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K‐Ar ages of clay‐sized mineral grains are used to determine the timing of activity on fossil seismogenic faults within the Cretaceous‐Paleogene Shimanto accretionary complex, southwest Japan. Samples were collected from three regional faults that separate hanging wall coherent rocks from footwall subduction mélange: the Goshikinohama Fault that caps the Yokonami mélange, the roof thrust of the Okitsu mélange, and the Nobeoka Thrust that caps the Hyuga mélange. The K‐Ar ages of fault rocks decrease with decreasing 2M₁illite polytype component, indicating a mixture of 1M_dand 2M₁illite polytypes. Based on illite dating analysis, the extrapolated ages of the pure 2M₁illite polytype from the Goshikinohama Fault, the roof thrust of the Okitsu mélange, and the Nobeoka Thrust are 79.3 ± 5.0, 66.1 ± 8.1, and 46.7 ± 8.2 Ma, respectively, similar to the depositional age of each host rock. Lower intercepts of regression lines, which correspond to samples containing 100% authigenic illite, are calculated as 50.7 ± 1.4, 18.4 ± 1.2, and 24.4 ± 1.4 Ma, respectively. These ages are significantly younger than both the depositional ages and the timing of accretion. These results indicate that authigenic illite associated with fault slip is not related to underthrusting along the subduction interface but rather formed during out‐of‐sequence thrusting in the upper plate. Early Miocene slip along faults of the northern Shimanto belt is coincident with major tectonic events along the convergent margin, including collision with elements of the Izu‐Bonin volcanic arc‐backarc system, and opening of the Japan Sea.

 

The ~180-km-diameter Chicxulub peak-ring crater and ~240-km multiring basin, produced by the impact that terminated the Cretaceous, is the largest remaining intact impact basin on Earth. International Ocean Discovery Program (IODP) and International Continental Scientific Drilling Program (ICDP) Expedition 364 drilled to a depth of 1335 m below the sea floor into the peak ring, providing a unique opportunity to study the thermal and chemical modification of Earth’s crust caused by the impact. The recovered core shows the crater hosted a spatially extensive hydrothermal system that chemically and mineralogically modified ~1.4 × 10 5 km 3 of Earth’s crust, a volume more than nine times that of the Yellowstone Caldera system. Initially, high temperatures of 300° to 400°C and an independent geomagnetic polarity clock indicate the hydrothermal system was long lived, in excess of 10 6 years.