Quantitative analogue modelling of the surface deformation associated with cone-sheet and dyke emplacement

International audienceInclined cone-sheets and sub-vertical dykes constitute the two principal types of magmatic sheet intrusionsproduced by volcanic systems. In active volcanic systems, the emplacement of sheet intrusions causes measurablesurface deformation, which is analyzed through geodetic models. Geodetic model output is classically the shape ofunderlying intrusions causing the surface deformation, however, the results of these models are not testable as thesubsurface intrusion is not accessible. Such test would only be doable with a physical system in which both (1) thesurface deformation pattern and (2) the 3D shape of the underlying intrusion are known. In addition, establishedgeodetic models only consider static magma intrusions, and do not account for emplacement and propagationprocesses. This would require combined good time- and space-resolution, which is not achievable with classicalgeodetic monitoring systems.We present a series of analogue models that may be a way of accurately linking surface deformation to theunderlying intrusions and associated emplacement processes. We systematically varied depth of intrusion, thecohesive properties of the silica powder representing the country rock and the velocity of injected magma.The pressure of the intruding vegetable oil was measured through time, and the model surface topography wasmonitored. The low viscosity magma was simulated by molten vegetable oil, which solidified after intrusion; thesolidified intrusion was then excavated and its shape was measured. By linking the development of the surfaceuplift in height, area, and volume with the pressure data from the onset of intrusion until the time of eruption,we identify characteristic laws of surface deformation. First results indicate that the pattern of uplift over timevaries, depending on whether deformation is caused by a dyke- or a cone-sheet-shaped intrusion. The results fromall experiments may enable us to distinguish the two intrusion types using surface deformation data alone. Inaddition, the different signatures of surface deformation patterns associated with dykes and cone sheets providestrong insights into their contrasting emplacement regimes: dykes mainly inject as open fractures, whereas localshear deformation is likely to control cone-sheet emplacement. This key can then be applied to presently activevolcanic areas where surface deformation is monitored