Search

We analyze seismic records collected at the oldest (170–180 Ma) Pacific seafloor using broadband dispersion array analysis. Using ambient noise and teleseismic waveforms, we measure Rayleigh‐wave phase velocities in a period range of 5–200 s that are inverted for array‐average one‐dimensional isotropic and azimuthally anisotropic shear‐wave velocity depth profiles from the crust to a depth of 300 km. The high‐velocity Lid and the low‐velocity zone are well‐resolved with velocity difference of ∼4%, whose transition occurs at depths between 80 and 100 km. The profile is compared with that obtained at the 130‐ and 140‐Ma seafloor. Accounting for the cooling effect due to the plate age difference, the low‐velocity zone of the oldest Pacific seafloor is ∼1.3% slower (∼110°C warmer) than that beneath the 140‐Ma seafloor, suggesting the occurrence of some reheating process beneath the oldest lithosphere. The azimuthal anisotropy at shallow depths (<50 km) is significantly different between the western and eastern areas of the array where the peak‐to‐peak amplitudes are estimated to be ∼2.8% and ∼1.6%, respectively. The fast direction is nearly parallel to the past seafloor spreading direction (perpendicular to the magnetic lineation) in the west, while it largely deviates in the east. The observed difference in azimuthal anisotropy may represent complicated evolution dynamics of the infant Pacific plate that involved a ridge‐ridge‐ridge triple junction. Plain Language Summary: One of the classical enigmas of plate tectonics is the cause of seafloor flattening where the seafloor depth becomes nearly constant for old ages (>∼70 Ma) while a simple cooling model of plate evolution predicts the age dependence. To solve this enigma, it is essential to resolve the subsurface structure of the oceanic plate and the asthenosphere below. As a part of an international effort, Pacific Array, that aims to cover the Pacific seafloor by arrays of ocean bottom seismometers, a team of Japanese and Korean scientists deployed an array in the oldest (170–180 Ma) Pacific seafloor. We determine isotropic and anisotropic shear‐wave velocity structures as a first step toward resolving the enigma. The results indicate that the asthenosphere beneath the oldest Pacific lithosphere is significantly slower than the prediction of the simple cooling and that the occurrence of some reheating process may be needed. Furthermore, azimuthal anisotropy (the dependence of the wave velocity on the horizontal propagation direction) in the shallow lithosphere shows that the anisotropy patterns differ between the eastern and western areas of the array, and simulation results indicate that the evolution process of the infant Pacific plate is not merely a simple triangular plate spreading. Key Points: Broadband Rayleigh‐wave dispersion array analysis has been conducted in the oldest (∼170 Ma) Pacific seafloorThe low‐velocity zone is significantly slower to indicate the occurrence of some reheating process beneath the oldest Pacific lithosphereThe spatial variation of the shallow azimuthal anisotropy indicates a complicated evolution of the infant Pacific plate [ABSTRACT FROM AUTHOR]

