Abstract

The timing and sense of the dip-slip component of a fault's motion can be quantified by using geohistory analysis. Geohistory analysis traditionally is used to quantify tectonic subsidence or uplift of marine basins, but is newly applied to evaluate phases of activity of individual faults, including blind faults. Intermittent fault activity and changing sense of dip slip emerge, particularly for faults reactivated under changing kinematic regimes. Through the use of outcrop or core data from two fault blocks on either side of a major fault, the vertical component of displacement can be isolated. Steps of geohistory analysis include generation of (1) a rock-accumulation curve consisting of stratal thicknesses as a function of age, (2) a decompacted sediment-accumulation curve as a function of age, and (3) a total-subsidence curve with paleobathymetry inferred from fossil assemblages and sedimentary structures. Because errors on the inferred water depths are considered to be 20% of the water depth, the method is limited to large-scale faults. The analysis is repeated on both sides of the fault to determine which block subsided or was uplifted faster, thus isolating both the sense of dip slip and its relative magnitude. If both blocks display parallel subsidence 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 curves cross. If the dip direction of the fault plane is interpreted from field or subsurface data, a change—for example, from an extensional component to a contractional component—also emerges, and the timing of basin inversion is detectable. As a case study, fault activity was evaluated for a mosaic 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 contrasting water depths across a fault zone.

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