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Mapping of rock types, structural geology, and hydrothermal alteration, supported by geochronology and thermochronology, sheds light on the original spatial relationships of hydrothermal systems to intrusions in the northern Shoshone Range in north-central Nevada. Rocks in the Hilltop district are tilted ~35–40°E, as indicated by orientations of flattened pumice fiamme and bedding in sedimentary rocks along a single set of presently low-angle normal faults that initiated at 60–70°W dips. New U-Pb zircon geochronology from two sets of dikes in the Lewis district could suggest late Eocene–early Oligocene extension, but definitive crosscutting relations are lacking to demonstrably support this potential earlier period of normal faulting. Reinterpretation of previously reported apatite fission-track cooling ages with a new palinspastic restoration in the Lewis mining district concurs with middle Miocene extension as documented to the south at the Caetano caldera; however, the depth of burial of the Lewis district—and thus the significance of the apatite fissiontrack cooling ages—is uncertain. The comparable orientations and tilting history, supported by fault scaling relations, suggest that the temporally coincident extension in the Caetano caldera to the south represents the along-strike continuation of the same system of normal faults as in the Hilltop and Lewis districts, with changes in observed offset, percent extension, and fault spacing attributed to the gradual tipping out of the fault system northward. [ABSTRACT FROM AUTHOR]

Middle Eocene to early Oligocene intrusions, widespread in the Ruby Mountains– East Humboldt Range metamorphic core complex, Nevada, USA, provide insights into a major Paleogene magmatic episode and its relation to tectonism in the northeastern Great Basin. These intrusions, well-exposed in upper Lamoille Canyon, range in composition from gabbro to leucomonzogranite. They form small plutons, sheets, and dikes that intrude the metamorphic and granitic infrastructure of the core complex. Two types of Paleogene monzogranite were recognized. The first is exemplified by two of the larger intrusive bodies, the Snow Lake Peak and Castle Lake intrusions, which occur as sheet- like bodies near and at the structural base of metamorphosed Neoproterozoic and Cambrian Prospect Mountain Quartzite where it is inverted above Cambrian and Ordovician marble of Verdi Peak in the Lamoille Canyon nappe. Swarms of dikes are associated with these intrusions. U-Pb (zircon) ages range ca. 40– 33 Ma and typically display relatively simple and minor inheritance. The rocks have the lowest εHf (zircon) and εNd (whole rock) of any of the middle Cenozoic granites. The second type of monzogranite, Overlook type, typically occurs as thin, isolated dikes and leucosome- like bodies in Late Cretaceous granites of the infrastructure, with no obvious relationship to the large monzogranite bodies. Overlook- type monzogranite displays complex zircon inheritance, yields igneous ages ca. 37– 32 Ma, and has εHf (zircon) and εNd (whole rock) identical to those of Late Cretaceous granites in the core complex. These isotopic and field data indicate that Overlook- type monzogranite formed in situ through anatexis of host Cretaceous granites. A pervasive thermal event was required to stimulate this crustal melting. Gabbros from Lamoille Canyon and quartz diorite dated from 32 km away signal mantle- derived magmatism ca. 39– 37 Ma (U-Pb, zircon) was a driver of crustal melting and hybridization. Eocene 40Ar/39Ar apparent ages on hornblende and biotite are consistent with syn- to post- magmatic extensional exhumation and decompression. Thus, the core complex provides a window into trans- crustal magmatism and insight into how such magmatism affected the Nevadaplano orogenic plateau. This Paleogene thermal pulse, which may relate to removal of the Farallon slab by delamination of mantle lithosphere, involved partial melting of the upper mantle and transfer of magma and heat to the Nevadaplano crust. Lower- crustal melting of Archean(?) to Paleoproterozoic rocks resulted in Snow Lake Peak– type magmas, and middle- crustal melting of granite in the infrastructure yielded Overlook- type magmas. This crustal magmatism, as exemplified by the intrusions in the core complex, likely played a role in destabilizing the Nevadaplano and its later collapse during middle Miocene extension. The Paleogene intrusions and associated structural features also provide insight into the evolution of the core complex through either the buoyant upwelling of a melt- rich diapir (gneiss- dome model) or buoyant upwelling of the melt- rich middle crust synchronous with a west- rooted mylonitic shear zone (extensional shear- zone model). We favor a hybrid that incorporates both models. [ABSTRACT FROM AUTHOR]

