Melt Supply Research Articles

Jettability of reactive nylon materials for additive manufacturing applications

An inkjet printing process has been suggested for producing nylon 6 in an additive manufacturing approach and this paper reports on the jettability of molten reactive materials made by caprolactam with activator and catalyst to study the feasibility of the approach in an inkjet system. The materials physical properties which are important for jetting were characterized and then the melt supply behaviour in the system was studied and finally jetting trials were monitored to investigate the effect of parameter settings on the stability of the jet array. It was found that the surface tension and viscosity of all materials were within the suitable range. However, with the catalyst mixtures, microcrystals of the undissolved salt of the catalyst complex were found to influence the melt supply behaviour considerably. The mixtures had a narrower range of parameters where stability occurred compared with caprolactam. Monitoring the jet stability recommended a suitable range of jetting parameters for the deposition stage of researching the new approach. A higher level of instability was observed with the catalyst mixture especially when jetting with multiple nozzles due to the microcrystals.

Crustal thickness and earthquake distribution south of the Logachev Seamount, Knipovich Ridge

During RV Polarstern cruise ARK‐XXIV/3 (2009), a geophysical study along the ultraslow spreading Knipovich Ridge was conducted. The survey, located in the rift valley south of the Logachev Seamount (∼76°36′N), provides a crustal thickness of ∼4.5 km in the amagmatic parts and 5.7 km underneath the seamount itself. The velocity‐depth function indicates the presence of a thick oceanic layer 2 (4.5 km), but no indication for a thick oceanic layer 3. The only exception is the area underneath the Logachev Seamount, where velocities higher than 6 km/s are detected. This indicates a stronger and focussed melt supply underneath the seamount. Local seismicity was analysed for two days. In total, 191 quakes were identified in the vicinity of the rift valley with magnitudes up to ML = 2.6. At least 48 of them are located in the upper mantle (up to 18 km below sea level), supporting models predicting a cold mantle or conductive cooling underneath ultraslow ridges to explain the reduced melt supply.

Refinement of equipment for amorphous-ribbon production

Systems for melt supply to the cooling disk in the production of amorphous and microcrystalline ribbon are considered. For large-scale industrial production, the optimal technology includes lateral supply and inductive melting of the initial blank (regardless of the melt volume). The design of the casting nozzle must be free of danger spots; the material employed will depend on the composition of the melt being cast. For an ampoule system, remote visualization of the nozzle-disk gap is recommended, as well as remote control of the crucible motion. The use of a composite cooling disk reduces wobble at its working surface and simplifies the design. The influence of vibration on ribbon quality must be taken into account in selecting the drive and bearings. The device for removing the ribbon from the disk surface must provide auxiliary ribbon cooling.

Direct evidence for dynamic magma supply fossilized in the lower oceanic crust of the Troodos ophiolite

[1] Temporal and spatial variabilities of mantle upwelling and melt supply in mid-ocean ridges (MORs) have long been documented. Such variabilities span a range of scales and have a profound effect on the structure as well as the composition of the oceanic crust. Previous seismic and gravity studies have suggested that the lower oceanic crust plays a major role in accommodating these changes in melt supply. Here we report the first direct evidence for a sharp transition from coherent sub-horizontal to near vertical magma flows frozen in the lower oceanic crust of the Troodos ophiolite at the segment edge near a fossil ridge-transform intersection. We constrain the preferred petrofabric lineation directions at 13 gabbroic sites using anisotropy of magnetic susceptibility (AMS) verified by electron backscatter diffraction. Pre-emplacement accretion-related rotations were corrected using magnetic remanence directions. We identify two provinces of nearly uniform susceptibility directions (principal axes) and attribute them to two magmatic episodes. A more focused mantle upwelling and melting episode near the segment midpoint may have resulted in lower crustal lateral magma flows along the fossil segment-edge, whereas uniform mantle upwelling and melt supply along the entire axis may have resulted in vertical magma flows at the segment-edge. Overall, our data verify the vital role of the lower oceanic crust in accommodating changes in mantle upwelling and melt supply beneath MORs.

