International Ocean Discovery Program Preliminary Report
The five primary sites proposed for International Ocean Discovery Program (IODP) Expedition 395, ... more The five primary sites proposed for International Ocean Discovery Program (IODP) Expedition 395, which was postponed because of the COVID-19 pandemic, were cored during IODP Expedition 395C. The Expedition 395C operations, shipboard measurements, and sampling were adjusted to account for the absence of a sailing science party. The Expedition 395/395C objectives are (1) to investigate temporal variations in ocean crust generation at the Reykjanes Ridge and test hypotheses for the influence of Iceland mantle plume fluctuations on these processes, (2) to analyze sedimentation rates at the Björn and Gardar contourite drifts, as proxies for Cenozoic variations of North Atlantic deepwater circulation, and for uplift and subsidence of the Greenland-Scotland Ridge gateway related to plume activity, and (3) to analyze the alteration of oceanic crust and its interaction with seawater and sediments. During Expedition 395C, basalt cores were collected at four sites: U1554, U1555, U1562, and U1563. Sediment cores were also collected from these sites as well as from Site U1564, and casing was installed to 602 m at Site U1554. The amount of recovered cores, their preliminary descriptions, and the analyses of shipboard samples show that the results of Expedition 395C will fulfill a significant part of the Expedition 395 objectives. Basalts were collected from two V-shaped ridge and trough pairs, which will allow the investigation of the variability in mantle source and temperature causing this ridge/trough pattern. Basalt cores span an expected age range of 2.8-13.9 Ma, which will allow us to investigate the hydrothermal weathering processes. Sediments from the Björn drift were cored to basement, along with the uppermost 600 m of sediments from the Gardar drift. The data provided by Expedition 395C are a major advancement in achieving the work of Expedition 395.
The George V and Tasman Transform Fault Systems (TFS) are major, right-stepping offsets of the So... more The George V and Tasman Transform Fault Systems (TFS) are major, right-stepping offsets of the South-East Indian Ridge between 140°E and 148°E. The George V TFS (~140°E) has an offset of about 300 km, and the Tasman TFS (~148°E) an offset of about 600 km. These TFS have multiple shear zones with intra-transform ridge segments (ITRS), mostly unmapped yet. We present the results of the analysis of geophysical and petrological data collected during the STORM cruise (South Tasmania Ocean Ridge and Mantle), completed with global data sets including satellite-derived gravity and bathymetry, and earthquake distribution. The swath bathymetry data cover some parts of the shear zones and only a few of ITRSs. They reveal a complex interaction between tectonic processes at the plate boundary and near-axis volcanic activity along and across the transform faults. In both the George V and Tasman TFS the western ITRS are shallower than the eastern ones, and they appear to receive a lot more magma s...
We studied the east intersection between the Romanche transform fault (TF) and the Mid-Atlantic r... more We studied the east intersection between the Romanche transform fault (TF) and the Mid-Atlantic ridge using bathymetry and gravity anomalies, to investigate the temporal evolution of the ultra-cold ridge-transform intersection. Our results reveal a complex ridge axis, with evidence of a significant decrease in the along-axis melt supply towards the RTI but also since ~10 Ma.Over a 100 km distance south of the RTI, the ridge axis is formed by three spreading segments offset by large non-transform discontinuities. Large detachment faults mark the present-day spreading style at the RTI, while magma supply increases away from the Romanche intersection. Axial and near-axis fault patterns reveal a marked obliquity, especially in the north and center of the study area.In lithosphere older than 10 Ma, the ridge axis appears to form a single spreading segment between the Romanche and Chain TFs, perpendicular to the spreading direction, with relatively regular abyssal hills. From around 10 to...
At mid-oceanic ridges, mantle temperature and magma supply influence the structure of the neo-vol... more At mid-oceanic ridges, mantle temperature and magma supply influence the structure of the neo-volcanic zone. Due to the large Romanche offset, a strong “cold edge” effect is present at its eastern intersection with the Mid-Atlantic Ridge. This effect decreases with the distance from the transform fault, making this region an ideal area to study the impact of the thermal gradient on the architecture of the neo-volcanic zone. We analyzed seafloor videos and photos from submersible dives, as well as bathymetry and backscatter data collected during the SMARTIES cruise (2019), from the Ridge-Transform Intersection (RTI) to approximately 80 km to the south of it. We produced maps at local and regional scales and quantified the morphology of volcanoes (height, diameter, height/diameter ratio, volume and surface). Visual observations have showed that the seafloor is mainly made up of pillows or elongated pillows and rare massive lava flows. Within 30 km of the RTI, the neo-volcanic area is ...
