GeoDaze 2007 Sponsors
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GeoDaze 2007 Sponsors GeoDaze 2007 thanks the following organizations and individuals for their support! This symposium has been made possible by their generous contributions. Organizations ChevronTexaco BP Corporation ConocoPhillips Errol L. Montgomery Associates ExxonMobil Newcrest Resources, Inc. Hydrogeophysics, Inc. (Bonnie Roripaugh) Golder Associates, Inc. Individuals Mark Melton Peter L. Kresan Donald P. Witter John & Helen Schaefer Donald R. Dietrich Elwood R. Brooks James F. Hays Raymond M. Turner Nancy D. Naeser Wade E. Miller William R. Keller John R. Matis Brian Earl Gross Ken Zonge Sally J. Meader-Roberts Douglas Silver Kevin L. Mosser Richard D. Jones Gary Huckleberry Terrence M. Gerlach Carlotta B. Chernoff Richard S. Pfirman Bernard Wallace Pipk, Trustee Anthony B. Ching Camille A. Holmgren Miles G. Shaw John P. Schloderer Cheryl Butler Corolla Hoag & Kevin Horstman Margaret E. Venable Mary Lin Windes Megan L. Anderson Louis H. Taylor John M. Sweet John & Nicea Wilder William W. Jenney Jr. Tom L. Heidrick Joseph R. Mitchell M. Stephen Enders Leslie D. McFadden The printing of this program has been underwritten by BP Corporation. i GeoDaze 2007 Committee Co-Chairs Amanda Reynolds John Volkmer Treasurer Shundong He Webmaster Lepolt Linkimer Field Trip Scott McBride Alex Strugatskiy Publications Jason Stein Stephanie McAfee Refreshments Alyson Thibodeau Kelley Stair Correspondence Audio-Visual Evaluations Slideshow Outreach Soledad Velasco Meg Blome James Girardi Adam Csank April Chiriboga Lisa Molofsky Sarah Thompson Andy Frassetto Ryan Porter Joel Saylor Advisory Board Sarah Thompson James Girardi Faculty Advisor Susan Beck Program and Poster Photograph Contributors: Dave Pearson, Jason Stein, Kevin Jones ii Co-Chairs’ Welcome GeoDaze 2007 Geoscience Symposium, The University of Arizona We wish to take this opportunity to welcome you to GeoDaze 2007, the Department of Geosciences’ 35th Annual Student Symposium. Organized by geoscience students and funded by the generosity of private donations, the symposium allows graduate and undergraduate students an opportunity to showcase their research. Every year, GeoDaze talks and posters cover a wide variety of topics: paleoclimatology, seismology, dendrochronology, isotope geochemistry, and economic and structural geology, to name a few. This year, we have divided the presentations into five general sessions: geodesy & seismology, economic geology, tectonics, surface processes & trace geochemistry, and paleoclimate. The 28 talks and 12 posters that are being presented exemplify the diversity and multidisciplinary nature of geoscience research within the department. You will find evaluation forms for both talks and posters at the registration booth, giving you an opportunity to share your unofficial comments and suggestions on student presentations. We encourage you to offer your feedback and help to make this symposium the most beneficial for our student presenters. We are pleased to have Dr. Carmala Garzione, a departmental alumna, as our keynote speaker this year. She comes to us from the University of Rochester, where she works on sedimentary basin evolution and their related tectonic histories, geochemical and petrologic provenance of clastic sedimentary rocks, and stable isotopes in paleoenvironmental studies. Her talk is entitled Sediment Accumulation and Surface Uplift in the Altiplano Basin: Geodynamic Implications for the Growth of Orogenic Plateaus. The keynote address is scheduled for Friday, April 13th, at 3 pm in the Arizona Ballroom A (North). Dr. Garzione is this year’s recipient of the department’s Outstanding Alumni Award. We would like to welcome her back to Tucson and thank her for agreeing to participate in GeoDaze 2007. Please also make room in your schedule for several events following Dr. Garzione’s talk. The Student Awards Ceremony will take place at 4 pm followed by the Annual Slide Show. In addition, we hope to see you at the Friday evening party hosted by Geosciences department head Dr. Susan Beck. The Saturday field trip will take us to the Santa Rita Mountains, south of Tucson. Dr. Charles Ferguson has agreed to design and lead the trip, which is entitled, Laramide multiphase folding, Late Cretaceous magmatism, and Cenozoic extension and sedimentation in the Mt. Fagan and Cienega Gap area, eastern margin of the Tucson basin. The University of Arizona GeoDaze Geoscience Symposium would not be possible without the generous financial support of friends of the department. We would like to thank the alumni for allowing GeoDaze to continue. In addition, we commend the faculty, staff, and students whose efforts made this year’s symposium so successful. We would also like to extend a special thank you to the student committee members…your hard work is appreciated and without you GeoDaze 2007 would not have been possible. Welcome to GeoDaze 2007! Amanda Reynolds & John Volkmer Co-Chairs 2007 Geoscience Symposium iii Table of Contents Schedule of Events . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Field Trip Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Abstracts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Session I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Session II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Session III . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Session IV . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Session V . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Session VI . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Map to the GeoDaze Party . . . . . . . . . . . . . . . . . . . . . . 50 Map of the UA Student Union . . . . . . . . . . . . . . . . . . . . . 51 iv Schedule of Events Thursday, April 11 8:15 Coffee and Pastries 8:45 GeoDaze Welcome Susan Beck SESSION I: Geodesy & Seismology 9:00 Moho Offsets on Major Continental Strike-Slip Faults: Fact or Fiction? Ryan Porter 9:15 Preliminary Results from North Anatolian Fault Passive Seismic Experiment: Receiver Functions and Anisotropy C. Berk Biryol 9:30 Seismic Anisotropy of the Lower Crust in the Central Sierra Nevada Andy Frassetto 9:45 Relationship between Peak Ground Acceleration and Modified Mercalli Intensity in Costa Rica Lepolt Linkimer 10:00 Coffee Break 10:15 Distributed Crustal Deformation Across the Eastern Basin and Range, Western USA M. Soledad Velasco 10:30 Vertical Land Motion (VLM) of the Dinarides Inferred from the Tide Gauge Data Goran Buble 10:45 Crustal Deformation in the Pacific Northwest: GPS Results from 1994-Present Sarah K. Thompson SESSION II: Economic Geology 11:00 A Belt of Coeval but Compositionally Diverse Intrusions in East-Central Nevada: Structural, Provincial, or Petrologic Control on Mineralization? Michael Takaichi 1 11:15 Structural Dismemberment of Copper Basin, Battle Mountain Mining District, NorthCentral Nevada David Keeler 11:30 The Dos Pobres Deposit: a Petrographic and Geochemical Study of Veins and Alteration Assemblages Daniel Russin 11:45 Application of VNIR Reflectance Spectroscopy to Hydrothermal Systems: Continuation of Spectral Classification of Mineralization near Copiapo, Chile Michael Strickler 12:00 Lunch Break SESSION III: Poster Session 1:00 Poster Presentations Booth # 1 2 Offset of a Bright, Shallow Moho across the Coast Shear Zone in the Coast Mountains Batholith, 52°-56° N Josh Calkins 3 New Insights into the Sierras Pampeanas Crust Using High Frequency Local Event Receiver Functions Christine R. Gans 4 Crustal Thickness in Northern Costa Rica: Preliminary Results using Receiver Function Analysis Lepolt Linkimer 5 6 2 Geophysics & Geodesy Measuring Crustal Strain Accumulation Associated with the Southern San Andreas Fault Zone Using GPS Dana Brodie Economic Geology Exploration of Genetic Links Between Breccia Pipes and Porphyry Copper Deposits in a Laramide Hydrothermal System, Sombrero Butte, Pinal County, Arizona Amy Eichenlaub Hydrothermal Alteration and its Effects on Slope Evolution in Alum Creek, Eastern San Juan Mountains, South-Central Colorado Douglas C. Kreiner Booth # 7 8 Geochemistry Provenance Interpretations from U-Pb Ages of Detrital Zircons in the Triassic Moenkopi Formation of the Colorado Plateau Carl E. Anderson Timing of Deformation within Tanque Verde Creek Jared Hamilton 9 U-Pb Detrital Zircon Studies from Cretaceous Strata on the Colorado Plateau to Determine Provenance Related to the Cordilleran Foreland Basin Jennifer L. McGraw 10 Age and Origin of Water in the Roswell Basin from Peñasco to Artesia Ryan Rodney 11 12 3:00 Navigating NAVDAT: Applications and Exploration of a Large Scale Petrologic Database Gabriel L. Rotberg Planetary Forward Modeling of the Topography of Ice on Mars to Infer Basal Shear Stress Conditions Maria Banks Coffee Break SESSION IV: Structure and Tectonics 3:15 P-T-t paths of Metapelites between Prince Rupert and Bella Coola, British Columbia: Implications for the Late Cretaceous-Early Tertiary Tectonic Evolution of the Coast Mountains Batholith David Pearson 3:30 Exhumation History of the Laramide Ranges Using (U-Th)/He Thermochronology Lynn Peyton 3:45 Early Cenozoic High Elevation in the Laramide Rocky Mountains: Estimate from Oxygen Isotopes of Freshwater Bivalves Majie Fan 4:00 Uplift, Damming and Sedimentation in the Zada Basin, Southwestern Tibet Joel Saylor 3 Friday, April 13 8:45 Coffee & Pastries 9:15 Welcome & Announcements SESSION V: Paleoclimate 9:15 Playing a Shell Game: Peruvian Marine Upwelling and Marine Chronometry from Carbon and Oxygen Isotopes Kevin B. Jones 9:30 A Cloudy Future? Simulated Variability in Neotropical Montane Forest Hydroclimatology in Response to Anthropogenic Climate Change Kevin J. Anchukaitis 9:45 Regional Tree Growth and Inferred Summer Climate in the Winnipeg River Basin, Canada since AD 1783 Scott St. George 10:00 Holocene Flood Histories in the Southwestern United States Tessa Harden 10:15 Coffee Break 10:30 Does the Arctic Oscillation Drive the Timing of Spring in the Southwest? Stephanie McAfee 10:45 Abrupt Changes in Eastern Equatorial Pacific Climate during the Holocene Jessica L. Conroy 11:00 North American Precipitation and the Cadence of Decadal Variability Scott St. George 11:15-1:00 Lunch Break SESSION VI: Surface Processes and Geochemical Tracers 1:00 4 Nutrient Sourcing of Nine Plant Species in the Southwestern US Using Strontium Isotopes Amanda C. Reynolds 1:15 A Detailed Strontium Isotope Study of the Bouse Formation, Lower Colorado River, USA: Implications for Colorado River History Jennifer A. Roskowski 1:30 Effects of Local Farming and Deforestation on Sediment Discharge Inferred From Lake Core Records and Stream Morphology, Coastal Lake Tanganyika, East Africa Michael Strickler 1:45 Coffee Break 2:00 Timing of Downcutting Events in the Upper Sutlej River, Tibet and Possible Anthropological Implications Jennifer Boerner 2:15 Lead and Strontium Isotopes as Tracers of Turquoise: A Pilot Study Alyson Thibodeau 2:30 Interactions Between Falling Chicxulub Ejecta and the Atmosphere: The Deposition of the K/T Boundary Layer Tamara Goldin 2:45 Presentation of Outstanding Alumni Award 3:00 Keynote Speaker: Dr. Carmala Garzione Sediment Accumulation and Surface Uplift in the Altiplano Basin: Geodynamic Implications for the Growth of Orogenic Plateaus 4:00 Student