Indiana University Bloomington

Most Recent Colloquia: Fall 2014

September 8: Herschel McDivitt, Indiana Department of Natural Resources. Title: Oil and Gas Production in Indiana How is it Regulated?

Synopsis: In my presentation will cover the following areas:

Rooney Landscapt

September 15: Tyrone Rooney Michigan State University. Title: From initiation to termination - the critical role of magma in rift evolution

Abstract:The evolution of the continental lithosphere during the rupture of continents involves a series of processes that requires a multi-disciplinary approach to their study. Geochemistry provides a powerful tool that can augment geophysical and tectonic observations and provide a unique control on the rifting process. In this talk we will examine the geochemical evidence for mantle plume contributions to rift magmatism, the consequences of lithospheric thinning on magma generation, and the long term impact of rifting processes on the upper mantle. Our focus will be on the East African Rift - the archetypical example of continental rifting, with application to magma-rich rifting worldwide.

September 22: Alan Chapman, Missouri University of Science and Technology. Title: Constraints on Plateau Architecture and Assembly From Deep Crustal Xenoliths, Northern Altiplano (SE Peru)

Abstract: Newly discovered xenoliths from southern Peru permit examination of lithospheric processes by which thick crust (60-70 km) and high average elevations (34 km) resulted within the Altiplano, the second most extensive orogenic plateau on earth. The most common petrographic groups of xenoliths studied here are igneous or metaigneous rocks with radiogenic isotopic ratios consistent with recent derivation form asthenospheric mantle (87Sr/86Sr = 0.704-0.709, 143Nd/144Nd = 0.5126-0.5129). A second group of high-grade metasedimentary xenoliths, the focus of this study, have more radiogenic compositions (87Sr/86Sr = 0.711-0.782, 143Nd/144Nd = 0.5122-0.5126) and are interpreted as metamorphosed cover units of the central Andes. Felsic granulites representing Mesozoic (Upper Triassic or younger) metasediments, based on the distribution of detrital zircon U-Pb ages, equilibrated at ˜0.9 GPa (˜30 km paleodepth) and 750°C. Equivalent non-metamorphosed rocks comprise the country rocks that Quaternary xenolith-hosting volcanic rocks intrude. A Sm-Nd garnet-whole rock isochron of 42 ± 5 Ma demonstrates that garnet growth took place during the late Eocene. Monazite grains associated with quenched anatectic melts from the same rocks yield a range of ion microprobe U-Pb ages from 3.2 ± 0.2 to 4.4 ± 0.3 Ma. These disparate geochronologic datasets are reconciled by a model wherein Mesozoic cover rocks were transferred to ˜30 km depth beneath the plateau in the Eocene and progressively heated until at least Pliocene time with no signs of unroofing, as evidenced by thermodynamic modeling, monazite chemistry, and homogenous garnet zonation. Mafic granulites and peridotites from the same xenolith suite apparently comprise the basement of the metasedimentary sequence, exhibiting isotopic characteristics of central Andean basement. Calculated equilibrium pressures for these basement rocks are as low as 1.1 GPa (36 km below the surface) suggesting that the basement-cover interface lies beneath the northernmost Altiplano at some 30-35 km below the surface. Together, these results indicate that the thickening of the crust under the northernmost Altiplano started earlier than the documented major uplift episodes of the northern plateau (latest Oligocene and Miocene) and was coeval with a flat slab-related regional episode of deformation. Total shortening must have been at least 10% more than previous estimates in order to satisfy the basement to cover depth constraints provided by the xenolith data. Sedimentary rocks at this depth require that Andean basement thrusts decapitated earlier Triassic normal faults, trapping Paleozoic and Mesozoic rocks below the main decollement. Magma loading from intense Cenozoic plutonism within the plateau may be responsible for part of the burial of the Paleozoic cover rocks to > 30 km as well as played a significant role in thickening the crust under the northern Altiplano. Fragments of Arequipa terrane basement or Brazilian craton crust were not recognized among the xenoliths investigated here.

September 29: Bill Ellsworth USGS Menlo Park, California

October 6: Audrey Sawyer, University of Kentucky Title: Hydrologic speed bumps: Heterogeneity in water and nutrient exchange between rivers and aquifers

Abstract In recent decades, nitrate loads to rivers and coasts have increased dramatically and contributed to eutrophication and hypoxia in coastal waters. A primary sink for nitrate in the environment is denitrification in aquatic sediments. In rivers and estuaries, surface water-groundwater exchange transports nitrate across the sediment-water interface and thus influences nitrogen fate. Due to heterogeneity in aquatic sediments, rates of water and nitrate exchange vary over orders of magnitude at scales ranging from centimeters to kilometers. Here, I use numerical models and field observations to quantify sedimentary controls on surface water-groundwater interactions and implications for nitrogen fluxes from aquifers to rivers and estuaries. At the core-scale, I show that the presence of small zones of organic-rich silt significantly increase the efficiency of nitrate removal in shallow aquatic sediments. Nitrate removal rates are as much as 100 times more efficient in heterogeneous sediments than equivalent homogeneous sediments. At larger scales, stratigraphy strongly controls the form and fluxes of nitrogen from aquifers to estuaries. Field studies from the Delaware Inland Bays (USA) show that peat-filled paleochannels divert fresh groundwater discharge to channel margins. Saltwater exchange and ammonium production are locally enhanced in sediments near the channel margins. In interfluves far from paleochannels, fresh groundwater discharges rapidly near the coast and carries a large nitrate load. I estimate that more than 99% of the groundwater-borne nitrate flux to the Delaware Inland Bays occurs within interfluve portions of coastline, and more than 50% of the ammonium flux occurs at paleochannel margins. The difficulty of characterizing heterogeneity in aquatic sediments remains one of the greatest challenges to measuring and predicting nitrogen fate in aquatic systems.

October 13: Becky Lange, University of Michigan

October 19-22: No colloquium, GSA Meeting in Vancouver BC, Canada

October 27: Bill Gilhooly, IUPUI

November 3: Jennifer Glass, Georgia Tech

November 10: Mickey Gunter, University of Idaho

November 17: Paul Knauth, Arizona State University

December 1: Terry Pavlis, University of Texas at El Paso

December 8: Wolfram Kuerschner, University of Oslo Title: Vegetation patterns during the End-Triassic mass extinction: did better genes increased survival changes of land plants?"