Easter Microplate Research Articles

Cross-sectional areas of mid-ocean ridge axes bounding the Easter and Juan Fernandez microplates

We have calculated cross-sectional areas for the ridges bounding the Easter and Juan Fernandez microplates, 22°–28°S and 31°–35°S, obtaining accurate results where complete bathymetric data exist and estimates in other regions with partial bathymetric coverage and predicted bathymetry. We consider the reliability and usefulness of global predicted bathymetry in these calculations and the possible application of this dataset in other localities. The spreading rates on ridges bounding these microplates span the range from slow to superfast, allowing an investigation of ridge axis inflation over most of the rates active on Earth today. The across-axis areas of the Easter microplate ridge axes range from −29 km2 to 7 km2, while the Juan Fernandez ridge axis areas range from −27 km2 to 8 km2. Positive values correlate with regions usually interpreted as magmatically robust. Negative values arise from calculations in areas of propagating rift tips and deep grabens, such as Pito and Endeavor Deeps. Geochemical trends of Easter microplate axial basalts show decreasing MgO toward propagating rift tips and slight positive correlations between variables such as MgO vs. cross-sectional area, Na8.0 vs. axial depth, and Na8.0 vs. cross-sectional area. We document the decrease in the axial area approaching segment ends and propagating rift tips along both the West and East ridges of the microplates. On the Easter microplate both East and West ridge systems undergo large variations in spreading rate from >130 km Myr−1 to <50 km Myr−1. Inflation on these ridge segments is highly variable and only weakly correlated with spreading rate. On the Juan Fernandez microplate, West ridge spreading rates vary only between ∼115–140 km Myr−1 and are systematically faster than on the East ridge, where rates vary between ∼10–35 km Myr−1. Cross axis areas are systematically greater and significantly less variable on the faster spreading West ridge. Overall, compared to oceanic spreading centers bounding major plates with similar spreading rates, the axial areas are smaller on the microplate ridge systems, possibly because their rapidly changing configurations create a lag in the mantle response to the rigid plate boundary.

Intraplate en echelon volcanic ridges in the South Pacific west of the Easter microplate

Satellite altimetric data for the south-central Pacific ocean show linear geoid anomalies extending from Pitcairn island (130°W-24°S) to the western border of the Easter microplate (118°W-26°S). Previously, these anomalies have been interpreted as a submarine volcanic chain more than 1300 km long, inclusive of Henderson island, Ducie island and Crough seamount, and constructed on young (< 30 Ma) oceanic lithosphere. Recent bathymetric data and side-scan sonar images (GLORIA) from the eastern part of this volcanic chain suggest that the anomalies correspond to a succession of elongate ridges arranged in a right-stepping en echelon pattern along a fracture zone. At least five volcanic ridges were recognized during the SO80a cruise (1992) between 123°W and 118°W at 25°S, trending 275°–280° (parallel to plate motion). The ridges range from 70 to 200 km long, 20 to 35 km wide and 1100 to 2900 m high. Their volumes, ∼ 600 to 1500 km 3, are equivalent to those of large intraplate submarine volcanoes from the Polynesian hotspots. Sediment thickness and the degree of alteration of samples increase progressively from east to west, suggesting that the chain grew from a volcanic centre, located near 118°W–25°S, close to the West Rift that forms the western border of the Easter microplate. The ridges are built along a fracture zone of the Pacific plate, trending N100°–110°, which shifts the magnetic anomalies more than 300 km to the left. The origin of this fracture zone is not well assessed but it strikes in the continuity of the present-day Orongo transform fault forming the southwestern boundary between the Easter and Pacific plates, east of 114°50′W. The crests of the volcanic ridges are oriented N055°–075°, at angles of 30°–50° with respect to the fracture zone. The arrangement of both features could be compared to that of right-stepping opening cracks (volcanic ridges) along a sinistral shear zone (transform fault). An alternative model is based on the orientation of the abyssal hills which is almost perpendicular to that of the ridges, especially in the eastern part of the volcanic alignment. The origin of the en echelon ridges could also be related to tension and fracturing in the youngest lithosphere along leaky transform faults.

