Magma Flow Direction Research Articles

Magma flow in dikes from rift zones of the basaltic shield of Tenerife, Canary Islands: Implications for the emplacement of buoyant magma

Dikes of rift zones in the Miocene basaltic shield of Tenerife have been studied along several profiles in order to record overall variations in lateral and vertical magma flow direction and sense. Dikes were sampled for Anisotropy of Magnetic Susceptibility (AMS) and other flow indicators and dike propagation indicators were also recorded at each sampling site. Bulk susceptibilities suggest a predominance of ferromagnetic minerals in the AMS fabrics and most of the sampled dikes show normal fabrics. Magnetic fabrics have been classified according to clustering and imbrication shown by the maximum susceptibility axis on each dike margin, a procedure that provides rapid information on the direction and sense of magma flow. The flow of magma tends to be upward and steep beneath the volcanic centers of the basaltic shield massifs. Along the axes of rift zones and in those rift zones located across the flanks of the volcanic edifices (roughly normal to the main rifts), magma is mainly emplaced laterally and downward. The flow of magma is mostly towards the external parts of the volcanic centers of the basaltic shield massifs and indicates that magma propagated toward regions of lower topography. This interpretation agrees with magma being emplaced above the level of neutral buoyancy and with buoyancy as the driving pressure gradient. The observed flow patterns can be explained in terms of density gradients within the volcanic edifices, in which magma flow trajectories are toward less dense regions.

Emplacement of multiple magma sheets and wall rock deformation: Trachyte Mesa intrusion, Henry Mountains, Utah

A detailed structural and rock magnetic study of the Trachyte Mesa intrusion and deformed sedimentary wall rocks, Henry Mountains, Utah, indicates that the intrusion grew vertically and horizontally by the accumulation of multiple horizontal magma sheets. 2–3 cm thick shear zones recognized by intensely cataclasized plagioclase phenocrysts define the contact between sheets. Sheets have bulbous and / or steep frontal terminations and are flat on top. The foliation within the interior of the sheets, near the frontal termination, is subvertical. This steep foliation rotates into the subhorizontal shear zones near the top and bottom contacts and provides a magma flow direction indicator. Away from the frontal termination, the interior foliation rotates to become subhorizontal, similar to the fabric in recent analog experimental studies. Sheets are interpreted as being emplaced as plug flows. Both the field fabric and the rock magnetic data collected from 103 locations on the top of the intrusion and from 73 locations along a vertical cross section exposed in a stream gorge support a multi-stage model of intrusion growth. Emplacement begins as narrow magma channels and magma spreads radially outward from the channels to form sheets. Sheets are stacked upon one another and stop at the same lateral termination. The deformation of the sandstones at the margin of the intrusion, which are rotated upward from the margin to become the roof, is partitioned into layer parallel extension, shearing and layer-parallel shortening components. Bulk strain within the thickest sandstone layer indicates ∼20% thinning and microstructures indicate that the thinning was accommodated by grain-scale fracture-induced porosity collapse. Extension occurred as the layer was stretched over the margin of the rising intrusion. Shearing and layer-parallel shortening are a result of coupling with the underlying sheets as they advanced and accommodated through numerous faults parallel to bedding and at low angles to bedding. The deformation of thinner sedimentary layers is consistent with the sedimentary layering immediately in front of an advancing sheet being translated upward and over the top of the sheet as the hinge zone migrates with the front edge of the advancing sheet.

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.

