Fracture Zone Traces Research Articles

Seismotectonics of the Blanco Transform Fault System, Northeast Pacific: Evidence for an Immature Plate Boundary

AbstractAt the Blanco transform fault system (BTFS) off Oregon, 138 local earthquakes and 84 double‐couple focal mechanisms from ocean‐bottom‐seismometer recordings jointly discussed with bathymetric features reveal a highly segmented transform system without any prominent fracture zone traces longer than 100 km. In the west, seismicity is focused at deep troughs (i.e., the West and East Blanco, and Surveyor Depressions). In the east, the BTFS lacks a characteristic transform valley and instead developed the Blanco Ridge, which is the most seismically active feature, showing strike‐slip and dip‐slip faulting. Sandwiched between the two main segments of the BTFS is the Cascadia Depression, representing a short intra‐transform spreading segment. Seismic slip vectors reveal that stresses at the eastern BTFS are roughly in line with plate motion. In contrast, stresses to the west are clockwise skewed, indicating ongoing reorganization of the OTF system. As we observed no prominent fracture zones at the BTFS, plate tectonic reconstructions suggest that the BTFS developed from non‐transform offsets rather than pre‐existing transform faults during a series of ridge propagation events. Our observations suggest that the BTFS can be divided into two oceanic transform systems. The eastern BTFS is suggested to be a mature transform plate boundary since ∼0.6 Ma. In contrast, the western BTFS is an immature transform system, which is still evolving to accommodate far‐field stress change. The BTFS acts as a natural laboratory to yield processes governing the development of oceanic transform faults.

Detailed reconstructions of India-Somalia plate motion, 60 Ma to present: Implications for Somalia plate absolute motion and India-Eurasia plate motion

Summary We estimate India-Somalia plate motion at 45 times since 60 Ma from ∼9,000 crossings of Carlsberg and northern Central Indian Ridge magnetic reversals C1 to C26 and numerous fracture zone crossings. The new rotations reveal at least seven significant spreading rate changes since ∼60 Ma, some previously unknown. The largest changes occurred before 46 Ma, when the forces acting on the India plate evolved rapidly due to the transition from subduction to continent-continent collision between India and Asia and the influence of the Reunion hotspot plume on the India plate. The new rotations reveal a gradual ∼50 percent decline in Carlsberg Ridge spreading rates from 57-52.7 Ma, but an end to the decline at ∼53 Ma, when spreading rates surged rapidly by up to 100 percent. From 52-46.7 Ma, Carlsberg Ridge spreading rates collapsed by ∼90 percent, possibly defining a protracted transition to continental collision between India and Asia. Significant kinematic events since 46.7 Ma have included a ∼25 percent spreading rate recovery from 42-40 Ma, ultraslow spreading from 38.6-33.2 Ma, a gradual rate doubling from 33-18 Ma, a ∼50 percent slowdown from 18-13 Ma, and apparently steady motion since 13 Ma. The new rotations successfully predict Carlsberg Ridge abyssal hill orientations for seafloor ages of 48-42 Ma and 20-0 Ma and Central Indian Ridge fracture zone traces for seafloor ages of 43 to 16 Ma, constituting useful tests of the rotation accuracies at these ages. When corrected for the movement of India relative to the Capricorn plate since 16 Ma, the new rotations also successfully restore magnetic lineations C13, C18, C20, and C21, and fracture zone segments from the Capricorn plate onto of their Somalia plate counterparts. This further confirms the accuracies of our new rotations back to C21n.1o (47.8 Ma), and validates a ∼16 Ma start date for India-Capricorn plate motion and published correction for India-Capricorn motion. Anticorrelated changes in India-Somalia and Antarctic-Somalia seafloor spreading rates from 37-18 Ma may be evidence that Somalia plate absolute motion changed during this period, possibly triggered by Somalia’s post-30-Ma detachment from the Arabian Peninsula or the kinematic effects of the Afar and/or Reunion mantle plumes on the Somalia plate. New India-Eurasia rotations that we estimate from an updated global plate circuit predict convergence rates from 53-47 Ma that are ∼30 percent faster than previous estimates and that decline ∼75 percent by ∼38 Ma. Changes in India-Somalia and India-Eurasia rates correspond closely with recently described Tibetan deformation pulses, consistent with linkages between all three. A joint inversion of Carlsberg and southern Central Indian ridge magnetic reversal and fracture zone data, including a correction for movement of the Capricorn plate relative to India, satisfactorily realigns the reconstructed magnetic lineations and fracture zones back to C23n.1n (50.7 Ma), but misfits some data by 100 km or more at earlier times. The misfits may be evidence for deformation within the IndoCapricorn and/or Somalia plates before 48 Ma or a misinterpretation of magnetic reversal and/or fracture zone data from times before 48 Ma.

