RECEIVER FUNCTION AND RAYLEIGH WAVE IMAGING OF THE LITHOSPHERE IN THE MENDOCINO TRIPLE JUNCTION REGION

We have made PdS and SdP receiver function images and inverted fundamental mode Rayleigh wave phase velocities for shear velocity structure to examine the structure of the lithosphere in the Mendocino Triple Junction region. We used USArray Transportable Array data together with the dataset acquired as part of FAME (the Flexible Array Mendocino Experiment). We made a 3D CCP stacked image of PdS receiver functions using more than 100 teleseismic earthquakes recorded at more than 80 broadband seismograph stations. The data were depth mapped and laterally migrated incorporating 3D travel time corrections determined from 3D P- and S-tomography models (Schmandt and Humphreys, unpublished). A smaller number of events were used to make a similar 3D SdP CCP image volume, and for Rayleigh wave tomography.

A NW-SE PdS CCP profile extending from the Cascadia accretionary wedge terranes into the San Andreas transform regime shows the subducting Gorda oceanic crust and Moho dipping southward to > 35 km depth at the Gorda plate’s southern edge, and then shallowing rapidly to 25 km in the transform regime to the south. The receiver function images are in close agreement with previous active source seismic results (Beaudoin et al., 1996; Henstock and Levander, 2000). The lithosphere-asthenosphere boundary at the base of the subducting Gorda plate is imaged at ~80 km depth, giving a plate thickness of about 55-60 km. Crossing the southern edge of the Gorda plate, the LAB is offset and shallows abruptly to 35-60 km beneath the transform regime, forming the slab window ~10 km below the Moho.

Another CCP profile from the PdS 3D image volume taken along the trend of Mt. Lassen and Mt. Shasta shows what we infer to be the top of the Gorda plate at ~90-100 km depth. Between the plate and the Moho, at ~40-45 km depth, the mantle wedge appears as a complex structure with alternating bright positive and negative events.