Ridge Propagation Research Articles

Mechanics of wrinkle/ridge transitions in thin film/substrate systems

The mechanics of the formation and propagation of ridges on compressed stiff film/compliant substrate systems is studied theoretically and experimentally. Ridges form on bilayer systems where the elastomeric substrate is subject to a significant pre-stretch prior to attachment of the film. When the bilayer is then subject to increasing overall compressive strain, sinusoidal wrinkles first form and subsequently become unstable giving way to localized ridges with relatively large amplitudes. Two-dimensional plane strain simulations for neo-Hookean film/substrate systems reveal the transition from wrinkles to ridges under increasing compression and the reverse transition from ridges to wrinkles when the overall compression is subsequently reduced. For a significant range of pre-stretch, the two transition strains differ, and a significant hysteresis response is observed in a complete cycle of loading and unloading. The Maxwell equal-energy condition has been identified associated with co-existence of wrinkles and ridges and with the three-dimensional steady-state propagation condition for the ridges. Experiments conducted with a specially designed film/substrate loading system have been performed that confirm the essential features of ridge formation and the hysteretic behavior in loading/unloading cycles that span the two transitions.

Upper crustal seismic structure of the Endeavour segment, Juan de Fuca Ridge from traveltime tomography: Implications for oceanic crustal accretion

The isotropic and anisotropic P wave velocity structure of the upper oceanic crust on the Endeavour segment of the Juan de Fuca Ridge is studied using refracted traveltime data collected by an active-source, three-dimensional tomography experiment. The isotropic velocity structure is characterized by low crustal velocities in the overlapping spreading centers (OSCs) at the segment ends. These low velocities are indicative of pervasive tectonic fracturing and persist off axis, recording the history of ridge propagation. Near the segment center, velocities within the upper 1 km show ridge-parallel bands with low velocities on the outer flanks of topographic highs. These features are consistent with localized thickening of the volcanic extrusive layer from eruptions extending outside of the axial valley that flow down the fault-tilted blocks that form the abyssal hill topography. On-axis velocities are generally relatively high beneath the hydrothermal vent fields likely due to the infilling of porosity by mineral precipitation. Lower velocities are observed beneath the most vigorous vent fields in a seismically active region above the axial magma chamber and may reflect increased fracturing and higher temperatures. Seismic anisotropy is high on-axis but decreases substantially off axis over 5–10 km (0.2–0.4 Ma). This decrease coincides with an increase in seismic velocities resolved at depths ≥1 km and is attributed to the infilling of cracks by mineral precipitation associated with near-axis hydrothermal circulation. The orientation of the fast-axis of anisotropy is ridge-parallel near the segment center but curves near the segment ends reflecting the tectonic fabric within the OSCs.

Cretaceous to present kinematics of the Indian, African and Seychelles plates

An iterative inverse model of seafloor spreading data from the Mascarene and Madagascar basins and the flanks of the Carlsberg Ridge describes a continuous history of Indian–African Plate divergence since 84 Ma. Visual-fit modelling of conjugate magnetic anomaly data from near the Seychelles platform and Laxmi Ridge documents rapid rotation of a Seychelles Plate about a nearby Euler pole in Palaeocene times. As the Euler pole migrated during this rotation, the Amirante Trench on the western side of the plate accommodated first convergence and later divergence with the African Plate. The unusual present-day morphology of the Amirante Trench and neighbouring Amirante Banks can be related to crustal thickening by thrusting and folding during the convergent phase and the subsequent development of a spreading centre with a median valley during the divergent phase. The model fits FZ trends in the north Arabian and east Somali basins, suggesting that they formed in India–Africa Plate divergence. Seafloor fabric in and between the basins shows that they initially hosted a segmented spreading ridge that accommodated slow plate divergence until 71–69 Ma, and that upon arrival of the Deccan–Réunion plume and an increase to faster plate divergence rates in the period 69–65 Ma, segments of the ridge lengthened and propagated. Ridge propagation into the Indian continental margin led first to the formation of the Laxmi Basin, which accompanied extensive volcanism onshore at the Deccan Traps and offshore at the Saurashtra High and Somnath Ridge. A second propagation episode initiated the ancestral Carlsberg Ridge at which Seychelles–India and India–Africa Plate motions were accommodated. With the completion of this propagation, the plate boundaries in the Mascarene Basin were abandoned. Seafloor spreading between this time and the present has been accommodated solely at the Carlsberg Ridge.