Are segmented fracture zones weak? Analytical and numerical models constrain anomalous bathymetry at the Clarion and Murray fracture zones

Between 150°W and 135°W, the Clarion Fracture Zone (FZ) coalesces from six discrete FZ traces to a single FZ over a period of ∼30 million years, coinciding with a change in plate motion that placed the associated transform fault (TF) in compression. Between 160° and 157°, the Murray FZ is also comprised of several closely-spaced (<50 km) discrete FZs and the associated TF experienced similar transpressional motion. Analysis of newly collected bathymetry data along the Clarion and Murray FZs reveals FZ structures that are inconsistent with predictions from a simple locked fault thermal subsidence model. These structures include FZ-bounded lithosphere that dips towards the old side of the FZ, whereas locked fault models predict it should dip towards the young side, and reversed fault scarp relationships where younger lithosphere lies deeper than older lithosphere. We investigate these anomalous observations using a combination of analytical and numerical models of two closely-spaced fracture zones to test how FZ strength and tectonic motions (compression or extension) affect the evolution of the intra-transform lithosphere. Our models predict that interior blocks within a segmented FZ, initially in isostatic equilibrium, can develop anomalous tilts and scarp depth relationships if the intervening FZs are relatively weak. Model results suggest that unusually low apparent coefficients of friction (<0.01) and tectonic compression are required to reproduce the magnitude of reverse tilt and scarp reversals observed in the Clarion and Murray FZs. We suggest that these low apparent friction coefficients might be the result of near lithostatic pore-fluid pressures associated with compression and alteration processes. Our results provide a tool to constrain histories of tectonic compression across FZs in places where rotation poles or other measures of plate motion may not be well constrained. Our results also imply that FZs are tectonically weak regions of hydrothermal alteration that may carry large volumes of volatiles into the mantle when subducted.

A new high‐resolution seafloor age grid for the South Atlantic

AbstractDigital grids of basement age of the world's oceans are essential for modern geodynamic and paleoceanographic studies. Any such grid is built using a plate kinematic model, whose accuracy and reliability directly influence the accuracy and reliability of the grid. We present a seafloor age grid for the South Atlantic based on a recent high‐resolution plate kinematic model. The grid is built from a data set of points whose ages are defined in or for the plate kinematic model, incorporating breaks at tectonic boundaries like fracture zones where the age function is discontinuous. We compare predictions of the new grid and of a previously published one, which is based on an older plate kinematic model, to magnetic isochron pick data sets. The comparison shows the new grid to provide a more reliable depiction of seafloor age in the South Atlantic. Numerical estimates of the new grid's uncertainty are determined by interpolation between (1) misfits at grid cells coinciding with magnetic isochron ages, (2) misfits implied by locational uncertainties in predicted isochrons propagated from uncertainties in the plate kinematic model, and (3) by the proximities of cells to fracture zone traces or ridge‐jump scars. Estimated total uncertainty is &lt;10 My for 94% of the grid and &lt;5 My for 72%, but much larger in areas where magnetic anomaly data are scarce (such as the Cretaceous Normal Superchron) and in the vicinity of long‐offset fracture zones.

