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Evidence for active folding and faulting at the northern Apennines mountain front near Bologna, Italy from high resolution seismic reflection profiling

2011; American Geophysical Union; Volume: 38; Issue: 16 Linguagem: Inglês

10.1029/2011gl047828

1944-8007

Pier Paolo Bruno, Frank J. Pazzaglia, Vincenzo Picotti,

Seismic Waves and Analysis

Geophysical Research LettersVolume 38, Issue 16 Solid EarthFree Access Evidence for active folding and faulting at the northern Apennines mountain front near Bologna, Italy from high resolution seismic reflection profiling Pier Paolo G. Bruno, Pier Paolo G. Bruno [email protected] Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Grottaminarda, ItalySearch for more papers by this authorFrank J. Pazzaglia, Frank J. Pazzaglia Department of Earth and Environmental Science, Lehigh University, Bethlehem, Pennsylvania, USASearch for more papers by this authorVincenzo Picotti, Vincenzo Picotti Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Bologna, ItalySearch for more papers by this author Pier Paolo G. Bruno, Pier Paolo G. Bruno [email protected] Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Grottaminarda, ItalySearch for more papers by this authorFrank J. Pazzaglia, Frank J. Pazzaglia Department of Earth and Environmental Science, Lehigh University, Bethlehem, Pennsylvania, USASearch for more papers by this authorVincenzo Picotti, Vincenzo Picotti Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Bologna, ItalySearch for more papers by this author First published: 16 August 2011 https://doi.org/10.1029/2011GL047828Citations: 9AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract [1] We have acquired and processed an ∼2 km long high-resolution seismic reflection profile across a segment of the Northern Apennine mountain front (Italy), west of the city of Bologna. The profile, constrained by several wells, targets a long-postulated shallow blind or emergent thrust called the Pede-Apenninic Thrust Fault. Despite decades of reflection seismology in this part of the Apennines, a shallow or emergent structure consistent with the surface geology has yet to be definitively identified, a problem likely caused by the topography of the Apennine front and the traditional focus on deep hydrocarbon targets where the first 0.5 km of strata is poorly imaged. Our seismic data show an ∼300 m deep high-resolution picture of the Po foreland as it meets the Apennine mountain front. All imaged reflectors are continuous at the mountain front and are foreland-dipping, showing clear growth relationships; higher-angle reflectors are interpreted as faults. Our interpretation includes a possible hinterland-dipping blind thrust and surface normal faults, which offset late Pleistocene-Holocene deposits as much as 60 m (long-term slip rates of 0.1–0.25 mm/yr) that disrupt, but do not conceal, the growth strata relationships. Vp tomographic imaging also suggests coseismic surface-faulting in Holocene colluvium. These results have implications relevant for the effective data collection and processing techniques for these kinds of shallow active structures as well as a re-evaluation of the seismogenic potential of densely populated cities like Bologna along the Apennine mountain front. Key Points High-resolution reflection seismology on rollback-retreat settings Seismic data collection and processing techniques in complex settings Fault kinematics and slip rates of faults inferred by seismic exploration 1. Introduction and Approach [2] A sharp, linear, and locally steep mountain front of debated geodynamic origin demarcates the northern Apennines, Italy from the Po foreland (reviewed by Picotti and Pazzaglia [2008]). Recent studies documenting geodetic strain at the Bologna segment of this front propose that it is the forelimb of an actively growing fold [Stramondo et al., 2007] cored presumably by a emergent or shallow blind thrust fault [Boccaletti et al., 1985] thought to be seismogenic [Chiarabba et al., 2005; Pondrelli et al., 2006]. At present, both the geometry of the fault and the potential hazard it presents to the densely populated Po foreland are incompletely understood. Present-day seismicity (Figure 1c), shows a gap in the Bologna area [see also Chiarabba et al., 2005] and does not yield definitive clues to recent activity for this sector. Deep thrust-sense earthquakes are the most common kind of seismicity documented for the northern Apennines. The distribution of the compressional seismicity may suggest that, beside the postulated blind trust, other middle crustal sources are patchily distributed both internally, within the underplating volume, and in the Po Plain, possibly associated with reactivation of basement structures [Picotti and Pazzaglia, 2008]. A number of normal faults active in the upper crust of the uplifting belt suggests a diffusion of the