Marie-Hélène Cormier

East Pacific Rise 18°-19°S : asymmetric spreading and ridge reorientation by ultrafast migration of axial discontinuities

A detailed bathymetric, side scan, and magnetic survey of the East Pacific Rise out to a seafloor age of 1 Ma has been carried out between 18° and 19°S. It reveals that some left-stepping axial discontinuities have been migrating southward at rates an order of magnitude faster than the spreading rates (1000 mm/a or higher). These rapid migration events have left on the Nazca plate discordant features striking nearly parallel to the ridge axis. A discontinuity with an offset of several kilometers has migrated in two stages at around 0.45 and 0.3 Ma, and has left two large discordant zones consisting of a series of unfaulted, hummocky basins bounded to the east by short ridges oriented about N-S, oblique to the ambient 013° fabric. The morphology and reflectivity characteristics of these discordant zones are akin to the overlap basins and abandoned ridge tips which make up the migration trails of large, slowly-migrating overlapping spreading centers. Between 18°35′ and 19°03′S, the ridge axis is flanked a few kilometers to the east by a prominent, sedimented ridge previously recognized as a recently abandoned ridge axis. The present ridge segment steadily deepens and narrows southward, which suggests the abandoned ridge has been rafted onto the Nazca plate during the ultrafast southward propagation of the ridge segment rather than by one discrete ridge jump. By transferring Pacific lithosphere to the Nazca plate, these migration events account for most of the asymmetric accretion observed (faster to the east). This process is consistent with the features common to asymmetric spreading, namely the sudden onset or demise of asymmetric spreading, and the ridge segment to ridge segment variability. Because the discordant zones left by these rapid migration events are near-parallel to the ambient seafloor fabric, they are unlikely to be detected by conventional bathymetry or magnetic surveys, and so-called “ridge jumps” may actually often represent ultrafast propagation of a ridge segment. Variations in fault azimuth with age show there has not been any significant change in spreading direction over the past 0.8 m.y. Instead, the counterclockwise trend of the East Pacific Rise relative to the Brunhes/Matuyama reversal (0.78 Ma) mostly reflects that ultrafast propagation of ridge segments has transferred a larger amount of the Pacific lithosphere to the Nazca plate at 18°S than at 19°S. In keeping with the regional features of the magnetic anomalies, we propose that an 8 to 10 km left-stepping discontinuity which was located between 17° and 17°30′S at 0.78 Ma has been recently redistributed into the present staircase of small left-stepping discontinuities between 16° and 19°S. This smoothing of the ridge geometry probably occurred through repeated small lateral steps of the ridge segments inside of the discontinuities during ultra-fast propagation episodes, and may be the consequence of a significant replenishment of the magma reservoir between 17° and 17°30′S during the past 1 m.y.

A map series of the Southern East Pacific Rise and its flanks, 15° S to 19° S

Four large-scale bathymetric maps of the Southern East Pacific Rise and its flanks between 15° S and 19° S display many of the unique features of this superfast spreading environment including abundant seamounts (the Rano Rahi Field), axial discontinuities, discontinuity migration, and abyssal hill variation. Along with a summary of the regional geology, these maps will provide a valuable reference for other sea-going programs on-and off-axis in this area, including the Mantle ELectromagnetic and Tomography (MELT) experiment.

Formation of large summit troughs along the East Pacific Rise as collapse calderas: An evolutionary model

Summit troughs wider than 500 m and 30-110 m deep are present along 15-20% of the fast spreading East Pacific Rise (EPR). They occur only along ridge segments with large cross-sectional areas, indicative of a time-averaged robust magma supply. Where available, seismic data confirm that these troughs are underlain by an axial magma chamber (AMC) 1.2-1.6 km below the seafloor. Furthermore, detailed investigation of the large summit troughs which notch the EPR between 17°56 and 18°35S indicates that the vertical relief of the troughs tends to be maximum where the AMC is shallowest. Both of these observations are inconsistent with the predictions from a model in which large summit troughs form by rifting of the brittle upper crust during phases of amagmatic extension. Rather, we propose that they represent elongated collapsed calderas that form when the melt supply to formerly inflated AMCs wanes or nearly ceases. Because seismic studies constrain the melt lens along the EPR to be only 30-80 m thick, the proposed existence of collapsed calderas 40-110 m deep implies that the entire magma reservoir comprising the melt lens and the underlying crystal mush zone deforms and compacts during periods of waning magma supply. In particular, we suggest that a voluminous crystal mush zone will stretch in response to steady state seafloor spreading when the magma supply temporarily decreases. The resulting caldera will further widen with each subsequent dike intrusion. When an abundant melt supply finally resumes, the associated tumescence of the neovolcanic zone and profuse lava flows will combine to smooth out the caldera.

