Subducting slab thermo-petrologic structure and intermediate-depth supra-slab earthquakes beneath southern Colombia

Abstract

We develop 2-D thermo-petrologic models for two transects through the southern Colombia subduction zone and compare the model-predicted temperature distributions and stability zones of hydrous minerals with seismological observations. Along the northern transect, intermediate-depth earthquakes occur above the relatively young subducting Nazca slab (10–12 Ma) at 80–160 km depths. These intermediate-depth supra-slab (IDS) earthquakes are interpreted to occur in an accreted oceanic terrane and/or in the overriding mantle wedge. Along the southern transect, supra-slab earthquakes are absent. The Vp/Vs in the slab are on average 1.78 and 1.76 along the northern and southern transects, respectively, with locally higher values of up to 1.95 along the northern transect. The Vp/Vs in the region of IDS earthquakes along the northern transect is ~1.78. These relatively high values likely indicate dehydration in the subducting slab and the addition of slab-derived fluids in the mantle wedge. Thermo-petrologic models indicate that to provide the relatively cool condition (<700ºC) for the observed seismicity, the maximum depth of decoupling (MDD) between the subducting slab and the overriding mantle needs to be anomalously deep, at ~180 and ~130 km depths for the northern and southern transects, respectively. With these MDDs, the model-predicted thermal fields result in slightly deeper peak dehydration in the subducting slab along the northern transect than the southern transect, affecting the distribution of slab-derived fluids in the subduction zone. The difference in the MDD between the two transects may explain the contrast in the distributions of intraslab and supra-slab seismicity between them.