Keywords: Arkansas, Prairie Creek lamproite field, paleosols, Franciscan Complex, Coast Range Ophiolite, Blue Mountains Province, western Mexico, Mazatzal Province: Burro Mountains, New Mexico, Eastern Ghats Belt, India, Ross Ice Sheet, Antarctica
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Archean lithospheric mantle beneath Arkansas: Continental growth by microcontinent accretion
W.L. Griffin et al., Australian Research Council (ARC) National Key Center for Geochemical Evolution and Metallogeny of Continents, Macquarie University, New South Wales 2109, Australia. Published online 12 Apr. 2011; doi: 10.1130/B30253.1.
Much of south-central North America is underlain by relatively young crust, formed 1.6 to 1.0 billion years ago as the continent grew southward. However, deep-seated diamond-bearing volcanic rocks (the Prairie Creek lamproite field) that erupted in Arkansas 100 million years ago have carried up fragments of the upper mantle from 100 to 150 km below the surface. The dating of sulfide minerals in these mantle samples shows they first formed more than 3 billion years ago, and the composition of the rock fragments links them to ancient continental roots, like those found under much of Canada. Seismic imaging shows a block with a deep, high-velocity mantle root lying under southern Arkansas and Louisiana, and the ancient rock fragments in the lamproites are interpreted as samples of the root of this "Sabine microcontinent." Ancient mantle roots are buoyant relative to the rest of the upper mantle, and the younger rocks of the upper crust probably have ridden this "life raft" until they were accreted to the growing North American continent. The findings by W.L. Griffin of the Macquarie University and colleagues suggest that the growth of individual continents is significantly affected by the accretion of older microcontinental blocks, and that the extent of early continental crust, therefore, may be greater than generally estimated.
Biogeochemical and ecosystem behavior interpreted from Late Cretaceous and early Paleocene paleosols and climates in the western interior of North America
Lee C. Nordt et al., Dept. of Geology, Baylor University, Waco, Texas 76798, USA. Published online 12 Apr. 2011; doi: 10.1130/B30365.1.
Ancient weathering zones, called paleosols, are routinely characterized by whole-rock geochemical properties because of limited sample disaggregation for physical and chemical laboratory analysis, compromising the interpretation of important biogeochemical information in deep time. As a consequence, Lee Nordt of Baylor University and colleagues have developed a series of modern transfer functions and applied them to a suite of Late Cretaceous and early Tertiary paleosols, providing the first glimpse into the colloidal world of lithified soils. Results reveal optimal water-holding potential computed from bulk densities and optimal fertility levels judging from high cation exchange capacity and base saturation, and limited salinity and sodicity. Unlike warm-temperate and forested soils with neutral pH from the early Tertiary, subtropical, and alkaline soils from the late Cretaceous appear to have supported a woodland plant formation adapted to low availability of iron, manganese, and phosphorous fixed in insoluble compounds. Carbon, nitrogen, phosphorous, and sulfur cycling through microbially mediated mineralization of soil organic matter was limited from low litter inputs. Results do not reveal demonstrable changes in soil characteristics during the extinction event of the Cretaceous-Tertiary transition.
Accretion of the Franciscan Complex attending Jurassic-Cretaceous geotectonic development of northern and central California
W.G. Ernst, Dept. of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-211, USA. Published online 12 Apr. 2011; doi: 10.1130/B30398.1.
