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Maksymowicz, A, Chadwell CD, Ruiz J, Trehu AM, Contreras-Reyes E, Weinrebe W, Diaz-Naveas J, Gibson JC, Lonsdale P, Tryon MD.  2017.  Coseismic seafloor deformation in the trench region during the Mw8.8 Maule megathrust earthquake. Scientific Reports. 7   10.1038/srep45918   AbstractWebsite

The M-w 8.8 megathrust earthquake that occurred on 27 February 2010 offshore the Maule region of central Chile triggered a destructive tsunami. Whether the earthquake rupture extended to the shallow part of the plate boundary near the trench remains controversial. The up-dip limit of rupture during large subduction zone earthquakes has important implications for tsunami generation and for the rheological behavior of the sedimentary prism in accretionary margins. However, in general, the slip models derived from tsunami wave modeling and seismological data are poorly constrained by direct seafloor geodetic observations. We difference swath bathymetric data acquired across the trench in 2008, 2011 and 2012 and find similar to 3-5 m of uplift of the seafloor landward of the deformation front, at the eastern edge of the trench. Modeling suggests this is compatible with slip extending seaward, at least, to within similar to 6 km of the deformation front. After the M-w 9.0 Tohoku-oki earthquake, this result for the Maule earthquake represents only the second time that repeated bathymetric data has been used to detect the deformation following megathrust earthquakes, providing methodological guidelines for this relatively inexpensive way of obtaining seafloor geodetic data across subduction zone.

Plattner, C, Malservisi R, Amelung F, Dixon TH, Hackl M, Verdecchia A, Lonsdale P, Suarez-Vidal F, Gonzalez-Garcia J.  2015.  Space geodetic observation of the deformation cycle across the Ballenas Transform, Gulf of California. Journal of Geophysical Research-Solid Earth. 120:5843-5862.   10.1002/2015jb011959   AbstractWebsite

The Gulf of California, Mexico, accommodates similar to 90% of North America-Pacific plate relative motion. While most of this motion occurs on marine transform faults and spreading centers, several fault segments in the central Gulf come close to peninsular Baja California. Here we present Global Positioning System and interferometric synthetic aperture radar data near the Ballenas transform fault, separating the peninsula from Angel de la Guarda Island. We observe interseismic motion between June 2004 and May 2009 and displacements associated with the 3 August 2009 M-w 6.9 earthquake. From the interseismic data we estimate a locking depth of 9-12.5km and a slip rate of 44.9-48.1mm/yr, indicating that faults east of Angel de la Guarda deform at negligible rates and that the Ballenas Transform accommodates virtually all of the relative motion between the North American plate and the Baja California microplate. Our preferred model for coseismic slip on a finite rectangular fault plane suggests 1.3m of strike-slip displacement along a vertical rupture plane that is 60km long and extends from the surface to a depth of 13km in the eastern Ballenas Channel, striking parallel to Baja California-North America relative plate motion. These estimates agree with the seismic moment tensor and the location of the major foreshock and aftershocks and are compatible with the fault location identified from high-resolution bathymetric mapping. The geodetic moment is 33% higher than the seismic moment in part because some afterslip and viscous flow in the first month after the earthquake are included in the geodetic estimate. Coulomb stress changes for adjacent faults in the Gulf are consistent with the location of smaller aftershocks following the 2009 main shock and suggest potential triggering of the 12 April 2012 M-w 6.9 Guaymas earthquake.

Duque-Trujillo, J, Ferrari L, Orozco-Esquivel T, Lopez-Martinez M, Lonsdale P, Bryan SE, Kluesner J, Pinero-Lajas D, Solari L.  2015.  Timing of rifting in the southern Gulf of California and its conjugate margins: Insights from the plutonic record. Geological Society of America Bulletin. 127:702-736.   10.1130/b31008.1   AbstractWebsite

The Gulf of California is a young example of crustal stretching and transtensional shearing leading to the birth of a new oceanic basin at a formerly convergent margin. Previous studies focused along the southwestern rifted margin in Baja California indicated rifting was initiated after subduction and related magmatism ceased at ca. 14-12.5 Ma. However, the geologic record on the Mexico mainland (Sinaloa and Nayarit States) indicates crustal stretching in the region began as early as late Oligocene. The timing of cooling and exhumation of pre-and synrift plutonic rocks can provide constraints on the timing and rate of rifting. Here, we present results of a regional study on intrusive rocks in the southern Gulf of California sampled along the conjugate Baja California and Nayarit-Sinaloa rift margins, as well as plutonic rocks now exposed on submerged rifted blocks inside the gulf. Forty-one samples were dated via U/Pb zircon and Ar-40/Ar-39 mineral ages, providing emplacement age and thermochronological constraints on timing and rate of cooling. We found an extensive suite of early and middle Miocene plutons emplaced at shallow depths within the basement Cretaceous-Paleocene Peninsular Range and Sinaloa-Jalisco Batholiths. Early Miocene granitoids occur in an elongated WNW-ESE belt crossing the entire southern gulf from southern Baja California to Nayarit and Sinaloa. Most have an intermediate composition (<67 SiO2 wt%), but a distinctive group of high-silica granites (>75 SiO2 wt%) was emplaced 20.1-18.3 Ma, near the end of the early Miocene. Age span and chemical composition of the early Miocene silicic plutons essentially overlap ignimbrites and domes exposed in the southern Sierra Madre Occidental and in southern Baja California, suggesting that eruptive sources for the early Miocene ignimbrite flare-up may also have been located within the southern Gulf of California. Early Miocene plutons cooled below the Ar-40-Ar-39 biotite closure temperature (350-400 degrees C) in less than 2.5 m.y., which we interpret as evidence of a regional extensional event leading to the opening of the Gulf of California. A less widely distributed suite of intermediate-composition, middle Miocene granitoids (15-13 Ma) was sampled from the central-western part of the gulf, west of the Pescadero Basin, and these correspond to an episode of scarce volcanism recorded by the middle and upper members of the onshore Comondu Group in Baja California. Our widely spaced sampling of the generally sediment-covered igneous crust suggests that middle Miocene primary volcanic rocks are much less abundant than implied by previous models in which the gulf was the site of a robust Comondu arc. Thermobarometry data also indicate a very shallow depth (<5 km) of emplacement for the middle Miocene plutonic rocks. Some of these rocks also show a distinctive inequigranular texture indicative of at least two crystallization stages at different pressure. Early and middle Miocene granitoids away from the gulf axis yielded Ar-40-Ar-39 cooling ages very close to U-Pb zircon ages, demonstrating rapid cooling to <350 degrees C, which we attribute to their shallow emplacement and, possibly, to exhumation soon after intrusion. Since Comondu-age and middle Miocene magmatism in the gulf region coincided with rapid cooling of young plutons that predate the end of subduction, we suggest that intense crustal stretching controlled the pattern and timing of Comondu-age magmatism, rather than the middle Miocene magmatism controlling the locus of <12 Ma extension.

