Geological and archaeological evidence of active faulting on the Martana Fault (Umbria–Marche Apennines,Italy) and its geodynamic implications

This paper examines the morphotectonic and structural–geological characteristics of the Quaternary Martana Fault in the Umbria–Marche Apennines fold‐and‐thrust belt. This structure is more than 30 km long and comprises two segments: a N–NNW‐trending longer segment and a 100°N‐trending segment. After developing as a normal fault in Early Pleistocene times, the N–NNW Martana Fault segment experienced a phase of dextral faulting extending from the Early to Middle Pleistocene boundary until around 0.39 Ma, the absolute age of volcanics erupted in correspondence to releasing bends. The establishment of a stress field with a NE–ENE‐trending σ₃ axis and NW–NNW σ₁ axis in Late Pleistocene to Holocene times resulted in a strong component of sinistral faulting along N–NNW‐trending fault segments and almost pure normal faulting on newly formed NW–SE faults. Fresh fault scarps, the interaction of faulting with drainage systems and displacement of alluvial fan apexes provide evidence of the ongoing activity of this fault. The active left‐lateral kinematic along N–NNW‐trending fault segments is also revealed by the 1.8 m horizontal offset of the E–W‐trending Decumanus road, at the Roman town of Carsulae. We interpret the present‐day kinematics of the Martana Fault as consistent with a model connecting surface structures to the inferred north‐northwest trending lithospheric shear zone marking the western boundary of the Adria Plate. Copyright © 2003 John Wiley & Sons, Ltd.     

Normal faulting, transcrustal permeability and seismogenesis in the Apennines (Italy)

Late Pliocene–Pleistocene tectonic evolution of the Apennines is driven by progressive eastward migration of extensional downfaulting superposed onto the Late Miocene–Early Pliocene compressional thrust belt. This process has led to distinct structural domains that show decreasing transcrustal permeability from conditions of pervasive mixing between deep and surface fluids in the hinterland (west) to conditions of restricted fluid circulation and overpressuring in the foreland (east). At present, the highest rates of normal faulting and the strongest seismicity occur in the area bounded by stretched, highly permeable crust to the west and thick, poorly permeable crust to the east. In this area, the seismogenic sources of the largest earthquakes (5<M_s<7) are potentially related to mature normal faults that deeply penetrate thick brittle upper crust, and act as transient high-permeability channels during seismic activity. In this framework, it is plausible that domains of overpressuring govern progressive inception of normal faulting and fluid redistribution in the crust, leading to eastward migration of the belt of maximum seismicity with time.     

Normal faulting along the western side of the Matese Mountains: Implications for active tectonics in the Central Apennines (Italy)

We provide new field data from geologic mapping and bedrock structural geology along the western side of the Matese Mts in central Italy, a region of high seismicity, strain rates among the highest of the entire Apennines (4–5 mm/yr GPS-determined extension), and poorly constrained active faults. The existing knowledge on the Aquae Iuliae normal fault (AIF) was implemented with geometric and kinematic data that better constrain its total length (16.5 km), the minimum long-term throw rate (0.3–0.4 mm/yr, post-late glacial maximum, LGM), and the segmentation. For the first time, we provide evidence of post-350 ka and possibly late Quaternary activity of the Ailano – Piedimonte Matese normal fault (APMF). The APMF is 18 km long. It is composed of a main 11 km-long segment striking NW–SE and progressively bending to the E–W in its southern part, and a 7 km-long segment striking E–W to ENE-WSW with very poor evidence of recent activity. The available data suggest a possible post-LGM throw rate of the main segment of ≳0.15 mm/yr. There is no evidence of active linkage in the step-over zone between the AIF and APMF (Prata Sannita step-over).An original tectonic model is proposed by comparing structural and geodetic data. The AIF and APMF belong to two major, nearly parallel fault systems. One system runs at the core of the Matese Mts and is formed by the AIF and the faults of the Gallo-Letino-Matese Lake system. The other system runs along the western side of the Matese Mts and is formed by the APMF, linked to the SE with the Piedimonte Matese – Gioia Sannitica fault. The finite extension of the APMF might be transferred to the NW towards the San Pietro Infine fault. The nearly 2–3 mm/yr GPS-determined extension rate is probably partitioned between the two systems, with a ratio that is difficult to establish due to poor GPS coverage. The proposed model, though incomplete (several faults/transfer zones need further investigations), aids in the seismotectonic interpretation of poorly-known earthquakes (e.g., 346/355 AD earthquake on the Ailano – Piedimonte Matese – Gioia Sannitica fault system), and stimulates and further orients seismotectonic investigations aimed at constraining the segmentation pattern and seismogenic potential of the area.     

