January 2000 – Emmons Lecture
“INTERACTIONS OF CLIMATE AND TECTONICS IN OROGENESIS”
Douglas W. Burbank
Pennsylvania State University
Recent geodynamic models that incorporate erosion at the earth’s surface have yielded a provocative hypothesis: climatically modulated erosion exerts a strong control on where strain occurs within a collisional mountain belts. In particular, these models predict that deformation will be greatest where erosion is most intense. In order to assess this hypothesis, numerous data sets must be assembled, including the spatial and temporal variations in erosion rates, deformation rates, and climate across an orogen. In this talk, I discuss different approaches that can be used to calibrate the interaction between climate, erosion, and tectonics. I move from local examples of the dissection of growing folds to the erosion and deformation of the Himalaya. In the northwestern Himalaya, I use a geomorphic perspective to illustrate how rates of river incision into bedrock, widespread bedrock landsliding, and glacial erosion interact within this landscape of immense topographic relief. In this region, it appears that deformation and erosion rates are closely coupled: the highest strain occurs where erosion is also highest. The balance between rock uplift and erosion suggest that the mountains could be in a long-term steady state. Perhaps surprisingly, the topography appears at least partially decoupled from spatial variations in tectonic rates. Instead the topography is more closely tied to regional climatic variations and the distribution of glacial snowlines.
FAULT ZONE ARCHITECTURE AND PERMEABILITY STRUCTURE
U.S. Geological Survey
Fault zones in the upper crust are typically composed of complex fracture networks and discrete zones of comminuted and geochemically altered fault rocks. Their three-dimensional architecture, as well as their response to in situ stress, can significantly impact the patterns and rates of fluid flow in and around them. A series of numerical simulations of fluid flow in a set of three-dimensional discrete fracture network models aids in identifying the primary controlling parameters of fault-related fluid flow. Conceptual models for fault-related permeability structures in many geologic settings are derived from field investigations, laboratory permeability measurements, and in situ flow tests. Brittle fault zones often comprise a complex arrangement of distinct structural and hydraulic components: a fault core (where strain is accommodated) and a damage zone (subsidiary structures related to growth). Fluid flow in individual fault zone components and full outcrop scale model domains are simulated using a finite element routine. Two idealized end-member fault zones are used to shed light on the myriad combinations of fault core and damage zone structures found in nature and how they control whether a fault zone will act as a fluid flow conduit, barrier, or combined conduit-barrier system. The simulations are done in idealized, but geologically realistic, fault zone architectural models based on fracture data collected along exposures of the Stillwater Fault Zone in Dixie Valley, Nevada and geometric data from a series of normal fault zones in east Greenland. The models are also constrained by an Andersonian model for mechanically compatible fracture networks associated with normal faulting.
Permeability contrasts between components and permeability anisotropy within components are identified as the major controlling factors in fault-related fluid flow. Additionally, the structural and hydraulic variations in these components are also major controls of flow at the scale of the full model domains (20m by 20m by 20m). The model results demonstrate that small changes in the architecture and hydraulic parameters of individual fault zone components can have very large impacts, up to five orders of magnitude, on the permeability structure of the full model domains. Changes in fault zone architecture can cause major changes in permeability structure that, in turn, significantly impact the magnitude and patterns of fluid flux and solute transport both within and near the fault zone. Future modeling efforts will include the impact of in situ stress on fault-related fluid flow in the discrete fracture network models.
* Excerpted from: Caine, J. S. and Forster, C. B., 1999, Fault zone architecture and fluid flow: Insights from field data and numerical modeling, in Haneberg et al., eds., Faults and subsurface fluid flow in the shallow crust, AGU Geophysical Monograph Series 113, p. 101-127 AND Caine, J. S., 1999, The architecture and permeability structure of brittle fault zones, Ph.D. Dissertation, University of Utah, Salt Lake City.
Jonathan is a U.S. Geological Survey Postdoctoral Fellow, he received his Ph.D. from the University of Utah in March, 1999.
JUPITER’S SATELLITE IO THE VOLCANIC MOON
John Reiss, Jr.
