Cenozoic exhumation patterns in the northern Andes: Constraints from the southern Bucaramanga Fault, Eastern Cordillera, Colombia
Introduction
The axial zone of the Eastern Cordillera of Colombia is characterized by regional faults such as the Soapaga, Boyacá and the southern termination of the Bucaramanga master fault, along which crystalline basement outcrops in the Santander and Floresta massifs. The Cordillera comprises a Mesozoic basin, which was inverted tectonically during the Cenozoic (Fabre, 1983; Colletta et al., 1990; Dengo and Covey, 1993; Cooper et al., 1995; Sarmiento, 2001; Toro et al., 2004; Sarmiento-Rojas et al., 2006; Tesón et al., 2013). The boundary between the Santander and Floresta massifs is not clearly defined, so the whole area is generally considered as uplifted Mesozoic basement known as the “Santander High”. Sarmiento (2001) suggested that the extension of this uplifted basement propagated from the Floresta Massif in the south to the Perijá Range in the north (Fig. 1).
To the south, tectonic inversion of the Santander High occurred mainly along the Boyacá and Soapaga faults (Cooper et al., 1995; Sarmiento, 2001; Toro et al., 2004; Tesón et al., 2013); whereas to the north, the inversion has been described along the La Salina and Suárez faults (Sarmiento, 2001; Tesón et al., 2013; Caballero et al., 2013). During the Andean orogeny, Cenozoic reactivation of the Bucaramanga, Río Servitá and Baraya faults, located at the center and to the east of the current Santander Massif was significant (Kammer, 1993; Corredor, 2003; Forero-Ortega et al., 2020). The origin of the Eastern Cordillera has been explained from the point of view of transpressional models by several authors (e.g., Kammer, 1999; Taboada et al., 2000; Sarmiento, 2001; Sarmiento-Rojas et al., 2006; Velandia et al., 2020). Transpression is especially pronounced at the southern termination of the sinistral Bucaramanga Fault (Velandia and Bermúdez, 2018; Bermúdez et al., 2021), the study area of this work.
Based on field observations and geo-thermochronological data, different tectonic models have been proposed to explain the evolution of the Santander and Floresta massifs (Shagam et al., 1984; Parra et al., 2009; Villagómez et al., 2011; Tesón et al., 2013; van der Lelij, 2013; van der Lelij et al., 2016a, b; Amaya et al., 2017; Siravo et al., 2019), which record exhumation events from the Paleocene for the Eastern Cordillera, related also with the initial shortening of its central part during the late Oligocene (Villamil, 1999; Mora et al., 2013; Egbue et al., 2014; Horton et al., 2020). Thermochronological studies in the Santander and Floresta massifs have a dispersed regional sampling approach (Shagam et al., 1984; Villagómez et al., 2011; van der Lelij, 2013; van der Lelij et al., 2016a), or are located too far from our study area (Saylor et al., 2012a,b; Parra et al., 2009; Ramírez-Arias et al., 2012; Amaya et al., 2017; Amaya–Ferreira et al., 2020). All these studies report cooling episodes, especially the Miocene-Pliocene Andean Orogeny. At the southern Bucaramanga Fault, only three thermochronological ages in the literature are from locations close to our study area: two reported by van der Lelij et al. (2016a) in San Joaquín and Onzaga, and one by Toro, 1990. These are insufficient however to understand the exhumation history of this part of the axial zone of the Eastern Cordillera, where besides compressional structures, strike-slip deformation is also reported (Velandia and Bermúdez, 2018).
Exhumation along strike-slip faults has been studied globally in several regions. Some of the processes that contribute to exhumation in these zones have been described by Cao and Neubauer (2016): (i) along transtensive and transpressive zones, (ii) overlapping of ancient strike-slip faults, (iii) reactivation of normal or reverse faults, and (iv) oblique shear with magma ascent and buoyancy by density contrasts. Exhumation along strike-slip fault terminations, some related to transpression, have been studied by Spotila et al. (2007a, b), Cruz et al. (2007), Furlong (2007), Umhoefer et al. (2007), Bermúdez et al. (2010; 2011), and Cochran et al. (2017). Our research constitutes another example of exhumation along a transpressive system where tectonics is the main driver for the exhumation (García-Delgado et al., 2021).
