scholarly journals U-Pb dating of interspersed gabbroic magmatism and hydrothermal metamorphism during lower crustal accretion, Vema lithospheric section, Mid-Atlantic Ridge

2015 ◽  
Vol 120 (4) ◽  
pp. 2093-2118 ◽  
Author(s):  
Matthew Rioux ◽  
Niels Jöns ◽  
Samuel Bowring ◽  
C. Johan Lissenberg ◽  
Wolfgang Bach ◽  
...  
Keyword(s):  
Crustal Accretion ◽  
Mid Atlantic Ridge ◽  
Hydrothermal Metamorphism ◽  
Lower Crustal

Rock and paleomagnetic evidence for the existence and nature of a Cayman Trough spreading center

1978 ◽  
Vol 15 (12) ◽  
pp. 1930-1940 ◽  
Author(s):  
M. J. Clark ◽  
J. M. Hall ◽  
J. W. Peirce
Keyword(s):  
Central Valley ◽  
Spreading Center ◽  
Small Scale ◽  
Low Temperature Oxidation ◽  
Crustal Accretion ◽  
Temperature Oxidation ◽  
Magnetic Parameters ◽  
Mid Atlantic Ridge ◽  
Curie Temperatures

Rock and paleomagnetic measurements have been made on a set of 54 basalts dredged from 17 stations located within the central valley of the Cayman Trough. Seventeen of the samples could be oriented with respect to the in situ vertical by the use of lava cooling ledges and stalactites.Peak remanent intensities in the Cayman Trough are lower than peak Mid-Atlantic Ridge values by a factor of 2 or 3 even after allowance is made for the latitudinal variation in geomagnetic field intensity. This difference is likely to be the result of the combined effects of relatively low saturation magnetization and more advanced low temperature oxidation of titanomagnetite in the Cayman Trough basalts.Five young, reversely magnetized basalts, similar to those found on the Mid-Atlantic Ridge, occur in the Cayman Trough sample set.Plots of the magnetic parameters of the pillow basalts with distance from the axis of the trough show broad highs or lows associated with the axis. Our interpretation is that crustal formation in the central valley has occurred recently, but it has either been rather diffuse or is now much disturbed tectonically on a small scale in comparison with the Mid-Atlantic Ridge. Analysis of the distribution of Curie temperatures suggests that crustal accretion has been slow (0.1–0.4 cm year−1 half-rate) and may have ceased in the area studied at about 0.6 Ma BP.

scholarly journals Seismicity trends and detachment fault structure at 13°N, Mid-Atlantic Ridge

Geology ◽  
2020 ◽  
Author(s):  
R. Parnell-Turner ◽  
R.A. Sohn ◽  
C. Peirce ◽  
T.J. Reston ◽  
C.J. MacLeod ◽  
...  
Keyword(s):  
Ocean Bottom ◽  
Crustal Rocks ◽  
Detachment Faults ◽  
Spatial Coverage ◽  
Mid Atlantic Ridge ◽  
Similar Survey ◽  
Linear Band ◽  
Ocean Bottom Seismographs ◽  
Plate Spreading ◽  
Lower Crustal

At slow-spreading ridges, plate separation is commonly partly accommodated by slip on long-lived detachment faults, exposing upper mantle and lower crustal rocks on the seafloor. However, the mechanics of this process, the subsurface structure, and the interaction of these faults remain largely unknown. We report the results of a network of 56 ocean-bottom seismographs (OBSs), deployed in 2016 at the Mid-Atlantic Ridge near 13°N, that provided dense spatial coverage of two adjacent detachment faults and the intervening ridge axis. Although both detachments exhibited high levels of seismicity, they are separated by an ~8-km-wide aseismic zone, indicating that they are mechanically decoupled. A linear band of seismic activity, possibly indicating magmatism, crosscuts the 13°30′N domed detachment surface, confirming previous evidence for fault abandonment. Farther south, where the 2016 OBS network spatially overlapped with a similar survey done in 2014, significant changes in the patterns of seismicity between these surveys are observed. These changes suggest that oceanic detachments undergo previously unobserved cycles of stress accumulation and release as plate spreading is accommodated.

