Insights into lava dome and spine extrusion using analogue sandbox experiments

<p>Lava dome formation is a common process at stratovolcanoes involving the shallow intrusion or extrusion of viscous lava and may lead to the rise of spines. Spines are protrusions observed to extrude episodically during lava dome growth, yet the structural and mechanical factors controlling their formation are only partially understood. Here, we provide new, detailed insight into lava dome growth and the production of spines using a novel set of analogue experiments extruding sand-plaster mixtures from a fixed-diameter conduit under isothermal conditions. We trace displacement and strain with photogrammetric methods for precise and detailed monitoring of the extrusion process. Results show initial dome growth forming a steep-sided and flat-topped shape through extrusion of new material, leading to slumping of oversteepening slopes, forming a talus. Spines are found to protrude at a later stage through the dome surface along discrete circular faults that originate from the conduit walls, starting a cycle of spine growth and collapse. As our spines only appear after prolonged extrusion, we relate their appearance to the compaction and strengthening of material within the conduit. We find that spine diameter, height and volume are positively correlated with increasing cohesion and therefore material strength. The spine diameter was also observed to be smaller or equal to the diameter of the underlying conduit, as shear extrusion occurs along vertical to outward-dipping fault planes. For natural domes, our findings imply that spine growth may be the consequence of compaction and densification via porosity loss, shearing and/or outgassing of conduit magma during ascent. More efficient compaction will yield wider and taller spines as a result of increasing rock strength. Our study further highlights the relevance of analogue experiments in the study of lava domes and spines, which remain one of the most hazardous and unpredictable features at dome-forming volcanoes worldwide.</p>

Evolution of extensional faults between two rift systems: insights from sandbox experiments

<p>The interference between two offset propagating rift systems creates fractures, with a sigmoid shape in map view and previously referred to as accommodation zones (Mc Clay et al, 2002). This peculiar kinematics may be observed in the Southeastern Brazilian margin in the Santos Basin, developed between the tips of two propagating, offset rifts. In this region, northward propagating rift was aborted during the southward propagation of another rift further to the east leading eventually to the opening of this segment of the South Atlantic. Could this structural setting explain the geometry and the position of the fracture zones in this basin?</p><p>To answer this question, we explore a range of geometrical and kinematic parameters with sandbox experiments to observe the deformation between these two propagating rift systems. The basement of the rift zones were modelled with rubber strips glued to rigid metal plates, following the setup of McClay et al, 2002. However, this setup suffers from the lateral contraction of the rubber due to its elastic extension (the Poisson’s effect). This introduces a spurious kinematics, and in particular an unrealistic opening at the contact between the two rift parts. A new device, whereby thin metallic strips are glued to the sides of the rubber sheet reduces very substantially the Poisson effect and therefore improves the simulation of the overall extension. </p><p>Two main parameters are varied: the offset between the two rifts (D) and the relative velocity of extension of each rift. Narrowly spaced cross –sections of two experiments are interpreted to build 3D patterns.</p><p>The main results from the sandbox experiments are:</p><p>- Major and minor faults with the rifting zone localized by the rubber base present dips approximately equal to 75°.</p><p>- To obtain sigmoid fault array in map view best resembling the structural interpretation of Lebreton (2012), the rifts must be offsets (D>0) and the extension must be synchronous.</p><p>- The 3D fault patterns reveal that fault planes are not continuous in the accommodation zone, between the two rifts.  If these major faults are not connected in the central zone as shown by the physical models, then the fluid flow will be certainly influenced. This central relay zone could also be considered as a diffuse strain zone.</p><p>Numerical models will be helpful to introduce further material heterogeneities in this key area. The experimental results provide the data to validate the numerical modeling and to guide in the selection of the boundary conditions.</p>

Testing in sandbox experiments the potentialities of active-Distributed Temperature Sensing to quantify distributed groundwater fluxes in porous media

<p>Active-Distributed Temperature Sensing is a new method that has been recently developed for quantifying groundwater fluxes in the sub-surface along fibre-optic cables with a great spatial resolution. It consists in measuring and modelling the increase of temperature due to a heat source, dissipated through heat conduction and heat advection, depending on groundwater fluxes. Here, we propose to estimate the applicability and limitations of the method using sandbox experiments where flow rate and temperature are well controlled. For doing so, active-DTS experiments have been achieved under different flow rates and experimental conditions. In addition, we compare three different and complementary methods to estimate in practice the spatial resolution of DTS measurements. </p><p>Active-DTS experiments have been conducted by deploying a fiber optic cable in a large PVC tank (1.6m long; 1.2 m width and 0.3 m height) and filled with 0.4-1.3 mm diameter sand. The height of water in water reservoirs on either side of the sandbox can be adjusted to control the head gradient and the flow rate through the sand. Heating was done by injecting during at least 8 hours for each experiment, a well-controlled electrical current along the steel armouring of the fiber optic cable. The three methods for estimating spatial resolution were applied and compared using FO-DTS measurements obtained on the same fiber-optic cable but with two different DTS units having different spatial resolution. Results show that a large range of groundwater fluxes may be estimated with a very good accuracy. Finally, we compare the advantages and complementarities of the different methods proposed for estimating the spatial resolution of measurements. In particular, the spatial resolution estimated using a temperature step change is both dependent on the effective spatial resolution of the DTS unit but also on heat conduction induced because of the high thermal conductivity of the cable. By showing the applicability of the method for a large range of flow rates and with an excellent spatial resolution, these experiments demonstrate the potentialities of the method for quantifying fluid fluxes in porous media for a large range of applications.</p>