Heat focussing, granite genesis and inverted metamorphic gradients in continental collision zones

Details Heat focussing, granite genesis and inverted metamorphic gradients in continental collision zones Heat focussing, granite genesis and inverted metamorphic gradients in continental collision zones

May 1985

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985e%26psl..73..385j&link_type=abstract

Earth and Planetary Science Letters, Volume 73, Issue 2-4, p. 385-397.

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Scientific paper

The thermal conductivity of sedimentary rocks is usually much lower than that of crystalline rocks, gneisses and granites. Continental collision zones are characterized by large-scale thrusts which superpose crystalline basement and sedimentary layers. This leads to heat refraction effects of significant magnitude. Sedimentary layers act as a barrier to the transport of heat. We show that the peculiar geometry of a gently sloping sedimentary layer thrust into more conductive rocks is very efficient in diverting the vertical flux of heat. Temperatures can vary by more than 50% along the horizontal and reach a marked maximum at the top of crystalline layers. These results are in general agreement with the main features of orogenic belts as exemplified by the Himalayas. They explain why the young leucogranites of the Main Central Thrust (MCT) are confined to a narrow belt at the top of the basement slab. They also account for the inverted metamorphic gradient observed there. In a crustal accretionary wedge, folding and thrusting modify the structure of the upper crust, by thickening the sedimentary cover and by introducing sedimentary rocks at depth. This leads to high temperatures at shallow depths in specific places.
Les zones de collision continentale se caractérisent par de grands chevauchements à faible pendage qui empilent les unes sur les autres des couches de socle cristallin et des couches sédimentaires. Cette structure particulière engendre d'importants effets de réfraction thermique. En effet, leur conductibilité thermique étant normalement bien plus faible que celle des roches cristallines, les formations sédimentaires font office de barrières pour le transfert thermique: le flux de chaleur se trouve fortement dévié. Dans un plan horizontal, la température peut varier de plus de 50%, les maxima étant situés à l'interface supérieur des dalles cristallines. En ces endroits précis, les isothermes profonds remontent très significativement. Ces résultats rendent bien compte de plusieurs faits essentiels observés dans l'Himalaya: zonéographie inverse du métamorphisme, localisation des leucogranites le long d'une bande étroite située au sommet de la dalle du Tibet.

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