We present data for glassy basalts from ~37 localities along the spreading axis of the Southeast ... more We present data for glassy basalts from ~37 localities along the spreading axis of the Southeast Indian Ridge (SEIR) between 126\ub0-140\ub0E, eastward of the Australian-Antarctic Discordance (AAD). Each of the five ridge segments (A1 to A5, west to east) show well-defined major element trends. An isotopic and negative axial depth anomaly is present, centered on the overlapping tips of segments A3 and A4 at ~135\ub0E. Segment A4 basalts have distinct radiogenic Pb and He isotopes plus enriched MORB-like \u3b5Hf, relative to segments to the west and east. Crystal fractionation is more extensive at the A3 and A5 overlapping segment tips adjacent to A4, and decreases both to the west and east. The along axis pattern suggests a mantle heterogeneity located beneath the A3-A4 segments. Pb-Pb isotopic co-variations for the 5 segments define two linear arrays, with a western trend (A1-A3) and an eastern trend (A4-A5) that intersects it at the composition of the anomalous A4 segment, at a 206Pb/204Pb ~ 19. The western trend has higher 208Pb/204Pb for a given 206Pb/204Pb, revealing a gradient in the asthenosphere, with 06208Pb/204Pb decreasing to the east away from the AAD. Overall, 206,207,208Pb/204Pb and 4He/3He of the A4 anomaly define trends that vector toward the fields for Cenozoic lavas from west Antarctica (Marie Byrd Land and Balleny Islands). West Antarctica has a history of mantle plume underplating and lithosphere modification by subduction [1,2], and there is a broad seismic anomaly below 250 km underlying the West Antarctic Rift system [3]. Our data supports a model in which flow of underplated material plus lithosphere may be guided by the underside topography of the lithosphere beneath the Transantarctic mountains. This flow emerges from beneath east Antarctica, where it leads to volcanism in the Balleny Islands [4]. The material apparently continues to flow northward to the SEIR at ~135\ub0E. The geochemical anomaly beneath Zone A is potentially explained by the presence of this residual plume/mobilized lithospheric material in the subridge mantle of the SEIR. [1] Hart et al., 1997, Chem Geol 139; [2] Aviado et al. 2015, G3 16; [3] Moelli and Danesi, 2004, GPC 42; [4] Sleep, 2006, ESR 77
Oceanic crust is formed by melt derived from the mantle at oceanic spreading centers. A small amo... more Oceanic crust is formed by melt derived from the mantle at oceanic spreading centers. A small amount of melting initiates at about 150-300 km depths in the presence of volatiles (CO2, H2O)1–3, but the extensive dry melting commences at 60-70 km depths due to the upwelling of the mantle as two diverging plates move apart4,5. However, how these melts migrate to the surface and what happens to these melts in the upper part of the mantle are still not understood. Using seismological data recorded by ocean-bottom seismometers, here we report the presence of deep earthquakes at 10-20 km depth in the hot mantle along the Mid-Atlantic Ridge axis, much below the brittle-ductile boundary, suggesting that these earthquakes are caused by a volume change associated with the CO2 degassing from the ascending melt. The geochemical analyses of basalts from the ridge axis show an abnormally high quantity of CO2 (>1.9 wt%) in the primitive melt, confirming the CO2 degassing hypothesis. The large co...