According to W.G. Ernst of Stanford University, the Jura-Cretaceous history of northern California included these geologic events: (1) Middle Jurassic transpressive underflow produced an Andean volcanic arc along the continental edge; its erosion supplied volcanogenic debris to Mariposa-Galice overlap strata. (2) Garnetiferous metabasaltic rocks formed ~170 to 155 million years ago in an inboard transpressive subduction zone; as tectonic blocks, they represent exotic interlopers in the much-younger Franciscan melange. (3) The Klamath Mountains salient moved ~200 km westward ~155 to 145 million years ago, preceding the Great Valley forearc basin and the Franciscan trench formation. (4) After step-out of the convergent junction on the south, stranding the Coast Range Ophiolite, clastic sediment arrived at the trench and forearc basin ~145 to 140 million years ago, and accumulated over the next 60 million years. The most voluminous accretion took place during paroxysmal igneous activity and rapid, orthogonal convergence ~125 to 85 million years ago. (5) Accompanying Franciscan metagraywackes, metabasalts formed low-grade blueschists as the subduction zone cooled ~135-85 million years ago. (6) Andean volcanism-plutonism ceased ~85 million years ago in northern California, signaling transition to subhorizontal plate underflow and Laramide orogeny far to the east. (7) Franciscan Coastal Belt strata were deposited in a tectonic realm unaffected by subduction; other than feeble recrystallization, these rocks are lithologically similar to clastic units making up Eastern and Central belts of the Franciscan Complex.
Early Mesozoic paleogeography and tectonic evolution of the western United States: Insights from detrital zircon U-Pb geochronology, Blue Mountains Province, northeastern Oregon
Todd A. LaMaskin et al., Dept. of Environmental Sciences, Wisconsin Geological and Natural History Survey, University of Wisconsin-Extension, 3817 Mineral Point Road, Madison, Wisconsin 53705-5100, USA. Published online 12 Apr. 2011; doi: 10.1130/B30260.1.
Accreted terrranes of the Blue Mountains Province in northeastern Oregon and western Idaho preserve one of the most complete and least deformed early Mesozoic stratigraphic records in the western U.S. Cordillera. Todd A. LaMaskin of the Wisconsin Geological and Natural History Survey and colleagues use petrographic and detrital zircon uranium-lead age data from Triassic and Jurassic sedimentary basins of the Blue Mountains Province to assess potential linkages to North America and place new constraints on tectonic and paleogeographic modes for the western United States. They present new data that are consistent with previously documented shifts in sandstone composition from Late Triassic to early Late Jurassic time. In particular, the data from LaMaskin and colleagues suggest that Jurassic basins of the Blue Mountains were linked to a Triassic-Jurassic transcontinental sediment-dispersal system. The presence of North American detrital zircon grains in sediments of the Blue Mountains indicates that far-traveled, transcontinental sediment was delivered to Cordilleran basins and that these basins were proximal to North America during deposition. LaMaskin and colleagues also note similar detrital zircon age distributions in Jurassic turbidites of numerous Cordilleran terranes and suggest that many Jurassic basins of the Cordillera are not far-traveled with respect to southwestern North America. Apparent differences between the tectonic setting of accreted terranes in Oregon, Nevada, and California during Middle and Late Jurassic time may be typical aspects of an evolving convergent margin.
Evolution of the Guerrero composite terrane along the Mexican margin, from extensional fringing arc to contractional continental arc
Elena Centeno-Garcia et al., Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Avenida Universidad 3000, Ciudad Universitaria, Mexico D.F. 04510, Mexico. Published online 12 Apr. 2011; doi: 10.1130/B30057.1.
Elena Centeno-Garcia of Universidad Nacional Autonoma de Mexico and colleagues describe the evolution of the paleography of western Mexico during the Cretaceous, based on reconstruction of the source of sediments that were deposited more than 100 million years ago in shallow seas that surrounded a chain of volcanic islands.
Syntectonic 1.46 Ga magmatism and rapid cooling of a gneiss dome in the southern Mazatzal Province: Burro Mountains, New Mexico
Jeffrey M. Amato et al., Shell Oil, 200 N. Dairy Ashford, Office 2183, Houston, Texas 77079, USA. Published online 12 Apr. 2011; doi: 10.1130/B30337.1.