Morphology and magnetic survey of the Rivera-Cocos plate boundary of Colima, Mexico

The propagation of the Pacific-Cocos Segment of the East Pacific Rise (EPR-PCS) has significantly altered the plate configuration at the north end of the Middle America Trench. This ridge propagation, the collision of the EPR-PCS with the Middle America Trench, the separation of the Rivera and Cocos plates and the formation of the Rivera Transform have produced a complex arrangement of morphotectonic elements in the area of Rivera-Cocos plate boundary, atypical of an oceanic transform boundary. Existing marine magnetic and bathymetric data has proved inadequate to unravel this complexity, thus, a dense grid of total field magnetic data were collected during campaigns MARTIC-04 and MARTIC-05 of the B/O EL PUMA in 2004 and 2006. These data have greatly clarified the magnetic lineation pattern adjacent to the Middle America trench, and have revealed an interesting en echelon, NE-SW oriented magnetic high offshore of the Manzanillo Graben. We interpret these new data to indicate that the EPR-PCS ridge segment reached the latitude (~18.3°N) of the present day Rivera Transform at about Chron 2A3 (~3.5Ma) and propagated further northward, intersecting the Middle America Trench at about 1.7 Ma (Chron 2). At 1.5 Ma spreading ceased along the EPR north of 18.3°N and the EPR-PCS has since retreated southward in association with a southward propagation of the Moctezuma Spreading Segment. North of 18.3°N the seafloor near the trench has been broken into small, uplifted blocks, perhaps due to the subduction of the young lithosphere generated by the EPR-PCS.

Reconciling geochemical and geophysical observations of magma supply and melt distribution at the 9°N overlapping spreading center, East Pacific Rise

Early studies of mid‐ocean ridge discontinuities, such as transform faults and overlapping spreading centers, suggested a lower magma supply compared to ridge segment centers. This is reflected in bathymetrically deeper ridge axes, decreased hydrothermal activity, and the eruption of more evolved lava compositions. While many signatures of lower magma supply are observed at the 9°N overlapping spreading center on the East Pacific Rise, geophysical studies indicate extensive sub‐surface melt in the region, suggesting that the present magmatic system is not diminished. Here major and trace element concentrations of erupted lavas are used to better understand magma supply at a large second‐order ridge discontinuity. We show that the wide range of lava compositions erupted at the 9°N overlapping spreading center is generally consistent with early petrologic models of ridge propagation and require variable degrees of fractional crystallization, extensive magma mixing, and in some instances crustal assimilation. Moderately evolved ferrobasalts and FeTi basalts erupted at the OSC indicate that crustal residence times are long enough for significant crystallization of all magmas within the region, but the presence of dacitic lavas reflects periods of even lower magma supply, where melt replenishment is subordinate to cooling and crystallization. The geophysical observations of extensive melt within the shallow crust are reconciled with the geochemistry of the lavas, if melts are supplied intermittently to the propagating ridge over relatively short timescales.

Tectono-stratigraphic setting of the Moreton’s Harbour Group and its implications for the evolution of the Laurentian margin: Notre Dame Bay, Newfoundland1This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

The Moreton’s Harbour Group lies along the Red Indian Line, the fundamental Iapetus suture that separates rocks of peri-Laurentian affinity with rocks of peri-Gondwanan affinity in the Newfoundland Appalachians. Characterization of age and environment of formation of the Moreton’s Harbour Group is an important constraint on evolution of the Laurentian margin during Ordovician closure of Iapetus Ocean and associated marginal basins. The Moreton’s Harbour Group comprises a fault-bounded ophiolitic sequence of layered gabbro, sheeted diabase, pillow basalt, and felsic intrusive rocks. It is offset by high-angle shear zones that were contemporaneous with a 477.4 ± 0.4 Ma syn-tectonic and syn-magmatic suite of trondhjemite and tonalite. Trace element data from the felsic suite indicate formation in a supra-subduction zone setting, although isotopic data from the felsic intrusive rocks (εNd (–5.02) to (–10.53), Tdm 1200–1800 Ma) indicate a significant amount of contamination from Mesoproterozoic or older continental crust. The age and tectonic setting of the Moreton’s Harbour Group suggest that it is the northernmost extent of the ca. 480 Ma Annieopsquotch Ophiolite Belt. We present a model in which the Moreton’s Harbour Group formed in response to propagation of the Annieopsquotch Ophiolite Belt spreading centre into the Dashwoods microcontinent. This ridge propagation model supports the formation of the Annieopsquotch Ophiolite Belt immediately outboard of Dashwoods and explains its rapid accretion to the composite Laurentian margin.

