The formation of the Mojave crustal province has been a persistent enigma in models of the Proter... more The formation of the Mojave crustal province has been a persistent enigma in models of the Proterozoic tectonic history of southwestern Laurentia. It is composed of similarly-aged 1.8-1.7 Ga rocks as the adjacent Yavapai province, yet shows evidence of much older (>2.2 Ga) lithospheric components in all isotopic systems. We present >700 new U-Pb analyses and >350 new Lu-Hf analyses of zircon from the oldest metasedimentary and plutonic rocks of the Mojave province to better understand its origin and evolution. Six metasedimentary rocks have detrital zircon age populations dominated by ~1.87 and 2.4-2.7 Ga grains. This age distribution is like the Vishnu Schist in Grand Canyon and we suggest that together they comprise a regional 2 turbidite basin that we name the Vishnu basin. Cross cutting relationships indicate that deposition of the Vishnu basin proceeded from west to east (present coordinates) from ~1.8-1.75 Ga. The Vishnu basin extends from central Arizona to the Transverse Ranges in California and possibly beyond the present boundaries of Laurentia to previously adjacent cratonic blocks. We propose the Mawson continent as the source of Vishnu basin detritus, and hence favor Nuna accretion at ~1.8 Ga. Paleoproterozoic plutons that intrude the Vishnu basin sampled in this study range in age from 1791±15 to 1691±15 Ma. Plutons show a systematic change in Hfisotope composition through time and space. The oldest plutons in the western Mojave province have the most isotopically evolved signatures and contain inherited zircon cores and xenocrysts with age and Hf-isotope characteristics that suggest they were derived from Vishnu basin sediments and/or 1.8-2.7 Ga lower crust. The Hf-isotope composition of plutons becomes more radiogenic (juvenile) from west-to-east and from old-to-young. The magnitude of Hf-isotope variation requires increased influence of depleted mantle sources through time. Hf-isotope compositions of the younger 1.7-1.68 Ga syn-to-post orogenic granites show more evolved compositions attributed to lower crustal melting due to crustal thickening during the Yavapai orogeny.
U-Pb dating of 1035 detrital zircons from 12 spatially distributed samples of the Paleoproterozoi... more U-Pb dating of 1035 detrital zircons from 12 spatially distributed samples of the Paleoproterozoic Vishnu Schist (Arizona, United States) reveals a bimodal 207 Pb/ 206 Pb age probability diagram with peaks at 1.8 Ga and 2.5 Ga. Surprisingly, only 13% of detrital zircon ages overlap with the published depositional age range of 1750-1741 Ma. The similarity of the age distributions in all samples constrains possible suturing of crustal blocks to pre-Vishnu Schist deposition rather than during the peak 1710-1680 Ma deformation. Of all grains analyzed, 15% overlap at 2σ with the 1.84 ± 1 Ga Elves Chasm orthogneiss of the western Grand Canyon. This supports fi eld evidence that the Vishnu Schist was deposited on 1.84 Ga arc basement rather than in a juvenile 1.75 Ga arc setting. Archean grains of 3.8-2.5 Ga compose 30% of all grains. A comparison of the ages older than 2.2 Ga from the Vishnu Schist (495 grains) with compilations of zircon ages from other cratons does not support provenances in the Wyoming, South China, or Siberian cratons; instead sources may be located in Gawler craton of Australia, North China craton, or Antarctica. If the detrital zircons were far-traveled, this is a new constraint for viable reconstructions of the Nuna supercontinent. However, given the high percentage of pre-1.8 Ga zircons, unexposed proximal basement sources are more likely, resulting in a model by which Vishnu sediments were derived from Mojave province crust that consists of Archean and 1.9-1.8 Ga crust, now in the subsurface, that was unroofed during Vishnu deposition.