strain that possibly inhibits the development of important extensional seismogenic sources in this sector of the chain [Picotti and Pazzaglia, 2008]. Figure 1Open in figure viewerPowerPoint (a) Inset map shows the general location of the study area in northern Italy. General Geological map of the northern Apennine mountain front near Bologna (see location box in Figure 1c: modified from Martelli et al. [2010]), draped upon a 10-m DEM (courtesy of Servizio Geologico, Sismico e dei Suoli, Regione Emilia Romagna). The seismic line is shown as the A-A′ line. Note the location of Ponte Ronca, wells constraining the stratigraphy in the seismic line, and the inferred location of the PTF [Boccaletti et al., 1985]. (b) Schematic, not to scale, composite cross section of the Bologna mountain front exposed in the surface geology west of the Reno River showing the Plio-Pleistocene growth allostratigraphic units separated by major unconformities. MIOS = marine oxygen isotope stage. Nomenclature, stratigraphy and sedimentology of PAA, QIMO, AEI, and AES units are described in the text. (c) 10-m DEM of the northern Apennines mountain front near Bologna showing the locations of epicenters from the 1981–2002 Italian seismicity catalogue (INGV: CSI 1.1) and the seismic moment tensors of the two 1997–2001 quakes with Mb > 5 which occurred in the area (RMCT - INGV catalogue). (d) Landscape of Ponte Ronca district (Bologna) on a satellite image (Google Earth) with the seismic line track and shot-points noted. [3] One of the key limitations in all studies that have struggled with the origin of mountain fronts in active orogenic wedges [Vann et al., 1986] and linking shallow, active structures to surface geology has been the inability to clearly image faults that offset and cause foreland-dipping growth relationships of young strata in the shallow subsurface. Fault systems and related geological structures in the Northern Apennines and elsewhere are commonly investigated by using industry seismic reflection data [e.g., Smith and Bruhn, 1984], aimed at constraining the fault geometry, dimensions and kinematics, which are also crucial information to assess seismogenic potential. However, mountain-front faults present significant challenges for seismic exploration. The main factors hindering seismic imaging are strong lateral velocity contrasts across steeply-dipping faults at the basin edges and unfavourable topographic and near-surface conditions along the mountain front, which can pose significant difficulties for data acquisition and processing. Poor quality and resolution mainly affect the shallow portion of reflection profiles (usually down to 500–1000 m depth) and consequently a data gap commonly exists between surface geology and the deeper industry seismic images of mountain-front faults. This problem can hamper the accurate imaging of the shallow architecture of the fault system, which is crucial to link morpho-structural and paleoseismological observations to deep faulted structures, in the perspective of seismogenic potential estimation. [4] In principle, the data gap can be filled by high-resolution near-surface reflection data [e.g., Bruno et al., 2010]. In this paper, we present a new high-resolution seismic reflection line, recently acquired for the Bologna mountain front that successfully images the first 200–350 m of the subsurface, critical for a clearer interpretation of mountain-front structures. For our acquisition, we used a dense wide-aperture acquisition layout as discussed by Bruno et al. [2010]. This acquisition geometry differs from typical common-midpoint small-aperture reflection surveying as it aims at recording both multi-fold reflection data spanning a large range of offsets (from small-offset near-vertical reflections to large-offset large amplitude post-critical reflections) and deep penetrating refracted waves, which are suitable for first-arrival traveltime tomography. Tomography not only contributes information about the subsurface structure but also provides a good control on the near-surface velocity structure that is crucial for improving the reliability of the static corrections and, ultimately, the stacking of shallow reflections [Bruno et al., 2010]. 