Implications for Active Normal Faulting

Geomorphology 201 (2013) 293–311

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Geomorphology

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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 locally 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 inferred 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

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, 1974); and (iii) knickpoint mapping, including their height, ge- reliable instrumental seismological data for strong earthquakes have ometry, and distribution. presented a long-standing challenge to the identification and charac- Particular attention has been paid to the knickpoints whose origin terization of active and possibly seismogenic faults that rupture to the can be associated to the fault activity rather than to lithologic or cli- surface. Here, we report new morphotectonic data extracted from the matic factors. We show that the knickpoints closest to the analyzed basins draining Lunigiana and Garfagnana in the context of, and fault traces are caused by recent fault activity and also that the D. Di Naccio et al. / Geomorphology 201 (2013) 293–311 295 along-strike variation of knickpoint heights records the expected dis- these continental deposits are similar in age and facies evolving placement profile of a fault, leading to insights on the segmentation from prevailing fine-grained fluvial, lacustrine, or swamp deposits pattern. Lastly, we inferred the late Quaternary throw rates of the (early Pliocene to early Pleistocene) to coarse-grained alluvial de- likely active faults by assuming that the footwall knickpoints closest posits (early Pleistocene) to terraced alluvial fan and fluvial deposits to the fault trace formed from late Pleistocene–Holocene faulting. (middle Pleistocene to Holocene) (Federici, 1978; Puccinelli, 1987; Bernini et al., 1990; Bertoldi, 1997; Bernini and Papani, 2002; Landi 2. Geological setting of the Lunigiana and Garfagnana grabens et al., 2002–2003; Perilli et al., 2004; Coltorti et al., 2008). The continental deposits are locally offset, or strongly controlled in their depo- The study area is located within the upper Magra River and sition (e.g., alluvial fans), by normal faults (e.g., Bernini and Papani, Serchio River valleys (Fig. 1) that extend along the western side of 2002) and are now incised by the drainage network caused by a the northern Apennines. This area is characterized by a nearly regional-scale Quaternary uplift that has accelerated during the last 80-km-long, NW–SE-oriented system of extensional structures that 1Ma(Argnani et al., 2003). crosscut and dissect the contractional structures of the northern The Lunigiana graben extends for ~ 45 km along the upper Magra Apennines (e.g., Elter et al., 1975). River valley, from Pontremoli to the northern side of the Apuane Alps The northern Apennines are a NW–SE-trending belt formed by (Fig. 1). Based on structural geology and distribution of continental NE-verging tectonic units stacked since the late Oligocene after the deposits two sub-basins, the Pontremoli and Aulla–Olivola, are recog- collision of the Corsica–Sardinia and Adria continental blocks. From nized, with the base of the Pontremoli continental deposits being top to bottom, the main tectonic units are (i) the Liguride allochthon; younger (middle Villafranchian land mammal age; i.e., early Pleisto- (ii) the Subligurian unit; and (iii) the Tuscan unit (for a comprehensive cene) than the base of the Aulla–Olivola deposits (early Pliocene) synthesis and review see Bortolotti et al., 2001; Carmignani et al., 2001; (Federici, 1978; Raggi, 1985; Bernini et al., 1991; Bertoldi, 1997; Castellarin, 2001; Vai and Martini, 2001, and references therein). Bernini and Papani, 2002). The Liguride allochthon, of oceanic affinity, was internally folded The Garfagnana graben is left-stepped compared to the Lunigiana and thrusted during the late Cretaceous–Eocene subduction of the graben and extends for ~27 km along the Serchio River valley, paral- Liguride–Piedmont oceanic lithosphere. In the studied area, it consists lel to the eastern margin of the Apuane Alps metamorphic core. of an upper Cretaceous stratigraphic succession (the so called external Again, two distinct sub-basins, but with similar continental sedimen- Ligurides) composed of basal ophiolite-bearing clastic rocks and tary sequences, are recognized: the Castelnuovo and Barga basins, olistoliths, resistant to erosion, passing upward to a thick, fairly separated by the Mt. Perpoli topographic high (Puccinelli, 1987; resistant succession of carbonate turbidites formed by alternating Landi et al., 2002–2003; Coltorti et al., 2008). marls, limestones, shales, and subordinate siliciclastic sandstones From a structural point of view, the first-order architecture of the (Helminthoid flysch Auctorum). The Subligurian unit originated in extensional system of both the Lunigiana and Garfagnana grabens is an intermediate position between the Ligurian oceanic domain and rather well known in the literature thanks to field data and seismic re- the Tuscan continental domain. This stratigraphic succession is Paleo- flection profiles (for geologic and geomorphologic data see Bartolini et cene–Oligocene in age and is formed by a lower part of alternating al., 1982; Raggi, 1985; Nardi et al., 1987; Puccinelli, 1987; Carmignani shales, limestones, and calcareous–marly turbidites and an upper and Kligfield, 1990; Bernini et al., 1991; Dallan et al., 1991; Castaldini part of siliciclastic turbidites. The rock erodibility varies along the suc- et al., 1998; Bernini and Papani, 2002; Brozzetti et al., 2007; Coltorti et cession, but on average this unit is poorly resistant to erosion. The al., 2008; DISS Working Group, 2010; for subsurface data see Artoni et Tuscan unit is internally divided into an upper non metamorphic al., 1992; Camurri et al., 2001; Argnani et al., 2003). In the field, the unit (Tuscan nappe) and a lower metamorphic unit (greenschist fa- NE-dipping faults generally have lower dip angles (30°–60°) com- cies; e.g., Apuane Alps metamorphic core complex). Within the gra- pared to the SW-dipping faults (50°–70°). Moreover, the NE-dipping bens, the non metamorphic Tuscan nappe crops out. This is formed faults generally have higher cumulative displacement (N 4 km) than by a strong, highly resistant to erosion, lower part (dolostone, lime- the SW-dipping faults (up to 2.5 km). The easternmost NE-dipping stone, cherty limestone, and chert; Triassic–early Cretaceous), an in- faults (i.e., the faults closest to the graben axis) and some of the termediate part consisting of lithologies with different erodibility, major SW-dipping faults (e.g., the easternmost faults in Lunigiana but on average poorly resistant to erosion (shale, marl, and limestone and the faults bordering the depression occupied by the continental of the Scaglia Toscana fm., early Cretaceous–late Oligocene), and an deposits in Grafagnana) are associated with distinct geomorphic and upper thick sequence of siliciclastic turbidites (Macigno Fm.; late topographic lineaments that suggest recent, possibly late Quaternary, Oligocene–early Miocene). The Macigno Fm. is the most widely ex- activity. Similarly, geomorphic, stratigraphic, and tectonic evidence of posed unit in the Lunigiana and Garfagnana grabens. It mainly con- post-middle Pleistocene normal faulting is documented, though sists of thick layers of sandstones with siltstones and subordinate discontinuously, along the system (Raggi, 1985; Nardi et al., 1987; pelitic interbeds. The erodibility of this siliciclastic formation varies, Puccinelli, 1987; Bernini et al., 1991; Dallan et al., 1991; Castaldini et depending on the relative thickness, ratio, and degree of alteration al., 1998; Bernini and Papani, 2002; Coltorti et al., 2008). At crustal of the arenitic and pelitic lithotypes; but on average it is less resistant scale, the NE-dipping and SW-dipping normal faults recognized in to erosion than the underlying Mesozoic carbonates. the field are synthetic and antithetic splays of a major NE-dipping The present tectonic setting of the Lunigiana–Garfagnana area is detachment fault (e.g., Argnani et al., 2003), which was inferred to the result of extensional tectonics that presumably started here be the northern termination of a regional-scale system of NE- 4–5 Ma ago (early Pliocene), determining the dissection of the former dipping, low-angle normal faults (Etrurian fault system in Boncio contractional structures, the exhumation and uplift of the Apuane et al., 2000) extending along the entire northern Apennines (Barchi Alps metamorphic core complex, and the formation of two NW– et al., 1998; Boncio et al., 1998; Lavecchia et al., 2009). SE-oriented large grabens bordered by NE-dipping and SW-dipping Between the southern termination of the Lunigiana graben and the normal faults (Elter et al., 1975; Bartolini et al., 1982; Raggi, 1985; northern termination of the Garfagnana graben, there is a nearly E– Carmignani and Kligfield, 1990; Bernini et al., 1991; Carmignani et W-striking, N-dipping fault zone with normal-oblique right-lateral ki- al., 2001; Bernini and Papani, 2002; Argnani et al., 2003). The central nematics (fault (k) in Fig. 1). This fault zone forms the northern part of the grabens is discontinuously covered by continental deposits boundary of the Apuane metamorphic core and has been interpreted that recorded their progressive, but unsteady evolution of the two as a presently active transfer fault between the Lunigiana and tectonic depressions from the early Pliocene to Quaternary (the Garfagnana extensional grabens (north Apuane transfer fault zone in base of Quaternary is considered here at 2.59 Ma ago). In general, Brozzetti et al., 2007). 296 D. Di Naccio et al. / Geomorphology 201 (2013) 293–311