Awards 4:30 Slide Show 5:00 Thank You's 7:00 GeoDaze Party Saturday, April 14 Andy Frassetto and Ryan Porter 8:00am Field Trip Departure Gould-Simpson 5 Field Trip Guide Laramide multiphase folding, Late Cretaceous magmatism, and Cenozoic extension and sedimentation in the Mt. Fagan and Cienega Gap area, eastern margin of the Tucson basin Field Trip led by Charles Ferguson, USGS and sponsored by ConocoPhillips 8:00 am Depart Gould Simpson Stop 1 (8:45am) Overview of Davidson Canyon and the north end of the Empire Mountains, Old Sonoita Highway at Andrada Ranch. Discuss regional geology using maps and cross-sections. Drive 15 minutes Stop 2 (9:20 am) Late Cretaceous angular unconformity. Overturned Bisbee Group is overlain by gently southeast-dipping Fort Crittenden Formation conglomerate in the floor of the Mt. Fagan caldera. Drive 5 minutes Stop 3 (10:10 am) Walk through gully bottom section of Mt. Fagan rhyolite megabreccia with spectacularly exposed megablocks of Fort Crittenden, and Bisbee Group. Meet vans at the bottom of the gully where it crosses Highway. Drive 30 minutes Stop 4 (12:10 pm) Lunch and folded angular unconformity at the base of the Glance Conglomerate (base of the Bisbee). Overlook of Rosemont district. Drive 10 minutes Stop 5 (1:50 pm) Mt. Fagan caldera margin and intracaldera mesobreccia. Topographic scarp of the Mt. Fagan is exposed in narrow gully where it overlies the Apache Canyon Formation of the Bisbee Group. Wonderful exposures of avalanche breccia with and without tuff matrix. Restroom facilities (other than bushes) available. Drive 60 minutes 4:20 pm 6 Arrive Gould Simpson 7 NOTES 8 Abstracts 9 SESSION I: GEODESY AND SEISMOLOGY MOHO OFFSETS ON MAJOR CONTINENTAL STRIKE-SLIP FAULTS: FACT OR FICTION? Ryan Porter, Arda Ozacar, and George Zandt Department of Geosciences, The University of Arizona An important question associated with a large transform fault is its lower crustal structure and how it is influenced by its regional tectonic setting. The Denali Fault located in southern Alaska is a major transform fault influenced by the subduction of the Pacific plate beneath the North American plate. Using data from the Broadband Experiment Across the Alaska Range (BEAAR), we computed receiver functions to image changes in crustal structure across the fault. Initial results show a complicated crustal structure with Vp/Vs variations across the fault, as well as changes in crustal thickness. We will validate our initial results by exploring the possible influence of dipping structures. These data will be compared to similar studies along the San Andreas fault in California and the Altyn Tagh fault in southeast Asia. We plan to study the deep structure of large continental strike-slip faults by investigating the occurrence of large Moho offsets and their relationship to regional tectonic settings. 10 PRELIMINARY RESULTS FROM NORTH ANATOLIAN FAULT PASSIVE SEISMIC EXPERIMENT: RECEIVER FUNCTIONS AND ANISOTROPY C. Berk Biryol and Arda Ozacar Department of Geosciences, The University of Arizona The North Anatolian Fault (NAF) is one of the world’s largest continental strike-slip faults. Despite much geological work at the surface, the deep structure of the NAF is relatively unknown. The North Anatolian Fault Passive Seismic Experiment is mainly focused on the lithospheric structure of this newly coalescing continental transform plate boundary. In the summer of 2005, we deployed five broadband seismic stations near the fault to gain more insight on the background seismicity, and in June 2006 we deployed 34 additional broadband stations along multiple transects crossing the main strand of the NAF and its splays. Based on the results of receiver functions computed for five stations deployed in 2005, crust is relatively thin (35 km) in a region of high topography and has a relatively high Vp/Vs ratio (>1.8) supporting the presence of partial melt in the crust and the substantial role of mantle in maintaining the elevation. Preliminary analysis of SKS splitting for the five stations indicates seismic anisotropy with delay times exceeding 1 sec. The fast polarization directions for these stations are primarily in NE-SW orientation, which remains uniform across the NAF. This direction is at a high angle to the surface trace of the fault and crustal velocity field, suggesting decoupling of lithosphere and mantle flow. Our SKS splitting observations are also similar to that observed from GSN station ANTO in central Turkey and stations across the Anatolian Plateau in eastern Turkey indicating relatively uniform mantle anisotropy throughout the region. 11 SEISMIC ANISOTROPY OF THE LOWER CRUST IN THE CENTRAL SIERRA NEVADA Andy Frassetto, George Zandt, Hersh Gilbert, Owen Hurd, Ian Bastow, Tom Owens, and Craig Jones Department of Geosciences, The University of Arizona The Sierra Nevada Earthscope Project (SNEP) is an Earthscope-funded broadband seismic deployment in the central and northern Sierra Nevada with the objective of investigating the processes of lithospheric foundering and corresponding uplift and deformation of the crust. We use teleseismic receiver functions to characterize crustal seismic anisotropy using a raytheoretical forward modeling technique (Frederiksen and Bostock, 2000). Our preliminary models require a 5-km thick zone of 15% slow-axis anisotropy above the crust-mantle boundary, which we interpret to result from a C-S fabric striking southwesterly, consistent with the inferred direction of removal of an eclogitic, negatively buoyant granitoid residue. Current efforts are focusing on refinement of the forward model and performing a neighborhood inversion of these receiver functions to constrain the trade-off between anisotropy parameters and thickness of the fabric throughout the central Sierra Nevada. These observations also correspond to a region beneath the western foothills of the Sierra which lacks a coherent P-S converted phase from the crust-mantle boundary and contains several clusters of anomalously deep seismicity. We believe that the absent Moho signature on receiver functions, combined with the presence of deep crustal seismicity, identifies a zone of actively foundering lithosphere. Our new observations of seismic anisotropy match well with modeled orientation of crustal shear induced by the delamination of a dense ultra-mafic root from beneath the eastern Sierra Nevada (Le Pourhiet et al., 2006) 12 RELATIONSHIP BETWEEN PEAK GROUND ACCELERATION AND MODIFIED MERCALLI INTENSITY IN COSTA RICA Lepolt Linkimer1 and Victor Schmidt2 1 2 Department of Geosciences, The University of Arizona Laboratory of Earthquake Engineering, University of Costa Rica We have developed the first relationship between Modified Mercalli Intensity (MMI) scale and both the horizontal (h) and vertical (z) components of Peak Ground Acceleration (PGA) for the Costa Rican region using regression analysis of 110 earthquakes (2.9 < Mw < 7.7) that occurred between 1983 and 2004. PGA and MMI were acquired from the Laboratory of Earthquake Engineering at the University of Costa Rica and the National Seismological Network of Costa Rica. Additionally, isoseismal maps were made for three significant events: the 2002 Bijagua (5.2 Mw), 2002 Burica (6.2 Mw), and 2004 Christmas (6.6 Mw) earthquakes. For each instrumentally recorded event, a single MMI value was assigned based on the general proximity to one or more MMI observations. A total of 429 values of PGA in both the horizontal and vertical components were associated to a single MMI value. The general correlation obtained was: MMI = 2.74 log (PGAh) + 0.1759 MMI = 2.30 log (PGAz) + 0.3739 Slightly different correlations were obtained when MMI and PGA were plotted separately for earthquakes occurring at different source depths (crustal and subduction earthquakes), local geology at the seismic station (rock, hard, soft), frequency range at which the strong motion instrument record, and six individual significant earthquakes. Previous studies in many regions of the world have shown that MMI and instrumental ground motion relationships are region specific and thus these relationships should be carefully assessed for each area. In Costa Rica the seismic intensity is the only available parameter from which to quantify the level of ground shaking of the historical earthquakes and is still the only ground shaking parameter observed in much of the country because the coverage of seismic instruments is limited. 13 DISTRIBUTED CRUSTAL DEFORMATION ACROSS THE EASTERN BASIN AND RANGE, WESTERN USA M. Soledad Velasco, Roy A. Johnson, and Rick A. Bennett Department of Geosciences, The University of Arizona Geodetic measurements observed across fault zones are usually represented by elastic strain accumulation models that relate strain along the locked seismogenic parts of the faults and strain patterns at depth. These patterns are highly dependent on fault geometry. The prediction of slip on a fault to match the GPS data may not always reflect the true subsurface fault geometry. However, the use of seismic reflection data can aid in building a more detailed structure model to interpret GPS data and could have a significant difference on the strain field predicted otherwise. In north-central Utah, USA, published geodetic measurements reveal east-west extensional deformation of about 3 mm/yr across a ~200-km-wide swath that spans the eastern Basin and Range and the Wasatch front (Friedrich et al., 2003; Niemi et al., 2004). The Wasatch fault has been considered the major, presently active tectonic structure accommodating this horizontal extension. However, Global Positioning System (GPS) results show a horizontal displacement rate of 1.6 ± 0.4 mm/yr across the Wasatch fault (Chang et al., 2006), accounting for only ~50% of the crustal deformation. Seismic reflection data from the Great Salt Lake (GSL) suggest that other major normal faults are active west of the Wasatch fault that could be accommodating part of the remaining 50% of the northern Basin and Range extensional deformation. Palinspastic reconstruction of depth-converted seismic sections involve approximately 83% of Tertiary extension across the GSL, and high fault slip rates in the Pliocene (~1.1 mm/yr) and Quaternary (~0.7 mm/yr), along with lake-floor bathymetric scarps, imply that these faults remain tectonically active. A more detailed structure model in this area can assist in determining differences in loading rates and better account for elastic strain accumulation within the upper and lower crust. 14 VERTICAL LAND MOTION (VLM) OF THE DINARIDES INFERED FROM THE TIDE GAUGE DATA Goran Buble Department of Geosciences, The University of Arizona This talk will present the results of the tide gauge data analysis for the region of the Eastern Adriatic Sea. Method developed by Davis et al. (1999) was applied to tide gauge data from the eight sites (in Italy, Slovenia, Croatia and Montenegro) with long time series (30-95 yr). This method models the observations (annualy averaged sea level data) and assumes for the interannual sealevel variations to be constant for all sites. Correlation ceofficents, determined between residuals for the each site and residuals for the Trieste site prove the interannual sealevel variations to be the constant. This method is unique in that it differences sea level estimates with respect to one site (Trieste, Italy), eliminating bias and resulting in relative vertical crustal rates with respect to geoid. Also, future plans regarding the continous GPS network planned to be built in summer 2007 on the Eastern Adriatic coast will be presented. 