Crustal Accretion Processes Research Articles

Melt to mush variations in crustal magma properties along the ridge crest at the southern East Pacific Rise

The determination of along-axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle, eruption history along the ridge crest, and the process of crustal accretion. Seismic reflection images1,2,3,4 have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths (250–4,500?m), but variations in its seismic properties and thickness have remained elusive, despite several attempts to constrain these properties5,6,7. Here we report that the melt sill along the southern East Pacific Rise, which is about 50?m thick, undergoes abrupt changes in its internal properties, ranging from pure melt to mush. The 60-km-long ridge-crest segment near 14°?00′?S is characterized by three 2–4-km sections containing pure melt embedded within a magma chamber rich in mush. These small pure melt pockets may represent a fresh supply of magma from the mantle, capable of erupting and forming the upper crust. Conversely, the 80–90% of the magma chamber which is mushy is unlikely to erupt and may influence the lower-crustal accretion.

Spatial and Temporal Variations in Mid-Ocean Ridge Basalt Geochemistry Along the 9°–10°N East Pacific Rise

Temporal constraints on mid-ocean ridge basalt (MORB) magmatism both on and off-axis are critically needed to better understand problems associated with MORB petrogenesis and the processes of crustal accretion at the MOR. Using measurements of U-series disequilibria, radiogenic isotopes and majorand trace-element compositions in a suite of MORBs, from 9~176 East Pacific Rise (EPR), we are addressing two questions related to the temporal component of MORB magmatism: 1) How does magma storage time (i.e. the time after which the melt is no longer in equilibrium with its source) affect the degree of crystal fractionation of MORBs and what is the time-scale of this storage? 2) Is MOR magmatism focused at the ridge or is there a significant component of off-axis volcanism?

Rift relocation — A geochemical and geochronological investigation of a palaeo-rift in northwest Iceland

A dominant process in the evolution of Iceland is the repeated eastward relocation of the spreading axis in response to westward migration of the plate boundary relative to the plume centre. Two major former rifts can be identified in western Iceland: the Snaefellsnes rift zone, which last erupted tholeiitic lavas at about 7 Ma, and an older spreading system, lava flows from which can be traced some 100 km along a SW-NE strike in the extreme northwest of Iceland. The extinction of the latter is marked by a 14.9 Ma unconformity with a laterite-lignite horizon representing a maximum 200 k.y. hiatus in the lava succession. Lavas below the unconformity dip northwest towards the older axis from which they were erupted, whereas lavas above the unconformity dip southeast towards their source in the younger Snaefellsnes axis. Thus, two nearly complete rift relocation cycles are preserved in western Iceland, each lasting about 8 m.y. as measured between rift extinction events, and for around 12 m.y. from initial propagation to extinction. In this paper we present major- and trace-element analyses, Sr, Nd and Pb isotope data, and 40Ar/ 39Ar dates on basalt samples from above and below the unconformity in northwest Iceland. The Icelandic Tertiary and Quaternary plateau basalts are remarkably homogeneous in composition, in contrast to the much more diverse compositions found in the presently active rift zone. However, basaltic lava flows beneath the unconformity in northwest Iceland show a wider range of incompatible element and radiogenic isotope ratios than do the younger plateau basalts. At least two mantle components, one depleted and the other less depleted with respect to bulk Earth, are required to explain the composition of post-15 Ma Icelandic basalt. The depleted end-member is chemically and isotopically distinct from the N-MORB source. Basalt from the northwest palaeo-rift, however, contains a significant North Atlantic N-MORB component, suggesting that depleted upper mantle can influence the composition of Icelandic basalt in a dying rift that is too far from the plume centre to be dominated by plume mantle. This may account for the periods of low magma productivity represented by troughs between the V-shaped ridges on the Reykjanes Ridge. We suggest that temporal variation in the composition of Icelandic basalt is better explained by crustal accretion and rift relocation processes than by variations in plume composition and temperature.