2. Local Setting [5] The geology and topography of the Bologna mountain front is representative of the northern Apennines (Figures 1a and 1b), but is also unique in several specific geologic and topographic ways. The Bologna mountain front is underlain by folded and faulted Cenozoic epi-Ligurian deposits that sit in wedge-top basins atop the older Ligurian nappe (Figure 1b). The bedrock units are variably consolidated and stratified sandstones, mudstones, and marls that are folded into a frontal anticline paired with a hinterward syncline that collectively span a ∼12 km wavelength. Neogene units up to and including the lower Pliocene are clearly involved in the folding as growth strata [Ori and Friend, 1984]. Younger, Pleistocene, poorly-consolidated, marine to continental gravel, sand and mud deposits that drape the forelimb of the frontal bedrock anticline are of particular interest to our study (Figures 1a and 1b). Based on the lithostratigraphy of the Casalecchio di Reno 1:50,000 geologic map [Martelli et al., 2010] and well information, we identify four major allostratigraphic units. [6] The Plio-Pleistocene "Argille Azzure" Formation (PAA) is the oldest stratigraphic unit in our imaged section. It consists mainly of marine mudstones, but locally includes sandstones, and calcarenites collectively several hundred meters thick. A rapid transitional package separates PAA from an overlying littoral sequence of sand, silty-sand, and mud that coarsens upwards to sand and pebbly sand. This succession is known as the "Sabbie di Imola" Fm (QIMO) and outcrops in the direct vicinity of our seismic line. QIMO is 70–85 m thick at the mountain front and imaged in many seismic lines as foreland propagating clinoforms [Pieri, 1987]. [7] QIMO is uncomfortably overlain by a 40–200 m foreland-thickening package of fluvial gravel and gravelly sand interbedded with lacustrine and brackish coastal silty sand and mud called Synthem "Emiliano-Romagnolo Inferiore" (AEI), which represents the onset of continental deposition for this part of the foreland and does not reach the outcrops at the foothills. Unconformably overlying AEI are several silty-clay to sandy-gravel coarsening-up packages, collectively called the Synthem "Emiliano-Romagnolo Superiore" (AES). These units drape the mountain front as inset alluvial fan and fluvial terrace deposits, each a few meters to tens of meters thick [Amorosi et al., 1996]. AES units thicken into the subsurface of the foreland where their distribution is well constrained by water well data. [8] The subsurface position of these deposits and their bounding unconformities are constrained by four wells located directly adjacent to the seismic line (courtesy of Paolo Severi: Figure 1d) and on the mountain front by core S1 and commercial well C1 (see locations in Figure 1a). 3. Data and Methods [9] We acquired the ∼2-km long seismic reflection profile near the village of Ponte Ronca, Comune of Zola Predosa (Figure 1d). The line stretches from the nearly flat Po Plain and ascends the mountain front with approximately 60 m of relief atop the foreland-dipping deposits of QIMO and AES units (Figures 1a, 1b, and 1d). The logistics of line acquisition had to accommodate two gaps of ∼100 m and ∼50 m where the line crosses a railroad and the main mountain front road respectively (Figure 1d). [10] P-wave seismic reflection data were collected using a single 6382-kg IVI-Minivib® truck. At each vibration point we stacked 3, 15-s-long, 10–200 Hz sweeps and generated a 1-s correlated record with 1.0 ms sample interval. Source move-up was 5 m. Single 10-Hz vertical geophones were also placed at 5 m intervals. Tight spacing of both geophone and vibration points ensured a very regular and dense subsurface coverage, with common midpoints spaced of only 2.5 m. We fixed the recording 96-channel array length to 475 m, and moved the source within the spread for 46 positions, as shown by Figure 2. Multiple shoots within the spread created redundant reciprocity relationships to ensure consistent first-arrival picking among different record sections. After the first 48 sweeps, the spread was shifted 48 geophone positions leaving an overlap area of 240 m between the two spread locations. The field procedure described above was expressly designed to keep maximum data redundancy at changeovers (Figure 2d). Figure 2Open in figure viewerPowerPoint Example of cross-correlated common-shot gathers acquired (a) on the Po river valley and (b and c) above the mountain front. (d) Fold variation along the line. (e) wide-aperture acquisition scheme (see text), consisting of seven overlapping patterns of the receiver array. Location of shots A, B and C are also highlighted. [11] Three representative common-shot panels in Figures 2a–2c show overall good data quality with high-frequency reflections and clear first arrivals in the whole offset range. Data quality is excellent in the Po river plain; however, it degrades above the mountain front (Figures 2b and 2c) because of the complexity of the structural settings and because of static problems caused by the topographic pattern (Figure 2e) and laterally heterogeneous near-surface velocity (Figure 3). Therefore, (tomo)statics and surface-consistent residual statics played an important role for improving the S/N ratio. The processing sequence also consisted of trace editing, Vibroseis whitening and conversion to minimum phase of the seismic wavelet, spiking and predictive deconvolution, automatic gain control, time varying band-pass filtering, first-arrival muting, elevation statics, surface-consistent