As the mantle plume rises from the deep mantle and reaches the base of a tectonic plate, it changes its traveling direction from vertical to horizontal. The horizontal spread of plume material is often radially asymmetric. An example is a plume found below the Canary Hotspot. Previous studies have suggested that the channeling of the Canary Plume toward the westernmost Mediterranean (Alboran Sea) and consequent lithospheric delamination may have contributed to the low‐velocity anomalies found beneath the Moroccan Atlas Mountains. Regional upwelling and edge‐driven convection have been proposed as other candidates to explain the origin of the low‐velocity anomalies. In this study, we incorporated anisotropy as an a priori constraint in teleseismic P‐wave travel‐time tomography as mantle flow can develop seismic anisotropy. We inverted a new set of travel‐time delays by removing the hypothetical anisotropy‐imposed travel‐time delays from the observations. Our improved results are more consistent with the hypothesis that the low‐velocity anomalies come from the mantle material of the Canary Plume. Plain Language Summary: The propagation velocity of seismic waves is sensitive to the temperature. Low seismic velocities in the mantle found from seismic tomography, a technique used to image the velocity structure of the inner Earth with seismic waves, are commonly interpreted as high‐temperature, hydrous regions, the presence of partial melting, changes in the compositional chemistry, and grain sizes of mantle minerals. Thus, seismic tomography has been the primary tool for revealing mantle structures, such as hot plumes rooted in the deep mantle that play significant roles in mantle dynamics. Previous studies have shown that rising plume material can drag the surrounding mantle and consequently cause directional dependence of the seismic velocities, which can also affect the results of seismic tomography. This study improved the quality of seismic tomography results by considering the directional dependence of wave speeds in the mantle to elucidate the evolution of the mantle plume and its interaction with the upper plate in Morocco. The imaged low‐velocity conduit below the high‐altitude Moroccan Atlas indicates the lateral travel of the mantle plume originating from the Canary Hotspot in northwest Africa, which may have dragged the surrounding mantle beneath the Moroccan Atlas. Key Points: Teleseismic P‐wave tomography is used in combination with anisotropy models to study the origin of the low‐velocity anomalies below the Moroccan AtlasWe incorporate anisotropy as an a priori constraint in tomography and show the reduction in low‐velocity anomalies below the Moroccan AtlasThe lateral travel of plume material from the Canary Hotspot may be the origin of low‐velocity anomalies [ABSTRACT FROM AUTHOR]

Abstract: Jeju Island (JI) is an intraplate volcanic field located at the continental margin of Northeast Asia. This volcanic island has been formed by multiple eruptions from the Pleistocene to the Holocene (~3.7 ka), which have yielded hundreds of monogenetic volcanic cones and a central basaltic shield. To understand the volcanic structures and mechanism beneath JI, we deployed 20 broadband temporary seismometers across the island for over two years (October 2013 to November 2015). We investigated the crustal and upper mantle structures in JI for the first time using the gathered data. Through teleseismic traveltime tomography, we obtained images of the lithospheric structure related to the volcanic system. A major finding was the identification of a prominent low‐velocity anomaly (<−0.3 km/s in P wave velocity relative to the surrounding high‐velocity region) beneath the summit of the central shield volcano at greater depths (50–60 km), which separates into low‐velocity zones at shallower depths (10–45 km). Based on previous geological observations, the anomalies were interpreted as a magmatic system, potentially with partial melting. Moreover, relatively high velocity zones were consistently imaged to the north, east, and west of the island, indicating relatively thick lithospheric structures at the southern margin of the continental lithosphere beneath the Korean Peninsula. Based on the geometries of the imaged structures, we suggest that a focused decompressional melting at sublithospheric depths and complex magma interactions within the lithosphere resulted in the characteristics of JI volcanism as intraplate magmatic activities that are isolated in space and confined in time. [ABSTRACT FROM AUTHOR]

Abstract: Recent GPS observations show that slow slip events in south central Alaska are segmented along strike. Here we review several mechanisms that might contribute to this segmentation and focus on two: along‐strike variation of slab geometry and effective normal stress. We then test them by running numerical simulations in the framework of rate‐and‐state friction with a nonplanar fault geometry. Results show that the segmentation is most likely related to the along‐strike variation of the effective normal stress on the fault plane caused by the Yakutat Plateau. The Yakutat Plateau could affect the effective normal stress by either lowering the pore pressure in Upper Cook Inlet due to less fluids release or increasing the normal stress due to the extra buoyancy caused by the subducted Yakutat Plateau. We prefer the latter explanation because it is consistent with the relative amplitudes of the effective normal stress in Upper and Lower Cook Inlet and there is very little along‐strike variation in Vp/Vs ratio in the fault zone from receiver function analysis. However, we cannot exclude the possibility that the difference in effective normal stress results from along‐strike variation of pore pressure due to the uncertainties in the Vp/Vs estimates. Our work implies that a structural anomaly can have a long‐lived effect on the subduction zone slip behavior and might be a driving factor on along‐strike segmentation of slow slip events. [ABSTRACT FROM AUTHOR]