No abstract

The Eastern Cordillera of Colombia rose to maximum elevations of >5 km during the Cenozoic by inversion of a Mesozoic rift basin. Previous studies proposed that the exhumation of the Eastern Cordillera increased from ~6 Ma to the present due to the interplay between tectonic shortening and climate. In this study, we integrate new field observations, structural data, low‐temperature thermochronology, thermobarometry, and vitrinite reflectance along a section through the Tablazo, Cocuy, and Llanos regions to estimate the amount of shortening and the exhumation history. Our results indicate that shortening started as early as the latest Maastrichtian‐Paleocene in the Tablazo and Cocuy regions. Exhumation migrated eastward, starting in the Paleocene in the west and continuing in the Miocene in the east. The amount and rate of exhumation peaked in the Cocuy region with values of <5 km and < 0.4 km/Ma, respectively. At the highest elevations in the Cocuy Sierra, we also found evidence of a low‐pressure/high‐temperature metamorphic overprint, possibly related to shallow and local magmatic intrusions that occurred in the Late Miocene. Our cross‐section interpretation suggests a low amount of shortening (13%) that is mainly accommodated by high‐angle inverted faults and by the frontal thrust system. The presence of shallow magmatic bodies, moderate exhumation, and low shortening raises questions about the processes (isostatic versus dynamic) that drove the topographic growth of the high Cocuy Sierra.

The Chiloé fore-arc sliver is an approximately N-S elongated crust block detached from South America along the dextral intra-arc Liquiñe-Ofqui fault zone (LOFZ). The sliver is internally dissected by active NW-SE sinistral faults whose relations with the LOFZ are speculative, also due to widespread fluvioglacial and volcanic blanket hiding the substratum. We focus on the northern LOFZ end and on the Biobio fault, supposedly the northernmost of the sinistral fault set, reporting on the results from field investigation and paleomagnetism of 48 (mostly Oligo-Miocene) volcanic sites. We find that the Biobio fault is an old inherited crust discontinuity that did not yield significant block rotation and deformation during the Cenozoic, thus testifying the end of sinistral shear at about 38°S. At the same latitudes, a northward transition from pure strike-slip to transpressive LOFZ deformation occurs. Intense tectonic deformation and >90°clockwise rotations characterize the main LOFZ strand. Conversely, a supposedly western LOFZ strand displays counterclockwise rotations, similar to the pattern previously documented in the forearc; thus, it does not represent a LOFZ segment. LOFZ and sinistral fault kinematics must be related, and we suggest that crust pushed northward west of the LOFZ escapes laterally toward the trench along the sinistral faults. We also speculate that the northward increasing age of the subducting Nazca plate implies a concomitant decrease of heat transfer on the upper plate, thus an increasing crust rigidity that eventually inhibits strain partitioning and sliver decoupling from 38°S.

Widespread wrench tectonics have been described along the northern Andes. The Bucaramanga fault, described as sinistral strike-slip, bounds the high Santander Massif. We combine structural and thermochronological data from the central-southern portion of the fault to investigate the vertical displacement. The structural survey data show old phases of activity preserved in the host rocks along the fault trace, with the superimposition of different generations of slickenlines, and both strike-slip and dip-slip kinematics indicators. New and previous thermochronological data show that differential exhumation of the fault walls has been ongoing for the last 50 Ma. The hanging wall, the Santander Massif, records, in the central portion, decreasing exhumation rates from the early Miocene to the mid- to late Miocene and, in the southern portion, constant rates from the Late Oligocene to the Pliocene. Combining such observations, the thermochronological offset resulting from the relative motion of the two fault walls is comparable with the observed drop in elevation across the fault, suggesting that the present topography of the Santander Massif is related to vertical movement along the Bucaramanga fault. We infer that the fault has a significant Neogene reverse component, consistent with the present day horizontal global positioning system vector data, long-term exhumation rates and the structural assemblage. Supplementary Material: The Supplementary Materials contain methodological information on the thermochronological procedures, a compilation of the previously published data used in this study and the structural geology dataset, https://doi.org/10.6084/m9.figshare.c.4620140

 20°-30° CW post-Late Miocene rotation of the Cordillera Occidental, no rotation of the Cordillera Real, southern Ecuador.  The southern Cordillera Occidental emplaced as a rotational nappe after late Miocene-Pliocene over the flexural Interandean Valley basin.  No paleomagnetic evidence for latitudinal terrane drift and orogen-parallel strike-slip activity since Mid-Late Eocene (~40 Ma).