Semiautomatic fracture zone tracking

AbstractOceanic fracture zone traces are widely used in studies of seafloor morphology and plate kinematics. Satellite altimetry missions have resulted in high‐resolution gravity maps in which all major fracture zones and other tectonic fabric can be identified, and numerous scientists have digitized such lineaments. We have initiated a community effort to maintain low‐cost infrastructure that allows seafloor fabric lineaments to be stored, accessed, and updated. A key improvement over past efforts is our processing software (released as a GMT5 supplement) that allows for semiautomatic corrections to previously digitized fracture zone traces given improved gridded data sets. Here we report on our seafloor fabric processing tools, which complement our database of seafloor fabric lineations, magnetic anomaly identifications, and plate kinematic models.

The Cretaceous opening of the South Atlantic Ocean

The separation of South America from Africa during the Cretaceous is poorly understood due to the long period of stable polarity of the geomagnetic field, the Cretaceous Normal Superchron (CNS, lasted between ∼121 and 83.6 Myr ago). We present a new identification of magnetic anomalies located within the southern South Atlantic magnetic quiet zones that have arisen due to past variations in the strength of the dipolar geomagnetic field. Using these anomalies, together with fracture zone locations, we calculate the first set of magnetic anomalies-based finite rotation parameters for South America and Africa during that period. The kinematic solutions are generally consistent with fracture zone traces and magnetic anomalies outside the area used to construct them. The rotations indicate that seafloor spreading rates increased steadily throughout most of the Cretaceous and decreased sharply at around 80 Myr ago. A change in plate motion took place in the middle of the superchron, roughly 100 Myr ago, around the time of the final breakup (i.e., separation of continental–oceanic boundary in the Equatorial Atlantic). Prominent misfit between the calculated synthetic flowlines (older than Anomaly Q1) and the fracture zones straddling the African Plate in the central South Atlantic could only be explained by a combination of seafloor asymmetry and internal dextral motion (<100 km) within South America, west of the Rio Grande fracture zone. This process has lasted until ∼92 Myr ago after which both Africa and South America (south of the equator) behaved rigidly. The clearing of the continental–oceanic boundaries within the Equatorial Atlantic Gateway was probably completed by ∼95 Myr ago. The clearing was followed by a progressive widening and deepening of the passageway, leading to the emergence of north–south flow of intermediate and deep-water which might have triggered the global cooling of bottom water and the end for the Cretaceous greenhouse period.

Spreading behaviour of the Pacific-Farallon ridge system since 83 Ma

We present improved rotations, complete with uncertainties, for the Pacific-Farallon Ridge (PFR) between geomagnetic chrons 34y (83 Ma) and 10y (28.28 Ma). Despite substantial shortening since ∼55 Ma, this ridge system and its remnants (e.g. the East Pacific Rise) have produced as much as 45per cent of all oceanic lithosphere created since the Late Cretaceous, but reconstructions face the twin challenges of extensive subduction of Farallon crust—which precludes reconstruction by fitting conjugate magnetic anomaly and fracture zone (FZ) traces—and asymmetric spreading behaviour for at least the past 51 Myr. We have calculated best-fit ‘half’-angle stage rotations between nine geomagnetic chron boundaries (34y, 33y, 29o, 24.3o, 20o, 18.2o, 17.1y, 13y and 10y) using combined anomaly and FZ data from both the northern and southern Pacific Plate. For rotations younger than chron 24.3o, estimates for spreading asymmetry, derived using anomaly picks from yet-to-be subducted Farallon/Nazca crust in the south Pacific, allow full stage rotations to be calculated. Between 50 and 83 Ma, where no direct constraints on spreading asymmetry are possible, a ‘best-fit’ full stage rotation was calculated based on the net Nazca:Pacific spreading asymmetry (Pacific spreading fraction f PAC =0.44) over the past 50 Myr, with conservative lower and upper bounds, based on variability in the degree of spreading asymmetry over periods of<15 Myr, assuming f PAC s of 0.5 and 0.36, respectively. Synthetic flowlines generated from our new stage rotation produce a better match to Pacific FZ trends than previously published rotations. With the exception of the chron 18o–20o rotation, the six stage poles for rotations between chrons 33y and 13y (74–33 Ma) all cluster tightly at 60–75°E, 60–68°N, consistent with the relatively constant trend of the major Pacific FZs. This stability spans at least one episode of Farallon Plate fragmentation caused by subduction of PFR segments beneath the Americas, at 55–48 Ma, which appears to have greatly accelerated divergence on the surviving ridge without significantly affecting the location of the instantaneous rotation pole. Together with quasi-periodic 15–20 Myr variations in the degree of spreading asymmetry that also appear to correlate with changes in spreading rate, this indicates that forces other than slab pull may be a factor in determining Pacific-Farallon Plate motions.