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

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

Magmatic-metering controls the stopping and restarting of eruptions

[1] We use time-series of magma efflux and GPS-derived surface deformation observations to constrain the transfer of compressible magma within the crustal plumbing of the Soufriere Hills volcano for three cycles of effusion and pause. Our system model has two vertically-stacked spherical chambers. Deep melt supply to the system is constrained to be continuous and steady, yielding a rate of 1.2 m3/s, which fixes the geometry of the dual interconnected chambers to depths of about 5 and 19 km. The eruptive volume change of the shallow chamber is in-phase and an order of magnitude smaller than the deep chamber. Significantly, the shallow chamber seems to control the periodic system behavior: surface magma efflux resumes when the shallow chamber reinflates to its initial threshold pre-eruptive volume (triggering re-opening of an eruptive feeder dike), and ceases when it has lost 14–22 Mm3 (106 m3) of its volume (sealing the conduit and staunching magma flow). These observations are consistent with eruption re-initiation and re-cessation controlled by magma overpressure thresholds.

Multiple melt injection along a spreading segment at Askja, Iceland

[1] Lower crustal earthquakes (12–25 km depth) have been detected since August 2005 in the Askja volcanic system along the north Iceland rift, in the normally ductile part of the crust. The earthquakes occur in three clusters, which have stable dimensions and locations through time and are interpreted as positions of repeated melt supply from the mantle to the lower crust. Seismic velocity Vp/Vs ratios are consistent with the presence of partial melt in the lower crust at Askja. The spatial separation of the clusters shows that there are multiple positions of melt injection within this one magmatic segment and all three positions are currently active. This pattern of melt supply is more like that observed on fast spreading ridges than slow spreading ridges and is probably a consequence of the increased melt production beneath Iceland compared to the rest of the Mid-Atlantic Ridge. However, the relative number of earthquakes in each cluster shows that two thirds of the melt is supplied to the central volcano Askja (i.e., segment center). During the last major rifting episode shallow lateral melt migration occurred from the magma chamber beneath the volcano. Therefore on long time scales melt supply is probably greater at the segment center, with melt redistribution in the upper crust, even though there are multiple points of lower crustal injection along the segment.

Ice-sheet acceleration driven by melt supply variability

Increased ice velocities in Greenland are contributing significantly to eustatic sea level rise. Faster ice flow has been associated with ice-ocean interactions in water-terminating outlet glaciers and with increased surface meltwater supply to the ice-sheet bed inland. Observed correlations between surface melt and ice acceleration have raised the possibility of a positive feedback in which surface melting and accelerated dynamic thinning reinforce one another, suggesting that overall warming could lead to accelerated mass loss. Here I show that it is not simply mean surface melt but an increase in water input variability that drives faster ice flow. Glacier sliding responds to melt indirectly through changes in basal water pressure, with observations showing that water under glaciers drains through channels at low pressure or through interconnected cavities at high pressure. Using a model that captures the dynamic switching between channel and cavity drainage modes, I show that channelization and glacier deceleration rather than acceleration occur above a critical rate of water flow. Higher rates of steady water supply can therefore suppress rather than enhance dynamic thinning, indicating that the melt/dynamic thinning feedback is not universally operational. Short-term increases in water input are, however, accommodated by the drainage system through temporary spikes in water pressure. It is these spikes that lead to ice acceleration, which is therefore driven by strong diurnal melt cycles and an increase in rain and surface lake drainage events rather than an increase in mean melt supply.

Greenland's glacial basics

Sliding of the Greenland ice sheet is affected by the production of surface meltwater. A new theory shows that whether the result is a long-term speed-up or slow-down of ice motion depends on the variability in melt input. See Letter p.803 Recent observations in Greenland have led to the notion that surface meltwater supply to the ice-sheet bed lubricates ice flow, suggesting that climate warming could lead to runaway glacial acceleration. Christian Schoof uses a physically based model that captures drainage channelization under the ice to challenge this view. The model shows that increased melt supply leads to channelization and a drop in water pressure. This causes ice flow to slow down rather than speed up. However, this effect can be overcome if melt supply is variable over short timescales, when temporary pressure spikes can lead to accelerated flow. The positive melt/dynamic thinning feedback is therefore still viable, but is heavily dependent on daily temperature variations and rain events that are largely ignored in current ice sheet models.

