Crustal Structure of the Incoming Iquique Ridge Offshore Northern Chile

Abstract

The Peru‐Chile subduction zone hosts M > 8 earthquakes as well as multiple ridges on the downgoing Nazca plate, making this region well suited for investigating the formation, evolution, and potential impacts of subducting features on seismogenesis. To evaluate the physical properties and structural variability of the Iquique Ridge offshore northern Chile, we present a P wave velocity model of the Nazca plate and Iquique Ridge outboard of the 2014 M 8.1 Iquique earthquake sequence. 2D tomographic inversions of P wave traveltime data from the 2016 PICTURES (Pisagua/Iquique Crustal Tomography to Understand the Region of the Earthquake Source) controlled‐source experiment indicate that the Iquique Ridge is characterized by significant crustal thickening relative to typical Nazca plate oceanic crust, with a maximum thickness of ∼13 km beneath the most prominent portion of the Iquique Ridge and outer rise. Reduced upper crustal seismic velocities extend from the ridge eastward to the trench, which may indicate increased fracturing and/or hydration during plate bending. Corresponding multi‐channel seismic reflection data show along‐profile structural variation as well, including an intermittent Moho reflector near the onset of crustal thickening and faulting along the landward slope. The structural heterogeneity entering the Peru‐Chile Trench has potential implications for slip behavior at depth assuming that the anomalous crustal structure continues beneath the forearc. We suggest that the increased buoyant normal force associated with discrete subducted Iquique Ridge seamounts similar to the zone of thick crust imaged here could lead to localized increased interplate coupling, while entrained sediment and fractured/altered crust could facilitate aseismic slip.

Megathrust reflectivity reveals the updip limit of the 2014 Iquique earthquake rupture

“…Although the incoming oceanic crust off northern Chile has not been drilled, recent seismic data suggest that the upper crustal velocity of the incoming plate beneath the outer rise is anomalously low, suggesting the presence of mineral hydration and/or pervasive fluid-filled cracks to a depth of ~5 km 40 . We conclude that the coherent highly reflective shallow plate boundary off Northern Chile, which behaved aseismic during the 2014 Iquique earthquake, is fluid-rich, and that the source of the fluid is intergranular and fracture porosity in seismic layer 2A 6 , 37 , 41 of the oceanic crust and mineral bound water released through dehydration of the weathered clay-bearing oceanic basalt.…”

“…The convergence of the Nazca and South America plate indicated by a black arrow 60 . The location of the seismic line defining the structure of the incoming plate 40 is shown by a red dashed line. …”

The Role of Subslab Low‐Velocity Anomalies Beneath the Nazca Ridge and Iquique Ridge on the Nazca Plate and Their Possible Contribution to the Subduction Angle

“…Roughly ∼700 km south of the Nazca Ridge, another SSLA is also imaged with seismic tomography beneath the subducted ∼200‐km‐wide Iquique Ridge (Portner et al., 2020; Scire et al., 2015) (Figures 2e and 2g). This ridge originated at the Foundation Hotspot (Bello‐González et al., 2018; Myers et al., 2022), but in this case, the subduction of the ridge does not appear to contribute to the slab flattening (Espurt et al., 2008; Portner et al., 2020; Scire et al., 2016). Intriguingly, although both slab segments include an aseismic ridge and an SSLA, only one of them experiences flat subduction.…”