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Sea-level changes have shaped the world's carbonate platform margins and continental shelves, leaving typical geomorphic imprints, such as drowned reef terraces. In this paper, we present the results of 112 scuba diving transects across seven different Maldivian atolls and one multibeam survey around Male Island, the capital of Maldives. We report on the occurrence of drowned reef terraces down to 120 m depth. In total, we identified six levels of submerged terraces that we consider as indicative of periods of time with stable or slowly rising sea level that can be attributed either to deceleration of the last deglacial sea-level rise or to Late Quaternary sea-level highstands. We compare our dataset to the depth of reef terraces reported globally, and we discuss the reasons why common global submerged terrace levels are difficult to identify in the field record.

Bathymetric and seismic surveys of the Florida Middle Ground area on the West Florida Shelf, aid in identifying the structure and origin of these unusual banks. Recent cores recovered from the area by other workers provide a useful context for interpreting geophysical data. Data indicate that high-relief banks may be largely biohermal, whereas low-relief banks are likely composed of clastic sediments with a vermetid gastropod carbonate caprock. The most seaward ridges reveal subtle internal aggradational stratal geometries indicating a possible origin as a subtidal shoal, or paleoshoreline, deposited during or immediately following the Younger Dryas. Bank locations appear to be related to the antecedent geology, specifically the occurrence of the highly weathered Miocene surface. High-resolution multibeam bathymetry and backscatter data reveal varied morphologies that suggest the influence of karst processes, linear trends suggestive of wave-reworked intertidal hardbottom, poorly lithified and benthic fauna-mantled beach ridges or sand waves, or perhaps surface-water flow patterns during lowstand exposure. Circular depressions on low banks are of the same scale and morphology as modern shallow karst features in the Everglades. An episodically active sand-wave field occurs in the northern study area, and provides sediment to the system. Scoured depressions surrounding carbonate banks also indicate intense, episodic current activity originating from shelf and wave currents. A partial modern analog for some of the morphology of this system may be the Ten Thousand Islands and portions of the Florida Everglades. Data indicate that the banks represent a brief period of reef growth (possibly mixed coral and vermetid) constrained by two rapid sea-level rise events, and impacted by changing water quality and trophic conditions.

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Red grouper Epinephelus morio act as ecosystem engineers by excavating depressions (or holes) in areas of flat sandy bottom, which provide suitable habitat for themselves and numerous other species. To understand the spatial extent of the holes, which serve as spawning habitat, and determine how that habitat changes, high-resolution multibeam sonar data were collected in overlap- ping areas in 2006 and 2009 within Steamboat Lumps Marine Reserve. This marine reserve was estab- lished in 2000 and is located in the eastern Gulf of Mexico. Vertical profiles of the holes visually iden- tified from the multibeam data were extracted to characterize hole shape and determine changes in the height, width, and slope of each hole over time and space. Results from this analysis indicated an increase in hole density from 110 to 141 holes km -2 from 2006 to 2009, respectively, with 181 holes detected in 2006 and 231 holes detected in 2009. Height and slope also increased between 2006 and 2009. The shape changes present in the 151 holes identified in the same location between the 2 survey years suggest that while hole shape varies due to red grouper maintenance, holes are constructed and maintained over time. The communication network determined from calculating a 70 m limit to red grouper acoustic communication showed an increase in communication overlap from 2006 to 2009, with over 95% of holes located within 70 m of their nearest neighbor. The increase in number and den- sity of holes from 2006 to 2009 demonstrates that multiyear habitat mapping using active acoustic sonar is an effective method to monitor the presence and extent of red grouper spawning populations.

The hypothesis that caves in the Florida Peninsula are tied to Quaternary sea levels was proposed by hydrogeologists, without data, some 40 years ago. The hypothesis is a version of glacial control of cave levels, which is the logical combination of the water-table theory of speleogenesis and the concept that base level positions the water table. At the USA type example of glacial control of cave levels—Mammoth Cave in the Paleozoic rocks of Kentucky—the intermediary is base level determined by rivers. By hypothesis, the intermediary for Florida is glacioeustatic sea level. This paper presents elevation data that supports this hypothesis. Recent cave surveys in the air-filled caves and spot elevations from archived maps reveal prominent levels of passages centered at 5, 12, 21, and 30 m above sea level over broad areas. They do not follow the large-scale structure of the Floridan aquifer. Instead, they align with nearby, coastal marine terraces identified as modal peaks on frequency plots from various topographic data bases. Levels matching with the three highest terraces—Wicomico, Penholoway, and Talbott—are particularly clear. Lower levels, if they accord with sea-level stands, are likely composites. Data from cavities encountered in drilled wells (e.g., bit drops) and spot elevations from archived underwater cave maps demonstrate passage levels at depths of 15, 30, 70, and 90–120 m below the modern water table. The depths below water table are similar to the depths below sea level of distant submerged terraces and paleoshoreline features identified using multibeam bathymetric data in the Gulf of Mexico. The cave, bit-drop, and terrace data are all consistent with the concept that Quaternary sea level is the fundamental control on the cave-scale porosity within the Floridan aquifer. This conclusion does not rule out the possibility that lithologically favored positions, paleokarst features and confining units, and mixing zones are also involved in the location of caves levels in this near-coastal environment.

We present a large-scale quantitative test of a hyperspectral remote-sensing reflectance algorithm. We show that coastal bathymetry can be adequately derived through model inversions using data from the Airborne Visible-Infrared Imaging Spectrometer instrument. Data are analyzed from a shore-perpendicular transect 5 km offshore Sarasota, Florida at water depths ranging from 10 m to 15.5 m. Derived bottom depths are compared to a high-resolution multibeam bathymetry survey. Model-derived depths are biased 4.9% shallower than the mean of the multibeam depths with an RMS error of 7.83%. These results suggest that the model performs well for retrieving bottom depths from hyperspectral data in subtropical coastal areas in water depths ranging from 10 m to 15.5 m.

High-resolution bathymetry surveys of Egmont Channel were conducted in 1999 and 2001 using a Kongsberg Simrad EM 3000 multibeam bathymetric system. These data were supplemented with other bathymetry data, seismic profiles, underwater scuba observations, and current velocity data, in order to investigate the geologic and hydrodynamic characteristics of Egmont Channel. Bounded to the north by a linear steep scarp (∼38°) and by a more gradual slope (>10°) to the south Egmont Channel is an asymmetric tidal inlet and the main shipping channel for Tampa Bay, Florida. The cross sectional area (17,964 m2) and the tidal prism (6×108 m3) for Egmont Channel were derived in this study. Currents measured at Egmont Deep and the Sunshine Skyway Bridge (∼11 km away) with Acoustic Doppler Current Profilers, have a high correlation (97%) indicating the current velocities at Sunshine Skyway Bridge can be used as a proxy for current velocities at Egmont Deep. Seismic profile data indicate that both the mouth of Tamp...

