Factors controlling the phases and styles of extension in the northern Rio Grande rift
— Paul Morgan and Matthew P. Golombek

The Rio Grande rift is the latest development in a complex series of tectonic and magmatic events in Colorado and New Mexico that began in the Late Cretaceous with Laramide orogeny and have continued to the present. Numerous authors have documented all or part of this complex geologic history in northern New Mexico (e.g., Baldridge and others, 1980; Baltz, 1978; Chapin, 1979; Chapin and Cather, 1981; Golombek and others, 1983; Lipman, 1981; Stearns, 1953a). Following a generalization and simplification of this series of events in southcentral and southwestern New Mexico by Seager and others (1984) and Morgan and Seager (1983 and unpubl.), we identify four basic Cenozoic tectonic—geologic events in northern New Mexico (Fig. 1): (1) Laramide (Cretaceous—Eocene) compressional deformation, essentially amagmatic, producing uplifts and compressional and strike—slip-related sedimentary basins (Baltz, 1978; Chapin and Cather, 1981; Stearns, I 953a); (2) major Oligocene—early Miocene volcanism (Bachman and Mehnert, 1978; Baldridge and others, 1980; Kautz and others, 1981; Lipman, 1981; Lipman and Mehnert, 1979; Osburn and Chapin, 1983; Stearns, 1953a, b; Tweto, 1979); (3) early-phase (late Oligocene—early Miocene) extension with locally large strain and formation of broad basins (Baldridge and others, 1981; Baltz, 1978; Chamberlin, 1983; Lipman, 1981; Manley and Mehnert, 1981; Rhoades and Callender, 1983; Stearns, 1953a); and, following a middle Miocene lull in volcanic activity (Chapin and Seager, 1975), (4) late-phase (approximately 13 m.y. to present with most extension from 12 to 5 m.y.), extensional block faulting with associated volcanism (Golombek and others, 1983; Gardner, 1983). Major volcanic and modern tectonic activities continue to the present (Aubele, 1979; Bachman and Mehnert, 1978; Doell and others, 1968; Kudo, 1982; Lipman and Mehnert, 1979). Recently, Morgan and Seager (1983 and unpubl.) used the tectonic and magmatic history of southcentral and southwestern New Mexico to study the evolution of geotherm and changing styles of extensional tectonism in the southern Rio Grande rift. In the present contribution we attempt to identify the factors which controlled the different tectonic styles of the two phases of Cenozoic extension in northern New Mexico.