Active deformation in the inner western Alps inferred from comparison between 1972-classical and 1996-GPS geodetic surveys

Eighteen geodetic points surveyed in 1972 by the French National Geographic Institute (IGN) were remeasured by GPS in 1996 in the Brianconnais and Pie mont Zones, east of the Pelvoux massif (French Western Alps). A displacemen t vector set was determined for the two surveys' common points. Calculation s of the strain-rate tensors associated with 15 triangular cells of the net work have been performed. Only four of them show a strain rate significant at a 95% level of confidence. These data suggest an E-W extension of about 2-4 mm/yr between the western and eastern part of the network (Pelvoux exte rnal crystalline massif and Queyras blueschists, respectively) associated w ith N-S shortening. This active deformation agrees with neotectonic and sei smotectonic data. The measured tectonic motion seems to be distributed thro ughout the central part of the Brianconnais zone, where the seismic activit y is concentrated. The local seismicity has been precisely surveyed since 1 989. It is moderate (M-1<4.7) and no larger earthquake occured in the 1972- 1989 period. The seismic deformation of the 1972-1996 period, extrapolated from the 1989-1996 local seismicity, accounts for less than 10% of the geod etic deformation. Thus, aseismic processes accommodated more than 90% of th e observed deformation during this period. This could correspond to accumul ation of elastic strain on locked faults, creep on faults or plastic deform ation in a large crustal volume. (C) 2000 Elsevier Science B.V. All rights reserved.