Umbria-Marche Apennines Research Articles

Integrated stratigraphy of the Oligocene pelagic sequence in the Umbria-Marche basin (northeastern Apennines, Italy): A potential Global Stratotype Section and Point (GSSP) for the Rupelian/Chattian boundary

The Oligocene represents an important time period from a wide range of perspectives and includes significant climatic and eustatic variations. The pelagic succession of the Umbria-Marche Apennines (central Italy) includes a complete and continuous sequence of marly limestones and marls, with volcaniclastic layers that enable us to construct an integrated stratigraphic framework for this time period. We present here a synthesis of detailed biostratigraphic, magnetostratigraphic, and chemostratigraphic studies, along with geochronologic results from several biotite-rich volcaniclastic layers, which provide the means for an accurate and precise radiometric calibration of the Oligocene time scale. From this study, the interpolated ages for the Rupelian/Chattian stage boundary, located in the upper half of Chron 10n at meter level 188 in the Monte Cagnero section, and corresponding to the O4/O5 planktonic foraminiferal zonal boundary, are 28.36 Ma (paleomagnetic interpolation), 28.27 ± 0.1 Ma (direct radioisotopic dating), and 27.99 Ma (astrochronological interpolation). These ages appear to be slightly younger than those reported in recent chronostratigraphic time scale compilations. The Monte Cagnero section is a potential candidate for defining the Chattian Global Stratotype Section and Point (GSSP) and some reliable criteria are here proposed for marking the Rupelian/Chattian boundary according to International Union of Geological Sciences (IUGS) recommendations.

A mechanical model for complex fault patterns induced by evaporite dehydration and cyclic changes in fluid pressure

Complex fault patterns, i.e. faults which exhibit a diverse range of strikes, may develop under conditions where a regional tectonic stress field is weak or absent (e.g. polygonal faults). The present paper considers a complex system of synsedimentary faults in the Umbria-Marche Apennines (Italy), geometrically similar to polygonal fault systems, developed during an early Jurassic extensional episode. This particular fault pattern differs from many extensional fault systems as it lacks structures that are developed with a classical bimodal conjugate “Andersonian” geometry. A conceptual and mechanical model is proposed to explain the development and evolution of the complex fault pattern in which it is proposed that faulting is primarily controlled by the development of volumetric instability in Triassic Evaporites due to dehydration processes (anhydritisation) during burial. The lithological architecture of the Triassic Evaporites, comprising interbedded Ca-sulphate layers and dolostones, played a fundamental role in controlling the deformation processes. Cyclic fluid overpressure build-up/release and the coexistence of brittle and brittle-ductile flow processes led to horizontal isotropic and non-plane extensional strain fields within the dehydrating rock mass, which favoured the development of complex deformation patterns. The mechanical model proposed shows that the studied fault pattern developed under a stress field consistent with almost homogeneous stress intensities within the horizontal plane. The data presented show that local strain fields and transient fluid pressure conditions have been dominant over weak regional extensional tectonics. The findings are relevant to many other areas where complex faulting patterns—including polygonal faults—occur in association with evaporite or clay-rich sedimentary sequences.

EARLY TO LATE OLIGOCENE CALCAREOUS NANNOFOSSIL BIOEVENTS IN THE MEDITERRANEAN (UMBRIA-MARCHE BASIN, CENTRAL ITALY)

