The largest and best studied concentration of porphyry-type deposits is the Southwest porphyry province in Arizona, southwestern New Mexico, and northern Sonora (Figure 1). Most of these deposits are in the 58 to 72 million-year-old range, with one 163 m.y. date and one 20 m.y. date. Several Mesozoic age porphyry copper deposits occur northwest of the main cluster, and mid-Tertiary age porphyry molybdenum and porphyry copper deposits are present in the Colorado mineral belt and at Bingham Canyon, Utah. A progressive decrease in age occurs from northwest to southeast among the Laramide age Southwest porphyries.Tertiary rocks contemporaneous with and younger than the Laramide mineralization appear to have been formed under conditions of strong deformation and crustal extension. Large displacement, flat, "gravity slide" faults are present in southern Arizona together with evidence of abundant horst-graben structure and rift faulting. A correlation of rock types, structures, and intrusive and extrusive activity suggests that "basin-range"-type extensional structures were formed throughout the Tertiary period and probably through much of the Mesozoic as well.In detail, mineralization tends to favor N. 65 degrees E. and N. 40 degrees W. structures, and the deposits in some instances appear to form belts with these alignments, but the cluster of deposits has an overall oval shape unlike the lineal porphyry provinces in Iran, Chile, and the Philippines.The individual porphyry belts in the Southwest province cross the Texas lineament with no apparent strike-slip displacement. However, current plate tectonic theory suggests that a large right-lateral displacement should exist if this structure corresponds to the Walker Line in Nevada as postulated by Jerome and Cook (1967). There is as yet little definite evidence for the presence of subduction zones in Arizona and west of Arizona during the period of formation of the Southwest porphyries, and evidence for the association of porphyry mineralization with plate tectonic processes as described in the literature is not convincing in the Southwest province. It is particularly unlikely that subducted oceanic crust was the source of metals in the Southwest porphyry deposits.An interpretation of depth of erosion in porphyry deposits can be attempted by comparing the alteration zones and mineral assemblages exposed at the present ground surface with an assumed model of vertical zonation. This suggests that erosion has cut deeply into the columns of Laramide mineralization in deposits in the northwestern part of the general cluster and that deposits to the southeast were left generally intact by post-Laramide erosion.