Papers by Alexander Peyve
Journal of Geophysical Research, 1999
The South American, African, and Antarctic lithospheric plates meet in the Bouvet Triple Junction... more The South American, African, and Antarctic lithospheric plates meet in the Bouvet Triple Junction (BTJ) located in the South Atlantic near the island of Bouvet. Multibeam, magnetic, gravimetric, and seismic reflection data have been used to understand the evolution of the three accretionary/transform boundaries that converge in the BTJ. The easternmost segments of the American-Antarctic Ridge (AAR) have a spreading full rate of 19.5 mm/yr for the last 8 m.y. They are oriented N-S, except for some NE-SW segments, probably created by magma-poor extension. The southernmost portion of Mid-Atlantic Ridge (MAR) (spreading full rate of 30.5 mm/yr for the last 9 m.y.) has elevated topographic anomalies; it is segmented by transform and overlapping discontinuities and shows evidence of axial propagation. The MAR axial valley bifurcates at its southern tip in two branches oriented 115 ø and 180 ø that are, or have been up to recently, loci of crustal accretion. The bifurcation represents a former ridge-ridge-ridge (RRR) triple junction. The westernmost segments of the Southwest Indian Ridge (SWIR) are anomalously high. The segment adjacent to the island of Bouvet (spreading rate 14.5 mm/yr) is shallower than normal by almost 1 km due to the influence of the Bouvet hot spot. The westernmost SWIR segment (Spiess Ridge) consists of a "swollen" volcanic ridge that reaches 320 rn below sea level and has a deep caldera on its summit. Spiess Ridge narrows and deepens to the NW; V-shaped topographic and magnetic lineations suggest that it propagates NW at a rate of 40 to 50 mm/yr. The Spiess magmatic event started at roughly 1 Ma, when it caused deactivation of the 115 ø spreading branch. Therefore the Antarctic, South American, and African plates meet presently not in a triple point but in a broad zone of diffuse deformation. An area of extensional deformation observed east of Spiess Ridge may be caused by excess crustal formation at Spiess Ridge that cannot be accommodated by motion of rigid plates. The evolution of the BTJ since 10 Ma involves stages of RRR, RFF and RRF configurations with highly variable geometry of the accretionary/transform boundaries. Topographic anomalies, anomalously thick crust and excess volcanism suggest that the upper mantle below this region is affected by widespread, strong thermal anomalies that have influenced the configuration of the BTJ, and determined indirectly intraplate deformation in wide areas of the BTJ region. The thermal anomaly that gave rise to the SWIR-Spiess excess magmatism is the prime cause of the recent disruption of a former RRR configuration, and of the imminent establishment of a new RRR Triple Junction to the north.
Geology, 2001
Dredged glass from the southern Mid-Atlantic Ridge near the Bouvet Triple Junction has unique maj... more Dredged glass from the southern Mid-Atlantic Ridge near the Bouvet Triple Junction has unique major element, trace element, and isotopic composition, distinct from typical mid-ocean ridge basalts. It is a high-Mg (Mg# 67.8), high-Ni (NiO 290 ppm) andesite depleted in highly incompatible and heavy rare-earth elements with an isotopic signature of ancient continental lithosphere (i.e., low 206 Pb/ 204 Pb and 143 Nd/ 144 Nd and high 87 Sr/ 86 Sr and ␦ 18 O). The origin of this glass is attributed to melting of a Precambrian garnetbearing, mafic lithology, possibly related to lower crustal blocks stranded in the upper mantle during breakup of Gondwana and opening of the Atlantic. This composition can be used to explain anomalous geochemical features of oceanic rocks in the Southern Hemisphere
Origin of submarine volcanism at the eastern margin of the central atlantic: Investigation of the alkaline volcanic rocks of the carter seamount (Grimaldi Seamounts)
Petrology, 2012
ABSTRACT This paper addresses the composition, geochemistry, isotopic characteristics, and age of... more ABSTRACT 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.
