Meinert Rahn

Miocene to Holocene exhumation of metamorphic crustal wedges in the NW Himalaya: Evidence for tectonic extrusion coupled to fluvial erosion

The Himalayan crystalline core zone exposed along the Sutlej Valley (India) is composed of two high-grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded, as a consequence of the foreland-directed propagation of crustal deformation in the Indian plate margin. The High Himalayan Crystalline Sequence (HHCS) is composed of amphibolite facies to migmatitic paragneisses, metamorphosed at temperatures up to 750°C at 30 km depth between Eocene and early Miocene. During early Miocene, combined thrusting along the Main Central Thrust (MCT) and extension along the Sangla Detachment induced the rapid exhumation and cooling of the HHCS, whereas exhumation was mainly controlled by erosion since middle Miocene. The Lesser Himalayan Crystalline Sequence (LHCS) is composed of amphibolite facies para- and orthogneisses, metamorphosed at temperatures up to 700°C during underthrusting down to 30 km depth beneath the MCT. The LHCS cooled very rapidly since late Miocene, as a consequence of exhumation controlled by thrusting along the Munsiari Thrust and extension in the MCT hanging wall. This renewed phase of tectonic extrusion at the Himalayan front is still active, as indicated by the present-day regional seismicity, and by hydrothermal circulation linked to elevated near-surface geothermal gradients in the LHCS. As recently evidenced in the Himalayan syntaxes, active exhumation of deep crustal rocks along the Sutlej Valley is spatially correlated with the high erosional potential of this major trans-Himalayan river. This correlation supports the emerging view of a positive feedback during continental collision between crustal-scale tectono-thermal reworking and efficient erosion along major river systems.

Climate-induced rebound and exhumation of the European Alps

Foreland basins record the regional isostatic compensation of mountain belts; during periods of crustal thickening, they subside, and when erosion unloads the mass of the mountains, the basins rebound and are eroded. In order to evaluate this mechanism for rebound, it is critical that the timing and magnitude of erosion are documented. We present data estimating the timing and magnitude of late orogenic or postorogenic erosion in the North Alpine Foreland Basin of Switzerland. Mineral cooling ages demonstrate that the basin underwent 1-3 km of erosion soon after 5 Ma. This erosion coincided with a decline in structural deformation in the Swiss Alps, and a doubling of sediment accu- mulation rates in surrounding depocenters. We propose that accelerated erosional un- roofing of the Swiss Alps triggered isostatic rebound and erosion of the foreland basin after 5 Ma. A projection of the isostatic rebound of the basin into the mountains suggests that at least 6.5 km of erosion should have occurred in the high topography of the Aar Massif. Accelerated erosion in the Swiss Alps at that time is explained by an increase in atmospheric moisture driven by an intensification of the Atlantic Gulf Stream at 4.6 Ma. Consequently, we propose that the changing erosional capacity of the climate triggered late orogenic to postorogenic mass reduction and isostatic rebound of the Swiss Alps and their neighboring foreland basin.

A zero-damage model for fission-track annealing in zircon

Abstract A zircon fission track-annealing model is calculated on the basis of annealing experiments from the literature with induced tracks in α-decay event damage-free zircon samples. Empirically derived parallel and fanning equations for this “zero-damage” model yield an excellent fit to the data, with the fanning model providing slightly better statistical parameters. A comparison between annealing models with fanning iso-annealing lines but different α-decay event damage densities reveals that annealing temperatures and closure temperatures for the estimated partial annealing zone are highest for the zero-damage model. Compilations of existing geologic constraints on the zircon partial-annealing zone on one hand and the zircon closure temperature on the other show that these constraints do not or only partly overlap with curves of proposed models for the zircon partial-annealing zone and closure temperature. This finding is consistent with the fact that the annealing behavior of zircon from long-duration temperature evolutions is increasingly influenced by the accumulated α-decay event damage. Zircon samples of young age or low U content show a behavior closest to the predictions of the zero-damage model, and are in the predicted range of published models with low α-decay event damage density. For thermal events of more than 10 myr duration, however, constraints from field studies show marked differences from proposed partial-annealing zone boundaries of the zero- or low-damage models. The applicability of the zero-damage model is threefold. (1) It predicts correct closure temperatures in the case of very rapid cooling across the partial annealing zone where basically no α-decay event damage is accumulated. (2) It predicts an uppermost boundary for complete annealing of a mixture of zircon components of different age, as found in sedimentary samples, and in this case may be used as a thermometer. (3) It represents an important reference for the establishment of a more comprehensive model of zircon fission-track annealing that also includes the influence of α-decay event damage. For such a model, two different equations are discussed. However, additional detailed experimental and field data are needed for a more robust annealing model that includes the influence of α-decay event-damage annealing.

