Sveva Corrado

Tectonic burial and 'young' (< 10 Ma) exhumation in the southern Apennines fold and thrust belt (Italy)

In the southern Apennines fold-and-thrust belt, thermal indicators record exhumation of sedimentary units from depths locally in excess of 5 km. The thrust belt is made of allochthonous sedimentary units that overlie a 6–8-km-thick, carbonate footwall succession. The latter, continuous with the foreland Apulian Platform, is deformed by reverse faults involving the underlying basement. Therefore, a switch from thin-skinned to thick-skinned thrusting occurred as the Apulian Platform carbonates—and the underlying thick continental lithosphere—were deformed during the latest shortening stages. Apatite fission track data, showing cooling ages ranging between 9.2 ± 1.0 and 1.5 ± 0.8 Ma, indicate that exhumation marks these late tectonic stages, probably initiating with the buttressing of the allochthonous wedge against the western margin of the Apulian Platform. Pliocene-Pleistocene foreland advancing of the allochthonous units exceeds the total amount of slip that, based on cross-section balancing and restoration, could be transferred to the base of the allochthon from the underlying thick-skinned structures. This suggests that emplacement of the allochthon above the western portion of the Apulian Platform carbonates was followed by gravitational readjustments within the allochthonous wedge, coeval—and partly associated with—thick-skinned shortening at depth. The related denudation processes are interpreted to have played a primary role in tectonic exhumation.

Time and space variability of “thin-skinned” and “thick-skinned” thrust tectonics in the Apennines (Italy)

In the Apennine fold and thrust belt of Italy, «thin-skinned» (i.e. detachment-dominated) and «thick-skinned» (i.e. crustal ramp-dominated) structures coexist, but with marked differences in both time and space. The external part of the northern Apennines and the deeper and younger portions (buried Apulian carbonates) of the thrust belt in the central and southern Apennines show limited amounts of shortening (in the range of 5–14 km). These result from similar deformation styles, involving the occurrence of relatively low-displacement, thick-skinned thrust ramps. The latter represent, at least in the northern Apennines, preexisting basement structures reactivated and inverted during contractional deformation. Interposed between the northern and southern parts of the fold and thrust belt, the central Apennines appear to constitute a transitional area in which strike-slip tectonics is relevant and carbonate platform units become predominant over pelagic basin ones, whereas the overall structure of the thrust belt becomes similar to that of the southern Apennines. In the latter, a peculiar structural style is revealed by the integrated analysis of surface and subsurface data. Structurally, the upper part of the thrust belt consists of allochthonous units made of Mesozoic peritidal carbonate platform and pelagic basin successions, and of Miocene foredeep sediments. These are completely detached from their original substratum and transported onto the 6–7 km thick, foreland carbonates of the Apulian platform. Based on available seismic data, the latter appears to be involved, together with the underlying Permo-Triassic clastics and, we infer, also the basement, in relatively low-displacement, thick-skinned structures. Therefore, in the southern Apennines, a transition from thin-to thick-skinned tectonics appears to have occurred through time. Thin-skinned structures characterise the shallower — and older — part of the thrust belt made of detached units, while a thick-skinned tectonic style is dominant in the buried Apulian carbonates of most recent accretion. The present boundary between the two different, superposed portions of the thrust belt consists of a low-angle, large-displacement thrust fault penetrated by numerous oil wells. Different styles and modes of contractional deformation in the investigated sectors of the Apennines appear to result from the geometrical requirement of maintaining strain compatibility and overall displacement continuity along a highly segmented orogen characterised by variable mechanical stratigraphy and southward increasing amounts of shortening.RiassuntoNella catena a pieghe e sovrascorrimenti dell’Appennino coesistono strutture che vedono coinvolto nella deformazione il basamento (tipo «thick-skin») e strutture scollate da questo (tipo «thin-skin»). La parte esterna dell’Appennino Settentrionale e le parti più profonde e più giovani (i carbonati della piattaforma Apula sepolta) dell’Appennino Centrale e Meridionale mostrano un raccorciamento limitato (compreso tra i 5 e i 14 km). Ciò deriva da stili deformativi simili, che producono rigetti relativamente bassi per la presenza di rampe di sovrascorrimento di tipo «thick-skin». Queste ultime rappresentano, almeno nell’Appennino Settentrionale, delle preesistenti strutture di basamento riattivate e invertite durante la deformazione contrazionale. Interposta tra le porzioni settentrionali e meridionali della catena, l’Appennino Centrale costituisce un’area di transizione in cui la tettonica trascorrente risulta di rilevante importanza e le unità carbonatiche di piattaforma divengono predominanti su quelle dei bacini pelagici, mentre la struttura generale della catena a pieghe e sovrascorrimenti diventa simile a quella dell’Appennino Meridionale. In quest’ultimo, lo stile strutturale tipico è rivelato dall’analisi integrata dei dati di superfice e di sottosuolo. Nella parte strutturalmente superiore di questo settore di catena sono presenti unità alloctone costituite da piattaforme a carbonati peritidali del Mesozoico e da successioni bacinali, sopra le quali sono presenti sedimenti miocenici di avanfossa e di bacini satelliti. Tale parte superiore è completamente scollata dal substrato di origine e trasportata sulla piattaforama carbonatica dell’avampaese apulo, che raggiunge potenze di circa 6–7 km. Sulla base dei profili sismici diponibili, la piattaforma apula appare coinvolta, insieme ai sottostanti sedimenti clastici permo-triassici e quindi anche al basamento, in strutture con basso rigetto di tipo «thick-skin». Di conseguenza, nell’Appennino Meridionale, un passaggio da un regime tettonico di tipo «thin-skin» ad uno di tipo «thick-skin» sembra essere avvenuto nel tempo. Le strutture «thin-skin» sono caratteristiche della parte più superficiale (e più vecchia) della catena costituita da unità scollate, mentre lo stile «thick-skin» è dominante nei carbonati apuli sepolti di più recente accrezione. Il limite attuale tra le due diverse, sovrapposte porzioni della catena è costituito da una importante superficie di sovrascorrimento a basso angolo, penetrata da numerosi pozzi petroliferi. Gli stili geometrici e le modalità di deformazione contrazionale differenziati nello spazio e nel tempo sembrano essere il risultato di esigenze di compatibilità geometrica della deformazione e di continuità dei rigetti lungo un orogene fortemente segmentato e caratterizzato da una stratigrafia variabile nelle sue caratteristiche meccaniche e da un aumento del raccorciamento verso Meridione.

