Rise Flanks Research Articles

Observed Mixing at the Flanks of Maud Rise in the Weddell Sea

AbstractMaud Rise is a seamount in the eastern Weddell Sea and the location of polynyas and a persistent halo of reduced sea ice. We present novel in situ data from two profiling floats with up to daily resolved hydrographic profiles in this region. The water properties below the mixed layer of the Maud Rise region are significantly correlated with bathymetric depth; thus, the Maud Rise flank defines the front between the Warm Deep Water of the abyssal ocean and the colder Taylor cap over Maud Rise. We analyze the spiciness curvature in density space to quantify the observed frequency or magnitude of intrusions, which are substantially increased along the flanks of Maud Rise. These intrusions are indicative of enhanced lateral and vertical mixing along heavily sloping isopycnals, creating favorable conditions for thermobaric and double diffusive convection and likely facilitating the formation of the Maud Rise halo and polynyas.

Petrology and Sr-Nd-Pb-He isotope geochemistry of postspreading lavas on fossil spreading axes off Baja California Sur, Mexico

Postspreading volcanism has built large seamounts and volcanic ridges along the short axes of a highly segmented part of the East Pacific Rise crest that ceased spreading at the end of the middle Miocene, offshore Baja California Sur, Mexico. Lava samples from Rosa Seamount, the largest volcano, are predominantly alkalic basalts, mugearites, and benmoreites. This lavas series was generated through fractional crystallization and is compositionally similar to the moderately alkalic lava series in many oceanic islands. Samples from volcanic ridges at three adjacent failed spreading axes include mildly alkalic, transitional, and tholeiitic basalts and differentiated trachyandesites and andesite. The subtle but distinct petrologic and isotopic differences among the four sites may be due to differences in the degree of partial melting of a common, heterogeneous source. Postspreading lavas from these four abandoned axes off Baja California Sur together with those from other fossil spreading axes and from seamount volcanoes that grew on the East Pacific Rise flanks define a compositional continuum ranging from normal mid-ocean ridge basalt (NMORB)-like to ocean island basalt (OIB)-like. We propose that the compositional spectrum of these intraplate volcanic lavas is due to different degrees of partial melting of the compositionally heterogeneous suboceanic mantle in the eastern Pacific. A large degree of partial melting of this heterogeneous mantle during vigorous mantle upwelling at an active spreading center produces NMORB melts, whereas a lesser degree of partial melting during weak mantle upwelling following cessation of spreading produces OIB-like melts. The latter melts have a low (<8 RA) 3He/4He signature indicating their formation is different from that of OIBs from major “hot spot” volcanoes in the Pacific with high 3He/4He ratios, such as Hawaii and Galapagos.

Hydrothermal mineral deposits and fossil biota from a young (0.1 Ma) abyssal hill on the flank of the fast spreading East Pacific Rise: Evidence for pulsed hydrothermal flow and tectonic tapping of axial heat and fluids

Heat flow data indicate that most hydrothermal heat loss from ocean lithosphere occurs on the flanks of the mid‐ocean ridge, but few ridge flank hydrothermal sites are known. We describe the first nonseamount, abyssal hill hydrothermal mineral deposits to be recovered from the fast spreading East Pacific Rise (EPR) flanks. Deposits were sampled at two sites on an abyssal hill ∼5 km east of the EPR axis, just north of Clipperton Fracture Zone at 10°20′N, on ∼0.1 Ma lithosphere. “Tevnia Site” is on the axis‐facing fault scarp of the hill, and “Ochre Site” is located ∼950 m farther east near the base of the outward‐facing slope. Clusters of fragile, biodegradable Tevnia worm tubes at both sites indicate that hydrothermal fluids carried sufficient H2S to sustain Tevnia worms, and that fluid flow waned too recently to allow time for tube destruction. Presence of microbial mats and other biota also are consistent with recent waning of flow. The deposits are mineralogically zoned, from nontronite‐celadonite to hydrous Fe‐oxide+opaline silica to Mn‐oxide (birnessite and todorokite). This places them into a distinctive class of Fe‐Si‐Mn hydrothermal deposits found along tectonic cracks and faults in young oceanic crust, and suggests that (1) deposits precipitated along an O2 gradient between ambient seawater and hydrothermal fluid; (2) fluid temperatures were &lt;150°C; and (3) undiluted fluids were Mg‐depleted, and Fe‐, K‐, Si‐ and Mn‐enriched. These fluids may derive from high temperature seawater‐basalt interaction ± phase separation proximal to the axial melt zone, and lose Cu and Zn before venting due to conductive cooling and/or pH increase. Ochre Site samples are purely hydrothermal; however, Tevnia Site samples incorporate volcanic, sedimentary, and fossil components, and exhibit at least three generations of fracturing and hydrothermal cementation. The Tevnia Site breccias accumulated on the exposed fault scarp, possibly during multiple slip events and hydrothermal pulses as the abyssal hill was uplifted. We hypothesize that frequent earthquakes rejuvenate young abyssal hill hydrothermal systems episodically over 104–105 years, tapping axial heat and hydrothermal fluids, sustaining biota, and likely helping to chill the margins of the axial melt zone.

