Spatial and temporal distribution of seismicity along the northern Mid‐Atlantic Ridge (15°–35°N)

Smith, Deborah; Escartı́n, J.; Cannat, Mathilde; Tolstoy, M.; Fox, Christopher G.; Bohnenstiehl, D. R.; Bazin, Sara

doi:10.1029/2002jb001964

Cited by 104 publications

(143 citation statements)

References 69 publications

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“…Minshull et al 2006), thinning in Layer 3 has been associated with reduced magma supply. This explanation would be consistent with the high levels of seismicity along the doomed rift (Smith et al 2003). However, it does not explain the thick crust observed in the gravity model to the west of the failed rift.…”

Section: Kinematics Of the Tammar Propagating Rift 1391supporting

confidence: 70%

“…The propagating segment has a narrow, well-defined, hourglass-shaped axial valley with relatively small, closely spaced abyssal hills, indicative of mainly magmatic spreading. This observation is supported by the presence of numerous small hummocks assumed to be volcanic in origin, as well as an almost complete lack of seismicity in the propagating rift (Smith et al 2003).…”

Section: Kinematics Of the Tammar Propagating Rift 1383supporting

confidence: 62%

“…In contrast, the doomed rift is extremely seismically active (one of the 'seismic stripes' recognised in Smith et al 2003) and has a broad and asymmetric morphology with a poorly defined axial centre. A large part of the spreading in the doomed rift is therefore assumed to be taken up by normal faulting.…”

Section: Kinematics Of the Tammar Propagating Rift 1383mentioning

confidence: 99%

“…3. Also on the right is seismicity (circles) from Smith et al (2003) with only those events with picks on at least five stations shown. The inset map (bottom right) gives an overview of the study location in the North Atlantic Ocean.…”

Section: Kinematics Of the Tammar Propagating Rift 1383mentioning

confidence: 99%

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Crustal structure and kinematics of the TAMMAR propagating rift system on the Mid-Atlantic Ridge from seismic refraction and satellite altimetry gravity

Kahle¹,

Tilmann

Grevemeyer

2016

Geophys. J. Int.

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: Crustal structure and kinematics of the TAMMAR propagating rift system on the Mid-Atlantic Ridge from seismic refraction and satellite altimetry gravity. -Geophysical Journal International, 206, 2, pp. 1382-1397 S U M M A R YThe TAMMAR segment of the Mid-Atlantic Ridge forms a classic propagating system centred about two degrees south of the Kane Fracture Zone. The segment is propagating to the south at a rate of 14 mm yr −1 , 15 per cent faster than the half-spreading rate. Here, we use seismic refraction data across the propagating rift, sheared zone and failed rift to investigate the crustal structure of the system. Inversion of the seismic data agrees remarkably well with crustal thicknesses determined from gravity modelling. We show that the crust is thickened beneath the highly magmatic propagating rift, reaching a maximum thickness of almost 8 km along the seismic line and an inferred (from gravity) thickness of about 9 km at its centre. In contrast, the crust in the sheared zone is mostly 4.5-6.5 km thick, averaging over 1 km thinner than normal oceanic crust, and reaching a minimum thickness of only 3.5 km in its NW corner. Along the seismic line, it reaches a minimum thickness of under 5 km. The PmP reflection beneath the sheared zone and failed rift is very weak or absent, suggesting serpentinisation beneath the Moho, and thus effective transport of water through the sheared zone crust. We ascribe this increased porosity in the sheared zone to extensive fracturing and faulting during deformation. We show that a bookshelf-faulting kinematic model predicts significantly more crustal thinning than is observed, suggesting that an additional mechanism of deformation is required. We therefore propose that deformation is partitioned between bookshelf faulting and simple shear, with no more than 60 per cent taken up by bookshelf faulting.

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Section: Kinematics Of the Tammar Propagating Rift 1391supporting

confidence: 70%

Section: Kinematics Of the Tammar Propagating Rift 1383supporting

confidence: 62%

Section: Kinematics Of the Tammar Propagating Rift 1383mentioning

confidence: 99%

Section: Kinematics Of the Tammar Propagating Rift 1383mentioning

confidence: 99%

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Crustal structure and kinematics of the TAMMAR propagating rift system on the Mid-Atlantic Ridge from seismic refraction and satellite altimetry gravity

Kahle¹,

Tilmann

Grevemeyer

2016

Geophys. J. Int.

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“…For example, shock-aftershock sequences generated along steep normal faults in the flanks of the MAR may result in elevated rates of hydrophone events along symmetrical segments, such as those near 24.25°N 12 , which are not observed in the longer-term teleseismic data (Figures 2b-c). Complex tectonic processes at segment ends and oblique zones can also significantly increase seismicity, even if segments show little or no seismicity at their centres (Figures 1b-c) 14 .Comparison of microseismicity results with regional seismic patterns allows us to constrain the nature of the zones of elevated and sustained seismicity. An 8-month microseismic experiment at the TAG hydrothermal field (26.1°N) shows deformation along the active detachment fault accommodated by continuous creep associated with steady hydroacoustic event rates (Figure 1d).…”

mentioning

confidence: 99%

Central role of detachment faults in accretion of slow-spreading oceanic lithosphere

