Cadomian S-type granites as basement rocks of the Variscan belt (Massif Central, France): Implications for the crustal evolution of the north Gondwana margin

International audienceFrom the Neoproterozoic to the early Paleozoic, the northern Gondwana margin was sequentially shaped by the Cadomian accretionary and the Variscan collisional orogens which offers the opportunity to investigate the relative extent of crust production/reworking in both geodynamic settings. In the eastern part of the Variscan French Massif Central (FMC), the Velay Orthogneiss Formation (VOF) represents a consistent lithological unit of the pre-Variscan basement and comprises augen gneisses and leucogneisses. Such rocks constitute a unique record of the pre-Variscan magmatic history and bear critical information on the crustal evolution of the northern Gondwana margin.Here, we present whole–rock major and trace element compositions indicating that: (i) the VOF shows a remarkable geochemical homogeneity; (ii) the protolith of the augen gneisses corresponds to strongly peraluminous, “S-type” porphyritic granites originating from partial melting of an Ediacaran sedimentary sequence; (iii) the leucogneisses are former leucogranites generated by fractionation of the magma at the origin of the porphyritic granites; and (iv) the whole suite emplaced at shallow crustal levels (< 7 km). U–Pb LA–(MC–)ICP–MS analyses on zircon yielded similar emplacement ages of c. 542 Ma and a narrow range of εHf(t) clustering around 0 for the protoliths of both augen and leucogneisses. This homogeneous Hf isotope signature, notably uncommon for S-type granites, would originate from a sequential process of: (i) inherited zircon dissolution during melting and ascent in the crust due to Zr-undersaturated conditions, (ii) isotopic homogenization of the melt by advection and elemental/isotopic diffusion, followed by (iii) early saturation upon emplacement owing to rapid cooling at shallow crustal levels.We propose that partial melting of Ediacaran sediments occurred during inversion of a Cadomian back-arc basin and was promoted by the high thermal gradient typical of thinned crust domains. Therefore, the VOF and other Cadomian S-type granitoids from the northern Gondwana margin are indicative of substantial crust reworking away from any proper continental collision zone

Understanding resilience and stability in complex ecological networks

Understanding the relationship between complexity and stability in natural systems is a long-standing issue of ecological research and an urgent need given the impact of human activities on natural resources. Network approaches offered ecologists new tools to investigate the relationship between complexity and stability in natural ecosystems. However, a major obstacle is to integrate theoretical prediction and empirical observations. My PhD project aims at assessing how the intrinsic structure of natural communities influences their overall stability and ability to respond to perturbations. In Chapter 1, I explored the complex relationships among species interactions, disturbance and stability in kelp forest communities in Southern California. For this analysis, I combined non-linear statistical approaches, network analysis and community matrix theory. This framework allowed me to track the strength of significant causal interspecific interactions through time and thus derive an indicator of network stability in control and manipulated communities. The results obtained in this work corroborated the growing evidence showing that natural systems can persist out of equilibrium due to time-varying interactions. Network theory predicts the potential of modularity to contain the propagation of local disturbances, but field experimental tests of this hypothesis are still lacking. In Chapters 3 and 4, I investigated the ability of modularity to buffer the propagation of disturbance through two manipulative field experiments. In the first experiment, I created modular networks using intertidal macroalgal assemblages associated with the canopy-forming alga Ericaria amentacea (C. Agardh) Molinari & Guiry (2020) as a model system to assess the role of modularity in confining the spread of algal turfs among modules. In Chapter 3, I further assessed the role of modularity by developing a metacommunity model which reproduced competitive dynamics between E. amentacea and algal turfs within a networked system resembling experimental networks. In the second experiment (Chapter 4), I used Posidonia oceanica (L.) Delile meadows as a model system to test the hypothesis that algal turfs can spread more easily in random than in modular networks. Overall, the results of these experiments and model simulations supported the existing literature showing that modularity limited the spreading of algal turfs within the perturbed module slowing down their diffusion to nodes in the other modules. Moreover, modular networks exhibited a larger resistance to turf spreading than random networks. Natural disturbances are major forces generating spatial and temporal heterogeneity in marine systems. However, complex spatial patterns may also emerge due to endogenous self-organized processes, such as small-scale disturbance-recovery dynamics. In Chapter 5, I explored the effect of different regimes of disturbance on the spatial synchrony of intertidal macroalgal assemblages dominated by the brown alga Cystoseira compressa (Esper) Gerloff & Nizamuddin. The application of network approaches allowed me to explicitly analyse the topology of spatial synchrony. Overall, experimental outcomes suggested that the observed patterns of synchrony were mainly imposed by exogenous disturbances, with predictable consequences on both synchrony topology and stability

