http://hdl.handle.net/1957/7889 Search the Channel search http://ir.library.oregonstate.edu/dspace/simple-search http://hdl.handle.net/1957/9020 Title: The geochemistry and geochronology of the Eocene Absaroka volcanic province northern Wyoming and southwest Montana, USA <br/> <br/>Abstract: The Absaroka volcanic province is the largest of Eocene volcanic fields in the northern Cordillera of the western U.S., and consists of 25,000 km2 of lava flows, shallow intrusions, ash-flow tuffs and volcaniclastic deposits. It is aligned with northwest-trending Precambrian lineaments, and includes the remains of at least ten volcanic centers. This study presents 40Ar/39Ar geochronological data, and major, trace element and isotopic compositional data which are the result of detailed mapping and field sampling of four representative volcanic centers, peripheral lava flows, intrusions, and ash-flow tuffs. Age data show that volcanism occurred between 53 and 43 Ma in a general northwest to southeast age progression, and have allowed significant revisions in regional correlations across the volcanic province. Local dike orientations from one volcanic center suggest that volcanism occurred during extensional faulting. Geochemical and Nd, Sr and Pb isotopic data show that mafic lavas are enriched in incompatible elements derived from an ancient source. Mafic rocks (<53% SiO2, >5% MgO) are characteristically potassic, and are typical products of early eruptions at each of the Hyalite, Crandall, Ishawooa, and Rampart volcanic centers, and not spatially restricted to any one region. Although products of volcanism are broadly similar, volcanic centers exhibit distinct mineralogic, compositional and isotopic characteristics. Least squares calculations based on mineralogical data indicate that shoshonites can be produced from mafic samples by fractionation of olivine and augite t plagioclase. More silicic samples have petrographic features and compositions which indicate they are derived from a mixture of sources including continental crust. Changing chemical and isotopic compositions suggest that early eruptions contain a lithospheric mantle component, with an increasing melt contribution from crustal sources with time. The last eruptions include rhyolite from the southern Absaroka volcanic province and basanite with an asthenospheric isotopic composition sampled from a peripheral lava flow in the northern Absaroka volcanic province. <br/> <br/>Description: Graduation date: 2000; Presentation date: 1999-03-30 http://hdl.handle.net/1957/9018 Title: Fold growth due to kink-band migration in repeated earthquakes, Sierra de Villicum, San Juan, Argentina <br/> <br/>Abstract: Growth strata geometries and deformed geomorphic surfaces reflect the shorttimescale growth of Sierra de Villicum (San Juan Province, Argentina). Accumulated deformation in repeated earthquakes is recorded by the structural geometry of deformed geomorphic surfaces. Through geologic mapping and topographic surveying, six strath terraces were identified on the eastern flank of Sierra de Villicum. A comparison between the modem channel and long terrace profiles indicates that recent deformation is restricted to the eastern half of the study area. In this region, the terrace surfaces are folded - 15° from their original orientation with the fold axes of younger terraces located progressively to the east. This folding has created stepped topography that is interpreted to reflect eastward migration of an active axial surface through repeated earthquakes. Ages for the terrace surfaces were obtained through physical correlation with similar surfaces to the south of the study area. These surfaces are dated at 18700, 6800, and 1500 yr BP using cosmogenic radionuclide exposure age dating (Siame et al., 2002) Deformation rates were calculated by measuring the displacement between the preserved growth axial surface and the active axial surface for each terrace level. On the basis of the inferred terrace ages, the measured axial surface positions yields average vertical displacement rates of 1.25±0.3 mm/yr and average horizontal displacement rates of 4.6±1.0 mm/yr. The close correlation between historically documented deformation and measured fold displacements suggests the discovery of new evidence for fold growth during a devastating Ms. 7.4 earthquake in 1944. <br/> <br/>Description: Graduation date: 2004 http://hdl.handle.net/1957/9017 Title: Feeder dikes to the Columbia River flood basalts : underpinnings of a large igneous province <br/> <br/>Abstract: Feeder dikes to the Columbia River Basalt Group (CRBG) large igneous province provide a rare opportunity to examine magma transport through the shallow crust during flood basalt eruptions. Over 