2015 IEEE/OES Acoustics in Underwater Geosciences Symposium (RIO Acoustics), 2015
The COLMEIA Project is an international collaboration between France and Brazil involving several... more The COLMEIA Project is an international collaboration between France and Brazil involving several universities and research institutes. One of the objectives of this project is to study the tectonic evolution and to understand the processes that result in the exhumation of the ultramafic body which composes the São Pedro e São Paulo archipelago, as wellasthe magmatism and tectonics of this part of the Mid-Atlantic Ridge, especially for the recent geologic time. The work is essential to understand the evolution of the Equatorial Atlantic Ocean. A fundamental part of thestudy targets the detailed analysis of sites that indicate recent off-axis deformation, since these faults could work as pathways for hydrothermal fluids, which is important to understand hydrothermalism in sediment environments. This paper presents the preliminary results of the analysis of the seismic data acquired during the COLMEIA cruise. The first step was to process the seismic lines in the north part of the area covered during the cruise, where the St. Peter peridotite ridge is located. The software used, Sispeed v5.5, was developed by the IFREMER and available for use at the laboratory. The seismic configuration on the COLMEIA cruise used 24 hydrophones and 2airguns. The value of 1500 m/s was assigned for the velocity of the sound wave on the water and sediments. After processing, interpretation was performed using a specific software called Kingdom 8.5. During the first part of the interpretation, the focus was to identify the most important seismic horizons, such as the basement and major seismic horizons inside the sediment infill. The seismic profiles showed a strongly rugged bathymetry. One can easily note that the area is full of geological depressions, representing fracture zones, forming small, elongated basins. These features, that during millions of years gathered sediments, most of them hemipelagic and volcanogenic, allow to infer the age of each sector of the study area and shows some deformation at their upper part. Some difficulties were faced, due to the strongly rugged bathymetry in the area, during the interpretation of the seismic profiles. Thus, some seismic attributes were used to find where is the true ocean floor. One method used was the envelope, since this seismic attribute represents the instantaneous energy of the signal and its magnitude is proportional to the reflection coefficient.
Logging data are measurements of physical properties of the formation surrounding a borehole, acq... more Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.
Images of the outside of hard rock whole-round sections were acquired using a linescan imager (Se... more Images of the outside of hard rock whole-round sections were acquired using a linescan imager (Section Half Imaging Logger [SHIL]) and a special holder that allows each 90 degree segment of the outer surface to be positioned properly. The images were taken at a resolution of 20 lines/mm (50 micropixels). JRSO staff take these quadrant images and compile them into a side-by-side \rollout\ photograph of the section. Composite images are available as both JPG and TIF image formats. Individual quadrant images are available as JPG images only through this report; contact the IODP-JRSO Data Librarian if quadrant TIF files (~160 MB) are needed.
Temporal fluctuations of magmatic processes during the last 800 kyr have been investigated for th... more Temporal fluctuations of magmatic processes during the last 800 kyr have been investigated for the slow spreading Central Indian Ridge. The fluctuations are recorded by variations in lava chemistry along a 40 km long profile across the ridge. The temporal relations of the basalts were accurately restored using magnetic microanomalies. We report on the occurrence of ancient lavas enriched in incompatible elements whereas on-axis samples are typical normal mid-ocean ridge basalts. The enriched lavas are symmetrically distributed on either side of the ridge, implying that enriched melts reached the seafloor at intervals of about 150-200 kyr. This periodicity is viewed as a characteristic time scale in the aggregation processes of the melts produced from a heterogeneous mantle source. Geochemical variations of zero-age mid-ocean Copyright 2010 by the American Geophysical Union 1 of 10 ridge basalts may primarily reflect such periodic processes rather than the spatial distribution of mantle heterogeneities.
International Ocean Discovery Program Preliminary Report, 2018
seismic reflection profiles, and can be correlated between the two sites using both seismic refle... more seismic reflection profiles, and can be correlated between the two sites using both seismic reflectors and biostratigraphy. Together with results from other holes previously drilled in the SCS, these samples will help to constrain changes in paleoceanographic conditions during the Miocene in this part of the SCS basin.
Journal of Geophysical Research: Solid Earth, 2015
Coring/logging data and physical property measurements from International Ocean Discovery Program... more Coring/logging data and physical property measurements from International Ocean Discovery Program Expedition 349 are integrated with, and correlated to, reflection seismic data to map seismic sequence boundaries and facies of the central basin and neighboring regions of the South China Sea. First-order sequence boundaries are interpreted, which are Oligocene/Miocene, middle Miocene/late Miocene, Miocene/Pliocene, and Pliocene/Pleistocene boundaries. A characteristic early Pleistocene strong reflector is also identified, which marks the top of extensive carbonate-rich deposition in the southern East and Southwest Subbasins. The fossil spreading ridge and the boundary between the East and Southwest Subbasins acted as major sedimentary barriers, across which seismic facies changes sharply and cannot be easily correlated. The sharp seismic facies change along the Miocene-Pliocene boundary indicates that a dramatic regional tectonostratigraphic event occurred at about 5 Ma, coeval with the onsets of uplift of Taiwan and accelerated subsidence and transgression in the northern margin. The depocenter or the area of the highest sedimentation rate switched from the northern East Subbasin during the Miocene to the Southwest Subbasin and the area close to the fossil ridge in the southern East Subbasin in the Pleistocene. The most active faulting and vertical uplifting now occur in the southern East Subbasin, caused most likely by the active and fastest subduction/obduction in the southern segment of the Manila Trench and the collision between the northeast Palawan and the Luzon arc. Timing of magmatic intrusions and seamounts constrained by seismic stratigraphy in the central basin varies and does not show temporal pulsing in their activities.
