By: Dr. Eugene I. Smith, Principal Investigator,
Shirley Morikawa and Alex Sanchez, Research Associates,
University of Nevada, Las Vegas
|Reader's Note: Dr. Eugene Smith is the Director of the Center for Volcanic and Tectonic Studies, University of Nevada, Las Vegas and a professor in the Geoscience Department, University of Nevada, Las Vegas. From 1986 through 1996, Dr. Smith and his associates conducted research related to the potential for a volcanic hazard at the Yucca Mountain proposed repository site for the Nevada Agency for Nuclear Projects. This report summarizes the volcanic hazard studies conducted and their conclusions.|
Established in 1986, the Center for Volcanic and Tectonic Studies (CVTS) coordinated the research efforts for the State of Nevada's Agency for Nuclear Projects to evaluate the hazard of volcanic activity near the proposed high-level nuclear waste repository at Yucca Mountain. The goal of the CVTS, as stated in the original proposal to the Agency, was "to understand basaltic eruptions near Yucca Mountain at Crater Flat in terms of regional volcanism and tectonism." In defense of using regional analogs, the proposal stated, "Analog studies of volcanoes having ages and tectonic settings comparable to those at Crater Flat are necessary for the construction of realistic models of emplacement, localization, and longevity of the Crater Flat volcanoes. Analog studies of this type are an important part of many geologic investigations, and they will be effective in the study of late Miocene and Pliocene volcanic rocks of the southern Great Basin." The research strategy of the CVTS was, therefore, to study volcanic analogs around the Yucca Mountain-Crater Flat area and then, with the experience gained from these analog areas, begin detailed geologic investigations at Crater Flat. CVTS has faithfully followed this research strategy. Originally criticized by DOE and their contractors, DOE-funded investigators actually adopted this approach in the early 1990's. Additionally, the CVTS, in cooperation with Dr. C.H. Ho of the Mathematics Department at UNLV, combined geological and statistical studies to estimate the probability of future volcanic eruptions near the proposed high-level nuclear waste repository.
NORTHERN COLORADO RIVER EXTENSIONAL CORRIDOR (NCREC)
Patterns of Volcanism in the NCREC
The NCREC is a 45 to 125 km wide area of extended crust between the Colorado Plateau to the east and the Spring Mountains, Clark Mountain, Old Women Mountains, and Mojave block to the west. The Lake Mead and Las Vegas Valley fault zones bound the corridor to the north. The corridor contains 27 volcanic centers, including sources for two regionally extensive ash-flow sheets (Peach Springs Tuff and Tuff of Bridge Spring), and 9 plutons. Volcanic centers and plutons occur in three east-west zones that extend across the corridor. The southern zone (Kingman-Castle zone:KCZ) extends from Kingman, AZ to the Castle Mountains, CA and contains volcanic centers at Kingman, Union Pass, Oatman, and in the Castle Mountains, as well as plutons in the Newberry Mountains. The source of the Peach Springs tuff has been inferred to lie within or near the Newberry Mountains. To the north of the KCZ is a magmatic-free zone 18 to 30 km wide. Although volcanic strata are common in parts of this area, no major volcanic centers or plutons have been identified with the exception of the Crescent Peak pluton. The central magmatic zone (Dolan Springs-Devil Peak zone: DDZ) extends from the White Hills, AZ to the Spring Mountains, NV. It includes volcanic centers in the Black, Eldorado, and Spring Mountains and plutons in the central Black and Eldorado Mountains.
The eastern part of the DDZ is bounded to the north by a 10 km wide magmatic-free zone extending from the Colorado Plateau to the Colorado River. The western part of the DDZ merges with the northern magmatic zone that extends from the White Hills to the McCullough range, NV (WMZ). The WMZ contains volcanic centers in the Lake Mead area and the McCullough range and plutons at Wilson Ridge and Boulder City. The source of the Tuff of Bridge Spring lies within the southern WMZ or northern DDZ in the Eldorado Mountains. Extensional strain appears to be partitioned differently in each magmatic zone. Several major normal faults terminate near the margins of magmatic zones or within the amagmatic belts. However, a major accommodation zone lies fully within the DDZ. Major Miocene magmatism migrated northward in the NCREC, beginning at 22 Ma in the south, 18 Ma in the central part, and 16 Ma in the northernmost areas. It preceded the northward advancing front of major extension by 2-3 Ma. Magmatism locally continued behind the northward moving magmatic front for as long as 10 Ma. Magmatism began with compositionally restricted calc-alkaline suites (basaltic-andesite) that are intruded by compositionally variable plutons or overlain by compositionally variable flows (andesite to rhyolite).
Fortification Hill Volcanic Field
Studies of the Pliocene Fortification Hill volcanic field have been the prime focus of the CVTS work in the NCREC. This work resulted in articles published in the Geological Society of America Bulletin (Feuerbach and others, 1993) and Isochron West (Feuerbach and others, 1992), geological maps (e.g., Mills (1994)), and numerous reports and abstracts. As part of Feuerbach's Ph.D. dissertation at the University of Iowa, he will produce a paper that discusses the petrogenesis of the Fortification Hill lavas. In addition to work on the Fortification Hill volcanic field, CVTS staff have studied the relationship of geologic structures to vent locations in the Hoover Dam area (Mills, 1994) and near Malpais Flattop (Faulds, 1996a and 1996b). Morikawa (1993) conducted an important topical study of a regional ash-flow sheet, the Tuff of Bridge Spring. Among many contributions, this thesis developed geochemical criteria for correlating ash-flow tuffs across complexly faulted terranes. In addition, Cascadden (1991) mapped several small basaltic-andesite shield-stratocones in the White Hills, north of Dolan Springs, Arizona as part of her study to describe the upper plate of the Salt Springs Wash detachment fault. Suggestions that Yucca Mountain and Crater Flat resided in the upper plate of a detachment structure prompted this study. The CVTS considered the Salt Springs Wash area a natural structural analog to the Yucca Mountain area.
Pliocene basalts in the Lake Mead area define the Fortification Hill volcanic field. Basalts of this field are subalkalic and alkalic and erupted between 5.89 and 4.7 Ma. Source areas are at Fortification Hill, Lava Cascade, Petroglyph Wash, and Malpais Flattop. Activity in the field ended with the formation of low-volume alkali basalt centers in northwest Arizona along U.S. 93 and in Petroglyph Wash and near Boulder Beach, Nevada (Campbell and Schenk, 1950; Smith, 1984; Feuerbach, 1986; Smith and others, 1990b).
