International audienc

Large impacts provide a mechanism for resurfacin g 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 peak s transition to peak ri ngs. 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. Ex pedition 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

[1] We present a two-dimensional velocity model to constrain crustal thickness and composition of the Yakutat terrane in the northern Gulf of Alaska. The model was constructed using seismic reflection and refraction data along a $455 km onshore-offshore profile. Our model shows that the crystalline crust composing the Yakutat terrane is wedge-shaped, with crustal thickness increasing west to east from $15 km to $30 km. Crustal velocity and structure are continuous across the terrane, with lower crustal velocities >7 km/s, suggesting that the Yakutat terrane is an oceanic plateau across its entire offshore extent rather than a composite oceanic-continental terrane as previously proposed. The thickest Yakutat crust is entering the adjacent St. Elias orogen where elevated exhumation rates and concentrated seismicity in this vicinity are likely influenced by incipient Yakutat-North America collision. Our model includes a $8 km thick low-velocity crustal cap extending across the eastern portion of the profile where shallow basement is imaged on marine seismic reflection data. We interpret this cap as a lithified, metamorphosed remnant accretionary prism, providing evidence of a previous attempt at Yakutat subduction along its eastern margin prior to current emplacement at the southern Alaska margin.

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We present a new synthesis of oceanic crustal structure from two‐dimensional seismic profiles to explore differences related to spreading rate and age. Primary results are as follows: (1) Layer 2 has an average thickness of 1.84 km but is thicker for young slow‐spreading crust and thinner for young superfast‐spreading crust. At faster‐spreading rates the layer 2/3 boundary likely corresponds to the lithologic boundary between dikes and gabbros. At slow‐spreading centers, the layer 2/3 boundary is interpreted to mark a change in porosity with depth within the dikes. (2) Total crustal thickness averages 6.15 km and is similar across all spreading rates. (3) Velocities at the top of layer 2 increase rapidly from 3.0 km/s at 0 Ma to 4.6 km/s at 10.5 Ma, with a slower increase to 5.0 km/s at 170 Ma. The rapid increase in velocity at young ages is attributed to crack closure by precipitation of hydrothermal alteration products; the increase at older ages suggests that this process persists as the oceanic crust evolves. (4) There is a correlation between velocities at the top of layer 2 and sediment thickness, with velocities of 5.8–5.9 km/s associated with a sediment thickness of 4.0–4.3 km. The thick sediment may collapse large‐scale features such as lava tubes and fractures. (5) Average velocities at the top of layer 3 are lower for young slow‐spreading and intermediate‐spreading oceanic crust (6.1–6.2 km/s) than for older or faster‐spreading oceanic crust (6.5–6.7 km/s). These low velocities are likely associated with faults penetrating into the sheeted dikes.

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The uppermost 2 km of the oceanic crust created at the fast spreading (135 mm yr À1 , full rate) equatorial East Pacific Rise (EPR) is exposed for tens of kilometers along escarpments bounding the Hess Deep Rift. Mosaics of large-scale digital images from the remotely operated vehicle (ROV) Argo II and direct observations from the submersible Alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading. Dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units. These rock units are characterized by extensive faulting, fine-scale fracturing, and rotations of coherent crustal blocks meters to tens of meters across. The uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces. Through most of the basaltic lava unit, however, lava flow contacts dip (208-708W) toward the EPR and generally increase in dip downward in the section. Dikes cutting the lavas and in the underlying sheeted dike unit generally dip (908 -408E) away from the EPR. Deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units. We interpret this upper crustal structure as the result of subaxial subsidence within 1-2 km of the EPR that accommodated the thickening of the basaltic lava unit to $500 m. Variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply. These results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the Hess Deep Rift. If this type of structure is typical of uppermost oceanic crust generated at the EPR, these processes may be common along fast spreading mid-ocean ridges.

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