How initial basin geometry influences gravity-driven salt tectonics: Insights from laboratory experiments

As a rifted margin starts to tilt due to thermal subsidence, evaporitic bodies can become unstable, initiating gravity-driven salt tectonics. Our understanding of such processes has greatly benefitted from tectonic modelling efforts, yet a topic that has however gotten limited attention so far is the influence of large-scale salt basin geometry on subsequent salt tectonics. The aim of this work is therefore to systematically test how salt basin geometry (initial salt basin depocenter location, i.e. where salt is thickest, as well as mean salt thickness) influence salt tectonic systems by means of analogue experiments. These experiments were analyzed qualitatively using top view photography, and quantitatively through Particle Image Velocimetry (PIV), and 3D photogrammetry (Structure-from-Motion, SfM) to obtain their surface displacement and topographic evolution. The model results show that the degree of (instantaneous) margin basin tilt, followed by the mean salt thickness are dominant factors controlling deformation, as enhancing basin tilt and/or mean salt thickness promotes deformation. Focusing on experiments with constant basin tilt and mean salt thickness to filter out these dominant factors, we find that the initial salt depocenter location has various effects on the distribution and expression of tectonic domains. Most importantly, a more upslope depocenter leads to increased downslope displacement of material, and more subsidence (localized accommodation space generation) in the upslope domain when compared to a setting involving a depocenter situated farther downslope. A significant factor in these differences is the basal drag associated with locally thinner salt layers. When comparing our results with natural examples, we find a fair correlation expressed in the links between salt depocenter location and post-salt depositional patterns: the subsidence distribution due to the specific salt depocenter location creates accommodation space for subsequent sedimentation. These correlations are applicable when interpreting the early stages of salt tectonics, when sedimentary loading has not become dominant yet

Seismic and structural characterization of fluid escape pipes using 3D and partial stack seismic from the Loyal field (UK) : A multiphase and repeated intrusive mechanism

Acknowledgements We thank an anonymous reviewer for the several constructive comments. The seismic interpretation and image processing was carried out in the SeisLab facility at the University of Aberdeen (sponsored by BG BP and Chevron). Seismic imaging analysis was performed using GeoTeric® (ffA), and analysis of seismic amplitudes was performed in Petrel® 2016 (Schlumberger). We would like to thank the Tuscany PhD Regional program and the Erasmus+exchange for funding the Aberdeen permanence of one of us (D.M.). Gazprom for supporting A.J PhD., BP for the release of the Loyal field seismic dataset utilized in this research paper and also N.Vanden Beukel (BP) and M. Gorling (BP) and his colleagues for their assistance.Peer reviewedPostprin

Assetto strutturale e tassi di deformazione lungo il margine esterno dell'Appennino Settentrionale, con implicazioni per la migrazione di fluidi profondi

