Abstract

A new fracture-mediated intrusion model resolves the sequence of magma and rock displacements generating a felsic magma system with a lower crustal source, central conduit and shallow sill pluton. Idealized intraplate conditions are assumed, to neglect regional tectonism and to focus on juvenile cracking by magma-intrinsic hydraulic and buoyant loads.

The magma source is conductively heated and develops by endothermic fluid-absent melting in approximately 10⁶ years. The idealized domical thermal anomaly and endothermic heat focusing yield a low aspect ratio source, with outer-porous and inner-permeable partial melt zones. An anatectic core region is unrealized owing to magma segregation.

Thermal stresses are readily relaxed and unimportant to source loading while crustal uplift generates tensile stress and, upon relaxation, lateral space for tensile fractures. Dilative melting generates buoyancy overpressure (ΔP_B) and a hydraulic contribution (ΔP_V) to the magma pressure (P_M). ΔP_V develops by elastic wall-rock compression as the ‘excess magma volume’, EMV, arises too abruptly for full relaxation by inelastic deformation, inducing a brittle response. Tensile rupture criteria are met in an effective tensile stress field with low differential stress induced by magma pore pressure and wedging by pressurized cracks, which initiate by source inflation and uplift. Preferred vein geometry reflects the starting stress field. For symmetric doming, radial vertical cracks with a central nexus form a natural conduit. A vertically extensive crack system, however, requires special explanation because wedging by ΔP_V reorients dykes to sills just above source. The solution is that volumetric crack growth accommodating non-relaxed EMV (EMV*) causes ΔP_V→0. Magma transport becomes buoyancy-driven and the ΔP_V problem does not arise. The critical sill intrusion depth, I, is where ΔP_B exceeds the regional vertical stress curve, where columns must intrude owing to ΔP_B alone. Sill growth is mainly by floor depression, involving ductile shear of lower crust, creating sill volume, suppressing roof uplift, expelling source contents, processing protolith through the melting zone, reducing stress, σ_H, and widening conduits for sustained flow.

Two intrusive regimes are identified; ΔP_V>0 (hydraulic inflation) and ΔP_V=0 (buoyancy pumping). Partitioning between three sinks for EMV – inelastic uplift (ϕ), crack growth (η) and a non-relaxed portion (EMV*) generating ΔP_V – defines four hydraulic subregimes. Disequilibrium dilation occurs during crustal relaxation prior to rupture, when η=0 and EMV partitions between ϕ and EMV*. Uplift occurs readily owing to the crust's weakness in flexure, so ϕ abruptly increases while EMV* decreases, causing abrupt variations in source failure mode, geometry and rate that smooth initial ΔP_V variations. During equilibrium dilation source swelling continues with ϕ dominant over EMV*. Dilatant loading rates mean that positive ΔP_V is always maintained however, keeping the source near-isotropically inflated and prepared for rupture. Disequilibrium cracking begins when uplift-driven horizontal stretching initiates rupture and crack growth (η). Crack volume is initially small, but readily enlarges as dykes propagate by conversion of stored ΔP_V. EMV is minimized better and faster by η than by ϕ, owing to crack-tip stress concentration, giving abrupt augmentation of η and decreases in ϕ and EMV* in a crack growth-surge until the uplift-modified stress field is balanced. In equilibrium cracking, once EMV* (and ΔP_V) decrease to incipient levels, each new increment of EMV* partitions directly into crack growth, while continued uplift maintains vertical rupture, generating a vertically extensive fracture system.

The absolute volumetric equivalence of EMV*, at most a few tens of km³, will be exhausted during dyking, sill intrusion or surface eruption. The system then becomes buoyancy-driven and, if depth I is reached, must intrude a sill. Relaxation of sill underburden initiates crustal decoupling and buoyancy pumping, where the downward underburden-flux drives and is balanced by upward magma flow.