A. A. Peyve

Diffuse impact of the Mid-Atlantic Ridge with the Romanche transform: an ultracold ridge-transform intersection

The Romanche is a long offset (∼950 km), slow slip (∼1.7 cm/yr) transform; thus a hot ridge axis should meet a ∼50-m.y.-old, thick and cold lithosphere at the ridge-transform intersection (RTI). A strong thermal/topographic “transform cold edge effect” is therefore predicted. A morphobathymetric, seismic reflection and petrologic study of the eastern Romanche RTI shows that as the Mid-Atlantic Ridge approaches the transform, a well-formed axial rift valley disappears about 80 km from the RTI and is substituted by short en echelon, poorly developed axial ridge segments; they too disappear about 30 km from the edge of the transform valley. The predicted gradual deepening of the ridge axis toward the transform was not observed. An active nodal deep and an “inside corner high” are also absent. These observations, and the distribution of earthquake epicenters, suggest a poorly developed, diffuse RTI. An inactive rift valley ∼80 km west of the present RTI suggests ridge jumping within the last ∼4 m.y. The present poorly developed RTI may reflect the attempts of an embryonic spreading axis to become established and to propagate toward the transform. We infer from bottom rock sampling that the basaltic crust is patchy or absent and mantle-derived serpentinized peridotites outcrop ubiquitously on the seafloor starting ∼30 km from the edge of the transform valley. The unusually deep (∼4 km below sea level) axial ridge segments, the lack of crust, and the chemistry of the peridotites suggest a prevalently amagmatic regime due to an ultracold upper mantle in this region. Absence of basaltic crust would favor massive serpentinization of a several kilometers thick peridotite column. Mass balance modeling suggests that the decrease of density and volume expansion resulting from serpentinization could explain the absence of the predicted deepening of the seafloor as it approaches the transform. These results suggest that the topographic effect of the transform edge thermal contrast may disappear at ultracold RTIs and that ultracold RTIs are magma starved, short lived, and unstable in time and space.

New data on the structure of the Vitoria-Trindade seamount chain (western Brazil basin, South Atlantic)

In 2008, during cruise 24 of the R/V Akademik Vavilov, much of our research work was focused on the central segment (Jaseur and Davis seamounts, Dogaressa Bank) of the Vitoria-Trindade seamount chain (west of the Brazil basin) extending along 20.5° S. Work was conducted to survey the upper part of the sedimentary cover and to perform subbottom profiling. The samples dredged on the seamount slopes are represented by volcanites and Fe-Mn crusts.

Tectonics and magmatism in eastern South America and the Brazil basin of the Atlantic in the Phanerozoic

The magmatic and tectonic activity of eastern South America and the western South Atlantic shows that extension of the continental crust is the determinant factor of magmatism. Heating of the upper mantle is a necessary condition of its manifestation. Ascending plume material is a source of additional heat. In the Early Mesozoic, Eastern Brazil was situated above a large, ascending and probably ramifying plume, which has supplied heat and material since the Triassic, creating favorable conditions for continental magmatism. Magmatic activity continued, gradually waning, until the Neogene as evidence for long-term retention of heat energy beneath the continental lithosphere after the plume ascent. It has been shown that heated mantle material can be displaced from the continent to the ocean for a significant distance beneath the lithosphere with the formation of linear tectonomagmatic rises of the oceanic crust. The structural elements inherited certain directions on the continent and in the ocean, beginning from the Neoproterozoic. These directions were reactivated and continued to control the younger structural grain and magmatic activity. In Southeastern Brazil, these were the structural units striking in the southeastern (about 120° SE) and northeastern directions parallel to the continent-ocean boundary. In Northeastern Brazil, the W-E- and N—S-trending structural units are predominant. All these directions are manifested in oceanic structural units (Rio Grande, Vitória-Trindadi, Fernando de Noronha, Pernambuco rises, etc.).

