David W. Mogk

Paleoproterozoic crust within the Great Falls tectonic zone: Implications for the assembly of southern Laurentia

The Great Falls tectonic zone and the Vulcan structure both have been proposed as the site of a Paleoproterozoic suture between the Archean Hearne and Wyoming provinces. Both hypotheses remain viable because all Precambrian rocks composing the Vulcan structure and much of the Great Falls tectonic zone are buried beneath Phanerozoic cover. The primary exceptions to this are the mafic to felsic igneous and metamorphic rocks of the Little Belt Mountains (Montana), previously considered the northernmost exposures of the Wyoming province. New U-Pb zircon ages from the late kinematic Pinto diorite (207Pb/206Pb age: 1864 ± 5 Ma) and a gneissic unit intruded by the Pinto (207Pb/206Pb age: 1867 ± 6 Ma), however, confirm their Paleoproterozoic age. These rocks exhibit an overall calc-alkaline affinity and the depletion in high field strength elements typical of convergent margin environments. Whole-rock Sm-Nd data (initial epsilon of −1 to +4) and a lack of premagmatic zircons indicate that the magmas were principally derived from a depleted mantle source, not from older crust. These data suggest that at least some rocks within the Great Falls tectonic zone originated at a convergent margin that developed during the closure of an ocean basin along the northwestern margin of the Wyoming craton ca. 1.9 Ga.

How Geoscientists Think and Learn

Decades ago, pioneering petroleum geologist Wallace Pratt pointed out that oil is first found in the human mind. His insight remains true today: Across geoscience specialties, the human mind is arguably the geoscientists most important tool. It is the mind that converts colors and textures of dirt, or blotches on a satellite image, or wiggles on a seismogram, into explanatory narratives about the formation and migration of oil, the rise and fall of mountain ranges, the opening and closing of oceans. Improved understanding of how humans think and learn about the Earth can help geoscientists and geoscience educators do their jobs better, and can highlight the strengths that geoscience expertise brings to interdisciplinary problem solving.

Teaching Methods in Undergraduate Geoscience Courses: Results of the 2004 on the Cutting Edge Survey of U.S. Faculty

A survey of U.S. geoscience faculty provides an integrated look at the geoscience courses currently being taught and the teaching methods that are used in these courses. The survey data indicate that there is a wide array of offerings both at the introductory level and for majors and thus no standard geoscience curriculum. While teaching methods remain dominated by lecture, most faculty use a range of more interactive methods. Most students are asked to solve problems including quantitative ones as part of their courses although relatively few explore problems of their own choosing. Writing and reading in the primary literature are used extensively in courses of all sizes at both the introductory level and in courses for majors. Strategies and tools for assessing student learning are strongly dependent on class size; however, students are more likely to be assessed through problem sets, oral presentations or papers in courses for majors. There is no question that research on learning and the resulting recommendations for best classroom practice that have emerged over the past decade have had an impact on geosciences classes. On the other hand, there is room for growth. Our data suggest that most faculty are still using these techniques infrequently. These results strongly support the continued offering of professional development activities that both bring new ideas to faculty and address the practicalities of widespread implementation of these techniques.

Extended history of a 3.5 Ga trondhjemitic gneiss, Wyoming Province, USA: evidence from UPb systematics in zircon

Abstract The Beartooth-Bighorn magmatic zone (BBMZ) and the Montana metasedimentary province (MMP) are two major subprovinces of the Archean Wyoming province. In the northwestern Beartooth Mountains, these subprovinces are separated by a structurally, lithologically and metamorphically complex assemblage of lithotectonic units that include: (1) a strongly deformed complex of trondhjemitic gneiss and interlayered amphibolites; and (2) an amphibolite facies mafic unit that occurs in a nappe that structurally overlies the gneiss complex. Zircons from a trondhjemitic blastomylonite in the gneiss complex yield concordant UPb ages of 3.5 Ga, establishing it as the oldest rock yet documented in the Wyoming province. Two younger events are also recorded by zircons in this rock: (1) an apparently protracted period of high-grade metamorphism and/or intrusion of additional magmas at ∼ 3.25 Ga; and (2) growth of hydrothermal zircon at ∼ 2.55 Ga, apparently associated with ductile deformation that immediately preceded structural emplacement of the gneiss. Although this latter event appears confined to areas along the BBMZ-MMP boundary, evidence of ∼ 3.25 Ga igneous activity is found in the overlying amphibolite (3.24 Ga) and throughout the MMP. These data suggest that this boundary first developed as a major intracratonic zone of displacement at or before 3.25 Ga. The limited occurrences of 2.8 Ga magmatic activity in the MMP suggest that it had a controlling influence on late Archean magmatism as well.

