We analyze the intrusion of the ll-krn-long lnyo Dike at the margins of Long Valley caldera, eastern California. The dike trends N07'W and is divided into at least three segments which are rotated by as much as 25' with respect to the main trend. The dike seems affected primarily by the regional stress field of right-lateral shear of the western United States and by the local thermal conditions of the crust; the dike seems unaffected by the preexisting caldera margins and Sierra-Nevada frontal fault system. The high heat flow in Long Valley caldera implies that crustal rocks below 3-4.5 krn deform by steady state creep under tectonic strain rate and support low to vanishing tectonic shear stresses. The upper rocks, above 3-4.5 krn, deform by frictional slip along fractures and may support tectonic shear stresses as high as 24 MPa. We demonstrate that deplh variations of tectonic stresses may have a profound effect on the segmentation and rotation of dikes, both at Long Valley and in other areas of high heat flow. The analys is places constraints on several tectonic conditions. The lnyo Dike intruded under a tectonic stress state with a horizontal maximum compression oriented N07•W. The maximum extensional fracture strength of the host rocks is 1-2.5 MPa, and the pressure drop within the propagating Inyo Dike was about 0.55 MPa/km. The volatile overpressure in the magma chamber was about 15 MPa during eruption of rhyolitic lavas at the Inyo Domes. INffiODUCTION Seismic activity and ground deformation since 1980 have focused attention on Long Valley caldera as a site of incipient volcanic activity. The last major volcanic event to occur in this area was the emplacement of the Inyo Domes and associated pyroclastic deposits approximately 550 years ago; these features were fed by an 11kin-long dike we will refer to as the lnyo Dike. The distribution of Holocene vents in Long Valley suggests that future eruptions will likely be fed by pipes situated along dikes. Thelnyo Dike provides a good opportunity to analyze the process of magma propagation from source area to the surface, thanks to detailed studies of the region. The three-dimensional geometry of thedikehas been resolved on the basis of surface structural features and mapping of the domes [Fink. and Pollard, 1983; Fink, 1985; Mastin and Pollard, in press]. Three research drill holes at thelnyo Domes provide subsurface control on the dike geometry and its composition [Eichelberger et al., 1985]. The vast body of geophysical measurements in Long Valley caldera further constrains the mechanical and rheological conditions of the upper crustal rocks here [e.g., Hill et al., 1985]. In this paper we explain the dike geometry in terms of the interaction between tectonic stresses and local variations in host rock rheology. We evaluate relationships between tectonic and magmatic stresses and estimate the magnitude of magmatic pressure at depth necessary to get the dike to the surface. Application of the model to other volcanoes is also discussed. THE 000 VOLCANIC CHAIN Bas...