RUOLO DEI PROCESSI ENDOGENI E DELLA VARIABILITA' DI EVENTI DI DISTURBO NEL PROMUOVERE LA DISTRIBUZIONE SPAZIALE DEI POPOLAMENTI ALGALI DI COSTA ROCCIOSA

Gli ecosistemi possono presentare una distribuzione non-omogenea e frammentata che può persistere per un ampio intervallo di scale spaziali e temporali. La formazione di pattern regolari (regular-spatial pattern) può essere determinata da due tipi di processi: processi esogeni, generalmente associati a fattori abiotici (es. temperatura, moto ondoso, disponibilità di risorse), e processi endogeni, per lo più associati a interazioni biotiche, intra- ed interspecifiche (ad es. competizione ed erbivoria). La teoria nota come “Spatial self-organization”, evidenzia come processi endogeni, interni al sistema, possano generare variabilità spazio-temporale nella distribuzione degli organismi, indipendentemente dall’azione di fattori esogeni al sistema. Secondo questa teoria, i processi endogeni agendo a piccola scala spaziale sono in grado di determinare pattern di distribuzione a grande scala spaziale. I sistemi naturali sarebbero quindi in grado di organizzarsi autonomamente nello spazio e nel tempo, acquisendo proprietà cosiddette emergenti, cioè che non possono essere comprese studiando singolarmente le componenti del sistema. Ad oggi sono stati identificati diversi sistemi ecologici nei quali i processi endogeni sembrano essere determinanti nel generare i pattern di distribuzione spaziale degli organismi, tra questi i più studiati sono: ecosistemi aridi e semi-aridi, ecosistemi umidi, come paludi e torbiere, letti di mitili che si sviluppano su fondi mobili della fascia intertidale. Il seguente lavoro di tesi ha l’obiettivo di investigare i processi che determinano le modalità di distribuzione spaziale dell’alga bruna Cystoseira compressa (Esper) Gerloff & Nizamuddin, caratteristica dell’intertidale roccioso del Mediterraneo. Da un precedente studio è emerso che C. compressa può presentare una distribuzione spaziale eterogenea a chiazze (patches), la cui taglia diminuisce lungo l’asse orizzontale della costa. L’eterogeneità spaziale è data dall’alternanza di chiazze di C. compressa con aree occupate da feltri algali e/o alghe incrostanti. Allo scopo di valutare il contributo dei fattori esogeni (mareggiate) ed endogeni (interazioni locali) nel generare il pattern spaziale regolare nella copertura di C. compressa, nell’autunno 2017 è stato allestito un esperimento manipolativo nella località di Calafuria (Livorno). Lungo la fascia intertidale sono stati selezionati 18 transetti (costituiti da 6 quadrati contigui di 30 x 30 cm) caratterizzati da una copertura omogenea di C. compressa, i quali sono stati sottoposti a tre tipologie di disturbo con differente distribuzione spaziale: 1) disturbo applicato in modo omogeneo (e casuale) lungo l’asse orizzontale del transetto (Disturbo Omogeneo), 2) disturbo applicato secondo un gradiente spaziale lungo l’asse orizzontale del transetto (Disturbo a Gradiente), 3) disturbo che alla prima data di applicazione viene distribuito casualmente e in modo omogeneo, ma alle applicazioni successive interessa uno dei margini delle chiazze precedentemente disturbate, andando così ad allargare le gap create in precedenza (Disturbo Localizzato). Tale tipologia di disturbo è motivata dal fatto che nei sistemi in cui è stata osservata la formazione di pattern spaziali autorganizzati (ad es. letti di mitili), i margini delle aree precedentemente disturbate sono risultati maggiormente suscettibili a nuovi eventi di disturbo. Nella pratica, il disturbo è consistito nella rimozione totale degli organismi