SCUOLA DOTTORALE IN GEOLOGIA DELL'AMBIENTE E DELLE RISORSE (SDiGAR) XVII CICLO Surface deformation and magma intrusion along divergent plate boundaries Dottorando: __________________ Daniele Trippanera firma Docente Guida: __________________ Valerio Acocella firma Coordinatore: __________________ Claudio Faccenna firma Co-Tutor: Dr. Joel Ruch: ricercatore presso King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia. Revisori: Dott. Marco Neri: primo ricercatore presso l’Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, Sezione di Catania, Unità Funzionale Gravimetria e Magnetismo. Dott.ssa Eleonora Rivalta: PI of an ERC Starting grant research group, project CCMP-POMPEI presso il GeoForschungsZentrum GFZ German Research Centre for Geosciences. Commissari: Prof.ssa Maria Rita Palombo: professore Associato SSD GEO/01 Dipartimento di Scienze della Terra Università Roma la Sapienza. Prof. Orlando Vaselli: professore Associato SSD GEO/08 Dipartimento di Scienze della Terra Università di Firenze. Dott. Marco Bonini: professore Associato SSD GEO/03, ricercatore CNR, Istituto di Geoscienze e Georisorse Dipartimento di Scienze della Terra, Università di Firenze. Table of Contents Abstract (in English) ........................................................................................................................................... 1 Riassunto (in Italian) .......................................................................................................................................... 2 Introduction ....................................................................................................................................................... 4 1.2 Methods .................................................................................................................................................... 5 1.2.1 Field surveys ...................................................................................................................................... 5 1.2.2 Analogue models ............................................................................................................................... 8 Chapter 2: Surface deformation along the divergent plate boundaries: understanding magmatic vs. tectonic processes ............................................................................................................................................ 11 Abstract ........................................................................................................................................................ 11 1. Introduction ............................................................................................................................................. 12 2. Tectonic setting ........................................................................................................................................ 14 3. Methods ................................................................................................................................................... 16 4. Results ...................................................................................................................................................... 17 4.1. Eruptive fissures ................................................................................................................................ 17 4.1.1 Lakagigar ..................................................................................................................................... 17 4.1.2 Eldgjá .......................................................................................................................................... 18 4.1.3 Bardarbunga .............................................................................................................................. 19 4.1.4 Sveinagja .................................................................................................................................... 20 4.1.5 Sveinar ....................................................................................................................................... 21 4.2. Rift segments ..................................................................................................................................... 21 4.2.1 The Krafla magmatic system ...................................................................................................... 21 4.2.2 Vogar rift zone ........................................................................................................................... 23 4.2.3 Thingvellir rift zone .................................................................................................................... 24 4.2.4 Fantale magmatic system .......................................................................................................... 24 5. Discussion ................................................................................................................................................ 25 5.1. Interpreting the collected data .......................................................................................................... 25 5.2. A general model ................................................................................................................................. 30 6. Conclusions .............................................................................................................................................. 32 References .......................................................................................................................................... 33 Figure and tables ................................................................................................................................ 39 Chapter 3: Experiments of dike-induced deformation: an application to divergent plate boundaries ....... 57 Abstract ........................................................................................................................................................ 57 1. Introduction ............................................................................................................................................. 58 2. Experimental setup and scaling ................................................................................................................ 59 2.1. Setup and methods............................................................................................................................ 59 2.2. Scaling and materials ......................................................................................................................... 