Segment‐scale and intrasegment lithospheric thickness and melt variations near the Andrew Bain megatransform fault and Marion hot spot: Southwest Indian Ridge, 25.5°E–35°E

We analyze bathymetric, gravimetric, and magnetic data collected on cruise KN145L16 between 25.5°E and 35°E on the ultraslow spreading Southwest Indian Ridge, where the 750 km long Andrew Bain transform domain separates two accretionary segments to the northeast from a single segment to the southwest. Similar along‐axis asymmetries in seafloor texture, rift valley curvature, magnetic anomaly amplitude, magnetization intensity, and mantle Bouguer anomaly (MBA) amplitude within all three segments suggest that a single mechanism may produce variable intrasegment lithospheric thickness and melt delivery. However, closer analysis reveals that a single mechanism is unlikely. In the northeast, MBA lows, shallow axial depths, and large abyssal hills indicate that the Marion hot spot enhances the melt supply to the segments. We argue that along‐axis asthenospheric flow from the hot spot, dammed by major transform faults, produces the inferred asymmetries in lithospheric thickness and melt delivery. In the southwest, strong rift valley curvature and nonvolcanic seafloor near the Andrew Bain transform fault indicate very thick subaxial lithosphere at the end of the single segment. We suggest that cold lithosphere adjacent to the eastern end of the ridge axis cools and thickens the subaxial lithosphere, suppresses melt production, and focuses melt to the west. This limits the amount of melt emplaced at shallow levels near the transform fault. Our analysis suggests that the Andrew Bain divides a high melt supply region to the northeast from an intermediate to low melt supply region to the southwest. Thus, this transform fault represents not only a major topographic feature but also a major melt supply boundary on the Southwest Indian Ridge.

Influence of impurities on the thermoelectric properties of layered anisotropic PbBi4Te7 compound: Experiment and calculations

The kinetic coefficients of high-quality single crystals of ternary layered n-PbBi4Te7 compounds have been measured in the temperature range of 77–400 K. These crystals, doped with electroactive Cd and Ag impurities, were grown by Czochralski pulling with melt supply through a floating crucible. A significant anisotropy of the thermoelectric properties is found. The means of incorporation of electroactive impurities into the ternary compound lattice is established. The experimental values of the Nernst—Ettingshausen coefficient have been analyzed together with the Seebeck, Hall, and conductivity data. The features of transport phenomena in PbBi4Te7 can be explained within the single-band model of nonparabolic energy spectrum and mixed mechanism of electron scattering from acoustic phonons and the Coulomb potential of impurities. It is suggested that acoustic phonon scattering is dominant along the cleavage plane, whereas the impurity scattering dominates along the trigonal axis.

Australian Antarctic Discordance as a simple mantle boundary

Several complex models require unique mantle conditions to explain the Australian Antarctic Discordance (AAD), an unusually deep and rugged section of the Southeast Indian Ridge (SEIR) between ∼120°–128°E. Seismic evidence suggests the AAD is instead the manifestation of two contrasting mantle domains converging along its eastern edge. Variations in axial morphology and flanking topographic relief along the SEIR arise as ridge segments to the west (Indian mantle) grade into a cooler melting regime while those to the east (Pacific mantle) are more magmatically robust. Seismic refraction data show crustal thickness decreases from the west into the AAD at a rate of 0.1 km/100 km, then rapidly increases from 4.8 ± 0.4 km to 7.3 ± 0.2 km across the eastern border. The AAD thus appears to be the terminal end of a long‐wavelength reduction in melt supply at what may be the simplest global example of a mantle boundary.

Spinel compositions and tectonic relevance of the Bibong ultramafic bodies in the Hongseong collision belt, South Korea

