Dissected caldera structures expose thick intracaldera tuff and, uncommonly, cogenetic shallow plutons, while remnants of correlative outflow tuffs deposited on the pre-eruption ground surface record elements of ancient landscapes. The Middle Fork caldera encompasses a 10 km × 20 km area of rhyolite welded tuff and granite porphyry in east-central Alaska, ∼100 km west of the Yukon border. Intracaldera tuff is at least 850 m thick. The K-feldspar megacrystic granite porphyry is exposed over much of a 7 km × 12 km area having 650 m of relief within the western part of the caldera fill. Sensitive high-resolution ion microprobe with reverse geometry (SHRIMP-RG) analyses of zircon from intracaldera tuff, granite porphyry, and outflow tuff yield U-Pb ages of 70.0 ± 1.2, 69.7 ± 1.2, and 71.1 ± 0.5 Ma (95% confidence), respectively. An aeromagnetic survey indicates that the tuff is reversely magnetized, and, therefore, that the caldera-forming eruption occurred in the C31r geomagnetic polarity chron. The tuff and porphyry have arc geochemical signatures and a limited range in SiO2 of 69 to 72 wt%. Although their phenocrysts differ in size and abundance, similar quartz + K-feldspar + plagioclase + biotite mineralogy, whole-rock geochemistry, and analytically indistinguishable ages indicate that the tuff and porphyry were comagmatic. Resorption of phenocrysts in tuff and porphyry suggests that these magmas formed by thermal rejuvenation of near-solidus or solidified crystal mush. A rare magmatic enclave (54% SiO2, arc geochemical signature) in the porphyry may be similar to parental magma and provides evidence of mafic magma and thermal input.
The Middle Fork is a relatively well preserved caldera within a broad region of Paleozoic metamorphic rocks and Mesozoic plutons bounded by northeast-trending faults. In the relatively downdropped and less deeply exhumed crustal blocks, Cretaceous–Early Tertiary silicic volcanic rocks attest to long-term stability of the landscape. Within the Middle Fork caldera, the granite porphyry is interpreted to have been exposed by erosion of thick intracaldera tuff from an asymmetric resurgent dome. The Middle Fork of the North Fork of the Fortymile River incised an arcuate valley into and around the caldera fill on the west and north and may have cut down from within an original caldera moat. The 70 Ma land surface is preserved beneath proximal outflow tuff at the west margin of the caldera structure and beneath welded outflow tuff 16–23 km east-southeast of the caldera in a paleovalley. Within ∼50 km of the Middle Fork caldera are 14 examples of Late Cretaceous (?)–Tertiary felsic volcanic and hypabyssal intrusive rocks that range in area from <1 km2 to ∼100 km2. Rhyolite dome clusters north and northwest of the caldera occupy tectonic basins associated with northeast-trending faults and are relatively little eroded. Lava of a latite complex, 12–19 km northeast of the caldera, apparently flowed into the paleovalley of the Middle Fork of the North Fork of the Fortymile River. To the northwest of the Middle Fork caldera, in the Mount Harper crustal block, mid-Cretaceous plutonic rocks are widely exposed, indicating greater total exhumation. To the southeast of the Middle Fork block, the Mount Veta block has been uplifted sufficiently to expose a ca. 68–66 Ma equigranular granitic pluton. Farther to the southeast, in the Kechumstuk block, the flat-lying outflow tuff remnant in Gold Creek and a regionally extensive high terrace indicate that the landscape there has been little modified since 70 Ma other than entrenchment of tributaries in response to post–2.7 Ma lowering of base level of the Yukon River associated with advance of the Cordilleran ice sheet.
Large calderas are the sources of voluminous ignimbrites and contain vast thicknesses of intracaldera tuff. Silicic magma of such ignimbrites is widely considered to be related to more voluminous crystal-rich magma or mush that eventually solidifies as a pluton (Smith, 1979; Bachmann et al., 2007; de Silva and Gregg, 2014). Yet directly relating an ignimbrite to a specific pluton is complicated by failure of erosion to both preserve volcanic rocks and expose subjacent plutons and by processes that affect plutons between the time of an eruption and when they eventually solidify. Deep erosion of caldera structures may expose shallow plutons, but commonly those plutons are significantly younger, and thus they are difficult to relate directly to ignimbrites (e.g., Questa, Zimmerer and McIntosh, 2012). However, resurgence of unerupted magma commonly produces structural doming of caldera floors soon after voluminous eruption and caldera collapse (Smith and Bailey, 1968). A few caldera structures are conveniently eroded to expose resurgent intrusions but not to have removed intracaldera tuff (e.g., Grizzly Peak, Fridrich et al., 1991). The Middle Fork caldera (Bacon and Lanphere, 1996), named for the Middle Fork of the North Fork of the Fortymile River, is such a resurgent caldera structure that encompasses a 10 km × 20 km area of rhyolite welded tuff and granite porphyry ∼100 km west of the Yukon border (Figs. 1 and 2). The Middle Fork caldera is situated within a broad region of Alaska and adjacent Yukon within the Yukon–Tanana Upland that contains Late Triassic, Early Jurassic, and Cretaceous plutons and, in the less deeply exhumed blocks, silicic volcanic rocks. Among the identified caldera structures, the Middle Fork is the only one known to be of latest Cretaceous age, the others that have been dated being mid-Cretaceous (Bacon et al., 1990; Mortensen, 2008). Preservation of thick caldera fill and, locally, of outflow tuff indicates that the Middle Fork caldera has not been subjected to major uplift and deep erosion since the caldera-forming eruption ca. 70 Ma.
The Fortymile mining district, which includes the Middle Fork caldera and much of the Eagle and Tanacross 1° × 3° quadrangles, has long been known for occurrences of placer gold and base- and precious-metal sulfides (Yeend, 1996; Werdon et al., 2004, and references therein). The region south of the caldera to the Mosquito Fork of the Fortymile River is one of active exploration for Zn-Pb-Ag-Cu-Au skarn and carbonate-replacement sulfide deposits (Dusel-Bacon et al., 2009). Many of the identified occurrences of sulfide mineralization are associated with igneous rocks similar in age to those of the Middle Fork caldera (Day et al., 2014). The intracaldera tuff and the granite porphyry illustrate what may have existed, prior to exhumation, above granitic plutons exposed nearby and throughout the district.
Volcanic rocks of the Middle Fork caldera and vicinity were mapped in reconnaissance by Foster (1976). On the basis of this geologic map, and on thin sections and field notes of Foster and coworkers, Bacon et al. (1990) identified the caldera structure. Bacon and Lanphere (1996) presented an overview of the geology of the caldera and reported a 40Ar/39Ar biotite age of 69.10 ± 0.19 (1σ) Ma for basal outflow tuff. In 2010, we revisited the Middle Fork caldera to check key relationships and sample for geochemistry and additional geochronology. Although exposures are more extensive than in lower-elevation portions of the Fortymile district, solifluction debris, colluvium, and vegetation obscure much of the fill and the wall rocks of the caldera. Traverses of ridge tops reveal mainly areas of rubble interspersed between tundra and low vegetation. True outcrops are common locally at higher elevations and, rarely, along margins of the valley of the Middle Fork of the North Fork of the Fortymile River.