Recent dueling propagation history at the fastest spreading center, the East Pacific Rise, 26°–32°S

The East Pacific Rise (EPR) 26°–32°S, located between the Easter microplate and the Juan Fernandez microplate, has the world's fastest spreading rate and tectonics characterized by dueling ridge propagation. A GLORI‐B and Sea Beam 2000 side scan/bathymetry survey, together with other geophysical data collection, was conducted aboard the R/V Melville covering an approximately 5° by 5° area centered on this part of EPR. The side scan and bathymetric data reveal the complete geometry of the propagator system. The western and eastern ridges both curve inwardly and are overlapped ∼120 km with an offset to overlap ratio of nearly one. Several abandoned ridges and rifts are clearly mapped on the east flank, and a few failed rifts are also mapped near the western ridge tip. A fracture zone discovered south of Easter Island indicates a stage of stable ridge‐transform intersection in the past. An inversion with magnetic anomaly vectors was carried out for the area. A resultant magnetic boundary strike map was interpreted with the downward component profiles to construct a magnetic isochron map. The spreading and propagation rates were estimated from the magnetic isochrons. The net southward propagation rate of the western ridge is ∼120 km/m.y. for the last 1.9 m.y. The spreading rate of the eastern ridge has been ∼150 km/m.y., and its current propagation rate is estimated as ∼500 km/m.y. toward the north. Intense asymmetric spreading as high as 30% is observed at both ridges. Together with the side scan and bathymetric data, the magnetic lineations provide significant constraints on the evolution of the propagator system, and a propagation model with cyclic rift failure was applied to model the tectonic evolution during the last 2 m.y. A normal ridge‐transform intersection evolved into a nontransform overlapping offset ∼1.95 Ma, and the eastern and the western ridges have propagated alternately. The dueling propagation history is characterized by the dominant southward propagation of the western ridge and the transitory propagation events of the eastern ridge with a variety of overlap lengths, widths, and ridge curvatures in the transition zone.

Petrology of the Easter microplate region in the South Pacific

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, < 0.7), intermediate (0.12–0.25, 0.7–1.2) and high (> 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 (< 2.5 Ma) Easter microplate (called EMP, comprising the East Rift Inner pseudofaults and Pito Deep west walls) and on thrust-fault crusts. The most enriched (T- and E-MORB) volcanics occur along the East Rift at 25 °50′–27 °S (called 26 °S East Rift) and on the Pito seamount located near the tip of the East Rift at 23 °00′–23 °40′S (called 23 °S East Rift). The diversity in incompatible element ratios of the basalts in relation to their structural setting suggests that the volcanics are derived from a similar heterogenous mantle which underwent variable degrees of partial melting and magma mixing. In addition the Pito seamount volcanics have undergone less crystal fractionation (< 20%) than the lavas from the other Easter microplate structures (up to 35–45%). The tectonic segmentation of the East Rift observed between 23 and 27 °S corresponds to petrological discontinuities related to Mg# variations and mantle melting conditions. The highest Mg# (> 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 Na 8 (2.5–2.8%) and Fe 8 (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 Na 8 and a low (8–9%) Fe 8 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 Na 8 (2.6–2.0%) and a high range of Fe 8 (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 ( P o=16kbar) and final melting ( P f=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).

Two-way exchange between the Easter mantle plume and the Easter microplate spreading axis

THE conventional model whereby plume volcanism forms linear age-progressive volcanic chains, with the youngest activity occurring nearest a spreading axis (at a 'hotspot'), has been challenged for the Easter seamount chain1–4. Whereas early work suggested the existence of a linear melting anomaly (a 'hotline')1,2, more recent studies3,4 have proposed a hotspot near Salas y Gomez island, connected with the Easter microplate spreading system by an ∼800-km-long, volcanically active plume channel. Here we use geochemical, geological and geochronological data to argue that the hotspot lies close to Easter Island. Moreover, new isotopic data for lavas from the seamount chain provide evidence for bidirectional flow between the spreading axis and the plume, thus supporting geophysical and fluid-dynamical models of mantle flow in a plume/spreading axis system5–7. Material balance and flux considerations show the Easter plume to be weak and cool compared with those beneath larger features such as Iceland, Hawaii and the Galapagos islands.

