Inclusion trail patterns in porphyroblasts from the Foothills Terrane, California: a record of orogenesis or local strain heterogeneity?

A major problem with the current use of porphyroblast–matrix microstructural relationships to infer orogenic histories, such as multiple orthogonal orogenic events, is that other evidence for these events is typically lacking. For example, a comparison of regional relationships and local structures formed in and adjacent to porphyroblasts present in contact aureoles in the Foothills Terrane, Sierra Nevada, California, shows that: (1) except in shear zones, contact aureoles and local zones along lithological contacts, the Foothills Terrane has a single regional cleavage, although locally formed by multiple processes; (2) the regional cleavage and locally developed porphyroblast inclusion trails have variable orientations, and neither dataset supports the formation of dominantly subhorizontal and subvertical cleavages in this orogen; (3) structural and metamorphic heterogeneities occur at all scales and can markedly affect inclusion trail patterns in porphyroblasts; (4) complex porphyroblast growth features and internal inclusion trail patterns can form in porphyroblasts that grow during short time intervals in contact aureoles, indicating that local complexity in porphyroblasts does not imply regional complexity. Because of these conclusions, multiple datasets, rather than data acquired only from porphyroblasts, should be considered when attempting to understand the evolution of orogens. Furthermore, using microstructural information preserved only in porphyroblasts to infer orogenic processes and plate motions is generally unjustified.