The southeastern component of a subtle ridge feature extending over 200 km along the western ramped margin of the south Florida platform, known as Pulley Ridge, is composed largely of a non-reefal, coastal marine deposit. Modern biostromal reef growth caps southern Pulley Ridge (SPR), making it the deepest hermatypic reef known in American waters. Subsurface ridge strata are layered, lithified, and display a barrier island geomorphology. The deep-water reef community is dominated by platy scleractinian corals, leafy green algae, and coralline algae. Up to 60% live coral cover is observed in 60–75 m of water, although only 1–2% of surface light is available to the reef community. Vertical reef accumulation is thin and did not accompany initial ridge submergence during the most recent sea-level rise. The delayed onset of reef growth likely resulted from several factors influencing Gulf waters during early stages of the last deglaciation (∼14 kyr B.P.) including; cold, low-salinity waters derived from discrete meltwater pulses, high-frequency sea-level fluctuations, and the absence of modern oceanic circulation patterns. Currently, reef growth is supported by the Loop Current, the prevailing western boundary current that impinges upon the southwest Florida platform, providing warm, clear, low-nutrient waters to SPR. The rare discovery of a preserved non-reefal lowstand shoreline capped by rich hermatypic deep-reef growth on a tectonically stable continental shelf is significant for both accurate identification of late Quaternary sea-level position and in better constraining controls on the depth limits of hermatypic reefs and their capacity for adaptation to extremely low light levels.

High-resolution side-scan mosaics, sediment analyses, and physical process data have revealed that the mixed carbonate/siliciclastic, inner shelf of west-central Florida supports a highly complex field of active sand ridges mantled by a hierarchy of bedforms. The sand ridges, mostly oriented obliquely to the shoreline trend, extend from 2 km to over 25 km offshore. They show many similarities to their well-known counterparts situated along the US Atlantic margin in that both increase in relief with increasing water depth, both are oriented obliquely to the coast, and both respond to modern shelf dynamics. There are significant differences in that the sand ridges on the west-central Florida shelf are smaller in all dimensions, have a relatively high carbonate content, and are separated by exposed rock surfaces. They are also shoreface-detached and are sediment-starved, thus stunting their development. Morphological details are highly distinctive and apparent in side-scan imagery due to the high acoustic contrast. The seafloor is active and not a relict system as indicated by: (1) relatively young AMS 14C dates (

Geophysical and coring data from the Dry Tortugas, Tortugas Bank, and Riley’s Hump on the southwest Florida margin reveal the stratigraphic framework and growth history of these carbonate banks. The Holocene reefs of the Dry Tortugas and Tortugas Bank are approximately 14 and 10 m thick, respectively, and are situated upon Pleistocene reefal edifices. Tortugas Bank consists of the oldest Holocene corals in the Florida Keys with earliest coral recruitment occurring at ∼9.6 cal ka. Growth curves for the Tortugas Bank reveal slow growth (

Located in the South Pacific, between latitudes 33°S and 38°S and longitudes 120°W and 125°W, the extinct Selkirk microplate was first described as originating from lithosphere transferred from the Nazca plate to the Pacific plate [Earth Planet. Sci. Lett. 113 (1992) 293]. Multibeam bathymetry data, backscatter imagery and magnetic data obtained in 1997 during the Foundation Hotline cruise permit a more complete and detailed characterisation of the evolution of this paleomicroplate. The western boundary of the paleomicroplate is a fossil-spreading axis composed by two segments presenting two southward propagations and fan-shaped opening. The eastern boundary of the paleomicroplate is formed by a northward-propagating rift. In the present-day, only the western part of the fan-shaped sequence is present in the Pacific plate. According to satellite altimetry data, the northern boundary is a complex compressive domain formed by N110 ridges and troughs. The southern boundary is located along the Mocha fracture zone. Our magnetic interpretation suggests that this microplate has been active between chron 6C (24 Ma) and chron 6Ar (anomaly 6A, reverse period; 20.8 Ma). The last stage of the Selkirk microplate evolution is marked by a compression in the southern boundary and a sinistral shearing along an associated N45 structure located in the microplate. These observations imply the transfer of the eastern southern boundary of the microplate, previously located along the Mocha fracture zone, to these complex structures. The observed structures imply a change of the microplate rotation axis for this last stage, leading to the locking of the microplate and the transfer of all the spreading to the eastern boundary.

Several mechanisms have been proposed to account for the rotation of the nearly north-south abyssal hill fabric formed on the East Pacific Rise north of the Easter Microplate to the nearly east-west trends in the northern microplate interior. Proposed mechanisms include rigid microplate rotation, transform fault – parallel shear, and bookshelf faulting during the transfer of lithosphere from the Nazca Plate to the microplate. We used a submersible magnetometer on a NAUTILE dive program to measure the magnetic vector rotation of a pillow basalt and dike spur near Pito Deep, the present location of the tip of the propagating rift system that created the microplate. Our results, although too limited to draw strong conclusions from, suggest clockwise rotations of the seafloor magnetic vectors inconsistent with the transform-parallel shear model, and larger than can be explained solely by rigid microplate rotation.

The processes responsible for hotspot seamount formation are complex, yet the cumulative frequency-volume distribution of hotspot seamounts in the Easter Island/Salas y Gomez Chain (ESC) is found to be well-described by an upper-truncated power law. We develop a model for hotspot seamount formation where uniform energy input produces events initiated on a self-similar distribution of critical cells. We call this model Self-Similar Criticality (SSC). By allowing the spatial distribution of magma migration to be self-similar, the SSC model recreates the observed ESC seamount volume distribution. The SSC model may have broad applicability to other natural systems.

Published mechanisms for rift tip propagation at spreading centers include extensional deformation and an initial period of slow spreading. We investigate whether the gravity signal and inferred crustal structure at pseudofaults formed in medium to superfast spreading environments resemble the gravity signal at fracture zones or nontransform discontinuities formed in slow spreading environments. We find that altimetry-based gravity anomalies on the Mathematician, Bauer, Easter, Juan Fernandez, and northern Chile Ridge pseudofaults, located in 75-150 mm/yr (full rate) seafloor spreading environments, are similar in amplitude and form to Atlantic fracture zones with 20-30 mm/yr spreading rates. A 5-15 mGal positive mantle Bouguer anomaly is observed on the pseudofault bounding the eastern Juan Fernandez microplate, comparable to those at some similar age-offset nontransform discontinuities in slow spreading environments. Our results suggest that the deeps associated with active propagating rift tips result from both a dynamic mantle component and anomalous crust, the latter of which remains frozen at pseudofaults. We predict that any pseudofaults with age offsets more than ∼1 m.y. and not coincident with hotspot volcanism will be associated with thin (and possibly unusually dense) crust, even in superfast seafloor spreading environments.