Mantle exhumation at slow-spreading ridges is favoured by extensional tectonics through low-angle... more Mantle exhumation at slow-spreading ridges is favoured by extensional tectonics through low-angle detachment faults 1-4 , and, along transforms, by transtension due to changes in ridge/transform geometry 5,6. Less common, exhumation by compressive stresses has been proposed for the large-o set transforms of the equatorial Atlantic 7,8. Here we show, using high-resolution bathymetry, seismic and gravity data, that the northern transform fault of the St Paul system has been controlled by compressive deformation since ∼10 million years ago. The long-lived transpression resulted from ridge overlap due to the propagation of the northern Mid-Atlantic Ridge segment into the transform domain, which induced the migration and segmentation of the transform fault creating restraining stepovers. An anticlockwise change in plate motion at ∼11 million years ago 5 initially favoured extension in the left-stepping transform, triggering the formation of a transverse ridge, later uplifted through transpression, forming the St Peter and St Paul islets. Enhanced melt supply at the ridge axis due to the nearby Sierra Leone thermo chemical anomaly 9 is responsible for the robust response of the northern Mid-Atlantic Ridge segment to the kinematic change. The long-lived process at the origin of the compressive stresses is directly linked to the nature of the underlying mantle and not to a change in the far-field stress regime. When Darwin stopped in the St Peter and St Paul islets in 1832, he recognized that the rocks exposed there were not volcanic and postulated that the mechanism for their formation was different from that building other oceanic islands 10. The islets, formed by variably serpentinized and mylonitized peridotites 11,12 , are currently uplifting at rates of 1.5 mm yr −1 (ref. 12). Previous work suggested that the exposure of such a large volume of ultramafic rocks resulted from an abnormally cold upper mantle 13-15 or cold lithosphere 16 in the equatorial Atlantic and that the islets were either part of an extensional flexural ridge, as observed in other transform faults 6,13 , or linked to compression 8. Our data reveal that the islets are part of a major uplift of the lithospheric mantle due to a 10-Myr-long period of transpression at the transform boundary 17. Similar push-ups exist along continental strike-slip faults 18,19 but this is the first time one has been fully mapped in the oceanic lithosphere near a presentday spreading centre. Understanding the processes responsible for its formation will shed new light on the behaviour of large transform boundaries and their response to changes in both local and far-field stresses as well as on mechanisms leading to mantle exhumation at spreading centres. Four transform faults and three intra-transform ridge segments, cumulating a 630 km offset, form the complex St Paul transform system (Fig. 1 and Supplementary Fig. 1). Tectonic patterns and lithology reveal that the St Peter and St Paul islets are part of a major, 200-km-long, 30-km-wide submarine shear zone that accommodates transpressive stresses along the northern transform fault of the St Paul system 17 (Fig. 1). Most dredged samples are breccias of mylonitic peridotite and ultramylonites that underwent various degrees of serpentinization and deformation (Supplementary Fig. 2). The morphology and the distribution of the deformation in the transform fault allow three tectonic domains in the shear zone to be defined: the western transfer zone (WTZ), the central transpressive zone (CTZ) and the eastern shear zone (ESZ) (Fig. 2). An important segmentation of the transform fault with associated push-up ridges resulting from dextral transpression characterizes the WTZ (Fig. 2a). Eastwards, a series of left-stepping restraining bends and offsets in the transform, associated with a large topography, form the distinct tectonic pattern of the CTZ. Thrust faults (Figs 1, 2b and 3) mark the base of its south flank, overlapping the crust formed at the north intra-transform segment. These thrust faults, imaged by seismic reflection data (Fig. 3), are associated with the exposure of mylonitized peridotites and deformed sediments at this crustal contact. They form positive flower structures resulting from transpression at the restraining offsets along the shear zone (Fig. 2b). The area is seismically very active and, while most of the events have strike-slip focal mechanisms, a few compressive events 20 near the islets confirm the presence of thrust faults. The ∼3,500-m-high Atobá Ridge, located at the largest offset of the transform fault, is a major push-up ridge
International Ocean Discovery Program Preliminary Report
