Controls on sill and dyke-sill hybrid geometry and propagation in the crust: The role of fracture toughness

Analogue experiments using gelatine were carried out to investigate the role of the mechanical properties of rock layers and their bonded interfaces on the formation and propagation of magma-filled fractures in the crust. Water was injected at controlled flux through the base of a clear-Perspex tank into superposed and variably bonded layers of solidified gelatine. Experimental dykes and sills were formed, as well as dyke-sill hybrid structures where the ascending dyke crosses the interface between layers but also intrudes it to form a sill. Stress evolution in the gelatine was visualised using polarised light as the intrusions grew, and its evolving strain was measured using digital image correlation (DIC). During the formation of dyke-sill hybrids there are notable decreases in stress and strain near the dyke as sills form, which is attributed to a pressure decrease within the intrusive network. Additional fluid is extracted from the open dykes to help grow the sills, causing the dyke protrusion in the overlying layer to be almost completely drained. Scaling laws and the geometry of the propagating sill suggest sill growth into the interface was toughness-dominated rather than viscosity-dominated. We define KIc* as the fracture toughness of the interface between layers relative to the lower gelatine layer KIcInt / KIcG. Our results show that KIc* influences the type of intrusion formed (dyke, sill or hybrid), and the magnitude of KIcInt impacted the growth rate of the sills. KIcInt was determined during setup of the experiment by controlling the temperature of the upper layer Tm when it was poured into place, with Tm < 24 °C resulting in an interface with relatively low fracture toughness that is favourable for sill or dyke-sill hybrid formation. The experiments help to explain the dominance of dykes and sills in the rock record, compared to intermediate hybrid structures

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

Decadal to monthly timescales of magma transfer and reservoir growth at a caldera volcano

International audienceCaldera-forming volcanic eruptions are low-frequency, highimpact events capable of discharging tens to thousands of cubic kilometres of magma explosively on timescales of hours to days, with devastating effects on local and global scales1. Because no such eruption has been monitored during its long build-up phase, the precursor phenomena are not well understood. Geophysical signals obtained during recent episodes of unrest at calderas such as Yellowstone, USA, and Campi Flegrei, Italy, are difficult to interpret, and the conditions necessary for large eruptions are poorly constrained2,3. Here we present a study of pre-eruptive magmatic processes and their timescales using chemically zoned crystals from the 'Minoan' caldera-formingeruption of Santorini volcano,Greece4, which occurred in the late 1600s BC. The results provide insights into how rapidly large silicic systems may pass from a quiescent state to one on the edge of eruption5,6. Despite the large volume of erupted magma4 (40-60 cubic kilometres), and the 18,000-year gestation period between the Minoan eruption and the previous major eruption, most crystals in the Minoan magma record processes that occurred less than about 100 years before the eruption. Recharge of the magma reservoir by large volumes of silicic magma (and some mafic magma) occurred during the century before eruption, and mixing between different silicicmagmabatches was still taking place during the final months. Final assembly of large silicic magma reservoirs may occur on timescales that are geologically very short by comparison with the preceding repose period, with major growth phases immediately before eruption. These observations have implications for the monitoring of long-dormant, but potentially active, caldera systems

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

Magnetic fabrics reveal three-dimensional flow processes within elongate magma fingers at the margin of the Shonkin Sag laccolith (MT, USA)

Unravelling magma flow in ancient sheet intrusions is critical to understanding how magma pathways develop and feed volcanic eruptions. Analyzing the shape preferred orientation of minerals in intrusive rocks can provide information on magma flow, because crystals may align parallel to the primary flow direction. Anisotropy of magnetic susceptibility (AMS) is an established method to quantify such shape preferred orientations in igneous sheet intrusions with weak or cryptic fabrics. However, use of AMS data to characterize how magma flows within the individual building blocks of sheet intrusions (i.e., magma fingers and segments), hereafter referred to as elements, has received much less attention. Here we use a high spatial resolution sampling strategy to quantify the AMS fabric of the Eocene Shonkin Sag laccolith (Montana, USA) and associated elongate magma fingers. Our results suggest that magnetic fabrics across the main laccolith reflect sub-horizontal magma flow, and inferred flow directions are consistent with an underlying NE-SW striking feeder dyke. Within the magma fingers, we interpret systematic changes in magnetic fabric shape and orientation to reflect the interaction between competing forces occurring during finger-parallel magma flow (i.e., simple shear) and horizontal and vertical inflation (i.e., pure shear flattening). For example, we highlight how local crossflow of magma between coalesced fingers increases the complexity of magma flow kinematics and related fabrics. Despite these complexities, the AMS data in coalesced magma fingers maintain their internal flow- and inflation-related fabrics, which suggests that magma flow within the fingers remains channelized after coalescence. Given that many sheet intrusions consist of amalgamated elements, our findings highlight the need to carefully consider element distribution and sample locations when interpreting magma flow based on AMS measurements

Granitic magma formation, transport and emplacement in the Earth's Crust

The origin of granites was once a question solely for petrologists and geochemists. But in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation, transport and emplacement of silica-rich melts in the continental crust. Accepted petrological models are being questioned, either because they require unrealistic rheological behaviours of rocks and magmas, or because they do not satisfactorily explain the available structural or geophysical data. Provided flow is continuous, mechanical considerations suggest that--far from being geologically sluggish--granite magmatism is a rapid, dynamic process operating at timescales of < or = 100,000 years, irrespective of tectonic setting