Objective classification of oceanic ridge-crest terrains using two-dimensional spectral models of bathymetry: Application to the Juan de Fuca Ridge

An important application of detailed bathymetric mapping is the interpretation of geological processes based on the nature of the fine-scale morphology of the seafloor. This interpretation is usually accomplished through qualitative analysis of contour maps. In this paper, an objective classification technique, based on a two-dimensional spectral model of bathymetry developed by Fox and Hayes (1985) is applied to detailed Sea Beam data from the Juan de Fuca Ridge. Parameters of the model can be directly related to seafloor properties corresponding to 1) isotropic (non-directionally dependent) roughness; 2) anisotropic (directionally dependent) roughness; 3) orientation of the anisotropic component; and 4) spectral rool-off ( ∼ fractal dimension), by fitting the model surface to two-dimensional amplitude spectra of bathymetry determined on a regular grid over the study area. A test area was selected which encompasses the southern volcanic rift zone of Axial Volcano and the northern terminus of the Vance Segment. Parameters of the model clearly define the contrast between the constructional volcanic terrain (rough, isotropic, with high fractal dimension) and the tectonic extensional terrain (smoother, anisotropic, with low fractal dimension). An agglomerative, hierarchical cluster analysis is applied to the data, independent of spatial information, to delineate groups of spectra with similar characteristics. Distinct, mappable regions, corresponding to volcanic and tectonic provinces, are objectively determined. Also, coherent sub-regions of consistent spectral properties occur within the larger volcanic/tectonic divisions. The classification is extended to the Juan de Fuca Ridge system from 44°30′ N to 47°20′ N through combining these results with an a priori technique (K-means clustering). Broad-scale physiographic regions of the Juan de Fuca Ridge are delineated by the technique, which may aid geologists in the interpretation of crustal accretion processes at seafloor spreading centers.

Ridge segmentation and the magnetic structure of the Southern Mid‐Atlantic Ridge 26°S and 31°–35°S: Implications for magmatic processes at slow spreading centers

Along‐axis profiles of three‐dimensional magnetic inversions for the Mid‐Atlantic Ridge (MAR) 31°–35°S show low magnetization near the middle of ridge segments and high magnetization at the segment tips for three adjacent spreading segments; thus there is an inverse relation between axial magnetization and axial topography. The ridge segment at 26°S on the MAR has the same inverse relationship between magnetization and topography. The common occurrence of this relationship suggests that it reflects a fundamental process of crustal accretion at the MAR. We analyze the rock magnetic properties from 42 locations within the four ridge segments in the South Atlantic to constrain the inherent trade‐off between source intensity and source thickness in the magnetization model. The natural remanent magnetization (NRM) intensities from the four ridge segments, averaged together, correlate with the magnetic inversion profiles. This finding implies that changes in the magnetization of the extrusives may account for much of the observed magnetic anomaly amplitude variation. A direct correlation of FeO content and magnetization suggests that magnetic anomaly amplitudes may be an indicator of FeTi‐rich basalts at the slow spreading MAR, even though the iron content of the basalts from high magnetization areas is not as high as observed at Pacific spreading centers. Despite the different magma plumbing systematics of the Pacific spreading centers and the MAR, it appears that the segment‐scale magma system of the MAR also results in segment‐scale crustal magnetization variations. Further evidence that the axial magnetic variations result from source intensity variations is that older isochrons have higher intensities near the ridge‐discontinuities, similar to the behavior on‐axis. Between 0 and 5 Ma the decay in magnetization is ∼50% independent of location within a spreading segment.

Uniform accretion of oceanic crust south of the Garrett transform at 14°15′S on the East Pacific Rise