The structure of the Knipovich-Mohns junction (North Atlantic)

In 2007, the Geological Institute of the Russian Academy of Sciences (GIN RAS) carried out investigations in the North Atlantic, in the southern part of the Knipovich Ridge extending for 600 km from the Mohns spreading ridge to the Molloy fault zone (chief of the cruise A.V. Zaionchek). The investigations were conducted according to the Program of the RAS Presidium entitled “Basic Problems of Oceanology: World Ocean Physics, Geology, Biology, Ecology” (Project “Regularities of the Structure and Formation of the Oceanic Crust in Characteristic Regions of the Atlantic Ocean: Tectonics, Magmatism, Composition and Genesis of Fe‐Mn Deposits,” supervisor Academician Yu.M. Pushcharovskii). The problem facing the expedition was to study the geological structure of the Knipovich‐Mohns junction. With the help of the R/V Akademik Nikolaj Strakhov (Cruise 25), there were conducted complex areal, medium-scale, regional geological study of the selected object, which involved echo-sounding with SeaBat 7150 multibeam sounder, continuous seismic profiling (CSP), high-frequency sounding with the Edgetech 3300 profiler, and bottom dredging (Fig. 1). Within the region of 74 ° N, south of the Greenland Fault Zone (FZ) extending southeastward, the Mohns spreading ridge passes into the north‐south-trending Knipovich Ridge. The peculiarity of both ridges is that these are unified extensive spreading structures not broken into segments by transform faults. The ridges differ in the time and conditions of formation. From the beginning of formation, regular and steady growth of the oceanic crust in the rift zone was characteristic of the Mohns Ridge, which is marked by the symmetrical and natural position of linear magnetic anomalies relative to the rift valley axis [1, 2]. The Knipovich Ridge began forming under unsteady geodynamic conditions, which was reflected in the disordered position and fragmentation of magnetic anomalies. The region of the Mohns and Knipovich junction attracts the attention of researchers in that this is a unique area where one spreading ridge passes into another with rift valley structures gradually bending by 40 ° without apparent transform faults serving as accommodation zones for stresses generated in the course of plate motions. Hence, the geodynamics of structures in this key region has been the subject of investigations.

Seamounts in the east of South Atlantic: Origin and correlation with Mesozoic-Cenozoic magmatic structures of West Africa

The Mesozoic and Cenozoic seamounts and submarine ridges in the east of the South Atlantic are considered and compared with the coeval tectonomagmatic structures of West Africa. The conclusion is drawn that within-plate magmatism of the Atlantic is a waning process related to the ascent of several large plumes beneath West Africa beginning from the Triassic and subsequent lateral spreading of their material. It is shown that the heated plume material can spread beneath the lithosphere for a great distance, mixing in various proportions with asthenospheric matter, forming melts variable in geochemistry and isotopic characteristics. Cooling of the material takes many tens of years with retention of small magma sources episodically supplying melts to the surface. Localization of permeable zones in the lithosphere, along which the melts ascend, is determined by global stress fields responsible for the formation of long-lived linear tectonic elements on continents, inherited by young oceanic tectonic lines.

Alkali volcanism of the Bathymetrists Seamounts chain (Central Atlantic): Description and comparison

were analyzed for major and trace elements at the chemical laboratories of the Geological Institute (Moscow) and the Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences (table). All volcanic rocks dredged from Carter Seamount are strongly altered. The losses of ignition vary from 10 to 14%. In terms of major element composition, the volcanic rocks of group B1 are ascribed to olivine melilitites, while B2 are classed with nephelinites. Olivine melilitites contain 3‐5% clay pseudomorphs (0.5‐ 5 mm in size) after olivine phenocrysts. Microphenocrysts (up to 0.1‐0.3 mm) of clinopyroxene (augite and Ti-augite) and melilite account for 2‐3%. The matrix is made up of lathlike microlites of melilite (20‐30%) and clinopyroxene (augite) (20‐30%), as well as equant grains of Ti-magnetite (about 5%) embedded in a flaky aggregate of secondary minerals (possibly chlorite and zeolite). About 5‐8% is occupied by 0.5‐3 mm pores filled with various zeolites. Nephelinites contain about 2‐3% microphenocrysts of greenish egirine‐augite 0.2‐1 mm in size. The groundmass consists of microlites of egirine‐augite (15‐20%) and nepheline (20‐30%) and grains of Ti-magnetite (about 5%) set in a flaky matrix of secondary minerals. In the oxide‐MgO variation diagrams, the composi

Origin of Submarine Volcanism at the Eastern Margin of the Central Atlantic: Investigation of the Alkaline Volcanic Rocks of the Carter Seamount (Grimaldi Seamounts)

