Timing of Accretion and Mountain-Building in The Northern Andes of Colombia through Low-Temperature Thermochonology

Abstract:

Orogenic configuration of the Northern Andes is closely associated to accretional processes since the Upper Cretaceous. In Colombia, the regional boundary between a Paleozoic continental domain to the east and Cretaceous accreted terrenes to the west is well exposed in several E-W sections near Medellin City and along the Cauca River, which occupies a major depression located between the Central and Western cordilleras. The area is dominated by the N-S trending Romeral Fault System (RFS) that can be traced to southern Ecuador. Relationships between the RFS and W-SW verging thrust system are unknown, although they represent key components of a transpressional orogeny. To understand timing of accretion and associated mountain building processes, we performed (U-Th)/He and fission track dating on samples derived from vertical profiles in cordilleran massifs.

Samples were collected along four vertical profiles on two distinct litho-tectonic units: (1) three vertical profiles in the older eastern realm corresponding to metamorphic basement rocks of the Paleozoic Paleo-continental margin and associated Cretaceous intrusives, and (2) one vertical profile in the Mande batholith, Eocene in age at the eastern portion of the Panama Chocó Block (PCB) .

The resulting zircon (U-Th)/He (ZHe) ages show a clear contrast between the ancient eastern realm (~50-60 Ma) and the Mande Batholith (~30-40 Ma). Apatite (U-Th)/He (AHe) ages also show a strong contrast with 23-42 Ma for the eastern realm and a well defined cluster at ~4 Ma for the Mande Batholith. These preliminary results suggest distinctive cooling histories for the two litho-tectonic blocks. The Mande batholith (western block) records both the late Eocene and Pliocene events whereas the ancient eastern block does not preserve any of these events. The Paleocene events recorded by the eastern block are probably related to the Laramic orogenetic phase. Finally, elevation-invariable ZHe ages from the ancient eastern block suggest a pulse of rapid exhumation during the Paleogene.