Calcareous nannofossil assemblages have been investigated by means of quantitative analyses in three Oligocene pelagic sections located in the Umbria-Marche Apennines (Central Italy). The studied sections mainly consist of marly limestones and marls belonging to the Scaglia Cinerea Formation, and include the interval between NP23 and NP25 representing a time interval of about 3.5 Ma. Biostratigraphic resolution is extremely low and only two standard bioevents are known, which are the FO of Sphenolithus ciperoensis and the LO of Sphenolithus distentus . The distribution patterns of poorly known or recently described calcareous nannofossils provided a valuable tool for improving the current biostratigraphic framework. The studied interval is characterized by significant changes in the calcareous nannofossil assemblages and by several extinction events. The last occurrence (LO) and/or the last common occurrence (LCO) here proposed are: the LO of Sphenolithus akropodus , the LO of Reticulofenestra circus , the LCO of Helicosphaera ethologa , the LCO of Helicosphaera compacta the LO of Discoaster tanii nodifer . The reversal in abundance between Sphenolithus predistentus and S. distentus provided an additional biostratigraphic constraint at the NP23-NP24 transition. In addition biometric criteria enabled the recognition of the first common occurrence (FCO) of Cyclicargolithus abisectus > 12 mm as potential bioevent within NP24. The quantitative distribution of Sphenolithus distentus suggests to rely on the LCO of the species rather than on the LO, for the identification of NP24-NP25 boundary. The identified bioevents is a first step towards the improvement of the present Mediterranean biostratigraphic framework of the Oligocene geological record. A preliminary correlation of the bioevents to the Geomagnetic Polarity Time Scale is presented.

Early orogenic normal faults and their reactivation during thrust belt evolution: the Gubbio Fault case study, Umbria-Marche Apennines (Italy)

Foreland (early-orogenic) and hinterland (late-orogenic) extensional domains have been widely documented in the Northern Apennines, where they are synchronous with contraction in the active part of the fold and thrust belt. The progressive eastward migration of the contractional front and the associated hinterland extensional field implies that early-orogenic extensional structures, developed in the foreland domains, may experience reactivation/inversion. We present new field data, integrated with seismic evidence, from the Gubbio normal fault, a 22 km long presently active fault, showing: a) evidence for early-orogenic extension since the Lower Miocene time; b) successive positive inversion during the Upper Miocene contraction; and c) renewed, late-orogenic, extension during the Quaternary age. Field data allowed two systems of mesoscale normal faults, respectively interpreted as early- and late-orogenic structures, to be recognised. Stress fields associated with contractional and early- late-orogenic extensional tectonic regimes, are characterised by an overall coaxiality, with directions of compression and tension consistently aligned NE-SW. The symmetry between the successive deformation stages is interpreted as a likely cause for the repeated reactivation of the Gubbio fault since the Miocene. Our analysis in the Gubbio area shows that normal faults play a key role during the evolution of a fold and thrust belt from the early-orogenic stages, when they influence the geometry and evolution of foredeep basins, to the late-orogenic stages when Quaternary activity, due to extension reactivation, controls the evolution of the intramountain basins.

Comparative high-resolution chemostratigraphy of the Bonarelli Level from the reference Bottaccione section (Umbria–Marche Apennines) and from an equivalent section in NW Sicily: Consistent and contrasting responses to the OAE2

The Bonarelli Level (BL) from the upper Cenomanian portion of the reference Bottaccione section (central Italy) is characterized by the presence of black shales containing high TOC concentrations (up to 17%) and amounts of CaCO 3 near to zero. In the absence of carbonate and, consequently, of relative carbon- and oxygen-isotopic data, the elemental geochemistry revealed to be a very useful tool to obtain information about the palaeoclimatic and palaeoceanographic evolution of the Tethys Ocean during the OAE2. Based on several geochemical proxies (Rb, V, Ni, Cr, Si, Ba), the BL is interpreted as a high-productivity event driven by increasingly warm and humid climatic conditions promoting an accelerated hydrological cycle. The enrichment factors of peculiar trace metals (Zn, Cd, Pb, Sb, Mo, U) provide further insight about the H 2S activity at the seafloor during the organic-rich sediment deposition and permitted us to evaluate the use of Ba as palaeoproductivity tracer in conditions of high rate of sulphate reduction. By comparing geochemical records from the reference Bottaccione section (central Italy) with those previously obtained for the coeval Calabianca section (northwestern Sicily), different degrees of oceanic anoxia were delineated and ascribed to different abundance and type (degradable or refractory) of organic matter, which are limiting factors in the bacterial sulphate reduction reactions and in subsequent euxinic conditions at seafloor in the Tethys realm. Based on a ciclostratigraphic approach, consistent fluctuations at 100 ky scale in the chemostratigraphic signals from the two sections are inferred to be expression of a strong orbital-climatic forcing driving changes in the oceanic environment during the BL deposition.

