Progressive development of blockÃ¢ - Wiley Online Library

TECTONICS, VOL. 31, TC1003, doi:10.1029/2011TC002924, 2012

Progressive development of block-in-matrix fabric in a shale-dominated shear zone: Insights from the Bobbio Tectonic Window (Northern Apennines, Italy) Kei Ogata,1 Gian Andrea Pini,2 Davide Carè,3 Mario Zélic,4 and Francesco Dellisanti2 Received 16 April 2011; revised 17 November 2011; accepted 25 November 2011; published 19 January 2012.

[1] Block-in-matrix is a common fabric characterizing highly deformed to apparently chaotic rocks originated by sedimentary, tectonic and mud-diapiric processes, in many exposed orogenic belts. A true mélange originates when this fabric is associated with mixing of rocks of different ages and provenance, as that characterizing the main décollement shear zone developed between the highly allochthonous Ligurian nappe and its substratum of foredeep deposits at the margins of the Bobbio Tectonic Window (Trebbia Valley, Northern Apennines). Mixing of rocks by both mass transport processes and synsedimentary thrusting occurs at the front and tectonic erosion at the base of the nappe during its emplacement. Evidence of polyphased deformations, spanning from liquefaction-related and hydroplastic structures, to pseudo-hydrofracturing features and mineralized veining, have been recognized within the marginal portions and along the sheared contacts of blocks encased within a scaly matrix. Crosscutting relationships testify a progressive deformation involving a transition from mesoscopic ductile to brittle conditions with the significant contribution of fluid overpressure. A novel evolutionary scheme for block-in-matrix fabric development is here proposed, involving the progressive dismembering of already highly deformed units, originally developed in non- to poorly lithified conditions, through a generalized simple shearing achieved during the evolution of the shear zone, coupled with strain hardening due to tectonic compaction, synkinematic diagenesis and fluid expulsion. Citation: Ogata, K., G. A. Pini, D. Carè, M. Zélic, and F. Dellisanti (2012), Progressive development of block-in-matrix fabric in a shale-dominated shear zone: Insights from the Bobbio Tectonic Window (Northern Apennines, Italy), Tectonics, 31, TC1003, doi:10.1029/2011TC002924.

1. Introduction [2] Although there have been many studies in the past four decades covering internally fragmented and chaotic rock units exposed in orogenic belts (see, e.g., the review papers and books of Gansser [1974], Raymond [1984], Cowan [1985], Horton and Rust [1989], Taira et al. [1992], Festa et al. [2010a, 2010b], Vannucchi and Bettelli [2010], and Wakabayashi and Dilek [2011]), uncertainty still remains about their geological significance and origin. A common feature characterizing all these units is the lost of lateral continuity of beds (stratal disruption) up to a block-in-matrix (BIM) fabric [Cowan, 1985], where chunks of beds and

1 Department of Arctic Geology, University Centre in Svalbard, Longyearbyen, Norway. 2 Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Bologna, Italy. 3 Exploration and Production Division, ENI S.p.A., San Donato Milanese, Italy. 4 SJS Resource Management, Canning Bridge, Western Australia, Australia.

Copyright 2012 by the American Geophysical Union. 0278-7407/12/2011TC002924

blocks of competent rocks are completely dispersed in a binder (matrix). [3] After its re-introduction by Hsü, [1968], the term “mélange” [Greenly, 1919] has been widely used to define these units. Following Silver and Beutner [1980, p. 32]: “‘Mélange’ is a general term describing a mappable (at 1:25,000 or smaller scale), internally fragmented and mixed rock body containing a variety of blocks, commonly in a pervasively deformed matrix.” We believe that the two key terms in defining a mélange are: “internally fragmented” and “mixed.” The latter term, in our opinion, implies a certain degree of mixing of rocks from different stratigraphic units, and/or of diverse age, state of consolidation and provenance. This concept is implic in several of the examples given in Silver and Beutner’s paper, as well as is pointed out in other papers [Hsü, 1974; Şengör, 2003]. Mélanges are thought to originate from en-masse sedimentary transport, tectonicinduced deformation, or any combination of these processes [Silver and Beutner, 1980], as well as from the ascent of finegrained sediments remobilized by fluid overpressure (mud diapirs and volcanoes) [see Orange, 1990; Camerlenghi and Pini, 2009], or the in situ liquefaction of sediments by earthquake-related shaking [Yamamoto et al., 2009]. The term mélange should be used as a descriptive one [Silver and

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OGATA ET AL.: BLOCK-IN-MATRIX FABRIC AND IMPLICATIONS

Beutner, 1980], and the adjectives “sedimentary,” “tectonic” and “diapiric” can be added to point out their origin. Clayrich sedimentary mélanges have been often called olistostromes or argillaceous breccias [Bettelli et al., 1996; Remitti et al., 2011]. [4] However, other stratally disrupted and apparently chaotic units differ from mélanges, since they lack the mixing of rocks, that is the presence of “exotic” components [see Hsü, 1974; Şengör, 2003], and maintain their internal stratigraphic order in spite of the complete stratal disruption (broken formations, Hsü [1974]; tectonosomes, Pini [1999]). These units resulted from thrust tectonics [see, e.g., Nelson, 1982; Needham, 1995; Cowan and Pini, 2001; Bettelli and Vannucchi, 2003], by gravity-related movements (mass-transport processes and/or slope tectonics) [Naylor, 1981; Cowan, 1982, 1985; Steen and Andresen, 1997; Alonso et al., 2006; Ogata et al., 2012], or in situ fluidization [Aung, 2007; Yamamoto et al., 2009]. [5] There are, therefore, two different categories of units showing BIM fabric, namely broken formation and mélanges. Their distinction depends from the concept of exotic and native components and the amount of rock mixing [Hsü, 1968, Pini, 1999; Şengör, 2003; Camerlenghi and Pini, 2009; Vannucchi and Bettelli, 2010; Festa et al., 2012]. Moreover, a clear-cut distinction between the two generating processes (stratal disruption only and stratal disruption plus mixing) is easy only from theoretical point of view: they are often genetically related and are triggered by the same tectonic, sedimentary and diapiric processes or their superposition. [6] The potential similarity of these products, especially when partially lithified materials are involved, has produced long-lasting debate in the international scientific community [e.g., Cowan, 1985; Osozawa et al., 2009; Festa et al., 2010a, 2010b]. The major interpretative problem concerns possible polyphase deformation and the mutual interaction of various processes like sedimentary mass transport events, diapirism of mud and loose sediments, in situ liquification (including both liquefaction and fluidization processes) [Allen, 1982] and thrusting at shallow structural depths [Elter and Trevisan, 1973; Vollmer and Bosworth, 1984; Barber et al., 1986; Kano and Konishi, 2001; Alonso et al., 2006; Yamamoto et al., 2009; Camerlenghi and Pini, 2009; Festa et al., 2010a, 2010b, and references therein]. Genetic processes and evolution of polygenetic BIM units developed through reworking of already disrupted and mixed rock masses are still poorly understood, although often suggested [Aalto, 1981; Abbate et al., 1981; Osozawa et al., 2009]. [7] In this framework, mass transport process represents an efficient mechanism to achieve a “primary” (i.e., sedimentary) mixing (i.e., combination of different components in terms of lithology, shape, size, age, provenances), as well as complex relationships with the host/conterminous rocks [see Ogata et al., 2012]. [8] Complexity of these rocks have a societal impact in geological engineering, influencing the strength and deformation properties of rocks [Sönmez et al., 2006; Coli et al., 2011] and, therefore, conditioning slope stability [Medley and Sanz Rehermann, 2004; Berti and Simoni, 2010] and influencing the building of tunnels and dams [Button et al., 2004]. They are also suspected to be the preferential loci of seismogenesis in accretionary wedges, hosting