Using migrated common depth point reflection profiles, we find the structural differences along the ultrafast spreading (>150 mm/yr) East Pacific Rise south of the Garrett fracture zone are second‐order, suggesting a remarkably uniform process of crustal accretion. The rise axis south of the Garrett transform is underlain by a narrow (<1.0 km) melt lens which shows great along‐strike continuity. The depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading East Pacific Rise at 9°30′N. This observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent. Melt sill width, however, shows little variation along the East Pacific Rise, suggesting no dependence of magma chamber size on spreading rate. The melt reservoir decreases in width toward/across the 14°27′S ridge axis discontinuity by a modest 250–300 m and appears to be continuous across this feature. Given the small aspect ratio (∼1.0 km by ∼50 m by tens of kilometers) of the axial melt lens, the previously recorded jump in MgO content across the 14°27′S offset is likely the result of a mixing boundary which is sustained through an along‐strike impedance in convection. Wide‐angle reflections originating at the base of seismic layer 2A, assumed to coincide with the extrusive layer, reveal a twofold to threefold increase (200–250 to 500–600 m) in thickness within 1–2 km of the rise axis. The pattern of extrusive thickening imaged south of the Garrett transform is similar to that observed along the slower spreading (110–120 mm/yr) East Pacific Rise at 9°N. Outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile. This implies a degree of temporal stability of the along‐strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone. The inferred uniformity of off‐axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14°27′S discontinuity are caused by long‐term reductions in magma supply. Second‐order differences in the style of extrusive thickening may be related to structural differences within the low‐velocity zone underlying the rise axis and/or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis. Images of Mono on cross‐axis profiles may be traced to within ∼1.0 km of the melt sill edge; this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber.

Himalayan-Tibetan analogies for the evolution of the Zimbabwe Craton and Limpopo Belt

The Limpopo Orogeny, which lasted from ∼ 2700 to ∼ 2600 Ma, was characterised by NNW-SSE directed shortening. This resulted in both crustal thickening, accommodated by folding and NNW-directed thrusting, and associated crustal extrusion whereby a number of crustal blocks, including the Central Zone of the Limpopo Belt, was displaced to the west-southwest along WSW-ENE trending shear zones. This was part of a regionally distributed phase of orogenic deformation which affected the adjoining Kaapvaal and Zimbabwe Cratons from the Witwatersrand Basin in the south to the Harare Greenstone Belt in the north. This deformation, which migrated northwards with time, postdated an earlier phase of crustal accretion during which the Archaean crustal unit now marked by the Kaapvaal and Zimbabwe Cratons and the Limpopo Belt, grew towards the north-northwest. Shortening of this unit during the Limpopo Orogeny may be regarded as the last stage of the crustal accretion process. This tectonic process of continental growth by accretion followed by crustal shortening with associated crustal extrusion has many features in common with Tibetan tectonics from the Mesozoic to the present.

Shallow intrusive directions of sheeted dikes in the Troodos ophiolite: Anisotropy of magnetic susceptibility and structural data

Sheeted dikes play a central role in the formation of oceanic crust. It is commonly assumed that sheeted dikes intrude vertically upward, from elongated mid-ocean ridge (MOR) magma chambers, but there are no direct observational data bearing on this hypothesis. This assumption contrasts with the intrusive behavior of subaerial volcanoes where magmas rise into shallow central magma chambers that laterally feed vertically oriented dikes. The authors have studied intrusive directions of sheeted dikes in a structural analogue to oceanic crust, the Troodos ophiolite. Structural and magnetic fabric data of 65 dikes provide consistent results and suggest a broad distribution of shallow (< 20[degree]) to nearly vertical, upward magma-transport directions. These data suggest that horizontal emplacement has to be considered for sheeted dikes at MORs, implying more centralized MOR plumbing systems than previously thought. Such plumbing systems provide ample opportunity for complex mixing, fractionation, and contamination of MOR lavas in magma chambers and tabular magma-storage volumes. Whether the MOR magma supply is linear or centralized also has a fundamental effect on crustal accretion processes and the geometry of hydrothermal convection systems.

Neoproterozoic crustal accretion in central Brazil

Recent geochronological studies have demonstrated the existence of several Neoproterozoic orthogneiss and metavolcanic are suites (ca. 900 to 600 Ma) in the southern part of the Tocantins province, a major Neoproterozoic orogenic area in central Brazil, exposed between the Amazon and Sao Francisco cratons. Nd and Sr isotopic characteristics of these metamorphic suites are primitive: initial 87 Sr/ 87 Sr ratios vary from ∼0.7024 to ∼0.7042 and initial ϵ Nd values are positive, varying between +0.2 and +6.9. The most primitive initial Sr and Nd isotopic compositions are very similar to model depleted- mantle compositions at the time of formation of the original magmas. The mantle- like characteristics of these are suites in western Goias contradict previous ensialic evolution models for the Tocantins province and reveal that crustal accretion processes were important in large areas of central Brazil during Neoproterozoic time.

