Boulder, Colo., USA - Understanding of the Yellowstone hotspot and its connection to flood basalts of the Columbia River Basalt province (western and northwestern USA) has grown tremendously over the past decades since the model was first proposed in 1972. Despite strong support for a plume origin of the entire Yellowstone-Columbia River Basalt magmatic province, new non-plume models have emerged to explain early flood basalt volcanism.
Unresolved issues of the early flood basalt stage include the location of crustal magma reservoirs feeding these voluminous eruptions and to what extent these were associated with contemporaneous silicic reservoirs.
This study focuses on the newly defined 15 to 16 million-year-old Dinner Creek Tuff Eruptive Center that overlaps in time and space with flood basalt volcanism of the Columbia River Basalt Group. New work on distribution, lithologic variations, geochemical compositions, and eruption ages indicates that the extensive Dinner Creek Welded Tuff (herein Dinner Creek Tuff) and associated mapped and unmapped ignimbrites include a minimum of four discrete cooling units that spread out over an area of approx. 25,000 square kilometers.
Widespread fallout deposits in northeast Oregon and the neighboring states of Nevada, Idaho, and Washington have now been compositionally correlated with the redefined Dinner Creek Tuff. Compositional coherence between the ignimbrite sheets and fallout deposits indicate a common source, herein referred to as the Dinner Creek Tuff eruptive center (DITEC).
Major and trace element compositions of the more mafic components match the compositions of nearby Grande Ronde Basalt (GRB) flows and dikes. Compositional similarities between cognate mafic components and GRB flows are direct evidence for coeval mafic and silicic magmatism linking DITEC and GRB eruptions.
Furthermore, finding GRB magmas as co-eruptive component in Dinner Creek Tuff suggests that GRB magmas were stored beneath Dinner Creek Tuff rhyolites, thereby providing the first direct evidence for the location of a storage site of Columbia River Basalt magmas. Shallow crustal rhyolitic reservoirs active during approx. 15 to 16 million years ago that yielded tuffs of the DITEC and other surrounding contemporaneous and widespread rhyolites of the area likely imposed control on timing and place of eruption of Columbia River Basalt Group lava flows.
Large, persistent rhyolitic magma reservoirs above Columbia River Basalt storage sites: The Dinner Creek Tuff Eruptive Center, eastern Oregon
Martin J. Streck et al., Portland State University, Portland, Oregon, USA. Published online on 17 Feb. 2015; http://dx.
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Yellowstone plume trigger for Basin and Range extension, and coeval emplacement of the Nevada-Columbia Basin magmatic belt
Victor E. Camp et al., San Diego State University, San Diego, California, USA. Posted online 17 Feb. 2015; http://dx.
Widespread extension began across the northern and central Basin and Range at 16 to 17 million years ago, contemporaneous with the initiation of dike intrusion and volcanism along a ~1000-km-long belt that extends from southern Nevada to the Columbia Basin of eastern Washington. The apparent extension direction of dike emplacement lies 45 degrees from the direction of coeval Basin and Range extension, and therefore cannot be attributed to prevailing stress in the upper crust. Instead, we attribute dike orientation to magmatic stress and the upward, forceful injection of magma rooted in an elongated sublithospheric melt zone. Melting was associated with rapid decompression of the Yellowstone mantle plume as it squeezed through a north-south propagating fracture in the Juan de Fuca slab. The southern half of mantle upwelling was emplaced beneath cratonic lithosphere with an elevated surface derived by Late Cretaceous to mid-Tertiary crustal thickening. By mid-Miocene time, this high Nevadaplano was primed for collapse with high gravitational potential energy under the influence of regional stress. Plume arrival at 16 to 17 million years ago was not the sole cause of Basin and Range extension, but rather the catalyst for extension of the Nevadaplano, which was already on the verge of regional collapse.
An upper-crustal fold province in the hinterland of the Sevier orogenic belt, eastern Nevada, U.S.A.: A Cordilleran Valley and Ridge in the Basin and Range
Sean P. Long, Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada, USA. Published online on 17 Feb. 2015; http://dx.
In Nevada, Tertiary extension of the crust has obscured the geological records of older deformation events, including the Mesozoic construction of the Cordilleran mountain belt. In this study, a paleo-geologic map of eastern Nevada, showing the ages and distributions of rocks that were at the surface at about 30 million years ago, illustrates the style and geometry of deformation associated with Cordilleran mountain building. The map reveals a region of folds in eastern Nevada that are similar in scale to those in the Valley and Ridge province of the Appalachian Mountains. The folds are interpreted to have formed during the Cretaceous period, as a result of eastward translation above a deep detachment surface that projects westward from the Sevier fold-and-thrust belt in Utah. These observations are synthesized with studies of other structural provinces of Nevada and western Utah, to propose a model for deformation during Cordilleran mountain building.
