The Northeast Nevada Volcanic Field: Magnetic properties and source implications

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B11, 2298, doi: /2001jb000690, 2002 The Northeast Nevada Volcanic Field: Magnetic properties and source implications H. C. Palmer Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada W. D. MacDonald Department of Geological Sciences, State University of New York, Binghamton, New York, USA Received 18 June 2001; revised 21 March 2002; accepted 26 March 2002; published 15 November [1] Magnetic susceptibility and remanence studies on late Eocene ash flow deposits of the Northeast Nevada Volcanic Field (NENVF) were undertaken at 39 sites in four sampling localities: Dolly Varden Mountains (20 sites), Nanny Creek area of the Pequop Mountains (11 sites), eastern Windermere Hills (7 sites), and southern Snake Mountains (1 site). These deposits span a narrow age range from to Ma based on 40 Ar/ 39 Ar data [Brooks et al., 1995a, 1995b] and on new K-Ar ages presented here. Characteristic remanences reveal that all ignimbrites sampled are reversely magnetized. The characteristic directions of remanent magnetization do not significantly deviate from the expected direction computed from the 42 Ma mean pole for North America, indicating that no major vertical axis rotations have affected the sampling regions since Eocene time. Anisotropy of magnetic susceptibility studies, undertaken to decipher flow trends and possible source areas for these ash flow tuff units of the NENVF, are consistent with a source region near the Toano Range. This places the ash flow source area east of Nanny Creek, the type area for the NENVF. The suspected source region lies close to the Wells fault system. However, Tertiary detachment faulting, Basin and Range normal faulting, erosion, and sedimentation have obscured the suspected source region. A more clearly defined zone of Eocene igneous activity, in the central Pilot Range to the east, cannot be ruled out as a potential source area. INDEX TERMS: 1518 Geomagnetism and Paleomagnetism: Magnetic fabrics and anisotropy; 8404 Volcanology: Ash deposits; 1525 Geomagnetism and Paleomagnetism: Paleomagnetism applied to tectonics (regional, global); 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: ignimbrites, Nevada, anisotropy of magnetic susceptibility, volcanic vents, paleomagnetism Citation: Palmer, H. C., and W. D. MacDonald, The Northeast Nevada Volcanic Field: Magnetic properties and source implications, J. Geophys. Res., 107(B11), 2298, doi: /2001jb000690, Introduction Copyright 2002 by the American Geophysical Union /02/2001JB000690$09.00 [2] The Northeast Nevada Volcanic Field (NENVF) [Brooks et al., 1995a] is a heterogeneous assemblage of andesite to dacite flows, flow domes and flow breccias, together with rhyolitic ignimbrites. The rocks of intermediate composition are presumed to have local vent sources whereas the ignimbrites have unknown source(s). Presently, the NENVF is represented by rocks in more than a dozen ranges, and the ignimbrites would have originally covered an area of 50,000 km 2. The 40 Ar/ 39 Ar ages for the rocks of the NENFV range from 42.6 to 39.0 Ma [Brooks et al., 1995a, 1995c] (Figure 1; see also Table 1). The age of the magnetization of the lowest ignimbrite at Nanny Creek, the type area for the NENVF, is thought to correspond to anomaly C18r. The stratigraphically higher ignimbrites at Nanny Creek and the ignimbrites at the other studied localities are thought to have been emplaced during the much shorter reverse polarity subchron C18n.1r for which Cande and Kent [1995] have indicated an age range from to Ma. [3] Paleomagnetic data (Tables 2 and 3) and anisotropy of magnetic susceptibility (AMS) data (Table 4) have been obtained from 39 sites in four areas separated from one another by distances of 40 to 80 km: (1) Dolly Varden Mountains, (2) Pequop Mountains, (3) Windermere Hills, and (4) southern Snake Mountains. Paleomagnetic data are used here to assess the possibility of relative differential rotation between the sampling regions and of synchronous emplacement of ignimbrites. The AMS data are used to decipher fabrics in the ignimbrites, specifically evidence of lineations which reflect ash flow paths. Patterns of flow from range to range, inferred from maximum AMS axes, are used in a triangulation method to infer source vents [Ellwood, 1982; MacDonald and Palmer, 1990; Hillhouse and EPM 4-1

2 EPM 4-2 PALMER AND MACDONALD: NORTHEAST NEVADA VOLCANIC FIELD Figure 1. Map and age distribution of units of the Northeast Nevada Volcanic field, modified from Brooks et al. [1995c]. Circles on the map indicate locations where age data have been obtained by Brooks et al. [1995a]. Areas of our magnetic and structural studies are DV, Dolly Varden Mountains; NC, Nanny Creek; WH, Windermere Hills; SSM, Southern Snake Mountains. Other areas to which we make reference are Toano Range (TR) and Pilot Range (PR.). Open oval against the Dolly Varden Mountains on the age distribution part of the figure depicts our new K-Ar age data (A. Hayatsu, personal communication, 1997).

3 PALMER AND MACDONALD: NORTHEAST NEVADA VOLCANIC FIELD EPM 4-3 Table 1. Radiometric Ages and K-Ar Analytical Data a Sample Mineral Age, Ma K, % 40 Ar rad, 10 6 cm 3 /g 40 Ar/ 36 Ar 40 K/ 36 Ar, 10 6 DV11 biotite DV11 feldspar DV12 biotite DV12 feldspar BDV b biotite ± 0.11 a The constants used for the other samples are as follows: l e = yr 1 ; l b = yr 1 ; atomic 40 K/K = b The age of this sample was obtained by the 40 Ar/ 39 Ar age spectrum technique [Brooks et al., 1995a, 1995c]. Wells, 1991; Palmer and MacDonald, 1999; LePennec, 2000]. 2. Geologic Background and Sampling Strategy [4] In the above four regions, our sampling emphasized localities where flow foliation could be measured. We avoided sites which seemed to be affected by secondary rheomorphic flow [Wolff and Wright, 1981] which contributes to spurious flow indications in regards to ash flow source directions. At each site, five or six structural attitudes were measured and oriented samples were collected within the area over which the structural measurements were made. Oriented core samples were drilled in the field using Table 2. Paleomagnetic Site Mean Directions for the Dolly Varden Formation, Tuffs of Nanny Creek and Windermere Hills a In Situ Foliation Tilt Corrected Site N A N T N S D 1, deg I 1, deg R k a 95, deg f, deg q, deg D 2, deg I 2, deg DV DV DV DV DV DV DV DV DV DV DV DV DV DV DV15A DV15B DV DV DV DV b W W W W W W W W W W W c W W W W W W W W a N A, number of specimens treated by AF demagnetization. N T, number of specimens treated by thermal demagnetization. N S, number of specimens that yielded stable end-points (SEPs), and that contribute to site-mean statistics. D 1 and I 1, in situ declination and inclination, respectively; D 2 and I 2, declination and inclination after tilt correction. R, k, and a 95, statistical parameters of Fisher [1953]. f and q, attitude of foliation expressed as azimuth of dip and value of dip, respectively; regional attitudes used for sites DV2 and DV12. b Includes one remagnetization circle and site mean calculated by the method of Bailey and Halls [1984]. c Includes two remagnetization circles and site mean calculated by the method of Bailey and Halls [1984].

