Abstract
The Nazas area, in the central-eastern portion of the state of Durango, is within the poorly studied southern half of the Basin and Range province in Mexico. At Nazas, high-angle normal faults that cut part of the mid-Tertiary volcanic sequence strike between N 20° and 70° W, with 40% of them between N 40° and 50° W. Tilting of faulted blocks varies from 5° to 35° to the northeast, most commonly around 25° NE. A few blocks are tilted to the southwest as much as 25°. Fault offsets range from 40 m to nearly 300 m. The earliest faulting occurred between 31 and 29 Ma, before the emplacement of mid-Tertiary ash-flow tuffs had ended. The Santa Inés Formation, a widespread fanglomerate, was deposited after the earliest faulting episode and is overlain by 24 to 20 Ma alkalic basalt flows. Although not cut by faults, the flows are adjacent to or cover normal faults. Some of the mapped faults could have been coeval with basalt eruption, as is the case in Trans-Pecos Texas, where alkalic basalt having similar age and composition to that in the Nazas area erupted contemporaneously with normal faulting. The Nazas alkalic basalt also has similar age and stratigraphic position as, but is compositionally distinct from, the Southern Cordilleran Basaltic Andesite (SCORBA), a widespread mafic suite in southwestern North America that has been linked to regional extension.