15 CRUSTAL DEFORMATION IN THE PACIFIC NORTHWEST: GPS RESULTS FROM 1994-PRESENT Sarah K. Thompson, Sigrun Hreinsdottir, and Rick A. Bennett Department of Geosciences, The University of Arizona Crustal deformation in the Pacific Northwest is the result of the superposition of several tectonic processes, culminating in a complex deformation field. Long-term deformation in Cascadia is the combination of interseismic, cosesismic, and transient deformation signals. This overall deformation field is attributed to large-scale tectonic processes, such as subduction and block rotation, which are in turn related to overall western US tectonics. Observed crustal deformation also has important links to the seismogenic cycle, including strain accumulation and slow slip events. Transient deformation in Cascadia, also known as Episodic Tremor and Slip (ETS) occurs every 14 months with maximum displacements ranging from 5-10 cm. How exactly this transient reversal of the interseismic deformation field relates to the seismogenic cycle is still not well known. Results from 80 continuous GPS stations in the Pacific Northwest, beginning in 1994, are presented, with an emphasis on how ETS relates to the interseismic deformation field. 16 SESSION II: ECONOMIC GEOLOGY A BELT OF COEVAL BUT COMPOSITIONALLY DIVERSE INTRUSIONS IN EAST-CENTRAL NEVADA: STRUCTURAL, PROVINCIAL, OR PETROLOGIC CONTROL ON MINERALIZATION? Michael Takaichi Center for Mineral Resources, Department of Geosciences, The University of Arizona Economic geologists have long been interested in the relative importance of faults and other tectonic features, crustal compositions, and petrologic factors in controlling the productivity of magmas and the metal ratios of associated mineral deposits (i.e., structural, provincial, and geochemical process controls, respectively). A series of Mid-Cretaceous plutons were emplaced through a 15-km thick sequence of Proterozoic and Paleozoic miogeoclinal rocks in east-central Nevada. These intrusions define a 150-km long, east-west trending belt through Eureka and White Pine Counties at 39°15’N latitude, perpendicular to the continental margin and behind the axis of the Cordilleran arc. The intrusions are dated at ~110 Ma but differ in their igneous compositions, their economic productivity, and the type of associated mineralization. A dacitic body in the Pancake Range (110 Ma) and several gneissic tonalite intrusions in the Snake Range (102 and 108 Ma) are barren of mineralization. A quartz monzonite pluton and associated feldspar porphyries in the Robinson district (109.5 Ma) generated a porphyry Cu-(Mo-Au) system, whereas quartz porphyry bodies associated with granodiorite (106.3 Ma) in the Eureka district produced a porphyry Mo-Cu system with base-metal skarn and replacement deposits. The Monte Cristo stock in the Hamilton district (106.6 Ma) likewise is a porphyry Mo-Cu system. Butte Valley is a covered porphyry copper deposit that closely resembles Robinson but is related to quartz porphyries of uncertain age. Limited published Nd and Sr isotopic data suggest that plutons associated with the porphyry Cu-(Mo-Au) systems such as Robinson have a strong mantle component, but porphyry Mo-Cu systems such as Monte Cristo have a substantial crustal component. The origin of the east-west trending belt is unknown, but possibilities include structural flaws in either the upper crust, the crystalline basement, or the downgoing slab during Mesozoic subduction. The apparent uniformity in the crustal composition within the miogeocline argues against a significant provincial control on mineralization. Differences in magma source suggest that the metallogeny of this belt is controlled more by complex petrologic processes than by structural or provincial factors. 17 STRUCTURAL DISMEMBERMENT OF COPPER BASIN, BATTLE MOUNTAIN MINING DISTRICT, NORTH-CENTRAL NEVADA David Keeler Department of Geosciences, The University of Arizona North-central Nevada is intensely mineralized, such that in some cases genetically unrelated systems partially overlap. In addition, the region is variably dismembered by Tertiary Basin and Range normal faults, complicating the interpretation of zoning patterns in mineral deposits. In the Copper Basin area, the age of the copper mineralization, the timing of the onset of extension, and the amount of tilting have been debated but remain to be resolved. The Copper Basin area is located approximately 15 km southwest of Battle Mountain, Nevada, and 9 km north of Copper Canyon (Newmont’s Phoenix project), within the Battle Mountain mining district. The Copper Basin area contains a Late Cretaceous (~95 Ma) porphyry molybdenum deposit (Buckingham) and several Eocene (35.4 – 39.9 Ma) porphyry-related gold ± copper deposits in skarn and silica-pyrite bodies (Labrador, Empire, Northern Lights, Surprise, Carissa), which are similar in age to the Copper Canyon porphyry Cu-Au deposit. The supergene copper deposits (Contention, Sweet Marie, Widow) are of less certain genetic affiliation. The copper in these supergene deposits could be remobilized either from Eocene Au-Cu deposits or from the copper-rich portion of the Late Cretaceous porphyry Mo-Cu systems. The area is structurally complex, with Paleozoic thrust faults cut by several sets of normal faults. The first set of normal faults strikes north-northwest and dips to the northeast and has displacements up to several hundred meters. Northeast striking normal faults dip steeply to the northwest and have displacements up to a few hundred meters. The displacement of an east-west striking fault that dips gently to the north is poorly constrained. Continuing work on geologic maps of the hanging wall and footwall surfaces of faults will more rigorously constrain the amount of displacement on major faults in the region. These maps, along with additional U-Pb dates on igneous rocks of interest, allow for a more detailed reconstruction and a better understanding of the timing of extension. This will better constrain the source of the copper in the supergene deposits and will benefit exploration in the district. 18 SESSION III: POSTERS THE DOS POBRES DEPOSIT: A PETROGRAPHIC AND GEOCHEMICAL STUDY OF VEINS AND ALTERATION ASSEMBLAGES Daniel Russin Department of Geosciences, The University of Arizona The Dos Pobres deposit in the Safford district of southeast Arizona is an andesite-hosted porphyry copper-gold-molybdenum system of Laramide age. It is nearly intact albeit tilted, and is cut by the post-mineral Butte Fault. The purpose of this study, funded by Phelps Dodge, is to use selected core logging and detailed petrography coupled with bulk geochemical and microprobe analyses to construct a model of the distribution of vein types, their associated alteration, and ore mineralogy. Like most porphyry systems, Dos Pobres shows multiple types and generations of veins. There are several sets of sulfide-absent veins: hairline biotite veins, wispy quartz veins, calcite veins, and veins consisting of one or more species of zeolites. The mineralized veins comprise several different types of quartz-sulfide dominated veins: quartz-sulfide veins with chlorite±calcite±epidote envelopes; chlorite±sericite envelopes; and potassic (biotite or Kfeldspar) envelopes. These veins are all dominated by quartz and sulfides but also contain various amounts of chlorite, epidote, biotite, anhydrite, etc. The magnetite component of these veins increases with depth. Additionally, these veins often contain molybdenite, especially in the deeper distal regions of the deposit. The sulfide-absent biotite and quartz veins are the earliest veins observed. They are cut by sulfide-bearing veins. In general, veins with chlorite±sericite and chlorite±calcite±epidote envelopes cut those with potassic envelopes. The zeolite and calcite veins post-date sulfide introduction. The distribution of ore minerals is simple; bornite is dominant in the center of the deposit; chalcopyrite surrounds the bornite core and gradually gives way to pyrite. This geometry gives a bulls-eye pattern in map view and concentric domes in cross-section. This deposit has been speculated to be an intermediate style that combines aspects of both porphyry and iron oxide-copper-gold (IOCG) deposits. This is largely based on observations in Dos Pobres of local magnetite abundances up to around 10%. Since IOCG deposits often have well over 50% iron oxides, this has led some to hypothesize that they and Dos Pobres may share some aspects of their origins. 19 APPLICATION OF VNIR REFLECTANCE SPECTROSCOPY TO HYDROTHERMAL SYSTEMS: CONTINUATION OF SPECTRAL CLASSIFICATION OF MINERALIZATION NEAR COPIAPO, CHILE Michael Strickler and Mark D. Barton Department of Geosciences, The University of Arizona Reflectance spectroscopy using visible and near-infrared wavelengths (VNIR) is a powerful tool for identifying many minerals and characterizing selected aspects of their compositions and structural states. Using publicly available data (USGS) and our own, previously characterized samples we developed a set of data libraries and routines to examine sheet silicates and other mafic minerals that occur in igneous, metamorphic, and hydrothermally altered rocks. The Applied Spectral Devices (ASD) FieldSpec Pro VNIR spectrometer belonging the mineral deposits group was used for this work. The FieldSpec Pro rapidly acquires reflectance (or other) spectral data at wavelengths between 350 and 2500 nm with variable resolution (2 nm increasing to 12 nm at long wavelengths). Given this resolution others have shown that it is possible to routinely identify many minerals and/or characterize compositions where other methods (e.g., Xray diffraction or electron microprobe) are much more time intensive. As a result, this method offers a rapid, semi-quantitative approach to mineral characterization – an essential step in mapping, interpreting, and understanding hydrothermal mineral deposits. The purpose of this study is to determine the nature and distribution of sheet silicates and mafic silicates that are indicators of the intensity and type of hydrothermal alteration. Key characteristics include not only the mineral ID, but also the mineral assemblages, and, to the extent possible, information on the structural state and compositions of the minerals. These results can then be used in better understanding zoning of hydrothermal minerals and the distribution of other rock types. Reflectance spectra have been obtained on multiple samples that were collected as part of an ongoing field project in the Copiapo batholith and surrounding rocks. This project has been sponsored by Phelps Dodge Exploration and contributes to a district-scale exploration program around their Candelaria deposit. We have examined a large number of located samples from the district and some from the deposits to determine: (1) the identity of the sheet silicates and related minerals, (2) variations in the compositions of mafic minerals which can indicate differences in conditions of origin, and (3) a simple spatial analysis of these distributions in light of existing mapping. Preliminary results from spectroscopy, mapping, and electron microprobe analyses show that there are systematic and major changes in mineral assemblages and compositions across the district. Significant reflectivity differences exist between different mineral associations in the Copiapo batholith, its metamorphic aureole, and the various parts of the hydrothermal systems. These results might also be used as a foundation for remote sensing applications using airborne or satellite hyperspectral datasets. 