Demerara Plateau - the structure and evolution of a transform passive margin

SUMMARY The Amazon Cone Experiment acquired two transects (Profile sD&A )across the Demerara Plateau, part of the French Guiana-Northeast Brazil continental margin, to better understand rift and transform-style margin evolution. Profile A images an intermediate-type margin formed as a result of trans-tensional extension. In this paper we describe the modelling of wide-angle and multichannel seismic and gravity data from Profile D, to reveal whole crustal structure and features exhibiting transform characteristics. Combining these results with other studies in the region and comparing our results with ‘young’ rift analogues, we develop a model of along-margin segmentation that explains the evolution of the west equatorial Atlantic. Interpretation of the velocity-depth model for Profile D shows a 35‐37 km thick continental crust which thins to 10‐11 km over a distance of 320 km. This thinning is accommodated in two regions. The narrowest region, associated with the ocean‐continent transition, is interpreted to have formed by dextral shearing of the margin along major transform zones that accommodated the initial break-up geometry of the Central Atlantic. Given the orientation of the margin relative to local fracture zone traces it is likely that the second region of thinning, located 162 km landward of the ocean‐continent transition, is a result of rifting suborthogonal to the profile orientation. There is no evidence of rotated faulted blocks, half graben structures or riftrelated magmatism, manifest as either seaward-dipping reflectors or as a high-velocity region within the lower crust. The Demerara Plateau is, therefore, interpreted as a margin segment comprising thinned continental crust bound to the north and south by transform-type zones in which trans-tensional extension is accommodated. In contrast to Profile A, modelling suggests that the eastern margin exhibits a relatively broad region of crustal thinning associated with extension consistent with a rift-type setting. Offshore, unusually thin oceanic crust of 3.3‐5.7 km thickness is identified which is consistent with similar observations further south. In the absence of identifiable magnetic anomalies, best estimates of the initial half-spreading rate of ∼20 mm yr −1 suggest that the thin crust throughout the region is unlikely to be a result of ultra-slow spreading but, instead, it is most likely due to a reduced magma supply at numerous, long-lived transform faults reflected by those presently offsetting the Mid-Atlantic Ridge in this equatorial setting.

New observations on mid-plate volcanism and the tectonic history of the Pacific plate, Tahiti to Easter microplate