Manifestations of hydrothermal discharge from young abyssal hills on the fast-spreading East Pacific Rise flank

Spectacular black smokers along the mid-ocean-ridge crest represent a small fraction of total hydrothermal heat loss from ocean lithosphere. Previous models of measured heat flow suggest that 40%–50% of oceanic hydrothermal heat and fluid flux is from young seafloor (0.1–5 Ma) on mid-ocean-ridge flanks. Despite evidence that ridge-flank hydrothermal flux affects crustal properties, ocean chemistry, and the deep-sea biosphere, few ridge-flank vent sites have been discovered. We describe the first known seafloor expressions of hydrothermal discharge from tectonically formed abyssal hills flanking a fast-spreading ridge. Seafloor manifestations of fluid venting from two young East Pacific Rise abyssal hills (0.1 Ma at 10°20′N, 103°33.2′W; 0.5 Ma at 9°27′N, 104°32.3′W) include fault-scarp hydrothermal mineralization and macrofauna; fault-scarp flocculations containing hyperthermophilic microbes; and hilltop sediment mounds and craters possibly created by fluid expulsion. These visible features can be exploited for hydrothermal exploration of the vast abyssal hill terrain flanking the mid-ocean ridge and for access to the subseafloor biosphere. Petrologic evidence suggests that abyssal hills undergo repeated episodes of transitory fluid discharge, possibly linked to seismic events, and that fluid exit temperatures can be briefly high enough to transport copper (≥250 °C).

Evidence for diffuse extension of the Pacific Plate from Pukapuka ridges and cross‐grain gravity lineations

Satellite altimeter measurements of marine gravity reveal 100 to 200‐km wavelength lineations over a wide area of the Pacific plate oriented roughly in the direction of absolute plate motion. At least three mechanisms have been proposed for their origin: small‐scale convective rolls aligned in the direction of absolute plate motion by shear in the asthenosphere; diffuse N‐S extension of the lithosphere resulting in lineated zones of extension (boudins); and minihotspots that move slowly with respect to major hotspots and produce intermittent volcanism. Recently, several chains of linear volcanic ridges have been found to be associated with the gravity lineations. Following ridgelike gravity signatures apparent in high‐resolution Geosat gravity measurements, we surveyed a series of volcanic ridges that extend northwest from the East Pacific Rise flank for 2600 km onto 40 Ma seafloor. Our survey data, as well as radiometric dates on samples we collected from the ridges, provide tight constraints on their origin: (1) Individual ridge segments and sets of ridges are highly elongate in the direction of present absolute plate motion. (2) The ridges formed along a band 50 to 70‐km‐wide in the trough of one of the more prominent gravity lineations. (3) Radiometric dates of the largest ridges show no hotspot age progression. Moreover, the directions predicted for minihotspot traces older than 24 Ma do not match observed directions of either the gravity lineations or the ridges. Based on this last observation, we reject the minihotspot model. The occurrence of the ridges in the trough of the gravity lineation is incompatible with the small‐scale convection model which would predict increased volcanism above the convective upwelling. We favor the diffuse extension model because it is consistent with the occurrence of ridges in the trough above the more highly extended lithosphere. However, the multibeam data show no evidence for widespread normal faulting of the crust as predicted by the model. Perhaps the fault scarps are buried under more than 30 m of sediments and/or covered by the elongated ridges. Finally, we note that if ridge‐push force is much smaller than trench‐pull force, then near the ridge axis the direction of maximum tensile stress must be perpendicular to the direction of absolute plate motion.

