Using stress deflections to identify slip events in fault systems

A new approach for identifying past slip events in strike-slip fault systems without geological markers is presented. A 2-D distinct element modeling code is used to simulate the stress distribution around a strike-slip fault zone after a major slip event. Mature and immature fault zones are investigated. Slip on the fault is responsible for stress rotations near the active fault strand. Series of experiments reveal that the stress deflections are qualitatively the same whatever the fault strike, fault friction, and far-field differential stresses. For a left-lateral fault, stresses are rotated clockwise in the two compressive quadrants near the fault tips and counterclockwise in the two extensive quadrants. The stress field signature of a slip event thus highlights the extent of the active fault strand. Conversely, the reconstruction of paleostress fields through inversion of minor striated faults that commonly follow slip on the master fault and the analysis of their deflections allow recognition of the fault strands that slipped in a close or remote past. Application to the left-lateral Pontarlier Fault Zone in the Jura fold-and-thrust belt (France), Late Cenozoic in age, shows that the fault activity has migrated northward, in agreement with the in-sequence development of the mountain belt.