The tectonic fabric of the ocean basins

[1] We present a global community data set of fracture zones (FZs), discordant zones, propagating ridges, V-shaped structures and extinct ridges, digitized from vertical gravity gradient (VGG) maps. We use a new semi-automatic FZ tracking program to test the precision of our hand-digitized traces and find a Mean Absolute Deviation of less than 3.4 km from the raw VGG minima that most clearly delineate each feature, and less than 5.4 km from the FZ location predicted by fitting model profiles to the VGG data that represent the morphology of the individual FZs. These offsets are small considering gravity data only provide an approximation for the underlying basement morphology. We further investigate the origin of non-FZ seafloor fabric by combining published abyssal hill heights computed from gravity anomalies with global half-spreading rates. A residual abyssal hill height grid, with spreading rate effects removed, combined with our interpreted tectonic fabric reveals several types of seafloor fabric distinct from typical abyssal hills. Where discordant zones do not overprint abyssal hill signals, residual abyssal hill height anomalies correspond to seafloor that accreted near mantle thermal anomalies or zones of melt-depletion. Our analysis reveals several areas where residual abyssal hill height anomalies reflect pseudo-faults and extinct ridges associated with ridge propagation and/or microplate formation in the southern Pacific Ocean.

A Comparison on Propagating Tectonic Evolution Characteristics Between the Gulf of Aden and the Southwestern Sub‐Basin, South China Sea

AbstractThe global spatial distribution of the ridges and the oceanic basins is summarized in this paper, which are formed by propagating rifting and spreading. A new view of the tectonic evolution in the Southwestern Sub‐basin (SWSB), as well as the new evidence of the tectonic evolution, is suggested by comparing the origins, characteristics of spreading center, volcanism, and magnetic anomalies. The SWSB is a basin with a process of rifting and ridge propagation, formed at the same period with the Eastern Sub‐basin (ESB). Propagating spreading and its orientation were possibly controlled by the mushroom‐shaped low‐velocity mantle plume in the South China Sea (SCS), the Indo‐Sinian mantle plume or Hainan mantle plume. Therefore the simple evolution model is not appropriate for the formation of the SCS, and it should be an integration of many kinds of models. Mantle plume interaction should be taken into account when such a model is constructed.

Crustal structure of the propagating TAMMAR ridge segment on the Mid-Atlantic Ridge, 21.5°N

Active ridge propagation frequently occurs along spreading ridges and profoundly affects ridge crest segmentation over time. The mechanisms controlling ridge propagation, however, are poorly understood. At the slow spreading Mid-Atlantic Ridge at 21.5°N a seismic refraction and wide-angle reflection profile surveyed the crustal structure along a segment controlled by rapid ridge propagation. Tomographic traveltime inversion of seismic data suggests that the crustal structure along the ridge axis is controlled by melt supply; thus, crust is thickest, 8 km, at the domed segment center and decreases in thickness toward both segment ends. However, thicker crust is formed in the direction of ridge propagation, suggesting that melt is preferentially transferred toward the propagating ridge tip. Further, while seismic layer 2 remains constant along axis, seismic layer 3 shows profound changes in thickness, governing variations in total crustal thickness. This feature supports mantle upwelling at the segment center. Thus, fluid basaltic melt is redistributed easily laterally, while more viscose gabbroic melt tends to crystallize and accrete nearer to the locus of melt supply. The onset of propagation seems to have coincided with the formation of thicker crust, suggesting that propagation initiation might be due to changes in the melt supply. After a rapid initiation a continuous process of propagation was established. The propagation rate seems to be controlled by the amount of magma that reaches the segment ends. The strength of upwelling may govern the evolution of ridge segments and hence ultimately controls the propagation length.