Segmentation and disruption of the East Pacific Rise in the mouth of the Gulf of California

Analysis of new multibeam bathymetry and all available magnetic data shows that the 340 km-long crest of the East Pacific Rise between Rivera and Tamayo transforms contains segments of both the Pacific-Rivera and the Pacific-North America plate boundaries. Another Pacific-North America spreading segment (“Alarcon Rise”) extends 60 km further north to the Mexican continental margin. The Pacific-North America-Rivera triple junction is now of the RRR type, located on the risecrest 60 km south of Tamayo transform. Slow North America-Rivera rifting has ruptured the young lithosphere accreted to the east flank of the rise, and extends across the adjacent turbidite plain to the vicinity of the North America-Rivera Euler pole, which is located on the plate boundary. The present absolute motion of the Rivera microplate is an anticlockwise spin at 4° m.y.−1 around a pole located near its southeast corner; its motion has recently changed as the driving forces applied to its margins have changed, especially with the evolution of the southern margin from a broad shear zone between Rivera and Mathematician microplates to a long Pacific-Rivera transform. Pleistocene rotations in spreading direction, by as much as 15° on the Pacific-Rivera boundary, have segmented the East Pacific Rise into a staircase of en echelon spreading axes, which overlap at lengthening and migrating nontransform offsets. The spreading segments vary greatly in risecrest geomorphology, including the full range of structural types found on other rises with intermediate spreading rates: axial rift valleys, split shield volcanoes, and axial ridges. Most offsets between the segments have migrated southward, but within the past 1 m.y. the largest of them (with 14–27 km of lateral displacement) have shown “dueling” behavior, with short-lived reversals in migration direction. Migration involves propagation of a spreading axis into abyssal hill terrain, which is deformed and uplifted while it occupies the broad shear zones between overlapping spreading axes. Tectonic rotation of the deformed crust occurs by bookshelf faulting, which generates teleseismically recorded strike-slip earthquakes. When reversals of migration direction occur, plateaus of rotated crust are shed onto the rise flanks.

Geomorphology and structural segmentation of the crest of the southern (Pacific‐Antarctic) East Pacific Rise

Geomorphology of the boundary between Pacific and Antarctic plates was mapped with a Sea Beam multibeam echosounder and a SeaMARC II bathymetric side scan sonar, from the southern end of Juan Fernandez microplate at 35°S to Heezen transform at 56°S. There are six spreading center systems separated by two large, left‐stepping nontransform offsets (at 36.5°S and 41.5°S), two right‐stepping transform‐fault systems, and a left‐stepping hybrid structure with equally long strike‐slip and nontransform offsets. Axial rift zones (spreading axes) are generally within 1° of normal to the relative motion predicted by current plate rotation models. Most axial rift zones crop out along the crests of typical East Pacific Rise (EPR) axial ridges which commonly have flat rather than humped long profiles. About 2% of this fast‐spreading (84–100 mm/yr) rise crest has an axial rift valley instead of an axial ridge. The longest rift‐valley segment is midway between Menard and Vacquier transforms near 51°S, where an anomalously deep rise crest may mark a zone of below‐average mantle upwelling. Nontransform offsets structurally similar to those on the tropical EPR, but mostly formed by differential asymmetric spreading, subdivide the four longest spreading center systems. On the northern, transform‐free two thirds of the rise, net rift propagation at migrating offsets has been into the faster Pacific plate. This causes right steps to migrate north, left steps to migrate south, and crust to be transferred to the east flank. This pattern has prevailed for several million years, judging from the oblique fracture zones (pseudofaults) mapped on the rise flanks with older marine data and Geosat altimetry. On this rise crest, the difference in stress on two plates with very different velocities may control the direction of offset migration.

Distribution of magma beneath the East Pacific Rise near the 9°03′N overlapping spreading center from forward modeling of common depth point data