Testing thrust tectonic models at mountain fronts : where has the displacement gone?

The alternative relationships that can exist between a mountain front and the adjacent foreland basin have been recognized for many years. However, seismic reflection data from such areas are commonly of poor quality and therefore structural models may contain large uncertainties. In view of scientific and commercial interest in mountain belts, we have reviewed the methods for discriminating between alternative interpretations using a case study from the Montagna dei Fiori in the central Apennines, Italy. In this area Mesozoic and Tertiary carbonate sediments are juxtaposed with a foredeep basin containing up to 7 km of Messinian and Plio-Pleistocene siliciclastic sediments. A new structural model for this area demonstrates how the structures in this area form a kinematically closed system in which displacement is transferred from the thrust belt to blind structures beneath the present-day foreland. Growth strata show that Pliocene shortening was initially rapid (15 mm a−1) followed by slower rates during the final stages of deformation. Variations in structural elevation indicate a component of basement involvement during thrusting, and this is further supported by magnetic modelling. The results illustrate the interaction of thin- and thick-skinned structures in the central Apennines, and the methods for discriminating between alternative structural models.

Thrusting and strike-slip tectonics in the Alto Molise region (Italy): implications for the Neogene-Quaternary evolution of the Central Apennine orogenic system

The Alto Molise area belongs to the Apennine orogenic belt, which has been developing since the Late Cretaceous as a result of the Europe–Africa collision. This area is characterized by the complex superposition of different palaeogeographic domains that developed during the Mesozoic along the passive southern margin of Tethys Ocean. These domains have been subdivided into four main structural units; from shallowest to deepest they are: the Sannio pelagic basin, the Latium-Abruzzi carbonate platform, the Molise pelagic basin and the Apulia carbonate platform Units. Until the present study, the tectonic relationship among these units was thought to have been mainly determined by Early Messinian to Late Pliocene thrusting. This paper reports the results of a new geometric and kinematic analysis of the Alto Molise area. This new interpretation of the Alto Molise structural style indicates that strike-slip tectonics played a primary role during post-Pliocene times, greatly modifying the previous Alto Molise thrust structures. Two main tectonic events are recorded, consisting of the Pliocene thrusting of the Molise domain onto the Apulia platform followed by the post-Pliocene disruption and rotation of the pre-existing compressional structures by N–S-oriented, high-angle, right-lateral strike-slip faults. These faults are manifest in the Apulia platform as narrow shear zones, but propagate towards the surface into wider belts of strike-slip and oblique-slip deformation. We have compared our results with published structural, palaeomagnetic and anisotropy of magnetic susceptibility (AMS) data from the Central Apennines. This comparison suggests that the superposition of the younger strike-slip tectonic deformation on the older fold and thrust structures in the Alto Molise area is consistent with block-rotation of a segmented orogen, applied to the whole Apennine system.