Riassunto Scopo principale di questa tesi di dottorato è l’analisi dell’assetto strutturale del margine esterno dell’Appennino Settentrionale Emiliano-Romagnolo e Marchigiano, nonché lo studio delle relazioni che intercorrono tra le strutture ivi presenti e la migrazione di fluidi profondi. Tale fenomeno, nell’area di studio comporta la formazione di cosiddetti vulcanelli di fango (mud volcanoes), ben conosciuti ed investigati nel settore Emiliano-Romagnolo, tuttavia meno studiati in quello Marchigiano. Inoltre, tale studio ha visto l’utilizzo di dataset sismici 3D, provenienti da altri contesti tettonici, utili per l’investigazione dettagliata delle strutture, geometrie, meccanismi di messa in posto e processi evolutivi di analoghi dei vulcanelli di fango. Il margine Appenninico Emiliano-Romagnolo è stato investigato con un approccio multidisciplinare, integrando dati strutturali, sedimentologici, pedologici e sismici. In aggiunta, si è proceduto, per due transetti individuati attraverso di esso, ad impostare una modellizzazione numerica -effettuata applicando il metodo deformativo del Trishear- atta a calcolare i tassi di deformazione della principale struttura del margine, il cosiddetto Thrust Pedeappenninico (PAT). L’analisi ha messo in evidenza come il settore studiato (compreso tra le Valli del Fiume Panaro e del Fiume Enza) sia interessato da un’attività tettonica recente. Il PAT infatti, spesso emergente o sub-emergente, deforma depositi Medio Pleistocenici. Tali depositi, investigati da un punto di vista sedimentologico, hanno messo in evidenza come le strutture del margine abbiano condizionato non solo l’evoluzione tettonica del margine stesso, ma anche il reticolo idrografico dei paleo-corsi d’acqua. Si è messo in evidenza infatti come durante il Pleistocene Medio questi fossero orientati parallelamente al margine Appenninico, almeno per alcuni tratti. Tale orientamento si riscontra anche in depositi estremamente recenti, e localizzati a fronte del margine in corrispondenza del Plateau di Ghiardo. Datazioni OSL di suoli ivi deformati hanno permesso di individuare fasi tettoniche relative a circa 60-80 kyr. La modellizzazione numerica ha permesso di calcolare i tassi di deformazione del PAT, ed in particolare gli slip rate, che sono risultati essere compresi nell’ordine di 0,68-0,79 mm/anno per gli ultimi 0,8-1,2 Ma. Nello stesso settore, e in quello immediatamente circostante la città di Bologna, si è dunque provveduto ad investigare le relazioni intercorrenti tra le strutture del margine e i fluidi responsabili della formazione dei mud volcanoes. Sezioni sismiche hanno permesso di mappare le strutture del sottosuolo e constatare la corrispondenza tra anticlinali e vulcanelli. In particolare, uno studio di dettaglio sul vulcanello denominato Dragone di Sassuno, ha messo in evidenza questa dipendenza, nonché l’ulteriore relazione intercorrente tra strutture e fluidi: questi, nella risalita verso la superficie, utilizzano infatti i sistemi di frattura associati all’anticlinale sul quale il vulcanello giace, e che probabilmente svolge in profondità il ruolo di trappola per i fluidi stessi. Come ulteriore approfondimento della tematica, si è provveduto ad indagare possibili correlazioni tra l’accadimento di terremoti storici e l’attivazione di tale vulcanello di fango. È stato possibile modellizzare le variazioni degli stress statici associati a tre eventi sismici di M> 4 occorsi tra il 1779 ed il 1780, in concomitanza di forti eruzioni del vulcanello di Sassuno, descritte dettagliatamente dall’Autore Serafino Calindri al tempo dell’eruzione. In questo caso si è osservato che tale variazione degli stress non può aver avuto un’influenza positiva sulle attivazioni, essendo i valori ottenuti negativi. Tali valori indicano infatti condizioni sfavorevoli all’apertura del complesso di fratture costituenti il feeder dyke system del vulcanello. Si è ipotizzato quindi, data la corrispondenza temporale tra eventi sismici ed attivazione del vulcanello, una possibile dipendenza di tali attivazioni dagli stress dinamici, transitori ed associati al passaggio delle onde sismiche. Vulcanelli di fango sono stati individuati ed investigati anche lungo il Margine Apenninico Marchigiano, con particolare focus su tre aree chiave: S.Paolo di Jesi, Montelone di Fermo e Offida. Anche in questo caso si è provveduto a caratterizzarne l’assetto strutturale con rilievo di campagna ed utilizzo di sezioni sismiche. Come nel caso dei vulcanelli emiliani, si è riscontrata una corrispondenza tra anticlinali -in questo caso sepolte-, strutture ad esse associate