SUBDUCTION TECTONICS, MAGMATISM AND SURFACE HEAT FLOW IN THE ANDEAN ARC

We have tested several heat flow models of the crust and upper mantle in the Andes magmatic arc to assess the relative roles of magmatism, asthenosphere flow, crustal thickness, and plate velocity in establishing the observed surface heat flow pattern and the pattern of volcanic activity during the past 40 ma. Observations along a 2,500 km long by 400 km wide transect perpendicular to the magmatic arc show that heat flow decreases sharply from about 200 mW m^-2 to 60 mW m^-2. The zone of highest heat flow, 200 mW m^-2, is approximately 100 km wide in the active volcanic section of the Andean arc. The heat flow models include a subduction zone roll back at 77 mm a^-1 with the descending slab sinking vertically at 44 mm a^-1 but maintaining a 30 degree dip of the seismic zone that defines the upper surface of the slab. Asthenosphere counter flow occurs along the surface of the slab parallel to the slab surface at 89 mm a^-1. The models indicate that frequent magma pulses of significant volumes into the upper crust are necessary to achieve the observed heat flow profile. Heating of the base of the crust by upwelling asthenosphere results in significant magma production through basalt injection and lower crustal melting, assimilation, storage and homogenization (MASH process). This lower crustal zone feeds upper crustal magmatic systems at 8 to 4 km that feed the active volcanoes. The transport of magma through the crust results in significant changes to the thermal profile and strength of the entire crust.