Subduction systems play a key role in plate tectonics, but the deformation of the crust and uppermost mantle during continental subduction remains poorly understood. Observations of seismic anisotropy can provide constraints on dynamic processes in the crust and uppermost mantle in subduction systems. The subduction zone beneath Peru and Bolivia, where the Nazca plate subducts beneath South America, represents a particularly interesting location to study subduction-related deformation, given the along-strike transition from flat to normally dipping subduction. In this study we constrain seismic anisotropy within and above the subducting slab (including the overriding plate) beneath Pent and Bolivia by examining azimuthal variations in radial and transverse component receiver functions. Because anisotropy-aware receiver function analysis has good lateral resolution and depth constraints, it is complementary to previous studies of anisotropy in this region using shear wave splitting or surface wave tomography. We examine data from long-running permanent stations NNA (near Lima, Peru) and LPAZ (near La Paz, Bolivia), and two dense lines of seismometers from the PULSE and CAUGHT deployments in Pent and Bolivia, respectively. The northern line overlies the Peru flat slab, while the southern line overlies the normally dipping slab beneath Bolivia. We applied harmonic decomposition modeling to constrain the presence, depth, and characteristics of dipping and/or anisotropic interfaces within the crust and upper mantle. We found evidence for varying multi-layer anisotropy, in some cages with dipping symmetry axes, underneath both regions. The presence of multiple layers of anisotropy with distinct geometries that change with depth suggests a highly complex deformation regime associated with subduction beneath the Andes. In particular, our identification of depth-dependent seismic anisotropy within the overlying plate crust implies a change in deformation geometry, dominant mineralogy, and/or theology with depth, shedding light on the nature of deep crustal deformation during orogenesis.

Subduction systems play a key role in plate tectonics, but the deformation of the crust and uppermost mantle during continental subduction remains poorly understood. Observations of seismic anisotropy can provide constraints on dynamic processes in the crust and uppermost mantle in subduction systems. The subduction zone beneath Peru and Bolivia, where the Nazca plate subducts beneath South America, represents a particularly interesting location to study subduction-related deformation, given the along-strike transition from flat to normally dipping subduction. In this study we constrain seismic anisotropy within and above the subducting slab (including the overriding plate) beneath Pent and Bolivia by examining azimuthal variations in radial and transverse component receiver functions. Because anisotropy-aware receiver function analysis has good lateral resolution and depth constraints, it is complementary to previous studies of anisotropy in this region using shear wave splitting or surface wave tomography. We examine data from long-running permanent stations NNA (near Lima, Peru) and LPAZ (near La Paz, Bolivia), and two dense lines of seismometers from the PULSE and CAUGHT deployments in Pent and Bolivia, respectively. The northern line overlies the Peru flat slab, while the southern line overlies the normally dipping slab beneath Bolivia. We applied harmonic decomposition modeling to constrain the presence, depth, and characteristics of dipping and/or anisotropic interfaces within the crust and upper mantle. We found evidence for varying multi-layer anisotropy, in some cages with dipping symmetry axes, underneath both regions. The presence of multiple layers of anisotropy with distinct geometries that change with depth suggests a highly complex deformation regime associated with subduction beneath the Andes. In particular, our identification of depth-dependent seismic anisotropy within the overlying plate crust implies a change in deformation geometry, dominant mineralogy, and/or theology with depth, shedding light on the nature of deep crustal deformation during orogenesis.

Seismic nests (or clusters) are typically defined as regions of high seismic activity at intermediate depths that do not move over time and are not associated with volcanic activity at the surface (e.g. Zarifi and Hayskov, 2003; Zarifi et al., 2007; Prieto et al., 2012). In this study, we focus on a seismic nest located in central Peru beneath the Amazon basin city of Pucallpa. The location of this nest is just beyond the eastern-most extent of the Peruvian flat slab, similar to the settings of two other South American clusters, the Bucaramanga and Pipanaco nests. While the Pucallpa nest is visible in the figures of many earlier papers on intermediate depth South American seismicity, it has not to our knowledge been described as a seismic nest before. We present the first detailed description of the Pucallpa nest, compare it to other established nests, and discuss its implications for our understanding of the Peruvian flat slab. Our results indicate that the Pucallpa nest demarcates the northern margin of a sag in the horizontally subducting Nazca plate in central Peru. The position of the nest along the projected location of the downgoing Mendana fracture zone is consistent with local variations in b-values and could help to explain both the nest's genesis and the co-located change in slab geometry. The nest is also spatially well correlated with the northern margin of the thick-skinned Shira Mountains and with the high heat flow associated with the Agua Caliente dome (Hermoza et al., 2006; Navarro Comet, 2018). Further study is needed to understand the effects of the complex Peruvian slab geometry on the formation of thick skinned deformation and heat flow anomalies on the overriding South American continent.

Seismic nests (or clusters) are typically defined as regions of high seismic activity at intermediate depths that do not move over time and are not associated with volcanic activity at the surface (e.g. Zarifi and Hayskov, 2003; Zarifi et al., 2007; Prieto et al., 2012). In this study, we focus on a seismic nest located in central Peru beneath the Amazon basin city of Pucallpa. The location of this nest is just beyond the eastern-most extent of the Peruvian flat slab, similar to the settings of two other South American clusters, the Bucaramanga and Pipanaco nests. While the Pucallpa nest is visible in the figures of many earlier papers on intermediate depth South American seismicity, it has not to our knowledge been described as a seismic nest before. We present the first detailed description of the Pucallpa nest, compare it to other established nests, and discuss its implications for our understanding of the Peruvian flat slab. Our results indicate that the Pucallpa nest demarcates the northern margin of a sag in the horizontally subducting Nazca plate in central Peru. The position of the nest along the projected location of the downgoing Mendana fracture zone is consistent with local variations in b-values and could help to explain both the nest's genesis and the co-located change in slab geometry. The nest is also spatially well correlated with the northern margin of the thick-skinned Shira Mountains and with the high heat flow associated with the Agua Caliente dome (Hermoza et al., 2006; Navarro Comet, 2018). Further study is needed to understand the effects of the complex Peruvian slab geometry on the formation of thick skinned deformation and heat flow anomalies on the overriding South American continent.