The transition from hydrostatic to greater than hydrostatic fluid pressure in presently active continental hydrothermal systems in crystalline rock

摘要: Fluid flow at hydrostatic pressure (P{sub h}) is relatively common through fractures in silicic and mafic crystalline rocks where temperatures are less than about 350-370C. In contrast, pore-fluid f}) > P{sub h} has been encountered the bottom of 3 geothermal exploration wells that attained >370C (at Larderello, Italy, Nesjavellir, Iceland, The Geysers, California). Chemical sealing by deposition minerals veins appears to have allowed development high f} above wells. upper limit for magnitude can be controlled either onset shear fracturing (where differential stress high) reopens clogged veins, or hydraulic opening new old low values stress). brittle-plastic transition occur as 370-400C tectonically active regions. regions high-temperature systems develop persist, it strain rates commonly range 10{sup {minus}12} {minus}13}, shallow crust generally behave rheologically more like wet quartz diorite Westerly granite.