Russian Journal of Earth Sciences, 2003
We have studied major and trace element (including REE) geochemistries of basalts and chilled bas... more We have studied major and trace element (including REE) geochemistries of basalts and chilled basaltic glasses from the MAR axial zone in the vicinity of the Sierra Leone FZ (5 • -7 • 10 N). The links of basalts of various compositions with particular oceanfloor geological structural features have been analyzed as well. Three basaltic varieties have been discriminated. Almost ubiquitous are high-Mg basalts that are derivatives of N-MORB tholeiitic melts and that are produced in the axial zone of spreading. Variety 2 is alkaline basalts widespread on the southwest flank of the MAR crestal zone in the Sierra Leone region, likely generated through deep mantle melting under plume impact. Variety 3 is basalts derivative from T-and P-MORB-like tholeiitic melts and originating through addition of a deeper mantle material to depleted upper mantle melts. Magma generation parameters, as calculated from chilled glass compositions, are different for depleted tholeiites (44-55 km, 1320-1370 • C) and enriched tholeiites (45-78 km, 1330-1450 • C). Mantle plume impact is shown to affect not only tholeiitic basalt compositions but also magma generation conditions in the axial spreading zone, resulting in higher Ti and Na concentrations in the melts parental to the rift-related basalts occurring near the plume. T-and P-MORBs are also developed near the areas where mantle plumes are localized. The high-Mg basalts are shown to come in several types with distinctive Ti and Na contents. Nearly every single MAR segment (bounded by sinistral strike slips and the Bogdanov FZ) is featured by its own basalt type suggesting that it has formed above an asthenospheric diapir with its unique magma generation conditions. These conditions are time variable. The likely causes of the temporal and spatial instability of mantle upwelling beneath this portion of the MAR are singular tectonic processes and plume activity. In the sulfide-bearing rift morphostructures (the so-called "Ore area" and the Markov Basin), basalts make up highly evolved suites generated through olivine and plagioclase fractionation, which is suggestive of relatively long-lived magma chambers beneath the sulfide-bearing rift morphostructures. The functioning of these chambers is a combined effect of a singular geodynamic regime and plume activity. In these chambers, melts undergo deep differentiation leading to progressively increasing concentration of the sulfide phase, eventually to be supplied to the hydrothermal plumbing system.
Geotectonics, 2011
The Mesozoic and Cenozoic seamounts and submarine ridges in the east of the South Atlantic are co... more 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 charac teristics. 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 ele ments on continents, inherited by young oceanic tectonic lines.
Russian Journal of Earth Sciences, 2001
New data on some major MAR structures: preliminary results of R/V Akademik Nikolaj Strakhov 22 cruise
We carried out in January-March 1998 a geological-geophysical cruise to the Vema fracture zone th... more We carried out in January-March 1998 a geological-geophysical cruise to the Vema fracture zone that offsets by 320 km the Mid Atlantic Ridge in the central Atlantic. This expedition (S19) was part of PRIMAR (Russian-Italian Mid Atlantic Ridge Project). The field work aimed at obtaining geophysical and petrological data from a prominent transverse ridge that runs on the southern side of the transform valley and constitutes a major topographic anomaly relative to the depth/square root of age relationship. Previous work had shown that a relatively undisturbed section of oceanic lithosphere is exposed on the northern side of the transverse ridge for roughly 270 km along a seafloor spreading flow line. Given an average spreading half rate of 16 mm/y, this length corresponds to over 16 My. One of the objectives of our expedition was to sample at close-spaced (~ 5 km) horizontal intervals the mantle ultramafic basal unit, in order to detect temporal variations of mantle composition and of ...
Structure of the Mid-Atlantic Ridge in the Area of the Bouvet Triple Junction
Geochmica et Cosmochimica Acta
Russian Journal of Earth Sciences, 1998
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Papers by Alexander Peyve