Influence of palaeogeography on thrust system geometries: an analogue modelling approach for the Abruzzi-Molise (Italy) case history

Abstract The Abruzzi–Molise sector in the Central Apennines is a part of a fold and thrust belt that has been deforming since the Late Cretaceous as a result of collision tectonics between the European and Adriatic plates. The superposition of different deformational styles highly reworked the originally complex palaeogeography of this portion of the southern Tethyan margin. Analogue modelling has been performed on thrusting mechanisms in the Abruzzi–Molise area in order to (1) reduce the number of admissible hypotheses regarding palaeogeographic setting, and (2) define thrusting mechanics. Both of these goals are crucial for hydrocarbon exploration purposes. The sandbox apparatus used to simulate the undeformed passive margin consisted of a thin compartment juxtaposed against a thick one along a linear boundary having variable geometries and mechanical stratigraphy; a rigid but mobile backstop was used to deform the stratigraphy in a Coulomb thrust wedge. Results from six experiments show that the geometric relationships between different structural units depend on the distribution of palaeogeographic domains. These domains are defined by mechanical and/or geometrical parameters, such as the orientation between the maximum compression direction and the palaeogeographic boundary, the mechanical stratigraphy and the thickness of the successions reproduced in the models. The present-day tectonic styles and Meso–Cenozoic palaeogeography of the Abruzzi–Molise area are discussed in terms of the mechanisms and structures analysed through the models.

Shallow burial and exhumation of the Peloritani Mountains (NE Sicily, Italy): Insight from paleothermal and structural indicators

We used vitrinite refl ectance and mixedlayered clay minerals to investigate levels of diagenesis of the Oligocene-Miocene basin developed on the nappes of the Alpine orogen exposed in the Peloritani Mountains (NE Sicily). Paleothermal indicators were integrated with stratigraphic and structural analyses and published apatite fi ssion-track and (U-Th-Sm)/He ages to defi ne the late evolutionary stages of the Peloritani Mountains. This multimethod approach allowed us to reconstruct the paleogeothermal gradient of the basin in Oligocene-Miocene times, to constrain its burial evolution, and discriminate between areas where it has been affected by sedimentary and/or tectonic load. In the southern area of the basin, organic and inorganic thermal parameters increase as function of depth, suggesting that their evolution was ruled by sedimentary burial. They record a decrease in paleogeothermal gradient values marking the evolution of the basin from a forearc to a thrust-top setting during the convergence-collision process between the Calabria-Peloritani Arc and the African plate. On the other hand, in the northern edge of the basin, vitrinite refl ectance values (0.46%‐0.58%) indicate that the thermal evolution of this area was controlled by tectonic burial related to late Langhian‐early Serravallian out-of-sequence thrust tec tonics. The tectonic overburden has been totally removed by extensional tectonics and/or erosion since the late Miocene. The short time span at maximum temperature (<2 m.y.) elapsing between thrust stack emplacement and the beginning of tectonic overburden removal has allowed only vitrinite refl ectance and thermochronological indicators to record this compressive reactivation.

Correlation of Diagenetic Data from Organic and Inorganic Studies in the Apenninic‐Maghrebian Fold‐and‐Thrust Belt: A Case Study from Eastern Sicily

Temperature‐dependent clay mineral assemblages and vitrinite reflectance data have been used to investigate levels of diagenesis from the Apenninic‐Maghrebian fold‐and‐thrust belt in eastern Sicily at the footwall of the Peloritani‐Calabride Arc. Data are from units sampled along a regional transect between the Nebrodi Mountains to the north and Mount Judica to the south. These units developed in very different tectonic settings from those of oceanic to passive continental margin domains deformed during the Cenozoic mountain building and related active margin deposits. The integration of organic and inorganic thermal indicators allowed us to distinguish among different tectonic settings, with thermal maturity generally decreasing from hinterland to foreland as a result of progressively less severe thermal evolution and/or tectonic loading during the mountain building. Specifically, the highest vitrinite reflectance (VRo%) values (ca. 0.60%–0.75%) and percentages of illite layers in illite‐smectite (I‐S; 60%–80%) are found in trench‐involved and accreted passive margin units. Lower VRo% values (0.20%–0.47%) and percentages of illite layers in I‐S (30%–60%) are found in thrust‐top and foredeep basin deposits and far‐traveled Sicilide units that have escaped involvement in trench evolution. Furthermore, either sedimentary or long‐lived tectonic burial (at least more than 5 m.yr.) seem to have affected levels of diagenesis of the studied successions. The correlation between organic and inorganic thermal indicators is satisfactory for most of the samples derived from hemipelagic and siliciclastic deposits, whereas it is poor for some proximal siliciclastics. A tentative calculation of paleotemperatures is also proposed for the studied tectonostratigraphic units.