70% of the CRBG erupted from the Chief Joseph dike swarm, which is exposed across southeastern Washington, eastern Oregon, and western Idaho. The four manuscripts of this dissertation examine physical, thermal, and compositional characteristics of dikes from the southern Chief Joseph swarm. The majority of CRBG dikes are chilled against their wallrock, however, rare dikes have induced partial melting in their wallrock. Melt zones in tonalite wallrock adjacent to the Maxwell Lake dike are up to 4 in thick and contain up to 47 volume percent quenched silicic melt produced from dehydration-melting reactions involvir biotite, hornblende, quartz, orthoclase, and plagioclase. Melt zones record the thermal history of basalt flow and cooling in the Maxwell Lake dike, a feeder to Wapshilla Ridge flows (Grande Ronde Basalt). Results of one- and two-dimensional numerical modeling suggest that basalt flowed in the Maxwell Lake dike for 3-4 years, yielding maximum eruption rates of 3.4-4.6 km3/day for typical Wapshilla Ridge flows. The Maxwell Lake dike likely represents an upper crustal exposure of a long-lived point source in the CRBG. Chief Joseph dikes are concentrated into sub-swarms of 7-12 dikes per km2. Based on transects through four sub-swarms, dikes become more aligned, more frequent, thinner, and more closely spaced from northwest to southeast across the southern Chief Joseph swarm. Fewer than 2% of dikes, and less than 0.5% of cumulative dike length, had caused extensive melting in their wallrock. In the Cornucopia sub-swarm, numerical modeling of cross-cutting and compound dikes suggests that magmatic activity occurred intermittently over 2-4 years. Compositional data collected from -250 southern Chief Joseph dikes indicate that most are Grande Ronde Basalt, although isolated Imnaha and Dodge (Eckler Mountain Member, Wanapum Basalt) dikes also occur. Imnaha dikes are compositionally primitive, whereas Grande Ronde and Dodge dikes are more evolved. Subtle compositional differences between sub-swarms of Grande Ronde dikes suggest that each sub-swarm represents a discrete episode of Grande Ronde volcanism. <br/> <br/>Description: Graduation date: 2005 http://hdl.handle.net/1957/9016 Title: Constraints of the origin of the middle Pleistocene transition from the glacial sedimentary record of the north-central United States <br/> <br/>Abstract: This dissertation focuses on the role of ice sheets in the transition during the middle Pleistocene (-1.2 Ma) from 41-kyr glacial cycles to 100-kyr glacial cycles. This research evaluates the hypothesis that the middle Pleistocene transition (MPT) was related to the glacial erosion of a regolith mantle and the subsequent exposure of fresh crystalline bedrock. This issue is addressed through the study of glacial sedimentary sequences in Iowa, Nebraska, Kansas, and Missouri. Testing the hypothesis first required the establishment of a stratigraphic framework for pre-Illinoian glacial sequences. A chronology developed around till compositional data and chronological constraints provided by paleomagnetic measurements on glacigenic sediments, and three volcanic ashes, indicate that the midcontinent tills can be grouped under three categories representing at least seven ice advances: two older groups of reverse-polarity tills containing a low and intermediate proportion of clasts and minerals derived from crystalline bedrock, respectively, and one younger group of normal-polarity tills enriched in crystalline materials. The bulk geochemistry of the silicate fraction of the midcontinent tills was then used to evaluate the character of the rock source eroded by ice sheets. The results show a general trend in which geochemical indices, from oldest to youngest tills, fall parallel to a mixing line defined by the composition of a weathered and fresh crystalline rock sources, respectively. The content in meteoric 10Be of tills also supports the existence of a regolith and its glacial erosion by -1.3 Ma, thus concordant with the onset of the MPT. Marine records of strontium, hafnium, and osmium isotopes provide additional support for the hypothesis. The nature of the till compositional changes was further addressed through a provenance study based on 504 40Ar/39Ar ages measured on individual K-feldspar grains retrieved from tills that span the last 2 Myr. Most samples yielded ages identical to those of numerical ages of the Churchill Province, thereby constraining deposition by ice from the western (Keewatin) sector of the Laurentide ice sheet. These results rule out a change in provenance, and thus show support for a change in the composition of the Churchill bedrock source during the late Cenozoic. <br/> <br/>Description: Graduation date: 2004