Space For Intrusions Research Articles

Experimental investigation on granite emplacement during shortening

We present analogue experiments performed to investigate the emplacement of granitic plutons in a shortening upper crust. The models were made of quartz–sand to simulate the brittle crust and a low-viscosity mixture of silicone and oleic acid to reproduce granitic magmas. Shortening of the models was obtained by a moving wall while a special injection apparatus allowed syn-kinematic magma intrusion from the base of the models. Experimental results show that: (1) space for intrusions is achieved during the movement along thrust faults and mostly coincides with low-pressure areas developed into the thrust–anticlines; (2) intrusion shapes are strictly dependent upon the competition between shortening rate (Sh) and injection rate (Inj). For high Sh/Inj values, plutons were elongated with the long axis parallel to the thrust surfaces; (3) magma migrates horizontally away from the injection point and towards the external sector in the direction of tectonic transport a longer distance for high Sh/Inj values; (4) syn-kinematic emplacement is also controlled by model thickness; an increase in this parameter results in an increase in the pluton plan-view aspect ratio. These results support that the final shape of orogenic plutons emplaced at shallow crustal levels may be strongly controlled by deformational features.

Subaerial-submarine evidence of structures feeding magma to Stromboli Volcano, Italy, and relations with edifice flank failure and creep

Structural field data, including parameters of 109 sheets (strike: mostly N10-55°E, average dip angle: mostly 70–90°; average thickness: two peaks at 1.2 m and 4.2 m), and of 14 eruptive fissures and vents, as well as bathymetric and numerical data, suggest that Stromboli experienced the development of a volcanic rift zone across the cone summit, with a tendency to widen towards the lower submarine flanks, during the last 100 ka. Inside this rift zone, intrusive sheets mostly strike NE and become more abundant, more steeply dipping and thicker on approaching the rift axis. This rift persistence reflects the edifice's state of stress, with a NW–SE horizontal least principal stress, on the magma intrusion paths. After 13 ka ago, sheets mainly intruded with a horseshoe-shaped geometry in plan view along the boundaries of the northwestern unstable volcano flank, where four main lateral collapses occurred; during the last 13 ka they also intruded with a NE-strike across the volcano summit and the northeastern volcano flank. The horseshoe-shaped geometry is influenced by the lateral creep of the volcano flank, which provides space for intrusion along the transtensional boundaries of the unstable sector where thinner sheets were emplaced. When the collapse depression was filled up by new deposits, NE-striking thicker dykes emplaced across the northwestern volcano flank indicating a rapid re-establishment of regional tectonic forces across the cone. Unbuttressing due to the present Sciara del Fuoco depression, and ongoing flank creeping, can account for intrusions also inside the Sciara del Fuoco (the latter in 2007).

Magma emplacement in a thrust ramp anticline: The Gavorrano Granite (northern Apennines, Italy)

Magma emplacement in the upper crust is often associated with crustal extension, whereby space for intrusions is made by dilation along transtensive and strike‐slip faults. The lenticular shape of several intrusions indicates that a further mechanism for magma intrusion is laccolith emplacement with roof uplift of the overburden. Additionally, melts exploit rheological discontinuities (e.g., sedimentary layering) during their ascent. We present an emplacement model for intrusion of the shallow level Gavorrano Granite (northern Apennine, Italy), which is located at the core of an open anticline. The shape of the intrusion and structural features of the host rocks are indicative of a synkinematic emplacement in a growing thrust anticline. Space was provided by the opening of dilatation zones at the core of anticline as a consequence of different amounts of translation between hanging wall and footwall units which were separated by evaporitic rocks. These evaporitic units acted as major décollement layers. Analog models provide results in good agreement with the structural setting of the anticline and emphasize the possibility that melts filled the dilatation zone, with the décollement layers further facilitating intrusion.