One of the major challenges in early Earth geology is the interpretation of the nature of the crust and tectonic processes due to the limited exposures of Archean rocks. This question is predominantly addressed by numerical modeling, structural geology, geochemical analyses, and petrological approaches. Here we report on the reconstruction of one of the oldest, well-preserved volcano-sedimentary sequences on Earth, the 3.28-3.22 Ga Fig Tree Group in the Barberton Greenstone Belt, South Africa, based on geochronology, provenance, and stratigraphy to provide new constraints on the nature of tectonic processes in the Archean. The Fig Tree basin was asymmetric and the onset of deposition varied across the greenstone belt. The Fig Tree Group is now preserved in east-west oriented bands of fault-bounded structural belts with those preserved in the southern parts of the greenstone belt showing an onset of deposition at 3.28 Ga, those in the center at 3.26 Ga, and those in the north at 3.24 Ga. Stratigraphically, the rocks display a general up-section trend from deeper to shallower-water deposition and/or from finer- to coarser-grained sedimentary rocks. Associated with this up-section stratigraphic trend, the sedimentary rocks show a change in provenance from more regionally similar to more locally variable, and an increase in felsic volcanic activity, especially toward the closure of Fig Tree deposition. The data is consistent with formation of the Fig Tree Group in a compressional tectonic setting by deposition in a foreland basin that experienced progressive accretion of crustal terranes onto a northward prograding fold-and-thrust belt. [ABSTRACT FROM AUTHOR]

Virtual reality concepts have been widely adapted to teach geoscientific content, most notably in virtual field trips--with increased developments due to recent travel restrictions and challenges of field access. On the spectrum between real and fully virtual environments are also combinations of digital and real content in mixed-reality environments. In this category, augmented-reality (AR) sandboxes have been used as a valuable tool for science outreach and teaching due to their intuitive and haptic interaction-enhancing operation. Most of the common AR-sandboxes are limited to the visualization of topography with contour lines and colors, as well as water simulations on the digital terrain surface. We show here how we can get beyond this limitation, through an open-source implementation of an AR-sandbox system with a versatile interface written in the free and cross-platform programming language Python. This implementation allows for creative and novel applications in geosciences education and outreach in general. With a link to a 3-D geo modelling system, we show how we can display geologic subsurface information such as the outcropping lithology, creating an interactive geological map for structural geology classes. The relations of subsurface structures, topography, and outcrop can be explored in a playful and comprehensible way. Additional examples include the visualizations of geophysical fields and the propagation of seismic waves, as well as simulations of Earth surface processes. We further extended the functionality with ArUco-marker detection to enable more precise and flexible interaction with the projected content. In combination, with these developments, we aim to make AR-sandbox systems, with the additional dimension of haptic interactions, accessible to a wider range of geoscientific applications for education and outreach. [ABSTRACT FROM AUTHOR]

To evaluate ground deformation resulting from large (~10 m) coseismic strike-slip displacements, we focus on deformation of the Kekerengu fault during the November 2016 Mw 7.8 Kaikōura earthquake in New Zealand. Combining post-earthquake field observations with analysis of high-resolution aerial photography and topographic models, we describe the structural geology and geomorphology of the rupture zone. During the earthquake, fissured pressure bulges ("mole tracks") initiated at stepovers between synthetic Riedel (R) faults. As slip accumulated, near-surface "rafts" of cohesive clay-rich sediment, bounded by R faults and capped by grassy turf, rotated about a vertical axis and were internally shortened, thus amplifying the bulges. The bulges are flanked by low-angle contractional faults that emplace the shortened mass of detached sediment outward over less-deformed ground. As slip accrued, turf rafts fragmented into blocks bounded by short secondary fractures striking at a high angle to the main fault trace that we interpret to have originated as antithetic Riedel (R') faults. Eventually these blocks were dispersed into strongly sheared earth and variably rotated. Along the fault, clockwise rotation of these turf rafts within the rupture zone averaged ~20°-30°, accommodating a finite shear strain of 1.0-1.5 and a distributed strike slip of ~3-4 m. On strike-slip parts of the fault, internal shortening of the rafts averaged 1-2 m parallel to the R faults and ~1 m perpendicular to the main fault trace. Driven by distortional rotation, this contraction of the rafts exceeds the magnitude of fault heave. Turf rafts on slightly transtensional segments of the fault were also bulged and shortened-relationships that can be explained by a kinematic model involving "deformable slats." In a paleoseismic trench cut perpendicular the fault, one would observe fissures, low-angle thrusts, and steeply dipping strike-slip faults-some cross-cutting one another-yet all may have formed during a single earthquake featuring a large strike-slip displacement. [ABSTRACT FROM AUTHOR]

The article offers information related to Sentinel Bluffs Member, which is the youngest unit of the Grande Ronde Basalt, having typical physical characteristics of most large-volume Columbia River Basalt Group (CRBG) sheet flows. Topics discussed include federal and state regional mapping projects associated with the U.S. Department of Energy's (USDOE) Hanford Site in southeastern Washington State; and data collection of the Sentinel Bluffs Member and chemical variations.

