Geosphere details the geology of North America with 6 new papers online

June 18, 2013

Boulder, Colo., USA - Each of the six new papers published in Geosphere on 13 June address geoscience compiled in specially themed issues: CRevolution 2: Origin and Evolution of the Colorado River System II; The 36-18 Ma southern Great Basin, USA, ignimbrite province and flareup: Swarms of subduction-related supervolcanoes; New Developments in Grenville Geology; and Origin and Evolution of the Sierra Nevada and Walker Lane.

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A post-6 Ma sediment budget for the Colorado River
Rebecca J. Dorsey, Department of Geological Sciences, University of Oregon, Eugene, Oregon, USA; and Greg Lazear, Grand Junction Geological Society, Cedaredge, Colorado, USA. Published 13 June 2013, Special issue: Crevolution 2: Origin and Evolution of the Colorado River System II.

Fundamental questions persist regarding the history of uplift and erosion on the Colorado Plateau and the fate of sediment that has been carried away by the Colorado River in the past 5 to 10 million years. Sediment exiting the mouth of the Colorado River enters deep basins in the Salton Trough and Gulf of California that are rapidly subsiding along the active boundary between the Pacific and North America plates. Previous studies suggest that the lithosphere beneath these basins has been fully ruptured and broken by continental spreading and that a new generation of crust is formed at depth by rapid burial, high heat flow, and metamorphism of Colorado River sediment. In this paper, Rebecca Dorsey and Greg Lazear calculate the volume of rock that has been eroded from the Colorado Plateau over the past ~6 million years and compare it to the volume of sediment sequestered in basins along the plate boundary. They find that the volume of eroded crust broadly matches the volume of sediment in the basins, but only if deeply buried metamorphic crust is included in the sediment volume estimate. This result lends new support to the model for lithospheric rupture beneath the Salton Trough. The long-term (geologic) rate of sediment discharge from the Colorado River is roughly equal to the modern sediment discharge that was measured at Yuma in the early 1900s, before construction of large dams along the river. The similarity of geologic and historical sediment discharge rates suggests operation of a positive feedback between erosion by the Colorado River and flexural-isostatic uplift on the Plateau. These findings highlight dynamic linkages among processes of erosion and uplift, fluvial transport, basin subsidence, and rapid accumulation of sediment along an active oblique-rift plate boundary.

Magmatism, ash-flow tuffs, and calderas of the ignimbrite flareup in the western Nevada volcanic field, Great Basin, USA
Christopher D. Henry and David A. John, Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557, USA. Published 13 June 2013, Special Issue: The 36-18 Ma southern Great Basin, USA, ignimbrite province and flareup: Swarms of subduction-related supervolcanoes.

The western Nevada volcanic field is the western third of a belt of calderas that extends into western Utah and that erupted huge volumes of ash-flow tuff during the mid-Cenozoic ignimbrite flareup of the Great Basin. This paper, the first comprehensive treatment of the western Nevada volcanic field, (a) provides a state-of-the-art summary of ignimbrite chronology, composition, caldera sources, associated lavas and intrusions, and associated mineral deposits based on extensive geologic mapping and precise geochronology; (b) documents the character of caldera-forming ignimbrite volcanism developed on the western slope of the Sevier orogenic plateau and commonly on oceanic crust west of Precambrian basement; and (c) uses extraordinary 3-D exposures of two major caldera systems to document fundamental characteristics of ignimbrite (or ash-flow) calderas that are rarely exposed elsewhere. The understanding provided by geologic mapping and other studies of these exceptionally exposed calderas can be applied worldwide and can further constrain models of ash-flow caldera genesis and the relationship of large-volume silicic volcanism to plutonism.

Denudation and flexural isostatic response of the Colorado Plateau and southern Rocky Mountains region since 10 Ma
Greg Lazear et al., 20508 Brimstone Road, Cedaredge, Colorado 81413, USA. Published 13 June 2013, Special Issue: CRevolution 2: Origin and Evolution of the Colorado River System II.