Structural development of the Neogene Radicondoli–Volterra and adjoining hinterland basins in Western Tuscany (Northern Apennines,Italy)

This paper presents a geological–structural study of some Neogene hinterland basins of the Northern Apennines, located on the Tyrrhenian side of the chain. These basins developed on the already delineated thrust-fold belt from middle–late Tortonian times. Their evolution has been commonly referred to an extensional tectonic regime, related to the opening of the Tyrrhenian Sea. New data have allowed us to hypothesize a different tectonic evolution for the chain, where compressive tectonics plays a major role both in the external and in the hinterland area. In this frame, the hinterland area located west of a major outcropping crustal thrust (Mid-Tuscany Metamorphic Ridge) has been the target of a geological–structural investigation. The field mapping and structural analysis has been focused on the syntectonic sediments of the Radicondoli–Volterra basin as well as on adjoining minor basins. These basins commonly display a synclinal structure and are generally located in between basement culminations, probably corresponding to thrust anticlines. Sediments of the hinterland basins have been affected by compressive deformation and regional unconformities separate stratigraphic units due to the activity of basement thrusts. In the study area, normal faulting either accommodates the thrusting processes or post-dates compressive deformation. A chronology of faulting and a six-stage evolution of this area are presented, providing further insights for the Neogene tectonic evolution of the Northern Apennines. Copyright © 1998 John Wiley & Sons, Ltd.     

Elastic modeling of the Alto Tiberina normal fault (central Italy): geometry and lithological stratification influences on the local stress field

Two-dimensional, elastic, plane-strain, finite element models (FEMs) are generated to study the state of stress and failure induced by a low-angle normal fault, the Alto Tiberina Fault of the Northern Apennines (central Italy): it is beyond the scope of the present work to show that slip can occur on such a fault. The numerical study was performed to evaluate the influences on the local stress field of the litho-mechanical stratification of formations surrounding the fault, and those induced by the geometry of the fault. The performed models have shown the important role played by evaporites and basement formations of the Umbria-Marche succession as seismogenetic layers. The model results have also underlined that the flat-ramp geometry of the fault induces high relative concentration of stresses in correspondence with the low-angle, east-dipping, synthetic normal faults observed today in shallow depth near the Alto Tiberina surface trace. The stress regime predicted by the final model, in which the behavior of the Alto Tiberina together with the antithetic normal fault of Gubbio was simulated, reconciles available geological and geophysical observable to a greater extent. The numerical results can assist interpretation of the tectonic evolution of the region.     

Genesis and evolution of a curved mountain front: paleomagnetic and geological evidence from the Gran Sasso range (central Apennines, Italy)

The Gran Sasso range is a striking salient formed by two roughly rectilinear E–W and N–S limbs. In the past 90° counterclockwise (CCW) rotations from the eastern Gran Sasso were reported [Tectonophysics 215 (1992) 335], suggesting west–east increase of rotation-related northward shortening along the E–W limb. In this paper, we report on paleomagnetic data from Meso-Cenozoic sedimentary dykes and strata cropping out at Corno Grande (central part of the E–W Gran Sasso limb), the highest summit of the Apennine belt. Predominant northwestward paleomagnetic declinations (in the normal polarity state) from both sedimentary dykes and strata are observed. When compared to the expected declination values for the Adriatic foreland, our data document no thrusting-related rotation at Corno Grande. The overall paleomagnetic data set coupled with the available geological information shows that the Gran Sasso arc is in fact a composite structure, formed by an unrotated-low shortening western (E–W trending) limb and a strongly CCW rotated eastern salient. Late Messinian and post-early Pliocene shortening episodes documented along the Gran Sasso front indicate that belt building and arc formation occurred during two distinct episodes. We suggest that the southern part of a late Messinian N–S front was reactivated during early–middle Pliocene time, forming a tight range salient due to CCW rotations and differential along-front shortening rates. The formation of a northward displacing bulge in an overall NW–SE chain is likely a consequence of the collision between the Latium-Abruzzi and Apulian carbonate platforms during northeastward propagation of the Apennine wedge, inducing lateral northward extrusion of Latium-Abruzzi carbonates towards ductile basinal sediment areas.     