Gravity lineaments of the Cocos Plate: Evidence for a thermal contraction crack origin

Lineaments in the gravity field with wavelengths of 100–200 km affect the south-central Pacific. Because they align with absolute plate motion, it has been proposed that they reflect small-scale convection cells beneath the lithosphere that become elongated by basal shear. Alternatively, it was suggested that they reflect channelized flow of low viscosity material following the base of the lithosphere toward the East Pacific Rise, or that they result from lithospheric-scale thermal contraction cracks. Here, we report about previously undetected gravity lineaments across the Cocos Plate. Similarly to the south-central Pacific lineaments, the Cocos lineaments affect a plate that is anomalously shallow, with seamounts aligning mostly within their troughs. However, the Cocos lineaments strike markedly oblique to absolute plate motion and follow instead trajectories that are perpendicular to seafloor isochrons, a characteristic best explained by the thermal contraction crack model. The presence of steep scarps at the base of seamounts and the seismic imaging of faults striking perpendicular to isochrons further support this interpretation. Assuming that the slow subsidence rates of the south-central Pacific and Cocos plates reflect a warmer upper mantle, we propose that the associated thinner elastic plates favor the formation of thermal contraction cracks. A thinner elastic plate may also explain the pattern of ridge propagation in both areas. At large ridge offsets with a history of steady migration, the propagating segments have been those cutting into the shallower flanks, consistent with the concept that a warmer, thinner plate is more easily cracked.

Gravity and seismic study of crustal structure along the Juan de Fuca Ridge axis and across pseudofaults on the ridge flanks

Variations in topography and seismic structure are observed along the Juan de Fuca (JdF) Ridge axis and in the vicinity of pseudofaults on the ridge flanks left by former episodes of ridge propagation. Here we analyze gravity data coregistered with multichannel seismic data from the JdF Ridge and flanks in order to better understand the origin of crustal structure variations in this area. The data were collected along the ridge axis and along three ridge‐perpendicular transects at the Endeavor, Northern Symmetric, and Cleft segments. Negative Mantle Bouguer anomalies of −21 to −28 mGal are observed at the axis of the three segments. Thicker crust at the Endeavor and Cleft segments is inferred from seismic data and can account for the small differences in axial gravity anomalies (3–7 mGal). Additional low densities/elevated temperatures within and/or below the axial crust are required to explain the remaining axial MBA low at all segments. Gravity models indicate that the region of low densities is wider beneath the Cleft segment. Gravity models for pseudofaults crossed along the three transects support the presence of thinner and denser crust within the pseudofault zones that we attribute to iron‐enriched crust. On the young crust side of the pseudofaults, a 10–20 km wide zone of thicker crust is found. Reflection events interpreted as subcrustal sills underlie the zones of thicker crust and are the presumed source for the iron enrichment.

Crustal structure beneath the Galápagos Archipelago from ambient noise tomography and its implications for plume-lithosphere interactions

[1] To constrain the seismic velocity structure of the crust beneath the Galapagos Archipelago, we conducted a tomographic study using high-frequency Rayleigh waves obtained from cross correlations of ambient noise. We analyzed waves with periods between 5 and 8.5 s, sensitive to shear wave velocity (VS) structure between about 3 and 10 km depth, after accounting for the effect of water depth. Crustal velocities are up to 25% lower than those of very young crust at the East Pacific Rise and are comparable to those of Hawaii. We attribute the lower than normal velocities to the combined effect of heating and the presence of melt in the crust above the Galapagos plume as well as the construction of a highly porous volcanic platform emplaced atop preexisting oceanic crust. On average, VS between 3 and 10 km depth beneath the western archipelago is up to 15% lower than beneath the eastern archipelago. We attribute the west-to-east velocity increase to a decrease in porosity of the volcanic platform and to cooling of the crust after its passage above the Galapagos plume. The results of this study, in combination with previous work, indicate that many of the unusual aspects of the Galapagos Archipelago are the result of variations in the thickness and internal structure of the chemical and thermal lithospheres. Our findings indicate that observed variations in the flexural response to loading observed in the Galapagos cannot be explained by the current thermal state of the lithosphere. Instead, the flexural response likely represents varying elastic strength at the time of loading. We also propose that the northwest and northeast trending alignments of volcanic centers found throughout the archipelago (the Darwinian lineations) may be associated with preexisting zones of weakness in the lithosphere formed during earlier episodes of ridge jumping and ridge propagation that were later reactivated by stresses generated by plume-lithosphere interactions.

