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• 1. [Article] Geology and hydrothermal alteration, Glass Buttes, Southeast Oregon

  The Glass Buttes volcanic complex consists of many domes and individual vents that erupted both rhyolitic and basaltic lavas during the late Miocene to early Pliocene. The east half of the complex, in ...

  Citation × Citation Citation Abstract Copy Title: Geology and hydrothermal alteration, Glass Buttes, Southeast Oregon Author: Berri, Dulcy Annette Year: 1982 The Glass Buttes volcanic complex consists of many domes and individual vents that erupted both rhyolitic and basaltic lavas during the late Miocene to early Pliocene. The east half of the complex, in the vicinity of Little Glass Butte, contains interfingering, finely flow-banded rhyolite and black obsidian flows. The youngest unit, an obsidian, has been dated at 4.9 m.y. East of Little Glass Butte lie two northwest-trending ridges, Antelope and Cascade Ridges, composed of two or more overlapping exogenous domes that formed along northwest-trending faults. Title: Geology and hydrothermal alteration, Glass Buttes, Southeast Oregon Author: Berri, Dulcy Annette Year: 1982 The Glass Buttes volcanic complex consists of many domes and individual vents that erupted both rhyolitic and basaltic lavas during the late Miocene to early Pliocene. The east half of the complex, in the vicinity of Little Glass Butte, contains interfingering, finely flow-banded rhyolite and black obsidian flows. The youngest unit, an obsidian, has been dated at 4.9 m.y. East of Little Glass Butte lie two northwest-trending ridges, Antelope and Cascade Ridges, composed of two or more overlapping exogenous domes that formed along northwest-trending faults. Close

• 2. [Article] Igneous Dikes at Long Valley, CA: Emplacement Mechanisms and Associated Geologic Structures

  Two hypotheses for dike emplacement are: (1) magma flows into and dilates pre-existing fractures; or (2) magma flows into and dilates self-generated fractures. In the first case dikes should be parallel ...

  Citation × Citation Citation Abstract Copy Title: Igneous Dikes at Long Valley, CA: Emplacement Mechanisms and Associated Geologic Structures Author: Pollard, David D., Fink, Jonathan H., Delaney, Paul T. Year: 1984 Two hypotheses for dike emplacement are: (1) magma flows into and dilates pre-existing fractures; or (2) magma flows into and dilates self-generated fractures. In the first case dikes should be parallel to an element of the rock fabric; in the second, they should be perpendicular to the least compressive stress. The two hypotheses suggest different dike intrusion and fissure eruption mechanisms and therefore different strategies for monitoring igneous events at Long Valley. We derive a method to distinguish the two mechanisms, a priori, from in-situ stress measurements and estimates of magma pressure. Estimates of relative dilation and slip across a dike plane from models constrained by surface displacement data provide a method to distinguish the two mechanisms, a posteriori. Joints cluster near dike contacts just as microcracks cluster near laboratory fractures. Such clusters define a process zone of secondary cracking that forms at the tip of a primary crack. For basaltic dikes in sedimentary host rocks of the Colorado Plateaus, the process zone size is about 10 m and the number of joints is in the range 10 to 100. The mechanical energy release rate for propagation of these dikes is estimated to be 10 to 100 times that for a single laboratory fracture. Data from .proposed drill holes through rhyolite dikes under the Inyo Domes will elucidate propagation mechanisms and process zone characteristics. As a dike nears the Earth's surface, two sets of ground cracks open parallel to the dike trend. A "rule of thumb" is that the depth to the dike top is one-third to one-half the spacing between the innermost surface cracks of each set. Surface structures on and near the Inyo Domes suggest a NNE trend for shallow (< several hundred meters) dike segments, but dome alignment suggests a NNW trend for the feeder dikes at depth. Title: Igneous Dikes at Long Valley, CA: Emplacement Mechanisms and Associated Geologic Structures Author: Pollard, David D., Fink, Jonathan H., Delaney, Paul T. Year: 1984 Two hypotheses for dike emplacement are: (1) magma flows into and dilates pre-existing fractures; or (2) magma flows into and dilates self-generated fractures. In the first case dikes should be parallel to an element of the rock fabric; in the second, they should be perpendicular to the least compressive stress. The two hypotheses suggest different dike intrusion and fissure eruption mechanisms and therefore different strategies for monitoring igneous events at Long Valley. We derive a method to distinguish the two mechanisms, a priori, from in-situ stress measurements and estimates of magma pressure. Estimates of relative dilation and slip across a dike plane from models constrained by surface displacement data provide a method to distinguish the two mechanisms, a posteriori. Joints cluster near dike contacts just as microcracks cluster near laboratory fractures. Such clusters define a process zone of secondary cracking that forms at the tip of a primary crack. For basaltic dikes in sedimentary host rocks of the Colorado Plateaus, the process zone size is about 10 m and the number of joints is in the range 10 to 100. The mechanical energy release rate for propagation of these dikes is estimated to be 10 to 100 times that for a single laboratory fracture. Data from .proposed drill holes through rhyolite dikes under the Inyo Domes will elucidate propagation mechanisms and process zone characteristics. As a dike nears the Earth's surface, two sets of ground cracks open parallel to the dike trend. A "rule of thumb" is that the depth to the dike top is one-third to one-half the spacing between the innermost surface cracks of each set. Surface structures on and near the Inyo Domes suggest a NNE trend for shallow (< several hundred meters) dike segments, but dome alignment suggests a NNW trend for the feeder dikes at depth. Close