The Canadian Cordillera as a modern analogue of Proterozoic crustal growth

Processes of crustal accretion and the real additions to continental volume occurring in the Canadian Cordillera 0.25 Ga to present are viewed as an analogue for major crustal addition episodes in Proterozoic 2.5–0.5 Ga time. The Canadian Cordillera represents a minimum of 20% (maximum 25%) of the global Phanerozoic crustal growth rate. This is similar to Proterozoic crustal genesis events such as the Grenville Belt and the Arabian‐Nubian Shield, but represents less crust production than the 1.9–1.7 Ga events of North America‐Greenland‐Europe. The Canadian Cordillera is a valid modern analogue for Proterozoic events chiefly on grounds other than crustal quantity, however. These grounds are (1) progressive accretion of terranes to the continent; (2) predominance of arc‐related rocks; (3) difficulty of identifying sutures between terranes; (4) high abundance of positive‐eNd, hence juvenile rocks; and possibly (5) presence of transcurrent motions leading to accumulation of terranes preferentially along one p...

Heterogeneity of the upper crust.

Recent research confirms that the uppermost several hundred meters of the young oceanic crust is a distinct unit with low compressional velocity (&amp;lt;2.5 km/s) and a shear velocity lower than that of the acoustic velocity of the overlying water column. The acoustic contrast at the bottom of this waveguide is larger than that at the seafloor. Seismic data from ocean spreading centers can also address the question of whether upper crustal heterogeneity is intrinsic to the crustal accretion process or is imprinted subsequently by hydrothermal and tectonic processes. Seismic lines collected along axis at the fast spreading, northern East Pacific Rise indicate that, within the narrow zone of emplacement, the crustal structure is remarkably homogeneous in terms of layer thickness and velocity. However, as the crust moves from the axis and is buried by subsequent lava flows, the integrated effect of differences in magmatic supply rates gives rise to a wide variation in the total thickness of the waveguide. The waveguide is approximately 150 m on axis, but can vary between 200 and 700 m off axis.

Inverse square-root dependence of mid-ocean-ridge flank roughness on spreading rate

THE topographic roughness of mid-ocean-ridge flanks is known to increase with decreasing spreading rate1–4, but the exact form of this relationship has not been established. As the topographic features that make up ridge flank roughness (abyssal hills) are created near the axes of mid-ocean ridges by faulting and volcan-ism5–7, the relationship between roughness and spreading rate may shed light on the process of crustal accretion at spreading centres. Here I present measurements of ridge flank roughness on profiles crossing the world mid-ocean-ridge system, which show that roughness is proportional to the inverse square-root of the spreading rate. This result is consistent with some very simple inferences of how the topographic roughness of mid-ocean-ridge flanks scales with the lithospheric thickness near ridge axes, as determined by thermal models.

Early Proterozoic crust-mantle interaction at a continental margin in northern Sweden

Abstract On the basis of recent geochemical and Sm-Nd isotopic data, alternative models for the early Proterozoic evolution of the Archaean craton of the Baltic Shield and its marginal areas in northern Sweden are discussed. Before the deposition of a craton cover and the subsequent Svecofennian orogenic activity, the Archaean areas probably formed a continuous domain. The 1.89-1.86 Ga old early Svecofennian granitoids found close to exposed Archaean rocks were created largely by the remobilization of Archaean crust. To the south of the Archaean Domain, however, chronologically equivalent granitoids are made up of matter with short crustal residence times. That area, the Skellefte district, represents a new addition to the continental crust of the Baltic Shield ∼1.90 Ga ago. The particulars of the crustal accretion process are still controversial. However, recent data indicate different tectonic developments in the Skellefte district and in the vicinity of the Archaean-Proterozoic boundary farther north. Apparently, the one-time edge of the Archaean craton was situated somewhere between these two areas, but appears to have had a rather more northerly position than previously assumed.

