Mid-Tertiary magamtism in southeastern Arizona
— M. Shafiqullah, P. E. Damon, D. J. Lynch, P. H. Kuck, and W. A. Rehrig

Southeastern Arizona is characterized by subparallel north-northwest trending mountain ranges, 5 to 30 km wide, separated by valley floors or alluviated basins. The area lies within the mountain region of the southern Basin and Range province (fig. 1). The Mesozoic and Cenozoic geology of the area is characterized by superimposed deformation, complex struc-tural relationships and localized zones of mineralization. More-over, it is difficult to distinguish between the effects of different tectonic episodes. Fortunately, the deformational episodes were accompanied by igneous activity, making possible radio-metric dating of Tertiary fossil-poor continental deposits. Some of the complex structural-lithologic relationships are partially exposed in fault-controlled, tilted mountain blocks, while others are buried under an extensive cover of Cenozoic sedimentary rocks in the alluviated basins. Margins of the basins and the pediments have become the focus of mineral exploration because most of the exposed deposits have already been found.

 

This paper deals with the chronology of post-Eocene vol-canism and tectonism in southeastern Arizona from 111° 30'W longitude to its eastern border, and from 32° 31 'N latitude south to the international border (fig. 2). Some of this data has already appeared in progress reports or other reports of limited circulation. K-Ar ages for 26 samples are listed in Table 1. To facilitate interlaboratory comparison, previously re-ported K-Ar ages have been recalculated using constants recommended by Steiger and Jager (1977). However, the analytical uncertainties associated with the previously reported samples have not been modified. Our work reassigns geologic ages to several units formerly correlated only by lithologic similarity.