Continuous attractors for dynamic memories

Episodic memory has a dynamic nature: when we recall past episodes, we retrieve not only their content, but also their temporal structure. The phenomenon of replay, in the hippocampus of mammals, offers a remarkable example of this temporal dynamics. However, most quantitative models of memory treat memories as static configurations, neglecting the temporal unfolding of the retrieval process. Here, we introduce a continuous attractor network model with a memory-dependent asymmetric component in the synaptic connectivity, which spontaneously breaks the equilibrium of the memory configurations and produces dynamic retrieval. The detailed analysis of the model with analytical calculations and numerical simulations shows that it can robustly retrieve multiple dynamical memories, and that this feature is largely independent of the details of its implementation. By calculating the storage capacity, we show that the dynamic component does not impair memory capacity, and can even enhance it in certain regimes

Structural analysis of a subduction-related contact in southern Sesia-Lanzo Zone (Austroalpine Domain, Italian Western Alps)

A new foliation trajectory map at 1:10000 scale, represented here with an interpretative structural map, is derived from an original field analysis at 1:5000 scale in the southern Sesia-Lanzo Zone (SLZ). It shows the relative chronology of overprinting foliations, characterised by the mineral assemblages that mark superposed fabrics in each rock type. This map and the associated cross-sections, which synthesise the 3D structural outline of the tectonic contact between the Eclogitic Micaschists Complex (EMC), the Rocca Canavese Thrust Sheets and the Lanzo Ultramafic Complex, allow the correlation of the structural and metamorphic imprints that developed in these crustal and mantle complexes during Alpine subduction. Furthermore, the map and cross-sections allow the immediate perception of the metamorphic environments in which the structural imprints developed in each complex successively under eclogite, blueschist and greenschist facies conditions. The represented structural and metamorphic evolution of the southern end of the SLZ (internal Western Alps) has been inferred based on multiscale structural analysis. The dominant fabrics at the regional scale are two superposed mylonitic foliations that developed under blueschist and greenschist facies conditions, respectively. Metamorphic assemblages underlying the successive fabrics in the different metamorphic complexes allow us to identify contrasting metamorphic evolutions indicating that the tectonic contacts between the EMC, the Rocca Canavese Thrust Sheets and the Lanzo Ultramafic Complex developed under blueschist facies conditions and were successively reactivated during the greenschist facies retrogression

Slant total electron content for Sirio-Mortelliccio ray path

The Total Electron Content (TEC) is used to indicate the ionisation of the ionosphere. TEC is a quantity that concern for predicting space weather effects on telecommunications, improving the accuracy of satellite navigation, fly control vehicles and other systems that use transionospheric signals, because the ionospheric layer affects the mentioned signals. In this work the Slant Total Electron Content (STEC) was calculated with a technique that uses so-called «auxiliaries stations model», and a Chapman layer with scale height equal to atomic oxygen scale height (CHO). The validity was checked with STEC measurements obtained from geosynchronous satellite signals, for SIRIO-Mortelliccio link considering solstices and equinox, in high solar activity period. In general, the deviations between predictions and measurements were lower than 30% for 16 h per day (average). The results suggest that additional studies for other links and solar activity are required in order to improve the model predictions

How many subductions in the Variscan orogeny? Insights from numerical models

We developed a 2D numerical model to simulate the evolution of two superposed ocean-continent-ocean subduction cycles with opposite vergence, both followed by continental collision, aiming to better understand the evolution of the Variscan belt. Three models with different velocities of the first oceanic subduction have been implemented. Striking differences in the thermo-mechanical evolution between the first subduction, which activates in an unperturbed system, and the second subduction, characterised by an opposite vergence, have been enlighten, in particular regarding the temperature in the mantle wedge and in the interior of the slab. Pressure and temperature (P-T) conditions predicted by one cycle and two cycles models have been compared with natural P-T estimates of the Variscan metamorphism from the Alps and from the French Massif Central (FMC). The comparative analysis supports that a slow and hot subduction well reproduces the P-T conditions compatible with data from the FMC, while P-T conditions compatible with data of Variscan metamorphism from the Alps can be reproduced by either a cold or hot oceanic subduction models. Analysing the agreement of both double and single subduction models with natural P-T estimates, we observed that polycyclic models better describe the evolution of the Variscan orogeny

