Analogue Models of the Effect of Long-Term Basement Fault Movement on Volcanic Edifices
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Bull Volcanol (2009) 71:1111–1131 DOI 10.1007/s00445-009-0289-3 RESEARCH ARTICLE Analogue models of the effect of long-term basement fault movement on volcanic edifices Luke Wooller & Benjamin van Wyk de Vries & Emmanuelle Cecchi & Hazel Rymer Received: 4 June 2004 /Accepted: 29 April 2009 /Published online: 16 May 2009 # Springer-Verlag 2009 Abstract Long-term fault movement under volcanoes can we graphically display the geometry of structures produced. control the edifice structure and can generate collapse The models are applied to volcanoes with known underly- events. To study faulting effects, we explore a wide range ing faults, and we demonstrate the importance of these of fault geometries and motions, from normal, through faults in determining volcanic structures and slope instabil- vertical to reverse and dip-slip to strike-slip, using simple ity. Using the knowledge of fault patterns gained from these analogue models. We explore the effect of cumulative sub- experiments, geological mapping on volcanoes can locate volcanic fault motions and find that there is a strong fault influence and unstable zones, and hence monitoring of influence on the structural evolution and potential instabil- unstable flanks could be carried out to determine the actual ity of volcanoes. The variety of fault types and geometries response to faulting in specific cases. are tested with realistically scaled displacements, demon- strating a general tendency to produce regions of instability Keywords Volcano-tectonics . Fault . Analogue modelling . parallel to fault strike, whatever the fault motion. Where Lateral collapse . Debris avalanche . Deformation there is oblique-slip faulting, the instability is always on the downthrown side and usually in the volcano flank sector facing the strike-slip sense of motion. Different positions of Introduction the fault beneath the volcano change the location, type and magnitude of the instability produced. For example, the Volcanoes have an inherent structural instability that results further the fault is from the central axis, the larger the in lateral collapse and debris avalanches, mobilising many destabilised sector. Also, with greater fault offset from cubic kilometres of material that is transported away from the central axis larger unstable volumes are generated. Such the source as a rapidly flowing mass (Siebert 1984; Siebert failures are normal to fault strike. Using simple geometric et al. 1987). Collapse events are not necessarily accompa- dimensionless numbers, such as the fault dip, degree of nied by eruptions, but may also occur at dormant or extinct oblique motion (angle of obliquity), and the fault position, centres (Siebert et al. 1987; Cecchi et al. 2004). It is important to further develop our understanding of Editorial responsibility: C. Kilburn the factors that influence and trigger volcano collapse. L. Wooller : H. Rymer These include the intrusion of shallow magmatic bodies Volcano Dynamics Group, Department of Earth Sciences, such as crypto-domes (e.g. Mount St. Helens, USA (Voight The Open University, et al. 1983; Donnadieu et al. 2001)) and dykes (e.g. Milton Keynes MK6 7AA, UK Stromboli, Italy (Tibaldi 2001, 2003)), deforming hydro- B. van Wyk de Vries (*) : E. Cecchi thermal systems (e.g. Casita Volcano, Nicaragua (van Wyk Laboratoire Magmas et Volcans, de Vries et al. 2000)), increased pore fluid pressures (Day Observatoire du Physique du Globe de Clermont, 1996) and local earthquakes (e.g. Bandai Volcano, Japan Université Blaise Pascal, 5 rue Kessler, (Mimura and Endo 1997; Glicken and Nakamura 1988)). 63038 Clermont-Ferrand, France An additional control on the direction of collapse may be e-mail: [email protected] the regional structural and lithological setting. Previous 1112 Bull Volcanol (2009) 71:1111–1131 work has suggested that collapse predominantly occurs at Belousov et al. (2005) looked at the influence of fault right angles to the regional tectonic orientation (Nakamura offset, applied to caldera boundary faults. They suggest that 1977; Siebert 1984; Francis and Wells 1988; Tibaldi 1995). the large fault offsets needed to cause volcano instability Factors influencing collapse close to the volcano include are most likely to occur at calderas and that in situations of the presence of ductile sub-strata, which allow gravitational tectonic faulting the edifice can “repair” itself between spreading of the edifice (van Wyk de Vries and Francis episodes of faulting. 