Geological Society of America Bulletin, Apr 15, 2015
I would like to acknowledge several institutions and individuals that have made this research pos... more I would like to acknowledge several institutions and individuals that have made this research possible. First I would like to extend my thanks to the University of New Mexico Department of Earth and Planetary sciences for the financial support that has made my graduate education possible. Second, I would like to thank the University of New Mexico Office of Career Services for financial support that allowed me to present this research at both the Geological Society of America Annual Meeting in 2013, and the American Geophysical Union Fall Meeting in 2013. Next, I acknowledge the National Science Foundation for providing funds to Dr. Karl Karlstrom and Dr. George Gehrels through NSF grant EAR-1145247. These funds provided the wherewithal to analyze dozens of samples and expand our understanding of the Proterozoic assembly of the North American continent. In addition, I am grateful to have had the Arizona Laserchron Center at my disposal. The ALC is a top notch analytical facility with an exceptionally knowledgeable and helpful staff. A special to thanks to the Laserchron staff who must be well sick of the Karlstrom/Holland group by now. As for professional acknowledgements, I must acknowledge my committee, but first and foremost extend my sincerest gratitude to my principal advisor, Dr. Karl Karlstrom. Karl has always been eager to answer questions, consider new ideas or interpretations, conduct fieldwork, and has been a powerful inspiration both personally and professionally. I look forward to continuing my graduate career as his student. I would also like to thank Dr. Laura Crossey, whose quick wit extends beyond geology and makes field excursions hilarious and unpredictable. Dr. Yemane Asmerom has been one iv of the most influential instructors I have ever had. He has challenged me intellectually and I have always felt stronger for it. Furthermore, I must extend my thanks to faculty at the University of Massachusetts, Dr. Michael Williams and Dr. Sheila Seaman, whose academic nurturing enabled me to pursue a graduate degree in geosciences. Thanks to Mike for his contagious enthusiasm for geology, and to Sheila who was quite right when she told me that majoring in geology would be a wonderful idea. Next, I thank my fellow students for support, comradery, and numerous discussions of our research and geology in general. I acknowledge:
The formation of the Mojave crustal province has been a persistent enigma in models of the Proter... more The formation of the Mojave crustal province has been a persistent enigma in models of the Proterozoic tectonic history of southwestern Laurentia. It is composed of similarly-aged 1.8-1.7 Ga rocks as the adjacent Yavapai province, yet shows evidence of much older (>2.2 Ga) lithospheric components in all isotopic systems. We present >700 new U-Pb analyses and >350 new Lu-Hf analyses of zircon from the oldest metasedimentary and plutonic rocks of the Mojave province to better understand its origin and evolution. Six metasedimentary rocks have detrital zircon age populations dominated by ~1.87 and 2.4-2.7 Ga grains. This age distribution is like the Vishnu Schist in Grand Canyon and we suggest that together they comprise a regional 2 turbidite basin that we name the Vishnu basin. Cross cutting relationships indicate that deposition of the Vishnu basin proceeded from west to east (present coordinates) from ~1.8-1.75 Ga. The Vishnu basin extends from central Arizona to the Transverse Ranges in California and possibly beyond the present boundaries of Laurentia to previously adjacent cratonic blocks. We propose the Mawson continent as the source of Vishnu basin detritus, and hence favor Nuna accretion at ~1.8 Ga. Paleoproterozoic plutons that intrude the Vishnu basin sampled in this study range in age from 1791±15 to 1691±15 Ma. Plutons show a systematic change in Hfisotope composition through time and space. The oldest plutons in the western Mojave province have the most isotopically evolved signatures and contain inherited zircon cores and xenocrysts with age and Hf-isotope characteristics that suggest they were derived from Vishnu basin sediments and/or 1.8-2.7 Ga lower crust. The Hf-isotope composition of plutons becomes more radiogenic (juvenile) from west-to-east and from old-to-young. The magnitude of Hf-isotope variation requires increased influence of depleted mantle sources through time. Hf-isotope compositions of the younger 1.7-1.68 Ga syn-to-post orogenic granites show more evolved compositions attributed to lower crustal melting due to crustal thickening during the Yavapai orogeny.
Geological Society of America Special Papers, 2012
