Inverse square-root dependence of mid-ocean-ridge flank roughness on spreading rate

Mid-ocean ridge basalts (MORB) in the South China Sea (SCS) record deep crust-mantle processes during seafloor spreading. We conducted a petrological and geochemical study on the MORBs obtained from the southwest sub-basin of the SCS at site U1433 and U1434 of the International Ocean Discovery Program (IODP) Expedition 349. Results show that MORBs at IODP site U1433 and U1434 are unaffected by seawater alteration, and all U1433 and the bulk of U1434 rocks belong to the sub-alkaline low-potassium tholeiitic basalt series. Samples collected from site U1433 and U1434 are enriched mid-ocean ridge basalts (E-MORBs), and the U1434 basalts are more enriched in incompatible elements than the U1433 samples. The SCS MORBs have mainly undergone the fractional crystallization of olivine, accompanied by the relatively weak fractional crystallization of plagioclase and clinopyroxene during magma evolution. The magma of both sites might be mainly produced by the high-degree partial melting of spinel peridotite at low pressures. The degree of partial melting at site U1434 was lower than at U1433, ascribed to the relatively lower spreading rate. The magmatic source of the southwest sub-basin basalts may be contaminated by lower continental crust and contributed by recycled oceanic crust component during the opening of the SCS.

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Assessing the impact of sedimentation on fault spacing at the Andaman Sea spreading center

Geology ◽

10.1130/g48263.1 ◽

2020 ◽

Author(s):

Clément de Sagazan ◽

Jean-Arthur Olive

Keyword(s):

Active Faults ◽

Spreading Rate ◽

Andaman Sea ◽

Spreading Center ◽

Normal Faults ◽

Mid Ocean Ridge ◽

Plate Separation ◽

Slow Spreading ◽

Ocean Ridge ◽

The Impact

The stabilizing effect of surface processes on strain localization, albeit predicted by several decades of geodynamic modeling, remains difficult to document in real tectonic settings. Here we assess whether intense sedimentation can explain the longevity of the normal faults bounding the Andaman Sea spreading center (ASSC). The structure of the ASSC is analogous to a slow-spreading mid-ocean ridge (MOR), with symmetric, evenly spaced axis-facing faults. The average spacing of faults with throws ≥100 m (8.8 km) is however large compared to unsedimented MORs of commensurate spreading rate, suggesting that sedimentation helps focus tectonic strain onto a smaller number of longer-lived faults. We test this idea by simulating a MOR with a specified fraction of magmatic plate separation (M), subjected to a sedimentation rate (s) ranging from 0 to 1 mm/yr. We find that for a given M ≥ 0.7, increasing s increases fault lifespan by ~50%, and the effect plateaus for s > 0.5 mm/yr. Sedimentation prolongs slip on active faults by leveling seafloor relief and raising the threshold for breaking new faults. The effect is more pronounced for faults with a slower throw rate, which is favored by a greater M. These results suggest that sedimentation-enhanced fault lifespan is a viable explanation for the large spacing of ASSC faults if magmatic input is sufficiently robust. By contrast, longer-lived faults that form under low M are not strongly influenced by sedimentation.

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The spreading rate dependence of three-dimensional mid-ocean ridge gravity structure

Geophysical Research Letters ◽

10.1029/91gl03041 ◽

1992 ◽

Vol 19 (1) ◽

pp. 13-16 ◽

Cited By ~ 208

Author(s):

Jian Lin ◽

Jason Phipps Morgan

Keyword(s):

Three Dimensional ◽

Spreading Rate ◽

Rate Dependence ◽

Mid Ocean Ridge ◽

Ocean Ridge

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Provenance, Stratigraphic Architecture, and Hydrogeologic Influence of Turbidites on the Mid-Ocean Ridge Flank of Northwestern Cascadia Basin, Pacific Ocean

Journal of Sedimentary Research ◽

10.2110/jsr.2005.012 ◽

2005 ◽

Vol 75 (1) ◽

pp. 149-164 ◽

Cited By ~ 48

Author(s):

M. B. Underwood ◽

K. D. Hoke ◽

A. T. Fisher ◽

E. E. Davis ◽

E. Giambalvo ◽

...

