sgr ms thesis - University of Maine

thinned lithosphere allows for partial melt. The partially melted lithosphere
becomes weakened and allows segregation and transport of the melt fraction
(e.g., Petford et al., 2000; Jackson et al., 2003; Bergantz and Barboza, 2005;
Aizawa et al., 2006). Asthenospheric melt occurs directly below the volcanic front
of the convergent margin, while lithospheric melting occurs below back-arc
extensional basins. Melts from both sources can undergo significant crystal
fractionation, producing chemically evolved magmas (e.g. Johnson and Jin,
2009).
The density anomaly produced by lithospheric melting provides a
buoyancy potential for the generated magma, which then rises upward into the
lithosphere. The dominant mechanism of magma transport is debatable.
Traditionally, diapirism was used to explain the mass transport of large igneous
bodies. This explanation is flawed for upper crustal emplacement because the
driving force of buoyancy cannot surpass the strength of brittle upper crustal
rock. Also, the large amounts of thermal energy spent on weakening the wall
rock to allow ascension reduces the potential magma transport distance before
solidification (Clemens and Mawer, 1992; Petford et al., 2000; Aizawa et al.,
2006). Diapirism may be plausible in the lower lithosphere where wall rock can
deform under viscous strain to accommodate ascent of large magmatic bodies
(Weinberg and Podladchikov, 1994), but magma transport by dike propagation is
thought to be the dominant mechanism for transport through much of the mid to
upper lithosphere (e.g., Gudmundsson et al., 1999; Accocella et al., 2006;
Johnson and Jin, 2009).
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