Abstract

Detached isoclinal folds, overturned asymmetric folds, and unbroken cascades of recumbent folds pervade sheets of sedimentary and metasedimentary rocks of Paleozoic and Mesozoic age that occur around the base of the Rincon Mountains near Tucson, Arizona. The sheets of folded rocks rest subconcordantly on the gently dipping surface of the granitic gneiss that composes much of the range. This surface, known as the Catalina fault, parallels the attitude of the foliation in the gneiss and is folded about two macroscopic upright antiforms and an intervening synform.

The low-angle tectonic displacement reflected in the folds was brought about by local gravitational tectonics. The slip-line directions inferred from the geometry of the fold arrays define a radial pattern centered on the Rincon Mountains. The forms of the folds are consistent with the characteristics of gravity-induced folds.

Most of the gravity-induced folding is interpreted to have accompanied the 28- to 24-m.y. uplift that ended the Tertiary metamorphism of gneiss in the Rincon Mountain complex. The Catalina fault is interpreted to be a décollement, above which the sedimentary and metasedimentary rocks folded independently of their substratum.

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