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pseudotachylytes or other zones of concentrated deformation thought to originated during seismic slip [Ikesawa et al., 2003; Kitamura et al., 2005; Rowe et al., 2005; Ujiie et al., 2007; Vannucchi et al., 2008; Fagereng and Sibson, 2010; Meneghini et al., 2010]. Moreover, mélanges, olistostromes and broken formations have been used as markers of tectonostratigraphic events [Delteil et al., 2006], or to unravel the kinematics of accretionary wedges [Onishi and Kimura, 1995; Kusky and Bradley, 1999; Ujiie, 2002]. Notwithstanding the importance of these rocks, and apart some exceptions [see Festa et al., 2010a, 2010b; Vannucchi and Bettelli, 2010], their settings, the meaning of their fabric and the internal structures have been often overlooked, and consequently the mechanism of stratal disruption and mixing are not yet completely understood. [9] In this paper we describe some structures characterizing shale-rich, polyphased BIM units located within a main shear zone cropping out in the Bobbio Tectonic Window of the Northern Apennines (Italy). This location provide the unique opportunity to study an exposed shear zone located at the main detachment between an highly allochthonous nappe, the Ligurian nappe, thrust over foreland successions, which have been deformed during the nappe emplacement and after, during the nappe stack and the exhumation processes building the Northern Apennines mountain chain. Data have been collected through extensive field mapping and detailed structural analysis of key-outcrops mainly from the northern rim of the Bobbio Tectonic Window. We will present some consideration upon origin and evolution of these units, focusing on two key mechanisms: (1) the development of the BIM fabric (i.e., fragmentation and dismembering of components) and (2) mixing of rock of different age, state of consolidation and provenance.

2. Geologic Setting 2.1. Northern Apennines: Overview [10] The Northern Apennines are the product of a complex tectonic stacking of different structural units, developed after the northeastward overthrusting of the Cretaceous-Eocene Ligurian accretionary prism on the western margin of the Adriatic continental block during the Oligocene-Miocene [Elter, 1994; Carmignani et al., 2001; Elter et al., 2003; Cerrina Feroni et al., 2004; Molli, 2008; Argnani, 2009] (Figures 1a and 1b). The structural units originated in the Ligurian accretionary prism (Ligurian units) [Vai and Castellarin, 1993; Vescovi et al., 1999; Catanzariti et al., 2007] are at the top of the tectonic pile (Figure 1c). They are mainly composed of sedimentary rocks (clay-rich sequences with carbonatic and siliciclastic turbidites and mass-transport deposits) and ophiolites, which are the remnants of the western part of the oceanic Alpine Tethys (the Ligurian ocean) and of the thinned margin of the Adriatic continental block (Figure 1d) [Elter, 1973; Boccaletti et al., 1980; Coward and Dietrich, 1989; Zanzucchi, 1994; Marroni and Treves, 1998; Vai and Martini, 2001; Argnani, 2009]. These units have subsequently thrust over the Subligurian units, which are interpreted to represent an “intermediate domain,” located between Ligurian and Tuscan paleogeographic domains at the margin of the Adriatic continental block (Figure 1d) [Bortolotti et al., 2001; Remitti et al., 2011].

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Figure 1. (a) Simplified geological map of the Northern Apennines (slightly modified from Marroni and Pandolfi [2001]): 1) Plio-Quaternary units, 2) Epiligurian units, 3) Tertiary Piedmont Basin units, 4) Voltri Group (Alpine HP/LT metamorphic units), 5) Ligurian units, 6) Subligurian units, 7) Tuscan units, and 8) Apuane Alps (Tuscan low-grade metamorphic units). (b) Schematic block diagram showing the general architecture and the main structural units of the Northern Apennine tectonic stack (slightly modified after Elter [1994]). (c) Conceptual cross section of the Northern Apennines (slightly modified from Montomoli et al. [2001]). The approximate path of transect is shown in Figure 1b. (d) Structural-stratigraphic diagram of the Northern Apennines showing the main structural units, the lithostratigraphic groups and the paleogeographic domains, with particular emphasis on the distribution of olistostromes and tectonosomes (modified after Pini [1999]). Keys to the abbreviations: Lf = Ligurian flysches (Helminthoid and Paleocene-Eocene flysches); om = Casanova, Vieri and Mt. Ragola complexes (ophiolitic mélanges); bc = Ligurian pre-flysch successions (“basal complexes”); cs = Triassic-Jurassic substratum from a continental margin (Adria?); op = ophiolites; Cc = Canetolo complex; ccs = carbonate and carbonate-clastic succession; ev = Triassic evaporites; Hb = Hercinian basament and continental deposits. [11] The Ligurian and Subligurian units are the main components of a large structural unit, the Ligurian nappe, which was emplaced over the diverse Tuscan units and the Romagna-Umbria fold-and thrust belt (see Figure 1). These units represent the deformed foreland successions recording the progressive deepening of the Adriatic continental margin, from relatively shallow- to deep-water settings (i.e., foredeep complexes) [Lucente and Pini, 2008, and references therein]. Most of this tectonic evolution took place in a submarine environment with synkinematic sedimentation in wedge-top basins atop the advancing Ligurian nappe (i.e., Epiligurian succession) [Ricci Lucchi and Ori, 1985], and in front of it, as northeastward-migrating foredeep basins. [12] The Oligocene to Miocene foredeep deposits are subdivided from west to east (i.e., from the oldest to the youngest) in Macigno, Modino, Cervarola and MarnosoArenacea units, according to their age, stratigraphy, petrographic composition and geographical location (Figure 1d) [Ricci Lucchi, 1986; Boccaletti et al., 1990]. These units

were progressively incorporated in the evolving orogen as thrust sheets and fold-and-thrust belts [Boccaletti et al., 1990; De Donatis and Mazzoli, 1994; Barchi et al., 2001]. Thick and laterally extensive mass-transport bodies (i.e., slide, slump and debris flow deposits) are interbedded within turbidite deposits of these foredeep successions at progressively younger stratigraphic levels from west to east [Abbate et al., 1970; Ricci Lucchi, 1986; Lucente and Pini, 2008]. These bodies, classically referred to as olistostromes, were mostly sourced by rocks and sediments from the Ligurian nappe, the Epiligurian succession and slope deposits representing the inner basin margins and the closure facies of the foredeeps [Lucente and Pini, 2003, 2008]. The olistostromes are interpreted as sedimentary precursors of the submarine emplacement of the Ligurian nappe on to the foredeep sequences (precursory olistostromes) [Elter and Trevisan, 1973]. [13] The contact between the Ligurian nappe and the foredeep has been interpreted in different ways: (1) as an