Arabia‐Somalia plate kinematics, evolution of the Aden‐Owen‐Carlsberg triple junction, and opening of the Gulf of Aden

New geophysical data collected at the Aden‐Owen‐Carlsberg (AOC) triple junction between the Arabia, India, and Somalia plates are combined with all available magnetic data across the Gulf of Aden to determine the detailed Arabia‐Somalia plate kinematics over the past 20 Myr. We reconstruct the history of opening of the Gulf of Aden, including the penetration of the Sheba Ridge into the African continent and the evolution of the triple junction since its formation. Magnetic data evidence three stages of ridge propagation from east to west. Seafloor spreading initiated ∼20 Myr ago along a 200 km‐long ridge portion located immediately west of the Owen fracture zone. A second 500 km‐long ridge portion developed westward up to the Alula‐Fartak transform fault before Chron 5D (17.5 Ma). Before Chron 5C (16.0 Ma), a third 700 km‐long ridge portion was emplaced between the Alula‐Fartak transform fault and the western end of the Gulf of Aden (45°E). Between 20 and 16 Ma, the Sheba Ridge propagated over a distance of 1400 km at an extremely fast average rate of 35 cm yr−1. The ridge propagation resulted from the Arabia‐Somalia rigid plate rotation about a stationary pole. Since Chron 5C (16.0 Ma), the spreading rate of the Sheba Ridge decreased first rapidly until 10 Ma and then more slowly. The evolution of the AOC triple junction is marked by a change of configuration around 10 Ma, with the formation of a new Arabia‐India plate boundary. Part of the Arabian plate was then transferred to the Indian plate.

Age of Seychelles–India break-up

Many continental flood basalt provinces are spatially and temporally linked with continental break-up. Establishing the relative timing of the two events is a key step in determining their causal relationship. Here we investigate the example of the Deccan Traps and the separation of India and the Seychelles. Whilst there has been a growing consensus as to the age of the main phase of the Deccan emplacement (65.5 ± 1 Ma, chron 29r), the age of the rifting has remained unclear. We resolve this issue through detailed seafloor magnetic anomaly modeling (supported by wide-angle and reflection seismic results) of the north Seychelles and conjugate Laxmi Ridge/Gop Rift margins, and geochemistry and 40Ar/ 39Ar geochronology of rocks from the north Seychelles margin. We show that syn-rift volcanics offshore the Seychelles Islands in the form of seaward-dipping reflectors were most likely erupted during chron 28n, and the first organized seafloor spreading at the Carlsberg Ridge also initiated during this chron at 63.4 Ma. The severing of the Seychelles occurred by a south-eastward ridge propagation that was completed by the start of chron 27n (~ 62 Ma). A brief, pre-28r phase of seafloor spreading occurred in the Gop Rift, possibly as early as 31r–32n (~ 71 Ma). Initial extension at the margin therefore preceded or was contemporaneous with the Deccan emplacement, and separation of the Seychelles was achieved less than 3.5 Ma afterwards. This is the shortest time interval between flood basalt emplacement and break-up yet reported for any continental flood basalt-rifted margin pair. A contributing factor to the apparently short interval in the Deccan case may be that rifting occurred by a ridge jump into already thinned continental lithosphere. However, we conclude that external plate-boundary forces, rather than the impact of a mantle plume, were largely responsible for the rifting of the Seychelles from India.