Large rotation of the Easter microplate as evidenced by oriented paleomagnetic samples from the ocean floor

One of the goals of the Pito93 cruise along the perimeter of the Easter microplate was to obtain oriented rock samples from the ocean floor for paleomagnetic study. Using a new orienting device, the Geocompass, 14 oriented blocks were obtained during 6 dives conducted using the Ifremer DSRV Nautile. Paleomagnetic analysis shows that 13 of these 14 blocks give consistent results in thermal and AF demagnetizations, and reveals a mean Characteristic Remanent Magnetization (ChRM) direction within each block, with both normal and reverse polarities. After elimination of data from 5 blocks situated outside the microplate, and one block which bears an anomalous direction, we have computed a mean ChRM direction at D m = 49.0°, I m = −39.0° (k = 10.2, α 95 = 19.8 °, n = 7). Although the mean inclination is in agreement with the inclination of the recent dipole magnetic field (D = 0°, I = −43°), the average declination is significantly different from this direction, which we interpret as due to a bulk clockwise rotation of 48.5° ± 11° (after the bivariate average) of the Easter microplate over the past 2–3 Ma. This value is in excellent agreement with previous estimates based on remote geophysical measurements and models.

A history of the Easter microplate, 5.25 Ma to present

GLORIA and other geophysical data are used to interpret the morphology and tectonic pattern of the Easter microplate. We model magnetic anomaly data and calculate spreading rates for use in Euler pole calculations. We calculate three‐plate closure and best fit poles for Nazca‐Pacific, Nazca‐Easter, and Pacific‐Easter plate motions and compare the results with earthquake focal mechanisms and seismicity. We use the three‐plate closure poles to rotate digital GLORIA and magnetic isochron data of the microplate and surrounding major plates back in time since the microplate's East Rift first started to propagate northward at around 5.25 Ma. The microplate has evolved in two stages. The first is from 5.25 Ma to around 2.5 Ma, and involved continuous northward propagation of the East Rift, while the West Rift became segmented along new transform faults as it adjusted to keep the overall Nazca‐Pacific velocities constant. The transpressive northern boundary migrated with the East Rift tip, while the transtensional southern boundary remained more or less still. At some time after 2.5 Ma the East Rift ceased northward propagation and the Southwest Rift began to open up along the transtensional Pacific‐Easter transform. A convergent northern boundary formed involving southward thrusting of the Nazca plate over the microplate. No further Nazca plate lithosphere was transferred from north of the microplate to the microplate interior, and the Pito Deep region opened by pervasively rifting apart preexisting Nazca plate lithosphere.

New observations on mid-plate volcanism and the tectonic history of the Pacific plate, Tahiti to Easter microplate

We describe the geology and tectonics of a continuous swathe of seafloor between Tahiti and the western edge of the Easter microplate imaged by GLORIA and Sea Beam on two separate cruise transits in 1987 and 1988. The data reveal that mid-plate volcanism is common in this region, even on deep seafloor hundreds of kilometres from major lines of seamounts and islands. This supports the idea of a thin weak lithosphere over the Pacific Superswell, and the idea that the tops of major mantle plumes may spread out over diameters of the order of 1000 km. The mid-plate volcanism occurs in two distinct forms. Over most of our traverse it appears as fields of relatively young and acoustically strongly backscattering lava flows, often accompanied by groups of numerous small, circular volcanoes. East of 122° W (about chron 5A), however, we observed a distinct form: major, sharp-crested, constructional volcanic ridges, many tens of kilometres long, individually trending ENE, but lying en-echelon along an E-W regional trend. These ridges appear morphologically identical to the ‘cross-grain ridges’ seen elsewhere in the Pacific. We attribute their formation to magma supplied from the regionally hot mantle leaking along tectonic lines of weakness. However, although these ridges are parallel to fracture zone trends seen farther west, they are morphologically very different from any known fracture zone. Moreover, individual ridges are somewhat oblique to the tectonic spreading fabric around them, and so do not seem to follow actual fracture zone traces. The whole line of en-echelon ridges lies along part of the predicted trace of Fracture Zone 2 of Okal and Cazenave [15], and is probably its morphological expression. However, nowhere did we see a convincing ‘conventional’ fracture zone trace in or following the predicted position or orientation. We suggest instead that magma from an independent source has used lines of weakness along minor fracture zones to produce these en-echelon features. The Austral Fracture Zone is the only major fracture zone crossed in our transit, and here is characterised by four fossil transform strands. Its marked position on the AAPG and GEBCO maps is found to be in error. Finally, we found that the expected change from NNW- to NNE-trending spreading fabric at chron 6C did not occur in a clear-cut way, as predicted by earlier tectonic histories of the Pacific. Instead, the post-chron 6C fabric oscillates in a confused way between NNE and NNW, suggesting to us that this area has been characterised by an unstable plate boundary, probably associated with a succession of propagating rifts or microplates from chron 6C to the present.

