Commentary on Jaillard, E., (2022): Late Cretaceous-Paleogene orogenic build-up of the Ecuadorian Andes: Review and discussion

Abstract

Although Jaillard's (2022) paper denotes the first attempt to propose a model for the orogeny build-up of the Ecuadorian Andes, it is based on endorsing the poorly constrained and uncertain Caribbean Colombia Oceanic Plateau (CCOP) from the Coastal Ranges (CR) to the Western Cordillera (WC). This model is inconsistent with the crustal thickness variation confirmed by seismic tomography profiles and Moho map (Araujo et al., 2021), unreliable REE chondrite-normalized flat patterns (Kerr, 2014), absent of HIMU Pb isotope anomaly and lack of W, He or Ne isotope information indicative of primordial mantle. Along the Coastal Ranges, Jaillard discusses the NE-oriented San Lorenzo Arc (SLA) within the context of a back-arc basin, incorporating the Chongón Colonche High (CCH) as a remnant arc orthogonal to the margin since its emplacement. Contrarily, he overlooks the widespread Cretaceous olistolith outcrops representing the cryptic and vanished Paleocene suture zone, unlike the undeformed Santa Elena Formation. The occurrence of CR lower continental crust, inferred from lower than 7 Km/s sec V_p velocity, decisively challenges the CCOP paradigm. To the east, while Jaillard describes west-vergent contractional deformation and the prowedge foredeep flysch, he geologically thickens the Western Cordillera by rooting thrusting near the crustal-mantle boundary, including the slender Guaranda Terrane (Totoras amphibolite). This is inconsistent with the 85 Ma crystallization age of HT metamorphism predating the collision event, further questioning the CCOP model. Additionally, Jaillard disregards the Pallatanga suture melange and the adakite Pujilí Granite obstructing subduction, which shifted westward post-collision, forming the Rio Cala intraoceanic arc synchronous with flysch foredeep deposition (Vallejo, 2007) and providing the stresses for contractional deformation. Along the foreland basin, Jaillard's distortion from lithostratigraphic to chronostratigraphic isopach maps impedes recognition of the molasse source, provenance, and deposition, coeval with the EC development. The Abitagua Granite, adjacent to the giant Pungarayacu heavy oil field, necessitates continuity of Cretaceous source rocks under the EC, also functioning as a detachment. The high gravitational potential energy achieved during strong collision dissipates through extensional gravitational collapse, modulating eastward as toe-thrust imbrication and utilizing the Cretaceous detachment to form the Eastern Cordillera retrowedge concurrently with WC contractional deformation, resulting in a bivergent fold and thrust belt.