The?Role?of?Temperature?in?the?Along-Margin? Distribution?of?Volcanism?and?Seismicity?in?Subduction? Zones:?Insights?From?3-D?Thermomechanical?Modeling? of?the?Central?Andean?Margin

Abstract

The distribution of volcanic and seismogenic zones is segmented along the trench-paralleldirection in the Central Andes, and factors controlling their clustering are not fully understood. Herewe present a 3-D thermomechanical model of the subduction zone at 18°–26°S to examine the role thattemperature and mantle flow play in the distribution of active volcanoes and seismicity. We applied asteady state approach in which solid-state flow is driven by a kinematically prescribed slab with realisticgeometry (including changes along the Bolivian Orocline) and using a 3-D model of the continental crustthickness. The obtained temperature distribution is consistent with proxies for isotherms derived fromindependent geophysical data, except below the Eastern Cordillera at 21°–23°S. The computed mantleflow pattern reveals the presence of along-margin dynamic pressure gradients. This 3-D preferential flowresults in mantle temperatures of 1200–1400°C at 80–100 km depth below the arc, with comparativelyhigher temperatures at ?22°–25°S. The obtained along-margin variations in temperature and in estimatedmelt velocity suggest that the subarc mantle south of 22°S exhibits more favorable conditions forgeneration and upward migration of partial melts. This segment coincides with the higher concentrationof active arc volcanoes and the presence of the Altiplano-Puna Volcanic Complex in the backarc.Intermediate-depth seismicity concentrates roughly below where the slab top is at 400–800°C, suggestingthat temperature exerts some control on the first-order distribution of intraslab seismicity. However,most intraslab seismicity occur at pressure-temperature conditions which are outside of the stability fieldexpected for key dehydration reactions in slabs.