Reverse Faulting Between the "Gonfolite Lombarda" and Pliocene Marine Clays at Monte Morello (Novazzano, Ticino): Remarks on the Chronology of Compressional Tectonics in the Southern Alps, and Implications for Seismic Hazard Assessment in the Insubria Region
by
Giancanio Sileo
Dipartimento di Scienze Chimiche e Ambientali, Università dell'Insubria, Via Vallegio 11, 22100, Como, Italia
Coauthors: F. Giardina, A.M. Michetti, E. Vittori

The Insubria Region lies in the Southern Alps amid Lake Como and Lake Maggiore, along the border between Lombardia (Italy) and Ticino (Switzerland). This is a very densely populated and developed area, ranking among the richest ones of the whole Europe. In this sector of the Southern Alps, relationships of Quaternary faults and folds with earthquake processes are still inadequately investigated (e.g., Giardina et al., 2004). In order to review the geological evidence for evaluating past seismicity, and provide fundamental input data for seismic hazard assessment, we are conducting field mapping, geomorphic survey and seismotectonic analyses along the most important tectonic structures in this region.

In particular, here we focus on the regional north-verging backthrust that borders the Como and Chiasso Plains ("Monte Olimpino backthrust"; Bernoulli et al., 1989; Gelati et al., 1992). The compressional activity along this structure reportedly ceased during the Late Miocene, when also the emplacement of the most external south-verging fronts of the Southern Alps beneath the central part of the Po Plain took place ("Milano belt" of Bernoulli et al., 1989).

We have made new observations near the junction between the Faloppia Valley and the Chiasso Plain, at the Lombardy-Ticino border, where the backthrust clearly controls the geomorphology of the Spina Verde mountain front. In a small valley along the northern slope of Monte Morello, we have carried out new field work on the outcrop already described by Felber (1993), where Pliocene clays are juxtaposed against the sandstones and conglomerates of the Gonfolite Group (Oligo-Miocene) with a nearly vertical contact, whose origin is likely tectonic, according to Felber (1993).

Our new data confirm such origin, being the contact definitely due to a high angle reverse fault. Few tens of meters away from the fault plane, bedding in the Pliocene clays is sub-horizontal; moving closer to the fault, Pliocene beds are firstly tilted ca 20^° northward, then become nearly vertical, and finally are clearly dragged against the fault plane and overturned. New palaeontological analyses show that the marine clays include a faunal assemblage typical of the Upper Pliocene (Pichezzi, personal communication).

Reconnaissance field work, structural interpretation and geomorphic analyses in a wide area surrounding this outcrop strongly suggest that this feature is not local, but typical of the whole region. In fact, post-Miocene activity of regional out-of-sequence back-thrusts seems to characterize all the structural belt between Lake Garda and Lake Maggiore.

In our opinion, these results illustrate the need for more detailed investigation on Quaternary tectonics and Holocene paleoseismicity in the Insubria Region, in order to verify if the now available seismic hazard estimates, essentially based on the historical catalogue (where seismic events are practically absent for this region), are in agreement with the geological evidence.

References

Bernoulli D., Bertotti G. & Zingg A. (1989). Northward thrusting of the Gonfolite Lombarda ("South-Alpine Molasse") onto the Mesozoic sequence of the Lombardian Alps; implications for the deformation history of the Southern Alps. Ecl. Geol. Helv., 82, 3, 841-856.

Felber M. (1993). La storia geologica del tardo terziario e del quaternario nel Mendrisiotto (Ticino Meridionale, Svizzera). PH.D. Thesis. Diss. ETH n. 10125, Zurich.

Gelati R., Napolitano A. & Valdisturlo A., (1992) Results of studies on the Meso-Cenozoic succession in the Monte Olimpino 2 tunnel; the tectono-sedimentary significance of the Gonfolite Lombarda. Riv. It. Pal. Str., 97, 3-4, 565-598.

Giardina F., Michetti A. M., Serva L. & Doglioni C. (2004). The seismic potential of the Insubria Region (Southern Alps): insights from topographic and rheological modelling. Boll. Geof. Teor. appl. 45, 1, 87-91.

Date received: July 22, 2005

Copyright © 2005 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caqy-54.