The Foundation hotspot-Pacific-Antarctic Ridge (PAR) system is the best documented case of a fast spreading ridge approaching a hotspot and interacting with it. The morphology, crustal structure inferred from gravity anomalies and the chemical composition of the lavas of the axial area of the PAR show evidence of the influence of the hotspot, that is presently located roughly 35 km west of the spreading ridge axis. Along-axis variation in the Mantle Bouguer anomaly is about 28 mGal, corresponding to a crustal thickening of 1.5 km where the hotspot is nearer to the PAR. Anomalous ridge elevation is 650 m and the along-axis width of the chemical anomaly is 200 km. A comparison of these axial parameters with those derived for other ridge-hotspot systems, suggests that the amount of plume material reaching the ridge axis is smaller for the Foundation-PAR system. This implies a weaker connection between the plume and the ridge. Cumulative effects of a fast spreading rate and of a fast ridge-hotspot relative motion can be responsible for this weak plume-ridge flow. The flow from the hotspot may be less efficiently channelled towards the ridge axis when a fast ridge is rapidly moving towards a hotspot.

Side-scan sonar, swath bathymetry and magnetic anomaly date define a detailed, three-phase history of the Juan Fernandez microplate. The ∼6 m.y. history is presented in a series of discrete time steps to document the growth and reorganization of propagating spreading centers and structural feature, and microplate kinematic evolution. Prior to the microplate, the East Pacific Rise at the Pacific-Antarctic-Nazca triple junction was offset by a long transform fault zone, likely the fastest slipping transform on Earth at anomaly 3A time. The microplate originated from an intratransform setting between anomaly 3A (5.95 Ma) and anomaly 3 (5.24 Ma) time. Its early development resembled a large propagating rift system, and microplate core structures suggest the entire offset zone may have experienced deformation. Fast propagation of the East Ridge dominated microplate growth until ∼2.6–1.9 Ma when seafloor spreading became the dominant growth process. The microplate rotation rate increased threefold (from 9 to 29° m.y.−1 average) from phase 1 (4.2–2.6 Ma) to phase 2 (2.6–1.1 Ma) of the microplate's history, then reduced fourfold (29 to 7° m.y. −1 average; phase 3, 1.1 Ma to Present). Phases 2 and 3 of the microplate's rotational history support the edge-driven model for microplate kinematics of Schouten and others to a good approximation. The Pacific-Nazca shear couple drove microplate rotation during phase 2, but development of the southeastern boundary enabled a transfer to the Nazca-Antarctic plate pair (phase 3). West Ridge propagation and reorganization of the south-western boundary may have decoupled the Pacific plate from the microplate, thus facilitating the shear couple transfer. The recent continued deceleration in microplate rotation rate and westward migration of the Pacific-Antarctic ridge axis relative to the microplate may indicate that the process of microplate “death” has begun. We speculate that the Juan Fernandez microplate will accrete to the Antarctic plate, perhaps within the next million years, like the extinct Friday microplate has done, thereby accomplishing another northward migration of the Pacific-Antarctic-Nazca triple junction. Our reconstructions illustrate that the Easter and Juan Fernandez microplates are more similar than previously thought in terms of their origin, growth, rift propagation, ridge segmentation and overall tectonic evolution.

Bathymetry and gravity data collected during Legs 5, 6, and 7 of the 1993 GLORIA Expedition and the recently released 2-min altimetry-derived global gravity grid are used to determine the effective elastic thickness of the lithosphere along the Easter Seamount Chain (ESC). Forward modeling, admittance, and coherence methods yield consistent restfits. With the exception of the eastern and western ends of the ESC the effective elastic thickness along the chain is ~1-4 km. The thin elastic thickness for the majority of the ESC seamounts is compatible with a young seafloor age at the time of loading derived from new radiometric ages of the seamounts along the chain and a magnetic isochron age interpretation of the Nazca plate seafloor age. The elastic thickness southeast of the Nazca fracture zone is ~6 km, apparently because of the seafloor age discontinuity across the fracture zone. The elastic thickness near the San Felix Island, at the eastern end of the ESC, is even greater (~ 11 km), which is compatible with the estimated seafloor age at the time of loading. A slight increase in the effective elastic thickness of the far western part of the ESC suggests dynamic compensation or less thermal weakening of lithosphere above a plume channel versus directly above the plume center. These findings combined with published geochemistry support a hotspot origin for the ESC, complicated by large-scale plate boundary reorganizations and channeling of plume material to the East Pacific Rise.

We investigate the morphology and distribution of a seamount population on a section of seafloor influenced by both superfast seafloor spreading and hotspot volcanism. The population under investigation is part of a broad chain of seamounts extending eastward from the East Pacific Rise, near Easter Island. In order to define the morphological variability of the seamounts, basal shape, cross-sectional area, volume, flatness, and flank slope are plotted against height for 383 seamounts with heights greater than 200 m, based on bathymetry data collected by GLORI-B and SeaBeam 2000, during three cruises onboard the R/V Melville in the spring of 1993. Nearly complete swath mapping coverage of the seamounts is available for the analysis of size and shape distribution. We quantitatively describe the seamount population of this active region, in which seamounts cover ∼27% of the seafloor, and account for ∼4.2% of the total crustal volume. Over 50% of the total volume (61,000 km3) of seamounts used in this study is made up by the 14 largest seamounts, and the remaining volume is made up by the 369 smaller seamounts (>200 m in height). Our analysis indicates there are at least two seamount populations in the Easter Island-Salas y Gomez Island (25°–29°S, 113°–104°W) study area. One population of seamounts is composed of short seamounts ( 1200 m), shield-like, pointy cones (flatness ∼1200 m) originate exclusively from a hotspot source, but only a portion of the smaller volcanoes (

A new bathymetry processing software package has been developed to postprocess new GLORI-B swath bathymetry data using preexisting techniques. GLORI-B bathymetry is calculated using an interferometry (phase delay) method using the modified GLORIA towfish which has parallel rows of transducers on both sides. We describe four types of artifacts observed during the first use of this new system during Legs 5 and 6 of the Gloria Expedition which surveyed the fastest spreading segment of the global seafloor spreading system and the broad chain of volcanoes near Easter Island. These artifacts include cross-track bias, along-track bias, a 'dropped edge' effect, and random noise. We describe and illustrate how we minimize these artifacts. We merge the SeaBeam 2000 bathymetry data with the GLORI-B bathymetry data to produce a final bathymetric mosaic which covers about 243,400 km2 and shows a different style of diffuse widely spread volcanism not previously observed along hotspot chains. The data are used in several studies describing seamount morphology, elastic thickness of the lithosphere, tectonic and geochemical evolution of the area, and mantle flow from a hotspot to a superfast seafloor spreading center.

A new side-scan processing software package has been developed to postprocess data from the new GLORI-B system and SeaBeam 2000 side-scan data using preexisting techniques. We describe the procedures used to process the side-scan data collected during Legs 5, 6 and 7 of the Gloria Expedition in the spring of 1993. The procedures included minimizing artifacts and merging the two types of side-scan data together. The final mosaic shows recent widespread volcanic flows over large areas of the seafloor, unlike any other volcanic chain surveyed on the seafloor. The side-scan data were also very useful in dredging young rocks which are described in recently submitted geochemical manuscripts. The unique style of widespread low-lying young volcanism is attributed to the combination of superfast seafloor spreading and an underlying hotspot channel.

