Ash-Flow Tuffs in the Nine Hill, Nevada, Paleovalley And
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Ash-fl ow tuffs in the Nine Hill, Nevada, paleovalley and implications for tectonism and volcanism of the western Great Basin, USA Christopher D. Henry James E. Faulds Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557, USA ABSTRACT Deposition of tuffs in paleovalleys pro- both tectonic and magmatic processes. The tuffs duced several features relevant to interpret- are commonly used as the dominant structural Cenozoic ash-fl ow tuffs are key units for ing the tectonic evolution of the region. Most markers to measure extension with the assump- analyzing the tectonic and magmatic evolu- tuffs show primary dips, commonly up to tions that their compaction foliation records tion of the Great Basin. The tuffs are com- ~20° and locally up to near vertical, because paleohorizontal and that any divergence from monly assumed to have spread nearly radi- they compacted against gentle to steep paleo- horizontal refl ects tilting and/or vertical-axis ally from source calderas and to have formed valley walls. Angular unconformities, unre- rotation during later deformation (Anderson, nearly continuous, fl at-lying deposits. Based lated to tectonism, are common where tuffs 1971; Proffett, 1977; Geissman et al., 1982; on detailed mapping and paleomagnetic were deposited on eroded tuffs that had Chamberlin, 1983). An obvious corollary is investigations of the Nine Hill paleovalley in primary dips. The tuffs are commonly inter- that any angular discordance between differ- western Nevada and analysis of the regional bedded with coarse clastic deposits that orig- ent tuffs must result from deformation that distribution of the 27–23 Ma tuffs that crop inated either as channel deposits in high dis- occurred between their depositions. Because out in the paleovalley, we fi nd numerous fea- charge, possibly high gradient rivers, or from ash-fl ow tuffs are the most voluminous volca- tures that differ markedly from these pre- fl oods induced by failure of dams, whereby a nic rocks of the Great Basin, especially during sumed characteristics. tuff temporarily blocked drainage. the mid-Cenozoic “ignimbrite fl are-up” (Best et Many individual ash-fl ow tuffs can be cor- Tuffs show highly asymmetric, elliptical al., 1989), estimates of their volumes are major related from their source calderas in the cen- distributions, preferentially west of their factors in evaluating magmatic processes such tral Nevada caldera belt westward into the source calderas, because they fl owed in the as mass and heat fl ux from the mantle, inter- Sierra Nevada of eastern California. Present- westward-draining paleovalleys. Although action with the crust, and assembly of crustal day distances from source to distal, primary deposited continuously along the paleoval- magma chambers (e.g., Best and Christiansen, tuff deposits are as much as ~295 km. Cor- leys, tuffs were subsequently eroded by the 1991; Johnson, 1991; Perry et al., 1993; Lip- rected for extension, original fl ow distances rivers, so that tuff distributions are highly man, 2007; Farmer et al., 2008). Volume esti- were as much as ~200 km. The tuffs fl owed, discontinuous both between and along indi- mates commonly assume that the tuffs were were deposited, and are preserved primar- vidual paleovalleys. Thicknesses of tuffs deposited as essentially continuous sheets with ily in deep (as much as 1.2 km) but wide (8– vary irregularly with distance from source, a near radial distribution around their sources, 10 km) paleovalleys. The tuffs were able and some tuffs are as much as 300 m thick both in the Great Basin and worldwide (Ekren to fl ow these great distances because they even 150 km (primary distance) from source. et al., 1980; Cas and Wright, 1987; Best et al., were channelized, did not disperse, and did Thickness variations probably are in part 1995). Distribution can, in turn, be used to infer not mix with air as much as would tuffs due to variations in width and depth along displacements on strike-slip faults (Faulds et that spread more radially. Tuffs probably paleovalleys. Estimates of tuff volumes al., 2000, 2005a, 2005b; Henry et al., 2002; spread more radially only within a few tens should not assume radial distribution or Hardyman et al., 2003), and asymmetric distri- of kilometers of source. The paleovalleys held continuous deposition between paleovalleys, butions have been used to evaluate the amount major rivers that drained from a “high” pla- which give unreasonably large volumes. In and direction of regional extension (Gans et al., teau in the present Basin and Range Prov- ideal cases where the source caldera is well 1989; Colgan et al., 2008). ince and Walker Lane and fl owed across the understood, the