Sedimentary and tectonic burial evolution of the Northern Apennines in the Modena-Bologna area: constraints from combined stratigraphic, structural, organic matter and clay mineral data of Neogene thrust-top basins

Vitrinite reflectance and mineralogical data from 55 samples collected from the Mt. Cervarola Sandstones and the Granaglione Sandstones (thrust-top basin Cervarola Successions) testify the severe thermal maturity of the present-day outcropping successions. This is due to the deep tectonic burial the successions underwent during Miocene chain building. Lower levels of thermal maturity are recorded in the Mt. Cervarola Sandstones. They form a thick upper thrust sheet (Ro% ranging between 1.0 and 1.2%; I% in I/S mixed layers ranging between 80 and 85%). Higher levels of thermal maturity characterise the structurally lower thrust sheets, made up mainly of Granaglione Sandstones (Ro% ranging between 1.2 and 1.5%; and I% in I/S mixed layers between 85 and 90%). Maturity generally decreases from hinterland to foreland. Integration of Ro% and I% in I/S data with published low-temperature indicators allowed to reconstruct a maximum tectonic burial ranging between ~3.5 on the upper thrust sheet and ~5 km on the lower ones. Burial was mainly due to the early emplacement of the Ligurian Unit and the Modino Unit on top of the studied units and to later piggy-back thrusting of the Mt. Cervarola Sandstones onto the Granaglione Sandstones.

Structural evolution of the sedimentary accretionary wedge of the alpine system in Eastern Sicily: Thermal and thermochronological constraints

Temperature-dependent clay-mineral assemblages, vitrinite reflectance, and apatite fission-track data have been used to investigate levels of diagenesis and time of exhumation of the double-verging Sicilide-Antisicilide accretion-ary wedge in Eastern Sicily. The integration of organic and inorganic thermal indicators allowed us to distinguish parts of the accre-tionary wedge with different thermal signature and evolution. We recognize a warmer core made up of the Mount Soro and Troina units and two colder rims (Antisicilide and far-traveled Sicilide units). The Antisicilide unit was thrust back toward the hinterland, and the far-traveled Sicilide units were gravity-driven toward the Hyblean Plateau. In detail, the highest percentages of vitrinite reflectance (VR o ) values (0.60%–0.96%) and percentages of illite layers in illite-smectite (I-S; 60%–85%) are found in the Mount Soro and Troina units. Apatite fission-track data, together with the paleotemperature estimates from vitrinite-reflectance data and clay-mineral–based geothermometers, indicate that fission tracks were partially to totally annealed during wedge accretion and that the subsequent exhumation occurred mainly in Burdigalian times. Low VR o values (0.35%–0.50%) and percentages of illite layers in I-S (30%–60%) occur in early thrust-top deposits (Reitano Flysch) that unconformably overlie the Sicilide Complex, as well as the far-traveled Sicilide and Antisicilide units. Apatite fission-track data for the Antisicilide unit confirm low paleo-temperature values. Thus the far-traveled Sicilide and Antisicilide units were probably at higher structural levels in the original accretionary prism and were remobilized since late Aquitanian–Burdigalian times.

Clay mineral assemblages and vitrinite reflectance in the laga basin (Central Apennines, Italy) : What do they record?

Temperature-dependent clay mineral assemblages and vitrinite reflectance data have been used to investigate levels of diagenesis from the Messinian Laga Basin in the Central Apennines developed at the footwall of the Sibillini Mts. and the Gran Sasso Massif. Data are from stratigraphic units forming the main siliciclastic basin fill up to Middle Messinian gypsum-arenites and its pre-orogenic substratum. Specifically, the largest Rom% values and percentages of illite layers in illite-smectie (I-S) are found in the basin depocenter and at the footwall of the main carbonate thrust sheets. Smaller Rom% values, and percentage of illite layers in I-S characterize less subsided sectors surrounding the depocenter.The X-ray diffraction data were treated using decomposition methods and the peaks identified were rationalized in terms of discrete and/or mixed-layer phases. Complex clay mineral assemblages were found in the Laga Fm. including three sub-populations of illitic material corresponding to authigenic and detrital components. I-S mixed layers record the maximum paleotemperature the Laga Fm. experienced, which is directly related to its burial history. Kübler index (KI) data, however, suggest higher temperatures related to detrital K-micas inherited from the uplift of the Alpine-Apennines chain.A tentative calculation of paleotemperatures from selected data of organic and inorganic parameters is also proposed and compared with recent sedimentological, stratigraphic and structural data. We conclude that the Laga Basin fill never experienced temperatures of >100–110°C, generally due to variable sedimentary loading, whereas localized anomalous heating is due to the effect of the tectonic emplacement and subsequent local erosion of the Sibillini and Gran Sasso thrust sheets.