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

Abstract

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 unjusti®ed.

Porphyroblast rotation during crenulation cleavage development: an example from the aureole of the Mooselookmeguntic pluton, Maine, USA

“…, 1992; Johnson, 1993), (2) quantifying elongation strain (Johnson & Williams, 1998), (3) quantifying shear strain and shear strain rate (Christensen et al. , 1989, 1994; Busa & Gray, 1992; Vance & O'Nions, 1992; Barker, 1994; Holcombe & Little, 2001; Biermeier & Stüwe, 2003), (4) quantifying vorticity (Passchier, 1987; Beam & Fisher, 1999; Holcombe & Little, 2001), (5) evaluating the effects of strain localization on rigid object behaviour (Ildefonse & Mancktelow, 1993; Marques & Coelho, 2001; Paterson & Vernon, 2001; ten Grotenhuis et al. , 2002; Mancktelow et al.…”

“…, 1992), object interaction where objects are closely spaced (Ildefonse et al. , 1992), measurement error (Steinhardt, 1989; Timms, 2003), late brittle–ductile deformation (Aerden, 1995), or heterogeneous distribution of strain at the thin‐section scale (Ilg & Karlstrom, 2000; Paterson & Vernon, 2001).…”

Multi-scale, open-system magmatic and sub-solidus processes contribute to the chemical and isotopic characteristics of the Jurassic Guadalupe Igneous Complex, Sierra Nevada, California, USA

“…3A) indicate that the BMFZ was active prior, during, and after the emplacement of the GIC under upper amphibolite-facies conditions (Paterson et al, 1987;Vernon et al, 1989). The BMFZ likely provided a pathway for low-δ 18 O seawater and/or meteoric waters (δ 18 O seawater : 0‰; Valley et al, 2005) to reach deeper parts of the crust during the marine Jurassic arc conditions, or seawater could also have been liberated from marine sediments of the Mariposa Formation during deformation (Paterson and Vernon, 2001). High deformation temperatures (>500 °C; Vernon et al, 1989) in the BMFZ would have allowed for water-rock oxygen isotope exchange to produce low-δ 18 O rocks in the vicinity of the lowermost units of the GIC.…”