20 PROVENANCE INTERPRETATIONS FROM U-Pb AGES OF DETRITAL ZIRCONS IN THE TRIASSIC MOENKOPI FORMATION OF THE COLORADO PLATEAU Carl E. Anderson Department of Geosciences, The University of Arizona Detrital zircon grains in the Moenkopi Formation, which rests on continental redbeds and marine limestones of Permian age, were sampled from fluvial sandstones of the Lower Triassic (Olenekian) Torrey Member in Red Canyon, Utah, and the Middle Triassic (Anisian) Holbrook Member near Winslow, Arizona. Paleocurrent indicators and the wide lateral extent of the Moenkopi Formation imply derivation of the sediment from sources lying to the southeast of the Colorado Plateau. U-Pb ages of 100 detrital zircon grains from each Moenkopi sample were analyzed by LA-ICP-MS using a beam diameter of 50 microns. Analytical data were then filtered to reject grains with >20% age discordance. Varied detrital zircon age populations include prominent clusters of Permian-Triassic and Neoproterozoic grains and several clusters of Mesoproterozoic grains (1800-1000 Ma) apparently derived from the Grenville and YavapaiMazatzal provinces. The Permian-Triassic grains were probably derived from the East Mexico magmatic arc, and Paleozoic and Neoproterozoic grains most likely from the OuachitaAppalachian orogen. Mesoproterozoic grains were probably derived from both the YavapaiMazatzal belt (1800-1600 Ma) of southwestern Laurentia, including granitic plutons (1500-1400 Ma) intruded into Yavapai-Mazatzal bedrock, and from Grenville rocks either flanking the Appalachian-Ouachita orogen or exposed beyond the Ouachita-Marathon suture in eastern Mexico (Oaxaquia). The variety of sources for the detrital zircons implies that headwaters of the Moenkopi fluvial systems embraced southwest Laurentia, the Marathon suture belt, and eastern Mexico of Gondwanan derivation south of the suture belt. 21 MEASURING CRUSTAL STRAIN ACCUMULATION ASSOCIATED WITH THE SOUTHERN SAN ANDREAS FAULT ZONE USING GPS Dana Brodie Department of Geosciences, The University of Arizona Using a dense network of GPS stations located in the eastern Transverse Ranges Province, a transitional zone between the southern San Andreas and eastern California shear zone, we determined crustal velocities at more than 40 sites. We revisited three sites in 2006 and plan to measure at least three more in March 2007 located along Interstate-10 in southern California, which had been previously measured in 2000 after the Hector Mine earthquake by the National Geodetic Survey. We combined this with existing continuous stations located in the study area and the campaign data from Joshua Tree National Park collected by the University of Arizona’s Geodesy Laboratory. The Joshua Tree National Park campaign data set consists of five measurements of 20 sites roughly equally spaced between September 2005 and February 2007. From the data, we were able to create a profile showing how the velocity rates decrease as the sites move further east from the San Andreas. We were also able to determine the velocity rates before the Hector Mine earthquake from previous visits to the sites along Interstate 10 provided by SCEC, which we compared to the current velocity rates. 22 OFFSET OF A BRIGHT, SHALLOW MOHO ACROSS THE COAST SHEAR ZONE IN THE COAST MOUNTAINS BATHOLITH, 52°-56° N Josh Calkins and George Zandt Department of Geosciences, The University of Arizona Common conversion point stacks of receiver functions from the passive seismic component of the Batholiths Continental Dynamics project reveal a ~7 km offset in the Moho discontinuity in the vicinity of the Coast Shear Zone (CSZ) in western British Columbia. Cross sections from the southern transect, near 52° North, reveal a bright, nearly flat Moho at 33 km east of the CSZ and at 26 km west of the CSZ. Analysis of well-recorded crustal multiples reveals a change in the Vp/Vs ratio coincident with the change in crustal thickness. The high Vp/Vs (~1.88) to the west of the CSZ indicates the average crustal composition is more mafic than that to the east, where Vp/Vs averages 1.76. A similar, if somewhat less distinct change in Moho depth and Vp/Vs ratio is observed near the CSZ on the northern line, between 53.5° and 55.5° North, indicating that the CSZ is a crustal scale structural feature in the study area. The high amplitude of the Moho converted phase throughout the Batholiths project area indicates a sharp velocity contrast between the lower crust and upper mantle, suggesting that a residual ultramafic root is not currently present beneath the Coast Mountains Batholith. 23 NEW INSIGHTS INTO THE SIERRAS PAMPEANAS CRUST USING HIGH FREQUENCY LOCAL EVENT RECEIVER FUNCTIONS Christine R. Gans and Josh Calkins Department of Geosciences, The University of Arizona Flat slab subduction of the Nazca plate beneath the western margin of South America is thought to control the tectonic deformation of the Andean Precordillera and Sierras Pampeanas between 30° and 32° S. Several arrays of broadband seismic instruments have been deployed in Chile and Western Argentina to study this phenomenon (e.g., CHARGE, 2000-2002; CHARSME, 11/2002 - 01/2003). In order to better constrain the crustal structure in the transition region between flat slab and normal subduction to the south, we have calculated local receiver functions from earthquakes occurring beneath the stations of the CHARGE and CHARSME arrays, and then compared results to published teleseismic receiver functions. Initially, 37 intermediatedepth local earthquakes recorded by the CHARGE array were used to calculate high frequency receiver functions at station JUAN. The improved vertical resolution afforded by the higher frequency content of the local earthquakes reveals complex layering throughout the crust below JUAN. Further, forward modeling of the combined local/teleseismic data set indicates a gradational increase in velocity in the lower crust. This initial study has since been expanded to the surrounding region using CHARGE station USPA, as well as data from the more densely spaced three month CHARSME array, which was deployed directly above a cluster of earthquakes occurring near 100 km depth. The new results allow us to image the crustal thickness in this region, as well as a high-velocity mid-crustal layer. 24 EXPLORATION OF GENETIC LINKS BETWEEN BRECCIA PIPES AND PORPHYRY COPPER DEPOSITS IN A LARAMIDE HYDROTHERMAL SYSTEM, SOMBRERO BUTTE, PINAL COUNTY, ARIZONA Amy Eichenlaub Department of Geosciences, The University of Arizona Bell Resources Corporation, Vancouver, Canada Breccia pipes represent high grade ore bodies that are of interest because of their possible association with porphyry copper deposits. The Sombrero Butte breccia pipe system is located in the Copper Creek Mining district, 40 miles north of Tucson in the Galiuro Mountains. This system of more than one dozen breccia pipes lies in a well known Laramide porphyry copper belt that runs northwest to southeast across the state of Arizona. Breccia pipes at Sombrero Butte outcrop as 10-20m circular-oval shaped bodies which extend to over 490m vertically and tend to narrow at depth. The breccia pipes vary between vertical to plunging 10-20 degrees. Mineralization is concentrated within the breccia pipes with higher grade material located along the edges in highly fractured ring structures surrounding the pipe. Mineralization drops significantly when leaving the breccia pipes and entering surrounding granodiorite. Breccia pipe formation is believed to be due to the forces involved with a suspected underlying magma chamber. Exsolved magmatic gases from such a hypothetical magma body rise and collect in a chamber or large cavity above the magma. As more gas accumulates and the gas bubble increases in size, the roof of the chamber at some point in time can no longer withstand the weight of the overlying rocks. Collapse of the overlying rocks begins, and the breccia pipe propagates upward toward the surface. This porous column of collapsed rock works as a conduit of open spaces and fractures, which aqueous magmatic fluids can easily travel through accounting for high concentrations of copper mineralization. The aqueous magmatic fluids are thought to be the same type that produce porphyry copper systems, leading to the conclusion that such a system might be present at depth beneath the breccia pipes. Breccia pipes at Sombrero Butte are dominated by angular to subangular clasts of granodiorite which vary between clast and matrix supported, with clast size varying from centimeters to tens of meters. Matrix composition can vary from chl-cal-spec hem-qtz-amethyst-kspar-rhodo to tourm-bio-chl to chl-ser-hem. The dominant alteration throughout the Campstool pipe is potassic-feldspar alteration, which becomes increasingly pervasive in the granodiorite (host) below the breccia pipes. The upper portions of many of the breccia pipes contain approximately 200 meters of strong, earthy, hematitic leached capping, followed by a deeper more highly mineralized 175 meter zone of cc-bn-cpy mineralization. In looking at the detailed mineralogy and alteration in a 645m section of drill core and in thin section there is evidence that the breccia pipes are underlain by a porphyry copper system. Evidence such as porphyry dikelets as breccia matrix, hypersaline fluid inclusions, andalusiteKspar-shreddy biotite-rutile alteration, and cp-bn mineralization are indications of a link to an underlying hydrothermal ore deposit. 25 TIMING OF DEFORMATION WITHIN TANQUE VERDE CREEK Jared Hamilton Department of Geosciences, The University of Arizona The Tanque Verde Creek area just east of Tucson provides an excellent exposure of ductily deformed rocks. Most of the exposed rock is gneissic in composition and exhibits mylonitization with S and C fabrics that display a top to the southwest sense of shear. Lineation in the mylonites is oriented shallowly dipping to the southwest. The location and orientation of these deformed rocks have led to this area to be identified as a shear zone associated with the Catalina metamorphic core complex. Despite the area’s proximity to Tucson and popularity with geologists a definitive date of deformation has not been determined. Numerous, generally granitic, intrusions are prevalent throughout the area, in the form of both dikes and sills. Some of these intrusions appear deformed while others do not display obvious deformation. Samples of these dikes have been collected and examined at both the hand sample and petrographic scale to determine the presence of deformation. Using zircons from these samples an age of deformation is being constrained using U-Pb in six samples from two elevations within the shear zone. This should give us more insight into the formation of this highly studied mountain range. 