Mid-Cenozoic orogenic andesites and ignimbrites of western Mexico, southwestern New Mexico, and Arizona are commonly capped by basaltic andesites, most from 29--20 Ma. We refer to these mafic lavas as the Southern Cordilleran Basaltic Andesite (SCORBA) suite, and they may constitute the most extensive Cenozoic basaltic suite in North America. The SCORBA suite has trace element and isotopic characteristics of orogenic (arc) rocks (i.g., Ba/Nb>40), and silica content (53--56% SiO2) like the Grande Ronde Basalt, which represents about 80% of the volume of the Columbia River Group. Geochemical and isotopic data are presented on SCORBA lavas and rare mafic lavas (PRE-SCORBA) interlayered with older ignimbrites from a 700-km-long NE-SW transect of southern Chihuahua, Mexico. SCORBA and PRE-SCORBA lavas with relatively low K/P (0.8).

The western mantle source contains less of the intraplate component and is more oceanic in character. Overprinting both the eastern and western sources is a Cenozoic subduction component that is responsible for the western radiogenic Pb, and this component fades out inland to the east. This transect crosses the inferred position of the Mojave-Sonora megashear, previosly proposed to be a major lithospheric boundary, separating Proterozoic basement to the east from Phanerozoic basement to the west at the latitude of the transect. Most chemical changes near the inferred position of the megashear are subtle, and they may be gradational rather than abrupt. The uniformity of Sr and Nd isotopic compositions across the inferred position of the megashear indicates that one or more of the following statements is true: (1) Phanerozoic and Proterozoic subcontinental lithospheres are essentially indistinguishable in Sr and Nd compositions in southern Chihuahua, (2) the megashear is not a lithospheric boundary separating Phanerozoic and Proterozoic crust in the vicinity of the transect, or (3) the isotopic signatures were acquired in the asthenosphere rather than in subcontinental lithosphere.

The principal difference between the SCORBA suite and the earlier mid-Cenozoic andesite to rhyolite orogenic suite is average SiO2 content. This difference reflects regional stress regimes at the time of eruption and magmatic plumbing. SCORBA was erupted in a more extensional tectonic environment than the orogenic suite. SCORBA magmas reached the surface more quickly and directly than most PRE-SCORBA basaltic magmas, and the suite experienced less differentiation. Although the tectonic setting of SCORBA was more extensional than the orogenic suite, it was less extensional than that of true flood basalts because SCORBA was erupted from central vents rather than major fissures. The SCORBA suite closely resembles the Grande Ronde Basalt in average SiO2 content, large ion lithophile/Nb ratios, voluminous nature, and Sr and Nd isotopic compositions.

Furthermore, SCORBA west of the megashear and the Grande Ronde have similar Pb isotopic ratios. Voluminous continental basalts include a spectrum of compositions from intraplate tholeiites to rocks with more orogenic affinities. The common feature that they share is erption in an extensional tectonic environment, be it intraplate, back arc, or perhaps intra-arc. The Grande Ronde is intermediate in the compositional spectrum, but SCORBA is at or near the orogenic extreme. ¿ American Geophysical Union 1989 The Springerville Field