John Reiss, Jr. and Associates, Inc.
Io is Jupiter’s third largest moon and is slightly larger than earth’s moon. Io’s surface is radically different from any other body in the solar system. It is seemingly the most volcanically active body of the solar system and has an amazing variety of terrains: calderas up to several kilometers deep, lakes of molten sulfur, mountains which are apparently not volcanoes, extensive flows hundreds of kilometers long consisting of some low viscosity fluid ( sulfur or silicate rock? ) and volcanic vents. This presentation will cover the details of the latest discoveries on Io by the Galileo spacecraft; the Hubble telescope and other earth based telescopes. (The slides, as provided by NASA scientists, are remarkable)
¨ John (a Colorado resident for 23 years) is president of the John Reiss, Jr. and Associates, Inc, environmental and geotechnical engineering consulting firm. John is also an amateur astronomer, and the JPL-NASA Ambassador for Colorado. He is a geological engineering graduate from the Missouri School of Mines, Rolla, Missouri, where he received a B.S. in 1971.
NEW DEVELOPMENTS IN CARBON CYCLE AND CLIMATE CHANGE
University of Colorado, Boulder
Humans continue to put ever-increasing amounts of carbon dioxide in the atmosphere every year via fossil fuel burning, deforestation and cement production. While it is reasonably clear that about half of that CO2 accumulates in the atmosphere, it is still a subject of debate how much of the other half goes into the ocean versus land plants. Atmospheric observations of CO2 concentrations and isotopic content indicate that land plants are a substantial sink for CO2, but a highly volatile one. Large sinks of CO2 have been documented in the early 1990’s, and a recent controversial study suggests that land plants in the United States may be capable of removing enough CO2 to offset all fossil fuel emissions in the United States for a short period of time. But do we have the observational network we need to make such conclusions? Will we ever have such a network? Looking farther into the past, studies of CO2 variability from ice cores tell us that the relationship between CO2 and climate is stronger than previously thought, with a remarkably strong relationship apparent over the past 400,000 years. However, CO2 levels never reached steady state in the Holocene, rising slowly over the past few millennia. With a growing number of studies pointing to major climate change as an abrupt, rather than slow process, the gaps in our knowledge of how carbon cycles in the environment need to be plugged and fast.
ROCKY MOUNTAIN FORELAND STRUCTURES
Compressive, basement-involved, fault-related structures are the primary structural style within the Rocky Mountain foreland province. The causal fault zones can be divided into 1) basin-boundary megathrusts that border regional arches or platforms, and 2) smaller, basement-involved thrust zones within the basins and on the platforms. Displacement along these thrust zones produced the celebrated anticlinal “oil-field” structures of the Rocky Mountain basins. Some natural examples will be illustrated by seismic profiles, structural cross sections, and contour maps. Also, if time permits, structures produced by wrench duplexing, secondary detachment thrusting, and non-tectonic mechanisms (laccolithic intrusion, meteor impact) will be included.
BLASTS FROM THE PAST: WHEN THERE WERE HUGE VOLCANOES IN COLORADO
U.S. Geological Survey, Menlo Park, CA
Since about 20 Ma, mountainous parts of Colorado have been sites of relatively small infrequent basaltic eruptions. In contrast, from 35 to 23 Ma at least 25 gigantic explosive eruptions generated enormous silicic ash flows that avalanched as much as 100 km from large caldera sources, mainly in the Sawatch and San Juan Mountain areas. Because of the size of these eruptions and the erosional dissection of internal structures of caldera volcanoes, the middle Tertiary volcanism in Colorado provides instructive comparisons and contrasts with recent explosives eruptions such as Mount St. Helens in 1980 and Pinatubo in 1991. Especially impressive, in terms of ash-flow volumes, caldera size, and magma-production rates is the 28-26 Ma central San Juan caldera cluster, where recent studies provide new insights into processes of silicic magma generation, pyroclastic eruptions, and caldera formation.
