Showing papers by "Paul Ryan published in 1993"

Orogenic uplift and collapse, crustal thickness, fabrics and metamorphic phase changes: the role of eclogites

Abstract: Coesite-bearing eclogites in several deep crustal metamorphic assemblages now exposed in extensionally-collapsed orogens indicate the tectonic denudation of more than 90km of crustal rocks and pre-collapsed crustal thicknesses of at least 120km. For mountain ranges and orogenic plateaux up to 5 km in elevation and average crustal densities of about 2.8, crustal thickness cannot exceed about 80 km unless pre-shortening crustal/ lithosphere thickness ratios were less than 0.135 or some way can be found to preferentially thicken the lithospheric mantle. This problem can be avoided and very thick orogenic crusts built up if granulite facies rocks transform to denser eclogite facies during shortening, where the petrographic Moho is continuously depressed below a density/seismic velocity Moho buffered at about 70 km and mountains at about 3 km. Advective thinning of the lithosphere combined with the resultant heating and eclogite to sillimanite-granulite/amphibolite transformation causes surface uplift of about 2 km, a rapid change in isostatic compensation level, and a switch from a shortening to an extensional/collapse regime. We have developed a simple numerical model based upon field observations in southwestern Norway in which coherent regional-scale transformation of lower crustal rocks to eclogite facies during lithospheric shortening is followed by heating, transformation of eclogite to amphibolite and granulite, extension, and crustal thinning by coaxial then non-coaxial mechanisms. The model also explains strong lower crustal layering (eclogite and other lenses in horizontallyextended amphibolites), regionally horizontal gneissic fabrics, rapid return from orogenic to 'normal' crustal thickness with minor erosion, the lateral and vertical juxtaposition of low-grade and high-grade rocks and rapid marine transgression shortly after orogeny. Young orogens have maximum regional average elevations of about 5 km; their crustal thicknesses, determined directly from seismic reflection/refraction studies or indirectly from gravity anomalies, do not exceed about 70 km (Meissner 1986). The pre-plate tectonic view was that the surface elevation (e) of a mountain belt in perfect isostatic balance is related to its compensating root (r) and crustal thickness (Cz) by the relative densities of crust (Pc) and mantle (Pro), the floating iceberg principle in which the mantle was considered to behave as a fluid (Stokes 1849; Airy 1855; Heiskanen & Vening Meinesz 1958). The plate tectonic view is that the crust forms part of the lithospheric boundary conduction layer (thickness lz) with a mantle density of Pm" The whole l z column of crust and mantle is compensated to the asthenospheric mantle of density Pa" The crust 'floats', whereas the mantle portion 'sinks', the lithosphere; therefore, the level at which the surface sits relative to the oceanic ridges depends upon l~ and CJl~. Also, the cooler upper portion of the boundary layer acts as a strong flexural beam to support loads, which allows a departure from pure zerostrength Airy isostasy and the support of loads and elevations higher than those for a pure Airy model and the development and maintenance of substantial positive gravity anomalies. The elevation of a mountain range is, therefore, a function of the vertical density distribution within and the flexural strength of the lithosphere, although, for most mountain belts, there is insufficient knowledge of these parameters yet to draw conclusions that clearly relate crustal thickness to elevation. In this paper, we wish to draw attention to some general principles of vertical density distribution in relation to elevation and crustal/lithospheric thickness that may be important in understanding orogenic evolution, although we emphasize that crust/lithospheric thickness beneath mountain ranges is poorly constrained. Assuming homogenous bulk strain and preshortening crustal (Cz) and lithospheric (lz) values of 32 km and 120 km respectively, crustal thickening, caused by vertical stretching, is buffered at about 70 km beneath a surface elevation of about 3 km by vertical compression caused by isostatic compensation in a thickened crust that From Prichard, H. M., Alabaster, T., Harris, N. B. W. & Neary, C. R. (eds), 1993, Magmatic Processes and Plate Tectonics, Geological Society Special Publication No. 76, 325-343. 