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Abstract. -- A vitric ash bed on the east bank of West Amarillo Creek, Potter County, of the Texas Panhandle occupies a portion of a paleovalley in the Ogallala Formation [...]

Unaltered silicic ash fall tuffs are abundant in Neogene sedimentary basins of the western U.S. and constitute an important record of explosive silicic volcanism in this region. In particular, ash fall tuffs from silicic volcanic centers along the Yellowstone hotspot track are common in these basins and provide a detailed record of explosive volcanism along the hotspot track. The available hotspot ash fall tuff record commences at ca. 16 Ma, shortly after the initiation of hotspot silicic volcanism at ca. 16.5 Ma, and continues through the most recent explosive eruptions in the late Pleistocene. Post-16 Ma hotspot silicic volcanism has been dominated by eruption of metaluminous ash flow tuffs and rhyolites, and ash fall tuffs produced by these eruptions dominate the Yellowstone hotspot ash fall tuff record. Evaluation of a well-dated composite sequence of 142 of these tuffs reveals 0systematic variation in magma composition, magma temperature, eruption frequency, and, possibly, volumetric discharge as the hotspot migrated eastward from the western edge of the North America craton to its current location in the Yellowstone Plateau. On the basis of these variations, three primary stages of metaluminous rhyolite magmatism (M stages) are recognized: M1 (16.0-15.2 Ma), M2 (15.2-7.5 Ma), and M3 (7.5-0 Ma). Each of these stages is marked by distinctive magma compositions, eruption frequencies, and magma temperature ranges and trends, with an overall decline in average eruption frequency and magma temperature from stage to stage. The partitioning of explosive hotspot volcanism into stages likely reflects variation in the style and intensity of the interaction between the mantle anomaly powering the hotspot magmatic systems and a spatially and temporally heterogeneous lithosphere along the hotspot track. Although the ash fall tuff record does not provide definitive constraints on the nature of these variable interactions, correlations between changes in eruption frequency, ash fall tuff discharge, and magma temperature point to variable input of mantle basalt into hotspot crustal magmatic systems as a first order control on intensity of explosive volcanism. As future studies reveal more about the processes controlling the variations in hotspot silicic volcanism, a fuller understanding of both the nature of silicic magmatism and the nature of the Yellowstone hotspot should emerge. Keywords: ash fall tuff, composition, eruption frequency, explosive volcanism, volumetric discharge, Yellowstone hotspot.

The latest Cenozoic (

A 900-m-thick section of tuffaceous sedimentary rock, vitric fallout tuff, and ash-flow tuff is well exposed along Trapper Creek in south-central Idaho. This section provides nearly continuous exposure through the fill of the Goose Creek basin, a major north-trending Miocene extensional basin located along the southern margin of the Snake River Plain volcanic province (SRPVP). Some 51 separate units of vitric fallout tuff are recognized in the Trapper Creek section. Petrographic and chemical characteristics of these vitric tuffs indicate that most are from SRPVP sources. New 40Ar/39Ar laser-fusion dating, along with prior isotopic age determinations, show that the Trapper Creek tuffs span the period ca. 13.9-8.6 Ma. Chemical correlation indicates that fallout tuffs in the central part of the Trapper Creek section (12.5-10.0 Ma) are from sources in the Bruneau-Jarbidge volcanic field of the SRPVP centered [approximately]100 km west of Trapper Creek. Underlying fallout tuffs may have had sources in the Owyhee-Humboldt field of the SRPVP centered [approximately]200 km west of Trapper Creek, while overlying fallout tuffs, interlayered with several ash-flow tuffs, had a relatively proximal source, possibly in the proposed Twin Falls volcanic field centered [approximately]60 km north of Trapper Creek. The Trapper Creek tuffs provide insight into the characteristics of explosive silicic volcanism within the SRPVP during middle-late Miocene time. From ca. 13.9 to ca. 9.5 Ma, major eruptions (those depositing [greater than or equal to]1.5 m of fallout tuff) were frequent (about one event per 200 k.y.); their products display a trend toward the eruption of progressively less evolved, higher temperature silicic magma after 12.5 Ma. This trend to higher temperature eruptions, termed the Cougar Point 'flare-up,' culminated in the eruption of high-temperature ([approximately]1000 [degrees] C), plagioclase-rich magma during the period 10.5-9.5 Ma. In contrast to these eruptions, later (

