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The northeastern San Juan Mountains, the least studied portion of this well-known segment of the Southern Rocky Mountains Volcanic Field are the site of several newly identified and reinterpreted ignimbrite calderas. These calderas document some unique eruptive features not described before from large volcanic systems elsewhere, as based on recent mapping, petrologic data, and a large array of newly determined high-precision, laser-fusion [sup.40]Ar/[sup.39]Ar ages (140 samples). Tightly grouped sanidine ages document exceptionally brief durations of 50-100 k.y. or less for individual Oligocene caldera cycles; biotite ages are more variable and commonly as much as several hundred k.y. older than sanidine from the same volcanic unit. A previously unknown ignimbrite caldera at North Pass, along the Continental Divide in the Cochetopa Hills, was the source of the newly distinguished 32.25-Ma Saguache Creek Tuff (~400-500 [km.sup.3]). This regionally distinctive crystal-poor alkalic rhyolite helps fill an apparent gap in the southwestward migration from older explosive activity, from calderas along the N-S Sawatch locus in central Colorado (youngest, Bonanza Tuff at 33.2 Ma), to the culmination of Tertiary volcanism in the San Juan region, where large-volume ignimbrite eruptions started at ca. 29.5 Ma and peaked with the enormous Fish Canyon Tuff (5000 [km.sup.3]) at 28.0 Ma. The entire North Pass cycle, including caldera-forming Saguache Creek Tuff, thick caldera-filling lavas, and a smaller volume late tuff sheet, is tightly bracketed at 32.25-32.17 Ma. No large ignimbrites were erupted in the interval 32-29 Ma, but a previously unmapped cluster of dacite-rhyolite lava flows and small tufts, areally associated with a newly recognized intermediate-composition intrusion 5 x 10 km across (largest subvolcanic intrusion in San Juan region) centered 15 km north of the North Pass caldera, marks a near-caldera-size silicic system active at 29.8 Ma. In contrast to the completely filled North Pass caldera that has little surviving topographic expression, no voluminous tufts vented directly from the adjacent Cochetopa Park caldera, which is morphologically beautifully preserved. Instead, Cochetopa Park subsided passively as the >500 [km.sup.3] Nelson Mountain Tuff vented at 26.9 Ma from an 'underfit' caldera (youngest of the San Luis complex) 30 km to the SW. Three separate regional ignimbrites were erupted sequentially from San Luis calderas within an interval of less than 50-100 k.y., a more rapid recurrence rate for large explosive eruptions than previously documented elsewhere. In eruptive processes, volcanic compositions, areal extent, duration of activity, and magmatic production rates and volumes, the Southern Rocky Mountains Volcanic Field represents present-day erosional remnants of a composite volcanic field, comparable to younger ignimbrite terranes of the Central Andes. Keywords: volcanism, ignimbrites, calderas, [sup.40]Ar/[sup.39]Ar ages, San Juan Mountains, eruptive processes.

A site on the shoulder of the ice-filled summit crater of Mount Moulton, located at an elevation of 2800 m in West Antarctica, exhibits a 400-m-long section of exposed blue ice and intercalated tephra layers. A total of 48 tephra layers are observed. Many are thick (up to 10 cm) and coarse (pumice up to 3 cm). Detailed dip measurements and global positioning system (GPS) mapping reveals a simple geometry of parallel tephra layers all exhibiting consistent dips. The simple stratigraphy of the tephra layers suggests that although the local ice is thinned, based on the boudinaged appearance of the thicker tephra layers, it is otherwise undeformed. Local measured ice motion and ablation rates are similar, averaging around 3 cm/yr for the 4-yr measurement period. Most of the Mount Moulton tephra layers are trachytic, and are derived from Mount Berlin, a still thermally active volcano located ~30 km away to the west. Eight of the tephra layers at the Mount Moulton site have been directly dated, using [sup.40]Ar/[sup.39]Ar geochronology of potassic feldspar phenocrysts, to (in stratigraphic order from top to base of the blue ice section) 10.5 [+ or -] 2.5; 24.7 [+ or -] 1.5; 92.1 [+ or -] 0.9; 104.9 [+ or -] 0.6; 118.1 [+ or -] 1.3; 135.6 [+ or -] 0.9; 225.7 [+ or -] 11.6; and 495.6 [+ or -] 9.7 ka. The depth-age curve for the Mount Moulton site, based on these radio-isotopic ages, is qualitatively similar to that of the Siple Dome ice core, also in West Antarctica. Although not part of the West Antarctic ice sheet, the Mount Moulton site certainly contains some of the oldest known ice in West Antarctica, and provides a long and detailed climate record. Keywords: Antarctica, tephrochronology, geochemistry, [sup.40]Ar/[sup.39]Ar, Marie Byrd Land, Mount Berlin volcano.