Deformation features within an active normal fault zone in carbonate rocks: The Gubbio fault (Central Apennines, Italy)

The Gubbio fault is an active normal fault defined by an important morphological scarp and normal fault focal mechanism solutions. This fault truncates the inherited Miocene Gubbio anticline and juxtaposes Mesozoic limestones in the footwall against Quaternary lacustrine deposits in the hanging wall. The offset is more than 2000 m of geological throw accumulated during a poly-phased history, as suggested by previous works, and has generated a complex zone of carbonate-rich fault-related structures. We report the results of a multidisciplinary study that integrates detailed outcrop and petrographic analysis of two well-exposed areas along the Gubbio fault zone, geochemical analysis (fluid inclusions, stable isotopes, and trace elements) of calcite-sealed fault-related structures and fault rocks, and biostratigraphic controls. Our aims are: (i) the characterization of the deformation features and their spatial–temporal relationships, and (ii) the determination of the P/T conditions and the fluid behaviour during deformation to achieve a better understanding of fluid–rock interaction in fault zones.We show that few of the observed structures can be attributed to an inherited shortening phase while the most abundant structures and fault rocks are related to extensional tectonics. The outcropping extensional patterns formed at depths less than 2.5–3 km, in a confined fluid system isolated from meteoric water, and the fault structures are the response to a small amount of cumulated displacement, 12–19% of the total geological throw.     

Fluid‐induced breakdown of monazite in medium‐grade metasedimentary rocks of the Pontremoli basement (Northern Apennines,Italy)

The last (decompression) stages of the metamorphic evolution can modify monazite microstructure and composition, making it difficult to link monazite dates with pressure and temperature conditions. Monazite and its breakdown products under fluid‐present conditions were studied in micaschist recovered from the cuttings of the Pontremoli1 well, Tuscany. Coronitic microstructures around monazite consist of concentric zones of apatite + Th‐silicate, allanite and epidote. The chemistry and microstructure of the monazite grains, which preserve a wide range of chemical dates ranging from Upper Carboniferous to Tertiary times, suggest that this mineral underwent a fluid‐mediated coupled dissolution–reprecipitation and crystallization processes. Consideration of the chemical zoning (major and selected trace elements) in garnet, its inclusion mineralogy (including xenotime), monazite breakdown products and phase diagram modelling allow the reaction history among accessory minerals to be linked with the reconstructed P–T evolution. The partial dissolution and replacement by rare earth element‐accessory minerals (apatite–allanite–epidote) occurred during a fluid‐present decompression at 510 ± 35 °C. These conditions represent the last stage of a metamorphic history consisting of a thermal metamorphic peak at 575 °C and 7 kbar, followed by the peak pressure stage occurring at 520 °C and 8 kbar. An anticlockwise P–T path or two clockwise P–T loops can fit the above P–T constraints. The former path may be related to a context of late Variscan strike‐slip‐dominated exhumation with minor Tertiary (Alpine‐related) reworking and fluid infiltration, while the latter requires an Oligocene–Miocene fluid‐present tectono‐metamorphic overprint on the Variscan paragenesis.     

