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ENREGISTREMENT DES SURRECTIONS LIEES AUX ACCRETIONS DE TERRAINS OCEANIQUES : LES SEDIMENTS CRETACE-PALEOGENES DES ANDES D'EQUATEUR

Abstract : The Northern Andes differ from the Central Andes by the presence of oceanic terranes accreted in the upper Cretaceous and Paleogene. As a consequence, the Andes of Ecuador comprise accreted oceanic terranes to the West (Costa, Cordillera Occidental), and the South American continental margin to the East (Cordillera Real, Subandean zone and Oriente basin).
This work aims to characterize the quartz-rich clastic sediments deposited during successive accretions and to propose a model for the chain uplift and erosion. In addition, this aims to verify the hypothesis of Guillier et al. (2001) et Jaillard et al. (2002), which propose that the relief of the Andes of Ecuador does not result from tectonic shortening of the continental margin, as in the Central Andes, but from the addition, by accretion and underplating during oceanic terranes accretions, of buoyant oceanic crustal material, exerting an Archimedes force responsible for the chain uplift, which then induced the reactivation of the erosion, and the deposition of quartz-rich clastic sediments, that seal the accretions.
We studied sandstones and conglomerates of Campanian to Eocene age, coeval with the accretion and underplating of oceanic terranes, in order to determine the tectonic consequences of these processes on the tectonic behaviour of the Ecuadorian continental margin, specially on the Occidental Cordillera and the Real Cordillera, but also on the back-arc and fore-arc zones.
Therefore, we analyzed : (1) the paleodepths of deposition of the syntectonic sediments, and the erosional discontinuities due to each accretion ; (2) the position of the paleo-shoreline and of the marine sedimentation on either side of the emergent margin ; (3) the topographic gradient between the source zones and the depocentres, expressed by the grain size variations of the sediments ; and (4) the exhumation of the source zones, through the petrographic evolution of the studied clastic sediments. Three basins of the Occidental Cordillera were studied in detail: the Cuenca basin to the South, and those of Saquisilí and Apagua in the Central part of Ecuador.
The vertical succession of sedimentary environments in the studied basins shows the presence of major erosive unconformities defining four main depositional sequences, and points out the accretion periods. These accretions are: (1) Late Campanian (≈ 75–72 Ma), (2) Late Maastrichtian (≈ 69–65 Ma), and (3) Late Paleocene (≈ 58–56 Ma) in age. A last tectonic event of Late Eocene age (Bartonian - Priabonian) was well recorded throughout the margin. From South to North of the Occidental Cordillera, the 4 sequences evidence, except for the Maastrichtian, a jerked shallowing upward trend of the paleodepths of deposition from the Campanian to the Eocene.
This study also evidences the retreat of the paleoshoreline during the upper Cretaceous - Paleogene interval. Indeed, the sedimentary basins of the Cordillera Occidental are younger, as they migrate to the west, and also northward. As a matter of fact, the southern Cuenca basin is infilled by middle Campanian to Paleocene p.p. deposits, the sediments of the Saquisilí basin are of Early Maastrichtian to early Middle Eocene age, and the Apagua basin contains eocene sediments. It is worth to note that farther Northwest, the La Cubera basin was infilled by sediments of Paleocene to Late Eocene age.
Each accretion corresponds to an increasing grain size of sandstones and conglomerates, in the western basins (Cordillera Occidental and Costa) as well as in the Eastern zones (Subandean Zone and Oriente basin), except for the upper Maastrichtian accretion, which did not provoke an increase of the grain size in the sediments, but an erosional hiatus and a noticeable change in the source zones.
In the Cuenca, Saquisilí and Apagua basins, a coarsening upward trend is recorded within each sequence (Campanian, Paleocene and Eocene), as well as in the succession of the sequences, confirming both the steepening of the margin paleoslope, and the uplift of the source area, essentially constituted by the Cordillera Real. Within each sequence, the increasing paleoslope is expressed by the rapid evolution from sandstones to conglomerates, which announces the tectonic event responsible for the overlying unconformity. For the same time-spans, similar evolutions are recorded, although in a more distal manner, on the Coast and in the eastern zones of Ecuador.
Since the Cretaceous until the Paleogene, the mineral assemblages of the clastic sediments of the Cordillera Occidental show a three-stage evolution : (1) surrection and exhumation of the Cordillera Real, from the Campanian to the Paleocene, and to a minor extent until the upper Eocene ; (2) increasing amount, from the Paleocene to the Eocene, of volcanic and sedimentary lithic fragments deriving from the deformation and erosion of the previously accreted terrains ; and (3) resumption of the magmatic arc activity from upper Eocene onward, indicated by the enrichment in plagioclase crystals. During stage 1, the sediments show a clear enrichment in stable minerals and lithic grains (mono- and polycrystaline quartz) at the expense of feldspars, thus reflecting a greater maturity of Paleogene sediments, with respect to those of the latest Cretaceous. At that time, the main source area was the Cordillera Real. From upper Paleocene on (stage 2), the increasing amount of lithic fragments deriving from the oceanic terranes shows that the deformed and eroded zone extends westward, and eventually includes the accreted terranes.
In addition, this analysis allowed to point out two original results.
The rectangle-shaped basins of the Cordillera Occidental (Cuenca, Saquisilí, Apagua, La Cubera) show tectonically active borders corresponding to major, NNE-trending dextral strike-slip faults, which reactivates accretionary sutures, and important subsidence rates, which suggest they are pull apart basins. We observe also an increase of deposition rates through time, suggesting an increasing instability of these basins located on the accretion sutures. The creation of these basins seems to coincide with the accretion of oceanic terranes, that would have occurred, therefore, in a dextral transpressive regime. The northward migration of these basins through time remains, however, to be explained.
The eastern areas of Ecuador are classically interpreted as representing the flexural back arc basin (foredeep) of the Andean chain since the upper Cretaceous. Nevertheless, our study shows that, at a regional scale: (1) the hiatuses and erosions are more important at the foot of the Cordillera Real than to the East ; (2) the thicknesses of the Campanian to Eocene deposits increase from West to the East ; and (3) the Maastrichtian, Paleocene and Eocene sequences seem to retrograde toward the West. This set of observations point out that this part of the margin underwent uplift, rather than flexural subsidence. We propose that the Andes of Ecuador were built up by isostatic uplift associated with the successive accretion and underplating, beneath the margin, of crustal oceanic material of relatively weak density. These terranes would have generated an Archimedes force, responsible for the isostatic surrection of the margin following each accretion. In this model, the viscous reaction of the uplifted and flexured lithosphere would had accentuated its curvature radius, allowing the retrogradation of back-arc clastic deposits toward the chain, and explaining the observed lag-time of the sedimentary response in the eastern areas.
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Contributor : Jorge Toro Alava <>
Submitted on : Friday, September 21, 2007 - 2:58:30 PM
Last modification on : Thursday, November 19, 2020 - 3:54:26 PM
Long-term archiving on: : Monday, June 27, 2011 - 4:55:56 PM

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Jorge Toro Alava. ENREGISTREMENT DES SURRECTIONS LIEES AUX ACCRETIONS DE TERRAINS OCEANIQUES : LES SEDIMENTS CRETACE-PALEOGENES DES ANDES D'EQUATEUR. Géologie appliquée. Université Joseph-Fourier - Grenoble I, 2006. Français. ⟨tel-00174095⟩

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