26 HYDROTHERMAL ALTERATION AND ITS EFFECTS ON SLOPE EVOLUTION IN ALUM CREEK, EASTERN SAN JUAN MOUNTAINS, SOUTH-CENTRAL COLORADO Douglas C. Kreiner Department of Geosciences, The University of Arizona Hydrothermal alteration may affect the competency of rocks, by changing the cohesive properties as a function of mineralogy. A hydrothermal system, centered on Alum Creek Basin, resulted in intense alteration in the 1.5 km2 study area. Two styles of alteration are exposed in the Alum Creek Basin: a proposed weak Cu-Mo porphyry system at lower levels and an acidsulfate epithermal system at upper levels (Bove et al., 1995). Seven different alteration mineralogies were mapped and analyzed using petrographic analyses. The primary alteration assemblages in Alum Creek are characteristic of acid-sulfate alteration and consist of quartzsericite-pyrite (QSP) in the core grading upwards into intermediate argillic (IARG), and at higher elevations and upper levels of the system is a clay-rich intermediate argillic (CIARG) assemblage. Advanced argillic (AARG) rocks are present at the highest levels of the system with propylitic rocks extending distally from the core of the system. Detailed analysis of the alteration facies through thin-section petrography, x-ray diffraction, and x-ray fluorescence indicated that CIARG have a quartz:clay ratio of 1.1, QSP rocks have a value of 1.6, and IARG rocks have a value of 1.3. Slopes on the CIARG assemblage have less than 15% vegetation and are characterized by the greatest number of slopes exceeding 36° (angle of repose for CIARG), while slopes on IARG and QSP are greater than 75% vegetated and have slopes at angles less than 30°. The angle of slopes in Alum Creek is controlled by the quartz:clay ratio of the mineral assemblage on individual slopes (of 10’s of meters). The initial process in slope retreat in Alum Creek is the failure of the CIARG rocks as a landslide. Quartz-rich rocks on the slope will then retreat in order to maintain an angle of repose based on their own physical properties. The entire hillslope is thus affected by the initial failure of the CIARG mineral assemblage. The presence of talus deposits (typically composed of QSP, AARG, and/or propylitic rocks) stabilizes slopes at the angle of repose of talus (42°), which is greater than would otherwise be stable for a slope composed of CIARG colluvium (36°). 27 CRUSTAL THICKNESS IN NORTHERN COSTA RICA: PRELIMINARY RESULTS USING RECEIVER FUNCTION ANALYSIS Lepolt Linkimer, Susan Beck, and George Zandt Department of Geosciences, The University of Arizona Costa Rica is located near the southern end of the Middle American Trench (MAT) in a complicated tectonic setting controlled by the interaction of four major crustal blocks: the Cocos, Caribbean, and Nazca plates and the Panama microplate. The oceanic Cocos plate subducts to the northeast underneath the Caribbean plate and the Panama microplate, creating a volcanic arc located 150 km away from MAT. In northern Costa Rica, the arc basement is represented by serpentinized peridotites, Albian silicic sediments, and Paleocene to Middle Eocene turbidites. Magmatic units vary from basalts to rhyolites including pyroclastic deposits, subvolcanic bodies, and lavas with the only known localities of mantle xenoliths in Central America. For this study, receiver functions have been calculated using telesesimic events (mb > 5.4) recorded by the Pocosol Seismic Network, an NSF-founded array of three stations deployed by Rutgers University in Northern Costa Rica between April of 2005 and January 2007. Preliminary results shows that Moho is visible at depths of 32 km beneath station JOC, and 27 km beneath station TIR. For the entire Costa Rican region, previous studies have determined a slightly thicker crust with average Moho depths of 34 km using Pn and Pg travel time residuals (Quintero and Kulhánek, 1998) and 40 km using 2-D forward modeling of seismic refraction data (Sallarès, et al., 2001). Future work will attempt to calculate receiver functions using more available broadband data from instruments located throughout Costa Rica, such as station JTS from the Global Seismograph Network, stations HDC and HDC2 from the GEOSCOPE Network, and 15 stations of the CORISUBMOD Experiment deployed between May 2005 to January 2007 by the Institut für Geophysik (ETH, Zürich). Understanding the nature and thickness of the crust is a relevant topic for the Costa Rican isthmus since it may help determine whether the basement is part of the Caribbean plateau or is a fragment of oceanic crust that was accreted to the margin. 28 U/Pb DETRITAL ZIRCON STUDIES FROM CREATCEOUS STRATA ON THE COLORADO PLATEAU TO DETERMINE PROVENANCE RELATED TO THE CORDILLERAN F ORELAND BASIN Jennifer L. McGraw and Carla M. Eichler Department of Geosciences, The University of Arizona Sandstones from several formations on the Colorado Plateau were sampled for detrital zircon U/Pb analysis as part of an integrated study pertaining to the Mesozoic succession of the plateau, and leads to our interpretation of sediment delivery into the Cordilleran foreland basin. The locations selected for sampling include three from a widespread Lower Cretaceous fluvial blanket (Cedar Mountain and Burro Canyon Formations), three from lower Upper Cretaceous (Turonian) deltaic tongues (Ferron, Toreva, Gallup), and four from upper Upper Cretaceous (Campanian) but pre-Laramide fluvial to deltaic complexes (Castlegate, Wahweap, Menefee). The purpose of this sampling was to test our hypothesis of provenance of sandy sediment from the Sevier thrust belt and the Mogollon highlands. Marine shoreface and shelf sandstones deposited basinward from the Upper Cretaceous units were not sampled to avoid possible longshore redistribution of detrital zircons by strandline processes or marine currents. A total of ten samples were collected and processed using a LA-ICP-MS to determine ages of 1000 grains. Some grains were excluded for high age discordance and the rest were used in the final age distribution. From the final grains, we see that the detrital zircons sampled had sources from Precambrian basement of the Mogollon highlands, Mesozoic volcano-plutonic assemblages of the Cordilleran magmatic arc, Paleozoic to Mesozoic strata within the Sevier thrust belt, and sedimentary cover over the Mogollon Highlands. The ages from the detrital zircons imply that each source interpreted was significant, and that both the Sevier thrust belt and the Mogollon highlands contributed sediment to the Cordilleran foreland basin on the Colorado Plateau. 29 AGE AND ORIGIN OF WATER IN THE ROSWELL BASIN FROM PEÑASCO TO ARTESIA Ryan Rodney and Christopher Eastoe Department of Geosciences, The University of Arizona Groundwaters in the southern Roswell basin, and surface water from the Pecos River, have isotopic characteristics indicating an origin of high-elevation precipitation from the Sacramento Mountains. Most samples plot on an evaporation trend, showing that some recharge has occurred from surface water originating in the Sacramento Mountains. The principal ions in the groundwater are calcium, bicarbonate and sulfate, consistent with dissolution of gypsum and calcite from the local stratigraphy. Sulfur isotopes are consistent with dissolution of Permian marine gypsum. Tritium content of groundwater indicates that groundwater west of Hope, NM, contains some water that fell as rain since 1953. East of Hope, on the Pecos River plain, groundwater has been resident since before 1953. Pecos River surface water has no detectable tritium, which indicates the main source of water is ground water that fell before 1953 and is now discharging into the river. West of Hope, groundwater contains 58 to 75 percent modern carbon (pMC), while east of Hope the groundwater contains 29 to 36 pMC. Thus there appear to be two groundwater domains of different groundwater age, the boundary in the study area being an escarpment near Hope. 30 NAVIGATING NAVADAT: APPLICATIONS AND EXPLORATION OF A LARGE SCALE PETROLOGIC DATABASE Gabriel L. Rotberg, Mihai N. Ducea, Mark D. Barton, Lukas Zürcher, Kelley N. Stair, and Michael Strickler Department of Geosciences, The University of Arizona The North American Volcanic and Intrusive Rock Database (NAVDAT) is an NSF-funded, spatially referenced database for geochemical and age data of Mesozoic and younger igneous rocks in western North America. Since data entry started in 2002, over 44,000 samples have been added to the database, which can be queried from the online web interface (navdat.kgs.ku.edu). Data included for each sample is extensive, and comprises items such as location, the results of one or more dating methods, geochemical and isotopic data, rock type and sample descriptions. In addition, the database can include a large amount of other data items that might be a part of the complete sample analysis, and can be expanded to encompass developments in methodology. The power of a large scale petrologic database such as NAVDAT comes in the potential to easily visualize and plot large amounts of complex data. Through a few case examples, using NAVDAT data from study areas that are relevant to current research at the University of Arizona, some of the potential applications of a temporally and spatially referenced geochemical database will be explored. One such application could be the use of previously collected geochemical data from NAVDAT to quickly establish the geologic context of a study area. Animations created from downloaded datasets could utilize geochemical and age data from samples to produce maps that evolve through time, expressing trends that would not otherwise be apparent. Trends and patterns in igneous activity identified through such applications could be correlated with local and regional investigations of tectonics, petrology, and structure to produce a more comprehensive study. 