis. A hypothesis is presented whereby ultramafic transverse protrusions are related to the formation of the offsets of the Mid-Atlantic Ridge and of the trenches associated with the offsets. During the early stages of development of the Atlantic Rift, the ultramafic blocks protruded from the upper mantle where the rift intersected preexisting east-west crustal fractures. Where the linear, north-south pattern of upwelling (pattern corresponding to the axis of the proto-Atlantic Rift) abutted against a transverse ultramafic protrusion, "normal" upwelling and basaltic volcanism were prevented; their "normal" north-south linearity resumed at one edge of the protrusive body. Consequently, separation of Africa and South America took place along an offset pattern. During further spreading, remnants of the transverse ultramafic protrusions may have been left as semistagnant blocks within the offsets of the Mid-Atlantic Ridge. The offsets of the Mid-Indian Ridge have features similar to those of the Mid-Atlantic Ridge, while the offsets of the East Pacific Rise in the equatorial and South Pacific are systematically different from the Atlantic-Indian-type offsets: for instance, they lack deep transverse valleys and ultramafic protrusions. The East Pacific Rift probably initiated along a roughly continuous line; the offsets developed gradually during subsequent crustal spreading. These differences between the equatorial-southern Pacific rift and the Atlantic-Indian rifts are explained by the fact that the former originated in a proto-oceanic region; the latter instead originated within a protocontinent.

Sedimentary and Tectonic History of Ouachita Mountains: ABSTRACT

The Ouachita Mountains of Oklahoma and Arkansas contain Paleozoic flysch-like geosynclinal rocks exposed by elongated east-west folds and thrust faults. Approximately 5,000 ft of Cambrian to Devonian flysch consists predominantly of dark slates and cherts, comprising a classical starved succession. Minor incursions of mature sands, apparently from the North American craton, invaded the trough at three different intervals. The succeeding Carboniferous, almost 40,000 ft thick, consists of proximal and distal turbidite sandstones, black shales, and minor interlayered wildflysch and volcanic ash. Sedimentary structures indicate southwestward, westward, and northwestward sand dispersal. Sandstone compositions suggest a cratonic, quartz-rich provenance as well as a fel spathic, lithic extracontinental source. The tectonic setting may well have been due to oceanic crust spreading northwestward, plunging under continental crust, and creating an island-arc-trench-subduction zone whose present location is overlapped by post-Paleozoic rocks. Northwest of the trench, a complex of slope, rise, and abyssal sediments formed upon the depressed outer margin of continental crust. East of the Ouachitas, continent-continent collision caused suturing of Africa and North America, which created source materials that were subsequently emplaced as a westward-building subsea cone during the Carboniferous. In the Ouachita area, continued subduction finally created a series of uplifted tectonic lands resulting in northward sliding of the sedimentary succession as continentward-directed folds and thrust sheets. ubsequent stress-field orientation changed so that the area then became dormant. End_of_Article - Last_Page 796------------

Crustal Spreading in Southern California

The current excitement among geologists and geophysicists stemming from the "new global tectonics" has led to a widespread, speculative reinterpretation of continental geology. The Gulf of California and its continuation into the Imperial Valley provide an excellent opportunity for studying the border zone between the North American and Pacific plates, and an interface of continental and oceanic tectonics. The Salton trough, the landward extension of the gulf, is a broad structural depression, comparable in size with the deeper marine basins of the southern part of the gulf, but here partially filled with sediments deposited by the Colorado River.

Surface structure and plate tectonics of Afar

The surface structural elements of Afar are indicative of crustal tension. The dominating feature is intense slicing of the Afar Neogene volcanics by early Pleistocene fault-belts, whose trends are related to those of the three rift systems converging on Afar. This faulting shows that crustal extension affected the whole width of Afar, rather than being confined to oceanic-type, axial zones. Nevertheless, the computed amounts of extension agree well with results of plate analysis of the Afar triple-junction. Refined plate analysis for this region yields data consistent with rotation of the Danakil horst, with inferred presence of cross-rift faults, with longitudinal shearing in eastern Afar, with the apparent asymmetry of spreading in the western Gulf of Aden, and with an early episode of north-south spreading in southernmost Afar. Crustal spreading in Afar seems to be a long-term stop-go phenomenon, initiated at the beginning of the Cainozoic (70 m.y. B.P.). The largest tectonic episode was probably related to the Upper Pliocene-Lower Pleistocene swell uplift of Ethiopia, and the splitting apart of the Danakil and Aisha horsts.