We have reprocessed six cross‐axis and three along‐axis common depth point (CDP) profiles near the 9°03′N overlapping spreading center (OSC) to understand the relationship between axial magma chamber (AMC) width and seafloor morphology. Travel time modeling of the AMC reflector reveals an asymmetric distribution of melt across the 9°03′N OSC. The variation of modeled AMC width beneath either OSC limb is minimal, but the width increases nearly fourfold across the offset attaining an estimated maximum width of 4.15 km near the 9°17′N ridge axis discontinuity. Additionally, melt distribution underlying the eastern rise limb is not symmetric with respect to the rise axis/neovolcanic zone but is displaced toward the western rise flank. Depth migration, based on a continuum velocity model consistent with postcritical reflections from the base of layer 2A, places the skewed AMC reflector beneath a nearly constant thickness sheeted dike section which dips ∼ 10° away from the rise axis. To confirm AMC continuity beneath the western rise flank, we use the Maslov synthetic seismogram method to show that amplitude enhancement of the AMC reflector is consistent with a continuous melt body underlying a thickening extrusive layer. Analysis of along‐strike CDP profiles indicates an AMC which is neither overlapping nor discontinuous when projected onto the along‐strike plane. Identifying intracrustal events on along‐axis CDP lines, however, requires extreme caution; we have modeled out‐of‐plane scattering using a Kirchhoff formulation, and we show that a coherent event identified beneath the overlap basin results from diffraction off the AMC which lies nearly 3 km to the west of the profile. We attribute the asymmetric pattern of melt to a decoupling of melt supply from preexisting weaknesses in the brittle upper crust. In this model, melt ascends upward (buoyancy forces) until deflected by the impermeable sheeted dike complex; melt then migrates updip, beneath the base of the sheeted dikes, toward the neovplcanic zone where fissuring produces a temporary conduit for emplacement. Discrete jumps in modeled AMC width toward the overlap basin represent a further displacement/defocusing of melt supply (western AMC edge) relative to the neovolcanic zone (eastern AMC edge). The asymmetric pattern of melt therefore represents a gradual, en‐echelon accommodation of melt supply across the 9 km of ridge axis offset at 9°03′N. Thus for asymmetric configurations, AMC width may not correlate solely with magmatic robustness but may signify the amount of decoupling which exists between melt supply and extrusive emplacement within the neovolcanic zone. Here we present a new model for OSC development which invokes a significant component of cross‐axis melt migration. Moreover, abrupt changes in AMC width near ridge axis discontinuities (e.g., 9°17′N deviation in axial linearity) suggest that any along‐axis melt migration is confined to subsegments of the ridge and seem to preclude the segment length migration of melt proposed in some current models. The transition of melt supply beneath the overlap basin might favor a continuous low‐velocity zone underlying this feature; if true, basin development may be related to the subsidence of a mechanically weak crustal lid. The proposed model for OSC development therefore views ridge axis discontinuities as the surficial response of misalignment and/or defocusing of melt supply in the uppermost mantle.

The renavigation of Sea Beam bathymetric data between 9� N and 10� N on the East Pacific Rise

We present a gridded Sea Beam bathymetric map of a 5100 km2 area between 9° and 10° N on the East Pacific Rise (included as a color separate accompanying this issue). The raw bathymetric data are renavigated using a technique for calculating smooth adjustments to navigation that incorporates absolute constraints from satellite fixes and acoustically-located explosive shots, and relative constraints from the misfit of bathymetric data at ship track crossovers. We describe a back-projection technique for gridding the bathymetric data that incorporates an approximation for the power distribution within a narrow-beam echo sounding system and accounts for the variable uncertainties associated with multi-beam data. The nodal separation of the resulting map is ~ 80 m in both latitude and longitude, and the sampling of grid points within a 60 × 85 km2 region is in excess of 99%. A formal analysis of variance is applied to the gridded bathymetric data. For each grid point, the difference between the variance of data from within a track versus data from between tracks provides an upper bound on the magnitude of bathymetric misfits arising from navigational errors. The renavigation results in an 88% reduction in this quantity. We also examine the effects of renavigation on the misfit of magnetic and gravity data at crossovers and compare our results with other bathymetric surveys. A striking feature of the final bathymetric map is the sinuous regional shape of the rise axis. In plan view, the local trend of morphology sometimes varies by up to 15° and the distances separating changes in morphological trend are about 10–20 km. In cross section the slopes of the rise flanks are notably asymmetric and show some correlation with the offset of the axial magmatic system as detected by seismic methods.

