Interpreting Earth in Space and Time
David W. Mogk, Montana State University
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Earthscope has, in name at least, a "Complementary Geophysics" component. Complementary Geophysics includes chiefly active-source seismology (reflection profiling, refraction), potential field geophysics (gravity, magnetics), magnetotellurics, and petrophysics. My main point today is that Complementary Geophysics is essential to understanding Earth structure and processes-an understanding that we will not have unless we take steps to make sure it is integrated in a fundamental way with the primary efforts of Earthscope, which include
PBO–modern deformation of crust
SAFOD–drilling at Parkfield, California
Complementary Geophysics is essential to USARRAY-which, as you know, is a passive-seismic array planned to roll across the US and collect an image primarily of the mantle with unprecedented areal coverage. USARRAY also includes a component called FlexArray, which will consist of scattered detailed studies of the mantle and crust. Money is not guaranteed for this component.
The case I will make today is for multidisciplinary studies of the crust and mantle that are integrated and lie along at least one corridor, or transect, across the US from east to west margins. These studies must be done in enough detail (close station and source spacing) to yield real understanding of what has happened over geologic time. The flagship of this effort should be seismic reflection profiling, as this yields the most detailed structural information of the crust. Reflection profiling on this scale will be expensive, but you get what you pay for: What is real understanding worth? Such an effort in not currently in the Earthscope plans.
A model of the kind of effort I am advocating has been achieved by the Canadians in a project called LITHOPROBE, that Ron Clowes, chief scientist/secretariat, will talk about later today. I will show you some samples of the Canadian data, that are almost self explanatory.
Why is the crust important? 1) We live on it, and we have our most detailed information on rock composition and structure from the crust. The US Public, which is funding Earthscope, also lives on the crust. 2) The crust preserves the most detailed history of the Earth. If we compare observables in the crust and the mantle, the crust has an order or magnitude more variability than mantle. Reflection profiling, in particular, is a key mode of investigation to reveal the structure and history of the crust. If one lists the greatest geophysical discoveries of all time, reflection profiling is responsible for ~ one half, and for discoveries relating to the crust, it is responsible for nearly all. I will show my list of these discoveries, but I encourage the audience to make its own lists. I will show briefly key LITHOPROBE results. (Ron Clowes will expand on these later.) and, finally, I will give a brief tutorial in seismic methods.Conclusions
- The crust is an important layer to image–perhaps the most important for understanding Earth processes present and past. Currently, Earthscope has no definite plans or money to image the crust in a systematic way.
- Reflection profiling is the chief tool for observing the crust.
- We need, however, multidisciplinary data collection, using all the methods of investigation included in Complementary Geophysics, to obtain the best constrained understanding of lithosphere.
- LITHOPROBE provides a good model for how to proceed. (Ron Clowes did not pay me to say this.)
- Our COCORP "transects" of the 1970's and 1980's need to be replaced with at least one transect across the U.S. from margin to margin, taking advantage of the tremendous improvements in hardware and software/processing since those days in order to produce modern images like the Canadians have done.
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GEON (GEOscience Network; www.geongrid.org) is a National Science Foundation funded research program to develop cyberinfrastructure for the solid Earth sciences. This effort is a multi-institutional partnership between Earth Scientists and Information Technology researchers designed to facilitate integration, analysis, visualization and modeling of 4-D data. GEON architecture is designed to facilitate access to data for earth scientists through standardized interfaces, and on the Web via a portal, while in the background significant, but transparent technology resources provide a stable infrastructure. Through the portal mechanism, GEON is able to provide access to a number of services and resources including advanced search and query of distributed, semantically-integrated databases, Web-enabled access to shared tools, and seamless access to distributed computational, storage, visualization resources and data archives.
GEON research in the use of knowledge representation techniques for Earth Science data indicates that an ontologic framework for data, along with semantic registration, can facilitate the management, integration and analysis of databases and other data objects in a web-based environment. Although the term "ontology" is derived from philosophical literature, the term as employed in science simply refers to specification of the meanings of (geologic) terms and relationships between terms. Thus, earth scientists, can use ontologies to represent knowledge in the science domain through a declarative formalism, where geologic objects and phenomena have describable relationships between them (e.g. subclass, part of, etc.). Using a common, community-accepted vocabulary of Earth science terms and taxonomic representation of classification schemes such as those for rocks, the geologic time scale, or geologic structures, ontologies provide an organizational structure for classifying data that can be discovered by computers. This is only possible because an ontology contains explicit definitions of terms used by scientists to associate meaning to the data or relationships between datasets.
GEON research recognizes that there are several facets to integration of databases, which include (1) schema merging, when the user is knowledgeable about the data organization (semantics of the schema) with the opportunity to actually merge the data from the independent databases, (2) view-based integration which involves the definition and creation of an integration schema that allows the user to address structural heterogeneity, and (3) ontology-based integration, which is accomplished by registering databases to ontologies. While the first two are relatively well-known Computer Science techniques, the ontology-based approach is relatively new, and GEON researchers have made major progress in this area. Ontology-based integration serves the needs of data integration in the case of varying semantics between different databases, and where the data being integrated are very different in nature.
GEON has developed technologies for data integration based on registering databases to data ontologies. In general, this allows integration of multiple heterogeneous databases into a single virtual database. GEON researchers are working with other members of the Earth Sciences community to use existing ontologies (e.g. NADM, North American Geological Data Model), or, where necessary, develop ontologies at the disciplinary, sub-disciplinary, or even individual database levels. These ontologies need to be "plugged into" a larger ontologic framework representing broader (higher level) concepts, for example, the SWEET ontology that has been developed at NASA (see http://sweet.jpl.nasa.gov/). We believe that the creation of such an integrated data environment will facilitate more integrative science and a "deeper appreciation of connections between different aspects of our environment" (www.earthscope.org).
