Axial Trend Research Articles

Flexural response of the oceanic lithosphere at an arc—arc junction: implication for the subduction of aseismic ridges

The deformation of the oceanic lithosphere subducting at the junction of two trenches is studied by means of a three-dimensional finite-element analysis. Results show that the existence of a junction (i.e. a change in trend of the trench axis) yields a specific shape of the outer topographic rise. In a convex junction area—such as the Japan and Kuril trenches, the topographic bulge presents a “dome”, whereas in a concave junction area—such as the Java and Sumatra trenches, this bulge is less pronounced. These theoretical results are confirmed by the bathymetry seaward of the junctions of the Japan and Kuril trenches and of the Peru-Chile trench. Moreover, the existence of the abnormal topographic dome in front of a convex junction contributes to the creation of normal faults which help the subduction of seamounts or of other bathymetric features in such areas.

Fault and kink-band relationships at Mystery Bay, Australia

Spectacular conjugate kink bands are exposed in a 400 m-wide kink zone of coastal outcrops of Ordovician turbidites, south of Mystery Bay on the New South Wales south coast. Within the kink zone, upright F 1 and F 2 folds with subhorizontal axial trends exhibit a segmented, counterclockwise rotation of approximately 55° about a near-vertical axis from trends outside the kink zone. The kink zone is cut by numerous conjugate faults with total offsets indicative of compression at high angles to the foliation. Kink-band development is directly controlled by fault location and is commonly concentrated in wedge-shaped fault blocks between non-parallel faults. Major faults separate regions of kink bands with distinctly different geometries and do not offset the kinks, and hence, were initiated prior to kinking. The opposing stress orientations inferred from the conjugate faults and kinks permit two models of the deformational history: 1. (1) Regional layer-normal compression causing fault movement followed by a single (or multiple) switch of regional principal stress orientations causing kink development, and 2. (2) Regional layer-normal compression causing fault movement with synchronous kink-band development resulting from local stresses imparted to the adjacent wedge-shaped fault blocks. Conclusive evidence to discriminate between the two models has not been observed, although circumstantial evidence supports the latter. The feasibility of a synchronous development is supported by a Mohr-circle stress analysis. Under certain conditions, reductions of both the shear stress on a fault plane and the normal stress acting parallel to the fault plane during stick-slip faulting will induce a transient stress system compatible with kink-band development. Kinking by local reorientation of principal compressive stress to lower angles to the orogen may thus be an important process accompanying late-stage faulting under regional compression normal to the orogen.

Strain-induced mineral growth in ductile shear zones and a preliminary study of ductile shearing in western Newfoundland

In the Fleur de Lys and the Central Gneiss terranes the presence of strain-induced mineral growth characteristic of ductile shear zones within zones of rocks with mylonitic fabrics indicates the existence of major belts of layer-parallel ductile shearing with complex evolutionary sequences. Kinematic markers in several of these shear belts indicate that shearing movements on initially probably gently inclined surfaces, directed not normal to the axial trend of the orogen but parallel to it, are tectonically important in western Newfoundland. The shear belts are in excess of 1 km thick, and one well-exposed example exhibits a pattern in which zones of the highest strain anastomose on the map scale.The base of the Fleur de Lys Supergroup is marked by one such major zone of shearing (décollement) in which kinematic indicators record movements directed to the north and to the south, before the regional attitude of the rocks was steepened. Along the Baie Verte Line, earlier north- and south-directed movements in the Fleur de Lys were succeeded by reverse movements towards the east, over the Dunnage Terrane.

