ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉We adapt the relative polarity method from 〈a href="https://pubs.geoscienceworld.org/bssa#rf29"〉Shelly 〈span〉et al.〈/span〉 (2016)〈/a〉 to compute focal mechanisms for microearthquakes associated with the 2014 hydroshearing stimulation at the Newberry volcano geothermal site. We focus the analysis on events relocated by 〈a href="https://pubs.geoscienceworld.org/bssa#rf2"〉Aguiar and Myers (2018)〈/a〉, who report that six event clusters predominantly comprise the 2014 sequence. Data quality allows focal mechanism analysis for four of the six event clusters. We use 〈a href="https://pubs.geoscienceworld.org/bssa#rf13"〉Hardebeck and Shearer (2002〈/a〉, 〈a href="https://pubs.geoscienceworld.org/bssa#rf14"〉2003〈/a〉; hereafter HASH) to compute focal mechanisms based on first‐motion polarities and 〈span〉S〈/span〉/〈span〉P〈/span〉 amplitude ratios. We manually determine 〈span〉P〈/span〉‐ and 〈span〉S〈/span〉‐wave polarities for a well‐recorded reference event in each cluster, then use waveform cross correlation to determine whether recordings of other events in the cluster are the same or reversed polarity at each network station. Most waveform polarities are consistent with the affiliated reference event, indicating similar focal mechanisms within each cluster. The deeper clusters are east–west‐striking normal faults, whereas the shallower clusters, close to the top of the open‐hole section of the borehole, are strike slip with east–west motion. Regional studies and prestimulation borehole breakouts find the maximum stress direction is vertical and maximum horizontal stress is approximately north–south. Fault geometry and focal mechanisms of microseismicity during the stimulation suggest that increased pressure from fluid injection predominantly caused changes in horizontal stress, consistent with predictions from numerical studies of stress change caused by fluid injection. At shallow depths, where previous studies suggest the difference between vertical and horizontal stress is lowest, injection appears to have rotated the direction of maximum stress from vertical to horizontal, resulting in strike‐slip motion. At greater depth, vertical stress continued to be the dominant direction during the stimulation, but fault orientation indicates either reactivation of pre‐existing fractures or rotation of the direction of maximum horizontal stress from approximately north–south to east–west.〈/span〉

Description: 〈span〉The Turkana Depression of northern Kenya and southern Ethiopia contains voluminous plume-related basalts that mark the onset of the Paleogene–recent East African Rift System (EARS) at ca. 45 Ma. Thus, the Turkana Depression is crucial to understanding the inception of intracontinental rifting. However, the precise chronology of early rift-basin formation in Turkana is poorly constrained. We present apatite fission-track and (U-Th-Sm)/He thermochronology data from basement rocks from the margins of the north-south–trending Lokichar Basin that constrain the onset of rift-related cooling. Thermal history modeling of these data documents pronounced Eocene to Miocene denudational cooling of the basin-bounding Lokichar fault footwall. These results, along with ~7 km of Paleogene to middle Miocene syn-rift strata preserved in the Lokichar fault hanging wall, suggest that formation of the Lokichar Basin began as early as ca. 45–40 Ma. Preexisting lithospheric heterogeneities inherited from earlier Mesozoic rifting and Eocene plume magmatism likely facilitated the broadly concurrent nucleation of strain in the Turkana Depression, up to ~15 m.y. earlier than EARS initiation elsewhere. Late Paleogene extension in the Lokichar Basin and other parts of Turkana significantly predate the Miocene creation of pronounced plume-related topography in East Africa, suggesting that other mechanism(s), such as far-field stresses or mantle basal drag, likely played a critical role during EARS inception.