28 research outputs found
Mid Ocean Ridge Processes at Very Low Melt Supply : Submersible Exploration of Smooth Ultramafic Seafloor at the Southwest Indian Ridge, 64 degree E - T32C-01
No full textMid-ocean ridges are the Earth's most extensive and active volcanic chains. They are also, particularly at slow spreading rates, rift zones, where plate divergence is in part accommodated by faults. Large offset normal faults, also called detachments, are characteristic of slow-spreading ridges, where they account for the widespread emplacement of mantle-derived rocks at the seafloor. In most cases, these detachments occur together with ridge magmatism, with melt injection and faulting interacting to shape the newly formed oceanic lithosphere. Here, we seek to better understand these interactions and their effects on oceanic accretion by studying the end-member case of a ridge where magmatism is locally almost absent.
The portion of the Southwest Indian ridge we are studying has an overal low melt supply, focused to discrete axial volcanoes, leaving almost zero melt to intervening sections of the axial valley. One of these nearly amagmatic section of the ridge, located at 64°E, has been the focus of several past cruises (sampling, mapping and seismic experiments). Here we report on the most recent cruise to the area (RV Pourquoi Pas? with ROV Victor; dec-jan 2017), during which we performed high resolution mapping, submersible exploration and sampling of the ultramafic seafloor and of sparse volcanic formations. Our findings are consistent with the flip-flop detachment hypothesis proposed for this area by Sauter et al. (Nature Geosciences, 2013; ultramafic seafloor forming in the footwall of successive detachment faults, each cutting into the footwall of the previous fault, with an opposite polarity). Our observations also document the extent and geometry of deformation in the footwall of a young axial detachment, the role of mass-wasting for the evolution of this detachment, and provide spectacular evidence for serpentinization-related hydrothermal circulation and for spatial links between faults and volcanic eruptions
Strain localization in oceanic detachment faults : the extreme case of a magma-starved slow spreading ridge
No full textLa partie orientale de la dorsale sud-ouest indienne est particulièrement pauvre en apports magmatiques et constitue de ce fait un laboratoire naturel pour étudier l'accrétion océanique dans une configuration extrême, pour laquelle la divergence des plaques est presque complètement accommodée par le jeu de failles normales à grand rejet. Ces failles, également appelées failles de détachement, exhument des péridotites serpentinisées d'origine mantellaire sur le plancher océanique. Les mécanismes de déformation des roches mantelliques au cours de leur exhumation, avant leur passage dans le domaine de circulation de fluides hydrothermaux, sont peu connus. Nous avons étudié les textures de déformation et les mécanismes de localisation, ainsi que les assemblages minéralogiques associés, dans un corpus de presque 400 échantillons de péridotites serpentinisées draguées pendant la campagne SMoothSeafloor (2010). Tous les échantillons présentent à des degrés variables une déformation hétérogène combinant des mécanismes de déformation cassants (fractures, kinks) et plastiques (extinction ondulante, sous-joints, recristallisation dynamique). L’une des manifestations de cette déformation est la recristallisation syntectonique d’olivines et de pyroxènes au sein de kinks et de microfractures. Ces textures résultent de conditions de hautes contraintes et hautes températures en base de lithosphère, dans la zone d’enracinement des détachements. Ces observations microstructurales ont été intégrées dans un modèle thermomécanique 2D du domaine axial des dorsales lentes, qui explore deux mécanismes d'affaiblissement observés dans les échantillons: la serpentinisation (800°C). La combinaison de ces deux mécanismes permet la reproduction de failles de détachement dans une lithosphère épaisse (20-25 km), avec un relief et une fréquence qui sont consistantes avec celles observées sur notre zone d'étude.The Southwest Indian Ridge in its eastern part has particularly low magmatic inputs and is therefore a natural laboratory to study oceanic accretion in an extreme configuration for which the divergence of the plates is almost completely accommodated by large offset normal faults, also called detachment faults. These faults exhume mantle-derived peridotites. Very little is known about the deformation mechanisms that operate at the lower levels of the lithosphere, prior to serpentinization.We studied the deformation textures, analyze strain localization mechanisms, as well as mineralogical assemblages associated with these mechanisms from a corpus of almost 400 samples of serpentinized peridotites dredged during the