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1.
The Sheep Mountain‐Little Sheep Mountain Anticlines, Bighorn Basin (USA) formed as basement‐cored Laramide structures in the formerly undeformed foreland of the thin‐skinned Sevier orogen. We take advantage of the well‐constrained microstructural network there to reconstruct differential stress magnitudes that prevailed during both Sevier and Laramide layer‐parallel shortening (LPS), before the onset of large‐scale folding. Differential stress magnitudes determined from tectonic stylolites are compared and combined to previous stress estimates from calcite twinning paleopiezometry in the same formations. During stress loading related to LPS, differential stress magnitudes transmitted from the distant Sevier thin‐skinned orogen into the sedimentary cover of the Bighorn basin (11–43 MPa) are higher than the differential stress magnitudes accompanying the early stage of LPS related to the thick‐skinned Laramide deformation (2–19 MPa). This study illustrates that the tectonic style of an orogen affects the transmission of early orogenic stress into the stable continental interior. 相似文献
2.
Peter H. Koehn 《Climatic change》2006,77(3-4):377-413
The United States and China are responsible for nearly 40 per cent of total annual greenhouse gas emissions. Global climatic stabilization cannot succeed without the participation of people and organizations in the PRC and the USA. Given the likelihood of continued political paralysis at the national level in both countries, it is important to assess the strengths, weaknesses, and potential of nongovernmental actors in bringing about durable reductions in GHG emissions. Consumption trajectories present one of the most intractable barriers to anthropogenic GHG-emissions reductions and to long-term climatic stabilization. Nordlund and Garvill's (2003) model is adapted and applied in assessing prospects for value change, specific problem awareness, and sustainable-consumption behavior in the PRC and USA. The contributions of nonstate actors are elucidated when issue bundling (specifically links with stress reduction and healthy futures) and principle bundling are incorporated into the explanatory framework. The involvement of transnationally competent activists in emissions-mitigation projects, value change, and durable sustainable-consumption practices provides a beacon for future possibilities. Chinese transnationals are a key overlooked and underestimated piece in the current emissions-reduction puzzle because of their critical position at the intersection of values, behavior change, and nongovernmental institutions directly and indirectly involved in climatic-change mitigation. Given their special U.S./P.R.C. leverage, other actors concerned with building a sustainable-consumption movement and with climatic stabilization are likely to benefit from devoting increased attention and responsibilities to this group. 相似文献
3.
D. Koehn D. G. A. M. Aerden P. D. Bons C. W. Passchier 《Journal of Metamorphic Geology》2001,19(2):217-231
Antitaxial non‐deforming strain fringes from Lourdes, France, show complex quartz, calcite and chlorite fibre patterns that grew around pyrite in a slate during non‐coaxial progressive deformation. Development of these fringes was modelled using a computer program ‘Fringe Growth 2.0’ which can simulate incremental growth of crystal fibres around core‐objects of variable shape. It uses object‐centre paths as input, which are obtained from fibre patterns in thin section. The numerical experiments produced fibre patterns that show complex intergrowth of displacement‐controlled, face‐controlled and intermediate fibres similar to those in the natural examples. The direction of displacement‐controlled growth is only dependent on the relative movement between core‐object and fringe, so that core‐object rotation with respect to the fringe influences the fibre patterns and produces characteristic asymmetric fibre curvature. Object‐centre paths should be used for kinematic analysis of strain fringes instead of single fibres since these paths represent the fringe as a whole. The length along the path can be interpreted in terms of finite strain and path curvature in terms of rigid body rotation of fringes with respect to an external reference frame. 相似文献
4.
Stress sensitivity of stylolite morphology 总被引:1,自引:0,他引:1
Marcus Ebner Daniel Koehn Renaud Toussaint François Renard Jean Schmittbuhl 《Earth and Planetary Science Letters》2009,277(3-4):394-398
Stylolites are rough surfaces that form by localized stress-induced dissolution. Using a set of limestone rock samples collected at different depths from a vertical section in Cirque de Navacelles (France), we study the influence of the lithostatic stress on the stylolites morphology on the basis of a recent morphogenesis model. We measured the roughness of a series of bedding-parallel stylolites and show that their morphology exhibits a scaling invariance with two self-affine scaling regimes separated by a crossover-length (L) at the millimeter scale consistent with previous studies. The importance of the present contribution is to estimate the stylolite formation stress σ from the sample position in the stratigraphic series and compare it to the crossover-length L using the expected relationship: L ~ σ ?2. We obtained a successful prediction of the crossover behavior and reasonable absolute stress magnitude estimates using relevant parameters: depth of stylolite formation between 300 to 600 m with corresponding normal stress in the range of 10–18 MPa. Accordingly, the stylolite morphology contains a signature of the stress field during formation and we thus suggest that stylolites could be used as paleo-stress gauges of deformation processes in the upper crust. 相似文献
5.
