Cyclicity of Intra-arc Deformation, Crustal Thickening and Strain Rates: Implications for Arc Tempos in the Mesozoic Sierra Nevada, California
Wenrong Cao, University of Southern California
Scott Paterson, University of Southern California
The Mesozoic Sierra Nevada Arc in the North America Cordillera records magmatic cyclicity. Its causes and links to other geological processes are long-standing questions. Existing models have emphasized processes in the fore- and retro-arc regions while downplaying the deformation within the arc itself. Our integrated research has led to the recognition of cyclic intra-arc deformation involving significant crustal thickening, pulsed material transfer processes at variable rates and of a likely role in the genesis of increased magmatism.
Field mapping and geochronology reveals three regional, angular unconformities in the central Sierra that formed during the early stages of the three magmatic flare-ups. The unconformities record episodic intra-arc deformation, erosion, and renewed volcanism/sedimentation. Structures in both host rocks and plutons in central Sierra indicate co-axial to transpressive contraction during most of the Mesozoic. These events led to at least 50-75% roughly plane strain shortening of the arc crust. Based on the geology in the host rock pendants and mass balance calculations, we propose that the intensity but not the kinematics of intra-arc deformation is cyclic mimicking a similar temporal pattern to arc magmatism. We argue that two complete and one incomplete cycles existed during the Mesozoic. A complete deformational cycle consists of a ca. 20 to 30-Myr-long higher-strain rate period (HSP, bulk shortening strain rate at least at 10-15/s) followed by a ca. 40 to 50-Myr-long lower-strain rate period (LSP, strain rate at 10-16/s or lower). The HSPs temporally coincide with the magmatic flare-ups and features the formation of thrusts, host rocks fabrics and strata tiling. The LSPs temporally coincide with the magmatic lulls, during which erosion, and renewed volcanism/sedimentation occurred. The styles of intra-arc deformational fields decoupled from the complication along the plate boundary during the Mesozoic indicating a possible viscosity-controlled strain partitioning. Thermobarometry in plutons indicates that the maximum exhumation of the central Sierra during the Mesozoic is about 5-7 km and thus requires significant downward transfer of host rocks (DTH). The strain rates of DTH are estimated to 6×10-15 to 4×10-14/s, close to the strain rate of the HSP, but more rapid than the strain rate of the contemporaneous fold-and thrust belt in foreland region, whose strain rate is about 4×10-16/s. We propose that thickening of arc crust invoked fertilization the magma source regions by the downward-transported crustal materials, which would contribute to the productivity of magma during flare-ups.