RHEOLOGICAL CHANGES ASSOCIATED WITH PERIDOTITE FABRIC DEVELOPMENT IN THE LITHOSPHERIC MANTLE

Spinel lherzolite and harzburgite xenoliths, erupted from the Pleistocene-Holocene San Quintín volcanic field in northwestern Baja California, Mexico, preserve a range of fabrics from nearly isotropic to strongly foliated and lineated. We interpret these variable fabrics to result from sampling of a strain gradient developed within a transcurrent shear zone in the lithospheric mantle that accommodates displacement adjacent and parallel to the Pacific-North American plate boundary.

These xenoliths preserve several fabrics representative of the peridotite deformation cycle – protogranular, porphyroclastic, and equigranular – proposed by Mercier and Nicolas (1975). We use calculated deformation conditions (e.g., P, T, water content) along with textural and microstructural analysis and existing flow laws to document changes in active deformation mechanism(s) and rheology with fabric evolution. Protogranular fabrics record the highest temperatures (1000 - 1100° C) and strongest crystallographic preferred orientations (CPO) in olivine. Deformation in protogranular rocks was likely dominated by dislocation creep. Porphyroclastic fabrics, characterized based on the presence of mm-scale pyroxenes within a fine grained, variably mixed matrix, record a large range of temperatures (900 - 1100° C) and olivine CPO intensity. Deformation in these rocks likely included both dislocation creep as well as grain size sensitive (GSS) processes, such as grain or phase boundary sliding, within the finer grained matrix. Equigranular fabrics are characterized by well mixed phases with grain sizes smaller than protogranular but larger than porphyroclastic matrix material. Olivine preserves only limited evidence for intragrain strain. These fabrics are generally restricted to the lowest temperatures (850 – 950° C) and the weakest intensity olivine CPO. These observations suggest that deformation in equigranular rocks was dominated by GSS processes with limited contribution of dislocation creep. Using laboratory derived flow laws, we show that fabric groups are each characterized by distinct viscosities that record only minor overlap. Collectively, these results shed light on the rheological evolution and stratification of the mantle deformed beneath lithosphere-scale fault systems.