Tectonic stress controls on ascent and emplacement of magmas

The tectonic stresses can significantly affect the propagation of a magma-f illed crack. It has been pointed out that the rheological boundaries contro l the emplacement of magmas through the effect of stress. However, it has n ot been clarified how the role of rheological boundaries depends on the reg ional tectonic and thermal states. We have evaluated the role of rheologica l boundaries under various tectonic and thermal conditions and found that t he level of magma emplacement may jump according to the changes in the tect onic force or the surface heat flow. The stress profiles were estimated by a simple model of lithospheric deformation. We employed a three-layer model of the Lithosphere; the upper crust, the lower crust and the upper mantle have different rheological properties. A constant horizontal force is appli ed to the lithosphere, and the horizontal strain is assumed to be independe nt of depth. When realistic tectonic forces (> 10(11) N/m) are applied, the rheological boundaries mainly control the emplacement of magma, The emplac ement is expected at the MOHO, the upper-lower crust boundary, and the brit tle-ductile boundary. For lower tectonic forces (< 10(11) N/m), the tectoni c stress no longer plays an important role in the emplacement of magmas, Wh en the tectonic stress controls the emplacement, the roles of rheological b oundaries strongly depend on the surface heat flow. When the surface heat f low is relatively high (> 80 mW/m(2)), the stress in the mantle is quite lo w and the MOHO cannot trap ascending magmas. For relatively low heat flow ( < 80 mW/m(2)), on the other hand, the MOHO acts as a magma trap, and the up per-lower crust boundary acts as a magma trap only when the magma supply ra te is sufficiently high. Our results suggest that the emplacement depth can change responding to the change in the tectonic force and/or that in the s urface heat flow. This may provide us a key to understand the relation betw een the evolution of a volcanic region and its tectonic and/or thermal hist ory. (C) 1999 Elsevier Science B.V. All rights reserved.