Abstract The occurrence and distribution of monogenic eruptive features in volcanic areas testify to the presence of deep-crustal or subcrustal magma reservoirs hydraulically connected to the surface via a fracture network. The spatial distribution of vents can be studied in terms of self-similar (fractal) clustering, described by a fractal exponent D and defined over a range of lengths ( l ) between a lower and upper cutoff, L co and U co , respectively. The computed U co values for several volcanic fields on Earth match the thickness of the crust between vents and magma reservoirs at depth. This analysis can thus be extended to other volcanic fields and volcanoes on rocky planets in the solar system where features such as vents and dykes occur, and for where complementary geophysical data are currently lacking. We applied this method to the Ascraeus Mons volcano on Mars, which presents hundreds of collapse pits similar to those observed on Earth volcanoes that are most likely related to feeder dykes. Based on structural mapping with High Resolution Stereo Camera data at 12 m/px and Context Camera data at 6 m/px mosaics, more than 2300 collapse pits and dyke traces were analysed, revealing two distinct fractal clustered populations. The obtained U co values reveal the presence and likely depth of both a deep magma reservoir ( c. 60 km deep) and a small shallower chamber ( c. 11 km deep). This analysis can help to better constrain the depth and time evolution of volcanic processes on Tharsis, and on terrestrial planets’ volcanoes in general.

Abstract An example of sheet-like intrusion emplacement at very shallow crustal levels on Elba Island, Italy, is described. The Eastern Elba Dyke Complex (EEDC) consists of decimetre- to metre-thick sheeted aplites emplaced within intensely folded low-grade metamorphic rocks. Field data indicate that sill and dyke emplacement was controlled by mechanical discontinuities, represented by fractures in the host rocks, and was strongly favoured by magma overpressure. The occurrence of angular fragments of host rocks in the dyke border zones and the branching of sills testify to hydraulic fracturing. Analysis of the spatial distribution and geometry of EEDC sills and dykes provides clues on fluid pressure conditions and the stress state at the time of magma emplacement, as well as on the depth of emplacement. The calculated stress ratio and driving pressure ratio were used to estimate a magma overpressure of 6–54 MPa at the time of emplacement of the EEDC at a depth of about 2 km.

Abstract The influence of pre-existing thrusts on the development of later normal faults was investigated using scaled laboratory analogue models. Experiments consisted of a phase of shortening followed by extension at variable angles of obliquity (a) to the shortening direction. Results suggest that the angle a has a major influence on the surface fault pattern and on the interaction between shortening-related structures and later extensional structures. Three different modes of interactions were identified depending upon the extension kinematics. (1) For orthogonal extension (α = 0°), shortening-related fold and thrust structures strongly influence the development of normal faults: graben structures nucleate within anticlines and the normal faults reactívate thrusts at depth (branching at depth mode of interaction) . (2) For highly oblique extension (α > 45°), shortening-related structures exert no influence on normal faults as extension-related steeply-dipping faults (characterized by an oblique component of movement) displace early thrusts (no interaction mode) . (3) For intermediate obliquity angles (a = 15°, 30°), an intermediate mode ofinter-action characterizes the experiments, where the no interaction and branching at depth modes coexist in different regions of models. Modelling results can be used to infer regional extension directions as is shown for the Northern Appenines (Italy).

Abstract Mechanical discontinuities within the crust, represented by tectonic structures (faults) or lithological heterogeneities, strongly control the emplacement of magmas as tabular intrusions within the middle-upper crust. The occurrence of mechanical layering is a common feature in fold and thrust belts. In the northern Apenniness, a Cenozoic fold-thrust belt affected in its inner part by Neogene magmatism, the Gavorrano laccolith (southern Tuscany) is a particularly suitable example for studying the relationships between magmatism and tectonic structures. New geological mapping, together with a large amount of subsurface data available from historical mining activity in the area, have allowed the reconstruction of: 1 the original relationships of the intrusion within the nappe pile, and 2 the laccolithic shape of the intrusion. Using the Gavorrano laccolith as an example, we propose that the emplacement of Neogene intrusions in southern Tuscany was strongly controlled by the occurrence of mechanical discontinuities represented by thrust zones in the nappe pile.