Fortification Hill basalt crops out south of the Lake Mead fault zone in a north-northeast-trending belt that extends from Malpais Flattop to Fortification Hill, Arizona. Vents occur in three groups (Fortification Hill-Lava Cascade, Petroglyph Wash, and Malpais Flattop) that are closely associated with major high-angle faults. Activity appeared to be focused at just a few centers along or near these faults.
The Fortification Hill-Lava Cascade group forms a chain of at least 6 vents with a length of 25 km. The chain appears to be controlled by north-northwest-striking faults that bound the west side of the Black Mountains. Vents occur both on the margin and in the interior of the range. At the Lava Cascade, the southernmost volcano of the chain, lava erupted from a cinder cone near the summit of the Black Mountains in an area nearly devoid of any major faults. Major north-northwest-trending high-angle faults cut the range just to the west of the cinder cone. Dikes related to the Lava Cascade cut both scoria and Precambrian basement. Fortification Hill, the northernmost volcano of the chain, is a steep-sided mesa just to the east of Hoover Dam. The escarpment that forms the cap of Fortification Hill is composed of over 100 flows of olivine basalt and scoria. Cinder cones aligned in a north-south direction are intruded by dikes and plugs and served as the source for the flows. The Fortification Hill cinder cones represent a node of intense volcanic activity along a north-trending en echelon dike system. Dike orientation is coplanar with the east-dipping mid-Miocene Fortification fault zone. The Fortification Hill-Lava Cascade zone also contains a 4.7 Ma alkali basalt center located about 12 km south of Hoover Dam just to the west of U.S. 93 (Campbell and Schenk, 1950). This center is associated with dikes, flows, and pyroclastic deposits and has a total volume of no more than 0.1 km3.
Petroglyph Wash Basalts
The Petroglyph Wash lavas crop out along the east side of the Black Mountains. Basalts of this group erupted from at least two vents. The Malpais Flattop basalts lie to the southwest of the Fortification Hill group and erupted from a northwest-striking dike.
Located in Petroglyph Wash is a diatreme (100 m in diameter) and a basalt plug. The diatreme is composed of tuff breccia intruded by a highly foliated alkali basalt plug. The breccia contains scoria and angular clasts of Wilson Ridge quartz monzonite. Small cylindrical pipes of basalt containing pyroxene megacrysts also intrude the breccia. The contact between the plug and breccia dips inward 40 to 60 degrees. The diatreme is 4.61 + 0.17 m.y. old (K-Ar plagioclase concentrate date) (Feuerbach and others, 1991).
The Mantle Boundary
In the Lake Mead area, the boundary between the amagmatic zone and the northern Colorado
River extensional corridor (NCREC) parallels a contact between ocean-island basalt (OIB) and
lithospheric mantle domains. The boundary formed between 11 and 6 Ma during and just
following the peak of extension (Feuerbach and others, 1993). Mafic lavas to the north of the
boundary have a lithospheric mantle (LM) signature (Nd= -3 to -9; 87Sr/86Sr=0.706-0.707). South
of the boundary, lavas have a source composed of OIB and HIMU-mantle (Nd=0 to +4;
87Sr/86Sr=0.703-0.705). The source of mafic lavas in the region prior to 11 Ma was the
lithospheric mantle. The change in source with time and the HIMU-like mantle component are
compatible with a plume beneath the NCREC during extension.
Mt. Perkins Pluton
CVTS funded studies of the Mt. Perkins pluton by Rodney Metcalf (UNLV) for the information the study could provide about the emplacement and evolution of magma in the upper crust during regional extension. A summary of this work is provided below.
The west-tilted Mt. Perkins block in the Black Mountains of northwestern Arizona exposes a cross-section of a mid-Miocene volcanic-plutonic complex from the Miocene volcanic section exposed in the west to the sub-volcanic Miocene Mt. Perkins pluton to the east. The Mt. Perkins pluton, which intrudes the Precambrian basement underlying the volcanic section, was emplaced at a depth of 7.5 km and represents the upper portion of a larger pluton at depth (Metcalf et al., 1995). Detailed mapping of the pluton revealed a four-phase emplacement history with each magma pulse solidifying prior to the emplacement of subsequent magmas. Phase 1 consists of gabbro and diorite. Phase 2 is dominated by mafic (diorite-quartz diorite) to intermediate (tonalite) compositions with mafic lithologies occurring as microgranitoid enclaves enclosed within granitoid hosts of intermediate to felsic composition. Phase 3 constitutes the largest volume of the pluton and is composed of a bimodal suite of granite and diorite. The diorite occurs as microgranitoid enclaves enclosed within the granite host; the latter makes up the largest volume of the pluton. Phase 4 consists of a suite of planar aphanitic dikes that strike north-northwest, similar to Miocene normal faults in the area. The dike suite exhibits bimodal, mafic-felsic compositions.
A detailed petrogenetic study of phases 2 and 3 was recently published (Metcalf et al., 1995). Field, petrographic, major element, trace element, and isotopic data were used to document the origin of phase 2 intermediate rocks by mixing (combined with fractional crystallization) of mafic and felsic magmas similar to bimodal phase 3 rocks. Trace element and isotopic data indicate that the mixing end-members were mafic magma derived from the asthenospheric mantle and felsic magma derived by partial melting of continental crust.
The volcanic section to the west of Mt. Perkins consists of a basal section of basalt overlain by a thick section of andesite and subordinate dacite, in turn overlain by a thick section of dacite and rhyolite flows and domes. The uppermost portion of the section consists of a bimodal suite of rhyolite tuff and basalt flows with intercalated conglomerates. Thus, the volcanic sections mirror the emplacement sequence observed in the pluton. A detailed geochronologic and structural study by Faulds et al. (1995) documented that volcanism began at 19.9 Ma and continued until about 11.4 Ma. The base of the volcanic section dips nearly 90. Westward dips decrease up section with the uppermost portions of the section dipping less than 10. These observations indicate that much of the volcanic section was deposited in a synextensional growth fault basin (Faulds et al. 1995). Extensional faulting began at about 15.7 Ma, some 4.2 million years after the onset of magmatism. Major extensional faulting continued until about 11.3 Ma. A 40Ar/39Ar biotite of 15.96 Ma was reported for a granite from phase 3 (Faulds et al., 1995), suggesting that much of the pluton pre-dates the onset of major extensional faulting. A massive dike swarm extends from the base of the rhyolite flows and domes, west towards the apex of the Mt. Perkins pluton. Geochemical, isotopic, and geochronologic data (Faulds et al., 1995; Metcalf et al., 1995) point to a correlation among the rhyolite flows and dikes and the pluton granites. Similar correlations may exist between the intermediate and mafic portions of the volcanic (basal) section and the pluton (phases 1 and 2).