e vulcanelli di fango. Per i vulcanelli di Monteleone di Fermo e di Offida, si è provveduto ad indagare la possibilità di attivazione a seguito di triggering sismico. Un caso particolarmente interessante è rappresentato dall’attivazione dei vulcanelli di Monteleone di Fermo a seguito degli eventi sismici dell’Italia Centrale, verificatisi a partire dal 24 Agosto 2016. Si è potuto verificare come i valori degli stress dinamici calcolati in corrispondenza dei vulcanelli attivati siano sufficientemente elevati (>4 bar) da essere considerati la principale causa di attivazione, considerando anche i non sufficienti valori degli stress statici (70-80 km) e l’innesco delle eruzioni. Come ultima analisi si è provveduto ad utilizzare due volumi sismici 3D provenienti dal Loyal Field (Scozia) e dal Bacino di Ceará (Brasile). Il primo dataset ha messo in evidenza fondamentali punti di similarità tra l’evoluzione delle cosiddette “fluid escape pipes” e i vulcanelli di fango. Entrambe le tipologie di oggetti geologici sono strettamente dipendenti dall’assetto strutturale delle anticlinali che ne ospitano i reservoir e dalle geometrie dei sistemi di frattura che favoriscono la risalita dei fluidi verso la superficie. In aggiunta, il fluido responsabile della formazione delle fluid escape pipes del Loyal field è risultato essere composto da una mistura contenente fango-acqua-idrocarburi, come nel caso dei vulcanelli di fango italiani. Ulteriore analogia risiede nell’evoluzione dell’attività delle fluid escape pipes e dei vulcanelli, entrambi alternanti periodi di quiescenza a periodi di intensa attività eruttiva. Nel caso del Bacino di Ceará, la presenza sul fondale marino di oggetti assimilabili a pockmarks, ovvero zone depresse indicanti l’emissione di fluidi, è risultata essere uno stimolo per l’analisi del dataset, nell’ottica di un’ulteriore analisi dei processi che guidano la migrazione dei fluidi stessi. Lo studio ha però messo in evidenza come queste depressioni siano in realtà associate a forme di fondo (sediment waves), impilate in maniera tale da formare condotti di migrazione preferenziale, tuttavia passiva, per i fluidi intrappolati nel sottostante reservoir, in maniera indipendente da strutture tettoniche. Abstract The principal aim of this Ph.D. thesis was the investigation of the structural setting of the external Northern Apennine margin, in the Emilia-Romagna and Marche Region, as well as the analysis of possible relations linking the structure of the margin and the migration of deep fluids. This last phenomenon is responsible for the formation of the so-called mud volcanoes, which are of particular interest for our study and that have been investigated with a multidisciplinary approach. First, I investigated the structural setting of the Emilia-Romagna pede-Apennine margin, including possible relations with the mud volcanoes of the area. The same study approach was used for the Marche foothills, and for their associated mud volcanoes. Furthermore, I studied two 3D seismic dataset from different tectonic contexts, to be used as analogues for investigation of mud volcanoes and -more in general- fluid migration processes. The Emilia Romagna Pede-Apennine Margin has been analysed using a multidisciplinary approach, integrating structural, sedimentological, pedological and seismic data. In addition, I carried, for two target transects crossing the margin (at Quattro Castella and Scandiano localities), cinematic 2D cross section numerical modelling of the main structure, the so-called Pede-Apennine Thrust (PAT). The implemented Trishear deformation mechanism, has shown how the studied sector (between the Enza and Panaro rivers) has been tectonically active in recent times. The PAT is poorly exposed and deforms at least Middle Pleistocene deposits. These semi-consolidated sediments, investigated from a sedimentological point of view, revealed that the PAT and its related structures influenced also the river drainage pattern, forcing paleo-rivers to flow parallel to the margin (which roughly strikes NW-SE), at least in some sectors during the Pleistocene. This trend is also visible in extremely recent deposits, outcropping in the vicinity of the margin at the Ghiardo Plateau, that have been dated using the OSL technique applied to deformed palaeosoils. Dating highlighted an extremely recent tectonic phase (~60-80 kyr). Numerical modelling allowed us to calculate PAT deformation rates. Of particular interest are the slip rates, which reach maximum values up to 0.68-0.79 mm/yr for the last 0.8-1.2 Ma. In the same sector, extended toward the