The Earth’s crustal stress field controls active deformation and reflects the processes driving plate tectonics. Here we present the first quantitative synthesis of relative principal stress magnitudes throughout North America together with hundreds of new horizontal stress orientations, revealing coherent stress fields at various scales. A continent-scale transition from compression (strike-slip and/or reverse faulting) in eastern North America to strike-slip faulting in the mid-continent to predominantly extension in western intraplate North America is likely due (at least in part) to drag at the base of the lithosphere. Published geodynamic models, incorporating gravitational potential energy and tractions from plate motions or relative mantle flow, successfully predict most large-wavelength stress rotations but not the shorter-wavelength (<~200 km) rotations observed in the western USA. The stresses resulting from glacial isostatic adjustment appear to be much smaller than the magnitude of ambient tectonic stresses in the crust at depth. The authors here present a stress map of the North American crust that gives a new view of dynamics of the continent. The results can be applied to probabilistic seismic hazard analysis and resource development as well as to provide constraints for theoretical models of crustal dynamics.

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Multiscale variations of the crustal stress field throughout North America.

Lipid biomarker assemblages preserved within the bitumen and kerogen phases of sedimentary rocks from the ca. 780-729 Ma Chuar and Visingso Groups facilitate paleoenvironmental reconstructions and reveal fundamental aspects of emerging mid-Neoproterozoic marine communities. The Chuar and Visingso Groups were deposited offshore of two distinct paleocontinents (Laurentia and Baltica, respectively) during the Tonian Period, and the rock samples used had not undergone excessive metamorphism. The major polycyclic alkane biomarkers detected in the rock bitumens and kerogen hydropyrolysates consist of tricyclic terpanes, hopanes, methylhopanes, and steranes. Major features of the biomarker assemblages include detectable and significant contribution from eukaryotes, encompassing the first robust occurrences of kerogen-bound regular steranes from Tonian rocks, including 21-norcholestane, 27-norcholestane, cholestane, ergostane, and cryostane, along with a novel unidentified C30 sterane series from our least thermally mature Chuar Group samples. Appreciable values for the sterane/hopane (S/H) ratio are found for both the free and kerogen-bound biomarker pools for both the Chuar Group rocks (S/H between 0.09 and 1.26) and the Visingso Group samples (S/H between 0.03 and 0.37). The more organic-rich rock samples generally yield higher S/H ratios than for organic-lean substrates, which suggests a marine nutrient control on eukaryotic abundance relative to bacteria. A C27 sterane (cholestane) predominance among total C26 -C30 steranes is a common feature found for all samples investigated, with lower amounts of C28 steranes (ergostane and crysotane) also present. No traces of known ancient C30 sterane compounds; including 24-isopropylcholestanes, 24-n-propylcholestanes, or 26-methylstigmastanes, are detectable in any of these pre-Sturtian rocks. These biomarker characteristics support the view that the Tonian Period was a key interval in the history of life on our planet since it marked the transition from a bacterially dominated marine biosphere to an ocean system which became progressively enriched with eukaryotes. The eukaryotic source organisms likely encompassed photosynthetic primary producers, marking a rise in red algae, and consumers in a revamped trophic structure predating the Sturtian glaciation.

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Free and kerogen-bound biomarkers from late Tonian sedimentary rocks record abundant eukaryotes in mid-Neoproterozoic marine communities.

The Sveconorwegian orogeny

Microtexture and U–Pb geochronology of detrital zircon grains in the Chachalacas beach, Veracruz State, Gulf of Mexico

Automatic detection of fractures and breakouts patterns in acoustic borehole image logs using fast-region convolutional neural networks

A meta-analysis of the Steptoean Positive Carbon Isotope Excursion: The SPICEraq database

U-Pb detrital zircon geochronology, petrography, and synthesis of the middle Neoproterozoic Visingsö Group, Southern Sweden

