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Oil and gas occur in the foreland margins of deformed belts around the world, concealed beneath the mountain-facing flank of a foreland syncline. Such a syncline is formed by wedging of blind, foreland-directed thrusts against an upper detachment zone that extends out to the synclinal axis. Examples include mature exploration areas such as the Carpathian foothills of Rumania and the foothills of the Canadian Cordillera, and the foreland margins of the Appalachians, Ouachitas, and Brooks Range. Other examples have been reported from several sectors of the Alpine-Himalayan and Andean orogens. The upper detachment was originally horizontal, uplifted by the blind thrusts beneath it. While there is no way of measuring how far into a thrust belt an upper detachment extended bef re it was removed by erosion, computer modeling can reconstruct thrust belts within the constraints imposed by inclusion of an upper detachment. An example from Canada shows that the entire southern Alberta foothills belt can be modeled this way. This is consistent with the observed plunge of the Alberta thrust belt along strike beneath the fold belt of northeastern British Columbia, where wells spudded in folds penetrate blind subsurface thrusts. These data suggest that, like folded faults, blind thrusts and upper detachments are common features of deformed belts. Failure to recognize them can result in severely underestimating the extent of thrusting, and consequently downgrading the hydrocarbon potential of a deformed belt.
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