dal substrato, mediante l’utilizzo di martello e scalpello, allo scopo di creare aree di roccia nuda di 6 x 6 cm (gap), le quali sono state rese nuovamente disponibili alla colonizzazione di larve e propaguli algali. Il trattamento di disturbo è stato allestito sia ad alta sia a bassa intensità, per un totale di 12 e 24 unità spaziali perturbate, rispettivamente in ogni transetto sperimentale. Sono stati selezionati, inoltre, 6 transetti di Controllo: 3 transetti caratterizzati da un popolamento di Cystoseira compressa omogeneo (Controllo Omogeneo), e 3 caratterizzati da chiazze di C. compressa la cui taglia diminuiva orizzontalmente lungo il transetto (Controllo Frammentato). I dati di abbondanza del popolamento algale e della fauna associata sono stati acquisiti a elevata risoluzione spaziale, mediante campionamento visivo, dopo circa 4 mesi dalla data di applicazione di ciascun trattamento. Dall’analisi dei dati è emerso che la distribuzione spaziale naturalmente frammentata del popolamento associato a C. compressa osservata lungo la costa di Calafuria è ricorrente nello spazio e permane nel tempo. Le cinture omogenee di C. compressa non tendono naturalmente verso un pattern spaziale a gradiente, possono però andare incontro ad un certo grado di frammentazione, probabilmente a causa dell’eterogeneità del substrato e del regime di disturbo naturale. Le analisi dei dati di copertura del popolamento algale non evidenziano differenze tra i due livelli di Intensità, Alta e Bassa, del disturbo. Questo risultato è in controtendenza con altri studi che hanno dimostrato come l’effetto del disturbo su una comunità dipenda anche dall’intensità stessa del disturbo. Infine, sebbene nessuna tipologia di disturbo (né a Bassa né ad Alta Intensità) sia stata in grado di determinare un calo significativo dell’abbondanza di C. compressa, sia il disturbo applicato ai margini delle gap create precedentemente sia il disturbo applicato secondo un gradiente orizzontale sono stati in grado di determinare una struttura spaziale analoga a quella osservata nei transetti naturalmente frammentati. Ciò indica che entrambe le tipologie di processi, endogeni ed esogeni, possano avere un ruolo e plausibilmente interagiscano nel determinare modalità di distribuzione a scale spaziali più ampie, riconducibili a quelle osservate nei transetti naturalmente frammentati. E’ necessario ricordare che i risultati discussi in questo lavoro di tesi sono preliminari e che come suggerisce l’assenza di una riduzione significativa della copertura di C. compressa in tutti i transetti trattati, due date di trattamento non sono sufficienti al fine di osservare un effetto significativo del disturbo, sarà quindi importante portare avanti l’esperimento affinché i trattamenti sperimentali siano in grado di generare dinamiche simili a quelle naturali, permettendo così di eseguire un test definitivo delle ipotesi

Cognitive Assemblages: Spatial Generation Through Wave Function Collapse and Reinforcement Learning

This research explores the integration of AI in an iterative decision process for the open-ended procedural generation of architectural spaces. Leveraging on state-of-the-art Deep Reinforcement Learning techniques, an Artificial Neural Network (ANN) is trained to perform local decisions selecting tiles in a Wave Function Collapse (WFC) algorithm, assembling discrete elements that build up a complex spatial organization, pursuing selected spatial qualities at the architectural scale

Training Spaces - Fostering machine sensibility for spatial assemblages through wave function collapse and reinforcement learning

