GIS evidences for low-angle segments in the Valnerina fault system (Central Apennines, Italy)

A long time ago, my PhD thesis was about the Valnerina line, a Cenozoic structural lineament in the Central Apennines of Italy, that runs parallel to the more important Olevano-Antrodoco line (Fig. 1), that is considered by many Authors to have played an major syn-sedimentary role during the Mesozoic pre-orogenic phase. The Valnerina line was investigated, among others, by Francesco Antonio Decandia (e.g., Decandia 1982), my thesis supervisor in Siena University. 

During the Cenozoic compression phase, both the Valnerina and the Olevano-Antrodoco lines would have been acted as oblique-dextral ramps in the Apenninic thrust-and-fold belt. This role would have derived from the reactivation of syn-sedimentary faults of the Mesozoic Umbrian basin (Decandia, 1982). 

Fig. 1. Map of the described zone. From Fig. 9 in Alberti, 2006.

I remember, in a field trip with students, that Decandia showed us a large fault slickenside between Jurassic Calcari Diasprini/Calcari a Posydonia and Cenozoic Scaglia tectonites in the Schioppo segment of the line. The slickenside was quite high angle, dipping 70° or more to the West (Fig. 2).

 

Fig. 2. Mesofaults with dextral movements in the footwall of the Schioppo fault. From Alberti, 1998.

 

In the Umbrian sector, the Valnerina line is composed of a few segments, mainly with a NNE-SSW trend. I studied two segments at the North of the Schioppo one, the Tassinare and the Grotti faults (Fig. 3). 

 

Fig. 3. Traces of Tassinare and Grotti segments of the Valnerina line. From Alberti, 2006.

Studying the slickensides and shear zones exposed along the trace of the Grotti fault, while top-to-NE movements were common, I didn't  find abundant examples of high-angle meso-faults (e.g., Fig. 4, 5).

Fig. 4. The Grotti faults (left) and observed meso-faults at structural stations (right). From Alberti, 2006.

 

Fig. 5. S-C calcareous mylonites, with calcite shear veins, in a shear zone in the Grotti area. Foto M. Alberti.

At the time, during the first half of '90, I was not aware of GIS tools and related quantitative digital techniques for studying geological surfaces. I just remember, during a stage in Basel University, the geologist Daniel Bernouilli, digitizing a structural surface at the table with the equivalent of a mouse.

Only after the PhD, while working in the Museo dell'Antartide in Siena, I began knowing and working with commercial GIS tools, i.e. ArcView and Arc/Info. Later I began using QGIS, Saga, Grass, i.e, the open source side of the GIS software.

With Python, a scripting language well integrated with QGIS, I started creating plug-ins devoted to structural analysis of geological field data. One of these plug-ins, qgSurf, includes a module, named 'DEM-plane intersection' that allows to calculate the expected intersections between a geological plane and a topography. 

When applying this module to the data of the Grotti fault, I was surprised to find that a very low angle plane (West-dipping and about 7° of dip angle) would approximate in a more than acceptable way the traces of both the Grotti fault and the southern portion of the Tassinare fault, even when considering that the Grotti fault is locally displaced by a few minor NW-SE normal faults  (Fig. 6).

Fig. 6. Map of traces (red lines) of the Grotti (NNE-SSW mean trend, central part) and Tassinare (broadly N-S trending, to the West) faults. The theoretical trace of the inferred geological plane with dip direction 269° and dip angle of 6.7° is superposed (semi-transparent thick orange line).

In Fig. 6 you may note that in the South-Eastern part a large klippe, plus a minor one to the North would be expected. There are no geological evidence of these klippen in the field (cf. Fig. 7), but it could be explained by the fact that the geological surface increases its dip to the South-East.

Fig. 7. Geological sketch of the Tassinare-Grotti zone (from Alberti, 1998).

 

To represent the inferred attitude of the plane with respect to the geological situation, I have modified the gsf and gst Python modules to allow plotting significant planes into parallel profiles, as visualized in the profiles below. 

The input data are geological outcrops, faults and a DEM of the zone. Analyses and plots were made within a Jupyter Notebook.

The five parallel lines in the map (Fig. 8, white lines), from North (# 1) to South (# 5), are shown as topographic profiles in Fig. 9, with geological formations (see legend) and fault traces (red dots) added.

The very low-angle geological plane 269°/06.7° is represented in these profiles by the thick semi-transparent orange line. 

It can be seen that it approximates quite well the mapped traces of the NNE-SSW trending Grotti segment. It is therefore possible that the Grotti segment is a low-angle fault, differently from the Schioppo segment of the Valnerina line.

Fig. 8. Topographic map of the studied zone, with fault traces (red lines) and paralell profiles (white lines). Created with gst and gsf Python modules.

Fig. 9. Topographic profiles as in Fig. 8, with geological formations and fault traces (red dots). The low-angle plane is represented by the thick orange line. Created with gst and gsf Python modules.

References

Alberti, M., 1998. Ruolo cinematico e dinamico di lineamenti sisedimentari mesozoici durante la tettogenesi Appenninica - Linea della Valneria, Umbria. Unpublished Phd thesis.

Alberti, M., 2006. Spatial structures in earthquakes and faults: quantifying similarity in simulated stress fields and natural data sets. Journal of Structural Geology, 28, 998–1018.

Decandia F.A., 1982. Geologia dei Monti di Spoleto (Prov. di Perugia). Boll. Soc. Geol. It., 101, 291-315.