Maria Laura Balestrieri

Contrasting styles of (U) HP rock exhumation along the Cenozoic Adria‐Europe plate boundary (Western Alps, Calabria, Corsica)

Since the first discovery of ultrahigh pressure (UHP) rocks 30 years ago in the Western Alps, the mechanisms for exhumation of (U)HP terranes worldwide are still debated. In the western Mediterranean, the presently accepted model of synconvergent exhumation (e.g., the channel-flow model) is in conflict with parts of the geologic record. We synthesize regional geologic data and present alternative exhumation mechanisms that consider the role of divergence within subduction zones. These mechanisms, i.e., (i) the motion of the upper plate away from the trench and (ii) the rollback of the lower plate, are discussed in detail with particular reference to the Cenozoic Adria-Europe plate boundary, and along three different transects (Western Alps, Calabria-Sardinia, and Corsica-Northern Apennines). In the Western Alps, (U)HP rocks were exhumed from the greatest depth at the rear of the accretionary wedge during motion of the upper plate away from the trench. Exhumation was extremely fast, and associated with very low geothermal gradients. In Calabria, HP rocks were exhumed from shallower depths and at lower rates during rollback of the Adriatic plate, with repeated exhumation pulses progressively younging toward the foreland. Both mechanisms were active to create boundary divergence along the Corsica-Northern Apennines transect, where European southeastward subduction was progressively replaced along strike by Adriatic northwestward subduction. The tectonic scenario depicted for the Western Alps trench during Eocene exhumation of (U)HP rocks correlates well with present-day eastern Papua New Guinea, which is presented as a modern analog of the Paleogene Adria-Europe plate boundary.

Morphostructural development of the Eritrean rift flank (southern Red Sea) inferred from apatite fission track analysis

[1] The Red Sea is one of the best exposed young rift basins in the world. Its flanks on both the African and Arabian sides are characterized by basement uplifts parallel to the margins and by active erosion. Through the integration of 37 new apatite fission track (FT) analyses and regional geology, we elucidate the uplift and denudational history of the Eritrean continental margin along the southern Red Sea and, in particular, the development, timing, and past and present morphostructural features of its onshore portion. FT data indicate that at around 20 Ma, the Eritrean margin was affected by a crustal cooling event due to a postrifting accelerated phase of denudation. This cooling has the same age as those already detected on the conjugate Arabian margin (Yemen and Saudi Arabia). FT ages increase from 10-20 to 300-400 Ma with increasing distance and elevation from the coastal areas toward the interior. This trend indicates a diminishing amount of eroded section in the same direction. We use FT and structural data to define the position of the main border fault along the margin where the original scarp was located. We estimate rates of vertical denudation of 190-200 m/Myr at the border fault and of <60-70 m/Myr on the plateau. On the basis of these new data, we argue against an Oligocene tectonic unroofing of the margin through a low-angle detachment assumed by previous authors, but we maintain the essential role of the erosional denudation in the development of present margin morphology.

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.

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.

Compression-to-extension record in the Late Pliocene-Pleistocene Upper Valdarno Basin (Northern Apennines, Italy): structural and thermochronological constraints

We use new structural and apatite fission-track data together with apatite fission-track and (U-Th)/He data from literature to examine the tectonic evolution of the continental Upper Valdarno Basin, in the hinterland sector of the Northern Apennines fold-and-thrust belt. This basin is located in-between two structural ridges, the Chianti Mountains to the southwest and the Pratomagno to the northeast. In our interpretation, the Upper Valdarno Basin developed at ca. 3.4-3.3 Ma as pop-down synformal-shaped depression bounded and controlled by oppositely-verging thrust-related structures, namely the thrust system lifting the Chianti Mountains and the southwest-facing backfolds at the base of the Pratomagno. This evolution is compatible with the accelerated exhumation rates at 4-5 Ma documented through apatite fission-track data along both the Pratomagno and Chianti ridges. Shortening suffered by basin deposits is clearly manifested by the outcrop-scale reverse faults and thrust-related folds affecting the Late Pliocene sediments (Castelnuovo dei Sabbioni Synthem), which are well exposed in the Santa Barbara mine. These strongly folded deposits are overlain unconformable by Early Pleistocene sediments (Montevarchi Synthem), which display evidence for syn-depositional normal faulting. This suggests that the Upper Valdarno Basin experienced a phase of normal faulting that started at the base of Pleistocene (ca. 2.6-2.5 Ma) and likely produced the large southwest-dipping “Trappola Fault”, which affects the southwestern margin of the Pratomagno displacing the earlier backthrusts and backfolds. Basin evolution can be thus basically framed into a two-phase history, with extensional tectonics superposed onto compressional structures that were deactivated by ca. 2.7 Ma. Being the Chianti Mountains part of the 250 km-long regional line of thrusts and thrust-related folds (the so-called “Tuscan Nappe front”), the results of this study may also involve regional implications, as they would also hint for the tectonic history of other sectors and basins settled along-strike this regional element.