A Nautile submersible investigation of Pito Seamount documents vigorous hydrothermal venting at 23° 19.65'S, 111° 38.41'W and at a depth of 2270 m. The data indicate the volcano is young and recently active, as predicted from analyses of SeaMARC II side-scan and swath bathymetry, and geophysical data. Pito Seamount lies near Pito Deep (5980 m), which marks the tip of the northwestward propagating East rift of the Easter microplate. Bathymetry surrounding Pito Seamount consists of a series of ridges and valleys with relief up to ∼ 4 km. The 4-km submersible-transect to the summit of Pito Seamount crossed areas of very glassy basalt with little or no sediment cover, suggesting the lava flows are very young. Most of the lava samples from Pito Seamount are depleted normal MORB (mid-ocean ridge basalt). Lava samples associated with active and dead hydrothermal vents consist of phyric and aphyric transitional and enriched MORB. Sulfides consist primarily of sphalerite and pyrite, with traces of chalcopyrite. The active hydrothermal chimney on Pito Seamount has a small, undiversified biological community similar to northern East Pacific Rise vent sites (alvinellid worms, bythograeid crabs and bythitid fishes) and western Pacific back-arc basin sites (alvinocaridid shrimps). No vestimentiferan worms were observed. Previous geophysical data, and new geochemical data and visual observations, suggest that the vigorous black smoker is a result of deep, extensive crosscutting faults formed by extensive tectonic thinning of Pito Deep, and a very robust magmatic supply being supplied from upwelling asthenosphere. Although no biological or vent fluid samples were obtained, geological and biological observations, such as the large number of inactive chimneys, old hydrothermal deposits, and starfish, as well as the occurrence of dead mollusks (gastropod and mussels), suggest a recent waning of hydrothermal activity near the summit. The speculative interpretation that Pito Seamount is acting as a focal point for the formation of a new seafloor spreading axis trending northwest (310°) from the seamount summit towards Pito Deep is supported by the new data and observations reported here. These include the similar geochemistry of young lava samples obtained from Pito Seamount and from a small volcanic mound within Pito Deep and the strong SeaMARC II side-scan backscatter amplitudes along most of the ∼ 50 km rift zone connecting the summit of Pito Seamount to Pito Deep.

Submersible investigations along the East Rift segments, the Pito Deep and the Terevaka transform fault of the Easter microplate eastern boundary, and on a thrust-fault area of the Nazca Plate collected a variety of basalts and dolerites. The volcanics consist essentially of depleted (N-MORB), transitional (T-MORB) and enriched (E-MORB) basalts with low (0.01−0.1, 0.25, > 1.2–2) K/Ti and(La/Sm)N ratios, respectively. The Fe-Ti-rich ferrobasalt encountered among the N-MORBs are found on the Pito Deep Central volcano, on the Terevaka intra-transform ridge, on the ancient ( 61) are found on topographic highs (2000–2300 m) and lower values (Mg# < 56) at the extremities of the East Rift segments (2500–5600 m depths). The deepest area (5600 m) along the East Rift is located at 23 °S and coincides with a Central volcano constructed on the floor of the Pito Deep. Three major compositional variabilities of the volcanics are observed along the East Rift segments studied: (1) the 26 °S East Rift segment where the volcanics have intermediate Na8 (2.5–2.8%) and Fe8 (8.5–11%) contents; (2) the 23 °S East Rift segment (comprising Pito seamount and Pito Deep Central volcano) which shows the highest (2.9–3.4%) values of Na8 and a low (8–9%) Fe8 content; and (3) the 25 °S (at 24 °50′–26 °10′S) and the 24 °S (at 24 °10′–25 °S) East Rift segments where most of the volcanics have low to intermediate Na8 (2.6–2.0%) and a high range of Fe8 (9–13%) contents. When modeling mantle melting conditions, we observed a relative increase in the extent of partial melting and decreasing melting pressure. These localized trends are in agreement with a 3-D type diapiric upwelling in the sense postulated by Niu and Batiza (1993). Diapiric mantle upwelling and melting localized underneath the 26, 25 and 23 °S (Pito seamount and Central volcano) East Rift segments are responsable for the differences observed in the volcanics. The extent of partial melting varies from 14 to 19% in the lithosphere between 18 and 40 km deep as inferred from the calculated initial (Po=16kbar) and final melting (Pf=7kbar) pressures along the various East Rift segments. The lowest range of partial melting (14–16%) is confined to the volcanics from 23 °S East Rift segment including the Pito seamount and the Central volcano. The Thrust-fault area, and the Terevaka intra-transform show comparable mantle melting regimes to the 25 and 26 °S East Rift segments. The older lithosphere of the EMP interior is believed to have been the site of high partial melting (17–20%) confined to the deeper melting area (29–50 km). This increase in melting with increasing pressure is similar to the conditions encountered underneath the South East Pacific Rise (13–20 °S).

A multibeam survey was conducted over San Antonio submarine canyon, near Valparaiso, Chile, in April and May 1993 using the SeaBeam 2000 system on the R/V Melville. The bathymetric data from this survey reveal a canyon with an overall sinuosity of 1.25, a broad, roughly U-shaped cross-section along most of its length, and an almost constant channel slope above the forearc structural high. The course of the canyon is deflected to the north by a prominent structural high opposite the town of San Antonio. SeaBeam 2000 side-scan sonar data reveal high backscatter material on the floor of the canyon with a longitudinal fabric, reminiscent of stream braiding, and point bars formed on the inside of channel bends. We interpret this high backscatter material to be coarse sediment, transported down the canyon as turbidity currents. The source of these turbidity currents is probably the Rio Maipo, which enters the ocean near the head of the canyon.

THE existence of rapidly spinning microplates along the southern East Pacific Rise has been documented by geophysical swath-mapping surveys1–6, and their evolution has been successfully described by an edge-driven kinematic model7. But the mechanism by which such microplates originate remains unknown. Proposed mechanisms1–10 have generally involved rift propagation11, possibly driven by hotspots or changes in direction of sea-floor spreading. Here we present geophysical data collected over the Earth's fastest spreading centre, the Pacific–Nazca ridge between the Easter and Juan Fernandez microplates (Fig. 1), which reveal a large-offset propagating rift presently reorganizing the plate boundary geometry. A recent episode of rapid 'duelling' propagation of the historically failing spreading centre in this system has created a 120 × 120 km overlap zone between dual active spreading centres, which may be the initial stage of formation of a new microplate.

