Crustal growth at active continental margins: Numerical modeling

The dynamics and melt sources for crustal growth at active continental margins are analyzed by using a
2D coupled petrological–thermomechanical numerical model of an oceanic-continental subduction zone.
This model includes spontaneous slab retreat and bending, dehydration of subducted crust, aqueous fluid
transport, partial melting, melt extraction and melt emplacement in form of extrusive volcanics and
intrusive plutons. We could identify the following three geodynamic regimes of crustal growth: (i) stable
arcs, (ii) compressional arcs with plume development, and (iii) extensional arcs. Crustal growth in a stable
subduction setting results in the emplacement of flattened intrusions in the lower crust. At first dacitic
melts, extracted from partially molten rocks located atop the slab (gabbros and basalts), intrude into
the lower crust followed by mantle-derived (wet peridotite) basaltic melts from the mantle wedge. Thus
extending plutons form in the lower crust, characterized by a successively increasing mantle component
and low magmatic addition rates (10 km3/km/Myrs). Compressional arcs are accomplished by the formation
and emplacement of hybrid plumes. In the course of subduction localization and partial melting of
basalts and sediments along the slab induces Rayleigh Taylor instabilities. Hence, buoyant plumes are
formed, composed of partially molten sediments and basalts of the oceanic crust. Subsequently, these
plumes ascend, crosscutting the lithosphere before they finally crystallize within the upper crust in form
of silicic intrusions. Additionally, intrusions are formed in the lower crust derived by partial melting of
rocks located atop the slab (basalts, gabbros, wet peridotite) and inside the plume (basalts, sediments).
Magmatic addition rates are somewhat higher compared to stable arcs (40–70 km3/km/Myrs). Subduction
in an extensional arc setting results in decompression melting of dry peridotite. The backward
motion of the subduction zone relative to the motion of the plate leads to thinning of the overriding plate.
Thus, hot and dry asthenosphere rises into the neck as the slab retreats, triggering decompression melting
of dry peridotite. Consequently large volumes of mafic (oceanic) crust are formed in the backarc region
with total magmatic addition rates being as high as 90–170 km3/km/Myrs.