We compute high-resolution seismic images from scattered wavefield to detect discontinuities beneath the High Lava Plains (HLP), using data recorded at a dense broadband array. Our images of the HLP and surrounding regions reveal (1) a prominent Moho discontinuity with varying depth, with thinnest crust of 35 km beneath the volcanic track, and thickened crust of similar to 45 km beneath the Owyhee Plateau (OP); (2) distinct intracrustal velocity reversals beneath regions of pre-2.0 Ma volcanism and within the OP; and (3) intermittent negative velocity discontinuities in the uppermost mantle beneath regions of Holocene volcanism and volcanic centers near Steens Mountain and Newberry volcano. These features exhibit remarkable similarity with those seen in the surface wave tomography and Ps receiver functions. We fail to find evidence for a ubiquitous regional lithosphere-asthenosphere boundary (LAB). In concert with petrological constraints on the equilibration depths of primitive basaltic melts, our results suggest that the present-day HLP mantle lithosphere is thin or absent, perhaps a consequence of episodes of extensive mantle inflow, lithospheric extension, and possibly melting induced by rapid slab rollback and trench retreat. It remains possible, however, that strong E-W seismic anisotropy reported across this region may reduce the effective S-wave velocity contrast to render the LAB less detectable. In contrast, the Owyhee Plateau exhibits a clear LAB, consistent with it being a block of older preexisting lithosphere. Our images demonstrate the complexity of mantle dynamics in the Cascadian back-arc and the close casual link between subduction-related processes and the origin of HLP volcanism.

We compute high-resolution seismic images from scattered wavefield to detect discontinuities beneath the High Lava Plains (HLP), using data recorded at a dense broadband array. Our images of the HLP and surrounding regions reveal (1) a prominent Moho discontinuity with varying depth, with thinnest crust of 35 km beneath the volcanic track, and thickened crust of similar to 45 km beneath the Owyhee Plateau (OP); (2) distinct intracrustal velocity reversals beneath regions of pre-2.0 Ma volcanism and within the OP; and (3) intermittent negative velocity discontinuities in the uppermost mantle beneath regions of Holocene volcanism and volcanic centers near Steens Mountain and Newberry volcano. These features exhibit remarkable similarity with those seen in the surface wave tomography and Ps receiver functions. We fail to find evidence for a ubiquitous regional lithosphere-asthenosphere boundary (LAB). In concert with petrological constraints on the equilibration depths of primitive basaltic melts, our results suggest that the present-day HLP mantle lithosphere is thin or absent, perhaps a consequence of episodes of extensive mantle inflow, lithospheric extension, and possibly melting induced by rapid slab rollback and trench retreat. It remains possible, however, that strong E-W seismic anisotropy reported across this region may reduce the effective S-wave velocity contrast to render the LAB less detectable. In contrast, the Owyhee Plateau exhibits a clear LAB, consistent with it being a block of older preexisting lithosphere. Our images demonstrate the complexity of mantle dynamics in the Cascadian back-arc and the close casual link between subduction-related processes and the origin of HLP volcanism.

The cause of seismic anisotropy exhibiting trench parallel fast directions in subduction systems has been the subject of significant recent research. We provide new constraints on the contributions of hydrous phases to seismic anisotropy from an unusually well-localized region of trench parallel fast directions in Rayleigh wave phase velocities near the Cascade arc at 45 to 66 s periods. We constrain the location of the anisotropic material to within or directly above the oceanic plate, using the depth sensitivity of Rayleigh waves as a function of frequency and the accurate slab imaging available for Cascadia from scattered wave studies. We infer that the likely source of trench-parallel anisotropy is either a thin layer of sheared hydrous material directly above the slab or hydrated outer rise faults in the upper part of the subducting plate. Similar contributions to trench parallel anisotropy from hydrous phases are likely stronger in other subduction zones.

The cause of seismic anisotropy exhibiting trench parallel fast directions in subduction systems has been the subject of significant recent research. We provide new constraints on the contributions of hydrous phases to seismic anisotropy from an unusually well-localized region of trench parallel fast directions in Rayleigh wave phase velocities near the Cascade arc at 45 to 66 s periods. We constrain the location of the anisotropic material to within or directly above the oceanic plate, using the depth sensitivity of Rayleigh waves as a function of frequency and the accurate slab imaging available for Cascadia from scattered wave studies. We infer that the likely source of trench-parallel anisotropy is either a thin layer of sheared hydrous material directly above the slab or hydrated outer rise faults in the upper part of the subducting plate. Similar contributions to trench parallel anisotropy from hydrous phases are likely stronger in other subduction zones.

To constrain mantle structure that might contribute to the topography of the southern Appalachian Mountains, Pn phases from regional earthquakes recorded in northern Georgia by EarthScope Southeastern Suture of the Appalachian Margin Experiment and Transportable Array stations were used to solve for shallow mantle P wave velocities. Mantle velocities vary laterally, with values of 7.6-7.8km/s beneath the higher elevations of the Blue Ridge terrane and northwestern flank of the Inner Piedmont terranes and values of 8.3-8.5km/s farther south where elevation is lower. The zone of low-velocity mantle could represent a source of buoyancy that helps to support the higher elevations, in addition to the root of thickened crust that also exists beneath the mountains.

