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.

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.

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.

scholarly journals Mantle rock exposures at oceanic core complexes along mid-ocean ridges

Geologos ◽  
2015 ◽  
Vol 21 (4) ◽  
pp. 207-231 ◽  
Author(s):  
Jakub Ciazela ◽  
Juergen Koepke ◽  
Henry J.B. Dick ◽  
Andrzej Muszynski
Keyword(s):  
Partial Melting ◽  
Geophysical Methods ◽  
Ex Situ ◽  
Crustal Rocks ◽  
New Class ◽  
Ocean Ridges ◽  
Common Structure ◽  
Spreading Ridges ◽  
Slow Spreading ◽  
Lower Crustal

Abstract The mantle is the most voluminous part of the Earth. However, mantle petrologists usually have to rely on indirect geophysical methods or on material found ex situ. In this review paper, we point out the in-situ existence of oceanic core complexes (OCCs), which provide large exposures of mantle and lower crustal rocks on the seafloor on detachment fault footwalls at slow-spreading ridges. OCCs are a common structure in oceanic crust architecture of slow-spreading ridges. At least 172 OCCs have been identified so far and we can expect to discover hundreds of new OCCs as more detailed mapping takes place. Thirty-two of the thirty-nine OCCs that have been sampled to date contain peridotites. Moreover, peridotites dominate in the plutonic footwall of 77% of OCCs. Massive OCC peridotites come from the very top of the melting column beneath ocean ridges. They are typically spinel harzburgites and show 11.3–18.3% partial melting, generally representing a maximum degree of melting along a segment. Another key feature is the lower frequency of plagioclase-bearing peridotites in the mantle rocks and the lower abundance of plagioclase in the plagioclase-bearing peridotites in comparison to transform peridotites. The presence of plagioclase is usually linked to impregnation with late-stage melt. Based on the above, OCC peridotites away from segment ends and transforms can be treated as a new class of abyssal peridotites that differ from transform peridotites by a higher degree of partial melting and lower interaction with subsequent transient melt.

Unveiling the volcanic evolution of the Mohns Ridge

2021 ◽  
Author(s):  
Håvard Stubseid ◽  
Anders Bjerga ◽  
Haflidi Haflidason ◽  
Rolf Birger Pedersen
Keyword(s):  
Autonomous Underwater Vehicles ◽  
Volcanic Eruptions ◽  
Integrated Approach ◽  
Hydrothermal Systems ◽  
Ocean Ridges ◽  
Spreading Ridges ◽  
Geological Evolution ◽  
Fast Spreading ◽  
Slow Spreading ◽  
Resolution Dataset

<p>Volcanic eruptions are far less common along slow-spreading ridges compared to fast-spreading ridges. Consequently, knowledge of the volcanic rejuvenation along close to 1/3 of the global mid-ocean ridges is poorly constrained. To determine the temporal evolution of the rift valley of one of the slowest spreading-ridges in the world, the Mohns Ridge in the Norwegian-Greenland Sea, we have interpreted more than 3000 km of sub-bottom profiles. Sedimentation rates derived from several core locations along the ridge are used to calculate the age of the underlying volcanic crust. Here we present a framework for understanding the geological evolution of rift valleys of slow-spreading ridges using an integrated approach combining geological and geophysical data. The high-resolution dataset acquired using autonomous underwater vehicles, cover more than 50% of the 575 km long Mohns Ridge. The results unravel large variation in sediment thickness inside the central rift area, from exposed basalts to several meters of sediments, within only a few hundreds of meters. Studied sub-bottom profiles reveal active volcanism in the deepest parts of the ridge, areas thought to be inactive, surrounded by significantly older crust covered in meters of sediments. We find that all axial volcanic ridge systems (AVRs) in our area completely renewed their surface within the last 30-50 ka. Detailed volcanological investigation of the central parts of an AVR reveal at least 72 individual eruptions during the last 20 ka ranging in size from 1.2x10<sup>3 </sup>m<sup>2</sup> - 2.6 x10<sup>5</sup> m<sup>2</sup>. These estimates have been verified with visual observations and sampling using an ROV. Our estimates indicate that more than 230 eruptions are required to renew the surface of an average AVR. Based on the acquired age assessments a volcanic eruption is anticipated to occur approximately every 200 years. Volcanic renewal is a first order control on the lifetime of magmatically driven hydrothermal systems.</p>

scholarly journals Magma Reservoir Formation and Evolution at a Slow-Spreading Center (Atlantis Bank, Southwest Indian Ridge)

