Erwan Hallot

Volcanism in a compressional Andean setting: A structural and geochronological study of Tromen volcano (Neuquén province, Argentina)

We document evidence for growth of an active volcano in a compressional Andean setting. Our data are surface structures and 39Ar-40Ar ages of volcanic products on Tromen volcano. Tromen is an active back-arc volcano in the Andean foothills of Neuquen province, Argentina. Its volcanic products are unconformable upon Mesozoic strata of the Neuquen basin. The volcano straddles a N-S trending pop-up, which formed during E-W shortening. The main underlying structures are eastward verging thrusts. Their traces curve around the eastern foot of the volcano. Minor folds and faults also occur in the volcanic cover of Tromen, as a result of E-W shortening. New 39Ar-40Ar ages for these volcanic rocks are younger than 2.27 ± 0.10 Ma and show that Tromen has been active almost continuously from the late Pliocene to the Holocene. We conclude that volcanism and thrusting have been coeval and that magma must have reached the surface in a tectonic setting of horizontal compression. Our results have wider implications for magmatic processes in such settings.

A Complex History for the Caribbean Plateau: Petrology, Geochemistry, and Geochronology of the Beata Ridge, South Hispaniola

The Beata Ridge is a prominent SSW‐trending topographic structure in the central Caribbean basin. It is characterized by unusually thick oceanic crust (up to 20 km) and is believed to form part of the Caribbean oceanic plateau. Samples recovered by submersible during the Nautica‐Beata cruise show the ridge to be composed mainly of gabbros, dolerites, and rare pillow basalts. Textures, which vary significantly, reflect differences in cooling rates and suggest a subsurface, hypabyssal environment. Major‐element compositions of gabbros and dolerites plot on simple trends that correspond to fractional crystallization of olivine, clinopyroxene, and plagioclase. Trace‐element ratios are close to chondritic [(Nb/Zr)N 0.85–1.1] and rare earth element patterns (REE) are almost flat [(La/Yb)N 0.63–1.02]. The source, however, was isotopically depleted (ϵNd +7.4 to +9.5). To explain these geochemical features, we propose that the magmas formed through pooling of fractional melts of spinel peridotite. The rare basalts recovered have higher trace‐element ratios and enriched REE patterns [(Nb/Zr)N 3.45; (La/Yb)N 28–30]. They possibly formed through lower‐degree melting of an isotopically less depleted source (ϵNd +5). Several samples were dated by the 40Ar‐39Ar method, either on whole rocks or separated plagioclases. Most samples have ages between 80 and 75 Ma, which are consistent with previous ages within the province, but others are surprisingly young, around 55 Ma. The chemical signature of the gabbro‐dolerite group is very similar to that of basalts from other parts of the Caribbean and from other oceanic plateaus. The persistence of this signature raises questions about the validity of generally accepted mantle‐plume models for the formation of oceanic plateaus. Alternative hypotheses are evaluated in the light of geodynamic reconstructions of the Caribbean plate. Two geodynamic models may account for the geochemical and isotopic characteristics of the Beata Ridge samples. In one interpretation, the Caribbean plateau formed ∼80–90 Ma in the Pacific south of the Galapagos hot spot, possibly above the Sala y Gomez hot spot. In this model, the 76‐Ma episode is related to the Galapagos plume. In the second interpretation, the Galapagos plume was responsible for the main plume‐related magmatic event at 90 Ma and the 76‐Ma episode is attributed to lithospheric thinning. In both interpretations, the 55‐Ma episode is related to lithospheric thinning localized on the Beata Ridge.

Kinematics of long‐lasting Paleoproterozoic transpression within the Thompson Nickel Belt, Manitoba, Canada

This paper presents a reappraisal of kinematics and tectonic history of the Thompson Nickel Belt, a major Paleoproterozoic deformation zone along the western boundary of the Archean Superior Province in Manitoba. The study, based on the analysis of foliation trajectory maps and associated shear zone arrays, emphasizes that the overall strain pa ttern results from transpression. Strains are large, ma rked by strong subvertical stretch and NW-SE subhorizontal shortening, combined with along-strike stretch. From these, we infer distributed crustal shortening, involving steeply plunging flow rather than large horizontal displacements, although kinematic indicators show that top-to-the-west motions were dominant. Along-strike shear indicators are poorly expressed but suggest a component of bulk dextral strike-slip. Synkinematic thermal indicators and new geochronological data indicate that transpression could have been a long-lasting, circa 100 Ma, event in the area

