Mass transfer into the leading edge of the mantle wedge: Initial results
from Oman Drilling Project Hole BT1B
Abstract
This paper provides an overview of research on core from Oman Drilling
Project Hole BT1B and the surrounding area, plus new data and
calculations, constraining processes in the Tethyan subduction zone
beneath the Samail ophiolite. The area is underlain by gently dipping,
broadly folded layers of allochthonous Hawasina pelagic sediments, the
metamorphic sole of the Samail ophiolite, and Banded Unit peridotites at
the base of the Samail mantle section. Despite reactivation of some
faults during uplift of the Jebel Akdar and Saih Hatat domes, the area
preserves the tectonic “stratigraphy” of the Cretaceous subduction
zone. Gently dipping listvenite bands, parallel to peridotite banding
and to contacts between the peridotite and the metamorphic sole, replace
peridotite at and near the basal thrust. Listvenites formed at less than
200°C and (poorly constrained) depths of 25 to 40 km by reaction with
CO2-rich, aqueous fluids migrating from greater depths,
derived from devolatilization of subducting sediments analogous to
clastic sediments in the Hawasina Formation, at 400-500°. Such processes
could form important reservoirs for subducted CO2.
Listvenite formation was accompanied by ductile deformation of
serpentinites and listvenites – perhaps facilitated by fluid-rock
reaction – in a process that could lead to aseismic subduction in some
regions. Addition of H2O and CO2 to the
mantle wedge, forming serpentinites and listvenites, caused large
increases in the solid mass and volume of the rocks. This may have been
accommodated by fractures formed as a result of volume changes, perhaps
mainly at a serpentinization front.