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2012-2013

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  • Thumbnail for Numerical modeling of late-Pleistocene glaciers  in the Front Range, Colorado :  insights into LGM paleoclimate and  post-LGM rates of climate change
    Numerical modeling of late-Pleistocene glaciers in the Front Range, Colorado : insights into LGM paleoclimate and post-LGM rates of climate change by Gall, Ryan D.

    A 2D numerical model developed by Plummer and Phillips (2003) was employed to reconstruct the Middle Boulder Creek (MBC) and North St. Vrain (NSV) paleoglaciers, of the Colorado Front Range. The model was used to investigate two climatic aspects of the late-Pleistocene Pinedale Glaciation (~30,000-12,000 ka): 1. The specific combinations of temperature and precipitation present in the Front Range that may have sustained the MBC and NSV glaciers during the Last Glacial Maximum (LGM, ~21 ka), and 2. The magnitudes and rates of climate change that drove the Front Range ice recession following the LGM. The ArcMap 3.3-based model has two components, an energy/mass balance component that calculates an annual mass balance grid from input climate data and valley topography, and an ice flow component that utilizes the mass balance grid to calculate the glacial flow according to ice flow laws and topography. After several hundred model years, the glacier reaches a steady state geometry that is in mass balance equilibrium with the input climate. Determining the input climate parameters that sustain a modeled glacier at field-mapped extents therefore provides quantified insight into the regional climate. Modeling of the NSV system did not provide reliable results, likely due to issues concerning input modern climate data collected from secondary sources for use in this study. However, results from modeling the MBC glacier to its LGM mapped-extent suggest the glacier may have been sustained with temperature depressions of 5.0˚C, 6.6˚C, and 8.6˚C, respectively coupled with modern precipitation factors of 1.5x, 1.0x, and 0.5x, and is agreement with results of earlier studies. The rate of climate change was determined by modeling the MBC glacier to multiple CRN-dated ice margin locations associated with the post-LGM deglaciation (from Ward et al., 2009), and by assessing the temperature changes between intervals assuming a constant precipitation equal to today’s. This modeling suggests an initial warming of ~0.1˚C/ky from 21 – ~18 ka, a slight cooling period (~0.2˚C) and glacial stillstand from ~18 – ~16 ka, followed by rapid warming of ~0.7˚C/ky from ~16 – 13.5 ka. This first ever attempt to quantify Pinedale deglaciation rate of temperature changes in the Rocky Mountains has error associated the selected CRN dates (up to +/- 2.7 ky), and does not account for potential changes in precipitation. Despite the potential error, the quantified results provide appreciable generalized insight into the natural rate of climate change associated with post-LGM deglaciation.

  • Thumbnail for Petrology and geothermometry of garnet amphibolite blocks, Santa Catalina Island, CA
    Petrology and geothermometry of garnet amphibolite blocks, Santa Catalina Island, CA by Seymour, Abigail

    In the Santa Catalina Island, CA subduction complex, there are garnetiferous mafic gneiss blocks, from a tholeiitic protolith, that occur in a serpentinite mélange, a unit that is poorly understood and has been little studied in the last twenty-five years. Study of the microstructural relationships and geothermometry of two hand samples of garnet-rich gneiss yield information about processes and conditions for high temperature metamorphism in the Catalina complex and possibly more broadly. Results of the study provide insight into 1) the nature of HT metamorphism in the subduction setting, and 2) the metamorphic history of disparate gneiss blocks within the Catalina serpentinite mélange. Thin section petrography and scanning electron microscope (SEM) analysis were performed on polished thin sections of samples to identify minerals and microstructures. Compositional maps of garnets and quantitative mineral analysis of garnet, garnet inclusions, and matrix phases were acquired using an electron microprobe (EMP). The Zr-in-rutile geothermometer (Watson et al., 2006) was used to calculate peak metamorphic temperatures based upon Zr concentrations (ppm) for rutile inclusions and matrix measured by EMP. One of the garnet gneisses exhibits compositional zoning with Mg increase / Ca decrease toward the rim, and by a pattern of distribution of inclusions. The other sample has unzoned, compositionally homogeneous garnet with an unusual abundance of rutile for a metabasalt. The excess of rutile is a possible indication of metasomatism. Zr-in-rutile thermometry yielded temperatures of 480 to 516 C for the study samples, in contrast to the results of Zack et al. (2004) of 764 to 800 C for clinopyroxene-bearing garnet amphibolite in mélange blocks. The differing Zr-in-rutile results provide a possible indication that blocks in the mélange originated at vastly different depths in the subduction zone, then were brought together during mélange formation. My findings indicate that the two gneiss blocks studied have distinct protoliths and metamorphic histories, suggesting that mélange blocks within the upper tectonic unit of Catalina Island derived from different locations within a subduction zone.

  • Thumbnail for Repeating earthquakes in the Darfield region, New Zealand
    Repeating earthquakes in the Darfield region, New Zealand by Armstrong, Ryan Scott

    The M 7.1 3 September 2010 Darfield, New Zealand, earthquake ruptured a previously unknown fault system. Fault-slip models (e.g., Beavan et al., 2010; Holden et al., 2011; Eliott et al., 2012) have been calculated using InSAR, GPS, and seismic data. They show that although the rupture initiated on a SW-dipping thrust fault, the majority of fault motion was right-lateral strike slip from the surface to 10 km depth. The InSAR data used in the geodetic model provide the cumulative ground motion due to the Darfield earthquake and some early aftershocks, while the seismic model utilizes waveforms for the mainshock, limiting the solution to slip during the initial rupture. This study utilizes cross correlation methods to identify repeating earthquakes within continuous seismic waveforms from the Canterbury region, New Zealand between September 2010 and January 2011. Repeating events indicate portions of fault segments that are not locked, possibly due to high pore pressure (Bisrat et al. 2012), and thus can indirectly identify locked areas of fault segments. Despite the fact that our method initially recognized 8 groups of potentially repeating earthquakes, a cross correlation check at a second station indicates that none of the identified earthquakes are truly repeating earthquakes. Our method provides negative results, which indicate repeating earthquakes may not be present within the Darfield fault complex, although it remains unclear whether they are truly absent or the methodology is not sufficient to detect them. While our method failed to identify repeating earthquakes, it possibly identified clusters of events with similar focal mechanisms In theory, our study shows a direct relationship between the compactness of a cluster and the similarity of focal mechanisms.