We describe the geology and tectonics of a continuous swathe of seafloor between Tahiti and the western edge of the Easter microplate imaged by GLORIA and Sea Beam on two separate cruise transits in 1987 and 1988. The data reveal that mid-plate volcanism is common in this region, even on deep seafloor hundreds of kilometres from major lines of seamounts and islands. This supports the idea of a thin weak lithosphere over the Pacific Superswell, and the idea that the tops of major mantle plumes may spread out over diameters of the order of 1000 km. The mid-plate volcanism occurs in two distinct forms. Over most of our traverse it appears as fields of relatively young and acoustically strongly backscattering lava flows, often accompanied by groups of numerous small, circular volcanoes. East of 122° W (about chron 5A), however, we observed a distinct form: major, sharp-crested, constructional volcanic ridges, many tens of kilometres long, individually trending ENE, but lying en-echelon along an E-W regional trend. These ridges appear morphologically identical to the ‘cross-grain ridges’ seen elsewhere in the Pacific. We attribute their formation to magma supplied from the regionally hot mantle leaking along tectonic lines of weakness. However, although these ridges are parallel to fracture zone trends seen farther west, they are morphologically very different from any known fracture zone. Moreover, individual ridges are somewhat oblique to the tectonic spreading fabric around them, and so do not seem to follow actual fracture zone traces. The whole line of en-echelon ridges lies along part of the predicted trace of Fracture Zone 2 of Okal and Cazenave [15], and is probably its morphological expression. However, nowhere did we see a convincing ‘conventional’ fracture zone trace in or following the predicted position or orientation. We suggest instead that magma from an independent source has used lines of weakness along minor fracture zones to produce these en-echelon features. The Austral Fracture Zone is the only major fracture zone crossed in our transit, and here is characterised by four fossil transform strands. Its marked position on the AAPG and GEBCO maps is found to be in error. Finally, we found that the expected change from NNW- to NNE-trending spreading fabric at chron 6C did not occur in a clear-cut way, as predicted by earlier tectonic histories of the Pacific. Instead, the post-chron 6C fabric oscillates in a confused way between NNE and NNW, suggesting to us that this area has been characterised by an unstable plate boundary, probably associated with a succession of propagating rifts or microplates from chron 6C to the present.

Magnetic anomalies in the canary basin and the Mesozoic evolution of the central North Atlantic

The data from a recent magnetic compilation by Verhoefet al. (1991) off west Africa were used in combination with data in the western Atlantic to review the Mesozoic plate kinematic evolution of the central North Atlantic. The magnetic profile data were analyzed to identify the M-series sea floor spreading anomalies on the African plate. Oceanic fracture zones were identified from magnetic anomalies and seismic and gravity measurements. The identified sea floor spreading anomalies on the African plate were combined with those on the North American plate to calculate reconstruction poles for this part of the central Atlantic. The total separation poles derived in this paper describe a smooth curve, suggesting that the motion of the pole through time was continuous. Although the new sea floor spreading history differs only slightly from the one presented by Klitgord and Schouten (1986), it predicts smoother flowlines. On the other hand, the sea floor spreading history as depicted by the flowlines for the eastern central Atlantic deviates substantially from that of Sundvik and Larson (1988). A revised spreading history is also presented for the Cretaceous Magnetic Quiet Zone, where large changes in spreading direction occurred, that can not be resolved when fitting magnetic isochrons only, but which are evident from fracture zone traces and directions of sea floor spreading topography.

Evidence for relative motions between the Indian and Australian Plates during the last 20 m.y. from plate tectonic reconstructions: Implications for the deformation of the Indo‐Australian Plate