Weber (1986) recognized deposits of six glacial advances in the Yukon–Tanana Upland, the oldest and most extensive of which she interpreted to be early Pleistocene in age and termed the Charley River advance. According to Weber and Wilson (2012), the peaks within the intracaldera tuff near its southern limit, as well as the Mount Veta area to the southeast, were sources of small alpine glaciers of the early (?) Wisconsin Eagle advance, and all of the major stream valleys contain glacial deposits of the Charley River advance. However, glacial erosion was insufficient to produce cirques or extensive cliff exposures in the vicinity of the Middle Fork caldera.
This paper presents new data that we use to reconstruct the Middle Fork caldera, including the extent of granite porphyry within the caldera structure, major- and trace-element geochemistry of the igneous rocks, and U-Pb zircon geochronology. We also summarize what is known about mapped Cretaceous (?)–Early Tertiary felsic volcanic rocks within 50 km of the caldera. We refine initial interpretations of the caldera geology, igneous geochemistry, and geomorphic significance in the context of the geologic history of the Fortymile region and the long-lived, post–Late Cretaceous land surface in this part of east-central Alaska.
The Middle Fork caldera lies within a region of Paleozoic metamorphic rocks and Mesozoic plutons cut by northeast-trending, high-angle faults (Dusel-Bacon et al., 2006, 2009) (Fig. 3). These regional faults are considered to have left-lateral strike-slip displacement, antithetic to the throughgoing Denali and Tintina right-lateral, strike-slip faults (Fig. 1; Page et al., 1995; O’Neill et al., 2010). Many of the northeast-trending faults also have a component of vertical offset (Dusel-Bacon and Murphy, 2001; Siron et al., 2010). The Middle Fork caldera (Figs. 3 and 4) occurs in the Eagle 1° × 3° quadrangle in a downdropped block, here termed the Middle Fork block, between northeast-trending faults in which the exposed crustal level is dominated by Paleozoic greenschist- and amphibolite-facies metamorphic rocks intruded by Early Jurassic and Cretaceous granitoids (Dusel-Bacon et al., 2013). To the northwest of the caldera, across the Mount Harper fault zone in the Mount Harper block, mid-Cretaceous granitic rocks are widely exposed, consistent with greater uplift and exhumation. Southeast of the Middle Fork block, the upthrown Mount Veta block exposes the same Paleozoic metamorphic units cut by Early Jurassic and Late Cretaceous granitoids. Southeast of the Kechumstuk fault zone, the geology of the Kechumstuk block is dominated by Early Jurassic and Late Triassic plutons and Paleozoic amphibolite-facies metamorphic rocks. In the Tanacross 1° × 3° quadrangle, ∼60–100 km south of the area of Figure 3, a larger area of felsic volcanic and Paleozoic metamorphic rocks (Foster, 1970) is bounded by northeast-trending faults. This crustal block contains three mid-Cretaceous calderas identified by Bacon et al. (1990), the ages of which were suggested by Mortensen (2008) to be ca. 108 Ma, similar in age to plutonic rocks to the southeast in Alaska and Yukon.
Geologic Units of the Middle Fork Caldera
The dimensions of the caldera structure are defined by the presence of intracaldera welded tuff (unit Kr, Fig. 4) because erosion has proceeded to a depth below the original topographic caldera basin. The densely welded intracaldera tuff has ≤4 mm quartz and feldspar phenocrysts, cm-sized fíamme visible in thin section and on some weathered joint surfaces, and common lithic fragments that typically are ≤1–2 cm. Maximum exposed thickness of intracaldera tuff, measured from the deepest valley to the highest peak, is 850 m (Fig. 5A). At a minimum, the extent of intracaldera tuff indicates the area of the structurally subsided caldera block. The original topographic caldera would have been larger, to the extent that it may not have been filled by ponded intracaldera tuff, owing to inward landsliding of the unstable caldera walls (Lipman, 1997). Contacts of intracaldera tuff with wall rocks are well defined only locally. The contact location is particularly uncertain in the low-elevation vegetated country on the north and east. There, and west of the high peaks on the south, it is unclear whether mapped metamorphic rocks are in fact outside of the caldera or are instead wall-rock blocks within megabreccia in which the tuff matrix between blocks is not exposed (compare Lipman, 1976a). One large marble block >100 m across (Fig. 5B), shown as unit Kb on the geologic map, swims in welded tuff near the southeast caldera boundary. Other possible areas of megabreccia are conservatively indicated by Kb? on the map. Because of poor exposure, other large areas of megabreccia could be present, such as that presently shown as unit Pzm between the northern mapped extent of intracaldera tuff and the Middle Fork of the North Fork of the Fortymile River (Fig. 4).
Areas of rhyolite welded tuff near the west edge of the geologic map (Figs. 2 and 4) are interpreted as remnants of proximal outflow (unit Kro) of the caldera-forming eruption. The tuff caps a northeast-trending ridge continuously for a distance of 4 km, as well as a ridge top to the south and an isolated hill to the northeast of the main exposure. The outcrop pattern indicates that the tuff has a subhorizontal base that represents the 70 Ma land surface, a relation that is illustrated particularly well on the north-northwest–trending ridge south of the largest area of outflow tuff that displays columnar joints above its base (Fig. 5C, leftmost outcrop). The sample from which Bacon and Lanphere (1996) reported a biotite 40Ar/39Ar age of 69.10 ± 0.19 (1σ) Ma was obtained from the west base of the northeast hill exposure. Densely welded basal tuff has well-developed columnar joints (Fig. 5D) and evidently rests on mid-Cretaceous granodiorite (unit Kgd) and Paleozoic metamorphic rocks (Pzm), although the actual contact is obscured by talus, scree, and colluvium. Here, the tuff has a preserved thickness of as much as 100 m. Immediately east of these areas of outflow tuff are poorly exposed granodiorite (?) and probable intracaldera tuff, which together suggest that the outflow tuff is a proximal deposit on what possibly is the original topographic caldera rim. Existing geologic mapping is insufficient to clearly delineate the west boundary of intracaldera tuff and the nature of the contact between granodiorite wall rock and intracaldera tuff. On ridges southeast of the mapped outflow tuff, outcrops interpreted as intracaldera tuff are less densely welded and locally contain centimeter-sized feldspar megacrysts, characteristics that would be consistent with proximity to the original west wall of the caldera. However, northeast projection of a regional northeast-trending fault shown by Foster (1976), part of the Mount Harper lineament of Wilson et al. (1985), would pass just southeast of the ridge-capping outflow tuff, so that a fault contact between granodiorite that underlies outflow tuff and the tuff on its southeast is possible (but note faceted spurs and linear fault trace northwest of outflow tuff in Fig. 2). Exposure appears sufficient such that foot traverses in this area might resolve the geometry of map units and the important question of the nature of this contact.
