Seismic Structure of the Upper Crust From 0–75 Ma in the Equatorial Atlantic Ocean on the African Plate Using Ultralong Offset Seismic Data

Audhkhasi, Pranav; Singh, S. C.

doi:10.1029/2019gc008577

Cited by 23 publications

(34 citation statements)

References 98 publications

(188 reference statements)

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“…P ‐wave velocities at the top of basement in the study region increase from the ridge axis to ages of 58 Ma; this velocity increase is attributed to infilling of pore space during crustal alteration and provides evidence that hydrothermal circulation in the upper crust persists to 58 Ma (Kardell et al, 2019). A study in the equatorial Atlantic found that P ‐wave velocities at the top of seismic layer 2B (mean depth 820 m below sediment‐basement interface) increased to ages of 46 Ma, which is associated with hydrothermally circulating fluids extending to this depth and age (Audhkhasi & Singh, 2019). Studies on the Juan de Fuca ridge flank show that circulation through basement outcrops drives hydrothermal fluid movement, even when outcrops 50 km apart bracket seafloor with low‐permeability sediment cover (Fisher et al, 2003).…”

Section: Discussionmentioning

confidence: 99%

South Atlantic Transect: Variations in Oceanic Crustal Structure at 31°S

Christeson

Reece

Kardell

et al. 2020

Geochem Geophys Geosyst

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We present an analysis of geophysical data acquired along a transect of 0–62 Ma crust located on the western flank of the Mid‐Atlantic Ridge at 31°S; all crust was formed at the same ridge segment. Crustal thickness, constrained by five wide‐angle profiles, has mean values of 5.6 km at 6.6 and 15.2 Ma, 7.0 km at 30.6 Ma, 5.5 km at 49.2 Ma, and 3.6 km at 61.2 Ma. Crustal thickness is uniform along each ridge‐parallel profile (standard deviations 0.1–0.3 km), indicating uniform along‐axis magmatic accretion over lateral distances of 40–60 km. The crustal structure of 61.2 Ma crust is not only anomalously thin compared to the other profiles but also contains regions with a linear velocity gradient from seafloor to Moho, which suggests that intense fracturing may extend to the base of the thin crust. Abyssal hill root‐mean‐square heights in the study region are 57–142 m and have an inverse correlation with spreading rate. These values are lower than the average root‐mean‐square height of 196 m elsewhere on the southern Mid‐Atlantic Ridge and indicate relatively high mantle temperatures in our study area. Unsedimented or lightly sedimented basement highs are prevalent at all ages; we argue that bottom currents scour the high topography, transporting sediment into adjacent basement lows. All drillsites planned for International Ocean Discovery Program Expeditions 390 and 393 are within 1–10 km of unsedimented or lightly sedimented basement highs, which should facilitate fluid flow and continued geochemical exchange between crust and seafloor.

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Section: Discussionmentioning

confidence: 99%

South Atlantic Transect: Variations in Oceanic Crustal Structure at 31°S

Christeson

Reece

Kardell

et al. 2020

Geochem Geophys Geosyst

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“…A total of 2,775 km of ultra‐deep reflection data was acquired along a stair‐like 2‐D profile (Audhkhasi & Singh, 2019; Mehouachi & Singh, 2018). Here, we focus our analysis and present results for one part of the data (∼250 km) collected along the line with a 347° azimuth sampling the region between the Romanche TF and ∼Charcot FZ, crossing the Chain FZ and propagator (Figure 1c).…”

Section: Methodsmentioning

confidence: 99%

“…The imaged Layer 2A/2B event is picked and depth converted. For the depth conversion we use a constant velocity of 4,000 m/s (Audhkhasi & Singh, 2019). .…”

Section: Transatlantic Ilab Mcs Data Setmentioning

confidence: 99%

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

et al. 2020

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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.