Structural evolution of a fold and thrust belt generated by multiple décollements: analogue models and natural examples from the Northern Apennines (Italy)

This study analyses the role played by the mechanical proprieties of the rocks involved in a thrust system, focusing on the problem of multiple décollements and of related compressional structures. Integration of geophysical (seismic reflection profiles) and geological data and scaled sandbox models are used to study the deformation style of two areas of the Northern Apennines (Italy): the Po Plain and the Umbria-Marche Apennines. Both areas are characterised by a complex stratigraphy, consisting of décollements located at different depths that influence the geometry and kinematics of the thrust system. The main characteristic of the models presented here is the presence of two décollement horizons, situated at the base and in an intermediate level of the models. During deformation of the models, these two décollement horizons generate two sets of structures, with different geometrical characteristics and significance. Through the joint analysis of geophysical and geological data and model results, wavelengths of the compressional structures analysed show that the structural style of the two analysed areas is almost similar. Moreover, model results prove that the final configuration of the thrust system follows the general rules of evolution of a wedge deformed above a weak décollement and is largely governed by the larger, deep seated structures.

Stress Changes due to Recent Seismic Events in the Central Apennines (Italy)

The Central Apennines, Italy, are characterized by moderate seismic activity on normal faults, oriented in directions parallel to the Apenninic chain. The subject of this study is the Umbria-Marche Apennines, a segment approximately 200-km long, where three main seismic events occurred in the last three decades. The 1979 Norcia earthquake was a Mw = 5.8 event, taking place at the south end of the considered segment. The 1984 Gubbio earthquake was a Mw = 5.6 event which took place at the north end. The 1997-1998 Colfiorito sequence constituted 8 main shocks with magnitudes Mw between 5 and 6 and epicenters located between the Gubbio and the Norcia earthquake areas. A model made of an elastic half-space is considered, in which the seismic sources are represented by rectangular dislocations which have the appropriate values of source parameters, and in which the static stress field produced by each event is calculated. The analysis of the Coulomb stress change (ΔC) as a function of time shows that the coseismic stress transfer and fault interaction played an important role in the region during the past three decades: 7 earthquakes of the 9 considered took place where ΔC>0. Such an interaction has been confirmed by the analysis of the aftershocks in the Colfiorito zone post September 26, 1997: about the 61% of the aftershocks considered took place where ΔC>0. The comparison between the ΔCs due to the coseismic stress transfer and the rate ΔĊt due to the tectonic stress allows us to quantify the time advance of the earthquakes. The ΔCs pattern shows positive values in two areas that can be regarded as historical seismic gaps.

Seismological, geological and geophysical constraints for the Gualdo Tadino fault, Umbria–Marche Apennines (central Italy)

The April 3, 1998 Mw = 5.1 Gualdo Tadino earthquake (central Italy) was the last significant event in the 6-month-long Umbria–Marche seismic crisis. This event and its aftershocks occurred in an area where active faulting produces no striking geological and geomorphological effects. In this study, we investigated the ruptured fault using detailed seismological data and a re-processed and re-interpreted seismic reflection profile. Aftershock location and focal mechanisms were used to constrain the geometry and kinematics of the ruptured fault and a comparison was made with the subsurface image provided by the seismic profile. We found that the 1998 Gualdo Tadino earthquake occurred on a WSW-dipping, normal fault, with a length of about 8 km and a relatively gentle dip (30°–40°), confined between 3.5 and 7 km in depth. Kinematics of the mainshock and aftershocks revealed a NE-trending extension, in agreement with the regional stress field active in the Northern Apennines belt. The Mw = 5.1 earthquake originated above the top of the basement and ruptured within the sedimentary cover, which consists of an evaporites–carbonates multilayer. We hypothesised that the active fault does not reach the surface (blind normal fault).