The East Pacific Rise and its flanks 8?18� N: History of segmentation, propagation and spreading direction based on SeaMARC II and Sea Beam studies

SeaMARC II and Sea Beam bathymetric data are combined to create a chart of the East Pacific Rise (EPR) from 8°N to 18°N reaching at least 1 Ma onto the rise flanks in most places. Based on these data as well as SeaMARC II side scan sonar mosaics we offer the following observations and conclusions. The EPR is segmented by ridge axis discontinuities such that the average segment lengths in the area are 360 km for first-order segments, 140 km for second-order segments, 52 km for third-order segments, and 13 km for fourth-order segments. All three first-order discontinuities are transform faults. Where the rise axis is a bathymetric high, second-order discontinuities are overlapping spreading centers (OSCs), usually with a distinctive 3:1 overlap to offset ratio. The off-axis discordant zones created by the OSCs are V-shaped in plan view indicating along axis migration at rates of 40–100 mm yr−1. The discordant zones consist of discrete abandoned ridge tips and overlap basins within a broad wake of anomalously deep bathymetry and high crustal magnetization. The discordant zones indicate that OSCs have commenced at different times and have migrated in different directions. This rules out any linkage between OSCs and a hot spot reference frame. The spacing of abandoned ridges indicates a recurrence interval for ridge abandonment of 20,000–200,000 yrs for OSCs with an average interval of approximately 100,000 yrs. Where the rise axis is a bathymetric low, the only second-order discontinuity mapped is a right-stepping jog in the axial rift valley. The discordant zone consists of a V-shaped wake of elongated deeps and interlocking ridges, similar to the wakes of second-order discontinuities on slow-spreading ridges. At the second-order segment level, long segments tend to lengthen at the expense of neighboring shorter segments. This can be understood if segments can be approximated by cracks, because the propagation force at a crack tip is directly proportional to crack length.

Temporal evolution of overlapping spreading centers at 16°20′N on the East Pacific Rise

A compilation of Sea Beam data on the right-stepping overlapping spreading center (OSC) at 16°20′N on the East Pacific Rise (EPR) and in the surrounding region is presented. Sea Beam data from the rise flanks at 16°20′N reveal three deep basins, at least one of which is flanked by a curved ridge whose morphology is similar to that at the offset. The widths of these offaxis basins range from 8 to 13 km and allow an estimate to be made of the sizes of the offsets at which they were formed. It is proposed that they represent traces on the rise flank which provide a record of the development of the offset and a basis for prediction of its future evolution. The repeated pattern of abandoned overlap basins and cuspate ridges supports the notion that OSCs are episodic on a time scale of approximately 200,000 years and are cyclic in nature. It also reveals an alternation of dominance between the southern and northern segments, which have shifted the location of the discontinuity along the axis so as to create zig-zag traces of basins on the ridge flanks.

The rise flank trails left by migrating offsets of the equatorial East Pacific Rise axis

Sea Beam and magnetic surveys of the young (&lt;1 Ma) rise flank around nontransform rise crest offsets at 5.5°N, 4.9°N, 3.4°N, 2.0°N, and 2.8°S mapped distinctive trails of obliquely lineated, highly magnetic crust that spreads from the characteristically curving overlapping rift zones. A Deep Tow survey around the small (1.5 km) 3.9°N offset found only normal rise flank terrain. Near‐bottom Deep Tow observations at the medium size (9 km) 5.5°N offset and the large (27 km) 2.0°N offset provided more detailed information on the formation of crust in the offset trails, and of the structure of tectonically modified crust at interrift rise crest sites. Patches of interrift crust, which forms deep overlap basins at medium size offsets and basins and plateaus of rotated abyssal hills at large offsets, are periodically shed from the axial zone, together with abandoned portions of overlapped rift zone, to become important elements of the rise flank trails. The oblique azimuths of the paired “fracture zone” trails that diverge from the rise crest offsets show that most have migrated along the rise crest at average speeds of 25–125 mm/yr. Net movement of the 27‐km 2.8°S offset has been slower, with alternating periods of rapid northward and southward migration. Information on the migration history of the surveyed offsets was supplemented by inferring the migration direction of another eight offsets on the equatorial East Pacific Rise (EPR) whose trails were less thoroughly mapped. Most offsets of the southern Pacific‐Cocos boundary have migrated south, and most on the northern Pacific‐Nazca boundary have migrated north; the patterns are not readily explained by the inferred distribution of mantle upwellings along the EPR axis.

