Mountain building at ocean-continent margins – linking mass flux, mechanics, and earthquakes at the Andean margin

Abstract:

Deformation at convergent plate boundaries involves various styles of mass flux and of backarc shortening. For the Andes, patterns appear obvious. Long-term mass flux style – i.e. accretionary versus erosive – shows a distinct relationship with forearc as well as backarc deformation mode. Neogene surface deformation exhibits tectonically uplifting areas along the coast driven by interseismically active reverse faulting. Moreover, seismic-cycle vertical displacement is not coincident with long-term vertical motion that probably is superseded by slow basal underplating (southern Chile) or tectonic erosion (northern Chile). Reconstruction clearly indicates that the Central Andean trench has always been underfilled with less than 500 m of sediment. In southern Chile, our data illustrate a similar trend during most of the Cenozoic with a shift around some 6-7 Ma to substantial sediment influx from glaciation of the Patagonian part of the Andean Cordillera. As a consequence, backarc shortening stopped at this latitude, while shortening velocity in the Central Andes was still accelerating.

Using latitudinal evolution and variations of shortening rate, orogenic strain accumulation and deformation partitioning in the Andes can be shown to be dominated by distinct factors. The Altiplano-Puna plateaux are characterized by a complete cycle of initial lateral spreading of deformation followed by subsequent localization and acceleration of bulk shortening rate. Estimates of strength evolution based on force balance calculations and critical wedge analysis suggest significant backarc weakening driving this change. Lithosphere-scale failure from strain weakening beyond a critical strain threshold (c. 20%) and fault coalescence with formation of a weak detachment in shales (µ_eff < 0.1) played a key role in the evolution of the Central Andes. Strain-related lithosphere weakening dominated over the impact of external forcing mechanisms, such as variations of plate convergence, mantle-assisted processes, or erosion. In summary, the climatically-controlled sediment flux into the trench and the subduction channel along with strain-dependent weakening of the upper plate appear to be the key parameters affecting the styles of subduction orogeny.