What Lies Beneath Modern New England? Mountain-building and the end of an ancient ocean

(Press-News.org) Boulder, Colo., USA – When and where did the ancient Iapetus Ocean suture (the most fundamental Appalachian structure) form? Is part of New England made up of ancient African-derived rocks? What is the Moretown terrane? This new GEOLOGY study by researchers from Harvard, Middlebury College, Boise State University, and Williams College finds new evidence for an earlier closing of the Iapetus that is farther west than previous studies have reported.

Mountain-building events, called "orogenies," in the northern U.S. Appalachia record the closure of the Iapetus Ocean, an ancient precursor to the Atlantic. The Iapetus separated continental fragments of ancestral North America and Africa more than 450 million years ago.

The mountain-building period that affected most of modern-day New England, known as the "Taconic orogeny," is commonly depicted as a collision during the Ordovician period (435 to 500 million years ago) of a North American-derived arc (the Shelburne Falls arc) and the North American margin, followed by accretion of African-derived terranes (groups of rocks with geologic histories different from surrounding rocks) during the Silurian period (410 to 435 million years ago).

New uranium-lead (U-Pb) zircon dating presented here by Harvard researcher Francis A. Macdonald and colleagues demonstrates instead that the Shelburne Falls arc was constructed on an African-derived terrane, which they have named the Moretown terrane. Their geochronologic data reveal that the main Iapetan suture, which marks the location of the Iapetus as it was consumed through subduction, is more than 50 km west than previously suspected.

Macdonald and colleagues conclude that the Moretown terrane lies below North American-derived volcanic and sedimentary rocks of the Hawley Formation, which proves a link between North American- and African-derived terranes. The Moretown terrane and Hawley Formation were both intruded by 475-million-year-old plutonic rocks (rocks formed by magma rising from great depths beneath Earth's surface), suggesting that these terranes were together by this time and that the Iapetus Ocean closed approx. 20 million years earlier than documented elsewhere.

Other GEOLOGY postings for 24 April (see details below) cover: 1. Sampling of water up to half a mile beneath the Greenland Ice Sheet 2. Learning more about how the presence of amphibole affects the strength of shear zones; 3. Analyzing the very first severe extinction of the Phanerozoic and an extended marine anoxia period; and 4. Examining results from volcanic glass hydrogen isotope ratios deposited over the last 35 million years that constrain the timing of uplift of the central Rocky Mountains and nearby Great Plains.

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FEATURED ARTICLE
A newly identified Gondwanan terrane in the northern Appalachian Mountains: Implications for the Taconic orogeny and closure of the Iapetus Ocean
F.A. Macdonald et al., Dept. of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. Published online on 24 April 2014; http://dx.doi.org/10.1130/G35659.1.

Chemical weathering under the Greenland Ice Sheet
Joseph A. Graly et al., Dept. of Geology and Geophysics, Dept. 3006, University of Wyoming, Laramie, Wyoming 82071, USA. Published online on 24 April 2014; http://dx.doi.org/10.1130/G35370.1.

Researchers from the University of Wyoming, the U.S. Geological Survey, and the University of Montana have directly sampled water from underneath the Greenland Ice Sheet. The waters were collected from boreholes drilled through as much as half a mile of ice to reach the ice sheet's bed. By examining the chemistry of the water, the researchers found that the rock underneath the ice is actively reacting with the Earth's atmosphere. These reactions, known as chemical weathering, are occurring at rates comparable to landscapes not covered by ice. The samples were collected in a region of the ice sheet known as the ablation zone, where surface meltwater is able to reach the ice sheet's bed. The interaction of the surface meltwater with rock ground to fresh, fine sediments by the ice's weight promotes the chemical weathering of the landscape. Similar rates of weathering had been previously found in small mountain glaciers. But the mechanisms proposed for weathering in the mountain environment had depended on rare but highly reactive minerals to do most of the chemical work. The research on Greenland shows that the most common rock forming minerals can weather at substantial rates in the ice sheet environment.