3. Channel and basin analysis

In the studied area, geomorphic indices were computed using a 10-m digital topographic base. The DEM was prepared by interpolat- ing 10-m-interval contour lines, obtained from 1:10,000 scale topographic base maps, published by the Tuscany Regional Authority in digital format (available at http://www.rete.toscana.it/sett/territorio/ carto/cartopage/index.htm). Our analysis is similar to that successfully completed for carbonate bedrock streams in central Italy (Whittaker et al., 2007; Attal et al., 2011). In particular, we specifically investigated the bedrock channels crossing the suspected active faults bordering the Lunigiana and Garfagnana grabens, paying particular attention to knickpoints. The stream profile parameters, such as channel elevation, distance, and drainage area were digitally extracted, utilizing the EasyProfile and Hydrology tools in the ArcGIS software, following the method developed by Snyder et al. (2000) and Kirby et al. (2003). The channel longitudinal profiles were smoothed by applying a first-order polyno- mial loess filter with nearest neighbors bandwidth method, choosing a 0.1 sampling proportion. It allowed us to remove artificial steps in- troduced in the DEM, but not natural steps or knickpoints. The stream network was extracted considering the break in co-variance in a slope-area graph (Fig. 2A; Flint, 1974; Tarboton et al., 1989; Montgomery and Foufoula-Georgiou, 1993; Stock and Die- trich, 2003; Wobus et al., 2006). The threshold drainage area values (Fig. 2A) range from 0.2 to 0.5 km2 in both grabens, and we omitted from the regression analysis all data points with area b0.5 km2. For 34 channels (14 in Lunigiana and 20 in Garfagnana), we measured steepness and concavity indices as the y-intercept and gradient, respectively, of the linear regression through the log–log plot of channel slope (S) vs. drainage basin area (A)(Fig. 2A; Mackin, 1948; Hack, 1957; Flint, 1974; Howard et al., 1994; Pazzaglia et al., 1998; Snyder et al., 2000; Duvall et al., 2004; Whipple, 2004). Because concavity and steepness indices co-vary, normalizing the steepness index (ksn) Fig. 2. Example of drainage basin-scale stream analysis. (A) W5 basin and its main trib- with respect to a reference concavity (Wobus et al., 2006) is neces- utary (light gray line) in the Lunigiana graben (see Fig. 5 for location). The knickpoint is sary. This allows an effective comparison of profiles of streams with identified as a steep zone between an upstream point (white circle) and a downstream different drainage areas (Wobus et al., 2006). We used, as reference point (triangle if it is in the footwall, square if it is in the hanging wall). The knickpoints in the footwall are distinguished if they cross lithological boundaries (light gray) or not concavity, a regional mean (0.45) obtained by averaging the concav- (dark gray). The graph of channel slope (S) against drainage area (A) is also reported −θ ity values of all channels. This mean value is in close agreement (S = ksA ). The θ (concavity index) and ks (steepness index) are the gradient and with theoretical predictions (Whipple and Tucker, 1999) and mea- y-intercept, respectively, of the best fit regression (gray line). The gray point repre- surements (Snyder et al., 2000) of the reference concavity (common- sents the threshold area. (B) Channel longitudinal profile smoothed by applying a fi fi ly found between 0.3 and 0.6) for equilibrium bedrock channels rst-order loess lter (black line), contributing drainage area (gray line) and stream length gradient (SL) index (dashed line). H = knickpoint height; Dtrunk = difference under conditions of uniform rock uplift rate, rock erodibility, and in elevation between knickpoint on trunk stream and junction point with tributary steady climate (Hack, 1957; Snyder et al., 2000; Kirby and Whipple, stream (e.g., Goldrick and Bishop, 1995, 2007); Fw1 = knickpoint in the footwall of 2001; Tucker and Whipple, 2002; Kirby et al., 2003; Duvall et al., the suspected active fault, the number 1 indicates that it is the first observed 2004; Whipple and Meade, 2004; Wobus et al., 2006). knickpoint moving from the fault trace upstream; Hw1 = knickpoint in the hanging wall of the suspected active fault, the number 1 indicates that it is the first observed To provide an objective measure in knickpoint identification, we knickpoint moving from the fault trace downstream; gray cross in the square = fi also calculated the SL-index for all rst-order streams and for the trunk–tributary junction point; gray cross in the circle = junction point of other trunk and tributary longitudinal profiles. The SL-index is defined as tributaries. the channel reach slope (ΔH/ΔL) multiplied by the total channel length under investigation (L), measured from the midpoint where the index is being calculated upstream to the divide (Hack, 1973). (knickpoints). Therefore, inspired by the method proposed by The SL-index is very sensitive to changes in channel slope (Keller Goldrick and Bishop (1995, 2007), built on the work of Hack (1973, and Pinter, 1996); and in the studied area, where the climate can cur- 1975), the local tectonic and lithologic effects were explored analyz- rently be considered steady and uniform, identifying recent tectonic ing pairs of knickpoints developed on trunk and tributary channels activity by looking for high values within a given rock type may cutting across the faults (the Ds method). For each knickpoint pair, help. Therefore, we calculated the SL-index along the first-order we measured the difference in elevation between the junction point streams (Strahler ordering). In fact, the low orders best reflect tecton- and the top of the knickpoint on the trunk stream (Dtrunk) and on its ically controlled variations in channel gradient (Merritts and Vincent, tributary (Dtrib)(Fig. 2B). In the original work by Goldrick and 1989). We interpolated the SL-index values of the first-order streams Bishop (1995), D is the vertical distance between the junction point by using the inverse distance weighted technique in ArcGIS. In this and the projection of the theoretical long profile, therefore it is slight- way we obtained an objective distribution of knickpoints indepen- ly different from that calculated here (Fig. 2B). Nevertheless, for the dently from their origin (tectonic, lithological, or eustatic). The purpose of this work (Dtrunk vs. Dtrib) and given the characteristics of high variability of lithology outcropping in the studied area makes the analyzed knickpoint pairs, we are confident that this difference difficult the separation and quantification of the role of relative fault in measuring D does not significantly alter the results. The height displacement and lithological resistance causing SL-index anomalies (D) associated to lithological variation typically yields different values D. Di Naccio et al. / Geomorphology 201 (2013) 293–311 297

