Los Zerpa Moraine & Boconó Fault

Mapping with a drone: Boconó Fault

After a first mapping test with my drone Phantom 2 Vision Plus (details here), I identified an area in the Venezuela Andes that would allow mapping a particularly interesting geologic feature. Here is the full story.

The Boconó fault is a major tectonic lineament oriented SW-NE and extending more than 300 km along the Venezuelan Mérida Andes.

It is part of the tectonic features that define the southern border of the Caribbean plate.

This tectonic plate is located between North and South America and is currently moving eastwards with respect to the two continents, at a rate of roughly 2 cm/year (see animation below).

The relative motion of the Caribbean plate defines a convergent edge to the east, characterized by compression, subduction of the Atlantic Ocean and growth of the Lesser Antilles volcanic arc.

The north and south edges of the plate are characterized by lateral slip: sinistral along the northern edge (Guatemala and Greater Antilles) and dextral along the southern edge (Northern Venezuela)

(see map below).

The Boconó fault, located by the southern edge, accomodates a dextral slip in the order of 1 cm/year.

The rate of this motion has been precisely measured in recent years (1994-2000) by means of GPS data (see article).

Another evidence of this motion is the displacement of landscape features that cross the fault.

One good example is represented by the “Los Zerpa” moraine system, located a few km NE of the locality “Apartaderos” and of the Mucubají lagune (see article).

The moraine formed during the last glaciacion that ended 10000 years ago; its northern tip crosses the Boconó fault and is displaced 100 m towards NE. This corresponds to a rate of 1 cm/year, which is consistent with the GPS measurements.

The project consisted in acquiring a grid of azimuthal images by means of a drone, over an area including the two moraine segments displaced by the Boconó fault. The images were to be processed in order to generate a georeferenced map, a digital elevation model and a 3D model, that would help highlighting the tectonic features of this particular area.

Image taken from "The Caribbean Plate Evolution: Trying to Resolve a Very Complicated Tectonic Puzzle", Giuseppe Giunta and Silvia Orioli, 2011 (see link).

A video taken with the drone Phantom 2VP over the Mucubají lagune, showing the post-glacial landscape typical of the area.

Mapping with a drone: Acquisition, Modeling and Interpretation

The acquisition was performed in August 2015, covering an area of 400 by 600 m over the moraine system.

The images were acquired at an initial altitude of 80 meters at take-off and with a 70% overlap.

A total of about 300 images were acquired in two flights 20 minutes long.

The picture above shows the location of these images and the topography generated by processing them with Pix4D.

The 3D model of the area was obtained by processing the images with Pix4D.

Processing and 3D visualization were possible thanks to Marco Casella and AeroDron (www.aerodron.com).

Play with the 3D model using these commands:

• Click on the picture to open the 3D model

• PC mouse: rotate (MB1), pan (MB2 or MB3), zoom (wheel)

• Tablet: rotate (one finger), pan (two fingers), zoom (pinch fingers)

Additional display options are available by clicking on the blue buttons.

This picture shows the 3D model color-coded according to relative elevation. 

Various geomorphic features related to the interaction between the Boconó fault and the moraine deposits are evident:

• the trace of the Boconó fault, marked by a series of aligned tectonic scarps

• the sharp bend of the Rio Los Zerpa, that runs down the glacial valley and deviates 90º when it intersects the fault trace

• the 100 m dextral displacement of the lateral moraines as they cross the fault

• two terraces witnessing past periods of fluvial infill within the glacial valley, later eroded when the fault activity opened a fluvial escape through the right lateral moraine

• the abandoned fluvial valley that used to drain the moraine system before it was breached by the fault

A detailed interpretation of the geological evolution of the "Los Zerpa" moraine was described by C. Schubert in 1983.

The full article "Tectonic and Sedimentation: an example from the Mérida Andes" can be downloaded from HERE.

A synthesis of the model can be seen in this presentation. Click on the Pause key and Left/Right arrows to flip the slides at ease.

A planar surface adjusted along the edge of the upper fluvial terrace, represents the fill with fluvio-glacial sediments as described by Schubert in the 3rd stage of his model.

This planar surface, extending towards the terminal moraine, reaches the same elevation as the abandoned drainage, thus supporting Schubert interpretation:

"An alluvial plain formed at an elevation of 20-25 m above the present-day river level, draining through the terminal moraine".

This side view highlights the fault scarps that are present along the trace of the Boconó fault (vertical scale exaggerated by 1.5).

The arrow shows the point of view of the next picture.

Field view (see arrow in the previous picture): the topography is intensely modeled by the action of the Boconó fault.

This animation shows an interpretation of the displacement along the fault and through the moraine system.

The bend in the fault trajectory presumably generated a small trans-tensional graben delimited by fault scarps.