SMoothSeafloor cruise (2010). All samples display heterogeneous deformation combining brittle (fracturation, kinks) and plastic mechanisms (undulose extinction, subgrain boundaries, dynamic recrystallization). For example, microfracturation and kinks are locally accompanied by partial recrystallization of the primary minerals into fine-grained aggregates along orthopyroxene grains. These textures result from high stress and high temperature conditions at the base of the lithosphere, where the root of the active detachment.We use these microstructural observations to constrain a 2D thermomechanical model of lithospheric extension, in which we explore two weakening mechanisms seen in the samples: serpentinization (temperatures 800°C). Combining the two, we develop detachment faults in a thick lithospheric context (20-25 km), with fault topography and offsets that are consistent with geological observations in the eastern SWIR
Cold cracking in DC-cast high strength aluminum alloy ingots: An intrinsic problem intensified by casting process parameters
No full textFor almost half a century the catastrophic failure of direct chill (DC) cast high strength aluminum alloys has been challenging the production of sound ingots. To overcome this problem, a criterion is required that can assist the researchers in predicting the critical conditions which facilitate the catastrophic failure of the ingots. This could be achieved at first glance by application of computer simulations to assess the level and distribution of residual thermal stresses. However, the simulation results are only able to show the critical locations and conditions where and when high stresses may appear in the ingots. The prediction of critical void/crack size requires simultaneous application of fracture mechanics. In this paper, we present the thermo-mechanical simulation results that indicate the critical crack size distribution in several DC-cast billets cast at various casting conditions. The simulation results were validated upon experimental DC-casting trials and revealed that the existence of voids/cracks with a considerable size is required for cold cracking to occur.Materials Science & EngineeringMechanical, Maritime and Materials Engineerin
Modes de localisation des déformations dans les Failles de détachement océaniques : le cas extrême d'une dorsale lente à très faible budget magmatique
No full textThe Southwest Indian Ridge in its eastern part has particularly low magmatic inputs and is therefore a natural laboratory to study oceanic accretion in an extreme configuration for which the divergence of the plates is almost completely accommodated by large offset normal faults, also called detachment faults. These faults exhume mantle-derived peridotites. Very little is known about the deformation mechanisms that operate at the lower levels of the lithosphere, prior to serpentinization.We studied the deformation textures, analyze strain localization mechanisms, as well as mineralogical assemblages associated with these mechanisms from a corpus of almost 400 samples of serpentinized peridotites dredged during the SMoothSeafloor cruise (2010). All samples display heterogeneous deformation combining brittle (fracturation, kinks) and plastic mechanisms (undulose extinction, subgrain boundaries, dynamic recrystallization). For example, microfracturation and kinks are locally accompanied by partial recrystallization of the primary minerals into fine-grained aggregates along orthopyroxene grains. These textures result from high stress and high temperature conditions at the base of the lithosphere, where the root of the active detachment.We use these microstructural observations to constrain a 2D thermomechanical model of lithospheric extension, in which we explore two weakening mechanisms seen in the samples: serpentinization (temperatures 800°C). Combining the two, we develop detachment faults in a thick lithospheric context (20-25 km), with fault topography and offsets that are consistent with geological observations in the eastern SWIR.La partie orientale de la dorsale sud-ouest indienne est particulièrement pauvre en apports magmatiques et constitue de ce fait un laboratoire naturel pour étudier l'accrétion océanique dans une configuration extrême, pour laquelle la divergence des plaques est presque complètement accommodée par le jeu de failles normales à grand rejet. Ces failles, également appelées failles de détachement, exhument des péridotites serpentinisées d'origine mantellaire sur le plancher océanique. Les mécanismes de déformation des roches mantelliques au cours de leur exhumation, avant leur passage dans le domaine de circulation de fluides hydrothermaux, sont peu connus. Nous avons étudié les textures de déformation et les mécanismes de localisation, ainsi que les assemblages minéralogiques associés, dans un corpus de presque 400 échantillons