for the trunk (Dtrunk) compared to its tributary (Dtrib)(Goldrick and data are considered comparable because they are within the Bishop, 1995, 2007). In contrast, if the knickpoints, observed on uni- uncertainties. form lithology, have a common origin sourced in a single base level If footwall knickpoints have a tectonic origin, the height (H) of the fall caused by faulting, the channel will incise an amount equal to knickpoint nearest to the fault trace, indicated as Fw1 (e.g., Fig. 2B), the base level lowering. During upstream migration, the knickpoint can be considered as a proxy of the throw of the youngest displace- height should in theory be the same for the trunk channel and its trib- ment produced by the fault. Moreover, in order to check possible cor- utary (Goldrick and Bishop, 1995, 2007). Moreover, to strengthen the relations between knickpoints and faults, we analyzed the distribution local tectonic origin of the knickpoint and to reasonably exclude of the knickpoint heights (the Fw1 H) along the fault strike. If the far-ﬁeld base fall of the axial rivers (e.g., climatic or regional tectonic knickpoint is caused by faulting, we expect an along-strike distribu- perturbations), channel segments in the hanging wall of the sus- tion of Fw1 H similar to that observed for along-strike displacement pected active faults are also analyzed (Hw1 in Fig. 2B). Comparable proﬁles. In fact, it has been widely recognized that fault displacement values upstream of a fault (i.e., in the footwall, indicated as Fw; varies along-strike, tapering to zero at fault tips and increasing to a e.g., Fw1 in Fig. 2B) and different values downstream of a fault maximum near the center of the fault (Watterson, 1986; Barnett et (i.e., in the hanging wall, indicated as Hw) possibly support the al., 1987). local tectonic origin of the footwall knickpoint. In order to further assess the control exerted by normal faulting on

The knickpoints can be recognized as a step in the longitudinal topography, we calculated the normalized steepness indices (ksn)of profile. For the identification of knickpoints, in this work we preferen- river segments cutting across the fault. The variations in H (normal- tially used the high peaks in the SL-index profiles (e.g., Fig. 2B; Molin ized for Hmax) of Fw1 and in ksn (normalized for ksn max) were then an- et al., 2004; Ferraris et al., 2012). The change in elevation between the alyzed along the fault strike (normalized for fault length, dmax). We upstream and downstream points delimiting the knickpoint zone de- chose normalized diagrams to compare the channel parameters inde- fines the knickpoint height (H in Fig. 2B). Furthermore, knickpoints pendently from basin size and fault length. Finally, we used swath to- crossing lithological changes were distinguished from those crossing pographic profiles as a general tool for the broadest-scale picture of homogeneous lithologies (Fig. 2A). We assigned a standard uncer- regional scale deformation (Gilchrist et al., 1994; Molin et al., 2004). tainty of ±10 m, based on the DEM resolution, to all elevation values In particular, we extracted two swath profiles using Easy Profiler in calculated for each reach channel (Dtrunk) and its tributary (Dtrib)in ArcGIS. The first swath profile was constructed across the Aulla– respect to their junction point. If Dtrunk − Dtrib is ≤20 m, the two Olivola basin in Lunigiana — obtained from 75 topographic profiles,

Fig. 3. The SL-index contour plot of the ﬁrst-order streams in the Lunigiana graben. 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. 298 D. Di Naccio et al. / Geomorphology 201 (2013) 293–311

5.9-km-long and spaced 20 m — and the second across the Compared to the SW-dipping Compione–Comano fault, the SL-index Castelnuovo basin in Garfagnana — obtained from 100 topographic is higher in the footwall than in the hanging wall, even though this profiles, 12.8-km-long, and spaced 40 m. For each topographic pro- latter is mostly formed by carbonate rocks of the Ligurian units, on av- file, 100 elevation values were extracted and reclassified to obtain erage highly resistant to erosion. In the western side of the graben, graphs of maximum, medium, and minimum elevation. the SL-index peaks are less evident than in the eastern side. An interesting alignment of high SL-index values is located close to the 4. Results alluvial plain, along the NE-dipping Mulazzo fault (d in Fig. 3). More to the west, where the area is cut by the NE-dipping Mt. Picchiara, 4.1. Lunigiana graben Mt. Grosso, and Mt. Carmuschia normal faults (a, b, and c in Fig. 3, respectively), the SL-index remains low, lacking a clear alignment of 4.1.1. SL-index and swath profile elevated values. The map of contoured first-order SL-index for the Lunigiana Similarly, along the NE-dipping Olivola fault (e in Fig. 3), there are (Fig. 3) shows concentrations of steep streams. In many cases, the no high SL-index values. Nevertheless, the fault is of particular interest SL-index increases close to the normal faults. Lithological control on because there is field evidence of fault activity post-dating at least the stream steepness is also evident, for example for the highly resistant early Pleistocene (Fig. 4). In fact, the Olivola fault offsets about 180 m metamorphic rocks of the Apuane Alps. High SL-index values are lo- the Caio flysch of the Ligurian unit (calcareous–marly turbidites) and cated especially along the eastern side of the graben and close to the bottom of the overlying early Pleistocene Olivola conglomerates the divide. In particular, high SL-index values are distributed sub- (Oc in Fig. 4; the stratigraphy and age of the continental units, includ- parallel to the strike of the SW-dipping Groppodalosio fault (fault i ing the Olivola conglomerates, are from the 1:50,000 Italian Geologic in Fig. 3) and mostly confined within the relatively homogeneous Map of the CARG project available at www.isprambiente.gov.it/ Macigno Fm. Moving southward, a broad zone with high SL-index Media/carg/234_FIVIZZANO/Foglio.html). The swath profile across values characterizes the stepover zone between the SW-dipping the NE-dipping Olivola fault (Fig. 4B) suggests a strong control of the Groppodalosio and Compione–Comano faults (i and j in Fig. 3). Olivola fault over the topography, as indicated by the steep gradient

Fig. 4. (A) Shaded relief map of the Aulla–Olivola basin in Lunigiana with main normal faults and continental deposits. The distribution of continental deposits is from Bernini and Papani (2002) and 1:50,000 Italian Geologic Map of the CARG project (www.isprambiente.gov.it/Media/carg/234_FIVIZZANO/Foglio.html), slightly modified on the basis of original field work. The stratigraphy and age of the Pliocene–early Pleistocene continental units is based on the revision performed within the CARG project, which differ from those proposed by Bernini and Papani (2002). The map pattern of the Olivola fault is modified from Bernini and Papani (2002) on the basis of original geologic and geomorphologic field work. Key for continental deposits: a = alluvial deposits (latest Pleistocene–Holocene); ta = Val Magra fanglomerates (middle Pleistocene, Eemian paleosol on top; Bernini and Papani, 2002) and late Pleistocene terraced alluvial deposits; Oc = Olivola conglomerates (early Pleistocene (late Villafranchian)); Ap = Aulla pelites (early Pliocene (Ruscinian) to early Pleistocene (base of late Villafranchian)). (B) 1.5-km-wide swath profile across the Olivola normal fault (white box in the map). (C) Geological cross section (black line in the map). D. Di Naccio et al. / Geomorphology 201 (2013) 293–311 299 of either the maximum, mean, or minimum topography in correspon- The diagrams of Fig. 6A, C, and E display analyzed pairs of dence of the fault trace. Furthermore, a fault control on the drainage is knickpoints in the footwall and in the hanging wall of the faults. We highlighted by straight streams, characterized by Holocene flooded report the knickpoint elevation measured on a trunk channel (Dtrunk) valleys, which flow toward a main fault-parallel river (Fig. 4A). The and on its tributary (Dtrib), with respect to their junction for each pair. lack of high SL-index values along the fault can be explained by the Many knickpoints coincide with mapped lithological variations; these fact that the area is of particularly low gradient and the fault dips in are highlighted by asterisks in the diagrams of Fig. 6 and by light gray the opposite direction of the drainage (i.e., NE-ward). triangles in the map plots of Fig. 5. In the footwall, along all of the channel profiles the lowest knickpoint occurs near the fault trace, in- 4.1.2. Knickpoints form and distribution dicated as Fw1 (e.g., Fig. 6B, D, and F). Locally, there are multiple In the Lunigiana graben, the main tributary channels of the upper knickpoints (e.g., Fw2 in Fig. 6B) up to a maximum of four, further up- Magra River commonly cross the normal faults at a high angle stream (Fw4 in Fig. 6E). The difference in elevation between the trunk through oversteepened reaches (Figs. 5 and 6). The knickpoints reach and its tributary (Dtrunk − Dtrib) indicates that in most cases, in- along the trunk channels are mainly confined upstream the faults, dependently from lithology, the knickpoints in the footwall lie at the toward the channel headwater (e.g., Fig. 6B). same elevation, within the 20-m uncertainty (Fw in Fig. 6A, C, and F).

