Abstract

Predicting present-day pore pressure and 3D in situ stress in ultra-deep fold-thrust belts is severely hindered by the inadequacies of traditional 1D vertical compaction models, which fail to account for massive lateral tectonic compression and continuous elastoplastic yielding. To overcome this, a 3D poro-elastoplastic analytical framework is developed based on the Modified Cam-Clay model to decode the irreversible “geomechanical memory” of deeply buried argillaceous rocks. Applied to the highly compressed Kelasu Thrust Belt, this method links volumetric strain with mean and deviatoric stresses in stress-invariant space to reconstruct the maximum paleo-pore pressure and 3D paleo-stress tensor during the Coulomb Failure Period (CFP). The quantitative decoupling reveals an extreme state of geopressure prior to macroscopic faulting (pore pressure ratio α = 0.85–0.89). Crucially, the mean stress surge is identified as the dominant driver, generating ~91% of the excess overpressure. Consequently, horizontal tectonic compression accounts for 80–90% of the total overpressure anomaly, fundamentally overturning the classical assumption that vertical undercompaction (10–20%) is the primary mechanism. Furthermore, it is demonstrated that during subsequent tectonic uplift, the heavily compacted, salt-capped mudstones follow an undrained unloading path; the reduction in lithostatic burden is almost entirely offset by fluid depressurization, maintaining a constant effective stress state. This physically decoupled framework provides a rigorous basis for optimizing pre-drill safe mud-weight windows, designing hydraulic fracturing in highly deviatoric stress regimes, and assessing caprock integrity for deep geo-energy storage.

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