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<jats:p>The Sevier and Laramide belts of the U.S. Cordillera are differentiated based on thin- and thick-skinned structural domains, commonly inferred to have formed under different plate-boundary conditions. However, spatial and temporal overlap in the Idaho-Montana fold-thrust belt suggests that thin- and thick-skinned thrust systems are kinematically linked. We present the first balanced and sequentially restorable cross section that integrates the Sevier and Laramide belts. Encompassing most of the width of the Cordilleran retroarc, our kinematic model accounts for at least 244 km of horizontal shortening, linking thin- and thick-skinned thrust systems. We hypothesize that thin strata overlying the Lemhi arch basement high determined the geometry and relative timing of the later thrusting. Early shortening (pre–ca. 90 Ma) was thin skinned, with the décollement of the Medicine Lodge–McKenzie thrust system following Devonian shales on top of the Lemhi arch unconformity. Displacement on upper thin- and lower thick-skinned thrusts overlapped between ca. 90 and 70 Ma as a mid-crustal décollement was activated, efficiently transmitting strain through the Lemhi arch to the Blacktail-Snowcrest uplift in the foreland. A regional-scale duplex (Patterson culmination) linked the lower and upper décollements, internally thickening and increasing the basal slope of the orogenic wedge. Thick-skinned thrusts of the Dillon cutoff (Hawley Creek, Cabin, and Johnson thrusts) eventually thickened the wedge and exhumed the abandoned upper décollement. Following this, the thick-skinned wedge advanced in-sequence from ca. 70 to 55 Ma. This kinematic model establishes continuity between thin- and thick-skinned thrust systems by a mid-crustal décollement. In this model, the stratigraphic thicknesses of sedimentary cover rocks limit the availability of décollement horizons, determining the style of mountain building and triggering a slow transition from thin- to thick-skinned thrusting.</jats:p>

<jats:p>The Dadeville Complex of Alabama and Georgia (southeastern United States) represents the largest suite of exposed mafic-ultramafic rocks in the southern Appalachians. Due to poor preservation, chemical alteration, and tectonic reworking, a specific tectonic origin for the Dadeville Complex has been difficult to deduce. We obtained new whole-rock and mineral geochemistry coupled with zircon U-Pb geochronology to investigate the magmatic and metamorphic processes recorded by the Dadeville Complex, as well as the timing of these processes. Our data reveal an up-stratigraphic evolution in the geochemistry of the volcanic rocks, from forearc basalts to boninites. Our new U-Pb zircon crystallization data—obtained from three amphibolite samples—place the timing of forearc/protoarc volcanism no later than ca. 467 Ma. New thermobarometry suggests that the Dadeville Complex rocks subsequently experienced deep, high-grade metamorphism, at pressure-temperature conditions of &amp;gt;7 kbar and &amp;gt;760 °C. The data presented here support a model for formation of the Dadeville Complex in the forearc region of a subduction zone during subduction initiation and protoarc development, followed by deep burial/underthrusting of the complex during orogenesis.</jats:p>

<jats:p>The Cretaceous intrusive units of the Sahwave and Nightingale ranges in northwestern Nevada, USA, located between the Sierra Nevada and Idaho batholiths, represent a critical segment of Cretaceous arc magmatism. U-Pb zircon age dating shows that the older, 104 Ma Power Line intrusive complex is dominantly granodioritic in composition, while the younger 94–88 Ma Sahwave Range intrusive suite (the Juniper Pass, Bob Springs, and Sahwave plutons) is similar in composition (tonalite to granodiorite) and age to the plutons of the Tuolumne intrusive suite of the east-central Sierra Nevada batholith. We present new field measurements, microstructural observations, and anisotropy of magnetic susceptibility analyses of the Power Line intrusive complex and Sahwave Range intrusive suite. The Power Line intrusive complex is characterized by a vertical, N–S-striking, solid-state foliation and down-dip lineation. Evidence of dextral shearing is observed on subhorizontal planes that are perpendicular to the lineation, which is consistent with pure shear-dominated transpression. This fabric is similar in style and timing to both the western Idaho shear zone of the Idaho batholith and mid-Cretaceous shear zones of the central Sierra Nevada. The plutons of the Sahwave Range intrusive suite are not affected by the pure shear-dominated transpressional fabric observed in the Power Line intrusive complex, which indicates that this deformation ceased by ca. 94 Ma. Rather, the Juniper Pass pluton contains an E–W-striking magmatic foliation fabric that rotates to a steep NW–SE-striking, solid-state foliation in the younger Sahwave pluton. These fabrics are strikingly similar to fabrics in the Tuolumne intrusive suite, Sierra Nevada, California, USA. Recent work in the western Idaho shear zone also indicates that late-stage deformation occurred there until ca. 85 Ma. Therefore, the intrusions of northwestern Nevada provide a tectonic link between the Sierra Nevada and Idaho batholiths, which suggests that two distinct phases of mid-Cretaceous, transpressional deformation occurred in at least three magmatic arc segments of the western U.S. margin.</jats:p>

