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

Geology ◽  
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.

On magma supply and spreading modes at slow and ultraslow mid-ocean ridges

2021 ◽  
Author(s):  
Mathilde Cannat
Keyword(s):  
Heat Carrier ◽  
Strain Localization ◽  
Normal Faults ◽  
Axial Region ◽  
Ocean Ridges ◽  
Magma Supply ◽  
Composition And Structure ◽  
Mid Ocean Ridge ◽  
Plate Separation ◽  
Ocean Ridge

<p>The availability of magma is a key to understand mid-ocean ridge tectonics, and specifically the distribution of the two contrasted spreading modes displayed at slow and ultraslow ridges (volcanically-dominated, and detachment fault-dominated). The part of the plate divergence that is not accommodated by magma emplaced as gabbros or basaltic dikes is taken up by normal faults that exhume upper mantle rocks, in many instances all the way to the seafloor. </p><p>Magma is, however, more than just a material that is, or is not, available to fill the gap between two diverging plates. It is the principal carrier of heat into the axial region and as such it may contribute to thin the axial lithosphere, hence diminishing the volume of new plate material formed at each increment of plate separation. Magma as a heat carrier may also, however, if emplaced in the more permeable upper lithosphere, attract and fuel vigorous hydrothermal circulation and contribute instead to overcooling the newly formed upper plate (Cochran and Buck, JGR 2001). </p><p>Magma is also a powerful agent for strain localization in the axial region: magma and melt-crystal mushes are weak; gabbros that crystallize from these melts are weaker than peridotites because they contain abundant plagioclase; and hydrothermally-altered gabbros, and gabbro-peridotite mixtures, are weaker than serpentinites because of minerals such as chlorite and talc. As a result, detachment-dominated ridge regions that receive very little magma probably have a stronger axial lithosphere than detachment-dominated ridge regions that receive a little more magma. </p><p>Because magma has this triple role (building material, heat carrier, and strain localization agent), and because it is highly mobile (through porosity, along permeability barriers, in fractures and dikes), it is likely that variations in magma supply to the ridge, in time and space, and variations in where this magma gets emplaced in the axial plate, cause a greater diversity of spreading modes, and of the resulting slow and ultraslow lithosphere composition and structure, than suggested by the first order dichotomy between volcanically-dominated and detachment-dominated spreading. </p><p>In this talk I illustrate these points using results of recent studies at the Mid-Atlantic and Southwest Indian ridges.</p>

scholarly journals On spreading modes and magma supply at slow and ultraslow mid-ocean ridges

2020 ◽  
Author(s):  
Mathilde Cannat
Keyword(s):  
Heat Carrier ◽  
Strain Localization ◽  
Normal Faults ◽  
Axial Region ◽  
Ocean Ridges ◽  
Magma Supply ◽  
Composition And Structure ◽  
Mid Ocean Ridge ◽  
Plate Separation ◽  
Ocean Ridge

<p> </p><p><span>The availability of magma is a key to understand mid-ocean ridge tectonics, and specifically the distribution of the two contrasted spreading modes displayed at slow and ultraslow ridges (volcanically-dominated, and detachment fault-dominated). The part of the plate divergence that is not accommodated by magma emplaced as gabbros or basaltic dikes is taken up by normal faults that exhume upper mantle rocks, in many instances all the way to the seafloor. </span></p><p><span>Magma is, however, more than just a material that is, or is not, available to fill the gap between two diverging plates. It is the principal carrier of heat into the axial region and as such it may contribute to thin the axial lithosphere, hence diminishing the volume of new plate material formed at each increment of plate separation. Magma as a heat carrier may also, however, if emplaced in the more permeable upper lithosphere, attract and fuel vigorous hydrothermal circulation and contribute instead to overcooling the newly formed upper plate (Cochran and Buck, JGR 2001). </span></p><p><span>Magma is also a powerful agent for strain localization in the axial region: magma and melt-crystal mushes are weak; gabbros that crystallize from these melts are weaker than peridotites because they contain abundant plagioclase; and hydrothermally-altered gabbros, and gabbro-peridotite mixtures, are weaker than serpentinites because of minerals such as chlorite and talc. As a result, detachment-dominated ridge regions that receive very little magma probably have a stronger axial lithosphere than detachment-dominated ridge regions that receive a little more magma. </span></p><p><span>Because magma has this triple role (building material, heat carrier, and strain localization agent), and because it is highly mobile (through porosity, along permeability barriers, in fractures and dikes), it is likely that variations in magma supply to the ridge, in time and space, and variations in where this magma gets emplaced in the axial plate, cause a greater diversity of spreading modes, and of the resulting slow and ultraslow lithosphere composition and structure, than suggested by the first order dichotomy between volcanically-dominated and detachment-dominated spreading. </span></p><p><span>In this talk I illustrate these points using results of recent studies at the Mid-Atlantic and Southwest Indian ridges.</span></p>