The goal of the present study is to characterize the exhumation and tectonic evolution at the southern termination of the Bucaramanga Fault, where the spatial and temporal pattern of rock exhumation is poorly constrained. For this purpose, we present new thermal history models based on low-temperature thermochronology data from bedrock along five “vertical” elevation profiles in that area. Since Velandia and Bermúdez (2018) described the structural style for the transpressive system, including the relationship with inverted faults, in this study we further elucidate the timing of the exhumation of this deformational system. A corollary of this work will be to understand the mechanism of how oblique motion is partitioned into vertical strain. Further, in the middle part of the Bucaramanga Fault a major cooling event occurring at ~25 Ma has been reported by Amaya et al. (2017) and Amaya–Ferreira et al. (2020). We will also evaluate whether such cooling extended southward to the study area, and if so, explain the implications in a regional context.
In this study we plan to address the following research questions: (i) what is the role of the Santa Marta-Bucaramanga strike-slip fault in the study area?, (ii) how does this fault influence the exhumation patterns?, (iii) to what degree are other faults (Boyacá and Soapaga) also involved in the exhumation?, (iv) will there be differences in terms of exhumation between the southern termination of the Bucaramanga fault and the Santander Massif or the Floresta Massif?, (v) is it possible to define tectonic blocks or structural domains that exhume at different rates and times?.
Section snippets
Geological setting
The core of the northern Eastern Cordillera of Colombia consists of the Floresta Massif, the Santander Massif and the Perijá Range (Fig. 1), which together formed the so-called Santander High during Mesozoic extension (De Freitas et al., 1997; Sarmiento, 2001; Sarmiento-Rojas et al., 2006). In the area where the Bucaramanga, Boyacá and Soapaga faults converge, crystalline basement comprises Paleozoic to Jurassic metamorphic and igneous rocks (Fig. 1). This area is bounded by folded Mesozoic
Methods
A total of 36 samples of crystalline basement (igneous and metamorphic rocks) were collected along the southern termination of the Bucaramanga Fault between the town of Cepitá and the Páramo (Fig. 1, Fig. 3). 21 of these samples belong to five age-elevation profiles (Fig. 3, Fig. 4), with samples collected as near as vertically as possible from the highest to the lowest part within the blocks. It is important to clarify that locally no faults cross or cut the blocks. These profile are: (i)
Fission/track analysis
Regarding the three AFT ages obtained using the external detector method, the late Oligocene-early Miocene age for FV-07 (Table 1) located near Cepitá town (Fig. 3), is comparable with an age reported by Amaya et al. (2017) and Amaya–Ferreira et al. (2020) for the same western block of the Bucaramanga Fault. The other two samples, which are separated by the Pueblo Viejo fault and sinistral Chaguacá fault (Fig. 3), yield early Miocene (FV-21 to the west) and late Miocene (FV-20 to the east)
Interpretation and discussion
A common approach to estimating exhumation rates is to collect and date a suite of samples within a tectonic block according to elevation, in certain cases (e.g. when there is not a combination of advection and isotherm compression, the effect of topography on near-surface isotherms, and where samples are collected across the topography or the topographic wavelength) the slope of these age-elevation relationship (AER) can be considered as the exhumation rate (Braun, 2002), where changes or
Conclusions
Numerical models based on low temperature thermochronology data suggest contrasting thermal histories between tectonic blocks prior to transpressional Bucaramanga Fault reactivation in the Eastern Cordillera of Colombia. Some blocks experienced slow cooling through the PAZ and PRZ, but some heating pulses occurred prior to the fault displacement, after that, short and fast cooling pulses are discerned, possibly as a result of Bucaramanga Fault activity.
Although Eocene exhumation of the Eastern
Author statement
Velandia: methodology, conceptualization, funding acquisition, software, formal analysis, writing, reviewing and editing. Bermúdez: methodology, conceptualization, software, validation, formal analysis, writing, reviewing and editing. Kohn: methodology, conceptualization, software, validation, formal analysis, writing, reviewing and editing. Bernet: methodology, conceptualization, writing and reviewing. Zuluaga: methodology, conceptualization and writing. Brichau: methodology, conceptualization
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work is part of the doctoral thesis of Francisco Velandia at the Universidad Nacional de Colombia, supported by Colciencias-Colfuturo grant (PDBC 6172) and a study commission given by the Universidad Industrial de Santander. We are indebted to María Isabel Marín and Wilton Echavarría (Universidad EAFIT), Héctor Enciso, Francisco Santaella, Sergio Amaya and Mary Luz Peña (laboratories of the Servicio Geológico Colombiano). We are grateful to Sergio Restrepo-Moreno for remarks on the first
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