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We present results from integrated field, microstructural, and textural analysis of the Burlington mylonite zone (BMZ) in eastern Massachusetts (northeastern USA) to establish a unified microkinematic framework for vorticity analysis in heterogeneous shear zones. Specifically, we develop a methodology for the structural analysis of polyphase lithologies that defines the vorticity-normal surface based on lattice-scale rotation axes calculated from electron backscatter diffraction data using orientation statistics. In doing so, we objectively identify a suitable reference frame for rigid grain methods of vorticity analysis that can be used in concert with field and microstructural methods of strain analysis and textural studies to constrain field- to plate-scale kinematics and deformation geometries without assumptions that may bias tectonic interpretations, such as relationships between kinematic axes and fabric-forming elements or the nature of the deforming zone (e.g., monoclinic versus triclinic shear zones).Rocks within the BMZ comprise a heterogeneous mix of quartzofeldspathic ± hornblende-bearing mylonitic gneisses and quartzites. Vorticity axes inferred from lattice rotations lie within the plane of mylonitic foliation perpendicular to lineation—a pattern consistent with monoclinic deformation geometries involving simple shear and/or wrench-dominated transpression. The mean kinematic vorticity number (〈span〉W〈/span〉〈sub〉m〈/sub〉) is calculated using rigid grain net analysis and ranges from 0.25 to 0.55, indicating dominant general shear. Using the calculated vorticity values and the dominant geographic fabric orientation, we constrain the angle of paleotectonic convergence between the Nashoba and Avalon terranes to ∼56°–75° with the convergence vector trending ∼142°–160° and plunging ∼3°–10°. Application of the quartz recrystallized grain size piezometer suggests differential stresses in the BMZ mylonites ranging from ∼44 to 92 MPa; patterns of quartz crystallographic preferred orientation are consistent with deformation at greenschist- to amphibolite-facies conditions. We conclude that crustal strain localization in the BMZ involved a combination of pure and simple shear in a sinistral reverse transpressional shear zone that was active at or near the brittle-ductile transition under relatively high stress conditions. Moreover, we demonstrate the utility of combined crystallographic and rigid grain methods of vorticity analysis for deducing deformation geometries, kinematics, and tectonic histories in polyphase shear zones.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Instead of using discrete values for properties that influence the volumetric calculation for recoverable reserves from the Middle Bakken, Pronghorn, and Three Forks reservoir rocks in the Williston Basin in North Dakota, an uncertainty-based assessment method was used. Various estimates have been published in the past that attempt to quantify recoverable reserves from the Bakken petroleum system. The Bakken–Three Forks trend is regarded as an unconventional tight oil play typical of a continuous-type basin-centered accumulation. However, production data reveal that areas are unequal and that certain regions stand out as sweet spots whereas others exhibit fairly high water cuts. This paper is based on 28 well models, which have been porosity-calibrated and adjusted for the prevalent thermal regime. The area of interest was delineated by geological parameters such as shale maturity and reservoir rock presence as well as existing production data. The purpose of this study is to use an uncertainty assessment method based on hundreds of basin model simulations that sample ranges of probable input parameters to quantify the recoverable reserves from the Bakken petroleum system in North Dakota. The results are displayed in reverse cumulative probability plots, tornado sensitivity charts, as well as in maps of the 10% chance, 50% chance (P50), 90% chance values. This means that there is an X% chance of success or an X probablity of realizing a certain amount of hydrocarbon. The P50 results of the uncertainty assessment indicate that approximately 4 billion bbl of oil and 3.6 tcf (102 billion m〈sup〉3〈/sup〉) of gas are recoverable from the Middle Bakken, Pronghorn, and Three Forks reservoir rocks in North Dakota. The Bakken–Three Forks trend appears to be an overcharged petroleum system, where the available pore space in reservoir rocks is the limiting factor for each accumulation.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉In this work, a decision tree is presented—constructed on the basis of hydrogeological characteristics (water table depth, freshwater thickness, surface area required and distance between wells)—in order to choose the optimal groundwater extraction method in the case of a coastal unconfined aquifer. A comparison is made of the groundwater extraction methods in a freshwater aquifer of limited thickness occurring in coastal dunes in the eastern region of the Province of Buenos Aires (Argentina). The negative effects brought about by the wrong use of the groundwater extraction methods are analysed, since—as a result of excessive extraction—such methods lead to the dramatic decrease of the freshwater reserves. The decision tree is a useful tool to assist decision-makers as it suggests the most suitable groundwater extraction method options (vertical wells or wellpoints), as well as identifying areas that are unsuitable for sustainable groundwater extraction. 〈/span〉