The five primary sites proposed for International Ocean Discovery Program (IODP) Expedition 395, ... more The five primary sites proposed for International Ocean Discovery Program (IODP) Expedition 395, which was postponed because of the COVID-19 pandemic, were cored during IODP Expedition 395C. The Expedition 395C operations, shipboard measurements, and sampling were adjusted to account for the absence of a sailing science party. The Expedition 395/395C objectives are (1) to investigate temporal variations in ocean crust generation at the Reykjanes Ridge and test hypotheses for the influence of Iceland mantle plume fluctuations on these processes, (2) to analyze sedimentation rates at the Björn and Gardar contourite drifts, as proxies for Cenozoic variations of North Atlantic deepwater circulation, and for uplift and subsidence of the Greenland-Scotland Ridge gateway related to plume activity, and (3) to analyze the alteration of oceanic crust and its interaction with seawater and sediments. During Expedition 395C, basalt cores were collected at four sites: U1554, U1555, U1562, and U1563. Sediment cores were also collected from these sites as well as from Site U1564, and casing was installed to 602 m at Site U1554. The amount of recovered cores, their preliminary descriptions, and the analyses of shipboard samples show that the results of Expedition 395C will fulfill a significant part of the Expedition 395 objectives. Basalts were collected from two V-shaped ridge and trough pairs, which will allow the investigation of the variability in mantle source and temperature causing this ridge/trough pattern. Basalt cores span an expected age range of 2.8-13.9 Ma, which will allow us to investigate the hydrothermal weathering processes. Sediments from the Björn drift were cored to basement, along with the uppermost 600 m of sediments from the Gardar drift. The data provided by Expedition 395C are a major advancement in achieving the work of Expedition 395.
The George V and Tasman Transform Fault Systems (TFS) are major, right-stepping offsets of the So... more The George V and Tasman Transform Fault Systems (TFS) are major, right-stepping offsets of the South-East Indian Ridge between 140°E and 148°E. The George V TFS (~140°E) has an offset of about 300 km, and the Tasman TFS (~148°E) an offset of about 600 km. These TFS have multiple shear zones with intra-transform ridge segments (ITRS), mostly unmapped yet. We present the results of the analysis of geophysical and petrological data collected during the STORM cruise (South Tasmania Ocean Ridge and Mantle), completed with global data sets including satellite-derived gravity and bathymetry, and earthquake distribution. The swath bathymetry data cover some parts of the shear zones and only a few of ITRSs. They reveal a complex interaction between tectonic processes at the plate boundary and near-axis volcanic activity along and across the transform faults. In both the George V and Tasman TFS the western ITRS are shallower than the eastern ones, and they appear to receive a lot more magma s...
We studied the east intersection between the Romanche transform fault (TF) and the Mid-Atlantic r... more We studied the east intersection between the Romanche transform fault (TF) and the Mid-Atlantic ridge using bathymetry and gravity anomalies, to investigate the temporal evolution of the ultra-cold ridge-transform intersection. Our results reveal a complex ridge axis, with evidence of a significant decrease in the along-axis melt supply towards the RTI but also since ~10 Ma.Over a 100 km distance south of the RTI, the ridge axis is formed by three spreading segments offset by large non-transform discontinuities. Large detachment faults mark the present-day spreading style at the RTI, while magma supply increases away from the Romanche intersection. Axial and near-axis fault patterns reveal a marked obliquity, especially in the north and center of the study area.In lithosphere older than 10 Ma, the ridge axis appears to form a single spreading segment between the Romanche and Chain TFs, perpendicular to the spreading direction, with relatively regular abyssal hills. From around 10 to...
At mid-oceanic ridges, mantle temperature and magma supply influence the structure of the neo-vol... more At mid-oceanic ridges, mantle temperature and magma supply influence the structure of the neo-volcanic zone. Due to the large Romanche offset, a strong “cold edge” effect is present at its eastern intersection with the Mid-Atlantic Ridge. This effect decreases with the distance from the transform fault, making this region an ideal area to study the impact of the thermal gradient on the architecture of the neo-volcanic zone. We analyzed seafloor videos and photos from submersible dives, as well as bathymetry and backscatter data collected during the SMARTIES cruise (2019), from the Ridge-Transform Intersection (RTI) to approximately 80 km to the south of it. We produced maps at local and regional scales and quantified the morphology of volcanoes (height, diameter, height/diameter ratio, volume and surface). Visual observations have showed that the seafloor is mainly made up of pillows or elongated pillows and rare massive lava flows. Within 30 km of the RTI, the neo-volcanic area is ...
