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Description
Highly folded, upper-greenschist-grade, Paleozoic rocks of cratonal affinity, as well as Precambrian basement and Mesozoic metasedimentary rocks, are present in the Big Maria Mountains. The style of deformation in the Big Maria Mountains is one of Penninic-style folding with large amplitude (> 1 km), overturned and recumbent folds associated with thrust faults. Limbs of major folds are highly attenuated to widths of only a few meters. Formational identity is still preserved despite this intense thinning. The purpose of this study is to examine the minor folds, lineations, foliations, boudins, and refolded folds and their relationship to the major folds. Minor features found in four domains within the range are examined to determine the direction of tectonic transport. A generalized geologic map is also constructed in order to reconstruct mid-Tertiary doming of the range and late Tertiary strike-slip faulting. A schematic cross section of the range is then constructed. Maxima of the fold axes orientations trend east-west or northwest-southeast for both large-and small-scale folds. Vergence is mostly to the south or southwest, and may indicate north-or northeast-directed underthrusting. Maxima of axial-surface orientations trend east-west to northwest-southeast and dip to the north or northeast. Two sets of folds are distinguishable on the basis of their orientations. The flow direction of the second set of folds lies within the axial surface of the first fold set. Outcrop patterns of these refolded folds are circular or elliptical in cross-section. A lineation is also developed parallel to this flow direction. Complex folding, or twisting, of early formed axial surfaces appears characteristic of many of the larger folds. This complex deformation may be a reflection of two separate folding events or the product of progressive flattening of early formed folds during progressive deformation. Intrusion of Mesozoic plutonic rocks and remobilization of Precambrian rocks appear at least partially responsible for producing the complex refolding of the initially developed folds. Late Tertiary detachment faulting, which is present both north and south of the Big Maria Mountains, appears to be superimposed on this multiply deformed Mesozoic terrane producing the overall morphology. Development of large-scale antiforms and synforms is probably responsible for the elongate, domed shape of the range and its relationship to surrounding valleys and ranges.