Magmatic evolution of the Easter microplate-Crough Seamount region (South East Pacific)

The Easter microplate-Crough Seamount region located between 25° S–116° W and 25° S–122° W consists of a chain of seamounts forming isolated volcanoes and elongated (100–200 km in length) en echelon volcanic ridges oriented obliquely NE (N 065°), to the present day general spreading direction (N 100°) of the Pacific-Nazca plates. The extension of this seamount chain into the southwestern edge of the Easter microplate near 26°30′ S–115° W was surveyed and sampled. The southern boundary including the Orongo fracture zone and other shallow ridges ( 0.25) MORBs which are similar in composition to other more recent basalts from the Southwest and East Rifts spreading axes of the Easter microplate. Incompatible element ratios normalized to chondrite values [(Ce/Yb)N = 1−2.5}, {(La/Sm)N = 0.4−1.2} and {(Zr/Y)N = 0.7−2.5} of the basalts are also similar to present day volcanism found in the Easter microplate. The volcanics from the Easter microplate-Crough region are unrelated to other known South Pacific intraplate magmatism (i.e. Society, Pitcairn, and Salas y Gomez Islands). Instead their range in incompatible element ratios is comparable to the submarine basalts from the recently investigated Ahu and Umu volcanic field (Easter hotspot) (Scientific Party SO80, 1993) and centered at about 80 km west of Easter Island. The oblique ridges and their associated seamounts are likely to represent ancient leaky transform faults created during the initial stage of the Easter microplate formation (≈ 5 Ma). It appears that volcanic activity on seamounts overlying the oblique volcanic ridges has continued during their westward drift from the microplate as shown by the presence of relatively fresh lava observed on one of these structures, namely the first Oblique Volcanic Ridge near 25° S–118° W at about 160 km west of the Easter microplate West Rift. Based on a reconstruction of the Easter microplate, it is suggested that the Crough seamount (< 800 m depth) was formed by earlier (7–10 Ma) hotspot magmatic activity which also created Easter Island.

High rates of hydrothermal activity at the west rift of the Easter Microplate deduced from metalliferous sediment composition

Based on the mineralogy and composition of sediment cores from three locations along the East Pacific Rise, sediments from 12°58’ N are shown to contain about 34% metalliferous sediment (on a carbonate‐ and salt‐free basis) whereas those from 23°50’ S and 27°24’ S each contain about 73% metalliferous sediment. The latter two cores are situated near the West Rift of the Easter Microplate. The metalliferous contents of the sediments at 23°50’ S and 27°24’ S are among the highest recorded on the East Pacific Rise. The distribution of elements with depth in these cores indicates that the hydrothermal activity has lasted more than 100,000 years. These data suggest that the West Rift of the Easter Microplate is a highly prospective site for submarine hydrothermal minerals. This is in accord with the high rate of ocean spreading (15–16 cm/yr) and the complex processes of plate extension and rotation there.

NAUDUR explorers discover recent volcanic activity along the East Pacific Rise

Surveying an ultra‐fast spreading ridge along the East Pacific Rise (EPR), explorers aboard the submersible Nautile examined features such as lava pillows and tubes, sulfide chimneys, black smokers, hot shimmering waters, and colonies of animals living in hydrothermal vents to learn more about the processes of accretion and tectonics on the ocean floor. Taken together, the observations of the EPR between 17°S and 19°S from the 1993 NAUDUR cruise (a French acronym for Nautile on Ultra‐fast Ridge) indicate recent volcanic eruptions occurring as frequently as every few years.The NAUDUR cruise was designed to study the interaction between magmatic, tectonic, and hydrothermal processes at an ultra‐fast spreading axis of the EPR. Researchers performing twenty three dives in five regions (Figure 1) along the axis of the Garrett fracture zone collected more than 150 rock samples and made 52 gravity measurements [Auzende et al., 1994]. The Garrett fracture zone (13°S) and the Easter Microplate limit a large segment of the East Pacific Rise where the accretion rate is near the upper limit for present‐day spreading values (141 to 162mm/yr) [Perram et al., 1993]. The five dive regions with distinct morphological characteristics represent different stages in the accretion process.

