Imaging the Upper Plate and Mantle Wedge in the Nicaraguan Subduction Zone with Sp Phases

Abstract:

The goal of this study is to better understand how subduction zone melting processes are manifest in the structure of the mantle wedge, and how they modify the upper plate lithosphere. This investigation focuses on the Nicaraguan segment of the Central American subduction zone, where the Cocos plate subducts beneath the Caribbean plate, and partial melting in the mantle wedge above the slab creates a well-defined volcanic arc. To characterize upper plate and mantle wedge structure beneath Nicaragua, we imaged mantle seismic velocity gradients using Sp converted waves. Broadband waveforms were employed from the Nicaraguan INETER network, the TUCAN broadband seismometer experiment, and other permanent and temporary broadband networks in Nicaragua and northern Costa Rica. Using periods of 4-100s and time-domain deconvolution, we calculated Sp receiver functions. With common-conversion point (CCP) stacking that employs weighting functions based on Sp sensitivity kernels, the receiver functions were mapped to three-dimensional positions in space. The resulting CCP stack reveals clear Sp phases that correspond to the Moho and multiple mantle velocity gradients. Sp arrivals that correspond to a negative velocity gradient mark the base of the Caribbean upper plate where it meets the warm mantle wedge. In the near back-arc, this velocity gradient lies at depths of approximately 75 km, but it deepens to 100 km depth further into the back-arc in northeastern Nicaragua. This result is consistent with the idea that interaction with warm mantle wedge that is rich in slab-derived fluids and partial melt thins the upper plate closer to the arc, while thicker and less-modified lithosphere lies beneath northeast Nicaragua. The CCP stack also shows positive gradients in the mantle wedge, and the strongest and most widespread of these occurs at depths near 150 km. This depth coincides with the lower boundary of a zone of high Vp/Vs from local body wave tomography (Syracuse et al., 2008) that has been interpreted as a zone with higher partial melt fractions that extends from the slab interface up towards the arc. The positive velocity gradient at 150 km likely represents the onset of partial melting in the deeper regions of the mantle wedge.