Dual controls on lithospheric dripping: The role of mantle flow and orogen scale

Rapid surface uplift or subsidence and voluminous magmatic activity have often been ascribed to regional-scale downwelling of lithospheric mantle. However, because lithospheric drips—sinking plumes of cold and dense lithosphere—are relatively small and transient features, direct evidence of their existence has been difficult to obtain. Moreover, the significant vertical mantle flow that should be associated with such structures has not been detected. Here we integrate seismic anisotropy data with tomographic images to identify and describe a lithospheric drip beneath the Great Basin region of the western United States. The feature is characterized by a localized cylindrical core of cooler material with fast seismic velocities and mantle flow that rapidly shifts from horizontal to vertical. Our numerical experiments suggest that the drip can be generated by gravitational instability resulting from a density anomaly of as little as 1% and a localized temperature increase of 10%. The drip tilts to the northeast—opposite to the motion of the North American plate in the hotspot reference frame—and thereby indicates northeast-directed regional mantle flow. Seismic anisotropy data for the Great Basin region of the western United States, coupled with tomographic images, help delineate a northeast-dipping lithospheric drip. Numerical experiments suggest that the drip could have formed owing to gravitational instability triggered by a density increase of about 1% and a temperature increase of about 10%.

Depth constraints on azimuthal anisotropy in the Great Basin from Rayleigh-wave phase velocity maps

SUMMARYSources of stress responsible for earthquakes occurring in the Central and Eastern United States (CEUS) include not only far-field plate boundary forces but also various local contributions. In this study, we model stress fields due to heterogeneities in the upper mantle beneath the CEUS including a high-velocity feature identified as a lithospheric drip in a recent regional P-wave tomography study. We calculate velocity and stress distributions from numerical models for instantaneous 3-D mantle flow. Our models are driven by the heterogeneous density distribution based on a temperature field converted from the tomography study. The temperature field is utilized in a composite rheology, assumed for the numerical models. We compute several geodynamic quantities with our numerical models: dynamic topography, rate of dynamic topography, gravitational potential energy (GPE), differential stress, and Coulomb stress. We find that the GPE, representative of the density anomalies in the lithosphere, is an important factor for understanding the seismicity of the CEUS. When only the upper mantle heterogeneities are included in a model, differential and Coulomb stress for the observed fault geometries in the CEUS seismic zones acts as a good indicator to predict the seismicity distribution. Our modelling results suggest that the upper mantle heterogeneities and structure below the CEUS have stress concentration effects and are likely to promote earthquake generation at preexisting faults in the region’s seismic zones. Our results imply that the mantle flow due to the upper-mantle heterogeneities can cause stress perturbations, which could help explain the intraplate seismicity in this region.

We use Rayleigh waves to invert for shear velocities in the upper mantle beneath southern California. A one‐dimensional shear velocity model reveals a pronounced low‐velocity zone (LVZ) from 90 to 210 km. The pattern of velocity anomalies indicates that there is active small‐scale convection in the asthenosphere and that the dominant form of convection is three‐dimensional (3‐D) lithospheric drips and asthenospheric upwellings, rather than 2‐D sheets or slabs. Several of the features that we observe have been previously detected by body wave tomography: these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern Sierra Nevada and Walker Lane region; sinking lithosphere beneath the southern Central Valley; upwelling beneath the Salton Trough; and downwelling beneath the Transverse Ranges. Our new observations provide better constraints on the lateral and vertical extent of these anomalies. In addition, we detect two previously undetected features: a high‐velocity anomaly beneath the northern Peninsular Range and a low‐velocity anomaly beneath the northeastern Mojave block. We also estimate the azimuthal anisotropy from Rayleigh wave data. The strength is ∼1.7% at periods shorter than 100 s and decreases to below 1% at longer periods. The fast direction is nearly E‐W. The anisotropic layer is more than 300 km thick. The E‐W fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms: pure shear in the lithosphere due to N‐S tectonic shortening and simple shear in sublithosphere mantle due to mantle flow.

The Anatolian Plateau, currently experiencing rapid uplift and westward escape, records both the termination of oceanic subduction and the conversion to continental collision. The crustal response to the transition of the subduction environment from eastern to western Anatolia can be inferred by the seismic velocity and attenuation structures. With this study, we construct a broadband Lg‐wave attenuation model for the Anatolian Plateau and use it to constrain lateral crust heterogeneities linked to this transition. Crustal Lg attenuation links late Cenozoic magmatism with asthenospheric upwelling by characterizing the lithospheric thermal structure. The widely distributed strong attenuation observed in eastern Anatolia may be related to the crustal partial melting due to mantle upwelling after the delamination and subsequent break‐off of the Bitlis slab. Lithospheric dripping in central Anatolia likely facilitates the mantle flows through the window between the Cyprus and Aegean slabs, which results in the piecemeal low anomaly in central Anatolia.

In Northeast Japan and Izu‐Bonin, arc volcanoes form in clusters or as cross‐arc chains. Their occurrence emphasizes the non‐uniform distributions of sub‐arc temperature and fluids that control the spacing of arc volcanoes. Here, using 3‐D numerical models, we show that the cessation of back‐arc spreading promotes volcano clustering by triggering the formation of nascent lithospheric drips – downward protrusions of cold and dense lithosphere‐adjacent to the thinned back‐arc lithosphere. The nascent drips interfere with the flow of the hot asthenospheric mantle from the back‐arc toward the arc, leading to gradual development of alternating hot and cold regions beneath the arc. The results indicate that along‐arc variation in the sub‐arc mantle temperature is largest not during back‐arc spreading but after its cessation, explaining the time offset by several million years between back‐arc spreading and volcano clustering in Northeast Japan and Izu‐Bonin.

Along-strike translation of a fossil slab

The oceanic lithosphere is a primary component of the plate tectonic system, yet its evolution and its asthenospheric interaction have rarely been quantified by in situ imaging at slow spreading systems. We use Rayleigh wave tomography from noise and teleseismic surface waves to image the shear wave velocity structure of the oceanic lithosphere‐asthenosphere system from 0 to 80 My at the equatorial Mid‐Atlantic Ridge using data from the Passive Imaging of the Lithosphere‐Asthenosphere Boundary (PI‐LAB) experiment. We observe fast lithosphere (VSV > 4.4 km/s) that thickens from 20–30 km near the ridge axis to ~70 km at seafloor >60 My. We observe several punctuated slow velocity anomalies (VSV < 4.1 km/s) in the asthenosphere between 50 and 150 km depth, not necessarily focused beneath the ridge axis. Some of the slow velocity regions are located within 100 km of the ridge axis, but other slow velocity regions are observed at distances > 400 km from the ridge. We observe a high velocity lithospheric downwelling drip beneath 30 My seafloor that extends to 80–130 km depth. The asthenospheric slow velocities likely require partial melt. Although melt is present off axis, the lack of off‐axis volcanism suggests the lithosphere acts as a permeability boundary for deeper melts. The punctuated and off‐axis character of the asthenospheric anomalies and lithospheric drip suggests small‐scale convection is active at a range of seafloor ages. Small‐scale convection and/or more complex mantle flow may be aided by the presence of large offset fracture zones and/or the presence of melt and its associated low‐viscosities and enhanced buoyancies.