Five drainage basins (E1, E2, E3, E4, and E5 in Fig. 5)and21bedrock In fact, only 1 of the 32 pairs of data displays Dtrunk − Dtrib value of channels were analyzed along the SW-dipping Groppodalosio and N20 m (22 m). This is observed along the SW-dipping Compione– Compione–Comano faults (i and j in Fig. 1, respectively). Similarly, we an- Comano fault (j in Fig. 1) within the E5 basin (see Fig. 5 for location), alyzed ﬁve drainage basins (W1, W2, W3, W5, and W6 in Fig. 5), including close to its southern tip, together with a lithologic variation. In 13 bedrock channels, for the NE-dipping Mulazzo fault (d in Fig. 1). contrast, the knickpoints located in the hanging wall, indicated as

Fig. 5. Drainage basins (W1–W9 and E1–E5) analyzed in the Lunigiana area and knickpoints identiﬁed in the footwall (triangle) and hanging wall (square) of the NE-dipping Mulazzo and SW-dipping Groppodalosio and Compione–Comano faults. The knickpoints in the footwall are distinguished if they lay at lithologic contacts (light gray triangle) or not (dark gray triangle). 300 D. Di Naccio et al. / Geomorphology 201 (2013) 293–311

Fig. 6. Left side: plot of knickpoint elevation and its error bar (±10 m) above tributary mouth for the analyzed bedrock channels (see Fig. 5 for location). The analysis was computed on pairs of knickpoints developed on trunk and tributary channels cutting across the SW-dipping Groppodalosio fault (A), the SW-dipping Compione–Comano fault (C), and the NE-dipping Mulazzo fault (E). Knickpoints in the footwall are indicated as Fw, the numbers (e.g., Fw1) are progressive moving from the fault trace upstream; knickpoints in the hanging wall are indicated as Hw, the numbers are progressive moving from the fault trace downstream. The asterisk highlights knickpoints crossing lithological boundaries. The analyzed basins and the fault trace crossing the channel are also reported. Right side: examples of stream analysis across the SW-dipping Groppodalosio fault (B), the SW-dipping Compione–Comano fault (D), and the NE-dipping Mulazzo fault (F). In each graph the black line indicates the longitudinal profile smoothed by applying a first-order loess filter; the gray line indicates the contributing drainage area; the dashed line indicates the stream length gradient (SL) index. For each knickpoint (e.g., Fw and Hw) is reported the knickpoint zone (Fw1, Fw2, Hw1), the knickpoint elevation in respect to the trunk–tributary junction point (Dtrunk or Dtrib), and the height (H) of the Fw1 knickpoint. Gray cross in the square = trunk–tributary junction point; gray cross in the circle = junction point of other tributaries. The fault trace crossing the channels is also indicated.

Hw (e.g., Fig. 6B and D), have Dtrunk − Dtrib values commonly N20 m Fig. 1), the Fw1 H values display a steep gradient close to the northern (Hw in Fig. 6A, C, and E). tip and a high of 87 m in the central part (Fig. 7C). From this point the The distribution of the Fw1 knickpoint heights (H; e.g., Fig. 6B) Fw1 H values decrease toward the southern termination of the fault. along-strike of the SW-dipping Groppodalosio fault (i in Fig. 1) The ksn index does not have a clear pattern; however, a peak is notice- shows an upward convex shape with values rising from the northern able at the southern termination of the fault where the Mesozoic tip of the fault to 103 m near the central part of the fault (Fig. 7A). rocks, on average highly resistant to erosion, crop out (Fig. 7D). The

Comparatively, normalized ksn–d graph has the lowest values at curve obtained along the NE-dipping Mulazzo fault (d in Fig. 1) for fault tips, and generally higher values in the central part or the fault the Fw1 H shows a maximum of 57 m shifted toward the northern (Fig. 7B). Along the SW-dipping Compione–Comano fault (j in tip of the fault and low values toward the southeastern fault D. Di Naccio et al. / Geomorphology 201 (2013) 293–311 301

Fig. 7. Plot of the Fw1 knickpoint height (H, normalized for Hmax) (left graphs) and of the normalized steepness index (ksn, normalized for ksn max) calculated for river segments crossing the fault trace (right graphs) vs. the distance along-strike the fault (d, normalized for fault length dmax) for the SW-dipping Groppodalosio fault (A) and (B), the SW-dipping Compione–Comano fault (C) and (D) and the NE-dipping Mulazzo fault (E) and (F). termination (Fig. 7E). We do not have data for the northwestern part, (Fig. 8). Wide zones of steep gradient are located, for example, in which is outside the Magra River watershed. The maximum of Fw1 H the Apuane Alps with its highly resistant metamorphic rocks or in concurs with the highest values of the ksn index, which decreases the Pania di Corfino and Mt. Mosca areas where the Mesozoic carbon- southward (Fig. 7F). ates crop out. In the eastern side of the graben, steep gradients are also located upstream toward the divide. The Macigno Fm. is the 4.2. Garfagnana graben most widely exposed unit in the area, and we observed elevated SL-index values in this bedrock where it is offset by the SW-dipping 4.2.1. SL-index and swath profile Colle Uccelliera fault (s in Fig. 8). There are higher values in the foot- In contrast to the Lunigiana graben, we find that most of the wall than in the hanging wall of the fault. Along the western side of SL-index anomalies are linked to lithology in the Garfagnana graben the graben, close to the alluvial plain, we observed an interesting 302 D. Di Naccio et al. / Geomorphology 201 (2013) 293–311

Fig. 8. The SL-index contour plot of the first-order streams in the Garfagnana graben. Fault names: 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. alignment of high SL-index values. The peaks are located in the foot- surface is carved on either bedrock (Macigno Fm.) and middle Pleisto- wall of the easternmost NE-dipping faults where the Mesozoic Tuscan cene conglomerates (ta in Fig. 9; middle–late Pleistocene according to succession, on average highly resistant to erosion, crops out. In partic- the 1:50,000 Italian Geologic Map of the CARG project, and references ular, along the NE-dipping Casciana–Sillicano–Mt. Perpoli fault (m in therein, available at http://www.isprambiente.gov.it/Media/carg/ Fig. 8), the high SL-index values are close to the locality of Colle; and 250_CASTELNUOVO/Foglio.html), the fault should have accumulated with respect to the NE-dipping Bolognana–Gioviano fault (n in Fig. 8), ~50 m of vertical displacement after the middle Pleistocene. they are distributed along the entire fault trace. We extracted a swath profile across the NE-dipping Casciana– 4.2.2. Knickpoints form and distribution Sillicano–Mt. Perpoli fault (m in Fig. 1) between Sillicano and Mt. In the Garfagnana graben, the main drainage network intersects Perpoli, where the fault is particularly evident from geologic and geo- the tectonic structures at high angles (Fig. 10), showing well- morphologic data (Fig. 9). The main fault is formed by two closely defined, oversteepened reaches (Fig. 11) and allowing us to study spaced parallel splays and a third minor synthetic splay that, on the the knickpoint distribution within the main tributary basins of the whole, down-throw the Macigno Fm. and the overlaying Liguride Serchio River and their possible relationship with normal faults unit against the older non metamorphic Tuscan units (Figs. 1 and (Figs. 11 and 12). 9). The three synthetic splays are reported on the swath profile and Along the NE-dipping Casciana–Sillicano–Mt. Perpoli and Bolognana– the most visible of them coincides with the middle one, which origi- Gioviano faults (m and n in Fig. 1), we analyzed 24 bedrock channels nates a clear step in the mean and maximum elevation profiles belonging to six basins (W2, W3, W4, W5, W6, and W7 in Fig. 10). (Fig. 9B). Also the SW-dipping Corfino fault (p in Fig. 1) is visible on Along the SW-dipping Corfino and Barga faults (p and q in Fig. 1)wean- the maximum elevation profile. Near the locality of Colle, the middle alyzed nine basins (E3, E4, E5, E8, E9, E10, E11, E12, and E13) and a total of splay forms an ~50-m-high escarpment (Fig. 9C and D). In the foot- 26 bedrock channels (Fig. 10). wall, a bedrock erosional paleo-land surface dips a few degrees to All of the longitudinal profiles show a well-defined footwall the east. A similar erosional surface, having the same dip to the east, knickpoint close to the fault trace (Fw1 in Fig. 11), and have a maxi- is in the hanging wall, suggesting that this is the same paleo-land sur- mum of two knickpoints located upstream from the fault (e.g., Fw2 face faulted and downthrown to the east by the middle splay of the and Fw3 in Fig. 11B). Nearly all the knickpoints fall at lithologic con- Casciana–Sillicano–Mt. Perpoli fault (Fig. 9D). If this interpretation is tacts highlighted by asterisks in the diagrams of Fig. 11 and light gray correct, and considering that in the hanging wall the paleo-land triangles in the map of Fig. 10. In the footwall of the analyzed faults, D. Di Naccio et al. / Geomorphology 201 (2013) 293–311 303