The Hongseong area in the western Gyeonggi Massif, South Korea, is characterized by the occurrence of several isolated ultramafic lenses in close association with metabasites within the granitic gneiss. Most of these bodies suffered various degrees of serpentinization and also have been highly deformed and metamorphosed. The Bibong ultramafic rock is assumed to be a mantle section of a probable Neoproterozoic supra-subduction zone (SSZ) and its evolution involved two igneous stages overprinted by later stage metamorphism. The compositions of the chromian spinel cores and olivines representative of the igneous stages were used to deduce the petrogenesis and tectonic environments for the formation of the Bibong ultramafic rocks. Spinel Cr# (100 ⁎ Cr/(Cr + Al)) for the igneous stages in lherzolite, harzburgite and chromitite are 24.5–33.3, 37.4–61.2 and 37.3–50.9, respectively. During the first stage, magma formed in the nascent back-arc basin spreading center by a small degree of partial melting of wedge mantle intruded the preexisting lithospheric mantle causing the melt/rock interaction that resulted in lherzolite with Al-rich chromian spinel. During the second stage more evolved magma formed in the mature back-arc basin by high degrees of partial melting due to the increased supply of fluid or melt from the subducting oceanic crust and intruded the lherzolitic mantle forming harzburgite with chromian spinel (with high Cr#) through the coupled process of olivine precipitation and dissolution of pyroxene. The Bibong ultramafic rocks have later undergone three metamorphic episodes. The first stage is evidenced by development of medium to fine grained olivine, orthopyroxene, clinopyroxene and amphibole around orthopyroxene porphyroblasts. The second stage is serpentinization which is followed by the third stage amphibolite facies metamorphism as evidenced by Fe-rich rims around Al-rich chromian spinel due to the diffusional exchange of Al, Mg, Cr, and Fe between chromian spinel and adjacent silicate minerals. The spinel composition shows that the transition from the nascent back-arc basin to mature back-arc basin tectonic setting may have been caused by trench roll back during the Neoproterozoic in the Hongseong area, and the serpentinization and amphibolite facies metamorphism could have occurred during uplift after Triassic collision between the North and South China blocks.

Crustal velocity structure of the Lucky Strike segment of the Mid‐Atlantic Ridge at 37°N from seismic refraction measurements

We estimate the seismic structure of the slow spreading Lucky Strike segment of the Mid‐Atlantic Ridge, located approximately 300 km south of the Azores platform, using seismic reflection and seismic refraction data acquired in June 2005 as a part of the Seismic Study for Monitoring of the Mid‐Atlantic Ridge (SISMOMAR) survey. The three‐dimensional velocity model shows an upper crustal low‐velocity anomaly running parallel to the ridge axis, which is limited by the median valley bounding faults. The velocity models also show a low‐velocity anomaly underlying the axial melt lens reflector located at the segment center below the Lucky Strike volcano. This lower crustal low‐velocity region can be explained by elevated temperatures and possibly small amounts of melt. The lower crustal low‐velocity anomaly and the axial melt lens reflector constrain the geometry of the magma chamber responsible for the construction of the Lucky Strike volcano. The presence of this magma chamber and thick crust at the segment center are consistent with a focused melt supply to the segment center.

Crustal structure of a rifted oceanic core complex and its conjugate side at the MAR at 5°S: implications for melt extraction during detachment faulting and core complex formation

We present results of a seismic refraction experiment which determines the crustal and upper-mantle structure of an oceanic core complex (OCC) and its conjugate side located south of the 5°S ridge–transform intersection at the Mid-Atlantic Ridge. The core complex with a corrugated surface has been split by a change in location of active seafloor spreading, resulting in two massifs on either side of the current spreading axis. We applied a joint tomographic inversion of wide-angle reflected and refracted phases for five intersecting seismic profiles. The obtained velocity models are used to constrain the magmatic evolution of the core complex from the analysis of seismic layer 3 and crustal thickness. An abrupt increase of crustal velocities at shallow depth coincides with the onset of the seafloor corrugations at the exposed footwall. The observed velocity structure is consistent with the presence of gabbros directly beneath the corrugated fault surface. The thickness of the high-velocity body is constrained by PmP reflections to vary along and across axis between <3 and 5 km. The thickest crust is associated with the central phase of detachment faulting at the higher-elevated northern portion of the massif. Beneath the breakaway of the OCC the crust is 2.5 km thick and reveals significantly lower velocities. This implies that the fault initially exhumed low-velocity material overlying the gabbro plutons. In contrast, crust formed at the conjugate side during OCC formation is characterized by an up to 2-km-thick seismic layer 2 overlying a 1.7-km-thick seismic layer 3. Obtained upper-mantle velocities range from 7.3 to 7.9 km s−1 and seem to increase with distance from the median valley. However, velocities of 7.3–7.5 km s−1 beneath the older portions of the OCC may derive from deep fluid circulation and related hydrothermal alteration, which may likely be facilitated by the subsequent rifting. Our velocity models reveal a strongly asymmetric velocity structure across the ridge axis, associated with the accretion of gabbros into the footwall of the detachment fault and upper-crustal portions concentrated at the conjugate side. Our results do not support a substantial increase in the axial ridge's melt supply related to the final phase of detachment faulting. Hence, the footwall rifting at 5°S may be a generic mechanism of detachment termination under very low melt conditions, as predicted by recent numerical models of Tucholke et al.

