Calibration and comparison of etching techniques for apatite fission-track thermochronology

Understanding time-temperature histories using apatite fission-track thermochronology involves sample preparation, analysis and then thermal modelling using an appropriate annealing algorithm. A subtle point in this sequence is ascertaining that the sample preparation utilized is compatible with the methodology used in obtaining the data for constructing the annealing data set. This issue is important if one wishes to utilize the relatively new multikinetic annealing algorithm of Ketcham et al. that is implemented in their AFTSolve and HeFTy models which is based on a different etching recipe than those previously used. A preliminary calibration step involves comparing published etch pit diameters for a suite of samples with those analysed by an operator. Results show that the operator can reliably reproduce the calibration data set. We then report a laboratory experiment using samples from Finland and Spain that compares the results obtained using two different etching methodologies (7% nitric acid with qualitative etching conditions and 5.5 M nitric acid at constant conditions). The two raw data sets yield variable results. Comparing the two etching methodologies reveals the influence of this procedure on the kinetic parameter

Reply to comment by Y. Rolland et al. on ''Alpine thermal and structural evolution of the highest external crystalline massif: The Mont Blanc''

International audience1. Introduction[1] Leloup et al. [2005] discussed the Cenozoic structural evolution of the Mont Blanc and Aiguilles Rouges ranges by combining new structural, 0Ar/39Ar, and fission track data with published P-T estimates and geochronological data. Our main conclusions were (1) Alpine exhumation of the Aiguilles Rouges was limited to the thickness of the overlying nappes (10 km), while rocks now outcropping in the Mont Blanc have been xhumed 15 to 20 km.(2) Uplift of the two massifs started 22 Myr ago; while at 12 Ma, the Mont Blanc shear zone (MBsz), a reverse fault with a slight right-lateral component, initiated bringing the Mont Blanc above the Chamonix synclinorium and the Aiguilles Rouges; total vertical throw on the MBsz isbetween 4 and 8 km. (3) Fission track data suggest that relative motion between the Aiguilles Rouges and the Mont Blanc stopped 4 Myr ago. Since that time, uplift of the Mont Blanc has mostly taken place along the Mont Blanc back thrust, a steep north dipping fault zone bounding thesouthern flank of the range. (4) The highest summits are located where the back thrust intersects the MBsz. (5) Exhumation of the Mont Blanc and Aiguilles Rouges occurred toward the end of motion on the Helvetic basal de´collement (HBD) at the base of the Helvetic nappes. Uplift is linked with a deeper, more external thrust that induced the formation of the Jura arc. [2] While acknowledging that our paper is ‘‘a good step forward in the tectonic comprehension of the Mont Blanc area and provides a good synthesis of preexisting data,’’Rolland et al. [2007] claim that the timing we propose for the thrust and back thrust events is not in agreement with new 40Ar/39Ar data that they publish in their comment. In fact, they raise two main arguments with our observations/ interpretations:[3] 1. Alpine deformation is penetrative within the Mont blanc granite and is not accommodated by the two localized shear zones we describe (the SE dipping Mont Blanc shear zone, or MBsz, in the north and the NW dipping back thrust in the south, Figure 1), but by numerous anastomosed shearzones in the way described by Choukroune and Gapais [1983] in the Aar massif and Gourlay [1986] in the Mont Blanc. All deformations within the Mont Blanc are thus coeval and the Mont Blanc is a transpressive pop-up structure at the rim of a large transpressive fault that runs from the Rhone dextral fault system. [4] 2. The timing of deformation cannot be obtained through 40Ar/39Ar thermochronology due to excess argon and intense fluid circulation. They instead provide a minimum age of 16 Ma for the initiation of top to the SE motions on the SE side of the Mont Blanc (back thrust) based on five phengites 40Ar/39Ar ages from three shear zones (their Figure 3). [5] We will take the opportunity of this reply to address these two points and, in a third point, we briefly discuss possible deformation models of the Mont Blanc range

From sea level to high elevation in 15 million years: Uplift history of the northern Tibetan Plateau margin in the Altun Shan