Role of mechanical stratigraphy on fracture development in carbonate reservoirs: Insights from outcropping shallow water carbonates in the Umbria–Marche Apennines, Italy

Studies on mechanical stratigraphy show that a link exists among facies, sedimentary cycles, diagenesis and fracturing. Understanding this link is fundamental for characterising fluid flow in natural reservoirs, especially carbonate ones. This work investigates the field of evidence through a case study in the Umbria–Marche Apennines: the Lower Jurassic shallow water carbonates of the Calcare Massiccio unit, a potential carbonate reservoir. A multidisciplinary approach is used, involving stratigraphy, sedimentology and structural geology. The studied succession crops out at the core of an ENE-verging anticline, located at Campolarzo, within the Umbria–Marche Apennines. The Calcare Massiccio here consists of high-frequency, metre-scale, shallowing upward, peritidal cycles. Three different cycles (thickness ranging from 0.3 to 3 m) have been recognised: type A (incomplete asymmetric cycle), type B (complete asymmetric cycle with sheet cracks) and type C (complete asymmetric cycle with tepees). Sedimentary textures vary according to the depositional environment. The subtidal facies is almost entirely mud-supported, with the exception of type A cycle, which locally can be grain-supported. The intertidal facies is from mud- to grain-supported and commonly laminated. The supratidal facies may be either absent (type A) or represented by vadose pisoid caliches, sheet cracks (type B) and tepees (type C). The overprinting of diagenetic facies (early diagenesis) is particularly evident in the supratidal intervals of type C cycle (tepee structures) and in the inter-supratidal intervals of type B cycle (sheet cracks). The fracture pattern within the Calcare Massiccio mainly consists of systematic joints organized in two prominent mutually perpendicular sets (orthogonal joints) and two subordinate non-perpendicular sets (diagonal joints). The orthogonal sets are interpreted as tensile joints developed sub-parallel (longitudinal joints) and sub-perpendicular (cross joints) to the NNW–SSE axial trend of the host anticline. The diagonal joints are interpreted as shear joints. In the study area, the fracture density is strongly controlled by heterogeneities of rock properties between and within facies, which in turn are determined by sedimentary textures and, dominantly, by the combination of sedimentary and diagenetic facies, e.g., by the petrofacies. The control played by the petrofacies distribution across the stratigraphic succession may justify strong variations in the fracture density (up to ca. 80% or more) within the same sedimentary cycle, as well as fracture terminations, independently from tectonic causes. A systematic decrease in fracture density going from subtidal to intertidal to inter-supratidal and tepee facies is evident. Minimum values are observed on tepee structures; fractures often terminate against supratidal intervals with tepee. The lowest fracture density characterises petrofacies where early diagenetic processes are more pronounced (i.e., the intertidal facies and the inter-supratidal facies with tepee, strongly affected by early dissolution cavities filled by early cements). No obvious correlation is observed between the fracture density and the thickness of the petrofacies layer.

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

AbstractThis 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.

Active tectonics in Central Italy: constraints from surface wave tomography and source moment tensor inversion

We investigate the lithosphere–asthenosphere structure and active tectonics of Central Italy, with emphasis on the Umbria–Marche area, by means of surface wave tomography and seismic moment tensor inversion. The data include: a large number of short period local and regional group velocity measurements sampling the Umbria–Marche Apennines and the Adria margin, respectively; incorporation of published phase velocity measurements sampling Italy and surroundings; results from deep seismic soundings which go through the Umbria–Marche area. The local group velocity maps, covering the area reactivated by the 1997–1998 Umbria–Marche earthquake sequence, suggest an intimate relationship between the lateral earth structure variations and the distribution of the active fault systems and related sedimentary basins. The upper crustal models reveal the importance of inherited compressional tectonics on the recent extensional deformation and associated seismic activity. Source inversion studies of the main events of the 1997 earthquake sequence show the dominance of normal faulting mechanisms, whereas selected aftershocks between the fault segments, at the step-over, reveal that the prevailing deformation is of strike-slip faulting type. At the regional scale, the crust exhibits clear layering and varies in thickness from about 25 km below the Tuscan Metamorphic Complex (TMC), to about 30 km below the Val Tiberina extensional thick sedimentary basin and reaches about 35 km below the Umbria–Marche geological domain (UMD). The lithospheric mantle (lid) is thin (about 30 km) below TMC, while it is about 70 km thick below UMD. A lithospheric root about 120 km wide, between the TMC and UMD, reaches a depth of at least 130 km. A low-velocity zone, defined mantle wedge ( V S less than 4.2 km/s) in the uppermost mantle overlying the high velocity lid is detached. This wedge is about 20 km thick and decouples the underlying lid from the crust. The retrieved crust and upper mantle structure beneath Central Italy is in agreement with Bouguer anomaly and heat flow data and supports a delamination process. The high velocity upper mantle underlying the mantle wedge is inferred to be subcrustal lithospheric material delaminated from the overlying crust.