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erosional subduction channel from Eocene to middle Miocene [Vannucchi et al., 2008] inherited from the phase of subduction of the Ligurian oceanic crust to the collision and the intracontinental deformation [Remitti et al., 2007], (2) as the base of a gravitational nappe [Elter and Trevisan, 1973], prevailingly made up of mass-transport deposits (olistostromes) often reactivated as diapirs [Borgia et al., 2006], as a carpet of coalescent mass-transport deposits supplied from the front of the Ligurian nappe in an intracontinental deformation [Lucente and Pini, 2008]. These different interpretations are suggested by the uncertainty that still exists about the age of the transition between oceanic subduction and continental collision in the Northern Apennines [see, e.g., Vai and Castellarin, 1993; Pini, 1999; Catanzariti et al., 2007; Argnani, 2002, 2009; Remitti et al., 2011]. [14] After the collision, the underthrusting of the Adria plate (lower plate) occurred below the Ligurian CretaceousEocene accretionary prism [Remitti et al., 2007; Vannucchi et al., 2008]. Subduction of the Adriatic continental crust is the favorite geodynamic model for this stage [see, e.g., Molli, 2008; Picotti and Pazzaglia, 2008; Argnani, 2009; Vannucchi et al., 2010, and references therein]. Other models have been also considered in the past and are still sustained by some authors [see Lavecchia, 1988; Lavecchia and Stoppa, 1996; Lavecchia et al., 2003; Tiberti et al., 2005]. Even the roll-back scenario of the continental subduction [Malinverno and Ryan, 1986] is thought to have developed since Miocene in a complex way, with more alternative models of slab deformation and wedge kinematics [Thomson et al., 2010], also pointing out that lateral segments of the orogen can behave in different way at the same geological time [Thomson et al., 2010]. [15] Because of the complex structural architecture of the western part of the orogenic wedge, there are only few places where the main tectonic contact of the Ligurian Nappe and the underthrust foredeep sequences are directly cropping out: among them, one of the best exposed is in the Bobbio Tectonic Window (location in Figure 1). In this framework, this area represents a singularity in that diverse units of the foredeep and intermediate units rest on a carpet of mass transport deposits from the Subligurian units [Elter et al., 1997; Agency for the Protection of the Environment and Technical Services (APAT), 1997]. 2.2. The Bobbio Tectonic Window (BTW) [16] The BTW [Ludwig, 1929] is a key-sector for the understanding of the tectonostratigraphical relationship between the Tuscan Oligo-Miocene foredeep sequences and the overlying allochthonous Ligurian nappe [Mutti, 1964; Ten Haaf, 1971; Reutter and Schlüter, 1968; Bellinzona et al., 1968; Plesi, 1974; Labaume, 1992; Labaume and Rio, 1994; APAT, 1997] (Figure 2). [17] Six main structural units crop out in this area, namely the (1) Cervarola, (2) Coli, (3) Sanguineto, and (4) Aveto units and the components of Ligurian nappe, the (5) Subligurian and (6) Ligurian units (Figures 2 and 3): [18] 1. The early Miocene Cervarola unit occupies the lowest position of the whole tectonic pile and represents the northwestern extension of the Tuscan foredeep sequence. This unit comprises fine-grained to coarse-grained turbidites deposits, the San Salvatore sandstone member of the Bobbio Formation, hosting shale-rich mass-transport deposits

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(olistostromes) in the fine-grained, topmost (i.e., Peli shale member) and lowermost portions (i.e., Brugnello shale member). The presence of these bodies is associated with the occurrence of synsedimentary growth-like stratal geometries and facies associations, lateral stratigraphic expansions and onlap relationships. This indicates a complex depositional setting confined between the slope and an inner foredeep margin [Mutti and Ghibaudo, 1972; Labaume, 1992; Labaume and Rio, 1994]. This foredeep succession stratigraphically overlies a shale-rich complex made of Subligurian-related mass-transport deposits and slumped slope deposits, the so-called Marsaglia complex, cropping out in the southern edge of the BTW. This unit represents the early Miocene base-of-slope succession (i.e., “chaotic apron”) developed at the front of the Ligurian nappe [Mutti and Ghibaudo, 1972; Labaume, 1992; Labaume and Rio, 1994] (Figure 3b). [19] In this paleogeographic context, the leading slope is located atop the frontal part of the advancing wedge and is “active” in terms of superficial processes (e.g., sedimentary mass transport/wasting), being characterized by: (1) increased seismicity, (2) progressive tectonic oversteepening, (3) focused fluid flow, (4) complex physiography and (5) continuous sediment supply from the inner part of the exposed portions of the prism. These features are recognizable in many modern submarine accretionary belts, especially those involving continental crust, such as SW Taiwan [e.g., Yu and Huang, 2006] and NW Borneo [e.g., Morley and Leong, 2008]. [20] All these sedimentary successions extensively crop out as a northeasterly vergent recumbent syncline, having been progressively tilted and overturned during synsedimentary folding, and then sheared during the superposition of the Ligurian nappe [Labaume and Rio, 1994]. The Ligurian nappe emplacement, which produced the shear zone studied in this work, is thought to occur at shallow crustal depths, in almost synsedimentary condition [Labaume et al., 1991]. [21] Paleotemperature ranges (Figure 3a) are inferred by Dellisanti et al. [2010] from the following mineralogical parameters of clays and organic matter maturity data: (1) KI = Kübler Index expressing the ordering of the illite structure [Kübler, 1967; Guggenheim et al., 2002], (2) %IS = the percentage of illite in illite-smectite mixed layers, marker of the progressive smectite → mixed-layers illite/smectite (I/S) → illite reaction [Reynolds, 1980], (3) Tmax = Rock-Eval pyrolysis temperature [Espitalié et al., 1985] and (4) R0 = vitrinite random reflectance [Teichmüller, 1958]. [22] Some of the raw data are shown in Figure 2b, projected along a geological cross section. The inferred maximum burial depth reached by the Bobbio Formation is of about 6 km at least, as suggested by the estimate paleotemperatures of 180–220°C (deep diagenesis-early anchizone) assuming a geothermal gradient of 30°C/km (samples 12, 13, 14, 15, 16 and 18; Figure 2b). In this framework, data coming from the Marsaglia complex are anomalous (samples 10 and 11 projected; Figure 2b), showing significantly lower paleotemperatures/shallower structural conditions (deep diagenetic zone and paleotemperatures between 100 and 150°C; see Figure 3a), comparable with those inferred for the Coli unit and the Subligurian units. This could be due

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Figure 2. Geologic framework and main structural-stratigraphic units of the Bobbio Tectonic Window. (a) Simplified geologic map and explanation (location of Figure 4 is labeled). (b) Schematic SW-NEdirected cross-section (modified after Labaume and Rio [1994]) with location (projected) of a selected set of the samples used for clay mineralogy and thermal maturity estimation of the organic matter (data from Dellisanti et al. [2010]): Ki = Kubler Index; %I/S = illite percentage in illite/smectite mixed layers; Tmax = peak temperature of Rock-Eval pyrolysis (oil = anomalously low values due to oil impregnation); R0 = vitrinite random reflectance (let = data from Reutter et al. [1983]). Approximate location of the study area is labeled. to the original structurally higher position of the Marsaglia complex during the thermal re-equilibrium, differential thermal flux operated by fluid circulation, and/or SWdipping, low angle normal faults displacing the southern margin of the BTW.

[23] A discontinuous succession of medium- to finegrained turbiditic sandstone with similar characteristics to the San Salvatore sandstone member crops out roughly to the S of the Marsaglia complex exposures (i.e., Aveto-Trebbia sandstone). This unit, showing complex relationships with

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Figure 3. (a) Chronostratigraphic scheme for the tectonostratigraphic units cropping out in the Bobbo Tectonic Window (see also Figures 1 and 2) (biostratigraphic data from Labaume [1992], Fornaciari and Labaume [1992], and Labaume and Rio [1994]; paleotemperature data from Dellisanti et al. [2010]). Geochronologic scale from Berggren et al. [1995]. (b) Schematic cartoon of the geodynamic setting of the Bobbio area during the early Miocene. The represented scenario shows the main features of a tectonic and sedimentary submarine system linking the front of the advancing allochthon (active frontal slope; see text) with the internal margin of the foredeep. It is worth noting that mass wasting-generated chaotic units (ol) accumulating at the base of the slope are progressively involved in the thrust system going along with the NE-verging tectonic transport of the nappe (modified after Labaume and Rio [1994]). the nearby lithologies, is interpreted as lateral (inner?) equivalents of the lowermost, fine-grained Cervarola unit (i.e., Brugnello shale) and the Marsaglia complex [Labaume, 1992; Labaume and Rio, 1994; APAT, 1997] (see Figures 2 and 3). [24] 2. Two minor tectonic units (namely the Coli I and II sub-units) were identified by Labaume [1992] and Labaume and Rio [1994] above the Cervarola foredeep deposits, made up of silty marls, shaly mass transport deposits and

hosting large blocks from other units (slide blocks). The two sub-units are in tectonic relationships by thrust surfaces, being the Coli I interposed between the Coli II and the foredeep deposits. The two sub-units have different composition: Coli I is composed of slope marlstones (Monte La Croce marlstone) with strong affinity with the slope deposits within the Marsaglia complex, whereas the Coli II shows the same stratigraphic units of the base of the Sanguineto unit (see below), which have been more