1997), regional slope (Wooller et al. 2004), and active Although these earlier studies clearly show the interac- tectonic faulting beneath, or adjacent to, the edifice tion between long-term fault movement and overlying (Lagmay et al. 2000; Vidal and Merle 2000; Acoccella volcanoes, they were limited to simple fault geometries 2005). This last factor is the subject of this study. and did not investigate the transition between the various Volcanoes can serve as stress loci to concentrate or end-members of faulting (fault position, fault angle, dip-slip nucleate tectonic structures (van Wyk de Vries and Merle and strike-slip). Our study attempts to marry these separate 1996, 1998; Branquet and van Wyk de Vries 2001). This studies into a single model. This work therefore aims to leads to an increase in the fault density and fault reassess the earlier models and to examine further aspects displacement around volcanoes. Sudden movement along of the influence of basement faulting on volcano evolution these faults can destabilise the edifice and trigger a lateral and instability. collapse, leading to a debris avalanche (Siebert 1984; Siebert et al. 1987; Merle et al. 2001). Also, at longer timescales, cumulative movement along faults during the Scaling and materials growth of an overlying volcanic edifice can generate shallow faulting within the volcano (Merle et al. 2001). Firstly, it should be noted that there was no time- or rate- These fractures can define the extent of unstable sectors and dependence in the models: as they were brittle, they were potentially control both the intrusion of magmatic bodies not affected by the phenomena of creep or stress relaxation, (Lagmay et al. 2000) and the extent of hydrothermal and were ruled by the Navier-Coulomb criterion of brittle alteration (van Wyk de Vries and Francis 1997), which failure (e.g. Merle et al. 2001). As in other experiments, further destabilise the cone. slow continuous slip was achieved along the basement fault Analogue modelling methods have been extensively so that experiments were aseismic. Thus, the role of applied to studying crustal faulting (e.g. Horsfield 1977; earthquakes was not taken into account as described in McClay 1990; Dauteuil and Mart 1998). The role of faulting Merle et al. (2001). It was the geometric organisation of as an underlying control on the evolution of volcanoes has structures responding to fault movement and the volcano been studied using models by Tibaldi (1995), Merle et al. topography that was considered important. (2001), van Wyk de Vries and Merle (1996, 1998), Branquet The principle geometric variables for the volcano were and van Wyk de Vries (2001), Lagmay et al. (2000, 2003) the height (H), radius (R), layering thickness (T)in and Acoccella (2005). Lagmay et al. (2000, 2003)demon- heterogeneous (alternating layers of sand and sand / plaster) strated that underlying strike-slip faulting produces a cones, substrata thickness (B), and the fault position sigmoidal structure across the cone with two regions parallel (distance from volcano centre to the fault) (Table 1, Fig. 1). to fault strike showing instability through bulging, fracturing In addition, there were three fault displacement compo- and avalanching. These models were applied to Iriga nents: vertical throw (Fv, where positive values were Volcano, Ancestral Mount Bao (Philippines) and Mount St. normal faulting and negative reverse faulting), horizontal Helens (USA). Norini et al. (2008) applied similar models to normal displacement (Fh) and along strike displacement Nevado de Toluca (Mexico). (Fs) (Fig. 1). These described the dip of the fault plane (tan Movement along vertical (dip-slip) basement faults Fv/Fh), the opening angle in transtension (tan Fs/Fh) and centred directly below volcanic cones was also shown to the angle of obliquity (tan Fv/Fs). The latter was particu- lead to faulting, which eventually induced a strike-normal larly relevant for vertical faults. The magnitude of fault failure of the downthrown area (Vidal and Merle 2000; movement was also gained from these parameters using Merle et al. 2001). Branquet and van Wyk de Vries (2001) Pythagoras: showed that regional sub-volcanic thrusts become deflected around the edifice, leading to instabilities accommodated Fault displacement magnitude ÀÁ through shallow failures. Thrust deflection may produce 1=2 ¼ Fs2 þ Fh2 þ Fv2 ð1Þ features similar to those often associated with basal thrusting at gravitationally spreading volcanoes. Acoccella In models where the fault was offset from the cone axis, the (2005) used a simple horizontal base plate set up and found horizontal distance from the cone centre to the fault (Df), similar results. varied from zero when the cone axis was directly above the Bull Volcanol (2009) 71:1111–1131 1113 Table 1 Representative values of geometric variables Parameter Symbol Equivalent Π–number Units Model Nature Scaling ratio Volcano height H Π1 m 0.1 2000 5×10−5 Volcano radius R – m 0.3 6000 5×10−5 Layer thickness T Π2m10−3 200 5×10−5 Basal layer thickness B Π3m10−2–5×10−2 200–1000 5×10−5 Fault horizontal disp Fh Π4m10−2–10−4 2–200 5×10−5 Fault vertical disp Fv Π5m10−2–10−4 2–200 5×10−5 Fault strike-slip disp Fs Π6m10−2–10−4 2–200 5×10−5 Fault magnitude Fm – m10−2–10−4 2–200 5×10−5 Fault position Df Π7m0–0.3 0–6000 5×10−5 The type volcano is taken to be 2,000 m high with a 6,000 m radius base.