Comment on “Structural controls on a carbon dioxide-driven mud volcano field in the Northern Apennines (Pieve Santo Stefano, Italy): Relations with pre-existing steep discontinuities and seismicity”

Bonini (2009, Structural controls on a carbon dioxide-driven mud volcano field in the Northern Apennines (Pieve Santo Stefano, Italy): relations with pre-existing steep discontinuities and seismicity. Journal of Structural Geology 31, 44–54) presents a 2D mechanical analysis to infer the failure conditions responsible for the seismicity distribution during an M_w = 4.6 seismic sequence nucleating during 2001 in the Northern Apennines. In my view the mechanical analysis presented in this paper has some weakness or is not well constrained, in particular: 1) the assumption of a dip angle of 50°, is not consistent with the activated structures; 2) the P_f = σ₃ condition, difficult to be attained along a cohesionless fault dipping at 50°; 3) the isotropic stress state, i.e. σ₂ = σ₃, that is not consistent with the active or recent stress field in the area.     

Miocene low-angle detachments and upper crust megaboudinage in the Mt. Amiata geothermal area (Northern Apennines,Italy)

Information from surface and subsurface geology (boreholes and seismic reflection lines) are used to depict the geometry of the extensional structures (low-angle normal faults and related Tuscan Nappe megaboudins) affecting the Mt. Amiata geothermal area and developed during the early stage of the extensional tectonics which affected the inner Northern Apennines and Tyrrhenian Sea from the Early-Middle Miocene. Normal faulting involved the thickened middle-upper crust after the collisional stage and, in the Mt. Amiata region, took place over relatively short periods (5-7 Ma) characterised by rapid extensional strain rates. Normal faults showing articulated geometry (flat-ramp-flat) characterised by subhorizontal detachments (flats) and synthetic ramps, caused widespread megaboudinage mainly in the sedimentary tectonic units and particularly in the Tuscan Nappe. Evaporites occurring at the base of the Tuscan Nappe, the deepest sedimentary tectonic unit of the Northern Apennines, controlled the geometry of the faults, and rift-raft tectonics may be the style of this first extensional phase. Three Tuscan Nappe extensional horses (megaboudins) have been recognised in the subsurface of the Mt. Amiata area. They are characterised, in map view, by elliptical shapes and show a mean NNW-SSE lengthening. They are delimited at the base and at the top by east-dipping flats, while their western and eastern margins coincide with east-dipping ramps. On the whole, considering their geometrical features, these megaboudins correspond to extensional horses belonging to an asymmetrical east-dipping extensional duplex system.

Rollover anticlines deformed the western ramp of the megaboudins and rotated the uppermost flat as well as all the structures previously developed, which became steeply-dipping to the west.     

Late Miocene fish otoliths from the Colombacci Formation (Northern Apennines,Italy): implications for the Messinian ‘Lago‐mare’ event

A fish otolith assemblage from the Messinian ‘Lago‐mare’ deposits of the Colombacci Formation cropping out in the Montecalvo in Foglia Syncline, Marche, central Italy, is described. The assemblage displays a low diversity and consists of seven taxa belonging to three families: the Gobiidae, Myctophidae and Sciaenidae. Sciaenid otoliths are the most abundant elements representing 88% of the entire assemblage. The palaeoecological analysis reveals a coastal shallow marine environment strongly influenced by continental outflow. The low diversity and high abundance of the euryecious sciaenids are indicative of a very simplified food web, which probably represented an ecological response to the fluctuating environmental parameters and available food resources. The fish remains documented here provide an unambiguous evidence that normal marine conditions were present in the Mediterranean, at least in the upper part of the ‘Lago‐mare’ event, and unquestionably demonstrate that the marine refilling preceded the Mio‐Pliocene boundary. These findings clearly demonstrate that fishes, because of their mobility and migratory behaviour, represent a useful tool for the large‐scale interpretation of the environmental conditions of the Messinian Mediterranean water body. The necessity of a new scenario of palaeoenvironmental evolution for the post‐evaporitic Messinian of the Mediterranean is also discussed. Copyright © 2006 John Wiley & Sons, Ltd.     

