Spectral-Element and Adjoint 3D Full-Wave Tomography for the Lithosphere of Central Italy

The primary objective of this thesis is to obtain a better understanding of the 3D velocity structure of the lithosphere in central Italy. To this end, I adopted the Spectral-Element Method to perform accurate numerical simulations of the complex wavefields generated by the 2009 Mw 6.3 L’Aquila event and by its foreshocks and aftershocks together with some additional events within our target region. For the mainshock, the source was represented by a finite fault and different models for central Italy, both 1D and 3D, were tested. Surface topography, attenuation and Moho discontinuity were also accounted for. Three-component synthetic waveforms were compared to the corresponding recorded data. The results of these analyses show that 3D models, including all the known structural heterogeneities in the region, are essential to accurately reproduce waveform propagation. They allow to capture features of the seismograms, mainly related to topography or to low wavespeed areas, and, combined with a finite fault model, result into a favorable match between data and synthetics for frequencies up to ~0.5 Hz. We also obtained peak ground velocity maps, that provide valuable information for seismic hazard assessment. The remaining differences between data and synthetics led us to take advantage of SEM combined with an adjoint method to iteratively improve the available 3D structure model for central Italy. A total of 63 events and 52 stations in the region were considered. We performed five iterations of the tomographic inversion, by calculating the misfit function gradient - necessary for the model update - from adjoint sensitivity kernels, constructed using only two simulations for each event. Our last updated model features a reduced traveltime misfit function and improved agreement between data and synthetics, although further iterations, as well as refined source solutions, are necessary to obtain a new reference 3D model for central Italy tomography.

Seismic tomographic full-waveform inversion for the Vrancea sinking lithosphere structure using the adjoint method.

The Vrancea region, at the south-eastern bend of the Carpathian Mountains in Romania, represents one of the most puzzling seismically active zones of Europe. Beside some shallow seismicity spread across the whole Romanian territory, Vrancea is the place of an intense seismicity with the presence of a cluster of intermediate-depth foci placed in a narrow nearly vertical volume. Although large-scale mantle seismic tomographic studies have revealed the presence of a narrow, almost vertical, high-velocity body in the upper mantle, the nature and the geodynamic of this deep intra-continental seismicity is still questioned. High-resolution seismic tomography could help to reveal more details in the subcrustal structure of Vrancea. Recent developments in computational seismology as well as the availability of parallel computing now allow to potentially retrieve more information out of seismic waveforms and to reach such high-resolution models. This study was aimed to evaluate the application of a full waveform inversion tomography at regional scale for the Vrancea lithosphere using data from the 1999 six months temporary local network CALIXTO. Starting from a detailed 3D Vp, Vs and density model, built on classical travel-time tomography together with gravity data, I evaluated the improvements obtained with the full waveform inversion approach. The latter proved to be highly problem dependent and highly computational expensive. The model retrieved after the first two iterations does not show large variations with respect to the initial model but remains in agreement with previous tomographic models. It presents a well-defined downgoing slab shape high velocity anomaly, composed of a N-S horizontal anomaly in the depths between 40 and 70km linked to a nearly vertical NE-SW anomaly from 70 to 180km.