Between ∼16 and ∼23 km east-southeast of the caldera in the Kechumstuk block are exposures of welded tuff mapped by Foster (1976) in Gold Creek (Fig. 3). The preserved thickness of the tuff may be >70 m (Fig. 5E). All of the characteristics of the tuff in Gold Creek are consistent with it being distal outflow tuff from the Middle Fork caldera. No other potential source, Tertiary or Cretaceous, is known in the region. Pyroclastic flows that deposited this ignimbrite would have traversed a land surface of low relief in comparison to today’s topography in the Mount Veta block between the caldera and the valley of Gold Creek. Exposure of the ca. 68–66 Ma pluton north of Mount Veta (Fig. 4; Day et al., 2014) and establishment of south-to-north drainage in the uplifted Mount Veta block between major northeast-tending, high-angle faults (Fig. 3) postdates the Late Cretaceous landscape. Survival of the welded tuff in the valley of Gold Creek, which has a limited catchment area and evidently was unaffected by Pleistocene glaciation (Weber and Wilson, 2012), requires protracted stability of the landscape there over the 70 m.y. since the caldera-forming eruption.
Much of the western and central parts of the area within the mapped extent of the Middle Fork caldera are occupied by K-feldspar megacrystic granite porphyry (unit Kgp, Fig. 4). The porphyry appears to be the only rock exposed within an ∼7 km × 12 km area and is easily recognized in float by ubiquitous skeletal quartz as large as 1 cm and alkali feldspar megacrysts typically as large as 1–2 cm (rarely 4 cm) in a fine-grained groundmass that weathers light tan. We interpret the porphyry as intruding intracaldera tuff, although we have not discovered a locality where the two rock types can be seen in contact. The porphyry appears to have crystallized from relatively uniform magma that forms a contiguous intrusive mass. No xenoliths were seen in outcrops or amongst float during foot traverses of ridges and only one mafic enclave (sample 10ADb20) was found. The porphyry intrusion has maximum topographic relief of ∼650 m (Fig. 2). The most continuous exposure may be the prominent cliff shown in Figure 5F east of the Middle Fork of the North Fork of the Fortymile River near the southernmost extent of map unit Kgp (Fig. 4). Differential erosion along widely spaced vertical joints has given this 600-m-long by 150-m-high cliff a fluted appearance that is visible from a great distance.
The porphyry intrusion is everywhere surrounded by intracaldera tuff except possibly at its poorly exposed northernmost limit (Fig. 4). There do not appear to be exposures of porphyry east of about longitude 143°07′, that is, within approximately the eastern one-third of the caldera fill. Calderas the size of the Middle Fork and larger commonly have a resurgent structural dome in caldera fill that forms by intrusion of magma beneath the floor soon after caldera collapse (Smith and Bailey, 1968). The resurgent dome can be eccentric to the caldera, such as in the 0.78 Ma Long Valley caldera in California (Bailey et al., 1976). On the basis of its map pattern, we infer that the granite porphyry within the Middle Fork caldera is the solidified intrusion that was responsible for producing a resurgent dome that has since been eroded away. Hence, we refer to the porphyry as a resurgent intrusion. Resurgent intrusions are exposed at eroded calderas elsewhere, such as the Lake City (Lipman, 1976b), Grizzly Peak (Fridrich et al., 1991), Turkey Creek (du Bray and Pallister, 1991), Chegem (Lipman et al., 1993), and Caetano (John et al., 2008) calderas. Compaction foliation of intracaldera tuff would be expected to dip steeply away from contacts with a resurgent intrusion, but we have not visited a sufficient number of appropriate outcrops of Middle Fork intracaldera tuff to test this hypothesis. As will be seen below, the resurgent intrusion interpretation is consistent with available petrographic, geochronologic, and whole-rock geochemical data.
The densely welded intracaldera tuff (Fig. 6A) contains phenocrysts diagnostic of its rhyolitic composition. Typical intracaldera tuff is crystal rich (Fig. 6B) and somewhat heterogeneous in hand specimen owing to the presence of lithic fragments and collapsed pumice clasts in a matrix of phenocrysts, crystal fragments, and welded shards that initially were glass but are now finely crystalline. A collapsed pumice clast that displays large intact feldspar crystals can be seen in more representative intracaldera tuff in Figure 6C. Less densely welded tuff near the caldera margins locally contains 1–2 cm K-feldspar megacrysts and pumice clasts to 6 cm.
In thin section, intracaldera tuff specimens (Figs. 7A–7E) have embayed quartz and variably altered plagioclase and alkali feldspar crystals, each as large as 2–4 mm. Plagioclase is more abundant than alkali feldspar. Quartz and feldspar in intracaldera tuff and outflow tuff commonly are fragments that formed by explosive vesiculation of melt inclusions or intergranular melt in crystal aggregates (Best and Christiansen, 1997). Biotite as large as 1.4 mm may be brown, variably altered, or chloritized. Accessory minerals are Fe-Ti oxides, apatite, and rare zircon. Eutaxitic texture is well preserved in some specimens, which carry relatively crystal-poor fíamme to >1 cm. Lithic fragments observed in thin section range in size from <1 to >20 mm and comprise foliated metamorphic rocks and probable felsic volcanic and porphyritic felsic plutonic rocks.
Outcrop-scale features, notably columnar jointing, distinguish proximal outflow tuff from intracaldera tuff more than texture and modal composition. Proximal outflow tuff is crystal rich, containing ∼50 vol% phenocrysts and fragments of mainly plagioclase and alkali feldspar as large as 3 mm and quartz, commonly embayed, as large as 2 mm. Plagioclase may be partially replaced by sericite and carbonate minerals. Brown biotite phenocrysts are euhedral hexagonal books up to 2 mm across and 1 mm thick. The biotite commonly is intergrown with quartz ± plagioclase ± Fe-Ti oxide and contains apatite and rare zircon inclusions. Opaque oxides and minor euhedral brown amphibole ≤0.5 mm long also are present, as are minor subhedral, partially chloritized, biotite xenocrysts. Lithic fragments as large as 6 cm are muscovite schist and altered biotite granite. The devitrified matrix has relict glass shards. Collapsed pumice clasts have ∼20 vol% crystals of the same phases, which may be representative of the bulk magma crystallinity as opposed to the crystal-enriched character of the bulk welded tuff.
Foster (1976) mapped welded tuff (Figs. 5E and 6D) that appears locally >70 m thick in the valley of Gold Creek from ∼16 km to ∼23 km east-southeast of the Middle Fork caldera. She reported embayed quartz, brown biotite, and rare lithic fragments of quartzite and greenstone. Thin sections of Foster’s three samples reveal abundant quartz, plagioclase, probable alkali feldspar, and brown biotite, all as large as 2 mm, together with minor Fe-Ti oxides and apatite in a honey-colored groundmass (Fig. 7F). Quartz phenocrysts are embayed and contain melt inclusions. The groundmass originally was vitric but has crystallized to fine-grained salic minerals, zeolites (?), and clay (?). Fíamme are well preserved, and porphyritic rhyolitic lithic fragments are common. Although crystal rich, this tuff contains a lesser fraction of crystals than intracaldera or proximal outflow tuff. Petrographic features are consistent with this extensive remnant of ash-flow tuff being outflow from the Middle Fork caldera.