A major hydrothermal alteration halo exists around and within the Mt. Perkins volcanic-plutonic complex. At the depth of the pluton, granulite facies mineral assemblages in the Precambrian country rock have been converted to greenschist and lower grade (argillic and propylitic) assemblages.
Little disrupted by faulting, the McCullough Range is a natural laboratory for the study of
Miocene volcanoes in the Basin and Range province. CVTS has funded (or partially funded) two
studies in the McCullough Range. The first dealt with the origin of a broad 10 km wide crater, the Sloan Sag. This work demonstrated that the sag is a caldera formed by lava fountaining of gas poor andesitic lava (Bridwell, 1991). Boland (1996) did a topical study on basaltic-andesite petrogenesis using the thick section of basaltic-andesite in the northern McCullough Range as her study area. Magma mixing of compositionally similar basaltic andesite parental magmas may be responsible for forming basaltic andesite dominated volcanoes in this area. To test the global applicability of her work, Boland collected samples at the Xitle volcano in the Trans-Mexican volcanic belt near Mexico City. Processes similar to those operating in the McCullough Range may have produced Xitle magmas.
Volcanic Centers in the McCullough Range
Late-Miocene volcanism in the McCullough Range is related to five volcanic centers, the
McCullough Pass caldera, Hidden Valley sag, Henderson Volcanic complex, Northern McCullough stratocones, and the Colony volcanic center. The McCullough Pass caldera in the central McCullough Range is about 3 km in diameter. Stratigraphically, rocks related to the caldera lie above the Tuff of Bridge Spring (15.1 Ma) and beneath the Hidden Valley volcanics (12-11 Ma). The caldera has many of the features of a large caldera including rhyolite domes, scalloped margins, and an associated rhyolite ash-flow tuff. Basalt crops out within the caldera, but may have erupted from nearby sources and then flowed into the caldera. The shape of the caldera was partially controlled by pre-existing structures. The northern part is a graben that strikes to the northwest. The southern part is elliptical and cuts regional structure. The caldera wall is locally cut by high-angle normal faults. The Hidden Valley sag is 13.5 km in diameter and is filled with andesite and dacite domes, flows, and thin pyroclastic units (Sloan volcanics). Sloan volcanics lie stratigraphically above the Hidden Valley volcanics (12-11 Ma) and are undeformed. They represent the youngest volcanic activity in the McCullough Range. The formation of the sag may be related to the eruption of the voluminous (min. 9 km3) aphyric
Mt. Hanna andesite by lava-fountaining. This andesite may be the high temperature and dry
equivalent of an ash-flow tuff.
The Henderson volcanic complex occupies a 15 km diameter semi-circular depression filled with dacite domes and flows and is associated with a thin outflow sheet of pumice-rich dacite tuff. Dacite erupted from vents along the southern margin of the complex and flowed to the north as steep-sided coulees. Recorded within the depression are at least two periods of eruption. Flows of the second episode commonly abut against flows of the first episode. The Henderson volcanic complex formed on the already tilted and eroded margin of the McCullough Range. Dacite filled the depression and locally overflowed into canyons cut in the stratigraphically lower Hidden Valley basalt. The Northern McCullough range stratocone formed by nearly 1200 m of calc-alkaline andesite and basaltic andesite was produced by the mixing together of several different mafic sources that are similar in chemistry but isotopically distinct. The Colony volcanic center, located along the eastern margin of the northern McCullough Range, sits stratigraphically between Precambrian basement and andesites of the Northern McCullough Range stratocone. The volcanic center is largely unknown, but seems to be composed of several large dacite domes and related flows, sections of debris flow breccia, and local flows of andesite.
TRANSITION ZONE AND COLORADO PLATEAU
CVTS has supported two studies in the transition zone and one on the Colorado Plateau. Cole (1989) and Sanchez (1995) worked on Quaternary (and Pliocene) alkali basalt volcanism in the transition zone. Cole's work in the Grand Wash trough demonstrated the difficulty of identifying source areas in some Pliocene-Quaternary volcanic fields and raised a question important to Probabilistic Volcanic Hazard Analysis (PVHA) of whether all volcanic vents have been located in the Yucca Mountain area. Sanchez' work in the Hurricane volcanic field in Utah demonstrated that small volume cinder cones can be polygenetic and that complex processes like lower crustal contamination, magma mixing, and fractional crystallization may all operate to produce magmas. Sanchez also demonstrated that, although major faults may cross a volcanic field, they may not control the location of cinder cones. Vents occur on relatively minor joints or may have no apparent structural control. Sunset Crater on the Colorado Plateau near Flagstaff, Arizona is one of the youngest cinder cones in North America and erupted 1064 to 1180 AD. Blaylock's study (in progress) concentrated on the chemistry of Sunset Crater and other coeval volcanoes that extend 10 km northeast of Sunset Crater. An important reason for choosing Sunset Crater for this type of study is that chemical differences can be directly related to volcanic stratigraphy and thus to a specific volcanic event. Two chemical and isotopic groups form Sunset Crater and the Sunset Crater chain, but the volcanoes are not polygenetic. Variable contamination by the lower crust produced the two magma batches. Prior fractional crystallization produced variability within each magma batch.
Hurricane Volcanic Field
The Hurricane volcanic field is a small volume (0.48 km3) mafic volcanic field in the St. George basin in southwestern Utah. Three rock groups: low-silica basanite (<42 wt. % SiO2), basanite (43-46% SiO2) and alkali basalt (>46% SiO2) erupted over a period of at least 100,000 years and originated from the partial melting of four isotopically distinct, garnet-free mantle sources. Limited mixing between two of the four types of magmas may explain intraelement variations of basanites and some alkali basalts. The range of La/Ba (0.13-0.15) and La/Nb (0.9-2.6) for rocks of the Hurricane volcanic field suggest the older lavas had a lithospheric mantle source with younger lavas becoming more like ocean island basalt (OIB) with time. Hurricane volcanic field lavas have 87Sr/86Sr = 0.703 to 0.7049 and relatively lower Nd = +1.6 to -7.5 values compared to Basin and Range basalts less than 5 m.y. old indicating that Hurricane volcanic field magmas interacted with lower crustal component(s) in one or two steps. Differences in chemical and isotopic features of volcanic rocks erupted from the Volcano Mountain vent complex in the Hurricane volcanic field indicate that the complex is polygenetic. Geochronological data indicate that the complex erupted over a period of at least 100,000 years and is also polycyclic (from Sanchez and others, 1996).