Description: 〈span〉The planet Mercury is sufficiently close to the Sun to pose a major challenge to spacecraft exploration. The 〈span〉Mariner 10〈/span〉 spacecraft flew by Mercury three times in 1974–1975 but viewed less than half of the surface. With the three flybys of Mercury by the 〈span〉MESSENGER〈/span〉 spacecraft in 2008–2009 and the insertion of that probe into orbit about Mercury in 2011, our understanding of the innermost planet substantially improved. In its four years of orbital operations, 〈span〉MESSENGER〈/span〉 revealed a world more geologically complex and compositionally distinctive, with a more dynamic magnetosphere and more diverse exosphere–surface interactions, than expected. With the launch of the 〈span〉BepiColombo〈/span〉 dual-orbiter mission, the scientific understanding of the innermost planet has moved another major step forward.〈/span〉

Description: 〈span〉We present results from integrated field, microstructural, and textural analysis of the Burlington mylonite zone (BMZ) in eastern Massachusetts (northeastern USA) to establish a unified microkinematic framework for vorticity analysis in heterogeneous shear zones. Specifically, we develop a methodology for the structural analysis of polyphase lithologies that defines the vorticity-normal surface based on lattice-scale rotation axes calculated from electron backscatter diffraction data using orientation statistics. In doing so, we objectively identify a suitable reference frame for rigid grain methods of vorticity analysis that can be used in concert with field and microstructural methods of strain analysis and textural studies to constrain field- to plate-scale kinematics and deformation geometries without assumptions that may bias tectonic interpretations, such as relationships between kinematic axes and fabric-forming elements or the nature of the deforming zone (e.g., monoclinic versus triclinic shear zones).Rocks within the BMZ comprise a heterogeneous mix of quartzo­feld­spathic ± hornblende-bearing mylonitic gneisses and quartzites. Vorticity axes inferred from lattice rotations lie within the plane of mylonitic foliation perpendicular to lineation—a pattern consistent with monoclinic deformation geometries involving simple shear and/or wrench-dominated transpression. The mean kinematic vorticity number (〈span〉W〈/span〉〈sub〉m〈/sub〉) is calculated using rigid grain net analysis and ranges from 0.25 to 0.55, indicating dominant general shear. Using the calculated vorticity values and the dominant geographic fabric orientation, we constrain the angle of paleotectonic convergence between the Nashoba and Avalon terranes to ~56°–75° with the convergence vector trending ~142°–160° and plunging ~3°–10°. Application of the quartz recrystallized grain size piezometer suggests differential stresses in the BMZ mylonites ranging from ~44 to 92 MPa; patterns of quartz crystallographic preferred orientation are consistent with deformation at greenschist- to amphibolite-facies conditions. We conclude that crustal strain localization in the BMZ involved a combination of pure and simple shear in a sinistral reverse transpressional shear zone that was active at or near the brittle-ductile transition under relatively high stress conditions. Moreover, we demonstrate the utility of combined crystallographic and rigid grain methods of vorticity analy­sis for deducing deformation geometries, kinematics, and tectonic histories in polyphase shear zones.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉A new species of the Paleozoic bryozoan genus 〈span〉Ptilotrypa〈/span〉 of the order Cryptostomata is described from the lower part of the Yong Limestone Formation, Katian, Upper Ordovician of the Kumaun Tethys Himalaya: 〈span〉Ptilotrypa bajpaii〈/span〉 new species. The presence of the genus 〈span〉Ptilotrypa〈/span〉 in the Tethyan Himalaya suggests paleogeographic connections to the Upper Ordovician of North America and, consequently, Upper Ordovician age for the lower part of the Yong Limestone Formation. This species displays a reticulate colony shape, which suggests an efficient filtering capacity in an environment with a high primary production. Morphological peculiarities and systematic assignment of the genus 〈span〉Ptilotrypa〈/span〉 are discussed.UUID: http://zoobank.org/898276c8-2924-4da2-ae96-3392cb2ebbc3〈/span〉