ALBERT

Description: Despite their abundance, diversity, and importance today, organisms with mineralized skeletons are a relatively recent introduction. For the first three billion years of its history, life was soft-bodied, inducing mineralized structures passively, if at all. Beginning ca. 550 Ma, however, more than two dozen clades—primarily animal, but also protistan—independently evolved mineralized skeletons within a geologically short interval of time (Fig. 1; Bengtson, 1992). Now a new report by Cohen et al. (2011; p. 539 in this issue of Geology) describing beautifully intricate scale-like microfossils from the Fifteenmile Group, Yukon Territory, provides definitive evidence for mineralized skeletons some 150–250 m.y. earlier. These scale-like microfossils were first reported over two decades ago (Allison and Hilgert, 1986), but neither their age nor their mineralogy were well constrained. Work by Cohen and her colleagues has now shown that these scales (which perhaps enveloped a single-celled green alga) are between ca. 717 and ca. 812 Ma in age and composed of primary phosphate (Macdonald et al., 2010; Cohen et al., 2011). This adds to earlier suggestive evidence for mineralization at this time: the ca. 770–742 Ma vase-shaped microfossil (VSM) Melicerion poikilon, interpreted on the basis of taphonomic models to be a euglyphid amoeba whose organic-walled test was embedded with mineralized scales, possibly siliceous (Figs. 1B and 1C; Porter and Knoll, 2000; Porter et al., 2003); the mid-Neoproterozoic Tenuocharta cloudii, a multicellular, sheet-like fossil whose calcareous cell walls may reflect primary (Horodyski and Mankiewicz, 1990) or early diagenetic (Knoll, 2003) mineralization; and ca. 650 Ma millimeter- to centimeter-scale asymmetric bodies permeated with a network of canals and interpreted to be sponge-like organisms perhaps lightly mineralized with carbonate (Maloof et al., 2010b)...

Description: Understanding lower-crustal deformational processes and the related features that can be imaged by seismic waves is an important goal in active tectonics. We demonstrate that teleseismic receiver functions calculated for broadband seismic stations in Southern California reveal a signature of pervasive seismic anisotropy in the lower crust. The large amplitudes and small move-out of the diagnostic converted phases, as well as the broad similarity of data patterns from widely separated stations, support an origin primarily from a basal crustal layer of hexagonal anisotropy with a dipping symmetry axis. We conducted neighborhood algorithm searches for depth and thickness of the anisotropic layer and the trend and plunge of the anisotropy symmetry (slow) axis for 38 stations. The searches produced a wide range of results, but a dominant SW-NE trend of the symmetry axis emerged. When the results are divided into crustal blocks and restored to their pre–36 Ma locations, the regional-scale SW-NE trend becomes more consistent, although a small subset of the results can be attributed to NW-SE shearing related to San Andreas transform motion. We interpret this dominant trend as a fossilized fabric within schists, created from top-to-the-SW sense of shear that existed along the length of coastal California during pretransform, early Tertiary subduction or from shear that occurred during subsequent extrusion. Comparison of receiver-function common conversion point stacks to seismic models from the active Los Angeles Regional Seismic Experiment shows a strong correlation in the location of anisotropic layers with “bright” reflectors, further affirming these results.

Description: Before the onset of the Neoproterozoic Snowball Earth glaciations, eukaryotes had begun diversifying, and in their aftermath, macroscopic life, including both animals and macroalgae, became abundant and widespread. Although glacially driven mass extinctions have been hypothesized, little is known about the biosphere during and between these glaciations. Here we present new data from organic-walled microfossil assemblages from five successions in Australia and Svalbard that collectively span the first (Sturtian) glaciation and interglacial interval and integrate them with data derived from a critical evaluation of the literature to produce a new estimate of eukaryotic diversity from 850 to 650 Ma. These new glacial and interglacial assemblages consist of only smooth-walled spheroids (leiosphaerids), aggregates of cells, and filaments, in contrast to the much more diverse organic-walled microfossil assemblages found in early Neoproterozoic rocks. This contrast is not attributed to biases in deposition or preservation, but is instead interpreted as reflecting an interval of lowered eukaryotic diversity that spanned the glaciations and that may have begun millions of years prior to their onset.