Volcanism, jump and propagation on the Sheba ridge, eastern Gulf of Aden: segmentation evolution and implications for oceanic accretion processes

The rifting between Arabia and Somalia, which started around 35 Ma, was followed by oceanic accretion from at least 17.6 Ma leading to the formation of the present-day Gulf of Aden. Bathymetric, gravity and magnetic data from the Encens-Sheba cruise are used to constrain the structure and segmentation of the oceanic basin separating the conjugate continental margins in the eastern part of the Gulf of Aden between 51 degrees E and 55.5 degrees E. Data analysis reveals that the oceanic domain along this ridge section is divided into two distinct areas. The Eastern area is characterized by a shorter wavelength variation of the axial segmentation and an extremely thin oceanic crust. In the western segment, a thicker oceanic crust suggests a high melt supply. This supply is probably due to an off-axis melting anomaly located below the southern flank of the Sheba ridge, 75 km east of the major Alula-Fartak transform fault. This suggests that the axial morphology is produced by a combination of factors, including spreading rate, melt supply and the edge effect of the Alula-Fartak transform fault, as well as the proximity of the continental margin. The oceanic domains have undergone two distinct phases of accretion since the onset of seafloor spreading, with a shift around 11 Ma. At that time, the ridge jumped southwards, in response to the melting anomaly. Propagating ridges were triggered by the melting activity, and propagated both eastward and westward. The influence of the melting anomaly on the ridges decreased, stopping their propagation since less than 9 Ma. From that time up to the present, the N025 degrees E-trending Socotra transform fault developed in association with the formation of the N115 degrees E-trending segment #2. In recent times, a counter-clockwise rotation of the stress field associated with kinematic changes could explain the structural morphology of the Alula-Fartak and Socotra-Hadbeen fracture zones.

Origin of the smooth zone in early Cretaceous North Atlantic magnetic anomalies

[1] Late Jurassic-Early Cretaceous marine magnetic anomalies observed in the North Atlantic exhibit an abrupt change in character in M5-M15 crust. The anomalies are smoother with low amplitudes, and are difficult to correlate among nearby profiles. The accepted explanation for the origin of this smooth zone is diminished resolution and anomaly interference due to slow spreading rates, which narrows the widths of polarity reversals in the crust and causes interference among sea-surface anomalies. Magnetic modeling of these anomalies indicates that neither slow spreading rates alone nor slow spreading rates in combination with a decrease in geomagnetic field intensity can explain the basic character of the smooth zone. Combined with other geophysical evidence, our study suggests that one consequence of slow spreading rates that is responsible for the magnetic “smooth zone” is a thinned crustal basalt layer or a non-basaltic magnetic source layer resulting from low melt supply during a period of ultra-slow spreading.

Oceanic corrugated surfaces and the strength of the axial lithosphere at slow spreading ridges

We analyse the topography and gravity signature of 39 corrugated surfaces formed over the past 26 myrs in the footwall of axial detachment faults at the eastern Southwest Indian Ridge. These corrugated surfaces appear to have formed at a melt supply about half the global melt supply average for mid-ocean ridges, and we find that their presently elevated topography, relative to adjacent non-corrugated seafloor, was mostly acquired at the end of their formation, at the “termination stage”. This configuration, which also characterizes many off-axis corrugated surfaces in other oceans, suggests that the plate flexural rigidity was very low during the formation of the corrugated surface, and increased significantly at the termination stage. Following Buck (1988), we hypothesize that stresses related to bending of the plate cause internal deformation and damage in the footwall of the fault, which is associated with weakening. As a possible mechanism for enhanced footwall weakening while corrugated surfaces form, we propose the formation of weak shear zones coated with hydrous minerals such as talc, amphibole, chlorite and serpentine, in mantle-derived ultramafics next to gabbro intrusions. If this hypothesis is correct, the amount of footwall weakening and roll-over along axial detachment faults at slow spreading ridges may be controlled both by access to hydrothermal fluids in the footwall of the detachment, and by the abundance and distribution of gabbros intrusions in exhumed ultramafics.