We use plate tectonic reconstructions to establish whether motions between India and Australia occurred since chron 18 (43 Ma). We test the Africa/Antarctica/Australia/India plate circuit closure at chrons 5 (10 Ma), 6 (21 Ma) and 13 (36 Ma) using a compilation of magnetic anomalies and fracture zone traces from the Southeast, Southwest, Central Indian and the Carlsberg ridges. Additional reconstructions at chrons 23 (55 Ma) and 26 (61 Ma) are used to estimate the overall motion between India and Australia. Relative motions between the Indian and Australian plates are estimated using the plate circuit India → Africa → Australia. A new statistical approach, based on spherical regression analyses, is used to assess the uncertainty of the “best‐fitting” finite rotations from the uncertainties in the data. The uncertainty in a rotation is described by a covariance matrix directly related to the geometry of the reconstructed plate boundary, to the distribution and estimated errors of the data points along it. Our parameterization of the rotations allows for simple combination of the rotation uncertainties along a plate circuit path. Results for chron 5 are remarkably consistent with present‐day kinematics in the Indian Ocean, except that the Arabian and Indian plates are found to be separate plates. Comparisons of the motions between the Indian and African plates across the Carlsberg Ridge with that between the Australian and African plates across the Central Indian Ridge evidence a significant counterclockwise rotation of the Australian plate relative to the Indian plate about a pole located in the Central Indian Basin. The determinations are consistent for chrons 26, 13, 6 and 5. Determination at chron 23 is different but questionable due to the small number of available data. We propose two alternative solutions that both predict convergence within the Wharton and Central Indian basins and extension in the vicinity of the Chagos‐Laccadive Ridge. The first solution assumes that all the deformation in the equatorial Indian Ocean started 7 Ma ago as found by Ocean Drilling Program Leg 116. Hence all the determinations at different times represent the total motion between India and Australia. The averaged India/Australia Euler vector (chrons 5, 6, 13, and 26: 11.1°S, 78.0°, ω=3.54°) lies within the Central Indian Basin and yields a N‐S contraction of 46±52 km at 85°, and 80±63 km at 90°. However, the difference of the India/Australia Euler vectors at chrons 5 and 6 suggests that the India/Australia convergence started between 10 and 21 Ma, following the continent‐continent collision of India with Asia in the Early Miocene. The second averaged solution (chrons 6, 13, and 26: 5.2°S, 74.3°E, ω=5.93°) predicts a total N‐S contraction of 123±73 km at 85°E, and 178±91 km at 90°E. Both models are compatible with the deformation pattern observed in the equatorial Indian Ocean.

High‐resolution inversion for South Atlantic Plate kinematics using joint altimeter and magnetic anomaly data

We present an inversion for plate kinematics that solves for finite rotation parameters using fracture zone (FZ) and magnetic anomaly location data jointly. We define misfit functions that incorporate properties unique to each data type; in particular, the FZ misfit function does not depend upon alignment of conjugate FZ traces in the same way as magnetic lineations under the finite rotations. This property is useful for FZ locations, in which the signals on conjugate sides of the ridge may include systematic differences, or where data from one side of the ridge are sparse or missing. Formal error bounds estimated for the pole parameters show that the magnetic and FZ data are complementary in their information content. Error bounds computed for the joint inversion are substantially smaller than for either the FZ or magnetics data used separately, indicating that simultaneous use of the data in an inversion is crucial. We apply this method to Seasat altimeter data and magnetic anomaly picks in the South Atlantic. We solve for finite poles corresponding to magnetic anomalies 5, 6, 8, 13, 21, 22, 25, 30, 33, 33r, and 34; these define a smooth path over the past 84 m.y. indicating that the plate motions have not been as erratic as found previously.