Relative and absolute elevations of the Sierra Nevada and adjacent Basin and Range province, timing of their differentiation, and location, amount, and timing of strike-slip movement between them are controversial. The provincial boundary near Reno developed in two stages. (1) At ca. 12 Ma, the [is greater than or equal to] 700 [km.sup.2] Verdi-Boca sedimentary basin formed across what was to become the boundary, probably as a result of a small-magnitude but regional extensional episode that affected much of the western Basin and Range. (2) At 3 Ma, the basin was complexly faulted and folded during a larger magnitude extensional episode that established the modern Sierran structural and topographic boundary in this area. The boundary is really a transition zone with a western edge along the Donner Pass, California, fault zone, which is farther west than previously placed. Both episodes appear to have resulted from east-west extension only, which suggests that northwest motion of the Sierra Nevada relative to the Basin and Range shown by geodetic data began after 3 Ma or was taken up farther east. Keywords: Sierra Nevada, Basin and Range, extension, [sup.40]Ar/[sup.39]Ar, tephrochronology, sedimentary basins.

The extensional history of the Malad and Bannock ranges in southeast Idaho and northern Utah involves multiple phases of Tertiary normal faulting and synextensional deposition. Detailed geologic mapping, structural and stratigraphic analyses, and geochronologic data from this study elucidate previously defined deformational events in the region, and define two new extensional episodes. The largest-magnitude extension took place along low-angle normal faults of the ˜10-4-Ma Bannock detachment system, with concurrent sedimentation of the Miocene-Pliocene Salt Lake Formation in a regionally continuous supradetachment basin. This basin developed by ˜10.2 Ma, and was preceded by two phases of smaller-magnitude, aerially restricted normal faulting and sedimentation. In the Henderson Creek quadrangle of the southern Malad Range, the earliest event involved ˜8% north-south extension during deposition of the Paleocene-Eocene Wasatch (?) Formation. This conglomerate unit was deposited in an asymmetric, southtilted half-graben bounded on the south by the syntectonic Willow Spring normal fault. The next pre-detachment extensional event produced a north-striking, east-tilted half-graben in which the Middle to Late Miocene Skyline Member of the Salt Lake Formation (˜11.9-10.2 Ma) was deposited as an ash-rich alluvial fan. This half-graben is bounded on the east and south by the syntectonic Red Knoll and Spring Trail normal faults. Detrital zircon age data from a tuffaceous sandstone bed in the Skyline Member suggest incorporation of reworked zircons from the ˜12.5-15-Ma Owhyee-Humboldt volcanic field in southwest Idaho with 10.3-Ma glass. The inception of the regional Bannock detachment system is recorded by the breakup and -16% west-southwest extension of its hanging wall by a set of north- to north-northwest-striking Late Miocene normal faults. These faults were associated with syntectonic deposition of the Cache Valley Member of the Salt Lake Formation (˜10.2-<9.2 Ma) in a regional-scale lake system. During deposition, the northeast-dipping Steel Canyon normal fault accommodated uplift of an intrabasinal horst that shed a wedge of Third Creek Member conglomerate eastward into the lake system between ˜10.0 and <9.2 Ma. The Third Creek Member interfingers with a Cache Valley Member lake-margin tufa-bearing facies, which changes eastward into a deeper-water, micritic limestone-bearing facies. The most recent extensional event in the Henderson Creek quadrangle involved ˜9% east-west extension on the north-striking Pliocene-Quaternary Wasatch fault. A segment boundary of the Wasatch fault, consisting of a 2.5-3.5-kin-wide relay ramp that formed between two right-stepping en echelon segments, lies just north of the Idaho-Utah border. In addition, a broad, north- to north-northwest-trending antiformal zone of extensional folds is present in the south half of the study area and is interpreted as a double-rollover anticline that formed progressively in Late Miocene and Pliocene-Quaternary time above two oppositely dipping listric normal faults. [ABSTRACT FROM AUTHOR]