Petrogenesis of picrites from the Caribbean Plateau and the North Atlantic magmatic province

We studied the petrography, mineralogy and geochemistry of picrites from three different regions: the island of Curacao which forms part of a Cretaceous oceanic plateau; Iceland, an active hot spot on the mid-Atlantic ridge; and the early Tertiary volcanic margin off the coast of Greenland, which formed during the rifting that created the Atlantic ocean. Using the compositions of olivine phenocrysts and relations between MgO and FeO, Al2O3 and Ni, we estimated compositions of parental liquids and the proportion of accumulated olivine in each rock. Picrites from Curacao formed mainly from a liquid with 12 wt.% MgO and they contain up to 55 wt.% excess olivine in the form of phenocrysts. A small proportion of more forsterite-rich olivine grains are xenocrysts from a more magnesian source. Picrites from Iceland formed from a slightly less magnesian liquid but one with also about 12 wt.% MgO and they contain both olivine and plagioclase in the accumulated assemblage. Picrites from the Greenland volcanic margin formed from a liquid that was distinctly more magnesian, with up to 20 wt.% MgO. In some of these rocks the proportion of accumulated olivine was minimal and in these the whole-rock composition is roughly equivalent to the liquid composition. The picrites from the three areas formed under different conditions and through contrasting melting processes. The Curacao picrites derive from pooled liquids formed through moderate degrees of melting at moderate depths beneath a relatively old and thick oceanic lithosphere. Iceland picrites, on the other hand, formed through advanced fractional melting of mantle that ascended almost to the base of the crust at the mid-ocean spreading center. An unusual combination of relatively high concentrations of incompatible trace elements and high MgO indicates that the Greenland picrites formed by relatively low degrees of melting at greater depths in the mantle.

Analog experimental insights into the formation of magmatic sulfide deposits

Abstract Magmatic sulfide deposits result from transport and settling of dense sulfide liquid droplets in basic magma. To model these processes, we carried out analog experiments in which dense silicone oil droplets were transported in a viscous aqueous polymer solution. The results of the experiments indicate that sulfide droplets can be carried in suspension by magma flowing at typical velocities. Due to the large surface tension of sulfide droplets, coalescence does not occur during transport. Transport ceases when upward magma velocity decreases because of conduit enlargement or change from vertical to horizontal flow. Coalescence of droplets is controlled by magma viscosity and the presence of crystals.

Dating the exhumation of a metamorphic dome: geological evidence for pre-Eocene unroofing of the Niğde Massif (Central Anatolia, Turkey)

The timing of exhumation of metamorphic rocks and granitoids of the Nigde metamorphic dome, at the southern tip of the Central Anatolian Crystalline Complex, is a matter of debate. According to some authors, the metamorphic rocks are overlain nonconformably by a sedimentary sequence of late Maastrichtian to Late Palaeocene age. In contrast, other authors recently argued that the Nigde dome represents an extensional core complex of Oligocene–Early Miocene age, finally unroofed during late Miocene times. On the one hand, the results of our study contradict the latter interpretation. A sedimentary sequence of earliest Eocene to early Middle Eocene age nonconformably overlies the high-grade rocks of the Nigde dome on its southeastern flank. Pebbles from the metamorphic rocks are ubiquitous in the conglomerates of this sequence. As a result, the Nigde metamorphic rocks must have reached the surface before Eocene times, or at the very beginning of the Eocene at the latest. The Uckapili granite, whose crystallization age has been inferred to be Early Miocene, has intruded the metamorphic complex during exhumation. The granite is also found as pebbles within the conglomerates of the Eocene sedimentary sequence and, thus, is actually older than the Eocene. Apatite fission track dates of 12–11 Ma across the Nigde dome do not indicate that the metamorphic rocks were still on their way to the surface at that time; instead, they must reflect a later event, which is most probably heating during late Neogene magmatism. Lastly, there is no ductile-then-brittle extensional detachment in the two areas where it has been invoked, that is, on the western and southern flanks of the dome. An extensional detachment nevertheless exists at the top of the Nigde dome, best documented in its northern part, where the detachment fault superposes a superficial unit made up of massive ophiolitic rocks onto the high-grade metamorphic sequence. Field evidence indicates that this detachment developed before Eocene times. On the other hand, our observations do not confirm the nonconformity of the sedimentary sequence dated as late Maastrichtian–Late Palaeocene onto the Nigde high-grade rocks. Field relations show either a tectonic contact between the two, or the direct nonconformity of the Eocene sediments onto the metamorphic rocks. The lack of coarse clasts originating from the Nigde high-grade rocks within the Maastrichtian–Palaeocene sequence further suggests that the metamorphic dome did not reach the surface before Late Palaeocene times. These results compare well with available data from the northwestern part of the Central Anatolian Crystalline Complex, suggesting that exhumation has been broadly synchronous on the scale of the massif, as a result of an episode of high magnitude extension that affected the region in Campanian to Palaeocene times.