61 2.3. Assumptions and limitations ............................................................................................................. 62 3. Results ...................................................................................................................................................... 63 3.1. Setup A: upward propagating intrusions. Experiment A2 ................................................................. 63 3.2. Setup B and C: thickening intrusions at constant depth ................................................................... 64 3.2.1 Experiment B1: shallow rectangular intrusive complex ............................................................. 64 3.2.2 Experiment B4: medium depth rectangular intrusive complex ................................................. 66 3.2.3 Experiment B6: deep rectangular intrusive complex ................................................................. 67 3.2.4 Experiment C1: shallow triangular intrusive complex ................................................................ 68 3.2.5 Experiment C2: medium depth triangular intrusive complex .................................................... 69 4. Discussions ............................................................................................................................................... 69 4.1. Overall deformation pattern and setup relevance ............................................................................ 69 4.2. Effect of the intrusions depth and geometry on the deformation.................................................... 72 4.3. Fault propagation .............................................................................................................................. 73 4.3.1 Normal faulting ........................................................................................................................... 73 4.3.2 Arcuate normal/reverse faults ................................................................................................... 74 4.4. Comparison to rift zones ................................................................................................................... 75 4.4.1 Single rifting episodes ................................................................................................................. 76 4.4.2 Multiple rifting episodes ............................................................................................................. 77 4.4.3 Intrusion depth vs. thickness ...................................................................................................... 79 5. Conclusions .............................................................................................................................................. 81 References .......................................................................................................................................... 82 Figure and tables ................................................................................................................................ 87 Chapter 4: Experiments of dike-induced deformation: an application to divergent plate boundaries ..... 105 Abstract ...................................................................................................................................................... 105 1. Introduction ........................................................................................................................................... 105 2. Field analysis ........................................................................................................................................... 105 3. Analogue models .................................................................................................................................... 107 4. Discussion and conclusions..................................................................................................................... 109 References ........................................................................................................................................ 110 Chapter 5: General conclusions ..................................................................................................................... 112 Abstract The interest in the role of magma in splitting plates at divergent plate boundaries has been recently re-enhanced. However, the peculiar mechanism by which the magma affects the geometry, the kinematics, and the temporal evolution of a rift is still poorly understood, especially in a long time perspective. Moreover, it is also necessary to better define how and to what extend the regional tectonics - by means of plate pull mechanism - affects the rift structures formation along these margins. The aim of this work is to address these issues through field survey and analogue modeling. Field survey consists of studying the surface deformation along different fissural portions of the magmatic systems at the Neovolcanic Zone of Iceland and the Main Ethiopian Rift, focusing mainly on: 1) single eruptive fissures (Laki, Eldgjá and Bardarbunga in Iceland) or narrow fissure zones hosting recent eruptive fissures (i.e., Sveinagjá and Sveinar in Iceland), 2) wider fissure zones where several rifting episodes occurred (i.e. Krafla, Vogar and Thingvellir in Iceland and Fantale in Ethiopia). In all these areas, fault and extension fracture geometries and kinematics have been systematically characterized, including the analysis of the structure of the fault’s lateral terminations, conceived as possible indicators of their propagation direction. In addition, these data have been integrated with the study of the roots (depth of about 1.5 km) of the fossil Alftafjordur magmatic system (Eastern Iceland). Analogue models have been used to test the effect of repeated dike intrusions at the surface, characterizing the geometry and the kinematics of dike-induced structures. In order to quantify and reconstruct the temporal evolution of the surface deformation, laser-scanner and Particle Image Velocimetry (PIV) techniques have been applied to the models. Field analysis show that at the surface, the eruptive fissures are bounded by normal faults forming a graben, thus suggesting a clear relationship between diking and surface deformation. Grabens, normal faults and extension fractures also characterize the surface of wider fissure zones. The faults usually exhibit an open structure, with locally tilted hanging wall and possible contraction at its base. However, the study of the fissure zones roots reveals that at depth the extension is accommodated by dikes, with almost no faulting. Analogue models results show that a graben forms above a dike complex gradually thickening at depth, whose structures geometry and kinematic depend on ratio between the intrusion depth and its cumulative thickness. This suggests, again, that diking may play a fundamental role in rift formation. 