Dating brittle faults by the growth of authigenic illite in fault gouge allows for direct testing of kinematic models of orogenic evolution, such as in‐sequence versus out‐of‐sequence thrusting, and the presence of orogenic “pulses” of deformation on multiple thrusts simultaneously. We present illite gouge ages from five thrust faults along the Ter and Freser rivers in the south‐eastern Pyrenees fold‐and‐thrust belt dated by Ar‐Ar methods. The ages show in‐sequence thrusting for the south‐eastern Pyrenees fold‐thrust belt during the Eocene and early Oligocene, ranging from 49.5 + 4.4 Ma for the inboard Freser Antiformal Stack to 31.9 ± 3.9 Ma for the frontal L’Escala Thrust. Foreland progression of ages is preserved, although overlapping of ages of the frontal thrusts permits the interpretation of a period of more rapid fault displacement around 35 Ma. Shortening rates for the Ter‐Freser section range from 1.2–2.7 km/my, with strain rates ranging from 7.0 × 10 −16 s −1 to 1.6 × 10 −15 s −1 The latest Paleozoic ages of the cataclastically derived component inferred from the gouge dating methodology are shown to be pooled 2M 1 illite/mica ages of wall rock sediments and have geological meaning. These inferred mica ages of the thrust wallrocks in the southeastern Pyrenees indicate Hercynian‐age micas were deposited in the Southern Pyrenean foreland basin as early as the late Cretaceous. Previous thermochronometer and Δ 47 temperatures from syn‐faulting calcite veins constrain temperature of clay gouge formation in the Ter‐Freser section at 50–210°C, with most likely temperatures of 50–160°C, consistent with previous studies.

AbstractIsotopic studies of Canadian Rocky Mountain thrust faults preserve the timing and identity of orogenic fluids and their fault zone pathways. Using previously dated samples, we measure the O‐ and H‐isotopic compositions of fault gouge. These nearly 100% neomineralized gouges and their associated damage zones act as primary orogenic fluid pathways. As such, they provide a specific and local look into the nature of the Late Jurassic to Early Eocene orogenic plumbing system in the Alberta Rockies. Considering clay polytype stability and regional temperature conditions, we obtain a range of geofluid isotopic compositions during Jurassic‐Eocene thrust faulting: δ18Ofluid ranged from ∼−3.3 to 9.2 ± 3.2‰; δDfluid ranged from −119 to −46 ± 13‰ VSMOW. The range of O‐ and H‐isotopic compositions reflects mixing of fluid sources, including the pervasive presence of surface‐sourced fluids (up to ∼90%). The interpreted prevalence of a surface fluid source in fault rocks is in agreement with regional isotopic trends previously observed in undated veins of fractured host rock. Our results confirm that thrust faults of the Alberta Rocky Mountains acted as major fluid‐focusing conduits during orogenic activity. We further show that these faults incorporated both deeply sourced and surface‐sourced fluids into zones of enhanced and dynamic permeability, heterogeneously distributing fluids along fault planes across the fold‐thrust belt, promoting the growth of fault‐zone weakening clay minerals.

The continental interior of the United States encompasses the region between the Rocky Mountain front and the Appalachia-Ouachita front. It is a region of cratonic platform that represents the southern third of the North American craton. Crust of the continental interior consists of Precambrian basement (formed mostly during Proterozoic accretionary orogeny) overlain, at the Great Unconformity (which formed due to late Proterozoic exhumation), by a relatively thin cover of Phanerozoic stratigraphic sequences that were deposited during transgressions and regressions of shallow seas. Nearer the margins of the interior, clastic wedges derived from bordering orogens spread out onto the craton. The continental interior has not undergone orogenic deformation for about 1 billion years, so it does not display the consequences of penetrative deformation or dynamothermal metamorphism, but it does display the consequences of tectonic activity. This Phanerozoic activity produced several epeirogenic domes, basins, and arches, as well as numerous midcontinent fault-and-fold zones. The fault-and-fold zones likely represent transpressional and transtensional reactivation of Proterozoic rift-related normal faults. The timing of Phanerozoic tectonism in the continental interior of the United States corresponds with the timing of orogenic activity in the continent's marginal orogens, suggesting that stress transmitted into the interior of the continent during orogeny caused continental-interior tectonism. Uplifts related to tectonic events also drove groundwater flow that produced distinct pulses of diagenesis. Though the topography of the continental interior is not as dramatic as occurs in orogenic belts, the region nevertheless contains a rich record of lithospheric evolution through the Phanerozoic.

