COLD CRUST IN A HOT-SPOT

Shear waves from locally-recorded Icelandic earthquakes exhibit very l ittle anelastic attenuation, even though they turn in 0-5 Ma lower (15 -25 km) crust just above the Moho. Lower crustal shear wave quality fa ctors are about Q=150-225 for paths that cross central Iceland and abo ut 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 pub lished experimental studies of the effect of temperature on shear wave attenuation in gabbro). These low temperatures are not consistent wit h existing models of crustal accretion, which predict partial melt at lower crustal depths.