Great thicknesses of eolian dune deposits of early Oligocene age crop out in the Chuska Mountains of northwestern New Mexico-Arizona (as much as 535 m thick) and in the Mogollon-Datil volcanic field of western New Mexico-Arizona (as much as 300 m thick). [sup.40]Ar/[sup.39]Ar ages of intercalated volcanic rocks indicate eolian deposition in these areas was approximately synchronous, with eolian accumulation beginning regionally at ca. 33.5 Ma and ending at ca. 27 Ma. Probable eolian sandstone of Oligocene age 483 m thick is also present in the subsurface of the Albuquerque Basin of the Rio Grande rift. The beginning of eolian deposition on the Colorado Plateau corresponds closely to the beginning of eolian (loessic) deposition in the White River Group of the Great Plains and major Oi-1 glaciation in Antarctica, suggesting possible global paleoclimatic control. Successions of Oligocene eolian sandstone on the Colorado Plateau are thicker than all of the better known Upper Paleozoic-Mesozoic eolianites in the region, except the Jurassic Navajo Sandstone. We suggest that the widely separated Oligocene eolianites in the Colorado Plateau region were probably originally continuous, and thus are erosional remnants of an extensive (~140,000 [km.sup.2]), regional sand sea (the Chuska erg). This interpretation is based on: (1) comparison with thickness trends of older eolianites in the Colorado Plateau region, (2) evaluation of regional topographic gradients of modern ergs, and (3) hydrologic modeling of a 300- to 400-m-thick zone of saturation that existed during eolian deposition in the Chuska Mountains. The Chuska erg represents the final episode of Paleogene aggradation on the central and southern Colorado Plateau. Aggradation was driven primarily by trapping of fluvial sediments on the plateau by development of major volcanic fields along the eastern plateau margin. These volcanic fields blocked earlier Laramide drainages that had previously transported sediments eastward off the plateau. Following a shift to widespread eolian deposition at ca. 33.5 Ma, constructional volcanic topography induced eolian accumulation upwind of developing volcanic fields. Stratal accumulation rates (not decompacted) of eolian deposits were ~28-82 m/m.y. The reconstructed top of the Chuska erg would lie at a present-day elevation of ~3000 m or more, and provides a datum for assessing subsequent erosion on the Colorado Plateau. Major exhumation ([greater than or equal to] 1230 m) occurred during the late Oligocene and early Miocene, following the end of Chuska deposition and prior to the onset of Bidahochi Formation deposition at ca. 16 Ma on the south-central part of the plateau. The Bidahochi Formation attained a thickness of ~250 m by ca. 6 Ma, followed by ~520 m of late Miocene and younger erosion in the valley of the Little Colorado River. The depth of late Oligocene-early Miocene (ca. 26-16 Ma) exhumation of the central and southern Colorado Plateau thus was more than twice that of the late Miocene-Holocene (ca. 6-0 Ma). The timing of initial deep erosion in the Colorado Plateau-Southern Rocky Mountains region suggests the beginning of major epeirogenic rock uplift occurred during post-Laramide magmatism. Keywords: eolian, erg, paleoclimate, Colorado Plateau, Oligocene, epeirogeny, exhumation.