Vanderkluysen, L, Mahoney JJ, Koppers AAP, Beier C, Regelous M, Gee JS, Lonsdale PF.  2014.  Louisville Seamount Chain: Petrogenetic processes and geochemical evolution of the mantle source. Geochemistry Geophysics Geosystems. 15:2380-2400.   10.1002/2014gc005288   AbstractWebsite

The Louisville Seamount Chain is a similar to 4300 km long chain of submarine volcanoes in the southwestern Pacific that spans an age range comparable to that of the Hawaiian-Emperor chain and is commonly thought to represent a hot spot track. Dredging in 2006 recovered igneous rocks from 33 stations on 22 seamounts covering some 49 Myr of the chain's history. All samples are alkalic, similar to previous dredge and drill samples, providing no evidence for a Hawaiian-type tholeiitic shield-volcano stage. Major and trace element variations appear to be predominantly controlled by small but variable extents of fractional crystallization and by partial melting. Isotopic values define only a narrow range, in agreement with a surprising long-term source homogeneity-relative to the length scale of melting-and overlap with proposed fields for the "C" and "FOZO" mantle end-members. Trace element and isotope geochemistry is uncorrelated with either seamount age or lithospheric thickness at the time of volcanism, except for a small number of lavas from the westernmost Louisville Seamounts built on young (<20 Ma old) oceanic crust. The Louisville hot spot has been postulated to be the source of the similar to 120 Ma Ontong Java Plateau, but the Louisville isotopic signature cannot have evolved from a source with isotopic ratios like those measured for Ontong Java Plateau basalts. On the other hand, this signature can be correlated with that of samples dredged from the Danger Islands Troughs of the Manihiki Plateau, which has been interpreted as a rifted fragment of the "Greater" Ontong Java Plateau.

Kluesner, J, Lonsdale P, Gonzalez-Fernandez A.  2014.  Late Pleistocene cyclicity of sedimentation and spreading-center structure in the Central Gulf of California. Marine Geology. 347:58-68.   10.1016/j.margeo.2013.11.001   AbstractWebsite

The interaction between climatic-driven fluctuations in sediment supply and dynamic changes in basin physiography is a fundamental process of rift basin evolution that is poorly understood. A high-resolution seismic profile collected across the southern Guaymas Basin spreading center reveals how cyclical changes in sedimentation interact with on-going axial rifting and accretion of oceanic crust. The 4 km-wide axial rift valley abuts the steep, canyon incised continental slope of Baja California. Alternating acoustically transparent and well-layered high-amplitude seismic units imaged on the accreted flanks record the filling and regeneration of the axial rift over the past similar to 200 ka. An approximate correlation of seismic units to nearby Deep Sea Drilling Program (DSDP) Site 478 indicates that transparent units are composed primarily of diatomaceous silty mud turbidites, whereas the intervening well-layered high-amplitude units represent mud turbidites interlayed with terrigenous silty sand. Age estimates using sparse biostratigraphy and plate stratigraphy suggest deposition of transparent units coincident with interglacial/high-stands, while deposition of well-layered high-amplitude units corresponds to glacial/low-stands. Analysis of seismic stratigraphy and imaged faults reveals that during glacial/low-stands, deposition of terrigenous-rich turbidites out-paced axial spreading and subsidence, filling the axial rift and spilling over onto the adjacent basin floor. However, during interglacial/high-stands, reduction in sediment supply resulted in reestablishment of the axial rift relief. We propose that increases in sediment supply and terrigenous material during glacial periods were caused by a combination of (i) enhanced erosion of Baja California during pluvial periods, corresponding to northern hemisphere glacial maxima, (ii) increased delivery of sediment from the Sierra Madre Occidental, and (iii) sediment bypass on the continental shelf and slope to the deep-water basins during relative sea-level low-stands. (C) 2013 Elsevier B.V. All rights reserved.

Ferrari, L, Lopez-Martinez M, Orozco-Esquivel T, Bryan SE, Duque-Trujillo J, Lonsdale P, Solari L.  2013.  Late Oligocene to Middle Miocene rifting and synextensional magmatism in the southwestern Sierra Madre Occidental, Mexico: The beginning of the Gulf of California rift. Geosphere. 9:1161-1200.   10.1130/ges00925.1   AbstractWebsite

