Large-scale mechanics of fracture-mediated felsic magma intrusion driven by hydraulic inflation and buoyancy pumping
G. J. Ablay, J. D. Clemens, N. Petford, 2008. "Large-scale mechanics of fracture-mediated felsic magma intrusion driven by hydraulic inflation and buoyancy pumping", Structure and Emplacement of High-Level Magmatic Systems, K. Thomson, N. Petford
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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 (ΔPB) and a hydraulic contribution (ΔPV) to the magma pressure (PM). ΔPV 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 ΔPV reorients dykes to sills just above source. The solution is that volumetric crack growth accommodating non-relaxed EMV (EMV*) causes ΔPV→0. Magma transport becomes buoyancy-driven and the ΔPV problem does not arise. The critical sill intrusion depth, I, is where ΔPB exceeds the regional vertical stress curve, where columns must intrude owing to ΔPB 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, σH, and widening conduits for sustained flow.
Two intrusive regimes are identified; ΔPV>0 (hydraulic inflation) and ΔPV=0 (buoyancy pumping). Partitioning between three sinks for EMV – inelastic uplift (φ), crack growth (η) and a non-relaxed portion (EMV*) generating ΔPV – 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 ΔPV variations. During equilibrium dilation source swelling continues with φ dominant over EMV*. Dilatant loading rates mean that positive ΔPV 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 ΔPV. 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 ΔPV) 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.
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Structure and Emplacement of High-Level Magmatic Systems
There are continual rounds of annual conferences, special sessions and other symposia that provide ample opportunity for researchers to convene and discuss igneous processes. However, the origins of laccoliths and sills continue to inspire and confound geologists.
In one sense, this is surprising. After all, don’t we know all we need to know about these rocks by now? As testified by the diverse range of topics covered in this volume, the answer is clearly ‘no’.
This book contains contributions on physical geology, igneous petrology, volcanology, structural geology, crustal mechanics and geophysics that cover the entire gambit of geological processes associated with the shallow emplacement of magma. High-level intrusions in sedimentary basins can also act as hydrocarbon reservoirs and as sources for thermal maturation.
In drawing together a diversity of perspectives on the emplacement of sills, laccoliths and dykes we hope to advance further our understanding of their behaviour.