Seismic structure of the St. Paul Fracture Zone and Late Cretaceous to Mid Eocene oceanic crust in the equatorial Atlantic Ocean near 18°W

Plate tectonics characterize transform faults as conservative plate boundaries where the lithosphere is neither created nor destroyed. In the Atlantic, both transform faults and their inactive traces, fracture zones, are interpreted to be structurally heterogeneous, representing thin, intensely fractured, and hydrothermally altered basaltic crust overlying serpentinized mantle. This view, however, has recently been challenged. Instead, transform zone crust might be magmatically augmented at ridge-transform intersections before becoming a fracture zone. Here, we present constraints on the structure of oceanic crust from seismic refraction and wide-angle data obtained along and across the St. Paul fracture zone near 18°W in the equatorial Atlantic Ocean. Most notably, both crust along the fracture zone and away from it shows an almost uniform thickness of 5-6 km, closely resembling normal oceanic crust. Further, a well-defined upper mantle refraction branch supports a normal mantle velocity of 8 km/s along the fracture zone valley. Therefore, the St. Paul fracture zone reflects magmatically accreted crust instead of the anomalous hydrated lithosphere. Little variation in crustal thickness and velocity structure along a 200 km long section across the fracture zone suggests that distance to a transform fault had negligible impact on crustal accretion. Alternatively, it could also indicate that a second phase of magmatic accretion at the proximal ridge-transform intersection overprinted features of starved magma supply occurring along transform faults. Key Points: - Seismic structure along the St. Paul fracture zone reflects magmatically accreted oceanic crust - Oceanic crust across St. Paul shows only small thickness variations, lacking evidence for regional crustal thinning near fracture zones - Magmatic nature of crust supports a mechanism where transform crust is augmented before being turned into a fracture zon

Mars Reflectivity by Ambient Noise Correlation

In the end of November 2018 the InSight mission landed on the Martian surface deploying a six axis broadband and short period seismometer, in order record marsquakes and ambient noise, which can be used to constrain the deeper interior structures of the planet. In this study we process the three components of the broad band seismometer and apply autocorrelation methods to more than a year of ambient noise data and to 250 selected marsquake waveforms, to retrieve the empirical Greens functions (EGFs), that are related to the subsurface impedance discontinuities below the lander. The most prominent impedance discontinuity is commonly represented by the crust-mantle boundary (Moho), which yet needs to be confirmed on Mars. A lot of care is taken to attenuate the spurious signals and the wind induced lander resonance, that contaminate the data, before the autocorrelation analysis. We further apply attribute filtering and reject data windows that exceed wind speed thresholds or reveal too large amplitudes at later lags, which are often related to residuals of artefacts or lander resonance. To further improve the signal to noise ratio of the EGFs we apply phase-weighted stacking. We observe a good agreement in the autocorrelograms of the ambient noise and the marsquakes and determine several stable arrivals on the vertical component, in particular at 10.8 and 12.7 s, that we interpret as trapped P-waves, which are reflected between the surface and the potential Moho or other layer interfaces. Using available velocity constraints we convert the arrival times to possible Moho depths of 19-28 and 22-33 km, respectively. In addition, we find that large part of the seismic energy during the quiet night period of Mars is related to the 2.4 Hz mode and that removing it from the autocorrelations the distinct arrivals can no longer be identified.Applied Geophysics | IDEA Leagu

Seismic refraction and wide-angle segy-data from profile iLAB-SPARC (north), equatorial Atlantic Ocean near 18°W

Seismic refraction and wide-angle data were acquired in the equatorial Atlantic Ocean along the rougly north-south running iLAB-SPARC profile (IS-01) near 18°W. Here we provide the data from 15 ocean-bottom-seismometer (OBS) stations representing the northern part (~300 km) of a seismic line with a total length of 850 km. Data were obtained aboard the French R/V Pourquoi Pas? in 2018. The repository contains two types of files for the OBS data and the MCS data, respectively

Seismic Crustal Structure and Morphotectonic Features Associated With the Chain Fracture Zone and Their Role in the Evolution of the Equatorial Atlantic Region

Oceanic transform faults and fracture zones (FZs) represent major bathymetric features that keep the records of past and present strike‐slip motion along conservative plate boundaries. Although they play an important role in ridge segmentation and evolution of the lithosphere, their structural characteristics, and their variation in space and time, are poorly understood. To address some of the unknowns, we conducted interdisciplinary geophysical studies in the equatorial Atlantic Ocean, the region where some of the most prominent transform discontinuities have been developing. Here we present the results of the data analysis in the vicinity of the Chain FZ, on the South American Plate. The crustal structure across the Chain FZ, at the contact between ∼10 and 24 Ma oceanic lithosphere, is sampled along seismic reflection and refraction profiles. We observe that the crustal thickness within and across the Chain FZ ranges from ∼4.6–5.9 km, which compares with the observations reported for slow‐slipping transform discontinuities globally. We attribute this presence of close to normal oceanic crustal thickness within FZs to the mechanism of lateral dike propagation, previously considered to be valid only in fast‐slipping environments. Furthermore, the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale. Our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower‐order Mid‐Atlantic Ridge segmentation around the equator.ISSN:2169-9313ISSN:0148-0227ISSN:2169-935