The active Mai'iu low-angle normal fault in the Woodlark rift dips as low as 16°-20° at the surface and has formed by extensional inversion of the Paleogene Owen-Stanley thrust fault. The Mai'iu fault has slipped at centimeter-per-year rates for at least 3.3 m.y., in the process exhuming a >29 km width of a largely uneroded fault surface, and uplifting the Suckling-Dayman metamorphic core complex to elevations up to 3.7 km. The exhumed fault surface is overlain by one rider block. This formed where the main fault locally became so shallowly dipping in a synformal megacorrugation that it was no longer frictionally viable for slip. Tectonic geomorphology, structural geology, and microseismicity provide evidence for a convex-upward shape and rolling hinge style of evolution for this strongly back-warped normal fault, and for an approximately Andersonian state of stress in the footwall at depth. Flexure of the exhuming footwall as a result of tectonic unloading apparently caused a late increment of extension-parallel horizontal contraction, together with a constrictional stress state. During exhumation, fault megacorrugations amplified as folds--even in the near surface. Glassy pseudotachylytes in a meter-thick foliated cataclasite unit yield 40Ar/39Ar maximum ages of 2.2-3.0 Ma. The pseudotachylytes and interleaved ultracataclasites developed in a zone of mixed-mode seismic/ aseismic slip behavior that formed near the brittle-ductile transition along a frictionally strong, high-stress ramp at 10-12 km depth. The exhumed mafic mylonite zone narrows upward and probably deformed at strain rates of up to 2.1 x 10-10 s-1 in its narrowest, upper part. Observed microseismicity at 10-25 km depth attests to current (interseismic) fault activity downdip of the fault trace. Rolling-hinge--style deformation causes the Mai'iu fault to dip at low angles (<22°) in the near surface. The shallowest part of the Mai'iu fault contains abundant saponite and is thus probably weak enough (µ <0.2) to slip at such a poorly oriented fault dip. At depth, this microseismically active structure slips at moderate dip angles (dip 30°-40°) akin to conventional normal faults. [ABSTRACT FROM AUTHOR]

Earth's only active natrocarbonatite volcanism, occurring at Oldoinyo Lengai (OL), Tanzania, suggests that natrocarbonatite melts are formed through a unique geological process. In the East African Rift, the extinct Kerimasi (KER) volcano is a neighbor of OL and also contains nephelinites hosting melt and fluid inclusions that preserve the igneous processes associated with formation of natrocarbonatite melts. Here, we present evidence for the presence of coexisting nephelinite melt, fluorine-rich carbonate melt, and alkali carbonate fluid. The compositions of these phases differ from the composition of OL natrocarbonatites; therefore, it is unlikely that natrocarbonatites formed directly from one of these phases. Instead, mixing of the outgassing alkali carbonate fluid and the fluorine-rich carbonate melt can yield natrocarbonatite compositions at temperatures close to subsolidus temperatures of nephelinite (<630-650 °C). Moreover, the high halogen content (6-16 wt%) in the carbonate melt precludes saturation of calcite (i.e., formation of calciocarbonatite) and maintains the carbonate melt in the liquid state with 28-41 wt% CaO at temperatures ≥600 °C. Our study suggests that alkali carbonate fluids and melts could have commonly formed in the geological past, but it is unlikely they precipitated calcite that facilitates fossilization. Instead, alkali carbonates likely precipitated that were not preserved in the fossil nephelinite rocks. Thus, alkali carbonate fluids and melts have been so far overlooked in the geological record because of the lack of previous detailed inclusion studies. [ABSTRACT FROM AUTHOR]

Few modern sediment dispersal pathways predate the breakup of Pangea. This suggests that river lifespan can be controlled by continental assembly and dispersal cycles, with the longest-lived river systems present during supercontinent regimes. Based on the strikingly similar age spectra and Hf isotopic array extracted from Paleozoic to early Mesozoic sedimentary sequences from the Paleo-Tethyan margin basins, we argue that a long-lived supercontinental- scale system, with headwaters originating in Antarctica, flowed northward to finally debouch on the margin with the Paleo-Tethys Ocean. Channel-belt thickness scaling relationships, which provide an estimate of drainage area, support the notion that this was a supercontinental-scale system. Sediments were eroded from Proterozoic orogenic belts and flanked resistant kernels of Archean cratons. Remnants of this system, which can still be traced today as topographic lows, controlled post-breakup drainage patterns in Gondwanan fragments in Western Australia. We conclude that supercontinental regimes allow sediment dispersal systems to be long-lived, as they provide both an abundant sediment supply, due to erosion of large-scale, collision-related internal mountain systems, and a stable, large-scale configuration that lasts until breakup. [ABSTRACT FROM AUTHOR]

Volcano seismicity is an important tool used to monitor volcanic hazards, as seismic signals are commonly associated with fracturing and the movement of volcanic fluids. In investigating the relation between fluids and seismic signals, we record acoustic emissions (AEs) in the laboratory that result from fracturing and fluid depressurization through the fractured rock. We vary the fluid viscosity to create field analogues of hydrothermal and magmatic fluids. While fracturing causes high-frequency volcano-tectonic (VT)-type AE signals, fluid venting results in VT and long-period AEs with variable but overall lower dominant frequencies. The viscosity of the vented fluid correlates (1) inversely with the peak dominant frequency, (2) inversely with the number of induced AEs, (3) proportionally to the onset time of AEs, and (4) inversely with the initial rate of AE generation. Tremor was observed only in the early stages of low-viscosity fluid venting and thus may be more associated with gases and hydrothermal fluids than with magma. [ABSTRACT FROM AUTHOR]