Relationships between olivine LPO and deformation parameters in
naturally deformed rocks and implications for mantle seismic anisotropy
Abstract
We analyze peridotites from a wide range of tectonic settings to
investigate relationships between olivine lattice preferred orientation
(LPO) and deformation conditions in naturally deformed rocks. These
samples preserve the five olivine LPO types (A through E-type) that rock
deformation experiments have suggested are controlled by water content,
temperature, stress magnitude, and pressure. The naturally deformed
specimens newly investigated here (65 samples) and compiled from an
extensive literature review (445 samples) reveal that these factors may
matter less than deformation history and/or geometry. Some trends
support those predicted by experimentally determined parametric
dependence, but several observations disagree — namely that all LPO
types are able to form at very low water contents and stresses, and that
there is no clear relationship between water content and LPO type. This
implies that at the low stresses typical of the mantle, LPO type more
often varies as a function of strain geometry. Because olivine LPO is
primarily responsible for seismic anisotropy in the upper mantle, the
results of this study have several implications. These include (1) the
many olivine LPO types recorded in samples from individual localities
may explain some of the complex seismic anisotropy patterns observed in
the continental mantle, and (2) B-type LPO – where olivine’s “fast
axes” align perpendicular to flow direction – occurs under many more
conditions than traditionally thought. This study highlights the need
for more experiments, and the difficulty in using olivine LPO in
naturally-deformed peridotites to infer deformation conditions.