This research explores the integration of Deep Reinforcement Learning (RL) and a Wave Function Collapse (WFC) algorithm for a goal-driven, open-ended generation of architectural spaces. Our approach binds RL to a distributed network of decisions, unfolding through three key steps: the definition of a set of architectural components (tiles) and their connectivity rules, the selection of the tile placement location, which is determined by the WFC, and the choice of which tile to place, which is performed by RL. The act of thinking becomes granular and embedded in an iterative process, distributed among human and non-human cognition, which constantly negotiate their agency and authorial status. Tools become active agents capable of developing their own sensibility while controlling specific spatial conditions. Establishing an interdependency with the human, that engenders the design patterns and becomes an indispensable prerequisite for the exploration of the generated design space, exceeding human or machinic reach alone

Exhumation of a migmatitic unit through self-enhanced magmatic weakening enabled by tectonic contact metamorphism (Gruf complex, Central European Alps)

The Central Alpine lower crustal migmatitic Gruf complex was exhumed in contact to the greenschist-grade Chiavenna ophiolite and gneissic Tambo nappe leading to a lateral gradient of similar to 70 degrees C/km within the ophiolite. The 14 km long, E-W striking subvertical contact now bridges metamorphic conditions of similar to 730 degrees C, 6.6 kbar in the migmatitic gneisses and similar to 500 degrees C, 4.2 kbar in the serpentinites and Tambo schists 2-4 km north of the contact. An obvious fault, mylonite or highly sheared rock that could accommodate the similar to 8.5 km vertical displacement is not present. Instead, more than half of the movement was accommodated in a 0.2-1.2 km thick orthogneiss of the Gruf complex that was heterogeneously molten. Discrete bands with high melt fractions (45-65%) now contain variably stretched enclaves of the adjacent MOR-derived amphibolite. In turn, the adjacent amphibolites exhibit tonalitic in-situ leucosomes and dikes i.e., were partially molten. The H2O necessary for fluid-assisted melting of the orthogneiss and amphibolites was likely derived from the tectonic contact metamorphism of the Chiavenna serpentinites, at the contact now in enstatite + olivine-grade. U-Pb dating of zircons shows that partial melting and diking occurred at 29.0-31.5 Ma, concomitant with the calc-alkaline Bergell batholith that intruded the Gruf. The major driving forces of exhumation were hence the strong regional North-South shortening in the Alpine collisional belt and the buoyancy provided by the Bergell magma. The fluids available through tectonic contact metamorphism led to self-enhanced magmatic weakening and concentration of movement in an orthogneiss, where melt-rich bands provided a low friction environment. Continuous heating of the originally greenschist Chiavenna ophiolite and Tambo gneisses + schists by the migmatitic Gruf complex during differential uplift explains the skewed temperature profile, with intensive contact heating in the ophiolite but little cooling in the portion of the now-exposed Gruf complex.ISSN:0010-7999ISSN:1432-096

Exogenous disturbances and endogenous self-organized processes are not mutually exclusive drivers of spatial patterns in macroalgal assemblages

Complex spatial patterns are common in coastal marine systems, but mechanisms underlying their formation are disputed. Most empirical work has focused on exogenous spatially structured disturbances as the leading cause of pattern formation in species assemblages. However, theoretical and observational studies suggest that complex spatial patterns, such as power laws in gap-size distribution, may result from endogenous self-organized processes involving local-scale interactions. The lack of studies simultaneously assessing the influence of spatially variable disturbances and local-scale interactions has fuelled the idea that exogenous and endogenous processes are mutually exclusive explanations of spatial patterns in marine ecosystems. To assess the relative contribution of endogenous and exogenous processes in the emergence of spatial patterns, an intertidal assemblage of algae was exposed for two years to various combinations of intensity and spatial patterns of disturbance. Localized disturbances impinging at the margins of previously disturbed clearings and homogenous disturbances without any spatial pattern generated heterogeneous distributions of disturbed gaps and macroalgal patches, characterized by a power-law scaling. Spatially varying disturbances produced a spatial gradient in the distribution of algal patches and, to a lesser extent, also a power-law scaling in both patch- and gap-size distributions. These results suggest that exogenous and endogenous processes are not mutually exclusive forces that can lead to the formation of similar spatial patterns in species assemblages