TECTONIC CONTROLS ON THE CAPTURE OF THE ORINOCO RIVER AND FORMATION OF THE CASIQUIARE CANAL, VENEZUELA
The capture of the Orinoco has been hypothesized to be recent, possibly related to Late Pleistocene or Holocene tilting, although no mechanism was proposed. In this work, we present a preliminary geomorphometric analysis of the Casiquiare region and propose a tectonic control of drainage pattern and captures. Analysis were carried out with GRASS-GIS using SRTM DEMs with 3-arcsec resolution.
In the studied area, present-day tributaries of the Orinoco and Negro show a clear structural control by linear NNE-SSW and NW-SE-oriented structures. The orientation of these structures suggest two families of conjugated shear fractures, at NNE-SSW and NW-SE, which indicates a maximum horizontal stress direction (SHmax) at NNW-SSE. This interpretation agrees with stress orientations interpreted for the Guiana Shield. In such scheme, the ENE-WSW direction is perpendicular to SHmax, and the preferential orientation of fold axes and cross-joints of the conjugated pair.
We identified at least four areas that indicate previous connections between the Orinoco and Guainia-Negro, such as the Atabapo river headwaters (Pimichin Isthmus). Three of these former connections are aligned at ENE-WSW, configuring a topographic high which could be related to folding and/or faulting perpendicular to SHmax.
The Casiquiare course is controlled by NW-SE and ENE-WSW structures. Upstream of the Casiquiare mouth, the Guainia-Negro river is aligned at NNW-SSE, which is the direction of SHmax and of extension fractures. This would facilitate fluvial carving and superimposition of the river’s course through the arching created by the regional stress.