Massimiliano Zattin

Apatite fission-track data for the Miocene Arabia-Eurasia collision

The collision between the Eurasian and Arabian plates along the 2400-km-long BitlisZagros thrust zone isolated the Mediterranean from the Indian Ocean and has been linked to extension of the Aegean, rifting of the Red Sea, and the formation of the North and East Anatolian fault systems. However, the timing of the collision is poorly constrained, and estimates range from Late Cretaceous to late Miocene. Here, we report the fi rst apatite fi ssiontrack (AFT) ages from the Bitlis-Zagros thrust zone. The AFT samples are distributed over the 450 km length of the Bitlis thrust zone in southeast Turkey and include metamorphic rocks and Eocene sandstones. Despite the disparate lithology and large distance, the AFT ages point consistently to exhumation between 18 and 13 Ma. The AFT ages, along with a critical appraisal of regional stratigraphy, indicate that the last oceanic lithosphere between the Arabian and Eurasian plates was consumed by the early Miocene (ca. 20 Ma). The results imply that Aegean extension predated the Arabia-Eurasia collision.

Steady-state exhumation of the European Alps

Fission-track grain-age distributions for detrital zircon are used in this study to resolve the late Cenozoic exhumation history of the European Alps. Grain-age distributions were determined for six sandstone samples and one modern river sediment sample, providing a record from 15 Ma to present. All samples can be traced to sources in the Western and Central Alps. The grain-age distributions are dominated by two components, P1 (8–25 Ma) and P2 (16–35 Ma), both of which show steady lag times (cooling age minus depositional age), with an average of 7.9 m.y. for P1 and 16.7 m.y. for P2. These results indicate steady-state exhumation in the source region at rates of ∼0.4–0.7 km/m.y. since at least 15 Ma.

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.

The carbonate tectonic units of northern Calabria (Italy): a record of Apulian palaeomargin evolution and Miocene convergence, continental crust subduction, and exhumation of HP-LT rocks.

In northern Calabria (Italy), the metasedimentary succession of the Lungro–Verbicaro tectonic unit preserves mineral assemblages suggesting underthrusting to depths in excess of 40 km. Internal deformation of these rocks occurred continuously during the following decompression. Index mineral composition associated with progressively younger tectonic fabrics indicates that a substantial part of the structural evolution took place within the blueschist-facies P–T field. Despite their tectonic and metamorphic history, the rocks of the Lungro–Verbicaro Unit preserve significant sedimentary and palaeontological features allowing correlations with successions included in adjacent thrust sheets and the reconstruction of the Mesozoic continental margin architecture. The subduction–exhumation cycle recorded by the Lungro–Verbicaro Unit is entirely of Miocene age. This portion of the Apulia continental palaeomargin was involved in convergence-related deformation not earlier than the Aquitanian. The integration of our results with available constraints on the tectonic evolution of the Apennine–Calabrian Arc system suggests that subduction and most of the subsequent exhumation of the Lungro–Verbicaro Unit occurred, up to Langhian time, at maximum vertical rates in excess of 15 mm a−1. The exhumation process was then completed, at much slower rates (<2 mm a−1) in Late Miocene time, as indicated by both apatite fission-track data and stratigraphic information.

Discriminating between tectonic and sedimentary burial in a foredeep succession, Northern Apennines

An extensive apatite fission‐track survey has been carried out on the Marnoso‐arenacea foredeep succession in the Northern Apennines. The data show a general decrease of the maximum paleotemperature undergone by the sediments toward the foreland areas. The maximum burial calculated by using a geothermal gradient of 20°C km−1 spans from more than 5 km to less than 2.5 km and indicates that the previously assessed total thickness of the Marnoso‐arenacea succession is not enough to justify the determined values. It is concluded that a now eroded Ligurian wedge, up to 5 km thick, was present on top of the Marnoso‐arenacea sediments.

An Oligocene ductile strike-slip shear zone: The Uludağ Massif, northwest Turkey—Implications for the westward translation of Anatolia

The Uludag Massif in northwest Turkey represents an exhumed segment of an Oligocene ductile strike-slip shear zone that is over 225 km long and has ~100 km of right-lateral strike-slip displacement. It forms a faultbounded mountain of amphibolite-facies gneiss and intrusive Oligocene granites. A shear-zone origin for the Uludag Massif is indicated by: (1) its location at the tip of the active Eskisehir oblique-slip fault, (2) pervasive subhorizontal mineral lineation in the gneisses with a right-lateral sense of slip, (3) foliation with a consistent strike, (4) the presence of a subvertical synkinematic intrusion, and (5) the alignment of the Eskisehir fault, synkinematic metagranite, and the strike of the foliation and mineral lineation. The shear zone nucleated in amphibolite-facies gneisses at peak pressure-temperature (P-T) conditions of 7.0 kbar and 670 °C, and it preserves Eocene (49 Ma) and Oligocene (36–30 Ma) Rb/Sr muscovite and biotite cooling ages. The shear zone was active during the latest Eocene and Oligocene (38–27 Ma), as shown by the crystallization and cooling ages from synkinematic granite. A 27 Ma postkinematic granite marks the termination of shear-zone activity. The 20–21 Ma apatite fi ssion-track (AFT) ages indicate rapid exhumation during the early Miocene. A 14 Ma AFT age from an Uludag gneiss clast deposited in a neighboring Neogene basin shows that the shear zone was on the surface by the late Miocene. Results of this study indicate that during the Oligocene, crustal-scale right-lateral strikeslip faults were transporting crustal fragments from Anatolia into the north-south– extending Aegean; this implies that the westward translation of Turkey, related to the Hellenic slab suction, started earlier than the Miocene Arabia-Eurasia collision.

