Along-strike segmentation of the Farallon-Phoenix midocean ridge: Insights from the Paleogene tectonic evolution of the Patagonian Andes between 45° and 46°30′S

GIANNI, GUIDO M.; PESCE, AGUSTINA; SOLER, S.; GARCÍA, H.P.A.; SANCHEZ, MARCOS; NAVARRETE, CESAR; ECHAURREN, ANDRES; ENCINAS, ALFONSO; FOLGUERA, ANDRES

andean tectonics

elsevier

Año: 2019; p. 651 - 673

Interactions between spreading centers and subduction zones are recognized as an eventual and inevitableprocess in plate tectonics (DeLong and Fox, 1977). During subduction, a midocean ridge may unzipinto a “slab window,” where the subslab mantle upwells reaching the base of the upper plate (Dickinsonand Snyder, 1979). At this point, subduction-related magmatism is interrupted in the slab-free area andreplaced by magmatic activity of differing nature, which extends from the forearc to backarc. In theformer area, MORB magmas intrude the forearc, triggering crustal anatexis, thermal metamorphism,hydrothermal activity, and in certain cases, high-T, low-P Buchan metamorphism (DeLong et al., 1979;Sisson et al., 2003a). In the backarc region, low- or eventually high percentages of mantle melting linkedto the asthenospheric upwelling generate alkalic or tholeiitic magmas with OIB affinities (e.g., Gorringet al., 1997; D’Orazio et al., 2000). Additionally, partial fusion of the slab window edges may producemagmas of adakitic composition (e.g., Kay et al., 1993; Johnston and Thorkelson, 1997). Mechanismsof exhumation and deformation in the upper plate during ridge subduction are diverse. In the forearcarea, local ophiolite obduction may take place in the triple junction zone and crustal slivers may developwhen ridges interact obliquely with the margin producing transpressive to transtensive deformation (e.g.,Nelson et al., 1993; see Sisson et al., 2003b, for a synthesis). In the backarc region, extensional to transtensionaltectonic activity may take place contemporaneous to intraplate magmatism (e.g., Thorkelson,1996 and references therein; Lagabrielle et al., 2007). Noteworthy, all of these effects are time transgressiveand are dictated by the kinematics of the triple junction (Thorkelson, 1996). According to recent numericalmodels, ridge subduction may not always lead to the development of a slab window. Accordingto the study of Quevedo et al. (2013) that tested scenarios of oblique subduction of midocean ridges in aglobal 3D spherical setting, there is a critical obliquity angle of 60° above which a compressive regimein the ridge, mainly due to Earth curvature effect, does not enable window formation.Late Cretaceous to Paleogene subduction of the Farallon-Phoenix (or Aluk) midocean ridge beneaththe Andean margin has been suggested in most plate kinematic reconstructions in the southeastPacific Ocean (Cande and Leslie, 1986; Somoza and Ghidella, 2012; Eagles et al., 2009; Eagles andScott, 2014) (Fig. 1). Nevertheless, the exact time-space evolution of this complex interaction remainssomehow uncertain (Somoza and Ghidella, 2012). Initial reconstructions of Cande and Leslie (1986)indicated a southward sweep of the ridge-trench interaction in Patagonia from 52 to 42 Myr (Fig. 1).In this line, Ramos and Kay (1992) placed this interaction in Patagonia south of 43°S based on geologicalconstraints linked to the finding of a Paleocene-Eocene arc shut-off and intraplate magmatismwith geochemical signatures compatible with slab window development. Aragón et al. (2011a, 2013),based on geochemical and seismic tomography evidence of slab detachment and slab window formation,suggested that this interaction started further north between 36°S and 44°S.Nevertheless, morerecent studies suggest that the regional tectonic setting could have been more complex than previouslyassumed. In this sense, recent works have documented that certain sectors experienced intermittent arcactivity in Eocene times (Pankhurst et al., 1999; Fernández et al., 2018) and contraction reaching thecontinental interior from Paleocene to Eocene, opposed to the expected deformational effects of slabwindow development (Navarrete et al., 2015; Gianni et al., 2015a, 2017).Several studies have demonstrated that geological information can be helpful to infer pastlocations of midocean ridge-trench interactions, even when oceanic plates have been completelysubducted (e.g., Atwater, 1990; Breitsprecher et al., 2003). In this contribution, with the aimto shed light into the complex Paleogene tectonic setting of Central Patagonia, we carry out asynthesis of latest works with special emphasis on a key orogenic segment between 45°S and46°30′S (Fig. 2). This updated geological history reveals a complex tectonic evolution that isthen compared to existing models linking the Paleogene tectonics of Patagonia to midocean ridgeinteractions.