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severely deformed by thrust-related repetitions. We consider, therefore, the Coli II unit as a highly deformed part of the Sanguineto unit and Coli I as separated structural units with the name of Coli unit. This interpretation is implied by Labaume and Rio [1994], where the Coli unit is a displaced portion of the Marsaglia complex, dragged and stacked above the foredeep deposits of the Cervarola unit (Figure 3b). [25] The paleotemperatures/burial conditions inferred for this unit are significantly lower/shallower (deep diagenetic conditions, estimate paleotemperature in the range 100°– 150°C; see Figure 3a) if compared to those measured in the underlying Bobbio Formation (samples 19 and 20; Figure 2b). [26] 3. The Coli unit is overthrust by the late Eocene-early Miocene deposits of the Sanguineto unit. The latter unit is characterized by a basal level debris flow deposits (i.e., olistostromes) and slide blocks composed of rocks belonging to the Subligurian units, which is stratigraphically overlain by medium to fine grained turbidites (Rio Fuino sandstone), and thin-bedded turbidites and hemipelagites (Salsominore Fm.) [Labaume, 1992; Labaume and Rio, 1994; Elter et al., 1997]. [27] Clay mineralogy and organic matter maturity data suggest for this unit almost the same paleotemperatures/ burial conditions inferred for the Bobbio formation, suggesting deep diagenesis-early anchizone with paleotemperatures in the range 180–200°C (samples 5, 6, 7, 8 and 17; Figures 2b and 3a). [28] 4. The units of the BTW are presently thrust over by another turbidite complex, classically associated to the Subligurian units [see Elter et al., 1992; Pandolfi and Marroni, 1996]: the early Oligocene Aveto unit, consisting of fine-grained turbidites and shales, conglomerates with coarse-grained volcaniclastic elements and medium to coarse-grained volcaniclastic sandstones. The Aveto unit has been recently re-interpreted as the oldest and southwesterly placed foredeep unit, possibly deposited above the deformational front of the Subligurian units, or above its gravitational collapse [Elter et al., 1999; Catanzariti et al., 2003; Marroni and Pandolfi, 2007]. [29] In this unit the clay mineralogy and organic matter maturity data suggest almost the same paleotemperatures/ burial conditions shared by the Bobbio Formation and the Sanguineto unit (samples 3 and 4; Figure 2b). The paleotemperature ranges from deep diagenesis (150°–180°) to anchizone (220°–250°) (see Figure 3a). [30] 5. The Subligurian units, made up of PaleoceneEocene calcareous and siliciclastic turbidites, and shale deposits (mainly belonging to the Canetolo Complex [Plesi, 1974; Zanzucchi, 1994; Bortolotti et al., 2001] tectonically overlie the Coli, Sanguineto and Aveto units. [31] The clay mineralogy and organic matter maturity data collected in the Subligurian units (samples 2 and 21; Figure 2b) show quite lower paleotemperatures (deep diagenetic zone and paleotemperatures in the range 100–150°C; see Figure 3a), comparable with those inferred for the Coli unit. [32] 6. The topmost element of this tectonic stack is represented by the Ligurian units, mainly composed by calcareous turbiditic successions (Helminthoid flysch), finegrained clastic successions (“basal complexes”) and resedimented ophiolitic rocks (Casanova, Monte Ragola and Veri

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complexes) [Elter et al., 1992; Pandolfi and Marroni, 1996; Marroni and Pandolfi, 2001]. [33] In these units the clay mineralogy and organic matter maturity data (samples 1 and 22; Figure 2b) suggest paleotemperatures between 150 and 180°C and deep diagenetic conditions (see Figure 3a).

3. The Peli Shear Zone (PSZ) 3.1. Outline [34] The data presented in this work derive from a 1:10,000 field mapping of the northern portion of the BTW. In this area the Peli Shear Zone (PSZ) has been identified as a distinct tectonic unit including a heavily deformed core zone bounded by two relatively less deformed damage zones that grade upward and downward into the lithologies belonging to the Coli and Cervarola units, respectively. Here the PSZ is discontinuously exposed for at least 5 km in E-W direction and shows an average total thickness up to 70 m. To better define the overall character and structures of the PSZ, three key-outcrops roughly representing the basal damage zone, intermediate core zone and upper damage zone have been chosen (see Figure 4). [35] Generally, the PSZ consists of cm-sized clasts, msized blocks (i.e., fragments or pieces of single beds) and up to tens of m-sized coherent slabs (i.e., variously disrupted bed packages) of lithified to partly lithified rocks hosted by a matrix (BIM) or by shales and thin bedded turbidites (slide blocks and slabs), or representing tectonic slices. Their composition is mainly represented by massive and brecciated limestones, silty marlstones, and subordinately by fineto coarse-grained sandstones. As confirmed by our observations, supported by biostratigraphical data of Labaume [1992] and Labaume and Rio [1994], these rocks are sourced from: (1) Subligurian units (as blocks and slabs coming from the Canetolo complex), (2) Marsaglia complex, (3) Sanguineto unit (Rio Fuino sandstones and Salsominore Formation), (4) Miocene slope deposits of the Coli unit (Monte La Croce marlstone and related chaotic units), and (5) the fine-grained, upper part of the Cervarola unit (Peli shale and the interbedded olistostromes [Labaume, 1992; E. Mutti et al., unpublished data, 2003] (see Figures 2 and 3). Since the youngest elements enclosed within the PSZ are represented by the poorly lithified silty-marls of the Coli unit (i.e., Monte La Croce marlstone), their inclusion within the shear zone should be attributed at least to the middle-late Miocene (e.g., latest Burdigalian-Langhian). [36] In its whole extension, the core zone of the PSZ shows well-developed, meso-scale block-in-matrix fabric. Two different types of BIM can be recognized. The first type shows blocks with angular to sub-angular (limestones) and sub-rounded shapes (siltstones, mudstones and sandstones) dispersed in a pervasive scaly matrix, mainly composed by dark-black shale. This fabric is typical of the broken formations constituting the deformed Canetolo complex of the Subligurian unit and characterizes large-scale slabs within the other component units of Coli and Cervarola units, which may be either slide blocks or tectonic slices. [37] The second type is characterized by a polymictic matrix, with a strong component of fine- to medium-grained sand, hosting sub-millimetric to centimetric clasts of indurated

