Double seismic zone of the Nazca plate in northern Chile: High-resolution velocity structure, petrological implications, and thermomechanical modeling

Abstract

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 the tomoDD doubledifference tomography method. The double seismic zone (DSZ) is observed between 80 and 140 km depth, and the two seismic planes is 20 km apart. Then, the chemical and petrologic characteristics of the oceanic lithosphere associated with this DSZ are deduced by using current thermal-petrological-seismological models and are compared to pressure-temperature conditions provided by a numerical thermomechanical model. Our results agree with the common hypothesis that seismicity in both upper and lower planes is related to fluid releases associated with 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 high-P (>2.4 GPa) and low-T (<570 degrees C) jadeite-lawsonite blueschists and, at greater depth (>130 km), lawsonite-amphibole eclogite conditions. The lower plane lying in the oceanic mantle can be associated with serpentinite dehydration reactions. The Vp and Vs characteristics of the region in between both planes are consistent with a partially (similar to 25-30 vol % antigorite, similar to 0-10% vol % brucite, and similar to 4-10 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 fore arc, thermomechanical models indicate that both seismic planes delimit the inner slab compressional zone around the 400 degrees C (+/-50 degrees C) isotherm. Lower plane earthquakes are predicted to occur in the slab's flexural neutral plane, where fluids released from surrounding metamorphic reactions could accumulate and trigger seismicity. Fluids migrating upward 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 tensile characteristics. Numerical tests indicate that the slab's thermal structure is not the only factor that controls the occurrence of inner slab compression. (1) A weak ductile subduction channel and (2) a cold mantle fore arc both favor inner slab compression by facilitating transmission of compressional stresses from the continental lithosphere into the slab. (3) Decreasing the radius of curvature of the slab broadens the depth of inner slab compression, whereas (4) decreasing upper plate convergence diminishes its intensity. All these factors indicate that if DSZs indeed contour inner slab compression, they cannot be linked only to slab unbending, but also to the transmission of high compressional stresses from the upper plate into the slab.