To constrain mantle structure that might contribute to the topography of the southern Appalachian Mountains, Pn phases from regional earthquakes recorded in northern Georgia by EarthScope Southeastern Suture of the Appalachian Margin Experiment and Transportable Array stations were used to solve for shallow mantle P wave velocities. Mantle velocities vary laterally, with values of 7.6-7.8km/s beneath the higher elevations of the Blue Ridge terrane and northwestern flank of the Inner Piedmont terranes and values of 8.3-8.5km/s farther south where elevation is lower. The zone of low-velocity mantle could represent a source of buoyancy that helps to support the higher elevations, in addition to the root of thickened crust that also exists beneath the mountains.

Polarities and amplitudes of intracrustal P-SV conversions (P waves converted to vertically polarized shear waves) in receiver functions from the Southeastern Suture of the Appalachian Margin Experiment array and USArray Transportable Array provide new constraints on the origin of seismic reflectivity delineating the Alleghanian detachment in the southern Appalachians(eastern United States). Forward modeling of receiver functions is consistent with a 3.5-km-thick, high shear-wave velocity (Vs = 3.9 km/s) section of deformed Paleozoic platform metasedimentary rocks beneath the Blue Ridge at 3-6.5 km depth. In the Inner Piedmont, conversions from the top and base of a low-Vs zone (3.1 km/s) at depths of 5-9 km are interpreted as a package of metasedimentary rocks or a shear zone characterized by radial anisotropy. The detachment continues to the southeast beneath the Carolina terrane, where high-amplitude negative conversions at 10-13 km depth are consistent with arc rocks (Vs = 4.0 km/s) overlying sheared rocks with lower Vs (3.2 km/s). Southeast-dipping conversions at 5-10 km depth mark the boundary between the Inner Piedmont and Carolina terrane. This study demonstrates that relatively high-frequency receiver functions (up to similar to 3 Hz), though still lower in frequency than P-wave energy analyzed for reflection profiling (>20 Hz), can provide important links between surface geology and active-source experiments to better constrain models of crustal structure.

Polarities and amplitudes of intracrustal P-SV conversions (P waves converted to vertically polarized shear waves) in receiver functions from the Southeastern Suture of the Appalachian Margin Experiment array and USArray Transportable Array provide new constraints on the origin of seismic reflectivity delineating the Alleghanian detachment in the southern Appalachians(eastern United States). Forward modeling of receiver functions is consistent with a 3.5-km-thick, high shear-wave velocity (Vs = 3.9 km/s) section of deformed Paleozoic platform metasedimentary rocks beneath the Blue Ridge at 3-6.5 km depth. In the Inner Piedmont, conversions from the top and base of a low-Vs zone (3.1 km/s) at depths of 5-9 km are interpreted as a package of metasedimentary rocks or a shear zone characterized by radial anisotropy. The detachment continues to the southeast beneath the Carolina terrane, where high-amplitude negative conversions at 10-13 km depth are consistent with arc rocks (Vs = 4.0 km/s) overlying sheared rocks with lower Vs (3.2 km/s). Southeast-dipping conversions at 5-10 km depth mark the boundary between the Inner Piedmont and Carolina terrane. This study demonstrates that relatively high-frequency receiver functions (up to similar to 3 Hz), though still lower in frequency than P-wave energy analyzed for reflection profiling (>20 Hz), can provide important links between surface geology and active-source experiments to better constrain models of crustal structure.

Flat-slab subduction occurs when the descending plate becomes horizontal at some depth before resuming its descent into the mantle. It is often proposed as a mechanism for the uplifting of deep crustal rocks ('thick-skinned' deformation) far from plate boundaries, and for causing unusual patterns of volcanism, as far back as the Proterozoic eon(1). For example, the formation of the expansive Rocky Mountains and the subsequent voluminous volcanism across much of the western USA has been attributed to a broad region of flat-slab subduction beneath North America that occurred during the Laramide orogeny (80-55 million years ago)(2). Here we study the largest modern flat slab, located in Peru, to better understand the processes controlling the formation and extent of flat slabs. We present new data that indicate that the subducting Nazca Ridge is necessary for the development and continued support of the horizontal plate at a depth of about 90 kilometres. By combining constraints from Rayleigh wave phase velocities with improved earthquake locations, we find that the flat slab is shallowest along the ridge, while to the northwest of the ridge, the slab is sagging, tearing, and re-initiating normal subduction. On the basis of our observations, we propose a conceptual model for the temporal evolution of the Peruvian flat slab in which the flat slab forms because of the combined effects of trench retreat along the Peruvian plate boundary, suction, and ridge subduction. We find that while the ridge is necessary but not sufficient for the formation of the flat slab, its removal is sufficient for the flat slab to fail. This provides new constraints on our understanding of the processes controlling the beginning and end of the Laramide orogeny and other putative episodes of flat-slab subduction.