4 EPM 4-4 PALMER AND MACDONALD: NORTHEAST NEVADA VOLCANIC FIELD Table 3. Paleomagnetic Mean Directions and Pole Statistics a Region Polarity U/C N R D, deg I, deg k, K a 95, A 95, deg Lat, N Long, E DV R U DV R C DV N U DV N C DV N + R C DV N + R NC R U NC R C NC R WH R U WH R C WH R NC + WH + SSM R U NC + WH + SSM R C NC + WH + SSM R DV + NC + WH + SSM R U DV + NC + WH + SSM R C DV + NC + WH + SSM R DV + NC + WH + SSM R + N U DV + NC + WH + SSM R + N C DV + NC + WH + SSM R + N a Regions as follows: DV, Dolly Varden Mountains; NC, Nanny Creek, Pequop Mountains; WH, Windermere Hills; SSM, Southern Snake Mountains. Polarity: R, reverse; N, normal; U/C: U, structurally uncorrected (in situ); C, tilt corrected (stratigraphic coordinates). N, number of site means used in the analysis; D and I, declination and inclination, respectively. R, k, and a 95, statistical parameters of Fisher [1953]. portable drilling equipment and oriented using a sun compass and clinometer. At site DV11, the deposit is poorly indurated and cubic samples were collected in oriented plastic boxes [Ellwood et al., 1993]. In total 263 oriented samples were collected at 39 sites. In the Dolly Varden formation, block samples for K-Ar age dating were collected at sites DV11 and DV12 to supplement the single sample from this area analyzed by Brooks et al. [1995a, 1995c] Dolly Varden Mountains [5] The Dolly Varden formation was mapped as the Dolly Varden Volcanic Series by Snow [1964]. Our emphasis on sampling here was placed on the ash flow tuff unit (sites 1,3,4,5,6,7,8,9,10,13,14, and 17, Figure 2), because the AMS of the ash flow tuff is likely to have regional significance in terms of source vent location(s). For better averaging of secular variation of the paleomagnetic data, units below and above the ignimbrite were also sampled. Snow [1964] mapped two welded tuff units: an ignimbrite and a quartz latite welded tuff with the latter being somewhat of a catch-all unit for a variety of lithologies. The latter unit was interpreted to be stratigraphically higher than, and in part equivalent in age to the ignimbrite. Our site DV17 is in Snow s quartz latite unit whereas the remainder of our sites are in his ignimbrite unit. For simplicity, his two ash flow tuff units are lumped together as ignimbrite in Figure 2. [6] A schematic stratigraphic section illustrates the relative thickness, facies variations, and site distributions in the Dolly Varden formation (Figure 3). Near the southern end of the range, an andesite flow unit at the base of the Cenozoic section rests unconformably on deformed sedimentary rocks of Carboniferous to Triassic age. Near our northern limit of sampling at site 20, a reddish flow breccia overlies nonconformably the Melrose granitoid intrusion of 165 ± 3 Ma age [Zamudio and Atkinson, 1995]. This, in turn, is overlain by a few decimeters of laminated vitric tuff at our sites 16 and 19. Snow [1964] subdivided the stratigraphically and structurally simple section along Black Rock Canyon road into three ignimbrite members separated by mappable vitric marker (vitrophyre) horizons. The Black Rock Canyon ignimbrite section is about 550 m thick and dips homoclinally 20 toward 072. The base rests unconformably on deformed Paleozoic strata; the top of the section is truncated by the Window Ridge fault. The vitric marker horizons disappear both SE and NW along strike, making regional correlations less certain. Faults to the SW (Black Rock fault), SE (Juniper fault), NE (Window Ridge fault), and NW (unnamed fault; Figure 2) further complicate the regional correlations [Snow, 1964]. [7] Structural complications and dramatic variations in thickness and lithology occur away from the simple section of Black Rock Canyon, making it difficult to place all sites in correct relative stratigraphic order. For example, sites 11, 15, 17, and 18 lie structurally above the Black Rock Canyon section northeast and east of the Window Ridge fault or its southerly extension. Sites 15 and 18 are at approximately equivalent stratigraphic levels in flow breccia, above the level of site 11 in vitric tuff. Site 11, however occurs in the lowest level of the formation, just above the unconformity with the Paleozoic, whereas site 15 is only about 100 m stratigraphically higher and near the top of the section. These relations suggest that the ignimbrite section is reduced by about 400 m in thickness across the Window Ridge fault. At the southeast end of the Dolly Varden range, several hundred meters of andesite flows lie below the basal unconformity of the DV ignimbrite section. Sites 2 and 12 lie near the middle of the andesite section. [8] Some vitric tuff lies below the andesite [Snow, 1964; Zamudio, 1992], but it is not clear if that vitric tuff correlates with similar vitric tuff at the base of the ignimbrite section in the north (i.e., site 16). Up to several

