Subducting oceanic basement roughness impacts on upper-plate tectonic structure and a backstop splay fault zone activated in the southern Kodiak aftershock region of the Mw 9.2, 1964 megathrust rupture, Alaska

In 1964, the Alaska margin ruptured in a giant Mw 9.2 megathrust earthquake, the second largest during worldwide instrumental recording. The coseismic slip and aftershock region offshore Kodiak Island was surveyed in 1977–1981 to understand the region’s tectonics. We re-processed multichannel seismic (MCS) field data using current standard Kirchhoff depth migration and/or MCS traveltime tomography. Additional surveys in 1994 added P-wave velocity structure from wide-angle seismic lines and multibeam bathymetry. Published regional gravity, backscatter, and earthquake compilations also became available at this time. Beneath the trench, rough oceanic crust is covered by ∼3–5-km-thick sediment. Sediment on the subducting plate modulates the plate interface relief. The imbricate thrust faults of the accreted prism have a complex P-wave velocity structure. Landward, an accelerated increase in P-wave velocities is marked by a backstop splay fault zone (BSFZ) that marks a transition from the prism to the higher rigidity rock beneath the middle and upper slope. Structures associated with this feature may indicate fluid flow. Farther upslope, another fault extends >100 km along strike across the middle slope. Erosion from subducting seamounts leaves embayments in the frontal prism. Plate interface roughness varies along the subduction zone. Beneath the lower and middle slope, 2.5D plate interface images show modest relief, whereas the oceanic basement image is rougher. The 1964 earthquake slip maximum coincides with the leading and/or landward flank of a subducting seamount and the BSFZ. The BSFZ is a potentially active structure and should be considered in tsunami hazard assessments.

A discussion of “hidden subduction” in orogenic belts

The McKenzie et al. (2019) model concerning the cause of the deep earthquakes in the Hindu Kush region in Asia greatly resembles the hidden subduction model proposed earlier. However, in the case of the Hindu Kush, the age of the disappearance of the Tethyan waters was early Jurassic and the sutures were overlain by early Cretaceous sedimentary cover. The question then becomes how long a “subcutaneous” oceanic lithosphere can survive within a continent. It seems that the “oceanic” basement of the North Caspian Depression has been there since the late Palaeozoic, which is encouraging for the McKenzie et al. model. Whether an already subducted slab can also survive for more than 100 million years attached to its continental continuation remains an unanswered question. In the examples with which we are familiar (eastern Turkey, Apennines, Magrebides, Betic and Rif cordilleras), subducted lithosphere became detached at most 25 million years after collision.

Viruses in the Oceanic Basement

ABSTRACT Microbial life has been detected well into the igneous crust of the seafloor (i.e., the oceanic basement), but there have been no reports confirming the presence of viruses in this habitat. To detect and characterize an ocean basement virome, geothermally heated fluid samples (ca. 60 to 65°C) were collected from 117 to 292 m deep into the ocean basement using seafloor observatories installed in two boreholes (Integrated Ocean Drilling Program [IODP] U1362A and U1362B) drilled in the eastern sediment-covered flank of the Juan de Fuca Ridge. Concentrations of virus-like particles in the fluid samples were on the order of 0.2 × 105 to 2 × 105 ml−1 (n = 8), higher than prokaryote-like cells in the same samples by a factor of 9 on average (range, 1.5 to 27). Electron microscopy revealed diverse viral morphotypes similar to those of viruses known to infect bacteria and thermophilic archaea. An analysis of virus-like sequences in basement microbial metagenomes suggests that those from archaeon-infecting viruses were the most common (63 to 80%). Complete genomes of a putative archaeon-infecting virus and a prophage within an archaeal scaffold were identified among the assembled sequences, and sequence analysis suggests that they represent lineages divergent from known thermophilic viruses. Of the clustered regularly interspaced short palindromic repeat (CRISPR)-containing scaffolds in the metagenomes for which a taxonomy could be inferred (163 out of 737), 51 to 55% appeared to be archaeal and 45 to 49% appeared to be bacterial. These results imply that the warmed, highly altered fluids in deeply buried ocean basement harbor a distinct assemblage of novel viruses, including many that infect archaea, and that these viruses are active participants in the ecology of the basement microbiome. IMPORTANCE The hydrothermally active ocean basement is voluminous and likely provided conditions critical to the origins of life, but the microbiology of this vast habitat is not well understood. Viruses in particular, although integral to the origins, evolution, and ecology of all life on earth, have never been documented in basement fluids. This report provides the first estimate of free virus particles (virions) within fluids circulating through the extrusive basalt of the seafloor and describes the morphological and genetic signatures of basement viruses. These data push the known geographical limits of the virosphere deep into the ocean basement and point to a wealth of novel viral diversity, exploration of which could shed light on the early evolution of viruses. IMPORTANCE The hydrothermally active ocean basement is voluminous and likely provided conditions critical to the origins of life, but the microbiology of this vast habitat is not well understood. Viruses in particular, although integral to the origins, evolution, and ecology of all life on earth, have never been documented in basement fluids. This report provides the first estimate of free virus particles (virions) within fluids circulating through the extrusive basalt of the seafloor and describes the morphological and genetic signatures of basement viruses. These data push the known geographical limits of the virosphere deep into the ocean basement and point to a wealth of novel viral diversity, exploration of which could shed light on the early evolution of viruses.

Pre-collision evolution of the Piñón oceanic terrane of SW Ecuador: stratigraphy and geochemistry of the “Calentura Formation”

The stratigraphic revision of the southern coastal Ecuadorian series makes possible the reconstruction of the pre-collision history of the Caribbean plateau accreted to the Ecuadorian margin. The Coniacian age of the oceanic basement (Piñón Fm) indicates that the latter is part of the Caribbean oceanic plateau. It is overlain by the Calentura Fm, which comprises from base to top: (i) 20 to 200 m of lavas and volcanic breccias of arc affinity (Las Orquídeas Mb), (ii) siliceous, organic rich black limestones of (middle?) Coniacian age, (iii) red, radiolarian rich, calcareous cherts ascribed to the Santonian-early Campanian, and (iv) marls, greywackes and island arc tuffs of Mid Campanian age. The latter are overlain by volcaniclastic turbidites of Mid to Late Campanian age (Cayo Fm), coeval to the Campanian-Maastrichtian island arc series locate farther west (San Lorenzo Fm). The Las Orquídeas magmatic unit is interpreted as resulting from the melting of the Caribbean plateau, rather than from an ephemeral subduction process. The transition from coniacian limestones to santonian red cherts would be related to the thermal subsidence of the Caribbean plateau. The uplift of the latter and the development of the San Lorenzo island arc in the Middle Campanian would be due to the collision of the Caribbean plateau with the Mexican margin. Early in the Late Maastrichtian, the collision of the Caribbean plateau with the Ecuadorian margin would have triggered the cessation of the San Lorenzo arc activity. In the Late Paleocene, the Caribbean plateau was split into two terranes: the western Piñón terrane, which collided with the eastern Guaranda terrane.