Fig. 9. (A) Shaded relief map of the Castelnuovo basin in Garfagnana with the main normal faults and continental deposits. The distribution and the age of continental deposits are from 1:50,000 Italian Geologic Map of the CARG project (available at http://www.isprambiente.gov.it/Media/carg/250_CASTELNUOVO/Foglio.html). Key for 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)). (B) A 3.4-km-wide swath proﬁle across the NE-dipping Casciana–Sillicano– Mt. Perpoli normal fault (white box in (A)). (C) Three-dimensional view of the NE-dipping Casciana–Sillicano–Mt. Perpoli fault. (D) Panoramic view of the fault near Colle; note the ~50-m-high escarpment separating two erosional paleo-land surfaces (highlighted by thick dashed line) interpreted as the same faulted surface.

the Dtrunk − Dtrib values (Fw in Fig. 11A, C, E, and G) are all small, with H = 61 m) is located at about 8 km from the northern tip of the values generally b15 m. Only 1 of the 32 pairs of data displays a value fault, close to the Colle locality (see Fig. 10 for location); and the sec- of N20 m. We observed this value along the SW-dipping Barga fault ond one (Fw1 H = 44 m) is at about 14 km from the northern tip. (q in Fig. 1), within the E10 basin (see Fig. 10 for location), for a The values sharply decrease from the maxima toward both sides. pair of knickpoints (Fw2) that developed in uniform lithology The ksn–d graph may be divided into two parts separated by an abrupt (Fig. 11G). Along the NE-dipping Casciana–Sillicano–Mt. Perpoli drop of the values at about 10 km from the northern tip of the fault fault (m in Fig. 1), the knickpoint elevation analysis was completed (Fig. 12B). Along the NE-dipping Bolognana–Gioviano fault (n in only within the W5 basin (see Fig. 10 for location), close to the south- Fig. 1), the Fw1 H–d curve displays a bi-lobed shape with a maximum ern tip of the fault, because of the absence of trunk–tributary channel value of 39 m (Fig. 12C). The values sharply decrease toward the fault pairs in the northwestern basins. terminations. The ksn index generally remains low, with a well- We observed hanging wall knickpoints (e.g., Hw1 in Fig. 11F and deﬁned peak at about 3 km from the northern tip and with an abrupt

H) only for the SW-dipping faults, where the obtained Dtrunk − Dtrib increase approaching the southern tip (Fig. 12D). Along the SW- values are often N30 m (Hw1 in Fig. 11E and G). dipping Corﬁno fault (p in Fig. 1) the Fw1 H–d curve exhibits a The distribution of the Fw1 knickpoint heights (Fw1 H) along- bi-lobate shape (Fig. 12E). The two peak values of 49 and 63.6 m strike of the NE-dipping Casciana–Sillicano–Mt. Perpoli fault (m in are located at about 3.4 and 6.5 km (respectively) from the northern Fig. 1) shows two maxima (Fig. 12A). The ﬁrst maximum (Fw1 tip of the fault. The values sharply decrease toward the fault 304 D. Di Naccio et al. / Geomorphology 201 (2013) 293–311

Fig. 10. Drainage basins (W1–W7 and E1–E13) analyzed in the Garfagnana area and knickpoints identiﬁed in the footwall (triangle) and hanging wall (square) of the NE-dipping Casciana–Sillicano–Mt. Perpoli and Bolognana–Gioviano faults and of the SW-dipping Corﬁno and Barga faults. The knickpoints in the footwall are distinguished if they lay at lithologic contacts (light gray triangle) or not (dark gray triangle).

terminations. The ksn index (Fig. 12F) has two well-defined peaks: the constraints on the segmentation pattern of the fault system. We first is located at about 3.4 km from the northern tip of the fault discuss these points in detail below. where the Mesozoic carbonates, on average highly resistant to erosion, crop out; and the second is located more to the south where (i) A first correspondence between the knickpoints and the faults the fault cuts the Macigno Fm. (see also Fig. 1). Along the mapped in the field or compiled from the literature (Fig. 1) SW-dipping Barga fault (q in Fig. 1) the Fw1 H–d curve exhibits a comes from the SL-index analysis (Figs. 3 and 8). However roughly tri-lobed shape (Fig. 12G) with two well-defined peaks of not all the faults display along-strike SL-index anomalies. For 80 and 75 m located in the middle part of the structure and near example, high SL-index anomalies are absent along the the southern termination (respectively). Toward the southern termi- SW-dipping Fivizzano and NE-dipping Mt. Picchiara and Mt. nation of the fault, the Fw1 H decreases (minimum value of 11 m); Grosso faults (h, a, and b in Fig. 3). This might indicate that while toward the northern termination the Fw1 H increases, the faults are possibly inactive structures. High values of suggesting the presence of a third relative maximum. For the ksn–d SL-index are absent also along the NE-dipping Olivola and graph, recognizing a trend is quite difficult (Fig. 12H). It displays a SW-dipping Arzengio and Mocrone faults (e, f, and g in first peak at about 3 km and a series of peaks from 4.5 km southward Fig. 3), probably because they traverse a foothill area of partic- in conjunction with lithologies more resistant to erosion cropping out ularly low gradient, and are underlain by easily erodible rock in the footwall and belonging to the Mesozoic carbonate rocks of the types. Nevertheless their recent activity is proposed in the lit- Tuscan unit. erature for the Arzengio and Mocrone faults (Bernini and Papani, 2002) and here suggested for the NE-dipping Olivola 5. Discussion fault by geological data and swath profile analysis (Fig. 4). In several cases, instead, we observed well-defined SL-index The morphometry of channel profiles in the Lunigiana and peaks arranged along-strike the normal faults (faults d, i, j, m, Garfagnana grabens indicates that not all the normal faults of the gra- n, p, and q in Figs. 3 and 8), suggesting a tectonic control on bens have the same geomorphic signature, as quantified by SL and ksn the fluvial system. Unfortunately, in the studied area there is indices; however, there is a general correspondence of channel great variability of lithologies, and the anomalies often pass knickpoints with many normal faults. For these faults, we discuss through contacts between adjacent lithological units with dif- the results of our analysis in terms of (i) tectonic origin of the ferent erodibility. Therefore, the SL-index analysis alone is knickpoints located closely upstream of the faults (Fw1 knickpoints); not sufficient to establish if the anomaly is from lithologic var- and (ii) existence of a possible systematic pattern in the variation of iations, faulting, or both. As a check of the tectonic origin of the the Fw1 height (H) along-strike the faults. The findings also allowed knickpoints, the Dtrunk − Dtrib values were computed in the us to discuss the possibility of (iii) considering Fw1 H as a proxy of footwall and hanging wall of the main and potentially active the cumulative throw during the youngest fault activity; (iv) inferring faults (Figs. 6 and 11). All of the tributary channels exhibit the late Quaternary throw rate of the analyzed faults; and (v) adding a well-defined knickpoint close to the fault (e.g., Fw1 in D. Di Naccio et al. / Geomorphology 201 (2013) 293–311 305

