Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere

Along ultraslow-spreading ridges, where oceanic tectonic plates drift very slowly apart, conductive cooling is thought to limit mantle melting and melt production has been inferred to be highly discontinuous. Along such spreading centres, long ridge sections without any igneous crust alternate with magmatic sections that host massive volcanoes capable of strong earthquakes. Hence melt supply, lithospheric composition and tectonic structure seem to vary considerably along the axis of the slowest-spreading ridges. However, owing to the lack of seismic data, the lithospheric structure of ultraslow ridges is poorly constrained. Here we describe the structure and accretion modes of two end-member types of oceanic lithosphere using a detailed seismicity survey along 390 kilometres of ultraslow-spreading ridge axis. We observe that amagmatic sections lack shallow seismicity in the upper 15 kilometres of the lithosphere, but unusually contain earthquakes down to depths of 35 kilometres. This observation implies a cold, thick lithosphere, with an upper aseismic zone that probably reflects substantial serpentinization. We find that regions of magmatic lithosphere thin dramatically under volcanic centres, and infer that the resulting topography of the lithosphere–asthenosphere boundary could allow along-axis melt flow, explaining the uneven crustal production at ultraslow-spreading ridges. The seismicity data indicate that alteration in ocean lithosphere may reach far deeper than previously thought, with important implications towards seafloor deformation and fluid circulation.

[1]  Mathilde Cannat,et al.  Regional seismicity of the Mid-Atlantic Ridge: observations from autonomous hydrophone arrays , 2010 .

[2]  V. Schlindwein Teleseismic earthquake swarms at ultraslow spreading ridges: indicator for dyke intrusions? , 2012 .

[3]  J. Gutt The expedition of the research vessel "Polarstern" to the Antarctic in 2013 (ANT-XXIX/3) , 2013 .

[4]  Urs Kradolfer,et al.  Initial reference models in local earthquake tomography , 1994 .

[5]  W. Jokat The expedition of the research vessel "Polarstern" to the Antarctic in 2013 (ANT-XXIX/5) , 2013 .

[6]  J. Escartín,et al.  Modes of seafloor generation at a melt-poor ultraslow-spreading ridge , 2006 .

[7]  B. Evans,et al.  Strength of slightly serpentinized peridotites: Implications for the tectonics of oceanic lithosphere , 2001 .

[8]  B. Reynard,et al.  Creep of phyllosilicates at the onset of plate tectonics , 2012 .

[9]  P. Molnar,et al.  Focal depths of intracontinental and intraplate earthquakes and their implications for the thermal and mechanical properties of the lithosphere , 1983 .

[10]  M. Cannat,et al.  The Ultraslow Spreading Southwest Indian Ridge , 2013 .

[11]  C. Langmuir,et al.  Discovery of abundant hydrothermal venting on the ultraslow-spreading Gakkel ridge in the Arctic Ocean , 2003, Nature.

[12]  P. Michael,et al.  MORB generation beneath the ultraslow spreading Southwest Indian Ridge (9–25°E): Major element chemistry and the importance of process versus source , 2008 .

[13]  D. L. Anderson Lithosphere, asthenosphere, and perisphere , 1995 .

[14]  C. Langmuir,et al.  Magmatic and amagmatic seafloor generation at the ultraslow-spreading Gakkel ridge, Arctic Ocean , 2003, Nature.

[15]  R. White,et al.  Crustal structure of the Southwest Indian Ridge at 66°E: seismic constraints , 2006 .

[16]  M. Cannat Emplacement of mantle rocks in the seafloor at mid‐ocean ridges , 1993 .

[17]  M. Cannat,et al.  Spreading rate, spreading obliquity, and melt supply at the ultraslow spreading Southwest Indian Ridge , 2008 .

[18]  H. Schouten,et al.  An ultraslow-spreading class of ocean ridge , 2003, Nature.

[19]  M. Pilkington,et al.  EMAG2: A 2–arc min resolution Earth Magnetic Anomaly Grid compiled from satellite, airborne, and marine magnetic measurements , 2009 .

[20]  S. Singh,et al.  Along‐axis variation in crustal thickness at the ultraslow spreading Southwest Indian Ridge (50°E) from a wide‐angle seismic experiment , 2015 .

[21]  Deborah K. Smith,et al.  Central role of detachment faults in accretion of slow-spreading oceanic lithosphere , 2008, Nature.

[22]  W. Jokat,et al.  Seismic gap beneath Logachev Seamount: Indicator for melt focusing at an ultraslow mid‐ocean ridge? , 2013 .

[23]  W. Jokat,et al.  Crustal thickness and earthquake distribution south of the Logachev Seamount, Knipovich Ridge , 2012 .

[24]  H. Fujimoto,et al.  Melt supply variations to a magma‐poor ultra‐slow spreading ridge (Southwest Indian Ridge 61° to 69°E) , 2003 .

[25]  L. Matias,et al.  Seismological constraints on the thermal structure along the Lucky Strike segment (Mid-Atlantic Ridge) and interaction of tectonic and magmatic processes around the magma chamber , 2009 .

[26]  M. Cannat,et al.  Magnetization of 0–26.5 Ma seafloor at the ultraslow spreading Southwest Indian Ridge, 61°–67°E , 2008 .

[27]  R. White,et al.  Variation with spreading rate of oceanic crustal thickness and geochemistry , 1994 .

[28]  W. J. Morgan,et al.  A nonlinear rheology model for mid‐ocean ridge axis topography , 1990 .

[29]  S. Humphris,et al.  Kinematics and geometry of active detachment faulting beneath the Trans-Atlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge , 2007 .

[30]  L. Montési,et al.  Controls on melt migration and extraction at the ultraslow Southwest Indian Ridge 10°–16°E , 2011 .

[31]  B. Reynard,et al.  Pressure-temperature estimates of the lizardite/antigorite transition in high pressure serpentinites , 2013 .

[32]  N. Sleep,et al.  Effect of latent heat of freezing on crustal generation at low spreading rates , 2014 .

[33]  R. White,et al.  Oceanic crustal thickness from seismic measurements and rare earth element inversions , 1992 .

[34]  L. Montési,et al.  Mantle flow and melting underneath oblique and ultraslow mid‐ocean ridges , 2007 .

[35]  K. Priestley,et al.  Thermal structure of oceanic and continental lithosphere , 2005 .

[36]  M. Cannat,et al.  Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years , 2013 .

[37]  M. Cannat How thick is the magmatic crust at slow spreading oceanic ridges , 1996 .

[38]  Johannes Schweitzer,et al.  HYPOSAT – An Enhanced Routine to Locate Seismic Events , 2001 .

[39]  Mathilde Cannat,et al.  Serpentinization of mantle‐derived peridotites at mid‐ocean ridges: Mesh texture development in the context of tectonic exhumation , 2014 .

[40]  M. Cannat,et al.  Formation of the axial relief at the very slow spreading Southwest Indian Ridge (49° to 69°E) , 1999 .