IASPEI 29th General Assembly
Thessaloniki,Hellas - 18-28 August 1997
Hansjürgen Meyer, Jorge A. Mejia
ABSTRACT
The North Andean Block is regarded as a microplate detached from the South America plate and moving as a wedge towards NE, driven by the Nazca-South America convergence. To the N and NW, the Block is also under compression from the Caribbean plate and the Panama microplate.
A common feature of most convergent margins are inland trench-parallel strike-slip faulting systems, taken as the expression of oblique convergence; in this region the most prominent structure of this kind is the Dolores Megashear (incl. Cauca and Romeral Faults).
However, even major traits of the structure and movements at the norther ending of South America's convergent margin are still a matter of debate. Most recently, opposing senses of motion on the northern and southern segments of W-Colombia's main strike-slip systems (Cauca, Romeral) have been proposed, calling for a transition zone with N-S stretching and/or shortening in central W-Colombia. This transition zone would coincide with the seismically most active volume of W-Colombia's continental faults and Wadati-Benioff Zone.
Our analysis, based mainly on single-event and composite first-motion focal mechanisms from observations with a regional seismic network between 2. 5 and 5 degrees N latitude, aims at detailing senses of motion and identifying patterns of the proposed transition zone. We also look for evidence of northward displacement of the Block at this latitude, which requires left-lateral sense somewhere on its western faults.
We find that the Dolores Megashear does not behave uniformly; its western part (Cauca Fault) showing mainly reverse motion while the Romeral system slips presently mainly in strike. In the study area the Atrato fault shows sinistral motion; however, this - as well as the sinistral sense found by others on the northern segment of the Caua-Romeral system - may be due to the compression exerted by the Panama-Costa Rica microplate. We also found evidence of N-S compression in the Wadati-Benioff zone, but as yet no clear signs of a transition zone.