• 3. [Article] The Mechanism of Intrusion of the Inyo Dike, Long Valley Caldera, California

  We analyze the intrusion of the 11-km-long lnyo Dike at the margins of Long Valley caldera, eastern California. The dike trends N07°W and is divided into at least three segments which are rotated by as ...

  Citation × Citation Citation Abstract Copy Title: The Mechanism of Intrusion of the Inyo Dike, Long Valley Caldera, California Author: Reches, Ze'ev, Fink, Jonathan H. Year: 1988 We analyze the intrusion of the 11-km-long lnyo Dike at the margins of Long Valley caldera, eastern California. The dike trends N07°W and is divided into at least three segments which are rotated by as much as 25° with respect to the main trend. The dike seems affected primarily by the regional stress field of right-lateral shear of the western United States and by the local thermal conditions of the crust; the dike seems unaffected by the preexisting caldera margins and Sierra-Nevada frontal fault system. The high heat flow in Long Valley caldera implies that crustal rocks below 3-4.5 km deform by steady state creep under tectonic strain rate and support low to vanishing tectonic shear stresses. The upper rocks, above 3-4.5 km, deform by frictional slip along fractures and may support tectonic shear stresses as high as 24 MPa. We demonstrate that depth variations of tectonic stresses may have a profound effect on the segmentation and rotation of dikes, both at Long Valley and in other areas of high heat flow. The analys is places constraints on several tectonic conditions. The lnyo Dike intruded under a tectonic stress state with a horizontal maximum compression oriented N07°W. The maximum extensional fracture strength of the host rocks is 1-2.5 MPa, and the pressure drop within the propagating Inyo Dike was about 0.55 MPa/km. The volatile overpressure in the magma chamber was about 15 MPa during eruption of rhyolitic lavas at the Inyo Domes. Title: The Mechanism of Intrusion of the Inyo Dike, Long Valley Caldera, California Author: Reches, Ze'ev, Fink, Jonathan H. Year: 1988 We analyze the intrusion of the 11-km-long lnyo Dike at the margins of Long Valley caldera, eastern California. The dike trends N07°W and is divided into at least three segments which are rotated by as much as 25° with respect to the main trend. The dike seems affected primarily by the regional stress field of right-lateral shear of the western United States and by the local thermal conditions of the crust; the dike seems unaffected by the preexisting caldera margins and Sierra-Nevada frontal fault system. The high heat flow in Long Valley caldera implies that crustal rocks below 3-4.5 km deform by steady state creep under tectonic strain rate and support low to vanishing tectonic shear stresses. The upper rocks, above 3-4.5 km, deform by frictional slip along fractures and may support tectonic shear stresses as high as 24 MPa. We demonstrate that depth variations of tectonic stresses may have a profound effect on the segmentation and rotation of dikes, both at Long Valley and in other areas of high heat flow. The analys is places constraints on several tectonic conditions. The lnyo Dike intruded under a tectonic stress state with a horizontal maximum compression oriented N07°W. The maximum extensional fracture strength of the host rocks is 1-2.5 MPa, and the pressure drop within the propagating Inyo Dike was about 0.55 MPa/km. The volatile overpressure in the magma chamber was about 15 MPa during eruption of rhyolitic lavas at the Inyo Domes. Close