Isotopic and geochronologic investigation of authigenic, K-bearing clays in the Appalachian Plateau of the northeastern U.S. Midcontinent yields new insights about the tectonic and diagenetic history of the North American sedimentary cover sequence. In situ texture analysis by High Resolution X-ray Texture Goniometry indicates preservation of bedding-parallel diagenetic fabrics with burial depths of 2-5 km, and illite mineralization temperatures are spatially variable, ranging from 80 to 190 degrees C, correlating to similar depths of 3-6 km. The mineralizing geofluid is surface derived, with delta D values ranging from -48 parts per thousand to -72 parts per thousand (in the range of predicted Pangea meteoric fluid). In addition, we find that mineralizing fluid delta D values increase away from the orogenic front, consistent with a rain shadow effect from the high elevation Appalachian orogen. The age of authigenic illite is constrained by(40)Ar/Ar-39 geochronology to 308-318 Ma, reflecting Upper Carboniferous diagenesis. We postulate that far-field stress transmission from continent-continent collision created regional permeability pathways for surface fluids, altering the hydrologic architecture of the brittle crust and allowing meteoric fluid infiltration into upper crustal rocks. This interpretation challenges the popular view of tectonically forced, lateral fluid flow from the Appalachian orogen (squeegee hypothesis).

The outgoing Editor in Chief of Earth’s Future reflects on the first 6 years of the open-access journal and the factors that have contributed to its growth and success.

Abstract The configuration of continents and oceans of our tectonically active planet is ever changing. Using new, high‐resolution paleogeographic base maps, we created a set of animations that examine key elements of plate tectonics. These time‐ and space‐based paleoglobe reconstructions illustrate continental rifting, continental breakup, ocean ridges and fracture zones, hot spot tracks, arc‐backarc systems, continental collision, terrane accretion, opening‐closing of ocean basins, supercontinent formation, plate velocities, and future Earth. Each animation is supported by a narrative that offers a brief topical overview, some observations to guide a user's exploration, and key references that formulated the main ideas and concepts that became the foundations of modern plate tectonics.

Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the coronavirus disease 2019 (COVID-19) pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record. Although the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This quiet period provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of human activities.

Few methods exist for directly dating orogenic events in the upper crust. Here we report in-situ dates from (similar to 10 mu m diameter) monazite grown in quartz-calcite fibrous veins, which precipitated at very low-grade metamorphic conditions (about 250 degrees C) in volcaniclastic and calcareous rocks of the Early Cretaceous Trancas Fm. in the Zimapan Basin, Central Mexico. Our integrated study includes regional and local structural and kinematic analysis of vein and folds, detailed textural and mineralogical characterization of vein-bearing minerals, and texturally-controlled Th/Pb geochronologic analysis of monazite by LA-MC-ICPMS. We find that monazite in the veins preferentially crystallized at the interface between vein-forming calcite and quartz fibers, as well as at the vein-host rock interface. Monazite ages range from 154 to 68 Ma, with the youngest sub-population yielding a weighted mean age of 76.8 +/- 0.8 Ma, coincident with the age of basin-wide folding as earlier constrained by Ar/Ar illite dating. The oldest monazite ages coincide with the timing of deposition of the volcaniclastic host-rock. Given that veins are ubiquitous in shallow crustal rocks, this approach to dating of veins with enclosed monazite has great potential to improve the constraints on temporal resolution of fluid-rock interaction during of deformation.