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Geochemical and petrologic affinities of late Proterozoic (~ 1Ga) bimodal igneous rocks of the Franklin Mountains, west Texas, suggest a rift origin. Scattered occurrences of similar rocks southward into the state of Chihuahua, Mexico, indicate a southerly trend for the feature. The feature is bounded by stable blocks: the stable craton of west Texas to the east and northeast, and the Sierra del Nido block to the west and southwest. Separation of the Sierra del Nido block from the craton occurred about 1 Ga. Gravity gradients mark the boundaries of the blocks, and a northwest-trending Bouger gravity high may mark the axis of the aulacogen. The aulacogen and the Sierra del Nido block are truncated to the south by the Mesozoic Mojave-Sonora discontinuity.
The aulacogen was reactivated, at least in part, in the late Paleozoic as the Pedregosa basin and in the Mesozoic as the Chihuahua trough. These reactivations were apparently not full-fledged rifting events, but did result in basin development. The Sierra del Nido block was a paleographic high throughout the Paleozoic, and the Aldama platform developed on this block during the Cretaceous. The most recent reactivation of the aulacogen is as the southern extension of the Rio Grande rift, as evidenced by trends of high heat flow, recent mafic magmatism, and regional extensional faulting.
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