The digital library for earth system education: building community, building the library

Science educators have called repeatedly for an information system that can effectively deliver quality educational materials in readily accessible formats, with a high degree of confidence in their usefulness, interest, and effectiveness [4]. In the past 18 months, the Earth system education community has begun development of the Digital Library for Earth System Education (DLESE). Earth system educators and key agency officials at NSF and NASA have recognized that the convergence of information and learning technologies , the maturation of basic digital library research, and the increasing ubiquity of the Web in classrooms has made the DLESE both possible and timely. Representatives of the Earth system education community came together in August 1999 to institute a governance system and an operational arm (the DLESE Program Center, or DPC) to design and develop a community-sponsored and community-owned digital library [3]. DLESE will serve the unique needs of Earth system educators and learners at all academic levels , in both formal and informal settings, by providing: Interfaces and tools to allow student exploration of geospatial materials and Earth data sets. Though a wealth of Earth data exists on the Web, much of it is difficult for educators to use. DLESE will provide student-friendly access to a wide variety of archived and real-time data sets. Rapid, sophisticated access to collections of peer-reviewed teaching and learning resources. Earth science educators have been frustrated in attempts to find high-quality teaching resources appropriate for their teaching style and educational level on the Web in a timely manner. This resource discovery challenge is being met with the creation of metadata schemas, controlled vocabularies, and cataloging best practice recommendations , all informed by community participation [2]. Services to help users effectively create and use materials. A full array of digital services and human-mediated services for both users and contributors to the library is critical to the vision of DLESE as an active organization that both builds and serves its community. A community center to facilitate sharing and collaboration. DLESE will serve as an intellectual commons for the global Earth system community by being the primary contact for educators, learners , and citizens who seek reliable information about the Earth. A federated collection of holdings. DLESE is being designed from the beginning to support resource discovery across a diverse, federated net

The nature of Archean terrane boundaries: an example from the northern Wyoming Province

Abstract The Archean northern Wyoming Province can be subdivided into two geologically distinct terranes, the Beartooth-Bighorn magmatic terrane (BBMT) and the Montana metasedimentary terrane (MMT). The BBMT is characterized by voluminous Late Archean (2.90-2.74 Ga) magmatic rocks (primarily tonalite, trondhjemite, and granite); metasedimentary rocks are preserved only as small, rare enclaves in this magmatic terrane. The magmatic rocks typically have geochemical and isotopic signatures that suggest petrogenesis in a continental magmatic arc environment. The MMT, as exposed in the northern Gallatin and Madison Ranges, is dominated by Middle Archean trondhjemitic gneisses (3.2–3.0 Ga); metasedimentary rocks, however, are significantly more abundant than in the BBMT. Each terrane has experienced a separate and distinct geologic history since at least 3.6 Ga ago based on differences in metamorphic and structural styles, composition of magmatic and metasupracrustal rocks, and isotopic ages; consequently, these may be described as discrete terranes in the Cordilleran sense. Nonetheless, highly radiogenic and distinctive Pb-Pb isotopic signatures in rocks of all ages in both terranes indicate that the two terranes share a significant aspect of their history. This suggests that these two Early to Middle Archean crustal blocks, that initially evolved as part of a larger crustal province, experienced different geologic histories from at least 3.6 Ga until their juxtaposition in the Late Archean (between 2.75 to 2.55 Ga ago). Consequently, the boundary between the BBMT and MMT appears to separate terranes that are not likely to be exotic in the sense of their Phanerozoic counterparts. Other Archean provinces do appear to contain crustal blocks with different isotopic signatures (e.g. West Greenland, India, South Africa). The use of the term exotic, therefore, must be cautious in situations where geographic indicators such as paleontologic and/or paleomagnetic data are not available. In these cases, isotopic signatures are one of the most useful features for assessing overall genetic relations amongst geologically distinct terranes.

Application of Auger Electron Spectroscopy (AES) to naturally weathered hornblende

Abstract The surface chemistry of naturally weathered hornblende has been analyzed using Auger Electron Spectroscopy. The high spatial resolution and depth profiling capabilities of this technique allow changes in the relative concentrations of cations to be determined over micron-scale areas and at depths resolvable on the sub-micron scale. These data indicate that during chemical weathering: 1) there is a systematic change in surface chemistry through a thickness up to 1200 angstroms, 2) complete cation depletion at the surface layer does not occur, 3) different components are leached to different depths to varying degrees, and 4) no new phase such as clay or smectite necessarily forms. A nonsteady state diffusion model is most consistent with these data.