Crustal movement along a collision boundary of plates (case of eastern Taiwan)

Recent crustal movements in eastern Taiwan were investigated based on geodetic and seismological data as a case study of the tectonics along a colliding plate boundary. Secular horizontal strain along the Longitudinal Valley, a probable arc-continent collision boundary between the Philippine Sea and Eurasion plates, is compressive with a large rate of 2 microstrains/year, and the maximum compression axis trends in NW-SE direction. This means that nearly two-thirds of the convergence rate between the two plates is consumed in the narrow zone along the valley and confirms the view that the valley is the plate boundary between the two. The amount of uplift in the Central Range, deduced from trigonometric leveling, reaches more than one meter during the past 60 years. Such active orogeny is caused by the high strain rate in this area. Horizontal displacement of the Coastal Range, a leading edge of the Philippine Sea plate, has a left-lateral component, suggesting that the boundary is a region of oblique collision and the range moves toward the north, producing subduction of the Philippine Sea plate off the Coast of northeastern Taiwan. An extensional strain pattern is found northeastern Taiwan near Hualien, in northern Taiwan near Ilan, and in southeastern Taiwan south Taitung. In northeastern Taiwan, the direction of the maximum extension axis is nearly parallel to the isodipth contours of deep-focus earthquakes that occurred under the southern part of the Ryukyu Trench. From a model calculation using a finite-element method, this pattern of strain is interpreted as an edge effect of the collision-subduction junction in northeastern Taiwan. The direction of the maximum extension in northern Taiwan forms high angles with the direction of the tectonic line near Ilan stretching from the Okinawa Trough. This fact is harmonious with the view that the Okinawa Trough is spreading. Extensional strain observed in southeastern Taiwan may be related to the eastward subduction since the area is situated on a transition zone, from collision to subduction.

Source mechanisms of three moderate California earthquakes of July 1986

Abstract We studied the source processes of three moderate size earthquakes in California: the M s = 6.0 event of July 8, 1986, near Palm Springs, the M s = 5.8 shock of July 13, 1986, offshore of Oceanside, and the M s = 6.2 event of July 21, 1986, in the Chalfant Valley, on the east side of the Sierra Nevada. The centroidal source parameters are estimated by inversion of long-period P and SH waves recorded at teleseismic distances by seismographs of worldwide networks. A more detailed description of the source is then obtained by analyzing broadband seismograms. The North Palm Springs earthquake had a simple rupture process. Most of the seismic moment was released in the first 3 sec in a single step. The strike of the source fault plane follows the average trend of the Banning fault and its dip agrees with the dip of the fault plane delineated by the aftershocks. The faulting mechanism of the main shock is right-lateral strike-slip with a sizable reverse component that is consistent with uplift north of the Banning fault. The earthquake offshore of Oceanside had a slightly more complex rupture process. The far-field source-time function consists of two pulses that represent the release of most of the moment in the first 3 sec. The source mechanism indicates reverse faulting with the compressional axis trending northeast, almost perpendicular to the trend of mapped faults offshore. The event does not appear to be associated with major mapped faults in the area, but the strike of one of the nodal planes agrees with the trend of the major mapped faults offshore and its mechanism is consistent with the missing component of plate convergence not taken up by the onshore strike-slip fault systems, and the idea of a weak San Andreas fault system. The rupture process of the Chalfant Valley earthquake was yet more complicated. The source-time function indicates that in a 4 sec time window three asperities were broken. The source mechanism indicates right-lateral strike-slip faulting, with a small normal-faulting component on a fault that dips approximately 50° southwest. Earthquake activity suggests that presently the deformation in the northern part of the Owens Valley (which includes the Chalfant Valley) is dominated by conjugate strike-slip faults which strike obliquely to the main range bounding normal faults. The waveforms indicate the presence of a low-velocity zone at a depth of about 20 km in the crust in the Chalfant Valley. A synthesis of information from waveform modeling, first-motion fault-plane solutions, and aftershock distributions indicates that the nucleation and/or termination of rupture for the North Palm Springs and the Chalfant Valley events were controlled by preexisting fault geometry.

