COLD CRUST IN A HOT-SPOT

Shear waves from locally-recorded Icelandic earthquakes exhibit very little anelastic attenuation, even though they turn in 0-5 Ma lower (15-25 km) crust just above the Moho. Lower crustal shear wave quality factors are about Q=150-225 for paths that cross central Iceland and about Q=300 for paths in southwestern Iceland, values that indicate that the temperature of the lower crust is at least 250-325-degrees-C below the solidus of gabbro (that is, about 875-950-degrees-C, based on published experimental studies of the effect of temperature on shear wave attenuation in gabbro). These low temperatures are not consistent with existing models of crustal accretion, which predict partial melt at lower crustal depths.