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A new category of large-scale volcanism, here termed Snake River (SR)-type volcanism, is defined with reference to a distinctive volcanic facies association displayed by Miocene rocks in the central Snake River Plain area of southern Idaho and northern Nevada, USA. The facies association contrasts with those typical of silicic volcanism elsewhere and records unusual, voluminous and particularly environmentally devastating styles of eruption that remain poorly understood. It includes: (1) largevolume, lithic-poor rhyolitic ignimbrites with scarce pumice lapilli; (2) extensive, parallel-laminated, medium to coarsegrained ashfall deposits with large cuspate shards, crystals and a paucity of pumice lapilli; many are fused to black vitrophyre; (3) unusually extensive, large-volume rhyolite lavas; (4) unusually intense welding, rheomorphism, and widespread development of lava-like facies in the ignimbrites; (5) extensive, fines-rich ash deposits with abundant ash aggregates (pellets and accretionary lapilli); (6) the ashfall layers and ignimbrites contain abundant clasts of dense obsidian and vitrophyre; (7) a bimodal association between the rhyolitic rocks and numerous, coalescing lowprofile basalt lava shields; and (8) widespread evidence of emplacement in lacustrine-alluvial environments, as revealed by intercalated lake sediments, ignimbrite peperites, rhyolitic and basaltic hyaloclastites, basalt pillow-lava deltas, rhyolitic and basaltic phreatomagmatic tuffs, alluvial sands and palaeosols. Many rhyolitic eruptions were high mass-flux, large volume and explosive (VEI 6-8), and involved H 2 O-poor, low-δ 18 O, metaluminous rhyolite magmas with unusually low viscosities, partly due to high magmatic temperatures (900-1,050°C). SR-type volcanism contrasts with silicic volcanism at many other volcanic fields, where the fall deposits are typically Plinian with pumice lapilli, the ignimbrites are low to medium grade (non-welded to eutaxitic) with abundant pumice lapilli or fiamme, and the rhyolite extrusions are small volume silicic domes and coulées. SR-type volcanism seems to have occurred at numerous times in Earth history, because elements of the facies association occur within some other volcanic fields, including Trans-Pecos Texas, EtendekaParaná, Lebombo, the English Lake District, the Proterozoic Keewanawan volcanics of Minnesota and the Yardea Dacite of Australia.

No abstract

We show here that transtensional rifting along the eastern boundary of the Sierra Nevada microplate (Walker Lane rift) began by ca. 12 Ma in the central Sierra Nevada (USA), within the ancestral Cascades arc, triggering voluminous high-K intermediate volcanism (Stanislaus Group). Flood andesite (i.e., unusually large-volume effusive eruptions of intermediate composition) lavas erupted from fault-controlled fi ssures within a series of grabens that we refer to as the Sierra Crest graben-vent system. This graben-vent system includes the following.1. The north-northwest-south-southeast Sierra Crest graben proper consists of a single 28-km-long, 8-10-km-wide full graben that is along the modern Sierra Nevada crest between Sonora Pass and Ebbetts Pass (largely in the Carson-Iceberg Wilderness). This contains fi ssure vents for the high-K intermediate lavas.2. A series of north-northwest-south-southeast half-grabens on the western margin of the full graben, which progressively disrupted an ancient Nevadaplano paleochannel that contains the type section of Stanislaus Group (Red Peak-Bald Peak area). These Miocene half-grabens are as much as 15 km west of the modern Sierra Nevada crest, and vented high-K lavas from point sources.3. Series of northeast-southwest grabens defi ne a major transfer zone along the northeast side of the Sierra Crest graben. These extend as much as ~30 km from the modern range crest down the modern Sierra Nevada range front, in a zone ~30 km wide, and vented high-K lavas and tuffs of the Stanislaus Group from point sources. Rangefront north-south and northeast-southwest faults to the south of that, along the southeast side of the Sierra Crest graben, did not vent volcanic rocks (although they ponded them); those will be described elsewhere.We present evidence for a dextral component of slip on the north-northwest-southsoutheast normal faults, and a sinistral component of slip on the northeast-southwest normal faults. The onset of transtension immediately preceded the high-K volcanism (within the analytical error of 40 Ar/ 39 Ar dates), and triggered the deposition of a debris avalanche deposit with a preserved volume of ~50 km 3 . The grabens are mainly fi lled with high-K lava fl ows, ponded to thicknesses of as much as 400 m; this effusive volcanism culminated in the development of the Little Walker caldera over a relatively small part of the fi eld. Trachydacite outfl ow ignimbrites from the caldera also became ponded in the larger graben-vent complex, where they interfi ngered with high-K lavas vented there, and escaped the graben-vent complex on its west margin to fl ow westward down two paleochannels to the western foothills.The Sierra Crest graben-vent system is spectacularly well exposed at the perfect structural level for viewing the controls of synvolcanic faults on the siting and styles of feeders, vents, and graben fi lls under a transtensional strain regime in an arc volcanic fi eld.