Flow lobes in granite: The determination of magma flow direction in the Trawenagh Bay Granite, northwestern Ireland, using anisotropy of magnetic susceptibility

Anisotropy of magnetic susceptibility (AMS) has been determined from 152 sites over the ∼64 km2 Trawenagh Bay Granite (part of the 400 Ma Caledonian Donegal Batholith). Microscopic observations, thermomagnetic analyses, high-field torque magnetometry, and heating experiments demonstrate that the AMS fabric is carried by biotite with varying amounts of mimetic magnetite in the biotite cleavage planes. The AMS fabric is interpreted as a flow fabric because microstructures indicate magmatic-state deformation: it defines large-scale macroscopic flow features, flow lobes, in this weakly deformed granite. At pluton scale the data set shows a gentle westward-plunging lineation. At a smaller scale many lobate magnetic foliation patterns can be discerned, in which the magnetic lineation is usually symmetrically disposed about the long axes of the lobes. In three dimensions the lobes are tongue-like, <1500 m wide (subunit scale), and elongated east-west (parallel with the general lineation). They close consistently westward, and younger lobes in the east appear to deflect older lobes in the west. The Trawenagh Bay Granite is west of a major synplutonic shear zone that hosts the Main Donegal Granite pluton. These structures are interpreted as frozen flow lobes and indicate that the magma flowed in a westward direction, from within the active shear zone of the Main Donegal Granite into the weakly deformed wall rock (Trawenagh Bay Granite). Intra-unit pulses or surges have long been suspected in granite plutons. However, this study reports the most detailed description of such structures to date.

The genetic relationship between mafic dike swarms and plutonic reservoirs in the mesozoic of central chile (30°–33°45′S): insights from AMS and geochemistry

Five mafic dike swarms between 30° and 33°45′S were studied for their geochemical signature and kinematics of magma flow directions by means of AMS data. In the Coastal Range of central Chile (33°−33°45′S), Middle Jurassic dike swarms (Concon and Cartagena dike swarms, CMDS and CrMDS, respectively) and an Early Cretaceous dike swarm (El Tabo Dike Swarm, ETDS) display the presence of dikes of geochemically enriched (high-Ti) and depleted (low-Ti) basaltic composition. These dikes show geochemical patterns that are different from the composition of mafic enclaves of the Middle Jurassic Papudo-Quintero Complex, and this suggests that the dikes were injected from reservoirs not related to the plutonic complex. The mantle source appears to be a depleted mantle for Jurassic dikes and a heterogeneous-enriched lithospheric mantle for Cretaceous dikes. In the ETDS, vertical and gently plunging magma flow vectors were estimated for enriched and depleted dikes, respectively, which suggest, together with variations in dike thickness, that reservoirs were located at different depths for each dike family. In the Elqui Dike Swarm (EDS) and Limari Mafic Dike Swarm (LMDS), geochemical patterns are similar to those of the mafic enclaves of the Middle Jurassic plutons. In the LMDS, east to west magma flow vectors are coherent with injection from neighbouring pluton located to the east. In the EDS, some dikes show geochemical and magma flow patterns supporting the same hypothesis. Accordingly, dikes do not necessarily come from deep reservoir; they may propagate in the upper crust from coeval shallow pluton chamber.

Dike intrusion under shear stress: Effects on magnetic and vesicle fabrics in dikes from rift zones of Tenerife (Canary Islands)

A theoretical model predicting how anisotropy of magnetic susceptibility (AMS) and vesicle fabrics are modified by shear stress resolved on the dike walls prior to the final cooling of magma is developed for vertical dikes. The resulting fabrics are asymmetric with respect to initial fabrics assumed to be symmetric. Application of this model together with collected data on magma flow direction, dike propagation direction and mechanism, and shear sense, allow us to interpret dike fabrics in terms of shear resolved on the dike walls during intrusion (en echelon arrangement, offsetting, and dike curvature). The interpretation of AMS and vesicle fabrics of the margins of four dikes shows a reasonable agreement with the proposed theoretical models, suggesting that asymmetric fabrics can be used to infer magma flow and may provide valuable information on the shear resolved on the dike walls during intrusion.