5 PALMER AND MACDONALD: NORTHEAST NEVADA VOLCANIC FIELD EPM 4-5 Table 4. Mean-Site AMS Parameters and Densities, Dolly Varden Formation, Tuffs of Nanny Creek and Windermere Hills a K 1 Site r SD K K 1 /K 2 K 2 /K 3 D, deg I, deg Semiaxes D, deg I, deg Semiaxes D, deg I, deg Semiaxes DV (50,22) (49,33) (37,21) DV (35,12) (35,24) (26,07) DV (60,14) (61,12) (17,14) DV (37,02) (37,03) (04,01) DV (53,05) (53,10) (10,07) DV (18,09) (19,14) (17,07) DV (39,33) (39,28) (39,28) DV (35,15) (34,28) (28,14) DV (39,10) (39,09) (13,06) DV (83,12) (83,05) (13,04) DV (55,21) (55,13) (23,16) DV (20,07) (20,07) (09,06) DV (43,04) (43,03) (05,03) DV (46,33) (56,37) (55,32) DV (56,13) (56,24) (25,18) DV (47,16) (46,33) (34,24) DV (16,05) (16,11) (11,04) DV18 b (57,12) (57,29) (31,15) DV (29,05) (42,14) (37,05) DV (46,16) (50,19) (34,16) W (47,10) (47,12) (13,09) W (48,19) (48,18) (22,17) W (28,13) (27,17) (20,11) W (53,29) (53,36) (42,34) W (10,09) (32,09) (32,09) W (11,10) (30,08) (30,10) W (49,12) (49,16) (18,11) W (67,12) (67,13) (16,11) W (59,39) (59,35) (43,31) W (73,31) (73,61) (66,27) W (42,06) (42,10) (11,07) W (29,03) (29,04) (05,03) W (45,04) (45,32) (32,14) W (22,05) (22,07) (08,06) W (46,03) (46,07) (07,03) W (54,17) (54,17) (18,17) W (61,17) (61,32) (36,17) W (36,07) (36,12) (14,06) W (65,22) (65,25) (35,12) a Notes: r is the average density of the samples at the site in g cm 3. SD is the standard deviation of this mean. K is bulk volume susceptibility in SI units K 1, K 2, K 3 are axes of maximum, intermediate, and minimum susceptibility, respectively. D, I are declination and inclination, respectively, after tilt correction. Values in parentheses are the maximum and minimum semiaxes of the 95% confidence ovals. K 1 /K 2 and K 2 /K 3 are ratios of susceptibilities along the axes indicated. AMS means and ratios are calculated by the normalized tensor method. b 092, 17 used for the tilt correction at this site; conventions as in Table 2. K 2 K 3 hundred meters of quartz-latite welded tuff immediately overlie the andesite; those were not sampled. However, quartz-latite welded tuff occurs at several widely separated locations, including site 17 where it appears to occur stratigraphically higher than ignimbrite which appears to correlate with the highest part of the main ignimbrite section (e.g., site 14). [9] The ignimbrite is a varicolored crystal vitric pumice tuff with varying amounts of lithic fragments. Rock densities (Table 4) have a wide range principally reflecting varying degrees of devitrification and welding. Primary crystals are principally quartz, sanidine, plagioclase and sparse biotite. Electron microprobe analyses of feldspars in samples of the ignimbrite at sites 4 and 10 indicate very homogeneous feldspar compositions: sanidines in the range Or67-Or 69 and plagioclase in the range Ab74-72, Or Nanny Creek, Pequop Mountains [10] Outcrops of volcanic rocks in the northern Pequop Mountains are centrally located with respect to known localities of the NENVF (Figure 1) and these rocks in the vicinity of Nanny Creek (Figure 4) have been designated as the type area for the NENVF [Brooks et al., 1995a, 1995c]. In the Nanny Creek area, the lowest widespread unit is a crystal-rich tuff (T1) which is overlain by an ash flow tuff poorer in crystal content (T2). Both of these ignimbrite units are rhyolite in composition [Brooks et al., 1995c, Figure 5]. These units, in turn are overlain by a flow breccia of intermediate composition, followed by flows and a more localized ash flow tuff unit (T3). Unit T1 is richer in biotite and our sites W3 and W5 are in this basal unit whereas sites W1, W2, W4, W13, W14, and W17 are in unit T2. Sites W15 and W18 are in unit T3. Brooks et al. [1995c] estimate the total thickness of the volcanic sequence at about 1200 meters but more than half of this is contributed by the flows and flow breccias which were not sampled by us Eastern Windermere Hills [11] Sampling in the eastern Windermere Hills was guided by the maps of Mueller [1993] and Mueller et

6 EPM 4-6 PALMER AND MACDONALD: NORTHEAST NEVADA VOLCANIC FIELD Figure 2. Geological map of the Dolly Varden Mountains showing sampling sites. Age of Ma is based on 40 Ar/ 39 Ar ratios in biotite [Brooks et al., 1995a, 1995c], whereas the ages of 39.8 and 39.5 Ma are conventional K-Ar biotite ages (this study). WRF, BRF, and JF are the Window Ridge, Black Rock, and Juniper faults, respectively. BRC and DVC are Black Rock Canyon and Dolly Varden Canyon sections, respectively. Map simplified from Snow [1964]. al. [1999]. The Eocene calc-alkaline volcanic rocks in this area are dominantly rhyolitic ash flow tuff but a dacitic flow is also present [Mueller et al., 1999]. These rocks are exposed in a series of west tilted fault blocks that overlie a major shallow detachment fault. Easiest access is provided along east-west trending creeks along one of which Mueller [1993] was able to estimate a thickness of about 270 m for an incomplete section of the volcanic rocks.

7 PALMER AND MACDONALD: NORTHEAST NEVADA VOLCANIC FIELD EPM 4-7 Figure 3. Schematic section illustrating relative stratigraphic position of DV sites, lithologies, and magnetic polarity intervals west of the Window Ridge fault in the Dolly Varden Formation (Figure 2).

