Geologia

TECTONICS, VOL. 31, TC2013, doi:10.1029/2011TC003036, 2012

Nazca–South America interactions and the late Eocene–late Oligocene flat-slab episode in the central Andes
Leland J. O’Driscoll,1 Mark A. Richards,2 and Eugene D. Humphreys1
Received 30 September 2011; revised 27 February 2012; accepted 28 February 2012; published 12 April 2012.

[1] The most prominent features of the Andean rangeare the Altiplano and Puna plateaus, which were constructed by crustal shortening and uplift over the past $45 Myr. The early construction of these plateaus may have controlled subsequent growth of the orogen. Proposed models have suggested that an abrupt acceleration in relative motion between the Nazca plate and the South American plate at $30 Ma may have led to compression of the continent.However, the major plate motion change occurred at 25–23 Ma, and paleomagnetic rotations and crustal shortening of the Andean forearc require that the Arica Bend formed prior to about 25 Ma. Inferred history of flat-slab subduction along the Altiplano section of the Andean margin and the structure of the adjacent South American cratonic shield combine to suggest an alternate scenario, based partlyupon geodynamic models of oceanic-continental plate interactions in subduction zones. We propose that central Andean tectonism may have been controlled by two distinct regimes of subduction: (1) oblique subduction along the central Andean margin during the late Eocene and Oligocene accompanied by downdip alignment with the center of the Amazonian Shield (flat-slab activity in this phase oforogenesis may have been caused by a combination of cratonic root enhanced tectonics and oceanic plateau subduction) and (2) an abrupt transition to trench-normal subduction after $25 Ma toward the more distal São Francisco Craton was accompanied by a return to normal angle subduction. Similar interactions are hypothesized to have occurred during the Laramide Orogeny in western North America.
Citation:O’Driscoll, L. J., M. A. Richards, and E. D. Humphreys (2012), Nazca–South America interactions and the late Eocene–late Oligocene flat-slab episode in the central Andes, Tectonics, 31, TC2013, doi:10.1029/2011TC003036.

1. Introduction and Background
[2] The localization in space and time of early ($50–40 Ma) tectonism in the central Andes must be considered when evaluating the subsequent regionalevolution. Proposed models have suggested that an abrupt acceleration in relative motion between the Farallon/Nazca plate and the South American plate at $30 Ma may have led to compression of the continent [Russo and Silver, 1994; Silver et al., 1998]. However, it appears that this relative motion change occurred at $25–24 Ma [Tebbens and Cande, 1997; Somoza, 1998], when westward acceleration ofSouth America also began. We propose an alternative model to explain strong compression of the central Andean margin and coincident flatslab formation during late Eocene to late Oligocene while reconciling relative plate motions, intense forearc shortening that predates 25 Ma [Arriagada et al., 2008], and the location of thick craton-bearing lithosphere. Flat-slab geometry has

been proposed forthis period beneath Peru and Bolivia [Isacks, 1988; James and Sacks, 1999], but the dynamic driving force has yet to be examined. We argue for the importance of the interaction between the South American cratonic lithosphere structure and the downgoing Nazca plate, and extend the modeling of O’Driscoll et al. [2009] to demonstrate the role of enhanced mantle wedge suction in Eocene and OligoceneAndean tectonics. 1.1. Andean Orogeny [3] The orogenesis that built the high-standing Andean tectonic belt occurred over the past 70 Myr [e.g., Coney and Evenchick, 1994; Sempere et al., 1997; Horton et al., 2001; McQuarrie et al., 2005], and the western South American margin has been in a state of compression since at least the early Paleocene. The modern ocean-continent convergent Andean...