The Miocene Grey's Landing ignimbrite reaches 70 m thick and covers at least 400 km 2 in the central region of the Snake River Plain. It shows particularly intense welding and rheomorphic deformation, and although parts are eutaxitic, most is lava-like with fl ow-banding and no fi amme. A nearubiquitous penetrative fl ow lamination, associated with a well-developed elongation lineation, is folded into small intrafolial tight to isoclinal oblique and sheath folds, which are refolded by larger folds in the upper parts. Structural and kinematic analysis reveals that welding and early deformation occurred rapidly during deposition from a very hot (≤1000 °C), high-mass-fl ux pyroclastic density current that fl owed westward across a graben-faulted landscape. As hot particles were deposited, they rapidly agglutinated and coalesced, and underwent noncoaxial shear in a subhorizontal ductile shear zone close to the current-deposit interface. The shear zone is interpreted to have been less than 2 m thick. It produced and deformed the rheomorphic fabric, and it migrated upward with the rising current-deposit interface during aggradation, so that it transiently affected all levels of the resultant thick ignimbrite. Deformation was progressive, and after the density current had dissipated, viscous spreading and downslope fl ow continued and involved an increasingly thick portion of the sheet. This folded the fl ow banding and F 1 intrafolial isoclines into larger sheath folds, and into more upright periclines near the top of the ignimbrite. We demonstrate that structural and kinematic analysis can elucidate the emplacement history of rheomorphic ignimbrites.

The 80 km long NNE-trending Rogerson

No abstract

The 1 Myr tephra records of IODP (International Ocean Discovery Program) Holes U1436A and U1437B in the Izu‐Bonin fore‐ and reararc were investigated in order to assess provenance and eruptive volumes, respectively. In total, 304 tephra samples were examined and 260 primary tephra layers were identified. Tephra provenance was determined by means of major and trace element compositions of glass shards and distinguished between Japan and Izu‐Bonin arc origin of the tephra layers. A total of 33 marine tephra compositions were correlated to the Japan arc and 227 to the Izu arc. Twenty marine tephra layers were correlated between the two drilling sites. Additionally, we defined eleven correlations of marine tephra deposits to major widespread Japanese eruptions; from the 1.05 Ma Shishimuta‐Pink Tephra to the 30 ka Aira‐Tn Tephra, both from Kyushu Island. These eruptions provide independent time markers within the sediment record and six correlations were used to date tephra layers from Japan in Hole U1436A to establish an alternative age model for this hole. Furthermore, the minimum distal tephra volumes of all detected events were calculated, which enabled the comparison of the tephra volumes that derived from the Japan and the Izu‐Bonin arcs. For some of the major Japanese eruptions these are the first volume estimations that also include distal deposits. All of the Japanese tephras derived from events with eruption magnitude Mv ≥ 5.6 and three of the investigated eruptions reach magnitudes Mv ≥ 7. Volcanic events of the Izu‐Bonin arc have mostly eruption magnitudes Mv ≤ 5.