Fault spacing enhanced by sedimentation at the Andaman Sea spreading center

2020 ◽  
Author(s):  
Clement de Sagazan ◽  
Jean-Arthur Olive
Keyword(s):  
Sedimentation Rate ◽  
Strain Localization ◽  
Andaman Sea ◽  
Spreading Center ◽  
Normal Faults ◽  
Plate Boundaries ◽  
Spreading Ridge ◽  
Seismic Reflection Data ◽  
Wide Range ◽  
The Impact

<p>Tectonic models commonly predict that erosion and sedimentation enhance strain localization onto a few major faults at subaerial plate boundaries such as orogens and continental rifts. By contrast, the influence of “seafloor-shaping processes” on the tectonic makeup of submarine plate boundaries has received far less attention. Submarine plate boundaries are however subjected to a wide range of sedimentation rates, and as such constitute excellent natural laboratories to investigate the influence of sediment deposition on seafloor shaping tectonics. Here we assess the impact of sedimentation on fault development at the Andaman Sea spreading center (ASSC), by comparing it to unsedimented mid-ocean ridges (MORs) of commensurate spreading rate (38 mm/yr).</p><p>Seafloor spreading has been occurring for the last ~4 Myrs along the ASSC, which is located at the center of a pull-apart basin in the back-arc domain of the Sumatra subduction. Recent bathymetric and seismic reflection data show that fault-induced topography at the ASSC is buried under a sedimentary layer of thickness up to 1.5–2 km. This massive sedimentary input is largely provided by the Irawaddy river, and amounts to an average deposition rate of ~0.5 mm/yr over the last 4 Myrs. The structure of the ASSC is analogous to an intermediate- / slow-spreading MOR, with symmetric, evenly spaced axis-facing normal faults. The characteristic spacing of these faults is however unusually large (8.8 km) and their dips are unusually shallow (~30º) compared to typical MORs.</p><p>We use numerical modeling to assess whether sedimentation can explain the unusual longevity of ASSC normal faults. We use the FLAC method to model a spreading ridge subjected to a sedimentation rate ranging from 0 to 1 mm/yr. In our models, a fraction <em>M </em>of plate separation (between 0.6 and 0.8) is taken up by magma injection. This allows the sequential growth of regularly-spaced, axis-facing faults. In the absence of sedimentation, fault lifespan and spacing decrease with increasing <em>M</em>. We find that, for a given <em>M</em> of 0.7 or above, increasing the sedimentation rate increases fault lifespan by as much as ~50%, and the effect plateaus for rates > 0.5 mm/yr. By contrast, we cannot resolve any significant effect of sedimentation on fault lifespan for <em>M </em>< 0.7. The effect of sedimentation is more pronounced on fault spacing, with rates as fast as 1 mm/yr nearly suppressing the decrease in spacing with increasing <em>M</em>.</p><p>We propose that 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 <em>M</em> fraction. Our simulations show that enhancement of fault lifespan by sediment blanketing is a viable explanation for the anomalously high spacing of normal faults at the ASSC. This could therefore constitute the first field evidence of topographic reworking promoting strain localization at a major plate boundary, a mechanism predicted by over two decades of geodynamic modeling.</p>

scholarly journals Lithogeochemistry of the Mid-Ocean Ridge Basalts near the Fossil Ridge of the Southwest Sub-Basin, South China Sea