Although Basin and Range-style extension affected large areas of western Mexico after the Late Eocene, most consider that extension in the Gulf of California region began as subduction waned and ended ca. 14-12.5 Ma. A general consensus also exists in considering Early and Middle Miocene volcanism of the Sierra Madre Occidental and Comond Group as subduction related, whereas volcanism after ca. 12.5 Ma is extension related. Here we present a new regional geologic study of the eastern Gulf of California margin in the states of Nayarit and Sinaloa, Mexico, backed by 43 new Ar-Ar and U-Pb mineral ages, and geochemical data that document an earlier widespread phase of extension. This extension across the southern and central Gulf Extensional Province began between Late Oligocene and Early Miocene time, but was focused in the region of the future Gulf of California in the Middle Miocene. Late Oligocene to Early Miocene rocks across northern Nayarit and southern Sinaloa were affected by major approximately north-south-to north-northwest- striking normal faults prior to ca. 21 Ma. Between ca. 21 and 11 Ma, a system of north-northwest-south-southeast high-angle extensional faults continued extending the southwestern side of the Sierra Madre Occidental. Rhyolitic domes, shallow intrusive bodies, and lesser basalts were emplaced along this extensional belt at 20-17 Ma. Rhyolitic rocks, in particular the domes and lavas, often show strong antecrystic inheritance but only a few Mesozoic or older xenocrysts, suggesting silicic magma generation in the mid-upper crust triggered by an extension-induced basaltic influx. In northern Sinaloa, large grabens were occupied by huge volcanic dome complexes ca. 21-17 Ma and filled by continental sediments with interlayered basalts dated as 15-14 Ma, a stratigraphy and timing very similar to those found in central Sonora (northeastern Gulf of California margin). Early to Middle Miocene volcanism occurred thus in rift basins, and was likely associated with decompression melting of upper mantle (inducing crustal partial melting) rather than with fluxing by fluids from the young and slow subducting microplates. Along the eastern side of the Gulf of California coast, from Farallon de San Ignacio island offshore Los Mochis, Sinaloa, to San Blas, Nayarit, a strike distance of > 700 km, flat-lying basaltic lavas dated as ca. 11.5-10 Ma are exposed just above the present sea level. Here crustal thickness is almost half that in the unextended core of the adjacent Sierra Madre Occidental, implying significant lithosphere stretching before ca. 11 Ma. This mafic pulse, with subdued Nb-Ta negative spikes, may be related to the detachment of the lower part of the subducted slab, allowing an upward asthenospheric flow into an upper mantle previously modified by fluid fluxes related to past subduction. Widespread eruption of very uniform oceanic island basalt-like lavas occurred by the late Pliocene and Pleistocene, only 20 m.y. after the onset of rifting and similar to 9 m.y. after the end of subduction, implying that preexisting subduction-modified mantle had now become isolated from melt source regions. Our study shows that rifting across the southern- central Gulf Extensional Province began much earlier than the Late Miocene and provided a fundamental control on the style and composition of volcanism from at least 30 Ma. We envision a sustained period of lithospheric stretching and magmatism during which the pace and breadth of extension changed ca. 20-18 Ma to be narrower, and again after ca. 12. 5 Ma, when the kinematics of rifting became more oblique.

Sutherland, FH, Kent GM, Harding AJ, Umhoefer PJ, Driscoll NW, Lizarralde D, Fletcher JM, Axen GJ, Holbrook SW, Gonzalez-Fernandez A, Lonsdale P.  2012.  Middle Miocene to early Pliocene oblique extension in the southern Gulf of California. Geosphere. 8:752-770.   10.1130/ges00770.1   AbstractWebsite

A multichannel seismic (MCS) experiment spanning 600 km across the Alarcon Rise and its conjugate rifted margins in the southern Gulf of California (western North America) provides insight into the spatial and temporal evolution of extension between Baja California and the mainland (Mexico). Stratigraphic analysis of multiple rift basins within the Alarcon spreading corridor indicates an initial stage of oblique extension starting ca. 14-12 Ma. This initial phase of extension was characterized by the formation of several large basins in the center of the gulf and on the southeast margin with negligible synrift sedimentation. A second phase of oblique extension, likely synchronous with large-scale basin opening in the central and northern Gulf of California, began ca. 8-5 Ma and was characterized by the formation of smaller half-grabens distributed across the conjugate margins that contain both synrift and postrift deposits. A key feature imaged within the MCS data is a highly reflective, ropey layer at the top of basement, interpreted to be either volcanic rocks from the 25-12 Ma Comondu Group, and/or early rifting volcanic rocks that are between 11 and 9 Ma, or younger. This volcanic layer is extensively faulted, suggesting that it predates the episode of early extension. Upper crustal extension appears to be equally distributed across conjugate margins, forming a symmetrical continental rift. Two styles of rifted basin are observed; older basins (estimated as 14-11 Ma using sedimentation rates) show distributed extension with extensive basement faulting. In contrast, the younger basins (likely post-6 Ma) are asymmetrical with synrift deposits thickening into the basin-bounding faults. The northeast-southwest geomorphic expression of the Tamayo bank and trough and other features provides additional evidence that northwest-southeast oblique extension began ca. 12 Ma. These new spatial and temporal constraints, when combined with a crustal thickness profile obtained across the entire Alarcon corridor, suggest that significant northwest-southeast oblique extension within the Gulf of California started well before 6 Ma, in contrast to earlier models.

Tian, LY, Castillo PR, Lonsdale PF, Hahm D, Hilton DR.  2011.  Petrology and Sr-Nd-Pb-He isotope geochemistry of postspreading lavas on fossil spreading axes off Baja California Sur, Mexico. Geochemistry Geophysics Geosystems. 12   10.1029/2010gc003319   AbstractWebsite

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

Koppers, AAP, Gowen MD, Colwell LE, Gee JS, Lonsdale PF, Mahoney JJ, Duncan RA.  2011.  New Ar-40/Ar-39 age progression for the Louisville hot spot trail and implications for inter-hot spot motion. Geochemistry Geophysics Geosystems. 12   10.1029/2011gc003804   AbstractWebsite

