Variation of magnetic properties in sedimentary rocks hosting the Foum Zguid dyke (southern Morocco): Combined effects of re-crystallization and Fe-metasomatism

The effects of dyke intrusion on the magnetic properties of host sedimentary rocks are still poorly understood. Therefore, we have evaluated bulk magnetic parameters of standard palaeomagnetic samples collected along several sections across the sediments hosting the Foum Zguid dyke in southern Morocco. The study has been completed with the evaluation of the magnetic fabric after laboratory application of sequential heating experiments.The present study shows that: (1) close to Foum Zguid dykes, the variations of the bulk magnetic parameters and of the magnetic fabric is strongly related with re-crystallization and Fe-metasomatism intensity. (2) The thermal experiments on AMS of samples collected farther from the dyke and, thus, less affected by heating during dyke emplacement, indicate that 300–400 °C is the minimum experimental temperature necessary to trigger appreciable transformations of the pre-existing magnetic fabrics. For temperatures higher than ca. 580 °C, the magnetic fabric transformations are fully realized, with complete transposition of the initial fabric to a fabric similar to that of samples collected close to the dyke. Therefore, measured variations of the magnetic fabric can be used to evaluate re-crystallization temperatures experienced by the host sedimentary rock during dyke emplacement. The distinct magnetic behaviour observed along the cross-sections strongly suggests that samples collected farther from the dyke margins did not experience thermal episodes with temperatures higher than 300 °C after dyke emplacement. (3) AMS data shows a gradual variation of the magnetic fabric with distance from the dyke margin, from sub-horizontal K₃ away from the dyke to vertical K₃ close to the dyke. Experimental heating shows that heat alone can be responsible for this strong variation. Therefore, such orientation changes should not be unequivocally interpreted as the result of a stress field (resulting from the emplacement of the dyke, for instance). (4) Magnetic studies prove to be a very sensitive tool to assess rock magnetic transformations, thermally and chemically induced by dyke intrusion in hosting sediments.     

Variation trends in basaltic rocks of the canary islands

More than two hundred new analyses of basaltic rocks from the Canary Islands are presented. The available data show that the earlier successions have similar variation trends throughout the archipelago; these trends represent all the intermediate types between basic differentiates (oceanites, ankaramites) and more salic differentiates (trachy-basalts, hawaiites). In the more recent volcanic series, the study shows that there is a magmatic diversification with different variation trends in the Eastern than in the Central Canary Islands. In the latter the alkaline character becomes stronger in the successive periods of volcanic activity which have been established in each island. In the Eastern islands the basaltic evolution is, on the contrary, towards basaltic rocks with tholeiitic affinities.     

Correlation between magnetic and mineralogical properties of some tertiary basaltic rocks from qatrani-fayum area,Egypt

This paper reports clear correlation between the magnetic and mineralogical properties of some basaltic rocks from Egypt. Although all the measured samples are reversely magnetized lavas, yet clear differences were found between them. A four-fold classification based on the nature of both titanomagnetite and discrete ilmenite grains is given. The classification is based mainly on the differentiation between the different stages of oxidation and these were found to be in accordance with the intensity of magnetization and susceptibility values.     

Dredged basaltic rocks from the seaward extensions of the Reykjanes and Snaefellsnes volcanic zones,Iceland

Data for basaltic rocks dredged on the Reykjanes Ridge and Jökulbanki off the southwest coast of Iceland are presented. The rocks from the Reykjanes Ridge, which are identified as recent lavas, are olivine tholeiites and quartz tholeiites very similar to those formerly described from this region. The heterogeneous Jökulbanki dredges were identified as erratics.In the light of the present and other recent data on tholeiitic rocks from Iceland and the Reykjanes Ridge, a distinct chemical gradient, together with increased scatter across Iceland is demonstrated for certain incompatible elements. This is in accordance with the mantle plume theory.     