Publisher Summary The Madison-Swanson Marine Protected Area (MPA) and Pulley Ridge Habitat Area of Particular Concern (HAPC), two protected areas off the Florida shelf in the eastern Gulf of Mexico, were established by the Gulf of Mexico Fishery Management Council and the US National Oceanic and Atmospheric Administration to protect benthos-associated organisms. However, the types of organisms and the reasons for the protection are quite different. Madison- Swanson is used by an economically valuable reef fish species, the gag grouper, as a spawning ground. Pulley Ridge contains the deepest hermatypic scleractinian coral colonies in the continental USA. Biological assessments of these protected areas were carried out in generally similar fashion: initial mapping with multibeam sonar followed by targeted observations with remote still and video cameras. The formation of the Florida carbonate platform began when North America and Africa rifted apart during the opening of the Atlantic Ocean. The principal biological interest in Madison-Swanson was a large and mobile apex predator, while in Pulley Ridge it was a sessile invertebrate. The mobility, or lack thereof, of these biological targets shaped the biological assessment strategies. In Madison-Swanson, the initial effort was complete multibeam bathymetric and backscatter mapping. These maps were used to stratify the entire reserve into seven regions. The bathymetry was used to segregate the high-relief areas, while the backscatter data were used to differentiate low-relief areas with different sediment types. Remotely Operated Vehicle (ROV) video transects were surveyed to confirm the habitat types within each strata.

This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published chapter [58] is copyrighted by Elsevier.

Artificial reefs (ARs) are becoming a popular biological and management component in shallow water environments characterized by soft seabed, representing both important marine habitats and tools to manage coastal fisheries and resources. Because of the unstable nature of sediments, they require a detailed and systematic investigation that acoustic systems can provide. An AR into the marine environment acts as an open system with exchange of material and energy, altering the physical and biological characteristics of the surrounding area. The AR stability will depend on the balance of scour, settlement, and burial resulting from ocean conditions over time. The acoustic systems are efficient tools in monitoring the environmental evolution around ARs, whereas water turbidity can limit visual dive and ROV inspections. High-frequency multibeam echosounder offers the potential of detecting fine-scale distribution of reef units, providing an unprecedented level of resolution, coverage, and spatial definition. In 2008 the EM3002 multibeam was used to investigate the first scientifically-planned AR in Italy deployed during 1974-75. After 33 years the AR is almost in perfect condition. A considerable scouring around the structures and some vertical/horizontal movements of the single units have occurred. Moreover a deepening of the whole area hosting the AR was observed.Os recifes artificiais (ARs) estão se tornando um componente biológico e de manejo de ambientes de áreas rasas com substratos característicos não rígidos, representando habitats marinhos importantes e ainda constituindo uma ferramenta para o manejo da pesca e dos recursos costeiros. Devido à natureza instável dos sedimentos, exigem um estudo detalhado e sistemático que os sistemas acústicos podem disponibilizar. Um RA atua no ambiente marinho como um sistema aberto com troca de materiais e energia, alterando as características físicas e biológicas das áreas que o cerca. A estabilidade do RA dependerá do equilíbrio entre a escavação, o assentamento e aterramento resultantes das ações do oceano no decorrer do tempo. Os sistemas acústicos são ferramentas eficientes no monitoramento da evolução ambiental ao redor dos ARs, uma vez que a turbidez da água pode limitar o mergulho visual e as inspeções ROV. A ecosonda multifeixe de alta frequência oferece meios para identificar com precisão a distribuição das unidades recifais, com nível de resolução, cobertura e definição especial. Em 2008, a sonda multifeixe EM3002 foi usada para estudar a primeira implantação cientificamente planejada de RA na Itália, durante 1974-75. Após 33 anos, o RA está praticamente em perfeitas condições. Foram observadas a ocorrência de escavação considerável ao redor das estruturas , alguns movimentos vertical/horizontal das unidades isoladas e ainda o aprofundamento de toda a area que abriga o RA.

In this paper we synthesize and discuss the results of recent detailed geophysical surveys and reconstructions of two active mid-ocean ridge microplates on the East Pacific Rise. We quantify their development in terms of the principal dimensions of the microplates and their boundaries, and the temporal changes in these dimensions. The results are discussed in the framework of the ‘roller-bearing’ microplate model of Schouten and others, which postulates that microplates are driven by shear couples imposed on them by the motions of their bounding major plates. We test the specific predictions of this model for the geometry of the ‘pseudofaults’ (traces of the propagating tips of the microplates’ spreading boundaries) and the histories of rotation of the microplates. Both microplates are currently rotating at the rates predicted by the model on the assumption of complete coupling (no slip or deformation) between the microplates and the major plates at instantaneous points of contact near the propagating tips of the spreading boundaries. Juan Fernandez microplate was also rotating at the predicted rate just before 2 Ma when it was driven by the Nazca–Pacific couple rather than the Nazca–Antarctic couple that currently applies. However, earlier rotations were slower than predicted, implying some degree of decoupling or deformation at earlier times. The microplates appear to have grown from moderate-sized ( c . 100 km), probably left-stepping offsets of the East Pacific Rise since about 5 Ma. The nature of these offsets is currently uncertain, but they may have been propagating rifts, which changed from shearing by ‘bookshelf faulting’ to rigid block (microplate) rotation on reaching a critical size. Once rigid block rotation began, the microplates grew by seafloor spreading, accompanied by propagation of their bounding spreading centres. The effects of the neighbouring Easter mantle plume and Pacific–Nazca–Antarctic triple junction can be discerned in the bathymetry and ridge propagation rates, but do not appear to have had a major influence on the microplates’ development.

A combined GLORIA sidescan and Hydrosweep multi-narrow-beam sonar survey shows striking evidence for fan-shaped spreading systems, bounding pseudofaults and compression ridges associated with 90° of clockwise rotation of the Juan Fernandez microplate in the past 4 Myr. A quantitative analysis of this evolution suggests that the microplate has rotated as a rigid 'roller bearing' for most of its history, driven at its edges by shear between the surrounding major plates. The remarkable similarity between the Juan Fernandez and Easter microplates, despite their different tectonic settings, suggests that this is a general model for microplate tectonics.

The Pito Rift area is the site of actively deforming oceanic lithosphere that has been primarily under extension for at least the past million years, based on kinematic reconstructions. The major morphologic features, Pito Deep and Pito Seamount, are aligned toward the Euler pole for relative motion between the Easter and Nazca plates. SeaMARC II side-scan and bathymetry data indicate that there are two general modes of faulting currently active in the Pito Rift area. One is associated with incipient rifting of old (∼3 Ma) Nazca lithosphere by large NW-SE normal faults, and the other is associated with a broad area of right-lateral transform shear between the Nazca and Easter plates. This transform shear is distributed over a broad region because of the northward growth of the East Rift and parallel tectonic rifting within the Pito Rift area. The majority of the Pito Rift area is composed of preexisting blocks of Nazca plate that are back-tilted away from Pito Deep and strike perpendicular to present and previous relative plate motions. This observation suggests that block-faulting and back-tilting are the primary mechanisms responsible for the distributed lithospheric extension, in agreement with gravity and magnetic analyses (Martinez et al., this issue). The only recent volcanic flows observed in side-scan data are from the Pito Seamount area and to the outside of the outer pseudofault of the East Rift. The significance of the young flows near the outer pseudofault is not understood. We interpret the flows extending northwest from the Pito Seamount as representing a newly formed seafloor spreading axis within the Pito Rift area. Gravity and magnetic analyses (Martinez et al., this issue) together with SeaMARC II bathymetry and side-scan data support this interpretation. Based on the tectonic evolution of the Easter microplate, we propose an evolutionary model for the formation of the Pito Rift area, where new ‘tectonic’ grabens form immediately west of the previous graben and with slightly more counterclockwise orientation. The duration and history of tectonic activity for each graben are not well constrained.

