Abstract

Low-temperature thermochronometry and crosscutting relationships identified in newly released reflection seismic data reveal a previously unrecognized zone of early Andean shortening in Colombia. Apatite fission-track data and thermal modeling help define a 60–50 Ma onset of rapid exhumation along the present boundary between the Magdalena Valley hinterland basin and Eastern Cordillera thrust belt. Subsurface angular unconformities localized above fold-thrust structures indicate Paleogene deposition in a wedge-top depozone containing doubly vergent reverse faults. Retrodeformation of a cross section based on interpreted seismic profiles and thermochronometric data indicates Paleocene to Early Eocene shortening and exhumation occurred through simultaneous activation of east- and west-directed reverse faults across a broad orogenic front. Subsequent deformation focused along west-directed inversion structures. These relationships reveal that deformation operated in a disparate manner, rather than following a systematic progression from hinterland to foreland. The northern Andes also exemplify the potential effects of hinterland sediment loading and fault strength on deformation advance in contractional orogens.

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