塔里木盆地中部满深1断裂带的多期断裂活动

满深1断裂带位于塔里木盆地中部的阿满过渡带,走向NNE-SSW,是三维地震解释发现的一条断裂带。精细的三维地震解释识别出6期断裂,从老到新包括：南华—震旦纪正断层、晚奥陶世—早志留世逆冲断层、中志留世—石炭纪正断层、二叠纪正断层、三叠纪逆冲断层和侏罗纪正断层。中志留世—石炭纪的正断层可以进一步划分为中志留世—中泥盆世正断层和晚泥盆世—石炭纪正断层两期。前者为主,是昆仑—阿尔金早古生代碰撞造山带的造山后构造;后者是前者的复活。中志留世—石炭纪正断层作用形成一系列近南北走向的正断层,组合成一条NNE-SSW走向的右列左旋张扭性断层带,形成满深1断裂带。二叠纪裂谷作用成因的正断层和岩浆活动显著改造断裂带,使之基本定型。三叠纪的冲断构造叠加于依合2构造之上。侏罗纪伸展作用对构造带有一定的调整影响。南华—震旦纪正断层和晚奥陶世—早志留世的冲断构造与满深1断裂带相交,对构造带的形成影响不大。侏罗纪正断层的发现,是塔里木盆地当时处于区域性伸展构造背景的重要证据。     

Phyllosilicate injection along extensional carbonate-hosted faults and implications for co-seismic slip propagation: Case studies from the central Apennines,Italy

We document phyllosilicates occurrence along five shallow (exhumed from depths < 3 km) carbonate-hosted extensional faults from the seismically-active domain of the central Apennines, Italy. The shallow portion of this domain is characterized by a sedimentary succession consisting of ∼5–6 km thick massive carbonate deposits overlain by ∼2 km thick phyllosilicate-rich deposits (marls and siliciclastic sandstones). We show that the phyllosilicates observed within the studied carbonate-hosted faults derived from the overlying phyllosilicate-rich sedimentary deposits and were involved in the faulting processes. We infer that, during fault zone evolution, the phyllosilicates downward injected into pull-aparts (i.e., dilational jogs) that were generated along staircase extensional faults. With further displacement accumulation, the clayey material was smeared and concentrated into localized layers along the carbonate-hosted fault surfaces. These layers are usually thin (a few centimeters to decimeters thick), but can reach also a few meters in thickness. We suggest that, even in tectonic settings dominated by high frictional strength rocks (e.g., carbonates), localized layers enriched in weak phyllosilicates can occur along shallow fault surfaces thus reducing the expected fault strength during earthquakes, possibly promoting co-seismic slip propagation up to the Earth's surface.     

Seismic-reflection profiles of the central part of the Clarendon–Linden fault system of western New York in relation to regional seismicity

Geological and geophysical research in upstate New York, with few exceptions, has not definitively associated seismicity with specific Proterozoic basement or Paleozoic bedrock structures. The central part of the Clarendon–Linden fault system (CLFS) between Batavia and Dale, NY is one of those exceptions where seismicity has been studied and has been spatially associated with structure. The CLFS is either a complex system of long faults with associated shorter branches and parallel segments, or a region of many short faults aligned north–south from the Lake Ontario shore southward to Allegany County, NY. Interpretation of 38 km of Vibroseis and approximately 56 km of conventional seismic-reflection data along 13 lines suggests that the CLFS is a broad zone of small faults with small displacements in the lower Paleozoic bedrock section that is at least 77 km long and 7–17 km wide and spatially coincident with a north-trending geophysical (combined aeromagnetic and gravity) lineament within the basement. The relative offset across the faults of the system is more than 91 m near Attica, NY. The CLFS is the expression of tectonic crustal adjustments within the Paleozoic rock above the boundary of two basement megablocks of differing petrologic provinces and differing earthquake characteristics that forms the eastern side of the Elzevir–Frontenac boundary zone. Deep seismic-reflection profiles display concave-eastward listric faults that probably merge at depth near the mid-crustal boundary layer. An interpretive vertical section provides the setting for refined definitions of the CLFS, its extensions at depth and its relation to seismicity. Most modern seismicity in western New York and the Niagara Peninsula of Ontario occurs in apparent patterns of randomly dispersed activity. The sole exception is a line of seven epicenters of small earthquakes that trend east from Attica, NY into the Rochester basement megablock. Earthquakes may be triggered at the intersections of north- and east-trending brittle faults within the Niagara basement megablock. Current interpretations of the mechanisms for earthquake generation in western New York and the Niagara Peninsula of Ontario require conservative estimates of seismic hazards that assume that an earthquake the size of the 1929 Attica, NY, event (M_b=5.2) or larger could occur anywhere in the Eastern Great Lakes Basin (EGLB). The broad zone of small-displacement faults that marks the CLFS in the lower Paleozoic sedimentary section and the uppermost basement may not provide the structural environment for generation of earthquakes in western New York. If this interpretation is correct, most seismicity is generated within the Niagara basement megablock beneath or west of the CLFS. Consequently, we may have to look to the deeper tectonic regime of basement megablocks to understand the distribution of modern seismicity in the EGLB.     

