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Many paleorifts and rift-faulted basins are characterized by high gravity, magnetic, and thermal anomalies, and high-density mantle cushions in the crust. Base metals are among the many important ore minerals and hydrocarbon accumulations occurring in the sedimentary formations and fracture zones of such rifts, and basins are commonly related to thermal processes and fluid migration through fractures, faults, and other micro or macro passages. Experimental and theoretical studies show that thermomechanical stresses owing to diapirism result in (1) development of fractures or faults and their patterns, (2) rejuvenation and opening of preexisting fractures, faults, or lineaments providing passages for migration of fluids or hydrothermal solutions, and (3) orientation of fra ture pattern of preexisting anisotropy in rocks. Experiments show that changing property from brittle to brittle-ductile to ductile influences the volume percentage of dilation of the preexisting fractures and exerts control on the orientation, patterns, and opening of fractures in the overlying rocks. Rock mechanics experiments also show that extensive en echelon fractures or faults that develop under high fluid pressure by brittle to brittle-ductile extensional fracturing provide additional passage for the migration of fluid during active thermal uplift or rift formation, but they close during subsidence or basin formation. However, marginal fractures or thrust faults formed during doming and uplift open during subsidence and rift-basin formation, and facilitate fluid migration and lat hydrothermal ore-mineral localization.
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