• 4. [Article] Geology of the Breitenbush Hot Springs area, Cascade Range, Oregon

  The Breitenbush Hot Springs area lies on the boundary of folded middle to late Tertiary Western Cascade rocks and younger High Cascade rocks. Within the mapped area the Western Cascade rocks are represented ...

  Citation × Citation Citation Abstract Copy Title: Geology of the Breitenbush Hot Springs area, Cascade Range, Oregon Author: Clayton, Clifford Michael Year: 1976 The Breitenbush Hot Springs area lies on the boundary of folded middle to late Tertiary Western Cascade rocks and younger High Cascade rocks. Within the mapped area the Western Cascade rocks are represented by four formations. The Detroit Beds, a sequence of interstratified tuffaceous sandstone, mudflow breccia, and tuff, is overlain unconformably by the Breitenbush Tuff. The Breitenbush Tuff consists of three units of welded pumice-rich crystal-vitric ash-flow tuffs interbedded with tuffaceous sedimentary rocks. The Outerson Formation unconformably overlies the Breitenbush Tuff and consists primarily of basaltic lava and breccia. The Outerson Formation includes three localized members: a basal, glassy, aphanitic basalt, the Lake Leone Sediments, and the Outerson Tuff. The Outerson Formation is cut by a number of feeder dikes and plugs and is unconformably overlain by the Cheat Creek Sediments, composed of volcanic sedimentary rocks and a distinctive basaltic tuff. The Western Cascade formations total more than 1660 m {5500 ft) of strata and range from Oligocene to Pliocene in age. The High Cascade rocks are represented by two formations: the Triangulation Peak Volcanics of basalt and andesite lava and breccia, lying unconformably atop the Cheat Creek Sediments; and unconformably beneath the Collowash Volcanics, a series of thin basaltic lava flows and breccias. The Western and High Cascade rocks are covered extensively by surficial deposits, primarily glacial drift. The High Cascade formations are at least 840 m (2800 ft) thick, ranging in age from Pliocene to Pliestocene. The Western Cascade rocks have been folded and faulted in the Breitenbush Hot Springs area, and form the eastern limb of the north-trending Breitenbush Anticline. The folded rocks and the erosional unconformities between the rock units probably represent two local episodes of orogeny: one in early to middle Miocene and another in late Pliocene to Pleistocene time. The Outerson Formation represents a depositional sequence between the periods of uplift and deformation. Faulting accompanied the orogenic sequences. The primary volcanic landforms in the area have been destroyed by erosion but skeletal remains of High Cascade volcanoes are recognized. Stream erosion and glaciation are responsible for the present landforms. Breitenbush Hot Springs occurs, in part, along basaltic dikes which channel the water through impermeable Breitenbush Tuff. The dikes are believed to be associated with the Outerson basalts. The Hot Springs discharge upwards at 3400 l/min. (900 gpm) of water at temperatures up to 92°C (198°F). Title: Geology of the Breitenbush Hot Springs area, Cascade Range, Oregon Author: Clayton, Clifford Michael Year: 1976 The Breitenbush Hot Springs area lies on the boundary of folded middle to late Tertiary Western Cascade rocks and younger High Cascade rocks. Within the mapped area the Western Cascade rocks are represented by four formations. The Detroit Beds, a sequence of interstratified tuffaceous sandstone, mudflow breccia, and tuff, is overlain unconformably by the Breitenbush Tuff. The Breitenbush Tuff consists of three units of welded pumice-rich crystal-vitric ash-flow tuffs interbedded with tuffaceous sedimentary rocks. The Outerson Formation unconformably overlies the Breitenbush Tuff and consists primarily of basaltic lava and breccia. The Outerson Formation includes three localized members: a basal, glassy, aphanitic basalt, the Lake Leone Sediments, and the Outerson Tuff. The Outerson Formation is cut by a number of feeder dikes and plugs and is unconformably overlain by the Cheat Creek Sediments, composed of volcanic sedimentary rocks and a distinctive basaltic tuff. The Western Cascade formations total more than 1660 m {5500 ft) of strata and range from Oligocene to Pliocene in age. The High Cascade rocks are represented by two formations: the Triangulation Peak Volcanics of basalt and andesite lava and breccia, lying unconformably atop the Cheat Creek Sediments; and unconformably beneath the Collowash Volcanics, a series of thin basaltic lava flows and breccias. The Western and High Cascade rocks are covered extensively by surficial deposits, primarily glacial drift. The High Cascade formations are at least 840 m (2800 ft) thick, ranging in age from Pliocene to Pliestocene. The Western Cascade rocks have been folded and faulted in the Breitenbush Hot Springs area, and form the eastern limb of the north-trending Breitenbush Anticline. The folded rocks and the erosional unconformities between the rock units probably represent two local episodes of orogeny: one in early to middle Miocene and another in late Pliocene to Pleistocene time. The Outerson Formation represents a depositional sequence between the periods of uplift and deformation. Faulting accompanied the orogenic sequences. The primary volcanic landforms in the area have been destroyed by erosion but skeletal remains of High Cascade volcanoes are recognized. Stream erosion and glaciation are responsible for the present landforms. Breitenbush Hot Springs occurs, in part, along basaltic dikes which channel the water through impermeable Breitenbush Tuff. The dikes are believed to be associated with the Outerson basalts. The Hot Springs discharge upwards at 3400 l/min. (900 gpm) of water at temperatures up to 92°C (198°F). Close