Detrital mineral chronology of the Uinta Mountain Group: Implications for the Grenville flood in southwestern Laurentia

Numerous studies have shown that large quantities of Grenville-age detritus dominate Neoproterozoic to Cambrian arenites in southwest Laurentia (southwestern United States). U-Pb ages and Hf isotopic compositions of zircons and 40 Ar/ 39 Ar ages of white mica from clastic sedimentary rocks of the Neoproterozoic Uinta Mountain Group also indicate significant Mesoproterozoic detritus mixed with a variably abundant Archean component. Zircons with ages representative of the Paleoproterozoic basement in the eastern Uinta Mountains or the younger Paleoproterozoic rocks of the adjacent Yavapai-Mazatzal terranes were not observed. A limited range of initial ϵ Hf (∼90% between –3 and +3) for Mesoproterozoic zircons suggests derivation from a source region (or regions) characterized by mixing between juvenile and reworked older crust during Grenville orogenesis. The enriched Grenville-age basement proposed to underlie much of southeastern North America may be this source based on similarities of Hf isotopic data from Mesoproterozoic zircons in Mississippi River sand and available paleocurrent data. If so, then disruption of this supply in the Cambrian may be related to Iapetan rifting and, perhaps, the separation of the Precordillera terrane from Laurentia.

Paleoproterozoic Metamorphism in the Northern Wyoming Province: Implications for the Assembly of Laurentia

U‐Pb ages measured on zircons from the Tobacco Root Mountains and monazite from the Highland Mountains indicate that the northwestern Wyoming province experienced an episode of high‐grade metamorphism at ∼1.77 Ga. Leucosome emplaced in Archean gneisses from the Tobacco Root Mountains contains a distinctive population of zircons with an age of 1.77 Ga but also contains zircons to ∼3.5 Ga; it is interpreted to have been derived primarily by anatexis of nearby Archean schist. A granulite facies mafic dike that cuts across Archean gneissic banding in the Tobacco Root Mountains contains two distinct populations of zircons. A group of small (<50 μm) nonprismatic grains is interpreted to be metamorphic and yields an age of 1.76 Ga; a group of slightly larger prismatic grains yields an age of 2.06 Ga, which is interpreted to be the time of crystallization of the dike. Monazite from a leucogranite from the Highland Mountains yields a well‐defined age of 1.77 Ga, which is interpreted as the time of partial melting and emplacement of the leucogranite. These results suggest that the northwestern Wyoming province, which largely lies within the western part of the Great Falls tectonic zone, experienced a metamorphic maximum at 1.77 Ga. This age is ∼100 m.yr. younger than the proposed time of Wyoming‐Hearne collision in the central Great Falls tectonic zone (1.86 Ga) and suggests that the northwestern Wyoming province may have been involved in a separate, younger collisional event at ∼1.77 Ga. An event at this time is essentially coeval with collisions proposed for the eastern and southeastern margins of the province and suggests a multiepisodic model for the incorporation of the Wyoming craton into Laurentia.

Paleoproterozoic evolution of the Farmington zone: Implications for terrane accretion in southwestern Laurentia

Precambrian rocks in the Farmington zone in northeastern Utah provide important constraints on the accretionary history of southwestern Laurentia because they lie at the orogenic intersection of the Wyoming, Yavapai-Colorado, and Mojave provinces. Approximately 200 U-Pb analyses of zircons from Paleoproterozoic rocks in the Wasatch Mountains (Farmington Canyon and Little Willow complexes) and Uinta Mountains (Owiyukuts and Red Creek complexes) indicate: (1) U-Pb ages of 2.446 ± 0.011 Ga for igneous zircons from a meta-igneous rock and 2.42 Ga for the youngest detrital zircon from a metasedimentary rock constrain the age of the Farmington Canyon complex and represent a heretofore unreported Paleoproterozoic event in southwestern North America; (2) Most lithologies in all areas are metasupracrustal and U-Pb ages of most detrital zircons and whole-rock Sm-Nd data clearly indicate a primarily Archean provenance for the metasedimentary rocks; and (3) Metamorphism, including partial melting, occurred at 1.674 ± 0.012 Ga (2σ) based on U-Pb ages of 36 of 38 zircons (overgrowths and whole grains with Th/U <0.1) from metamorphic rocks, including leucosomes, from both the Wasatch and Uinta Mountains. These observations combined with previous work suggest a geologic history that begins with development of a Paleoproterozoic passive or rifted margin along the southwestern edge of the Wyoming craton and terminates with Paleoproterozoic accretion of Mojavia (±Yavapai-Colorado) to the Wyoming Province. Crystallization and model ages of the Farmington sequence suggest a possible genetic link between Mojavia, the Farmington zone, and the Wyoming Province, which would provide new constraints on proposed Neoproterozoic conjugates and the role of the Cheyenne belt in the accretionary tectonics of southwestern Laurentia.