Sunset Crater. Arizona
Sunset Crater (A.D. 1064-65 to A.D. 1180) is part of a N60W trending, 10 km long chain (Sunset Crater Volcanic Chain-SCVC) of coeval vents that includes Rows of Cones, Gyp Crater, and Vent 512 (total magma volume is about 0.13 km3). Also part of the SCVC are the Bonito, Kana'a, and Vent 512 lava flows. A question regarding the evolution of Sunset Crater is whether the volcanic system is polygenetic, complex history/multiple magma sources; simple monogenetic, simple history/single magma source; or complex monogenetic, complex history/single source. Major element, trace element, and isotopic data allow division of SCVC alkali basalts into two chemical groups. The first (older) group, comprised of Gyp Crater and Vent 512, has values of 87Sr/86Sr of 0.73329 to 0.703385, Nd of 1.69 to 2.27 and generally higher abundances of incompatible trace elements. The second group (87Sr/86Sr=0.703339 to 0.703413, Nd=-0.12 to 0.87) consists of Sunset Crater and the Bonito and Kana'a flows. Isotopically, both groups lie below the mantle array and together form a line that points toward a field of lower crustal mafic granulites (87Sr/86Sr=0.7038 to 0.7042, Nd=-14 to -17). The older and younger groups formed by differing degrees of contamination (~20% and ~30% respectively) of an OIB-like parental magma by lower crustal mafic granulites and were modified by fractional crystallization of clinopyroxene and olivine. Vents and flows of the SCVC form a complex monogenetic volcanic system (from Blaylock and others, 1996).
REVEILLE AND PANCAKE RANGES
Northern Reveille Range
Naumann and others (1991) recognized two episodes of Pliocene volcanism in the Reveille Range. Pliocene activity began with a large volume of alkalic basalt (Episode 1: 5.1 to 5.9 Ma) followed by a second lower volume basalt (Episode 2: 3 to 4.7 Ma). Trachytic volcanism at 4.3 Ma locally occurred between eruptions of Episode 1 and 2 lavas. Episode 1 basalt has a volume of about 8 km3 and erupted from about 52 vents. Episode 2 comprises a minimum volume of 1 km3 and erupted from 14 vents located only in the northeastern part of the Reveille Range. Martin and Naumann (1995) mapped the Reveille 7½ minute Quadrangle. Although mapped in some detail by Akron and others (1973), Martin and Naumann demonstrated that many of the faults mapped by Akron are stratigraphic contacts. Martin and Naumann's mapping also clearly delineated the margin of the caldera of the Tuff of Northern Reveille Range and showed that at least some Pliocene cinder cones occur along the caldera margin. Rash (1996) mapped the northern tip of the Reveille Range and part of the southern Pancake Range. He extended the caldera of the Tuff of Northern Reveille Range from the northern boundary of the Reveille Quadrangle to the Twin Spring Ranch at the southern tip of the Pancake Range. Located along the wall of the caldera in this location is an impressive megabreccia deposit. Rash also verified the location of the Goblin Knobs caldera, earlier described by Akron and others (1973), and suggested that the Tuff of Northern Reveille Range caldera is nested within it. Naumann and others (1991) and Rash (1996) suggested that mafic volcanism began in the Reveille Range in Middle to Late Miocene time. Similar mafic rocks near the town of Rachel were dated at about 14 Ma (Naumann and others, 1991). These basaltic andesites occur in the northwestern Reveille Range and have an isotopically enriched signature suggesting a source in the lithosphere mantle. In comparison, Pliocene and Quaternary alkalic basalts were derived by partial melting of the asthenospheric mantle. Yogodzinski completed a detailed study of Reveille Range basalts and suggested (Yogodzinski and others, 1996) that "geochemical variations among the Pliocene-age basalts in the Reveille range require the addition of an upper crustal component to episode 1 and trachyte samples. The crustal component was probably incorporated through assimilation of carbonate-rich sedimentary wall rock in a well-developed magmatic plumbing system in the upper crust. The geochemical stratigraphy indicates that between 3 and 6 m.y. ago, the volcanic field developed in at least two eruptive cycles of approximately equal duration. Each eruptive cycle was apparently tied to discrete melting events in the mantle. Available information on the whole volcanic field indicates that through time, basaltic magmas were produced in diminishing volumes, they were stored prior to eruption at greater depth, and they traveled to the surface at higher ascent velocities with the younger basalt containing abundant mantle-derived xenoliths. The time-space eruptive pattern indicates that melting anomalies responsible for the formation of the volcanic field were initially large and covered a broad area in the Pliocene (3-6 m.y. ago), but progressively diminished in size so that by late Pleistocene time, volcanism had become localized in a relatively small area near the northern end of the field. Overall, it appears from this and previous studies, the volcanic activity in the Reveille Range area has declined progressively since its inception 5-6 m.y. ago (Yogodzinski and others, 1996)."
Work in progress on Citadel Mountain in the Lunar Crater volcanic field by Dickson shows that: (1) Citadel Mountain is a structural and lithological analog to Yucca Mountain. Miocene-aged ash-flow tuffs on Citadel Mountain are tilted 15-20 degrees by normal faulting and form a simple north rotated fault block. The top of Citadel Mountain stands nearly 300 m above the surrounding basins. (2) Cinder cones occur from the top of Citadel Mountain to its base. (3) Four of the cones that produced the most voluminous lava flows are near the summit of Citadel Mountain. (4) Topographic gradients and low-lava viscosity cooperated to produce long lava flows (2-3 km). Flows are thick (3 m) several kilometers from their vents. (5) Although previously mapped by Akron and others (1973), these authors did not recognize all of the volcanic vents. Dickson has discovered two cinder cones, a dike scoria complex, and a broad scoria cone filled with a collapsed lava lake. (6) Considering the degree of erosion of the vent areas, volcanism on Citadel Mountain continued for a considerable period of time. As an indication of the span of volcanism, K-Ar dates reported in Foland and others (1995) indicate that lava flows exposed in the Lunar Crater maar range in age from 1.2 to 3.9 Ma.
