The Dutung-Thaktote extensional fault zone and nappe structures documentedby illite crystallinity and clay-mineral paragenesis in the Tethys Himalaya between Spiti river and Tso Morari, NW India

Numerous measurements by XRD of the Scherrer width at half-peak height (001 reflection of illite), coupled with analyses of clay-size assemblages, pro vide evidence for strong variations in the conditions of low temperature me tamorphism in the Tethyan Himalaya metasediments between the Spiti river an d the Tso Morari. Three sectors can be distinguished along the Spiti river-Tso Morari transec t. In the SW, the Takling and Parang La area is characterised by a metamorp hism around anchizone-epizone boundary conditions. Further north, in the Du tung area, the metamorphic grade abruptly decreases to weak diagenesis, wit h the presence of mixed-layered clay phases. At the end of the profile towa rds the NE, a progressive metamorphic increase up to greenschist facies is recorded, marked by the appearance of biotite and chloritoid. The combination of these data with the structural. observations permits to propose that a nappe stack has been crosscut by the younger Dutung-Thaktote extensional fault zone (DTFZ). The change in metamorphism across this zone helps to assess the displacements which occurred during synorogenic extens ion. In the SW and NE parts of the studied transect, a burial of 12 km has been estimated, assuming a geothermal gradient of 25 degrees C/km. In the S W part, this burial is due to the juxtaposition of the Shikar Beh and Mata nappes and in the NE part, solely to burial beneath the Mata nappe. In the central part of the profile, the effect of the DTFZ is to bring down diagen etic sediments in-between the two aforesaid metamorphic zones. The offset a long the Dutung-Thaktote normal faults is estimated at 16 km.