Magma flow paths and strain patterns in magma chambers growing by floor subsidence: a model based on magnetic fabric study of shallow-level plutons in the Štiavnica volcano–plutonic complex, Western Carpathians

Abstract

The Miocene Stiavnica volcano–plutonic complex, Western Carpathians, exposes two nearly coeval intra-caldera plutons, their roof (basement of a stratovolcano), and associated volcanic rocks. The complex thus provides insights into mechanisms of magma chamber growth beneath large volcanoes. As inferred from the anisotropy of magnetic susceptibility (AMS), these plutons were emplaced through significantly different processes: the diorite as a discordant stock with steep fabric and the granodiorite as a tabular, bell-jar pluton. In detail, we interpret that the latter was assembled in two stages. First, an upper “layer” intruded as a thin sill along a major subhorizontal basement/cover detachment. The subhorizontal magnetic fabric and strongly oblate AMS ellipsoid in this layer record intrusive strain where the magma flow paths were subparallel to the pluton roof. Second, in the lower “layer” of the pluton, magnetic foliations dip moderately to the ∼NW and ∼WNW to vertical and are associated with down-dip to subhorizontal lineations and prolate to weakly oblate shapes of the AMS ellipsoids. Such a fabric pattern is compatible with piecemeal floor subsidence, where magma flowed along multiple subsiding fault-bounded blocks. Based on this case example, we develop a conceptual model for magma flow paths and strain patterns for four main modes of floor subsidence: (1) piston (cauldron) subsidence is characterized by convergent flow and radial principal stretching above the magma chamber floor; (2) the piecemeal floor subsidence leads to steep to inclined magma flow paths in conduits along fault-bounded blocks; (3) asymmetric (trapdoor) subsidence produces first divergent flow paths near the conduit sides, changing into convergent paths in the narrower space near the kinematic hinge; and (4) symmetric cantilever (funnel) subsidence will lead to divergent flow from a central feeder and thus circumferential principal stretching of the magma. If the growing pluton develops a “two-layer” structure, all the flow paths and associated strains are affected by the flat-lying pluton roof and will convert into horizontal flattening as the roof is approached.