Miguel Doblas

Sr and O isotope constraints on source and crustal contamination in the high-K calc-alkaline and shoshonitic neogene volcanic rocks of SE Spain

The Neogene volcanic province of SE Spain NVPS is characterized by calc-alkaline CA , high-K calc-alkaline KCA , . . . shoshonitic SH , ultrapotassic UP , and alkaline basaltic AB volcanic series. All these series, except the AB, have high LILErLREE, LILErHFSE and BrBe ratios and high but variable Sr, Pb and O isotope compositions. The KCA and SH lavas contain metapelitic xenoliths whose mineralogical and chemical composition are typical of anatectic restites. The geochemical characteristics of CA, KCA, SH and UP series suggest that they originated from the lithospheric mantle, previously contaminated by fluids derived from pelagic sediments. Additionally, the presence of restite xenoliths in the KCA and SH lavas indicates some sort of interaction between the mantle-derived magmas and the continental crust. Trace element and isotope modeling for the KCA and SH lavas and the restites, point towards the existence of two mixing stages. During the first stage, the lithospheric mantle was contaminated by 1-5% of fluids derived from pelagic sediments, which produced

Alkalic (ocean-island basalt type) and calc-alkalic volcanism in the Mexican volcanic belt: A case for plume-related magmatism and propagating rifting at an active margin?

The Mexican volcanic belt has been traditionally regarded as a classic case of subduction-related calc-alkalic volcanism. However, a series of geologic, geophysical, and petrological arguments makes this simple relationship doubtful. A seismic gap beneath the belt, a large-scale mantle anomaly, a graben triple-junction domain, and the presence of volumetrically important oceanic-island basalt (OIB) volcanism throughout the belt suggest a more complex tectonic scenario involving plume- and subduction-related processes. We here propose a model involving the development of a propagating rift opening from west to east in response to plume activity. The process started in Miocene time within the western sector of the belt (Guadalajara) and gave rise to a graben triple junction and OIB-type and calc-alkalic volcanism. Extension and volcanism proceeded to the east, giving rise to progressively younger ages for the initiation of OIB-type volcanism: (1) Miocene in the west (e.g., Guadalajara), (2) Pliocene in the central zone (e.g., Michoacan-Guanajuato), and (3) Quaternary farther east (e.g., Chichinautzin). Geochemical evidence suggests that part of the modern calc-alkalic volcanism (e.g., Chichinautzin) may be derived from magma mixing between the OIB mafic magmas and silicic, crust-derived magmas. However, we do not preclude some influence of the subducting slab in the generation of other (e.g., Jorullo) calc-alkalic volcanic rocks. Our model suggests a currently unrooted upper plume attached to the subcontinental lithosphere, which defines a hot zone beneath the Mexican volcanic belt.

Opening of the central Atlantic and asymmetric mantle upwelling phenomena: Implications for long-lived magmatism in western North Africa and Europe

The Mesozoic to present evolution of the central Atlantic realm is interpreted as a two-stage tectonomagmatic scenario involving long-lived asymmetric mantle upwelling phenomena and magmatism within its eastern margin. The first, a pre-drift tholeiitic stage of Triassic-Jurassic age, resulted from the interaction between several elements: (1) a thinned, weakened corridor along the collapsed southern branch of the Variscan belt between eastern North America and western North Africa; (2) a central Atlantic plume located in the triple junction between Africa, North America, and South America; (3) a progressive asymmetric continental breakup between northwest Africa and North America characterized by detachment systems; and (4) a highly thinned European realm pervaded by rift-type basins that we interpret as a large thin-spot-type domain. These conditions would have induced north-northeast–directed large-scale sublithospheric plume channeling from the central Atlantic plume site to the European large thin-spot, leading to widespread early tholeiitic magmatism (Triassic-Jurrassic) within a giant irregular zone of ∼3000 × 4000 km. Plume activity and chaneling continued afterward during a Cretaceous to present second tectonomagmatic stage (passive margin alkaline stage) leading to the onset of alkaline magmatism along a general north-northeast trend in the eastern Atlantic margin (Late Cretaceous) and Europe (Paleocene–Oligocene). As a whole, a north-northeasst–directed propagating magmatic vector can be defined from the Mesozoic to the present.