31 FORWARD MODELING OF THE TOPOGRAPHY OF ICE ON MARS TO INFER BASAL SHEAR STRESS CONDITIONS Maria Banks and Jon Pelletier Department of Geosciences, The University of Arizona Understanding the history of ice caps on Mars could reveal important information about Martian geologic and climatic history. To do this, an ice-reconstruction model is needed that operates in 3D over complex topography and can be constrained with a limited number of free parameters. In this study we developed a threshold-sliding model for ice-cap morphology based on the classic Bingham (perfectly-plastic) model of Nye later extended to 3D geometries with complex bed topography by Reeh and colleagues. We have updated the Nye-Reeh model with a new numerical algorithm. Although the model was originally developed to model perfectly-plastic deformation, it is applicable to any ice body that deforms when a threshold basal shear stress is exceeded. The model requires three inputs: a DEM of bed topography, a "mask" grid that defines the position of the ice terminus, and the threshold basal shear stress. To test the robustness of the model, the morphology of the Greenland ice sheet is reconstructed using an empirical equation between threshold basal shear stress and ice-surface slope. The model is then used to reconstruct the morphology of ice draping impact craters on the margins of the South Polar Layered Deposits using an inferred constant basal shear stress of ~0.6 bars for the majority of the examples. This inferred basal shear stress value is almost 1/3 of the average basal shear stress calculated for the Greenland ice sheet. What causes this lower basal shear stress value on Mars is unclear but could involve the strain-weakening behavior of ice. 32 SESSION IV: STRUCTURE P-T-T PATHS OF METAPELITES BETWEEN PRINCE RUPERT AND BELLA COOLA, BRITISH COLUMBIA: IMPLICATIONS FOR THE LATE CRETACEOUS-EARLY TERTIARY TECTONIC EVOLUTION OF THE COAST MOUNTAINS BATHOLITH David Pearson1, Mihai N. Ducea1, and Margaret Rusmpore2 1 Department of Geosciences, The University of Arizona 2 Department of Geology, Occidental College The Coast Mountains Batholith of western British Columbia is a composite magmatic arc that contains exceptionally large amounts of magmatism of Jurassic through Early Tertiary age. Virtually the entire length of the arc is cut by the Coast shear zone, a northwest-striking zone of ductile deformation that continues for >1200 km along the western margin of North America (Rusmore et al., 2001) and separates blocks with significantly different conditions of metamorphism and extents of Late Cretaceous-Early Tertiary magmatism (Crawford and Hollister, 1982). On a large scale, new thermobarometric results from metapelites generally indicate that rocks from a ~30 km wide by ~200 km long region just west of the Coast shear zone record metamorphism at mid-crustal depths (~6 kbar). Research to the north, in southeast Alaska and northern coastal British Columbia, documented an inverted metamorphic gradient with increasing pressure eastward toward the Coast shear zone (e.g., Hollister and Crawford, 1982). No such pressure gradient is apparent in thermobarometric data in the current study farther south. In Mussel Inlet, ~25 km west of the Coast shear zone and ~100 km northwest of Bella Coola, zircons from an orthogneiss yielded an Early Devonian U-Pb crystallization age, suggesting a correlation of rocks in Mussel Inlet with the Scotia-Quaal metamorphic belt ~120 km to the north. A syntectonically deformed tonalite ~4 km west of this Early Devonian orthogneiss yielded a U-Pb zircon age of 85.7+1.8 Ma. Additional, unpublished U-Pb zircon ages in Mussel Inlet (G. Gehrels, pers. comm., 2007) confirm that magmatism and deformation south of Douglas Channel continued after 90 Ma, the latest documented age of deformation recorded by rocks to the north. Deformation may have persisted longer in the south, or may just not be recorded by northern rocks. This work provides evidence that rocks west of the Coast shear zone generally share a common tectonic history with rocks to the north, but there are significant along-strike variations in magmatism and the style of deformation. 33 EXHUMATION HISTORY OF THE LARAMIDE RANGES USING (U-Th)/He THERMOCHRONOLOGY Lynn Peyton Department of Geosciences, The University of Arizona Almost all previous low-temperature thermochronology studies in the Rocky Mountain region have used apatite fission-track (AFT) dating to document Laramide-age (late Cretaceous through middle Eocene) exhumation, presumably related to uplift of the ranges. (U-Th)/He dating has been used by other authors in studies of the Bighorn Mountains and the Wind River Basin of Wyoming. The Bighorn study shows very slow exhumation rates from samples that resided in a pre-Laramide partial retention zone, implying that exhumation occurred more recently than the age of the youngest sample (~65 Ma). The Wind River Basin study indicated late Cenozoic thrusting, erosion and cooling < 15 Ma, an unexpected result. This study uses (U-Th)/He dating of apatite from petroleum well cuttings. All of the wells in this study penetrate Precambrian basement rocks in the hanging walls of Laramide-age uplifts. Cuttings have been obtained from at least one well through each of the Wind River, Beartooth, Bighorn, Laramie, Granite, and Medicine Bow ranges, as well as the Uncompahgre uplift. The use of subsurface well samples provides sampling profiles that are longer than those from surface samples alone, and are close to vertical. Initial results from the Wind River Range, in combination with previous AFT results, indicate multiple episodes of exhumation. (U-Th)/He results show rapid exhumation from ~80-70 Ma. Another episode of rapid exhumation occurred in the late Miocene, ~10 Ma, with removal of at least 630 m of overburden. The goal of this project is to document the distribution, timing, rate and amount of both Miocene and Laramide-age exhumation of the Rocky Mountains. 34 EARLY CENOZOIC HIGH ELEVATION IN THE LARAMIDE ROCKY MOUNTAINS: ESTIMATE FROM OXYGEN ISOTOPES OF FRESHWATER BIVALVES Majie Fan and David L. Dettman Department of Geosciences, The University of Arizona Based on the oxygen isotope ratios of widely distributed and well preserved unionid bivalves, we mapped the isotopic composition of Late Cretaceous to Early Eocene river systems in the Laramide Rocky Mountain perimeter basins. Our sample localities range from southern Alberta to northern Colorado. The δ18O values of fossil bivalves vary between -22 ‰ and -5‰ (VPDB). Using an empirically determined relationship between the oxygen isotope ratio of shell and river waters in temperate climates, we determined that these rivers had mean annual δ18O values that ranged from -21‰ to -5‰ (VSMOW). The δ18O values of paleo-river water follow a trend from more negative values in the north to more positive values in the south, with relatively small variation in most basins. By comparison to the modern latitude - δ18O values of river water relationship, we show that the trend is larger than expected due to the latitudinal movement of North America since the Late Cretaceous. In the Powder River basin and Hell Creek region, the δ18O values of the river water are much more negative than the trend observed in the other basins, with mean annual δ18O values as negative as -21‰ (VSMOW). At present, river water in Wyoming and Montana basins is a mixture of high elevation snowmelt with very negative δ18O values and local rainfall with more positive δ18O values. Seasonal shell δ18O data reveals a similar system in the Paleogene in the region. Therefore, we suggest that the negative δ18O values of the river water in Powder River basin and Hell Creek region results from the large elevation difference between the basins and the river sources. The estimated topographic relief of the mountain ranges that provide snowmelt to Hell Creek region is 1-2 km in the latest Cretaceous; and that of the mountain ranges close to Powder River basin is 2-3 km in the late Paleocene. This work documents the timing, magnitude and distribution of uplifted regions in the Laramide Rocky Mountains and helps test models explaining the formation of the Rocky Mountains. 35 UPLIFT, DAMMING AND SEDIMENTATION IN THE ZADA BASIN, SOUTHWESTERN TIBET Joel Saylor Department of Geosciences, The University of Arizona Thick (~800m) fluvio-lacustrine deposits in the Zada Basin in southwestern Tibet archive ~ 4 million years of environmental changes on this high part (3700-4500 m) of the Tibetan Plateau. The basin deposits formed by partial damming of the paleo-Sutlej River, and are fluvial at the base and top and lacustrine in the middle. Magnetostratigraphic and biostratigraphic dating establish that depostion in the basin spanned ~ 4 Myr. Paleocurrent indicators show a uniformly west to northwest direction for the main trunk of the river with tributaries flowing from the north and sourth. Detrital zircon analyses show that the sediment in Zada basin is almost exclusively derived from the Gangdese batholith to the north. Uplift in the Leo Pargil/Qusum range to the west was largely synchronous with Zada basin deposition. There is also a clear and anomalous windgap in the Leo Pargil/Qusum range to the west of Zada basin. This combination of facts suggests a paleo-environmental reconstruction in which the paleo-Sutlej river was a through flowing river from the Kailash, through the present Zada basin area and continuing to the northwest. Uplift in the Leo Pargil/Qusum range dammed the river resulting in deposition of the Zada formation. The river eventually created a breach to the southwest and began the process of downcutting through the Zada formation and Tethyan basement. 36 SESSION V: PALEOCLIMATE PLAYING A SHELL GAME: PERUVIAN MARINE UPWELLING AND MARINE CHRONOMETRY FROM CARBON AND OXYGEN ISOTOPES Kevin B. Jones and Gregory W. L. Hodgins Department of Geosciences, The University of Arizona Marine upwelling along coastal Peru can be intense and variable, complicating radiocarbon dating of marine and coastal systems. Historical and proxy records of upwelling along coastal Peru are few, and the coastal waters are too cold for corals. Mollusk shell carbonate, however, can record the marine ventilation age and seasonal changes in this age over the life of an organism. If large, these seasonal oscillations would make radiocarbon dating marine organisms difficult. To examine this possibility we profiled for δ13C, δ18O, and 14C a set of pre-bomb Argopecten purpuratus shells collected from coastal Peru during 1908 and 1926. Intrashell variations of over 200 14C yr were noted, generally correlating positively with δ18O although 14C variations tended to lag corresponding fluctuations in δ18O. Here, we report 14C and stable C and O isotope profiles for three A. purpuratus shells and discuss their implications for radiocarbon dating of marine systems and the potential for identifying of El Niño events and upwelling changes during the Holocene using ancient mollusks. 37 A CLOUDY FUTURE? SIMULATED VARIABILITY IN NEOTROPICAL MONTANE FOREST HYDROCLIMATOLOGY IN RESPONSE TO ANTHROPOGENIC CIMATE CHANGE Kevin J. Anchukaitis Department of Geosciences and Laboratory of Tree-Ring Research, The University of Arizona The disappearance and presumed extinction of the Monteverde Golden Toad (Bufo periglenes) has focused considerable attention on the potential sensitivity and response of neotropical montane cloud forest ecosystems to anthropogenic climate change. Earlier results from single general circulation model (GCM) experiments, forced by doubled concentrations of atmospheric CO2, suggested that the unique hydroclimatic conditions currently associated with tropical cloud forests could be displaced upward by up to several hundred meters, potentially endangering the very existence of this ecosystem across large parts of the New World tropics. Here, we use a multimodel ensemble consisting of state-of-the-art coupled atmosphere-ocean general circulation models developed for the IPCC Fourth Assessment Report (2007) to investigate the consequences of predicted future greenhouse gas emissions on the hydroclimatology of the Monteverde Cloud Forest in Costa Rica. The six models selected here reproduce many of the important features of intra-annual and inter-annual tropical climate variability. Simulations of temperature, precipitation, and relative humidity during the 20th Century (20c3m) for the model grid corresponding to Monteverde are first evaluated against observational and reanalysis data, with several models demonstrating skill in reproducing features of the modern climatology. We find, however, that model experiments using a midrange future emissions scenario (SRES A1B) with atmospheric CO2 concentrations stabilizing at 720 ppm show a considerable range of forecast local climatic changes at Monteverde. In particular, relative humidity demonstrates model-dependent changes that differ in sign, seasonality, and magnitude. The two models which best reproduce the observed climatology of the 20th century both predict reductions in relative humidity at cloud forest elevations, but still vary in the overall and intra-annual pattern and amplitude of change. The range of forecast climate changes in neotropical cloud forest hydroclimatology are discussed in light of recent observations of cloud forest climate and paleoclimate variability from associated research, as well as model representations of radiative forcing, teleconnection patterns, and tropical climate dynamics. 