From ductile shearing to brittle reactivation of a tectonic suture: The evolution of the Cauca–Romeral Fault System, Northern Andes

Andean-type orogens are characterized by prolonged subduction, in which the upper plate can record alternation between contractional and extensional phases. Subduction may involve the accretion of anomalies in the subducted slab, resulting in upper plate deformation and mountain building. The collision of the Caribbean Large Igneous Province (CLIP) with the western margin of South America between 75 - 62 Ma marked the evolution of the Northern Andes.

Despite this structure playing a fundamental role as an inherited structure that could control reactivations during the construction of the Northern Andes, the timing of this collisional event has been constrained through field-based structural observations and cross-cutting relationships. There is still no direct dating of the deformation, nor an understanding of the deformation mechanisms and metamorphic conditions of the shear zone during the collision and the subsequent reactivations. To address this, we propose an integrated study combining field observations, petrographic, geochronological, and mineral chemical analysis.

Extending approximately 2,000 km from Colombia to Ecuador, the Cauca–Romeral Fault System (CRFS) is the tectonic suture between the continental basement of South America and allochthonous oceanic terranes associated with the Caribbean Plate. Along this system, a mylonitic belt with well-developed ductile fabrics locally overprinted by brittle structures reflects a complex history of deformation and reactivation, which is used in this work to constrain the timing and conditions of the deformational phases.

Field relationships and petrographic observations suggest multiple deformation phases, including at least two ductile events and brittle reactivations. The ductile deformation is evidenced by shear zones with well-developed mylonitic fabrics affecting both oceanic and continental domains. The mylonites exhibit a first fabric defined by rotated and fractured hornblende and plagioclase porphyroclasts, with grain boundary migration (GBM) textures in quartz. This fabric is overprinted by a second foliation with neoformed chlorite and titanite, and subgrain rotation (SGR) textures in quartz. Both ductile fabrics are cross-cut by multiple fracture sets filled with epidote and calcite, which are fractured and displaced, as well as by extensive feldspar alteration to sericite, associated with brittle conditions.

These observations are consistent with the chemical compositions of chlorite, which indicate deformation under greenschist facies conditions, whereas hornblende porphyroclasts preserve inherited chemical signatures from the protolith or earlier metamorphic stages.

Apatite fission-track ages of 27.1 ± 1.6 and 28.0 ± 1.57 Ma from mylonites in the western fault of the CRFS suggest that the hanging wall of this fault reached temperatures above ~120°C before the Miocene.

The CRFS records a polyphase deformational history marked by ductile shearing and subsequent brittle reactivation. Textural and chemical evidence in quartz, hornblende, and chlorite suggest that the mylonitic deformation occurred under amphibolite facies conditions (~600°C), possibly associated with the collision of the CLIP, and was subsequently overprinted by lower-temperature deformation under greenschist facies (~350-450°C). These phases are overprinted by hydrothermal alteration, veining, and faulting, reflecting brittle deformation, which can be related to the AFT ages that indicate cooling to shallow crustal levels by the Oligocene.

This work is part of project 111193, funded by Minciencias, Colombia.