Reply to a paper by C. G. A. Harrison: "Comments on a paper by Watkins and Richardson: 'Intrusives, Extrusives and Linear Magnetic Anomalies'"

Harrison agrees with us on the following points: that extrusives exist on the sea floor; that triangular cross-sections are probably realistic for such extrusives; and that it is worthwhile to consider their magnetic effect. For example, Hamson states that formation of a dyke causes spreading: on the contrary, it is quite conceivable, or even probable, that spreading is the cause, rather than the result of dike injection. The major disagreements involve the models employed. In our original publication we were careful to point out that our computations were for what we considered to be a probable net configuration of some extrusives on some parts of the ocean floor. These models were based on speculations and bathymetric data by several authors. By no means was the possibility of other configurations excluded. To do so would be (to say the least) a claim to substantial intuitive powers. Harrison negates our conclusions, however, completely on the basis of an inferred universal applicability of his model, thus rejecting all other alternatives. Such confidence, while impressive, is in our opinion somewhat unrealistic. Our model deliberately employed a minimum of artistic liberties, whereas Hamson invokes a relatively complex (though elegant) configuration. As he emphasizes, the models have constant upper and lower surfaces, and normal polarity fraction, and our model does not. (We find ourselves unhappy at the use of flat upper surfaces for detailed analyses, however reasonable such a practice may be for large scale conventional crustal spreading analyses) . This is quite true. But why should our models suffer these limitations, which Hamson believes are obvious? Nature has not provided these restrictions, so we do not understand why Harrison should, particularly when we were not attempting to explain the magnetic effect of all extrusive configurations. We were clearly not considering the magnetic effects of various arrangements of a fixed volume of material, but rather the effect of extrusive accumulations of different volumes, but similar shapes. It is a fact that the extrusives in our models produce a sharpening of the surface magnetic anomaly, relative to that due to a dike alone, provided the widths do not exceed one ocean depth. Harrison’s extrusives do not produce this effect. We do not believe that our point is invalidated because Harrison has a different model, unless he has evidence to show that his models are realistic and ours are not. While we find the geometry of Hamson’s models attractive, we cannot readily accept the detailed extrusiveintrusive relationships, particularly in his model 3. Hamson emphasizes that it is impossible to describe the sharpness of the magnetic anomaly produced by a group of extrusives and intrusives, by examining the anomaly produced by one. In our original paper, we clearly stated that ‘ The body could of course be composed of a series of smaller units. This would be likely to be the case, as

A Discussion on volcanism and the structure of the Earth - Structure of the Icelandic basalt plateau and the process of drift

Conventional stratigraphic mapping of parts of the Icelandic flood basalt succession and regional studies of the palaeomagnetic stratigraphy suggest that in at least two areas the basalt pile is composed of large lenticular shield-like lava units. Each unit is related spatially to its own feeding dyke swarm and is the result of a protracted period of dominantly fissure volcanism from a single fissure zone. The geophysical evidence suggests that the most important seismic discontinuity is the boundary between layers 2 and 3: P seismic velocities 5.1 and 6.3 km/s respectively. This seismic discontinuity has been mapped over large areas of Iceland by Palmason who has shown that it is generally at a depth of between 2 and 5 km below sea level. In eastern Iceland the discontinuity is approximately horizontal and markedly discordant with the observed dip of the individual lavas at sea level. It is suggested that, under Iceland, layer 3 is composed of intrusive dykes and gabbroic masses, whereas layer 2 is made up of extrusives cut by dykes and smaller intrusions. The observed relationships of the lava lenses constituting layer 2 are compatible with a crustal spreading model. The drift away from the axial zone, largely accommodated by dyke injection, appears to be at about 1 cm/year, a rate comparable to that observed on the adjacent Reykjanes Ridge.

Magnetic anomalies south of the Murray Fracture Zone: New evidence for a secondary sea-floor spreading center and strike-slip movement

Detailed analyses of marine magnetic data that were collected between California and Hawaii south of the Murray fracture zone and north of the Molokai fracture zone reveal an apparent former center for secondary crustal spreading that trends north-south along the 133°W meridian in this area. This secondary crustal spreading center probably originated as a flank fissure and center of volcanic eruption west of the main center of the East Pacific rise at the time of magnetic anomaly 13 (about 38 m.y. ago) and ceased to be active at about the time of anomaly 7 (about 27 m.y. ago). The center appears to have migrated west at a rate of 2.9 cm/yr during its period of activity, while new crust was spreading from the center at the same rate. This movement, in combination with the movement of 4.0 cm/yr from the primary center of crustal spreading, resulted in a total crustal movement rate of 9.8 cm/yr south of the Murray fracture zone and west of the secondary spreading center. During the 11 m.y. of its existence, the center increased the original 150-km right lateral transform-fault displacement along the Murray fracture zone to 680 km by right lateral strike-slip movement along the fracture. The strike-slip movement gave rise to such structures as apparent tension gashes across the Murray fracture zone.