38 REGIONAL TREE GROWTH AND INFERRED SUMMER CLIMATE IN THE WINNIPEG RIVER BASIN, CANADA SINCE AD 1783 Scott St. George, David M. Meko, and Michael N. Evans Department of Geosciences, The University of Arizona This study uses a network of fifty-four ringwidth chronologies to estimate changes in summer climate within the Winnipeg River basin, Canada since AD 1783. The basin drains parts of northwestern Ontario, northern Minnesota and southeastern Manitoba, and is a key area for hydroelectric power production. Most chronologies were developed from Pinus resinosa and P. strobus, with a limited number of collections made from Thuja occidentalis, Picea glauca and Pinus banksiana. The dominant pattern of regional tree growth can be recovered using as few as nine chronologies, and is not affected by the method used to remove variability related to age or stand dynamics from individual trees. Tree growth is significantly, but weakly, correlated with both temperature and precipitation during summer. Simulated ringwidth chronologies produced by a process model of tree-ring growth exhibit similar relationships with summer climate variables. High and low growth across the region is associated with cool/wet and warm/dry summers respectively; this relationship is supported by comparisons with archival records from early 19th century fur-trading posts. The tree-ring record indicates that summer droughts were more persistent in the 19th and late 18th century, but there is no evidence that drought was more extreme prior to the onset of direct monitoring. 39 HOLOCENE FLOOD HISTORIES IN THE SOUTHWESTERN UNITED STATES Tessa Harden¹, Mark Macklin², and Victor Baker³ ¹ Department of Geosciences, The University of Arizona ²Institute of Geography and Earth Sciences, University of Wales, Aberysthwyth ³Department of Hydrology and Water Resources, The University of Arizona Rivers and streams in the southwestern United States are some of the most extensively studied arid land fluvial systems in the world. Since the 1960’s research projects have been conducted to assess the climate history by interpreting deposits from alluvial reaches throughout the Southwest. Additionally, the slackwater deposit technique of analyzing large floods in bedrock reaches was pioneered in the Southwest by Dr. Victor Baker and his students led to a large and thorough inventory of paleoflood frequency and magnitude. Nevertheless, past research has tended to focus on one or the other method of studying rivers in order to assess the flood history. This study indicates that individually these two methods present only a partial picture of change in the Holocene. When combined, a more complete record is produced. This study presents a probability-based record of flooding in the southwestern United States for the last 12,000 yrs. It combines the extensive alluvial archive with flood records from bedrock reaches. Using more than 700 14C-dated fluvial units, the analysis produces a high-resolution flood probability record--the first of its kind for the American Southwest. The combined record indicates seven episodes of increased flooding: 11250-10400, 8800-8350, 8230-7600, 67005700, 5600-4820, 4550-3320, and 2000-0 cal. BP. Major peaks occur around 10150, 8470, 7800, 6300, 5380, 4850, 1310, and 300 cal. BP. Bedrock systems tend to be better indicators of the flood record from the mid to late Holocene and indicate a higher probability of floods during this time. However, alluvial records show a higher flood probability occurring during the early to mid Holocene. This bias in the records likely reflects preservation factors. The results of this study suggest that the fluvial archive, given its very large number of dated units, can produce a sensitive record of recent climate change as reliable as that of most previously established proxy records in the Southwest. 40 DOES THE ARCTIC OSCILLATION DRIVE THE TIMING OF SPRING IN THE SOUTHWEST? Stephanie McAfee Department of Geosciences, The University of Arizona Throughout the western United States early snowmelt and peak streamflow have been associated with warmer spring temperatures (Knowles et al., 2006; Stewart et al., 2004). Earlier and warmer springs are also linked to larger and more severe fires (Westerling et al., 2006). The Arctic Oscillation (AO) describes variability in the strength of the polar vortex, and consequently in the position and coherence of the winter jet. Over the past two decades, the AO has shifted into a predominantly positive phase. One result has been a 3 - 5° northward migration of the jet over the Pacific Ocean (Thompson and Wallace, 1998). Although most research on the impact of the AO has focused on the northeastern United States and Europe, winter precipitation in the Southwest is strongly dependent on a southerly displaced jet directing storms across the region. In addition, the jet tends to limit the southward flow of cold air, so that much of the mid-latitudes are warmer during high index winters. Here we demonstrate that spring temperature and precipitation in the southwestern United States are correlated with the mid-winter AO index. Furthermore, the impact is substantial enough to reduce vegetative growth later in the spring and early summer, as evidenced by a negative correlation between the winter AO index and NDVI, a satellite measure of greenness. 41 ABRUPT CHANGES IN EASTERN EQUATORIAL PACIFIC CLIMATE DURING THE HOLOCENE Jessica L. Conroy1 and Jonathan T. Overpeck1, 2 1 2 Department of Geosciences, The University of Arizona Institute for the Study of Planet Earth, The University of Arizona Paleoclimate records from the tropical Pacific suggest the early- to mid-Holocene was a period of reduced El Niño/Southern Oscillation (ENSO) variability, with increased ENSO frequency occurring some time in the last few thousand years. However, the precise timing of this shift remains uncertain. Here we present a new view of millennial-scale climate variability for the eastern tropical Pacific using a well-dated lake sediment core from the Galápagos Islands. Percent clay and silt abundance in the Galápagos core indicate an abrupt, two-step transition from drier to wetter conditions at 3200 and 2100 cal years BP. Based on modern climatology, this transition indicates a shift from cooler eastern tropical Pacific sea-surface temperature (SST) to warmer eastern tropical Pacific SST. The record of percent sand abundance from El Junco shows similar variability compared to other paleoclimate reconstructions of El Niño event frequency, and suggests increased event frequency beginning ~4000 cal years BP. The Galápagos core and three other ENSO records also reveal that the period of greatest El Niño event frequency during the Holocene occurred from ~2000-1000 cal years BP. Carbon/nitrogen ratios from the Galápagos core also reveal a trend toward higher lake levels from the early Holocene to present, indicating a trend toward enhanced precipitation and warmer SST. 42 NORTH AMERICAN PRECIPITATION AND THE CADENCE OF DECADAL DARIABILITY Scott St. George and Toby Ault Department of Geosciences, The University of Arizona We investigate the importance of long-term (decadal-scale) variability in precipitation across North America. Several recent studies have suggested that understanding this type of variability is critical to manage water supplies and project drought risks over the next several decades. In this study we use a data adaptive method (singular spectrum analysis) to identify the regions and seasons where decadal signals in precipitation are important. Our results indicate that this type of variability is most prominent in northern California and northern Minnesota during winter, and the upper Colorado River basin (CRB) in autumn. In the CRB region, we suggest that this behavior is connected to changes in Pacific Ocean sea surface temperatures. However, over most of North America, decadal variability accounts for a small fraction of the total variance in precipitation. These findings imply that projecting decadal-scale changes in precipitation may only be relevant to select locations. 43 SESSION VI: SURFACE PROCESSES AND TRACE GEOCHEMISTRY NUTRIENT SOURCING OF NINE PLANT SPECIES IN THE SOUTHWESTERN US USING STRONTIUM ISOTOPES Amanda C. Reynolds1, Jay Quade1, and Julio L. Betancourt2 1 Department of Geosciences, The University of Arizona 2 US Geological Survey, Tucson, AZ This study uses the 87Sr/86Sr isotopic system as a proxy for plant Ca intake to trace nutrient inputs to nine plant species (Pinus ponderosa, Pinus edulis, Juniperus monosperma, Juniperus scopulorum, Chrysothamus nauseosus, Fallugia paradoxa, Rhus trilobata, Bouteloua gracilis, and lichen) in northern New Mexico. Specifically, we examined the influence of atmospheric dust and parent material on the 87Sr/86Sr ratio of the soil water and the resulting plant 87Sr/86Sr as a function of the soil profile, age of the bedrock substrate, and local dust sources. We sampled a variety of plants that exhibit different rooting depths from six localities that vary in bedrock type (granite, basalt, limestone, and sandstone) and bedrock age (Quaternary to Proterozoic). Plant 87 Sr/86Sr ratios suggest that atmospheric inputs dominate the 87Sr/86Sr ratio of shallowly rooted plants, Bouteloua and lichen, whereas more deeply rooted plants showed stronger dependence on bedrock 87Sr/86Sr. Basalts older than 0.009 Ma contributed close to half of their 87Sr/86Sr bedrock ratio to the soil water. Plants on granite bedrock displayed the highest range of 87Sr/86Sr ratios at one site (N34º 59.021’ W108º 00.725’, 2254 m) possibly suggesting more mineral weathering input or more variation in the 87Sr/86Sr of groundwater. The results provide us with a better understanding of plant nutrient resources and of soil developmental processes within arid environments of the southwestern USA. 44 A DETAILED STRONTIUM ISOTOPE STUDY OF THE BOUSE FORMATION, LOWER COLORADO RIVER, USA: IMPLICATIONS FOR COLORADO RIVER HISTORY Jennifer A. Roskowski, P. Jonathan Patchett, Jon E. Spencer, Philip A. Pearthree, James E. Faulds, and Amanda C. Reynolds Department of Geosciences, The University of Arizona The mechanisms and timing for integration of the Colorado River are the subject of much controversy, and the origin of the Bouse Formation has important implications for the Colorado River debate. The upper Miocene to Pliocene Bouse Formation exposed along the Lower Colorado River was originally interpreted as a Miocene embayment of the Gulf of California on the basis of paleontology; however, recent isotopic work suggests that the Bouse Formation is lacustrine. Rather than a single lake, the data suggest a lake-overflow model involving a chain of three separate lake basins for the integration of the lower Colorado River. Past studies have established an estimate for the overall geochemical character of the Bouse Formation, with data from a wide variety of sample types and locations. Strontium isotope data from Bouse Formation carbonates give 87Sr/86Sr values around 0.7108, close to that of the present-day Colorado River (0.71075), and well above late Neogene marine strontium (0.7090). However, these studies were undertaken in order to establish a general pattern for isotopic values in Bouse Formation, and the samples have no clear age relations to each other. To obtain Bouse Formation samples with discernable age relationships, a more detailed and systematic sampling was performed in this study. By sampling the basal 10 cm of marl over a 1500-foot range of elevations in the three paleo-lake basins, a time evolution of lake waters is established. Comparisons are also made with the Hualapai Limestone, located in the Grand Wash Cliffs area. The Hualapai Limestone has significantly higher 87Sr/86Sr (0.714 to 0.720), and the transition to the lower strontium values that characterize the Bouse Formation is under investigation. A sample taken from near Frenchman Mountain NV, thought perhaps to be Hualapai, instead shows a Sr isotope ratio very similar to that of the Bouse Formation. The Sr isotope results from Bouse Formation in the Mohave basin, immediately to the South, also corroborate the general range of previous values. These data illuminate lake-overflow and mixing processes as well as the evolution of the Colorado River drainage basin. 