THE SIERRA NEVADA, CALIFORNIA- AN EXAMPLE OF BOTTOM-DOWN MOUNTAIN BUILDING
Department of Geological Sciences and CIRES, University of Colorado
Recent geologic and geochemical studies of the Sierra Nevada in California have resurrected the debate regarding the origin and evolution of the range’s high topography. Early (circa 1960’s) studies based on geologic and paleontological information suggested that the high topography is relatively young (dating from <10 Ma) and that the high topography is supported by a deep crustal “root”. In contrast, recent seismic tomography studies show no evidence of a thick root beneath the Sierra and suggest instead that the modern topography is supported by upwelling asthenosphere (Wernicke et al., Science, 1996). In addition, estimates of ancient topography based on U-Th/He ages (House et al., Nature, 1998) and oxygen isotope data from altered ash-flow tuffs east of the Sierran crest (Chamberlain and Poage, Geology, 2000) suggest that the high topography is significantly older than 10 m.y. and may have been initiated in the Late Cretaceous during subduction-related magmatism and tectonism. To add to the general confusion, trace element and isotopic data from mantle and lower crustal xenoliths entrained in Cenozoic volcanic rocks present in the southern Sierra have been interpreted to indicate that “delamination” of the mafic, eclogitic, root of the Sierra occurred between 8 and 3 Ma (Ducea and Saleeby, JGR, 1996). The removal of the deep lithosphere, perhaps triggered by NW-SE directed lithospheric extension and the opening of a “mantle” window beneath the southwest U.S., may have resulted in asthenospheric upwelling and increased the uplift of portions of the southern Sierra. Such a “bottom-down” mountain building event seems consistent with new and existing age and geochemical data from the Cenozoic volcanic rocks, themselves. While Miocene volcanic rocks are mainly high-K basalts and andesites that span a range of ages from 12-8 Ma, widespread Pliocene volcanic rocks (which lack eclogite xenoliths) are ultrapotassic, include high MgO absarokites, and span a very narrow age range from about 3.2 to 3.5 Ma. The Pliocene rocks also have distinctly lower epsilon Nd values (-6 to -9) than either Miocene or Quaternary volcanics, suggesting that the Pliocene rocks tapped a source unavailable to other Cenozoic volcanic rocks. While the exact magma source is unknown, the fact that the Pliocene volcanism was short-lived and unique in composition supports a model in which deep lithosphere delamination triggers magmatism from a source related to the delaminated lithosphere itself, as suggested for potassic magmatism in the Tibetan plateau. Future work will be centered on reconciling the geologic and chemical evidence outlined above for a Pliocene-Miocene delamination event with the growing evidence that the Sierra Nevada may have existed as a topographic high for as long as 70 m.y.
THE SELF-DESTRUCTIVE NATURE OF MIDDLE-LATITUDE ANDEAN VOLCANOES
The Tatara-San Pedro Complex is a large frontal arc volcanic center of the Andean Southern Volcanic Zone, dissected on all flanks by glacial valleys exposing the eruptive products of seven edifices ranging in age from 930 ka to Holocene. Deposits older than 200 ka are remnants of spatially overlapping volcanoes reduced in volume and aerial extent by glaciation and sector collapse. Preserved remnants represent only10-50% of eruptive volumes, based on estimates of original edifice geometry, and generally record short eruptive episodes relative to intervening lacunae. The internal stratigraphy of several of these edifices has been reconstructed on the basis of geochemical data from 650 samples collected mainly in 25 canyon wall sections and accompanying geochronologic and paleomagnetic data for a subset of these sampled lavas. Digital photogrammetric projections of canyon wall stratigraphy portray geometries of erosion surfaces in far greater detail and more accurately than conventional field mapping would permit due to inaccessibility of steep canyon walls. Combined with detailed photogrammetric and field mapping, three-dimensional correlation of flow stratigraphy and delineation of major erosion surfaces is possible. The resulting composite stratigraphy is far more complete than the records present in any single section due to the eccentric distributions of the products of consecutive eruptive events and the effects of erosion and extensive glaciation. Many stratigraphic successions record complex temporal variations lacking in evidence for long-or short-term progressive differentiation, and many preserve evidence for variable parent magma compositions. The resulting constraints on petrologic models are far different than they would be if apparently co-magmatic lavas were assumed to reflect single-stage differentiation.