Multichannel seismic evidence for variations in crustal thickness across the Molokai Fracture Zone in the Mid‐Pacific

Coincident multichannel seismic reflection and refraction data from a N–S transect near Oahu, Hawaii, provide evidence for thickening of the Pacific crust by 1–2 ± 1 km south of the large‐offset (16 m.y.) Molokai Fracture Zone (FZ). Tau‐p stacks, tau‐sum inversions, and forward modeling of the refraction data indicate that the crustal thickening occurs primarily within the lower portion of seismic layer 2. Assuming isostatic balance, the differences in crustal thickness predict that seafloor having the same age will have different elevations across the FZ. Observations of sea‐floor depths across the FZ east of the Hawaiian Islands are consistent with this prediction implying that the processes which have generated the crustal differences have been stable for over 50 m.y. Previous correlations between the chemical composition of ridge crest basalts, crustal thickness, and ridge crest elevation have been attributed to variations in the thermal regime of the upper mantle under mid‐ocean spreading centers. In accord with this hypothesis, we propose that the observed differences in crustal structure across the Molokai FZ may have been produced by small (25°C) differences in the thermal regime of the upper mantle beneath the ancestral East Pacific Rise. Discontinuous intracrustal reflections located about 1.6 s below the sediment/basement interface are observed in migrated reflection data south of Oahu. These reflections are similar in character to the lower crustal “Horizon R” event observed in the western North Atlantic. Shallower intracrustal reflections, possibly from within seismic layer 2, are also observed. The observation of these intracrustal reflections in both the Atlantic and Pacific oceans suggests that they are a fundamental signature of the crustal accretion process at a variety of spreading rates and that they are mappable using modern seismic reflection/refraction methods.

Hydrothermal chimneys and associated fauna in the Manus Back‐Arc Basin, Papua New Guinea

Between December 6, 1985, and January 9, 1986, the Hawaii Institute of Geophysics Research Vessel Moana Wave surveyed the Manus Basin, north of New Britain, Papua New Guinea. This basin occupies a back‐arc position with respect to the New Britain arc‐trench system and contains an active plate boundary (Figure 1; also Taylor [1979]). Organized under the aegis of the Committee for Coordination of Joint Prospecting for Mineral Resources in South Pacific Offshore Areas (CCOP/SOPAC), the cruise was part of a program to assess the mineral potential of southwest Pacific marginal basins and included a National Science Foundation‐sponsored study of crustal accretion processes in a fastspreading (greater than 100 mm/yr) back‐arc basin. The tectonic fabric of the Manus Basin was mapped and defined with the Sea MARC II sidescan sonar and bathymetric mapping system. Then dredge samples and cores were collected, and a towed bottom camera system was used to search for signs of hydrothermal activity.

Implications of magmatic epidote-bearing plutons on crystal evolution in the accreted terranes of northwestern North America

Magmatic epidote in a plutonic rock signifies that the rock probably crystallized at pressures of at least 8 kbar (25–30 km depth). In western North America, undeformed Late Cretaceous plutons containing magmatic epidote occur within a mobile belt extending from northern California to southeastern Alaska; this belt has been interpreted as consisting of accreted terranes. Because the present crustal thickness within this belt is about 30 km, the plutons imply a crustal thickness at the time of emplacement of 55–60 km, comparable to that in the central Andes. The plutons further imply paleogeothermal gradients in the host country rock at the time of intrusion of not more than 20 °C/km and may be closer to 12 °C/km. Such cool, thick crust suggests that subduction and piling of thrust sheets were probably the principal processes of crustal accretion in western North America. The implications regarding the depth of crystallization of the plutons can be used to estimate the minimum rate of regional uplift; it is on the order of 1 mm/yr. The rapid uplift of the mobile belt by and large did not affect the craton.