This paper addresses the composition, geochemistry, isotopic characteristics, and age of rocks from the Carter Seamount of the Grimaldi seamount group at the eastern margin of the Central Atlantic. The age of the seamount was estimated as 57–58 Ma. Together with other seamounts of the Grimaldi system and the Nadir Seamount, it forms a “hot line” related to the Guinea Fracture Zone, which was formed during the late Paleocene pulse of volcanism. The Carter Seamount is made up of olivine melilitites, ankaramites, and analcime-bearing nepheline tephrites, which are differentiated products of the fractional crystallization of melts similar to an alkaline ultramafic magma. The volcanics contain xenoliths entrained by melt at different depths from the mantle, layer 3 of the oceanic crust, which was formed at 113–115 Ma, and earlier magma chambers. The rocks were altered by low-temperature hydrothermal solutions. The parental melts of the volcanics of the Carter Seamount were derived at very low degrees of mantle melting in the stability field of garnet lherzolite at depths of no less than 105 km. Anomalously high Th, Nb, Ta, and La contents in the volcanics indicate that a metasomatized mantle reservoir contributed to the formation of their primary melts. The Sr, Pb, and Nd isotopic systematics of the rocks show that the composition of the mantle source lies on the mixing line between two mantle components. One of them is a mixture of prevailing HIMU and the depleted mantle, and the other is an enriched EM2-type mantle reservoir. These data suggest that the formation of the Carter Seamount volcanics was caused by extension-related decompression melting in the Guinea Fracture Zone of either (1) hot mantle plume material (HIMU component) affected by carbonate metasomatism or (2) carbonated basic enclaves (eclogites) ubiquitous in the asthenosphere, whose isotopic characteristics corresponded to the HIMU and EM2 components. In the former case, it is assumed that the melt assimilated during ascent the material of the metasomatized subcontinental mantle (EM2 component), which was incorporated into the oceanic lithospheric mantle during rifting and the breakup of Pangea.

New data on composition and structure of the Pernambuco Seamounts, Brazil basin, south Atlantic region

330 Research vessel Akademik Ioffe performed investi gation of the Pernambuco Seamounts in the Brazil basin in the profile of cruise 33 in 2011. The chain of seamounts is oriented in the northwestern direction and consists of individual roughly NS , roughly EW , NE , and NW trending segments. The two highest seamounts in the northern part of the chain were investigated. They characterize roughly NS and EW trending segments conjugated at a right angle. The structure of the upper part of the sedimentary cover was studied with a SES 2000 deep seismoacoustic profilograph, while the bottom relief was investigated with an ELAC echosounder. Judging from the mor phology of slopes, the seamounts are characterized by a three level structure reflecting a few stages of their formation. Formation of swells was followed by devel opment of volcanic edifices emergening above sea level. Extinction of volcanic activity and marine abra sion of volcanic summits were followed by growth of carbonate platforms. On the slopes and at the feet of paleovolcanoes, there are abundant landslide and coarse grained debris deposits overlapped by pelagic stratified sediments. The latter are occasionally bro ken by neotectonic deformations.

Central Atlantic Igneous Province: Origin and Mechanisms of Formation

The paper considers various aspects of the formation of the Central Atlantic Igneous Province. Generation and eruption of enormous bodies of basic magmas over a short time interval (a few million years) are explained by the accumulation of thermal energy beneath the continental lithosphere. The thermal energy was transferred by heated matter or fluid flows from the lower mantle along separate channels or permeable zones and accumulated beneath the continental lithosphere of Pangea over a long period of time (tens of millions of years) over a vast area. The large thickness ofthe lithosphere hindered melting. A change in the geodynamic regime with the onset of the breakdown of Pangea resulted in extension, formation of linear permeable zones, local decompression, and, as a consequence, in generation and ascent of huge magma bodies along extended linear tectonically weakened zones. The homogeneity of igneous rocks is explained by the short time interval favorable for magma generation, when all the stored thermal energy had been exhausted.

New data on the composition and age of rocks from the Bathymetrists Seamounts (Eastern margin of the equatorial Atlantic)

The petrology, geochemistry, and isotope ratios of volcanics dredged during the 43rd cruise of R/V Academik Ioffe on the Bathymetrists Seamounts in the eastern equatorial Atlantic have been studied. These are alkaline volcanics of basic and ultramafic compositions. Spider diagrams of the trace elements of volcanic rocks demonstrate strong fractionation, indicating formation of their primary melts from an enriched mantle source at garnet depth facies. Considering the isotope ratio values of 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, and 87Sr/86Sr and the character of their variations, the volcanic mantle source was chemically heterogeneous: for various volcanic rocks it was a mixture of the mantle components HIMU with EM–1 or EM–2. Limestones dredged together with the volcanics yielded microfossils suggesting a Middle Eocene age of their formation in a carbonate platform environment.