Fig. 6B), and sometimes have multiple upstream knickpoints or throw, but rather appears to respond primarily to bedrock (e.g., Fw2 in Fig. 6B). Multiple knickpoints might indicate suc- lithology. In contrast, the elevations of the knickpoints in the cessive tectonic inputs perturbing the fluvial system and mi- footwall are systematically arranged sub-parallel to the fault grating upstream. In the footwall, the trunk and tributary trace, irrespective of bedrock lithology, consistent with a tec- knickpoints are at the same elevation (within the uncertainty). tonic, fault-surface rupture origin. In fact, the difference is in general b10 m, and only 2 of the 62 (ii) The fault-related origin of knickpoints is further confirmed by analyzed data pairs display differences in elevation of N20 m. the along-strike variation of the Fw1 knickpoint heights (H) Furthermore, 17 data pairs cross lithological boundaries with- (Figs. 7, 12, and 13). For all the analyzed faults, we observed out influencing the result on the whole. In contrast and to fur- a first-order systematic pattern of the along-strike Fw1 H dis-

ther illustrate the point, Dtrunk − Dtrib values for knickpoints tribution, which is an increase of the Fw1 H toward the fault located downstream in the hanging wall, indicated as Hw center. This pattern mimics the along-strike displacement pro- (e.g., Hw1 in Fig. 6B) are very dissimilar and range from 24 to ﬁle expected for faults, with fault displacement growing from 161 m. Owing to a fault dislocation, a knickpoint forms at the the fault tips toward the fault center (e.g., Watterson, 1986; fault and migrates upstream (i.e., in the footwall) along trunk see also point v). This suggests that the Fw1 H can be consid-

and tributary channels with comparable Dtrunk and Dtrib values, ered as a proxy of the youngest vertical component of the while the equilibrium of downstream channel segments fault displacement. Alternatively (e.g., knickpoints caused by (i.e., in the hanging wall) is not influenced. If the perturbation erosional exhumation of fault-line scarps), the Fw1 H should was related to a far-field base level fall of the axial river have a more irregular distribution, not correlated with the (e.g., climatic or regional tectonic perturbations), it should in- fault trace pattern, or even a nearly constant value indepen- volve the basin on the whole. Also in this case, the knickpoints dent from the position within the fault. This convex-upward, should record the basin disequilibrium; but we should observe first-order pattern of the Fw1 H along-strike profiles is locally

comparable Dtrunk and Dtrib values independently from their complicated by isolated maxima or minima. For example, two position (footwall or hanging wall). In summary, in the ana- isolated maxima are observed near the southern tip of the lyzed area the elevation of the knickpoints in the hanging Groppodalosio fault (Fig. 7A) and near the northern tip of the wall displays no discernible correspondence to fault location Compione–Comano fault (Fig. 7B) but it is difﬁcult to establish

Fig. 11. Left side: plot of knickpoint elevation and its error bar (±10 m) above tributary mouth for the analyzed bedrock channels. The analysis was computed on pairs of knickpoints developed on trunk and tributary channels cutting across the NE-dipping Casciana–Sillicano–Mt. Peroli fault (A), the NE-dipping Bolognana–Gioviano fault (C), the SW-dipping Corﬁno fault (E), and the SW-dipping Barga fault (G). Knickpoints in the footwall are indicated as Fw and the numbers (e.g., Fw1) are progressive moving from the fault trace upstream; knickpoints in the hanging wall are indicated as Hw, the numbers are progressive moving from the fault trace downstream. The analyzed basins (see Fig. 10 for location) and the fault trace crossing the channel are also indicated. Right side: examples of stream-analysis across the NE-dipping Casciana–Sillicano–Mt. Peroli fault (B), the NE-dipping Bolognana–Gioviano fault (D), the SW-dipping Corﬁno fault (F), and the SW-dipping Barga fault (H). For each knickpoint (e.g., Fw and Hw) is reported the knickpoint zone (Fw1, Fw2, Hw1,…), the knickpoint elevation in respect to the trunk–tributary junction point (Dtrunk or Dtrib), and the height (H) of the Fw1 knickpoint. The fault trace crossing the channels is also indicated. Other symbols are as in Fig. 6. 306 D. Di Naccio et al. / Geomorphology 201 (2013) 293–311

Fig. 11 (continued).

if they are outliers or if they have a structural-geologic expla- of cumulative, recent surface ruptures. Unfortunately, there nation, such as local structural complexities in the stepover are few offset geomorphic or stratigraphic markers to con- zone between the two faults. Internal minima are observed strain the age of the scarps. A post-middle Pleistocene activity along the Garfagnana faults (Fig. 12), but these minima do along the normal fault system analyzed here was more or less not seem to alter the ﬁrst-order pattern of growing of Fw1 H explicitly proposed in the literature on the basis of geomor- toward the central part of the fault. Local internal minima phic, stratigraphic, and tectonic observations (Raggi, 1985; could be explained in terms of linkage of former shorter seg- Nardi et al., 1987; Puccinelli, 1987; Bernini et al., 1991; ments (see point v). A further check of the hypothesis that Dallan et al., 1991; Castaldini et al., 1998; Bernini and Papani, the Fw1 H is a record of the youngest fault activity comes 2002; Coltorti et al., 2008), even though the neotectonic evi-

also from the along-strike variation of the ksn index (Figs. 7 dence proposed in the literature is fairly discontinuous along and 12) that is depending on rock uplift (Snyder et al., 2000; the fault system with only loose constrains on the age of Kirby and Whipple, 2001; Tucker and Whipple, 2002; Kirby faulting. A geological section across the NE-dipping Casciana– et al., 2003; Wobus et al., 2006). If the uplift is locally con- Sillicano–Mt. Perpoli fault close to the Colle locality (Fig. 9) trolled by faulting, and if faulting is the origin of the Fw1, a suggests that the fault accumulated ~50 m of vertical offset ﬁrst-order correlation between the along-strike variation of after the middle Pleistocene. This inferred vertical displace-

ksn (ksn–d graphs) and the along-strike variation of Fw1 H ment is very close to the maximum elevation of the Fw1 H (Fw1 H–d graphs) should be observed. The ksn–d graphs are calculated around the same place (60 m; Fig. 12A). Therefore, much more articulated than Fw1 H–d graphs (Figs. 7 and 12). on the basis of the available data and with the awareness of