Upward and downward flow in a camptonite dike as recorded by deformed vesicles and the anisotropy of magnetic susceptibility (AMS)

A 1.6-m wide Cretaceous camptonite dike on Higby Mountain, Connecticut, contains deformed vesicles that record an initial brief period of steeply upward intrusion followed by a prolonged vertical downward flow of magma prior to solidification. The camptonite is unusual in that it is almost aphyric, but it does contain abundant skeletal microphenocrysts (∼ 0.3 mm) of magnetite, which give the rock a strong magnetic susceptibility (∼ 127 × 10 − 3 SI). These crystals are slightly elongate, and their long axes became preferentially aligned parallel to the dike walls but perpendicular to the magma flow direction, which is indicated by the stretched vesicles. Consequently, the rock has a strong anisotropy of magnetic susceptibility (AMS) with the mean value of P′ being 1.03. The AMS ellipsoid is everywhere prolate ( T = − 0.9 to − 0.02), with the maximum susceptibility axis being almost parallel to the strike of the dike, that is, parallel to the walls of the dike but perpendicular to the magma flow direction. In the outer part of the dike where the deformed imbricate vesicles indicate upward flow at an inclination of 70–75°, the maximum susceptibility direction plunges ∼ 11°, perpendicular to the flow direction. In the interior of the dike where the vesicles indicate almost straight downward flow, the maximum susceptibility direction is almost horizontal and again is perpendicular to the flow. In many, but not all samples, the sense of flow (i.e., up or down) is indicated by imbrication of the intermediate magnetic susceptibility axis. Although the major magnetic susceptibility axis is commonly assumed to parallel the direction of flow of magma in dikes, the present study indicates that, at least in this camptonite dike, the maximum susceptibility axis is perpendicular to the flow of magma.

Mid-crustal shear zone evolution in a syn-tectonic late Hercynian granitoid (Sila Massif, Calabria, southern Italy)

By means of petrogrological, meso- and microstructural analyses, the fabric of a syn-tectonic late Hercynian K-feldspar megacryst-bearing granodiorite is described in this paper. The granodiorite was emplaced at 293 Ma within migmatitic paragneisses which had reached the regional peak metamorphic conditions at 304–300 Ma. The granodiorite and the migmatitic paragneisses are both affected by the same ductile shear zone. In the core of the shear zone, mylonites show a clear grain-size reduction and microstructures related to deformation at high to medium temperature conditions. Migmatitic paragneisses, foliated granodiorites and mylonites mostly show concordant lineation and foliation orientations. In addition, the preferred orientation of euhedral feldspars in granodiorites indicates that the fabric anisotropy started to develop in the magmatic state. These features strongly suggest that shear deformation was active during crystallisation of granitoids and continued under subsolidus conditions. In wall rocks and mylonites, kinematic indicators such as σ- and δ-type porphyroclasts, S/C fabrics, shear bands and quartz (c) axis orientations suggest a top-to-the-W sense of shear. This is similar to the magma flow direction indicated by the tiling of euhedral feldspar megacrysts in granodiorites. Shear deformation developed, preferentially, by partitioning of strain in the granodioritic crystal mush. Geobarometry indicates that deformation took place at middle crustal levels (P=400–500 MPa). Whole rock-white mica Rb/Sr geochronological analysis of an undeformed pegmatite, crosscutting the mylonitic foliation, provided an age of 265 Ma. Timing of deformation is therefore bracketed between 293 Ma and 265 Ma.

Theoretical aspects of particle movement in flowing magma: implications for the anisotropy of magnetic susceptibility of dykes