1 Models and nature share several common features (i.e., graben with downward propagating normal faults, contraction at the base of the tilted hanging wall, subsidence above the dike complex and uplift to its sides), suggesting that most deformation along divergent plate boundaries may be acquired through repeated dike injection, requiring no direct tectonic contribution. However, the extension due to the regional tectonics remains still important for the long-term evolution of a divergent plate boundary: on one hand, it provides the required conditions for the rise and focusing of magma along the axial zone of the margin, on the other, it enhances the fault activity in more distal areas from the axial zones and during the inter-rifting periods. Riassunto L’interesse rispetto al ruolo del magma nella separazione delle placche lungo i margini divergenti si è recentemente rinvigorito. Tuttavia, il meccanismo preciso con il quale il magma influenza la geometria, la cinematica e l’evoluzione temporale di un rift è ancora poco chiaro, soprattutto nel lungo termine. Inoltre, resta da definire quanto e come la tettonica regionale, attraverso il meccanismo di plate pull, influisce nella formazione delle strutture di rift lungo tali margini. Questo lavoro ha lo scopo di far luce su tali problematiche, attraverso analisi di terreno e modellazione analogica. L’analisi di terreno comprende lo studio della deformazione in superficie lungo diverse porzioni fissurali dei sistemi magmatici della Neovolcanic Zone in Islanda e del Main Ethiopian Rift, in particolare focalizzandosi su: 1) singole fessure eruttive (Laki, Eldgjá e Bardarbunga in Islanda) o strette zone fissurali che ospitano recenti fessure eruttive (Sveinagjá e Sveinar in Islanda), 2) ampie zone fissurali dove sono avvenuti numerosi episodi di rifting (Krafla, Vogar e Thingvellir in Islanda e Fantale in Etiopia). In queste aree, la geometria e la cinematica di faglie e fratture estensionali sono state caratterizzate in maniera sistematica, includendo l’analisi della struttura delle terminazioni laterali delle faglie come possibile indicatore del loro senso di propagazione. Inoltre, tali analisi sono state integrate con lo studio delle radici (paleo-profondità di circa 1.5 km) del sistema magmatico fossile di Alftafjordur (Islanda orientale). Attraverso i modelli analogici, invece, si è testato l’effetto dell’intrusione ripetuta di dicchi sulla superficie, caratterizzando la geometria e la cinematica delle strutture da essi indotte. Per quantificare la deformazione e ricostruirne l’evoluzione nel tempo sono state applicate ai modelli lo scansionatore laser e la tecnica di Particle Image Velocimetry (PIV). 2 Le analisi di terreno mostrano che in superficie le fessure eruttive sono bordate da faglie normali che formano un graben, suggerendo così una chiara relazione tra dicchi e deformazione in superficie. Graben, faglie normali e fratture estensionali caratterizzano anche la superficie delle zone fissurali più ampie e mature dove più episodi di rifting sono avvenuti. Le faglie mostrano generalmente una struttura aperta, con il tetto della faglia localmente basculato e con la possibile presenza di contrazione alla sua base. Tuttavia, lo studio delle radici dei rift rivela che in profondità l’estensione è accomodata quasi esclusivamente da dicchi, con una ridotta fagliazione. I risultati dei modelli analogici mostrano che al di sopra di un complesso di dicchi che si inspessisce gradualmente, si ha la formazione di un graben, le cui geometria e cinematica delle strutture interne dipendono dal rapporto tra la profondità di intrusione ed il suo spessore cumulato. Questo suggerisce, ancora una volta, che l’intrusione di dicchi può avere un effetto diretto sulla superficie e quindi un ruolo fondamentale nella formazione dei rift. Modelli e natura mostrano molte caratteristiche comuni (ad es.: graben con faglie normali che si propagano verso il basso, contrazione alla base del tetto di faglia basculato, subsidenza al di sopra del complesso di dicchi e sollevamento ai suoi lati), suggerendo che la maggior parte della deformazione lungo i margini divergenti di placca può essere acquisita attraverso ripetute iniezioni di dicchi, non richiedendo un contributo diretto della tettonica regionale. L’estensione dovuta alla tettonica regionale rimane tuttavia importante per l’evoluzione a lungo termine di un margine divergente, sia perché essa fornisce le condizioni necessarie per la risalita e la focalizzazione del magma lungo la zona assiale del margine, sia perché promuove l’attività di fagliazione nelle aree più distali dalle zone assiali e durante i periodi di inter-rifting. 3 1. Introduction Most seismic and volcanic activity on Earth is present along the divergent plate boundaries. A peculiar aspect concerning these boundaries provides a definition of the mechanism which characterizes the separation process of plates. According to traditional theories, the phenomenon of separation occurs through tectonic processes at large scale (i.e., plates pull), linked to extensional faults’ activity. However, the recent rifting episodes which occurred in the last decades, highlighted the crucial role the magma can play in shaping the divergent boundaries. These episodes occurred in the oceanic, as well as in the continental and transitional crusts, allowing to undertake an accurate research on surface deformation directly inducted by magma emplacement, through dikes intrusion. In particular, geodetic measurements (mainly InSAR and GPS) show that during intrusive episodes a characteristic deformation occurs at the surface. It is composed of a narrow (few km large) strip above the dike/dikes that is subject to subsidence, while an uplift occurs at both sides; both subsidence and uplift may reach up to few meters [i.e., Rubin, 1992; Sigmundsson, 2006; Wright et al., 2006]. Moreover, perpendicularly to the dike, a relevant horizontal opening occurs, up to several meters [i.e., Wright et al., 2006; Biggs et al., 2009]. Such a deformation is also obtained by means of numerical and analytical modeling of a dike that opens at depth [i.e. Dieterich and Decker, 1975; Mastin and Pollard, 1988; Rubin and Pollard, 1988; Rubin, 1992; Dvorak and Dzurisin, 1997]. Observations in the