Preseismic Creep Strains Revealed by Ground Tilt Measurements in Central Italy on the Occasion of the 1997 Umbria-Marche Apennines Earthquake Sequence

— Daily averaged tilt component data from four sites of the Central Apennines (Italy) revealed intermediate-term tilts of a few months as possible precursors of the seismic sequence occurred in the Umbria-Marche region during 1997. A change was also observed in the secular tectonic trend recorded at one site and referred to the same seismic sequence. The observed intermediate-term preseismic tilts are considered as the manifestation of aseismic creep episodes in the fault materials close to the tilt sites. The mechanism refers to a strain field slowly propagating from the dilatancy (focal) area to the tiltmeters, through rigid crustal blocks separated by weak transition zones with viscoelastic rheology. This propagation is thought to be the cause of the local aseismic fault slip recorded by tiltmeters. In one case such creep strains revealed to have larger amplitude and higher frequency content than those accepted for fault materials, and this was attributed to an amplified response of the heterogeneous ground surface at the site. The existence of a propagating strain field is confirmed by the different onset time delays in the preseismic tilt signals recorded at different distances from the same earthquake. In particular, the onset time delays observed at each tilt site appear to increase with increasing distance of such site from the epicentral area. At greatest distances, where the preseismic strain becomes negligible, the characteristic intermediate-term ground tilts vanish completely.

Myths and legends in the correlation between the Boreal and Tethyan Realms. Implications on the dating of the Early Toarcian mass extinctions and the Oceanic Anoxic Event

This study constitutes a revision of the biostratigraphy of Early Toarcian age ammonites coming from different successions in the NW Europe and Mediterranean areas, complemented by new, important data from the Migiana di M. Malbe section (Perugia, Italy). The research has demonstrated that the problems of correlation between some Zones of the Standard and Mediterranean biostratigraphic scales in this period are doubtful. On the negative side, procedural problems and defects have emerged that greatly detract from the validity of some boundaries which were, almost unanimously, held to be synchronous. Undervaluation of the problems brought to light in this study has had repercussions both on the construction of the eustatic curves and on the dating of the Oceanic Anoxic Event (OAE), and consequently on the evolutionary models for the various areas of the Tethys and of NW Europe. The various datings obtained by different writers have led to the belief that the anoxic conditions became established diachronically in the two areas. However, ammonite dating of the black shale-like facies, made for the first time in the Umbria–Marche Apennines, shows instead that, at least between this area and Germany, the diachronism was only partial. In both areas, the establishment of “anoxic” conditions took place in the Dactylioceras semicelatum Subzone. Problems linked to ammonite faunal provinciality and evolution, complicated by tectono-eustatic phenomena that produce hiatuses in NW Europe, are at the root of the lack of continuity in tracing the Pliensbachian–Toarcian boundary between the NW Europe area and the rest of the world.

Miocene fault-controlled sedimentation and thrust propagation in the previously faulted external zones of the Umbria-Marche Apennines, Italy