de péridotites serpentinisées draguées pendant la campagne SMoothSeafloor (2010). Tous les échantillons présentent à des degrés variables une déformation hétérogène combinant des mécanismes de déformation cassants (fractures, kinks) et plastiques (extinction ondulante, sous-joints, recristallisation dynamique). L’une des manifestations de cette déformation est la recristallisation syntectonique d’olivines et de pyroxènes au sein de kinks et de microfractures. Ces textures résultent de conditions de hautes contraintes et hautes températures en base de lithosphère, dans la zone d’enracinement des détachements. Ces observations microstructurales ont été intégrées dans un modèle thermomécanique 2D du domaine axial des dorsales lentes, qui explore deux mécanismes d'affaiblissement observés dans les échantillons: la serpentinisation (800°C). La combinaison de ces deux mécanismes permet la reproduction de failles de détachement dans une lithosphère épaisse (20-25 km), avec un relief et une fréquence qui sont consistantes avec celles observées sur notre zone d'étude
Hydrous fluids down to the semi-brittle root zone of detachment faults in nearly amagmatic ultra-slow spreading ridges
No full textAt the Eastern part of the Southwest Indian Ridge (SWIR), plate divergence is accommodated by large offset normal faults, also called detachment faults, that exhume mantle-derived rocks on the seafloor. A third of the ultramafic samples dredged on- and off-axis in this nearly amagmatic ridge setting present amphibole-bearing secondary mineralogical assemblages indicative of hydration, and for the most part predating the growth of serpentine minerals. The deepest evidence of hydration is the occurrence of small amounts of syn-kinematic amphibole in microshear zones with strongly reduced grain size, which record deformation at high stress and high temperatures (>800 °C) at the root zone of the detachment. The composition of these amphiboles is consistent with a hydrothermal origin, suggesting that seawater derived fluids percolated down to the root of detachment faults, at the Brittle-Ductile Transition (BDT). We propose that the constant exhumation of new mantle material to the seafloor, and the limited lifetime of each detachment (1–3 Myrs) prevent a more pervasive deep hydration of mid-ocean ridge detachment root regions, as proposed at transform fault plate boundaries
How do detachment faults form at ultraslow mid-ocean ridges in a thick axial lithosphere?
No full textInternational audienc
Erratum to "How do detachment faults form at ultraslow mid-ocean ridges in a thick axial lithosphere?" [Earth Planet. Sci. Lett. 533 (2020) 116048]
No full textThe Publisher regrets that an error occurred in the representation of Fig. 8, this should appear as follows
The Role of the Performing Arts in Postwar Phoenix, Arizona: Patrons, Performers, and the Public
No full textabstract: Civic leadership in Phoenix, Arizona promoted the city's performing arts as part of a deliberate plan towards the larger growth agenda after World War II. From the 1940s through the late 1960s, the business and professional leaders who controlled city government served on boards for performing arts groups, built venues, offered financial support, and sometimes participated as artists in order to attract high-technology firms and highly skilled workers to the area. They believed one aspect of Phoenix's urban development included a need for quality, high-culture performing arts scene that signaled a high quality of life and drew more residents. After this era of boosterism ended and control shifted from business and professional leaders to city government, performing arts support fluctuated with leadership's attitudes and the local, state, and national economies. The early civic leaders were successful in their overall mission to expand the city - now the sixth largest in the nation - and many of the organizations and venues they patronized still serve the community; however, the commitment to developing a quality arts and culture scene waned. Today's public, private, and arts and culture leaders are using the same argument as Phoenix tries once again to become a high-technology center. The theory that arts and culture stimulate the economy directly and indirectly is true today as it was in the 1940s. Although the plan was effective, it needed fully committed supporters, strong infrastructure, and continued revising in order to move the vision into the twenty-first century.Dissertation/ThesisM.A. History 201
Olivine-quartz association in a gabbro-peridotite hybrid rock of the Kane Fracture Zone: evidence for hydrous Si-rich melt percolation in abyssal context.
No full textStrain Localization in the Root of Detachment Faults at a Melt‐Starved Mid‐Ocean Ridge: A Microstructural Study of Abyssal Peridotites From the Southwest Indian Ridge
No full textInternational audienc
