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Abstract The southern San Andreas fault is in its interseismic period, occasionally releasing some stored elastic strain during triggered slow slip events (SSEs) at <2.5 km depth. A distinct, shallowly exhumed gouge defines the fault and is present at SSE depths. To evaluate if this material can host SSEs, we characterize its mineralogy, microstructures, and frictional behavior with water‐saturated deformation experiments near‐in situ conditions, and we compare laboratory healing rates to natural SSEs. Our results show that slip localizes along clay surfaces in both laboratory and natural settings. The gouge is weak (coefficient of friction of ∼0.29), exhibits low healing rates (<0.001/decade), and transitions from unstable to stable behavior at slip rates above ∼1 μm/s. Healing rate and friction drop data from laboratory instabilities are comparable to geodetically‐constrained values for SSEs. Collective observations indicate this gouge could host shallow SSEs and/or localize slip facilitating dynamic rupture propagation to the surface. 

Abstract Low‐temperature thermochronometric data can reveal the long‐term evolution of erosion, uplift, and thrusting in fold‐thrust belts. We present results from central Idaho and southwestern Montana, where the close spatial overlap of the Sevier fold‐thrust belt and Laramide style, basement‐involved foreland uplifts signify a complex region with an unresolved, long‐term tectono‐thermal history. Inverse QTQt thermal history modeling of new zircon (U‐Th)/He (ZHe,n = 106), and apatite (U‐Th)/He dates (AHe,n = 43) collected from hanging walls of major thrusts systems along a central Idaho to southwestern Montana transect, and apatite fission track results from 6 basement samples, reveal regional thermal and spatial trends related to Sevier and Laramide orogenesis. Inverse modeling of foreland basement uplift samples suggest Phanerozoic exhumation initiated as early as ∼80 Ma and continued through the early Paleogene. Inverse modeling of interior Idaho fold‐thrust belt ZHe samples documents Early Cretaceous cooling at ∼125 Ma in the Lost River Range (western transect), and a younger cooling episode in the Lemhi Arch region (mid‐transect) at ∼90–80 Ma through the late Paleogene. This cooling in the Lemhi Arch temporally overlaps with cooling in southwestern Montana's basement‐cored uplifts, which we interpret as roughly synchronous exhumation related to contractional tectonics and post‐orogenic collapse. These data and models, integrated with independent timing constraints from foreland basin strata and previously published thermochronometric results, suggests that middle Cretaceous deformation of southwestern Montana's basement‐cored uplifts was low magnitude and preceded tectonism along the classic Arizona‐Wyoming Laramide “corridor.” In contrast, Late Cretaceous and Paleogene thrust‐related exhumation was more significant and largely complete by the Eocene. The basement‐involved deformation was contemporaneous with and younger than along‐strike Sevier belt thrusting in central Idaho. 

Abstract The material properties and distribution of faults above the seismogenic zone promote or inhibit earthquake rupture propagation. We document the depths and mechanics of fault slip along the seismically active Hurricane fault, UT, with scanning and transmission electron microscopy and hematite (U‐Th)/He thermochronometry. Hematite occurs as mm‐scale, striated patches on a >10 m²thin, mirror‐like silica fault surface. Hematite textures include bulbous aggregates and cataclasite, overlain by crystalline Fe‐oxide nanorods and an amorphous silica layer at the slip interface. Textures reflect mechanical, fluid, and heat‐assisted amorphization of hematite and silica‐rich host rock that weaken the fault and promote rupture propagation. Hematite (U‐Th)/He dates document episodes of mineralization and fault slip between 0.65 and 0.36 Ma at ∼300 m depth. Data illustrate that some earthquake ruptures repeatedly propagate along localized slip surfaces in the shallow crust and provide structural and material property constraints for in models of fault slip. 

Abstract High‐spatial resolution textural and geochemical data from thin slip surfaces in exhumed fault zones archive thermal and rheological signatures of past fault slip. A network of minor, glossy, iridescent silica fault mirrors (FMs) cut Paleoproterozoic gneiss in the Wasatch fault zone (WFZ), Utah. We report field to nanoscale observations from scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy with energy‐dispersive X‐ray spectroscopy of a silica FM to infer deformation mechanisms during FM development. The FM volume comprises a ∼40–90 μm‐thick basal layer of sintered, µm‐ to nm‐diameter silica particles with polygonal to anhedral morphologies, pervasive crystalline Ti‐bearing phases containing measurable N, and µm‐ to nm‐scale void spaces. Silica particles lack shape and crystallographic preferred orientation and some are predominantly amorphous with internal crystalline domains. The basal layer is overlain by a ∼10–130 nm‐thick, chemically heterogeneous, amorphous film at the FM interface. Mass balance calculations of Ti in the basal layer and host rock indicate the FM volume can be sourced from the underlying gneiss. Multiple textural and geochemical lines of evidence, including N substitution in Ti‐bearing phases, support temperature rise during deformation, associated amorphization of host gneiss, and creation of the FM volume. During thermal decay, interstitial anatase and titanite fully crystallized, silica textures capture their incipient crystallization, and some residual elements are solidified in the nanofilm. Our results support a mechanism of weakening and re‐strengthening of silica FM during fault slip and, together with data from adjacent hematite FMs, record shallow, ancient microseismicity in the WFZ. 

Abstract In many regions globally, snowmelt‐recharged mountainous karst aquifers serve as crucial sources for municipal and agricultural water supplies. In these watersheds, complex interplay of meteorological, topographical, and hydrogeological factors leads to intricate recharge‐discharge pathways. This study introduces a spatially distributed deep learning precipitation‐runoff model that combines Convolutional Long Short‐Term Memory (ConvLSTM) with a spatial attention mechanism. The effectiveness of the deep learning model was evaluated using data from the Logan River watershed and subwatersheds, a characteristically karst‐dominated hydrological system in northern Utah. Compared to the ConvLSTM baseline, the inclusion of a spatial attention mechanism improved performance for simulating discharge at the watershed outlet. Analysis of attention weights in the trained model unveiled distinct areas contributing the most to discharge under snowmelt and recession conditions. Furthermore, fine‐tuning the model at subwatershed scales provided insights into cross‐subwatershed subsurface connectivity. These findings align with results obtained from detailed hydrogeochemical tracer studies. Results highlight the potential of the proposed deep learning approach to unravel the complexities of karst aquifer systems, offering valuable insights for water resource management under future climate conditions. Furthermore, results suggest that the proposed explainable, spatially distributed, deep learning approach to hydrologic modeling holds promise for non‐karstic watersheds. 

Abstract EarthScope's USArray Transportable Array has shortcomings for the purpose of interpreting geologic features of wavelengths less than the Transportable Array station spacing, but these can be overcome by using higher spatial resolution gravity data. In this study, we exploit USArray receiver functions to reduce nonuniqueness in the interpretation of gravity anomalies. We model gravity anomalies from previously derived density variations of sedimentary basins, crustalV_p/V_svariation, Moho variation, and upper mantle density variation derived from body wave imaging informed by surface wave tomography to estimateV_p/V_s. Although average densities and density contrasts for these seismic variations can be derived, the gravity anomalies modeled from them do not explain the entire observed gravity anomaly field in the United States. We use the unmodeled gravity anomalies (residuals) to reconstruct local variations in densities of the crust associated with geologic sources. The approach uses velocity‐density relationships and differs from density computations that assume isostatic compensation. These intracrustal densities identify geologic sources not sampled by and, in some cases, aliased by the USArray station spacing. We show an example of this improvement in the vicinity of the Bloomfield Pluton, north of the bootheel of Missouri, in the central United States.