The Chongon/Colonche orocline transrotational bending and the coeval opening of the Progreso basin in southwestern Ecuador

摘要

Integration of geological and geophysical studies of Progreso Basin in SW Ecuador provided a unique opportunity to propose an unorthodox model for the Progreso Basin opening within a strike slip regime. Shearing couple transrotation of the Chong acute accent on/Colonche (C/C) orocline bending accounted for the atypical, isosceles triangle shape, Progreso Basin opening during the Oligocene oblique convergence amid the Farallon and South America plates. Transrotation commenced with the sinistral strike slip reactivation of the Paleocene Chong acute accent on-Jipijapa-Jama (CJJ) Suture. This suture is inferred based on field kinematics of Cretaceous outcrops in the Santa Elena Peninsula and published high resolution potential field maps. The reactivated suture worked as the sinistral La Cruz (LCF) Fault and their "en echelon" synthetic Riedels C/CF, Consuelo (COF), and Carrizal (CAF) faults accommodated most of the basin heterogeneous transtension and transrotation. While the synthetic Riedels migrated northward along the La Cruz Fault, a narrow segment of the Paleogene foredeep and the intra-oceanic arc sequence of the C/C High attached to the Estancia Hills was scrapped off and rotated. The shearing couple between the "en echelon" Riedels enhanced the transrotation and transtension and triggered the counterclock-wise crustal rotation of the C/C High. The subvertical block rotation involved significant lithospheric decoupling of this high that set off the C/C Orocline development and coeval opening of the Progreso Basin. Early Zapotal Formation deposition characterized by alluvial fan aggradation rapidly filled the graben be-tween the Consuelo and Carrizal faults as the Consuelo Fault underwent strain hardening and confining. In doing so, the sediment input by the active alluvial fan complex spilled over the Carrizal Fault displaying down lapping and progradation along the hanging wall. Its listric behavior on other hand, set off the tilting of the pre-extensional Paleogene sequences, which in turn caused the onlapping and diachronous deposition of the Zapotal Formation toward the La Cruz Fault. Protracted marine incursion and deposition of a fine grained sili-ciclastic sequence that encompassed the whole Oligocene took place around the already opened basin toward the south. Early Miocene breakup of the Farallon Plate not only increased the convergence rate but also gradually changed the convergence from oblique to orthogonal. Multiple intraformational unconformities and high sedi-mentation and subsidence rates in the Dos Bocas Formation documented this change. Recurrent deposition of the diatomaceous Villingota Formation suggested episodic upwelling in the basin enriched by the high siliceous tuffs supply from the coeval Saraguro Arc volcanism in the Western Cordillera. Basin transtension terminated during Late Miocene inception of the Rodeo and Playas synthetic Riedels to the reactivated Pallatanga Suture known as the dextral Chingual-Cosanga-Pallatanga-Puna (CCPP) fault system. However, as this fault continued into the Gulf of Guayaquil, the transtension was transferred to this area. Hitherto, the Progreso Basin underwent a sagging phase characterized by the onlapping of the Subibaja and Progreso formations against the La Cruz Fault High as well as a localized, subtle tectonic inversion due to space problems linked to the synthetic Riedels inception.