[1] Oligocene-Miocene strata preserved in synclinal outcrop belts of the western Alborz Mountains record the onset of Arabia-Eurasia collision-related deformation in northern Iran. Two stratigraphic intervals, informally named the Gand Ab and Narijan units, represent a former basin system that existed in the Alborz. The Gand Ab unit is composed of marine lagoonal mudstones, fluvial and alluvial-fan clastic rocks, fossiliferous Rupelian to Burdigalian marine carbonates, and basalt flows yielding [sup.40]Ar/[39.sup.Ar] ages of 32.7 [+ or -] 0.3 and 32.9 [+ or -] 0.2 Ma. The Gand Ab unit is correlated with the Oligocene-lower Miocene Qom Formation of central Iran and is considered a product of thermal subsidence following Eocene extension. The Narijan unit unconformably overlies the Gand Ab unit and is composed of fluvial-lacustrine and alluvial fan sediments exhibiting contractional growth strata. We correlate the Narijan unit with the middle to upper Miocene Upper Red Formation of central Iran on the basis of lithofacies similarities, stratigraphic position, and an 8.74 [+ or -] 0.15 Ma microdiorite dike ([sup.40]Ar/[39.sup.Ar]) that intruded the basal strata. Deformation timing is constrained by crosscutting relationships and independent thermochronological data. The Parachan thrust system along the eastern edge of the ancestral Taleghan-Alamut basin is cut by dikes dated at 8.74 [+ or -] 0.15 Ma to 6.68 [+ or -] 0.07 Ma ([sup.40]Ar/[39.sup.Ar]). Subhorizontal gravels that unconformably overlie tightly folded and faulted Narijan strata are capped by 2.86 [+ or -] 0.83 Ma ([sup.40]Ar/[39.sup.Ar]) andesitic lava flows. These relationships suggest that Alborz deformation had migrated southward into the Taleghan-Alamut basin by late Miocene time and shifted to its present location along the active range front by late Pliocene time. Data presented here demonstrate that shortening in the western Alborz Mountains had started by late middle Miocene time. This estimate is consistent with recent thermochronological results that place the onset of rapid exhumation in the western Alborz at ~12 Ma. Moreover, nearly synchronous Miocene contraction in the Alborz, Zagros Mountains, Turkish-Iranian plateau, and Anatolia suggests that the Arabia-Eurasia collision affected a large region simultaneously, without a systematic outward progression of mountain building away from the collision zone. doi: 10.1029/2006TC002091.

The Snake River Plain (SRP) developed over the last 16 Ma as a bimodal volcanic province in response to the southwest movement of the North American plate over a fixed melting anomaly. Volcanism along the SRP is dominated by eruptions of explosive high-silica rhyolites and represents some of the largest eruptions known. Basaltic eruptions represent the final stages of volcanism, forming a thin cap above voluminous rhyolitic deposits. Volcanism progressed, generally from west to east, along the plain episodically in successive volcanic fields comprised of nested caldera complexes with major caldera-forming eruptions within a particular field separated by ca. 0.5-1 Ma, similar to, and in continuation with, the present-day Yellowstone Plateau volcanic field. Passage of the North American plate over the melting anomaly at a particular point in time and space was accompanied by uplift, regional tectonism, massive explosive eruptions, and caldera subsidence, and followed by basaltic volcanism and general subsidence. The Heise volcanic field in the eastern SRP, Idaho, represents an adjacent and slightly older field immediately to the southwest of the Yellowstone Plateau volcanic field. Five large-volume (>0.5 [km.sup.3]) rhyolitic ignimbrites constitute a time-stratigraphic framework of late Miocene to early Pliocene volcanism for the study region. Field relations and high-precision [sup.40]Ar/[sup.39]Ar age determinations establish that four of these regional ignimbrites were erupted from the Heise volcanic field and form the framework of the Heise Group. These are the Blacktail Creek Tuff (6.62 [+ or -] 0.03 Ma), Walcott Tuff (6.27 [+ or -] 0.04 Ma), Conant Creek Tuff (5.51 [+ or -] 0.13 Ma), and Kilgore Tuff (4.45 [+ or -] 0.05 Ma; all errors reported at [+ or -] 2[sigma]). The fifth widespread ignimbrite in the region is the Arbon Valley Tuff Member of the Starlight Formation (10.21 [+ or -] 0.03 Ma), which erupted from a caldera source outside of the Heise volcanic field. These results establish the Conant Creek Tuff as a distinct and widespread ignimbrite in the Heise volcanic field, eliminating former confusion resulting from previous discordant K/Ar and fission-track dates. New [sup.40]Ar/[sup.39]Ar determinations, when combined with geochemical, lithologic, geophysical, and field data, define the volcanic and tectonic history of the Heise volcanic field and surrounding areas. Volcanic units erupted from the Heise volcanic field also provide temporal control for tectonic events associated with late Cenozoic extension in the Snake Range and with uplift of the Teton Range, Wyoming. In the Snake Range, movement of large ([greater than or equal to] 0.10 [km.sup.3]) slide blocks of Mississippian limestone exposed 50 km to the east of the Heise field occurred between 6.3 and 5.5 Ma and may have been catastrophically triggered by the caldera eruption of the 5.51 [+ or -] 0.13-Ma Conant Creek Tuff, This slide block movement of ~300 vertical meters indicates that the Snake Range had significant relief by at least 5.5 Ma. in Jackson Hole, the distribution of outflow facies of the 4.45 [+ or -] 0.05-Ma Kilgore Tuff related to eruption from the Kilgore caldera in the Heise volcanic field on the eastern SRP indicates that the northern Teton Range was not a significant topographic feature at this time. Keywords: eastern Snake River Plain, Heise volcanic field, uplift of northern Teton Range, Snake Range detachment, Arbon Valley Tuff, Heise Group.

