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Half-graben structures: Balanced models of extensional fault-bend folds

¹ Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487-1945

One common structural style in extensional regimes is a half graben bounded on one side by a master normal fault and on the other side by a domain of beds dipping toward the master fault. This geometry is modeled as being caused by a bend in the master fault. The hanging-wall beds dipping toward the master fault are bounded by axial surfaces formed as the hanging wall moves past the bend, resulting in an extensional fault-bend fold. The footwall beds remain undeformed and unrotated. The major assumptions used in the derivation are that the geometry is area balanced and that the axial surfaces in the hanging wall have dips equal and opposite to the dip of the master fault above the bend. Important consequences of the model include a relationship between master-fault dip above and below a bend and the amount of hanging-wall dip. The horizontal width of the dipping beds in the hanging wall is twice the heave on the master fault above the bend. The asymmetry of the axial surfaces in the hanging wall requires that the rotated beds be strained. Hanging-wall beds dipping toward the master fault are predicted for a fault bend wherein the dip decreases downward. Hanging-wall beds dipping away from the master fault are predicted for a bend wherein the dip increases downward. The model fits measurements taken from a published ramp-and-flat clay model by E. Cloos and is used to develop a balanced and restorable cross section of the Schell Creek fault, a Basin and Range structure in Nevada.

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