Magnetostratigraphy and geochronology of northwest Iceland

The NW peninsula of Iceland is built mainly by tholeiitic, low K flood basalts that show a regional southeasterly dip, generally of less than 10°. From the lavas mapped and drilled for paleomagnetic measurements (a total of 1261 flows), two continuous composite sections were constructed, one on the western side of the peninsula 4055 m thick comprising 456 lavas and the other adjacent to the east coast, 3165 m thick comprising 396 lavas. K‐Ar age measurements were made on more than 70 lavas distributed throughout the two sections. These two lava sequences are about 90 km apart along strike, and although direct stratigraphic correlation is not possible between them, the evidence indicates that the eastern section is stratigraphically younger than that in the west, with some overlap. The results show that the lava sequences were erupted over an interval extending from about 14 to 8 Ma ago in the middle to late Miocene. Remarkably linear relations are found between the measured age and cumulative stratigraphic thickness for both sections, indicating rather uniform local rates of growth of the lava piles. For the western composite section a growth rate of 1820 m/Ma is found, contrasting with a much lower growth rate of 670 m/Ma for the eastern sequence. The average thickness of individual lavas in both sections is close to 8 m. The average time between eruptions of lavas in the western section is 5000 years, compared with a value of 12,000 years for the eastern section. The regional geology together with our measurements are interpreted in terms of crustal accretion processes by which the NW peninsula of Iceland was constructed by eruption of lavas from a spreading center of northeasterly strike. It is not clear whether the differences in rates of growth of the lava pile in the western and eastern sections are because of variation in eruptive activity with time or because eruptive activity has varied along the strike of the spreading center. Paleomagnetic measurements on the lavas provide a detailed history of polarity of the geomagnetic field during the interval 14–8 Ma ago. For the eastern composite section there is excellent and unambiguous correlation of the observed polarity pattern with the marine magnetic anomaly record. From these data we obtain an age of 9.6 Ma for the younger boundary of marine magnetic anomaly 5Table 1 is available with, entire article on microfiche. Order from American Geophysical Union, 2000 Florida Ave., N.W., Washington, D.C. 20009.

Structure and variability of oceanic crust on the flanks of the East Pacific Rise between 11° and 13°N

We use over 500 seismograms collected by five ocean bottom seismometers to examine the structure and variability of 0.5‐m.y.‐old crust in the Rivera Ocean Seismic Experiment (ROSE) area of the East Pacific Rise between 11° and 13°N. There are no significant differences among the first arrival travel times for 10 independent refraction line segments along the isochron. Amplitude patterns recorded on the individual lines, however, suggest that structural boundaries within the upper crust vary by hundreds of meters in depth over lateral distances of tens of kilometers within this 200‐km segment of lithosphere. On the basis of the modeling of amplitude patterns at ranges of 5–10 km and of comparisons with the upper crust sampled at Deep Sea Drilling Project hole 504B and in ophiolite complexes, layer 2 can be divided into three units: 2A corresponds to extrusive volcanics with a steep velocity gradient, 2B to extrusives with a near‐zero velocity gradient, and 2C to a complex region of transition from extrusives to sheeted dikes. The total thickness of layer 2 varies by about 0.7 km over the length of the line. Another amplitude peak which varies in range between 14 and 21 km may be related to the downward transition from sheeted dikes to isotropic gabbros within layer 3. The variability of these two amplitude patterns provides direct evidence that the along‐strike structure of oceanic crust in the ROSE area is definably heterogeneous and that the process of crustal accretion is likewise variable on the scale resolved by amplitude and travel time information.