west to also include the Bologna area, I performed an analysis of the relationships between fluid migration and structural framework, which is believed to be directly responsible for the formation of the locally outcropping mud volcanoes. Subsurface structures were mapped using seismic sections, confirming a straightforward correspondence between anticlines and mud volcanoes, being generally located on the anticline surface projection (roughly close to the anticline axis or in the anticline’s forelimb/backlimb). In particular, I investigated the Dragone di Sassuno mud volcano, evidencing this correlation and the link between the upward migrating fluids and the fracture array associated to the associated anticline: fluids exploit systematic fractures to reach the surface. An in depth analysis was carried out to investigate possible relationships linking the seismic activity of the PAT and the mud volcano eruptions. I investigated three seismic events with M > 4 occurred between 1779 and 1780 and I modelled the variation of static stresses associated to these earthquakes, that occurred almost contemporaneously to large eruption of the Dragone di Sassuno mud volcano, as described by the Author Serafino Calindri at the time of eruption. I observed that static stress changes have not been responsible for triggering the mud volcano eruption, being the calculated values negative. Negative values imply that the fractures forming the feeder dyke systems were maintained closed by the stresses, inhibiting the fluid flow. Therefore, I propose a correspondence between the dynamic stresses associated with these earthquakes and the mud volcano eruptions: dynamic stresses, shaking the reservoir, are able to increase overpressures and therefore favour the eruptions. Mud volcanoes have been investigated also along the Marche Apennine foothill, with particular focus on the mud volcano systems located in three key areas: S. Paolo di Jesi, Monteleone di Fermo and Offida. As for the Emilia-Romagna mud volcanoes, I characterized the structural setting of these systems using field and seismic data. Also in these cases, a correspondence between mud volcano and anticline structures has been observed. For the Monteloene di Fermo and Offida mud volcanoes eruptions, I investigated the possibility of seismic triggering. An interesting case study was offered by the activation of the Monteloene di Fermo mud volcanoes coincident with three major earthquakes of the 2016-2017 Central Italy seismic sequence. I verified that the dynamic stress associated with the seismic events and calculated at the mud volcano location was high enough (>4 bar) to be considered the main trigger of mud volcano activation. Normal stress changes were too small (less than 0.1 bar) to have had any influence on the triggering. Summarizing, I verified a “direct” relationship between structures (anticlines) and mud volcanoes, which steers their location, and an “indirect” one, linking the activation of seismic structures (also far from the mud volcanoes location, ~70-80 km) and the mud volcano eruptions. As a final investigation, I used two 3D seismic analogues from the Loyal Filed (Scotland) and from the Caerá Basin (Brazil) as seismic analogues for fluid migration mechanisms. The first dataset, highlighted fundamental similarities between the so called “fluid escape pipes” -here widely distributed- and mud volcanoes. Both are linked to anticline structures, and fluids responsible for their formation exploit systematic fractures to migrate upward. In addition, the fluid responsible for the Loyal Field fluid escape pipes formation, revealed to be composed of a mud-water-hydrocarbons mixture, as in the case of the Italian mud volcanoes. A further analogy is represented by the evolution and activity of these geological objects, alternating periods of large eruptions to periods of quiescence. The second dataset evidenced the presence of a very significant amount of depressions resembling pockmarks at the seabottom, generally indicating the presence of a strong fluid flux. Therefore, it has been analysed in order to obtain further information on a seismic analogue on fluid migration mechanisms and possible relation with structures. Nonetheless, this analogue case study from a different tectonic context highlighted the dependence of these fluid migration pathways on large-scale sediment waves, vertically stacking their permeable trough to form preferential paths for fluid “passive” migration. Therefore, in this case I did not highlight any correspondence between tectonic structures and fluid migration