We present data for glassy basalts from ~37 localities along the spreading axis of the Southeast ... more We present data for glassy basalts from ~37 localities along the spreading axis of the Southeast Indian Ridge (SEIR) between 126\ub0-140\ub0E, eastward of the Australian-Antarctic Discordance (AAD). Each of the five ridge segments (A1 to A5, west to east) show well-defined major element trends. An isotopic and negative axial depth anomaly is present, centered on the overlapping tips of segments A3 and A4 at ~135\ub0E. Segment A4 basalts have distinct radiogenic Pb and He isotopes plus enriched MORB-like \u3b5Hf, relative to segments to the west and east. Crystal fractionation is more extensive at the A3 and A5 overlapping segment tips adjacent to A4, and decreases both to the west and east. The along axis pattern suggests a mantle heterogeneity located beneath the A3-A4 segments. Pb-Pb isotopic co-variations for the 5 segments define two linear arrays, with a western trend (A1-A3) and an eastern trend (A4-A5) that intersects it at the composition of the anomalous A4 segment, at a 206Pb/204Pb ~ 19. The western trend has higher 208Pb/204Pb for a given 206Pb/204Pb, revealing a gradient in the asthenosphere, with 06208Pb/204Pb decreasing to the east away from the AAD. Overall, 206,207,208Pb/204Pb and 4He/3He of the A4 anomaly define trends that vector toward the fields for Cenozoic lavas from west Antarctica (Marie Byrd Land and Balleny Islands). West Antarctica has a history of mantle plume underplating and lithosphere modification by subduction [1,2], and there is a broad seismic anomaly below 250 km underlying the West Antarctic Rift system [3]. Our data supports a model in which flow of underplated material plus lithosphere may be guided by the underside topography of the lithosphere beneath the Transantarctic mountains. This flow emerges from beneath east Antarctica, where it leads to volcanism in the Balleny Islands [4]. The material apparently continues to flow northward to the SEIR at ~135\ub0E. The geochemical anomaly beneath Zone A is potentially explained by the presence of this residual plume/mobilized lithospheric material in the subridge mantle of the SEIR. [1] Hart et al., 1997, Chem Geol 139; [2] Aviado et al. 2015, G3 16; [3] Moelli and Danesi, 2004, GPC 42; [4] Sleep, 2006, ESR 77
Oceanic crust is formed by melt derived from the mantle at oceanic spreading centers. A small amo... more Oceanic crust is formed by melt derived from the mantle at oceanic spreading centers. A small amount of melting initiates at about 150-300 km depths in the presence of volatiles (CO2, H2O)1–3, but the extensive dry melting commences at 60-70 km depths due to the upwelling of the mantle as two diverging plates move apart4,5. However, how these melts migrate to the surface and what happens to these melts in the upper part of the mantle are still not understood. Using seismological data recorded by ocean-bottom seismometers, here we report the presence of deep earthquakes at 10-20 km depth in the hot mantle along the Mid-Atlantic Ridge axis, much below the brittle-ductile boundary, suggesting that these earthquakes are caused by a volume change associated with the CO2 degassing from the ascending melt. The geochemical analyses of basalts from the ridge axis show an abnormally high quantity of CO2 (>1.9 wt%) in the primitive melt, confirming the CO2 degassing hypothesis. The large co...