325 326 J .F . DEWEY E T A L . balances horizontal compression caused by plate convergence (Dewey 1988; England & Houseman 1988, 1989). Elevations above 3 km may be achieved by crustal underplating or by mechanisms that thin the lithosphere without thinning the crust such as hot-spot jetting, delamination, or rapid advective thinning. The general picture that has emerged for the wide Tibetan Himalayan zone of crustal thickening is one of Palaeogene vertical plane strain crustal thickening to about 65 km, caused by the India/Eurasia convergence, succeeded by a phase of horizontal plane strain followed, in turn, by a phase of post-Miocene uplift to the present 5 km average elevation accompanied and followed by lithospheric extension and magmatism (Dewey et al. 1988), a phase of so-called orogenic extensional collapse (Dewey 1988). Many Phanerozoic orogens appear to follow this general pattern of shortening followed by extensional collapse (Dewey 1988), which is related probably to lithospheric shortening followed by the rapid advective removal of the lithospheric mantle root. (Houseman et al. 1981; England & Houseman 1988, 1989). During shortening by roughly vertical plane strain and vertical bulk stretching, buoyant crustal and negatively buoyant lithospheric mantle roots develop, rocks are progressively buried and geothermal gradient and heat flow are reduced. During shortening, the principal axis of compression is horizontal and the axis of least compression vertical. Crustal thickness is buffered at about 70 km, and the intermediate axis of compression becomes vertical, a wrench regime is developed and shortening must spread laterally if convergence continues, one possible explanation of the lateral progradation of thrust sheet complexes. Advective removal of the mantle root leads to uplift to about 5 km, a rapid increase in geothermal gradient and vertical shortening/extensional collapse in a stress regime of vertical compression. There is sufficient seismic data to indicate that the Moho, as defined by that data, does not exceed depths of about 70 km beneath Cenozoic mountain ranges (Meissner 1986). If 70 km is the limit to which normal density (2.8) continental crust may be thickened by bulk vertical stretching, there are substantial implications for the structural and metamorphic history of orogenic belts. First, gross orogenic shortening values achieved by bulk vertical plane strain expressed by orogenic structure and fabrics cannot exceed about 50% if we start with a 'normal' thickness continental crust of about 35km. Many orogenic belts have shortening values, at least locally, greatly in excess of 50%. Of course, shortening may be increased in several ways such as by substantial erosion during shortening, and/or by starting with a thin crust, which would be expected at a colliding rifted margin. Both of these have operated in the Himalayas; some 20 km has been lost over large areas by erosion and some of the highest thrust sheets probably had a thin starting crust, although these factors alone are insufficient to account for Himalayan shortening. Horizontal plane strain by lateral escape tectonics will also allow increased shortening values without increasing C z although it is likely to generate steep structures and cannot generate thrust regime structures and fabrics. A C z limit of 70 km places even greater constraints on crustal metamorphism. Rocks now at the surface in older orogens have up to 35 km of continental crust below them so that, if orogenic thickness was 70 km, erosional denudation can have removed a maximum of only 35 km and extensional collapse reduces this figure still further. Tectonic denudation mechanisms such as motion towards the surface in the footwalls of major normal faults above zones of 'replacive' lower crustal flow may allow deeper crustal levels to be exposed but we are still limited by a C z maximum of 70 km. Therefore, we would expect the regional exposure of metamorphic rocks of greenschist/amphibolite granulite facies recording pressures of around 10-12kb with localized zones of blueschist/eclogite facies indicating maximum pressures of about 20 kb. The rocks of the Tauern window in Austria experienced peak pressures of 20 kb and yet appear to be part of a regionally-coherent metamorphic terrain now at the surface of a 50km crust suggesting a peak Cz of 100 km. In the western Alps, Chopin (1984, 1987) and Chopin et al. (1991) have described coesite/pyrope-bearing rocks and, in the Dabie Shan, Enami & Zhang (1990), Hirajima et al. (1990), Okay & Seng6r (1992), Wang & Liou (1991), Wang et al. (1989, 1992) and Xu et al. (1992) have recorded and described diamondand coesite-bearing eclogites indicating pressures of 39-40 kb. In southwestern Norway, coesite-bearing eclogites form part of the regionally-coherent Western Gneiss Region (Smith 1984; Austrheim 1987, 1991; Smith & Lappin, 1989; Andersen & Jamtveit 1990; Andersen et al. 1991), and experienced minimum peak pressures of 28 kb (Fig. 1) and have 35 km of subjacent crust indicating a peak C~ of at least 120 kin. We are, therefore, forced into the ~pparent paradox that observation and theory indicate that C~ cannot exceed about 70 km and yet metamorphic rocks indicate C~ values of at least 120 km. The problem is how to get rocks down OROGENIC UPLIFT & COLLAPSE: THE ROLE OF ECLOGITES 327

International Comparisons of Vocational Education and Training for Intermediate Skills

Abstract: Papers from a seminar held at the University of Manchester in September 1989. These papers document various aspects of the inadequacy in current British practice, concentrating on intermediate skills. The book includes: an outline of the strengths and weaknesses of comparative research; discussion of the use made of it by policy makers in Britain with practical comparisons; vocational preparation in connection with productivity; studies of the organisation of skills and work and the finance of training in the EEC as a whole; training in relation to labour market structures.