Seafloor at fast-spreading centers y5 is often concentrated

Seafloor spreading is a process
which forms oceanic crust and occurs at underwater mountainous region which is
known as mid-ocean ridges or spreading centers.  y1 Spreading centers are situated at
the boundary between two plates that are moving apart, which are defined as
divergent plate boundaries. Here, magma from mantle rise up and when
reaches the water it is cool and makes new oceanic crust.y2 
The
difference between fast spreading centers and slow spreading centers is that in
fast centers valley rifts are narrow and are surrounded by flat topography but
in slow spreading centers, valley rifts are large and wide and are encompassed
with rugged train.   y3 

y4  Magmatic accretion of oceanic crust at fast-spreading
centers y5 is often concentrated
in a narrow zone beneath the ridge axisy6 ,
where an
axial melt lens or axial magma chamber is situated on the top of partially
molten zone of lower crust which is specified by
high attenuation and low velocity.y7 y8 
This kind of magmatic accretion is called axial or on-axis magmatism and have
been studied in various research (Han et al., 2014;
Dunn et al., 2000; Kent et al., 1993). On the other hand, we can find another kind
of magmatic accretion which is known as off-axis magmatism. In this type, melt
sills are located above partial melt zone but are outside of axial zone or
ridge axis.y9 
Several
kinds of off-axis magmatic accretion have been reported. Garmany (1989) mentioned the presence of melt
sills 22 kilometer far from ridge axis at 12 N on the Earth Pacific Rise (EPR).
Crawford and web (2002) discovered a near Moho
magma lens and a low velocity zone in the lower crust 10 km east of the ridge
axis at 9 48′ and 2.5 km east of
the ridge axis at 9 08′ on EPR. 9 20′ N and 20 km east of the ridge
axis, a melt body underlain by low velocity crust was reported by Doorant and Toomy (2009) based on high seismic
attenuation, p to s conversions and p-wave refractions. Young lavas with
various composition have been discovered 30 km far from the ridge axis and on
the ridge flanks (Goldstein et al., 1994;Perfit
et al., 1994; Sims et al., 2003; Turner et al., 2011; Zouet al., 2002)y10Han2014-page3

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