| dc.description.abstracteng | The Middle Magdalena Valley (MMV) and the Eastern Cordillera (EC) are regional tectonic units of the northern Andes in Colombia that comprise a total area of about 100.000 km2. The MMV is an intermontane basin located between the Central Cordillera in the west and the EC in the east, bounded by the San Lucas range to the west and the La Salina fault system to the east. Its sedimentary infill records the uplift of the two adjacent cordilleras in Late Cretaceous to Recent time. The eastern margin of the MMV is affected by thick- and thin-skinned thrusts associated with the La Salina system. The EC today is a large pop-up structure uplifted between the La Salina system and the Guaicaramo system which forms its border to the undeformed Llanos foreland in the east. Both the MMV and EC are underlain by a Mesozoic rift basin that was inverted during the Andean Cenozoic orogeny to obtain its present-day thrust belt architecture. This project integrates geological and geophysical information from the MMV as well as surface and geochronological information from the EC with the primary aim to reconstruct the extensional (rift) setting during the Mesozoic.
As a basis for a regional overview of the tectonic evolution in the MMV and EC, we integrated pre-existing data including stratigraphy, geochemistry, age dates and structural information to compile a new Mesozoic chronostratigraphic chart. Two field campaigns were conducted to study the Mesozoic volcanism in the area. We constrained the possible onset of extension in the MMV and EC through new field observations and new geochronological data.
Based on that information, we conclude that extension in the MMV and EC began in the Late Triassic. Geochemical data suggest a close link of volcanism to a magmatic arc and thus an intra-arc setting for the early rift basins. The regional Santa Marta-Bucaramanga strike-slip fault has played an essential role in the Mesozoic basin development and the distribution of volcanics since the Late Triassic. According to the NW, transpressive trending in relation to the NE trending Boyacá and Soapaga normal faults are interpreted as splays structures of the Santa Marta-Bucaramanga fault and defined as a horsetail that started its extension since the Late Triassic.
Our geochronological analyses from the Mesozoic tuff layers along the strike of the EC basin reveal different asynchronous volcanic pulses whose spatial distribution and timing may be linked to northwestward slab migration and retreat of the magmatic arc. We therefore interpret that most of the Mesozoic basin infill was deposited in a back-arc tectonic setting in accordance with previous studies and the new data as a basis for a regional overview of the tectonic evolution in the MMV and EC, we integrated pre-existing data including stratigraphy, geochemistry, age dates and structural information to compile a new Mesozoic chronostratigraphic chart. Two field campaigns were conducted to study the Mesozoic volcanism in the area. We constrained the possible onset of extension in the MMV and EC through new field observations and new geochronological data. from this project.
3100 km of 2D seismic reflection data from the MMV and EC were analyzed and interpreted in the time domain. The analysis and interpretation were constrained using data such as well logs, additional geophysical data (gravity measurements), and geological surface data (dips and thicknesses). Using the velocity logs and the seismic attributes, pseudo-3D velocity models were generated to perform the time-depth domain conversion. The final interpretation in the depth domain and surface information allowed us to construct serial cross-sections along the basin. We tested all structural interpretations through geometric forward modeling. In this way we constrained structural geometries in the MMV and EC and validated the previous ideas about thick-skinned and thin-skinned thrusting domains. One principal conclusion is related to forebulge migration and the influence of inherited Mesozoic structures in the present configuration. From north to south the cross-sections exhibit variations in structural domains and styles. Our analysis suggests these are linked to variations in the magnitude and timing of the evolving orogenic load. The proportions of thick- vs. thin-skinned thrusting and fault displacements also vary. The displacement on the La Salina fault decreases southward. The thin-skinned domain appears probably linked to a weakness of the sedimentary cover that detached from the basement associated with a weak basement fault.
Triassic-middle Cretaceous sediments were deposited in an intra-arc/back arc setting, which was intruded by granitoid linked to the back-arc configuration. At the end of this extensional event, the basins experienced the maximum depth probably linked to the sag and the post-rift starting.
