Ductile deformation and the origin of layered gabbro in ophiolites

Abstract

Layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3–6 km deep. Although large, steady state magma chambers provide a possible explanation for the limited diversity of mid‐ocean ridge basalts, their existence is not supported by geophysical observations. Sinton and Detrick (1992) recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small (1–4 km wide) magma chambers perched on large (∼8 km wide) zones of low‐velocity material, which are interpreted to consist of partially molten crystal cumulates. In this paper, we compare the geology of the gabbroic rocks of the Samail ophiolite, which is thought to have formed by fast spreading, with results of a finite element analysis of the Sinton‐Detrick model for fast spreading centers. Our results demonstrate that flow, similar to that postulated by Sleep [1975], within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the Samail gabbros. As a consequence of this deformation, (1) layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward the spreading center, (2) the gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime, let alone the magma chamber; (3) feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering, but those that invade farther from the ridge will be less deformed and retain crosscutting relationships; and (4) total deviatoric strain should increase down section toward the Moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros. Upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in the gabbros and help maintain the magma chamber in a compositional steady state.

Expedition 335 summary

“…Black arrows show the movement of the solid lower crust; blue arrows show the dominant zones where hydrothermal circulation will remove latent and sensible heat; red arrows show the movement of magma-this is unknown in all models. A. Gabbro glacier ductile flow model (e.g., Henstock et al, 1993;Phipps Morgan and Chen, 1993;Quick and Denlinger, 1993). Ductile flow down and outward from a high-level axial magma chamber constructs the lower crust.…”

Root zone of the sheeted dike complex in the Oman ophiolite

“…It is where the floor and the roof of the melt lens were squeezed together when the melt lens closed, due to drifting away from the ridge axis. After settling on the floor of the lens, the gabbro mush subsided within the magma chamber and rotated, developing a steep foliation which was frozen when this mush solidified as a gabbro and drifted out of the magma chamber [Quick and Denlinger, 1993;Chenevez et al, 1998] (Figure 13a). The uppermost, crude and steep foliation taken as the top of the RZSDC has been developed right at the limit of the melt lens.…”

Formation of fast-spreading lower oceanic crust as revealed by a new Mg–REE coupled geospeedometer

“…Many crustal formation models have been proposed based on petrological observations, deformation structures, and geodynamic simulations at various ocean ridge settings and ophiolites (e.g., Nicolas et al, 1988;Phipps Morgan and Chen, 1993;Quick and Denlinger, 1993;Kelemen et al, 1997;Dick et al, 2008). Among these, two end-member models for fast spreading mid-ocean ridges are actively debating whether cumulates of the lower oceanic crust crystalize mainly in a shallow melt lens (e.g., Phipps Morgan and Chen, 1993;Quick and Denlinger, 1993;Coogan et al, 2007;Faak et al, 2015) or in situ solidify largely from magma sills at various depths (e.g., Nicolas et al, 1988;Kelemen et al, 1997;Lissenberg et al, 2004;Maclennan et al, 2005;VanTongeren et al, 2008VanTongeren et al, , 2015Natland and Dick, 2009). The former postulates much slower cooling in the deeper crust (i.e., near-conductive cooling), whereas the latter necessitates efficient heat removal by hydrothermal circulations throughout the entire crust.…”