This is the first volume in this series dealing with a petrological subject and contains the contributions of the lectures given at the 5th School of the European Mineralogical Union (EMU) on "Ultrahigh Pressure Metamorphism" held in Budapest from 21 to 25 July 2003. The topic of UHPM was selected because this extreme type of metamorphism, initially considered as a petrographic oddity by the geologic community, has now become recognised as a normal feature of continental plate collisional orogens and important to understanding just how deep the upper part of the continental lithosphere can subduct. We note that this School took place just twenty years from the first report by Christian Chopin of coesite in exposed orogenic metamorphic rocks of the continental crust. The lectures given at this school benefited by the scientific results of the research promoted by the ILP Task Groups III-6 and III-8, active on UHPM from 1994 to 1998 and 1999 to 2004, respectively, and published in a number of monographs and special issues of international journals. It is our strong belief that this petrologic topic should be recognised to be of paramount importance in the education of students and young researchers in Earth Science.
We present a series of three‐dimensional numerical models investigating the effects of metamorphic strengthening and weakening on the geodynamic evolution of convergent orogens that are constrained by observations from an exposed mid‐crustal section in the New England Appalachians. The natural mid‐crustal section records evidence for spatially and temporally variable mid‐crustal strength as a function of metamorphic grade during prograde polymetamorphism. Our models address changes in strain rate partitioning and topographic uplift as a function of strengthening/weakening in the middle crust, as well as the resultant changes in deformation kinematics and potential exhumation patterns of high‐grade metamorphic rock. Results suggest that strengthening leads to strain rate partitioning around the zone and suppressed topographic uplift rates whereas weakening leads to strain rate partitioning into the zone and enhanced topographic uplift rates. Deformation kinematics recorded in the orogen are also affected by strengthening/weakening, with complete reversals in shear sense occurring as a function of strengthening/weakening without changes in plate boundary kinematics.
Erosion rates in the hanging wall of the Alpine Fault are high, keeping pace with rock uplift over time frames of 104–106 years. On shorter time frames, prediction of temporal and spatial distribution of erosion is challenging and must account for local conditions and parameters including rock strength, topographic stresses and failure conditions. Constrained by field observations of rock strength, we use 3D mechanical models to predict where and by what mechanism slope failure and erosion are likely to take place along the Waikukupa section of the Alpine Fault. Shear failure is favoured along the base of slopes and where pore pressure is high. Tensile failure is favoured along ridges, higher on slopes and when pore pressure is moderate. A dry material with a high degree of rock strength heterogeneity promotes bedrock gully development, whereas distributed failure is more likely to occur when the material is saturated.
We reconstructed the former ice cap of the Wind River Range, Wyoming, using a glaciological model with scaled modern temperature and precipitation inputs to examine probable climate during the local Last Glacial Maximum (LGM) (or Pinedale glaciation). A key result is that temperature anomalies of - 10 °C, -8.5 °C, -6.5 °C, and -5 °C must compensate respective precipitation values of 50%, 100%, 200%, and 300% that of modern in order for the maximum glacier system to attain equilibrium. In further sensitivity tests, we find that ice-cap area and volume shrink by 75% under a climate forcing 50% modern and 50% LGM. The glacier system disappears altogether in ∼100 years when subjected to sustained modern conditions. Our results are consistent with other interpretations of western U.S. LGM climate, and demonstrate that the Wind River Ice Cap could have disintegrated rapidly during the first phase of the termination. In future work we will simulate glacier-climate evolution as constrained by emerging 10Be moraine chronologies.
Is erosion important to the structural and petrological evolution of mountain belts?The nature of active metamorphic massifs colocated with deep gorges in the syntaxes at each end of the Himalayan range, together with the magnitude of erosional fluxes that occur in these regions, leads us to concur with suggestions that erosion plays an integral role in collisional dynamics.At multiple scales, erosion exerts an influence on a par with such fundamental phenomena as crustal thickening and extensional collapse.Erosion can mediate the development and distribution of both deformation and metamorphic facies, accommodate crustal convergence, and locally instigate high-grade metamorphism and melting.
This paper examines the development of a subvolcanic magmatic breccia located along the contact of a granitic intrusion using fractal analysis and thermal‐elastic modeling. The breccia grades from clast‐supported, angular clasts adjacent to unfractured host rock to isolated, rounded clasts supported by the granitic matrix adjacent to the intrusion. Field observations point to an explosive breccia mechanism, and clast size distribution analysis yields fractal dimensions (D s > 3) that exceed the minimum value known to result from explosion (D s > 2.5). Field observations, clast size distribution data, clast circularity data, and boundary roughness fractal dimension data suggest that the clast sizes and shapes reflect post‐brecciation modification by partial melting and thermal fracture. Numerical modeling is employed to explore the possible thermal‐elastic effects on the size distribution of clasts. Instantaneous immersion is assumed for metasedimentary clasts of a fractal size distribution in a superheated granitic matrix for different matrix volume percentages. Thermal analysis is restricted to conductive heat transfer corrected for latent heat. Partial melting of metasedimentary clasts is an effective secondary modification process that was probably responsible for markedly altering the clast size distribution for clast populations adjacent to the intrusion. Diabase clasts experienced late‐stage fracture due to the instantaneous thermal pulse in which angular clasts with high surface area to volume ratios were preferentially fractured, although this process does not appear to have markedly influenced the clast size distribution.
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