8 EPM 4-8 PALMER AND MACDONALD: NORTHEAST NEVADA VOLCANIC FIELD Figure 4. Geologic map (simplified from Brooks et al. [1995c]) showing W series sampling sites within the ash flow tuffs of the Nanny Creek (NC) area. [14] The samples were crushed and from the 40 to 80 mesh fraction, biotite and feldspar concentrates were obtained using magnetic and heavy liquid separation methods. Potassium contents were determined, all in duplicate, by flame photometry with a lithium internal standard. Argon analyses were performed by the isotope dilution technique. The yields of biotite were small and for the biotite concentrates only single determinations of argon and argon ratios could be made. [15] The biotite ages of DV11 and DV12 (Table 1 and Figure 2) are concordant within the analytical errors. They suggest little time difference between the andesite below the ignimbrite and the volcanic units above the ash flow tuff. However, the K content of the biotite in DV12 is very low. This suggests that the biotite concentrate in DV12 is contaminated by hornblende, the major mafic phenocryst in this rock. Nevertheless it is not significantly different from the 40 Ar/ 39 Ar age of Ma from the same map unit [Brooks et al.,1995a, 1995c]. The feldspar concentrates yield discordant and younger ages. This age pattern is common in conventional K-Ar dating and in 40 Ar/ 39 Ar dating [Haggart et al., 1993; McDougall and Harrison, 1999]. The biotite age from DV11 is slightly older than either of the ages from the basal andesite-dacite unit. As previously noted the biotite yield was too small for duplicate argon analysis. An average of the three biotite ages is Our sites in this area are W6-W10, and W19 and W20 (Figure 5) Southern Snake Mountains [12] Fieldwork in the southern Snake Mountains was guided by Thorman et al. [1990] where they map two rhyolitic welded ash flow tuffs separated by intermediate composition lava flows. We found the lower tuff to be too poorly exposed for sampling purposes and collected samples at one site only (W21) in the upper ash flow tuff in this region. 3. Geochronology of the Dolly Varden Formation [13] At the time of our fieldwork in the Dolly Varden formation there were no published ages for these rocks. Subsequently, Brooks et al. [1995a, 1995c] obtained a wellconstrained 40 Ar/ 39 Ar biotite plateau age of ± 0.11 Ma from an andesite-dacite near our site 2 [Zamudio, 1992, p.39]. To better define the age and age range in these rocks we collected biotite-bearing samples at sites 11 and 12. Site DV11 is conformably below the uppermost flow breccia unit whereas site DV12 is in the basal andesite. Unfortunately, the stratigraphic position of site DV11 relative to the main ignimbrite section is unclear. The vitric tuff of DV11 lies unconformably above pre-cenozoic rocks, and is separated from the main ignimbrite section by the Window Ridge thrust fault. Elsewhere in the Dolly Varden Mountains, vitric tuff occurs both at the base of, and above, the main section of ignimbrite. The ignimbrite itself is biotitepoor. These samples were analyzed by A. Hayatsu at the University of Western Ontario. Figure 5. Map of the eastern Windermere Hills area (WH) locating W series sampling sites. Map generalized after Mueller [1993].

9 PALMER AND MACDONALD: NORTHEAST NEVADA VOLCANIC FIELD EPM 4-9 Figure 6. (a) Poles to foliation at sampling sites for units above the basal andesite of the Dolly Varden Formation (DVF). (b) Poles to foliation in the basal andesite of the Dolly Varden Formation; data from the map of Snow [1964] with addition of DV sites 2 and 12. (c) Poles to bedding in Paleozoic rocks unconformably below DVF and located within half km of the contact with the basal andesite; data from map of Snow [1964]. (d) Poles to foliation in ignimbrites at Nanny Creek (triangles), Windermere Hills (circles), and site W21 in southern Snake Mountains (square) ± 0.4 Ma which is the best estimate for the Dolly Varden formation. 4. Structure [16] Field-measured attitudes were averaged by an eigenvector method to obtain site-mean structural attitudes. These were subsequently used for tilt-correcting paleomagnetic vectors and AMS axes. We first consider the Dolly Varden formation. [17] Foliation poles for units above the basal andesite are shown (Figure 6a). Foliation attitudes are given in Table 2 and in Table 4 for site 18. The main ash flow unit possesses a fairly uniform shallow easterly dip. The average structural attitude of all sites exclusive of 2 and 12 in the basal andesite is 333 /18 E. This compares well with a regional estimate, based on 38 observations along the east half of the range, of 342, 20 E which is used for constructing Figure 3. Site 14 is an outlier (Figure 6a); it is in a small fault block with northwesterly dip. [18] At site 2 in the basal andesite, foliation dips eastnortheast and thus appears conformable with the overlying pyroclastic units. At site 12, however, andesite foliation dips steeply southeast. Andesite paleomagnetic vectors, corrected for these tilts, degrade the formational statistics. To better understand the structure of the basal andesite, 10 additional attitudes from the andesite were extracted from the geological map of Snow [1964]. The resulting poles to foliation (Figure 6b) display a dispersed pattern possibly consistent with strong folding about a WNW axis. However, a tectonic interpretation for this distribution is unlikely. Neither the underlying nor overlying rocks exhibit a similar pattern (Figure 6c). Structural attitudes of the Paleozoic rocks exposed within 0.5 km of the Paleozoic-andesite contact, taken from the map of Snow [1964] show a more pronounced foliation girdle, consistent with gentle folding about northwest trending axis. The foliation pole plots also suggest that the asymmetry in limb dip directions is a consequence of postignimbrite deformation (compare Figures 6a and 6c). The variability in andesite foliations (Figure 6b) is therefore attributed to variable initial dip. Snow [1964] indicates that the andesite unit was extruded on an irregular terrain with local relief up to several hundred feet. Although imperfect, the regional attitude (strike 333, dip 18 east), derived from the overlying units, is used to correct the paleomagnetic unit vectors at sites 2 and 12 in the basal andesite (Table 2). [19] Local structure in the three other areas (Figure 6d) is also simple with homoclinal dips dominating. All sampling sites at Nanny Creek are characterized by steep (60 ) easterly dipping foliations. In the Windermere Hills, dip values are all less than 42, but all have a westerly direction in contrast to the ignimbrites at Dolly Varden and Nanny Creek. Field measured foliations at our sites in the Windermere Hills have much more variation in strike direction than either Dolly Varden ash flow tuff sites (Figure 6a) or those at Nanny Creek. Given that the outcrop pattern of the Windermere tuffs (Figure 5) shows relatively little deviation from north-south strike, some of the variation in fieldmeasured strike (Figure 6d) may include a component of initial dip. The uppermost rhyolite unit in the southern Snake Mountains is characterized by southeasterly dip (Figure 6d). 5. Magnetic Methods [20] One or more standard paleomagnetic specimens were prepared from each oriented core sample. Measured volumes and masses for ideally shaped cylindrical specimens were used to calculate dry weight densities, the site means of which are recorded in Table 4. Subsequent to measurements of initial natural remanent magnetization (NRM) but before any demagnetization experiments, the anisotropy of low-field magnetic susceptibility (AMS) was measured. For the DV series of specimens a Sapphire Instruments SI-2 was used whereas for the W series specimens a Sapphire Instruments SI-2B instrument was used. A four-repeat measurement of each of six orientations was used on each of 338 specimens that were measured. An additional 47 specimens of the DV series with low bulk susceptibilities from sites 1,3,11,14,19, and 20 were measured employing a KLY-Kappabridge. The site-mean AMS axes were computed by the Hext/Jelinek method as implemented by Lienert [1991].