GEOLOGIC EVOLUTION OF TOBAGO, WEST INDIES
Arthur W. Snoke
Department of Geology and Geophysics, University of Wyoming
Tobago, West Indies, is a basement high that forms part of the northeasternmost corner of the present-day South American continental shelf. The pre-Cenozoic history of Tobago indicates that it has affinities with a Cretaceous oceanic-arc system, and it is part of an allochthonous terrane (Tobago terrane) that occurs along the South American-Caribbean plate-boundary zone. Mesozoic oceanic-arc crust is exposed on Tobago and can be conveniently divided into three east-west-striking lithologic belts that transect the island: North Coast Schist (NCS), ultramafic to tonalitic plutonic suite, and Tobago Volcanic Group (TVG). A mafic dike swarm widely intruded the plutonic suite and TVG, whereas only scattered premetamorphic and post-metamorphic dikes occur in the NCS belt. The plutonic-volcanic-dike complex is Albian based on paleontological and radiometric age data.
Detailed geologic mapping indicates that the NCS was wall rock for the plutonic suite. A selvage of amphibolitic rocks (<<300 m structural thickness) forms a mappable belt between the subjacent NCS and superjacent ultramafic rocks of the plutonic suite. Metamorphic grade in this amphibolite-facies aureole decreases with increasing structural depth, thereby exhibiting an inverted metamorphic gradient. The greenschist-grade penetrative foliation of NCS rocks, commonly containing a plunging, low-angle, west-southwest-east-northeast stretching lineation, is overprinted by a dynamothermal foliation with a down-dip hornblende mineral lineation, related to the emplacement of the ultramafic rocks. Scattered kinematic indicators yield a consistent sense-of-shear that is down-the-plunge of the lineation (i.e., normal sense in present orientation). These data coupled with evidence of mechanical mixing (e.g., dunite-clinopyroxenite breccia) within the ultramafic rocks suggest a history of subsidence of the ultramafic mass late in its crystallization history. This foundering of dense cumulate rocks may be related to intra-arc spreading.
A brittle, normal fault system (Central Tobago fault system) is interpreted as a younger, upper crustal manifestation of the same extensional regime that led to plutonic foundering and development of a normal-sense, amphibolite-facies shear zone along the contact between the ultramafic rocks and rocks of the NCS. Even younger, north-northwest-striking, oblique-slip faults transect the normal fault system. The Southern Tobago fault system is a buried fault system at the southwestern end of the island that has been documented through offshore seismic-reflection profiles and on-shore borehole data.
RIVERS: TYPES, EXPLORATION, TRADE
Robert J. Weimer
Colorado School of Mines, Golden, CO
Of all the forces that shape the land, running water is the most powerful. Gentle rain, melting snow, and trickles become rushing torrents, lazy streams and mighty rivers; water, running inexorably downhill, changes the face of the earth.
Geology, topography and climate influence a river’s run-off and sediment load. Because of these variables, rivers differ greatly in their physical characteristics and general behavior, and no two are alike. Geologists interpret ancient rivers, and predict their distribution for economic development, by studying the processes and deposits of modern rivers.
Most people have one or more rivers running through their lives. Because rivers have been vital to sustain life and to maintain the centers of old and new cultures, they have been objects of art works for centuries.
Foothills Art Center mounted an exhibit early this year-“Rivers: the Song of Life” -featuring paintings, sculpture, and photographs. Originally given in relation to a display on the “Science of Rivers” by Virginia Mast, Graham Curtis, and me, this talk uses space photography, photographs, sketches and paintings to illustrate river types and the role of rivers in early western U.S. exploration and trade.