Geochemical mapping based on geological units: A case study from the Marnoso-arenacea formation (Northern Apennines,Italy)

Geochemical maps can provide us with much information on geology, earth surface processes and anthropogenic pressure and are valuable tools for ore prospecting and land management. Stream sediments represent an integral of the various possible sources of sediments upstream from the sampling point therefore there can be multiple signal sources but generally the prevailing signal source is the one related to bedrock geology. Stream sediments collected from active second-order channels including singular geological units, were selected in order to determine the geochemical characteristics of each unit. The aim of this study was to analyse their potential for using them to integrate geological interpretation and produce a geologically-oriented geochemical map. From the 770 samples collected for a regional geochemical mapping program, we selected 149 samples whose catchment basin included only one of the members recognized within the Marnoso–Arenacea formation. This Middle-Upper Miocene (Langhian–Tortonian) turbiditic unit forms the backbone of the Romagna Apennines and has been subdivided into 14 members according to age and lithostratigraphic criteria. The results indicate that there are marked differences in the composition of the members of the Marnoso Arenecea formation which indicate the provenance of the sediment and the palaeogeographic evolution of the units. By means of univariate and multivariate statistical analyses (Factor analyses) two main types of sediment compositions are identified: Tortonian members are characterized by sialic coarse grain- sediments while the Langhian–Serravallian members are richer in carbonate fraction, slightly enriched in a mafic contribution. This study elaborated the geochemical data from a geological point of view by integrating the information available in literature to spatially extend the interpretation based on limited site observation as for petrographic studies. In general, the geochemical map based on a geological unit could be a useful tool for carrying out the geological reconstruction of a complex area.     

Passive‐roof thrusting in the Messinian Vena del Gesso Basin (Northern Apennines,Italy): constraints from field data and analogue models

We present the results of a study of the Vena del Gesso Basin (Romagna Apennines, Italy) integrating field analyses and analogue modelling. This basin represents one of the best‐preserved top‐thrust basins in the Northern Apennines foreland and is one of the few examples where primary evaporites, related to the Messinian salinity crisis of the Mediterranean, widely crop out. The structural style affecting the Messinian gypsum is examined to get insights into the mechanism responsible for the overall deformation features recognizable in the area. The evaporites are completely detached at the base and widespread back‐thrusts, repeatedly doubling these deposits, strongly contrast with the regional forelandward vergence of structures in the Apennines. On the basis of the comparison between field data and experimental results, the features characterising this area can be described as the result of the deformation linked to the sequential activation of an obliquely propagating passive‐roof duplex. Analogue models evidenced the major role played (1) by syntectonic erosion that promoted the development of passive‐roof duplex style, as well as (2) the role of décollement level pinch‐out that determined an oblique progression of deformation. Finally our data lead to reconsider the palaeoenvironmental reconstruction concerning the onset of the Messinian salinity crisis in the Mediterranean. Copyright © 2007 John Wiley & Sons, Ltd.     