field prove that, together with these kind of episodes or intrusive events, several fractures generate at the surface. This phenomenon is also associated with the formation or re-activation of normal faults (with metric throws), generating a graben centered and parallel respect to the dike/dikes [i.e., Bjornsson, 1977; Acocella and Neri, 2003; Rowland et al., 2007; Pallister et al., 2010]. Moreover, the generation of these grabens with inward dipping normal faults, related to dike propagations, has been also demonstrated through analytical, numerical and analogue models [Mastin and Pollard, 1988; Rubin and Pollard; 1988; Rubin, 1992; Gudmundsson, 2003; Gudmundsson, 2005]. Thus, all these observations have newly enhanced the interest towards divergent plate boundaries, reopening the whole question on traditional theories regarding plates’ separation process. In particular, it has been observed that the opening during rifting episodes can reach some meters of width across a short time period (less than few tens of years). These rates are definitely abundant in comparison with regional extension rate, which is at the size of mm/yr normally. Such evidences clearly show that in the short time period magma plays a 4 primary role in generating rift structures and extensional processes in general, by means of episodic and rapid dike intrusions. On the contrary, regional tectonic seems to play a very restricted role [Sigmundsson, 2006; Wright et al., 2006; Ayele et al., 2007; Ebinger et al., 2010]. However, data directly concerning the effect of the magma intrusion along divergent boundaries is limited to the last decades. Therefore, the relative influence of magma and tectonics in controlling the whole extensional processes and the formation of rift structures along divergent boundaries is still under debate, especially on a longer term view. 1.2 Methods In order to better investigate on the arguments previously introduced, two different methods have been applied: field survey and analogue modeling. 1.2.1 Field surveys Rift generation along divergent plate boundaries occurs both in continental crust (where the separation process among plates is at its first stage) and oceanic crust (where the process is at its advanced level). One of the most representative divergent plate boundary in the continental crust is the East African Rift System (EARS), which develops from Afar depression to Monzambique, separating the Nubia plate (to the West) from the Somalia plate (to the East) with an extension rate of 4-7 mm/yr [DeMets et al., 2010, and references therein]. In particular, in this work, a field survey has been carried out in the portion of the EARS coinciding with the Main Ethiopian Rift (MER), representing an excellent site for investigating on the present structure of a continental rift. Divergent plate boundaries at their mature stage are mainly associated with oceanic ridges. In particular, this work focuses on the Middle Atlantic Ridge (MAR). The most accessible portion of MAR geographically corresponds to Iceland, where the ridge arises at the surface. Here, the active portion of the ridge is several tens of kilometres wide and separates the North American plate (to the West) from the Eurasian one (to the Est), with a rate of ~2 cm/yr [DeMets et al., 2010, and references therein]. The axial active zones of the divergent boundaries are characterized by the presence of several magmatic systems (20-100 km long), which are their fundamental elements. Magmatic systems are composed of a polygenic central volcano, generally with a summit caldera, and a relevant fissural zone, characterized by open normal faults, extensional fractures, eruptive fissures and monogenic aligned cones [Fig. 1; Einarsson e Saemundsson, 1987; Gudmundsson, 1987, 1995, 2000]. At depth, they are mainly formed by two types of dike swarms: local, limited to the area involving the central volcano, and 5 regional, which spread outside the area of the volcano and are parallel to the fissural zone of the system. Local dikes are characterized by circular shapes, depending on the stress of the volcanic structure and its magmatic chamber [Gudmundsson, 1995; Tibaldi, 2008], while regional dikes concentrate mostly in swarms 5-10 km wide and ~ 50 km long, whose direction is mainly dependent on regional stress [i.e., Walker, 1959; 1960; Gudmundsson, 1995; Paquet et al., 2007]. The field surveys carried out in this work aim at analyzing the fissural portions of different magmatic systems distinguishing two main domains. The first is related to simple eruptive fissures with a narrow deformation zone (hundreds of m wide, as Eldgjá, Laki, Sveinagja, Sveinar and Bardarbunga, in Iceland), related to one or very few diking events. Since the eruptive fissures are generated by feeder dikes, the study of geometry and kinematics of deformation pattern associated with them is focused on understanding the direct impact of the magma on the surface deformation, with a negligible contribute of the regional tectonics. In particular, the eruptive fissures of Edgjà (933-941 A.C.) and Lakagigar, (1783-1784) are the two greatest rifting episodes in Iceland history, with an eruptive volume of 18.3 km3 and 14.7 km3 [Thordarson et al., 1993; Thordarson, et al., 2001]. The Svenagià and Sveinar grabens have been recently reactivated by a rifting episode (1875), associated with a relevant caldera collapse of the Askja central volcano [i.e., Gudmundsson and Backstrom, 1991] and are characterized by a narrow deformed zone (<1.5 km). Indeed, the most recent Bardarbunga rifting episode started back in August 2014 and still on-going: it represents a unique opportunity for investigating on deformation related to active dike intrusions. This event is also one of the biggest occurred in Iceland across the last 100 years and may be directly compared with the oldest Edgjá and Lakagigar fissures. The second domain concerns the geometric and kinematics investigation of grabens, normal faults and extensional fractures, characterizing more complex and mature rift segment, hosting several eruptive fissures within a broader deformed area (several km wide, as Krafla, Vogar, Thingvellir in Iceland and Fantale in Ethiopia) and in which magma and tectonic induced structures may be mixed. In particular, the fissural areas of the Krafla magmatic system is well known for a recent rifting episode, during which ~20 dike intrusions occurred in ~20 years (1975-1984) [i.e. Opheim and Gudmundsson, 1989 and references therein]. For further details, Chapter n. 2 provides more information on geology, structure and eruptive activity of each single area. 6