We present results from a SeaMARC II bathymetry, gravity, and magnetics survey of the northern end of the large-offset propagating East Rift of the Easter microplate. The East Rift is offset by more than 300 km from the East Pacific Rise and its northern end has rifted into approximately 3 Ma lithosphere of the Nazca Plate forming a broad (70–100 km) zone of high (up to 4 km) relief referred to as the Pito Rift. This region appears to have undergone distributed and asymmetric extension that has been primarily accommodated tectonically, by block faulting and tilting, and to a lesser degree by seafloor spreading on a more recently developed magmatic accretionary axis. The larger fault blocks have dimensions of 10–15 km and have up to several km of throw between adjacent blocks suggesting that isostatic adjustments occur on the scale of the individual blocks. Three-dimensional terrain corrected Bouguer anomalies, a three-dimensional magnetic inversion, and SeaMARC II backscatter data locate the recently developed magmatic axis in an asymmetric position in the western part of the rift. The zone of magmatic accretion is characterized by an axis of negative Bouguer gravity anomalies, a band of positive magnetizations, and a high amplitude magnetization zone locating its tip approximately 10 km south of the Pito Deep, the deepest point in the rift area. Positive Bouguer gravity anomalies and negative magnetizations characterize the faulted area to the east of the spreading axis supporting the interpretation that this area consists primarily of pre-existing Nazca plate that has been block faulted and stretched, and that no substantial new accretion has occurred there. The wide zone of deformation in the Pito Rift area and the changing trend of the fault blocks from nearly N-S in the east to NW-SE in the west may be a result of the rapidly changing kinematics of the Easter microplate and/or may result from ridge-transform like shear stresses developed at the termination of the East Rift against the Nazca plate. The broad zone of deformation developed at the Pito Rift and its apparent continuation some distance south along the East Rift has important implications for microplate mechanics and kinematic reconstructions since it suggests that initial microplate boundaries may consist in part of broad zones of deformation characterized by the formation of lithospheric scale fault blocks, and that what appear to be pseudofaults may actually be the outer boundaries of tectonized zones enclosing significant amounts of stretched pre-existing lithosphere.

[1] Much historical perspective about terrestrial and marine processes can be obtained from examination of fluvial valleys formed during lower sea level on surfaces that are now continental shelves. The continental shelf near the mouth of the Fly River (Gulf of Papua) has three incised valleys, which were not extensively modified or filled during the Holocene Transgression. Multibeam mapping of the valleys documents their morphology; seismic profiling reveals stratigraphy of sediment fill; and coring within and near the valleys suggests mechanisms of filling. Kiwai Valley is deep (20–50 m relief), narrow (∼1 km wide), steep-sided and meandering, due to river flows that caused its excavation through competent sedimentary deposits. Purutu Valley is shallow (10–20 m relief) and broad (>2 km wide). Umuda Valley is widest (∼14 km) and has multiple channels with variable incision depths, suggesting more extensive fluvial activity than the other two valleys. Valley filling has occurred in several ways, reflecting valley morphology and location relative to the present river mouth. Kiwai and Umuda valleys reveal three stages of infill: (1) hemipelagic sedimentation at distal sites, (2) gravity-driven flows spreading down the valley, and (3) subsequent clinoform progradation that completely fills the valley. Purutu Valley fill is dominated by clinoform progradation. Clinoform progradation in Umuda Valley is driven by intense sediment reworking on the surrounding topset regions, and its large width allows progradation from the sides as well as down its axis. Most shelf valleys around the world were filled long ago, and available techniques have severe limitations for documenting the details of morphology and the mechanisms of filling. The shelf valleys described in this paper provide a unique perspective to terrestrial and marine processes before, during, and after the Holocene Transgression.

SeaMARC II side-scan sonar data reveal that a large area of seafloor north and west of Easter Island has been disrupted by recent submarine volcanism. A large volcanic area begins approximately 60 km WNW of the island and extends for over 130 km to the west. The volcanic field is characterized by high backscatter intensity in the side-scan sonar records and is elevated 400–1000 m above the N-S seafloor fabric that surrounds it. This field, the Abu Volcanic Field, covers at least 2500 km2 and appears to consist of recent lava flows and small volcanoes. Backscatter intensity of the Abu Volcanic Field is similar to that of the adjacent ridge flank which is less than 0.4 Ma, suggesting a similar age for its formation.

[1] We have collected 12 kHz SeaBeam bathymetry and 120 kHz DSL-120 side-scan sonar and bathymetry data to determine the tectonic and volcanic segmentation along the fastest spreading (∼150 km/Myr) part of the global mid-ocean ridge system, the southern East Pacific Rise between the Easter and Juan Fernandez microplates. This area is presently reorganizing by large-scale dueling rift propagation and possible protomicroplate tectonics. Fracture patterns observed in the side-scan data define structural segmentation scales along these ridge segments. These sometimes, but not always, correlate with linear volcanic systems defining segmentation in the SeaBeam data. Some of the subsegments behave cohesively, with in-phase tectonic activity, while fundamental discontinuities occur between other subsegments. We also collected hydrothermal plume data using sensors mounted on the DSL-120 instrument package, as well as CTDO tow-yos, to determine detailed structural and volcanic controls on the hydrothermal vent pattern observed along 600 km of the Pacific-Nazca axis. Here we report the first rigorous correlation between coregistered hydrothermal plume and high-resolution marine geophysical data on similar scales and over multisegment distances. Major plume concentrations were usually found where axial inflation was relatively high and fracture density was relatively low. These correlations suggest that hydrothermal venting is most active where the apparent magmatic budget is greatest, resulting in recent eruptions that have paved over the neovolcanic zone. Areas of voluminous acoustically dark young lava flows produced from recent fissure eruptions correlate with many of the major hydrothermal vent areas. Increased crustal permeability, as gauged by increased fracture density, does not enhance hydrothermal venting in this area. Axial summit troughs and graben are rare, probably because of frequent volcanic resurfacing in this superfast spreading environment, and are not good predictors of hydrothermal activity here. Many of the hydrothermal areas are found in inflated areas near the ends of segments, suggesting that abundant magma is being supplied to these areas.

