Ghost glaciers and cosmic trips: New GSA Bulletin postings for July 2013

(Press-News.org) Boulder, Colo., USA – July 2013 GSA Bulletin postings cover the solid Earth's influence on the sea; the diverging geologic histories of the North America Cordillera; "ghost glaciers" in Greenland; the Picuris Orogeny, New Mexico, USA; the Corner Brook Lake Block in the Appalachian orogen of western Newfoundland; the Cryogenian Perry Canyon Formation in Utah, USA; geochronology of the Bighorn Basin of Wyoming, USA; and "A cosmic trip: 25 years of cosmogenic nuclides in geology."

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The solid Earth's influence on sea level
Clinton P. Conrad, Dept. of Geology & Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii at Mānoa, Honolulu, Hawaii 96822, USA. Posted online 1 July 2013; http://dx.doi.org/10.1130/B30764.1.

Sea level lies at the intersection of Earth's solid, liquid, and gaseous components, and thus forms a fundamental boundary on our planet that affects both biology and geology. Human society must adjust to changes in this boundary, which is now rising 2-3 mm per year. Although climatological factors such as seawater warming and glacial melting are major contributors to sea level rise, deformation of the solid earth also exerts an important, and often dominating, influence on sea level. Over decades, the movement of mass from glaciers to oceans drives ground surface deformations that cause rates of sea level change to vary from place to place. Over thousands of years, the solid earth responds to past deglaciations, causing slow but large sea level adjustments. Over millions of years, plate tectonics, sedimentation and seafloor volcanism have driven a slow sea level drop of 150-300 meters, but with a coastal expression that depends on local patterns of tectonic uplift and subsidence. Over billions of years, ocean water is probably lost to larger reservoirs stored within the earth's deep interior. Understanding these solid earth processes is essential to predicting patterns of future sea level change, some of which will impact society significantly.

Strain partitioning in accretionary orogens, and its effects on orogenic collapse: Insights from western North America
Steve A. Israel et al., Yukon Geological Survey, 2099 2nd Avenue, Whitehorse, Yukon Y1A 1B5, Canada. Posted online 1 July 2013; http://dx.doi.org/10.1130/B30777.1.

Mountain belts around the world have been viewed as the products of the interaction between two or more tectonic plates throughout time. However, how the mountain belts react to these changes is only part of the story. In the case of the western North American Cordillera, a portion of the mountain belt underwent diverging geologic histories approximately 80 million years ago. This divergence was created by a change in how the mountain belt accommodated deformation along its axis. A portion of the belt continued to rise and another transferred deformation into strike-slip faults. This variation in deformation regimes led to a change in the overall geologic architecture. As the belt evolved, the portion that continued to rise became gravitationally unstable eventually leading to large-scale collapse of the mountain belt. The portion of the belt that underwent strike-slip deformation stayed stable, with only small regions of the underlying crust being exposed through extension related to the strike-slip faults. Through examination of the Cordilleran mountain belt, Steve A. Israel and colleagues are able to determine that the way in which mountain belts throughout the world evolve depend not only on the changing tectonic plate interactions, but also on the geologic architecture of the belts themselves.

Constraining landscape history and glacial erosivity using paired cosmogenic nuclides in Upernavik, northwest Greenland
Lee B. Corbett et al., Dept. of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA; and Dept. of Geology, University of Vermont, Burlington, Vermont 05405, USA. Posted online 23 July 2013; http://dx.doi.org/10.1130/B30813.1.

The evolution of landscapes in the high Arctic is a complex process that takes place over long timescales and by multiple mechanisms. Here, Lee B. Corbett and colleagues investigate the age and history of the landscape in northwestern Greenland in order to understand how it has evolved over time and how effectively glaciers have shaped it. They use beryllium-10 and aluminum-26, two rare isotopes that are produced in rocks when they are exposed to bombardment by high-energy cosmic rays from space. By quantifying the concentrations of these two isotopes, they make inferences about how much time the landscape has spent buried beneath glacial ice versus how much time it has spent exposed. Corbett and colleagues conclude that the landscape in Upernavik is very old; some locations preserve a record of almost one million years. This contrasts greatly with many other landscapes in Greenland, which date back only to about 10,000 years ago (the end of the last glacial period). They also conclude that the land surfaces in Upernavik have been preserved beneath non-erosive glacial ice throughout many glacial periods over the course of geologic time. These so-called "ghost glaciers" cover the landscape but are incapable of eroding it, leaving behind no geologic evidence of their presence.

Detrital zircon evidence for non-Laurentian provenance, Mesoproterozoic (ca. 1490 Ma) deposition and orogenesis in a reconstructed orogenic belt, northern New Mexico, USA: Defining the Picuris orogeny
Christopher G. Daniel et al., Dept. of Geology, Bucknell University, Lewisburg, Pennsylvania 17837, USA. Posted online 23 July 2013; http://dx.doi.org/10.1130/B30804.1.