Abstract Most studies of the anisotropy of magnetic susceptibility (AMS) of dykes have assumed that the axes of maximum susceptibility ( k max ) should define an opposed imbrication pointing along the direction of magma flow, and that this orientation should be preserved along the dyke. This assumption is partially based on a model predicting the orientation of ellipsoidal particles floating in a moving liquid although the model actually predicts a cyclic movement of the particles that has been overlooked in msot AMS studies without further justification. The consequences of considering the full rotation of the ellipsoidal particles, as actually predicted by the theory, in the expected AMS of dykes are examined in this work. The complete version of the motion of ellipsoidal particles is then incorporated in a model of magma movement that takes into consideration the distribution of shear deformation within the dyke as predicted from the velocity gradient of the moving magma. Results of this model show that both particle elongation and the amount of shear that is sampled will affect the quality of the AMS results. By paying attention to the systematic variations of the AMS predicted by the theory, however, it is possible to devise sampling schemes that can be used to add more confidence to the interpretation of the AMS results. Although based on an idealized scenario of magma movement within a dyke, the model developed here explains satisfactorily the sometimes observed variation of AMS along flow direction in one dyke, and provides a simple explanation for many of the ‘abnormal’ magnetic fabrics that have been reported in dyke swarms around the world.

Magma flow directions of post Jotnian dolerite sills in central-east Sweden: a magnetic fabric and paleomagnetic survey

An anisotropy of magnetic susceptibility (AMS) and paleomagnetic study has been performed on post-Jotnian basic sills of the Ulvö complex, of the Central Scandinavian Dolerite Group (CSDG). The CSDG intruded some 1.27 Ga ago in the central part of the Fennoscandian Shield and extends over an area of c. 150 000 km2. Previously presented AMS data of the CSDG sills from south-central and northern Sweden indicated a NW or SE directed magma flow. In this study new measurements of the AMS reveal a fairly uniform pattern of NW-SE lineation directions also for the dolerites of the Ulvö complex. While the magma is flowing in a dyke or a sill, elongated particles become imbricated against the chilled margins, and the AMS can be used to determine not only the lineation but also the true direction of the magma flow. An imbrication pattern defined for one of the sills suggests that the magma propagated from the present NW towards SE. The established paleomagnetic pole position (Plat=−3° and Plon=157°) is in very good agreement with other paleomagnetic data from the Ulvö complex and from other parts of the CSDG. The new AMS data from the Ulvö complex presented in this study fit well in to the magnetic fabric pattern previously presented for the CSDG sills and support a plate tectonic model suggesting that the CSDG is a related to the break up of Baltica from Laurentia at c. 1.27 Ga ago.

Magma flow inferred from AMS fabrics in a layered mafic sill, Insizwa, South Africa

The Insizwa sill, is a 25-km-diameter, >1000-m-thick layered mafic intrusion, part of the Karoo Igneous Province in South Africa. The peridotitic and gabbronoritic rocks are undeformed and mineral fabrics demonstrably result from magma flow. A horizontal, centimeter-scale model layering is visible in numerous outcrops. Plagioclase crystals are both tabular and elongated. Their preferred orientation, parallel to the layering, forms a foliation and a NW–SE lineation, respectively interpreted as the magma flow plane and flow direction. Throughout the 78 stations of this study (699 specimens), magnetic susceptibilities ( K m) range from 750 to 10,000×10 −6 SI. The magnetic anisotropy ( P j ) ranges from 1.03 to 1.08. Magnetic ellipsoids are both prolate and oblate (average T j ≈0). Anisotropy of magnetic susceptibility (AMS) fabrics are dominated by multidomain to pseudo-single domain magnetite. High-field magnetic experiments indicate that the paramagnetic contribution from the mafic silicates is less than 50% for low susceptibility rock types. The anisotropy results from magnetite grain shape solely as shown by no significant increase in P j with increasing K m. The magnetic lineation (305°, 05°) is consistent throughout the sill at various scales and coincides with the mineral lineation in average. In contrast, the magnetic foliation (125° NE 10°) is generally perpendicular to the mineral foliation and to the layering. Several explanations for this odd configuration are discussed. The variations of magnetic parameters across the layering and field observations point to a multiple injection. The magnetic lineation is consistent with the presence of a single feeder dike situated to the SE of the sill.