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Figure 4. Detailed geologic map slice of the study area with explanation (see Figure 2a for location). Positions of key-outcrops are marked. clays (“brecciated” fabric [see Ogniben, 1953; Rigo de Righi, 1956; Abbate et al., 1981]). This kind of matrix texture has been classically considered as diagnostic of an origin from mass-transport processes (olistostromes [see, e.g., Abbate et al., 1981; Bettelli and Panini, 1985; Pini, 1999]). These bodies (hereafter defined to as olistostromes) are mud-rich mass-transport deposits interbedded within, or in tectonic contact with the other component units. 3.2. Outcrop Descriptions 3.2.1. Key-Outcrop N°1 (Peli Section, 44°43′31.24″N/9°25′44.36″E) [38] This outcrop is located roughly 15 m below the PSZ core zone, within the tectonized stratigraphic top of the Cervarola unit (location in Figure 4). It comprises two main lithofacies corresponding to a shaly mass-transport body (olistostrome) intercalated in the silty marls (thin-bedded turbidites) of the Peli shale [Labaume, 1992; Mutti et al., unpublished data, 2003]. The olistostrome typically shows a polygenic breccia made of siliciclastic (medium- to coarsesandstones) and calcareous (massive and brecciated limestones) elements enclosed within a prevalent shaly, brecciated matrix and sourced from the Canetolo complex of Subligurian units and partly by the underlying Cervarola unit. The polygenic breccia crops out in the lower portion of the outcrop and passes upward to the pervasively deformed silty marls. They are separated by an evident sedimentary contact, representing the original bedding, and dipping about 45° in an easterly direction (Figure 5). The overall bedding (S0) of the Peli shale dips 20° toward the NE. [39] A single set of spaced cleavage (S1) has been identified within the silty marls, dipping toward WNW. In the basal breccia a single set of foliation is evidenced by a welldeveloped scaly fabric of the matrix (S1sf) and the common alignment of blocks’ and clasts’ long axes (pseudo-bedding dipping toward ESE). The preferential orientation of elongated clasts (L1) is roughly directed WNW-ESE.

[40] Within the silty marls at least three sets of shear systems characterized by mutual crosscutting relationships are usually recognizable. Such systems are made of roughly parallel micro-faults and kink/deformation band-like structures (Figure 6a). Their shearing sense has been interpreted on the basis of kinematic indicators (e.g., sigmoidal and SC-type structures, poorly developed slickensides, kink band sense of displacement and relative geometric offset of contacts and sedimentary markers). The first set (Sh) is represented by a dm- to m-spaced system of mm- to cmthick shears trending roughly parallel to the S1sf foliation in the underlying shales. The sedimentary contact between the two lithologies is reactivated/reworked accordingly to this shear system as well. An associated set of shears (Sh′) with opposite sense of movement is also present. This system, composed of mm- to cm-thick shears, is more closely spaced compared to Sh and clearly cuts the S1 foliation of the silty marls. The last set (ShL) comprises low-angle, mmto cm-thick shears characterized by a m-sized spacing and roughly developed along the S1 foliation of the silty marls. Since the eastward tilting of the bedding (see above), the Sh system dips roughly toward ENE while the Sh′ system steeply dips toward WSW. The ShL system is dipping in an opposite direction in respect to the Sh system (toward WSW). [41] The contact between silty marls and the polygenic breccia is deformed by cm- to m-sized upward convex inflections (see Figure 5). These structures show the incipient intrusions of the underlying brecciated material into the overlying silty marls, with the development of dome-like structures. From the apex of these structures, cm-thick pseudo-deformation bands characterized by incipient scaly fabric and localized fluid-induced alteration, seem to propagate into the silty marl portion through the structural discontinuities marked by the above-described shear systems. The cores of these dome-like structures are also the loci of major concentration of mineralized veins. These are

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Figure 5. Key-outcrop n. 1: photomosaic and interpretation, integrated by stereographic diagrams of analyzed structural features (see explanation). In the interpretation, the gray shaded areas represent damaged portions affected by pseudo-hydrofracturing banding. White arrows indicate passive injections (i.e., dome-like structures; see text). Locations of Figures 6a, 6b, and 13 are indicated. organized in a web of fractured network systems made of millimetric, fibrous veins. Similar features are also recognizable in a more subordinate amount within the shaly portions of the polygenic breccia, along the anastomosing surfaces of the scaly fabric, especially close to the lithologic contact (Figure 6b).

3.2.2. Key-Outcrop N°2 (Coli Section, 44°44′31.88″N/9° 24′13.98″E) [42] This outcrop has been chosen due to its position, roughly located in the central portion of the PSZ, and therefore well representative for the core zone. In this case, the overall composition is of Subligurian provenance, with a typical BIM fabric of m-sized, lozenge-shaped limestone

Figure 6. Detail of key-outcrop n. 1. (a) Kink/deformation band-like structure developed along a Sl shear plane; these discontinuities cutting silty-marls are expressed by discrete cm-thick bands of pseudo-scaly appearance (coin for scale; 2.2 cm in diameter). Location shown in Figure 5. (b) Calcite vein network characterizing the shaly polymictic breccia in the core portions of a passive injections (knife for scale; 5 cm long). Location shown in Figure 5. 9 of 21

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Figure 7. Key-outcrop n. 2: photomosaic and interpretation, integrated by stereographic diagrams of analyzed structural features (see explanation). In the interpretation, the gray shaded areas represent damaged portions affected by pseudo-hydrofracturing banding. White arrows indicate passive injections (i.e., dome-like structures; see text) and a single active injection in the middle (i.e., pipe-like structures; see text). Locations of Figures 8 and 12 are indicated. blocks enclosed within a black shale matrix characterized by a pervasive scaly fabric (Figure 7). The overall monomictic composition and the lack of a clearly defined brecciated matrix allow us to interpret these lithologies as belonging to a Subligurian broken formation. [43] A weak S1 foliation is recognizable in limestone blocks, while a relatively well-developed S1sf foliation is identifiable in the shaly matrix, which is gently dipping toward the northern sectors. The preferential orientation of elongated clasts (L1) is roughly directed NNE-SSW. [44] Portions of enhanced fissility developed around stiffer blocks (e.g., limestones). These narrow zones, showing concentrated scaly fabric, are located along blocks boundaries, surrounding the blocks, and sometimes thickening and wedging at the opposite edges of the enveloped element, geometrically resembling generalized shear-related pressure shadow-like structures. [45] Some folds affecting the original bedding surfaces (S1) has been observed along the lower boundary of the blocks. Such folds, developed close to the dome- and pipetype injections, show soft sediment deformation signature (e.g., thickening of hinge zone). Structural data on these features has been plotted for this location (see Figure 7). [46] The Sh system is here represented by mm- to cmthick shear surfaces and kink/deformation-like bands with a tens of cm-wide spacing, affecting also the margins of the blocks. The Sh′ system consists of well developed mm- to

cm-thick shear surfaces with an overall spacing of about 40–60 cm, which seem confined within Sh and ShL surfaces. The mm- to cm-thick shears and kink/deformation bands of ShL system is better developed in this area and clearly crosscuts the other two systems. [47] Due to the 20° tilting of the host rock stratification (S0) toward the NE, the restored dip of the Sh system is 40° to the northern sectors, while the Sh′ system is roughly subvertical. The ShL system instead gently dips toward S-SW with an overall inclination of 20° (see Figure 7). [48] This exposure is characterized by well-developed pipe-like structures affecting the margins of limestone blocks, and grossly corresponding to the edges of Sh′ system surfaces. These punctual structures show an overall wedgelike shape, with widths of few centimeters, and penetrate into the blocks for around one meter. Their infilling consists of a scaly black shale with a pervasive foliation trending parallel to the margins, and with scattered cm-sized calcareous fragments, probably ripped from fracture boundaries (e.g., clastic dykes) (see Figure 12). [49] Detailed observations on the core of these structures highlight the presence of a network of mm-sized calcite veins, following scaly fabric surfaces and roughly defining some S-C-type structure (Figure 8). Coupled with the strong weathering of limestone fragments and fracture margins, this should indicate a significant fluid circulation within these pipe-like structures. Moreover, crosscutting relationships