45 EFFECTS OF LOCAL FARMING AND DEFORESTATION ON SEDIMENT DISCHARGE INFERRED FROM LAKE CORE RECORDS AND STREAM MORPHOLOGY, COASTAL LAKE TANGANYIKA, EAST AFRICA Michael Strickler Department of Geosciences, The University of Arizona Lake Tanganyika is the oldest and largest of the East African rift lakes and vital to the economy of the surrounding villages and countries. Deforestation around the lake causes many negative effects from an increased sediment flux and has changed dramatically as a function of population. Increases in inorganic sediment flux from deforested watersheds and an associated decrease in organic sediment flux may negatively impact the viability of littoral habitats in the lake. This study examines the interrelationship between deforestation on streams that feed into Lake Tanganyika and sedimentation offshore. We mapped stream morphology of two distinct streams: Kalande (Gombe Stream National Park), and Ngelwa (deforested). An offshore core was collected from the lake bottom just offshore of the deforested Ngelwa stream at a water depth of 106m and dated using 210Pb geochronologic methods. In addition, multiple laboratory analyses were performed on the core and within the streams to help constrain sedimentation and deforestation effects. Three distinct units within the core correlate to reflect changes in sedimentation (higher rates as deforestation continues) in addition to a shift from dry, low lake level conditions to wet, high lake levels as evidenced by shifts in core lithologies, diatom taxonomy and content, magnetic susceptibility, and clastic particle size. Streambed and suspended sediment analyses reflect high levels of erosion in deforested watersheds coupled with changes in dissolved nutrients in the water, which may suggest an overall diminished level of ecosystem function in deforested and heavily farmed watersheds. 46 TIMING OF DOWNCUTTING EVENTS IN THE UPPER SUTLEJ RIVER, TIBET AND POSSIBLE ANTHROPOLOGICAL IMPLICATIONS Jennifer Boerner1, Jon Pelletier1, and Ronald J. Goble2 1 2 Department of Geosciences, The University of Arizona Department of Geosciences, University of Nebraska-Lincoln Along the upper Sutlej River in western Tibet, the collapse and fragmentation of the Purang polities ca. AD 1200–1400 has been blamed on dramatic climate change linked to changes in Asian Monsoon intensity. Initial geomorphic work on the problem focuses on the drainage of the upper Sutlej, and the reconstruction of a solid record of Late Pleistocene to Holocene fluctuations of the main river at two points: in the Zada Basin and along a stretch of river near the ancient town of Kyunglung. In both locations a series of well-preserved alluvial terraces have been abandoned by channel downcutting. Dates for these downcutting events will be obtained through both diffusive modeling of the terrace slopes, and radiometric techniques including radiocarbon dating, cosmogenic surface dating, and optically stimulated luminescence (OSL). Preliminary results from the radiometric techniques used to constrain the K value in the diffusion equation of the Zada Basin indicate that K is higher than the initially assumed 1.0. 1.0 is the default value assigned to the American Southwest, an environment similar to that of the Zada Basin and thus a starting point in the diffusion model. A higher K value means a higher rate of erosion through environmental factors such as material of the slope, amount and intensity of precipitation, and type of vegetation cover. A K value specific to the conditions of the area allows dating of terraces with no appropriate material for radiometric or OSL dating. A wellknown Late Pleistocene to Holocene chronology of upper Sutlej River behavior allows us to begin to investigate the theory of changes in monsoon intensity around the period of AD 1200– 1400. 47 LEAD AND STRONTIUM ISOTOPES AS TRACERS OF TURQUOISE: A PILOT STUDY Alyson Thibodeau, Joaquin Ruiz, and John Chesley Department of Geosciences, The University of Arizona Turquoise [CuAl6(PO4)4(OH)8·4H2O] is a mineral that was widely traded among prehistoric societies in the American Southwest and Mexico. Since the 1960’s, there have been numerous efforts to link turquoise found in the archaeological record to specific geologic sources (e.g., Mathien and Olinger, 1992; Sigleo, 1975; Weigand et al., 1977). Attempts to chemically source turquoise have been hindered both by the chemical variability of individual deposits, abundant impurities, and the susceptibility of turquoise to alteration when exposed to surface conditions (Hull 2006). This pilot study was designed to determine if the isotopic signatures of lead and strontium could provide a basis for discriminating between turquoise sources and identifying the provenance of prehistoric turquoise artifacts. Samples of turquoise from five deposits in Arizona and New Mexico were analyzed for both lead and strontium isotopes. Lead and strontium were separated from individual turquoise samples and measured by solution MC-ICPMS and TIMS, respectively. These initial results confirm that the isotopic composition of lead and strontium varies significantly between sources and show inter-source variation is greater than the intrasource variation. Using isotopic results from turquoise deposits located in the Cerrills Hills mining district of central New Mexico, we suggest it is also possible to distinguish between individual mines within large turquoise mining districts. Extensive isotopic analyses of turquoise from the Sleeping Beauty deposit near Globe, AZ reveal highly variable and extremely radiogenic signatures of both lead and strontium. Analyses of leached fractions of turquoise from this deposit show that variable isotopic domains are contributing to the “bulk” isotopic signature of individual samples. This variability may be the result of a) impurities, b) transient nonstoichiometric release c) surficial weathering processes or d) long-term formation of turquoise from different sources. Because southwestern turquoise occurs as a cryptocrystalline material and can contain many impurities, both leaching experiments and petrographic examination of samples may be necessary to fully understand isotopic variation within deposits. Despite this observed variability, turquoise from this locale can still be isotopically distinguished from all other turquoise examined in this study. We conclude that lead and strontium isotopes can be used to differentiate between turquoise deposits, and may provide a way to trace archaeological turquoise back to its geologic source. 48 INTERACTIONS BETWEEN FALLING CHICXULUB EJECTA AND THE ATMOSPHERE: THE DEPOSITION OF THE K/T BOUNDARY LAYER Tamara Goldin1 and H. Jay Melosh2 Department of Geosciences, The University of Arizona Lunar and Planetary Laboratory, University of Arizona The mechanics of impact ejecta deposition are not well understood, especially for planets with atmospheres where complex interactions occur between the ejected particles and the surrounding atmosphere. The K/T boundary ejecta layer is found world-wide and is linked to the 65-Ma Chicxulub impact. We modeled a simplified Chicxulub scenario using KFIX-LPL, a two-phase fluid flow code, which allows us to simulate deposition of the K/T boundary layer through the atmosphere. Air is modeled as a perfect gas and the impact spherules (condensed from the vapor plume) are modeled as a simple incompressible fluid with the properties of basaltic glass. The particles fall through the thin upper atmosphere, pushing the atmosphere downwards until the particles decelerate due to drag and increasing atmospheric pressure. The particles accumulate at ~50-km altitude and the deceleration heats the atmosphere around the particles (>700 K), causing expansion of the atmosphere and creating a sharp transition between hot dense atmosphere below the deceleration boundary and cool thin atmosphere above. These dramatic changes to the atmosphere have important environmental implications and provide the starting conditions and timeframes for chemical models examining the environmental consequences of Chicxulub. The results also support the delivery of significant thermal radiation to the Earth’s surface possibly igniting global wildfires. Several theories have been suggested to explain the double ejecta layer observed at North American K/T localities (including a second impact) and our models shed light on this mystery. Adding an initial injection of ejecta curtain material into our model atmosphere, as expected at such intermediate distances from the impact, produces two distinct layers due to the alteration of the atmosphere’s structure. Deposition of the lower terrestrial layer on the ground begins at ~80 minutes and that of the upper fireball layer begins at ~130 minutes; thus deposition of both layers occurred over a timescale of hours. The double layer observed in North American localities does not require two impacts, and is, in fact, expected from a single impact. 49 Map to GeoDaze party Directions to Beck/Zandt house: 4535 W. Camino Nuestro From the University, W on Speedway, N on Silverbell, W on Sweetwater. Just past Robin Elementary school, turn N onto Oxbow (dirt road), past speed bump turn left onto Nuestro. At “Y” keep left until you reach driveway of tan Santa Fe (4535 on lamppost). (Please note the star on the map below is not quite in the correct place). 50 Map of the Student Union Directions to the Grand Ballroom: From the ground level (main level-2nd floor), take the northwestern stairwell to the 3rd level. This stairwell is directly north of the U-Mart. Once on the 3rd floor, you will be right next to the glass doors that lead to the Diamond Atrium. It is in this area where the welcoming/registration booths will be set-up. 51
