Søren B. Nielsen

Glacial effects limiting mountain height

The height of mountain ranges reflects the balance between tectonic rock uplift, crustal strength and surface denudation. Tectonic deformation and surface denudation are interdependent, however, and feedback mechanisms—in particular, the potential link to climate—are subjects of intense debate. Spatial variations in fluvial denudation rate caused by precipitation gradients are known to provide first-order controls on mountain range width, crustal deformation rates and rock uplift. Moreover, limits to crustal strength are thought to constrain the maximum elevation of large continental plateaus, such as those in Tibet and the central Andes. There are indications that the general height of mountain ranges is also directly influenced by the extent of glaciation through an efficient denudation mechanism known as the glacial buzzsaw. Here we use a global analysis of topography and show that variations in maximum mountain height correlate closely with climate-controlled gradients in snowline altitude for many high mountain ranges across orogenic ages and tectonic styles. With the aid of a numerical model, we further demonstrate how a combination of erosional destruction of topography above the snowline by glacier-sliding and commensurate isostatic landscape uplift caused by erosional unloading can explain observations of maximum mountain height by driving elevations towards an altitude window just below the snowline. The model thereby self-consistently produces the hypsometric signature of the glacial buzzsaw, and suggests that differences in the height of mountain ranges mainly reflect variations in local climate rather than tectonic forces.

How Epigallocatechin Gallate Can Inhibit α-Synuclein Oligomer Toxicity in Vitro

Background: Protein oligomers are implicated as cytotoxic membrane-disrupting agents in neurodegenerative diseases. Results: The small molecule EGCG, which inhibits α-synuclein oligomer toxicity, moderately reduces membrane binding and immobilizing the oligomer C-terminal tail. Conclusion: The α-synuclein oligomer may disrupt membranes by vesicle destabilization rather than pore formation. Significance: Limited reduction of oligomer membrane affinity may be sufficient to prevent cytotoxicity. Oligomeric species of various proteins are linked to the pathogenesis of different neurodegenerative disorders. Consequently, there is intense focus on the discovery of novel inhibitors, e.g. small molecules and antibodies, to inhibit the formation and block the toxicity of oligomers. In Parkinson disease, the protein α-synuclein (αSN) forms cytotoxic oligomers. The flavonoid epigallocatechin gallate (EGCG) has previously been shown to redirect the aggregation of αSN monomers and remodel αSN amyloid fibrils into disordered oligomers. Here, we dissect EGCGs mechanism of action. EGCG inhibits the ability of preformed oligomers to permeabilize vesicles and induce cytotoxicity in a rat brain cell line. However, EGCG does not affect oligomer size distribution or secondary structure. Rather, EGCG immobilizes the C-terminal region and moderately reduces the degree of binding of oligomers to membranes. We interpret our data to mean that the oligomer acts by destabilizing the membrane rather than by direct pore formation. This suggests that reduction (but not complete abolition) of the membrane affinity of the oligomer is sufficient to prevent cytotoxicity.

Heat flow density values and paleoclimate determined from stochastic inversion of four temperature-depth profiles from the Superior Province of the Canadian Shield

Abstract It has long been recognized that terrestrial heat flow density observations from boreholes may be biased due to non-steady surface temperature conditions. This calls for paleoclimatic corrections which have conventionally been obtained by solving a forward problem based on the equation of heat conduction and models of the surface temperature history and the thermal properties structure. While computationally convenient, the forward method fails 1. (1) to provide a stringent quantitative representation of all available data with confidence limits 2. (2) to extract all available information in an optimum manner 3. (3) to provide confidence limits on the parameters of interest. A stochastic inverse method which overcomes these shortcomings is described. The method treats the surface temperature history, the thermal conductivity structure and the background heat flow as unknowns which have been constrained a priori with soft bounds. The bounds ensure numerical stability and prevent unrealistic solutions when the a-priori hypothesis is safe. The technique is applied to geothermal data from four 600-m deep boreholes in the Superior Province of the Canadian Shield. The examples show how it is possible to constrain simultaneously the recent surface temperature history and the steady-state background heat flow from shallow geothermal data, and furthermore show how uncertainty about the average ground temperature during the Wisconsin Glaciation may be taken into account.

Dynamics of Mid-Palaeocene North Atlantic rifting linked with European intra-plate deformations.