Escartı́n

Smith

Cann

et al. 2008

Nature

421

374

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Enhanced seismicity is probably generated along detachment faults accommodating a sizeable proportion of the total plate separation. In contrast, symmetrical segments have lower levels of seismicity, which concentrates primarily at their ends. Basalts erupted along asymmetrical segments have compositions that are consistent with crystallization at higher pressures than basalts from symmetrical segments, and with lower extents of partial melting of the mantle. The large fields of detachment surfaces recently identified in oceanic crust formed along the slow spreading MAR and ultra-slow spreading South-West Indian Ridge (SWIR) 3,6 demonstrate the involvement of these structures in the accretion of a larger portion of the oceanic lithosphere than previously inferred from seafloor corrugated planes alone 7 . The resulting seafloor morphology and lithospheric structure on the flanks of the ridge axis are strongly asymmetrical 3 and differ from the more regular and roughly symmetrical axis-parallel abyssal hill fabric believed to characterize 'normal' slow-spreading seafloor. The abyssal hill morphology is caused by ridge-parallel, highangle faulting of volcanic seafloor 8 (Figures 1a-c). In contrast, detachment-related terrain is caused by long-lived steep, normal faults initiated beneath the rift valley floor that rotate to low angles as their footwalls are exposed 7,8 . Distinctive narrow ridges with steep outward-facing slopes that are often curved in plan view develop near exposed detachments at the seafloor, and bound deep swales 7 (Figures 1d-e), producing blocky and chaotic terrain 7,9 . The asymmetric nature of accretion in the presence of detachments is also observed in the overall lithospheric structure, composition and geophysical character wherever data are available 3,4,10 . The MAR lacks the broad ridges only found along the melt-poor SWIR, likely a manifestation of detachment faulting that is different from striated fault planes and associated structure 3 . There is an excellent correlation between mode of accretion and seismicity at the ridge axis. This section of the MAR was hydroacoustically monitored between January 1999 and September 2003 11 . The hydroacoustic catalogue is complementary to the >30 year teleseismic catalogue, as it records smaller magnitude events (magnitude of completeness of 3 and 5, respectively 12 ), over a shorter period of time (<5 vs. >30 years). Both seismic catalogues show that detachment-dominated, asymmetrical ridge sections host ~75% more hydroacoustic events and ~65% more teleseismic events than 4 symmetrical segments (Figure 2b and c). The concentration of seismicity at segments shown to have active detachment faults (Figures 1d-e), such as the Logachev massif south of the Fifteen-Twenty Fracture zone and the TAG detachment fault near 26°N 6,7,13 , is thus a general pattern. Active detachments also control the zones of sustained seismicity, which lack shock-aftershock sequences that were previously identified along the northern MAR 14 . Differences between the hyd...

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Tectonic structure, lithology, and hydrothermal signature of the Rainbow massif (Mid-Atlantic Ridge 36°14′N)

Andréani

Escartı́n

Delacour

et al. 2014

Geochem. Geophys. Geosyst.

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Rainbow is a dome-shaped massif at the 36 14 0 N nontransform offset along the Mid-AtlanticRidge. It hosts three ultramafic-hosted hydrothermal sites: Rainbow is active and high temperature; Clamstone and Ghost City are fossil and low temperature. The MoMARDREAM cruises (2007, 2008) presented here provided extensive rock sampling throughout the massif that constrains the geological setting of hydrothermal activity. The lithology is heterogeneous with abundant serpentinites surrounding gabbros, troctolites, chromitites, plagiogranites, and basalts. We propose that a W dipping detachment fault, now inactive, uplifted the massif and exhumed these deep-seated rocks. Present-day deformation is accommodated by SSW-NNE faults and fissures, consistent with oblique teleseismic focal mechanisms and stress rotation across the discontinuity. Faults localize fluid flow and control the location of fossil and active hydrothermal fields that appear to be ephemeral and lacking in spatiotemporal progression. Markers of high-temperature hydrothermal activity (350 C) are restricted to some samples from the active field while a more diffuse, lower temperature hydrothermal activity (<220 C) is inferred at various locations through anomalously high As, Sb, and Pb contents, attributed to element incorporation in serpentines or microscalesulfide precipitation. Petrographic and geochemical analyses show that the dominant basement alteration is pervasive peridotite serpentinization at 160-260 C, attributed to fluids chemically similar to those venting at Rainbow, and controlled by concomitant alteration of mafic-ultramafic units at depth. Rainbow provides a model for fluid circulation, possibly applicable to hydrothermalism at oceanic detachments elsewhere, where both low-temperature serpentinization and magmatic-driven high-temperature outflow develop contemporaneously, channeled by faults in the footwall and not along the detachment fault.

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Spatial and temporal distribution of seismicity along the northern Mid‐Atlantic Ridge (15°–35°N)

Cited by 104 publications

References 69 publications

Crustal structure and kinematics of the TAMMAR propagating rift system on the Mid-Atlantic Ridge from seismic refraction and satellite altimetry gravity

Crustal structure and kinematics of the TAMMAR propagating rift system on the Mid-Atlantic Ridge from seismic refraction and satellite altimetry gravity

Central role of detachment faults in accretion of slow-spreading oceanic lithosphere

Tectonic structure, lithology, and hydrothermal signature of the Rainbow massif (Mid-Atlantic Ridge 36°14′N)

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