<jats:p>The Cross-Hosgri slope is a bathymetric lineament that crosses the main strand of the Hosgri fault offshore Point Estero, central California. Recently collected chirp seismic reflection profiles and sediment cores provide the basis for a reassessment of Cross-Hosgri slope origin and the lateral slip rate of the Hosgri fault based on offset of the lower slope break of the Cross-Hosgri slope. The Cross-Hosgri slope is comprised of two distinct stratigraphic units. The lower unit (unit 1) overlies the post–Last Glacial Maximum transgressive erosion surface and is interpreted as a Younger Dryas (ca. 12.85–11.65 ka) shoreface deposit based on radiocarbon and optically stimulated luminescence (OSL) ages, Bayesian age modeling, seismic facies, sediment texture, sediment infauna, and heavy mineral component. The shoreface was abandoned and partly eroded during rapid sea-level rise from ca. 11.5 to 7 ka. Unit 2 consists of fine sand and silt deposited in a midshelf environment when the rate of sea-level rise slowed between ca. 7 ka and the present. Although unit 2 provides a thin, relatively uniform cover over the lower slope break of the older shoreface, this feature still represents a valuable piercing point, providing a Hosgri fault slip rate of 2.6 ± 0.8 mm/yr. Full-waveform processing of chirp data resulted in significantly higher resolution in coarser-grained strata, which are typically difficult to interpret with more traditional envelope processing. Our novel combination of offshore radiocarbon and OSL dating is the first application to offshore paleoseismic studies, and our results indicate the utility of this approach for future marine neotectonic investigations.</jats:p>

<jats:p>The rate and location at depth of fault creep are important, but difficult to characterize, parameters needed to assess seismic hazard. Here we take advantage of the magnetic properties of serpentinite, a rock type commonly associated with fault creep, to model its depth extent along the Bartlett Springs fault zone, an important part of the San Andreas fault system north of the San Francisco Bay, California (western United States). We model aeromagnetic and gravity anomalies using geologic constraints along 14 cross sections over a distance of 120 km along the fault zone. Our results predict that the fault zone has more serpentinite at depth than inferred by geologic relationships at the surface. Existing geodetic models are inconsistent and predict different patterns of creep along the fault. Our results favor models with more extensive creep at depth. The source of the serpentinite appears to be ophiolite thrust westward and beneath the Franciscan Complex, an interpretation supported by the presence of antigorite, a high-temperature serpentine mineral stable at depth, in fault gouge near Lake Pillsbury.</jats:p>

<jats:p>We present a new tectonic plate reconstruction that suggests substantial revisions to events associated with development of the Peruvian flat slab and resolves several long-standing issues regarding the subduction of bathymetric highs in the region. The Tuamotu Plateau is widely considered to be the product of Easter Plume magmatism, and plate reconstructions suggest it formed following initial plume ascent at ca. 55 Ma. The Nazca Ridge is also linked to the Easter Plume and is an obvious candidate to be the spreading ridge conjugate to the Tuamotu Plateau. Models for the paired evolution of the two ridges, however, generally stop at ca. 33 Ma because of the inability of plate reconstructions to associate the two ridges across a spreading center prior to this time. In addition, seafloor magnetic data demonstrate that the Tuamotu Plateau developed at a complexly shaped and evolving mid-oceanic ridge that precluded development of a simple mirror image conjugate of the type commonly employed in Nazca Ridge reconstructions. Seafloor isochrons also suggest that a ridge jump separated the Tuamotu Plateau from its conjugate at ca. 42 Ma. Global plate models offer an alternative approach to assessing conjugate development, by showing how a hypothetical conjugate to the Tuamotu Plateau is built up over time. Using such a model, we found that the conjugate that developed during the main stage of Tuamotu growth (55 Ma to 42 Ma) cannot be the Nazca Ridge, which appears to have initiated at ca. 42 Ma, when the Easter Plume diverted volcanism southward. We named the newly recognized conjugate the Enigma Ridge. Importantly, subduction of this ridge starting from ca. 17 Ma on the north Peruvian trench can account for the missing slab buoyancy previously attributed to the hypothesized, but controversial, Inca Plateau. The Enigma Ridge must still be providing far more buoyancy over a much greater area than the Nazca Ridge, which only began to subduct rather recently.</jats:p>

