THE MYSTERIOUS SEISMOTECTONICS OF THE INTERIOR OF THE SIERRA NEVADA-GREAT VALLEY PLATE

To first order, the Sierra Nevada-Great Valley block is a rigid plate within the broadly transform Pacific-North America boundary. However, seismicity within the block has characteristics that suggest a more complex system. Deviations in the orientation of principal strain rate axes indicate the presence of a buoyant basal load in the Tulare Lakebed in the San Joaquin Valley (Unruh et al., 2014). Seismicity near Yosemite indicates shortening and thickening of the lower crust not far from extensional faults ~70 km to the east (Ryan et al., 2020).

Internal deformation has been anticipated from studies suggesting that the Sierra rose as lower lithosphere descended into the mantle. As modelled by LePourheit and colleagues (2006; Saleeby et al., 2012) in 2-D, sub-Sierran lower lithosphere is removed through a combination of delamination and gravitational instability. As further sketched by Jones et al. (2014), material moving west coalesces into two main downwellings, one east of the Tulare Lakebed, the other the south end of the downgoing Gorda slab. Thus it seems subsidence should be continuing near the Tulare Lakebed from the antibuoyant Isabella Anomaly, and the remaining deep lithosphere under near Yosemite should be stretching and thinning as material migrates to north or south. And yet observations are nearly exactly opposite expectations: the load near the Tulare Lakebed is buoyant, not antibuoyant, and the seismicity of the lower crust near Yosemite shows thickening, not thinning.

Vertical gradients in horizontal strain rates caused by vertically varying viscosities in the lithosphere could resolve this puzzle (e.g., Molnar, Tectonics, 2015). Subsidence induced by descending lithosphere could entrain lower crust, thickening the crust where subsidence was occurring; this might be the case in the southwesternmost Sierra (Levandowski and Jones, 2015). As the descending body begins to detach from the lithosphere, stresses driving the crust together are reduced, leading to a net upward buoyancy such as observed by Unruh et al. Similarly, thickening near Yosemite might be a response to the changing geodynamics. These curious observations suggest that careful examination of the very low strain rate deformation in this block might yield constraints on lithospheric evolution in the presence of mantle loads.