The nature of magnetic anomalies in subduction zones

The analysis of the magnetic survey data suggests the presence of a frontal zone of intense magnetic anomalies in a number of the Pacific island-arc systems. These zones with amplitudes of 100–300 nT are observed within the Kuril–Kamchatka and Aleutian island arc systems, in Southern and Central America, and Alaska. As demonstrated by the solution of the inverse problem and petromagnetic investigation of the rocks, these zones are presumably related to the serpentinite bodies which form as a result of the hydration of the upper mantle peridotites by the oceanic water penetrating through a system of cracks and fractures into the subducting slab at its bend. The rock magnetic studies show that magnetite is the main carrier of magnetization in these serpentinite bodies. Hydration of the subducting slab also causes hydration of the mantle rocks of the overriding plate with the formation of the magnetized serpentinite wedge. The decompaction of ultrabasic rocks under hydration is marked by a decrease in the gravity field and velocities of elastic waves. As the subducting plate loses water, it becomes embrittled and becomes the localization region for the epicenters of the strongest earthquakes. Magnetic survey can be used for revealing the potential sources of catastrophic earthquakes.     

Some magnetic fabric characteristics of sheared zones

In this paper the magnetic susceptibility anistropyy technique has been evaluated for the purpose of identifying shear zones in deformed regions and for detecting movements on shear planes. The magnitude parameters and orientations of the susceptibility ellipsoid show distinct characteristic behaviour on traversing deformed zones. The actual shear lines are characterized by the very high values of anistrropy and anisttropy gradients in their vicinities. The orientational data give clear indication of any relative rotations across questionalbe boundaries. Furthermore, a semi-analytical method is outlined for determining detailed pice-meal movements along shear lines as well as giving the regional movement direction. It is not possible to quantify the movements since too many parameters are unknown.     

Effect of hydrostatic pressure upto 1.5 GPa on the magnetic parameters of basaltic rocks

Summary The effect of hydrostatic pressure upto 1.5 GPa on the remanent magnetic polarization of samples, representing three groups of basalts with different ratios of the reversible and irreversible component of overall changes is studied. It is shown that, as a result of permanent changes of the transfer properties of the non-magnetic matrix, the patterns of pressure changes of the remanent magnetic polarization differ after applying a pressure of 1.5 GPa, however, the ratio of the reversible and irreversible component does not change.     

嵊泗岛辉绿岩墙群的侵位方式:来自磁组构的证据

岩墙磁组构能反映岩浆的侵位方式.中国东部嵊泗岛广泛发育了晚白垩世辉绿岩岩墙群.我们对其中8条不同走向岩墙进行了采样,沿岩墙两边部及横跨岩墙剖面获得共273个独立定向岩芯样品.岩石磁学分析表明辉绿岩的主要携磁矿物为多畴贫钛磁铁矿,可能含少量磁赤铁矿.各条岩墙的磁组构均具有低的各向异性度Pj＜1.2,且主轴的空间方位各不相...     

Anisotropy of magnetic susceptibility in welded tuffs: application to a welded-tuff dyke in the tertiary Trans-Pecos Texas volcanic province,USA

Consideration of published anisotropy of magnetic susceptibility (AMS) studies on welded ignimbrites suggests that AMS fabrics are controlled by groundmass microlites distributed within the existing tuff fabric, the sum result of directional fabrics imposed by primary flow lineation, welding, and (if relevant) rheomorphism. AMS is a more sensitive indicator of fabric elements within welded tuffs than conventional methods, and usually yields primary flow azimuth estimates. Detailed study of a single densely welded tuff sample demonstrates that the overall AMS fabric is insensitive to the relative abundances of fiamme, matrix and lithics within individual drilled cores. AMS determinations on a welded-tuff dyke occurring in a choked vent in the Trans-Pecos Texas volcanic field reveals a consistent fabric with a prolate element imbricated with respect to one wall of the dyke, while total magnetic susceptibility and density exhibit axially symmetric variations across the dyke width. The dyke is interpreted to have formed as a result of agglutination of the erupting mixture on a portion of the conduit wall as it failed and slid into the conduit, followed by residual squeezing between the failed block and in situ wallrock. Irrespective of the precise mechanism, widespread occurrence of both welded-tuff dykes and point-welded, aggregate pumices in pyroclastic deposits may imply that lining of conduit walls by agglutionation during explosive volcanic eruptions is a common process.     