Evolution of the East Pacific Rise at 16°–19° S since 5 Ma: Bisection of overlapping spreading centers by new, rapidly propagating ridge segments

Nearly complete side-scan, bathymetry and magnetic coverage documents the evolution of the geometry of the East Pacific Rise (EPR) between 16° and 19° S since 5 Ma. Lineaments visible in SeaMARC II, H-MR1 and Sea Beam 2000 side-scan data correspond dominantly to normal fault scarps which have developed in the axial region perpendicular to the least compressive stress. Except near overlapping spreading centers (OSCs), the lineament orientations are taken to represent the perpendicular to the instantaneous Pacific-Nazca spreading direction. Their dominant orientation in the axial region is 012°, in good agreement with the prediction of the current model of relative plate motion (DeMets et al., 1994). However, the variations of the lineament azimuths with age show that there has been a small (3°–5°) clockwise change in the Nazca-Pacific relative motion since 5 Ma. There is also a distinct population of lineaments which strike counterclockwise to the ambient orientation. These discordant lineaments form somewhat coherent patterns on the seafloor and represent the past migration tracks of several left-stepping OSCs. Concurrent analysis of these discordant zones and the magnetic anomalies, reveals that up to 1 Ma, the EPR was offset by a few large, left-stepping OSCs. These OSCs were bisected into smaller OSCs by new spreading segments forming within their overlap basins. The smaller OSCs proceeded to migrate rapidly and were further bisected by newly spawned ridge segments until the present staircase of small, left-stepping OSCs was achieved. By transferring lithosphere from one plate to the other, these migration events account remarkably well for the variable spreading asymmetry in the area. Between 16° and 19° S, the present EPR is magmatically very “robust”, as evidenced by its inflated morphology, the profuse volcanic and hydrothermal activity observed from submerisbles and towed cameras, the geochemistry of axial basalts, and seismic and gravity data. Since 1 Ma, all the OSCs have migrated away from the shallowest, most robust section of the ridge between 17° and 17°30′ S, which was previously offset by a large OSC. We propose that the switch from a presumed starved magmatic regime typically associated with large OSCs to the presently robust magmatic regime occurred when the EPR overrode a melt anomaly during its westward migration relative to the asthenosphere. The resulting increase in melt supply at 17°–17°30′ S has fed the migration of axial discontinuities for this section of the southern EPR since 1 Ma.

A three-dimensional gravity analysis of the East Pacific Rise from 18° to 21°30'S

Multibeam bathymetry and gravity coverage of the East Pacific Rise (EPR) between 18 o and 21 o 30S is used to investigate the relation between melt supply and tectonic segmentation at ultrafast spreading rates. The long-wavelength features in the residual anomaly show a good correlation with those in the bathymetry. The highest residual anomaly values occur over the broad discordant zone of the 20 o 40S overlapping spreading center (OSC), for seafloor ages of 0 Ma to at least 1.5 Ma. We interpret the deepening of the bathymetry and the increase of the residual anomaly toward that discordant zone as due to a decrease of 500±200 m of the crustal thickness. Hence the 20 o 40S OSC has been associated with a reduced magmatic budget for at least the past 1.5 m.y. and represents a persistent segmentation of the EPR. This is consistent with models in which mantle upwelling, even at the fastest spreading centers, is enhanced between large discontinuities rather than evenly distributed along axis. However, this decrease of the crustal thickness toward an axial discontinuity is several times smaller than that typically documented for slow spreading ridges, which suggests that mantle upwelling is less focused at fast spreading ridges, or that along-axis transport of crustal material is more efficient, or both. Across the study area, the residual anomaly decreases toward the NW by 15-20 mGal. This regional gradient can be modeled with lateral temperature variations in the upper mantle of up to 60 o C, increasing toward the NW. This interpretation is consistent with the numerous seamounts present to the NW and the robust magmatic budget of the ridge between 17 o and 18 o S, and it could also explain why the ridge segments defined by the smaller OSCs between 18 o and 19 o S propagate very rapidly away from the robust area. Similar patterns of ridge propagation away from the shallowest section of a ridge have been documented near the Galapagos and Easter Island hot spots. Hence these shorter ridge segments may not be associated with significant individual melt sources. Rather, they may represent a superficial segmentation due to the interaction between the EPR and a mantle heterogeneity located between 17 o and 18 o S