The process of continental break-up provides a large-scale experiment that can be used to test causal relations between plate tectonics and the dynamics of the Earth’s deep mantle. Detailed diagnostic information on the timing and dynamics of such events, which are not resolved by plate kinematic reconstructions, can be obtained from the response of the interior of adjacent continental plates to stress changes generated by plate boundary processes. Here we demonstrate a causal relationship between North Atlantic continental rifting at ∼62 Myr ago and an abrupt change of the intra-plate deformation style in the adjacent European continent. The rifting involved a left-lateral displacement between the North American-Greenland plate and Eurasia, which initiated the observed pause in the relative convergence of Europe and Africa. The associated stress change in the European continent was significant and explains the sudden termination of a ∼20-Myr-long contractional intra-plate deformation within Europe, during the late Cretaceous period to the earliest Palaeocene epoch, which was replaced by low-amplitude intra-plate stress-relaxation features. The pre-rupture tectonic stress was large enough to have been responsible for precipitating continental break-up, so there is no need to invoke a thermal mantle plume as a driving mechanism. The model explains the simultaneous timing of several diverse geological events, and shows how the intra-continental stratigraphic record can reveal the timing and dynamics of stress changes, which cannot be resolved by reconstructions based only on plate kinematics.

Small-Scale Mantle Convection Produces Stratigraphic Sequences in Sedimentary Basins

Changes in the Rocks Changing sea level or major tectonic events, such as continental collisions, shift stratigraphic sequences by changing the depositional environment where certain rock types form. For example, a deep marine environment where limestone formation is favored may shift relatively quickly to a near-shore environment favoring sandstone formation because the relative sea level has dropped several meters. Petersen et al. (p. 827; see the Perspective by Müller), however, suggest that small-scale convection in the mantle may also induce appreciable changes in the sequence of sedimentary deposits. Using a modeling approach, they found that this is possible on a small scale (that is, just a few hundreds of kilometers) over variable time scales. Thus, while the co-occurrence of sedimentary deposit sequences at regional and global scales can allow sedimentary rocks to serve as markers of marine environments, it should be kept in mind that local changes in surface movements may also manifest themselves in the rock record. Sedimentary stratigraphic sequences may result from sea-level changes that were induced by small-scale mantle convection. Cyclic sedimentary deposits link stratigraphic sequences that are now geographically distant but were once part of the same depositional environment. Some of these sequences occur at periods of 2 to 20 million years, and eustatic sea-level variations or regional tectonic events are likely causes of their formation. Using numerical modeling, we demonstrate that small-scale mantle convection can also cause the development of stratigraphic sequences through recurrent local and regional vertical surface movements. Small-scale convection-driven stratigraphic sequences occur at periods of 2 to 20 million years and correlate only at distances up to a few hundred kilometers. These results suggest that previous sequence stratigraphic analyses may contain erroneous conclusions regarding eustatic sea-level variations.

A numerical dynamic model for the Norwegian–Danish Basin

Abstract A 2D numerical dynamic model for the Late Palaeozoic and Mesozoic formation and evolution of the Norwegian–Danish Basin (NDB) is presented. This basin developed in the eastern part of the Northern Permian Basin within the system of Northwest European sedimentary basins. The basin forming processes are modelled in a lithospheric plane-strain model along a 400-km profile across the basin. Layered rheology is used with elasto-plastic behaviour in the brittle parts and elasto-viscous behaviour at higher temperatures and pressures. The mechanical equations of equilibrium and the heat equation are solved by the finite element technique. The model contains two tectonic events. Basin initiation in Late Carboniferous and Early Permian times is modelled by combined lithospheric heating and lithospheric extension. Additional lithospheric extension, however, less pronounced and taking place over a longer period, is introduced during the Triassic. Isostatic effects associated with lithospheric extension combined with thermal contraction and sediment loading accounts for up to 6 km of sediments of which 2–3 km are of Triassic age. The model shows a generally good agreement with observations in terms of basin geometry, sediment layer thicknesses, stratigraphy, crustal thickness, and heat flow. In the central parts of the basin, the model implies a crustal thinning factor of about 1.5—from 35 km to about 23 km, close to that observed in seismic data.

Plate-wide stress relaxation explains European Palaeocene basin inversions

During Late Cretaceous and Cenozoic times, many Palaeozoic and Mesozoic rifts and basin structures in the interior of the European continent underwent several phases of inversion (the process of shortening a previously extensional basin). The main phases occurred during the Late Cretaceous and Middle Palaeocene, and have been previously explained by pulses of compression, mainly from the Alpine orogen. Here we show that the main phases differed both in structural style and cause. The Cretaceous phase was characterized by narrow uplift zones, reverse activation of faults, crustal shortening, and the formation of asymmetric marginal troughs. In contrast, the Middle Palaeocene phase was characterized by dome-like uplift of a wider area with only mild fault movements, and formation of more distal and shallow marginal troughs. A simple flexural model explains how domal, secondary inversion follows inevitably from primary, convergence-related inversion on relaxation of the in-plane tectonic stress. The onset of relaxation inversions was plate-wide and simultaneous, and may have been triggered by stress changes caused by elevation of the North Atlantic lithosphere by the Iceland plume or the drop in the north–south convergence rate between Africa and Europe.