Extensional deformation structures within a convergent orogen: The Val di Lima low-angle normal fault system (Northern Apennines,Italy)

A low-angle extensional fault system affecting the non metamorphic rocks of the carbonate dominated Tuscan succession is exposed in the Lima valley (Northern Apennines, Italy). This fault system affects the right-side-up limb of a kilometric-scale recumbent isoclinal anticline and is, in turn, affected by superimposed folding and late-tectonic high-angle extensional faulting.The architecture of the low-angle fault system has been investigated through detailed structural mapping and damage zone characterization. Pressure-depth conditions and paleofluid evolution of the fault system have been studied through microstructural, mineralogical, petrographic, fluid inclusion and stable isotope analyses. Our results show that the low-angle fault system was active during exhumation of the Tuscan succession at about 180°C and 5 km depth, with the involvement of low-salinity fluids. Within this temperature - depth framework, the fault zone architecture shows important differences related to the different lithologies involved in the fault system and to the role played by the fluids during deformation. In places, footwall overpressuring influenced active deformation mechanisms and favored shear strain localization.Our observations indicate that extensional structures affected the central sector of the Northern Apennines thrust wedge during the orogenic contractional history, modifying the fluid circulation through the upper crust and influencing its mechanical behavior.     

Seismotectonics of strike–slip earthquakes within the deep crust of southern Italy: Geometry, kinematics, stress field and crustal rheology of the Potenza 1990–1991 seismic sequences (Mmax 5.7)

We present a revision and a seismotectonic interpretation of deep crust strike–slip earthquake sequences that occurred in 1990–1991 in the Southern Apennines (Potenza area). The revision is motivated by: i) the striking similarity to a seismic sequence that occurred in 2002  140 km NNW, in an analogous tectonic context (Molise area), suggesting a common seismotectonic environment of regional importance; ii) the close proximity of such deep strike–slip seismicity with shallow extensional seismicity (Apennine area); and iii) the lack of knowledge about the mechanical properties of the crust that might justify the observed crustal seismicity. A comparison between the revised 1990–1991 earthquakes and the 2002 earthquakes, as well as the integration of seismological data with a rheological analysis offer new constraints on the regional seismotectonic context of crustal seismicity in the Southern Apennines. The seismological revision consists of a relocation of the aftershock sequences based on newly constrained velocity models. New focal mechanisms of the aftershocks are computed and the active state of stress is constrained via the use of a stress inversion technique. The relationships among the observed seismicity, the crustal structure of the Southern Apennines, and the rheological layering are analysed along a crustal section crossing southern Italy, by computing geotherms and two-mechanism (brittle frictional vs. ductile plastic strength) rheological profiles. The 1990–1991 seismicity is concentrated in a well-defined depth range (mostly between 15 and 23 km depths). This depth range corresponds to the upper pat of the middle crust underlying the Apulian sedimentary cover, in the footwall of the easternmost Apennine thrust system. The 3D distribution of the aftershocks, the fault kinematics, and the stress inversion indicate the activation of a right-lateral strike–slip fault striking N100°E under a stress field characterized by a sub-horizontal N142°-trending σ1 and a sub-horizontal N232°-trending σ3, very similar to the known stress field of the Gargano seismic zone in the Apulian foreland. The apparent anomalous depths of the earthquakes (> 15 km) and the confinement within a relatively narrow depth range are explained by the crustal rheology, which consists of a strong brittle layer at mid crustal depths sandwiched between two plastic horizons. This articulated rheological stratification is typical of the central part of the Southern Apennine crust, where the Apulian crust is overthrusted by Apennine units. Both the Potenza 1990–1991 and the Molise 2002 seismic sequences can be interpreted to be due to crustal E–W fault zones within the Apulian crust inherited from previous tectonic phases and overthrusted by Apennine units during the Late Pliocene–Middle Pleistocene. The present strike–slip tectonic regime reactivated these fault zones and caused them to move with an uneven mechanical behaviour; brittle seismogenic faulting is confined to the strong brittle part of the middle crust. This strong brittle layer might also act as a stress guide able to laterally transmit the deviatoric stresses responsible for the strike–slip regime in the Apulian crust and may explain the close proximity (nearly overlapping) of the strike–slip and normal faulting regimes in the Southern Apennines. From a methodological point of view, it seems that rather simple two-mechanism rheological profiles, though affected by uncertainties, are still a useful tool for estimating the rheological properties and likely seismogenic behaviour of the crust.