scholarly journals In situ lower crustal accretion by melt sill injection revealed by seismic layering

2021 ◽  
Author(s):  
Peng Guo ◽  
Satish Singh ◽  
Venkata Vaddineni ◽  
Ingo Grevemeyer ◽  
Erdinc Saygin
Keyword(s):  
Oceanic Crust ◽  
Lower Crust ◽  
Velocity Variation ◽  
Main Process ◽  
Upper Crust ◽  
Crustal Accretion ◽  
Slow Spreading ◽  
Lower Crustal ◽  
Lower Oceanic Crust

Abstract Oceanic crust is formed at mid-ocean spreading centres by a combination of magmatic, tectonic and hydrothermal processes. The crust formed by magmatic process consists of an upper crust generally composed of basaltic dikes and lava flows and a lower crust presumed to mainly contain homogeneous gabbro whereas that by tectonic process can be very heterogeneous and may even contain mantle rocks. Although the formation and evolution of the upper crust are well known from geophysical and drilling results, those for the lower crust remain a matter of debate. Using a full waveform inversion method applied to wide-angle seismic data, here we report the presence of layering in the lower oceanic crust formed at the slow spreading Mid-Atlantic Ridge, ~7-12 Ma in age, revealing that the lower crust is formed mainly by in situ cooling and crystallisation of melt sills at different depths by the injection of magma from the mantle. These layers are 400-600 m thick with alternate high and low velocities, with ± 100-200 m/s velocity variation, and cover over a million-year old crust, suggesting that the crustal accretion by melt sill intrusions beneath the ridge axis is a stable process. We also find that the upper crust is ~400 m thinner than that from conventional travel-time analysis. Taken together, these discoveries suggest that the magmatism plays more important roles in the crustal accretion process at slow spreading ridges than previously realised, and that in-situ lower crustal accretion is the main process for the formation of lower oceanic crust.

The Xigaze ophiolite: fossil ultraslow-spreading ocean lithosphere in the Tibetan Plateau

2020 ◽  
pp. jgs2020-208
Author(s):  
Tong Liu ◽  
Chuan-Zhou Liu ◽  
Fu-Yuan Wu ◽  
Henry J.B. Dick ◽  
Wen-Bin Ji ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
Spreading Rate ◽  
Southwest Indian Ridge ◽  
The Tibetan Plateau ◽  
Crustal Accretion ◽  
Ocean Ridges ◽  
Magma Supply ◽  
Spreading Ridges ◽  
Ocean Lithosphere ◽  
Lower Crustal

The crust and mantle in both ophiolites (fossil ocean lithosphere) and in modern oceans are enormously diverse. Along-axis morphology and lower crustal accretion at ultraslow-spreading ocean ridges are fundamentally different from those at faster-spreading ridges, and are key to understanding how crustal accretion varies with spreading rate and magma supply. Ultraslow-spreading ridges provide analogs for ophiolites, to identify those that may have formed under similar conditions. Parallel studies of modern ocean lithosphere and ophiolites therefore can uniquely inform the origin and genesis of both. Here we report the results of structural and petrological studies on the Xigaze ophiolite in the Tibetan Plateau, and compare it to the morphology and deep drilling results at the ultraslow-spreading Southwest Indian Ridge. The Xigaze ophiolite has a complete but laterally discontinuous crust, with discrete diabase dikes/sills cutting both mantle and lower crust. The gabbro units are thin (∼350 m) and show upward cyclic chemical variations, supporting for an episodic and intermittent magma supply. These features are comparable to the highly focused magmatism and low magma budget at modern ultraslow-spreading ridges. Thus we suggest that the Xigaze ophiolite represents an on-land analog of ultraslow-spreading ocean lithosphere.

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