On-fault earthquake energy density partitioning from shocked garnet in an exhumed seismic midcrustal fault.

The energy released during an earthquake is mostly dissipated in the fault zone and subordinately as radiated seismic waves. The on-fault energy budget is partitioned into frictional heat, generation of new grain surface by microfracturing, and crystal-lattice distortion associated with dislocation defects. The relative contribution of these components is debated and difficult to assess, but this energy partitioning strongly influences earthquake mechanics. We use high-resolution scanning-electron-microscopy techniques, especially to analyze shocked garnet in a fault wall-rock, to provide the first estimate of all three energy components for a seismic fault patch exhumed from midcrustal conditions. Fault single-jerk seismicity is recorded by the presence of pristine quenched frictional melt. The estimated value of energy per unit fault surface is ~13 megajoules per square meter for heat, which is predominant with respect to both surface energy (up to 0.29 megajoules per square meter) and energy associated with crystal lattice distortion (0.02 megajoules per square meter).

The Role of Adria Plate Lithospheric Structures on the Recent Dynamics of the Central Mediterranean Region.

The Tertiary tectonic evolution of the Central Mediterranean has been relatively well constrained by abundant geological data. Yet, several uncertainties persist about the mechanisms that led to the present-day surface morphology and deep slab geometry. Here, we combine geodynamic and seismological numerical modeling techniques to reproduce the recent large-scale evolution of the Central Mediterranean and the associated strain-induced upper mantle fabrics and seismic anisotropy. 3D thermo-mechanical subduction models were designed and calibrated according to paleogeographic-tectonic reconstructions and seismological observations available in the literature. It is found that, although the opening of back-arc extensional basins in response to the retreat of the Ionian slab is a common feature in all models, structural heterogeneities within the Adria plate and/or the geometry of its Tyrrhenian passive margin profoundly impact on the segmentation of the subducting slab and the amount of Ionian trench retreat. More, in general, this study highlights the importance of coupling geodynamic and seismological modeling to better constrain the tectonic evolution of complex convergent margins such as the Central Mediterranean.

Extrinsic Anisotropy of Two-Phase Newtonian Aggregates: Fabric Characterization and Parameterization.

Rocks of the Earth's crust and mantle commonly consist of different minerals with contrasting mechanical properties. During progressive, high-temperature (ductile) deformation, these rocks develop extrinsic mechanical anisotropy linked to strain partitioning between different minerals, amount of accumulated strain, and bulk strain geometry. Extrinsic anisotropy plays an important role in a wide range of geodynamic processes up to the scale of mantle convection. However, the evolution of grain- and rock-scale fabrics causing this anisotropy cannot be directly simulated in large-scale numerical simulations. For two-phase aggregates-a good rheological approximation of most Earth's rocks-we propose a method to indirectly approximate the extrinsic viscous anisotropy by combining (a) 3D mechanical models of rock fabrics, and (b) analytical effective medium theories. Our results confirm that weak inclusions induce substantial weakening by forming a network of weak thin layers with limited lateral connectivity. Consequently, even when the inclusion phase is extremely weak, structural weakening is not larger than 30-60%, less than in previous estimates. On the other hand, the presence of strong inclusions does not have a profound impact on the effective strength of the aggregate, and lineated fabrics only develop at relatively low viscosity contrasts. When rigid inclusions become clogged, however, the aggregate viscosity can increase over the theoretical upper bound. We show that the modeled grain-scale fabrics can be parameterized as a function of the bulk deformation and material phase properties and combined with analytical solutions to approximate the anisotropic viscous tensor.

Intraplate volcanism originating from upwelling hydrous mantle transition zone.

Most magmatism occurring on Earth is conventionally attributed to passive mantle upwelling at mid-ocean ridges, to slab devolatilization at subduction zones, or to mantle plumes. However, the widespread Cenozoic intraplate volcanism in northeast China1-3 and the young petit-spot volcanoes4-7 offshore of the Japan Trench cannot readily be associated with any of these mechanisms. In addition, the mantle beneath these types of volcanism is characterized by zones of anomalously low seismic velocity above and below the transition zone8-12 (a mantle level located at depths between 410 and 660 kilometres). A comprehensive interpretation of these phenomena is lacking. Here we show that most (or possibly all) of the intraplate and petit-spot volcanism and low-velocity zones around the Japanese subduction zone can be explained by the Cenozoic interaction of the subducting Pacific slab with a hydrous mantle transition zone. Numerical modelling indicates that 0.2 to 0.3 weight per cent of water dissolved in mantle minerals that are driven out from the transition zone in response to subduction and retreat of a tectonic plate is sufficient to reproduce the observations. This suggests that a critical amount of water may have accumulated in the transition zone around this subduction zone, as well as in others of the Tethyan tectonic belt13 that are characterized by intraplate or petit-spot volcanism and low-velocity zones in the underlying mantle.