Rifting above a mantle plume: structure and development of the Iceland Plateau

The interaction of the Mid-Atlantic Ridge with the North Atlantic Mantle Plume has produced a magmatic plateau centred about Iceland. The crust of this plateau is 30 km thick on average. This abnormal thickness implies that, unlike other slow-spreading ridges, addition of magmatic material to the crust is not balanced by crustal stretching. The thermal effect of the plume also reduces the strength of the lithosphere. Both mechanisms affect the rifting process in Iceland. A structural review, including new field observations, demonstrates that the structure of the Iceland plateau differs from that of other slow-spreading oceanic ridges. Lithospheric spreading is currently accommodated in a 200 km wide deformation strip, by the development of a system of half-grabens controlled by growth faults. Similar extinct structures, with various polarities, are preserved in the lava pile of the Iceland plateau. These structures are identified as lithospheric rollover anticlines that developed in hanging walls of listric faults. We introduce a new tectonic model of accretion, whereby the development of the magmatic plateau involved activation, growth and decay of a system of growth fault/rollover systems underlain by shallow magma chambers. Deactivation of a given extensional system, after a lifetime of a few My, was at the expense of the activation of a new, laterally offset, one. Correspondingly, such systems formed successively at different places within a 200 km wide diffuse plate boundary. Unlike previous ones, this new model explains the lack of an axial valley in Iceland, the dip pattern of the lava pile, the complex geographical distribution of ages of extinct volcanic systems and the outcrops of extinct magma chambers.

Dynamics of Surface Deformation Induced by Dikes and Cone Sheets in a Cohesive Coulomb Brittle Crust

The analysis of surface deformation associated with intruding magma has become an established method to study subsurface processes and intrusion architecture. Active subsurface magmatism induces deformation that is commonly modeled using static elastic models. To what extent, Coulomb failure of the crust affects surface deformation remains, so far, largely unexplored. In this contribution we present quantitative laboratory results of surface deformation induced by the emplacement of simulated dikes and cone sheets in a cohesive Coulomb material. The analysis of the experimental surface deformation shows that these intrusion types produce distinct and characteristic surface deformation signatures, which reflect the evolution of the intrusion at depth. Generally, dikes show a two-phase evolution while cone sheets develop gradually. In comparison, cone sheets induce larger uplifted areas and volumes than dikes relative to the depth of the injection source. Dike formation is, in turn, is likely accommodated, to a larger degree than cone sheets, by lateral opening of the host consistent with our current understanding of dike emplacement mechanics. Notably, only surface uplifts develop above the experimental dikes, consistent with a viscous indenter propagation mechanism, that is, a dike pushing ahead. The measured surface deformation patterns associated with dikes starkly contrast with established static, elastic models that predict local subsidence above the tip of a dike. This suggests that Coulomb failure of crustal rocks may considerably affect surface deformation induced by propagating igneous intrusions. This is especially relevant when a relatively high viscosity magma intrudes a weak host, such as unconsolidated sedimentary and volcaniclastic rocks.

Experimental Constraints on Forecasting the Location of Volcanic Eruptions from Pre-eruptive Surface Deformation

Volcanic eruptions pose a threat to lives and property when volcano flanks and surroundings are densely populated. The local impact of an eruption depends firstly on its location, whether it occurs ...