NATURE AND TIMING OF FAULTING AND SYNEXTENSIONAL MAGMATISM IN THE SOUTHERN BASIN AND RANGE, CENTRAL-EASTERN DURANGO, MEXICO

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 pr ovince in Mexico. At Nazas, high-angle normal faults that cut part of the mid-Tertiary volcanic sequence strike between N 20-degrees and 70- degrees-W, with 40% of them between N 40-degrees and 50-degrees W. Til ting of faulted blocks varies from 5-degrees to 35-degrees to the nort heast, most commonly around 25-degrees NE. A few blocks are tilted to the southwest as much as 25-degrees. Fault offsets range from 40 m to nearly 300 m. The earliest faulting occurred between 31 and 29 Ma, bef ore the emplacement of mid-Tertiary ash-flow tuffs had ended. The Sant a Ines Formation, a wide-spread fanglomerate, was deposited after the earliest faulting episode and is overlain by 24 to 20 Ma alkalic basal t flows. Although not cut by faults, the flows are adjacent to or cove r normal faults. Some of the mapped faults could have been coeval with basalt eruption, as is the case in Trans-Pecos Texas, where alkalic b asalt having similar age and composition to that in the Nazas area eru pted contemporaneously with normal faulting. The Nazas alkalic basalt also has similar age and stratigraphic position as, but is composition ally distinct from, the Southern Cordilleran Basaltic Andesite (SCORBA ), a widespread mafic suite in southwestern North America that has bee n finked to regional extension.