Author(s): Sileshi Semaw (corresponding author) [1]; Scott W. Simpson [2, 3]; Jay Quade [4]; Paul R. Renne [5, 6]; Robert F. Butler [4]; William C. McIntosh [7, 8]; Naomi Levin [...]

The Silver Peak extensional complex, located in the Silver Peak Range of west-central Nevada, is a displacement-transfer system linking the Furnace Creek-Fish Lake Valley fault system and transcurrent faults of the central Walker Lane. Late Neogene, northwest-directed motion of an upper plate, composed of lower Paleozoic sedimentary rocks and late Tertiary volcanic and volcaniclastic strata, exhumed a lower-plate assemblage of metamorphic tectonites with Proterozoic and Mesozoic protoliths. Paleomagnetic investigation of Miocene-Pliocene pyroclastic and sedimentary rocks of the upper plate and Miocene mafic dikes in the lower plate reveals modest horizontal-axis tilting (northwest-side-up) and vertical-axis rotation (clockwise) within the extensional complex. Eight to ten samples from each of 123 sites were demagnetized; 95 sites yielded interpretable results. Dual-polarity results from one population of mafic dikes in the lower-plate assemblage indicate moderate, northwest-side-up tilting (declination D = 329[degrees], inclination I = 37[degrees], [[alpha].sub.95] = 4.3[degrees] number N = 30 sites; in situ) ([[alpha].sub.95] = the confidence limit for the calculated mean direction expressed as an angular radius from the calculated mean direction). Some dikes yield exclusively normal-polarity results that are interpreted to indicate modest clockwise vertical-axis rotation (D = 021[degrees], I = 57[degrees], [[alpha].sub.95] = 4.3[degrees], N = 19 sites; in situ) concurrent with uplift of the lower-plate rocks, and nine sites yield magnetization directions that are north-directed with positive inclinations of moderate steepness, similar to an expected Miocene field. Late Miocene pyroclastic rocks in the upper plate yield normal-polarity magnetizations suggestive of moderate, clockwise, vertical-axis rotation (D = 032[degrees], I = 53[degrees], [[alpha].sub.95] = 8.8[degrees], N = 10 sites). The apparent clockwise rotation is unlikely to result from incomplete sampling of the geomagnetic field, because the overall dispersion of the VGP (virtual geomagnetic pole) positions is high for the latitude of the site location. Middle Miocene sedimentary rocks probably were remagnetized shortly after deposition. Of eight [sup.40]Ar/[sup.39]Ar determinations from mafic dikes in the lower plate, five groundmass concentrates yield saddle-shaped age spectra, and one separate provided a plateau date of low confidence. Isochron analysis reveals that all six groundmass concentrates contain excess Ar. If rapid cooling and Ar retention below ~250[degrees] C are assumed, the preferred age estimate for mafic intrusions is provided by isochron dates and suggests emplacement between 12 and 10.5 Ma. The [sup.40]Ar/ [sup.39]Ar age-spectrum data are consistent with existing fission-track cooling and K-Ar isotopic age information from lower-plate granitic rocks and indicate rapid cooling of the lower-plate assemblage from well above 300 [degrees] C to 100[degrees] C between 13 and 5 Ma. Rapid cooling may explain the overall distribution of paleomagnetic results from lower-plate intrusions such that the earliest acquired magnetizations reflect both northwest-side-up tilt and clockwise rotation and the younger magnetizations reflect northwest-side-up tilt. Overall, the paleomagnetic data from the Silver Peak extensional complex are interpreted to suggest that vertical-axis rotation of crustal-scale blocks, associated with displacement transfer in the central Walker Lane, may play an integral part in accommodating strain within a continental displacement-transfer system. Keywords: ash-flow tuffs, extensional tectonics, geochronology, metamorphic core complexes, paleomagnetism, Walker Lane.

