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Evidence for coseismic temperature rise that induces dynamic weakening is challenging to directly observe and quantify in natural and experimental fault rocks. Hematite (U-Th)/He (hematite He) thermochronometry may serve as a fault-slip thermometer, sensitive to transient high temperatures associated with earthquakes. We test this hypothesis with hematite deformation experiments at seismic slip rates, using a rotary-shear geometry with an annular ring of silicon carbide (SiC) sliding against a specular hematite slab. Hematite is characterized before and after sliding via textural and hematite He analyses to quantify He loss over variable experimental conditions. Experiments yield slip surfaces localized in an ∼5–30-µm-thick layer of hematite gouge with 71% ± 1% (1σ) and 18% ± 3% He loss, respectively. Documented He loss requires short-duration, high temperatures during slip. The spatial heterogeneity and enhanced He loss from FM zones are consistent with asperity flash heating (AFH). Asperities >200–300 µm in diameter, producing temperatures >900 °C for ∼1 ms, can explain observed He loss. Results provide new empirical evidence describing AFH and the role of coseismic temperature rise in FM formation. Hematite He thermochronometry can detect AFH and thus seismicity on natural FMs and other thin slip surfaces in the upper seismogenic zone of Earth’s crust.

Strain localization across the brittle-ductile transition is a fundamental process in accommodating tectonic movement in the mid-crust. The tectonically active Cordillera Blanca shear zone (CBSZ), a ∼200-km-long normal-sense shear zone situated within the footwall of a discrete syn-convergent extensional fault in the Peruvian Andes, is an excellent field laboratory to explore this transition. Field and microscopic observations indicate consistent top-down-to-the-southwest sense of shear and a sequence of tectonites ranging from undeformed granodiorite through mylonite and ultimately fault breccia along the detachment.Using microstructural analysis, two-feldspar and Ti-in-quartz (TitaniQ) thermometry, recrystallized quartz paleopiezometry, and analysis of quartz crystallographic preferred orientations, we evaluate the deformation conditions and mechanisms in quartz and feldspar across the CBSZ. Deformation temperatures derived from asymmetric strain-induced myrmekite in a subset of tectonite samples are 410 ± 30 to 470 ± 36 °C, consistent with TitaniQ temperatures of 450 ± 60 to 490 ± 33 °C and temperatures >400 °C estimated from microstructural criteria. Brittle fabrics overprint ductile fabrics within ∼150 m of the detachment that indicate that deformation continued to lower-temperature (∼280–400 °C) and/or higher-strain-rate conditions prior to the onset of pervasive brittle deformation. Initial deformation occurred via high-temperature fracturing and dissolution-precipitation in feldspar. Continued subsolidus deformation resulted in either layering of mylonites into monophase quartz and fine-grained polyphase domains oriented subparallel to macroscopic foliation or the interconnection of recrystallized quartz networks oriented obliquely to macroscopic foliation. The transition to quartz-controlled rheology occurred at temperatures near ∼500 °C and at a differential stress of ∼16.5 MPa. Deformation within the CBSZ occurred predominantly above ∼400 °C and at stresses up to ∼71.4 MPa prior to the onset of brittle deformation.

The Yukon–Alaska Highway corridor in southern Yukon is subject to geohazards ranging from landslides to floods and earthquakes on faults in the St. Elias Mountains and Shakwak Valley. Here we discuss the late Holocene seismic history of the Denali fault, located at the eastern front of the St. Elias Mountains and one of only a few known seismically active terrestrial faults in Canada. Holocene faulting is indicated by scarps and mounds on late Pleistocene drift and by tectonically deformed Pleistocene and Holocene sediments. Previous work on trenches excavated against the fault scarp near the Duke River reveals paleoseismic sediment disturbance dated to ∼300–1,200, 1,200–1,900, and 3,000 years ago. Re-excavation of the trenches indicates a fourth event dated to 6,000 years ago. The trenches are interpreted to show a negative flower structure produced by extension of sediments by dextral strike-slip fault movement. Nearby Crescent Lake is ponded against the fault scarp. Sediment cores reveal four abrupt sediment and diatom changes reflecting seismic shaking at ∼1,200–1,900, 1,900–5,900, 5,900–6,200, and 6,500–6,800 years ago. At the Duke River, the fault offsets sediments, including two White River tephra layers (∼1,900 and 1,200 years old). Late Pleistocene outwash gravel and overlying Holocene aeolian sediments show in cross section a positive flower structure indicative of post-glacial contraction of the sediments by dextral strike-slip movement. Based on the number of events reflecting ∼M6, we estimate the average recurrence of large earthquakes on the Yukon part of the Denali fault to be about 1,300 years in the past 6,500–6,800 years.