2015 IEEE/OES Acoustics in Underwater Geosciences Symposium (RIO Acoustics), 2015
The COLMEIA Project is an international collaboration between France and Brazil involving several... more The COLMEIA Project is an international collaboration between France and Brazil involving several universities and research institutes. One of the objectives of this project is to study the tectonic evolution and to understand the processes that result in the exhumation of the ultramafic body which composes the São Pedro e São Paulo archipelago, as wellasthe magmatism and tectonics of this part of the Mid-Atlantic Ridge, especially for the recent geologic time. The work is essential to understand the evolution of the Equatorial Atlantic Ocean. A fundamental part of thestudy targets the detailed analysis of sites that indicate recent off-axis deformation, since these faults could work as pathways for hydrothermal fluids, which is important to understand hydrothermalism in sediment environments. This paper presents the preliminary results of the analysis of the seismic data acquired during the COLMEIA cruise. The first step was to process the seismic lines in the north part of the area covered during the cruise, where the St. Peter peridotite ridge is located. The software used, Sispeed v5.5, was developed by the IFREMER and available for use at the laboratory. The seismic configuration on the COLMEIA cruise used 24 hydrophones and 2airguns. The value of 1500 m/s was assigned for the velocity of the sound wave on the water and sediments. After processing, interpretation was performed using a specific software called Kingdom 8.5. During the first part of the interpretation, the focus was to identify the most important seismic horizons, such as the basement and major seismic horizons inside the sediment infill. The seismic profiles showed a strongly rugged bathymetry. One can easily note that the area is full of geological depressions, representing fracture zones, forming small, elongated basins. These features, that during millions of years gathered sediments, most of them hemipelagic and volcanogenic, allow to infer the age of each sector of the study area and shows some deformation at their upper part. Some difficulties were faced, due to the strongly rugged bathymetry in the area, during the interpretation of the seismic profiles. Thus, some seismic attributes were used to find where is the true ocean floor. One method used was the envelope, since this seismic attribute represents the instantaneous energy of the signal and its magnitude is proportional to the reflection coefficient.
Logging data are measurements of physical properties of the formation surrounding a borehole, acq... more Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.
Images of the outside of hard rock whole-round sections were acquired using a linescan imager (Se... more Images of the outside of hard rock whole-round sections were acquired using a linescan imager (Section Half Imaging Logger [SHIL]) and a special holder that allows each 90 degree segment of the outer surface to be positioned properly. The images were taken at a resolution of 20 lines/mm (50 micropixels). JRSO staff take these quadrant images and compile them into a side-by-side \rollout\ photograph of the section. Composite images are available as both JPG and TIF image formats. Individual quadrant images are available as JPG images only through this report; contact the IODP-JRSO Data Librarian if quadrant TIF files (~160 MB) are needed.
Temporal fluctuations of magmatic processes during the last 800 kyr have been investigated for th... more Temporal fluctuations of magmatic processes during the last 800 kyr have been investigated for the slow spreading Central Indian Ridge. The fluctuations are recorded by variations in lava chemistry along a 40 km long profile across the ridge. The temporal relations of the basalts were accurately restored using magnetic microanomalies. We report on the occurrence of ancient lavas enriched in incompatible elements whereas on-axis samples are typical normal mid-ocean ridge basalts. The enriched lavas are symmetrically distributed on either side of the ridge, implying that enriched melts reached the seafloor at intervals of about 150-200 kyr. This periodicity is viewed as a characteristic time scale in the aggregation processes of the melts produced from a heterogeneous mantle source. Geochemical variations of zero-age mid-ocean Copyright 2010 by the American Geophysical Union 1 of 10 ridge basalts may primarily reflect such periodic processes rather than the spatial distribution of mantle heterogeneities.
International Ocean Discovery Program Preliminary Report, 2018
seismic reflection profiles, and can be correlated between the two sites using both seismic refle... more seismic reflection profiles, and can be correlated between the two sites using both seismic reflectors and biostratigraphy. Together with results from other holes previously drilled in the SCS, these samples will help to constrain changes in paleoceanographic conditions during the Miocene in this part of the SCS basin.
Journal of Geophysical Research: Solid Earth, 2015
Coring/logging data and physical property measurements from International Ocean Discovery Program... more Coring/logging data and physical property measurements from International Ocean Discovery Program Expedition 349 are integrated with, and correlated to, reflection seismic data to map seismic sequence boundaries and facies of the central basin and neighboring regions of the South China Sea. First-order sequence boundaries are interpreted, which are Oligocene/Miocene, middle Miocene/late Miocene, Miocene/Pliocene, and Pliocene/Pleistocene boundaries. A characteristic early Pleistocene strong reflector is also identified, which marks the top of extensive carbonate-rich deposition in the southern East and Southwest Subbasins. The fossil spreading ridge and the boundary between the East and Southwest Subbasins acted as major sedimentary barriers, across which seismic facies changes sharply and cannot be easily correlated. The sharp seismic facies change along the Miocene-Pliocene boundary indicates that a dramatic regional tectonostratigraphic event occurred at about 5 Ma, coeval with the onsets of uplift of Taiwan and accelerated subsidence and transgression in the northern margin. The depocenter or the area of the highest sedimentation rate switched from the northern East Subbasin during the Miocene to the Southwest Subbasin and the area close to the fossil ridge in the southern East Subbasin in the Pleistocene. The most active faulting and vertical uplifting now occur in the southern East Subbasin, caused most likely by the active and fastest subduction/obduction in the southern segment of the Manila Trench and the collision between the northeast Palawan and the Luzon arc. Timing of magmatic intrusions and seamounts constrained by seismic stratigraphy in the central basin varies and does not show temporal pulsing in their activities.