Heterogeneous seismic velocity structure of the upper lithosphere at Kane oceanic core complex, Mid‐Atlantic Ridge

The Kane oceanic core complex (OCC) is a large, corrugated megamullion that was formed by a long‐lived detachment fault at the axis of the Mid‐Atlantic Ridge adjacent to Kane Fracture Zone between 2.1 and 3.3 Ma. We use refracted arrivals recorded along a 6‐km‐long hydrophone streamer during a multichannel seismic survey to constrain the shallow seismic velocity structure of the OCC. Results are presented in high‐resolution traveltime seismic tomographic models along six lines that cover all of the main morphological features of the megamullion. The models show large lateral variability in P wave velocity within the upper ∼0.5–2.0 km of the lithosphere, and these variations correlate to first order with observed variations in lithology, documented by in situ basement samples and seafloor morphology. Lithological interpretation of the velocity models indicates that there is marked lateral variability in distribution of gabbroic intrusions, serpentinized peridotites, and basalts at scales of a few kilometers to ∼10 km. Serpentinized peridotites appear to dominate the central and older parts of the OCC. High‐velocity gabbros are consistently (but not exclusively) present closer to the termination of the Kane detachment fault and toward the ends of the OCC. The structure of the lithosphere exhumed by the Kane detachment fault is far from the standard ophiolite‐based Penrose model, and it does not show segment‐centered magmatism that is commonly interpreted at slow spreading ridges. If the gabbros exhumed toward the termination of the OCC were emplaced deep (∼10 km) beneath the spreading axis, they may have constituted a weak zone that focused initiation of the Kane detachment fault. Alternately, as the OCC footwall was being exhumed the gabbros may have been emplaced because of dynamic changes in melt supply, changes in mantle fertility, or decompression melting. Late stage volcanism is clearly associated with a major high‐angle normal fault that cuts the detachment surface; this volcanism may have been stimulated or enhanced by bending stresses in the bending footwall. The shape of the large‐scale corrugated morphology of the OCC is nearly invariant in the dip direction across major changes in basement lithology, indicating that once established, the form of the Kane detachment fault was highly resistant to modification.

Along‐ and across‐axis variations in crustal thickness and structure at the Mid‐Atlantic Ridge at 5°S obtained from wide‐angle seismic tomography: Implications for ridge segmentation

Two end‐member styles of crustal accretion are observed at two adjacent spreading segments at the Mid‐Atlantic Ridge at 5°S: focused accretion to the segment center with rapid crustal thinning toward the transform in the northern segment and crustal thickening toward the transform at an oceanic core complex in the southern segment. Our results were obtained by tomographic inversion of wide‐angle seismic reflection and refraction data collected along three intersecting profiles. The segment north of the 5°S fracture zone is characterized by a well‐developed median valley with a pronounced seafloor bulge in the segment center. A discrete portion of anomalously low velocities (−0.4 to −0.5 km/s relative to average off‐axis structure) at depths of ∼2.5 km beneath this bulge is possibly related to the presence of elevated temperatures and perhaps small portions of partial melt. This suggests that this segment is currently in a magmatically active period, which is confirmed by the observation of fresh lava flows and ongoing high‐temperature hydrothermal activity at the seafloor. Close to the current spreading axis, the crust thins rapidly from 8.5 km beneath the segment center to less than 3 km beneath the transform fault which indicates that melt supply here is strongly focused to the segment center. The reduction in crustal thickness is almost exclusively accommodated by the thinning of velocity portions indicative of seismic layer 3. The transform fault is characterized by more uniform velocity gradients throughout the entire crustal section and very low upper mantle velocities of 7.2–7.3 km/s indicating that serpentinization could be as much as 25% at 3.5 km depth. In contrast, ∼4.1 Ma old crust of the northern segment shows only minor thinning from the segment center toward the segment end. Here, the transform is characterized by a normal seismic layer 2/3 transition suggesting robust melt supply to the segment end at those times. In the adjacent southern segment, the crust thickens from ∼2.5 km beneath the flank of an oceanic core complex to ∼5.0 km at the segment boundary. The observed changes in crustal thickness show a significant temporal and lateral variability in melt supply and suggest a more complex crustal emplacement process than predicted by models of focused melt supply to the segment centers.