Approximately 1300 in of Oligocene-Miocene clastic strata are exposed along the Miran River in the southeastern Tarim basin, where the adjacent Altun Shan form the topographic escarpment of the northern Tibetan Plateau. The sedimentary section is faulted against Proterozoic rocks of the Altun Shan in the footwall of the south-dipping, oblique reverse Northern Altyn Tagh fault. Oligocene-Lower Miocene strata consist of fine-grained rocks that record low~gradient depositional systems. Mid-Miocene and younger rocks consist of coarse conglomerate, derived from the Altun Shan and deposited by high-gradient depositional systems. The change to coarse, high-gradient depositional systems with detrital source areas coincident with the modern Miran River drainage is interpreted to mark the onset of uplift of the Altun Shan on the Northern Altyn Tagh fault and its erosional exhumation. The age of the change from pre-orogenic to synorogenic sedimentation is constrained by a foraminifera assemblage at the base of the conglomeratic section that includes Early-Middle Miocene planktonic foraminifera. This interpretation is also supported by apatite fission track and (U-Th)/He ages and thermal models that indicate rapid Miocene cooling, and hence, rapid exhumation of the Altun Shan. In addition to defining the age of the synorogenic section, the foraminifera assemblage contains planktonic taxa, indicating a connection to open marine waters, and benthic assemblages typical of brackish to near-sea level paleobathymetry. Thus, micropaleontologic evidence demonstrates that the Miran River locality, now at ∼1400 in elevation, was at sea level approximately 15 million years ago. Thus, in addition to constraining the age of surface uplift and exhumation of the Altun Shan, the principal mountain range of the Tibetan Plateau in this region, as ∼ 15 to 16 Ma, the foraminifera assemblage indicates that the SE Tarim basin, off the northern edge of the plateau, had an average surface uplift rate of nearly 100 m/m.y. for the past 15 million years. These results suggest that shortening in the Altun Shan and uplift of the range significantly post-dated the initiation of large-scale strike-slip on the Altyn Tagh fault, and that regional surface uplift mechanisms operated in the Tarim basin, beyond the margins of the Tibetan Plateau

Does the topographic distribution of the central Andean Puna Plateau result from climatic or geodynamic processes?

Orogenic plateaus are extensive, high-elevation areas with low internal relief that have been attributed to deep-seated and/or climate-driven surface processes. In the latter case, models predict that lateral plateau growth results from increasing aridity along the margins as range uplift shields the orogen interior from precipitation. We analyze the spatiotemporal progression of basin isolation and filling at the eastern margin of the Puna Plateau of the Argentine Andes to determine if the topography predicted by such models is observed. We find that the timing of basin filling and reexcavation is variable, suggesting nonsystematic plateau growth. Instead, the Airy isostatically compensated component of topography constitutes the majority of the mean elevation gain between the foreland and the plateau. This indicates that deep-seated phenomena, such as changes in crustal thickness and/or lateral density, are required to produce high plateau elevations. In contrast, the frequency of the uncompensated topography within the plateau and in the adjacent foreland that is interrupted by ranges appears similar, although the amplitude of this topographic component increases east of the plateau. Combined with sedimentologic observations, we infer that the low internal relief of the plateau likely results from increased aridity and sediment storage within the plateau and along its eastern margin

Tectonic Evolution of the Pamir Recorded in the Western Tarim Basin (China):Sedimentologic and Magnetostratigraphic Analyses of the Aertashi Section

The northward indentation of the Pamir salient into the Tarim basin at the western syntaxis of the India-Asia collision zone is the focus of controversial models linking lithospheric to surface and atmospheric processes. Here we report on tectonic events recorded in the most complete and best-dated sedimentary sequences from the western Tarim basin flanking the eastern Pamir (the Aertashi section), based on sedimentologic, provenance, and magnetostratigraphic analyses. Increased tectonic subsidence and a shift from marine to continental fluvio-deltaic deposition at 41 Ma indicate that far-field deformation from the south started to affect the Tarim region. A sediment accumulation hiatus from 24.3 to 21.6 Ma followed by deposition of proximal conglomerates is linked to fault propagation into the Tarim basin. From 21.6 to 15.0 Ma, increasing accumulation rates of fining upward clastics is interpreted as the expression of a major dextral transtensional system linking the Kunlun to the Tian Shan ahead of the northward Pamir indentation. At 15.0 Ma, the appearance of North Pamir-sourced conglomerates followed at 11 Ma by Central Pamir-sourced volcanics coincides with a shift to E-W compression, clockwise vertical-axis rotations and the onset of growth strata associated with the activation of the local east vergent Qimugen thrust wedge. Together, this enables us to interpret that Pamir indentation into Tarim had started by 24.3 Ma, reached the study location by 15.0 Ma and had passed it by 11 Ma, providing kinematic constraints on proposed tectonic models involving intracontinental subduction and delamination