Although ksn is linearly related to uplift, it is inﬂuenced also the large uncertainties on recent fault ages, it seems to us rea- by ﬂuvial dynamics, channel morphology, rock types, and cli- sonable to assume that the Fw1 H is a proxy of the fault throw mate (Duvall et al., 2004; Wobus et al., 2006). Therefore, the accumulated during the late Pleistocene–Holocene (i.e., late interpretation is not always straightforward. Nevertheless, a Quaternary). This seems consistent with the fact that the Fw1 coarse correlation can be found. Where the Fw1 knickpoints H should mostly record the fault scarp formation since the

have the greatest heights, the ksn is high (or the highest), last major bedrock channel erosional phase that likely occurred which means that the observed reach is far from equilibrium during the last interglacial (Eemian, ~125 ka ago). and steeper. Moreover, a coarse convex-upward shape of the (iv) After these considerations, the maximum Fw1 H observed

ksn–d proﬁles can be recognized for the central part of some along each fault has been used to infer the late Quaternary analyzed faults. throw rate, assuming that the fault scarp formed during the (iii) If the Fw1 H can be viewed as a proxy of the fault throw, it is last 125 ka (Table 1). Besides the uncertainties on faulting useful to consider that the escarpments have grown because ages, it is important to stress that Fw1 H must be considered D. Di Naccio et al. / Geomorphology 201 (2013) 293–311 307

as only an approximation of the fault throw. In fact, the throw (Fig. 4C) allowed us to estimate a minimum throw rate of coincides with the vertical scarp height (H) only for vertical about 0.2 mm/a, averaged since 800 ka ago (basal age of mid- faults or horizontal faulted surfaces. For a given H, the throw dle Pleistocene, Table 1). decreases with decreasing fault dip and with increasing dip (v) It has been widely recognized that fault displacement varies of the faulted surface. For the faults analyzed here, we estimat- within the fault surface and three main types of along-strike ed that the Fw1 H might overestimate the fault throw up to displacement profiles have been observed, characterizing 5–10% (Mulazzo, Compione–Comano and Barga faults), isolated, interacting, and linked faults (Watterson, 1986; 5–15% (Corfino fault), 10–15% (Bolognana–Gioviano fault), Peacock and Sanderson, 1991; Cowie and Scholz, 1992; 10–25% (Casciana–Sillicano–Mt. Perpoli fault), and 25–30% Cartwright et al., 1995; Peacock, 2002; Roberts et al., 2004; (Groppodalosio fault). The range of throw rates reported in Kim and Sanderson, 2005; Whittaker et al., 2007; Attal et al., Table 1 accounts for these possible overestimated values. The 2011). For isolated faults in uniform rock type, the predicted inferred late Quaternary throw rates range from 0.4 to shape of the displacement profile is a symmetrical semi- 0.8 mm/a in the Lunigiana graben and from 0.3 to 0.6 mm/a ellipse. The dislocation is zero at the fault tips and increases in the Garfagnana graben. The NE-dipping normal faults have, to a maximum at the center of the fault. For interacting faults on average, lower throw rates (0.3–0.5 mm/a) than the the displacement profile is an asymmetric semi-ellipse, with SW-dipping faults (0.4–0.8 mm/a). This might be explained the maximum displacement shifted toward the interaction by considering that the SW-dipping faults have on average zone (e.g., a stepover zone of en échelon faults); consequently, higher dip angles, at the surface and at depth (e.g., Argnani et the displacement gradient is steeper near the interaction zone. al., 2003). In fact, under the assumption of a constant horizon- For linked faults, which are faults formed by hard linkage of tal extension rate, equally partitioned between NE- and smaller segments, the displacement profile displays more max- SW-dipping normal faults, the throw rate of the steeper faults ima, separated by relative minima. The minima correspond to should be higher. Along the NE-dipping Olivola fault the interaction-and-linkage zones of the former smaller seg- (Lunigiana), we do not have Fw1 H data. Nevertheless, the off- ments. In the studied area, the along-strike variation of the set of about 180 m of the early Pleistocene continental deposits Fw1 H displays strong similarity with the along-strike variation

Fig. 12. Plot of the Fw1 knickpoint height (H, normalized for Hmax) (left graphs) and of the normalized steepness index (ksn, normalized for ksn max) calculated for river segments crossing the fault trace (right graphs) vs. the distance along-strike the fault (d, normalized for fault length dmax) for the NE-dipping Casciana–Sillicano–Mt. Perpoli fault (A) and (B), the NE-dipping Bolognana–Gioviano fault (C) and (D), the SW-dipping Corﬁno fault (E) and (F) and the SW-dipping Barga fault (G) and (H). 308 D. Di Naccio et al. / Geomorphology 201 (2013) 293–311

Fig. 12 (continued).

of displacement profiles described above. This observation 12-km-long, NE-dipping Bolognana–Gioviano fault, the Fw1 H–d allowed us to use the Fw1 H–d graphs for better constraining graph has a smoothed bi-lobed shape (Fig. 12C) that suggests a the segmentation pattern of the analyzed faults (Fig. 13). hard-linkage of two originally distinct segments. The steep gradients of the Fw1 H–d profiles of both the Mt. Perpoli and Bolognana– In the Lunigiana graben, the SW-dipping Groppodalosio and Gioviano faults toward the zone where they are approaching each Compione–Comano faults are arranged in an en échelon pattern other suggest a possible reciprocal interaction or linkage. Overall, with an overlap of about 5 km, highlighted by a broad zone of high the segmentation pattern of the NE-dipping Garfagnana faults is not SL-index (Figs. 3 and 6D). Overall, the Fw1 H–d profiles exhibit an straightforward. The first-order geological and morphological fea- asymmetric semi-elliptical shape, with their maxima shifted toward tures of the area clearly indicate the presence of two major faults bor- the overlapping zone. This suggests interaction, but probably not link- dering as many major tectonic depressions, which are the Casciana– age, between the two major faults which are 18.6 (Groppodalosio) Sillicano and Bolognana–Gioviano faults bordering the Castelnuovo and 23 (Compione–Comano) km-long. The Fw1 H–d graph of the and Barga basins, respectively. Instead, the Mt. Perpoli segment is lo- NE-dipping Mulazzo fault is poorly sampled toward the northern cated in a relative topographic high separating the two basins (see and southern tips, but the semi-elliptical shape can still be recog- Fig. 9). The available geological data and the results of our analysis nized. The maximum is slightly shifted toward the northern tip, might indicate that the Casciana–Sillicano and Bolognana–Gioviano suggesting that the fault growth might be inhibited NW-ward, possi- faults acted as independent faults during most of their history, con- bly by interaction with a northern fault (not analyzed in this work). trolling the evolution of the two hanging wall basins. More recently Southward, the Fw1 H decreases regularly toward the fault tip. The (e.g., during the late Quaternary) the two faults might have con- fault length is about 28.5 km. nected through linkage with the central Mt. Perpoli segment, forming In the Garfagnana graben, all of the Fw1 H–d graphs display more a larger, ~28-km-long single fault. than one maximum, suggesting interactions and/or linkages between The Fw1 H–d graphs of the SW-dipping Corfino and Barga faults adjacent fault segments. Along the NE-dipping Casciana–Sillicano– (Fig. 12E and G) are characterized by two well-defined maxima, Mt. Perpoli fault we observe two well-defined peaks with a relative which again suggest possible linkage of former shorter segments. minimum located ~11 km from the northern tip (Fig. 12A). This min- Interestingly, the Fw1 H along the SW-dipping Barga fault (Fig. 12G) imum corresponds to the transition from the 10.7-km-long Casciana– increases toward the northern tip rather than progressively tapering Sillicano segment to the 5.8-km-long Mt. Perpoli segment. Along the off. This suggests that the fault might be longer than usually mapped, D. Di Naccio et al. / Geomorphology 201 (2013) 293–311 309