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Figure 8. Detail of key-outcrop n. 2: calcite vein network characterizing the scaly shales within the active injections core portions (coin for scale; 2 cm in diameter); note a sub-rounded limestone clast altered by fluid circulation on the right side of the structure. An evident S-C-type fabric is highlighted by two almost regular calcite vein network, indicating a sinistral shear (compatible with the movement of Sh′ shears). Location shown in Figure 7. show that these fractures are older than Sh, Sh′ and ShL shear systems. 3.2.3. Key-Outcrop N°3 (Bobbio-Coli Road Section, 44°44′36.58″N/9°24′11.19″E) [50] The third analyzed outcrop is located 20 m above the core of the PSZ, within the basal portion of the Coli unit, which is represented by a thick stratified succession (100 m) of marlstones, siltstones and fine sandstones belonging to the Monte La Croce marlstone (see above). [51] This lower part of the exposure consists of an olistostrome hosting limestone blocks (bed fragments up to 2 m sized) and sparse sandstone elements, mostly sourced from Subligurian units. The olistostrome is overlain by deformed and fractured silty marls of Monte La Croce marlstone with a NE-gently dipping and smooth contact of stratigraphic origin, though characterized by evidence of later tectonic reactivation. In the upper part of the outcrop the silty marls host a 60 cm thick olistostrome with a m-sized limestone block, rapidly wedging out toward the NNE (Figure 9). [52] In this location the silty marls, especially those overlying the upper olistostromal intercalation show some particular features: (1) an enrichment of mm- to cm-sized angular shaly clasts, characterized by fibrous calcite veining along the margins and within the original internal foliation, and (2) cm- to dm-sized, fluidal, convolute folds characterized by thickening of hinge zone and thinning of the limbs, which deform the thin silty laminae interbedded with the marls (Figure 10). [53] The S1 foliation recognizable in the silty marls dips toward SW with generally weak inclinations and is clearly crosscut by the shear systems, while the S1sf trend of the shales is almost horizontal and gently dipping toward northern sectors. The preferential orientation of elongated clasts (L1) is roughly directed NE-SW.

[54] The Sh system affects either the BIM shales or the silty marls, mainly comprising mm- to cm-thick shear surfaces and deformation-like bands with an overall spacing of 1 m. In this location the Sh system is much better developed and it crosscuts all the other shear systems. Particularly, the Sh shear surfaces develop along the lithologic boundaries between shales and silty marls, with a subparallel trend to S1sf foliation. [55] The Sh′ system, comprising mm- to cm-thick shear surfaces and deformation-like bands up to 10 cm thick, shows a wider spacing up to 2 m and more complex trending in respect to the other outcrops. The ShL system, composed of cm-thick shear zones and deformation-like bands up to 10 cm thick, is more closely spaced (50–70 cm) and results severely dissected by surfaces of Sh and Sh′ systems. [56] The stratigraphic reference frame (i.e., overall bedding attitude) in this area is gently dipping (10°) toward N-NE and this influence the overall dips of the shear systems that are: 30° toward NE for the Sh system, subvertical (mainly steeply dipping toward SW) for the Sh′ system, and 50° toward SW for the ShL system. [57] This exposure is characterized by the presence of well-developed dome-like structures, closely associated to the sub-vertical Sh′ system surfaces. These structures are represented by shale-cored bulges with heights up to 1 m and width up to 2 m, which deform the basal boundary of the silty marl portion (see Figure 11). [58] The cores of these structures present a dense network of mm-thick fibrous calcite veins, characterized by connected and disconnected (fractured) systems, with roughly lenticular shapes and short lateral continuity. These vein systems are developed along the anastomosing surfaces defined by the S1sf foliation of the shales, which show a locally complex

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Figure 9. Key-outcrop n. 3: photomosaic and interpretation, integrated by stereographic diagrams of analyzed structural features (see explanation). In the interpretation, the gray shaded areas represent damaged portions affected by pseudo-hydrofracturing banding. White arrows indicate passive injections (i.e., dome-like structures; see text). Locations of Figures 10, 11a, and 11b are indicated.

Figure 10. Detail of key-outcrop n. 3: calcite vein network characterizing the shaly polygenic breccia within the core portions of a passive injection (coin for scale; 2.2 cm in diameter). Location in Figure 9.

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Figure 11. (a and b) Examples of hydroplastic folds within the Monte La Croce marlstone testifying gravity-related deformations in non- to poorly lithified slope sediments. Locations in Figure 9. pattern compared to the underlying more regular arrangement (see Figure 12).

4. Discussion 4.1. Structures of the Block-in-Matrix Units Composing the PSZ 4.1.1. Scaly Fabric and Foliation [59] The fine-grained matrix of BIM rocks (both olistostromes and broken formations) composing the PSZ is pervaded

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by closely spaced, polished and sometimes faintly striated, anastomosing surfaces (scaly cleavage) that split the whole rock in progressively smaller lenses (scaly fabric [see, e.g., Vannucchi et al., 2003]). At a mesoscopic observation, the main direction of lenses (scaly fabric envelope [Pini, 1999]) defines a foliation (S1sf), which is roughly coplanar with planar distribution of blocks and clasts defining an outcropscale foliation (pseudo-bedding). The preferential orientation (stretching direction) of the main axis of blocks and clasts (L1) develops in NE-SW direction inside the plane of S1sf/pseudo-bedding. The latter is sub-parallel to the bounding surfaces of the PSZ, the main shear zone boundaries, and roughly coincident with the general bedding attitude shown by the underlying lithologies (S0). Another foliation is faintly recognizable within the more competent lithologies (i.e., silty marls and calcareous blocks) and is represented by discontinuous, slightly undulated cleavage (S1). [60] The general setting of S1sf foliation and pseudobedding, coupled with the geometric disposition of the enclosed elements (i.e., unidirectional elongation) and the kinematic indicators (e.g., asymmetric detached boudins and lithons) allow us to attribute this feature to a low-angle to layer parallel shearing developed along with the tectonic overthrusting of the Coli unit on the Cervarola unit. Although the evident deformation by simple shearing, a partial role of tectonic/sedimentary compaction (i.e., pure shearing) cannot be excluded (for and extensive review on the scaly fabric, see Vannucchi et al. [2003]). In this framework the S1 foliation within the blocks assumes a different significance from the scaly fabric for the BIM shales. Since the consistent angle shown by the Sl foliation compared to the other structures and the S1sf, it can be attributed to the original bedding/lamination signature, or otherwise it can be interpreted as structures developed perpendicularly to the main principal stress axis, such as pressure solution cleavage or “compaction” foliation developed in the plane s2-s3. [61] To summarize, the scaly fabric observed in the broken formations can be considered an original (primary) feature due to simple shearing (and folding?), while the one observed in the olistostromes can be considered a later feature, developed through the simple shearing of preceding tectonic and/or sedimentary compaction structures. This scaly fabric, pervasively splitting the whole rock continuity, is thought to give an overall, homogeneous, macroscopic ductile behavior to the entire shear zone. [62] Portions of enhanced fissility developed around stiffer blocks (e.g., limestones). These narrow zones are marked by a concentrated pervasiveness of the scaly fabric located along blocks boundaries, surrounding the blocks and sometimes thickening and wedging at the opposite edges of the enveloped element. Such zones of less developed deformation and diffraction of the scaly fabric form pressure shadow-like structures elongated in parallel way to the tectonic pseudo-bedding trend (see above). 4.1.2. Shear Surface Systems [63] At least three sets of cm- to m-spaced, mm- to dmthick shear systems characterized by mutual crosscutting relationships affect the relatively more competent lithologies (i.e., silty marls and calcareous blocks) and are usually recognizable as roughly parallel micro-faults and kink/