International audience; We present a petrological, geochronological and geochemical study of Neoproterozoic ophiolitic units exposed in the Pan-African orogenic belt (Moroccan Anti-Atlas). These units comprise two main complexes: (i) the Khzama ophiolite (in the Sirwa window) to the west and (ii) the Aït Ahmane ophiolite to the east (in the Bou Azzer inlier). Both complexes mainly consist in serpentinized ultramafics associated with rare chromite pods and pyroxenites, which are in tectonic contact with mafic units made of isotropic and layered metagabbros and metabasalts dykes. Khzama ophiolite has been dated at 762 Ma (U-Pb zircon dating on plagiogranite dykes) and we dated Aït Ahmane ophiolite at 745.9 ± 7.4 Ma (U-Pb zircon dating on a layered metagabbro). The precursors of Aït Ahmane and Khzama serpentinites are spinel harzburgites associated with minor dunitic lenses, both entirely serpentinized. Bulk rock composition shows low contents in incompatible major and trace elements (Al2O3: 0.2-1.3 wt.% and Ti: 3-38 ppm) and in HREE ([Yb]N

Ongoing climate change focuses attention on the Arctic cryosphere’s responses to past and future climate states. Although it is now recognized the Arctic Ocean Basin was covered by ice sheets and their associated floating ice shelves several times during the Late Pleistocene, the timing and extent of these polar ice sheets remain uncertain. Here we relate a relict barrier-island system on the Beaufort Sea coast of northern Alaska to the isostatic effects of a previously unrecognized ice shelf grounded on the adjacent continental shelf. A new suite of optically stimulated luminescence dates show that this barrier system formed during one or more marine transgressions occurring late in Marine Isotope Stage 5 (MIS 5) between 113 ka and 71 ka. Because these transgressions occurred after the warmest part of the last interglacial (ca. 123 ka) and did not coincide with the global eustatic sea-level maximum during MIS 5e, this indicates Arctic ice sheets developed out-of-phase with lower-latitude sectors of the Laurentide and Fennoscandian ice sheets. We speculate that Arctic ice sheets began development during full interglacial conditions when abundant moisture penetrated to high latitudes, and low summer insolation favored glacier growth. These ice sheets reached their full extents at interglacial-glacial transitions, then wasted away at the heights of mid-latitude glaciations because of moisture limitations.

Fault rock low-temperature thermochronometry can inform the timing, temperature, and significance of hydrothermal fluid circulation in fault systems. We demonstrate this with combined hematite (U-Th)/He (He) dating, and sandstone apatite fission-track (AFT) and apatite and zircon (U-Th)/He (He) thermochronometry from fault-related fissures on the Gower Peninsula, Wales. Hematite He dates from 141 ± 5.1 Ma to 120 ± 5.0 Ma overlap with a 131 ± 20 Ma sandstone infill AFT date. Individual zircon He dates are 402–260 Ma, reflecting source material erosion, and imply a maximum Late Permian infill depositional age. Burial history reconstruction reveals modern exposures were not buried sufficiently in the Triassic–Early Cretaceous to have caused reheating to temperatures necessary to reset the AFT or hematite He systems, and thus these dates cannot reflect cooling due to erosion alone. Hot fluids circulating through fissures in the Early Cretaceous reset the AFT system. Hematite was either also reset by fluids or precipitated from these fluids. Similar hematite He dates from fault-related mineralization in south Glamorgan (Wales) and Cumbria (England) imply concomitant regional hot groundwater flow along faults. In this example, hydrothermal fluid circulation, coeval with North Atlantic rifting, occurred in higher-permeability fissures and fault veins long after they initially formed, directly influencing local and regional geothermal gradients.

International audience; The European Alps are a mountain belt that is characterized by a series of discrete orogenic events, which have long been recognized. Despite the inherent episodic nature of orogenic evolution, the Alps have been continuously exhumed, mainly by erosion, but also by normal faulting. Since continental collision started in the late Eocene/Early Oligocene evidence for ongoing erosional exhumation has been preserved in synorogenic sediments that accumulated in basins adjacent to the pro- and retro-side of this double-vergent mountain belt. This long-term erosion record can be used to determine exhumation rates. Lag-times calculated from fission-track (FT) ages of detrital zircon from synorogenic sediments are fairly constant for the European Alps since the Oligocene–Late Miocene. Although the fast exhuming areas were unroofed at rates of 0.4–0.7 km Myr−1, the overall average exhumation rate is between 0.2 and 0.3 km Myr−1 on a regional scale. The detrital and bedrock zircon FT data of the Alps do not detect the increase in erosion rates since the Pliocene over the past ∼5 Myr, as shown elsewhere. This increase cannot be detected yet with the detrital zircon FT method because not enough rock has been removed to widely expose zircons with Pliocene or younger cooling ages in the Alps. Long term (30 Myr) exhumation rates appear to have been approximately constant when averaged over a sliding time window of about 8 Myr, or depth window of 5 to 10 km (ZFT closure depths); shorter-term fluctuations are not identified using this method.