Morphotectonics of the Carlsberg Ridge between 62°20′ and 66°20′E, northwest Indian Ocean

The segmentation pattern of 440 km long section of the Carlsberg Ridge between 62°20′E and 66°20′E, in the northwest Indian Ocean, has been studied using multibeam bathymetry and magnetic data. A well-defined transform fault is the only first order discontinuity documented in this section of the ridge. Four segments have been identified based on the topographic fabric, along-axis depth variations and the presence of off-axis traces of transform and non-transform discontinuities. These segments constitute four distinct domains representing magmatic and sparsely magmatic crustal accretion. Magnetic model studies show this section of the ridge is a slow spreading ridge with spreading half rates varying from 11 to 16 mm/yr. Along-axis variations in the magnetic anomalies and crustal magnetization, the presence of axial volcanic ridges on the inner valley floor, variations in the depth and geometry of the rift valley, and recovery of mantle derived peridotites from the inner valley floor suggest distinct variations in the accretionary processes along the ridge. It is suggested that the linear section in the east with ridge-parallel topographic fabric, continuous magnetic lineations, and the segment with a shallow valley floor with along-axis ridge propagation fabric, represent a magmatic phase of crustal accretion. The wide rift valley, recovery of mantle rocks, deformed topographic fabric, along-axis depth maxima and discontinuous magnetic lineations documented along the middle part of the ridge and the existence of short ridges east of the transform fault represent sparsely magmatic sections with dominance of tectonic extension.

Tectonic core of a sedimentary drift: a potential ridge propagation feature beneath the Blake Outer Ridge

The Blake Outer Ridge is a 480–kilometer long linear sedimentary drift ridge striking perpendicular to the North American coastline. By modeling free-air gravity anomalies we tested for the presence of a crustal feature that may control the location and orientation of the Blake Outer Ridge. Most of our crustal density models that match observed gravity anomalies require an increase in oceanic crustal thickness of 1–3 km on the southwest side of the Blake Outer Ridge relative to the northeast side. Most of these models also require 1–4 km of crustal thinning in zone 20–30 km southwest of the crest of the Blake Outer Ridge. Although these features are consistent with the structure of oceanic fracture zones, the Blake Outer Ridge is not parallel to adjacent known fracture zones. Magnetic anomalies suggest that the ocean crust beneath this feature formed during a period of mid-ocean ridge reorganization, and that the Blake Outer Ridge may be built upon the bathymetric expression of an oblique extensional feature associated with ridge propagation. It is likely that the orientation of this trough acted as a catalyst for sediment deposition with the start of the Western Boundary Undercurrent in the mid-Oligocene.

Luminescence chronometry and geomorphic evidence of active fold growth along the Kachchh Mainland Fault (KMF), Kachchh, India: Seismotectonic implications

The Kachchh region of Western India is a pericratonic basin experiencing periodic high magnitude earthquakes events. In 2001 a catastrophic seismic event occurred at Bhuj measuring M w = 7.7. The epicenters of both the 1956 and 2001 earthquakes were along the Kachchh Mainland Fault (KMF), proximal to the eastern end of the Northern Hill Range (NHR). The latter is a topographic expression of an active fault related fold on the hanging wall, and is controlled by a south dipping blind thrust. The present study deals with the eastern sector of NHR and uses optical dating to reconstruct the chronology of tectonically caused incisions. Along the backlimb of the NHR, incision ages on, channel fills and valley fill terraces progressively decrease from ∼ 12 ka to 4.3 ka. This age progression along with geomorphic evidences (decrease in topographic relief, drainage capture and drainage migration across the fold nose) suggests an active vertical and lateral fold growth along the KMF. Optical ages suggest that during the Late Holocene, the average uplift rate along the eastern NHR was 10 ± 1 mm/a. Recent GPS based estimates on crustal shortening are ∼ 12 mm/a. The KMF and the South Wagad Fault (SWF) represent the bounding faults of a transtensional basin that formed during the initial rifting. This basin is termed as the Samakhiali basin. The compressive stresses on account of structural inversion from normal to reverse phase resulted in lobate-shaped anticlines along KMF and SWF zone. These anticlines subsequently coalesced and formed linked and overlap segments. The present study suggests that eastward lateral deformation across the eastern portion of KMF has continued and has now resulted in its interaction with a left step over transfer fault called the South Wagad Master Fault (SWMF). This implies an increasing transpersional deformation of the Samakhiali basin. We therefore, suggest that the eastward NHR ridge propagation along KMF resulted in the thrust faulting on the south dipping SWMF resulting in the Bhuj 2001 event. The increasing strain on this basin may cause enhanced seismicity in the future along the eastern KMF and Wagad region.