Low-temperature hydrothermal alteration of natural metamict zircons from the Eastern Desert, Egypt

Abstract The chemical and structural alteration of metamict zircon crystals from a 619±17 (2σ) Ma old, post-tectonic granite in the southern part of the Eastern Desert, Egypt was studied. The crystals show simple oscillatory growth zones with metamictization-induced fractures, which provided pathways for fluid infiltration. Electron and ion microprobe analyses reveal that metamict, i.e. U and Th-rich, areas are heavily enriched in Ca, Al, Fe, Mn, LREE, and a water species, and have lost Zr and Si as well as radiogenic Pb. These chemical changes are the result of an intensive reaction with a low-temperature (120-200°C) aqueous solution. The chemical reactions probably occurred within the amorphous regions of the metamict network. During the zircon-fluid interactions the metamict structure was partially recovered, as demonstrated by micro-Raman and -infrared measurements. A threshold degree of metamictization, as defined empirically by an α-decay dose, Dc, was necessary for zircons to undergo hydrothermal alteration. It is proposed that Dc marks the first percolation point, where the amorphous domains start to form percolating clusters in the metamict network and where bulk chemical diffusion is believed to increase dramatically. The time of the hydrothermal alteration is determined by a lower intercept age of a U-Pb SHRIMP discordia of 17.9+6.9-7.4 (2σ) Ma, which is in good agreement with an apatite fission track age of 22.2+5.4-4.8 (2σ) Ma. The hydrothermal alteration event occurred contemporaneously with the main rifting phase of the Red Sea and widespread low-temperature mineralizations along the Red Sea coast.

Metamorphic rates in collisional orogeny from in situ allanite and monazite dating

The prograde sequence of rare earth minerals recorded in metapelites during regional metamorphism reveals a series of irreversible reactions among silicates and phosphates. In individual samples from the northern Lepontine (Central Alps), allanite is partly replaced by monazite at 560–580 °C. Relic allanite retains its characteristic growth zoning acquired at greenschist facies conditions (430–450 °C). Coexisting monazite and allanite were dated in situ to delimit in time successive stages of the Barrovian metamorphism. In situ sensitive high-resolution ion microprobe (SHRIMP) U-Th-Pb dating of allanite (31.5 ± 1.3 and 29.2 ± 1.0 Ma) and monazite (18.0 ± 0.3 and 19.1 ± 0.3 Ma) constrains the time elapsed between 430–450 °C and 560–580 °C, which implies an average heating rate of 8–15 °C/m.y. Combined with new fission track ages (zircon, 10–9 Ma; apatite, 7.5–6.5 Ma), metamorphic rates of the entire orogenic cycle, from prograde to final cooling, can be reconstructed.

Thermal history of the central Gotthard and Aar massifs, European Alps: Evidence for steady state, long‐term exhumation

[1] Quantifying long‐term exhumation rates is a prerequisite for understanding the geodynamic evolution of orogens and their exogenic and endogenic driving forces. Here we reconstruct the exhumation history of the central Aar and Gotthard external crystalline massifs in the European Alps using apatite and zircon fission track and apatite (U‐Th)/He data. Age‐elevation relationships and time‐temperature paths derived from thermal history modeling are interpreted to reflect nearly constant exhumation of ∼0.5 km/Ma since ∼14 Ma. A slightly accelerated rate (∼0.7 km/Ma) occurred from 16 to 14 Ma and again from 10 to 7 Ma. Faster exhumation between 16 and 14 Ma is most likely linked to indentation of the Adriatic wedge and related thrusting along the Alpine sole thrust, which, in turn, caused uplift and exhumation in the external crystalline massifs. The data suggest nearly steady, moderate exhumation rates since ∼14 Ma, regardless of major exogenic and endogenic forces such as a change to wetter climate conditions around 5 Ma or orogen‐perpendicular extension initiated in Pliocene times. Recent uplift and denudation rates, interpreted to be the result of climate fluctuations and associated increase in erosional efficiency, are nearly twice this ∼0.5 km/Ma paleoexhumation rate.

