Geological record of along-strike segmentation of the Farallon-Phoenix mid-ocean ridge: Insights from the Paleogene tectonic evolution of the Patagonian Andes between 45° and 46°30?S

GUIDO GIANNI; AGUSTINA PESCE; HÉCTOR GARCÍA; CÉSAR NAVARRETE; ANDRÉS ECHAURREN; ANDRÉS FOLGUERA

Congreso; XV Congreso Geológico Chileno; 2018

Plate kinematic reconstructions in the southeast Pacific Ocean indicate that the Farallon-Phoenix (or Aluk) mid-ocean ridge should have subducted someplace beneath South America during Late Cretaceous to Paleogene times (Cande and Leslie, 1986; Somoza and Ghidella, 2012; Eagles and Scott, 2014; Eagles and Jokat, 2014). However, the precise location of this oceanic feature through time remains somehow uncertain (Somoza and Ghidella, 2012). Supporting previous inferences of Cande and Leslie (1986), geological constraints based on the finding of a Paleogene arc shut-off and 57-40 Myr intraplate magmatism with geochemical signatures compatible with slab window development between 46°15?S to 50°S, allowed placing this interaction in Central Patagonia (Ramos and Kay, 1992; Kay et al., 2004; Espinoza et al., 2005).More recently, Aragón et al. (2011a) argued that id-ocean ridge collision took place further north, between 36°S to 44°S, based on geochemical signatures on 60-40 Myr back-arc bimodal volcanic rocks and seismic tomography evidences of Paleocene-Eocene Phoenix/Aluk slab detachment. Nevertheless, additional complexity has been provided by recent works showing that the Andean segment between 45º-46º30?S experienced intermittent arc activity in Eocene times (Pankhurst et al., 1999) and orogeny from the Andes to the intraplate area from Paleocene to Eocene indicated by the age of syncontractional deposits (62-40 Myr), contrarily to what is expected for slab window processes (Paredes et al., 2006; Ceballos et al., 2016; Navarrete et al., 2015; Gianni et al., 2015; 2017). According to Gianni et al. (2017), the compressional stress-field originated from the active margin to the west as indicated by the record of partially coeval subduction-related magmatic arc activity between ~45 to 39 Myr. Noteworthy, Pankhurst et al. (1999) related this stage of ~45-39 Myr arc magmatism to a period of rapid orthogonal convergence. Thus, the presence of two separate areas with simultaneous ~60-40 Myr slab window-related magmatism disconnected by a sector registering plate-wide contraction with minimum within-plate magmatism impedes proposing a single ridge-trench interaction (Figure 1).A detailed geological record can give us helpful information about the potential past locations and kinematics of mid-ocean ridges, even when oceanic plates have been totally subducted (e.g. Atwater, 1990). In this work, an updated evolution including latest results from geological studies is used as constraint in a recent plate kinematic model to propose a hypothetical scenario that explains enigmatic features in Paleogene times, such as simultaneous slab-window synextensional magmatism at two different latitudes and subduction orogenesis between them.