Most inner continental shelves are dynamic environments, subject to current flows on a variety of temporal and spatial scales that are relevent to processes of mine scour and burial. We propose to quantify the response of a non-cohesive seabed to these flows on spatial scales of 10-2 to 104 m and on temporal scales of seconds to years by using a combination of acoustic remote sensing and in situ instrumentation. These studies will be undertaken in support of the ONR Mine Scour and Burial Study Site west of Indian Rocks Beach (IRB), on the west-central Florida coast.

Papers on Geomagnetism and Paleomagnetism Marine Geology and Geophysics.-- 14 pages, 10 figures, The Galapagos Spreading Center (GSC) is marked by systematic changes in axial morphology between the Inca Fracture Zone (FZ) at 85.5°W and the 95.5°W propagator. We analyze these changes using new swath bathymetry and magnetic data acquired aboard the B/O Hespérides during the Galápagos'96 experiment. Within ~350 km of the Galapagos hotspot the ridge axis is associated with an East Pacific Rise (EPR)-like axial high. At increasing distance from the hotspot the axial high broadens and deepens forming a distinctive transitional axial morphology (TAM). The axis in this transitional region is typically a broad zone (~20 km wide) consisting of very rough volcanic and fault-generated topography. West of 95°W, this TAM evolves into a 20-40 km wide, 400-1500 m deep axial valley typical of the slow spreading MidAtlantic Ridge (MAR). There is not an abrupt change from axial high to rift valley along the GSC, but a distinct TAM occurs over a distance of ~200-300 km along-axis and is accompanied by a gravity-estimated crustal thickening of>1-2 km. The boundary between an axial high and this TAM is quite abrupt and occurs along a segment that is less than 9 km long. These changes in axial morphology are primarily caused by variations in magma supply along the GSC due to the entrainment and dispersal of plume mantle from the Galapagos hotspot. However, the changes in morphology are not symmetric about the Galapagos FZ at 91°W. The axial high topography extends farther east of the 91°W FZ than to the west, and the rift valley which develops west of 94°W is not found at comparable distances along the GSC east of the hotspot. Axial depth variations are also asymmetric across the 91°W FZ. This asymmetry in both morphology and axial depth variation is attributed to a full spreading rate increase along the GSC from 46 mm/yr at 97°W to 64 mm/yr at 85°W. Off-axis depth changes are symmetric about the 91°W FZ and suggest that 15-40% of on-axis depth variation is dynamically supported

Fluvial sediment fills the coastal ocean, and sea level rise floods river valleys. This epic battle of terrestrial and marine processes occurs along all shorelines, and the complexities are especially well revealed in the Gulf of Papua, a foreland basin on the southern coast of New Guinea. Two hundred to four hundred million tons of sediment are supplied each year by the Fly and other rivers to a continental shelf that has been dissected by ancestors of these same rivers. The new sediment builds a large depositional feature known as a clinoform, which grows seaward and buries the record of earlier environments.

[1] During March/April 1998 we conducted detailed mapping and sampling of hydrothermal plumes along six segments of Earth's fasting spreading mid-ocean ridge, 27.5°–32.3°S on the East Pacific Rise. We compared the distribution and chemistry of hydrothermal plumes to geological indicators of long-term (spreading rate) and moderate-term (ridge inflation) variations in magmatic budget. In this large-offset, propagating rift setting, these geological indices span virtually the entire range found along fast spreading ridges worldwide. Hydrothermal plumes overlaid ∼60% of the length of superfast (>130 km/Myr) spreading axis surveyed and defined at least 14 separate vent fields. We observed no plumes over the slower spreading propagating segments. Finer-scale variations in the magmatic budget also correlated with hydrothermal activity, as the location of the five most intense plumes corresponded to subsegment peaks in ridge inflation. Along the entire ridge crest, the more inflated a ridge location the more likely it was to be overlain by a hydrothermal plume. Plume chemistry mostly reflected discharge from mature vent fields apparently unperturbed by magmatic activity within the last few years. Plume samples with high volatile/metal ratios, generally indicating recent seafloor volcanism, were scarce. Along-axis trends in both volatile (3He; CH4; ΔpH, a proxy for CO2; and particulate S) and nonvolatile (Fe, Mn) species showed a first-order agreement with the trend of ridge inflation. Nevertheless, a broad correspondence between the concentration of volatile species in plumes and geological proxies of magma supply identifies a pervasive magmatic imprint on this superfast spreading group of ridge segments.

We describe a model for episodic, open-ocean triple-junction migration based on observations of actual triple-junction evolutions. Migration progresses by repeated episodes of rift propagation, microplate formation, and microplate accretion to an adjacent, larger plate. These episodes may be highly variable in space and time depending on triple-junction geometry, velocity triangle, and other factors affecting local thermal and rheological conditions. Resulting tectonic features may include an abandoned transform fault, straight and potentially curving pseudofaults, sheared and potentially rotated abyssal-hill fabric, and a paleomicroplate with no associated failed rift. This model, developed mainly from the evolution of the Pacific-Antarctic-Nazca triple junction, may be relevant for other types of triple junctions such as the Bouvet and Azores triple junctions and ridge-ridge-ridge triple junctions in the Indian and Pacific Oceans. Episodic migration is found to occur even when the triple junction is kinematically stable. This model highlights the difference between predicted kinematic stability of triple junctions and observations of their true tectonic history.

We present evidence that the Easter and Juan Fernandez microplates originated by rift propagation from within transform fault zones at locations far from ridge-transform intersections. An intratransform spreading center may have served as the focal point for each new propagation. Both events appear coincident with a circum-Pacific plate reorganization dated at 5.6 Ma, which may have been related to a change in Pacific absolute plate motion and a counterclockwise rotation of Pacific-Nazca relative plate motion. This intratransform model may be applicable to the generation of other microplates and may thus serve as a general model for microplate origins.

SeaMARC II side scan and bathymetric data, combined with magnetic anomaly and Sea Beam bathymetric data, define a large (∼90–120 km) nontransform offset of the East Pacific Rise (EPR) between the Easter and Juan Fernandez microplates near 29°S, where one or more transforms had been suggested by the seismicity pattern. On the basis of morphology, structural, and tectonic features, three major regions are delineated: an eastern spreading axis, a western spreading axis, and a central transfer zone. These two spreading axes overlap by at least 35 km and possibly as much as 100 km. The eastern axis is characterized by an axial ridge that narrows and deepens toward the tip (28°38′S, 112°08′W). Approaching the tip, the spreading axis azimuth changes from approximately N5°E to N25°W, and axial magnetic anomaly values increase to over 700 nT 37 km south of the rift tip. A pair of deep (>600 m) bathymetric troughs on either side of the ridge axis, trending N11°W and N44°W, converge on the ridge tip and define the outermost boundaries of ridge-parallel structures. These structures are interpreted as pseudofaults resulting from northward rift propagation. The western spreading axis is approximately 2400 m deep and corresponds to a young volcanic area. The central lithospheric transfer zone, west of the inner pseudofault of the eastern spreading axis, is characterized by oblique abyssal hill and magnetic fabric interpreted to result from shear between overlapping spreading centers and a dispersed pattern of teleseismic earthquake epicenters. The eastern spreading axis is propagating rapidly northward at up to 265 km/m.y, based on a 158 km/m.y spreading rate and the 33° angle between the pseudofaults. However, complex patterns of seafloor fabric, which differ from that predicted by the simple propagating rift model, suggest that multiple propagation events may have occurred. Furthermore, southward propagation of the western limb may be dominant in the long term. The absence of transform faults along this superfast spreading EPR segment documents that transforms do not presently exist at such superfast spreading rates.