Physical explanation of the formation and evolution of inversion zones and marginal troughs

Inversion zones are elongate structures, some tens of kilometers wide and up to hundreds of kilometers long, that have deformed in response to compression and produced topography. Inversion zones in the Alpine foreland are mainly associated with Mesozoic grabens and troughs, and although very important in the geologic picture, the conditions of their formation and evolution and their regional geologic significance are not entirely understood. The internal structure of inversion zones is variable and depends on details in the pre-inversion setting, the inversion-inducing stress field, and the sedimentary fill. However, on a larger scale, most inversion zones share certain principal observational features, which sample the physical structure and the rheologic properties of the lithosphere and thereby provide an opportunity to test hypotheses of lithospheric rheology and dynamics. The quantitative model presented in this paper explains how inversion zones and the associated marginal troughs are related to lithospheric zones of differential shortening and regional isostatic compensation of the induced topography.

The post-Triassic evolution of the Sorgenfrei–Tornquist Zone — results from thermo-mechanical modelling

Abstract The Sorgenfrei–Tornquist Zone (STZ) is part of the Fennoscandian Border Zone separating the Danish Basin from the Fennoscandian Shield. The STZ as a structural element is of Palaeozoic origin and represents the north-westerly segment of the Tornquist–Teisseyre Zone, which separates the younger West European crust from the older East European Platform and extends from the Black Sea to the eastern North Sea area. The STZ was reactivated in Triassic–Jurassic extension and Late Cretaceous and Paleogene compression. This paper investigates the regional geological consequences of the reactivations by quantitative modelling along a profile across the STZ in the Danish area. The numerical model invokes elastic, viscous and plastic deformations of the lithosphere as well as surface processes governed by erosion, sedimentation and lateral transport under the influence of eustatic sea level variations and regional isostatic compensation. Surface processes and lithospheric mechanics are coupled through thermal blanketing effects and loading. The results, in general, address the regional geological consequences of the existence of intracontinental zones of structural weakness. More specifically the results show that the Late Cretaceous and Paleogene chalk depocentres in the Danish Basin are a direct consequence of the inversion of the STZ, and that the STZ inversion together with falling sea level in Cenozoic time are amongst the principal controlling factors in the geological evolution in the eastern North Sea area.

Paleocene initiation of Cenozoic uplift in Norway

Abstract The timing of Cenozoic surface uplift in NW Europe relies on the assumption that the sedimentary response in basins is synchronous with tectonic processes in the source areas. However, many of the phenomena commonly used to infer recent uplift may as well be a consequence of climate change and sea-level fall. The timing of surface uplift therefore remains unconstrained from the sedimentary record alone, and it becomes necessary to consider the constraints imposed by physically and geologically plausible tectonic mechanisms, which have a causal relation to an initiating agent. The gradual reversal of the regional stress field following the break-up produced minor perturbations to the thermal subsidence on the Norwegian Shelf and in the North Sea. Pulses of increased compression cannot be the cause of Cenozoic land surface uplift and accelerated Neogene basin subsidence. Virtually deformation-free regional vertical movements could have been caused by changes in the density column of the lithosphere and asthenosphere following the emplacement of the Iceland plume. A transient uplift component was produced as the plume displaced denser asthenosphere at the base of the lithosphere. This component decayed as the plume material cooled. Permanent uplift as a result of igneous underplating occurred in areas of a thin lithosphere (some Palaeozoic and Mesozoic basins) or for lithosphere under extension at the time of plume emplacement (the ocean-continent boundary). In areas of a thicker lithosphere (East Greenland, Scotland and Norway) plume emplacement may have triggered a Rayleigh-Taylor instability, causing partial lithospheric delamination and associated transient surface uplift at a decreasing rate throughout Cenozoic time. A possible uplift history for the adjacent land areas hence reads: initial transient surface uplift around the break-up time at 53 Ma caused by plume emplacement, and permanent tectonic uplift caused by lithospheric delamination and associated lithospheric heating. The permanent tectonic uplift increased through Cenozoic time at a decreasing rate. Denudation acted on this evolving topography and reduced the average surface elevation, but significantly increased the elevation of the summit envelope. The marked variations in the sedimentary response in the basins were caused by climatic variations and the generally falling eustatic level. This scenario bridges the gap between the ideas of Paleocene-Eocene uplift versus repeated Cenozoic tectonic activity: the tectonic uplift history was initiated by the emplacement of the Iceland plume, but continued throughout Cenozoic time as a consequence of early plume emplacement, with climatic and eustatic control on denudation. The mechanism is consistent with topography, heat flow, crustal structure, and the Bouguer gravity of Norway, and may be applicable also to East Greenland.