Electrolytic depletion interactions

We consider the interactions between two uncharged planar macroscopic surfaces immersed in an electrolyte solution which are induced by interfacial selectivity. These forces are taken into account by introducing a depletion free-energy density functional, in addition to the usual mean-field Poisson-Boltzmann functional. The minimization of the total free-energy functional yields the density profiles of the microions and the electrostatic potential. The disjoining pressure is obtained by differentiation of the total free energy with respect to the separation of the surfaces, holding the range and strength of the depletion forces constant. We find that the induced interaction between the two surfaces is always repulsive for sufficiently large separations, and becomes attractive at shorter separations. The nature of the induced interactions changes from attractive to repulsive at a distance corresponding to the range of the depletion forces.Comment: 17 pages, 4 Postscript figures, submitted to Physical Review

Influence of subduction geometry and mantle wedge hydration in ocean/continent convergent systems : a numerical simulation

To analyze the effects of hydration mechanism on continental crust recycling a parametric study using a 2D finite elements thermo-mechanical model is here presented. We implemented oceanic slab dehydration and consequent mantle wedge hydration using a dynamic method; hydration is accomplished by Lawsonite and Serpentine break- down. Topography is treated as a free surface. Results of a set of numerical simulations investigate the influence of subduction rate, slab dip and mantle rheology changes on thermal regime variations, exhumation rate and amount of recycled crust. At this purpose subduction rates of 1, 3, 5, 7.5 and 10 cm/yr, slab angles of 30\ub0, 45\ub0 and 60\ub0 and a mantle rheology represented by dry dunite and dry olivine flow laws, have been taken into account during successive numerical experiments. Model predictions pointed out that a direct relationship between mantle rheology and the amount of recycled crustal material exists: the larger is the viscosity contrast between hydrated and dry mantle the larger is the percentage of recycled material into the mantle wedge. A quite impact on recycling is consequent to slab dip variation. Metamorphic evolutions of recycled material are influenced by subduction style: Tmax of Pmax, generally accomplished under eclogite-facies conditions, is sensible to slab dip changing and the increasing subduction rate induces a decrease in Tmax of Pmax values. A direct relationship between subduction rate and exhumation rate results for different slab dips, independent of mantle flow law used. Thermal regimes predicted by different numerical models are compared with PT paths followed by continental crustal slices involved in ancient and actual subduction zones

Numerical simulation of ocean/continent convergent systems : influence of subduction geometry and mantle wedge hydration on crustal recycling

