Friedhelm von Blanckenburg

Slab breakoff: A model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens

We present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision (slab breakoff), allowing an explanation of syn- to post-collisional magmatism and metamorphism. Continental collisions are preceded by subduction of dense oceanic lithosphere, and followed by attempted subduction of buoyant continental lithosphere. This situation of opposing buoyancy forces leads to extensional deformation in the subducted slab. A narrow rifting mode of deformation will result if there is strain localization. Slab breakoff results. We have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere, and have compared it with the change in buoyancy force during continental sbduction. Whether breakoff will occur, and the depth at which it will occur, is a strong function of temperature and hence the subduction velocity. For a subduction velocity of 1 cm/yr breakoff could occur at depths of between 50 and 120 km, while at higher velocities it is still likely to occur, but at deeper depths. As a result of the rifting during breakoff, the asthenosphere upwells into the narrow rift, and following breakoff it impinges on the mechanical lithosphere of the overriding plate. The resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere, producing basaltic magmatism that leads to granitic magmatism in the crust. Dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca. 50 km. Breakoff removes the force at the downdip side of the continental crust, while the enhanced heating leads to a reduction of the strength of the underlying crust. Both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface, leading to the rapid exhumation of eclogite facies continental crust. The cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen. We have tested the variety of predicted expressions of slab breakoff in the geological record of the Alps, the Aegean Islands, and the Dabie Shan. A comparison of the various belts highlights (1) that the magmatism and metamorphism are found near the suture and the centre of the orogen, demonstrating the general steepening of suture during collision, (2) that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn-orogenic magmatism, and (3) that the melts can be emplaced in both compressive and extensional environments. We suggest that slab breakoff is an important step in the evolution of many orogenic belts, and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra-high-pressure metamorphics.

Molybdenum isotope records as a potential new proxy for paleoceanography

New high-precision isotope ratios of dissolved Mo in seawater from different ocean basins and depths show a homogeneous isotope composition (‘mean ocean water 98 Mo/ 95 Mo’ (MOMO)), as expected from its long ocean residence time (800 kyr). This composition appears to have been constant for the past 60 Myr at a 1^3 Myr time resolution as indicated from thick sections of Fe^Mn crusts from the Atlantic and Pacific. These records yield a constant offset from MOMO (average of 33.1 and 32.9x). They are similar to our new data on recent oxic Mo sinks: pelagic sediments and six Fe^Mn crust surface layers range from 32.7 to 32.9x and 32.7 to 33.1x, respectively. Recent suboxic Mo sinks from open ocean basins display heavier and more variable isotope ratios (30.7 to 31.6x relative to MOMO). Crustal Mo sources were characterized by measuring two granites (and a mild acid leach of one granite), seven volcanic rocks and two clastic sediments. All show a narrow range of compositions (32.0 to 32.3x). These data indicate that isotope fractionation by chemical weathering and magmatic processes is insignificant on a global scale. They therefore represent good estimates of the composition of dissolved Mo input to the oceans and that of the average continental crust. Thus, the Mo input into the oceans appears to be distributed into lighter oxic sinks and heavier reducing sinks. This is consistent with steady-state conditions in the modern ocean. The constant isotope offset between oxic sediments and seawater suggests that the relative amounts of oxic and reducing Mo removal fluxes have not varied by more than 10% over the last 60 Myr. An equilibrium fractionation process is proposed assuming that Mo isotope fractionation occurs between (dominant) MoO 23 4 and (minor) Mo(OH)6 species in solution, of which the latter is preferentially scavenged. @ 2003 Elsevier Science B.V. All rights reserved.

Long-term stability of global erosion rates and weathering during late-Cenozoic cooling

Over geologic timescales, CO2 is emitted from the Earth’s interior and is removed from the atmosphere by silicate rock weathering and organic carbon burial. This balance is thought to have stabilized greenhouse conditions within a range that ensured habitable conditions. Changes in this balance have been attributed to changes in topographic relief, where varying rates of continental rock weathering and erosion are superimposed on fluctuations in organic carbon burial. Geological strata provide an indirect yet imperfectly preserved record of this change through changing rates of sedimentation. Widespread observations of a recent (0–5-Myr) fourfold increase in global sedimentation rates require a global mechanism to explain them. Accelerated uplift and global cooling have been given as possible causes, but because of the links between rates of erosion and the correlated rate of weathering, an increase in the drawdown of CO2 that is predicted to follow may be the cause of global climate change instead. However, globally, rates of uplift cannot increase everywhere in the way that apparent sedimentation rates do. Moreover, proxy records of past atmospheric CO2 provide no evidence for this large reduction in recent CO2 concentrations. Here we question whether this increase in global weathering and erosion actually occurred and whether the apparent increase in the sedimentation rate is due to observational biases in the sedimentary record. As evidence, we recast the ocean dissolved 10Be/9Be isotope system as a weathering proxy spanning the past ∼12 Myr (ref. 14). This proxy indicates stable weathering fluxes during the late-Cenozoic era. The sum of these observations shows neither clear evidence for increased erosion nor clear evidence for a pulse in weathered material to the ocean. We conclude that processes different from an increase in denudation caused Cenozoic global cooling, and that global cooling had no profound effect on spatially and temporally averaged weathering rates.