Description: The upper Tonian Chuar, Uinta Mountain, and middle Pahrump (ChUMP) groups of present-day western Laurentia collectively record the early breakup of Rodinia, large-scale perturbations in the carbon cycle, and eukaryotic evolution, all of which preceded the onset of global glaciation by tens of millions of years. The spectacularly preserved and shale-rich Chuar Group of the Grand Canyon Supergroup stands out as one of the best global records of this time period, particularly for paleobiology. A new U-Pb age of 782 Ma on detrital zircons ( n = 14 young grains) from the underlying Nankoweap Formation refines the Chuar Group’s maximum depositional age to younger than 782 Ma. A new 40 Ar/ 39 Ar age of 764 ± 16 Ma (2) from K-feldspar within early diagenetic marcasite nodules from the upper Chuar Group (Awatubi Member) helps calibrate the rich Chuar microfossil record and constrain the large-magnitude shift in 13 C org (up to 18; referred to here as the Awatubi positive carbon-isotope excursion or APCIE) to between ca. 764 and ca. 742 Ma, the date of an ash near the top of the Chuar Group. In addition to the maximum depositional age of ca. 782 Ma, U-Pb detrital zircon analyses ( n = 826 grains) on sandstone beds from the underlying Nankoweap Formation indicate the presence of multiple older Laurentian age peaks. The similarity of detrital zircon populations and sedimentary character to that of the overlying Chuar Group ( n = 764 grains) suggests that the Nankoweap Formation should be included as the lowermost unit in the Chuar Group. This revised geochronological framework indicates a 300 Ma unconformity between the Chuar Group (including the Nankoweap Formation) and the underlying 1.1 Ga Cardenas Basalt of the Unkar Group. Chuar Group detrital zircon populations share similarities with those of the Uinta Mountain Group and especially the middle Pahrump Group, including ca. 780 Ma grains. Biostratigraphic correlation using microfossils enhances the ChUMP connection and shows a trend of higher acritarch diversity in the lower Chuar and Uinta Mountain groups, and the presence of vase-shaped microfossils in the upper intervals of all three ChUMP units. Comparisons of 13 C org and 13 C carb among ChUMP successions suggest a combination of local and regional controls. Thus, ChUMP successions are coeval within the 780–740 Ma range, show similar fossil and C-isotope trends, and derived sediments from similar Laurentian sources or source types. In light of recent age constraints and compiled paleontology in other Neoproterozoic basins, our high-resolution correlation of ChUMP successions can be extended to the Callison Lake dolostone of NW Canada and the Akademikerbreen-Polarisbreen groups of Svalbard. Biostratigraphic correlation with poorly age-constrained strata such as the Akademikerbreen-Polarisbreen groups and, farther afield, the Visingsö Group of Baltica suggests that ChUMP units record continentwide—and perhaps global—evolutionary patterns. The 13 C org and 13 C carb values in the Chuar Group and its equivalents in Canada and Svalbard show broadly similar trends, including the APCIE, suggesting that 13 C org values from organic-rich shale record variations in the C-isotope composition of late Tonian oceans. Intracratonic basins and contiguous rift margins of ChUMP age are inferred to have been important locations for microbial productivity and significant organic carbon burial that induced large positive shifts in 13 C and changes in global carbon balance prior to the onset of snowball Earth.

Description: 〈span〉〈div〉Abstract〈/div〉The ability of models designed to use near-surface structural information to predict the deep geometry of a faulted block is tested for a thick-skinned thrust by matching the surface geometry to the crustal structure beneath the Wind River Range, Wyoming, USA. The Wind River Range is an ∼100-km-wide, thick-skinned rotated basement block bounded on one side by a high-angle reverse fault. The availability of a deep seismic-reflection profile and a detailed crustal impedance profile based on teleseismic receiver-function analysis makes this location ideal for testing techniques used to predict the deep fault geometry from shallow data. The techniques applied are the kinematic models for a circular-arc fault, oblique simple-shear fault, shear fault-bend fold, and model-independent excess area balancing. All the kinematic models imply that the deformation cannot be exclusively rigid-body rotation but rather require distributed deformation throughout some or all of the basement. Both the circular-arc model and the oblique-shear models give nearly the same best fit to the master fault geometry. The predicted lower detachment matches a potential crustal detachment zone at 31 km subsea. The thrust ramp is located close to where this zone dies out to the southwest. The circular-arc model implies that the penetrative deformation could be focused at the trailing edge of the basement block rather than being distributed uniformly throughout and thus helps to explain the line of second-order anticlines along the trailing edge of the Wind River block.Key points: (1) The circular-arc fault model and the oblique-shear model predict a lower detachment for the Wind River rotated block to be ∼31 km subsea, consistent with the crustal structure as defined by teleseismic receiver-function analysis. The thrust ramp starts where this zone dies out. (2) The kinematic models require distributed internal deformation within the basement block, probably concentrated at the trailing edge. (3) The uplift at the trailing edge of the rotated block is explained by the circular-arc kinematic model as a requirement to maintain area balance of a mostly rigid block above a horizontal detachment; the oblique-shear model can explain the uplift as caused by displacement on a dipping detachment.〈/span〉

