Kurt Stüwe

The influence of eroding topography on steady-state isotherms. Application to fission track analysis

The influence of surface topography on the form of steady-state isotherms during erosion-driven denudation is investigated. This is of particular interest to the interpretation of low-temperature geochronological data, for example fission track data, because this rests generally on the untested assumption that isotherms are not perturbed by topography and, therefore, that the data can be interpreted with one-dimensional models. In order to calculate isotherms and investigate the potential errors introduced by this assumption we use an approximate analytical solution of the diffusion-advection equation in two dimensions. It is shown that, for realistic geothermal gradients and a topography amplitude of H = 3 km and wavelength of w = 20 km, the 100°C isotherm may be perturbed to an amplitude of 1000 m/Ma. The effect increases for larger H and smaller w. For the interpretation of denudation histories derived from fission tracks, these relationships imply that the denudation rate of a terrain may be substantially overestimated, if it is only inferred from the slope of data points in a plot of sample elevation versus fission track age and if the sampling profile is not exactly vertical. A simple relationship is discussed that can be used to estimate the real denudation rate from the apparent denudation rate in an elevation-age plot.

PT Paths from modal proportions: application to the Koralm Complex, Eastern Alps

Thermodynamic pseudosections portray those parts of a petrogenetic grid that are relevant to a given bulk composition and the reactions appearing on them can therefore be used directly to infer the PT path that the rock followed. However, for many ‘normal’ bulk compositions the use of pseudosections is hampered by the fact that they display only few large fields of high thermodynamic variance in the PT range of interest. Here it is discussed how modal information on reaction progress within these fields can be used to determine PT path information for thermodynamically high variant metamorphic assemblages. We use this information on reaction progress to contour pseudosections for modal proportions of minerals using the software package THERMOCALC. The approach is applied to di- tri- and quadrivariant assemblages from the Koralm complex in the eastern Alps. A PT path for these rocks is derived from modal considerations and compared with interpretations of mineral composition contours on the same pseudosection and with conventional thermobarometry. It is shown that at least part of the complex must have cooled initially near isobarically from prevalent peak conditions around 700°C and 14 kbar before the rocks commenced a Barroviantype decompression path.

Episodic metamorphism and deformation in low-pressure, high-temperature terranes

In many cases, low-pressure, high-temperature metamorphism must be the consequence of advective heat transfer, e.g., the ascent of granitic magma. Whereas such heating mechanisms are necessarily short-lived, the lifetime of the underlying orogenic processes is likely to be much longer, and it is conceivable that, during the evolution of an orogen, repeated transient heating at shallow crustal levels is caused by episodes of segregation and upward passage of melts generated at deep crustal levels. The number and timing of these events would depend on such factors as critical melt segregation volume, Moho temperature, and strain-rate evolution. We investigate some of the controls on this multiple-event superposition using a simple thermomechanical model designed to predict, simultaneously, the strain-rate and the isostatic and thermal evolution of convergent orogenesis, subject to a constant driving force. An appealing aspect of this formulation is that, in contrast to models that have attempted to explain event cyclicity through episodic processes acting on the orogenic boundaries, it accounts for the repeated occurrence of events through processes inherent to the orogenic system itself.

Soviet geological maps of the Prince Charles Mountains, East Antarctic Shield

The results of the Soviet geological mapping program in the Prince Charles Mountains (Australian Antarctic Territory) are presented as the first major publication of the Soviet mapping program from 1983 and 1991. Except for a map of Else Platform, the maps published in this paper represent all maps completed by the 28th to 35th Soviet Antarctic Expeditions in the Prince Charles Mountains. The maps cover four suggested tectonic provinces, the Beaver Belt, Lambert Province, Fisher Belt and Ruker Terrane. The four terranes show a continuous increase in metamorphic grade from south to north. The Ruker Terrane is of a greenschist and lower amphibolite facies metamorphic grade, the Lambert Province consists of retrogressively metamorphosed Beaver Belt granulite and prograde metamorphosed greenschist of the Ruker Terrane and the Beaver Belt consists of high grade granulite. The Fisher Belt is a greenstone belt which is interpreted to be thrust later against the Lambert Province. Rock‐types of all four terranes a...

On the timing relationship between fluid production and metamorphism in metamorphic piles: Some implications for the origin of post-metamorphic gold mineralisation

Abstract The relative timing of metamorphism at different crustal levels is quantified for a range of metamorphic environments associated with convergent deformation. We characterise different metamorphic environments by making simple assumptions about the nature of heat input and the nature of heat withdrawal from the terrane. If internal heat production is the major heating agent during crustal thickening, then deep crustal levels reach their metamorphic temperature peak later than shallow levels. In terranes heated from below, such relationships apply only if erosion commenced early in the thermal history or if erosion rates are fast. Quantitative knowledge of these relations can be used together with simple assumptions about prograde fluid release to predict the likely timing of fluid passage through rocks higher in the column. In terranes with deep-later characteristics the evidence for fluid penetration may be preserved in the form of mineralised quartz veins or late stage alteration. Within our simple parameterisation of crustal heat sources we can use the grade of a metamorphic rock together with field evidence on the timing of metamorphism, igneous intrusion and deformation in order to constrain the time scale of the underlying thermal perturbation and erosion process for a given terrane. We demonstrate that time intervals up to several tens of million years may separate metamorphism and fluid emplacement in model evolutions that are scaled to describe greenschist facies metamorphic terranes that formed at low pressure. Such timing relationships are in fact recorded in many greenschist facies metamorphic terranes that host late stage mesothermal gold deposits. We suggest therefore that there is no conflict between the observation that the emplacement of many gold deposits occurred demonstrably after peak metamorphism of their host rocks and the interpretation that metamorphic fluids are responsible for the mineralisation.

