The role of the slab pull force in the late Oligocene to early Miocene extension in the Southern Central Andes (27°-46°S): Insights from numerical modeling

IANNELLI, SOFIA B.; FENNELL, LUCAS M.; LITVAK, VANESA D.; FENNELL, LUCAS M.; LITVAK, VANESA D.; QUINTEROS, JAVIER; FOLGUERA, ANDRÉS; QUINTEROS, JAVIER; FOLGUERA, ANDRÉS; IANNELLI, SOFIA B.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2018 vol. 87 p. 174 - 187

0895-9811

Although extensional deformation plays a significant part of Andean history, the causes behind itsdriving mechanisms and its impact throughout the geological record remain controversial. Through theaid of numerical modeling of subduction zone dynamics, we were able to reproduce a brief period ofintra-arc basin formation that affected the Southern Central Andes (27-46S) during late Oligocene andearly Miocene times. The results of the model show that, after a period of slow subduction (6e8 cm/yr),the oceanic plate approaches the mantle transition zone at ca. 23 Ma, triggering the slab pull force. Theaddition of this slab pull force generates a progressive increase in convergence velocity (reaching ~20 cm/yr) and the retreat of the trench hinge away from the upper plate, resulting in the steepening of the slab.Effects observed in the upper plate are the formation of a basin located 200e300 km east of the trenchand an asthenospheric influx beneath an 800 km wide zone east of the oceanic and continental plate´sboundary. A series of parameters extracted from our model, such as the basin depth and the stretchingfactor, indicate that crustal stretching, basin formation, convergence velocity and asthenospheric influxwould have reached their climax approximately at 20 Ma. These results are in good correlation with theconvergence rate obtained through plate reconstructions and the geological record along the SouthernCentral Andes, where a series of extensional intra-arc basins were created and mantle derived magmaticprocesses affected a wide area ranging between the present fore-arc and retroarc areas during lateOligocene to early Miocene times. However, differences in extension magnitude, magma compositionand basin fill depositional environment are observed, indicating that the impact of the slab pull force wasstronger towards the southern basins. Possible causes that could explain these differences are variationsin crustal thickness before the influence of the slab pull force and the effect of toroidal mantle flow nearthe southern lateral slab edge. This would indicate that although the main parameter controlling tectonicregime is the absolute motion of the overriding plate, the slab pull force may leave its imprint along theevolution of subduction-type orogens such as the Ande