Metamorphic Processes in Rodingites of the Zermatt-Saas Ophiolites

Three types of rodingites can be distinguished in the serpentinite complex of the Zermatt-Saas ophiolites based on mineral assemblages, texture, and chemical characteristics of bulk-rock and mineral compositions. These three types of rodingites show a genetic relationship on the basis of their mineral evolution. The mineral assemblage Czo-Hgrs-Chl-Di ± Uvr (rodingite I) was developed during ocean-floor metamorphism and represents the earliest rodingitic rock. A second phase of oceanic alteration is characterized by the formation of andradite-rich hydrogarnet. Subsequent progressive metamorphism resulted in formation of vesuvianite and continued formation of hydroandradite, producing the assemblage Hgrs-Vsv-Hadr-Chl-Di (rodingite II). The low-variance mineral assemblage Hadr-Vsv-Chl (rodingite III) represents equilibrium conditions, and the most completely rodingitized rock. Complex variation in chemical composition and mineral assemblages (i.e., distribution of rodingite) is the result of variation in protoliths and degrees of rodingitization.

Fission track and numerical thermal modeling of differential exhumation of the Glarus thrust plane (Switzerland)

Abstract Geometry and differences in metamorphic grade along the Glarus thrust (external European Alps) can be explained by a restoration of the hanging wall some 10 km toward the south, and back rotation of the thrust plane into a ramp–flat–ramp geometry. The apatite fission track (FT) age pattern from the area suggests that isotherms were parallel to the thrust plane during cooling of the rocks along the thrust plane below the apatite closure temperature. Two forward modeling programs that both produce time–temperature paths are used to constrain the thermal and structural evolution of differential exhumation along the Glarus thrust. (a) A two-dimensional finite-difference thermal model is used to examine differential exhumation of the thrust plane, starting from an assumed horizontal position of the middle part of the Glarus thrust at some 10 Ma when parallel to the paleo-isotherms. Five locations in the crustal section corresponding to locations on a final horizontal and vertical sampling traverse record their time–temperature ( t – T ) history during exhumation along particle paths. The near-surface temperature distribution is strongly influenced by the development of a simplified step-like topographic relief, which causes all modeled samples to reach the surface at the end of the modeled time period. (b) The calculated t – T paths of these five modeled samples were used to forward model apatite FT data based on an existing annealing model. By comparison between synthetically generated FT data and actual measured FT samples, the exhumation history of the Glarus Alps was evaluated with respect to its thermal and temporal brackets. Several parameters such as the location of the rotation point, the thermal gradient, and the topography have significant influence on model results. Best-fit models produce results that are consistent with independent geological and geophysical information.

Rotation and exhumation of a thrust plane: Apatite fission-track data from the Glarus thrust, Switzerland

Apatite fission-track ages and track-length measurements are presented from 22 samples from the Glarus Alps, eastern Switzerland, where maximum temperatures of 100 to 350 °C along an Alpine thrust plane were reached during late Tertiary metamorphism. With the exception of the northernmost sample, apatite ages are fully reset and range between 5 and 12 Ma. Apatite fission track data along a vertical profile and a horizontal profile indicate that isothermal planes were oriented parallel to the Glarus thrust plane and that the thrust plane cooled below a mean apatite fission track closure temperature of 110 °C at 9 Ma. The metamorphic pattern and fission-track data indicate a south-dipping thrust plane during the times of peak metamorphic conditions, postmetamorphic thrusting over a distance of ∼ 10 km, and gradational exhumation of the plane to a horizontal position at ca. 9 Ma. Ongoing rotation of the plane is reflected by the present dip of about 7°NNW.

Linking the northern Alps with their foreland: The latest exhumation history resolved by low-temperature thermochronology

The evolution of the Central Alpine deformation front (Subalpine Molasse) and its undeformed foreland is recently debated because of their role for deciphering the late orogenic evolution of the Alps. Its latest exhumation history is poorly understood due to the lack of late Miocene to Pliocene sediments. We constrain the late Miocene to Pliocene history of this transitional zone with apatite fission track and (U-Th)/He data. We used laser ablation inductively coupled mass spectrometry for apatite fission track dating and compare this method with previously published and unpublished external detector method fission track data. Two investigated sections across tectonic slices show that the Subalpine Molasse was tectonically active after the onset of folding of the Jura Mountains. This is much younger than hitherto assumed. Thrusting occurred at 10, 8, 6–5 Ma and potentially thereafter. This is contemporaneous with reported exhumation of the External Crystalline Massifs in the central Alps. The Jura Mountains and the Subalpine Molasse used the same detachments as the External Crystalline Massifs and are therefore kinematically coupled. Estimates on the amount of shortening and thrust displacement corroborate this idea. We argue that the tectonic signal is related to active shortening during the late stage of orogenesis.