Centroid Depths of Mid‐Ocean Ridge Earthquakes: Dependence on Spreading Rate

From inversions of teleseismic P and SH waveforms we have determined the source parameters of 50 large earthquakes that occurred during 1962–1983 on slowly spreading mid‐ocean ridges. All events are characterized by predominantly normal faulting on planes that dip at approximately 45° and strike parallel to the local trend of the ridge axis. Centroid depths range from 1 to 6 km beneath the seafloor. The P waves from these earthquakes show strong water column reverberations, suggesting that fault rupture extended to the seafloor. Under the assumption that the centroid depth marks the mean depth of fault slip, earthquake faulting extended to a depth of 2–10 km for these earthquakes. The water depths inferred from the predominant periods of the water column reverberations constrain the epicenters of these earthquakes to lie within the relatively deep inner floor of the median valley. The maximum centroid depths of ridge crest earthquakes decrease with increasing spreading rate, and the maximum seismic moment may also decrease with increasing spreading rate. These results indicate a general decrease with spreading rate in maximum thickness of the mechanically strong layer beneath the ridge axis region. The concentration of large earthquakes within the deepest parts of the median valley and the depth extent of seismic faulting support lithospheric necking models for the origin of the median valley.

Apparent tectonic rotations, declination anomaly equations, and declination anomaly charts

The conventional method for tilt correction of paleomagnetic data assumes that rock bodies rotate about horizontal axes during deformations. Apparent tectonic rotations, which may produce anomalous declination readings of paleomagnetic directions and other linear elements, can result from the use of such a procedure if the rock bodies deform about inclined axes. The error (declination anomaly) resulting from two apparent tectonic rotations, (1) single rotations about an oblique axis and (2) continuous rotations about a precessing fold axis, can be determined algebraically for given orientations of rotation axis and angles of rotation. The declination anomaly does not depend on the orientation of the paleomagnetic vector or the trend of the rotation axis; rather, it is a function of the plunge of the rotation axis and the angle of rotation. A declination anomaly chart (DAC) is constructed for each of the two processes to determine graphically whether anomalous paleomagnetic data obtained from a study area can result from the process. In the DACs the plunge of the bedding‐pole vector and the declination anomaly resulting from the apparent tectonic rotations are plotted against the plunge of the rotation axis and the angle of rotation. The declination anomaly for an individual site is taken as the deviation of the measured remanent magnetic direction at the site from the reference direction for coeval rocks. Each data point is plotted at the intersection of the declination anomaly curve and the beddingpole curve in the DACs. If the dispersion in remanent magnetic directions over a study area results from one of the apparent tectonic rotations, the data points will lie along a straight line in the corresponding DAC. Potential applications of the DACs include testing the nature of anomalous paleomagnetic directions, determining the deformation style of folds, and restoring structural orientations of tilted blocks.

Crustal movements along a collision boundary of plates (case of Eastern Taiwan)

Abstract Recent crustal movements in eastern Taiwan were investigated based on geodetic and seismological data as a case study of the tectonics along a colliding plate boundary. Secular horizontal strain along the Longitudinal Valley, a probable arc-continent collision boundary between the Philippine Sea and Eurasion plates, is compressive with a large rate of 2 microstrains/year, and the maximum compression axis trends in NW-SE direction. This means that nearly two-thirds of the convergence rate between the two plates is consumed in the narrow zone along the valley and confirms the view that the valley is the plate boundary between the two. The amount of uplift in the Central Range, deduced from trigonometric leveling, reaches more than one meter during the past 60 years. Such active orogeny is caused by the high strain rate in this area. Horizontal displacement of the Coastal Range, a leading edge of the Philippine Sea plate, has a left-lateral component, suggesting that the boundary is a region of oblique collision and the range moves toward the north, producing subduction of the Philippine Sea plate off the Coast of northeastern Taiwan. An extensional strain pattern is found northeastern Taiwan near Hualien, in northern Taiwan near Ilan, and in southeastern Taiwan south Taitung. In northeastern Taiwan, the direction of the maximum extension axis is nearly parallel to the isodipth contours of deep-focus earthquakes that occurred under the southern part of the Ryukyu Trench. From a model calculation using a finite-element method, this pattern of strain is interpreted as an edge effect of the collision-subduction junction in northeastern Taiwan. The direction of the maximum extension in northern Taiwan forms high angles with the direction of the tectonic line near Ilan stretching from the Okinawa Trough. This fact is harmonious with the view that the Okinawa Trough is spreading. Extensional strain observed in southeastern Taiwan may be related to the eastward subduction since the area is situated on a transition zone, from collision to subduction.

