Group C, Poster #055, Seismology

Deep lower crustal seismicity and fabric in the Sierra Nevada: Implications for the fossil slab versus delamination debate

Vera Schulte-Pelkum, & Debi Kilb
Poster Image: 

Poster Presentation

2022 SCEC Annual Meeting, Poster #055, SCEC Contribution #12471 VIEW PDF
The Sierra Nevada and Great Valley are a non-deforming block within the transform plate boundary, but a debate remains about the relationship between uplift of the Sierra and deep structure including the high-velocity Isabella mantle anomaly. Here, we use seismicity and receiver function arrivals from contrasts in anisotropy to investigate the region.

In the southern Sierra, no lower crustal seismicity is observed. However, receiver functions image a strong anisotropic contrast near or below Moho depths, with dipping fabric at the contrast showing ~N-S strikes parallel to the nearby Kern Canyon fault.

In the western foothills of the central Sierra, an anomalou...
sly deep (to 50 km) and narrow band of seismicity parallels the foothills for ~200 km. The deep seismicity band is located near a Moho welt imaged as reaching more than 50 km depth by Frassetto et al. (2011). Across-strike depth sections hint at dipping lower bounds to the deep seismicity. The band lies in between two previously imaged high-velocity mantle anomalies in latitude, the Isabella anomaly to the south (interpreted as a fossil slab, delaminating lithosphere, or a mixture of both) and the Redding anomaly to the north (associated with active subduction). Receiver functions show fabric strikes paralleling the seismicity band at a range of crustal depths, with dips to the NE if the contrast is assumed to be an upper boundary.

We interpret these observations as follows. In the southern Sierra, the lack of lower crustal seismicity suggests a steady state. Imaged fabrics near the Moho may be of magmatic origin from the time of emplacement during Farallon subduction. Alternatively, these fabrics may be attributed to shear during removal of lithospheric mantle, but would suggest the shear was westward rather than southwestward as proposed previously.

The deep band of seismicity under the western foothills suggests present-day deformation in unusually thick and cold lower crust. Focal mechanisms of nearby deep clusters were previously interpreted as horizontal compression and vertical extension (Ryan et al., 2020). The geometry apparent in our cross sections in conjunction with the Moho geometry proposed by Frassetto et al. (2011) hints at a crustal welt with maximum depths paralleling the foothills. It is possible that this lower crustal deformation is associated with a delamination process. A relationship to a fossil slab (e.g. Dougherty et al., 2020) appears more difficult to explain.