Session I
Interpreting Earth in Space and Time

The Northern Rockies EarthScope workshop provides an opportunity for a diverse array of earth scientists to meet and discuss opportunities for utilizing the northern Rocky Mountains as a natural laboratory for enhancing our understanding of continental evolution. Geographically, the workshop focuses on the area generally within northern Wyoming, SW Montana, and eastern Idaho. This region has an incredibly diverse and extensive geologic history that is recorded in rocks that range from ancient (>3.5 Ga) gneisses to the modern Yellowstone hotspot/Snake River Plain. This portion of the North American continent provides an ideal environment for studying the physical and chemical evolution of the crust-mantle system because it records multiple generations of crustal evolution involving a variety of tectonic environments (e.g., continent-continent collisions, rifting, the formation of a major sedimentary basin (Belt basin), and impingement of a modern mantle plume). The region provides an opportunity, therefore, to examine two of the most challenging problems in the study of the formation and evolution of continental crust: 1) how newly segregated, low-density crust and lithosphere (most commonly formed in island arcs and along continental margins) is integrated into compositionally and structurally mature continents and 2) how this newly formed crust and lithosphere evolves within the continental environment and how its structure and composition influence the subsequent evolution of the continent itself. These are two of the most basic questions in continental crustal evolution because modern continents are clearly not the geochemical or structural equivalent of modern island arcs. In addition, it is clear that Precambrian lithospheric features influence Phanerozoic continental evolution, particularly in western North America. To understand the processes by which this combined crust-lithosphere system forms and evolves requires the application of many disciplines (geochemistry, petrology, geophysics, etc.). In particular, EarthScope resources can play a pivotal role in elucidating the complex history of the northern Rocky Mountains, and thereby add significantly to our understanding of both crustal genesis and continental evolution, by providing images of crustal and mantle structures in terms of the ages and compositions of material added to the continental crust.

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

USARRAY--Bigfoot
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

1. 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.
2. Reflection profiling is the chief tool for observing the crust.
3. We need, however, multidisciplinary data collection, using all the methods of investigation included in Complementary Geophysics, to obtain the best constrained understanding of lithosphere.
4. LITHOPROBE provides a good model for how to proceed. (Ron Clowes did not pay me to say this.)
5. 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.

The EarthScope Plate Boundary Observatory (PBO) is providing an unprecedented level of GPS and related resources to the Earth science community. The heart of PBO is an extensive network of permanent GPS and strainmeter stations spanning the North American plate boundary zone. Supplementing the permanent network is a new, state-of-the-art, portable campaign receiver pool that is available from UNAVCO for individual research projects. With large memory, low weight, and low power requirements, these GPS systems can be deployed in large numbers and for long periods of time, allowing investigators to tackle a new class of research problems. Dense campaign deployments are particularly suitable for tectonic, magmatic, or earthquake deformation studies. GPS is also a tool for precise mapping and provides geodetic control for ground-based, aerial, and satellite surveys such as LIDAR and InSAR. The acquisition of aerial and satellite imagery and geochronology is part of EarthScope-funded research that will examine the strain field beyond the decade time scales available from the PBO geodetic instrumentation. We will give an overview of GPS that is aimed at all users and potential users and will discuss how the research community can access and use PBO equipment, data, and products. For more information on UNAVCO PBO and GeoEarthScope resources see www.unavco.org.

In order to meet the science goals of the EarthScope initiative, there is a critical need to develop an earth science infrastructure for semantic discovery and integration of disciplinary databases. It can be demonstrated that the rapid growth of data rich resources associated with geology, such as maps created by in-situ and remote sensing techniques, as well as spatial and aspatial relational databases, is driving new requirements for an information infrastructure that will facilitate scientific discovery. The emergence of web based access to data has clearly facilitated our ability to download data, but our ability to explore the more important and usually more complex geologic problems is limited, because we are forced to work at the syntactic level, rather than at a higher scientific, or semantic, level. The missing element in enabling the higher-level interconnections is the technology related to knowledge representation, and the use of ontologies and semantically aware interfaces between science components, thus leading to the possibility of machine-to-machine communication across multiple disciplines. The ability to more easily integrate data, with minimum human intervention will clearly support scientists seeking to unravel the evolutionary history of continents, develop better assessment of hazards or a more precise estimate of natural resources (www. earthscope.org) .

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"

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. http://www.earthscope.org/education

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.

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.

Mantle

Xenoliths from the lithospheric mantle are generally composed of peridotite, with lesser amounts of pyroxenite and/or eclogite. Equilibration temperatures for these lithologies can generally be determined on the basis of two-pyroxene thermometery; precise depths of equilibration are much harder to estimate unless the samples contain garnet. The crystallization ages of mantle xenoliths are also usually difficult to constrain, as zircon is a rare phase in most upper mantle lithologies and most xenoliths have resided above the blocking temperature of other radiogenic isotope systems (Rb-Sr, Sm-Nd, Lu-Hf) for a significant fraction of their histories. The Re-Os isotope system provides arguably the best means of determining the crystallization age of mantle xenoliths, but, like most model age approaches, carries significant uncertainty.

Crust

Xenoliths from the lower continental crust can be extremely heterogeneous in composition, but mafic compositions dominate in a number of regions. Equilibration T and P may determined from coexisting phases and, in some cases, thermal histories deduced from presence of frozen metamorphic reactions (e.g., coronas). The presence of zircon and other U-bearing accessory phases provides the opportunity to determine the thermal evolution of the lower crust (e.g., Schmitz and Bowring, 2003, CMP), which, combined with other isotopic studies (e.g., Sm-Nd, Lu-Hf and Pb-Pb) can be used to understand the development and evolution of the lithospheric section.