In this study we present 42 new Ar-40/Ar-39 incremental heating age determinations that contribute to an updated age progression for the Louisville seamount trail. Louisville is the South Pacific counterpart to the Hawaiian-Emperor seamount trail, both trails representing intraplate volcanism over the same time interval (similar to 80 Ma to present) and being examples of primary hot spot lineaments. Our data provide evidence for an age-progressive trend from 71 to 21 Ma. Assuming fixed hot spots, this makes possible a direct comparison to the Hawaiian-Emperor age progression and the most recent absolute plate motion (APM) model (WK08G) of Wessel and Kroenke (2008). We observe that for the Louisville seamount trail the measured ages are systematically older relative to both the WK08G model predictions and Hawaiian seamount ages, with offsets ranging up to 6 Myr. Taking into account the uncertainty about the duration of eruption and magmatic succession at individual Louisville volcanoes, these age offsets should be considered minimum estimates, as our sampling probably tended to recover the youngest lava flows. These large deviations point to either a contribution of inter-hot spot motion between the Louisville and Hawaiian hot spots or to a more easterly location of the Louisville hot spot than the one inferred in the WK08G model. Both scenarios are investigated in this paper, whereby the more eastern hot spot location (52.0 degrees S, 134.5 degrees W versus 52.4 degrees S, 137.2 degrees W) reduces the average age offset, but still results in a relatively large maximum offset of 3.7 Myr. When comparing the new ages to the APM models (S04P, S04G) by Steinberger et al. (2004) that attempt to compensate for the motion of hot spots in the Pacific (Hawaii) or globally (Hawaii, Louisville, Reunion and Walvis), the measured and predicted ages are more in agreement, showing only a maximum offset of 2.3 Myr with respect to the S04G model. At face value these more advanced APM models, which consider both plate and hot spot motions, therefore provide a better fit to the new Louisville age data. The fit is particularly good for seamounts younger than 50 Ma, a period for which there is little predicted motion for the Louisville hot spot and little inter-hot spot motion with Hawaii. However, discrepancies in the Louisville age-distance record prior to 50 Ma indicate there is an extra source of inter-hot spot motion between Louisville and the other Pacific hot spots that was not corrected for in the global S04G model. Finally, based on six new Ar-40/Ar-39 age dates, the 169 W bend in the Louisville seamount trail seems to have formed at least 3 Myr before the formation of the Hawaiian-Emperor bend. The timing of the most acute parts of both bends thus appears to be asynchronous, which would require other processes (e. g., plume motions) than a global plate motion change between 50 and 47 Ma to explain these two observations.

Castillo, PR, Clague DA, Davis AS, Lonsdale PF.  2010.  Petrogenesis of Davidson Seamount lavas and its implications for fossil spreading center and intraplate magmatism in the eastern Pacific. Geochemistry Geophysics Geosystems. 11   10.1029/2009gc002992   AbstractWebsite

Seafloor spreading causes abundant magmatism along active ocean spreading centers, but the cause of magmatism along fossil spreading centers is enigmatic. Samples collected from Davidson Seamount, a typical volcanic ridge along an abandoned spreading center in the eastern Pacific, consist of an alkalic basalt to trachyte lava series; transitional basalts were sampled from another part of the abandoned axis, 20 km from the seamount. All samples experienced complex fractional crystallization prior to eruption, but they all share a common, compositionally heterogeneous mantle source. The parental magmas of the transitional basalts were produced from this source at higher degree of melting than those of the alkalic lava series. The composition of Davidson lavas overlaps with those of ridges along other fossil spreading centers and isolated near-and off-ridge seamounts in the eastern Pacific. Together they define a compositional continuum ranging from tholeiitic, normal mid-ocean ridge basalt (MORB)-like to alkalic, ocean island basalt (OIB)-like, similar to lavas that form linear island chains and ridges. We propose that this entire compositional spectrum of intraplate lavas that do not form linear volcanic chains in the eastern Pacific results from variations in the degree of partial melting of a common, compositionally heterogeneous mantle source. This source consists of more easily melted, geochemically enriched components of varying sizes and amounts embedded in a depleted lherzolitic matrix. Large degree of partial melting produces normal MORB-like melts represented by some near-ridge seamount lavas, whereas small degree of melting produces OIB-like fossil spreading center lavas. The small degree of partial melting beneath recently abandoned spreading centers results from either buoyancy-driven decompression melting of the hot lithospheric and asthenospheric mantle material beneath active spreading centers or rapid motion, with respect to the underlying asthenosphere, of abandoned spreading axes that are thickening over a fertile mantle. Mid-Tertiary volcanic rocks in coastal California, which are compositionally akin to intraplate lavas, are interpreted to be small degree partial melts of the same compositionally heterogeneous sub-Pacific mantle that has upwelled through windows in a subducted slab.

Castillo, PR, Lonsdale PF, Moran CL, Hawkins JW.  2009.  Geochemistry of mid-Cretaceous Pacific crust being subducted along the Tonga-Kermadec Trench: Implications for the generation of arc lavas. Lithos. 112:87-102.   10.1016/j.lithos.2009.03.041   AbstractWebsite

The Pacific Plate is currently being subducted at a rapid rate beneath the Indo-Australian Plate along the Tonga-Kermadec Trench in the southwestern Pacific. It has long been assumed that the lithosphere being subducted is relatively old and homogeneous in composition. Basaltic lavas dredged from the upper crust of the incoming lithosphere along the length of the trench are mid- to late-Cretaceous in age. Although the samples are mainly N-MORB, they range from tholeiitic to alkalic basalts. Concentrations of incompatible trace elements show a high degree of variability (e.g., Ba = 8 to 270 ppm, Rb = 0.4 to 39 ppm, Sr = 60 to 625 ppm, and Nd = 2.3 to 25 ppm). Neodymium, Sr and Pb isotopic data also show wide ranges (epsilon(Nd)(T) = 5.7-11.2; (87)Sr/(86)Sr(i) = 0.70224-0.70311; (206)Pb/(204)Pb(i) = 17.84-20.22). More importantly, the basaltic crust being subducted displays a latitudinal compositional variation that is similar to that shown by the Tonga-Kermadec arc lavas. Previous studies have proposed that the variably depleted sub-arc mantle, which was preconditioned through a backarc melt extraction or displacement process, is mainly responsible for the latitudinal variation in the Tonga-Kermadec arc lavas. However, our new results suggest a greater role of the lithospheric input into the source of arc lavas. The three end-member possibilities linking the latitudinal variation of the lithospheric input to the source of arc lava output are: (I) the mantle wedge beneath the volcanic arc on the west side of the trench, the main source of Tonga-Kermadec arc lavas, is a western extension of the Cretaceous Pacific upper mantle east of the trench; (2) the altered oceanic crust melts and the resultant slab melt modifies the mantle source of arc lavas; and (3) fluids dehydrated from the altered oceanic crust effectively transfer the compositional signature of the subducted slab into the mantle source of arc lavas. (C) 2009 Elsevier B.V. All rights reserved.