10 EPM 4-10 PALMER AND MACDONALD: NORTHEAST NEVADA VOLCANIC FIELD [Potter and Stephenson, 1988]. To estimate the effective magnetic grain size a small subset of specimens was subjected to increasing direct fields in twelve to seventeen steps to a maximum of 2 T producing SIRM buildup curves. Prior to the IRM acquisition all specimens were AF demagnetized in a 100 mt field to reduce the NRM to a minimum. The resulting SIRMs were subjected to steps of alternating field (AF) demagnetization (seven to ten steps up to a maximum available field of 170 mt) with intervening measurements of remanence. The resulting plots were then compared to the type curves of Symons and Cioppa [2000]. Figure 7. Orthogonal vector diagrams of in situ vectors for pairs of samples from sites (a and b) DV4 and (c and d) DV10. Figures 7a and 7b illustrate a site in which the initial NRM direction is little different from the final characteristic direction, whereas Figures 7c and 7d illustrate a site with low coercivity and low unblocking components that are quite different from the final higher coercivity, higher unblocking characteristic (ChRM) component. [21] Progressive alternating field demagnetization (AF) was performed on 3 or more specimens from each of the 39 sites, up to a maximum field of 100 mt, using a Schonstedt GSD-5 demagnetizer and a Schonstedt DSM-1 magnetometer. One to five (Table 2) specimens from each site (except site DV11, in plastic cases) were subjected to progressive thermal demagnetization. Most of the thermal demagnetization was done using a large volume furnace (1400 cm 3 sample space) enclosed in a 1.5 m square threeaxis field cancelling array and with magnetizations measured using a Molspin magnetometer. Characteristic remanence directions (ChRM) were obtained using orthogonal vector diagrams [Zijderveld, 1967] (Figure 7) and least squares line regression [Kirschvink, 1980]. Site-mean remanence directions (Table 2) were obtained by Fisher [1953] statistics. [22] The interpretation of anisotropy of magnetic susceptibility data is complicated by the effects of domain state 6. Paleomagnetic Results 6.1. Dolly Varden Formation [23] Rocks from sites DV2 and DV12 in the basal andesite have high susceptibilities (Table 4) consistent with their more mafic character. Sites in the ignimbrite with indications of hydrothermal alteration have very low susceptibilities and/or low remanent intensities. These include sites DV1, DV3, DV6, and DV14 in which fiamme and feldspars are replaced by clay minerals producing a chalky appearance. Site DV19 in white tuffaceous sediment also has low susceptibility and NRM intensity. [24] Some samples have initial NRM directions which are little different from the characteristic directions (ChRM) achieved after AF or thermal demagnetization (Figures 7a and 7b). Others reveal a characteristic remanence after low coercivity-low unblocking temperature components are removed (Figures 7c and 7d). [25] In situ site-mean characteristic directions are plotted in Figure 8a. The stratigraphically lowest sites, DV2 and DV12 in the basal andesite and DV16 and DV20 in the basal flow breccias are the only sites of normal polarity. The usual site-by-site tilt correction increases dispersion of the normal group, probably because of large initial dip components in the andesite and breccia. Therefore a regional attitude (333 /18 east, Table 2) derived from the overlying units was used to tilt-correct the andesite sites (Figure 8b). This regional attitude improves the formation-mean directional statistics. [26] All sites above the stratigraphic level of the andesite are of reversed polarity (Figures 3 and 8a). Site DV11 is a modest outlier and DV15B is a distinct outlier and both remain as outliers with structural tilt (Figure 8b). No internally consistent foliation could be found at site DV11, which shows abundant evidence of soft-sediment deformation in poorly consolidated volcaniclastic sediments; the foliation of the overlying andesite-dacite flow was used for structural correction and it probably provides an imperfect estimate for paleohorizontal. Alternatively, site DV11 may have been reoriented by movement on the Window Ridge fault. At site DV15, six core samples were taken in two groups of three with each group separated by 5 m of section. Each of the two groups of three samples have high internal consistency of characteristic directions (k = 254 and k = 372; Table 2) but differ significantly from one another. The site has been divided into two, one of which (DV15B) remains as a distinct outlier with structural correction (Figure 8b). It is possible that the blocks constituting this flow breccia continued to rotate at temperatures below the magnetic blocking temperatures. Anomalously high

11 PALMER AND MACDONALD: NORTHEAST NEVADA VOLCANIC FIELD EPM 4-11 Figure 8. Equal-area projections showing characteristic site-mean remanence directions from all sites. Solid (open) dots are projections on the lower (upper) hemisphere. (a) In situ site-mean paleomagnetic directions from the Dolly Varden Formation. (b) Tilt-corrected paleomagnetic directions from the Dolly Varden formation. (c) In situ directions from the Nanny Creek area are represented by triangles, Windermere Hills by open circles and the single site from the Snake Mountains (W21) by a square. (d) The corresponding tilt-corrected directions. Plus indicates the mean direction for each polarity group excluding sites DV11 and DV15B. Solid square is the 42 Ma expected field direction calculated from the 42 Ma pole of Besse and Courtillot [1991]. initial intensities of NRM and high bulk susceptibility were found for site DV18 (Table 4). Neither AF nor thermal demagnetization was successful in isolating a coherent within-site magnetization in this stratigraphically highest site; a lightning strike at this site is likely. [27] Excluding sites DV11, DV15B, and DV18, the reversed directions form a moderately coherent cluster (k = 40) which increases in precision with structural tilt correction (k = 52, Table 3). Although improvement of precision is not statistically significant [McFadden, 1990], it is as good as might be expected from a sequence of rocks with low variability in foliation attitudes (Figure 6a). The asymmetry of the normal and reversed mean directions (Figure 8b and Table 3) is mainly in declination. If caused by a secondary overprint it would require a subhorizontal field with ENE declination. No Tertiary field with this orientation is known for North America [Van der Voo, 1993]. We attribute the nonantipodal mean directions to the imperfect structural corrections for the normal polarity sites. A contributing factor might be incomplete averaging of secular variation of one or both polarities. The angle between the normal and the antipode of the reverse means is Because of the small number of directions in the normal group and their high dispersion, the reversal test of

