Late-Hercynia granitoid plutons emplaced along a deep crustal shear zone. A case study from the S. Lucia Nappe (Alpine Corsica, France)

The S. Lucia Nappe (Alpine Corsica, France) is located in the Corte area. This tectonic unit recorded a polyphase Alpine evolution developed under lower greenschist facies conditions. In addition, the basement recorded an older (i.e. late-Paleozoic) tectono-metamorphic history, which mainly developed under granulite to amphibolite facies conditions. The S. Lucia basement represents an example of Ivrea-type lower crustal section, including a segment of a Km-scale granulite-facies shear zone made up of sheared Permian gabbros and enclosed metasediments. The occurrence of a thick (i.e. at least 1 Km-wide) high-grade shear zone indicates that the late-Palaeozoic deformation in the lower crust was homogeneously distributed on a large scale. The non-coaxial character of the deformation and the invariable sinistral shear sense, generalized along the whole section, recalls the models of uniform-sense, crustal-scale normal shear zones. Two distinct granitoid plutons were emplaced at the margins of the S. Lucia shear zone. Field relations, microstructures, quartz LPO fabric and mineral chemistry data document the syntectonic emplacement with respect to the shear zone. Indeed, both plutons experienced a nearly continuum down-temperature deformation, during emplacement and subsequent syndeformational cooling. The observed structural style suggests that the granitoids probably accommodated large strain through magmatic to submagatic flow. In both cases, the pattern of magmatic and subsequent solid-state fabric was strongly controlled by the stress field imposed by the S. Lucia shear zone. Moreover, the bulk composition of the two plutons likely played a fundamental role in the tectonic evolution, influencing the structural style and the spatial distribution of the solid-state overprint. Finally, the occurrence of magmas along the shear zone may have promoted strain partitioning within the melt-rich areas, allowing the exhumation and the preservation of granulite-facies rocks. The Mesozoic syn-rift exhumation of the S. Lucia basement is suggested by the occurrence of undeformed MORB-type dolerites, by the Middle Jurassic thermal anomaly and by the Cretaceous sedimentary cover. However, although the P-T-t evolution of the S. Lucia section is only partly constrained, several lines of evidence suggest that the Permian granulites had already been incorporated into the upper crust prior to the Mesozoic. Several paleogeographic reconstructions propose that, in the Alpine region, Permian granulites were intimately associated with the Mesozoic ophiolites at the ocean-continent transition. However, the case study proposed here points out that the occurrence of granulites alone cannot be taken as an indication of a syn-rift exhumation of the lower crust along the continental margin

Greenstone burial–exhumation cycles at the late Archean transition to plate tectonics

Converging lines of evidence suggest that, during the late Archean, Earth completed its transition from a stagnant-lid to a plate tectonics regime,although how and when this transition occurred is debated. The geological record indicates that some form of subduction, a key component of plate tectonics—has operated since the Mesoarchean, even though the tectonic style and timescales of burial and exhumation cycles within ancient convergent margins are poorly constrained.Here, we present a Neoarchean pressure–temperature–time (P–T–t) path from supracrustal rocks of the transpressional Yilgarn orogen (Western Australia), which documents how sea-floor-altered rocks underwent deep burial then exhumation during shortening that was unrelated to the episode of burial. Archean subduction, even if generally short-lived, was capable of producing eclogites along converging lithosphere boundaries, although exhumation processes in those environments were likely less efficient than today, such that return of high-pressure rocks to the surface was rare

The influence of phase and grain size distribution on the dynamics of strain localization in polymineralic rocks

Highlights: • Rheological evolution of the rock is influenced by its original and evolving structure. • Strain localizes at boundary between domains of different assemblages (qtz-, fsp-rich). • Strain localizes in original and recrystallized/precipitated finer-grained phases. • Formation of interconnected meso- and microshear zones deformed by diffusion creep. • The rheology of studied example is controlled by Newtonian flow. Abstract: Deformation microstructures of a quartzo-feldspathic pegmatite deformed at mid-crustal levels allow the study of the dynamics of strain localization in polymineralic rocks. Strain localization results from (i) difference in grain sizes between phases, both original and obtained during fluid present reactions and (ii) initial compositional banding. Due to original difference in grain size stress concentrates in the initially finer-grained phases resulting in their intense grain size reduction via subgrain rotation recrystallization (SGR). When the grain size is sufficiently reduced through either deformation or interphase coupled dissolution–precipitation replacement of the coarse grained feldspar, aggregates start to deform by dominantly diffusion accommodated grain boundary sliding (GBS). Phase mixing inhibits grain growth and sustains a grain size allowing GBS. Consequently, discontinuous microscale shear zones form locally within initially coarse grained areas. At the same time difference in strain rate between feldspar-rich and quartz-rich domains needs to be accommodated at domain boundaries. This results in the formation of continuous mesoscale shear zones deformed by GBS. Once these are formed, deformation in the coarse grained parts is arrested and strain is mainly accommodated in the mesoscale shear zones resulting in “superplastic” behaviour consistent with diffusion creep