Erosione e sollevamento nell'arco calabro-peloritano

The Calabrian-Peloritani arc represents key site to unravel evolution of surface processes on top of subducting lithosphere. During the Pleistocene, in fact the arc uplifted at rate of the order of about 1mm/yr, forming high-standing low-relief upland (figure 2). Our study is focused on the relationship between tectonic and land evolution in the Sila Massif, Messina strait and Peloritani Mts. Landforms reflect a competition between tectonic, climatic, and surficial processes. Many landscape evolution models that explore feedbacks between these competing processes, given steady forcing, predict a state of erosional equilibrium, where the rates of river incision and hillslope erosion balance rock uplift. It has been suggested that this may be the final constructive stage of orogenic systems. Assumptions of steady erosion and incision are used in the interpretation of exhumation and uplift rates from different geologic data, and in the formulation of fluvial incision and hillslope evolution models. In the Sila massif we carried out cosmogenic isotopes analysis on 24 samples of modern fluvial sediments to constrain long-term (~103 yr) erosion rate averaged on the catchment area. 35 longitudinal rivers profiles have been analyzed to study the tectonic signal on the landscape evolution. The rivers analyzed exhibit a wide variety of profile forms, diverging from equilibrium state form. Generally the river profiles show at least 2 and often 3 distinct concave-up knickpoint-bounded segments, characterized by different value of concavity and steepness indices. River profiles suggest three main stages of incision. The values of ks and θ in the lower segments evidence a decrease in river incision, due probably to increasing uplift rate. The cosmogenic erosion rates pointed out that old landscape upland is eroding slowly at ~0.1 mm/yr. In the contrary, the flanks of the massif is eroding faster with value from 0.4 to 0.5 mm/yr due to river incision and hillslope processes. Cosmogenic erosion rates mach linearly with steepness indices and with average hillslope gradient. In the Messina area the long term erosion rate from low-T thermochronometry are of the same order than millennium scale cosmogenic erosion rate (1-2 mm/yr). In this part of the chain the fast erosion is active since several million years, probably controlled by extensional tectonic regime. In the Peloritani Mts apatite fission-track and (U-Th)/He thermochronometry are applied to constraint the thermal history of the basement rock. Apatite fission-track ages range between 29.0±5.5 and 5.5±0.9 Ma while apatite (U-Th)/He ages vary from 19.4 to 1.0 Ma. Most of the AFT ages are younger than the overlying terrigenous sequence that in turn postdates the main orogenic phase. Through the coupling of the thermal modelling with the stratigraphic record, a Middle Miocene thermal event due to tectonic burial is unravel. This event affected a inner-intermediate portion of the Peloritani belt confined by young AFT data (<15 Ma) distribution. We interpret this thermal event as due to an out-of–sequence thrusting occurring in the inner portion of the belt. Young (U-Th)/He ages (c. 5 Ma) record a final exhumation stage with increasing rates of denudation since the Pliocene times due to postorogenic extensional tectonics and regional uplift. In the final chapter we change the spatial scale to insert digital topography analysis and field data within a geodynamic model that can explain surface evidence produced by subduction process.

Deep geometry of subduction below the Andean belt of Colombia as revealed by seismic tomography

In this study new tomographic models of Colombia were calculated. I used the seismicity recorded by the Colombian seismic network during the period 2006-2009. In this time period, the improvement of the seismic network yields more stable hypocentral results with respect to older data set and allows to compute new 3D Vp and Vp/Vs models. The final dataset consists of 10813 P- and 8614 S-arrival times associated to 1405 earthquakes. Tests with synthetic data and resolution analysis indicate that velocity models are well constrained in central, western and southwestern Colombia to a depth of 160 km; the resolution is poor in the northern Colombia and close to Venezuela due to a lack of seismic stations and seismicity. The tomographic models and the relocated seismicity indicate the existence of E-SE subducting Nazca lithosphere beneath central and southern Colombia. The North-South changes in Wadati-Benioff zone, Vp & Vp/Vs pattern and volcanism, show that the downgoing plate is segmented by slab tears E-W directed, suggesting the presence of three sectors. Earthquakes in the northernmost sector represent most of the Colombian seimicity and concentrated on 100-170 km depth interval, beneath the Eastern Cordillera. Here a massive dehydration is inferred, resulting from a delay in the eclogitization of a thickened oceanic crust in a flat-subduction geometry. In this sector a cluster of intermediate-depth seismicity (Bucaramanga Nest) is present beneath the elbow of the Eastern Cordillera, interpreted as the result of massive and highly localized dehydration phenomenon caused by a hyper-hydrous oceanic crust. The central and southern sectors, although different in Vp pattern show, conversely, a continuous, steep and more homogeneous Wadati-Benioff zone with overlying volcanic areas. Here a "normalthickened" oceanic crust is inferred, allowing for a gradual and continuous metamorphic reactions to take place with depth, enabling the fluid migration towards the mantle wedge.

Thermochronological evolution of the Eastern Pontides and the Eastern Anatolian Plateau and NW Lesser Caucasus (Turkey, Georgia, Armenia)

The analysis of apatite fission tracks is applied to the study of the syn- and post-collisional thermochronological evolution of a vast area that includes the Eastern Pontides, their continuation in the Lesser Caucasus of Georgia (Adjara-Trialeti zone) and northern Armenia, and the eastern Anatolian Plateau. The resulting database is then integrated with the data presented by Okay et al. (2010) for the Bitlis Pütürge Massif, i.e. the western portion of the Bitlis-Zagros collision zone between Arabia and Eurasia. The mid-Miocene exhumation episode along the Black Sea coast and Lesser Caucasus of Armenia documented in this dissertation mirrors the age of collision between the Eurasian and Arabian plates along the Bitlis suture zone. We argue that tectonic stresses generated along the Bitlis collision zone were transmitted northward across eastern Anatolia and focused (i) at the rheological boundary between the Anatolian continental lithosphere and the (quasi)oceanic lithosphere of the Black Sea, and (ii) along major pre-existing discontinuities like the Sevan-Akera suture zone.The integration of both present-day crustal dynamics (GPS-derived kinematics and distribution of seismicity) and thermochronological data presented in this paper provides a comparison between short- and long-term deformation patterns for the entire eastern Anatolia-Transcaucasian region. Two successive stages of Neogene deformation of the northern foreland of the Arabia-Eurasia collision zone can be inferred. (i) Early and Middle Miocene: continental deformation was concentrated along the Arabia-Eurasia (Bitlis) collision zone but tectonic stress was also transferred northward across eastern Anatolia, focusing along the eastern Black Sea continent-ocean rheological transition and along major pre-existing structural discontinuities. (ii) Since Late-Middle Miocene time the westward translation of Anatolia and the activation of the North and Eastern Anatolian Fault systems have reduced efficient northward stress transfer.