residual statics, normal moveout, dip moveout, CDP stacking and finally Kirchoff post-stack depth migration. Tomographic velocities were also used to guide the normal-moveout correction for data from the shallow part of the mountain front where strong ground roll and refractions hamper the standard velocity analysis technique based on the semblance. At larger Two-Way Travel Times (TWTT), and above the Po River plain, stacking velocities were instead estimated with the semblance. Figure 3Open in figure viewerPowerPoint Final tomogram (CDP 550–1900) with, below, a checkerboard test performed using a cell dimension of 40 × 20 m. [12] For refraction tomography data analysis over 32,000 first-arrival travel times were hand-picked on the common source gathers. Uncertainty on travel time readings ranges from a minimum of 2 ms (i.e. 1/8 of the dominant period of the P-pulse) to 10 ms. For travel time modelling we used an optimized ray-tracing technique described by Zhang and Toksoz [1998]. Inversion was achieved by a nonlinear least-squares approach [Scales, 1987]. The code accounts for nonlinearity by adding a variable damping parameter in the Gauss-Newton method. The 2D starting models were determined via traditional refraction analysis procedures using travel-time curves from several source locations within the spreads. The inversion process halted after 12 iterations. The final model has a RMS traveltime residual equal to 3.4 ms. A checkerboard test performed using a sinusoidal perturbation pattern with cell size of 20 × 40 m and extreme values of ±10% of final models (Figure 3) reveals that resolution of the velocity image rapidly deteriorates at depth. The model is well resolved down to about 40 m depth from the surface, but provides valuable information on the shallower structure (Figure 3). 4. Results and Line Interpretation [13] The seismic reflection/refraction survey (Figures 3 and 4) effectively images ∼350 m beneath the Po foreland valley and ∼200 m below the mountain front. Interpretation of the results relies on the complementary information provided by the seismic tomography and migrated section. The most noticeable feature of the tomographic model is the abrupt southward deepening of two prominent high-Vp regions (2000–2300 m/s) at 1100 m and 1350 m distance (Figure 3). Both these features are outlined by an abrupt change in depth of the 1500–2000 m/s contours, showing a vertical separation of ∼20 m, and are accompanied upward by a gradual southward thickening of the regions with low Vp (700–1500 m/s) that can be interpreted as near-surface colluvium. Both steps, at 1100 m and at 1350 m distance, spatially correlate with two high-angle normal faults, visible on the migrated section of Figure 4, as we shall discuss below. Figure 4Open in figure viewerPowerPoint Geological Interpretation of the depth-migrated reflection section with the seismic tomographic image of Figure 3 (simplified) shown on top for comparison. Nomenclature, stratigraphy and sedimentology of PAA, QIMO, AEI, and AES units are described in the text. QCOL: Quaternary, fault-related colluvia. Two alternative interpretations are presented for the structure at position 1600–1800 m: a low-wavelength growing fold in the pre-Middle Pleistocene units, possibly cored by a shallow blind thrust fault; a north-dipping normal fault. [14] The principal seismo-stratigraphic feature of the reflection data is a multicyclic event that separates two seismic facies: a basal, discontinuous to nonreflective package, distinct from the overlying more continuous and high-amplitude reflections. Using the constraints provided by wells west of the Reno River valley (Figure 4), we take this reflector to represent the transition between the underlying PAA and overlying QIMO that becomes less conformable toward the foothills as well as westward along the mountain front. [15] QIMO is characterized by high-amplitude reflectors and good continuity at its base, located around 200 m below sea-level at the northern end (beginning) of the line. As a whole, the thickness of this package decreases south-westward towards the mountain front from around 100 m to 40 m (Figure 4). All allostratigraphic units show offsets across 1100 m and 1350 m distance, what we interpret to be high-angle normal faults in agreement with the interpretations of Bertotti et al. [1997] and Picotti et al. [2009]. These faults clearly dissect late Pleistocene-Holocene deposits, and disrupt, but do not conceal the growth-strata relationships. [16] Progressive rotation of sedimentary units and unconformities beneath the Po Plain indicates growth for a fold active since the Early Pleistocene. The unconformity at the base of QIMO is rotated 18° towards the foreland indicating a forelimb tilt rate of ∼23°/Ma. The morphology of the reflectors at position 1600–1800 m may suggest two alternative interpretations. First: a low-wavelength growing fold in the pre-Middle Pleistocene units, possibly cored by a shallow blind thrust fault, that collectively indicates a superficial compressional regime that is no longer documented in the Pleistocene reflectors. This compressive structure coexists with the high-angle south-dipping normal faults located ∼300 m and 600 m to the north. However, based on seismic data, it also reasonable to interpret the structure at position 1600–1800 m simply as a north-dipping normal fault. The compressive structure responsible for the tilting of the Pleistocene reflectors and for the mountain front uplift, has been proved to be much deeper, tipping at around 17 km, by Picotti and Pazzaglia [2008]. 