Implications for Active Normal Faulting
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Geomorphology 201 (2013) 293–311 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph Morphotectonic analysis of the Lunigiana and Garfagnana grabens (northern Apennines, Italy): Implications for active normal faulting Deborah Di Naccio a,⁎, Paolo Boncio b, Francesco Brozzetti b, Frank J. Pazzaglia c, Giusy Lavecchia b a Istituto Nazionale di Geofisica e Vulcanologia, Dipartimento di Sismologia e Tettonofisica, L'Aquila, Italy b Università G. d'Annunzio di Chieti-Pescara, Dipartimento di Scienze P.U.Ter., Sez. Geologia e Archeologia, Italy c Lehigh University, Department of Earth and Environmental Sciences, Bethlehem, PA, USA article info abstract Article history: This work integrates existing structural geology data with new detailed geomorphic analyses of the fluvial Received 27 October 2012 network to characterize active and potentially seismogenic faults bordering the Lunigiana and Garfagnana Received in revised form 25 June 2013 basins in the northern Apennines of Italy. These two basins are NW–SE-oriented asymmetric grabens, Accepted 2 July 2013 bordered by several normal faults with a poorly known, but probable recent slip history. Several strong earth- Available online 10 July 2013 quakes (M 5.0–6.5) have occurred in the area in the last millennium, demonstrating that this is one of the most seismically active areas of the northern Apennines. However, the lack of reliable instrumental data Keywords: Northern Apennines for strong earthquakes, generally low deformation rates, and poor exposures of faulted Quaternary sediments Active fault render the characterization of active, seismogenic faults problematic. Normal fault Here, we quantify the relationships between faults and watershed-scale geomorphology using 10-m digital Tectonic geomorphology topography to extract channel and basin metrics, such as steepness, concavity, and stream length-gradient Knickpoint indices of modeled river longitudinal profiles. In particular, convex segments of longitudinal profiles Geomorphic indices (knickpoints) are investigated in the spatial context of suspected active faults. Several knickpoints arise local- ly from juxtaposed rock types of different erodibility; however, many others mapped along major normal faults have a clear tectonic origin. In fact, the height of the footwall knickpoints (the closest to the fault trace) varies along-strike the fault, increasing toward the fault center and tapering off toward the fault tips, mimicking the expected displacement profile of a fault. In these cases, we consider the knickpoint height as a proxy of the fault throw accumulated by the youngest fault activity, probably during the late Quaternary. The along-strike distribution of knickpoint heights helps in defining the likely segmentation pattern of the fault system. The identified active normal fault segments have lengths ranging from 9.5 to 28.5 km. The in- ferred late Quaternary throw rate ranges from 0.3 to 0.8 mm/a; however, the absence of any offset datable material limits our ability to assign precise numeric ages and rates of offset to the faulting. © 2013 Elsevier B.V. All rights reserved. 1. Introduction 1983), transmitting base level fall throughout an entire basin (Rosenbloom and Anderson, 1994; Whipple and Tucker, 1999; A number of recent studies describe how the fluvial network Crosby and Whipple, 2006; Wobus et al., 2006; Harkins et al., 2007; responds to active tectonic processes that build landscapes. These Haviv et al., 2010; Whipple et al., 2013). studies are rooted in the idealized shape and form of river longitudi- In this work we extract quantitative tectonic information directly nal profiles and have been successfully used in defining the relation- from channel longitudinal profile exploiting the manner and the de- ship between longitudinal profile form and crustal deformation gree to which the river network responds to variations in rock erod- (Whipple and Tucker, 1999; Snyder et al., 2000; Kirby and Whipple, ibility and tectonic forcing. We do this analysis in the Lunigiana and 2001; Wobus et al., 2006). Specifically and pertinent to the study Garfagnana basins (northwestern Italy), a region of active tectonics, here, a primary mechanism of geomorphic response to fault offset seismicity, and suspected seismogenic reactivation of preexisting in bedrock rivers is the creation of knickpoints or if stretched out normal faults, but lack clear offset of very young Quaternary deposits along the profile, knickzones. Collectively, these features are tran- (Bartolini et al., 1982; Raggi, 1985; Nardi et al., 1987; Puccinelli, 1987; sients that evolve through parallel retreat and reclining (Gardner, Dallan et al., 1991; Castaldini