Magnetic fabrics study and inferred flow directions of lavas of the Old Pali Road, O′ahu, Hawaii

A section 600–700 m deep in the Koolau tholeiitic lava-shield volcano consists of mostly thin pahoehoe and aa flows cut by occasional dikes and outward-dipping gravity-driven intrusive sheets. A magnetic fabric study of 250 specimens from 18 sites shows a tight clustering of K 1 axes for some of the lava flows and is thought to fairly reliably define the lava flow direction. A systematic plunge of the K 1 axes appears to define an imbrication and yields a flow azimuth. The azimuth differs by 60° from that anticipated but is readily explained by a change in shape of the Koolau Volcano during growth. The tightest clustering is in massive aa lava flows 5 m thick. Samples collected from near the flow base give the most consistent orientations. Compound pahoehoe lavas with many small flow units give the least tightly clustered anisotropy of magnetic susceptibility (AMS) axes and, in some flows, the orientation is random. The gravity-driven sheet has an AMS fabric indicating a down-dip magma flow direction. After the Koolau Volcano was deeply eroded, a cinder cone and lava flows of nephelinite were formed in the rejuvenation stage of volcanism. The nephelinite is normally magnetized and the AMS fabric indicates a generally downslope flow.

Ropy flow structures: A neglected indicator of magma-flow direction in sills and dikes

Ropy flow structures that form in dikes and sills are very similar to ropy lava structures (pahoehoe), but are of intrusive origin. They are unusual among magma-flow indicators in that they provide a frozen record of the magma flow. Our observations suggest that they occur in large vesicles, which form as a result of local repeated pressure drops during propagation and emplacement of the magma. Adiabatic gas expansion causes a ductile rim to form around a vesicle; this rim is then sheared by the underlying flow. Anisotropy of magnetic susceptibility data from the contact zones confirm the flow data from the ropy flow structures, but the results from deeper within the body, both locally and regionally, yield different, yet consistent flow orientations. This discrepancy suggests that a distinction can be made between the magma flow close to the contact and the deeper regional flow patterns in sills and dikes.

Magma flow directions of shallow dykes from the East Greenland volcanic margin inferred from magnetic fabric studies

The role played by plume-generated crustal magmatic complexes in the segmentation of volcanic margins is highlighted by a preliminary study of magma flow directions in shallow intrusives from the East Greenland volcanic margin. We investigate the magmatic texture using anisotropy of magnetic susceptibility for eight dykes of tholeitic affinity belonging to a dyke swarm associated with the Tertiary opening of the North Atlantic Ocean. The thickness of the sampled dykes ranges from 3 to 37 m. The dykes are of doleritic texture and contain up to 12% of opaque minerals and 35% of plagioclase laths. Dykes showing a magnetic foliation plane within the dyke plane, i.e. a magnetic fabric usually attributed to magmatic processes, represent 40% of the samples from the studied swarm. These dykes have a low degree of anisotropy and their ellipsoid of magnetic susceptibility is strongly oblate. Inverse magnetic fabrics, where the maximum principal susceptibility axis lies near the pole to the dyke account for 60% of the data. We interpret these inverse fabrics either as alteration minerals with highly prolate ellipsoids of magnetic susceptibility or primary titanomagnetites with oblate ellipsoid of low anisotropy. The flow direction is inferred for normal fabric dykes using the mirror imbrication of magnetic lineation. Analysis of thin sections shows a good agreement between magnetic fabric directions and phenocryst preferred orientations. The inferred flow directions are predominantly horizontal, throwing a new light on volcanic margin development.