Fission track data obtained during the last two decades was compiled and organized according to the proximity of the cross-sections. This information was employed to reconstruct the exhumation magnitude and style over time. According to other authors' thermochronological data and stratigraphic sequence analysis, the basins experienced uplift since the Late Cretaceous-early Paleocene; different uplifting events have been identified during the Andean Cenozoic Orogeny, where the major shortening is linked since the late Miocene.Basin modeling was employed to test and further constrain the validity of the geometries and the events previously identified. Stepwise kinematic restoration of two cross-sections spanning both the rifting and the inversion phase was used to construct transformation ratio, vitrinite reflectance models employing the stratigraphic, geochemical data i.e., TOC (Total organic content), kerogen, hydrogen index; and heat flow scenarios divided in different tectonic settings, the initial extension phase started from a basal a heat flow of 60~45 mWm-2q with a highest peak to 80 mWm-2q. Followed by a gradual thermal subsidence with a basal heat flow of 75 ~60 mWm-2q. Episodes of compression, uplift, erosion and cooling started since the Paleocene to the present-day with a basal heat flow of 70~25 mWm-2q.
The best-fitting models suggests an initial pulse of hydrocarbon generation in the Paleocene, when the inversion phase started in the northern area.
The main generation phase occurred during the Oligocene, but the absence of accumulations at this time was associated to the trapping and seal formation, the structures presented in the basins had not significant cut-offs to preserve hydrocarbons in most of the areas. Moreover, the sedimentary environment was fluvial mainly and the seal generation is associated with intra-lagoon environments.
In addition and based on these restrictions, is important to highlight the relevance of Mesozoic inverted structures that play a role in the migration path and trapping; these structures allowed to generate the potential traps, as is the case of the La Salina Fault, where its compression domain with an ideal juxtaposition between layers to preserve hydrocarbons.The unconformities generated during the inversion phase served as migration paths and were in charge to transport the hydrocarbons from the source to the intermontane MMV basin. Furthermore, the absence of Cenozoic reservoirs in the Eastern Cordillera is attributed to the inversion and uplift at that time.
The improved understanding of the inversion in fold and thrust belts requires integrating different data and methodologies to reduce uncertainty in such complex areas. This work was carried out to provide new insights in order to contribute to the basin development comprehension.
The new perspectives obtained in this work are the initial extension and its configuration started from the Late Triassic, and the extension continues during the Mesozoic with different peaks linked to the magmatic activity. Moreover, the basin development began with an intra- arc to a back-arc configuration, and during the Late Cretaceous the basin experienced the compression and inversion attributed to the Cenozoic Orogeny by different authors. We identified the relevance of the Mesozoic inherited structures in the present-day configuration, and the fundamental role of the Inversion fold and thrust belts in the sedimentary development. The depozones in the MMV are controlled by the orogenic load migration to the south and its variations in size and weight. The double vergence structures in the MMV reflects the Central and Eastern Cordilleras uplifitng. The basement high in the MMV could be generated by an east verging deep crustal blind thrust, that propagates during the Central Cordillera uplift, nevertheless another hypotheses is associated with the forebulge migration due the coeval orogenic load. According to the kinematic restorations we considered that the extensional phase ended during the middle Cretaceous, but the increase of thickness data will support it. During the Late Cretaceous the basin experienced a cooling event and it is associated to the thermal subsidence, then at the end of that time and early Paleocene the basin experienced the compression and inversion attributed to the Central Cordillera and Eastern Cordillera uplift. The major deformation occurred during the late Miocene, attributed to the Eastern Cordillera uplifting event. Our basin modeling indicates that the principal organic matter transformation occurred during the Oligocene and this generation is due the lithostatic chart and the basal heat flow variation. The vitrinite reflectance models suggest that the maturity of the hydrocarbon source rocks is affected by the deformation stage since the Miocene and the basal heat flow variations, moreover the source rocks located in the intermontane basin are in oil window generation in relation to the the hinterland source rocks that are mature or overmature for the present-day configuration. In summary the inversion of the Mesozoic extensional configuration played a fundamental role in the development of the structures such as folds, faults that preserved the hydrocarbon in the MMV. | de |