Project HOTSPOT, the Snake River Scientific Drilling Project (International Continental Scientific Drilling Program), tested for deep geothermal resources and examined the petrology of volcanic rocks with three drillholes in the central and western Snake River Plain (western USA). The MH-2 drillhole targeted fractured crystalline and hydrothermally altered basalt in the area of the Mountain Home Air Force Base (Idaho) to a total depth of 1821 m. At 1745 m depth the drillhole encountered flowing artesian hydrothermal fluids of at least 150 °C. We integrate geological analyses of core, image log, and borehole geophysical data, and in situ stress analyses to describe the structural environment that produces permeability for artesian flow. The rocks in the lower 540 m of the drillhole consist of basalt flows as much as 30 m thick, altered basalt, and thin sedimentary horizons. The mechanical stratigraphy is defined by nine mechanical horizons that are in three ranges of rock strength on the basis of experimentally determined strength data, core logging, and geophysical log signatures. Hydrothermal alteration products and mineralization in the core are associated with three highly faulted sections; the lowermost section is associated with the zone of flowing thermal water. Shear slip indicators on faults observed in core indicate slip ranging from pure strike slip to normal failure mechanisms in the stronger horizons. The borehole breakouts indicate that the maximum horizontal stress, SH, is oriented 047° ± 7°, and drilling-induced tensile fractures indicate that SH is oriented at 67° ± 21°. The in situ stress orientations exhibit little variation over the depth of the measured interval, but the SH magnitude varies with depth, and is best explained by an oblique normal fault stress regime. The geomechanical model indicates that if pore pressures at depth are elevated above the normal hydrostatic gradient, as observed here, the system has the potential to deform by mixed normal and strike-slip failure. Our observations and interpretations suggest that the MH-2 borehole was drilled into oblique normal faults that intersect a buried 300°-trending fault block masked by the basaltic volcanic complex. These data indicate that the transition from the central to western Snake River Plain is characterized by complex structures developed in response to a transitional stress state related to Snake River Plain and western Basin and Range stress regimes. The western Basin and Range stress and tectonic regime may extend from northern Nevada into western Idaho and may enhance the potential for geothermal resources by creating interconnected fracture and fault-related permeability at depth.

/pdf/geology-and-in-situ-stress-of-the-mh-2-borehole-idaho-usa-2dg2ixqvkm.pdf

Geology and in situ stress of the MH-2 borehole, Idaho, USA: Insights into western Snake River Plain structure from geothermal exploration drilling

A contiguous record of the SPICE event, sea-level change and the first appearance of Fenghuangella laevis in Shandong Province, North China

The Silurian Waukesha Lagerstätte of Wisconsin, USA, hosts a fauna of distinctive soft-bodied organisms, including a range of graptolites, worms, trilobites and uncommon arthropods, but otherwise lacks most of the shelly fauna typical of contemporaneous deposits. While several species have been described, much of the Waukesha biota remains understudied. Here, we examine numerous specimens of a previously unnamed and undescribed vermiform arthropod that bears a cephalic carapace, a suite of specialized head appendages, a long, segmented trunk, and a battery of small swimming appendages. The taxonomic and phylogenetic evaluations presented herein establish this taxon as a novel genus and species, Acheronauta stimulapis gen. et sp. nov., plausibly positioned within the basal mandibulates. Our phylogenetic analyses suggest that a new arthropod clade may be warranted, comprising Acheronauta gen. nov., Captopodus, and the thylacocephalans. https://zoobank.org/urn:lsid:zoobank.org:pub:B572831B-1A8A-4E4E-9258-34D91AA22A74nk

Mikaela A. Pulsipher

Papers

A meta-analysis of the Steptoean Positive Carbon Isotope Excursion: The SPICEraq database

U-Pb detrital zircon geochronology, petrography, and synthesis of the middle Neoproterozoic Visingsö Group, Southern Sweden

Geology and in situ stress of the MH-2 borehole, Idaho, USA: Insights into western Snake River Plain structure from geothermal exploration drilling

A contiguous record of the SPICE event, sea-level change and the first appearance of Fenghuangella laevis in Shandong Province, North China

Description of Acheronauta gen. nov., a possible mandibulate from the Silurian Waukesha Lagerstätte, Wisconsin, USA