5. Discussion and Conclusions [17] Our interpretation of the seismic profile finds continuity of the Pleistocene reflectors across the mountain front of the Apennines near Bologna, therefore documenting that the compressional structure responsible for the topography of the mountain front has to be blind and deep. Furthermore, activity of this thrust is documented by the divergent pattern of the reflectors starting from the base of QIMO, i.e., from the base of the Middle Pleistocene. This geometry documents clear growth relationships over a foreland-dipping limb of a fold, that has been modelled by Picotti and Pazzaglia [2008] as the frontal limb of a large fold related to the presence of a middle crustal thrust (>15 km of depth). Several studies show that the mountain front – Po Plain transition appears to have components of both deep (>10 km, reviewed by Picotti and Pazzaglia [2008]) and shallow (<10 km, reviewed by Ponza et al. [2010]) compressive deformation. [18] Our results place some constraints on the shallow component near Bologna where it has not been previously well-defined. The shallow blind thrust, or alternatively the north-dipping normal fault, coexists with south-dipping normal faults, which offset the late Pleistocene-Holocene deposits, and disrupt, but do not conceal the growth-strata relationships. These high-angle faults have a maximum vertical throw up to 60 m, as indicated by the displacement of the top of QIMO in Figure 4, and present a somewhat less well understood or constrained seismic hazard to large Po Plain cities like Bologna. These faults indicate modest stretching in the carapace of the frontal anticline and are similar to faults recently reported by Morley [2007] for the frontal anticlines of active collisional wedges. Map-length distribution [Bertotti et al., 1997; Capozzi and Picotti, 2002; Picotti et al., 2009] of normal faults outcropping in the Ponte Ronca area show them to be several km to a few tens of km long and hypothetically to extend to depths of more than 4 km. Locally, such as south of Bologna, such high angle faults steepen the dips of the forelimb such that QIMO can dip in excess of 10 degrees and the mountain front has a particularly abrupt topography. [19] Following the outcomes of multidisciplinary surveys carried out on active basin-bounding normal faults in the western U.S. [Morey and Schuster, 1999; Sheley et al., 2003; Mattson, 2004], we can tentatively infer recent coseismic surface faulting from our survey. Indeed, as these authors point out by comparing shallow tomography and reflection profiling with paleoseismological trenches, the coexistence of (1) steps in high-Vp bodies, (2) thickening of low-Vp colluvium in the hangingwall and (3) deformation of reflectors in the very near-surface, can be diagnostic of recent coseismic surface-faulting episodes. [20] Based on the observed offset, long-term slip rates on high-angle faults are in the range of 0.1 to 0.25 mm/yr. However, displacements of about 20 m, measured on the 1.5 and 2.0 km/s isovelocity lines of the tomogram of Figure 3 argue for Holocene slip rates an order of magnitude faster, indicative of a recent increase in fault activity, or long-term unsteadiness in the fault slip rate. On the other hand, the seismic gap around Bologna (Figure 1c), also revealed by recent seismological studies [Chiarabba et al., 2005] may hint that such faults are not seismogenic and behave by some aseismic creep as they accommodate stretching in the carapace of the frontal anticline. Alternatively, the recurrence interval for earthquakes on these faults is on the order of 104 years and if they are seismogenic, their strike length and their recent slip rate suggests that they are capable of generating earthquakes with moment magnitudes approaching 6 [e.g., Wells and Coppersmith, 1994]. At a more general scale, this study proves the efficacy of the dense wide-aperture profiling, for the shallow, high-resolution exploration of mountain-front faults. Acknowledgments [21] The authors would like to acknowledge the help and cooperation from Paola Lipparini, Annamaria Todisco and Giancarlo Borsari of the Comune di Zola Predosa. 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