Tests of crustal divergence models for Aphrodite Terra, Venus

Aphrodite Terra, the largest highland region of Venus, is a likely site of mantle upwelling, active volcanism, and extensional tectonics. We examine three alternative kinematic models for the interaction of mantle convection and the surface: one in which little horizontal surface displacement results from mantle flow (“vertical tectonics”), one in which shear strain from large horizontal displacements is accommodated only in narrow zones of deformation (“plate divergence”), and one in which strain from large horizontal motions is broadly accommodated (“distributed deformation”). In support of a divergent plate model for Aphrodite, Head and Crumpler have cited the presence of an organized system of lineaments held to be analogues to oceanic fracture zones; small‐scale bilateral symmetry of topographic elements, hypothesized to be rifted and separated relief analogous to some terrestrial oceanic plateaus; and subsidence of topography proportional to the square root of distance, consistent with that expected for a divergent thermal boundary layer. We undertake quantitative tests of each of these assertions, and we compare the results with those from similar tests of the Mid‐Atlantic Ridge. We find that apart from the long‐wavelength symmetry of broadly elevated regions, there is no convincing evidence for regional bilateral symmetry of features several hundred kilometers in size on either planet. The fit of the topography of Aphrodite to that of a thermal boundary layer is in general much poorer than for the Earth, and so other mechanisms such as dynamic uplift or crustal thickness variations must dominate the topography. The broad saddle‐shaped region between Thetis and Atla Regiones shows the best fit to the root‐distance relation, yielding apparent spreading half rates of a few centimeters per year, but the goodness of fit and apparent spreading rates are quite sensitive to the distance range analyzed. Calculation of a single pole of relative motion for the entire postulated system of transform faults shows that the inferred fracture zone traces are not consistent with a simple two‐plate model, regardless of past pole motions. A multiple‐plate geometry is therefore called for, and one or more of the lineaments must act as a plate boundary along nearly its full length, if the plate divergence hypothesis is correct. Such a boundary would be distinguishable from other lineaments by geological evidence for recent nontransform motion in forthcoming Magellan radar images. In the absence of such evidence, as long as the existence and distribution of the lineaments are verified, then broad disruption of a thin lithosphere is favored. In such a model, lineaments may be surface manifestations of mantle convective flow. A model dominated by primarily vertical tectonics is tenable only if the lineaments are not confirmed by Magellan.

Evolution of intra-oceanic arc-trench systems

Intra-oceanic island arc-trench systems represent a locus of crustal thickening through the addition of mantle-derived magmas which form the volcanoes and subjacent stratiform plutons. The crust of immature island arcs and adjacent back-arc basins is oceanic in chemistry in that it is dominated by mafic (e.g., tholeiitic) magmas. A magma sequence is recognized in many arc systems which progress from boninite to arc-tholeiite to calc-alkaline and, in some arcs, to shoshonites. The adjacent back-arc basins are formed of MORB-like tholeiite. Arc systems may be dominated by extension rather than compression if the subducted and overriding plates are decoupled. Extension of the arc system may be associated with seaward retreat or “ roll back” of the trench axis. Alternate cycles of extension and compression of the arc system can lead to both tectonic erosion of the fore-arc and accretion of fragments from the incoming sea floor. The Izu-Bonin, Mariana, Tonga, Yap and Palau trench-arc systems are examples of different stages in a dynamic process which starts when subduction begins along old fracture zone traces. Crustal extension causes both fore-arc subsidence and opening of back-arc basins. Island arc volcanic chains and back-arc basins are new crust formed largely of fractional melts of the mantle. The evolution of an arc system may stop when blocking of the subduction zone, or changes in relative plate motion, causes overthrusting of back-arc lithosphere on to former volcanic arcs. Many ophiolites preserve remnants of arc system crust and upper mantle.

Mesozoic magnetic lineations and the magnetic quiet zone off northwest Africa

An extensive compilation of recently acquired geophysical reconnaissance data has allowed the Mesozoic magnetic lineations (The Eastern Keathley sequence) to be identified and mapped in detail for the area off northwest Africa lying between Madeira and the Cape Verde Islands. These anomalies were generated as one limb of a symmetric spreading center (Paleo Mid-Atlantic Ridge) from about 107 to 153 m.y.B.P. Offsets in the lineation pattern serve to identify fracture zone traces whose trends are approximately east-west. The seaward boundary of the marginal quiet zone does not precisely define an isochron due to the presence of a variable width transition zone of intermediate amplitude magnetic anomalies. Crust underlying the marginal quiet zone was generated, at least in part, during the Jurassic, Graham normal polarity epoch. The quiet zone boundary is not offset significantly on opposite sides of the Canaries lineament as previously suggested. A possible counterpart of the U.S. east coast magnetic anomaly is observed in some areas near the shelf/slope break of Spanish Sahara and Mauritania. The presence of relatively high-amplitude (but not-correlatable) magnetic anomalies seaward of the Mesozoic sequence and presumably generated during the Cretaceous, Mercanton normal polarity epoch remains a paradox.