Spatio-temporal evolution of intraplate strike-slip faulting: The Neogene–Quaternary Kuh-e-Faghan Fault, central Iran

Central Iran provides an ideal region in which to study the long-term morphotectonic response to the nucleation and propagation of intraplate faulting. In this study, a multidisciplinary approach that integrates structural and stratigraphic field investigations with apatite (U + Th)/He (AHe) thermochronometry is used to reconstruct the spatio-temporal evolution of the Kuh-e-Faghan Fault in northeastern central Iran. The Kuh-e-Faghan Fault is a narrow, ~80-km-long, deformation zone that consists of three main broadly left-stepping, E-W–trending, dextral fault strands that cut through the Mesozoic–Paleozoic substratum and the Neogene–Quaternary sedimentary cover. The AHe thermochronometry results indicate that the intrafault blocks along the Kuh-e-Faghan Fault experienced two major episodes of fault-related exhumation at ca. 18 Ma and ca. 4 Ma. The ca. 18 Ma faulting/exhumation episode is chiefly recorded by the structure and depositional architecture of the Neogene deposits along the Kuh-e-Faghan Fault. A source-to-sink scenario can be reconstructed for this time frame, where topographic growth caused the synchronous erosion/exhumation of the pre-Neogene units and deposition of the eroded material in the surrounding fault-bounded continental depocenters. Successively, the Kuh-e-Faghan Fault gradually entered a period of relative tectonic quiescence and, probably, of regional subsidence, during which a thick pile of fine-grained onlapping sediments was deposited. This may have caused resetting of the He ages of apatite in the pre-Neogene and the basal Neogene successions. The ca. 4 Ma faulting episode caused the final exhumation of the fault system, resulting in the current fault zone and topography. The two fault-related exhumation episodes fit with regional early Miocene collision-enhanced uplift/exhumation, and the late Miocene–early Pliocene widespread tectonic reorganization of the Iranian Plateau. The reconstructed long-term, spatially and temporally punctuated fault system evolution in intraplate central Iran during Neogene–Quaternary times may reflect states of far-field stress changes at the collisional boundaries.

Insights on the thermal evolution of the Ligurian Apennines (Italy) through fission-track analysis

Apatite fission-track analysis has been applied on 24 samples from the geometrically upper units (Internal and External Ligurids) of the Northern Apennines nappe pile. They yield ages between c. 20 Ma and c. 6 Ma. Two samples were also dated with fission tracks in zircon (ages of c. 155 Ma and c. 199 Ma). Together the data reveal that during post-Late Cretaceous compression the Internal Ligurids reached temperatures to completely anneal fission tracks in apatite (T>c. 120 °C) but not in zircon (T<c. 300°C). After Eoccne continental collision these rocks were cooled (below c. 120°C) and started again to retain fission tracks in apatite. Since the Oligocene a further burial under episutural sediments ensued that caused a partial to total apatite fission-track annealing. During a final phase of cxhumation the sedimentary cover was removed. This exhumation can be linked to Late Miocenc extensional tectonics, that started at c. 8–9 Ma in the central and southern portions of the study area.

A PC compatible Brazilian software for obtaining thermal histories using apatite fission track analysis

Abstract In this work a software developed in the Instituto de Fisica Gleb Wataghin, IFGW, UNICAMP, Campinas, SP, Brazil for obtaining thermal histories using apatite fission track analysis is presented. This software works in Microsoft-Windows environment. It will be freely disposable in the web site of the Departamento de Raios Cosmicos, IFGW, UNICAMP. Thermal histories obtained through this software are compared with those deduced using Monte Trax the software compatible with Apple Macintosh developed by Gallagher.

FISSION-TRACK DATING OF A TEPHRA LAYER IN THE ALAT FORMATION OF THE DANDIERO GROUP (DANAKIL DEPRESSION, ERITREA)

Attempts to date a biotite separate from a tephra layer recognized near Buia (Danakil Depression, Eritrea) in the liwer part of the Homo remains – bearing Dandiero group (formerly attributed to the Danakil Formation) using the 39Ar/40Ar method failed because of xenocrystic contamination. For this reason it was applied the fission-track method on glass, since no other phases datable with this technique were present. The quality of glass was very poor for fission-track dating, because of the small size of grains. In addition, after polishing only few glass shards showed useful surfaces for track counting and only 25 spontaneous tracks were counted. The determined fission-track age - 0.75 +/- 0.16 Ma - is a rejuvenated age due to the presence of a certain amount of annealing of spontaneous tracks. An attempt to apply the plateau method for correcting this apparent age failed. A corrected age of 1.3 +/- 0.3 Ma was computed using the size-correction method. In spite of its low precision, this fission-track age represents a significant result, since it corroborates the attribution to Jaramillo Subchron of the normal magnetozone near the base of which the tephra is located.

An extensive apatite fission-track study throughout the Northern Apennines nappe belt

Abstract This paper takes into consideration more than 100 apatite fission-track analyses on samples coming from an approximately west-east cross-section throughout the Northern Apennines. This collisional chain is made of structural units and nappes (Ligurian and Tuscan Nappe) accreted to the Adriatic Foreland during the Neogene, which overthrust the Miocene turbiditic successions of the Cervarola and Marnoso-arenacea Formations. Different cooling ages and degrees of annealing delineate different evolution histories for these units. Exhumation of the western outcrops of the Ligurian Nappe can be placed at 8 Ma and follows a first denudation event occurred in Eocene times. Timing of exhumation decreases eastwards. A break in this general trend is shown by the Apuan Alps, that occupy an intermediate position and yielded the youngest cooling ages. In the external part of the Marnoso-arenacea foredeep deposits this tendency could not be tested because total annealing of the apatite system has not been reached. In this case, modeling of data allows evaluating maximum burial temperatures.