et al., 1998; Bernini and Papani, 2002; Argnani et al., 2003; Coltorti et al., 2008; DISS Working Group, 2010). The Lunigiana and Garfagnana basins are part of a series of early Pliocene to Quaternary, NW–SE-oriented extensional grabens in the ⁎ Corresponding author. Tel.: +39 0862 709122; fax: +39 0862 709109. E-mail addresses: [email protected] (D. Di Naccio), [email protected] northern Apennines (Fig. 1; see Argnani et al. (2003) and references (P. Boncio). therein for a review). The overall tectonic setting is rather well 0169-555X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geomorph.2013.07.003 294 D. Di Naccio et al. / Geomorphology 201 (2013) 293–311 Fig. 1. Structural map of the Lunigiana and Garfagnana grabens showing the normal fault systems, synthetic bedrock geology, continental deposits and historical seismicity from CPTI 11 catalog (Rovida et al., 2011). Owing to scale problems, the relations between fault traces and outcrops of Quaternary deposits are not detailed; the fault traces are locally simplified in order to highlight the first-order structural pattern. Fault names: a) Mt. Picchiara, b) Mt. Grosso, c) Mt. Carmuschia, d) Mulazzo, e) Olivola, f) Mocrone, g) Arzengio, h) Fivizzano, i) Groppodalosio, j) Compione–Comano, k) north Apuane transfer fault zone, l) Minucciano, m) Casciana–Sillicano–Mt. Perpoli, n) Bolognana–Gioviano, o) Verrucole–S. Romano, p) Corfino, q) Barga, r) Mt. Prato, s) Colle Uccelliera, t) Montefegatesi–Mt. Memoriante, u) Mt. Mosca. Continental deposits: a = alluvial deposits (latest Pleistocene– Holocene); ta = terraced alluvial deposits and fanglomerates (middle–late Pleistocene); PQ = clays, sands, and conglomerates of lacustrine and alluvial environment (early Pliocene (Ruscinian)–to early Pleistocene (late Villafranchian)). This map is from different sources (Carmignani et al., 2000; Bernini and Papani, 2002; Coltorti et al., 2008;1:10,000geologic maps of the Tuscany Regional Authority available at http://159.213.57.103/geoweb/listmet/lista_metadati_10k.htm; 1:50,000 Italian Geologic Map of the CARG project available at http://www.isprambiente.gov.it/MEDIA/carg/toscana.html), modified on the basis of original, partly published (Brozzetti et al., 2007), photo-geologic and field data. known at the surface and at depth, with systems of NE-dipping nor- compared to, available geological data. The available geological data mal faults along the western side of the grabens and systems of used in this work mostly consist of published geologic maps and SW-dipping normal faults along the eastern side (Elter et al., 1975; associated publications (particularly, Carmignani et al., 2000; Eva et al., 1978; Bartolini et al., 1982; Raggi, 1985; Carmignani and Bernini and Papani, 2002; 1:10,000 geologic maps of the Tuscany Re- Kligfield, 1990; Bernini et al., 1991; Artoni et al., 1992; Carmignani gional Authority available at http://159.213.57.103/geoweb/listmet/ et al., 2000; Camurri et al., 2001; Carmignani et al., 2001; Bernini lista_metadati_10k.htm; 1:50,000 Italian Geologic Map of the CARG and Papani, 2002; Argnani et al., 2003; Brozzetti et al., 2007). project available at http://www.isprambiente.gov.it/MEDIA/carg/ In the past few centuries, several large and destructive earth- toscana.html). Our analysis focuses on channels crossing normal quakes impacted this part of Italy, confirming that it is one of the faults, with particular attention paid to those that are suspected to most active areas of the northern Apennines. The most important be active on the basis of previously published data (Bartolini et al., historical earthquakes occurred on 7 May 1481 (Imax VIII MCS; 1982; Raggi, 1985; Nardi et al., 1987; Puccinelli, 1987; Dallan et al., Mw ~ 5.6); 14 February 1834 (Imax IX MCS; Mw ~ 5.8); 11 April 1991; Castaldini et al., 1998; Bernini and Papani, 2002; Argnani et 1837 (Imax X MCS; Mw ~ 5.8); and 7 September 1920 (Imax X MCS; al., 2003; Coltorti et al., 2008; DISS Working Group, 2010) and origi- Mw ~ 6.5) (CPTI 11 catalog, Rovida et al., 2011). The epicenters of nal, partly published (Brozzetti et al., 2007), photo-geologic and the strongest historical earthquakes are all located within the field data. Lunigiana graben or close to the transfer zone between the Lunigiana The geomorphic indices we collect include: (i) bedrock channel and Garfagnana grabens (Fig. 1). longitudinal profiles, contributing drainage area, and stream-length Unfortunately, the overall low rates of deformation coupled with gradient index (SL) (Hack, 1973); (ii) longitudinal profile slope-area the highly erodible rocks, the near absence of young surficial deposits analysis, including steepness and concavity indices (Hack, 1957; to serve as stratigraphic or geomorphic markers, as well as the lack of Flint,