Proterozoic deformation of the East Saharan Craton in Southeast Libya, South Egypt and North Sudan

The basement areas in Southeast Libya, South Egypt and North Sudan, west of the Nile, between Gebel Uweinat and the Bayuda Desert, are part of an approximately 1000-km-wide, complexly folded, polymetamorphic zone with a regional N-NNE-NE-ENE trend of foliation and fold axis. Since this belt extends southwestward into the area of Zalingei in the southern Darfur block (West Sudan), it is named the Northern Zalingei fold zone. Sr and Nd isotopic studies suggest that this zone is older than Pan-African and further indicate that, apart from Archean rocks in the Gebel Uweinat area, this belt is of Early-Middle Proterozoic age. An Early-Middle Proterozoic three-stage deformational and anatectic event established the present-day fold and fault geometry in the western parts of this zone in the Gebel Uweinat—Gebel Kamil area. The Pan-African tectono-thermal episode was most effective in the eastern part of the belt, near the boundary with the Nubian Shield volcano-sedimentary-ophiolite-granitoid assemblages. It caused migmatization, granite emplacement, mylonitization and large-scale wrench faulting which was related to Late Proterozoic accretionary and collisional events of the Arabian-Nubian Shield with the margin of the East Saharan Craton.

Oriented olivine crystals in the upper mantle: a test from the inversion of multimode surface-wave data

A large Love-Rayleigh wave discrepancy of phase velocity has been observed for mantle overtones in the Pacific Ocean. To test if preferred orientation of olivine crystals can be the cause for such an anisotropic behaviour, we have considered that upper mantle material might be represented by a medium with hexagonal symmetry and a horizontal symmetry axis oriented in the direction of preferred orientation of the olivine crystal a-axis. A set of Love and Rayleigh wave data that are related to paths crossing the whole Pacific plate have been inverted within this framework considering three parameters varying with depth: S-velocity, density and the proportion of oriented crystals. Crystals are found to be preferentially oriented to 400 km depth. Different orientations of the symmetry axis have been tested. A good fit to the data is obtained for an axis trending E-W to SE-NW, in qualitative agreement with the average trend of the absolute plate motion along the investigated paths. The model presented in this study predicts azimuthal variations of fundamental mode Rayleigh wave velocities in good agreement with estimates published in the literature.

The North Gower, Ontario, Earthquake of 11 October 1983: Focal Mechanism and Aftershocks

Abstract An earthquake near Ottawa (45.20°N, 75.75°W, focal depth 12 km) of unusually large size for the region's seisrhicity (mb(Lg) = 4.1) provided good P-wave first-motion data for a focal-mechanism solution. The maximum intensity was V(MM) and the area of perceptibility in Canada about 80,000 km2. The first and largest recorded aftershock occurred nine minutes after the main shock with magnitude mb(Lg) = 1.7. Two further small aftershocks ware recorded by a field network. The mechanism is thrust faulting with a predominantly horizontal pressure axis trending 154°. Thrust mechanisms have been found for other earthquakes in southeastern Canada and the northeastern U.S., but orientations of their stress axes are different, and so the North Gower earthquake may reflect a local and not a regional stress field. The nodal planes have strike 71°, dip 75°, and strike 221°, dip 17°. The spatial distribution of aftershocks suggests the gently-dipping nodal plane could be the fault plane. There is uncertainty about the seismotectonics of the region, and the orientation of neither nodal plane correlates with known geological features. More earthquake fault-plane solutions are required to interpret the seismotectonics and the stress regime.