Another study currently in progress by Tom Wickham describes the geology of the Fang Ridge area in the central Reveille Range. Wickham located the northern margin of the Kawich Caldera, the source of the 24.5 Ma Paranaghat tuff. In this locality, rhyolite domes in the caldera moat contact the Tuff of Goblin Knobs outside of the caldera. The moat of the caldera is nearly 5 km wide and contains rhyolite domes, intracaldera tuff, megabreccia, and lacustrine sedimentary units. Pliocene basalt erupted near the caldera wall and flowed through a canyon along the caldera margin. Another cinder cone formed at the contact between the moat and thick deposits of intracaldera fill (resurgent dome?) and flowed through a canyon along this contact.
CRATER FLAT AND ADJACENT AREAS
Summary of Age dates from Crater Flat
K/Ar plagioclase separate dates were completed for all volcanic centers in Crater Flat and the
Lathrop Wells cone by the Isotope Geochronology Laboratory at the University of Arizona in
March, 1990. These dates are summarized below.
Volcanic Center Sample Number Age (Ma)
Black Cone C9-1-26-LN 0.71 ± 0.06
Red Cone C9-2-27-LN 0.98 ± 0.10
Red Cone C9-2-26-LN 1.01 ± 0.06
Red Cone C9-2-37-LN 0.95 ± 0.08
Northern Cone C9-3-47-LN 1.05 ± 0.07
Little Cone C9-4-48-LN 0.77 ± 0.04
Lathrop Wells C9-5-50-LN 0.06 ± 0.03
3.7 Ma basalt C9-6-54-LN 2.54 ± 0.09
Note: The 2.54 Ma date on the Pliocene basalts in southeastern Crater Flat is not a valid age.
Subsequent dating indicates that these flows are indeed 3.7 m.y. old.
Black Cone Cinder Cone
Lava and scoria from Black Cone covers nearly 4 km2. The center is composed of alkali basalt flows and pyroclastic deposits of scoria, agglutinate, ash, and bombs that erupted from at least 10 separate vents. Lava was deposited on alluvium sloping to the south. As a result, basalt flowed preferentially to the south. The following description of Feuerbach's work is abstracted from the 1990 CVTS annual report. The geology of the cones is presented on the geologic map of the Crater Flat quadrangle (Faulds and others, 1994).
The most prominent land form at Black Cone is a cinder cone that stands approximately 60 m above the surrounding flows. Oval in plan view, the cone has a long axis oriented north-south. Except for a well-developed lava lake at its summit and numerous rootless lava flows, the cone is composed of pyroclasts of scoria, agglutinated scoria, ash, and bombs. The northwest half of the cone retains its mantle of scoria and has a near-pristine profile. This slope is slightly modified by rills and has a distinct cone apron. Erosion has removed the scoria mantle from the southeast side of Black Cone revealing the internal stratigraphy of the cone. In a section from the southern base of Black Cone to its summit, there are four packages of scoria exposed. The lowest scoria unit (Scl) appears to represent the lower flanks of the cone. The unit dips gently away from the summit (9 to 12 degrees). Locally, Scl contains concentrations of large (>0.5 m in size) bombs. These beds may represent deposits at the base of the cone where bombs rolled down the cone and accumulated (McGetchin and others, 1974).
The next higher package of scoria (Sc2) is a cliff-forming section that may have been deposited outside and below the crater rim. The unit strikes N45E and dips away from the crater rim (20 degrees). Within this unit, bedded scoria reflects as many as 10 pulses of Strombolian eruption. In addition to bedded scoria, the deposit contains bombs and zones of agglutinated and welded scoria and rootless flows. These deposits suggest that the style of volcanism fluctuated between mildly explosive Strombolian to Hawaiian-type lava fountaining .
Separating Sc2 from Sc3 is an angular unconformity marked by a 2 cm thick zone of caliche. Sc3, a 4 m thick section of scoria similar in lithology to Sc2, strikes toward the summit crater and dips 22 to 33 degrees to the west-southwest. Sc3 may be a section of Sc2 that collapsed during an event that oversteepened the walls of the summit crater or the flank of the cinder cone (possibly a crater breaching event).
Another unconformity separates Sc3 from Sc4, the uppermost section of scoria. This unconformity can be traced around the south and east sides of the cone and may represent the lip of a pre-Sc4 crater. Bedding in Sc4 dips radially inward toward the summit and represents pyroclastic deposition in a summit crater. Sc4 records a style of eruption that fluctuated between Strombolian and Hawaiian-type lava fountaining; interbedded with rootless flows and agglutinated spatter is bedded scoria. The ratio of flow to scoria increases upward, suggesting that lava fountaining became more important late in the eruption of Sc4. Sc4 is capped by a 2-3 m thick lava lake formed by massive flows of olivine basalt.
Although the cinder cone is the most prominent topographic feature associated with the Black Cone complex, it may only account for a small volume of flows. A larger volume of flows erupted from at least 10 vents located north, south, and east of Black Cone. These vents are commonly represented by scoria mounds that are composed of cinder, ash, and large bombs. Vents are aligned along two sub-parallel zones that strike approximately N40E. One zone includes Black Cone and 4 scoria mounds (Black Cone zone); the other zone lies 300 m to the southeast of Black Cone and contains at least 7 mounds (Southeast zone). A feeder dike within the Southeast zone is exposed in the interior of a scoria mound cut by a wash. The dike is 3 m wide and strikes N36E and dips 81 degrees to the south. The attitude of the dike at this locality reflects the northeast trend of the southeast zone. The interpretation of these mounds as vents came under intense scrutiny during the Geomatrix PVHA meetings in 1995. Several of the experts suggested that the mounds were instead rafted pieces of the cone, and the dikes represented upturned margins of Aa flows.
Block and Aa olivine-basalt flows surround Black Cone. The basalt flows are vesicular to massive and contain olivine as the dominant phenocryst. Flow ramps and topographic benches are the major features on the surface of the flows. The flows were subdivided into four groups based on spatial relationships and stratigraphy. They are: northern flows; scoria-mound flows; southwest flows; and south-cone flow.
The northern flows are composed of at least three flow units. They may have erupted from two scoria mounds near the northern base of Black Cone. These flows traveled nearly a kilometer from their source toward the north, east, and west. Some of the older northern flows may have erupted from the northernmost part of the Southeast zone.
The scoria-mound flows are exposed south and southeast of Black Cone. These short and stubby flows can be traced back to seven coalescing cinder mounds that lie along the Southeast zone. Flows traveled primarily to the south. The longest unit flowed nearly 1 km from its source.