Conditions for the arrest of a vertical propagating dyke

Magma ascent towards the Earth’s surface occurs through dyke propagation in the vast majority of cases. We investigate two purely mechanical effects unrelated to cooling or solidification that lead to the arrest of propagation, so that no eruption occurs. The first is that the input of magma from the source is not maintained continuously, such that a fixed volume of magma is released. Laboratory experiments show that, in this case, the dyke stops at a finite distance from the source. This behaviour is specific to the fracturing process in 3-D. We derive a relationship for the minimum magma volume required for an eruption as a function of magma buoyancy and source depth. When large magma volumes are available, eruption may also be prevented by a thick low density layer in the upper crust. Numerical studies of dyke propagation show that the dyke continues to rise even though it is negatively buoyant. Magma accumulates in a swollen nose region at the interface between the low density layer and the dense basement. Magma overpressure is largest at this interface and increases with increasing penetration into the upper layer. It may become large enough to induce horizontal fractures in the dyke walls and lateral intrusion of a sill, which prevents eruption. This requires that the thickness of the low density layer exceeds a threshold value that depends on the density contrast between magma and host rock. If the magma volume is smaller than a threshold value, neither sill intrusion nor eruption are possible and magma gets stored in a horizontal blade-shaped dyke straddling the interface. Scaling laws for variations of ascent rate and for the minimum magma volume allow diagnosis of a failed eruption.     

Magma expansion and fragmentation in a propagating dyke

The influence of magma expansion due to volatile exsolution and gas dilation on dyke propagation is studied using a new numerical code. Many natural magmas contain sufficient amounts of volatiles for fragmentation to occur well below Earth's surface. Magma fragmentation has been studied for volcanic flows through open conduits but it should also occur within dykes that rise towards Earth's surface. The characteristics of volatile-rich magma flow within a hydraulic fracture are studied numerically. The mixture of melt and gas is treated as a compressible viscous fluid below the fragmentation level and as a gas phase carrying melt droplets above it. The numerical code solves for elastic deformation of host rocks, the flow of the magmatic mixture and fracturing at the dyke tip. With volatile-free magma, a dyke fed at a constant rate in a uniform medium adopts a constant shape and width and rises at a constant velocity. With volatiles involved, magma expands and hence the volume flux of magma increases. With no fragmentation, this enhanced flux leads to acceleration and thinning of the dyke. Simple scaling laws allow accurate predictions of dyke width and ascent rate for a wide range of conditions. With fragmentation, dyke behaviour is markedly different. Due to the sharp drop of head loss that occurs in gas-rich fragmented material, large internal overpressures develop below the dyke tip and induce swelling of the nose region, leading to deceleration of the dyke. These results are applied to the two-month long period of volcanic unrest that preceded the May 1980 eruption of Mount St Helens. An initial phase of rapid earthquake migration from the 7–8 km deep reservoir to shallow levels was followed by very slow progression of magma within the edifice. Such behaviour can be accounted for by magma fragmentation at the top of a dyke.     