Clague, DA, Paduan JB, Duncan RA, Huard JJ, Davis AS, Castillo PR, Lonsdale P, DeVogelaere A.  2009.  Five million years of compositionally diverse, episodic volcanism: Construction of Davidson Seamount atop an abandoned spreading center. Geochemistry Geophysics Geosystems. 10   10.1029/2009gc002665   AbstractWebsite

Davidson Seamount, a volcano located about 80 km off the central California coast, has a volume of similar to 320 km(3) and consists of a series of parallel ridges serrated with steep cones. Davidson was sampled and its morphology observed during 27 ROV Tiburon dives. During those dives, 286 samples of lava, volcaniclastite, and erratics from the continental margin were collected, with additional samples from one ROV-collected push core and four gravity cores. We report glass compositions for 99 samples and (40)Ar-(39)Ar incremental heating age data for 20 of the samples. The glass analyses are of hawaiite (62%), mugearite (13%), alkalic basalt (9%), and tephrite (8%), with minor transitional basalt (2%), benmoreite (2%), and trachyandesite (2%). The lithologies are irregularly distributed in space and time. The volcano erupted onto crust inferred to be 20 Ma from seafloor magnetic anomalies. Ages of the lavas range from 9.8 to 14.8 Ma. The oldest rocks are from the central ridge, and the youngest are from the flanks and southern end of the edifice. The compositions of the 18 reliably dated volcanic cones vary with age such that the oldest lavas are the most fractionated. The melts lost 65% to nearly 95% of their initial S because of bubble loss during vesiculation, and the shallowest samples have S contents similar to lava erupted subaerially in Hawaii. Despite this similarity in S contents, there is scant other evidence to suggest that Davidson was ever an island. The numerous small cones of disparate chemistry and the long eruptive period suggest episodic growth of the volcano over at least 5 Myr and perhaps as long as 10 Myr if it began to grow when the spreading ridge was abandoned.

Lonsdale, P.  2005.  Creation of the Cocos and Nazca plates by fission of the Farallon plate. Tectonophysics. 404:237-264.   10.1016/j.tecto.2005.05.011   AbstractWebsite

Throughout the Early Tertiary the area of the Farallon oceanic plate was episodically diminished by detachment of large and small northern regions, which became independently moving plates and microplates. The nature and history of Farallon plate fragmentation has been inferred mainly from structural patterns on the western, Pacific-plate flank of the East Pacific Rise, because the fragmented eastern flank has been subducted. The final episode of plate fragmentation occurred at the beginning of the Miocene, when the Cocos plate was split off, leaving the much reduced Farallon plate to be renamed the Nazca plate, and initiating Cocos-Nazca spreading. Some Oligocene Farallon plate with rifted margins that are a direct record of this plate-splitting event has survived in the eastern tropical Pacific, most extensively off northern Peru and Ecuador. Small remnants of the conjugate northern rifted margin are exposed off Costa Rica, and perhaps south of Panama. Marine geophysical profiles (bathymetric, magnetic and seismic reflection) and multibeam sonar swaths across these rifted oceanic margins, combined with surveys of 30-20 Ma crust on the western rise-flank, indicate that (i) Localized lithospheric rupture to create a new plate boundary was preceded by plate stretching and fracturing in a belt several hundred kin wide. Fissural volcanism along some of these fractures built volcanic ridges (e.g., Alvarado and Sarmiento Ridges) that are 1-2 km high and parallel to "absolute" Farallon plate motion; they closely resemble fissural ridges described from the young western flank of the present Pacific-Nazca rise. (ii) For 1-2 m.y. prior to final rupture of the Farallon plate, perhaps coinciding with the period of lithospheric stretching, the entire plate changed direction to a more easterly ("Nazca-like") course; after the split the northern (Cocos) part reverted to a northeasterly absolute motion. (iii) The plate-splitting fracture that became the site of initial Cocos-Nazca spreading was a linear feature that at least through the 680 kin of ruptured Oligocene lithosphere known to have avoided subduction, did not follow any pre-existing feature on the Farallon plate, e.g., a "fracture zone' trail of a transform fault. (iv) The margins of surviving parts of the plate-splitting fracture have narrow shoulders raised by uplift of unloaded footwalls, and partially buried by fissural volcanism. (v) Cocos-Nazca spreading began at 23 Ma; reports of older Cocos-Nazca crust in the eastern Panama Basin were based on misidentified magnetic anomalies. There is increased evidence that the driving force for the 23 Ma fission of the Farallon plate was the divergence of slab-pull stresses at the Middle America and South America subduction zones. The timing and location of the split may have been influenced by (i) the increasingly divergent northeast slab pull at the Middle America subduction zone, which lengthened and reoriented because of motion between the North America and Caribbean plates; (ii) the slightly earlier detachment of a northern part of the plate that had been entering the California subduction zone, contributing a less divergent plate-driving stress; and (iii) weakening of older parts of the plate by the Galapagos hotspot, which had come to underlie the equatorial region, midway between the risecrest and the two subduction zones, by the Late Oligocene. (c) 2005 Elsevier B.V. All rights reserved.

Mofield, HO, Massell Symons C, Lonsdale P, Gonzalez FI, Titov VV.  2004.  Tsunami scattering and earthquake faults in the deep Pacific ocean. Oceanography. 17:38-46.   10.5670/oceanog.2004.65   Abstract

Tectonic processes in the deep ocean occur over an immerse range of temporal and spatial scales. The shortest in time are from seconds to minutes during which earthquakes and landslides occur. The recurrence interval between earthquakes, submarine landslides, and volcanic eruptions may be tens, hundreds, or thousands of years. At the far end of the range are the tens of millions of years over which oceanic plates form at the spreading centers and drift, sometimes thousands of kilometers with speeds of a few centimeters per year, to the subduction zones. It is also over these long time scales that chains of volcanic islands and seamounts form and over even longer time scales that major asteroids may impact the Earth. These processes create a host of smaller-scale geological features, including rift valleys, earthquake fault scarps, submarine landslide deposits, and abyssal hills, which cover vast areas of the ocean floor. Using a wide variety of methods, scientists are developing a better understanding of these geological features and the processes that create them.

