Dynamic Compressive Strength and Fragmentation in Felsic Crystalline Rocks

Brittle deformation in rocks depends upon loading rate; with increasing rates, typically greater than ~102 s‐1, rocks become significantly stronger and undergo increasingly severe fragmentation. Dynamic conditions required for rate‐dependent brittle failure may be reached during impact events, seismogenic rupture, and landslides. Material characteristics and fragment characterization of specific geomaterials from dynamic loading are only approximately known. Here we determine the characteristic strain rate for dynamic behavior in felsic crystalline rocks, including anisotropy, and describe the resulting fragments. Regardless of the type of felsic crystalline rock or anisotropy, the characteristic strain rate is the same within uncertainties for all tested materials, with an average value of 229 ± 81 s‐1. Despite the lack of variation of the critical strain rate with lithology, we find that the degree of fragmentation as a function of strain rate varies depending on material. Scaled or not, the fragmentation results are inconsistent with current theoretical models of fragmentation. Additionally, we demonstrate that conditions during impact cratering, where the impactor diameter is less than ~100 m, are analogous to the experiments carried out here, and therefore that dynamic strengthening and compressive fragmentation should be considered as important processes during impact cratering

Extraordinary rocks from the peak ring of the Chicxulub impact crater: P-wave velocity, density, and porosity measurements from IODP/ICDP Expedition 364

Joint International Ocean Discovery Program and International Continental Scientific Drilling Program Expedition 364 drilled into the peak ring of the Chicxulub impact crater. We present P-wave velocity, density, and porosity measurements from Hole M0077A that reveal unusual physical properties of the peak-ring rocks. Across the boundary between post-impact sedimentary rock and suevite (impact melt-bearing breccia) we measure a sharp decrease in velocity and density, and an increase in porosity. Velocity, density, and porosity values for the suevite are 2900–3700 m/s, 2.06–2.37 g/cm3, and 20–35%, respectively. The thin (25 m) impact melt rock unit below the suevite has velocity measurements of 3650–4350 m/s, density measurements of 2.26–2.37 g/cm3, and porosity measurements of 19–22%. We associate the low velocity, low density, and high porosity of suevite and impact melt rock with rapid emplacement, hydrothermal alteration products, and observations of pore space, vugs, and vesicles. The uplifted granitic peak ring materials have values of 4000–4200 m/s, 2.39–2.44 g/cm3, and 8–13% for velocity, density, and porosity, respectively; these values differ significantly from typical unaltered granite which has higher velocity and density, and lower porosity. The majority of Hole M0077A peak-ring velocity, density, and porosity measurements indicate considerable rock damage, and are consistent with numerical model predictions for peak-ring formation where the lithologies present within the peak ring represent some of the most shocked and damaged rocks in an impact basin. We integrate our results with previous seismic datasets to map the suevite near the borehole. We map suevite below the Paleogene sedimentary rock in the annular trough, on the peak ring, and in the central basin, implying that, post impact, suevite covered the entire floor of the impact basin. Suevite thickness is 100–165 m on the top of the peak ring but 200 m in the central basin, suggesting that suevite flowed downslope from the collapsing central uplift during and after peak-ring formation, accumulating preferentially within the central basin

Sphene Emotional: How Titanite Was Shocked When the Dinosaurs Died

Accessory mineral geochronometers such as zircon, monazite, baddeleyite, and xenotime are increasingly being recognized for their ability to preserve diagnostic microstructural evidence of hypervelocity processes. However, little is known about the response of titanite to shock metamorphism, even though it is a widespread accessory phase and U-Pb geochronometer. Here we report two new mechanical twin modes in titanite within shocked granitoids from the Chicxulub impact structure, Mexico. Titanite grains in the newly acquired International Ocean Discovery Program Site expedition 364 M0077A core preserve multiple sets of polysynthetic twins, most commonly with composition planes (K1), = ~{111}, and shear direction (1) = , and less commonly with the mode K1 = {130}, 1 = ~. In some grains, {130} deformation bands have formed concurrently with shock twins, indicating dislocation glide with Burgers vector b = [341] can be active at shock conditions. Twinning of titanite in these modes, the presence of planar deformation features in shocked quartz, and lack of diagnostic shock microstructures in zircon in the same samples highlights the utility of titanite as a shock indicator for a shock pressure range between ~12 and ~17 GPa. Given the challenges of identifying ancient impact evidence on Earth and other bodies, microstructural analysis of titanite is here demonstrated to be a new avenue for recognizing impact deformation in materials where other impact evidence may be erased, altered, or did not manifest due to low shock pressure

Probing the hydrothermal system of the Chicxulub impact crater

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 × 105 km3 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 106 years

The formation of peak rings in large impact craters

Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust

Preliminary Results From the Chicxulub Post-Impact Sediments: XRF and Physical Properties Data

In spring 2016, joint IODP/ICDP Expedition 364 drilled into the peak ring of the Chicxulub crater, offshore the Yucatan Peninsula, Mexico. A continuous core was drilled (Hole M0077A) and recovered a sequence of Paleogene post-impact rocks, suevites, impact-melt rocks and granitic basement between 505.7 m and 1334.7 m below sea floor (bsf). The Chicxulub crater was formed ~66 million years ago by an impact. This catastrophic event was directly linked to a major mass extinction. For this study, we concentrate on the post-impact sediments (505.7 to 617.3 m bsf; ~48 to ~66 Ma). The main goal of drilling the post-impact section was to study the pace and mode of recovery of life in the ocean after the impact, and to analyze the paleoenvironmental changes across the Paleocene and Eocene. The late Paleocene and Eocene are characterized by a series of transient warming events, so-called hyperthermals that were associated with increased atmospheric pCO2. Here, we present preliminary geochemical and physical properties data from the ~112 m of Paleogene sediments. XRF data show high log (Ca/Ti) values between ~617 and ~598 m bsf (Paleocene and early Eocene), and lower values between ~598 and 505 m bsf. In particular the upper part is characterized by high-frequency fluctuations in log (Ca/Ti) reflecting repeated changes in lithology. These were presumably caused by Milankovitch cycles. Low log (Ba/Ti) values characterize the lowermost part of the record between ~617 and ~610, followed by a gradual increase to higher values, presumably indicating an increase in primary productivity towards the end of the Paleocene. Values remain at this higher level between ~605 and ~540 m bsf. Hyperthermals are characterized by strong positive log (Ba/Ti) peaks, likely pointing at highly elevated primary productivity levels during these short-lived events. Between ~540 and ~505 m bsf, log (Ba/Ti) values are more variable and drop occasionally to values as low as were encountered in the lowermost part. Similar to the log Ca/Ti curve, the log Ba/Ti curve is superimposed by high-frequency fluctuations. These fluctuations are also strongly visible in color reflectance measurements