Seismic measurements of the internal properties of fault zones

The internal properties within and adjacent to fault zones are reviewed, principally on the basis of laboratory, borehole, and seismic refraction and reflection data. The deformation of rocks by faulting ranges from intragrain microcracking to severe alteration. Saturated microcracked and mildly fractured rocks do not exhibit a significant reduction in velocity, but, from borehole measurements, densely fractured rocks do show significantly reduced velocities, the amount of reduction generally proportional to the fracture density. Highly fractured rock and thick fault gouge along the creeping portion of the San Andreas fault are evidenced by a pronounced seismic low-velocity zone (LVZ), which is either very thin or absent along locked portions of the fault. Thus there is a correlation between fault slip behavior and seismic velocity structure within the fault zone; high pore pressure within the pronounced LVZ may be conductive to fault creep. Deep seismic reflection data indicate that crustal faults sometimes extend through the entire crust. Models of these data and geologic evidence are consistent with a composition of deep faults consisting of highly foliated, seismically anisotropic mylonites.     

Variation in peperite textures associated with differing host-sediment properties

Peperites formed by mixing of magma and wet sediment are well exposed along Punta China, Baja California, Mexico, where two sills intrude a section of lava flows, limestones, and volcaniclastic rocks. Irregular lobes and dikes extend from the sills several meters into host sediments, including highly comminuted flow top breccias (lithic lapilli tuff breccias) and shelly micrites, whereas intrusive contacts with lava flows are sharp and planar. Where one sill intruded both coarse-grained volcaniclastic rock and fine-grained limestone, textural differences between the hosts produced strikingly different styles of peperite. Blocky masses of the basaltic intrusions up to 1 m in size were dispersed for distances up to 3 m into host lithic lapilli tuff breccias; the blocks consequently underwent in situ fragmentation as they were rapidly quenched. The high degree of dispersion resulted from steam explosions as the magma enveloped pockets of water in the coarse-grained permeable host. Elutriation of fine-grained material from vertical pipes in tuff breccia above the lower sill provides evidence for meter-scale fluidization of the host. The contact zone between the basaltic magma and the shelly micrite host resembles a mixture of two viscous, immiscible fluids (fluidal peperite). Intrusion occurred behind a stable vapor film which entrained lime mud particles and carried them off grain by grain as magma advanced into the host. Thin-section-scale elutriation pipes formed. Microglobular peperite represents a frozen example of a fuel-coolant interaction (FCI) between basaltic magma and fluidized micrite host. The intimate intermixing of magma and host at the submillimeter level is attributed to fluid instabilities developed along the magma-vapor-host interface. Such intimate intermixing of magma and water-bearing fragmental debris is commonly a precursory step toward explosive hydrovolcanism.     

Three-dimensional modeling of pollutants transportation in the flow field around a spur dyke

Pollutants are carried by sediment-laden flow dissolved in water or adsorbed on sediment particles. A transient three-dimensional model based on a compressible VOF(volume of fluid) method was developed to simulate the transport of dissolved and particulate pollutants.VOF is a numerical technique for acquiring and tracking the free surface of water flow.Local scouring,deposition and re-suspension were simulated and the processes of adsorption and desorption of pollutants on suspended sediment were analyzed.A series of experiments and numerical simulations were performed to study the transportation and dispersion of pollutants in the flow around a non-submerged spur dyke in a straight flume of rectangular shape.The simulation results agreed well with the experimental results.A certain volume of pollutants solution was released into the flow at upstream of the spur dyke.The concentration reduced with time.The concentration reduction was slower in the circumfluence zone than in the main flow.The ratio of adsorption to desorption coefficients was different for pollutants on suspended sediment with different diameter.The peak concentration of dissolved pollutants increased with the ratio of the adsorption to the desorption coefficients.The angle of the spur dyke affected the peak concentration around the dyke.The effect of the angle on dissolved pollutants decreased with the ratio of adsorption to desorption coefficients.The adsorption and desorption coefficients,as well as the saturated adsorption capacity had no effect on the concentration of particulate pollutants.     