Time calibration of a PT-path from the Western Tauern Window, Eastern Alps: the problem of closure temperatures

AbstractNew Hornblende K-Ar and 39Ar-40Ar and mica Rb-Sr and K-Ar ages are used to place specific timemarks on a well-constrained pressure-temperature path for the late Alpine metamorphism in the Western Tauern Window. After identification of excess 40Ar, the closure behavior of Ar in hornblende is compared with that of Sr and Ar in phengite and biotite. Samples were collected in three locations, whose maximum temperatures were 570° C (Zemmgrund), 550° C (Pfitscher Joch), and 500–540° C (Landshuter Hütte).The average undisturbed age sequence found is: Phengite Rb-Sr (20 Ma)>hornblende K-Ar (18 Ma)>phengite K-Ar (15 Ma)>biotite Rb-Sr, K-Ar (13.3 Ma)>apatite FT (7 Ma). Except for the phengite Rb-Sr age, the significance of which is debatable, all ages are cooling ages. No compositional effects are seen for closure in biotite. Additionally, Rb-Sr phengite ages from shearzones possibly indicate continuous shearing from 20 to 15 Ma, with reservations regarding the validity of the initial Sr correction and possible variations of the closure temperatures. The obviously lower closure temperature (Tc) for Ar in these hornblendes than for Sr in the unsheared phengites indicates that the Tc sequence in the Western Tauern Window is different from those observed in other terrains. In spite of this discrepancy, valuable geological conclusions can be drawn if the application of closure temperatures is limited to this restricted area with similar T, P and

Restoring dense vegetation can slow mountain erosion to near natural benchmark levels

P_{H_2 O}

Modeling novel stable isotope ratios in the weathering zone

: (1) All ages of samples located on equal metamorphic isotherms decrease from east to west by about 1 Ma which is the result of a westward tilting of the Tauern Window during uplift. (2) In a PT-path, the undisturbed cooling ages yield constantly decreasing uplift rates from 3.6 mm/a to 0.1 mm/a. (3) Use of recently published diffusion data for Ar in hornblende (Tc=520° C) and biotite (Tc=320° C) suggests an extrapolated phengite closure temperature for Sr at 550° C. This suggests that the prograde thermal metamorphism at this tectonic level of the Tauern Window lasted until some 20 Ma ago.

Nd-, Sr-, O-isotopic and chemical evidence for a two-stage contamination history of mantle magma in the Central-Alpine Bergell intrusion

We quantify the difference between the human-caused sediment yield and the natural rates of soil production and bedrock erosion in a now largely deforested tropical highland. The present-day rate of soil loss in the Upper Mahaweli catchment, Sri Lanka, is calculated by using suspended river-load fluxes. These data provide spatially averaged sediment yields of 130-2100 t-km - 2 .yr - 1 . Local rates of soil loss from agricultural plots on hillslopes are as high as 7000 t.km - 2 .yr - 1 . By comparison, natural rates of sediment generation, as determined by measuring cosmogenic 1 0 Be in quartz from sediments and soils, are only 13-30 t.km - 2 .yr - 1 . The natural rates presented here provide a benchmark against which recent erosion rates, determined by various sediment gauging techniques, can be referenced. In the Sri Lankan highlands, these results suggest that soil is now being lost 10-100 times faster from agriculturally utilized areas than it is being produced.

Argon retentivity and argon excess in amphiboles from the garbenschists of the Western Tauern Window, Eastern Alps

Tropical mountain areas may undergo rapid land degradation as demographic growth and intensified agriculture cause more people to migrate to fragile ecosystems. To assess the extent of the resulting damage, an erosion rate benchmark against which changes in erosion can be evaluated is required. Benchmarks reflecting natural erosion rates are usually not provided by conventional sediment fluxes, which are often biased due to modern land use change, and also miss large, episodic events within the measuring period. To overcome this, we combined three independent assessment tools in the southern Ecuadorian Andes, an area that is severely affected by soil erosion. First, denudation rates from cosmogenic nuclides in river sediment average over time periods of 1–100 k.y. and establish a natural benchmark of only 150 ± 100 t km −2 yr −1 . Second, we find that land use practices have increased modern sediment yields as derived from reservoir sedimentation rates, which average over periods of 10–100 yr to as much as 15 × 10 3 t km −2 yr −1 . Third, our land cover analysis has shown us that vegetation cover exerts first-order control over present-day erosion rates at the catchment scale. Areas with high vegetation density erode at rates that are characteristically similar to those of the natural benchmark, regardless of whether the type of vegetation is native or anthropogenic. Therefore, our data suggest that even in steep mountain environments sediment fluxes can slow to near their natural benchmark levels with suitable revegetation programs. A set of techniques is now in place to evaluate the effectiveness of erosion mitigation strategies.