Fluid flow patterns during Pyrenean thrusting

Geochemical data for fluid circulation patterns during thrusting in the Pyr enees are reviewed. New halogen data from fluid inclusions suggests that br ines responsible for metasomatic alteration in shear zones, and expelled al ong the Gavarnie Thrust, were of evaporitic origin. These brines mixed with more dilute formation waters in a topographically driven flow system at hi gher levels. The brines probably formed in the Triassic and were also invol ved in che formation of Mesozoic Pb-Zn deposits. Dense fluids are hard to c ompletely expel from upper crustal rocks, and recycling of such fluids thro ugh several metasomatic events is probably a common process. (C) 2000 Elsev ier Science B.V. All rights reserved.