Himalaya is a mountain range that follows globally characteristics of the Coulomb prism. Upon them, the deformation is globally localized at the front of the range, and transported thrusts show few out of sequence activity. The northern western Himalayan syntax is different from the rest of the range. In this area, several ranges with different orientations interact; from east to west the Pir Panjal Range, orientated NW-SE, the Salt Range globally orientated E-W and the Sulaiman Range orientated NE-SW. The normal sequence of deformation in the syntax is not respected: an important out of sequence deformation has been measured in the Salt Range by structural studies. Moreover, Pir Panjal Range also presents out of sequence deformation as illustrated by the Mw 7.6 earthquake on 8th October 2005. It occurred on the Balakot-Bagh fault (northern extremity of the Medlicott Wadia Thrust, MWT), located more than 100 km from the front. These observations arise two questions: (i) does out of sequence deformation observed in the syntax at seismic and long term time scale exists at intermediate time scale? (ii) does the geometry of the syntax favor the observed out of sequence deformation? To address these questions we characterized the deformation in the Pir Panjal Range through a morphotectonic study. Along the Chenab River, we mapped 7 strath terraces at the hanging wall of the MWT, and dated 3 regional alluviation events using 3 methods (53 10Be samples, 6 OSL and 4 14C). These events correspond to the end of maximum monsoon phases. We measured the topography of alluvial terraces with kinematic GPS and total station and their deformation revealed that in the Pir Panjal Range deformation is localized along two structures: the frontal one that leads to a growing anticline, and a major thrust situated 20 km in internal position, called the Riasi Thrust (segment of the MWT). Most of the 16 mm/yr convergence in the area is absorbed by the Riasi Thrust which fault displacement rate is comprised between 10 and 14 mm/yr. The frontal structure absorbs the remaining part of the convergence. Also, we looked the more internal thrust called Main Boundary Thrust, and showed that this structure is not active since at least 15 kyr. In parallel, we designed a set of 24 analogic experiments using a sand box which borders geometry reproduced the external part of the northern western Himalayan syntax and changing the basal rheology and thickness of materials. These experiments showed an important shortening perpendicular to the convergence direction, and an influence of this geometry on the deformation sequence: the out of sequence deformation percentage is higher than in cylindrical prism. Our study shows that in the northern western Himalayan syntax, out of sequence deformation also exists at intermediate time scale between seismic cycle and long term time scale. It can be partly explained by the geometry of the syntax. Other mechanism such as erosion, due to intense local precipitations and structural heritage could also be considered.