Eakins, BW, Lonsdale PF.  2003.  Structural patterns and tectonic history of the Bauer microplate, Eastern Tropical Pacific. Marine Geophysical Researches. 24:171-205.   10.1007/s11001-004-5882-4   AbstractWebsite

The Bauer microplate was an independent slab of oceanic lithosphere that from 17 Ma to 6 Ma grew from 1.4 x 10(5) km(2) to 1.2 x 10(6) km(2) between the rapidly diverging Pacific and Nazca plates. Growth was by accretion at the lengthening and overlapping axes of the (Bauer-Nazca) Galapagos Rise (GR) and the (Pacific-Bauer) East Pacific Rise (EPR). EPR and GR axial propagation to create and rapidly grow the counter-clockwise spinning microplate occurred in two phases: ( 1) 17 - 15 Ma, when the EPR axis propagated north and the GR axis propagated south around a narrow (100- to 200-km-wide) core of older lithosphere; and ( 2) 8 - 6 Ma, when rapid northward propagation of the EPR axis resumed, overlapping similar to 400 km of the fast-spreading Pacific-Nazca rise-crest and appending a large (200- to 400-km-wide) area of the west flank of that rise as a 'northern annex' to the microplate. Between 15 and 8 Ma the microplate grew principally by crustal accretion at the crest of its rises. The microplate was captured by the Nazca plate and the Galapagos Rise axis became extinct soon after 6 Ma, when the south end of the Pacific-Bauer EPR axis became aligned with the southern Pacific-Nazca EPR axis and its north end was linked by the Quebrada Transform to the northern Pacific-Nazca EPR axis. Incomplete multibeam bathymetry of the microplate margins, and of both flanks of the Pacific-Bauer and Bauer-Nazca Rises, together with archival magnetic and satellite altimetry data, clarifies the growth and (counter-clockwise) rotation of the microplate, and tests tectonic models derived from studies of the still active, much smaller, Easter and Juan Fernandez microplates. Our interpretations differ from model predictions in that Euler poles were not located on the microplate boundary, propagation in the 15 - 8 Ma phase of growth was not toward these poles, and microplate rotation rates were small (5 degrees/m.y.) for much of its history, when long, bounding transform faults reduced coupling to Nazca plate motion. Some structures of the Bauer microplate boundary, such as deep rift valleys and a broad zone of thrust-faulted lithosphere, are, however, similar to those observed around the smaller, active microplates. Analysis of how the Bauer microplate was captured when coupling to the Pacific plate was reduced invites speculation on why risecrest microplates eventually lose their independence.

Castillo, PR, Hawkins JW, Lonsdale PF, Hilton DR, Shaw AM, Glascock MD.  2002.  Petrology of Alarcon Rise lavas, Gulf of California: Nascent intracontinental ocean crust. Journal of Geophysical Research-Solid Earth. 107:2222.   10.1029/2001gl000666   AbstractWebsite

[1] The Alarcon Basin in the southern Gulf of California originated by intracontinental rifting of the southwestern margin of North America and has evolved into an intercontinental ocean basin by 3.7 m.y. of seafloor spreading. Lava samples collected on the axial ridge along Alarcon Rise, two of the many near-axis seamounts on the northwest flank of the rise, and an abyssal hill on 0.6 Ma crust on the southeast flank were examined petrographically and analyzed for major and trace element and He-Sr-Nd-Pb isotope composition. Most samples are typical mid-ocean ridge basalt (MORB), but rocks at the axial ridge site closest to continent are more differentiated. Rocks from the axial ridge and abyssal hill are genetically related through separate episodes of fractional crystallization. Seamount samples are relatively more primitive than seafloor samples and may have been formed by smaller degree of melting of the same mantle source as the seafloor lavas. Tamayo transform fault has no apparent effect on the geochemistry of axial ridge lavas erupted close to it; this is contrary to the postulated transform fault effect. Alarcon Basin differs from the Salton Trough and Guaymas Basin in that its youngest seafloor shows no evidence for contamination with continental crust or a subduction component. We propose that Pacific MORB source mantle underlies the Gulf of California because the western North American margin has overridden the Pacific suboceanic mantle. Miocene rifting and Pliocene spreading in Alarcon Basin have created a window allowing the most recent MORB melts to rise uncontaminated to the surface.

Mackiernan, G, Lonsdale P, Shany N, Cooper B, Ginsburg P.  2001.  Observations of seabirds in Peruvian and Chilean waters during the 1998 El Nino. Cotinga. 15:88-94. AbstractWebsite
Fisher, AT, Giambalvo E, Sclater J, Kastner M, Ransom B, Weinstein Y, Lonsdale P.  2001.  Heat flow, sediment and pore fluid chemistry, and hydrothermal circulation on the east flank of Alarcon Ridge, Gulf of California. Earth and Planetary Science Letters. 188:521-534.   10.1016/s0012-821x(01)00310-7   AbstractWebsite

New seismic, heat flow, sediment and pore fluid geochemistry data from the east flank of Alarcon Ridge. at the mouth of the Gulf of California, provide evidence for vigorous hydrothermal circulation within young oceanic crust formed at a moderate-rate spreading center. Data and samples were collected 9-20 km from the ridge axis to assess the hydrologic state of 0.30-0.65 Ma seafloor. Conductive heat now values are 15-55% of that input at the base of the lithosphere. Heat flow is highest near the center of a sediment-covered trough, and lowest along the trough margins, suggesting that trough-bounding faults and basement exposures may help to focus hydrothermal recharge. Sediment and pore fluid geochemistry data, in combination with reactive transport modeling. indicate that conditions within the shallow sediments are dominantly diffusive and reactive in two locations, but that bottom seawater recharges through the thin sediment layer with velocities on the order of 2-10 mm/yr at other sites. Seafloor heat now appears to be entirely conductive, which is consistent with the slow rate of seepage inferred from pore fluid observations and modeling. Fluid recharge through sediments requires that basement is underpressured relative to hydrostatic conditions. We interpret these observations and inferences to indicate vigorous fluid flow in basement. and secondary seepage through overlying sediments. The heat flow deficit along the 11-km Alarcon Basin transect averages 440 mW/m(2), equivalent to heat output of 5 MW per kilometer of spreading axis. This heat output is similar to the combined focused and diffusive heat output of a single basement outcrop on the east flank of Juan de Fuca Ridge. and suggests that sites of concentrated heat and fluid outflow may exist on the east flank of Alarcon Ridge. (C) 2001 Published by Elsevier Science B.V.