Mantle exhumation at slow-spreading ridges is favoured by extensional tectonics through low-angle... more Mantle exhumation at slow-spreading ridges is favoured by extensional tectonics through low-angle detachment faults 1-4 , and, along transforms, by transtension due to changes in ridge/transform geometry 5,6. Less common, exhumation by compressive stresses has been proposed for the large-o set transforms of the equatorial Atlantic 7,8. Here we show, using high-resolution bathymetry, seismic and gravity data, that the northern transform fault of the St Paul system has been controlled by compressive deformation since ∼10 million years ago. The long-lived transpression resulted from ridge overlap due to the propagation of the northern Mid-Atlantic Ridge segment into the transform domain, which induced the migration and segmentation of the transform fault creating restraining stepovers. An anticlockwise change in plate motion at ∼11 million years ago 5 initially favoured extension in the left-stepping transform, triggering the formation of a transverse ridge, later uplifted through transpression, forming the St Peter and St Paul islets. Enhanced melt supply at the ridge axis due to the nearby Sierra Leone thermo chemical anomaly 9 is responsible for the robust response of the northern Mid-Atlantic Ridge segment to the kinematic change. The long-lived process at the origin of the compressive stresses is directly linked to the nature of the underlying mantle and not to a change in the far-field stress regime. When Darwin stopped in the St Peter and St Paul islets in 1832, he recognized that the rocks exposed there were not volcanic and postulated that the mechanism for their formation was different from that building other oceanic islands 10. The islets, formed by variably serpentinized and mylonitized peridotites 11,12 , are currently uplifting at rates of 1.5 mm yr −1 (ref. 12). Previous work suggested that the exposure of such a large volume of ultramafic rocks resulted from an abnormally cold upper mantle 13-15 or cold lithosphere 16 in the equatorial Atlantic and that the islets were either part of an extensional flexural ridge, as observed in other transform faults 6,13 , or linked to compression 8. Our data reveal that the islets are part of a major uplift of the lithospheric mantle due to a 10-Myr-long period of transpression at the transform boundary 17. Similar push-ups exist along continental strike-slip faults 18,19 but this is the first time one has been fully mapped in the oceanic lithosphere near a presentday spreading centre. Understanding the processes responsible for its formation will shed new light on the behaviour of large transform boundaries and their response to changes in both local and far-field stresses as well as on mechanisms leading to mantle exhumation at spreading centres. Four transform faults and three intra-transform ridge segments, cumulating a 630 km offset, form the complex St Paul transform system (Fig. 1 and Supplementary Fig. 1). Tectonic patterns and lithology reveal that the St Peter and St Paul islets are part of a major, 200-km-long, 30-km-wide submarine shear zone that accommodates transpressive stresses along the northern transform fault of the St Paul system 17 (Fig. 1). Most dredged samples are breccias of mylonitic peridotite and ultramylonites that underwent various degrees of serpentinization and deformation (Supplementary Fig. 2). The morphology and the distribution of the deformation in the transform fault allow three tectonic domains in the shear zone to be defined: the western transfer zone (WTZ), the central transpressive zone (CTZ) and the eastern shear zone (ESZ) (Fig. 2). An important segmentation of the transform fault with associated push-up ridges resulting from dextral transpression characterizes the WTZ (Fig. 2a). Eastwards, a series of left-stepping restraining bends and offsets in the transform, associated with a large topography, form the distinct tectonic pattern of the CTZ. Thrust faults (Figs 1, 2b and 3) mark the base of its south flank, overlapping the crust formed at the north intra-transform segment. These thrust faults, imaged by seismic reflection data (Fig. 3), are associated with the exposure of mylonitized peridotites and deformed sediments at this crustal contact. They form positive flower structures resulting from transpression at the restraining offsets along the shear zone (Fig. 2b). The area is seismically very active and, while most of the events have strike-slip focal mechanisms, a few compressive events 20 near the islets confirm the presence of thrust faults. The ∼3,500-m-high Atobá Ridge, located at the largest offset of the transform fault, is a major push-up ridge
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