The Lithospheric Structure of the Mendocino Triple Junction from Receiver Function Analysis

The Lithospheric Structure of the Mendocino Triple Junction from Receiver Function Analysis, Figure 1 Figure 1.
Coast-range-parallel receiver function cross-section. P-wave velocity model of MTJSE line 9 is shown in the upper panel, superimposed by the depth migrated single-fold section. The black solid line on top of the CCP image schematically shows the surface elevation along the cross-section, whereas the red dashed line schematically shows the heat flow. The red triangles denote projected stations, and the black dots denote the projected earthquake hypo- centers since 1985 with M > 3.0. The three strong events in the CCP image in order of depth are the Moho, LAB, and first Moho multiple respectively. The black lines mark the Moho depth determined by the RF image, and white lines mark the top and bottom of the Gorda slab. The basalt intrusions near Lake Pillsbury are represented by strong reflections of the depth migrated section from the lower crust and Moho. Black dashed line illustrates the southern edge of the Gorda plate, which dips to the south.
The Mendocino Triple Junction (MTJ), which occurs offshore from Cape Mendocino at ~ 40.50N in northern California, is the intersection of the Gorda plate, Pacific plate, and North American plate. It was formed ~28 Ma when the Pacific spreading ridge first contacted the western margin of the North American plate. In rigid plate frame, the northward migrating MTJ leaves in its wake a slab-free region filled by upwelling asthenospheric mantle just south of the Gorda plate, widely known as the “slab window” model.
</p><p>To examine the lithospheric structure in the MTJ region, especially the structure of the “slab window”, we generated a 3D PdS receiver function CCP image. We used 186 earthquakes recorded at 111 broadband stations of the Flexible Array Mendocino Experiment (FAME) together with the Berkeley Digital Seismic Network and USArray Transportable Array. The data were depth mapped and laterally migrated incorporating 3D traveltime corrections determined from 3D P-and S-tomography models.
</p><p>The resulting image confirms the crustal structure of the Mendocino Triple Junction Seismic Experiment (MTJSE) and reveals detailed lithospheric structure. The top and bottom of the Gorda slab are identified by the Moho and Lithosphere-Asthenosphere Boundary (LAB) in the subduction regime, showing that the thickness of the Gorda slab is ~ 40 km, comparable with that predicted by half-space cooling model. The “slab window” in the transform regime has a complex structure, but its top can be traced continuously to the Gorda LAB, although the transition is more abrupt than that suggested by the refraction velocity model. The LAB is shallowest beneath Clear Lake volcano and Lake Pillsbury where high heat flow and basalt intrusions are observed respectively. Under the western part of northern Great Valley, the Moho signal is absent, likely due to the hydration and serpentinization of the upper mantle during the subduction of the Gorda slab ~2Ma. The Great Valley ophiolite is observed at the crustal depth instead.
Zhai, Y., and A. Levander (2010), The Lithosphere-Asthenosphere Boundary and the Slab Window in the Mendocino Triple Junction Region, in preparation.
</p><p>Acknowledgements: This project is supported by NSF grants EAR-0642474 and EAR-0746379.</p>


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