GROUNDWATER VULNERABILITY TO AGRICULTURAL CONTAMINATION IN COLORADO AND THE HIGH PLAINS
John McCray, Stephanie Schlosser, and Joseph McCarthy
Colorado School of Mines, Golden, CO
The importance of ground water as a resource in the state of Colorado motivates research to investigate the vulnerability of unconfined aquifers to contamination from agricultural sources. Review of previous vulnerability studies indicates that hydrogeologic factors, chemical transport characteristics, land-use, and land-management factors contribute to the vulnerability of ground water to chemical contamination. However, few vulnerability studies have been rigorously validated using large, reliable groundwater chemical data sets. In addition, the impacts of data-collection scale on the accuracy of the predicted vulnerabilities have not been assessed. For this research, we use cell-based geographic information systems to analyze the sensitivity and vulnerability of Colorado ground water to agricultural contamination (e.g., pesticides and nitrates) in Rio Grande and Weld Counties. Subjective and physics-based mathematical expressions that represent transport of contaminants in vadose zones are developed and included in the cell-based GIS calculation scheme. Specific goals of the research are to: assess the impact of data-collection scale on vulnerability-model results and accuracy; develop vulnerability models that account for pesticide-specific transport and management data; and validate the vulnerability models using ground water quality data. The results of this study will allow us to develop a methodology for evaluating the vulnerability of ground water to contamination from land-surface-applications of agrochemicals.
GEOCHEMISTRY OF THE LEAKING RADIOACTIVE WASTE TANKS AT HANFORD, WASHINGTON
Department of Geological Sciences, University of Colorado at Boulder
The Hanford radioactive waste tanks, located in east central Washington adjacent to the Columbia River, contain a mixture of solutions and solids from the reprocessing of nuclear fuel. Early tanks, constructed of concrete lined with a single shell of iron, have failed resulting in leakage of radioactive elements such as cesium and technetium into the underlying sediments during the last 50 years. Some evidence has indicated that cesium has been transported to the depth of the water table, approximately 150 feet below the surface, but it is not clear how such transport has occurred in a relatively short time. Leaking tanks also release highly caustic high-temperature aluminum-nitrate solutions to the sediments resulting in unusual geochemical reactions that affect the transport of the radionuclides. Evidence for what these reactions are and how fast they can occur will be presented with the goal of understanding longer-term aspects of the transport of radionuclides through the unsaturated sediments to the water table.
STRATIGRAPHIC ANOMALIES ASSOCIATED WITH A TECTONICALLY ACTIVE EMBAYMENT IN THE CRETACEOUS WESTERN INTERIOR SEAWAY: A STORY OF THE LEWIS SHALE AND FOX HILLS SANDSTONE, WYOMING
David R. Pyles
University of Colorado, Department of Geological Sciences, Boulder, CO
The lower Maastrichtian Lewis Shale and Fox Hills Sandstone were deposited in a tectonically active embayment during the final major transgression and regression of the Cretaceous Western Interior seaway. Localized uplifts along the margins of the embayment, coupled with changes in sediment supply into the embayment, had a profound effect on basin physiography, water depths, stacking patterns, facies and sediment accumulation rates. Four uplift events impacted Lewis/Fox Hills deposition. The first three occurred at the ancestral Lost Soldier anticline, creating a shelf-slope-basin physiography, which would remain through Lewis time. The fourth uplift occurred at the Sierre Madre uplift. Sediment supply from the north increased during late Lewis and Fox Hills time. The combination of localized uplifts at the ancestral Lost Soldier anticline and increasing sediment supply into the depositional basin resulted in aggradational/progradational stacking patterns. High-density turbidites and debrites, which punctuate the pelagic and hemipelagic mudstones in the slope and basin facies tracts, began with the development of the shelf-slope-basin physiography. Compacted sediment accumulation rates in the depositional basin range from 0.15 mm/year during non-tectonically active times and up to 1.2 mm/year during tectonically active times. The shelf-slope-basin physiography and high-density turbidites and debris flows are anomalous to all other Cretaceous Western Interior deposits.
