Quantifying the timing and sense of fault dip slip: New application of biostratigraphy and geohistory analysis

The timing and sense of the dip-slip component of a fault's motion can be q uantified by using geohistory analysis. Geohistory analysis traditionally i s used to quantify tectonic subsidence or uplift of marine basins, but is n ewly applied to evaluate phases of activity of individual faults, including blind faults. Intermittent fault activity and changing sense of dip slip e merge, particularly for faults reactivated under changing kinematic regimes . Through the use of outcrop or core data from two fault blocks on either s ide of a major fault, the vertical component of displacement can be isolate d. Steps of geohistory analysis include generation of (1) a rock-accumulati on curve consisting of stratal thicknesses as a function of age, (2) a deco mpacted sediment-accumulation curve as a function of age, and (3) a total-s ubsidence curve with paleobathymetry inferred from fossil assemblages and s edimentary structures. Because errors on the inferred water depths are cons idered to be 20% of the water depth, the method is limited to large-scale f aults. The analysis is repeated on both sides of the fault to determine whi ch block subsided or was uplifted faster, thus isolating both the sense of dip slip and its relative magnitude. If both blocks display parallel subsid ence curves within the error margins, a period of fault inactivity or pure strike slip is inferred. If the sense of dip slip changed, the subsidence c urves cross. If the dip direction of the fault plane is interpreted from fi eld or subsurface data, a change-for example, from an extensional component to a contractional component-also emerges, and the timing of basin inversi on is detectable. As a case study, fault activity was evaluated for a mosai c of faults cutting the Messaras basin of Crete, Greece, a Neogene forearc basin that underwent extension followed by transpression, This approach is applicable to neotectonics or ancient tectonics and pertains to subsurface or exhumed faults as well as active faults offshore that generate contrasti ng water depths across a fault zone.