Description: 〈span〉The ability of models designed to use near-surface structural information to predict the deep geometry of a faulted block is tested for a thick-skinned thrust by matching the surface geometry to the crustal structure beneath the Wind River Range, Wyoming, USA. The Wind River Range is an ∼100-km-wide, thick-skinned rotated basement block bounded on one side by a high-angle reverse fault. The availability of a deep seismic-reflection profile and a detailed crustal impedance profile based on teleseismic receiver-function analysis makes this location ideal for testing techniques used to predict the deep fault geometry from shallow data. The techniques applied are the kinematic models for a circular-arc fault, oblique simple-shear fault, shear fault-bend fold, and model-independent excess area balancing. All the kinematic models imply that the deformation cannot be exclusively rigid-body rotation but rather require distributed deformation throughout some or all of the basement. Both the circular-arc model and the oblique-shear models give nearly the same best fit to the master fault geometry. The predicted lower detachment matches a potential crustal detachment zone at 31 km subsea. The thrust ramp is located close to where this zone dies out to the southwest. The circular-arc model implies that the penetrative deformation could be focused at the trailing edge of the basement block rather than being distributed uniformly throughout and thus helps to explain the line of second-order anticlines along the trailing edge of the Wind River block.Key points: (1) The circular-arc fault model and the oblique-shear model predict a lower detachment for the Wind River rotated block to be ∼31 km subsea, consistent with the crustal structure as defined by teleseismic receiver-function analysis. The thrust ramp starts where this zone dies out. (2) The kinematic models require distributed internal deformation within the basement block, probably concentrated at the trailing edge. (3) The uplift at the trailing edge of the rotated block is explained by the circular-arc kinematic model as a requirement to maintain area balance of a mostly rigid block above a horizontal detachment; the oblique-shear model can explain the uplift as caused by displacement on a dipping detachment.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉In this work, we compile several seismic velocity models publicly available from the Incorporated Research Institute for Seismology (IRIS) Earth Model Collaboration (EMC) and compare subcrustal mantle velocities in the models to each other and to the timing of tectonism across the continent. This work allows us to assess the relationship between the time elapsed since the most recent thermotectonic event and uppermost mantle temperatures. We apply mineral- and physics-based models of velocity-temperature relationships to calculate upper-mantle temperatures in order to determine cooling rates for the lower-crust and uppermost mantle following thermotectonic activity. Results show that most of the cooling occurs in the ∼300–500 million years following orogeny. This work summarizes current estimates of upper-mantle shear velocities and provides insights on the thermal stabilization of continental lithosphere through time.〈/span〉

Description: 〈span〉In this work, we compile several seismic velocity models publicly available from the Incorporated Research Institute for Seismology (IRIS) Earth Model Collaboration (EMC) and compare subcrustal mantle velocities in the models to each other and to the timing of tectonism across the continent. This work allows us to assess the relationship between the time elapsed since the most recent thermotectonic event and uppermost mantle temperatures. We apply mineral- and physics-based models of velocity-temperature relationships to calculate upper-mantle temperatures in order to determine cooling rates for the lower-crust and uppermost mantle following thermotectonic activity. Results show that most of the cooling occurs in the ~300–500 million years following orogeny. This work summarizes current estimates of upper-mantle shear velocities and provides insights on the thermal stabilization of continental lithosphere through time.〈/span〉