Magnetic fabric and inferred flow direction of dikes, conesheets and sill swarms, Isle of Skye, Scotland

We have measured the AMS of 16 dikes, 35 conesheets and three sills associated with the Cuillin Hills magmatic center of the Isle of Skye in the Inner Hebrides of Scotland and of nine dikes located in the regional dike swarm of Skye. Sixty-three intrusives, totalling 734 samples, were studied to determine the plumbing of the Skye volcanic system. Low-field susceptibility versus temperature ( k– T) identified three different mineral phases in the area, namely Ti-rich magnetite, pyrrhotite and titanomaghemite. The petrofabrics of the 63 intrusives yielded coherent flow azimuths regardless of their time of emplacement. Three main types of magnetic fabric, (A–C) were found. Fabric Type A (plane K max– K int parallel to the dike plane) represents the magma-flow direction within the intrusives and is the dominant fabric (55% of all the intrusives) within the Cuillin Hills magmatic center and its regional dike swarm. The K max inclinations show that 55% of the intrusives were fed by horizontal to sub-horizontal (AMS inclination of the K max axis is equal or less than 30°) magma fluxes and the rest of them were fed by inclined to vertical fluxes. Horizontal magma flow means lateral magma injection inside fractures and becomes more probable as the source is located further away.

Post Jotnian basic Intrusions in the Fennoscandian Shield, and the break up of Baltica from Laurentia: a palaeomagnetic and AMS study

A palaeomagnetic and anisotropy of magnetic susceptibility (AMS) study has been performed on dolerite sills of the Central Scandinavian Dolerite Group (CSDG) in the Fennoscandian Shield. The dolerites occur in four previously known complexes in central Sweden and Finland and from the results of this palaeomagnetic study another complex has been identified in northern Sweden. These complexes cover an area of at least 100 000 km 2 and the palaeomagnetic data suggest a small difference in time between the intrusion of the dolerites. The measurements of anisotropy of magnetic susceptibility reveal a magnetic fabric with almost horizontal foliation planes and lineations that indicate fairly uniform ca NW or SE directed magma flows. The dolerites of the CSDG are geochemically rather uniform and have compositions typical of mantle derived melts formed in continental tensional settings. In a palaeomagnetic reconstruction of Baltica versus Laurentia at ca 1.27 Ga the two continents were joined, with NE Greenland attached to NW Baltica. AMS data from a few dolerites and a basalt in NE Greenland indicate magma flow directions that in the tectonic reconstruction are more or less parallel to the flow of the dolerites in Sweden. This may suggest a common magma source located at the reconstructed contact between Baltica and Laurentia. Both the dolerites in Greenland and those in Sweden are of tholeitic composition indicating an intraplate origin, which supports the interpretation of joined continents at that time. The tensional regime, that is reflected by the huge sill complexes, is in our interpretation related to the break up of Baltica from Laurentia at ca 1.27 Ga ago.

Thermal and mechanical aspects of magma emplacement in giant dike swarms

We consider the thermal history and dynamics of magma emplacement in giant feeder dikes associated with continental flood basalts. For driving pressure gradients inferred for giant dike swarms, thicknesses of &lt;10 m would enable dikes to transport magma laterally over the distances observed in the field (up to thousands of kilometers) without suffering thermal lock‐up. Using time‐dependent numerical solutions for the thermal evolution of a dike channel under laminar and turbulent flow conditions in the presence of phase transitions, we investigate the possibility that the observed dike thicknesses (of the order of 100 m) result from thermal erosion of the country rocks during dike emplacement. This implies that the observed range of dike widths in giant dike swarms may reflect variations in the source volume and not the excess magma pressure. It is found that the total volume of intruded magma required to produce an order of magnitude increase in dike width via wall rock melting broadly agrees with the estimated volumes of individual flows in continental flood basalts. The presence of chilled margins and apparently low crustal contamination characteristics of some giant dikes may be consistent with turbulent magma flow and extensive melt back during dike emplacement. In this case, measurements of the anisotropy of magnetic susceptibility most likely indicate magma flow directions during the final stages of dike intrusion. Shear stresses generated at the dike wall when the dike starts to freeze strongly decrease with increasing dike width, which implies that thicker dikes may have less tendency to produce consistent fabric alignment. Our results suggest that if the dike was propagating downslope off a plume‐related topographic swell, the mechanism responsible for flow termination could possibly have been related to underpressurization and collapse (implosion) of the shallow magma plumbing system feeding the intrusion. Radial dikes that erupted at the periphery of the topographic uplift might have increased (rather than decreased) extensional stresses in the crust within the topographic uplift upon their solidification.

