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41.
On the role of subducting oceanic plateaus in the development of shallow flat subduction 总被引:2,自引:0,他引:2
Oceanic plateaus, aseismic ridges or seamount chains all have a thickened crust and their subduction has been proposed as a possible mechanism to explain the occurrence of flat subduction and related absence of arc magmatism below Peru, Central Chile and at the Nankai Trough (Japan). Their extra compositional buoyancy could prohibit the slab from sinking into the mantle. With a numerical thermochemical convection model, we simulated the subduction of an oceanic lithosphere that contains an oceanic crustal plateau of 18-km thickness. With a systematic variation, we examined the required physical parameters to obtain shallow flat subduction. Metastability of the basaltic crust in the eclogite stability field is of crucial importance for the slab to remain buoyant throughout the subduction process. In a 44-Ma-old subducting plate, basalt must be able to survive a temperature of 600–700 °C to keep the plate buoyant sufficiently long to cause a flat-slab segment. We found that the maximum yield stress in the slab must be limited to about 600 MPa to allow for the necessary bending to the horizontal. Young slabs show flat subduction for larger parameter ranges than old slabs, since they are less gravitationally unstable and show less resistance against bending. Hydrous weakening of the mantle wedge area and lowermost continent are required to allow for the necessary deformation of a change in subduction style from steep to flat. The maximum flat slab extent is about 300 km, which is sufficient to explain the observed shallow flat subduction near the Nankai Trough (Japan). However, additional mechanisms, such as active overthrusting by an overriding continental plate, need to be invoked to explain the flat-slab segments up to 500 km long below Peru and Central Chile. 相似文献
42.
Numerical investigation of alternative fracture stiffness measures and their respective scaling behaviours
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We study the mechanical deformation of fractures under normal stress, via tangent and specific fracture stiffnesses, for different length scales using numerical simulations and analytical insights. First, we revisit an equivalent elastic layer model that leads to two expressions: the tangent stiffness is the sum of an “intrinsic” stiffness and the normal stress, and the specific stiffness is the tangent stiffness divided by the fracture aperture at current stress. Second, we simulate the deformation of rough fractures using a boundary element method where fracture surfaces represented by elastic asperities on an elastic half‐space follow a self‐affine distribution. A large number of statistically identical “parent” fractures are generated, from which sub‐fractures of smaller dimensions are extracted. The self‐affine distribution implies that the stress‐free fracture aperture increases with fracture length with a power law in agreement with the chosen Hurst exponent. All simulated fractures exhibit an increase in the specific stiffness with stress and an average decrease with increase in length consistent with field observations. The simulated specific and tangent stiffnesses are well described by the equivalent layer model provided the “intrinsic” stiffness slightly decreases with fracture length following a power law. By combining numerical simulations and the analytical model, the effect of scale and stress on fracture stiffness measures can be easily separated using the concept of “intrinsic” stiffness. We learn that the primary reason for the variability in specific stiffness with length comes from the fact that the typical aperture of the self‐affine fractures itself scales with the length of the fractures. 相似文献
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Jakob Wallinga Adrie J.J. Bos Pieter Dorenbos Andrew S. Murray Jeroen Schokker 《Quaternary Geochronology》2007,2(1-4):216-221
Infrared-stimulated luminescence (IRSL) dating of feldspars has the potential to date deposits beyond the age range of quartz optical (OSL) dating. Successful application of feldspar IRSL dating is, however, often precluded due to anomalous-fading, the tunnelling of electrons from one defect site to another. In this paper we test procedures proposed for anomalous-fading correction by comparing feldspar IRSL and quartz OSL dating results on a suite of samples from continental deposits from the southeastern Netherlands. We find that even after anomalous-fading correction IRSL ages underestimate the burial age of the deposits and argue that this may be a consequence of a dependency of anomalous fading rate on the dose rate and on the absorbed dose. 相似文献
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Philip J. Ward Hans Renssen Jeroen C. J. H. Aerts Peter H. Verburg 《Climatic change》2011,106(2):179-202
We used a calibrated coupled climate–hydrological model to simulate Meuse discharge over the late Holocene (4000–3000 BP and
1000–2000 AD). We then used this model to simulate discharge in the twenty-first century under SRES emission scenarios A2
and B1, with and without future land use change. Mean discharge and medium-sized high-flow (e.g. Q99) frequency are higher in 1000–2000 AD than in 4000–3000 BP; almost all of this increase can be attributed to the conversion
of forest to agriculture. In the twentieth century, mean discharge and the frequency of medium-sized high-flow events are
higher than in the nineteenth century; this increase can be attributed to increased (winter half-year) precipitation. Between
the twentieth and twenty-first centuries, anthropogenic climate change causes a further increase in discharge and medium-sized
high-flow frequency; this increase is of a similar order of magnitude to the changes over the last 4,000 years. The magnitude
of extreme flood events (return period 1,250-years) is higher in the twenty-first century than in any preceding period of
the time-slices studied. In contrast to the long-term influence of deforestation on mean discharge, changes in forest cover
have had little effect on these extreme floods, even on the millennial timescale. 相似文献
48.
Flood risks of deltaic areas increase because of population growth, economic development, land subsidence and climatic changes such as sea-level rise. In this study, we analyze trends in flood exposure by combining spatially explicit historical, present, and future land-use data with detailed information on the maximum flood inundation in the Netherlands. We show that the total amount of urban area that can potentially become inundated due to floods from the sea or main rivers has increased six-fold during the 20th century, and may double again during the 21st century. Moreover, these developments took, and probably will take, place in areas with progressively higher potential inundation depths. Potential flood damage has increased exponentially over the 20th century (16 times) and is expected to continue to increase exponentially (∼ten-fold by 2100 with respect to 2000) assuming a high economic growth scenario. Flood damages increase more moderately (two- to three-fold by 2100 with respect to 2000) assuming a low growth scenario. The capacity to deal with catastrophic flood losses - expressed as the ratio damage/GDP - will, however, decrease slightly in the low growth scenario (by about 20%). This trend deviates from the historical trend of the 20th century, which shows an increasing capacity to cope with flood damage (almost doubling). Under the high growth scenario the capacity to deal with such losses eventually increases slightly (by about 25%). These findings illustrate that, despite higher projections of potential flood damage, high economic growth scenarios may not necessarily be worse than low growth scenarios in terms of the impact of floods. 相似文献
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