Spatial patterns of deformation and paleoslope estimation within the marginal and central portions of a basin-floor mass-transport deposit, Taranaki Basin, New Zealand
Glenn R. Sharman et al., Stanford University, Stanford, California, USA. Published online on 17 Feb. 2015; http://dx.
This study presents an in-depth examination of deposits associated with a large, ancient submarine landslide observed along the coastline of New Zealand. Submarine landslides, also known as mass-transport deposits (MTDs), form within deep ocean basins and thus are difficult to study in detail. It is relatively common to observe portions of fossil MTDs exposed on land in the geologic record, but it is uncommon for rock outcrops to preserve a complete example. This study documents a rare occurrence of a fossil MTD with nearly complete preservation across its width. Detailed study of folds and faults within the MTD shows that the orientations of these features are different between the marginal and central portions of the MTD. We hypothesize that these differences relate to edge effects, possibly lateral compression, that occurred along the margins of the MTD during emplacement onto the seafloor. Our analysis has implications for how fold and fault orientations are used to reconstruct the direction of motion for ancient MTDs, and this study has improved our understanding of the ancient geography of the northeastern Taranaki Basin.
Devils Tower (Wyoming, USA): A lava coulée emplaced into a maar-diatreme volcano?
P. Závada et al., Institute of Geophysics of the Czech Academy of Sciences, Prague, Czech Republic. Published online ahead of print on 17 Feb. 2015; http://dx.
ABSTRACT: We have investigated the mode of emplacement of iconic Devils Tower, which is a phonolite porphyry monolith in the state of Wyoming in the western United States. Our field survey of this structure and its geological setting, its radiometric dating, and the tectonomagmatic evolution of the region suggest a new genetic interpretation of the volcaniclastic rocks in the area and provide a basis for a new hypothetical emplacement scenario for Devils Tower. This interpretation was inspired by an analogy of the tower with a similar phonolite butte in the Cenozoic volcanic region of the Czech Republic and analogue modeling using plaster of paris combined with finite element thermal numerical modeling. Our results indicate that Devils Tower is a remnant of a coulée or low lava dome that was emplaced into a broad phreatomagmatic crater at the top of a maar-diatreme volcano.
Origin, structural geometry, and development of a giant coherent slide: The South Makassar Strait mass transport complex
Cipi Armandita et al., SKK Migas, Jakarta, Indonesia. Published online on 17 Feb. 2015; http://dx.
ABSTRACT: The South Makassar Strait mass transport complex (MTC) covers an area of at least 9000 km2 and has a total volume of 2438 cubic kilometers. It is composed of a shale-dominated sedimentary unit with high water content. Seismic reflection data across the South Makassar Strait MTC show that it displays relatively coherent internal sedimentary stratigraphy that in the toe region is deformed into well-defined thrust-related structures (imbricates, ramps and flats, fault bend folds). It is one of the largest known coherent MTCs. The bowl-shaped central core region is as much as 1.7 km thick, and is confined to the west and east by 355 degrees and 325 degrees trending (respectively) lateral ramps in the upper slope area that pass via oblique ramps into a northeast-southwest-trending frontal ramp area. The core area passes via the lateral, oblique, and frontal ramps into an extensive, thin (tens of meters thick) region of the MTC (the lateral and frontal apron areas) that are internally deformed by thrusts, normal faults, and thrust faults reactivated as normal faults. The MTC anatomy can be divided into extension headwall, translational, toe, flank, lateral apron, and frontal apron domains. The headwall region is located in the upper slope area of the Paternoster platform; the main body of the slide is in the deep-water region of the Makassar Strait. The complex is interpreted to be triggered by uplift of the platform area (accompanied by inversion), and/or basin subsidence, which caused seaward rotation of ~2 degrees of the Paternoster platform in the Pliocene. Variable uplift promoted sliding dominantly from the eastern and western margins of the headwall. The internal fault patterns of the MTC show that extension in the upper slope to lower slope in the core area changes downslope to compressional structures in the toe domain and apron. Later extensional collapse of parts of the compressional toe area occurred with negative inversion on some faults. The coherent internal stratigraphy, and evidence for multiphase extension in the eastern headwall area, suggests that the ~6 to 7 km of shortening in the toe region of the MTC occurred at a slow strain rate. Therefore, this type of MTC does not have the potential to generate tsunamis.