Thrust propagation through previously faulted continental margins may result in fold and thrust belts whose structure is strongly controlled by the inherited basin archi- tecture. A detailed geological study has been carried out in the external zone of the Umbria-Marche Apennines, from Monte San Vicino to the north, to Montagna dei Fiori to the south. Stratigraphic and structural data, together with the construction of a series of balanced and restored geological sections, point out the fundamental role played by the pre- orogenic basin architecture in controlling the geometry and evolution of the fold and thrust belt. Pre-thrusting structures include not only those inherited from the Mesozoic rifted continental margin, but also synsedimentary faults associated with Miocene extension which occurred ahead of the advanc- ing thrust front. The latter structures produced important fa- cies and thickness variations in the units deposited during the late Burdigalian-early Messinian, pre-evaporitic stages of foredeep development. In the southern sector (Montagna dei Fiori), high values of Messinian regional subsidence, bathymetry and sedimentation rate overcome the effects of synsedimentary extensional tectonics, which is best recorded in pre-Messinian sequences. On the other hand, Messinian regional subsidence was significantly less in the northern sec- tor (Monte San Vicino). Here, several minor sub-basins de- veloped within the foredeep, generally reaching evaporitic conditions during the middle Messinian (marked by the de- position of the Gessoso-solfifera Fm). In this area, a ma- jor control by pre-thrusting normal faults on sedimentation is recorded in the foredeep siliciclastic sequences. Late Burdigalian-early Messinian extension, possibly associated with flexure of the foreland lithosphere, peripheral bulge up- lift and/or foreland tectonic activity, was followed by a late Messinian (post-evaporitic) contractional episode of regional extent. During shortening, inversion of preexisting Miocene

BIOERODED AND OR ENCRUSTED AMMONITE MOULDS AND THEIR TAPHONOMIC IMPLICATIONS

Encrustation and/or bioerosion have been observed on both sides of several ammonites sampled from Lower-Middle Toarcian Rosso Ammonitico of the Umbria-Marche Apennines (Central Italy). The majority of these features are due to the activity of polychaete worms: the encrusting s.s. is due to serpulids, whilst bioerosion s. s. is similar to Maeandropolydora decipiens Voigt, and a third category of polychaetes has produced bioerosion-encrustation structures. The ornamentation features of the samples studied, allowed us to establish that the upper sides are preserved as composite moulds and the lower sides as internal moulds. The overprinting of the external shell structures on the internal infillings took place during burial diagenesis; the deformation style is plastic and only vertical, lateral deformation s being impeded. The presence of boring and/or encrustation on the lower side of internal moulds is classically interpreted as evidence of reelaboration. Regarding these Toarcian specimens, the nature of the composite moulds would seem to confirm that the phenomena of encrusting and/or boring by polichaete worms occurred before the dissolution of the shells. Except in the cases of pure bioerosion, it is impossible to ascertain whether the shell was already infilled or not. The encrustation by polichaete worms probably occurred during post-mortem drifting or when the shell was exposed on the sea floor. The similarity between the observed bioerosion phenomena and other hard and/or firm material borers gives us reason to think that the complete re-exhumation of the shell is not necessary to permit the boring action: however, a partial re-exhumation might have helped the borer to detect the presence of a shell test.

A structural model for active extension in Central Italy

This work presents a structural model for earthquake faulting in the Umbria-Marche Apennines (Central Italy). The model is derived by an integrated analysis of geological, geophysical and seismological data. At regional scale, the distribution and character of the seismicity appear to be mainly controlled by a low-angle east-dipping normal fault (Altotiberina fault, AF). The latter is the lower boundary of an active, continuously deforming hangingwall block moving toward NE. Moderate magnitude earthquakes (4 < M < 6), such as the Norcia 1979 ( M = 5.9), the Gubbio 1984 ( M = 5.2) and the Colfiorito 1997 ( M max = 5.9), occur within the active hangingwall block and are related to the activity of major west-dipping normal faults detaching on the AF. The geometry of the deep seismogenic structures is listric (as in the case of Colfiorito) or more complex, because of local reactivation of pre-existing low-angle thrust (e.g. Gubbio) or high-angle strike-slip faults (e.g. Norcia). For all the analysed earthquakes the rupture nucleation is located at the base of the aftershock volumes, near the line of intersection between the SW-dipping normal faults and the east-dipping AF basal detachment. The progressive increase in depth of the earthquake foci from the north–west (e.g. Gubbio, 6–7 km) to the south–east (e.g. Norcia, 11–12 km) appears to be related to the eastward deepening of the basal detachment. These seismotectonic features are relevant for determining the seismogenic potential of the Apennine active faults, which depends not only on the length of the faults, but also on the depth of the detachment zone as well.