Is this magnetic fabric normal? A review and case studies in volcanic formations

When studying weakly anisotropic rocks non-standard relationships between magnetic and expected petrofabric axes are sometimes observed, in particular the so-called inverse or intermediate fabrics. A review of possible explanations for these effects is presented and leads to the conclusion that a majority of abnormal magnetic fabrics, often encountered in magnetite-rich volcanic rocks, are due to abnormal preferred orientations or distributions of magnetite grains. Through case studies in dikes (from Skye Island, Yemen and the Iberian Peninsula), and magma and ash flows (from the Ethiopian traps and southern Corsica) the various possible origins of such fabrics are examined. It is possible to retrieve reliable magma flow directions from these sites provided proper data selection is performed, leading to non-unique interpretations.

Dike surface lineations as magma flow indicators within the sheeted dike complex of the Troodos Ophiolite, Cyprus

Mesoscopic flow lineations and anisotropy of magnetic susceptibility (AMS) have been measured for dikes within the Cretaceous‐age Troodos ophiolite with the goal of comparing the direction of initial magma flow through dike conduits immediately following crack propagation with that of flow of subsequent magma emplaced during later stages of dike growth. Dike margin indicators of flow include cusp axes and elongate vesicles found high in the ophiolite pseudostratigraphy and ridge‐and‐groove structures termed hot slickenlines found throughout the complex. A unique flow direction is determined where elongate vesicles near dike margins display imbrication with respect to the margin. Significant changes in vesicle elongation directions across dikes likely indicate either changes in magma flow direction after dike propagation or back‐flow of magma during the waning stages of intrusion. Surface lineations generally lie subparallel to the direction of flow inferred from AMS determinations on cores within 5 cm of dike margins. Surface lineations also lie subparallel to the long axis (ε1) of the orientation ellipsoid defined by long axes of groundmass plagioclase phenocrysts measured in sections from AMS cores. Correlation of surface lineations with interior indicators of flow (AMS, plagioclase trachytic texture) indicate that the surface features are good proxies for grain‐scale magma flow directions during dike propagation in Troodos dikes. Orientations of surface flow features in the dikes of the Troodos ophiolite indicate an approximately equal mix of subhorizontal to near‐vertical magma flow, contradicting the paradigm of primarily vertical flow of magma beneath continuous axial magma chambers at oceanic spreading centers. Our data are consistent with a model of magma emplacement both vertically and horizontally away from isolated magma chambers beneath axial volcanoes spaced along a ridge crest.

Magnetic anisotropy produced by magma flow: theoretical model and experimental data from Ferrar dolerite sills (Antarctica)

SUMMARY Volcanic rocks forming sills, dykes or lava flows may display a magnetic anisotropy derived from the viscous flow during their emplacement. We model a sill as a steadystate flow of a Bingham fluid, driven by a pressure gradient in a horizontal conduit. The magma velocity as a function of depth is calculated from the motion and constitutive equations. Vorticity and strain rate are determined for a reference system moving with the fluid. The angular velocity and the orientation of an ellipsoidal magnetic grain immersed in the fluid are calculated as functions of time or strain. Magnetic susceptibility is then calculated for a large number of grains with a uniform distribution of initial orientations. It is shown that the magnetic lineation oscillates in the vertical plane through the magma flow direction, and that the magnetic foliation plane changes periodically from horizontal to vertical. The results are compared with the magnetic fabric of Ferrar dolerite sills (Victoria Land, East Antarctica) derived from low-field susceptibility measurements.