Supplemental Material: Reconciling plate motion and faulting at a Rift-Rift-Rift triple junction

The Supplemental Material reports text, table, and figures showing details about analog model setup and scaling, particle image velocimetry analysis, as well as additional comparison with the natural case study.  </p

Quaternary slip rates along the frontal thrust system of the Pede-Apennine margin, between the Enza and Panaro valleys, Emilia Romagna, Italy

The study area is located on the Emilia Pede-Apennine Margin, between the Reno and the Enza valleys. This area is marked by the Pede-Apennine Thrust Front (PTF) (Boccaletti et al., 1985), which separates the internal sectors of the Northern Apennines from the external thrust fronts (ETF), buried underneath the Quaternary deposits of the Po Plain. Growth strata layouts, related to PTF activity, can be observed in discontinuous outcrops along the Pede-Apennine Margin. In this study, the reconstruction of Quaternary deformation rates was addressed by means of the analysis of growth strata and structural field data. Therefore, we carried out a structural and geological survey of the stratigraphic units that present a growth strata layout. We collected a dataset made up of mesoscopic joints and faults to which the application of a stress inversion procedure yielded a sub-horizontal N-NE-oriented s1. Two sets of numerical models (for a total of 104) were built considering two transects across the PTF, referred herein to as Quattro Castella and Scandiano, where the SSW-dipping Pede-Apennine thrust (PAT, which is the main PTF structure), is nearly surfacing along the margin. The Trishear Fault-propagation Folding mechanism (Erslev, 1991) was applied to the considered transects by using the software Fault Fold (Allmendinger, 1998). For both transects a best fit model was chosen, by comparing the geological data with the obtained outputs. With the data provided by the best fit models, we then obtained the PAT deformation rates in the study area. At Quattro Castella, averaged minimum values over the last 1,07 Myr, are: slip rate 0,79 mm/yr, propagation rate 1,96 mm/yr, shortening rate 0,50 mm/yr, and uplift rate 0,62 mm/yr. At Scandiano averaged minimum values over the last 800 kyr are: slip rate 0,70 mm/yr, propagation rate 2,10 mm/yr, shortening rate 0,44 mm/yr, and uplift rate 0,59 mm/yr. The obtained values are compatible with others determined in adjoining areas, and thus can be considered representative of the PAT in the analysed sector. Finally, our models suggest that, along the considered transects, the PAT is best depicted as a high-angle thrust fault (> 50°). Allmendinger R.W. 1998. Inverse and forward numerical modelling of trishear fault propagation-folds. Tectonics, 17, 640-656. Boccaletti M., Coli M., Eva C., Ferrari G., Giglia G., Lazzarotto A., Merlanti F., Nicolich R., Papani G. & Postpichl D. 1985. Consideration on the seismotectonics of the Northern Apennines. Tectonophysics, 117, 7-38. Erslev E.A. 1991. Trishear fault-propagation folding. Geology, 19, 617-620

Digital image correlation data from analogue modelling experiments addressing the influence of basin geometry on gravity-driven salt tectonics at the Tectonic Modelling Lab of the University of Rennes (F)

This data set includes the results of digital image correlation of 35 brittle-viscous experiments on gravitational salt tectonics performed at the Tectonic Modelling Lab of the University of Rennes 1 (UR1). The experiments demonstrate the influence of basin geometry on gravity-driven salt tectonics. Detailed descriptions of the experiments can be found in Zwaan et al. (2021) to which this data set is supplementary. The data presented here consist of movies and images displaying the cumulative analogue model surface displacement, digital elevation models as well as profiles of the downslope cumulative displacements and surface elevation

Analog and numerical modeling of Rift-Rift-Rift triple junctions

Rift-Rift-Rift triple junctions are key features of emergent plate boundary networks during fragmentation of a continent. A key example of such a setting is the Afar triple junction where the African, Arabian and Somalian plates interact. We performed analog and numerical models simulating continental break-up in a Rift-Rift-Rift setting to investigate the resulting structural pattern and evolution. We modified the ratio between plate velocities, and we performed single-stage (with all plates moving at the same time) and two-stage (where one plate first moves alone and then all the plates move simultaneously) models. Additionally, the direction of extension was changed to induce orthogonal extension in one of the three rift branches. Our models suggest that differential extension velocities in the rift branches determine the localization of the structural triple junction, which is located closer to the rift branch experiencing slower extension velocities. Furthermore, imposed velocities affect the deformation resulting in end-member fault patterns. The effect of applying similar velocities in all rift arms is to induce a symmetric fault pattern (generating a Y-shaped geometry). In contrast, a faster plate generates structures trending orthogonal to dominant velocity vectors, while faults associated with the movement of the slower plates remain subordinate (generating a T-shaped pattern). Two-stage models reveal high-angle faults interacting at the triple junction, confirming that differential extension velocities strongly affect fault patterns. These latter models show large-scale similarities with fault patterns observed in the Afar triple junction, providing insights into the factors controlling the structural evolution of this area