Till Sachau Daniel Koehn D. Sarah Stamps Michael Lindenfeld 《International Journal of Earth Sciences》2016,105(6):1729-1740
The Rwenzori Mountains in western Uganda form an active rift-transfer zone in the western branch of the East African Rift System. Here we quantify local stress fields in high resolution from field observations of fault structures to shed light on the complex, polyphase tectonics expected in transfer zones. We apply the multiple inverse method, which is optimized for heterogeneous fault-slip data, to the northern and central Rwenzori Mountains. Observations from the northern Rwenzori Mountains show larger heterogeneity than data from the central Rwenzori, including unexpected compressional features; thus the local stress field indicates polyphase transpressional tectonics. We suggest that transpression here is linked to rotational and translational movements of the neighboring Victoria block relative to the Rwenzori block that includes strong overprinting relationships. Stress inversions of data from the central Rwenzori Mountains indicate two distinct local stress fields. These results suggest that the Rwenzori block consists of smaller blocks. 相似文献
6.
Klemens Link Daniel Koehn Matthias G. Barth John V. Tiberindwa Erasmus Barifaijo Kevin Aanyu Stephen F. Foley 《International Journal of Earth Sciences》2010,99(7):1559-1573
Western Uganda is a key region for understanding the development of the western branch of the East African rift system and
its interaction with pre-existing cratonic lithosphere. It is also the site of the topographically anomalous Rwenzori Mountains,
which attain altitudes of >5000 m within the rift. New structural and geochronological data indicate that western Uganda south
and east of the Rwenzori Mountains consists of a WSW to ENE trending fold and thrust belt emplaced by thick-skinned tectonics
that thrust several slices of Proterozoic and Archaean units onto the craton from the south. The presence of Archaean units
within the thrust stack is supported by new Laser-ICP-MS U–Pb age determinations (2637–2584 Ma) on zircons from the Rwenzori
foothills. Repetition of the Paleoproterozoic units is confirmed by mapping the internal stratigraphy where a basal quartzite
can be used as marker layer, and discrete thrust units show distinct metamorphic grades. The thrust belt is partially unconformably
covered by a Neoproterozoic nappe correlated with the Kibaran orogenic belt. Even though conglomerates mark the bottom of
the Kibaran unit, intensive brittle fault zones and pseudotachylites disprove an autochthonous position. The composition of
volcanics in the Toro-Ankole field of western Uganda can be explained by the persistence of a cratonic lithosphere root beneath
the northwardly thrusted Archaean and Palaeoproterozoic rocks of westernmost Uganda. Volcanic geochemistry indicates thinning
of the lithosphere from >140 km beneath Toro-Ankole to ca. 80 km beneath the Virunga volcanic field about 150 km to the south.
We conclude that the western branch of the East African rift system was initiated in an area of thinner lithosphere with Palaeoproterozoic
cover in the Virunga area and has propagated northwards where it now abuts against thick cratonic lithosphere covered by a
thrust belt consisting of gneisses, metasediments and metavolcanics of Neoarchaean to Proterozoic age. 相似文献
7.
8.
Faulting of the lithosphere during extension and related rift-flank uplift: a numerical study 总被引:1,自引:1,他引:0
In this contribution, we present a new model of passive rifting and related rift-flank uplift. The numerical model is based
on a lattice spring network coupled with a viscous particle model so that we can simulate visco-elasto-plastic behaviour with
dynamic fault development. In our model, we show that rift-flank uplift can be achieved best when extension in the crust is
localized and the lower crust is strong so that major rift faults transect the whole crust. Uplift of rift flanks follows
a smooth function whereas down-throw in the rift basin takes place in steps. The geometry of the developing faults has also
an influence on the uplift; in this case, displacement along major rift faults produces higher flanks than distributed displacement
on many faults. Our model also shows that the relative elastic thickness of the crust has only a minor influence on the uplift
since fault depth and elastic thickness are not independent. In addition, we show with a second set of simulations and analytically
that a strain misfit between the upper and lower boundaries of a stretched crust, which is created by the horizontal extension,
leads to an active uplift driven by elastic forces. We compare the numerical simulations, the analytical solution and real
surface data from the Albertine rift in the East African Rift System and show that our new model can reproduce realistic features.