The porphyry carries 40–50 vol% of larger phenocrysts of the same phases found in the tuffs (Figs. 8A, 8B, and 9A–9C). Porphyry samples have skeletal or embayed quartz as large as 8 mm (rarely to 1.2 cm), plagioclase to 3 mm, and euhedral to subrounded alkali feldspar commonly 1–2 cm, rarely to 2 cm × 4 cm. Brown biotite is the most abundant mafic mineral, is as much as 3 mm across, and commonly is chloritized. Sparse prismatic hornblende pseudomorphs as long as 7 mm are locally visible in hand specimen. Opaque Fe-Ti oxides are as large as 0.4 mm. Apatite and zircon (Fig. 9A) are ubiquitous accessory phases. The fine-grained groundmass (Fig. 9D) is more coarsely crystalline than those of the tuffs and is characterized by abundant 50–100 μm quartz crystals.
Mafic Magmatic Enclave
The single mafic magmatic enclave (sample 10ADb20; Fig. 8C), obtained from a porphyry float block, measures 8 cm × 12 cm. Although the enclave is subangular, and therefore is a fragment of a larger object, its texture indicates that it is a sample of mafic magma that was incorporated into cooler magma, presumably that of the granite porphyry, and crystallized rapidly in an undercooled state (Walker and Skelhorn, 1966; Vernon, 1984). Such enclaves are examples of less evolved magma that was coeval with the host granitic magma.
In thin section (Figs. 9E and 9F), the enclave displays brown biotite as large as 2–4 mm and more abundant green hornblende to 2 mm. Euhedral plagioclase with clear rims and saussuritized cores is abundant (to 1.5 mm but mostly 0.2–0.8 mm). Several examples of late-crystallized ∼1 mm quartz and one 4 mm cluster of ≤3 mm sieved plagioclase are present. Plagioclase, together with hornblende, biotite, and accessory Fe-Ti oxides and apatite, give the enclave an intergranular groundmass texture indicative of crystallization of phenocryst-poor melt. Very fine grained matrix between groundmass crystals, such as in the center of Figure 9F, is the product of crystallization of residual liquid.
Radiometric ages have been determined for samples of intracaldera and outflow tuff and for resurgent granite porphyry using the Stanford–U.S. Geological Survey (USGS) sensitive high-resolution ion microprobe with reverse geometry (SHRIMP-RG) at Stanford University. The primary ion beam extracted material from inclusion- and crack-free areas of interior mantles that display fine concentric oscillatory zoning in cathodoluminescence (CL) (Fig. 10). The analytical crater was ∼25–30 µm in diameter and ∼0.5–1 µm deep. Isotopic data were reduced using Squid 2 (Ludwig, 2009) and plotted using Isoplot 3 (Ludwig, 2003). Calculated ages for samples in this study are weighted averages of 14–15 selected individual 206Pb/238U ages. Uncertainties are quoted at the 95% confidence level. Analytical methods are described in detail in the Supplemental File1, and results of individual spot analyses are tabulated in the Supplemental Table2.
Crystallization ages of zircon crystals are 70.0 ± 1.2 Ma for intracaldera tuff (sample 10ADb22), 69.7 ± 1.2 Ma for granite porphyry (sample 10ADb17), and 71.1 ± 0.5 Ma for welded tuff in the valley of Gold Creek (sample 71AFr965) (Fig. 10). These ages agree within analytical uncertainty.
Bacon and Lanphere (1996) reported a 40Ar/39Ar weighted mean age of 69.10 ± 0.19 (1σ) for five total-fusion measurements on individual biotite phenocrysts from rhyolite outflow tuff immediately west of the caldera (sample 91ADb29). Recalculating the biotite age using 28.02 Ma (Renne et al., 1998) for the age of the Fish Canyon Tuff sanidine fluence monitor yields 69.94 ± 0.52 Ma at 95% confidence, in agreement with the U-Pb zircon data.
The age determinations are consistent with the view that outflow and intracaldera tuff formed during the same explosive eruption. The dated zircon crystals carried by erupted rhyolite tuff and intruded porphyry may have grown from the same or similar magmas prior to—perhaps as much as a few hundred k.y.—eruption, caldera formation, and resurgence (cf. Reid, 2008). Note that in the porphyry, at least some zircons are enclosed within large plagioclase crystals that occur in clusters (Fig. 9A). Concordance of the 40Ar/39Ar biotite age for outflow tuff and the zircon ages indicates that the zircon data reflect crystallization in the igneous system that produced the caldera-forming eruption and porphyry intrusion, even if there may be uncertainty as to when in the magmatic history the dated zircon crystallized. At 70 Ma, precision of SHRIMP-RG 206Pb/238U ages is insufficient to resolve potential differences between times of zircon growth recorded within crystals and times of rhyolite eruption or porphyry solidification that might be recorded by the outermost micrometers of zircon in contact with melt (cf. Chamberlain et al., 2014). The geochronologic data show that the caldera-forming eruption and intrusion of the porphyry took place closely in time, in a geological sense at least, and that the erupted and intruded magmas likely originated during the same episode of igneous activity.
An aeromagnetic map of the Eagle 1° × 3° quadrangle provides additional insight into the distribution and ages of tuff and porphyry. The aeromagnetic survey was conducted in 1972 with three-quarter–mile flight line spacing, at 1000 feet above ground where possible; after removing the regional field, contours of magnetic intensity were shown by Veach (1973). Bacon et al. (1990) noted that the aeromagnetic map has an elliptical ring of comparatively large amplitude magnetic lows surrounding a central magnetic high within the area of the Middle Fork caldera and suggested that the lows are best explained by reversely magnetized rock. Comparing the aeromagnetic map with new geologic mapping (Fig. 11), the distribution of intracaldera tuff is coincident with magnetic lows with values of ∼4240–4850 nT, the central magnetic high over the porphyry reaches nearly 5500 nT, and the western lobe of porphyry reaches nearly 5300 nT. Terrain outside of the area of intracaldera tuff has values generally >5000 nT, with highs near 5300 nT. Moreover, outflow tuff west of the caldera coincides with a magnetic trough that has values as low as ∼4000 nT. The outflow tuff in the valley of Gold Creek (Fig. 3) also is associated with a magnetic trough with minima of ∼4700–4800 nT between highs of ∼5000–5500 nT.