Over the past 10 million years, the high-relief landscapes of the Colorado Plateau-southern Rocky Mountains region have been shaped by erosional processes. Incision rates have increased in the southern Rocky Mountains, the Colorado River system has been superimposed across buried Laramide structures as it was integrated from the Rocky Mountains to the Gulf of California, the modern Grand Canyon formed, and there has been widespread denudation of the Canyonlands region of the Colorado Plateau. Greg Lazear and colleagues examine the spatial and temporal distribution of erosion and its associated isostatic rebound since 10 million years ago. This paper is an advance over previous studies that focused just on the Colorado Plateau. Here Lazear and colleagues evaluate isostatic response to erosion in an extended region that includes parts of the Basin and Range, Colorado Plateau, southern Rocky Mountains, and eastern piedmont of the Rocky Mountains. They find that erosion of the southern Rocky Mountains and eastern piedmont is comparable to that of the Colorado Plateau and that the flexural isostatic rebounds of all these regions are coupled and cannot be considered in isolation.

Geochemistry and geochronology of the 1.3 Ga metatonalites from the Central Metasedimentary Belt boundary thrust zone in southern Ontario, Grenville Province, Canada
Kenjo S. Agustsson et al., Natural Resources and Environmental Management Department, California Polytechnic State University, San Luis Obispo, CA 93407, USA. Published 13 June 2013, Special Issue: New Developments in Grenville Geology.

Studies of ancient mountain-building episodes (now with much of their steep topography eroded away exposing the cores of former mountains) offer insight into the processes occurring within the deeper levels of active mountain ranges. This study by Kenjo Agustsson and colleagues provides new age and geochemical data from tonalitic and amphibolitic rocks from the vicinity of Haliburton, Ontario, Canada. These ages support previous studies' findings that the Central Metasedimentary Belt boundary thrust zone (CMBbtz) is geochemically and temporally related to far traveled terranes -- near Parry Sound, Ontario, and the Adirondack Highlands, New York -- and new data presented in this manuscript are instrumental in supporting this relationship.

The Goldstein Peak Formation, central California: Record of a nonmarine intra-arc basin within the Early Cretaceous Sierra Nevada arc
Diane Clemens-Knott et al., Department of Geological Sciences, California State University, Fullerton, 800 N. State College Blvd., Fullerton, California 92834, USA. Published 13 June 2013; doi: Special Issue: Origin and Evolution of the Sierra Nevada and Walker Lane.

This paper by Diane Clemens-Knott and colleagues reports the discovery of a new package of sedimentary and volcanic rocks that document the presence of river, lake, and alluvial fan environments ~140 million years ago in what is now central California. These rocks record the earliest non-marine landscape in this part of the continental margin, a landscape that developed along the flanks of an arc volcanic chain that stretched along the entire west coast of North America. Few comparable intra-arc basin sequences exist in the geologic record. Below the volcanic arc, new continental crust formed as magma was transferred from the Earth's mantle upwards into the nascent crust. Heat from these magmas caused the young Goldstein Peak deposits to recrystallize, partially obscuring their original identities but also possibly helping to preserve this unusual rock sequence as a metamorphic relict sandwiched between the granitic plutons that dominate the Sierra Nevada mountain range.

The 36-18 Ma Indian Peak-Caliente ignimbrite field and calderas, southeastern Great Basin, USA: Multicyclic super-eruptions
Myron G. Best et al., Department of Geological Sciences, Brigham Young University, Provo, Utah 84602-4606, USA. Published 13 June 2013, Special Issue: The 36-18 Ma southern Great Basin, USA, ignimbrite province and flareup.

The Indian Peak-Caliente caldera complex and its surrounding ignimbrite field were a major focus of explosive silicic activity in the eastern sector of the subduction-related southern Great Basin ignimbrite province during the middle Cenozoic (36-18 Ma) ignimbrite flareup. Caldera-forming activity migrated southward through time in response to rollback of the subducting lithosphere. Nine partly exposed, separate to partly overlapping source calderas and an equal number of concealed sources compose the Indian Peak-Caliente caldera complex. Calderas have diameters to as much as 60 km and are filled with as much as 5000 m of intracaldera tuff and wall-collapse breccias.

Geological Society of America

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