According to Jeffrey M. Amato of Shell Oil Company and colleagues, it is difficult to determine the timing and sequence of events of Precambrian rocks because they formed prior to the widespread preservation of fossils that marks the Phanerozoic. They note that the only way to accurately determine their history is to use radiometric dating. In southern New Mexico, Precambrian rocks are generally exposed as small slivers of crust at the base of fault blocks, but in the Burro Mountains of southwest New Mexico, two uplifted fault blocks consist entirely of Precambrian rocks. The majority of the rocks are plutons that formed 1.46 billion years ago as part of a belt of magmatic activity that stretches across the entire southern part of the North American continent. These plutons intrude older sedimentary rocks that were buried to 10 to 15 km. The plutons are deformed with the same orientation as the fabrics in the surrounding rocks, indicating that deformation occurred before the oldest plutons, but younger than the youngest plutons that are similar in age, thus tightly constraining the timing of deformation. Deformation 1.46 billion years ago has been documented in other areas in the southwest United States; this additional area provides insight into the orientation of tectonic activity responsible for deforming the rocks.
India-Antarctica-Australia-Laurentia connection in the Paleoproterozoic-Mesoproterozoic revisited: Evidence from new zircon U-Pb and monazite chemical age data from the Eastern Ghats Belt, India
Sankar Bose et al., Dept. of Geology, Presidency University, Kolkata 700 073, India. Published online 18 Apr. 2011; doi: 10.1130/B30336.1.
The correlation of events in the India-east Antarctica sector in the Precambrian is problematic in absence of high-resolution geochronological data from India. Sankar Bose of Presidency University, India, and colleagues present data from the Eastern Ghats Belt (EGB) of India, which acts as a crucial link in this correlation. The results show a contrasting time frame for tectonothermal imprints of southern and central parts of the EGB. Discovery of events that occurred 1.76 to 1.60 billion years ago in the southern EGB implies prolonged orogenic activities culminated in eventual cratonization with India. The central part of EGB, on the other hand, witnessed major orogenic activities 1.03 to 0.90 billion years ago before being cratonized to India although inheritance of events that occurred 1.76 to 1.70 billion year ago is also recorded in metasediments. This clearly established the link between the EGB and the Rayner Complex of east Antarctica, evolved as a composite mobile belt suturing India and east Antarctic cratons within the supercontinent Rodinia. Though probable cratonization of different parts might have occurred in a different period, the record of events that occurred 1.76 to 1.60 billion years ago from different segments of the EGB, for the first time, allows modeling for the Paleo-Mesoproterozoic transcontinental correlation. The accretionary orogenic processes in the supercontinent Columbia encompassed Australia, Antarctica, Laurentia, and India.
Early and middle Miocene Antarctic glacial history from the sedimentary facies distribution in the AND-2A drill hole, Ross Sea, Antarctica
S. Passchier et al., Dept. of Earth and Environmental Studies, Montclair State University, Montclair, New Jersey 07043, USA. Published online 18 Apr. 2011; doi: 10.1130/B30334.1.
S. Passchier of Montclair State University and colleagues document the dynamics of the Antarctic ice sheets during a time of global climate change in the Miocene Epoch, 20 to 14 million years ago. For the first time, they are able to document in detail how the ice sheet responded to warming during the middle Miocene climatic optimum from a geological sediment archive near the Antarctic coast. Paleoenvironments near Antarctica are reconstructed from an 1138-m-long geological drill core, using the properties of sediments and fossils, which were deposited grain by grain in an environment with grounded and floating glaciers, icebergs, sea ice, mud plumes, microscopic algae, and seafloor dwelling organisms. Shifts in the accumulation of the different types of sediment provide a record of changing environmental and climatic conditions on the East Antarctic coast. Passchier and colleagues demonstrate that the ice sheet retreated in response to warming 17 to 15 million years ago. They also propose that the ice sheet grew to larger than modern proportions 14 million years ago and that this preceded global cooling observed elsewhere in the world, confirming the important role of the Antarctic ice sheets in the global climate system.
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