Digital geologic maps that use a virtual globe interface, like Google Earth (GE), are a relativel... more Digital geologic maps that use a virtual globe interface, like Google Earth (GE), are a relatively new medium for presenting geologic data and interpretations. This format incorporates signifi cant advantages over traditional paper geologic maps and cross sections, including: • A user-friendly and intuitive interface for novice users, which enhances the utility of geologic information for students and the general public; • The ability to view multiple maps simultaneously and seamlessly transition between maps by zooming or panning; • The option of displaying cross sections in situ on geologic maps as vertical interpretations of above ground or subsurface geology; and • A facility for integrating map interpretations with individual outcrop and fi eld data, which traditionally has been relegated to fi eld books. This paper outlines a digital maps package, composed of geologic maps of regions of Virginia, as a proof of concept and template for possible future expansion beyond state boundaries or into the realm of soils, geomorphological or hydrological maps. Through collaboration between universities, state agencies, and federal organizations we have assembled a multi-layered, fully interactive map accessible through two portals: the stand-alone Google Earth application, and as a web page using the GE web browser plug-in (GE API). All maps within this package have selectable polygons, polylines ("paths"), and points ("placemarks"), many of which contain associated metadata, such as lithologic descriptions, fault information, outcrop orientation data, etc. At the smallest scale, a generalized geologic map of Virginia is displayed with a selectable overlay of regional physiographic provinces. As users pan and zoom, the maps automatically transition from generalized statewide maps to more refi ned
The formation of the Mojave crustal province has been a persistent enigma in models of the Proter... more The formation of the Mojave crustal province has been a persistent enigma in models of the Proterozoic tectonic history of southwestern Laurentia. It is composed of similarly-aged 1.8-1.7 Ga rocks as the adjacent Yavapai province, yet shows evidence of much older (>2.2 Ga) lithospheric components in all isotopic systems. We present >700 new U-Pb analyses and >350 new Lu-Hf analyses of zircon from the oldest metasedimentary and plutonic rocks of the Mojave province to better understand its origin and evolution. Six metasedimentary rocks have detrital zircon age populations dominated by ~1.87 and 2.4-2.7 Ga grains. This age distribution is like the Vishnu Schist in Grand Canyon and we suggest that together they comprise a regional 2 turbidite basin that we name the Vishnu basin. Cross cutting relationships indicate that deposition of the Vishnu basin proceeded from west to east (present coordinates) from ~1.8-1.75 Ga. The Vishnu basin extends from central Arizona to the Transverse Ranges in California and possibly beyond the present boundaries of Laurentia to previously adjacent cratonic blocks. We propose the Mawson continent as the source of Vishnu basin detritus, and hence favor Nuna accretion at ~1.8 Ga. Paleoproterozoic plutons that intrude the Vishnu basin sampled in this study range in age from 1791±15 to 1691±15 Ma. Plutons show a systematic change in Hfisotope composition through time and space. The oldest plutons in the western Mojave province have the most isotopically evolved signatures and contain inherited zircon cores and xenocrysts with age and Hf-isotope characteristics that suggest they were derived from Vishnu basin sediments and/or 1.8-2.7 Ga lower crust. The Hf-isotope composition of plutons becomes more radiogenic (juvenile) from west-to-east and from old-to-young. The magnitude of Hf-isotope variation requires increased influence of depleted mantle sources through time. Hf-isotope compositions of the younger 1.7-1.68 Ga syn-to-post orogenic granites show more evolved compositions attributed to lower crustal melting due to crustal thickening during the Yavapai orogeny.
U-Pb dating of 1035 detrital zircons from 12 spatially distributed samples of the Paleoproterozoi... more U-Pb dating of 1035 detrital zircons from 12 spatially distributed samples of the Paleoproterozoic Vishnu Schist (Arizona, United States) reveals a bimodal 207 Pb/ 206 Pb age probability diagram with peaks at 1.8 Ga and 2.5 Ga. Surprisingly, only 13% of detrital zircon ages overlap with the published depositional age range of 1750-1741 Ma. The similarity of the age distributions in all samples constrains possible suturing of crustal blocks to pre-Vishnu Schist deposition rather than during the peak 1710-1680 Ma deformation. Of all grains analyzed, 15% overlap at 2σ with the 1.84 ± 1 Ga Elves Chasm orthogneiss of the western Grand Canyon. This supports fi eld evidence that the Vishnu Schist was deposited on 1.84 Ga arc basement rather than in a juvenile 1.75 Ga arc setting. Archean grains of 3.8-2.5 Ga compose 30% of all grains. A comparison of the ages older than 2.2 Ga from the Vishnu Schist (495 grains) with compilations of zircon ages from other cratons does not support provenances in the Wyoming, South China, or Siberian cratons; instead sources may be located in Gawler craton of Australia, North China craton, or Antarctica. If the detrital zircons were far-traveled, this is a new constraint for viable reconstructions of the Nuna supercontinent. However, given the high percentage of pre-1.8 Ga zircons, unexposed proximal basement sources are more likely, resulting in a model by which Vishnu sediments were derived from Mojave province crust that consists of Archean and 1.9-1.8 Ga crust, now in the subsurface, that was unroofed during Vishnu deposition.