Amphibole fabric formation during diffusion creep and the rheology of shear zones
A.J. Getsinger, Dept. of Geological Sciences, Brown University, Box 1846, 324 Brook Street, Providence, Rhode Island 02912, USA; and G. Hirth. Published online on 24 April 2014; http://dx.doi.org/10.1130/G35327.1.

Geologic studies indicate that the mineral amphibole is abundant in the lower continental crust. Amphibole is stable over a wide range of pressure and temperature, and highly deformed shear zones, such as observed near plate tectonic boundaries, often show significantly more amphibole than undeformed rocks observed nearby. Despite these observations, very little is known about how the presence of amphibole affects the strength of shear zones. To examine the effect of amphibole on the strength of these rocks, A.J. Getsinger and colleagues conducted experiments at lower crust temperatures and pressures. The experimentally produced rock textures are strikingly similar to those observed in natural shear zones and indicate that viscous deformation is accommodated by diffusion of mineral components along grain boundaries. Although this mechanism had been suggested for naturally deformed rocks, this is the first experimental study to demonstrate the viability of this mechanism. Getsinger and colleagues show that the viscosity of these amphibole-bearing rocks is comparable to that estimated from similar experiments on wet plagioclase, which is another common mineral in the lower continental crust. Thus, both their experiments and field analyses indicate that wet plagioclase rheology provides a good constraint on the viscosity of lower crust, especially along plate boundaries.

High-precision dating of the Kalkarindji large igneous province, Australia, and synchrony with the Early–Middle Cambrian (Stage 4-5) extinction
F. Jourdan et al., Western Australian Argon Isotope Facility, Dept. of Applied Geology and JdL-CMS, Curtin University, Perth, Western Australia 6845, Australia. Published online on 24 April 2014; http://dx.doi.org/10.1130/G35434.1.

The Early–Middle Cambrian (Stage 4-5) boundary which is approx. 510 million years old, marks the very first severe extinction of the Phanerozoic and an extended marine anoxia period. In this study, F. Jourdan and colleagues use a combination of 40Ar/39Ar and U-Pb dating techniques to demonstrate that the Kalkarindji large igneous province (about two million square kilometers), Australia, was emplaced over a relatively short period of time 510-511 million years ago. The temporal synchrony between the Kalkarindji eruption of and the Early-Middle Cambrian boundary approx. 510 million years ago is another example of a well-defined temporal correlation between volcanism, climate shifts, and extinction and extends such a relationship to the beginning of the Phanerozoic demonstrating a direct causation. Geochemical analyses show that the likely factors responsible for the Early-Middle Cambrian extinction are climate shifts due to emission of mantle gases (SO2 and possibly CO2) dissolved in the magma, and gases (in particular CH4 and SO2) generated by the interaction between magma and abundant evaporite layers and oil-rich rocks.

Middle Cenozoic uplift and concomitant drying in the central Rocky Mountains and adjacent Great Plains
Majie Fan et al., Dept. of Earth and Environmental Sciences, University of Texas at Arlington, Arlington, Texas 76019, USA, Published online on 24 April 2014; http://dx.doi.org/10.1130/G35444.1.

The central Rocky Mountains and adjacent Great Plains of western North America is a region of high topography that runs along the backbone of the continent. When and how the region reached its current elevation is the subject of debate. To date, most studies have focused on the last major mountain building event that occurred between approx. 45 and 75 million years ago. More recent constraints on uplift history are sparse. In this paper, Majie Fan and colleagues present results from volcanic glass hydrogen isotope ratios deposited over the last 35 million years that constrain the timing of uplift. Their samples were collected along a transect following the gradual eastward decrease of elevation and increase of surface water hydrogen isotope ratios. They show that the present gradient of surface-water hydrogen isotope ratios was established before 35 million years ago. These results, when considered with other published paleoelevation studies, suggest that the central Rocky Mountains and adjacent Great Plains underwent uplift during the late Eocene, and have not undergone any large-magnitude (i.e., greater than about 500 m) surface uplift since that time. They show that this differential uplift caused regional drying in the region over the past 35 million years.

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