2020 ◽  
Vol 8 ◽  
Author(s):  
Marine Boulanger ◽  
Lydéric France ◽  
Jeremy R.L. Deans ◽  
Carlotta Ferrando ◽  
C. Johan Lissenberg ◽  
...  
Keyword(s):  
Magma Reservoir ◽  
Southwest Indian Ridge ◽  
Spreading Center ◽  
Atlantis Bank ◽  
Slow Spreading ◽  
Formation And Evolution ◽  
Indian Ridge

scholarly journals 13 million years of seafloor spreading throughout the Red Sea Basin

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Nico Augustin ◽  
Froukje M. van der Zwan ◽  
Colin W. Devey ◽  
Bryndís Brandsdóttir
Keyword(s):  
Red Sea ◽  
Gravity Data ◽  
Ocean Basin ◽  
Gravity Gradient ◽  
Ocean Ridges ◽  
Spreading Ridges ◽  
Vertical Gravity Gradient ◽  
Slow Spreading ◽  
Vertical Gravity ◽  
High Resolution Bathymetry

AbstractThe crustal and tectonic structure of the Red Sea and especially the maximum northward extent of the (ultra)slow Red Sea spreading centre has been debated—mainly due to a lack of detailed data. Here, we use a compilation of earthquake and vertical gravity gradient data together with high-resolution bathymetry to show that ocean spreading is occurring throughout the entire basin and is similar in style to that at other (ultra)slow spreading mid-ocean ridges globally, with only one first-order offset along the axis. Off-axis traces of axial volcanic highs, typical features of (ultra)slow-spreading ridges, are clearly visible in gravity data although buried under thick salt and sediments. This allows us to define a minimum off-axis extent of oceanic crust of <55 km off the coast along the complete basin. Hence, the Red Sea is a mature ocean basin in which spreading began along its entire length 13 Ma ago.

scholarly journals Microseismicity and lithosphere thickness at a nearly amagmatic mid-ocean ridge

2021 ◽  
Author(s):  
Jie Chen ◽  
Wayne Crawford ◽  
Mathilde Cannat
Keyword(s):  
Hanging Wall ◽  
Oceanic Lithosphere ◽  
Spreading Rate ◽  
Southwest Indian Ridge ◽  
Flip Flop ◽  
Detachment Faults ◽  
Ocean Ridges ◽  
Tectonic Earthquakes ◽  
Mid Atlantic Ridge ◽  
Ocean Ridge

Abstract Successive flip-flop detachment faults in a nearly-amagmatic region of the ultraslow-spreading Southwest Indian Ridge (SWIR) at 64°30'E accommodate ~100% of plate divergence, with mostly ultramafic seafloor. As magma is the main heat carrier to the oceanic lithosphere, the nearly-amagmatic SWIR 64°30'E is expected to have a very thick lithosphere. Here, our microseismicity data shows a 15-km thick seismogenic lithosphere, actually thinner than the more magmatic SWIR Dragon Flag detachment with the same spreading rate. This challenges current models of how spreading rate and melt supply control the thermal regime of mid-ocean ridges. Microearthquakes with normal focal mechanisms are colocated with seismically imaged damage zones of the detachment and reveal hanging-wall normal faulting, which is not seen at more magmatic detachments at the SWIR or the Mid-Atlantic Ridge. We also document a two-day seismic swarm, interpret as caused by an upward-migrating melt intrusion in the detachment footwall (6-11 km), triggering a sequence of shallower (~1.5 km) tectonic earthquakes in the detachment fault plane. This points to a possible link between sparse magmatism and tectonic failure at melt-poor ultraslow ridges.

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