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The evolution and characteristics of narrowcontinental rifting are illustrated in this paper through a review of recent lithospheric-scale analog models of continental extension compared with selected examples from the East African Rift System. Rift location is controlled by reactivation of lithospheric-scale pre-existing weaknesses; in these areas, the initial phases of rifting correspond to the activation of few, large-offset boundary faults that accommodate basin subsidence, which can be at places strongly asymmetric. The plan-view geometry of rift faults is primarily related to the relative orientation of the lithospheric weakness with respect to the extension direction: orthogonal rifting gives rise to long, extension-orthogonal boundary faults with associated pronounced subsidence, whereas oblique rifting results in a general en-echelon arrangement of faults and basins with less subsidence. Inherited fabrics having variable orientation with respect to the rift trend may control rift architecture at both regional and local scales. In these initial phases, widespread magmatism may encompass the rift, with volcanic activity localized along major boundary faults, transfer zones and limited portions of the rift shoulders (off-axis volcanism). Progressive extension leads to a change in deformation style from the few, large-offset boundary faults at the rift margins to dense fault swarms - with limited vertical motions - affecting the rift floor where the magmatic activity is concentrated. In these areas of focused tectono-magmatic activity (the so-called magmatic segments) the thinned lithosphere is strongly modified and weakened by the extensive magma intrusion, and extension is facilitated and accommodated by a combination of magmatic intrusion, dyking and faulting. The feedback between strain localization, magma injection and lithospheric weakening is selfreinforcing, facilitating the rupture of the continental lithosphere. At this stage, magmatic segments (as for instance in the Northern Main Ethiopian Rift) act as incipient slow-spreading mid-ocean ridges, developing within a lithosphere that is transitional between continental and oceanic. © 2011 Published by Elsevier B.V.

Bonini M.,CNR Institute of Geosciences and Earth Resources
Earth-Science Reviews | Year: 2012

This study examines the use of specific mud volcano features (i.e., elongated calderas, aligned vents and elongated volcanoes) as potential indicators of tectonic stress orientation. The stress indicator principles, widely recognised for magmatic systems, have been discussed and applied to mud volcano settings such as in the Northern Apennines and the Azerbaijan Greater Caucasus, as well as in other instances where the analysis was fully based on a remote sensing study. The results of these applications are promising, the obtained maximum horizontal stress (S H) directions generally showing a good correlation with those determined in the upper crust by classical methods (i.e., earthquake focal mechanism solutions, well bore breakouts). Therefore, stress information from mud volcanoes could be used as a proxy for stress orientation (1) where stress data is lacking, (2) where settings are inaccessible (i.e., underwater or the surface of planets), or simply (3) as supplementary stress indicators. This study also pays special attention to structural elements that may control fluid expulsion at various length scales, and pathways that should have spawned the mud volcanoes and controlled their paroxysmal events and eruptions. Different types of sub-planar brittle elements have been found to focus fluid flow rising up-through fold cores, where the vertical zonation of stresses may take part in this process by creating distinctive feeder fracture/fault sets. On a regional scale, mud volcanoes in active fold-and-thrust belts may occur over wider areas, such as the prolific mud volcanism in Azerbaijan, or may cluster along discrete structures like the steep Pede-Apennine thrust in the Northern Apennines, where the generation of overpressures is expected to establish a positive feedback loop allowing for fault movement and mud volcanism. © 2012 Elsevier B.V.

Scambelluri M.,University of Genoa | Tonarini S.,CNR Institute of Geosciences and Earth Resources
Geology | Year: 2012

Serpentinites formed by alteration of oceanic and forearc mantle are major volatile and fl uid-mobile element reservoirs for arc magmatism, though direct proof of their dominance in the subduction-zone volatile cycles has been elusive. Boron isotopes are established markers of fl uid-mediated mass transfer during subduction. Altered oceanic crust and sediments have been shown to release in the subarc mantle 11B-depleted fl uids, which cannot explain 11B enrichment of many arcs. In contrast to these crustal reservoirs, we document high 11B values retained in subduction-zone Alpine serpentinites. No 11B fractionation occurs in these rocks with progressive burial: the released 11B-rich fl uids uniquely explain the elevated 11B of arc magmas. B, O-H, and Sr isotope systems indicate that serpentinization was driven by slab fl uids that infi ltrated the slab-mantle interface early in the subduction history. © 2012 Geological Society of America.

