Jörg Robl

Garnet zoning in high pressure granulite-facies metapelites, Mozambique belt, SE-Kenya constraints on the cooling history

Three metapelitic samples from the granulite facies Taita Hills, part of the Neoproterozoic Mozambique belt in SE-Kenya, contain nearly pure almandine-pyrope garnets. These garnets show a diffusional zoning of X Fe = Fe/(Fe+Mg) at the rim over a distance of ∼200-500 μm if in contact with biotite. Garnet-biotite Fe-Mg exchange thermometry yields closure temperatures between 530–735°C. Diffusion zoning profiles in garnets are used to estimate cooling rates using a numerical model. For the calculations a metamorphic peak temperature and pressure of 820°C and 1.15 GPa are obtained from mafic granulites. Matching of numerically modelled and observed zoning profiles indicates cooling rates between 1–3°C/my. Comparison with cooling rates estimated with the analytical approach of Ehlers & Powell (1994) and with geochronologically derived cooling rates shows that the volumetric ratio of biotite to garnet was about 0.5 during closure. This is consistent with the volumetric ratio observed in thin section, but inconsistent with microprobe analyses that indicate that only biotite in the immediate vicinity of garnet equilibrated with garnet. Conversely, significant garnet zoning only occurs where in contact with biotite. We suggest that these inconsistencies can be explained with changes in the grain boundary processes during cooling: in the thermal evolution above the closure temperature around 735°C a fast grain boundary model applied so that all biotite in the thin section equilibrated with garnet. At lower temperatures local zoning developed, but did not influence the composition of the garnet grain centers. The change in grain boundary process from fast to slow diffusing grain boundaries may correlate with the solidus temperature of the rock.

Extension during continental convergence in the Eastern Alps: The influence of orogen-scale strike-slip faults

In the Miocene, the European Eastern Alps extruded laterally along orogen-scale strike-slip faults due to both extensional gravitational collapse and compressional tectonic forcing. Horizontal extension is most prominently evidenced by detachments east and west of the Tauern Window; it is commonly explained by a retreating slab beneath the Carpathian arc hundreds of kilometers east of the orogen. Horizontal compression is shown by north-south shortening in the Tauern Window and the entire Eastern Alps in response to the convergence of the Adriatic plate with Europe. It is interesting that analogue and numerical models for the Eastern Alps designed to describe the east-directed lateral extrusion have failed to explain the extensional regime in the region of the Tauern Window. Using a numerical model for plan-view deformation that considers internal faults, we show here that orogen-scale strike-slip faults are mechanically required to cause extension during plate convergence in the Miocene Eastern Alps. We test the idea by coupling this model with a landscape evolution model and by comparing modeled and observed drainage system geometries.

THERMAL HISTORY: A new software to interpret diffusive zoning profiles in garnet

Mineral grains can record the cooling history of metamorphic terrains by preserving characteristic chemical zoning profiles caused by diffusion. A range of analytical and numerical models have been used to describe the relationship between the cooling rate and the shape of chemical zoning profiles. Most of these models are characterized by a deficit of usability to external users. This problem is overcome by the code THERMAL HISTORY presented here. The code is platform independent and runs without compilation or a hard disk install. Model results are stored in a basic database and displayed graphically. The code is controlled by an intuitive graphical user interface and uses a very fast diffusion algorithm. THERMAL HISTORY can be used to model zoning profiles as a function of a series of cooling histories, and is written so that it is particularly applicable for the Fe-Mg exchange between garnet and biotite. The code takes into account mass balance so that the volumetric ratio of garnet and biotite can be considered explicitly and it provides a facility to calculate sections through the grains. As some of these facilities have never been published before, the impact of cooling histories, mass balance and section position is explained in some detail. As an application example, THERMAL HISTORY is used to demonstrate that highly non-linear cooling histories, small biotite-garnet ratios and the section effect may result in zoning profiles that are misleading if interpreted in terms of the cooling history.

Chemical denudation rates of a small torrential catchment in the Northern Calcareous Alps

Abstract In this study, we present chemical denudation rates derived from measuring the dissolved load of an alpine catchment located in Salzburg, Austria. The catchment has a drainage area of about 7 km2 and is predominantly covered by limestone- rich glacial deposits and carbonate rocks that are characteristic of the Northern Calcareous Alps. To obtain catchment-wide chemical denudation rates, we integrated discharge time series that were measured by a permanent water gauge of the Austrian Service for Torrent and Avalanche Control to compute the total discharge of the investigated catchment over a period of one year. During the same period, samples were taken during several campaigns to consider variations of the dissolved load. Samples were collected at high and low runoff conditions to study the effect of precipitation and at different locations along the tributaries to account for lithological variations of the river beds on the dissolved load. For the investigation period of one year, 2.97 ×106 m3 of discharge was measured at the catchment outlet. The summed cation-concentration varies between about 85 mg/l for dry-conditions and 75 mg/l for rainy-conditions at the gauge and consists predominantly of Ca2+and Mg2+ cations. Based on the total discharge of the river integrated over a period of one year, and the average dissolved load determined from water samples, we obtained a chemical denudation rate of 0.094 mm/a. The results imply that chemical denudation is a significant driver for redistributing mass in carbonate-dominated catchments and might be the dominant erosional process in such settings.