Lower-crustal cracking front at fast-spreading ridges: Evidence from the East Pacific Rise and the Oman Ophiolite

Abstract

The earliest fracturing of lower-crustal gabbros in the Oman ophiolite and the East Pacific Rise occurred in a distributed, microscopic, semibrittle crack network that formed primarily at grain boundaries. The newly created permeability provided conduits that first delivered fluid to the lower crust and drove fluid + rock reactions. The abundance of metamorphic amphibole and modified magmatic plagioclase so produced ranges from trace quantities to >50 modal percent. The initial crack network differs in many respects from previous models. Amphibole and plagioclase compositions show that temperatures of initial cracking are high, ranging from ∼700 °C near the dike-to-gabbro transition to ∼825 °C near the petrologic Moho. An increase in temperatures of initial cracking with depth implies that strain rate increased with depth or that mineralogic controls on rheology or hydrolytic-weakening behavior varied through the crust. Comparison with thermal models shows that the semibrittle cracking front moves away from the ridge axis as it penetrates downward in the oceanic crust. For example, models involving rapid lower crustal cooling imply that the cracking front penetrates the dike-gabbro transition within ∼1 km of the ridge and reaches the Moho within ∼6 km. We suggest that microcracking occurred episodically because continuous advance of a cracking front over these distances implies velocities that are slow compared to reaction rates. The microscopic fracturing scale and inferred low fluid flux suggest that associated heat flow was conductive. These observations provide new insights into the brittle-plastic transition and fluid-rock interaction in the lower oceanic crust formed at fast-spreading ridges.