Author(s): Tim R. Naish (corresponding author) [1]; Ken J. Woolfe [2, 3]; Peter J. Barrett [4]; Gary S. Wilson [5]; Cliff Atkins [4]; Steven M. Bohaty [6]; Christian J. Bücker [...]

Sanidine 40Ar/39Ar ages of lapilli tuffs and the mammalian fauna of Pipestone Springs strata provide a high-resolution chronostratigraphy for upper Eocene (Priabonian) rock units in southwestern Montana. Two felsic lapilli tuffs with weighted-mean 40Ar/39Ar single crystal sanidine ages of 37.50±0.02 Ma and 36.00±0.20 Ma both fall within the Priabonian, late Eocene. These tuffs occur within the basal to upper part of the 55 m of exposed Pipestone Springs strata. The uppermost 15 m yield a diverse and abundant assemblage of mostly small-bodied middle Chadronian (Priabonian, late Eocene) mammals. The older lapilli tuff is an ashfall tuff, whereas the younger lapilli tuff exhibits minor aeolian reworking. The new 40Ar/39Ar age constraints significantly increase the age range of Pipestone Springs strata to include uppermost Duchesnean-lowermost Chadronian (Priabonian, upper Eocene) deposits in addition to its well-known middle Chadronian vertebrate assemblage. These new 40Ar/39Ar ages combined with its mammalian fauna further support Pipestone Springs strata as age-correlative to the Flagstaff Rim section in central Wyoming, and provide a basis for better determining late Eocene mammalian paleogeography and regional paleolandscapes in the United States Rocky Mountain to Great Plains areas. Loessites intercalated with paleosols dominate Pipestone Springs deposits. The recognition of loessites comprising these strata is a new depositional interpretation of Pipestone Springs strata, making these loessites some of the oldest known aeolian Eocene strata in the Great Plains-Rocky Mountains region. Pipestone Springs paleosols developed on lapilli tuffs are vertisols. Alfisols and inceptisols, developed from a parent material of volcanic glass mixed with non-volcanic grains, are the remaining paleosols within the loessite strata. Additionally, a new and important discovery in this project is the recognition that all paleosols are extensively bioturbated, containing trace fossils similar to Rebuffoichnus and newly identified trace fossils resembling Feoichnus, Eatonichnus, Fictovichnus, and Coprinisphaera. [ABSTRACT FROM AUTHOR]

During systematic sampling of volcanic ash (tephra) layers at a well-known Antarctic meteorite collection site (the Allan Hills main ice field), a band of unusually dark and rounded (many spheroidal) particles was discovered. This debris layer (BIT-58) extends parallel to the stratigraphy of the ice established from the tephra bands, apparently marking a single depositional event. The shapes, internal texture, major element composition, and levels of cosmogenic nuclides of particles from within BIT-58 all strongly suggest that this material represents ablation debris from the passage of a large H-group ordinary chondrite. Preliminary cosmogenic isotope dating suggests an age of 2.8 Ma, implying that the East Antarctic ice sheet has been stable since that time. The relationship of the Bit-58 layer to known impact events is not clear.