<jats:p>The analysis of Coulomb stress changes has become an important tool for seismic hazard evaluation because such stress changes may trigger or delay subsequent earthquakes. Processes that can cause significant Coulomb stress changes include coseismic slip and transient postseismic processes such as poroelastic effects and viscoelastic relaxation. However, the combined influence of poroelastic effects and viscoelastic relaxation on co- and postseismic Coulomb stress changes has not been systematically studied so far. Here, we use three-dimensional finite-element models with arrays of normal and thrust faults to investigate how pore fluid pressure changes and viscoelastic relaxation overlap during the postseismic phase. In different experiments, we vary the permeability of the upper crust and the viscosity of the lower crust or lithospheric mantle while keeping the other parameters constant. In addition, we perform experiments in which we combine a high (low) permeability of the upper crust with a low (high) viscosity of the lower crust. Our results show that the coseismic (i.e., static) Coulomb stress changes are altered by the signal from poroelastic effects and viscoelastic relaxation during the first month after the earthquake. For sufficiently low viscosities, the Coulomb stress change patterns show a combined signal from poroelastic and viscoelastic effects already during the first postseismic year. For sufficiently low permeabilities, Coulomb stress changes induced by poroelastic effects overlap with the signals from viscoelastic relaxation and interseismic stress accumulation for decades. Our results imply that poroelastic and viscoelastic effects have a strong impact on postseismic Coulomb stress changes and should therefore be considered together when analyzing Coulomb stress transfer between faults.</jats:p>

<jats:p>Development and evaluation of models for tectonic evolution in the Cascadia forearc require understanding of along-strike heterogeneity of strain distribution, uplift, and upper-plate characteristics. Here, we investigated the Neogene geologic record of the Klamath Mountains province in southernmost Cascadia and obtained apatite (U-Th)/He (AHe) thermochronology of Mesozoic plutons, Neogene graben sediment thickness, detrital zircon records from Neogene grabens, gravity and magnetic data, and kinematic analysis of faults. We documented three aspects of Neogene tectonics: early Miocene and younger rock exhumation, development of topographic relief sufficient to isolate Neogene graben-filling sediments from sources outside of the Klamath Mountains, and initiation of mid-Miocene or younger right-lateral and reverse faulting. Key findings are: (1) 10 new apatite AHe mean cooling ages from the Canyon Creek and Granite Peak plutons in the Trinity Alps range from 24.7 ± 2.1 Ma to 15.7 ± 2.1 Ma. Inverse thermal modeling of these data and published apatite fission- track ages indicate the most rapid rock cooling between ca. 25 and 15 Ma. One new AHe mean cooling age (26.7 ± 3.2 Ma) from the Ironside Mountain batholith 40 km west of the Trinity Alps, combined with previously published AHe ages, suggests geographically widespread latest Oligocene to Miocene cooling in the southern Klamath Mountains province. (2) AHe ages of 39.4 ± 5.1 Ma on the downthrown side and 22.7 ± 3.0 Ma on the upthrown side of the Browns Meadow fault suggest early Miocene to younger fault activity. (3) U-Pb detrital zircon ages (n = 862) and Lu-Hf isotope geochemistry from Miocene Weaverville Formation sediments in the Weaverville, Lowden Ranch, Hayfork, and Hyampom grabens south and southwest of the Trinity Alps can be traced to entirely Klamath Mountains sources; they suggest the south-central Klamath Mountains had, by the middle Miocene, sufficient relief to isolate these grabens from more distal sediment sources. (4) Two Miocene detrital zircon U-Pb ages of 10.6 ± 0.4 Ma and 16.7 ± 0.2 Ma from the Lowden Ranch graben show that the maximum depositional age of the upper Weaverville Formation here is younger than previously recognized. (5) A prominent steep-sided negative gravity anomaly associated with the Hayfork graben shows that both the north and south margins are fault-controlled, and inversion of gravity data suggests basin fill is between 1 km and 1.9 km thick. Abrupt elevation changes of basin fill-to-bedrock contacts reported in well logs record E-side-up and right-lateral faulting at the eastern end of the Hayfork graben. A NE-striking gravity gradient separates the main graben on the west from a narrower, thinner basin to the east, supporting this interpretation. (6) Offset of both the base of the Weaverville Formation and the cataclasite-capped La Grange fault surface by a fault on the southwest margin of the Weaverville basin documents 200 m of reverse and 1500 m of right-lateral strike-slip motion on this structure, here named the Democrat Gulch fault; folded and steeply dipping strata adjacent to the fault confirm that faulting postdated deposition of the Weaverville Formation.</jats:p> <jats:p>Based on these findings, we suggest that Miocene rock cooling recorded by AHe ages, accompanying graben formation, and development of topographic relief record early to middle Miocene initiation of underplating or “subcretion” in the southern Cascadia subduction zone beneath the southern Klamath Mountains.</jats:p>

