Gravitational collapse structures in the Venezuelan Andes revealed by SAR JERS imagery

Damien Dhont, Guillaume Backé, Yves Hervouët

UMR 5831 : Imagerie Géophysique, Université de Pau et des Pays de l'Adour, Avenue de l’Université, BP 1155, 64013 Pau cedex, France

damien.dhont@univ-pau.fr

The geometry of extensional gravitational collapse structures is described for the first time in the active setting of the Venezuelan (or Merida) Andes using remote sensing imagery. We favoured the use of a mosaic of Synthetic Aperture Radar (SAR) scenes of the JERS-1 satellite assisted by complementary remote sensing devices (Landsat TM, SPOT panchromatic and Digital Elevation Models) to make a structural analysis at regional scale. SAR JERS imagery is sensitive to textural or topographic contrasts. It is very efficient in the Venezuelan Andes where dense vegetation and frequent cloud covering earlier lent difficulties to remote sensing studies. The current tectonic setting is responsible for strain partitioning in the Merida Andes, the foothills being shortened transversely in the NW-SE direction whereas the Bocono fault accommodates rigth-lateral motion along the axial part of the chain. The left-lateral strike-slip Valera fault is another main active structure, which branches to the Bocono fault in a triple-junction geometry. These two faults individualize the Trujillo triangular block extruded to the NNE as a consequence of the relative convergence between the Maracaibo block and the Guyana shield.

Using satellite imagery, numerous faults and basins which were previously unknown have been observed for the first time. More importantly, in an area where on-going compression and strike-slip deformations occur, gravitational collapse structures can be detected independently from previous knowledge. The geometry of such structures is that of a rotating elongate tilted block, which dimension can reach several tens of kilometres in length. The blocks are bounded by curved faults in plan view, the concavity being turned toward lower elevated areas such as main structural valleys or basins. The detached gravitational crustal blocks extend at the same time in order to occupy the free surface of the basins, and may ultimately overthrust the front side of the mountain belt. Most of the gravitational structures have been detected in the central part of the Merida Andes.

We propose that gravitational collapse in this area is related to the geometry of the Bocono fault. Extensional structures such as the Rio Mucujun and Sierra Nevada blocks occur in an area characterised by right-stepping offset of the fault. This peculiar geometry allow the opening of an extensional zone between two purely dextral strike-slip fault segments in the SW and in the NE. This zone was subsequently and currently filled by portions of orogen that gravitationally collapse and spread in. Gravity collapse also occurs at the junction between the Bocono and Valera faults. This is the case of the Tuname block which rotates clockwise toward the Bocono fault as a consequence of the lateral escape of the Trujillo block.

This analysis in the Venezuelan Andes may serve as a case example for recognition of gravitational collapse structures using SAR imagery in other mountain belts.