20 My hiatus in the Jurassic of Umbria-Marche Apennines (Italy): carbonate crisis due to eutrophication

A hiatus ranging from Early Bajocian to late Early Kimmeridgian is recorded on swells of Umbria-Marche Apennines (Italy). In morphostructural basins this time-interval coincides with both biosiliceous sedimentation and a drop in the biodiversity. On swells, precursor microfacies of the hiatus, whose beginning coincides with aδ 13C positive peak, are characterized by high density oligotypical associations alternatively dominated by crinoids, protoglobigerinids and Bositra buchii. The hiatus, the onset of biosiliceous sedimentation and the fall of biodiversity are manifestations of a carbonate productivity crisis, induced by highest eutrophication, which affected the Western Tethys during the Middle Jurassic.

Drainage on evolving fold‐thrust belts: a study of transverse canyons in the Apennines

Anticlinal ridges of the actively deforming Umbria–Marche Apennines fold‐thrust belt are transected by deep gorges, accommodating a drainage pattern which almost completely ignores the presence of pronounced anticlinal mountains. Because the region was below sea level until the folds began to form, simple antecedence cannot explain these transverse canyons. In addition, the fold belt is too young for there to have been a flat‐lying cover from which the rivers could have been superposed.In 1978, Mazzanti &amp; Trevisan proposed an explanation for these gorges which deserves wider recognition. They suggested that the Apennine fold ridges emerged from the sea in sequence, with the erosional debris from each ridge piling up against the next incipient ridge to emerge, gradually extending the coastal plain seaward. The new coastal plain adjacent to each incipient anticline provided a flat surface on which a newly elongated river could cross the fold, positioning it to cut a gorge as the fold grew. Their mechanism is thus a combination of antecedence and superposition in which folds, overlying sedimentary cover and downstream elongations of the rivers all form at the same time.A study of Apennine drainage, using the sequence of older‐to‐younger transected Apennine folds as a proxy for the historical evolution of drainage cutting through a single fold, shows that transverse drainage forms when sedimentation dominates at the advancing coastline. Longitudinal drainage forms when uplift dominates, the folds first emerge as offshore islands and the Mazzanti–Trevisan mechanism is suppressed.Complicating factors include several departures from steady‐state growth of the fold‐thrust belt, a possible case of precursory submarine drainage, early emergence of anticlinal culminations and the location of several transverse canyons at the structurally highest point along anticlinal axes.

Normal faults in thrust sheets: pre-orogenic extension, post-orogenic extension, or both?

In fold-and-thrust belts that experienced both pre-orogenic and post-orogenic extension, it may be difficult to establish whether observed normal faults pre-dated, post-dated, or were synchronous with thrusting. Geometrical structural patterns may be insufficient to constrain the relative chronology of extensional and contractional deformations. The systematic use of kinematic criteria makes it possible to unequivocally define the timing relationships of reverse and normal fault development, and hence to correctly unravel complex structural evolutions. Kinematic analysis in the southernmost Umbria–Marche Apennines of Italy, where both normal and thrust faults are present, revealed a history of repeated tectonic inversion, characterised by two distinct stages of extension separated by an episode of folding and thrusting. Structural overprinting relationships observed at thrust–normal fault intersections were useful for: (i) removing sequentially younger deformations; and hence (ii) separating and quantifying the effects of orogenic contraction from those of both pre-orogenic and post-orogenic extension.

Active tectonics and seismicity in the area of the 1997 earthquake sequence in central Italy: A short review

A first tentative comparison between the structural framework related to the active tectonics and the long-term seismicity of the Umbria–Marche Apennines (affected by the 1997 seismic sequence) has provided some insight for discussing the seismotectonic characteristics of the area. This Apennine sector is affected by 15 to 20-km-long active fault systems, consisting of minor fault-segments. Each of these fault-segments may be responsible for earthquakes characterised by magnitudes ranging between 5.5 and 6.0 (such as those occurred in 1599, 1730, 1838, 1859, 1979). However, the occurrence of one large-magnitude event (1703, Ms = 6.7) and of seismic sequences (1747–1751; 1997–1998) indicate that an entire fault system may be activated suddenly (at least in the southern part of the investigated area) or during seismic crises which may last many months. The comparison between the active faulting framework and the long-term seismicity also indicates that no significant earthquakes may be related to the Mt. Vettore Fault System since 1000 AD.