Subduction of young oceanic lithosphere and extensional orogeny in southwestern North America during Mid‐Tertiary time

Abstract

An “extensional orogeny” deformed the Basin and Range province, probably beginning in the late Eocene (about 40 ± 3 Ma). Its characteristics include partial melting of the continental lithosphere during the “ignimbrite flareup,” massive ductile extension (including detachment faulting), and rise of metamorphic core complexes. The affected zone became about 1200 km wide, possibly double its original width. It rose an average of 1–2 km, despite crustal thinning. Locally, some of the highest mountains of North America, up to 4.3 km high, rose through resurgence of ignimbrite cauldrons and isostatic uplift of underlying plutons. The climax of extension occurred prior to the development of the present basin and range topography. Modeling of major and trace elements and Sr and Pb isotopes strongly suggests that mid‐Tertiary volcanic magmas equilibrated, and probably originated, in the continental lithosphere. Components attributable to subducted oceanic lithosphere have not yet been identified. The rocks seem to belong to two provinces, separated by the quartz diorite boundary line of Moore (1959), which also marks the western limit of North America at the end of the late Paleozoic Sonoman orogeny. To the west, low‐K rocks rest on a basement of predominantly oceanic accreted terranes; to the east, high‐K rocks rest on an autochthonous sialic basement. Within the high‐K province, potassium variations can be correlated with crustal thickness; there is no need to invoke a K‐h relationship. Conventional models of plate convergence and back arc extension which involve subduction of old, rigid, cool, and dense oceanic lithosphere may not apply to the mid‐Tertiary Basin and Range province. The overridden Farallon plate is more likely to have been young, hot, ductile, buoyant, and no denser than continental asthenosphere, having been generated in a spreading center close to North America. Under these conditions, motion of the subducting plate slows and slab‐pull is likely to approach zero. Even prior to ridge‐trench collision, overridden oceanic lithosphere may have become underplated beneath the continental lithosphere and ruptured by rising mantle diapirs. Subducted oceanic lithosphere no longer acted as a heat sink, which could partly account for the great width of the affected zone and the anomalous thermal gradients required for partial melting, extension, and metamorphism. Had these processes not died down, after ridge‐trench collision, the western segment of the Cordillera might have separated from North America to form a Japanlike archipelago, while the Basin and Range province foundered into an analog to the Sea of Japan. Instead of rupturing completely, the Basin and Range province fractured into fault blocks.

Chapter 10: Tectonic evolution of the Intermontane System; Basin and Range, Colorado Plateau, and High Lava Plains

“…Isotopic ages, together with isotopic systems reset to Cenozoic characteristics, show that the zones of extreme extension and midcrustal excision tend to follow igneous episodes closely in time (Coney, 1980). Elston (1984), Zoback and others (1981), and Eaton (1982) interpreted the igneous and tectonic events as manifestations of widespread extensional orogenesis, both the inception and main phases of which swept through the province along regular paths without regard to known preexisting fundamental tectonic features, such as the presence or absence of Precambrian crust, Cordilleran miogeocline, or Mesozoic volcanicplutonic arc.…”

“…6). The large-volume nested calderas and subjacent plutons of the Mogollon-Datil field of New Mexico formed during this magmatism (Elston, 1984). In many parts of the linear belts of isochronous rocks that mark the migratory trends, magmatism was cyclic, beginning with andesitic lavas and evolving through quartz latitic to rhyolitic ash-flow tuffs to high-silica rhyolitic ash-flow tuffs and lavas (McKee and Silberman, 1970;McKee, 1971;Noble, 1972).…”

Geochemical homogeneity of a long-lived, large silicic system; evidence from the Cerro Galán caldera, NW Argentina

“…Each eruption records the state of a batholith-scale magma body at an instant in its evolution. Lipman (1984) and Elston (1984) proposed that these super-eruptions eviscerate the tops of granite batholiths and as such, provide a vital window into the plutonic system. Smith (1979) and Hildreth (1981) first commented in detail on the nature of these large-volume crystalline felsic deposits, termed "monotonous intermediates".…”

Laccolith complexes of southeastern Utah: Time of emplacement and tectonic setting — Workshop proceedings

“…Furthermore, when applied to the Western United States, such terms leave unconsidered the location of the concurrent magmatic arc west of any back arc. Thus Elston (1984) instead proposed "extensional orogen," but we prefer the name "magmatic arc." Neglecting such semantic matters, however, the point to be made here is that mi~dle Miocene calc-alkaline magmatism and late Cenozoic bimodal magmatism in the Great Basin are associated with significant extension, possibly along many northnorthwest-to north-northeast-trending, perhaps diffuse, central axes, and thus a spreading analogy can be made.…”

“…In fact, extension is a characteristic feature of subduction-related arc magmatism (Hamilton, 1988c(Hamilton, , 1989(Hamilton, , 1995. Like Elston (1984), we consider that in some parts of the Basin and Range province, including Caliente, middle Cenozoic extension was greater than late Cenozoic extension. Basins containing clastic sedimentary fill, smaller than those of the later basin-range episode, have been identified in some areas (for example, Seedorf, 1991;Axen and others, 1993).…”