Nimis P.,University of Padua | Nimis P.,CNR Institute of Geosciences and Earth Resources | Grutter H.,BHP Billiton
Contributions to Mineralogy and Petrology | Year: 2010

Mutual relationships among temperatures estimated with the most widely used geothermometers for garnet peridotites and pyroxenites demonstrate that the methods are not internally consistent and may diverge by over 200°C even in well-equilibrated mantle xenoliths. The Taylor (N Jb Min Abh 172:381-408, 1998) two-pyroxene (TA98) and the Nimis and Taylor (Contrib Mineral Petrol 139:541-554, 2000) single-clinopyroxene thermometers are shown to provide the most reliable estimates, as they reproduce the temperatures of experiments in a variety of simple and natural peridotitic systems. Discrepancies between these two thermometers are negligible in applications to a wide variety of natural samples (≤30°C). The Brey and Köhler (J Petrol 31:1353-1378, 1990) Ca-in-Opx thermometer shows good agreement with TA98 in the range 1,000-1,400°C and a positive bias at lower T (up to +90°C, on average, at T TA98 = 700°C). The popular Brey and Köhler (J Petrol 31:1353-1378, 1990) two-pyroxene thermometer performs well on clinopyroxene with Na contents of ~ 0.05 atoms per 6-oxygen formula, but shows a systematic positive bias with increasing Na Cpx (+150°C at Na Cpx = 0.25). Among Fe-Mg exchange thermometers, the Harley (Contrib Mineral Petrol 86:359-373, 1984) orthopyroxene-garnet and the recent Wu and Zhao (J Metamorphic Geol 25:497-505, 2007) olivine-garnet formulations show the highest precision, but systematically diverge (up to ca. 150°C, on average) from TA98 estimates at T far from 1,100°C and at T < 1,200°C, respectively; these systematic errors are also evident by comparison with experimental data for natural peridotite systems. The older O'Neill and Wood (Contrib Mineral Petrol 70:59-70, 1979) version of the olivine-garnet Fe-Mg thermometer and all popular versions of the clinopyroxene-garnet Fe-Mg thermometer show unacceptably low precision, with discrepancies exceeding 200°C when compared to TA98 results for well-equilibrated xenoliths. Empirical correction to the Brey and Köhler (J Petrol 31:1353-1378, 1990) Ca-in-Opx thermometer and recalibration of the orthopyroxene-garnet thermometer, using well-equilibrated mantle xenoliths and TA98 temperatures as calibrants, are provided in this study to ensure consistency with TA98 estimates in the range 700-1,400°C. Observed discrepancies between the new orthopyroxene-garnet thermometer and TA98 for some localities can be interpreted in the light of orthopyroxene-garnet Fe 3+ Partitioning systematics and suggest localized and lateral variations in mantle redox conditions, in broad agreement with existing oxybarometric data. Kinetic decoupling of Ca-Mg and Fe-Mg exchange equilibria caused by transient heating appears to be common, but not ubiquitous, near the base of the lithosphere. © Springer-Verlag 2009.

This paper discusses the palaeoenvironmental significance of the "Orbitolina Level", the microbial carbonates and the Salpingoporella dinarica-rich deposits encased in the Aptian/Albian shallow water carbonate platform strata of Monte Tobenna and Monte Faito (Southern Italy). These facies show a peculiar field appearance due to their color and/or fossil content. In the shallow water carbonate strata, the Late Aptian "Orbitolina Level" was formed during a period of decreasing accommodation space. Microbial carbonates occur in different levels in the composite section. They reach their maximum thickness around the sequence boundaries just above the "Orbitolina Level" and close to the Aptian-Albian transition, and were not deposited during maximum flooding. S. dinarica-rich deposits occur in the lower part of the Monte Tobenna-Monte Faito composite section, in both restricted and more open lagoonal sediments. S. dinarica has its maximum abundance below the "Orbitolina Level" and disappears 11m above this layer. On the basis of δ13C and δ18O values recorded at Tobenna-Faito, the succession has been correlated to global sea-level changes and to the main volcanic and climatic events during the Aptian. Deterioration of the inner lagoon environmental conditions was related to high trophic levels triggered by volcano-tectonic activity. Microbial carbonates were deposited especially in periods of third-order sea level lowering. In such a scenario, periods of increased precipitation during the Gargasian induced the mobilization of clay during flooding of the exposed platform due to high-frequency sea-level changes, with consequent terrigenous input to the lagoon. This and the high nutrient levels made the conditions unsuitable for the principle carbonate producers, and an opportunistic biota rich in orbitolinids (Mesorbitolina texana and M. parva) populated the platform. In the more open marine domain, the increased nutrient input enhanced the production of organic matter and locally led to the formation of black shales (e.g. the Niveau Fallot in the Vocontian Basin). It is argued that the concomitant low Mg/Ca molar ratio and high concentration of calcium in seawater could have favoured the development of the low-Mg calcite skeleton of the S. dinarica green algae. During third-order sea-level rise, no or minor microbial carbonates formed in the shallowlagoonal settings and S. dinarica disappeared. Carbonate neritic ecosystems were not influenced by the environmental changes inferred to have been induced by the mid-Cretaceous volcanism. The "Orbitolina Level", the microbial carbonates and the Salpingoporella dinarica-rich deposits in the studied Aptian/Albian shallow water carbonate strata are interpreted to be the response to environmental and oceanographic changes in shallow-water and deeper-marine ecosystems. © Author(s) 2012. CC Attribution 3.0 License.

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