The scavenging of U, 230Th and 231Pa during pulsed hydrothermal activity at 20°S, East Pacific Rise

Uranium, Th and Pa isotopes were measured in eight box-cores from two transects to the west of the East Pacific Rise at 10°S and 20°S. Sediment geochemistry (Al, Ca, Fe, Mn) and water column paniculates (Mn) suggest that highly metalliferous plumes, rich in Fe and Mn, are emanating from the rise crest at 19°S. Measurements of 230Th xs and 231Pa xs indicate decreasing sediment accumulation rates from the rise flank (0.4 cm ky −1) to the Tiki Basin (0.23 cm ky −1). Over this geographical area Al is found to have a constant flux of 3.6 × 10 −4 g cm −2 ky −1. Application of an Al constant flux model to ridge crest metalliferous sediment reveals two 1,000 year pulses of hydrothermal activity during the Holoccne (since 11,000 y BP). These hydrothermal pulses are characterised by hydrothermal Fe/Mn ratios (3.5:1), enhanced 230Th and 231Pa scavenging, and low 230Th xs/ 231Pa xs ratios (3–4). The similarity of the 230Th xs and 231Pa xs ratio to the dissolved seawaterrauo suggests that in fracuonation of the isotopes occurs during scavenging. Supplementary supply of dissolved 230Th and 231Pa in a westwards-moving water mass is suggested.

Nontransform offsets of the Pacific-Cocos plate boundary and their traces on the rise flank

A multi-beam bathymetric reconnaissance of the fast-spreading crest of the East Pacific Rise between 7°N and 2.6°N found the axial ridge to be interrupted by four 4–13 km non-transform offsets. They have probably persisted since initiation by a small change in spreading direction 3.5 Ma. At each offset, the volcanic rift zones veer 15° toward each other and overlap for 5–25 km around deep basins. Intervening lateral displacements of the rift zones are too short (<2.5 km) to break the linear axial volcanoes but cause local bends in their plan shapes; some examples have a morphology that mimics the larger intervolcano offsets. Interpretation of a thorough bathymetric and magnetic survey around the 9 km, 5.5°N offset indicates that the mainly one-sided spreading from the overlapped sections of rift zone occurs by a combination of highly asymmetric dike accretion and episodic small-scale jumps of the eruptive zone. The northern rift zone is elongating (propagating) at the expense of the southern one. The resulting along-axis migration of this and other examples, at speeds comparable to spreading rates, causes the rise-flank scars of the nontransform offsets (equivalent to the fracture zones of transform faults) to be oblique to the spreading direction. These traces are recognizable within the highly lineated fault-block terrain of the rise flanks because of the distinctive morphology of the 5–25-km-wide bands that accreted at overlapped sections of the rift zones. Here, lineation is less marked and oblique to the spreading direction, and volcanic relief is more important than relief caused by faulting. The west-flank trace of the 5.5°N offset is occupied by a line of volcanoes and shallow volcanic ridges. Several other volcanic chains, previously identified as hot-spot traces, seem to have grown in similar tectonic settings.

Nontransform offsets of the Pacific-Cocos plate boundary and their traces on the rise flank : Lonsdale, Peter, 1985. Geol. Soc. Am. Bull., 96(3):313–327

Nontransform offsets of the Pacific-Cocos plate boundary and their traces on the rise flank : Lonsdale, Peter, 1985. Geol. Soc. Am. Bull., 96(3):313–327

A geological transect across the crest of the East Pacific Rise at 21�N latitude made from the deep submersible ALVIN

This paper is a report of geological observations made using the submersible ALVIN on the crest of the East Pacific Rise near 21°N. The profile is 6 km long and crosses a 5–10 km wide plateau which rises 100 m±above the rise flanks. At the axis are exposed fresh glassy pillow lavas with no sediment accumulation in a region termed the neovolcanic zone. This zone is about one kilometer wide and includes elongate ridges of pillow lavas and seventeen hydrothermal vent fields in the study area. Outside the neovolcanic zone the seafloor is extensively fissured in another zone which is up to two kilometers wide. The neovolcanic zone and the fissured zone are included within a rift valley or graben about 3 to 5 km wide and 50 m±deep. This rift valley is asymmetrically located on the west side of the axial plateau; the neovolcanic zone in the study area is asymmetrically located on the east side of the rift graben. Fissured crust is not common outside the rift graben or in the neovolcanic zone; similarly, large throw faults such as those which form the edges of the graben are not found outside of it. These observations can be interpreted according to a volcanic-tectonic cycle in which volcanic eruptions and hydrothermal circulation are followed by a tectonic phase which includes fissuring and vertical movements. When a new cycle starts it may involve a lateral shift of the spreading axis. Lavas along the dive profile are suggested to be no older than a few thousand years based on sediment accumulation. In contrast, seafloor spreading rates here predict crust up to 105 yr old. This observation suggests that lavas from the neovolcanic zone can spread laterally about a kilometer or more and overlap on older crust.

