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We define subducting plate geometries in the Nazca subduction zone by (re)locating intra-slab earthquakes in southern Peru (14–18°S) and using previously published converted phase analysis results to clarify the slab geometry and inferred relationships to the seismicity. We also provide both P - and S -wave velocity models of the subducting Nazca Plate and mantle layer above the slab using double-difference tomography to understand upper-plate volcanism and subduction zone process. The double-difference constraints for determining the hypocenters and velocity model ensure high accuracy of the relative location of earthquakes with respect to velocity structure. The relocated seismicity shows a smooth contortion in the slab-dip transition zone for ∼400 km between the shallow (30°)-to-flat dipping interface to the northwest and the 30°-dipping interface to the southeast. We find a significant slab-dip difference (up to 10°) between our results and previous slab models along the profile region sampling the horizontal slab at a depth of ∼85–95 km. Robust features in both P - and S -wave tomography inversions are both arc-normal and along-arc velocity variations. In the arc-normal direction, all profile results show that the slab velocities beneath the forearc (down to a depth of ∼100 km) transition to higher velocities beneath the backarc (at ∼100–140 km depth). In the along-arc direction, velocities of the subducting flat slab are shown to be depressed compared to those of the normal-dip slab. In addition, high shear-wave velocities and low Vp / Vs are detected in the mantle layer above the flat slab, indicating its cold and dry environment. Such differences in the velocity structures for the slab and mantle wedge between the two regions may indicate different hydration states, which greatly affects the upper-plate structure of southern Peru. [ABSTRACT FROM AUTHOR]

The horizontal Nazca slab, extending over a distance of ∼800 km along the trench is one of enigmatic features in Peruvian subduction zone. Increased buoyancy of the oceanic lithosphere alone due to the subduction of Nazca Ridge is insufficient to fully explain such a lengthy segment. We use data from the recent seismic experiment in southern Peru to find that the subduction-related hydration plays a major role in controlling shear wave velocities within the upper part of the oceanic crust and overlying materials. We observe substantial velocity reductions of ∼20–40% near the top plate interface along- and perpendicular-to the trench from ∼40–120 km depths. In particular, significant shear wave velocity reductions and subsequently higher P-to-S velocity ratio (exceeding 2.0) at the flat slab region suggest that the seismically probed layer is fluid-rich and mechanically weak. The dominant source of fluid comes from metasediments and subducted crust (Nazca Ridge). Long-term supply of fluid from the southward migrating Nazca Ridge provides additional buoyancy of the subducting oceanic lithosphere and also lowers the viscosity of the overlying mantle wedge to drive and sustain the flat plate segment of ∼800 km along the trench. Also, by comparing calculated seismic velocities with experimentally derived mineral physics data, we additionally provide mechanical constraints on the possible changes in frictional behavior across the subduction zone plate interface. Observed low seismic velocities in the seismogenic zone suggest a presence of low strength materials that may be explained by overpressured pore fluids (i.e., accreted sediment included in the subduction channel). [ABSTRACT FROM AUTHOR]

Abstract: Geophysical evidence shows the presence of low-seismic velocity material at the surface of slabs in subduction zones. In the central Mexican subduction zone this appears as a thin (∼4 km) low-velocity zone that absorbs nearly all of the strain. The P-to-S velocity ratio as a function of S wave velocity distinguishes among the various candidate hydrous (low-strength) minerals; the thin layer in the flat-slab region is most consistent with a layer showing enrichment in talc overlying normal MORB-like gabbro. Based on available thermodynamic data for equilibria for talc, its generation at the trench is nearly impossible, and hence we propose it originates from the mantle wedge during the slab flattening process coupled with trench rollback. The evolution of this low-strength zone has important implications for the dynamics of the slab-flattening process as well as the geochemistry of the mantle wedge and arc in central Mexico. [Copyright &y& Elsevier]

Abstract: Teleseismic converted phases are used to probe the composition of the downgoing oceanic crust as a function of depth along the Cocos slab in central and southern Mexico. Previously, modeling of the receiver function (RF) conversion amplitude of the flat Cocos slab beneath central Mexico at 45km depth revealed a thin low-velocity upper oceanic crust of a thickness of 4±1km, which has much lower seismic velocities ( reduction in shear wave velocities) than (normal) lower crust. High Vp/Vs ratio () also suggested a large concentration of hydrous minerals such as talc in combination of high pore-fluid pressure in the horizontal segment. We extend this previous effort to examine seismic properties of both the steeply subducting Cocos oceanic crust beneath the Trans-Mexican Volcanic Belt (TMVB) in central Mexico and the shallowly dipping crust beneath southern Mexico. Inverted seismic velocities using the converted amplitudes at the top and bottom of the dipping oceanic crust are compared with experimentally constrained seismic velocities of candidate mineral phases in a range of likely pressures and temperatures. The composition of the oceanic crust downdip in the steep part of slab beneath the TMVB includes the minerals such as lawsonite and zoisite at 60–100km depth, and the eclogitization occurs around 100km depth. This is related to arc volcanism in the TMVB directly above the slab as well as the slab rollback. In contrast, the dominant mineral phase in the upper oceanic crust of southern Mexico beneath the Isthmus of Tehauntepec is amphibole on top of unaltered gabbroic oceanic crust. The difference in mineral assemblages of the subducted oceanic crust may help explain the difference in slab geometries between central and southern Mexico. [Copyright &y& Elsevier]