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Abstract: Using a new experimental approach, fission-track formation has been simulated, for the first time, under crustal conditions by exposing natural zircon, at a pressure of 7.5 kbar and a temperature of 250 °C, to a beam of relativistic heavy ions. The latent tracks were investigated using high-resolution transmission electron microscopy, and the diameters of several hundred tracks were measured. The mean values (±σ) of the track diameters were 5.2±0.5 nm and 5.4±0.4 nm for zircon at ambient and elevated pressure-temperature, respectively. Based on the number of measurements, this represents a statistically significant difference between the tracks at ambient vs. high-pressure/temperature conditions. The slightly larger size of the tracks at elevated pressure can be understood in terms of the increased efficiency of the damage process in a strained crystal lattice. This slight variation in track diameter (~0.2 nm) at high pressure probably will not affect the dimensions of etched tracks. [Copyright &y& Elsevier]

Abstract: The source of huge volumes of quartzofeldspathic detritus feeding Oligo-Miocene foredeep turbidites of the Apennines has long remained an unsolved geological enigma. In this article we reinterpret the available sandstone petrography, fission-track, stratigraphic, and geomorphological evidence to demonstrate that the Adriatic foredeep was dominantly sourced by a “Himalayan-type tectonic aneurysm” growing in the Central Alps since the late Oligocene: the Lepontine Dome. The Lepontine Dome is the envelope of distinct subdomes that formed stepwise from east to west during northwestward indentation of Adria beneath the axial Alps. Focused erosion of domal culminations cut by antecedent drainage started suddenly in the late Oligocene, with unroofing of recently-emplaced plutons and young amphibolite-facies gneisses. Exhumation was tectonically driven, and possibly accelerated by positive feedback between erosion and uplift of hot weak rocks from depth. The huge volumes of resulting detritus (Gonfolite–Macigno clastic wedge) were transported southward for hundreds of kilometers along the rapidly subsiding foredeep, formed since middle Eocene times in front of the Alpine retrobelt and nascent Apennine forebelt. Such massive sediment influx, following 15±2 Myr of starved-foredeep sedimentation, testifies to rapid formation of mountain relief in the Central Alps, synchronous to dextral activity along the Insubric Fault. The Lepontine Dome remains today as the only deeply-dissected region in the Alps, where medium-grade metamorphic rocks of the axial belt are extensively exposed. The relationships between geological structures and drainage patterns still mirror those formed originally at Oligocene times. River headwaters have not been significantly displaced since then, and their thalwegs constrain finite lateral displacement relative to the Southalpine retrowedge to a maximum of 10–20 km. Our reconstruction has wide implications, not only for a better understanding of the Tertiary evolution of the Alps–Apennines orogenic couple, but also for general relationships among tectonic activity, relief formation, drainage development, erosion distribution, and long-distance sediment transfer during continental collision and orogenic growth. [Copyright &y& Elsevier]

Abstract: The dating of the uplift onset of the Mongolian mountain ranges, the northernmost relief associated with the India-Eurasia convergence, is a fundamental issue to better understand the mechanisms of propagation of the Cenozoic transpressive deformation in Central Asia. Using apatite fission tracks we determined the timing and strain rates of the tectonics affecting the Gurvan Bogd system, in the Gobi-Altay, since the Middle Mesozoic to the Late Cenozoic. The region was firstly affected by a Lower–Middle Jurassic tectonic phase, characterized by a vertical crustal movement larger than 2 km. Then followed a protracted period without major crustal vertical movements until the last uplift phase. The peneplanation of the Jurassic relief produced an erosional surface that has undergone negligible denudation or sedimentation for more than 100 Ma. This same surface corresponds to the present summit plateaux of the massifs, standing about 2000 m above the surrounding region, which corresponds to the vertical movement produced by the ongoing uplift. Modelling of fission track data from the massifs of Ih Bogd and Baga Bogd shows that this uplift phase probably started at 5±3 Ma. The Gobi-Altay mountain range appears therefore as one of the youngest mountain ranges in Central Asia, which is consistent with the idea of a northward propagation of the transpressional deformation from the Himalayan front to the Siberian craton. The Cenozoic uplift rate of the massifs is estimated to be between 0.25 and 1 mm/yr, which is slightly higher than the upper Pleistocene vertical slip rates of the bordering faults. This suggests that thrust faults observed within the massifs would increase the uplift rate inside the massifs compared to the uplift rate determined at their boundaries. [Copyright &y& Elsevier]

Abstract: Apatite (U–Th)/He and fission-track thermochronometers are combined to constrain models of denudation and escarpment development of the high elevation Eritrean margin along the southern Red Sea. (U–Th)/He ages have been determined for apatites from two coast-perpendicular transects that were previously used for apatite fission-track analysis [E. Abbate, M.L. Balestrieri, G. Bigazzi, Morphostructural development of the Eritrean rift flank (southern Red Sea) inferred from apatite fission track analysis, J. Geophys. Res., 107, B11, doi: 10.1029/2001JB001009, (2002)]. In central Eritrea near Asmara, He ages increase from c. 7 Ma at the coast to 152 Ma on the plateau along a transect. Further south, He ages from the margin border to the base of the escarpment span a narrower range (7 to 12 Ma). One sample from the top of a marginal high (2950 m) yields older ages (22–43 Ma). Forward modeling suggests that the He age distribution across the margin should allow scarp retreat and downwearing mechanisms to be distinguished in the case of a margin formed in the last twenty million years. The distribution of He ages suggests that the escarpment evolved by downwearing and that post-break up erosion of the escarpment was facilitated by in situ excavation rather than parallel retreat. This implies the existence of an inland drainage divide located seaward of the present day escarpment rim. We envisage that the inland divide formed during or immediately after continent break up, and that the eastern marginal belt represents a remnant of the apex structure. The short distance between the eastern marginal belt and the rim (c. 10 km) represents the total retreat of the escarpment. Comparison of the measured He ages from the coastal plain with those predicted from forward modelling indicates that the main phase of post-break up erosion started at about 15 Ma, closer to the initiation of seafloor spreading in the Red Sea than was thought previously. [Copyright &y& Elsevier]

Fission track analysis has seen a major expansion in application to general geological problems reflecting its advances in understanding the temperature dependence of track annealing and track length distributions. However, considerable uncertainties still persist, in particular concerning the stability of fission tracks subjected to the interaction of environmental physical parameters (e.g. pressure, temperature, stress) and in extrapolation of laboratory data to geological time scales. In this work, we studied the fading behavior of spontaneous fission tracks in basic apatite [hexagonal Ca5(PO4)3(OH, F, Cl)] when exposed simultaneously to laboratory pressures, temperatures and stress over varying time spans. The experiments showed that track fading is a complex recovery mechanism, which is extremely sensitive to the coupling of these three parameters. In particular, a strong decrease in the fission track fading rate was observed as a function of increasing pressure. And a nearly temperature-independent dramatic increase in fission track recovery was observed as a function of stress. Consequently, (1) the stability field of fission tracks in apatite increases towards temperatures higher than 110°C depending on the absolute pressure; (2) closure ages in apatite are underestimated (>100% for an ideal geothermobarometric gradient); (3) related exhumation and erosion rates are overestimated above the closure temperature and underestimated below the closure temperature; and (4) since the widely applied statistical description of thermally induced fading kinetics does not account for the influences of either pressure or stress and is based on fission track annealing data produced at ambient pressure, the accuracy in extrapolating fission track data to geological time scales and in their application to dynamical systems must be cast into doubt. [Copyright &y& Elsevier]