CALCULATING RATES OF SYNDEPOSITIONAL NORMAL FAULTING IN THE WESTERN MARGIN OF THE MESOZOIC SUB-ALPINE BASIN (SOUTH-EAST FRANCE)

A method is developed to quantify the rate of fault movement, with a v ery fine time-resolution, so that relevant histories of fault movement s can be obtained. The study subject is a Triassic-Jurassic syndeposit ional normal fault located at the margin of an intracratonic deep basi n, the Subalpine basin of south-eastern France. The fault has recently been identified and specifically investigated by a seismic survey alo ng with drilling (Geologie profonde de la France Program). The investi gation is based on correlation of time-lines on both sides of the stru cture through a period of about 70 Myr. Correlations have been made us ing variable approaches depending on the stratigraphic interval: recog nition of laterally continuous marker-beds, biostratigraphic informati on and application of genetic stratigraphy concepts. In the case of bi ostratigraphic data, absolute ages are assigned to selected lines of c orrelation to determine time lengths and calculate velocities of fault movements. A specific backstripping procedure is established. The dif ferential subsidence history between the two sites is restored not as a simple subtraction made after conventional backstripping on each sit e but as the sum of discrete differential subsidence increments calcul ated for each chronostratigraphic interval. The originality of the wor k lies in the completion of the supporting data base, implementation o f high-resolution correlations within the large time-span of the inves tigation and development of a method to calculate the differential sub sidence. Even though unassessable errors and uncertainties are still a ssociated with the stratigraphic correlations, the backstripping proce dure and the chronological control, the overall method offers a certai n validity because the calculated and the observed differential subsid ences are close. Beyond the obvious control on depositional geometries and localization of some reservoirs at the toe of the fault, the kine tic regime of the normal fault played an indirect but crucial part in the differential burial-related alteration of the reservoirs recorded on both sides of the fault. The high accuracy of the calculation has r evealed that: (1) the growth pattern of the fault does not result from a continuous thermomechanical process but is composed of a series of rifting and sliding events related to gravity-driven extension; (2) th e spectacular differential stratigraphic record on both sides of the f ault is associated with fairly low values of the fault growth rate (ma ximum of 165 m Myr(-1)). The method for measuring the growth of struct ures can be applied to any tectonic and sedimentary environment and of fers a wide range of applications.