Definition and use of Cyberinfrastructure (modified from Futrell, J. and the AC-ERE, May 2003, NSF):
"Cyberinfrastructure as a suite of critical enabling tools and research essential to the study of complex earth systems"
"Cyberinfrastructure provides tools for storing, finding, analyzing, and synthesizing a diverse array of data"
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The EarthScope project is a 15 year long experiment involving the instrumentation of North America with GPS stations, strainmeters, drill holes, and seismometers. It is hoped that earth scientists will integrate the complementary data sets into a new and deeper understanding of plate tectonics processes, such as the recently discovered episodic tremor and slip events in Cascadia. Similarly, EarthScope's education and outreach program must integrate the contributions of existing and vibrant programs on the local as well as the national level to create a new and vital program for exciting students and the general public about earth science.
By the end of this project, an EarthScope instrument will have been emplaced in virtually every county in the United States. This provides us with an unprecedented opportunity to create "teachable moments' about earth science on both a national and local scale over the entire educational cycle of an educational cohort. By using EarthScope's progress and discoveries as a means of raising interest in the content and concepts of earth science, we hope to have as profound an impact on science education as on the science of plate tectonics. In order to be successful, this effort must be effective on all scales: national, regional, and local. One method for accomplishing this is to integrate the national message with the regional and local concerns and capabilities. For example, Yellowstone National Park is unsurpassed in the world for the variety and quality of its geology and its personnel. Local geologists have already made plans to use EarthScope's data and results to provide park visitors and local students with a deeper understanding of the geology underlying this national treasure and its relevance to their lives. As this program proceeds, its successes will be exploited by other regions. On the national level, we anticipate interacting with programs such as this one in order to facilitate a rapid sharing of best practices and local resource information. This presentation will touch on some of the challenges in developing the EarthScope E&O program, and will also describe future plans for and provide information on getting involved at the local and regional levels. www.earthscope.org/education
Integrating isotopic and geochemical tracers, petrology and geochronology into earthscope research: Examples from the Wyoming Province
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Because of its >3.5 billion year history, the Wyoming province is an ideal place to address fundamental problems relating to the origin and evolution of continental crust. Its present-day three-dimensional crustal structure, as imaged by geophysical data sets, is the result of this lengthy process of crust formation and modification. Information from surface geology, petrology, geochemistry, isotope geology and geochronology is crucial to understanding the processes that over time formed the crustal structure of the Wyoming province imaged by geophysics.
Although relatively few surface exposures in the Wyoming province date from Early and Middle Archean, the isotopic compositions of Late Archean plutons require that a sizeable continent was present by 3 Ga. Something of the character of this crustal block can be inferred from the petrogenesis of the voluminous 2.75-2.95 Ga plutonic rocks of the Bighorn-Beartooth magmatic domain. These include granitoids of both TTG and calc-alkalic affinity. The TTG suite appears to have formed by partial melting of hydrous basaltic crust, a feature that, if it has survived, should be present on geophysical images of the lithosphere. Periods of Late Archean active tectonism modified the southern and western margin of the craton. The petrology, geochemistry and isotopic compositions of the igneous rocks formed at this time indicate the presence of an active continental margin in the area now occupied by the Teton Range and Wind River, Granite and Laramie Mountains. Subduction-related magmatism was accompanied by accretion of isotopically juvenile terranes at South Pass, Rattlesnake Hills, Bradley Peak and Sierra Madre, and by high P metamorphism related to continent-continent collision in the Teton Range. Both of these magmatic and accretionary processes may be reflected in present-day geophysical profiles. Proterozoic events that may have contributed to crustal structure include 2.0 Ga rifting along the southern and eastern margins of the province, 1.86 to 1.74 Ga events related to collision along the Great Falls tectonic zone and Cheyenne belt, and 1.5-1.43 Ga rifting and mafic magmatism associated with the formation of the Belt basin to the northwest. In addition, petrologic, geochemical and isotopic studies of Mesoproterozoic A-type magmatism in southeastern Wyoming and throughout the SW USA have shown that it is the product of partial melting of mantle-derived tholeiitic rock. A high velocity lower crustal layer imaged by CD-ROM beneath the Proterozoic terranes of the SW USA may represent this basaltic underplate. Finally, crustal scale faulting during the Laramide orogeny, Tertiary magmatism in the Absaroka, Rattlesnake Hills and Black Hills, Basin and Range extension, and magmatism associated with the Yellowstone hotspot all may have contributed to present-day crustal structure.
Deep Probe, Lithoprobe SAREX and CD-ROM seismic data sets have provided important information about the structure of the Wyoming province. However, there is a critical need for EarthScope projects that collect additional geophysical data and develop more detailed images of Wyoming province crust. Integration of these geophysical models with ongoing petrologic, geochemical and isotopic studies has the best potential to achieve the EarthScope goal of elucidating the processes of crust formation and evolution.
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Studies of xenoliths provide a depth dimension to surface geology studies, and, in favorable circumstances, also provide the fourth dimension of time. Such studies, when combined with information derived from seismological investigations and heat flow measurements, allow a clearer picture of the composition, structure and thermal evolution of deep continental lithosphere. In particular, geochemical studies of xenoliths provide insights into the processes that formed and modified the deep lithosphere (e.g., melting, metamorphism, fluid infiltration, basaltic underplating) and when they occurred. While xenoliths can provide a glimpse of the types of lithologies present at depth and how they formed, they cannot be assumed to be representative of the deep lithosphere, and inferences regarding the dominant lithologies present in the lower crust or upper mantle must be tempered by geophysical constraints on bulk physical properties of these regions.