Fig. 13. Map of the Lunigiana–Garfagnana normal faults with Fw1 H–d profiles along the analyzed fault traces. perhaps because an obscure segment connects the Barga fault with an erodibility, and lack of faulted very young continental deposits — additional northwestern segment (Castelnuovo segment in Fig. 13). make the identification and characterization of active faults problem- In this case, the total length of the Barga fault would be ~14 km. atic if only classical geological field data are used. Instead, the Furthermore the Fw1 H along the SW-dipping Corfino fault morphotectonic analysis from channel longitudinal profiles, in con- (Fig. 12E) decreases steeply toward the southern fault tip, suggesting junction with basic geological data (e.g., geological maps), allowed that its southward growth is inhibited by interaction with a southern us to (i) discriminate the youngest, likely active normal faults within fault segment (Castelnuovo segment). Northward, the Fw1 H the grabens, (ii) infer their late Quaternary throw rate, and (iii) assess decreases regularly toward the fault tip. The fault length is about their likely segmentation pattern. All of these data are essential for 9.5 km. geology-based seismic hazard assessments and can help in address- Unfortunately, we could not analyze the north Apuane transfer ing more local-scale, detailed investigations (e.g., paleoseismologic fault zone (fault (k) in Fig. 1; NAFZ), because of the very steep and ar- studies). ticulated morphology of the northern side of the Apuane Alps that The Lunigiana and Garfagnana grabens are affected by long-lived prevented us to identify clear knickpoints. The NAFZ has been extensional tectonics, active since the early Pliocene and responsible interpreted by Brozzetti et al. (2007) as an active normal-oblique for the formation of several gently, NE-dipping and steeply, right-lateral transfer fault between the Lunigiana and Garfagnana SW-dipping normal faults bordering the grabens westward and east- NE-dipping normal faults (e.g., between the Olivola and Casciana– ward, respectively. On the basis of our analysis, we suggest that the Sillicano–Mt. Perpoli faults; Fig. 13). We do not have additional mor- most likely active NE-dipping normal faults are those located closest phometric data to further support the present activity of the NAFZ, to the graben axis, corresponding to the Mulazzo and Olivola faults but if the Olivola and Casciana–Sillicano–Mt. Perpoli faults are late in the Lunigiana graben and to the Casciana–Sillicano–Mt. Perpoli Quaternary, probably active normal faults, as suggested by our and Bolognana–Gioviano faults in the Garfagnana graben. The work, it seems logical to consider also the NAFZ as a probably active normal-oblique transfer fault connecting the NE-dipping Lunigiana transfer fault. and Garfagnana faults along the northern side of the Apuane Alps, though not analyzed here, is also considered as a likely active fault, 6. Conclusions at least along the segment connecting the Olivola and Casciana– Sillicano–Mt. Perpoli faults. Among the SW-dipping normal faults, We present the results of a study in the Lunigiana and Garfagnana the most likely active ones are the Groppodalosio and Compione– grabens (northern Apennines) that illustrates the utility of the analy- Comano faults in Lunigiana and the Corfino and Barga faults in sis of digital topography for extracting quantitative tectonic informa- Garfagnana. Along these faults, the knickpoints analysis allowed us tion on active faulting directly from channel longitudinal profiles. In to conclude that the knickpoints are tectonic in origin and that the particular, the analysis proved to be a powerful tool in this area be- height of the footwall knickpoint closest to the fault trace (Fw1 H) cause the concurrence of several geologic and morphologic factors can be considered as a proxy of the fault throw accumulated during — such as the generally low rates of active tectonics, variable bedrock the youngest fault activity. 310 D. Di Naccio et al. / Geomorphology 201 (2013) 293–311

Table 1 12-km-long Bolognana–Gioviano faults), which might be linked to Estimated throw rates for the analyzed faults in the Lunigiana and Garfagnana each other through an intermediate segment (5.8-km-long Mt. Perpoli grabens⁎. segment) forming a large, ~28-km-long single fault. Normal fault Fault length Max Fw1 H Late Quaternary (km) (m) throw rate Acknowledgments (since 125 ka) (mm/a) This work was funded by the Ministero dell'Istruzione, dell'Università Lunigiana graben Mulazzo fault 28.5 57 0.5–0.4 e della Ricerca (MIUR) (ex 60% grants to P. Boncio, F. Brozzetti and Groppodalosio fault 18.6 103 0.8–0.6 G. Lavecchia). We acknowledge the Editor, R. A. Marston, and four Compione–Comano fault 23.0 87 0.7–0.6 anonymous referees of Geomorphology for the constructive revision of the manuscript. Normal fault Fault length Post-early Throw rate (km) Pleistocene (since 800 ka) offset (m) (mm/a) References Olivola fault 9.5 (a) 180 ± 20 0.2–0.25 Argnani, A., Barbacini, G., Bernini, M., Camurri, F., Ghielmi, M., Papani, G., Rizzini, F., – 15 18 (b) Rogledi, S., Torelli, L., 2003. Gravity tectonics driven by Quaternary uplift in the northern Apennines: insight from the La Spezia–Reggio Emilia geo-transect. Normal fault Fault length Max Fw1 H Late Quaternary Quaternary International 101–102, 13–26. (km) (m) throw rate Artoni, A., Bernini, M., Papani, G., Vescovi, P., Zanzucchi, G., 1992. Sezione geologica (since 125 ka) schematica Bonassola (SP)–Felino (PR). Studi Geologici Camerti Special Volume (mm/a) 1992 (2), 61–63. Attal, M., Cowie, P.A., Whittaker, A.C., Hobley, D., Tucker, G.E., Roberts, G.P., 2011. Testing Garfagnana graben fluvial erosional models using the transient response of bedrock rivers to tectonic – – – Casciana Sillicano 10.7 (c) 61 0.5 0.4 forcing in the Apennines, Italy. Journal of Geophysical Research 116. http:// Mt. Perpoli fault 5.8 (d) dx.doi.org/10.1029/2010JF001875 (F02005). 16.5 (e) Barchi, M., Minelli, G., Pialli, G., 1998. The Crop 03 profile: a synthesis of result on deep Bolognana–Gioviano fault 12.0 39 0.3 structures of the northern Apennines. Memorie della Società Geologica Italiana 52, Corfino fault 9.5 64 0.5–0.4 383–400. Barga fault 10.0 (f) 80 0.6 Barnett, J.A.M., Mortimer, J., Rippon, J.H., Walsh, J.J., Watterson, J., 1987. Displacement 14.0 (g) geometry in the volume containing a single normal fault. Bulletin of the American Association of Petroleum Geologists 71, 925–937. a = most constrained fault trace. Bartolini, C., Bernini, M., Carloni, G.C., Costantini, A., Federici, P.R., Gasperi, G., b = including an inferred segment up to the intersection with the north Apuane Lazzarotto, A., Marchetti, G., Mazzanti, R., Papani, G., Pranzini, G., Rau, A., transfer zone (dashed fault in Fig. 1). Sandrelli, F., Vercesi, P.L., Castaldini, D., Francavilla, F., 1982. Carta neotettonica c = Casciana–Sillicano segment. dell'Appennino settentrionale. Note illustrative Bollettino della Società Geologica d = Mt. Perpoli segment. Italiana 101, 523–549. e = total length considering a hard linkage between the Casciana–Sillicano and Mt. Bernini, M., Papani, G., 2002. La distensione della fossa tettonica della Lunigiana nord- Perpoli segments. occidentale (con carta geologica alla scala 1:50,000). Bollettino della Società – f = Barga fault in a strict sense (fault p in Fig. 1). Geologica Italiana 121, 313 341. Bernini, M., Boccaletti, M., Moratti, G., Papani, G., Sani, F., Torelli, L., 1990. Episodi g = total length considering a possible linkage with the Castelnuovo segment. compressivi neogenici-quaternari nell'area estensionale tirrenica nord-orientale. ⁎ The late Quaternary throw rate is calculated assuming that the Fw1 H formed during Dati in mare e a terra. Memorie della Società Geologica Italiana 44, 577–589. the last 125 ka; the range of late Quaternary throw rates accounts for the fact that Bernini, M., Papani, G., Dall'Asta, M., Lasagna, S., Heida, P., 1991. The upper Magra valley the maximum Fw1 H might overestimate the maximum fault throw, depending on the extensional basin: a cross section between Orsaro Mt. and Zeri (Massa province). – near-surface fault dip and the dip of the faulted channel, up to 5 10% (Mulazzo, Bollettino della Societa Geologica Italiana 110, 451–458. Compione–Comano and Barga faults), 5–15% (Corfino fault), 10–15% (Bolognana–Gioviano Bertoldi, R., 1997. 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