12 EPM 4-12 PALMER AND MACDONALD: NORTHEAST NEVADA VOLCANIC FIELD Figure 9. Equal-area projection for Nanny Creek site W13 showing three stable endpoint directions (large circles). Specimens 5-1 and 3-1 fail to yield characteristic directions under AF demagnetization but converge along great circles, marked by small circles, toward the characteristic direction for the site which is shown by the large square. The three stable endpoint and two remagnetization circle data were combined by the method of Bailey and Halls [1984]. McFadden and McElhinny [1990] is indeterminate. The mean tilt-corrected direction for 18 sites (D = 341.5, I = 53.3, a 95 =7.1 ; Table 3) is not very different from the expected field (D = 349.5, I = 59.2 ) computed from the 42 Ma North American reference pole [Besse and Courtillot, 1991]. We assess the possibility of vertical axis rotations in our sampling areas later in this section Nanny Creek, Windermere Hills, and Southern Snake Mountains [28] The majority of samples examined by AF and thermal demagnetization from Nanny Creek and the eastern Windermere Hills behaved similarly to those of the Dolly Varden formation and characteristic components could be selected with ease. However, five sites (W2, W6, W7, W15, and W18) have poor within-site consistency of ChRMs manifested by low within-site precision parameters (Table 2). Samples at four of these sites appear to have suffered hydrothermal alteration as manifested by either physical appearance and/or very low bulk susceptibilities (Table 4). They are not considered further in the analysis. Judging from anomalously high remanent intensities, samples at site W15 have been affected by lightning and the data from this site are also rejected. Some samples at site W13 appear to have been partially affected by lightning but acceptable data from this site could be obtained by combining stable end point and remagnetization circle data (Figure 9). Samples at site W5 exhibit reasonably good internal directional consistency (Table 2), do not have anomalous magnetic properties nor physical appearance, but yield a site-mean direction very different from all other Nanny Creek and Windermere Hills sites (Figures 8c and 8d). The large difference (135 ) between this direction and all other sites cannot reasonably be attributed to initial dip. The direction recorded at site W5 implies an equatorial paleolatitude which it clearly is not. Speculatively, it may have recorded a brief transitional field. W5 data are excluded from formational mean calculations. [29] Nanny Creek in situ site-mean directions (Figure 8c) are far removed from the expected Eocene field direction at this locality. With tilt correction they move toward the expected Eocene field direction shown as the square in Figure 8d. Within-unit precision does not change significantly (Table 3) which is to be expected given the coherence of structural attitudes (Figure 6d). Windermere Hills in situ site-mean directions (Figure 8c) appear to be Fisherian whereas the tilt-corrected equivalents form a partial great circle girdle (Figure 8d). This pattern may indicate variable amounts of unremoved secondary components. Alternatively, initial dip may be contributing to this curious result. Viewing the Dolly Varden, Nanny Creek, and the Windermere Hills remanence data collectively there is a small degree of crossover geometry [Hudson et al., 1989] when the tilt-corrected site means are compared to the in situ sitemean directions (Figures 8b, 8c, and 8d). This pattern may be a reflection of synfolding magnetization, unresolved prefolding and postfolding magnetization components or be a consequence of penetrative strain [Hudson et al., 1989]. In the case of the Nanny Creek-Windermere Hills data, the crossover geometry would disappear with about a 90% untilting correction as opposed to the 100% tilt correction that was applied. We suppose that the small degree of crossover geometry is the result of the fieldmeasured foliations being imperfect estimates of the actual paleohorizontal. Unresolved pretilting and posttilting components are not very likely given that AF and thermal demagnetization (e.g., Figure 7) yield the same characteristic magnetizations. The low degree of magnetic anisotropy (Table 4) coupled with the lack of a visible cleavage in these rocks argues against internal strain as being the cause of the crossover geometry. [30] The magnitude of possible vertical axis rotations of the ranges of this study is important for the interpretation of flow patterns as deduced from anisotropy of magnetic susceptibility axes. We use the poles DV, NC, and WH (Table 3) to calculate the rotation angles necessary to bring them into coincidence with the 42 Ma North American reference pole of 82 N, 157 E [Besse and Courtillot, 1991]. Clockwise rotations are taken as positive and the confidence limits are as given by Cox and Hart [1986] and Butler [1992] incorporating the corrections suggested by Demarest [1983]. They are as follows: Dolly Varden 8.0 ± 6.1, Windermere Hills 2.2 ± 15.7, and Nanny Creek (northern Pequop Mountains) 19.9 ± These results imply that the Nanny Creek area has undergone a 9 clockwise rotation whereas the Dolly Varden Mountains has undergone a 2 counter clockwise rotation relative to the craton. However, we are hesitant to accept these results literally because of the uncertainties noted above concerning unremoved secondary components, imperfect estimates of paleohorizontal, and imperfect averaging of secular variation.

