WHAT IS THE ROLE OF ANDEAN MARGIN PROCESSES IN THE EVOLUTION OF PALEOZOIC NORTHERN APPALACHIAN OROGENIC BELTS?

The Andean margin is the type example of an arc formed on continental crust and provides a Neogene laboratory for examining magmatic and tectonic processes that could have relevance to older continental margins like those that existed in the northern Appalachians. Among important processes are (a) periods of broadening and narrowing of the magmatic arc and the deformational front into the foreland related to shallowing and steepening of the subducting slab: (b) periodic foreland advances of the frontal magmatic arc linked to loss of forearc crust and lithosphere in peaks of forearc tectonic erosion; and (c) delamination of thickened continental crust and lithosphere. Among key clues to these processes are magmatic patterns. Signals of shallowing subduction zones include the eruption of magmas with subduction-like trace element signatures across the backarc. Signals of steepening include magmatic flare-ups spurred by decompression melting in an expanding mantle wedge. Lithospheric thinning can occur in both the shallowing and steepening stages.

An important question with regard to constructing older orogenic events is how much of the continental record has been lost to forearc subduction erosion. A Neogene Andean example is the large volume of forearc continental crust suggested to have been lost as the arc front was displaced ~50 km eastward over at least 800 km along the margin from the northern Southern Volcanic Zone across the Chilean flat-slab to the southern Central Volcanic Zone. The estimated removal rate is over 150 km³/my/km over this region between 8 and 3 Ma. This is just part of the estimated total loss with many estimates suggesting more than 250 km of loss on parts of the Andean margin since the Jurassic. This material is hard to account for in local mass balance models, consistent with a large portion being recycled into the mantle. Geochemical indicators for removed crust reaching the arc magma source include transient steep REE patterns in erupted adakitic magmas that cannot be explained by melting of the down going slab or thickened crust, and temporal isotopic changes in erupted magmas. Regions of the most intense changes along Andean margins are often related to subduction of anomalies on the seafloor including active and aseismic ridges.