Clearly, the tuff of the Middle Fork caldera is reversely magnetized. Reverse magnetization is established by consistent strong magnetic lows over areas of tuff and by very high natural remanent magnetic intensity (NRM) and relatively low magnetic susceptibility measured for two cores from a non-oriented sample of intracaldera tuff that result in unusually high Koenigsberger ratios (Q) of remanent to induced magnetic intensity (Table 1; cf. Bath et al., 1972). The porphyry either is normally magnetized or contemporary induced magnetization has overwhelmed its remanent magnetization. The most recent version of the paleomagnetic time scale (Ogg, 2012) shows normally polarized chron C31n spanning 68.369–69.269 Ma and reversely polarized chron C31r encompassing 69.269–71.449 Ma. Because the outflow tuff and the intracaldera tuff are reversely magnetized, our independent geochronologic results require that the caldera-forming eruption must have occurred during chron C31r. The U-Pb zircon crystallization age for the porphyry, though nominally 69.7 Ma, is within analytical uncertainty of the presently accepted age of chron C31n. However, because studies of resurgent calderas indicate that it is likely that the comagmatic porphyry was intruded relatively soon after caldera formation (e.g., Bailey et al., 1976; Hildreth, et al., 1984), it is important to consider induced magnetization as the source of the magnetic highs associated with the porphyry. Magnetic measurements on cores from two non-oriented porphyry samples yield low NRM, high susceptibility, and thus low Q (Table 1) typical of felsic intrusive rocks (e.g., Ponce and Langenheim, 1993). It is therefore likely that the aeromagnetic highs over granite porphyry simply reflect magnetization induced by the present normal field (57.4 × 103 nT when the survey was flown) and that the tuff and porphyry did not capture the C31r-to-C31n reversal.
Chemical analyses of rock specimens characterize magmas and reveal genetic relationships between the tuff and porphyry. Major-element oxide and trace-element concentrations were determined for one sample of proximal outflow tuff, three of intracaldera tuff, three of granite porphyry, and one mafic enclave by a combination of X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) at the GeoAnalytical Laboratory at Washington State University (Table 2). In spite of likely crystal concentration in bulk tuff and possible melt loss from porphyry, the seven Middle Fork caldera felsic rocks are compositionally similar rhyodacite-rhyolite and granite. Although minor amounts of corundum (0.3%–1.3%) appear in CIPW norms calculated with all Fe as Fe2+ (FeO*), accounting for a fraction of the Fe as Fe3+ indicates that the samples are metaluminous. The felsic rocks have SiO2 contents that range from 69 wt% to 72 wt% (major-element oxides recalculated to sum to 100% volatile free). The composition of proximal outflow tuff sample 91ADb29 is virtually identical to that of intracaldera tuff sample 10ADb11. The seven felsic rock compositions define a limited differentiation series within the range of ca. 68 Ma to 66 Ma felsic intrusive rocks from the Fortymile area just south of the Middle Fork caldera on variation diagrams (Fig. 12). The mafic enclave from the porphyry contains 54 wt% SiO2 and is basaltic trachyandesite on the basis of the silica versus total alkalis classification scheme (Le Bas et al., 1986; not shown).
Trace-element concentrations of the felsic samples are similar to those of 68–66 Ma felsic intrusive rocks from nearby in the Fortymile area (Fig. 13). Multi-element patterns relative to primitive mantle composition (McDonough and Sun, 1995) have the familiar shape characteristic of evolved magmas that formed in convergent plate margins. Importantly, the mafic enclave, itself differentiated relative to a primary melt of the upper mantle, provides an approximation to the parent or the agent of melting for the felsic magmas. The widely used Ta-Yb tectonic discrimination diagram (Fig. 14), originally developed for granitic rocks (Pearce et al., 1984), shows the felsic rocks of the caldera in the volcanic arc field near the boundary with syncollisional granites.
The mafic enclave has a multi-element pattern (Fig. 13A) typical of moderately evolved, subduction-related basaltic andesite in which notable relative depletion in Nb and Ta and enrichment in Pb are superimposed on a negatively sloping curve from high values for elements strongly incompatible in mantle minerals on the left to lower values for moderately incompatible elements on the right. Relative depletions in P and Ti reflect fractionation of apatite and titanomagnetite. The companion rare-earth element (REE) pattern for the enclave normalized to CI chondritic meteorites (Fig. 13B; McDonough and Sun, 1995) is smooth and has a gentle negative slope from light to heavy REE. Lack of a negative Eu anomaly implies that the enclave magma did not experience significant plagioclase fractionation. The enclave composition plots well within calc-alkalic arc fields on the Th-Hf-Ta (Wood, 1980) and La-Y-Nb (Cabanis and Lecolle, 1989) tectonic discrimination diagrams (not shown).
In comparison to the mafic enclave, the felsic samples have multi-element and REE patterns that show enrichment in the more incompatible elements and depletion in elements compatible in feldspars (negative Eu anomaly), hornblende (middle to heavy REE), and accessory phases (P and Ti) that are qualitatively consistent with derivation of the felsic magmas from enclave-like magma by crystallization differentiation. Trace-element abundances also may be consistent with assimilation or partial melting of crustal rocks. Data for the enclave provide evidence of mafic magma and thermal input for either crystallization differentiation or partial melting scenarios. The small compositional range of the tuff samples possibly could be ascribed to varying degrees of syneruptive crystal concentration owing to selective loss of glass shards to the eruption column (Smith, 1960). Similarly, the small variation in porphyry compositions may be related to accumulation of crystals brought about by migration of residual melt.
OTHER CRETACEOUS–EARLY TERTIARY FELSIC VOLCANICS
Several areas of felsic volcanic and hypabyssal intrusive rocks within ∼50 km of the Middle Fork caldera, as well as the area of the caldera itself, were mapped as unit Tf in the Eagle (Foster, 1976) and Big Delta (Weber et al., 1978) 1° × 3° quadrangles (Fig. 15) and, lacking radiometric dates, were inferred to be Tertiary (?) in age. Map patterns and archived thin sections of specimens collected during reconnaissance geologic mapping by Foster and coworkers allow us to suggest the mode of occurrence of unit Tf in most of these areas (Table 3). It should be borne in mind that exposures typically are limited to ridge tops, valley floors and gentle slopes are obscured by surficial deposits and vegetation, and not every ridge in the 48 1:63,360 scale quadrangles (12 of which are shown in Fig. 15) was traversed on foot. It thus is likely that the total area of Tf rock is greater than shown on the geologic maps.
The ∼60 km2 area of rhyolite lava and tuff at the east edge of the Big Delta quadrangle (Fig. 15, loc. 1) is the only dated Tf occurrence other than those with a known Middle Fork caldera source. The nominal ∼5 m.y. age difference between dome clusters may be real, as the lavas have different mafic phenocrysts. The northeastern Tf patch may be an intracanyon lava flow that delineates a Paleocene valley. The southeastern boundary of the volcanic field coincides with northeast-trending, high-angle faults of the Black Mountain tectonic zone (Day et al., 2007; O’Neill et al., 2010). Although this rhyolite field has dimensions similar to small calderas, the depression in which it lies evidently is tectonic. Local vitrophyre high on the domes and preservation of a pyroclastic apron indicate that this volcanic field has not been greatly eroded.