13 PALMER AND MACDONALD: NORTHEAST NEVADA VOLCANIC FIELD EPM 4-13 [31] Considering the 27 site means from all ignimbrite sites from the Dolly Varden Mountains, Nanny Creek, the Windermere Hills, and the southern Snake Mountains (Table 3) the in situ precision parameter k increases from 11 to 22 with tilt correction which can be considered statistically significant at the 99% confidence level under the conservative test of McElhinny [1964]. The mean direction for these 27 reverse polarity ignimbrite sites (D = 169.3, I = 53.2, a 95 = 6.1 ) is not significantly different from the expected field (D = 169.4, I = 59.4 ) computed from the 42 Ma North American reference pole [Besse and Courtillot, 1991]. 7. AMS Results [32] These rocks are typical of volcanic rocks in terms of the degree of magnetic anisotropy which ranges from 0.4% to 5.0% (Table 4). Most sites have oblate fabrics except sites DV15, a flow breccia, and W5, a tuff in the Nanny Creek area both of which are weakly prolate. Agreement between poles to field-measured foliation and K min axes is good at most sites (Figure 10). Principal anisotropy axes were restored to estimated paleohorizontal by using site-mean foliations (given in Table 2) as estimates of tectonic tilt. A single example is given (Figures 11a and 11b) and tectonically restored axes for all sites are given in Table 4. [33] A feature of the AMS data in these rocks and in ignimbrites in general [e.g., Palmer and MacDonald, 1999, Figure 2] is that K min axes are better grouped than either the intermediate and maximum susceptibility axes. This reflects the fact that the foliation components of ash flow tuff fabrics are better developed than the lineation components. The feature may also have a geological explanation in that at some levels during ash flow emplacement more grains are aligned by rolling about axes perpendicular to flow than are aligned by shear gradients parallel to flow. A third explanation, also of a geological nature, may involve fluctuations in flow azimuth during emplacement possibly as a consequence of short-order turbulent fluctuations. A fourth reason may be due to compaction accentuating foliation fabrics but compaction is unlikely to affect lineation fabrics. A fifth reason for the relatively large scatter in K max axes is owing to variations in the proportion of single domain to multidomain ferromagnetic grains; this can result in the interchange of K max and K int axes as explained next. [34] A rock containing exclusively single domain magnetites with a preferred shape orientation would have the minimum susceptibility axis aligned with the long axis [Potter and Stephenson, 1988]. This arises because nonequant single domain grains are theoretically saturated in the direction of the long dimension and no additional magnetization can be induced along that dimension. To explore the possibility that the single domain effect perturbs some or all of our distributions several specimens were chosen from sites where the majority of K max and K int axes were well grouped but where one of two of the specimens in the site had K int axes lying within the K max group (Figures 11a and 11c). In these specimens (W14-3-2, and W20-1-1) we performed anisotropy of isothermal remanence (AIRM) experiments. Isothermal remanent magnetization (IRM) was imposed in nine directions following Girdler [1961] using a 15 cm diameter coil in a pulse Figure 10. AMS axes in in situ coordinates for andesite site DV12 and ash flow tuff site DV17 from the Dolly Varden formation. Field measured foliation (F) is in good agreement with K min axes (solid circles). The fieldmeasured foliation in the andesite is thought to be a poor proxy for paleohorizontal (see Figure 6b). magnetizer at 21.6 mt. After an initial alternating field demagnetization at 50 mt, subsequent 40 mt demagnetization remanences were vector-subtracted from remanences acquired in the 21.6 mt pulse field. The nine pulsed remanences (resultants) were used to calculate the anisotropy tensor. The AIRM eigenvectors are compared with the AMS axes in Figure 11. [35] In site W14, specimen 3-2 is an AMS outlier in that the intermediate and maximum axes are interchanged with respect to the overall site groupings. The AIRM axes lie

14 EPM 4-14 PALMER AND MACDONALD: NORTHEAST NEVADA VOLCANIC FIELD Figure 12. Normalized IRM acquisition curves, together with the corresponding AF demagnetization curves for five NENVF specimens. The Dolly Varden and Nanny Creek specimens possess acquisition and demagnetization curves that intersect in the mt range indicating that the majority of the magnetic carriers are in the multidomain to pseudo-single domain range of effective magnetic grain size for magnetite [Symons and Cioppa, 2000]. Specimen W20-1-1, in contrast, has characteristics of hematite or goethite. close to the AMS and therefore the single domain effect is not an explanation in this case. Specimen W20-1-1, however, does exhibit IRM maximum and intermediate axes near their respective AMS clusters but far removed from the position of the corresponding AMS axes for this specimen. W has much higher coercivity of NRM than does W (mdf of 50 mt versus 7mT) and therefore could have a significant proportion of single domain magnetite. However, the results of the SIRM experiments described next suggest that the anomalous AMS pattern and higher remanence coercivity in W is due to the large proportion of hematite in this specimen. [36] Additional evidence that the bulk magnetic characteristics of the majority of tuff samples are those of multidomain aggregates is derived from the saturation isothermal remanent magnetization (SIRM) experiments and the progressive AF demagnetization of these SIRMs (Figure 12). Figure 11. (opposite) (a and c) In situ AMS K max, K int, and K min axes are shown as squares, triangles, and circles, respectively, for Nanny Creek site W14 and for site W20 from the Windermere Hills. The K min axes cluster about the pole to field-measured foliation (star). The K min AMS axes for specimens 3-2 and 1-1 are in good agreement with the field measured foliation pole as are the AIRM minimum axes. The K max and K int axes for these specimens are interchanged with the site-mean groupings. In both parts of the figure the AIRM axes are plotted as larger symbols where it can be seen that specimen W s AIRM axes are in concordance with the main AMS groupings. The higher median destructive field of NRM in W is consistent with a single domain effect on this AMS pattern. In contrast, W has both its AMS and AIRM maximum and intermediate axes interchanged with the main AMS groups. (b) The K min and K max axes for site W14 are shown after tilt correction.