Neogene extensional collapse in the western Mediterranean (Betic-Rif Alpine orogenic belt): Implications for the genesis of the Gilbraltar Arc and magmatic activity

The Betic-Rif orogenic belt crops out in the western Mediterranean and constitutes the westernmost part of the Alpine edifice. The building up of this belt by compressional nappe tectonics occured during a series of Alpine deformational episodes. Subsequent Neogene extension resulted from the collapse of the previous compressional edifice through major extensional detachment systems. This episode was accompanied by cafe-alkaline to shoshonitic and lamproitic volcanism, tectonic denudation of metamorphic core complexes, and emplacement of peridotite bodies as a final consequence of the evolution of detachment systems. Furthermore, we relate the generation of the Gibraltar Arc and the opening of the Alboran Sea to this major extensional event. Finally, two main stages of tectonic denudation of these detachment systems are defined: (1) Nevado Filabride stage, with ex-humation of metamorphic core complexes, and (2) Ronda-Beni Boussera stage, with a more advanced degree of denudation, leading to the exhumation of upper mantle peridotites, which may be tentatively termed mantle core complexes.

“Mantle core complexes” and Neogene extensional detachment tectonics in the western Betic Cordilleras, Spain: an alternative model for the emplacement of the Ronda peridotite

Abstract An alternative model for the emplacement of the Ronda peridotite (Betic Alpine orogenic belt, southern Spain) is proposed, involving mantle uprising during a major Neogene extensional event. The development of a detachment system, and its further evolution by isostatic rebound and upward arching, would have triggered the tectonic denudation of these ultramafic bodies, forming what may be termed “mantle core complexes”. Mantle core complexes would represent a final step in the core-complex forming processes once a complete separation of the upper and lower blocks has been achieved in a detachment system, marking probably the transition from an intracontinental to a proto-oceanic rift environment.

Mantle insulation beneath the West African craton during the Precambrian-Cambrian transition

At the time of the Precambrian-Cambrian transition, the West African craton underwent widespread magmatism, hydrothermal activity, and thermal rejuvenation. This tectonothermal event gave rise to an anorogenic ‘‘ring of fire’’ along the rim of this craton, following the Pan-African‐Brasiliano belt that was reactivated by extension and transtension. The thermal phenomena were due to the progressive peripheral release of mantle heat that had built up beneath this craton because of strong insulating conditions. The West African craton at the Precambrian-Cambrian transition can thus be envisioned in terms of a gigantic pressure-cooker with a thick blanketing lithospheric lid. These insulation processes triggered an unusually hot mantle that was channeled by edge-driven convection toward the peri‐West African craton extensional corridors and released through magmatic pressure-relief valves. Massive ice melting and outgassing of volcanic CO2 gave rise to a planet-scale sea-level rise, a greenhouse effect, and the end of the icehouse snowball Earth. These processes played an important role in the Phanerozoic explosion of life on Earth.

Permo-Carboniferous volcanism in Europe and northwest Africa: a superplume exhaust valve in the centre of Pangaea?

Abstract The Permo-Carboniferous transition in the European-northwest African province was characterised by widespread volcanism (calc-alkaline with crustal and/or mantle lithospheric characteristics followed by alkaline/subalkaline with HIMU-type signature) with a maximum in the Stephanian-Autunian (Late Pennsylvannian to Early Permian). Extrusion of volcanics was accompanied by massive S-type synextensional granitic intrusions and hydrothermal mineralisation. The geotectonic framework involved the gravitational collapse of the Variscan Belt by extensional detachment tectonics, and its final disruption by wrench faulting. The characteristics of this volcanism are explained within progressively evolving extensional processes. It is suggested that this Permo-Carboniferous Pangaean volcanic province might be interpreted in terms of a superplume impinging on the base of the lithosphere. This model envisages that magmatism acted as an exhaust valve releasing the heat accumulated beneath the Pangaean supercontinent by insulation and blanketing processes which triggered large-scale mantle-wide upward convection and general instability of the supercontinent.

