M. Obata et S. Karato, ULTRAMAFIC PSEUDOTACHYLITE FROM THE BALMUCCIA PERIDOTITE, IVREA VERBANO ZONE, NORTHERN ITALY, Tectonophysics, 242(3-4), 1995, pp. 313-328
This paper reports an occurrence of ultramafic pseudotachylite from a
spinel Iherzolite mass in the Ivrea-Verbano zone, northern Italy. The
pseudotachylite occurs as discordant dikes and veins crosscutting prim
ary structures of the peridotite and as thin (similar to 1 cm) layers
in the middle of shear zones. The pseudotachylite has an ultramafic co
mposition similar to the host peridotite. In thin section it consists
of fine-grained euhedral olivine and clinopyroxene crystals densely pa
cked in a glassy matrix with minor amounts of elastic olivine grains a
nd lithic mylonitic clasts. Microstructural observations on the olivin
es using TEM yields evidence for deformation in the wall rock and oliv
ine clasts at a differential stress level of similar to 300 MPa. Petro
logical analysis of the host rock and pseudotachylite combined with an
energy calculation for shear heating suggests that the pseudotachylit
e originated by a near total melting of the host peridotite followed b
y rapid crystallization on a time scale of 100 s or less, conditions c
onsistent with an earthquake. The presence of an ultramafic pseudotach
ylite implies a very significant shear heating due presumably to a hig
h shear stress on the fault plane. Faulting in an ultramafic rock asso
ciated with melting will result in a large stress drop and resultant l
arge moment release, because resistance to sliding will almost complet
ely be lost upon melting due to the very low viscosity of ultramafic m
elts.