Geological Society of America Bulletin, Apr 15, 2015
I would like to acknowledge several institutions and individuals that have made this research pos... more I would like to acknowledge several institutions and individuals that have made this research possible. First I would like to extend my thanks to the University of New Mexico Department of Earth and Planetary sciences for the financial support that has made my graduate education possible. Second, I would like to thank the University of New Mexico Office of Career Services for financial support that allowed me to present this research at both the Geological Society of America Annual Meeting in 2013, and the American Geophysical Union Fall Meeting in 2013. Next, I acknowledge the National Science Foundation for providing funds to Dr. Karl Karlstrom and Dr. George Gehrels through NSF grant EAR-1145247. These funds provided the wherewithal to analyze dozens of samples and expand our understanding of the Proterozoic assembly of the North American continent. In addition, I am grateful to have had the Arizona Laserchron Center at my disposal. The ALC is a top notch analytical facility with an exceptionally knowledgeable and helpful staff. A special to thanks to the Laserchron staff who must be well sick of the Karlstrom/Holland group by now. As for professional acknowledgements, I must acknowledge my committee, but first and foremost extend my sincerest gratitude to my principal advisor, Dr. Karl Karlstrom. Karl has always been eager to answer questions, consider new ideas or interpretations, conduct fieldwork, and has been a powerful inspiration both personally and professionally. I look forward to continuing my graduate career as his student. I would also like to thank Dr. Laura Crossey, whose quick wit extends beyond geology and makes field excursions hilarious and unpredictable. Dr. Yemane Asmerom has been one iv of the most influential instructors I have ever had. He has challenged me intellectually and I have always felt stronger for it. Furthermore, I must extend my thanks to faculty at the University of Massachusetts, Dr. Michael Williams and Dr. Sheila Seaman, whose academic nurturing enabled me to pursue a graduate degree in geosciences. Thanks to Mike for his contagious enthusiasm for geology, and to Sheila who was quite right when she told me that majoring in geology would be a wonderful idea. Next, I thank my fellow students for support, comradery, and numerous discussions of our research and geology in general. I acknowledge:
The formation of the Mojave crustal province has been a persistent enigma in models of the Proter... more The formation of the Mojave crustal province has been a persistent enigma in models of the Proterozoic tectonic history of southwestern Laurentia. It is composed of similarly-aged 1.8-1.7 Ga rocks as the adjacent Yavapai province, yet shows evidence of much older (>2.2 Ga) lithospheric components in all isotopic systems. We present >700 new U-Pb analyses and >350 new Lu-Hf analyses of zircon from the oldest metasedimentary and plutonic rocks of the Mojave province to better understand its origin and evolution. Six metasedimentary rocks have detrital zircon age populations dominated by ~1.87 and 2.4-2.7 Ga grains. This age distribution is like the Vishnu Schist in Grand Canyon and we suggest that together they comprise a regional 2 turbidite basin that we name the Vishnu basin. Cross cutting relationships indicate that deposition of the Vishnu basin proceeded from west to east (present coordinates) from ~1.8-1.75 Ga. The Vishnu basin extends from central Arizona to the Transverse Ranges in California and possibly beyond the present boundaries of Laurentia to previously adjacent cratonic blocks. We propose the Mawson continent as the source of Vishnu basin detritus, and hence favor Nuna accretion at ~1.8 Ga. Paleoproterozoic plutons that intrude the Vishnu basin sampled in this study range in age from 1791±15 to 1691±15 Ma. Plutons show a systematic change in Hfisotope composition through time and space. The oldest plutons in the western Mojave province have the most isotopically evolved signatures and contain inherited zircon cores and xenocrysts with age and Hf-isotope characteristics that suggest they were derived from Vishnu basin sediments and/or 1.8-2.7 Ga lower crust. The Hf-isotope composition of plutons becomes more radiogenic (juvenile) from west-to-east and from old-to-young. The magnitude of Hf-isotope variation requires increased influence of depleted mantle sources through time. Hf-isotope compositions of the younger 1.7-1.68 Ga syn-to-post orogenic granites show more evolved compositions attributed to lower crustal melting due to crustal thickening during the Yavapai orogeny.
Geological Society of America Special Papers, 2012
Digital geologic maps that use a virtual globe interface, like Google Earth (GE), are a relativel... more Digital geologic maps that use a virtual globe interface, like Google Earth (GE), are a relatively new medium for presenting geologic data and interpretations. This format incorporates signifi cant advantages over traditional paper geologic maps and cross sections, including: • A user-friendly and intuitive interface for novice users, which enhances the utility of geologic information for students and the general public; • The ability to view multiple maps simultaneously and seamlessly transition between maps by zooming or panning; • The option of displaying cross sections in situ on geologic maps as vertical interpretations of above ground or subsurface geology; and • A facility for integrating map interpretations with individual outcrop and fi eld data, which traditionally has been relegated to fi eld books. This paper outlines a digital maps package, composed of geologic maps of regions of Virginia, as a proof of concept and template for possible future expansion beyond state boundaries or into the realm of soils, geomorphological or hydrological maps. Through collaboration between universities, state agencies, and federal organizations we have assembled a multi-layered, fully interactive map accessible through two portals: the stand-alone Google Earth application, and as a web page using the GE web browser plug-in (GE API). All maps within this package have selectable polygons, polylines ("paths"), and points ("placemarks"), many of which contain associated metadata, such as lithologic descriptions, fault information, outcrop orientation data, etc. At the smallest scale, a generalized geologic map of Virginia is displayed with a selectable overlay of regional physiographic provinces. As users pan and zoom, the maps automatically transition from generalized statewide maps to more refi ned
Uploads
Papers by Owen Shufeldt