Mid-Tertiary strata near Gray Butte, central Oregon, record volcanism and sedimentation on the margin of a west-tilted basin that was subsequently disrupted by a northeast-striking fault system. Compositional characteristics of the volcanic rocks support correlation of the section to the upper Eocene-Oligocene part of the John Day Formation. The [approximately]1.2-km-thick section contains five fossil floras documenting climatic change in late Eocene-early Oligocene time and a progression between better known Eocene and Oligocene floras in the region. The presence of the transitional floras is a consequence of the subsidence of the Gray Butte basin to accommodate a section that is nearly four times thicker than better studied correlative strata [approximately]50 km to the northeast that consist almost entirely of volcanic rocks. The lowest flora is within flood-plain facies, whereas the other four are hosted in lacustrine strata. Although alteration of volcanic rocks most closely associated with the floras precludes development of a precise isotopic-age chronology, regional correlations and several accurate isotopic-age determinations indicate that the principal interval of climatic cooling may have been in late Eocene time (ca. 38-39 Ma) rather than at the Eocene-Oligocene boundary. The paleoclimate interpretation is tempered, however, by the low diversity of the floras (12-24 species) and possible taphonomic biases in comparing flood-plain and lacustrine environments. Mapping established the presence of the Cyrus Springs fault zone, a large (1.2 km vertical displacement, possibly >7 km dextral offset) shear zone that is possibly a surface expression of the Klamath-Blue Mountain gravity-anomaly lineament. The orientations of this fault zone, subsidiary sinistral structures, dikes, and fold axes suggest that the presumed Mesozoic structure marked by the lineament was reactivated as a dextral normal fault by east-northeast-west-southwest-oriented compressive stress. This stress is consistent with early Oligocene North America-Farallon convergence but is inconsistent with northwest-southeast to north-south compression suggested by structures farther east. Stress either varied temporally or was partitioned into complex local strain domains. The Cyrus Springs fault zone may have become active at about 28-30 Ma, resulting in uplift and southward tilting of part of the Gray Butte basin fill, and ceased activity before deposition of the horizontal upper Miocene-Pliocene Deschutes Formation. The Gray Butte area was also an eruptive center for rhyolitic and alkaline-mafic lava and tuff both before and after initiation of movement on the Cyrus Springs fault. Mid-Tertiary volcanism and sedimentation near the western end of the Blue Mountains, heretofore not clearly related to active structures, may have taken place within a regional transtensional regime associated with stress orientations different from those of Neogene time. Local basins with higher subsidence rates accumulated relatively thick sequences of lacustrine tuffaceous strata, and their fossil floras show progressive climate change through the Eocene-Oligocene boundary interval.

[sup.40]Ar/[sup.39]Ar dating of fine-grained alunite that formed during cave genesis provides ages of formation for the Big Room level of Carlsbad Cavern [4.0 to 3.9 million years ago (Ma)], the upper level of Lechuguilla Cave (6.0 to 5.7 Ma), and three other hypogene caves (11.3 to 6.0 Ma) in the Guadalupe Mountains of New Mexico. Alunite ages increase and are strongly correlative with cave elevations, which indicates an 1100-meter decline in the water table, apparently related to tectonic uplift and tilting, from 11.3 Ma to the present. [sup.40]Ar/[sup.39]Ar dating studies of the hypogene caves have the potential to help resolve late Cenozoic climatic, speleologic, and tectonic questions., Carlsbad Cavern and Lechuguilla Cave are world renowned for their size, geology, and mineral decorations. These and other related caves are located in the Permian Capitan Limestone, Goat Seep Dolomite, [...]