The southwest flows traveled approximately 0.75 km to the south and west from their probable source near the western base of Black Cone. The source for these flows is not exposed; however, flow direction indicators such as ramp structures allow these flows to be traced toward their source.
Flow direction of lava flows from the Black Cone complex was controlled by the south dipping slope of the floor of Crater Flat. Only the northern flows traveled to the north. CVTS suggests that the northern flows erupted after a substantial mound of scoria and flows formed to the south, related to the eruption of the scoria mounds and the Black Cone cinder cone. The northern flows traveled down a reversed topographic slope off the northern flank of this mound to the north, northeast, and northwest. Clearly, the northern flows and south-cone flow are younger than the scoria mound and southwestern flows, but the relative age of the northern flows and the south-cone flow is unknown. The relative age of the scoria-mound flows and the southwestern flows is also unknown.
The Red Cone complex is composed of a large cone (Red Cone), 13 scoria mounds, and basalt flows that cover about 3 km2.
Red Cone proper is a highly eroded block and scoria cone built primarily of blocks of lava that are locally welded together and minor amounts of scoria, bombs, and ash. The top of the cone is capped by numerous rootless flows that may have ponded in a summit crater. A dike exposed on the west side of the summit may be the feeder for some of the summit eruptions. Small pods of amphibole-bearing lava crop out on the west flank of Red Cone. These lavas are either intrusive into the cone or are flows that erupted from the western flank of the cone.
Red Cone proper did not serve as a source for any of the major lava flows at the center. Most of the lava flows originated from vents expressed by scoria mounds that crop out south and east of Red Cone. The scoria mounds contain cinder, scoria, ash, bombs, and zones of agglutinated scoria. These units were deposited by Strombolian eruptions and by lava fountaining of the Hawaiian type. Dikes are exposed in the interiors of some of the more highly eroded scoria mounds. The scoria mounds are aligned in three zones: two trend approximately N45E and a third subordinate zone strikes N50W. The two northeast trends may represent two sub-parallel fissures that are approximately 200 m apart. Red Cone lies on the northernmost of these trends. A degraded scoria mound at the intersection of the southernmost northeast-striking fissure and the northwest trend may have been the source for many of the flows that lie southeast of Red Cone. Directly south of this locality, along strike of the fissure, is a linear outcrop of agglutinated scoria and dikes in scoria. This appears to be a zone where lava fountaining occurred.
Basalt flows crop out west, east, and south of Red Cone. They are primarily vesicular to massive blocks and Aa flows that contain varying amounts of olivine phenocrysts. The flows were divided into three units based on spatial relationships and stratigraphy: Qbl - a stack of flows that erupted from scoria mounds southwest of Red Cone; Qb2 - flows that erupted from the mounds south and southeast of Red Cone; and Qb3 - a small flow that erupted from the base of Red Cone. Using topographic benches as indicators of flow fronts, at least three pulses of lava formed the Qb1 stack of flows. Contacts between flows are usually covered by blocky talus making it difficult to observe soil horizons between flows. The southward topographic gradient of the floor of Crater Flat may explain the southerly flow of lava and the lack of flows to the north of Red Cone. Stratigraphy of the flow units is ambiguous because of poor exposure of the contacts; however, Qb3 flowed toward the north because of the volcanic barrier produced by the eruption of Qb2, so we feel that Qb3 represents the latest flow unit.
The Crater Flat Quadrangle Map
A geologic map of the Crater Flat Quadrangle was completed as a joint effort between CVTS and the Nevada Bureau of Mines and Geology. Bedrock geology was completed by Jim Faulds and Dan Feuerbach and surficial geology by John Bell and Allan Ramelli. The map includes the detailed geology of Red and Black Cones, the Quaternary geology of Crater Flat, and bedrock structure and lithology on Yucca Mountain. The major structural features in the map area are: (1) several narrow, generally east-tilted fault blocks; (2) closely spaced, moderately to steeply dipping, north- to northeast-striking normal faults that bound the fault blocks; (3) the Crater Flat basin; and (4) the east dipping Bare Mountain fault. Most of the closely spaced normal faults dip moderately to steeply westward. The east-tilted fault blocks and west-dipping normal faults give way to west-tilted fault blocks and east-dipping normal faults westward and southward across the map area. The boundary between east- and west-tilted fault blocks is essentially a large open anticline, which can be described as an interference accommodation zone. Several of the faults mapped within the Miocene bedrock also cut Quaternary alluvial units within Crater Flat (e.g., Windy Wash fault) (Faulds and others, 1994). The authors proposed that "the Crater Flat basin essentially represents a large synform produced by displacement maxima and associated displacement gradients along the east-dipping Bare Mountain fault and the system of west-dipping normal faults flanking Yucca Mountain."
Geochemistry by Bradshaw of Red Cone and Black Cone
Bradshaw used mainly trace elements to develop a petrogenetic history of Red Cone and Black Cone in Crater Flat. The following paragraph is taken from Bradshaw and Smith (1994).
"Alkali basalts erupted during the Quaternary at Crater Flat, Nevada, record a complex history of polycyclic and polygenetic volcanism. Magmas from the two main centers (Black Cone and Red Cone) are petrographicaily and geochemically similar, although field evidence suggests a number of separate eruptive events. High incompatible element concentrations, low Nb/La, and high Zr/Y indicate that the magmas were derived by small degrees of partial melting from the lithospheric mantle. At Red Cone, a significant range of Sr, La, Ce, Ba, and Th concentrations is observed with time (e.g., Sr range 1308-1848 ppm), the youngest samples having the more elevated values. However, there is only limited variation in the compatible trace elements (e.g., Sc and Ni). The array of compositions at Red Cone could not have been produced by changes in the degree of partial melting or by fractional crystallization. Rather, a model of magma mixing is proposed between relatively enriched and depleted end-members. The cluster of Black Cone data falls consistently at the least-enriched end of the Red Cone sample arrays, suggesting that the Black Cone magma represents one of the mixing end-members. The modeling indicates that the magmatic plumbing systems of the two centers were linked, at least during the early stages of volcanism. Moreover, volcanic activity may have occurred at a number of sites along the length of the magmatic feeder zone during a single eruptive phase. This could have significant implications for volcanic hazard assessment in the region around Yucca Mountain and the proposed nuclear waste repository."