These interpretations were formulated from a seventy mile long, north-south trending, depositional dip-oriented, chronostratigraphic cross section that runs along the eastern margins of the Great Divide and Washakie basins. The data incorporated into the chronostratigraphic cross section consists of outcrop data comprising facies descriptions, ammonite localities and gamma ray logs; subsurface data consisting of well logs and cores; and published stratigraphic cross sections.
GPRELIMINARY ASSESSMENT OF THE SUITABILITY OF LASER ABLATION ICPMS AND ENGELMAN SPRUCE AS A TECHNIQUE TO DETERMINE TRENDS IN TRACE METAL MOVEMENT
Colorado School of Mines, Department of Geology and Geological Engineering, Golden, CO
Abandoned mine lands throughout the Rocky Mountains are potential sources of acid, metal-rich drainage. This drainage potentially impacts flora and fauna that utilize affected ground and surface waters. Such is the case for the abandoned mine tailings at the Waldorf mine, approximately 6 miles southwest of Georgetown, Colorado. Conifers (Engelman Spruce) are the dominant tree species growing in many areas covered with transported mill tailings. Cores taken from Engelman spruce were analyzed with laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to determine suitability of this species and this technique to monitor trace metal movement related to mill tailings deposition. Development of spiked cellulose standard was undertaken to accurately determine concentrations of metals within individual tree rings. Tree cores analyzed using LA-ICPMS indicated distinct differences in Zn, Fe, Mn, and Sr uptake by a tree growing in the transported tailings vs a control tree. Both cores had similar concentrations of As, Pb, Cd, Bi, and Mg.
INCLUSIONS AND CHEMICAL HETEROGENEITIES IN GARNET: PROBLEMS, PITFALLS, AND POTENTIAL FOR SM-ND- GEOCHRONOLOGY
Colorado State University, Department of Earth Resources, Fort Collins, CO
Garnet is one of the most important minerals used for deciphering the thermal and chemical history of metamorphic rocks. Based on a favorable Sm/Nd ratio, dating of garnets by Sm-Nd geochronology has proven possible for rocks from numerous settings. Advances in in-situ techniques have shown that the systematics of Sm and Nd in garnet are complex. Our work investigates the complexities introduced by inclusions and REE heterogeneities in garnet.
It has been demonstrated that the actual levels of Sm and Nd hosted in the garnet lattice are often quite low and numerous microscopic to submicroscopic inclusions can contain many times those levels of REEs. Not only does the presence of inclusions lower the 147Sm/144Nd ratio, thereby lowering the precision of the isochron date, but the presence of inclusions may also reduce the accuracy of the date. Breakdown of REE-rich xenotime during garnet growth may account for such zoning. If such zoning produces heterogeneities in the isotopic signature, the Sm-Nd isochron may be erroneous. The above factors all may need to be examined closely before any Sm-Nd garnet date or closure temperature estimate is considered.
PALEOMAGNETISM AS A STRUCTURAL TOOL
U.S. Geological Survey and CSS President
Paleomagnetism, the study of ancient Earth magnetic fields trapped in rocks, has long been used in tectonic studies. Initially focused on plate-tectonic-scale reconstructions, technological advances now make paleomagnetism practical at scales of mountain ranges to individual fault blocks, the realm structural geology. General deformation of material is broken into 4 components (rotation, translation, dilation, and distortion), but most strain in the brittle upper crust is accommodated by coordinated block rotation and fault translation. Thus, a major application of paleomagnetism in structural geology is to constrain the three-dimensional geometry of fault block rotations. Paleomagnetic data can be used to detect tilt of igneous plutons and massive lavas that lack field markers of paleohorizontal. Coupled with isotopic dating, such paleomagnetically determined tilts provide tight constraints on styles and ages of upper crustal deformation. Paleomagnetic vectors are particularly useful as structural markers for detection of steep-axis rotations; unambiguous field markers of such rotations are rare. Paleomagnetic data integrated with geologic mapping and fault studies have documented broad, previously unrecognized zones that accommodated distributed strike-slip strain via vertical axis rotation and discontinuous faulting. Case studies from the Basin and Range and Rocky Mountains provinces illustrate these paleomagnetic applications and their tectonic implications.