Our two-layer beam model with strain misfit can also explain why a thick crust in the simulations can have an even higher
rift flank than a thin crust even though the thin crust topography has a higher curvature. We discuss the implications of
our simulations for real rift systems and for the current theory of rift-flank uplift. 相似文献
9.
This study aims at showing how far pre-existing crustal weaknesses left behind by Proterozoic mobile belts, that pass around cratonic Archean shields (Tanzania Craton to the southeast and Congo Craton to the northwest), control the geometry of the Albertine Rift. Focus is laid on the development of the Lake Albert and Lake Edward/George sub-segments and between them the greatly uplifted Rwenzori Mountains, a horst block located within the rift and whose highest peak rises to >5000 m above mean sea level. In particular we study how the southward propagating Lake Albert sub-segment to the north interacts with the northward propagating Lake Edward/George sub-segment south of it, and how this interaction produces the structures and geometry observed in this section of the western branch of the East African Rift, especially within and around the Rwenzori horst. We simulate behaviour of the upper crust by conducting sandbox analogue experiments in which pre-cut rubber strips of varying overstep/overlap connected to a basal sheet and oriented oblique and/or orthogonal to the extension vector, are placed below the sand-pack. The points of connection present velocity discontinuities to localise deformation, while the rubber strips represent ductile domain affected by older mobile belts. From fault geometry of developing rift segments in plan view and section cuts, we study kinematics resulting from a given set of boundary conditions, and results are compared with the natural scenario. Three different basal model-configurations are used to simulate two parallel rifts that propagate towards each other and interact. Wider overstep (model SbR3) produces an oblique transfer zone with deep grabens (max. 7.0 km) in the adjoining segments. Smaller overlap (model SbR4) ends in offset rift segments without oblique transfer faults to join the two, and produces moderately deep grabens (max. 4.6 km). When overlap doubles the overstep (model SbR5), rifts propagate sub-orthogonal to the extension direction and form shallow valleys (max. 2.9 km). Relative ratios of overlap/overstep between rift segments dictate the kind of transition zone that develops and whether or not a block (like the Rwenzoris) is captured and rotates; hence determining the end-member geometry. Rotation direction is controlled by pre-existing fabrics. Fault orientation, fault kinematics, and block rotation (once in play) reinforce each other; and depending on the local kinematics, different parts of a captured block may rotate with variable velocities but in the same general direction. Mechanical strength anisotropy of pre-structured crust only initially centres fault nucleation and propagation parallel to the grain of weakness of the basement, but at later stages of a protracted period of crustal extension, such boundaries are locally defied. 相似文献
10.
We present an experimental investigation on the dissolution of uniaxially stressed crystals of NaClO3 in contact with brine. The crystals are immersed in a saturated fluid, stressed vertically by a piston and monitored constantly in situ with a CCD camera. The experiments are temperature-controlled and uniaxial shortening of the sample is measured with a high-resolution capacitance analyzer. Once the crystal is stressed it develops dissolution grooves on its free surface. The grooves are oriented with their long axis perpendicular to the direction of compressive stress and the initial distance between the parallel grooves is in accordance with the Asaro-Tiller-Grinfeld instability. We observe a novel, transient evolution of this roughness: The grooves on the crystal surface migrate upwards (against gravity), grow in size and the inter-groove distance increases linearly with time. During the coarsening of the pattern this switches from a one-dimensional geometry of parallel grooves to a two-dimensional geometry with horizontal and vertical grooves. At the end of the experiment one large groove travels across the crystal and the surface becomes smooth again. Uniaxial shortening of the crystal by pressure solution creep decays exponentially with time and shows no long term creep within the range of the resolution of the capacitance analyzer (accuracy of 100nm over a period of 14 days). This indicates that, while active, the fast transient processes on the free surface increase the solution concentration and thereby significantly slow down or stop pressure solution at the top of the crystal. This novel feedback mechanism can explain earlier results of cyclic pressure solution creep and demands development of a more complex theory of pressure-solution creep including processes that act on free surfaces. 相似文献