Geodetic GPS measurements in south Iceland: Strain accumulation and partitioning in a propagating ridge system

GPS observations in south Iceland between 1994 and 2003 are compared with two‐dimensional elastic half‐space and viscoelastic coupling models for two parallel rift zones, representing the Western and Eastern volcanic zones (WVZ, EVZ). GPS data from the Hreppar block, between the WVZ and EVZ, fit a rigid block model within uncertainties. Spreading rates across the WVZ increase from 2.6 ± 0.9 mm/yr in the northeast to 7.0 ± 0.4 mm/yr in the southwest. Conversely, spreading rates in the EVZ decrease from 19.0 ± 2.0 mm/yr in the northeast to 11.0 ± 0.8 mm/yr in the southwest, the direction of ridge propagation. Summed extension rates across the two rift zones are approximately constant and equal to the total plate rate, ∼18–20 mm/yr, consistent with a simple propagating ridge model whereby the WVZ is deactivating in the direction of EVZ propagation. The coupling model confirms results from the simple elastic half‐space model, including relatively shallow locking depths (<5 km) beneath the rift zones, and allows for an estimate of mean viscosity (∼1019–1020 Pa s) beneath the elastic layer. The location of maximum surface velocity gradient in the EVZ, presumably the locus of subsurface magma accumulation and future rifting, does not coincide with the 1783–1784 Lakagígar fissure eruption but is 20 km to the west, on the Bárðabunga‐Veidivötn fissure swarm. This had a small volume eruption in 1862–1864 but a major eruption in 1477 A.D.

Subsurface tectonic structure between overlapping mid-ocean ridge segments

Our results from seismic anisotropy analyses reveal for the first time the complex spatial variability of the characteristics of subsurface tectonic structures associated with ridge propagation. The significance lies in the fact that these variations are found at a locality with few lineaments or fissures at seafloor level. The overlap region between mid-ocean ridge segments at 98N on the East Pacific Rise is characterized by aligned cracks that are structurally more closely related to the propagating-ridge segment. These aligned cracks, which are approximately parallel to the ridge segments, provide conclusive observational evidence for establishing the nontransform nature of overlapping spreading centers, especially those with overlap basins covered by volcanic edifices. The aligned cracks of the 98039N overlapping spreading center are more similar to the ridge-parallel lineaments observed between overlapping axial-summit collapse troughs than those found at larger overlapping spreading centers. Our results therefore suggest that the lithospheric deformation between overlapping ridge segments depends on ridge offset and that this dependency may be thermally related.

Palaeomagnetism and K-Ar and40Ar/39Ar ages in the Ali Sabieh area (Republic of Djibouti and Ethiopia): constraints on the mechanism of Aden ridge propagation into southeastern Afar during the last 10 Myr

SUMMARY A new detailed palaeomagnetic study of Tertiary volcanics, including extensive K-Ar and 40 Ar/ 39 Ar dating, helps constrain the deformation mechanisms related to the opening processes of the Afar depression (Ethiopia and Djibouti). Much of the Afar depression is bounded by 30 Myr old flood basalts and floored by the ca 2 Myr old Stratoid basalts, and evidence for pre-2 Ma deformation processes is accessible only on its borders. K-Ar and 40 Ar/ 39 Ar dating of several mineral phases from rhyolitic samples from the Ali Sabieh block shows indistinguishable ages around 20 Myr. These ages can be linked to separation of this block in relation to continental breakup. Different amounts of rotation are found to the north and south of the Holhol fault zone, which cuts across the northern part of the Ali Sabieh block. The southern domain did not record any rotation for the last 8 Myr, whereas the northern domain experienced approximately 12 ± 9 ◦ of clockwise rotation. We propose to link this rotation to the counter-clockwise rotation observed in the Danakil block since 7 Ma. This provides new constraints on the early phases of rifting and opening of the southern Afar depression in connection with the propagation of the Aden ridge. A kinematic model of propagation and transfer of extension within southern Afar is proposed, with particular emphasis on the previously poorly-known period from 10 to 4 Ma.