Thermal buffering effects at the solidus. Implications for the equilibration of partially melted metamorphic rocks

Abstract A simple thermal model is used to investigate to what extent buffering effects of latent heat of fusion may prolong the thermal evolution of a one-dimensional, statically cooling, partially or completely melted heat source. For a melting model appropriate for crustal rocks, the time of cooling of rocks through the solidus may be about tripled by this process. Depending on the melting model, cooling may halt completely near the solidus for time spans comparable to the thermal time constant of the heat source. In order to test the influence of this buffering effect on the equilibration of partially melted metamorphic rocks, these thermal model results are coupled with a simple diffusion model that relates the degree of equilibration of a simple assemblage to temperature and cooling rate. It is shown that, for a broad range of initial heat source temperatures, freezing of selected mineral equilibria may occur in a comparably narrow temperature range at and around the solidus. The calculations may have some relevance to low- P high- T metamorphic terranes in as much as they may provide an explanation for the widely observed equilibration and partial re-equilibration of rocks near the crystallisation temperatures of partial melts.

Thermometrically inferred cooling rates from the Plattengneis, Koralm region, Eastern Alps

Abstract Closure of FeMg exchange between garnet and biotite is used to estimate cooling rates from the Koralm region of the Austro-Alpine nappe complex east of the Tauern window in the Eastern Alps. Maximum cooling rates for a 700 m thick mylonitic shear zone, the Plattengneis, are at least an order of magnitude higher than those of surrounding country rocks. The cooling rate history inferred for the Plattengneis suggests exponentially decreasing cooling rates, whilst the cooling history of the surrounding country rocks is inferred to involve an approximately constant cooling rate. The fast cooling rates in the shear zone are interpreted as evidence for an additional energy input from an internal heat source, which may have been heat that was released during deformation, related to the Alpine nappe-stacking processes.

Mantle-lithospheric deformation and crustal metamorphism with some speculations on the thermal and mechanical significance of the Tauern Event, Eastern Alps

Changes in the thickness of the mantle lithosphere beneath convergent orogens through processes such as the pervasive thickening of the lithosphere and the convective thinning of the thermal boundary layer dictate the heat flow through the base of the crust and thus may profoundly influence the thermal evolution of crustal metamorphic terrains. Calculations based on one-dimensional thermal-energy balances show that when mantle lithosphere is substantially thickened and remains intact through the orogenic cycle, then low-temperature facies series, including blueschist and eclogite facies, may be preserved throughout the crust. In contrast, crust thickened above attenuated mantle lithosphere will develop much higher-temperature facies series. Potential-energy arguments suggest that changes in the thickness in the mantle lithosphere induce changes in the elevation and the potential energy stored within the lithosphere. Therefore, the response to mantle-lithospheric deformation observable in crustal metamorphic terrains should not only be recorded in the thermal regime but also in changes in the incremental strain history in as much as it reflects the force balance operating within the orogen. For example, mantle-lithospheric thinning beneath thickened crust may cause an increase in crustal temperatures synchronous with the termination of convergent strain, or, for large reductions in mantle-lithospheric thickness, with the onset of extensional deformation. Such histories are recorded in a number of young metamorphic terrains, for example the Eastern Alps which may provide an important thermal and isostatic record of the mantle-lithospheric response to convergent deformation. Here, we show how application of these ideas may lead to new insights into some outstanding problems concerning the thermal and mechanical evolution of the Eastern Alps during the Tertiary Tauern Metamorphic Event.

Geological observations in high‐grade mid‐Proterozoic rocks from Else Platform, northern Prince Charles mountains region, east Antarctica

Granulite facies rocks on Else Platform in the northern Prince Charles Mountains, east Antarctica, consist of metasedimentary gneiss extensively intruded by granitic rocks. The dominant rock type is a layered garnetbiotite‐bearing gneiss intercalated with minor garnet‐cordierite‐sillimanite gneiss and calc‐silicate. Voluminous megacrystic granite intruded early during a mid‐Proterozoic (ca 1000 Ma) granulite event, M1, widely recognized in east Antarctica. Peak metamorphic conditions for M1 are in the range of 650–750 MPa at ∼800°C and were associated with the development of a gneissic foliation, S1 and steep east‐plunging lineation, L1. Strain partitioning during progressive non‐coaxial deformation formed large D2 granulite facies south‐dipping thrusts, with a steep, east‐plunging lineation. In areas of lower D2 strain, large‐scale upright, steep east‐plunging fold structures formed synchronously with the D2 high‐strain zones. Voluminous garnet‐bearing leucogneiss intruded at 940 ±20 Ma and was deformed ...

Geology and structure of Depot Peak, MacRobertson Land. More evidence for the continuous extent of the 1000 Ma event of East Antarctica

The 1000 Ma event of East Antarctica is a granulite facies metamorphic event that affected rocks underlying more than 1000 km of the Antarctic coastline between 50°E and 70°E. The limits of the belt are defined by isotopic dating but attempts to define the belt on metamorphic grounds are sparse. Depot Peak provides an important link for determining the history of this belt as it is the only outcrop linking the Prince Charles Mountains to the Framnes Mountains and the western parts of the Rayner Complex. Much of the Prince Charles Mountains are also 1000 Ma old but much of their metamorphic history is yet to be established. The rocks at Depot Peak are garnet and cordierite‐bearing gneisses of low pressure granulite facies that were intruded by a large sheet of garnet granite prior to metamorphism and deformation. They experienced two deformation phases, D1 and D2, and they record two different metamorphic events around 560 MPa, 700°C (M1) and 510 MPa, 600°C (M2). It is difficult to demonstrate to what exte...