Unlike other Quaternary volcanic centers in Crater Flat, the Northern Cone center does not contain a cinder cone. Comprised of Aa flows and scoria, the center forms a low rise with no more than 10 m of topographic relief. A vent area consisting of agglutinated scoria and ribbon bombs is at the northern tip of the volcanic complex. Pyroclastic material is interbedded with toe-like Aa flows. Pyroclastic material also occurs along the eastern margin of the volcanic complex and in a north-south wash in the central part of Northern Cone. Stacked Aa flows form the western margin of the complex. The field work suggests that Northern Cone may be comprised of one or two vent areas with flows extending to the south and southwest.
Geochemistry of the Crater Flat Chain (Shirley Morikawa)
Two geochemically distinct suites comprise the Quaternary volcanic rocks in the vicinity of Crater Flat. These are: basalts at Northern Cone, Little Cones, Sleeping Butte, and Lathrop Wells; and basalts at Black Cone and Red Cone. Geochemical modeling suggests that the two suites are related by the mixing of three chemically distinct magma end-members that were repeatedly tapped at different vents throughout the formation of the field. These end-members are represented by ( 1 ) black cone basalt, (2) trace-element enriched Red Cone basalt, and (3) basalt from the northeastern Little Cone center. Such an interpretation is consistent with the geochemical models for Red Cone and Black Cone previously presented in Bradshaw and Smith
(1994) that call upon two-component mixing to explain the chemical variation observed in rocks from these centers.
On a Sr versus La/Y plot, basalts from Crater Flat and vicinity form two intersecting linear trends. Red Cone/Black Cone analyses form a steeply sloping trend of increasing Sr and La/Y; Black Cone analyses cluster at the low Sr and low La/Y end of the trend; and Red Cone samples form a data cluster that points toward higher Sr and La/Y. Little Cones, Northern Cone, Sleeping Butte, and Lathrop Wells basalts define a nearly horizontal data trend that displays slightly decreasing concentrations of Sr with increasing La/Y. The low Sr end of this trend formed by Northern Cone basalt intersects the Black Cone end of the Black Cone-Red Cone trend. Basalt from the northeast Little Cones vents defines the high Sr end of this trend. Two samples with high Sr were collected from the southwestern Little Cones vents (samples C9-4-74 and C9-4-48).
Fractional crystallization models were not successful in modeling either the Red Cone-Black Cone or the Little Cones-Northern Cone trends. Development of fractional crystallization models for the removal of olivine, hornblende, plagioclase, and clinopyroxene indicate that none of the fractionation vectors correspond to the data arrays, effectively ruling out fractional crystallization as the process to explain chemical variation. The interpretation is that the two data arrays are due to magma mixing. The Red Cone-Black Cone trend was produced by mixing Black Cone sample C9-1-07 with Red Cone sample C9-2-3 1. Mixing of Little Cones sample C9-4-49 and Black Cone sample C9-1-07 produced the Little Cones-Northern Cone trend. This model implies that the Black Cone magma type is common to Northern Cone, Red Cone, and Black Cone. Little Cones is the only center in Crater Flat that lacks the Black Cone basalt component; however, this magma type may have mixing with it to produce the basalts at Lathrop Wells and Sleeping Buttes. Both the Black Cone and Little Cone end-members appear to be regional in extent. Only the Red Cone type is restricted in area to Crater Flat.
Comparison of Crater Flat Basalts with those of the Caldera Cycle (Shirley Morikawa)
An important question is whether or not the Quaternary basalts of the Yucca Mountain area represent a magma type involved in the previous caldera cycle. Farmer et al. (1991) implied that the 16-9 Ma old Timber Mountain-Oasis Valley trachytes/felsic rocks (TMOV) are cogenetic with both 10.5 Ma and Quaternary basalts. Comparisons of analyses from Farmer et al. (1989; 1991) and Crater Flat basalts/basaltic andesites of Bradshaw and Smith (1994) and unpublished CVTS data collected over the period 1990 to 1996 agree with the observations of Farmer et al. (1991). It further suggests that mafic volcanism at Crater Flat represents the youngest expression of a mafic magma type that has played an important part in the petrogenesis of magmas in this region for the last 15 Ma. An important implication of this observation is that volcanism related to the TMOV did not terminate in the Miocene, but continued throughout the Quaternary and includes volcanism in Crater Flat, at Lathrop Wells, and at Sleeping Buttes. Quaternary volcanism in the Yucca Mountain area, therefore, may represent the latest eruptions of a magma type that has persisted intermittently for nearly 15 Ma.
Plotted on Figure 1 are Nd and Sr isotopic ratios of (1) Quaternary Crater Flat basalts; (2) tuffs, trachytes, and basalts associated with the TMOV caldera system; (3) andesites from the Wahmonie-Salyer volcanic complex; (4) Timber Mountain-Oasis Valley caldera complex trachytes and coeval felsic rocks; (5) basalts of the silicic cycle (terminology of Crowe et al., 1993): Dome Mountain, 10.5 Ma Crater Flat basalts, Black Mountain Caldera basalts; and (6) the older and younger postcaldera basalts (8.8 to 2.87 Ma) (terminology of Crowe et al., 1993): Pahute Mesa, Paiute Ridge, Rocket Wash, Nye Canyon, Thirsty Mesa, 3.7 Ma Crater Flat basalts, and the Buckboard Mesa basaltic andesites. Chemical data on Figure 1 is divisible into three groups: ( 1) a high Nd group formed by three of the older postcaldera basalts (Paiute Ridge, Nye Canyon, and Rocket Wash) and the younger postcaldera basalt from Thirsty Mountain; (2) volcanic rocks of the pre-caldera Wahmonie-Salyer volcanic field (13.9 Ma) (Farmer et al., 1991), early lavas associated with Dome Mountain and Dome Mountain basalts; and (3) tuffs and associated trachytes from the Ammonia Tanks, Rainier Mesa, Tiva Canyon, and Topopah Spring volcanic cycles of the Timber Mountain-Oasis Valley caldera complex (Farmer et al.. 1991). The low 87Sr/86Sr end of group 3 overlaps the data cluster formed by the Quaternary-aged Crater Flat analyses.