Distribution of isolated volcanoes on the flanks of the East Pacific Rise, 15.3°S–20°S

Volcanic constructions, not associated with seamount (or volcano) chains, are abundant on the flanks of the East Pacific Rise (EPR) but are rare along the axial high. The distribution of isolated volcanoes, based on multibeam bathymetric maps, is approximately symmetric about the EPR axis. This symmetry contrasts with the asymmetries in the distribution of volcano chains (more abundant on the west flank), the seafloor subsidence rates (slower on the west flank), and the distribution of plate-motion-parallel gravity lineaments (more prominent on the west flank). Most of the isolated volcanoes complete their growth within ∼14 km of the axis on crust younger than 0.2 Ma, while seamount chain volcanoes continue to be active on older crust. Volcanic edifices within 6 km of the ridge axis are primarily found adjacent to axial discontinuities, suggesting a more sporadic magma supply and stronger lithosphere able to support volcanic constructions near axial discontinuities. The volume of isolated near-axis volcanoes correlates with ridge axis cross-sectional area, suggesting a link between the magma budget of the ridge and the eruption of near-axis volcanoes. Within the study area, off-axis volcanic edifices cover at least 6% of the seafloor and contribute more than 0.2% to the volume of the crust. The inferred width of the zone where isolated volcanoes initially form increases with spreading rate for the Mid-Atlantic Ridge (<4 km), northern EPR (<20 km), and southern EPR (<28 km), so that isolated volcanoes form primarily on lithosphere younger than 0.2 Ma (< 4-6 km brittle thickness), independent of spreading rate. This suggests some form of lithospheric control on the eruption of isolated off-axis volcanoes due to brittle thickness, increased normal stresses across cracks impeding dike injection, or thermal stresses within the newly forming lithosphere.

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.

Extensional faulting and caldera collapse in the axial region of fast spreading ridges : Analog modeling

The axial high of the East Pacific Rise (EPR) is bounded by ridge-parallel lateral grabens that develop 2-8 km off-axis. These troughs appear to lengthen away from the ridge crest, suggesting that tectonics is active at least 10 km away from the axis. Along 15-20% of the length of the ridge the axial high is notched by a summit trough 500 to 1800 m wide. These large axial troughs represent elongated collapse calderas that form when the EPR magma reservoir (comprising the melt lens and the underlying crystal mush zone) deforms and compacts during periods of waning magma supply under continuous stretching. We report the results of analog experiments performed in order to constrain the tectonic-magmatic evolution of the crestal region of fast spreading ridges and more particularly the possible link that may exist between the development of axial caldera and the creation of lateral grabens along the crestal region. We used inflatable elongated balloons filled with water as an analog of the magma reservoir. The balloon is capped with a silicone layer representing hot rocks below the brittle-ductile transition and is covered by a sand layer representing the brittle crust. The sand surface was given a dome shape that approximates the morphology of a fast spreading ridge. Mobile walls activated by a stepping motor allowed us to conduct the deflation experiments during continuous extension. Combination of deflation and extension leads to the creation of two lateral depressions and one axial trough. The lateral depressions are controlled by normal faults, while the central trough is delineated by reverse faults. During balloon deflation, tectonic extension is accommodated away from the axis due to the presence of the ductile silicone layer, whereas the deflation of the axial reservoir is accommodated by the collapse of the overlaying brittle crust. This accounts (1) for the development of axial troughs as collapse calderas, and (2) for the ubiquitous formation of lateral grabens of the flanking tectonic province in response to the deformation of the crystal mush.

Combined SeaBeam 2000 bathymetry and sidescan on the Melville: East Pacific Rise 15-19 S

Preliminary results are presented from an expedition to the ultra-fast spreading segment on the East Pacific Rise. The combined multibeam and sidescan sonar worked extremely well, and provided the authors with surprising discoveries of abundant off-axis volcanism. Individuals from several institutions have contributed to a collection of public domain software for processing and displaying SeaBeam 2000 data, much of which is being used to visualize the East Pacific Rise.<<ETX>>