Rise and Fall of a Multi-sheet Intrusive Complex, Elba Island, Italy

Elba Island intrusive complex: multisheet laccoliths, sheeted pluton, mafic dyke swarm. Laccolith magma fed from dykes and emplaced in crustal discontinuities (traps). Pluton growth by downward stacking of three magma pulses. Laccoliths and plutons: different outcomes of similar processes in different conditions. Emplacement of excess magma in a short time led to massive gravity slide

Structures Related to the Emplacement of Shallow-Level Intrusions

A systematic view of the vast nomenclature used to describe the structures of shallow-level intrusions is presented here. Structures are organised in four main groups, according to logical breaks in the timing of magma emplacement, independent of the scales of features: (1) Intrusion-related structures, formed as the magma is making space and then develops into its intrusion shape; (2) Magmatic flow-related structures, developed as magma moves with suspended crystals that are free to rotate; (3) Solid-state, flow-related structures that formed in portions of the intrusions affected by continuing flow of nearby magma, therefore considered to have a syn-magmatic, non-tectonic origin; (4) Thermal and fragmental structures, related to creation of space and impact on host materials. This scheme appears as a rational organisation, helpful in describing and interpreting the large variety of structures observed in shallow-level intrusions

Linear volcanic segments in the central Sunda Arc, Indonesia, identified using Hough Transform analysis: Implications for arc lithosphere control upon volcano distribution

Hough Transform analysis is used as an objective means to constrain volcano distribution in the central Sunda Arc, Indonesia. Most volcanoes in the arc define four en echelon, linear segments, each of 500–700 km length. Javan volcanoes that do not lie on these segments either (i) formed at an early stage in the history of the arc and erupted products that are petrologically and geochemically distinct from typical arc magma, or (ii) lie along other mapped structures. The en echelon distribution of volcanoes in the central Sunda Arc is best explained as originating from two possible sources. First, interaction with the subducting Indo-Australian Plate may induce stress in the arc lithosphere generating pathways for magma to exploit. Second, downward flexure of the arc lithosphere, as a result of mantle flow or loading by the arc, would also establish arc-normal tension towards the base of the lithosphere, where magma is supplied to volcanic systems. To the west and east of the central Sunda Arc deviations from the distribution of long, en echelon, linear segments can be understood as responses to specific stress fields in the arc lithosphere of Sumatra and eastern Nusa Tenggara, respectively. Control of volcano distribution by arc lithosphere explains why there are large variations in the depth from volcanoes to the zone of slab seismicity in the central Sunda Arc, where there is little variation in slab geometry or the rate of plate convergence

No Unique Scaling Law for Igneous Dikes

In linear elastic fracture mechanics (LEFM), veins, dikes, and sills grow in length when the stress intensity factor (Formula presented.) at the tip reaches a critical value: the host rock fracture toughness (Formula presented.). This criterion is applied broadly in LEFM models for crack growth and it is often assumed that the pressure inside the crack is uniform. When applied to intrusion length versus thickness scaling, a significant issue arises in that derived (Formula presented.), which is about 100–1,000 times that of measured (Formula presented.) values for rocks at upper crustal depths. The same scaling relationships applied to comparatively short mineral vein data gives (Formula presented.), approaching the expected range. Here we propose that intrusions preserve non-equilibrated pressures as cracks controlled by kinetics, and therefore cannot be treated in continuum with fracture-controlled constant pressure (equilibrium) structures such as veins, or many types of scaled analogue model. Early stages of dike growth (inflation) give rise to increasing length and thickness, but magma pressure gradients within intrusions may serve to drive late-stage lengthening at the expense of maximum thickness (relaxation). For cracks in 2D, we find that intrusion scaling in non-equilibrium growth is controlled by the magma injection rate and initial dike scaling, effective (2D) host rock modulus, magma viscosity and cooling rate, which are different for all individual intrusions and sets of intrusions. A solidified intrusion can therefore achieve its final dimensions via many routes, with relaxation acting as a potentially significant factor, hence there is no unique scaling law for dike intrusions