Extensional tectonics in the central Iberian Peninsula during the Variscan to Alpine transition

Abstract The passage from the Variscan cycle to the Early Alpine framework in the central part of the Iberian Peninsula can be explained in terms of a transitional process involving four clearly differentiated tectonic episodes. 1. (1) A first Variscan compressional stage (VI, Middle Devonian to Early Carboniferous) dominated by compressional conditions leading to the building-up of the orogenic edifice. The stress regime was relevant to what might be called “Variscan-type” compression (E-W-oriented). This stage was characterized by major Himalayan-type tectonics with frontal nappes, thrusts, overturned folds, lateral transcurrent ramps, and localized anatectic magmatism. Minor synorogenic extension and plutonism was also recorded during this stage in the Tormes Granitic Dome. 2. (2) A second Variscan stage (V2, Early to Middle Carboniferous) was characterized by increasing extensional conditions leading to widespread plutonism (adamellites, granodiorites). Wanning compressional conditions were restricted to the eastern and southern realms of central Iberia (the eastern part of the Spanish Central System, and the Toledo Mountains). 3. (3) A third stage, here defined as Late Variscan (LV), developed from Middle Carboniferous to Early Permian, as a result of N-S late-orogenic extension. This episode is relevant to detachment tectonics and the gravitational collapse of the Variscan orogenic edifice under combined simple/ pure-shear conditions. Plutonism (granites and leucogranites) was still of major importance. Early Permian andesitic to dacitic volcanism and sedimentary basins developed within the eastern part of the Spanish Central System. 4. (4) A fourth stage, here defined as Early Alpine (EA, Early Permian to Triassic) marks the onset of the Alpine framework. This stage was characterized by what might be called an “Early Alpine-type” regional stress regime i.e. E-W extension and N-S compression, within a simple-shear model, and resulted in the configuration of the Iberian Peninsula into two contrasted realms: a western inherited Variscan block, and an eastern Alpine block subjected to post-orogenic extension. Elements developed during this event include N-S high-angle normal faults, NW-SE and NE-SW conjugate strike-slip faulting, and asymmetric rifting involving listric low-angle detachments.

Slickenside and fault surface kinematic indicators on active normal faults of the Alpine Betic Cordilleras, Granada, southern Spain

Abstract Twenty four mesoscopic slickenside kinematic indicators are described here for the Pliocene-Quaternary active normal faults of the Alpine Betic Cordilleras, Granada, southern Spain. The indicators are classified into twelve groups depending on the down- or up-slope asymmetric orientation of features such as concavities, damaged versus sharp grain-borders or step-edges, V-shaped markings, fractures, and trailed material. Seventeen of the indicators were not previously described in the literature. The new kinematic indicators include two types of asymmetric grains, a carrot-shaped marking, six substructures of congruous and incongruous steps, trailed fault material, a drop-shaped figure, a V-shaped marking, three types of synthetic hybrid fractures with congruous steps, and two varieties of pluck holes. We think that some of these kinematic indicators provide information on the amount of displacement, aseismic versus seismic slip rates, cataclastic flow, frictional wear, or surface polishing.

Cenozoic intra-plate volcanism related to extensional tectonics at Calatrava, central Iberia

The Calatrava Volcanic Province is characterized by an intra-plate alkaline undersaturated magmatic association of melilitites, nephelinites, alkali olivine basalts and leucitites that were extruded in the central Iberian Peninsula (Spain) during Late Miocene–Quaternary time. This volcanism can be related to the following two-stage scenario: (1) an initial mantle-diapir stage, during which magma generation would have occurred at the lithosphere/asthenosphere boundary triggering minor accumulation processes at the crust/mantle interface, and consequent weakening/extension of the upper crust; and, (2) a final indentation stage related to disruption effects caused by the tectonic welding of the Prebetic Arc onto the Iberian foreland.