15 PALMER AND MACDONALD: NORTHEAST NEVADA VOLCANIC FIELD EPM 4-15 Figure 13. The sterogram shows structurally tilted K min and K max axes for Nanny Creek site W14 and the associated ovals of 95% confidence about the mean directions. From the azimuths of the limits of the confidence oval (25 80, in this example) about the K max mean, a map representation is made in the form of a bidirectional double fan or a unidirectional fan. Where the bow tie representation is used, the more heavily patterned arc indicates the downplunge sense of K max. The center of the bow tie or apex of the fan is located at the sampling site in subsequent diagrams. nearly to the mean uncertainty in the K max azimuth. In young ash flow deposits the K max axes are typically inclined slightly toward the source direction [MacDonald and Palmer, 1990; Palmer and MacDonald, 1999] presumably due to clast imbrication. However, in the ignimbrites of the Dolly Varden Formation we do not observe a significant plunge in the K max axes. This might possibly be a characteristic of the distal ash flow environment. Because the sampling region is near the south limit of this Eocene volcanic field, a northern source area is more probable (Figure 1). [38] The 11 ignimbrite sites in the Nanny Creek area yield tilt-corrected K max axes defined with somewhat better precision than those in the Dolly Varden formation (Figure 15). A Bingham mean K max axis has a trend of 237 and a plunge of 09 with a 95% azimuthal error of 18. Nanny Creek strata have the steepest dips of all units studied (Figure 6). If these are overestimates of paleohorizontal due to the presence of initial dip, as is perhaps suggested by the paleomagnetic data (Figures 8c and 8d), the downplunge sense of the mean K max might be NE instead of SW. [39] Although the within-site error ovals about K max axes in the ash flow tuffs of the Windermere Hills at most sites are large (Figure 16), the between-site trends of K max axes exhibit high formational directional consistency. Excluding site W10, which has low bulk susceptibility and which is magnetically isotropic (Table 4), the six site-mean K max axes define a Bingham mean with a trend of 122 and a plunge of 18 with an azimuthal uncertainty of 38. Five of Dolly Varden ignimbrite specimens and W have acquisition curves that intersect the demagnetization curves in the mt range corresponding to rocks dominated by multidomain to pseudosingle domain magnetite [Symons and Cioppa, 2000]. W20-1-1, in contrast, did not reach saturation in the highest field available (2 T). Comparison with the results of Symons and Cioppa [2000] indicates a dominance of hematite or goethite in this specimen. The effect of the above perturbations is to degrade the precision with which direction of flow is estimated from the direction of K max axes. 8. Regional Patterns of Flow as Deduced From AMS K max Axes [37] To assess the regional pattern of K max axes we have used a modification of the double fan representation used by Hillhouse and Wells [1991]. The principle of the representation is given in Figure 13. Although we measured AMS in all sites in the Dolly Varden formation, only the sites in the ignimbrites are likely to have regional significance in terms of inferred flow patterns. Bingham statistics on the sitemean K max axes give K max means with shallow inclinations and N-S trends. The K max mean (for the 12 ash flow tuff sites only, Figure 14) has I = 06.1, D = for in situ axes and I = 03.1, D = for the tilt-corrected axes. The tilt-corrected directions have slightly less dispersion than the in situ directions. Because the Bingham mean of the K max axes is nearly horizontal (I =3 ) and because the long axis of its 95% error oval is nearly horizontal, then the maximum radius of that error oval (28.6 ) corresponds very Figure 14. On the simplified map of the Dolly Varden Mountains, the downplunge trend of structurally corrected K max from ignimbrite sites is plotted at the center of an arc the apex of which is at the collecting site. The implied flow during emplacement of the Dolly Varden ignimbrites is north-south but with large directional uncertainty.

16 EPM 4-16 PALMER AND MACDONALD: NORTHEAST NEVADA VOLCANIC FIELD Figure 15. Geological map of the Nanny Creek area, showing bidirectional fans. The implied flow axis, derived as the mean of the K max axes, is The downplunge sense of the K max axis has the darker pattern. the six K max axial means (Figure 16) have easterly downplunge sense. [40] Our single site in the southern Snake Mountains (W21) occurs in the same outcrop area as does the chemically analyzed sample and dated sample (88T 36) of Brooks et al., [1995c]. The chemical analysis indicates substantial hydrothermal alteration in the form of silicification and K metasomatism which seemingly has not adversely affected the remanence at our site W21 (Figure 8), but the site does have the lowest bulk susceptibility of the entire collection and the anisotropy of magnetic susceptibility is not well defined (Table 4). [41] Projections of formation mean K max trends converge in the vicinity of the Toano Range. Assuming a single source area, the geometrically most probable source would be close to the cluster of intersections of the mean K max axes, indicated by the heavily shaded pattern in Figure 17. We return to an assessment of possible source vent-source caldera locations in section Discussion [42] One of the principal results of this investigation is a contribution toward the understanding of the source areas of the NENVF. Basically, the inferred flow directions from four widely separated areas support a source area east of the type area for the NENVF. That type area is near the Nanny Creek region of the Pequop Mountains [Brooks et al., 1995a, 1995c]. Brooks and colleagues noted that the NENVF coincides in time and space with the Tuscarora magmatic belt of Christiansen and Yeats [1992], and suggested that perhaps some of the NENVF rhyolite ash flows might have a source in the Eocene caldera complex near Tuscarora, approximately 110 km west of the Pequop Mountains. However, a closer source, in the opposite direction from the type area, is clearly implied by the somewhat surprising results presented here. We should note here that we also sampled numerous sites around the Tuscarora caldera complex, with unsatisfactory results. In that area, pervasive hydrothermal alteration has caused widespread variable remagnetization. Furthermore, leaching of iron minerals has resulted in anomalously low magnetic susceptibilities. Because of these complications, we abandoned our study of the magnetic properties of the tuffs in the Tuscarora region. However, before discussing details of the source region implications of the AMS results, we discuss the remanence results. [43] The time interval represented in the deposition of the Dolly Varden formation is believed to have been sufficient to average secular variation adequately for paleofield purposes, for several reasons. Erosional surfaces are present at several stratigraphic levels [Snow, 1964]. Unconformities separate the ignimbrite sequence from the underlying andesite and from the overlying units. Waterlaid stratified tuffs occur at numerous levels throughout the ignimbrite section [Snow, 1964]. The abrupt changes in facies and thickness are thought to be owing not only to structural movements accompanying volcanism, but also to erosion. [44] In contrast, the sections that we have sampled in the Nanny Creek area and in the Windermere Hills are exclusively ignimbrite and at neither of these areas are there interflow sedimentary rocks. The stratigraphically sequential T1, T2, and T3 ignimbrite units in the type area of the NENVF span 1.5 Ma [Brooks et al., 1995a] (Figure 1) and appear to represent three discrete pulses of ash flow activity. Figure 16. Simplified map of the eastern Windermere Hills showing bidirectional fans in six sites in the ash flow tuffs. The downplunge sense of the K max axis has the darker pattern. This plunge implies a flow from 122. The double fans are slightly displaced from actual site locations for visual clarity.