The third group containing TMOV silicic and trachytic rocks forms a continuous trend with relatively constant Nd and variable 87Sr/86Sr. Based upon the similarities of isotopic compositions of TMOV rhyolites and 0-10 Ma southern Nevada basalts, Farmer et al. (1991; Figure 5) proposed that the felsic rocks of the TMOV caldera complex were generated by partial melting of upper mantle and/or lower crust. Farmer et al. ( 1991 ) further suggested that the trend of approximately constant Nd and variable 87Sr/86Sr formed by TMOV lavas represents the production of rhyolites from mafic magmas that have incorporated approximately 20-40 wt.% upper crustal wall rocks. CVTS suggests that the common occurrence of the Crater Flat magma type as a component of older magmatic systems in the Yucca Mountain area indicates that this magma type has persisted since the Miocene (at least since 15 Ma) and has contributed to magma generation region wide. Though volumes are low at the Quaternary centers in Crater Flat, at Lathrop Wells and Sleeping Buttes, the basalt magma type represented by these eruptions is linked to older, larger volume eruptions responsible for the caldera cycle. The Quaternary volcanoes represent the last episode of the formation of the TMOV caldera complex.
Figure 1: Epsilon Nd v.s. 87Sr/86Sr plot of volcanic rocks of the Southern Nevada volcanic field. Open squares=Timber Mountain (TMOV) ash-flow tuffs; filled diamonds=Wahmonie-Salyer volcanic complex; filled circles=Red Cone and Black Cone (data cluster at 87Sr/86Sr=0.707 and epsilon Nd of -10); *=Lathrop Wells, X=basalt of Rocket Wash; +=Buckboard Mesa; Y=3.7 Ma flows in Crater Flat; open circles=Sleeping Buttes.
Feuerbach and Naumann spent four days in the field at Buckboard Mesa during the winter of 1990. A northeast-striking lineament in the northern part of Buckboard Mesa is mapped as a single fault on the Geologic Map of Timber Mountain. These studies suggest that this structure is actually a 2 km wide zone composed of en echelon fault segments that strike N10E. This zone cuts the entire mesa and cinder cone (Scrugham Peak). Relationships of offset strata from the Timber Mountain tuff and the lavas at Buckboard Mesa suggest that this fault zone was active before, during, and after eruption of the basaltic andesite 2.8 m.y. ago. Several small plugs intrude the zone northeast and southwest of the main vent at Scrugham Peak. Collectively, these observations indicate that the magmas utilized the fault zone as the primary structure for emplacement. This is significant because it demonstrates that northeast-striking high-angle faults provided the dominant structural control for the emplacement of magmas at Buckboard Mesa, the same style of structural control as at other volcanic centers in the NTS area (Sleeping Butte, Crater Flat, Lathrop Wells).
Date of the Solitario Canyon Dike
A groundmass feldspar concentrate K-Ar date on a dike in Trench 10 along the Solitario Canyon fault was completed by the Geochronology Laboratory, University of Arizona. The basalt dike is 11.66± 0.27 m.y. old.
AMRV AND AVIP
CVTS used a two-step method to do probabilistic volcanic hazard analysis (PVHA) at Yucca Mountain. First, areas of higher volcanic risk by geological and geochemical studies were established and then PVHA models were developed that were tightly constrained by geological data. The proposed area of most recent volcanism (AMRV) includes all known post- 6 Ma volcanoes in the Yucca Mountain area and encompasses the four volcanic centers in Crater Flat, the Lathrop Wells cone, two volcanic centers at Sleeping Butte, and a center at Buckboard Mesa. There is some controversy as to whether Buckboard Mesa should be considered in PVHA models. Arguments against including Buckboard Mesa include its higher SiO2 content and its location within the Timber Mountain Caldera. Studies indicate that the higher SiO2 values are due to the presence of numerous quartz xenocrysts. In fact, Buckboard Mesa lavas are very similar to Crater Flat basalts in incompatible element ratios and have nearly identical Sr and Nd isotopic values. The work also suggests that magmas used any available shallow crustal structure to rise to the surface. Therefore, the nature of the structure control can not be used to eliminate a volcano from the AMRV. CVTS concluded that Buckboard Mesa is an integral part of the AMRV and must be included in any PVHA model.
High "risk" zones were identified about the youngest volcanoes in the AMRV. The dimensions and orientations of high "risk" zones were based on structural and volcanic chain length data obtained by detailed studies of bedrock geology and of Plio-Pleistocene centers at Yucca Mountain and in selected analog areas. The AMRV "risk" zone concept is also based on the assumptions that some of the cinder cones near Yucca Mountain are polygenetic and possibly polycyclic and that northeast-striking structures control the location of volcanoes. Analog studies provide important information on the nature of structural control of volcanism, the length of volcanic chains, and the geometry of conduits. A good analog should be Plio-Pleistocene or Quaternary in age, in the tectonic setting of Basin and Range, deeply eroded to expose vents, extinct, and similar in volume to Crater Flat-Lathrop Wells (individual centers and the entire field). The Fortification Hill field, Arizona; the Reveille Range, Nevada; Hurricane field, Utah; and the Sunset Crater chain, Arizona were used as analog areas.
Pliocene and younger basalts in the Yucca Mountain area including Crater Flat, Lathrop Wells, Buckboard Mesa, and the Sleeping Butte cones define a regionally distinctive isotopic end member with 87Sr/86Sr ~0.707 and Nd = -8.5 to -11.9. The Yucca Mountain area data forms a relatively tight cluster with a total range in Nd isotopes of less that 4 epsilon units. This cluster contrasts with data arrays for all adjacent volcanic fields that are isotopically less enriched (higher Nd and lower 87Sr/86Sr) and more diverse (range in Nd isotopes of 6 to 14 epsilon units). Isotopic data for adjacent volcanic fields also generally show a strong negative correlation between Nd and Sr isotopes that is not evident in the data cluster for the Yucca Mountain area.
Only Pliocene and younger basalts of the Death Valley (Black and Greenwater Mountains) have Nd and Sr isotopic compositions that fall largely or entirely within the Yucca Mountain area data cluster. The southeastern Death Valley and Yucca Mountain areas form a continuous isotopic province centered on the Amargosa Valley. This area is termed the Amargosa Valley Isotopic Province (AVIP).
The AVIP is elongated north-south, stretching from Buckboard Mesa and Sleeping Butte on the north to the southeastern end of Death Valley on the south. The shape of the isotopic province suggests north-south control on volcanism, but the southern half of the area is cut by northwest- striking strike-slip faults (Furnace Creek and Walker Lane). Time-space patterns of volcanism within the AVIP are not well defined, although most of the activity in the southern part of the AVIP appears to be 4 to 8 Ma, and activity in the Crater Flat and to the north is generally less than 4 Ma.
The boundary around the AVIP encompasses the magmatic system that has produced Pliocene and younger basalts in the Yucca Mountain area. This natural boundary should be considered in PVHA models at Yucca Mountain.