The plate tectonic history of the Easter microplate has been reconstructed by “closing” the microplate in a series of steps using the Pacific-Nazca magnetic anomalies north and south of the microplate and the NUVEL 1 global plate motion model. After each step, the Easter microplate was rotated rigidly to match the Nazca and Pacific anomalies. Gaps and overlaps formed by this kinematic treatment indicate compressional and tensional deformation, respectively, and show that rigid plate motions are insufficient to explain the complete tectonic evolution. Analysis of the magnetic anomaly data was guided by contoured SeaMARC II, Sea Beam, and 3.5-kHz bathymetry data and a lineament map derived from SeaMARC II side scan and Sea Beam bathymetry data. The patterns of lineaments and bathymetric structures suggest that rotational deformation of the Nazca plate is the general mechanism that accommodates the space problems arising from transfer of the Nazca plate to the microplate and rapid rotation of the microplate against the Nazca plate. Similar but smaller amounts of deformation are predicted along the southern boundary of the microplate. Prior to the origin of the microplate, the East Pacific Rise (EPR) was offset in at least two places according to the older magnetic anomalies, yet there is no evidence of linear fracture zones within the sparse data set except for occasional small consistent changes in regional depth across these age offsets. The magnetic, bathymetry, and satellite altimetry data indicate that the microplate initially formed at (or perhaps southeast of) Easter Island near a left-lateral offset of the EPR sometime between anomaly 3 and 3'. The East Rift started propagating north from the present location of Easter Island at ∼4.5 Ma, which is ∼1.5 m.y. earlier than previously proposed. However, the magnetic data that support this interpretation are sparse and complicated by recent volcanic flows and associated rough bathymetry west of Easter Island. The geometry of the microplate changes very rapidly during its evolution. At the initial stages of development, the microplate resembles a large propagating rift system, suggesting that deformation may have been occurring throughout most of its interior up to about 2.47 Ma. At this time, the length to width of overlap ratio of the two rifts reaches a value of 3, the northward propagation slows down, the curved opening of the Southwest Rift becomes well established, and rigid rotation of the previously deformed transferred lithosphere probably starts to predominate. At this time, the offset distance between the two overlapped rifts starts to increase. Some time after 2.47 Ma and before 1 Ma, the East Rift starts propagating northwestward, probably in response to the microplate rotation, and continues up until present. Also during this time period, the East Rift breaks into a series of northward propagating rifts, each propagating into the microplate interior, thereby transferring lithosphere from the microplate to the Nazca plate and reducing the total growth rate of the microplate.

The Easter (Rapanui) microplate is a case example of a large dual spreading center system in a region where the fastest seafloor spreading on Earth is occurring today. Recent theoretical models of the tectonic evolution of dual spreading center systems have explored the effects of shear and rigid rotation on the boundaries and internal structure of microplates but the models must be critically constrained by improved relative motion and structural fabric data sets. During the January 1987 Rapanui expedition on the N/O “Jean Charcot” we conducted a Sea Beam/magnetics/ gravity survey of a portion of the microplate boundaries. The method that was used was to fully map selected portions of the boundaries in order to establish precise structural relationships. The northern terminus of the East Rift or eastern boundary of the microplate is expressed as a series of parallel NW-SE trending valleys including what appears to be, with 5890 m depth, the deepest active rift axis mapped in the Pacific today (Pito Rift). The northern end of the Pito Rift merges with an E-W to 083° narrow band of linear faults interpreted to be a transform fault between the Nazca and Easter (Rapanui) plates. The northern triple junction between the Easter (Rapanui), Nazca and Pacific plates is a RFF type with the two transform faults colinear along an approximately E-W direction. The southwestern boundary of the Easter (Rapanui) microplate is marked by a series of en-echelon offsets, outlined by depressions, which merge into an approximately E-W zone where shear must be predominant. The southern triple junction is a RRF junction with an overlapping ridge system. The structural data acquired during the survey provide strong constraints for kinematic models of the microplate. The structural data need to be combined with crustal age determinations in order to derive a model for the evolution of the microplate.

There is a growing Easter microplate between the Pacific and Nazca plates. The eastern and western boundaries of the microplate are actively spreading and propagating north and south, respectively. The northern and southern boundaries are broad transform zones. Most of the interior of the microplate contains the sort of oblique sheared structures characteristic of zones of transferred lithosphere between propagating and failing spreading centres.

During a 1983 Seabeam cruise, we surveyed the Easter-Nazca spreading center which forms the eastern boundary of the Easter microplate. We have analyzed all available bathymetry and magnetic anomaly data to determine the present fine-scale tectonic configuration and recent tectonic evolution of this spreading center. The narrowing of the axial (Brunhes) anomaly is best explained by a young propagating rift growing northward at an average rate of 150 mm/yr assuming an average spreading rate of 120 mm/yr. There are indications of a short episode of faster propagation (350 mm/yr) prior to 0.4 Ma. These are the fastest rates of rift propagation yet documented. The tip of the propagating rift is at about 25°S, 112°25′W, ∼180 km south of the previously hypothesized position. The recent spreading direction is N85°W. A large well-defined “sheared zone” to the west of the western pseudofault bounding the crust formed by the propagating rift suggests that the Easter-Pacific spreading center (360 km to the west) is being replaced by the fast propagator.

Knowledge of the locations and sizes of seamounts is of great importance in applications such as inertial navigation and ocean mining. The quality and density of bathymetry data in the equatorial regions and the southern hemisphere are, unfortunately, highly variable. Our present knowledge of bathymetry, and in particular of seamount locations and characteristics, is based upon ship surveys, which are both time-consuming and expensive. It is likely that a significant number of uncharted seamounts exist throughout the oceans, and remote-sensing techniques may be the most effective means of locating them.

Oceanic transform faults with slip rates greater than ∼145 km/m.y. do not currently exist along the East Pacific Rise where sea-floor spreading rates range from 145 to 160 km/m.y. Instead, offsets of the the very fast spreading East Pacific Rise are accommodated by microplates, propogating rifts, or overlapping spreading centers. This suggests that there might be a speed limit above which transform faults do not exist. A physical reason for a speed limit is not known, but it might be related to unstable stress fields near the rifts tips, causing them to episodically propagate and prevent a transform fault from being formed. The spreading rates quoted are from our new (0-0.73 Ma) relative-motion model for the Pacific and Nazca plates.