Castillo, PR, Natland JH, Niu YL, Lonsdale PF.  1998.  Sr, Nd and Pb isotopic variation along the Pacific-Antarctic risecrest, 53-57°S: Implications for the composition and dynamics of the South Pacific upper mantle. Earth and Planetary Science Letters. 154:109-125.   10.1016/s0012-821x(97)00172-6   AbstractWebsite

Sr, Nd and Pb isotope data for basalts from spreading axes and off-axis volcanoes near the Pacific-Antarctic risecrest, from Vacquier transform to just south of Udintsev transform, reveal an isotopically heterogeneous upper mantle. The isotopic composition of the mantle is represented by three end-members: (1) the 'depleted' source of the bulk of Pacific normal-type mid-ocean ridge basalts (N-MORB); (2) an 'enriched' source that produces basalts of the Hollister Ridge; and (3) a source, restricted to two adjacent sample locales, similar to that of Indian MORE. The distribution of these isotopic heterogeneities along the Pacific-Antarctic risecrest suggests two alternative hypotheses on the nature and dynamics of the south Pacific upper mantle. The whole area could be a single N-MORB mantle domain that shows a weak but continuous increase in Nd-143/Nd-144 from northeast to southwest across more than 2000 km of sea floor. The gradient is unrelated to the Louisville hotspot because Louisville basalts have low Nd-143/Nd-144 and the hotspot's influence along the ridge is spatially limited and near the high Nd-143/Nd-144 southwestern end of the gradient. The gradient appears consistent with a southwestward flow of the Pacific N-MORB-type mantle that has been proposed mainly on the basis of ridge morphology. That the N-MORB mantle domain is continuous across Heezen suggests that large-scale magmatic segmentation is not Sr-87/Sr-86, Delta Nd and Delta 8/4 of samples related to the largest structural offsets of the Pacific ridges. Alternatively, the higher from southwest of the Heezen transform relative to those from the northeast could result from southwestward pumping of both plume and Indian Ocean-type mantle material by the Louisville hotspot. The Heezen transform forms a prominent tectonic and mantle domain boundary that prohibits the Louisville-and Indian Ocean-type mantle from flowing towards and contaminating the depleted Pacific-type source in the northeast. (C) 1998 Elsevier Science B.V.

Chen, YJ, Enriquez KD, Lonsdale P.  1996.  Does the mid-ocean ridge propagate concurrently both on the seafloor and at depth? Implications from a gravity study of a large nontransform offset at 36.5°S, East Pacific Rise Journal of Geophysical Research-Solid Earth. 101:28281-28289.   10.1029/96jb02723   AbstractWebsite

Analyses of gravity data collected at a large nontransform offset (NTO) at 36.5 degrees S on the East Pacific Rise indicate that the southward ridge propagation there does not occur concurrently both on the seafloor and at depth. A large negative axial mantle Bouguer anomaly commonly observed at the East Pacific Rise does not extend from the north into the overlapping zone following the ridge axis observed in seafloor bathymetry data. We speculate that the southward ridge propagation at this offset is probably an episodic process: it starts with rapid rise-crest (dikes) propagation on the seafloor and is followed by slow propagation of the melt supply sources associated with the mantle upwelling. The gravity ata collected at the 36.5 degrees S NTO provide new insight on the tectonic evolution of a large propagating nontransform offset and also provide important constraints on ridge-NTO models.

Lonsdale, P.  1995.  Segmentation and disruption of the East Pacific Rise in the mouth of the Gulf of California. Marine Geophysical Researches. 17:323-359.   10.1007/bf01227039   AbstractWebsite

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

Cervenka, P, Demoustier C, Lonsdale PF.  1994.  Geometric corrections on sidescan sonar images based on bathymetry - application with Seamarc II and Sea Beam data. Marine Geophysical Researches. 16:365-383.   10.1007/bf01203973   AbstractWebsite

Acoustic backscatter images of the seafloor obtained with sidescan sonar systems are displayed most often using a flat bottom assumption. Whenever this assumption is not valid, pixels are mapped incorrectly in the image frame, yielding distorted representations of the seafloor. Here, such distortions are corrected by using an appropriate representation of the relief, as measured by the sonar that collected the acoustic backscatter information. In addition, all spatial filtering operations required in the pixel relocation process take the sonar geometry into account. Examples of the process are provided by data collected in the Northeastern Pacific over Fieberling Guyot with the SeaMARC II bathymetric sidescan sonar system and the Sea Beam multibeam echo-sounder. The nearly complete (90%) Sea Beam bathymetry coverage of the Guyot serves as a reference to quantify the distortions found in the backscatter images and to evaluate the accuracy of the corrections performed with SeaMARC II bathymetry. As a byproduct, the processed SeaMARC II bathymetry and the Sea Beam bathymetry adapted to the SeaMARC II sonar geometry exhibit a 35m mean-square difference over the entire area surveyed.