The North Gower, Ontario, Earthquake of 11 October, 1983: Focal Mechanism And Aftershocks

An earthquake near Ottawa (45.20°N, 75.75°W, focal depth 12 km) of unusually large size for the region9s seisrhicity (m b (Lg) = 4.1) provided good P-wave first-motion data for a focal-mechanism solution. The maximum intensity was V(MM) and the area of perceptibility in Canada about 80,000 km 2 . The first and largest recorded aftershock occurred nine minutes after the main shock with magnitude m b (Lg) = 1.7. Two further small aftershocks ware recorded by a field network. The mechanism is thrust faulting with a predominantly horizontal pressure axis trending 154°. Thrust mechanisms have been found for other earthquakes in southeastern Canada and the northeastern U.S., but orientations of their stress axes are different, and so the North Gower earthquake may reflect a local and not a regional stress field. The nodal planes have strike 71°, dip 75°, and strike 221°, dip 17°. The spatial distribution of aftershocks suggests the gently-dipping nodal plane could be the fault plane. There is uncertainty about the seismotectonics of the region, and the orientation of neither nodal plane correlates with known geological features. More earthquake fault-plane solutions are required to interpret the seismotectonics and the stress regime.

Thrust faults in southern Illinois basin—Result of contemporary stress?

Many north-trending thrust faults have been observed in coal mines of southern Illinois and adjacent western Kentucky. Displacement ranges from less than an inch to about 3 m (10 ft). No known ancient tectonic stress field or nontectonic process has been found to have produced these faults; they correspond to the contemporary tectonic stress field in the region—maximum horizontal compression along an axis trending slightly north of east. This compression is thought to account for the preferred orientation of roof failure in underground coal mines, jointing in roof shales, and minor earthquakes in southern Illinois.

Rupture characteristics of normal faults: an example from the Wasatch fault zone, Utah

Summary The Salt Lake fault segment is one of several independent rupture segments in the Wasatch normal fault zone, Utah. The segment is about 35 km long and consists of several, approximately linear fault sections that intersect in geometrical barriers defined by bends and branching of the fault trace. The dips of these fault sections vary based on a preliminary model of the fault zone, with estimated dips ranging from 45 to 90°. The palaeo-stress tensor was characterized by sub-vertical maximum principal compressive stress, a minimum principal stress axis trending about 050° and φ ≈ 0.2−0.5. The average slip direction across the fault segment was ≈240°. Three large, non-conservative barriers occur within the fault segment—at the northern and southern ends of the segment, and a branch point in the centre of the segment. Bifurcation of the rupture zone into two primary fault systems has presumably increased the fracture toughness of the northern part of the segment due to the overlap of secondary faulting adjacent to the primary fault planes. Two large historic earthquakes in the eastern Basin and Range and Rocky Mountain transition region have ruptured bifurcated fault zones similar in structure to the Salt Lake segment; the Hebgen Lake, Montana earthquake ( M = 7.5) in 1959 and the Borah Peak, Idaho earthquake ( M = 7.3) in 1983. The earthquake foci were located at depths of 12–18 km and the ruptures propagated towards the regions of fault-zone bifurcation in both cases. A characteristic earthquake ( M ≈ 7−7.5) may initiate at either the centre or the southern end of the Salt Lake segment. In the first case, the rupture would propagate bilaterally, but in the latter case the rupture would propagate unilaterally to the N. A unilateral propagation direction is preferred based on inferences drawn from the structural geology of the segment and direct comparison with the rupture traces of the Borah Peak and Hebgen Lake earthquakes.

