STRUCTURAL STYLE VARIATIONS ALONG N-NE REACTIVATED FAULTS FROM THE MIDDLE MAGDALENA VALLEY (VMM) TO THE SANTANDER MASSIF COLOMBIA. PRELIMINARY RESULTS.

The Eastern Cordillera is a fold and thrust belt surrounded by the Magdalena Valley and the Llanos foreland basins. The Eastern Cordillera shows variations in structural styles, from pure shear structures on both flanks to strike-slip system in the axial zone and in the Santander Massif. The Triassic-Cretaceous sedimentary sequences preserve evidence concerning the basin configuration and geologic evolution of the Eastern Cordillera and Santander Massif. The lack of reliable ages data in Triassic to Cretaceous sequences, mapping scales, and poor subsurface data has led to uncertainties in tectonic models.  We used reliable U/Pb ages of Jurassic sequences, surface information, 2D reflection seismic (Post-Stack), cross sections and seismicity data around the Middle Magdalena Valley Basin, and the Santander Massif.

In the Santander Massif, deformation has been related to the Bucaramanga Fault (Velandia et al., 2020). Exhumation and uplift of the Santander Massif started around 10 Ma (Siravo et al., 2019). The Upper Jurassic-Lower Cretaceous Girón Group strata form the core of the Los Yariguíes anticlinorium, while east-verging reverse faults (Suárez faults) deform the forelimb. The back limb of the Los Yariguíes anticlinorium is deformed by west-verging reverse faults, affecting the Lower Cretaceous units (Jiménez et al., 2021). In the Middle Magdalena Valley, the Nuevo Mundo Syncline preserves the Cenozoic sedimentary sequence. The footwall of the Bucaramanga Fault is characterized by structural flat-blocks of sedimentary sequences of Upper Triassic, black mudstones (Bocas Formation) (Alarcón et al., 2020), and volcaniclastic red beds of the Lower Jurassic (Jordán Formation) and continental and transitional sandstones of Late Jurassic-Early Cretaceous (Girón Group) (Jiménez et al., 2021). Thickness variation of the Girón Group, changes from >4 km in the hanging wall to <100 m in the footwall of the Suárez Fault. In the Santander Massif a thin (< 1km) and undifferentiated Triassic-Jurassic succession overlies unconformably the Paleozoic rocks (Bayona et al., 2020; Alarcón et al., 2020).

The Santander Massif and the Los Yariguíes Anticlinorium are characterized by thick-skinned deformation style with right lateral displacements, and high angle reverse faults (Suarez, El Monje, Suratá, Perchiquéz). These types of faults are related to Neogene tectonic inversion of Jurassic normal faults. The western flank of the Los Yariguíes and the Middle Magdalena Valley are affected by thin-skinned, west vergent reverse faults deforming the Cretaceous to Neogene strata. The U–Pb detrital zircon geochronology for provenance analysis was carried out on six sedimentary rocks of the Jurassic Jordan Formation. Samples were collected around the NNE Sisota Fault in the Santander Massif. The age peaks include Paleo to Neoproterozoic, and Cambrian ages with less abundant Frasnian (378 Ma) to Tournaisian age (350 Ma). The new U/Pb ages reported in this work for Jurassic rocks suggest provenances of recycled Proterozoic to Paleozoic rocks, while U/Pb ages reported in the footwall of Bucaramanga Fault (Alarcón et al., 2020) suggest coeval sedimentation and vulcanism suggesting the N-NE faults controlled sedimentation during Jurassic.

The improved structural style in the study area links the reactivated NNE structures and seismicity. Major faults that have experienced tectonic inversion are localized along the Eastern Cordillera. During Neogene, in the western flank of the Eastern Cordillera and in the Santander Massif occurred tectonic inversion of Mesozoic normal faults as N-NE right-lateral faults. These structures controlled the deposition of sedimentary sequences and basin geometry in the Santander Massif.