Gravity study of the Pitcairn‐Easter Hotline

Shipboard free air gravity and bathymetric anomalies with an extension of 400 km were identified across the Pitcairn‐Easter hotline in the South Pacific. The anomalies are associated with one of the positive geoid undulations observed in the area from satellite data. Several smaller topographic features, volcano‐tectonic ridges oriented N65°E, are superimposed on the topographic high. Admittance computations and direct modeling show that the swell topography is compensated by a low density zone within the lithosphere, 4 to 8 km below the crust The volcano tectonic ridges are locally compensated in a classical Airy sense. The swell and the associated ridges were probably created by the action of a thermal anomaly resulting from the interaction of the Easter Island hotspot and of the Easter Microplate accretion centers.

Geology of young submarine volcanoes west of Easter Island, Southeast Pacific

Submarine volcanoes, probably related to the Easter hotspot, were found between the Easter Microplate and Easter Island near 26°15′S, 111°W. Hydrosweep swath mapping accompanied by deep-towed camera observations and sampling revealed the presence of fresh lava flows confined to two distinct fields, the previously-known Ahu and the newly discovered Umu Volcanic Fields, covering a combined area of more than 4300 km 2. These lava fields form constructional ridges with volcanic cones as well as isolated seamounts. Another large volcanic area, probably also belonging to the Easter hotspot, was found 100 km south of the Ahu/Umu region. Thus the young hotspot region has a diameter of about 200 km and its centre probably lies underneath the Umu Volcanic Field rather than under Ahu Volcanic Field. The fresh volcanics recovered from these areas are olivine and plagioclase phyric enriched and, more rarely, depleted tholeiites. The enriched lavas are comparable to E-MORB from the Easter Microplate spreading axes and volcanic ridges and seamounts west of the microplate.

Helium isotope ratios in Easter microplate basalts

3He/ 4He ratios in Easter Microplate basalt glasses show clear evidence of the effects of a mantle plume. The East Rift of the microplate between 26 and 28°S, identified by La/Sm, Sr and Pb isotopes and ridge crest elevation as the region of maximum plume influence, has 3He/ 4He ratios spanning the entire range from 7.5 to 11.7 R A. The Easter Microplate is the only section of the entire East Pacific Rise that is associated with a known ‘hotspot’ track (mantle plume) and has elevated 3He/ 4He ratios. Although most of the West Rift basalts contain MORB helium (8.0–8.7 R A), the basalt closest to the East Rift has an elevated 3He/ 4He ratio (11.3 R A), consistent with a significant plume component. The diversity in isotopic signatures also indicates that homogenization of isotopic anomalies does not occur, even in this region of ‘super-fast’ spreading. The overall 3He/ 4He/ 206Pb/ 204Pb and 3He/ 4He 87Sr/ 86Sr trends have positive correlations, although the high 3He/ 4He, 87Sr/ 86Sr and 206Pb/ 204Pb component is not a single value but rather a broad band. The apparent ‘decoupling’ between the He and Sr isotope distribution is modeled in the context of a plume source-migrating ridge sink. During channeling of the plume toward the ridge, helium is preferentially lost from the center of the channeled plume, resulting in lower He/Pb and He/Sr concentration ratios in the high 3He/ 4He component. Mixing trajectories in He Sr isotopic space between a LILE depleted asthenosphere and a variably degassed plume component provide a reasonably good fit to the data and may explain the isotope systematics of plume-ridge interactions in the context of modern theories of plume dynamics.

Magnetic and tectonic studies of the dueling propagating spreading centers at 20°40′S on the East Pacific Rise: Evidence for crustal rotations

We present the results of a magnetic study of a 225 km by 240 km area centered on the dueling propagating spreading centers located at 20°40′S on the East Pacific Rise. A majority of the data used were collected during a cruise aboard the R/V Moana Wave during which continuous SeaMARC II coverage was obtained. These data were combined with additional data to produce an anomaly map which extends to anomaly‐2‐aged crust. A three‐dimensional inversion in the presence of bathymetry was carried out for the area. The resulting magnetization distribution was interpreted and compared to side scan sonar and bathymetry data sets in order to determine the recent history of the discontinuity. The results indicate consistent asymmetric spreading faster to the east, discontinuous high magnetizations in the discordant zone associated with the discontinuity, and southward migration of the feature at a rate of 90–100 mm/yr between Jaramillo and Brunhes time (0.95 to 0.73 Ma) with slowing during the Brunhes to less than 10 mm/yr. There is also an overlapping Jaramillo isochron on the west flank and a gap in that isochron on the east flank indicating a transfer of crust during this time period from the Nazca to the Pacific plate. In addition, areas of oblique lineations possibly representing rotated crust were modelled using an inverse method which enables the specification of a nonuniform magnetization unit vector. Results from this second model support the presence of highly rotated pre‐Brunhes Nazca crust within Brunhes Pacific crust which has been deformed by bookshelf faulting. This indicates at least two episodes of crustal transfer from the Nazca plate to the Pacific plate. The discontinuity appears to mark the boundary between rigid plate tectonics to the north and deformation within the Nazca plate between the discontinuity and the Easter microplate to the south. The detailed history of the discontinuity involves dueling propagation with a great deal of variation in the amount of overlap of the two ridges as well as inward and outward cutting and abandonment of the tips of both ridges.

