Chapter 4 - Folding of a weak boundary zone between an axial zone and a frontal belt, Chicamocha Valley, Eastern Cordillera of Colombia

Abstract

The northern part of the Eastern Cordillera of Colombia is marked by a change in trend from a margin-parallel, strongly folded intermediate segment to a northern, fault-bounded NNW-striking leg, which forms the hanging wall of the Bucaramanga Fault and exposes the North Andean basement of the Santander Massif. This structural pattern is governed by inherited NE-trending Late Triassic normal faults, which merge into the NNW-trending Bucaramanga Fault at the western margin of the Santander Massif and define, by their Cenozoic reactivation, an axial domain. The shape of this kinked fault array is outlined by a more easterly Cordilleran lobe, referred to as the Cocuy Salient, which defines, by its structural organization, an independent Cordilleran tract and for which a late Cenozoic exhumation history has been determined. The axial domain and the Cocuy Salient are separated by an intensely folded structural depression that acted as a buffer zone, mitigating east-vergent folding emerging from the axial domain and west-vergent folding and faulting of the western flank of the Cocuy Salient. The folding of this boundary zone initiated at a central pair of anticlines, which, upon reaching steeply inclined flanks, ceased to grow by bedding-parallel flexural slip and became locked. As a consequence of this strain hardening, deformation was transferred to marginal parts. The aim of this chapter is to document the fold style of an initial phase of buckling and present evidence for its locking and an outward stepping of the zone of active deformation. An understanding of these fold mechanisms helps to reevaluate the longstanding question about the relative contribution of basement and cover tectonics in the shortening of this mountain belt.