Magma chamber construction and deformation at oceanic spreading centers

The focused emplacement of magmas at spreading ridges represents the primary means by which more than two-thirds of the earth's crust is generated. Yet the mechanisms by which the partially molten lower crust is accreted and deformed are poorly understood. This dissertation presents field and laboratory studies conducted on two ophiolites (Josephine and Oman) and results from participation in drilling of the lower crust of the Southwest Indian Ridge. Together, these studies provide constraints on the modes of crustal construction and deformation that occur at the ridge axis.; Observations from all three “field areas” demonstrate the role that episodic melt migration, magma chamber construction, and deformation may play during the accretion of basaltic magma at the ridge axis. The lower crust of the Josephine ophiolite and the Southwest Indian Ridge, representing intermediate and slow-spreading oceanic crust, respectively, is characterized by multiple mafic intrusions. Recent work in Oman also demonstrates that sills may form a significant portion of the lower crust. Microstructural analyses indicate that extensional deformation in the lower crust of the Josephine and Oman ophiolites was accommodated by hypersolidus flow in a predominantly pure shear environment. In contrast, the lower crust of the Southwest Indian Ridge is dominated by hypersolidus to subsolidus shear zones that formed during bulk simple shear.; These data are interpreted to indicate that (a) construction of the lower crust may occur by emplacement of numerous intrusions ranging in thickness from mm to ∼1 km; (b) deformation and magmatism may occur in cycles in which during periods of higher magma supply, extensional strain is accommodated by hypersolidus flow, whereas during periods of lower magma supply larger magnitudes of extensional strain are accommodated by localized, on-axis faults; (c) the rate of magma supply (i.e., heat) will control the operative deformation mechanisms (i.e., hypersolidus/subsolidus flow, brittle faulting, etc.) at the ridge axis; and (d) during periods of higher magma supply, the presence of localized magma bodies may decouple the stress field at the ridge axis, resulting in partitioning of deformation between the upper and lower crust.