THE MODE OF PLUTON EMPLACEMENT IN THE LATE JURASSIC SIERRA NEVADA ARC IS CONTROLLED BY STRAIN PARTITIONING

Strain partitioning related to the oblique convergence between the Farallon and North American plates is recorded in the mode of pluton emplacement and structures in the Late Jurassic Sierra Nevada continental arc of eastern California. The two structural components are the East Sierran thrust system (ESTS), roughly on the eastern boundary of the locus of Jurassic magmatism, and sinistral oblique deformation within the arc coincident with the intrusion of the Independence Dike swarm (IDS). We present a more detailed analysis of the partitioning of strain and coeval pluton emplacement using high-precision geochronology and structural analysis of syntectonic igneous rocks from about 152 to 148 Ma.

The ESTS is a Middle to early Late Jurassic belt of east-directed brittle and ductile thrust faults and related structures exposed for over 150 km along strike from the Inyo Mountains into the Mojave Desert. Late Jurassic plutonism was synchronous with deformation, indicating a contractional setting. Plutonic bodies from this time are commonly small stocks and roughly foliation parallel sills of intermediate to felsic composition. Dikes are commonly oriented to the northwest, oblique to the more northerly overall trend of the swarm. Dikes are dominantly mafic to intermediate in composition, although locally are felsic or intermediate with magma-mingling textures. Sinistral fabrics developed locally during dike emplacement.

From these observations, we propose a model for partitioning both strain and pluton emplacement. The ESTS is east-directed shortening at the edge of the arc, while the IDS represents sinistral shear and arc-parallel extension within the arc. Such strain partitioning is common in modern arcs with oblique convergence (e.g., Sumatra). Modes of pluton emplacement are likewise partitioned. The IDS is dominated by mafic dikes, meaning that mantle melts had mostly unobstructed pathways along fractures to the surface. This would indicate that the vertical stress was probably σ₂ with σ₃ slightly oblique and more westerly than the locus of the arc. In the ESTS, the vertical stress was probably σ₃ with σ₁ oriented perpendicular to the arc. In this environment, ascending magmas would stall and differentiate toward more felsic compositions and smaller volume intrusions with more sill-like forms.