Formation of caldera periphery faults: an experimental study

Changing stresses in multi-stage caldera volcanoes were simulated in scaled analogue experiments aiming to reconstruct the mechanism(s) associated wit h caldera formation and the corresponding zones of structural weakness. We evaluate characteristic structures resulting from doming (chamber inflation ), evacuation collapse (chamber deflation) and cyclic resurgence (inflation and deflation), and we analyse the consequential fault patterns and their statistical relationship to morphology and geometry. Doming results in radi al fractures and subordinate concentric reverse faults which propagate dive rgently from the chamber upwards with increasing dilation. The structural d ome so produced is characterised bysteepening in the periphery, whereas the broadening apex subsides. Pure evacuation causes the chamber roof to colla pse along adjacent bell-shaped reverse faults. The distribution of concentr ic faults is influenced by the initial edifice morphology; steep and irregu lar initial flanks result in a tilted or chaotic caldera floor. The third s et of experiments focused on the structural interaction of cyclic inflation and subsequent moderate deflation. Following doming, caldera subsidence pr oduces concentric faults that characteristically crosscut radial cracks of the dome. The flanks of the edifice relax, resulting in discontinuous circu mferential faults that outline a structural network of radial and concentri c faults: the latter form locally uplifted and tiltedwedges (half-grabens) that grade into horst-and-graben structures. This superimposed fault patter n also extends inside the caldera. We suggest that major pressure deviation s in magma chamber(s) are reflected in the fault arrangement dissecting the volcanoflanks and may be used as a first-order indication of the processes and mechanisms involved in caldera formation.