The largest area mapped by Foster (1976) as unit Tf in the Eagle quadrangle, apart from the Middle Fork caldera, is a 4–8 km × 18 km cluster of rhyolite porphyry domes or intrusions bisected by the Charley River (Fig. 15, loc. 2). Groundmass textures verging on granophyric and ubiquitous sericitization of plagioclase are suggestive of intrusions or deeply eroded domes. According to Weber and Wilson (2012), glaciers of the early (?) Pleistocene Charley River advance and, to a lesser extent, the middle (?) Pleistocene Mount Harper and early (?) Wisconsin Eagle advances occupied the valleys here, consistent with relatively intense erosion. An isolated 0.3 km2 patch of Tf (Fig. 15, loc. 3) also would have been affected by multiple glacial advances. In contrast, the 2 km × 3 km cluster of five or more domes and adjacent clastic sedimentary rocks downdropped in a wedge-shaped graben along Ruby Creek (Fig. 15, loc. 4) escaped erosive destruction. Weber and Wilson (2012) show deposits of the Charley River glaciation in the valley of Ruby Creek west and north of the domes, approximately the eastern limit (generalized in Fig. 15) of major ice streams of that age.
Northeast of the caldera is an ∼20 km2 area of Tf with maximum relief of 500 m (Fig. 15, loc. 5). The only available thin section is of latite vitrophyre from breccia at the confluence of Manila Creek and the Middle Fork of the North Fork of the Fortymile River, ∼60 m below early Pleistocene terrace deposits (Foster, 1976; Weber and Wilson, 2012). Because of probable sluggish hydration of volcanic glass in a subarctic environment, the high water content (7.4 wt% H2Ototal) of the vitrophyre suggests that it is significantly older than Pleistocene. Unit Tf also is mapped 13–17 km east of the caldera (Fig. 15, loc. 6), but no samples or field notes are available. Close association of rhyolite and alkali basalt (Werdon et al., 2000) in the four ∼3–10 km2 areas of unit Tf near Bullion Creek 26–33 km east-northeast of the caldera (Fig. 15, loc. 7) suggest this area is a moderately eroded bimodal volcanic field likely of early Tertiary age.
Near Montana Creek, ∼24 km east of the caldera, are five small occurrences of unit Tf that intrude or rest on Paleozoic metasedimentary rocks (Fig. 15, loc. 8). Werdon et al. (2000) described the tiny hilltop Tf patch south of the head of Montana Creek as biotite-bearing, crystal-lithic tuff and reported compositions that broadly resemble those of welded tuffs of the Middle Fork caldera; thus, it is conceivable that this locality preserves outflow tuff from that source. Six small areas of Tf are mapped (Foster, 1976) south of the Middle Fork caldera (Fig. 15, locs. 9–14). Of these, occurrences of Tf in a valley bottom (loc. 10) and on a broad topographic high (loc. 13) are candidates worthy of investigation as potential outflow tuff from the Middle Fork source. Although the topographic high of locality 13 is within an extensive relative low on the aeromagnetic map (Veach, 1973), no striking magnetic lows that might be additional large areas of outflow tuff are evident.
Calderas are significant for their formation during catastrophic explosive volcanic eruptions, favorable structures and thermal histories for localization of economic mineral deposits, and association with cogenetic plutons. The Middle Fork caldera is notable because its degree of exhumation is sufficient to reveal a comagmatic granite porphyry intrusion, yet remnants of the outflow tuff sheet are locally preserved. These characteristics provide evidence for the history of landscape evolution relative to vertical motion on throughgoing northeast-trending faults. The Middle Fork also is significant in being the only preserved Late Cretaceous caldera in a region of east-central Alaska where the distribution of plutons (Foster et al., 1994) suggests the possibility that other “supervolcanoes” have been lost to uplift and erosion. Geologic mapping by Foster and coworkers in the late 1960s and early 1970s documented the felsic volcanic rocks, and their field notes hinted at the porphyry intrusion, at a time when studies of younger and better exposed calderas that provided a conceptual framework for interpretation of map patterns were only starting to appear in the literature (e.g., Smith and Bailey, 1968; Lipman, 1976a). Armed with that framework and modern geochronological and geochemical tools, we can now interpret the geology of the Middle Fork caldera with much greater confidence.
The elliptical map pattern of intracaldera tuff (Fig. 4), reinforced by a ring of magnetic lows (Fig. 11) over reversely magnetized tuff, defines the limits of the foundered cauldron block of the Middle Fork caldera. The block may extend farther north and (or) east, if vegetated and poorly exposed metamorphic rocks mapped at low elevations actually are blocks in megabreccia. The original topographic walls of the caldera may have been located outboard of the present area of intracaldera tuff, except on the west, because it is well known from studies of younger calderas that landsliding into a caldera during syneruptive collapse enlarges the topographic basin (Lipman, 1997). By analogy with deeply eroded calderas, the original thickness of caldera fill, consisting of in-falling juvenile pyroclasts and landslide breccia from the failing walls, may have been as much as ∼3–5 km (Lipman, 1997). Some of this thickness of intracaldera tuff has been eroded away to expose the granite porphyry intrusion. By analogy with Quaternary resurgent calderas (e.g., Bailey et al., 1976; Hildreth et al., 1984; Kennedy et al., 2012), this intrusion probably domed the caldera fill during a geologically short time after the caldera-forming eruption. The resurgent structural dome was west of the caldera center, similar to the Pleistocene Long Valley caldera in California (Bailey et al., 1976). Between the topographic caldera walls and the resurgent dome, a moat, such as described by Smith and Bailey (1968), likely guided subsequent erosion along the course of the present Middle Fork of the North Fork of the Fortymile River, although the river probably has migrated somewhat over the past 70 m.y.
Geochronology, petrography, and geochemistry tie the intracaldera tuff, outflow tuff, and the resurgent intrusion to one igneous system that, by analogy with Cenozoic calderas, probably was active for at least a few hundred thousand years ca. 70 Ma. The caldera-forming eruption and intrusion of the porphyry may have occurred very closely in time because the intracaldera tuff apparently contains clasts of compositionally and mineralogically similar porphyry magma. Moreover, ubiquitous resorption of quartz and feldspar in tuff and intrusive porphyry suggests that these magmas resided in the upper crust as a body of crystal mush that was rejuvenated shortly before the caldera-forming eruption. Resorption of crystals may have resulted from advection of heat or gas transfer associated with subjacent infusion of mafic magma, such as proposed for the Fish Canyon Tuff of southern Colorado (Lipman et al., 1997; Bachmann et al., 2002). Alternatively, heating and remobilization of the mush may have been a consequence of addition of the most recent of many batches of less-evolved silicic magma from a deeper source (Moore and Sisson, 2008). The felsic magma that accumulated as crystal mush formed by processes related to plate convergence through mantle melting, crystallization differentiation, and interaction with continental crust. Physical evidence for a mantle-derived component is present as a rare basaltic trachyandesite enclave in the porphyry that has a clear subduction-related geochemical signature. In comparison to many large caldera systems (e.g., Timber Mountain, Christiansen et al., 1977; Cerro Galan, Sparks et al., 1985), the Middle Fork appears to have had a relatively simple magmatic and eruptive history that represents a single cycle of caldera formation, resurgent intrusion, and cooling (e.g., Creede, Steven and Lipman, 1976; Long Valley, Bailey et al., 1976; Calabozos, Hildreth et al., 1984). Beneath the present level of exposure, there is likely a related equigranular granodiorite-granite pluton of areal dimensions at least as large as those of the caldera fill and possibly contiguous with the 68–66 Ma intrusion to the south (Fig. 3).
