The Mount Perkins block, northwestern Arizona: An exposed cross section of an evolving, preextensional to synextensional magmatic system

Abstract

Abstract. The steeply tilted Mount Perkins block, northwestern Arizona, exposes a cross section of a magmatic system that evolved through the onset of regional extension. New 4øAr/39Ar ages of variably tilted (0-90 ø) volcanic strata bracket extension between 15.7 and 11.3 Ma. Preextensional intrusive activity included emplacement of a composite Miocene laccolith and stock, trachydacite dome complex, and east striking rhyolite dikes. Related volcanic activity produced an -18-16 Ma stratovolcano, cored by trachydacite domes and flanked by trachydacite-trachyandesite flows, and -16 Ma rhyolite flows. Similar compositions indicate a genetic link between the stratovolcano and granodioritic phase of the laccolith. Magmatic activity synchronous with early regional extension (15.7-14.5 Ma) generated a thick, felsic volcanic sequence, a swarm of northerly striking subvertical rhyolite dikes, and rhyolite domes. Field relations and compositions indicate that the dike swarm and felsic volcanic sequence are cogenetic. Modes of magma emplacement changed during the onset of extension from subhorizontal sheets, east striking dikes, and stocks to northerly striking, subvertical dike swarms, as the regional stress field shifted from nearly isotropic to decidedly anisotropic with an east-west trending, horizontal least principal stress. Preextensional trachydacitic and preextensional to synextensional rhyolitic magmas were part of an evolving system, which involved the ponding of mantle-derived basaltic magmas and ensuing crustal melting and assimilation at progressively shallower levels. Major extension halted this system by generating abundant pathways to the surface (fractures), which flushed out preexisting crustal melts and hybrid magmas. Remaining silicic melts were quenched by rapid, upper crustal cooling induced by tectonic denudation. These processes facilitated eruption of mafic magmas. Accordingly, silicic magmatism at Mount Perkins ended abruptly during peak extension -14.5 Ma and gave way to mafic magmatism, which continued until extension ceased.

Oligocene–Miocene tectonic evolution of the South Fiji Basin and Northland Plateau, SW Pacific Ocean: Evidence from petrology and dating of dredged rocks

“…Samples were dated by conventional argon step heating at the University of Santa Barbara using techniques similar to Faulds et al (1996). Samples were irradiated in the cadmium-lined tube at Oregon State University in three separate packages for 3-8 MW-h.…”

Miocene extension in the East Range, Nevada: A two-stage history of normal faulting in the northern Basin and Range

“…During this period, deformation was generally focused into discrete zones of high strain separated by unextended crustal blocks. Extended domains were characterized by high strain rates; closely spaced, highly tilted fault blocks; and, locally, the development of metamorphic core complexes (e.g., Proffett, 1977;Fitzgerald et al, 1991;Faulds et al, 1995;Miller et al, 1999;Snow and Wernicke, 2000;Stockli, 2005;Carter et al, 2006). In westcentral Nevada the East Range (this study) and the adjacent Tobin Range (Gonsior, 2006) constituted a middle Miocene extensional domain that was separated from the extended Shoshone-Toiyabe Range domain to the east by the intact Fish Creek Mountains block (Fig.…”

Tracking magmatic processes through Zr/Hf ratios in rocks and Hf and Ti zoning in zircons: An example from the Spirit Mountain batholith, Nevada

“…Magmatism swept northward through the northern part of the corridor in the early to middle Miocene, followed closely by and generally outlasting extension related-faulting (Faulds et al, 1995;Gans and Bohrson, 1998;Howard et al, 1996). At this latitude, magmatic activity spanned the interval 18 to 5 Ma (Gans and Bohrson, 1998;Faulds et al, 2001), but exposed plutons were all emplaced between ~17.5 and 15.5 Ma (Faulds et al, 1995;Cates et al, 2003;.…”