Brittle Strain Localization and Thrust Fault Geometry Within Thrust-Cored Folds: Causative Principal Stresses and Insights From Coulomb Stress Change

Details Brittle Strain Localization and Thrust Fault Geometry Within Thrust-Cored Folds: Causative Principal Stresses and Insights From Coulomb Stress Change Brittle Strain Localization and Thrust Fault Geometry Within Thrust-Cored Folds: Causative Principal Stresses and Insights From Coulomb Stress Change

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.t51a0851o&link_type=abstract

American Geophysical Union, Fall Meeting 2001, abstract #T51A-0851

Physics

1242 Seismic Deformations (7205), 5475 Tectonics (8149), 8005 Folds And Folding, 8020 Mechanics, 8164 Stresses: Crust And Lithosphere

Scientific paper

Arrays of thrust faults, with intricate geometries of synthetic and antithetic thrusts of various inclinations, underlie the structure and topography of thrust-cored folds. Coulomb stress changes around a single slipped thrust fault can induce brittle strain localization within the surrounding fold and initiate formation of an array of secondary slip surfaces around the primary fault. We relate magnitudes and ratios of causative principal stresses to the geometry of the resulting thrust fault array by using numerical models of Coulomb stress changes around a slipped blind thrust fault. Our analysis assumes that faults are localized in areas of enhanced Coulomb stress. Coulomb slip is evaluated for a range of friction coefficients from 1 to 0, which correlates to fault dips ranging from 45 degrees to 22.5 degrees. The magnitude of the principal stresses required for Coulomb slip is determined by evaluating the Coulomb failure criterion in principal stress form (assuming S1 is horizontal). Fault displacement due to Coulomb slip is determined by first resolving the critical principal stresses onto the fault plane. The resulting driving stress is then related to displacement by assuming elastic behavior for the wall rock, strong end zones bounding the fault tips, and maximum displacement at the center of the fault, tapering to zero at the tips with an elliptical slip distribution. A ratio of the maximum fault displacement to the down-dip fault length (Dmax/L) yields the value of critical shear strain that is required for thrust faults to form through Coulomb slip. Representative values of Dmax/L calculated here for a 30° dipping thrust fault on the Earth are 0.001, 0.0004 on Mars, and 0.0002 on the moon. We use the COULOMB dislocation software to map the post-slip Coulomb stress changes that are induced around active blind thrust faults under these conditions. Our results show systematic, dip-dependent variations in the extent and magnitude of Coulomb stress localizations around a thrust fault. A prominent lobe of positive Coulomb stress change is predicted within the hanging wall of a thrust fault dipping at 22.5 degrees. The magnitude of this stress change is sufficient to predict Coulomb slip along additional failure surfaces (secondary thrust faults). At progressively steeper dips, this lobe of hanging wall deformation migrates toward the upper tip of the fault and significantly reduces in extent and magnitude. At a fault dip of 45 degrees, the hanging wall is dominated by negative Coulomb stress change (faulting in the hanging wall is inhibited). Thus, the locations of secondary synthetic and antithetic thrust faults within a thrust-cored fold correlate with the dip of the primary fault. The Coulomb failure criterion shows that the dip of the primary fault is a function of the principal stress ratio. Therefore, the orientations of the secondary synthetic and antithetic thrust faults, as well as the primary fault, strongly depend on the principal stress ratio at the time of faulting. These results are consistent with field observations of thrust fault geometries within fault-cored folds. Furthermore, this analysis shows that the Coulomb failure criteria can be used to ascertain the horizontal driving stresses, and resulting strains, for mapped thrust faults that formed under known vertical loading conditions (e.g. lithostatic load).

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