The region between the Denali and Tintina faults in east-central Alaska known as the Yukon–Tanana Upland is characterized by moderate to low relief (Fig. 1). Evidence of Pleistocene glacial erosion is subtle except in the higher elevation, alpine portions (Weber, 1986). Preservation of the Middle Fork caldera and correlated outflow tuff, as well as of Late Cretaceous–Paleocene lava domes and intracanyon flows, constrains interpretation of the evolution of this landscape and the degree of exhumation in blocks bounded by northeast-trending, high-angle faults. The Middle Fork caldera occupies a crustal block, here termed the Middle Fork block, between the Mount Harper block to the west and the Mount Veta block to the east (Fig. 15). Whereas ca. 70 Ma intracaldera tuff and the resurgent intrusion are exposed in the Middle Fork block, the Mount Harper block contains mid-Cretaceous equigranular, and apparently no younger, plutonic rocks. Any calderas once associated with Mount Harper block plutons have been removed by uplift and erosion, in contrast to mid-Cretaceous calderas in the Tanacross quadrangle to the south that were downdropped across additional northeast-trending fault zones (Bacon et al., 1990). Proximal outflow tuff from the Middle Fork caldera rests on the ca. 70 Ma land surface immediately west of the caldera fill (Figs. 4, 5C, and 5D), although it presently is unclear if that surface is the northwestern limit of the Middle Fork block or the southeastern limit of the Mount Harper block. The Mount Veta block contains Early Jurassic plutons but also ca. 68–66 Ma equigranular granodiorite-granite compositionally akin to the Middle Fork caldera tuffs and porphyry (Day et al., 2014), much as the ca. 23 Ma Rio Hondo pluton is exposed adjacent to the 25.4 Ma Questa caldera fill (Lipman, 1988; Zimmerer and McIntosh, 2012). The Mount Veta block therefore has been uplifted and exhumed post–70 Ma to a somewhat greater extent than the Middle Fork block.
The crustal block southeast of the Mount Veta block across the northeast-trending Kechumstuk fault zone, here called the Kechumstuk block, has Late Triassic and Early Jurassic plutons but no exposed Cretaceous plutons (Fig. 3). Importantly for understanding landscape evolution in this region, the Kechumstuk block contains the extensive remnant of rhyolitic welded ash-flow tuff, here considered distal outflow from the Middle Fork caldera, in the valley of Gold Creek (Fig. 3). The tuff locally is at least 70 m thick (Foster, 1976) and originally must have been considerably thicker, having ponded in the paleovalley of Gold Creek (cf. Loma Seca Tuff, Hildreth et al., 1984). This occurrence requires stability of low-lying areas of the Kechumstuk block landscape and, in contrast, significant differential uplift and erosion of the Mount Veta block in order to expose a ca. 68–66 Ma pluton between Gold Creek and the caldera.
Consistent with Kechumstuk block stability, the high terrace along the Mosquito Fork of the Fortymile River (to the south and east) is correlated by Weber and Wilson (2012) with the terrace on Gold Creek (a tributary of the Mosquito Fork) that is immediately downstream from the outflow tuff (Fig. 15). The tuff evidently is preserved on either side of Gold Creek because long-term local base level was that of the regional high terrace. The high terrace deposits are shown by Weber and Wilson (2012) as both predating the early (?) Pleistocene Charley River glacial advance and including Charley River–age outwash deposits. Remnants of the terrace were identified by Weber and Wilson (2012) in all of the major drainages southeast of the Mount Harper lineament in the Eagle quadrangle, including the Middle Fork of the North Fork of the Fortymile River as far upstream as the northwest quadrant of the caldera fill and granite porphyry (Fig. 15).
The high terrace is evidence of regional lowering of base level that has driven cutting of the present stream channels following a long period of stability that ended with deposition of Charley River–age outwash gravels. Although downcutting of the Fortymile and other rivers in east-central Alaska might be attributed to tectonic uplift (e.g., Weber, 1986), it has become clear that climatic factors likely played the principal role. As a tributary of the Yukon River, the history of the Fortymile River is tied to adjustments in grade of the Yukon, and it is significant that the youngest fluvial deposits on the high terrace date from the Charley River glaciation. Lowering of base level and regional downcutting of the Yukon and its tributaries in east-central Alaska began with the earliest glaciation—the Charley River glaciation in the Fortymile area. During initial regional glaciation, the Cordilleran ice sheet forced capture of the south-flowing ancestral Yukon by the north-flowing Kwikhpak River, establishing the modern Yukon River drainage network (Fig. 16; Tempelman-Kluit, 1980; Duk-Rodkin et al., 2001, 2010). Hence, the high terrace on the Fortymile River preserves a long-established paleovalley system that has been entrenched by many tens of meters since the onset of Cordilleran glaciation. The beginning of full glacial conditions in the Yukon Territory occurred in the Gauss normal chron >2.58 Ma (Froese et al., 2000) and probably ca. 2.7–2.6 Ma (Haug et al., 2005; Miller et al., 2010). The Middle Fork outflow tuff in Gold Creek demonstrates that low-lying elements of the landscape in the Kechumstuk block were established by 70 Ma and were preserved with little modification until ca. 2.7 Ma.
Cretaceous–Early Tertiary lava domes within ∼50 km of the Middle Fork caldera commonly retain elements of their original constructional morphology. Some clearly occupy local downfaulted basins, such as near Slate and Ruby Creeks and probably also the Charley River cluster (Fig. 15, locs. 1, 4, and 2, respectively). The Slate Creek and Charley River domes are immediately northwest of the northeast-trending Black Mountain tectonic zone identified by Day et al. (2007) and O’Neill et al. (2010), northwest of the relatively deeply exhumed Mount Harper block. A large remnant of an intracanyon lava flow preserved near Slate Creek provides further evidence of only minor downcutting since the Paleocene. The evidently less completely preserved felsic volcanic landforms northeast of the Middle Fork caldera (Fig. 15, locs. 5–8) are largely, if not entirely, within the projected Mount Veta block, which we have argued was uplifted and exhumed to a greater extent post–70–66 Ma than the adjacent Middle Fork block. This higher standing ground provided less protection from erosive forces than basins that host the domes near Slate and Ruby Creeks and the Charley River.