best way to estimate erupted The assumptions of “paleohorizontality” and Sierra Nevada to the Pacifi c Ocean in what is volume can be from the volume of caldera essentially continuous, semiradial sheets are true now the Great Valley. The Basin and Range– collapse (i.e., area within ring fracture × as long as ash-fl ow tuffs fl owed over surfaces Sierra Nevada structural and topographic amount of subsidence). with little relief. However, volcanologists have boundary did not exist before 23 Ma, the age long recognized that tuffs are density currents of the youngest tuff in the Nine Hill paleoval- INTRODUCTION whose fl ow is strongly infl uenced by topography ley. Any faulting in western Nevada before and that tuffs drape topography and compact 23 Ma was insuffi cient to disrupt the paleo- Cenozoic ash-fl ow tuffs of the Great Basin against the underlying surface (Ross and Smith, drainages other than temporarily. and many other regions are key indicators of 1961; Chapin and Lowell, 1979; Walker, 1983). Geosphere; August 2010; v. 6; no. 4; p. 339–369; doi: 10.1130/GES00548.1; 13 fi gures; 3 tables; 2 plates. For permission to copy, contact [email protected] 339 © 2010 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/4/339/3338420/339.pdf by guest on 25 September 2021 Henry and Faulds Such relations are particularly well established Hill paleovalley, a well-exposed segment of a tuffs in western Nevada were initially com- around Quaternary calderas, where tuffs fl owed paleovalley in western Nevada. We use the geol- bined into a single unit (Hartford Hill Rhyolite; down surrounding drainages, and present-day ogy of the Nine Hill paleovalley, supplemented e.g., Bingler, 1978a); on the other, Best et al. topography is little changed since the time with published and our new regional data about (1995) point out that the same ash-fl ow tuff in of tuff eruption (e.g., Aso, Japan, Matumoto, the tuffs, to document that (1) ash-fl ow tuffs in central Nevada had been assigned four different 1943; Lipman, 1967; Long Valley, California, western Nevada generally fl owed in and were names in different ranges, a common problem Bailey et al., 1976, Bailey, 1989; Taupo, New deposited in paleovalleys, (2) the tuffs com- for western Nevada tuffs also. Zealand, Wilson and Walker, 1985). Matumoto pacted against sloping paleovalley walls, so Work for this study shows that the six ash- (1943) described the distribution of the Aso Tuff commonly have primary dips, as much as 80° in fl ow tuffs present in the Nine Hill paleovalley as “likened to the pseudopods of an amoeba... extreme examples, (3) deposition of subsequent (and one just outside) are regional units wide- branching away on account of pre-existing tuffs on variably eroded tuffs with primary spread in western and central Nevada (Table 1; mountain masses and projecting radially, far dips generated angular unconformities that are Fig. 1). The tuffs were initially subdivided and along the rivers and valleys.” Tuffs can have sig- unrelated to tectonism, (4) a combination of partly correlated between Yerington (Proffett nifi cant primary dips where deposited against normal fl uvial processes and “dam-burst” type and Proffett, 1976; Proffett and Dilles, 1984) slopes (Chapin and Lowell, 1979; Geissman et fl oods resulting from blockage of drainages by and the Nine Hill paleovalley (Bingler, 1977, al., 1982; Hagstrum and Gans, 1989; Henry et ash-fl ow tuffs generated coarse clastic deposits 1978a). We recognize further correlations based al., 2003; Brooks et al., 2008). Many geologists without tectonism, (5) tuffs are generally lat- on geologic mapping, stratigraphy, petrography, recognized that ash-fl ow tuffs and interbedded erally discontinuous, both between and along and especially on new 40Ar/39Ar ages. Distal, sedimentary deposits in western Nevada and valleys, because they were deposited and rap- paleovalley deposits and intracaldera or near elsewhere were deposited in large paleovalleys idly eroded in paleovalleys, (6) tuffs generally source tuffs have the same stratigraphic posi- with up to 1.2 km of relief and that thicknesses have elliptical distributions, offset to the west tion, indistinguishable ages, the same pheno- of individual ash-fl ow tuffs varied from as much from their source calderas because they fl owed cryst assemblages, and distinctive petrographic as 400 m to 0 across short distances (Proffett preferentially westward down the paleodrain- features (Table 1). The overall distributions of and Proffett, 1976; Bingler, 1977, 1978a; Geiss- ages, and (7) tuff volumes, although commonly the seven tuffs are variably known because of man et al., 1982; Proffett and Dilles, 1984; Baer ≥1000 km3, are much less than would be cal- variable coverage by geologic mapping and et al., 1997; Henry et al., 2004;