Exhumation of the northern Sub-Andean Zone of Ecuador and its source regions: a combined thermochronological and heavy mineral approach

Resumen

The Ecuadorian Andean Amazon Basin (AAB) developed along the eastern Andean margin as a response to the (1) interaction of the subducting Farallon/Nazca plates and the South American Craton, and (2) accretionary events since Early Cretaceous.The Aptian to Recent sediments contain clastic material which was eroded from the craton and its Paleozoic cover to the east as weil as the rising Andean Cordilleras in the west.Previous work established a quantitative thermal history record of exhumation in the bounding orogen to the west although the depths of rock exhumed limit the temporal extent of this record.Often information cannot be continuously traced within the orogens because it has been removed by erosion or overprinted.Hence the details of source region tectonics can be recognized in sedimentary sequences using geochronology.An advanced thermochronological methodology based on pattern of changes in lagtime upwards in the stratigraphie column, combined with heavy mineral analysis, allowed events in the source regions of the AAB to be distinguished through dating of the eroded material present in the northern SAZ and Pastaza depression where uplift has exposed both the Jurassie basement and its Aptian to Recent sedimentary cover.In a preliminary study, the thermal history of the northern SAZ was constrained using fission-track analysis on both zircon and apatite from 25 sampies from the basement of the region.The results reveal that the basement of the sediments, dominantly Jurassie igneous sequences, has never undergone temperatures higher than 100C since emplacement.Consequently, the region was never heated to temperatures that would have modified zircon detrital fission-track ages in the overlying sedimentary sequences (zircon has a closure temperature of -270 40C).Hence, the dated zircon fission-track ages from the Aptian to Recent siliciclastic sediments are true detrital ages and express the varied thermal histories of the source regions.70 detrital zircon fission-track age populations were statistically extracted from 1082 zircon grains, and show a significant variation from 579 65 Ma to 22.9 1.2 Ma.The abundance of DZFT populations with Proterozoic to Early Cretaceous ages in the Aptian-Albian Hollin Fm. suggests that several distinct source regions, which may have been significantly geographically dispersed, contributed to the early infilling of the AAB.The old populations (600-250 Ma) indicate a probable sourcing in the Guyana-Brazilian Shield regions to the east.Early Cretaceous detrital zircon fissiontrack populations suggest a Late Jurassic-Early Cretaceous phase of exhumation along the Ecuadorian margin (Peltetec event?), which may be responsible for the 60 My hiatus observed in some locations between the Jurassie Misahualli volcanic are and the overlying Aptian-Albian sediments (Hollin Fm.).A rapid exhumation within the source region greater than 2mm/yr during the Coniacian/Santonian to Paleocene was recognised.This is most Iikely from within the Cordillera and may have been due to the accretion of an oceanic plateau, referred to as the Pallatanga Terrane, against the Ecuadorian margin.This is also manifested by the first appearance of metamorphie detritus into the Maastrichtian Tena Fm.