REMOVING THE BACKBONE OF THE AMERICAS

INTRODUCTION: The Pacific edge of the basement backbone of much of the Americas is being stripped away by subduction erosion. Subduction erosion inserts upper plate material into the mantle-bound subduction channel separating the forearc from the underthrusting oceanic plate. The principal evidence for subduction erosion is a narrow or completely missing forearc of Mesozoic or early Tertiary age, landward march of the magmatic front over tens of millions of years, and documentation of on-going crustal thinning and commensurate subsidence of the submerged forearc on the order of 4-5 km. Excluding Alaska, except for Panama, tectonic trimming of the western side of the Americas extends from Mexico ~8000 km southward to Tierra del Fuego.

THE REMOVED BACKBONE: Forearc truncation causes the trench axis to migrate landward toward a fix onshore reference. Long-term (i.e., >10-15 Myr) measurements of truncation for the Mexico, Guatemala, Costa Rica, Peru, northern Chile, and central Chile margin average ~2.5 km/Myr. Truncation over the long term is a pokey tectonic process, trimming the margin back at an average rate only 1/30-1/50 of the rate of orthogonal convergence. But over long periods of time a wide strip of forearc basement is destroyed. For example, since the breakup of Pangea a ~400-km-wide strip of western South American's basement and included younger arc magmatic bodies has been removed. Since the earlier breakup of Rodinia and the subsequence establishment of a Pacific subduction zone, an even wider belt (at least 600 km) of basement rock--no matter how acquired after Rodinian breakup--has been struck from the western margin of South America.

FOSSIL TRUNCATION SCARS: It would be bizarre if the forming truncation scar from Mexico to Tierra del Fuego is a unique occurrence for the western Americas. It is far more likely that fossil truncations caved by subduction erosion are included within the Americas' western rock framework. Candidate scars are the Sur-Nacimiento fault of western California (marks contact of the Franciscan accretionary complex and its Juro-Cret arc basement), sectors of the Border Ranges fault of south central Alaska (marks contact of the Jurassic Talkeena arc and its accretionary prism), and at least the northwestern part of the much-debated Mojave-Sonora megashear of western North America.