• 5. [Article] Plagioclase Populations and Zoning in Dacite of the 2004–2005 Mount St. Helens Eruption: Constraints for Magma Origin and Dynamics

  We investigated plagioclase phenocrysts in dacite of the 2004–5 eruption of Mount St. Helens to gain insights into the magmatic processes of the current eruption, which is char­acterized by prolonged, ...

  Citation × Citation Citation Abstract Copy Title: Plagioclase Populations and Zoning in Dacite of the 2004–2005 Mount St. Helens Eruption: Constraints for Magma Origin and Dynamics Author: Streck, Martin J., Broderick, Cindy A., Thornber, Carl R., Clynne, Michael A., Pallister, John S. Year: 2008 We investigated plagioclase phenocrysts in dacite of the 2004–5 eruption of Mount St. Helens to gain insights into the magmatic processes of the current eruption, which is char­acterized by prolonged, nearly solid-state extrusion, low gas emission, and shallow seismicity. In addition, we investigated plagioclase of 1980–86 dacite. Light and Nomarski microscopy were used to texturally characterize plagioclase crystals. Electron microprobe analy­ses measured their compositions. We systematically mapped and categorized all plagioclase phenocrysts in a preselected area according to the following criteria: (1) occurrence of zones of acicular orthopyroxene inclusions, (2) presence of dissolution surface(s), and (3) spatial association of 1 and 2. Phenocrysts fall into three main categories; one category con­tains four subcategories. The range of anorthite (An) content in 2004–5 plagio­clase is about An57–35 during the last 30–40 percent crystal­lization of plagioclase phenocrysts. Select microphenocrysts (10–50 μm) range from An30 to An42. Anorthite content is lowest near outermost rims of phenocrysts, but zonation pat­terns between interior and rim indicate variable trends that correlate with textural features. Crystals without dissolution surfaces (about 14 percent of total) show steadily decreas­ing An content outward to the crystal rim (outer ~80 μm). All other crystals are banded as a consequence of dissolu­tion; dissolution surfaces are band boundaries. Such crystals display normal outward An zoning within a single band that, following dissolution, is then overgrown abruptly by high-An material of the next band. Swarms of acicular orthopyroxene inclusions in plagioclase are characteristic of 2004–5 dacite. They occur mostly inward of dissolution surfaces, where band composition reaches lowest An content. The relative propor­tions of the three crystal types are distinctly different between 2004–5 dacite and 1980s dome dacite. We propose that crystals with no dissolution surfaces are those that were supplied last to the shallow reservoir, whereas plagioclase with increasingly more complex zoning patterns (that is, the number of zoned bands bounded by dissolution surfaces) result from prolonged residency and evolution in the reservoir. We propose that banding and An zoning across multiple bands are primarily a response to thermally induced fluctuations in crystallinity of the magma in combination with recharge; a lesser role is ascribed to cycling crystals through pressure gradients. Crystals without dissolution surfaces, in contrast, could have grown only in response to steady(?) decompression. Some heating-cooling cycles probably postdate the final eruption in 1986. They resulted from small recharge events that supplied new crystals that then experi­enced resorption-growth cycles. We suggest that magmatic events shortly prior to the current eruption, recorded in the outermost zones of plagioclase phenocrysts, began with the incorporation of acicular orthopyroxene, followed by last resorption, and concluded with crystallization of euhedral rims. Finally, we propose that 2004–5 dacite is composed mostly of dacite magma that remained