The high terrace on the Middle Fork of the North Fork of the Fortymile River apparently is floored by felsic lava near Manila Creek (Fig. 15, loc. 5), northeast of the caldera. Curiously, latite vitrophyre crops out at the confluence, which is difficult to reconcile with Pleistocene downcutting of the terrace if the vitrophyre represents the front of a lava flow. A plausible explanation may lie in the change in character of the valley of the Middle Fork of the North Fork of the Fortymile River 5 km northeast, near Portage Creek (Fig. 15), where the river leaves a broad alluviated valley to enter a narrow canyon downstream that traverses Triassic granodiorite yet preserves antecedent meanders. The terrace surface has virtually the same ∼650 m elevation at Manila Creek as downstream where the Middle Fork of the North Fork joins the North Fork proper (Fig. 15) but the modern river descends from ∼600 m to ∼550 m. Possibly, pre-Pleistocene downfaulting west of the granodiorite along a cryptic fault raised local base level and caused aggradation upstream to bury the lava flow front; subsequently, the low gradient represented by the present terrace became established and persisted until early Pleistocene regional base-level lowering resulted in renewed downcutting, terrace formation, and exhumation of the vitrophyre, itself an intracanyon lava flow emplaced much earlier. On balance, the morphologies and topographic positions of the felsic volcanics are consistent with a long period of stability of at least the low-lying parts of the landscape, as demonstrated by the Middle Fork outflow tuff in the valley of Gold Creek and the regional high terrace on Yukon River tributaries.
Further constraints on uplift and exhumation in the Fortymile region are provided by apatite fission-track (AFT) data. Dusel-Bacon and Murphy (2001) presented weighted-mean ages along with single-grain and confined track-length AFT results for five plutonic and seven metamorphic rocks from the Yukon–Tanana Upland. The majority of samples indicate rapid cooling at ca. 40 Ma for 3–5 m.y. through the apatite partial annealing zone (PAZ; ∼110–60 °C). These results were interpreted to reflect heating during Eocene extensional magmatism and exhumation from >3.8 km to 2.7 km depth, assuming geothermal gradients of 32–45 °C/km. Samples that yield weighted mean AFT ages >50 Ma have confined track lengths and single-grain age distributions that suggest incomplete Eocene annealing from temperatures ≤110 °C at depths of 3.3–2.0 km; these samples represent rock bodies that were closer to the Eocene ground surface. In the Fortymile region, one sample with a Late Cretaceous (ca. 76 ± 7 Ma; 1σ uncertainty) weighted mean AFT age (Dusel-Bacon and Murphy, 2001) is from the Mount Harper block immediately west of the Mount Harper fault where mid-Cretaceous granitic rocks are widely exposed (Fig. 15, loc. A). This sample has a broad distribution of single-grain AFT ages that suggests slow cooling and partial annealing prior to final cooling between 50 and 25 Ma. Two samples from the Mount Veta block (Fig. 15, locs. B and C) have AFT age distributions that suggest rapid cooling at ca. 40 Ma and final cooling after ca. 40 Ma, respectively (Dusel-Bacon and Murphy, 2001). The widespread geographic distribution of ca. 40 Ma AFT ages suggests that differential exhumation on the order of 2–4 km of the sampled material and uplift across northeast-trending faults, such as of the Mount Veta block, postdates cooling through the PAZ. Reconciliation of AFT results with preservation of Middle Fork outflow tuff in the valley of Gold Creek and Paleocene lava domes in local basins may require appealing to (1) Cretaceous establishment of canyon floors and base level within a high-relief landscape (e.g., Eocene Nevada, Henry, 2008), followed by (2) rapid late Paleocene–early Eocene degradation of interfluves during that interval of warm and wet high-latitude climate (Jahren and Sternberg, 2003; Moran et al., 2006; Greenwood et al., 2010) to account for exhumation and rapid cooling of AFT-sampled rock bodies (now exposed on relatively high ground), succeeded by (3) slower adjustment of interfluves and slopes to produce the modern subdued landscape.
The Middle Fork caldera serves as a clear example of voluminous eruption of rhyolitic magma that was comagmatic with a spatially associated K-feldspar megacrystic granite porphyry intrusion. The caldera structure is defined by a 10 km × 20 km area of intracaldera rhyolite tuff. The 8 km × 12 km porphyry intrusion was responsible for asymmetric resurgent doming of caldera fill. Erosion has destroyed the original topographic caldera, removed perhaps 1–3 km of intracaldera tuff, and exposed the porphyry. Radiometric ages of tuffs and porphyry are analytically indistinguishable at ca. 70 Ma. Despite its Late Cretaceous age, outflow tuff is preserved at the west margin of the caldera and 16–23 km east of the caldera in the valley of Gold Creek. Similar whole-rock chemical compositions and phenocryst mineralogy indicate that the tuffs and porphyry are products of one magmatic system that produced a single caldera-forming eruption and resurgent intrusion, likely from thermally rejuvenated crystal mush. Geochemical data for rhyolite tuffs, granite, and a mafic enclave are consistent with continental arc magmatism. An aeromagnetic survey (Veach, 1973) and magnetic property measurements indicate that magnetic lows over the tuffs are produced by reverse remanent magnetization and that magnetic highs over the porphyry result from magnetization induced by the contemporary field. Reverse remanent magnetization and the ca. 70 Ma age of the tuffs are consistent with eruption and cooling during chron C31r.
Northeast-trending faults divide the landscape into a set of crustal blocks, some of which are relatively deeply exhumed and exhibit more rugged terrain and others that have been stable since Late Cretaceous or Early Tertiary time. Like mid-Cretaceous calderas to the south, the Middle Fork caldera is preserved in a crustal block that is downdropped along regional northeast-trending, high-angle faults within a wide region of low to moderate topographic relief. Uplift and exhumation have been greater for the adjacent Mount Veta block to the south in which ca. 68–66 Ma equigranular plutonic rocks are now exposed. At the west edge of the caldera, proximal outflow tuff rests on the 70 Ma ground surface. Farther southeast, the northern Kechumstuk block has been vertically stable such that low-lying elements of the Late Cretaceous landscape are preserved as evidenced by presence of distal outflow tuff in the valley of Gold Creek. Elsewhere in the region, Cretaceous (?)–Paleocene rhyolitic lava domes and an intracanyon lava flow are present in local basins, downdropped adjacent to northeast-trending fault zones, and as isolated remnants of lava, tuff, and small hypabyssal intrusions. Preservation of Middle Fork outflow tuff in Gold Creek and of rhyolite domes and intracanyon lavas shows that many paleovalleys are little modified since Late Cretaceous–Early Tertiary time except by comparatively recent incision that produced a regional terrace on tributaries of the Yukon River. Formation of this regional terrace apparently was driven by rapid lowering of Yukon River base level ca. 2.7 Ma when the Cordilleran ice sheet induced capture of the south-flowing ancestral Yukon by the Kwikhpak River to form the modern north-flowing Yukon River system.
Our work on the Middle Fork caldera, the resurgent intrusion, and felsic volcanic rocks in the region has benefited greatly from access to the original field notes, map sheets, and thin sections of H.L. Foster and coworkers. Renee Pillers separated zircon for U-Pb geochronology. Kate Gans and Joel Robinson helped with geographic information system preparation of the geologic map of the caldera. Jon Hagstrum kindly measured magnetic properties of rocks. We appreciate constructive reviews by Joe Colgan (for USGS), Ryan Mills, an anonymous reviewer, and Associate Editor Lang Farmer that led to clarification and improvement of the manuscript.