Tectonic pumping of pervasive granitic melts

Finite element 2D models are used to study how tectonic stresses pump perva sive granitic melts within migmatites. We start by assuming elliptical melt -filled veins (of different orientations) interconnected by the assumption that they share the same pressure. The melt is then redistributed in the Ve in array by the application of lateral compression or extension. The effect ive permeability of vein networks is thought to be up to 13 orders of magni tude higher than the matrix permeability of 10(-19) m(2). This allows us to approximate pressure equilibration and neglect irreversible compaction in the matrix. This simple configuration allows solution of constitutive equat ions for the equation of state in the viscoelastic matrix, the melt in the vein array, and the mechanical equilibrium along with force balance on the walls of the deforming veins. A rectangular box is pressurized by a constan t load applied to one side and continuously deformed by shift of another si de with constant velocity. New veins cannot initiate and pre-existing veins cannot migrate bodily down pressure gradients. However, potential flaws are cut in-line with the rip of every pre-defined vein. This means that. veins not only close, open, and shear as they rotate, but can also undergo limited propagation at rates di ctated by the bulk deformation of the matrix. We use constant viscosities e ta (s), in the range 10(17)-10(18) Pa s, deformation rates (e) over dot var y between 10(-10) and 10(-9) s(-1), and a total strain up to 4-5%. Isolated veins parallel and normal to sigma (1) have melt pressures between P-0 and P-0 + 4 eta (s)(e) over dot. The mean vein pressure differs from t hese extremes and is equilibrated by driving melt from shrinking veins into veins parallel to sigma (1) which widen and lengthen. The above assumption s result in an asymptotic pattern of stress distribution and the amount of melt redistributed at given strain approaching the kinematic limit with tim e. This occurs whatever the viscosity of the matrix and the strain rate. Mu ltilayered systems are modeled by pre-defining a single vertical vein cross ing the planar horizontal boundary between two uniform media. Compression p arallel to the layering expels melt from the part of the vein in the more v iscous layer to open and extend that part of the same vein in the less visc ous layer. Melt moves in the opposite direction during lateral extension, i f sheet-like bodies become sufficiently tall to become buoyant, horizontal sheets resulting from lateral compression are likely to rise to higher crus tal levels as diapirs while vertical sheets resulting from lateral extensio n are more likely to ascend as dikes. (C) 2001 Elsevier Science B.V. All ri ghts reserved.