Geologic and seismic velocity structure of the crust/mantle transition in the Bay of Islands Ophiolite Complex

Geological investigations of the Bay of Islands Ophiolite Complex show that while a typical layered ophiolite suite is present, the thickness of major lithologic units is extremely variable from place to place. The composition and internal structure of map‐scale (kilometers across) lithologic units as well as the contacts that bound them are laterally variable. Inferred velocity‐depth functions of the crust/mantle (“Moho”) transition reconstructed for this terrane as oceanic lithosphere suggest an extremely complex internal seismic structure. The crust/mantle transition from mafic to ultramafic lithologies varies across the ophiolite from a sharp geologic and seismic velocity discontinuity to a complexly interlayered transition zone as much as 3 km thick. The mafic/ultramafic transition is characterized by laterally discontinuous lithologic units on the order of hundreds of meters to a few kilometers thick and up to several kilometers long. These units (megalenses) may be composed of lithologies with higher or lower seismic velocities than those of the surrounding units. Solid‐state deformation has produced seismically anisotropic materials in the upper mantle and lower crustal units. Substantial relief and slopes of up to at least 12° over lateral distances of about 10 km occur on the top of the crust/mantle transition zone as well as other geologic unit contacts. Although these types of geological features are striking to the field geologist working in ophiolites and are significant to the understanding of crustal accretion processes, many of them occur on such a small scale that they might go undetected in different types of seismic experiments in contemporary oceanic lithosphere.

Investigation of a Vine‐Matthews Magnetic Lineation from a submersible: The source and character of marine magnetic anomalies

We report here the first attempt to map directly the boundary of a Vine‐Matthews magnetic stripe on the seafloor. Our objectives are to study the processes of oceanic crustal accretion as recorded in the reversal transition zone and to investigate the formation of the magnetic source of Vine‐Matthews magnetic anomalies. Our deep‐tow and ALVIN‐based magnetic studies focus on the Matuyama/Brunhes reversal transition on the flanks of the East Pacific Rise near 21°N. While the sea level magnetic anomalies are less than average in clarity for the East Pacific Rise, a ‘three‐dimensional’ inversion of deep‐tow data reveals a sharp, strike‐linear polarity transition less than 1.8 km wide (Macdonald et al., 1980a). These measurements have been augmented by mounting a vertical magnetic gradiometer on ALVIN and making 280 reliable polarity determinations along and across the polarity transition zone. Even on long traverses across both sides of the boundary, we find that nearly every magnetic target has the correct polarity, i.e., the same polarity as the regional magnetic lineation. This homogeneity in polarity of the magnetic lineations is surprising. The magnetic polarity transition in the outcropping volcanic section is sharp and linear along strike, delineated in some cases by a clear geologic contact of opposing flow fronts of different ages. Several weakly magnetized outcrops mapped within the transition zone may have erupted during the time in which the geomagnetic field was reversing. The reversal boundary mapped on the seafloor from ALVIN is displaced 250 m to 500 m NW away from the spreading axis relative to the position of the average boundary as derived from inversion of the deep‐tow and sea level magnetic data. This offset provides a means for estimating the spillover of lava flows away from the spreading axis during the time the crust was formed. The combination of deep‐tow and ALVIN measurements suggests that circa 0.7 m.y. ago the crustal accretion zone (magnetized volcanic, intrusive, and plutonic rocks) was 2000–2800 m wide, while the zone of recent volcanism alone was only 1000–2000 m wide. The determination for the most recent reversal agrees well with submersible observations at the present spreading center where the zone of recent volcanism (neovolcanic zone) varies between 600 and 2000 m in width. This very orderly picture for the formation of magnetic lineations and crustal accretion processes appears to conflict with complex Deep Sea Drilling Project (DSDP) magnetic results from the Atlantic. We suggest that the crustal generating processes and resulting magnetic structure vary significantly with spreading rate. On the slow‐spreading Mid‐Atlantic Ridge, major episodes of volcanism are likely to be infrequent (∼104 years), the magma chamber may be non‐steady state, and the neovolcanic zone shifts or varies in width considerably. This sporadic, start and stop spreading process will contribute to a highly heterogeneous and complex crustal and magnetic structure as seen in DSDP holes. In addition, significant faulting and tilting may disrupt slow‐spreading crust. For intermediate‐ to fast‐spreading centers, more frequent volcanism (∼50–600 years), a nearly steady state magma chamber, and a narrow, stable neovolcanic zone will create a less complex magnetic and crustal structure both as seen from ALVIN and as inferred from clear sea level magnetic anomalies in the Pacific.