On the tectonic evolution of the Ligurian accretionary complex in southern Italy

Unraveling the deformation pattern characterizing the transition from final oceanic subduction stages to early stages of deformation of a foreland continental margin is crucial for a better understanding of the geodynamic processes taking place at convergent plate boundaries. In particular, the combined role of internal wedge dynamics and continental-margin architecture in controlling the tectonic evolution of an accretionary complex during its final emplacement onto the foreland continent is discussed in this study. To this purpose, we conducted integrated structural, stratigraphic, and low-temperature thermochronometric analyses on the Ligurian accretionary complex units exposed in the Campania region (Italy) and on continental-margin successions located in their footwall, as well as on related foredeep and wedge-top basin deposits. Our results point out a series of late early Miocene (Burdigalian) shortening events, also involving buttressing of the accretionary wedge against the crustal ramp of the foreland continental margin. Emplacement of the overthickened accretionary complex onto the distal part of the continental margin was followed by horizontal extension and wedge thinning, aiding the development of wedge-top depocenters. Extension may have been either related to reduced subduction rates during the middle Miocene, or to a period of subduction erosion (known to have occurred in the Northern Apennines in the same time frame). Early Miocene NW-SE shortening recorded by Ligurian accretionary complex units was completely unrelated to later (late Miocene to Pleistocene) NE-directed thrusting in the Apennines, which was coeval with backarc extension in the Tyrrhenian Sea. Therefore, our results emphasize the occurrence of a major discontinuity in the Neogene geodynamic evolution of the Southern Apennines, the tectonic history of which may be clearly subdivided, from a kinematic point of view, into pre- and syn-Tyrrhenian backarc extension stages.

Detrital Fission-Track Analysis and Sedimentary Petrofacies as Keys of Alpine Exhumation: The Example of the Venetian Foreland (European Southern Alps, Italy)

Sedimentary petrography and detrital apatite fission-track analysis of sediments from the Oligo-Miocene Venetian foreland succession (NE Italy) reveal a multi-stage evolution of the Alpine source areas. During the first stage of sedimentation (Chattian to Langhian), the foreland basin was filled by sediments derived from erosion of mainly crystalline Austroalpine units exhumed north of the Periadriatic Lineament. These sediments are characterized by litharenites with mean quartz content of about 61% and by a mean detrital apatite fission-track peak of about 30 Ma. The Permian-Paleogene sedimentary cover of the Southalpine crystalline basement (Dolomite region) was bypassed by the sediments and made only a minor input. From the Serravallian onwards, the foreland basin was incorporated into the Southalpine thrust belt. The compressional events of the Valsugana phase resulted in thrusting and uplift of the Southalpine domain, as documented by Tortonian fission-track ages in the hanging wall of the major regional thrust (Valsugana thrust fault). The Southalpine crystalline basement, unaffected by Alpine metamorphism, and its sedimentary cover became the main source of sediments, as revealed by the increase in extrabasinal carbonate grains and detrital apatite grains with Late Triassic-Jurassic fission-track ages. During this final stage of basin evolution, about 3000 m of sediments were deposited. These strata subsequently have been uplifted and eroded since the Pliocene, indicating a southward migration of the Alpine deformation front.

Focused erosion in the Alps constrained by fission-track ages on detrital apatites

Abstract Fission-track dating on detrital apatites from modern sands of the Po Delta is used for a provenance study of sediments in the Po River basin. Analysed samples show a fission-track grain-age distribution characterized by two prominent peaks at 7.7 Ma and 17 Ma. The youngest peak accounts for 46% of the total population of dated grains. This young component in the grain-age distribution is consistent with bedrock cooling ages observed in the Western Alps between the External Massifs and the Houiller unit, as well as in the Lepontine dome of the Central Alps and in the Miocene foredeep units of the Apennines, that overall represent only 12% of the orogenic source area. Results suggest that most of the sediment load in the last 102–105 years was supplied by focused erosion of relatively small areas that experienced short-term erosion rates one order of magnitude higher than in the rest of the belt.

Low-angle normal faulting and focused exhumation associated with late Pliocene change in tectonic style in the southern Apennines (Italy)

In the southern Apennines, low-temperature thermochronometry data indicate that exhumation of previous tectonically buried sedimentary units started at around 10 Ma and took place mostly during the last 6 Ma. Relatively high exhumation rates are obtained from apatite fission track (AFT) and (U-Th)/He (AHe) analysis, pointing to a substantial contribution of tectonic processes to rock exhumation besides erosion. Exhumation rates derived from new apatite (U-Th)/He data (AHe) for the last 3 Ma are generally lower than rates determined by AFT data and almost in line with erosion rates inferred from cosmogenic nuclides and sediment yield, thus suggesting that tectonic exhumation was dominant during the older exhumation stages of this region. However, younger cooling ages in the Monte Alpi area from both AFT and AHe analyses point out focused exhumation during the last 3 Ma. Structural and morphotectonic analyses indicate that fast exhumation occurred specifically in this area—where the Apulian Platform reservoir carbonates, elsewhere buried beneath a several kilometer-thick allochthonous cover, are exposed at the surface—as a result of a complex interplay between steep-rooted reverse faulting and shallow low-angle extension. This deformation involved the development of foreland-dipping low-angle normal faults affecting the allochthonous cover units during the late stages of reverse fault-related anticlinal growth in the underlying buried carbonates. Extension of the region triggered focused exhumation in the footwall of the extensional low-angle faults, which was followed by widespread crustal extension and associated development of high-angle normal faults, leading to surface uplift of Monte Alpi.