Minerals ◽  
2020 ◽  
Vol 10 (5) ◽  
pp. 465 ◽  
Author(s):  
Kai Sun ◽  
Tao Wu ◽  
Xuesong Liu ◽  
Xue-Gang Chen ◽  
Chun-Feng Li
Keyword(s):  
South China Sea ◽  
Partial Melting ◽  
Fractional Crystallization ◽  
South China ◽  
Spreading Rate ◽  
Tholeiitic Basalt ◽  
Spinel Peridotite ◽  
China Sea ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

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.

scholarly journals Constraining the maximum depth of brittle deformation at slow- and ultraslow-spreading ridges using microseismicity

Geology ◽  
2019 ◽  
Vol 47 (11) ◽  
pp. 1069-1073 ◽  
Author(s):  
Ingo Grevemeyer ◽  
Nicholas W. Hayman ◽  
Dietrich Lange ◽  
Christine Peirce ◽  
Cord Papenberg ◽  
...  
Keyword(s):  
Spreading Rate ◽  
Maximum Depth ◽  
Southwest Indian Ridge ◽  
Spreading Center ◽  
Ocean Ridges ◽  
Spreading Ridges ◽  
Brittle Layer ◽  
Full Rate ◽  
Slow Spreading ◽  
The Caribbean

Abstract The depth of earthquakes along mid-ocean ridges is restricted by the relatively thin brittle lithosphere that overlies a hot, upwelling mantle. With decreasing spreading rate, earthquakes may occur deeper in the lithosphere, accommodating strain within a thicker brittle layer. New data from the ultraslow-spreading Mid-Cayman Spreading Center (MCSC) in the Caribbean Sea illustrate that earthquakes occur to 10 km depth below seafloor and, hence, occur deeper than along most other slow-spreading ridges. The MCSC spreads at 15 mm/yr full rate, while a similarly well-studied obliquely opening portion of the Southwest Indian Ridge (SWIR) spreads at an even slower rate of ∼8 mm/yr if the obliquity of spreading is considered. The SWIR has previously been proposed to have earthquakes occurring as deep as 32 km, but no shallower than 5 km. These characteristics have been attributed to the combined effect of stable deformation of serpentinized mantle and an extremely deep thermal boundary layer. In the context of our MCSC results, we reanalyze the SWIR data and find a maximum depth of seismicity of 17 km, consistent with compilations of spreading-rate dependence derived from slow- and ultraslow-spreading ridges. Together, the new MCSC data and SWIR reanalysis presented here support the hypothesis that depth-seismicity relationships at mid-ocean ridges are a function of their thermal-mechanical structure as reflected in their spreading rate.

Longest continuously erupting large igneous province driven by plume-ridge interaction

Geology ◽  
2020 ◽  
Author(s):  
Qiang Jiang ◽  
Fred Jourdan ◽  
Hugo K.H. Olierook ◽  
Renaud E. Merle ◽  
Joanne M. Whittaker
Keyword(s):  
Short Pulse ◽  
Large Igneous Province ◽  
Igneous Provinces ◽  
Mid Ocean Ridge ◽  
Slow Spreading ◽  
Magma Flux ◽  
And Migration ◽  
Main Construct ◽  
Ocean Ridge

Large igneous provinces (LIPs) typically form in one short pulse of ~1–5 Ma or several punctuated ~1–5 Ma pulses. Here, our 25 new 40Ar/39Ar plateau ages for the main construct of the Kerguelen LIP—the Cretaceous Southern and Central Kerguelen Plateau, Elan Bank, and Broken Ridge—show continuous volcanic activity from ca. 122 to 90 Ma, a long lifespan of >32 Ma. This suggests that the Kerguelen LIP records the longest, continuous high-magma-flux emplacement interval of any LIP. Distinct from both short-lived and multiple-pulsed LIPs, we propose that Kerguelen is a different type of LIP that formed through long-term interactions between a mantle plume and mid-ocean ridge, which is enabled by multiple ridge jumps, slow spreading, and migration of the ridge. Such processes allow the transport of magma products away from the eruption center and result in long-lived, continuous magmatic activity.

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