A tectonic redefinition of the Southern Rocky Mountains

Abstract The Southern Rocky Mountains of the Western United States, physiographically defined and described by N.M. Fenneman nearly 60 yrs ago, are tectonically redefined and extended. They are shown to constitute the crestal range of a mammoth, continental, arch-like feature here named the Alvarado ridge. Its axis trends south from Casper, Wyoming at least as far south as El Paso, Texas, beyond which the ridge begins to lose morphological identity. Maximum elevations along the crest of the ridge exceed 4.2 km. The summit ranges are bordered on either side by gently sloping rises that extend outward for at least 1300 km, falling to elevations of less than 400 m. Modest rift structures along the ridge axis continue beyond the southern terminus of the mountains before playing out. A major sediment-filled axial graben exists over the southern two-thirds of the ridge, but equivalent parts of it farther north were stripped of their Neogene fill by erosion in the headwaters areas of the Colorado and North Platte rivers. The maximum elevation of earlier Laramide mountains in this area has been estimated to have been no more than 2 km in Colorado, half that of the present range. Related topography was nearly obliterated by erosion prior to late Eocene time. Lateral stream planation produced a southeast-sloping, major late Eocene erosion surface across the region that had what was probably an isostatically adjusted, average maximum elevation of less than 900 m. Present day elevations and relief on the ridge crest are the result of steep crestal normal faulting, pronounced block uplift, and regional arching, with extensional strain limited to an axial corridor less than 200 km wide. This episode of mountain building began in middle Miocene time (17 to 12 Ma), culminating in latest Miocene and early Pliocene time, between 7 and 4 Ma ago. Debris from the newly elevated range (the Southern Rocky Mountains, sensu lato) was shed along the full length of the Neogene Alvarado ridge down parts of both rises, as well as into its medial graben. The uplifted and tilted east rise (the Great Plains ramp) has an extensive aggradational cover that spread eastward to distances of 700 km and more in middle Miocene to earliest Pliocene time. Its fluviatile, lacustrine and eolian sediments are undergoing dissection today, but a sufficient portion remains to define the unmodified profile of the east rise, as well as the original length of the ridge. It is coincidentally similar in configuration and scale to the west rise of the mid-Atlantic ridge. The topography of the crestal mountain ranges on both structures is also remarkably similar. The Southern Rocky Mountains are as high as the Alps, but are not the product of lithospheric compression and crustal thickening. Rather they reflect profound epeirogenic uplift driven by lithospheric thinning and accompanied by extensional strain and differential vertical jostling along the crest of the ridge.

Resistivity measurements for groundwater investigation in the Umm Al-Aish area of northern Kuwait

With a view to testing the possibilities and limitations of geoelectrical methods in exploration and evaluation of groundwater resources in Kuwait, vertical electrical sounding measurements (VES), utilizing the Schlumberger array, were conducted in the Umm Al-Aish area. Here, localized fresh water zones occur in the upper portions of Pleistocene sand and gravel aquifers. Iterative automatic inversion of VES data, coupled with borehole information, resolved with some confidence the dry/saturated and fresh/saline water interfaces and revealed some correspondence between lithostratigraphical units and resistivity values. Maps of resistivity and transverse resistance values agree reasonably with the general hydrogeological condition in the area. The results confirm the hypothesis that potable groundwater is associated with low relief, mainly along the central axial trend of the basin, where seasonal recharge is likely. Correlations have been established between aquifer resistivity and water salinity and to a lesser extent between aquifer transverse resistance and potential area(s) for fresh groundwater production. These correlations could be used to assist in exploration and evaluation of groundwater resources in new target areas in Kuwait where lithostratigraphical and hydrogeological information is very limited.

Considerations about Hercynian thrusting in the Marimanya massif (central Pyrenees)

We can distinguish four main units within the Marimanya massif: the Cambro-Ordovician detrital series; a carbonate unit, mainly made up of massive limestones dated as Silurian-Devonian; dark slates and limestones dated as Middle-Upper Devonian; and the late Hercynian Marimanya intrusive, mainly composed of granites and granodiorites. Mapping has shown that the carbonate units lie on different levels of the Cambro-Ordovician series and even on the dark slates and limestones. This abnormal contact is demonstrated to be a thrust. The thrusting surface is affected by Hercynian folds with vertical axial planes, a fold axis trending E-W and an associated cleavage. Moreover, it is cut by the late Hercynian intrusive. Consequently, considering paleontological and structural data as well as the regional context, the most reasonable interpretation is that this structure represents a Hercynian thrust, probably with southward displacement.

