Tectonic oblique inversion in the Northern Andes from thermochronological results. Insights for mountain building construction of the Eastern Cordillera (Colombia) and evolution of the Amazon Basin.

The deformation history of the Andes has occurred in the context of the Cenozoic subduction of the Nazca plate under the South American plate, accommodating crustal shortening and mountain-building in a system of fold and thrust belts advancing towards the east with several structural styles including thin-skinned, thick-skinned or both. In contrast to the most typical orthogonal collision between these plates in the Central Andes, the oblique collision of the Caribbean plate against the South American plate has characterized the mode of deformation for the Northern Andes margin in the last 70 Ma. The highly oblique convergence between these plates during the Paleocene –Eocene and the subsequent orthogonal convergence of the Nazca plate in the Neogene, as well as tectonic inheritance, influence the mode of deformation, shortening rates, and thermal history of the Eastern Cordillera (EC) in Colombia. In its northern domain (4°N), the thermokinematic history is well documented indicating a late Oligocene age for the onset of exhumation and an acceleration for the last 5 m.y. In contrast, its thermokinematic evolution in the southern domain (Garzon Massif) is barely known.

In this work, two 2D geological cross sections, about 55 km long and 50 km-long from the southern EC domain, at 1°N Lat and 1.7° N Lat, respectively, are presented. These sections extend from the dextral strike-slip Algeciras Fault System located in the western flank of the EC, eastwards into the Putumayo Basin.  A total of 30 new and 14 previously published thermochronometric ages (ZHe, AFT, AHe) have been utilized to assess the thermokinematic history of the Garzon Massif. The first cross-section located in the southernmost segment (1°N) of this massif exhibits a structural style characterized by thick-skinned deformation involving a Jurassic plutonic-volcanic basement and Cretaceous-Cenozoic sedimentary sequences. One-dimensional multi-sample thermal modeling for both sections suggests asynchronous onset of cooling and rates in different structural domains, ranging from  55Ma to 10 Ma with rates of 30°C/Myr to 3.4°C/Myr respectively, which correspond to 1.3 km/Myr and 0.13 km/Myr for a thermal gradient of 25°C/km. A pulse of acceleration in the exhumation rates on the frontal faults towards the east over the past 10 m.y. resembles acceleration documented in the northern domain of EC.

The second cross-section (1.7°N) across the Garzon Massif also shows a thick-skinned structural style, but the basement involved consists of Mesoproterozoic high-grade metamorphic rocks and Cretaceous-Cenozoic sedimentary sequences. Unlike in the southern cross-section, here the distribution of thermochronometric ages is controlled by the Algeciras Fault System. In contrast to the northern EC domain , the older thermochronometric ages (AFT= 220Ma to 30Ma, ZHe = 76Ma,  AHe = 55Ma to 15 Ma) are located on the easters flank of Garzon Massif. One dimensional thermal models also suggest an asynchronous onset of exhumation, but ages ranging between 68Ma to 10Ma with rates of 1.2°C/Myr (0.04Km/Myr) to 5°C/Myr (0.2Km/Myr) along different structural domains respectively.  We explore the exhumation and shortening rates implied by these histories using 2D thermokinematic modeling.

The onset of the inversion of the Garzon Massif in the Eastern Cordillera is mainly controlled by inherited structures at times and with rates different from those previously documented in the northernmost domain. These are related to the oblique collision of the Caribbean Plate in the Paleogene and the Nazca Plate in the Neogene. Understanding how deformation occurs in the Northern Andes has implications for the evolution of the Amazon basin and the petroleum systems of the Putumayo basin.

The project is funded by FAPESP JP2 Project 2018-15613-1 to MP and also linked to the PhD scholarship (GN) of the PRH 43.1, financed by the Brazilian National Agency for Petroleum, Natural Gas and Biofuels (ANP).