Variation of magnetic directions within pillow basalts

Relative directions of magnetization have been measured within individual pillow basalts collected from the Atlantic Ocean and Caribbean Sea. The angle between the magnetic directions was determined and is referred to as the directional difference. Although one pillow contained a directional difference of 44°, the remaining ten pillows had differences less than 14°. The maximum orientation and measurement error was 7°. Dispersion on the scale found in these fine-grained pillow basalts would not appreciably affect the magnetic anomaly pattern on the sea floor. We detected no reversals of magnetization despite the sometimes large and variable low-temperature oxidation. Comparison of directions within homogeneous segments of the pillow, viscous remanent magnetization (VRM) acquisition experiments, and alternating field (AF) demagnetization indicate a large portion of the dispersion was due to the acquisition of a viscous component in the larger grained, less oxidized portion of the pillows. Evidence from one variably weathered pillow suggests that extreme low-temperature oxidation may lead to the acquisition of a secondary component with high coercivities (20–80 mT). We could not determine whether this was a chemical remanent magnetization (CRM) or a VRM acquired by single domain grains near the superparamagnetic threshold. Hysteresis properties confirmed by microscopic examination indicated that the magnetic grain size in all the pillows was at least as small as pseudo-single domain.     

Magma flow revealed by magnetic fabric in the Okavango giant dyke swarm, Karoo igneous province, northern Botswana

To determine the magma flow direction of the giant, 179 Ma Okavango dyke swarm of northern Botswana, we measured the anisotropy of magnetic susceptibility (AMS) of 23 dykes. Dykes are located in two sections (Shashe and Thune Rivers), which are about 300 km and 400 km from the presumed magma source respectively; the Nuanetsi triple point. We collected samples from the margins of the dykes in order to use the imbrication of magnetic foliation to determine magma flow direction. About half of the magnetic fabric in the dykes is inverse, i.e. with the magnetic foliation perpendicular to the dyke plane. Lateral flow to the west and vertical flow is in evidence in the Shashe section. However, the overall analysis of normal and inverse magnetic fabric data supports that lateral flow to the west was dominant in the Shashe section. Across the Thune section, a poorly defined imbricated magnetic foliation also suggests lateral flow to the west.     

Numerical models of subduction of the oceanic crust with basaltic plateaus

Light continents and islands characterized by a crustal thickness of more than 30 km float over a convective mantle, while the thin basaltic oceanic crust sinks completely in subduction zones. The normal oceanic crust is 7 km thick. However, anomalously thick basaltic plateaus forming as a result of emplacement of mantle plumes into moving oceanic lithospheric plates are also pulled into the mantle. One of the largest basaltic plateaus is the Ontong Java plateau on the Pacific plate, which arose during the intrusion of a giant superplume into the plate ～100 Myr ago. Notwithstanding its large thickness (averaging ～30 km), the Ontong Java plateau is still experiencing slow subduction. On the basis of numerical modeling, the paper analyzes the oceanic crust subduction process as a function of the mantle convection vigorousness and the density, thickness, viscosity, and shape of the crust. Even a simplified model of thermocompositional convection in the upper mantle is capable of explaining the observed facts indicating that the oceanic crust and sediments are pulled into the mantle and the continental crust is floating on the mantle.     

Variations in magnetic anisotropy and opaque mineralogy along a kilometer deep profile within a vertical dyke of the syenogranite porphyry at Cínovec (Czech Republic)

Vertical variations in magnetic fabric and paragenesis of the ferrimagnetic minerals for a virtually upright dyke of the Altenberg syenogranite porphyry were investigated using profile data of borehole E-16 to a depth of 922.7 m (Eastern Kru né hory Mts., NW Bohemia). It was revealed that this dyke likely consists of two magma pulses indicated both by magnetic fabric and by opaque mineralogy. In most of the profile, the magnetite grains are oriented parallel to the sub-vertical dyke. Only at the base of the upper magma pulse, in a depth interval of 200–400 m, are these grains oriented sub-horizontally being thus perpendicular to the dyke walls. This pattern can be interpreted as a consequence of a static vertical compaction of the magma of the basal portions of the upper pulse due to the pressure of the ascending lower pulse. The large planes of the ferrimagnetic minerals are oriented perpendicular to the shortening direction.