Active collision in eastern Taiwan: The Coastal Range

A detailed analysis of Late Cenozoic faulting has been carried out in the Coastal Range of eastern Taiwan. 4000 measurements have been collected in 60 sites where faults affect formations that range from upper Miocene to Pleistocene and scattered Quaternary deposits. This large data set enables us to accurately determine paleostress axes in the area. During the major tectonic event (the Pleistocene Coastal Range orogeny), the direction of the maximum compressional stress σ 1 has remained constant. 25% of reconstructed σ 1 axes trend approximately 120; 90% trend between 090 and 140. Taking into account the internal deformation in the Coastal Range during folding and thrusting, these reconstructed directions of compression are remarkably homogeneous. Present faulting in the Longitudinal Valley is consistent with the paleostress pattern recorded in the adjacent Coastal Range formations. The Longitudinal Valley fault is a thrust, with a minor left-lateral strike-slip component (20% of the total movement). This quantitative estimate of motion is compatible with reconstructed Pleistocene paleostress and more general reconstructions of recent plate motion in the vicinity of Taiwan. The analysis of the focal mechanisms of earthquakes provides similar results. Finally, the analysis of the Coastal Range faults systems related to the collision of the volcanic Luzon arc (including the Luzon trough) with the Central Range of Taiwan provides a consistent picture of the distribution of stress patterns and deformations. The strikes of folds and thrusts in the Coastal Range are mainly controlled by the direction of convergent plate motion (125), whereas the overall trend of the Coastal Range is similar to that of the Central Range (020). The obliquity of convergent motion has resulted in the complex en échelon structure of the Coastal Range. The relative plate motion is not the sole factor controlling the development of a collision chain such as the Coastal Range; the existence of major pre-existing structures also plays an important role.

Small non-overlapping offsets of the East Pacific Rise

High-resolution bathymetric surveys1,2 of the East Pacific Rise (EPR) reveal that in addition to overlapping spreading centres (OSCs), the crest of the EPR is offset tiny amounts by smaller features. Recently, Langmuir et al. have shown that despite their subtle bathymetric expression, these small offsets, bends and kinks of the EPR, collectively termed deviations from axial linearity (devals), represent important petrological and tectonic boundaries. Some devals are small non-overlapping offsets (SNOOs) that offset the EPR axis by up to a few hundred metres. Bathymetric contours near SNOOs are usually disturbed from their axis-parallel trend in ways that we interpret as evidence for strike-slip faulting and volcanism at high angles to the trend of EPR axis. We offer a model for the formation of SNOOs and their associated faults that invokes independent magma supply, stochastic active spreading and limited non-rigid behaviour of the neovolcanic zone.

The northern Red Sea and the Dead Sea rift

The results of geophysical measurements at sea, when combined with mesostructural measurements on land, demonstrate systematic extensional features in the region of the northern Red Sea and the Dead Sea rift. Rifts and diapirs were encountered in the northern Red Sea, showing an arcuate trend. Whereas the faulted continental margins at that region are oriented NW-SE, the rifts and the diapirs changed their trend northwards from NNW-SSE to N-S. Systems of rifts and diapirs are considered to be extensional features, therefore the tectonic extensional axis probably changed its orientation accordingly. A series of rifts and diapirs was found also in the Gulf of Elat, trending approximately parallel to the coastlines, and is interpreted to be the northern extension of the Red Sea rift and diapir system. Mesostructural measurements that were carried out in the western flank of the Dead Sea rift indicated further evidence for structural extension, trending E-W to ENE-WSW, in addition to measurements of left-lateral offset of approximately 10 km. All these data, when combined with the geophysical measurements of the characteristics of the earth crust underneath the Dead Sea rift, suggest a tectonic regime of oblique extension in the region of the Red Sea and the Dead Sea rift. This tectonic regime indicates that the Red Sea spreading center extends northwards along the Dead Sea rift, and the clockwise change of the axial trend imposed a sinistral offset to the tectonic extension. Thus it is suggested that the Dead Sea rift is an incipient oblique spreading center.