Lonsdale, P.  1994.  Geomorphology and structural segmentation of the crest of the southern (Pacific-Antarctic) East Pacific Rise. Journal of Geophysical Research-Solid Earth. 99:4683-4702.   10.1029/93jb02756   AbstractWebsite

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

Lonsdale, P.  1994.  Structural geomorphology of the Eltanin fault system and adjacent transform faults of the Pacific-Antarctic plate boundary. Marine Geophysical Researches. 16:105-143.   10.1007/bf01224756   AbstractWebsite

Seabeam and SeaMARC II swath-mapping surveys, with ancillary magnetic and gravity profiling, describe the fast-slipping (84-86 mm year-1) 380 km-long Heezen transform (56-degrees-S) and the 145 km-long Raitt transform (54-degrees-S), together with the youngest parts of their rise-flank fracture zones. Archived seismicity, satellite altimetry, and older geophysical traverses extend these descriptions, constrain the structural interpretations, and allow preliminary interpretations of the adjacent Tharp, Hollister, and Udintsev transforms. At Heezen transform, Pacific-Antarctic plate motion is partitioned between the principal strike-slip fault zone in a deep transform valley and a marginal zone of rifting 30-40 km north of the transform axis, where a zone of secondary Riedel shearing evolved into a belt of crustal extension following a Pliocene change in relative plate motion. Crustal extension and lithospheric rupture along this belt has opened rift valleys, allowed the eruption of high volcanic ridges, and suppressed uplift of a transverse ridge along the north side of the transform valley. The south side has a high transverse ridge that is probably a flexural response to the mass deficiency of the valley; it subsides and vanishes along the eastern part of the valley, which has been infilled with recent volcanism. At the eastern risecrest intersection is another uplift of old lithosphere, an intersection high raised by transfer of heat from a curved and transform-parallel overshot ridge that prolongs the axial ridge of the East Pacific Rise (EPR). Tharp transform appears to be a mirror-image of Heezen transform, but with less evidence of volcanism at the marginal rifting site. Raitt transform responded differently to the Pliocene change in plate motion: a single strike-slip zone was replaced-with an en echelon pair of newly oriented faults, connected by a 10 km-long mid-Raitt spreading axis which has accreted rough, obliquely lineated crust. Transverse ridges have been raised along both sides of the transform, probably in response to the mass deficiency of the strip of mid-Raitt crust and to heating at the mid-Raitt axis. The intersections of Raitt transform with the EPR crest lack long overshot ridges, but periodically have tall, narrow intersection highs probably raised mainly by intrusion across the transform into old lithosphere. Udintsev transform adjusted to the change in slip direction by segmenting like Raitt transform, but the mid-Udintsev spreading axis grew within a widened transtensional transform valley bordered on both sides by high transverse ridges. Volcanism at the intersections with the rifted crest of the Pacific-Antarctic Ridge does not infill and close off the transform valley, so the Udintsev transverse ridges extend past the intersections to become part of the rise-flank fracture zones. At faster separating parts of the Pacific-Antarctic boundary, and on most of the rest of the EPR, fracture zone structure is mainly inherited from the variable arrangement of volcanic ridges and tectonic uplifts at the risecrest intersections, rather than from structures formed at the transform valley.

Hekinian, R, Bideau D, Francheteau J, Cheminee JL, Armijo R, Lonsdale P, Blum N.  1993.  Petrology of the East Pacific Rise crust and upper mantle exposed in Hess Deep (eastern equatorial Pacific). Journal of Geophysical Research-Solid Earth. 98:8069-8094.   10.1029/92jb02072   AbstractWebsite

The Hess Deep, a rifted oval-shaped depression located east of the Galapagos Triple Junction at the tip of the Cocos-Nazca ridge (about 101-degrees-W, 2-degrees-N), was explored in 1988 during 21 submersible dives. A total of 11 dives were devoted to the exploration of the E-W trending Intrarift ridge (15 km in length, 3000-5400 m in depth) north of the Hess Deep depression. The Intrarift ridge represents, an outcrop of recent (1 m.y.) crustal and subcrustal material created at the axis of the East Pacific Rise (EPR), and emplaced during the lithospheric extention responsible for the westward propagation of the Cocos-Nazca rift (Francheteau et al., 1990). The lithospheric block has undergone cataclastic deformation and was dislocated by tectonic activity as attested to by the mixed and erratic distribution of rock types and by the occurrence of polygenic breccias and gabbroic mylonites. The samples are metamorphosed to varying degrees, but their protolith textures are generally well preserved. Their relic mineralogy indicates that they consist of harzburgites, dunites, gabbroic cumulates (gabbronorites and olivine gabbros), isotropic gabbros, dolerites, and basalts. Some samples of refractory harzburgites and most dunitic cumulates (with local accumulation of chromite) have been impregnated by wehrlitic and gabbroic primitive melts similar to those described from the mantle-crust transition zone of the Samail ophiolite complex (Oman). The mineral chemistry indicate that the ultramafics partly reequilibrated with the magmatic impregnations in the liquidus-solidus temperature range of 980-1100-degrees-C. The dolerites and basalts have been derived from mid-ocean ridge basalt primary melts presenting a broad range of incompatible element composition which suggests intermittent cycles of magmatic processes involving a progressive melting of a composite source with discontinous extraction of liquids as proposed for the EPR volcanics near 13-degrees-N (Hekinian et al., 1989). Most of the rocks underwent partial retrograde metamorphism and recorded several episodes of recrystallization from the upper greenschist facies (ultramafics and gabbros) to diagenetic alteration (volcanics). The cumulate gabbronorites, the isotropic gabbros, and some dolerites were partially albitized and amphibolitized during the penetration of seawater in the ocean crust prior to serpentinization. Several samples of unfoliated amphibolites are believed to be completely metamorphosed gabbroic rocks. The gabbroic cumulates and the plagioclase-rich melt impregnations were variably rodingitized (presence of various Ca-silicates such as epidote, prehnite, hydrogarnet, and zeolite) in relation to the serpentinization of the peridotites. One dive located on the scarps forming the northern wall of the Hess Deep to the east of the explored area, revealed the presence of in situ outcrops of isotropic gabbros, doleritic dikes, and extrusives and permitted to observe the contact between the sheeted dike complex and the high level isotropic gabbros.