The Nazca Drift System – palaeoceanographic significance of a giant sleeping on the SE Pacific Ocean floor

The evolution and resulting morphology of a contourite drift system in the SE Pacific oceanic basin is investigated in detail using seismic imaging and an age-calibrated borehole section. The Nazca Drift System covers an area of 204 500 km2 and stands above the abyssal basins of Peru and Chile. The drift is spread along the Nazca Ridge in water depths between 2090 and 5330 m. The Nazca Drift System was drilled at Ocean Drilling Program Site 1237. This deep-water drift overlies faulted oceanic crust and onlaps associated volcanic highs. Its thickness ranges from 104 to 375 m. The seismic sheet facies observed are associated with bottom current processes. The main lithologies are pelagic carbonates reflecting the distal position relative to South America and water depth above the carbonate compensation depth during Oligocene time. The Nazca Drift System developed under the influence of bottom currents sourced from the Circumpolar Deep Water and Pacific Central Water, and is the largest yet identified abyssal drift system of the Pacific Ocean, ranking third in all abyssal contourite drift systems globally. Subduction since late Miocene time and the excess of sediments and water associated with the Nazca Drift System may have contributed to the Andean orogeny and associated metallogenesis. The Nazca Drift System records the evolution in interactions between deep-sea currents and the eastward motion of the Nazca Plate through erosive surfaces and sediment remobilization.

4 Ma-long uplift cycles of the southern Peru forearc since Late Miocene

<p>We explore the coastal morphology along an uplifting 500 km-long coastal segment of   the Central Andes, between the cities of Chala (Peru) and Arica (Chile). We use accurate DEM and field surveys to extract sequences of uplifted shorelines along the study area. In addition, we consider continental pediment surfaces that limit both the geographical and vertical extent of the marine landforms. We establish a chronology based on published dates for marine landforms and pediment surfaces. We expand this corpus with new <sup>10</sup>Be data on uplifted shore platforms. The last 12 Ma are marked by three periods of coastal stability or subsidence dated ~12-11 Ma, ~8-7 Ma and ~5-2.5 Ma ago. The uplift that accumulated between these stability periods has been ~1000 m since 11 Ma; its rate can reach 0.25 mm/a (m/ka). For the last period of uplift only, during the last 800 ka, the forearc uplift has been accurately recorded by the carving of numerous coastal sequences. Within these sequences, we correlated the marine terraces with the sea level highstands (interglacial stages and sub-stages) up to MIS 19 (790 ka), i.e., with a resolution of ~100 ka. The uplift rate for this last period of uplift increases westward from 0.18 mm/a at the Peru-Chile border to ~0.25 mm/a in the center of the study area. It further increases northwestward, up to 0.45 mm/a, due to the influence of the Nazca Ridge. In this study, we document an unusual forearc cyclic uplift with ~4 Ma-long cycles. This periodicity corresponds to the predictions made by Menant et al. (2020) based on numerical models, and could be related to episodic tectonic underplating (subducting slab stripping) beneath the coastal forearc area.</p>

Seismic Velocity Structure in the Area of the 2007, Mw 8.0, Pisco-Peru Earthquake: Implications for the Mechanics of Subduction in the Vicinity of the Nazca Ridge

In this study, we present a velocity model for the area of the 2007 Pisco-Peru earthquake ( Mw = 8.0 ) obtained using a double-difference tomography algorithm that considers aftershocks acquired for 6 months. The studied area is particularly interesting because it lies on the northern edge of the Nazca Ridge, in which the subduction of a large bathymetric structure is the origin of geomorphological features of the central coast of Peru. Relocated seismicity is used to infer the geometry of the subduction slab on the northern flank of the Nazca Ridge. The results prove that the geometry is continuous but convex because of the subduction of the ridge, thereby explaining the high uplift rates observed in this area. Our inferred distribution of seismicity agrees with both the coseismic and postseismic slip distributions.

A new species of Eunice Cuvier, 1817 (Polychaeta: Eunicidae) from the slope of the Desventuradas Islands and seamounts of the Nazca Ridge, southeastern Pacific Ocean

A new species of Eunicidae, Eunice decolorhami sp. n., from the southeastern Pacific Ocean, is described. The species was collected at the slope of the Desventuradas Islands (San Félix and San Ambrosio) and in three nearby seamounts of the Nazca Ridge, in dead coral rubble bottoms from 180 to 340 m depth and inhabiting inside parchment-like branched tubes. Eunice decolorhami sp. n. can be distinguished from other species of the genus, mainly by the coloration pattern of the subacicular hooded hooks along the body segments, the beginning of the subacicular hooded hooks, the beginning of the branchiae, the maximum number of branchial filaments, and the maxillary formula. A key for the seven Eunice species recorded off the coast of Chile and Peru, including the new species described herein, is provided.

Lithospheric Structure of the Central Andes Forearc from Gravity Data Modeling: Implication for Plate Coupling

Geodetic and seismological data indicates that the Central Andes subduction zone is highly coupled. To understand the plate locking mechanism within the Central Andes, we developed 2.5-D gravity models of the lithosphere and assessed the region’s isostatic state. The densities within the gravity models are based on satellite and surface gravity data and constrained by previous tomographic studies. The gravity models indicate a high-density (~2940 kg m-3) forearc structure in the overriding South American continental lithosphere, which is higher than the average density of the continental crust. This structure produces an anomalous pressure (20-40 MPa) on the subducting Nazca plate, contributing to intraplate coupling within the Central Andes. The anomalous lithostatic pressure and buoyancy force may be controlling plate coupling and asperity generation in the Central Andes. The high-density forearc structure could be a batholith or ophiolite emplaced onto the continental crust. The isostatic state of the Central Andes and Nazca plate is assessed based on residual topography (difference between observed and isostatic topography). The West-Central Andes and Nazca ridge have ~0.78 km of residual topography, indicating undercompensation. The crustal thickness beneath the West-Central Andes may not be sufficient to isostatically support the observed topography. This residual topography may be partially supported by small-scale convective cells in the mantle wedge. The residual topography in the Nazca ridge may be attributed to density differences between the subducting Nazca slab and the Nazca ridge. The high density of the subducted Nazca slab has a downward buoyancy force, while the less dense Nazca ridge provides an upward buoyancy force. These two forces may effectively raise the Nazca ridge to its current-day elevation.