Cooling history and emplacement of a pyroxenitic lava as proxy for understanding Martian lava flows.

Terrestrial analogues are often investigated to get insights into the geological processes occurring on other planetary bodies. Due to its thickness and petrological similarities, the pyroxenitic layer of the 120m-thick magmatic pile Theo's Flow (Archean Abitibi greenstone belt Ontario, Canada), has always been regarded as the terrestrial analogue for Martian nakhlites. However, its origin and cooling history and, as a consequence those of nakhlites, have always been a matter of vigorous debate. Did this lava flow originate from a single magmatic event similar to those supposed to occur on Mars or do the different units derive from multiple eruptions? We demonstrate, by a combination of geothermometric constraints on augite single crystals and numerical simulations, that Theo's Flow has been formed by multiple magma emplacements that occurred at different times. This discovery supports the idea that the enormous lava flows with similar compositions observed on Mars could be the result of a process where low viscosity lavas are emplaced during multiple eruptions. This has profound implications for understanding the multiscale mechanisms of lava flow emplacement on Earth and other planetary bodies.

Extrinsic Elastic Anisotropy in a Compositionally Heterogeneous Earth's Mantle.

Several theoretical studies indicate that a substantial fraction of the measured seismic anisotropy could be interpreted as extrinsic anisotropy associated with compositional layering in rocks, reducing the significance of strain-induced intrinsic anisotropy. Here we quantify the potential contribution of grain-scale and rock-scale compositional anisotropy to the observations by (i) combining effective medium theories with realistic estimates of mineral isotropic elastic properties and (ii) measuring velocities of synthetic seismic waves propagating through modeled strain-induced microstructures. It is shown that for typical mantle and oceanic crust subsolidus compositions, rock-scale compositional layering does not generate any substantial extrinsic anisotropy (<1%) because of the limited contrast in isotropic elastic moduli among different rocks. Quasi-laminated structures observed in subducting slabs using P and S wave scattering are often invoked as a source of extrinsic anisotropy, but our calculations show that they only generate minor seismic anisotropy (<0.1-0.2% of Vp and Vs radial anisotropy). More generally, rock-scale compositional layering, when present, cannot be detected with seismic anisotropy studies but mainly with wave scattering. In contrast, when grain-scale layering is present, significant extrinsic anisotropy could exist in vertically limited levels of the mantle such as in a mid-ocean ridge basalt-rich lower transition zone or in the uppermost lower mantle where foliated basalts and pyrolites display up to 2-3% Vp and 3-6% Vs radial anisotropy. Thus, seismic anisotropy observed around the 660-km discontinuity could be possibly related to grain-scale shape-preferred orientation. Extrinsic anisotropy can form also in a compositionally homogeneous mantle, where velocity variations associated with major phase transitions can generate up to 1% of positive radial anisotropy.

Upper- and mid-mantle interaction between the Samoan plume and the Tonga-Kermadec slabs.

Mantle plumes are thought to play a key role in transferring heat from the core-mantle boundary to the lithosphere, where it can significantly influence plate tectonics. On impinging on the lithosphere at spreading ridges or in intra-plate settings, mantle plumes may generate hotspots, large igneous provinces and hence considerable dynamic topography. However, the active role of mantle plumes on subducting slabs remains poorly understood. Here we show that the stagnation at 660 km and fastest trench retreat of the Tonga slab in Southwestern Pacific are consistent with an interaction with the Samoan plume and the Hikurangi plateau. Our findings are based on comparisons between 3D anisotropic tomography images and 3D petrological-thermo-mechanical models, which self-consistently explain several unique features of the Fiji-Tonga region. We identify four possible slip systems of bridgmanite in the lower mantle that reconcile the observed seismic anisotropy beneath the Tonga slab (V(SH)>V(SV)) with thermo-mechanical calculations.