Ethiopian rift and plateaus: Some volcanic petrochemical differences

Volcanism on the Arabo-Ethiopian swell has accompanied the development of the three traversing spreading zones conjoining at Afar: the Red Sea, Gulf of Aden, and African rift systems. The Red Sea and Gulf of Aden floors are formed by oceanic tholeiites, but Afar and the main Ethiopian rift show a wider range of more alkaline volcanics. related to slower crustal spreading rates. The high plateaus between the rifts have had a strongly alkaline volcanism. The plateau basalts and especially the intermediate lavas are much more undersaturated than their rift equivalents, and are enriched in potash and Al/Fe. The silicic volcanics of the rift are soda-iron enriched relative to the plateau silicics. Magmatic composition can be equated with depth of melting, but the voluminous silicic volcanics are not the products of fractionation, direct partial melting, or anatexis of continental crust: the accretion hypothesis of Battey is supported and developed.

Temperature field and geophysical effects of a downgoing slab

The factors affecting the thermal behavior of a lithospheric slab descending into the mantle are so numerous and complicated that only numerical methods can accurately account for them. A series of finite-difference calculations of the temperature field, including the effects of viscous dissipation, adiabatic compression, phase changes and radioactive heat generation are carried out, and the relative effects of different parameters are investigated. An analysis of the stability and convergence of the numerical method indicates that the errors are small and can be reduced to any desired level by varying the grid size and the time steps. At a crustal spreading rate of 8 cm/yr, with all heat sources, the slab reaches thermal equilibrium with the surrounding mantle at a depth of about 650 km. Among observable geophysical quantities, seismic travel times and amplitudes provide the most information about the slab. Surface heat flow is sensitive to subsurface conditions that are at relatively shallow depths, and gravity anomalies are broad and are masked by crustal effects. Three dimensional calculations predict strong bending of seismic rays near slabs, which causes strong focusing and produces shadow zones. Analysis of travel-time data for the Tonga-Fiji region indicates that waves propagating down the slab from shallow events are advanced by about 4 sec. Observations and theoretical travel-time anomalies based on calculated temperature fields are in general agreement.

North Atlantic Crustal Genesis in the Vicinity of the Azores

In terms of current knowledge of crustal genesis in the Atlantic Ocean, several unique or highly anomalous features exist in the vicinity of the Azores. These include the seismically active East Azores Fracture Zone extending from Gibraltar to the Mid-Atlantic Ridge; the seismically inactive West Azores Fracture Zone which is offset northwards from the trend of the East Azores Fracture Zone; the transverse island chain of the Azores islands which trends southeast-northwest across the Mid-Atlantic Ridge; the marked change in direction of the Mid-Atlantic Ridge from northeast-southwest to north-south; and the broadening of the Mid-Atlantic Ridge to the east. Bathymetric and magnetic data from surveys of the Mid-Atlantic Ridge in the Azores area by R.V. Trident, the U.S.Naval Oceanographic Office, and other sources have been compiled. These, together with the previously published data are compatible with a basic model consisting of a Mid-Atlantic Ridge migrating eastwards at the local crustal spreading rate, which is greater to the north than to the south of an east-west transverse fracture system. Superimposed on this is the development of a northwest-southeast trending secondary spreading centre or triple junction, within a ‘leaky transform’ system (Menard & Atwater 1968) which would have developed as the result of a change in the local crustal spreading direction south of the east-west transverse fracture system from mainly east-west to mainly northwest-southeast. Simple geometrical considerations, when combined with the length of a newly proposed ‘Terceira Rift’ and independent estimates of local crustal spreading rates suggest that the change in crustal spreading direction and triple junction development began at least 45 My ago.

A tectonic analysis of the south-west Pacific

Abstract The regional structure of the south-west Pacific is attributed to the spreading and segmentation by sub-crustal convection currents of a Paleozoic—Mesozoic geosynclinal belt marginal to the Australian craton, with migration of the separated sialic fragments northward and eastward into the Pacific Basin. The source of the convection currents presumably lay beneath the mid-ocean rises (Indian-Antarctic and Pacific-Antarctic Ridges) south of Australia and New Zealand. During Cenozoic times Australia has moved northward relative to the New Zealand Plateau, the migration of the latter having a greater eastward component that allowed opening of the Tasman Basin. As a result of these differential movements the geosynclinal belt extending from New Zealand across the north-east margin of the Australian craton has been subjected not only to crustal spreading and stretching in a north-easterly direction with anticlockwise rotation, but also to north-south extension and tensional rifting. The New Guinea sect...

Structural Evolution of the Rift Zones of the Middle East—a Comment

ROBERTS1 has explained differences in the rate and direction of crustal spreading in the Red Sea and Gulf of Aden by postulating approximately east–west extension across the Ethiopian rift at a rate of 0.7 cm yr−1 per limb. This rate would produce 140 km of widening of the Ethiopian rift during the last 10 m.y., an amount approximately twice the width of this rift2 and twenty times the crustal extension that can be deduced from the observed structure. The eastern rift of Africa differs from the Red Sea and Gulf of Aden rifts in its small amount and rate of crustal extension, the importance of alkaline and silicic volcanism, and distinctive Bouguer gravity profiles3. These three branches of the rift system ought not to be regarded as three convergent spreading axes with comparable rates of spreading.