after 1986 and under­went subsequent magmatic evolution but, more importantly, contains a component of new dacite from deeper in the mag­matic system, which may have triggered the new eruption. Title: Plagioclase Populations and Zoning in Dacite of the 2004–2005 Mount St. Helens Eruption: Constraints for Magma Origin and Dynamics Author: Streck, Martin J., Broderick, Cindy A., Thornber, Carl R., Clynne, Michael A., Pallister, John S. Year: 2008 We investigated plagioclase phenocrysts in dacite of the 2004–5 eruption of Mount St. Helens to gain insights into the magmatic processes of the current eruption, which is char­acterized by prolonged, nearly solid-state extrusion, low gas emission, and shallow seismicity. In addition, we investigated plagioclase of 1980–86 dacite. Light and Nomarski microscopy were used to texturally characterize plagioclase crystals. Electron microprobe analy­ses measured their compositions. We systematically mapped and categorized all plagioclase phenocrysts in a preselected area according to the following criteria: (1) occurrence of zones of acicular orthopyroxene inclusions, (2) presence of dissolution surface(s), and (3) spatial association of 1 and 2. Phenocrysts fall into three main categories; one category con­tains four subcategories. The range of anorthite (An) content in 2004–5 plagio­clase is about An57–35 during the last 30–40 percent crystal­lization of plagioclase phenocrysts. Select microphenocrysts (10–50 μm) range from An30 to An42. Anorthite content is lowest near outermost rims of phenocrysts, but zonation pat­terns between interior and rim indicate variable trends that correlate with textural features. Crystals without dissolution surfaces (about 14 percent of total) show steadily decreas­ing An content outward to the crystal rim (outer ~80 μm). All other crystals are banded as a consequence of dissolu­tion; dissolution surfaces are band boundaries. Such crystals display normal outward An zoning within a single band that, following dissolution, is then overgrown abruptly by high-An material of the next band. Swarms of acicular orthopyroxene inclusions in plagioclase are characteristic of 2004–5 dacite. They occur mostly inward of dissolution surfaces, where band composition reaches lowest An content. The relative propor­tions of the three crystal types are distinctly different between 2004–5 dacite and 1980s dome dacite. We propose that crystals with no dissolution surfaces are those that were supplied last to the shallow reservoir, whereas plagioclase with increasingly more complex zoning patterns (that is, the number of zoned bands bounded by dissolution surfaces) result from prolonged residency and evolution in the reservoir. We propose that banding and An zoning across multiple bands are primarily a response to thermally induced fluctuations in crystallinity of the magma in combination with recharge; a lesser role is ascribed to cycling crystals through pressure gradients. Crystals without dissolution surfaces, in contrast, could have grown only in response to steady(?) decompression. Some heating-cooling cycles probably postdate the final eruption in 1986. They resulted from small recharge events that supplied new crystals that then experi­enced resorption-growth cycles. We suggest that magmatic events shortly prior to the current eruption, recorded in the outermost zones of plagioclase phenocrysts, began with the incorporation of acicular orthopyroxene, followed by last resorption, and concluded with crystallization of euhedral rims. Finally, we propose that 2004–5 dacite is composed mostly of dacite magma that remained after 1986 and under­went subsequent magmatic evolution but, more importantly, contains a component of new dacite from deeper in the mag­matic system, which may have triggered the new eruption. Close