ABSTRACT:

Volumetric effects of magma chamber solidification in the upper crust produces stresses in the surrounding rocks and causes surface uplift and subsidence. We study this process using a FEM that accounts for the effects of the magma contraction by perfect dissolved fluid release. The numerical model uses a viscoelastic rheology with strain weakening based on Dynamic Power Law. In the numerical simulation, it is assumed that the chamber roof is intersected by a normal fault represented by a friction-less contact surface. Upon fault slip, immediate gravity sliding of the hanging wall occurs causing a pressure drop in the magma. With time, the effect of slow contraction begins to work and the foot wall starts to subside. As subsidence proceeds above the contracting magma chamber, maximum tensile stresses develop near the surface and shear stresses concentrates in the arch zone above the BT transition isotherm. In the thermal aureole of the magma chamber, stresses gradually relax. Zones of the strain localization appear on the boundary of the chamber in contact with the magma. An additional subhorizontal shear zone develops below the BT transition zone in connection with fault termination. This shear zone in the thermal aureole can act as a host for a deep geothermal reservoir. A strong seismic reflector found in the Coso geothermal field at a depth about 6.5 km can be viewed as an example of this type of a shear zone, which facilitate transfer of the deeper magmatic fluid in the Coso geothermal system.

1. INTRODUCTION

Mechanics of a periodically replenished magma chamber in relation to surface deformations, eruption frequencies, and magma volume accumulation has been the subject of many studies. Meanwhile, the behavior of a solidifying magma chambers has not been considered in great detail. Solidification of the magma at depth is accompanied by fluid accumulation in the melt and its release upon saturation. The net volume effect of solidification is negative since generally minerals have higher density than the melt. However, fluid of mainly water composition not entering water-bearing minerals (such as mica or amphibole) is accumulated in the melt and eventually released as a separate phase with density ¿fl ~0.3-0.5 g/cm3 (for P=1-2 kbar and T=700-900°C). Accumulation of the fluid causes increase of pressure and eventually mechanical failure of the solidified margins and fluid migration into the surrounding rocks. This work focuses on the contraction stage of the solidification when the extra fluid escapes the intrusion and its impact on stress distribution and deformation of the surrounding rock. The problem is treated numerically using a finite element method. This complex geologic process is first simplified while maintaining the essential features such as a fault in the chamber roof, viscoelastic rheology, corresponding to the thermal field around the magma chamber, and Dynamic Power Law modification of the viscoelastic rheology.

1.1 Temperature-Dependant Rheology

It is known that solid rocks become softer when heated due to activation of the dislocation migration, subgrain formation, and recrystallization in the presence of a fluid.

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