Edge‐driven microplate kinematics

It is known from plate tectonic reconstructions that oceanic microplates undergo rapid rotation about a vertical axis and that the instantaneous rotation axes describing the microplate's motion relative to the bounding major plates are frequently located close to its margins with those plates, close to the tips of propagating rifts. We propose a class of edge‐driven block models to illustrate how slip across the microplate margins, block rotation, and propagation of rifting may be related to the relative motion of the plates on either side. An important feature of these edge‐driven models is that the instantaneous rotation axes are always located on the margins between block and two bounding plates. According to those models the pseudofaults or traces of disrupted seafloor resulting from the propagation of rifting between microplate and major plates may be used independently to approximately trace the continuous kinematic evolution of the microplate back in time. Pseudofault geometries and matching rotations of the Easter microplate show that for most of its 5 m.y. history, block rotation could be driven by the drag of the Nazca and Pacific plates on the microplate's edges rather than by a shear flow of mantle underneath.

Intraplate thrusting near the Easter microplate

GLORIA long-range sidescan sonar images from a region of the Nazca plate immediately to the north of the Easter microplate have revealed a prominent series of anomalous ridges. These extend from lat 22°S, long 114°W, near the East Pacific Rise, to 23°S, 112°W, near Pito, deep at the northeast corner of the microplate. They are 20 to 50 km long and trend east-west, subparallel to the Pacific-Nazca spreading direction. Swath bathymetric data indicate that the ridges dip steeply to the south and gently to the north, and in places reliefs exceed 1 km. Earthquake focal mechanisms and predicted plate motions point to thrust faulting as the most likely mechanism for producing these ridges, and their morphology suggests that the crust to the north has over-ridden that to the south. The Nazca-Easter Euler pole predicts a maximum present-day convergence of about 50 km/m.y. at the western end of the Easter microplate9s northern boundary, and we estimate that about one-third of this has been taken up by the intraplate thrusts during the past 2.5 m.y.

GLORIA and other geophysical studies of the tectonic pattern and history of the Easter Microplate, southeast Pacific

Abstract In October–November 1988 the RRS Charles Darwin surveyed the Easter Microplate using the GLORIA (long-range sidescan) sonar system and also obtained magnetic, bathymetric and gravity data. The aim of the cruise was to produce a sidescan mosaic covering most of the microplate, in order that a greater understanding of the region could be achieved. Previous geophysical studies of the area had enabled most of the microplate boundaries to be determined to a reasonable extent, but still little was known about the interior of the microplate, or its tectonic history. The major objectives of the GLORIA survey were to image the whole microplate including its boundary, discover the type of plate boundary element throughout (e.g. fast or slow spreading, transform or compressional zone), calculate the spreading directions and rotation poles, and determine the complex history of the microplate from the tectonic spreading fabric, fracture zone and propagating rift traces, and magnetic anomaly data. The GLORIA survey imaged virtually all of the Easter microplate in just 18 days, and revealed unprecedented detail and complexity in its tectonic fabric and history. There is clear evidence of rapid microplate rotation, of rapidly evolving plate boundaries and of complex deformation near the northern and southern microplate boundaries. Examination of GLORIA and magnetic data to the west of the East Rift of the Easter Microplate indicate that spreading may have been taking place for over 6 Ma and the calculation of poles of rotation for different stages of the Easter Microplate evolution have enabled a simplified history to be reconstructed.

Three-dimensional SeaMARC II, gravity, and magnetics study of large-offset rift propagation at the Pito Rift, Easter microplate

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.

Tectonic evolution of the Easter Microplate

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.