Studies on high-pressure (HP) and ultrahigh-pressure (UHP) rocks exposed in orogenic belts linked to collisional margin show that nappes of oceanic and continental deeply subducted crust can be exhumed to shallow structural levels. In particular, during ocean-continent-type subduction, the crustal material dragged into subduction channel is composed chie\ufb02y by ocean and trench sediments, crustal slices belonging to subducting plate [microcontinent (Ring & Layer, 2003) or linked to early continental collision (Chemenda et al., 1995)] or crustal slices tectonically eroded from the overriding plate (ablative subduction) (Tao & O\u2019Connell, 1992, Marotta & Spalla, 2007). Several models have been developed, during last 20 years, to analyse exhumation of subducted crustal material. They can be resume on \ufb01ve main mechanisms: a) crustal-mantle delamination (Chemenda et al., 1995), b) slab break-off (Ernst et al., 1997), c) slab retreat (Ring & Layer, 2003) and roll-back slab (Brun & Faccenna, 2008) and d) decoupling of two main ductile layers (Yamato et al., 2008), in which the exhumation is mainly driven by negative buoyancy and/or faulting and e) subduction-channel \ufb02ow (Gerya & Stockhert, 2005) in which the exhumation is driven by the upwelling \ufb02ow developed in low-viscosity mantle wedge. Only channel \ufb02ow takes into account recyrculation of crustal slices dragged to high depth by ablation in pre-collisional subduction zones. To study the effects of subduction rate, slab dip and mantle rheology changes on channel flow efficiency a parametric analysis is made. We present the results of a set of numerical simulation with different subduction rates, slab dips and mantle rheology represented by dry dunite and dry olivine \ufb02ow laws. Numerical model predictions are \ufb01nally compare to some PT paths obtained from ancient and actual subduction zones with different slab dips and convergence velocities. A general good agreement between natural data and model predictions emerges from the comparison: exhumation rates obtained from complete PTt-paths (total exhumation rates) are more compatible with natural rates rather than maximum exhumation rates; the thermal states predicted by ablative subduction simulations with a hydrated mantle wedge justify the natural PT estimates obtained on continental crust units involved in ocean/continent subduction systems. For these reasons, we propose ablative subduction of the upper continental plate linked to hydrated mantle wedge as a good alternative pre-collisional mechanism, with respect to the collisional mechamisms as the slab break-off, slab-retreat and roll-back slab

Influenza dell'idratazione del cuneo di mantello sull'evoluzione di un sistema di subduzione oceano/continente : una simulazione numerica

The evolution of an ocean/continent subduction system is simulated by using a 2D finite elements thermomechanical model. The effects of hydration in mantle wedge on mantle flow and on crustal recycling is studied for different hydration rates and maximum depth of dehydration of the oceanic crust for two selected typical subduction velocities (1 cm/a and 5 cm/a). We found a direct relationship between the amount of recycled material and the maximum depth of dehydration. Moreover, the hydration rate and hydration depth have an important impact on peak pressure and temperature of recycled continental crust for a subducion rate of 5 cm/a

Permian geodynamics of the central Southalpine by tectono-thermal record in post-Variscan conglomerates

The central Southern Alps consist of the pre-Alpine basement and Permian-Mesozoic covers, both affected by the Alpine fold-and-thrust belt. The pre-Alpine basement recorded heterogeneous structural and metamorphic evolutions and therefore consists of different tectono-metamorphic units related to different stages of the Variscan evolution. To the east, rocks recorded the effects of the Variscan tectonic burial and escaped the subsequent collision, whereas the units outcropping westward recorded both the effects of Variscan tectonic burial and collision and the westernmost basement rocks even host late-Variscan intrusives and recorded the effects of lithosphere thinning-related Triassic high-temperature (Spalla et al., 2014). Lower Permian volcanoclastic sequences infill intermontane basins and are the oldest sedimentary rocks uncoformably capping the basement (Berra et al., 2016; Zanoni & Spalla, 2018 and refs therein). These sequences consist of volcanites overlaid by lacustrine sandstone and alluvial fan conglomerates. According to radiometric constraints, the age of the conglomerates is more recent westward. These conglomerates contain pebble- to boulder-sized crystalline clasts. The metamorphic evolution recorded in clasts are related to the Variscan orogeny and revealed that the thermal maturity of orogenic traces increases westward, likewise the general record in the metamorphic basement, indicating that conglomerates were fed by the erosion of tectono-metamorphic units similar to those exposed today. In the westernmost conglomerate, clasts recorded high-temperature metamorphism and some derive even from late-Variscan intrusives and later tourmalinite-breccia. Since the conglomerates rejuvenate westward with the increase of orogenic maturity in clasts, we speculate that the post-Variscan lithosphere was affected by westward propagating extension, also responsible for intermontane wrenching. To test this hypothesis, we started 2D numerical simulations on the thermo-mechanical evolution of the lithosphere affected by westward propagating extension