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“…As newly formed oceanic crust migrates away from the ridge axis it cools and subsides (Stein & Stein 1992;Hasterok 2013), primarily as a result of heat loss through hydrothermal circulation (Stein & Stein 1994) and, as sedimentation increases, thermal conduction (Fisher et al 2003). These processes alter the upper crust (Audhkhasi & Singh 2019) and effectively infill fractures, reducing porosity and permeability inherited at formation and during early development (Shaw 1994). Although this change is gradual, over time cracks are sealed and hydrothermal circulation becomes negligible.…”

Section: Introductionmentioning

confidence: 99%

“…Bratt & Solomon 1984;Collier & Singh 1998), two-dimensional profiles (e.g. Van Avendonk et al 1998;Audhkhasi & Singh 2019;Peirce et al 2019a,b;Wilson et al, 2019;Christeson et al 2020;Davy et al 2020), and three-dimensional tomographic grids (e.g. Toomey et al 1990;Gregory 2018;Robinson et al 2020;Simao et al 2020), that have become more detailed through the use of greater numbers of instrumentation deployed at smaller spatial intervals and able to sample at higher temporal rates.…”

Section: Introductionmentioning

confidence: 99%

Evolution and properties of young oceanic crust: constraints from Poisson's ratio

Funnell

Robinson

Hobbs

et al. 2021

Geophysical Journal International

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Summary The seismic velocity of the oceanic crust is a function of its physical properties that include its lithology, degree of alteration, and porosity. Variations in these properties are particularly significant in young crust, but also occur with age as it evolves through hydrothermal circulation and is progressively covered with sediment. While such variation may be investigated through P-wave velocity alone, joint analysis with S-wave velocity allows the determination of Poisson's ratio, which provides a more robust insight into the nature of change in these properties. Here we describe the independent modelling of P- and S-wave seismic datasets, acquired along an ∼330 km-long profile traversing new to ∼8 Myr-old oceanic crust formed at the intermediate-spreading Costa Rica Rift (CRR). Despite S-wave data coverage being almost four-times lower than that of the P-wave dataset, both velocity models demonstrate correlations in local variability and a long-wavelength increase in velocity with distance, and thus age, from the ridge axis of up to 0.8 and 0.6 km s−1, respectively. Using the Vp and Vs models to calculate Poisson's ratio (σ), it reveals a typical structure for young oceanic crust, with generally high values in the uppermost crust that decrease to a minimum of 0.24 by 1.0–1.5 km sub-basement, before increasing again throughout the lower crust. The observed upper crustal decrease in σ most likely results from sealing of fractures, which is supported by observations of a significant decrease in porosity with depth (from ∼15 to < 2 per cent) through the dyke sequence in Ocean Drilling Program borehole 504B. High Poisson's ratio (>0.31) is observed throughout the crust of the north flank of the CRR axis and, whilst this falls within the ‘serpentinite’ classification of lithological proxies, morphological evidence of pervasive surface magmatism and limited tectonism suggests, instead, that the cause is porosity in the form of pervasive fracturing and, thus, that this is the dominant control on seismic velocity in the newly formed CRR crust. South of the CRR, the values of Poisson's ratio are representative of more typical oceanic crust, and decrease with increasing distance from the spreading centre, most likely as a result of mineralisation and increased fracture infill. This is supported by borehole observations and modelled 3-D seismic anisotropy. Crustal segments formed during periods of particularly low half-spreading rate (<35 mm yr−1) demonstrate high Poisson's ratio relative to the background, indicating the likely retention of increased porosity and fracturing associated with the greater degrees of tectonism at the time of their formation. Across the south flank of the CRR, we find that the average Poisson's ratio in the upper 1 km of the crust decreases with age by ∼0.0084 Myr−1 prior to the thermal sealing of the crust, suggesting that, to at least ∼7 Myr, advective hydrothermal processes dominate early CRR-generated oceanic crustal evolution, consistent with heat flow measurements.

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Seismic Structure of the Upper Crust From 0–75 Ma in the Equatorial Atlantic Ocean on the African Plate Using Ultralong Offset Seismic Data

Cited by 23 publications

References 98 publications

South Atlantic Transect: Variations in Oceanic Crustal Structure at 31°S

South Atlantic Transect: Variations in Oceanic Crustal Structure at 31°S

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

Evolution and properties of young oceanic crust: constraints from Poisson's ratio

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