Supporting Information for [Neoarchean synmagmatic crustal extrusion in the transpressional Yilgarn Orogen]

Supplementary file for a Manuscript Submitted To "Tectonics" (Agu Journal

Late Palaeozoic tectonics in Central Mediterranean: a reappraisal

A revision of late Palaeozoic tectonics recorded in Tuscany, Calabria and Corsica is here presented. We propose that, in Tuscany, upper Carboniferous-Permian shallow-marine to continental sedimentary basins, characterized by uncon- formities and abrupt changes in sedimentary facies, coal-measures, red fanglomerate deposits and felsic magmatism, may be related with a transtensional setting where upper-crustal splay faults are linked with a mid-crustal shear zone. The remnants of the latter can be found in the deep-well logs of Pontremoli and Larderello-Travale in northern and southern Tuscany respectively. In Calabria (Sila, Serre and Aspromonte), a continuous pre-Mesozoic crustal section is exposed, where the lower-crustal portion mainly includes granulites and migmatitic paragneisses, together with subordinate marbles and metabasites. The mid-crustal section, up to 13 km-thick, includes granitoids, tonalitic to granitic in composition, emplaced between 306 and 295 Ma. They were progressively deformed during retrograde extensional shearing, with a final magmatic activity, between 295 ± 1 and 277 ± 1 Ma, when shallower dykes were emplaced in a transtensional regime. The section is completed by an upper crustal portion, mainly formed by a Palaeozoic sedimentary succession deformed as a low-grade fold and thrust belt, and locally overlaying medium- grade paragneiss units. As a whole, these features are reminiscent of the nappe zone domains of the Sardinia Vari- scan Orogen. In Corsica, besides the well-known effusive and intrusive Permian magmatism of the “Autochthonous” domain, the Alpine Santa Lucia Nappe exposes a kilometer-scale portion of the Permian lower to mid-crust, exhibit- ing many similarities to the Ivrea Zone. The distinct Mafic and Granitic complexes characterizing this crustal domain are juxtaposed through an oblique-slip shear zone named Santa Lucia Shear Zone. Structural and petrological data witness the interaction between magmatism, metamorphism and retrograde shearing during Permian, in a tempera- ture range of c. 800–400 °C. We frame the outlined paleotectonic domains within a regional-scale, strain–partitioned, tectonic setting controlled by a first-order transcurrent/transtensional fault network that includes a westernmost fault (Santa Lucia Fault) and an easternmost one (East Tuscan Fault), with intervening crustal domains affected by exten- sional to transtensional deformation. As a whole, our revision allows new suggestions for a better understanding of the tectonic framework and evolution of the Central Mediterranean during the late Palaeozoic

Upper Carboniferous-Permian tectonics in Central Mediterranean: an updated revision

An updated revision of the upper Carboniferous-Permian tectonics recorded in Corsica, Calabria and Tuscany is here proposed. We combine our and literature data to document how the sedimentary, tectono-metamorphic and magmatic upper Carboniferous-Permian record fits with a regional-scale tectonic scenario characterized by trascurrent fault systems associated with stretched crustal domains in which extensional regional structures, magmatism and transtensional basins developed. In Corsica, altogether with well-known effusive and intrusive Permian magmatism, the alpine S.Lucia nappe exposes a kilometer-scale portion of the Permian lower to mid-crust, with many similarities to the Ivrea-Verbano zone. The two distinct Mafic and Leucogranitic complexes, which characterize this crustal domain are juxtposed by an oblique-slip shear zone named as S.Lucia Shear Zone. Structural and petrological data document interaction between magmatism, metamorphism and shearing during Permian in the c. 800-400 °C temperature range. In Calabria (Sila, Serre and Aspromonte), a continuous pre-Mesozoic crustal section is exposed. The lower crust portion of such section is mainly made up of granulites and migmatitic paragneisses with subordinate marbles and metabasites. The mid-crustal section includes an up to 13 km thick sequence of granitoids of tonalitic to granitic composition, emplaced between 306 and 295 Ma and progressively deformed during retrograde extensional shearing to end with a final magmatic activity between 295 and 277 Ma, consisting in the injection of shallower dykes in a transtensional regime. The section is completed by an upper crustal portion mainly formed by a Paleozoic succession deformed as a low-grade fold and thrust belt, locally overlaying medium-grade paragneiss units, and therefore as a whole reminiscent of the external/nappe zone domains of Sardinia Hercynian orogen. In Tuscany we document, how late Carboniferous/Permian shallow marine to continental sedimentary basins characterized by unconformity and abrupt change in sedimentary facies (coal-measures, red fanglomerate deposits) and acid magmatism well fit a transtensional setting with a mid-crustal shear zone linked with a system of E-W trending (in present orientation) upper crust splay faults. We will frame the whole dataset in a regional framework of first-order transcurrent shear zones network which includes a westernmost S.Lucia Shear Zone and an easternmost East Tuscan Shear Zone, with intervening crustal domains in which extensional to transtensional shearing occured