Keyword(s):

Pacific Ocean ◽

Stratigraphic Architecture ◽

Mid Ocean Ridge ◽

Ocean Ridge ◽

Ridge Flank

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Mixing-Driven Mean Flows and Submesoscale Eddies over Mid-Ocean Ridge Flanks and Fracture Zone Canyons

Journal of Physical Oceanography ◽

10.1175/jpo-d-19-0174.1 ◽

2020 ◽

Vol 50 (1) ◽

pp. 175-195 ◽

Cited By ~ 3

Author(s):

Xiaozhou Ruan ◽

Jörn Callies

Keyword(s):

Numerical Experiments ◽

Water Mass ◽

Mixing Layers ◽

Global Ocean ◽

Mid Ocean Ridge ◽

Mixing Rates ◽

Ocean Ridge ◽

Ridge Flank ◽

Abyssal Mixing ◽

Water Mass Transformation

AbstractTo close the abyssal overturning circulation, dense bottom water has to become lighter by mixing with lighter water above. This diapycnal mixing is strongly enhanced over rough topography in abyssal mixing layers, which span the bottom few hundred meters of the water column. In particular, mixing rates are enhanced over mid-ocean ridge systems, which extend for thousands of kilometers in the global ocean and are thought to be key contributors to the required abyssal water mass transformation. To examine how stratification and thus diabatic transformation is maintained in such abyssal mixing layers, this study explores the circulation driven by bottom-intensified mixing over mid-ocean ridge flanks and within ridge-flank canyons. Idealized numerical experiments show that stratification over the ridge flanks is maintained by submesoscale baroclinic eddies and that stratification within ridge-flank canyons is maintained by mixing-driven mean flows. These restratification processes affect how strong a diabatic buoyancy flux into the abyss can be maintained, and they are essential for maintaining the dipole in water mass transformation that has emerged as the hallmark of a diabatic circulation driven by bottom-intensified mixing.

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Magnetostratigraphic effects and artifacts of an inverse redox zonation in bottom-up oxygenated East Pacific mid-ocean ridge flank sediments

10.5194/egusphere-egu21-9458 ◽

2021 ◽

Author(s):

Adrian Höfken ◽

Tilo von Dobeneck ◽

Sabine Kasten

Keyword(s):

Pore Water ◽

Planetary Science ◽

Ex Situ ◽

Sediment Geochemistry ◽

Magnetic Minerals ◽

Diagenetic Alteration ◽

Mid Ocean Ridge ◽

Redox Zonation ◽

Ocean Ridge ◽

Ridge Flank

Shipborne ex-situ oxygen measurements in mid-ocean ridge flank sediment cores from the eastern low-latitude North Pacific (Clarion-Clipperton Zone) revealed a downward increase of pore-water oxygen above the sediment-crust interface (Mewes et al., 2016, Kuhn et al., 2017). This inverse redox zonation is caused by an upward diffusion of oxygen (and other solutes) from fluids circulating through the underlying 20 Mio. Year old and still cooling ocean crust. In consequence, these sediments experience a cyclic change in redox-conditions from oxic seafloor conditions at the top through mostly suboxic conditions throughout the sediment column back to oxygen-rich pore water in the last few sediment meters above the rock basement. We studied paleomagnetic records and bulk magnetic properties of three gravity cores from such settings that were collected during RV Sonne expedition SO-240 in 2015 and obtained high-quality magnetostratigraphic records covering the past 3.2 Ma. The generally very good preservation and interpretability of our reversal and RPI records, however, conflicts with a well-defined, but irregular &#8216;ghost event&#8217; of normal polarity within the upper Gilbert reversed C2Ar section. This magnetic polarity and intensity artifact cannot be explained by sediment tectonics, but coincides with the present depth of the lower suboxic-to-oxic redox boundary. Although chemical overprinting could be considered as an obvious explanation of such findings, bulk magnetic analyses (FORCs, thermomagnetics) infer no diagenetic alteration of the magnetic minerals. Over the entire paleomagnetic record, bacterial magnetite appears to be the predominant NRM carrier. We therefore introduce a novel conceptual model of secondary biogenic magnetite formation at crustal depth, hypothesizing that microaerophilic magnetotactic bacteria live and biomineralize not only in the shallow subsurface, but also near the deep oxygen above the sediment-crust interface.&#160;References Mewes, K., Mogoll&#243;n, J.M., Picard, A., R&#252;hlemann, C., Eisenhauer, A., Kuhn, T., Ziebis, W., Kasten, S., 2016. Diffusive transfer of oxygen from seamount basaltic crust into overlying sediments: An example from the Clarion-Clipperton Fracture Zone. Earth and Planetary Science Letters 433, 215-225.Kuhn, T., Versteegh, G.J.M., Villinger, H., Dohrmann, I., Heller, C., Koschinsky, A., Kaul, N., Ritter, S., Wegorzewski, A.V., Kasten, S., 2017. Widespread seawater circulation in 18-22 Ma oceanic crust: Impact on heat flow and sediment geochemistry. Geology 45, 799-802.&#160;&#160;&#160;

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