Analogue models of the effect of long-term basement fault movement on volcanic edifices |
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Authors: | Luke Wooller Benjamin van Wyk de Vries Emmanuelle Cecchi Hazel Rymer |
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Institution: | 1.Volcano Dynamics Group, Department of Earth Sciences,The Open University,Milton Keynes,UK;2.Laboratoire Magmas et Volcans, Observatoire du Physique du Globe de Clermont,Université Blaise Pascal,Clermont-Ferrand,France |
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Abstract: | Long-term fault movement under volcanoes can control the edifice structure and can generate collapse events. To study faulting
effects, we explore a wide range of fault geometries and motions, from normal, through vertical to reverse and dip-slip to
strike-slip, using simple analogue models. We explore the effect of cumulative sub-volcanic fault motions and find that there
is a strong influence on the structural evolution and potential instability of volcanoes. The variety of fault types and geometries
are tested with realistically scaled displacements, demonstrating a general tendency to produce regions of instability parallel
to fault strike, whatever the fault motion. Where 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 the fault beneath
the volcano change the location, type and magnitude of the instability produced. For example, the further the fault is from
the central axis, the larger the destabilised sector. Also, with greater fault offset from the central axis larger unstable
volumes are generated. Such failures are normal to fault strike. Using simple geometric dimensionless numbers, such as the
fault dip, degree of oblique motion (angle of obliquity), and the fault position, we graphically display the geometry of structures
produced. The models are applied to volcanoes with known underlying faults, and we demonstrate the importance of these faults
in determining volcanic structures and slope instability. Using the knowledge of fault patterns gained from these experiments,
geological mapping on volcanoes can locate fault influence and unstable zones, and hence monitoring of unstable flanks could
be carried out to determine the actual response to faulting in specific cases. |
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