Upper mantle structure in the south central Chilean subduction zone (30° to 36°S)

Abstract

[1] Regional P and S wave travel time data were used to obtain three-dimensional seismic tomography models for V p , V s , and V p /V s above the subducting slab in central Chile and Argentina. In this region, there is an abrupt change from a normal subduction geometry south of 33°S to a flat subduction geometry to the north. We find low V p , low V s , and high V p /V s ratios in the southern half of our study area directly beneath the modern active volcanic arc, which we interpret as localized pockets of melt. In the northern half of our study area, above where the subducting Nazca plate flattens at 100 km depth, we find low V p , high V s , and low V p /V s ratios. These unusual results point to a lack of melt or hydrated mineralogies such as serpentine, both of which are characterized by high V p /V s values. The only mantle rocks that have low V p /V s and high V s are Mg-rich compositions, such as dehydrated serpentinite or orthopyroxenite. We suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite, formed by a transient fluxing of silica-rich fluids. Such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at $10 Ma. This would imply that the hydration of mantle material above a flat slab can be a transient phenomenon, which leaves little residual-free water behind but significantly alters the mantle chemistry.

Low Vp/Vs ratios in the crust and upper mantle beneath the Sea of Okhotsk inferred from teleseismic pMP, sMP, and sMS underside reflections from the Moho

“…[30] Anomalous V p /V s ratios are not novel in subduction zones [e.g., Zandt et al, 1994;Wagner et al, 2005]. However, in most cases, low V p and high V p /V s ratios are found, which is usually attributed to either partial melting or mantle serpentinization.…”

Imaging Poisson's Ratio of the Uppermost Mantle beneath China

“…This approach assumes that the ray paths of P waves and S waves are identical, which is true only when V P =V S is constant. However, for regional-scale tomography, the difference between the predicted S-P times along identical ray paths and those along independent P-wave and S-wave ray paths was much smaller than the uncertainty of the inverted model, and thus the assumption has no significant impact on the tomography results (Wagner et al, 2005). For Pn and Sn, if we rely on the classical assumption that these phases are headwaves along the mantle lid, V pn =V sn and Poisson's ratio of the uppermost mantle can be obtained by using Sn-Pn traveltime differences.…”

Receiver function images of the Hellenic subduction zone and comparison to microseismicity

“…Anomalous Vp / Vs ratios are frequently shown in subduction zones (e.g. Zandt et al, 1994;Wagner et al, 2005;Zheng and Lay, 2006) and provided useful information on the composition of crustal rocks. Although presence of high values of Vp / Vs ratio in subduction zones can be associated with rocks of more mafic composition, low values of Vp / Vs ratio may reveal felsic crustal composition and high upper-crustal temperatures, which correspond to rocks which contain large amounts of silica.…”