FLAT SLAB SUBDUCTION IN LATIN AMERICA IN THE CONTEXT OF ONGOING SUBDUCTION OF TERTIARY OCEANIC LITHOSPHERE, SUBDUCTING SEAMOUNTS AND RIDGES, AND CONTINENTAL UPPER PLATES: IMPLICATIONS FOR SLAB MORPHOLOGY, SEISMICITY STRUCTURE AND FOCAL MECHANISMS, AND SEISMIC HAZARDS

Differences exist between oceanic plates subducting in the eastern and northern Pacific Ocean (the Americas), and the Pacific Plate subducting under the western Pacific (e.g., age of subducting lithosphere, the nature of original plume magmatic sources penetrating and subducting with the plate, and the young sediment fluxes entering their trenches). These and other variables have important consequences among subduction observables. Five of the seven largest interplate-thrust earthquakes since 1900 in the Pacific Basin have occurred in the Americas, as have many of the largest trans-Pacific earthquake-induced tsunamis. For reasons rooted in human history, building practices, and effects of climatic and geographic drivers on population distribution, many of the largest and most destructive intermediate-depth earthquakes (deeper than 40 km) along the Pacific Rim since 1900 have occurred under the Americas. Likewise, arc volcanoes under the Americas near population centers are among the most closely watched and feared along the Pacific Rim.

The combination of young oceanic plate age, relatively fast subduction, continental upper plates, and abundant offshore volcanic ridges and seamounts produces a unique suite of slab morphologies and profiles of seismic hazards in Latin America. True flat-slab subduction zones are presently unique to Latin America. In the south, three of the four boundaries that define these zones correlate with present-day or past collisions of large volcanic ridges and/or seamount chains. Several such chains are colliding with the South American forearc along the Chile Trench between the Juan Fernandez and Nazca Ridges. Additional evidence of prior collisions is found in trench-slope re-entrants. Extreme clustering of intermediate-depth earthquakes east of the Chile trench is also thought to be a consequence of seamount/ridge subduction. Further aspects of flat-slab discussed are: (1) Closely-spaced double seismic seismic zones, including paired crustal and mantle events near the outer rise and the roles of subducting seamounts and ridges. (2) High flexural strain rates (double flexure). (3) Forearc basin subsidence. (4) Large, shallow high-frequency intraslab earthquakes that have caused more fatalities since 1900 than all the great interplate thrust events.