Geochronological results from igneous and detrital zircon show that the Proterozoic Pilar and Piedra Lumbre Formations exposed in the Picuris Mountains, New Mexico, USA, were deposited between about 1500 and 1460 million years ago, some 200 million years younger than previously thought. Following deposition, these rocks and older Paleoproterozoic units were buried in the middle crust and experienced regional metamorphism and deformation at conditions near 550 degrees Celsius, 0.4 Gpa. Previous workers attributed this metamorphism and deformation to the approx. 1650 million-year-old Mazatzal Orogeny. According to Christopher G. Daniel and colleagues, this interpretation is no longer viable. The deposition of sediments at about 1500-1450 million years ago followed by regional metamorphism, deformation and significant regional magmatism are the hallmarks of an orogenic event that Daniel and colleagues propose to call the Picuris Orogeny. This orogenic belt extends east across the Truchas Peaks and Rio Mora areas and to the west across the Tusas Mountains and likely extends further to the southwest and is broadly correlative with the Mesoproterozoic Yankee Joe/Blackjack formations in Arizona. The Picuris Orogeny may reflect either intracratonic tectonic processes, or alternatively, may be associated with the accretion of the Mazatzal crustal province in the Mesoproterozoic, ca. 1500 to ca. 1400 million years ago.

The Corner Brook Lake block in the Newfoundland Appalachians: A suspect terrane along the Laurentian margin and evidence for large-scale orogen-parallel motion
Shoufa Lin et al., Dept. of Earth and Environmental Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada. Posted online 23 July 2013; http://dx.doi.org/10.1130/B30805.1.

Isotopic dating and geological data indicate that the Corner Brook Lake Block, a geological entity in the Appalachian orogen of western Newfoundland, has unique characteristics. Rocks of about 1.0 billion years old, which are typical for the western margin of the Appalachian orogen, are absent. The block also has a distinct geological history from about 470 to 430 million years ago. Available evidence indicates that the block has moved 400 km or more parallel to the orogeny. The recognition of large-scale orogeny-parallel motion indicates that presently neighboring geological blocks might have been far apart and thus has significant implications for tectonic interpretation of the Appalachian orogeny.

Stratigraphic, geochronologic, and geochemical record of the Cryogenian Perry Canyon Formation, northern Utah: Implications for Rodinia rifting and snowball Earth glaciation
E.A. Balgord et al. (corresponding author Adolph Yonkee), Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Posted online 23 July 2013; http://dx.doi.org/10.1130/B30860.1.

The Cryogenian Period (800-635 million years ago) represents a time of profound and interrelated global tectonic, climatic, and biologic changes, involving the breakup of the supercontinent Rodinia, extreme glaciations, and a drastic turnover in the diversity and morphology of life on Earth. The Cryogenian Perry Canyon Formation of northern Utah contains diamictite- and volcanic-bearing strata that record glacial events and an early phase of rifting along the western margin of Laurentia. Sedimentologic, geochronologic, and geochemical information from the formation and overlying strata provide important insights on the paleogeographic evolution of western Laurentia. Detailed analyses of strata reveal a distinct change from narrow fault-bound basins with local sediment sources and associated basalt, trachyte, and rhyolite volcanism, to a regionally integrated, slowly subsiding basin with distal and recycled sediment sources, recording evolution from an active rift to a passive margin setting. Maximum depositional ages of diamictite and locally preserved cap carbonate in the Perry Canyon Formation are consistent with two glacial events at older than 710 million years and about 685 to 670 million years, interpreted to be correlative with glacial deposits preserved on other continents.

Detrital zircon geochronology from the Bighorn Basin, Wyoming, USA: Implications for tectonostratigraphic evolution and paleogeography
Steven R. May et al., ExxonMobil Upstream Research Company, P.O. Box 2189, Houston, Texas 77252, USA. Posted online 23 July 2013; http://dx.doi.org/10.1130/B30824.1.

Radiometric ages from zircon grains within the Bighorn Basin of Wyoming provide a record of sediment origin and dispersal for the past 510 million years. More than 4,000 new ages reported in this paper provide a fingerprint of where the grains that make up Cambrian through Eocene sedimentary rocks in the basin originated, were transported, and deposited. These data provide one of the most complete records of detrital zircon age distributions from this part of North America. Patterns recognized within these data reflect the evolution of northwest Wyoming through a series of tectonic and paleogeographic environments. Detrital zircon ages from certain parts of the stratigraphic record provide evidence for linkages between ancient plate tectonic processes along the margin of North America and sedimentation on the continent. During these times, detrital zircons provide a potentially useful tool for determining the ages of sedimentary rocks.

A cosmic trip: 25 years of cosmogenic nuclides in geology
Darryl E. Granger et al., Dept. of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, Indiana 47907, USA. Posted online 23 July 2013; http://dx.doi.org/10.1130/B30774.1.

Terrestrial cosmogenic nuclides, produced by secondary cosmic-ray interactions in the atmosphere and in situ within minerals in the shallow lithosphere, are widely used to date surface exposure of rocks and sediments, to estimate erosion and weathering rates, and to date sediment deposition or burial. Their use has transformed geomorphology and Quaternary geology, for the first time allowing landforms to be dated and denudation rates to be measured over soil-forming time scales. The application of cosmogenic nuclides to geology began soon after the invention of accelerator mass spectrometry (AMS) in 1977 and increased dramatically with the measurement of in situ–produced nuclides in mineral grains near Earth's surface in the 1980s. The past 25 years have witnessed the development of cosmogenic nuclides from their initial detection to their prevalence today as a standard geochronological and geochemical tool. This review by Darryl E. Granger and colleagues covers the major developments of the past 25 years by comparing the state of the field in 1988 with that of today, and by identifying key advances in that period that moved the field forward. Granger and colleagues emphasize the most commonly used in situ-produced nuclides measured by AMS for geological applications, but also discuss other nuclides where their applications overlap. Their review covers AMS instrumentation, cosmogenic nuclide production rates, the methods of surface exposure dating, measurement of erosion and weathering, and burial dating, and meteoric 10Be.

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