Where is the “residue” from which the plutons of the batholith were extracted?
If it delaminated in the past 10 Ma we might be lucky enough to catch a snapshot of the delamination event similar to the one under the Southern Sierras (Ruppert et al., 1998). If a negatively buoyant lower crust did detach (Jull and Kelemen, 2001) and sink into the mantle shortly after formation, then all previous layering associated with the creation, suturing and compression of the CPC crust would be erased. Thus, we would expect truncation of mantle layering. Of particular interest in this respect is the structure beneath the sharp magmatic front where we speculate that the delamination event may have nucleated. However, if the root did not delaminate, but instead remains hidden beneath the CPC’s seismic Moho, then continuity of mantle layering is expected, but the high density of this garnet rich root should produce an observable gravity anomaly. An example of the quality of the sub-crustal layering image we expect from the proposed passive seismic experiment is shown in Figure 16
