Fluctuation in the fluid and tectonic pressures in the South Almora Thrust Zone (SATZ), Kumaun Lesser Himalaya; paleoseismic implications

Abstract

This study uses veins, striated faults, and fluid inclusions (FIs) for deciphering the tectonic stresses and the role of fluids in the evolution of the South Almora Thrust Zone (SATZ). The paleostress analysis implies that the SATZ bears a record of four different stress states. The syntectonic fluids preserved in the fluid inclusions in the quartz veins are predominantly aqueous-carbonic saline with a minor amount of methane detected in Raman spectroscopy. FIs showing the variable and uniform degrees of fill were entrapped as the immiscible and miscible aqueous-carbonic fluids, respectively. Microthermometry on FIs implies a considerable fluctuation in fluid pressure during the evolution of the SATZ. The absolute paleostress magnitude, estimated by the 3-D Mohr circle reconstruction, substantiates fluctuation in the ambient stress, indicating the role of tectonic stress. The implosion and explosion textures in fluid inclusions, fluid pressure fluctuations, and tectonic stress fluctuations are attributed to multiple seismic events that occurred at a depth of 7–13 km during the Late Oligocene-Early Miocene (21–25 Ma).