Transform Tectonics and Non-Volcanic Oceanic Islands

Oceanic islands can be divided, according to their origin, in volcanic and tectonic. Volcanic islands are due to excess volcanism. Tectonic islands are mainly formed due to vertical tectonic motions of blocks of oceanic lithosphere along transverse ridges flanking transform faults at slow and ultraslow mid-ocean ridges. Vertical tectonic motions are due to a reorganization of the geometry of the transform plate boundary, with the transition from a transcurrent tectonics to a transtensive and/or transpressive tectonics, with the formation of the transverse ridges. Tectonic islands can be located also at the ridge–transform intersection: in this case the uplift is due by the movement of the long-lived detachment faults located along the flanks of the mid-ocean ridges. The "Vema" paleoisland (equatorial Atlantic) is at the summit of the southern transverse ridge of the Vema transform. It is now 450 m bsl and it is capped by a carbonate platform 500 m-thick, dated by 87Sr/86Sr at 10 Ma. Three tectonic paleoislands are on the summit of the transverse ridge flanking the Romanche megatrasform (equatorial Atlantic). They are now about 1,000 m bsl and they are formed by 300 m-thick carbonate platforms dated by 87Sr/86Sr, between 11 and 6 Ma. The tectonic paleoisland “Atlantis Bank" is located in the South-Western Indian Ridge, along the Atlantis II transform, and it is today 700 m bsl. The only modern example of oceanic tectonics island is the St. Paul Rocks (equatorial Atlantic), located along the St. Paul transform. This archipelago is the top of a peridotitic massif that it is now a left overstep undergoing transpression. Oceanic volcanic islands are characterized by rapid growth and subsequent thermal subsidence and drowning; in contrast, oceanic tectonic islands may have one or more stages of emersion related to vertical tectonic events along the large oceanic fracture zones.

Structural and stratigraphic control on hypogene dissolution in carbonate rocks: case studies of cave analogues and implications for karst reservoirs

The aim of this research is to improve the understanding of the factors that control the formation of karst porosity in hypogene settings and its associated patterns of void-conduit networks. Subsurface voids created by hypogene dissolution may span from few microns to decametric tubes providing interconnected conduit systems and forming highly anisotropic permeability domains in many reservoirs. Characterizing the spatial-morphological organization of hypogene karst is a challenging task that has dramatic implications for the applied industry, given that only partial data can be acquired from the subsurface by indirect techniques. Therefore, two outcropping cave analogues are examined: the Cavallone-Bove Cave in the Majella Massif (Italy), and the karst systems of the Salitre Formation (Brazil). In the latter, a peculiar example of hypogene speleogenesis associated with silicification has been studied, providing an analogue of many karstified reservoirs hosted in cherts or cherty-carbonates within mixed sedimentary sequences. The first part of the thesis is focused on the relationships between fracture patterns and flow pathways in deformed units in: 1) a fold-and-thrust setting (Majella Massif); 2) a cratonic block (Brazil). These settings represent potential playgrounds for the migration and accumulation of geofluids, where hypogene conduits may affect flow pathways, fluid storage, and reservoir properties. The results indicate that localized deformation producing cross-formational fracture zones associated with anticline hinges or fault damage zones is critical for hypogene fluid migration and karstification. The second part of the thesis deals with the multidisciplinary study of hydrothermal silicification and hypogene dissolution in Calixto Cave (Brazil). Petrophysical analyses and a geochemical characterization of silica deposits are used to unravel the spatial-morphological organization of the conduit system and its speleogenesis. The novel results obtained from this cave shed new light on the relationship between hydrothermal silicification, hypogene dissolution and the development of multistorey cave systems in layered carbonate-siliciclastic sequences.