The Double Seismic Zone of the Nazca Plate in Northern Chile:

This paper presents an interdisciplinary study of the Northern Chile Double Seismic Zone. First, a high resolution velocity structure of the subducting Nazca Plate has been obtained by a new double­difference tomography method. The double seismic zone (DSZ) is observed between 80 and 140 km depth and the two seismic planes are 20 km apart. Then, the chemical and petrologic characteristics of the oceanic lithosphere associated to this DSZ are deduced by using current thermal­petrological­seismological models, and are compared to pressure­temperature conditions provided by a numerical thermo­ mechanical model. Our results agree with the common hypothesis that seismicity in both upper and lower planes are related to fluid releases associated to metamorphic dehydration reactions. In the seismic upper plane located within the upper crust, these reactions would affect material of basaltic (MORB) composition and document different metamorphic reactions occurring within high­P (> 2.4 GPa) and low­ T (< 570°C) jadeite­lawsonite blueschists and, at greater depth (> 130 km), lawsonite­amphibole eclogite conditions. The lower plane lying in the oceanic mantle can be associated to serpentinite dehydration reactions. The Vp and Vs characteristics of the region in between both planes are consistent with a partially (~25­30 vol.% antigorite, ~0­10% vol. % brucite and ~4­10 vol. % chlorite) hydrated harzburgitic material. Discrepancies persist that we attribute to complexities inherent to heterogeneous structural compositions. While various geophysical indicators evidence particularly cold conditions in both the descending Nazca plate and the continental fore­arc, thermo­mechanical models indicate that both seismic planes delimit the inner­slab compressional zone around the 400°C (±50°C) isotherm. Lower plane earthquakes are predicted to occur in the slabs flexural neutral plane, where fluids released from surrounding metamorphic reactions could accumulate and trigger seismicity. Fluids migrating upwards from the tensile zone below could be blocked in their ascension by the compressive zone above this plane, thus producing a sheeted layer of free fluids, or a serpentinized layer. Therefore earthquakes may present either down­dip compression and down­dip extension characteristics. Numerical tests indicate that inner­slab compression is not only favored by the slab’s thermal structure such as plate age. i) A weak ductile subduction channel, and ii) a cold mantle fore­arc both favor inner­slab compression by facilitating transmission of compressional stresses from the continental lithosphere into the slab. iii) Decreasing the radius of curvature of the slab broadens the depth of inner­slab compression, whereas iv) decreasing upper plate convergence diminishes its intensity. All these factors indicate that if indeed DSZs contour inner­slab compression, they cannot only be linked to slab unbending, but also to the transmission of high compressional stresses from the upper plate into the slab.