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Figure 12. Picture and interpretation of an active injection (pipe-like structure; key-outcrop n. 2, location shown in Figure 7). Knife for scale; 5 cm long. deformation band-like structures. The shearing sense of these structures has been interpreted on the basis of kinematic indicators (e.g., sigmoidal and SC-type structures, poorly developed slickensides, kink band sense of displacement and relative geometric offset of contacts and sedimentary markers). Two of these sets are conjugate (synthetic and antithetic) systems of dm-spaced planar surfaces arranged at low- (20°–30°) and high-angle (70°–80°) to the foliations. The other synthetic set is represented by dm- to mspaced surfaces and cm- to dm-thick bands oriented at a low angle (20°–30°) to the foliations and with opposite dipping direction to the other sets. [64] These systems of mutually intersecting synthetic (Sh), antithetic (Sh′) and low-angle synthetic shear surfaces (ShL) are herein interpreted as Riedel-type systems (i.e., R, R′ and P shears, respectively [Skempton, 1966; Tchalenko, 1970]), which reflect an overall sense of general shear of the whole PSZ toward E-NE, coherent with the general tectonic trend proposed by Labaume [1992] and Labaume and Rio [1994]. The recognition of similar shear-related structures has been performed also for other chaotic rock assemblages (i.e., Californian Franciscan Complex [Meneghini and Moore, 2007]). [65] The kink/deformation band-like structures marking these shear surfaces within the silty marl portions, are interpreted as pseudo-hydrofracturing bands developed when these sediments were not completely lithified (shallow burial conditions), as they show fluid-related alteration of the host-rock and a strong genetic relationship with the injection-related structures. [66] Their complex crosscutting and offsetting relationships are evidence of stepwise reactivation and slight rotation of each set, likely consistent with a polyphase deformation. Since their persistence, their outcrop-scale continuity and the

overprinting character, these structures probably represent the products of the main tectonic event(s). [67] It is also worthy to note that the frequency and the relative abundance of the R′ are by far higher than the ones typical of the fault rock evolution in increasing shear strain. R′ shears are considered as the primary product of the first step of deformation together with the low-angle R, their importance decreases as far as the strain increases and the R-P association predominates [Tchalenko, 1970; Pini, 1992]. This abundance of R′ shears may be related to a first stage of extensional flattening, as recognized by Vannucchi et al. [2008], and Remitti et al. [2007, 2011], in tectonosedimentary complexes between the Ligurian nappe and the foredeep deposits in the eastern part of the Northern Apennines. This interpretation could explain the higher degree of dispersion that R′ shows in Figures 5, 7, and 9. 4.1.3. Dome- and Pipe-Like Structures [68] Clastic injection-related structures deform lithologic contacts producing scaly matrix-cored “bulges” (domes) or cuspidate intrusions of scaly shale (pipes). These structures penetrate into the blocks or into the more competent rocks in contact with the BIM. Their apical portions are directly connected to the discontinuities created by the Sh (R), Sh′ (R′) and ShL (P) shear systems affecting the blocks (see above) (Figure 13). [69] Dome-like structures show bulge-shaped, convex upward geometries, cored with fine-grained material, which shows an alignment of the scaly fabric to the margins of the structure and less deformation of the internal elements (i.e., stretching). These structures are probably generated by the passive, macroscopic “flowage” of relatively ductile material (i.e., scaly fine-grained lithologies; see above) into newly created space in response to the differential movement due to R′-type shears dislocation (see Figure 14).

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Figure 13. Picture and interpretation of a passive injection (dome-like structure; key-outcrop n. 1, location shown in Figure 5). Knife for scale, 5 cm long. [70] Pipe-like structures show instead narrow, cuspidate shapes cored with more homogenous shaly material and share soft sediment deformation of the margins (e.g., folding), resembling the typical appearance of sediment-filled dikes (e.g., Neptunian dykes) (Figure 13). These structures are therefore interpreted as active injections of liquefied material (i.e., fluid escape structures formed in overpressured and shallow level crustal conditions). Similar features have been classically interpreted as diagnostic evidence of mud diapirism [Barber et al., 1986; Wakita, 1988; Lewis and Byrne, 1996]. [71] The calcite vein network characterizing the core portions of these structures are composed of antitaxial fibers disposed perpendicular to the vein-walls, testifying a slow opening of the fracture and episodic syntectonic crystal growth [Durney and Ramsay, 1973]. Given their complex arrangement (i.e., fractured, bended and lenticular vein networks developed along the surfaces defined by the scaly fabric), these vein systems probably developed through a crack-and-seal mechanism [Ramsay, 1980; Ramsay and Huber, 1983], consistent with a polyphased deformation history. Moreover the punctual occurrence of these vein networks within the cores of dome- and pipe-like structures seems to testify a localized accumulation of mineralizing fluids, and this could be attributed, at least for the dome-like structures, to an additional “suction effect” caused by the progressive displacement of the R′-type shears. 4.2. Origin and Evolution of the Block-in-Matrix Fabric [72] The BIM fabric is interpreted as first originated in the Eocene clays and limestones sequences (Argille e Calcari) of the Canetolo complex belonging to the Subligurian units. The deformation of these rocks occurred before the middle Eocene and caused pinch-and-swell and boudinage of limestones beds, folds and the scaly fabric of the matrix, leading to stratal disruption and to block-in-matrix fabric (Subligurian broken formation) [see Remitti et al., 2011]. These units are currently mixed with younger lithologies (Miocene silty marls; i.e., Monte La Croce marlstone) and jointly further deformed. Mixing occurred prevailingly by masstransport processes involving the already deformed Subligurian broken formations with their consequent re-deposition into the basins (Figure 15, stage 1).

[73] The described structure associations, their location and their relationships allow us to hypothesize a possible evolutionary scheme of the block-in-matrix fabric shown by the units bordering the northern rim of the BTW. 4.2.1. First Deformational Stage [74] The first stage of deformation is responsible for the development of cuspidate structures and sediment-filled fractures where the fine-grained material (i.e., brecciated matrix) is actively injected into the blocks (Figure 14a). The high mobility of the fine-grained material, the brecciated texture, the high degree of lithologic mixing and the common evidence of soft sediment deformations (e.g., hydroplastic folding, ductile shearing, flow-related features) allow the interpretation of these structures as created by overpressure-driven injections of a fluid phase (i.e., watersediment mixture) into relatively more competent material. Therefore, this deformational stage is probably related to the combination of superficial to very shallow crustal processes, as submarine landsliding, mud diapirism and incipient thrusting, typical in this kind of geodynamic setting (i.e., internal margin of a foredeep prone to be overridden by a tectonic nappe [Festa et al., 2010a, 2010b]) (Figure 15a). Moreover, the synsedimentary origin of this initial deformation seems to be supported by the concordance of these BIM bodies with the overall stratigraphic arrangement and the occurrence of original, lately reworked sedimentary contacts. [75] Given these lines of evidence, the primary origin of such BIMs is thought to be sedimentary (i.e., olistostromes), recycling/reworking and incorporating material and already internally deformed blocks coming from Subligurian broken formations. During this process, the original scaly fabric is destroyed with the subsequent production of loose material, which constitutes the elements composing the brecciated matrix of the olistostromes. Such sedimentary BIMs are compatible with cohesive debris/blocky flow processes [Mutti et al., 2006; Ogata, 2010]. The increased lozenge/ phacoidal shapes and internal deformations (i.e., detached, symmetric and asymmetric boudinage) shared by blocks, could be attributed to subsequent coaxial/coplanar shearing and layer-parallel extension, while the enhanced lithologic fragmentation of the matrix is thought to be possibly due to shallow tectonic and/or diapiric reworking [Ogata et al., 2007].

