A new methodology for paleostress reconstruction using theory, field observations and petrophysical data

The measurement of crustal stress magnitude is always challenging and generally poorly constrained. This is particularly significant in active fault zones where the knowledge of stress magnitude is crucial for understanding fault mechanics during earthquakes nucleation. In this work we propose a workflow using laboratory and field data as a proxy for quantitative paleostress reconstruction along active fault zone. We studied the exhumed Olevano-Antrodoco Thrust Fault (OATF) in Central Italy consisting of a SW-dipping thrust fault that juxtapose middle Miocene carbonates in the hangingwall above upper Miocene foredeep sandstones, W-SW-dipping, in the footwall. We collected 26 samples of footwall sandstones approaching progressively the OATF, from the undeformed deposits (1 km away to the E) to the tectonically deformed sandstones close (50 m far) to the OATF. Field data highlighted that the footwall sandstones dips towards W-SW, thus moving towards the OATF, shallower strata progressively crop out, hence from the stratigraphical point of view, porosity should increase due to the decreasing in burial depth. On the other hand laboratory measurements revealed the opposite. Using a permeameter we measured porosity, permeability, and P wave velocity both at ambient pressure and at increasing confining pressure up to 100 MPa, simulating an increase in burial depth up to 4 km. Porosity measured at ambient pressure decreases moving towards the OATF as well as permeability, whilst P wave velocity increased. P wave velocities obtained during depressurization from 100 MPa to ambient pressure were always higher than those recorded during pressurization suggesting inelastic compaction. In order to reconstruct the paleostresses we started from the Athy's exponential porosity-depth relationship. We calculate the initial porosity at the time of deposition for undeformed sandstones 1 km away from OATF (11.1%) Using stratigraphic and geometrical relationships we calculated that the maximum burial depth of sandstones close to the OATF was about 1500 m. We then calculated that the porosity of sandstones close to the OATF related only to sedimentary load was about 7.4 %. This value is higher than the present-day porosity that is 3.7%. The difference (Δϕ = 3.7%, equal to inferred porosity minus measured porosity) is thought to be caused by the tectonic load and inelastic compaction associated with the activity of the OATF that changed permanently the petrophysical properties inherited from sedimentation and diagenesis as confirmed by laboratory measurements. The stress needed to reduce porosity from the theoretical value of 7.4% to the measured value of 3.7% at 1500 m depth, is 64.8 MPa. This value represents the maximum differential stress (Δσ) that acted close to the fault plane (tectonic load). Since field data indicated a compressional regime; this implies that the horizontal stress is σ1 and the vertical stress is σ3. By using the density-depth relationship, it resulted that, close to the OATF at a depth of 1500 m, σ3=37.7 MPa. Consequently, σ1, calculated as σ1=(σ3+Δσ), is 102.5 MPa. Assuming a coefficient of friction for sandstones of 0.71 and overburden-related inelastic compaction in the proximity of the fault plane, it results that the so calculated stresses are exactly the stress needed to reach critical conditions for slip. Since the OATF has more than 500 m of displacement, critical conditions for slip should have been maintained for long time; this strengthens our methodology that can thus be potentially applied for other tectonically deformed zones.