<jats:p>The 39.8-ka Campanian Ignimbrite was emplaced during a large caldera-forming eruption of Campi Flegrei near Naples, Italy. The ignimbrite is found up to 80 km from the caldera, and co-ignimbrite ash-fall deposits occur 3200 km away. The proximal and distal stratigraphy of the Campanian Ignimbrite has not been definitively correlated due to the dissimilar appearance of the proximal and distal deposits, a lack of medial exposures, and the inconsistency and heterogeneity of the proximal stratigraphy. Here, we document the major-element glass-shard chemistry, matrix componentry, and lithic componentry of the proximal and distal stratigraphic sequences of the ignimbrite to attempt to correlate the units. The results of these disparate observations taken together suggest that the established stratigraphic units cannot be directly and uniquely correlated between the proximal and distal regions and that neither the proximal nor distal stratigraphy provides a record of the entire eruptive sequence. However, the characteristics studied can be used to demarcate eruptive phases that are connected to some of the defined units in the proximal and distal stratigraphy.</jats:p>

<jats:p>In the eastern North China Craton, the Zibo bauxitic clay deposits are situated between Permian sandstones. These deposits exhibit distinct characteristics in two horizons. The lower horizon consists of disordered kaolinite with anhedral–subhedral, rounded morphologies that indicate a detrital origin. The upper horizon, however, contains ordered kaolinite. Detrital zircon grains in the lower horizon indicate a unimodal age spectrum with a mean age of ca. 290 Ma and εHf(t) values ranging from −20.8 to −6.0. These findings suggest a continental volcanic arc source on the northern margin of the North China Craton. In contrast, detrital zircon grains in the upper section exhibit a multi-modal detrital age spectrum with significant age peaks at 2500 Ma, 1850 Ma, and 310 Ma that originates from the local basement. The zircon dating establishes a maximum depositional age of ca. 280 ± 3 Ma, which indicates denudation of the source area in the northern North China Craton during the Artinskian stage. The relative abundance of detrital kaolinite indicates a warm and humid climate during the late Artinskian (ca. 283 Ma) to Early Kungurian (ca. 280 Ma), while cold and dry conditions prevailed during the mid–late Kungurian (ca. 277 Ma). The northern North China Craton, which supplied source material to the lower section of Zibo bauxitic clay, experienced rapid uplift and exhumation and underwent intense weathering under high humidity and warm temperatures during the late Artinskian to Early Kungurian. However, the source area shifted from the north to a more central region as the climate transitioned to cold and dry conditions in the mid–late Kungurian. Considering that detrital clay formation is indicative of specific climatic conditions, the Permian bauxitic clay deposits in Zibo provide a valuable record of environmental changes during the late Paleozoic ice age (LPIA; ca. 360–260 Ma).</jats:p>