MAGMATIC AND SOLID-STATE FABRIC DEVELOPMENT IN MID-CRUSTAL SHEETED PLUTONS, EASTERN TRANSVERSE RANGES, SOUTHERN CALIFORNIA

Sheeted plutons have been recognized throughout many of the Cretaceous and Paleogene magmatic arcs in the western Cordillera (Ingram and Hutton, 1994; Manduca et al., 1993; Miller and Paterson, 2001). In the Eastern Transverse Ranges of southern California, a 10x70km zone of sheeted plutons has been recognized at the base of a tilted crustal section (Barth et al, 2004). Compared to the weak magmatic fabrics that characterize upper crustal plutons, the mid-crustal sheeted plutons show well-developed, gently to moderately dipping magmatic fabrics. These fabrics record the presence of a long-lived deviatoric stress within the mid-crustal portions of the tilted section. This study evaluates the timing of magmatic and solid-state fabrics within these sheeted plutons to better understand the long-lived deviatoric stresses within the tilted section.

Petrography reveals that magmatic microstructures and fabrics are dominant in the sheeted plutons. Magmatic microstructures are defined by euhedral to subhedral plagioclase, hornblende, and biotite that do not show significant internal crystal-plastic deformation. Magmatic fabrics observable at the outcrop scale are defined by shape preferred orientations of hornblende and biotite. Solid-state overprinting is defined by grain-boundary migration (GBM) recrystallization of quartz, kinking of biotite, and microfracturing of feldspar.

U-Pb geochronology (Needy et al, submitted) reveals that mid-crustal sheeted plutons are synchronous with upper crustal plutonism. The strong fabrics in the mid-crustal plutons suggest that a strain field was localized in the mid-crust, owing to the development of strong magmatic fabrics in the mid-crust but not in the contemporaneous upper crustal plutons. Geochronology also shows that 83-80 Ma sheeted plutons contain strong magmatic fabrics, whereas 79-75Ma sheeted plutons have weaker magmatic fabrics and stronger solid-state fabrics. Ar/Ar thermochronlogy (USGS open file) suggests that cooling of sheeted plutons to biotite closure occurred by 70Ma. This cooling event, coupled with increased solid-state fabric development, corresponds well to the onset of Laramide tectonics in southern California and the refrigeration of the western Cordillera lithosphere (Dumitru et al., 1991; Grove et al., 2003; Jacobson et al., 2006).