P and S waveform modeling of continental sub‐lithospheric detachment at the eastern edge of the Rio Grande Rift

East of the Rio Grande Rift, tomographic images of teleseismic data have revealed a SE dipping, slab‐like structure underneath the western edge of the Great Plains in the southwestern United States. However, finite difference synthetics require an amplified tomographic model to reproduce the waveform distortions as observed in broadband waveform data recorded along the LA RISTRA Transect. In addition to travel time anomalies, Song and Helmberger (2007) demonstrated how to use S waveforms and their amplitude patterns to further constrain the magnitude of the anomalous structure. Their preferred S velocity model suggests that the slab‐like structure is about 4% fast, 120 km thick and dipping 70–75° to the SE to about a depth of 600 km. We adapt the preferred S wave model from Song and Helmberger (2007) and scale the P wave model using a suite of scaling factors (SF ≡ dln V s /dln V p ). We find that synthetics from the P model generated by SF ≈ 1.25 can distort P waveforms and fit the amplitude pattern best. Such a low SF indicates that the slab‐like anomaly is not only cold but also compositionally distinct. We make use of SF and the S wave anomaly simultaneously to obtain corresponding V p / V s anomaly and infer the origin of the slab‐like anomaly. Our result suggests that the observed sub‐lithospheric detachment is 310 ± 20°C colder and more depleted than the adjoining mantle asthenosphere by 3 units of Mg# (Mg / (Mg + Fe) × 100). It is negatively buoyant and is geodynamically consistent with the observed foundering of the continental lithosphere at the eastern edge of the Rio Grande Rift. In short, we demonstrate the importance of seismic waveform anomalies in geochemical and geodynamic inferences.