The primary structures and fabrics of the upper mantle and lower oceanic crust from ophiolite complexes

Abstract

The structures and textures of areas from the lower sequences of the Oman Ophiolite, the Troodos Ophiolite, Cyprus, and the Shetland Ophiolite have been studies in order to gain an understanding of the structural and kinematic history of the upper mantle and lowermost oceanic crust at accretionary margins. In all areas the mantle has similar petrological features. It is composed largely of harzburgite which is often compositionally banded into orthopyroxene-rich and olivine-rich segregation layers. Dunitic sheets and pods are common in some of the areas studied. They are highly variable in size and shape (up to 1 1/2km diameter in Troodos) and some contain concentrations of chromite ore. The mantle is strongly foliated in all areas, the foliations being defined by the alignment of chrome-spinel and orthopyroxene grains. The foliations cross-cut all the mantle petrological features except for a few late-stage dykes, and in some areas extend into the lowermost crustal cumulates. Lineations are sometimes measurable on the foliation planes. Emplacement of the different ophiolite sequences has caused only localised shearing and brittle faulting, these structures being clearly identifiable. from the primary, ocean-spreading related foliations. In some areas two primary foliations (S1 and S2) are measurable from the same outcrop. A detailed crystallographic and shape fabric analysis shows that they have,been formed by two different ductile shearing episodes. (D1 and D2). Both D1 and D2 are essentially simple shearing events. Individual olivine and orthopyroxene crystals deform by a combination of slip along discrete slip planes and by recrystallisation processes. In many specimens which have undergone D2 deformation, some olivine grains preserve a partially rotated relict D1 crystallographic orientation. Changes in the D2 shear sense form a shear folding pattern of the S1 planes. The shapes and orientations of the petrological features are strongly controlled by the intensity of D1 and D2 shearing. Both the D1 and D2 shearing events are the remnants of mantle asthenospheric flow at an oceanic spreading centre which have been fossilised into the lithospheric mantle at the asthenosphere-lithosphere boundary. This fossilisation occurs during the offaxis, dominantly horizontal, flow of the asthenosphere. A study of the orientations of the D1 and D2 shearing structures from the different areas shows that they have similar orientations within a single area but that their orientations are highly variable between areas. This can be related to the horizontal flow of mantle material away from diapiric uprise centres along a ridge-axis. The interaction between the diverging flow from an earlier diapiric centre (S1 planes) and those from a later centre (S2 planes) will cause the earlier flow lines to be deformed.