Late Tertiary/Quaternary magnetostratigraphy and biostratigraphy of Vema Channel sediments

Twenty-one piston cores in a transect of the Vema Channel near 31°S were analyzed magnetostratigraphically and biostratigraphically in order to ascertain the depositional continuity in the late Tertiary and Quaternary. The cores span a depth interval of 2941–4235 m and represent three distinct physiographic provinces, each of which shows a relatively consistent accumulation pattern. The deepest cores are from the broad terraces which flank the Vema Channel axis on both the west (∼3650 mm) and east (4150–4250 m); these cores show the highest accumulation rates in the region (>20 m m.y. −2) and no evidence of unconformities. Twelve cores from the flanks of the Rio Grande Rise (2941–3950 m) show depositional continuity in the late Quaternary and normal pelagic accumulation rates (5–10 m m.y. −2), with evidence in two of the cores for minor erosional hiatuses on a local scale, perhaps due to slumping. A relatively narrow transition zone between the Rio Grande Rise flanks and the eastern terrace (3950–4150 m) shows buried manganese pavements and near-outcrops of Miocene/Pliocene strata, indicating intermittent erosion during the late Tertiary and Quaternary, perhaps due to episodes of intensified AABW flow. Modern flow within this transition zone and over the eastern terrace is relatively weak ( \\ ̄ gu ≈ 2 cm s −2 ) and southward. The presence of well-developed sediment waves on the terrace and buried erosional unconformities upslope to the east suggests substantially higher current velocities during the late Tertiary than those present today.

Overlapping rift zones at the 5.5°S offset of the East Pacific Rise

A Seabeam and magnetometer survey of the Pacific‐Nazca plate boundary around 5.5°S mapped a “nontransform offset” whose geology and kinematics seem typical of a whole class of structures formed where fast‐spreading rises are laterally offset for distances less than the width of a subaxial magma chamber. Though the spreading center is abruptly displaced by 15 km right laterally, there is no trace of strike‐slip faulting. Instead the ends of two 100 km‐long axial volcanic ridges veer 15° toward each other and overlap for more than 20 km. In this overlapping region, between 5°22′S and 5°32′S, accretion of the upper oceanic crust by dike injection and eruption is partitioned between two adjacent and parallel rift zones with oblique and one‐sided spreading. A continuous but dog‐legged magma chamber is inferred to underlie both rift zones and an intervening 9‐km wide volcano‐studded basin. A strong magnetic signature from the crust, which accreted around this oblique chamber identifies the trace of the offset on the adjacent rise flank; in the past 0.5×106 years the southern axial ridge has propagated north at 40 mm/yr, 55% of the spreading half‐rate.

Laccoliths(?) and small volcanoes on the flank of the East Pacific Rise

Clusters of low circular domes and discs, each a few hundred metres high and a few kilometres across, have been mapped but not sampled during a Seabeam traverse of old (9–18 m.y.) oceanic crust at (at 19–20°S in the South Pacific, where they replace the typical lineated fault-block terrain for several hundred square kilometres. The landforms are probably produced by laccolith intrusion beneath a sediment cover in an off-axis volcanic province. Their overall shapes are distinctly different from the small volcanic shields that also dot the rise flank, and from small satellite cones that are abundant on archipelagic aprons, though some of the domes do bear small cratered shields and cones on their peripheries, and even shallow summit depressions.

Ocean drilling surveys planned

As a continuation of the International Phase of Ocean Drilling (IPOD), the Glomar Challenger is slated to drill in the Pacific and North Atlantic oceans during 1982–83. In preparation for the drilling, the Joint Oceanographic Institutions (JOI), Inc. will manage the site survey program during 1981–82. These site surveys will be focused to support four programs: a hydrogeology study on the equatorial East Pacific Rise flank; a study of Mesozoic sediments in the western Pacific; a study in sedimentation of the equatorial Pacific basin; and a study of the geochemistry of the North Atlantic ocean crust.JOI has issued a request for proposals for the United States site survey program. Proposal deadline is March 5. For additional information, contact JOI, Inc., 2600 Virginia Avenue, N.W., Suite 512, Washington, D.C. 20037.