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Figure 14. (a–c) Proposed conceptual evolutionary scheme of the development and evolution of the block-in-matrix fabric within the PSZ. The polyphased deformation phases leading to progressive element fragmentation are expressed by Figures 14a, 14b, and 14c. See text for explanation. [76] Due to the inferred superficial conditions and the roughly continuous stratigraphic relationships, these deformations should be attributed to the early Miocene (Aquitanianlower Burdigalian), coeval with the synsedimentary deformational stages characterizing the lower foredeep deposits of the Cervarola unit (i.e., Marsaglia complex and Brugnello shale) (Figure 15). [77] Similar evidence reported from other exposed accretionary complexes have been related to early phases of prism advancement and footwall overloading (incipient tectonic

compaction and layer parallel mineralized veins [Meneghini et al., 2009]), mud diapirism, and/or gravity mass movement [Barber et al., 1986; Byrne, 1985; Byrne and Fisher, 1990; Kano and Konishi, 2001; Alonso et al., 2006]. 4.2.2. Second Deformational Stage [78] A second stage of strictly tectonic deformation developed during to the NE-vergent overthrusting of the Subligurian (frontal) portion of the Ligurian nappe, with the consequent incorporation of these units in the main shear zone. At this stage the Subligurian units were not yet

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Figure 15. General schematic cartoon showing an attempt to attribute the deformational phases described in Figure 14 to the different steps of the tectonostratigraphic evolution of the western Northern Apennines. completely covered by the Ligurian units (at least not in the frontal part of the wedge), as confirmed by the Subligurian provenance of the precursory olistostromes emplaced within the uppermost Cervarola unit (i.e., Peli shale). [79] In this framework, generalized shearing allows the mechanical reactivation of stratigraphic contacts, the development of pervasive scaly fabric in clayey lithologies and foliation in stiffer blocks (i.e., S1 foliation). Along with the progression of the deformation, the first linked development of Riedel-type shears and P planes takes place inside and along the boundaries of these relatively more competent blocks, with the subsequent formation of related passive injections (i.e., dome-like structures) (Figure 14b). [80] The development of early generations of fibrous calcite vein networks probably represent a significant contribution of mineralizing fluids, precipitating calcite during a slow overpressure-related opening of previous developed discontinuities such as anastomosing planes of the scaly fabric. In this case, overpressured conditions may be due to fluid generation (i.e., water) from saturated clayey lithologies by pore collapse; further increase in interstitial pressure is probably due to coupled and cyclic mechanisms of fluid migration from a deeper and more internal source (i.e., fluid pumping), hydrocarbon generation and dehydration/transformation reaction of clay minerals [Vannucchi et al., 2008]. Fluid transfer should be favored by lateral dynamic permeability of scaly clays (i.e., horizontal connectivity along the tectonic pseudo-bedding), which constitutes preferential pathway systems for fluid migration. Therefore, overpressured fluids migrate into blocks through structural discontinuities (i.e., Riedel-type shear and P planes), using active and passive injections as preferred pathways,

developing incipient pseudo-hydrofracturing, which products are expressed by fractured calcite vein networks in the shales, and within the blocks by narrow fracture systems characterized by bands of marked scaly fabric and fluidrelated alteration. [81] Tectonic shearing also produces bands of intense deformation arranged as haloes around competent blocks; these cm- to dm-thick bands are characterized by stronger and more pervasive scaly fabric, probably developed in response to relatively high rheologic contrast between blocks and surrounding shale matrix. [82] In the our scheme, these deformational stages represent the earlier stages of the PSZ development, and the related units should be referred to as broken formations, for those located in the uppermost Cervarola unit (i.e., Peli shale) and the lowermost Coli unit, and mélange those characterizing the incipient core zone. As already pointed out, mélanges are of tectonic origin, but often rework previous sedimentary mélanges (olistostromes). [83] This second tectonic phase should be middle Miocene in age, during the main overriding phases of the advanced Subligurian portion of the Ligurian nappe, and attributable to the development of the PSZ (Figure 15). 4.2.3. Third Deformational Stage [84] Proceeding along with the tectonic deformation, fluid overpressure, shear-induced element rotation and reactivation of previous formed discontinuities cause the progressive loss of internal integrity of the blocks and allow opening of connected fractures (Figure 14c). Passive flowage of enveloping shales due to the overall macroscopic ductility of the scaly fabric allow widening of injection structures and detachment of internal discontinuities within the blocks.

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This incipient fragmentation of blocks in smaller elements continues until it reaches a complete disruption, resulting in an enhanced pervasiveness of the block-in-matrix fabric. Moreover, vein formation (i.e., crystallization pressure) within scaly clays-filled fractures may contribute to the element separation process. [85] This third deformational stage marks the latest stages of the PSZ activity, marked by an increase in thickness and in decoupling of the core zone (more brittle behavior), reworking the involved units in an almost mesoscopically homogeneous mélange belt. The timing of development should be attributed to the middle-late Miocene, and be the consequence of the protracted northeastward movement of the entire Ligurian nappe (including Subligurian and Ligurian units, and the Epiligurian deposits). The maximum burial temperatures of the Aveto, Sanguineto, Coli and Cervarola units are reached in this stage [Dellisanti et al., 2010]. [86] After this stage, the Aveto and Sanguineto units are stacked onto the southwestern part of the BTW and a general antiformal stack geometry is assumed by the entire BTW structure, possibly related to in-depth duplexing. This geometry is could be responsible for the differential uplift of the BTW during the incipient post-orogenic (gravitational?) stages of the Northern Apennines (Figure 15). Evidence of this latter deformation is also recognizable in the southern exposures of the BTW, within the BIM units of the Marsaglia complex.

5. Conclusions [87] The BIM rock assemblages characterizing the PSZ represent the final product of a protracted deformational history, leading to the amalgamation of different, already deformed/disrupted units: generally, from (1) Eocene broken formations to (2) olistostromes, passing through (3) diapirs and (4) Miocene broken formations, and evolving to (5) tectonosedimentary mélanges. [88] The diverse lithologic components are characterized by different styles of deformation, reflecting different states of lithification, and therefore implying different deformational processes and burial depths (i.e., from superficial, soft-sediment, ductile deformations to progressively more brittle and deeper mechanisms). The combined development and the progressive evolution of these structural associations within the PSZ, actively produce a diffuse fragmentation of the more competent elements enclosed within rheological different lithologies, leading to the achievement of a continuously enhanced block-in-matrix texture characterized byprogressively smaller elements coming along with the overall deformation and increasing of the lithification degree. [89] The rheologic contrast (i.e., competence contrast) between enveloping material and blocks seems to be maintained during the various deformational phases. This fact is evident from the overall ductile behavior of the shales compared to the more competent behavior of the silty-marls and limestones along with compaction, burial and tectonic deformation. Among the generating processes, the fluid overpressure seems to having played an important role in contributing to the fragmentation of elements during the whole evolution of the shear zone, from the earlier stages (e.g., soft sediment deformations) to the latest ones (e.g., pseudohyrofracturing, calcite veining processes).

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[90] From a wider point of view the recognition of these structural associations and the subsequent discrimination of units with different origin, are useful to get more information regarding the development of the BTW, better constraining its geodynamic evolution, from the early stages of the nappe emplacement to the latest exumation/unroofing phases of the entire structure. [91] Acknowledgments. The authors are deeply indebted to E. Mutti for having introduced us to the complex geology of the Bobbio area and for the financial and intellectual support. The authors are also grateful to A. Festa and G. Codegone for the fundamental discussions on the field; A. Artoni and A. Braathen for having carefully reviewed an early version of the manuscript; and T. Byrne, K. Barber, R. E. Holdsworth, and T. Needham for their criticism and useful suggestions. The journal reviewers, C. Fergusson and P. Vannucchi, are kindly acknowledged for the stimulating and constructive reviews. The study of the paleotemperature has been funded with the PRIN 2005–045211 grants (G. A. Pini responsible). This study has been supported by research funds of the Centro di Documentazione e Studio per la Geologia dell’Appennino Settentrionale, Bobbio (PC) (E. Mutti grants).

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