Moho-Depth Dike Swarms and Rifting of the Sierra Nevada Microplate, Northeast California

Moho-depth earthquake swarms below N. Lake Tahoe (LT) in late 2003 and, beneath southern Sierra Valley (SV), California in late 2011/early 2012, ~50 km to the north, define nearly identical dike injection events along the same Moho-depth structure. Each swarm lasts ~6 months, outlines an ~7x7 km fault area, initiates at its deepest extent, shows dominantly reverse slip mechanisms, and aligns along an ~N45W striking, ~50 degree east dipping Moho-depth structure. About 1600 events were located in the LT swarm (maximum M 2.2; b-value 2.0) and nearly 2200 have been located under SV (maximum M 1.9; b-value 1.7); an NSF-Earthscope supported deployment in SV includes five broadband instruments that are currently operating within one focal depth - three directly above the sequence. A Long-Period earthquake (LP), located at a depth of 30 km along the dipping structure (9/29/2011) was located midway between the two sequences. This is the first LP identified outside of the Long Valley volcanic center along the northeastern Sierra. These diking events define a northward propagating rift boundary along the northeastern boundary of the Sierran Microplate. Extension driven decompression melting of upper mantle magmas and weakening of a stronger (~5 km thick) upper-Moho lid drives dike injection and faulting of the rift structure. Following failure of the strong upper mantle layer, footwall unloading and flexural response help bring the Sierran block elevations into equilibrium with the upper-mantle and crustal profile. Forces driving plate boundary processes, slab roll back, westward escape of the southern Cascadia upper plate, and northward evolution of the Mendocino Triple Junction (MTJ) result in northwestward motion of the Sierra block (e.g., Atwater and Stock, 1988) and extension east of the Sierra, driving northwestward rift propagation. These overall processes are reflected in the historical earthquake record in northeast California and western Nevada. Shear dominated San Andreas transform motion, along northwestern California, tracks seismicity trends east of the Sierra. Northwest of the SV region more northwesterly alignments of earthquakes reflect by a stress field increasingly influenced by Cascadia domain motions. The spatial and temporal progression of the diking processes can be mapped in detail with high precision earthquake locations. These magmas may have similar chemistry to young volcanics in the north LT area. Structurally, the top of the LT sequence, at ~24 km depth, is ~5 km shallower than the upper extent of SV sequence (~29 km depth), suggesting an ~10% Moho-depth gradient over 50 km. Diking most likely represents emplacement of differentiated upper-mantle lower density magmas into a weakened higher density upper mantle layer. Each sequence is also associated with increased upper-crustal seismicity, including Mw 4.5 (2004; LT), Mw 4.8 (2005; LT), and Mw 4.7 (2011; SV) events. Extrapolating rift initiation of Walker Lane faulting near Bridgeport, CA at ~10 Ma (Putrika and Busby, 2009), returns a northwest rift propagation rate of ~18 mm/yr, within 30 % of the current Sierran rate. This interpreted continental rifting process is fundamental to the tectonic evolution of the eastern Sierra and has important implications for volcanic and seismic hazards of the Lake Tahoe-Truckee-Reno area.